Enhanced Factor IX Activity following Administration of AAV5-R338L "Padua" Factor IX versus AAV5 WT Human Factor IX in NHPs

Correlation of antigen expression with epigenetic modifications after rAAV delivery of a human factor IX variant in mice and rhesus macaques

We investigated long-term human coagulation factor IX (huFIX) expression of a novel variant when delivered into mice and rhesus macaques and compared transduction efficiencies using two different adeno-associated virus (AAV) capsids. In hemophilic mice injected with KP1-packaged recombinant AAV (rAAV) expressing the hyperactive FIX variant specific activity plasma levels were 10-fold or 2-fold enhanced when compared with wild-type or Padua huFIX injected mice, respectively. In rhesus macaques AAV-LK03 capsid outperformed AAV-KP1 in terms of antigen expression and liver transduction. Two animals from each group showed sustained low-level huFIX expression at 3 months after administration, while one animal from each group lost huFIX mRNA and protein expression over time, despite comparable vector copies. We investigated whether epigenetic differences in the vector episomes could explain this loss of transcription. Cut&Tag analysis revealed lower levels of activating histone marks in the two animals that lost expression. When comparing rAAV genome associated histone modifications in rhesus macaques with those in mice injected with the same vector, the activating histone marks were starkly decreased in macaque-derived episomes. Differential epigenetic marking of AAV genomes may explain different expression profiles in mice and rhesus macaques, as well as the wide dose response variation observed in primates in both preclinical and human clinical trials.

Hemophilia B and gene therapy: a new chapter with etranacogene dezaparvovec

ABSTRACT

The US Food and Drug Administration (FDA)'s authorization of etranacogene dezaparvovec (Hemgenix) is a significant milestone, constituting not only the first FDA approval of a gene therapy for hemophilia but also the first approval of a liver-targeted adeno-associated virus vector gene therapy. This review summarizes the nonclinical studies and clinical development that supported regulatory clearance. Similar to other gene therapies for single gene disorders, both the short-term safety and the phenotypic improvement were unequivocal, justifying the modest-sized safety and efficacy database, which included 57 participants across the phase 2b (3 participants) and phase 3 (54 participants) studies. The most common adverse reactions included liver enzyme elevation, headache, flu-like symptoms, infusion-related reactions, creatine kinase elevation, malaise, and fatigue; these were mostly transient. One participant had hepatocellular carcinoma on a study-mandated liver ultrasound conducted 1 year after vector infusion; molecular analysis of the resected tumor showed no evidence of vector-related insertional mutagenesis as the etiology. A remarkable 96% of participants in the phase 3 trial were able to stop factor IX (FIX) prophylaxis, with the study demonstrating noninferiority to FIX prophylaxis in terms of the primary end point, annualized bleeding rate. Key secondary end points such as the annualized infusion rate, which declined by 97%, and the plasma FIX activity level at 18 months after infusion, with least squares mean increase of 34.3 percentage points compared with baseline, were both clinically and statistically significant. The FDA's landmark approval of Hemgenix as a pioneering treatment for hemophilia stands on the shoulders of >20 years of gene therapy clinical research and heralds a promising future for genomic medicines.

AAV mediated gene therapy for haemophilia B: From the early attempts to modern trials

Early gene therapy clinical trials for the treatment of Haemophilia B have been instrumental to our global understanding of gene therapy and have significantly contributed to the rapid expansion of the field. The use of adeno-associated viruses (AAVs) as vectors for gene transfer has successfully led to therapeutic expression of coagulation factor IX (FIX) in severe haemophilia B patients. Expression of FIX has remained stable following a single administration of vector for up to 8 years at levels that are clinically relevant to reduce the incidence of spontaneous bleeds and have permitted a significant change in the disease management with reduction or elimination of the need for coagulation factor concentrates. These trials have also shed light on several concerns around AAV-mediated gene transfer such as the high prevalence of pre-existing immunity against the vector capsid as well as the elevation of liver transaminases that is associated with a loss of FIX transgene expression in some patients. However, this field is advancing very rapidly with the development of increasingly more efficient strategies to overcome some of these obstacles and importantly raise the possibility of a functional cure, which has been long sought after. This review overviews the evolution of gene therapy for haemophilia B over the last two decades.

