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Fluorescent Dyes Used in Polymer Carriers as Imaging Agents in Anticancer Therapy | OMICS International
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Medicinal Chemistry

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Fluorescent Dyes Used in Polymer Carriers as Imaging Agents in Anticancer Therapy

Beata Miksa*

Centre of Molecular and Macromolecular Studies, Polish Academy of Science, Lodz Sienkiewicza, Poland

*Corresponding Author:
Beata Miksa
Centre of Molecular and Macromolecular Studies
Polish Academy of Science, Lodz Sienkiewicza 112, 90-363, Poland
Tel: +48426815832
E-mail: [email protected]

Received date: October 22, 2016; Accepted date: November 12, 2016; Published date: November 15, 2016

Citation: Miksa B (2016) Fluorescent Dyes Used in Polymer Carriers as Imaging Agents in Anticancer Therapy. Med Chem (Los Angeles) 6:611-639. doi:10.4172/2161-0444.1000406

Copyright: © 2016 Miksa B. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

This review highlights the role of fluorescent dyes as active “molecular photoswiches” focusing on the application in bioimaging and anticancer therapies in the field of therapeutic delivery. The author describes the development of prodrugs targeted to specific cell types and polymeric nanocarriers (capsules, micelles and silica nanoparticles) used as fluorescent probes which offer advantages in the integration of “smart” features of fluorescent dyes into synthetic materials. The incorporation of biologically responsive components that cleave upon changes in pH or light irradiation is fundamental to the successful design of such carriers with capabilities to load and release therapeutics specifically at a target site.

Keywords

Fluorescent; Polymer; Nanocarriers; Theranosticscapsules; Silica dots; NIR dyes

Introduction

Optical fluorescence technology has been widely applied in recent biological analysis and bioimaging, owing to its high sensitivity, high resolution, and the availability of simple and rapid noninvasive analytical systems. Along with advances in fluorescence detection which is the basic method for direct visualization of dynamic protein interactions and material transport in living cells, live-cell imaging has become an essential tool for the study of changes in cell morphology. Moreover, fluorescence imaging methods are currently being developed to image specific molecular targets in vivo, thus fluorescent labeling is a central tool for studying localization, trafficking, and expression levels of biomolecules in live cells. So dyes can be attached to targeting molecules to make fluorescent probes due to a specific interaction between carriers and cell or tissue components. These probes have been recently used in active targeting to image receptors, antigens, antibodies, DNA and enzymes [1]. Despite the fact that dyes as imaging agents offer advantages in screening by enabling visual detection of the probe-target interaction, an ideal labeling reagent would remain nonfluorescent until bound to its target. The conjugates of fluorescent chromophores with anticancer therapeutics can be used also as prodrugs and nanoprobes for application them towards diagnosis and therapy in live cells. The different dyes including derivatives of fluorescence scaffolds (e.g., rosamine, BODIPY, chalcone, and xanthone) that cover a very wide range of near-infrared (NIR) and visible fluorescence spectra (400-800 nm) can provide microscopic details of biological molecules in vivo and in vitro [2]. Moreover, NIR light can penetrate skin and tissue to a few millimeters, enabling fluorescence detection in dermatological or in vivo diagnostic devices [3,4]. Therefore, multiphotochromic molecular systems should be used to build highly functional materials due to their potential multi-addressability and/or multi-response properties [5]. This review focuses on the progress made in developing fluorescent polymer capsules and the dye-doped silica nanoparticles (SiNPs) with tunable properties such as size, surface chemistry composition, morphology and functionality in respect of imaging and cellular detection, which is crucial in targeted drug delivery for biomedical and diagnostic applications [6,7]. Additionally, this review is designed to give an overview of passive targeting with fluorescent prodrugs that either works as active targeting systems. Furthermore, polymer nanocapsules [8,9] due to the polymer molecular weight or size have come to refer to the accumulation of drug by using physicochemical or pharmacological factors [1]. The integration of “smart” features i.e., chemically responsive components (which cleave upon changes in pH or redox potential) into synthetic materials is fundamental to the successful design of such carriers with capabilities to load and release therapeutics specifically at a target site. Thus, therapeutic molecules can be sequestered and/or released in a controlled manner from polymeric carries that labeled with pH- or light sensitive dyes which may regulate a process of diffusion. Polymer dye-doped capsules with compartmentalized, water-filled cavities and the synthetic amphiphilic shell have tremendous potential applications in nanomedicine because enable to create biomimetic nano- or multi-compartment reactors and this system could function also as a biomolecular analysis tool. The general methods to produce polymer capsules are described in literature [10,11].

Fluorescent small molecules as cell-type-specific imaging probes accomplished active targeting functions

Fluorescent dyes applied in biological labeling have many advantages, such as fast detection speed, good repetitiveness, low dosage, and non-radiation [12]. Molecules of dyes can label compounds by a covalent bonding using an active group (-SH, -COOH, -NH2) or might be inserted into the double-helix structure of DNA by affinity, as in the case of thiazole orange, oxazole yellow and their dimers [13,14]. Antibodies (their size is about 12 nm) endowed with fluorescent dyes (e.g., Alexa Fluor 488 λEm=525 nm and Texas Red λEm=609 nm) [15- 17] were used in the development of a wide range of effective targeted therapies [18-22] but they create clustering artifacts due to limited antibody penetration, which have an impact on insufficient labeling density. Thus, ligands that have been exploited for chemotherapeutic agent targeting systems can include monoclonal antibodies [23-28] but also low molecular weight receptor-binding conjugates such as dyelabeled peptides (arginine-glycine-aspartic acid-Cyt5,5) [29], peptide hormones [30,31], receptor antagonist and agonists [32], aptamers [33-35], transferrin [36-37], oligosaccharides [38], glycoconiugates [39], polyunsaturated fatty acids [40], oligopeptides [41,42], vitamin B12 [43], folic acid [44-54] and hyaluronic acid [55,56]. These ligands can be regarded as a tumor-specific receptor to construct a “guided molecular missile” with different therapeutic warheads. In an effort to contribute to the development of optical imaging modalities, a variety of fluorescent dyes e.g., fluorescein, Texas Red, the rhodamines, the cyanines (indocyanine green, thiazole orange) [57-59], polymethine cyanine dyes (the Cy3, Cy5, Cy5.5, Cy7 series) and the Alexa Fluors have been conjugated to folic acid [60-64]. Nowadays, the development of fluorescent dyes with longer excitation wavelengths (λEx>500 nm) transparent to human tissues [65,66] shows great promise, because imaging of malignant cells by optical means is not hazardous for living organisms [67]. Moreover, with improved optical sensors that can excite fluorophores deep in a tumor mass under low light intensities but with the higher fluorescence quantum yields detection [68] the problems associated with tissue autofluorescence and tissue absorption/scattering of light [69] might be neglected. Another advantage of ligand-targeted therapies is the ability to design a cognate imaging agent with the same targeting ligand [70,71]. Furthermore, ligand (2-[3-(1,3-dicarboxypropyl)ureido]pentanedioic acid) (DUPA) can selectively bind to a cell surface glycoprotein and it is known as prostate-specific membrane antigen [72,73]. The efficiency of drug release from respective DUPA-conjugates with chemotherapeutics was assessed by treating the conjugate with either glutathione (GSH, tripeptide) or dithiothreitol (DTT) to reduce the disulfide bonds. In addition, whenever membrane impermeable drugs must be delivered, ligand-targeted therapies are generally preferred, since a good targeting ligand will often carry its attached cargo into the target cell by receptormediated endocytosis.

Photosensitizers as selective therapeutic and imaging agents

The spectral work outside the range of autofluorescence of cell and tissue (350-700 nm) is the main advantage of using fluorophores with longer excitation and emission wavelengths for labeling of the biomolecules [74]. Near-infrared fluorescence (NIR, region 700- 900 nm) is expected to have a major impact on biomedical imaging of specific targeting. In the NIR range the absorbance spectra for all biomolecules reach their minima and it allows to avoid interference by endogenous chromophores within the body. NIR fluorescence technology can minimize background interference, improve tissue depth penetration and tumor contrast. Shi et al. [75] reported the newly developed NIR dyes or NIR dye-encapsulated nanoparticles by conjugation with tumor specific ligands and their potential applications in cancer targeting and imaging. The potential of NIR dyes was assessed in photodynamic therapy (PDT) for a skin cancer model in vivo. PDT is a minimally invasive procedure based on the lightinduced excitation of a photosensitizer (light-activatable chemical) to the singlet state due to the absorption of photons. Irradiation of photosensitizers can generate highly reactive singlet oxygen species or superoxide radical anion (1O2 and/or O2-) that causes destruction in malignant tissue [76-78]. Light-responsive compounds offer the promise of targeted therapy, which has several benefits including: prolonged action at the target site, overall reduced systemic dosage, reduced adverse effects, and localized delivery of multiple agents [43]. An efficient PDT photosensitizer requires the following features: a high quantum yield for singlet oxygen generation (??), efficient uptake into the diseased tissue (be preferentially retained by the target tissue), and localization in regions of the cell which are vulnerable to singlet oxygen damage [79-81]. In addition, other important requirements such as low dark toxicity (only be cytotoxic in the presence of light) and good pharmacokinetics (be rapidly excreted from the body to provide low systemic toxicity) are needed. Some examples of common organic dyes used as photosensitizers and imaging agents are illustrated in Table 1.

medicinal-chemistry-photosensitizers

Table 1: The example of dyes used as photosensitizers or imaging agents.

