6-Thioguanine blocks SARS-CoV-2 replication by inhibition of PLpro

Summary The emergence of SARS-CoV-2 has led to a global health crisis that, in addition to vaccines and immunomodulatory therapies, calls for the identification of antiviral therapeutics. The papain-like protease (PLpro) activity of nsp3 is an attractive drug target as it is essential for viral polyprotein cleavage and for deconjugation of ISG15, an antiviral ubiquitin-like protein. We show here that 6-Thioguanine (6-TG), an orally available and widely available generic drug, inhibits SARS-CoV-2 replication in Vero-E6 cells with an EC50 of approximately 2 μM. 6-TG also inhibited PLpro-catalyzed polyprotein cleavage and de-ISGylation in cells and inhibited proteolytic activity of the purified PLpro domain in vitro. We therefore propose that 6-TG is a direct-acting antiviral that could potentially be repurposed and incorporated into the set of treatment and prevention options for COVID-19.


INTRODUCTION
Coronavirus Disease-2019 (COVID-19) is caused by SARS-CoV-2, a betacoronavirus highly similar to SARS-CoV-1 (now SARS-CoV-1) (Coronaviridae Study Group of the International Committee on Taxonomy of Viruses, 2020; Gralinski and Menachery, 2020). An urgent need exists for additional treatment strategies, and repurposed FDA-approved drugs with existing supply chains and well characterized pharmacologic properties represent a rapid and efficient approach toward COVID-19 therapeutics (Guy et al., 2020).
Enzymatic activities encoded by SARS-CoV-2 are attractive drug targets, including PLpro, an essential cysteine protease within the large multi-domain nsp3 protein (Harcourt et al., 2004). PLpro cleaves the pp1a polyprotein at three sites and is required to generate the mature nsp1, 2, 3 and 4 proteins. These proteins modulate host cell translation (nsp1) and contribute to formation of viral replication/transcription complexes (nsp2, 3, and 4) (V'kovski et al., 2021). The PLpro recognition consensus site within pp1a (LXGG) is also found at the C-terminus of ubiquitin and ISG15, an antiviral ubiquitin-like modifier, although PLpro of MERS-CoV, SARS-CoV-1, and SARS-CoV-2 PLpro preferentially catalyzes de-ISGylation over ubiquitylation (Daczkowski et al., 2017;Freitas et al., 2020). The fact that these viruses encode de-ISGylase activity suggests they are sensitive to the antiviral effects of ISGylation. Therapeutic inhibition of PLpro would therefore be predicted to have two antiviral effects: inhibition of viral replication by blocking polyprotein cleavage and restoration of the antiviral effects of ISGylation. Further, de-ISGylation by PLpro, through generation of free (unconjugated) ISG15, enhances the secretion and extracellular signaling function of ISG15, which in turn promotes pro-inflammatory cytokine production from cells of the immune system (Swaim et al., 2020). Therefore, a potential third effect of inhibiting PLpro would be a decrease in the pro-inflammatory ''cytokine storm'' associated with COVID-19 (Tay et al., 2020). Here, we show that an inhibitor of SARS-CoV-2 PLpro, 6-thioguanine (6-TG), inhibits virus replication at low micromolar concentrations. 6-TG is a widely available generic drug that could therefore potentially be repurposed as a betacoronavirus therapeutic in the current and future pandemics. catalytic domain in vitro (Cheng et al., 2015;Chou et al., 2008); however, there was no further follow up of its ability to inhibit de-ISGylation or viral polyprotein cleavage in cells or its ability to inhibit viral replication. 6-TG was therefore tested for its ability to inhibit SARS-CoV-2 replication in Vero-E6 African Green Monkey kidney cells. As shown in Figure 1B, 6-TG inhibited viral replication with a half-maximal effective concentration (EC50) of 2.13 mM. Remdesivir, a SARS-CoV-2 therapeutic and an inhibitor of the viral RNA-dependent RNA polymerase (Yin et al., 2020), inhibited replication with a very similar EC50, 2.16 mM. GRL0617, a previously reported inhibitor of SARS-CoV-2 PLpro inhibited replication with an EC50 approximately 15-fold higher than 6-TG (35.43 mM), consistent with previously reported results (Freitas et al., 2020). Azathioprine, a thiopurine related to 6-TG that is also used clinically for IBD (Bayoumy et al., 2020), was approximately 10-fold less effective at inhibiting viral replication than 6-TG (EC50 19.16 mM). The cytotoxicity of these compounds was determined, and the CC50 (50% cytotoxic concentration) ranged from approximately 35 mM for 6-TG to 60 mM for Remdesivir ( Figure S1).

