Safe and Sustained Expression of Human Iduronidase After Intrathecal Administration of Adeno-Associated Virus Serotype 9 in Infant Rhesus Monkeys

Lumbar Intrathecal Injection in Adult and Neonatal Mice

This article describes a step-by-step process of lumbar intrathecal injection of Evans blue dye and AAV9-EGFP in adult (2-month-old) and neonatal (postnatal day 10) mice. Intrathecal injection is a clinically translatable technique that has already been extensively applied in humans. In mice, intrathecal injection is considered a challenging procedure that requires a trained and experienced researcher. For both adult and neonatal mice, lumbar intrathecal injection is directed into the L5-L6 intervertebral space. Intrathecally injected material enters the cerebrospinal fluid (CSF) within the intrathecal space from where it can directly access the central nervous system (CNS) parenchyma. Simultaneously, intrathecally injected material exits the CSF with pressure gradient and enters the endoneurial fluid and ultimately the peripheral nerves. While in the CSF, the injectable material also enters the bloodstream and systemic circulation through the arachnoid villi. A successful lumbar intrathecal injection results in adequate biodistribution of the injectable material in the CNS, PNS, and peripheral organs. When correctly applied, this technique is considered as minimally invasive and non-disruptive and can be used for the lumbar delivery of any solute. © 2024 Wiley Periodicals LLC. Basic Protocol 1: C57BL/6 adult and P10 mice lumbar intrathecal injection Basic Protocol 2: Tissue collection and preparation for evaluating Evans blue dye diffusion Basic Protocol 3: Tissue collection and preparation for immunohistochemistry staining Basic Protocol 4: Tissue collection and vector genome copy number analysis.

Positron Emission Tomography Quantitative Assessment of Off-Target Whole-Body Biodistribution of I-124-Labeled Adeno-Associated Virus Capsids Administered to Cerebral Spinal Fluid

Based on studies in experimental animals demonstrating that administration of adeno-associated virus (AAV) vectors to the cerebrospinal fluid (CSF) is an effective route to transfer genes to the nervous system, there are increasing number of clinical trials using the CSF route to treat nervous system disorders. With the knowledge that the CSF turns over four to five times daily, and evidence in experimental animals that at least some of CSF administered AAV vectors are distributed to systemic organs, we asked: with AAV administration to the CSF, what fraction of the total dose remains in the nervous system and what fraction goes off target and is delivered systemically? To quantify the biodistribution of AAV capsids immediately after administration, we covalently labeled AAV capsids with iodine 124 (I-124), a cyclotron generated positron emitter, enabling quantitative positron emission tomography scanning of capsid distribution for up to 96 h after AAV vector administration. We assessed the biodistribution to nonhuman primates of I-124-labeled capsids from different AAV clades, including 9 (clade F), rh.10 (E), PHP.eB (F), hu68 (F), and rh91(A). The analysis demonstrated that 60-90% of AAV vectors administered to the CSF through either the intracisternal or intrathecal (lumbar) routes distributed systemically to major organs. These observations have potentially significant clinical implications regarding accuracy of AAV vector dosing to the nervous system, evoking systemic immunity at levels similar to that with systemic administration, and potential toxicity of genes designed to treat nervous system disorders being expressed in non-nervous system organs. Based on these data, individuals in clinical trials using AAV vectors administered to the CSF should be monitored for systemic as well as nervous system adverse events and CNS dosing considerations should account for a significant AAV systemic distribution.

Advances in therapies for neurological lysosomal storage disorders

Lysosomal Storage Disorders (LSDs) are a diverse group of inherited, monogenic diseases caused by functional defects in specific lysosomal proteins. The lysosome is a cellular organelle that plays a critical role in catabolism of waste products and recycling of macromolecules in the body. Disruption to the normal function of the lysosome can result in the toxic accumulation of storage products, often leading to irreparable cellular damage and organ dysfunction followed by premature death. The majority of LSDs have no curative treatment, with many clinical subtypes presenting in early infancy and childhood. Over two-thirds of LSDs present with progressive neurodegeneration, often in combination with other debilitating peripheral symptoms. Consequently, there is a pressing unmet clinical need to develop new therapeutic interventions to treat these conditions. The blood-brain barrier is a crucial hurdle that needs to be overcome in order to effectively treat the central nervous system (CNS), adding considerable complexity to therapeutic design and delivery. Enzyme replacement therapy (ERT) treatments aimed at either direct injection into the brain, or using blood-brain barrier constructs are discussed, alongside more conventional substrate reduction and other drug-related therapies. Other promising strategies developed in recent years, include gene therapy technologies specifically tailored for more effectively targeting treatment to the CNS. Here, we discuss the most recent advances in CNS-targeted treatments for neurological LSDs with a particular emphasis on gene therapy-based modalities, such as Adeno-Associated Virus and haematopoietic stem cell gene therapy approaches that encouragingly, at the time of writing are being evaluated in LSD clinical trials in increasing numbers. If safety, efficacy and improved quality of life can be demonstrated, these therapies have the potential to be the new standard of care treatments for LSD patients.

Assessment of Safety and Biodistribution of AAVrh.10hCLN2 Following Intracisternal Administration in Nonhuman Primates for the Treatment of CLN2 Batten Disease

CLN2 disease is a fatal, childhood autosomal recessive disorder caused by mutations in ceroid lipofuscinosis type 2 (CLN2) gene, encoding tripeptidyl peptidase 1 (TPP-1). Loss of TPP-1 activity leads to accumulation of storage material in lysosomes and resultant neuronal cell death with neurodegeneration. Genotype/phenotype comparisons suggest that the phenotype should be ameliorated with increase of TPP-1 levels to 5-10% of normal with wide central nervous system (CNS) distribution. Our previous clinical study showed that intraparenchymal (IPC) administration of AAVrh.10hCLN2, an adeno-associated vector serotype rh.10 encoding human CLN2, slowed, but did not stop disease progression, suggesting that this may be insufficient to distribute the therapy throughout the CNS (Sondhi 2020). In this study, we assessed whether the less invasive intracisternal delivery route would be safe and provide a wider distribution of TPP-1. A study was conducted in nonhuman primates (NHPs) with intracisternal delivery to cerebrospinal fluid (CSF) of AAVrh.10hCLN2 (5 × 1013 genome copies) or phosphate buffered saline (PBS). No abnormal behavior was noted. CNS magnetic resonance imaging and clinical chemistry data were all unremarkable. Histopathology of major organs had no abnormal finding attributable to the intervention or the vector, except that in one out of two animals treated with AAVrh.10hCLN2, dorsal root ganglia showed mild-to-moderate mononuclear cell infiltrates and neuronal degeneration. In contrast to our previous NHP study (Sondhi 2012) with IPC administration where TPP-1 activity was >2 × above controls in 30% of treated brains, in the two intracisternal treated NHPs, the TPP-1 activity was >2 × above controls in 50% and 41% of treated brains, and 52% and 84% of brain had >1,000 vector genomes/μg DNA, compared to 0% in the two PBS NHP. CSF TPP1 levels in treated animals were 43-62% of normal human levels. Collectively, these data indicate that AAVrh.10hCLN2 delivered by intracisternal route is safe and widely distributes TPP-1 in brain and CSF at levels that are potentially therapeutic. Clinical Trial Registration: NCT02893826, NCT04669535, NCT04273269, NCT03580083, NCT04408625, NCT04127578, and NCT04792944.

Pre-clinical delivery of gene therapy products to the cerebrospinal fluid: challenges and considerations for clinical translation

While the majority of gene therapy studies in neurological indications have focused on direct gene transfer to the central nervous system (CNS), there is growing interest in the delivery of therapeutics using the cerebrospinal fluid (CSF) as a conduit. Historically, direct CNS routes-of-administration (RoAs) have relied on tissue dynamics, displacement of interstitial fluid, and regional specificity to achieve focal delivery into regions of interest, such as the brain. While intraparenchymal delivery minimizes peripheral organ exposure, one perceived drawback is the relative invasiveness of this approach to drug delivery. In this mini review, we examine the CSF as an alternative RoA to target CNS tissue and discuss considerations associated with the safety of performing such procedures, biodistribution of therapeutics following single administration, and translation of findings given differences between small and large animals. These factors will help delineate key considerations for translating data obtained from animal studies into clinical settings that may be useful in the treatment of neurological conditions.

