RT Journal Article
SR Electronic
T1 Identification of Human Cytochrome P-450 Isoforms Involved in Metabolism of <em>R</em>(+)- and <em>S</em>(−)-Gallopamil: Utility of In Vitro Disappearance Rate
JF Drug Metabolism and Disposition
JO Drug Metab Dispos
FD American Society for Pharmacology and Experimental Therapeutics
SP 1254
OP 1259
VO 27
IS 11
A1 Suzuki, Akio
A1 Iida, Izumi
A1 Tanaka, Fumie
A1 Akimoto, Masayuki
A1 Fukushima, Kiyomi
A1 Tani, Masayoshi
A1 Ishizaki, Takashi
A1 Chiba, Kan
YR 1999
UL http://dmd.aspetjournals.org/content/27/11/1254.abstract
AB Isoforms of cytochrome P-450 (CYP) involved in the metabolism of gallopamil enantiomers were identified by measuring the disappearance rate of parent drug from an incubation mixture with human liver microsomes and recombinant human CYPs. Mean (± S.D.) intrinsic clearances (CLint) of R(+)- andS(−)-gallopamil in human liver microsomes were 0.320 ± 0.165 and 0.205 ± 0.107 ml/min/mg protein, respectively. These values were highly correlated with the 6β-hydroxylation activity of testosterone, a marker substrate of CYP3A4 (r = 0.977 and 0.900 forR(+)- and S(−)-gallopamil, respectively,p &lt; .001). Ketoconazole and troleandomycin, selective inhibitors of CYP3A4, and polyclonal antibodies raised against CYP3A4/5 markedly reduced the CLint of gallopamil enantiomers in human liver microsomes. Among the 10 recombinant human CYP isoforms, CYP3A4 exhibited the highest CLint of gallopamil enantiomers, and CYP2C8 and CYP2D6 also exhibited appreciable activity. When the contribution of CYP3A4 to the total metabolic clearance of gallopamil enantiomers in human liver microsomes was estimated by relative activity factor, the mean (± S.D.) contributions were 92 ± 18 and 68 ± 19% forR(+)- and S(−)-gallopamil, respectively. These values were comparable to the rates of immunoinhibition by antibodies raised against CYP3A4/5 observed in human liver microsomes. The present study suggests that CYP3A4 is a major isoform involved in the overall metabolic clearance of gallopamil enantiomers in the human liver, and that the present approach based on disappearance rate may be applicable to identify major isoforms of CYP involved in the metabolism of a drug in human liver microsomes. The American Society for Pharmacology and Experimental Therapeutics