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574 Background: Epidermal growth factor receptor (EGFR) monoclonal antibodies (mAbs) are approved in RAS WT mCRC; however, patients (pts) will develop resistance to these agents. Alterations in glutamine (Gln) metabolism play a critical role in cancer cell growth. In cancers such as CRC, EGFR and Gln cooperate to provide signals and fuel for mitogen activated protein kinase-dependent cell growth. Our in vitro data show that Gln abrogates EGFR inhibition, and blockade of Gln transport restores sensitivity. We also observed a greater antitumor response in vivo with EGFR mAb plus CB-839, an inhibitor of a rate-limiting enzyme of Gln metabolism, than either agent alone. We designed a phase I/II study (NCT03263429) to evaluate CB-839 + panitumumab + irinotecan in anti-EGFR refractory RAS WT mCRC. Methods: Dose escalation used a Bayesian continual reassessment method targeting a 25% toxicity probability. CB-839 (600 mg or 800 mg twice daily [BID]) were evaluated with panitumumab (6 mg/kg) and irinotecan (180 mg/m2). Irinotecan was included in phase I to establish a future phase II dose of the triplet. Prior EGFR mAb treatment (tx) was not required for phase I. Dose-limiting toxicity (DLT) was any tx-related non-hematologic ≥Gr 3 toxicity (except fatigue, rash, or elevated liver enzymes) or ≥Gr 4 hematologic toxicity during the first 28 days. Results: Nine pts have been enrolled; 2 were not evaluable for DLT and replaced. Zero DLTs were observed at dose level 1 (n = 3) or 2 (n = 4); 2 more pts are needed to confirm the maximum tolerated dose (MTD). Most frequent toxicities were anemia and hypomagnesemia (88%) and elevated alkaline phosphatase, nausea, and rash (75%), most ≤Gr 2. One of 7 evaluable pts (14%) has an ongoing partial response, and 5 pts had stable disease (SD; 71%). Three pts have been on tx > 6 months, and 3 pts with prior EGFR mAb tx achieved SD. Conclusions: Triplet combination was tolerable at full doses of each drug, and preliminary antitumor activity was observed in a majority of pts. Phase II will begin after phase I completion and will evaluate efficacy of CB-839 (800 mg BID) and panitumumab (6 mg/kg). Imaging studies using investigational PET tracers to evaluate Gln metabolism as a function of tumor response are planned. Clinical trial information: NCT03263429.