Vol. 57, Issue 3, 595-601, March 2000

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

Christine T. Murphy, Andrew M. Riley, Steve J. Mills, Catherine J. Lindley, Barry V. L. Potter, and John Westwick

Department of Pharmacy and Pharmacology, University of Bath, Bath, UK

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-trisphosphorothioate [Ins(1,4,6)PS₃], prepared by total synthesis, were examined in Ca²⁺ 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-inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] receptor, releasing less than 20% of the Ins(1,4,5)P₃-sensitive Ca²⁺ store. D-Ins(1,4,6)PS₃ displaced specifically bound [³H]Ins(1,4,5)P₃ from rat cerebellar membranes, although displacement was some 34-fold weaker than by D-Ins(1,4,5)P₃. D-Ins(1,4,6)PS₃ displaced [³H]Ins(1,4,5)P₃ from cerebellar membranes with roughly twice the affinity of DL-Ins(1,4,6)PS₃ (IC₅₀ value = 1.4 ± 0.35 µM compared with 2.15 ± 0.13 µM), whereas D-Ins(1,3,6)PS₃ displaced [³H]Ins(1,4,5)P₃ with roughly twice the affinity of DL-Ins(1,3,6)PS₃ (IC₅₀ value = 17.5 ± 5.8 µM compared with 34 ± 10 µM), confirming that the activity of both these phosphorothioates resides in their D-enantiomers. Increasing concentrations of either D-Ins(1,3,6)PS₃ or D-Ins(1,4,6)PS₃ were able to partially antagonize Ca²⁺ release induced by submaximal concentrations of Ins(1,4,5)P₃, an inhibition that could be overcome by increasing the concentration of Ins(1,4,5)P₃, suggesting competition for binding at the Ins(1,4,5)P₃-R. The only low-efficacy partial agonists at the Ins(1,4,5)P₃-R discovered 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)P₃-R but maintains the lowest efficacy of all the partial agonists thus far identified. As such, it may be a useful tool for pharmacological intervention in the polyphosphoinositide pathway and an important lead compound for the development of further Ins(1,4,5)P₃-R antagonists.