Imaging neuroendocrine tumours with radiolabelled somatostatin analogues and X-ray computed tomography: a comparative study

Various endocrine neoplasms, including a number of carcinoid and pancreatic islet cell tumours, express somatostatin receptors. These tumours may be difficult to localize using conventional techniques. Radiolabelled somatostatin analogues 123I-Tyr-3-octreotide and 111In-pentatreotide have been used for imaging these tumours. In a retrospective study of 24 patients, the sensitivities of somatostatin receptor scintigraphy and X-ray computed tomography (CT) were compared. Ten patients were scanned using 123I-Tyr-3-octreotide and 14 with 111In-pentatreotide. All patients had CT scans. Twenty patients had neuroendocrine tumours with a total of 45 lesions in the anatomical areas included on both scans. CT detected 42 while somatostatin receptor scanning detected 31 of these lesions. Three lesions were missed by CT which were detected by scintigraphy, and there were six CT false positive lesions. In a further four patients investigated for suspected neuroendocrine tumours, no lesion was shown using either modality and subsequent investigations and clinical follow-up have shown no evidence of a tumour. We conclude that while CT is more sensitive than somatostatin receptor scintigraphy, the techniques are complementary, especially in patients with disseminated pathology, equivocal lesions on CT, or a negative CT and strong clinical or biochemical evidence of a neuroendocrine tumour.

Radioiodinated somatostatin analogue RC-160: preparation, biological activity, in vivo application in rats and comparison with [123I-Tyr3]octreotide

We have evaluated the potential usefulness of the radioiodinated octapeptide RC-160, a somatostatin analogue, which might serve as a radiopharmaceutical for the in vivo detection of somatostatin receptor-positive tumours. For this purpose, iodine-123 and iodine-125 labelled RC-160 was tested for biological activity and applied in vivo in rats bearing the transplantable rat pancreatic tumour CA20948, which expresses somatostatin receptors. Our group has recently described the in vivo visualization of such tumours in rats and in humans with the radioiodinated somatostatin analogue [Tyr3]octreotide. Like [123I-Tyr3]octreotide, 123I-RC-160 showed uptake in and specific binding in vivo to somatostatin receptor-positive organs and tumors. However, blood radioactivity (background) was higher, resulting in a lower tumour to blood (background) ratio. We therefore conclude that in this animal model 123I-RC-160 has no advantage over [123I-Tyr3]octreotide as a radiopharmaceutical for the in vivo use as a somatostatin receptor imager, although, like [123I-Tyr3]octreotide, 123I-RC-160 shows specific binding to different somatostatin receptor-positive organs. Recently differences were reported in affinity between somatostatin and its analogues for somatostatin receptors expressed in different human cancers, like those of the breast, ovary, exocrine pancreas, prostate and colon. Therefore 123I-RC-160 might be of interest for future use in humans as a radiopharmaceutical for imaging octreotide receptor-negative tumours.

Hepatobiliary handling of iodine-125-Tyr3-octreotide and indium-111-DTPA-D-Phe1-octreotide by isolated perfused rat liver

Radiolabeled bioactive peptides may show receptor-mediated binding to tumors, making them suitable for scintigraphic imaging. The liver is an important organ for peptide clearance. To gain insight into the uptake and intracellular processing of somatostatin analogs, we compared the hepatobiliary handling of 125I-Tyr3-octreotide and 111In-DTPA-D-Phe1-octreotide, which are successfully used to image somatostatin receptor-positive tumors in vivo in isolated recirculating perfused rat livers. Sixty minutes following administration of the radiolabeled peptides, perfusion medium and biliary radioactivity were analyzed. Radioiodinated Tyr3-octreotide was rapidly cleared by the liver and 60% of the dose was excreted intact into the bile after 60 min. In contrast, 111In-DTPA-D-Phe1-octreotide was not cleared by the liver; medium radioactivity levels remained about constant and only 2% of the dose was found in the bile. These results are in agreement with in vivo findings in rats and humans. We concluded that isolated rat liver perfusion is a good system to rapidly gain insight into the hepatic handling of radiopharmaceuticals.

Intraoperative localization of neuroendocrine tumors with 125I-TYR(3)-octreotide and a hand-held gamma-detecting probe

BACKGROUND

This study evaluates a novel method of intraoperative localization of endocrine gastroenteropancreatic tumors with a hand-held gamma-detecting probe to detect in situ tumor binding of the radioiodinated somatostatin analog 125I-TYR(3)-octreotide.

