© 2015 American Association for Cancer Research. Purpose: The emerging need for rational combination treatment approaches led us to test the concept that cotargeting MEK and CDK4/6 would prove efficacious in KRAS-mutant (KRASmt) colorectal cancers, where upregulatedCDK4 and hyperphosphorylated retinoblastoma (RB) typify the vast majority of tumors. Experimental Design: Initial testing was carried out in the HCT-116 tumor model, which is known to harbor a KRAS mutation. Efficacy studies were then performed with five RB patient-derived colorectal xenograft models, genomically diverse with respect to KRAS, BRAF, and PIK3CA mutational status. Tolerance, efficacy, and pharmacodynamic evaluation of target modulation were evaluated in response to daily dosing with either agent alone or concurrent coadministration. Results: Synergy was observed in vitro when HCT-116 cells were treated over a broad range of doses of trametinib and palbociclib. Subsequent in vivo evaluation of this model showed a higher degree of antitumor activity resulting from the combination compared to that achievable with single-agent treatment. Testing of colorectal patient-derived xenograft (PDX) models further showed that combination of trametinib and palbociclib was well tolerated and resulted in objective responses in all KRASmt models tested. Stasis was observed in a KRAS/BRAF wild-type and a BRAFmt model. Conclusions: Combination of trametinib and palbociclib was well tolerated and highly efficacious in all three KRASmutant colorectal cancer PDX models tested. Promising preclinical activity seen here supports clinical evaluation of this treatment approach to improve therapeutic outcome for patients with metastatic colorectal cancer.