Quantitation of fusidane antibiotics in pharmaceuticals using the surfactant-dye binding degree method

The determination of fusidic acid (FA)/sodium fusidate (SF) in dosage forms based on their modification of the binding degree of the cationic surfactant didodecyldimethylammonium bromide (DDABr) to the anionic dye Coomassie Brilliant Blue G (CBBG) was proposed. The formation of mixed DDABr-CBBG aggregates was monitored from changes in the spectral features of the dye. Addition of fusidane antibiotic to DDABr-CBBG aqueous mixtures caused a decrease in the surfactant-dye binding degree as a result of the formation of DDABr-drug aggregates. At the working pH (7.0), FA and SF were as fusidate anion; therefore, both hydrophobic and attractive electrostatic interactions were responsible for the formation of these mixed surfactant-drug aggregates. Based on the mathematical expression previously derived for determining surfactants, a linear calibration in the fusidane antibiotic concentration range 1.9-70 mg L−1 was obtained. The surfactant to dye binding degree (SDBD) method offered important advantages over conventional method used for quality control of fusidane antibiotics in terms of sensitivity (detection limit = 0.6 mg L−1), selectivity, which permitted a minimum sample treatment (only dissolution of dosage forms), precision [relative standard deviations for the whole analytical process = 1.5-2.0% (n = 6)], rapidity, because of the speed of both sample treatment and determination step, required instrumentation (a photometric titrator) and cost. Pharmaceutical samples analyzed included tablets, pomades and creams.