Scientific and Regulatory Policy Committee Technical Review: Biology and Pathology of Ganglia in Animal Species Used for Nonclinical Safety Testing

Dorsal root ganglia (DRG), trigeminal ganglia (TG), other sensory ganglia, and autonomic ganglia may be injured by some test article classes, including anti-neoplastic chemotherapeutics, adeno-associated virus-based gene therapies, antisense oligonucleotides, nerve growth factor inhibitors, and aminoglycoside antibiotics. This article reviews ganglion anatomy, cytology, and pathology (emphasizing sensory ganglia) among common nonclinical species used in assessing product safety for such test articles (TAs). Principal histopathologic findings associated with sensory ganglion injury include neuron degeneration, necrosis, and/or loss; increased satellite glial cell and/or Schwann cell numbers; and leukocyte infiltration and/or inflammation. Secondary nerve fiber degeneration and/or glial reactions may occur in nerves, dorsal spinal nerve roots, spinal cord (dorsal and occasionally lateral funiculi), and sometimes the brainstem. Ganglion findings related to TA administration may result from TA exposure and/or trauma related to direct TA delivery into the central nervous system or ganglia. In some cases, TA-related effects may need to be differentiated from a spectrum of artifactual and/or spontaneous background changes.

CNS-dominant human FMRP isoform rescues seizures, fear, and sleep abnormalities in Fmr1-KO mice

Fragile X syndrome is a neurodevelopmental disorder caused by the absence of the mRNA-binding protein fragile X messenger ribonucleoprotein (FMRP). Because FMRP is a highly pleiotropic protein controlling the expression of hundreds of genes, viral vector-mediated gene replacement therapy is viewed as a potential viable treatment to correct the fundamental underlying molecular pathology inherent in the disorder. Here, we studied the safety profile and therapeutic effects of a clinically relevant dose of a self-complementary adeno-associated viral (AAV) vector containing a major human brain isoform of FMRP after intrathecal injection into wild-type and fragile X-KO mice. Analysis of the cellular transduction in the brain indicated primarily neuronal transduction with relatively sparse glial expression, similar to endogenous FMRP expression in untreated wild-type mice. AAV vector-treated KO mice showed recovery from epileptic seizures, normalization of fear conditioning, reversal of slow-wave deficits as measured via electroencephalographic recordings, and restoration of abnormal circadian motor activity and sleep. Further assessment of vector efficacy by tracking and analyzing individual responses demonstrated correlations between the level and distribution of brain transduction and drug response. These preclinical findings further demonstrate the validity of AAV vector-mediated gene therapy for treating the most common genetic cause of cognitive impairment and autism in children.

Safety and Efficacy of Intravenous and Intrathecal Delivery of AAV9-Mediated ARSA in Minipigs

Metachromatic leukodystrophy (MLD) is a hereditary neurodegenerative disease characterized by demyelination and motor and cognitive impairments due to deficiencies of the lysosomal enzyme arylsulfatase A (ARSA) or the saposin B activator protein (SapB). Current treatments are limited; however, gene therapy using adeno-associated virus (AAV) vectors for ARSA delivery has shown promising results. The main challenges for MLD gene therapy include optimizing the AAV dosage, selecting the most effective serotype, and determining the best route of administration for ARSA delivery into the central nervous system. This study aims to evaluate the safety and efficacy of AAV serotype 9 encoding ARSA (AAV9-ARSA) gene therapy when administered intravenously or intrathecally in minipigs, a large animal model with anatomical and physiological similarities to humans. By comparing these two administration methods, this study contributes to the understanding of how to improve the effectiveness of MLD gene therapy and offers valuable insights for future clinical applications.

Adeno-associated virus vectors and neurotoxicity-lessons from preclinical and human studies

Over 15 years after hepatotoxicity was first observed following administration of an adeno-associated virus (AAV) vector during a hemophilia B clinical trial, recent reports of treatment-associated neurotoxicity in animals and humans have brought the potential impact of AAV-associated toxicity back to prominence. In both pre-clinical studies and clinical trials, systemic AAV administration has been associated with neurotoxicity in peripheral nerve ganglia and spinal cord. Neurological signs have also been seen following direct AAV injection into the brain, both in non-human primates and in a clinical trial for late infantile Batten disease. Neurotoxic events appear variable across species, and preclinical animal studies do not fully predict clinical observations. Accumulating data suggest that AAV-associated neurotoxicity may be underdiagnosed and may differ between species in terms of frequency and/or severity. In this review, we discuss the different animal models that have been used to demonstrate AAV-associated neurotoxicity, its potential causes and consequences, and potential approaches to blunt AAV-associated neurotoxicity.

Immune transgene-dependent myocarditis in macaques after systemic administration of adeno-associated virus expressing human acid alpha-glucosidase

Immune responses to human non-self transgenes can present challenges in preclinical studies of adeno-associated virus (AAV) gene therapy candidates in nonhuman primates. Although anti-transgene immune responses are usually mild and non-adverse, they can confound pharmacological readouts and complicate translation of results between species. We developed a gene therapy candidate for Pompe disease consisting of AAVhu68, a clade F AAV closely related to AAV9, that expresses an engineered human acid-alpha glucosidase (hGAA) tagged with an insulin-like growth factor 2 variant (vIGF2) peptide for enhanced cell uptake. Rhesus macaques were administered an intravenous dose of 1x1013 genome copies (GC)/kg, 5x1013 GC/kg, or 1 x 1014 GC/kg of AAVhu68.vIGF2.hGAA. Some unusually severe adaptive immune responses to hGAA presented, albeit with a high degree of variability between animals. Anti-hGAA responses ranged from absent to severe cytotoxic T-cell-mediated myocarditis with elevated troponin I levels. Cardiac toxicity was not dose dependent and affected five out of eleven animals. Upon further investigation, we identified an association between toxicity and a major histocompatibility complex class I haplotype (Mamu-A002.01) in three of these animals. An immunodominant peptide located in the C-terminal region of hGAA was subsequently identified via enzyme-linked immunospot epitope mapping. Another notable observation in this preclinical safety study cohort pertained to the achievement of robust and safe gene transfer upon intravenous administration of 5x1013 GC/kg in one animal with a low pre-existing neutralizing anti-capsid antibodies titer (1:20). Collectively, these findings may have significant implications for gene therapy inclusion criteria.

Biodistribution of Adeno-Associated Virus Gene Therapy Following Cerebrospinal Fluid-Directed Administration

Adeno-associated virus (AAV)-based gene therapies, exemplified by the approved therapy for spinal muscular atrophy, have the potential to deliver disease-course-altering treatments for central nervous system (CNS) indications. However, several clinical trials have reported severe adverse events, including patient deaths following high-dose systemic administration for muscle-directed gene transfer, highlighting the need to explore approaches utilizing lower doses when targeting the CNS. Animal models of disease provide insight into the response to new AAV therapies. However, translation from small to larger animals and eventually to humans is hampered by anatomical and biological differences across the species and their impact on AAV delivery. We performed a literature review of preclinical studies of AAV gene therapy biodistribution following cerebrospinal fluid (CSF) delivery (intracerebroventricular, intra-cisterna magna, and intrathecal lumbar). The reviewed literature varies greatly in the reported biodistribution of AAV following administration into the CSF. Differences between studies, including animal model, vector serotype used, method used to assess biodistribution, and route of administration, among other variables, contribute to differing outcomes and difficulties in translating these preclinical results. For example, only half of the published AAV-based gene therapy studies report vector copy number, the most direct readout following administration of a vector; none of these studies reported details such as the empty:full capsid ratio and quality of encapsidated genome. Analysis of the last decade's literature focusing on AAV-based gene therapies targeting the CNS underscores limitations of the body of knowledge and room for continued research. In particular, there is a need to understand the biodistribution achieved by different CSF-directed routes of administration and determining if specific cell types/structures of interest will be transduced. Our findings point to a clear need for a more systematic approach across the field to align the assessments and elements reported in preclinical research to enable more reliable translation across animal models and into human studies.

Gene therapy for liver diseases - progress and challenges

Gene therapy is poised to revolutionize modern medicine, with seemingly unlimited potential for treating and curing genetic disorders. For otherwise incurable indications, including most inherited metabolic liver disorders, gene therapy provides a realistic therapeutic option. In this Review, we discuss gene supplementation and gene editing involving the use of recombinant adeno-associated virus (rAAV) vectors for the treatment of inherited liver diseases, including updates on several ongoing clinical trials that are producing promising results. Clinical testing has been essential in highlighting many key translational challenges associated with this transformative therapy. In particular, the interaction of a patient's immune system with the vector raises issues of safety and the duration of treatment efficacy. Furthermore, several serious adverse events after the administration of high doses of rAAVs suggest greater involvement of innate immune responses and pre-existing hepatic conditions than initially anticipated. Finally, permanent modification of the host genome associated with rAAV genome integration and gene editing raises concerns about the risk of oncogenicity that require careful evaluation. We summarize the main progress, challenges and pathways forward for gene therapy for liver diseases.

