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Long-acting local anesthetics are beneficial for the management of postoperative pain and chronic pain. The authors recently reported that a single injection of N-β-phenylethyl-lidocaine (tonicaine), a quaternary lidocaine derivative, effectively blocks rat sciatic nerve function four to nine times longer than lidocaine, with a predominance of sensory versus motor blockade. The purposes of this study were to measure directly the potency of this charged drug by internal perfusion of cultured neuronal cells, and to evaluate the differential blockade of sensory versus motor function via spinal route in rats.The tonic and additional use-dependent blockade of Na+currents by internal tonicaine was assayed in cultured GH3 cells during whole cell voltage-clamp conditions. In addition, tonicaine was injected into the intrathecal space of rats at intervertebral space L4–L5, and the proprioceptive, motor, and sensory functions, and tissue integrity, subsequently were evaluated.Internal application of tonicaine in GH3 cells revealed that it was ∼80 times more potent in blocking Na+ currents than was externally applied lidocaine. In vivo testing in a rat neuraxial anesthesia model showed that tonicaine at 0.5 mm produced blockade that lasted much longer than that produced by bupivacaine even at ∼a 55 times higher concentration (28.8 mm). Tonicaine spinal block also produced a longer duration of sensory than motor blockade (112.5 ± 16.3 min vs. 45.8 ± 7.1 min). Evidence of neurotoxicity was seen at a concentration of 1.0 mm.In vitro testing shows that tonicaine displays a higher affinity for the local anesthetic binding site than does lidocaine;in vivo testing indicates that tonicaine elicits sensory blockade of a duration significantly longer than that elicited by bupivacaine. Tonicaine, however, has a narrow therapeutic index, with substantial neurotoxicity at 1 mm in rats, and may have limited clinical value.