Integration and the risk of liver cancer-Is there a real risk?

Adeno-associated virus (AAV)-based gene therapies are in clinical development for haemophilia and other genetic diseases. Since the recombinant AAV genome primarily remains episomal, it provides the opportunity for long-term expression in tissues that are not proliferating and reduces the safety concerns compared with integrating viral vectors. However, AAV integration events are detected at a low frequency. Preclinical studies in mouse models have reported hepatocellular carcinoma (HCC) after systemic AAV administration in some settings, though this has not been reported in large animal models. The risk of HCC or other cancers after AAV gene therapy in clinical studies thus remains theoretical. Potential risk factors for HCC after gene therapy are beginning to be elucidated through animal studies, but their relevance to human studies remains unknown. Studies to investigate the factors that may influence the risk of oncogenesis as well as detailed investigation of cases of cancer in AAV gene therapy patients will be important to define the potential risk of AAV genotoxicity.

Hemophilia Healing with AAV: Navigating the Frontier of Gene Therapy

Gene therapy for hemophilia has advanced tremendously after thirty years of continual study and development. Advancements in medical science have facilitated attaining normal levels of Factor VIII (FVIII) or Factor IX (FIX) in individuals with haemophilia, thereby offering the potential for their complete recovery. Despite the notable advancements in various countries, there is significant scope for further enhancement in haemophilia gene therapy. Adeno-associated virus (AAV) currently serves as the primary vehicle for gene therapy in clinical trials targeting haemophilia. Subsequent investigations will prioritize enhancing viral capsid structures, transgene compositions, and promoters to achieve heightened transduction efficacy, diminished immunogenicity, and more predictable therapeutic results. The present study indicates that whereas animal models have transduction efficiency that is over 100% high, human hepatocytes are unable to express clotting factors and transduction efficiency to comparable levels. According to the current study, achieving high transduction efficiency and high levels of clotting factor expression in human hepatocytes is still insufficient. It is also crucial to reduce the risk of cellular stress caused by protein overload. Despite encountering various hurdles, the field of haemophilia gene therapy holds promise for the future. As technology continues to advance and mature, it is anticipated that a personalized therapeutic approach will be developed to cure haemophilia effectively.

Stable and durable factor IX levels in patients with hemophilia B over 3 years after etranacogene dezaparvovec gene therapy

Etranacogene dezaparvovec (AMT-061) is a recombinant adeno-associated virus serotype 5 (AAV5) vector containing a codon-optimized Padua variant human factor IX (FIX) transgene with a liver-specific promoter. Here, we report 3-year outcomes from a phase 2b, open-label, single-dose, single-arm, multicenter trial conducted among adults with severe or moderately severe hemophilia B (FIX ≤2%). All participants (n = 3) received a single intravenous dose (2 × 1013 gene copies per kg) and will be followed up for 5 years. The primary end point of FIX activity ≥5% at 6 weeks was met. Secondary end points included bleed frequency, FIX concentrate use, joint health, and adverse events (AEs). All participants required routine FIX prophylaxis and had neutralizing antibodies to AAV5 before etranacogene dezaparvovec treatment. After administration, FIX activity rose to a mean of 40.8% in year 1 and was sustained in year 3 at 36.9%. All participants discontinued FIX prophylaxis. Bleeding was completely eliminated in 2 out of 3 participants. One participant required on-demand FIX replacement therapy per protocol because of elective surgical procedures, for 2 reported bleeding episodes, and twice for a single self-administered infusion because of an unreported reason. One participant experienced 2 mild, self-limiting AEs shortly after dosing. During the 3-year study period, there were no clinically significant elevations in liver enzymes, no requirement for steroids, no FIX inhibitor development, and no late-emergent safety events in any participant. Etranacogene dezaparvovec was safe and effective in adults with hemophilia B over 3 years after administration. This trial was registered at www.clinicaltrials.gov as #NCT03489291.

