Animal models for acute radiation syndrome drug discovery

Introduction: Although significant scientific advances have been made over the past six decades in developing safe, nontoxic and effective radiation/medical countermeasures (MCMs) for acute radiation syndrome (ARS), no drug has been approved by the US FDA. The availability of adequate animal models is a prime requisite under the criteria established by the FDA ‘animal rule’ for the development of novel MCMs for ARS and the discovery of biomarkers for radiation exposure.

Areas covered: This article reviews the developments of MCMs to combat ARS, with particular reference to the various animal models (rodents: mouse and rat; canine: beagle; minipigs and nonhuman primates [NHPs]) utilized for the in-depth evaluation. The objective, pathways and challenges of the FDA Animal Efficacy Rule are also discussed.

Expert opinion: There are a number of well-defined animal models, the mouse, canine and NHP, that are being used for the development of MCMs. Additional animal models, such as the minipig, are under development to further assist in the identification, efficacy testing and approval of MCMs under the FDA Animal Efficacy Rule.     

The efficacy and safety of amifostine for the acute radiation syndrome

ABSTRACT

Introduction: A radiation countermeasure that can be used prior to radiation exposure to protect the population from the harmful effects of radiation exposure remains a major unmet medical need and is recognized as an important area for research. Despite substantial advances in the research and development for finding nontoxic, safe, and effective prophylactic countermeasures for the acute radiation syndrome (ARS), no such agent has been approved by the United States Food and Drug Administration (FDA).

Area covered: Despite the progress made to improve the effectiveness of amifostine as a radioprotector for ARS, none of the strategies have resolved the issue of its toxicity/side effects. Thus, the FDA has approved amifostine for limited clinical indications, but not for non-clinical uses. This article reviews recent strategies and progress that have been made to move forward this potentially useful countermeasure for ARS.

Expert opinion: Although the recent investigations have been promising for fielding safe and effective radiation countermeasures, additional work is needed to improve and advance drug design and delivery strategies to get FDA approval for broadened, non-clinical use of amifostine during a radiological/nuclear scenario.     

Radiation countermeasure agents: an update (2011 – 2014)

Introduction: Despite significant scientific advances over the past 60 years towards the development of a safe, nontoxic and effective radiation countermeasure for the acute radiation syndrome (ARS), no drug has been approved by the US FDA. A radiation countermeasure to protect the population at large from the effects of lethal radiation exposure remains a significant unmet medical need of the US citizenry and, thus, has been recognized as a high priority area by the government.

Area covered: This article reviews relevant publications and patents for recent developments and progress for potential ARS treatments in the area of radiation countermeasures. Emphasis is placed on the advanced development of existing agents since 2011 and new agents identified as radiation countermeasure for ARS during this period.

Expert opinion: A number of promising radiation countermeasures are currently under development, seven of which have received US FDA investigational new drug status for clinical investigation. Four of these agents, CBLB502, Ex-RAD, HemaMax and OrbeShield, are progressing with large animal studies and clinical trials. G-CSF has high potential and well-documented therapeutic effects in countering myelosuppression and may receive full licensing approval by the US FDA in the future.     

Medical countermeasures for unwanted CBRN exposures: part II radiological and nuclear threats with review of recent countermeasure patents

Introduction: The global threat of a chemical, biological, radiological, or nuclear (CBRN) disaster is an important priority for all government agencies involved in domestic security and public health preparedness. Radiological/nuclear (RN) attacks or accidents have become a larger focus of the United States Food and Drug administration (US FDA) over time because of their increased likeliness. Clinical signs and symptoms of a developing acute radiation syndrome (ARS) are grouped into three sub-syndromes named for the dominant organ system affected, namely the hematopoietic (H-ARS), gastrointestinal (GI-ARS), and neurovascular systems. The availability of safe and effective countermeasures against radiological/nuclear threats currently represents a significant unmet medical need.

Areas covered: This article reviews the development of RN threat medical countermeasures and highlights those specific countermeasures that have been recently patented and approved following the FDA Animal Rule. Patents for such agents from 2015 have been presented.

Expert opinion: Two granulocyte colony-stimulating factor (G-CSF)-based radiation countermeasures (Neupogen® (Amgen, Thousand Oaks, CA) and Neulasta® (Amgen, Thousand Oaks, CA)) have recently been approved by the FDA for treatment of H-ARS and both these agents are radiomitigators, used after radiation exposure. To date, there are no FDA-approved radioprotectors for ARS.     

