AAV2-mediated gene delivery to monkey putamen: evaluation of an infusion device and delivery parameters

Genome concentration, characterization, and integrity analysis of recombinant adeno-associated viral vectors using droplet digital PCR

Precise, reproducible characterization of AAV is critical for comparing preclinical results between laboratories and determining a safe and effective clinical dose for gene therapy applications. In this study, we systematically evaluated numerous parameters to produce a simple and robust ddPCR protocol for AAV characterization. The protocol uses a low ionic strength buffer containing Pluronic-F68 and polyadenylic acid to dilute the AAV into the ddPCR concentration range and a 10-minute thermal capsid lysis prior to assembling ddPCR reactions containing MspI. A critical finding is that the buffer composition affected the ITR concentration of AAV but not the ITR concentration of a double stranded plasmid, which has implications when using a theoretical, stoichiometric conversion factor to obtain the titer based on the ITR concentration. Using this protocol, a more comprehensive analysis of an AAV vector formulation was demonstrated with multiple ddPCR assays distributed throughout the AAV vector genome. These assays amplify the ITR, regulatory elements, and eGFP transgene to provide a more confident estimate of the vector genome concentration and a high-resolution characterization of the vector genome identity. Additionally, we compared two methods of genome integrity analysis for three control sample types at eight different concentrations for each sample. The genome integrity was independent of sample concentration and the expected values were obtained when integrity was determined based on the excess number of positive droplets relative to the number of double positive droplets expected by chance co-encapsulation of two DNA targets. The genome integrity was highly variable and produced unexpected values when the double positive droplet percentage was used to calculate the genome integrity. A protocol using a one-minute thermal capsid lysis prior to assembling ddPCR reactions lacking a restriction enzyme used the non-ITR assays in a duplex ddPCR milepost experiment to determine the genome integrity using linkage analysis.

Improving the Efficacy and Accessibility of Intracranial Viral Vector Delivery in Non-Human Primates

Non-human primates (NHPs) are precious resources for cutting-edge neuroscientific research, including large-scale viral vector-based experimentation such as optogenetics. We propose to improve surgical outcomes by enhancing the surgical preparation practices of convection-enhanced delivery (CED), which is an efficient viral vector infusion technique for large brains such as NHPs’. Here, we present both real-time and next-day MRI data of CED in the brains of ten NHPs, and we present a quantitative, inexpensive, and practical bench-side model of the in vivo CED data. Our bench-side model is composed of food coloring infused into a transparent agar phantom, and the spread of infusion is optically monitored over time. Our proposed method approximates CED infusions into the cortex, thalamus, medial temporal lobe, and caudate nucleus of NHPs, confirmed by MRI data acquired with either gadolinium-based or manganese-based contrast agents co-infused with optogenetic viral vectors. These methods and data serve to guide researchers and surgical team members in key surgical preparations for intracranial viral delivery using CED in NHPs, and thus improve expression targeting and efficacy and, as a result, reduce surgical risks.

Reduction of Recombinant Adeno-Associated Virus Vector Adsorption on Solid Surfaces by Polyionic Hydrophilic Complex Coating

Recombinant adeno-associated virus (rAAV) vectors have proven efficacy as gene therapy vehicles. However, non-specific adsorption of these vectors on solid surfaces is encountered during production, storage, and administration, as well as in quantification processes. Such adsorption has been reported to result in the loss of up to 90% of vector particles and can also result in high variability in vector genome quantification. In this study, we demonstrate the effective decrease of recombinant adeno-associated virus vector adsorption by application of a polyionic hydrophilic complex polymer coating on the surfaces of the tools used in viral vector quantification analyses [i.e., pipette tips, cryotube vials, and quantitative polymerase chain reaction (qPCR) plates]. qPCR analyses showed efficient recovery of vector particles from tools with this coating, with up to 95% of vector particle loss being prevented, leading to a higher transduction efficiency in vitro. Thus, the tested coating has the potential to be widely used in material processing in the gene therapy field.

Synthetic amyloid-β oligomers drive early pathological progression of Alzheimer's disease in nonhuman primates

As an insidious and slowly progressive neurodegenerative disorder, Alzheimer’s disease (AD) uniquely develops in humans but fails in other species. Therefore, it has been challenged to rebuild human AD in animals, including in non-human primates. Here, we bilaterally delivered synthetic Aβ oligomers (AβOs) into the cerebral parenchyma of cynomolgus monkeys, which rapidly drove the formation of massive Aβ plaques and concomitant neurofibrillary tangles in the cynomolgus brain. The amyloid and tau pathology as well as their co-occurrence in AβO-monkeys were reminiscent of those in patients with AD. In addition, the activated astrocytes and microglia surrounding Aβ plaques indicated the triggered neuroinflammation. The degenerative neurons and synapses around Aβ plaques also emerged in cynomolgus brain. Together, soluble AβOs caused the cascade of pathologic events associated with AD in monkeys as occurred in patients at the early phase, which could facilitate the development of a promising animal model for human AD in non-human primates.

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The Aβ oligomers (AβOs) drive to develop massive Aβ plaque in the monkey brain

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Neurofibrillary tangles form in multiple brain regions of AβO-monkeys

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The co-occurrence of amyloid and tau pathology in AβO-monkeys as in patients with AD

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The neuroinflammation and neurodegeneration are triggered in AβO-monkeys

Of rAAV and Men: From Genetic Neuromuscular Disorder Efficacy and Toxicity Preclinical Studies to Clinical Trials and Back

Neuromuscular disorders are a large group of rare pathologies characterised by skeletal muscle atrophy and weakness, with the common involvement of respiratory and/or cardiac muscles. These diseases lead to life-long motor deficiencies and specific organ failures, and are, in their worst-case scenarios, life threatening. Amongst other causes, they can be genetically inherited through mutations in more than 500 different genes. In the last 20 years, specific pharmacological treatments have been approved for human usage. However, these "à-la-carte" therapies cover only a very small portion of the clinical needs and are often partially efficient in alleviating the symptoms of the disease, even less so in curing it. Recombinant adeno-associated virus vector-mediated gene transfer is a more general strategy that could be adapted for a large majority of these diseases and has proved very efficient in rescuing the symptoms in many neuropathological animal models. On this solid ground, several clinical trials are currently being conducted with the whole-body delivery of the therapeutic vectors. This review recapitulates the state-of-the-art tools for neuron and muscle-targeted gene therapy, and summarises the main findings of the spinal muscular atrophy (SMA), Duchenne muscular dystrophy (DMD) and X-linked myotubular myopathy (XLMTM) trials. Despite promising efficacy results, serious adverse events of various severities were observed in these trials. Possible leads for second-generation products are also discussed.

