Smallpox (Variola Virus) 
Infection:  Developing 
Drugs for Treatment or 
Prevention 
Guidance for Industry 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Drug Evaluation and Research (CDER) 
 
November 2019 
Clinical/Antimicrobial 
 
 
 
   
Smallpox (Variola Virus) 
Infection:  Developing 
Drugs for Treatment or 
Prevention 
Guidance for Industry 
 
 
 
Additional copies are available from: 
 
Office of Communications, Division of Drug Information 
Center for Drug Evaluation and Research 
Food and Drug Administration 
10001 New Hampshire Ave., Hillandale Bldg., 4th Floor  
Silver Spring, MD  20993-0002  
Phone: 855-543-3784 or 301-796-3400; Fax: 301-431-6353; Email: druginfo@fda.hhs.gov 
https://www.fda.gov/drugs/guidance-compliance-regulatory-information/guidances-drugs 
 
 
 
 
 
 
 
 
 
 
 
 
 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Drug Evaluation and Research (CDER) 
 
November 2019 
Clinical/Antimicrobial 
 
 
 
   
TABLE OF CONTENTS 
 
I.  INTRODUCTION............................................................................................................. 1 
II.  BACKGROUND ............................................................................................................... 2 
III.  DEVELOPMENT PROGRAM ....................................................................................... 4 
A.  Multidisciplinary Considerations for Studies in Animal Models Using Orthopoxvirus ......... 4 
1. Considerations for Preliminary Assessments of Antiviral Activity in Animal Models ..................... 4 
2. Key Study Design Considerations for Animal Efficacy Studies to Support Potential NDA 
Submission Under the Animal Rule ................................................................................................. 5 
3. Selection of an Effective Dosing Regimen in Humans ..................................................................... 7 
B.  Pharmacology/Toxicology Considerations .................................................................................. 7 
C.  Nonclinical Virology Considerations ............................................................................................ 8 
D.  Clinical Considerations ................................................................................................................. 9 
1. Healthy Volunteer Safety Trials ....................................................................................................... 9 
2. Safety Data from Non-Smallpox Clinical Experience .................................................................... 10 
3. Clinical Trials in the Event of a Public Health Emergency ........................................................... 10 
4. Expanded Access IND for Emergency Use .................................................................................... 11 
5. Emergency Use Authorization ....................................................................................................... 11 
E.  Clinical Pharmacology Considerations ...................................................................................... 12 
F.  Chemistry, Manufacturing, and Controls Considerations ....................................................... 12 
REFERENCES ............................................................................................................................ 13 
 
  Contains Nonbinding Recommendations 
 
Smallpox (Variola Virus) Infection: 
Developing Drugs for Treatment or Prevention 
Guidance for Industry1 
 
 
This guidance represents the current thinking of the Food and Drug Administration (FDA or Agency) on 
this topic.  It does not establish any rights for any person and is not binding on FDA or the public.  You 
can use an alternative approach if it satisfies the requirements of the applicable statutes and regulations.  
To discuss an alternative approach, contact the FDA office responsible for this guidance as listed on the 
title page.   
 
 
 
I.  INTRODUCTION  
 
The purpose of this guidance is to assist sponsors in the clinical development of drugs for 
treatment or prevention of smallpox (variola virus) infection.2  Clinical efficacy trials of drugs 
for treating or preventing smallpox are not feasible3 and challenge studies in healthy subjects are 
unethical; therefore, drugs for these indications should be developed and approved under the 
regulations commonly referred to as the Animal Rule (21 CFR part 314, subpart I, for drugs and 
21 CFR part 601, subpart H, for biologics).  This guidance serves as a focus for continued 
discussions among the Division of Antiviral Products (DAVP), pharmaceutical sponsors, the 
academic community, and the public.4 
 
This guidance focuses on drugs that are expected to inhibit variola virus replication.  Although 
antiviral drugs are the primary focus of this guidance, therapeutic proteins or monoclonal 
antibodies also may be eligible for evaluation under the Animal Rule.5  Sponsors interested in 
developing small molecules, therapeutic proteins, or monoclonal antibodies for use against 
                                                 
1 This guidance has been prepared by the Division of Antiviral Products in the Center for Drug Evaluation and 
Research (CDER) at the Food and Drug Administration. 
 
2 For the purposes of this guidance, unless otherwise specified, all references to drugs include both human drugs and 
therapeutic biological products (such as therapeutic proteins and monoclonal antibodies) that are regulated by 
CDER.  References to approval include new drug application approval for drugs or biologics license application 
licensure for therapeutic proteins and monoclonal antibodies. 
 
