Somatostatin receptor subtypes sst1, sst2, sst3 and sst5 expression in human pituitary, gastroentero-pancreatic and mammary tumors: comparison of mRNA analysis with receptor autoradiography

Bis(Disulfide)-Bridged Somatostatin-14 Analogs and Their [111In]In-Radioligands: Synthesis and Preclinical Profile

The overexpression of one or more somatostatin receptors (SST1-5R) in human tumors has provided an opportunity for diagnosis and therapy with somatostatin-like radionuclide carriers. The application of "pansomatostatin" analogs is expected to broaden the clinical indications and upgrade the diagnostic/therapeutic efficacy of currently applied SST2R-prefering radioligands. In pursuit of this goal, we now introduce two bicyclic somatostatin-14 (SS14) analogs, AT5S (DOTA-Ala1-Gly2-c[Cys3-Lys4-Asn5-c[Cys6-Phe7-DTrp8-Lys9-Thr10-Cys11]-Thr12-Ser13-Cys14]) and AT6S (DOTA-Ala1-Gly2-c[Cys3-Lys4-c[Cys5-Phe6-Phe7-DTrp8-Lys9-Thr10-Phe11-Cys12]-Ser13-Cys14]), suitable for labeling with trivalent radiometals and designed to sustain in vivo degradation. Both AT5S and AT6S and the respective [111In]In-AT5S and [111In]In-AT6S were evaluated in a series of in vitro assays, while radioligand stability and biodistribution were studied in mice. The 8/12-mer bicyclic AT6S showed expanded affinity for all SST1-5R and agonistic properties at the SST2R, whereas AT5S lost all affinity to SST1-5R. Both [111In]In-AT5S and [111In]In-AT6S remained stable in the peripheral blood of mice, while [111In]In-AT6S displayed low, but specific uptake in AR4-2J tumors and higher uptake in HEK293-SST3R tumors in mice. In summary, high radioligand stability was acquired by the two disulfide bridges introduced into the SS14 motif, but only the 8/12-mer ring AT6S retained a pansomatostatin profile. In consequence, [111In]In-AT6S targeted SST2R-/SST3R-positive xenografts in mice. These results call for further research on pansomatostatin-like radioligands for cancer theranostics.

Highlighting Fibroblasts Activation in Fibrosis: The State-of-The-Art Fibroblast Activation Protein Inhibitor PET Imaging in Cardiovascular Diseases

Fibrosis is a common healing process that occurs during stress and injury in cardiovascular diseases. The evolution of fibrosis is associated with cardiovascular disease states and causes adverse effects. Fibroblast activation is responsible for the formation and progression of fibrosis. The incipient detection of activated fibroblasts is important for patient management and prognosis. Fibroblast activation protein (FAP), a membrane-bound serine protease, is almost specifically expressed in activated fibroblasts. The development of targeted FAP-inhibitor (FAPI) positron emission tomography (PET) imaging enabled the visualisation of FAP, that is, incipient fibrosis. Recently, research on FAPI PET imaging in cardiovascular diseases increased and is highly sought. Hence, we comprehensively reviewed the application of FAPI PET imaging in cardiovascular diseases based on the state-of-the-art published research. These studies provided some insights into the value of FAPI PET imaging in the early detection of cardiovascular fibrosis, risk stratification, response evaluation, and prediction of the evolution of left ventricular function. Future studies should be conducted with larger populations and multicentre patterns, especially for response evaluation and outcome prediction.

225Ac-Labeled Somatostatin Analogs in the Management of Neuroendocrine Tumors: From Radiochemistry to Clinic

The widespread use of peptide receptor radionuclide therapy (PRRT) represents a major therapeutic breakthrough in nuclear medicine, particularly since the introduction of ¹⁷⁷Lu-radiolabeled somatostatin analogs. These radiopharmaceuticals have especially improved progression-free survival and quality of life in patients with inoperable metastatic gastroenteropancreatic neuroendocrine tumors expressing somatostatin receptors. In the case of aggressive or resistant disease, the use of somatostatin derivatives radiolabeled with an alpha-emitter could provide a promising alternative. Among the currently available alpha-emitting radioelements, actinium-225 has emerged as the most suitable candidate, especially regarding its physical and radiochemical properties. Nevertheless, preclinical and clinical studies on these radiopharmaceuticals are still few and heterogeneous, despite the growing momentum for their future use on a larger scale. In this context, this report provides a comprehensive and extensive overview of the development of ²²⁵Ac-labeled somatostatin analogs; particular emphasis is placed on the challenges associated with the production of ²²⁵Ac, its physical and radiochemical properties, as well as the place of ²²⁵Ac-DOTATOC and ²²⁵Ac-DOTATATE in the management of patients with advanced metastatic neuroendocrine tumors.

Prognostic factors of primary neuroendocrine breast cancer: A population-based study

Background

Primary neuroendocrine breast carcinomas (NEBCs) are an extremely rare and underrecognized subtype of mammalian carcinoma. The prognostic factors for NEBCs remain controversial.

Methods

In this multicenter retrospective study, the prognostic factors for patients with primary NEBCs who underwent surgery and had a pathologically confirmed diagnosis of neuroendocrine carcinoma in China and the United States were examined. The endpoints were disease‐free survival (DFS) and overall survival (OS).

Results

A total of 51 Chinese patients and 98 US patients were included. In the Chinese cohort, tumor grade and Ki‐67 levels were prognostic factors for DFS in univariate analysis (hazard ratio [HR] = 5.11 [1.67–15.60], p = 0.004; HR = 57.70 [6.36–523.40], p < 0.001, respectively) and multivariate analysis (HR = 100.52 [1.33–7570.21], p = 0.037; HR = 31.47 [1.05–945.82], p = 0.047, respectively). In the US cohort, age was an important prognostic factor for OS in univariate analysis (HR = 1.09 [1.04–1.15], p = 0.001). The random effects model for the combined cohorts revealed age and positive expression of estrogen receptor (ER) as potential prognostic factors for OS (HR = 1.08 [1.01–1.14], p = 0.015; HR = 0.10 [0.02–0.44], p = 0.003, respectively).

Conclusions

Tumor grade and Ki‐67 levels are important prognostic factors for DFS of patients with primary NEBCs. Age and ER status are important prognostic factors for OS of patients with primary NEBCs.

