Additive effects of mirabegron on muscarinic receptor binding and on relaxation of cholinergic detrusor muscle contraction by antimuscarinics

Mirabegron is the first selective β₃-adrenoceptor agonist, to be developed as an alternative to antimuscarinic therapy for patients with overactive bladder (OAB). This agent exerts off-target effects on muscarinic receptors. Its combination with solifenacin results in a higher incidence of anticholinergic effects, such as dry mouth, than solifenacin alone. The present study investigated whether the combination of mirabegron and antimuscarinics exerted additive effects on muscarinic receptors and cholinergic contraction in rat tissues. Muscarinic receptor binding activity and inhibitory effect on carbachol-induced contraction in rat tissues were evaluated by radioreceptor assays using [N-methyl-³H]scopolamine chloride (3HNMS) and the organ-bath procedure, respectively. The muscarinic receptor binding activities of solifenacin and imidafenacin in the rat brain increased when administered in combination with mirabegron. Moreover, the inhibitory effects of solifenacin and imidafenacin on carbachol-induced contractions in rat isolated bladder strips were increased by mirabegron, particularly at lower concentrations of solifenacin and imidafenacin. This is the first study to suggest that mirabegron in combination with antimuscarinics exerts additive effects for muscarinic receptor binding and inhibitory effects on cholinergic contractions in bladder strips. The results obtained also showed that these additive effects may contribute to enhanced therapeutic effects on OAB, but also cholinergic adverse effects.