111Indium (DTPA-octreotide) scintigraphy in patients with cerebral gliomas

Somatostatin and Somatostatin Receptors in Tumour Biology

Somatostatin (SST), a growth hormone inhibitory peptide, is expressed in endocrine and non-endocrine tissues, immune cells and the central nervous system (CNS). Post-release from secretory or immune cells, the first most appreciated role that SST exhibits is the antiproliferative effect in target tissue that served as a potential therapeutic intervention in various tumours of different origins. The SST-mediated in vivo and/or in vitro antiproliferative effect in the tumour is considered direct via activation of five different somatostatin receptor subtypes (SSTR1-5), which are well expressed in most tumours and often more than one receptor in a single cell. Second, the indirect effect is associated with the regulation of growth factors. SSTR subtypes are crucial in tumour diagnosis and prognosis. In this review, with the recent development of new SST analogues and receptor-specific agonists with emerging functional consequences of signaling pathways are promising therapeutic avenues in tumours of different origins that are discussed.

Somatostatin receptor-based PET/CT of intracranial tumors: a potential area of application for 68 Ga-DOTA peptides?

OBJECTIVE

Similar to neuroendocrine tumors (NETs) at other sites, a wide array of intracranial tumors also express somatostatin receptors (SSTRs). This expression can be exploited for both imaging and therapy. The introduction of (68)Ga-labeled tetraazacyclododecanetetraacetic acid (DOTA)-peptide PET/CT has given new dimension to SSTR-based imaging because of its improved sensitivity and excellent spatial resolution.

CONCLUSION

However, in contrast to gastropancreatic and bronchopulmonary NETs, limited literature is available regarding the use of (68)Ga-DOTA-peptide PET/CT in intracranial tumors. Here, we briefly review the available literature and highlight the potential role that (68)Ga-DOTA-peptide PET/CT can play in the management of intracranial tumors.

SPECT and PET imaging of meningiomas

Meningiomas arise from the meningothelial cells of the arachnoid membranes. They are the most common primary intracranial neoplasms and represent about 20% of all intracranial tumors. They are usually diagnosed after the third decade of life and they are more frequent in women than in men. According to the World Health Organization (WHO) criteria, meningiomas can be classified into grade I meningiomas, which are benign, grade II (atypical) and grade III (anaplastic) meningiomas, which have a much more aggressive clinical behaviour. Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) are routinely used in the diagnostic workup of patients with meningiomas. Molecular Nuclear Medicine Imaging with Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) could provide complementary information to CT and MRI. Various SPECT and PET tracers may provide information about cellular processes and biological characteristics of meningiomas. Therefore, SPECT and PET imaging could be used for the preoperative noninvasive diagnosis and differential diagnosis of meningiomas, prediction of tumor grade and tumor recurrence, response to treatment, target volume delineation for radiation therapy planning, and distinction between residual or recurrent tumour from scar tissue.

Clinical utility of somatostatin receptor scintigraphic imaging (octreoscan) in esthesioneuroblastoma: a case study and survey of somatostatin receptor subtype expression

BACKGROUND

For tumors that express somatostatin receptors (SSTR), radiolabeled somatostatin analogs, such as 111In-pentetreotide, can demonstrate the presence of tumor by radioligand uptake using somatostatin receptor scintigraphy (SRS). The use of 111In-pentetreotide for SRS depends on the specific high affinity of octreotide for SSTR subtypes 2, 3, and 5. Of these, SSTR2 has the greatest affinity for octreotide and the greatest relevance for tumor detection with Octreoscan imaging. Discriminating between postoperative changes and residual or recurrent tumor after extensive skull base surgery is often difficult, but in a case of recurrent esthesioneuroblastoma (ENB) we found the use of Octreoscan imaging clinically useful. To better define the general relevance of this imaging technique in this setting, we analyzed SSTR subtype expression in a panel of ENB tumors.

METHODS

The case history and correlations between MRI and 111In-pentetreotide SRS of a patient with recurrent ENB were reviewed. The expression pattern of the SSTR subtypes in a panel of ENB tumors was then analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR) to better define the potential of more general use of Octreoscan for imaging ENB. To correlate SSTR2 protein expression with 111In-pentetreotide uptake, immunohistochemistry to detect SSTR2 was performed on tumor samples from regions of increased uptake on Octreoscan.

RESULTS

The SSTR2 message was expressed at high levels in all five ENB tumor samples, and either SSTR2 protein or histologic findings typical for ENB were found in all tumor tissue obtained from regions of increased 111In-pentetreotide uptake. Furthermore, Octreoscan imaging in this case proved useful in clinical decision making.

