Competitive inhibition of platelet thromboxane A2 receptor binding by picotamide

On the basis of indirect pharmacological evidence, picotamide, a methoxy derivative of 4-hydroxy-isophthalic acid (N,N'bis(3-picolyl)-4-methoxy-isophthalamide) has been postulated to inhibit platelet aggregation by competitively interfering with the thromboxane A2 (TxA2) platelet receptor. In the present study the interaction between picotamide and TxA2 receptors on human platelets was investigated by a direct radioligand assay method with [125I]PTA-OH and [3H]U46619 as labelled radioligands. The ONO11120 and U46619 inhibitory constants (Ki) for [125I]PTA-OH binding were 19 +/- 4 and 17 +/- 3 nM, respectively. Picotamide displaced [125I]PTA-OH binding with a Ki of 1472 +/- 321 nM. The Ki for ONO 11120 and U46619 on [3H]U46619 binding were 42 +/- 12 and 16 +/- 5 nM, respectively, whereas the Ki for picotamide was 1648 +/- 431 nM. These data provide evidence that picotamide can directly inhibit the TxA2 platelet receptor.