NSAH is a cell penetrant, potent and selective nonnucleoside reversible inhibitor of large subunit (hRRM1) of hRR (human ribonucleotide reductase). NSAH binds to catalytic site of hRRM1. It inhibits hRR in cells. NSAH exhibit potent cytotoxicity toward cancer cells, while demonstrated very moderate toxicity against normal blood progenitor cells.