Neurologic Recovery in MPS I and MPS II Mice by AAV9-Mediated Gene Transfer to the CNS After the Development of Cognitive Dysfunction

The mucopolysaccharidoses (MPS) are a group of recessively inherited conditions caused by deficiency of lysosomal enzymes essential to the catabolism of glycosaminoglycans (GAG). MPS I is caused by deficiency of the lysosomal enzyme alpha-L-iduronidase (IDUA), while MPS II is caused by a lack of iduronate-2-sulfatase (IDS). Lack of these enzymes leads to early mortality and morbidity, often including neurological deficits. Enzyme replacement therapy has markedly improved the quality of life for MPS I and MPS II affected individuals but is not effective in addressing neurologic manifestations. For MPS I, hematopoietic stem cell transplant has shown effectiveness in mitigating the progression of neurologic disease when carried out in early in life, but neurologic function is not restored in patients transplanted later in life. For both MPS I and II, gene therapy has been shown to prevent neurologic deficits in affected mice when administered early, but the effectiveness of treatment after the onset of neurologic disease manifestations has not been characterized. To test if neurocognitive function can be recovered in older animals, human IDUA or IDS-encoding AAV9 vector was administered by intracerebroventricular injection into MPS I and MPS II mice, respectively, after the development of neurologic deficit. Vector sequences were distributed throughout the brains of treated animals, associated with high levels of enzyme activity and normalized GAG storage. Two months after vector infusion, treated mice exhibited spatial navigation and learning skills that were normalized, that is, indistinguishable from those of normal unaffected mice, and significantly improved compared to untreated, affected animals. We conclude that cognitive function was restored by AAV9-mediated, central nervous system (CNS)-directed gene transfer in the murine models of MPS I and MPS II, suggesting that gene transfer may result in neurodevelopment improvements in severe MPS I and MPS II when carried out after the onset of cognitive decline.

Current advances in gene therapy of mitochondrial diseases

Mitochondrial diseases (MD) are a heterogeneous group of multisystem disorders involving metabolic errors. MD are characterized by extremely heterogeneous symptoms, ranging from organ-specific to multisystem dysfunction with different clinical courses. Most primary MD are autosomal recessive but maternal inheritance (from mtDNA), autosomal dominant, and X-linked inheritance is also known. Mitochondria are unique energy-generating cellular organelles designed to survive and contain their own unique genetic coding material, a circular mtDNA fragment of approximately 16,000 base pairs. The mitochondrial genetic system incorporates closely interacting bi-genomic factors encoded by the nuclear and mitochondrial genomes. Understanding the dynamics of mitochondrial genetics supporting mitochondrial biogenesis is especially important for the development of strategies for the treatment of rare and difficult-to-diagnose diseases. Gene therapy is one of the methods for correcting mitochondrial disorders.

AAVrh10 vector corrects pathology in animal models of GM1 gangliosidosis and achieves widespread distribution in the CNS of nonhuman primates

GM1 gangliosidosis is a rare, inherited neurodegenerative disorder caused by mutations in the GLB1 gene, which encodes the lysosomal hydrolase acid β-galactosidase (β-gal). β-gal deficiency leads to toxic accumulation of GM1 ganglioside, predominantly in the central nervous system (CNS), resulting in progressive neurodegeneration. LYS-GM101 is an AAVrh.10-based gene therapy vector carrying the human GLB1 cDNA. The efficacy of intra-cerebrospinal fluid injection of LYS-GM101 analogs was demonstrated in GM1 mouse and cat models with widespread diffusion of β-gal and correction of GM1 ganglioside accumulation in the CNS without observable adverse effects. Clinical dose selection was performed, based on a good-laboratory-practice study, in nonhuman primates (NHPs) using the clinical LYS-GM101 vector. A broadly distributed increase of β-gal activity was observed in NHP brain 3 months after intra-cisterna magna injection of LYS-GM101 at 1.0 × 10¹² vg/mL CSF and 4.0 × 10¹² vg/mL CSF, with 20% and 60% increases compared with vehicle-treated animals, respectively. Histopathologic examination revealed asymptomatic adverse changes in the sensory pathways of the spinal cord and dorsal root ganglia in both sexes and at both doses. Taken as a whole, these pre-clinical data support the initiation of a clinical study with LYS-GM101 for the treatment of GM1 gangliosidosis.

Immunogenicity assessment of AAV-based gene therapies: An IQ consortium industry white paper

Immunogenicity has imposed a challenge to efficacy and safety evaluation of adeno-associated virus (AAV) vector-based gene therapies. Mild to severe adverse events observed in clinical development have been implicated with host immune responses against AAV gene therapies, resulting in comprehensive evaluation of immunogenicity during nonclinical and clinical studies mandated by health authorities. Immunogenicity of AAV gene therapies is complex due to the number of risk factors associated with product components and pre-existing immunity in human subjects. Different clinical mitigation strategies have been employed to alleviate treatment-induced or -boosted immunogenicity in order to achieve desired efficacy, reduce toxicity, or treat more patients who are seropositive to AAV vectors. In this review, the immunogenicity risk assessment, manifestation of immunogenicity and its impact in nonclinical and clinical studies, and various clinical mitigation strategies are summarized. Last, we present bioanalytical strategies, methodologies, and assay validation applied to appropriately monitor immunogenicity in AAV gene therapy-treated subjects.

Intrathecal Drug Delivery: Advances and Applications in the Management of Chronic Pain Patient

Advances in our understanding of the biology of spinal systems in organizing and defining the content of exteroceptive information upon which higher centers define the state of the organism and its role in the regulation of somatic and automatic output, defining the motor response of the organism, along with the unique biology and spatial organization of this space, have resulted in an increased focus on therapeutics targeted at this extracranial neuraxial space. Intrathecal (IT) drug delivery systems (IDDS) are well-established as an effective therapeutic approach to patients with chronic non-malignant or malignant pain and as a tool for management of patients with severe spasticity and to deliver therapeutics that address a myriad of spinal pathologies. The risk to benefit ratio of IDD makes it a useful interventional approach. While not without risks, this approach has a significant therapeutic safety margin when employed using drugs with a validated safety profile and by skilled practioners. The present review addresses current advances in our understanding of the biology and dynamics of the intrathecal space, therapeutic platforms, novel therapeutics, delivery technology, issues of safety and rational implementation of its therapy, with a particular emphasis upon the management of pain.

A Historical Review of Brain Drug Delivery

The history of brain drug delivery is reviewed beginning with the first demonstration, in 1914, that a drug for syphilis, salvarsan, did not enter the brain, due to the presence of a blood-brain barrier (BBB). Owing to restricted transport across the BBB, FDA-approved drugs for the CNS have been generally limited to lipid-soluble small molecules. Drugs that do not cross the BBB can be re-engineered for transport on endogenous BBB carrier-mediated transport and receptor-mediated transport systems, which were identified during the 1970s-1980s. By the 1990s, a multitude of brain drug delivery technologies emerged, including trans-cranial delivery, CSF delivery, BBB disruption, lipid carriers, prodrugs, stem cells, exosomes, nanoparticles, gene therapy, and biologics. The advantages and limitations of each of these brain drug delivery technologies are critically reviewed.

Circulating neurofilament light chain as a promising biomarker of AAV-induced dorsal root ganglia toxicity in nonclinical toxicology species

Adeno-associated virus (AAV)-induced dorsal root ganglia (DRG) toxicity has been observed in several nonclinical species, where lesions are characterized by neuronal degeneration/necrosis, nerve fiber degeneration, and mononuclear cell infiltration. As AAV vectors become an increasingly common platform for novel therapeutics, non-invasive biomarkers are needed to better characterize and manage the risk of DRG neurotoxicity in both nonclinical and clinical studies. Based on biological relevance, reagent availability, antibody cross-reactivity, DRG protein expression, and assay performance, neurofilament light chain (NF-L) emerged as a promising biomarker candidate. Dose- and time-dependent changes in NF-L were evaluated in male Wistar Han rats and cynomolgus monkeys following intravenous or intrathecal AAV injection, respectively. NF-L profiles were then compared against microscopic DRG lesions on day 29 post-dosing. In animals exhibiting DRG toxicity, plasma/serum NF-L was strongly associated with the severity of neuronal degeneration/necrosis and nerve fiber degeneration, with elevations beginning as early as day 8 in rats (≥5 × 10¹³ vg/kg) and day 14 in monkeys (≥3.3 × 10¹³ vg/dose). Consistent with the unique positioning of DRGs outside the blood-brain barrier, NF-L in cerebrospinal fluid was only weakly associated with DRG findings. In summary, circulating NF-L is a promising biomarker of AAV-induced DRG toxicity in nonclinical species.

New Indications for Hematopoietic Stem Cell Gene Therapy in Lysosomal Storage Disorders

Lysosomal storage disorders (LSDs) are a heterogenous group of disorders due to genetically determined deficits of lysosomal enzymes. The specific molecular mechanism and disease phenotype depends on the type of storage material. Several disorders affect the brain resulting in severe clinical manifestations that substantially impact the expectancy and quality of life. Current treatment modalities for LSDs include enzyme replacement therapy (ERT) and hematopoietic cell transplantation (HCT) from allogeneic healthy donors, but are available for a limited number of disorders and lack efficacy on several clinical manifestations. Hematopoietic stem cell gene therapy (HSC GT) based on integrating lentiviral vectors resulted in robust clinical benefit when administered to patients affected by Metachromatic Leukodystrophy, for whom it is now available as a registered medicinal product. More recently, HSC GT has also shown promising results in Hurler syndrome patients. Here, we discuss possible novel HSC GT indications that are currently under development. If these novel drugs will prove effective, they might represent a new standard of care for these disorders, but several challenges will need to be addresses, including defining and possibly expanding the patient population for whom HSC GT could be efficacious.