Current NIR probes with much improved photostability, high fluorescence intensity and long fluorescent life include three categories: i) organic chromophores (with squaraine, phtalocyanines, indocyanine green, porphyrin derivatives, BODIPY - borondipyrromethane anologues) [82], ii) inorganic silica nanoparticles (SiNPs) with NIR dyes (trimethine Cy3, pentamethine Cy5, Cy5.5, or heptamethine Cy7) [83] and iii) polymer dye-labeled capsules [Chapter 6]. Farther, fluorescence efficiency is greatly increased upon binding of cyanine dyes with nucleic acids or proteins as a result of the rigidization of the fluorophores [84-87]. Thus, Strekowski et al. [88] have developed bis(heptamethine cyanine) dyes containing two cyanine subunits, which exhibit a strong increase in the fluorescence (1000-fold) upon binding with protein. Gragg [89] reported a zwitterionic heptamethine indocyanine dye (Cy7), which has superior advantages for in vivo applications, such as no serum protein binding, rapid renal clearance, ultra-low non-specific tissue uptake, and high signal-to-background ratio after conjugation with tumor targeting ligands. Finally, Cy5.5 (λAb.max=675 nm, λEm.max=694 nm) is a promising contrast agent for the optical imaging studies in vivo providing a demarcation line between healthy and tumor cells [29]. However, cyanine dyes are far from perfect and have several limitations such as moderate to poor photostability, undesired reactivity with nucleophiles, and propensity to self-quench [41,90]. Therefore, different NIR dyes as squaraines [91] (i.e., bis(3,5-diiodo-2,4,6-trihydroxyphenyl)squaraine) that emit in the “optical window of tissue” at wavelengths above 800 nm in aqueous solution was assessed in PDT therapy. Squaraine derivatives consist of an oxocyclobutenolate core with aromatic or heterocyclic components at both ends of the molecules [92]. The bis-squaraine dye in which two squaraine units are bridged by a phenyl, biphenyl, thiophene or pyrene spacer exhibit fluorescence enhancements with red emissions in trizma buffer and present noncovalent labeling of human serum albumin (HSA) or bovine serum albumin (BSA) [93]. Water-soluble NIR fluorescent labeling probes named KSQ-3 and KSQ-4 substituted with four sulfo groups (-SO3-) based on the squaraine backbone exhibited significant fluorescence emission at λ=817 nm. In the presence of BSA which was covalently labeled with KSQ-4 the conjugate showed a strong absorption peak at λ=787 nm and fluorescence emission at λ=812 nm [94,95]. These compounds have satisfying optical characteristics and functionality for bioanalyses thus they enable the development of noninvasive and simple diagnosis techniques. Moreover, squaraine rotaxanes were developed as fluorescence imaging probes and chemosensors [96]. They can be converted into bright and highly stable deep-red fluorescent probes useful in biomedical imaging analyses. The depth of penetration doubles from 4 mm observed between λ=(500 - 600 nm) to 8 mm at λ=850 nm, where light penetration is most effective [81]. Furthermore, deep-red light can potentially penetrate several centimeters (2.5 cm) through skin and tissue [97]. The photosensitizers with high fluorescence detection are phtalocyanines, used to generate fluorescence contrast for the selective identification of diseased tissue. Phtalocyanines, are two-dimensional 18π-electron aromatic porphyrin derivatives, consisting of four isoindole rings bridged by four imino nitrogen atoms at the mesoposition. Their unique properties as a result of the π electrons strongly delocalized around their perimeter make phatlocyanines and porphyrin analogues (hematoporphyrins) well-known for molecular materials used in biomedicine [98]. Despite the fact that a long π-π electronic conjugation provides phtalocyanine high photostability and strong absorption bands, most of them exhibit absorption and emission maxima below λ=700 nm. Studies have demonstrated that the addition of benzene groups or multiple electron-donating substituents moves the absorption bands from the visible to the NIR spectral region. A series of conjugated porphyrin dimmers with intense absorptions ranging from 650 to 800 nm and fluorescence emission from 700 to 800 nm have been used in imaging studies [99]. In porphyrins, the Soret band in the 400 nm range is typically excited for diagnosis, taking advantage of the large Stokes shift between excitation and emission bands. Besides imaging applications as biosensors, phatocyanines and porphyrin derivatives have also been known as photosensitizers for photodynamic therapy of cancers and “photodynamic diagnosis”, since this kind of dye can be taken up in malignant and dysplastic tissue and generate singlet oxygen (highly toxic for tumor cells) when the tumor area is exposed to light [79]. Therefore, with excellent stability and suitable photophysical properties, versatility of newly developed phtalocyanines and porphyrin derivatives such as bacteriochlorins, gives them great potential to be extensively applied in clinics and evaluated for their application for both biological imaging and cancer photodynamic therapy [81]. The development of organic chromophores based on the BF2 chelated-tetraarylazadipyrromethene scaffold and they related structural analogues with spectral emission in the NIR and visible regions (650-900 nm) offers optical benefits by utilizing them in vitro and in vivo as imaging agents [100] with controllable photophysicochemical properties. These highly emissive dyes, which are derivatives of 4,4’-difluoro-4-bora-3a,4a-diaza-s-indacene (hereafter abbreviated to BODIPY; boron-dipyrromethane) have been widely utilized in various polymers [101-103] because they tend to be strongly UVabsorbing small molecules and emit relatively sharp fluorescence peaks with high quantum yields. This system provides the framework for more elaborate receptors to detect other analytes [104,105]. The comprehensive review summarizing the basic chemistry and spectroscopic properties common BODIPY-derivatives probes intended to intracellular imaging was prepared by Burgess et al. [101]. BODIPY dyes depending on the electronic structure of conjugated aromatic groups can indicate fluorescence in a broad range of wavelengths from green to red. A BODIPY derivative labeling of recombinant protein molecules FABP7-expressing 293HEK cells (i.e., a protein encoded by a gene) are well-known as intracellular markers of radial glial cells [106,107]. Chang et al. [108,109] discovered BODIPY which selectively stain Langerhans islets composed of glucagonsecreting α-cells (in the presence of glucagon the fluorescent compound was named Glucagon Yellow) and insulin-secreting β-cells in pancreas tissue sections. They also developed chemical imaging probes that specifically stain live microglia of brain cells [110]. The coordination capsules prepared from derivatives of phenylanthracen molecules linked by Pt(II) can co-encapsulate BODIPY with coumarin 153 or Nile red. These structures indicated that the emission color of the fluorescent guest within a cavity of such capsules was readily modulated upon pairwise encapsulation with planar aromatic molecules [111]. The encapsulated complex enclosed by the shell having Pt (II)- phenylanthracen demonstrated the guest-guest interaction within synthetic molecular host-capsules. So, the emission color of BODIPYguest is sensitive to the identity of the co-encapsulated guest (Figure 1). Thus, pair-selective encapsulation of two substrates modulates the intensity of fluorescence and fine-tuning of the emission properties of dyes. The observed unusual color changes most probably stemmed from the exclusively formation of excimer-like complexes in the confined nanospace of the capsule. The non-covalent approach for the straightforward preparation of aqueous fluorescent nanocomposites capable of modulating the emission color upon encapsulation could lead to the development of new photofunctional sensing and labeling materials.

medicinal-chemistry-various-fluorescent-dyes

Figure 1: The highly emissive (with good quantum yield, ?F) hostguest complex prepared upon encapsulation of various fluorescent dyes (e.g., BODIPY with non-substituted antracene (λmax=554 nm, yellow) and 9-methylantracene (λmax=577 nm, orange) by a Pt(II) ions linked coordination capsule with bent bispyridine ligands substituted phenylantracene in water.

Fluorescent dyes as “molecular photoswiches” and molecular chromophores for imaging

The interesting progress has been made in disulfide-based cleavage systems in thiol detection connected with biological applications [112]. The disulfide cleavage reactions have a great impact on chemical sensing of drug delivery involved in the polymer and pharmaceutical chemistry. The concentration of contrasting glutathione (GSH) inside cells and in plasma play an important role in the cellular antioxidant defense system and detoxification of reactive oxygen species [113,114]. The regulation of cellular redox status i.e., the equilibrium established between reduced free thiols (GSH) and oxidized disulfides (GSSG) is associated with a healthy condition and some kind of imbalance due to their abnormal levels are often related to diseases [115,116]. Moreover, intracellular GSH concentrations is 4-fold higher in tumor cells than in the corresponding normal cells and 1000-fold higher than in blood plasma [117-120]. More precisely, GSH can keep the cysteine thiol group in proteins in the reduced state and protect the cells from oxidative stress by trapping free radicals that damage DNA and RNA. Imbalance of GSH/GSSG ratio in cells associated with oxidative stress can be estimated by various fluorescent probes. The breakdown of the disulfide bond upon attack by specific thiols can cause rapid response and changes in the fluorescent signals of the probes. The GSH/GSSG cellular ratios can be estimated by using targeting fluorescent probes that allow for fluorescence imaging of reduced thiols both in vitro and in vivo in liver cells. To develop a new fluorescent tool for analysis of GSH content in cells and tissue Zhai et al. [121] obtained a convenient compound named Glutathione Green (λEx/λEm=565/585 nm) which shows a significant hypsochromic shift (λEx/λEm=512/522 nm) in response to GSH but not to the oxidized form of glutathione (GSSG). This compound has a BODIPY scaffold with a furan ring as a small functional group, which is likely to be a binding site for GSH (Figure 2).

medicinal-chemistry-Glutathione-Green

Figure 2: Glutathione Green: spectra (a) absorbance and (b) fluorescence, (c) structure, (d) a picture with GSH (right) and without (left) under UV irradiation at λ=365 nm.

Kim et al. [122] described a tool embedded with a single galactose subunit (which guides the probe to hepatocytes) and a disulfide-linked 4-amino-1,8-naphthalimide moiety. The probe provides an easy-tomonitor fluorescent change when exposed to endogenous thiols as the result of disulfide cleavage. The signal of an emission probe at λ=473 nm (yellow) decreases gradually with increasing concentrations of GSH while the emission at λ=540 nm (green) increases linearly towards GSH (Figure 3). Thus, upon exposure to free thiols the tool can be useful in diagnostics and in the screening of new potential drug agents. The same group of researchers described a probe bearing triphenylphosphinenaphthalimide with a disulfide moiety in order to measure the activity of the mitochondrial thioredoxin (Trx) level in cells. Such information may also indicate illnesses related to cancer [123].

medicinal-chemistry-hepatocyte-targeted

Figure 3: Schematic representation of hepatocyte-targeted imaging and cleavage of disulfide bond to induce fluorescence changes.

The triphenylphosphonium headgroup guides the probe to the mitochondria and also improves its water solubility. The fluorescence emission was induced by the cleavage of a disulfide bond of the probe, resulting from a reaction with Trx and subsequent intramolecular cyclization by the released thiolate, thus a green fluorescence product was obtained. A fluorescence off-on change of the maxima at λ=376 nm and λ=472 nm shifts a new emission band centered at λ=482 nm (yellow) and λ=540 nm (green) respectively (Figure 4). In this case linear correlation between the concomitantly increasing fluorescence intensity at λ=540 nm and increasing concentration of Trx was retained. So, the naphthalimide unit provides a suitable fluorescent scaffold owing to its excellent spectroscopic characteristics and structural flexibility for introducing various modifications [124].

medicinal-chemistry-Mitochondria-Naphthalimide

Figure 4: Proposed mechanism of the reaction of Mitochondria Naphthalimide with Thioredoxin.

The fluorescent thiol probes have been obtained in reactions of conjugates endowed with thiols together with maleimides (α,β- unsaturated ketones) [125]. The sensitive fluorogenic probe which works preferentially with the cysteine (Cys) was obtained by condensation of coumarinaldehyde and diethyl malonate [126]. The coumarin-based chemodosimeter selectively recognizes thiols through the 1,4-addition reaction (Michael type) to the α,β-unsaturated carbonyl group in the conjugate of the coumarin analogues and formed thioeters. As a result fluorescent spectra were enhanced 107-fold (with a green emission of the maximum at λ=502 nm) (Figure 5). The probe did not react with GSH, homocysteine (Hcy) and amino acid due to an intermolecular charge transfer blocking event.

medicinal-chemistry-fluorescent-turn

Figure 5: The design concept of fluorescent turn-on probe for thiol adduct.

Lin et al. [127] reported the new NIR fluorescent probe chloro-hydroxyl-merocyanine-thiol or (CHMC-thiol) containing 2,4-dinitrobenzenesulfonate moiety as a fluorescence quencher. The nucleophilic substitution by thiols of the chloro moiety in the chloro-substituted cyanine conjugate can induce a ratiometric mode in fluorescence of the probe CHMC-thiol. In this way, the unique probe CHMC-thiol containing both the high-sensitivity site (the 2,4-dinitrobenzenesulfonate moiety) and the low-sensitivity site (the chloro moiety) can provide a turn-on fluorescence response upon interactions with thiols. So the probe may serve as a powerful molecular tool useful for the estimation of the thiols concentration in biological and clinical studies in living cells (Figure 6). The different chemodosimeter which can effectively discriminate Cys from Hcy and GSH in living cells with notable fluorescence enhancement was reported by Peng et al. [128]. They synthesized the colorless and weakly fluorescent Rhodamine-6G-conjugate including a spirolactam-ring with an unsaturated aldehyde. This probe provides a facile way to visualize the changes of Cys concentration depending on the cyclization of N-terminal Cys residues (endowed with -SH and -NH2 groups) which reacts with the aldehyde group to form thiazolidine ring.

medicinal-chemistry-fluorescent-probe

Figure 6: The structure of NIR fluorescent probe chloro-hydroxylmerocyaninethiol (CHMC-thiol) containing 2,4dinitrobenzenesulfonate moiety. It can induce a ratiometric mode in fluorescence which depends on the concentration of thiols.

The Rhodamine-6G-COOH was released from the probe embedded with a thiazolidine derivative due to the hydrolysis and the ringopening reaction. The interaction of the probe with Cys accompanied by a color change from colorless to pink and strong fluorescence intensity which increased linearly (peaking at λEm=552 nm) with the Cys concentration (Figure 7).

medicinal-chemistry-mechanism-discrimination

Figure 7: Proposed mechanism of discrimination of Cys by using rhodamine 6G-conjugate with unsaturated aldehyde group. In the presence of Cys the color developed from colorless to pink by rhodamine 6G- COOH.

Wender et al. [129] reported that conjugation of luciferin through a disulfide-carbonate linker with an octaarginine, cyclosporine A or oligoguanidine transporter, produces conjugates that are water soluble and readily enter cells and tissue [130]. The linker-luciferin system allowed to assess the uptake of various transporters in cells and also released cargo only after cell entry. The decomposition of the conjugate (luciferin-linker-transporter) mediated by GSH caused the disulfide cleavage and developed a strong luminescence of released luciferin (Figure 8).

medicinal-chemistry-ariginine-reach-transpoters

Figure 8: The scheme of conjugates of ariginine-reach transpoters with luciferin, which can be released by luciferase.

