Comparison between recombinant P450s and human liver microsomes in the determination of cytochrome P450 Michaelis–Menten constants

Abstract

1. Non-linear dose–exposure (supra-proportionality) occurs when plasma drug concentrations increase in a non-linear fashion with increasing dose. To predict the likelihood of this, an understanding is required of the K_M, which reflects a drug ability to saturate a specific enzyme involved in its metabolism.

2. This study assessed the accuracy of K_M and V_max determinations for compounds using a substrate-depletion approach with those determined using the product-formation approach, using both recombinant human cytochrome P450 (CYP) enzymes and human liver microsomes.

3. For the vast majority of the compounds studied, the K_M’s using recombinant CYPs and human liver microsomes in the two approaches predicted within two-fold. Further comparisons between the K_M and V_max-values were made between those measured using the product-formation approach and those estimated following simultaneous fitting of the Michaelis–Menten equation to all substrate depletion plots. In each case values were comparable.

4. In conclusion, the current study showed the substrate-depletion approach can be used to estimate K_M and V_max using both human liver microsomes and recombinant P450s. Estimation of these parameters during early discovery will aid in the understanding of dosages at which non-linearity may occur, but potentially aid predictions of likely clinical drug–drug interactions.

Keywords::

• Cytochrome P450
• supra-proportionality
• Michaelis–Menten
• metabolism
• drug–drug interaction

Acknowledgements

The authors gratefully acknowledge Dr Maurice Dickins and Dr Barry Jones for critically reviewing this manuscript and Dr Hannah Jones for her technical support around use of WinNonlin with multiple depletion curve data.

Declaration of interest

The authors report no conflict of interest. The authors alone are responsible for the content and writing of the paper.