Efficacy and Safety of a Krabbe Disease Gene Therapy

Krabbe disease is a lysosomal storage disease caused by mutations in the gene that encodes galactosylceramidase, in which galactosylsphingosine (psychosine) accumulation drives demyelination in the central and peripheral nervous systems, ultimately progressing to death in early childhood. Gene therapy, alone or in combination with transplant, has been developed for almost two decades in mouse models, with increasing therapeutic benefit paralleling the improvement of next-generation adeno-associated virus (AAV) vectors. This effort has recently shown remarkable efficacy in the canine model of the disease by two different groups that used either systemic or cerebrospinal fluid (CSF) administration of AAVrh10 or AAV9. Building on our experience developing CSF-delivered, AAV-based drug products for a variety of neurodegenerative disorders, we conducted efficacy, pharmacology, and safety studies of AAVhu68 delivered to the CSF in two relevant natural Krabbe animal models, and in nonhuman primates. In newborn Twitcher mice, the highest dose (1 × 10¹¹ genome copies [GC]) of AAVhu68.hGALC injected into the lateral ventricle led to a median survival of 130 days compared to 40.5 days in vehicle-treated mice. When this dose was administered intravenously, the median survival was 49 days. A single intracisterna magna injection of AAVhu68.cGALC at 3 × 10¹³ GC into presymptomatic Krabbe dogs increased survival for up to 85 weeks compared to 12 weeks in controls. It prevented psychosine accumulation in the CSF, preserved peripheral nerve myelination, ambulation, and decreased brain neuroinflammation and demyelination, although some regions remained abnormal. In a Good Laboratory Practice-compliant toxicology study, we administered the clinical candidate into the cisterna magna of 18 juvenile rhesus macaques at 3 doses that displayed efficacy in mice. We observed no dose-limiting toxicity and sporadic minimal degeneration of dorsal root ganglia (DRG) neurons. Our studies demonstrate the efficacy, scalability, and safety of a single cisterna magna AAVhu68 administration to treat Krabbe disease. ClinicalTrials.Gov ID: NCT04771416.

Characterization of AAV-mediated dorsal root ganglionopathy

Recent studies in non-human primates administered recombinant adeno-associated viruses (rAAVs) have shown lesions in the dorsal root ganglia (DRG) of unknown pathogenesis. In this study, rAAV9s manufactured using different purification methods alongside a non-expressing Null AAV9 vector was administered to groups of cynomolgus monkeys followed by neuropathological evaluation after 4 weeks. Lesions, including neuronal degeneration, increased cellularity, and nerve fiber degeneration, were observed in the DRG, regardless of purification methods. Animals did not develop any neurological signs throughout the study, and there was no loss of function observed in neuro-electrophysiological endpoints or clear effects on intraepidermal nerve fiber density. However, magnetic resonance imaging (MRI) of animals with axonopathy showed an increase in short tau inversion recovery (STIR) intensity and decrease in fractional anisotropy. In animals administered the Null AAV9 vector, DRG lesions were not observed despite vector DNA being detected in the DRG at levels equivalent to or greater than rAAV9-treated animals. This study further supports that DRG toxicity is associated with transgene overexpression in DRGs, with particular sensitivity at the lumbar and lumbosacral level. The data from this study also showed that the nerve fiber degeneration did not correlate with any functional effect on nerve conduction but was detectable by MRI.

Durability of transgene expression after rAAV gene therapy

Recombinant adeno-associated virus (rAAV) gene therapy has the potential to transform the lives of patients with certain genetic disorders by increasing or restoring function to affected tissues. Following the initial establishment of transgene expression, it is unknown how long the therapeutic effect will last, although animal and emerging human data show that expression can be maintained for more than 10 years. The durability of therapeutic response is key to long-term treatment success, especially since immune responses to rAAV vectors may prevent re-dosing with the same therapy. This review explores the non-immunological and immunological processes that may limit or improve durability and the strategies that can be used to increase the duration of the therapeutic effect.

An in vivo Cell-Based Delivery Platform for Zinc Finger Artificial Transcription Factors in Pre-clinical Animal Models

Zinc finger (ZF), transcription activator-like effectors (TALE), and CRISPR/Cas9 therapies to regulate gene expression are becoming viable strategies to treat genetic disorders, although effective in vivo delivery systems for these proteins remain a major translational hurdle. We describe the use of a mesenchymal stem/stromal cell (MSC)-based delivery system for the secretion of a ZF protein (ZF-MSC) in transgenic mouse models and young rhesus monkeys. Secreted ZF protein from mouse ZF-MSC was detectable within the hippocampus 1 week following intracranial or cisterna magna (CM) injection. Secreted ZF activated the imprinted paternal Ube3a in a transgenic reporter mouse and ameliorated motor deficits in a Ube3a deletion Angelman Syndrome (AS) mouse. Intrathecally administered autologous rhesus MSCs were well-tolerated for 3 weeks following administration and secreted ZF protein was detectable within the cerebrospinal fluid (CSF), midbrain, and spinal cord. This approach is less invasive when compared to direct intracranial injection which requires a surgical procedure.

Biodistribution and Tolerability of AAV-PHP.B-CBh-SMN1 in Wistar Han Rats and Cynomolgus Macaques Reveal Different Toxicologic Profiles

Recombinant adeno-associated viruses (AAVs) have emerged as promising vectors for human gene therapy, but some variants have induced severe toxicity in Rhesus monkeys and piglets following high-dose intravenous (IV) administration. To characterize biodistribution, transduction, and toxicity among common preclinical species, an AAV9 neurotropic variant expressing the survival motor neuron 1 (SMN1) transgene (AAV-PHP.B-CBh-SMN1) was administered by IV bolus injection to Wistar Han rats and cynomolgus monkeys at doses of 2 × 10¹³, 5 × 10¹³, or 1 × 10¹⁴ vg/kg. A dose-dependent degeneration/necrosis of neurons without clinical manifestations occurred in dorsal root ganglia (DRGs) and sympathetic thoracic ganglia in rats, while liver injury was not observed in rats. In monkeys, one male at 5 × 10¹³ vg/kg was found dead on day 4. Clinical pathology data on days 3 and/or 4 at all doses suggested liver dysfunction and coagulation disorders, which led to study termination. Histologic evaluation of the liver in monkeys showed hepatocyte degeneration and necrosis without inflammatory cell infiltrates or intravascular thrombi, suggesting that hepatocyte injury is a direct effect of the vector following hepatocyte transduction. In situ hybridization demonstrated a dose-dependent expression of SMN1 transgene mRNA in the cytoplasm and DNA in the nucleus of periportal to panlobular hepatocytes, while quantitative polymerase chain reaction confirmed the dose-dependent presence of SMN1 transgene mRNA and DNA in monkeys. Monkeys produced a much greater amount of transgene mRNA compared with rats. In DRGs, neuronal degeneration/necrosis and accompanying findings were observed in monkeys as early as 4 days after test article administration. The present results show sensory neuron toxicity following IV delivery of AAV vectors at high doses with an early onset in Macaca fascicularis and after 1 month in rats, and suggest adding the autonomic system in the watch list for preclinical and clinical studies. Our data also suggest that the rat may be useful for evaluating the potential DRG toxicity of AAV vectors, while acute hepatic toxicity associated with coagulation disorders appears to be highly species-dependent.

Real-time MR tracking of AAV gene therapy with βgal-responsive MR probe in a murine model of GM1-gangliosidosis

Transformative results of adeno-associated virus (AAV) gene therapy in patients with spinal muscular atrophy and Leber's congenital amaurosis led to approval of the first two AAV products in the United States to treat these diseases. These extraordinary results led to a dramatic increase in the number and type of AAV gene-therapy programs. However, the field lacks non-invasive means to assess levels and duration of therapeutic protein function in patients. Here, we describe a new magnetic resonance imaging (MRI) technology for real-time reporting of gene-therapy products in the living animal in the form of an MRI probe that is activated in the presence of therapeutic protein expression. For the first time, we show reliable tracking of enzyme expression after a now in-human clinical trial AAV gene therapy (ClinicalTrials.gov: NTC03952637) encoding lysosomal acid beta-galactosidase (βgal) using a self-immolative βgal-responsive MRI probe. MRI enhancement in AAV-treated enzyme-deficient mice (GLB-1^-/-) correlates with βgal activity in central nervous system and peripheral organs after intracranial or intravenous AAV gene therapy, respectively. With >1,800 gene therapies in phase I/II clinical trials (ClinicalTrials.gov), development of a non-invasive method to track gene expression over time in patients is crucial to the future of the gene-therapy field.

