Imaging pharmacodynamic efficacy using FDG-PET. Baseline FDG-PET imaging reveals FDG avidity in tumors (arrows) arising in a genetically engineered mouse model of lung cancer driven by the L858R mutant epidermal growth factor receptor (EGFR). After 2 d of treatment with the irreversible EGFR inhibitor WZ4002, repeat FDG-PET imaging shows loss of FDG avidity. PET images (color scale) are combined with CT (gray scale) for anatomic reference, demonstrating no change in tumor size. These results show the utility of FDG-PET imaging to demonstrate rapidly pharmacodynamic efficacy (metabolic change) preceding any change in tumor volume. SUV, standardized uptake value.