Thus, the development of probes with disulfide-based luciferin conjugates with a variable distance of releasable linkers between carbonyl groups and the proximal disulfide linkage of octaarginine would allow to release in a controllable way a drug attached to transporters. Moreover, these conjugates incubated with the PC3 prostate cancer cell line were transfected with a luciferase gene (PC3Mluc- C6) [131,132] and the emission of light was caused by luciferase activity which utilizing luciferin from the disulfide cleavage of the conjugates. Yu et al. [133-135] prepared a different fluorescent probe using a porphyrin-coumarin scaffold for detection thiols based on F?rster fluorescence resonance energy transfer (FRET) which is a “spectroscopic ruler” between a donor and an acceptor species due to photoinduced electron transfer. FRET requires two fluorescent dyes: one (the fluorescent donor) emits high-energy photons and the other (the fluorescent acceptor) emits low-energy photons. Thus, within the F?rster distance, fluorescent color switches from the donor to the acceptor, or from high- to low-energy photons. The excitation energy of a coumarin fluorophore (an energy donor) can be transferred through a radiationless transfer of energy to a neighboring porphyrin fluorophore (an energy acceptor) within 100 Å, if there is good overlap between the donor emission and acceptor absorption spectra [129]. In this case the orientation of the donor-acceptor dipole moment is not perpendicular and the donor molecule can transfer energy via a dipoledipole interaction to the acceptor. Then the acceptor can emit a photon at a longer wavelength than the fluorescence of the donor [5]. The FRET off-on sensing system provides advantages over conventional fluorescence detection methods based on single fluorophores, because it has greater sensitivity and less background interference. Upon excitation of the probe (which composed of a porphyrin attached to a coumarin scaffold through a disulfide linker) at λ=350 nm, the characteristic emission of porphyrin at λ=658 nm appeared with little fluorescence. In the presence of thiols significant fluorescence enhancement was observed at λ=459 nm due to the coumarin emission and a concomitant decrease in emission at λ=658 nm was evident (Figure 9).

medicinal-chemistry-ratiometric-fluorescent

Figure 9: The scheme of a ratiometric fluorescent probe for thiols based on a tetrakis(4-hydroxyphenyl)porphyrin-coumarin scaffold.

Thus, the FRET was turned off by disulfide cleavage reaction induced by the thiol. This probe was suitable for ratiometric fluorescence imaging of thiols in living HeLa cells [133]. The photoactive fluorophores (caged dyes) are useful tools as fluorescent marks in dynamic, biological systems [136]. They serve like powerful imaging probes to follow the trajectory, speed, and timing of molecular and cellular movements and to provide information regarding spatiotemporal resolution in living tissue [137-139]. The nonfluorescent caged fluorophore can be homogeneously distributed within the system of interest and a mark is generated by UV photolysis, which can be controlled spatially and temporally with good precision. These caged dyes are weakly fluorescent when key functional groups of fluorophores are masked by photoremovable protecting groups like 2-nitrobenzyl derivatives. The 1-(2-nitrophenyl)ethyl or (NPE) is by far the most common photolabile group (Figure 10) [140].

medicinal-chemistry-permeable-caged-coumarin

Figure 10: The example of a cell permeable caged coumarin fluorophore for in vivo cellular imaging applications.

Photoactivation with ultraviolet (UV) light removes the protecting group (uncage) and abruptly switches on the fluorescence of parent dyes. The key requirement for caged fluorophores is that parent fluorophores should be photostable to resist photobleaching. This class of cage coumarins was developed by Li et al. [141]. They improved properties of coumarin analogues like good water solubility, biocompatibility, photostability of parent fluorophores, and high photolytic efficiency by both one photon and multiphoton excitation. Since the used coumarins emit only blue light these NPE-coumarin cages were conjugated with other fluorescent dyes which had a longer wavelength of the emitted light. This method expanded the fluorescent imaging window for multicolor simultaneously imaging in live cells. This group of researchers designed a cage coumarin-calcein green fluorophore used as a FRET probe. The NPE-cage coumarin 3-carboxyamide and a calcein 6-carboxyamide (as the energy acceptor) connected by a cyclohexyl linker led to an extensive spectral overlap between coumarin emission and fluorescein excitation (Figure 11). The efficient energy transfer from coumarin to calcein caused the increase of emission fluorescence intensity at λ=520 nm more than 14- fold using λ=410 nm excitation (of the coumarin maximum absorbance) [141].

medicinal-chemistry-caged-coumarin

Figure 11: The photoactivable FRET probe based on a caged coumarin and a calcein analogue, which can be efficiently photolyzed by ultraviolet light.

The rational design of highly fluorescent fluorophore compounds with optical switches for high-contrast optical lock-in detection used in imaging microscopy in living cells was reported by Marriott et al. [142,143]. They designed a highly fluorescent probe consisted of tetramethylrhodamine (TMR) and spironaphthoxazine (NISO) or (TMR-NISO) which was an optical switch to improve the image contrast. The TMR-NISO molecule integrated a reversible excited-state and a transition is carried out between a colorless spiro (SP)-state and a colored merocyanine (MC)-state in response to irradiation with near ultraviolet (λ=365 nm) or with visible (λ>530 nm) light respectively (Figure 12). In the MC-state, the TMR (donor) emission is almost completely extinguished by FRET to the MC probe (acceptor), whereas in the colorless SP-state, the quantum yield for TMR fluorescence is maximal [142]. Irradiation of TMR-NISO with a defined sequence of λ=365 nm and λ=546 nm manipulates the levels of SP and MC with concomitant modulation of FRET efficiency and the TMR fluorescence signal. This cell permeable probe presents greatly improved quantum yields for both fluorescence emission and optical switching in the same molecule.

medicinal-chemistry-fluorescence-intensity

Figure 12: Optical control of TMR fluorescence intensity in TMRNISO probes. Optical switching between SP- and MC-state of NISO generates the modulation of TMR fluorescence intensity by FRET.

Kikuchi et al. [144] developed a no-wash fluorogenic protein labeling system by exploiting FRET-based probes of fluoresceincephalosporin- azopyridinium (i.e., the synthetic β-lactam probe) and a mutant of 29-kDa TEM-1 β-lactamase tag (BL-tag) (Figure 13). This technique is useful for labeling cellular proteins with synthetic molecules under physiological conditions [145,146]. The labeling mechanism in live cells imaging involves two steps: an initial noncatalytic enzyme reaction with the probe and subsequent quencher elimination via a self-immolative reaction. Elimination rates of the leaving group (2-(4-dimethylaminophenylazo)pyridinium) which works as a quencher then converted into the neutral azopyridine form is accelerated by a decrease in the pKa (4.3) value of the conjugate acid. The hydrolysis of the β-lactam ring induces fast elimination from 3’-position and the leaving quencher allows to switch on the probe fluorescence [147,148].

medicinal-chemistry-fluorogenic-labeling-system

Figure 13: The scheme of a no-wash fluorogenic labeling system by exploiting fluorescence resonance energy transfer (FRET)-based fluoresceincephalosporin- azopyridinium probes and a mutant βlactamase tag.

The absorption spectrum of the azopyridinium shows the maximum at λ=581 nm and broadly ranges from λ=500 nm to λ=600 nm. So this compound works as an efficient quencher for fluorescein and rhodamines. This technology enables adequate discrimination of the location of proteins labeled with the same protein tag, in conjunction with different color probes, by dual-color fluorescence [149]. These systems operate in a nonspecific manner and do not allow specific analysis at the level of individual organs.

Non-fluorescent prodrugs/converted into fluorescent active drugs

Prodrugs by definition are precursors (derivatives) of drugs that are metabolized or activated in the body to release or generate the active pharmacological agent. Efficiency of chemotherapy crucially depends on the concentration of the effective agent in the cancer tissue (resulting in the therapeutic effect) and on the concentration in healthy tissues (resulting in side effects). The instantaneous report on the release of drugs in vivo is a great-benefit, especially if the reported signal could be detected in a non-invasive manner (as in the fluorescence detection technique). This type of cancer treatment with biodistribution that can be imaged in real time has been called theranostic (therapy combined with diagnostics). Chemotherapeutic prodrugs which provide a signal that allows determination of locations and activation of the drug upon selective cell internalization are being widely investigated for cancer treatment. The disulfide bond is used as a cleavable linker of a drug to a conjugate as an inactive prodrug, which can be converted into the corresponding active parent drug by an activation process. Ojima et al. [150] developed a prodrug conjugate with biotin (vitamin H or B-17) as the tumor-targeting molecule [151] bearing a self-immolative disulfide linker with a second-generation taxoid (SB-T-1214) as the cytotoxic agent (Figure 14). The conjugate can be internalized efficiently into tumor cells (e.g., L1210FR leukemia cells) overexpressing cell-surface biotin receptors via receptor-mediated endocytosis (RME). Taxoid was released via GSH-triggered disulfide cleavage. In order to monitor and validate the RME of drug release and drug binding to the target protein, three different probes with fluorescent or fluorogenic molecular chromophores were synthesized: biotin-fluorescein (FITC), biotinlinker- coumarin, and biotin-linker-taxoid-fluorescein. Kim et al. [124] studied direct fluorescence monitoring of a delivery and cellular uptake of a cancer-targeted cyclic peptide RGD (with sequence Arg-Gly-Asp). The multifunctional molecule containing a disulfide bond as a cleavable linker was built from a naphthalimide moiety as a fluorescent reporter, RGD as a cancer targeting unit and camptothecin (CPT) as a model active anticancer reagent. Thus, a peptide-appended naphthalimide pro-camptothecin conjugate was a theranostic prodrug (Figure 15).

medicinal-chemistry-generation-self-immolative

Figure 14: Second-generation self-immolative disulfide linkers with biotin as tumor-targering module and a fluorescent taxoid.

medicinal-chemistry-theranostic-prodrug

Figure 15: The scheme of the theranostic prodrug of a naphthalimide procamptothecin molecule with a disulfide bond as a cleavable linker. The Rdescribes the group of an RGD cyclic peptide.

Upon reaction with free thiols in aqueous media at pH 7.4 disulfide cleavage occurs. This leads to release of the free CPT drug in endoplasmic reticulum, as well as the production of a red-shifted fluorescence emission at λmax=535 nm. Low et al. [152] synthesized a folate-peptide-camptothecin conjugate prodrug with releasable disulfide carbonate linker. The poor water solubility of CPT is a drawback of its therapeutic agent. Therefore, a folate conjugate linked with the drug via a hydrophilic peptide spacer (Pt-γ-Glu-Asp-Arg- Arg-Asp-Asp-Cys) containing a disulfide releasable carbonate linker resolves the water solubility problems and provides efficient release of unmodified CPT via endosomal disulfide reduction. The cytotoxicity of folate-peptide-CPT in vitro was evaluated using KB cells (a human cervical cancer cell line) and revealed inhibition of cells proliferation. Shabat et al. [153] designed a prodrug composed of a specific triggering substrate (phenylacetamide) attached to a self-immolative linker with a pair of identical fluorophores (fluorescein moieties) and CPT. Cleavage of the phenylacetamide group by the enzyme penicillin-G-amidase (PGA) and subsequent elimination reactions released the free CPT moiety and fluorescein molecules (Figure 16).

medicinal-chemistry-theranostic-prodrug-activated

Figure 16: The self-immolative theranostic prodrug activated by the enzyme penicillin-G-amidase (PGA) can release two fluorescein derivatives as reporters and the active drug camptothecin (CPT).

Coumarin derivatives as a phototrigger have been widely used in drug delivery system mainly due to the strong fluorescence nature and their ability to the fast disintegration and the rapid release. Singh et al. [154,155] synthesized photoresponsive and pH sensitive theranostic conjugates based on benzothiazole-coumarin derivatives linked with the anticancer drug chlorambucil. In a physiological milieu (pH=7.4) fluorescence of the conjugate was only in the blue range (λ=406 nm). In such conditions the coumarin moiety occurs in the phenolic form (with –OH group). At acidic pH green fluorescence from the tautomeric keto form with a protonated benzotiazole group was turned on (λ=516 nm) resulting from the intramolecular proton transfer. Photolysis of anticancer drug chlorambucil was carried out using soft UV irradiation at λ ≥ 365 nm. in vitro studies showed that a coumarin-benzothiazolechlorambucil conjugate can act both as an imaging agent due to its highly fluorescent nature and a phototrigger for controlled drug release (Figure 17).

medicinal-chemistry-Coumarin-BenzothizoylChlorambucil

Figure 17: The enol and keto forms of Coumarin-BenzothizoylChlorambucil conjugate.

