Issues regarding gene therapy products for Parkinson's disease: The development of CERE-120 (AAV-NTN) as one reference point

Great expectations: virus-mediated gene therapy in neurological disorders

Gene therapy (GT) has tremendous potential for the treatment of neurological disorders to transform patient care. The successful application of virus-mediated GT to treat spinal muscular atrophy is a significant milestone, serving to accelerate similar progress in a spectrum of neurological conditions, with more than 50 clinical trials currently underway, across neurodevelopmental, neurodegenerative, muscular dystrophy, epilepsy, chronic pain and neoplastic diseases. This review provides an overview of the key features of virus-mediated GT, paradigms of delivery and dosing, potential risks and highlights ongoing research to optimise safe and effective delivery of vectors into the nervous system. Examples of the application of GT in various neurological diseases alongside clinical development challenges will be presented. As the development and translation of GTs gain pace, success can only ultimately be realised for patients following implementation in the health system. The challenges and controversies of daunting costs, ethics, early diagnosis and health system readiness will require innovative pricing schemes, regulatory policies, education and organisation of a skilled workforce to deliver of high-quality care in clinical practice as we prepare for advanced therapeutics in neurology.

Delivering therapy to target: improving the odds for successful drug development

The direct delivery of drugs and other agents into tissue (in contrast to systemic administration) has been used in clinical trials for brain cancer, neurodegenerative diseases and peripheral tumors. However, continuing evidence suggests that clinical efficacy depends on adequate delivery to a target. Inadequate delivery may have doomed otherwise effective drugs, through failure to distinguish drug inefficacy from poor distribution at the target. Conventional pretreatment clinical images of the patient fail to reveal the complexity and diversity of drug transport pathways in tissue. We discuss the richness of these pathways and argue that development and patient treatment can be sped up and improved by: using quantitative as well as 'real-time' imaging; customized simulations using data from that imaging; and device designs that optimize the drug-device combination.

Convection-Enhanced Delivery

Convection-enhanced delivery (CED) is a promising technique that generates a pressure gradient at the tip of an infusion catheter to deliver therapeutics directly through the interstitial spaces of the central nervous system. It addresses and offers solutions to many limitations of conventional techniques, allowing for delivery past the blood-brain barrier in a targeted and safe manner that can achieve therapeutic drug concentrations. CED is a broadly applicable technique that can be used to deliver a variety of therapeutic compounds for a diversity of diseases, including malignant gliomas, Parkinson's disease, and Alzheimer's disease. While a number of technological advances have been made since its development in the early 1990s, clinical trials with CED have been largely unsuccessful, and have illuminated a number of parameters that still need to be addressed for successful clinical application. This review addresses the physical principles behind CED, limitations in the technique, as well as means to overcome these limitations, clinical trials that have been performed, and future developments.

The role of growth factors as a therapeutic approach to demyelinating disease

A variety of growth factors are being explored as therapeutic agents relevant to the axonal and oligodendroglial deficits that occur as a result of demyelinating lesions such as are evident in Multiple Sclerosis (MS). This review focuses on five such proteins that are present in the lesion site and impact oligodendrocyte regeneration. It then presents approaches that are being exploited to manipulate the lesion environment affiliated with multiple neurodegenerative diseases and suggests that the utility of these approaches can extend to demyelination. Challenges are to further understand the roles of specific growth factors on a cellular and tissue level. Emerging technologies can then be employed to optimize the use of growth factors to ameliorate the deficits associated with demyelinating degenerative diseases.

