Studies on controlled release of hydrophilic drugs from W/O high internal phase ratio emulsions

Formation of high internal phase ratio emulsions (HIPREs) has been studied in water/Cremophor WO7/soybean oil and water/Cremophor WO7/liquid paraffin systems. Two hydrophilic model drugs, clindamycin hydrochloride (CH) and theophylline (TP), were incorporated in HIPREs with a water concentration of 90% and an oil/surfactant (O/S) weight ratio of 60:40 and their release was determined in vitro at 25 degrees C. The release of both model drugs from HIPREs was much slower than from aqueous solutions. In aqueous solution the release pattern of both actives was identical. In contrast, a clearly distinct release pattern from HIPREs was observed: The release of CH, which is freely soluble in water, was very slow, regardless of the emulsion system, while the release of TP, which is slightly soluble in water, was faster. By changing the pH of the dispersed phase of HIPREs, which in turn affects solubility, drug release was modulated. An increase in the solubility of TP in the dispersed phase by a factor of roughly 4.5 produced a decrease in the diffusion coefficient of two orders of magnitude. These results show for the first time the key role of drug solubility in the release from W/O-HIPREs.