Complexities of glucuronidation affecting in vitro in vivo extrapolation.

Glucuronidation is responsible for the clearance of a diverse range of drug and chemicals whose topology confers properties that complicate in vitro-in vivo clearance correlations as compared to those possible for oxidative metabolism. The active site of the UGTs faces the inside of the luminal space of the endoplasmic reticulum, thus presenting diffusional barriers for substrates, the cosubstrate, UDPGA, and resultant glucuronide products. Transport processes for the cosubstrate UDPGA and glucuronidated products likely contribute to the well-known latency phenomena in which exogenous detergents or alamethicin are required for maximal UGT activity in microsomes. This complicates the extrapolation of results of in vitro clearance studies to the in vivo situation. Even with activation, the microsomal-based clearance values still underestimate the actual in vivo UGT-mediated clearance; therefore latency is not the only explanation for the poor correlation. Recent data indicate that hepatocytes are a promising in vitro system that can be used for the early evaluation of human clearance behavior of drug candidates. Both induction and inhibition of UGT-mediated clearance are a source of clinical drug-drug interactions. Emerging evidence indicates that the same mechanisms identified in the regulation of CYP enzymes also are involved in regulation of the UGTs, i.e., CAR, AH and probably PXR mediate regulation of UGT1A1, 1A6 and UGT2B7, respectively. In contrast to CYP-mediated interactions, with a few exceptions, the magnitude of UGT-mediated interactions are less than 2-fold because of the relatively high UGT Km values and substrate overlap among the multiple isozymes.