Anagrelide, a potent inhibitor of platelet aggregation, inhibits inositol phosphate generation and elevates cAMP

The effects of the potent platelet aggregation inhibitor, anagrelide (6,7-dichloro-1,5-dihydroimidazo(2,1-6)quinazolin-2(/sup 3/H)-one monohydrochloride; BL-4162A), on polyphosphoinositide metabolism in human platelets was investigated. Anagrelide maximally inhibited the thrombin-stimulated increase in inositol phosphates (IPn) at approximately 3.3 x 10/sup -6/M. The time of incubation required for a maximal response was between 15 and 30 min. Since anagrelide has been reported to inhibit low-Km cAMP phosphodiesterase, the authors compared its ability to inhibit stimulated IPn to that of the well known cAMP phosphodiesterase inhibitor IBMX. Both compounds inhibited IPn similarly except in experiments in which the IP phosphatase inhibitor, lithium chloride, was omitted. In these experiments, anagrelide totally inhibited, while IBMX only partially inhibited, the thrombin-stimulated elevation of IPn. The effect of anagrelide on platelet cyclic nucleotide levels was also measured. After 30 min incubation, concentrations of anagrelide which maximally inhibit aggregation induced a 2-fold increase in cAMP without significantly altering cGMP. These results suggest the mechanism of platelet aggregation inhibition includes an anagrelide-induced elevation of cAMP.