Potent inhibition of the cytochrome P-450 3A-mediated human liver microsomal metabolism of a novel HIV protease inhibitor by ritonavir: A positive drug-drug interaction.

ABT-378 is a potent in vitro inhibitor of the HIV protease and is currently being developed for coadministration with another HIV protease inhibitor, ritonavir, as an oral therapeutic treatment for HIV infection. In the present study, the effect of ritonavir, a potent inhibitor of cytochrome P-450 (CYP) 3A, on the in vitro metabolism of ABT-378 was examined. Furthermore, the effect of ABT-378-ritonavir combinations on several CYP-dependent monooxygenase activities in human liver microsomes was also examined. ABT-378 was found to undergo NADPH- and CYP3A4/5-dependent metabolism to three major metabolites, M-1 (4-oxo) and M-3/M-4 (4-hydroxy epimers), as well as several minor oxidative metabolites in human liver microsomes. The mean apparent K(m) and V(max) values for the metabolism of ABT-378 by human liver microsomes were 6.8 +/- 3.6 microM and 9.4 +/- 5.5 nmol of ABT-378 metabolized/mg protein/min, respectively. Ritonavir inhibited human liver microsomal metabolism of ABT-378 potently (K(i) = 0.013 microM). The combination of ABT-378 and ritonavir was much weaker in inhibiting CYP-mediated biotransformations than ritonavir alone, and the inhibitory effect appears to be primarily due to the ritonavir component of the combination. The ABT-378-ritonavir combinations (at 3:1 and 29:1 ratios) inhibited CYP3A (IC(50) = 1.1 and 4.6 microM), albeit less potently than ritonavir (IC(50) = 0.14 microM). Metabolic reactions mediated by CYP1A2, CYP2A6, and CYP2E1 were not affected by the ABT-378-ritonavir combinations. The inhibitory effects of ABT-378-ritonavir combinations on CYP2B6 (IC(50) = >30 microM), CYP2C9 (IC(50) = 13.7 and 23.0 microM), CYP2C19 (IC(50) = 28.7 and 38.0 microM), and CYP2D6 (IC(50) = 13.5 and 29.0 microM) were marginal and are not likely to produce clinically significant drug-drug interactions.

In vitro metabolism of the HIV-1 protease inhibitor ABT-378: species comparison and metabolite identification.

HIV protease inhibitor ABT-378 (ABT-378) was metabolized very extensively and rapidly by liver microsomes from mouse, rat, dog, monkey, and humans. The rates of NADPH-dependent metabolism of ABT-378 ranged from 2.39 to 9.80 nmol.mg microsomal protein-1.min-1, with monkey liver microsomes exhibiting the highest rates of metabolism. ABT-378 was metabolized to 12 metabolites (M-1 to M-12), which were characterized by mass and NMR spectroscopy. The metabolite profile of ABT-378 in liver microsomes from all five species was similar, except that the mouse liver microsomes did not form M-9, a minor secondary metabolite. The predominant site of metabolism was the cyclic urea moiety of ABT-378. In all five species, the major metabolites were M-1 (4-oxo-ABT-378) and M-3 and M-4 (4-hydroxy-ABT-378). Metabolite M-2 (6-hydroxy-ABT-378) was formed by rodents at a faster rate than by dog, monkey, and human liver microsomes. Metabolites M-5 to M-8 were identified as monohydroxylated derivatives of ABT-378. Metabolites M-9 and M-10 were identified as hydroxylated products of M-1. Metabolites M-11 and M-12 were identified as dihydroxylated derivatives of ABT-378. The metabolite profile in human hepatocytes and liver slices was similar to that of human liver microsomes. The results of the current study indicate that ABT-378 is highly susceptible to oxidative metabolism in vitro, and possibly in vivo, in humans.

ABT-378, a highly potent inhibitor of the human immunodeficiency virus protease.

The valine at position 82 (Val 82) in the active site of the human immunodeficiency virus (HIV) protease mutates in response to therapy with the protease inhibitor ritonavir. By using the X-ray crystal structure of the complex of HIV protease and ritonavir, the potent protease inhibitor ABT-378, which has a diminished interaction with Val 82, was designed. ABT-378 potently inhibited wild-type and mutant HIV protease (Ki = 1.3 to 3.6 pM), blocked the replication of laboratory and clinical strains of HIV type 1 (50% effective concentration [EC50], 0.006 to 0.017 microM), and maintained high potency against mutant HIV selected by ritonavir in vivo (EC50, 50-fold after 8 h. In healthy human volunteers, coadministration of a single 400-mg dose of ABT-378 with 50 mg of ritonavir enhanced the area under the concentration curve of ABT-378 in plasma by 77-fold over that observed after dosing with ABT-378 alone, and mean concentrations of ABT-378 exceeded the EC50 for >24 h. These results demonstrate the potential utility of ABT-378 as a therapeutic intervention against AIDS.

Pharmacokinetics and metabolism of [14C]dichloroacetate in male Sprague-Dawley rats. Identification of glycine conjugates, including hippurate, as urinary metabolites of dichloroacetate.

Pathways of metabolism of dichloroacetate (DCA), an investigational drug for the treatment of lactic acidosis in humans and a rodent hepatocarcinogen, are poorly understood. In this study, rats were given, by gavage, one or two 50 mg/kg doses of NaDCA. DCA labeled with 14C (carboxy carbon) or 13C (both carbons) was used in studies of disposition and pharmacokinetics, respectively. The effect of fasting for 14 hr before dosing was studied. Expired air, urine, feces, and tissues were collected from [14C]DCA-dosed rats. Urine was analyzed by HPLC, GC/MS, and NMR spectroscopy. Plasma samples were analyzed by GC/MS. DCA plasma elimination half-lives were 0.1 +/- 0.02 and 5.4 +/- 0.8 hr in young adult rats (180-265 g, 3-4 months of age) given one or two doses of DCA, respectively, and 9.7 +/- 1 hr in large, 16-month-old rats given two DCA doses. The percentage of the DCA dose excreted as CO2 varied from 17 to 46% and was lower (p < 0.001) in fed rats, compared with rats fasted overnight before dosing. Urine contained DCA and DCA metabolites, including oxalate, glyoxylate, and conjugated glycine (mainly hippurate and phenylacetylglycine). More unchanged DCA was excreted by large rats pretreated with DCA (mean, 20.2% of the dose) than by young adult rats given one dose of DCA (mean, 0.5%). This study confirmed that CO2, glycine, and oxalate are major products of DCA metabolism, it demonstrated that one dose of DCA altered the elimination of a subsequent dose, and it showed that age or body size, as well as access to food, significantly affected DCA metabolism in rats.