Safety Assessment of AAV2-hGDNF Administered Via Intracerebral Injection in Rats for Treatment of Parkinson's Disease

Glial cell line-derived neurotrophic factor (GDNF) is a potent neuroprotective biologic in Parkinson's disease models. Adeno-associated viral vector serotype 2 (AAV2)-human GDNF safety was assessed in rats treated with a single intracerebral dose of vehicle, 6.8 × 108, 6.8 × 109, or 5.2 × 1010 vector genomes (vg)/dose followed by interim sacrifices on day 7, 31, 90, and 376. There were no treatment-related effects observed on food consumption, body weight, hematology, clinical chemistry, coagulation parameters, neurobehavioral parameters, organ weights, or serum GDNF and anti-GDNF antibody levels. Increased serum anti-AAV2 neutralizing antibody titers were observed in the 5.2 × 1010 vg/dose group. Histopathological lesions were observed at the injection site in the 6.8 × 109 vg/dose (day 7) and 5.2 × 1010 vg/dose groups (days 7 and 31) and consisted of gliosis, mononuclear perivascular cuffing, intranuclear inclusion bodies, and/or apoptosis on day 7 and mononuclear perivascular cuffing on day 31. GDNF immunostaining was observed in the injection site in all dose groups through day 376 indicating no detectable impacts of anti-AAV2 neutralizing antibody. There was no evidence of increased expression of calcitonin gene-related peptide or Swann cell hyperplasia in the cervical and lumbar spinal cord or medulla oblongata at the 5.2 × 1010 vg/dose level indicating lack of hyperplastic effects. In conclusion, no systemic toxicity was observed, and the local toxicity observed at the injection site appeared to be reversible demonstrating a promising safety profile of intracerebral AAV2-GDNF delivery. Furthermore, an intracerebral dose of 6.8 × 108 AAV2-GDNF vg/dose was considered to be a no observed adverse effect level in rats.

Data-driven evolution of neurosurgical gene therapy delivery in Parkinson's disease

Loss of nigrostriatal dopaminergic projection neurons is a key pathology in Parkinson's disease, leading to abnormal function of basal ganglia motor circuits and the accompanying characteristic motor features. A number of intraparenchymally delivered gene therapies designed to modify underlying disease and/or improve clinical symptoms have shown promise in preclinical studies and subsequently were evaluated in clinical trials. Here we review the challenges with surgical delivery of gene therapy vectors that limited therapeutic outcomes in these trials, particularly the lack of real-time monitoring of vector administration. These challenges have recently been addressed during the evolution of novel techniques for vector delivery that include the use of intraoperative MRI. The preclinical development of these techniques are described in relation to recent clinical translation in an adeno-associated virus serotype 2-mediated human aromatic L-amino acid decarboxylase gene therapy development programme. This new paradigm allows visualisation of the accuracy and adequacy of viral vector delivery within target structures, enabling intertrial modifications in surgical approaches, cannula design, vector volumes and dosing. The rapid, data-driven evolution of these procedures is unique and has led to improved vector delivery.

GDNF and Parkinson's Disease: Where Next? A Summary from a Recent Workshop

The concept of repairing the brain with growth factors has been pursued for many years in a variety of neurodegenerative diseases including primarily Parkinson's disease (PD) using glial cell line-derived neurotrophic factor (GDNF). This neurotrophic factor was discovered in 1993 and shown to have selective effects on promoting survival and regeneration of certain populations of neurons including the dopaminergic nigrostriatal pathway. These observations led to a series of clinical trials in PD patients including using infusions or gene delivery of GDNF or the related growth factor, neurturin (NRTN). Initial studies, some of which were open label, suggested that this approach could be of value in PD when the agent was injected into the putamen rather than the cerebral ventricles. In subsequent double-blind, placebo-controlled trials, the most recent reporting in 2019, treatment with GDNF did not achieve its primary end point. As a result, there has been uncertainty as to whether GDNF (and by extrapolation, related GDNF family neurotrophic factors) has merit in the future treatment of PD. To critically appraise the existing work and its future, a special workshop was held to discuss and debate this issue. This paper is a summary of that meeting with recommendations on whether there is a future for this therapeutic approach and also what any future PD trial involving GDNF and other GDNF family neurotrophic factors should consider in its design.

Magnetic resonance imaging-guided phase 1 trial of putaminal AADC gene therapy for Parkinson's disease

OBJECTIVE

To understand the safety, putaminal coverage, and enzyme expression of adeno-associated viral vector serotype-2 encoding the complementary DNA for the enzyme, aromatic L-amino acid decarboxylase (VY-AADC01), delivered using novel intraoperative monitoring to optimize delivery.

METHODS

Fifteen subjects (three cohorts of 5) with moderately advanced Parkinson's disease and medically refractory motor fluctuations received VY-AADC01 bilaterally coadministered with gadoteridol to the putamen using intraoperative magnetic resonance imaging (MRI) guidance to visualize the anatomic spread of the infusate and calculate coverage. Cohort 1 received 8.3 × 10¹¹ vg/ml and ≤450 μl per putamen (total dose, ≤7.5 × 10¹¹ vg); cohort 2 received the same concentration (8.3 × 10¹¹ vg/ml) and ≤900 μl per putamen (total dose, ≤1.5 × 10¹² vg); and cohort 3 received 2.6 × 10¹² vg/ml and ≤900 μl per putamen (total dose, ≤4.7 × 10¹² vg). (18)F-fluoro-L-dihydroxyphenylalanine positron emission tomography (PET) at baseline and 6 months postprocedure assessed enzyme activity; standard assessments measured clinical outcomes.

