In a first, FDA approves SIGA’s smallpox treatment

SIGA has a $472 million contract to supply the U.S. government with Tpoxx. (FDA)

The FDA has approved the first treatment for smallpox. SIGA Technologies tested the drug in 359 healthy volunteers to demonstrate safety and relied on animal data to show efficacy.

Smallpox was eradicated in 1980, but the presence of the virus in laboratories in the U.S. and Russia, plus the periodic discovery of lost vials at other sites, has left lingering concerns that it could be used as a bioweapon. These concerns fuel government demand for smallpox vaccines and led SIGA to develop a drug to treat people infected with the virus.

The nature of smallpox means the drug, an antiviral branded as Tpoxx, took an unusual path to market. No cases of smallpox occur naturally, and it would be highly unethical to infect people for research purposes. As such, SIGA has never tested whether its treatment clears the virus in humans.


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Rather, SIGA went to the FDA with data from animals infected with viruses similar to smallpox. These animal studies linked Tpoxx to a higher rate of survival than placebo, enabling SIGA to clear the efficacy bar established by the FDA’s Animal Rule. The rule allows drugs to win approval on the basis of data from well-controlled animal studies if it would be unethical or unfeasible to run efficacy trials in humans.

SIGA did assess the safety of Tpoxx in humans, though, administering the drug to 359 healthy volunteers. Some participants experienced headache, nausea and abdominal pain after receiving Tpoxx, but overall the FDA saw the safety data as clean enough to support approval. The FDA approval follows a unanimous recommendation from the agency’s advisory committee. 

In return for its efforts, SIGA has become the first company to receive a Material Threat Medical Countermeasure priority review voucher. The saleable voucher grants the holder a speedy regulatory review for a future filing. SIGA has a $472 million contract to supply the U.S. government with Tpoxx. The U.S. Department of Defense's involvement in the development and commercialization of the antiviral is one of several ways the government has supported SIGA to protect public health. 

“To address the risk of bioterrorism, Congress has taken steps to enable the development and approval of countermeasures to thwart pathogens that could be employed as weapons,” FDA Commissioner Scott Gottlieb, M.D., said in a statement. “Today’s approval provides an important milestone in these efforts. This new treatment affords us an additional option should smallpox ever be used as a bioweapon.”

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