Disaggregation and reaggregation of 'irreversibly' aggregated platelets: a method for more complete evaluation of anti-platelet drugs.

Anti-platelet drugs are generally screened by evaluating their ability to influence initiation and development of irreversible aggregation of platelets. However, to fully characterize the inhibitory effects of an agent, it is essential to determine if it can also disperse irreversibly aggregated cells, whether drug-treated, dispersed platelets are refractory, sensitive to some but not all aggregating agents, as sensitive to all agents as before initial exposure, hypersensitive or whether they can be restored to a sensitive state by modulation of the platelet membrane. We have developed an in vitro system for evaluating the influence of a variety of agents on disaggregation and reaggregation of aggregated blood platelets. Results demonstrate that some agents which inhibit aggregation can also cause disaggregation, while others cannot. The ability to disaggregate is often selective, revealing a dependence on the nature of the agent causing aggregation, and the time after irreversible aggregation that the inhibitor is added. Agents that elevate intracellular cyclic adenosine monophosphate levels were potent inducers of platelet dissociation in aggregates caused by adenosine diphosphate. On the other hand, antimalarials, calmodulin complexing agents and phospholipase inhibitors caused dissociation of platelet aggregates, irrespective of the agonist used. By and large, the dissociated cells were refractory to the action of agonists. Restoration of the sensitivity of disaggregated platelets by treatment with epinephrine demonstrate an ability of inhibitor treated, refractory platelets to recover full functional capacity almost immediately. Thus, careful study of the effects of inhibitors on disaggregation, recovery and reaggregation may reveal features critical to the selection of anti-platelet drugs for clinical utilization.