No abstract

The non-invasive detection of insulinomas remains a diagnostic problem that is not solved by means of somatostatin receptor scintigraphy. We investigated the biokinetics and specificity of uptake and degradation of the incretin hormone glucagon-like peptide-1 (GLP-1) in a rat insulinoma cell line (RINm5F) in order to ascertain whether radiolabelled GLP-1 may be suitable for specific visualisation of insulinomas in vivo. GLP-1 (7-36)amide was radioiodinated according to the iodogen method. The specificity of the uptake of [(125)I]GLP-1(7-36)amide by RINm5F cells was investigated. Degradation products of GLP-1 (7-36)amide in the cell medium were purified by HPLC. Their masses and amino acid sequences were determined by (252)Cf-plasma desorption mass spectrometry. Lysosomal degradation was inhibited and after differential centrifugation the amount of radiotracer incorporated into lysosomes was determined. Biodistribution studies were performed in a rat insulinoma model (NEDH rats and RINm5F cells) with [(123)I]GLP-1(7-36)amide and its more stable agonist [(123)I]exendin 3. The uptake of radiotracer into insulinoma cells reached a maximum within 5 min. It was inhibited by an excess of unlabelled peptide. [(125)I]GLP-1(7-36)amide accumulated in the cells if lysosomal degradation was inhibited. Degradation products of the peptide were found in the cell medium. We determined their mass and derived their amino acid sequence. Radiolabelling of exendin 3 was more difficult than that of GLP-1 because of the lack of tyrosine in its primary structure. Biodistribution studies showed rapid blood clearance and uptake of the radiotracer into the tumour and the pancreas. It was also possible to detect insulinomas in an animal model by external scintigraphy using radioiodinated GLP-1 (7-36)amide and exendin 3. GLP-1 (7-36)amide is specifically internalised into insulinoma cells by a receptor-mediated mechanism. Our results demonstrate that GLP-1 receptor-directed scintigraphy may be a new method for the detection of insulinomas in vivo. Due to the short half-life of GLP-1, its more stable analogue exendin 3 may better suit this purpose in vivo.

A prospective study was performed to determine the efficacy of octreotide (Sandostatin; SMS 201-995) 200 micrograms tid in controlling tumor growth. The study included 21 patients with metastasized endocrine GEP tumors: 6 gastrinomas, 8 carcinoid syndromes, 7 nonfunctioning tumors. Treatment was performed for 3 to 59 months (median 15 months). Evaluation of the response to octreotide was facilitated in 12 patients by a pretreatment observation period of 3 to 47 months (median 17 months) during which the natural growth behavior was determined. Based on the presence or absence of a control period prior to treatment, 5 patients were considered to be responders, 7 as questionable responders (no pretreatment phase available), and 9 as nonresponders. None of the 21 patients had documented shrinkage of the tumor mass. The most favorable response was tumor standstill. In all but one responder an escape to an initially favorable response occurred after 6 to 28 months (median 14 months). Proved inhibition of growth was paralleled by a reduction of serum and urine hormone parameters, whereas unaltered progression of tumor growth was observed also in the presence of hormone suppression. Tumor growth and hormone release was inhibited in the absence and presence of somatostatin receptors on the tumor. It is concluded that octreotide exerts a limited effect on metastatic GEP tumor growth. The evaluation of a response to octreotide is facilitated by an observation period prior to the drug that provides information on growth characteristics of the tumor. The presence of octreotide receptors does not predict the success of therapy.

Aim: Many studies describe the sensitivities and specificities of computed tomography (CT), magnetic resonance imaging (MRI), and somatostatin receptor scintigraphy (SRS) in patients with gastrointestinal neuroendocrine tumors (GNT). We performed a study to evaluate the influence of these techniques on the therapeutic management of patients with advanced stages of GNT. Patients and Methods: The results of either CT/MRI scans or SRS were reviewed by two independent observers who decided on the therapy of a patient. They then had to determine whether the results of the complementary imaging modality would change the decision. The study design was a matched cross-over study with two groups matching in respect to tumor type, imaging modality known first to the observer, and number of patients. For further analysis, patients were divided into three subgroups dependent on tumor stage (group 1, without metastases, group 2, liver metastases, group 3, recurrent disease/extrahepatic metastases). Results: 188 patients were included into the study. If SRS was known to the observers first, CT/MRI changed the therapeutic management in 16.2, 13.9 and 11.4% of the patients (subgroups 1–3). SRS changed the therapeutic management in 13.5, 12.5 and 10.3%. Overall, CT/MRI would have changed the management in 13.3% and SRS in 11.7% of the patients. Conclusion: Though the patients studied mainly suffered from already advanced stages of the disease, all imaging techniques change the therapeutic management to a comparable extent. Our results support the importance of combined imaging in the management of patients with GNT.

There is a considerable discrepancy in the literature concerning the sensitivity of parathyroid scintigraphy (PS) with 99mTc-MIBI. We therefore analyzed our own data and compared them to the literature in a metaanalysis. All patients who received 99mTc -MIBI scintigraphy and subsequent surgery in our department for the detection of enlarged parathyroid glands in primary (pHPT) or secondary (sHPT) hyperparathyroidism between 1991 and 1999 were included in our retrospective analysis. The results of surgery served as the gold standard. For a true positive result, the scintigraphy had to predict the exact location of parathyroid adenoma (PA) or parathyroid hyperplasia (PH). We then compared these data to the results of a nonstatistical systematic metaanalysis of the literature. Patients (178) underwent PS between 1991 and 1999; 139 were operated on and included in this study. Of these, 109 had pHPT and 30 had sHPT. The sensitivity and specificity of the PS were found to be 45%/94% for pHPT and 39%/40% for sHPT. Fifty-two studies concerning PS were included in the metaanalysis. Sensitivities reported varied from 39% to >90%. Consideration of the different possible techniques used for PS could not explain these discrepancies. Our data show that the sensitivity of PS in clinical routine may be lower than expected from the literature. Our data are consistent with other studies and with partially unpublished clinical observations from other university hospitals. We believe that a well-designed and properly conducted prospective study is necessary to evaluate the reasons for the differences observed.