Pharmacological evaluation of both enantiomers of (R,S)-BM-591 as thromboxane A2 receptor antagonists and thromboxane synthase inhibitors.

The aim of this work is to evaluate the anti-thromboxane activity of two pure enantiomers of (R,S)-BM-591, a nitrobenzene sulfonylurea chemically related to torasemide, a loop diuretic. The drug affinity for thromboxane A2 receptor (TP) of human washed platelets has been determined. In these experiments, (R)-BM-591 (IC50 = 2.4 ± 0.1nM) exhibited a significant higher affinity than (S)-BM-591 (IC50 = 4.2 ± 0.15nM) for human washed platelets TP receptors. Both enantiomers were stronger ligands than SQ-29548 (IC50 = 21.0 ± 1.0nM) and sulotroban (IC50 = 930 ± 42nM), two reference TXA2 receptor antagonists. Pharmacological characterisations of (S)-BM-591 and (R)-BM-591 were compared in several models. Thus, (R)-BM-591 strongly prevented platelet aggregation induced by arachidonic acid (AA) (600μM) and U-46619 (1μM) while (S)-BM-591 showed a lower activity. On isolated tissues pre-contracted by U-46619, a stable TXA2 agonist, (S)-BM-591 was more potent in relaxing guinea-pig trachea (EC50 = 0.272 ± 0.054μM) and rat aorta (EC50 = 0.190 ± 0.002μM) than (R)-BM-591 (EC50 of 9.60 ± 0.63μM and 0.390 ± 0.052μM, respectively). Moreover, at 1μM, (R)-BM-591 totally inhibited TXA2 synthase activity, expressed as TXB2 production from human platelets, while at the same concentration, (S)-BM-591 poorly reduced the TXB2 synthesis (22%). Finally, in rats, both enantiomers lost the diuretic activity of torasemide. In conclusion, (R)-BM-591 exhibits a higher affinity and antagonism on human platelet TP receptors than (S)-BM-591 as well as a better thromboxane synthase inhibitory potency. In contrast, (S)-BM-591 is more active than the (R)-enantiomer in relaxing smooth muscle contraction of rat aorta and trachea guinea pig. Consequently, (R)-BM-591 represents the best candidate for further development in the field of thrombosis disorders.