RESULTS

MRI-guided administration of ascending VY-AADC01 doses resulted in putaminal coverage of 21% (cohort 1), 34% (cohort 2), and 42% (cohort 3). Cohorts 1, 2, and 3 showed corresponding increases in enzyme activity assessed by PET of 13%, 56%, and 79%, and reductions in antiparkinsonian medication of -15%, -33%, and -42%, respectively, at 6 months. At 12 months, there were dose-related improvements in clinical outcomes, including increases in patient-reported ON-time without troublesome dyskinesia (1.6, 3.3, and 1.5 hours, respectively) and quality of life.

INTERPRETATION

Novel intraoperative monitoring of administration facilitated targeted delivery of VY-AADC01 in this phase 1 study, which was well tolerated. Increases in enzyme expression and clinical improvements were dose dependent. ClinicalTrials.gov Identifier: NCT01973543 Ann Neurol 2019;85:704-714.

Glial cell line-derived neurotrophic factor (GDNF) is an endogenous protector in the mesolimbic system against excessive alcohol consumption and relapse

Moderate social consumption of alcohol is common; however, only a small percentage of individuals transit from social to excessive, uncontrolled alcohol drinking. This suggests the existence of protective mechanisms that prevent the development of alcohol addiction. Here, we tested the hypothesis that the glial cell line-derived neurotrophic factor (GDNF) in the mesolimbic system [e.g. the nucleus accumbens (Acb) and ventral tegmental area (VTA)] is part of such a mechanism. We found that GDNF knockdown, by infecting rat Acb neurons with a small hairpin RNA (shRNA) targeting the GDNF gene, produced a rapid escalation to excessive alcohol consumption and enhanced relapse to alcohol drinking. Conversely, viral-mediated overexpression of the growth factor in the mesolimbic system blocked the escalation from moderate to excessive alcohol drinking. To access the mechanism underlying GDNF's actions, we measured the firing rate of dopaminergic (DAergic) neurons in the VTA after a history of excessive alcohol intake with or without elevating GDNF levels. We found that the spontaneous firing rate of DAergic neurons in the VTA was reduced during alcohol withdrawal and that GDNF reversed this alcohol-induced DA deficiency. Together, our results suggest that endogenous GDNF in the mesolimbic system controls the transition from moderate to excessive alcohol drinking and relapse via reversal of alcohol-dependent neuro-adaptations in DAergic VTA neurons.

Translational fidelity of intrathecal delivery of self-complementary AAV9-survival motor neuron 1 for spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neuromuscular disease caused by mutations in survival motor neuron 1 (SMN1). Previously, we showed that central nervous system (CNS) delivery of an adeno-associated viral (AAV) vector encoding SMN1 produced significant improvements in survival in a mouse model of SMA. Here, we performed a dose-response study in SMA mice to determine the levels of SMN in the spinal cord necessary for efficacy, and measured the efficiency of motor neuron transduction in the spinal cord after intrathecal delivery in pigs and nonhuman primates (NHPs). CNS injections of 5e10, 1e10, and 1e9 genome copies (gc) of self-complementary AAV9 (scAAV9)-hSMN1 into SMA mice extended their survival from 17 to 153, 70, and 18 days, respectively. Spinal cords treated with 5e10, 1e10, and 1e9 gc showed that 70-170%, 30-100%, and 10-20% of wild-type levels of SMN were attained, respectively. Furthermore, detectable SMN expression in a minimum of 30% motor neurons correlated with efficacy. A comprehensive analysis showed that intrathecal delivery of 2.5e13 gc of scAAV9-GFP transduced 25-75% of the spinal cord motor neurons in NHPs. Thus, the extent of gene expression in motor neurons necessary to confer efficacy in SMA mice could be obtained in large-animal models, justifying the continual development of gene therapy for SMA.

Distribution of nanoparticles throughout the cerebral cortex of rodents and non-human primates: Implications for gene and drug therapy

When nanoparticles/proteins are infused into the brain, they are often transported to distal sites in a manner that is dependent both on the characteristics of the infusate and the region targeted. We have previously shown that adeno-associated virus (AAV) is disseminated within the brain by perivascular flow and also by axonal transport. Perivascular distribution usually does not depend strongly on the nature of the infusate. Many proteins, neutral liposomes and AAV particles distribute equally well by this route when infused under pressure into various parenchymal locations. In contrast, axonal transport requires receptor-mediated uptake of AAV by neurons and engagement with specific transport mechanisms previously demonstrated for other neurotropic viruses. Cerebrospinal fluid (CSF) represents yet another way in which brain anatomy may be exploited to distribute nanoparticles broadly in the central nervous system. In this study, we assessed the distribution and perivascular transport of nanoparticles of different sizes delivered into the parenchyma of rodents and CSF in non-human primates.

