Efficacy and safety of long-term prophylaxis in severe hemophilia A dogs following liver gene therapy using AAV vectors

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.

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.

Gene therapy for hemophilia, a clinical viewpoint

Gene therapy for hemophilia has been investigated for decades but no breakthroughs were made until Nathwani et al. achieved a significant and sustainable factor IX increase in hemophilia B patients in 2011. About eleven years later, in August 2022, the first hemophilia A gene therapy product was approved by the European Commission and hemophilia treatment entered a new era. This review does not focus on the newest advances but rather the practical aspects of gene therapy aiming to provide an overview for physicians who treat hemophiliacs who did not participate in the clinical trials. The current status of gene therapy, focusing particularly on products likely to be clinically available soon, are reviewed and summarized. Currently, possible limitations of gene therapy are pre-existing neutralizing antibodies toward the vector, liver health, age, and inhibitor status. Possible safety concerns include infusion reactions, liver damage, and adverse effects from immune suppressants or steroids. In summary, generally speaking, gene therapy is effective, at least for several years, but the exact effect may be unpredictable and intensive monitoring for several months is needed. It can also be considered safe with careful practice on selected patients. In its current form, gene therapy will not replace all hemophilia treatments. Advances in non-factor therapy will also improve hemophilia care greatly in the future. We envisage that gene therapy may be included in multiple novel therapies for hemophilia and benefit some hemophilia patients while novel non-factor therapies may benefit others, together fulfilling the unmet needs of all hemophilia patients.

Restoring Ciliary Function: Gene Therapeutics for Primary Ciliary Dyskinesia

Primary ciliary dyskinesia (PCD) is a genetic disease characterized by defects in motile cilia, which play an important role in several organ systems. Lung disease is a hallmark of PCD, given the essential role of cilia in airway surface defense. Diagnosis of PCD is complicated due to its reliance on complex tests that are not utilized by every clinic and also its phenotypic overlap with several other respiratory diseases. Nonetheless, PCD is increasingly being recognized as more common than once thought. The disease is genetically complex, with several genes reported to be associated with PCD. There is no cure for PCD, but gene therapy remains a promising therapeutic strategy. In this review, we provide an overview of the clinical symptoms, diagnosis, genetics, and current treatment regimens for PCD. We also describe PCD model systems and discuss the therapeutic potential of different gene therapeutics for targeting the intended cellular target, the ciliated cells of the airway.

Progress, and prospects in the therapeutic armamentarium of persons with congenital hemophilia. Defining the place for liver-directed gene therapy

In persons with congenital severe hemophilia A (HA) living in high-income countries, twice weekly intravenous infusions of extended half-life (EHL) factor VIII (FVIII) products, or weekly/biweekly/monthly subcutaneous injections of emicizumab are the gold standard home treatments to grant days without hurdles and limitations. Once weekly/twice monthly infusions of EHL Factor IX (FIX) products achieve the same target in severe hemophilia B (HB). Gene therapy, which is likely to be licensed for clinical use within 1-2 years, embodies a shift beyond these standards. At an individual patient level, a single functional gene transfer leads to a > 10-yr almost full correction of the hemostatic defect in HB and to a sustained (3-6-yrs) expression of FVIII sufficient to discontinue exogenous clotting factor administrations. At the doses employed, the limited liver toxicity of systemically infused recombinant adeno-associated virus (rAAV) vectors is documented by long-term (12-15 yrs) follow-ups, and pre-existing high-titer neutralizing antibodies to the AAV5 vector are no longer an exclusion criterion for effective transgene expression with this vector. A safe durable treatment that converts a challenging illness to a phenotypically curable disease, allows persons to feel virtually free from the fears and the obligations of hemophilia for years/decades. Along with patient organizations and health care professionals, communicating to government authorities and reimbursement agencies the liberating potential of this substantial innovation, and disseminating across the Centers updated information on benefits and risks of this strategy, will align expectations of different stakeholders and establish the notion of a potentially lifelong cure of hemophilia.

Multiplexing AAV Serotype-Specific Neutralizing Antibodies in Preclinical Animal Models and Humans

The accurate assessment of AAV-specific pre-existing humoral immunity due to natural viral infection is critical for the efficient use of clinical gene therapy. The method described in the present study applies equivalent infection conditions to each AAV serotype (AAV1, AAV2, AAV3, AAV5, AAV6, AAV7, AAV8, AAV9, AAV10, and AAVAnc80L65). In the current study, we validated the assay by assessing AAV-neutralizing antibody titers in a limited cohort of random human donors and well-established preclinical large animal models, including dogs and non-human primates (NHPs). We achieved a rapid and accurate evaluation of neutralizing titers for each individual subject that can be used for clinical enrollment based on specific AAV serotypes and individualized selection of the most suitable AAV serotype for each specific patient.

Gene therapy for hemophilia: looking beyond factor expression

Hemophilia A (factor VIII [FVIII] deficiency) and hemophilia B (factor IX [FIX] deficiency) are the X-linked recessive bleeding disorders that clinically manifest with recurrent bleeding, predominantly into muscles and joints. In its severe presentation, when factor activity is less than 1% of normal, hemophilia presents with spontaneous musculoskeletal bleeds and may progress to debilitating chronic arthropathy. Management of hemophilia has changed profoundly in the past decades. From on-demand to prophylactic factor concentrate replacement, the treatment goal shifted from controlling bleeds to preventing bleeds and improving quality of life. In this new scenario, gene therapy has arisen as a paradigm-changing therapeutic option, a one-time treatment with the potential to achieve sustained coagulation FVIII or FIX expression even within the normal range. This review discusses the critical impact of adeno-associated virus (AAV) gene transfer in hemophilia care, including the recent clinical outcomes, changes in disease perceptions, and its treatment burden. We also discuss the challenging scenario of the AAV-directed immune response in the clinical setting and potential strategies to improve the long-lasting efficacy of hemophilia gene therapy efficacy.

The Arrival of Gene Therapy for Patients with Hemophilia A

Historically, the standard of care for hemophilia A has been intravenous administration of exogenous factor VIII (FVIII), either as prophylaxis or episodically. The development of emicizumab, a humanized bispecific monoclonal antibody mimicking activated FVIII, was a subsequent advance in treatment. However, both exogenous FVIII and emicizumab require repeated and lifelong administration, negatively impacting patient quality of life. A recent breakthrough has been the development of gene therapy. This allows a single intravenous treatment that could result in long-term expression of FVIII, maintenance of steady-state plasma concentrations, and minimization (or possibly elimination) of bleeding episodes for the recipient's lifetime. Several gene therapies have been assessed in clinical trials, with positive outcomes. Valoctocogene roxaparvovec (an adeno-associated viral 5-based therapy encoding human B domain-deleted FVIII) is expected to be the first approved gene therapy in European countries, including Italy, in 2022. Some novel challenges exist including refining patient selection criteria, managing patient expectations, further elucidation of the durability and variability of transgene expression and long-term safety, and the development of standardized 'hub and spoke' centers to optimize and monitor this innovative treatment. Gene therapy represents a paradigm shift, and may become a new reference standard for treating patients with hemophilia A.

