We report the in vivo efficacy, in tumor-bearing mice, of cancer prodrugs consisting of poly(methacryloyloxyethyl phosphorylcholine) (polyMPC) conjugated to doxorubicin (DOX). Our synthesis of polyMPC-DOX conjugates established prodrugs with tunable drug loading, pH sensitive release kinetics, and a maximum tolerated dose in the range of 30-50 mg/kg (DOX equivalent) in healthy mice. Here we show prolonged circulation of polyMPC-DOX, with a measured in vivo half-life (t1/2) 8 times greater than that of the free drug. We observed reduced drug uptake in healthy tissue, and 2-3 times enhanced drug accumulation in tumors for polyMPC-DOX prodrugs compared to free DOX, using BALB/c mice bearing 4T1 tumors. Prolonged survival and reduced tumor growth were observed in mice receiving the polyMPC-DOX prodrug treatment. Moreover, we evaluated immunogenicity of polyMPC-DOX prodrugs by examining complete blood count (CBC) and characteristic cytokine responses, demonstrating no apparent innate or adaptive immune system response.