Etranacogene dezaparvovec-drlb gene therapy for patients with hemophilia B (congenital factor IX deficiency)

INTRODUCTION

Congenital hemophilia B (HB) is an X-linked bleeding disorder resulting in Factor IX (FIX) deficiency and bleeding of variable severity. There is no cure for HB. Typical management consists of prophylactic intravenous (IV) recombinant or plasma-derived FIX infusions. Etranacogene dezaparvovec-drlb (Hemgenix, AMT-061) is an adeno-associated virus serotype 5 (AAV5) vector containing a codon-optimized Padua variant of the human F9 gene with a liver-specific promoter. Etranacogene dezaparvovec-drlb received FDA approval on 22 November 2022 for the treatment of HB in adult patients who use FIX prophylaxis therapy, have current or historical life-threatening hemorrhage, or have experienced repeated, serious spontaneous bleeding episodes.

AREAS COVERED

This drug profile discusses the safety and efficacy of etranacogene dezaparvovec-drlb in patients with HB.

EXPERT OPINION

Etranacogene dezaparvovec-drlb therapy results in stable and sustained expression of near-normal to normal FIX levels in patients with HB regardless of neutralizing antibodies to AAV5 up to a titer of 678. Its use has led to significant reduction in bleeding and FIX prophylaxis. Etranacogene dezaparvovec-drlb was well tolerated; however, 17% of patients required corticosteroid therapy for alanine aminotransferase (ALT) elevation. Etranacogene dezaparvovec-drlb therapy marks the beginning of an exciting era in HB treatment and opens questions regarding treatment longevity and long-term safety.

Etranacogene dezaparvovec for the treatment of adult patients with severe and moderately severe hemophilia B

INTRODUCTION

Etranacogene dezaparvovec is the first gene therapy approved for treatment of adults with severe and moderately severe hemophilia B.

AREAS COVERED

This review describes the results of the clinical trial program of AMT-060 and etranacogene dezaparvovec, outlining the pharmacokinetic, clinical efficacy and safety data. With the entry of etranacogene dezaparvovec into the market, this review summarizes the treatment landscape in hemophilia B and discusses the current unknowns in the field.

EXPERT OPINION

Gene therapy appears to be a feasible option for adults with severe and moderately severe hemophilia B. Etranacogene dezaparvovec enables most patients to reach stable factor IX (FIX) levels after a single intravenous infusion, eliminating the need for regular prophylaxis; thus, drastically reducing treatment burden and avoiding variable bleeding risk owing to fluctuating FIX activity levels. Efficacy of etranacogene dezaparvovec has been demonstrated even in the presence of preexisting neutralizing antibodies (up to a titer of 1:678), with a relative low risk of transaminitis and its associated potential loss of transgene expression. However, long-term data are required to ascertain the durability of FIX levels achieved and safety. The cost-effectiveness and adoption of innovative payment models for reimbursement are key in choosing gene therapy over existing treatments.

Etranacogene Dezaparvovec: First Approval

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.

Gene Therapy with Etranacogene Dezaparvovec for Hemophilia B

BACKGROUND

Moderate-to-severe hemophilia B is treated with lifelong, continuous coagulation factor IX replacement to prevent bleeding. Gene therapy for hemophilia B aims to establish sustained factor IX activity, thereby protecting against bleeding without burdensome factor IX replacement.

METHODS

In this open-label, phase 3 study, after a lead-in period (≥6 months) of factor IX prophylaxis, we administered one infusion of adeno-associated virus 5 (AAV5) vector expressing the Padua factor IX variant (etranacogene dezaparvovec; 2×10¹³ genome copies per kilogram of body weight) to 54 men with hemophilia B (factor IX activity ≤2% of the normal value) regardless of preexisting AAV5 neutralizing antibodies. The primary end point was the annualized bleeding rate, evaluated in a noninferiority analysis comparing the rate during months 7 through 18 after etranacogene dezaparvovec treatment with the rate during the lead-in period. Noninferiority of etranacogene dezaparvovec was defined as an upper limit of the two-sided 95% Wald confidence interval of the annualized bleeding rate ratio that was less than the noninferiority margin of 1.8. Superiority, additional efficacy measures, and safety were also assessed.