Single-shot AAV-vectored vaccine against SARS-CoV-2 with fast and long-lasting immunity

Due to the insufficient long-term protection and significant efficacy reduction to new variants of current COVID-19 vaccines, the epidemic prevention and control are still challenging. Here, we employ a capsid and antigen structure engineering (CASE) strategy to manufacture an adeno-associated viral serotype 6-based vaccine (S663V-RBD), which expresses trimeric receptor binding domain (RBD) of spike protein fused with a biological adjuvant RS09. Impressively, the engineered S663V-RBD could rapidly induce a satisfactory RBD-specific IgG titer within 2 weeks and maintain the titer for more than 4 months. Compared to the licensed BBIBP-CorV (Sinopharm, China), a single-dose S663V-RBD induced more endurable and robust immune responses in mice and elicited superior neutralizing antibodies against three typical SARS-CoV-2 pseudoviruses including wild type, C.37 (Lambda) and B.1.617.2 (Delta). More interestingly, the intramuscular injection of S663V-RBD could overcome pre-existing immunity against the capsid. Given its effectiveness, the CASE-based S663V-RBD may provide a new solution for the current and next pandemic.

Evaluating the state of the science for adeno-associated virus integration: An integrated perspective

On August 18, 2021, the American Society of Gene and Cell Therapy (ASGCT) hosted a virtual roundtable on adeno-associated virus (AAV) integration, featuring leading experts in preclinical and clinical AAV gene therapy, to further contextualize and understand this phenomenon. Recombinant AAV (rAAV) vectors are used to develop therapies for many conditions given their ability to transduce multiple cell types, resulting in long-term expression of transgenes. Although most rAAV DNA typically remains episomal, some rAAV DNA becomes integrated into genomic DNA at a low frequency, and rAAV insertional mutagenesis has been shown to lead to tumorigenesis in neonatal mice. Currently, the risk of rAAV-mediated oncogenesis in humans is theoretical because no confirmed genotoxic events have been reported to date. However, because insertional mutagenesis has been reported in a small number of murine studies, there is a need to characterize this genotoxicity to inform research, regulatory needs, and patient care. The purpose of this white paper is to review the evidence of rAAV-related host genome integration in animal models and possible risks of insertional mutagenesis in patients. In addition, technical considerations, regulatory guidance, and bioethics are discussed.

Pharmacokinetic analysis identifies a factor VIII immunogenicity threshold after AAV gene therapy in hemophilia A mice

Advances in the development of novel treatment options for hemophilia A are prevalent. However, the anti-factor VIII (FVIII) neutralizing antibody (inhibitor) response to existing FVIII products remains a major treatment challenge. Although some novel products are designed to function in the presence of inhibitors, they do not specific address the immunogenicity risk or mechanistic causes of inhibitor development, which remain unclear. Furthermore, most preclinical studies supporting clinical gene therapy programs have reported immunogenicity signals in animal models, especially at higher vector doses and sometimes using multiple vector designs. In these settings, immunogenicity risk factor determination, comparative immunogenicity of competing vector designs, and the potential for obtaining meaningful prognostic data remain relatively unexplored. Additionally, there remains the opportunity to investigate clinical gene therapy as an alternative to standard immune tolerance induction therapy. The current study was designed to address these issues through longitudinal dose-response evaluation of 4 adeno-associated viral (AAV) vector candidates encoding 2 different FVIII transgenes in a murine model of hemophilia A. Plasma FVIII activity and anti-FVIII antibody data were used to generate a pharmacokinetic model that (1) identifies initial AAV-FVIII product expression kinetics as the dominant risk factor for inhibitor development, (2) predicts a therapeutic window where immune tolerance is achieved, and (3) demonstrates evidence of gene therapy-based immune tolerance induction. Although there are known limitations to the predictive value of preclinical immunogenicity testing, these studies can uncover or support the development of design principles that can guide the development of safe and effective genetic medicines.

Preclinical assessment of an optimized AAV-FVIII vector in mice and non-human primates for the treatment of hemophilia A

Extensive clinical data from liver-mediated gene therapy trials have shown that dose-dependent immune responses against the vector capsid may impair or even preclude transgene expression if not managed successfully with prompt immune suppression. The goal of this preclinical study was to generate an adeno-associated viral (AAV) vector capable of expressing therapeutic levels of B-domain deleted factor VIII (FVIII) at the lowest possible vector dose to minimize the potential Risk of a capsid-mediated immune response in the clinical setting. Here, we describe the studies that identified the investigational agent SPK-8011, currently being evaluated in a phase 1/2 study (NCT03003533) in individuals with hemophilia A. In particular, the potency of our second-generation expression cassettes was evaluated in mice and in non-human primates using two different bioengineered capsids (AAV-Spark100 and AAV-Spark200). At 2 weeks after gene transfer, primates transduced with 2 × 1012 vg/kg AAV-Spark100-FVIII or AAV-Spark200-FVIII expressed FVIII antigen levels of 13% ± 2% and 22% ± 6% of normal, respectively. Collectively, these preclinical results validate the feasibility of lowering the AAV capsid dose for a gene-based therapeutic approach for hemophilia A to a dose level orders of magnitude lower than the first-generation vectors in the clinic.

Chimeric Mice Engrafted With Canine Hepatocytes Exhibits Similar AAV Transduction Efficiency to Hemophilia B Dog

Adeno-associated virus (AAV) mediated gene therapy has been successfully applied in clinical trials, including hemophilia. Novel AAV vectors have been developed with enhanced transduction and specific tissue tropism. Considering the difference in efficacy of AAV transduction between animal models and patients, the chimeric xenograft mouse model with human hepatocytes has unique advantages of studying AAV transduction efficiency in human hepatocytes. However, it is unclear whether the results in humanized mice can predict AAV transduction efficiency in human hepatocytes. To address this issue, we studied the AAV transduction efficacy in canine hepatocytes in both canine hepatocyte xenografted mice and real dogs. After administration of AAV vectors from different serotypes into canine hepatocyte xenograft mice, AAV8 induced the best canine hepatocyte transduction followed by AAV9, then AAV3, 7, 5 and 2. After administration of AAV/cFIX (cFIX-opt-R338L) vectors in hemophilia B dogs, consistent with the result in chimeric mice, AAV8 induced the highest cFIX protein expression and function, followed by AAV9 and then AAV2. These results suggest that mice xenografted with hepatocytes from different species could be used to predict the AAV liver transduction in real species and highlight this potential platform to explore novel AAV variants for future clinical applications.

