5  DRUGS REPOSITIONING APPROACH
Drug repurposing is an attractive strategy for finding new indications for already well‐established, marketed drugs or highly characterized compounds. It is a fast way to identify of new therapeutic options directly available for clinical use or eligible for accelerated approval for various diseases and disorders. An extensive effort has been made in repurposing approved drugs since the outbreak of SARS‐CoV‐1. Below, we summarize selected drug repositioning strategies for anticoronaviral therapy and their results.
The host's innate interferon (IFN) response is one key for controlling viral replication. The IFN response can be increased by administering artificial IFNs and IFN inducers. The recombinant IFN‐α and ‐β inhibited the replication of SARS‐ and MERS‐CoVs in animal models. 231  Several studies also described the combination of IFNs with antiviral drugs like ribavirin (180) or lopinavir‐ritonavir for treating SARS. 232 ,  233
In 2004, SARS patients in an open‐label study had better clinical outcomes when treated with ribavirin in combination with lopinavir‐ritonavir (400 and 100 mg, respectively) than the control group receiving only ribavirin. 227  A study in SARS patients found that viral replication could not be blocked at ribavirin concentrations achievable in human serum. 234  Nevertheless, the combination of ribavirin with IFN‐β had a synergistic effect on the inhibition of SARS‐CoV‐1 replication. The effects of PEGylated IFN together with ribavirin against SARS‐CoV‐2 are being studied in clinical trials. 87
Nitazoxanide (178; Figure 39), a broad‐spectrum antiparasitic drug, was reported to inhibit SARS‐CoV‐2 (EC50, 2.12 μM in Vero E6 cells). 215  It is also an IFN‐inducing agent, and it is being studied for treating a wide range of infections.
Figure 39  Selected structures of drugs suitable for repositioning against SARS‐CoV‐1 and 2. SARS‐CoV, severe acute respiratory syndrome coronavirus The antiarrhythmic drug amiodarone (179, Figure 39) also inhibited SARS‐CoV‐1 replication in infected Vero cells. 235  The drug appears to alter the endocytotic pathway, thus inhibiting endosomal viral entry.
Glycyrrhizin inhibited viral replication in Vero cells with an EC50 value of 300 mg/L, possibly by blocking viral entry as well. 232
As nitric oxide (NO) has been associated with antiviral activity, the NO donor, S‐nitroso‐N‐acetylpenicillamine (180; Figure 39) was reported to inhibit SARS‐CoV‐1 replication in a dose‐dependent manner. 236
In a search for potential antiviral agents against SARS‐CoV‐1, the screening of a library of 8000 approved drugs identified cinanserin (150; Figure 39), a serotonin antagonist, as a potential inhibitor of SARS‐CoV‐1 targeting its Mpro with IC50 value 4.0 µM. 189
A virtual screening and docking study identified the calmodulin antagonist calmidazolium as a SARS‐CoV‐1 Mpro inhibitor (K i, 61 µM). 237
In 2014, Dyall et al. reported an array of pharmaceutical drugs with antiviral activity against MERS‐CoV, and SARS‐CoV‐1 (Table 2, chemical structure of all drugs were indicated in Figure S1). 121  The agents were grouped according to their modes of action. Hits inhibited both investigated CoVs.
Table 2  Compounds with inhibitory activity at MERS‐CoV and SARS‐CoV‐1
Drugs  Class  MERS‐CoVEC50 (µM)  SARS‐CoV‐1EC50 (µM)
Emetine  Antibacterial agent  0.014  0.051
Chloroquine  Antiparasitic agent  6.27  6.53
Hydroxychloroquine  Antiparasitic agent  8.27  7.96
Mefloquine  Antiparasitic agent  7.41  15.55
Amodiaquine  Antiparasitic agent  6.21  1.27
Loperamide  Antidiarrheal agent  4.8  5.90
Lopinavir  HIV‐1 inhibitor  8.0  24.4
E‐64‐D  Cathepsin inhibitor  1.27  0.76
Gemcitabine  DNA metabolism inhibitor  1.21  4.95
Tamoxifen  Estrogen receptor inhibitor  10.11  92.88
Toremifene  Estrogen receptor inhibitor  12.91  11.96
Terconazole  Sterol metabolism inhibitor  12.20  15.32
Triparanol  Sterol metabolism inhibitor  5.28  ‐
Anisomycin  Protein‐processing inhibitor  0.003  0.19
Cycloheximide  Protein‐processing inhibitor  0.189  0.04
Homoharringtonine  Protein‐processing inhibitor  0.071  ‐
Benztropine  Neurotransmitter inhibitor  16.62  21.61
Fluspirilene  Neurotransmitter inhibitor  7.47  5.96
Thiothixene  Neurotransmitter inhibitor  9.29  5.31
Chlorpromazine  Neurotransmitter inhibitor  9.51  12.97
Fluphenazine  Neurotransmitter inhibitor  5.86  21.43
Promethazine  Neurotransmitter inhibitor  11.80  7.54
Astemizole  Neurotransmitter inhibitor  4.88  5.59
Chlorphenoxamine  Neurotransmitter inhibitor  12.64  20.03
Thiethylperazine  Neurotransmitter inhibitor  7.86  ‐
