Structure–Activity Relationships of Di-2-pyridylketone, 2‑Benzoylpyridine, and 2‑Acetylpyridine Thiosemicarbazones for Overcoming Pgp-Mediated Drug Resistance

Multidrug
resistance (MDR) mediated by P-glycoprotein (Pgp) represents
a significant impediment to successful cancer treatment. The compound,
di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT), has
been shown to induce greater cytotoxicity against resistant cells
than their nonresistant counterparts. Herein, the structure–activity
relationships of selected thiosemicarbazones are explored and the
novel mechanism underlying their ability to overcome resistance is
further elucidated. Only thiosemicarbazones with electron-withdrawing
substituents at the imine carbon mediated Pgp-dependent potentiated
cytotoxicity, which was reversed by Pgp inhibition. Treatment of resistant
cells with these thiosemicarbazones resulted in Pgp-dependent lysosomal
membrane permeabilization (LMP) that relied on copper (Cu) chelation,
reactive oxygen species generation, and increased relative lipophilicity.
Hence, this study is the first to demonstrate the structural requirements
of these thiosemicarbazones necessary to overcome MDR. We also demonstrate
the mechanism that enables the targeting of resistant tumors, whereby
thiosemicarbazones “hijack” lysosomal Pgp and form redox-active
Cu complexes that mediate LMP and potentiate cytotoxicity.