The somatostatin receptor, which is overexpressed by many neuroen-docrine tumors, is a well-known target for molecular imaging and peptide receptor radionuclide therapy. Recently, ⁵⁷Co-labeled DOTA-TOC, an octreotide analog, was shown to have the highest affinity yet found for somatostatin receptor subtype 2. The aim of this study was to evaluate the biologic effects of novel cobalt-labeled octreotide analogs targeting the somatostatin receptor to identify promising candidates for molecular imaging and Auger electron-based radionuclide therapy. Methods: Cobalt-labeled DOTATATE, DOTATOC, and DOTANOC were prepared with ⁵⁷Co or ^58mCo for SPECT or Auger electron-based therapy, respectively. The cellular uptake and intracellular distribution of the radioligands were characterized with the pancreatic tumor cell line AR42J in vitro, including assessment of the therapeutic effects of ^58mCo-DOTATATE via DNA double-strand break and proliferation assays. Comparisons with the therapeutic effects of ¹¹¹In-and ¹⁷⁷Lu-DOTATATE were also performed. Tumor uptake and normal tissue uptake were characterized in a subcutaneous pancreatic tumor mouse model. Results: All 3 cobalt-conjugated peptides resulted in time-dependent and receptor-specific uptake, with a high level (≥88%) of cellular in-ternalization in vitro of the total cell-associated radioactivity. The DNA double-strand break yield showed a dose-dependent increase with activity, whereas cell survival showed a dose-dependent decrease. ^58mCo- DOTATATE was significantly more efficient in cell killing per cumulated decay than ¹¹¹In- and ¹⁷⁷Lu-DOTATATE. The in vivo phar-macokinetic studies showed a high level of receptor-specific tumor uptake. Conclusion: All cobalt-labeled radioligands showed a high level of receptor-specific uptake both in vitro and in vivo in tumor-bearing mice. Furthermore, ^58mCo-DOTATATE showed considerable therapeutic effects in vitro and, thus, could be an effective agent for eradicating disseminated tumor cells and micrometastases. COPYRIGHT