Triflupromazine  Neurotransmitter inhibitor  5.75  6.39
Clomipramine  Neurotransmitter inhibitor  9.33  13.23
Imatinib  Kinase signaling inhibitor  17.68  9.82
Dasatinib  Kinase signaling inhibitor  5.46  2.10
John Wiley & Sons, Ltd. This article is being made freely available through PubMed Central as part of the COVID-19 public health emergency response. It can be used for unrestricted research re-use and analysis in any form or by any means with acknowledgement of the original source, for the duration of the public health emergency. In particular, the protein‐processing inhibitors cycloheximide and anisomycin showed strong inhibitory activities against both CoVs. The HIV protease inhibitor lopinavir was more effective against SARS‐CoV‐1 than against MERS‐CoV. The antidiarrheal agent loperamide showed moderate inhibitory activitiy against both CoVs. The anti‐protozoal and emetic alkaloid with antibacterial properties, emetine, showed strong antiviral activity against MERS‐CoV. The antiparasitic drugs chloroquine, hydroxychloroquine, and mefloquine showed moderate antiviral activities against both CoVs. Cathepsin inhibitor, E‐64‐D, inhibited both as well. Two of the neurotransmitter inhibitors, chlorpromazine and triflupromazine also blocked both viruses (see Section 2) The DNA synthesis inhibitor gemcitabine was able to inhibit SARS‐CoV‐1 and MERS‐CoV with an EC50 value of 1.2 and 4.9 µM, respectively. Toremifene is an estrogen receptor 1 antagonist that inhibited both MERS‐CoV and SARS‐CoV‐1 (EC50, 12.9 and 11.97 µM, respectively).
Kinase signaling pathway inhibitors imatinib and dasatinib were active against both MERS‐CoV and SARS‐CoV‐1. Imatinib was reported to act at an early stage of viral infection by hampering the fusion of viral particles with the endosome. 53
Niclosamide (181; Figure 39), an anthelmintic drug, exhibited very potent antiviral activity against SARS‐CoV‐1 replication and stopped viral antigen synthesis at 1.56 μM concentrations. 238  It prevented the cytopathic effect of SARS‐CoV‐1 at low concentrations of 1 μM and halted SARS‐CoV‐1 replication with an EC50 less than 0.1 μM in Vero E6 cells. 188  Gassen et al. demonstrated that niclosamide inhibits SKP2 activity, increases the lysine‐48‐linked polyubiquitination of the Benclin 1 level, boosts autophagy, and effectively impedes MERS‐CoV replication. 239  Niclosamide inhibited MERS‐CoV replication by up to 1000‐fold at 48 h p.i. at 10 μM. 239
Jeon et al. conducted a screening of FDA approved drugs in Vero cells to discover promising antiviral drug candidates against SARS‐CoV‐2 infection. 240  They reported 24 drugs that exhibited antiviral efficacy with IC50 values between 0.1 and 10 µM.
Among them two approved drugs, niclosamide (181) and ciclesonide (182; Figure 39), exhibited notable inhibitory activities against virus replication in Vero cells. Niclosamide exhibited very potent antiviral activity against SARS‐CoV‐2 (IC50, 0.28 µM). The action of niclosamide might be attributed to autophagy as it was reported for MERS‐CoV. 239  Ciclesonide (182; Figure 39) is another interesting drug candidate with far lower antiviral potency (IC50, 4.33 µM) compared to niclosamide. It is a cortisol derivative used to treat asthma and allergic rhinitis. 241  A recent report by Matsuyama et al. confirmed ciclesonide as a possible antiviral drug against SARS‐CoV‐2. 242  A treatment report of three COVID‐19 patients (https://www3.nhk.or.jp/nhkworld/en/news/20200303_20/) merits further clinical investigation of this drug. The molecular target of ciclesonide's antiviral activity was revealed to be NSP15, a viral riboendonuclease. Together with its well‐established anti‐inflammatory effects, ciclesonide could offer an interesting option for the control of COVID‐19 symptoms.
Azithromycin showed a synergistic effect in combination with hydroxychloroquine in vitro against SARS‐CoV‐2 at realistic concentrations reachable in the human lung. Clinical trials with this antibiotic were initiated in New York on 24 March 2020. 243  Very recently, however, the clinical benefit of the drug in COVID‐19 patients was called into question. 82
Studies for colchicine as an anti‐SARS‐CoV‐2 agent are currently ongoing with the aim of curtailing inflammation and lung complications in mild COVID‐19 cases. 244
Famotidine has been proposed as a therapeutic against COVID‐19, and a clinical trial is underway. 245  It is used to treat peptic ulcers and gastroesophageal reflux disease, among others. Cimetidine is a similar drug and has also been suggested as a treatment for COVID‐19.
