2-Aminoethyl Diphenylborinate (2-APB) Analogues: Part 4-Poly-Boron Compounds: Regulators of Ca2+ Release and Consequent Cellular Processes

Inhibitory activities of 74 poly-boron compounds for SOCE and IICR were measured. Activities of poly-boron compounds were compared with 2APB, mono-boron and bis-boron compounds. The IC50 of best poly-boron compound 1042 was 2 μM. This value was almost same as IC50 3 μM of 2-APB. Poly (aminoethoxyboryldiphenylether) 1042 is best candidate for regulation of Ca2+ release and consequent cellular processes in this paper. 2-Aminoethyl Diphenylborinate (2-APB) Analogues: Part 4 Poly-Boron Compounds: Regulators of Ca2+ Release and Consequent Cellular Processes

We synthesized several 2-APB analogues and measured their inhibitory activities on Store-Operated Calcium Entry (SOCE) and IP 3 Induced Calcium Release (IICR). We found that bis boron compound DBP 161 and DBP 163 were 10 times more effective than 2-APB [38]. Previously, we studied bis-boron compounds in more detail [39,40]. We extended these studies and synthesized 493 2-APB analogues and measured their inhibitory activities on SOCE and IICR [38][39][40][41][42][43][44]. The numbers of compounds and data obtained are so many. We decided to report the results by dividing into three part.; Part 1 (mono-boron compounds), Part 2 (bis-boron compounds), and Part 3 (poly -boron compounds). We have reported about mono-boron compounds [45] and bis-boron compounds recently [46]. This time, we report about poly-boron compounds. Here we analyzed SOCE inhibitory activities and IICR inhibitory activities of our poly-boron compounds collection.
We believe that if we would regulate Ca 2+ release and associated cellular processes by boron compounds with various Ca 2+ releaserelated activities, we may therapeutically intervene in many diseases, such as heart diseases and Alzheimer`s disease.
Preparation of poly (aminoethoxyboryldiphenylether) 8001: 7142 (48.8 mg) was dissolved in ethanol 1.5 ml. Ethanolamine 15.7 mg was added and stirred overnight. N-hexane 10 ml was added and filtered to get 8001 11.7 mg as white solid. dissolved in ethanol 0.2 ml and water 2 ml Ethanolamine 19 mg was added. The reaction mixture was heated for 17 hrs at 80°C. Ether 10 ml was added to get 1024 17 mg as white precipitate.
Solution A and solution B was mixed and the solution was gradually warmed and stirred at room temperature for 15 hr. The mixture was acidified with 1N hydrochloric acid, and the organic layer was washed with water and dried and concentrated to give the title compound 178 mg.

Methods
We have assayed the inhibitory activity of the 2-APB analogues for SOCE and IICR using our improved assays described previously [45].

Results and Discussion
We measured inhibitory activities of poly-boron compounds for SOCE and IICR. The results are shown in Figure 1, shown as supplementary file.
From Figure 1, typical 18 poly boron compounds are selected as follow.

Comparison of 2APB, mono-boron compounds, bis-boron compounds and poly-boron compounds
• The IC 50 of best poly-boron compound 1042 at this paper is 2 µM.
• The IC 50 of 2-APB for SOCE inhibition is 3 µM. That is, the IC 50 of poly-boron compounds showed almost same activity as 2-APB.
• The IC 50 of best mono-boron compound 919 at first paper [45] is 0.2 µM. The IC 50 of best bis-boron compound 1024 reporting at previous paper [46] is 0.2 µM.
That is, the mono-boron compounds and bis-boron compounds showed almost 10 times strong activity than poly-boron compounds. Poly-boron compound does not fit well to IP 3 receptor, because of bulkiness of the molecule and does not inhibit IP 3 -induced calcium release strongly.
These compounds can thus regulate the Ca 2+ release and consequent cellular response. Some of these compounds were shown to inhibit the calcium dependent enzyme transglutaminase [44]. Transglutaminase inhibitors block the abnormal cross-link of protein [43,44] and therefore they may slow down or even stop the progression of diseases caused by misfolded proteins, such as Huntington`s disease.
The 2-APB analogues presented in this study could be proven to be excellent lead compounds for many human diseases including heart disorders [53], Alzheimer`s [54,55] and Huntington`s disease [56,57].
We have shown widely different kinds of active compounds with IC 50 ranging 0.2 to 50 µM from mono-boron, bis-boron and poly-boron compounds. By choosing the compound we can control the release of Ca 2+ and regulate many cellular processes such as secretion, cardiac contraction, fertilization, proliferation, synaptic plasticity, atrial arryhythmiss [31], inhibition of calcium entry channel [25], excitationcontraction coupling in the heart [32], arrhythmogenic action of endothelin-1 on ventricular cardiac myocytes [34], dysreguration of neural calcium signaling in Alzheimer disease [55], Huntington aggregation [56,57] and protein cross-link by transglutaminase [43].
We believe that many investigators will find these reagents regulating Ca 2+ release and related cellular processes very useful.