Myobloc: Is it worth a shot?

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After years of delays, countless false starts, and many disappointments among patients with dystonia, the U.S. Food and Drug Administration finally approved in December, 2000, the new Elan Pharmaceuticals botulinum toxin, Myobloc. Long anticipated by many, it had taken on a mythical aura for those with ST who had found no improvement with the only other available botulinum toxin, Botox. Many had written this magazine with inquiries of its arrival. They had been hoping for the reserves to rescue them from their siege. Many had initially responded to Botox but had over time found it less effective. Botox had never helped others at all. Both hoped that Myobloc would be useful to them.

I have now had a year’s experience with Myobloc, and I think I can tell you a little about it. First, a little background. Both Myobloc and Botox are products of complex molecules produced by a species of bacteria called Clostridium botulinum , a bacteria that lives in soil but does not tolerate oxygen well, and which makes a toxin that, when ingested by animals, paralyzes muscles by shutting down the nerves that activate them. Just as there are many breeds of dogs, so are there several strains of C. botulinum . These different strains produce slightly different toxins; each one assigned a letter, A through F. Only toxin types A, B, and F have significant effects in humans. Botulinum toxin type A is the one used to make Botox (and another brand called Dysport, not available in the US yet). Botulinum toxin type B is the one used to make Myobloc. Type F is very short acting and has not been commercially developed. These several types have somewhat different mechanisms of action, and that can be exploited in the treatment of dystonia and other disorders of excessive muscle contraction. For example, type A binds to a portion of the nerve ending called SNAP-25, and type B binds to another portion called synaptobrevin. The end result is the same. That is, the nerve cannot release a substance called acetylcholine (see previous Mini Medical School article for more information on the physiology of the motor unit) which causes the muscle to contract, and so there is more mobility. Because the targets of the different toxin strains are not related to each other, and because there is a significant difference in the structure of each toxin type, an immune response to one type of toxin should not interfere with the mechanism or effects of another type. Indeed, that is what has been found in studies of the two products. That also explains why most people who are “immune” to Botox still respond to Myobloc.

Which one should you use? Well, if you have become resistant to Botox the answer is easy. Myobloc is effective in most patients who initially responded to Botox but who now find no relief with it. If you have never been treated with neurotoxin before, then either type should help you. My experience is that both brands are very good at relaxing muscles for about 10 weeks in most cases. Both are well tolerated in most people with equal incidences of weakness in the neck and about the same amount of swallowing disturbances (perhaps 10-20%). At higher doses, especially when used for the first time, Myobloc appears to cause more mouth dryness than Botox, and some of my patients have found this to be troublesome. For some reason, the degree of xerostomia, as it is called, is much less severe in subsequent injections, so the effect may be transient. The great advantage of Myobloc to the physician is that it does not need to be frozen or reconstituted with saline and is ready to inject from the vial, and that it comes in three different sizes of vials. Overall, however, both products seem to be very effective for cervical dystonia. For those who are doing well with Botox, I do not recommend changing to Myobloc for the simple adage that “if it ain’t broke don’t fix it.” Besides, you may one day become resistant to Botox, and saving Myobloc for later is a prudent thing. Likewise, if you are doing well on Myobloc there is no need to change to Botox. To my knowledge there are no reports of Myobloc resistance yet, but this could be because it is just too early.

So now you have some idea of the two types of available botulinum toxin. Although a long time coming, Myobloc adds another alternative in treatment. When you realize that just a little more than a decade ago there was no effective treatment for spasmodic torticollis, it is encouraging that we now have two good treatments for this distressing syndrome.

Matthews Gwynn, MD

993-F Johnson Ferry Rd NE Suite 120

Atlanta, GA 30305 w

My mom and I wanted to thank you for hosting such a great symposium this year. This was our third year and we are looking forward to next years. E. Mathews