At the time of writing this manuscript, a rapidly evolving international outbreak of SARS- CoV2 is spreading daily to new communities and the search for effective drugs and vaccines is ongoing, but will require time. Even when a new drug is discovered and manufactured, demand may rapidly outstrip supply if the epidemic continues to grow. Identifying and repositioning existing drugs to treat COVID–19 infections may provide a rapid, inexpensive option with known toxicities. [10, 12, 14, 17, 22]
The purpose of our study was to evaluate the levels of expression of genes in patients while they are sick and compare them to healthy controls and convalescent patients. Genes with different levels of expression reflect pathways that are used by the host in response to viral infection (sick vs. Convalescent) or pathways that are dysregulated by the infection (sick vs. Healthy).[22] Similar to all bioinformatics pipelines, parameter and algorithm selection impact results, and may not be optimal even after tuning, which makes use of a black-box approach for drug repositioning less appealing. [23] However, despite limitations, this approach to identify drugs that interact with differentially expressed genes has proven useful in many cases.[20] The algorithm we used in this paper, cogena, overcomes several limitations of this approach. It clusters genes and enriches the clusters using established ontologic pathways, and thus is more likely to find clinically-relevant drugs. In pursuing this analysis, two questions must be addressed: First, which patient group is more suitable for comparison with sick patients, healthy or convalescent patients? One may argue that healthy controls were not exposed to the virus and identifying genes with different levels of expression in this group does not reflect the physiologic pathways that lead to recovery, which are of great importance in this current problem. The second question concerns the selection of the number of differentially expressed genes for drug repositioning. Obviously, including more genes will yield more pathway enrichment. However, increasing the number of used genes will add more confusion to the model. Many differentially expressed genes reflect pathways that are related to other physiologic processes (e.g. improved tissue nutrition, oxygenation and perfusion). To cover all these possibilities we decided to include 4 analysis sets of gene expression.
It is very impressive that different drug repositioning approaches lead to similar results. For example, our study did not show chloroquine, which was suggested before to be active in treating SARS-FCoVs. [4] Interestingly, other drugs with antimalarial activity were identified, including sirolimus, chlorpromazine, sulfadiazine, pheniramine, colchicines, metformin, ionomycin and calcium pantothenate. Additionally, when the gene set of sick vs. convalescent (analysis set 1) was altered to include the top 50 DEG (differentially expressed genes) only, chloroquine appeared on the list, possibly as the focus on the very top differentially expressed genes eliminated less important pathways. Again, this shows the robustness of this approach, but the need to refine its work. We claim that our choice for the cogena framework is justified as it provides an unmatched level of transparency.
SARS-FCoVs obtain entry into the cells by attaching to ACEII surface protein. Moving the virus into the cell depends on clathrin-mediated endocytosis.[9] Identified drugs that interfere with endocytosis in our study are ikarugamycin, triflupromazine, clozapine and chlorpromazine; the last three drugs are typically used as antipsychotics. Acidification of vacuoles to eliminate the spike protein is an essential step in activating the virus.[9] This can be repressed by inhibiting v-ATPase activity to block the vacuolar protein pump. Omeprazole, an H+K+ proton pump inhibitor inhibits v-ATPase at higher concentration.[24] Additionally, chloroquine is known to increase the pH of endosomes.[25] Colchicine and vinca alkaloids interact with cellular microtubules, which cause inhibition of lysosomal formation.[26] Non-steroidal anti-inflammatory drugs (e.g.Nabumetone) exhibit their antiviral/antitumor effect by via inhibition of endoplasmic-reticulum-resident protein kinase (PERK) mediated phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2A (eIF2a). This leads to protein synthesis inhibition and cellular death. [27] Etacrynic acid, a drug identified in analysis set 1, was shown before to be a suitable inhibitor of the main coronavirus protease (M pro) due to its electrophilic binding of its active domain. This binding inhibits the function of this vital protease which cleaves viral proteins at 9 specific sites in preparation for viral assembly. [28] Amiloride derivatives interacts with protein ion channel (protein E of coronavirus) which prevents viral formation and stops replication, at least in “in-vitro” models.[29] Figure 1 shows a simplified representation of some of the drugs we identified and their mode of action.
One drug of great interest that was identified in our study was captopril. This commonly used antihypertensive medication from the Angiotensin-converting enzyme (ACE) inhibitor class works by blocking the active domain of this enzyme, leading to decreased conversion of angiotensin I to angiotensin II. Another rarely used ACE inhibitor appeared in analysis set 3, rescinnamine; which is a vinca alkaloid derivative. While the SARS-FCoVs get access to cells via the ACE-II protein, which carries similarity to ACE-I protein as more than 60% of their sequences are identical. We wonder why captopril was never studied in-vitro or in-vivo to treat coronaviruses. Using this drug in particular, despite proposed benefit, should be only attempted in non-human models first. Prolonged use of captopril is associated with increasing levels of ACE-II. Whether this could possibly lead to enhanced entry of the virus is a question that can only be answered through careful preclinical tests.
Geldanamycin and its derivatives (tanespimycin and alvespimycin) were identified in analysis sets 3 and 4. Geldanamycin is an antibiotic that is believed to be targeting the ADP/ATP binding site of the Heat-shock protein (HSP90). It was shown to be targeting many viruses, including SARS coronavirus, where it inhibits SARS replication with a selectively index of >300 and an EC50 of 0.91uM; for comparison the EC50 for chloroquine is 8.8uM. The drug was submitted for a Chinese patent (application 03146591, July 2003).[30] Interestingly, geldanamycin is produced by soil bacteria, streptomyces hygroscopicus, the same source of sirolimus.
