Abstract Doxorubicin is a widely used anticancer drug used in the treatment of various cancers. It acts as an intercalator of DNA and inhibits topoisomerase II thus preventing cell replication. Its side effects include nausea, myelosuppression, alopecia, and dose-limiting cardiotoxicity. Dox is nonspecific, and high doses are needed to treat cancer cells. These high doses, however, are also harmful to normal cells. Because of this toxicity to normal cells, there is a great need for a delivery system that will target dox to the tumor site. In our research, we have developed a tumor targeted drug delivery system comprising the elastin-like polypeptide (ELP) which is a thermally responsive macromolecule that remains soluble in physiologic temperature but aggregates upon heating at 42 °C in the tumor. The N-terminus of ELP was modified with the cell-penetrating peptide SynB1. The SynB1-ELP construct was further modified by the addition of an acid-sensitive hydrazone derivative of doxorubicin (DOXO), by conjugation with a 6-maleimidocaproyl moiety on the drug to a terminal cysteine residue on ELP. SynB1-ELP-DOXO was found to have a thermal transition temperature of 41 °C. This construct was 2-fold more effective in inhibiting MCF-7 breast cancer cell proliferation at 42 °C than at 37 °C, indicating that the hyperthermia-induced phase transition of ELP increases the local concentration of the drug. This inhibition may be due to apoptosis since apoptosis was enhanced 2.3-fold in MCF-7 cells when SynB1-ELP-DOXO aggregation was induced by hyperthermia. SynB1-ELP-DOXO had a similar accumulation inside the cell as free Dox as shown by confocal fluorescence imaging, and the uptake into the cell was increased by hyperthermia. In the in vivo studies, a marked reduction in tumor size was seen with SynB1-ELP-DOXO exposed to hyperthermia in the E0771 syngeneic breast cancer model in C57BL/6 mice. The efficacy of the construct was further proved by the animal's body weight increasing over time as compared to treatment with free Dox and a decreasing body weight. An ongoing study uses the combination of free Dox and SynB1-ELP-DOXO at lower doses to reduce tumor size. The ELP macromolecule can also be applied to other small molecule therapeutics opening a potential platform technology for this thermally responsive biopolymer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4771. doi:1538-7445.AM2012-4771