WASHINGTON: US health officials are warning against overuse of the only drug available to treat monkeypox, saying even a small mutation in the virus could render the pills ineffective.
The Food and Drug Administration updated its guidelines this week to Tpoxxwhich 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 significantly impact the antiviral activity of Tpoxx.” Since viruses are constantly evolving to overcome barriers to infection, including drugs, regulators have stressed that doctors must be “judicious” in prescribing drugs.
The call to reduce the use of Tpoxx follows 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 variola virus, and its use against monkeypox is considered experimental and tightly controlled by federal authorities.
Doctors wishing to prescribe the drug must submit a request to the Centers for Disease Control and Prevention, documenting their patient’s needs and agreeing to track their results and any side effects. Authorities have shipped 37,000 courses of the drug to doctors.
Tpoxx works by targeting a single protein found on monkeypox, smallpox and similar viruses. The FDA said this week that several laboratory, animal and human reports suggest multiple ways monkeypox could develop resistance to therapy.
The update came as federal officials on Thursday expressed cautious optimism about the trajectory of the outbreak, noting that new cases have fallen about 50% since their peak in August.
During a briefing at the White House, CDC director Dr. Rochelle Walensky attributed the decline to vaccinations, awareness and individuals reducing behaviors related to the spread. The vast majority of cases in the United States 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 using antiviral drugs.
“That’s why we’re uncomfortable when you only have one drug,” Fauci told reporters. He added that a recently launched study on Tpoxx and 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 US sites.
Last month, the Biden administration invoked rare emergency powers to expand the country’s limited supply of monkeypox vaccines. And last week, a separate statement accelerated the use of experimental tests for the virus.
But no changes have been made to allow emergency use of Tpoxx, fueling complaints from groups representing gay and bisexual men.
The US Government’s National Stockpile contains over 1.7 million courses of Tpoxx, originally manufactured for use in a possible 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 impractical. Smallpox was declared eradicated in 1980 by the World Health Organization, ruling out the possibility of human studies.
Even though the drug was approved for smallpox, its effectiveness was measured in monkeys infected with monkeypox, considered a reasonable predictor of the effect of smallpox on humans. Animals receiving Tpoxx survived at higher rates than those receiving placebo. But FDA officials have warned that the results in animals need to be confirmed by human testing.
“Without human trials, we don’t know if Tpoxx is beneficial for humans with monkeypox,” FDA Commissioner Dr. Robert Califf told Senate lawmakers during a hearing this week.
The CDC reported last week that 3.5% of patients followed through its Tpoxx program reported side effects, mostly headaches and nausea.
The agency only recovered about 200 forms from physicians documenting the patient’s early symptoms and results, representing less than 1% of doses shipped since the outbreak began.

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