Coagulation Factor VIII: Biological Basis of Emerging Hemophilia A Therapies.

Coagulation factor (F) VIII is essential for hemostasis. After activation, it combines with activated FIX (FIXa) on anionic membranes to form the intrinsic tenase enzyme complex, responsible for activating FX in the rate-limiting step of sustained coagulation. Hemophilia A and hemophilia B are due to inherited deficiencies in the activity of FVIII and FIX, respectively. Treatment of hemophilia A over the last decade has benefited from improved understanding of FVIII biology, including its secretion pathway, its interaction with von Willebrand factor in circulation, the biochemical nature of its FIXa cofactor activity, the regulation of FVIIIa by inactivation pathways, and its surprising immunogenicity. This has facilitated biotechnology innovations with first-in-class examples of several new therapeutic modalities recently receiving regulatory approval for hemophilia A, including FVIII mimetic bispecific antibodies and recombinant adeno associated viral (rAAV) vector-based gene therapy. Biological insights into FVIII are also guiding the development and use of gain-of-function FVIII variants aimed at addressing limitations of first-generation rAAV vectors for hemophilia A. Several gain-of-function FVIII variants designed to have improved secretion are currently incorporated in second-generation rAAV vectors and have recently entered clinical trials. Continued mutually reinforcing advancements in the understanding of FVIII biology and treatments for hemophilia A will be necessary to achieve the ultimate goal of hemophilia therapy: normalizing hemostasis and optimizing well-being with minimal treatment burden for all patients worldwide.

Enhanced procoagulant activity of select hemophilia B causing factor IX variants with emicizumab.

ABSTRACT: Emicizumab improves the procoagulant activity of select loss-of-function factor IX (FIX) variants with likely dysfunctional assembly of the intrinsic Xase complex, resulting in hemophilia B (HB). FVIII mimetics may represent an alternative nonfactor therapy for select patients with HB.

Hemolysis After Medication Exposure in Pediatric Patients With G6PD Deficiency.

Hemolysis in glucose-6-phosphate dehydrogenase (G6PD) deficiency varies by mutation status and the oxidative stressor. Although classified by percent of enzymatic deficiency, variability in normal G6PD values clouds assessment of hemolysis risk by level. This was a retrospective, single institution, cohort study assessing risk of postexposure medication-induced hemolysis in G6PD deficient patients. Exposures occurred in 87 of 1415 deficient patients. Only 2 of 87 medication-exposed patients had hemolytic episodes and both had very low enzymatic activity. No hemolytic events occurred with G6PD levels >7 units/g hemoglobin. Correlation of levels with mutation may improve predictive capacity for hemolysis in G6PD deficiency.

Diagnosis and Management of Pediatric Venous Thromboembolism: New Therapies on the Horizon.

ABSTRACT: The incidence of venous thromboembolism (VTE) is increasing in pediatric patients. Prompt recognition and evaluation of VTE in young patients could prevent significant morbidity or mortality. In contrast to VTE in adults, current treatment guidelines are largely based on expert opinion as limited randomized controlled trial data exist about the appropriate management in pediatric patients with traditional anticoagulants. However, recently approved direct-acting oral anticoagulants in adults are also being investigated in pediatric VTE and these data could inform future evidence-based treatment principles. Thus, healthcare providers must be well informed about the management of pediatric VTE and the data from these trials to date. This continuing medical education article will provide a summary of management of pediatric VTE with particular emphasis on emerging direct-acting oral anticoagulants.

Novel approaches to hemophilia therapy: successes and challenges.

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.

Gene therapy for hemophilia: Progress to date and challenges moving forward.

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.

Iliopsoas hematoma in a patient with sickle cell disease.

Although musculoskeletal pain in patients with sickle cell disease (SCD) is most frequently the result of vaso-occlusive episodes, clinicians often consider other etiologies including osteomyelitis, avascular necrosis, and trauma. In this study, we report the case of a young female with SCD with hip and back pain secondary to a nontraumatic iliopsoas periosteal hematoma with evidence of adjacent bone infarction. The pathophysiology, diagnostic considerations, and management of periosteal hematomas in SCD are reviewed. This case highlights the need for recognition of unusual causes of musculoskeletal pain in SCD.

A major determinant of the immunogenicity of factor VIII in a murine model is independent of its procoagulant function.

A main complication of treatment of patients with hemophilia A is the development of anti-factor VIII (fVIII) antibodies. The immunogenicity of fVIII potentially is a function of its procoagulant activity, which may result in danger signals that drive the immune response. Alternatively, intrinsic structural elements in fVIII may be particularly immunogenic. Finally, VWF, the carrier protein for fVIII in plasma, may play a role in immune recognition. We compared the immunogenicity of wild-type (wt) B domain-deleted fVIII and 2 inactive fVIII molecules, R372A/R1689A fVIII and V634M fVIII in fVIII(-/-) and fVIII(-/-)/VWF(-/-) mice. R372A/R1689A fVIII lacks proteolytic recognition sites and is not released from VWF. In contrast, V634M fVIII undergoes proteolytic cleavage and dissociation from VWF. No significant difference was observed in the immunogenicity of wt fVIII and V634M fVIII. R372A/R1689A fVIII was slightly less immunogenic in a subset of immunization regimens tested. High doses of wt fVIII were required to produce an immune response in fVIII(-/-)/VWF(-/-) mice. Our results indicate that a main component of the immune response to fVIII is independent of its procoagulant function, is both positively and negatively affected by its association with VWF, and may involve intrinsic elements of fVIII structure.

