Biotransformation profiling of [(14)C]ixabepilone in human plasma, urine and feces samples using accelerator mass spectrometry (AMS).

Ixabepilone (BMS-247550, Ixempra is a semi-synthetic analog of the natural product epothilone B and marketed for its use in the treatment of cancer. A conventional human ADME study using decay counting methods for (14)C detection could not be conducted due to the radiolytic instability of [(14)C]ixabepilone at a typical specific activity (generally 1-10 microCi/mg). However, [(14)C]ixabepilone was sufficiently stable at low specific activity (1-2 nCi/mg). These low levels required the use of accelerator mass spectrometry (AMS) for radioactivity detection. The metabolic fate of this compound was investigated in eight patients following single intravenous doses of [(14)C]ixabepilone (70 mg, 80 nCi; specific activity: 1.14 nCi/mg). Metabolite profiles in pooled urine, feces and plasma samples were determined by HPLC-AMS analysis. The major radioactive component in urine and plasma was [(14)C]ixabepilone. Feces had a small amount of ixabepilone. There were numerous other drug-related components in both urine and fecal extracts (each <6% of the administered dose). LC/MS analysis of plasma, urine and feces samples showed the presence of three degradants of ixabepilone and several oxidative metabolites (M+16, M+14 and M-2 metabolites). This study demonstrates the utility of AMS in investigating the metabolite and excretion profiles of [(14)C]ixabepilone following administration to humans.

Biotransformation of carbon-14-labeled muraglitazar in male mice: interspecies difference in metabolic pathways leading to unique metabolites.

Muraglitazar (Pargluva; Bristol-Myers Squibb), a dual alpha/gamma peroxisome proliferator-activated receptor activator, is under development for treatment of type 2 diabetes. This study describes the biotransformation profile of carbon-14-labeled muraglitazar in plasma, urine, feces, and bile samples from male CD-1 mice [intact and bile duct cannulation (BDC)] after single oral doses of 1 and 40 mg/kg. The major drug-related component circulating in mouse plasma was the parent compound for up to 4 h postdose. Similar to excretion profiles of muraglitazar in humans, monkeys, and rats, urinary excretion was the minor and fecal excretion via the biliary route was the major elimination pathway for muraglitazar in mice. The parent compound was a minor component in urine, bile, and feces, indicating that muraglitazar was extensively metabolized in mice. Major biotransformation pathways of muraglitazar in mice included taurine conjugate formation, acyl glucuronidation, hydroxylation, and O-dealkylation. In addition to those metabolites previously identified in humans, monkeys, and rats (M1-M21), several unique metabolites identified in mice included taurine conjugates (M24, M25, M26a,b,c, and M31), oxazole-ring-opened metabolites (M27 and M28), glutathione conjugates (M29a,b and M30), a dihydroxylated metabolite (M32), hydroxylated metabolites (M33 and M35), and a dehydrogenated metabolite (M34). The taurine conjugate of muraglitazar, M24, was a major metabolite in mice, accounting for 12 to 15% of the total dose in BDC mice or 7 to 12% of the total dose in intact mice. None of these taurine and glutathione conjugates were found in the bile samples of humans, monkeys, or rats.

Glucuronidation as a major metabolic clearance pathway of 14c-labeled muraglitazar in humans: metabolic profiles in subjects with or without bile collection.

The metabolism and disposition of 14C-labeled muraglitazar (Pargluva), a novel dual alpha/gamma peroxisome proliferator-activated receptor activator, was investigated in eight healthy male subjects with and without bile collection (groups 1 and 2) after a single 20-mg oral dose. Bile samples were collected for 3 to 8 h after dosing from group 2 subjects in addition to the urine and feces collection. In plasma, the parent compound was the major component, and circulating metabolites, including several glucuronide conjugates, were minor components at all time points. The exposure to parent drug (Cmax and area under the plasma concentration versus time curve) in subjects with bile collection was generally lower than that in subjects without bile collection. The major portion of the radioactive dose was recovered in feces (91% for group 1 and 51% for group 2). In addition, 40% of the dose was recovered in the bile from group 2 subjects. In this 3- to 8-h bile, the glucuronide of muraglitazar (M13, 15% of dose) and the glucuronides of its oxidative metabolites (M17a,b,c, M18a,b,c, and M20, together, 16% of dose) accounted for approximately 80% of the biliary radioactivity; muraglitazar and its O-demethylated metabolite (M15) each accounted for approximately 4% of the dose. In contrast, fecal samples only contained muraglitazar and its oxidative metabolites, suggesting hydrolysis of biliary glucuronides in the intestine before fecal excretion. Thus, the subjects with and without bile collection showed different metabolic profiles of muraglitazar after oral administration, and glucuronidation was not observed as a major pathway of metabolic clearance from subjects with the conventional urine and fecal collection, but was found as a major elimination pathway from subjects with bile collection.

In vitro inhibition of UDP glucuronosyltransferases by atazanavir and other HIV protease inhibitors and the relationship of this property to in vivo bilirubin glucuronidation.

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 microM. The IC50 values found for all compounds for UGT1A6, 1A9, and 2B7 were >100 microM. The inhibition (IC50) 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 microM versus 2.5, 68, and 5.0 microM, 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 (Ki = 1.9 and 47.9 microM, 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]/Ki predicts that atazanavir and indinavir are more likely to induce hyperbilirubinemia than other HIV protease inhibitors studied when a free Cmax drug concentration was used. Our current study provides a unique example of in vitro-in vivo correlation for an endogenous UGT-mediated metabolic pathway.