Purpose: The objective of this study was to develop two different in vitro release testing methods (Franz diffusion cells and USP apparatus 2) for in situ forming gels, and to evaluate their discriminatory capability. At present, there is no standard in vitro release testing method suggested in the US Pharmacopeia for in situ forming gels. Therefore, it is necessary to develop an appropriate testing method, not only to assess in vitro formulation performance, but also for the quality control of the final products.
Methods: Three bupivacaine hydrochloride in situ gel formulations were prepared by dissolving different amounts of bupivacaine hydrochloride, Pluronic F127 (F127) and Pluronic F68 (F68) in 4°C water. Gelation temperature(s) were determined using a tube inversion method (test-tubes containing the formulations were inverted at different temperatures and the time until no fluidity was measured). Gelation time was measured from different storage conditions (4°C and 25°C) to 37°C. The in vitro drug release testing of the formulations were performed with pH 5.8 phosphate buffered saline at 37°C using two different apparati, vertical Franz diffusion cells with a volume of 12 mL and USP apparatus 2 with enhancer cells (Figure 1). First order and Higuchi models were used to analyze the release kinetics of the formulations.
Results: Three bupivacaine hydrochloride in situ gel formulations were successfully prepared. As shown in Table 1, addition of F68 in the formulation resulted in higher gelation temperature and longer gelation time. Low storage temperature (4°C) increased the gelation time and prolonged the gel formation process. The in vitro release data from both Franz diffusion cells and USP apparatus 2 showed a better fit with the Higuchi model better than with the First order model (Table 2). The Franz diffusion cell method provided better differentiation between the formulations compared with USP apparatus 2.
Conclusion: F68 may be added in Pluronic-based in situ gel formulations to modulate the gelation temperature and gelation time. Low temperature slows down the gelation process, therefore the storage temperature should be carefully considered and tightly controlled when optimizing the gelation time. The Higuchi model showed a good fit with the in vitro release profiles obtained using the Franz diffusion cells and USP apparatus 2, suggesting diffusion controlled release. Compared to USP apparatus 2, Franz diffusion cells possessed better discriminatory capability for the three in situ gel formulations. Although the Franz diffusion cell method appeared to be more promising than USP apparatus 2 to differentiate the three formulations, further studies correlating in vitro with in vivo data are necessary.
Rajesh Lalla– Associate Professor, University of Connecticut Health Center, Section of Oral Medicine, Connecticut
Diane Burgess– Distinguished Professor of Pharmaceutics, University of Connecticut, Storrs, Connecticut