The Europeans showed chloroquine worked against SARS-1 in 2004. CDC confirmed their findings the next year

https://www.sciencedirect.com/science/article/pii/S0006291X0401839X

In vitro inhibition of severe acute respiratory syndrome coronavirus by chloroquine

Author links open overlay panelElsKeyaertsLeenVijgenPietMaesJohanNeytsMarc VanRanst

https://doi.org/10.1016/j.bbrc.2004.08.085Get rights and content

Abstract

We report on chloroquine, a 4-amino-quinoline, as an effective inhibitor of the replication of the severe acute respiratory syndrome coronavirus (SARS-CoV) in vitro. Chloroquine is a clinically approved drug effective against malaria. We tested chloroquine phosphate for its antiviral potential against SARS-CoV-induced cytopathicity in Vero E6 cell culture. Results indicate that the IC50 of chloroquine for antiviral activity (8.8 ± 1.2 μM) was significantly lower than its cytostatic activity; CC50 (261.3 ± 14.5 μM), yielding a selectivity index of 30. The IC50 of chloroquine for inhibition of SARS-CoV in vitro approximates the plasma concentrations of chloroquine reached during treatment of acute malaria. Addition of chloroquine to infected cultures could be delayed for up to 5 h postinfection, without an important drop in antiviral activity. Chloroquine, an old antimalarial drug, may be considered for immediate use in the prevention and treatment of SARS-CoV infections.