New therapies for hemophilia A and hemophilia B will likely continue to change clinical practice. Ranging from extended half-life to nonfactor products and gene therapy, these innovative approaches have the potential to enhance the standard of care by decreasing infusion frequency to increase compliance, promoting prophylaxis, offering alternatives to inhibitor patients, and easing route of administration. Each category has intrinsic challenges that may limit the broader application of these promising therapies. To date, none specifically address the challenge of dispersing treatment to the developing world.

After successful efforts in adeno-associated virus (AAV) gene addition for hemophilia B gene therapy, the development of valoctocogene roxaparvovec (Roctavian; Biomarin) over the past decade represents a potential new hemophilia A (HA) treatment paradigm. Roctavian is the first licensed HA gene therapy that was conditionally approved in Europe in August 2022 and approved in the United States in June 2023. Beyond Roctavian, there are ongoing pivotal trials of additional AAV vectors for HA, others that are progressing through preclinical development or early-phase clinical trial, as well as non-AAV approaches in clinical development. This review focuses on the clinical development of Roctavian for which the collective clinical trials represent the largest body of work thus far available for any licensed AAV product. From this pioneering clinical development, several outstanding questions have emerged for which the answers will undoubtedly be important to the clinical adaptation of Roctavian and future efforts in HA gene therapy. Most notably, unexplained year-over-year declines in factor VIII (FVIII) expression after Roctavian treatment contrast with stable FVIII expression observed in other AAV HA gene therapy clinical trials with more modest initial FVIII expression. This observation has been qualitatively replicated in animal models that may permit mechanistic study. The development and approval of Roctavian is a landmark in HA therapeutics, although next-generation approaches are needed before HA gene therapy fulfills its promise of stable FVIII expression that normalizes hemostasis.

Adeno-associated virus (AAV) vector liver-directed gene therapy (GT) for hemophilia B (HB) is limited by a vector dose–dependent hepatotoxicity. Recently, this obstacle has been partially circumvented by the use of a hyperactive factor IX (FIX) variant, R338L (Padua), which has an 8-fold increased specific activity compared with FIX-WT. FIX-R338L has emerged as the standard for HB GT. However, the underlying mechanism of its hyperactivity is undefined; as such, safety concerns of unregulated coagulation and the potential for thrombotic complications have not been fully addressed. To this end, we evaluated the enzymatic and clotting activity as well as the activation, inactivation, and cofactor dependence of FIX-R338L relative to FIX-WT. We observed that the high specific activity of FIX-R338L requires factor VIIIa (FVIIIa) cofactor. In a potentially novel system using emicizumab, an FVIII-mimicking bispecific antibody, the hyperactivity of both recombinant FIX-R338L and AAV-mediated transgene-expressed FIX-R338L from HB GT subjects was ablated without FVIIIa activity. We conclude that the molecular regulation of activation, inactivation, and cofactor dependence of FIX-R338L is similar to FIX-WT but that the FVIIIa-dependent hyperactivity of FIX-R338L is the result of a faster rate of factor X activation. This mechanism helps mitigate safety concerns of unregulated coagulation and supports the expanded use of FIX-R338L in HB therapy.

Mechanisms thought to regulate activated factor VIII (FVIIIa) cofactor function include A2-domain dissociation and activated protein C (APC) cleavage. Unlike A2-domain dissociation, there is no known phenotype associated with altered APC cleavage of FVIII, and biochemical studies have suggested APC plays a marginal role in FVIIIa regulation. However, the in vivo contribution of FVIIIa inactivation by APC is unexplored. Here we compared wild-type B-domainless FVIII (FVIII-WT) recombinant protein with an APC-resistant FVIII variant (FVIII-R336Q/R562Q; FVIII-QQ). FVIII-QQ demonstrated expected APC resistance without other changes in procoagulant function or A2-domain dissociation. In plasma-based studies, FVIII-WT/FVIIIa-WT demonstrated dose-dependent sensitivity to APC with or without protein S, whereas FVIII-QQ/FVIIIa-QQ did not. Importantly, FVIII-QQ demonstrated approximately fivefold increased procoagulant function relative to FVIII-WT in the tail clip and ferric chloride injury models in hemophilia A (HA) mice. To minimize the contribution of FV inactivation by APC in vivo, a tail clip assay was performed in homozygous HA/FV Leiden (FVL) mice infused with FVIII-QQ or FVIII-WT in the presence or absence of monoclonal antibody 1609, an antibody that blocks murine PC/APC hemostatic function. FVIII-QQ again demonstrated enhanced hemostatic function in HA/FVL mice; however, FVIII-QQ and FVIII-WT performed analogously in the presence of the PC/APC inhibitory antibody, indicating the increased hemostatic effect of FVIII-QQ was APC specific. Our data demonstrate APC contributes to the in vivo regulation of FVIIIa, which has the potential to be exploited to develop novel HA therapeutics.

Background Limited information exists regarding the factor IX (FIX) coagulant activity (FIX:C) measured by different assays following FIX‐Padua gene therapy. Objective Assess for the first time FIX:C in 5 commonly used coagulation assays in plasma samples from hemophilia B subjects receiving FIX‐Padua gene transfer. Methods FIX:C was compared between central (n=1) and local laboratories (n=5) in the study, and across 4 commonly used FIX:C one‐stage assays and one FIX:C chromogenic assay. For comparison, samples of pooled congenital FIX‐deficient plasma spiked with purified recombinant human FIX (rHFIX)‐Padua protein or rHFIX (nonacog alfa) to obtain FIX:C concentrations from ≈20% to ≈40% were tested. Results FIX:C results at local laboratories strongly correlated with central laboratory results. However, absolute values at the central laboratory were consistently lower than those at local laboratories. Across 5 different FIX:C assays, a consistent pattern of FIX:C was observed for subjects receiving fidanacogene elaparvovec‐expressed gene transfer. Use of Actin FSL aPTT reagent in the central laboratory resulted in lower FIX:C values compared with other aPTT reagents tested. The chromogenic assay determined lower FIX:C than any of the one‐stage assays. The rHFIX‐Padua protein–spiked samples showed similar results. In contrast, FIX:C results for rHFIX‐nonacog alfa measured within 25% of expected for all one‐stage assays, and below 25% in the chromogenic assay. Conclusions Assay‐based differences in FIX:C were observed for fidanacogene elaparvovec transgene product and rHFIX‐Padua protein, suggesting the variable FIX:C determined with different assay reagents is inherent to the FIX‐Padua protein and is not specific to gene therapy–derived FIX‐Padua.

The high-specific-activity factor IX (FIX) variant Padua (R338L) is the most promising transgene for hemophilia B (HB) gene therapy. Although R338 is strongly conserved in mammalian evolution, amino acid substitutions at this position are underrepresented in HB databases. We therefore undertook a complete 20 amino acid scan and determined the specific activity of human (h) and canine (c) FIX variants with every amino acid substituted at position 338. Notably, we observe that hFIX-R338L is the most active variant and cFIX-R338L is sevenfold higher than wild-type (WT) cFIX. This is consistent with the previous identification of hFIX-R338L as a cause of a rare X-linked thrombophilia risk factor. Moreover, WT hFIX and cFIX are some of the least active variants. We confirmed the increased specific activity relative to FIX-WT in vivo of a new variant, cFIX-R338I, after gene therapy in an HB dog. Last, we screened 232 pediatric subjects with thromboembolic disease without identifying F9 R338 variants. Together these observations suggest a surprising evolutionary pressure to limit FIX activity with WT FIX rather than maximize FIX activity.