Abstract
In vitro studies were conducted to identify the hepatic cytochrome P450 (CYP) forms involved in the oxidative metabolism of [14C]ABT-761 and itsN-dehydroxylated metabolite, [14C]ABT-438, by human liver microsomes. The two compounds were metabolized by parallel pathways, to form the corresponding methylene bridge hydroxy metabolites. There was no evidence of sulfoxidation and/or ring hydroxylation. Over the ABT-761 and ABT-438 concentration ranges studied (1–300 μM), the rate of NADPH-dependent hydroxylation was linear with respect to substrate concentration ([S]) and did not conform to saturable Michaelis-Menten kinetics. Under these conditions ([S] <KM ), the intrinsic clearance (Vmax/KM ) of ABT-438 was 10-fold higher than that of ABT-761 (1.7 ± 0.8vs. 0.17 ± 0.06 μl/min/mg, mean ± SD,N = 3 livers). The hydroxylation of both compounds was shown to be highly correlated (r = 0.83,p < 0.01, N = 11 different human livers) with CYP3A-selective erythromycin N-demethylase activity, and the correlation between ABT-761 hydroxylation and tolbutamide hydroxylase (CYP2C9-selective) activity (r= 0.63, p < 0.05, N = 10) was also statistically significant. Ketoconazole (2.0 μM), a CYP3A-selective inhibitor, inhibited the hydroxylation of both compounds by 53–67%, and sulfaphenazole (CYP2C9-selective) decreased activity by 10–20%. By comparison, α-naphthoflavone, a known activator of CYP3A, stimulated the hydroxylation of ABT-761 (8-fold) and ABT-438 (4-fold). In addition, the abundance-normalized rates of cDNA-expressed CYP-dependent metabolism indicated that hydroxylation was largely mediated (66–86%) by CYP3A(4). Therefore, it is concluded that the hydroxylation of ABT-761 and ABT-438 (≤10 μM) is primarily mediated by CYP3A, although CYP2C9 may play an ancillary role.
Footnotes
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Send reprint requests to: J. M. Machinist, Ph.D., Department 46V, Building AP9, Abbott Laboratories, 100 Abbott Park Road, Abbott Park, IL 60064.
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↵1 Present address: Drug Metabolism, Merck Research Laboratories, P.O. Box 4, WP26A 2044, Sumneytown Pike, West Point, PA 19486-0004.
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↵3 Inhibition studies were carried out at a defined substrate concentration ([S] ≈ KM). Assuming competitive inhibition, for [S] ≈KM, IC50/2 ≈Ki. In addition,KM ≈ Kifor a competitive inhibitor (cosubstrate).
- Abbreviations used are::
- CYP
- cytochrome P450
- COU
- coumarin
- HPLRC
- high performance liquid radiochromatography
- Received January 8, 1998.
- Accepted May 12, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
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