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Fidanacogene elaparvovec, an adeno-associated virus (AAV) gene-therapy vector for hemophilia B containing a high-activity human factor IX variant (FIX-R338L/FIX-Padua), was associated with sustained factor IX activity in a phase 1-2a study. We conducted a phase 3 open-label study of fidanacogene elaparvovec at a dose of 5×10 vector genome copies per kilogram of body weight. Men 18 to 65 years of age with hemophilia B and a factor IX level of 2% or less were eligible for screening if they had received at least 6 months of therapy with prophylactic factor IX concentrate. The primary end point, tested for noninferiority, was the annualized bleeding rate (treated and untreated bleeding episodes) from week 12 to month 15 after treatment with fidanacogene elaparvovec as compared with the prophylaxis lead-in period. Superiority, additional efficacy end points, and safety were also assessed. Of 316 men who underwent screening for the lead-in study, 204 (64.6%) were not eligible; 188 (59.5%) of those were ineligible owing to the presence of anti-AAV neutralizing antibodies. Of the 45 participants who received fidanacogene elaparvovec, 44 completed at least 15 months of follow-up. The annualized rate of bleeding for all bleeding episodes decreased by 71%, from 4.42 (95% confidence interval [CI], 1.80 to 7.05) at baseline to 1.28 (95% CI, 0.57 to 1.98) after gene therapy, a treatment difference of -3.15 episodes (95% CI, -5.46 to -0.83; P = 0.008). This result shows the noninferiority and superiority of fidanacogene elaparvovec to prophylaxis. At 15 months, the mean factor IX activity was 26.9% (median, 22.9%; range, 1.9 to 119.0) by one-stage SynthASil assay. A total of 28 participants (62%) received glucocorticoids for increased aminotransferase levels or decreased factor IX levels (or both) starting between 11 and 123 days. No infusion-related serious adverse events, thrombotic events, development of factor IX inhibitors, or malignant conditions were observed. Fidanacogene elaparvovec was superior to prophylaxis for the treatment of participants with hemophilia B, leading to reduced bleeding and stable factor IX expression. (Funded by Pfizer; BENEGENE-2 ClinicalTrials.gov number, NCT03861273.).

A fusion protein comprising factor IX and the dimeric Fc domain of IgG1 has a half-life that is five times as long as that of native factor IX, allowing prophylactic injections to be spaced as far as 2 weeks apart while maintaining levels of factor IX that are sufficient to prevent bleeding. In patients with severe hemophilia B, recurrent bleeding leads to painful hemarthroses, disabling hemophilic arthropathy, and other sequelae. 1 , 2 Prophylactic replacement of coagulation factor IX is associated with improved clinical outcomes 3 – 7 ; however, the relatively short half-lives of currently available factor IX products necessitate frequent intravenous injections (two or three times weekly) to maintain protective levels (at or above 1 IU per deciliter). 8 , 9 The frequency of injections is a considerable burden, cited by patients as a key deterrent to undertaking prophylactic treatment. 10 Various strategies to reduce this burden and improve the treatment of hemophilia B are under investigation, . . .

An AAV5 vector containing the factor IX Padua allele was administered to 54 men with hemophilia B. Factor IX expression increased to approximately 39% of normal, and the annualized bleeding rate was decreased to 1.5.

An adeno-associated viral vector was used to introduce a FIX gene with enhanced biologic activity in 10 participants with hemophilia B. The annualized bleeding rate was 11.1 events per year before therapy versus 0.4 afterward. Steady-state factor IX levels were 33.7% of normal.

Adeno-associated virus (AAV)-mediated gene therapy has emerged as a promising treatment for hemophilia B. Data on safety and durability from 13 years of follow-up in a cohort of patients who had been successfully treated with scAAV2/8-LP1-hFIXco gene therapy are now available. Ten men with severe hemophilia B received a single intravenous infusion of the scAAV2/8-LP1-hFIXco vector in one of three dose groups (low-dose: 2×10 vector genomes [vg] per kilogram of body weight [in two participants]; intermediate-dose: 6×10 vg per kilogram [in two]; or high-dose: 2×10 vg per kilogram [in six]). Efficacy outcomes included factor IX activity, the annualized bleeding rate, and factor IX concentrate use. Safety assessments included clinical events, liver function, and imaging. Participants were followed for a median of 13.0 years (range, 11.1 to 13.8). Factor IX activity remained stable across the dose cohorts, with mean factor IX levels of 1.7 IU per deciliter in the low-dose group, 2.3 IU per deciliter in the intermediate-dose group, and 4.8 IU per deciliter in the high-dose group. Seven of the 10 participants did not receive prophylaxis. The median annualized bleeding rate decreased from 14.0 episodes (interquartile range, 12.0 to 21.5) to 1.5 episodes (interquartile range, 0.7 to 2.2), which represented a reduction by a factor of 9.7. Use of factor IX concentrate decreased by a factor of 12.4 (interquartile range, 2.2 to 27.1). A total of 15 vector-related adverse events occurred, primarily transient elevations in aminotransferase levels. Factor IX inhibitor, thrombosis, or chronic liver injury did not develop in any participant. Two cancers were identified but were deemed by the investigators, together with an expert multidisciplinary team, as being unrelated to the vector. A liver biopsy that was conducted in 1 participant 10 years after the infusion revealed transcriptionally active transgene expression in hepatocytes without fibrosis or dysplasia. Levels of neutralizing antibodies to AAV8 remained high throughout follow-up, thus indicating potential barriers to readministration of the vector. A single administration of scAAV2/8-LP1-hFIXco gene therapy resulted in durable factor IX expression, sustained clinical benefit, and no late-onset safety concerns over a period of 13 years. These data support the long-term efficacy and safety of AAV gene therapy for severe hemophilia B. (Funded by the U.K. Medical Research Council and others; ClinicalTrials.gov number, NCT00979238; EudraCT number, 2005-005711-17.).