3 The determination of infeasibility of field trials can change.  Should circumstances change such that field trials 
become feasible (e.g., after accidental exposure to or intentional release of variola virus occurs), the sponsor should 
discuss its development plans with CDERs Division of Antiviral Products. 
 
4 In addition to consulting guidances, sponsors are encouraged to contact DAVP to discuss specific issues that arise 
during the development of drugs for treating or preventing smallpox. 
 
5 See 21 CFR 601.91(a). 
 
1  Contains Nonbinding Recommendations 
 
smallpox are encouraged to discuss their approach with the FDA as early as possible in 
development and to communicate with the FDA through the Pre-IND Consultation Program.6 
 
This guidance does not address the treatment of bacterial complications of smallpox or the 
development of biological therapies (other than monoclonal antibodies) to treat or prevent 
smallpox.  Sponsors interested in developing other types of biological products, such as vaccines 
and immunoglobulin preparations (other than monoclonal antibodies), should contact the 
appropriate review division in the Center for Biologics Evaluation and Research. 
 
Nor does this guidance address the general issues of statistical analysis or clinical trial design.  
Those topics are addressed in the ICH guidances for industry E9 Statistical Principles for 
Clinical Trials (September 1998) and E10 Choice of Control Group and Related Issues in 
Clinical Trials (May 2001), respectively.7 
 
This guidance finalizes the draft guidance of the same name issued on July 11, 2018.  This 
guidance also clarifies the recommended immunological characterization of animals in key 
studies and includes minor editorial changes. 
 
In general, FDAs guidance documents do not establish legally enforceable responsibilities.  
Instead, guidances describe the Agencys current thinking on a topic and should be viewed only 
as recommendations, unless specific regulatory or statutory requirements are cited.  The use of 
the word should in Agency guidances means that something is suggested or recommended, but 
not required. 
 
 
II.  BACKGROUND 
 
Ordinary smallpox, the most common form of smallpox caused by infection with variola major 
virus, had a reported mortality rate ranging from 5 percent to 50 percent in different outbreak 
situations (Fenner et al. 1988).  This form is the principal source of concern regarding potential 
bioterrorist uses of smallpox and therefore is the most relevant to this guidance.  Worldwide 
efforts at case identification, containment, and vaccination eventually led the World Health 
Organization in 1980 to declare that smallpox was eradicated.  Retention of variola virus stocks 
is limited by international agreement to two sites, one in Russia and the other at the U.S. Centers 
for Disease Control and Prevention (CDC) in Atlanta, Georgia.  However, concerns exist that 
variola virus could remain in undeclared or unknown locations or could be developed and used 
as a weapon of bioterrorism. 
 
                                                 
6 For more information, see the Getting Started with the Division of Antiviral Products Pre-IND Process web page at 
https://www.fda.gov/Drugs/DevelopmentApprovalProcess/HowDrugsareDevelopedandApproved/ApprovalApplicat
ions/InvestigationalNewDrugINDApplication/Overview/ucm077546.htm and the Investigational New Drug (IND) 
Application web page at https://www.fda.gov/drugs/types-applications/investigational-new-drug-ind-application. 
 
7 We update guidances periodically.  To make sure you have the most recent version of a guidance, check the FDA 
guidance web page at https://www.fda.gov/regulatory-information/search-fda-guidance-documents. 
 
2  Contains Nonbinding Recommendations 
 
The first line of defense against smallpox infection is vaccination with vaccinia virus (Petersen et 
al. 2015; Petersen et al. 2016).8  However, the usefulness of vaccination in a biothreat situation 
depends on the safety profile of the vaccine, on the ability to vaccinate exposed and at-risk 
persons in a timely manner, and on whether vaccine immunity will be able to protect against a 
variola virus strain used in a terrorist attack.  Because routine smallpox vaccination in the United 
States was discontinued in the 1970s and there is no natural disease exposure, most of the U.S. 
population is immunologically naive to smallpox. 
 
Historically, treatment for smallpox was supportive (Dixon 1962).  It is not known what effect 
technologically advanced supportive care might have on mortality and morbidity.  Generally, the 
mode of death in fatal cases was unclear and could have been multifactorial (Fenner et al. 1988; 
Dixon 1962). 
 
