Molecular pharmacology of somatostatin receptor subtypes

Structural basis for activation of somatostatin receptor 5 by cyclic neuropeptide agonists

Somatostatin receptor 5 (SSTR5) is an important G protein-coupled receptor and drug target for neuroendocrine tumors and pituitary disorders. This study presents two high-resolution cryogenicelectron microscope structures of the SSTR5-Gi complexes bound to the cyclic neuropeptide agonists, cortistatin-17 (CST17) and octreotide, with resolutions of 2.7 Å and 2.9 Å, respectively. The structures reveal that binding of these peptides causes rearrangement of a "hydrophobic lock", consisting of residues from transmembrane helices TM3 and TM6. This rearrangement triggers outward movement of TM6, enabling Gαi protein engagement and receptor activation. In addition to hydrophobic interactions, CST17 forms conserved polar contacts similar to somatostatin-14 binding to SSTR2, while further structural and functional analysis shows that extracellular loops differently recognize CST17 and octreotide. These insights elucidate agonist selectivity and activation mechanisms of SSTR5, providing valuable guidance for structure-based drug development targeting this therapeutically relevant receptor.

Peptide Therapeutics: Unveiling the Potential against Cancer-A Journey through 1989

The United States Food and Drug Administration (FDA) has approved a plethora of peptide-based drugs as effective drugs in cancer therapy. Peptides possess high specificity, permeability, target engagement, and a tolerable safety profile. They exhibit selective binding with cell surface receptors and proteins, functioning as agonists or antagonists. They also serve as imaging agents for diagnostic applications or can serve a dual-purpose as both diagnostic and therapeutic (theragnostic) agents. Therefore, they have been exploited in various forms, including linkers, peptide conjugates, and payloads. In this review, the FDA-approved prostate-specific membrane antigen (PSMA) peptide antagonists, peptide receptor radionuclide therapy (PRRT), somatostatin analogs, antibody-drug conjugates (ADCs), gonadotropin-releasing hormone (GnRH) analogs, and other peptide-based anticancer drugs are analyzed in terms of their chemical structures and properties, therapeutic targets and mechanisms of action, development journey, administration routes, and side effects.

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.

The Role of Receptor-Ligand Interaction in Somatostatin Signaling Pathways: Implications for Neuroendocrine Tumors

Neuroendocrine tumors (NETs) arise from neuroendocrine cells and manifest in diverse organs. Key players in their regulation are somatostatin and its receptors (SSTR1-SSTR5). Understanding receptor-ligand interactions and signaling pathways is vital for elucidating their role in tumor development and therapeutic potential. This review highlights SSTR characteristics, localization, and expression in tissues, impacting physiological functions. Mechanisms of somatostatin and synthetic analogue binding to SSTRs, their selectivity, and their affinity were analyzed. Upon activation, somatostatin initiates intricate intracellular signaling, involving cAMP, PLC, and MAP kinases and influencing growth, differentiation, survival, and hormone secretion in NETs. This review explores SSTR expression in different tumor types, examining receptor activation effects on cancer cells. SSTRs' significance as therapeutic targets is discussed. Additionally, somatostatin and analogues' role in hormone secretion regulation, tumor growth, and survival is emphasized, presenting relevant therapeutic examples. In conclusion, this review advances the knowledge of receptor-ligand interactions and signaling pathways in somatostatin receptors, with potential for improved neuroendocrine tumor treatments.

A selective nonpeptide somatostatin receptor 5 (SST5) agonist effectively decreases insulin secretion in hyperinsulinism

Congenital hyperinsulinism (HI), a beta cell disorder most commonly caused by inactivating mutations of beta cell K_ATP channels, results in dysregulated insulin secretion and persistent hypoglycemia. Children with K_ATP-HI are unresponsive to diazoxide, the only FDA-approved drug for HI, and utility of octreotide, the second line therapy, is limited because of poor efficacy, desensitization, and somatostatin receptor type 2 (SST2)-mediated side effects. Selective targeting of SST5, an SST receptor associated with potent insulin secretion suppression, presents a new avenue for HI therapy. Here, we determined that CRN02481, a highly selective nonpeptide SST5 agonist, significantly decreased basal and amino acid-stimulated insulin secretion in both Sur1^-/- (a model for K_ATP-HI) and wild type mouse islets. Oral administration of CRN02481 significantly increased fasting glucose and prevented fasting hypoglycemia compared to vehicle in Sur1^-/- mice. During a glucose tolerance test, CRN02481 significantly increased glucose excursion in both WT and Sur1^-/- mice compared to control. CRN02481 also reduced glucose- and tolbutamide-stimulated insulin secretion from healthy, control human islets similar to the effects observed with SS14 and peptide somatostatin analogs. Moreover, CRN02481 significantly decreased glucose- and amino acid-stimulated insulin secretion in islets from two infants with K_ATP-HI and one with Beckwith-Weideman Syndrome-HI. Taken together, these data demonstrate that a potent and selective SST5 agonist effectively prevents fasting hypoglycemia and suppresses insulin secretion not only in a K_ATP-HI mouse model, but also in healthy human islets and islets from HI patients.

Molecular basis for the selective G protein signaling of somatostatin receptors

G protein-coupled receptors (GPCRs) modulate every aspect of physiological functions mainly through activating heterotrimeric G proteins. A majority of GPCRs promiscuously couple to multiple G protein subtypes. Here we validate that in addition to the well-known G_i/o pathway, somatostatin receptor 2 and 5 (SSTR2 and SSTR5) couple to the G_q/11 pathway and show that smaller ligands preferentially activate the G_i/o pathway. We further determined cryo-electron microscopy structures of the SSTR2‒G_o and SSTR2‒G_q complexes bound to octreotide and SST-14. Structural and functional analysis revealed that G protein selectivity of SSTRs is not only determined by structural elements in the receptor-G protein interface, but also by the conformation of the agonist-binding pocket. Accordingly, smaller ligands fail to stabilize a broader agonist-binding pocket of SSTRs that is required for efficient G_q/11 coupling but not G_i/o coupling. Our studies facilitate the design of drugs with selective G protein signaling to improve therapeutic efficacy.

Immunomodulatory Role of Neuropeptides in the Cornea

The transparency of the cornea along with its dense sensory innervation and resident leukocyte populations make it an ideal tissue to study interactions between the nervous and immune systems. The cornea is the most densely innervated tissue of the body and possesses both immune and vascular privilege, in part due to its unique repertoire of resident immune cells. Corneal nerves produce various neuropeptides that have a wide range of functions on immune cells. As research in this area expands, further insights are made into the role of neuropeptides and their immunomodulatory functions in the healthy and diseased cornea. Much remains to be known regarding the details of neuropeptide signaling and how it contributes to pathophysiology, which is likely due to complex interactions among neuropeptides, receptor isoform-specific signaling events, and the inflammatory microenvironment in disease. However, progress in this area has led to an increase in studies that have begun modulating neuropeptide activity for the treatment of corneal diseases with promising results, necessitating the need for a comprehensive review of the literature. This review focuses on the role of neuropeptides in maintaining the homeostasis of the ocular surface, alterations in disease settings, and the possible therapeutic potential of targeting these systems.

Differences in tumor-to-normal organ SUV ratios measured with 68 Ga-DOTATATE PET compared with 177 Lu-DOTATATE SPECT in patients with neuroendocrine tumors

BACKGROUND

Our goal is to quantitatively compare radiotracer biodistributions within tumors and major normal organs on pretherapy 68 Ga-DOTATATE PET to post-therapy 177 Lu-DOTATATE single-photon emission computed tomography (SPECT) in patients receiving peptide receptor radionuclide therapy (PRRT).