Etranacogene dezaparvovec (etranacogene dezaparvovec-drlb; Hemgenix ® ) is an adeno-associated virus vector-based gene therapy being developed by uniQure and CSL Behring for the treatment of haemophilia B. In November 2022, etranacogene dezaparvovec was approved in the USA for the treatment of haemophilia B [congenital factor IX (FIX) deficiency] in adults who are currently using FIX prophylaxis therapy, have current or historical life-threatening haemorrhage or have repeated, serious spontaneous bleeding episodes. In December 2022, etranacogene dezaparvovec also received positive opinion in the EU for the treatment of haemophilia B. This article summarizes the milestones in the development of etranacogene dezaparvovec leading to this first approval.

Emicizumab is a humanized bispecific antibody that mimics the cofactor function of factor VIII. In a dose-escalation study in Japanese persons with hemophilia A, including those with factor VIII inhibitors, emicizumab markedly reduced the number of bleeding episodes. Hemophilia A is a serious bleeding disorder caused by a deficiency of clotting factor VIII. Approximately 50% of patients have severe hemophilia A, 1 defined as less than 1% residual factor VIII activity (<1 IU per deciliter). 2 Such patients have severe bleeding from early childhood, and without appropriate treatment, recurrent bleeding into joints can lead to irreversible hemoarthropathy. 3 , 4 Standard treatment for hemophilia A includes regular prophylaxis and episodic treatment with recombinant or plasma-derived factor VIII. The goals of prophylaxis with factor VIII are to increase factor VIII activity to at least a moderate level (1 to 5 IU per deciliter) . . .

Nine of 10 patients with hemophilia B who received a liver-directed adeno-associated viral vector containing a hyperactive variant of factor IX maintained factor IX activity for up to 42 months. Thrombosis developed in 1 patient.

Prophylactic treatment for hemophilia B necessitates lifelong, regular intravenous factor IX infusions. Gene therapy offers the possibility of a single-dose treatment that produces durable endogenous factor IX expression and disease control. Etranacogene dezaparvovec comprises an adeno-associated virus serotype 5 (AAV5) vector and the highly active Padua factor IX variant. The primary analysis of this study showed that etranacogene dezaparvovec reduced annualized bleeding rates and adverse events were mainly of low-grade severity. Final data from 5 years of follow-up are now available. In this open-label, phase 3 study, after a lead-in period (≥6 months) of factor IX prophylaxis, we administered a single infusion of etranacogene dezaparvovec in men with hemophilia B (factor IX activity level, ≤2 IU per deciliter), regardless of preexisting AAV5 neutralizing antibodies. The prespecified 5-year analyses included adjusted annualized bleeding rates (difference between the post-treatment period of months 7 through 60 after gene therapy and the lead-in period), factor IX expression, and safety outcomes. In the full analysis population (54 participants), the adjusted annualized bleeding rate for all bleeding events was 4.16 during the lead-in period and 1.52 during months 7 through 60 after gene therapy, a reduction of 63% (95% confidence interval, 24 to 82). During the 5-year follow-up period, endogenous factor IX expression remained stable; at 5 years, the mean (±SD) factor IX activity level was 36.1±15.7 IU per deciliter and the mean exogenous factor IX consumption for routine prophylaxis and treatment of bleeding events had decreased by 96%, from 257,339 IU per year during the lead-in period to 10,924 IU per year during months 7 through 60 after gene therapy. Efficacy did not differ substantially between participants with and those without AAV5 neutralizing antibodies at baseline. Adverse events that were possibly related to treatment were rare after month 6. Sustained endogenous factor IX expression and low annualized bleeding rates over a 5-year period were observed after an infusion of etranacogene dezaparvovec. (Funded by uniQure and CSL Behring; HOPE-B ClinicalTrials.gov number, NCT03569891.).

In this long-term follow-up study, men with hemophilia B had steady production of functional factor IX after gene transfer with an adeno-associated viral vector. Patients had a marked reduction in bleeding episodes and factor IX replacement with minimal toxicity. Hemophilia B, an X-linked recessive bleeding disorder, results from a defect in the gene encoding coagulation factor IX, a serine protease that is critical for blood clotting. Patients with functional plasma levels of factor IX that are less than 1% of the normal value (1 IU per deciliter) have a severe phenotype characterized by frequent spontaneous bleeding episodes that result in chronic, debilitating arthropathy and occasionally death. 1 Current treatment to prevent these bleeding episodes entails lifelong intravenous injections of factor IX every 2 or 3 days. Although this treatment is effective in preventing spontaneous bleeding episodes, it is not curative . . .