Study of the stability of erythromycin in a hydrophilic creme basis by liquid chromatography

The stability of the macrolide antibiotic erythromycin, incorporated at a 2% m/m concentration in a hydrophilic creme basis containing 2% m/m of chlorocresol, was monitored over a period of 2 months using liquid chromatography as the analytical method. Extracts of the creme were analysed using wide-pore poly(styrene-divinylbenzene) PLRP-S 1000 A as the stationary phase and a mixture of 2-methyl-2-propanol-acetonitrile-potassium phosphate buffer (pH 11.0; 0.02 M)-water (165:30:50:755, v/v/v/v) as the mobile phase. The method showed good selectivity towards chlorocresol, erythromycin A, its related substances and degradation products. As the pH of the creme containing erythromycin was 8.6, alkaline degradation products were expected to be formed. The presence of pseudoerythromycin A enol ether was observed after storage of the creme for 1 week at a temperature of 25 degrees C. After 1 month the content of erythromycin was still more than 95%.