Background: DFP-11207 combines a 5-fluorouracil (5-FU) pro-drug with a reversible dihydropyrimidine dehydrogenase inhibitor and a potent inhibitor of orotate phosphoribosyl transferase resulting in enhanced pharmacological activity of 5-FU with decreased gastrointestinal and myelosuppressive toxicity. Methods: Patients with advanced solid tumors were treated in this single arm, dose escalation study. Accelerated dose escalation using single pt cohorts was followed until drug-related Gr2 adverse events occurred; then a 3+3 design was followed to determine maximum tolerated dose (MTD). Pts were dosed daily in 28-day cycles until intolerable toxicity or disease progression. PK sampling was performed for DFP-11207 metabolites on all pts and a 6 pt cohort received DFP-11207 in fed and fasted states to determine drug bioavailability. Results: Primary tumors among the 23 enrolled pts included esophageal, colorectal, gastric, pancreatic and gallbladder. Seventeen pts were treated at 8 dose levels of oral DFP-11207 administered daily, ranging from 40 to 440 mg/m²/d. At 440 mg/m²/d one pt experienced drug-related Gr3 dehydration and mucosal inflammation, and Gr4 febrile neutropenia; a second pt experienced Gr4 febrile neutropenia. One DLT of Gr3 vomiting occurred among 6 pts treated at 330 mg/m²/d dosed q12hrs confirming the MTD. Six pts were administered 300 mg DFP-11207 q12 hrs in a fed or fasted state to determine drug bioavailability. Among all pts, the most frequently reported drug-related TEAEs were fatigue (47.8%), nausea (47.8%), decreased appetite (39.1%), diarrhea (26.1%), vomiting (21.7%), anemia (13.0%), dysgeusia (13.0%), mucosal inflammation (13.0%) and palmar-plantar erythrodysaesthesia syndrome (13.0%). PK analyses of pts treated at 330 and 440 mg/m²/d indicate 5-FU concentrations of ~20 ng/mL throughout the dose cycle. There was no substantial difference in DFP-11207 bioavailability among pts in a fed or fasted state. Out of 21 efficacy evaluable pts, 7 pts had stable disease (33.3%), of which 2 had prolonged stable disease of > 6 months. No pts achieved CR or PR. Conclusions: DFP-11207 at the dose of 330 mg/m²/d q12hrs is well-tolerated in pts with solid tumors with mild myelosuppressive and gastrointestinal side effects, and results in circulating 5-FU levels conducive to an anti-tumor effect. DFP-11207 can be explored as monotherapy or substitute for 5-FU, capecitabine or S-1 in combination treatment regimens. Clinical trial information: NCT02171221