The different coumarin conjugates e.g., coumarin-quinoline are also pH-dependent. The above mentioned compound indicates a significant red-shift of the maximum intensity in fluorescence emission from λ=544 nm to λ=644 nm before and after protonation of a quinoline ring in neutral and acidic conditions respectively. Unfortunately, the two emission peaks are poorly resolved because of relatively broad profiles which cover each other [156]. Moreover, a coumarin phototrigger was used as a photoremovable protecting group for nucleobases [157]. Furthermore, in recent years coumarino-pyrimidine conjugates have been studied for their diverse pharmacological properties (antitumor activity) [158-160]. A different delivery strategy based on the nontoxic B-subunit of Shiga toxin (termed STxB) which is produced by intestinal pathogenic bacteria was reported by Schmidt et al. [161]. The prodrug consisted of SN-38 (that belongs to the class of camptothecin derivatives) linked via a disulfide bond to STxB. Such structures can bind to the glycolipid Gb3 cellular toxin receptor and then internalized into tumor cells e.g., colorectal carcinoma. This strategy termed the retrograde route was used for delivery prodrugs targeting to membranes of the biosynthetic/secretory pathway, by using STxB (Figure 18).

medicinal-chemistry-retrograde-delivery

Figure 18: Principle of retrograde delivery.

Perez et al. [162] synthesized a folate receptor targeting theranostic molecular probe with both imaging and therapeutic properties due to covalently linked doxorubicin (DOX). The prodrug containing a disulfide linker i.e., dithiobis(succinimidyl propionate) conjugated with a folate unit and the DOX moiety retained fluorescence and cytotoxic ability. The disulfide bond linking DOX with the targeting folate unit (DOX-S-S-Fol) is cleaved within the cell by gluthationemediated dissociation. Thus, it allowed to turn on fluorescence (at λEx=497 nm, λEm=594 nm) and activated cellular toxicity of the probe by subsequent release of DOX-SH (Figure 19).

medicinal-chemistry-specific-cytotoxicity

Figure 19: The targetable theranostic prodrug (DOX-S-S-Fol) leading to target specific cytotoxicity and fluorescence emission (ON) due to cleavage of the disulfide bond by GSH.

Latent fluorophores (pro-fluorophores) [163] are attractive candidates as labels for the imaging strategy due to coupling their activation to the drug release event in a delivery system [164-166]. In recent years, the quinone-methide elimination has proven to be a valuable tool for drug delivery, molecular probe design, signal amplification, stimulus supramolecular assembly, and self-immolative dendritic and polymeric molecular systems [167]. In this manner, intramolecular charge transfer (electron rearrangement) can lead to formation of π-conjugated quinone-methide-type dye compounds (fluorochrome) with long-wavelength emission of fluorescence. These dyes (e.g., QCy7 derivatives) [168,169] can also be applied as latent fluorophore linkers of theranostic prodrugs that provide a turn-on NIR fluorescence response upon activation (Figures 20 and 21).

medicinal-chemistry-specific-analyte

Figure 20: Activation of a QCy7 molecular probe by a specific analyte to produce a fluorescent turn-on response.

medicinal-chemistry-hydrogen-peroxide

Figure 21: Oxidation of the phenylboronic acid derivative by hydrogen peroxide releases fluorophore sulfo-QCy7.

The mitochondria-targeting carrier QCy7HA (with two positive charges on each molecules) is illustrated in Figure 22. The prodrug based on a QCy7HA scaffold endowed with the covalently attached doxorubicin (DOX) was studied by Lee et al. [170]. Such therapeutics are useful for treatment of drug-resistant cancers.

medicinal-chemistry-mitochondrial-targeting

Figure 22: The scheme of synthesis of the mitochondrial-targeting conjugate QCy7HA-DOX (by using NHS: N-hydroxyl succinimide; DCC: N,N’- dicyclohexylcarbodiimide).

For clinical use, the homo and heterodimeric prodrug [171] with a latent fluorophore was designed due to monitoring of drug-release in activated therapy for inhibition of Molt-3 leukemia cells by taking the catalytic antibody 38C2. Moreover, Shabat et al. [171] synthesized a theranostic prodrug based on 7-hydroxycoumarin which was covalently bound with melphalan and the dipeptide Phe-Lys. This linker is the triggering substrate for Cathepsin B. The conjugate was disintegrated due to cleavage of the amide bond at the C-terminus of the lysine initiated by Cathepsin B. As a result the free melphalan and a fluorescent coumarin derivative were released (Figure 23). However, fluorescent emission at λEm=460 nm is in the blue region, which is not practical used in studies in vivo.

medicinal-chemistry-Melphalan-theranostic

Figure 23: Melphalan theranostic prodrug activated by cathepsin B.

To summarize, new chemotherapeutic prodrugs that can report on the localization and activation of the drug upon internalization into select cells are being widely investigated for cancer treatment [172]. Thus, a new molecular design for a theranostic prodrug based on a self-immolative linker attached to a pair of FRET dyes that produces a fluorescent signal upon disassembly are still very interesting.

The design of polymer membranes with controlled pore sizes by using different fluorescent moieties

Synthetic membranes containing pores facilitate transport therefore, they are desired in biological applications [173-180]. Recent developments in methods for the preparation of smart responsive polymer membranes and the mechanism of their response to external stimuli with particular attention to the behavior of light responsive polymer membranes was reported in reviews [181,182]. The possibility of an opening or a closing of pores inside the membrane upon the light irradiation or by interaction with different pH of the surrounding solution [183] was taken to the design of polymer capsules used as carriers of therapeutics. The polymer material building the shell of capsules should be sensitive to external actions but not hazardous for the patient. Additionally, the shell should keep capsules intact without their destruction during “smart” activities such as drug release. The transport across the shell by using light to modulate the channel’s activity is possibly due to the application of photoswitchable copolymers [184-187] incorporated into lipids bilayers [187] or the doping them with photosensitive amphiphiles [188-195] and using photoswichable lipids (e.g., containing malachite green) [196] or cerasomes with a porous silicate framework containing azobenzene moieties [197]. Light is a useful trigger to manipulate ion channels externally, because does not change the chemical environment of the studied system. The synthetic molecules called “molecular photoswitches” e.g., photocaged small molecules like coumarin derivatives [198] or azobenzenes [199], spiropyranes [200], and malachite green leuconitriles [196] can change their structure and properties upon light irradiation. These dyes are often characterized by having two or more isomeric forms. They are adapting to changing conditions in a reversible manner during illumination with light of a specific wavelength. The presence of dyes in phospholipid bilayers or polymer membrane can modify the shell of capsules by creating light-sensitive ion channels. In the result of a photoinduced charge separation the channel can be opened. For example, the bacterial α-helical channel protein (with 3 nm pore) was converted into a pH-sensitive valve by chemical modulators [201]. The chemical modulator (i.e., sulfhydryl reactive molecules with a photocleavable group as the cage-modulator) was attached to the protein (linked with glycine-22 mutated into cysteine (G22C)). After removal of the protecting group from small photocaged molecules by long-wavelength UV (365 nm) irradiation and photolysis, the channel becomes responsive to the pH of the environment [202]. Furthermore, UV-irradiation of azobenzene molecules promotes the trans-cis isomerization at the central double bond, resulting in considerable changes in geometry and polarity [203,204]. Azobenzene is known to exist in cis (loose spacing) and trans (tight spacing) configurations in dark and light conditions, respectively [205,206]. This reversible process is controlled both thermally and by irradiation with visible light [207]. The isomerization involves a decrease in the distance between the para carbon atoms in azobenzene from about 9.0 Å in the trans-form to 5.5 Å in the cis-form. The modification of the capsule shell by incorporation of light-sensitive molecules such as polymer derivatives of azobenzene [208-211] or coumarin [212] caused changes in the capsule diameter and their illumination. Moreover, the absorbed light was transferred into the local heating of the capsules resulting in increasing permeability of their membranes. Additionally, metal nanoparticles (i.e., Au) inside the shell [213] and IR dyes which doped the outer layer of capsules (e.g., IR-860) [214] allowed to increase the release of the encapsulated compounds. Besides, spiropyrane molecules are very promising as switches. In spiropyrans (SP) [181] the CSP–O bond undergoes heterolytic cleavage upon UV-irradiation (360-370 nm) resulting in the formation of a polar zwitterionic conjugates system as merocyanine (MC) with a large change in polarity. The ring opening can be reversed both thermally and photochemically by irradiation with visible (>460 nm) light. It causes a dramatic increase in surface wettability and the hydrophilicity of the pore lining. This approach is a promising non-invasive method for controlled delivery of therapeutics. Farther, infrared light (750-1000 nm) has been used for releasing encapsulated a hydrophobic molecule from a micellar system containing 2-diazo-1,2- naphthoquinones (DNQ) [215,216]. These groups constitute an attractive trigger because UV light induced Wolff rearrangement of DNQ to a 3-indenecarboxylic acid group with hydrophilic properties enhancing water solubility of that compound. Moreover, infrared light can be applied in photodynamic therapy [217- 220] and liposome degradation [221]. Furthermore, mechanopores along the polymer poly(methyl acrylate) backbone (e.g., cyclobutane mechanopores) can be activated in polymers containing a coumarin dimer probe by mechanochemical scisson under pulsed ultrasound conditions (sonochemical) in solid state [222]. The use of forcereactive functional units (mechanopores) has become a promising approach to the development of new stress-responsive materials. Finally, the light activation of hollow capsules with bacteriorhodopsin (bR, 26 kDa) molecules [223] incorporated into the shell was reported by Erokhin et al. [224]. Capsules were prepared by the polyelectrolyte self-assembly technique [225] using alternate layers of positively charged polyallylamine hydrochloride (PAH) with negative charge polystyrene sulfonate (PSS). The captured into the shell the bR (PAH/ PSS-bR) caused destruction of a polymer membrane [226]. The LbL adsorption technique demonstrated that the biological activity of the bR embedded in PAH is preserved and not denatured after deposition, because the characteristic absorption of bR at around 552 nm is still observed distinctly. The shell permeability was induced under the illumination with low light intensity (daylight illumination) which caused the local pH change, necessary for the opening of pores and the releasing of encapsulated compounds [227,228]. The average diameter of pores about 400 nm is an important parameter that can be useful for the prediction of the release rate [229]. The bR is polyanion (purple membrane fragments of Halobacterium salinarium or Halobacterium halobium) [230] with a single-chain polypeptide folded into seven α-helices which contain a single covalently bound chromophore present in every helix [231-233]. Thus, the bR is wellknown transmembrane protein working as a proton pump creating a light-driven pH gradient between intracellular and extracellular parts of purple bacteria [234-238]. The bright ion channels embedded in a lipid bilayer (approximately 5 nm thick) control the selective passage of ions and their movements from one side of the cell to the other [239].

The role of caged fluorophores in a polymer carrier design

Chalcone (1,3-diphenyl-2-propen-1-one) moieties upon illumination with ultraviolet (UV=365 nm) light induced photodimerization of two chromophores due to the [2π + 2π] electron transfer cycloaddition reaction [240] can modify polymers resulting in their cross-linking (Figure 24). These versatile molecules can generate photoactivatable fluorescent probes which strategies rely on the photo- “uncaging” of “caged” fluorophores [241]. Fluorescent chalcones give useful potential applications in photoresponsive polymer technology as well as in medicinal prospects in the synthesis of antitumor therapeutics (i.e., nitrogen mustard-linked chalcones) [242].

medicinal-chemistry-Crosslinking-reaction

Figure 24: Crosslinking reaction of chalcone groups by UV irradiation.

The most well-known cage molecules belonging to coumarin derivatives are applied in polymeric materials obtained by using dynamic covalent chemistry [243]. The photoreactive coumarin derivative units underwent dimerization (λ=300-350 nm) and the reverse-dimerization (λ=254 nm) by photoirradiation, thus are usually used as a reversible cross-linking point in polymers (Figure 25).

medicinal-chemistry-reverse-dimerization

Figure 25: Molecular distance before and after photodimerization and the reverse-dimerization of coumarin derivatives.