Efficient Expression of Igf-1 from Lentiviral Vectors Protects In Vitro but Does Not Mediate Behavioral Recovery of a Parkinsonian Lesion in Rats

Gene therapy approaches delivering neurotrophic factors have offered promising results in both preclinical and clinical trials of Parkinson's disease (PD). However, failure of glial cell line-derived neurotrophic factor in phase 2 clinical trials has sparked a search for other trophic factors that may retain efficacy in the clinic. Direct protein injections of one such factor, insulin-like growth factor (IGF)-1, in a rodent model of PD has demonstrated impressive protection of dopaminergic neurons against 6-hydroxydopamine (6-OHDA) toxicity. However, protein infusion is associated with surgical risks, pump failure, and significant costs. We therefore used lentiviral vectors to deliver Igf-1, with a particular focus on the novel integration-deficient lentiviral vectors (IDLVs). A neuron-specific promoter, from the human synapsin 1 gene, excellent for gene expression from IDLVs, was additionally used to enhance Igf-1 expression. An investigation of neurotrophic effects on primary rat neuronal cultures demonstrated that neurons transduced with IDLV-Igf-1 vectors had complete protection on withdrawal of exogenous trophic support. Striatal transduction of such vectors into 6-OHDA-lesioned rats, however, provided neither protection of dopaminergic substantia nigra neurons nor improvement of animal behavior.

Gene transfer provides a practical means for safe, long-term, targeted delivery of biologically active neurotrophic factor proteins for neurodegenerative diseases

Efforts to develop neurotrophic factors to restore function and protect dying neurons in chronic neurodegenerative diseases like Alzheimer's (AD) and Parkinson's (PD) have been attempted for decades. Despite abundant data establishing nonclinical proof-of-concept, significant delivery issues have precluded the successful translation of this concept to the clinic. The development of AAV2 viral vectors to deliver therapeutic genes has emerged as a safe and effective means to achieve sustained, long-term, targeted, bioactive protein expression. Thus, it potentially offers a practical means to solve those long-standing delivery/translational issues associated with neurotrophic factors. Data are presented for two AAV2 viral vector constructs expressing one of two different neurotrophic factors: nerve growth factor (NGF) and neurturin (NRTN). One (AAV2-NGF; aka CERE-110) is being developed as a treatment to improve the function and delay further degeneration of cholinergic neurons in the nucleus basalis of Meynert, the degeneration of which has been linked to cognitive deficits in AD. The other (AAV2-NRTN; aka CERE-120) is similarly being developed to treat the degenerating nigrostriatal dopamine neurons and major motor deficits in PD. The data presented here demonstrate: (1) 2-year, targeted, bioactive-protein in monkeys, (2) persistent, bioactive-protein throughout the life-span of the rat, and (3) accurately targeted bioactive-protein in aged rats, with (4) no safety issues or antibodies to the protein detected. They also provide empirical guidance to establish parameters for human dosing and collectively support the idea that gene transfer may overcome key delivery obstacles that have precluded successful translation of neurotrophic factors to the clinic. More specifically, they also enabled the AAV-NGF and AAV-NRTN programs to advance into ongoing multi-center, double-blind clinical trials in AD and PD patients.

A phase1 study of stereotactic gene delivery of AAV2-NGF for Alzheimer's disease

BACKGROUND

Nerve growth factor (NGF) is an endogenous neurotrophic-factor protein with the potential to restore function and to protect degenerating cholinergic neurons in Alzheimer's disease (AD), but safe and effective delivery has proved unsuccessful.

METHODS

Gene transfer, combined with stereotactic surgery, offers a potential means to solve the long-standing delivery obstacles. An open-label clinical trial evaluated the safety and tolerability, and initial efficacy of three ascending doses of the genetically engineered gene-therapy vector adeno-associated virus serotype 2 delivering NGF (AAV2-NGF [CERE-110]). Ten subjects with AD received bilateral AAV2-NGF stereotactically into the nucleus basalis of Meynert.

RESULTS

AAV2-NGF was safe and well-tolerated for 2 years. Positron emission tomographic imaging and neuropsychological testing showed no evidence of accelerated decline. Brain autopsy tissue confirmed long-term, targeted, gene-mediated NGF expression and bioactivity.

