Atlanta—Even though more than 22,000 cases of monkeypox had been confirmed in the U.S. as of mid-September, no FDA-approved treatments for human monkeypox are currently available.

The most promising appears to be Tpoxx (tecovirimat), according to a new study. Tecovirimat is an antiviral drug that has demonstrated efficacy in animal studies and is FDA-approved for treating smallpox. No safety or efficacy data regarding its use in patients with monkeypox virus infection has been available, however, according to an article in the Morbidity & Mortality Weekly Report.

The researchers from the CDC reported that—among 549 patients with monkeypox virus infection treated with tecovirimat under an Expanded Access Investigational New Drug protocol—99.8% received it orally as outpatients with few adverse events reported.

“Tecovirimat is generally well tolerated, and these data support continued access to treatment with tecovirimat during the current monkeypox outbreak,” according to the researchers.

The article pointed out that the use of tecovirimat for the treatment of monkeypox in the U.S. is permitted only through an FDA-regulated Expanded Access Investigational New Drug (EA-IND) mechanism. The CDC holds a nonresearch EA-IND protocol that facilitates access to and use of tecovirimat for the treatment of monkeypox.

The study team noted that the protocol included patient treatment and adverse event reporting forms to monitor safety and ensure intended clinical use in line with the FDA EA-IND requirements. The protocol added that the outbreak has predominantly affected gay, bisexual, and other men who have sex with men (MSM). Intake forms were available for 549 patients treated with tecovirimat, while outcome forms were available for 369.

As of August 20, 2022, patients treated with tecovirimat for monkeypox were 97.7% male, with a median age of 36.5 years. Among patients with available data, 38.8% were reported to be non-Hispanic white people. Nearly 99.8% were prescribed oral tecovirimat, and 93.1% were not hospitalized.

The researchers pointed out that about one-half of patients with monkeypox virus infection who received tecovirimat were living with HIV infection. The median interval from initiation of tecovirimat to subjective improvement was 3 days and did not differ by HIV-infection status.

The authors added that adverse events were reported in only 3.5% of patients, and all but one adverse event were nonserious.

The study adds that two other investigational treatments for orthopoxviruses (cidofovir and brincidofovir), “have demonstrated substantial toxicity with limited efficacy data.”

For patients treated under the EA-IND protocol and included in this report, the median time to subjective improvement was 3 days after receiving tecovirimat, but researchers cautioned that no control group was available for comparison, making it impossible to draw conclusions about effectiveness.

The article cited a report of a small U.S. cohort treated with tecovirimat (also under the EA-IND) finding complete resolution of lesions by Day 21 in 23 (92%) of 25 patients.

“Although this report could not evaluate the efficacy, clinicians are encouraged to continue following CDC guidelines for tecovirimat use in patients with severe disease or at risk for severe disease,” the authors advised. “Because there is the potential for false-positive test results, tecovirimat should be considered only in those with a high pretest probability of being infected with Monkeypox virus to avoid unnecessary treatment or implementation of other public health measures. Inappropriate uses could potentially lead to resistance.”

They also urged ongoing monitoring to assess the safety of tecovirimat in patients with Monkeypox virus infection under the EA-IND during the current monkeypox outbreak.

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