Bispecific Antibodies and Advances in Non-Gene Therapy Options in Hemophilia

Regular prophylaxis has markedly improved the treatment for patients with hemophilia A, especially after the introduction of highly purified factor VIII (FVIII) concentrates. However, frequent intravenous infusions and the development of FVIII inhibitors remain as unsolved difficulties. To overcome these unmet needs, a bispecific antibody mimicking activated FVIII has been developed in Japan. This bispecific antibody, emicizumab, recognizes activated factor IX (FIXa) and activated factor X (FXa), and promotes FIXa-catalyzed activation of FX in the absence of FVIII. Emicizumab initially reacts with FIXa generated by the action of factor VIIa/tissue factor complexes. Subsequently, thrombin generation is enhanced in the presence of higher amounts of FIXa derived from FXIa-dependent mechanisms. Hence, emicizumab-driven FXa and thrombin generation is maintained by a FXI activation loop in the intrinsic coagulation pathway. Reactions downstream of emicizumab are regulated by natural anticoagulants including activated protein C, antithrombin, and tissue factor pathway inhibitor. Phase 3 studies (HAVEN 1-4 and HOHOEMI studies) demonstrated a remarkable reduction in bleeding rates together with a high percentage of patients with zero treated bleeds irrespective of the presence of inhibitors. In general, emicizumab proved to be well tolerated, although isolated thromboembolic and thrombotic microangiopathic complications were observed in the HAVEN 1 studies, and 3 out of a total of 400 patients developed neutralizing antidrug antibodies. In addition, several questions remain to be discussed with respect to open-use clinical practice, including when to start treatment, how to monitor therapy, and optimum dosage for surgical procedures and immune tolerance induction.

Efficacy of emicizumab prophylaxis versus factor VIII prophylaxis for treatment of hemophilia A without inhibitors: network meta-analysis and sub-group analyses of the intra-patient comparison of the HAVEN 3 trial

Objectives: To compare the efficacy of emicizumab prophylaxis with that of factor VIII (FVIII) prophylaxis in patients with hemophilia A without inhibitors using two approaches: network meta-analyses (NMA) and additional sub-group analyses from the HAVEN 3 trial.Methods: The NMA used data from trials identified using a systematic literature review and compared bleed rates in patients receiving emicizumab prophylaxis and patients receiving FVIII prophylaxis using a Bayesian, random effects generalized linear model with log link Poisson likelihood. Additional sub-groups of the HAVEN 3 trial included here were defined as patients whose dose-taking behavior met either European label or World Federation of Hemophilia guidelines. A negative binomial regression model was used to conduct an intra-patient comparison of bleed rates within the sub-groups, during treatment with FVIII prophylaxis before entering HAVEN 3 and treatment with emicizumab prophylaxis during HAVEN 3.Results: Four studies were included in the base-case NMA. Evidence showed that the total treated bleed rate was lower with emicizumab prophylaxis compared with FVIII prophylaxis (rate ratio [RR] = 0.36 [95% credible interval (CrI) = 0.13-0.95]). Similar associations were observed in sensitivity analyses. The additional HAVEN 3 analyses also showed lower rates of treated bleeds with emicizumab prophylaxis than with FVIII prophylaxis (RRs [95% confidence interval (CI)] = 0.380 [0.186-0.790] and 0.472 [0.258-0.866] in two sub-groups). These results confirm the original HAVEN 3 intra-patient comparison findings.Conclusions: Combined findings from NMA and additional sub-group analyses of HAVEN 3 support the superiority of emicizumab prophylaxis over FVIII prophylaxis in patients with hemophilia A without inhibitors.