Inhibition of thrombocyte aggregation by oral motapizone and other drugs.

Ten healthy subjects took single oral doses of placebo, 8.8 +/- 1.8 mg motapizone, 40 +/- 13 mg captopril, 25 mg dihydralazine, 20 mg nifedipine and 4.5 +/- 1.1 mg prazosin in random order, and, as the last preparation 500 mg acetylsalicylic acid. Thrombocyte aggregation induced "ex-vivo" with collagen, ADP and adrenaline was measured before and after 60 min. Immediately before each dose, the "threshold concentration" of each agent was determined in each subject, i.e. the concentration producing about 90% of maximal aggregation. After the preparation had been taken, aggregation was induced with 1-, 2- and 4-times the threshold concentration. Both motapizone and also acetylsalicylic acid caused marked inhibition of aggregation at up to 4-times the threshold concentration; the dose ratio was about 1:50. Motapizone produced greater inhibition of the aggregation induced by ADP and acetylsalicylic acid than of that due to collagen. The inhibitory actions after captopril, dihydralazine, nifedipine and prazosin were weak and did not significantly differ from placebo.