Siga's smallpox antiviral Tpoxx misses the mark in another mpox study

It’s déjà vu for Siga Technologies and its smallpox antiviral Tpoxx. At an interim analysis of a second mpox trial, the drug again missed the mark on a key measure of disease resolution.

In patients with the clade 2 mpox strain, Tpoxx failed to demonstrate efficacy on the endpoint of time to skin and mucosal lesion reduction, the National Institute of Allergy and Infectious Diseases (NIAID) reported this week. The trial enrolled patients with mild to moderate cases of mpox. 

Based on the findings and the recommendation of the trial's data monitoring board, NIAID ended enrollment in the randomized arms of the study. The agency further closed enrollment in the open-label arm, which enrolled severe mpox patients and those at risk of developing severe disease.

In August, similar results emerged from a preliminary analysis of a separate study that weighed the drug’s benefits in mild to moderate patients with the clade 1 mpox strain in the Democratic Republic of the Congo (DRC). In that trial, Tpoxx did not meet the primary endpoint of improving time to lesion resolution within 28 days post-randomization.

Given the prior data and the similarities between the two studies, the latest results were “not unexpected,” Siga’s CEO Diem Nguyen noted in a company release. Antivirals are "most effective when administered early in the course of an infection and tend to demonstrate the greatest benefit in patients with more severe disease," the CEO noted. 

The company expects that currently enrolling studies across the globe will likely yield similar results.

Tpoxx became the first FDA-approved treatment for smallpox back in 2018, but it is not approved to treat mpox in the U.S. Regulators in other countries have signed off on the mpox use for the drug.