Abstract
In this study, clotrimazole (CTZ) and ketoconazole (KTZ) were evaluated for their inhibition of testosterone metabolism catalyzed by rat hepatic microsomes differentially expressing certain cytochrome P450 enzymes. The objective was to compare the inhibitory potencies using hepatic microsomes from adult female rats treated with dexamethasone (F-DEX) and hepatic microsomes from vehicle-treated adult male rats (M-VEH), which are known to contain high levels of isozymes CYP3A1 (3A23) and 3A2, respectively. The results demonstrate that CTZ is a very potent and selective inhibitor of the 6β-hydroxylation of testosterone, a CYP3A-mediated reaction, in all rat metabolic systems tested. The IC50 value was 9.7 nM in F-DEX, and 6.7 nM in M-VEH for CTZ. The in vitro inhibitory potency for CTZ significantly exceeds the same parameters for KTZ, a well established specific inhibitor of human CYP3A-mediated reactions. It was found that the IC50 values of KTZ in F-DEX and M-VEH were 69 and 780 nM, respectively. These values for KTZ are 10-fold and 100-fold higher, respectively, than for CTZ. CTZ, at the concentration that inhibits 90% and more of CYP3A-mediated reactions (40 nM), has less than a 10% inhibitory effect on the activities of other rat liver enzymes, such as CYP1A1, -1A2, -2A1, -2B1, -2B2, -2C11, and -2E1. In summary, CTZ is a more potent and selective inhibitor of all CYP3A-mediated reactions than KTZ in rat hepatic microsomes.
Footnotes
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Send reprint requests to: Dr. Paul E. Thomas, Rutgers, The State University of New Jersey, Laboratory for Cancer Research, 164 Frelinghuysen Rd., Piscataway, NJ 08854. E-mail:pethomas{at}eohsi.rutgers.edu
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This research was funded by National Institute of Environmental Health Sciences Training Grant in Environmental Toxicology ESO 7148-13 and by National Institutes of Health R01 GM44982.
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Presented in part at the 39th Annual Meeting of the Society of Toxicology, March 2000, Philadelphia, PA.
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↵2 Confusion over whether CYP3A1 and -3A23 refer to different proteins or the same protein has been evident over the last few years. Recently, Nagata et al. (1999) conclude from their data examining this issue that CYP3A1 and -3A23 refer to the same protein encoded by P450/6βB. It is with this background that we use the original designation “CYP3A1” in this manuscript.
- Abbreviations used are::
- P450
- cytochrome P450
- CTZ
- clotrimazole
- KTZ
- ketoconazole
- 4-MA
- 17β-N,N-diethylcarbamoyl-4-methyl-4-aza-5α-androstan-3-one
- VEH
- vehicle
- DEX
- dexamethasone
- PB
- phenobarbital
- 3-MC
- 3-methylcholanthrene
- ISN
- isoniazid
- F-DEX or M-VEH
- liver microsomes from adult female or male Sprague-Dawley rats treated with DEX or VEH
- HPLC
- high-performance liquid chromatography
- Received December 6, 2000.
- Accepted February 27, 2001.
- The American Society for Pharmacology and Experimental Therapeutics
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