METHODS

Seven patients with biochemical and radiologic evidence of a specific endocrine tumor, one patient with biochemical evidence of gastrinoma but no tumor localized by conventional imaging techniques, and four patients with equivocal preoperative biochemical or radiologic study results but suspected of harboring a neuroendocrine tumor underwent abdominal exploration with intraoperative injection of 125I-TYR(3)-octreotide. 298 +/- 63 microCi. A hand-held gamma-detecting probe was used during operation to determine whether gross tumor accumulated the radiolabeled analog and occult tumor could be detected. Positive uptake was defined as tumor/background ratios exceeding 2:1.

RESULTS

The tumor in all seven patients with gross disease accumulated 125I-TYR(3)-octreotide. Occult tumor beyond that appreciated with preoperative imaging or by routine operative exploration was detected in a patient with carcinoid tumor. In the patient with the occult gastrinoma the probe detected the lesion within the duodenal bulb before duodenotomy and also predicted what proved histologically to be positive peripancreatic adenopathy. There was a single false-positive reading from the stomach in a patient with suspected carcinoid tumor in whom no tumor could be found grossly or histologically. A pancreatic mass that probed negative proved to be an adenocarcinoma of ductal origin.

CONCLUSIONS

Tumor-specific peptide-receptor binding can be detected in situ with 125J-TYR(3)-octreotide and a hand-held gamma-detecting probe. This technique may facilitate neuroendocrine tumor localization and operative cytoreduction.

Imaging of renal cell cancer with radiolabelled octreotide

Recently the presence of somatostatin receptors on human renal cell carcinomas has been demonstrated by autoradiographic techniques on surgically removed kidneys. In a prospective study we evaluated, by means of 111In-labelled octreotide scintigraphy, the in vivo tumour imaging in a group of patients with biopsy proven renal cell carcinomas at different tumour stages. Seven patients were studied. In three of them (43%) pathological tracer accumulation was demonstrated. In these patients 20 out of 23 known tumour localizations were clearly visualized. Tracer uptake could be inhibited by prior administration of cold octreotide. We conclude that 111In-octreotide scintigraphy can be used to demonstrate, in vivo, metastatic renal cell carcinoma.

The role of iodine-123-Tyr-3-octreotide scintigraphy in the staging of small-cell lung cancer

The purpose of this study is to investigate the role of 123I-Tyr-3-octreotide scintigraphy in staging small-cell lung cancer (SCLC), its efficacy for the discrimination of limited and extensive disease stages and its regional sensitivity for different metastatic locations. Twenty patients with histologically confirmed SCLC and 50 radiologically staged tumors sites were investigated by an imaging protocol including dynamic (0-30 min p.i.), static (30 min, 90 min, 4 hr, 24 hr p.i.) and SPECT (90 min p.i.) studies. The primary tumor site was visualized in 84%, whereas the best delineation was noted in early planar (15-30 min p.i.) and SPECT studies, due to a rapidly decreasing tumor-to-background ratio. Lymph node metastases were seen in 73%, but SPECT was needed for anatomical localization. All three adrenal metastases could be identified in sequential planar images. One clinically unsuspected brain metastasis was seen, whereas a second clinically overt metastasis was not visualized. The global and regional sensitivity for liver and bone metastases was unsatisfactory. In summary, 78% (7/9) of the patients with extensive disease were correctly identified by scintigraphy alone. We conclude that 123I-Tyr-3-octreotide scintigraphy is a substantial tool in the staging work-up of SCLC if it is performed initially to allow fast identification of patients with extensive disease stages and save additional radiological or invasive examinations. Yet, 123I-Tyr-3-octreotide scintigraphy cannot substitute liver sonography or conventional bone scanning in patients who have no scintigraphic evidence of distant tumor spread.

