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.

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.

A review of the rationale for gene therapy for hemophilia A with inhibitors: one-shot tolerance and treatment?

The therapeutic landscape for people living with hemophilia A (PwHA) has changed dramatically in recent years, but many clinical challenges remain, including the development of inhibitory antibodies directed against factor VIII (FVIII) that occur in approximately 30% of people with severe hemophilia A. Emicizumab, an FVIII mimetic bispecific monoclonal antibody, provides safe and effective bleeding prophylaxis for many PwHA, but clinicians still explore therapeutic strategies that result in immunologic tolerance to FVIII to enable effective treatment with FVIII for problematic bleeding events. This immune tolerance induction (ITI) to FVIII is typically accomplished through repeated long-term exposure to FVIII using a variety of protocols. Meanwhile, gene therapy has recently emerged as a novel ITI option that provides an intrinsic, consistent source of FVIII. As gene therapy and other therapies now expand therapeutic options for PwHA, we review the persistent unmet medical needs with respect to FVIII inhibitors and effective ITI in PwHA, the immunology of FVIII tolerization, the latest research on tolerization strategies, and the role of liver-directed gene therapy to mediate FVIII ITI.

Adeno-Associated Virus Gene Therapy for Hemophilia.

In vivo gene therapy is rapidly emerging as a new therapeutic paradigm for monogenic disorders. For almost three decades, hemophilia A (HA) and hemophilia B (HB) have served as model disorders for the development of gene therapy. This effort is soon to bear fruit with completed pivotal adeno-associated viral (AAV) vector gene addition trials reporting encouraging results and regulatory approval widely anticipated in the near future for the current generation of HA and HB AAV vectors. Here we review the clinical development of AAV gene therapy for HA and HB and examine outstanding questions that have recently emerged from AAV clinical trials for hemophilia and other monogenic disorders.

Appendix to the Society of Interventional Radiology Consensus Guidelines for the Periprocedural Management of Thrombotic and Bleeding Risk in Patients Undergoing Percutaneous Image-Guided Interventions: Pediatric Considerations.

PURPOSE: To provide guidance on the use of anticoagulant and antithrombotic agents in pediatric patients undergoing interventional radiology procedures. MATERIALS AND METHODS: A multidisciplinary writing group conducted a comprehensive literature search to identify studies on the topic of interest. Recommendations were developed for procedural risk and medication dosage and withholding. A modified Delphi technique was used to achieve consensus agreement on the recommendations. RESULTS: A total of 24 studies, including systematic reviews and meta-analyses, randomized controlled trials, and prospective and retrospective cohort studies, were identified as relevant. The expert writing group agreed on procedural risk categorization, laboratory testing thresholds, and medication dosage and withholding recommendations specific to pediatric practice. They additionally described the nuances of anticoagulation in clinical conditions specific to pediatrics. CONCLUSIONS: The Society of Interventional Radiology recommends following the guidance provided in the document when developing multidisciplinary management protocols for anticoagulation and antithrombotic treatment in pediatric patients undergoing interventional radiology procedures.

Digital haemophilia: Insights into the use of social media for haemophilia care, research and advocacy.

INTRODUCTION: The haemophilia community on Twitter is diverse, consisting of advocacy groups, patients, physicians, researchers and other users. However, the scope of this community is uncharacterized, and limited data is available regarding effective participation in this community. AIM: To assess the types of users active in the haemophilia community on Twitter, as well as major themes present in haemophilia-related tweets. METHODS: Forty-nine thousand five hundred and twelve tweets between September 2019 and September 2021 were classified using regular expressions. A subset of the classified tweets was manually analysed to identify prevalent discussion themes. RESULTS: Among the top 250 users by post count, the largest categories of users were support and advocacy groups, people with bleeding disorders and healthcare providers. The largest thematic categories of tweets were gene therapy, contaminated haemophilia blood products, haemophilia research, clinical management of haemophilia and COVID-19. While misinformation was rare, negative and incorrect perceptions of haemophilia were present among the general public. CONCLUSION: Our results demonstrate patterns of effective Twitter usage for patient care, research and advocacy purposes among the haemophilia community.

Multiyear Factor VIII Expression after AAV Gene Transfer for Hemophilia A.

