PGP and leukemia: Among the tested compounds, derivatives bearing coumarin scaffolds 12 and 13 (Figure 6), incorporating an (E)-3-(3,4,5-trimethoxyphenyl)vinyl moiety linked to a 7-methylenes spacer, showed the best synergistic MDR inhibitory profile, being more potent on LoVo/DOX colon cancer cells that overexpressed both P-gp and CA XII than on K562/DOX leukemia cells overexpressing only P-gp.