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Given, in addition, the increased probability of late stage clinical trial failures for central nervous system CNS drugs Bunnage, , it is not surprising that leadership within industry is increasingly investing elsewhere. The greater risk and cost associated with treatment discovery and development for human brain disorders is directly related to the scientific challenges. In directing investment away from neurology and psychiatry, industry leadership points to several hurdles: 1 target identification and validation have lagged compared with other disease areas such as metabolism, infectious disease, and cancer; 2 significant disillusionment with use of animal models to predict efficacy; and 3 lacking biomarkers for most brain disorders, stratification of populations for clinical trial is difficult and often impossible, and endpoints must often be measured using subjective rating scales.

Across all of industry, companies are experimenting with ways of making drug development more successful and efficient. What is critically needed for brain disorders is acceleration of scientific progress: better understandings of basic disease mechanisms and improved ability to translate such discoveries into biomarkers and therapeutics. The challenges and opportunities that the IOM has explored are summarized in a recent graphic reproduced in Figure 1 and are briefly discussed below. Although there is a high burden associated with nervous system disorders, development of new therapeutics remains stagnant.

Over the last decade, fewer new drugs for nervous system disorders have garnered approval in comparision to other therapeutic areas. Current data suggest that drug development, from the start of a discovery program to regulatory approval, can take an average of 12 to 15 years. This familiar statistic prompts an equally familiar question: Can the therapeutic development timeline be improved and accelerated by addressing challenges and developing opportunities? The challenges and opportunities presented in this graphic are not related and are presented in no certain order. The opportunities listed should not be interpreted as solutions to the challenges identified.

Statements, recommendations, and opinions expressed are those of the individual participants and are not necessarily endorsed or verified by the IOM, and should not be construed as reflecting any group consensus. IND, investigational new drug; IP, intellectual property. Adapted and reprinted with permission from the Institute of Medicine. The prevalence and burden of brain disorders ensure that leadership within industry, academia, and government does not forget them. As alluded to, however, to spur new investment in this international enterprise advances in science and new approaches to translation are needed.

The human brain is arguably the most complex object of biological study, has significant recent evolutionary changes that are poorly modeled in animals, and cannot readily be studied in life. In contrast to other disease areas, such as cancer, surgical procedures that yield relevant tissue can almost never be undertaken, and in any case, most brain disorders are not cell autonomous. Given the large number of cell types in the brain, their diverse synaptic connections, and the complexity of neural circuit structure and function, deep understandings of disease mechanism remain difficult to achieve.

Because the pathophysiology of brain disorders is generally poorly understood, it is difficult to identify promising molecular targets and validate them. Without better understanding of disease mechanisms, it is also challenging to construct predictive animal models.

Systematic Reviews of Animal Models: Methodology versus Epistemology

AD is an area of intense focus in both academia and industry, with a major current effort to generate therapeutics based on the beta amyloid pathway, while investigating other promising mechanisms. These efforts notwithstanding, neither the normal nor possible pathogenic roles of beta amyloid peptides are well understood.

Moreover, animal models that overexpress beta amyloid generally lack the key property of human neurodegenerative disorders, cell death. Mechanisms other than beta amyloid are also not well understood and are at an earlier stage. While there is no substitute for relevant scientific discoveries, the production of better tools to interrogate brain function as well as disease mechanisms should prove generally useful as will the judicious use of large-scale collaborative organization of science.

Important tools that are under development include enumeration and characterization of neural cell types, a goal of the Brain Research through Advancing Innovative Neurotechnologies BRAIN initiative The White House, and characterization of neural connectivity that yields for small and large-scale circuits. Enhanced sharing of data and larger-scale collaboration, as has been increasingly modeled in the genetics community, could help advance basic scientific knowledge about diverse diseases.

For example, both genetics and cellular findings have revealed potential commonalities across neurodegenerative diseases that could contribute to development of therapeutics that may address more than one neurodegenerative disease IOM, b.

Target identification is a critical step in the drug discovery and development pipeline. Across all of medicine, genetics continues to provide important molecular clues to disease pathogenesis, but for brain disorders in which defects in synaptic communication and functional connectivity represent the primary pathology e.

What causes central nervous system disorders ? - Best Health FAQ Channel

These provide the greatest new opportunity to target identification in a generation IOM, , b , a , b. Among the challenges that exist for putting emerging genetic discovery to work in understandings of disease mechanism and target identification is the fact that almost all risk alleles have limited penetrance.

Even rare variants that have arisen more recently in human population history and have been less subject to evolutionary pressure than older common variants, rarely act in Mendelian or near-Mendelian fashion. Thus systems biology approaches, often requiring new cell-type specific molecular information, are likely to prove critical. Target validation is an iterative process of increasing confidence in a target, which can be conceptualized as continuing through phase 3 clinical trials.

For CNS disorders, however, the increasing concerns about the predictive validity of many current animal models must be addressed if industry is to re-invest. To put the concern directly, many companies have come to the view that current putative brain disease models and animal-based assays of drug action are likely screening out potentially efficacious drugs, and screening in drugs that will not demonstrate efficacy in clinical trials. There is growing agreement that there are no animal models of psychiatric disorders such as schizophrenia or depression that capture the relevant pathophysiology.

