FLT1 and cancer: Their antiproliferative potency was tested against suitable sunitinib‐sensitive cancer cell lines which were previously evaluated for their VEGFR‐1, PDGFR β, as well as cathepsin B expression and cathepsin B activity by Western blot analysis and zymography, respectively, showing similar activity of 8 compared to the original drug sunitinib, while the non‐cleavable reference drug 14 was distinctly less active.