In vivo and in vitro interaction of the novel selective histamine H1 receptor antagonist mizolastine with H1 receptors in the rodent.

The interaction of mizolastine (CAS 108612-45-9, SL 85.0324) with histamine H1 receptors has been evaluated in the rodent. Mizolastine inhibited with high affinity (IC50 = 47 nmol/l) the binding of [3H]pyrilamine to histamine H1 receptors in guinea pig cerebellar membranes and sections. The order of potency of mizolastine and various H1 antagonists in this binding assay was the following: cyproheptadine > pyrilamine > mequitazine > mizolastine > astemizole > terfenadine > cetirizine > loratadine. Mizolastine also potently antagonized the contractile effects of histamine in the guinea pig ileum (pA2 = 8.5) and histamine-induced stimulation of phosphoinositide turnover in rat cortical slices (IC50 = 0.35 mumol/l). In contrast, this compound displayed very low affinity for serotonergic, noradrenergic and muscarinic cholinergic receptors as evidenced in both binding assays and functional tests. In guinea pig cerebellar membranes, [3H]mizolastine labelled in a saturable and reversible manner a single population of binding sites with Kd and Bmax values of 1.1 nmol/l and 635 fmol/mg protein, respectively. [3H]Mizolastine binding in guinea pig cerebellar membranes was inhibited by histamine (IC50 = 30 mumol/l) and by drugs that possess affinity for the H1 receptor such as pyrilamine (IC50 = 1 nmol/1), DL-chlorphenyramine (IC50 = 6.4 nmol/l) terfenadine (IC50 = 6 nmol/l) and loratadine (IC50 = 50 nmol/l). At concentrations lower than 10 mumol/l, the H2 receptor ligands dimaprit and cimetidine and the H3 receptor ligands burimamide and 4-methyl-histamine failed to displace [3H]mizolastine binding.(ABSTRACT TRUNCATED AT 250 WORDS)