Evaluation of methoxsalen, tranylcypromine, and tryptamine as specific and selective CYP2A6 inhibitors in vitro.

CYP2A6 is the principle enzyme metabolizing nicotine to its inactive metabolite cotinine. In this study, the selective probe reactions for each major cytochrome P450 (P450) were used to evaluate the specificity and selectivity of the CYP2A6 inhibitors methoxsalen, tranylcypromine, and tryptamine in cDNA-expressing and human liver microsomes. Phenacetin O-deethylation (CYP1A2), coumarin 7-hydroxylation (CYP2A6), diclofenac 4'-hydroxylation (CYP2C9), omeprazole 5-hydroxylation (CYP2C19), dextromethorphan O-demethylation (CYP2D6), 7-ethoxy-4-trifluoromethylcoumarin deethylation (CYP2B6), p-nitrophenol hydroxylation (CYP2E1), and omeprazole sulfonation (CYP3A4) were used as index reactions. Apparent K(i) values for inhibition of P450s' (1A2, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1, and 3A4) activities showed that tranylcypromine, methoxsalen, and tryptamine have high specificity and relative selectivity for CYP2A6. In cDNA-expressing microsomes, tranylcypromine inhibited CYP2A6 (K(i) = 0.08 microM) with about 60- to 5000-fold greater potency relative to other P450s. Methoxsalen inhibited CYP2A6 (K(i) = 0.8 microM) with about 3.5- 94-fold greater potency than other P450s, except for CYP1A2 (K(i) = 0.2 microM). Tryptamine inhibited CYP2A6 (K(i) = 1.7 microM) with about 6.5- 213-fold greater potency relative to other P450s, except for CYP1A2 (K(i) = 1.7 microM). Similar results were also obtained with methoxsalen and tranylcypromine in human liver microsomes. R-(+)-Tranylcypromine, (+/-)-tranylcypromine, and S-(-)-tranylcypromine competitively inhibited CYP2A6-mediated metabolism of nicotine with apparent K(i) values of 0.05, 0.08, and 2.0 microM, respectively. Tranylcypromine [particularly R-(+) isomer], tryptamine, and methoxsalen are specific and relatively selective for CYP2A6 and may be useful in vivo to decrease smoking by inhibiting nicotine metabolism with a low risk of metabolic drug interactions.