Somatostatin and Its Receptor System in Colorectal Cancer

Somatostatin (SST)/somatotropin release-inhibiting factor (SRIF) is a well-known neuropeptide, widely distributed in the central and peripheral nervous systems, that regulates the endocrine system and affects neurotransmission via interaction with five SST receptors (SST1-5). In the gastrointestinal tract, the main SST-producing cells include intestinal enteroendocrine cells (EECs) restricted to the mucosa, and neurons of the submucosal and myenteric plexuses. The action of the SRIF system is based on the inhibition of endocrine and exocrine secretion, as well as the proliferative responses of target cells. The SST1–5 share common signaling pathways, and are not only widely expressed on normal tissues, but also frequently overexpressed by several tumors, particularly neuroendocrine neoplasms (NENs). Furthermore, the SRIF system represents the only peptide/G protein-coupled receptor (GPCR) system with multiple approved clinical applications for the diagnosis and treatment of several NENs. The role of the SRIF system in the histogenesis of colorectal cancer (CRC) subtypes (e.g., adenocarcinoma and signet ring-cell carcinoma), as well as diagnosis and prognosis of mixed adenoneuroendocrine carcinoma (MANEC) and pure adenocarcinoma, is poorly understood. Moreover, the impact of the SRIF system signaling on CRC cell proliferation and its potential role in the progression of this cancer remains unknown. Therefore, this review summarizes the recent collective knowledge and understanding of the clinical significance of the SRIF system signaling in CRC, aiming to evaluate the potential role of its components in CRC histogenesis, diagnosis, and potential therapy.

Peptide-drug conjugate-based novel molecular drug delivery system in cancer

Drug delivery systems are generally believed to comprise drugs and excipients. A peptide-drug conjugate is a single molecule that can simultaneously play multiple roles in a drug delivery system, such as in vivo drug distribution, targeted release, and bioactivity functions. This molecule can be regarded as an integrated drug delivery system, so it is called a molecular drug delivery system. In the context of cancer therapy, a peptide-drug conjugate comprises a tumor-targeting peptide, a payload, and a linker. Tumor-targeting peptides specifically identify membrane receptors on tumor cells, improve drug-targeted therapeutic effects, and reduce toxic and side effects. Payloads with bioactive functions connect to tumor-targeting peptides through linkers. In this review, we explored ongoing clinical work on peptide-drug conjugates targeting various receptors. We discuss the binding mechanisms of tumor-targeting peptides and related receptors, as well as the limiting factors for peptide-drug conjugate-based molecular drug delivery systems.

Peptide-Based Strategies for Targeted Tumor Treatment and Imaging

Cancer is one of the leading causes of death worldwide. The development of cancer-specific diagnostic agents and anticancer toxins would improve patient survival. The current and standard types of medical care for cancer patients, including surgery, radiotherapy, and chemotherapy, are not able to treat all cancers. A new treatment strategy utilizing tumor targeting peptides to selectively deliver drugs or applicable active agents to solid tumors is becoming a promising approach. In this review, we discuss the different tumor-homing peptides discovered through combinatorial library screening, as well as native active peptides. The different structure–function relationship data that have been used to improve the peptide’s activity and conjugation strategies are highlighted.

Investigation of somatostatin receptor profile of neuroendocrine carcinomas of the breast

Carcinomas of the breast with neuroendocrine features are rare primary neoplasms positive for neuroendocrine markers. According to the WHO classification of tumours of the breast they are divided into three morphologically distinct categories. They comprise neuroendocrine tumour (NET), neuroendocrine carcinoma (NEC) and carcinoma with neuroendocrine differentiation (NED). The purpose of this study was to investigate for the first time the full spectrum of sstr expression status in breast carcinomas with neuroendocrine features. Fifteen primary breast carcinomas with histological and immunohistochemical neuroendocrine features were studied. Four of them were classified as NETs and two as NECs, and the remaining 9 as carcinomas with NED. All six types of somatostatin receptor (sstr) types (sstr1, sstr 2A, sstr2B, sstr3, sstr4 and sstr5) were investigated by immunohistochemistry. To assess the distribution and intensity of membranous receptor immunoreactivity, a four-scale scoring system was used. Overall predominant receptors were sstr2A, sstr2B, sstr3 and sstr5 showing the highest membranous staining scores 3+ and 2 + . The sstr1 was not detected. Given that carcinomas with neuroendocrine features represent distinct entities, patients with such tumours may benefit from sstr targeting therapies. Immunohistochemistry for sstrs can predict the effectiveness of administration of SST analogues to those patients, thus contributing to achieve the maximum therapeutic outcome, particularly in NETs and NECs with scores 2+ and 3 + .

State of the Art and Recent Developments of Radiopharmaceuticals for Pancreatic Neuroendocrine Tumors Imaging

Background:

Neuroendocrine Tumors (NETs) are relatively rare tumors, mainly originating from the digestive system, that tend to grow slowly and are often diagnosed when metastasised. Surgery is the sole curative option but is feasible only in a minority of patients. Among them, pancreatic neuroendocrine tumors (pancreatic NETs or pNETs) account for less than 5% of all pancreatic tumors. Viable therapeutic options include medical treatments such as biotherapies and more recently Peptide Receptor Radionuclide Therapies (PRRT) with radiolabeled somatostatin analogues. Molecular imaging, with main reference to PET/CT, has a major role in patients with pNETs.

Objective:

The overexpression of specific membrane receptors, as well as the ability of cells to take up amine precursors in NET, have been exploited for the development of specific targeting imaging agents.

Methods:

SPECT/CT and PET/CT with specific isotopes such as [68Ga]-1,4,7,10-tetra-azacyclododecane- N,N’,N’’,N’’’-tetra-acetic acid (DOTA)-somatostatin analogs, [18F]-FDG and [18F]-fluorodopa have been clinically explored.

Results:

To overcome the limitations of SSTR imaging, interesting improvements are connected with the availability of new radiotracers, activating with different mechanisms compared to somatostatin analogues, such as glucagon-like peptide 1 receptor (GLP-1 R) agonists or antagonists.

Conclusion:

This paper shows an overview of the RPs used so far in the imaging of pNETs with insight on potential new radiopharmaceuticals currently under clinical evaluation.

Clinical and functional implication of the components of somatostatin system in gastroenteropancreatic neuroendocrine tumors

PURPOSE

Gastroenteropancreatic neuroendocrine tumors (GEP-NETs) comprise a heterogeneous group of malignancies often presenting with metastasis at diagnosis and whose clinical outcome is difficult to predict. Somatostatin (SST) analogs (SSAs) provide a valuable pharmacological tool to palliate hormonal symptoms, and control progression in some NETs. However, many patients do not respond to SSAs or develop resistance, and there are many uncertainties regarding pathophysiology of SST and its receptors (sst1-sst5) in GEP-NETs.

METHODS

The expression of SST system components in GEP-NETs was determined, compared with that of non-tumor adjacent and normal tissues and correlated with clinical and histological characteristics. Specifically, 58 patients with GEP-NETs and 14 normal samples were included. Cell viability in NET cell lines was determined in response to specific SSAs.

RESULTS

Normal samples and non-tumor adjacent tissues presented a similar expression profile, with appreciable expression of sst2 and sst3, and a lower expression of the other receptors. In contrast, cortistatin, sst1, sst4, and sst5 were overexpressed in tumors, while sst3 and sst4 seemed overexpressed in less differentiated tumors. Some SST system components were related to vascular/nerve invasion and metastasis. In vitro, sst1 and sst3 agonists reduced viability in BON-1 cells, while they, similar to octreotide and pasireotide, increased viability in QGP-1 cells.

