Rbio-05. mitotic enrichment as an efficient radiosensitization strategy for intracranial tumors

Abstract Radiotherapy remains one of the most effective modalities for anticancer treatment and part of standard of care for many primary and metastatic brain tumors. Boosting the efficacy of radiotherapy is therefore a logical avenue to improve patient survival. We have developed a radiosensitization strategy called ‘induction of mitotic enrichment’. It has long been known that the radiosensitivity of a cell depends on the phase of the cell cycle and that mitotic cells are especially vulnerable. Enriching the tumor for mitotic cells by arresting them during division prior to each radiotherapy fraction should therefore render the tumor population more sensitive to irradiation. Ideally, induction of mitotic enrichment should be reversible and non-cytotoxic to prevent healthy tissue toxicity and be compatible with clinically applied fractionated radiotherapy regimens. We have now identified an orally available targeted tubulin polymerization inhibitor that can achieve repeated and reversible mitotic enrichment for up to 10 hours prior to radiotherapy, without causing cytotoxicity in vitro or healthy tissue toxicity in vivo. This tubulin inhibitor (ABT-751) efficiently radiosensitizes a range of preclinical tumor models representing adult and pediatric, primary and metastatic brain tumors. Importantly, ABT-751 also improves survival in mouse models of adult glioblastoma (GBM), but only in a mitotic enrichment setup when given several hours prior to radiotherapy to allow accumulation in mitosis. Of note, induction of mitotic enrichment does not exacerbate healthy tissue toxicity. To conclude, induction of mitotic enrichment by ABT-751 radiosensitizes adult GBM to fractionated radiotherapy. We are currently expanding our preclinical development of mitotic enrichment as a radiosensitization strategy to other targets and cancers and have designed a phase 0/I trial for adult GBM to demonstrate induction of mitotic enrichment and tolerability when combined with radiotherapy. We expect to start enrollment in this trial soon.