In vitro biotransformation of sildenafil (Viagra) in the male rat: the role of CYP2C11.

To assess the suitability of the male rat model for human studies on sildenafil metabolism, we examined the biotransformation of sildenafil in male rat liver microsomes and identified the role of specific cytochrome P450s (P450) using inhibitory antibodies and cDNA-expressed P450s. Rates of formation of the major circulating metabolite of sildenafil, UK-103,320, were 11-fold greater in the male rat than in human liver microsomes at 36 microM sildenafil, whereas substrate concentration corresponding to 50% V(max) (K(m) values) were 2.9-fold lower in the male rat. Although sildenafil is largely metabolized by CYP3A isoforms in humans, coincubation of rat liver microsomes with immunoinhibitory antibodies (CYP1A1/2, 2B1/2, 2C11, 2E1, and 3A1/2) revealed that metabolite formation was inhibited only by an antirat CYP2C11 antibody. Incubation of sildenafil with a cDNA-expressed CYP2C11 produced 10-fold higher levels of UK-103,320 than other P450s (CYP1A1, 1A2, 2B1, 2C6, 2C12, 2C13, 2E1, 3A1, and 3A2). Thus CYP2C11 contributes in a major way to the metabolism of sildenafil in the male rat. P450 isoforms mediating sildenafil biotransformation differ substantially between humans and the male rat, thereby limiting the applicability of this species as a model for sildenafil metabolism and drug interactions in humans.