The reversible photodimerization of coumarin moieties undergoes [2π + 2π] cycloaddition during the formation of a cyclobutane ring causes a decline of olefinic moieties (-C=C-) in chromophores and a diminution of the fluorescence emission. Static quenching typically resulted from the formation of non-fluorescent cyclobutane rings between two fluorophores increasing with increasing illumination time. The cleavage of the resulting cyclobutane ring by the irradiation of the corresponding wavelength of light (λ=254 nm) causes de-crosslinking of polymers. Photodimerization of coumarin derivatives units proceeded faster in lower Tg (glass transition temperature) polymers [244]. The Tg describes the mobility of a main copolymer chain and it is an essential factor to affect the rate of photodimerization [245-247]. The intramolecular photodimerization is preferred in high Tg polymers because of the low mobility of a main chain, thus the higher conversion to dimers i.e.,, larger number of cross-linking points [247,248]. Instead, in low Tg polymers (used e.g., 2-ethyhexyl methacrylate or n-butyl acrylate monomers) the intermolecular photodimerization competed with the intramolecular photodimerization and a performed network is loosely cross-linked. Photodimerization of coumarin derivative units in low Tg polymers tended to proceed faster than in high Tg. The release of small molecules via the network of cross-linked materials coincides with a change in the mobility of polymer chains which is important to limit passive drug release [249]. The tuning of a polymer’s Tg via copolymerization with hydrophobic monomers and varying network cross-linking density has been successfully used to create clinically applicable products such as self-tying sutures and implantable stents. This approach can be investigated for the triggered release of drugs either. Thus, polymer capsules should limit passive diffusion, and release a drug in a repeatable manner with light exposure. So the coumarin-containing amphiphilic diblock copolymer has been explored for functional photocontrollable polymer micelles [250] and nanoparticles [251]. Such micelles could be reversibly crosslinked and de-cross-linked using light at two different wavelengths for the reversible photoisomerization and photocleavage of coumarin derivatives. Kim proposed [252] to modify of the surface of liposomes with the copolymer poly(hydroxyethyl acrylate-co-coumaryl acrylateco- 2-ethylhexylacrylate). Agarwal et al. [253] synthesized micelledrug conjugates of {PEO-b-poly(n-butyl methacrylate-co-4-methyl- [7-(methacryloyl)-oxyethyloxy]coumarin)} by covalent bonding of anticancer drug 5-fluorouracil to the coumarin units under UV irradiation. Furthermore, Miksa et al. [198] prepared nanocapsules for 5-fluorouracil delivery decorated with a poly(2-ethylhexyl methacrylateco- 7-(4-trifluoromethyl)coumarin acrylamide) cross-linked wall which fulfilled the versatile strategy of release drug combining a long shelflife with the ability for rapid release using external factors such as light (Figure 26). In this case a monomer of 2-ethyhexyl methacrylate was used as a “linactant” - line active molecule that reduce the line tension thereby modulating the lipid bilayer membrane texture [254-257]. Zhao et al. [257] reported the near-infrared light sensitive polypeptide block copolymer PEO114-b-P(LGA0.62-co-COU0.38)34 with coumarin moieties which self-assembled into micelles for drug delivery of an antibacterial Rifampicin and an anticancer Paclitaxel. They studied the biocompatible block copolymer composed of poly(ethylene oxide) (PEO) and poly(L-glutamic acid) (PLGA) containing a number of 6-bromo-7-hydroxycoumarin-4-ylmethyl groups. The micelles could be disrupted by λ=794 nm NIR light excitation via two-photon absorption [258,259]. Disruption followed from the NIR light-induced removal of coumarin groups from the polypeptide block that shifted the hydrophilic-hydrophobic balance toward the destabilization of the micelles in aqueous solution [260].

medicinal-chemistry-scanning-electron-microscopy

Figure 26: The scanning electron microscopy (SEM) image of nanocapsules with the copolymer wall from a poly(2-ethylhexyl methacrylate-co-7-(4- trifluoromethyl)coumarin acrylamide). The polymer nanocapsules were successfully generated due to dimerization of the coumarin moiety, whose content has a large impact on cross-linking and nanocapsules formation. The diameter of the capsules is 250 nm.

Thereafter, spiropyran (SP) is the exciting dye useful for polymeric carriers because it enables a single molecule dual-color fluorescence on-off photoswitching. One molecule alternately emits two distinct fluorescent colors and photoexcitation can switch one fluorescence color to the other [261-263]. The UV illumination induces the spiropyran (SP) to merocyjanine (MC) conversion and switches on fluorescence (Figure 27).

medicinal-chemistry-structures-spiropyran

Figure 27: Isomeric structures of spiropyran (SP) and two canonical forms of merocyanine (MC). The dimeric complex of MC with metal ions is also shown.

The SP ring-closed form exposure to UV-irradiation (360- 370 nm) undergoes heterolytic cleavage and converts to MC ringopened mero form resulting in the formation of a polar, zwitterionic conjugated system, which absorbs strongly at λAb.max=570 nm and emits vivid red fluorescence at λEm.max=665 nm [264]. The visible photoexcitation of the MC accelerates the back conversion and returns the photoswiching dye into the off “dark” state. Thus, photoswitchable derivatives of SP were used to fabricate feature-packed polymer nanoparticles (NPs), whose fluorescence color can be optically switched either from one color to another. Li et al. [265] obtained polymer NPs from poly(styrene-co-2,2’-N-isopropylacrylamide) (PS-NIPAM) containing SP-MC photoswitches and employed the whole polymer particle as a switching unit. NPs are colorless in the SP form and turn to dark blue under UV irradiation at λ=365 nm to emit a strong red fluorescence. These photochromic NPs are capable of undergoing reversible fluorescence photoswitching and demonstrate that onoff switching can be controlled with specific frequencies of light. In aqueous solutions, the fluorophores were protected in the hydrophobic (polystyrene) core of the particle and their fluorescence was unlikely to be quenched by components of the biological milieu. The hydrophilic shells of NPs consisting of NIPAM moieties and the water-soluble carboxylic groups from the initiator (4,4’-azobis-(4-cyano-valeric acid)) at the end of polymer chains allowed to incorporate them into the polar cavities of liposomes. Using liposomes as a delivery vehicle, NPs were introduced into HEK-293 cells as biological markers. Li also prepared NPs containing butyl acrylate and acrylamide monomers with a SP single chromophore [266,267]. These optically switchable dualcolor NPs binding with HMGA1 proteins undergo endocytosis and they can be identified from autofluorescence of live cells, thus improve a tool for imaging. Furthermore, the SP molecules were grafted onto a diblock copolymer polystyrene-co-polyacrylic acid chain (PSPAA-SP) obtained from atom-transfer radical polymerization (ATRP) by using monomers of styrene and tert-butylacrylate [268]. Thus, copolymers of PSPAA-SP containing both the hydrophilic and hydrophobic segments self-assembled into nanomicelles in nonpolar solvent (toluene). In this synthesis a dye was embedded in the interior of high polar cores inside the micelles, which can promote the concentration of the MC form. The PSPAA-SP micelles loaded with rhodamine 6G as a model might be used for controlled release of drug substances [269]. Moreover, self-assemblies of amphiphilic poly(ethylene glycol-block-spiropyrane methacrylate) (PEG-b-PSPMA) [270-272] were a salt-induces (e.g., FeCl3 used as inorganic ions) micelles obtained in a DMF/water (v/v 10 : 1) mixture [273]. The inorganic ions were gathered inside the hydrophobic core which was surrounded by the PEG chains formed the hydrophilic coronae. Upon UV irradiation, the hydrophobic SP units in the cores were isomerized into hydrophilic MC forms. The complex formation between inorganic ions and MC isomers plays a key role in the self-assembly process. The different amphiphilic micelles synthesized from block copolymer spiropyrane-poly(2- methacryloyloxyethyl phosphorylcholine) (SP-PMPC) exhibited superior biocompatibility and were used as smart drug nanocarriers with doxorubicin [274]. The photostable amphiphilic BODIPYphotoswichable fluorescent NPs with SP derivatives were obtained as a result of copolymerization of the methyl methacrylate (MMA) in the presence of PEO as a macro-RAFT agent in the miniemulsion polymerization (Figure 28). Both chromophores were covalently incorporated into polymer backbones and NPs displayed reversible distinct dual-color (green-red) fluorescence by irradiation of UV or visible light via occurring switchable interparticle FRET [275]. The fluorescence switching conversion of NPs occurs also thermally in the living A549 cells (human lung carcinoma).

medicinal-chemistry-combining-polymerizably

Figure 28: Schematic illustration of amphiphilic NPs via covalently combining polymerizably dyes BODIPY-methacrylate with photochromic derivatives spiropyrane methacrylate.

The photoresponsive reversible aggregation and dissolution of rod-coil polypeptide i.e., poly(L-glutamic acid-b-PEO) diblock copolymers containing SP as side-chain motif (PLGASP20-b-PEO460) was reported by Mezzenga et al. [276]. The PLGA with SP can undergo conformational changes (from α-helix to random coil and vice versa) with UV and visible light respectively [277-279]. So, polypeptides are attractive in the biomedical field due to their unique protein-mimetic properties thus micelles can be used in the drug delivery. Recently, the new bi-functional compounds was synthesized, in which chalcone was connected with indolinospiropyran moieties and doped in poly(methyl methacrylate) to increase the stability of photochromic materials [280].

Stimulus Responsive Shell-tuning Capsules for Drug Delivery

The delivery of encapsulated drugs to the target position in live cells requires specifically targeted capsules. Such capsules sensitive to external conditions e.g., magnetic field should contain magnetic particles [281] or they have special receptor molecules like antibodies (Ab) attached to the outer side of the shell [282]. The Ab can be adsorb by electrostatic interactions [283] or covalently liked to surfaces [284-288]. For uncharged materials like poly(ethylene glycol) (PEG) or poly(N-vinylpyrrolidone) (PVPON) covalent attachment of the Ab is required since electrostatic attachment is not effective for lowfouling materials because they inhibit nonspecific protein binding.

Moreover, pH-responsive capsules [289,290] linked with glutamic acid residues and containing disulfide groups within the shell or on the surface of polymeric capsules can be gathered into endosomes. The disulfide acts as a linker to attach different functional moieties to the surface of polymer capsules via a covalent or noncovalent interaction. Additionally, the cytotoxicity of drugs can be largely decreased by encapsulating them into polymer capsules. Such carriers improve the enhanced permeability retention effect (EPR) of the drugs [112]. The interesting example of self-assembly of molecular components into nanometer scale capsules with irreversible covalent bond formation using photoinitiated thiole-ene “click” reactions was reported by Kim et al. [291]. The synthesis was carried out with (allyloxy)12cucurbit[6] uril as a disk shape monomer embedded with multiple polymerizable groups at the periphery and oligoethylene oxide or alkyl-based dithiols (Figure 29).

medicinal-chemistry-polymer-nanocapsules

Figure 29: The scheme of the synthesis of polymer nanocapsules from (allyloxy)12cucurbit[6]uril as a disc shape monomer and alkyl-based dithiols.

