Halofantrine metabolism in microsomes in man: Major role of CYP 3A4 and CYP 3A5

We have clarified the contribution of the different enzymes involved in the N-debutylation of halofantrine in liver microsomes in man. The effect of k etoconazole and cytochrome P450 (CYP) 3A substrates on halofantrine metabol ism has also been studied. The antimalarial drug halofantrine is metabolized into one major metabolite , N-debutylhalofantrine. In microsomes from nine livers from man, N-debutyl ation of halofantrine was highly variable with apparent Michaelis-Menten co nstant V-max and K-m values of 215 +/- 172 pmol min(-1) mg(-1) and 48 +/- 2 6 mu mol L-1, respectively, (mean +/- standard deviation). Formation of N-d ebutylhalofantrine was cytochrome P450 (CYP)-mediated. Studies using select ive inhibitors of individual CYPs revealed the role of CYP 3As in the forma tion of N-debutylhalofantrine. alpha-Naphthoflavone, a CYP 3A activator, in creased metabolite formation. In microsomes from 12 livers from man the rat e of N-debutylation of halofantrine correlated strongly with CYP 3A4 relati ve levels (P = 0.002) and less strongly, but significantly, with CYP 2C8 le vels (P = 0.025). To characterize CYP-mediated metabolism of halofantrine f urther, incubations were performed with yeast microsomes expressing specifi c CW 3A4, CYP 3A5, CYP 2D6, CYP 2C8 and CYP 2C19 from man. The rate of form ation of N-debutylhalofantrine was six- and twelvefold with CYP 3A4 than wi th CYP 3A5 and CYP 2C8, respectively. CYP 2D6 and CYP 2C19 did not mediate the N-debutylation of halofantrine, but, because in-vivo CYP 2C8 is present at lower concentrations than CYP 3A in the liver in man, the involvement o f CYP 3As would be predominant. Diltiazem, erythromycin, nifedipine and cyc losporin (CYP 3A substrates) inhibited halofantrine metabolism. Similarly, ketoconazole inhibited, noncompetitively, formation of N-debutylhalofantrin e with an inhibition constant, K-i, of 0.05 mu M. The theoretical percentag e inhibition of halofantrine metabolism in-vivo by ketoconazole was estimat ed to be 99%. These results indicate that both CYP 3A4 and CTP 3A5 metabolize halofantrin e, with major involvement of CYP 3A4. In-vivo, the other CYPs have a minor role only. Moreover, strong inhibition, and consequently increased halofant rine cardiotoxicity, might occur with the association of ketoconazole or ot her CYP 3A4 substrates.