Effect of the admixture of tetracycline and nadifloxacin ointments on their stability and their antibacterial activity

Impetigo contagiosa staphylogenes is commonly treated by administering a combination of nadifloxacin and tetracycline ointments. However, it is not clear whether nadifloxacin and tetracycline are stable after mixing. The purpose of this study was to evaluate the stability of these agents in combination. We also evaluated changes in antibacterial activity after mixing. Mixing the two ointments caused tetracycline to change from yellow to brown in the admixture. Furthermore, the tetracycline content in the ointment decreased in a time-dependent manner, to about 40% at 288 h after mixing. In addition, the nadifloxacin content in the ointment did not change 288 h after mixing. In an alkaline environment (pH 9.0 and 11.0), the tetracycline content decreased and the color of tetracycline changed to brown. These results suggest that sodium hydroxide, which is an additive in nadifloxacin ointment, influences the content of tetracycline. We evaluated the chemical sensitivity of Staphylococcus aureus using disk tests. Nadifloxacin and tetracycline ointment showed the largest radius of inhibition circle, followed by the admixture 0 h after mixing and the admixture 72 h after mixing. These results suggest that the antibacterial activity is inhibited by the admixture. We propose that pharmacists should avoid mixing nadifloxacin with tetracycline ointment in the treatment of impetigo contagiosa staphylogenes and should take care to avoid interactions caused by additives in the ointments.