Polymer micelles and capsules loaded with fluorescent therapeutic complex-cargoes

The self-assemblies (micelles or vesicles) fabricated from polymeric amphiphiles have a large capacity for guest molecules. Although they are often thermally stable but it has been a great challenge to construct regular capsules with a monomodal distribution. In general, the layer-by-layer (LbL) assembly based on the sequential adsorption of charged species onto an oppositely charged surface (using electrostatic as the main driving force) is the facile method for the fabrication of polyelectrolyte multilayer films on homogenous particle supports from sacrificial spherical substrates [8]. Moreover, the same size distribution of capsules can be obtained after removal of their template. The combination of multilayer structures of capsules providing a high level of control over film properties such as thickness, composition, and nanostructure. This system of the assembly makes the modification of the surface relatively simple for inclusion of compounds with stimuli-responsive properties and enables loading of drugs inside the capsules [287]. The polyelectrolyte capsules for intracellular purposes comprising both therapeutic as well as imaging applications need to be biodegradable. Thus, several degradable multilayers of bio-polyelectrolytes, such as polysaccharides, polypeptides or polynucleotides have been reported by Picart et al. [292]. Lynn et al. [293-301] introduced poly α,β-aminoesters for the fabrication of capsules with degradable multilayers. Harashima et al. [302] reported multifunctional envelope-type-nanocapsules including a nucleic acid core complex with polycations or lipid aggregates and also different polymeric shell structures equipped with specific functional groups binding ligand-targeted agents and cell-penetrating proteins. Polycations are commonly used for gene delivery because they self-assemble and condense DNA into nanoparticles that can be internalized readily by cells [303]. Several vehicles have been created for improving efficiency of delivery small interfering RNA (siRNA) to animal tissues [304]. For example, Davis et al. [305] reported linear cyclodextrin-based polymer (CDP) nanoparticles containing camptothecin. The CDP are embedded with a highly water soluble, neutrally charged backbone containing polyethylene glycol (PEG) with incorporated β-cyclodextrins cyclic sugar moieties. Such structures get the ability to form inclusion complexes with hydrophobic drugs within the center of its cup-ring. As a result macromolecular prodrugs selfassemble into nanoparticles. Moreover, Davis’s group of researchers developed of effective anticancer therapies (against the EWS-FLII gene) by using a cyclodextrin-containing polycation (endowed with imidazole groups) binding with siRNA and a human transferrin protein (Tf) as a target ligand. Terminal imidazole groups assist in the intracellular trafficking and release of the nucleic acid [306]. The formulation of particles rely on mixing the components together and allowing for further the self-assembly process. The surface of colloidal particles was covered with PEG via inclusion of terminal adamantine (PEG-AD) into cyclodextrins. Some of PEG chains contained targeting ligands (AD-PEG-Tf) for specific interactions with cell-surface receptors (Figure 30).

medicinal-chemistry-Schematic-illustration

Figure 30: Schematic illustration of delivery system. A) Components: The CDP condenses siRNA and protects it from nuclease degradation. The AD-PEG conjugate stabilizes the particles in physiologic fluids via inclusion compound formation. The Tf-PEG-AD conjugate confers the targeting ligand to particles (CDP), promoting their uptake by cells. B) assembly of nontargeting and targeting polyplex particles.

Additionally, the stabilization of the surface of particles via thiolated polyethylene glycol (PEGSH) with the terminal adamantine allowed to bind cyclodextrins with a more multivalent way. The administration of siRNA using a targeting ligand Tf was possible with a 5 nm gold particle (λEm=507 nm) that was capped with the thiol group of PEGSH at the end distal to the adamantine (AD-PEGAu) [307]. So, specific hydrophobic drugs are suitable dwellers in β-CDs cavities (e.g., doxorubicin [308,309], camptothecin, paclitaxel, fluorouracil) [310]. The introduction of light sensitive chromophores like azo-groups, or a redox ferrocene (Fc) moiety, and pH- sensitive dansyl residues, or a benzyl moiety allowed to control CDs conjugates by temperature and obtained stimuli-responsive switching sol-gel systems [311]. The reversible association/dissociation of the polymer sol-gel transition under an external stimulus can alter the cross-link density of CDs conjugates [312-314]. There are a few examples of nanoassemblies prepared from diblock hydrophilic copolymers i.e., the PEG-b-polyaspartamide carrying β-CDs units on the side chain which created inclusion complexes with hydrophobic molecules (pyrene, coumarin 102 and indomethacin – an anti-infammatory drug) [310]. Furthermore, the hydrophobic poly(ε-caprolactone) (PCL) unit was connected with the hydrophilic diblock poly(N-isopropylacrylamideco- N-acryloxysuccinimide) segment by the α-β-CDs dimer used as a “bride” [315]. These components formed noncovalently integrated self-assembly micelles obtained through host-guest interactions between the α-β-CDs dimer and the guest molecules (rhodamine B and fluorescein isothiocyanate). The both dyes were functionalized respectively with the hydrophilic (PEG-FITC) and the hydrophobic (PCL-RhB) segments and switched on after removing the PEG segment in sites of a tumor because of the thermoinduced phase transition (T>37°C, pH<6.8). The addition of a short peptide (Arg-Gly-Asp) as a target ligand to these micelles improved their cell uptake efficacy [315]. Moreover, dextran (Dex) based microgels e.g., dextran-hydroxyethyl methacrylate (Dex-HEMA) could easily be loaded with macromolecules such as proteins and dyes (FITC) as well as latex nanoparticles (NPs, 20kDa) labeled with dyes [316-319]. The bio-polycation i.e., a poly-Larginine (pARG) labeled with rodamine B isothiocyanate (RITC) or tertramethylrodamine isothiocyanate (TRITC) can form coatings on the Dex-HEMA microgel yielding a clear red fluorescence [320,321]. The microgels with a cationic surface charge due to the addition of dimethyl aminoethyl methacrylate (DMAEMA) can be subsequently coated with four bilayers of poly(styrene sulfonate)/diazoresin (PSS/DAR)4. Upon light irradiation the cationic diazo groups of DAR form a covalent bond with the anionic sulfonate groups of PSS resulting in a cross-linked multilayer structure with a semipermeable membrane [322]. When the microgel core degrades through the hydrolysis of the carbonate esters which link the polymerized methacrylate groups to the dextran backbone, the swelling pressure increases. Thus, the surrounding rigid membrane ruptures and the encapsulated material is released from the exploding microcapsule. Such self-exploding microcapsules may find advanced applications in many fields for example as a “single shot” vaccine delivery system and release antigens enclosed in microgels [323]. Additionally, cationic nanogels for therapeutics were prepared in an inverse miniemulsion photopolymerization method from dextran and [(2-(methacryloyloxy)ethyl)trimethylammonium chloride] with 2-aminoethyl methacrylate hydrochloride. These nanogels were loaded with a negatively charged conjugate of AlexaFluor 488-labeled 25/27–mer siRNA [324]. Then, neutral nanogels were consecutively PEGylated by using NHS (N-hydroxysuccinimide) activated methoxyPEG5000 propionic acid and such hemocompatible dextran nanogels were used in the intravenous delivery [324-326]. Furthermore, the efficient delivery of deoxyribonucleic acid plasmid DNA (pDNA) to the nucleous of cells becomes easier especially in the presence of chromatin targeting peptides, which enhance the nuclear inclusion and a probability that DNA can be passed on during subsequent cell divisions in gene therapy. The preparation of the red fluorescent pDNA labeled with an intercalating hydrophobic far-red cyanine dye Cy5 (mono-reactive NHS ester) allowed to obtain the complex of Cy5- pDNA/peptides which was encapsulated and co-delivery with lipidbased carriers. The pre-condensation of pDNA with peptides improves lipid-based delivery systems and transfection efficiencies [326]. At last, by using LbL method Caruso et al. [284-288] synthesized redoxactive polymeric microcapsules with the cross-linked shell containing disulfide bonds for the delivery of a siRNA targeting to PC-3 prostate cancer cells. The polycation-polylysine (PLL) for a siRNA complexation was infiltrated into the pores of nanoporous silica particles prior to an assembly of layer from the negatively charged poly(methacrylic acid) with thiol-modified groups (PMASH). The core was dissolved after the cross-linking of deposited the PMASH layer. In these PMASHPLL microcapsules, the layer of PMASH was labeled with AlexaFluor 488 (with a green fluorescence) and PLL by using TRITC (with red fluorescence). The negatively charged siRNA cargo was postloaded into performed microcapsules by a diffusion through the multilayer film and formed complex with the positively charged amino groups of PLL. The complexation of siRNA inside the cavity of microcapsules prevents it from leaking, thus the PLL/siRNA polyplex covered with the multilayer film was shielded from the direct effects of a milieu. The PLL-siRNA complexes can be released when disulfide bonds in the membrane are broken by pore-forming GSH, then the PLL can be enzymatically degraded within cells into the amino acid lysine [288]. According with the LbL method the similar redox capsules were prepared but aminefunctionalized silica particles as templates were used for coating by alternately assembled the PMASH layer and poly(vinylpyrrolidone) (PVPON). In the acidic conditions at pH 4 the integrated shell with hydrogen-bonded networks was formed. The siRNA cargo was preloaded onto the template particle before a preparation of multilayer films. The disulfide bridge was produced by oxidation thiols using chloramine T and after cores removal (with HF) the film was washed into a neutral pH solution, in which the PVPON was released. In this way microcapsules were filled with an uncomplexed siRNA [288]. In addition, Jin et al. [327] reported magnetically vectored nanocapsules based on polystyrene seeds with incorporated Fe3O4 surrounded by porous SiO2 shells. The polystyrene core was removed by dissolving or burning polymeric materials within the capsule and anticancer drugs (CPT or DOX) were loaded. Such obtained capsules for tumor penetration allowed a controlled on-off switchable release of an anticancer cargo. Thus, the remote RF magnetic field induces magnetic particle (Fe3O4) heating and makes the localized temperature inside the capsule rise. The resultant temperature gradient between inside versus outside of the capsule is likely to be the key element that stimulates accelerated diffusion and drug release.

Polymer capsules containing the fluorescent shell

One of the most interesting feature of the LbL technique is that it allows different macromolecular species for molecular selfassembly on a nanometer length scale. This method enables the arrangement of “reactants” (e.g., optically active molecules with chemically reactive functional groups) at a molecular level to create theranostic tools with co-operative chemical sensitive and optical fragile properties. For example, the incorporation of a dye (e.g., anionic Congo Red) [328] into layers is affected by size, number of charge and spatial orientation of dye molecules, thus enables one to use it as a probe to obtain detailed information about local features such as a polarity and molecular mobility [329,330]. Further works in this area demonstrate that the formation of capsules with a polymer shell containing different segments can enhance drugs loading and modify the release of drugs by sequestering their molecules in both the core and shell compartments. For instance, decomposable hollow capsules were obtained by adsorption of sperimidine (which is a low molecular weight organic polyamine) alternately with DNA on colloids (MnCO3 particles). The multilayers DNA/spermidine formed the shell of capsules then a templated core was removed with acid [331-335]. DNA is negatively charged due to the phosphate group in its backbone, while a sperimidine in its ionized form contains three positive charges. Sperimidine forms complexes with DNA which can be suppressed by increasing the monovalent salt (NaCl) concentration [336-339]. Thus, a decomposition of hollow bio-capsules can occur readily upon exposure to the sodium chloride solution. Such capsules may be exploited for the release of many other encapsulated compounds such as dyes or drugs. So, selecting a composition of the capsules wall by using e.g., rhodaminelabeled DNA [332] can provide tailored delivery vehicles with a specific targeting capability. Moreover, the anionic dye pyrene chromophore (i.e., pyrenetetrasulfonic acid) (4-PSA) which exhibits four negative charges forms a complex with cationic polyelectrolyte chains with a very high binding constant [340]. Thus, 4-PSA was sequentially deposited in polyelectrolyte-dye thin films consisted of alternating poly(allylamine hydrochloride) (PAH) with poly(styrene sulfonate) (PSS) multilayers films [341,337]. So, binding of 4-PSA moieties to the polyelectrolytes causes changes in their fluorescence emission. The excitation spectra of probe molecules which form excimers in polymer systems were seen as a broad structureless fluorescence band centered around λEm.=500 nm. This phenomena was explained by sequestering 4-PSA in the polyelectrolyte which caused an increase of the local concentration of probe molecules along the polymer chain that promoted an excimer formation and a quenching of dyes fluorescence emission [340]. The similar phenomena connected with excimer formation was observed in the case of coumarin 6 dye molecules incorporated within the crosslinked poly(styrene-co-divinylbenzene) shell of polymer capsules which can be used for imaging [187] (Figure 31).

medicinal-chemistry-cross-linked

Figure 31: Nanolipopolymersomes with a nanometer-thick fluorescent film from cross-linked poly(styrene-codivinylbenzene) embedded with coumarin 6 moieties.

