Investigation on the chemical stability of erythromycin in solutions using an optimization system.

Erythromycin, an antibiotic commonly used topically in the treatment of acne, is unstable in solution. The stability is influenced by the pH and by the presence of water. The influence of the pH on the stability of erythromycin was investigated even with the use of dimethyl isosorbide as co-solvent instead of water. The addition of zinc was attempted to ameliorate erythromycin stability as suggested in the literature. To investigate these three factors and their interactions, an optimization technique was carried out consisting of a 2(3) factorial analysis and a rotative central composite design. The erythromycin solutions were analysed with an HPLC method. The pH and the concentration of dimethyl isosorbide had a significant influence on the stability of erythromycin but the addition of zinc was not a significant factor. Moreover, there was a significant interaction between the pH and dimethyl isosorbide.