Spasticity

Immunoresistance and Antibody Detection

Development of resistance to BTX-A therapy is an important clinical issue. Resistance is characterized by absence of any beneficial effect and by lack of muscle atrophy following the injection. Antibodies against the toxin are presumed to be responsible for most cases of resistance. While early studies reported no detectable antibodies in patients exposed either by intestinal colonization or for therapeutic indications, more recent reports have shown that small numbers of patients do develop antibodies with repeated BTX-A treatment.

Several types of assays are available to detect the presence of antibody in serum. The most widely used is the in vivo mouse neutralization assay, available through North-view Pacific Labs (Berkeley, Calif.). In this assay, BTX-A is titrated with human serum suspected of harboring anti-BTX antibodies, and then injected into mice. Binding of antibodies with toxin protects the mice from the toxin's lethal effects. Death of the mice indicates no antibodies (or insufficient antibodies) are present.

Immunosorbent assays have also been used to detect anti-BTX antibody, including an enzyme linked immunosorbent assay (ELISA) an enzyme-linked coagulation assay, a sphere-linked immunodiagnostic assay, a western blot assay, and most recently, a combined fluorescein- and enzyme-linked assay. However, correlation has not been established between antibodies detected by these assays and clinical resistance to BTX-A injections, and it is likely that some patients may develop antibodies to functionally unimportant regions of the toxin. For instance, using the sphere-linked immunodiagnostic assay, antibodies were detected in more than half of all patients treated with BTX-A, including those who continued to respond to toxin treatment.

On the other hand, studies using the mouse lethality assay report much lower rates of antibody production, but probably under estimate the number of cases of immunoresistance. It is possible that the minimum antibody titer sufficient to cause human resistance is too low to protect the mouse, or that immunological differences between patients make some more susceptible than others to the effects of very low antibody levels. While a positive antibody result appears to correlate well with poor clinical response to treatment, a negative result is not necessarily predictive of a good clinical response. The prevalence of antibody production as determined by the mouse assay varies somewhat among different reports. In one study, 3 of 96 (3%) cervical dystonia patients had developed antibodies to Dysport®, the form of BTX-A produced in England.

In a study of patients with cervical dystonia, Greene et al. found that at least 24 of 559 (4.3%) developed anti-BTX-A antibodies; however, not all nonresponders were tested, and the authors suggested that the true prevalence of antibodies may be more than 7%. In this study, all seropositive patients were resistant to BTX-A treatment. The authors noted that some originally responsive patients later developed resistance without serological evidence of antibody production.

These authors then went on to study a second cohort of 76 patients, 8 of whom (10.5%) had developed clinical resistance to BTX treatments. Compared to nonresistant patients, resistant patients had had a shorter interval between injections, more "boosters," a higher dose per 3-month interval, and a higher dose at the "nonbooster" injection. Of these 8 patients, 3 were seropositive, while 5 were not.

Jankovic and Schwartz analyzed resistance among 1321 patients treated for cervical or oromandibular dystonia, including some patients treated only once. Sixty patients, or 4.5%, had inadequate or no response to treatment. They then tested sera from these patients and 22 patients randomly chosen as controls. They detected antibodies in the sera of 20 of the 60 resistant patients, but in none of the others. Compared to the controls, seropositive patients had an earlier age of onset of dystonia, higher mean dose per visit, and a higher total cumulative dose.

Lack of response to BTX in seronegative patients may reflect test insensitivity to antibodies actually present, but alternatively may be technical, resulting from inadequate dose or choice of injection site.

Two clinical assays of BTX resistance have been described. Resistance can be tested functionally using the frontalis test (FTAT). Approximately 15 U BOTOX® are injected into two sites of one side of the corrugator muscle. If the muscle does not move within 2 weeks, and the patient cannot furrow that side of the brow, the patient is "not resistant;" if the corrugator moves properly, the patient is "resistant." The nonresistant patient may choose to be injected on the opposing side as well to maintain expression symmetry until corrugator function returns.

Kessler and Benecke have reported a correlation between the results of a challenge test and the antibody status of dystonia patients found clinically resistant to BTX-A therapy. They measured compound muscle action potentials (CMAPs) elicited by electrical stimulation of the peroneal nerve before and after BTX-A injection into the extensor digitorum brevis. In seropositive nonresponders, CMAP amplitude remained roughly unchanged 4 weeks after injection. In seronegative nonresponders, a marked decrease in CMAP amplitude was detected, indicating the patients remained susceptible to the effects of BTX despite their clinical resistance, and suggesting that dose or injection site may have been suboptimal. If these results are borne out by further studies, this test may also prove to be a useful adjunct to the antibody test in the diagnosis of resistant patients.

A number of studies have confirmed that patients with BTX-A resistance may benefit from injections with other serotypes. In these studies, the benefits of BTX-F seem to last approximately 1 month. Both seropositive and seronegative patients may benefit. The preliminary data in healthy volunteers suggest that BTX-B provides clinical effects similar to BTX-A, but with a shorter duration of effect as well. A recent report suggests that BTX-C has a duration of effect similar to that of BTX-A. We concur with Greene et al. in these recommendations to minimize immunoresistance: 1) use the smallest possible effective dose, 2) extend the interval between treatment as long as possible, with at least 3 months between treatments, and 3) avoid using booster injections. In our practice we limit dose to no more than 300 to 400 U per 3-month period and rarely perform booster injections.