Gabapentin: An impurity profiling approach with hydrogen peroxide induced thermal oxidation in autoclave

Predicting unconventional pathways in long term stability studies can be very challenging, especially in solid state degradation. This is of vital importance for enhancement in the level of degradation and to further enable the isolation of unknown degradation products. Predicting the mechanism or pathway also help in designing control strategies to minimise the potential chances of impurity formation. In this context, the lagtime of most oxidative degradation mechanisms is extremely important, which allows enough requisite of free radicals etc., one of the unconventional pathways. This is difficult to replicate in degradation studies involving usual solution form. In this investigation, we demonstrate that employing autoclave degradation with hydrogen peroxide conditions provide an enhanced level of an unknown degradation product, which was found to be futile in traditional forced degradation studies. The conditions used probably bypass the lag time. This impurity has been isolated using preparative liquid chromatography (Prep-HPLC) method and spectroscopic techniques like high-resolution mass spectrometry (HRMS) and nuclear magnetic resonance NMR (¹H, ¹³C, DEPT-135, NOESY, APT) for elucidation of the molecular structure. The impurity was identified as a dimer of gabapentin impurity-A (impurity (2,2′-diaza[l,2′-bispiro[4.5]decane]-3,3′-dione) and named it as impurity-U. A plausible mechanism for the formation of isolated impurity is proposed. HPLC methods have been developed and reported for the determination of gabapentin and its related substances using UV detection.