Antiviral drugs may be a valuable adjunct for exposure situations in which vaccination is not 
feasible or fails to provide adequate protection.  Drug development to evaluate the safety and 
efficacy of treating or preventing smallpox has numerous challenges, including the following: 
 
  The absence of smallpox cases for decades because of the successful smallpox 
eradication program 
 
  The absence of detailed information on the pathophysiology of human smallpox itself, 
including the mode of death 
 
  The lack of any previously recognized effective drug in controlled clinical trials 
conducted prior to the eradication of smallpox 
 
  Ethical issues that preclude human smallpox challenge studies 
 
  Restriction of variola virus samples to two designated maximum containment facilities 
 
  The exceptionally narrow host range of variola virus, which contributes to a lack of 
pathogenicity in most animal species after variola virus exposure 
 
  Lack of consistent reproducibility of data from current nonhuman primate (NHP) models 
using variola virus, and failure to mimic what is known about human smallpox disease 
 
  The possibility of antiviral drug interference with effects of the live-virus vaccine 
 
  The known and potential differences between variola virus and other orthopoxviruses in 
disease characteristics, drug susceptibility, and host range 
 
In light of these challenges, many specifics of the approaches to drug development for smallpox 
are likely to differ even from the approaches to other situations involving rare and life-
                                                 
8 These citations contain recommendations for vaccination of certain personnel considered to be at risk of 
occupational exposure to orthopoxviruses (2016) and for broader use if a smallpox event were to occur (2015). 
3  Contains Nonbinding Recommendations 
 
threatening diseases.  Because of the unique complexities of drug development in this area, 
extensive discussion with multiple stakeholders has taken place, including a public workshop in 
2009 and an advisory committee meeting in 2011.9  These discussions helped the FDA formulate 
the regulatory pathway for smallpox drug development that is described in this guidance. 
 
 
III.  DEVELOPMENT PROGRAM 
 
A.  Multidisciplinary Considerations for Studies in Animal Models Using 
Orthopoxvirus 
 
Because of the unique characteristics of smallpox disease and variola virus mentioned above and 
discussed further below, animal studies with several related viruses play a much larger role in 
drug development for smallpox than is the case for many other infectious diseases.10 
 
1.  Considerations for Preliminary Assessments of Antiviral Activity in Animal 
Models 
 
We recommend that compounds found to be active in cell culture be studied in several lethal 
animal models using multiple different orthopoxviruses, including vaccinia virus, monkeypox 
virus, and other orthopoxviruses as appropriate depending on the breadth of antiviral activity in 
cell culture. 
 
Vaccinia virus should be studied because it is related to variola virus, and studies of vaccinia 
virus also might be relevant to the development of drugs to treat complications of vaccination.  
Sponsors should consider conducting studies in vaccinia virus-infected immunocompromised or 
immunosuppressed animals to support using the drug in immunocompromised people with either 
variola virus infection or complications caused by vaccination. 
 
Antiviral activity against monkeypox virus should also be studied in one or more animal models.  
Monkeypox virus infection of NHPs is a potentially useful model of human smallpox for 
establishing drug efficacy.  In addition, human monkeypox itself can be severe and is endemic in 
certain regions, and preliminary studies in monkeypox virus-infected animals may inform the 
design of studies evaluating the drug as a treatment for human monkeypox. 
 
Sponsors should initially use small animal models to characterize the preliminary antiviral 
activity of the drug and should evaluate the effects of a wide range of study variables, including 
drug doses, dosing regimens, treatment times relative to viral exposure and evolution of disease, 
                                                 
9 See 2011 advisory committee materials at https://wayback.archive-
it.org/7993/20170404145348/https://www.fda.gov/AdvisoryCommittees/CommitteesMeetingMaterials/Drugs/Antiv
iralDrugsAdvisoryCommittee/ucm247236.htm.  
 
10 Approval under the Animal Rule requires adequate and well-controlled animal efficacy studies; however, we 
support the principles of the 3Rs, to reduce, refine, and replace animal use in testing when feasible.  We encourage 
sponsors to consult with the FDA if they wish to use a nonanimal testing method they believe is suitable, adequate, 
validated, and feasible.  We will consider if such an alternative method could be assessed for equivalency to an 
animal test method. 
4  Contains Nonbinding Recommendations 
 
differences in viral species, strain and inoculum, and route of viral exposure.  Results of such 
studies may help both in estimating the possible effect of these variations and in setting priorities 
for using resources (such as NHPs and/or more pathogenic viruses) that are less readily available 
or more difficult to work with.  We recommend that sponsors consider selecting and assessing 
NHP models in later stages of animal investigations after initial results become available from 
small animal models. 
 
2.  Key Study Design Considerations for Animal Efficacy Studies to Support 
Potential NDA Submission Under the Animal Rule 
 
Selecting the animal models in which to test the efficacy of an investigational drug is critically 
important for drugs developed under the Animal Rule.11  Sponsors are strongly encouraged to 
obtain concurrence from DAVP on the animal models and the design of the adequate and well-
controlled efficacy studies before initiating a study. 
 
