This proposal outlines a body of research to evaluate the cytotoxicity and therapeutic efficacy of radiolabeled alpha-melanocyte stimulating hormone (a-MSH) peptide analogs that target a-particle emitting radioisotopes to the nuclei of melanoma cells. The selective targeting of alpha-particle emitting radioisotopes for cancer therapy is emerging as an exciting and potentially powerful new treatment strategy in targeted radiotherapy. Alpha-radiation is so densely ionizing that it forms destructive radicals directly, independent of oxygen content of the tissue and its cytotoxicity is independent of the dose rate, resulting in potentially powerful therapeutic index. The pathlengths of a-particles are short, on the order of 55-80 fm (~4-6 cell diameters), therefore, the cytotoxic properties of a-particles are highly focused, which minimizes collateral radiation damage of healthy tissues. Based on microdosimetry studies on a-particle cytotoxicity, it appears that a-particles must traverse the cell nucleus to be effective in causing cell death. Designing a biological targeting vehicle that would deposit an a-emitter in the nucleus of a cell would be highly advantageous. To that end a prototype nuclear targeting a-MSH peptide (DOTA-NT-MSH) was developed and shown to localize in the nuclei of melanoma cell by fluorescence imaging. The goal of the proposed research is to determine the efficacy targeting a- emitting radionuclides to the nucleus in vitro at the cellular level and in vivo in melanoma animal models. The specific objectives of this research are: 1) optimize the radiolabeling efficiency and purification of the nuclear targeting peptide with 212 Pb the parent of the alpha emitter 212 Bi, 2) examine the in vitro subcellular distribution and cytotoxicity of 212 Pb-labeled DOTA-NT-MSH and a control DOTA-nonspecific peptide, 3) determine the biodistribution of DOTA-NT-MSH and DOTA-NDPMSH peptides in melanoma bearing mice. Malignant melanoma is a serous health problem due to an increase in incidence and resistance to conventional chemotherapeutics and external beam radiation. In spite of recent advances in cancer therapy, the median life expectancy for patients with disseminated metastatic disease remains at 12-15 months. There is a clear need for the development of new and efficacious therapy agents to attack malignant melanoma. The development of targeted a-therapy for malignant melanoma is attractive since the short particle pathlength of a-emitters is particularly well match small clusters of disseminated tumor cells that characterize metastatic disease. Preliminary results from our laboratory have demonstrated that peptide targeted a-emitters are very effective in treating melanoma in pre-clinical therapy studies. Refinement of the peptide-targeted a-therapy approach through nuclear localization of radionuclides should result in improved therapeutic efficacy at potentially lower doses due to higher cell killing efficiencies. Peptide targeted alpha-therapy for metastatic melanoma has the potential to provide tumor-specific and efficacious treatment of advanced stage disease, which is resistant to current therapeutic approaches