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.

Postmortem Analysis in a Clinical Trial of AAV2-NGF Gene Therapy for Alzheimer's Disease Identifies a Need for Improved Vector Delivery

Nerve growth factor (NGF) gene therapy rescues and stimulates cholinergic neurons, which degenerate in Alzheimer's disease (AD). In a recent clinical trial for AD, intraparenchymal adeno-associated virus serotype 2 (AAV2)-NGF delivery was safe but did not improve cognition. Before concluding that NGF gene therapy is ineffective, it must be shown that AAV2-NGF successfully engaged the target cholinergic neurons of the basal forebrain. In this study, patients with clinically diagnosed early- to middle-stage AD received a total dose of 2 × 10¹¹ vector genomes of AAV2-NGF by stereotactic injection of the nucleus basalis of Meynert. After a mean survival of 4.0 years, AAV2-NGF targeting, spread, and expression were assessed by immunolabeling of NGF and the low-affinity NGF receptor p75 at 15 delivery sites in 3 autopsied patients. NGF gene expression persisted for at least 7 years at sites of AAV2-NGF injection. However, the mean distance of AAV2-NGF spread was only 0.96 ± 0.34 mm. NGF did not directly reach cholinergic neurons at any of the 15 injection sites due to limited spread and inaccurate stereotactic targeting. Because AAV2-NGF did not directly engage the target cholinergic neurons, we cannot conclude that growth factor gene therapy is ineffective for AD. Upcoming clinical trials for AD will utilize real-time magnetic resonance imaging guidance and convection-enhanced delivery to improve the targeting and spread of growth factor gene delivery.

Targeted Delivery and Tolerability of MRI-Guided CED Infusion into the Cerebellum of Nonhuman Primates

This study explored the feasibility of intraparenchymal delivery (gadoteridol and/or Serotype 5 Adeno-Associated Viral Vector-enhanced Green Fluorescent Protein [AAV5-eGFP]) into the cerebellum of nonhuman primates using real-time magnetic resonance imaging-guided convection enhanced delivery (MRI-CED) technology. All animals tolerated the neurosurgical procedure without any clinical sequela. Gene expression was detected within the cerebellar parenchyma at the site of infusion and resulted in transduction of neuronal cell bodies and fibers. Histopathology indicated localized damage along the stem of the cannula tract. These findings demonstrate the potential of real-time MRI-CED to deliver therapeutics into the cerebellum, which has extensive reciprocal connections and may be used as a target for the treatment of neurological disorders.

Glutamatergic Tuning of Hyperactive Striatal Projection Neurons Controls the Motor Response to Dopamine Replacement in Parkinsonian Primates

Dopamine (DA) loss in Parkinson's disease (PD) alters the function of striatal projection neurons (SPNs) and causes motor deficits, but DA replacement can induce further abnormalities. A key pathological change in animal models and patients is SPN hyperactivity; however, the role of glutamate in altered DA responses remains elusive. We tested the effect of locally applied AMPAR or NMDAR antagonists on glutamatergic signaling in SPNs of parkinsonian primates. Following a reduction in basal hyperactivity by antagonists at either receptor, DA inputs induced SPN firing changes that were stable during the entire motor response, in clear contrast with the typically unstable effects. The SPN activity reduction over an extended putamenal area controlled the release of involuntary movements in the "on" state and therefore improved motor responses to DA replacement. These results demonstrate the pathophysiological role of upregulated SPN activity and support strategies to reduce striatal glutamate signaling for PD therapy.

Advances in optogenetic and chemogenetic methods to study brain circuits in non-human primates

Over the last 10 years, the use of opto- and chemogenetics to modulate neuronal activity in research applications has increased exponentially. Both techniques involve the genetic delivery of artificial proteins (opsins or engineered receptors) that are expressed on a selective population of neurons. The firing of these neurons can then be manipulated using light sources (for opsins) or by systemic administration of exogenous compounds (for chemogenetic receptors). Opto- and chemogenetic tools have enabled many important advances in basal ganglia research in rodent models, yet these techniques have faced a slow progress in non-human primate (NHP) research. In this review, we present a summary of the current state of these techniques in NHP research and outline some of the main challenges associated with the use of these genetic-based approaches in monkeys. We also explore cutting-edge developments that will facilitate the use of opto- and chemogenetics in NHPs, and help advance our understanding of basal ganglia circuits in normal and pathological conditions.

Gene Therapy in the Nervous System: Failures and Successes

Genetic disorders, caused by deleterious changes in the DNA sequence away from the normal genomic sequence, affect millions of people worldwide. Gene therapy as a treatment option for patients is an attractive proposition due to its conceptual simplicity. In principle, gene therapy involves correcting the genetic disorder by either restoring a normal functioning copy of a gene or reducing the toxicity arising from a mutated gene. In this way specific genetic function can be restored without altering the expression of other genes and the proteins they encode. The reality however is much more complex, and as a result the vector systems used to deliver gene therapies have by necessity continued to evolve and improve over time with respect to safety profile, efficiency, and long-term expression. In this chapter we examine the current approaches to gene therapy, assess the different gene delivery systems utilized, and highlight the failures and successes of relevant clinical trials. We do not intend for this chapter to be a comprehensive and exhaustive assessment of all clinical trials that have been conducted in the CNS, but instead will focus on specific diseases that have seen successes and failures with different gene therapy vehicles to gauge how preclinical models have informed the design of clinical trials.