Seven-year follow-up of durability and safety of AAV CNS gene therapy for a lysosomal storage disorder in a large animal

Delivery of adeno-associated viral vectors (AAVs) to cerebrospinal fluid (CSF) has emerged as a promising approach to achieve widespread transduction of the central nervous system (CNS) and peripheral nervous system (PNS), with direct applicability to the treatment of a wide range of neurological diseases, particularly lysosomal storage diseases. Although studies in small animal models have provided proof of concept and experiments in large animals demonstrated feasibility in bigger brains, there is not much information on long-term safety or durability of the effect. Here, we report a 7-year study in healthy beagle dogs after intra-CSF delivery of a single, clinically relevant dose (2 × 10¹³ vg/dog) of AAV9 vectors carrying the canine sulfamidase, the enzyme deficient in mucopolysaccharidosis type IIIA. Periodic monitoring of CSF and blood, clinical and neurological evaluations, and magnetic resonance and ultrasound imaging of target organs demonstrated no toxicity related to treatment. AAV9-mediated gene transfer resulted in detection of sulfamidase activity in CSF throughout the study. Analysis at tissue level showed widespread sulfamidase expression and activity in the absence of histological findings in any region of encephalon, spinal cord, or dorsal root ganglia. Altogether, these results provide proof of durability of expression and long-term safety for intra-CSF delivery of AAV-based gene transfer vectors encoding therapeutic proteins to the CNS.

Efficacy of AAV serotypes to target Schwann cells after intrathecal and intravenous delivery

To optimize gene delivery to myelinating Schwann cells we compared clinically relevant AAV serotypes and injection routes. AAV9 and AAVrh10 vectors expressing either EGFP or the neuropathy-associated gene GJB1/Connexin32 (Cx32) under a myelin specific promoter were injected intrathecally or intravenously in wild type and Gjb1-null mice, respectively. Vector biodistribution in lumbar roots and sciatic nerves was higher in AAVrh10 injected mice while EGFP and Cx32 expression rates and levels were similar between the two serotypes. A gradient of biodistribution away from the injection site was seen with both intrathecal and intravenous delivery, while similar expression rates were achieved despite higher vector amounts injected intravenously. Quantified immune cells in relevant tissues were similar to non-injected littermates. Overall, AAV9 and AAVrh10 efficiently transduce Schwann cells throughout the peripheral nervous system with both clinically relevant routes of administration, although AAV9 and intrathecal injection may offer a more efficient approach for treating demyelinating neuropathies.

Best of most possible worlds: Hybrid gene therapy vectors based on parvoviruses and heterologous viruses

Parvoviruses and especially the adeno-associated virus (AAV) species provide an exciting and versatile platform for the rational design or molecular evolution of human gene-therapy vectors, documented by literature from over half a century, hundreds of clinical trials, and the recent commercialization of multiple AAV gene therapeutics. For the last three decades, the power of these vectors has been further potentiated through various types of hybrid vectors created by intra- or inter-genus juxtaposition of viral DNA and protein cis elements or by synergistic complementation of parvoviral features with those of heterologous, prokaryotic, or eukaryotic viruses. Here, we provide an overview of the history and promise of this rapidly expanding field of hybrid parvoviral gene-therapy vectors, starting with early generations of chimeric particles composed of a recombinant AAV genome encapsidated in shells of synthetic AAVs or of adeno-, herpes-, baculo-, or protoparvoviruses. We then dedicate our attention to two newer, highly promising types of hybrid vectors created via (1) pseudotyping of AAV genomes with bocaviral serotypes and capsid mutants or (2) packaging of AAV DNA into, or tethering of entire vector particles to, bacteriophages. Finally, we conclude with an outlook summarizing critical requirements and improvements toward clinical translation of these original concepts.

Scientific and Regulatory Policy Committee Points to Consider: Nonclinical Research and Development of In Vivo Gene Therapy Products, Emphasizing Adeno-Associated Virus Vectors

Sequencing of the human genome and numerous advances in molecular techniques have launched the era of genetic medicine. Increasingly precise technologies for genetic modification, manufacturing, and administration of pharmaceutical-grade biologics have proved the viability of in vivo gene therapy (GTx) as a therapeutic modality as shown in several thousand clinical trials and recent approval of several GTx products for treating rare diseases and cancers. In recognition of the rapidly advancing knowledge in this field, the regulatory landscape has evolved considerably to maintain appropriate monitoring of safety concerns associated with this modality. Nonetheless, GTx safety assessment remains complex and is designed on a case-by-case basis that is determined by the disease indication and product attributes. This article describes our current understanding of fundamental biological principles and possible procedures (emphasizing those related to toxicology and toxicologic pathology) needed to support research and development of in vivo GTx products. This article is not intended to provide comprehensive guidance on all GTx modalities but instead provides an overview relevant to in vivo GTx generally by utilizing recombinant adeno-associated virus-based GTx-the most common in vivo GTx platform-to exemplify the main points to be considered in nonclinical research and development of GTx products.

Quantitative Whole-Body Imaging of I-124-Labeled Adeno-Associated Viral Vector Biodistribution in Nonhuman Primates

A method is presented for quantitative analysis of the biodistribution of adeno-associated virus (AAV) gene transfer vectors following in vivo administration. We used iodine-124 (I-124) radiolabeling of the AAV capsid and positron emission tomography combined with compartmental modeling to quantify whole-body and organ-specific biodistribution of AAV capsids from 1 to 72 h following administration. Using intravenous (IV) and intracisternal (IC) routes of administration of AAVrh.10 and AAV9 vectors to nonhuman primates in the absence or presence of anticapsid immunity, we have identified novel insights into initial capsid biodistribution and organ-specific capsid half-life. Neither I-124-labeled AAVrh.10 nor AAV9 administered intravenously was detected at significant levels in the brain relative to the administered vector dose. Approximately 50% of the intravenously administered labeled capsids were dispersed throughout the body, independent of the liver, heart, and spleen. When administered by the IC route, the labeled capsid had a half-life of ∼10 h in the cerebral spinal fluid (CSF), suggesting that by this route, the CSF serves as a source with slow diffusion into the brain. For both IV and IC administration, there was significant influence of pre-existing anticapsid immunity on I-124-capsid biodistribution. The methodology facilitates quantitative in vivo viral vector dosimetry, which can serve as a technique for evaluation of both on- and off-target organ biodistribution, and potentially accelerate gene therapy development through rapid prototyping of novel vector designs.

Intravenous delivery of adeno-associated viral gene therapy in feline GM1 gangliosidosis

GM1 gangliosidosis is a fatal neurodegenerative disease caused by a deficiency of lysosomal β-galactosidase. In its most severe form, GM1 gangliosidosis causes death by 4 years of age, and no effective treatments exist. Previous work has shown that injection of the brain parenchyma with an adeno-associated viral vector provides pronounced therapeutic benefit in a feline GM1 model. To develop a less invasive treatment for the brain and increase systemic biodistribution, intravenous injection of AAV9 was evaluated. AAV9 expressing feline β-galactosidase was intravenously administered at 1.5x1013 vector genomes/kilogram body weight to six GM1 cats at approximately 1 month of age. The animals were divided into two cohorts: 1) a long-term group, which was followed to humane endpoint, and 2) a short-term group, which was analyzed 16-weeks post treatment. Clinical assessments included neurological exams, cerebrospinal fluid and urine biomarkers, and 7-Telsa magnetic resonance imaging and spectroscopy. Postmortem analysis included β-galactosidase and virus distribution, histological analysis, and ganglioside content. Untreated GM1 animals survived 8.0 ± 0.6 months while intravenous treatment increased survival to an average of 3.5 years (n = 2) with substantial improvements in quality of life and neurologic function. Neurological abnormalities, which in untreated animals progress to the inability to stand and debilitating neurological disease by 8 months of age, were mild in all treated animals. Cerebrospinal fluid biomarkers were normalized, indicating decreased central nervous system cell damage in the treated animals. Urinary glycosaminoglycans decreased to normal levels in the long-term cohort. Magnetic resonance imaging and spectroscopy showed partial preservation of the brain in treated animals, which was supported by postmortem histological evaluation. β-galactosidase activity was increased throughout the central nervous system, reaching carrier levels in much of the cerebrum and normal levels in the cerebellum, spinal cord and cerebrospinal fluid. Ganglioside accumulation was significantly reduced by treatment. Peripheral tissues such as heart, skeletal muscle, and sciatic nerve also had normal β-galactosidase activity in treated GM1 cats. GM1 histopathology was largely corrected with treatment. There was no evidence of tumorigenesis or toxicity. Restoration of β-galactosidase activity in the central nervous system and peripheral organs by intravenous gene therapy led to profound increases in lifespan and quality of life in GM1 cats. This data supports the promise of intravenous gene therapy as a safe, effective treatment for GM1 gangliosidosis.

