Drazen Raucher, Gene Bidwell and Eddie Perkins

Biochemistry, University of Mississippi Medical Center, Jackson, MS 39216, United States

Abstract:

Although surgical resection with adjuvant chemotherapy and/or radiotherapy are used to treat tumors, normal tissue tolerance, development of metastases, and inherent tumor resistance to radiation or chemotherapy can hinder a successful outcome. Therefore, it is necessary to consider alternative targeted therapeutic approaches for adjuvant therapy that would significantly reduce undesired side effects in normal tissues. We have developed a thermally responsive polypeptide (CPP-ELP-H1) that inhibits cancer cell proliferation by blocking the activity of the oncogenic protein c-Myc. The structure of this anti-tumor agent is based on elastin-like polypeptide (ELP), which is a thermally responsive polymer that reversibly forms aggregates at temperatures just above body temperature. When administered systemically, ELP polypeptides remain soluble and eventually are cleared from circulation at normal body temperature. But, at the tumor site where mild external hyperthermia is applied, ELP aggregates and accumulates. ELP is modified with a cell penetrating peptide (CPP) to allow escape from the tumor vasculature and entry into the tumor cells, and with an inhibitory peptide that blocks transcriptional activation by c-Myc (H1).

When administered IV or IP into mice bearing orthotopic, syngeneic breast tumors, CPP-ELP-H1 exhibited a long plasma half life and accumulated in tumor tissue. The tumor levels of the CPP-ELP delivered H1 peptide were enhanced when hyperthermia was applied to the tumor site. Daily administration (7 days) of CPP-ELP-H1 followed by tumor heating induced over 70% reduction in tumor volume compared to unheated controls or controls lacking the H1 inhibitory peptide.

We also tested the CPP-ELP-H1 polypeptide in a rat model of glioma. Delivery of drugs to gliomas is particularly challenging due to the tumors' high resistance to chemotherapeutic agents, poor penetration of drugs across the blood brain barrier (BBB), and damaging effects of chemotherapy and radiation to normal neural tissue. Sprague Dawley rats were implanted with C6 tumors by intracerebral injection. Pharmacokinetics, tumor deposition, and biodistribution of the CPP-ELP-delivered therapeutic peptide were determined by quantitative fluorescence analysis. Brain tumor progression following CPP-ELP-H1 therapy was monitored using MRI. We demonstrated that the brain tumor targeting of ELP following systemic administration could be enhanced up to 5-fold by the use of cell penetrating peptides, and thermal targeting can enhance brain tumor uptake by an additional 3-fold. When the lead ELP-fused c-Myc inhibitor was combined with focused hyperthermia of the brain tumors, we observed 80% reduction in tumor volume, delayed onset of tumor-associated neurological deficits, and significant extension of survival including complete regression in 80% of animals. This work provides a new modality for targeted delivery of a specific oncogene inhibitor. In addition to the peptide's intrinsic tumor targeting due to its oncogene inhibition, ELP can be physically targeted to the tumor site by application of external hyperthermia. Specific targeting of these therapeutic polypeptides by local hyperthermia to breast or brain tumors or to areas of potential metastasis following tumor resection would increase specificity and efficacy of treatment and reduce the cytotoxicity in normal tissues.