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
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
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 1H and 13C 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.