ABSTRACT:Several human immunodeficiency virus (HIV) protease inhibitors, including atazanavir, indinavir, lopinavir, nelfinavir, ritonavir, and saquinavir, were tested for their potential to inhibit uridine 5-diphospho-glucuronosyltransferase (UGT) activity. Experiments were performed with human cDNA-expressed enzymes (UGT1A1, 1A3, 1A4, 1A6, 1A9, and 2B7) as well as human liver microsomes.All of the protease inhibitors tested were inhibitors of UGT1A1, UGT1A3, and UGT1A4 with IC 50 values that ranged from 2 to 87 M. The IC 50 values found for all compounds for UGT1A6, 1A9, and 2B7 were >100 M. The inhibition (IC 50 ) of UGT1A1 was similar when tested against the human cDNA-expressed enzyme or human liver microsomes for atazanavir, indinavir, and saquinavir (2.4, 87, and 7.3 M versus 2.5, 68, and 5.0 M, respectively). By analysis of the double-reciprocal plots of bilirubin glucuronidation activities at different bilirubin concentrations in the presence of fixed concentrations of inhibitors, the UGT1A1 inhibition by atazanavir and indinavir was demonstrated to follow a linear mixed-type inhibition mechanism (K i ‫؍‬ 1.9 and 47.9 M, respectively). These results suggest that a direct inhibition of UGT1A1-mediated bilirubin glucuronidation may provide a mechanism for the reversible hyperbilirubinemia associated with administration of atazanavir as well as indinavir. In vitro-in vivo scaling with [I]/K i predicts that atazanavir and indinavir are more likely to induce hyperbilirubinemia than other HIV protease inhibitors studied when a free C max drug concentration was used. Our current study provides a unique example of in vitro-in vivo correlation for an endogenous UGT-mediated metabolic pathway.The HIV protease is an essential enzyme that cuts the viral gag-pol polyprotein into its functional subunits. Atazanavir, indinavir, saquinavir, lopinavir, ritonavir, and nelfinavir are HIV protease inhibitors. The structures of these HIV protease inhibitors are shown in Fig. 1. As opposed to atazanavir, which is an azapeptide protease inhibitor (Goldsmith and Perry, 2003), other listed HIV protease inhibitors are peptidomimetics sharing the same structural determinant, i.e., a hydroxyethylene or a hydroxyethylamine moiety, which makes them nonscissile HIV protease substrate analogs. These HIV protease inhibitors are metabolized primarily by hepatic CYP3A enzymes, and they are also inhibitors of CYP3A enzymes (Flexner, 2000;de Maat et al., 2003;Goldsmith and Perry, 2003;Ernest et al., 2005). All of these HIV protease inhibitors have a high protein binding (Ͼ98%), except indinavir and atazanavir, which have a protein binding of 60 and 86%, respectively. Most of the HIV protease inhibitors are bound to ␣1-acid glycoprotein instead of albumin (de Maat et al., 2003). There are no literature reports indicating that HIV protease inhibitors are good substrates for human UGT enzymes, although there is evidence to support indinavir as a substrate of UGTs (Balani et al., 1996).Glucuronidation represents a major pathway for the elimin...

Absolute oral bioavailability and disposition characteristics of irbesartan, an angiotensin II receptor antagonist, were investigated in 18 healthy young male volunteers. Subjects received [14C] irbesartan as a 30-minute intravenous infusion (50 mg), [14C] irbesartan orally as a solution (50 mg or 150 mg), or irbesartan capsule (50 mg). Irbesartan was rapidly and almost completely absorbed after oral administration, and exhibited a mean absolute oral bioavailability of 60% to 80%. Mean total body clearance was approximately 157 mL/min, and renal clearance was 3.0 mL/min. Volume of distribution at steady state was 53 L to 93 L, and terminal elimination half-life was approximately 13 to 16 hours. Hepatic extraction ratio was low (0.2). There were no major circulating metabolites, and approximately 80% of total plasma radioactivity was attributable to unchanged irbesartan. Regardless of route of administration, approximately 20% of dose was recovered in urine and the remainder in feces.

An LC-MS/MS-based approach that employs authentic radioactive metabolites as reference standards was developed to estimate metabolite exposures in early drug development studies. This method is useful to estimate metabolite levels in studies done with non-radiolabeled compounds where metabolite standards are not available to allow standard LC-MS/MS assay development. A metabolite mixture obtained from an in vivo source treated with a radiolabeled compound was partially purified, quantified, and spiked into human plasma to provide metabolite standard curves. Metabolites were analyzed by LC-MS/MS using the specific mass transitions and an internal standard. The metabolite concentrations determined by this approach were found to be comparable to those determined by valid LC-MS/MS assays. This approach does not requires synthesis of authentic metabolites or the knowledge of exact structures of metabolites, and therefore should provide a useful method to obtain early estimates of circulating metabolites in early clinical or toxicological studies.

]BMS-690514 to rats and dogs, the majority of the radioactive dose (61-71%) was recovered in the feces, whereas 18 to 20% was eliminated in urine. In bile duct-cannulated rats, 83 and 17% of the administered radioactivity was recovered in the bile and urine, respectively, suggesting that biliary secretion was a major route for the elimination of BMS-690514-derived radioactivity in rats.The parent compound underwent extensive metabolism in both species, with <12% of the administered radioactivity recovered as BMS-690514 in the excreta samples. Metabolite profiles in plasma were qualitatively similar in rats, rabbits, and dogs. Unchanged BMS-690514 was a prominent drug-related component in the plasma profiles from all the species. However, multiple metabolites contributed significantly to the circulating radioactivity, particularly for rabbit and dog, in which metabolites comprised 73 to 93% of the area under the time curve (0-8 h). Circulating metabolites included M6, a direct Oglucuronide conjugate; M1, a hydroxylated metabolite; and glucuronide conjugates of hydroxylated and O-demethylated metabolites. Overall, the results from these studies suggested that BMS-690514 was well absorbed and highly metabolized through multiple pathways in these preclinical species.

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