The 3C-like proteinase (3CLpro) of severe acute respiratory syndrome-associated coronavirus (SARS-CoV)
is one of the most promising targets for anti-SARS-CoV drugs due to its crucial role in the viral life cycle. In
this study, a database containing structural information of more than 8,000 existing drugs was virtually
screened by a docking approach to identify potential binding molecules of SARS-CoV 3CLpro. As a target for
screening, both a homology model and the crystallographic structure of the binding pocket of the enzyme were
used. Cinanserin (SQ 10,643), a well-characterized serotonin antagonist that has undergone preliminary
clinical testing in humans in the 1960s, showed a high score in the screening and was chosen for further
experimental evaluation. Binding of both cinanserin and its hydrochloride to bacterially expressed 3CLpro of
SARS-CoV and the related human coronavirus 229E (HCoV-229E) was demonstrated by surface plasmon
resonance technology. The catalytic activity of both enzymes was inhibited with 50% inhibitory concentration
(IC50) values of 5 M, as tested with a fluorogenic substrate. The antiviral activity of cinanserin was further
evaluated in tissue culture assays, namely, a replicon system based on HCoV-229E and quantitative test assays
with infectious SARS-CoV and HCoV-229E. All assays revealed a strong inhibition of coronavirus replication
at nontoxic drug concentrations. The level of virus RNA and infectious particles was reduced by up to 4 log
units, with IC50 values ranging from 19 to 34 M. These findings demonstrate that the old drug cinanserin is
an inhibitor of SARS-CoV replication, acting most likely via inhibition of the 3CL proteinase.