CONCLUSIONS

These results provide novel information on SST system pathophysiology in GEP-NETs, including relevant associations with clinical-histological parameters, which might help to better understand the intrinsic heterogeneity of NETs and to identify novel biomarkers and/or targets with potential prognostic and/or therapeutic value for GEP-NETs patients.

Peptides for optical medical imaging and steps towards therapy

Optical medical imaging is a rapidly growing area of research and development that offers a multitude of healthcare solutions both diagnostically and therapeutically. In this review, some of the most recently described peptide-based optical probes are reviewed with a special emphasis on their in vivo use and potential application in a clinical setting.

Evaluation of agonist and antagonist radioligands for somatostatin receptor imaging of breast cancer using positron emission tomography

Background

The somatostatin receptor subtype 2 (sstr2) is expressed on a majority of luminal breast cancers, however SPECT and scintigraphy imaging with agonistic sstr2 probes has been sub-optimal. High affinity antagonists can access more binding sites on the cell surface, resulting in higher tumor uptake and improved sensitivity. We compared the tumor uptake and biodistribution of the antagonist ⁶⁸Ga-NODAGA-JR11 with two agonists ⁶⁸Ga-DOTA-Tyr³-octreotide (⁶⁸Ga-DOTATOC) and ⁶⁸Ga-DOTA-Tyr³-octreotate (⁶⁸Ga-DOTATATE), in the human, sstr2-positive, luminal breast cancer model: ZR-75-1.

Results

Peptides were assayed for binding affinity using a filtration-based competitive assay to sstr2. ^natGa-DOTATOC and ^natGa-DOTATATE had excellent affinity (inhibition constant K_i: 0.9 ± 0.1 nM and 1.4 ± 0.3 nM respectively) compared to ^natGa-NODAGA-JR11 (25.9 ± 0.2 nM). The number of binding sites on ZR-75-1 cells was determined in vitro by saturation assays. Agonist ^67/natGa-DOTATOC bound to 6.64 ± 0.39 × 10⁴ sites/cells, which was 1.5-fold higher than ^67/natGa-NODAGA-JR11 and 2.3-fold higher than ^67/natGa-DOTATATE. All three ⁶⁸Ga-labeled peptides were obtained in good decay-corrected radiochemical yield (61-68%) and were purified by high performance liquid chromatography to ensure high specific activity (137 – 281 MBq/nmol at the end of synthesis). NOD scid gamma mice bearing ZR-75-1 tumors were injected intravenously with the labeled peptides and used for PET/CT imaging and biodistribution at 1 h post-injection. We found that ⁶⁸Ga-DOTATOC had the highest tumor uptake (18.4 ± 2.9%ID/g), followed by ⁶⁸Ga-DOTATATE (15.2 ± 2.2%ID/g) and ⁶⁸Ga-NODAGA-JR11 (12.2 ± 0.8%ID/g). Tumor-to-blood and tumor-to-muscle ratios were also higher for the agonists (>40 and >150 respectively), compared to the antagonist (15.6 ± 2.2 and 45.2 ± 11.6 respectively).

Conclusions

The antagonist ⁶⁸Ga-NODAGA-JR11 had the lowest tumor uptake and contrast compared to agonists ⁶⁸Ga-DOTATOC and ⁶⁸Ga-DOTATATE in ZR-75-1 xenografts.

The main contributing factor to this result could be the use of an endogenously expressing cell line, which may differ from previously published transfected models in the number of low-affinity, antagonist-specific binding sites. The relative merit of agonists versus antagonists for sstr2 breast cancer imaging warrants further investigation, first in preclinical models with other sstr2-positive breast cancer xenografts, and ultimately in luminal breast cancer patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s41181-017-0023-y) contains supplementary material, which is available to authorized users.

Somatostatin and dopamine receptor regulation of pituitary somatotroph adenomas

Somatostatin and dopamine receptors are expressed in normal and tumoral somatotroph cells. Upon receptor stimulation, somatostatin and the somatostatin receptor ligands octreotide, lanreotide, and pasireotide, and to a lesser extent, dopamine and the dopamine analogs bromocriptine and cabergoline, suppress growth hormone (GH) secretion from a GH-secreting pituitary somatotroph adenoma. Somatostatin and dopamine receptors are G_αi-protein coupled that inhibit adenylate cyclase activity and cAMP production and reduce intracellular calcium concentration and calcium flux oscillations. Although their main action on somatotroph cells is acute inhibition of GH secretion, they also may inhibit GH production and possibly somatotroph proliferation. These receptors have been reported to create complexes that exhibit functions distinct from that of receptor monomers. Somatostatin suppression of GH is mediated mainly by somatostatin receptor subtype 2 and to a lesser extent by SST5. Human somatostatin receptor subtype 5 has also been shown to harbor mutations associated with GH levels, somatotroph tumor behavior, and somatostatin receptor ligand (SRL) responsiveness. Reviewing current knowledge of somatostatin and dopamine receptor expression and signaling in normal and tumoral somatotroph cells offers insights into mechanisms underlying SRL and dopamine agonist effectiveness in patients with acromegaly.

Somatostatin Receptor Expression in GH-Secreting Pituitary Adenomas Treated with Long-Acting Somatostatin Analogues in Combination with Pegvisomant

BACKGROUND

Growth hormone-secreting pituitary adenomas (somatotroph adenoma) predominantly express somatostatin receptors (SSTRs) subtypes 2 and 5. Higher SSTR2 expression on somatotroph adenomas results in a better response to somatostatin analogues (SSAs), which preferentially bind, but also downregulate, SSTR2. The effect of the combined treatment with SSAs and the GH receptor antagonist pegvisomant (PEGV) on SSTR expression in somatotroph adenomas is currently unknown.

AIM OF THE STUDY

To assess SSTR2 and SSTR5 expression in three groups of somatotroph adenomas: drug-naive, treated with long-acting (LA) SSA monotherapy, or LA-SSA/PEGV combination therapy before surgery. Additionally, we evaluated the required PEGV dose to achieve insulin-like growth factor I (IGF-I) normalization in relation to the SSTR expression.

MATERIALS AND METHODS

At our Pituitary Center Rotterdam, we selected acromegalic patients who underwent transsphenoidal neurosurgery. All patients were eventually treated with LA-SSA/PEGV combination therapy during their medical history. SSTR2 and SSTR5 expression in somatotroph adenoma tissues was determined using immunohistochemistry.

RESULTS

Out of 39 somatotroph adenoma tissue samples, 23 were drug-naive, 9 received pretreatment with LA-SSA and 7 LA-SSA/PEGV combined treatment. SSTR2 expression was significantly higher in treatment-naive compared to combined treatment somatotroph adenomas (p = 0.048), while SSTR5 expression did not differ. Noteworthy, SSTR2 expression in naive somatotroph adenoma tissues was inversely correlated with the required PEGV dose to achieve IGF-I normalization during postsurgical medical treatment (ρ = -0.538, p = 0.024).

