Background: RAISE-XT (NCT04225871; Phase 3 study) showed clinically meaningful and sustained improvements in myasthenia gravis (MG)-specific outcomes with zilucoplan, a macrocyclic peptide complement component 5 inhibitor, in patients with acetylcholine receptor autoantibody-positive generalised MG. Methods: Adults self-administered once-daily subcutaneous zilucoplan 0.3mg/kg. This post hoc analysis assessed durability of response to Week 120 in MG-Activities of Daily Living (MG-ADL) and Quantitative MG (QMG) responders at Week 1 of two double-blind studies (NCT03315130, NCT04115293). Responder definitions: improvements of ≥3-points (MG-ADL) or ≥5-points (QMG) (interim data cut: 11 November 2023). Results: 93 patients were randomised to zilucoplan 0.3mg/kg in the double-blind studies; 43.0% (n=40/93) and 33.3% (n=31/93) were MG-ADL and QMG responders, respectively, at Week 1. Week 1 responders spent a median #of 98.9% (5.8–99.2) and 99.0% (2.5–99.2) time in response up to Week 120 for MG-ADL and QMG. Week 1 non-responders spent# a median #of 84.6% (0.0–98.3) and 66.7% (0.0–98.9) time in response up to Week 120 for MG-ADL and QMG, with most responding later in the study. Conclusions: Among early (Week 1) zilucoplan responders, time in response remained high (99%) up to Week 120. These data demonstrate rapid and sustained efficacy with long-term zilucoplan treatment.#