RESULTS

The annualized bleeding rate decreased from 4.19 (95% confidence interval [CI], 3.22 to 5.45) during the lead-in period to 1.51 (95% CI, 0.81 to 2.82) during months 7 through 18 after treatment, for a rate ratio of 0.36 (95% Wald CI, 0.20 to 0.64; P<0.001), demonstrating noninferiority and superiority of etranacogene dezaparvovec as compared with factor IX prophylaxis. Factor IX activity had increased from baseline by a least-squares mean of 36.2 percentage points (95% CI, 31.4 to 41.0) at 6 months and 34.3 percentage points (95% CI, 29.5 to 39.1) at 18 months after treatment, and usage of factor IX concentrate decreased by a mean of 248,825 IU per year per participant in the post-treatment period (P<0.001 for all three comparisons). Benefits and safety were observed in participants with predose AAV5 neutralizing antibody titers of less than 700. No treatment-related serious adverse events occurred.

CONCLUSIONS

Etranacogene dezaparvovec gene therapy was superior to prophylactic factor IX with respect to the annualized bleeding rate, and it had a favorable safety profile. (Funded by uniQure and CSL Behring; HOPE-B ClinicalTrials.gov number, NCT03569891.).

Gene Therapy in Combination with Nitrogen Scavenger Pretreatment Corrects Biochemical and Behavioral Abnormalities of Infant Citrullinemia Type 1 Mice

Citrullinemia type I (CTLN1) is a rare autosomal recessive disorder caused by mutations in the gene encoding argininosuccinate synthetase 1 (ASS1) that catalyzes the third step of the urea cycle. CTLN1 patients suffer from impaired elimination of nitrogen, which leads to neurotoxic levels of circulating ammonia and urea cycle byproducts that may cause severe metabolic encephalopathy, death or irreversible brain damage. Standard of care (SOC) of CTLN1 consists of daily nitrogen-scavenger administration, but patients remain at risk of life-threatening decompensations. We evaluated the therapeutic efficacy of a recombinant adeno-associated viral vector carrying the ASS1 gene under the control of a liver-specific promoter (VTX-804). When administered to three-week-old CTLN1 mice, all the animals receiving VTX-804 in combination with SOC gained body weight normally, presented with a normalization of ammonia and reduction of citrulline levels in circulation, and 100% survived for 7 months. Similar to what has been observed in CTLN1 patients, CTLN1 mice showed several behavioral abnormalities such as anxiety, reduced welfare and impairment of innate behavior. Importantly, all clinical alterations were notably improved after treatment with VTX-804. This study demonstrates the potential of VTX-804 gene therapy for future clinical translation to CTLN1 patients.

Phase 1-2 Trial of AAVS3 Gene Therapy in Patients with Hemophilia B

BACKGROUND

FLT180a (verbrinacogene setparvovec) is a liver-directed adeno-associated virus (AAV) gene therapy that uses a synthetic capsid and a gain-of-function protein to normalize factor IX levels in patients with hemophilia B.

METHODS

In this multicenter, open-label, phase 1-2 trial, we assessed the safety and efficacy of varying doses of FLT180a in patients with severe or moderately severe hemophilia B (factor IX level, ≤2% of normal value). All the patients received glucocorticoids with or without tacrolimus for immunosuppression to decrease the risk of vector-related immune responses. After 26 weeks, patients were enrolled in a long-term follow-up study. The primary end points were safety and efficacy, as assessed by factor IX levels at week 26.

RESULTS

Ten patients received one of four FLT180a doses of vector genomes (vg) per kilogram of body weight: 3.84×10¹¹ vg, 6.40×10¹¹ vg, 8.32×10¹¹ vg, or 1.28×10¹² vg. After receiving the infusion, all the patients had dose-dependent increases in factor IX levels. At a median follow-up of 27.2 months (range, 19.1 to 42.4), sustained factor IX activity was observed in all the patients except one, who resumed factor IX prophylaxis. As of the data-cutoff date (September 20, 2021), five patients had normal factor IX levels (range, 51 to 78%), three patients had levels from 23 to 43%, and one had a level of 260%. Of the reported adverse events, approximately 10% were related to FLT180a and 24% to immunosuppression. Increases in liver aminotransferase levels were the most common FLT180a-related adverse events. Late increases in aminotransferase levels occurred in patients who had received prolonged tacrolimus beyond the glucocorticoid taper. A serious adverse event of arteriovenous fistula thrombosis occurred in the patient with high factor IX levels.

