Relative Bioavailability of Chewable and Conventional Film-Coated Tablet Formulations of Sildenafil 100 mg in Healthy Volunteers Under Fasting Conditions

Sildenafil citrate is a phosphodiesterase type 5 inhibitor. In countries worldwide, including Mexico, sildenafil citrate is approved for the treatment of erectile dysfunction as conventional film-coated tablets of 50-mg and 100-mg strengths for oral administration with water (Viagra®, Pfizer Inc, New York, NY). Viagra is rapidly absorbed, reaching a mean maximum plasma concentration (Cmax) of 514 ng/mL within 30 to 120 minutes (median, 60 min) after oral administration of the 100-mg dose in the fasted state [1]. Mean terminal half-life (t1⁄2) is approximately 4 hours [1]. The absolute average bioavailability after oral administration is about 41% [1,2]. For doubling the dose, across the range of 25–200 mg, a small and clinically insignificant degree of non-proportionality was observed in predicted increases in Cmax (2.1fold) and in area under the plasma concentration versus time curve (AUC, 2.2-fold) [1].


Introduction
Sildenafil citrate is a phosphodiesterase type 5 inhibitor. In countries worldwide, including Mexico, sildenafil citrate is approved for the treatment of erectile dysfunction as conventional film-coated tablets of 50-mg and 100-mg strengths for oral administration with water (Viagra ® , Pfizer Inc, New York, NY). Viagra is rapidly absorbed, reaching a mean maximum plasma concentration (C max ) of 514 ng/mL within 30 to 120 minutes (median, 60 min) after oral administration of the 100-mg dose in the fasted state [1]. Mean terminal half-life (t ½ ) is approximately 4 hours [1]. The absolute average bioavailability after oral administration is about 41% [1,2]. For doubling the dose, across the range of 25-200 mg, a small and clinically insignificant degree of non-proportionality was observed in predicted increases in C max (2.1fold) and in area under the plasma concentration versus time curve (AUC, 2.2-fold) [1].
A chewable tablet formulation of sildenafil citrate has been developed that does not have to be swallowed whole. Because citrate salts have a bitter taste, taste masking was essential to improve the palatability of the chewable tablet formulation.
The primary aim of this study was to evaluate the pharmacokinetics and bioavailability of sildenafil from its chewable tablet formulation taken with or without water relative to the marketed conventional filmcoated tablet taken with water. Secondary objectives were to assess the tolerability and palatability of the new formulation.

Study design
This was a randomized, open-label, 3-period, 6-sequence, crossover trial in 30 healthy male volunteers aged 18 to 40 years, with body mass indices between 18 and 27 kg/m 2 . Subjects were given single oral doses of the following 3 treatments after an overnight fast: Viagra 100-mg tablet with 250 mL of water; sildenafil citrate 100-mg chewable tablet with 250 mL of water, and sildenafil citrate 100-mg chewable tablet without water. Treatments were separated by 1-week washout periods between consecutive doses. Subjects were instructed to chew the chewable tablet until full disintegration and then swallow it, with or without water, depending on the treatment. Each subject was randomized to receive the 3 treatments in 1 of 6 possible sequences.
Health status was assessed based on physical examinations, medical records, electrocardiograms, and clinical laboratory results at *Corresponding author: Fermin Valenzuela, Investigación Farmacológica y Biofarmacéutica, S.A. de C.V., Av. Periférico Sur 4927, Arenal Tepepan, Tlalpan, México D.F., CP 14610, Mexico, Tel: +52-55-24583115; Fax: +52-55-24583118; E-mail: fvalenzuela@ifab.com.mx, fvgg@prodigy.net.mx the screening visit. Key exclusion criteria included use of prescription or nonprescription drugs, vitamins, or dietary supplements within 7 days or 5 half-lives (whichever is longer) before the first dose of study medication, except for acetaminophen at doses of ≤1 g/day; any clinically significant disease or drug allergies; febrile illness within the 5 days before first administration of study medication; sensitivity to heparin or heparin-induced thrombocytopenia; a positive test result for a drug of abuse; regular consumption of alcohol exceeding 14 drinks per week within the 6 months before the screening visit; excessive tobacco or nicotine use (equivalent to 5 cigars per day); an electrocardiogram showing QTc >450 msec at the screening visit; phosphodiesterase type 5 inhibitor use within the 4 days before first administration of the medicine under study; and treatment with nitrates or nitric oxide donors, either regularly or intermittently.
Plasma concentrations of sildenafil were determined at Investigación Farmacológia y Biofarmacéutica S.A. de C.V. (Mexico D.F., Mexico) using an in-house developed and validated high-performance liquid chromatography method with ultraviolet detection. The plasma samples were extracted using liquid-liquid extraction, propranolol was used as the internal standard, and separation was through a reverse-phase column. The analytical methodology was developed and validated according to the requirements of the Official Mexican Standard [3]. Validation of the method was shown by determination of stability of standard solutions and stability of analyte in biological fluid under conditions of processing and during the storage period, and by selectivity tests, accuracy tests, precision tests, calculation of the quantification limit, regression model, recovery, and validation of the quality-control samples in 3 levels of concentration (30 ng/mL [low], 1250 ng/mL [medium], and 2000 ng/mL [high]). The dynamic range of the assay was 10 ng/mL to 2500 ng/mL. During performance of the analytical runs, the percentage relative SD (% CV) of the quality control samples was ≤2.76% and the percentage relative error ranged from 2.23% to 8.29%, across the range of tested nominal concentrations.
Vital signs were assessed at 1, 2, 4, 6, 10, and 16 hours postdose. Safety was assessed through adverse event reports and changes in vital signs and clinical laboratory test results.
The design of this study complied with the provisions of the Official Mexican Standard [3]. The study was approved by the site's Ethics and Research Committee and conducted in accordance with the Declaration of Helsinki, Japan Revision, and with Good Clinical Practices. All volunteers provided written and verbal informed consent before study enrollment.

