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    Experts break down differences in botulinum toxin products

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    Because the pharmacokinetics of popular botulinum toxin products differ, there's no shortcut to determining equivalent therapeutic doses, according to one expert.

    Botox (onabotulinumtoxinA, Allergan) first earned Food and Drug Administration (FDA) approval in 1989. Dysport (abobotulinumtoxinA, Medicis) was approved in 2009. Xeomin (botulinum toxin A/BTX-A, Merz) earned approval from Health Canada in 2009. It is not yet approved in the United States, where it is known as NT-201.


    Dr. Smith
    Among FDA-approved products, "There's no easy way to convert patients from one to the other. You can't just multiply the Botox dosage by 2.5, put patients on Dysport and hope for the best," says Kevin C. Smith, M.D., an Ontario-based dermatologist in private practice. "It's not just a matter of the dose." Because these products diffuse differently, achieving comparable results when switching from one drug to the other may require alteration in injection points, he says.

    If converting from Botox to Dysport to treat glabellar frown lines, Dr. Smith says physicians might want to inject higher above the eyebrow. If one injects 2.5 units of Dysport in the same spot where one would inject Botox, he says, "Because Dysport spreads a little more, there's a chance that it will diffuse downward into the upper eyelid, causing it to droop. You might also have to adjust the dose up or down. If you mechanically give everyone 2.5 times the Botox dose, you'll be high in some cases and low in others. It's like working with a different material entirely."

    Neurotoxins diffuse differently in part because Botox and Dysport have protective proteins clustered around the active part of the molecule, while Xeomin has no protective proteins. "Botox has a full complement of protective proteins and weighs about 900 kD," Dr. Smith says. "Dysport is a mixture of 500 kD and 300 kD complexes of protective proteins and botulinum toxin A." Being heavier, these complexes migrate more slowly than Xeomin/NT-201, which is BTX-A without protective proteins. "Xeomin/NT-201 is going to migrate probably faster and farther because there's nothing dragging it down. Conversely, you'll have less precision," Dr. Smith says.

    Additionally, "The protective proteins cluster around the C-terminal end of the BTX-A molecule, which is the part that binds to the post-synaptic nerve terminal. That's the way botulinum toxin exists in nature, and in Botox and Dysport," Dr. Smith says. These protective proteins also may shield the active part of the molecule from the immune system. This way, "While it is being protected by protective proteins, the binding site part of the molecule may not stimulate the immune system. And if there are antibodies against that part of the molecule floating around, they would have a harder time getting at it," he says.

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    John Jesitus
    John Jesitus is a medical writer based in Westminster, CO.

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