CONCLUSION

The expression pattern of SSTR2 and the specificity of the Octreoscan for regions of active tumor growth support further investigation of the utility of Octreoscan imaging in the diagnosis and surveillance of ENB. Recent advances in novel therapies based on SSTR ligand binding also provide the rationale to consider such novel therapeutic approaches in patients with ENB.

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.

Somatostatin receptor subtype 2 is expressed by supratentorial primitive neuroectodermal tumors of childhood and can be targeted for somatostatin receptor imaging

PURPOSE

Although gliomas predominate among central nervous system (CNS) neoplasms in adulthood, embryonal tumors are the most common malignant brain tumors in children. Despite novel treatment approaches, including improved radiotherapy and high-dose chemotherapy, survival rates remain unsatisfactory. The timely diagnosis of residual or recurrent embryonal CNS tumors and thus the earliest possible time point for intervention is often hampered by inaccuracies of conventional imaging techniques. Novel and refined imaging methodologies are urgently needed.

EXPERIMENTAL DESIGN

We have previously demonstrated the use of somatostatin receptor imaging (SRI) in the diagnosis of recurrent and residual medulloblastomas. Here, we evaluated somatostatin receptor type 2 (sst(2)) expression using an antibody in an array of CNS tumors of childhood. Eight high-grade gliomas, 4 atypical teratoid/rhabdoid tumors, 7 supratentorial primitive neuroectodermal tumors (stPNET), 1 medulloepithelioma (ME), and 8 ependymomas were screened. Tumors positive in vitro were additionally analyzed in vivo using SRI.

RESULTS

Abundant expression of somatostatin receptor type 2 in stPNET, a ME, and ependymomas warranted in vivo imaging of 7 stPNET, 1 rhabdomyosarcoma, 3 ependymomas, 1 ME, and 1 glioblastoma. Although SRI was positive in 6/7 stPNET, 1 rhabdomyosarcoma, and 1 ME, none of the ependymomas nor the glioblastoma could be imaged using SRI. In selected cases SRI was more sensitive in the detection of relapse than conventional imaging by magnetic resonance imaging and computed tomography.

CONCLUSIONS

SRI should be considered in the evaluation of residual or recurrent embryonal CNS tumors, especially stPNET. The strengths of SRI lie in the differentiation of reactive tissue changes versus residual or recurrent tumor, the detection of small lesions, and possibly in the distinction of stPNET from gliomas.

Study of the 111In-DTPA complex by the electromigration method

The electrophoretic behavior of the 111In-DTPA radiopharmaceutical has been investigated. The stability constant, diffusion coefficient and effective charge of the complex as well as the temperature dependence of the electrophoretic mobility were determined. No-carrier-added 111In with high specific activity was used in the electrophoretic experiments in ultramicroconcentrations (10(-9)M).

Neuroendocrine tumors in the brain

Somatostatin and other neuropeptides are expressed in tumors originating from neuronal precursors and paraganglia, namely medulloblastoma, central Primitive Neuro-Ectodermal Tumors (cPNETs), neurocytoma, gangliocytoma. olfactory neuroblastoma, paraganglioma. In medulloblastoma, the most common malignant tumor in childhood, there is an extensive expression of somatostatin in addition to somatostatin receptors (SSTR) type 2. Although density of SSTR-2 and intensity of expression of somatostatin genes have no prognostic significance in medulloblastoma. their presence may bring along important information on oncogenesis and relate medulloblastoma to cPNETs. Radio-labeled octreotide scintigraphy may be useful in the follow-up of these patients. allowing differentiation between scar and tumoral tissue. Moreover, on the basis of octreotide-induced inhibition of cell proliferation in medulloblastoma, a trial with octreotide in patients with recurrent or high-risk tumor is warranted. Meningiomas and low-grade astrocytic gliomas, even if not displaying a clear neuroendocrine phenotype, have high levels of SSTR-2. In meningiomas, SSTRs-scintigraphy is not part of the routine pre-operative assessment; moreover, a therapeutic trial with somatostatin-analogues in patients with recurrent or inoperable meningiomas should be carried-out with great caution, because somatostatin and octreotide slightly increase cell proliferation in cultured meningiomatous cells. Low-grade gliomas (WHO grade 2), and a smaller fraction of anaplastic astrocytomas, express SSTR-2, while glioblastomas usually do not. Unfortunately, radiolabeled-octreotide scintigraphy is not useful in the differential diagnosis of gliomas, because the results are altered by the disruption of the blood brain barrier (BBB); in addition, radionuclide-labeled somatostatin analogues are not useful in the therapy of low-grade gliomas, because the intact BBB prevents them from reaching the target SSTR-2. Recently, a pilot study in gliomas, has proposed the use of a radio-labeled somatostostatin analogue with a loco-regional approach in order to overcome the intact BBB.