Promoter CAG is more efficient than hepatocyte‑targeting TBG for transgene expression via rAAV8 in liver tissues

The recombinant adeno‑associated virus 8 (rAAV8) vector is a widely used tool in basic research and clinical trials. The cytomegalovirus immediate‑early enhancer/chicken β‑actin (CAG) promoter is a synthetic promoter used in adenoviral constructs with a wide spectrum and notable efficiency. The thyroxine binding globulin (TBG) promoter is a liver‑specific promoter, which directs transgene expression in hepatocytes. However, the transduction efficiency of the rAAV vector is dependent on both the administration routes and the promoter elements. In the present study, the transduction efficiency in the liver following intraperitoneal (IP) and intravenous (IV) injections of rAAV8 with the CAG, TBG669 and TBG410 promoters was compared. Enhanced green fluorescent protein (EGFP) expression was used as the biomarker to indicate efficiency. Among the three different promoters, CAG exhibited the highest efficiency from both IV and IP injections. Following IV administration, EGFP expression, induced by the CAG promoter, was 67‑fold higher compared with that in the TBG410 promoter group and 26‑fold higher compared with that in the TBG669 promoter group. EGFP protein expression was higher with IV injection compared with that for IP injection for both the CAG and TBG669 promoters (P<0.05). With the CAG promoter, EGFP protein expression was 1.5‑fold higher with the use of IV injection than with IP injection. With the TBG410 promoter, no differences were observed between the two administrations. In conclusion, these findings demonstrated that the CAG promoter was much more efficient at driving gene expression in the liver compared with that for the TBG promoters in rAAV8. In addition, IP administration produced comparable efficiency for gene delivery via the rAAV8 vector, particularly with the promoter TBG410.

Determining the Minimally Effective Dose of a Clinical Candidate AAV Vector in a Mouse Model of Hemophilia A

Hemophilia A, a bleeding disorder, affects 1:5,000 males and is caused by a deficiency of human blood coagulation factor VIII (hFVIII). Studies in mice and macaques identified AAVhu37.E03.TTR.hFVIIIco-SQ.PA75 as a clinical candidate gene therapy vector to treat hemophilia A. In this study, we sought to determine the minimally effective dose (MED) of this vector in a hemophilia A mouse model. Mice received one of four vector doses (3 × 10¹¹–1 × 10¹³ genome copies [GCs]/kg) via intravenous tail vein injection; one cohort received vehicle as a control. Animals were monitored daily after vector/vehicle administration. Blood samples were collected to evaluate hFVIII activity levels and anti-hFVIII antibodies. Animals were sacrificed and necropsied on days 28 and 56; tissues were harvested for histopathological examination and blood was collected for serum chemistry panel analysis. We found no significant differences in liver transaminase levels in mice administered any vector dose compared to those administered vehicle (except for one group administered 3 × 10¹¹ GC/kg). Total bilirubin levels were significantly elevated compared to the vehicle group following two vector doses at day 56 (1 × 10¹² and 1 × 10¹³ GC/kg). We observed no vector-related gross or histological findings. Most microscopic findings were in the vehicle group and considered secondary to blood loss, an expected phenotype of this mouse model. Since we observed no dose-limiting safety markers, we determined that the maximally tolerated dose was greater than or equal to the highest dose tested (1 × 10¹³ GC/kg). Since we detected hFVIII activity in all cohorts administered vector, we conclude that the MED is 3 × 10¹¹ GC/kg—the lowest dose evaluated in this study.

Immune function in X-linked retinoschisis subjects in an AAV8-RS1 phase I/IIa gene therapy trial

This study explored systemic immune changes in 11 subjects with X-linked retinoschisis (XLRS) in a phase I/IIa adeno-associated virus 8 (AAV8)-RS1 gene therapy trial (ClinicalTrials.gov: NCT02317887). Immune cell proportions and serum analytes were compared to 12 healthy male controls. At pre-dosing baseline the mean CD4/CD8 ratio of XLRS subjects was elevated. CD11c⁺ myeloid dendritic cells (DCs) and the serum epidermal growth factor (EGF) level were decreased, while CD123⁺ plasmacytoid DCs and serum interferon (IFN)-γ and tumor necrosis factor (TNF)-α were increased, indicating that the XLRS baseline immune status differs from that of controls. XLRS samples 14 days after AAV8-RS1 administration were compared with the XLRS baseline. Frequency of CD11b⁺CD11c⁺ DCc was decreased in 8 of 11 XLRS subjects across all vector doses (1e9-3e11 vector genomes [vg]/eye). CD8⁺human leukocyte antigen-DR isotype (HLA-DR)⁺ cytotoxic T cells and CD68⁺CD80⁺ macrophages were upregulated in 10 of 11 XLRS subjects, along with increased serum granzyme B in 8 of 11 XLRS subjects and elevated IFN-γ in 9 of 11 XLRS subjects. The six XLRS subjects with ocular inflammation after vector application gave a modestly positive correlation of inflammation score to their respective baseline CD4/CD8 ratios. This exploratory study indicates that XLRS subjects may exhibit a proinflammatory, baseline immune phenotype, and that intravitreal dosing with AAV8-RS1 leads to systemic immune activation with an increase of activated lymphocytes, macrophages, and proinflammatory cytokines.

Restoration of FVIII Function and Phenotypic Rescue in Hemophilia A Mice by Transplantation of MSCs Derived From F8-Modified iPSCs

Hemophilia A (HA), an X-linked recessive congenital bleeding disorder, affects 80%–85% of patients with hemophilia. Nearly half of severe cases of hemophilia are caused by a 0.6-Mb genomic inversion (Inv22) that disrupts F8. Although viral-based gene therapy has shown therapeutic effects for hemophilia B (HB), this promising approach is not applicable for HA at the present stage; this limitation is mainly due to the large size of F8 cDNA, which far exceeds the adeno-associated virus (AAV) packaging capacity. We previously reported an in situ genetic correction of Inv22 in HA patient-specific induced pluripotent stem cells (HA-iPSCs) by using TALENs. We also investigated an alternative strategy for targeted gene addition, in which cDNA of the B-domain deleted F8 (BDDF8) was targeted at the rDNA locus of HA-iPSCs using TALENickases to restore FVIII function. Mesenchymal stem cells (MSCs) have low immunogenicity and can secrete FVIII under physiological conditions; in this study, MSCs were differentiated from F8-corrected iPSCs, BDDF8-iPSCs, and HA-iPSCs. Differentiated MSCs were characterized, and FVIII expression efficacy in MSCs was verified in vitro. The three types of MSCs were introduced into HA mice via intravenous injection. Long-term engraftment with restoration of FVIII function and phenotypic rescue was observed in HA mice transplanted with F8-corrected iMSCs and BDDF8-iMSCs. Our findings suggest that ex vivo gene therapy using iMSCs derived from F8-modified iPSCs can be feasible, effective, and promising for the clinical translation of therapeutic gene editing of HA and other genetic birth defects, particularly those that involve large sequence variants.

