Ku80-Targeted pH-Sensitive Peptide-PNA Conjugates Are Tumor Selective and Sensitize Cancer Cells to Ionizing Radiation

The development of therapeutic agents that specifically target cancer cells while sparing healthy tissue could be used to enhance the efficacy of cancer therapy without increasing its toxicity. Specific targeting of cancer cells can be achieved through the use of pH-low insertion peptides (pHLIP), which take advantage of the acidity of the tumor microenvironment to deliver cargoes selectively to tumor cells. We developed a pHLIP-peptide nucleic acid (PNA) conjugate as an antisense reagent to reduce expression of the otherwise undruggable DNA double-strand break repair factor, KU80, and thereby radiosensitize tumor cells. Increased antisense activity of the pHLIP-PNA conjugate was achieved by partial mini-PEG sidechain substitution of the PNA at the gamma position, designated pHLIP-alpha Ku80(g). We evaluated selective effects of pHLIP-alpha Ku80(gamma) in cancer cells in acidic culture conditions as well as in two subcutaneous mouse tumor models. Fluorescently labeled pHLIP-alpha Ku80(gamma) delivers specifically to acidic cancer cells and accumulates preferentially in tumors when injected i.v. in mice. Furthermore, pHLIP-alpha Ku80(gamma) selectively reduced KU80 expression in cells under acidic conditions and in tumors in vivo. When pHLIP-alpha Ku80(gamma) was administered to mice prior to local tumor irradiation, tumor growth was substantially reduced compared with radiation treatment alone. Furthermore, there was no evidence of acute toxicity associated with pHLIP-alpha Ku80(gamma) administration to the mice. These results establish pHLIP-alpha Ku80(gamma) as a tumor-selective radiosensitizing agent.