Depot lidocaine-loaded microemulsion for prolonged local anesthesia: Different efficacy model studies

Topical delivery of lidocaine has been an area of interest, considering the barrier properties of the skin and the high clearance rate of lidocaine (duration of action < 2 h). In this study, a lidocaine-loaded microemulsion was prepared to enable sustained local anesthesia. The lidocaine-loaded microemulsion was prepared using isopropyl myristate, Pluronic F68, Tween 20, and water, based on the characteristics identified in pseudo ternary diagrams. Microemulsions containing Pluronic F68:Tween 20 at a ratio of 1:4 showed a larger microemulsion area than those with a ratio of 1:6. An ex-vivo study indicated a sustained release of lidocaine from the microemulsion for up to 12 h, compared with 5 h from a commercially available ointment. The radiant heat tail-flick and hot water tail-flick models showed a maximum response at 1 h, while the pin-prick test in rabbits showed a relatively rapid response, due to the high permeability of lidocaine through the dorsal skin of the rabbit compared to the thick skin of the rat tail. The sciatic nerve block model showed a prolonged effect up to 7 h compared to the hotplate sensory test (up to 3 h). This was due to the depot application of lidocaine around the sciatic nerve. This study demonstrated the potential application of microemulsions to prolong local anesthesia, but also raised concerns about the selection of suitable animal models to investigate the efficacy of local anesthetics.