Chemical stability of 4'-azidocytidine and its prodrug balapiravir
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Updated: Jun 20, 2026

Preparation of Stable Bicyclic Aziridinium Ions and Their Ring-Opening for the Synthesis of Azaheterocycles
Published on: August 22, 2018
Fujun Li1, Xiaoyang Wu, Xu Hadig
1XenoPort, Inc., Santa Clara, CA 95051, USA. fujun_li@yahoo.com
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.
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