Pharmacokinetic Evaluation of Efanesoctocog Alfa under Emicizumab in Acquired Haemophilia A: Case-Based Observation

Introduction Acquired haemophilia A (AHA) requires individualised peri-operative haemostasis. We report peri-operative FVIII kinetics under efanesoctocog alfa in a patient on background emicizumab, illustrating the impact of inhibitor titre on dose–response and apparent half-life. Method Single-patient observational analysis. An 83-year-old woman (80 kg) presented with relapse of AHA (initial inhibitor 19 BU/mL). Emicizumab started on day 10 after relapse (target 30–38 µg/mL). Efanesoctocog alfa was used for bleeding control, core needle biopsy, and two breast surgeries. FVIII:C was measured by a chromogenic FVIII assay (bovine substrate; Siemens) to minimise emicizumab interference. Doses and sampling times were abstracted from the electronic record. Pharmacokinetic parameters included incremental recovery, apparent half-life from log-linear decay segments after the last dose of a dosing block (t1/2=ln(2)×Δt / ln(C1/C2)), and AUC0–24 h. Results Bleeding phase (April 2025; inhibitor 19 BU/mL): FVIII peaked at 47% and declined to 11% by 24 h, consistent with rapid neutralisation (early-phase t1/2≈12 h). First surgery (June 2025; inhibitor 3.7 BU/mL): early post-bolus declines showed short apparent t1/2 (≈3–7 h); the terminal log-linear segment 24–26 June (60.5→28.1%) gave t1/2≈42 h, indicating reduced inhibition and effective FVIII persistence. Second surgery (July 2025; inhibitor 0.9 BU/mL): overlapping dosing precluded precise t1/2, but FVIII remained>100% for>48 h, compatible with extended-half-life exposure ([ Fig. 1 ]). Fig. 1 FVIII:C kinetics under Emicizumab during bleeding episode and two subsequent surgical phases after s Total FVIII exposure (AUC) increased from 1,446%·h (bleeding) to 41,958%·h (first surgery) and 75,639%·h (second surgery), reflecting a marked rise in effective FVIII availability as inhibitors declined. No thrombotic events occurred. Minor delayed wound bleeding after the second operation resolved after FVIII optimization [ 1 ] [ 2 ]. Conclusion Efanesoctocog alfa achieved measurable, clinically relevant FVIII activity in AHA. As inhibitor titres fell, FVIII recovery increased and apparent half-life lengthened, consistent with re-establishment of extended-half-life kinetics. Chromogenic FVIII monitoring under emicizumab supports individualised dosing and safe transition from bypassing therapy to FVIII replacement in AHA. Publication History Article published online: 17 February 2026 © 2026. Thieme. All rights reserved. Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany