Binding kinetics and antiplatelet activities of picotamide, a thromboxane A2 receptor antagonist.

1 Picotamide was shown to inhibit platelet binding of thromboxane A2 (TxA2)-mimetics and to cause a reduction of TxA2 platelet receptors after in vivo administration. The present study aimed to investigate directly [3H]-picotamide binding to human platelets and in particular the relationship between binding kinetics and antiaggregating properties. 2 [3H]-picotamide time-dependently bound to a single class of platelet TxA2 receptors with a KD of 325 nmol I` at equilibrium. The binding was displaceable by TxA2 analogues U46619 and ONO11120 (Ki 19 and 28 nmol I-I respectively) but not by prostacyclin (PGI2), prostaglandin E2 (PGE2) and TxB2. Antiaggregating activity and TxA2 formation inhibition paralleled with binding kinetics. 3 By prolonging the incubation time from 30 to 120 min, picotamide showed a progressively increasing non-displaceable binding, whereas specific displaceable binding decreased in comparison to the values reached at 30 min. Non displaceable binding was specific, temperature-dependent, saturable and followed a Michaelis-Menten kinetic (V1,app = 130 fmol per i01 platelets h-', KMaPP = 330 nmol 1-'). Picotamide progressively underwent a specific stable interaction with its platelet receptor. 4 In conclusion, after an initial reversible binding, a progressive stabilization of picotamide binding takes place resulting in a progressively more stable interaction with platelets.