myo-Inositol 1,4,6-trisphosphorothioate and myo-inositol 1,3,6-trisphosphorothioate: partial agonists with very low intrinsic activity at the platelet myo-inositol 1,4,5-trisphosphate receptor

Racemic mixtures and enantiomerically pure D-isomers of both myo-inositol 1 ,3,6-trisphosphorothioate [Ins(1,3,6)PS3] and myo-inositol 1,4,6-trisphosph orothioate [Ins(1,4,6)PS3], prepared by total synthesis, were examined in C a2+ flux and binding assays. Both D-Ins(1,3,6)PS3 and D-Ins(1,4,6)PS3 were shown to be low intrinsic activity partial agonists at the platelet myo-ino sitol 1,4,5-trisphosphate [Ins(1,4,5)P-3] receptor, releasing less than 20% of the Ins(1,4,5)P-3-sensitive Ca2+ store. D-Ins(1,4,6)PS3 displaced speci fically bound [H-3]Ins(1,4,5)P-3 from rat cerebellar membranes, although di splacement was some 34-fold weaker than by D-Ins(1,4,5)P-3. D-Ins(1,4,6)PS3 displaced [H-3]Ins(1,4,5)P-3 from cerebellar membranes with roughly twice the affinity of DL-Ins(1,4,6)PS3 (IC50 value = 1.4 +/- 0.35 mu M compared w ith 2.15 +/- 0.13 mu M), whereas D-Ins(1,3,6)PS3 displaced [H-3]Ins(1,4,5)P -3 with roughly twice the affinity of DL-Ins(1,3,6)PS3 (IC50 value = 17.5 /- 5.8 mu M compared with 34 +/- 10 mu M), confirming that the activity of both these phosphorothioates resides in their D-enantiomers. Increasing con centrations of either D-Ins(1,3,6)PS3 or D-Ins(1,4,6)PS3 were able to parti ally antagonize Ca2+ release induced by submaximal concentrations of Ins(1, 4,5)P-3, an inhibition that could be overcome by increasing the concentrati on of Ins(1,4,5)P-3, suggesting competition for binding at the Ins(1,4,5)P- 3-R. The only low-efficacy partial agonists at the Ins(1,4,5)P-3-R discover ed to date have been phosphorothioates; the novel D-Ins(1,3,6)PS3 and D-Ins (1,4,6)PS3 can now be added to this small group of analogs. However, D-Ins( 1,4,6)PS3 has a relatively high affinity for the Ins(1,4,5)P-3-R but mainta ins the lowest efficacy of all the partial agonists thus far identified. As such, it may be a useful tool for pharmacological intervention in the poly phosphoinositide pathway and an important lead compound for the development of further Ins(1,4,5)P-3-R antagonists.