Comparison of in vitro selectivity profiles of solifenacin succinate (YM905) and current antimuscarinic drugs in bladder and salivary glands: a Ca²⁺ mobilization study in monkey cells

Abstract

We investigated the effects of the new muscarinic receptor antagonist solifenacin succinate [YM905; (+)-(1S,3′R)-quinuclidin-3′-yl 1-phenyl-1,2,3,4-tetrahydroisoquinoline-2-carboxylate monosuccinate] and the current antimuscarinic drugs for the treatment of overactive bladder (oxybutynin, tolterodine and darifenacin) on intracellular Ca²⁺ mobilization in response to M₃ muscarinic receptor activation in bladder smooth muscle and submandibular gland cells isolated from Cynomolgus monkeys. Solifenacin concentration-dependently inhibited carbachol-induced Ca²⁺ mobilization, with affinity constant values (pKi) of 8.5 ± 0.053 in bladder smooth muscle cells and 8.2 ± 0.051 in submandibular gland cells (n = 5). The pKi value of solifenacin was almost equivalent to the values of oxybutynin, tolterodine and darifenacin in bladder smooth muscle cells (8.7, 8.5 and 8.4, respectively), while being lower than those in submandibular gland cells (9.0, 8.7 and 8.8, respectively). The bladder-selectivity index (Ki ratio: submandibular gland/bladder) for solifenacin (2.1) was statistically higher, moreover, than those for oxybutynin, tolterodine and darifenacin (0.51, 0.65 and 0.46, respectively). These findings consequently indicate solifenacin's unique profile in terms of its selectivity for bladder smooth muscle cells over salivary gland cells in non-human primates, relative to oxybutynin, tolterodine and darifenacin. Solifenacin may, therefore, confer a promising therapeutic advantage for reducing adverse effects, such as dry mouth, exhibited by current antimuscarinic therapy for overactive bladder.

Introduction

Muscarinic receptors are responsible for the autonomic regulation of urinary bladder contractility. Although five subtypes (M₁–M₅) of muscarinic receptors have been identified by both molecular biological and pharmacological investigations Caulfield and Birdsall, 1998, Hulme et al., 1990, it is well known that the M₂ and M₃ subtypes are located postsynaptically on bladder smooth muscle Eglen et al., 1994, Eglen et al., 1996, Ehlert et al., 1997. M₃ receptors belong to a minority population in that tissue, but they have been proven to play a predominant role in mediating bladder smooth muscle contraction Eglen et al., 1994, Eglen et al., 1996, Ehlert et al., 1997. Current antimuscarinic drugs are consequently the mainstay of drug therapy to control overactive bladder, operating mainly through blockage of M₃ receptors. There is concern that they cause unwanted adverse effects, such as xerostomia/dry mouth Chapple, 2000, Feinberg, 1993, however, because they block not only bladder smooth muscle M₃ receptors but also the M₃ receptors in salivary glands that are pivotal for eliciting salivary secretory responses. New antimuscarinic agents which offer such therapeutic advantages as lowered effects on salivary gland functions have therefore been sought for clinical use.

Solifenacin succinate [YM905; (+)-(1S,3′R)-quinuclidin-3′-yl 1-phenyl-1,2,3,4-tetrahydroisoquinoline-2-carboxylate monosuccinate, Fig. 1] is a newly synthesized muscarinic receptor antagonist Ikeda et al., 2002, Kobayashi et al., 2001b that is currently undergoing clinical development for the treatment of overactive bladder. It has been reported that solifenacin binds to recombinant M₁, M₂ and M₃ muscarinic receptor subtypes with affinity constants (pKi) of 7.6, 6.9 and 8.0, respectively, and behaves as a competitive antagonist against muscarinically-induced contractile response in the isolated guinea pig bladder (Ikeda et al., 2002). It is interesting that solifenacin inhibited bladder contraction more preferentially than salivary secretion in anesthetized rats, in contrast to oxybutynin, a traditional class of clinical antimuscarinic drug used for treating overactive bladder (Ikeda et al., 2002), although those in vivo responses are mediated equally through the activation of M₃ receptors. However, the pharmacological basis for this action of solifenacin remains unknown.

Recently, new antimuscarinc drugs such as tolterodine Malone-Lee et al., 2001, Abrams et al., 1998 and darifenacin (Bohm, 2001) have undergone clinical evaluations for the treatment of overactive bladder. In vivo pharmacological data have suggested that they also exhibit a selectivity profile for bladder contraction over salivary gland function relative to oxybutynin Nilvebrant et al., 1997, Williamson et al., 1997. There has, however, still been only limited investigation to permit us to contrast the pharmacological profile of solifenacin with those of tolterodine and darifenacin in bladder and salivary glands. No data have been available, furthermore, regarding M₃ receptor antagonism by those drugs in the bladder and salivary gland tissues of non-human primates, which could be useful pharmacological tools for predicting the drugs' efficacy in humans. The present study was therefore undertaken to examine the tissue selectivity profiles of solifenacin, oxybutynin, tolterodine and darifenacin in bladder and salivary glands using non-human primate cells in vitro. In brief, the effects of those antagonists were evaluated on carbachol-induced intracellular Ca²⁺ mobilization in bladder smooth muscle and submandibular gland cells isolated from Cynomolgus monkeys, and the ratios of the antagonist's affinity constants (bladder vs. submandibular gland), defined as an index of bladder-selectivity profile, were statistically compared.