Merck spokesperson Melissa Moody said Merck and the Indian companies had defined “moderate” disease differently.
Merck's trials are based on U.S. Food and Drug Administration definitions, which for moderate COVID-19 describe blood oxygen levels as no lower than 93%. It defines blood oxygen levels for severe COVID-19 as 93% or lower.
The trials in India define moderate COVID-19 blood oxygen levels as 90% to 93%, according to the trial documents for the two companies.
Aurobindo and MSN are continuing to conduct studies of molnupiravir in patients with mild COVID-19 who have not been hospitalized, according to trial documents and the website of the Indian drug regulator's internal expert committee.
Merck and partner Ridgeback Biotherapeutics last week said molnupiravir had nearly halved the risk of hospitalization or death in at-risk non-hospitalized patients with mild-to-moderate COVID-19, results hailed by experts as potentially a major advance in fight against the illness.
The Indian drug regulator's committee also disclosed on its website that Aurobindo and MSN had presented interim clinical trial data for moderate COVID-19 patients and asked to end the trials.
A source with the Drug Controller General of India said the pill has not shown "significant efficacy" against moderate COVID-19, though it was having success against mild cases.
An Aurobindo spokesperson did not comment on the effectiveness of the drug in its trial. The spokesperson said the company had "faced challenges in patient recruitment at this juncture" for its trial in moderate COVID-19 patients, adding "we are evaluating the further course of action."
Aurobindo started the planned 100-patient trial of its generic version of the Merck pill against moderate COVID-19 in August, and is also running a trial that hopes to include 1,200 patients testing the pill against mild disease.
MSN did not respond to a request for comment.
Merck, whose shares closed down 1.6% at $80.63, has entered into voluntary licensing agreements with at least eight Indian drugmakers for molnupiravir, with an aim to turn the South Asian nation into a manufacturing hub for the drug to supply low- and middle-income nations.
Five of the eight Indian companies - Dr Reddy's Laboratories, Cipla, Sun Pharma, Torrent Pharmaceuticals and Emcure Pharmaceuticals - are conducting a joint trial for the antiviral drug only in mild COVID-19 patients in an outpatient setting.
The other licensed company, Hetero, in early July announced interim data from its own late-stage trial of generic molnupiravir in mild COVID-19 patients and submitted an application to regulators for its emergency use.
Hetero is separately conducting a study on moderate COVID-19 patients and has said those results will be presented at a future date.
(Reporting by Shivani Singh and Anuron Kumar Mitra in Bengaluru and Neha Arora in New Delhi, additional reporting by Deena Beasley; Editing by Caroline Humer and Bill Berkrot) ((ShivaniSingh1@thomsonreuters.com; +91 89 6969 2349))