Human pancreatic cancer cell lines do not express receptors for somatostatin

Somatostatin receptors in non-neuroendocrine malignancies: the potential role of somatostatin analogs in solid tumors

Somatostatin receptors (sstrs) are G-protein-coupled receptors that mediate various physiological effects when activated by the neuropeptide somatostatin or its synthetic analogs. In addition to the well-documented antisecretory effects of sstr2-preferential somatostatin analogs octreotide and lanreotide, ligand binding to sstr initiates an inhibitory action on tumor growth. This effect may result from both indirect actions (suppression of growth factors and growth-promoting hormones [e.g., GH/IGF-1 axis] and inhibition of angiogenesis) and direct actions (activation of antigrowth activities [e.g., apoptosis]). As solid tumor cells express multiple sstrs, there is a rationale to evaluate the potential antitumor effects of pasireotide (SOM230), a multireceptor-targeted somatostatin analog with high binding affinity for sstr1–3 and sstr5. Pasireotide reduces systemic IGF-1 levels more potently than currently available somatostatin analogs and has been well tolerated in clinical trials.

Treatment of transplanted tumor of lung adenocarcinoma A549 transfected by human somatostatin receptor subtype 2 (hsstr2) gene with 188Re-RC-160

BACKGROUND AND AIM

Radionuclide-labeled somatostatin analogues selectively target somatostatin receptor (SSTR)-expressing tumors as a basis for diagnosis and treatment of these tumors. To those tumors without somatostatin receptor expressed, the hSSTR2 gene was transfected. Express of the hSSTR2 receptor was imaging and the radiotherapeutic effect was evaluated with (188)Re-RC-160.

METHODS

The stable hSSTR2-expressing A549 cells (pcDNA3-hSSTR2 A549) and non-somatostatin receptor expressing A549 cells (pcDNA3 A549) were selected by western blot. Later, a corresponding animal tumor model was established. Expression of the hSSTR2 reporter was imaged using (188)Re-RC-160 recognition. Tumors were evaluated for somatostatin receptor expression using immunohistochemistry. The distribution of (188)Re-RC-160 in the animal tumor model was measured and the inhibitory effects of (188)Re-RC-160 were evaluated by measurement of tumor growth and hematoxylin and eosin and TdT mediated dUTP nick end labeling (TUNEL) staining.

RESULTS

In vivo radioimaging revealed specific targeting of (188)Re-RC-160 to tumors derived from pcDNA3- hSSTR2 A549 cells, compared to those from pcDNA3 A549 cells. pcDNA3- hSSTR2 A549 tumor growth inhibition was significantly higher in the single 7.4 MBq (188)Re-RC-160 treatment group than in the 2×7.4 MBq rhenium-188, RC-160 group, control group, and pcDNA3 A549 tumors (P<.05). Furthermore, treatment fractionation group (2 × 7.4 MBq (188)Re-RC-160), induced significantly increased tumor-growth inhibition compare with single 7.4 MBq (188)Re-RC-160 treatment (P<.05).

CONCLUSION

These studies showed that (188)Re-RC-160 could be effectively used for targeting therapy the A549-derived tumors exogenously expressing hSSTR2, which will offers a potential therapeutic strategy for the treatment of somatostatin receptor-negative cancers.

Radioiodine and 211At-labeled guanidinomethyl halobenzoyl octreotate conjugates: potential peptide radiotherapeutics for somatostatin receptor-positive cancers

Derivatives of the somatostatin analogues octreotide and octreotate labeled with radioiosotopes are used in the diagnosis and therapy of somatostatin receptor (SSTR)-positive tumors. A method has been devised to synthesize {N-(4-guanidinomethyl-3-iodobenzoyl)-Phe1-octreotate (GMIBO). Receptor binding assay and scatchard analysis yielded a Kd of 4.83 +/- 0.19 nM for this peptide. Derivatives of this peptide labeled with radioiodine ([*I]GMIBO) and the alpha-particle-emitting radiohalogen 211At N-(3-[211At]astato-4-guanidinomethylbenzoyl)-Phe1-octreotate; [211At]AGMBO} were prepared in a single step from a tin precursor in radiochemical yields of 30-35% and 15-20%, respectively. Paired-label internalization assays performed with the SSTR-positive D341 Med human medulloblastoma cell line demonstrated that [125I]GMIBO and [211At]AGMBO were specifically internalized 20-40% more than Nalpha-(1-deoxy-D-fructosyl)-[131I]I-Tyr3-octreotate ([131I]I-Glu-TOCA), the radioiodinated octreotide derivative previously shown to exhibit maximum internalization in this cell line. Uptake of [131I]GMIBO in D341 Med subcutaneous xenografts in a murine model (8.34 +/- 1.82 versus 8.10 +/- 2.23% ID/g at 1h) and SSTR-expressing normal tissues was comparable to that of [125I]I-Glu-TOCA and was shown to be specific. However, the uptake of [131I]GMIBO also was substantially higher in liver (16.9 +/- 3.15 versus 1.39 +/- 0.45% ID/g at 1 h) and in kidneys (44.33 +/- 6.47 versus 3.44 +/- 0.68% ID/g at 1h) compared to that of [125I]I-Glu-TOCA. These data suggest that these novel peptide conjugates retain their specificity for SSTR both in vitro and in vivo; however, because of their higher accumulation in normal tissues they would be best applied in settings amenable to loco-regional administration such as medulloblastoma neoplastic meningitis.

Emerging biological therapies for pancreatic carcinoma

AIMS

The incidence of pancreatic carcinoma remains approximately equal to its mortality, with the vast majority of patients having advanced disease at presentation. This review is an update of the promising novel approaches involving biological therapy that may be used in conjunction with new chemotherapeutic agents in the near future.

METHODS

A literature review was performed using the National Library of Medicine's Pubmed database, combined with recently published data from the AGA and ASCO conferences.

RESULTS

Rapid progress is being made in gene and molecular technology potentially enabling us to inhibit pancreatic carcinogenesis and to reduce disease progression. Different targets include signal transduction inhibitors, gene therapy, genetic prodrug activation therapy, antisense therapy, immunotherapy, matrix metalloproteinase and cyclo-oxygenase-2 inhibition and hormonal manipulation.

CONCLUSION

A variety of biological agents are currently undergoing clinical trials, targeting different areas of the pancreas'neoplastic process. .

The significance of somatostatin analogues in the antiproliferative treatment of carcinomas

Somatostatin is a cyclic tetradecapeptide hormone. It was initially isolated from bovine hypothalami. Somatostatin inhibits endocrine and exocrine secretion, as well as tumor cell growth, by binding to specific cell-surface receptors. Its potent inhibitory activity is limited, however, by its rapid enzymatic degradation and the consequently short plasma half-life. Octreotide is a short somatostatin analogue with increased duration of action compared with somatostatin. Preclinical studies have focused on the anticancer effects of octreotide and the related somatostatin analogues. In vitro, at nanomolar concentrations, these analogues inhibit the growth of tumor cells that express high-affinity somatostatin receptors. Accordingly, such analogues potently inhibit the growth of somatostatin receptor-positive tumors in various rodent models. The range of cancers susceptible to octreotide and related somatostatin analogues includes mammary, pancreatic, gastric, colorectal, prostate, thyroid, and lung carcinomas. Moreover, an indirect antiproliferative effect of somatostatin analogues is achievable in somatostatin receptor-negative tumors whose growth is driven by factors (e.g., gastrin, insulin-like growth factor-1) that become down-regulated by somatostatin. The clinical effect of somatostatin analogues in terms of tumor response in cancer patients is a subject of controversy, however. Most responses have been seen in patients with pancreatic cancers.

Lipophilization of somatostatin analog RC-160 with long chain fatty acid improves its antiproliferative and antiangiogenic activity in vitro

