There is an outstanding need for broadly acting antiviral drugs to combat emerging viral diseases. Here, we report that thiopurines inhibit the replication of the betacoronaviruses HCoV-OC43 and SARS-CoV-2. 6-thioguanine (6-TG) disrupted early stages of infection, limiting accumulation of full-length viral genomes, subgenomic RNAs and structural proteins. In ectopic expression models, we observed that 6-TG increased the electrophoretic mobility of Spike from diverse betacoronaviruses, matching the effects of enzymatic removal of N-linked oligosaccharides from Spike in vitro. SARS-CoV-2 virus-like particles (VLPs) harvested from 6-TG-treated cells were deficient in Spike. 6-TG treatment had a similar effect on production of lentiviruses pseudotyped with SARS-CoV-2 Spike, yielding pseudoviruses deficient in Spike and unable to infect ACE2-expressing cells. Together, these findings from complementary ectopic expression and infection models strongly indicate that defective Spike trafficking and processing is an outcome of 6-TG treatment. Using biochemical and genetic approaches we demonstrated that 6-TG is a pro-drug that must be converted to the nucleotide form by hypoxanthine phosphoribosyltransferase 1 (HPRT1) to achieve antiviral activity. This nucleotide form has been shown to inhibit small GTPases Rac1, RhoA, and CDC42; however, we observed that selective chemical inhibitors of these GTPases had no effect on Spike processing or accumulation. By contrast, the broad GTPase agonist ML099 countered the effects of 6-TG, suggesting that the antiviral activity of 6-TG requires the targeting of an unknown GTPase. Overall, these findings suggest that small GTPases are promising targets for host-targeted antivirals.

There is an outstanding need for broadly acting antiviral drugs to combat emerging viral diseases. Here, we report that thiopurines inhibit the replication of the betacoronaviruses HCoV-OC43 and SARS-CoV-2, and to a lesser extent, the alphacoronavirus HCoV-229E. 6-Thioguanine (6-TG) disrupted early stages of infection, limiting synthesis of full-length and subgenomic HCoV RNAs. Furthermore, consistent with our previous report on the effects of thiopurines on influenza A virus (IAV) glycoproteins, we observed that 6-TG inhibited accumulation of Spike glycoproteins from diverse HCoVs. Specifically, 6-TG treatment decreased the accumulation of Spike proteins and increased their electrophoretic mobility to match the properties of Spike following enzymatic removal of N-linked oligosaccharides with Peptide:N-glycosidase F (PNGaseF). SARS-CoV-2 virus-like particles (VLPs) harvested from 6-TG-treated cells were deficient in Spike. 6-TG treatment had a similar effect on lentiviruses pseudotyped with SARS-CoV-2 Spike; lentiviruses could be harvested from cell supernatants, but they were deficient in Spike and unable to infect human cells bearing ACE2 receptors. Together, these findings from complementary ectopic expression and infection models strongly indicate that defective Spike trafficking and processing is an outcome of 6-TG treatment. At low micromolar doses, the primary known mode of action of 6-TG is selective inhibition of the small GTPase Rac1. However, we show that selective chemical inhibitors of the small GTPases Rac1, CDC42 and Rho had no effect on Spike processing and accumulation, whereas the broad GTPase agonist ML099 was able to counter the effects of 6-TG, suggesting that an unknown GTPase could be the relevant 6-TG-target protein involved in regulating Spike processing and accumulation. Overall, these findings provide important clues about the mechanism of action of a candidate antiviral that can broadly target HCoVs and suggest that small GTPases may be promising targets for host-targeted antivirals.

Background: Anaphylaxis is a potentially life-threatening condition caused by the sudden release of inflammatory mediators into the systemic circulation. Among this condition’s etiologies, corticosteroid-induced anaphylaxis, despite being uncommon, should receive due consideration given the frequency of steroid use in various settings. Any patient that presents with shortness of breath, wheezing, hypotension, urticaria, or other characteristic signs of anaphylaxis following the administration of steroids should be promptly evaluated. Because of the potentially fatal nature of anaphylaxis, clinicians must be familiar with the presentation, diagnosis, and management of the reaction. Case Report: The primary objective of this case report is to discuss an example of such a reaction in a 21-year-old female with a past medical history of anxiety, depression, and alcoholism who presented with anaphylaxis following prednisone use, as well as the proposed pathophysiology and management thereafter. She was managed with intravenous epinephrine and diphenhydramine with complete resolution of her symptoms. She was subsequently discharged with an EpiPen, cetirizine, and advised to establish care with an allergist for follow up and additional allergy testing. To complete this case report, we performed a review of current primary literature on the subject. Conclusions: Though uncertain, many potential mechanisms of sensitization to corticosteroids were identified, including haptenization, preservatives, excipients, and conjugated esters. Various means exist to aid in diagnosis, such as skin testing, immunoCAP assays, lymphocyte transformation tests, basophil activation tests, and graded drug challenges, though these tests are associated with a high false negative rate. Accurate identification of the causative agent is crucial in facilitating avoidance or rapid desensitization prior to future corticosteroid use.