Microbial Proteins as Novel Industrial Biotechnology Hosts to Treat Epilepsy

Epilepsy is characterized by the hyperexcitability of various neuronal circuits that results due to the imbalance between glutamate-mediated excitation of voltage-gated cation channels and γ-amino butyric acid (GABA)-mediated inhibition of anion channels leading to aberrant, sporadic oscillations or fluctuations in neuronal electrical activity. Epilepsy with a risk of mortality and around 65 million sufferers of all ages all over the world is limited therapeutically with high rates of adverse reactions, lack of complete seizure control, and over 30% patients with refractory epilepsy. The only alternative to medicines is to identify and surgically remove the seizure foci in the brain or to abort the seizures just as they begin using an implanted cerebral electrode. However, these alternatives are unable to precisely aim aberrant neuronal circuits while leaving others unaltered. Epilepsy animal models also constitute the identical constraint. Thus, a better target-specific approach is needed to study and treat epilepsy. Unicellular green algae Chlamydomonas reinhardtii expresses a channelrhodopsin-2 (ChR2) sodium ion channel protein that controls the phototaxis movement of algae in response to blue light. Similarly, archaeon Natronomonas pharaonis (NpHR) expresses a monovalent Cl^- channel protein halorhodopsin that responds to yellow light. These features of ChR2 and NpHR proteins can be used in optogenetic techniques to manipulate the bi-directional firing pattern of neuronal circuits in an attempt to better understand the pathophysiology of epileptic seizures as well as to discover novel potential drugs to treat epilepsy.

Adeno-Associated Virus-Based Gene Therapy for CNS Diseases

Gene therapy is at the cusp of a revolution for treating a large spectrum of CNS disorders by providing a durable therapeutic protein via a single administration. Adeno-associated virus (AAV)-mediated gene transfer is of particular interest as a therapeutic tool because of its safety profile and efficiency in transducing a wide range of cell types. The purpose of this review is to describe the most notable advancements in preclinical and clinical research on AAV-based CNS gene therapy and to discuss prospects for future development based on a new generation of vectors and delivery.

Optogenetic tools for modulating and probing the epileptic network

Epilepsy affects roughly 1% of the population worldwide. Although effective treatments with antiepileptic drugs are available, more than 20% of patients have seizures that are refractory to medical therapy and many patients experience adverse effects. Hence, there is a continued need for novel therapies for those patients. A new technique called "optogenetics" may offer a new hope for these refractory patients. Optogenetics is a technology based on the combination of optics and genetics, which can control or record neural activity with light. Following delivery of light-sensitive opsin genes such as channelrhodopsin-2 (ChR2), halorhodopsin (NpHR), and others into brain, excitation or inhibition of specific neurons in precise brain areas can be controlled by illumination at different wavelengths with very high temporal and spatial resolution. Neuromodulation with the optogenetics toolbox have already been shown to be effective at treating seizures in animal models of epilepsy. This review will outline the most recent advances in epilepsy research with optogenetic techniques and discuss how this technology can contribute to our understanding and treatment of epilepsy in the future.

Viral vector delivery of neurotrophic factors for Parkinson's disease therapy

Parkinson's disease (PD) is a neurodegenerative disorder characterised by the progressive loss of midbrain dopaminergic neurons, which causes motor impairments. Current treatments involve dopamine replacement to address the disease symptoms rather than its cause. Factors that promote the survival of dopaminergic neurons have been proposed as novel therapies for PD. Several dopaminergic neurotrophic factors (NTFs) have been examined for their ability to protect and/or restore degenerating dopaminergic neurons, both in animal models and in clinical trials. These include glial cell line-derived neurotrophic factor, neurturin, cerebral dopamine neurotrophic factor and growth/differentiation factor 5. Delivery of these NTFs via injection or infusion to the brain raises several practical problems. A new delivery approach for NTFs involves the use of recombinant viral vectors to enable long-term expression of these factors in brain cells. Vectors used include those based on adenoviruses, adeno-associated viruses and lentiviruses. Here we review progress to date on the potential of each of these four NTFs as novel therapeutic strategies for PD, as well as the challenges that have arisen, from pre-clinical analysis to clinical trials. We conclude by discussing recently-developed approaches to optimise the delivery of NTF-carrying viral vectors to the brain.

AAV-Mediated Gene Therapy for Research and Therapeutic Purposes

Adeno-associated virus (AAV) is a small, nonenveloped virus that was adapted 30 years ago for use as a gene transfer vehicle. It is capable of transducing a wide range of species and tissues in vivo with no evidence of toxicity, and it generates relatively mild innate and adaptive immune responses. We review the basic biology of AAV, the history of progress in AAV vector technology, and some of the clinical and research applications where AAV has shown success.

Injection parameters and virus dependent choice of promoters to improve neuron targeting in the nonhuman primate brain

We, like many others, wish to use modern molecular methods to alter neuronal functionality in primates. For us, this requires expression in a large proportion of the targeted cell population. Long generation times make germline modification of limited use. The size and intricate primate brain anatomy poses additional challenges. We surved methods using lentiviruses and serotypes of adeno-associated viruses (AAVs) to introduce active molecular material into cortical and subcortical regions of old-world monkey brains. Slow injections of AAV2 give well-defined expression of neurons in the cortex surrounding the injection site. Somewhat surprisingly we find that in the monkey the use of cytomegalovirus promoter in lentivirus primarily targets glial cells but few neurons. In contrast, with a synapsin promoter fragment the lentivirus expression is neuron specific at high transduction levels in all cortical layers. We also achieve specific targeting of tyrosine hydroxlase (TH)- rich neurons in the locus coeruleus and substantia nigra with a lentvirus carrying a fragment of the TH promoter. Lentiviruses carrying neuron specific promoters are suitable for both cortical and subcortical injections even when injected quickly.

