A Lack of Antinociceptive or Antiinflammatory Effect of Botulinum Toxin A in an Inflammatory Human Pain Model

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Abstract
Several in vitro and in vivo investigations have shown that botulinum toxin A (BoNT/A) can inhibit the release of substance P and excitatory amino acids. Recently, a marked antinociceptive effect of BoNT/A and inhibition of glutamate release was observed in an animal pain model with inflammatory sensitization. In the present study, we tested the antiinflammatory and antihyperalgetic effect of BoNT/A in a well-characterized human inflammatory pain model. Using a randomized, double-blind, paired study design, we compared the effects of 100 mouse units of BoNT/A versus pure saline. Thermal and mechanical pain testings and superficial skin blood flow measurements were performed at baseline, at 48 h (in normal skin), and at 72 h (in inflamed skin) thereafter. Ultraviolet B irradiation resulted in a local inflammation with significant primary and secondary hyperalgesia. However, despite the evidence of efficacy on sudomotor function, BoNT/A had no effect on pain measures in either normal or inflamed skin. Signs of inflammation and primary and secondary hyperalgesia were found to be unaffected by BoNT. We have confirmed that BoNT/A has no direct effect on acute, noninflammatory pain. Furthermore, despite highly promising data from animal research, we have not observed antiinflammatory or antinociceptive effects of BoNT/A in human inflammatory pain.