CONCLUSIONS

This trial provides important evidence that bilateral stereotactic administration of AAV2-NGF to the nucleus basalis of Meynert is feasible, well-tolerated, and able to produce long-term, biologically active NGF expression, supporting the initiation of an ongoing multicenter, double-blind, sham-surgery-controlled trial.

Trophic factor gene therapy for Parkinson's disease

Parkinson's disease (PD) is a chronic and progressive neurodegenerative movement disorder for which there is presently no cure. Pharmacological remedies targeting the dopaminergic network are relatively effective at ameliorating motor deficits, especially in the early stages of the disease, but none of these therapies are curative and many generate their own problems. Recent advances in PD research have demonstrated that gene delivery of trophic factors, glial cell line-derived neurotrophic factor (GDNF) and neurturin, in particular, can provide structural and functional recovery in rodent and nonhuman primate models of PD. Similar success has been gleaned in open-label clinical trials, although this has yet to be realized in double-blinded analyses. This work reviews the field of trophic factor gene delivery for PD.

Advancing neurotrophic factors as treatments for age-related neurodegenerative diseases: developing and demonstrating "clinical proof-of-concept" for AAV-neurturin (CERE-120) in Parkinson's disease

Neurotrophic factors have long shown promise as potential therapies for age-related neurodegenerative diseases. However, 20 years of largely disappointing clinical results have underscored the difficulties involved with safely and effectively delivering these proteins to targeted sites within the central nervous system. Recent progress establishes that gene transfer can now likely overcome the delivery issues plaguing the translation of neurotrophic factors. This may be best exemplified by adeno-associated virus serotype-2-neurturin (CERE-120), a viral-vector construct designed to deliver the neurotrophic factor, neurturin to degenerating nigrostriatal neurons in Parkinson's disease. Eighty Parkinson's subjects have been dosed with CERE-120 (some 7+ years ago), with long-term, targeted neurturin expression confirmed and no serious safety issues identified. A double-blind, controlled Phase 2a trial established clinical "proof-of-concept" via 19 of the 24 prescribed efficacy end points favoring CERE-120 at the 12-month protocol-prescribed time point and all but one favoring CERE-120 at the 18-month secondary time point (p = 0.007 and 0.001, respectively). Moreover, clinically meaningful benefit was seen with CERE-120 on several specific protocol-prescribed, pairwise, blinded, motor, and quality-of-life end points at 12 months, and an even greater number of end points at 18 months. Because the trial failed to meet the primary end point (Unified Parkinson's Disease Rating Scale motor-off, measured at 12 months), a revised multicenter Phase 1/2b protocol was designed to enhance the neurotrophic effects of CERE-120, using insight gained from the Phase 2a trial. This review summarizes the development of CERE-120 from its inception through establishing "clinical proof-of-concept" and beyond. The translational obstacles and issues confronted, and the strategies applied, are reviewed. This information should be informative to investigators interested in translational research and development for age-related and other neurodegenerative diseases.

Translating the therapeutic potential of neurotrophic factors to clinical 'proof of concept': a personal saga achieving a career-long quest

