Matchmaking gene therapy with mimetic antibodies Free

In this issue of Blood, Muczynski et al¹ report stable correction of the bleeding diathesis in hemophilia A mice following gene therapy with an AAV vector encoding a novel single-chain factor VIII (FVIII) mimetic antibody expressed from hepatocytes. Combining gene therapy with FVIII-mimetic antibodies may help overcome some of the limitations of currently approved hemophilia A gene therapies.

The first liver-directed, AAV-based gene therapies for hemophilia A and B have recently gained regulatory approval.^2-4 These therapies promise long-term correction of bleeding disorders with a single administration. In severe hemophilia B, gene therapy typically results in stable hepatic expression of factor IX (FIX), leading to markedly reduced annualized bleeding rates (ABR) and factor use.⁵ In hemophilia A, gene therapy can similarly achieve multiyear FVIII expression, reducing ABR and factor replacement needs.⁶ However, in the case of valoctocogene roxaparvovec (Roctavian), FVIII expression is less durable and tends to decline over time, limiting long-term efficacy.^6,7 However, it is not certain if these expression kinetics are unique to this particular product as other clinical trial results suggest a more sustainable FVIII expression pattern, but will require longer-term confirmation.⁸ 

The mechanisms underlying FVIII decline associated with valoctocogene roxaparvovec (Roctavian) are not fully defined.^6,7,9,10 Limited liver biopsy data from treated patients suggest transcriptional silencing of the FVIII transgene may play a role.⁹ Alternatively, ectopic expression of FVIII in hepatocytes—normally synthesized in endothelial cells—could trigger cellular stress responses that reduce expression.^7,10 Furthermore, the FVIII coding sequence exceeds the AAV packaging limit (∼4.7 kb), leading to fragmented or incomplete genomes that may impair expression kinetics.

To address these challenges, Muczynski et al developed an alternative gene therapy strategy that avoids FVIII transgene expression entirely. They engineered an AAV vector encoding Bi8, a hepatocyte-driven single-chain FVIII-mimetic antibody. Bi8 mimics FVIII function by binding FIX and FX simultaneously, facilitating formation of a ternary complex that enhances FIXa-mediated FX activation. This mechanism resembles that of emicizumab, a clinically validated FVIII-mimetic bispecific antibody used successfully in patients with severe hemophilia A. In FVIII-deficient mice, a single IV injection of AAV8-Bi8 corrected the bleeding diathesis as measured by tail vein transection. Blood loss reduction was comparable to emicizumab-treated (5 mg/kg), FVIII-treated (2.5 IU/ml), and wild-type controls. Importantly, despite being a foreign antigen, Bi8 did not elicit detectable antidrug antibodies, possibly due to hepatic immune tolerance, as observed with other therapeutic transgenes.² 

Current gene therapy is not an option for hemophilia A patients with high-titer anti-FVIII inhibitors. Emicizumab has proven valuable in this subgroup because it bypasses FVIII. Encouragingly, purified Bi8 protein also restored hemostasis in human plasma spiked with high-titer FVIII inhibitors (200 BU), suggesting that AAV8-Bi8 therapy could benefit this difficult-to-treat patient population.

This strategy combines the long-term durability of AAV gene transfer with the favorable hemostatic properties of FVIII-mimetic antibodies. It bypasses major challenges of FVIII transgene therapy, including transcriptional repression, oversized vector cassettes, and FVIII-induced cellular stress. Moreover, unlike emicizumab, which requires lifelong repeated dosing, AAV8-Bi8 could provide a sustained therapeutic effect after a single administration.

Nevertheless, important safety and efficacy considerations remain. Although no thrombotic complications were detected, Bi8 interacts only with human clotting factors, limiting the predictive value of mouse models. Thorough evaluation is needed given prior reports of increased thrombosis when emicizumab-treated patients received activated prothrombin complex concentrate (aPCC) for breakthrough bleeds. Another limitation is the high AAV dose required to achieve therapeutic efficacy, which exceed doses used in clinical hemophilia trials and are associated with risks such as liver inflammation, complement activation, and other severe adverse events.² Enhancing the therapeutic index will likely require vector or transgene optimization. For example, Bi8 has a short half-life due to its single-chain format, unlike emicizumab’s long-lived immunoglobulin G structure. Fusion to albumin-binding proteins—shown to extend single-chain antibody half-life—may improve durability. Finally, because emicizumab does not fully normalize bleeding risk and patients remain susceptible to breakthrough or trauma-induced bleeding, Bi8-based gene therapy may face similar limitations.

In summary, the study by Muczynski et al introduces a potentially attractive alternative in hemophilia A gene therapy by harnessing a hepatocyte-expressed FVIII-mimetic antibody instead of the FVIII transgene itself. This innovative approach addresses key limitations of conventional FVIII-based strategies, while offering the potential for durable, single-dose treatment—even in patients with preexisting inhibitors. Although challenges remain regarding vector dosing, long-term safety, and translational efficacy, the AAV8-Bi8 platform provides a compelling foundation for next-generation therapies for individuals with hemophilia A, matchmaking the advantages of gene therapy with those of FVIII-mimetic antibodies.

Conflict-of-interest disclosure: T.V. served as consultant or speaker for Roche, CSL Behring, BioMarin, and Pfizer; and is an inventor on patent applications in the gene therapy field. M.K.C. is an inventor on patent applications in the gene therapy field.