Hemophilia Gene Therapy: Approaching the First Licensed Product

The clinical potential of hemophilia gene therapy has now been pursued for the past 30 years, and there is a realistic expectation that this goal will be achieved within the next couple of years with the licensing of a gene therapy product. While recent late phase clinical trials of hemophilia gene therapy have shown promising results, there remain a number of issues that require further attention with regard to both efficacy and safety of this therapeutic approach. In this review, we present information relating to the current status of the field and focus attention on the unanswered questions for hemophilia gene therapy and the future challenges that need to be overcome to enable the widespread application of this treatment paradigm.

A long-term study of AAV gene therapy in dogs with hemophilia A identifies clonal expansions of transduced liver cells

Nine dogs with hemophilia A were treated with adeno-associated viral (AAV) gene therapy and followed for up to 10 years. Administration of AAV8 or AAV9 vectors expressing canine factor VIII (AAV-cFVIII) corrected the FVIII deficiency to 1.9-11.3% of normal FVIII levels. In two of nine dogs, levels of FVIII activity increased gradually starting about 4 years after treatment. None of the dogs showed evidence of tumors or altered liver function. Analysis of integration sites in liver samples from six treated dogs identified 1,741 unique AAV integration events in genomic DNA and expanded cell clones in five dogs, with 44% of the integrations near genes involved in cell growth. All recovered integrated vectors were partially deleted and/or rearranged. Our data suggest that the increase in FVIII protein expression in two dogs may have been due to clonal expansion of cells harboring integrated vectors. These results support the clinical development of liver-directed AAV gene therapy for hemophilia A, while emphasizing the importance of long-term monitoring for potential genotoxicity.

In Situ Detection of Adeno-associated Viral Vector Genomes with SABER-FISH

Gene therapy with recombinant adeno-associated viral (AAV) vectors is a promising modality for the treatment of a variety of human diseases. Nonetheless, there remain significant gaps in our understanding of AAV vector biology, due in part to the lack of robust methods to track AAV capsids and genomes. In this study, we describe a novel application of signal amplification by exchange reaction fluorescence in situ hybridization (SABER-FISH) that enabled the visualization and quantification of individual AAV genomes after vector administration in mice. These genomes could be seen in retinal cells within 3 h of subretinal AAV delivery, were roughly full length, and correlated with vector expression in both photoreceptors and the retinal pigment epithelium. SABER-FISH readily detected AAV genomes in the liver and muscle following retro-orbital and intramuscular AAV injections, respectively, demonstrating its utility in different tissues. Using SABER-FISH, we also found that retinal microglia, a cell type deemed refractory to AAV transduction, are in fact efficiently infected by multiple AAV serotypes, but appear to degrade AAV genomes prior to nuclear localization. Our findings show that SABER-FISH can be used to visualize AAV genomes in situ, allowing for studies of AAV vector biology and the tracking of transduced cells following vector administration.

Gene Therapy for Hemophilia: Facts and Quandaries in the 21st Century

Therapy for hemophilia has evolved in the last 40 years from plasma-based concentrates to recombinant proteins and, more recently, to non-factor therapeutics. Along this same timeline, research in adeno-associated viral (AAV) based gene therapy vectors has provided the framework for early phase clinical trials initially for hemophilia B (HB) and now for hemophilia A. Successive lessons learned from early HB trials have paved the way for current advanced phase trials. Nevertheless, questions linger regarding 1) the optimal balance of vector dose to transgene expression, 2) amount and durability of transgene expression required, and 3) long-term safety. Some trials have demonstrated unique findings not seen previously regarding transient elevation of liver enzymes, immunogenicity of the vector capsid, and loss of transgene expression. This review will provide an update on the clinical AAV gene therapy trials in hemophilia and address the questions above. A thoughtful and rationally approached expansion of gene therapy to the clinics would certainly be a welcome addition to the arsenal of options for hemophilia therapy. Further, the global impact of gene therapy could be vastly improved by expanding eligibility to different patient populations and to developing nations. With the advances made to date, it is possible to envision a shift from the early goal of simply increasing life expectancy to a significant improvement in quality of life by reduction in spontaneous bleeding episodes and disease complications.

The Evolution of Gene Therapy in the Treatment of Metabolic Liver Diseases

Monogenic metabolic disorders of hepatic origin number in the hundreds, and for many, liver transplantation remains the only cure. Liver-targeted gene therapy is an attractive treatment modality for many of these conditions, and there have been significant advances at both the preclinical and clinical stages. Viral vectors, including retroviruses, lentiviruses, adenovirus-based vectors, adeno-associated viruses and simian virus 40, have differing safety, efficacy and immunogenic profiles, and several of these have been used in clinical trials with variable success. In this review, we profile viral vectors and non-viral vectors, together with various payloads, including emerging therapies based on RNA, that are entering clinical trials. Genome editing technologies are explored, from earlier to more recent novel approaches that are more efficient, specific and safe in reaching their target sites. The various curative approaches for the multitude of monogenic hepatic metabolic disorders currently at the clinical development stage portend a favorable outlook for this class of genetic disorders.

Gene Therapy for Hemophilia A: Where We Stand

Hemophilia A (HA) is a hereditary hemorrhagic disease caused by a deficiency of coagulation factor VIII (FVIII) in blood plasma. Patients with HA usually suffer from spontaneous and recurrent bleeding in joints and muscles, or even intracerebral hemorrhage, which might lead to disability or death. Although the disease is currently manageable via delivery of plasma-derived or recombinant FVIII, this approach is costly, and neutralizing antibodies may be generated in a large portion of patients, which render the regimens ineffective and inaccessible. Given the monogenic nature of HA and that a slight increase in FVIII can remarkably alleviate the phenotypes, HA has been considered to be a suitable target disease for gene therapy. Consequently, the introduction of a functional F8 gene copy into the appropriate target cells via viral or nonviral delivery vectors, including gene correction through genome editing approaches, could ultimately provide an effective therapeutic method for HA patients. In this review, we discuss the recent progress of gene therapy for HA with viral and nonviral delivery vectors, including piggyBac, lentiviral and adeno-associated viral vectors, as well as new raising issues involving liver toxicity, pre-existing neutralizing antibodies of viral approach, and the selection of the target cell type for nonviral delivery.

FVIII activity following FVIII protein infusion or FVIII gene transfer predicts the bleeding risk in hemophilia A rats

BACKGROUND

Prophylactic replacement therapy in hemophilia A (HA) patients does not adequately prevent bleeds and arthropathic complications. A more refined understanding of the relationship between coagulation factor VIII (FVIII) levels and bleeding risk during protein prophylaxis, or with gene therapy, is needed to improve patient care.

