Chloroquine (and a derivative, hydroxychloroquine) has been used for years in the treatment of malaria. The drug is also known to block the entry of many viruses into cells. A small clinical trial has revealed it to be effective in reducing viral loads in COVID-19 patients.
Entry of enveloped viruses into cells requires fusion of viral and cell membranes to allow release of the viral genome into the cytoplasm. In many cases, after virus particles bind to cell surface receptors, they are taken into cells via the endocytic pathway. As these vesicles move toward the nucleus, their pH drops, which catalyzes fusion of viral and cell membranes. When cells are treated with chloroquine the pH of the endosome is elevated, which prevents fusion and blocks viral infection. The reproduction of many viruses is known to be inhibited by chloroquine.
Reproduction of SARS-CoV-1 in cell culture was shown to be blocked by chloroquine in 2005, by which time there were no human infections. Recently reproduction of the newly emerged SARS-CoV-2 in cells was found to be inhibited by chloroquine.