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 l-1 at equilibrium. The binding was displaceable by TxA2 analogues U46619 and ONO11120 (Ki 19 and 28 nmol l-1 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 (Vmaxapp = 130 fmol per 10(8) platelets h-1, KMapp = 330 nmol l-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.