OBJECTIVES

Investigate this relationship in the HA rat, a model exhibiting spontaneous bleeds and development of arthropathy similar to HA patients.

METHODS

Human B domain-deleted FVIII was delivered to HA rats via adeno-associated virus (AAV)-mediated gene transfer or multiple intravenous protein injections.

RESULTS AND CONCLUSIONS

After 12 weeks of observation, both approaches significantly reduced bleeds per animal and increased the proportion of bleed-free animals compared with controls (43% vs 0%, respectively [AAV]; 75% vs 8%, respectively [injection]). Both approaches resulted in an anti-FVIII inhibitory response in 20% to 37% of treated animals, similar to HA patients. Inhibitory antibodies were refractory to clinical improvement (reduction of bleeds) only in the AAV-based prophylaxis. An integrated model-based analysis of data on FVIII exposure and bleeding events was performed. This predicted the bleeding risk at any given circulating FVIII activity. Specifically, 4.8 or 10 IU/dL FVIII (0.048 and 0.1 IU/mL, respectively) were predicted to reduce bleeding risk by 90% or 95%, respectively, compared with untreated controls. Our data establish the utility of the HA rat model in FVIII prophylaxis studies and describe how FVIII activity affects bleeding risk in this setting. These enable further studies on FVIII prophylaxis focusing on disease complications for an optimized treatment of HA patients.

Translational Potential of Immune Tolerance Induction by AAV Liver-Directed Factor VIII Gene Therapy for Hemophilia A

Hemophilia A (HA) is an X-linked bleeding disorder due to deficiencies in coagulation factor VIII (FVIII). The major complication of current protein-based therapies is the development of neutralizing anti-FVIII antibodies, termed inhibitors, that block the hemostatic effect of therapeutic FVIII. Inhibitors develop in about 20-30% of people with severe HA, but the risk is dependent on the interaction between environmental and genetic factors, including the underlying F8 gene mutation. Recently, multiple clinical trials evaluating adeno-associated viral (AAV) vector liver-directed gene therapy for HA have reported promising results of therapeutically relevant to curative FVIII levels. The inclusion criteria for most trials prevented enrollment of subjects with a history of inhibitors. However, preclinical data from small and large animal models of HA with inhibitors suggests that liver-directed gene therapy can in fact eradicate pre-existing anti-FVIII antibodies, induce immune tolerance, and provide long-term therapeutic FVIII expression to prevent bleeding. Herein, we review the accumulating evidence that continuous uninterrupted expression of FVIII and other transgenes after liver-directed AAV gene therapy can bias the immune system toward immune tolerance induction, discuss the current understanding of the immunological mechanisms of this process, and outline questions that will need to be addressed to translate this strategy to clinical trials.

Mouse γ-Synuclein Promoter-Mediated Gene Expression and Editing in Mammalian Retinal Ganglion Cells

Optic neuropathies are a group of optic nerve (ON) diseases caused by various insults including glaucoma, inflammation, ischemia, trauma, and genetic deficits, which are characterized by retinal ganglion cell (RGC) death and ON degeneration. An increasing number of genes involved in RGC intrinsic signaling have been found to be promising neural repair targets that can potentially be modulated directly by gene therapy, if we can achieve RGC specific gene targeting. To address this challenge, we first used adeno-associated virus (AAV)-mediated gene transfer to perform a low-throughput in vivo screening in both male and female mouse eyes and identified the mouse γ-synuclein (mSncg) promoter, which specifically and potently sustained transgene expression in mouse RGCs and also works in human RGCs. We further demonstrated that gene therapy that combines AAV-mSncg promoter with clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 gene editing can knock down pro-degenerative genes in RGCs and provide effective neuroprotection in optic neuropathies.SIGNIFICANCE STATEMENT Here, we present an RGC-specific promoter, mouse γ-synuclein (mSncg) promoter, and perform extensive characterization and proof-of-concept studies of mSncg promoter-mediated gene expression and clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 gene editing in RGCs in vivo To our knowledge, this is the first report demonstrating in vivo neuroprotection of injured RGCs and optic nerve (ON) by AAV-mediated CRISPR/Cas9 inhibition of genes that are critical for neurodegeneration. It represents a powerful tool to achieve RGC-specific gene modulation, and also opens up a promising gene therapy strategy for optic neuropathies, the most common form of eye diseases that cause irreversible blindness.

The Immune Response to the fVIII Gene Therapy in Preclinical Models

Neutralizing antibodies to factor VIII (fVIII), referred to as "inhibitors," remain the most challenging complication post-fVIII replacement therapy. Preclinical development of novel fVIII products involves studies incorporating hemophilia A (HA) and wild-type animal models. Though immunogenicity is a critical aspect of preclinical pharmacology studies, gene therapy studies tend to focus on fVIII expression levels without major consideration for immunogenicity. Therefore, little clarity exists on whether preclinical testing can be predictive of clinical immunogenicity risk. Despite this, but perhaps due to the potential for transformative benefits, clinical gene therapy trials have progressed rapidly. In more than two decades, no inhibitors have been observed. However, all trials are conducted in previously treated patients without a history of inhibitors. The current review thus focuses on our understanding of preclinical immunogenicity for HA gene therapy candidates and the potential indication for inhibitor treatment, with a focus on product- and platform-specific determinants, including fVIII transgene sequence composition and tissue/vector biodistribution. Currently, the two leading clinical gene therapy vectors are adeno-associated viral (AAV) and lentiviral (LV) vectors. For HA applications, AAV vectors are liver-tropic and employ synthetic, high-expressing, liver-specific promoters. Factors including vector serotype and biodistribution, transcriptional regulatory elements, transgene sequence, dosing, liver immunoprivilege, and host immune status may contribute to tipping the scale between immunogenicity and tolerance. Many of these factors can also be important in delivery of LV-fVIII gene therapy, especially when delivered intravenously for liver-directed fVIII expression. However, ex vivo LV-fVIII targeting and transplantation of hematopoietic stem and progenitor cells (HSPC) has been demonstrated to achieve durable and curative fVIII production without inhibitor development in preclinical models. A critical variable appears to be pre-transplantation conditioning regimens that suppress and/or ablate T cells. Additionally, we and others have demonstrated the potential of LV-fVIII HSPC and liver-directed AAV-fVIII gene therapy to eradicate pre-existing inhibitors in murine and canine models of HA, respectively. Future preclinical studies will be essential to elucidate immune mechanism(s) at play in the context of gene therapy for HA, as well as strategies for preventing adverse immune responses and promoting immune tolerance even in the setting of pre-existing inhibitors.

