Add time:09/02/2019         Source:sciencedirect.com

The affinity of pizotifen, ketotifen and other tricyclic antimuscarinic drugs for different muscarinic receptor subtypes was investigated in vitro in functional experiments with field-stimulated vas deferens of the rabbit (M1 and M2 receptors) and with ileum and trachea of the guinea-pig (M3 receptors). All compounds were competitive antagonists in the three tissues. Like the close analogue cyproheptadine (pA2 = 7.99−8.08), pizotifen (pA2 = 7.23−7.81) and ketotifen (pA2 = 6.34−6.99) were devoid of selectivity for the receptor subtypes studied. Thiazinamium, although exhibiting high for muscarinic receptors (pA2 = 7.83−8.51), was found to be non-selective. In contrast, the novel pirenzepine analogue nuvenzepine was selective for M1 receptors (pa2 = 6.63−7.74). The lack of selectivity of cyproheptadine, pizotifen and kitotifen is reflected in the chemical structures of these drugs. All three antagonists are composed of a very similar tricyclic ring system linked to a 1-methyl-4- piperidylene ring. The finding that thiazinamium, pizotifen and cyproheptadine were potent muscarinic antagonists and possessed non-selective affinity characteristics may have therapeutic implications.

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