Statistical analyses
Noncompartmental pharmacokinetic analysis was performed using WinNonlin version 5.2 software (Pharsight Corporation, Mountain View, CA, USA), and the following pharmacokinetic parameters were calculated:  (2)/K e Natural log-transformed AUC parameters (AUC 0-t and AUC 0-inf ) and C max of sildenafil were analyzed using a mixed-effect model with sequence, period, and comparator as fixed effects and subject within sequence as a random effect. Using geometric mean values, estimates of the adjusted mean differences (test -reference) and corresponding 90% CIs were obtained from the model. The adjusted mean differences and 90% CIs for the differences were exponentiated to provide estimates of the ratio of adjusted means (test/reference) and 90% CIs for the ratios. Comparisons were made between conventional oral Viagra (reference) and the chewable tablet with or without water (test).
The pharmacokinetic parameters AUC 0-t , AUC 0-inf , C max , T max , and t ½ of sildenafil were summarized descriptively by treatment. Mean profiles of the concentration versus time data were plotted by treatment.
The average bioequivalence statistics assessed the difference between the comparators administered orally, by a logarithmic comparison of the pharmacokinetic parameters C max , AUC 0-t , and AUC 0-inf . A statistical analysis was made on the bioequivalence of the medicines based on the construction of classic and Westlake CIs, which result favorably to bioequivalence if the limits calculated fall within the pre-established interval from 80% to 125% for data logarithmically converted [4]. These data were analyzed with a 90% confidence level, with statistical power of >0.8.
Limit tests were based on the rejection of the null hypothesis of nonbioequivalence, to conclude, with an alpha significance level (0.05) and confidence level to 90%, that the comparators are bioequivalent. Limit tests applied were Schuirmann unilateral double t test and the Anderson-Hauck test on the quotient between average of comparator under study and comparator of reference, for pharmacokinetic parameters of C max , AUC 0-t , and AUC 0-inf [5,6]. The limit test results would conclude bioequivalence if there was a <0.05 probability that the quotient is <80% and >125%.

Results
All 30 men who were enrolled completed the study. The mean ± SD age for the study population was 24.4±3.7 years (range, 20-35 y), mean weight was 69.9±7.6 kg, mean height was 170.5±7.0 cm, and mean body mass index was 24.1±2.2 kg/m 2 .