CONCLUSIONS

Sustained factor IX levels in the normal range were observed with low doses of FLT180a but necessitated immunosuppression with glucocorticoids with or without tacrolimus. (Funded by Freeline Therapeutics; ClinicalTrials.gov numbers, NCT03369444 and NCT03641703; EudraCT numbers, 2017-000852-24 and 2017-005080-40.).

Dexamethasone Transiently Enhances Transgene Expression in the Liver When Administered at Late-Phase Post Long-Term Adeno-Associated Virus Transduction

Glucocorticoids have anti-inflammatory and immunosuppressive functions and have commonly been used for preventing liver toxicity after the systemic application of a high dose of adeno-associated virus (AAV) vector for gene therapy. Clinical studies have reported that glucocorticoids have rescued factor IX (FIX) expression in patients with hemophilia B who showed a reduced FIX expression at 6 to 10 weeks post-AAV vector administration. In this study, we explored whether glucocorticoids could affect transgene expression in AAV targeted livers in animal models. When dexamethasone was applied before AAV9/FIX vector administration in the wild-type C57BL/6 mice, FIX expression was much higher than that of the control mice at any time point. More importantly, FIX expression transiently increased after dexamethasone was administered at week 6 or later post-AAV injection regardless of the various dexamethasone treatments applied. The transient enhancement in transgene expression was observed once there were one to several consecutive dexamethasone treatments completed. A similar result was also achieved in other wild-type BALB/c and hemophilia B mice that were treated with AAV9/FIX and dexamethasone. This mechanism study demonstrated that the administration of dexamethasone did not change either AAV genome copy number or transgene expression at the transcription level but transiently decreased interferon beta (IFN-β) and tumor necrosis factor alpha (TNF-α) expression in the livers of mice at a later time after AAV injection. Next, we studied the effect of dexamethasone on late transgene expression in hemophilia B dogs. Dexamethasone was administered 1 year after AAV9/FIX injection. Inconsistent with the results in mice, no significant change of FIX expression was observed in hemophilia B dogs. In summary, the results from this study indicate that dexamethasone may have various effects on transgene expression in AAV-transduced livers in different species, which provides valuable information about the rational application of dexamethasone in future clinical studies.

The Perspective of DMPK on Recombinant Adeno-Associated Virus-Based Gene Therapy: Past Learning, Current Support, and Future Contribution

Given the recent success of gene therapy modalities and the growing number of cell and gene-based therapies in clinical development across many different therapeutic areas, it is evident that this evolving field holds great promise for the unmet medical needs of patients. The recent approvals of Luxturna® and Zolgensma® prove that recombinant adeno-associated virus (rAAV)-based gene therapy is a transformative modality that enables curative treatment for genetic disorders. Over the last decade, Takeda has accumulated significant experience with rAAV-based gene therapies, especially in the early stage of development. In this review, based on the learnings from Takeda and publicly available information, we aim to provide a guiding perspective on Drug Metabolism and Pharmacokinetics (DMPK) substantial role in advancing therapeutic gene therapy modalities from nonclinical research to clinical development, in particular the characterization of gene therapy product biodistribution, elimination (shedding), immunogenicity assessment, multiple platform bioanalytical assays, and first-in-human (FIH) dose projection strategies.

