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Cemdisiran siRNA Monotherapy Significantly Reduces Symptom Burden in Generalised Myasthenia Gravis
In a phase 3 placebo-controlled trial, cemdisiran monotherapy produced a statistically significant and clinically meaningful improvement in daily functioning compared with placebo, with a placebo-adjusted least-squares mean difference in MG-ADL score of –2.3 points (95% CI –3.6 to –1.0; p=0.0005). Combination therapy with pozelimab also showed significant benefit, though the effect size was modestly smaller, and the monotherapy’s quarterly subcutaneous dosing regimen represents a potentially distinctive treatment schedule.
What Was Studied
The trial investigated whether cemdisiran, an siRNA that silences complement component 5 (C5), could improve functional outcomes in patients with generalised myasthenia gravis when used as a monotherapy or in combination with a C5-targeting antibody, pozelimab. The primary outcome was change from baseline in the Myasthenia Gravis-Activities of Daily Living (MG-ADL) total score at 24 weeks, a validated measure of disease impact on day-to-day function.
How It Was Studied
NIMBLE was a randomised, double-blind, placebo-controlled phase 3 trial conducted across 86 centres in 13 countries. Adults aged 18 years or older with generalised myasthenia gravis, confirmed anti-AChR or anti-LRP4 antibody positivity, and a baseline MG-ADL score of at least 6 were eligible. A total of 284 patients were randomly allocated to one of four arms: cemdisiran monotherapy (600 mg every 12 weeks subcutaneously), pozelimab monotherapy (200 mg every 4 weeks subcutaneously), a combination of both agents, or matching placebo—all delivered subcutaneously over a 24-week double-blind period. Of the 277 who received at least one dose, 263 (95%) completed the double-blind treatment period. The primary analysis was performed in a modified intention-to-treat (mITT) set comprising the first 245 randomised patients with at least one post-baseline assessment.
What Was Observed
- Cemdisiran monotherapy produced a statistically significant reduction in MG-ADL scores. The cemdisiran group achieved a least-squares mean change from baseline of –4.5 (SE 0.4) versus –2.2 (SE 0.5) in the placebo group, yielding a placebo-adjusted difference of –2.3 points (95% CI –3.6 to –1.0; p=0.0005), indicating a moderate but meaningful improvement in daily functioning.
- Combination therapy with pozelimab also significantly outperformed placebo, though with a somewhat smaller effect. The combination group showed a placebo-adjusted MG-ADL difference of –1.7 points (95% CI –3.0 to –0.4; p=0.0086), a statistically robust but numerically slightly lower result than cemdisiran alone.
- Cemdisiran monotherapy had a favourable adverse event profile relative to other active arms. Treatment-emergent adverse events occurred in 69% of patients in the cemdisiran group, compared with 81% in the combination group, 82% in the pozelimab group, and 77% in the placebo group. The most common event in the cemdisiran arm was upper respiratory tract infection (12%), occurring at a rate comparable to placebo (11%).
- No serious infections or meningococcal events occurred in the cemdisiran monotherapy group, and no deaths were recorded during the double-blind period. Two deaths occurred after the double-blind period concluded, one of which the investigator considered possibly treatment-related, though the sponsor did not concur.
Why This Matters
This trial provides the first phase 3 evidence that RNA interference targeting C5 can function effectively as a standalone therapy in complement-mediated generalised myasthenia gravis. The abstract identifies complement activation as central to the disease mechanism in AChR antibody-positive patients, and the demonstration that siRNA-mediated C5 silencing is sufficient to reduce disease burden—without requiring a concurrent antibody therapy—establishes RNA interference as a viable monotherapy strategy in this setting. The quarterly subcutaneous administration schedule, if confirmed in full follow-up, could offer a logistically advantageous option compared with more frequent dosing regimens.
How to Read This Result
Although the trial is high quality in design, the primary analysis was limited to the first 245 randomised patients rather than all enrollees, the trial remained active at the data cut-off point, and full long-term safety and efficacy data have yet to be reported, warranting measured interpretation of the current findings.
Limitations
The mITT primary analysis set encompassed only the first 245 randomly allocated participants, excluding later-enrolled patients and potentially reducing generalisability. Pozelimab monotherapy was not formally statistically tested against placebo per the prespecified hierarchical testing strategy, limiting conclusions about its independent efficacy. Because the trial was still active at the July 2025 data cut-off, the durability of benefit and the completeness of long-term safety surveillance remain uncertain. The inclusion of both anti-AChR and anti-LRP4 antibody-positive patients introduces potential immunopathological heterogeneity that could affect the consistency of treatment response across the population.