2591Background: Despite widespread clinical integration, refinement of treatment with sunitinib is actively pursued. Sub-therapeutic blood levels rather than true resistance and tumor adaptation through drug accumulation have been accounted as reasons for treatment failure. Based on our preclinical data with high dose sunitinib and prospective analyses supporting the concept of intermittent dosing, we designed a phase 1 trial to investigate the feasibility and tolerability of high dose, once weekly (1w) or once every two weeks (2w) sunitinib (NCT02058901). Methods: Eligible were patients (pts) with advanced solid tumors, refractory to standard treatment, measurable disease, WHO ≤ 1. Sunitinib was administered orally 1w or 2w. Starting dose was 200 mg, with cohorts escalating in 100 mg steps until maximum tolerated dose (MTD). Response was evaluated by RECIST 1.1. Treatment continued until progression or unacceptable toxicity. Dedicated PK sampling was performed. Sunitinib plasma concentration was measured...