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Introduction Emicizumab is a recombinant, humanized, bispecific monoclonal antibody that restores the function of missing activated factor VIII (FVIII) by bridging activated FIX and FX in persons with hemophilia A (PwHA). Prophylaxis with emicizumab once weekly or every two weeks resulted in significant reductions in bleeds, including joint bleeds, and a favorable safety profile in PwHA without FVIII inhibitors in the HAVEN 3 study (NCT02847637; Mahlangu et al. 2018). Recurrent joint bleeds in PwHA can lead to hemophilic arthropathy, and hemophilia A has been associated with decreased bone mineral density (Kempton et al. 2014). To explore the potential effect of emicizumab prophylaxis on bone and joint health beyond bleed prevention, we measured joint health scores and bone and joint biomarkers in HAVEN 3. Methods Hemophilia joint health scores (HJHS; v2.1) were evaluated at baseline and Week 49 of emicizumab prophylaxis in 107 PwHA in HAVEN 3. Biomarkers of bone formation (osteocalcin [OC], N-terminal propeptide of type I procollagen [P1NP]), bone resorption (C-terminal telopeptide of type I collagen [CTX-I]), osteoblasts (osteoprotegerin), osteoclastogenesis (soluble receptor activator of nuclear factor- kappaB Ligand [sRANKL]), cartilage turnover (cartilage oligomeric matrix protein [COMP]), and inflammation (interleukin 1 beta, interleukin 6, and tumor necrosis factor) were measured in 117 PwHA (Table 1) receiving emicizumab at baseline and after 3, 6, 12, and 18 months of treatment. In all, 94 of 117 PwHA with samples for biomarker analysis were part of the HJHS evaluation. Results PwHA previously on FVIII prophylaxis and those with no target joints at study entry had lower (indicating healthier) HJHS scores at baseline. Mean improvements from baseline of −2.25 (95% confidence interval [CI]: −4.12, −0.39) in total HJHS and −2.23 (95% CI: −4.07, −0.38) in HJHS joint-specific domain (excluding gait) were observed after 49 weeks of emicizumab prophylaxis in PwHA with one or more target joints at study entry (n=71). Improvement was consistent across HJHS for different locations (knee, ankle, elbow). No significant differences in the measured biomarkers between PwHA previously on FVIII prophylaxis or on on-demand treatment, or in those with or without target joints, were seen at baseline. Mean baseline values of most bone and joint biomarkers were within normal ranges, or similar to published levels in healthy individuals, although large variability was observed between individuals. None of the measured biomarkers changed significantly during emicizumab prophylaxis. Higher OC, P1NP, and CTX-I levels were observed in adolescent vs adult PwHA at all time points, which is consistent with reported increases of these biomarkers during skeletal growth. Data suggest a potential association of COMP levels with HJHS scores at baseline (Pearson correlation coefficient 0.46, p=0.0001). Data on two additional cartilage biomarkers, CTX-II (C-terminal telopeptide of type II collagen) and CS-846 (a chondroitin sulfate epitope) are being generated. Conclusions Reduction in joint bleeds was previously reported in HAVEN 3, including over 99% target joint resolution with long-term follow up (Callaghan et al. 2019). This analysis provides further evidence of the positive effect of emicizumab on joint health, showing significant and clinically relevant improvements in HJHS (defined as a ≥2-point reduction in HJHS joints domain [Kuijlaars et al. 2017]) after as few as 49 weeks of emicizumab prophylaxis. The biomarkers measured in blood as surrogates of bone and joint health did not show significant changes over the first 18 months of emicizumab prophylaxis. This may reflect heterogeneity between individuals, and effects on the measured biomarkers by factors other than joint health. However, improvement in bone and joint biomarkers would have been unexpected as the observed means at baseline were already similar to levels reported in healthy individuals. Although data from animal models have suggested that FVIII may play a role in bone health beyond protection against bleeds, in this study we observed no indication of worsening in any of the measured bone and joint health markers that might have resulted from reduced exposure to FVIII in PwHA who switched to emicizumab prophylaxis. Additional data are needed to better understand the long-term effect of emicizumab prophylaxis on bone and joint health. Kiialainen: F. Hoffmann-La Roche Ltd: Employment, Equity Ownership. Niggli:F. Hoffmann-La Roche Ltd: Employment. Kempton:Novo Nordisk: Research Funding; Octapharma: Honoraria; Pfizer: Honoraria; Genentech, Inc.: Honoraria. Castaman:Sanofi: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Bayer: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Uniqure: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Kedrion: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Werfen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Takeda (SHIRE): Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Sobi: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Roche: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Pfizer: Research Funding; CSL Behring: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Novo Nordisk: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Chang:Genentech/Roche: Equity Ownership; Genentech, Inc.: Employment. Paz-Priel:Genentech, Inc.: Employment. Adamkewicz:F. Hoffmann-La Roche Ltd: Equity Ownership; Genentech, Inc.: Employment. Levy:F. Hoffman La Roche: Equity Ownership; Genentech, Inc: Employment.