SAFETY AND TOLERABILITY OF MRI-GUIDED INFUSION OF AAV2-hAADC INTO THE MID-BRAIN OF NON-HUMAN PRIMATE

Aromatic L-amino acid decarboxylase (AADC) deficiency is a rare, autosomal-recessive neurological disorder caused by mutations in the DDC gene that leads to an inability to synthesize catecholamines and serotonin. As a result, patients suffer compromised development, particularly in motor function. A recent gene replacement clinical trial explored putaminal delivery of recombinant adeno-associated virus serotype 2 vector encoding human AADC (AAV2-hAADC) in AADC-deficient children. Unfortunately, patients presented only modest amelioration of motor symptoms, which authors acknowledged could be due to insufficient transduction of putamen. We hypothesize that, with the development of a highly accurate MRI-guided cannula placement technology, a more effective approach might be to target the affected mid-brain neurons directly. Transduction of AADC-deficient dopaminergic neurons in the substantia nigra and ventral tegmental area with locally infused AAV2-hAADC would be expected to lead to restoration of normal dopamine levels in affected children. The objective of this study was to assess the long-term safety and tolerability of bilateral AAV2-hAADC MRI-guided pressurized infusion into the mid-brain of nonhuman primates. Animals received either vehicle, low or high AAV2-hAADC vector dose and were euthanized 1, 3, or 9 months after surgery. Our data indicate that effective mid-brain transduction was achieved without untoward effects.

Adeno-associated virus type 6 is retrogradely transported in the non-human primate brain

We recently demonstrated that axonal transport of adeno-associated virus (AAV) is serotype-dependent. Thus, AAV2 is anterogradely transported (e.g., from cell bodies to nerve terminals) in both rat and non-human primate (NHP) brain. In contrast, AAV6 is retrogradely transported from terminals to neuronal cells bodies in the rat brain. However, the directionality of axonal transport of AAV6 in the NHP brain has not been determined. In this study, two Cynomolgus macaques received an infusion of AAV6 harboring green fluorescent protein (GFP) into the striatum (caudate and putamen) by magnetic resonance (MR)-guided convection-enhanced delivery. One month after infusion, immunohistochemical staining of brain sections revealed a striatal GFP expression that corresponded well with MR signal observed during gene delivery. As shown previously in rats, GFP expression was detected throughout the prefrontal, frontal, and parietal cortex, as well as substantia nigra pars compacta and thalamus, indicating retrograde transport of the vector in NHP. AAV6-GFP preferentially transduced neurons, although a few astrocytes were also transduced. Transduction of non-neuronal cells in the brain was associated with upregulation of the major histocompatibility complex-II (MHC-II) and lymphocytic infiltration as previously observed with AAV1 and AAV9. This contrasts with highly specific neuronal transduction in the rat brain. Retrograde axonal transport of AAV6 from a single striatal infusion permits efficient transduction of cortical neurons in significant tissue volumes that otherwise would difficult to achieve.

Safety study of adeno-associated virus serotype 2-mediated human acid sphingomyelinase expression in the nonhuman primate brain

Niemann-Pick disease is a lysosomal storage disorder resulting from inherited deficiency in acid sphingomyelinase (ASM). Use of adeno-associated virus serotype 2 (AAV2) to deliver human acid sphingomyelinase (hASM) is currently being explored as a means to treat the devastating neurological features of NPD, which are refractory to traditional enzyme replacement therapy. In this study, we evaluated the long-term efficacy and safety of AAV2-hASM after direct infusion into the CNS of nonhuman primates. First, we confirmed the efficacy of AAV2-hASM in naive rats, which exhibited increased ASM expression and enzyme activity after infusion, without evidence of local or systemic toxicity. Next, the model was adapted to naive nonhuman primates (NHPs) with various doses of AAV2-hASM or saline delivered into the brainstem and both thalami. Strikingly, NHPs that received a high dose of AAV2-hASM displayed significant motor deficits that were not seen in low-dose animals in both the short-term (3-month) and long-term (9-month) treatment groups. In treated NHPs, ASM expression and activity were elevated with associated alterations in the sphingolipidomic profile in brain regions transduced with AAV2-hASM. Initial histological analysis indicated marked inflammatory reactions, and immunohistochemical analysis confirmed a robust inflammatory response. Importantly, pronounced upregulation of the chemokine CCL5, a target of ASM-mediated inflammatory signaling, was detected that correlated with the inflammatory response, providing a possible mechanism for hASM-associated toxicity. This study defines dose-dependent and dose-independent toxicities of AAV2-hASM in the naive primate brain, and reveals potential challenges in the design of a clinical trial.

Gene therapy for misfolding protein diseases of the central nervous system

Protein aggregation as a result of misfolding is a common theme underlying neurodegenerative diseases. Accordingly, most recent studies aim to prevent protein misfolding and/or aggregation as a strategy to treat these pathologies. For instance, state-of-the-art approaches, such as silencing protein overexpression by means of RNA interference, are being tested with positive outcomes in preclinical models of animals overexpressing the corresponding protein. Therapies designed to treat central nervous system diseases should provide accurate delivery of the therapeutic agent and long-term or chronic expression by means of a nontoxic delivery vehicle. After several years of technical advances and optimization, gene therapy emerges as a promising approach able to fulfill those requirements. In this review we will summarize the latest improvements achieved in gene therapy for central nervous system diseases associated with protein misfolding (e.g., amyotrophic lateral sclerosis, Alzheimer's, Parkinson's, Huntington's, and prion diseases), as well as the most recent approaches in this field to treat these pathologies.

