MEK inhibition but not gemcitabine elicits a tumor response in CKP mice. (a) Photograph of a CKP animal showing a locally advanced PDAC and H&E staining demonstrating typical histological appearance of PDAC with epithelial-glandular differentiation and strong desmoplastic reaction. (b) Representative T2w images before and after therapy initiation with gemcitabine or refametinib. Tumor is delineated with a dotted line. A decrease in tumor size is visible in refametinib but not gemcitabine treated animals. (c) Waterfall plot of tumor volume change showing no response in Gemcitabine and significant response in refametinib treatment group (Gemcitabine: 7 days upon treatment onset, 2 doses in total; refametinib: 5–9 days upon treatment onset, 5–7 doses in total). Tumor volumes calculated based on T2w-MR images and expressed relative to the pre-treatment tumor volume. Each column represents one mouse.