The thiosemicarbazone, DpC, broadly synergizes with multiple anti-cancer therapeutics and demonstrates temperature- and energy-dependent uptake by tumor cells

Background: The di-2-pyridylketone thiosemicarbazones, di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone (Dp44mT) and di-2-pyridylketone-4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC), demonstrate potent and selective anti-tumor activity. In fact, DpC entered Phase I clinical trials for advanced and resistant tumors. Methods: This investigation examined the activity of these thiosemicarbazones in five tumor cell-types compared to nine clinically used chemotherapeutics and also in combination with these drugs. Results: Dp44mT and especially DpC demonstrated potent anti-proliferative activity that was significantly greater than a range of standard anti-cancer therapeutics. As most anti-cancer drugs are given in combination, further studies were performed to examine the synergistic activity of DpC or Dp44mT with these chemotherapeutics. Combination experiments revealed broad synergy between Dp44mT or DpC upon addition of these drugs, with a sequential protocol of treating first with standard chemotherapies followed by incubation with the thiosemicarbazones being optimal. However, combining DpC and Dp44mT resulted in a pronounced antagonistic drug interaction. To dissect the mechanism of this latter effect, custom-prepared C-14-DpC was implemented and examined for its uptake by cells. The avid uptake of C-14-DpC by tumor cells observed at 37 degrees C was suppressed at 4 degrees C and by the metabolic inhibitor, sodium fluoride, suggesting a temperature-and energy-dependent mechanism. Furthermore, competition studies using an excess of unlabeled Dp44mT or DpC inhibited 14C-DpC or C-14-Dp44mT uptake, respectively, suggesting these ligands utilize the same carrier/receptor, antagonizing the internalization of each other. Conclusions and general significance: These studies demonstrate the potent and broad anti-proliferative activity of Dp44mT and particularly DpC, and are important for establishing optimized combinations with standard chemotherapies.