Convection-enhanced delivery improves distribution and efficacy of tumor-selective retroviral replicating vectors in a rodent brain tumor model

In the present study, we compared the therapeutic effect of tumor-selective retroviral replicating vectors (RRV) expressing the yeast cytosine deaminase (CD) delivered by CED or simple injection, followed by systemic administration of the pro-drug, 5-fluorocytosine (5-FC). Treatment with RRV-CD and systemic 5-FC significantly increased survival in rodent U87MG glioma model in comparison to controls (p<0.01). Interestingly, CED of RRV-CD followed by 5-FC further enhanced survival in this animal model in comparison to intra-tumoral injection of RRV-CD followed by systemic 5-FC (p<0.05). High expression levels of Ki-67 were found in untreated tumors compared to treated. Untreated tumors were also much larger than treated. CED resulted in excellent distribution of RRV while only partial distribution of RRV was obtained after injection. Furthermore, RRV-CD and cytosine deaminase were also found in tumors from treated rats at study end-points. These results demonstrated that RRV vectors may efficiently transduce and stably propagate in malignant human glioma, thereby achieving a significant in-situ amplification effect after initial administration. We conclude that delivery of RRV into the glioma by CED provides much wider vector distribution than simple, injection, and this correlated with better therapeutic outcomes.

Strong cortical and spinal cord transduction after AAV7 and AAV9 delivery into the cerebrospinal fluid of nonhuman primates

The present study builds on previous work showing that infusion of adeno-associated virus type 9 (AAV9) into the cisterna magna (CM) of nonhuman primates resulted in widespread transduction throughout cortex and spinal cord. Transduction efficiency was severely limited, however, by the presence of circulating anti-AAV antibodies. Accordingly, we compared AAV9 to a related serotype, AAV7, which has a high capsid homology. CM infusion of either AAV7 or AAV9 directed high level of cell transduction with similar patterns of distribution throughout brain cortex and along the spinal cord. Dorsal root ganglia and corticospinal tracts were also transduced. Both astrocytes and neurons were transduced. Interestingly, little transduction was observed in peripheral organs. Our results indicate that intrathecal delivery of either AAV7 or AAV9 directs a robust and widespread cellular transduction in the central nervous system and other peripheral neural structures.

Guided delivery of adeno-associated viral vectors into the primate brain

In this review, we discuss recent developments in the delivery of adeno-associated virus-based vectors (AAV), particularly with respect to the role of axonal transport in vector distribution in the brain. The use of MRI-guidance and new stereotactic aiming devices have now established a strong foundation for neurological gene therapy to become an accepted procedure in interventional neurology.

Adeno-associated virus serotype 9 transduction in the central nervous system of nonhuman primates

Widespread distribution of gene products at clinically relevant levels throughout the CNS has been challenging. Adeno-associated virus type 9 (AAV9) vector has been reported as a good candidate for intravascular gene delivery, but low levels of preexisting antibody titers against AAV in the blood abrogate cellular transduction within the CNS. In the present study we compared the effectiveness of vascular delivery and cerebrospinal fluid (CSF) delivery of AAV9 in transducing CNS tissue in nonhuman primates. Both delivery routes generated similar distribution patterns, although we observed a more robust level of transduction after CSF delivery. Consistent with previous reports administering AAV9, we found greater astrocytic than neuronal tropism via both routes, although we did find a greater magnitude of CNS transduction after CSF delivery compared with intravascular delivery. Last, we have demonstrated that delivery of AAV9 into the CSF does not shield against AAV antibodies. This has obvious implications when developing and/or implementing any clinical trial studies.

Safety and tolerability of magnetic resonance imaging-guided convection-enhanced delivery of AAV2-hAADC with a novel delivery platform in nonhuman primate striatum

Degeneration of nigrostriatal neurons in Parkinson's disease (PD) causes progressive loss of aromatic l-amino acid decarboxylase (AADC), the enzyme that converts levodopa (l-DOPA) into dopamine in the striatum. Because loss of this enzyme appears to be a major driver of progressive impairment of response to the mainstay drug, l-DOPA, one promising approach has been to use gene therapy to restore AADC activity in the human putamen and thereby restore normal l-DOPA response in patients with PD. An open-label phase I clinical trial of this approach in patients with PD provided encouraging signs of improvement in Unified Parkinson's Disease Rating Scale scores and reductions in antiparkinsonian medications. However, such improvement was modest compared with the results previously reported in parkinsonian rhesus macaques. The reason for this discrepancy may have been that the relatively small volume of vector infused in the clinical study restricted the distribution of AADC expression, such that only about 20% of the postcommissural putamen was covered, as revealed by l-[3-(18)F]-α-methyltyrosine-positron emission tomography. To achieve more quantitative distribution of vector, we have developed a visual guidance system for parenchymal infusion of AAV2. The purpose of the present study was to evaluate the combined magnetic resonance imaging-guided delivery system with AAV2-hAADC under conditions that approximate the intended clinical protocol. Our data indicate that this approach directed accurate cannula placement and effective vector distribution without inducing any untoward effects in nonhuman primates infused with a high dose of AAV2-hAADC.

