Development and validation of a stability indicating method for S-carboxymethyl-L-cysteine and related degradation products in oral syrup formulation

A stability-indicating method for the determination of S-carboxymethyl-L-cysteine and related degradation impurities in Exputex® 250mg/5mL syrup was developed in anion-exchange liquid chromatography mode. A forced degradation study supported the method development to ensure stability indicating conditions. Aqueous solutions of the active pharmaceutical ingredient and syrup samples at different pH-values were stress-tested in different thermal, light exposure and headspace conditions. One degradation product was detected in thermal stress studies at 60°C and 80°C in the pH range 5.0-7.0 and was identified by mass spectrometry as 5-oxo-thiomorpholine-3-carboxylic acid (lactam of carbocysteine). A second degradation product was only generated in moderately strong oxidizing conditions (0.5% H2O2 aqueous solution) and was identified as S-carboxymethyl-L-cysteine-(R/S)-sulphoxide (carbocysteine sulphoxide). The method was developed on a Zorbax SAX column, in isocratic mode. The mobile phase consisted of 200mM phosphate solution at pH 4.0 and acetonitrile (50:50 v/v) and UV detection was performed at a wavelength of 205nm. The method was linear for carbocysteine (R>0.9982) over a concentration range of 2.5-50μg/mL and 0.4-0.6mg/mL. Linearity for the impurities was shown from the LOQ to 50μg/mL. Specificity was verified and accuracy demonstrated for the active ingredient and its degradation products in syrup samples at 3 levels around their respective specification limits. Repeatability, intermediate precision and inter-laboratory reproducibility were assessed on three commercial batches, analyzed in triplicate by two operators at both the transferring and the receiving site and demonstrated a successful method transfer to the manufacturing quality control laboratory.