Because of its well-studied and documented effectiveness in treating malaria, there is every reason to believe that chloroquine may also slow or impair the entry of coronavirus into cells.
This from the link that JimT posted:
"The available data are thin. The drugs [chloroquine and hydroxychloroquine] work by decreasing the acidity in endosomes, compartments inside cells that they use to ingest outside material and that some viruses can coopt to enter a cell. But the main entryway for SARS-CoV-2 is a different one, using its so-called spike protein to attach to a receptor on the surface of human cells. Studies in cell culture have suggested chloroquines have some activity against SARS-CoV-2, but the doses needed are usually high—and could cause serious toxicities.
Encouraging cell study results with chloroquines against two other viral diseases, dengue and chikungunya, didn’t pan out in people in randomized clinical trials. And nonhuman primates infected with chikungunya did worse when given chloroquine. “Researchers have tried this drug on virus after virus, and it never works out in humans. The dose needed is just too high,” says Susanne Herold, an expert on pulmonary infections at the University of Giessen."