Two cases of metastatic colorectal cancer in which epidermal growth factor receptor gene amplification benefited from panitumumab monotherapy.

Panitumumab, an anti-epidermal growth factor receptor (EGFR) antibody, is a standard drug used in patients with wild-type (WT) metastatic colorectal cancer (mCRC). Studies have reported that the mutation is a negative predictive biomarker; however, gene amplification may be a predictive biomarker for the response to anti-EGFR antibodies. We encountered two patients with gene amplification (one with mutation) who responded to panitumumab monotherapy after failure of standard chemotherapy. Case 1 was an 85-year-old woman with WT transverse colon cancer with liver metastases. After receiving capecitabine monotherapy as first-line therapy, S-1 plus irinotecan plus bevacizumab therapy as second-line, trifluridine/tipiracil (FTD/TPI) monotherapy as third-line, and regorafenib monotherapy as fourth-line therapy, the patient received panitumumab monotherapy as fifth-line therapy. After that, comprehensive gene panel testing showed gene amplification. This therapy continued for 6.9 months without progressive disease (PD), and liver metastases shrank by up to 72%. Case 2 was a 65-year-old man with WT (initially) sigmoid colon cancer and multiple liver metastases. After receiving mFOLFOX6 plus panitumumab therapy as first-line therapy, FOLFIRI plus bevacizumab as second-line, he underwent conversion surgery. After 3 months, multiple liver metastases were detected on CT scan, then, comprehensive gene panel testing was done, and it showed high tumor mutational burden (TMB) and amplification. As third-line therapy, he received pembrolizumab monotherapy. After PD for pembrolizumab, OncoBEAM™ was done and it showed neo- mutation. Regardless of that result, panitumumab was resumed as a fourth-line treatment and administered for 5.2 months without PD; liver metastases shrank by up to 50%. We encountered two patients with mCRC and gene amplification who responded to panitumumab monotherapy. gene amplification may be a potential biomarker for anti-EGFR antibodies, regardless of status.