ABSTRACT:Glucuronidation is a listed clearance mechanism for 1 in 10 of the top 200 prescribed drugs. The objective of this article is to encourage those studying ligand interactions with UDP-glucuronosyltransferases (UGTs) to adequately consider the potential consequences of in vitro UGT inhibition in humans. Spurred on by interest in developing potent and selective inhibitors for improved confidence around UGT reaction phenotyping, and the increased availability of recombinant forms of human UGTs, several recent studies have reported in vitro inhibition of UGT enzymes. In some cases, the observed potency of UGT inhibitors in vitro has been interpreted as having potential relevance in humans via pharmacokinetic drug-drug interactions. Although there are reported examples of clinically relevant drug-drug interactions for UGT substrates, exposure increases of the aglycone are rarely greater than 100% in the presence of an inhibitor relative to its absence (i.e., AUC i /AUC <2). This small magnitude in change is in contrast to drugs primarily cleared by cytochrome P450 enzymes, where exposures have been reported to increase as much as 35-fold on coadministration with an inhibitor (e.g., ketoconazole inhibition of CYP3A4-catalyzed terfenadine metabolism). In this article the evidence for purported clinical relevance of potent in vitro inhibition of UGT enzymes will be assessed, taking the following into account: in vitro data on the enzymology of glucuronide formation from aglycone, pharmacokinetic principles based on empirical data for inhibition of metabolism, and clinical data on the pharmacokinetic drug-drug interactions of drugs primarily cleared by glucuronidation.

1 Several selective 5-HT reuptake inhibitors (SSRIs) are inhibitors of the genetically polymorphic drug metabolizing enzyme, CYP2D6. We studied the interaction of venlafaxine, a new SSRI, with CYP2D6 in human liver microsomes. 2 Venlafaxine was a less potent inhibitor of this enzyme activity in vitro than other SSRIs tested. The average apparent K i values determined using CY P2D6-dependent dextromethorphan 0-demethylation were: 33, 52 and 22 PM for rac-venlafaxine, R( +)-venlafaxine and S (-)-venlafaxine, respectively, us 0.065 to 1.8 p~ for paroxetine, fluoxetine, norfluoxetine, fluvoxamine and sertraline. 3 Microsomes from human livers ( n = 3 ) and from yeast transformed with an expression plasmid containing human CYP2D6 cDNA catalyzed the 0-demethylation of venlafaxine, which is the major metabolic pathway in uiuo. Intrinsic metabolic clearance values ( Vmax/Km) indicated that S( -)-venlafaxine was cleared preferentially via this pathway. 4 In microsomes from CYP2D6-deficient livers (n = 2), Vmax/Km of O-demethylation of venlafaxine was one to two orders of magnitude lower and was similar to the rate of N-demethylation. 5 Studies with chemical probes which preferentially inhibit P450 isoforms suggested that CYP3A3/4 is involved in venlafaxine N-demethylation. 6 These in vitro findings predict phenotypic differences in the kinetics of venlafaxine in vivo, although the clinical importance of this is unclear as 0-demethylvenlafaxine is pharmacologically similar to the parent drug. The findings also predict relatively limited pharmacokinetic interaction between venlafaxine and other CYP2D6 substrates.

This promotor region polymorphism does not appear to play a major role in determining constitutive CYP3A4 expression.

Cytochrome P450s play a central role in the metabolism and disposition of an extremely wide range of drugs and chemical carcinogens. Individual differences in the expression of these enzymes may be an important determinant in susceptibility to adverse drug reactions, chemical toxins and mutagens. In this paper, we have measured the relative levels of expression of cytochrome P450 isoenzymes from eight gene families or subfamilies in a panel of twelve human liver samples in order to determine the individuality in their expression and whether any forms are co-regulated. Isoenzymes were identified in most cases on Western blots based on the mobility of authentic recombinant human cytochrome P450 standards. The levels of the following P450 proteins correlated with each other: CYP2A6, CYP2B6 and a protein from the CYP2C gene subfamily, CYP2E1 and a member of the CYP2A gene subfamily, CYP2C8, CYP3A3/A4 and total cytochrome P450 content. Also, the levels of two proteins in the CYP4A gene subfamily were highly correlated. These correlations are consistent with the relative regulation of members of these gene families in rats or mice. In addition, the level of expression of specific isoenzymes has also been compared with the rate of metabolism of a panel of drugs, carcinogens and model P450 substrates. These latter studies demonstrate and confirm that the correlations obtained in this manner represent a powerful approach towards the assignment of the metabolism of substrates by specific human P450 isoenzymes.

No abstract