August 1, 2007 (Vol. 27, No. 14)
Colleen Jonsson Ph.D.
Opportunities and Challenges with Models for Emerging and Biodefense-related Pathogens
While a number of promising drug therapies and vaccines have been identified for safeguarding bioterrorism organisms and diseases, further development requires well-characterized animal models for testing these candidates.
The NIAID program—the In Vitro and Animal Models for Emerging Infectious Diseases and Biodefense Program—provides targeted screening and evaluation of potential therapeutic and prevention modalities for emerging infectious agents and bioterrorism pathogens using in vitro, small animal, and nonhuman primate models to test safety and efficacy.
In vitro and animal models promote development and testing of vaccines, therapeutics, and diagnostics, while preclinical safety testing speeds the development of new generation products. In the past few years there have been many opportunities for innovation and considerable challenges in development of these models in BSL3 containment.
Several major issues must be considered when establishing animal models for discovery and development of new therapeutics that require BSL3 laboratories—regulatory issues (e.g., Select Agent Rule, GLP compliance, CDC and USDA permits); personal protective equipment; and containment of animals, animal tissues, and fluids (e.g., BioBubbles, biosafety cabinets, necropsy work stations). Additionally, the processes for work flow among the various laboratories must be thoroughly reviewed to ensure high-quality results and biosafety.
As one moves through the various milestones, it is important to understand the utility of animal models at each stage leading to an IND (investigational new drug). One can break the process into three major phases—lead discovery, lead optimization, and preclinical research and development (Figure).
Even in the very early stages of therapeutic discovery, validation of the antiviral activity of lead candidates relies on robust, small animal models that have a well-characterized natural history of the infection process of the pathogen and other key elements. These types of animal models, usually mice or hamsters, are used for general screening of antiviral activity of lead candidates.
Unfortunately, for most emerging pathogens and many biodefense-related pathogens, well-characterized models are not usually available. Minimally, it is important that such models are characterized with respect to the temporal dynamics of the levels of the pathogen in the animal in the major target organs.
In early screening, additional clinical and pathology information from the animal model of the infectious disease can be informative, however, evaluation of efficacy often follows selection of the most potent therapeutic or vaccine candidates.
Regardless of the purpose of the model, screening or efficacy, working with these animals in BSL3 requires active and on-going risk assessment for containment of the pathogen.
Numerous options have come available in recent years, however, each system has advantages and disadavantages. The greatest challenges lie in finding a flexible unit with appropriate airflow, one that can be used with more than one animal model, and is easy to access for daily clinical evaluations and final decontamination.
In addition to the animal screening model, there are two additional complementary activities for antiviral testing that require animal models—dose range and optimal route of administration of the therapeutic.
The maximum tolerated dose is determined for candidate compounds using the screening model, but not infected with the pathogen. The basic premise is to extrapolate from any known in vitro data for the candidate and administer various concentrations of drug by at least two routes at two concentrations to determine the effects of treatment.
With the recent two-animal ruling by the FDA for the licensing of drugs or vaccines directed against diseases of low or no incidence, the ferret represents an inexpensive, small, nonrodent animal model for evaluating efficacy. The ferret is attractive for pulmonary research studies because of the long trachea, large lung capacity, and bronchiolar branching. This has been particularly evident in influenza and SARS CoV research in the Southern Research Institute (www.sri.org) laboratories.
The two-animal rule states that a therapeutic may be licensed if it meets two criteria. First, the therapeutic should show adequate protection in a challenge of infection in two species of animals. Secondly, the agent must be shown to be safe in humans.
To evaluate efficacy, a comprehensive set of analyses with well-defined end points is ideal. Considerations for development and standardization of animal efficacy models include: species selection; challenge strain and dose; route of exposure, i.e., parenteral, e.g., subcutaneous; intravenous versus respiratory, i.e., intranasal, intratracheal or aerosol; clinical endpoints, i.e., mimic human disease; and challenge strain and dose effects.
At this stage it is important that standard operating protocols of the assays are qualified. Further, technical personnel are expected to qualify on the assays. One of the major challenges in conducting the efficacy studies often lies in the sheer number of animals required to obtain statistically significant data for each of these end points.
In effect, the containment of the animals in the BSL3, and the number of samples taken at each necropsy will often limit the number of animals one can employ. The transition of the efficacy model to GLP for the preclinical research stage will require the involvement of study coordination, quality assurance, and quality-control personnel. Often, these people are new to working in biocontainment and so adequate consideration needs to be made in biosafety training.
Early-phase efficacy studies are crucial for evaluation of proof of concept and immunogenicity data of candidate vaccines.
Promising vaccine candidates are further evaluated in animal models to provide additional immunogenicity data and define dose range and vaccine regimen. Animal immunogenicity data is generated with validated immunological assays and is used to bridge human immunogenicity. Together with animal efficacy data, correlates of vaccine-elicited immune protection can be identified. Finally, definitive or pivotal vaccine efficacy studies are required to be conducted under GLP to provide supporting data for licensure of vaccines.
Before a new drug or vaccine entity can be approved by the FDA for administration to humans, the developer must demonstrate not only that it is efficacious for its intended use, but also that it exhibits an acceptable margin of safety within the dose range, the route and pattern of administration, and the target population expected in the clinical setting.
Such demonstration of safety includes testing of the drug in appropriate in vitro and animal models to show that the drug has no unacceptable genetic, reproductive, immunologic, cardiovascular, neurologic, or general toxicity, as well as demonstrate the kinetics of absorption, distribution, metabolism, and excretion of the drug and its metabolites.
Safety testing is performed using standard animal models and study designs that have been accepted by the FDA and other international regulatory agencies. Initial testing will include single-dose range-finding (RF) studies in rodents (rats or mice) and large animals (dogs or primates) to determine the maximum dose that can be administered by the intended clinical route without producing serious and irreversible toxicity. These studies evaluate a minimum number of animals and endpoints, and the results are used to select dose levels for subsequent safety studies.
If the intended clinical dosing regimen is expected to involve administration of multiple doses, the singe-dose RF studies will be followed by multiple-dose RF studies using the anticipated clinical dosing regimen. Finally, the doses that are selected based on the results of the RF studies will be used to conduct definitive safety studies for each drug in each species. The latter studies include a larger number of animals and evaluation of multiple parameters of toxicity and are performed in strict compliance with GLP regulations.
Conventional human vaccines progress through various stages of review and regulation required for an IND application. Vaccine animal models for emerging and biodefense-related pathogens should be conducted alongside clinical studies at each stage. Phase I, II, and III trials, providing safety, dose ranging, immunogenicity and efficacy data in support of a BLA, lead to Phase IV, manufacturing, and eventual lot release.
Colleen Jonsson, Ph.D. (firstname.lastname@example.org), is program leader for emerging infectious disease research, and Peter Silvera Ph.D. (email@example.com), is senior project leader, vaccine research program, in the department of
infectious disease research at Southern Research Institute. Web: www.sri.org.
Phone: (205) 581-2000.