Abstract
In early discovery, compounds are often eliminated because of their potential to undergo metabolic activation and/or cytochrome P450 time-dependent inactivation (TDI). The blockbuster drug raloxifene is an example of a compound that would have been eliminated in the current paradigm. Despite raloxifene's in vitro bioactivation and TDI of CYP3A4, it is well tolerated in patients with no drug-drug interactions. This discordance is attributed to its presystemic glucuronidation, thereby decreasing the amount of unchanged raloxifene available for CYP3A inactivation. The current study used raloxifene as a model to assess the effect of hepatic and intestinal glucuronidation on the kinetic parameters of CYP3A4 inactivation. Therefore, a simple multistaged time-dependent inactivation using UDP-glucuronosyltransferase-enabled and -absent reactions was built to understand the impact of the gut metabolism on inactivation potential. The results of these experiments demonstrated a 2.7-fold change in inactivation efficiency of CYP3A4. Incorporation of these results into a simulated midazolam drug-drug interaction study showed very little change in the pharmacokinetic parameters of the victim drug. In contrast, the absence of glucuronidation resulted in a 4.1-fold increase in the area under the curve (AUC) of midazolam, when in the presence of raloxifene, hence providing an understanding of the impact of intestinal glucuronidation on raloxifene's time-dependent inhibition of CYP3A4 and also providing a validation of a simple in vitro experiment to assess the influence of gut metabolism on time-dependent inhibitors at the discovery phase.
Footnotes
Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
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ABBREVIATIONS:
- TDI
- time-dependent inactivation
- P450
- cytochrome P450
- UDPGA
- UDP-glucuronic acid
- DDI
- drug-drug interaction
- HLM
- human liver microsomes
- UGT
- UDP glucuronosyltransferase
- AUC
- area under the curve
- PK
- pharmacokinetic
- HIM
- human intestinal microsomes.
- Received October 7, 2011.
- Accepted November 23, 2011.
- Copyright © 2012 by The American Society for Pharmacology and Experimental Therapeutics
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