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Defective Activity of Recombinant Cytochromes P450 3A4.2 and 3A4.16 in Oxidation of Midazolam, Nifedipine, and Testosterone

Department of Clinical Pharmacology, Gunma University Graduate School of Medicine (M.M., K.N., R.H., K.Y.) and Department of Pharmacy, Gunma University Hospital (Y.F., R.H., K.Y.), Gunma, Japan; and Department of Biochemistry and Center in Molecular Toxicology (F.P.G.), Vanderbilt University School of Medicine, Nashville, Tennessee

Cytochrome P4503A4 (CYP3A4) is the most abundant cytochrome P450 in adult human liver and small intestine and oxidizes numerous clinically, physiologically, and toxicologically important compounds. The metabolic activity of CYP3A4 in patients varies at least 10-fold in vivo, and CYP3A4 genetic variants are considered one of the causes of individual differences. The cDNAs for the CYP3A4^*2 (S222P), ^*7 (G56D), ^*16 (T185S), and ^*18 (L293P) mutant alleles, found in high frequencies in Caucasians or Asians, were constructed by site-directed mutagenesis and expressed in an Escherichia coli expression system. Midazolam (MDZ), testosterone (TST), and nifedipine (NIF) were used to assess the catalytic activities of the CYP3A4 wild type (CYP3A4.1) and its variants. The catalytic activities of CYP3A4.2 and CYP3A4.16 were reduced (lower V_max and increased K_m relative to CYP3A4.1) for all substrates. The CYP3A4.7 showed lower V_max values for MDZ and NIF (60 and 84%, respectively) and a higher K_m (2-fold) for TST but not for MDZ or NIF. Although CYP3A4.18 showed low V_max values for MDZ, NIF, and TST (88, 72, and 80% of CYP3A4.1, respectively), no significant differences were identified in the ratio V_max/Km. In summary, CYP3A4.2 and CYP3A4.16 exhibited significantly lower activity for MDZ, TST, and NIF oxidations than CYP3A4.1. Therefore, drugs metabolized by only CYP3A should be carefully administered to patients with these alleles.

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