English: The natural somatostatin receptor ligand, the 14 amino acid peptide somatostatin (A), was abridged to the biologically more stable 8 amino acid peptide Octreotide (OC, B), which is used for the treatment of symptomatic neuroendocrine tumors. Introduction of a tyrosine into the 3rd position of the Octreotide sequence resulted in Tyr3-Octreotide (TOC, C), which allows for iodination of the tyrosine residue with the γ-emitter 123I and subsequent somatostatin receptor targeted imaging. For the use in somatostatin receptor targeted radiotherapy TOC was coupled with the chelator DOTA, which led to the study drug, the octapeptide DOTA-TOC (D).