Animal models for ebolavirus countermeasures discovery: what defines a useful model?

Introduction: Ebolaviruses are highly pathogenic filoviruses, which cause disease in humans and nonhuman primates (NHP) in Africa. The Zaire ebolavirus outbreak in 2014, which continues to greatly affect Western Africa and other countries to which the hemorrhagic fever was exported due to travel of unsymptomatic yet infected individuals, was complicated by the lack of available licensed vaccines or therapeutics to combat infection. After almost a year of research at an increased pace to find and test vaccines and therapeutics, there is now a deeper understanding of the available disease models for ebolavirus infection. Demonstration of vaccine or therapeutic efficacy in NHP models of ebolavirus infection is crucial to the development and eventual licensure of ebolavirus medical countermeasures, so that safe and effective countermeasures can be accelerated into human clinical trials.

Areas covered: The authors describe ebolavirus hemorrhagic fever (EHF) disease in various animal species: mice, guinea pigs, hamsters, pigs and NHP, to include baboons, marmosets, rhesus and cynomolgus macaques, as well as African green monkeys. Because the NHP models are supremely useful for therapeutics and vaccine testing, emphasis is placed on comparison of these models, and their use as gold-standard models of EHF.

Expert opinion: Animal models of EHF varying from rodents to NHP species are currently under evaluation for their reproducibility and utility for modeling infection in humans. Complete development and licensure of therapeutic agents and vaccines will require demonstration that mechanisms conferring protection in NHP models of infection are predictive of protective responses in humans, for a given countermeasure.     

Drug discovery strategies for acute radiation syndrome

Introduction: There are at the minimum two major, quite different approaches to advance drug discovery. The first being the target-based drug discovery (TBDD) approach that is commonly referred to as the molecular approach. The second approach is the phenotype-based drug discovery (PBDD), also known as physiology-based drug discovery or empirical approach.

Area covered: The authors discuss, herein, the need for developing radiation countermeasure agents for various sub-syndromes of acute radiation syndromes (ARS) following TBDD and PBDD approaches. With time and continuous advances in radiation countermeasure drug development research, the expectation is to have multiple radiation countermeasure agents for each sub-syndrome made available to radiation exposed victims.

Expert opinion: The majority of the countermeasures currently being developed for ARS employ the PBDD approach, while the TBDD approach is clearly under-utilized. In the future, an improved drug development strategy might be a ‘hybrid’ strategy that is more reliant on TBDD for the initial drug discovery via large-scale screening of potential candidate agents, while utilizing PBDD for secondary screening of those candidates, followed by tertiary analytics phase in order to pinpoint efficacious candidates that target the specific sub-syndromes of ARS.     

Nonhuman primates as models for the discovery and development of ebolavirus therapeutics

Introduction: Ebolaviruses are human pathogenic Category A priority pathogens for which no vaccines or therapeutics are currently licensed; however, several therapeutic agents have shown promising efficacy in nonhuman primate models of infection and are potential candidates for use in humans. Demonstration of efficacy in nonhuman primate models of ebolavirus infection will probably be central to the development and eventual licensure of ebolavirus medical countermeasures given the ethical and feasibility constraints of human efficacy assessments.

Areas covered: The authors describe ebolavirus hemorrhagic fever (EHF), with an emphasis on comparing human and nonhuman primate pathophysiology. Published data examining human and animal clinical disease parameters, histopathological findings, and immune responses in fatal and nonfatal cases are synthesized and evaluated. Importantly, the authors also introduce and describe the FDA Animal Efficacy Rule as well as recent advances in antiviral drug development strategies for the treatment of EHF.

Expert opinion: Well-characterized models of ebolavirus infection are currently under development and scrutiny as to their accuracy and utility for modeling fatal infection in humans. The advanced development and eventual licensure of therapeutic agents will require demonstration that mechanisms conferring protection in nonhuman primate models of infection are predictive of protective responses in humans.     

Development of transplant immunosuppressive agents – considerations in the use of animal models

ABSTRACT

Introduction: The development of all immunosuppressant agents to date has involved the experimental use of large and small animal models. Over the last half-century, immunosuppressive drugs have extended the lives of transplant patients worldwide. However, the use of animal models in the development of these drugs is not perfect, and this has brought to light a number of issues including idiosyncratic reactions that are found in animal models but not in humans. The 2006 highly publicized case of the ‘elephant man’ TGN 1412 drug trial highlights the importance of being cogent of the limitations of animal models.

Areas covered: This review covers the utility and limitations of the use of animal models for the development of immunosuppressant agents. This includes both large and small animal models, particularly rodent models in the transplant setting.

Expert opinion: The use of animal models represents a critical stage in the development of immunosuppressive drugs. Limitations include physiological differences to humans; this is especially true of immunologically naïve lab rodents with small memory cell populations. Toxic drug levels may differ widely between species. Animal models are also costly and raise ethical concerns. However, there is currently no way to recreate the complex environment of the human immune system purely in vitro.     

Medical countermeasures for unwanted CBRN exposures: Part I chemical and biological threats with review of recent countermeasure patents

Introduction: The threat of chemical, biological, radiological, and nuclear (CBRN) warfare has been addressed as the uppermost risk to national security since the terrorist attacks on 11 September 2001. Despite significant scientific advances over the past several decades toward the development of safe, non-toxic and effective countermeasures to combat CBRN threats, relatively few countermeasures have been approved by the US Food and Drug Administration (US FDA). Therefore, countermeasures capable of protecting the population from the effects of CBRN attack remain a significant unmet medical need. Chemical and biological (CB) threat agents can be particularly hazardous due to their effectiveness in small quantities and ease of distribution.

Area covered: This article reviews the development of countermeasures for CB threats and highlights specific threats for which at least one countermeasure has been approved following the FDA Animal Rule. Patents of CB countermeasures since 2010 have been included.

Expert opinion: Nine CB countermeasures have received FDA approval for use in humans following the Animal Rule, and a number of promising CB countermeasures are currently under development. In the next few years, we should expect to have multiple countermeasures approved by the FDA for each indication allowing for more flexible and effective treatment options.     

美国FDA“动物（效应）法规”在生物防御药物开发中的应用及启示

目的： 在应对重大公共卫生事件中，为我国相关生物防御药物的上市审批和应用储备提供参考依据和解决思路。方法： 结合审批实例，对美国FDA"动物（效应）法规"的主要内容、应用范围和实施状况进行综述，针对美国和我国现阶段应对重大公共卫生事件的药品审批机制进行讨论。结果与结论： 在针对可能发生的生物恐怖袭击事件及重大疫情开展的药物研发中，评价尺度可以与普通药物有所区别，应视情做好相应生物防御药物的战略储备，为应对突发重大公共卫生事件提供系统性支持。     

Pharmacological management of ionizing radiation injuries: current and prospective agents and targeted organ systems

ABSTRACT

Introduction: There is a limited array of currently available medicinals that are useful for either the prevention, mitigation or treatment of bodily injuries arising from ionizing radiation exposure.

Area covered: In this brief article, the authors review those pharmacologic agents that either are currently being used to counter the injurious effects of radiation exposure, or those that show promise and are currently under development.

Expert opinion: Although significant, but limited progress has been made in the development and fielding of safe and effective pharmacotherapeutics for select types of acute radiation-associated injuries, additional effort is needed to broaden the scope of drug development so that overall health risks associated with both short- and long-term injuries in various organ systems can be reduced and effectively managed. There are several promising radiation countermeasures that may gain regulatory approval from the government in the near future for use in clinical settings and in the aftermath of nuclear/radiological exposure contingencies.     

Animal models in tuberculosis research – where is the beef?

Introduction: Tuberculosis (TB) is a major global health problem, and new drugs and vaccines are urgently needed. As clinical trials in humans require tremendous resources, preclinical drug and vaccine development largely relies on valid animal models that recapitulate the pathology of human disease and the immune responses of the host as closely as possible.

Areas covered: This review describes the animal models used in TB research, the most widely used being mice, guinea pigs and nonhuman primates. In addition, rabbits and cattle provide models with a disease pathology resembling that of humans. Invertebrate models, including the fruit fly and the Dictyostelium amoeba, have also been used to study mycobacterial infections. Recently, the zebrafish has emerged as a promising model for studying mycobacterial infections. The zebrafish model also facilitates the large-scale screening of drug and vaccine candidates.

Expert opinion: Animal models are needed for TB research and provide valuable information on the mechanisms of the disease and on ways of preventing it. However, the data obtained in animal studies need to be carefully interpreted and evaluated before making assumptions concerning humans. With an increasing understanding of disease mechanisms, animal models can be further improved to best serve research goals.     