Escape or Fight: Inhibitors in Hemophilia A

Replacement therapy with coagulation factor VIII (FVIII) represents the current clinical treatment for patients affected by hemophilia A (HA). This treatment while effective is, however, hampered by the formation of antibodies which inhibit the activity of infused FVIII in up to 30% of treated patients. Immune tolerance induction (ITI) protocols, which envisage frequent infusions of high doses of FVIII to confront this side effect, dramatically increase the already high costs associated to a patient's therapy and are not always effective in all treated patients. Therefore, there are clear unmet needs that must be addressed in order to improve the outcome of these treatments for HA patients. Taking advantage of preclinical mouse models of hemophilia, several strategies have been proposed in recent years to prevent inhibitor formation and eradicate the pre-existing immunity to FVIII inhibitor positive patients. Herein, we will review some of the most promising strategies developed to avoid and eradicate inhibitors, including the use of immunomodulatory drugs or molecules, oral or transplacental delivery as well as cell and gene therapy approaches. The goal is to improve and potentiate the current ITI protocols and eventually make them obsolete.

Long-Term Delivery of an Anti-SIV Monoclonal Antibody With AAV

Long-term delivery of anti-HIV monoclonal antibodies using adeno-associated virus (AAV) holds promise for the prevention and treatment of HIV infection. We previously reported that after receiving a single administration of AAV vector coding for anti-SIV antibody 5L7, monkey 84-05 achieved high levels of AAV-delivered 5L7 IgG1 in vivo which conferred sterile protection against six successive, escalating dose, intravenous challenges with highly infectious, highly pathogenic SIVmac239, including a final challenge with 10 animal infectious doses (1). Here we report that monkey 84-05 has successfully maintained 240-350 μg/ml of anti-SIV antibody 5L7 for over 6 years. Approximately 2% of the circulating IgG in this monkey is this one monoclonal antibody. This monkey generated little or no anti-drug antibodies (ADA) to the AAV-delivered antibody for the duration of the study. Due to the nature of the high-dose challenge used and in order to rule out a potential low-level infection not detected by regular viral loads, we have used ultrasensitive techniques to detect cell-associated viral DNA and RNA in PBMCs from this animal. In addition, we have tested serum from 84-05 by ELISA against overlapping peptides spanning the whole envelope sequence for SIVmac239 (PepScan) and against recombinant p27 and gp41 proteins. No reactivity has been detected in the ELISAs indicating the absence of naturally arising anti-SIV antibodies; moreover, the ultrasensitive cell-associated viral tests yielded no positive reaction. We conclude that macaque 84-05 was effectively protected and remained uninfected. Our data show that durable, continuous antibody expression can be achieved after one single administration of AAV and support the potential for lifelong protection against HIV from a single vector administration.

Gene Therapy for Liver Cancers: Current Status from Basic to Clinics

The liver is a key organ for metabolism, protein synthesis, detoxification, and endocrine function, and among liver diseases, including hepatitis, cirrhosis, malignant tumors, and congenital disease, liver cancer is one of the leading causes of cancer-related deaths worldwide. Conventional therapeutic options such as embolization and chemotherapy are not effective against advanced-stage liver cancer; therefore, continuous efforts focus on the development of novel therapeutic options, including molecular targeted agents and gene therapy. In this review, we will summarize the progress toward the development of gene therapies for liver cancer, with an emphasis on recent clinical trials and preclinical studies.

Advances and challenges for hemophilia gene therapy

Hemophilia is an X-linked inherited bleeding disorder, resulting from defects in the F8 (hemophilia A) or F9 (hemophilia B) genes. Persons with hemophilia have bleeding episodes into the soft tissues and joints, which are treated with self-infusion of factor VIII or IX concentrates. Hemophilia provides an attractive target for gene therapy studies, due to the monogenic nature of these disorders and easily measurable endpoints (factor levels and bleed rates). All successful, pre-clinical and clinical studies to date have utilized recombinant adeno-associated viral (AAV) vectors for factor VIII or IX hepatocyte transduction. Recent clinical data have presented normalization of factor levels in some patients with improvements in bleed rate and quality of life. The main toxicity seen within these studies has been early transient elevation in liver enzymes, with variable effect on transgene expression. Although long-term data are awaited, durable expression has been seen within the hemophilia dog model with no late-toxicity or oncogenesis. There are a number of phase III studies currently recruiting; however, there may be some limitations in translating these data to clinical practice, due to inclusion/exclusion criteria. AAV-based gene therapy is one of a number of novel approaches for treatment of hemophilia with other gene therapy (in vivo and ex vivo) and non-replacement therapies progressing through clinical trials. Availability of these high-cost novel therapeutics will require evolution of both clinical and financial healthcare services to allow equitable personalization of care for persons with hemophilia.

Five Years of Successful Inducible Transgene Expression Following Locoregional Adeno-Associated Virus Delivery in Nonhuman Primates with No Detectable Immunity

Anti-transgene immune responses elicited after intramuscular (i.m.) delivery of recombinant adeno-associated virus (rAAV) have been shown to hamper long-term transgene expression in large-animal models of rAAV-mediated gene transfer. To overcome this hurdle, an alternative mode of delivery of rAAV vectors in nonhuman primate muscles has been described: the locoregional (LR) intravenous route of administration. Using this injection mode, persistent inducible transgene expression for at least 1 year under the control of the tetracycline-inducible Tet-On system was previously reported in cynomolgus monkeys, with no immunity against the rtTA transgene product. The present study shows the long-term follow-up of these animals. It is reported that LR delivery of a rAAV2/1 vector allows long-term inducible expression up to at least 5 years post gene transfer, with no any detectable host immune response against the transactivator rtTA, despite its immunogenicity following i.m. gene transfer. This study shows for the first time a long-term regulation of muscle gene expression using a Tet-On-inducible system in a large-animal model. Moreover, these findings further confirm that the rAAV LR delivery route is efficient and immunologically safe, allowing long-term skeletal muscle gene transfer.

AAV-mediated expression of 3TSR inhibits tumor and metastatic lesion development and extends survival in a murine model of epithelial ovarian carcinoma

An integral step in the development of solid tumors is the recruitment of blood vessels to fuel tumor growth. Antiangiogenic therapies can inhibit this process and control solid tumor growth. Thrombospondin-1 is an antiangiogenic protein possessing three type I repeats (3TSR) near the center of the protein and a CD47-binding peptide (CD47) in its C-terminus. Previously, we showed that treatment with recombinant 3TSR induces tumor regression, normalizes tumor vasculature, and improves uptake of chemotherapy drugs in an orthotopic, syngeneic mouse model of advanced stage epithelial ovarian cancer (EOC). While effective, this intervention required daily intraperitoneal injections. To circumvent this, here we employ adeno-associated virus (AAV) gene therapy vectors to express 3TSR alone or in combination with the CD47-binding peptide of TSP-1 and evaluate the impact on tumor development and survival in a mouse model of EOC. A single intraperitoneal injection of 1 × 10¹¹ vg of AAV expressing 3TSR, CD47-binding peptide, or 3TSR + CD47 effectively suppressed primary tumor growth; however, only AAV-3TSR was able to inhibit development of secondary lesions at 90-days post-tumor implantation and significantly improve survival. Taken together, AAV-mediated expression of 3TSR appears safe and effective at inhibiting tumor development and represents a novel, less invasive approach for treating ovarian carcinoma.