METHODS

PET/CT at ~ 60 min postinjection of Ga-68 DOTATATE and research 177 Lu-SPECT/CT imaging ~ at 4 h (SPECT1) and ~ 24 h (SPECT2) post-cycle#1 were available. Manual contours of lesions on baseline CT or MRI were applied to co-registered SPECT/CT and PET/CT followed by deep learning-based CT auto-segmentation of organs. Tumor-to-normal organ ratios (TNR) were calculated from standardized uptake values (SUV) mean and SUV peak for tumor, and SUV mean for non-tumoral liver (nliver), spleen and kidney.

RESULTS

There were 90 lesons in 24 patients with progressive metastatic neuroendocrine tumor. The correlation between PET and SPECT SUV TNRs were poor/moderate: PET versus SPECT1 R 2  = 0.19, 0.21, 0.29; PET versus SPECT2 R 2  = 0.06, 0.16, 0.33 for TNR nliver ,TNR spleen ,TNR kidney , respectively. Across all patients, the average value of the TNR measured on PET was significantly lower than on SPECT at both time points ( P  < 0.001). Using SUV mean for tumor, average TNR values and 95% confidence intervals (CI) were PET: TNR nliver  = 3.5 [CI: 3.0-3.9], TNR spleen  = 1.3 [CI, 1.2-1.5], TNR kidney  = 1.7 [CI: 1.6-1.9]; SPECT1: TNR nliver  = 10 [CI: 8.2-11.7], TNR spleen  = 2.9 [CI: 2.5-3.4], TNR kidney  = 2.8 [CI: 2.3-3.3]; SPECT2: TNR nliver  = 16.9 [CI: 14-19.9], TNR spleen  = 3.6 [CI: 3-4.2], TNR kidney  = 3.6 [CI: 3.0-4.2]. Comparison of PET and SPECT results in a sphere phantom study demonstrated that these differences are not attributed to imaging modality.

CONCLUSIONS

Differences in TNR exist for the theranostic pair, with significantly higher SUV TNR on 177 Lu SPECT compared with 68 Ga PET. We postulate this phenomenon is due to temporal differences in DOTATATE uptake and internalization in tumor as compared to normal organs.

Theranostics in Brain Tumors

Theranostic nuclear oncology, mainly in neuro-oncology (neurotheranostics), aims to combine cancer imaging and therapy using the same targeting molecule. This approach tries to identify patients who are most likely to benefit from tumor molecular radionuclide therapy. The ability of radioneurotheranostic agents to interact with cancer cells at the molecular level with high specificity can significantly improve the effectiveness of cancer therapy. A variety of biologic targets are under investigation for treating brain tumors. PET-based precision imaging can substantially improve the therapeutic efficacy of radiotheranostic approach in brain tumors.

The impact of SST2 trafficking and signaling in the treatment of pancreatic neuroendocrine tumors

Pancreatic neuroendocrine tumors (Pan-NETs), are heterogeneous neoplasms, whose incidence and prevalence are increasing worldwide. Pan-NETs are characterized by the expression of somatostatin receptors (SSTs). In particular, SST2 is the most widely distributed SST in NETs, thus representing the main molecular target for somatostatin analogs (SSAs). SSAs are currently approved for the treatment of well-differentiated NETs, and radionuclide-labeled SSAs are used for diagnostic and treatment purposes. SSAs, by binding to SSTs, have been shown to inhibit hormone secretion and thus provide control of hypersecretion symptoms, when present, and inhibit tumor proliferation. After SSA binding to SST2, the fate of the receptor is determined by trafficking mechanisms, crucial for the response to endogenous or pharmacological ligands. Although SST2 acts mostly through G protein-dependent mechanism, receptor-ligand complex endocytosis and receptor trafficking further regulate its function. SST2 mediates the decrease of hormone secretion via a G protein-dependent mechanism, culminating with the inhibition of adenylyl cyclase and calcium channels; it also inhibits cell proliferation and increases apoptosis through the modulation of protein tyrosine phosphatases. Moreover, SST2 inhibits angiogenesis and cell migration. In this respect, the cross-talk between SST2 and its interacting proteins, including Filamin A (FLNA) and aryl hydrocarbon receptor-interacting protein (AIP), plays a crucial role for SST2 signaling and responsiveness to SSAs. This review will focus on recent studies from our and other groups that have investigated the trafficking and signaling of SST2 in Pan-NETs, in order to provide insights into the mechanisms underlying tumor responsiveness to pharmacological treatments.

Effect of Obesity on the Expression of Nutrient Receptors and Satiety Hormones in the Human Colon

Background: Receptors located on enteroendocrine cells (EECs) of the colon can detect nutrients in the lumen. These receptors regulate appetite through a variety of mechanisms, including hormonal and neuronal signals. We assessed the effect of obesity on the expression of these G-protein coupled receptors (GPCRs) and hormones at both mRNA and protein level. Methods: qPCR and immunohistochemistry were used to examine colonic tissue from cohorts of patients from the Netherlands (proximal and sigmoid tissue) and the United Kingdom (tissue from across the colon) and patients were grouped by body mass index (BMI) value (BMI < 25 and BMI ≥ 25). Results: The mRNA expression of the hormones/signaling molecules serotonin, glucagon, peptide YY (PYY), CCK and somatostatin were not significantly different between BMI groups. GPR40 mRNA expression was significantly increased in sigmoid colon samples in the BMI ≥ 25 group, but not proximal colon. GPR41, GPR109a, GPR43, GPR120, GPRC6A, and CaSR mRNA expression were unaltered between low and high BMI. At the protein level, serotonin and PYY containing cell numbers were similar in high and low BMI groups. Enterochromaffin cells (EC) showed high degree of co-expression with amino acid sensing receptor, CaSR while co-expression with PYY containing L-cells was limited, regardless of BMI. Conclusions: While expression of medium/long chain fatty acid receptor GPR40 was increased in the sigmoid colon of the high BMI group, expression of other nutrient sensing GPCRs, and expression profiles of EECs involved in peripheral mechanisms of appetite regulation were unchanged. Collectively, these data suggest that in human colonic tissue, EEC and nutrient-sensing receptor expression profiles are not affected despite changes to BMI.

Somatostatin Receptor as a Molecular Imaging Target in Human and Canine Cushing Disease

OBJECTIVES

Fluorescence-guided surgery may improve completeness of resection in transsphenoidal surgery for Cushing disease (CD) by enabling visualization of residual tumor tissue at the margins. In this review we discuss somatostatin receptors (SSTRs) as targets for fluorescence-guided surgery and overview existing SSTR-specific imaging agents. We also compare SSTR expression in normal pituitary and corticotrophinoma tissues from human and canine CD patients to assess canines as a translational model for CD.

METHODS

A PubMed literature search was conducted for publications containing the terms canine, somatostatin receptor, Cushing's disease, and corticotroph adenoma. SSTR expression data from each study was documented as the presence or absence of expression or, when possible, the number of tumors expressing a given SSTR subtype within a group of tumors being studied. Studies that used reverse transcription polymerase chain reaction to quantify SSTR expression were selected for additional comparative analysis.