Minipigs as models in drug discovery

ABSTRACT

Introduction: Influenza continues to be a major public health concern. Antivirals play an important role in limiting the burden of disease and preventing infection and/or transmission. The developments of such agents are heavily dependent on pre-clinical evaluation where animal models are used to answer questions that cannot be easily addressed in human clinical trials. There are numerous animal models available to study the potential benefits of influenza antivirals but each animal model has its own pros and cons.

Areas covered: In this review, the authors describe the advantages and disadvantages of using mice, ferrets, guinea pigs, cotton rats, golden hamsters and non-human primates to evaluate influenza therapeutics.

Expert opinion: Animals used for evaluating influenza therapeutics differ in their susceptibility to influenza virus infection, their ability to display clinical signs of illness following viral infection and in their practical requirements such as housing. Therefore, defining the scientific question being asked and the data output required will assist in selecting the most appropriate animal model.     

Small animal models of filovirus disease: recent advances and future directions

ABSTRACT

Introduction: Small animal models have played a critical role in understanding the pathogenesis and transmission of disease caused by filoviruses. Notably, small animals have served to identify and validate many different approaches to countering infection with these highly pathogenic viruses. Nonetheless, predictive efficacy between each model does not appear to be equivalent as higher order animals seem to be more prognostic and therefore successful in the evaluation of medical countermeasures (MCM).

Areas covered: This review comprehensively details the available small animal models of filovirus infection and discusses the benefits and shortcomings of each model with respect to the development of MCM. An up-to-date evaluation of mouse, hamster, guinea pig, and ferret models is provided.

Expert opinion: The recent development of the domestic ferret model for ebolavirus offers a small animal model that faithfully reproduces most features of human disease without the need for viral adaptation or an immunocompromised host. That being said, choosing a small animal model to evaluate a particular MCM must consider potential confounders associated with each model. These confounding issues include incomplete host immune systems or mutations in the challenge virus that enables the disease.     

Current tools for norovirus drug discovery

ABSTRACT

Introduction: Rapid transmission of norovirus often occurs due to its low infectious dosage, high genetic diversity and its short incubation time. The viruses cause acute gastroenteritis and may lead to death. Presently, no effective vaccine or selective drugs accepted by the United States Food and Drug Administration (FDA) are available for the treatment of norovirus. Advances in the development of norovirus replicon cell lines, GII.4-Sydney HuNoV strain human B cells, and murine and gnotobiotic pig norovirus models have facilitated the discovery of effective small molecule inhibitors in vitro and in vivo.

Areas covered: This review gives a brief discussion of the biology and replication of norovirus before highlighting the discovery of anti-norovirus molecules. The article coverage includes: an overview of the current state of norovirus drug discovery, the targeting of the norovirus life cycle, the inhibition of structural and nonstructural proteins of norovirus such as proteases and polymerase, and the blockage of virus entry into host cells. Finally, anti-norovirus drugs in the clinical development stage are described.

Expert opinion: The current approach for the counteraction of norovirus focuses on the inhibition of viral RNA polymerase, norovirus 3C-like protease and the structural proteins VP1 as well as the blockade of norovirus entry. Broad-spectrum anti-norovirus molecules, based on the inhibition of 3C-like protease, have been developed. Other host factors and ways to overcome the development of resistance through mutation are also being examined. A dual approach in targeting viral and host factors may lead to an effective counteraction of norovirus infection. Current successes in developing norovirus replicon harboring cells and norovirus infected human cells, as well as murine norovirus models and other animal models such as piglets have facilitated the discovery of effective drugs and helped our understanding of its mechanism of action.     

Developing models for cachexia and their implications in drug discovery

Introduction: Cachexia is a complex metabolic syndrome associated with underlying illness and characterized by loss of muscle with or without loss of fat mass. Systemic inflammation plays a central role in its pathophysiology. As millions of patients are in a cachectic state of chronic disease, cachexia is one of the major causes of death worldwide. Difficulties in the recruitment and follow-up of clinical trials mean that well-characterized animal models are of great importance in developing cachexia therapies. However, some of the widely used animal models have limitations in procedural reproducibility or in recapitulating in the cachectic phenotype, which has warranted the development of novel models for cachexia.

Areas covered: This review focuses on some of the currently developing rodent models designed to mimic each co-morbidity in cachexia.

