GS-646089

GS-646089 is a broad-spectrum antiviral nucleoside analog that exhibits significant inhibitory activity against respiratory syncytial virus (RSV), human metapneumovirus (hMPV), rhinovirus (enterovirus), and enterovirus. It targets RNA-dependent RNA polymerase (RdRp) with an IC50 of 43-46 nM. Within cells, GS-646089 is converted into a triphosphate metabolite, competitively incorporating with ATP into nascent RNA chains to act as an immediate chain terminator, thereby blocking viral replication. GS-646089 is the parent compound of the double prodrug GS-7682 and has been utilized in studies of acute respiratory virus infections and related pathogens.

GS-646089 (compound 2) effectively inhibits RSV replication in both NHBE and HEp-2 cells, with EC₅₀ values of 6.23 μM and 6.83 μM respectively, and shows no cytotoxicity up to 50 μM in HEp-2 and MT4 cells. Its active triphosphate metabolite (2-NTP) potently inhibits the RSV RNP complex with an IC₅₀ of 0.050 μM, demonstrating high selectivity for human mitochondrial polymerase and DNA polymerase. GS-646089 strongly suppresses both cell-free RSV-A2 RNP complexes and RdRp enzyme activity, with IC₅₀ values of 46 nM and 43 nM respectively. In NHBE cells, it inhibits RSV replication with an EC₅₀ of approximately 2-7 μM, maintaining non-cytotoxicity up to 50 μM. Similarly, it inhibits RSV replication in HEp-2 cells, again displaying an EC₅₀ of about 2-7 μM and no cytotoxicity for HEp-2 or MT4 cells at concentrations up to 50 μM. When administered at 10 μM for 2 hours, it generates low levels of intracellular active 2-NTP metabolite, suggesting limited initial phosphorylation of the nucleoside.

GS-646089 (compound 2) (1.6-7.6 mg/kg; i.t.; once daily; 6 days) is the active metabolite of the inhaled compound GS-7682. It significantly decreases the RSV viral load in the bronchoalveolar lavage (BAL) of RSV-infected African green monkeys by an average of 3.6 log 10, effectively inhibiting lower respiratory tract viral replication. This reduction is observed on day 5 post-infection and remains below solvent controls until day 11.