Efficacy and safety of subcutaneous abatacept + standard treatment for active idiopathic inflammatory myopathy: phase III randomised controlled trial

Objective To evaluate the efficacy and safety of subcutaneous (SC) abatacept and standard of care (SOC) for the treatment of idiopathic inflammatory myopathy (IIM) over 52 weeks. Methods In this randomised, double‐blind, placebo‐controlled phase III trial, patients with treatment‐refractory IIM received SC abatacept (125 mg weekly) + SOC (abatacept group) or placebo + SOC (placebo group) (NCT02971683). A 24‐week double‐blind period was followed by an open‐label period to assess outcomes from continued therapy with abatacept and initiation with abatacept (placebo‐to‐abatacept switch group) from 24 to 52 weeks. The primary endpoint was International Myositis Assessment and Clinical Studies definition of improvement (IMACS DOI) at week 24. Secondary efficacy and safety endpoints were assessed. Results Overall, 148 (double‐blind) and 133 (open‐label) patients were treated. Baseline demographics were well‐balanced between treatment groups and disease subtypes. At 24 weeks, improvement per IMACS DOI was 56.0% for the abatacept group and 42.5% for the placebo group (p=0.083); at 52 weeks, improvement was 69.8% (continued abatacept) and 69.0% (placebo‐to‐abatacept switch). IMACS DOI rate at 24 weeks was greater in the non‐dermatomyositis (DM) group (abatacept: 57.1%; placebo: 32.3%; p=0.040) than the DM group (abatacept: 55.0%; placebo: 50.0%; p=0.679). The observed safety profile was similar in both groups. Conclusion The proportion of patients who met improvement criteria after 24 weeks was similar between abatacept and placebo groups. However, analysis by IIM subtype suggested there may be sustained benefit of SC abatacept for patients with non‐DM subtypes.