Abstract
Anastrozole (2,2′[5-(1H-1,2,4-triazol-1-ylmethyl)-1,3-phenylene]bis(2-methylproprionitrile)) is a potent third-generation inhibitor of aromatase, currently marketed as a treatment for postmenopausal women with advanced breast cancer. While its potency and selectivity for inhibition of estrogen synthesis has been established in both preclinical and clinical studies, this study used in vitro methods to examine the effects of anastrozole on several drug metabolizing CYP enzymes found in human liver. Human liver microsomes were co-incubated with anastrozole and probe substrates for CYP1A2 (phenacetin), CYP2A6 (coumarin), CYP2C9 (tolbutamide), CYP2D6 (dextromethorphan), and CYP3A (nifedipine). The formation of the CYP-specific metabolites following co-incubation with various anastrozole concentrations was determined to establish IC50 and Kivalues for these enzymes. While anastrozole did not inhibit CYP2A6 and CYP2D6 activities at concentrations below 500 μM, this compound inhibited CYP1A2, CYP2C9, and CYP3A activities withKi values of 8, 10, and 10 μM, respectively. Dixon plots used to determine theKi values for the inhibition of CYP1A2 and CYP3A activities by anastrozole were biphasic, indicating additional lower affinity Ki values. Major metabolites of anastrozole did not retain the ability to inhibit the metabolism of nifedipine (CYP3A). The results of this study indicate that, although anastrozole can inhibit CYP1A2, 2C9, and 3A-mediated catalytic activities, this compound would not be expected to cause clinically significant interactions with other CYP-metabolized drugs at physiologically relevant concentrations achieved during therapy with Arimidex (Zeneca, Ltd., Macclesfield, UK) 1-mg.
Footnotes
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Send reprint requests to: S. W. Grimm, LW245, Drug Disposition and Metabolism Department, Zeneca Pharmaceuticals, Zeneca, Inc., 1800 Concord Pike, Wilmington, DE 19850.
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↵2 The kinetics of phenacetinO-deethylation by the microsomes used in this investigation was best fit to a 2-enzyme model with aKm of 59 μM for the high affinity site. This result suggests that more than one enzyme contributes to thisO-deethylation reaction and may potentially explain the appearance of biphasicity in the Dixon plot. In contrast and although other CYP3A enzymes are known to metabolize nifedipine, the kinetics of dehydronifedipine formation was fitted best to a 1-enzyme Michaelis-Menten model (Km ∼25 μM).
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↵3 The plasma protein binding of anastrozole was determined in vitro to be 40% (unpublished results). Therefore, the free (unbound) concentration of anastrozole would be approximately 0.6 times the total concentrations in plasma.
- Abbreviations used are::
- CYP
- cytochrome P450
- BSA
- bovine serum albumin
- BMPN-benzoic acid
- 3,5-bis-(2-methyl-propionitrile)-benzoic acid
- PMSF
- phenylmethyl-sulfonylfluoride
- HEPES
- 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
- TFA
- trifluoroacetic acid
- TCA
- trichloroacetic acid
- HCl
- hydrochloric acid
- MTBE
- methyl-tert-butyl ether
- Cmax
- maximum concentration
- Received October 25, 1996.
- Accepted January 23, 1997.
- The American Society for Pharmacology and Experimental Therapeutics
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