Background: Immune-mediated necrotizing myopathy (IMNM) is an autoimmune myopathy characterised by proximal muscle weakness, high creatine kinase (CK) values, and autoantibodies recognizing 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) or the signal recognition particle (SRP). There are currently no approved therapies for IMNM and many patients experience active disease despite off-label treatment with intravenous immunoglobulin, glucocorticoids, and immunosuppressants. Detection of complement-activating anti-HMGCR and anti-SRP autoantibodies and the presence of complement deposition on the sarcolemma of non-necrotic myofibers led to the hypothesis that complement activation may be pathogenic in IMNM, therefore zilucoplan, a complement component 5 (C5) inhibitor, could be a potential therapy.
Methods: IMNM01, a phase 2, multicenter, randomised, double-blind, placebo-controlled study (NCT04025632) at 15 sites (four countries) evaluated efficacy, safety, and tolerability of zilucoplan in adult participants with anti-HMGCR or anti-SRP autoantibody-positive IMNM. Participants were randomised 1:1 to receive daily subcutaneous zilucoplan (0·3mg/kg) or placebo for eight weeks; with optional enrolment in the study open-label extension. Primary efficacy endpoint was percent change from baseline to Week 8 in CK levels. Secondary endpoints included safety.
Findings: Between 07 November 2019 and 07 January 2021, 27 participants (13 female and 14 male) received zilucoplan (n=12) or placebo (n=15) and completed the 8-week main study. At Week 8 there were no clinically relevant or statistically significant differences, despite target engagement based on mode of action, between treatment arms in mean percent change (standard deviation) of CK levels versus baseline (-9·86% [26·06] versus -20·72% [31·22] in zilucoplan [n=10] and placebo arms [n=14], p=0·46, respectively) and no clinically relevant improvement over time within the treatment arm. There were no unexpected adverse safety or tolerability findings. Treatment emergent adverse events (TEAEs) and serious TEAEs were reported in n=9 (75·0%) vs n=13 (86·7%) and n=0 (0%) and n=3 (20·0%) participants, respectively. The most frequent TEAEs were headache (n=4 in both groups [33·3% and 26·7%, respectively]) and nausea (n=3 in both groups [25·0% and 20·0%, respectively]).
Interpretation: C5 inhibition does not appear to be an effective treatment modality for IMNM. Rather than driving myofiber necrosis, complement activation may be secondary to muscle injury.
Funding: Study funded by Ra Pharmaceuticals (now part of UCB Pharma).