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IDX184 is a phosphoramidate prodrug of 2′-methylguanosine-5′-monophosphate, developed to treat patients infected with hepatitis C virus. A mass balance study of radiolabeled IDX184 and pharmacokinetic studies of IDX184 in portal vein-cannulated monkeys revealed relatively low IDX184 absorption but higher exposure of IDX184 in the portal vein than in the systemic circulation, indicating >90 % of the absorbed dose was subject to hepatic extraction. Systemic exposures to the main metabolite, 2′-methylguanosine (2′-MeG), were used as a surrogate for liver levels of the pharmacologically active entity 2′-MeG triphosphate, and accordingly, systemic levels of 2′-MeG in the monkey were used to optimize formulations for further clinical development of IDX184. Capsule formulations of IDX184 delivered acceptable levels of 2′-MeG in humans; however, the encapsulation process introduced low levels of the genotoxic impurity ethylene sulfide (ES), which necessitated formulation optimization. Animal pharmacokinetic data guided the development of a tablet with trace levels of ES and pharmacokinetic performance equal to that of the clinical capsule in the monkey. Under fed conditions in humans, the new tablet formulation showed similar exposure to the capsule used in prior clinical trials.

AZT-P-ddI is an antiviral heterodimer composed of one molecule of 3'-azido-3'-deoxythymidine (AZT) and one molecule of 2',3'-dideoxyinosine (ddI) linked through their 5' positions by a phosphate bond. The metabolic fate of the dimer was studied with isolated rat, monkey, and human hepatocytes and was compared with that of its component monomers AZT and ddI. Upon incubation of double-labeled [14C]AZT-P-[3H]ddI in freshly isolated rat hepatocytes in suspension at a final concentration of 10 microM, the dimer was taken up intact by cells and then rapidly cleaved to AZT, AZT monophosphate, ddI, and ddI monophosphate. AZT and ddI so formed were then subject to their respective catabolisms. High-performance liquid chromatography analyses of the extracellular medium and cell extracts revealed the presence of unchanged dimer, AZT, 3'-azido-3'-deoxy-5'-beta-D-glucopyranosylthymidine (GAZT), 3'-amino-3'-deoxythymidine (AMT), ddI, and a previously unrecognized derivative of the dideoxyribose moiety of ddI, designated ddI-M. Trace extracellular but substantial intracellular levels of the glucuronide derivative of AMT (3'-amino-3'-deoxy-5'-beta-D-glucopyranosylthymidine [GAMT]) were also detected. Moreover, the extent of the formation of AMT, GAZT, and ddI-M from the dimer was markedly lower than that with AZT and ddI alone by the hepatocytes. With hepatocytes in primary culture obtained from rat, monkey, and human, large interspecies variations in the metabolism of AZT-P-ddI were observed. While GAZT and ddI-M, metabolites of AZT and ddI, respectively, as well as AZT 5'-monophosphate (MP) and ddI-MP were detected in the extracellular media of all species, AMT and GAMT were produced only by rat and monkey hepatocytes. No such metabolites were formed by human hepatocytes. The metabolic fate of the dimer by human hepatocytes was consistent with in vivo data recently obtained from human immunodeficiency virus-infected patients.

