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Study of ibuzatrelvir alone or combined with remdesivir in adults with COVID-19 who have severely compromised immune systems

Fast replyInvestigationalNo placebo
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What is this trial about?

A plain-language summary of the goals, design and what participants do

This study focuses on treating people with COVID-19 who have severely weakened immune systems. The research evaluates two medications: ibuzatrelvir, which is taken as a tablet by mouth, and remdesivir, which is given through a vein as an infusion. Some participants will receive ibuzatrelvir alone, some will receive both medications together, and others will receive placebo.

The purpose of this research is to determine how well ibuzatrelvir works when used alone or combined with remdesivir in treating COVID-19 symptoms and reducing the amount of virus in people with weakened immune systems. The study includes people who have received organ transplants, those with blood cancers, individuals who have received certain cancer treatments, or those taking medications that affect their immune system.

During the study, participants will receive the study medication for up to three days. They will be monitored for changes in their COVID-19 symptoms and the amount of virus in their body. The study will track whether participants need emergency care, hospital treatment, or experience other health problems related to COVID-19. The total study period is 38 days, during which participants will have regular check-ups to monitor their health and any side effects.

The research process

The trial runs in 5 steps – from screening to follow-up. Each step says what happens and what the team monitors.

  1. Step 1

    Initial evaluation and confirmation

    Your COVID-19 infection must be confirmed by a rapid antigen test (RAT) from a nasal sample taken within 2 days before starting the study

    Your symptoms must have started no more than 5 days before joining the study

    You must have at least one active COVID-19 symptom on the day of joining

  2. Step 2

    Treatment assignment

    You will be randomly assigned to receive one of three treatment options:

    Option 1: Ibuzatrelvir tablets taken by mouth + saline solution by intravenous infusion

    Option 2: Ibuzatrelvir tablets taken by mouth + Remdesivir given by intravenous infusion

    Option 3: Placebo tablets + saline solution by intravenous infusion

  3. Step 3

    Treatment period

    The study will monitor your progress for 38 days

    Regular nasal swabs will be collected to measure virus levels

    Your COVID-19 symptoms will be tracked throughout the study period

  4. Step 4

    Safety monitoring

    Your health will be monitored for any side effects or adverse reactions

    Emergency department visits related to COVID-19 will be recorded

    Any need for additional treatments such as oxygen, antibiotics, or other medications will be documented

  5. Step 5

    Final evaluation

    The study will assess how quickly your symptoms improved

    Final virus measurements will be taken from nasal samples

    Any hospital visits or health complications during the study period will be recorded

Who can join the trial?

8 criteria

  • Must be 18 years or older
  • Must have a confirmed COVID-19 infection through a rapid antigen test (RAT) or other approved test from a nasal sample taken within 2 days before starting the study
  • Must have started experiencing COVID-19 symptoms within 5 days before joining the study and still have at least one symptom on the day of joining
  • Must be severely immunocompromised due to one of these conditions:
    • Having received an organ or islet cell transplant and currently taking medications that suppress the immune system
    • Having an active blood cancer (such as chronic lymphocytic lymphoma, non-Hodgkin lymphoma, multiple myeloma, or acute leukemia)
    • Having received special immune cell therapy (CAR-T-cell therapy) or stem cell transplant within the last 2 years, or currently taking medications that suppress the immune system
    • Currently receiving treatments that reduce B-cells (a type of immune cell) such as rituximab
    • Both men and women can participate in the study

Who cannot join the trial?

14 criteria

  • Patients under 18 years of age cannot participate
  • Individuals who have received COVID-19 antiviral treatment in the past 7 days
  • People with severe liver problems (defined as liver enzymes more than 5 times above normal levels)
  • Patients with severe kidney problems (requiring dialysis or with severely reduced kidney function)
  • Pregnant or breastfeeding women
  • Individuals with known allergies to ibuzatrelvir or remdesivir (the study medications)
  • Patients who are participating in other clinical trials
  • People who have received a COVID-19 vaccine within the last 14 days
  • Individuals with uncontrolled heart conditions
  • Patients with severe mental health conditions that could affect their ability to follow study procedures
  • People who cannot provide informed consent
  • Individuals with a history of severe reactions to similar medications
  • Patients taking medications that could interact with the study drugs
  • People with unstable medical conditions that could interfere with the study results
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Investigated drugs

Ibuzatrelvir is an antiviral medication being studied for treating COVID-19 in people with weakened immune systems. It works by blocking a protein that the virus needs to multiply in the body. This medication is taken orally to help fight the coronavirus infection. Remdesivir is an established antiviral medication that is already approved for treating COVID-19. It works by interfering with the virus's ability to copy itself. This medication is given through an intravenous (IV) line directly into the bloodstream. The study also includes a combination therapy of ibuzatrelvir and remdesivir together, to see if using both medications provides better results than using either medication alone in people with severely compromised immune systems who have COVID-19.

What is already known about the treatment

  • Ibuzatrelvir

    An investigational oral antiviral medication being studied for the treatment of COVID-19 in severely immunocompromised patients. This drug is currently undergoing Phase 3 clinical trials and works by targeting specific viral proteins to inhibit SARS-CoV-2 replication. It is being evaluated both as a standalone treatment and in combination with other antivirals, particularly remdesivir.

  • Remdesivir

    An FDA-approved antiviral medication administered through intravenous infusion for treating COVID-19 in hospitalized patients. It works by interfering with the virus's ability to replicate its genetic material, effectively slowing down viral multiplication in the body. The drug belongs to the nucleoside analog class of antivirals and has shown particular promise in treating patients with severe COVID-19 infections, especially in immunocompromised individuals.

Investigated diseases

COVID-19 – A respiratory disease caused by the SARS-CoV-2 virus that primarily affects the lungs and respiratory system. The infection typically begins with symptoms such as fever, cough, fatigue, and loss of taste or smell. The virus spreads mainly through respiratory droplets when infected people breathe, talk, cough, or sneeze. The disease can range from mild to severe, with some people experiencing difficulty breathing and decreased oxygen levels. In immunocompromised individuals, the infection may last longer and symptoms can be more persistent.
Trial detailsLast updated 2 Oct 2026
Age18+ yearsPhasePhase IIITrial ID2024-517671-21-00Protocol codeC5091018Estimated enrolment300 patientsSponsorPfizer Inc.

sourced from the EU Clinical Trials Register and site verification

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On this site, “treatment” means an investigational medicine being studied in a clinical trial. Its safety and efficacy for the use being studied have not yet been confirmed, some participants may receive a placebo or a comparator medicine, and taking part does not guarantee any health benefit. The decision to take part is made by the doctor at the research site. This site is for information only and does not replace medical advice.

This service is not affiliated with the European Commission, the EMA, or the official CTIS system. Most information comes from publicly available international clinical-trial registries, supplemented by data from academic sites, national regulators and commercial sponsors. On this site, “treatment” and “therapy” mean a medicine being tested in a clinical trial. Its safety and effectiveness in the use being studied are not yet confirmed, some participants may receive a placebo or a comparator, and taking part does not guarantee a health benefit. The doctor at the research site decides who can take part. This site provides information, not medical advice. Certain content and visual elements on this website have been generated or enhanced using artificial intelligence (AI).