Development challenges associated with rAAV-based gene therapies

The number of gene therapies in development continues to increase, as they represent a novel method to treat, and potentially cure, many diseases. Gene therapies can be conducted with an in vivo or ex vivo approach, to cause gene augmentation, gene suppression, or genomic editing. Adeno-associated viruses are commonly used to deliver gene therapies, but their use is associated with several manufacturing, nonclinical and clinical challenges. As these challenges emerge, regulatory agency expectations continue to evolve. Following administration of rAAV-based gene therapies, nonclinical toxicities may occur, which includes immunogenicity, hepatotoxicity, neurotoxicity, and the potential risks for insertional mutagenesis and subsequent tumorgenicity. The mechanism for these findings and translation into the clinical setting are unclear at this time but have influenced the nonclinical studies that regulatory agencies are increasingly requesting to support clinical trials and marketing authorizations. These evolving regulatory expectations and toxicities, as well as future nonclinical considerations, are discussed herein.

MicroRNA-mediated inhibition of transgene expression reduces dorsal root ganglion toxicity by AAV vectors in primates

Delivering adeno-associated virus (AAV) vectors into the central nervous system of nonhuman primates (NHPs) via the blood or cerebral spinal fluid is associated with dorsal root ganglion (DRG) toxicity. Conventional immune-suppression regimens do not prevent this toxicity, possibly because it may be caused by high transduction rates, which can, in turn, cause cellular stress due to an overabundance of the transgene product in target cells. To test this hypothesis and develop an approach to eliminate DRG toxicity, we exploited endogenous expression of microRNA (miR) 183 complex, which is largely restricted to DRG neurons, to specifically down-regulate transgene expression in these cells. We introduced sequence targets for miR183 into the vector genome within the 3' untranslated region of the corresponding transgene messenger RNA and injected vectors into the cisterna magna of NHPs. Administration of unmodified AAV vectors resulted in robust transduction of target tissues and toxicity in DRG neurons. Consistent with the proposal that immune system activity does not mediate this neuronal toxicity, we found that steroid administration was ineffective in alleviating this pathology. However, including miR183 targets in the vectors reduced transgene expression in, and toxicity of, DRG neurons without affecting transduction elsewhere in the primate's brain. This approach might be useful in reducing DRG toxicity and the associated morbidity and should facilitate the development of AAV-based gene therapies for many central nervous system diseases.

Hurdles in treating Hurler disease: potential routes to achieve a "real" cure

Mucopolysaccharidoses (MPSs) are multiorgan devastating diseases for which hematopoietic cell transplantation (HCT) and, to a lesser extent, enzyme replacement therapy have substantially altered the course of the disease. Furthermore, they have resulted in increased overall survival, especially for Hurler disease (MPS-1). However, despite the identification of clinical predictors and harmonized transplantation protocols, disease progression still poses a significant burden to patients, although at a slower pace. To design better therapies, we need to understand why and where current therapies fail. In this review, we discuss important aspects of the underlying disease and the disease progression. We note that the majority of progressive symptoms that occur in "hard-to-treat" tissues are actually tissues that are difficult to reach, such as avascular connective tissue or tissues isolated from the circulation by a specific barrier (eg, blood-brain barrier, blood-retina barrier). Although easily reached tissues are effectively cured by HCT, disease progression is observed in these "hard-to-reach" tissues. We used these insights to critically appraise ongoing experimental endeavors with regard to their potential to overcome the encountered hurdles and improve long-term clinical outcomes in MPS patients treated with HCT.

From traditional pharmacological towards nucleic acid-based therapies for cardiovascular diseases

Nucleic acid-based therapeutics are currently developed at large scale for prevention and management of cardiovascular diseases (CVDs), since: (i) genetic studies have highlighted novel therapeutic targets suggested to be causal for CVD; (ii) there is a substantial recent progress in delivery, efficacy, and safety of nucleic acid-based therapies; (iii) they enable effective modulation of therapeutic targets that cannot be sufficiently or optimally addressed using traditional small molecule drugs or antibodies. Nucleic acid-based therapeutics include (i) RNA-targeted therapeutics for gene silencing; (ii) microRNA-modulating and epigenetic therapies; (iii) gene therapies; and (iv) genome-editing approaches (e.g. CRISPR-Cas-based): (i) RNA-targeted therapeutics: several large-scale clinical development programmes, using antisense oligonucleotides (ASO) or short interfering RNA (siRNA) therapeutics for prevention and management of CVD have been initiated. These include ASO and/or siRNA molecules to lower apolipoprotein (a) [apo(a)], proprotein convertase subtilisin/kexin type 9 (PCSK9), apoCIII, ANGPTL3, or transthyretin (TTR) for prevention and treatment of patients with atherosclerotic CVD or TTR amyloidosis. (ii) MicroRNA-modulating and epigenetic therapies: novel potential therapeutic targets are continually arising from human non-coding genome and epigenetic research. First microRNA-based therapeutics or therapies targeting epigenetic regulatory pathways are in clinical studies. (iii) Gene therapies: EMA/FDA have approved gene therapies for non-cardiac monogenic diseases and LDL receptor gene therapy is currently being examined in patients with homozygous hypercholesterolaemia. In experimental studies, gene therapy has significantly improved cardiac function in heart failure animal models. (iv) Genome editing approaches: these technologies, such as using CRISPR-Cas, have proven powerful in stem cells, however, important challenges are remaining, e.g. low rates of homology-directed repair in somatic cells such as cardiomyocytes. In summary, RNA-targeted therapies (e.g. apo(a)-ASO and PCSK9-siRNA) are now in large-scale clinical outcome trials and will most likely become a novel effective and safe therapeutic option for CVD in the near future. MicroRNA-modulating, epigenetic, and gene therapies are tested in early clinical studies for CVD. CRISPR-Cas-mediated genome editing is highly effective in stem cells, but major challenges are remaining in somatic cells, however, this field is rapidly advancing.

Comparative Effectiveness of Intracerebroventricular, Intrathecal, and Intranasal Routes of AAV9 Vector Administration for Genetic Therapy of Neurologic Disease in Murine Mucopolysaccharidosis Type I

Mucopolysaccharidosis type I (MPS I) is an inherited metabolic disorder caused by deficiency of the lysosomal enzyme alpha-L-iduronidase (IDUA). The two current treatments [hematopoietic stem cell transplantation (HSCT) and enzyme replacement therapy (ERT)], are insufficiently effective in addressing neurologic disease, in part due to the inability of lysosomal enzyme to cross the blood brain barrier. With a goal to more effectively treat neurologic disease, we have investigated the effectiveness of AAV-mediated IDUA gene delivery to the brain using several different routes of administration. Animals were treated by either direct intracerebroventricular (ICV) injection, by intrathecal (IT) infusion into the cerebrospinal fluid, or by intranasal (IN) instillation of AAV9-IDUA vector. AAV9-IDUA was administered to IDUA-deficient mice that were either immunosuppressed with cyclophosphamide (CP), or immunotolerized at birth by weekly injections of human iduronidase. In animals treated by ICV or IT administration, levels of IDUA enzyme ranged from 3- to 1000-fold that of wild type levels in all parts of the microdissected brain. In animals administered vector intranasally, enzyme levels were 100-fold that of wild type in the olfactory bulb, but enzyme expression was close to wild type levels in other parts of the brain. Glycosaminoglycan levels were reduced to normal in ICV and IT treated mice, and in IN treated mice they were normalized in the olfactory bulb, or reduced in other parts of the brain. Immunohistochemical analysis showed extensive IDUA expression in all parts of the brain of ICV treated mice, while IT treated animals showed transduction that was primarily restricted to the hind brain with some sporadic labeling seen in the mid- and fore brain. At 6 months of age, animals were tested for spatial navigation, memory, and neurocognitive function in the Barnes maze; all treated animals were indistinguishable from normal heterozygous control animals, while untreated IDUA deficient animals exhibited significant learning and spatial navigation deficits. We conclude that IT and IN routes are acceptable and alternate routes of administration, respectively, of AAV vector delivery to the brain with effective IDUA expression, while all three routes of administration prevent the emergence of neurocognitive deficiency in a mouse MPS I model.

