E3040 sulphate, a novel thromboxane synthase inhibitor, blocks the Cl − secretion induced by platelet‐activating factor in isolated rat colon

E3040 (6‐hydroxy‐5,7‐dimethyl‐2‐methylamino‐4‐(3‐pyridylmethyl)benzothiazole), is a novel dual inhibitor of 5‐lipoxygenase (5‐LOX) and thromboxane synthase (Tx synthase). Here, we examined the effects of E3040 sulphate, a sulphate conjugate of E3040, on these enzyme activities in cell‐free systems and on the thromboxane A 2 (TxA 2 )‐mediated Cl − secretion induced by platelet‐activating factor (PAF) in isolated rat colons. E3040 sulphate inhibited Tx synthase activity in a concentration‐dependent manner (IC 50 =0.013 μ M ), whereas it induced little effects on 5‐LOX and cyclo‐oxygenase activities (IC 50 >100 μ M ) with the cell‐free enzyme assay. With isolated rat colonic mucosa, E3040 sulphate in a concentration‐dependent manner (IC 50 =1.8 μ M ) inhibited the Cl − secretion induced by 10 μ M PAF. On the other hand, E3040 sulphate (30 μ M ) induced no effect on the prostaglandin E 2 (0.5 μ M )‐ and leukotriene D 4 (1 μ M )‐induced Cl − secretion in the colon. PAF (10 μ M ) increased a release of TxB 2 , a stable metabolite of TxA 2 , from the colonic mucosa. This increase was significantly inhibited by subsequent addition of E3040 sulphate (30 μ M ). Probenecid (100 μ M ), an inhibitor of organic anion transporter, abolished the inhibitory effect of E3040 sulphate on the PAF‐induced Cl − secretion. Another inhibitor, sulphobromophthalein (30 μ M ) partially but significantly attenuated the effect of E3040 sulphate. p ‐Aminohippuric acid (1 m M ) had no effect. These findings suggest that E3040 sulphate is a novel Tx synthase inhibitor, and that E3040 sulphate taken up into the colonic cells by organic anion transporters inhibits the PAF‐induced Cl − secretion by blocking a release of TxA 2 .British Journal of Pharmacology (2002) 136 , 383–390; doi: 10.1038/sj.bjp.0704723