The team from Tokyo University of Science and others mathematically modelled the effectiveness of mefloquine to predict its potential real-world impact, if applied to treat Covid-19.

They found that mefloquine could reduce the overall viral load in affected patients to under 7 per cent and shorten the "time-till-virus-elimination" by 6.1 days. Their findings are published in Frontiers in Microbiology.

To identify drugs with higher antiviral potency than existing antivirals, the team first screened approved anti-parasitic/anti-protozoal drugs.

They found that mefloquine had the highest anti-SARS-CoV-2 activity among the tested compounds. Upon testing it against other quinoline derivatives, such as hydrochloroquine, in a cell line mimicking the cell-based environments of human lung cells, they found it to be better.

"In our cell assays, mefloquine readily reduced the viral RNA levels when applied at the viral entry phase but showed no activity during virus-cell attachment. This shows that mefloquine is effective on SARS-COV-2 entry into cells after attachment on cell surface," said lead scientist Koichi Watashi, from the varsity.

To bolster mefloquine's antiviral activity, the scientists looked into the possibility of combining it with a drug that inhibits the replication step of SARS-CoV-2: Nelfinavir.

Interestingly, they observed that the two drugs acted in "synergy" and the drug combination showed greater antiviral activity than either showed alone, without being toxic to the cells in the cell lines themselves.

While the study must be succeeded by clinical trials, "the world can hope that mefloquine becomes a drug used to effectively treat patients with Covid-19," the researchers noted.