Japan's pharmaceutical regulatory agency has granted approval for tecovirimat (TPOXX) to treat mpox, smallpox, and cowpox, raising eyebrows in the scientific community after two major clinical trials demonstrated the drug's ineffectiveness. The decision, made earlier this month, has left researchers questioning the evidence base for the approval.
Clinical Trial Results Challenge Drug's Efficacy
Two recent large-scale clinical trials have definitively shown that tecovirimat fails to provide meaningful benefits to mpox patients. The STOMP trial, which enrolled participants across multiple countries including the United States, Japan, Latin America, South Africa, and Thailand, was terminated early after showing clear evidence of inefficacy. Similarly, the PALM007 trial in the Democratic Republic of the Congo (DRC) found no improvement in patient outcomes.
"To approve it now is very confusing," stated Dr. Jason Zucker, an infectious disease specialist at Columbia University and co-leader of the STOMP trial. The PALM007 study revealed that skin healing rates and mortality (1.7%) remained unchanged whether patients received the drug or placebo.
Regulatory Decisions and Investment
SIGA Technologies, the drug's manufacturer, justified Japan's approval citing favorable results from 15 clinical trials involving 800 healthy volunteers. However, these trials only assessed safety and pharmacokinetics, not therapeutic efficacy. The company claims Japan's review process did not include the recent STOMP trial results.
The U.S. government has invested over $600 million in 1.5 million doses of tecovirimat for its Strategic National Stockpile, while the European Union and United Kingdom approved the drug in 2022 under "exceptional circumstances."
Scientific Mechanism and Previous Approvals
Tecovirimat works by blocking interactions between cellular proteins and a surface protein common to orthopoxviruses, theoretically preventing the release of new virus particles. The drug's initial approval for smallpox by the FDA in 2018 was based on animal studies under the "Animal Rule," as human efficacy trials for smallpox are unfeasible due to the disease's eradication.
Implications for Global Regulatory Framework
The European Medicines Agency (EMA) is now reviewing its earlier decision in light of the new trial data. Marco Cavaleri, head of EMA's office of biological health threats and vaccines, expressed concern about Japan's approval timing, stating he "would have had a problem" recommending approval given the current evidence.
Dr. Timothy Wilkin, study chair of STOMP and infectious disease clinician at the University of California-San Diego, concluded, "This is pretty convincing evidence that when used alone, it's not going to be efficacious."
Future Considerations
The failed trials may have broader implications, potentially affecting the drug's approval status for smallpox treatment. While the FDA maintains that mpox trial results don't necessarily predict smallpox efficacy, some experts, including Dr. Wilkin, suggest reconsidering the approval for smallpox treatment. The situation highlights the challenges in balancing urgent medical needs with scientific evidence in drug approval processes.