Differential expression of somatostatin receptors in ependymoma: implications for diagnosis

Somatostatin receptors (SSts) have been found in a variety of brain tumors, e.g., meningiomas, medulloblastomas and astrocytomas. Our aim was to investigate their expression in ependymomas. Using RT-PCR, expression of mRNA for the different SSt subtypes was analyzed and quantified in 28 ependymomas and correlated with different variables (age, tumor location, histological grade, recurrence and survival). In addition, in 8 cases, protein expression was studied in vitro, using immunohistochemistry, and in vivo, by somatostatin scintigraphy. mRNAs for all 5 subtypes were variably expressed in each ependymoma. The Southern blotting signal obtained after SSt(1) and SSt(2) amplification was higher than that for the other receptor subtypes. No significant correlation was seen between the level of SSt(1) and SSt(2) mRNA expression and age, location, histological grading, recurrence or survival. In the 8 cases, SSt(1) staining was negative in 3 and low in 5. Staining for SSt(2A) was positive but low in every specimen analyzed. SSt(1) and SSt(2) immunoreactivity was seen only in the cytoplasm of tumoral cells. Somatostatin scintigraphy showed clear uptake, which agreed with MRI data in the majority of cases. However, no correlation was seen between tracer uptake intensity and histological grade, SSt(1) and SSt(2) mRNA expression or immunostaining intensity. This evidence for the expression of SSt(2) receptors in ependymomas opens interesting prospects for their follow-up.

Somatostatin inhibits the production of vascular endothelial growth factor in human glioma cells

In various cell types, the neuro- and endocrine peptide somatostatin induces inhibitory and anti-secretory effects. Since somatostatin receptors, especially of the subtype sst2A, are constantly over-expressed in gliomas, we investigated the influence of somatostatin and the receptor subtype-selective peptide/non-peptide agonists octreotide and L-054,522 on the secretion of the most important angiogenesis factor produced by gliomas, vascular endothelial growth factor (VEGF). Cultivated cells from solid human gliomas of different stages and glioma cell lines secreted variable amounts of VEGF, which could be lowered to 25% to 80% by co-incubation with somatostatin or sst2-selective agonists (octreotide and L-054,522). These effects were dose-dependent at nanomolar concentrations. Stimulation with different growth factors (EGF, bFGF) or hypoxia considerably increased VEGF production over basal levels. Growth factor-induced VEGF synthesis could be suppressed to <50% by co-incubation with somatostatin or an sst2-selective agonist; this was less pronounced in hypoxia-induced VEGF synthesis. The effects were detected at the protein and mRNA levels. These experiments indicate a potent anti-secretory action of somatostatin or sst2 agonists on human glioma cells that may be useful for inhibiting angiogenesis in these tumors.

Influence of the somatostatin receptor sst2 on growth factor signal cascades in human glioma cells

The somatostatin receptor subtype sst2A is highly expressed, non-mutated and functionally active in gliomas. After stimulation of cultivated human U343 glioma cells with somatostatin, octreotide (sst2-, sst3- and sst5-selective peptide agonist) or the sst2-selective non-peptide agonist L-054,522 multiple signal transduction pathways are induced: elevated cAMP levels are reduced, protein tyrosine phosphatases (especially SHP2) are activated and mitogen-activated protein kinases are inhibited. Stimulation of the phosphatases resulted in dephosphorylation of activated receptors for EGF and PDGF (epidermal and platelet-derived growth factor), and as a consequence the mitogen-activated protein kinases ERK 1 and 2 (p42/p44) were de-phosphorylated in co-stimulation experiments. Furthermore, somatostatin or sst2-selective agonists reduced EGF-stimulated expression of the AP-1 complex (c-jun/c-jun) on the transcriptional and translational level. These experiments show that the interaction of stimulatory and inhibitory receptors are important mechanisms for the regulation of signal cascades and gene expression.

