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Merck said Thursday that it has seen “encouraging” results in a clinical trial of an antiviral pill to treat Covid-19 early in the disease’s course, but the pill failed to help hospitalized patients and must be used very early in the disease.

At the same time, the company said it will stop efforts to develop a second medicine for patients who have already been hospitalized with the disease.


The drug giant licensed the anti-Covid pill, molnupiravir, from Ridgeback Biotherapeutics last July. In November, Merck spent $425 million to buy OncoImmune, in order to obtain an immune-modifying treatment that already seemed to have a benefit for patients hospitalized with Covid-19.

Roy Baynes, the chief medical officer at Merck Research Laboratories, told STAT that proving the OncoImmune drug’s benefit to regulators would require another study, and that there is still work to be done on manufacturing the therapy. “Given where we are, we feel we can bring more value focusing on the full development of molnupiravir,” Baynes said.

Baynes said that a 302-patient study of molnupiravir in people with Covid-19 who have not been hospitalized has shown signs of benefit for the drug.  The percentage of patients who were hospitalized or died in the treatment groups was lower than in the placebo group, but Merck said “the number of events reported are not sufficient to provide a meaningful measure of clinical effect.” Baynes said the medicine showed “a very clear antiviral effect.” Actual data will be presented at a medical meeting.


Merck said that although there was not enough data to determine whether the medicine helps prevent patients who are newly infected from becoming hospitalized, although the benefits were most pronounced in those who received treatment within five days of developing symptoms.

“The biggest impact is for folks early on in their disease course in terms of the antiviral effect,” Baynes said. The Phase 3 portion of the study will be changed to say that in order to enroll, patients will need to be within five days of the onset of their symptoms, not seven. Merck will test the highest dose of the treatment, 800 milligrams given twice a day. Phase 3 results are expected in September or October.

Merck said that the number of adverse events with molnupiravir appeared similar to placebo.

However, the company said that it did not see a sign of a benefit giving the treatment to patients who were already hospitalized, and will stop studying molnupiravir for that population.

A  randomized controlled trial of the OncoImmune drug in 203 patients had been completed before Merck purchased the smaller company. But in February, the company revealed that the Food and Drug Administration wanted more data on the medicine. Baynes said that both the need to conduct additional studies and to create manufacturing capacity would mean the experimental medicine might not be available until 2022. Merck will continue to test the OncoImmune treatment in other diseases, he said.

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