Belimumab treatment of adult idiopathic inflammatory myopathy

Objective: To evaluate belimumab addition to the standard of care in patents with refractory idiopathic inflammatory myopathy (IIM). Methods: We conducted a 40-week multicentre, randomized, double-blind, placebo-controlled trial with 1:1 IV belimumab 10 mg/kg or placebo randomization and a 24-week open-label extension. Clinical responses were measured by the definition of improvement (DOI) and total improvement score (TIS). Flow cytometry analyses were performed on available samples before randomization, at 24 and 60-64 weeks. Descriptive statistics, t-test, Fisher's exact test and analysis of variance tests were used. Results: A total of 17 patients were randomized, 15 received five or more doses of belimumab or placebo and were included in the intention-totreat analysis. More belimumab patients vs placebo attained a TIS ≥40 [55.5% vs 33.3%; P=non-significant (NS)] and achieved the DOI (33.3% vs 16.7%; P=NS) at weeks 40 and 64; the mean TIS was similar among groups. Two patients achieved major responses (TIS=72.5) after week 40 in the belimumab arm and none in the placebo arm. No improvement in the placebo arm after switching to the open-label phase was observed. There was no steroid-sparing effect. No new safety signals were detected. Although total B cells were not reduced, belimumab induced naïve B cell depletion while enhancing the number and frequency memory B cells. Conclusion: The study did not meet the primary endpoint and no statistically significant differences were observed in clinical responses between arms. More patients achieved sustained TIS ≥40 and reached the DOI. Most patients who received belimumab for >40 weeks had clinical improvement. Phenotypic changes in B cell populations were not associated with clinical responses.