FDA Approves First Drug to Treat Smallpox

On July 13, 2018, FDA approved Tpoxx (tecovirimat) for the treatment of smallpox. Tpoxx is a small-molecule antiviral treatment for human smallpox disease caused by variola virus and is the first therapy approved for this indication. The drug, available for both oral and intravenous administration, was developed by Siga Technologies, a commercial-stage pharmaceutical company focused on the health security market

Tpoxx’s effectiveness against smallpox was established by studies done with animals infected with viruses that are closely related to the smallpox virus and was based on measuring the animals’ survival at the end of the studies. FDA reports that more animals treated with Tpoxx lived compared to the animals treated with placebo. Tpoxx was approved under FDA’s animal rule, which allows efficacy findings from adequate and well-controlled animal studies to support an FDA approval when it is not feasible or ethical to conduct efficacy trials in humans.

According to the agency, though the World Health Organization declared smallpox eradicated in 1980, there have been longstanding concerns that the disease could be used as a bioweapon.

“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,” said FDA Commissioner Scott Gottlieb, MD, in an agency press release. “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.”

FDA granted Siga’s request for a priority review voucher (PRV), which may be used to obtain an accelerated FDA review of a future Siga product or sold to a third party. This is the first PRV awarded under the Material Threat Countermeasure PRV program enacted by the 2016 21^st Century Cures Act.  In addition, FDA approved a seven-year expiry for Tpoxx, according to Siga.

Under Siga’s existing contract with the Biomedical Advanced Research and Development Authority (BARDA), the company will ask BARDA to exercise an option for a $50 million payment to the company based on this extended shelf-life determination. According to Siga, the exercise of this option is at the sole discretion of BARDA.

Tpoxx will be available initially only through the United States government’s Strategic National Stockpile (SNS), the US national repository of antibiotics, vaccines, chemical antidotes, antitoxins, and other medical equipment and supplies. Siga has a $472-million procurement and development contract with BARDA, under which two million courses of oral Tpoxx have been delivered to the SNS.

With the approval, Siga reports that it is entitled to a $41-million hold-back payment under the existing contract with BARDA, provided that BARDA confirms that there is no difference between the approved product and the courses of Tpoxx that have already been delivered to the SNS.

During the May 2018 FDA advisory committee, several members of the committee also suggested that there could be benefit in exploring additional indications post-FDA approval, including prophylactic use in individuals exposed to smallpox and other orthopoxviruses, such as monkeypox, which also infects humans. Committee members cited the likely use of Tpoxx as post-exposure prophylaxis in the event of a smallpox outbreak and a 15­­­–20% mortality rate in people infected with monkeypox. Siga will examine these potential markets and the feasibility of expanding the Tpoxx label to include these indications worldwide.

Source: FDA, Siga Technologies