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Evaluation of somatostatin receptors in human cancer
T Pantev1, I Virgolini, N Neuhold
1Department of Nuclear Medicine, University of Vienna.
Abstract:
The binding of 123I-Tyr-3-octreotide (SDZ-204-090; specific activity 1 mCi/nmol), a new somatostatin-receptor binding radiopharmaceutical, to human tumour membrane fractions was evaluated in presence of unlabeled Tyr-3-octreotide and octreotide (SMS-201-995; Sandostatin). Tumour tissue was obtained intraoperatively from 15 patients with different endocrine tumours (insulinoma, carcinoide, phaechromocytoma, hypophysal adenoma) and breast cancer. In equilibrium experiments, membrane fractions (200 micrograms protein/ml) were incubated with increasing concentrations of 123I-Tyr-3-octreotide (0.03-30 nM) in presence or absence of 5 microM of unlabeled agonist. Binding capacities ranged from 1-20 pmol/mg protein (Kd 4-100 nM). The IC50 values (2.5-112 nM versus 0.02-69 nM) were different for the octreotide and Tyr-3-octreotide indicating that octreotide was the better competitor as Tyr-3-octreotide for 123I-Tyr-3-octreotide binding sites. In ductal breast cancer high numbers of in vitro binding sites for the radiolabel were found. In initial clinical studies 123I-Tyr-3-octreotide was i.v.-injected (3 mCi) to 5 acromegaly patients with hypophyseal adenomas. Following rapid uptake by the liver, positive tumour imaging was obtained in 3 patients which correlated to computer tomographic findings. Positive images were obtained just some minutes after injection. Our results support recent data suggesting that the 123I-Tyr-3-octreotide would be a suitable receptor-radiopharmaceutical for the localization of endocrine tumours.
Insights
123I-Tyr-3-octreotide is a promising somatostatin receptor imaging agent for endocrine tumors. It showed good binding affinity and positive tumor visualization in initial clinical studies.
Area of Science:
- Nuclear Medicine
- Radiopharmaceutical Chemistry
- Oncology
Background:
- Somatostatin receptors are overexpressed in various endocrine tumors.
- Radiolabeled somatostatin analogs are valuable tools for tumor imaging.
- 123I-Tyr-3-octreotide is a novel radiopharmaceutical targeting somatostatin receptors.
Purpose of the Study:
- To evaluate the binding characteristics of 123I-Tyr-3-octreotide to human tumor membrane fractions.
- To assess the potential of 123I-Tyr-3-octreotide as a diagnostic imaging agent for endocrine tumors.
Main Methods:
- In vitro binding assays using human tumor membrane fractions and radioligands.
- Competition studies with unlabeled Tyr-3-octreotide and octreotide.
- Initial clinical imaging studies in patients with acromegaly and hypophyseal adenomas.
Main Results:
- 123I-Tyr-3-octreotide demonstrated specific binding to somatostatin receptors in various endocrine tumors and breast cancer.
- Octreotide was a more potent competitor than Tyr-3-octreotide for the binding sites.
- Initial clinical studies showed positive tumor imaging in acromegaly patients, correlating with CT findings.
Conclusions:
- 123I-Tyr-3-octreotide exhibits favorable binding properties for somatostatin receptors.
- The radiopharmaceutical shows potential for the in vivo localization of endocrine tumors.
- Further studies are warranted to confirm its clinical utility in a broader patient population.