SUMMARY There are currently no FDA-approved drugs available targeting the ERK pathway in which RAS mutations drive ERK activity. The long-term goal is to help develop therapeutically useful ERK inhibitors for the clinical treatment of malignancies. The overall objectives in this application are to (i) characterize the chemical and allosteric mechanism and develop a novel class of covalent ERK inhibitors, (ii) elucidate the molecular mechanism(s) by which they induce anti-proliferative activity in combination with FDA-approved drugs vertically targeting the ERK pathway and (iii) determine the in vivo anti-tumor efficacy of combination treatments using patient-derived organoid and tumor models. The central hypothesis is that combination therapy using covalent, allosteric inhibitors of ERK can be developed to possess suitable potency to induce apoptotic cell death and promote tumor regression in patient-derived tumor models predictive of clinical efficacy. The project's rationale is that developing a new chemical strategy to inhibit ERK and determination of its preclinical therapeutic efficacy and associated mechanisms is likely to offer a robust scientific framework for developing new cancer therapeutic approaches. Testing the central hypothesis occurs by pursuing two specific aims: 1) The design and elucidation of the mechanism of ERK recruitment site inhibitors and 2) investigating covalent ERK inhibitors in patient-derived CRC Tumor Models. The first aim is to optimize a new class of covalent allosteric ERK inhibitors and delineate allosteric binding and inhibition mechanisms to identify highly specific leads. The second aim delineates ERK inhibition's mechanisms and consequences by leads in RAS- and RAF- transformed patient-derived models and identifies agents that, combined with FDA-approved drugs targeting the ERK pathway, induce tumor regression. In the applicant's opinion, the research proposed in this application is innovative because it focuses on developing leads from a new class of ERK inhibitor compounds that covalently target a site of protein binding on ERK. These compounds will exhibit improved pharmacodynamics and block compensatory feedback signals from ERK signaling and induce durable ERK inhibition to promote robust cytotoxic anti- cancer effects. The proposed research is significant because it is expected to provide substantial scientific justification for the continued development and future clinical trials of novel ERK inhibitor therapies. Ultimately, such knowledge can offer new opportunities for the development of innovative therapies to treat cancer.