While the therapeutic potential of neurotrophic factors has been well-recognized for over two decades, attempts to translate that potential to the clinic have been disappointing, largely due to significant delivery obstacles. Similarly, gene therapy (or gene transfer) emerged as a potentially powerful, new therapeutic approach nearly two decades ago and despite its promise, also suffered serious setbacks when applied to the human clinic. As advances continue to be made in both fields, ironically, they may now be poised to complement each other to produce a translational breakthrough. The accumulated data argue that gene transfer provides the 'enabling technology' that can solve the age-old delivery problems that have plagued the translation of neurotrophic factors as treatments for chronic central nervous system diseases. A leading translational program applying gene transfer to deliver a neurotrophic factor to rejuvenate and protect degenerating human neurons is CERE-120 (AAV2-NRTN). To date, over two dozen nonclinical studies and three clinical trials have been completed. A fourth (pivotal) clinical trial has completed all dosing and is currently evaluating safety and efficacy. In total, eighty Parkinson's disease (PD) subjects have thus far been dosed with CERE-120 (some 7 years ago), representing over 250 cumulative patient-years of exposure, with no serious safety issues identified. In a completed sham-surgery, double-blinded controlled trial, though the primary endpoint (the Unified Parkinson's Disease Rating Scale (UDPRS) motor off score measured at 12 months) did not show benefit from CERE-120, several important motor and quality of life measurements did, including the same UPDRS-motor-off score, pre-specified to also be measured at a longer, 18-month post-dosing time point. Importantly, not a single measurement favored the sham control group. This study therefore, provided important, well-controlled evidence establishing 'clinical proof of concept' for gene transfer to the CNS and the first controlled evidence for clinical benefit of a neurotrophic factor in a human neurodegenerative disease. This paper reviews the development of CERE-120, starting historically with the long-standing interest in the therapeutic potential of neurotrophic factors and continuing with selective accounts of past efforts to translate their potential to the clinic, eventually leading to the application of gene transfer and its role as the 'enabling technology'. Because of growing interest in translational R&D, including its practice in industry, the paper is uniquely oriented from the author's personal, quasi-autobiographic perspective and career-long experiences conducting translational research and development, with a focus on various translational neurotrophic factor programs spanning 30+ years in Big Pharma and development-stage biotech companies. It is hoped that by sharing these perspectives, practical insight and information might be provided to others also interested in translational R&D as well as neurotrophic factors and gene therapy, offering readers the opportunity to benefit from some of our successes, while possibly avoiding some of our missteps.

NTS-Polyplex: a potential nanocarrier for neurotrophic therapy of Parkinson's disease

Nanomedicine has focused on targeted neurotrophic gene delivery to the brain as a strategy to stop and reverse neurodegeneration in Parkinson's disease. Because of improved transfection ability, synthetic nanocarriers have become candidates for neurotrophic therapy. Neurotensin (NTS)-polyplex is a "Trojan horse" synthetic nanocarrier system that enters dopaminergic neurons through NTS receptor internalization to deliver a genetic cargo. The success of preclinical studies with different neurotrophic genes supports the possibility of using NTS-polyplex in nanomedicine. In this review, we describe the mechanism of NTS-polyplex transfection. We discuss the concept that an effective neurotrophic therapy requires a simultaneous effect on the axon terminals and soma of the remaining dopaminergic neurons. We also discuss the future of this strategy for the treatment of Parkinson's disease.

FROM THE CLINICAL EDITOR

This review paper focuses on nanomedicine-based treatment of Parkinson's disease, a neurodegenerative condition with existing symptomatic but no curative treatment. Neurotensin-polyplex is a synthetic nanocarrier system that enables delivery of genetic cargo to dopaminergic neurons via NTS receptor internalization.

Properly scaled and targeted AAV2-NRTN (neurturin) to the substantia nigra is safe, effective and causes no weight loss: support for nigral targeting in Parkinson's disease