It has been argued see below that at least in the near term models of specific disease components may be possible if attention is paid to the evolutionary conservation of relevant mechanisms to the human. For example, the longstanding failure to develop analgesic drugs with new mechanisms e.

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Improving and Accelerating Drug Development for Nervous System Disorders

In psychiatry, antipsychotic and antidepressant drugs were discovered serendipitously when prototype drugs were administered to humans for other indications. Approaches to produce better predictive models, whether based on cells, animals, or human biology has been a critical area of discussion within the meetings organized by IOM Neuroscience Forum. Along with the process of target validation, it is critical to establish that therapeutic levels of a drug can be reliably delivered to the brain, that at those levels the drug binds its target, and modifies the disease pathway in the desired directions.

Absent such information, a clinical trial cannot test the target validation hypothesis. While this may seem obvious, there are many clinical trials in recent CNS research in which companies have been uncertain as to whether a prior failure truly tested a hypothesis.

Lack of attention to these factors, along with failures to publish underlying data, leads to costly and futile repetition of failures. Beyond drug properties, validated biomarkers are critical to confirming nervous system targets IOM, From the point of view of cost and efficiency, the ability to convincingly invalidate targets is nearly as important as validation IOM, a.

For targets that are well along in the validation process, first-in-human trials might be used to generate validation data, a concept that will be explored further on, might be another approach for accelerating the drug development pipeline IOM, a. The failure of animal models to predict accurately the efficacy of drugs with new mechanisms for nervous system disorders has been a central problem in drug development IOM, c.

Using animals to study nervous system disorders can be especially difficult at least in part due to differences between animals and humans in cell types, transmitter function and anatomy. Given the heterogeneity of common human diseases and evolutionary difference between humans and laboratory animals, it is highly unlikely that any single model, tool or technique could provide a complete picture of a disease.

Reliance on single models as efficacy gates or on a suite of models that is too narrow has likely contributed to failures in CNS drug development. Developing and integrating new approaches that utilize combinations of animal and non-animal models of disease mechanisms, along with new tools, technologies and techniques, might illuminate the underlying biological mechanisms of diseases and improve target identification, validation and therapeutic development IOM, a , c.

Animal models may be valuable at capturing a particular aspect of a disease or studying the function of a specific molecular target, but animal models cannot be expected to recapitulate the full mechanism or symptomatology of a human disease. Therefore, animal models are better thought of more narrowly, for example, as models of a particular disease mechanism and not a complete model of a disease IOM, a , c. Doing so will help ensure that an animal model serves as one of many important tools to study and validate disease mechanisms and targets, and that results derived from animal model studies are not over enthusiastically interpreted, and publicized, as evidence that the investigational agent will provide therapeutic benefit in the human disease.

Although animal models can reasonably assist in the prioritization of compounds for a validated target, they are not always as useful in prioritizing compounds aimed at novel targets. The absence of an animal model for recurring mood disorders e.

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Animal models do not always accurately predict dose, efficacy, and research priority. Some of these failures relate to the lack of understanding of the mechanisms for disease; how can successful animal models be created based on unknown mechanisms? Another explanation for these failures might not be poor animal models but rather researchers prematurely moving forward with answers from models without systemically validating the data across multiple animal and non-animal disease models.

In addition, translation of behaviors is especially problematic and presents a challenge in deciphering results; this challenge can be a significant barrier to developing drugs for nervous system disorders with a behavioral component e. Emerging tools and technologies e. Humanized animal models — developed by engrafting human tissue stem cells into mice - may help improve understanding of nervous system disorders and identify mechanisms of disease.

In addition, it is important to take into account potential sex differences in animal models, which have been shown to impact data reproducibility and their utility Clayton and Collins, , IOM, Computational neuroscience in conjunction with neuroimaging might aid in understanding the underlying neurobiological mechanisms of diseases; imaging technologies might be helpful in this regard as well IOM, a , Manji et al.

However, it is important to note that, like animal models, these tools and technologies do not fully mimic or recapitulate human diseases and disorders.

Microdosing and Other Phase 0 Clinical Trials: Facilitating Translation in Drug Development

An initial focus on human phenotypes rather than animal models might also provide an opportunity to better inform the drug discovery process IOM, a. A specific issue addressed by workshop IOM, a was under what circumstances it would be both ethical and practical Further, to ensure the humane care and use of animals, numerous laws, policies, and regulations are in place governing the use of animals in research, and certain animal regulations have implications specific to neuroscience research.

However, there is minimal harmonization of these rules between different countries, which can inherently create challenges for the international research enterprise IOM, For many mental and neurological disorders, determining the prospects of a drug requires clinical testing, which is difficult and expensive and beleaguered by such challenges as patient heterogeneity, a lack of biomarkers, subjective and insensitive rating scales, enrolling patients at the earliest stage in their disease progression and lengthy trial durations.

Success rates for new drugs in Phase II clinical trials have fallen to less than 20 percent, whereas the number of preclinical drugs needed to yield one approved drug has more than doubled Arrowsmith, Unlike in other fields, detailed clinical phenotyping and endotyping are not always present, even though failures of clinical trials are almost always predictable due to the known heterogeneity of the patient population. This heterogeneity necessitates larger, more complex, and thus more expensive clinical trials.