Antibodies, Severity Predict Myositis Response to Rituximab

Aggarwal R, Bandos A, Reed AM, et al., Predictors of Clinical Improvement in Rituximab-Treated Refractory Adult and Juvenile Dermatomyositis and Adult Polymyositis. Arthritis & Rheumatology. (2014) 66: 740–749. [Published online: 25 FEB 2014] DOI: 10.1002/art.38270

A study of patients treated with rituximab (Rituxan, RTX) for refractory myositis suggests that the presence of two key autoantibodies, coupled with lower disease damage, strongly predict clinical improvement. Also, children appear more likely than adults to respond to treatment.

The multi-center study, involved almost 200 patients -- 75 with adult polymyositis (PM), 72 with adult dermatomyositis (DM), and 48 with juvenile DM. All were non-responsive to prior treatment (mostly prednisone).

Patients in the Rituximab in Myositis (RIM) clinical trial, mostly white females, were randomized either to a “rituximab early” arm (n=98; RTX at weeks 0 and 1, placebo weeks 8 and 9) or “rituximab late” (n=102; placebo weeks 0 and 1, RTX weeks 8 and 9). But all received the B-cell depleting drug at some point in the 20-week trial.

Patient data were analyzed by subgroup (e.g., juvenile DM) and by clinical improvement, defined as 20% improvement in at least 3 of 6 measures of disease activity:

•   Physician's and patient's/parent's global assessment of disease activity
•   Manual muscle testing
•   Physical function
•   Muscle enzymes
•   Extramuscular disease activity
•   Myositis autoantibodies (or no antibodies)  

Patients with myositis antibodies -- especially antisynthetase and anti–Mi-2, both associated with lower disease activity – were 2 to 3 times more likely to improve, and got better more quickly than others. Those with no detectable autoantibodies had worse outcomes.

Children with DM and patients with lower physician's global assessment scores were also more likely to improve, but those associations were reduced by 20 weeks. Muscle damage and atrophy were associated strongly with poor outcomes.

There were no differences between the RTX early and late groups, but since all patients received active treatment it’s unclear whether or not the drug had a direct effect, the authors say.