Recent analyses of autopsied brains from subjects previously administered AAV2-neurturin (NRTN) gene transfer argues that optimizing the effects of neurotrophic factors in Parkinson's disease (PD) likely requires delivery to both the degenerating cell bodies (in substantia nigra) and their terminals (in striatum). Prior to implementing this novel dosing paradigm in humans, we conducted eight nonclinical experiments with three general objectives: (1) evaluate the feasibility, safety and effectiveness of targeting the substantia nigra (SN) with AAV2-NRTN, (2) better understand and appraise recent warnings of serious weight loss that might occur with targeting the SN with neurotrophic factors, and (3) define an appropriate dose of AAV2-NRTN that should safely and effectively cover the SN in PD patients. Toward these ends, we first determined SN volume for rats, monkeys and humans, and employed these values to calculate comparable dose equivalents for each species by scaling each dose, based on relative SN volume. Using this information, we next injected AAV2-GFP to monkey SN to quantify AAV2-vector distribution and confirm reasonable SN coverage. We then selected and administered a ~200-fold range of AAV2-NRTN doses (and a single AAV2-GDNF dose) to rat SN, producing a wide range of protein expression. In contrast to recent warnings regarding nigra targeting, no dose produced any serious side effects or toxicity, though we replicated the modest reduction in weight gain reported by others with the highest AAV2-NRTN and the AAV2-GDNF dose. A dose-related increase in NRTN expression was seen, with the lower doses limiting NRTN to the peri-SN and the highest dose producing mistargeted NRTN well outside the SN. We then demonstrated that the reduction in weight gain following excessive-doses can be dissociated from NRTN in the targeted SN, and is linked to mistargeted NRTN in the diencephalon. We also showed that prior destruction of the dopaminergic SN neurons via 6-OHDA had no impact on the weight loss phenomenon, further dissociating neurotrophic exposure to the SN as the culprit for weight changes. Finally, low AAV2-NRTN doses provided significant neuroprotection against 6-OHDA toxicity, establishing a wide therapeutic index for nigral targeting. These data support targeting the SN with AAV2-NRTN in PD patients, demonstrating that properly targeted and scaled AAV2-NRTN provides safe and effective NRTN expression. They also provided the means to define an appropriate human-equivalent dose for proceeding into an ongoing clinical trial, using empirically-based scaling to account for marked differences in SN volume between species.

Cell transplantation and gene therapy in Parkinson's disease

Parkinson's disease is a progressive neurodegenerative disorder affecting, in part, dopaminergic motor neurons of the ventral midbrain and their terminal projections that course to the striatum. Symptomatic strategies focused on dopamine replacement have proven effective at remediating some motor symptoms during the course of disease but ultimately fail to deliver long-term disease modification and lose effectiveness due to the emergence of side effects. Several strategies have been experimentally tested as alternatives for Parkinson's disease, including direct cell replacement and gene transfer through viral vectors. Cellular transplantation of dopamine-secreting cells was hypothesized as a substitute for pharmacotherapy to directly provide dopamine, whereas gene therapy has primarily focused on restoration of dopamine synthesis or neuroprotection and restoration of spared host dopaminergic circuitry through trophic factors as a means to enhance sustained controlled dopamine transmission. This seems now to have been verified in numerous studies in rodents and nonhuman primates, which have shown that grafts of fetal dopamine neurons or gene transfer through viral vector delivery can lead to improvements in biochemical and behavioral indices of dopamine deficiency. However, in clinical studies, the improvements in parkinsonism have been rather modest and variable and have been plagued by graft-induced dyskinesias. New developments in stem-cell transplantation and induced patient-derived cells have opened the doors for the advancement of cell-based therapeutics. In addition, viral-vector-derived therapies have been developed preclinically with excellent safety and efficacy profiles, showing promise in clinical trials thus far. Further progress and optimization of these therapies will be necessary to ensure safety and efficacy before widespread clinical use is deemed appropriate.

AAV and compacted DNA nanoparticles for the treatment of retinal disorders: challenges and future prospects

Gene therapy based on delivery of viral and nonviral vectors has shown great promise for the treatment of human ocular diseases; however, limitations have consistently prevented its widespread clinical application. Viral vectors have generally been better in terms of efficiency but have safety concerns. Nonviral vectors, on the other hand, offer safety but have often been disappointing in terms of efficiency of nuclear delivery and gene expression. Extensive animal studies have reported significant progress using both systems, but thus far only a few studies have shown promise in human clinical trials. This article reviews both viral and nonviral work with focus on two candidates for clinical ocular application--AAV and nanoparticles. Of particular interest are various requirements for successful clinical application of these technologies including vector trafficking, delivery, specific gene expression, and treatment safety, and tolerance.