Glial-derived neurotrophic factor gene transfer for Parkinson's disease: anterograde distribution of AAV2 vectors in the primate brain

Delivery of neurotrophic factors to treat neurodegenerative diseases has not been efficacious in clinical trials despite their known potency for promoting neuronal growth and survival. Direct gene delivery to the brain offers an approach for establishing sustained expression of neurotrophic factors but is dependent on accurate surgical procedures to target specific anatomical regions of the brain. Serotype-2 adeno-associated viral (AAV2) vectors have been investigated in multiple clinical studies for neurological diseases without adverse effects; however the absence of significant clinical efficacy after neurotrophic factor gene transfer has been largely attributed to insufficient coverage of the target region. Our pre-clinical development of AAV2-glial-derived neurotrophic factor (GDNF) for Parkinson's disease involved real-time image guided delivery and optimization of delivery techniques to maximize gene transfer in the putamen. We have demonstrated that AAV2 vectors are anterogradely transported in the primate brain with GDNF expression observed in the substantia nigra after putaminal delivery in both intact and nigrostriatal lesioned primates. Direct midbrain delivery of AAV2-GDNF resulted in extensive anterograde transport to multiple brain regions and significant weight loss.

An α-synuclein AAV gene silencing vector ameliorates a behavioral deficit in a rat model of Parkinson's disease, but displays toxicity in dopamine neurons

Effects of silencing ectopically expressed hSNCA in rat substantia nigra (SN) were examined as a novel therapeutic approach to Parkinson's disease (PD). AAV-hSNCA with or without an AAV harboring a short-hairpin (sh)RNA targeting hSNCA or luciferase was injected into one SN. At 9weeks, hSNCA-expressing rats had reduced SN dopamine (DA) neurons and exhibited a forelimb deficit. AAV-shRNA-SNCA silenced hSNCA and protected against the forelimb deficit. However, AAV-shRNA-SNCA also led to DA neuron loss suggesting undesirable effects of chronic shRNA expression. Effects on nigrostriatal-projecting neurons were examined using a retrograde tract tracer. Loss of striatal-projecting DA neurons was evident in the vector injection site, whereas DA neurons outside this site were lost in hSNCA-expressing rats, but not in hSNCA-silenced rats. These observations suggest that high levels of shRNA-SNCA were toxic to DA neurons, while neighboring neurons exposed to lower levels were protected by hSNCA gene silencing. Also, data collected on DA levels suggest that neurons other than or in addition to nigrostriatal DA neurons contributed to protection of forelimb use. Our observations suggest that while hSNCA gene silencing in DA neurons holds promise as a novel PD therapy, further development of silencing technology is required.

Novel platform for MRI-guided convection-enhanced delivery of therapeutics: preclinical validation in nonhuman primate brain

BACKGROUND/AIMS

A skull-mounted aiming device and integrated software platform has been developed for MRI-guided neurological interventions. In anticipation of upcoming gene therapy clinical trials, we adapted this device for real-time convection-enhanced delivery of therapeutics via a custom-designed infusion cannula. The targeting accuracy of this delivery system and the performance of the infusion cannula were validated in nonhuman primates.

METHODS

Infusions of gadoteridol were delivered to multiple brain targets and the targeting error was determined for each cannula placement. Cannula performance was assessed by analyzing gadoteridol distributions and by histological analysis of tissue damage.

RESULTS

The average targeting error for all targets (n = 11) was 0.8 mm (95% CI = 0.14). For clinically relevant volumes, the distribution volume of gadoteridol increased as a linear function (R(2) = 0.97) of the infusion volume (average slope = 3.30, 95% CI = 0.2). No infusions in any target produced occlusion, cannula reflux or leakage from adjacent tracts, and no signs of unexpected tissue damage were observed.

CONCLUSIONS

This integrated delivery platform allows real-time convection-enhanced delivery to be performed with a high level of precision, predictability and safety. This approach may improve the success rate for clinical trials involving intracerebral drug delivery by direct infusion.

Magnetic resonance imaging-guided delivery of adeno-associated virus type 2 to the primate brain for the treatment of lysosomal storage disorders

Gene replacement therapy for the neurological deficits caused by lysosomal storage disorders, such as in Niemann-Pick disease type A, will require widespread expression of efficacious levels of acid sphingomyelinase (ASM) in the infant human brain. At present there is no treatment available for this devastating pediatric condition. This is partly because of inherent constraints associated with the efficient delivery of therapeutic agents into the CNS of higher order models. In this study we used an adeno-associated virus type 2 (AAV2) vector encoding human acid sphingomyelinase tagged with a viral hemagglutinin epitope (AAV2-hASM-HA) to transduce highly interconnected CNS regions such as the brainstem and thalamus. On the basis of our data showing global cortical expression of a secreted reporter after thalamic delivery in nonhuman primates (NHPs), we set out to investigate whether such widespread expression could be enhanced after brainstem infusion. To maximize delivery of the therapeutic transgene throughout the CNS, we combined a single brainstem infusion with bilateral thalamic infusions in naive NHPs. We found that enzymatic augmentation in brainstem, thalamic, cortical, as well subcortical areas provided convincing evidence that much of the large NHP brain can be transduced with as few as three injection sites.