Somatostatin receptors in gliomas

Gliomas differ from non-malignant glial cells in the overexpression or mutations of genes involved in cell cycle or growth regulation. One example is the overexpression of the somatostatin receptor subtype 2 (sst2), especially of the splice variant sst2A. The reasons for this overexpression are not known. However, the coding sequence and part of the promoter region is not mutated. In accordance to this, the sst2 is functionally active and is internalised upon agonist stimulation. Immunoelectronmicroscopic studies show that the activated sst2 is internalised via caveolin-positive endosomal vesicles and later accumulates in multivesicular bodies and lysosomal compartments. The activated sst2 is found to be co-localised with the inhibitory G-protein Gialpha at the plasma membrane and in early endosomal vesicles. Multiple signal transduction pathways are induced. Stimulation of sst2 lowers cAMP levels elicited by forskolin and activates the protein tyrosine phosphatase SHP-2. In contrast to other sst2-expressing cells a long term antiproliferative effect of somatostatin or sst2-selective agonists are not detected in cultivated glioma cells. However, continuous stimulation of sst2 decreases the expression of genes promoting tumour survival.

Somatostatin and somatostatin receptors in the diagnosis and treatment of gliomas

Somatostatin analogues are in clinical use for the diagnosis and treatment of several oncological indications, namely pituitary adenomas and endocrine gastrointestinal tumors. In addition for a variety of malignancies their potential value is being studied. It has been speculated that somatostatin plays a role in the homeostasis of gliomas, and that gliomas could be susceptible to antiproliferative effects of somatostatin analogues. These assumptions were tested in 20 human cell lines derived from malignant gliomas and 4 glioblastoma tissue specimens, which were analyzed for their expression of the five known somatostatin receptor genes (SSTR1-5) and for the receptor function. Using semiquantitative PCR techniques, SSTR2 transcripts were found in all 20 cell lines and 4 glioblastomas, SSTR1 transcripts were detected in 9 cell lines and 4 glioblastomas, and SSTR3 transcripts were noted in 7 cell lines and 1 glioblastoma. SSTR4 and SSTR5 transcripts were only rarely detected. Gene expression profiles in glioblastoma tissue specimens resembled those of the cell lines in quality as well as quantity, with average transcript levels being highest for the SSTR2, followed by SSTR1 and SSTR3. However, when compared to GH3 anterior pituitary tumor cells, the relative amounts of PCR amplified DNA fragments were found to be at least 120 fold lower in glioblastoma cell lines and tumor specimens. Binding studies indicated that glioblastoma derived cells contained only minute amounts of SSTRs. No inhibition of proliferation was observed when 10 selected cell lines were incubated with somatostatin-14 (SST-14) or octreotide (SMS 201-995) at concentrations ranging from 10(-9) M to 10(-6) M, however, the proliferation of two cell lines was weakly stimulated after 6 days of incubation with 10(-6) M octreotide. The activity of adenylate cyclase, stimulated by forskolin, was inhibited by maximally 25% at 10(-6) M SST-14 or octreotide in one of 5 selected glioblastoma cell lines. Somatostatin peptides do not seem to exert anti-proliferative effects on glioblastoma cells and therefore appear to be of no obvious value for glioblastoma therapy. Most likely the amount of cell surface SSTRs is not sufficient to mediate antiproliferative effects. Since it has been described that SSTRs are detectable on most differentiated gliomas as well as astrocytes, it may be speculated that SSTRs may be relevant only in the context of well differentiated cellular programs but lose their significance with progressive dedifferentiation.

Receptor scintigraphy of non-neuroendocrine cancers with In-111 pentetreotide

Somatostatin receptors (SR) are surface markers characterizing not only APUDomas associated with neuroendocrine identities but also malignancies without neuroendocrine expression. Recently, the somatostatin analog pentetreotide was labeled with In-111 (OctreoScan 111, Mallinckrodt Medical BV, Petten, Holland) and introduced for the in vivo visualization in man of SR-positive tissues. In the present report, SR-specific scintigraphy is evaluated as a clinical tool for tissue characterization in correlation with histological and radiological examinations. Scintigraphy was focused and performed in cancer types without neuroendocrine tissue expression such as brain (n = 6) and breast tumors (n = 9) and lymphomas (n = 5). Scintigraphy was performed for comparison at 6 and 22 h after i.v. application of 111 MBq (3 mCi) of In-111-Pentetreotide. In the breast cancer group, the primary tumor was visualized in all 9 women as well as in all 4 cases with palpable axillary lymph nodes. Three women with a negative axillary node scan and impalpable nodes had positive biopsy. In two cases, mediastinal lymph node involvement was observed. So far the role of SR-positive breast cancer (BC) scans remains unknown. It is tempting to speculate that in resected women who are histologically and scintigraphically SR positive, it might be of value in the early detection of symptom-free recurrences. High densities of SR were present within both meningiomas, the high-grade astrocytoma and the craniopharyngioma. Differentiation of low- and high-grade astrocytomas could not be successfully achieved because both grades showed intense radioactivity uptake, even though high-grade tumors lack SR. The latter might be due to the damaged blood-brain barrier and the poor radioactivity washout observed in high-grade astrocytomas. All five lymphomas could be detected due to the presence of activated lymphocytes and macrophages that express SR at a sufficient density. In conclusion, SR scintigraphy in non-neuroendocrine malignancies does not seem to be reliable for an initial tumor staging but rather more suitable for a tissue characterization and extremely useful for monitoring changes of SR expression after treatment.

