Molecular aspects of cloricromene (AD₆) distribution in human platelets and its pharmacological effects

Cloricromene (AD₆) is an investigational drug which inhibits platelet aggregation and release reaction. We studied the relationship between its action and its distribution and metabolism in platelets. Incubation of anticoagulated whole blood or platelet-rich plasma (PRP) without exogenous aggregating agents resulted in a progressive decrease of platelet count with a concomitant increase of β-thromboglobulin (BTG) release. AD₆ (20-50 μmol/1), but not acetylsalicylic acid (ASA), incubated with whole blood or PRP, prevented the fall in platelet count and the release of BTG for at least 150 min. Moreover, incubation of PRP with AD₆ (50 μmol/1) and subsequent stimulation by ADP at threshold concentrations resulted in a significant reduction (about 30%) in aggregation for at least 90 min. AD₆ (20 μmol/1) added to PRP was rapidly metabolized by hydrolysis of an ester bond to AD₆ acid, a stable catabolite pharmacologically inactive in platelets. Significant amounts of AD₆ acid (up to 13.26 ± 2.80 pmol/10⁶ platelets) were associated with the platelets after incubation either at 37°C or 4°C. The amount of AD₆ acid in the platelet pellet was proportional to AD₆ concentration (2 to 100 μmol/1). PRP incubation with AD₆ acid (20 μmol/1) resulted in very low levels (< 1 pmol/10⁶ platelets) of the same compound in the platelet pellet after 1, 5 or 30 min. These data suggest that AD₆ is taken up as an ester and converted to its acid catabolite with a consequent long-lasting inhibition of platelet function.