During the December 1415, 2011, Antiviral Drugs Advisory Committee meeting on the 
development of drugs to treat variola virus infection, the advisory committee agreed with the 
FDAs assessment that current NHP models using variola virus are not consistently reproducible 
and do not mimic what is known about human smallpox disease.  Because scientific limitations 
of these available variola virus models preclude definitive efficacy assessments, and uncertainty 
exists whether an adequate variola virus model can be developed, the FDA and the advisory 
committee agreed that data from a combination of other lethal animal models using surrogate 
orthopoxviruses (e.g., NHP studies with monkeypox virus, rabbit studies with rabbitpox virus, 
mouse studies with ectromelia virus) could be used as evidence along with, or potentially instead 
of, animal studies using variola virus. 
 
Based on multiple discussions with stakeholders, DAVP recommends the following:  (1) data 
from at least two lethal animal models of orthopoxvirus infection should be obtained to evaluate 
drug efficacy; (2) orthopoxvirus animal models proposed for use in adequate and well-controlled 
efficacy studies should be well-characterized and generate reproducible results that are 
reasonably expected to predict efficacy in variola virusinfected or exposed humans; and (3) 
mortality, based on prospectively defined criteria for euthanasia, should be the primary endpoint 
for efficacy studies.  The recommendation for using multiple orthopoxvirus animal models 
acknowledges the unique challenges and uncertainties associated with smallpox drug 
development and the fact that no single orthopoxvirus animal model is known to be the best 
predictor of human responses to treatments for smallpox. 
 
As discussed in the guidance for industry Product Development Under the Animal Rule, 
(October 2015) [e]uthanasia criteria should be prospectively specified and agreed to by DAVP 
before sponsors conduct animal studies intended to support regulatory decision-making.  
Sponsors should include a detailed documentation of the euthanasia decision in the study report 
for each animal euthanized during the study.  The documentation should include, but is not 
limited to, how the animal met the euthanasia criteria and whether there were any deviations 
from the prespecified criteria.  The euthanasia documentation and methods for ensuring data 
                                                 
11 For general discussion of the Animal Rule and general guidance for developing products under this regulation, see 
the guidance for industry Product Development Under the Animal Rule. 
5  Contains Nonbinding Recommendations 
 
quality and integrity (including modifications to data handling due to high-containment facility 
requirements) should also be discussed with DAVP before study conduct.  See the guidance for 
industry Product Development Under the Animal Rule regarding data quality and integrity 
expectations for the adequate and well-controlled animal efficacy studies and the 
pharmacokinetic (PK) and/or pharmacodynamic (PD) studies used to select a dose and regimen 
in humans. 
 
The design of these animal studies should be based on the general principles of human clinical 
trial design as well as past experience with characterizing animal models and results from the 
nonclinical natural history and exposure-response studies.  Animals used in natural history and 
efficacy studies should have been demonstrated to be immunologically naive to the 
orthopoxvirus challenge agent. Sponsors should perform this assessment by direct serologic 
analysis for larger animals or by indirect approaches (e.g., using sentinel animals) for rodents 
with low blood volume to monitor for any prior orthopoxvirus exposures in the test animals.  
Assessments based on cellular immune assays should also be considered when feasible and 
appropriate.  Animal study protocols should include detailed clinical observations and laboratory 
evaluations in the animals, similar to clinical and laboratory monitoring that might be performed 
in human clinical trials in drug development programs for other types of serious illnesses.  
Furthermore, demonstrating consistency and reproducibility of results using the same model at 
different animal facilities can assist in characterizing the model.  Blinding for studies should 
follow recommendations outlined in the guidance for industry Product Development Under the 
Animal Rule.  The protocol should also include details about treatment assignment and 
randomization procedures. 
 
In addition to the primary endpoint of mortality (i.e., proportion of animals succumbing to rather 
than recovering from disease), sponsors are encouraged to evaluate secondary endpoints that 
could be associated with or predictive of outcomes in the animal models under development. 
 
Other important study design considerations include using a range of drug doses, durations, and 
start times, including treatment started both before and after infection and symptomatology have 
become clinically established.  In addition, differences in the time course of disease development 
between human smallpox and orthopoxvirus animal models should be considered in protocol 
design. 
 
Animal study protocols should also include methods for quantifying viral burden or viral 
shedding (both virus and viral DNA) and evaluating the relationship between these quantitative 
measurements and clinical outcomes of disease and treatment.  Viral isolates from animals 
failing treatment or with extended virus shedding should be evaluated for the development of 
drug resistance. 
 
