The formation of mutations in cellular DNA lies at the heart of cancer and its treatment. Patients diagnosed with the deadliest solid tumors undergo treatment based on the alterations of DNA sequence in their cancer, and further mutations that occur during treatment cause all-too-common adverse outcomes, including the emergence of drug resistance and metastasis. Of course, these DNA alterations are responsible for the genesis of malignancies in the first place, as accumulated mutations in driver genes lead to uncontrolled growth. Strategies for suppressing mutagenesis can be important for preventing cancer in at-risk populations, and for limiting the emergence of drug resistance and metastasis in existing cancer patients. Here we propose to test a new, molecularly targeted approach to agents that suppress this adverse mutagenesis. Our strategy is based on the most common molecular origins of these cancers: namely, point mutations that arise from specific forms of DNA damage. Our specific aims for the four-year term of the project are to develop new probes to quantify DNA damage in cells and tissues; to identify and develop new small- molecule activators of the repair enzymes that repair the most common sources of mutations; to test whether upregulating DNA repair can suppress the emergence of cancer drug resistance; and to test whether we can lower the incidence of cancer in tumor-prone mice. In progress leading up to this proposal, we have devised several novel and sensitive chemical probes as first-in-class reporters that can measure the cellular activities of multiple DNA repair enzymes. We have employed these probes in clinically relevant studies of cell and tumor specimens, and in investigating connections between inflammation and DNA repair in animal models of disease. In addition, we have used these probes to develop new small-molecule modulators of these pathways, including, excitingly, the only known activators of some of these enzymes. Putting our experience together, we have developed new hypotheses regarding how upregulating the activities of these pathways via small molecules can provide biologically important, and potentially clinically useful, outcomes in cancer. This research is important because it addresses the possibility of preventing common and deadly cancers that remain difficult to treat. In addition, our team will develop molecular tools, including probes, assays, and cell lines, that are likely to be useful to the cancer research community as a whole. Our research plan is innovative in several ways: it will develop and apply new molecular tools for assessing damage and repair pathways; it will lead to the development of the first small-molecule activators of multiple repair enzymes; and it tests new hypotheses regarding how modulating repair activities will be helpful in treatment - and even prevention - of these serious malignancies.