6-Thioguanine inhibits PLpro activity in cells and in vitro
An essential role of the PLpro domain of nsp3 is to generate the mature nsp1-4 proteins from the pp1a polyprotein through self-catalyzed cleavage reactions. To determine if 6-TG inhibited polyprotein cleavage (B) Vero-E6 cells were treated with 6-TG, Remdesivir, GRL0617, and Azathioprine to assess their ability to restrict SARS-CoV-2 replication (MOI: 0.01). Drug concentrations for 6-TG, GRL0617 and Azathioprine ranged from 0.1 pM to 100 mM at 10-fold increments; concentrations for Remdesivir ranged from 5 nM to 100 mM at 3-fold increments. Percent inhibition of viral replication was calculated by comparing plaques in drug-treated samples to those in DMSO treated samples and shown as mean G SED for either four replicates (6-TG, GRL0617 and Azathioprine) or two replicates (Remdesivir). The calculated EC50 for the compounds are shown in each panel with 95% confidence limits. iScience Article events in HEK293T cells, we expressed by plasmid transfection an N-terminally TAP-tagged pp1a protein consisting of the full-length nsp1, 2 and 3 proteins (TAP-nsp123 WT ; Figure 2A). We also expressed a version of the protein with the active-site cysteine within the PLpro domain of nsp3 altered to an alanine (TAP-nsp123 CÀA ). The fusion proteins were detected by immunoblotting of cell lysates with anti-TAP antibody, and TAP-nsp1 was expressed separately as a size marker for the processed fusion protein. Expression of TAP-nsp123 WT resulted primarily in a product the size of TAP-nsp1 (42 kD), as expected if the fusion protein was fully processed, whereas expression of TAP-nsp123 CÀA resulted in the expected full-length protein (336 kD) along with several breakdown products of the full-length protein. Increasing concentrations of 6-TG inhibited PLpro-mediated processing of the TAP-nsp123 WT polyprotein, with loss of the TAP-nsp1 product and accumulation of the full-length fusion protein, with the same pattern of breakdown products iScience Article seen with the TAP-nsp123 CÀA protein ( Figure 2A). Half-maximal inhibition of polyprotein cleavage occurred at approximately 0.5 mM 6-TG (quantitation shown in Figure S2A). In a related experiment (Figure 2B), the isolated PLpro domain (residues 746-1061 of nsp3) was co-expressed in trans with a TAPtagged nsp2-nsp3 fragment (residues 727-918 of pp1a), spanning the nsp2-3 PLpro cleavage site. PLpro efficiently processed the TAP-nsp2-3 protein, dependent on the active-site cysteine, and this was again inhibited with increasing amounts of 6-TG, with half-maximal inhibition at approximately 1.0 mM 6-TG (quantitation shown in Figure S2B).
In addition to polyprotein cleavage activity, SARS-CoV-2 PLpro deconjugates ISG15 from proteins that have been modified by this antiviral ubiquitin-like protein (Klemm et al., 2020;Shin et al., 2020;Swaim et al., 2020). The E1, E2, and E3 enzymes for conjugation of human ISG15 are Uba7, Ube2L6, and Herc5, respectively, and expression of these enzymes with ISG15 results in robust protein ISGylation in cells (Dastur et al., 2006). Figure 3A shows that co-expression of PLpro (residues 746-1061 of nsp3) with ISG15 and the ISG15 conjugation enzymes in HEK293T cells resulted in nearly complete loss of ISG15 conjugates, whereas iScience Article expression of the active-site C-to-A variant of PLpro (PLpro CÀA ) did not. Addition of 6-TG resulted in a dose-dependent inhibition of PLpro de-ISGylation activity, with half-maximal inhibition at approximately 0.1 mM (quantitation shown in Figure S2C). To demonstrate that 6-TG acts directly on PLpro, an in vitro cleavage assay was established based on an ISG15 fusion protein. Like most ubiquitin-like proteins, ISG15 is synthesized in cells as a precursor protein with a C-terminal extension that must be proteolytically removed prior to conjugation. We expressed and purified the precursor form of ISG15 with an additional C-terminal HA tag (Pro-ISG15-HA), and this was used as a substrate for the purified PLpro domain (nsp3 residues 746-1061). As shown in Figure 3B, purified PLpro proteolytically removed the HA-tagged C-terminal extension, and increasing amounts of 6-TG inhibited the reaction, dependent on the input amount of PLpro enzyme (quantitation shown in Figure S2D). These results indicate that 6-TG directly inhibits SARS-CoV-2 PLpro.