Expert opinion: Through developing cancer models, researchers have been seeking more targets for intervention. In cardiac cachexia, technical issues have been overcome by transgenic models. Furthermore, the development of new animal models has enabled the elucidation of the roles of inflammation, anabolism/catabolism in muscle/fat tissue and anorexia on cachexia. As metabolic and inflammatory pathways in cachexia may compromise cardiac muscle, the analysis of cardiac function/tissue in non-cardiac cachexia may be a useful component of cachexia assessment common to different underlying diseases and pave the way for novel drug discovery.     

美国FDA药品监管体系发展分析

目的：为发展和完善我国药品监管体系提供建议。方法：介绍美国FDA药品监管体系的发展,分析美国FDA监管系统改革举措。结果与结论：美国FDA药品监管体系建立了系统、连续的体系,监管方面的改革具有一定先进性和灵活性,我国可借鉴美国的相关经验,在监管体系改革过程中做出相关改进。     

Evaluation of the carcinogenicity of inorganic arsenic

Abstract

Inorganic arsenic (iAs) at high exposures is a human carcinogen, affecting mainly the urinary bladder, lung and skin. We present an assessment of the mode of action (MOA) of iAs’s carcinogenicity based on the United States Environmental Protection Agency/International Programme on Chemical Safety (USEPA/IPCS) framework, focusing primarily on bladder cancer. Evidence is presented for a MOA involving formation of reactive trivalent metabolites interacting with critical cellular sulfhydryl groups, leading to cytotoxicity and regenerative cell proliferation. Metabolism, kinetics, cell transport, and reaction with specific proteins play a critical role in producing the effects at the cellular level, regardless of cell type, whether urothelium, lung epithelium or epidermis. The cytotoxicity induced by iAs results in non-cancer toxicities, and the regenerative cell proliferation enhances development of epithelial cancers. In other tissues, such as vascular endothelium, different toxicities develop, not cancer. Evidence supporting this MOA comes from in vitro investigations on animal and human cells, from animal models, and from epidemiological studies. This MOA implies a non-linear, threshold dose-response relationship for both non-cancer and cancer end points. The no effect levels in animal models (approximately 1?ppm of water or diet) and in vitro (>0.1?µM trivalent arsenicals) are strikingly consistent. Cancer effects of iAs in humans generally are not observed below exposures of 100–150?ppb in drinking water: below these exposures, human urine concentrations of trivalent metabolites are generally below 0.1?µM, a concentration not associated with bladder cell cytotoxicity in in vitro or animal models. Environmental exposures to iAs in most of the United States do not approach this threshold.     

Modeling and simulation in dose determination for biodefense products approved under the FDA animal rule

Development of effective medical countermeasures for biodefense is vital to United States biopreparedness and response in the age of terrorism, both foreign and domestic. A traditional drug development pathway toward approval is not possible for most biodefense-related indications, creating the need for alternative development pathways such as the FDA’s Animal Rule. Under this unique regulatory mechanism, FDA-approval is based on adequate and well-controlled animal studies when it is neither ethical nor feasible to conduct human efficacy studies. Translation of animal efficacy findings to humans is accomplished by use of modeling and simulation techniques. Pharmacokinetic and exposure–response modeling allow effective dosing regimens in humans to be identified, which are expected to produce similar benefit to that observed in animal models of disease. In this review, the role of modeling and simulation in determining the human dose for biodefense products developed under the Food and Drug Administration’s Animal Rule regulatory pathway is discussed, and case studies illustrating the utility of modeling and simulation in this area of development are presented.     

Animal models for anti-emphysema drug discovery

Introduction: Emphysema is characterized by an abnormal and permanent enlargement of airspaces accompanied by destruction of their walls. Up to now, there is no cure for emphysema, and animal models may be important for new drug discovery.

Areas covered: Herein, the authors review animal models of emphysema since the protease-antiprotease hypothesis as well as the results obtained with compounds tested in these models. Of particular importance are animal models of cigarette smoke exposure since it is the most important risk factor of emphysema. The authors also analyze two approaches to drug testing, that is, the approach aimed at preventing emphysema and the one aimed at reversing it.

Expert opinion: It has been suggested that early and late interventions do not have the same protective effect and that late interventions are much more likely to reveal treatments beneficial in humans. However, this is not always the case, and a compound that prevents emphysema when administered as an early intervention can also have the same protective effect when given as a late intervention. Furthermore, the fact that a compound detected by means of early intervention is now in clinical practice shows that early intervention studies can be predictive for efficacy in humans.