Neuroendocrine tumors (NETs) are rare tumors with variable malignant behavior. The majority of NETs express increased
levels of somatostatin (SST) receptors, particularly SST2 receptors. Radiolabeled peptides specific for the SST2 receptors may be used for
diagnosis of NETs and for peptide receptor radionuclide therapy (PRRT). [111In-DTPA0]-octreotide has been the first peptide used for
PRRT. This radiolabeled peptide, emitting Auger electrons, often induced symptomatic relief, but objective morphological responses
were rarely documented. After the introduction of the chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) other
peptides, primarily [DOTA(0),Tyr(3)]octreotate (DOTATATE) and [DOTA(0),Tyr(3)]octreotide (DOTATOC) were labeled with 90Y or
177Lu and used for therapy applications. The rate of objective response obtained with these radiolabeled peptides ranges between 6% and
46%, owing to differences in inclusion criteria adopted in different studies, length and type of therapy, and criteria of evaluation of the
response. The present data in the literature do not allow defining the most suitable peptide and radionuclide for the treatment of NETs.
Instead emerging evidence indicates that a combination of nuclides with different physical characteristics might be more effective than
the use of a single nuclide. Kidney and bone marrow toxicity are the limiting factors for PRRT. Mild toxicity is often encountered while
severe toxicity is rarer. Toxicity could be reduced and therapeutic efficacy enhanced by patient-specific dosimetry. Future directions
include different issues of PRRT, such as defining the most suitable treatment scheme, evaluation of new peptides with different affinity
profiles to other SST receptor subtypes, and reduction of toxicity.
Keywords: Neuroendocrine tumors, Peptide receptor radionuclide therapy, Somatostatin receptors, Epidemiology, Diagnosis, Radiopharmaceuticals, Radionuclides, Plane waves, octreotide, spleen
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