Evaluation of Pharmacological Strategies Targeting the Glycolytic Phenotype of Cancer

The presence of hypoxia within tumours has significant biological and pharmacological implications leading to resistance to therapy and poor prognosis. The eradication of hypoxic cells has been a major goal for many years but despite decades of research, no hypoxia specific therapies have been approved for use in humans. The purpose of this thesis is to explore the possibility of targeting the glycolytic phenotype of cancers with the aim of specifically eradicating hypoxic cells.

Initial studies using a series of glycolytic inhibitors (3-bromopyruvate, 2-deoxyglucose and sodium oxamate) did not show hypoxia selectivity in vitro and in several cell lines, resistance under hypoxia (0.1% oxygen) occurred. In the case of the pyruvate dehydrogenase kinase 1 (PDK1) inhibitor dichloroacetate (DCA) however, hypoxia selectivity was observed in vitro but the magnitude of the sensitisation was much less than that of the hypoxia activated prodrug tirapazamine. Nevertheless, DCA did inhibit PDK1 leading to reduced phosphorylation of the pyruvate dehydrogenase complex and hypoxia selective cell kill.

These results questioned whether glucose was indeed important for the survival of hypoxic cells but studies where glucose was removed from culture media confirmed that glucose is required for the survival of hypoxic cells in vitro. Using this model, mannose was also shown to be an important monosaccharide that could support the growth of cells under hypoxia opening up a novel avenue for future research. Finally, a series of novel organometallic complexes based on the metal silver were explored as potential glycolytic inhibitors. The silver N-heterocyclic carbene Ag8 was shown to be a potent and selective inhibitor of lactate dehydrogenase A (LDH-A) leading to reduced export of lactate from cells. Ag8 has a complex mechanism of action with inhibition of LDH-A being a novel mechanism.

In conclusion, this study has demonstrated that targeting key enzymes in the glycolytic pathway does not generate hypoxia selective cytotoxic effects. This conclusion is surprising and reflects the complexity of targeting cellular metabolism. The identification of mannose as an important fuel for hypoxic cells opens up new avenues for research and the inhibition of LDH-A by organometallic complexes such as Ag8 requires further studies.