CONCLUSION

In our specific cohort, the SSTR2 expression was lower in patients pretreated with LA-SSA/PEGV compared to the drug-naive acromegalic patients. Additionally, the SSTR2 expression in treatment-naive somatotroph adenoma tissues was inversely correlated with the required PEGV dose to achieve IGF-I normalization.

LIMITATIONS OF CURRENT APPROACHES FOR THE TREATMENT OF ACROMEGALY

OBJECTIVE

Acromegaly is a rare disease characterized by hypersecretion of growth hormone (GH), typically from a benign pituitary somatotroph adenoma, that leads to subsequent hypersecretion of insulin-like growth factor 1 (IGF-1). Patients with acromegaly have an increased risk of mortality and progressive worsening of comorbidities. Surgery, medical therapy, and radiotherapy are currently available treatment approaches for patients with acromegaly, with overall therapeutic goals of lowering GH levels and achieving normal IGF-1 levels, reducing tumor size, improving comorbidities, and minimizing mortality risk. Although surgery can lead to biochemical remission in some patients with acromegaly, many patients will continue to have uncontrolled disease and require additional treatment.

METHODS

We reviewed recently published reports and present a summary of the safety and efficacy of current treatment modalities for patients with acromegaly.

RESULTS

A substantial proportion of patients who receive medical therapy or radiotherapy will have persistently elevated GH and/or IGF-1. Because of the serious health consequences of continued elevation of GH and IGF-1, there is a need to improve therapeutic approaches to optimize biochemical control, particularly in high-need patient populations for whom current treatment options provide limited benefit.

CONCLUSION

This review discusses current treatment options for patients with acromegaly, limitations associated with each treatment approach, and areas within the current treatment algorithm, as well as patient populations for which improved therapeutic options are needed. Novel agents in development were also highlighted, which have the potential to improve management of patients with uncontrolled or persistent acromegaly.

Peptide Receptor Radionuclide Therapy (PRRT) in a Patient Affected by Metastatic Breast Cancer with Neuroendocrine Differentiation

BACKGROUND

Breast cancer (BC) is the most frequent cancer in European women with nearly 30% of the patients eventually developing metastases. Neuroendocrine differentiation is a rare event, but overexpression of somatostatin receptors in BC has been reported in many studies.

CASE REPORT

A patient with liver metastases from BC was treated with peptide receptor radionuclide therapy (PRRT). Computed tomography scan and biochemical examinations showed a clear response to radionuclide therapy.

CONCLUSION

PRRT may be useful in metastatic BC patients.

[DOTA]Somatostatin-14 analogs and their (111)In-radioligands: effects of decreasing ring-size on sst1-5 profile, stability and tumor targeting

Multiple somatostatin receptor (sst)-subtype expression has been manifested in several human tumors. Hence, the availability of radiopeptides retaining the full pansomatostatin profile of the native hormone (SS14) is expected to increase the sensitivity and broaden the clinical indications of currently applied sst2-preferring cyclic octapeptide radioligands, like OctreoScan(®) ([(111)In-DTPA]octreotide). On the other hand, SS14 has been excluded from clinical use due to its rapid in vivo degradation. We herein present a small library of seven novel cyclic SS14-mimics carrying at their N-terminus the universal chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) for stable binding of medically useful radiometals, like (111)In. By decreasing the number of amino acids composing the ring in their structure from 12 up to 6 AA, we induced important changes in key-biological parameters in vitro and in vivo. In particular, we observed unexpected changes and even total loss of sst1-5-affinity (6AA-ring), as well as weaker sst2-internalization efficacy as the ring size decreased. In contrast, in vivo stability increased with decreasing ring size, reaching its maximum in the 6AA-ring analogs. Interestingly, only the 12AA- and 9AA-ring members of this series showed sst2-specific uptake in AR4-2J tumors in mice revealing the prominent role of ring size on the biological response of tested SS14-derived radioligands.

Production and quality control of [(90)Y]DOTATOC for treatment of metastatic neuroendocrine tumors: results of 85 syntheses

The aim of this paper was to describe the optimized labeling protocol and quality control measures used in the production of [(90)Y]DOTATOC, starting from three different radioactivity levels to treat one, two, or three patients per therapeutic session. We investigated three different starting radioactivity levels. For the low radioactivity preparation we used 5138±280 MBq of [Y] isotope, for the medium radioactivity preparation we used 8893±900 MBq, and for the high radioactivity preparation we used 11250±715 MBq. The radiochemical purity levels for the low radioactivity preparation, medium radioactivity preparation, and high radioactivity preparation were 99.95±0.09, 99.84±0.34, and 99.84±0.53%, respectively, and the radiochemical yields of the labeling procedures were 77.52±1.28, 75.53±3.72, and 78.00±3.20%, respectively. Media fill validation of the process was performed, and the parameters of pH, bacterial endotoxins, sterility, and osmolality were tested at process control. All radiopharmaceutical preparations satisfied the predetermined specifications fixed in our protocol regardless of the starting radioactivity level. The validation of the method guaranteed the safety and quality of the final products, contributing to providing the basis for constructing an informative and successful clinical trial.

Somatostatin subtype-2 receptor-targeted metal-based anticancer complexes

Conjugates of a dicarba analogue of octreotide, a potent somatostatin agonist whose receptors are overexpressed on tumor cells, with [PtCl(2)(dap)] (dap = 1-(carboxylic acid)-1,2-diaminoethane) (3), [(η(6)-bip)Os(4-CO(2)-pico)Cl] (bip = biphenyl, pico = picolinate) (4), [(η(6)-p-cym)RuCl(dap)](+) (p-cym = p-cymene) (5), and [(η(6)-p-cym)RuCl(imidazole-CO(2)H)(PPh(3))](+) (6), were synthesized by using a solid-phase approach. Conjugates 3-5 readily underwent hydrolysis and DNA binding, whereas conjugate 6 was inert to ligand substitution. NMR spectroscopy and molecular dynamics calculations showed that conjugate formation does not perturb the overall peptide structure. Only 6 exhibited antiproliferative activity in human tumor cells (IC(50) = 63 ± 2 μM in MCF-7 cells and IC(50) = 26 ± 3 μM in DU-145 cells) with active participation of somatostatin receptors in cellular uptake. Similar cytotoxic activity was found in a normal cell line (IC(50) = 45 ± 2.6 μM in CHO cells), which can be attributed to a similar level of expression of somatostatin subtype-2 receptor. These studies provide new insights into the effect of receptor-binding peptide conjugation on the activity of metal-based anticancer drugs, and demonstrate the potential of such hybrid compounds to target tumor cells specifically.

[111In-DOTA]Somatostatin-14 analogs as potential pansomatostatin-like radiotracers - first results of a preclinical study

Background

In this study, we report on the synthesis, radiolabeling, and biological evaluation of two new somatostatin-14 (SS14) analogs, modified with the universal chelator DOTA. We were interested to investigate if and to what extent such radiotracer prototypes may be useful for targeting sst_1-5-expressing tumors in man but, most importantly, to outline potential drawbacks and benefits associated with their use.

