Favorable decay properties, availability and affordability make 99mTc the radionuclide of choice for single photon emission computed tomography (SPECT) imaging. Chelating 99mTc to a biomolecule, such as a somatostatin receptor (SSTR)-seeking peptide, allows targeted delivery of this imaging radionuclide to neuroendocrine tumors expressing high densities of SSTRs relative to healthy tissues. Rhenium is the third row congener of technetium and has similar chemical properties. Thus, translation of 99mTc chemistry to the therapeutic radionuclide 186Re can potentially create a matched radionuclide pair, for both diagnostic and therapeutic applications in cancer management. In this study, SSTR-targeting peptide sst2-ANT (4-NO2-Phe-c(DCys-Tyr-DTrp-Lys-Thr- Cys)-DTyr-NH2) was successfully conjugated to a tridentate NSN-type ligand (L). A monocationic 99mTc tricarbonyl core was used to label L-sst2-ANT, producing two isomers with an 85% yield. Technetium-99m product identities were confirmed by HPLC co-elution with the analogous non-radioactive Re complexes, and their masses were confirmed with LC-MS. Full characterization of these complexes is currently being completed. Initial in vitro stability studies of 99mTc(CO)3(L-sst2-ANT) were promising, with stabilities of 89% in histidine and 60% in rat serum over 24 hours at physiological conditions. Future competitive receptor binding affinity studies will demonstrate the potential of 99mTc(CO)3(L-sst2-ANT) to target neuroendocrine tumors with high specificity.

Synthesis of a somatostatin receptor-seeking peptide labeled with rhenium and technetium-99m tricarbonyl cores

247th National Meeting (2014)

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