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.
Recent phase I/II adeno-associated viral vector-mediated gene therapy clinical trials have reported remarkable success in ameliorating disease phenotype in hemophilia A and B. These trials, which ...highlight the challenges overcome through decades of preclinical and first in human clinical studies, have generated considerable excitement for patients and caregivers alike. Optimization of vector and transgene expression has significantly improved the ability to achieve therapeutic factor levels in these subjects. Long-term follow-up studies will guide standardization of the approach with respect to the combination of serotype, promoter, dose, and manufacturing processes and inform safety for inclusion of young patients. Certain limitations preclude universal applicability of gene therapy, including transient liver transaminase elevations due to the immune responses to vector capsids or as yet undefined mechanisms, underlying liver disease from iatrogenic viral hepatitis, and neutralizing antibodies to clotting factors. Integrating vectors show promising preclinical results, but manufacturing and safety concerns still remain. The prospect of gene editing for correction of the underlying mutation is on the horizon with considerable potential. Herein, we review the advances and limitations that have resulted in these recent successful clinical trials and outline avenues that will allow for broader applicability of gene therapy.
Background
Hemophilia A (HA) inhibitor patients that fail traditional immune tolerance induction (ITI) have increased morbidity and mortality. Preclinical studies support factor VIII (FVIII) ...tolerance induction with a combined approach of anti‐CD20 mediated transient B cell depletion and rapamycin mediated regulatory T cell (Treg) induction.
Methods
Two refractory HA inhibitor patients were treated with rituximab, rapamycin, and FVIII ITI. Their clinical course, anti‐FVIII immunoglobulins, cytokines, and select lymphocytes were followed.
Results
One patient achieved complete and the other partial FVIII tolerance; both had marked annualized bleeding rate improvement. FVIII‐specific immunoglobulins, but not total Treg counts, correlated with tolerance induction. IL‐6 and IL‐21 correlation with complete tolerance induction may support that down‐regulation of T effectors and IgG4 production, respectively, contribute to the pathogenesis of tolerance induction.
Conclusions
This regimen may be considered to induce FVIII tolerance in HA patients with refractory inhibitors. Further characterization of the FVIII‐specific immune response is necessary to clarify the mechanism of immune tolerance.
Over the past decades hemophilia has been transformed from a debilitating disease to a manageable condition. However, the current treatment options are expensive, complex, and inaccessible to a large ...portion of the global population. Moreover, the development of antibodies to replacement factors, termed inhibitors, is a common complication that not only renders conventional prophylaxis regimens ineffective but also increases the annual bleeding rate in affected patients. Fortunately, much progress has been made toward developing a curative gene therapy treatment for hemophilia and these efforts have led to a series of human trials with promising results. This review seeks to address some of the new issues raised by recent progress in the field, including the differences between available recombinant adeno-associated viral (rAAV) vectors, the etiology of transaminitis following vector administration, and techniques to induce long-term factor expression. We also address other unresolved questions, including strategies to overcome pre-existing neutralizing antibodies to AAV, approaches that can make vector re-administration possible, and whether gene therapy can be used to induce factor tolerance and treat inhibitors. Finally, we discuss logistical and ethical issues related to hemophilia gene therapy including how to accurately measure therapeutic outcomes, when to consider treatment of pediatric patients, and how to equitably price the medication to ensure fair compensation while maximizing accessibility. As the field marches forward from clinical trials towards clinical application, answers to these questions will determine the future of gene therapy for hemophilia.
Therapy for hemophilia has evolved in the last 40 years from plasma-based concentrates to recombinant proteins and, more recently, to non-factor therapeutics. Along this same timeline, research in ...adeno-associated viral (AAV) based gene therapy vectors has provided the framework for early phase clinical trials initially for hemophilia B (HB) and now for hemophilia A. Successive lessons learned from early HB trials have paved the way for current advanced phase trials. Nevertheless, questions linger regarding 1) the optimal balance of vector dose to transgene expression, 2) amount and durability of transgene expression required, and 3) long-term safety. Some trials have demonstrated unique findings not seen previously regarding transient elevation of liver enzymes, immunogenicity of the vector capsid, and loss of transgene expression. This review will provide an update on the clinical AAV gene therapy trials in hemophilia and address the questions above. A thoughtful and rationally approached expansion of gene therapy to the clinics would certainly be a welcome addition to the arsenal of options for hemophilia therapy. Further, the global impact of gene therapy could be vastly improved by expanding eligibility to different patient populations and to developing nations. With the advances made to date, it is possible to envision a shift from the early goal of simply increasing life expectancy to a significant improvement in quality of life by reduction in spontaneous bleeding episodes and disease complications.