Fusion of engineered albumin with factor IX Padua extends half-life and improves coagulant activity

The short half‐life of coagulation factor IX (FIX) for haemophilia B (HB) therapy has been prolonged through fusion with human serum albumin (HSA), which drives the neonatal Fc receptor (FcRn)‐mediated recycling of the chimera. However, patients would greatly benefit from further FIX‐HSA half‐life extension. In the present study, we designed a FIX‐HSA variant through the engineering of both fusion partners. First, we developed a novel cleavable linker combining the two FIX activation sites, which resulted in improved HSA release. Second, insertion of the FIX R338L (Padua) substitution conferred hyperactive features (sevenfold higher specific activity) as for FIX Padua alone. Furthermore, we exploited an engineered HSA (QMP), which conferred enhanced human (h)FcRn binding [dissociation constant (K_D) 0·5 nM] over wild‐type FIX‐HSA (K_D 164·4 nM). In hFcRn transgenic mice, Padua‐QMP displayed a significantly prolonged half‐life (2·7 days, P < 0·0001) versus FIX‐HSA (1 day). Overall, we developed a novel FIX‐HSA protein with improved activity and extended half‐life. These combined properties may result in a prolonged functional profile above the therapeutic threshold, and thus in a potentially widened therapeutic window able to improve HB therapy. This rational engineering of both partners may pave the way for new fusion strategies for the design of engineered biotherapeutics.

Treatment of a Hemophilia B Mouse Model with Platelet-Targeted Expression of Factor IX Padua

Targeting the coagulation factor IX (FIX) expression in platelets has been shown to be effective in ameliorating bleeding in hemophilia B (HB) mice. To improve the therapeutic effects and evaluate the safety of this gene therapy strategy, we generated a transgenic mouse model on an HB background with FIX Padua target expressed in platelets. The transgenic mice exhibited stable expression and storage of FIX Padua in platelets. The platelet-stored FIX Padua could be released with the activation of platelets, and the proportion of platelet-stored FIX Padua in whole blood was the same as that of platelet-stored wild-type human FIX. The platelet-derived FIX Padua showed substantially increased specific activity compared with wild-type FIX. Reduced bleeding volume in the FIX Padua transgenic mice demonstrated that bleeding in the mice was improved. Levels of thrombin-antithrombin complex, fibrinogen, D-Dimer, and blood cell counts were normal in the transgenic mice, suggesting that thrombotic risk was not increased in this mouse model. However, the leakage and failure to overcome the presence of inhibitor to wild-type FIX is also observed with FIX Padua, as expected. Taken together, our results support the conclusion that targeting FIX Padua expression in platelets may be an effective and safe gene therapy strategy for HB, and could provide an ideal model to evaluate the safety of platelet-targeted gene therapy for treating hemophilia.

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.

CRISPR-Cas9-Mediated In Vivo Gene Integration at the Albumin Locus Recovers Hemostasis in Neonatal and Adult Hemophilia B Mice

Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 loaded by vectors could induce high rates of specific site genome editing and correct disease-causing mutations. However, most monogenic genetic diseases such as hemophilia are caused by different mutations dispersed in one gene, instead of an accordant mutation. Vectors developed for correcting specific mutations may not be suited to different mutations at other positions. Site-specific gene addition provides an ideal solution for long-term, stable gene therapy. We have demonstrated SaCas9-mediated homology-directed factor IX (FIX) in situ targeting for sustained treatment of hemophilia B. In this study, we tested a more efficient dual adeno-associated virus (AAV) strategy with lower vector dose for liver-directed genome editing that enables CRISPR-Cas9-mediated site-specific integration of therapeutic transgene within the albumin gene, and we aimed to develop a more universal gene-targeting approach. We successfully achieved coagulation function in newborn and adult hemophilia B mice by a single injection of dual AAV vectors. FIX levels in treated mice persisted even after a two-thirds partial hepatectomy, indicating stable gene integration. Our results suggest that this CRISPR-Cas9-mediated site-specific gene integration in hepatocytes could transform into a common clinical therapeutic method for hemophilia B and other genetic diseases.

Etranacogene dezaparvovec (AMT-061 phase 2b): normal/near normal FIX activity and bleed cessation in hemophilia B

