Background: KRAS, NRAS, and BRAF mutations constitutively activate and dysregulate the MAPK pathway in colorectal cancer. Pre-clinical data suggest that MEK 1/2 inhibition radiosensitizes RAS mutant cancers. Thus, we conducted a phase I study with trametinib, a MEK1/2 inhibitor, in combination with 5-FU CRT in patients with LARC. Methods: A phase I study for patients with stage II or III rectal cancer that harbor KRAS, NRAS, or BRAF mutations. It is a standard 3+3 design with 3 dose levels of trametinib: 0.5mg, 1mg, and 2mg daily, given in combination with 5-FU 225mg/m²/day (5 days week) and a total of 50.4 Gy radiation (1.8 Gy daily fractions). Trametinib is administered with a 5-day lead-in, and for the duration of CRT. Patients then undergo surgery 6-10 weeks later. There is an expansion cohort planned for a total of 12 patients at the maximum tolerated dose (MTD). The primary endpoint is to determine the MTD of trametinib with CRT. Results: Thirty-five patients have been screened and 10 patients (8 male, 2 female) enrolled. Median age is 54 years (range 38-64). Nine patients are evaluable for toxicities thus far. Patients have been enrolled to all 3 dose levels without dose-limiting toxicities, and the 10^th patient is receiving trametinib 2mg. There are no grade 4 toxicities, and the most common toxicities observed are shown (Table). Skin rash due to trametinib (2mg) led to dose-reduction in 1 patient and holding trametinib in another, both for the last 3 days of CRT. There were no interruptions in radiation therapy; 5FU was held for 3 days in 1 patient due to mucositis. Conclusions: Trametinib in combination with CRT has been well-tolerated. Additional patient enrollment and longer follow-up time are needed to detect activity of MTD on rates of pathologic response, local control, distant relapse, and survival. Clinical trial information: NCT01740648