Recent reexamination of autopsy specimen place estimates of putaminal coverage by NRTN at around 20%,37,43which due to NRTN-antibody limitations, is likely underestimated.5,44Perhaps more importantly, and getting to the second concern of brain transport, the more recent phase 2b trial increased the NRTN dose by three- to fourfold and increased the volume per injection by 10-fold in the putamen alone. importantly, these studies also demonstrated controlled, highly persistent generation of biologically active proteins targeted to structures deep in the human brain. Therefore, a renewed, focused emphasis must be placed on advancing clinical efficacy by improving clinical trial design, patient selection and outcome measures, developing more predictive animal models to support clinical testing, carefully performing retrospective analyses, and most importantly moving forwardbeyond our past limits. == Background: Impetus and Outcomes with Parkinson’s Gene Therapy (GT) Trials == Parkinson’s disease (PD) is a chronic, progressive neurodegenerative disease most widely recognized for the profound degeneration of mid-brain dopamine nigrostriatal neurons linked to serious motor symptoms.1However, PD is far more Rabbit polyclonal to MBD3 complex than commonly appreciated, with multiple etiologic variables and pathogenic pathways, complex pathologies, and a wide range of central nervous system (CNS) and non-CNS symptoms (Table 1).2,3Moreover, wide gaps in our understanding still exist at each disease level (i.e., etiology, pathogenesis, pathology, symptoms), and the cause-effect relationships between them remain especially obscure. Arguably, the most well-characterized relationship exists with regard to nigrostriatal degeneration linked to the key motor symptoms; currrent oral dopaminergic pharmaceuticals are effective in controlling these symptoms at early disease stages. However, the drugs’ effectiveness decline with progressive pathology, leading to gradual incapacitation of patients by increased off time (i.e., periods of no symptomatic relief) and increasing side effects such as peak-dose dyskinesias.1Thus, adequate treatment of the nigrostriatal-mediated motor impairments continues to represent a significant unmet medical need, affecting over 4 million people worldwide.4Though a number of solutions have been conceived to improve the function of the degenerating dopaminergic system, translating these biopharmaceutical concepts to the clinic has been challenging due to obstacles associated with delivering macromolecules to the central nervous system AM 0902 in a persistent and targeted fashion. == Table 1. Complex etiology, pathogenesis, pathology, and symptomatology of AM 0902 Parkinson’s disease. == Progress achieved in the realm of gene therapy (GT) over the past decade has offered solutions to many of the delivery constraints,5and several aspects of PD present it overtly as an ideal clinical indication to target using GT: (i) the well-defined, localizable, and targetable neuronal systems involved with major motor symptoms, (ii) the need for relatively small titer and volume of vector targeted to those sites, which avoids the systemic circulation of immunogenic materials, and (iii) the large and increasing demand for improved therapeutics with an aging population,4which in whole bolsters impact and financial support for research and development. Given this AM 0902 rationale, PD has, for better or worse, become a key exemplar for CNS GT. To date, the results of completed PD GT trials have supported the safety of GT targeting in the brain and many have further confirmed the successfully targeted expression of bioactive proteins in specific brain sites. However, none of the programs has yet produced sufficiently robust or reliable efficacy data to enable initiation of a pivotal phase 3 trial required for regulatory approval. We attempt to integrate the many successes and formidable challenges of GT treatment of PD, in an effort to seek the best path forward for PD and CNS GT as a whole. == The Look-See Approach == As compared with conventional small molecule drug testing, GT in the CNS is limited with respect to establishing initial dosing, quantifying targeting success, and accessing a comprehensive gauge of transgene production and localization, all of which provide the basis for iterative improvement with traditional drug development. While we expect that such limitations are attributable to a short-lasting gap between demand and current technological capability, the ramifications of such limitations include poor predictive power and a loss of continuity and progress AM 0902 between successive trials. Whereas continuing progress is being made in the visualization of viral vectors as they are delivered to the brain (e.g., the use of gadolinium-containing liposomes coupled with magnetic resonance imaging),6,7there is currently no direct means to determine the distribution or magnitude of transgene expression, since protein expression with CNS GT trials is both generated and produced entirely within the confines of the human brain. Therefore, it is not yet possible to confirm proper dose selection or monitor dosing levels, as would normally be done using standard pharmacokinetic (i.e., plasma level) methods. This.