Synthesis, structure–activity relationships, and SARS-CoV-2 antiviral activity of 3,5-disubstituted isothiazolo[4,3-b]pyridines as PIKfyve inhibitors

Introduction: 3-Alkynyl-6-aryl-isothiazolo[4,3-b]pyridines have previously been shown to be potent inhibitors of the lipid kinase FYVE finger-containing phosphoinositide kinase (PIKfyve), displaying broad-spectrum antiviral activity. Methods: To further study their structure-activity relationship (SAR), an efficient synthesis toward 3- bromo-5-chloro-isothiazolo[4,3-b]pyridine was established. It allowed to introduce structural modifications at positions 3 and 5 by palladium-catalyzed cross-coupling reactions and nucleophilic aromatic substitutions. Results and discussion: It led to the generation of a focused library of 3,5-disubstituted isothiazolo[4,3-b]pyridines. Several derivatives exhibited potent PIKfyve inhibition (in the low nM range) in a biochemical assay and antiviral activity against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) (in the low μM range). To gain an insight in their binding mode, molecular modeling was applied, indicating that these 3,5- disubstituted isothiazolo[4,3-b]pyridines bind to the ATP-binding site of PIKfyve, although with a different binding mode from that of the 3,6- disubstituted isothiazolo[4,3-b]pyridines.