Investigation and Characterization of New Impurity in Clorsulon Drug Substance

Background: Clorsulon is a drug substance in other words Active Pharmaceutical Ingredient (API). It is listed in the United States Pharmacopoeia (USP). The related substances analysis as per USP is by Thin Layer Chromatography (TLC). It has been extremely difficult for the analyst to take decision on quality as the test was not able to reproduce the result and hence the purity by HPLC was planned. It was observed from the HPLC analysis that there was presence of one unknown impurity at Relative Retention Time (RRT) of about 1.15 minute. The same impurity was observed in several batches and resulted out of specification (OOS). The light is thrown to investigate the impurity and establish the impurity profile. Objective: Any unknown impurities present in the drug substance are toxic to animal safety and affect the drug efficacy. The research work was aimed to identify and characterize the impurity and understand the structure. It helps toxicological evaluation and based on level of toxicity, the limit for the specific impurity is arrived and recommended for monograph. This work helps pharmaceutical manufacturers and regulators to comply with qualification requirements of both USP and VICH (International cooperation on Harmonisation of Technical Requirements for Registration of Veterinary Medicinal Products). Methods: HPLC method was used for related substances analysis. Preparative HPLC was used to isolate the impurity. LC-MS was used to identify the mass and further NMR was used for characterization. Results: The impurity isolated by preparatory HPLC was targeted and confirmed by spike study by regular HPLC method. The purity of the impurity was 96.9%. The molecular formula of the impurity is C9H10Cl3N3O4S2 and Mass is 394.38. The NMR data of both H-1 and C-13 characterization deduce the structure of the compound as "4-(Methylamino)-6-(trichloroethenyl) benzene-1, 3-disulfonamide". Conclusion: The extensive investigational data confirm the structure of the impurity. The specification limit for this impurity is applied based on toxicological data.