Adeno-associated viral gene delivery in neurodegenerative disease

The advent of viral gene therapy technology has contributed greatly to the study of a variety of medical conditions, and there is increasing promise for clinical translation of gene therapy into human treatments. Adeno-associated viral (AAV) vectors provide one of the more promising approaches to gene delivery, and have been used extensively over the last 20 years. Derived from nonpathogenic parvoviruses, these vectors allow for stable and robust expression of desired transgenes in vitro and in vivo. AAV vectors efficiently and stably transduce neurons, with some strains targeting neurons exclusively in the brain. Thus, AAV vectors are particularly useful for neurodegenerative diseases, which have led to numerous preclinical studies and several human trials of gene therapy in patients with Parkinson's disease, Alzheimer's disease, and pediatric neurogenetic disorders. Here, we describe an efficient and reliable method for the production and purification of AAV serotype 2 vectors for both in vitro and in vivo applications.

Bioactivity of AAV2-neurturin gene therapy (CERE-120): differences between Parkinson's disease and nonhuman primate brains

BACKGROUND

AAV2-neurturin (CERE-120) is designed to deliver the neurotrophic-factor, neurturin, to the striatum to restore and protect degenerating nigrostriatal neurons in Parkinson's disease (PD). A common hypothesis is that following expression in the striatum, neurotrophic-factors like neurturin (NRTN) will be transported from degenerating terminals to their cell bodies in the substantia nigra pars compacta (SNc).

METHODS

We tested this concept using immunohistochemistry, comparing the bioactivity of AAV2-neurturin in brains of PD patients versus those of nonhuman primates similarly treated.

RESULTS

NRTN-immunostaining in the targeted striatum was seen in all PD cases (mean putaminal coverage: ∼15% by volume); comparable expression was observed in young, aged, and parkinsonian monkeys. In the SNc cell bodies, however, only rare evidence of neurturin was seen in PD, while ample evidence of intense nigral-NRTN was observed in all monkeys. NRTN-expression was associated with occasional, sparse TH-induction in the striatum of PD, but nothing apparent in the SNc. In primates, NRTN produced robust TH-induction throughout the nigrostriatal neurons.

DISCUSSION

These data provide the first evidence that gene therapy can increase expression of a neurotrophic-factor deep in the PD brain and that clear but modest enhancement of degenerating neurons can be induced. They also provide important insight regarding deficiencies in the status of nigrostriatal neurons in advanced PD, suggesting that serious axon-transport deficits reduced the bioactivity of AAV2-NRTN by limiting the protein exposed to the cell body. Thus, future efforts using neurotrophic-factors to treat neurodegenerative diseases will need to target both the terminal fields and the cell bodies of degenerating neurons to assure maximal benefit is achieved.

Gene therapy in Parkinson's disease: rationale and current status

Parkinson's disease is the second most common age-related neurodegenerative disorder, typified by the progressive loss of substantia nigra pars compacta dopamine neurons and the consequent decrease in the neurotransmitter dopamine. Patients exhibit a range of clinical symptoms, with the most common affecting motor function and including resting tremor, rigidity, akinesia, bradykinesia and postural instability. Current pharmacological interventions are palliative and largely aimed at increasing dopamine levels through increased production and/or inhibition of metabolism of this key neurotransmitter. The gold standard for treatment of both familial and sporadic Parkinson's disease is the peripheral administration of the dopamine precursor, levodopa. However, many patients gradually develop levodopa-induced dyskinesias and motor fluctuations. In addition, dopamine enhancement therapies are most useful when a portion of the nigrostriatal pathway is intact. Consequently, as the number of substantia nigra dopamine neurons and striatal projections decrease, these treatments become less efficacious. Current translational research is focused on the development of novel disease-modifying therapies, including those utilizing gene therapeutic approaches. Herein we present an overview of current gene therapy clinical trials for Parkinson's disease. Employing either recombinant adeno-associated virus type 2 (rAAV2) or lentivirus vectors, these clinical trials are focused on three overarching approaches: augmentation of dopamine levels via increased neurotransmitter production; modulation of the neuronal phenotype; and neuroprotection. The first two therapies discussed in this article focus on increasing dopamine production via direct delivery of genes involved in neurotransmitter synthesis (amino acid decarboxylase, tyrosine hydroxylase and GTP [guanosine triphosphate] cyclohydrolase 1). In an attempt to bypass the degenerating nigrostriatal pathway, a third clinical trial utilizes rAAV2 to deliver glutamic acid decarboxylase to the subthalamic nucleus, converting a subset of excitatory neurons to GABA-producing cells. In contrast, the final clinical trial is aimed at protecting the degenerating nigrostriatum by striatal delivery of rAAV2 harbouring the neuroprotective gene, neurturin. Based on preclinical studies, this gene therapeutic approach is posited to slow disease progression by enhancing neuronal survival. In addition, we discuss the outcome of each clinical trial and discuss the potential rationale for the marginal yet incremental clinical advancements that have thus far been realized for Parkinson's disease gene therapy.

Neurotrophic factor therapy for Parkinson's disease

Parkinson's disease (PD) is a chronic, progressive neurodegenerative movement disorder for which there is currently no effective therapy. Over the past several decades, there has been a considerable interest in neuroprotective therapies using trophic factors to alleviate the symptoms of PD. Neurotrophic factors (NTFs) are a class of molecules that influence a number of neuronal functions, including cell survival and axonal growth. Experimental studies in animal models suggest that members of neurotrophin family and GDNF family of ligands (GFLs) have the potent ability to protect degenerating dopamine neurons as well as promote regeneration of the nigrostriatal dopamine system. In clinical trials, although no serious adverse events related to the NTF therapy has been reported in patients, they remain inconclusive. In this chapter, we attempt to give a brief overview on several different growth factors that have been explored for use in animal models of PD and those already used in PD patients.

Viral vectors for neurotrophic factor delivery: a gene therapy approach for neurodegenerative diseases of the CNS

The clinical manifestation of most diseases of the central nervous system results from neuronal dysfunction or loss. Diseases such as stroke, epilepsy and neurodegeneration (e.g. Alzheimer's disease and Parkinson's disease) share common cellular and molecular mechanisms (e.g. oxidative stress, endoplasmic reticulum stress, mitochondrial dysfunction) that contribute to the loss of neuronal function. Neurotrophic factors (NTFs) are secreted proteins that regulate multiple aspects of neuronal development including neuronal maintenance, survival, axonal growth and synaptic plasticity. These properties of NTFs make them likely candidates for preventing neurodegeneration and promoting neuroregeneration. One approach to delivering NTFs to diseased cells is through viral vector-mediated gene delivery. Viral vectors are now routinely used as tools for studying gene function as well as developing gene-based therapies for a variety of diseases. Currently, many clinical trials using viral vectors in the nervous system are underway or completed, and seven of these trials involve NTFs for neurodegeneration. In this review, we discuss viral vector-mediated gene transfer of NTFs to treat neurodegenerative diseases of the central nervous system.

Tight Long-term dynamic doxycycline responsive nigrostriatal GDNF using a single rAAV vector

Glial cell line-derived neurotrophic factor (GDNF) gene transfer is being developed as a treatment for Parkinson's disease (PD). Due to the potential for side effects, external transgene regulation should enhance this strategy's safety profile. Here, we demonstrate dynamic control during long-term expression of GDNF using a recombinant adeno-associated virus (rAAV)-based bicistronic tetracycline (tet)-off construct. Nigrostriatal GDNF overexpression induces body weight alterations in rodents, enabling longitudinal in vivo tracking of GDNF expression after nigral vector delivery. Regulated GDNF expression was highly sensitive to dietary doxycycline (DOX), displaying undetectable striatal GDNF levels at serum DOX levels below those required for antimicrobial activity. However, in the absence of DOX, striatal GDNF levels exceeded levels required for efficacy in PD models. We also demonstrate the absence of a series of known GDNF-associated side effects when using direct intrastriatal vector delivery. Therefore, this single rAAV vector system meets most of the requirements for an experimental reagent for treatment of PD.