The therapeutic potential of the somatostatin analogue RC-160 having antiproliferative activity, is limited by its short serum half life. To overcome this limitation, fatty acids namely butanoic acid and myristic acid were conjugated to the N-terminal residue of RC-160. The lipophilized derivatives of RC-160 were synthesized, purified by reverse phase HPLC and characterized by ES-mass spectroscopy. The antiproliferative activity of lipophilized derivatives of RC-160 on the growth of MIA-PaCa2 (human pancreatic carcinoma), DU145 (human prostate carcinoma), ECV304 (human umbilical chord endothelioma), as well as their antiangiogenic activity was evaluated in vitro. The relative stability of myristoyl-RC-160 towards degradation by proteases and serum was also determined. Myristoyl-RC-160 exhibited significantly higher antiproliferative efficacy than RC-160, on the above cell lines (P<0.01). Receptor binding assays, demonstrated that the affinity of RC-160 towards somatostatin receptors remains unaltered by myristoylation. Unlike RC-160, the myristoylated derivative was found to have significantly greater resistance to protease and serum degradation (P<0.01). Myristoyl-RC-160 exhibited significantly greater antiproliferative activity on ECV304, than RC-160 (P<0.01). Myristoyl RC-160 could also inhibit capillary tube formation more efficiently than RC-160 in a dose dependent manner, suggesting that it possessed enhanced antiangiogenic activity in vitro (P<0.001). Lipophilization of RC-160 with long chain fatty acids like myristic acid endows it with improved antiproliferative and antiangiogenic activity, stability and therapeutic index. British Journal of Pharmacology (2000) 109, 101 - 109

Gastrointestinal hormones as potential adjuvant treatment of exocrine pancreatic adenocarcinoma

CONCLUSION

Gastrointestinal hormones and their antagonists can alter the growth of pancreatic adenocarcinoma in vitro and in vivo. The potential clinical benefit of this approach deserves further study.

BACKGROUND

Epithelial cell growth is normally under hormonal control. Hormones also affect the growth of many epithelial cancers, and this fact is used to modify tumor growth. Pancreatic epithelial cell growth is under the influence of gastrointestinal hormones. This article reviews experiments designed to determine the effect of gastrointestinal hormones on the growth of pancreatic adenocarcinoma.

METHODS

Eighty-eight articles were identified from a Medline search using the terms pancreatic adenocarcinoma and the individual names of gastrointestinal hormones. The experimental design and results of these studies are reviewed.

RESULTS

In general, somatostatin, vasoactive intestinal polypeptide, pancreatic polypeptide, and pancreastatin inhibit pancreatic adenocarcinoma growth. Cholecystokinin, secretin, bombesin, gastrin, EGF, TGF-alpha, insulin, and IGF-1 have a growth-promoting effect.

The inhibitory effect of an EGF receptor-specific tyrosine kinase inhibitor on pancreatic cancer cell lines was more potent than inhibitory antibodies against the receptors for EGF and IGF I

CONCLUSION

Epidermal growth factor (EGF) increased the cell number of the two pancreatic cancer cell lines, MiaPaCa-2 and LN-36, in vitro. A blockade of the EGF-R tyrosine kinase with tyrphostin was more efficient in reducing the cell number than inhibiting receptor antibodies. IGF-1 increased the cell number, and blockade of the IGF-1-R initially decreased the cell number that later was followed by an increase in LN-36.

BACKGROUND/AIM

The receptors and ligands of EGF and insulin-like growth factor-1 (IGF-1) are overexpressed in pancreatic cancer tissue. The aim of the present experiments was to study the effects of EGF and IGF-1 on the cell number in two pancreatic cancer cell lines.

MATERIAL AND METHODS

MiaPaCa-2 cells were grown in 0.2% fetal calf serum (FCS) and the newly established LN-36 cells in serum-free medium (SFM). The cell number was measured with the XTT method. The effects of EGF and IGF-1 were studied in combination with inhibiting receptor antibodies and an EGF-R-specific tyrosine kinase inhibitor, tyrphostin B56.

RESULTS

MiaPaCa-2 responded with increased cell number to stimulation with EGF, and at 10(-8) M or higher concentrations a dose-response pattern was seen. Administration of B56 to MiaPaCa-2 decreased the cell number by 87%. The inhibiting EGF-R-Ab only inhibited EGF-induced increase in cell number. IGF-1 doubled the cell number of MiaPaCa-2 and increased the cell growth induced by EGF. The inhibiting IGF-1-R-Ab reduced the cell number by 10%. The LN-36 cell line responded to EGF with an increased cell number with a maximum at 5 x 10(-9) M after 96 h. B56 reduced the cell number by 90% at 10(-5) M, with less effect during stimulation with EGF. In contrast to B56, the inhibiting EGF-R-Ab in the same experiment did not reduce the cell number. LN-36 responded to IGF-1 with an increased cell number, but EGF-stimulated growth was not influenced. The inhibiting IGF-1-R-Ab reduced the cell number and suppressed the IGF-1 stimulated increase after 24 h and later it induced an increased cell number.

