Increasing the Hydrophobic Component of Poloxamers and the Inclusion of Salt Extend the Release of Bupivacaine from Injectable In Situ Gels, While Common Polymer Additives Have Little Effect

Various strategies have been applied to reduce the initial burst of drug release and sustain release from poloxamer-based thermoresponsive gels. This work focussed on investigating different formulation approaches to minimise the initial burst of release and sustain the release of the small hydrophilic drug bupivacaine hydrochloride from poloxamer-based thermoresponsive gels. Various in situ gel formulations were prepared by varying the polypropylene oxide (PPO)/polyethylene oxide (PEO) ratio and by adding additives previously described in the literature. It was observed that increasing the PPO/PEO ratio from 0.28 to 0.30 reduced the initial burst release from 17.3% ± 1.8 to 9.1% ± 1.2 during the first six hours and extended the release profile from 10 to 14 days. Notably, the inclusion of sodium chloride (NaCl 0.4% w/w) further reduced the initial burst release to 1.8% ± 1.1 over the first 6 h. Meanwhile, physical blending with additive polymers had a negligible effect on the burst release and overall release profile. The findings suggest that extended release of bupivacaine hydrochloride, with reduced initial burst release, can be achieved simply by increasing the PPO/PEO ratio and the inclusion of NaCl.