Differential effects of two MRI contrast agents on the integrity and distribution of rAAV2 and rAAV5 in the rat striatum

Intraoperative magnetic resonance imaging (MRI) has been proposed as a method to optimize intracerebral targeting and for tracking infusate distribution in gene therapy trials for nervous system disorders. We thus investigated possible effects of two MRI contrast agents, gadoteridol (Gd) and galbumin (Gab), on the distribution and levels of transgene expression in the rat striatum and their effect on integrity and stability of recombinant adeno-associated virus (rAAV) particles. MRI studies showed that contrast agent distribution did not predict rAAV distribution. However, green fluorescent protein (GFP) immunoreactivity revealed an increase in distribution of rAAV5-GFP, but not rAAV2-GFP, in the presence of Gd when compared with viral vector injected alone. In contrast, Gab increased the distribution of rAAV2-GFP not rAAV5-GFP. These observations pointed to a direct effect of infused contrast agent on the rAAV particles. Negative-stain electron microscopy (EM), DNAase treatment, and differential scanning calorimetry (DSC) were used to monitor rAAV2 and rAAV5 particle integrity and stability following contrast agent incubation. EMs of rAAV2-GFP and rAAV5-GFP particles pretreated with Gd appear morphologically similar to the untreated sample; however, Gab treatment resulted in surface morphology changes and aggregation. A compromise of particle integrity was suggested by sensitivity of the packaged genome to DNAase treatment following Gab incubation but not Gd for both vectors. However, neither agent significantly affected particle stability when analyzed by DSC. An increase in T m was observed for AAV2 in lactated Ringer's buffer. These results thus highlight potential interactions between MRI contrast agents and AAV that might affect vector distribution and stability, as well as the stabilizing effect of lactated Ringer's solution on AAV2.

Convection-enhanced drug delivery to the brain: therapeutic potential and neuropathological considerations

Convection-enhanced delivery (CED) describes a direct method of drug delivery to the brain through intraparenchymal microcatheters. By establishing a pressure gradient at the tip of the infusion catheter in order to exploit bulk flow through the interstitial spaces of the brain, CED offers a number of advantages over conventional drug delivery methods-bypass of the blood-brain barrier, targeted distribution through large brain volumes and minimization of systemic side effects. Despite showing early promise, CED is yet to fulfill its potential as a mainstream strategy for the treatment of neurological disease. Substantial research effort has been dedicated to optimize the technology for CED and identify the parameters, which govern successful drug distribution. It seems likely that successful clinical translation of CED will depend on suitable catheter technology being used in combination with drugs with optimal physicochemical characteristics, and on neuropathological analysis in appropriate preclinical models. In this review, we consider the factors most likely to influence the success or failure of CED, and review its application to the treatment of high-grade glioma, Parkinson's disease (PD) and Alzheimer's disease (AD).

The art of gene therapy for glioma: a review of the challenging road to the bedside

Glioblastoma multiforme (GBM) is a highly invasive brain tumour that is unvaryingly fatal in humans despite even aggressive therapeutic approaches such as surgical resection followed by chemotherapy and radiotherapy. Unconventional treatment options such as gene therapy provide an intriguing option for curbing glioma related deaths. To date, gene therapy has yielded encouraging results in preclinical animal models as well as promising safety profiles in phase I clinical trials, but has failed to demonstrate significant therapeutic efficacy in phase III clinical trials. The most widely studied antiglioma gene therapy strategies are suicide gene therapy, genetic immunotherapy and oncolytic virotherapy, and we have attributed the challenging transition of these modalities into the clinic to four major roadblocks: (1) anatomical features of the central nervous system, (2) the host immune system, (3) heterogeneity and invasiveness of GBM and (4) limitations in current GBM animal models. In this review, we discuss possible ways to jump these hurdles and develop new gene therapies that may be used alone or in synergy with other modalities to provide a powerful treatment option for patients with GBM.

Gene-based therapy of Parkinson's Disease: Translation from animal model to human clinical trial employing convection enhanced delivery

The existing treatment of Parkinson's disease (PD) is directed towards substituting dopamine loss with either dopamine replacement therapy or pharmacological therapies aimed at increasing dopamine at the synapse level. Emerging viable alternatives include the use of cell-based and gene-based therapeutics. In this review, we discuss efforts in developing in vitro and in vivo models and their translation to human clinical trials for gene-based therapy of this distressing and prevalent neurodegenerative disorder. Given the mismatch between expectations from preclinical data and results of human pivotal trials, drug delivery has been identified as the key emerging area for translational research due to limitation of limited efficacy. The chief highlights of the current topic include use of improved delivery methods of gene-based therapeutic agents. Convection-enhanced delivery (CED), an advanced infusion technique with demonstrated utility in ex vivo and in vivo animal models has recently been adopted for PD gene-based therapy trials. Several preclinical studies suggest that magnetic resonance imaging (MRI)-guided navigation for accurately targeting and real time monitoring viral vector delivery (rCED) in future clinical trials involving detection of gene expression and restoration of dopaminergic function loss using pro-drug approach will greatly enhance these PD treatments.

Long-term behavioral, electrophysiological, and neurochemical monitoring of the safety of an experimental antiepileptic implant, the muscimol-delivering Subdural Pharmacotherapy Device in monkeys

Object

The authors evaluated the extent to which the Subdural Pharmacotherapy Device (SPD), chronically implanted over the frontal cortex to perform periodic, localized muscimol-delivery/CSF removal cycles, affects overall behavior, motor performance, electroencephalography (EEG) activity, and blood and CSF neurochemistry in macaque monkeys.

Methods

Two monkeys were used to adjust methodology and 4 monkeys were subjected to comprehensive testing. Prior to surgery, the animals' behavior in a large test chamber was monitored, and the motor skills required to remove food pellets from food ports located on the walls of the chamber were determined. The monkeys underwent implantation of the subdural and extracranial SPD units. The subdural unit, a silicone strip integrating EEG electrodes and fluid-exchange ports, was positioned over the right frontal cortex. The control unit included a battery-powered, microprocessor-regulated dual minipump and radiofrequency module secured to the cranium. After implantation, the SPD automatically performed periodic saline or muscimol (1.0 mM) deliveries at 12-hour intervals, alternating with local CSF removals at 6-hour intervals. The antiepileptic efficacy of this muscimol concentration was verified by demonstrating its ability to prevent focal acetylcholine-induced seizures. During SPD treatment, the monkeys' behavior and motor performance were again monitored, and the power spectrum of their radiofrequency-transmitted EEG recordings was analyzed. Serum and CSF muscimol levels were measured with high-performance liquid chromatography electrochemical detection, and CSF protein levels were measured with turbidimetry.