The application of the 6-carboxyfluorescein (6-CF) dye in the presence of polyelectrolytes such as: PAH, poly(diallyldimethylammonium chloride) or (PDADMAC), poly(diallyldimethylammonium chlorideco- acrylamide) or (DADMA-acrylamide) copolymers can induce a fluorescence self-quenching of 6-CF due to increase probe-probe interactions [341]. Furthermore, the FITC dye can label PAH polymers and the following FITC-PAH was deposited in the polyelectrolyte poly(styrene sulfonate) PSS films [342]. The permeability of the PAH/ PSS film was measured after incorporation of the FITC molecules to the polymer in which fluorescent moieties were embedded in a defined, variable depth inside the film. The dye rhodamine B (RhB) was chosen as the quencher molecule for penetrating barriers in the film. The diffusion of probes RhB through layers caused a decrease of fluorescence intensity thus gave information on its depth and transport phenomena in films. The FITC emission (at λmax.=527 nm) decayed simultaneously with an increase of rhodamine fluorescence (at λmax=583 nm) and the energy transfer occurred from fluorescein to rhodamine via dipole-dipole interactions with an interaction radius about 60 Å (Förster energy transfer). The different mechanism of quenching was obtained with the smaller molecule 4-hydroxy-2,2,6,6- tetramethylpiperidine 1-oxide (TEMPOL) as a quencher (fluorescent intensity was integrated at λEm=515-550 nm). The spin label TEMPOL enhances the spin-orbit coupling in the FITC dye with an interaction radius of 6 Å (paramagnetic quenching). So, electrostatic attraction and a binding of ionic probes to oppositely charged polyelectrolytes is manifested by quenching of the fluorescence emission, the appearance of excimer emission, and changes in the probe fluorescence excitation and absorption spectra. Besides, a combination of LbL assemblies with “click chemistry” allowed to obtain covalently stabilized hollow capsules from biodegradable polymers with modified alkyne and azide moieties e.g., poly(L-lysine) (PLL) with properties promoting a cell adhesion and biocompatible poly(L-glutamic acid) (PGA) [285]. Capsules were postfunctionalized due to monolayers deposition from heterobifunctional poly(ethylene glycol, PEG) i.e., NHS-PEG3500- biotin and NHS-PEG2300-OMe (with N-hydroxy-succinimidyl ester at one end and methoxy-terminated group) that provided lowfouling properties and simultaneously enhanced specific protein binding. The combination of “click chemistry” and LbL provides flexilibility to design materials with tailored properties to better target these materials for various diagnostic and therapeutic applications. Thus, PLL-Alk, PGA-Alk (with alkyne functionality) and PLL-Az, PGA-Az (with azide functionality) were fluorescently labeled with red rhodamine B isothiocyanate (RITC) or green AlexaFluor 488 N-hydroxy-succinimidyl ester respectively. The low level of adhesion drug molecules to blood proteins and their long circular times in the bloodstream are very important in drug delivery applications. Therefore, adsorption of proteins e.g., fluorescent labeled bovine serum albumin (BSAFITC) (the most abundant plasma protein in coow’s blood) and streptavidin (SAFITC) (due to its high affinity to the biovine) on the surface of capsules was analyzed by the use of flow cytometry [285]. In addition, an antibody (Ab) was functionalized with an azide (Az) by coupling a linear bifunctional NHS-PEG2000- Az polymer with a succinimidyl ester group at one end which was active toward the lysine. Moreover, efficiency of the functionalization of polymer capsules from poly(vinylpyrrolidone) (PVPON) endowed with alkyne moieties (PVPONAlk) with azide-functionalized human monoclonal antibodies (huA33mAbAz) by using “click chemistry” was qualitatively monitored by flow cytometry [286]. The activity of the immobilized antibody was confirmed by incubating these capsules with the AlexaFluor 488-labeled A33 antigen that could bind specifically to huA33 mAb. The efficient click functionalization of capsules in the presence of a Cu(I) chelator prevented antibodies from aggregation. These functionalized capsules showed highly specific binding to cancer cells. Furthermore, silica “nanobubbles” similar to hollow nanospheres were synthesized by using the core-shell/gold- SiNPs method [343]. These structures used as nanocontainers for drugs could also be applied to a wide range of fluorescent probes. For instance, FITC molecules approximately in 25% modified the surface of colloidal gold nanoparticles by reversible dyes chemisorption. The rest of the available surface was covered with bifunctional aminosilane molecules (i.e., 3-aminopropyltrimethoxysilane, APS) used as a “primer” which introduced hydroxyl groups prior to silane deposition but the amine group of the primer molecule bound to the gold particle. Hydrolysis of the surface-bonded siloxane moieties formed silane triols which allowed condensation of silica over the particles. The gold core was removed by diffusion of cyanide ions (NaCN) through the porous silica shell. Thus, the fluorescent molecules remained inside the water-filled silica capsules were isolated from the external environment [344]. In other words, the direct attachment of a probe molecule to the gold surface before construction of the silica shell, allows the probe to remain inside the silica shell at the internal water-silica interface, after dissolution of the gold particle.

Bio-inspired poly(dopamine) capsules with a fluorescent shell

The metal-organic film i.e., a natural polyphenol [345,346] containing tannic acid (TA) and Fe+3 due to non-covalent coordination complexes can fulfil the criteria required for a dual-functional shell of capsules (Figure 32) [347]. Capsules with the metal-phenolic network (MPN) shell were synthesized by the complexation between the organic ligand deposited on removable - sacrificial templates and inorganic metal ions as a cross-linker that produced robust nanoscale coordination layers [348,349]. Such films are structurally rigid but could respond to external stimuli and they can be degraded under cytocompatible physiological conditions.

medicinal-chemistry-complexation-state

Figure 32: pH-responsive FeIII-TA complexation state in which R represents the remainder of the TA molecule.

The fluorescent shell of capsules from poly(dopamine) (F-PDA) [350,351] was obtained via the polymerization of dopamine i.e., 3-hydroxytyramine hydrochloride (Figure 33) on sacrificial templates (CaCO3 or SiO2 nanoparticles) in the presence of H2O2. The removal of templates (by using ethylenediaminetetraacetic acid (EDTA) or HF respectively) allowed capsules to retain their spherical shape and the well-defined size. The F-PDA shell was pH- dependent and exhibited a broad range of intrinsic fluorescence from λ=350 nm to λ=600 nm with the maximum of fluorescence intensity at pH 3.

medicinal-chemistry-suggested-structure

Figure 33: The suggested structure of polydopamine and the response to pH.

However, PDA slowly degrades, thereby its application in therapeutic delivery is limited. Therefore, a polyanion endowed with catechol groups i.e., a poly(acrylic acid-co-dopamine) was used as a building layer for LbL assembly together with a polycation i.e., poly(allylamine hydrochloride) [352]. Furthermore, the stimuliresponsive (pH-induced) capsules with a doxorubicin (DOX) conjugated to a thiolated poly(methacrylic acid) (PMASH) were obtained due to the alternately deposition of conjugate and PDA. So, a maleimide derivative of DOX with hydrazine groups was attached to PMASH by a pH-cleavable hydrazone bond [353]. Then, the polymerdrug conjugate was immobilized on the PDA wall of capsules via the thiol-catechol robust reaction. Thus, in a milieu at pH 5.0 only the DOX was released. Moreover, the adjunctive fluorescent labeling of PDA-DOX capsules by AlexaFluor-488 C5-maleimide (AF488mal) allowed to indicate locations of capsules after cellular uptake due to their direct visualization. The labeling also helped to circumvent the problem with broadband absorption of PDA in the ultraviolet and visible light regions [353]. Therefore, modifications of PDA by reactions with thiols and amines via Michael addition or Schiff base formation influence on excellent biocompatibility and low cytotoxicity of that polymer and enable to use polymer capsules in an anti- cancer therapy (Figure 34) [354-357]. Besides, CaCO3 particles mixing with DOX and covered with the poly(styrene sulfonate) (PSS) layer formed templates which were stabilized by different: electrostatic, hydrogen bonding and hydrophobic interactions. Then, the outer layer on their surface was formed by subsequently deposited TA-Al+3 molecules. The template was dissolved in a typical way and monodisperse DOX-loaded MPN capsules (DOX-Al+3-TA) were created.

medicinal-chemistry-responsive-release

Figure 34: Immobilization and pH-responsive release of DOX from PDA capsules. The red dots represent DOX.

The formation of coordination bonds between polyphenols and Al+3 is pH-dependent, and the stability of the capsules Al+3-TA is generally decreasing in pH 5.0, thus at physiologically relevant pH values (7.4 - 5.0). The strong fluorescence of capsules came from encapsulated DOX (red) and also from the Al+3-TA shell labeling with FITCbovine serum albumin (green). Moreover, other drugs (Irinotecan, Topotecan, Verapamil) can also be loaded within MPN capsules by using this method. The ability to incorporate multiple metals into MPN capsules demonstrates their diverse applications including drug delivery, positron emission tomography (PET), magnetic resonance imaging (MRI), biomedical imaging and catalysis [358]. Furthermore, a synthetic PEG-polyphenol derivative was prepared as a result of the PEG-NHS reaction with dopamine hydrochloride and then used it as a building layer [359]. Such obtained the conjugated polymer was labeled with AlexaFluor 488-cadaverine (AF488cad) to visualize the polymer shell after deposition on CaCO3 particles which were loaded dextran- FITC or a AF488-biovine serum albumin. After removal of CaCO3 templates by using ethylenediaminetetraacetic acid (EDTA) obtained capsules were covered with the PEG shell endowed with multiple catechol groups. They allowed the formation of MPN films due to the multivalent coordination Fe+3 bonding. The color of the mixed PEGpolyphenol- Fe+3 complexes changed progressively from colorless, green, blue, purple to red when the pH was gradually increasing from 3 to 11. The color change was attributed to the transition of coordination complexes from mono- to bis- to tris-catechol complexes. The pHdependent stoichiometry of the PEG-polyphenol-Fe+3 complex allowed the formation of stable coordination networks at physiological pH 7.4 and enabled their disassembly under mild acidic conditions. The pH-dependent encapsulation and release of small molecules (e.g., methyl orange, alizarin red, rhodamine 6G) were observed as a model [360,361]. At last, a reversible addition fragmentation chain transfer (RAFT) polymerization based on dopamine as a catechol-terminated RAFT agent and used subsequent oligo(ethylene glycol methyl ether methacrylate) (OEGMEMA) after chain extension with methyl methacrylate monomer (MMA) allowed to form the block copolymer POEGMEMA32-b-PMMA71 which self-assembled into micelles (Figure 35) [362].

medicinal-chemistry-Micelles-containing

Figure 35: Micelles containing catechol groups on the surface conjugated to 9-amino acridine via boronic acid ester.

Micelles with dopamine functionalities (containing catechol groups) on the surface can readily react with an analog of 9-amino acridine dye functionalized with a phenol boronic ester. The reaction between phenylboronic acid and 1,2-dihydroxyl functionalities (catechols) via a reversible boronate ester formation is described in literature [363]. Such labeling and traceability of capsules allowed direct monitoring of the yellow fluorescent complex in the cell. Thus, boronic acid ester conjugate polymer with dopamine was used as a tool for bioconjugation and visualization of cell apoptosis [364]. Moreover, these micelles loaded with highly efficient anti-cancer drugs (Oxoplatin and Abendazole) are recommended in therapy.