Dipyridamole was proposed as a treatment for COVID‐19 as well, and a clinical study is being conducted. 246  It is a nucleoside transport and PDE3 inhibitor that prevents blood clot formation.
Sildenafil was proposed as treatment for COVID‐19, and it is currently being investigated in a small trial. 247  It is a medication used to treat erectile dysfunction and pulmonary arterial hypertension.
Fenofibrate and bezafibrate have been suggested for the treatment of COVID‐19. Fenofibrate is a blood lipid‐lowering medicine of the fibrate class.  248 ,  249  Bezafibrate is a related lipid‐lowering agent.
The HIV‐protease inhibitor nelfinavir (183; Figure 39) strongly inhibited replication of SARS‐CoV‐1 in Vero cells with an EC50 value of 0.048 µM. It was suggested to exert its effect at the post‐entry step of SARS‐CoV‐1 infection. 250  Recently, Yamamoto et al reported that nelfinavir also potently inhibited replication of SARS‐CoV‐2 among nine other Anti‐HIV drugs tested (IC50, 1.13 µM; CC50, 24.32 µM; SI = 21.52). 251  The measured serum concentrations of nelfinavir were 3–6 times higher than the reported EC50 of this drug. This indicates that it is a promising drug candidate for the management of COVID‐19. Other drugs tested against SARS‐CoV‐2 replication were amprenavir (EC50, 31.32 µM; CC50 > 81 µM; SI > 2.59), darunavir (EC50, 46.41 µM; CC50 > 81 µM; SI > 1.75), and indinavir (EC50, 59.14 µM; CC50 > 81 µM; SI > 1.37). Tipranavir inhibited SARS‐CoV‐2 replication as well (EC50, 3.34 µM; CC50, 76.80 µM; SI = 5.76). Ritonavir (EC50, 8.63 µM; CC50, 74.11 µM, SI = 8.59), saquinavir (EC50, 8.83 µM; CC50, 44.43 µM; SI = 5.03), and atazanavir (EC50, 9.36 µM; CC50 > 81 µM; SI > 8.65) suppressed SARS‐CoV‐2 at less than 10 µM. Lopinavir, which was studied in SARS and COVID‐19 patients, also potently inhibited SARS‐CoV‐2 replication with the highest selectivity index (EC50, 5.73 µM; CC50, 74.44 µM; SI = 12.99).
De Wilde et al. identified four drugs—chloroquine (8), chlorpromazine (15), loperamide (184), and lopinavir (185)—by screening of an FDA approved drugs library (for structures, see Figure 40). 252  All of them blocked SARS‐CoV‐1, MERS‐CoV, and HCoV‐229E replication at small concentrations, suggesting potential as broad‐spectrum virostatic agents.
Figure 40  Drugs repurposed for MERS‐ and SARS‐CoV infections. MERS, Middle East respiratory syndrome; SARS‐CoV, severe acute respiratory syndrome coronavirus Chloroquine (8) was able to inhibit SARS‐CoV‐2 viral replication in vitro, but a recent study found no clinical benefit for COVID‐19 patients who had received the drug 82  (the drug is discussed in detail in the section viral entry inhibitors).
Chlorpromazine (15) stopped the replication of SARS‐CoV‐1, MERS‐CoV, and HCoV‐229E. It is a neuroleptic drug used against schizophrenia; 253  here, it interferes with clathrin‐mediated endocytosis. Since, clathrin‐mediated endocytosis is a crucial port for viral entry into the host cell, used by MHV, 254 ,  255 ,  256  SARS‐CoV‐1, 99  and MERS‐CoV, 102  future clinical trials could help elucidate this drug's therapeutic potential against COVID‐19.
Loperamide (184), an opioid receptor agonist against diarrhea, 257  inhibited the replication of SARS‐CoV‐1, MERS‐CoV, and HCoV‐229E.
Lopinavir (185) is an HIV protease inhibitor and was previously shown to block SARS‐CoV‐1 Mpro. 258
Shin et al. analyzed a library of 2334 approved medications and bioactive molecules to find possible antiviral compounds against MERS‐CoV. 259  A series of hit compounds was identified, categorized as anticancer (189, 190), antipsychotics (191, 192), and antidepressant (193) with inhibition activity between 2.1 and 14.4 µM (Figure 38). Saracatinib (189) was especially interesting, as it had remarkable anti‐MERS‐CoV activity (EC50 of 2.9 µM, CC50 > 50 µM). It is a small molecule drug with oral bioavailability used in the management of malignant neoplasms via Src‐family tyrosine kinases (SFKs) inhibition. It also suppressed other CoVs such as SARS‐CoV‐1 (EC50, 2.4 µM), HCoV‐229E (EC50, 5.1 µM), and FIPV (EC50, 7.0 µM) at nontoxic concentrations. Drugs 190 to 193 showed moderate antiviral activities.