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The unit measurement is made by finding out how much of the toxin is lethal to 50 percent of mice in a study. But that amount may vary when new production processes are introduced.
The neurotoxin will be known commercially in the United States as Reloxin, once approved by the Food and Drug Administration, and will offer an alternative to Botox (Allergan). However, challenges include appropriate conversion factors, diffusion and other dose and efficacy matters, according to Joel Schlessinger, M.D., who spoke at a recent meeting of the American Society of Cosmetic Dermatology and Aesthetic Surgery (ASCDAS). Dr. Schlessinger is president-elect of the ASCDAS.
Dysport is already used in about 60 countries for treatments including the ocular areas, spasmodic torticollis and cerebral palsy. The toxin is also approved in Europe for treatment of the glabellar area. Its side effects - similar to those of Botox - include headache and other symptoms, including blepharoptosis, pain, burning or stinging, facial pain or erythema.
Despite widespread use of Botox around the country, the conversion issue is still a key point of contention.
"Dysport is available in 500-unit vials, and the biggest issue right now is how much will 500 units of Dysport equal when compared to Botox," says Dr. Schlessinger, an Omaha-based solo private practitioner who is board-certified in dermatology and general cosmetic surgery.
Studies in European literature have reported anywhere from a 2.5-to-1 ratio to a 4-to-1 ratio when converting 500 units of Dysport relative to 100 units of Botox.
Looking at the studies
One study on palmar hyperhidrosis using a 4-to-1 ratio showed more efficacy with Dysport; however, side effects were also greater. The study specifically involved a dosage of 67 units of Botox, compared with 283 units of Dysport, and while Dysport was more effective and longer-lasting, there was a higher incidence of adverse effects (British J Derm. 2003).
"It really appears clear that a 4-to-1 ratio is a little too much and it probably is going to be around 3- to 3.5-to-1," Dr. Schlessinger says.
Another study regarding treatment for cervical dystonia compared a 3-to-1 ratio of Dysport to Botox, and efficacy results were generally equal, however adverse events for Dysport were 58 percent, compared to 69 percent for Botox (J Neurol Neurosurg Psychiatry, Jan. 1998;64:6-12).
A study on glabellar rhytids using 25, 50 and 75 units of Dysport concluded that 50 units was the optimal dose, with no blepharoptosis noted (J Am Acad Dermatol. 2004;51:223-233).
Units not static
One factor that can further complicate efforts in working out a conversion factor, however, is the nature of units themselves, which may not be absolute.
"One of the biggest secrets in the studies of Botox or other neurotoxins is that a unit is not necessarily this spectacularly reproducible thing. It's not as scientific as many may think," Dr. Schlessinger explains.
The unit measurement is made by finding out how much of the toxin is lethal to 50 percent of mice in a study. But that amount may vary when new production processes are introduced, he says.
"Most dermatologists and plastic surgeons don't realize it, but batches can vary from one to another and the concentration may vary significantly," Dr. Schlessinger says. "It's actually very illusory and that's something we all need to know."
Diffusion risks are another concern with botulinum toxin type A, presenting potential side effects such as the spreading of the neurotoxin into intraorbital muscles. But a study comparing a Dysport-to-Botox ratio of 2.5-to-1 on the upper part of frontal muscles and lateral periorbital area resulted in no diffusion differences (JAAD. 2005).