Merck & Co Inc on Thursday said it plans a large study of what could become the first pill to target the coronavirus in people at risk of severe COVID-19, but will no longer pursue use of the experimental antiviral drug in hospitalized patients.

The company said it would study the drug molnupiravir in a Phase 3 trial among COVID-19 patients out of hospitals who have at least one risk factor for poor outcomes, such as advanced age, obesity or diabetes.

Trial participants can have had symptoms for no more than five days, shortened from seven days in a prior study.

"We had some encouraging signals in the outpatient trial. That was not the case in the inpatient trial, but that was not unexpected," Roy Baynes, Merck's chief medical officer, told Reuters. He said COVID-19 patients who need hospital treatment are often in the "inflammatory" phase of the disease, which is too late for an antiviral drug to provide much help.

Molnupiravir, which Merck is developing with Ridgeback Biotherapeutics, is designed to interfere with the ability of the coronavirus to replicate.

Merck also said it discontinued development of an experimental immune modulator, known as MK-7110, for treating COVID-19 after concluding that clinical trial and manufacturing preparations would take too long.

The drugmaker earlier this year stopped development of two COVID-19 vaccines, citing data showing they were less effective than vaccines being developed by rivals.

Merck said its trials of molnupiravir found no drug-related adverse events that led participants to discontinue treatment and there were no drug-related deaths.

Patients in the Phase 3 trial will be given 800 mg of molnupiravir twice a day for five days.

Merck said it plans to present at an upcoming medical meeting details from earlier studies of the drug in both hospitalized and non-hospitalized patients. Results of the Phase 3 trial are expected by September or October, but interim analyses are planned.

Baynes said the company has ramped up production of molnupiravir and aims to have at least 10 million courses of therapy on hand later this year.

Merck also plans to study molnupiravir for preventing coronavirus infection in people exposed to the disease, starting in the second half of the year.