Indirect antiproliferative effect of the somatostatin analog octreotide on MIA PaCa-2 human pancreatic carcinoma in nude mice

Analogs of somatostatin (SRIF) such as octreotide exert antiproliferative effects that are mediated directly by tumoral SRIF receptors or indirectly by down-modulation of factors that stimulate tumor growth. Direct and indirect antiproliferative effects have been demonstrated in certain SRIF receptor-positive and -negative human breast cancer models in nude mice, respectively. These antiproliferative mechanisms are also being explored in other cancer types including pancreatic cancer. While clinical pilot studies have indicated that a fraction of pancreatic adenocarcinomas respond to high-dose octreotide treatment, it is known from receptor autoradiographic and scintigraphic studies that human pancreatic carcinomas fail to express SRIF receptors, in contrast to rat pancreatic carcinomas or human endocrine pancreatic cancer. Studies on the potential anticancer effect of octreotide on the growth of experimental human pancreatic cancer and its SRIF receptor status have been controversial. Therefore, we investigated in vivo the effects of octreotide on the growth of MIA PaCa-2 human pancreatic carcinomas raised from cultured cells with a low passage number after receipt from the American Type Culture Collection. Nude mice bearing MIA PaCa-2 tumors were treated with a single injection of the recently developed octreotide long-acting release formulation, "SMS pa LAR." This treatment was well tolerated and resulted in a highly significant inhibition of tumor growth during weeks three and eight after administration. MIA PaCa-2 tumors were removed after eight weeks and processed for RT-PCR analysis using probes specific for each of the five somatostatin receptor subtypes sst1-sst5. This analysis revealed that MIA PaCa-2 tumors, like human pancreatic adenocarcinomas, do not express any of the five SRIF receptor subtypes, suggesting an indirect mode of tumor growth inhibition. In summary, the depot formulation SMS pa LAR exerted long-lasting antiproliferative effects in SRIF receptor-negative human pancreatic carcinomas in nude mice.

AR4-2J cells: a model to study polypeptide hormone receptors

The AR4-2J cell line is derived from a transplantable tumour of the exocrine rat pancreas. Acinar in origin, this cell line contains significant amounts of amylase and can be grown in continuous culture. Many in vitro studies have been done using these cells; these studies were often complemented with in vivo experiments on animals. Particularly, many polypeptide hormones interacting with specific receptors located on the cell membrane have been analysed. The accurate knowledge of the hormone-receptor interactions has allowed to design interesting analogs of these hormones. In several cases, these compounds are powerful antagonists and are able to control cell proliferation induced by the corresponding polypeptide hormones. Other cell lines are useful to understand human pancreatic cancer. These human cell lines (Capan 1, Panc-1 for example) are of ductal origin and differ from AR4-2J cells, especially regarding the distribution of several polypeptide hormone and growth factor receptors. Both models are important for basic studies of neuropeptides, gastrointestinal peptides and their receptors, as well as for a better understanding of the underlying mechanisms of human pancreatic cancer.

Effect of gastrointestinal hormones and synthetic analogues on the growth of pancreatic cancer

The effects of hormones and synthetic analogues have been examined on the growth of 2 human pancreatic cancer cell lines, MiaPaCa2 a well-established cell line and PANI which was derived in our own laboratories from a tumour specimen. The hormones/growth factors included gastrin (G-17), epidermal growth factor (EGF) and bombesin, while the synthetic analogues used were a gastrin receptor antagonist (CR 1718), a somatostatin analogue (RC-160) and a bombesin receptor antagonist (ICI 216,140). Cell proliferation was assessed by the [75Se]selenomethionine uptake method which has been shown to correlate with cell counts. The effect of each hormone or growth factor on growth was expressed as a percentage of the untreated control. There were 5 replicates in each experiment, and each one was repeated at least 3 times. In vitro growth of both cell lines was unaffected by gastrin, bombesin or the respective antagonists (CR1718 and ICI 216140). The somatostatin analogue RC-160 also had no effect on basal growth. Significant growth stimulation of both MiaPaCa2 and PANI was seen with epidermal growth factor. We tested the hypothesis that somatostatin analogues may inhibit EGF-stimulated growth on both MiaPaCa2, a somatostatin receptor positive cell line, and on PANI which is negative for somatostatin receptors. RC-160 did not inhibit EGF-stimulated growth of either MiaPaCA2 or PANI. Both cell lines were established in vivo as xenografts in nude mice. The effect of RC-160 on tumour growth was measured. RC-160 inhibited the growth of MiaPaCa2, the somatostatin receptor-positive cell line, but not of PANI.

