Distribution and elimination of the somatostatin analogue (111In-DTPA-D-Phe1)-Octreotide (OctreoScan111)

213Bi-[DOTA0, Tyr3]Octreotide Peptide Receptor Radionuclide Therapy of Pancreatic Tumors in a Preclinical Animal Model

PURPOSE

The somatostatin analogue [DOTA0, Tyr3]octreotide (DOTATOC) has previously been labeled with low linear energy transfer (LET) beta-emitters, such as 177Lu or 90Y, for tumor therapy. In this study, DOTATOC labeled with the high-LET alpha-emitter, 213Bi, was evaluated.

EXPERIMENTAL DESIGN

The radiolabeling, stability, biodistribution, toxicity, safety, and therapeutic efficacy of 213Bi-DOTATOC (specific activity 7.4 MBq/microg) were investigated. Biodistribution studies to determine somatostatin receptor specificity were done in Lewis rats at 1 and 3 hours postinjection. Histopathology of various organs was used to evaluated toxicity and safety. Therapeutic efficacy of 4 to 22 MBq 213Bi-DOTATOC was determined in a rat pancreatic carcinoma model.

RESULTS

Radiolabeling of the 213Bi-DOTATOC was achieved with radiochemical purity >95% and an incorporation yield > or = 99.9%. Biodistribution data showed specific binding to somatostatin receptor-expressing tissues. Administration of free 213Bi, compared with 213Bi-DOTATOC, resulted in higher radioactivity accumulation at 3 hours postinjection in the kidneys [34.47 +/- 1.40% injected dose/g (ID/g) tissue versus 11.15 +/- 0.46%, P < 0.0001] and bone marrow (0.31 +/- 0.01% ID/g versus 0.06 +/- 0.02%, P < 0.0324). A significant decrease in tumor growth rate was observed in rats treated with >11 MBq of 213Bi-DOTATOC 10 days postinjection compared with controls (P < 0.025). Treatment with >20 MBq of 213Bi-DOTATOC showed significantly greater tumor reduction when compared with animals receiving <11 MBq (P < 0.02).

CONCLUSIONS

213Bi-DOTATOC showed dose-related antitumor effects with minimal treatment-related organ toxicity. No acute or chronic hematologic toxicities were observed. Mild, acute nephrotoxicity was observed without evidence of chronic toxicity. 213Bi-DOTATOC is a promising therapeutic radiopharmaceutical for further evaluation.

Receptor-Mediated Tumor Targeting with Radiopeptides. Part 1. General Concepts and Methods: Applications to Somatostatin Receptor-Expressing Tumors

Radiolabeled peptides have become important tools in nuclear oncology, both as diagnostics and more recently also as therapeutics. They represent a distinct sector of the molecular targeting approach, which in many areas of therapy will implement the old "magic bullet" concept by specifically directing the therapeutic agent to the site of action. In this three-part review, we present a comprehensive overview of the literature on receptor-mediated tumor targeting with the different radiopeptides currently studied. Part I summarizes the general concepts and methods of targeting, the selection of radioisotopes, chelators, and the criteria of peptide ligand development. Then, the >400 studies on the application to somatostatin/somatostatin-release inhibiting factor receptor-mediated tumor localization and treatment will be reviewed, demonstrating that peptide radiopharmaceuticals have gained an important position in clinical medicine.

Anatomical and Functional Imaging of Metastatic Pheochromocytoma

Although in the majority of patients with pheochromocytoma the tumor is localized in the adrenal, up to 26% of patients have malignant/metastatic disease. Metastatic disease should be ruled out before initial surgery is attempted. Anatomical imaging modalities (computed tomography or magnetic resonance imaging) should be done first over the adrenals, and if negative over the abdomen and if no tumor is found, then the chest and neck should be covered. Regardless of the anatomical imaging results functional imaging with [123-I]- or [131-I]-metaiodobenzylguanidine (MIBG) scintigraphy should be done to corroborate the diagnosis. Negative MIBG scans should be followed by positron emission tomography (PET) studies with specific ligands like [18-F]-dopamine. Persistently negative evaluations should be followed by PET studies with non-specific ligands such as [18-F]-deoxyglucose or somatostatin receptor scintigraphy.

Somatostatin receptor scintigraphy in patients with pituitary adenoma

Presence of high-affinity somatostatin (SST) receptors in most endocrine tumor cells allow in vivo scintigraphic visualization of these neoplasms after intravenous administration of a radionuclide-labeled somatostatin analog. 111In-octreotide is at present the most often used substance for imaging of the SST receptor expression in vivo. The aim of this study is to investigate the correlation between presence of in vivo scintigraphically detectable SST receptors in pituitary tumors and clinical parameters such as patients' age, tumor size, hormonal hypersecretion, and response to octreotide therapy. Forty-two-consecutive patients were enrolled in this trial. Twenty-five of them had nonsecreting pituitary tumors, 11 were acromegalic, and 6 had macro- or microprolactinoma. Scintigraphic images of the head were obtained at 10 min and 24 hours after injection of the radionuclide. In 23 patients, no specific binding of 111In-octreotide was found. Five patients showed a weak positive, 5 had a positive, and 9 a strong positive signal in the region of interest. Uptake of octreotide was significantly correlated with tumor size and age (p < 0.01). Small-size pituitary adenomas were most likely to be scintigraphically receptor-negative, while large suprasellar tumors tended to exhibit a rather strong receptor positivity. Statistical analysis of the data could not confirm the hypothesized correlation between endocrine activity of the pituitary tumors and the scintigraphically proven SST receptor expression in vivo. A positive Octreoscan was not predictive for the result of octreotide therapy.