The emergence of a new highly pathogenic virus in animal as
well as human populations presents a unique challenge for both veterinarians
and physicians alike since vaccines more often than not are not readily
available, leaving antiviral treatments the only option to contain an outbreak.
Pharmaceuticals however take time to be developed and tested thus the only
option available is to identify the antiviral pathways targeted by viral
proteins in the hope that existing drugs are available and effective in activating
these pathways and thus suppress viral replication. In the meantime, clinicians
can only offer supportive care and use serum from convalescent patients - often
a scarce commodity and not readily available. As an alternative however
pharmaceuticals used and approved for the treatment of other viral diseases or
indeed for other diseases might be repurposed.
The recent emergence of both the SARS-CoV in 2002 and
MERS-CoV in 2012 have lead to substantial increase in Coronaviruses as a
potential human pathogen. Following the emergence of MERS-CoV, the
International Respiratory and Emerging Infection Consortium (ISARIC) compiled a
list of pharmaceuticals available to physicians based on the experience gained
during the SARS-CoV epidemic in 2002/2003, with the most promising drugs being
Interferon and Ribavirin, which had been used in combination as well as
separate to treat SARS-CoV and pandemic Influenza A/2009 patients. Indeed, both
drugs are effective to prevent MERS-CoV replication in a rhesus macaque model
but failed to be effective in patients with a severe infection. A screen of
chemical library of 1280 pharmaceuticals known to be effective against
Influenza A was also assessed for their ability to reduce viral yield and
prevent the cytopathic effect following the infection of cells with MERS-CoV
confirmed that at least under laboratory conditions MERS-CoV is sensitive to
Interferon as well as to two antiretroviral drugs, nefinavir and lopinavir. At
first it may seem surprising that two antiretroviral drugs can prevent the
replication of a Coronavirus. Both drugs were developed to prevent the
replication of HIV by targeting the HIV protease. As discussed before however,
the Coronavirus genome encodes for a protease, 3CLpro which is
required for the processing of the orf1ab polyprotein and has been shown
sensitive to nefinavir and lopinavir due to the inhibition of the viral 3CLpro protease. Both drugs are non-specific for
MERS-CoV and also effective in treating SARS-CoV related infections.
Other targets of antiviral therapy most certainly include
preventing viral entry. As discussed in a previous post, monoclonal antibodies against the
viral S protein and small molecules binding to the receptor-binding site of the
S protein have been shown to be effective to neutralize viral particles.
Another possibility is to target the release of the viral genome into the
cytoplasm of the cell, which is dependent on a low pH within the endosome. The
application of a lysosomotropic agent such as Chloroquine/Hydroxychloroquine (the
protonated form of Chloroquine) (an antimalarial drug) or NH4Cl might therefore
prevent the fusion of the virus with the endosome by raising the pH. Indeed the
application of low doses of Chloroquine to cells infected with SARS-CoV or
MERS-CoV as well as Influenza A have shown to prevent viral replication. In
addition to prevent the fusion of the viral particle with the endosome,
Chloroquine might also prevent the glycosylation of ACE2, the receptor for
SARS-CoV and thus prevent binding of the SARS-CoV S1 subunit to its receptor
(it remains to be seen if this is the case with DPP4, the receptor for
MERS-CoV). The glycosylation of proteins is targeted by inhibiting
glycosyltransferases, namely quinone reductase 2, which is involved in the
biosynthesis of sialic acid, a component of cellular receptors. Sialic acid
moieties are also present within the glycoproteins of HIV-1 glycoproteins, the
SARS-CoV receptor ACE2, the MERS-CoV receptor DPP4/CD26, Coronavirus S proteins
as well in the receptors for Influenza A thus explaining the broad spectrum
activity of Chloroquine. Quinone reductase 2 inhibitors therefore reduce the
glycosylation of SARS S proteins although it seems that the reduction has no
effect on viral infectivity (or only a marginal effect).
So far however its effectiveness has not been demonstrated
in the animal model of MERS and studies with Influenza A have shown that
Chloroquine -although effective in cell lines- is not effective in humans thus
adding some caution. Apart from being a potential pharmaceutical against a
variety of human Coronaviruses, Chloroquine is well tolerated and better known
in treating in Malaria patients at therapeutic doses in micro molar
concentrations. Pharmaceuticals
effective specifically against MERS-CoV, two pharmaceuticals emerged recently,
mycophenolic acid (MPA) and IFN-β, both of which have been
discussed previously.
As outlined previously, the polyprotein 1ab is processed
further by auto proteolysis that generates a number of nonstructural proteins
varying among the Coronaviridae, which includes not the RNA dependent RNA
Polymerase (RdRp) but also an NTPase/Helicase known as nsp 12 and 13
respectively. In simian Vero E6 cells
infected with SARS-CoV these are located within perinuclear double membrane
bound vesicles representing replication-transcription complexes containing
nascent viral subgenomic RNAs, RdRp as well as viral positive strand RNA and
dsRNA intermediates which are resolved by the viral Helicase. Although the
precise mechanism and specific function of the Coronavirus Helicase is not
known, the replication of SARS-CoV, MERS-CoV, and the murine MHV can
effectively inhibited by a small compound, SSYA10-001, targeting the Helicase
at amino acid residues K508, R507, and Y277 respectively, thus offering a
potential broad spectrum inhibitor of Coronavirus mediated infections and highlighting the importance of the Coronavirus Helicase for viral replication since inhibition of the SARS-CoV Helicase by Bismuth has been shown to inhibit SARS-CoV replication in the past. In
addition to its wide spectrum of antiviral activity, SSYA10-001 exhibits only
minimal cytotoxicity if applied to cells. The viral RdRp itself can be inhibited by combination of Ribavirin and 5-Flourouracil, the latter being mutagenic and thus sensitizing infected cells to Ribavirin treatment (Ribavirin itself being ineffective).
