In vivo demonstration of M3 muscarinic receptor subtype selectivity of darifenacin in mice

A novel muscarinic receptor antagonist, darifenacin, inhibited specific binding of [N-methyl-3H]scopolamine ([3H]NMS) in the mouse bladder, submaxillary gland and heart in a concentration-dependent manner. The inhibitory effect was most potent in the submaxillary gland, followed by the bladder and heart. In addition, darifenacin inhibited specific [3H]NMS binding in the membranes of CHO-K1 cell lines expressing muscarinic M2 and M3 receptor subtypes, and the potency was significantly (22-fold) greater at the M3 than at the M2 subtype. At 0.5 to 12 h after oral administration of darifenacin, a significant increase in Kd values for specific [3H]NMS binding was seen in the bladder, submaxillary gland and lung of mice, compared with control values. Also, there was a sustained decrease in the Bmax values in the submaxillary gland. These data suggest that muscarinic receptor binding of oral darifenacin is rapid in onset and of a long duration. On the other hand, oral darifenacin exerted only temporary or little binding of muscarinic receptors in the heart and colon. Pilocarpine-induced salivary secretion in mice was continuously suppressed by oral darifenacin. The time-course of suppression coincided well with that for the muscarinic receptor binding in the submaxillary gland. The antagonistic effect of darifenacin against the dose–response curves for pilocarpine appeared to be insurmountable. In conclusion, the present study has shown that oral darifenacin may exert a pronounced and long-lasting binding of muscarinic receptors in tissues expressing the M3 subtype.