Palosuran inhibits binding to primate UT receptors in cell membranes but demonstrates differential activity in intact cells and vascular tissues

Background and purpose: The recent development of the UT ligand palosuran (1‐[2‐(4‐benzyl‐4‐hydroxy‐piperidin‐1‐yl)‐ethyl]‐3‐(2‐methyl‐ quinolin‐4‐yl)‐urea sulphate salt) has led to the proposition that urotensin‐II (U‐II) plays a significant pathological role in acute and chronic renal injury in the rat. Experimental approach: In the present study, the pharmacological properties of palosuran were investigated further using a series of radioligand binding and functional bioassays. Key results: Palosuran functioned as a ‘primate‐selective’ UT ligand in recombinant cell membranes (monkey and human UT Ki values of 4±1 and 5±1 nM), lacking appreciable affinity at other mammalian UT isoforms (rodent and feline Ki values >1 μM). Paradoxically, however, palosuran lost significant (10‐ to 54‐fold) affinity for native and recombinant human UT when radioligand binding was performed in intact cells (Ki values of 50±3 and 276±67 nM). In accordance, palosuran also exhibited diminished activity in hUT (human urotensin‐II receptor)‐CHO (Chinese hamster ovary) cells (IC50 323±67 nM) and isolated arteries (Kb>10 μM in rat aorta; Kb>8.5 μM in cat arteries; Kb>1.6 μM in monkey arteries; Kb 2.2±0.6 μM in hUT transgenic mouse aorta). Relative to recombinant binding Ki values, palosuran was subjected to a 392‐ to 690‐fold reduction in functional activity in monkey isolated arteries. Such phenomena were peculiar to palosuran and were not apparent with an alternative chemotype, SB‐657510 (2‐bromo‐N‐[4‐chloro‐3‐((R)‐1‐methyl‐ pyrrolidin‐3‐yloxy)‐phenyl]‐4,5‐dimethoxybenzenesulphonamide HCl). Conclusions and implications: Collectively, such findings suggest that caution should be taken when interpreting data generated using palosuran. The loss of UT affinity/activity observed in intact cells and tissues cf. membranes offers a potential explanation for the disappointing clinical efficacy reported with palosuran in diabetic nephropathy patients. As such, the (patho)physiological significance of U‐II in diabetic renal dysfunction remains uncertain. British Journal of Pharmacology (2008) 155, 374–386; doi:10.1038/bjp.2008.266; published online 30 June 2008