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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.).

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.

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.

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 . . .

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.

Safe and efficient delivery of messenger RNAs for protein replacement therapies offers great promise but remains challenging. In this report, we demonstrate systemic, in vivo, nonviral mRNA delivery through lipid nanoparticles (LNPs) to treat a Factor IX (FIX)-deficient mouse model of hemophilia B. Delivery of human FIX (hFIX)mRNA encapsulated in our LUNAR LNPs results in a rapid pulse of FIX protein (within 4–6 h) that remains stable for up to 4–6 d and is therapeutically effective, like the recombinant human factor IX protein (rhFIX) that is the current standard of care. Extensive cytokine and liver enzyme profiling showed that repeated administration of the mRNA–LUNAR complex does not cause any adverse innate or adaptive immune responses in immune-competent, hemophilic mice. The levels of hFIX protein that were produced also remained consistent during repeated administrations. These results suggest that delivery of long mRNAs is a viable therapeutic alternative for many clotting disorders and for other hepatic diseases where recombinant proteins may be unaffordable or unsuitable.

Using techniques to target the liver, enhance gene expression, and avoid eliciting an immune response, the authors administered an adenovirus-associated virus containing the gene for factor IX (FIX) in six patients with hemophilia B. The result was either reduction or elimination of the need for FIX prophylaxis. Hemophilia B is an X-linked bleeding disorder that results from a defect in the gene encoding coagulation factor IX (FIX), a serine protease that is critical for blood clotting. Persons with severe hemophilia B have functional FIX levels that are less than 1% of normal values and have frequent bleeding episodes, which are associated with crippling arthropathy and early death. 1 , 2 Current treatment involves frequent intravenous injections of FIX protein concentrate (i.e., two to three times a week). However, this treatment is prophylactic rather than curative, is extremely expensive, and is associated with inhibitor formation. Somatic gene therapy for hemophilia . . .

In a long-term follow-up study, a single dose of fidanacogene elaparvovec was found to lead to sustained factor IX expression in liver cells and dramatically lowered annualized bleeding rates in 14 patients with hemophilia B.