Photochemical stability of 4'-azido-2'-deoxy-2'-methylcytidine hydrochloride: structural elucidation of major degradation products by LC-MS and NMR analysis.

The photochemical stability of (1'R,2'S,3'S,4'R)-4'-azido-2'-deoxy-2'-methylcytidine hydrochloride, a new anti-HCV agent, was investigated. Aqueous solutions and bulk drug powder of the drug candidate were exposed to UV-visible light, complying with ICH requirements. The nucleoside analog decomposed via loss of nitrogen to yield products derived from a highly reactive azide intermediate. Major photolysis products were identified by LC-MS and NMR analysis, revealing three main photodegradation pathways. The first one led to the formation of a ring-expanded imidate ester. The other degradation pathways involved exocyclic or endocyclic bond cleavage with imine or imino lactone formation. The latter were prone to rapid hydrolysis, eventually resulting in the release of cytosine, 2-methyl malonaldehyde and (E)-cytosyl-2-methylpropenal.