First European real-world-Experience with anti-TFPI-Prophylaxis (Marstacimab) in a Patient with Hemophilia B and Hemophilic Arthropathy – improving Therapy Adherence and halting the Progression of the Arthropathy controlled by standardized Joint Ultrasonography (HEAD-US).

Introduction Hemophilia B is a rare X-linked disorder caused by factor IX deficiency, resulting in spontaneous bleeding, especially in joints and muscles. Marstacimab, an anti-TFPI-antibody, provides a new subcutaneous (s.c.) option for patients unable to receive intravenously (i.v.) clotting factor concentrates, improving adherence and quality of life for those with venous access challenges. Consistent prophylaxis is essential for joint health, as regular treatment reduces joint bleeds – a main cause of chronic damage [ 1 ]. Studies show that better adherence leads to improved joint function [ 2 ]. Method A 29-year-old male with severe Hemophilia B and no history of inhibitors struggled with adherence to i.v. FIX therapy, missing about 30% of scheduled infusions due to poor venous access and obesity (BMI 38.53), which prevented learning self-injection. To receive his factor infusions, he had to visit his general practitioner. Over time, he developed Hemophilic Arthropathy, particularly in both ankles. After approval of marstacimab as a new rebalancing therapy [ 3 ] the informed patient decided to switch therapy to this new s.c. anti-TFPI-antibody with an autoinjector due to his living circumstances and the difficulties he had in undergoing regular i.v. substitution of the clotting factor concentrate. After the patient has been shown only once how to use the autoinjector, he was able to inject the anti-TFPI-antibody marstacimab by himself at every week. Joint status of the ankle was regularly monitored by standardized joint ultrasound (HEAD-US) [ 4 ] before and after switching [[ Fig. 1 ]]. Fig. 1 Standardized Ultrasonography of the ankle (HEAD-US) Images from the actual edition of our brochure on the HEAD-US method Results Switching from FIX-EHL to marstacimab improved the patient’s adherence by allowing s.c. self-injection weekly. Early after the switch, suspected joint bleeds / aura in the left ankle were managed with single FIX-EHL doses ( 26 IU/kg ), but no swelling, pain, or functional loss followed. According to data no dose switching was made. Standardized joint ultrasonography (HEAD-US) at the time of switching and six months later [[ Fig. 2 ]] showed no progression of arthropathy, reduced synovial hypertrophy, and no signs for joint bleeds during marstacimab therapy. The patient reported a noticeably reduced burden in daily life after switching to one weekly s.c. treatment. Fig. 2 Ultrasound of the left ankle before and after switchiing to marstacimab Conclusion Switching therapy in a patient with Haemophilia B (PwH B ) and poor adherence from clotting factor concentrates to rebalancing therapy with marstacimab significantly improved adherence and reduced self-assessed treatment burden over 8-month treatment period. Suspected joint bleed during therapy were not confirmed, indicating that future interventions for joint symptoms should be carefully evaluated. Whether long-term marstacimab treatment will further improve joint health remains to be seen. Espacially in switching therapies in PwH the joint status should be regularly monitored according guide lines [ 5 ]. Publication History Article published online: 17 February 2026 © 2026. Thieme. All rights reserved. Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany