Animal Models — Essential Elements to 

Address Efficacy Under the Animal Rule 

CONCEPT PAPER 

September 2008 

 

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TABLE OF CONTENTS 

I. INTRODUCTION............................................................................................................. 1

II. ANIMAL RULE CONSIDERATIONS .......................................................................... 1

III. DISCUSSION OF ESSENTIAL DATA ELEMENTS OF AN ANIMAL MODEL ... 3

A. Characteristics of CBRN Agent that Influence the Disease or Condition ................................3

1. The Challenge Agent........................................................................................................................3

2. Pathogenic Determinants.................................................................................................................4

3. Route of Exposure............................................................................................................................4

4. Quantification of Exposure..............................................................................................................4

B. Host Susceptibility and Response to Etiologic Agent .................................................................4

C. Natural History of Disease: Pathophysiologic Comparability...................................................5

1. Time to Onset of Disease/Condition...............................................................................................6

2. Time Course of Progression of Disease/Condition ........................................................................6

3. Manifestations (signs and symptoms)..............................................................................................6

D. Trigger for Intervention................................................................................................................6

E. Characterization of Medical Intervention ...................................................................................7

1. Product Class...................................................................................................................................7

2. Mechanism of Action........................................................................................................................7

3. In vitro Activity ................................................................................................................................8

4. Activity in Disease/Condition of Similar Pathophysiology..............................................................8

5. Pharmacokinetics (PK) in Unaffected Animals/Humans.................................................................8

6. PK/PD (Pharmacokinetics/Pharmacodynamics) in Affected Animals/Humans ..............................8

7. PK Interactions With Medical Products Likely to Be Used Concomitantly ....................................9

8. Synergy or Antagonism of Medical Products Likely to Be Used in Combination ...........................9

F. Design Considerations for Efficacy Studies.................................................................................9

1. Endpoints .......................................................................................................................................10

2. Timing of intervention....................................................................................................................10

3. Route of Administration .................................................................................................................11

4. Dosing Regimen.............................................................................................................................11

G. Available Safety Information......................................................................................................11

IV. ESSENTIAL DATA ELEMENTS OF AN ANIMAL MODEL ................................. 13

ATTACHMENT: ACRONYMS AND ABBREVIATIONS .................................................. 14

REFERENCES............................................................................................................................ 15

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Concept Paper 

Animal Models — Essential Elements to Establish 

 Efficacy Under the Animal Rule 

I. INTRODUCTION 

FDA's regulations concerning the approval of new drugs or biological products when human 

efficacy studies are not ethical or feasible are known as "the Animal Rule" (21 CFR 314.600 for 

drugs; CFR 601.90 for biologics). The Animal Rule states that in selected circumstances, when it 

is unethical or infeasible to conduct human efficacy studies, the FDA may grant marketing 

approval based on adequate and well-controlled animal studies when the results of those studies 

establish that the drug or biological product is reasonably likely to produce clinical benefit in 

humans. Demonstration of the product’s safety in humans is still necessary (see section IV.G). 

This concept paper is intended to identify the critical characteristics of an animal model that 

should be addressed when efficacy of the product under development will be established under 

the Animal Rule. It should also help determine whether an animal model can be considered 

sufficiently well-characterized to propose that the effect demonstrated in a single animal species 

can be used to support approval/licensure. We anticipate that this concept paper will be further 

developed and issued as a draft guidance for public input. 

 

The critical characteristics discussed in section III of the concept paper identify the elements to 

be fully explored as an animal model is developed. All elements may not be achievable for each 

etiologic agent1

 and intervention2 being studied. Early and frequent interactions between the 

FDA and the sponsor are recommended to discuss these elements and any issues encountered by 

the sponsor. Current FDA requirements for establishing the safety of a product in humans 

continue to apply. Although the following discussion touches on clinical safety, it is not meant to 

address all requirements for assurance of human safety. 

II. ANIMAL RULE CONSIDERATIONS 

To develop an animal model to demonstrate efficacy, the sponsor should obtain information on 

the natural history of the disease or condition in both humans and animals, on the etiologic agent, 

and on the proposed intervention. Data from the human experience with the etiologic agent or 

with the intervention, if available, may support applicability of the animal model. 

The Animal Rule states that FDA can rely on the evidence from animal studies to provide 

substantial evidence of effectiveness only when: 

1

 For this document the terms agent, threat agent, or etiologic agent refer to chemical, biological, radiological or 

nuclear (CBRN) substances, as well as to any potentially lethal or permanently disabling toxic substance or 

organism in which efficacy studies in humans are not ethical or feasible. The term challenge agent refers to the 

CBRN material used in the animal studies. 

2

 The terms treatment and therapy refer to any intervention that prevents or mitigates the toxicity of these etiologic 

agents. 

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1. There is a reasonably well-understood pathophysiological mechanism of the toxicity of 

the (chemical, biological, radiological, or nuclear) substance and its prevention or 

substantial reduction by the product 

2. The effect is demonstrated in more than one animal species expected to react with a 

response predictive for humans, unless the effect is demonstrated in a single animal 

species that represents a sufficiently well-characterized animal model (meaning the 

model has been adequately evaluated for its responsiveness) for predicting the response 

in humans 

3. The animal study endpoint is clearly related to the desired benefit in humans, generally 

the enhancement of survival or prevention of major morbidity 

4. The data or information on the (pharmaco) kinetics and pharmacodynamics of the 

product or other relevant data or information, in animals and humans allows selection of 

an effective dose in humans 

(21 CFR 314.610(a)(1)-(4); 601.91(a)(1)-(4)) 

If these criteria are met, it is reasonable to expect the effectiveness of the product in animals to 

be a reliable indicator of its effectiveness in humans. 

Although the Animal Rule allows approval based on a single animal species, if the animal model 

is sufficiently well-characterized, the usual expectation is that efficacy will be demonstrated in 

more than one species. If one animal species is to be considered sufficient, in general more than 

one efficacy study using that species should be conducted to demonstrate reproducibility of the 

results. 

Data from animal studies to demonstrate dose-response and to support the dose selected for the 

animal efficacy studies are expected as is the case for traditional product development. Sponsors 

of products approved for other indications may be asked to provide additional nonclinical and/or 

clinical data to support approval/licensure of the proposed product for the indication under 

consideration. 

If another regulatory pathway to approval (i.e., one using human data) is feasible, that pathway 

must be used (21 CFR 314.600; 601.90). Although the Animal Rule allows development of 

products that would otherwise not have any route to approval, the rule reflects the Agency’s 

recognition that many treatments that appeared effective in animals have not proved to be 

effective in humans. Consequently, developing animal models that will yield efficacy results 

that can be expected to be predictive for humans is challenging. The animal studies should use 

the pertinent features of an adequate and well-controlled clinical study, such as a detailed 

protocol with randomization and adequate blinding and a statistical plan as described in 21 CFR 

314.126. 

Early and frequent interactions between the FDA and the sponsor are recommended to discuss 

the applicability of the Animal Rule and specific areas of concern, as well as to enable the review 

of, and comment on, protocols prior to study initiation. FDA may seek Advisory Committee 

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consultation before approval and/or early in the development process to discuss whether the 

concept of using certain animal data to support efficacy is reasonable (67 FR 37992). 

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All studies subject to the Animal Ruse must be carried out under the procedures and controls 

outlined in the good laboratory practices (GLP) regulations ( 21 CFR 58). FDA recognizes that 

conforming to GLP regulations in the conduct of studies on CBRN agents may present 

challenges. Such issues and their possible impact on study results and conclusions, should be 

discussed with the review division prior to conduct of the studies. In addition, the studies must 

comply with the Animal Welfare Act ( 7 U.S.C. 2131). For certain infectious agents, sponsors 

should adhere to the Select Agent Rule3

 and comply with standards on the use of Biosafety 

Level (BSL) laboratory faciliti 4

The number of animals available for research, especially nonhuman primates (NHP), is finite. 

The animal efficacy studies conducted under the Animal Rule will use a significant number of 

animals. Sponsors should submit detailed protocols and provide for frequent monitoring 

throughout the study period (see 21 CFR 312.23(a)(6)). The FDA strongly encourages sponsors 

to submit a development plan and to communicate frequently with the Agency when developing 

products under the Animal Rule. The protocols for the animal efficacy studies should be 

discussed with the FDA, with sufficient time for FDA review and comment, prior to the study 

being conducted. 

III. DISCUSSION OF ESSENTIAL DATA ELEMENTS OF AN ANIMAL MODEL 

This section provides further information on the Table, Essential Data Elements of Animal 

Model, found in section IV. 

A. Characteristics of CBRN Agent that Influence the Disease or Condition 

Some characteristics of the specific chemical, biological, radiological, and/or nuclear (CBRN) 

agent that influence the disease or condition under study include: the challenge agent, pathogenic 

determinants, the route of exposure, and quantification of exposure. 

1. The Challenge Agent 

The challenge agent used in animal studies should be identical to the etiologic agent that 

causes the human disease. The purity of the challenge preparation should be documented 

when appropriate. If the challenge agent is different from the etiologic agent known to 

cause human disease, the sponsor should provide justification for the use of this challenge 

agent and explain why, when used in the proposed animal model, it should be considered 

suitable for establishing effectiveness of the intervention in humans. For example, for an 

animal efficacy study to support approval of a radiation countermeasure, a sponsor will 

probably not be able to predict the actual radiation exposure that would follow a nuclear 

3

 See Select Agent Rule (42 CFR Parts 72 & 73) available at http://www.cdc.gov/od/sap/final_rule.htm. 4

 See 5th Edition of Biosafety in Microbiological and Biomedical Laboratories (BMBL), available at 

http://www.cdc.gov/od/ohs/biosfty/bmbl5/bmbl5toc.htm. 

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detonation or the subsequent fallout. In such a case, the sponsor should provide a detailed 

explanation of the appropriateness of the type of radiation and dose used in the study and 

its relevance to the clinical situation. 

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2. Pathogenic Determinants 

 

It should be demonstrated that the pathogenic determinants of disease in the animal 

model are similar to those understood for humans. Pathogenic determinants can include 

toxin production, target organs or enzyme systems, or type of radiation. For example, 

although mice and guinea pigs are susceptible to Bacillus anthracis, the pathogenesis and 

mechanism of toxicity are different from those in humans, so that these rodent species 

may not be appropriate efficacy models for anthrax.5

 Animal species that are not 

susceptible to the agent, or do not demonstrate the endpoint of interest (i.e., potential for 

mortality or major morbidity that might be reduced or prevented by sufficiently effective 

interventions) are not suitable for the efficacy studies. 

3. Route of Exposure 

In general, the animal models developed should use a route of exposure to the challenge 

agent that is the same as the anticipated human exposure route. This is especially 

important for conditions for which the route of exposure is directly related to 

pathogenesis. For example, human infection with Yersinia pestis through flea bite, the 

intravenous (IV) route, or aerosol exposure results in the development of bubonic, 

septicemic, or pneumonic plague, respectively. If a sponsor is proposing a route of 

exposure to the etiologic agent in animals that is different from what is expected in 

humans, scientific justification should be provided. The FDA strongly recommends that 

if such an approach is being considered, it should be discussed with the FDA before the 

start of the animal studies. 

4. Quantification of Exposure 

Reliable quantification and reproducibility of the challenge dose should be demonstrated. 

If appropriate, the sponsor should describe the scalar relationship of the animal dose to 

that anticipated in human disease. If large differences are observed, then potential 

implications for interpretation of comparative pathogenesis, pathophysiology, and study 

results should be discussed with the FDA. It is possible that there may be standardization 

of the challenge dose in the future such that comparison studies can be conducted. 

B. Host Susceptibility and Response to Etiologic Agent 

The animal model chosen for development should be susceptible to the threat agent. FDA 

recognizes there may be species differences. For example, an animal species being used to study 

efficacy for a radiation countermeasure may require a different threshold of radiation exposure to 

develop acute radiation syndrome, but the animal species may still be appropriate for study if the 

5

 Leffel, E.K. and Pitt, L.M., Anthrax. In Biodefense: Research Methodology and Animal Models. Swearengen, J.R. 

ed. Boca Raton, FL. CRC Press, 2006, 77-93. 

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resulting illness and course are similar in the animal species and humans. However, if this 

threshold differs greatly from the human threshold, the suitability of the animal model may be 

called into question. The factor that determines differences in susceptibility to the threat agent 

should be described to the best extent possible (e.g., see the discussion of pyridostigmine and 

soman in section E.2). 

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The response to the etiologic agent (resulting illness or injury) manifested by the animal species 

exposed to the threat agent should be similar to the illness or injury seen in humans. For 

example, mustard gas typically produces extensive blistering to exposed human skin. If the 

animal species evaluated does not have blistering as a prominent feature of exposure to mustard 

gas, it is unlikely that this animal model would be acceptable to the Agency. If the sponsor 

believes that such a model is supportive to the study of their investigational drug, the model 

should be discussed with the Agency and a justification should be provided. 

C. Natural History of Disease: Pathophysiologic Comparability 

The natural history of disease in animals and in humans should be characterized, compared, and 

discussed with the Agency before the sponsor initiates intervention studies in animals. In some 

instances, use of several different models in the same development plan can be considered. 

Experimental parameters may need to be modified to create a condition that more closely mimics 

the disease in humans. For example, variola virus causes human smallpox, and humans are the 

only known natural host. Nonhuman primate animal models that have been studied using variola 

virus as the challenge agent require a large inoculum, and often the IV route of administration is 

used. FDA recommends that compounds found to be active in vitro against orthopoxviruses be 

studied in several animal models using multiple different orthopoxviruses initially. Based on data 

from initial studies and availability of suitably characterized models, the next step may be to 

assess the appropriateness of additional study in an animal model using variola.6

 Sponsors who 

plan to use an animal model that involves exposure to a challenge agent that is different from the 

known etiologic agent in humans should discuss this with the Agency along with their planned 

protocols and any major differences in, or limitations of, the animal model. 

 

When comparing the disease in animals with the disease in humans, sponsors should include 

time to onset of disease/condition; time course of progression of disease; and manifestations, that 

is, signs and symptoms (severity, progression, clinical and pathologic features, laboratory 

parameters, the extent of organ involvement, morbidity, and outcome of disease). A single 

animal model may not reflect the entire spectrum of human disease. The time to onset of disease, 

progression of disease, and the manifestations/outcome can be influenced by many factors, 

including concentration and type of etiologic agent, virulence or lethal potential of the etiologic 

agent, route of exposure, and other host factors including immune status. 

6

 See FDA’s draft guidance for industry Smallpox (Variola) Infection: Developing Drugs for Treatment or 

Prevention. Once finalized this guidance will represent the Agency’s thinking on this topic. 

Also, we update guidances periodically. To make sure you have the most recent version of a guidance, check the 

appropriate (CDER or CBER) guidance Web site. 

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The time to onset of disease/condition in animals should be reasonably similar to that in 

humans. Factors such as strain of the infective microorganism, route of exposure, and/or 

the level of exposure (i.e., concentration of the chemical, radiological, or other etiologic 

agent(s)) may influence time to onset. 

2. Time Course of Progression of Disease/Condition 

The progression of the disease/condition in animals should be similar to the disease in 

humans to allow for observation of the effects of intervention. Hamsters challenged with 

anthrax have an extremely rapid disease progression. Thus, this species is not useful for 

testing the efficacy of products for the treatment of anthrax. Furthermore, the clinical 

course of disease in the animal may be more rapid than that in the human as a result of 

experimental conditions, such as the route of exposure. For example, an IV route of 

exposure may alter many characteristics including the time course of disease. The change 

in the clinical course may result in making disease recognition, intervention, and 

assessment of outcome more difficult. Showing the effect of an intervention may be 

more challenging when the time between onset of disease and death is short. 

3. Manifestations (signs and symptoms) 

The disease manifestations, including clinical signs and their known time course, 

laboratory parameters, histopathology, gross pathology, and the outcome (morbidity or 

mortality), should be compared between untreated animals and untreated humans (e.g., 

historical information). Differences should be clearly noted and explained based on the 

understanding of the pathophysiologic differences between the species, with due 

acknowledgment of the limitations that may arise where this level of understanding is 

limited. Because certain disease manifestations in humans (e.g., fever and shortness of 

breath) may be difficult to discern in animals through clinical observation, a sponsor may 

need to use more refined techniques, such as telemetry, to evaluate affected animals. 

Animals in the natural history as well as the efficacy studies should be observed with 

greater frequency over the entire course of the day than would be typical of most 

nonclinical (pharmacology/toxicology) animal studies. This is especially true when the 

primary endpoint is mortality and animals are being evaluated in the context of 

prospectively-defined euthanasia criteria. With a mortality endpoint, animal welfare and 

sample integrity need to be addressed. Sample integrity (e.g., cultures, histology) may be 

compromised if not obtained just prior to or immediately after death or euthanasia. Study 

results may be influenced by the criteria used. Study personnel should be blinded to 

treatment and should follow observation and euthanasia criteria to minimize the 

possibility of unnecessary suffering of moribund animals.7

 

D. Trigger for Intervention 

7

Refer to Animal Welfare Act (7 U.S.C. 2131).

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Identification of the trigger for intervention in the animal studies is critical to defining the timing 

of the intervention. Because animals cannot simulate the health-seeking behavior manifested by 

humans, the trigger for intervention should be accurately defined in the animal model. If signs 

and symptoms in the animal model closely resemble those in humans, these can serve as the 

trigger for intervention when they are recognized in the individual animal. However, in the 

absence of disease-defining manifestations, certain biological parameters should be used to 

identify the time for initiation of treatment if they are known to be relevant to the diagnosis of 

human disease and if a relationship to the likely diagnostic process and timing in human use of 

the product can be shown. For example, presence of bacteremia has been used in some efficacy 

studies in humans for initiation of intervention with antimicrobial drug products.

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 The utility of 

biological parameters/biomarkers should be demonstrated, including an analysis of the time 

course of the appearance of the biomarkers in animals and the onset of disease and availability of 

diagnostic information in humans. 

When a biomarker is used as a trigger for intervention in animal studies, both the assay 

methodology for the biomarker and its performance characteristics should be adequately 

characterized. The materials and methods for the assay, as well as the raw data and results from 

the actual testing, should be provided for FDA review. Summary data are not sufficient. 

Sponsors are encouraged to initiate early discussion with the FDA regarding the utility of the 

chosen triggers for intervention, particularly when the signs and symptoms of disease in the 

animal differ from those in humans. 

E. Characterization of Medical Intervention 

Efficacy studies should reflect the expected clinical use and indication. A particular dosage form 

may not be suitable for the proposed indication, so the product’s dosage form should be 

considered in planning the development of the product. For example, an oral dosage form is 

preferred for postexposure prophylaxis for large populations, while an IV dosage form may be 

necessary for seriously ill patients. If the product is already approved for human use, there may 

be information on which to base the expected dose and regimen, but if there is no proven human 

use, the animal result will need to be translated for human use, generally requiring some PK/PD 

assessment. The following specific information should be submitted on the product and its 

characteristics in humans and in animals. 

1. Product Class 

The product’s therapeutic class should be identified. Information that is available about 

other members of the class can be used to help identify potential animal models and 

predict/evaluate safety and efficacy issues in the proposed animal model. 

2. Mechanism of Action 

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 Refer to package insert for Cubicin, NDA No. 021572, accessible at Drugs@FDA: 

http://www.accessdata.fda.gov/scripts/cder/drugsatfda/. 

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Understanding the mechanism of action may help to identify specific safety and efficacy 

issues in the proposed animal model and to identify what additional studies should be 

performed. The animal studies to support the approval of pyridostigmine as a 

pretreatment for exposure to the nerve agent soman highlight the importance of 

understanding the mechanism of action of the drug and host factors in each animal 

species evaluated. Pretreatment with pyridostigmine was shown to decrease the lethality 

of soman in rhesus monkeys. However, pretreatment with pyridostigmine produced small 

and inconsistent effects on mortality in studies using rats, mice, and rabbits. The effect of 

pyridostigmine was masked in these latter species because of high serum levels of the 

enzyme carboxylesterase, which eliminates soman from the blood and makes these 

species naturally highly resistant to the nerve agent. Rhesus monkeys and humans have 

little or no carboxylesterase. To elucidate the mechanism of pyridostigmine and bridge 

the data to the human experience, a study was conducted in rats pretreated with 

pyridostigmine as well as a carboxylesterase inhibitor prior to exposure to soman. In this 

study, pyridostigmine demonstrated a mortality benefit in the rats similar to that seen in 

the rhesus monkeys. 

3. In vitro Activity 

Understanding the in vitro activity of the product will supplement known information on 

the mechanism of action and provide early screening information. 

4. Activity in Disease/Condition of Similar Pathophysiology 

If a candidate product is targeted at a common pathway in the pathophysiologic cascade, 

information may be available on the candidate product’s use for diseases that possess a 

similar pathway. Information for a product approved for the treatment of neutropenia 

secondary to chemotherapy in cancer patients may provide useful data to support 

studying this product for the reduction of mortality in patients with neutropenia 

secondary to acute radiation syndrome. This information in the related condition, 

although not required, lends further support to the candidate product’s efficacy for the 

indication to be studied. 

5. Pharmacokinetics (PK) in Unaffected Animals/Humans 

PK studies should be done in unaffected animals and humans to characterize the PK 

profile in each and to propose dosing regimens that provide comparable drug exposures 

in the animals and humans. Early interaction with the FDA is critical to justify and 

establish the appropriate dosing regimen for the pivotal animal studies. 

6. PK/PD (Pharmacokinetics/Pharmacodynamics) in Affected Animals/Humans 

PK information in affected animals should be compared to PK information obtained from 

unaffected animals to establish whether the pathophysiology of a disease affects the PK 

(e.g., changes in metabolic parameters may alter the pharmacokinetics). Measures of 

treatment response (PD measurements such as clinical outcome or exploratory 

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biomarkers) should be proposed for discussion based on both animal studies and any 

available human information. If a candidate product has been used in humans for other 

indications, PK/PD information for the alternate indications may be supportive. It should 

be noted that the animal model may not predict specific disease/drug interactions. Such 

interactions may not be observed until the disease is treated in humans, reinforcing the 

critical need for postmarket clinical studies in the event of human disease. 

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7. PK Interactions With Medical Products Likely to Be Used Concomitantly 

The absorption, distribution, metabolism, and excretion (ADME)9, 10 of a candidate 

product should be studied and understood. The sponsor, with knowledge of the ADME 

of the investigational product, should discuss with the FDA other medical products that 

are likely to be co-administered based on the clinical scenario. Potential combinations 

should be considered for interaction studies that may affect the PK of either product. If a 

candidate drug is metabolized via the cytochrome P450 system, safety or efficacy of the 

candidate drug could be compromised by cytochrome P450 inhibitors or inducers used 

concomitantly. Such drug/drug interactions should be evaluated. 

8. Synergy or Antagonism of Medical Products Likely to Be Used in Combination 

Candidate products should be evaluated within the context that reflects anticipated 

clinical use. The sponsor, in consultation with FDA, should consider other products that 

are likely to be used and evaluate whether the activity of either product, when used in 

combination, is affected (i.e., synergy or antagonism). Examples of potential interactions 

include drug/drug interactions and drug/vaccine interactions. For example, it should be 

known whether the use of an anthrax antitoxin monoclonal will have an effect on the 

activity of the antimicrobials used for the treatment of disseminated anthrax disease. This 

potential interaction should therefore be evaluated in the animal model. This information 

is especially important when the therapeutic intervention is expected to include more than 

one medical product. 

F. Design Considerations for Efficacy Studies 

Assessment of efficacy in animals should be as robust as possible. Adequate and well-controlled 

animal efficacy studies, with endpoints that demonstrate substantial clinical benefit, generally the 

enhancement of survival or prevention of major morbidity, are expected. The time course of 

observation should be optimized to assess the true treatment effect. At a minimum, placebocontrolled animal studies should be performed. If a product approved for the same indication is 

available, it should be used as an active comparator in addition to the investigational drug and 

placebo arms. The study should also be blinded to the extent feasible; any situation in which 

study staff might become aware of treatment assignments should be discussed in advance in view 

9

 See guidance for industry: Drug Metabolism/Drug Interaction Studies in the Drug Development Process: Studies 

In Vitro.

10 See guidance for industry: Drug Interaction Studies – Study Design, Data Analysis, and Implications for Dosing 

and Labeling.

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of the potential for major effects on study interpretability. Animals of both sexes should be 

included. FDA recognizes that there are significant supply constraints on using mature or older 

animals of certain animal species. The issue of the age and the immune status of the animals used 

in efficacy studies as compared to the intended human population should be addressed by the 

sponsor, when relevant. Study procedures should be uniformly applied to all study groups, and 

potential bias should be reduced by prespecifying the criteria for euthanasia and discussing their 

potential effects on interpretation of results. 

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Studies should be designed to mimic the clinical scenario and achieve meaningful outcomes 

comparable to the endpoints desired in humans. In some instances, supportive care should be 

administered to the animals as part of the study design. In such cases, demonstration of a 

product’s benefit over supportive care (i.e., supportive care plus investigational drug arm should 

be demonstrated to be superior to the supportive care plus placebo arm) will be required for 

approval or licensure. Early discussion between the sponsor and the review division regarding 

the type, timing, and choice of supportive care to be administered is highly recommended. 

In addition to the design characteristics discussed above, the following parameters should be 

addressed in the study protocols. We recommend that study protocols be prepared and submitted 

to FDA with enough time for FDA to review the protocols and provide feedback to the sponsor 

before the animal studies are initiated. The sponsor can submit these protocols with a request for 

review under the Special Protocol Assessment (SPA) provisions.11

1. Endpoints 

The product studied in the animal model should demonstrate a beneficial effect analogous 

to the intended outcome in humans. Primary study endpoints, which should be 

specifically discussed with the review division, generally are the enhancement of survival 

or prevention of major morbidity. The dose response for these endpoints should be 

explored fully and established. Although secondary endpoints can provide useful 

information about the animal model and the activity of the product as studied in the 

animal model, ordinarily, only primary endpoints can serve as the basis of approval. 

2. Timing of intervention 

The time to initiate intervention should support the specific indication sought for a 

product. If the intent is to develop the product for a treatment indication, intervention 

before disease is established may overestimate the effect that is likely to be seen in 

humans and may indeed show an effect when none would be seen in humans. A 

reasonable understanding of the disease course and a trigger for intervention defined by 

the natural history studies will be needed to design the animal efficacy studies for a 

treatment indication; it is important to establish the relationship of time after exposure to 

effectiveness. With this information, the timing for intervention can be defined, thus 

differentiating postexposure prophylaxis from treatment. A product to be used for 

11 See guidance for industry: Special Protocol Assessment. 

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postexposure prophylaxis should be administered within a reasonable window after 

exposure to the threat agent, but before onset of disease, with a time relationship that is 

adequately justified with respect to administration of the product to humans. Proposals 

for pre-exposure prophylaxis should be described and discussed in advance on a case-bycase basis. 

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3. Route of Administration 

The route of administration should reflect the indication being sought and the anticipated 

clinical scenario, such as mass casualty. For example, if a large number of people were 

exposed to anthrax, an oral dosage form would be preferred over an injectable for 

postexposure prophylaxis. It may be important to study multiple routes. 

4. Dosing Regimen 

The determination of the dosing regimen relies on sufficient PK and PD data or other 

relevant product information in animals and/or humans. The goals are to (a) determine a 

regimen in animals that is safe and effective for the indication studied; (b) determine the 

corresponding exposure in animals that is yielded by that dosing regimen; and (c) 

calculate a dosing regimen in humans that will give an equivalent exposure to that seen in 

the animal. This will enable initial extrapolation from a dosing regimen found to be 

efficacious in the animal model to one expected to produce a similar benefit in humans, 

assuming similar exposure–response relationships. Different dosing regimens in animals 

and humans may be needed to provide equivalent exposure to the product and thus should 

be discussed with the Agency. However, for vaccines, the goal should be to develop 

regimens that are safe and that provide an adequate protective immune response. For 

vaccines, these goals are typically achieved without extrapolation based on PK or relative 

PD, as the full human dose should be used in the dosing strategy when feasible. A shorter 

dosing interval between inoculations can be incorporated into the nonclinical study 

design as compared to the proposed clinical dosing interval. The dosing interval that is 

selected for the nonclinical toxicity study should maximize the immune response.12 

In summary, the indication being sought drives the study design. The desired outcomes of the 

study (i.e., product’s effect) should be determined early and carefully factored into the study 

design to ensure that the study meets both scientific and regulatory objectives. 

G. Available Safety Information 

The body of available human safety data, including data from the product’s evaluation and use in 

other indications, is a critical component of any product’s development plan and influences the 

risk/benefit considerations. FDA may ask for additional human safety trials to complete the 

12 See WHO Technical Report Series, No. 927, 2005, Annex 1, WHO guidelines on nonclinical evaluation of 

vaccines, World Health Organization, available at

http://www.who.int/biologicals/publications/trs/areas/vaccines/nonclinical_evaluation/en/. 

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safety profile of the product. Healthy human volunteers should be enlisted when there is no 

known significant risk in the administration of the product. If the risk is significant, study in a 

patient population with a similar disease should be considered if a population can be identified 

for which the risk/benefit balance of the study is appropriate. Sponsors should propose selection 

and justification of the appropriate study population in advance for FDA review and feedback. 

 

The size of the required clinical safety database depends on many factors. Existing safety data 

would generally be satisfactory for products that are already marketed for another indication 

and known to have an acceptable safety profile in the populations that would receive the product 

for the new indication. When the new indication requires a longer duration of use or higher 

dose, additional safety data must be obtained (21 CFR 314.50(d)(5)(v)). The type of indication 

being sought is another factor. For example, a product that will be used as prophylaxis in large 

numbers of people should have a larger safety database than a product developed for treatment of 

patients who are symptomatic with a disease of known high mortality. In prophylaxis scenarios, 

it is likely that some proportion of humans will receive the product without having been exposed 

to the threat agent. An adequate safety database is needed to reduce the risk of serious harm in a 

healthy population. 

The timing and design of clinical safety studies should be coordinated with exploration of the 

efficacious dose and regimen in animals to plan adequate studies to characterize the safety of the 

intended human dose, formulation, route of administration, and duration of use. 

Preclinical safety information should guide the choice of additional safety assessments of interest 

in the human safety studies. This is particularly useful for products with no prior human safety 

data, or when the anticipated human dosing regimen has not been previously studied or 

approved. 

FDA may request that products with significant toxicity show greater evidence of efficacy. For 

example, the use of an extremely nephrotoxic product, whose administration would likely lead to 

the requirement for chronic dialysis, could potentially be justified if animal models showed very 

robust evidence of effectiveness in a disease with significant mortality and no approved 

treatments. 
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IV. ESSENTIAL DATA ELEMENTS OF AN ANIMAL MODEL 

The essential data elements for the development and evaluation of animal models are listed in the 

table below. These elements serve as a guide. They may be modified or revised as new scientific 

information relevant to the condition under study becomes available. Early and frequent 

interactions between the sponsor and FDA are critical for feedback on proposals and appropriate 

discussion of uncertainties and the risk/benefit balance. 