Somatostatin analog RC-160 inhibits growth of CFPAC-1 human pancreatic cancer cells in vitro and intracellular production of cyclic adenosine monophosphate

The effect of somatostatin analog RC-160 on the growth of CFPAC-1 human pancreatic cancer cells in vitro was investigated. RC-160 effectively inhibited the proliferation of CFPAC-1 cells in culture, inducing a time- and dose-dependent decrease in the number of treated cells. A significant suppression of cell growth was observed after 48 and 72 hr of the exposure to (1 microM) RC-160, the cell number being decreased by 38% and 46%, respectively. RC-160 was more potent than SS-14 or SMS201-995 in inhibiting the growth of CFPAC-1 cells, and after 48-hr treatment the cell number decreased by 49% for RC-160 compared with 12% for SS-14 and 27% for SMS201-995. Binding experiments demonstrated that specific receptors for somatostatin were present on CFPAC-1 cells. SS-14 showed a high binding affinity for [125I]-Tyr11-SS-14 receptors on CFPAC-1 cells. Scatchard analysis indicated the presence of 2 classes of somatostatin binding sites on the cells, one with high binding affinity and low capacity and the other with low binding affinity and high capacity. RC-160 could bind to somatostatin receptors on these cells with an affinity similar to SS-14 but significantly higher than that of SMS201-995. Radioimmunoassay of intracellular cAMP showed that RC-160 could powerfully inhibit forskolin-stimulated cAMP production in CFPAC-1 cells. Addition of forskolin to the cultures increased cAMP concentrations in the cellular lysate of treated cells. RC-160 attenuated or nullified in a dose-dependent manner the cAMP production stimulated by forskolin. Our observations indicate that somatostatin analog RC-160 inhibits the proliferation of CFPAC-1 human pancreatic cancer cells in vitro and that this effect may involve the intracellular cAMP pathway.

Expression of somatostatin receptors in normal, inflamed, and neoplastic human gastrointestinal tissues

The multiple actions of somatostatin are mediated by specific membrane-bound receptors present in all somatostatin target tissues, such as brain, pituitary, pancreas, gastrointestinal tract, and kidney. For instance, in the human gastrointestinal tract, three different types of tissue compartments express somatostatin receptors: the gastrointestinal mucosa, the peripheral nervous system, and the gut-associated lymphoid tissue, where the receptors are preferentially located in germinal centers. In all these cases, somatostatin binding is of high affinity and specific for bioactive somatostatin analogues. Somatostatin receptors are also expressed in pathological states, such as cancers. A particular abundance is found in neuroendocrine tumors of the gastrointestinal tract. Ninety percent of the carcinoids and a majority of islet cell carcinomas, including their metastases, usually have a high density of somatostatin receptors. Several different somatostatin-receptor subtypes can be expressed by these tumors, the SSTR2 subtype being the most frequently and abundantly expressed. The somatostatin receptors in tumors are identified with in vitro-binding methods, molecular biology techniques, or in vivo-imaging techniques; the latter allow the precise localization of the tumors and their metastases in the patients. Because somatostatin receptors in human gastroenteropancreatic tumors are functional, their identification can be used to predict the therapeutical efficacy of octreotide to inhibit excessive hormone release. Of differential diagnostic importance is the fact that other pathological processes in the gastrointestinal tract may be associated with a high density of somatostatin receptors. Ninety percent of lymphomas, including those with intestinal involvement express somatostatin receptors. Furthermore, a moderate number of colorectal carcinomas contain somatostatin receptors, whereas exocrine pancreatic carcinomas do not. Finally, an increased expression of SS receptors in nonneoplastic conditions, such as in intestinal veins in inflammatory bowel disease, has been recently observed. These observations demonstrate the ability of the human body to regulate SS receptors in a wide number of tissues and conditions.

