Nuclear medicine imaging of somatostatin receptors using radiolabeled octreotide analogs

Octreotide, an analog of the hormone somatostatin (SS), is an eight amino acid peptide that has a growth inhibitory effect on many types of tumors, including breast, lung and tumors of the neuroendocrine system. Analogs of octreotide have been radiolabeled with I-123 and In-111 and used in gamma camera imaging of somatostatin receptor-positive tumors. Positron emission tomography (PET) offers advantages over gamma camera imaging, since a quantitative assessment of tracer accumulation within tissues is achievable. Quantitating SS receptors would be useful in predicting the eventual clinical response of patients who would subsequently undergo octreotide therapy. Octreotide analogs have been labeled with the short-lived positron emitters Ga-68 (T1/2 = 68 min) and F-18 (T1/2 = 110 min). The authors are interested in Cu-64 (T1/2 = 12.8h), a positron-emitting radionuclide that has applications in both PET imaging and radiotherapy. They have synthesized two macrocyclic chelate-octreotide conjugates, CPTA-octreotide and TETA-octreotide and labeled them with Cu-64. The synthesis, in vitro receptor binding and animal biodistribution data of these Cu-64-labeled octreotide conjugates are described