Project Abstract Despite decades of research into targeted therapeutics, the most effective treatments in glioma remain DNA damaging agents: radiation and the alkylating agents temozolomide and nitrosureas like CCNU. In this project’s prior cycle, we found that mismatch repair deficiency (MMRd) is a common source of temozolomide resistance; and that unlike other cancers, gliomas that gain temozolomide resistance through MMRd tend not to respond to immune checkpoint inhibition. But they often do respond to CCNU. We hypothesize that a fuller understanding of the different resistance mechanisms to TMZ and CCNU will enable 1) improved knowledge of when and how to use these agents, including clinically useful biomarkers, and 2) optimization of combined strategies using targeted and immunotherapies developed over the last decade. Although extensive work has been done to understand how CCNU damages DNA and to detect genes and pathways involved in repairing this damage, the field lacks a unified understanding of how CCNU effects vary across gliomas with different DNA damage response (DDR) characteristics, how resistance arises, and how the effects of CCNU interact with other agents including DNA damaging agents such as temozolomide and radiation, as well as therapeutics targeting specific DDR functions and pathways. As a result, we lack biomarkers that can accurately guide clinicians to prescribe CCNU to patients who are likely to respond, do not know the optimal combined therapeutic approaches involving CCNU, and clinical practice varies widely. We propose to pursue a systematic evaluation of the genomic effects and potential therapeutic roles of CCNU. A major innovation in our proposal is our systematic approach to evaluating the effects of CCNU on cancer survival and proliferation and genome integrity: when used alone and in combination with temozolomide, RT, and agents targeting DNA damage response pathways; and across a wide variety of DNA damage response contexts. For this, we will leverage a living tissue biobank of over 250 gliomas in vivo and in vitro models and state-of-the-art technologies for functional genomics and genome characterization across treatment conditions and DDR backgrounds. Our Aims are: Aim 1: Test the hypothesis that MMRd based resistance to TMZ within a GBM indicates relative sensitivity to CCNU and RT and can be detected through plasma cell-free DNA. Aim 2: Test the hypothesis that defects in proteins involved in repair of CCNU-induced ICLs determine resistance to CCNU and strategies to overcome. Aim 3: Test the hypothesis that intentional manipulation of mutational profiles and clonal dynamics by coordinating TMZ, CCNU, RT, and DDR pathway inhibition can increase the effectiveness of immunotherapy. DNA damaging agents remain the most effective agents in glioma and all other cancers, the unified understanding of their effects in isolation and combination across the varied DDR contexts in this proposal will shape the use o...