Silica Nanoparticles for Bioimaging in Therapy

Dye-doped SiNPs as labeling reagents have been used for imaging and detecting biological agents due to the attachment of sensitive biorecognition molecules (e.g., antibodies) to the silica surface through bioconjugation chemistries [365-369]. One of the greatest advantage of dye-doped SiNPs is a large number of fluorescent dye molecules encapsulated into nanoparticles, thus they provide high-quality luminescent signals and can serve as a probe with excellent photostability. The synthesis of organic-dye-doped SiNPs as dots with an uniform shape and size can be carried out by a) the Str?ber method [370] and b) the reverse-micelle method which is known as the waterin- oil (W/O) microemulsion system with a negatively charged surfactant (e.g., polyoxyethylene (10) isooctylphenyl ether (Triton X-100), dioctyl sulfosuccinate sodium salt (AOT), polyoxyethylene (5) nonylphenylether (NP-5)) [367]. In both methods, the dye-doped SiNPs are photostable and exhibit amplification of an emission signal of chromophores. Str?ber’s method [371] depends on the hydrolysis process of a silica alkoxide precursor [Si(OR)4] (i.e., tetraethyl orthosilicate, TEOS) in ethanol with addition of ammonium hydroxide in a water solution. The silicic acid is produced during hydrolysis and, when its concentration is above its solubility in ethanol, it nucleates homogeneously and forms silica nanoparticles. So, Van Blaaderen et al. [372] according with this method synthesized organic-dye-doped SiNPs. They incorporated hydrophobic rhodamine 6G molecules inside the SiNPs by using two compounds as precursors with a different lipophilicity i.e., more hydrophobic phenyltriethoxysilane (PTES) [PhSi(OCH3)] which keeps the organic dye in the silica matrix and [Si(OR)4] (TEOS) which can turn into a hydrophilic comopound, thus it allows obtained SiNPs to be dispersed in an aqueous solution [373]. All SiNPs have approximately the same size and composition but selected dyes (e.g., fluorescein, pyrene) [374,375] might exhibit different photophysical properties [376]. The silica matrix provides embedded dyes an effective barrier to protect them against the surrounding environment and both photobleaching and photodegradation phenomena can be minimized [377]. Van Blaaderen et al. [372] used also direct coupling of FITC dye molecules to the silane reagent (3-aminopropyl)triethoxysilane (APTS) by a covalent bonding to incorporate chromophore into the silica matrix. Moreover, different tracer nanoparticles embedded with photochromic dyes (e.g., orthonitrostilbene, rhodamine B, coumarin 343, pyrene) that molecules were covalently bound with reactive groups of (chlorobenzyl) trimethoxysilane as a core precursor can be synthesized and they form SiNPs with a core-shell architecture [378]. Therefore, the fluorescence intensity of the labeled SiNPs depends on dye concentrations, since reabsorption effects due to an overlap of absorption band influence on the fluorescence emission spectrum (“self-quenching”). However, the mobility of dye molecules i.e., their rotation inside the SiNPs is hindered by the silica matrix (caging effects) [376]. Moreover, covalent immobilization of dyes may cause an increase of the fluorescence quantum yield due to suppression of the radiationeless-decay pathways. Thus, the fluorescence intensity per dye molecule increases as the thickness of the protective shell rises, because of lower mobility of the dye labels incorporated inside the spheres. As a result, the photoemission intensity of dyes increases due to the loss of their molecular degrees of freedom [379]. In addition, the local environment (polarity effects) also influence on the intensity of fluorescence spectra. The synthesis of fluorophore-doped core-shell SiNPs can protect the fluorophore from environmental factors because dyes were shielded inside a certain shell matrix, thus the fluorophore still demonstrated the fluorescent characteristics. On the contrary, poly(organosiloxanes) spheres with the porous shell and a dye molecule inside a pore did not change the fluorescent emission characteristic of the free dye, because the local polarity of the pore environment is not differ very much from the polarity of the pure solvent. Thus, SiNPs containing a few labels protected by a relatively thick poly(organosiloxane) shell and therefore well embedded within the network of the core, show the highest fluorescence yield per single label [376]. In contrast, SiNPs with thinner shells containing dyes closer to the particle surface are more mobile, thus can exhibit a decrease of the fluorescence yield. This method to prepare the well-defined SiNPs is much simpler than the preparation of micelles from dye-labeled block copolymers or other nanosized tracers with a core-shell topology in which dyes were loaded through the diffusion [380]. Moreover, the major disadvantage of dye-labeled tracers with labels attached directly to the particle surface would allow dyes to escape by a simple diffusion, thus the synthesis of particles with core-shell topology according with Van Blaaderen method allowed to minimize such effects [372]. Different types of functional groups like e.g., amine in APTS and thiols in (3-mercaptopropyl)triethoxysilane (MPTS) can be introduced onto the surface of SiNPs and conjugated biomolecules and reactive dyes containing functional groups [375]. In addition, the properties of the silica surface makes SiNPs chemically inert and physically stable [374], so they are excellent labeling reagents for bioanalysis and bioimaging [381-385]. The second strategy of the preparation SiNPs [386] depends on the polycondensation of TEOS in a water-in-oil (W/O) microemulsion with the addition of surfactants (e.g., Triton X-100). In this process water nanodroplets in solvents (e.g., cyclohexane) work as nanoreactors to form reversed micelles which are converted into nanoparticles [385]. Thus, the ammonium hydroxide solution (NH4OH) initiates the hydrolysis of Si(OR)4 and the polycondensation process of the silane species carries out in “water pools” of reverse micelles. The TEOS molecules which are not amphiphilic are compartmentalized in the bulk oil phase but after hydrolysis they are translocated to the micellar phase. So, the Si(OR)3(OH) species would be located in the palisade surfactant layer (e.g., Triton X-100) but Si(OH)4 in the water pool. Depending on the lipophilicity of dyes they can be entrapped in an unaggregated form inside the silica pores like e.g., hydrophobic tertamethylrodamine (TMRA) linked to a hydrophilic dextran molecule (TMRA-dextran) [383]. Likewise, luminophore tris(2,2’-bipyridyl)dichlororuthenium (II) hexahydrate (Rubpy, λEx=458 nm λEm=594 nm) with methylene blue which formed a FRET pair of the model donor-aceptor [368,382] doped SiNPs. This model was used as a probe of leukemia cells [387]. Furthermore, analogues of cyanine dye-APTS were usefully employed to produce fluorescent SiNPs [379,387]. Derivatives of cyanine NHSester dyes containing sulfonic groups on the indoleninic rings introduce a hydrophilicity to fluorophores that labeled APTS. In this case SiNPs doped with cyanine dyes e.g., Cy5, NIR-664-N-succinimidyl ester (λEx=672 nm λEm=694 nm), AlexaFluor 670 and squaraines which have a fluorescence maximum excitation greater than λ=650 nm can be widely used in life science because in the NIR region an autofluorescence of bio-markers is much reduced. The versatility of this approach enables incorporation of various classes of fluorophores that cover the entire UV-vis absorption and emission spectrum [388]. The SiNPs with incorporated organic dyes: AlexaFluors (350, 488, 555, 568, 680, 750), N-(7-(dimethylamino)-4-methylcoumarin-3-yl), FITC, TRITC, Texas Red are illustrated in Figure 36. These highly fluorescent SiNPs with the core/shell architecture consisting of a fluorophore-rich center protected within a siliceous shell fulfilled requirements of a quantum dot brightness level (referred to as CU) [371,389]. They exhibit more than two orders of magnitude improvement in fluorescence intensity compared with a single dye molecule [390].

medicinal-chemistry-transmission-electron

Figure 36: (A) Schematic representation of the synthesis of coreshell fluorescent silica nanoparticles. (B) The image of 30 nm particles prepared by transmission electron microscopy (TEM) C) Fluorescent core-shell SiNPs doped organic dyes described respectively from left to right: Alexa 350, N-(7- (dimethylamino)4-methylcoumarin-3-yl), Alexa 488, FITC, TMRITC, Alexa 555, Alexa 568, Texas Red, Alexa 680, Alexa 750.

The potential of these silica dots as markers for biological imaging was demonstrated to label of Fc?RI receptor of rat basophilic leukemia (RBL) mast cells [388]. The positively charged biomolecules (e.g., IgG, polylysine) conjugated Cy5 were used as a core material which was coated with a silica shell [391]. These Cy5-IgG-doped-core/shell-SiNPs as a NIR fluorescent marker (λEx=653 nm, λEm=667 nm) selectively recognized the breast cancer cells. The biological modification of SiNPs depends on the activation of their surface by using the cyanogen bromide (CNBr) method then coupled the C-erb-B2 antibody [392]. The simlar method [393] was developed with the Au colloid core which exhibited strong chemisorption of the thiol functional groups of oligonucleotides labeled with cyanine dyes Cy5 (λEx=630 nm) and Cy3 (λEx=543 nm) molecules. The probes were formed by coupling of the fluorophore with e.g., alkanethiol oligonucleotides (5’-thiolated (dT)20-Cy5 and 5’-thiolated (dT)20-Cy3) to Au seeds. The use of oligonucleotides (dT)20 as a spacer arm allowed to avoid the proximity of fluorophores to the Au surface thus effectively prevented fluorescence quenching. Moreover, the class of cyanine dye doped-gold/core covered with silica/shell NPs provided a well-built doping mechanism which allowed to keep the problems of dye leaking off and limited a different amount of dye molecules in individual SiNPs. So, the host colloidal gold stability is preserved by the deposition of a thin silica shell which was produced from the hydrolysis of organosilane MPTS/TEOS. These SiNPs were directly coupled with the signaling probes of DNA by using the heterobifunctional cross-linker sulfo-KMUS endowed with NHS ester at one end (reacted with primary amine formed amide bond) and a maleimide group at the other end (reacted with a thiol to form a stable thioether bond). However, this method is complicated and expensive because of the thiolated oligonucleotide. Other silica probes doped with far-red fluorescent cyanine dyes (Cy5 and FR670) were also reported [394]. At last, the MnO nanoparticles offer an excellent matrix to accommodate optical imaging modalities by coniugation NIR dyes [395]. The carboxyl silane N-(trimethoxysilylpropyl)ethylene diamine triacetic acid trisodium salt) (TETT) can modify of the manganese oxideoleate complex (MnO-AO) to MnO-TETT. Obtained nanoparticles were coniugated to the water-dispersible PEG-Cy5.5 (achieved as a result of the reaction H2N-PEG500-NH2 with Cy5.5-NHS) in the presence of ethyl(dimethylaminopropyl) carbodiimide (EDC) and N-hydroxy succinimide (NHS). These MnO-PEG-Cy5.5 nanoparticles were used in magnetic resonance (MR) and NIR fluorescence as dualmodal imaging of brain gliomas, due to their preferential accumulation in the tumor region [395]. However, the sensitivity of MR imaging is relatively low in comparison to other biomedical imaging techniques, so NIR is the most relevant imaging modality in line with MR imaging. The biocompatibility of red fluorescent MnO-PEG-Cy5.5 (λEx=675 nm, λEm=710 nm) nanoparticles proved that in the bio-imaging applications the cell proliferation was dose-dependent. However, the cell viability higher than 90% suggested no significant cytotoxicity. Such MR/NIR dual-modal nanoprobes enabled to obtain both anatomical and physiological information from deep inside the body and simultaneously acquire more sensitive information at the cellular level.

Conclusions

There have been many recent advances in the field of molecular imaging, particularly in the utilization of different strategies for the discovery and development of new ‘smart’ polymeric carriers of therapeutics agents (e.g., by using LbL or self-assembly methods) [396]. Introducing new photoswichable on-off fluorescent probes with optimized properties to the polymer capsules can help boosting individual molecules or group of moieties strong fluorescence and it allows for high emission enhancement. Polymer capsules with the photo-responsive fluorescent wall provide a fascinating mechanism for an efficient molecular detection which can be applied to improve imaging in live cells by using optical fluorescence. The significant progress in the ability to prepare polymeric capsules with precisely designed various aspects of macromolecular structures including material properties and architectures using facile coupling techniques allowed to develop the field of therapeutics in medicine. Moreover, the tailored polymer capsules as carriers constituted the important tool for advance diagnostics thus improved results in the treatment. Therefore, the development of polymer capsules endowed with an excellent targeting ability to deliver drugs for a better therapeutic efficiency is still desired in the clinical usage. The versatile nature of a broad range of polymer building blocks-labeled with fluorescent dyes sensitive to biological stimuli allowed to understand the interactions between cells and polymeric markers. Although great achievements have been attained on the self-assembly of different amphiphilic polymers, there still exists plenty of room to expand sophisticated self-assemblies library with multifunctional properties and new recognition motifs which search different biomedical applications. The new ‘theranostic’ platform which combines diagnostic and therapeutic functions requires multifunctional hybrid self-assembly materials as nanocarriers with fluorescence specific recognition ability, multiple stimuli-responsiveness photodynamic/photothermal properties to endow this field of vigorous vitality. Both ‘activatable’ and ‘biomarker’ imaging strategies have been found useful for tracking of a mechanism of action along the drug development. This kind of research will greatly expand because the number of available imaging probes and carrier constructs can design new concepts of multimodal devices.

Acknowledgements

The author thanks the National Centre for Science in Poland, Grant no. NN 209 762 440 for the financial support. I am grateful to Deborah and Mike Reppert and Patrycja Miksa (University of Warsaw, Institute of Applied Linguistcs) for valuable guides.

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