Somatostatin receptor scintigraphy with [111In-DTPA-D-Phe1]- and [123I-Tyr3]-octreotide: the Rotterdam experience with more than 1000 patients

Various tumours, classically specified as either neuroendocrine or non-neuroendocrine, contain high numbers of somatostatin receptors, which enable in vivo localization of the primary tumour and its metastases by scintigraphy with the radiolabelled somatostatin analogue octreotide. In addition granulomas and autoimmune processes can be visualized because of local accumulation of somatostatin receptor-positive activated mononuclear leucocytes. In many instances a positive scintigram predicts a favourable response to treatment with octreotide. It is tempting to speculate that octreotide labelled with an appropriate radionuclide might be used in cancer therapy. The successful application of radiolabelled octreotide in scintigraphy indicates the possible usefulness of other radiolabelled peptides, either native peptides or derivatives of these, in, for example, nuclear oncology. The small size of these peptides, e.g. bombesin and substance P, is of the utmost importance for a relatively fast blood clearance, thus leading to low background radioactivity. In this way peptides are powerful alternatives to (fragments of) monoclonal antibodies, the application of which to scintigraphic localization of specific cell surface antigen-bearing tumours is plagued by slow blood clearance and, hence, high background levels.

Inhibitory effect of intraduodenal administration of somatostatin analogue SDZ CO 611 on rat pancreatic exocrine secretion

The inhibitory effect of the intraduodenal administration of a new somatostatin analogue, SDZ CO 611 (SDZ), on pancreatic secretion was evaluated in the rat. The exocrine pancreas was stimulated by either intravenous infusion of both secretion (0.06 CU/kg/h) and cholecystokinin octapeptide (CCK-8) (0.03 microgram/kg/h) or intraduodenal infusion of oleic acid (0.25 mmol/h) or casein (200 mg/h). Intraduodenal administration of SDZ in three different doses, 0.5, 1.0, and 2.0 mg/kg/h, resulted in dose-related inhibition of pancreatic secretion, including juice volume, bicarbonate, and amylase, stimulated by exogenous secretin and CCK. Intraduodenal oleic acid- and casein-induced increases in pancreatic secretion in terms of juice volume, bicarbonate, and amylase were also significantly suppressed by intraduodenal administration of SDZ in a dose of 2.0 mg/kg/h. Moreover, SDZ (2.0 mg/kg/h) significantly inhibited basal pancreatic secretion. It is concluded that the intraduodenal administration of SDZ inhibits pancreatic secretion stimulated by exogenous secretin and CCK-8 in physiological doses and by intestinal nutrients in the rat.

Somatostatin receptors in meningiomas: a scintigraphic study using 111In-DTPA-D-Phe-1-octreotide

111In-octreotide (Octreoscan) planar scintigraphy was performed in 12 patients with suspected meningioma. The scan was positive in 10 patients with meningioma and negative in two patients with acoustic neurinoma assisting in the differential diagnosis. Good quality images were obtained as early as 2 h after injection and there was no increase in diagnostic quality at 24 h. No adverse effects were observed to radiopharmaceutical administration. The following conclusions are drawn: 111In-octreotide is a safe and fast test which can increase the specificity of traditional neuroimaging procedures.

Use of isotope-labeled somatostatin analogs for visualization of islet cell tumors

The results of visualization of the islet cell tumors of 25 patients after intravenous administration of two isotope-labeled somatostatin analogs (123I-Tyr3-octreotide and 111In-octreotide) are described. The primary tumors, as well as previously often unrecognized distant metastases were visualized in 20 of the 25 patients (80%). Parallel in vitro detection of somatostatin receptors on those tumors that had also been visualized in vivo indicates that the ligand binding to the tumor in vivo indeed represents binding to specific somatostatin receptors. The detection of somatostatin receptors on these tumors in vivo predicted a good suppressive effect of octreotide on hormonal hypersecretion by these tumors. It is an easy, quick, harmless procedure that is valuable for localization of primary endocrine pancreatic tumors and their often radiologically and clinically not yet recognized metastases.

Localization of neuroendocrine tumours and insulinomas using radiolabelled somatostatin analogues, 123I-Tyr3-octreotide and 111In-pentatreotide

OBJECTIVE

A number of neoplasms are known to express somatostatin receptors; the use of somatostatin receptor imaging in their localization has recently been described, but the resolution and discrimination of the isotopes used remains sub-optimal. We have looked at the use of a new 111In-labelled analogue of somatostatin, pentatreotide, in the visualization and functional characterization of a number of neoplastic conditions.

PATIENTS

Thirteen patients with proven neoplasms were scanned using this agent. Planar and single-photon-emission computerized tomographic (SPECT) images of the relevant part of the body were obtained using a gamma-camera at 10 minutes and 4 and 21 hours after injection of the radiopharmaceutical. In six patients (three carcinoid, three insulinomas) scanning was also performed using 123I-Tyr-3-octreotide.

