Zilucoplan

A synthetic 15-amino-acid macrocyclic peptide that binds complement C5 and blocks terminal complement activation, FDA approved in October 2023 as Zilbrysq, a once-daily subcutaneous injection for acetylcholine receptor antibody-positive generalized myasthenia gravis.

Also referenced as: Zilbrysq, RA101495, RA-101495, Zilucoplan sodium, ZLP

Also appears in: Other

What is Zilucoplan?

Zilucoplan is a synthetic 15-amino-acid macrocyclic peptide (molecular weight about 3,562 Da, carrying a palmitoyl lipid tail) that binds complement component C5 with high affinity and prevents it from being cleaved into C5a and C5b, shutting down the terminal complement cascade. It was developed by Ra Pharmaceuticals (as RA101495) and carried through Phase 3 and approval by UCB after its 2020 acquisition of Ra. It is marketed as Zilbrysq, a once-daily subcutaneous self-injection approved in the United States on October 17, 2023 for adults with acetylcholine receptor (AChR) antibody-positive generalized myasthenia gravis (gMG), making it the first peptide C5 inhibitor and the first self-administered once-daily C5 inhibitor in that disease.

How it works

In AChR antibody-positive gMG, autoantibodies bound at the neuromuscular junction activate complement, and deposition of the membrane attack complex (C5b-9) damages the postsynaptic membrane and impairs neuromuscular transmission. Zilucoplan intervenes at the C5 step:

• Blocks C5 cleavage by C5 convertases — zilucoplan binds C5 and prevents its cleavage, so neither the anaphylatoxin C5a nor C5b is generated (Tang et al., Frontiers in Immunology, 2023)
• A second, downstream block on C5b6 assembly — the peptide remains bound to C5b and interferes with C5b6 formation, which also suppresses non-canonical, plasmin-mediated C5 activation that bypasses the convertases (Tang et al., Frontiers in Immunology, 2023)
• Activity against eculizumab-resistant C5 variants — in vitro, zilucoplan fully inhibited activation of clinical C5 R885 variants that respond poorly to the monoclonal antibody eculizumab, consistent with it binding a site on C5 distinct from the antibody’s epitope (Tang et al., Frontiers in Immunology, 2023)
• Reduced C5b-9 deposition at the neuromuscular junction — the label states the precise mechanism in gMG is unknown but presumed to involve reduction of C5b-9 deposition at the junction; clinically, benefit on functional scores was detectable within the first week of dosing (Howard et al., The Lancet Neurology, 2023)

Because it is a small macrocyclic peptide rather than an antibody, zilucoplan also crossed a reconstituted basement membrane more readily than a monoclonal C5 antibody, which the authors proposed as one explanation for its rapid onset.

Research status

Zilucoplan has a complete clinical development program in gMG and two documented failures in other indications.