Methods

AT1S and AT2S (DOTA-Ala¹-Gly²-c[Cys³-Lys⁴-Asn⁵-Phe⁶-Phe⁷-Trp⁸/DTrp⁸-Lys⁹-Thr¹⁰-Phe¹¹-Thr¹²-Ser¹³-Cys¹⁴-OH], respectively) were synthesized on the solid support and labeled with ¹¹¹In. The sst_1-5 affinity profile of AT1S/AT2S was determined by receptor autoradiography using [Leu⁸,dTrp²²,¹²⁵I-Tyr²⁵]SS28 as radioligand. The ability of AT2S to stimulate sst₂ or sst₃ internalization was qualitatively analyzed by an immunofluorescence-based internalization assay using hsst₂- or hsst₃-expressing HEK293 cells. Furthermore, the internalization of the radioligands [¹¹¹In]AT1S and [¹¹¹In]AT2S was studied at 37 °C in AR4-2J cells endogenously expressing sst₂. The in vivo stability of [¹¹¹In]AT1S and [¹¹¹In]AT2S was tested by high-performance liquid chromatography analysis of mouse blood collected 5 min after radioligand injection, and biodistribution was studied in normal mice. Selectively for [¹¹¹In]AT2S, biodistribution was further studied in SCID mice bearing AR4-2J, HEK293-hsst_2A⁺, -hsst₃⁺ or -hsst₅⁺ tumors.

Results

The new SS14-derived analogs were obtained by solid phase peptide synthesis and were easily labeled with ¹¹¹In. Both SS14 conjugates, AT1S, and its DTrp⁸ counterpart, AT2S, showed a pansomatostatin affinity profile with the respective hsst_1-5 IC₅₀ values in the lower nanomolar range. In addition, AT2S behaved as an agonist for sst₂ and sst₃ since it stimulated receptor internalization. The ¹¹¹In radioligands effectively and specifically internalized into rsst_2A-expressing AR4-2J cells with [¹¹¹In]AT2S internalizing faster than [¹¹¹In]AT1S. Ex vivo mouse blood analysis revealed a rapid degradation of both radiopeptides in the bloodstream with the DTrp⁸ analog showing higher stability. Biodistribution results in healthy mice were consistent with these findings with only [¹¹¹In]AT2S showing specific uptake in the sst₂-rich pancreas. Biodistribution of [¹¹¹In]AT2S in tumor-bearing mice revealed receptor-mediated uptake in the AR4-2J (1.82 ± 0.36 %ID/g - block 0.21 ± 0.17 %ID/g at 4 h post injection (pi)), the HEK293-hsst_2A⁺ (1.49 ± 0.2 %ID/g - block 0.27 ± 0.20 %ID/g at 4 h pi), the HEK293-hsst₃⁺ (1.24 ± 0.27 %ID/g - block 0.32 ± 0.06 %ID/g at 4 h pi), and the HEK293-hsst₅⁺ tumors (0.41 ± 0.12 %ID/g - block 0.22 ± 0.006 %ID/g at 4 h pi). Radioactivity washed out from blood and background tissues via the kidneys.

Conclusions

This study has revealed that the native SS14 structure can indeed serve as a motif for the development of promising pansomatostatin-like radiotracers. Further peptide stabilization is required to increase in vivo stability and, consequently, to enhance in vivo delivery and tumor targeting.

Comparison of ⁶⁸Ga-DOTATATE and ⁶⁸Ga-DOTANOC PET/CT imaging in the same patient group with neuroendocrine tumours

PURPOSE

Recent studies have suggested that positron emission tomography (PET) imaging with (68)Ga-labelled DOTA-somatostatin analogues (SST) like octreotide and octreotate is useful in diagnosing neuroendocrine tumours (NETs) and has superior value over both CT and planar and single photon emission computed tomography (SPECT) somatostatin receptor scintigraphy (SRS). The aim of the present study was to evaluate the role of (68)Ga-DOTA-1-NaI(3)-octreotide ((68)Ga-DOTANOC) in patients with SST receptor-expressing tumours and to compare the results of (68)Ga-DOTA-D-Phe(1)-Tyr(3)-octreotate ((68)Ga-DOTATATE) in the same patient population.

METHODS

Twenty SRS were included in the study. Patients' age (n = 20) ranged from 25 to 75 years (mean 55.4 ± 12.7 years). There were eight patients with well-differentiated neuroendocrine tumour (WDNET) grade1, eight patients with WDNET grade 2, one patient with poorly differentiated neuroendocrine carcinoma (PDNEC) grade 3 and one patient with mixed adenoneuroendocrine tumour (MANEC). All patients had two consecutive PET studies with (68)Ga-DOTATATE and (68)Ga-DOTANOC. All images were evaluated visually and maximum standardized uptake values (SUV(max)) were also calculated for quantitative evaluation.

RESULTS

On visual evaluation both tracers produced equally excellent image quality and similar body distribution. The physiological uptake sites of pituitary and salivary glands showed higher uptake in (68)Ga-DOTATATE images. Liver and spleen uptake values were evaluated as equal. Both (68)Ga-DOTATATE and (68)Ga-DOTANOC were negative in 6 (30 %) patients and positive in 14 (70 %) patients. In (68)Ga-DOTANOC images only 116 of 130 (89 %) lesions could be defined and 14 lesions were missed because of lack of any uptake. SUV(max) values of lesions were significantly higher on (68)Ga-DOTATATE images.

CONCLUSION

Our study demonstrated that the images obtained by (68)Ga-DOTATATE and (68)Ga-DOTANOC have comparable diagnostic accuracy. However, (68)Ga-DOTATATE seems to have a higher lesion uptake and may have a potential advantage.

GPCR somatostatin receptor extracellular loop 2 is a key ectodomain for making subtype-selective antibodies with agonist-like activities in the pancreatic neuroendocrine tumor BON cell line

OBJECTIVES

The extracellular loop 2 (ECL2) ectodomain of the G protein-coupled receptor class A is thought to function like an inactivation "lid." We created polyclonal somatostatin receptor ECL2 (anti-SSTR ECL2) antibodies to target this lid and to examine if these antibodies can selectively activate the SSTR.

METHODS

Western blots and live-cell immunofluorescence microscopy determined anti-SSTR ECL2 antibody receptor binding selectivity. 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium assay (MTS assay) and cell cycle assay (fluorescence-activated cell sorting) checked for antibody effect on antiproliferation. Nexin assay examined the antibody's ability to induce apoptosis. LANCE cAMP kit (Perkin Elmer) detected antibody-dependent cAMP decrease. Enzyme-linked immunosorbent assay measured antibody effect on suppressing serotonin secretion. Ligand-receptor binding interference assay with the fluorescent somatostatin (FAM-SST) was used to examine antibody interference to SST-SSTR binding.

RESULTS

Anti-SSTR ECL2 antibodies are SSTR subtype selective and agonist-like, and they suppress cell proliferation via cell cycle arrest and apoptosis. In addition, these antibodies decrease cAMP production and inhibit serotonin secretion. Interestingly, these antibodies do not interfere with SST-SSTR binding.

