PGP and neoplasm: The lead compound, bearing a methylenedioxyphenyl moiety, and a diselenide functionality, showed a good activity (GI50 = 0.88‒2.0 μM) and selectivity towards tumour cell lines (selectivity index: 14‒32); moreover, compounds considered herein were not substrates for the P-gp efflux pump, thus avoiding the development of chemoresistance coming from such mechanism, commonly found for widely-used chemotherapeutic agents.