DISCUSSION
The results shown here indicate that 6-TG is a direct-acting SARS-CoV-2 antiviral that functions by inhibiting the PLpro activity of the nsp3 protein. 6-TG inhibited both pp1a polyprotein processing in cells and the ISG15 deconjugation activity of PLpro in cells and in vitro. Pp1a polyprotein cleavage is essential for generation of the nsp1-4 viral proteins, and therefore essential for virus replication. Inhibition of ISG15 conjugation is likely to have a less important effect in cell-based replication assays but may have important consequences during an infection in an animal or human host. The fact that MERS-CoV and SARS-CoV-1 PLpro also have de-ISGylase activity suggests these viruses are particularly sensitive to the antiviral effects of ISG15. Although the critical targets of ISG15 conjugation in infected cells are not yet known, we speculate that these may include newly translated viral proteins, as suggested earlier (Durfee et al., 2010), and perhaps the pp1a protein, itself.
6-TG is an orally-delivered generic drug on the World Health Organization list of essential medicines, suggesting that it could be potentially re-purposed as an inexpensive and widely available therapeutic for COVID-19. Clinical trials are essential for determining the effectiveness of 6-TG against COVID-19. Four clinically approved hepatitis C protease inhibitors (Semeprevir, Paritaprevir, Vaniprevir, and Grazoprevir) have also recently been shown to inhibit SARS-CoV-2 PLpro in vitro and virus replication in Vero-E6 cells (Bafna et al., 2021). The EC50s for inhibition of viral replication for these drugs ranged from 4.25 to 10.8 mM, while that of 6-TG was determined here to be 2.13 mM. Although 6-TG directly inhibited PLpro activity, it should be noted that the mechanism of action of 6-TG as a chemotherapeutic and immunosuppressant is that it is converted into 6-Thioguanine ribonucleotides and deoxynucleotides and incorporated into RNA and DNA. It is therefore possible that incorporation of 6-TG-containing ribonucleotides into SARS-CoV-2 RNA could be a secondary antiviral effect of 6-TG.
Current 6-TG dosing in patients varies significantly, from 0.3 mg/kg/day for long-term treatment of inflammatory diseases, to up to 3 mg/kg/day in acute lymphocytic and myelogenous leukemia treatments (Seinen et al., 2010). Hepatotoxicity and bone marrow suppression are significant side effects of high dose treatment (2-3 mg/kg/day); however, this is typically not seen before two courses of treatment (2 months). 6-TG as an IBD therapeutic involves continuous doses of 0.3 mg/kg/day for more than a year, a dosage that results in fewer cases of liver and myeloid toxicities (Meijer et al., 2016;Mulder et al., 2014). Although it may seem counterintuitive to use a immunosuppressive as an antiviral, evidence to date indicates that IBD patients on thiopurine therapies do not have higher rates of COVID-19 than the general population (Higgins et al., 2020). Further, as a potential COVID-19 treatment, we anticipate that 6-TG treatment courses would be on the order of 5-10 days, like Remdesivir, and therefore toxicities and immune-suppression associated with prolonged or high dose 6-TG treatment are unlikely to be relevant. Also like Remdesivir, we anticipate that 6-TG would be most effective if administered early in the course of a SARS-CoV-2 infection, during the peak viral replication phase. We note that unlike 6-TG, Remdesivir must be delivered intravenously. We propose that the results presented here warrant the initiation of human clinical trials of 6-TG as a SARS-CoV-2 therapeutic. As PLpro is a conserved and essential enzymatic activity of the beta coronaviruses, 6-TG may prove useful in the current and future coronavirus pandemics and as a complement to other antivirals in use and in development. iScience Article given different IC50 values that would be more relevant in considering possible human clinical trials. An additional limitation of the study is that the effects of 6-TG were not tested in a whole animal model of SARS-CoV-2 infection. Human clinical trials will ultimately be necessary to determine the effectiveness of 6-TG as a COVID-19 therapeutic.

Lead contact
Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Jon Huibregtse (huibregtse@austin.utexas.edu).

Material availability
All newly generated reagents for this study are available at the institution of the lead contact.

Data and code availability
Not applicable.

METHOD DETAILS
Transfections and drug treatments HEK293T cells were plated in a 6 well plate at a density of 3x10 4 cells/mL. The next day cells were transfected using X-tremeGENE HP DNA transfection reagent (Roche). Plasmids were transfected at the following amounts: 0.35 mg FLAG-ISG15, 0.25 mg Uba7, 0.25 mg Ube2L6, 0.35 mg HA-Herc5, 1 mg TAP-nsp123 (WT or CA), 0.2 mg TAP-PLpro (WT or CA) Cells were treated with 6-TG at the indicated concentrations immediately following transfection (100 mM 6-TG stock in DMSO and diluted in PBS).

Protein purification
Sars-CoV-2 PLpro was purified as a GST fusion protein in BL12 E. coli. Overnight starter cultures were grown at 37 C for 16 hours. Cultures were diluted 1:10 and cultured with shaking for 2 hours at 37 C. Expression of protein was induced with 100 mM Isopropyl b-D-1-thiogalactopyranoside (IPTG) for 24 hours at 16 C. Cells were collected by centrifugation, resuspended in 10 mL TBS with 0.00025% Tween 20, 0.01 mM DTT and 5% glycerol (Binding Buffer), and sonicated for 1 minute. Lysates were spun at 15,000 x g for 10 minutes and supernatants were incubated with 100 mL per liter of culture Glutathione Sepharose (GE Healthcare) for ll