Overview of potential and existing antiviral strategies to treat Coronavirus infections |
In conclusion, whilst future outbreaks of novel respiratory
viruses cannot prevented, pharmaceuticals which are already available might be
used in the treatment during a pandemic or an epidemic whilst bioinformatics in
conjunction with the identification of ways that viral proteins interact with
the host cell might identify effective broad spectrum inhibitors which target
highly conserved proteins. A recent screen of potential antiviral
pharmaceuticals revealed that even antipsychotic drugs can have an antiviral
effect against MERS-CoV, revealing the hidden potential of many drugs already
approved.
Further reading
Dyall J, Coleman CM, Hart BJ, Venkataraman T, Holbrook MR, Kindrachuk J, Johnson RF, Olinger GG Jr, Jahrling PB, Laidlaw M, Johansen LM, Lear CM, Glass PJ, Hensley LE, & Frieman MB (2014). Repurposing of clinically developed drugs for treatment of Middle East Respiratory Coronavirus Infection. Antimicrobial agents and chemotherapy PMID: 24841273
Falzarano D, de Wit E, Rasmussen AL, Feldmann F, Okumura A, Scott DP, Brining D, Bushmaker T, Martellaro C, Baseler L, Benecke AG, Katze MG, Munster VJ, & Feldmann H (2013). Treatment with interferon-α2b and ribavirin improves outcome in MERS-CoV-infected rhesus macaques. Nature medicine, 19 (10), 1313-7 PMID: 24013700
Falzarano D, de Wit E, Martellaro C, Callison J, Munster VJ, & Feldmann H (2013). Inhibition of novel β coronavirus replication by a combination of interferon-α2b and ribavirin. Scientific reports, 3 PMID: 23594967
Chan, J., Chan, K., Kao, R., To, K., Zheng, B., Li, C., Li, P., Dai, J., Mok, F., Chen, H., Hayden, F., & Yuen, K. (2013). Broad-spectrum antivirals for the emerging Middle East respiratory syndrome coronavirus Journal of Infection, 67 (6), 606-616 DOI: 10.1016/j.jinf.2013.09.029
Kilianski A, & Baker SC (2014). Cell-based antiviral screening against coronaviruses: developing virus-specific and broad-spectrum inhibitors. Antiviral research, 101, 105-12 PMID: 24269477
Al-Tawfiq JA, Momattin H, Dib J, & Memish ZA (2014). Ribavirin and interferon therapy in patients infected with the Middle East respiratory syndrome coronavirus: an observational study. International journal of infectious diseases : IJID : official publication of the International Society for Infectious Diseases, 20, 42-6 PMID: 24406736
Smith EC, Blanc H, Vignuzzi M, & Denison MR (2013). Coronaviruses lacking exoribonuclease activity are susceptible to lethal mutagenesis: evidence for proofreading and potential therapeutics. PLoS pathogens, 9 (8) PMID: 23966862
Hart BJ, Dyall J, Postnikova E, Zhou H, Kindrachuk J, Johnson RF, Olinger GG Jr, Frieman MB, Holbrook MR, Jahrling PB, & Hensley L (2014). Interferon-β and mycophenolic acid are potent inhibitors of Middle East respiratory syndrome coronavirus in cell-based assays. The Journal of general virology, 95 (Pt 3), 571-7 PMID: 24323636
Coleman CM, Liu YV, Mu H, Taylor JK, Massare M, Flyer DC, Glenn GM, Smith GE, & Frieman MB (2014). Purified coronavirus spike protein nanoparticles induce coronavirus neutralizing antibodies in mice. Vaccine, 32 (26), 3169-74 PMID: 24736006
Keyaerts E, Li S, Vijgen L, Rysman E, Verbeeck J, Van Ranst M, & Maes P (2009). Antiviral activity of chloroquine against human coronavirus OC43 infection in newborn mice. Antimicrobial agents and chemotherapy, 53 (8), 3416-21 PMID: 19506054
Savarino A, Di Trani L, Donatelli I, Cauda R, & Cassone A (2006). New insights into the antiviral effects of chloroquine. The Lancet infectious diseases, 6 (2), 67-9 PMID: 16439323
Vincent MJ, Bergeron E, Benjannet S, Erickson BR, Rollin PE, Ksiazek TG, Seidah NG, & Nichol ST (2005). Chloroquine is a potent inhibitor of SARS coronavirus infection and spread. Virology journal, 2 PMID: 16115318
van Hemert, M., van den Worm, S., Knoops, K., Mommaas, A., Gorbalenya, A., & Snijder, E. (2008). SARS-Coronavirus Replication/Transcription Complexes Are Membrane-Protected and Need a Host Factor for Activity In Vitro PLoS Pathogens, 4 (5) DOI: 10.1371/journal.ppat.1000054
Adedeji AO, Singh K, Kassim A, Coleman CM, Elliott R, Weiss SR, Frieman MB, & Sarafianos SG (2014). Evaluation of SSYA10-001 as a Replication Inhibitor of SARS, MHV and MERS Coronaviruses. Antimicrobial agents and chemotherapy PMID: 24841268
Yang N, Tanner JA, Wang Z, Huang JD, Zheng BJ, Zhu N, & Sun H (2007). Inhibition of SARS coronavirus helicase by bismuth complexes. Chemical communications (Cambridge, England) (42), 4413-5 PMID: 17957304
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