Gene therapy in hemophilia A: a cost-effectiveness analysis

Gene therapy provides a potential phenotypic cure for hemophilia, yet the cost of this novel treatment is high, tempering enthusiasm and raising questions regarding cost vs benefit. To evaluate the cost-effectiveness of gene therapy treatment of severe hemophilia A compared with prophylaxis with factor VIII (FVIII), we developed a Markov state-transition model to estimate the costs and effectiveness of severe hemophilia A treatment strategies from a United States health care system perspective. Quality-adjusted life-years (QALYs) were the effectiveness measure. In the base case, hypothetical cohorts of 30-year-old patients received gene therapy or FVIII prophylaxis. We obtained model probabilities and utilities from the literature and costs from Medicare reimbursement data. One-way and probabilistic sensitivity analyses were performed to test the robustness of results. Over a 10-year time horizon, total per-person gene therapy strategy costs were $1.0M and resulted in 8.33 QALYs, whereas prophylaxis cost $1.7M and resulted in 6.62 QALYs. Thus, gene therapy dominated prophylaxis (costs less and was more effective). Gene therapy remained dominant unless initial costs exceeded $1.6M and were <$100 000 per 1 QALY gained compared with prophylaxis if initial costs were <$1.7M. Results were not sensitive to variation of all other parameters over clinically plausible ranges. In a probabilistic sensitivity analysis simultaneously varying all parameters 3000 times over parameter distributions, gene therapy was dominant in 92% of model iterations. Treatment of severe hemophilia A with gene therapy is likely to be cost-saving or cost-effective compared with FVIII prophylaxis.

AAV-Mediated Gene Delivery to the Liver: Overview of Current Technologies and Methods

Adeno-associated virus (AAV) vectors to treat liver-specific genetic diseases are the focus of several ongoing clinical trials. The ability to give a peripheral injection of virus that will successfully target the liver is one of many attractive features of this technology. Although initial studies of AAV liver gene transfer revealed some limitations, extensive animal modeling and further clinical development have helped solve some of these issues, resulting in several successful clinical trials that have reached curative levels of clotting factor expression in hemophilia. Looking beyond gene replacement, recent technologies offer the possibility for AAV liver gene transfer to directly repair deficient genes and potentially treat autoimmune disease.

Protein-Engineered Coagulation Factors for Hemophilia Gene Therapy

Hemophilia A (HA) and hemophilia B (HB) are X-linked bleeding disorders due to inheritable deficiencies in either coagulation factor VIII (FVIII) or factor IX (FIX), respectively. Recently, gene therapy clinical trials with adeno-associated virus (AAV) vectors and protein-engineered transgenes, B-domain deleted (BDD) FVIII and FIX-Padua, have reported near-phenotypic cures in subjects with HA and HB, respectively. Here, we review the biology and the clinical development of FVIII-BDD and FIX-Padua as transgenes. We also examine alternative bioengineering strategies for FVIII and FIX, as well as the immunological challenges of these approaches. Other engineered proteins and their potential use in gene therapy for hemophilia with inhibitors are also discussed. Continued advancement of gene therapy for HA and HB using protein-engineered transgenes has the potential to alleviate the substantial medical and psychosocial burdens of the disease.

Evaluation of the activity levels of rat FVIII and human FVIII delivered by adeno-associated viral vectors both in vitro and in vivo

The development of a novel coagulation factor VIII (FVIII) expression cassette with an enhanced activity for gene therapy of hemophilia A (HA) is essential. The biological properties of several non-human FVIII sequences, such as porcine and canine, have been evaluated. Here, we compared the activity level of rat FVIII (rFVIII) and human FVIII (hFVIII) by using single-chain and dual-chain strategies in 293 T cells and the HA mice. In both in vitro and hydrodynamic injection studies, the activity of rFVIII detected by the activated partial thromboplastin time assay was higher than that of hFVIII both by single-chain (~2.96-fold and ~1.72-fold, respectively) and dual-chain (~7.69-fold and ~2.35-fold, respectively). Moreover, the dual chain exerted a potentially higher delivery efficacy compared with the single chain (~4.96-fold and ~2.99-fold, respectively). The blood loss of HA mice administrated with rFVIII was less than those with hFVIII. AAV-delivered rFVIII and hFVIII also exerted long-term therapeutic effects on HA mice and caused a transient ALT elevation. These data might help to the development of novel, optimized FVIII expression cassettes based on the amino acid difference between rFVIII and hFVIII. These data indicate that the dual-chain strategy would likely enhance the delivery efficiency of the AAV-mediated FVIII gene therapy.

Gene therapy for hemophilia: what does the future hold?

Recent phase I/II adeno-associated viral vector-mediated gene therapy clinical trials have reported remarkable success in ameliorating disease phenotype in hemophilia A and B. These trials, which highlight the challenges overcome through decades of preclinical and first in human clinical studies, have generated considerable excitement for patients and caregivers alike. Optimization of vector and transgene expression has significantly improved the ability to achieve therapeutic factor levels in these subjects. Long-term follow-up studies will guide standardization of the approach with respect to the combination of serotype, promoter, dose, and manufacturing processes and inform safety for inclusion of young patients. Certain limitations preclude universal applicability of gene therapy, including transient liver transaminase elevations due to the immune responses to vector capsids or as yet undefined mechanisms, underlying liver disease from iatrogenic viral hepatitis, and neutralizing antibodies to clotting factors. Integrating vectors show promising preclinical results, but manufacturing and safety concerns still remain. The prospect of gene editing for correction of the underlying mutation is on the horizon with considerable potential. Herein, we review the advances and limitations that have resulted in these recent successful clinical trials and outline avenues that will allow for broader applicability of gene therapy.

