Background: CX-4945 is a first-in-class casein kinase-2 (CK2) threonine kinase small molecule inhibitor. CK2 is pleiotropic and regulates key cellular processes such as proliferation and DNA damage repair, and has been shown to enhance efficacy of gemcitabine (G) and cisplatin (C) in cell line and xenograft CCA model systems. A Phase IB study of G+C with CX-4945 was conducted in advanced cholangiocarcinoma (CCA) patients. Methods: A multi-center “3+3” dose escalation study was conducted in advanced CCA patients. Eligible patients had ECOG PS 0-1 and adequate hematologic, hepatic, and renal function. Primary endpoints were maximum tolerated dose (MTD) and recommended Phase 2 dose (RP2D). Secondary endpoints included pharmacokinetic (PK) analyses, overall toxicity evaluation and preliminary assessment of efficacy. Routine clinical dosing for G (1000 mg/m2) and C (25 mg/m2), both on days 1 and 8 every 21 days, was used. Dose escalation cohort were CX-4945 given twice daily on days 0,1,2 and 7,8,9 at 200 mg (DL1), 400mg (DL2), 600 mg (DL3), 800 mg (DL4) and 1000 (DL5) mg. NCI CTCAE v4.03 was used for toxicity assessment and RECIST v1.1 for efficacy evaluation. Results: Nineteen patients were enrolled (DL1:n = 3; DL2:n = 4 DL3:n = 3; DL4:n = 4; DL5:n = 5). No dose limiting toxicities were encountered. MTD and RP2D were ascertained to be DL5. No grade 3 or 4 toxicities at prevalence > 10% were encountered and the overall toxicity profile of the combination was reflective of standard G+C toxicity profile. At data cut-off, preliminary efficacy evaluation demonstrated disease control rate (CR+PR+SD) of 64% (PR:32%, SD:32% and CR:0%), along with median progression-free survival (PFS) of 5 months (range 0-14 months). PK analyses showed no evidence of drug-drug interactions between G, C and CX-4945. Conclusions: RP2D for G+C with CX-4945 is DL5 (CX-4945 dosed on days 0,1,2 and 7,8,9 at 1000 mg PO bid with standard G and C dosing). Preliminary clinical efficacy and tolerability of the regimen support planned controlled, randomized Phase 2 evaluation. Clinical trial information: NCT02128282.