Siga Technologies Inc.’s antiviral Tpoxx, which has mainly been shown to fight monkeypox in animal studies, will undergo human testing by University of Oxford researchers as countries look for ways to tackle a growing outbreak with limited vaccine supply.

The study will enroll at least 500 people across the U.K. who will take either take the antiviral or a placebo twice a day for two weeks at home, according to the team of scientists leading the trial. The U.K. trial is a world first in testing the efficacy of an antiviral in monkeypox patients, said the researchers who previously led Britain’s Recovery trial of Covid-19 treatments.

Investigators will assess whether Tpoxx helps heal the skin and mucosal lesions produced by the disease, as well as the time it takes infected people to test negative for the virus. The drug’s ability to prevent hospitalization will also be studied.

Company representatives for Siga didn’t immediately respond to an email requesting comment.

Initially approved by the U.S. Food and Drug Administration for treating smallpox, which belongs to the same family of viruses, Tpoxx has been quickly identified as a promising antiviral against monkeypox among researchers. The treatment comes in a tablet form that remains stable at room temperatures with a good safety profile.

“It’s clearly the front-runner in terms of the drugs most likely to work,” said Peter Horby, an infectious disease professor at Oxford and one of the trial’s lead investigators.


The U.K. drug regulator earlier this year approved the use of Tpoxx for people suffering from severe complications of monkeypox, based on studies conducted on animals and healthy people. Yet so far there’s no clear evidence from human studies that the antiviral is an effective treatment for the disease.

The aim is to find a treatment that’s readily available and helps reduce the symptoms and complications so patients can get better and “get out of quarantine quicker by reducing their infectiousness,” Horby told reporters at a briefing on Tuesday.

Researchers said they hope to have results on how the drug works by Christmas but it will depend on the pace of recruitment. Both adults and children with laboratory-confirmed monkeypox infections and active skin and mucosal lesions are eligible for participation in the study.

Studies on Tpoxx and other candidates are also being planned elsewhere in Europe, the U.S. and Canada. The U.K. trial comes amid a monkeypox vaccine supply shortage around the world, which is forcing some countries to recommend fractional doses to help meet demand.

There is only one approved monkeypox vaccine in the world — Jynneos, also known as Imvanex, made by Bavarian Nordic — and supplies are running low. The U.K. government said last week that it cannot book any new vaccination appointments until more shots arrive in September. Bavarian is close to finalizing a deal with a U.S. production partner as it seeks to ramp up production.

People infected with monkeypox usually experience a rash of painful blisters, and in some cases fever, muscle pain and swollen lymph glands. Most recover within weeks, but there is also risk of severe illness.

The infection, which has only caused sporadic outbreaks in parts of Africa over the past few decades, broke out in the U.K. in May and has since spread around the world. The rapid proliferation prompted the World Health Organization to declare it a public health emergency of international concern. Some 38,000 cases have been reported across over 90 countries, mostly among men who have sex with men.