CONCLUSIONS

The ECL2 is an important ectodomain for G protein-coupled receptor activation and required for ligand binding selectivity. The anti-SSTR2, anti-SSTR3, and anti-SSTR5 ECL2 antibodies independently inhibited BON proliferation and decreased hormone secretion. Unlike octreotide, our antibodies do not interfere with SST-SSTR binding.

Pituitary somatostatin receptor signaling

Somatotropin-release inhibitory factor (SRIF) is a major regulator of pituitary function, mostly inhibiting hormone secretion and to a lesser extent pituitary cell growth. Five SRIF receptor subtypes (SSTR1-5) are ubiquitously expressed G-protein coupled receptors. In the pituitary, SSTR1, 2, 3 and 5 are expressed, with SSTR2 and SSTR5 predominating. As new SRIF analogs have recently been introduced for treatment of pituitary disease, we evaluate the current knowledge of cell-specific pituitary SRIF receptor signaling and highlight areas of future research for comprehensive understanding of these mechanisms. Elucidating pituitary SRIF receptor signaling enables understanding of pituitary hormone secretion and cell growth, and also encourages future therapeutic development for pituitary disorders.

Somatostatin and somatostatin receptors: implications for neoplastic growth and cancer biology

Somatostatin agonists (SM-As) are capable of achieving durable symptomatic relief and significant clinical responses in certain tumours. Herein, we review the diverse direct and indirect mechanisms of antineoplastic activity elicited by SM-As as well as the hurdles that complicate their use as monotherapies in a broader range of malignancies. Emphasis is placed on recent clinical attempts to neutralise the IGF-mediated survival factor effects in the bone metastasis microenvironment in advanced prostate cancer. The first clinical trials of this 'anti-survival factor manipulation' strategy utilised the ability of SM-As to suppress the growth hormone-dependent liver-derived IGF-I bioavailability in combination with other drugs, such as dexamethasone, zolendronate and oestrogens, acting systemically and at the bone metastasis microenvironment. These regimens restored androgen ablation responsiveness in stage D3 prostate cancer patients and successfully produced objective clinical responses while only mild toxicities were observed. Furthermore, we focus on the preclinical experimental data of a targeted SM-A coupled to the super-potent doxorubicin derivative AN-201. The resulting conjugate (AN-238) has shown increased antitumour potency with a favourable toxicity profile. The potential use of novel SM-As as anticancer drugs is discussed in relation to data suggesting other direct and indirect treatment approaches pertaining to the somatostatin system.

Somatostatin-dopamine ligands in the treatment of pituitary adenomas

Somatostatin receptors (sst1-5) and dopamine receptor 2 (D2DR) are well expressed and co-localized in several human pituitary adenomas, suggesting possible functional interactions in the control of hormonal hypersecretion and tumor cell growth. The present review describes the expression and functionality of these receptors in the different classes of human pituitary adenomas. The sst2 agonists, octreotide and lanreotide, control GH hypersecretion and tumor growth in about 65% of somatotropinomas. The D2DR agonists, bromocriptine and cabergoline, control about 90% of prolactinomas. Such drugs are much less effective in the control of the others pituitary adenomas also expressing ssts and D2DR receptors. The second part summarizes the current knowledge on new chimeric compounds with sst2, sst5, and D2DR affinity. Such ligands bearing distinct ssts and DRD2 pharmacophores may synergistically produce an increased control of secretion and/or of proliferation in the different types of pituitary adenomas. The mechanisms of action of such chimeric molecules through increased binding affinities, prolonged bioavailability, ligand-induced modulation of receptors heterodimerization, are discussed.

Biology of somatostatin in breast cancer

The biological effects of the neuropeptide somatostatin (SST) are mediated via a family of five somatostatin receptors (SSTRs) belonging to a family of G-protein-coupled receptors (GPCRs). SSTR regulate the secretion of hormones, growth factors, neurotransmission and cell growth in receptor-specific manner. In addition, SST plays an inhibitory role in several mammary cancer models. These effects are mediated both indirectly through inhibition of hormones and growth factors which promote tumor growth as well as directly via SSTRs present on tumor cells to inhibit mitogenic signaling of growth factor receptor kinases leading to growth arrest and induction of apoptosis. Here, we present an overview on the role of SST and its analogs in breast cancer.

Relevance of coexpression of somatostatin and dopamine D2 receptors in pituitary adenomas

Dopamine and somatostatin are both involved in the negative control of normal pituitary cells. Dopamine subtype 2 receptor (D2DR) and somatostatin receptor (sst) agonists, mainly directed to sst2, are used in the treatment of pituitary adenomas. Nevertheless, a majority of corticotroph and gonadotroph adenomas and a third of somatotroph adenomas are still not sufficiently controlled by these treatments. D2DR and sst1, 2, 3 and 5 are present in most pituitary adenomas. These receptors may interact by heterodimerization as shown for sst1-sst5, sst5-D2DR, sst2-sst3 and sst2-D2DR suggesting possible additive effects. D2DR and sst2 agonist cotreatment showed limited additivity on GH secretion in acromegaly. Moreover, new chimeric compounds with sst2, D2DR and sst5 affinity have shown an increased control of secretion and/or proliferation of different types of pituitary adenomas in cell culture. Together with the multi-sst ligand drugs recently developed, these dopamine-somatostatin ligands represent a new opportunity in the combinatory treatment of pituitary adenomas.

Diagnostic targeting of colon cancer using a novel fluorescent somatostatin conjugate in a mouse xenograft model

Colorectal carcinoma is one of the more prevalent, highly malignant human tumors, occurring in about 7% of the population. However, if diagnosed and treated in its early stages, colon cancer is curable. In our study, we used a mouse xenograft model to investigate the capability of a fluorescent conjugate of a novel synthetic somatostatin (SST) analog to improve detection of human colorectal tumors that are characterized by over-expressed SST receptors. Human HT-29 colon carcinomas were induced in nude mice. After administration of the fluorescent SST conjugate, in vivo low- and high-magnification fluorescence microscopy, as well as high-resolution spectrally resolved imaging were performed, and the time-dependent biodistribution was determined quantitatively (using fiber-optic spectroscopy). Administration of the conjugate (at concentrations of 6 mg/kg body weight) enabled targeting small (1-5 mm diameter) tumors with high sensitivity and selectivity. Toxicity studies at dosages up to 1,000 mg/kg body weight did not reveal any drug related abnormalities. In conclusion, the SST conjugate significantly enhanced the detection of HT-29 colon tumors by fluorescence imaging because of a 5- to 8-fold increase in the contrast between malignant and normal tissues.

Preparation and biological studies of 125I-DOTA-TATE

DOTA-TATE, a somatostatin analog was radiolabeled with (125)I in good yields and high radiochemical purity. The product exhibited good stability in vitro. Pharmacokinetic studies in normal Swiss mice showed rapid blood clearance with low thyroid uptake. Biodistribution studies in murine melanoma showed 3.0+/-1.3% ID/g uptake in tumor at 3h post injection (p.i.), with negligible reduction at 24h p.i. Inhibition studies carried out in vivo using cold DOTA-TATE confirmed the tumor specificity of the product.