Results

The SPD was well tolerated in all monkeys for up to 11 months. The behavioral study revealed that during both saline and muscimol SPD treatment, the monkeys could achieve the maximum motor performance of 40 food-pellet removals per session, as before surgery. The EEG study showed that local EEG power spectra were not affected by muscimol treatment with SPD. The neurochemical study demonstrated that the administration of 1.0 mM muscimol into the neocortical subarachnoid space led to no detectable levels of this compound in the blood and cisternal CSF, as measured 1–125 minutes after delivery. Total protein levels were within the normal range in the cisternal CSF, but protein levels in the cortical-site CSF were significantly higher than normal: 361 ± 81.6 mg/dl. Abrupt discontinuation of 3-month, periodic, subdural muscimol treatments induced withdrawal seizures, which could be completely prevented by gradually tapering off the subdural muscimol concentration from 1.0 mM to 0.12–0.03 mM over a period of 2 weeks. The monkeys' general health and weight were maintained. Infection occurred only in one monkey 9 months after surgery.

Conclusions

Long-term, periodic, transmeningeal muscimol delivery with the SPD is essentially a safe procedure. If further improved and successfully adapted for use in humans, the SPD can be used for the treatment of intractable focal neocortical epilepsy affecting approximately 150,000 patients in the US.

Convection-enhanced delivery of therapeutic agents into the brain

CED of therapeutic agents remains a promising strategy for treating malignant gliomas and non-neoplastic neurological diseases. Although initial clinical trials have failed to show survival benefit for new agents delivered via this approach, multiple earlier stage trials have addressed only a fraction of the myriad of technical and technological issues that surround this novel approach. Development of CED has been limited by the fact that both new technologies and novel therapeutic agents are being developed simultaneously.New trials are being planned to investigate agents that can be coinfused with radiographic tracers, as well as novel catheters that avoid problems with backflow and potentially will provide more reliable drug distribution.

Six-month partial suppression of Huntingtin is well tolerated in the adult rhesus striatum

Huntington's disease is caused by expression of a mutant form of Huntingtin protein containing an expanded polyglutamine repeat. One possible treatment for Huntington's disease may be to reduce expression of mutant Huntingtin in the brain via RNA interference. Unless the therapeutic molecule is designed to be allele-specific, both wild-type and mutant protein will be suppressed by an RNA interference treatment. A key question is whether suppression of wild-type as well as mutant Huntingtin in targeted brain regions can be tolerated and result in a net benefit to patients with Huntington's disease. Whether Huntingtin performs essential functions in the adult brain is unclear. Here, we tested the hypothesis that the adult primate brain can tolerate moderately reduced levels of wild-type Huntingtin protein for an extended period of time. A serotype 2 adeno-associated viral vector encoding for a short hairpin RNA targeting rhesus huntingtin messenger RNA (active vector) was bilaterally injected into the striatum of four adult rhesus monkeys. Four additional animals received a comparable vector encoding a scrambled control short hairpin RNA (control vector). General health and motor behaviour were monitored for 6 months. Upon termination, brain tissues were sampled and assessed blindly for (i) huntingtin messenger RNA knockdown; (ii) Huntingtin protein expression; and (iii) neuropathological changes. Reduction in wild-type huntingtin messenger RNA levels averaging ∼30% was measured in the striatum of active vector recipients 6 months post-injection. A widespread reduction in Huntingtin protein levels was also observed by immunohistochemistry in these animals, with an average protein reduction of ∼45% relative to controls measured by western blot analysis in the putamen of active vector recipients. As with control vector recipients, no adverse effects were observed behaviourally, and no neurodegeneration was found on histological examination of active vector recipients. Our results suggest that long-term partial suppression of wild-type Huntingtin may be safe, and thus if a comparable level of suppression of mutant Huntingtin is beneficial, then partial suppression of both wild-type and mutant Huntingtin may result in a net benefit in patients with heterozygous Huntington's disease.

AAV8(gfp) preferentially targets large diameter dorsal root ganglion neurones after both intra-dorsal root ganglion and intrathecal injection

Adeno-associated viral vectors (AAV) are increasingly used to deliver therapeutic genes to the central nervous system (CNS) where they promote transgene expression in post mitotic neurones for long periods with little or no toxicity. In adult rat dorsal root ganglia (DRG), we investigated the cellular tropism of AAV8 containing the green fluorescent protein gene (gfp) after either intra-lumbar DRG or intrathecal injection and showed that transduced DRG neurones (DRGN) expressed GFP irrespective of the delivery route, while non-neuronal cells were GFP(-). After intra-DRG delivery of AAV8(gfp), the mean DRGN transduction rate was 11%, while intrathecal delivery transduced a mean of 1.5% DRGN. After intra-DRG injection, 2% of small DRGN (<30 μm in diameter) were GFP(+) compared with 32% of large DRGN (>60 μm in diameter). Axons of transduced DRGN were also GFP(+); no intra-spinal neurones were transduced. A small number of contralateral DRGN were transduced after intra-DRG injection, suggesting that AAV8 may diffuse from injected DRG into the spinal canal. Microglia and astrocytes were highly ramified with increased GFAP(+) immunoreactivity (i.e. activated) in the neuropil around GFP(+) DRG axon projections within the cord after intra-DRG injection. This study showed that after both intra-DRG and intrathecal delivery, strong preferential AAV8 tropism exists for large DRGN unassociated with cell death, but GFP(+) axons projecting in the spinal cord induced local glial activation. These results open up opportunities for targeted delivery of therapeutics such as neurotrophic factors to the injured spinal cord.

Intrastriatal convection-enhanced delivery results in widespread perivascular distribution in a pre-clinical model

BACKGROUND

Convection-enhanced delivery (CED), a direct method for drug delivery to the brain through intraparenchymal microcatheters, is a promising strategy for intracerebral pharmacological therapy. By establishing a pressure gradient at the tip of the catheter, drugs can be delivered in uniform concentration throughout a large volume of interstitial fluid. However, the variables affecting perivascular distribution of drugs delivered by CED are not fully understood. The aim of this study was to determine whether the perivascular distribution of solutes delivered by CED into the striatum of rats is affected by the molecular weight of the infused agent, by co-infusion of vasodilator, alteration of infusion rates or use of a ramping regime. We also wanted to make a preliminary comparison of the distribution of solutes with that of nanoparticles.