The diagnostic utility of somatostatin receptor scintigraphy in oncology

Somatostatin receptor scintigraphy (SRS) with the diethylenetriaminopentaacetic-acid-conjugated somatostatin analogue [111In-DTPA-D-Phe1] octreotide, also known as 111In-pentetreotide, is a new non-invasive modality for the evaluation of tumours that express receptors for somatostatin. These receptors are present on neuroendocrine and other tumours, including lymphomas and some breast cancers. In oncology SRS is a promising diagnostic tool for localizing primary tumours, staging, control and follow-up after therapy, and for identification of patients who may benefit from therapy with unlabelled octreotide or, in the future, with radiolabelled octreotide. In the past few years many small and large studies investigating various aspects of SRS have been reported. In this review the value of SRS in the management of individual tumour types is explored. For many tumours the best sensitivity in lesion detection is only achieved by very careful imaging after the administration of at least 200 MBq 111In-pentetreotide. On the basis of the current experience the main value of SRS in oncology is in the staging and evaluation of gastroenteropancreatic tumours, paragangliomas, small-cell lung cancer and lymphomas. Promising areas for SRS are the evaluation of breast cancer, non-medullary thyroid cancer and melanoma, and initial results with targeted radionuclide therapy using radiolabelled octreotide have been reported.

Somatostatin and cancer

The potential role of somatostatin (SRIF) in the diagnosis and treatment of nonendocrine human cancers is reviewed. There have been many reports of the growth-inhibitory activity of SRIF on normal and transformed cells in vitro. Many processes involved in malignant tumor growth depend on autocrine growth mechanisms, and somatostatin receptors (ssts) are present on many human cancers. It is possible that mutations in ssts result in a loss of check on proliferation in cancer cells. SRIF analogs may have a number of roles in clinical oncology. Use of radiolabeled tracers enables imaging of tumors bearing ssts; newer agents may enable positron emission tomography (PET) analyses or may be used to deliver lethal radiation doses to cells bearing a unique subset of sst. Although the ability of SRIF and its analogs to inhibit cellular proliferation has been shown in vitro, it has yet to be demonstrated in humans with cancer. Clinical improvements seen with SRIF or its analogs in cancer patients may be related to indirect effects, such as pain relief, reduction of gastrointestinal side effects of chemotherapeutic agents, effects on local production of growth factors, and inhibition of tumoral angiogenesis. Thus, with regard to their potential therapeutic role, SRIF analogs are likely to be used only in conjunction with other approaches, such as radiation, immunotherapy, chemotherapy, and growth factor modulation. Further research into the fundamental functions of each of the ssts and the intracellular actions of SRIF analogs will be needed to assess the potential usefulness of the latter in slowing the progression of human cancers.

Clinical relevance of somatostatin receptor scintigraphy in patients with skull base tumours

The differential diagnosis of tumours in the skull base is often difficult. With the experience that various intracranial tumours differ in their expression of somatostatin binding sites (SBS) somatostatin receptor scintigraphy (SRS) with the somatostatin analogue octreotide can give additional information of the tumour entity. Seventy patients with various tumours of the skull base were examined with 111Indium-labelled DTPA-octreotide injected i.v.. Planar and tomographic images were obtained with a gamma camera 4-6 and 24 hours after injection. All of the meningiomas (unifocal and multifocal tumours in various locations) showed a high density of SBS whereas in none of the examined neurinomas SR were found. Pituitary adenomas revealed in only 50% SR in different concentrations and independent of the endocrine activity. SRS can help in the differential diagnosis between meningiomas and other tumours, postoperative scar or radionecrosis at the skull base. A dural infiltration with meningioma tissue ("meningeal sign") may be discriminated from a reactive hypervascularisation in lesions with a diameter > 0.5 cm. We conclude that SRS can offer additional diagnostic aspects in the pre- and postoperative management of patients with skull base tumours.