• Phase 2 gMG (NCT03315130): Howard et al. (2020) randomized 44 AChR antibody-positive adults at 25 North American sites to placebo, 0.1 mg/kg or 0.3 mg/kg daily for 12 weeks. The higher dose produced a mean 6.0-point reduction in the Quantitative Myasthenia Gravis (QMG) score, a placebo-corrected change of −2.8 (p = 0.05 at the prespecified one-sided α of 0.10), with a placebo-corrected MG-ADL change of −2.3; no patient on 0.3 mg/kg needed rescue therapy versus three on placebo, and the 0.3 mg/kg dose was carried into Phase 3 (JAMA Neurology, 77(5):582–592).
• RAISE Phase 3 (NCT04115293): Howard et al. (2023) randomized 174 patients at 75 sites in Europe, Japan and North America to zilucoplan 0.3 mg/kg (n = 86) or placebo (n = 88) for 12 weeks. The MG-ADL score fell by a least-squares mean 4.39 points with zilucoplan versus 2.30 with placebo (difference −2.09, 95% CI −3.24 to −0.95, p = 0.0004). Treatment-emergent adverse events occurred in 77% versus 70%, most commonly injection-site bruising (16% vs 9%); serious adverse events and serious infections were similar between arms, and one death in each arm (COVID-19 on zilucoplan, cerebral hemorrhage on placebo) was judged unrelated (The Lancet Neurology, 22(5):395–406). This trial supported approvals in Japan (September 2023), the United States (October 17, 2023) and the European Union (December 4, 2023).
• RAISE-XT open-label extension (NCT04225871): Howard et al. (2024) reported an interim analysis of 200 patients with a median 1.2 years of exposure (data cut September 8, 2022). 94% had at least one treatment-emergent adverse event, most commonly MG worsening (26%) and COVID-19 (25%); MG-ADL improvements from double-blind baseline reached about −6 points by week 24 and were maintained through week 60 in both the continued-zilucoplan and placebo-switch groups (Therapeutic Advances in Neurological Disorders, 17, article 17562864241243186). The extension study completed on July 31, 2026.
• Immune-mediated necrotizing myopathy (NCT04025632) — failed: Mammen et al. (2023) randomized 27 adults with anti-HMGCR or anti-SRP antibodies at 15 sites in four countries to zilucoplan 0.3 mg/kg (n = 12) or placebo (n = 15) for 8 weeks. Mean creatine kinase change from baseline was −9.9% versus −20.7% (p = 0.46), with no clinically relevant difference between groups; the study was terminated after the primary-endpoint analysis at the first data lock and the program was discontinued, the authors concluding that complement activation is likely secondary to muscle injury in this disease rather than a driver of it (The Lancet Rheumatology, 5(2):e67–e76).
• COVID-19 acute hypoxic respiratory failure (ZILU-COV, NCT04382755) — did not meet its primary endpoint: De Leeuw et al. (2022) ran an academic (Ghent University Hospital-sponsored), open-label, 2:1 randomized Phase 2 trial in 81 hospitalized patients (55 zilucoplan, 26 control) at Belgian hospitals, with antibiotic prophylaxis in both arms. The primary oxygenation endpoint did not reach statistical significance (PaO2/FiO2 improvement at day 6 of 56.4 vs 20.6 mmHg, p = 0.12); day-28 mortality was 9% versus 21% with a wide confidence interval, and serum C5b-9 and interleukin-8 fell with treatment. The authors called for larger trials, which were not pursued (Respiratory Research, 23:202).
• Ongoing: a Phase 2/3 adolescent gMG study in patients aged 12–17 (NCT06055959, started October 2024, still recruiting with an estimated completion of December 2026) and its open-label extension (NCT06435312, estimated completion 2027). A Phase 3b autoinjector usability study (NCT06471361, 31 patients) completed in February 2025 and, together with a Phase 1 bioequivalence crossover study in 14 healthy volunteers, supported FDA approval of an autoinjector presentation on October 15, 2026 (NDA 216834 supplement 15, “new route of administration”; announced by UCB on October 16, 2026).

Common dosage forms

Zilbrysq is a prescription-only product dispensed through a restricted REMS program. It is supplied as single-dose prefilled syringes at a fixed 40 mg/mL concentration in three strengths — 16.6 mg/0.416 mL, 23 mg/0.574 mL and 32.4 mg/0.81 mL — distinguished by color-coded plungers (rubine red, orange, dark blue); strength is selected by body weight. Each strength is packaged as 28 syringes (four cartons of seven), stored refrigerated with a single allowance for room-temperature storage of up to three months. An autoinjector presentation that delivers a predefined dose when activated against the skin was approved by the FDA in October 2026, with commercial availability expected in the first quarter of 2027; the January 2026 label does not yet describe it. The compound is not a typical research-vendor listing; where it appears it is as lyophilized peptide, and such material has no relationship to the approved sterile product.

Key considerations

• Boxed warning for meningococcal infection. Like all terminal complement inhibitors, zilucoplan increases the risk of life-threatening Neisseria meningitidis infection. The label requires meningococcal vaccination (serogroups A, C, W, Y and B) at least two weeks before the first dose unless the delay would pose greater risk, and the product is available only through the Zilbrysq REMS program, with certified prescribers and pharmacies and a patient safety card carried for two months after the last dose. Susceptibility to other encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae) and to disseminated gonococcal infection is also increased.
• Pancreatitis and pancreatic cysts appear in the warnings and precautions, with baseline lipase and amylase recommended; this signal is not carried on the labels of the monoclonal C5 inhibitors used in gMG.
• Narrow approved population. The FDA Approved status reflects Zilbrysq’s October 2023 approval for adults with AChR antibody-positive gMG. MuSK antibody-positive and seronegative gMG were not studied, and complement-independent disease mechanisms would not be expected to respond. Adolescent data are still being generated.
• Evidence of class limits. Two randomized trials outside gMG — immune-mediated necrotizing myopathy and COVID-19 respiratory failure — failed to show statistically significant benefit, and neither program was pursued. The gMG pivotal trial was 12 weeks long; durability data come from an open-label extension without a comparator.
• Naming. The development code was RA101495 (Ra Pharmaceuticals); the approved brand is Zilbrysq. Zilucoplan is distinct from the C3 inhibitor pegcetacoplan (systemic and intravitreal) and from the C5 aptamer avacincaptad pegol, and from UCB’s other gMG product, rozanolixizumab, an FcRn blocker with a different mechanism that was approved in the same year.