Novel Combinatorial MicroRNA-Binding Sites in AAV Vectors Synergistically Diminish Antigen Presentation and Transgene Immunity for Efficient and Stable Transduction

Recombinant adeno-associated virus (rAAV) platforms hold promise for in vivo gene therapy but are undermined by the undesirable transduction of antigen presenting cells (APCs), which in turn can trigger host immunity towards rAAV-expressed transgene products. In light of recent adverse events in patients receiving high systemic AAV vector doses that were speculated to be related to host immune responses, development of strategies to mute innate and adaptive immunity is imperative. The use of miRNA binding sites (miR-BSs) to confer endogenous miRNA-mediated regulation to detarget transgene expression from APCs has shown promise for reducing transgene immunity. Studies have shown that designing miR-142BSs into rAAV1 vectors were able to repress costimulatory signals in dendritic cells (DCs), blunt the cytotoxic T cell response, and attenuate clearance of transduced muscle cells in mice to allow sustained transgene expression in myofibers with negligible anti-transgene IgG production. In this study, we screened individual and combinatorial miR-BS designs against 26 miRNAs that are abundantly expressed in APCs, but not in skeletal muscle. The highly immunogenic ovalbumin (OVA) transgene was used as a proxy for foreign antigens. In vitro screening in myoblasts, mouse DCs, and macrophages revealed that the combination of miR-142BS and miR-652-5pBS strongly mutes transgene expression in APCs but maintains high myoblast and myocyte expression. Importantly, rAAV1 vectors carrying this novel miR-142/652-5pBS cassette achieve higher transgene levels following intramuscular injections in mice than previous detargeting designs. The cassette strongly inhibits cytotoxic CTL activation and suppresses the Th17 response in vivo. Our approach, thus, advances the efficiency of miRNA-mediated detargeting to achieve synergistic reduction of transgene-specific immune responses and the development of safe and efficient delivery vehicles for gene therapy.

Optimization of AAV vectors to target persistent viral reservoirs

Gene delivery of antiviral therapeutics to anatomical sites where viruses accumulate and persist is a promising approach for the next generation of antiviral therapies. Recombinant adeno-associated viruses (AAV) are one of the leading vectors for gene therapy applications that deliver gene-editing enzymes, antibodies, and RNA interference molecules to eliminate viral reservoirs that fuel persistent infections. As long-lived viral DNA within specific cellular reservoirs is responsible for persistent hepatitis B virus, Herpes simplex virus, and human immunodeficiency virus infections, the discovery of AAV vectors with strong tropism for hepatocytes, sensory neurons and T cells, respectively, is of particular interest. Identification of natural isolates from various tissues in humans and non-human primates has generated an extensive catalog of AAV vectors with diverse tropisms and transduction efficiencies, which has been further expanded through molecular genetic approaches. The AAV capsid protein, which forms the virions' outer shell, is the primary determinant of tissue tropism, transduction efficiency, and immunogenicity. Thus, over the past few decades, extensive efforts to optimize AAV vectors for gene therapy applications have focused on capsid engineering with approaches such as directed evolution and rational design. These approaches are being used to identify variants with improved transduction efficiencies, alternate tropisms, reduced sequestration in non-target organs, and reduced immunogenicity, and have produced AAV capsids that are currently under evaluation in pre-clinical and clinical trials. This review will summarize the most recent strategies to identify AAV vectors with enhanced tropism and transduction in cell types that harbor viral reservoirs.

Krabbe disease successfully treated via monotherapy of intrathecal gene therapy

Globoid cell leukodystrophy (GLD; Krabbe disease) is a progressive, incurable neurodegenerative disease caused by deficient activity of the hydrolytic enzyme galactosylceramidase (GALC). The ensuing cytotoxic accumulation of psychosine results in diffuse central and peripheral nervous system (CNS, PNS) demyelination. Presymptomatic hematopoietic stem cell transplantation (HSCT) is the only treatment for infantile-onset GLD; however, clinical outcomes of HSCT recipients often remain poor, and procedure-related morbidity is high. There are no effective therapies for symptomatic patients. Herein, we demonstrate in the naturally occurring canine model of GLD that presymptomatic monotherapy with intrathecal AAV9 encoding canine GALC administered into the cisterna magna increased GALC enzyme activity, normalized psychosine concentration, improved myelination, and attenuated inflammation in both the CNS and PNS. Moreover, AAV-mediated therapy successfully prevented clinical neurological dysfunction, allowing treated dogs to live beyond 2.5 years of age, more than 7 times longer than untreated dogs. Furthermore, we found that a 5-fold lower dose resulted in an attenuated form of disease, indicating that sufficient dosing is critical. Finally, postsymptomatic therapy with high-dose AAV9 also significantly extended lifespan, signifying a treatment option for patients for whom HSCT is not applicable. If translatable to patients, these findings would improve the outcomes of patients treated either pre- or postsymptomatically.

Mucopolysaccharidosis Type I: Current Treatments, Limitations, and Prospects for Improvement

Mucopolysaccharidosis type I (MPS I) is a lysosomal disease, caused by a deficiency of the enzyme alpha-L-iduronidase (IDUA). IDUA catalyzes the degradation of the glycosaminoglycans dermatan and heparan sulfate (DS and HS, respectively). Lack of the enzyme leads to pathologic accumulation of undegraded HS and DS with subsequent disease manifestations in multiple organs. The disease can be divided into severe (Hurler syndrome) and attenuated (Hurler-Scheie, Scheie) forms. Currently approved treatments consist of enzyme replacement therapy (ERT) and/or hematopoietic stem cell transplantation (HSCT). Patients with attenuated disease are often treated with ERT alone, while the recommended therapy for patients with Hurler syndrome consists of HSCT. While these treatments significantly improve disease manifestations and prolong life, a considerable burden of disease remains. Notably, treatment can partially prevent, but not significantly improve, clinical manifestations, necessitating early diagnosis of disease and commencement of treatment. This review discusses these standard therapies and their impact on common disease manifestations in patients with MPS I. Where relevant, results of animal models of MPS I will be included. Finally, we highlight alternative and emerging treatments for the most common disease manifestations.

Systemic Treatment of Fabry Disease Using a Novel AAV9 Vector Expressing α-Galactosidase A

Fabry disease is a rare X-linked disorder affecting α-galactosidase A, a rate-limiting enzyme in lysosomal catabolism of glycosphingolipids. Current treatments present important limitations, such as low half-life and limited distribution, which gene therapy can overcome. The aim of this work was to test a novel adeno-associated viral vector, serotype 9 (AAV9), ubiquitously expressing human α-galactosidase A to treat Fabry disease (scAAV9-PGK-GLA). The vector was preliminary tested in newborns of a Fabry disease mouse model. 5 months after treatment, α-galactosidase A activity was detectable in the analyzed tissues, including the central nervous system. Moreover, we tested the vector in adult animals of both sexes at two doses and disease stages (presymptomatic and symptomatic) by single intravenous injection. We found that the exogenous α-galactosidase A was active in peripheral tissues as well as the central nervous system and prevented glycosphingolipid accumulation in treated animals up to 5 months following injection. Antibodies against α-galactosidase A were produced in 9 out of 32 treated animals, although enzyme activity in tissues was not significantly affected. These results demonstrate that scAAV9-PGK-GLA can drive widespread and sustained expression of α-galactosidase A, cross the blood brain barrier after systemic delivery, and reduce pathological signs of the Fabry disease mouse model.

Intravenous delivery for treatment of mucopolysaccharidosis type I: A comparison of AAV serotypes 9 and rh10

Mucopolysaccharidosis type I (MPS I) is an inherited metabolic disorder caused by deficiency of alpha-L-iduronidase (IDUA), resulting in accumulation of heparan and dermatan sulfate glycosaminoglycans (GAGs). Individuals with the most severe form of the disease (Hurler syndrome) suffer from neurodegeneration, intellectual disability, and death by age 10. Current treatments for this disease include allogeneic hematopoietic stem cell transplantation (HSCT) and enzyme replacement therapy (ERT). However, these treatments do not address CNS manifestations of the disease. In this study we compared the ability of intravenously administered AAV serotypes 9 and rh10 (AAV9 and AAVrh10) for delivery and expression of the IDUA gene in the CNS. Adult C57BL/6 MPS I mice were infused intravenously with either AAV9 or AAVrh10 vector encoding the human IDUA gene. Treated animals demonstrated supraphysiological levels and widespread restoration of IDUA enzyme activity in the plasma and all organs including the CNS. High levels of IDUA enzyme activity were observed in the plasma, brain and spinal cord ranging from 10 to 100-fold higher than heterozygote controls, while levels in peripheral organs were also high, ranging from 1000 to 10,000-fold higher than control animals. In general, levels of IDUA expression were slightly higher in peripheral organs for AAVrh10 administered animals although these differences were not significant except for the lung. Levels of IDUA expression between AAV 9 and rh10 were roughly equivalent in the brain. Urinary and tissue GAGs were significantly reduced starting at 3 weeks after vector infusion, with restoration of normal GAG levels by the end of the study in animals treated with either AAV9 or rh10. These results demonstrate that non-invasive intravenous AAV9 or AAVrh10-mediated IDUA gene therapy is a potentially effective treatment for both systemic and CNS manifestations of MPS I, with implications for the treatment of other metabolic and neurological diseases as well.

