Botox and Dysport: is there a dose conversion ratio in dermatology and aesthetic medicine?

To the Editor: The use of botulinum neurotoxin A is the most frequent intervention in aesthetic medicine. Sales of the two main preparations—Botox (Allergan Inc, Irvine, CA) and Dysport (Ipsen Ltd, Slough, Berkshire, UK)—amounted to USD $1 billion in 2007, and the trend is still growing. Even after 15 years of clinical use, the unit equivalence between these two main botulinum neurotoxin A products is still a matter of discussion. Because of the different excipients used to dilute the test toxins, the UK assay used to test Dysport is more sensitive than the US assay used for Botox, resulting in a different efficacy per unit in both formulations. For the mouse assay used to standardize each batch of Dysport, the toxin is diluted in a phosphate buffer containing gelatine that stabilizes the toxin at low concentrations, whereas saline is used as the diluent for the Botox assay, which gives a loss of potency because of dilution artefacts. This was first shown by Hambleton and Pickett, who measured different samples of Botox and Dysport using the two assays: a Botox unit was approximately 3 times more potent in the Dysport assay, and a Dysport unit was approximately 2.5 times less potent (activity declined to 39.7%) in the saline assay. In the Dysport assay, a unit of Botox was equivalent to 2.87 units of Dysport. In the saline assay, one Dysport unit was equivalent to a nominal 0.4 units of Botox, suggesting a potency ratio of 1:2.5. The clinical literature on dose equivalence is extensive but confusing, and many published studies differ in rigor and validity. Ratios from 6:1 down to 1:1 can be found in the literature, but the more recently published literature suggests that one unit of Botox is equivalent to about two to four units of Dysport (ratio, 2:1 to 4:1). A recently published review identified only four key papers on headto-head comparison of Botox and Dysport that are of sufficient quality to fulfill the criteria of evidencebased medicine. In these studies, unit ratios of 4:1 and 3:1 Dysport:Botox were tested in patients with blepharospasm or torticollis, and the joint conclusion was that 3:1 is more appropriate than 4:1, but that the two products are not equivalent at this ratio. In fact, the effect of Dysport was consistently marginally greater and longer lasting in two of these studies that tested 3:1 and in a more recent doubleblind controlled independent study at a ratio of 3:1 in