RESULTS

Primary tumours or metastases were visualized in six of the seven patients with neuroendocrine tumours, and three of six patients with insulinoma. One patient with an insulinoma who had a positive scan showed absent uptake when rescanned after tumour removal. A rise in blood glucose (more than twice basal) in response to octreotide was seen only in those insulinoma patients with positive scans. In cases where both 111In-pentratreotide and 123I-Tyr-3-octreotide scans were performed, both radiopharmaceuticals identified the same 4/6 tumours; however, tumour definition (reflecting high tumour to background ratio) was better with pentatreotide on the 21-hour images with minimum biliary and gut activity, allowing better resolution of the tumour image.

CONCLUSION

It appears that 111In-pentatreotide scintigraphy is a rapid and safe procedure for the visualization of neuroendocrine tumours possessing somatostatin binding sites. A positive scan may be predictive of neuroendocrine responsiveness to octreotide therapy. In addition, it also appears that 111In-pentatreotide has superior kinetics compared to 123I-Tyr-3-octreotide, typically achieving more satisfactory tumour to background ratios, and may thus be more useful in the localization of endocrine tumours.

Enteral absorption of octreotide: absorption enhancement by polyoxyethylene-24-cholesterol ether

1. The somatostatin octapeptide-analogue octreotide was absorbed as an intact peptide from the gastro-intestinal tract with an absolute bioavailability of about 0.3% in rats. Administration of octreotide in the presence of polyoxyethylene (24)-cholesterol-ether (POECE) resulted in an about 23 fold increase of bioavailability. 2. In vitro studies with Caco-2 cells showed a dose-dependent increase in octreotide permeation with increasing doses of coadministered POECE. The use of [3H]-polyethyleneglycol (PEG) 4000 as an extracellular marker also indicated that higher doses of POECE may partly enhance paracellular transport of macromolecules. 3. By means of fluorescence microscopy it was shown that transepithelial transport of the fluorescent octreotide analogue (4-nitrobenzo-2-oxa-1,3-diazol [NBD] labelled octreotide) was enhanced by the addition of POECE. Besides an increased enterocyte uptake, there was evidence of enhanced partition of NBD-octreotide into the intercellular space between enterocytes after co-administration of POECE. In addition, there appeared to be changes in the hepatic topographic disposition of NBD-octreotide when it was given together with POECE compared with its administration alone. 4. In a study in healthy volunteers, 16 mg POECE significantly enhanced by 8 fold the absorption of octreotide after oral administration.

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

The distribution and elimination characteristics of the 111In-labelled somatostatin analogue OctreoScan111 were studied in 23 patients with malignant tumours. The substance exhibited a rapid blood elimination following a bi-phasic pattern. The initial part of the elimination curves showed a t1/2a of between 0.27 and 3.6 h. The patients investigated had creatinine clearance rates ranging from 33 to 124 ml/min. However, within this range, no apparent correlation was found between the OctreoScan111 elimination rate and kidney function. Also no correlation was observed between the amount of administered activity and the elimination rate of OctreoScan111. The serum radioactivity of 6 patients was analyzed with respect to molecular size. These experiments showed that OctreoScan111 circulated unbound in serum. About 3% of the radioactivity, most probably representing 111In-chloride of DTPA-111In-chloride, circulated protein-bound. The elimination of OctreoScan111 radioactivity in urine displayed a bi-phasic pattern. Size separation of the radioactivity appearing in the urine after 24 h showed a higher molecular weight when compared with OctreoScan111, indicating the existence of a metabolite of the injected substance. The results obtained are discussed in the light of a potential role for the substance in systemic radiotherapy.

Somatostatin receptor scintigraphy in malignant lymphoma: first results and comparison with glucose metabolism measured by positron-emission tomography

Somatostatin receptor scintigraphy with 111In-labeled octreotide was performed in 22 patients with suspected or known malignant lymphoma. The majority of extra-abdominal lesions (21/24 = 87.5%) were correctly localized, however, only one of nine intraabdominal lymphomas could be detected. No significant correlation was found between scintigraphic results and histological type of lymphoma. Metabolic imaging by positron-emission tomography with 18F-labeled deoxyglucose yielded a higher rate of detection of lymphoma manifestations (92% vs 64%) and better tumor contrast. Further prospective studies are needed to establish the clinical relevance of somatostatin receptor scintigraphy in malignant lymphoma.

