Elucidation of the Anticancer Mechanism of Action of a Novel Di-Palladium Helicate

Abstract

Metal-based therapeutics are utilised in up to 50% of anticancer treatments. However, the therapeutic use of these agents is limited by the lack of target specificity and the development of patient resistance to therapy. TZ21 is a novel organometallic drug candidate that is being pursued for its potent cytotoxic activity against a host of cell lines. The complex represents the first complex of its class to be evaluated for anticancer activity. Preliminary data has suggested that TZ21 induces cell death via a mechanism distinctive from other organometallic therapeutics. Subsequently, this thesis sought to elucidate the mechanism of cell death following TZ21 administration in an in vitro lung cancer cell model.

TZ21 induced rapid cell death within 10 minutes concurrent with both membrane damage and severe depletion of intracellular ATP. Ultrastructural studies demonstrated that TZ21 induced a morphologically distinctive mode of cell death dissimilar from those induced by clinically used anticancer agents. The characteristic changes pertaining to TZ21-induced cell death included total cell and nuclear condensation, cell blebbing and the apparent excision of cytoplasmic remnants from the cell body. Further experiments sought to elucidate the mechanisms culminating into this morphological profile. Probing of the cell cytoskeleton, as a marker of membrane damage, revealed that perturbation of the cytoskeleton preceded changes to intracellular ultrastructure, indicating that the cell membrane was the primary target of the TZ21 helicate. Validation of the membrane as a putative target was confirmed by the ability of the TZ21 helicate to disrupt membrane integrity of an isolated membrane model (red blood cells). Further ultrastructural assessment of changes to the cell membrane revealed that TZ21 treatment was associated with the formation of a large central pore. The characteristic crater formed was largely empty space. It could therefore be speculated that initial perturbation of the cell membrane led to the organised extrusion of cellular components and eventual cell demise.

Taken together, the peculiar ultrastructural changes following TZ21 administration were comparable to a rare form of cell death termed autoschizis, a mode of cell death which has exhibited promising outcomes in clinical trials. While autoschizic cell death is reported to be executed through oxidative stress dependent mechanisms, TZ21 cell death was not prevented by antioxidant pre-treatment. The promising results of compounds which induce autoschizic death in clinical trials suggests the findings from this study provide another avenue for drug development, exploiting a novel mechanism to potentially target autoschizic cell death for cancer therapy.