Expression of type 5 somatostatin receptor in TSH-secreting pituitary adenomas: a possible marker for predicting long-term response to octreotide therapy

In TSH-secreting pituitary adenomas (TSHoma), octreotide (OCT) therapy reduces tumor size and TSH secretion in some cases but not in others. As OCT acts through various types of somatostatin receptors (SSTRs), the different responses of TSHoma to OCT might be explained by the differences of SSTR expression. We therefore studied the expression of subtype-specific SSTR mRNA transcripts in tumor tissues by RT-PCR. Type 2 (SSTR2) mRNA transcripts were detected in all 8 tumors but those of SSTR3 and SSTR5 were demonstrated only in 5 of them. Serum TSH levels were decreased by OCT administration test in all patients but OCT therapy was effective in two patients out of three. SSTR5 mRNA was detected in two tumors from the responder, but not in one tumor that was resistant to OCT. These observations suggest that the temporal decrease of TSH by OCT may be mediated by SSTR2, and that the long term response to OCT therapy may be related with the expression of SSTR5. Therefore, the expression of SSTR5 in TSHoma may be a useful marker for predicting the outcome of the therapy, but further studies with larger numbers of patients are necessary.

Mapping of somatostatin receptor types in GH or/and PRL producing pituitary adenomas

BACKGROUND

Somatostatin is a tetradecapeptide exerting inhibitory action on endocrine and exocrine cell secretion and proliferation. Somatostatin receptors (SST) are widely expressed in various neoplasms including endocrine tumours. Using immunohistochemistry, the expression of SST(1), SST(2A), SST(2B), SST(3), SST(4), and SST(5) was studied in tissue microarrays (TMAs), using a series of 90 human pituitary adenomas producing growth hormone and/or prolactin, including 30 of each somatotroph, lactotroph, and mixed somatotroph/lactotroph adenoma type.

METHODS

For immunohistochemistry, the standard avidin biotin complex method enhanced by tyramide was used, using polyclonal antisera for all SST types. A four point scoring system was used to assess the membranous immunopositivity.

RESULTS

All SST types were positive in all tumour types, showing varying immunoreactivity scores. SST(5) and SST(2A) were the predominant receptors, showing strong expression in high frequency in all three adenoma types. Strong expression of SST(1) was higher in lactotroph adenomas than in other tumour types.

CONCLUSIONS

The immunohistochemical results of SST expression are in agreement with most findings of previous molecular studies. The fact that SST(2A) expression is predominant suggests that pharmaceutical octapeptide somatostatin analogues may act through this receptor, while the role of SST(2B) may be merely synergistic.

Glucose-responsive and octreotide-sensitive insulinoma

Insulinoma is the most common cause of fasting hypoglycemia resulting from autonomous insulin hypersecretion. We describe herein a unique case with paradoxical hypoglycemic episodes induced by hyperglycemia. A 55-year-old female had repeated hypoglycemic episodes after meal or during increased physical activity. Although fasting (10 hr) failed to provoke hypoglycemia, oral glucose tolerance test (GTT) caused an exaggerated insulin response (885 microU/ml) at 30 min, followed by hypoglycemia (36 mg/dl) after 90 min. Moreover, intravenous GTT also induced an exaggerated insulin response (>2900 microU/ml) at 10 min, followed by hypoglycemia (34 mg/dl) after 40 min. Although MRI and CT scan of the abdomen failed to detect any mass lesions in the pancreas, Octreoscan revealed increased radioactive uptake around the pancreatic head region. Treatment with a daily injection of octreotide (100 microg) alleviated her hypoglycemic episodes. At surgery, two islet cell adenomas were identified in the pancreas and resected. Postoperatively, she was free from hypoglycemic episodes after meal. Postoperative oral and intravenous GTT did not induce hypoglycemia. Thus, this is a very rare case of glucose-responsive and octreotide-sensitive insulinoma in whom GTT and octreotide proved to be a useful provocation and treatment for hypoglycemic episodes.

Camptothecin-somatostatin conjugates inhibit the growth of small cell lung cancer cells

The effects of camptothecin-somatostatin (CPT-SS) conjugates were investigated on small cell lung cancer (SCLC) cells. CPT was coupled to a SS agonist (SSA), c(Cys-Phe-DTrp-Lys-Thr-Cys)Thr-NH2 using the built in nucleophile assisted-releasing group (L1) N-methyl-aminoethyl-Gly-Dser-Nle-Dtyr-Dser or (L2) aminoethyl-Gly-Dser-Nle-Dtyr-Dser. The resulting CPT-L1-SSA and CPT-L2-SSA inhibited the specific binding of [125I-Tyr11]SS to NCI-H69 cell membranes with IC50 values of 0.2 and 2.1 nM, respectively. [125I]CPT-L1-SSA was internalized by SCLC cells at 37 degrees C but not at 4 degrees C. CPT-L1-SSA and CPT-L2-SSA inhibited in a dose-dependent manner the increase in adenylylcyclase activity caused by 25 microM forskolin. In vitro, 0.3 microM CPT-L1-SSA half-maximally inhibited the clonal growth of SCLC cells and 1 microM CPT-L1-SSA strongly inhibited 3H-thymidine incorporation into DNA and trypan-blue exclusion. These results suggest that CPT conjugated peptides such as CPT-L1-SSA may prove useful for exploring the efficacy of receptor-directed cytotoxicity to inhibit the proliferation of SCLC cells.

Somatostatin receptors in primary human breast cancer: quantitative analysis of mRNA for subtypes 1–5 and correlation with receptor protein expression and tumor pathology

Somatostatin receptors (SSTRs) have been identified in most hormone-producing tumors as well as in breast cancer. In the present study, we determined SSTR1-5 expression in primary ductal NOS breast tumors through semi-quantitative RT-PCR and immunocytochemistry. The results from the analysis of 98 samples were correlated with several key histological markers and receptor expression. All five SSTR subtypes are variably expressed at the mRNA level in breast tumors with 91% of samples showing SSTR1, 98% SSTR2, 96% SSTR3, 76% SSTR4, and 54% SSTR5. SSTR1-5 are localized to both tumor cells and the surrounding peritumoral regions as detected by immunocytochemistry. Levels of SSTR mRNA, when corrected for beta-actin levels, were highest for SSTR3 followed by SSTR1, SSTR2, SSTR5, and SSTR4. Furthermore, there was good correlation between mRNA and protein expression with 84% for SSTR1, 79% for SSTR2, 89% for SSTR3, 68% for SSTR4, 68% for SSTR5, and 78% for all five receptors. SSTR1, 2 and 4 were correlated with ER levels whereas SSTR2 showed an additional correlation with PR levels. These correlations were independent of patient age and histological grade. Moreover, using immunocytochemistry, blood vessels exhibited receptor-specific localization for SSTR2 and SSTR5. Our results indicate significant correlations between mRNA and protein expression along with receptor-specific correlations with histological markers as well as ER and PR levels. Differential distribution of SSTR subtypes in tumors and receptor-specific expression in vascular structures may be considered as a novel diagnosis for breast tumors with receptor subtype agonists.

