Thiopurines inhibit coronavirus Spike protein processing and incorporation into progeny virions

Most antiviral drugs inhibit viral enzymes. A good example of this is Paxlovid, the front-line antiviral drug of the COVID-19 pandemic that has an active ingredient that directly inhibits the activity of a SARS-CoV-2 protease enzyme, thereby inhibiting an essential early step in viral replication. These direct-acting antivirals (DAAs) have advantages in terms of specificity, but sometimes viruses can rapidly evolve to gain resistance to these drugs. For this reason, host-targeted antivirals (HTAs) can be an attractive complementary approach in antiviral drug development. Rather than directly inhibiting viral enzymes, HTAs exploit the reliance of viruses on host cell processes to indirectly impede viral replication and/or stimulate host antiviral responses. Here, we describe a new antiviral mechanism of action for an FDA-approved thiopurine known as 6-thioguanine (6-TG) that establishes it as an HTA. We demonstrate that 6-TG is a pro-drug that must be metabolized by host enzymes to gain antiviral activity. We show that it can inhibit the replication of human coronaviruses, including SARS-CoV-2, at least in part by interfering with the processing and accumulation of Spike glycoproteins, thereby impeding assembly of infectious progeny viruses. In fact, we observed that 6-TG treatment not only reduced the production of infectious coronaviruses, but the few viral particles that were released from the cell lacked Spike, which we could readily observe by the lack of the characteristic Spike protein “corona” on transmission electron micrograph images. We dug into the mechanism of action of 6-TG and learned that these antiviral effects are linked to inhibition of cellular GTPase enzymes. Together, these findings suggest that certain host GTPase enzymes are required for efficient virus replication and that further development of GTPase inhibitors as HTAs is warranted.

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