Hydrolysis of an orally active platelet inhibitory prostanoid amide in the plasma of several species

The prostanoid 3-oxa-4,5,6-trinor-3,7-inter-m-phenylene-PPGE₁-amide (OI-PGE₁-amide) has a prolonged duration of oral platelet aggregation inhibitory activity when compared to the parent free acid (OI-PGE₁) in the rat. When incubated in rat plasma at 1μg/ml for 30 seconds prior to addition of ADP, OI-PGE₁-amide inhibits in vitro rat platelet aggregation ≈50%. OI-PGE₁ inhibits at 1 ng/ml. Inhibition of platelet aggregation by plasma incubated with OI-PGE₁-amide (1μg/ml) increases with time and the rate of this increase differs with species. Incubation of OI-PGE₁ in plasma does not result in an increase of platelet inhibitory activity with time. The increase of platelet inhibitory activity was assumed to indicate hydrolysis of OI-PGE₁-amide to the more active OI-PGE₁. A compound, different from OI-PGE₁-amide, was isolated by an ion exchange/silica gel separation sequence from an incubation of OI-PGE₁-amide in rat plasma. It had potent platelet aggregation inhibitory activity. This material was shown to be OI-PGE₁ by thin-layer chromatography, gas chromatography and mass spectral analysis. Studies with ³H]-OI-PGE₁-amide confirmed the formation of OI-PGE₁ in plasma incubations. Amide hydrolytic activity was significantly different between species, the rank order being: rat > guinea pig > monkey human > dog. This relationship corresponded with that determined by measuring the increase in platelet inhibitory activity with time in plasma incubations of OI-PGE₁-amide reported above. Present data indicate that (a) OI-PGE₁-amide is hydrolyzed to the parent acid by plasma enzymes of several species and (b) hydrolytic activity of plasma varies widely between species.