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.

In a pivotal, multicenter, open-label study, 25 patients aged 12-54 years with moderately severe/severe hemophilia B received on-demand nonacog alfa (6 months; dose at investigator's discretion) followed by once-weekly prophylaxis with nonacog alfa 100 IU/kg (12 months). During prophylaxis, patients had a median spontaneous annualized bleeding rate (sABR) of 1.0 and significant reductions in ABR (P < 0.0001). This post hoc analysis examined the time of onset of spontaneous bleeding events (sBEs) and spontaneous target joint bleeding events (sTJBEs). The postdosing day (D) of onset of sBEs observed during prophylaxis and steady-state FIX activity data (FIX:C) between 144 and 196 h postdose were collected at weeks 26 and 78. Twelve patients (48%) had no sBEs; the remaining 13 (52%) had the following onset of sBEs: less than 1 D (0%), 1 to less than 2D (5%), 2 to less than 3 D (22%), 3 to less than 4 D (9%), 4 to less than 5D (22%), 5 to less than 6D (23%), 6 to less than 7D (11%), and at least 7D (8%). Reductions in sBEs and sTJBEs during on-demand versus prophylaxis treatment were experienced by all 13 patients. Target joint sABR during prophylaxis was 0 for 5/13 patients. ABR reduction ranged from 66.1% (27.2→9.2) to 97.8% (46.2→1.0); sTJBE reductions ranged from 6.2% (2.1→2.0) to 100% (from 40.1, 19.1, 3.9, 9.0, 6.1--0). During prophylaxis, 47% (8/17) of trough FIX activity samples were more than 2%. In sBE patients, ABR and number of TJBEs were reduced with once-weekly nonacog alfa. When sBEs occurred, they followed no apparent pattern for day of occurrence. Clinicaltrials.gov identifier: NCT01335061.

Neutralizing antibodies (inhibitors) against factor VIII/IX (FVIII/FIX) poses a serious and challenging complication in the hemophilia treatment. Allergic reaction is more common in hemophilia B and always companion with FIX inhibitors, but it is rare in hemophilia A (HA). So far only few cases demonstrated FVIII-specific allergic response in hemophilia A. Coexistence of allergic reactions with inhibitors was contraindicated for immune tolerance induction (ITI) regimen which is the only proven therapy to eliminate inhibitor. We report a rare case of a 11-year-old boy with moderate HA who developed high titer inhibitor and severe allergic reaction to both plasma derived and recombinant FVIII concentrates. Inhibitor was eliminated with the use of prednisone. Further desensitization protocol by administering rFVIII of increasing does from 0.01 IU/kg to 40 IU/kg with a pre-determined time schedule allowed patient tolerance to the normal dose and infusion time to FVIII.

Hemophilia is a rare bleeding disorder that needs plasma or clotting factor concentrate transfusion. Therefore chances of blood-borne pathogens like HCV transmission increase due to high prevalence in healthy donors. This study was aimed to determine the prevalence of HCV genotypes and associated risk factors in hemophilia patients of Khyber Pakhtunkhwa, Pakistan. Blood samples and data were collected from 672 hemophiliacs after proper consent obtained from each patient. Samples were analyzed for anti-HCV, HCV RNA and HCV genotype/s detection. Of the total, 22.32% (150) were anti-HCV positive, of which HCV RNA was detected in 18.45% (124) individuals. HCV genotype 3a was found with significantly higher prevalence (p<0.05) (19.35%) as compared to 2a (16.13%) and 1a (12.90%). HCV-3b and HCV-4 were found each in 3.22% samples. Dual infection of genotypes was found in 22.58% of individuals and 22.58% HCV RNA positive sampels were not typed. A total of 572 (85.12%) subjects had hemophilia A and 100 (14.88%) had hemophilia B. In hemophiliacs A the most dominant genotype was 3a (19.27%) while in hemophilia B, genotype 1a was prevalent (26.67%). Whole blood and plasma transfusion were observed as the main risk factors of HCV. It is concluded that HCV genotype 3a and 2a are prevalent in hemophilia patients of Khyber Pakhtunkhwa Pakistan and the main risk factor observed was an unscreened whole blood transfusion.

