Early FDG/PET scanning as a pharmacodynamic marker of anti-EGFR antibody activity in colorectal cancer.

Panitumumab is an anti-EGF receptor (EGFR) antibody approved for use in treatment of chemotherapy-refractory colorectal cancers lacking K-RAS mutations. Despite overall response rates approximating 10%, no marker predictive of clinical benefit has been identified. We describe a chemotherapy-refractory patient whose clinical condition necessitated rapid identification of an effective agent in whom we used (18)F-fluorodeoxyglucose-positron emission tomography (FDG-PET)/computed tomographic scanning 48 hours after an initial dose of panitumumab to document a pharmacodynamic response to the antibody. The initial 46% ± 2.7% drop in SUV(max) of four target lesions correlated with a partial response by Response Evaluation Criteria in Solid Tumors and a >90% drop in serum carcinoembryonic antigen at 8 weeks, indicating that an early decrease in FDG uptake may predict subsequent clinical benefit in response to anti-EGFR antibody therapy in colorectal cancer.