Neurotoxicity of adjuvants used in perineural anesthesia and analgesia in comparison with ropivacaine

Clonidine, buprenorphine, dexamethasone, and midazolam (C, B, D, M) have been used to prolong perineural local anesthesia in the absence of data on the influence of these adjuvants on local anesthetic-induced neurotoxicity. Therefore, the impact of these adjuvants on ropivacaine (R)-induced death of isolated sensory neurons was assessed. The trypan blue exclusion assay was used to assess death of sensory neurons isolated from adult male Sprague-Dawley rats. Drugs were applied, alone or in combination, for 2 or 24 hrs at 37°C. Neuronal viability was halved by 24-hr exposure to R (2.5 mg/mL), far exceeding the neurotoxicity of C, B, D, or M (at 2-100 times estimated clinical concentrations). Plain M at twice the estimated clinical concentration produced a small but significant increase in neurotoxicity at 24 hrs. After 2-hr exposure, high concentrations of B, C, and M increased the neurotoxicity of R; the combination of R + M killed more than 90% of neurons. Estimated clinical concentrations of C + B (plus 66 μg/mL D) had no influence on (i) R-induced neurotoxicity, (ii) the increased neurotoxicity associated with the combination of R + M, or (iii) the neurotoxicity associated with estimated clinical concentrations of M. There was increased neurotoxicity with 133 μg/mL D combined with R + C + B. Results with R reaffirm the need to identify ways to mitigate local anesthetic-induced neurotoxicity. While having no protective effect on R-induced neurotoxicity in vitro, future research with adjuvants should address if the C + B + D combination can enable reducing R concentrations needed to achieve equianalgesia (and/or provide equal or superior duration, in preclinical in vivo models).