Abstract: BBRC Biochemical and Biophysical Research Communications 323 (2004) 264–268 www.elsevier.com/locate/ybbrc In vitro inhibition of severe acute respiratory syndrome coronavirus by chloroquine Els Keyaerts, Leen Vijgen, Piet Maes, Johan Neyts, Marc Van Ranst* Rega Institute for Medical Research, University of Leuven, 3000 Leuven, Belgium Received 11 August 2004 Available online 28 August 2004 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 lM) was significantly lower than its cytostatic activity; CC50 (261.3 ± 14.5 lM), 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. Ó 2004 Elsevier Inc. All rights reserved. Keywords: SARS-CoV; Severe acute respiratory syndrome; Coronavirus; Chloroquine; Antiviral activity Severe acute respiratory syndrome (SARS) has recently emerged as a new highly contagious human disease with a major impact all over the world [1]. The global SARS epidemic started in the Guangdong Province in southern China, where several cases of atypical pneumonia of unknown etiology were reported at the end of November 2002. A novel member of the Coronaviridae family has been identified as the causative agent of SARS [2–7]. Three other human coronaviruses (HCoV) OC43, 229E, and the recently characterized NL63 are important causes of upper respiratory tract illnesses. In late fall and winter they are responsible for approximately one-third of the common colds. During the epidemic in 2003, treatment of SARS was empirical due to the limited understanding of this new disease. Protease inhibitors (lopinavir/ritonavir) in combination with ribavirin may be of benefit as antiviral * Corresponding author. Fax: +32 16 347900. E-mail address: Marc.VanRanst@uz.kuleuven.ac.be (M.V. Ranst). 0006-291X/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.bbrc.2004.08.085 therapy, when given in the early phase of the illness [8,9]. The role of interferon and systemic steroids in preventing immune-mediated lung injury requires further investigation [10,11]. Since the epidemic, a lot of effort has been put into antiviral research to find compounds effective against SARS-CoV. Glycyrrhizin (an active component of liquorice roots), niclosamide (an antihelminthic drug), nelfinavir (a human immunodeficiency deficiency virus (HIV) protease inhibitor), and SNAP (a nitric oxide donor) were reported to have an antiviral effect against SARS-CoV [12–15]. Savarino et al. [16] hypothesized that chloroquine might be of some use for the clinical management of SARS. Chloroquine is known as an antimalarial agent and elicits also antiviral effects against several viruses including HIV type 1 (HIV-1) [17–19], hepatitis B virus [20], herpes simplex virus..