DC661 is capable of deacidifying the lysosome and inhibiting autophagy significantly better than hydroxychloroquine (HCQ).Treatment of melanoma cells with DC661 results in a more striking accumulation of the autophagic vesicle marker LC3B-II at lower concentrations compared with either Lys05 or HCQ, reflecting more pronounced accumulation of autophagic vesicles at concentrations between 0.1 and 10 μmol/L. All cells die at concentrations above 10 μmol/L for DC661 in contrast to Lys05 and HCQ. Compared with HCQ or Lys05, DC661 treatment induces a significantly more potent inhibition of autophagic flux in melanoma cells expressing the mCherry-eGFP-LC3B reporter, and significantly higher levels of free GFP in melanoma cells expressing GFP-LC3B. DC661 treatment resulted in significantly greater lysosomal deacidification compared with either HCQ or Lys05. The IC50 of DC661 in 72-hour MTT assays is 100-fold lower than that of HCQ across multiple cancer cell lines including colon and pancreas cancer cell lines. DC661 suppresses long-term clonogenic growth of melanoma cells more effectively and induces significantly more apoptosis than Lys05, HCQ, or combined BRAF and MEK inhibition in BRAF-mutant melanoma cells.