ICH guidance in practice: degradation behaviour of oseltamivir phosphate under stress conditions

Oseltamivir phosphate was subjected to stress degradation conditions prescribed by ICH guideline Q1A (R2). A total of five degradation products (Os I to Os V) were generated under hydrolytic (acid and alkaline) stress conditions. Their unambiguous structural elucidation was carried out using LC-MS, LC-NMR and HR-NMR data. First, accurate masses of Os I, Os II, Os IV and Os V were determined by LC-MS/TOF. Subsequently, (1)H and COSY NMR studies were carried on the drug and these four degradation products using LC-NMR. The structure of Os III was elucidated after preparative isolation and purification, followed by MS/TOF and HR-NMR studies. The degradation products, Os II, Os IV and Os V were characterized as 4-acetamido-5-amino-3-(pentan-3-yloxy)cyclohex-1-ene carboxylic acid, 4,5-diamino-3-(pentan-3-yloxy)cyclohex-1-ene carboxylic acid and ethyl 4,5-diamino-3-(pentan-3-yloxy)cyclohex-1-ene carboxylate, respectively. Os I and Os III were identified as positional isomers of Os II and the drug, respectively, involving N,N-acyl migration from 4-amino to 5-amino position in the ring. Two degradation products (Os IV and Os V) were found to be new and previously unreported. The degradation pathway for all five was outlined and justified mechanistically. In silico toxicity of the drug and degradation products was also assessed using TOPKAT and DEREK software and compared.