The goal of the adequate and well-controlled animal studies is to demonstrate that the 
investigational drug is statistically significantly superior to placebo and confers a treatment or 
prevention effect considered likely to be clinically meaningful.  Sponsors should discuss power 
considerations and a proposed statistical analysis plan with the FDA before initiating planned 
studies. 
 
6  Contains Nonbinding Recommendations 
 
3.  Selection of an Effective Dosing Regimen in Humans 
 
To support human dose selection for an investigational drug, sponsors should characterize the 
PK profile of the drug in healthy humans and both the PK profile and the PD profile of the drug 
in the surrogate orthopoxvirus animal models that are used to demonstrate efficacy.  In addition, 
the PK profile of the drug in infected animals should be compared with the PK profile of the 
drug in healthy animals to determine whether the specific orthopoxvirus infection affects the 
drugs PK profile.  It is critical that the PK data in humans and the PK and PD data in animals 
are obtained in well-controlled studies using fully validated bioanalytical assays for determining 
drug concentrations.  For each of the surrogate orthopoxvirus animal models used to establish 
efficacy, the exposure-response relationship of the drug should be established and the fully 
effective dose and the drug exposure associated with the fully effective dose should be 
determined.  Furthermore, sponsors should consider interspecies differences in absorption, 
distribution (including plasma protein binding), metabolism, and excretion when determining the 
human dose. 
 
The determination of treatment duration will depend on several factors including, but not limited 
to, the pharmacokinetic characteristics of the drug, the proposed indication (i.e., treatment or 
prophylaxis), and the pathophysiology and disease course in animal models of orthopoxvirus 
infection versus humans with smallpox. 
 
As described in the guidance for industry Product Development Under the Animal Rule, sponsors 
should select human doses that provide exposures that exceed the exposures in animals 
associated with the fully effective dose (ideally by severalfold if the drugs safety profile 
supports such dosing).  This selection serves to accommodate any uncertainties relating to the 
similarity of the exposure-response relationship between humans and animals. 
 
B.  Pharmacology/Toxicology Considerations 
 
Pharmacology/toxicology considerations for safety evaluation should follow the standard drug 
development paradigms for small molecules as outlined in the ICH guidance for industry M3(R2) 
Nonclinical Safety Studies for the Conduct of Human Clinical Trials and Marketing 
Authorization for Pharmaceuticals (January 2010) or for biologics as outlined in the ICH 
guidance for industry S6(R1) Preclinical Safety Evaluation of Biotechnology-Derived 
Pharmaceuticals (July 1997).  Historical clinical data suggest that some patients (e.g., 
immunocompromised) with variola virus infection may have had clinical disease lasting longer 
than 2 weeks; therefore, we recommend that initial toxicology and safety studies take this 
possibility into account.  Duration of studies to support investigational new drug application 
(IND) and new drug application (NDA)/biologics license application (BLA) filings are outlined 
in the respective ICH guidances. 
 
We do not anticipate that carcinogenicity studies will be needed for drugs that might be used 
only to treat established smallpox because the administration of such drugs will not, in most 
cases, exceed 6 months.  However, if there is a cause for concern (e.g., positive genotoxicity or 
other risks for carcinogenicity), then follow-up discussions with DAVP may be warranted.  
Lastly, see the guidances for industry Product Development Under the Animal Rule and 
7  Contains Nonbinding Recommendations 
 
Providing Regulatory Submissions in Electronic Format  Standardized Study Data (December 
2014) regarding requirements for electronic submission of nonclinical pharmacology/toxicology 
as well as nonclinical efficacy datasets. 
 
C.  Nonclinical Virology Considerations 
 
Study reports for the investigational drug should provide results and analyses describing its 
mechanism of action, establish its specific antiviral activity in cell culture and animal models, 
provide data on the development and potential mechanisms of viral drug resistance (or reduced 
susceptibility of the virus to the drug) in cell culture and animal models, and assess its 
cytotoxicity and mitochondrial toxicity.  Additional information on virology studies can be found 
in the guidance for industry Antiviral Product Development  Conducting and Submitting 
Virology Studies to the Agency (June 2006). 
 
We recommend that sponsors evaluate the investigational drugs antiviral activity against a broad 
panel of orthopoxviruses, including vaccinia virus and other orthopoxviruses expected to be used 
in animal models (e.g., monkeypox virus, rabbitpox virus, ectromelia virus).  Such assessments 
constitute a broad-based orthopoxvirus testing strategy to screen for potential relevance to 
variola virus and assess the potential of the investigational drug to treat vaccine complications or 
human monkeypox.  Ultimately, sponsors should explore the potential appropriateness of testing 
the antiviral activity of the investigational drug against variola virus isolates if other data are 
sufficiently promising to proceed to this stage. 
 
