G₂ DNA damage checkpoint : inhibitors and there potential for anticancer treatment

In response to DNA damage, cell survival can be enhanced by activation of DNA repair mechanisms and of checkpoints that delay cell cycle progression to allow more time for DNA repair. G₂ checkpoint inhibitors can force cells arrested in G₂ phase by DNA damage to enter mitosis thereby enhancing the killing of cancer cells by DNA-damaging approaches. Checkpoint inhibitors represent a promising class of therapeutic agents in the treatment of cancer. The goals of my research were to (i) characterize novel checkpoint inhibitors in terms of their therapeutic potential and, if possible, mode of action, (ii) increase the therapeutic properties of checkpoint inhibitors by combining them with DNA repair inhibitors, and (iii) develop a therapeutic use of the checkpoint inhibitor, isogranulatimide. Two novel checkpoint inhibitors were psilostachyin and cryptofolione. The target of psilostachyin may be Wee1, a kinase which inhibits CDK1. Cryptofolione shares structural and biological properties with leptomycin B, a highly specific inhibitor of Crm1-mediated nuclear export. This suggests nuclear export inhibition may be a novel means to achieve checkpoint inhibition. Inhibiting both repair and the checkpoint with drugs might cause cancer cells to undergo cell division in the presence of lethal amounts of un-repaired DNA. However, interfering with DNA repair via inhibition of DNA-dependent protein kinase (DNA-PK) reduces the ability of checkpoint inhibitors to abrogate G₂ arrest, as well as their radio sensitizing activity. Components of the checkpoint pathway were highly over-activated in this situation. This suggests that combining checkpoint inhibition with targeted DNA repair inhibition may not be therapeutically viable. Varying the time of addition of checkpoint inhibitors following DNA damage revealed that G₂ checkpoint abrogation alone is insufficient for radio sensitization. Instead, checkpoint inhibitors must be present for the duration of S-phase progression to achieve radio sensitization. Taken together, these studies describe the search for and development of checkpoint inhibitors, ideally for future therapeutic use. These results have relevance to the development of G₂ checkpoint inhibitors as experimental therapeutic approaches to the treatment of cancer. This research represents a major step forward in the pre-clinical development of checkpoint inhibitors as anticancer agents.Medicine, Faculty ofBiochemistry and Molecular Biology, Department ofGraduat