99mTc depreotide scan compared with 99mTc-MDP bone scintigraphy for the detection of bone metastases and prediction of response to hormonal treatment in patients with breast cancer

BACKGROUND

The purpose of this study was to determine the potential role of Tc depreotide scintigraphy for the evaluation of bone metastases compared with Tc methylenediphosphonate (MDP) bone scintigraphy and for the prediction of treatment response in breast cancer patients in whom first- or second-line hormonal therapy was to be initiated.

METHODS

Twelve patients with a diagnosis of advanced breast cancer were included. All patients underwent both a bone scan and a depreotide scan and at least one other conventional imaging procedure, including plain film radiography (n=11), computed tomography (n=6) or magnetic resonance imaging (n=5), for confirmation of metastatic disease. The mean time interval between the bone scan and the depreotide scan was 30.6 days. Follow-up data were retrieved from routine clinical evaluation by means of physical examination, imaging and blood analysis.

RESULTS

On a patient basis we found a sensitivity, specificity and accuracy of, respectively 100%, 50% and 83.3% for the bone scan and 62.5%, 100% and 75% for the depreotide scan in the diagnosis of bone metastasis. In eight patients with available follow-up data two with a positive depreotide scan remained stable and five of six patients with a negative depreotide scan had progressive disease.

CONCLUSION

In this small series of breast cancer patients Tc depreotide scintigraphy proves less sensitive but more specific as compared to Tc-MDP bone scintigraphy in measuring the extent of bone metastasis. On the other hand Tc depreotide scintigraphy elucidates, non-invasively, tumour characteristics and may be indicative for prognosis and response to hormonal treatment.

Effects of camptothecin conjugated to a somatostatin analog vector on growth of tumor cell lines in culture and related tumors in rodents

Two CPT-SSA conjugates, JF-10-71 and JF-10-81, containing a chemically adjustable release-rate carbamate linker, have been reported previously by us to potently inhibit growth of human neuroblastoma IMR32 cells overexpressing somatostatin receptor type II (SSTR2) but are stable under buffer incubation conditions or in rat plasma. Further experiments now reveal that the conjugates performed well against many additional cell lines, particularly somatostatin receptor containing rat pancreatic CA20948 cells that were actually more sensitive to the conjugates than free camptothecin itself. JF-10-71 and JF-10-81 also were examined for their inhibitory effects on the growth of this and several other tumors transplanted into rats (CA20948) or nude mice. CA20948 tumors, known to overexpress SSTR2 and grown in Lewis rats, were treated, respectively, with nontoxic 400 nmol/kg intraperitoneal (i.p.) doses of JF-10-71 or JF-10-81. Also, SSTR2-positive human SCLC NCI-H69 tumors transplanted in nude mice were treated in a similar fashion. Human prostate PC-3 tumors, which do not contain high concentrations of SSTR2, also were grown in nude mice and treated with a 400 nmol/kg ip dose of JF-10-71. Both cytotoxic conjugates significantly inhibited growth of SSTR2-specific pancreatic and SCLC tumors, but JF-10-81 did not significantly affect PC-3 tumor growth. These experimental results suggested that CPT-SSA conjugates can effectively target and kill tumor cells growing in vivo and that the effect is mediated by somatostatin receptors resulting in either release of camptothecin at the cell surface or, more likely, after receptor-mediated cellular internalization.

Radiolabeled peptides in nuclear oncology: influence of peptide structure and labeling strategy on pharmacology

Radiometallo-labeled analogs of SS have shown great benefit in the in vivo localization and targeted radiotherapy of human tumors. The progress and innovation in this clinical application came from the change in strategy, leaving the most widely used radiohalogens for a coordination chemistry approach. The use of chelators appended to the biologically active peptide which convey high thermodynamic and kinetic stability to the radiopeptides did not only improve the pharmacokinetics and pharmacodynamics of the molecules, but surprisingly the biological potency as well. The most urgent problem to be solved in the field is to improve the kidney clearance of the radiopeptides. The kidney turned out to be the dose-limiting organ in this type of targeted radiotherapy. Coordination chemical strategies have already paved the way to a successful clinical application; it is most likely that chelator modification will further help to improve the renal handling of radiometallopeptides.

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.

Oestrogen-mediated regulation of somatostatin receptor expression in human breast cancer cell lines assessed with 99mTc-depreotide

Investigating three somatostatin receptor (SSTR)-positive (+) human breast cancer cell lines, Xu et al. found a time- and dose-dependent up- or down-regulation of SSTR2 mRNA expression by 17beta-oestradiol (E(2)) or the anti-oestrogen tamoxifen, respectively, in the two oestrogen receptor-positive (ER+) cell lines but not in the oestrogen receptor-negative (ER-) cell line. This study aimed to confirm the findings of Xu et al. at the protein level by means of western blotting and saturation binding studies using (99m)Tc-depreotide (NeoSpect). The ER+/SSTR+ ZR75-1 and T47D and SSTR+/ER- MDA MB231 breast cancer cell lines were exposed to 1 n M E(2) or a combination of 1 n M E(2) plus 100 n M tamoxifen or ICI 182 780 (Faslodex) for 48 h. Exposed and non-exposed controls were incubated with increasing concentrations of (99m)Tc-depreotide (0.5 n M-15 n M) in the absence and the presence of 20 micro M of octreotide. Scatchard-Rosenthal plots were derived using commercially available software. SSTR subtypes responsible for E(2)-induced changes in (99m)Tc-depreotide binding were identified by means of western blotting. Mean K(d) values for (99m)Tc-depreotide were 13 n M, 7 n M and 4 n M for T47D, ZR75-1 and MDA MB231 cells, respectively. After stimulation with E(2), the ER+ cell line T47D demonstrated a mean increase of 81% ( P<0.05) in (99m)Tc-depreotide binding. Adding the partial agonist tamoxifen and full antagonist ICI 182 780 to E(2) blocked the induced increase in T47D cells, either reducing SSTR expression or restoring it to control levels. ZR75-1 cells stimulated with E(2) showed a mean decrease in (99m)Tc-depreotide binding of 36% as compared to control cells; this difference, however, proved to be not statistically significant. Similarly, B(max) values did not change in ZR75-1 cells exposed to E(2) in combination with an ER antagonist as compared to control cells. Finally, no influence of E(2) on (99m)Tc-depreotide binding was observed in the ER- cell line MDA MB231. Both SSTR2 and SSTR5 were expressed at high levels in T47D cells and ZR75-1 cells. SSTR5 drastically increased in the absence of E(2) and was restored to the original detection level after E(2) treatment. The presented findings support an oestrogen-dependent regulation of SSTR expression in breast cancer cell lines.