People with hemophilia (PWH) may require some surgical procedures (orthopedic surgery, cardiac surgery, neurosurgery, et cetera) throughout their lives.

Hemophilia A and B are inherited hemorrhagic disorders that result from alterations in the coagulation cascade. Aside from spontaneous bleeding, the main complication of hemophilia is hemarthrosis. Progress over the last three decades, specifically prophylaxis using recombinant factor, has prevented hemarthrosis and lengthened patient life expectancies. However, many treatments require frequent dosing up to three times a week, and alloantibodies (inhibitors) against replacement factor continues to be an issue. These problems call for novel treatments for patients with hemophilia. Although there has been progress in extended half-life factors and mimetics of factor VIII, an alternative treatment methodology is to rebalance the activities of pro- and anticoagulant factors through inhibition of the natural anticoagulants: antithrombin, tissue factor pathway inhibitor, protein C, and protein S. This review will explore the efficacy of targeting these inhibitory pathways from preclinical development through clinical trials, and delve into concerns of thrombotic risk.

Concizumab is a novel subcutaneous prophylactic therapy for hemophilia. It is a hemostatic rebalancing agent that binds to the Kunitz-2 domain of tissue factor pathway inhibitor (TFPI), one of the molecules that contributes to downregulation of coagulation thereby preventing TFPI from binding to and blocking the factor Xa (FXa) active site. When the TFPI inhibitory activity is decreased, sufficient FXa is produced by the FVIIa-tissue factor complex to achieve hemostasis. On the basis of this mechanism of action, concizumab is expected to be equally effective in hemophilia A and B, regardless of inhibitor status. Moreover, the concizumab mechanism of action does not interfere with the regulation of coagulation downstream of TFPI. Results from 2 phase 2 trials in patients with hemophilia A or B with and without inhibitors demonstrated that concizumab had a favorable safety profile, with no deaths, no thromboembolic events, and no adverse events leading to withdrawal. Clinical proof of concept in prevention of bleeding episodes was confirmed in both concizumab phase 2 trials across all hemophilia subtypes assessed, with a statistically significant and clinically relevant reduction in annualized bleeding rates observed in inhibitor patients compared to those who received on-demand treatment. On the basis of phase 2 results, the US Food and Drug Administration granted concizumab Breakthrough Therapy designation for hemophilia B with inhibitor patients, a rare and vulnerable patient subgroup that currently has the highest unmet medical need. In the ongoing concizumab phase 3 trials, an optimized dosing regimen will be administered in patients with hemophilia A or B with and without inhibitors.

Current treatments in the field of haemophilia are changing the phenotype of many patients with severe haemophilia to that of mild haemophilia. Despite this improvement, those with mild-to-moderate haemophilia A and haemophilia B continue to experience unmet needs. Whereas some patients with mild-to-moderate haemophilia experience similar complications to those of patients with severe haemophilia, they possess several unique attributes. These include a challenging diagnosis and variability in bleeding symptoms and treatment needs. In addition, haemophilia is an under-recognized condition in women even though many women with mild-to-moderate haemophilia experience the same symptoms and complications as men with haemophilia. These women also have their own unique challenges with this disease. This supplement highlights many of the unmet needs in men and women with mild-to-moderate haemophilia. The conclusions of each of these papers reinforce the need for additional research and resources for this patient population.

[This corrects the article DOI: 10.1371/journal.pgen.1008690.].

Hemophilia is caused by a lack of antihemophilic factor(s), for example, factor VIII (FVIII; hemophilia A) and factor IX (FIX; hemophilia B). Low bone mass is widely reported in epidemiological studies of hemophilia, and patients with hemophilia are at an increased risk of fracture. The detailed etiology of bone homeostasis imbalance in hemophilia is unclear. Clinical and experimental studies show that FVIII and FIX are involved in bone remodeling. However, it is likely that antihemophilic factors affect bone biology through thrombin pathways rather than via their own intrinsic properties. In addition, among patients with hemophilia, there are pathophysiological processes in several systems that might contribute to bone loss. This review summarizes studies on the association between hemophilia and bone remodeling, and might shed light on the challenges facing the care and prevention of osteoporosis and fracture in patients with hemophilia.