• T Suto, H Obata, M Tobe, H Oku, H Yokoo, Y Nakazato, and S Saito.
• Department of Anesthesiology and.
• Br J Anaesth. 2012 Dec 1;109(6):957-67.

BackgroundSingle applications of sustained-release local anaesthetics may provide prolonged pain relief without requiring indwelling catheters, but have not yet been investigated for epidural postoperative pain management. We synthesized injectable sustained-release lidocaine particles (SRLPs) from biodegradable polymers and examined their effect in a rat model of postoperative pain.MethodsTwo types of polylactic acid particles, SRLP-10 and SRLP-25, containing 10% or 25% lidocaine, respectively, were generated and the lidocaine release was evaluated in vitro for 14 days. The SRLPs were then injected epidurally in the male Sprague-Dawley rats immediately before they received a hindpaw incision (the postoperative pain model), and hindpaw hypersensitivity was evaluated with the von Frey test. Motor paralysis and coordination were also assessed using a paralysis score and rota-rod test. Neurotoxicity and inflammation of the spinal cord, cauda equina, and tissue surrounding the injection site were histologically evaluated.ResultsIn vitro, SRLP-10 and SRLP-25 released lidocaine over 7 and 3 days, respectively. The in vivo injection of SRLP-10 (80 mg) produced anti-hypersensitivity with no evidence of motor paralysis for 7 days after the paw incision, and SRLP-25 (60 mg) inhibited postoperative hypersensitivity for 7 days. Temporary motor paralysis (15 min) was observed after the injection of SRLP-25 (even with 40 mg). Foreign body reactions were observed around the SRLP injection site at 1 and 4 weeks after injection. No histopathological changes were observed at 1 or 4 weeks.ConclusionsThe epidural injection of SRLPs produced prolonged anti-hypersensitivity in a rat model of postoperative pain with no major complications.

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