3.2. Viral Proteases
The 3-chymotrypsin-like protease (3CLPro) and a papain-like protease
(PLPro) function to process NSPs including RdRp, and therefore,
inhibition of these proteases should interfere the activity of RdRp
(Rathnayake et al., 2020). The antiviral and cell protection efficacy of
3CLPro inhibition has been illustrated in simian Vero cells infected by
SARS-CoV-2 (Jin et al., 2020).
Several candidates have been found to have inhibitory effects on 3CLPro.
Anti-hepatitis C virus (HCV) drug ravidasvir has the ability to bind and
inhibit the 3CLPro of SARS-CoV-2 (Bera, 2021). Similarly, HCV protease
inhibitors paritaprevir and simeprevir were also identified as potential
inhibitors of SARS-CoV-2 3CLPro (Alamri et al., 2020). Using
computational molecular modeling to screen FDA approved drugs and
subsequent studying for their inhibitory effects on SARS-CoV-2 3CLpro
enzyme in vitro , boceprevir, ombitasvir, paritaprevir,
tipranavir, ivermectin, and micafungin were found to exhibit inhibitory
effect towards 3CLpro enzymatic activity (Mody et al., 2021).
The efficacy of 3CLPro inhibitors in COVID-19 need to be proved.
Lopinavir is a highly potent inhibitor of the human immunodeficiency
virus (HIV) protease essential for intracellular HIV assembly.
Concomitant oral administration of lopinavir and ritonavir, which blocks
the metabolism of lopinavir, increases the anti-viral potency of
lopinavir. However, COVID-19 patients receiving the combined treatment
lopinavir and ritonavir yielded no significant benefit compared to
patients treated with standard-care (Cao et al., 2020a). More specific
inhibitors of proteases for SARS-CoV-2 should be evaluated in COVID-19,
since a study indicated that lopinavir and ritonavir did not significant
inhibit 3ClPro in an in vitro enzymatic assays (Mahdi, Motyan,
Szojka, Golda, Miczi & Tozser, 2020). The aforementioned 3CLPro
inhibitors may benefit COVID-19 patients, which is needed to be proved
in future clinical trials.
In regard of PLPro, activity profiling and crystal structures of
inhibitor study indicated that there is a very high level of sequence
and structural similarity between SARS-CoV and SARS-CoV-2 PLPro in the
substrate binding pocket, suggesting that SARS-CoV PLPro inhibitors can
possibly inhibit SARS-CoV-2 PLPro (Rut et al., 2020). Non-covalent small
molecule SARS-CoV PLPro inhibitors has been shown to inhibit SARS-CoV-2
PLPro and display antiviral activity in a SARS-CoV-2 infection model
(Klemm et al., 2020). Biochemical, structural, and functional
characterization investigation revealed that SARS-CoV and SARS-CoV-2
PLPro share 83% sequence identity but exhibit different host substrate
preferences. SARS-CoV-2 PLPro preferentially bind to the ubiquitin-like
interferon-stimulated gene 15 protein (ISG15), cleaving ISG15 from
interferon responsive factor 3 (IRF3) and attenuating type I interferon
responses. Whereas SARS-CoV PLPro predominantly targets ubiquitin
chains. Inhibition of SARS-CoV-2 PLPro impairs the virus-induced
cytopathogenic effect, maintains the antiviral interferon pathway and
reduces viral replication in infected cells (Shin et al., 2020).