hCAIX/XII-IN-13 is an inhibitor of human carbonic anhydrases ( hCA ). hCAIX/XII-IN-13 shows good inhibitory activity against the tumor-related CA subtypes IX and XII, with K i values of 0.08 µM and 0.06 µM, respectively. Under hypoxic conditions, hCAIX/XII-IN-13 can restore the cytotoxicity of Doxorubicin on MCF-7 cells, increasing G 2 /M phase cell cycle arrest and apoptosis.