Abstract: Screening of an FDA-Approved Compound Library Identifies Four Small-Molecule Inhibitors of Middle East Respiratory Syndrome Coronavirus Replication in Cell Culture Adriaan H. de Wilde,a Dirk Jochmans,b Clara C. Posthuma,a Jessika C. Zevenhoven-Dobbe,a Stefan van Nieuwkoop,c Theo M. Bestebroer,c Bernadette G. van den Hoogen,c Johan Neyts,b Eric J. Snijdera Molecular Virology Laboratory, Department of Medical Microbiology, Leiden University Medical Center, Leiden, Netherlandsa; Rega Institute for Medical Research, KU, Leuven, Belgiumb; Department of Viroscience, Erasmus Medical Center, Rotterdam, Netherlandsc I n June 2012, a previously unknown coronavirus was isolated from a patient who died from acute pneumonia and renal failure in Saudi Arabia (1, 2). Since then, the virus, now known as the Middle East respiratory syndrome coronavirus (MERS-CoV) (3), has been contracted by hundreds of others in geographically distinct locations in the Middle East, and evidence for limited human-to-human transmission has accumulated (4). Travel-related MERS-CoV infections were reported from a variety of countries in Europe, Africa, Asia, and the United States, causing small local infection clusters in several cases (http://www.who.int/csr/disease /coronavirus_infections/en/). About 200 laboratory-confirmed human MERS cases were registered during the first 2 years of this outbreak, but recently, for reasons that are poorly understood thus far, this number has more than tripled within just 2 months’ time (April-May 2014 [5]). This sharp increase in reported infections has enhanced concerns that we might be confronted with a repeat of the 2003 severe acute respiratory syndrome (SARS) episode, concerns aggravated by the fact that the animal reservoir for MERS-CoV remains to be identified with certainty (6–9). Furthermore, at about 30%, the current human case fatality rate is alarmingly high, even though many deaths were associated with underlying medical conditions. MERS-CoV infection in humans can cause clinical symptoms resembling SARS, such as high fever and acute pneumonia, although the two viruses were reported to use different entry receptors, dipeptidyl peptidase 4 (DPP4) (10) and angiotensin-converting enzyme 2 (ACE2) (11), respectively. Coronaviruses are currently divided across four genera: Alphacoronavirus, Betacoronavirus, Gammacoronavirus, and Deltacoronavirus (12). MERS-CoV was identified as a member of lineage C August 2014 Volume 58 Number 8 of the genus Betacoronavirus (2), which also includes coronaviruses of bat (13, 14) and hedgehog (6) origin. Following the 2003 SARS epidemic, studies into the complex genome, proteome, and replication cycle of coronaviruses were intensified. Coronaviruses are enveloped viruses with a positive-sense RNA genome of unprecedented length (25 to 32 kb [12, 15, 16]). The crystal structures of a substantial number of viral nonstructural and structural proteins were solved, and targeted drug design was performed for some of those (reviewed in reference 17). Unfortunately, thus far, none of these efforts resulted in antiviral drugs that were advanced beyond the preclinical phase (18). The 2003 SARS-CoV epidemic was controlled within a few months after its onset, and since then, the virus has not reemerged, although close relatives continue to circulate in bat species (14). Consequently, the interest in anticoronavirus drug development has been limited, until the emer- Received 10 April 2014 Returned for..