Anterograde axonal transport of AAV2-GDNF in rat basal ganglia

We elucidated the effects of parkinsonian degeneration on trafficking of AAV2-GDNF in the nigro-striatum (nigro-ST) of unilaterally 6-hydroxydopamine (6-OHDA)-lesioned rats. Vector infused into striatum (ST) was transported to substantia nigra (SN), both pars compacta (SNc), and pars reticulata (SNr). In the lesioned hemisphere, glial cell line-derived neurotrophic factor (GDNF) immunoreactivity was only found in SNr consistent with elimination of SNc dopaminergic (DA) neurons by 6-OHDA. Further analysis showed that striatal delivery of AAV2-GDNF resulted in GDNF expression in globus pallidus (GP), entopeduncular nucleus (EPN), and subthalamic nucleus (STN) in both lesioned and unlesioned hemispheres. Injection of vector into SN, covering both SNc and SNr, resulted in striatal expression of GDNF in the unlesioned hemisphere but not in the lesioned hemisphere. No expression was seen in GP or EPN. We conclude that adeno-associated virus serotype 2 (AAV2) is transported throughout the nigro-ST exclusively by anterograde transport. This transport phenomenon directs GDNF expression throughout the basal ganglia in regions that are adversely affected in Parkinson's disease (PD) in addition to SNc. Delivery of vector to SN, however, does not direct expression of GDNF in ST, EPN, or GP. On this basis, we believe that striatal delivery of AAV2-GDNF is the preferred course of action for trophic rescue of DA function.

Regeneration of the MPTP-lesioned dopaminergic system after convection-enhanced delivery of AAV2-GDNF

Clinical studies to date have failed to establish therapeutic benefit of glial cell-derived neurotrophic factor (GDNF) in Parkinson's disease (PD). In contrast to previous nonclinical neuroprotective reports, this study shows clinically relevant and long-lasting regeneration of the dopaminergic system in rhesus macaques lesioned with 1-methy-4-phenyl-1,2,3,6-tetrahydropyridine 3-6 months before GDNF gene delivery (AAV2-GDNF). The observed progressive amelioration of functional deficits, recovery of dopamine, and regrowth of fibers to the striatal neuropil demonstrate that high GDNF expression in the putamen promotes restoration of the dopaminergic system in a primate model of advanced PD. Extensive distribution of GDNF within the putamen and transport to the severely lesioned substantia nigra, after convection-enhanced delivery of AAV2-GDNF into the putamen, indicates anterograde transport via striatonigral connections and is anticipated to occur in PD patients. Overall, these data demonstrate nonclinical neurorestoration after putaminal infusion of AAV2-GDNF and suggest that clinical investigation in PD patients is warranted.

Real-time MR imaging with Gadoteridol predicts distribution of transgenes after convection-enhanced delivery of AAV2 vectors

Gene therapies that utilize convention-enhanced delivery (CED) will require close monitoring of vector infusion in real time and accurate prediction of drug distribution. The magnetic resonance imaging (MRI) contrast agent, Gadoteridol (Gd), was used to monitor CED infusion and to predict the expression pattern of glial cell line-derived neurotrophic factor (GDNF) protein after administration of adeno-associated virus type 2 (AAV2) vector encoding human pre-pro-GDNF complementary DNA. The nonhuman primate (NHP) thalamus was utilized for modeling infusion to allow delivery of volumes more relevant to planned human studies. AAV2 encoding human aromatic L-amino acid decarboxylase (AADC) was coinfused with AAV2-GDNF/Gd to confirm regions of AAV2 transduction versus extracellular GDNF diffusion. There was a close correlation between Gd distribution and GDNF or AADC expression, and the ratios of expression areas of GDNF or AADC versus Gd were both close to 1. Our data support the use of Gd and MRI to monitor AAV2 infusion via CED and to predict the distribution of GDNF protein after AAV2-GDNF administration.

Safety evaluation of AAV2-GDNF gene transfer into the dopaminergic nigrostriatal pathway in aged and parkinsonian rhesus monkeys

We evaluated neuropathological findings in two studies of AAV2-GDNF efficacy and safety in naive aged (>20 years) or MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-lesioned rhesus macaques. In the first study, a total of 17 animals received one of two doses of AAV2-GDNF into either putamen or substantia nigra (SN). To control for surgical variables, all animals received identical putaminal and nigral infusions in which phosphate-buffered saline was substituted for vector as appropriate. All 17 aged monkeys were studied for 6 months before necropsy. In a separate study, 11 MPTP-lesioned rhesus macaques with extensive lesions in the right SN and mild lesions in the left SN received bilateral infusions of AAV2-GDNF (9.9 x 10(11) vector genomes) or PBS into the putamen and were then studied for up to 14 months. In the current analysis, we addressed safety issues regarding AAV2-GDNF administration. An extensive series of assessments of in-life behavioral and clinical parameters was conducted. No overt histopathology or immune responses were detected in any experimental monkey. However, the delivery of AAV2-GDNF to the SN of aged monkeys caused a marked and significant loss of body weight (-19.4%). No weight loss was observed in the MPTP-lesioned monkeys despite bilateral axonal transport of glial cell line-derived neurotrophic factor (GDNF) to the SN from the putamen. These findings indicate that putaminal administration of AAV2-GDNF by convection-enhanced delivery shows therapeutic promise without any apparent side effects. Importantly, nigral administration of AAV2-GDNF caused significant weight loss that raises substantial concern for clinical application of this approach.