SDZ CO 611: a highly potent glycated analog of somatostatin with improved oral activity

To obtain orally active octreotide (Sandostatin, SMS 201-995) analogs a new class of glycated somatostatin derivatives were synthesized by the Amadori reaction (Maillard reaction). The synthesis, chemical and biological characterization of a series of new compounds is described. These oligopeptides bind with high affinity to somatostatin receptors and retain full biological activity. Whereas generally polypeptide hormones are almost completely inactive after oral administration, we report here for the first time that these analogs show remarkably high activity by the oral route. Thus for example SDZ CO 611, the D(+)-maltose Amadori derivative of octreotide, has about 10 times higher oral effect bioavailability than octreotide while maintaining the selectivity, metabolic stability and long duration of action of the parent compound.

Somatostatin receptor scintigraphy of carcinoid tumours using the [111In-DTPA-D-Phe1]-octreotide

Somatostatin-receptor scintigraphy using the 111In-labelled somatostatin-analogue octreotide ([111In-DTPA-D-Phe1]-octreotide) was performed in 40 patients with carcinoid tumours. In 31/40 patients, this scintigraphy proved positive compared with the 33/40 patients whose tumours were disclosed on CT scans. In addition, 18 previously unidentified lesions were detected with this scintigraphy. Two of these lesions represented previously undetectable primary tumours. It is concluded that somatostatin receptor scintigraphy using [111In-DTPA-D-Phe1]-octreotide has a future role in the staging of patients with carcinoid disease.

Use of radiolabeled somatostatin analogs in the identification and treatment of somatostatin receptor-bearing tumors

Radiolabeled somatostatin analogs are potentially of considerable value in both the localization and treatment of somatostatin receptor-bearing tumors. Whole body 123I-labeled Tyr3-octreotide scintigraphy is capable of detecting and localizing primary tumors and metastatic disease that may not be detectable by other methods. Although it is possible that this technique may be applicable to a wide variety of neoplasms, 123I-Tyr3-octreotide scans appear particularly useful for imaging various gut neuroendocrine tumors, meningiomas, and paragangliomas. This primarily reflects the level of expression of somatostatin receptors by these lesions. In addition to whole body scintigraphy, intraoperative localization by receptor identification utilizing a hand-held gamma detector probe may prove to be an effective means of identifying spread and micrometastasis of somatostatin receptor-bearing tumors. Aside from identification and topographic localization of disease, the detection of an 123I-Tyr3-octreotide labeled tumor may be utilized as in vivo assay for the presence of somatostatin receptors. Such observations may prove useful in predicting the susceptibility of an individual tumor to octreotide therapy. Additionally, it may prove possible to deliver a therapeutic dose of radiation to receptor-bearing tumors by the administration of specific radiolabeled somatostatin analogs.

111In-octreotide scintigraphy in oncology

Various tumors of neuroendocrine origin that have amine precursor and decarboxylation (APUD) characteristics can be visualized in vivo after intravenous injection of the somatostatin analogue [123I-Tyr3]-octreotide. However, the relatively short effective half-life of this compound and the high background of radioactivity in the abdomen are drawbacks in its application. Therefore, an 111In-coupled somatostatin analogue ([111In-DTPA-D-Phe1]-octreotide) was developed. This analogue is excreted mainly via the kidneys, 90% of the dose being present in the urine 24 h after injection. Using 111In-octreotide scintigraphy, 7 out of 7 gastrinomas, 4 out of 7 insulinomas, 1 out of 1 glucagonoma, 3 out of 3 unclassified apudomas, but none out of 18 exocrine pancreatic carcinomas were visualized. Also, 19 out of 19 carcinoids, 15 out of 15 glomus tumors, 8 out of 12 medullary thyroid carcinomas, 6 out of 6 small cell lung carcinomas, 4 out of 4 growth hormone-producing and 6 out of 9 clinically nonfunctioning pituitary adenomas were visualized. Apart from APUD-cell-derived tumors, 111In-octreotide scintigraphy was also successfully applied to visualize breast cancer, lymphomas and granulomas. In 39 out of 50 patients with breast carcinoma, 10 out of 11 patients with non-Hodgkin lymphomas, 3 out of 3 patients with Hodgkin's disease, and 8 out of 8 patients with sarcoidosis, tumor sites accumulated radioactivity during octreotide scintigraphy. In a considerable number of patients with carcinoids and glomus tumors, but also in patients with granulomas and lymphomas, 111In-octreotide scintigraphy revealed more tumor sites than did conventional imaging techniques.(ABSTRACT TRUNCATED AT 250 WORDS)

