PURPOSE: EGFR-targeting monoclonal antibodies are essential for managing RAS WT metastatic colorectal cancer (mCRC), but their limited efficacy necessitates exploring immunological and metabolic factors influencing response. This study evaluated glutamine metabolism targeting with EGFR inhibition to identify response biomarkers in patients with prior anti-EGFR treatment progression. EXPERIMENTAL DESIGN: We conducted a phase I/II trial in KRAS WT mCRC patients, combining panitumumab and CB-839, hypothesizing that dual inhibition of glutamine metabolism and MAPK signaling would enhance outcomes. As study correlatives, we investigated the B cell activation signature 'Bscore' and glutamine PET as potential treatment response biomarkers. RESULTS: The combination of panitumumab and CB-839 was tolerable with manageable side effects, including Grade 4 hypomagnesemia in four patients, a known panitumumab-related event. Two patients achieved partial response (PR), and five had stable disease (SD), with a 41% disease control rate (DCR). Median progression-free survival (PFS) and overall survival (OS) were 1.84 and 8.87 months, respectively. A positive correlation between 'Bscore' and lesion size reduction suggested its association with clinical benefit (PR and SD). Lower 'Bscore' correlated with greater tumor avidity for glutamine by PET, indicating B cell activation sensitivity to glutamine depletion. CONCLUSIONS: The combination of CB-839 and panitumumab showed safety and promising preliminary responses, but the study closed early due to CB-839 development termination. The B cell activation signature 'Bscore' emerged as a potential biomarker for EGFR and glutaminase inhibition in mCRC, warranting further studies. These findings suggest opportunities to improve immune response and therapies in glutaminolysis-dependent tumors.