Background: Evidence suggests that hyperactivated fibroblast growth factor 4 (FGFR4) signaling pathway leads to enhanced tumor growth. Targeting FGFR4 may have therapeutic benefit in tumors with altered FGF19 signaling. A phase I study (NCT02834780) was undertaken to assess H3B-6527, a highly selective covalent FGFR4 inhibitor, in patients with HCC/ICC. Methods: Adults with advanced HCC/ICC, ECOG PS 0-1, well compensated liver function, who progressed after > one prior therapy, received H3B-6527 po daily (QD) or twice-daily (bid) on a 21-day cycle following a 3+3 design. Doses ranged from 300-2000mg QD or 500-700mg BID. Patients in dose escalation were treated regardless of FGF19 status. Patients in expansion had FGF19+ tumors by mRNA testing. Adverse events (AEs), and pharmacokinetics (PK) were assessed. Response was determined by RECIST 1.1/mRECIST imaging every 6 weeks. Results: Study enrollment is complete at 128 patients. Ninety HCC patients were treated (QD = 48, bid = 42). ICC enrollment was suspended after 38 patients due to limited efficacy. No dose-limiting toxicities were seen and no grade 4-5 treatment related AEs have been observed. Recommended Phase II dose for H3B-6527 is 1000mg QD based upon safety, efficacy, and PK data. Grade 3 TEAEs have occurred in 12.5% of patients on QD dosing. Treatment related TEAEs were seen in 62.5% of patients on the QD schedule, with diarrhea (45.8%), fatigue (12.5%), and nausea (12.5%) most frequent. Drug discontinuation due to AEs for QD dosing was 8.3%. Interim data analysis shows that, for HCC patients with >2 prior lines of therapy treated on QD schedule, overall survival was 10.6m, progression-free survival 4.1m, overall response rate 16.7% (all partial responses), and clinical benefit rate 45.8% (responders + durable stable disease >17 weeks). H3B-6527 Cmax and AUC were lower at 300 mg dose but then similar across 500–2000 mg doses. Following oral administration of 1000 mg fasted, H3B-6527 plasma concentration reached peak at a Tmax of ̃2-3 hours and then decayed exponentially, with terminal half-life of ̃4-5 hours. There was no accumulation following QD dose. Dosing with food did not meaningfully change H3B-6527 plasma exposure. Conclusions: H3B-6527 was well tolerated and demonstrated a favorable toxicity and safety profile and encouraging clinical activity in heavily pretreated HCC patients. Final trial results will be presented at conference. Clinical trial information: NCT02834780