RESULTS

SSTR5 is strongly expressed in human corticotroph adenomas and weakly expressed in surrounding pituitary parenchyma, a pattern not conclusively observed in canine patients. SSTR2 mRNA expression is similar in human normal pituitary and corticotrophinoma cells but may be significantly higher in canine normal pituitary tissue than in corticotroph tumoral tissue. Limited data were available on SSTR subtypes 1, 3, and 4.

CONCLUSIONS

Further studies must fill the knowledge gaps related to species-specific SSTR expression, so using canine CD as a translational model may be premature. We do conclude that the expression profile of SSTR5 (i.e., high local expression in pituitary adenomas relative to normal surrounding tissues) makes SSTR5 a promising molecular target for FGS.

Expression of Somatostatin Receptor Subtypes (SSTR-1-SSTR-5) in Pediatric Hematological and Oncological Disorders

Hematological and oncological disorders represent leading causes of childhood mortality. Neuropeptide somatostatin (SST) has been previously demonstrated in various pediatric tumors, but limited information exists on the expression and characteristics of SST receptors (SSTR) in hematological and oncological disorders of children. We aimed to investigate the expression of mRNA for SSTR subtypes (SSTR-1–5) in 15 pediatric hematological/oncological specimens by RT-PCR. The presence and binding characteristics of SSTRs were further studies by ligand competition assay. Our results show that the pediatric tumor samples highly expressed mRNA for the five SSTR subtypes with various patterns. The mRNA for SSTR-2 was detected in all specimens independently of their histological type. A Hodgkin lymphoma sample co-expressed mRNA for all five SSTR subtypes. SSTR-3 and SSTR-5 were detected only in malignant specimens, such as rhabdomyosarcoma, Hodgkin lymphoma, acute lymphoblastic leukemia, and a single nonmalignant condition, hereditary spherocytosis. The incidence of SSTR-1 and SSTR-4 was similar (60%) in the 15 specimens investigated. Radioligand binding studies demonstrated the presence of specific SSTRs and high affinity binding of SST analogs in pediatric solid tumors investigated. The high incidence of SSTRs in hematological and oncological disorders in children supports the merit of further investigation of SSTRs as molecular targets for diagnosis and therapy.

Synthesis, in vitro biological activity, hydrolytic stability and docking of new analogs of BIM-23052 containing halogenated amino acids

One of the potent somatostatin analogs, BIM-23052 (DC-23-99) D-Phe-Phe-Phe-D-Trp-Lys-Thr-Phe-Thr-NH₂, has established in vitro growth hormone inhibitory activity in nM concentrations. It is also characterized by high affinity to some somatostatin receptors which are largely distributed in the cell membranes of many tumor cells. Herein, we report the synthesis of a series of analogs of BIM-23052 containing halogenated Phe residues using standard solid-phase peptide method Fmoc/OtBu-strategy. The cytotoxic effects of the compounds were tested in vitro against two human tumor cell lines-breast cancer cell line and hepatocellular cancer cell line, as well as on human non-tumorigenic epithelial cell line. Analogs containing fluoro-phenylalanines are cytotoxic in μM range, as the analog containing Phe (2-F) showed better selectivity against human hepatocellular cancer cell line. The presented study also reveals that accumulation of halogenated Phe residues does not increase the cytotoxicity according to tested cell lines. The calculated selective index reveals different mechanisms of antitumor activity of the parent compound BIM-23052 and target halogenated analogs for examined breast tumor cell lines. All peptides tested have high antitumor activity against the HepG2 cell line (IC₅₀ ≈ 100 μM and SI > 5) compared to breast cells. This is probably due to the high permeability of the cell membrane and the higher metabolic activity of hepatocytes. In silico docking studies confirmed that all obtained analogs bind well with the somatostatin receptors with preference to ssrt3 and ssrt5. All target compounds showed high hydrolytic stability at acid and neutral pH, which mimic physiological condition in stomach and human plasma.

Prognostic impact of somatostatin receptor expression in advanced bladder cancer

INTRODUCTION AND OBJECTIVES

Somatostatin receptors (SSTR) recently have been identified as potential targets for treatment of solid tumors. Furthermore, they have been shown to be of high relevance for tumor biology and prognosis in various types of cancer. However, there is a lack of clinical data for SSTR in bladder cancer (BC). Aim of this study was to determine the expression of all relevant somatostatin receptor subtypes in benign urothelium and tumor tissue of patients with muscle invasive BC. Furthermore, their potential role as prognostic factor for cancer-specific survival (CSS) and overall survival (OS) was evaluated.

METHODS

The collective included BC and benign urothelium tissue of 103 patients (Median age 69; range 32-84, 79 male, 24 female) who underwent a radical cystectomy. A tissue microarray with subsequent immunohistochemical staining was used to assess membranous expression of SSTR1-5. Results were correlated to clinical and histopathological data as well as CSS and OS.

RESULTS

Expressions of SSTR1-4 were significantly decreased in BC compared to benign urothelium (P < 0.002 each), whereas SSTR5 expression was increased (P = 0.0017). Expression of SSTR1 was associated with organ-confined disease (≤pT2) (P = 0.0477). No correlation between SSTR1-5 expression and N- and M-stage was observed. Univariate analyses showed a significantly longer CSS and OS in patients with high expression of SSTR3 (P = 0.0316 and 0.0044). Multivariate analyses confirmed SSTR3 expression as independent marker of improved CSS and OS (P = 0.0324 and 0.0076).

CONCLUSIONS

The majority of somatostatin receptor subtypes exhibit decreased expression in BC compared to benign bladder tissue. Expression of SSTR3 is an indicator for favorable prognosis in patients with muscle-invasive BC. These results support preclinical investigations using somatostatin receptor analogues such as octreotide to influence BC growth.

In vitro anti-echinococcal activity of octreotide: Additive effect of metformin linked to autophagy

Cystic echinococcosis (CE) is a worldwide zoonosis caused by the Echinococcus granulosus larval stage. The currently available therapy for this disease is based on benzimidazoles, which are rarely curative and cause several adverse effects. Therefore, new treatment options are needed. Octreotide (Oct) is a somatostatin analogue which exhibits anti-proliferative and anti-secretory effects over several cancer cell lines expressing somatostatin receptors. Here, we assessed the in vitro pharmacological effect of Oct against the E. granulosus larval stage. The drug caused a significant dose-dependent decrease in the viability of both protoscoleces and metacestodes. SEM and TEM analysis showed ultrastructural damage in both larval forms under drug treatment. Based on this, we investigated the possible presence of an Oct binding receptor in the parasite. The putative somatostatin/allatostatin-like receptor (Eg-s/ast) conserves the characteristic topology and signature sequences of the prototype somatostatin receptor common to vertebrates and is expressed in both metacestodes and protoscoleces. Moreover, Oct treated-parasites showed the presence of autophagic structures and a significant increase in transcriptional expression of autophagy key genes such as Eg-atg6, Eg-atg8, Eg-atg12 and Eg-atg16. In addition, by in toto immunolocalization assays, an increase in the punctate pattern and Eg-Atg8 protein expression was detected in Oct-treated metacestodes. Subsequently, the combination of Oct and Met had an additive effect on the viability of both larval forms. Our results provide additional evidence for the participation of PI3K/AKT/TOR/autophagy pathway in the Echinococcus survival and suggest the concomitant use of these drugs as potential therapeutic agents in treating of CE.

