Abstract: Since January 2020 Elsevier has created a COVID-19 resource centre with free information in English and Mandarin on the novel coronavirus COVID19. The COVID-19 resource centre is hosted on Elsevier Connect, the company's public news and information website. Elsevier hereby grants permission to make all its COVID-19-related research that is available on the COVID-19 resource centre - including this research content - immediately available in PubMed Central and other publicly funded repositories, such as the WHO COVID database with rights for unrestricted research re-use and analyses in any form or by any means with acknowledgement of the original source. These permissions are granted for free by Elsevier for as long as the COVID-19 resource centre remains active. International Journal of Antimicrobial Agents 55 (2020) 105932 Contents lists available at ScienceDirect International Journal of Antimicrobial Agents journal homepage: www.elsevier.com/locate/ijantimicag Hot Topic Chloroquine and hydroxychloroquine as available weapons to fight COVID-19 Repositioning of drugs for use as antiviral treatments is a critical need [1]. It is commonly very badly perceived by virologists, as we experienced when reporting the effectiveness of azithromycin for Zika virus [2]. A response has come from China to the respiratory disease caused by the new coronavirus (SARS-CoV-2) that emerged in December 2019 in this country. Indeed, following the very recent publication of results showing the in vitro activity of chloroquine against SARS-CoV-2 [3], data have been reported on the efficacy of this drug in patients with SARS-CoV-2-related pneumonia (named COVID-19) at different levels of severity [4,5]. Thus, following the in vitro results, 20 clinical studies were launched in several Chinese hospitals. The first results obtained from more than Table 1 Main results of studies on the activity of chloroquine or hydroxychloroquine on coronavirusesa Reference Compound(s) Targeted virus System used for antiviral activity screening [12] Chloroquine SARS-CoV [16] [17] Chloroquine Chloroquine, chloroquine monophosphate, chloroquine diphosphate Vero E6 Vero Vero SARS-CoV (four strains) (African green monkey kidney) cells E6 cells 76 cells BALB/c mice [18] Chloroquine, hydroxychloroquine SARS-CoV Vero cells Feline coronavirus Crandell–Reese feline kidney (CRFK) cells Human epithelial lung cells (L132) [19] Chloroquine HCoV-229E [20] Chloroquine HCoV-OC43 [21] Chloroquine [22] Chloroquine [3] Chloroquine Feline infectious peritonitis virus (FIPV) SARS-CoV MERS-CoV HCoV-229E-GFP (GFP-expressing recombinant HCoV-229E) SARS-CoV-2 HRT-18 cells Newborn C57BL/6 mice; chloroquine administration transplacentally and via maternal milk Felis catus whole fetus-4 cells Antiviral effect EC50 = 8.8 ± 1.2 μM EC50 = 4.4 ± 1.0 μM Chloroquine: EC50 = 1–4 μM Chloroquine monophosphate: EC50 = 4–6 μM Chloroquine diphosphate: EC50 = 3–4 μM Intraperitoneal or intranasal chloroquine administration, beginning 4 h prior to virus exposure: 50 mg/kg but not 10 mg/kg or 1 mg/kg reduced for the intranasal route (but not the intraperitoneal route) viral lung titres from mean ± S.D. of 5.4 ± 0.5 to 4.4 ± 1.2 in log10 CCID50 /g at Day 3 (considered as not significant) Chloroquine: EC50 = 6.5 ± 3.2 μM Hydroxychloroquine: EC50 = 34 ± 5 μM Chloroquine: EC50 > 0.8 μM Hydroxychloroquine: EC50 = 28 ± 27 μM Chloroquine at concentrations of 10 μM and 25 μM inhibited..