BACKGROUND: The goal of gene therapy for patients with hemophilia A is to safely impart long-term stable factor VIII expression that predictably ameliorates bleeding with the use of the lowest possible vector dose. METHODS: In this phase 1-2 trial, we infused an investigational adeno-associated viral (AAV) vector (SPK-8011) for hepatocyte expression of factor VIII in 18 men with hemophilia A. Four dose cohorts were enrolled; the lowest-dose cohort received a dose of 5 × 1011 vector genomes (vg) per kilogram of body weight, and the highest-dose cohort received 2 × 1012 vg per kilogram. Some participants received glucocorticoids within 52 weeks after vector administration either to prevent or to treat a presumed AAV capsid immune response. Trial objectives included evaluation of the safety and preliminary efficacy of SPK-8011 and of the expression and durability of factor VIII. RESULTS: The median safety observation period was 36.6 months (range, 5.5 to 50.3). A total of 33 treatment-related adverse events occurred in 8 participants; 17 events were vector-related, including 1 serious adverse event, and 16 were glucocorticoid-related. Two participants lost all factor VIII expression because of an anti-AAV capsid cellular immune response that was not sensitive to immune suppression. In the remaining 16 participants, factor VIII expression was maintained; 12 of these participants were followed for more than 2 years, and a one-stage factor VIII assay showed no apparent decrease in factor VIII activity over time (mean [±SD] factor VIII activity, 12.9±6.9% of the normal value at 26 to 52 weeks when the participants were not receiving glucocorticoids vs. 12.0±7.1% of the normal value at >52 weeks after vector administration; 95% confidence interval [CI], -2.4 to 0.6 for the difference between matched pairs). The participants had a 91.5% reduction (95% CI, 88.8 to 94.1) in the annualized bleeding rate (median rate, 8.5 events per year [range, 0 to 43.0] before vector administration vs. 0.3 events per year [range, 0 to 6.5] after vector administration). CONCLUSIONS: Sustained factor VIII expression in 16 of 18 participants who received SPK-8011 permitted discontinuation of prophylaxis and a reduction in bleeding episodes. No major safety concerns were reported. (Funded by Spark Therapeutics and the National Heart, Lung, and Blood Institute; ClinicalTrials.gov numbers, NCT03003533 and NCT03432520.).

Real-world cost estimates of initiating emicizumab in US patients with haemophilia A.

INTRODUCTION: Emicizumab is the first approved non-factor therapy for haemophilia A. It provides superior prophylactic bleeding control compared to other products in both patients with and patients without inhibitors. However, there is no real-world data about the monetary consequences of starting emicizumab. AIM: To examine the estimated costs of starting emicizumab in a cohort of real-world haemophilia A patients with and without inhibitors. METHODS: The cost of haemostatic therapy for 6 months before and after initiating emicizumab for participants in a multicentre observational study was calculated based on the type of product and dosing that was used for prophylaxis and treating breakthrough bleeds, the number of treated bleeds and the participant weight. RESULTS: Ninety-two patients were included, 18 with an active inhibitor. The median age was 8.7 years. The median total cost for all patients decreased from $176,720 to $128,099 (p = .04) after initiating emicizumab, largely because of decrease in the total cost of high-cost outliers. The cost of prophylaxis and the total cost of bleeds also significantly decreased after starting emicizumab, both for patient with and patients without inhibitors. CONCLUSIONS: Starting or switching to prophylaxis with emicizumab results in decreased costs for the treatment of patients with haemophilia A. This real-world data could inform on payer decisions as well as future cost-effective analysis.

Evolutionary insights into coagulation factor IX Padua and other high-specific-activity variants.

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.

Factor IX assay discrepancies in the setting of liver gene therapy using a hyperfunctional variant factor IX-Padua.

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 five 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 four 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 five different FIX:C assays, a consistent pattern of FIX:C was observed for subjects receiving fidanacogene elaparvovec-expressed gene transfer. Use of Actin FSL activated partial thromboplastin time (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.

Gene Therapy for Inherited Bleeding Disorders.

Decades of preclinical and clinical studies developing gene therapy for hemophilia are poised to bear fruit with current promising pivotal studies likely to lead to regulatory approval. However, this recent success should not obscure the multiple challenges that were overcome to reach this destination. Gene therapy for hemophilia A and B benefited from advancements in the general gene therapy field, such as the development of adeno-associated viral vectors, as well as disease-specific breakthroughs, like the identification of B-domain deleted factor VIII and hyperactive factor IX Padua. The gene therapy field has also benefited from hemophilia B clinical studies, which revealed for the first time critical safety concerns related to immune responses to the vector capsid not anticipated in preclinical models. Preclinical studies have also investigated gene transfer approaches for other rare inherited bleeding disorders, including factor VII deficiency, von Willebrand disease, and Glanzmann thrombasthenia. Here we review the successful gene therapy journey for hemophilia and pose some unanswered questions. We then discuss the current state of gene therapy for these other rare inherited bleeding disorders and how the lessons of hemophilia gene therapy may guide clinical development.

Activated protein C has a regulatory role in factor VIII function.

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.

Conservative Management of Alloimmune Hemolysis and Cholestasis With Extreme Laboratory Abnormalities.

Alloimmune hemolytic disease of the fetus or newborn (HDFN) is a rare cause of neonatal cholestasis. HDFN-associated cholestasis has most often been reported secondary to anti-D alloimmunization. In utero transfusions are also an identified risk factor. A variety of diagnostic and therapeutic strategies have been described, mostly in case reports. Here, we report 2 cases of HDFN-associated cholestasis that were notable for extreme laboratory abnormalities including a peak ferritin of 24 700 ng/mL and a peak alanine aminotransferase of 1406 U/L (33.5-fold upper limit of normal). One case was due to alloimmunization other than anti-D. These cases help define the range of laboratory derangements that are consistent with HDFN-associated cholestasis, including extreme hyperferritinemia. Although in a number of cases, researchers have reported the use of iron chelation in these infants, herein, we describe successful management without iron chelation.