Chemical stability of 4'-azidocytidine and its prodrug balapiravir.

BACKGROUND: R1479, a 4'-azidocytidine nucleoside analog, was developed for the treatment of Hepatitis C virus infection. Balapiravir (R1626) is the tri-isobutyrate ester prodrug of R1479 under clinical development to improve exposure of R1479 upon oral administration. OBJECTIVE: The chemical stability and the rate of azide release of R1479 and balapiravir were studied. METHODS: R1479 and balapiravir solutions were prepared at different pH values and stored at various temperatures. An ion pair high-performance liquid chromatography (HPLC) method with gradient elution was employed to analyze the prodrug, parent, and degradation products. Azide was measured using a reversed phase HPLC method with UV detection after formation of the 3,5-dinitrobenzoyl azide derivative with 3,5-dinitrobenzoyl chloride. The data were analyzed using initial rate and conventional first-order kinetic methods. RESULTS: R1479 degrades to cytosine and azide in aqueous solutions, whereas balapiravir mainly degrades to R1479 and mono- and diesters of R1479. The rates of azide release from R1479 and balapiravir were generally comparable with the corresponding amount formed of cytosine. CONCLUSION: Azide release is pH dependent and is faster in acidic solutions than in neutral solutions. The amount of azide released is significantly less from balapiravir than that from R1479, suggesting a potential advantage of the prodrug over the parent drug.

Physicochemical properties of the nucleoside prodrug R1626 leading to high oral bioavailability.

The nucleoside analog R1479 is a potent and highly selective inhibitor of NS5b-directed hepatitis C virus (HCV) RNA polymerase in vitro. Because of its limited permeability, lipophilic prodrugs of R1479 were screened. Selection of the prodrug involved optimization of solubility, permeability, and stability parameters. R1626 has dissociation constant, intrinsic solubility, log partition coefficient (n-octanol water), and Caco-2 permeability of 3.62, 0.19 mg/mL, 2.45, and 14.95 x 10(-6) cm/s, respectively. The hydrolysis of the prodrug is significantly faster in the Caco-2 experiments than in hydrolytic experiments, suggesting that the hydrolysis is catalyzed by enzymes in the cellular membrane. Using GastroPlus, the physical properties of R1626 successfully predict the dose dependence of the pharmacokinetics in humans previously studied. The program predicts that if the particle size of R1626 is less than 25 microm, it will be well absorbed. Prodrugs with a solubility of greater than 100 microg/mL and permeability in the Caco-2 assay greater than 3 x 10(-6) cm/s are expected to achieve a high fraction absorbed.