LAUSR

e22502 Background: Eltrombopag (EP) is a small molecule, thrombopoietin receptor (TPO-R) agonist indicated for the treatment of patients with chronic immune thrombocytopenia and severe aplastic anemia. EP is a polyvalent cation chelator and inhibits leukemia cell proliferation via depletion of intracellular iron. Recent studies show EP inhibits the proliferation of osteosarcoma cells lines via depletion of polyvalent cations. The in vivo effects of EP were studied in osteosarcoma patient derived xenograft models by the Pediatric Preclinical Testing Consortium (PPTC). Methods: The in vivo anticancer effects of EP were assessed in a panel of six osteosarcoma PPTC PDX models with limited MPL mRNA expression (OS2, OS9, OS31, OS33, OS36, OS60). EP was administered at an oral dose of 5 mg/kg/day given for 5 days each week with planned treatment period of 4 weeks. High dose EP (50mg/kg/day) was also tested in 2 PDX models (OS2, OS9) on the same schedule. A control cohort that received vehicle was included for each PDX model. Tumor volumes were measured and responses defined utilizing the PPTC statistical analyses. Results: EP at 5 mg/kg failed to inhibit tumor growth or induce significant differences in event-free survival (EFS) in any of the 6 osteosarcoma PDX models. At the higher dose of 50 mg/kg a significant prolongation in time to event in the EP-treated group was observed, but the effect was small with the ratio of the median time to event for treated versus control animals (EFS T/C) being only 1.2 in the 2 OS PDX models tested. No objective responses were observed with EP at either dose, with all models demonstrating progressive disease. Conclusions: EP did not exhibit significant antitumor activity against the PPTC osteosarcoma PDX panel. However, EP also did not enhance tumor growth. EP’s lack of anti-tumor activity against the OS PDX models suggests leukemia and osteosarcoma cells likely have different dependencies on intracellular polyvalent cations. Given EPs effect on stimulating platelet production, and the demonstration that EP does not stimulate in vivo growth of osteosarcoma, EP may be considered in patients with osteosarcoma as a supportive care agent in supporting platelet recovery.