Abstract: ANTIMICROBIAL AGENTS AND CHEMOTHERAPY, Aug. 2009, p. 3416–3421 0066-4804/09/$08.00⫹0 doi:10.1128/AAC.01509-08 Copyright © 2009, American Society for Microbiology. All Rights Reserved. Vol. 53, No. 8 Antiviral Activity of Chloroquine against Human Coronavirus OC43 Infection in Newborn Mice䌤 Els Keyaerts, Sandra Li, Leen Vijgen, Evelien Rysman, Jannick Verbeeck, Marc Van Ranst,* and Piet Maes Laboratory of Clinical Virology, Department of Microbiology and Immunology, Rega Institute for Medical Research, University of Leuven, Leuven, Belgium Received 12 November 2008/Returned for modification 24 December 2008/Accepted 9 April 2009 Until recently, human coronaviruses (HCoVs), such as HCoV strain OC43 (HCoV-OC43), were mainly known to cause 15 to 30% of mild upper respiratory tract infections. In recent years, the identification of new HCoVs, including severe acute respiratory syndrome coronavirus, revealed that HCoVs can be highly pathogenic and can cause more severe upper and lower respiratory tract infections, including bronchiolitis and pneumonia. To date, no specific antiviral drugs to prevent or treat HCoV infections are available. We demonstrate that chloroquine, a widely used drug with well-known antimalarial effects, inhibits HCoV-OC43 replication in HRT-18 cells, with a 50% effective concentration (ⴞ standard deviation) of 0.306 ⴞ 0.0091 ␮M and a 50% cytotoxic concentration (ⴞ standard deviation) of 419 ⴞ 192.5 ␮M, resulting in a selectivity index of 1,369. Further, we investigated whether chloroquine could prevent HCoV-OC43-induced death in newborn mice. Our results show that a lethal HCoV-OC43 infection in newborn C57BL/6 mice can be treated with chloroquine acquired transplacentally or via maternal milk. The highest survival rate (98.6%) of the pups was found when mother mice were treated daily with a concentration of 15 mg of chloroquine per kg of body weight. Survival rates declined in a dose-dependent manner, with 88% survival when treated with 5 mg/kg chloroquine and 13% survival when treated with 1 mg/kg chloroquine. Our results show that chloroquine can be highly effective against HCoV-OC43 infection in newborn mice and may be considered as a future drug against HCoVs. PHEV was remarkably divergent from HCoV-OC43 and BCoV (30, 31, 33). HCoVs cause respiratory infections, but gastroenteritis and neurological disorders can also occur (3, 20). Until now, five HCoVs have been described. HCoV-OC43 and HCoV-229E are responsible for 10 to 30% of all common colds, and infections occur mainly during the winter and early spring (21). In 2003, a novel HCoV displaying only distant antigenic and genetic similarities to the two previously known HCoVs was identified as the causal agent of SARS, and this causes severe lung disorder, leading in some cases to systemic infection and eventually death in about 10% of cases (19). During the two years after the SARS outbreak, two additional previously unrecognized coronaviruses affecting humans, HCoV-NL63 and HCoV-HKU1, were identified (29, 35). HCoV-NL63 infection is related to acute respiratory dysfunction in infected individuals. Furthermore, HCoV-NL63 was identified as the major pathogen responsible for croup in young children (9, 10, 22). The clinical features of HCoV-NL63 infections appear to be more severe than those commonly attributed to infections by HCoV-OC43 and HCoV-229E (4, 12, 29). Infection with HCoV-HKU1 is mostly associated with bronchiolitis and pneumonia (35,..