An Observational Study from Long-Term AAV Re-administration in Two Hemophilia Dogs

Adeno-associated virus (AAV) vectors have been successfully applied in hemophilia clinical trials. However, this approach is limited to patients without AAV-neutralizing antibodies (NAbs). In this study, we explored the feasibility of AAV re-administration in hemophilia A dogs treated initially 8 years ago with AAV8.canine FVIII. After the re-administration in two NAb-negative dogs with AAV8 vectors carrying human factor VIII (hFVIII), along with the proteasome inhibitor bortezomib, we observed a phenotypic improvement in both dogs that persisted in one dog. Phenotypic improvement disappeared at 59 days after re-administration in the other dog, and specific cytotoxic T lymphocytes (CTLs) to the capsid were detected at day 17, but not to hFVIII. hFVIII inhibitors were observed at day 59 and gradually increased. Mechanistic studies demonstrated an increase in pro-inflammatory cytokines, a decrease in immunomodulatory cytokines, as well as lower Tregs after re-administration. These results suggest that hFVIII inhibitor development may contribute to the therapeutic failure via immune response activation. Interestingly, it takes about 30–50 days for AAV NAb titers to decrease by half. Collectively, this study suggests that re-administration of the same AAV serotype after long-term follow-up is feasible and that the study of AAV NAb kinetics will provide important information for predicating the efficacy of re-administration.

Double-stranded RNA innate immune response activation from long-term adeno-associated virus vector transduction

Data from clinical trials for hemophilia B using adeno-associated virus (AAV) vectors have demonstrated decreased transgenic coagulation factor IX (hFIX) expression 6-10 weeks after administration of a high vector dose. While it is likely that capsid-specific cytotoxic T lymphocytes eliminate vector-transduced hepatocytes, thereby resulting in decreased hFIX, this observation is not intuitively consistent with restored hFIX levels following prednisone application. Although the innate immune response is immediately activated following AAV vector infection via TLR pathways, no studies exist regarding the role of the innate immune response at later time points after AAV vector transduction. Herein, activation of the innate immune response in cell lines, primary human hepatocytes, and hepatocytes in a human chimeric mouse model was observed at later time points following AAV vector transduction. Mechanistic analysis demonstrated that the double-stranded RNA (dsRNA) sensor MDA5 was necessary for innate immune response activation and that transient knockdown of MDA5, or MAVS, decreased IFN-β expression while increasing transgene production in AAV-transduced cells. These results both highlight the role of the dsRNA-triggered innate immune response in therapeutic transgene expression at later time points following AAV transduction and facilitate the execution of effective strategies to block the dsRNA innate immune response in future clinical trials.

Emerging Issues in AAV-Mediated In Vivo Gene Therapy

In recent years, the number of clinical trials in which adeno-associated virus (AAV) vectors have been used for in vivo gene transfer has steadily increased. The excellent safety profile, together with the high efficiency of transduction of a broad range of target tissues, has established AAV vectors as the platform of choice for in vivo gene therapy. Successful application of the AAV technology has also been achieved in the clinic for a variety of conditions, including coagulation disorders, inherited blindness, and neurodegenerative diseases, among others. Clinical translation of novel and effective "therapeutic products" is, however, a long process that involves several cycles of iterations from bench to bedside that are required to address issues encountered during drug development. For the AAV vector gene transfer technology, several hurdles have emerged in both preclinical studies and clinical trials; addressing these issues will allow in the future to expand the scope of AAV gene transfer as a therapeutic modality for a variety of human diseases. In this review, we will give an overview on the biology of AAV vector, discuss the design of AAV-based gene therapy strategies for in vivo applications, and present key achievements and emerging issues in the field. We will use the liver as a model target tissue for gene transfer based on the large amount of data available from preclinical and clinical studies.

Hemophilia gene therapy comes of age

Concurrent with the development of recombinant factor replacement products, the characterization of the F9 and F8 genes over 3 decades ago allowed for the development of recombinant factor products and made the hemophilias a target disease for gene transfer. The progress of hemophilia gene therapy has been announced in 3 American Society of Hematology scientific plenary sessions, including the first "cure" in a large animal model of hemophilia B in 1998, first in human sustained vector-derived factor IX activity in 2011, and our clinical trial results reporting sustained vector-derived factor IX activity well into the mild or normal range in 2016. This progression to clinically meaningful success combined with numerous ongoing recombinant adeno-associated virus (rAAV)-mediated hemophilia gene transfer clinical trials suggest that the goal of gene therapy to alter the paradigm of hemophilia care may soon be realized. Although several novel therapeutics have recently emerged for hemophilia, gene therapy is unique in its potential for a one-time disease-altering, or even curative, treatment. This review will focus on the prior progress and current clinical trial investigation of rAAV-mediated gene transfer for hemophilia A and B.

Hemophilia gene therapy comes of age

Concurrent with the development of recombinant factor replacement products, the characterization of the F9 and F8 genes over 3 decades ago allowed for the development of recombinant factor products and made the hemophilias a target disease for gene transfer. The progress of hemophilia gene therapy has been announced in 3 American Society of Hematology scientific plenary sessions, including the first "cure" in a large animal model of hemophilia B in 1998, first in human sustained vector-derived factor IX activity in 2011, and our clinical trial results reporting sustained vector-derived factor IX activity well into the mild or normal range in 2016. This progression to clinically meaningful success combined with numerous ongoing recombinant adeno-associated virus (rAAV)-mediated hemophilia gene transfer clinical trials suggest that the goal of gene therapy to alter the paradigm of hemophilia care may soon be realized. Although several novel therapeutics have recently emerged for hemophilia, gene therapy is unique in its potential for a one-time disease-altering, or even curative, treatment. This review will focus on the prior progress and current clinical trial investigation of rAAV-mediated gene transfer for hemophilia A and B.

Innovative Approaches for Immune Tolerance to Factor VIII in the Treatment of Hemophilia A

Hemophilia A (coagulation factor VIII deficiency) is a debilitating genetic disorder that is primarily treated with intravenous replacement therapy. Despite a variety of factor VIII protein formulations available, the risk of developing anti-dug antibodies (“inhibitors”) remains. Overall, 20–30% of patients with severe disease develop inhibitors. Current clinical immune tolerance induction protocols to eliminate inhibitors are not effective in all patients, and there are no prophylactic protocols to prevent the immune response. New experimental therapies, such as gene and cell therapies, show promising results in pre-clinical studies in animal models of hemophilia. Examples include hepatic gene transfer with viral vectors, genetically engineered regulatory T cells (Treg), in vivo Treg induction using immune modulatory drugs, and maternal antigen transfer. Furthermore, an oral tolerance protocol is being developed based on transgenic lettuce plants, which suppressed inhibitor formation in hemophilic mice and dogs. Hopefully, some of these innovative approaches will reduce the risk of and/or more effectively eliminate inhibitor formation in future treatment of hemophilia A.

From IgG Fusion Proteins to Engineered-Specific Human Regulatory T Cells: A Life of Tolerance

Recent efforts have concentrated on approaches to expand and “specify” human regulatory T cells (Tregs) and to apply them to modulate adverse immune responses in autoimmunity and hemophilia. We have used retroviral transduction of specific T-cell receptor, single chain Fv, or antigen domains in Tregs to achieve this goal. Each of these approaches have advantages and disadvantages. Results with these engineered T cells and evolution of the research developments and paths that led to the development of specific regulatory approaches for tolerance are summarized.