[Somatostatin receptor scintigraphy. A new imaging procedure for the specific demonstration of carcinoids of the small intestine]

Scintigraphy with a radiolabelled somatostatin analog represents a new highly specific approach in the diagnostic work-up of receptor-positive APUD tumours and their metastases. We present our preliminary results with somatostatin receptor scintigraphy in 15 patients with histologically proven midgut carcinoid. 5 out of 6 primary tumour sites (83%) and 90% of the known metastatic lesions could be detected; unknown metastatic lesions were seen in 5 patients. Compared with other nuclear medicine procedures somatostatin receptor scintigraphy is able to detect all tumour sites within hours. This advantage will promote the acceptance of this sensitive and specific imaging modality by the clinicians with regard to preoperative work-up and symptomatic therapy with a somatostatin analog.

The visualization of gastroenteropancreatic endocrine tumors

In this review, we evaluate radiological techniques currently used to localize gastroenteropancreatic (GEP) endocrine tumors. We also describe the visualization, using intravenous administration of two isotope-labelled somatostatin analogs (123-I-Tyr3-octreotide and 111In-DTPA-octreotide), of islet cell tumors in 25 patients and carcinoids in 39 patients. The primary tumor as well as previously unrecognized distant metastases were visualized in 20 of the 25 patients (80%) and in 37 of the 39 (95%). Parallel in vitro detection of somatostatin receptors on those tumors also visualized in vivo showed that ligand binding to the tumor in vivo represents binding to specific somatostatin receptors. The detection of somatostatin receptors on tumors in vivo predicted a good suppressive effect of octreotide on hormonal hypersecretion by these tumors. It is an easy, quick and harmless procedure, valuable in the localization of primary endocrine pancreatic tumors and their often radiologically and clinically unrecognized metastases. Future prospective controlled studies comparing this procedure with other radiological investigative techniques should demonstrate its sensitivity and specificity and determine the place of somatostatin receptor imaging in the localization of GEP endocrine tumors.

Three-dimensional OctreoScan111 SPECT of abdominal manifestation of neuroendocrine tumours

In the present study we have investigated the three-dimensional (3D) reconstruction of OctreoScan111 SPECT (single photon emission tomography) images in 20 patients with neuroendocrine tumours. All patients had at least 2 tumour lesions as assessed from computerized tomography (CT) and SPECT. The 3D rendering was performed using a software, which produces images by implementing direct rendering from voxels without an intermediate surface data structure. The software has options for a free choice of thresholding and possibilities of clipping in coronal, transversal and sagittal planes. The results obtained showed that 3D reconstruction with volume rendering (3Dvr) gave a superior topographical localization of tumour uptakes when compared with SPECT. The 3Dvr technique was also combined with transversal clipping in rendered volumes (3Dvr+c). The major advantage with the 3Dvr+c technique was found to be an improved visualization of anatomical references as well as improved diagnostic information in a particular, selected, transversal slice, thus facilitating the identification and comparison of individual tumour lesions.

Preferential location of somatostatin receptors in germinal centers of human gut lymphoid tissue

Somatostatin receptors were evaluated in four human gut-associated lymphoid tissues (palatine tonsils, ileal Peyer patches, vermiform appendix, and colonic solitary lymphatic follicles) using receptor autoradiography on tissue sections incubated with 125I[Tyr3]octreotide. All four tissues were somatostatin-receptor positive; the receptors were preferentially located in the germinal centers, with the luminal part of the center more strongly labeled than the basal part. The corona of the follicles and the primary follicles without germinal centers did not display somatostatin receptors. The receptors were of high affinity (Kd = 1.3 +/- 0.6 nmol/L) and specific for somatostatin. Displacement by nanomolar concentrations of somatostatin 14, somatostatin 28, and octreotide was observed, as was guanosine triphosphate dependency. The gastrointestinal mucosa and the plexus submucosus and myentericus also contained somatostatin receptors. These data strongly suggest that the germinal centers of the gut-associated lymphoid tissue are a site of action of somatostatin. It possibly mediates antiproliferative effects and inhibits immunoglobulin synthesis in the activated lymphoid cells. The human gut represents a multifaceted target for somatostatin action, in which at least three different tissues (mucosa, nerve plexus, and lymphoid tissue) are involved.