Elsevier

Biochemical Pharmacology

Volume 71, Issue 11, 28 May 2006, Pages 1531-1539
Biochemical Pharmacology

Commentary
In vitro–in vivo correlation for drugs and other compounds eliminated by glucuronidation in humans: Pitfalls and promises

https://doi.org/10.1016/j.bcp.2005.12.019Get rights and content

Abstract

Enzymes of the UDP-glucuronosyltransferase (UGT) superfamily are responsible for the metabolism of many drugs, environmental chemicals and endogenous compounds. Identification of the UGT(s) involved in the metabolism of a given compound (‘reaction phenotyping’) currently relies on multiple confirmatory approaches, which may be confounded by the dependence of UGT activity on enzyme source, incubation conditions, and the occurrence of atypical glucuronidation kinetics. However, the increasing availability of substrate and inhibitor ‘probes’ for the individual UGTs provides the prospect for reliable phenotyping of glucuronidation reactions using human liver microsomes or hepatocytes, thereby providing data directly relevant to drug metabolism in humans. While the feasibility of computational prediction of UGT substrate selectivity has been demonstrated, the development of easily interpretable and generalisable models requires further improvement in the datasets available for analysis. Quantitative prediction of the hepatic clearance of glucuronidated drugs and the magnitude of inhibitory interactions based on in vitro kinetic data is more problematic. Intrinsic clearance (CLint) values generated using human liver microsomes under-predict in vivo hepatic clearance, typically by an order of magnitude. In vivo clearances of glucuronidated drugs are also generally under-predicted by CLint values from human hepatocytes, but to a lesser extent than observed with the microsomal model. While it is anticipated that systematic analysis of the potential causes of under-prediction may provide more reliable in vitro–in vivo scaling strategies, mechanistic interpretation of in vitro–in vivo correlation more broadly awaits further advances in our understanding of the structural and cellular determinants of UGT activity.

Section snippets

The reaction phenotyping of glucuronidated compounds

Multiple approaches have been adopted for the reaction phenotyping of CYP substrates, most frequently using human liver microsomes as the enzyme source, including [1], [2], [3]: (i) impairment of metabolism by enzyme-selective chemical and antibody inhibitors; (ii) competitive inhibition of the metabolism of enzyme-selective substrates by the compound, with Km matching Ki; (iii) a significant correlation between rates of metabolism of the compound and immunoreactive enzyme contents or

The quantitative prediction of drug glucuronidation kinetics in vivo

Two approaches may be adopted for the calculation of in vitro CLint, which may subsequently be extrapolated to in vivo CLH and EH[4]. Measurement of product formation (e.g. a drug glucuronide) under initial rate conditions over an appropriate substrate concentration range allows calculation of Km and Vmax for that pathway (in the absence of atypical kinetics; see below), and then CLint (as Vmax/Km). The second approach measures the rate of substrate depletion with time (at a low substrate

Summary and conclusions

The reaction phenotyping of drugs and other chemicals metabolised by glucuronidation is currently feasible, but generally requires the application of multiple experimental approaches. Even so, problems with the interpretation of data may arise due to the dependence of UGT activity in vitro on incubation conditions, enzyme source and the occurrence of atypical glucuronidation kinetics, emphasising the need for careful experimental design. However, UGT reaction phenotyping with human liver

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