A method to measure the solubility of drugs in ointment bases

The solubility of a drug in an ointment base is an important factor determining the efficacy of the formulation. However, it is difficult to measure the solubility of drugs in ointment bases. Thus, a rapid and simple method to determine the solubility of drugs in ointment bases was investigated. Using oxybenzone as a model drug, the bleeding liquid which leaked out from various ointments was collected, and the drug concentration in the bleeding liquid was measured. The drug concentrations in the bleeding liquid collected from soluble type ointments in which the drug is dissolved in bases were in accord with that in the ointments. On the other hand, the drug concentrations in the bleeding liquid collected from crystal dispersion type ointments in which drug crystal are dispersed in bases were the same in spite of the variance in total drug concentrations in the ointments. It was confirmed by microscopic examination that crystals do not flow out into bleeding liquid in the crystal dispersion type ointment. It was also confirmed that the solubility of the drug obtained in the present study was in a solubility range consistent with microscopic examination. Furthermore, an increase in the drug solubility was detected by this method when the drug solubility in the base was raised by adding a detergent into the base. And the results from this method were in accord with the results from microscopic examination. These results suggest that the drug concentration in bleeding liquid represents the solubility of the drug in an ointment.