Pharmacology of botulinum toxin: differences between type A preparations

Different types of botulinum neurotoxin (BoNT) block different proteins of the soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) protein complex within cholinergic nerve terminals, producing blockade of cholinergic neuromuscular and autonomic synapses. Animal studies indicate the longest duration of action for BoNT type A (BoNTA) followed by types B, F, and E. Diffusion to adjacent and remote muscles may be related to protein composition, dilutions, volume, target muscle selection, and injection technique. A review of head-to-head, randomized, controlled trials of BoNTA preparations (Botox and Dysport) suggests that Dysport tends to have higher efficacy, longer duration, and higher frequency of adverse effects. Conversion factors between the preparations varied, however, and remain controversial. In clinical settings, a Botox:Dysport conversion ratio of 1:3 may be appropriate. Animal studies suggest a conversion ratio of 1:2.5-3. When therapeutic effects between these preparations are attempting to be equalized, Dysport seems to produce more adverse effects. In mice, Botox appears to have a better safety margin than Dysport and BoNTB. In rats, diffusion margins are similar for Botox and Dysport. Jitter derived from stimulation single-fiber EMG of injected and remote muscles show no differences between Botox and Dysport. Atrophy of extrafusal muscle fibers of injected and remote muscles do not differ between the BoNTA preparations.