METHODS

We analysed the perivascular distribution of 4, 10, 20, 70, 150 kDa fluorescein-labelled dextran and fluorescent nanoparticles at 10 min and 3 h following CED into rat striatum. We investigated the effect of local vasodilatation, slow infusion rates and ramping on the perivascular distribution of solutes. Co-localisation with perivascular basement membranes and vascular endothelial cells was identified by immunohistochemistry. The uptake of infusates by perivascular macrophages was quantified using stereological methods.

RESULTS

Widespread perivascular distribution and macrophage uptake of fluorescein-labelled dextran was visible 10 min after cessation of CED irrespective of molecular weight. However, a significantly higher proportion of perivascular macrophages had taken up 4, 10 and 20 kDa fluorescein-labelled dextran than 150 kDa dextran (p < 0.05, ANOVA). Co-infusion with vasodilator, slow infusion rates and use of a ramping regime did not alter the perivascular distribution. CED of fluorescent nanoparticles indicated that particles co-localise with perivascular basement membranes throughout the striatum but, unlike soluble dextrans, are not taken up by perivascular macrophages after 3 h.

CONCLUSIONS

This study suggests that widespread perivascular distribution and interaction with perivascular macrophages is likely to be an inevitable consequence of CED of solutes. The potential consequences of perivascular distribution of therapeutic agents, and in particular cytotoxic chemotherapies, delivered by CED must be carefully considered to ensure safe and effective translation to clinical trials.

Distribution properties of lentiviral vectors administered into the striatum by convection-enhanced delivery

Before the successful use of lentiviral vectors in clinical trials it is essential that strategies for direct vector delivery into the brain be evaluated in vivo, particularly as these vectors are significantly larger than the brain extracellular space. To date no such studies have been undertaken. In this study, convection-enhanced delivery (CED) was employed in an attempt to achieve widespread lentiviral delivery in the striatum. Infusions of equine infectious anemia virus (EIAV) and HIV vector constructs expressing the reporter gene β-galactosidase (β-Gal) were undertaken into the striatum at a range of flow rates and viral titers. In rats, all EIAV and HIV infusions led to the extensive transduction of cells in perivascular spaces throughout the brain. Although infusions were performed under standardized conditions, the number and volume of distribution of transduced cells were highly variable, with approximately one-third of EIAV infusions leading to no concentrated cell transduction in the striatum. Heparin coinfusion had no effect on EIAV distribution, although coinfusion of nimodipine resulted in a significant reduction in the number and volume of distribution of transduced cells. Intrastriatal EIAV delivery in pigs led to extensive transduction of mainly neurons, which could be effectively visualized in real time by T(2)-weighted magnetic resonance imaging. No infusions were associated with a significant inflammatory response. Therefore, despite its large size, lentiviral vectors can be administered by CED to the striatum in both small and large animal models. However, the variability in vector distribution under standardized conditions and widespread vector distribution through the perivascular spaces raise serious concerns regarding the practicality of lentivirus-mediated gene therapy in the brain in clinical practice.

An evaluation of site-specific immune responses directed against first-generation adenoviral vectors administered by convection-enhanced delivery

BACKGROUND

Direct adenoviral vector injections into the brain have been used in clinical trials to treat patients with high-grade gliomas. However, a recent phase 3 trial using first-generation vectors failed to demonstrate significant survival benefits. Malignant gliomas infiltrate extensively through the white matter, making them difficult to treat, and chemotherapy is at best partially effective. Convection enhanced delivery (CED) represents a rationale approach for achieving widespread targeting of infiltrating tumour cells. Previous studies have demonstrated that infusions of particle numbers above a threshold level [10(8) plaque-forming units (pfu)] are associated with a pronounced inflammatory response in rat grey matter, although no such comparisons have been made with CED infusions into the white matter.

METHODS

In the present study, we investigated the distribution and immune response after the administration of 10(7) and 10(9) pfu of a first-generation adenoviral vector (Ad.CMV.EGFP) by CED in both small and large animal models.

RESULTS

We show that Ad.CMV.EGFP can be efficiently distributed by CED over large volumes of brain. A threshold vector dose of between 10(7) and 10(9) pfu was seen in both rat striatum and white matter, above which transgene expression was lost at 30 days. Furthermore, all adenoviral infusions were associated with evidence of significant tissue damage, as demonstrated by loss of neurones and astrocytes or the presence of extensive astrocytosis.

CONCLUSIONS

These results indicate that CED is capable of mediating widespread adenoviral vector distribution, although these vectors are associated with significant tissue toxicity that may render their safe application in clinical trials unfeasible.

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.

Potential therapeutic uses of BDNF in neurological and psychiatric disorders

The growth factor brain-derived neurotrophic factor (BDNF) and its receptor tropomyosin-related kinase receptor type B (TRKB) are actively produced and trafficked in multiple regions in the adult brain, where they influence neuronal activity, function and survival throughout life. The diverse presence and activity of BDNF suggests a potential role for this molecule in the pathogenesis and treatment of both neurological and psychiatric disorders. This article reviews the current understanding and future directions in BDNF-related research in the central nervous system, with an emphasis on the possible therapeutic application of BDNF in modifying fundamental processes underlying neural disease.

Rescuing neurons in prion disease

One of the major current challenges to both medicine and neuroscience is the treatment of neurodegenerative diseases, which pose an ever-increasing medical, social and economic burden in the developed world. These disorders, which include Alzheimer's, Huntington's and Parkinson's diseases, and the rarer prion diseases, are separate entities clinically but have common features, including aggregates of misfolded proteins and varying patterns of neurodegeneration. A key barrier to effective treatment is that patients present clinically with advanced, irreversible, neuronal loss. Critically, mechanisms of neurotoxicity are poorly understood. Prevention of neuronal loss, ideally by targeting underlying pathogenic mechanisms, must be the aim of therapy. The present review describes the rationale and experimental approaches that have allowed such prevention, rescuing neurons in mice with prion disease. This rescue cured animals of a rapidly fatal neurodegenerative condition, resulting in symptom-free survival for their natural lifespan. Early pathological changes were reversed; behavioural, cognitive and neurophysiological deficits were recovered; and there was no neuronal loss. This was achieved by targeting the central pathogenic process in prion disease rather than the presumed toxic species, first by proof-of-principle experiments in transgenic mice and then by treatment using RNA interference for gene knockdown. The results have been a new therapeutic target for prion disease, further insight into mechanisms of prion neurotoxicity and the discovery of a window of reversibility in neuronal damage. Furthermore, the work gives rise to new concepts for treatment strategies for other neurodegenerative disorders, and highlights the need for clinical detection of early neuronal dysfunction, so that similar early rescue can also be achieved for these disorders.