Functional effects of AAV2-GDNF on the dopaminergic nigrostriatal pathway in parkinsonian rhesus monkeys

We investigated the safety and neuroregenerative potential of an adeno-associated virus (AAV2) containing human glial cell line-derived neurotrophic factor (GDNF) in an MPTP primate model of Parkinson's disease. Dopaminergic function was evaluated by positron emission tomography with 6-[(18)F]fluoro-l-m-tyrosine (FMT) before and after AAV2-GDNF or phosphate-buffered saline infusion bilaterally into the putamen. FMT uptake was significantly increased bilaterally in the putamen of AAV2-GDNF but not phosphate-buffered saline-treated animals 6 months after infusion, indicating increased dopaminergic activity in the nigrostriatal pathways. AAV2-GDNF-treated animals also showed clinical improvement without adverse effects. These findings are consistent with our previous report in aged nonhuman primates that showed evidence of enhanced use of striatal dopamine and dopaminergic nigrostriatal innervation. Clinical improvement and evidence of functional recovery in the nigrostriatal pathway, and the absence of adverse effects, support the safety of this approach for the delivery of GDNF over a 6-month period.

Controlled dissemination of AAV vectors in the primate brain

Adeno-associated viral (AAV) vectors are currently the preeminent gene therapy vehicles for neurological application. However, issues regarding the trafficking of AAV vectors within the primate brain, and consequently control over the targeting of transgene expression, remain a matter of investigation. Studies in nonhuman primates have shown that distribution of AAV vectors is largely mediated by the flow of cerebrospinal fluid within perivascular space, trafficking of vector along axonal projections, and AAV receptor binding. Together these processes can result in transduction of cells in areas distant from the parenchymal site of infusion. Additionally, we have addressed the unique surgical issues concerning delivery of AAV vectors by convection-enhanced delivery and are working toward tailored delivery by means of real-time MRI.

AAV-mediated expression of Bcl-xL or XIAP fails to induce neuronal resistance against quinolinic acid-induced striatal lesioning

Apoptotic mechanisms have been proposed to contribute to the selective loss of medium spiny striatal projection neurons in Huntington's disease (HD). This raises the question as to whether enhancing the expression of anti-apoptotic factors in vulnerable striatal projection neurons can reduce their susceptibility to neurotoxic processes occurring in the HD brain. In this study AAV 1/2 vectors encoding either the anti-apoptotic factor Bcl-xL or XIAP were used to transduce striatal neurons prior to an intrastriatal injection of the excitotoxic glutamate analogue quinolinic acid (QA). AAV 1/2 vector treated rats were observed in behavioural tests undertaken to assess whether anti-apoptotic factor expression provided amelioration of motor function impairment following unilateral QA-induced striatal lesioning. AAV-XIAP treated rats displayed complete amelioration of an ipsilateral forelimb use bias relative to control animals. However, neither AAV-XIAP nor AAV-Bcl-xL treated rats demonstrated an improvement in sensorimotor neglect compared to control animals. Furthermore, we did not observe a significant reduction of QA-induced pathology in assessed neuronal populations of the basal ganglia. These results indicate that sole enhancement of XIAP or Bcl-xL is not sufficient to counteract QA-induced excitotoxic insult of striatal neurons.

AAV-BDNF mediated attenuation of quinolinic acid-induced neuropathology and motor function impairment

Maintenance and plasticity of striatal neurons is dependent on brain-derived neurotrophic factor (BDNF), which is depleted in the Huntington's disease striatum due to reduced expression and disrupted corticostriatal transportation. In this study we demonstrate that overexpression of BDNF in the striatum attenuates motor impairment and reduces the extent of striatal damage following quinolinic acid lesioning. Transfer of the BDNF gene to striatal neurons using serotype 1/2 adeno-associated viral vectors enhanced BDNF protein levels in the striatum, but induced weight loss and seizure activity following long-term high-level expression. Lower concentration BDNF expression supported striatal neurons against excitotoxic insult, as demonstrated by enhanced krox-24 immunopositive neuron survival, reduction of striatal atrophy and maintenance of the patch/matrix organization. Additionally, BDNF expression attenuated motor impairment in the forelimb use cylinder test, sensorimotor neglect in the corridor food selection task and reversed apomorphine-induced rotational behaviour. Direct correlations were shown for the first time between BDNF-mediated attenuation of behavioural impairment and the integrity of the globus pallidus, seemingly independent from the severity of striatal lesioning. These results demonstrate that BDNF holds considerable therapeutic potential for alleviating both neuropathological and motor function deficits in the Huntington's disease brain, and the critical role of pallidal neurons in facilitating motor performance.

Temporal profile of subventricular zone progenitor cell migration following quinolinic acid-induced striatal cell loss

A number of studies have demonstrated directed migration of neural progenitor cells to sites of brain injury and disease, however a detailed examination of when a cell is "born" in relation to injury induction and the migratory response of that cell has not previously been determined. This study therefore examined the temporal correlation between progenitor cell proliferation ("birth") and neuroblast migratory response into the damaged striatum following quinolinic acid (QA) lesioning of the adult rat striatum. Retroviral labeling of subventricular zone (SVZ)-derived progenitor cells demonstrated that cell loss in the QA-lesioned striatum increased progenitor cell migration through the rostral migratory stream (RMS) for up to 30 days. In addition, a population of dividing cells originating from the SVZ generated doublecortin positive neuroblasts that migrated into the damaged striatum in response to cell loss invoked by the QA lesion. Quantification of bromodeoxyuridine (BrdU)-labeled cells co-expressing doublecortin revealed that the majority of cells present in the damaged striatum were generated from progenitor cells dividing within 2 days either prior to or following the QA lesion. In contrast, cells dividing 2 or more days following QA lesioning, migrated into the striatum and exhibited a glial phenotype. These results demonstrate that directed migration of SVZ-derived cells and neuroblast differentiation in response to QA lesioning of the striatum is acute and transient. We propose this is predominantly due to a reduced capacity over time for newly generated neuroblasts to respond to the lesioned environment due to a loss or inhibition of migratory cues.

Verification of functional AAV-mediated neurotrophic and anti-apoptotic factor expression

The use of viral vectors for gene delivery offer many advantages for both basic research and therapeutic application through the continuous expression of a gene product within a target region. It is vital however that any gene product is correctly expressed in a biologically active form, and this should be confirmed prior to large scale in vivo studies. Using adeno-associated viral (AAV) vectors to direct the expression of either a neurotrophic factor or an anti-apoptotic protein, we have developed a range of in vitro assays to verify functional transgenic protein expression. Brain-derived neurotropic factor (BDNF) activity was confirmed by demonstrating enhanced generation of GABAergic neurons in embryonic (E15) striatal cultures and AAV-mediated glial-derived neurotrophic factor (GDNF) function using an assay for dopaminergic differentiation of embryonic (E14) ventral mesencephalic cultures. To assess functional anti-apoptotic factor expression we designed cell-survival assays, using embryonic cortical cultures to confirm Bcl-x(L) activity and the HT1080 cell-line for X-linked inhibitor of apoptosis protein (XIAP) activity following AAV-mediated expression. This study demonstrates that the use of functional assays provides valuable confirmation of desired biotherapeutic expression prior to extensive investigation with new gene delivery vectors.

AAV-mediated gene delivery of BDNF or GDNF is neuroprotective in a model of Huntington disease

Huntington disease (HD) is a neurodegenerative disorder that results in the progressive loss of GABAergic medium spiny projection neurons in the striatum. Neurotrophic factors have demonstrated neuroprotective actions on striatal neurons, suggesting that increased neurotrophic factor expression may prevent or reduce neuronal loss in the HD brain. We investigated whether enhanced expression of brain-derived neurotrophic factor (BDNF) or glial cell line-derived neurotrophic factor (GDNF), achieved by adeno-associated viral (AAV) vector-mediated gene delivery, could protect striatal neurons in the quinolinic acid (QA) rodent model of HD. Adult Wistar rats received unilateral intrastriatal injections of AAV-BDNF, AAV-GDNF, AAV-GFP, or PBS. Three weeks later, the rats were lesioned with QA, a toxin that induces striatal neuron death by an excitotoxic process. Both AAV-BDNF and AAV-GDNF significantly reduced the loss of both NeuN- and calbindin-immunopositive striatal neurons 2 weeks after lesion compared to controls. AAV-BDNF also provided significant neurotrophic support to NOS-immunopositive striatal interneurons, while AAV-GDNF-treated rats demonstrated significant protection of parvalbumin-immunopositive striatal interneurons compared to controls. These results indicate that AAV-mediated gene transfer of BDNF or GDNF into the striatum provides neuronal protection in a rodent model of HD.

Neurogenesis in the striatum of the quinolinic acid lesion model of Huntington's disease

The presence of ongoing neurogenesis in the adult mammalian brain raises the exciting possibility that endogenous progenitor cells may be able to generate new neurons to replace cells lost through brain injury or neurodegenerative disease. We have recently demonstrated increased cell proliferation and the generation of new neurons in the Huntington's disease human brain. In order to better understand the potential role of endogenous neuronal replacement in neurodegenerative disorders and extend our initial observations in the human Huntington's disease brain, we examined the effect of striatal cell loss on neurogenesis in the subventricular zone (SVZ) of the adult rodent forebrain using the quinolinic acid (QA) lesion rat model of Huntington's disease. Cell proliferation and neurogenesis were assessed with bromodeoxyuridine (BrdU) labeling and immunocytochemistry for cell type-specific markers. BrdU labeling demonstrated increased cell proliferation in the SVZ ipsilateral to the QA-lesioned striatum, resulting in expansion of the SVZ in the lesioned hemisphere. Quantification revealed that QA lesion-induced striatal cell loss produced a significant increase in the area of BrdU-immunoreactivity in the SVZ ipsilateral to the lesioned hemisphere between 1 and 14 days post-lesion compared with sham-lesioned animals, with the greatest increase observed at 7 days post-lesion. These changes were associated with an increase in cells in the anterior SVZ ipsilateral to the lesioned striatum expressing the antigenic marker for SVZ neuroblasts, doublecortin (Dcx). Importantly, we observed Dcx-positive cells extending from the SVZ into the QA-lesioned striatum where a subpopulation of newly generated cells expressed markers for immature and mature neurons. This study demonstrates that loss of GABAergic medium spiny projection neurons following QA striatal lesioning of the adult rat brain increases SVZ neurogenesis, leading to the putative migration of neuroblasts to damaged areas of the striatum and the formation of new neurons.