Overcoming Scale-Dependent Impurity Escalation in Epalrestat Synthesis: Identification, Mechanistic Elucidation, and Robust Process Control

Epalrestat is a clinically vital aldose reductase inhibitor for diabetic neuropathy. An unknown impurity exhibited an unacceptable increase from laboratory batches to approximately 2% in 100 kg pilot-scale batches, accompanied by a 20% yield reduction. This study investigates the impurity’s origin, structure, and control strategies. Through high-resolution mass spectrometry (HRMS) and NMR analyses, the impurity was identified as 4-methyl-5-phenyl-2-thiophenecarboxylic acid, a previously unreported impurity in Epalrestat synthesis. Mechanistic studies revealed that the impurity forms through base-mediated hydrolysis of rhodanine-N-acetic acid, followed by a Michael addition with α-methylcinnamaldehyde, and subsequent cyclization with concurrent hydrolysis and oxidation, ultimately leading to the impurity. Corresponding control strategies were established. Critical process parameters (CPPs), including NH3·H2O equivalents, reaction temperature, and EtOH volume, were optimized to suppress the impurity generation. Implementing these control strategies in scale-up lab batches suppressed this unknown impurity to undetectable levels while achieving >95% yield and >97% purity, and enabled transfer to pilot-scale facility. This systematic investigation of impurity profiling, combined with mechanism-driven process optimization, presents an effective strategy for pilot-scale process development of Epalrestat.