Identification of somatostatin receptors using labeled PEGylated octreotide, as an active internalization

Numerous normal and tumors cells are well-known to express the somatostatin receptors (SSTRs) on their surface which makes the receptor be useful for tumor scintigraphy. Thus, the identification of SSTRs is beneficial, especially SSTR₂. The somatostatin analog, Octreotide (OCT), was chosen as a ligand, as it is known to selectively bind to SSTR₂. Moreover, polyethylene glycol (PEG), 8armPEG, was used as a branched PEG to provide a low nonspecific cell binding and easily chemical modification. OCT and fluorescein (Flu) were conjugated to branched PEG using a water-soluble carbodiimide (EDC) and N-hydroxy succinimide (NHS) so as to activate its carboxylic acid group. 8armPEG-tagged Flu and OCT was characterized by gel permeation chromatography (GPC) to proof the conjugation of OCT to 8armPEG. Finally, cellular uptake was studied using pancreatic cancer cells with well-expressed somatostatin receptors using a confocal laser scanning microscope (CLMS) and fluorescence activated cell sorting (FACS). GPC showed increases in molecular mass since it showed a difference in elution time of 8armPEG itself and 8armPEG labeled with Flu. CLMS and FACS showed high binding with the positive SSTR₂ cells expression and showed negative results with negative expressing SSTR₂. These bindings were decreased when the receptors were occupied with free OCT which confirms the specific binding to SSTR₂. Therefore, we formulated a novel model to easily identify SSTR₂ and other receptors which serves as a promising platform for identification of tumor cells overexpressing the SSTR₂, which would be a hopeful target for cancer therapy and tumor scintigraphy.

An update on PET-based molecular imaging in neuro-oncology: challenges and implementation for a precision medicine approach in cancer care

PET imaging using novel radiotracers show promises for tumor grading and molecular characterization through visualizing molecular and functional properties of the tumors. Application of PET tracers in brain neoplasm depends on both type of the neoplasm and the research or clinical significance required to be addressed. In clinical neuro-oncology, ¹⁸F-FDG is used mainly to differentiate tumor recurrence from radiation-induced necrosis, and novel PET agents show attractive imaging properties. Novel PET tracers can offer biologic information not visible via contrast-enhanced MRI or ¹⁸F-FDG PET. This review aims to provide an update on the complementary role of PET imaging in neuro-oncology both in research and clinical settings along with presenting interesting cases in this context.

Synthesis, in vitro biological activity and docking of new analogs of BIM-23052 containing unnatural amino acids

Somatostatin (SST) is an endogenous cyclic tetradecapeptide hormone that exerts multiple biological activities via a family of five receptors. BIM-23052 (DC-23-99) D-Phe-Phe-Phe-D-Trp-Lys-Thr-Phe-Thr-NH2 is a linear SST analog with established in vitro GH-inhibitory activity and high affinity to sstr5, sstr3 and sstr2. The different SSTR subtypes are expressed in different tissues and in some tumor cells. Based on this finding, a series of new analogs of BIM-23052 with expected antitumor activity have been synthesized. The Thr at position 6 in BIM-23052 was replaced by the conformationally hindered Tle, Aib, Ac5c and Ac6c of the new analogs. The peptides were synthesized by standard solid-phase peptide chemistry methods, Fmoc strategy. The cytotoxic effects of the compounds were tested in vitro against a panel of tumor cell lines: HT-29, MDA-MB-23, Hep-G2, HeLa and the normal human diploid cell line Lep-3. All five somatostatin receptor subtypes were modeled and docking was performed to determine the binding affinity of the analogs. The new peptides exhibited different concentration-dependent antiproliferative effect on the tumor cell lines after 24 h of treatment. The compound 3B (Aib⁶) demonstrated the most pronounced antiproliferative effects on HepG-2 cells with the IC₅₀ = 0.01349 nM. Docking confirmed that all compounds bind well to SST receptors with preference to sstr3 and sstr5, which is most probably the reason for the observed biological effects.

Surgical-pharmacological interactions in the treatment of acromegaly

INTRODUCTION

Acromegaly requires a multimodal treatment approach that includes surgery by an expert pituitary neurosurgeon, pharmacological treatment with one or more of the available drugs and radiation therapy. These treatment alternatives are not mutually exclusive but rather complement each other when properly indicated in the individual patient. In this review, we summarize and analyze the available data concerning the choice of the surgical approach (microscopy vs. endoscopy) and the interactions between medical treatment with somatostatin analogs and pituitary surgery.

AREAS COVERED

Technical aspects, complications and outcome of transsphenoidal surgery (TSS); Advantages and disadvantages of the microscopic and endoscopic approaches; Safety and efficacy of somatostatin analogs (SSA); Primary pharmacological therapy versus primary TSS; Benefits of the preoperative treatment with SSA; and the effect of surgical tumor debulking in the therapeutic response to SSA.

EXPERT COMMENTARY

Continuing efforts at improving surgical techniques and at generating more efficacious pharmacological therapies for acromegaly are likely to improve the outcome of these patients. However, an integral approach of the patient aimed not only at achieving biochemical criteria of cure but also at treating the individual comorbidities is mandatory to improve the quality of life of these patients and to reduce their mortality rate.

Dynamic Core-Shell Bioconjugates for Targeted Protein Delivery and Release

Dynamic covalent chemistry is a versatile and powerful tool that integrates both stable chemical bonds and stimulus responsiveness into the construction of smart biotherapeutics. With minimalistic molecular design, a dynamic covalent protein assembly that incorporates selective targeting and intracellular release upon pH stimulus is presented. The construct comprises an active enzymatic protein core (cytochrome c) self-assembled with cancer cell targeting motifs (somatostatin) through boronic acid/salicylhydroxamate chemistry. The bioorthogonal assembly takes place rapidly under neutral aqueous conditions while the release of the protein is initiated under acidic conditions found within cellular vesicles during uptake. By demonstrating that these modular components act in synergy, we show the broad applicability of such chemical strategies to advance the frontier of modern nanomedicine.

Somatostatin receptors as a new active targeting sites for nanoparticles

The delivery of nanoparticles through receptor-mediated cell interactions has nowadays a major attention in the area of drug targeting applications. This specific kind of targeting is mediated by localized receptors impeded into the target site with subsequent drugs internalization. Hence, this type of interaction would diminish side effects and enhance drug delivery efficacy to the target site. Somatostatin receptors (SSTRs) are one type of G protein-coupled receptors, which could be active targeted for various purposes. There are five SSTRs types (SSTR1-5) which are localized at various organs in the body and spread into different tissues. SSTRs could be considered as a promising target to various nanoparticles which is facilitated when nanoparticles are modified through specific ligand or coating to allow better binding. This review discusses the exploration of SSTRs for active targeting of nanoparticles with certain emphasize on their interaction at the cellular level.

Novel Somatostatin Receptor Ligands Therapies for Acromegaly

Surgery is considered the treatment of choice in acromegaly, but patients with persistent disease after surgery or in whom surgery cannot be considered require medical therapy. Somatostatin receptor ligands (SRLs) octreotide (OCT), lanreotide, and the more recently approved pasireotide, characterized by a broader receptor ligand binding profile, are considered the mainstay in the medical management of acromegaly. However, in the attempt to offer a more efficacious and better tolerated medical approach, recent research has been aimed to override some limitations related to the use of currently approved drugs and novel SRLs therapies, with potential attractive features, have been proposed. These include both new formulation of older molecules and new molecules. Novel OCT formulations are aimed in particular to improve patients' compliance and to reduce injection discomfort. They include an investigational ready-to-use subcutaneous depot OCT formulation (CAM2029), delivered via prefilled syringes and oral OCT that uses a "transient permeability enhancer" technology, which allows for OCT oral absorption. Another new delivery system is a long-lasting OCT implant (VP-003), which provide stable doses of OCT throughout a period of several months. Finally, a new SRL DG3173 (somatoprim) seems to be more selective for GH secretion, suggesting possible advantages in the presence of hyperglycemia or diabetes. How much these innovations will actually be beneficial to acromegaly patients in real clinical practice remains to be seen.

