Background: 2^nd-line therapy of KRASm CRC is limited; targeting downstream signal transduction enzymes is rational here. Use of the MEK inhibitor SEL is supported by preclinical and clinical evidence. We designed a dose-finding/phase II study of IRI plus SEL in KRASm CRC. Methods: Eligibility included: KRASm or BRAFm CRC with measurable disease progressing after 1st-line therapy with an oxalipatin-based chemo + bevacizumab regimen, PS 0-1, acceptable organ function. Patients were treated with IRI 180 mg/m2 iv q2w and SEL 50-75 mg po bid. First 3 patients of run-in portion were treated with SEL 50 mg and if no DLTs, next 3-6 patients at 75 mg po bid. If no DLTs, then phase II dose of SEL 75 mg po bid would be used. Primary endpoint was RECIST 1.0, investigator determined response rate (RR). As compared to the historical RR of 4% (and median PFS 2.5 mos) for 2^nd-line FOLFIRI (Tournigand), with alpha 0.10 & beta 0.90, a sample size of 45 would have the power to detect improvement in RR to 15%, and 79% power to demonstrate improved med PFS to 4.0 mos. Early stopping would occur for responses of 0 of 20 patients. Results: 32 patients were entered and treated. Median age was 54 (40-71) yrs, 18 were male and 22 Caucasian, all KRASm. The first 3 tolerated SEL 50 mg bid without SAE and the remaining 29 were treated at 75 bid. Median number of cycles on study was 3.5 and median TTP approximately 4.0 months. Observed grade 3 AEs included: diarrhea 3, fatigue 2, neutropenia 2, and 1 each PLTS, enteritis, GI bleed, rash; one grade 4 ANC. Grade 2 AEs: diarrhea 12, rash 8 pts. Best response (investigator reported) included 3 (9%) confirmed PR and 15 (47%) SD [including 2 unconfirmed PR] of 32 entered. Six patients were on study for > 6 months (6, 6, 8, 9, 12.5, 14.5 months). The study was terminated early due to non-protocol considerations. These data are not yet verified. Conclusions: Despite early termination, the higher RR and PFS noted for 32 patients with KRASm CRC treated with IRI and SEL as 2^nd-line therapy of CRC (and treated for up to 14.5 months), are promising compared with historical controls. The strategy of MEK inhibition in KRAS mutated CRC should be explored further. Clinical trial information: NCT01116271.