Table: Essential Data Elements of an Animal Model 

 

Data Elements Animal(s) Human 

A. Characteristics of the CBRN Agent that Influence the Disease or Condition

1. The challenge agent 

2. Pathogenic determinants 

3. Route of exposure 

4. Quantification of exposure 

B. Host Susceptibility and Response to Etiologic Agent 

C. Natural History of Disease: Pathophysiologic Comparability 

1. Time to onset of disease/condition 

 2. Time course of progression of disease/condition 

 3. Manifestations (signs and symptoms) 

D. Trigger for Intervention 

E. Characterization of the Medical Intervention 

 1. Product class 

 2. Mechanism of action 

 3. In vitro activity 

 4. Activity in disease/condition of similar pathophysiology 

 5. PK in unaffected animals/humans 

6. PK/PD in affected animals/humans 

7. PK interactions with medical products likely to be used 

concomitantly 

8. Synergy or antagonism of medical products likely to be used 

in combination 

F. Design Considerations for Efficacy Studies

1. Endpoints 

2. Timing of intervention 

3. Route of administration 

4. Dosing regimen 

G. Available Safety Information 

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511 ATTACHMENT: ACRONYMS AND ABBREVIATIONS 

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ADME Absorption, distribution, metabolism, and excretion 

BSL Biosafety Level 

CBER Center for Biologics Evaluation and Research 

CBRN Chemical, Biological, Radiological, or Nuclear 

CDER Center for Drug Evaluation and Research 

FDA Food and Drug Administration 

GLP Good Laboratory Practices 

IV Intravenous 

NHP Nonhuman Primate 

PD Pharmacodynamics 

PK Pharmacokinetics 

SPA Special Protocol Assessment 

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REFERENCES 

21 CFR, Part 58. Good Laboratory Practice for Nonclinical Laboratory Studies (accessible at 

http://www.nal.usda.gov/awic/legislat/21cfr97.htm). 

42 CFR, Parts 72 and 73. “Possession, Use and Transfer of Select Agents and Toxins,” (CDC) 

Federal Register 70 (March 18, 2005): 13293 – 13325. 

(accessible at http://www.cdc.gov/od/sap/final_rule.htm) 

FDA, “New Drug and Biological Drug Products; Evidence Needed to Demonstrate Effectiveness 

of New Drugs When Human Efficacy Studies Are Not Ethical and Feasible,” Federal Register 

67 (31 May 2002): 37988-37998. 

FDA, Center for Drug Evaluation and Research and Center for Biologics Evaluation and 

Research, guidance for industry, Drug Interaction Studies – Study Design, Data Analysis, and 

Implications for Dosing and Labeling. 

FDA, Center for Drug Evaluation and Research and Center for Biologics Evaluation and 

Research, guidance for industry, Drug Metabolism/Drug Interaction Studies in the Drug 

Development Process: Studies in Vitro. 

 

FDA, Center for Drug Evaluation and Research and Center for Biologics Evaluation and 

Research, guidance for industry, Special Protocol Assessment. 

FDA, Center for Drug Evaluation and Research, draft guidance for industry, Smallpox (Variola) 

Infection: Developing Drugs for Treatment or Prevention. 

Leffel, E.K. and Pih, L.M. (2006), Anthrax. In Swearengen, J.R. (Ed.). Biodefense: Research 

Methodology and Animal Models. (pp. 77-93). Boca Raton, FL: CRC Press. 

USDA Animal Welfare Act, Public Law 89-544, as amended, 7 U.S.C. 2131 et.seq. (accessible 

at http://www.nal.usda.gov/awic/legislat/awa.htm). 

U.S. Department of Health and Human Services, Centers for Disease Control and Prevention and 

National Institute of Health, Biosafety in Microbiological and Biomedical Laboratories (BMBL), 

5th Edition, (Washington, DC: U.S. Government Printing Office, 2007): (accessible at 

http://www.cdc.gov/od/ohs/biosfty/bmbl5/bmbl5toc.htm). 

WHO guidelines on nonclinical evaluation of vaccines, Geneva, World Health Organization, 

2005 (WHO Technical Report Series, No. 927, Annex 1) (accessible at 

http://www.who.int/biologicals/publications/trs/areas/vaccines/nonclinical_evaluation/en/). 

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