Somatostatin receptor ligands in acromegaly: clinical response and factors predicting resistance

INTRODUCTION

Somatostatin (SST) receptor ligands (SRL), in particular those of first generation (Octreotide and Lanreotide), are widely used in medical treatment of acromegaly, but they assure biochemical control of disease (and the possibility of an improvement of clinical symptoms and tumor shrinkage), only in a subset of patients.

DISCUSSION

The mechanisms underlying the so called "SRL resistance" are various and involve in particular SST receptor expression and molecular pathways of signal transduction. Different predictors of SRL response have been reported, including clinical and biochemical features (gender, age, growth hormone and insulin-like growth factor-I levels at diagnosis), and tumor characteristic (both at preoperative magnetic resonance imaging study and histopathology) as well as expression of SST receptors. In some cases, only a "partial resistance" to SST can be detected, probably due to the presence of other impaired molecular mechanisms involved in signal transduction, which compromise specific pathways and not others. This may explain some cases of dissociated response between biochemical control and tumor shrinkage.

Clinical utility of lanreotide Autogel® in gastroenteropancreatic neuroendocrine tumors

Somatostatin analogs (SSAs), which were initially used to control hormonal syndromes associated with neuroendocrine neoplasms (NENs), have been successfully proposed as antiproliferative agents, able to control tumor growth in patients affected by gastroenteropancreatic (GEP)-NENs. The development of long-acting formulations of SSAs which require only weekly or monthly injections can improve patient compliance. In particular, lanreotide (LAN) Autogel^®, which is a viscous aqueous formulation supplied in ready-to-use prefilled syringes, can be administered every 28–56 days. Since its introduction in the clinical practice, several studies evaluated the clinical utility of LAN Autogel in the medical treatment of GEP-NENs. Although there is no evidence of an overall survival benefit, these studies confirm the efficacy of LAN Autogel in terms of benefit in progression-free survival, and in more than half of cases, a reduction of tumor markers can be observed during treatment with this drug. Moreover, LAN Autogel is widely recognized to be effective in controlling tumor-related symptoms in the majority of patients affected by GEP tumors, especially in patients affected by carcinoid syndrome, improving considerably patients’ quality of life.

Activation of Muscarinic Acetylcholine Receptor Subtype 4 Is Essential for Cholinergic Stimulation of Gastric Acid Secretion: Relation to D Cell/Somatostatin

Background/Aim: Muscarinic acetylcholine receptors exist in five subtypes (M₁∼M₅), and they are widely expressed in various tissues to mediate diverse autonomic functions, including gastric secretion. In the present study, we demonstrated, using M1∼M5 KO mice, the importance of M4 receptors in carbachol (CCh) stimulation of acid secretion and investigated how the secretion is modulated by the activation of M4 receptors.

Methods: C57BL/6J mice of wild-type (WT) and M1–M5 KO were used. Under urethane anesthesia, acid secretion was measured in the stomach equipped with an acute fistula. CCh (30 μg/kg) was given subcutaneously (s.c.) to stimulate acid secretion. Atropine or octreotide (a somatostatin analog) was given s.c. 20 min before the administration of CCh. CYN154806 (a somatostatin SST2 receptor antagonist) was given i.p. 20 min before the administration of octreotide or CCh.

Results: CCh caused an increase of acid secretion in WT mice, and the effect was totally inhibited by prior administration of atropine. The effect of CCh was similarly observed in the animals lacking M1, M2 or M5 receptors but significantly decreased in M3 or M4 KO mice. CYN154806, the SST2 receptor antagonist, dose-dependently and significantly reversed the decreased acid response to CCh in M4 but not M3 KO mice. Octreotide, the somatostatin analog, inhibited the secretion of acid under CCh-stimulated conditions in WT mice. The immunohistochemical study showed the localization of M₄ receptors on D cells in the stomach. Serum somatostatin levels in M4 KO mice were higher than WT mice under basal conditions, while those in WT mice were significantly decreased in response to CCh.

Conclusions: These results suggest that under cholinergic stimulation the acid secretion is directly mediated by M3 receptors and indirectly modified by M4 receptors. It is assumed that the activation of M4 receptors inhibits the release of somatostatin from D cells and minimizes the acid inhibitory effect of somatostatin through SST2 receptors, resulting in enhancement of the acid response mediated by M3 receptors on parietal cells.

Novel gold nanoparticles coated with somatostatin as a potential delivery system for targeting somatostatin receptors

Targeting of G-protein coupled receptors (GPCRs) like somatostatin-14 (SST-14) could have a potential interest in delivery of anti-cancer agents to tumor cells. Attachment of SST to different nano-carriers e.g. polymeric nanoparticles is limited due to the difficulty of interaction between SST itself and those nano-carriers. Furthermore, the instability problems associated with the final formulation. Attaching of SST to gold nanoparticles (AuNPs) using the positive and negative charge of SST and citrate-AuNPs could be considered a new technique to get stable non-aggregated AuNPs coated with SST. Different analyses techniques have been performed to proof the principle of coating between AuNPs and SST. Furthermore, cellular uptake studies on HCC-1806, HELA and U-87 cell lines has been investigated to show the ability of AuNPs coated SST to enter the cells via SST receptors. Dynamic light scattering (DLS) indicated a successful coating of SST on the MUA-AuNPs surface. Furthermore, all the performed analysis including DLS, SDS-PAGE and UV-VIS absorption spectra indicated a successful coating of AuNPs with SST. Cellular uptake studies on HCC-1806, HELA and U-87 cell lines showed that the number of AuNPs-SST per cell is signiflcantly higher compared to citrate-AuNPs when quantified using inductively coupled plasma spectroscopy. Moreover, the binding of AuNPs-SST to cells can be suppressed by addition of antagonist, indicating that the binding of AuNPs-SST to cells is due to receptor-specific binding. In conclusion, AuNPs could be attached to SST via adsorption to get stable AuNPs coated SST. This new formulation has a potential to target SST receptors localized in many normal and tumor cells.

Genetic Predictors of Response to Different Medical Therapies in Acromegaly

In the era of predictive medicine, management of diseases is evolving into a more personal and individualized approach, as more data are available regarding clinical, biochemical, radiological, molecular, histopathological, and genetic aspects. In the particular setting of acromegaly, which is a rare, chronic, debilitating, and disfiguring disease, an optimized approach deems even more necessary, especially because of an associated increased morbidity and mortality, the impact on patients' quality of life, and the increased cost of frequently necessary life-long treatments. In this paper, we review the available studies that address potential genetic influences on acromegaly, their role in the outcome, and response to treatments, as well as their contribution to the risk of developing side effects. We focus mainly on pharmacogenetic factors involved during treatment with dopamine agonists, somatostatin analogs, and pegvisomant. Specifically, mutations in dopamine receptors, somatostatin receptors, growth hormone receptors, and metabolic pathways involved in growth hormone action; polymorphisms in the insulin-like growth factor and the insulin-like growth factor binding proteins; and polymorphisms in other genes that may determine differences in the frequency of developing adverse events.

