Purpose DFP-10917 is a novel deoxycytidine analog with a unique mechanism of action. Brief exposure to high concentrations of DFP-10917 inhibits DNA polymerase resulting in S-phase arrest, while prolonged exposure to DFP-10917 at low concentration causes DNA fragmentation, G2/M-phase arrest, and apoptosis. DFP-10917 demonstrated activity in tumor xenografts resistant to other deoxycytidine analogs. Experimental design Two phase I studies assessed the safety, pharmacokinetic, pharmacodynamic and preliminary efficacy of DFP-10917. Patients with refractory solid tumors received DFP-10917 continuous infusion 14-day on/7-day off and 7-day on/7-day off. Enrollment required age > 18 years, ECOG Performance Status 0-2 and adequate organ function. Results 29 patients were dosed in both studies. In 14-day infusion, dose-limiting toxicities (DLT) consisting of febrile neutropenia and thrombocytopenia occurred at 4.0 mg/m2/day. At 3.0 mg/m2/day, 3 patients experienced neutropenia in cycle 2. The dose of 2.0 mg/m2/day was well tolerated in 6 patients. In 7-day infusion, grade 4 neutropenia was DLT at 4.0 mg/m2/day. The maximum tolerated dose was 3 mg/m2/day. Other toxicities included nausea, vomiting, diarrhea, neutropenia, and alopecia. Eight patients had stable disease for >12 weeks. Paired comet assays performed for 7 patients showed an increase in DNA strand breaks at day 8. Pharmacokinetic data showed dose-proportionality for steady-state concentration and AUC of DFP-10917 and its primary metabolite. Conclusion Continuous infusion of DFP-10917 is feasible and well tolerated with myelosuppression as main DLT. The recommended doses are 2.0 mg/m2/day and 3.0 mg/m2/day on the 14-day and 7-day continuous infusion schedules, respectively. Preliminary activity was suggested. Pharmacodynamic data demonstrate biological activity at the tested doses.