Coagulation factor VIII (FVIII) and von Willebrand factor (VWF) circulate as a noncovalent complex, but each has its distinct functions. Binding of FVIII to VWF results in a prolongation of FVIII's ...half-life in circulation and modulates FVIII's immunogenicity during hemophilia therapy. However, the biological effect of FVIII and VWF interaction on VWF homeostasis is not fully understood.
To determine the effect of FVIII in VWF proteolysis and homeostasis in vivo.
Mouse models, recombinant FVIII infusion, and patients with hemophilia A on a high dose FVIII for immune tolerance induction therapy or emicizumab for bleeding symptoms were included to address this question.
An intravenous infusion of a recombinant B-domain less FVIII (BDD-FVIII) (40 and 160 μg/kg) into wild-type mice significantly reduced plasma VWF multimer sizes and its antigen levels; an infusion of a high but not low dose of BDD-FVIII into Adamts13
and Adamts13
mice also significantly reduced the size of VWF multimers. However, plasma levels of VWF antigen remained unchanged following administration of any dose BDD-FVIII into Adamts13
mice, suggesting partial ADAMTS-13 dependency in FVIII-augmented VWF degradation. Moreover, persistent expression of BDD-FVIII at ∼50 to 250 U/dL via AAV8 vector in hemophilia A mice also resulted in a significant reduction of plasma VWF multimer sizes and antigen levels. Finally, the sizes of plasma VWF multimers were significantly reduced in patients with hemophilia A who received a dose of recombinant or plasma-derived FVIII for immune tolerance induction therapy.
Our results demonstrate the pivotal role of FVIII as a cofactor regulating VWF proteolysis and homeostasis under various (patho)physiological conditions.
Inhibitors of factor VIII (FVIII) remain the most challenging complication of FVIII protein replacement therapy in hemophilia A (HA). Understanding the mechanisms that guide FVIII-specific B cell ...development could help identify therapeutic targets. The B cell-activating factor (BAFF) cytokine family is a key regulator of B cell differentiation in normal homeostasis and immune disorders. Thus, we used patient samples and mouse models to investigate the potential role of BAFF in modulating FVIII inhibitors. BAFF levels were elevated in pediatric and adult HA inhibitor patients and decreased to levels similar to those of noninhibitor controls after successful immune tolerance induction (ITI). Moreover, elevations in BAFF levels were seen in patients who failed to achieve FVIII tolerance with anti-CD20 antibody-mediated B cell depletion. In naive HA mice, prophylactic anti-BAFF antibody therapy prior to FVIII immunization prevented inhibitor formation and this tolerance was maintained despite FVIII exposure after immune reconstitution. In preimmunized HA mice, combination therapy with anti-CD20 and anti-BAFF antibodies dramatically reduced FVIII inhibitors via inhibition of FVIII-specific plasma cells. Our data suggest that BAFF may regulate the generation and maintenance of FVIII inhibitors and/or anti-FVIII B cells. Finally, anti-CD20/anti-BAFF combination therapy may be clinically useful for ITI.
The hemophilias are the most common severe inherited bleeding disorders and are caused by deficiency of clotting factor (F) VIII (hemophilia A) or FIX (hemophilia B). The resultant bleeding ...predisposition significantly increases morbidity and mortality. The ability to improve the bleeding phenotype with modest increases in clotting factor levels has enabled the development and regulatory approval of adeno-associated viral (AAV) vector gene therapies for people with hemophilia A and B. The canine hemophilia model has proven to be one of the best predictors of therapeutic response in humans. Here, we report long-term follow-up of 12 companion dogs with severe hemophilia that were treated in a real-world setting with AAV gene therapy. Despite more baseline bleeding than in research dogs, companion dogs demonstrated a 94% decrease in bleeding rates and 61% improvement in quality of life over a median of 4.1 years (range 2.6–8.9). No new anti-transgene immune responses were detected; one dog with a pre-existing anti-FVIII inhibitor achieved immune tolerance with gene therapy. Two dogs expressing 1%–5% FVIII post gene therapy experienced fatal bleeding events. These data suggest AAV liver-directed gene therapy is efficacious in a real-world setting but should target expression >5% and closely monitor those with levels in the 1%–5% range.
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Doshi and colleagues demonstrate that AAV gene therapy in companion dogs in a real-world setting is safe and efficacious with decreased bleeding, no anti-transgene immune responses, and improved quality of life over a median of 4.1 years of follow-up. Bleeding rates are lower when factor VIII or IX expression is >5%.
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