In a recent study that evaluated 87 drugs effective in MCF7 breast cancer epithelial cells, 17-allyloamino-geldanamycin, LY294002, valproic acid, trichostatin A and sirolimus (in addition to wortmannin), were evaluated using pathway gene enrichment. Two pathways (ERBB_signaling and Insulin_signaling) were the 2 most combined pathways.[31] Wortmannin, another PI3K inhibitor, similar to LY294002, is known to inhibit RSV replication by inhibiting autophagy, a process that favors viral replication by inhibiting apoptosis.[32] This finding is also supported by wortmannin inhibition of coxsackievirus replication in infected mice treated with a histone-deacetylase which induced replication by inhibiting autophagy and inducing apoptosis.[33]
Geldanamycin is a benzoquinone with known antimalrial activity. It belongs to a group of drugs called ansamycins. One of the potent antiviral ansamycins is rifampin. This commonly used antibiotic has a known antiviral function due to its inhibition of DNA- dependent RNA polymerase.[34] This selective action makes rifampin of no value in treating RNA viruses, like coronavirus. To the contrary, geldanamycin is a potent HSP90 inhibitor and works on a broad spectrum of viruses through modifying host response.[30] While effective in treating resistant multiple cancer types, hepatotoxicity of geldanamycin limited its use and lead to the development of derivatives with less toxicity and improved oral bioavailability, namely tanespimycin and alvespimycin.[35]
Autophagy, a process where the internal cellular components degenerate, with or without integration with lysosomes, resulting in autophagosomes, which are hijacked by some viruses and used as safe sanctuary. This is opposed by programmed cell death, or apoptosis, which programmatically destroys the cell and calls for its clearance. Several studies showed the ability of SARS-CoV to induce autophagy.[36, 37] Notably, autophagy plays a role in cancer resistance to chemotherapy and modulation of its components may restore drug sensitivity.[38] Chloroquine and hydroxychloroquine show promising results in restoring chemotherapy sensitivity by modulating autophagy.[39] It is suggested that chloroquine function as antimalarial and antitumor drug can be linked to Hsp90 inhibition with secondary modulation of autophagy, among other pathways. [40]
An estrogen (alpha-estradiol), an anti-androgenic drug (nilutamide) and an aromatase inhibitor (exemestane) appeared on the analysis set 1 and set 2. This is noteworthy, particularly as the SARS-FCoVs are known to cause more severe infections in males and are associated with higher mortality; a phenomenon that proved true in male mice and female mice treated with estrogen receptor antagonist.[41] Complex interaction between estrogens, Hsp90 and lysine methyltransferase (SMYD2) point to the interaction of sex hormones with the function of this molecular chaperone in modulating autophagy.[42] Antidiabetic drugs (metformin, pioglitazone and possibly acetohexamide) restrict viral replication by activating AMP-activated protein kinase which modulates autophagy.[43, 44]
Multiple sulfonamides were identified, including antibiotics (sulfaphenazole, sulfaguanidine, sulfadiazine). In addition, diphemanil metisulfate and copper sulfate are sulfur containing drugs. Of note, the family of sulfonamides includes multiple drugs that are potent antivirals.[45] Many sulfonamides are known to inhibit the ATP-binding pocket of Hsp90.[46] Sirlomus, also known as rapamycin, is a macrolide mTOR inhibitor and was reported in analysis sets 1 and 2. Sirolimus is known to have antiviral effect on multiple viruses, including CMV, HIV, EBV and others.[47–49] Interestingly, the mTOR pathway functions in parallel with Hsp90 in modulating autophagy.[50] Inhibition of both pathways is synergistic in infected cells. The suggested link between these pathways involves viral-induced activation of the Hsp90 client, Akt, which in turn phosphorylates mTOR to facilitate translation of viral mRNA.[51] All of these identified drugs highlight Hsp90 as a druggable target that deserves more attention (Fig 2).
Based on our findings, we suggest adding more drugs to the experimental arsenal deployed against SARS-FCoVs. We suggest using nontoxic drugs that are readily available for wide scale prophylaxis or the treatment of mild cases of COVID–19. These drugs would include omeprazole, nonsteroidal anti-inflammatory drugs, colchicine, sulfoanamide antibiotics, antimalarial drugs and clozapine/chlorpromazine. For patients with serious COVID–19 illnesses, the above drugs can be combined with immune modulators like sirolimus, and direct Hsp90 inhibitors under well-designed clinical trials. Given the high pathogenicity of the current ongoing epidemic and its potential to become a pandemic,[52] it seems prudent to attempt to use multiple drugs in combination if low risk for toxicity and drug-drug interaction is established. Choosing a combination of drugs identified in this study or other drug-repositioning studies and careful reporting of the efficacy of these experiences can result in rapid accumulation of knowledge.
In conclusion, we showed that a drug repositioning framework using simple and unambiguous methodology can yield a plethora of drugs that are active for SARS-FCoVs. We believe that a combination of some of these relatively nontoxic drugs can be worth trying (Fig 3). Serious efforts should be shifted to Hsp90 and autophagy pathways. Multiple candidate drugs can be of great value; most notably, geldanamycin and its derivatives. Hsp90 inhibitors are being studied for cancer patients. It seems there is an urgent need to focus on this important pathway to overcome the ongoing outbreak.