AAV gene therapy in companion dogs with severe hemophilia: Real-world long-term data on immunogenicity, efficacy, and quality of life.

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.

Coagulation factor VIII regulates von Willebrand factor homeostasis invivo.

BACKGROUND: 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. OBJECTIVES: To determine the effect of FVIII in VWF proteolysis and homeostasis in vivo. METHODS: 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. RESULTS: 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. CONCLUSION: Our results demonstrate the pivotal role of FVIII as a cofactor regulating VWF proteolysis and homeostasis under various (patho)physiological conditions.

Analysis of vector genome integrations in multicentric lymphoma after AAV gene therapy in a severe hemophilia A dog.

Adeno-associated viral (AAV) vectors have traditionally been viewed as predominantly nonintegrating, with limited concerns for oncogenesis. However, accumulating preclinical data have shown that AAV vectors integrate more often than previously appreciated, with the potential for genotoxicity. To understand the consequences of AAV vector integration, vigilance for rare genotoxic events after vector administration is essential. Here, we investigate the development of multicentric lymphoma in a privately owned dog, PC9, with severe hemophilia A that was treated with an AAV8 vector encapsidating a B domain-deleted canine coagulation F8 gene. PC9 developed an aggressive B cell lineage multicentric lymphoma 3.5 years after AAV treatment. Postmortem analysis of the liver, spleen, and lymph nodes showed the expected biodistribution of the AAV genome. Integration events were found both in PC9 and a second privately owned hemophilia A dog treated similarly with canine F8 gene transfer, which died of a bleeding event without evidence of malignancy. However, we found no evidence of expanded clones harboring a single integration event, indicating that AAV genome integrations were unlikely to have contributed to PC9's cancer. These findings suggest AAV integrations occur but are mostly not genotoxic and support the safety profile of AAV gene therapy.

Acquired Hemophilia A: Current Guidance and Experience from Clinical Practice.

In acquired hemophilia A (AHA), autoantibodies to coagulation factor VIII (FVIII) neutralize FVIII activity leading to a potentially severe bleeding diathesis that carries a high rate of morbidity and mortality. This disorder is rare and occurs mainly in adults over 60 years of age or in the postpartum period. The diagnosis should be suspected in patients with new-onset bleeding without a personal or family history of bleeding and can be confirmed via specific assays for FVIII inhibitors. Treatment involves both hemostatic therapies to decrease bleeding and immune modulation strategies to re-establish immune tolerance to FVIII. There are limited data on treatment for refractory disease, based mostly on small case series. Registry studies have informed consensus guidelines for optimal hemostatic therapies and initial immunosuppressive therapies. Additional studies are needed to evaluate novel hemostatic agents and develop biomarkers to risk-stratify treatment while limiting adverse events.

B cell-activating factor modulates the factor VIII immune response in hemophilia A.

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.

Gene Therapy for Hemophilia: Facts and Quandaries in the 21st Century.

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.

Combined anti-CD20 and mTOR inhibition with factor VIII for immune tolerance induction in hemophilia A patients with refractory inhibitors.

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.

Utility of repeat testing in the evaluation for von Willebrand disease in pediatric patients.

BACKGROUND: Von Willebrand disease (VWD) is the most common inherited bleeding disorder and is caused by quantitative and qualitative defects in von Willebrand factor (VWF). The laboratory diagnosis of VWD in pediatric patients is complicated by VWF interassay and intra-assay variability, stress-induced elevations in VWF levels, and a lack of significant bleeding history with which to correlate test results. OBJECTIVE: Guidelines recommend repeat testing in patients with a high suspicion of VWD and unclear laboratory assay results; however, no studies have evaluated the utility of repeat VWF testing in pediatric patients. METHODS: This retrospective single-center cohort study aimed to determine clinical variables associated with requiring more than one test to diagnose VWD and to establish a cutoff VWF value above which further testing is not informative. RESULTS: Of 811 patients evaluated for a suspected bleeding disorder, 22.2% were diagnosed with VWD, with ~70% diagnosed on the first test. Patients with VWD were younger (5.8 vs. 8.5 years, P = .002) and more likely to have a family history of VWD (38% vs. 22%, P < .001) than those without VWD. Univariate analysis failed to identify any clinical variables that correlated with needing multiple tests for a VWD diagnosis. A cutoff of 100 IU/dL for VWF antigen or activity on the first test yielded negative predictive values >95%. CONCLUSIONS: We demonstrate that the majority of pediatric patients had diagnostic VWF values on the first set of testing. Pediatric patients without a family history of VWD and VWF levels >100 IU/dL may not need further testing to rule out the diagnosis of VWD.

Gene therapy for hemophilia: what does the future hold?

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.

Emerging therapies for hemophilia: controversies and unanswered questions.

Several new therapies for hemophilia have emerged in recent years. These strategies range from extended half-life factor replacement products and non-factor options with improved pharmacokinetic profiles to gene therapy aiming for phenotypic cure. While these products have the potential to change hemophilia care dramatically, several challenges and questions remain regarding broader applicability, long-term safety, and which option to pursue for each patient. Here, we review these emerging therapies with a focus on controversies and unanswered questions in each category.