Effects of olopatadine, a new antiallergic agent, on human liver microsomal cytochrome P450 activities.

Olopatadine, a new histamine H(1) receptor-selective antagonist, is a tricyclic drug containing an alkylamino moiety. Some compounds containing a similar alkylamino group form a cytochrome p450 (p450) -iron (II)-nitrosoalkane metabolite complex [metabolic intermediate complex (MIC)], thereby causing quasi-irreversible inhibition of the p450. There was concern that olopatadine might also form MICs, therefore, the present investigation was undertaken to explore this possibility. We identified the enzymes catalyzing olopatadine metabolism and investigated the effect of olopatadine on human p450 activities. During incubation with human liver microsomes in the presence of a NADPH-generating system, olopatadine was metabolized to two metabolites, M1 (N-monodemethylolopatadine) and M3 (olopatadine N-oxide) at rates of 0.330 and 2.50 pmol/min/mg protein, respectively. Troleandomycin and ketoconazole, which are both selective inhibitors of CYP3A, significantly reduced M1 formation but specific inhibitors of other p450 isozymes did not decrease M1 formation. Incubation of olopatadine with cDNA-expressed human p450 isozymes confirmed that M1 formation was almost exclusively catalyzed by CYP3A4. The formation of M3 was enhanced by N-octylamine and was inhibited by thiourea. High specific activity of M3 formation was exhibited by cDNA-expressed flavin-containing monooxygenase (FMO)1 and FMO3. Olopatadine did not inhibit p450 activities when it was simultaneously incubated with substrates for different p450 isozymes. Also, p450 activities in human liver microsomes were unaffected by pretreatment with olopatadine or M1. Furthermore, spectral analysis revealed that neither olopatadine nor M1 formed an MIC. Therefore, it is unlikely that olopatadine will cause drug-drug interactions involving p450 isozymes.