Differential somatostatin receptor (SSTR) 1-5 expression and downstream effectors in histologic subtypes of growth hormone pituitary tumors

PURPOSE

The aim of this study was to examine whether differential expression of somatostatin receptors (SSTR) 1-5 and downstream effectors are different in densely (DG) and sparsely (SG) granulated histological growth hormone (GH) pituitary tumor subtypes.

METHODS

The study included 33 acromegalic patients with 23 DG and 10 SG tumors. SSTR1-5 were measured by qPCR and immunoblotting. Signaling candidates downstream of SSTR2 were also assessed.

RESULTS

SSTR2 mRNA and protein levels were significantly higher in DG compared to SG tumors. Downstream of SSTR2, p27(kip1) was decreased (2.6-fold) in SG compared to DG tumors, suggesting a potential mechanism of SSA resistance in SG tumors with intact SSTR2 expression. Re-expression of E-cadherin in GH pituitary cell increased p27(kip1) levels.

CONCLUSIONS

Histological subtyping correlated with SSTR2, E cadherin and p27(kip) protein levels and these may serve as useful biomarkers in GH tumors to predict behavior and response to therapy with SSA.

A Transcriptome-Led Exploration of Molecular Mechanisms Regulating Somatostatin-Producing D-Cells in the Gastric Epithelium

The stomach epithelium contains a myriad of enteroendocrine cells that modulate a range of physiological functions, including postprandial secretion of regulatory peptides, gastric motility, and nutrient absorption. Somatostatin (SST)-producing D-cells are present in the oxyntic and pyloric regions of the stomach, and provide a tonic inhibitory tone that regulates activity of neighboring enteroendocrine cells and gastric acid secretion. Cellular mechanisms underlying the effects of regulatory factors on gastric D-cells are poorly defined due to problems in identifying primary D-cells, and uncertainty remains about which stimuli influence D-cells directly. In this study, we introduce a transgenic mouse line, SST-Cre, which upon crossing with Cre reporter strains, facilitates the identification and purification of gastric D-cells, or cell-specific expression of genetically encoded calcium indicators. Populations of D-cells from the gastric antrum and corpus were isolated and analyzed by RNA sequencing and quantitative RT-PCR. The expression of hormones, hormone receptors, neurotransmitter receptors, and nutrient receptors was quantified. Pyy, Gipr, Chrm4, Calcrl, Taar1, and Casr were identified as genes that are highly enriched in D-cells compared with SST-negative cells. Hormone secretion assays performed in mixed gastric epithelial cultures confirmed that SST secretion is regulated by incretin hormones, cholecystokinin, acetylcholine, vasoactive intestinal polypeptide, calcitonin gene-related polypeptide, oligopetides, and trace amines. Cholecystokinin and oligopeptides elicited increases in intracellular calcium in single-cell imaging experiments performed using cultured D-cells. Our data provide the first transcriptomic analysis and functional characterization of gastric D-cells, and identify regulatory pathways that underlie the direct detection of stimuli by this cell type.

Can we predict long-term remission after somatostatin analog withdrawal in patients with acromegaly? Results from a multicenter prospective trial

Somatostatin analogs (SSAs) represent the mainstay of therapy in acromegaly. One of the potential disadvantages is the expected need to maintain therapy indefinitely in previously non-irradiated patients. The aim of this multicenter prospective open trial was to evaluate the likelihood of successful discontinuation of SSA therapy in well-controlled acromegalic patients who fulfilled very strict criteria: two or more years of treatment with the long-acting SSA octreotide LAR (OCT-LAR), a stable dose and injections interval every 4 weeks or longer for the previous year, GH levels <2.5 ng/ml and normal IGF-1 levels for age, a tumor remnant <10 mm, no history of radiotherapy, and no use of cabergoline or pegvisomant over the previous 6 months. Disease recurrence was defined as an increase of IGF-1 to levels above 1.2-fold the upper limit of normal (ULN). Out of 220 patients, 20 patients (12 women and 8 men; mean age, 48.1 ± 10.3 years; age range, 27-64) treated for 2.74 ± 0.64 years (range, 2.0-4.4) were included in this prospective study and OCT-LAR therapy was stopped. Four patients (20 %) remained without clinical and biochemical/neuroradiological evidence of disease recurrence after 12-18 months of follow-up. Sixteen patients (80 %) relapsed biochemically within 9 months after drug withdrawal and restarted OCT-LAR at the same previous dose. Compared to recurring subjects, non-recurring patients had significantly lower mean IGF-1 (× ULN) levels but there were some overlapping values in both groups. No other characteristic could be identified as a predictor of successful OCT-LAR discontinuation. Our findings demonstrated that OCT-LAR withdrawal, though rare, is possible in well-selected acromegalic patients treated for at least 2 years and considered optimally controlled in hormonal and neuroradiological terms.

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.

Somatostatin receptor-2 negatively regulates β-adrenergic receptor mediated Ca(2+) dependent signaling pathways in H9c2 cells

In the present study, we report that somatostatin receptor 2 (SSTR2) plays a crucial role in modulation of β1AR and β2AR mediated signaling pathways that are associated with increased intracellular Ca(2+) and cardiac complications. In H9c2 cells, SSTR2 colocalizes with β1AR or β2AR in receptor specific manner. SSTR2 selective agonist inhibits isoproterenol and formoterol stimulated cAMP formation and PKA phosphorylation in concentration dependent manner. In the presence of SSTR2 agonist, the expression of PKCα and PKCβ was comparable to the basal condition, however SSTR2 agonist inhibits isoproterenol or formoterol induced PKCα and PKCβ expression, respectively. Furthermore, the activation of SSTR2 not only inhibits calcineurin expression and its activity, but also blocks NFAT dephosphorylation and its nuclear translocation. SSTR2 selective agonist abrogates isoproterenol mediated increase in cell size and protein content (an index of hypertrophy). Taken together, the results described here provide direct evidence in support of cardiac protective role of SSTR2 via modulation of Ca(2+) associated signaling pathways attributed to cardiac hypertrophy.

Octreotide modulates the effects on fibrosis of TNF-α, TGF-β and PDGF in activated rat hepatic stellate cells

BACKGROUND AND AIMS

Somatostatin and its analogs may influence hepatic fibrosis interfering through several mechanisms. The aim of this study was to investigate the effect of octreotide on cytokine activated hepatic stellate cells (HSC).

METHODS

Primary HSCs were isolated from rats and were cultured on plastic for activation. Expression of somatostatin receptors (SSTR) was investigated in cultured HSCs by immunofluorescence and western blot. The effect of octreotide on cellular proliferation was studied with the MTT assay and western blot for α1-procollagen (α1-PROC) production in TNFα, TGF-β1 or PDGF treated HSCs. Phosphotyrosine (PTP) and phosphoserine-phosphothreonine (STP) phosphatases inhibition was performed with sodium orthovanadate and okadaic acid respectively.

RESULTS

Activated HSC express SSTR subtypes 1, 2A, 2B, 3 and 4 and their expression is enhanced by further HSC activation. Octreotide did not have an effect on HSC proliferation but inhibited plastic induced α1-PROC production. Interestingly, it enhanced PDGF-induced HSC proliferation but inhibited PDGF and TGFβ1 dependent expression of α1-PROC, while an opposite effect was observed in TNFα-induced cell proliferation and collagen production. PTP inhibition reversed the inhibitory effect of octreotide on α1-PROC, but potentiated its effect on PDGF and TGFβ1 dependent α1-PROC production. Finally, STP inhibition profoundly inhibited α1-PROC expression in all cases suggesting that both STP and PTP phosphatases are important regulators of pro-fibrotic mechanisms.

