Background: Mutant KRAS tumors show a dependency on WT-H/N-Ras for activation of ATR/Chk1-mediated G2 DNA damage response (Grabocka, Cell, 2015). We have shown in vitro that the Wee1 kinase inhibitor AZD1775, which acts to abrogate the G2 DNA damage checkpoint and induces replication stress during S-phase, selectively sensitizes RAS/RAF mutant cells to the DNA damaging agent irinotecan. Up to 65% of metastatic colorectal cancers harbor RAS or BRAF mutations and these patients have limited treatment options following first line therapy. Methods: This is an open label, single-arm, phase Ib study using a modified 3+3 dose-escalation schedule with expansion cohort. Primary objective is to determine the MTD of AZD1775 in combination with irinotecan as 2^nd-line therapy in patients with metastatic KRAS, NRAS or BRAF mutated colorectal cancer. Up to 18 patients will be enrolled in the dose escalation portion. Standard dose irinotecan is given on day 1 of every 2 week cycle. AZD1775 is administered PO twice daily for 3 to 5 days of each cycle, starting cycle 2. The maximum tolerated dose (MTD) is defined as the highest dose level at which ≤1 of 6 patients experience a dose limiting toxicity. Once the MTD is reached and/or recommended dose for expansion is determined, a dose expansion cohort of 14 patients will be enrolled. Secondary endpoints include characterizing the safety profile at the MTD, obtaining a preliminary estimate of efficacy for the combination (measured by overall response rate, progression-free and overall survival rates), and obtaining pharmacokinetic parameters. Pre- and on-treatment biopsies will be collected from the expansion cohort to determine: adequate target engagement of Wee1, changes in markers of DNA damage, TP53 mutation status, and changes in gene expression profiles in order to identify potential biomarkers of response. At February 2017, 2 patients have been enrolled on this study. Clinical trial information: NCT02906059