Overlooked aspects of scaling enzyme activity through abundance across tissues and individuals: Insights from kcat measurements in matched liver and intestinal samples.

Prediction of human intestinal metabolism within physiologically based pharmacokinetic models is now well established within drug development. Extrapolation of in vitro kinetic parameters accounts for differences in abundance between different in vitro systems and tissues. The existing data assume that the activity of CYP3A4 is consistent between the intestine and liver once adjusted for its tissue-specific expression level. However, the validity of this assumption for other enzymes and other tissues remains uncertain. In the current study, indicators of "activity per unit of enzyme," namely, turnover number (k_cat) or specificity constant (k_sp), were measured for 7 enzymes (CYP2C9, CYP2C19, CYP2D6, CYP3A4, UGT1A6, UGT2B7, and UGT2B17) in microsomes prepared from 4 paired (same donor) intestine and liver tissue samples. After excluding 1 donor with low intestinal activity, the intestinal k_cat and k_sp for the studied CYPs were within 2-fold of the liver values, with the exception of 1 donor with 4-fold lower CYP2D6 k_cat in the intestine compared with the liver. Conversely, the UGT1A1 k_sp and UGT2B7 k_cat were 5-fold and 7-fold higher in intestinal microsomes compared with liver microsomes, respectively. Trends in interdonor variability in k_cat were noted and require further evaluation in a larger set of donors. The current paradigm of extrapolation of hepatic metabolism data to predict in vivo first-pass metabolism in the intestine using tissue abundances appears to be valid for CYPs but should be approached with caution when predicting intestinal glucuronidation. SIGNIFICANCE STATEMENT: This study assessed whether hepatic metabolism data can predict intestinal metabolism in physiologically based pharmacokinetic models by comparing enzyme abundance and activity in matched liver and intestine microsomes from 4 donors. Seven key drug-metabolizing enzymes were quantified. While CYP-mediated intestinal metabolism could generally be predicted from liver data after adjusting for tissue abundance, caution is warranted for enzymes involved in intestinal glucuronidation, where assumptions of equivalent activity across tissues may not hold.