Differential eudismic ratios in the antagonism of human platelet function by phenoxy- and thiophenoxyacetic acids.

The antilipidemic drug clofibric acid (CPIB) exhibits antiplatelet effects. In order to examine the role of enantioselectivity and hydrophobicity, the mono(desmethyl) enantiomers of 2-(4-chlorophenoxy)propanoic acid (CPPA), related butanoate homologs, 2-(4-chlorophenoxy)butanoic acid (CPBA), thioether relatives, 2-(4-chlorothiophenoxy)propanoic acid (CTPA) and corresponding butanoate homologs, 2-(4-chlorothiophenoxy)butanoic acid (CTBA) were studied. All compounds inhibit prostaglandin-dependent human platelet aggregation and serotonin secretion induced by ADP. For the phenoxy acid series, (+)-(R)-CPPA is 5-fold more potent than either (-)-(S)-CPPA or CPIB giving a rank order potency of (+)-(R)-CPPA > (+)-(R)-CPBA > (-)-(S)-CPPA > (-)-(S)-CPBA. With the exception of (-)-(S)-CTPA, all thiophenoxy acid enantiomers are less potent than their respective phenoxy acid isomers [(+)-(R)-CTPA > (-)-(S)-CTPA > (+)-(R)-CTBA > (-)-(S)-CTBA]. The same rank order of potencies are observed against responses induced by arachidonic acid (AA) with the exception of CPIB which is inactive. However, inhibition of thrombin-induced [3H]-AA release by phenoxyacetic acids is not stereoselective but correlates with hydrophobicity.