Revisitingin vitrorelease test for topical gel formulations: The effect of osmotic pressure explored for better bio-relevance
Release test methods for topical dosage forms including pharmacopeial tests require a large volume of release media, with limited application for high throughput screening. In the present study, we evaluated Transwell assay to miniaturize the release test method for optimization of thermoreversible topical gel formulations. We also explored the osmotic effect on the in vitro release rates from gel formulations to understand the bio-relevance of release media. An extreme vertices type of mixture design in Minitab®16 generated eleven formulations composed of poloxamer 407, poloxamer 188, and phosphate buffered saline (PBS). A quadratic equation adequately described the composition dependence of gelation temperature. Epidermal growth factor (EGF) and trypan blue were used as model drugs for proteins and small molecules, respectively. Cumulative release in PBS containing 30% sucrose exhibited linear correlation with respect to the gel compositions, while PBS without sucrose did not differentiate various compositions. Higher release rates in PBS than in sucrose media are attributable to the osmotic water flow from PBS into the donor phase, and subsequent increase in diffusivity. The time course of in vivo near-infrared fluorescence imaging of topical EGF gels on the wound sites were consistent with the in vitro release profiles measured with PBS as the release media. To the best of our knowledge, this is the first study to propose a release test method suitable for high throughput screening of topical formulations with emphasis on the osmotic pressure effect. Bio-relevant release media composition for a topical formulation would vary depending on its clinical application because the osmotic water flow through the normal skin would be negligible compared to compromised skin.