Orthopoxvirus DNA polymerases replicate their viral genomes with high fidelity, complicating 
the genotypic analysis of resistance in animal studies.  To aid in the interpretation of drug 
resistance results from animal studies, sponsors should include plans to evaluate the performance 
of their nucleotide sequencing assay(s) to help distinguish between nucleotide sequence changes 
caused by their resistance assay versus those occurring in vivo and possibly contributing to drug 
resistance. 
 
Sponsors should submit information on sample collection, assays performed, and validation 
approaches for these assays.  Using a specific procedure, method, or test system in an 
investigational protocol for a nonclinical laboratory study, or as a laboratory procedure 
supporting a clinical trial, does not constitute FDA endorsement of that procedure, method, or 
test system, or FDA approval for clinical laboratory use.12 
 
The FDA performs independent assessments of virologic and resistance data.  Sponsors should 
consult with DAVP before submitting virology datasets to obtain information on the most recent 
recommended formats.  In the case of next-generation sequence analysis, sponsors should refer 
to the guidance for industry and technical specification document Submitting Next Generation 
Sequencing Data to the Division of Antiviral Products (July 2019), which describes the 
                                                 
12 Submission of an investigational device exemption to the Center for Devices and Radiological Health may be 
warranted if an investigational assay is used in a clinical trial. 
 
 
8  Contains Nonbinding Recommendations 
 
procedure for submission of files in the FASTQ format (containing sequence nucleotides and 
quality information). 
 
Laboratory work with certain orthopoxviruses must comply with applicable regulations (e.g., the 
select agent regulations found at 42 CFR part 73)13 and should incorporate relevant biosafety and 
biosecurity procedures as appropriate to the viruses studied.  Sponsors should contact relevant 
government agencies, such as the CDC and the National Institutes of Health, for more 
information regarding biosafety procedures.14 Sponsors should also be aware that all studies of 
variola virus must be conducted in collaboration with CDC and will require approval from the 
World Health Organization.  
 
D.  Clinical Considerations 
 
For the FDA to approve a drug for treating or preventing smallpox under the Animal Rule, the 
safety of the drug must be established.15  However, the Animal Rule does not provide specific 
provisions for evaluating safety.  Therefore, the FDA evaluates these drugs under preexisting 
NDA/BLA regulations for establishing the safety of new drugs or biological products.  Under 
most conditions, the human safety data for smallpox drugs will come from healthy volunteer 
studies and/or relevant human safety data for the same drugs developed for other indications.  
Evaluating important drug-drug interactions also may involve healthy volunteer studies.  In the 
event of a smallpox public health emergency, human safety and efficacy data also can be 
obtained by using investigational smallpox drugs in clinical field trials.  For drugs approved 
under the Animal Rule, postmarketing clinical trials are required when feasible and ethical (21 
CFR part 314, subpart I, for drugs and 21 CFR part 601, subpart H, for biologics). 
 
1.  Healthy Volunteer Safety Trials 
 
Outside of a public health emergency, the safety evaluation of drugs developed solely for 
treating or preventing smallpox largely depends on safety trials in healthy volunteers.  
Nonclinical safety and activity data of the investigational drug should be available before the 
initiation of human trials to support safety and to guide clinical trial design (e.g., dose, duration), 
as outlined in the respective ICH guidances for small molecules (ICH M3(R2)) or biologics (ICH 
S6(R1)).  Sponsors should discuss any concerns related to the safety or ethics of healthy 
volunteer trials with the FDA early in the drug development program. 
 
The size and composition of the human safety database needed to support smallpox drug 
approval depend on issues such as the indication (e.g., treatment, post-exposure prophylaxis, or 
prophylaxis), the drugs toxicity, and the extent of the FDAs experience with a particular drug 
(and possibly with related drugs).  For a drug intended to treat smallpox, greater known risks or 
greater uncertainty about undefined risks may be acceptable if a drug offers a potential for 
benefit to smallpox patients, given the serious nature of the disease.  In general, a safety database 
                                                 
13 Information on the Federal Select Agent Program can be found at https://www.selectagents.gov. 
 
14 Information on biosafety can be found at https://www.cdc.gov/biosafety/publications/bmbl5/index.htm and 
https://www.nih.gov/research-training/safety-regulation-guidance. 
15 See 21 CFR 314.610(a) and 601.91(a). 
9  Contains Nonbinding Recommendations 
 
of at least 300 individuals is needed for a 95 percent confidence interval to rule out a 1 percent 
rate of a specific adverse reaction if that specific adverse reaction did not occur in the population 
studied.  For drugs intended to prevent smallpox infection that might therefore be administered 
to large numbers of healthy individuals with uncertain risk of smallpox disease, a larger safety 
database may be needed.  Sponsors should discuss with DAVP the appropriate safety database 
size for their drugs. 
 
