No medicines are proven to treat the new coronavirus which emerged from China late last year and has since jumped to scores of other countries.

With little else but supportive care to check the virus’ rapid spread, doctors and scientists have launched hundreds of trials in search of an effective therapy, testing mostly repurposed HIV or flu drugs.

Hopes are highest for remdesivir, an experimental antiviral developed by Gilead for use against Ebola but since pressed into study for the new coronavirus.

“There’s only one drug right now that we think may have real efficacy. And that’s remdesivir.” said Bruce Aylward, a senior advisor and international leader of the World Health Organization’s joint mission to China, at a Feb. 24 press conference.

Clinical trials are now underway in the U.S., China and, soon, other Asian countries where high numbers of people are infected by the virus, now named SARS-CoV-2. Preliminary results from the first of those studies could come by April, Gilead expects.

Other drugs designed specifically to treat SARS-CoV-2 are now in development but, for the next few months at least, much of the focus will be on remdesivir. Here’s a primer on how it works, its potential against the illness COVID-19, as well as why it’s no sure thing.

What kind of drug is remdesivir?

Remdesivir is designed to slow the infection of healthy cells by blocking viral replication, checking the ability of an invading virus to co-opt the body’s cellular machinery to reproduce itself.

In technical terms, it’s known as a nucleotide analog — meaning it mimics a genetic building block, but with an unusual molecular group tacked on. The thinking goes that, as the invading virus tries to replicate itself, the drug gets in the way and stops the process.

Remdesivir emerged from work done by Gilead and U.S. government in the mid-2010s to test compounds discovered by the biotech against emerging viruses. Initial animal experiments seemed to show the drug could work against Ebola, leading Gilead to start Phase 1 testing in 2015 and advance it into a mid-stage trial the next year.

A large Phase 3 study conducted in the Democratic Republic of the Congo, however, showed remdesivir to be less effective in preventing deaths from the virus than two other drugs, leaving Gilead’s therapy with an uncertain future.

But the outbreak of SARS-CoV-2 made suddenly relevant a series of experiments in human cell cultures and mice that indicated remdesivir could have activity against coronaviruses like MERS and, possibly, the new one now spreading.

What evidence suggests remdesivir could treat COVID-19?

Gilead has taken pains to emphasize, when announcing new plans for remdesivir, that it lacks human data on whether the drug actually works to treat COVID-19.

What it does have is preclinical data showing the drug to be active against the MERS and SARS viruses, which share enough genetic similarities to SARS-COV-2 that researchers and the company think remdesvir could work against it too.

Testing by National Institutes of Health researchers, published in January, demonstrated remdesivir prevented MERS from developing in monkeys, and helped improve the condition of those animals that were already infected.

“Our results, together with replication inhibition by remdesivir of a wide range of coronaviruses in vitro and in vivo, may further indicate utility of remdesivir against the novel coronavirus […] emerging from Wuhan, China,” they wrote.

Most recently, an in vitro study conducted by scientists at the Wuhan Institute of Virology found remdesivir and an anti-malarial called chloroquine more likely to be effective against SARS-CoV-2 than a handful of other potential treatments.​

As remdesivir has drawn attention, many have focused on its use in treating a Washington man who tested positive for SARS-CoV-2 after returning from Wuhan. One week into his hospitalization, doctors gave him remdesivir through a compassionate use program that Gilead has been supporting.

The man’s condition began to improve the next day, sparking some optimism about remdesivir’s benefit. But it’s not clear if the patient had already begun to recover, as viral levels were declining before the drug’s use.

How is remdesivir being tested now?

Early last month, doctors in Wuhan and Beijing opened two Phase 3 studies of remdesivir in coronavirus-infected patients with either mild to moderate illness or with severe disease. The former began enrolling on Feb. 6 and the latter a week later, on Feb. 13.

Together the trials aim to treat about 850 people and measure whether intravenous remdesivir can improve or resolve the symptoms of COVID-19 over the course of roughly a month.