From an mRNA Display Hit to Zilucoplan (3): How Did a Macrocycle Become a Once-Daily Subcutaneous Drug?
Compound 38 already had strong C5-inhibitory activity, but the team still had to maintain sufficient in vivo exposure without compromising target binding. Lipidation and spacer design advanced the optimized molecule toward clinical development and ultimately once-daily subcutaneous zilucoplan.
From an mRNA Display Hit to Zilucoplan · Part 3
← Previous: How Were Two Hits Combined into a More Stable Macrocycle?
Potency Was Strong Enough—How Long Could It Persist In Vivo?
The previous article ended with Compound 38: it achieved an IC₅₀ of 0.9 nM in the red-blood-cell lysis assay and showed a degree of stability in human plasma. Yet in vitro activity and plasma stability could not answer a more practical question: once the drug entered the body, could it maintain its effect for long enough?
The research team evaluated the pharmacokinetics of Compound 38 in monkeys and measured an intravenous half-life of approximately 20.7 hours. This result supported continued development toward once-daily subcutaneous administration, but in vivo exposure still needed to be increased. In other words, the next design objective was no longer only to make the molecule more potent, but to help it remain in the circulation for longer.
Adding “Circulation-Time Design” to the Peptide Terminus
The research team chose to lipidate the C terminus of the peptide. Earlier experiments had shown that the C-terminal norvaline was not required to maintain activity, making this position an available exit point for structural modification. The researchers introduced lysine as the attachment point, then combined a C16 fatty-acid chain, a γ-glutamic-acid linker, and a PEG spacer. The fatty-acid chain promotes albumin binding and thereby improves pharmacokinetic performance, while the PEG spacer was intended to reduce spatial interference from albumin binding with target engagement.
This type of modification must balance two effects: increasing albumin binding can prolong circulating exposure, but potency may decline if the lipid chain or linker prevents the peptide from approaching C5. The final Compound 45 combined a PEG24 spacer, γ-glutamic-acid linker, and C16 fatty-acid chain while retaining strong C5-inhibitory activity. The paper reported an IC₅₀ of 2.9 nM and an SPR-measured binding affinity (KD) of 0.43 nM for full-length human C5. Compound 45 subsequently became the drug substance of zilucoplan.
Figure 1. Complete structures of Compound 38 and Compound 45, highlighting the C-terminal lysine attachment point, γ-glutamic acid, PEG24 spacer, and C16 fatty-acid chain. The colors identify structural regions only. Redrawn from the structural data reported by Ye et al. and its Supporting Information; the source article is licensed under CC BY 4.0.
Why Choose AChR-Antibody-Positive Myasthenia Gravis?
C5 is an important component of the complement cascade. In anti-acetylcholine-receptor (AChR) antibody-positive generalized myasthenia gravis, antibody-associated complement activation damages the neuromuscular junction. Inhibiting C5 therefore provides a therapeutic direction connected to the disease mechanism in these patients. The clinical development of zilucoplan focused on adults who were AChR-antibody positive; it cannot simply be extrapolated to every type of myasthenia gravis.
The pivotal Phase 3 RAISE trial enrolled 174 patients and compared once-daily subcutaneous zilucoplan with placebo over 12 weeks. The primary endpoint was change in the Myasthenia Gravis Activities of Daily Living (MG-ADL) score: the least-squares mean improvement was 4.39 points with zilucoplan and 2.30 points with placebo, a between-group difference of 2.09 points that was statistically significant. This result moved evaluation of the drug's effect from laboratory measurements to a clinical assessment of patients' daily functioning.
The U.S. FDA approved zilucoplan on October 17, 2023, for adults with AChR-antibody-positive generalized myasthenia gravis. The prescribing information specifies once-daily subcutaneous administration, and patients or caregivers may inject the drug after training. At the same time, complement inhibition introduces infection risk: the FDA label carries a warning about serious meningococcal infections and requires vaccination and risk management as specified. Once a drug enters clinical use, efficacy, route of administration, and safety management must be considered together.
From a Screening Hit to a Drug
Zilucoplan development did not produce a finished drug directly from a single screen. mRNA display supplied the C5-inhibitor starting points; the research team then combined structural features from two hits and progressively optimized cyclization, metabolic stability, and pharmacokinetic performance. Clinical studies subsequently tested whether this design could improve patients' disease symptoms. Compound 45 becoming the drug substance of zilucoplan marked the completion of a path from screening hit to approved drug.
Scientific Sources
Ye P, Hammer RP, Wang Z, et al. Discovery of Zilucoplan: A Complement C5 Inhibitor for Treatment of Anti-Acetylcholine Receptor (AChR) Antibody-Positive Generalized Myasthenia Gravis (gMG). Journal of Medicinal Chemistry. 2025;68(24):25772–25782. https://doi.org/10.1021/acs.jmedchem.5c02537
Howard JF Jr, Bresch S, Genge A, et al. Safety and efficacy of zilucoplan in patients with generalised myasthenia gravis (RAISE): a randomised, double-blind, placebo-controlled, phase 3 study. The Lancet Neurology. 2023;22(5):395–406. https://doi.org/10.1016/S1474-4422(23)00080-7
U.S. Food and Drug Administration. Drug Trials Snapshot: ZILBRYSQ (zilucoplan). Original approval date: October 17, 2023. FDA Drug Trials Snapshot
U.S. Food and Drug Administration. ZILBRYSQ (zilucoplan) Prescribing Information. Current FDA label accessed during implementation. 2026 label, NDA 216834/S-013