Evaluation of transferrin and gallium-pyridoxal isonicotinoyl hydrazone as potential therapeutic agents to overcome lymphoid leukemic cell resistance to gallium nitrate.

Gallium nitrate is active against lymphoma and bladder cancer; however, little is understood about tumor resistance to this drug. Transferrin, the iron transport protein, increases gallium uptake by cells, whereas pyridoxal isonicotinoyl hydrazone (PIH), an iron chelator, transports iron into cells. Therefore, we examined whether these metal transporters would increase the cytotoxicity of gallium in gallium nitrate-resistant CCRF-CEM cells. Transferrin, in increasing concentrations, enhanced the cytotoxicity of gallium nitrate. One mg/ml transferrin decreased the 50% inhibitory concentration of gallium nitrate from 1650 to 75 micrometer in gallium-resistant cells and from 190 to 150 micrometer in gallium-sensitive cells. Transferrin also enhanced the cytotoxicity of gallium even at drug concentrations that were not growth inhibitory. The gallium chelate Ga-PIH inhibited the growth of both gallium nitrate-resistant and -sensitive cells. Fifty micrometer Ga-PIH inhibited cellular proliferation by 50%, whereas similar concentrations of PIH or gallium nitrate were not growth inhibitory. However, because higher concentrations of PIH also inhibited cell growth, the cytotoxicity of Ga-PIH was greater than PIH only at concentrations of <100 micrometer. Cross-titration experiments demonstrated that the cytotoxicity of PIH was partially reversed by gallium nitrate, whereas the cytotoxicity of gallium nitrate was enhanced by PIH. Our studies suggest that Ga-PIH warrants further evaluation as a potential antineoplastic agent. Because transferrin increases the cytotoxicity of gallium nitrate in transferrin receptor-bearing, gallium nitrate-resistant cells, future clinical trials of this drug should incorporate the development of strategies to increase plasma transferrin levels.