Effects of sulphasalazine and its metabolites on prostaglandin synthesis, inactivation and actions on smooth muscle

1 We have investigated the effects of sulphasalazine and of its principal colonic metabolites (5-aminosalicylic acid and sulphapyridine) on prostaglandin inactivation, synthesis and actions on gastrointestinal smooth muscle.

2 Sulphasalazine inhibits prostaglandin F_2α breakdown in 100,000 g supernatants in all organs so far tested from 7 species with an ID₅₀ of approx. 50 μM; it has a selective action on prostaglandin 15-hydroxydehydrogenase and does not inhibit prostaglandin Δ-13 reductase, prostaglandin 9-hydroxydehydrogenase or `enzyme X' at millimolar concentrations. Enzyme activities were measured radiochemically or by bioassay.

3 Sulphapyridine and 5-aminosalicylic acid do not inhibit prostaglandin inactivation in vitro (4 species tested). A methyl analogue of sulphasalazine is a more potent inhibitor than the parent compound. Rabbit colon prostaglandin F_2α metabolism in vitro was inhibited by the following drugs with ID₅₀ values (μM) of: diphloretin phosphate 20, sulphasalazine 50, indomethacin 220, frusemide 1000 and aspirin 10,000. A similar rank order of potencies was obtained with rabbit kidney.

4 Sulphasalazine at 50 to 100 μM inhibited inactivation of prostaglandin E₂ in the perfused rat and guinea-pig lung by 3 to 40% (rat) and 32 to 100% (guinea-pig) when measured by superfusion cascade bioassay and of prostaglandin F_2α by 43.6 ± 6.5% in rat lung perfused with 50 μM sulphasalazine and assayed radiochemically.

5 Prostaglandins E₁ and E₂ were 97.0 ± 8.2% and 92.3 ± 6.8% inactivated in the lungs after intravenous injection in the anaesthetized rat as measured by reference to their vasodepressor potencies when injected intra-arterially. Prostaglandin A₂ was not similarly inactivated. Pulmonary inactivation was prevented in the presence of an intravenous infusion of 16.3 μg kg^-1 min^-1 sulphasalazine and partially inhibited at a lower infusion rate.

6 Prostaglandin biosynthesis from arachidonic acid was measured in microsomal preparations from four sources by bioassay and radiochemical methods. Indomethacin was a potent inhibitor (ID₅₀ 0.8 to 4.1 μM) but sulphasalazine and its methyl analogue were very weak inhibitors (ID₅₀ 1500 to > 5000 μM), 5-aminosalicylic acid was weaker still and sulphapyridine inactive.

7 Sulphasalazine at 50 μM did not affect the actions of prostaglandins on five smooth muscle preparations; at 500 μM there was a rapidly reversible and probably non-specific antagonism of responses to low doses of prostaglandins.

8 The specificity and selectivity of the interaction of sulphasalazine and its metabolites with the formation, breakdown and actions of prostaglandins are discussed.