The adverse-event grading scale used in safety trials should be appropriate for healthy adult 
human volunteers.  Sponsors should characterize safety signals identified from animal studies or 
human trials and, if necessary, incorporate specific study design elements in the proposed 
nonclinical and clinical protocols. 
 
Evaluating certain drug-drug interactions also may involve healthy volunteer trials.  Sponsors 
should be prepared to address the potential interaction between a smallpox therapeutic and 
smallpox vaccination and should discuss with DAVP the conduct and timing of animal studies 
and any appropriate human trials for this purpose. 
 
2.  Safety Data from Non-Smallpox Clinical Experience 
 
Safety information to support approval of a smallpox drug can be derived from clinical trials of 
the same drug for a non-smallpox indication.  In the case of approved drugs, safety information 
can include safety data generated both pre- and postapproval.  For drugs in development for non-
smallpox indications, safety data acquired in all stages of development can support approval 
under the Animal Rule.  Because patients with smallpox disease may be expected to be acutely 
ill, safety data from clinical trials for non-smallpox indications associated with acute illness may 
be particularly relevant.  And because clinical trials in related viruses may provide additional 
support for a drugs activity as well as its safety, sponsors can consider simultaneously 
developing a drug for another poxvirus infection such as molluscum contagiosum virus, vaccinia 
virus, or monkeypox virus.16 
 
3.  Clinical Trials in the Event of a Public Health Emergency 
 
Sponsors developing smallpox drugs under the Animal Rule should design one or more clinical 
trials to assess the safety and efficacy of the investigational drugs in the event of a human 
smallpox outbreak.  Sponsors should discuss important trial design elements and potential 
smallpox emergency scenarios with the FDA and other relevant stakeholders early in the trial 
design process.  The trial or trials should be designed to evaluate the most appropriate 
therapeutic use or uses for the drug (treatment, post-exposure prophylaxis, or prophylaxis) based 
on results of nonclinical studies.  Depending on the strength of the data, efficacy and safety 
results from an emergency clinical trial could be used to support approval of a drug that was in 
the process of being developed under the Animal Rule. 
 
                                                 
16 Sponsors are strongly encouraged to discuss drug development for nonvariola indications with DAVP as early as 
possible, especially in circumstances in which the drug has potential to fill an unmet need by pursuing those other 
indications. 
 
10  Contains Nonbinding Recommendations 
 
The Animal Rule stipulates that all drugs approved using the Animal Rule should be evaluated 
for efficacy and safety through clinical trials if circumstances arise in which that would be 
feasible and ethical.  Therefore, smallpox drug approval under the Animal Rule will include a 
requirement to conduct one or more human postmarketing trials if a smallpox outbreak occurs, 
and the marketing application must include a plan or approach to meet this requirement (21 CFR 
part 314, subpart I, for drugs and 21 CFR part 601, subpart H, for biologics).  The drug-approval 
letter will include a milestone date for submission of the final clinical protocol, ready for 
implementation should the need arise. 
 
4.  Expanded Access IND for Emergency Use 
 
For sporadic events such as smallpox vaccine complications or accidental laboratory exposures 
to orthopoxviruses, treatment of a patient under an individual patient expanded access IND for 
emergency use may be appropriate if the drug under development is expected to have activity 
against the orthopoxvirus and if the patient is not able to participate in a clinical trial.17  If a 
situation arises in which it is necessary to treat a patient under an expanded access IND for 
emergency use, sponsors should collect data to the extent feasible while recognizing that the data 
collected may be of limited utility.  If frequent sporadic uses of an investigational drug are 
anticipated, sponsors should develop an appropriate clinical trial protocol. 
 
