Background: RAS/BRAF mutations constitutively activate the MAPK pathway in colorectal cancer, and may promote resistance to CRT. We propose that trametinib, a MEK1/2 inhibitor, in combination with 5FU CRT for patients with LARC will improve outcome. Methods: Phase I study with standard 3+3 design in patients with Stage II/III rectal cancer with 3 dose levels of trametinib: 0.5, 1, and 2mg daily with 5FU 225mg/m²/day and 50.4 Gy. Trametinib was given over a 5-day lead-in and continued through the course of CRT. Patients undergo surgery 6-10 weeks after the completion of CRT. An expansion cohort at the maximum tolerated dose (MTD) with 12 patients is ongoing. Tumor tissue is collected prior to therapy, at day 4/5 of trametinib, and at surgery. The primary endpoint is to determine the MTD of trametinib with CRT. Results: 15 patients (10 males, 5 females) have been enrolled and 14 patients are evaluable for toxicities to date. Median age is 53 years (range 35-74). Patients have completed enrollment to all dose levels, with 1 dose-limiting toxicity of diarrhea attributed to 5FU CRT. No grade 4/5 toxicities, and toxicities are shown in the table. Generalized skin rash led to trametinib discontinuation after 2 weeks of therapy for 1 patient, and was held 3 days for 2 patients. At the trametinib dose level of 2mg, 3 out of 7 (43%) patients had a pathological complete response and the average neoadjuvant rectal (NAR) score is 8.1. Tumor RAS/BRAF mutation status is determined. Analysis of tumor tissue shows dose-dependent decrease in phosphorylated-ERK1/2 with increasing doses of trametinib. Conclusions: Initial evaluation shows that the combination of trametinib with 5FU CRT is tolerable and effective, and warrants further investigation in LARC. Clinical trial information: NCT01740648