The United States warns that monkeypox could mutate to resist the antiviral drug

WASHINGTON (AP) — U.S. health officials are warning against overuse of the lone drug available to treat monkeypox, saying even a small mutation in the virus could render the pills ineffective.

The Food and Drug Administration this week updated its guidance for Tpoxx, which has been prescribed to tens of thousands of patients with the virus.

In an online update, FDA officials warned that a single molecular change in monkeypox “could have a major impact on the antiviral activity of Tpoxx.” Because viruses are constantly evolving to overcome obstacles to infection, including drugs, regulators stressed that doctors should be “cautious” when prescribing the medication.

Meanwhile, the Centers for Disease Control and Prevention said Thursday that Tpoxx should no longer be given to healthy adults who are not suffering from severe symptoms.

“For most patients with healthy immune systems, supportive care and pain management may be sufficient,” agency officials said in a statement.

The moves to reduce the use of Tpoxx follow weeks of criticism from HIV advocates and other patient groups who have urged the Biden administration to make the antiviral drug more widely available. Tpoxx is approved for the related smallpox virus, and its use against smallpox is considered experimental and closely monitored by federal officials.

Doctors who want to prescribe the drug must submit an application to the Centers for Disease Control and Prevention, documenting their patient’s need and agreeing to track their results and side effects. Officials have sent 37,000 courses of the drug to doctors.

Tpoxx works by targeting a single protein found in monkeypox, pox and similar viruses. The FDA said this week that research in labs, animals and people suggests several ways monkeypox could develop resistance to therapy.

The update came Thursday as federal officials expressed cautious optimism about the outbreak’s trajectory, noting that new cases have fallen about 50 percent since their peak in August.

During a briefing at the White House, CDC Director Dr. Rochelle Walensky attributed the decrease to vaccinations, educational outreach and reduction in behaviors related to the spread of individuals. The vast majority of US cases have been in men who have sex with men, although officials stress that the virus can infect anyone.

Dr. Anthony Fauci, the nation’s top infectious disease official, noted that resistance is always a risk when antiviral drugs are used.

“That’s why we’re uncomfortable when you only have one drug,” Fauci told reporters. He added that a recently launched study of Tpoxx supported by the National Institutes of Health will track signs of mutation that could lead to resistance. The study is expected to enroll more than 500 patients at 60 sites across the United States.

Last month, the Biden administration invoked rare emergency powers to stretch the nation’s limited supply of monkeypox vaccine. And last week an independent statement accelerated the use of experimental tests for the virus.

But no changes were made to allow emergency use of Tpoxx, prompting complaints from groups representing gay and bisexual men.

The US government’s national stockpile contains more than 1.7 million courses of Tpoxx, originally manufactured for use during a potential bioterrorist attack.

The FDA approved the drug in 2018 under its “animal rule,” which allows approval based on animal data when human testing is unethical or unfeasible. Smallpox was declared eradicated in 1980 by the World Health Organization, ruling out the possibility of human studies.

Although the drug was approved for smallpox, its effectiveness was measured in monkeys infected with smallpox, considered a reasonable predictor of the effect of smallpox in humans. Animals given Tpoxx survived at higher rates than those given a placebo. But FDA officials have cautioned that animal results must be confirmed in human trials.

“Without human trials, we don’t know if Tpoxx is beneficial to humans with monkeypox,” FDA Commissioner Dr. Robert Califf told Senate lawmakers at a hearing this week.

The CDC reported last week that 3.5 percent of patients screened through its Tpoxx program reported side effects, mainly headache and nausea.

The agency has recovered only about 200 forms from doctors documenting the patient’s initial symptoms and results, representing less than 1% of the doses sent since the outbreak began.

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Stobbe reported from New York

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The Department of Health and Science of the Associated Press is supported by the Department of Science Education at the Howard Hughes Medical Institute. The AP is solely responsible for all content.

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