Development and validation of an improved liquid chromatographic method for the analysis of dirithromycin

The official method for the determination of dirithromycin and related substances in the European Pharmacopoeia (Ph. Eur.) and in the United States Pharmacopeia (USP) is an isocratic liquid chromatographic (LC) method using an ODS column. With this method, the separation of the main component dirithromycin from its epimer is not complete. Moreover, this method suffers sometimes from drift of the baseline and from subsequent quantitation problems. The required resolution is not easy to obtain. Using an adapted method derived from the one prescribed in the pharmacopoeias, the selectivity of a set of more than 40 reversed-phase columns towards dirithromycin components was investigated. The selection of the most suitable column was achieved by the chromatographic response function (CRF) approach. Several changes were introduced to the method in order to improve the separation and to overcome the baseline drift problem. The resulting method uses a Zorbax Extend column maintained at 30 degrees C and a mobile phase containing acetonitrile, methanol, 2-propanol, water and a phosphate buffer at pH 7.5. The method allows a good separation of dirithromycin components, which is much better than that obtained with the existing methods. Several impurities of unknown identity are also separated. The method shows good repeatability, linearity and sensitivity, and it is robust. In addition, it proved to be applicable to a wide number of C18 reversed-phase columns.