Structural Elucidation of Unknown Oxidative Degradation Products of Mycophenolate Mofetil Using LC-MSn

Unknown degradation products of mycophenolate mofetil formed under oxidative stress conditions have been characterized via LC-MSn, using an RP-LC column (50 mm length, 2.1 mm i.d., 1.9 µm). The mobile phase consisted of 26 % acetonitrile and 74 % 9 mM ammonium acetate pH 5.87, the column temperature was set at 40 °C, and the flow rate of the mobile phase was 400 µL min−1. Four degradation products formed under the influence of 15 % hydrogen peroxide after an hour. Two of the degradation products had previously been identified as mycophenolic acid (m/z 321) and the N-oxide of mycophenolate mofetil (m/z 450). The other two unknown degradation products, denoted DP I (m/z 319) and DP II (m/z 466), were investigated in depth in order to identify their structures. The fragmentation patterns of DP I and DP II were established, analyzed, and compared to the fragmentation patterns of mycophenolate mofetil and the N-oxide of mycophenolate mofetil, which aided the elucidation of the structures of the unknown degradation products. The unknown degradation product DP I (m/z 319.2) was proposed to be an oxidized unsaturated mycophenolate aldehyde which also appeared as an ammonium adduct ion at m/z 336.3. The second unknown degradation product, DP II (m/z 466.3), was characterized as a doubly oxidized derivative of mycophenolate mofetil. A possible degradation pathway of mycophenolate mofetil under the influence of an oxidizing agent was also proposed.