Salicylate, Sulphinpyrazone, Indomeihacin And Aspirin May Interact With Platelet Cyclooxygenase At A Site Different From The Substrate Binding Site

In vitro studies have shown that salicylate(SA), inactive by itself, prevents the inhibitory effect of aspirin (ASA)on platelet prostaglandin production. The purpose of this study was to verify this interaction in vivo and the ability of other non-steroidal anti-inflammatory drugs to counter ASA effect on platelet cyclooxygenase. Groups of male CD-COBS rats received one of the following drugs : SA (25-200 mg/kg i.p.),sulphinpyrazone(200 mg/kg p.o.), indomethacin (0.5 mg/kg i.p.) or the corresponding vehicles, followed after 30 min by ASA(1-50 mg/kg i.p.)as its soluble lysine salt. One or 24 hours later platelet-rich plasma was prepared and MDA (spec- trophotanetric assay) or TxB₂ (RIA) generated on stimulation with 0.4-1 mM Na arachidonate were measured. All 3 compounds blunted ASA inhibitory action. In particular: 1. The preverting effect of SA was dose-dependent and could be overcome by increasing the dose of ASA. At a dose ratio (SA:ASA) of 20 the effect of ASA was reduced by about 50%, but no interaction was apparent at a ratio below 5. 2. Sulphinpyrazone had a greater preventing effect when it was administered 6 h (100%) than 3 h or 30 min (60%) before ASA (5 mg). 3. Indo-methacin reduced by 50% the effect of 5 mg of ASA but not of 10 mg. Tests were made 24 h after ASA administration, when the inhibitory effect of indcmethacin was no longer detectable. 4. 200 mg/kg SA reduced by 50% the inhibitory effect of indcmethacin.

This data suggests that the 4 drugs interact with the same enzyme and that competition does not necessarily involve the substrate arachidonic acid. This supports the existence of a supplementary binding site which regulates cyclooxygenase activity, but is not directly involved with the substrate site. The clinical implications of these results include the interference of ASA with its hydrolysis product SA and a reappraisal of the pharmacological basis for the association of sulphinpyrazone and ASA in thrombosis prevention trials.