Abstract
Models of the time course of the effect of P450 induction on substrate clearance have previously only considered induction through enhanced synthesis of protein. Induction of CYP2E1 does not always conform to this model, in that many chemicals induce the enzyme through stabilization of the protein apparently by binding to the active site. While such binding protects the enzyme from degradation, it also results in competitive inhibition of substrate clearance. We present a model based on experimental studies of chemical induction of CYP2E1 by ligand stabilization through which this mechanism of induction can be translated into its pharmacokinetic consequence with regard to clearance of substrate. CYP2E1 is considered to be localized in two pools: Pool 1 at which two mechanisms of degradation, fast and slow, operate and pool 2, at which only the slower mechanism operates. Binding of substrate to enzyme in pool 1 stabilizes it from degradation by the fast process, leaving only the slow process. Ligand stabilization therefore results in induction of CYP2E1 as enzyme accumulates as a consequence of unchanged synthesis. Binding of ligand to the active site results in competitive inhibition of the clearance of substrate. Model-based computer simulations show that the time course of interaction between inhibitor/inducer and substrate can be predicted from knowledge of I/Ki and S/Km and the synthesis and degradation kinetics of CYP2E1. The simulations demonstrate further that as long as inhibitor/inducer administration is not interrupted, the clearance of substrate will always be less than the value observed at low concentration of substrate even if the substrate concentration is raised to displace inhibitor/inducer from the active site. On the other hand, the degree of inhibition of clearance is less than would be seen if induction had not taken place. Clearance of substrate is observed to rise above the value observed in the absence of the inhibitor/inducer only after the inhibitor/inducer concentration declines low enough for substrate to gain access to the active site of the enzyme. The model-based simulations agree with reports of the interaction between isoniazid and acetaminophen in humans.
Footnotes
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Send reprint requests to: John T. Slattery, Ph.D., Department of Pharmaceutics, Box 357610, University of Washington, Seattle, WA 98195-7610. E-mail: jts{at}u.washington.edu.
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This work was supported in part by grants GM 32165 and GM 48349 from the National Institutes of Health.
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↵2 Since the inducing ligand induces a spin-shift consistent with occupation of the active site (36), the mechanism of inhibition most likely is competitive, as has been demonstrated with isoniazid in human liver microsomes and in a reconstituted system with expressed CYP2E1 (53). The equations for fu and fb are derived from Scatchard’s equilibrium binding equations (54).
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↵3 For ease of comparison to steady-state simulations, the time course simulations are identified with values of I/Ki achieved at steady-state. Infusion rate of inhibitor/inducer was varied to achieve the indicated steady-state I/Ki; I/Ki did change over the course of infusion.
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↵4 Induction of CYP3A isoforms byN-substituted imidazoles and macrolides may represent variants of this mechanism (66-68).
- Abbreviations used are::
- CYP2E1
- cytochrome P450 2E1
- NAPQI
- N-acetyl-p-benzoquinone imine
- Received March 31, 1997.
- Accepted June 19, 1997.
- The American Society for Pharmacology and Experimental Therapeutics
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