Adeno-Associated Virus-Induced Dorsal Root Ganglion Pathology

The administration of adeno-associated virus (AAV) vectors to nonhuman primates (NHP) via the blood or cerebrospinal fluid (CSF) can lead to dorsal root ganglion (DRG) pathology. The pathology is minimal to moderate in most cases; clinically silent in affected animals; and characterized by mononuclear cell infiltrates, neuronal degeneration, and secondary axonopathy of central and peripheral axons on histopathological analysis. We aggregated data from 33 nonclinical studies in 256 NHP and performed a meta-analysis of the severity of DRG pathology to compare different routes of administration, dose, time course, study conduct, age of the animals, sex, capsid, promoter, capsid purification method, and transgene. DRG pathology was observed in 83% of NHP that were administered AAV through the CSF, and 32% of NHP that received an intravenous (IV) injection. We show that dose and age at injection significantly affected the severity whereas sex had no impact. DRG pathology was minimal at acute time points (i.e., <14 days), similar from one to 5 months post-injection, and was less severe after 6 months. Vector purification method had no impact, and all capsids and promoters that we tested resulted in some DRG pathology. The data presented here from five different capsids, five different promoters, and 20 different transgenes suggest that DRG pathology is almost universal after AAV gene therapy in nonclinical studies using NHP. None of the animals receiving a therapeutic transgene displayed any clinical signs. Incorporation of sensitive techniques such as nerve-conduction velocity testing can show alterations in a minority of animals that correlate with the severity of peripheral nerve axonopathy. Monitoring sensory neuropathies in human central nervous system and high-dose IV clinical studies seems prudent to determine the functional consequences of DRG pathology.

Challenges for Therapeutic Applications of Opsin-Based Optogenetic Tools in Humans

As the technological hurdles are overcome and optogenetic techniques advance to have more control over neurons, therapies based on these approaches will begin to emerge in the clinic. Here, we consider the technical challenges surrounding the transition of this breakthrough technology from an investigative tool to a true therapeutic avenue. The emerging strategies and remaining tasks surrounding genetically encoded molecules which respond to light as well as the vehicles required to deliver them are discussed.The use of optogenetics in humans would represent a completely new paradigm in medicine and would be associated with unprecedented technical considerations. To be applied for stimulation of neurons in humans, an ideal optogenetic tool would need to be non-immunogenic, highly sensitive, and activatable with red light or near-infrared light (to maximize light penetration while minimizing photodamage). To enable sophisticated levels of neuronal control, the combined use of optogenetic actuators and indicators could enable closed-loop all-optical neuromodulation. Such systems would introduce additional challenges related to spectral orthogonality between actuator and indicator, the need for decision making computational algorithms and requirements for large gene cassettes. As in any gene therapy, the therapeutic efficiency of optogenetics will rely on vector delivery and expression in the appropriate cell type. Although viral vectors such as those based on AAVs are showing great potential in human trials, barriers to their general use remain, including immune responses, delivery/transport, and liver clearance. Limitations associated with the gene cassette size which can be packaged in currently approved vectors also need to be addressed.

In Vivo Gene Therapy for Mucopolysaccharidosis Type III (Sanfilippo Syndrome): A New Treatment Horizon

For most lysosomal storage diseases (LSDs), there is no cure. Gene therapy is an attractive tool for treatment of LSDs caused by deficiencies in secretable lysosomal enzymes, in which neither full restoration of normal enzymatic activity nor transduction of all cells of the affected organ is necessary. However, some LSDs, such as mucopolysaccharidosis type III (MPSIII) diseases or Sanfilippo syndrome, represent a difficult challenge because patients suffer severe neurodegeneration with mild somatic alterations. The disease's main target is the central nervous system (CNS) and enzymes do not efficiently cross the blood-brain barrier (BBB) even if present at very high concentration in circulation. No specific treatment has been approved for MPSIII. In this study, we discuss the adeno-associated virus (AAV) vector-mediated gene transfer strategies currently being developed for MPSIII disease. These strategies rely on local delivery of AAV vectors to the CNS either through direct intraparenchymal injection at several sites or through delivery to the cerebrospinal fluid (CSF), which bathes the whole CNS, or exploit the properties of certain AAV serotypes capable of crossing the BBB upon systemic administration. Although studies in small and large animal models of MPSIII diseases have provided evidence supporting the efficacy and safety of all these strategies, there are considerable differences between the different routes of administration in terms of procedure-associated risks, vector dose requirements, sensitivity to the effect of circulating neutralizing antibodies that block AAV transduction, and potential toxicity. Ongoing clinical studies should shed light on which gene transfer strategy leads to highest clinical benefits while minimizing risks. The development of all these strategies opens a new horizon for treatment of not only MPSIII and other LSDs but also of a wide range of neurological diseases.

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.

A Cure for Sanfilippo Syndrome? A Summary of Current Therapeutic Approaches and their Promise

Mucopolysaccharidoses III (MPS III, Sanfilippo syndrome) is a subtype of the Mucopolysaccharidoses (MPS), a group of inherited lysosomal disorders caused by a deficiency of lysosomal enzymes responsible for catabolizing glycosaminoglycans (GAGs). Although MPS III is rare, MPS diseases as a group are relatively frequent with an overall incidence of approximately 1 in 20,000 - 25,000 births. MPS III are paediatric diseases, which cause learning difficulties, behavioural disorders and dementia, as well as skeletal deformities and ultimately result in premature death. There are currently no approved treatments for MPS III, but a number of therapeutic approaches are under development. In the past 30 years, research using cellular and animal models have led to clinical trials involving enzyme replacement therapy (ERT), substrate reduction therapy (SRT) and gene therapy, while stem cells approaches remain at the pre-clinical stage. Although safety and clinical efficacy in animal models have shown promise, the results of clinical trials have proved costly and shown limited therapeutic effects. In this review, we describe the most recent results from clinical trials. While ERT and gene therapy are the most developed therapies for MPS III, we highlight the work that needs to be done to bring us closer to a real treatment for these devastating diseases.

Sexually Divergent Mortality and Partial Phenotypic Rescue After Gene Therapy in a Mouse Model of Dravet Syndrome

Dravet syndrome (DS) is a neurodevelopmental genetic disorder caused by mutations in the SCN1A gene encoding the α subunit of the NaV1.1 voltage-gated sodium channel that controls neuronal action potential firing. The high density of this mutated channel in GABAergic interneurons results in impaired inhibitory neurotransmission and subsequent excessive activation of excitatory neurons. The syndrome is associated with severe childhood epilepsy, autistic behaviors, and sudden unexpected death in epilepsy. Here, we compared the rescue effects of an adeno-associated viral (AAV) vector coding for the multifunctional β1 sodium channel auxiliary subunit (AAV-NaVβ1) with a control vector lacking a transgene. We hypothesized that overexpression of NaVβ1 would facilitate the function of residual voltage-gated channels and improve the DS phenotype in the Scn1a^+/− mouse model of DS. AAV-NaVβ1 was injected into the cerebral spinal fluid of neonatal Scn1a^+/− mice. In untreated control Scn1a^+/− mice, females showed a higher degree of mortality than males. Compared with Scn1a^+/− control mice, AAV-NaVβ1-treated Scn1a^+/− mice displayed increased survival, an outcome that was more pronounced in females than males. In contrast, behavioral analysis revealed that male, but not female, Scn1a^+/− mice displayed motor hyperactivity, and abnormal performance on tests of fear and anxiety and learning and memory. Male Scn1a^+/− mice treated with AAV-NaVβ1 showed reduced spontaneous seizures and normalization of motor activity and performance on the elevated plus maze test. These findings demonstrate sex differences in mortality in untreated Scn1a^+/− mice, an effect that may be related to a lower level of intrinsic inhibitory tone in female mice, and a normalization of aberrant behaviors in males after central nervous system administration of AAV-NaVβ1. The therapeutic efficacy of AAV-NaVβ1 in a mouse model of DS suggests a potential new long-lasting biological therapeutic avenue for the treatment of this catastrophic epilepsy.

Blood-Brain Barrier and Delivery of Protein and Gene Therapeutics to Brain

Alzheimer’s disease (AD) and treatment of the brain in aging require the development of new biologic drugs, such as recombinant proteins or gene therapies. Biologics are large molecule therapeutics that do not cross the blood-brain barrier (BBB). BBB drug delivery is the limiting factor in the future development of new therapeutics for the brain. The delivery of recombinant protein or gene medicines to the brain is a binary process: either the brain drug developer re-engineers the biologic with BBB drug delivery technology, or goes forward with brain drug development in the absence of a BBB delivery platform. The presence of BBB delivery technology allows for engineering the therapeutic to enable entry into the brain across the BBB from blood. Brain drug development may still take place in the absence of BBB delivery technology, but with a reliance on approaches that have rarely led to FDA approval, e.g., CSF injection, stem cells, small molecules, and others. CSF injection of drug is the most widely practiced approach to brain delivery that bypasses the BBB. However, drug injection into the CSF results in limited drug penetration to the brain parenchyma, owing to the rapid export of CSF from the brain to blood. A CSF injection of a drug is equivalent to a slow intravenous (IV) infusion of the pharmaceutical. Given the profound effect the existence of the BBB has on brain drug development, future drug or gene development for the brain will be accelerated by future advances in BBB delivery technology in parallel with new drug discovery.