CONCLUSIONS

The net effect of octreotide on HSCs and therefore liver fibrosis is subject to the cytokine microenvironment of these cells. This effect is modulated by PTPs and STPs inhibition. Especially in the case of STPs their profibrotic effects could be an interesting new therapeutic target in liver fibrosis.

Investigation of responsiveness to thyrotropin-releasing hormone in growth hormone-producing pituitary adenomas

Objective. The aim of this study was to investigate how the paradoxical response of GH secretion to TRH changes according to tumor volumes. Methods. Patients with newly diagnosed acromegaly were classified as either TRH responders or nonresponders according to the results of a TRH stimulation test (TST), and their clinical characteristics were compared according to responsiveness to TRH and tumor volumes. Results. A total of 41 acromegalic patients who underwent the TST were included in this study. Between TRH responders and nonresponders, basal GH, IGF-I levels, peak GH levels, and tumor volume were not significantly different, but the between-group difference of GH levels remained near significant over the entire TST time. ΔGHmax-min during the TST were significantly different according to the responsiveness to TRH. Peak GH levels and ΔGHmax-min during the TST showed significantly positive correlations with tumor volume with higher levels in macroadenomas than in microadenomas. GH levels over the entire TST time also remained significantly higher in macroadenomas than in microadenomas. Conclusion. Our data demonstrated that the paradoxical response of GH secretion to TRH in GH-producing pituitary adenomas was not inversely correlated with tumor volumes.

Free somatostatin receptor fraction predicts the antiproliferative effect of octreotide in a neuroendocrine tumor model: implications for dose optimization

Somatostatin receptors (SSTR) are highly expressed in well-differentiated neuroendocrine tumors (NET). Octreotide, an SSTR agonist, has been used to suppress the production of vasoactive hormones and relieve symptoms of hormone hypersecretion with functional NETs. In a clinical trial, an empiric dose of octreotide treatment prolonged time to tumor progression in patients with small bowel neuroendocrine (carcinoid) tumors, irrespective of symptom status. However, there has yet to be a dose optimization study across the patient population, and methods are currently lacking to optimize dosing of octreotide therapy on an individual basis. Multiple factors such as total tumor burden, receptor expression levels, and nontarget organ metabolism/excretion may contribute to a variation in SSTR octreotide occupancy with a given dose among different patients. In this study, we report the development of an imaging method to measure surface SSTR expression and occupancy level using the PET radiotracer (68)Ga-DOTATOC. In an animal model, SSTR occupancy by octreotide was assessed quantitatively with (68)Ga-DOTATOC PET, with the finding that increased occupancy resulted in decreased tumor proliferation rate. The results suggested that quantitative SSTR imaging during octreotide therapy has the potential to determine the fractional receptor occupancy in NETs, thereby allowing octreotide dosing to be optimized readily in individual patients. Clinical trials validating this approach are warranted.

Somatostatin receptor pathophysiology in the neuroendocrine system

The actions of somatostatin (SRIF) are mediated by specific G protein-coupled receptors, named SRIF receptor (SSTR) subtypes 1, 2, 3 and 5. SRIF binding to SSTR activates a series of second messenger systems, resulting in the inhibition of calcium channels and adenylate cyclase activity, ultimately leading to inhibition of hormone secretion, while stimulation of other second messengers, such as phosphotyrosine phosphatases play a role in the control of cell growth. The SSTR and dopamine receptor families share a 30% sequence homology and appear to be structurally related. The knowledge on the pathophysiology of these two families of G protein-coupled receptors in neuroendocrine tumors has progressively increased due to the new insights in receptor dimerization, internalization and trafficking. Depending on the expression of different SSTRs in tissues, their combinations and interactions affect the functionality of the subtypes expressed and the influence of the microenvironment, the response to ligands and, by consequence, the response to treatment can be very different.

Translation of molecular pathways into clinical trials of neuroendocrine tumors

Current treatment options for neuroendocrine tumors (NET) include somatostatin analogs, interferon-α, peptide receptor-targeted therapy and cytotoxic chemotherapy. Most patients undergo sequential therapies since these drugs are active only in subpopulations of patients and for a limited period of time. There is a need for novel drugs that are capable of amelioration of symptomatology (syndromic control) and/or tumor growth control. A number of diverse signaling pathways are involved in the pathogenesis of NET and tumor growth, thus many potential targets are available for drug targeting. Targeted therapies therefore represent an appropriate developmental therapeutic strategy given the multiplicity of potential targets in NET. These include but are not limited to: inhibitory or activating G protein-coupled receptors, receptor tyrosine kinases, ligands, and intracellular targets such as the mammalian target of rapamycin (mTOR). Numerous drugs that utilize single or multiple targets are currently in clinical development. Recently, two target-directed agents, the multiple tyrosine kinase inhibitor sunitinib and the mTOR inhibitor everolimus, have been approved for the treatment of progressive pancreatic NET. This review provides a broad overview of established and potential molecular targets in NET, summarizes data from phase II and III clinical trials with targeted drugs and outlines future therapeutic directions.

Contemporary methods of therapy and follow-up of neuroendocrine tumours of the gastrointestinal tract and the pancreas

The growing interest in neuroendocrine tumours is due to the dynamic growth of detection of this type of cancer. Neuroendocrine tumours (neuroendocrine neoplasms - NENs / neuroendocrine tumours - NETs) derive from glands, groups of endocrine cells and diffuse neuroendocrine system cells. Mainly they derive from the gastrointestinal tract (gastroenteropancreatic-neuroendocrine tumours - GEP-NETs). Currently the modified WHO classification from 2010 is widely used. An important element in the choice of treatment is histological maturity based on mitotic activity and on assessment of proliferation activity (Ki-67). The treatment of choice is surgery. In most cases, complete surgical removal is impossible because of the advanced staging at the time of diagnosis. In well-differentiated neoplasms where the expression of somatostatin receptors is expected, patients are qualified for somatostatin analogues therapy. Poorly differentiated lesions are qualified for chemotherapy. In the guidelines of ENETS (European Neuroendocrine Tumor Society) from 2007 the rules concerning monitoring depending on the WHO classification were specified.

Identification of critical residues involved in ligand binding and G protein signaling in human somatostatin receptor subtype 2

G protein signaling through human somatostatin receptor subtype 2 (SSTR2) is well known, but the amino acids involved in stimulation of intracellular responses upon ligand binding have not been characterized. We constructed a series of point mutants in SSTR2 at amino acid positions 89, 139, and 140 in attempts to disrupt G protein signaling upon ligand binding. The aspartic acid changes at position 89 to either Ala, Leu, or Arg generated mutant receptors with varying expression profiles and a complete inability to bind somatostatin-14 (SST). Mutations to Asp 139 and Arg 140 also led to varying expression profiles with some mutants maintaining their affinity for SST. Mutation of Arg 140 to Ala resulted in a mutated receptor that had a B(max) and dissociation constant (K(d)) similar to wild-type receptor but was still coupled to the G protein as determined in both a cAMP assay and a calcium-release assay. In contrast, mutation of Asp 139 to Asn resulted in a mutated receptor with B(max) and K(d) values that were similar to wild type but was uncoupled from G protein-mediated cAMP signaling, but not calcium release. Thus, we identified mutations in SSTR2 that result in either receptor expression levels that are similar to wild type but is completely ablated for ligand binding or a receptor that maintains affinity for SST and is uncoupled from G protein-mediated cAMP signaling.

