Advancement of a poxvirus antiviral

Project Summary/Abstract Poxviruses are a large group of human pathogens that include variola (the causative agent of smallpox), mpox, and cowpox. Recent global outbreaks of increasingly virulent mpox clades have underscored the growing need for both prophylactic vaccination and small molecule therapeutics to control the spread and severity of poxviral disease. To date, only two antiviral drug classes (tecovirimat and cidofovir/brincidofovir) have been FDA-approved for treating smallpox; both were developed and approved for poxviruses according to the Animal Rule with no efficacy testing in humans. The WHO, CDC and other agencies have stated a strong desire for at least two safe, effective small molecule therapeutics that broadly target poxviruses due to the high perceived risk of poxviral disease both from endemic exposure as well as the potential purposeful release of smallpox as a bioterror agent. Ongoing compassionate/emergency use of tecovirimat and brincidofovir against mpox has raised significant concerns about their spectrum of efficacy and safety, calling into question whether this goal has been met. We have established a program to develop a new class of non-nucleoside small molecules called pyridopyrimidinones ("PDPMs") that show broad spectrum antipoxviral activity with little-to-no host cell toxicity. Our studies to define the mechanism of PDPMs have determined that these inhibitors suppress viral mRNA production via targeting of the poxvirus RNA polymerase (RNAP), an ideal target that is highly conserved across all poxviruses and orthogonal to existing drug classes. Through targeted hit-to-lead medicinal chemistry optimization we have in hand multiple advanced PDPM candidates that are potent, orally bioavailable, and well-tolerated in mice. Now, we seek to continue our development of PDPMs as promising therapeutic candidates for poxviral disease. So far, our medicinal chemistry program has focused predominantly on defining PDPM structure-activity relationships (SAR) in vitro with vaccinia as a high-throughput, non-infectious surrogate for variola and mpox. Going forward, our lead optimization will include rigorous interrogation of all PDPMs in newly-developed in vitro assays against two emerging, clinically relevant clades of mpox (clade I and clade II). We will continue our efforts to use genetic, biochemical and chemical approaches to define mechanism by which PDPMs block viral replication. Lastly, we will for the first time validate the efficacy of these molecules in animal models of poxviral disease. When these efforts are completed, they will enable advanced (towards first-in-human) testing of a new class of poxvirus inhibitor - an inhibitor that has a mechanism of action complementary to existing approved compounds and candidates, and a broad protection profile, fulfilling the need for multi-compound protection from these significant human pathogens.