Inhibition of pancreatic cancer cell growth in vitro by the tyrphostin group of tyrosine kinase inhibitors

Tyrphostins are a group of low molecular weight synthetic inhibitors of protein tyrosine kinases (PTK). The intracellular domains of the receptors for epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha), insulin-like growth factor 1 (IGF-1) possess PTK activity. Since EGF, TGF-alpha and IGF-1 are considered to play an important role in the proliferation of pancreatic cancer cells, we studied the effects of tyrphostins on the growth of three human pancreatic cancer cell lines (MiaPaCa-2, Panc-1 and CAV). The tyrphostins AG17, T23 and T47 all inhibited EGF and serum-stimulated DNA synthesis. AG17 was found to be the most potent of these agents and caused a dose-dependent but reversible inhibition of cell growth. Furthermore using an immunoblotting procedure we also found AG17 to inhibit EGF-induced tyrosine phosphorylation in the MiaPaCa-2 cell line. Tyrosine kinase inhibitors may prove to be useful agents for the treatment of pancreatic cancer.

Somatostatin analogs for diagnosis and treatment of cancer

Somatostatin (SRIF) is a cyclic tetradecapeptide hormone initially isolated from ovine hypothalami. It inhibits endocrine and exocrine secretion, as well as tumor cell growth, by binding to specific cell surface receptors. Its potent inhibitory activity, however, is limited by its rapid enzymatic degradation and the consequent short plasma half-life. Octreotide is a short SRIF analog with increased duration of action compared to SRIF. Octreotide is approved for the treatment of acromegaly, amine precursor uptake and decarboxylation-omas, complications of pancreatic surgery and severe forms of diarrhea. Preclinical studies have focussed on the anticancer effects of octreotide and the related SRIF analogs BIM 23014 and RC-160. In vitro at nanomolar concentrations, these analogs inhibit the growth of tumor cells that express high affinity SRIF receptors. Accordingly, SRIF analogs, such as octreotide, potently inhibit the growth of SRIF receptor-positive tumors in various rodent models, and, in particular, xenotransplanted human tumors in nude mice. The range of cancers susceptible to octreotide and related SRIF analogs includes mammary, pancreatic, colorectal and lung malignancies. Moreover, an indirect antiproliferative effect of SRIF analogs is achievable in SRIF receptor-negative tumors, whose growth is driven by factors (gastrin, insulin-like growth factor-1, etc.) that are downregulated by SRIF. The use of radiolabeled somatostatin analogs represents a new diagnostic approach. [111In-DTPA]octreotide was developed for gamma camera imaging of SRIF receptor-positive malignancies, such as gasteroenteropancreatic tumors. Visualization of SRIF receptor-positive tumors in humans is emerging as an important methodology, both in tumor staging and predicting therapeutic response to octreotide. Recently, five SRIF receptor subtypes (SSTR1-5) have been cloned, all of which bind SRIF with high affinity. In contrast, SRIF receptor subtypes 1-5 have different binding profiles for short SRIF analogs. Octreotide, SSTR5, show moderate affinity for SSTR3 and fail to bind with high affinity to the other subtypes (SSTR1 and 4). Accordingly, the oncological profile of these three analogs is apparently similar. In conclusion, somatostatin analogs are a promising class of compounds for diagnosis and treatment of cancer. Current work is focussed on the identification of further SRIF receptor subtype-selective analogs with potential in oncology.

Somatostatin receptors in the gastrointestinal tract in health and disease

The multiple actions of somatostatin are mediated by specific membrane-bound receptors present in all somatostatin target tissues, such as brain, pituitary, pancreas, and gastrointestinal tract. Three different types of tissues in the human gastrointestinal tract express somatostatin receptors: (1) the gastrointestinal mucosa, (2) the peripheral nervous system, and (3) the gut-associated lymphoid tissue, where the receptors are preferentially located in germinal centers. In all these cases, somatostatin binding is of high affinity and specific for bioactive somatostatin analogs. Somatostatin receptors are also expressed in pathological states, particularly in neuroendocrine tumors of the gastrointestinal tract. Ninety percent of the carcinoids and a majority of islet-cell carcinomas, including their metastases, usually have a high density of somatostatin receptors. Only 10 percent of the colorectal carcinomas and none of the exocrine pancreatic carcinomas, however, contain somatostatin receptors. The somatostatin receptors in tumors are identified with in vitro binding methods or with in vivo imaging techniques; the latter allow the precise localization of the tumors and their metastases in the patients. Since somatostatin receptors in gastroenteropancreatic tumors are functional, their identification can be used to assess the therapeutic efficacy of octreotide to inhibit excessive hormone release in the patients.