What is Zilbrysq’s mechanism of action?
Zilbrysq’s mechanism of action is blocking the complement protein C5 to prevent harmful overactivation of the immune system in people with generalized myasthenia gravis (gMG).
Understanding Complement Protein C5
Complement C5 plays an important role in the immune system’s complement pathway, which helps fight infections and clear damaged cells. When activated (turned on), C5 is split into two parts: C5a and C5b.
- C5a acts as a potent signaling molecule, triggering inflammation and attracting immune cells to sites of injury, which is helpful in fighting infections but can cause harm if uncontrolled.
- C5b sets the stage for building the membrane attack complex (MAC), a structure that punctures cell membranes and destroys cells.
Zilbrysq (active ingredient: zilucoplan) is designed to attach directly to C5, preventing it from splitting into C5a and C5b. This helps calm a part of the immune system, stopping it from damaging healthy cells and tissues unnecessarily.
Clinical Relevance in Myasthenia Gravis
In generalized myasthenia gravis, the immune system mistakenly attacks connections between nerves and muscles, interfering with signals that enable muscle movement. The complement pathway (especially MAC) contributes to this attack, worsening muscle weakness and causing disease flares. By blocking C5, Zilbrysq protects these nerve-muscle connections, leading to improved muscle strength and fewer symptoms for people living with gMG.
Zilbrysq is given as an injection under the skin once per day. Currently, it is only approved for adults.
Summary
Zilbrysq, a daily injection for adults with myasthenia gravis, calms immune overactivity by blocking a protein in the immune system called C5. As a result, it shields muscles from the body’s own immune attack.
References
- Beloor Suresh A, Asuncion RMD. Myasthenia Gravis. [Updated 2023 Aug 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Accessed on September 11, 2025 at https://www.ncbi.nlm.nih.gov/books/NBK559331/
- Tang, G. Q., Tang, Y., Dhamnaskar, K., Hoarty, M. D., Vyasamneni, R., Vadysirisack, D. D., Ma, Z., Zhu, N., Wang, J. G., Bu, C., Cong, B., Palmer, E., Duda, P. W., Sayegh, C., & Ricardo, A. (2023). Zilucoplan, a macrocyclic peptide inhibitor of human complement component 5, uses a dual mode of action to prevent terminal complement pathway activation. Frontiers in immunology, 14, 1213920. https://doi.org/10.3389/fimmu.2023.1213920
- Zilbrysq [package insert]. Updated July 2025. UCB, Inc. Accessed on September 11, 2025 at https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=9f1d53d6-e495-4a3a-8bb9-680a15d25034
Read next
How long before Zilbrysq works?
Zilbrysq starts working within the first week of treatment, with patients showing improvements in myasthenia gravis symptoms and a reduction of more than 2 points in the MG-ADL score. By the end of the 12-week Phase 3 RAISE trial, symptoms continued to improve with a 4.39 reduction of the daily functioning score.
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How does Mestinon help with myasthenia gravis?
Mestinon is a prescription medicine that can help relieve symptoms of myasthenia gravis (MG) such muscle weakness, double vision, blurred vision, drooping eyelids, loss of facial expressions, and difficulties with talking, chewing food, swallowing and breathing. There is no cure for MG, but treatments can help with symptoms. Continue reading
Can I stop taking Mestinon (pyridostigmine)?
Mestinon is a common treatment for myasthenia gravis but 25% of all patients who take it discontinue it because of side effects, and many discontinue it because it does not make any difference to their symptoms. Because it is short-acting, it could theoretically be stopped without tapering it, but you should always talk to your doctor about stopping it before you do so. Continue reading
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Drug information
- Zilbrysq Information for Consumers
- Zilbrysq prescribing info & package insert (for Health Professionals)
- Side Effects of Zilbrysq (detailed)
- Zilbrysq user reviews (2)
Related support groups
- Zilbrysq (2 questions, 4 members)
- Myasthenia Gravis (20 questions, 97 members)