The Phase I trial of the therapy with 20 healthy subjects indicated no drug-related serious adverse events. Credit: Mongkolchon Akesin/Shutterstock.

SIGA Technologies has gained Japanese regulatory approval for TEPOXX (tecovirimat) 200mg capsules, known as TPOXX in the US, for the treatment of smallpox, mpox and cowpox, and complications following smallpox vaccination for adults and for children weighing a minimum of 13kg.

This is the first approval by the Pharmaceuticals and Medical Devices Agency and the Japan Ministry of Health, Labour and Welfare for an antiviral therapy targeting orthopoxviruses.

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In collaboration with its exclusive distributor, Japan Biotechno Pharma, the company has fulfilled an order of TEPOXX to contribute to the country’s national stockpile.

The approval is underpinned by data from 15 clinical trials involving more than 800 healthy volunteers. This includes a pivotal Phase I pharmacokinetics trial with 20 healthy subjects conducted in Japan, which indicated no drug-related serious adverse events and quantifiable pharmacokinetics within the efficacious dose ranges.

SIGA Technologies CEO Diem Nguyen stated: “The approval of TEPOXX in Japan marks another significant milestone in our mission to expand access to this critical antiviral treatment worldwide to support an effective response to an orthopoxvirus outbreak.

“This approval highlights the importance of stockpiling effective antiviral therapies as a cornerstone of innovative solutions to safeguard public health against serious and potentially devastating infectious diseases, such as smallpox.”

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Further clinical evidence of the therapy from four pivotal studies in non-human primates (NHPs) and two in rabbits demonstrated significant reductions in death rate and load of the virus.

Studies in NHPs infected with the variola virus, the causative agent of smallpox, showed that the therapy improved survival rates and reduced lesion counts.

TEPOXX inhibits the VP37 protein on the surface of orthopoxviruses and prevents the virus leaving infected cells, thus slowing the spread of infection. It has received approval in Canada and the US for smallpox treatment.

Under the name Tecovirimat-SIGA, it is approved for treating smallpox, mpox, cowpox and complications post-smallpox vaccination in the European Union and the UK.

In July 2024, the US Department of Health and Human Services exercised a procurement option for TPOXX.