FDA Instructs Pfizer to Test Second Course of Paxlovid

Pfizer_Pierre Suu/Getty Images

Pierre Suu/Getty Images

The FDA has instructed Pfizer to evaluate an additional course of its antiviral combination therapy Paxlovid in people who experienced COVID-19 rebound infections. 

Pfizer is to report initial data from a randomized trial of the second course of therapy by Sept. 30, 2023. The original communication between the agency and Pfizer was dated Aug. 5.

Paxlovid (nirmatrelvir and ritonavir tablets) was granted Emergency Use Authorization (EUA) by the FDA for high-risk adults and pediatric patients 12 years and older on Dec. 22, 2021. The EUA was granted for patients at high risk of progression to severe COVID-19, which includes hospitalization or death. It is an oral therapy and, in the original Phase II/III EPIC-HR trial, it significantly decreased the risk of hospitalization or death by 89% compared to placebo in non-hospitalized, high-risk adults with COVID-19 within three days of symptom onset.

Rebound infections have been observed in people who are almost recovered after receiving Paxlovid, but the condition recurs. Cases of rebound COVID-19 appear to be rare.

In a study out of Case Western University that evaluated 13,644 patients who received either Paxlovid or Merck's Lagevrio (molnupiravir), the 7-day and 30-day rebound rates associated with Paxlovid, respectively, were 3.53% and 5.4% for reinfection; 2.31% and 5.87% for symptoms; and 0.44% and 0.77% for hospitalizations.

The rebound rates for patients receiving Lagevrio were slightly higher, with 7-day and 30-day rebound rates of 5.86% and 8.59% for infection, respectively; 3.75% and 8.21% for symptoms; and 0.84% and 1.39% for hospitalizations.

The study's authors wrote, "Patients who rebounded from either therapy had a significantly higher prevalence of underlying medical conditions than those without."

However, Pamela Davis, M.D., co-author of the study, told BioSpace, "There is no underlying condition that stands out. We had thought (the rebound group would be composed of) immunocompromised people, but heart disease, hypertension, mood disorders and so on, all are increased in the rebound group."

The FDA indicates the study's design is expected to be finalized this month. A company spokesperson said Pfizer is "working with the FDA to finalize a protocol to study patients who may be in need of retreatment" and will provide further details when available.

The FDA letter calls for "Pfizer to conduct a clinical trial in patients with 'COVID-19 rebound' and a clinical trial evaluating different durations of treatment in immunocompromised patients with mild-to-moderate COVID-19. The Fact Sheet for Patients, Parents, and Caregivers was also revised to include additional clarifying information now how to take Paxlovid, which now includes pictures of packaging and tablets for both dosing presentations."

The letter also reiterated, "There is no adequate, approved, and available alternative to the emergency use of Paxlovid for the treatment of mild-to-moderate COVID-19 in adults and pediatric patients (12 years of age and older weighing at least 40 kg) with positive results of direct SARS-COV-2 viral testing, and who are at high risk for progression to severe COVID-19, including hospitalization or death."

Although the biggest cash cow for Pfizer has been its COVID-19 vaccine developed with BioNTech, bringing in $8.8 billion in revenue in the second quarter alone, Paxlovid also brought in $8.1 billion for the quarter and is projected to draw $22 billion for the year.

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