Synthesis and evaluation of 3,5-disubstituted isothiazolo[4,5-b]pyridines, pyrazolo[4,3-b]pyridines and isothiazolo[3,4-b]pyrazines as inhibitors of PIKfyve.

Various 3,5-disubstituted isothiazolo[4,3-b]pyridines were previously shown to be potent inhibitors of the lipid kinase PIKfyve, displaying broad-spectrum antiviral activity. To further study their structure-activity relationship and to discover novel skeletons as antivirally active PIKfyve inhibitors, a scaffold hopping strategy was applied yielding isothiazolo[4,5-b]pyridines, pyrazolo[4,3-b]pyridines and isothiazolo[3,4-b]pyrazines. Among the newly synthesized scaffolds, the isothiazolo[3,4-b]pyrazines were the most promising, displaying potent and selective PIKfyve inhibition in a biochemical assay, and, in addition, showing antiviral activity against SARS-CoV-2 (in the low μM range). Finally, molecular docking of the various scaffolds in the ATP-binding site of PIKfyve allowed to rationalize their differences in PIKfyve inhibitory activity.