Background: FOLFOX (5-flourouracil, leucovorin, oxaliplatin) and FOLFIRI (fluorouracil, leucovorin, irinotecan) in combination with targeted agents are standard-of-care options for patients for mCRC with response rates >50% in first line. In the second line setting, efficacy of chemotherapy and targeted agents are much lower with response rates of 4% for FOLFIRI + bevacizumab and treatment options are limited in particular for the 50% of patients harboring a RAS mutation. Polo-like kinase 1 (PLK1) is a serine/threonine kinase, master regulator of G2/M cell-cycle progression and genome wide RNAi screens identified PLK1 to be synthetic lethal for KRAS mutated tumor cells inducing cell cycle arrest and apoptosis. Onvansertib is an oral, highly selective PLK1 inhibitor that demonstrated single agent activity and synergistic activity with irinotecan in preclinical CRC models. Additionally, KRAS mutated cells showed higher sensitivity to onvansertib than isogenic KRAS wild-type cells. PLK1 inhibition is a potential target in KRAS-mutated mCRC and onvansertib + FOLFIRI may provide a new second-line treatment option. Methods: The primary objective of this single-arm Phase 1b/2 study (NCT03829410) is to assess the safety and preliminary efficacy of onvansertib in combination with FOLFIRI and bevacizumab in the second line setting for KRAS-mutated mCRC patients. The phase 1b will determine the MTD or RP2D using a traditional 3+3 design, with onvansertib initial dose at 12 mg/m². The phase 2 will enroll 26 patients at the RP2D to further assess the safety of the combination and to evaluate preliminary anti-tumor activity measured by objective response rate (ORR, RECIST v1.1). Based on a one-sided one sample log-rank test with 10% Type I error, there will be at least 90% power to detect an improvement in ORR from 5% to 20% with 26 patients. Exploratory studies include quantitation of KRAS circulating tumor DNA (ctDNA) and genomic studies of circulating tumor cells and ctDNA to determine altered pathways associated with patient response. Clinical trial information: NCT03829410