Response to anti-EGFR therapy in chemo-refractory right-sided RAS wild-type metastatic colorectal cancer: a case report and literature review.

Background: Anti-epidermal growth factor receptor (EGFR) therapies are important targeted agents in the treatment of metastatic colorectal cancer (CRC). However, clinical benefit is limited to patients with left-sided primary tumors and RAS wild-type (WT) disease. In right-sided chemo-refractory settings, response to anti-EGFR therapy has not been reported to date. Case Description: We present a case of a 70-year-old man with metachronous metastatic ascending colon adenocarcinoma who experienced an exceptional response to FOLFIRI (fluorouracil, leucovorin, and irinotecan) plus panitumumab after failing multiple lines of therapy. He was initially diagnosed with stage IIIB (pT4aN1M0) disease and underwent hemicolectomy followed by adjuvant FOLFOX (fluorouracil, leucovorin, and oxaliplatin). Nine months after completion of adjuvant therapy, disease recurred in the liver, peritoneum, and mesenteric lymph nodes. Subsequent treatments included FOLFIRI plus bevacizumab and FOLFOX with eventual progression. Tumor genomic profiling revealed RAS/RAF WT disease, and in the absence of anti-EGFR therapy resistance mutations, the patient was offered treatment with FOLFIRI plus panitumumab. He achieved immediate palliation of his abdominal pain after one cycle, followed by normalization of his tumor markers and significant tumor regression of his hepatic, peritoneal, lung, and distant lymph node metastases within four cycles. Conclusions: Treatment options for right-sided RAS-WT metastatic CRC are limited, particularly after progression on standard chemotherapies. While anti-EGFR antibodies have demonstrated detrimental survival impact in the first-line setting for right-sided CRC, their performance in later lines is less well-characterized. This case challenges the notion of right-sided disease as uniformly resistant to EGFR inhibition and highlights the need for additional biomarker studies to identify the subset of right-sided CRC that may benefit from EGFR targeted strategies. Emerging evidence suggests that more stringent genomic criteria for EGFR resistance, beyond RAS mutation status alone, may refine patient selection for benefit from anti-EGFR therapies.