Background: Dasatinib is a potent oral ATP competitive multi-targeted kinase inhibitor of multiple members of the Src kinase family, known to be involved in gliomagenesis and invasion. N0877 is a phase I/randomized phase II trial evaluating the combination of dasatinib, radiation (RT) and temozolomide (TMZ) in newly diagnosed GBM. The phase I portion has been completed and is the focus of this report. Methods: A cohorts-of-3 design was used to assess the safety of dasatinib in combination with RT and concomitant TMZ, and establish the phase II dose of the combination. Dasatinib was given orally for 42 days, beginning with the first day of RT (total dose 60 Gy) and first dose of TMZ (75 mg/m2/d). A 24 - 42 day rest (cycle 2) followed the RT/TMZ/dasatinib. Patients (pts) were observed for DLT to the end of cycle 2. Patients then received 6 cycles (28 day cycles) of dasatinib (once daily) and TMZ (days 1-5). At the completion of 6 cycles of TMZ + dasatinib, pts stay on dasatinib only (28 day cycles) until progressive disease. Results: Phase I component is complete with 13 patients (3 at dose level 0, 3 at dose level 0-A, 7 at dose level 1). One patient in dose level 1 had to be replaced due to the development of unrelated medical issues. One DLT (grade 4 pancytopenia) was observed in 1 patient at dose level 0 (50mg bid) and one DLT (grade 3 rash) was observed in 1 patient at dose level 1 (150mg qd). MTD of dasatinib was determined to be 150mg daily. Most common adverse events throughout the entire study period were hematologic with the most common toxicity being lymphopenia (grade 3 in 69% of patients, grade 4 in 8%). Other toxicities attributed to treatment and occurring in > 10% of patients included anemia (31%), neutropenia (15%), and fatigue(15%). Best response was stable disease in 10 patients, progressive disease in 1 patient, and not evaluable in 2. Conclusions: MTD for dasatinib in combination with TMZ and RT in newly diagnosed GBM patients is 150mg daily. Toxicity was primarily hematologic with minimal non-hematologic events. This dose is currently being used in the ongoing placebo-controlled, randomized phase II trial.