Current chemotherapies for solid tumors remain severely limited due to toxicity and the development of drug resistance. Hence, new agents are vital to explore in order to improve patient outcomes.
We have identified the novel chelator Dp44mT as a potent and selective anti‐tumor agent that overcomes resistance to standard chemotherapeutics (Whitnall, M., Howard, J., Ponka P. and Richardson, D.R. PNAS USA 2006; 103:14901–6).
To understand its mechanism of action and potential anti‐angiogenic activity, we have investigated its ability to interact with cellular copper (Cu). In contrast to its marked ability to potently mobilize cellular 59Fe, Dp44mT caused marked intracellular 64Cu accumulation. Cellular fractionation demonstrated that 64Cu was concentrated in a fraction containing lysosomes and mitochondria. Moreover, the lysosomotropic character of Dp44mT suggested that it may become concentrated in lysosomes. To test this, studies with the lysosomotropic agent, acridine orange, demonstrated that the Cu‐Dp44mT complex potentiated loss of lysosomal integrity, indicating that the lysosome was a target of the compound.
This was confirmed using the fluorescent probe, pepstatin A BODIPY FL, which demonstrated relocalisation of the lysosomal enzyme cathepsin D from the lysosome to the cytosol after incubation with the Cu‐Dp44mT complex. Further, the altered lysosomal membrane permeability was demonstrated by cleavage of the pro apoptotic Bcl2 family member, Bid. The Cu‐Dp44mT complex was markedly more redox active than the Fe complex at lysosomal pH. This indicated that Cu mediated redox events were responsible for the cytotoxicity observed, in particular the induction of apoptosis induced by lysosomal damage. Further, the Cu chelator, tetrathiomolybdate potently attenuated Dp44mT mediated cell death.
Our results indicate the potent anti‐tumor and anti‐angiogenesis activity of Dp44mT is mediated not only by Fe chelation, but also via its binding of cellular Cu, leading to the generation of a cytotoxic complex.
Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B245.