Etranacogene dezaparvovec (AMT-061) is a recombinant AAV5 vector including a gene cassette containing the factor IX (FIX) Padua variant under the control of a liver-specific promoter. A phase 2b study was conducted to confirm that a single dose of 2 × 1013 genome copies per kilogram of etranacogene dezaparvovec will result in FIX activity ≥5% 6 weeks after dosing. Secondary end points included FIX activity at other time points, bleed frequency, FIX replacement, and safety. Etranacogene dezaparvovec was administered as a single IV infusion to 3 adults with severe to moderately severe hemophilia B. Before treatment, participants had low levels of preexisting neutralizing antibodies to AAV5. This article reports a planned 26-week interim assessment. At week 6, mean FIX activity was 31% (23.9%-37.8%), increasing to 47% (33.2%-57.0%) at 26 weeks, with 2 subjects exhibiting sustained activity >40%. Consistent with the FIX activity, etranacogene dezaparvovec was associated with a complete bleed cessation with no need for FIX replacement therapy up to 26 weeks. Etranacogene dezaparvovec was generally well tolerated. No clinically significant elevations in levels of liver enzymes or inflammatory markers were observed, and no use of corticosteroids related to treatment was required. In individuals with severe to moderately severe hemophilia B, etranacogene dezaparvovec resulted in clinically relevant increases in FIX activity, cessation of bleeds, and abrogation of the need for FIX replacement, despite the presence of preexisting anti-AAV5 neutralizing antibodies detected by using a highly sensitive luciferase assay. Consistency of results in the 3 participants supported an expanded evaluation of the safety/efficacy of etranacogene dezaparvovec in the HOPE-B (Health Outcomes With Padua Gene; Evaluation in Hemophilia-B) phase 3 trial. The current trial was registered at www.clinicaltrials.gov as #NCT03489291.

Timing of Intensive Immunosuppression Impacts Risk of Transgene Antibodies after AAV Gene Therapy in Nonhuman Primates

Adeno-associated virus (AAV) vector gene therapy is a promising treatment for a variety of genetic diseases, including hemophilia. Systemic administration of AAV vectors is associated with a cytotoxic immune response triggered against AAV capsid proteins, which if untreated can result in loss of transgene expression. Immunosuppression (IS) with corticosteroids has limited transgene loss in some AAV gene therapy clinical trials, but was insufficient to prevent loss in other studies. We used a nonhuman primate model to evaluate intensive T cell-directed IS combined with AAV-mediated transfer of the human factor IX (FIX) gene. Early administration of rabbit anti-thymocyte globulin (ATG) concomitant with AAV administration resulted in the development of anti-FIX antibodies, whereas delayed ATG by 5 weeks administration did not. The anti-FIX immune response was associated with increases in inflammatory cytokines, as well as a skewed Th17/regulatory T cell (Treg) ratio. We conclude that the timing of T cell-directed IS is critical in determining transgene-product immunogenicity or tolerance. These data have implications for systemically administered AAV gene therapy being evaluated for hemophilia A and B, as well as other genetic diseases.

Long-Term Follow-Up of the First in Human Intravascular Delivery of AAV for Gene Transfer: AAV2-hFIX16 for Severe Hemophilia B

Adeno-associated virus (AAV) vectors are a leading platform for gene-based therapies for both monogenic and complex acquired disorders. The success of AAV gene transfer highlights the need to answer outstanding clinical questions of safety, durability, and the nature of the human immune response to AAV vectors. Here, we present longitudinal follow-up data of subjects who participated in the first trial of a systemically delivered AAV vector. Adult males (n = 7) with severe hemophilia B received an AAV2 vector at doses ranging from 8 × 10¹⁰ to 2 × 10¹² vg/kg to target hepatocyte-specific expression of coagulation factor IX; a subset (n = 4) was followed for 12-15 years post-vector administration. No major safety concerns were observed. There was no evidence of sustained hepatic toxicity or development of hepatocellular carcinoma as assessed by liver transaminase values, serum α-fetoprotein, and liver ultrasound. Subjects demonstrated persistent, increased AAV neutralizing antibodies (NAbs) to the infused AAV serotype 2 (AAV2) as well as all other AAV serotypes tested (AAV5 and AAV8) for the duration of follow-up. These data represent the longest available longitudinal follow-up data of subjects who received intravascular AAV and support the preliminary safety of intravascular AAV administration at the doses tested in adults. Data demonstrate, for the first time, the persistence of high-titer, multi-serotype cross-reactive AAV NAbs for up to 15 years post- AAV vector administration. Our observations are broadly applicable to the development of AAV-mediated gene therapy.