Efficient intracerebral delivery of AAV5 vector encoding human ARSA in non-human primate

Metachromatic leukodystrophy (MLD) is a lethal neurodegenerative disease caused by a deficiency in the lysosomal arylsulfatase A (ARSA) enzyme leading to the accumulation of sulfatides in glial and neuronal cells. We previously demonstrated in ARSA-deficient mice that intracerebral injection of a serotype 5 adeno-associated vector (AAV) encoding human ARSA corrects the biochemical, neuropathological and behavioral abnormalities. However, before considering a potential clinical application, scaling-up issues should be addressed in large animals. Therefore, we performed intracerebral injection of the same AAV vector (total dose of 3.8 x 10(11) or 1.9 x 10(12) vector genome, three sites of injection in the right hemisphere, two deposits per site of injection) into three selected areas of the centrum semiovale white matter, or in the deep gray matter nuclei (caudate nucleus, putamen, thalamus) of six non-human primates to evaluate vector distribution, as well as expression and activity of human ARSA. The procedure was perfectly tolerated, without any adverse effect or change in neurobehavioral examination. AAV vector was detected in a brain volume of 12-15 cm(3) that corresponded to 37-46% of the injected hemisphere. ARSA enzyme was expressed in multiple interconnected brain areas over a distance of 22-33 mm. ARSA activity was increased by 12-38% in a brain volume that corresponded to 50-65% of injected hemisphere. These data provide substantial evidence for potential benefits of brain gene therapy in patients with MLD.

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.

Optimized cannula design and placement for convection-enhanced delivery in rat striatum

The stereotactic delivery of therapeutic agents into brain has been problematic because of reflux and leakage of the delivered agent. Good distribution of infusates by convection-enhanced delivery (CED) depends very much on cannula design, precise cannula placement and infusion rates. We have recently published cannula targeting data for the non-human primate (NHP) putamen in which we defined infusion parameters referred to as "red", "blue", and "green" zones for cannula placements that result in poor, sub-optimal and optimal volumes of distribution (Vd), respectively. Therefore, we applied our observations in NHP putamen to the rat brain. Initially, trypan blue dye was infused into agarose gels to evaluate distribution and reflux characteristics of a scaled-down cannula without step and 1-mm stepped cannula. "Stepped" means a sharp transition from a wider stent to a narrower tip; thus the distance of the cannula tip to the larger diameter attachment defines the step distance. Reflux was contained with the stepped design even with an infusion rate of 3.0 microl/min and large infusion volumes in the agarose gel study. Infusions of a recombinant growth factor, GDNF, into rat striatum demonstrated that the presence of a 1-mm stepped cannula prevented reflux and resulted in excellent distribution of GDNF in the striatum. We conclude that a stepped cannula with a 1-mm tip is important for achieving reliable distribution of infused agents in rat brain. It should be considered when local therapies such as gene transfer, local protein administration or cellular replacement are evaluated in rodent models.

Safety and tolerability of putaminal AADC gene therapy for Parkinson disease

BACKGROUND

In Parkinson disease (PD), the benefit of levodopa therapy becomes less marked over time, perhaps because degeneration of nigrostrial neurons causes progressive loss of aromatic l-amino acid decarboxylase (AADC), the enzyme that converts levodopa into dopamine. In a primate model of PD, intrastriatal infusion of an adeno-associated viral type 2 vector containing the human AADC gene (AAV-hAADC) results in robust response to low-dose levodopa without the side effects associated with higher doses. These data prompted a clinical trial.

METHODS

Patients with moderately advanced PD received bilateral intraputaminal infusion of AAV-hAADC vector. Low-dose and high-dose cohorts (5 patients in each) were studied using standardized clinical rating scales at baseline and 6 months. PET scans using the AADC tracer [(18)F]fluoro-L-m-tyrosine (FMT) were performed as a measure of gene expression.

RESULTS

The gene therapy was well tolerated, but 1 symptomatic and 2 asymptomatic intracranial hemorrhages followed the operative procedure. Total and motor rating scales improved in both cohorts. Motor diaries also showed increased on-time and reduced off-time without increased "on" time dyskinesia. At 6 months, FMT PET showed a 30% increase of putaminal uptake in the low-dose cohort and a 75% increase in the high-dose cohort.

CONCLUSION

This study provides class IV evidence that bilateral intrastriatal infusion of adeno-associated viral type 2 vector containing the human AADC gene improves mean scores on the Unified Parkinson's Disease Rating Scale by approximately 30% in the on and off states, but the surgical procedure may be associated with an increased risk of intracranial hemorrhage and self-limited headache.

Differential transduction following basal ganglia administration of distinct pseudotyped AAV capsid serotypes in nonhuman primates

We examined the transduction efficiency of different adeno-associated virus (AAV) capsid serotypes encoding for green fluorescent protein (GFP) flanked by AAV2 inverted terminal repeats in the nonhuman primate basal ganglia as a prelude to translational studies, as well as clinical trials in patients with Parkinson's disease (PD). Six intact young adult cynomolgus monkeys received a single 10 microl injection of AAV2/1-GFP, AAV2/5-GFP, or AAV2/8-GFP pseudotyped vectors into the caudate nucleus and putamen bilaterally in a pattern that resulted in each capsid serotype being injected into at least four striatal sites. GFP immunohistochemistry revealed excellent transduction rates for each AAV pseudotype. Stereological estimates of GFP+ cells within the striatum revealed that AAV2/5-GFP transduces significantly higher number of cells than AAV2/8-GFP (P < 0.05) and there was no significant difference between AAV2/5-GFP and AAV2/1-GFP (P = 0.348). Consistent with this result, Cavalieri estimates revealed that AAV2/5-GFP resulted in a significantly larger transduction volume than AAV2/8-GFP (P < 0.05). Each pseudotype transduced striatal neurons effectively [>95% GFP+ cells colocalized neuron-specific nuclear protein (NeuN)]. The current data suggest that AAV2/5 and AAV2/1 are superior to AAV2/8 for gene delivery to the nonhuman primate striatum and therefore better candidates for therapeutic applications targeting this structure.

