Activation Pathway of a Nucleoside Analog Inhibiting Respiratory Syncytial Virus Polymerase

Human respiratory syncytial virus (RSV) is a negative-sense RNA virus and a significant cause of respiratory infection in infants and the elderly. No effective vaccines or antiviral therapies are available for the treatment of RSV. ALS-8176 is a first-in-class nucleoside prodrug inhibitor of RSV replication currently under clinical evaluation. ALS-8112, the parent molecule of ALS-8176, undergoes intracellular phosphorylation, yielding the active 5'-triphosphate metabolite. The host kinases responsible for this conversion are not known. Therefore, elucidation of the ALS-8112 activation pathway is key to further understanding its conversion mechanism, particularly given its potent antiviral effects. Here, we have identified the activation pathway of ALS-8112 and show it is unlike other antiviral cytidine analogs. The first step, driven by deoxycytidine kinase (dCK), is highly efficient, while the second step limits the formation of the active 5'-triphosphate species. ALS-8112 is a 2'- and 4'-modified nucleoside analog, prompting us to investigate dCK recognition of other 2'- and 4'-modified nucleosides. Our biochemical approach along with computational modeling contributes to an enhanced structure-activity profile for dCK. These results highlight an exciting potential to optimize nucleoside analogs based on the second activation step and increased attention toward nucleoside diphosphate and triphosphate prodrugs in drug discovery.