5.  Emergency Use Authorization 
 
In the event of a smallpox emergency, the FDA may issue an emergency use authorization 
(EUA)18 to provide emergency access to unapproved drugs (or approved drugs for unapproved 
indications) after the Secretary of Health and Human Services issues the requisite declaration19 
that circumstances exist justifying the authorization of emergency use of the drugs, provided 
other statutory criteria are met.  For example, the FDA must conclude that based on the totality 
of scientific evidence available, it is reasonable to believe that a drug may be effective to treat or 
prevent smallpox, the known and potential benefits outweigh the known and potential risks of 
the drug, and there is no adequate, approved, and available alternative.  Sponsors who think their 
drugs may warrant EUA consideration are encouraged to submit relevant information and initiate 
pre-EUA discussions rather than wait for a potential emergency to arise; however, the issuance 
of an EUA is not considered an appropriate final goal for drug development or a substitute for 
generating data to support an NDA or a BLA. 
 
                                                 
17 The requirements and procedures for expanded access INDs for emergency use can be found in 21 CFR part 312, 
subpart I, and in the guidance for industry Expanded Access to Investigational Drugs for Treatment Use  
Questions and Answers (June 2016). 
 
18 The requirements and procedures for EUAs can be found in section 564 of the Federal Food, Drug, and Cosmetic 
Act (FD&C Act) (21 U.S.C. 360bbb-3) and in the guidance for industry and other stakeholders Emergency Use 
Authorization of Medical Products and Related Authorities (January 2017). 
 
19 The declaration of the Secretary of Health and Human Services must be based on one of four determinations 
(including a material threat determination), as described in statute (section 564(b)(1) of the FD&C Act (21 U.S.C. 
360bbb-3(b)(1))). 
11  Contains Nonbinding Recommendations 
 
E.  Clinical Pharmacology Considerations  
 
See section III. A. 3, Selection of an Effective Dose in Humans, for a discussion on obtaining 
exposure-response data for the investigational drug from at least two well-characterized animal 
models to aid in determining an effective dose in humans.  Sponsors should follow the standard 
drug development paradigms for clinical pharmacology.  Intrinsic and extrinsic factors (such as 
organ impairment, food effect, or drug interactions) that may affect the pharmacokinetics of an 
investigational drug should be well characterized and the effective dose in humans should be 
adjusted if necessary.  Because of human subject protection considerations for conducting 
clinical trials in specific populations, such as pregnant women and pediatric patients (45 CFR 
part 46, subparts B and D and 21 CFR part 50 subpart D), administering an investigational drug 
solely for the purpose of collecting PK data may not be ethical.  For such specific populations, it 
may be possible to obtain PK data if there are situations in which a drug is already being used for 
reasons other than solely for obtaining PK data.  In some circumstances, modeling and 
simulation methods can be used to determine effective doses; sponsors should discuss using such 
methods with the FDA. 
 
F.  Chemistry, Manufacturing, and Controls Considerations 
 
Sponsors should pay particular attention to developing formulations for patients who are unable 
to swallow solid oral dosage formulations (e.g., development of oral solutions and powders for 
pediatric patients, parenteral formulations for extremely ill patients). 
 
It is likely that drugs for treating or preventing smallpox infection may be stockpiled for long 
periods of time in anticipation of a sudden outbreak, so an expiration dating period (shelf life) 
that is longer than usual may be desirable.  To generate the stability data needed to support a 
long expiration dating period, it may be advantageous to place in the long-term stability testing 
program larger amounts of drug than is usual. 
 
12  Contains Nonbinding Recommendations 
 
1  REFERENCES 
2   
3  Dixon, CW, 1962, Complications. Treatment and Nursing. Sequelae, in Smallpox, J. & A. 
4  Churchill LTD, London, available at 
5  https://www.nlm.nih.gov/nichsr/esmallpox/smallpox_dixon.pdf. 
6   
7  Fenner, F, DA Henderson, I Arita, Z Jezek, and ID Ladnyi, 1988, Smallpox and Its Eradication, 
8  World Health Organization, Geneva, available at 
9  http://whqlibdoc.who.int/smallpox/9241561106.pdf. 
10   
11  Petersen, BW, IK Damon, CA Pertowski, D Meaney-Delman, JT Guarnizo, RH Beigi, KM 
12  Edwards, MC Fisher, SE Frey, R Lynfield, and RE Willoughby, 2015, Clinical Guidance for 
13  Smallpox Vaccine Use in a Postevent Vaccination Program, MMWR Recomm Rep, 64(02):1
14  26. 
15   
16  Petersen, BW, TJ Harms, MG Reynolds, and LH Harrison, 2016, Use of Vaccinia Virus 
17  Smallpox Vaccine in Laboratory and Health Care Personnel at Risk for Occupational Exposure 
18  to Orthopoxviruses  Recommendations of the Advisory Committee on Immunization 
19  Practices, MMWR Morb Mortal Wkly Rep, 65(10):257262. 
13