Short Communication The Contributions of Cytochromes P450 3A4 and 3A5 to the Metabolism of the Phosphodiesterase Type 5 Inhibitors Sildenafil, Udenafil, and Vardenafil

The role of the genetically polymorphic CYP3A5 in the metabolism of CYP3A substrates is unclear. We investigated the contributions of the CYP3A4 and CYP3A5 isoforms to the metabolism of the phosphodiesterase type 5 inhibitors (PDE5Is) sildenafil, udenafil, and vardenafil. In vitro incubation studies of sildenafil N-demethylation, udenafil N-dealkylation, and vardenafil N-deethylation were conducted using recombinant CYP3A enzymes and 15 human liver microsome (HLM) preparations with predetermined CYP3A5 genotypes. Recombinant CYP3A4 and CYP3A5 both produced N-desalkyl metabolites of sildenafil, udenafil, and vardenafil. The catalytic efficiency (Clint Vmax/apparent Km) of the rCYP3A5 isoform for vardenafil N-deethylation was about 3.2-fold that of rCYP3A4, whereas the intrinsic clearance rates for N-dealkylation of both sildenafil and udenafil were similar between rCYP3A5 and rCYP3A4. The metabolite formation activity was higher in HLMs heterozygous for the CYP3A5*3 allele (n 9) than in HLMs homozygous for CYP3A5*3 (n 6). These findings suggest that CYP3A5 and CYP3A4 play a significant role in the metabolism of PDE5Is. The genetic polymorphism of CYP3A5 may contribute to interindividual variability in the disposition of PDE5Is, especially vardenafil. Further in vivo studies are needed to confirm the effects of CYP3A5 genotypes on the pharmacokinetics of PDE5Is. Sildenafil, udenafil, and vardenafil are potent, selective inhibitors of cyclic GMP-specific phosphodiesterase type 5 (PDE5) in the smooth muscle cells lining blood vessels, especially in the corpus cavernosum of the penis. These drugs are used for effective p.o. treatment of erectile dysfunction. Previous studies have reported large interindividual variability in the pharmacokinetic disposition of sildenafil, udenafil, and vardenafil (Klotz et al., 2001; Rajagopalan et al., 2003; Shim et al., 2003; Bischoff, 2004; Gupta et al., 2005; Mehrotra et al., 2007). Vardenafil showed the highest interindividual variation among three PDE5 inhibitors (PDE5Is), with a 14-fold variability among subjects receiving 20 mg of vardenafil (Klotz et al., 2001; Rajagopalan et al., 2003). The mechanisms of these large interindividual variations in vivo have not been elucidated. Sildenafil, udenafil, and vardenafil undergo N-dealkylation in the liver and intestine, and cytochrome P450 (P450) 3A is primarily involved in their metabolism (Fig. 1) (Ji et al., 2004; Kivisto et al., 2004; Mehrotra et al., 2007). CYP3A is most abundant human hepatic P450 and is involved in the metabolism of approximately 50% of commonly administered drugs (Evans and Relling, 1999). In adults, CYP3A4 and CYP3A5 are predominant among the four known isoforms (CYP3A4, CYP3A5, CYP3A7, and CYP3A43) in the liver and intestine (Nelson et al., 1996). CYP3A5 is similar to CYP3A4 with regard to sequence and substrate selectivity (Kuehl et al., 2001). There are 84% amino acid sequence similarity and overlapping substrate specificities between CYP3A4 and CYP3A5 (Aoyama et al., 1989; Wrighton and Stevens, 1992). However, some differences have been reported in enzymatic properties of CYP3A4 and CYP3A5, including substrate specificity and inhibition (Gibbs et al., 1999; Cook et al., 2002; Patki et al., 2003; Emoto and Iwasaki, 2006). Unlike CYP3A4, CYP3A5 is polymorphically expressed in human liver (Koch et al., 2002; Xie et al., 2004). The rate of CYP3A5 is 6 to 68% of the entire hepatic CYP3A content (Wrighton et al., 1990). The frequency of CYP3A5 is higher in the livers of African-Americans than in those of Caucasians (Kuehl et al., 2001). Several genetic variants have been described for CYP3A5; the most common, the CYP3A5*3 allele, causes the loss of CYP3A5 activity. The CYP3A5*3 genotype reportedly affects the disposition of some drugs and influences the plasma concentrations of simvastatin and alprazolam (Kivisto et al., 2004; Park et al., 2006; Kim et al., 2007). The importance of the CYP3A enzyme in the metabolism of sildenafil, udenafil, and vardenafil has been shown in vitro (Hyland et al., 2001; Bischoff, 2004; Ji et al., 2004). However, until now, the relative contributions of CYP3A4 and CYP3A5 were unknown. In the present study, we evaluated the relative contributions of human CYP3A4 and CYP3A5 to the metabolism of three PDE5Is: sildenafil, udenafil, and vardenafil. Materials and Methods Chemicals and Reagents. Udenafil and N-desalkyludenafil were supplied by the Dong-A Pharmaceutical Company (Yongin, Korea). Midazolam, 1 hydroxymidazolam, sildenafil, N-desmethylsildenafil, vardenafil, and N-desThis study was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (No. R13-2007-023-