Glioblastoma (GBM) ranks among the most common, aggressive, and least curable cancers due to a strong tendency for intracranial dissemination, high proliferation potential and inherent tumor resistance to radiation or chemotherapy. Current treatments face the critical challenge of adverse treatment effects in normal tissue within the brain itself and also when the agent cannot adequately penetrate the blood brain barrier (BBB) to reach the tumor microenvironment. We have developed a system to selectively deliver chemotherapeutic doxorubicin to GBM tumors. This carrier is based on elastin-like polypeptide (ELP), which is soluble at physiological temperatures but undergoes a phase transition and accumulates at tumor sites with externally applied, mild (40-41°C) hyperthermia. The CPP-ELP-Dox conjugate consists of a cell penetrating peptide (CPP), which facilitates transcytosis through the BBB and cell entry, and the 6-maleimidocaproyl hydrazone derivative of doxorubicin (Dox) at the C-terminus of ELP. The acid-sensitive hydrazone linker ensures release of Dox in the lysosomes/endosomes after cellular uptake of the drug conjugate. We have shown that CPP-ELP-Dox effectively inhibits cell proliferation in three GBM cell lines; GBM6, D54, and U251 with IC50 values of 250 nM, 60nM, and 30nM, respectively. Both the free drug and CPP-ELP-Dox conjugate exhibited similar in vitro cytotoxicity, although their subcellular localization was considerably different. The Dox conjugate was mainly dispersed in the cytoplasm, while free drug was largely dispersed in the cytoplasm but also had partial nuclear accumulation. The accumulation of free Dox in GBM cells was measured by flow cytometry to determine whether the cytotoxic activity of free Dox or CPP- ELP-Dox is related to intracellular Dox levels. Intracellular Dox concentration was increased in the CPP-ELP-Dox cells compared to that in the free Dox cells, which positively correlates with cytotoxic activity. Flow cytometry was also used to quantitatively investigate apoptosis of GBM cells. After incubation for 24 h, the total apoptosis percentage of free Dox was 1.5% in GBM6, 6% in D54, and 7% in U251 cells. However, the total apoptosis rates of CPP-ELP-Dox treated cells were 5%, 12% and 5% in GBM6, D54, and U251 cells respectively, which confirms increased apoptosis compared to free Dox treated cells. Cell cycle analysis of GBM cells treated with free Dox showed enhanced arrest of the cells in G1 phase, while there was increased percentage arrest of cells in G2 phase in cells treated with CPP-ELP-Dox.In summary, our findings demonstrate that CPP-ELP-Dox effectively kills GBM cells. Development of such a drug carrier has the potential to greatly improve current therapeutic approaches for GBM by increasing the specificity and efficacy of treatment and reducing cytotoxicity in normal tissues.
Citation Format: Sonja Dragojevic, Rebecca B. Mackey, Felix Kratz, Drazen Raucher. Targeted delivery of doxorubicin to glioblastomas by thermally sensitive polypeptides [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3708.