In the Chinese trial remdesivir, given by intravenous infusion, failed to improve patients' condition or reduce the pathogen's presence in the bloodstream, according to draft documents published accidentally by the World Health Organization (WHO).
But details were thin and suggested limitations in interpreting the data that has not yet been fully reviewed.
A screenshot of the WHO posting, captured by the medical news
website STAT before it was taken down, said the trial enrolled 237 patients with 158 receiving remdesivir compared with 79 who got a placebo.
The rate of death was similar at 13.9% for remdesivir versus 12.8% in the control group.
"It's still not that large of a study and therefore the statistics coming out of the trial aren't exactly robust," Mizuho analyst Salim Syed said in a research note.
Gilead in a statement said the WHO posting included inappropriate characterizations of the study, which was terminated early due to low enrollment and, as a result, cannot be used to make statistically meaningful conclusions.
"The study results are inconclusive, though trends in the data suggest a potential benefit for remdesivir, particularly among patients treated early in disease," the company said without providing details to back up that assertion.
Doctors have speculated that an antiviral drug like remdesivir would likely be most effective when administered as early as possible in the course of the disease as it is designed to help keep the virus from replicating in the blood.
"You can put out a campfire, but once it becomes a wildfire it's hard to control," said Dr. Kevin Grimes, an infectious disease specialist at Houston Methodist Hospital who was not involved in the study.
The WHO said the draft manuscript is undergoing peer review before it will be officially released.
David Katz, chief investment officer at Matrix Asset Advisors, which owns Gilead shares, noted that multiple clinical studies remained ongoing for remdesivir and said, "we would not be selling (the stock) into weakness today."
Gilead is awaiting results of a trial of remdesivir, which previously failed as an Ebola treatment, in 400 patients hospitalized with severe Covid-19, with data expected later this month.
A separate trial in China testing the drug in patients with more moderate symptoms last week was also suspended due to a lack of eligible patients.
Brad Loncar, whose Loncar Investments runs a cancer immunotherapy ETF, was not ready to draw conclusions from the latest China data.
"Just as it was too much of a reach to be optimistic about positive anecdotes we have heard, I think it's too pessimistic to write it off based on this (incomplete) data."