Background: Tumor suppressor proteins (TSPs) are inactivated by their export from the nucleus by Exportin1 (XPO1/CRM1). The oral selective inhibitor of nuclear export selinexor (KPT-330)restores the nuclear localization and function of TSPs and shows a broad anti-tumor activity in animal models. Here we report on the treatment of a subset of patients (pts) with metastatic colorectal cancer (CRC) in a phase I trial of selinexor. Methods: Objectives were to determine the recommended phase 2 dose, evaluate safety, pharmacokinetics (PK), pharmacodynamics (PD), and tumor response (RECIST 1.1) of selinexor administered in two different schedules with 8 or 10 doses in a 28-day cycle. An expansion cohort of 15 pts with CRC was planned. Results: Twenty-seven pts with CRC were treated across eight doses (3-40 mg/m2) including 15 pts in the expansion cohort (30-35 mg/m2). Median age was 61 yrs and median number of prior regimens was 4. MTD of the 10-dose schedule was 30 mg/m2. MTD for the 8-dose schedule has not been reached. Thirteen pts experienced drug related gr 3-4 adverse events (AEs) including fatigue (n = 6), hyponatremia (n = 5), thrombocytopenia (n = 3), anorexia (n = 3), dehydration (n = 2), anemia (n = 2), hyperglycemia (n = 1), pulmonary embolism (n = 1), and hypotension (n = 1). The most common gr 1-2 drug related AEs were nausea (74 %), anorexia (67 %), fatigue (67 %), vomiting (59 %), dysgeusia (52 %), and weight loss (48 %). Cmax increased with dose to ≈ 1.4 µM with T½ of 4.0 – 7.4 hrs. Significant increases (2-20x) in PD marker XPO1 mRNA in circulating leukocytes were observed at all doses. Analysis of tumor biopsies confirmed nuclear localization of TSPs and induction of apoptosis following selinexor. CEA decreased in 4 of 12 pts. One pt had a partial response at 23 mg/m2, 6 patients had stable disease ≥8 weeks, and 3 patients had stable disease ≥24 weeks in 25 evaluable pts Conclusions: Preliminary signals of antitumor activity in CRC pts were observed. Selinexor is generally well tolerated and prolonged drug exposure is feasible. Selinexor induces Exportin in leucocytes and apoptosis in tumor biopsies with restoration of the nuclear location of TSPs. Clinical trial information: NCT01607905.