By Rose Pagano and Suzanne M. Sensabaugh, MS, MBA


The purpose of the guidance: Smallpox (Variola Virus) Infection: Developing Drugs for Treatment or Prevention is to help sponsors involved in the clinical development of drugs for the treatment or prevention of smallpox (variola virus). Drugs relating to this disease need to be developed and approved following the Animal Rule regulations. All in all, the guidance is meant to address conversations between CDER Division of Antiviral Products (DAVP), pharmaceutical sponsors, academia, and the public.

Mainly, the guidance refers to drugs that are meant to inhibit variola virus replication. Variola major is the most severe form of smallpox, with a mortality ranging from five percent to fifty percent in various outbreak situations. This is the primary source of concern relating to possible bioterrorist uses of smallpox and is the most pertinent form addressed this guidance.

In 1980, the World Health Organization determined that smallpox was eradicated. There are variola virus stocks in Russia and the United States Center for Disease Control, which was approved by an international agreement. Regardless, there are concerns that the variola virus might be used as a weapon for bioterrorism.

Vaccination with the vaccinia virus is the first line of defense against smallpox infection.  However, routine vaccination in the United States was discontinued in the 1970s and there is no natural disease exposure, so a bioterrorist threat could be catastrophic.

Antiviral drugs could be a valuable asset for exposure situations when vaccination is not possible or inadequate. Drug development for the safety and efficacy of smallpox treatment and prevention is influenced by the absence of smallpox cases, lack of information about pathophysiology of smallpox, moral concerns, lack of variola virus samples, insufficient nonhuman models, etc.

These challenges with smallpox require a unique approach for drug development. The FDA recommends that active compounds in cell culture should be studied in lethal animal models through different non-variola orthopoxviruses, such as vaccinia virus.

Vaccinia virus needs to be studied due to its relationship to variola virus and the vaccinia virus should also be investigated relating to vaccination complications. Small animal models should be used in analyzing preliminary antiviral activity of the drug and serve as a means to evaluate drug doses, dosing regimens, evolution of disease, effects of viral species, and other characteristics. These studies could help with determining the effect of smallpox variations and determining the necessary resources to combat smallpox.

For the animal models, it is recommended that sponsors gain concurrence from DAVP for the animal models and study design. After numerous meetings with stakeholders, DAVP advises that drug efficacy should be based on information from at least two lethal animal models of non-variola orthopoxvirus infection. Also, non-variola animal models need to be well-characterized and generate reproducible results, and mortality.

In concordance with Product Development Under the Animal Rule, animal euthanasia criteria must be described and approved by DAVP before the study and include the necessary documentation (how animals met the euthanasia criteria and if there were any deviations from this criteria).

It is important that these animal studies are designed according to the general principles of human clinical trial design and past experience with characterization of animal models and results from the nonclinical natural history and exposure-response studies.

In order to pursue human dose selection for an investigational drug, sponsors need to create a pharmacokinetic profile of the drug for healthy humans and a pharmacokinetic profile and pharmacodynamics of the drug in surrogate orthopoxvirus animal models.

Additionally, study reports for the investigational drug should include data relating to the mechanism of action, antiviral activity in both cell culture and animal models, give data on development and possible mechanisms of viral drug resistance, and analyze cytotoxicity and mitochondrial toxicity.

The FDA recommends that sponsors evaluate the investigational drug’s antiviral activity against a variety of different orthopoxviruses. Sponsors should also submit data on sample collection, assays conducted, and validation on approaches for the assays.

Finally, the FDA performs independent assessments of virologic and resistance data. Sponsors should consult with DAVP before submitting virology data to help with future filing procedure.


At HartmannWillner LLC we understand the regulatory requirements for development of products through the Animal Rule. With our help, you can develop a realistic strategy that will move your product closer to approval as you reach each regulatory milestone.  Contact us today to learn more about our services and how we can help you.


Suzanne M. Sensabaugh has advanced the development of over 130 biologics, biosimilars, and combination products since 2009. She can be reached at