Primaquine metabolism by human liver microsomes: effect of other antimalarial drugs.

A number of drugs have been studied for their effect on the metabolism of the antimalarial drug primaquine by human liver microsomes (N = 4) in vitro. The only metabolite generated was identified as carboxyprimaquine by co-chromatography with the authentic standard. Ketoconazole, a known inhibitor of cytochrome P450 isozymes, caused marked inhibition of carboxyprimaquine formation with IC50 and K(i) values of 15 and 6.7 microM, respectively. This finding and the dependency of metabolite formation on NADPH indicates that cytochrome P450 isozyme(s) catalysed metabolite production. Of compounds actually or likely to be coadministered with primaquine to malaria patients, only mefloquine produced any inhibition (K(i) = 52.5 microM). Quinine, artemether, artesunate, halofantrine and chloroquine did not significantly inhibit metabolite formation. It seems unlikely that the concurrent administration of mefloquine, or other antimalarials, with primaquine will lead to appreciably altered disposition.