The inhibition of arachidonic acid mobilization in human platelets by R59 022, a diacylglycerol kinase inhibitor.

R59 022 (6-[2-[4-[(4-fluorophenyl)phenylmethylene]-1- piperidinyl]ethyl]-7-methyl-5H-thiazolo[3,2-a]pyrimidin-5-one) has been suggested as an inhibitor of diacylglycerol kinase in erythrocyte membranes and intact platelets. In the present study, we have investigated the effects of this drug on arachidonic acid mobilization occurring in response to thrombin in intact human platelets. Our results indicate that release of arachidonic acid from membrane phospholipids such as phosphatidylcholine and phosphatidylinositol was severely impaired by R59 022 and the extent of inhibition amounted to 77% and 84%, respectively, as compared to controls. This resulted in a dramatic decrease in the accumulation of free arachidonic acid (labeled/unlabeled) and the percent inhibition of free arachidonic acid accumulation amounted to 80-90% as compared to controls. Furthermore, the drug caused a significant accumulation of thrombin-induced diacylglycerol (labeled) without affecting the formation of labeled phosphatidic acid (PA). We found no significant changes in the radioactivity of either phosphatidylethanolamine or phosphatidylserine following stimulation with thrombin in the presence or absence of R59 022. We conclude that the observed inhibition of thrombin-induced arachidonic acid mobilization by R59 022 may be due to its effects on the activities of diacylglycerol lipase/phospholipase A2. In addition, the failure of further stimulation of thrombin-induced PA by R59 022 may indicate that PA-specific phospholipase A2 is either not involved in the release of arachidonic acid or is not a major source for arachidonic acid release in thrombin-stimulated human platelets. These findings may prove to be important when this drug is used as a selective inhibitor of diacylglycerol kinase.