Pharmaceutical Chemistry

Pharmaceutical Drug Impurities

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Impurities can be closely related to the product that is formed during the synthesis of a bulk drug or it can be decomposition product formed during the storage of the drug. The International Conference on Harmonization (ICH) has published guidelines on impurities in new drug substances, products and residual solvents. According to ICH guidelines, an impurity should not exceed 0.1 % and total impurity should not exceed 1.0 % in manufacturing each batch of the drug. Impurities present in excess of 0.1 % should be identified and quantified by sufficiently selective methods. The process of identification and quantification of impurities is known as impurity profiling. The procedure of impurity profiling begins with the detection of impurities using thin layer chromatography, high-performance liquid chromatography or gas chromatography. The presence of impurities in bulk drug can be identified by using the impurity reference standard, which includes the products of predictable side reactions or degradation products. If the retention time of both (impurities present in the bulk drug and impurity reference standard) match, then the impurities present will be easily identified. In case of unsuccessful identification, an analytical method, either LC/MS or GC/MS, is used. Based on MS information, the structure of the impurity will be proposed. The important step in impurity profiling is the synthesis of the material (impurity standard) with the proposed structure. Retention and spectral matching of the synthesized material with the impurity present in the bulk drug are useful for the analytical method development and method validation. It is essential to know the structure of impurities present in the bulk drug in order to alter the drug reaction conditions and to reduce the quantity of impurity to an acceptable level.