Antibodies and Botulinum Toxin Type A Therapy
Antibodies are proteins produced by the immune system to help fight infections or dispose of other foreign agents that enter the body. In some individuals treated with BoNT-A, antibodies may develop, bind to the drug, and inactivate it. This renders the toxin ineffective in weakening muscle contractions associated with dystonia.
Early studies estimated that approximately five percent of individuals with cervical dystonia who had been treated regularly with relatively higher doses of BoNT-A developed antibodies. More recently, with newer formulations of toxin, that figure is believed to be lower.
Once a patient forms antibodies to a particular serotype of BoNT (immunoresistance), further injections of that particular serotype of BoNT are typically ineffective. Physicians should, therefore, use the smallest amount of BoNT-A necessary to achieve therapeutic benefit; extend the time interval between treatment sessions as long as possible (with at least three months between treatments); and, if possible, avoid the use of "booster" injections.
On occasion, a patient may not respond to therapy with BoNT-A. So-called "primary non-responders" are patients who do not respond to their first injection of BTX. Secondary non-response may occur as the result of a technical problem, such as an inappropriate site of injection into the wrong muscle, a dose that is inadequate to provide a clinical effect, or disease progression. The toxin may weaken the muscle; however, the degree of relaxation may not provide symptomatic relief for the patient. In addition, some patients on combination therapies for segmental or generalized dystonias may fail to take their oral medications, leading to a general increase in symptoms (masking the local effects of BoNT). If failure to respond continues, it is possible that the patient has antibody-mediated resistance (immunoresistance).
It is important that patients work with their physicians to set appropriate treatment goals and tailor the course of treatment to meet these goals. There is no formal "recipe" that works for every patient. Each individual is unique and responds differently to BoNT-A therapy.