Our laboratory has shown that human liver microsomes metabolize the anti-HIV drug 3′-azido-3′-deoxythymidine (AZT) via a P450-type reductive reaction to a toxic metabolite 3′-amino-3′-deoxythymidine (AMT). In the present study, we examined the role of specific human P450s and other microsomal enzymes in AZT reduction. Under anaerobic conditions in the presence of NADPH, human liver microsomes converted AZT to AMT with kinetics indicative of two enzymatic components, one with a low K m (58–74 μM) and V max (107–142 pmol AMT formed/min/mg protein) and the other with a high K m (4.33–5.88 mM) and V max (1804–2607 pmol AMT formed/min/mg). Involvement of a specific P450 enzyme in AZT reduction was not detected by using human P450 substrates and inhibitors. Antibodies to human CYP2E1, CYP3A4, CYP2C8, CYP2C9, CYP2C19, and CYP2A6 were also without effect on this reaction. NADH was as effective as NADPH in promoting microsomal AZT reduction, raising the possibility of cytochrome b 5 ( b 5 ) involvement. Indeed, AZT reduction among six human liver samples correlated strongly with microsomal b 5 content ( r 2 = 0.96) as well as with aggregate P450 content ( r 2 = 0.97). Upon reconstitution, human liver b 5 plus NADH: b 5 reductase and CYP2C9 plus NADPH:P450 reductase were both effective catalysts of AZT reduction, which was also supported when CYP2A6 or CYP2E1 was substituted for CYP2C9. Kinetic analysis revealed an AZT K m of 54 μM and V max of 301 pmol/min for b 5 plus NADH: b 5 reductase and an AZT K m of 103 μM and V max of 397 pmol/min for CYP2C9 plus NADPH:P450 reductase. Our results indicate that AZT reduction to AMT by human liver microsomes involves both b 5 and P450 enzymes plus their corresponding reductases. The capacity of these proteins and b 5 to reduce AZT may be a function of their heme prothestic groups.

3'-Azido-3'-deoxythymidilyl-(5',5')-2',3'-dideoxy-5'-inosinic acid (AZT-P-ddI,IVX-E-59. Scriptene) is a heterodimer composed of one molecule of 3'-azido-3'-deoxythymidine (zidovudine or AZT) and one molecule of 2',3'-dideoxyinosine (didanosine or ddl) linked through their 5' positions by a phosphate bond. AZT-P-ddl exhibits enhanced antiviral activity and selectivity in vitro compared with AZT and ddl alone. The pharmacokinetics of AZT-P-ddI were studied in 12 patients with human immunodeficiency virus (HIV) who had CD4 + cell counts higher than 200 cells/mm 3 . Isotopic preparations of ( 14 C)-AZT-P-( 3 H)-ddI were administered intravenously (50 mg and 100 mg) to eight patients; 1 month later these patients were crossed over to oral administration (100 mg and 200 mg). A second group of patients (n = 4) received only a 450-mg oral dose of AZT-P-ddl. Plasma levels of unchanged AZT-P-ddI after intravenous infusion declined rapidly and were undetectable 0.75 hours after the end of infusion, whereas the parent compound was not detected after oral administration, indicative of a very rapid metabolism. The parent entity was enzymatically cleaved in viva yielding the two constituent drugs AZT and ddI, which were subsequently subjected to their respective pharmacokinetic and metabolic processes. The β-glucuronide derivatide of AZT (GAZT) represented the major metabolite of AZT, but there were no detectable levels of the toxic metabolite 3'-amino-3'-deoxythymidine (AMT). A major and previously unrecognized in vivo metabolite of ddI, referred as ddl-M, was detected in plasma and urine. Analysis by high-field proton nuclear magnetic resonance and mass spectrometry led to the identification of ddI-M as being R(-)-dihydro-5-(hydroxymethyl)-2(3H)-furanone. The formation of AZT and ddl metabolites was increased after oral administration of AZT-P-ddI compared with the intravenous infusion, with an area under the concentration- time curve (AUC) ratio of metabolite to AZT and metabolite to ddl being 7.7 and 5.7 (oral) and 3.8 and 1.1 (intravenous), respectively. The newly identified ddI-M exhibited sustained plasma levels for extended time periods with an apparent elimination half-life (t 1/2 ) of approximately 10 hours after oral administration of AZT-P-ddl. Recovery of radioactivity associated with 3 H and 14 C in urine was essentially complete within 48 hours after oral and intravenous administration of AZT-P-ddI. The oral bioavailability of AZT (64.7-67.3 %) and ddI (33.6-42.9 %) and the other pharmacokinetic parameters were consistent with previous data reported with each nucleoside analog alone or in combination therapy.