Abstract only 10012 Background: Sorafenib, an oral multitargeted kinase inhibitor, is indicated for treatment of adults with refractory renal cell or hepatocelluar carcinoma. We performed a phase I trial to determine the toxicities, maximum tolerated dose (MTD), pharmacokinetics (PK), and pharmacodynamics (PD) of sorafenib in children with refractory solid tumors. Methods: Sorafenib was administered q12h for 28 consecutive day cycles. Cohorts of 3–12 patients were enrolled at 105, 130, 150, 200, and 250 mg/m 2 /dose dose levels. Results: 34 eligible pts 16M, median age 14.6 yrs, (range, 5–21) with osteosarcoma (n = 8), rhabdomyosarcoma (n = 3), other sarcomas (n = 13), hepatoblastoma (n = 3), or other solid tumors (n = 7) received 1–22 cycles (median 2). Grade 3 dose-limiting toxicity (DLT) occurred in 4/6 pts at the starting dose (150 mg/m 2 ) and included hypertension (n = 1), rash/urticaria (n = 1), back pain (n = 1), thrombocytopenia (n = 1) and ALT/AST (n = 1). No DLTs were observed at 105 (n = 6) or 130 (n = 3) mg/m 2 , and the dose was re-escalated to 150 mg/m 2 with modified eligibility criteria (normal ALT) and revised guidelines for grading and management of hypertension. Gr 3 DLTs occurred in 1/6 pts (lipase) at 150 mg/m 2 and 2/2 pts (hyponatremia, hand-foot syndrome) at 250 mg/m 2 . At 200 mg/m 2 only 1/6 pts experienced DLT (gr 3 ALT). No objective responses were observed, but 2 pts had tumor shrinkage. Sorafenib AUC did not increase proportionally with dose - the mean AUC 0–24h was similar at 150 mg/m 2 (28±24 μg · h/mL, n = 9) and 200 mg/m 2 (28±17 μg · h/mL, n = 4). T max was prolonged and variable (10±11 h, n = 19). Plasma VEGFR (n = 13) decreased from 9.9±1.6 ng/mL at baseline to 8.3±1.7 ng/mL by d 28 (p < 0.001). Conclusions: The MTD of sorafenib in children with solid tumors is 200 mg/m 2 , similar to the adult recommended dose (400 mg). No significant financial relationships to disclose.