Optimal region of the putamen for image-guided convection-enhanced delivery of therapeutics in human and non-human primates

Optimal results in the direct brain delivery of brain therapeutics such as growth factors or viral vector into primate brain depend on reproducible distribution throughout the target region. In the present study, we retrospectively analyzed MRI of 25 convection-enhanced delivery (CED) infusions with MRI contrast into the putamen of non-human primates (NHP). Infused volume (V(i)) was compared to total volume of distribution (V(d)) versus V(d) within the target putamen. Excellent distribution of contrast agent within the putamen was obtained in eight cases that were used to define an optimal target volume or "green" zone. Partial or poor distribution with leakage into adjacent anatomical structures was noted in 17 cases, defining "blue" and "red" zones, respectively. Quantitative containment (99±1%) of infused gadoteridol within the putamen was obtained when the cannula was placed in the green zone, 87±3% in the blue zone and 49±0.05% in the red zone. These results were used to determine a set of 3D stereotactic coordinates that define an optimal site for putaminal infusions in NHP and human putamen. We conclude that cannula placement and definition of optimal (green zone) stereotactic coordinates have important implications in ensuring effective delivery of therapeutics into the putamen utilizing routine stereotactic MRI localization procedures and should be considered when local therapies such as gene transfer or protein administration are being translated into clinical therapy.

Novel drug delivery strategies in neuro-oncology

Treatment of malignant gliomas represents one of the most formidable challenges in oncology. Despite treatment with surgery, radiation therapy, and chemotherapy, the prognosis remains poor, particularly for glioblastoma, which has a median survival of 12 to 15 months. An important impediment to finding effective treatments for malignant gliomas is the presence of the blood brain barrier, which serves to prevent delivery of potentially active therapeutic compounds. Multiple efforts are focused on developing strategies to effectively deliver active drugs to brain tumor cells. Blood brain barrier disruption and convection-enhanced delivery have emerged as leading investigational delivery techniques for the treatment of malignant brain tumors. Clinical trials using these methods have been completed, with mixed results, and several more are being initiated. In this review, we describe the clinically available methods used to circumvent the blood brain barrier and summarize the results to date of ongoing and completed clinical trials.

Therapy for prion diseases: Insights from the use of RNA interference

Insights into the molecular basis and the temporal evolution of neurotoxicity in prion disease are increasing, and recent work in mice leads to new avenues for targeting treatment of these disorders. Using lentivirally mediated RNA interference (RNAi) against native prion protein (PrP), White et al. report the first therapeutic intervention that results in neuronal rescue, prevents symptoms and increases survival in mice with established prion disease.(1) Both the target and the timing of treatment here are crucial to the effectiveness of this strategy: the formation of the neurotoxic prion agent is prevented at a point when diseased neurons can still be saved from death. But the data also give new insights into the timing of treatment in the context of the pattern of spread of prion infection throughout the brain, with implications for developing the most effective treatments.

Novel drug delivery strategies in neuro-oncology

Treatment of malignant gliomas represents one of the most formidable challenges in oncology. Despite treatment with surgery, radiation therapy, and chemotherapy, the prognosis remains poor, particularly for glioblastoma, which has a median survival of 12 to 15 months. An important impediment to finding effective treatments for malignant gliomas is the presence of the blood brain barrier, which serves to prevent delivery of potentially active therapeutic compounds. Multiple efforts are focused on developing strategies to effectively deliver active drugs to brain tumor cells. Blood brain barrier disruption and convection-enhanced delivery have emerged as leading investigational delivery techniques for the treatment of malignant brain tumors. Clinical trials using these methods have been completed, with mixed results, and several more are being initiated. In this review, we describe the clinically available methods used to circumvent the blood brain barrier and summarize the results to date of ongoing and completed clinical trials.

Adeno-associated virus-mediated gene delivery approaches for the treatment of CNS disorders

Over the last few years, a large number of preclinical and clinical studies have demonstrated the potential of gene therapy applications using adeno-associated viral (AAV) vectors. Gene transfer via AAV vectors has been particularly successful for the treatment or adjunct therapy of several CNS disorders. The present review summarizes the progress on AAV gene delivery models for three different CNS disorders. In particular, we discuss advances in AAV-mediated gene transfer strategies in animal models of Parkinson's disease, Alzheimer's disease and spinal cord trauma and summarize the results from the first clinical studies using AAV systems.

Gene and cell delivery to the degenerated striatum: status of preclinical efforts in primate models

Significant progress has been achieved in developing restorative neurosurgical strategies for movement disorders on the basis of preclinical gene and cell therapy experiments in primates. Because of the unique similarities between human and primate anatomy and physiology, experiments in primate models are the critical step in translating these innovative neurosurgical treatment concepts into successful human applications. To clarify progress toward this goal, we have examined recent preclinical data regarding the delivery of gene and cell therapy to the lesioned primate striatum. Improved behavioral outcomes after in vivo gene transduction, achieved by brain delivery of adeno-associated vectors, have resulted in the initiation of ongoing clinical trials. Cell transplantation experiments are transitioning from the grafting of fetal tissue, which has met with mixed clinical success, to the grafting of expanded neural stem cells, for which preliminary results in primates are encouraging. Careful attention to the surgical delivery parameters for these agents in primate studies, along with the ability to realistically model imaging and behavioral outcomes in these animals, is essential for optimizing the restoration of function for patients. The authors review data obtained from primate models that form the basis for ongoing clinical trials to consider how new preclinical models should be developed to answer questions that arise from experimental clinical data.