Pharmacokinetics and bioequivalence
Mean sildenafil plasma concentration versus time profiles were nearly superimposable for Viagra and the chewable tablet taken with water; the chewable tablet taken without water resulted in a somewhat slower absorption rate, as indicated by lower C max compared with the chewable tablets or Viagra taken with water ( Figure 1). The sildenafil t ½ averaged approximately 2.9 to 3.5 hours and did not appear to be treatment dependent, particularly when the standard deviation is taken into consideration (Table 1). Median T max was lowest for the chewable tablet taken with water (0.75 h) compared with Viagra (1.0 h) or the chewable tablet taken without water (1.75 h) ( Table 1).
The classic and Westlake CIs (asymmetric and symmetric intervals around 100%, respectively) for logarithmically converted AUC 0-t and AUC 0-inf ratios were assessed as measures of the extent of systemic exposure. The intervals were contained within 80% to 125% for the chewable tablet taken with or without water, each compared with Viagra. Consistent with these statistical tests, the Schuirmann and Anderson-Hauck tests showed a high probability that AUC values fall within 80% to 125% (P<0.05), for which no statistically significant differences were found among the 3 comparators (Table 2).
For C max ratios of chewable tablet taken with water compared with Viagra, the lower limit of the classic CI was <80% and the upper limit AUC 0-inf =area under the plasma concentration vs time curve from time 0 extrapolated to infinity; AUC 0-t =area under the plasma concentration vs time curve from time 0 to time t; C max =maximum plasma concentration; CV=coefficient of variation; K e =elimination rate constant; t ½ =elimination half life; T max =time to reach C max . CT=chewable sildenafil 100-mg tablet. a Viagra conventional film-coated tablet. b Least squares geometric mean.  was contained within 125%; the Westlake CI was contained within the bioequivalence limits of 80% to 125%. The Schuirmann test results for the lower limit were consistent with the classic CI results, whereas the Anderson-Hauck test failed to meet the established criteria. For the chewable tablet taken without water, both the classic and Westlake CIs were not contained within 80% to 125%; the C max of the chewable tablet taken without water was about 22% lower than that of Viagra (Table 2).

Safety
A total of 17 adverse events were reported by 12 subjects following Viagra administration, 15 adverse events were reported by 13 subjects following chewable tablet administration with water, and 18 adverse events were reported by 13 subjects following chewable tablet administration without water. The most common adverse events that were considered probably related to the medicine were somnolence and headache (Table 3). Only a few of the 30 subjects in this study reported bitter taste with the chewable formulation (1 subject following administration with water and 4 subjects following administration without water; Table 3), indicating that unpleasant taste was successfully masked.
No statistically or clinically significant changes were noted in clinical laboratory test results compared with baseline values. Statistically significant decreases in systolic and diastolic blood pressure were recorded 1 to 4 hours after taking Viagra or chewable sildenafil with water and 4 to 6 hours after taking chewable sildenafil without water. These changes were not considered clinically meaningful.

Discussion
Administration of sildenafil 100 mg as conventional filmcoated Viagra or a chewable tablet with water resulted in a similar pharmacokinetics, with rapid absorption (T max within 1 h) and elimination (t ½ about 3-3.5 h). The peak and extent of systemic exposures for the 2 treatments were similar, with the 90% CIs of the test/reference ratios contained within 80% to 125%, establishing the existence of a fairly similar pharmacokinetic profile when administered with water. Chewable tablet taken without water showed a longer T max (median prolonged by about 0.75 h) and lower C max (mean reduced by about 22%), albeit with no statistically significant difference in the AUC, which means that the rate of absorption was slower but the total dose absorbed was equivalent. Therefore, for best results, it is recommended that doses of sildenafil citrate chewable tablet be taken with water. This may be particularly relevant for patients taking the 50-mg dose of sildenafil as the chewable tablet. For patients taking the 100-mg dose of the chewable tablet, the C max reduction of about 22% when taken without water may not be clinically relevant because of the demonstrated efficacy of the 50-mg dose of Viagra, the standard recommended dose [7][8][9].  Adverse events reported with chewable sildenafil were similar whether taken with or without water and comparable with those of Viagra. Oral sildenafil 100 mg, regardless of formulation, resulted in clinically insignificant decreases in systolic and diastolic blood pressure, an effect that is well documented with Viagra [10][11][12]. These changes were not considered clinically meaningful.
In conclusion, sildenafil citrate chewable tablet taken with water has a pharmacokinetic profile similar to the conventional film-coated tablet (Viagra) taken with water. When taken without water, the chewable tablet has somewhat lower C max (reduced by about 22%), delayed T max (1.75 h vs. 1.0 h), and equivalent AUC compared with Viagra taken with water. The taste-masking technology used in this formulation was successful in making the chewable sildenafil citrate formulation palatable. In this study in healthy volunteers, the sildenafil citrate 100-mg chewable tablet formulation was well tolerated, with reported adverse events that were consistent with the tolerability profile of Viagra.