9-(tetrahydro-2-furyl)-adenine and dazoxiben

The effects of aspirin and dazoxiben were determined on platelet behaviour in platelet-rich plasma (PRP) from 20 volunteers. Dazoxiben prevented aggregation and the release reaction induced by arachidonic acid (AA) in nine of the samples; in the other eleven aggregation and the release reaction still occurred. Aspirin always prevented aggregation and release but higher concentrations were needed in some of the samples of PRP than with others. When the platelets were sensitive to dazoxiben they were relatively sensitive to aspirin; when they were insensitive to dazoxiben they were relatively insensitive to aspirin. The effects of agents that alter production of cAMP on the sensitivity of platelets to aspirin and dazoxiben were determined. Increasing the intracellular level of cAMP rendered platelets more sensitive to the inhibitory effects of both aspirin and dazoxiben; lowering the level of cAMP made the platelets less sensitive to both agents.

Topics: Adenine; Adenylyl Cyclase Inhibitors; Adenylyl Cyclases; Arachidonic Acids; Aspirin; Blood Platelets; Cyclic AMP; Drug Interactions; Epoprostenol; Humans; Imidazoles; In Vitro Techniques; Platelet Aggregation; Serotonin

This study shows that dazoxiben, a selective inhibitor of thromboxane A2-synthetase in human platelets, inhibited arachidonic acid-induced platelet aggregation in platelet-rich plasma samples from four out of 16 healthy volunteers. In these four "responder" samples, the anti-aggregating effect of dazoxiben was prevented by the compound SQ 22536, a 9-substituted adenine analogue, endowed with an inhibitory activity on adenylate-cyclase. The compound SQ 22536 also counteracted the antiaggregating effect of prostaglandin D2, a known activator of platelet adenylate-cyclase. When platelet thromboxane A2-synthetase was blocked by dazoxiben, a marked increase of prostaglandin D2 was concomitantly observed both in "responder" and "non responder" samples. The compound SQ 22536 blunted the increase in platelet cAMP caused by either dazoxiben and sodium arachidonate or prostaglandin D2. It is suggested that the antiaggregating effect of dazoxiben is mediated by newly synthesized prostaglandin D2. The latter acts by stimulating adenylate-cyclase and increasing cAMP levels. The compound SQ 22536 prevents both phenomena. In "non responder" samples some factors--still to be defined--might counteract similarly to the compound SQ 22536 the antiaggregating activity of PGD2.

Topics: Adenine; Adenylyl Cyclase Inhibitors; Blood Platelets; Cyclic AMP; Humans; Imidazoles; Oxidoreductases; Platelet Aggregation; Prostaglandin D2; Prostaglandins D; Thromboxane A2; Thromboxane-A Synthase

Dazoxiben, a thromboxane synthase inhibitor, inhibits arachidonic acid induced aggregation in platelet-rich plasma from some donors only ("responders"). We have studied the effect of dazoxiben in vitro on platelet aggregation and prostaglandin (PG) metabolism and the influence of the incubation period and of exogenously added serum albumin (SA). SA, which increases the production of anti-aggregatory PGD2 from cyclic endoperoxides, induced "non-responder" human platelets to respond. With rabbit platelets, however, that are insensitive to PGD2, exogenous SA failed to potentiate dazoxiben-induced inhibition. The ratio between PGD2 and TXB2 + PGE2 formed was crucial in determining the response of human platelets to dazoxiben: whenever this ratio was high, platelet aggregation was inhibited. SQ 22536, an adenylate cyclase inhibitor, and NO164, a PGD2 antagonist, reversed the inhibition by dazoxiben in human platelet-rich plasma, stressing the importance of a PGD2 mediated rise of cyclic AMP for the effectiveness of a thromboxane synthase inhibitor.

Topics: Adenine; Adult; Arachidonic Acid; Arachidonic Acids; Blood Platelets; Female; Humans; Imidazoles; Male; Middle Aged; Oxidoreductases; Platelet Aggregation; Prostaglandin D2; Prostaglandins D; Serum Albumin; Thromboxane-A Synthase

The effects of several selective thromboxane synthetase inhibitors, alone and in combination with inhibitors of cyclic AMP phosphodiesterase, on the aggregation of human platelets in response to collagen, ADP, 1-0-alkyl-2-acetyl-sn-glycerophosphocholine (1-alkyl-2-acetyl-GPC), 9,11-azo-PGH2 and sodium arachidonate were studied in vitro. The thromboxane synthetase inhibitors caused little or no inhibition of aggregation at the concentrations used, while the phosphodiesterase inhibitors caused partial to complete inhibition of aggregation induced by all of the aggregating agents, with anagrelide being the most potent inhibitor and sodium arachidonate the agent most susceptible to inhibition. Combination of inhibitors of thromboxane synthetase with inhibitors of cyclic AMP phosphodiesterase, at concentrations of each which caused little or no effect when used alone, produced marked inhibition of platelet aggregation induced by collagen or arachidonic acid but not by the other aggregating agents studied. This interaction between the two classes of inhibitors appears to be due to the accumulation of cyclic AMP as it can be reversed by 9-(tetrahydro-2-furyl) adenine, an inhibitor of adenylate cyclase. Furthermore, as no synergistic inhibition was found using aggregating agents that are poor inducers of thromboxane formation, or using collagen after the ingestion of aspirin, it is most likely that activation of adenylate cyclase occurs because of diversion of thromboxanes into prostaglandins particularly PGD2. The present results suggest that combined use of inhibitors of thromboxane synthetase and inhibitors of cyclic AMP phosphodiesterase may provide a new approach to antithrombotic therapy.

Topics: Adenine; Adenosine Diphosphate; Collagen; Depression, Chemical; Drug Synergism; Humans; Imidazoles; Oxidoreductases; Phosphodiesterase Inhibitors; Platelet Aggregation; Quinazolines; Thromboxane-A Synthase