Background: Cell cycle dysregulation is a hallmark of most cancers. At the G1/S transition, cyclin dependent kinase (CDK) 4 and CDK 6 proteins associate with cyclin D proteins to allow cell cycle progression. Tumor cells that are driven by abnormalities of the cell cycle through activation of the cdk4/6-cyclin D axis showed increased phosphorylation of retinoblastoma protein (Rb), the major substrate for CDKs at the G1/S transition. Palbociclib is a potent specific inhibitor of cdk 4/6, with preclinical data showing G1 arrest in Rb-positive cells. Since activation of the MAP kinase pathway impacts directly on cdk-cyclin activation, we performed a phase II trial of palbociclib in patients with metastatic, KRAS-mutant colorectal cancer (mCRC). Methods: We screened 36 patients with KRAS mutant mCRC for Rb expression and 35 (97%) were Rb positive. We enrolled 15 patients, 9 (60%) of which were male and 6 (40%) female, median age was 62, 10 (67%) colon, 4 (27%) rectal (27%) rectal, and 1 (7%) appendiceal malignancy. Patients received 125mg daily of palbociclib for 21 days of a 28-day cycle, the recommended phase II dose. Results: Six patients (33%) experienced grade 3 neutropenia, 1 patient had grade 3 neutropenic fever (6%), and 2 patients (11%) had grade 3 AST/ALT elevation. There were no responses, but 5 patients (33%) had stable disease by RECIST criteria after 2 cycles of treatment, and 1 patient had stable disease for 8 cycles. The median number of cycles was two. As a pharmacodynamic marker of activity, 9 patients underwent FLT-PET at baseline and after 1 cycle of therapy. FLT-PET uses the tracer 3’-deoxy-3’[¹⁸F]-fluorothymidine, uptake of which is proportional to cellular proliferation. Six of the 9 patients (67%) had a decrease in uptake in target lesions, with an average 20% decrease in uptake in target lesions. FLT uptake in the bone marrow decreased by an average 4.85% and this correlated with a decrease in absolute neutrophil count of 62.3% (r² 0.59). Conclusions: While palbociclib has limited activity in KRAS mutant mCRC as a single agent, there is a clear pharmacodynamic effect as determined by FLT-PET imaging. Given the limited toxicity profile, combinations with non-myelosuppressive agents hold great promise. Clinical trial information: NCT01037790