Discontinuation of octreotide LAR after long term, successful treatment of patients with acromegaly: is it worth trying?

BACKGROUND

Somatostatin analogs (SA) have been used for over 25 years in the treatment of acromegaly. A major disadvantage is the need to continue therapy indefinitely.

OBJECTIVE

To evaluate the feasibility of discontinuing therapy in well-controlled patients with acromegaly treated chronically with SA.

DESIGN AND METHODS

Of the 205 subjects on octreotide LAR, we selected those who met the following criteria: two or more years of treatment, a stable dose and injection interval of 20  mg every 8 weeks or longer for the previous year, no history of radiation, no cabergoline for the previous 6 months, a GH <1.5  ng/ml, and an IGF1 <1.2×upper limit of normal (ULN). Octreotide LAR was stopped and both GH and IGF1 were measured monthly for 4 months; a glucose-suppressed GH value and magnetic resonance imaging were obtained at the 4th month, thereafter, basal GH and IGF1 were measured q. 3 months, for 12-18 months. Patients were removed from the study if GH or IGF1 rose to 1.5  ng/ml or 1.2×ULN respectively.

RESULTS

Twelve patients (ten women, mean age 48±13 years) were studied. Seven patients (58.3%) relapsed biochemically within 1 year of having stopped the SA; two patients relapsed by GH and IGF1 criteria, the remaining five patients kept GH levels within target. Five patients (41.7%) remain in remission after 12 months of follow-up. Non-recurring patients were on longer injection intervals but no other characteristic was associated with a successful withdrawal.

CONCLUSION

Withdrawal of SA is possible in a small but distinct subset of patients, particularly in those who are very well controlled on relatively low doses administered at long intervals.

The role of somatostatin and dopamine D2 receptors in endocrine tumors

Somatostatin (SS) and dopamine (DA) receptors have been highlighted as two critical regulators in the negative control of hormonal secretion in a wide group of human endocrine tumors. Both families of receptors belong to the superfamily of G protein-coupled receptors and share a number of structural and functional characteristics. Because of the generally reported high expression of somatostatin receptors (SSTRs) in neuroendocrine tumors (NET), somatostatin analogs (SSA) have a pronounced role in the medical therapy for this class of tumors, especially pituitary adenomas and well-differentiated gastroenteropancreatic NET (GEP NET). Moreover, NET express not only SSTR but also frequently dopamine receptors (DRs), and DA agonists targeting the D(2) receptor (D(2)) have been demonstrated to be effective in controlling hormone secretion and cell proliferation in in vivo and in vitro studies. The treatment with SSAs combined with DA agonists has already been demonstrated efficacious in a subgroup of patients with GH-secreting pituitary adenomas and few reported cases of carcinoids. The recent availability of new selective and universal SSA and DA agonists, as well as the chimeric SS/DA compounds, may shed new light on the potential role of SSTR and D(2) as combined targets for biotherapy in NET. This review provides an overview of the latest studies evaluating the expression of SSTR and DR in NET, focusing on their co-expression and the possible clinical implications of such co-expression. Moreover, the most recent insights in SSTR and D(2) pathophysiology and the future perspectives for treatment with SSA, DA agonists, and SS/DA chimeric compounds are discussed.

Cross-talk and modulation of signaling between somatostatin and growth factor receptors

The process of homo- and/or heterodimerization of G-protein coupled receptors (GPCRs) and receptor tyrosine kinase (RTK) families are crucial for implicating the fundamental properties of receptor proteins including receptor expression, trafficking, and desensitization as well as signal transduction. The members of GPCR and RTK family constitute largest cell surface receptor proteins and regulate physiological functions of cells in response to external and internal stimuli. Notably, GPCRs and RTKs play major role in regulation of several key cellular functions which are associated with several pathological conditions including cancer biology, neurodegenerative and cardiovascular diseases. The focus of this review is to highlight the recent findings on the possible cross-talk between somatostatin receptors (members of GPCR family) and growth factor receptors like epidermal growth factor receptors (members of RTK family). Furthermore, functional consequences of such an interaction in modulation of signaling pathways linked to pathological conditions specifically in cancer are discussed.

Resistance to somatostatin analogs in acromegaly

Somatostatin analogs (SA) are widely used in acromegaly, either as first-line or adjuvant treatment after surgery. First-line treatment with these drugs is generally used in the patients with macroadenomas or in those with clinical conditions so severe as to prevent unsafe reactions during anesthesia. Generally, the response to SA takes into account both control of GH and IGF-I excess, with consequent improvement of clinical symptoms directly related to GH and IGF-I excess, and tumor shrinkage. This latter effect is more prominent in the patients treated first-line and bearing large macroadenomas, but it is also observed in patients with microadenomas, even with little clinical implication. Predictors of response are patients' gender, age, initial GH and IGF-I levels, and tumor mass, as well as adequate expression of somatostatin receptor types 2 and 5, those with the highest affinity for octreotide and lanreotide. Only sporadic cases of somatostatin receptor gene mutation or impaired signaling pathways have been described in GH-secreting tumors so far. The response to SA also depends on treatment duration and dosage of the drug used, so that a definition of resistance based on short-term treatments using low doses of long-acting SA is limited. Current data suggest that response to these drugs is better analyzed taking together biochemical and tumoral effects because only the absence of both responses might be considered as a poor response or resistance. This latter evidence seems to occur in 25% of treated patients after 12 months of currently available long-acting SA.

Biological targeted therapies in patients with advanced enteropancreatic neuroendocrine carcinomas

Enteropancreatic (EP) neuroendocrine carcinomas (NECs) represent relatively rare and heterogeneous malignancies. They are the most common group among neuroendocrine tumors (NETs). In most cases they are advanced at diagnosis and slow-growing, therefore conditioning a better prognosis compared with non neuroendocrine carcinomas from the same sites. No standard medical therapy exists, except for somatostatin analogs in functioning tumors, and octreotide LAR in functioning or non functioning well differentiated NECs from small bowel. Several systemic therapeutic options exist, including chemotherapy, somatostatin analog, interferon, peptide receptor radionuclide therapy (PRRT), and molecular targeted drugs. Among them some therapies have specific biological tumor targets and can be defined as "biological targeted therapies". This review focuses on the status of EP NECs targeted therapies in the light of recent advances. Somatostatin receptors (SSTRs) are the first therapeutic target detected in EP NECs. Through them SS analogs and PRRT act, producing symptomatic, biochemical, and, to a lesser extent, antiproliferative effects. New SS analogs, covering a higher number of SSTR subtypes, were developed, including pasireotide (SOM230), which controls 25% of carcinoid syndromes resistant to full dose octreotide LAR. Chimeric analogs, which bind SSTR2/SSTR5 and dopamine-2 receptor subtype (D2), are in preclinical phase of development. Among the numerous molecular targeted agents investigated in NETs, mTOR inhibitors and VEGF/VEGFR/PDGFR inhibitors are in most advanced clinical phase of investigation. In particular, everolimus, sunitinib, and bevacizumab are all studied in phase III trials. Both everolimus and sunitinib produced significant survival benefit versus placebo in advanced progressing well-differentiated pancreatic NECs. Sunitinib data have been presented at the last ASCO in June 2010, and everolimus data will be presented at next ESMO in September 2010.

From targets to treatments: a review of molecular targets in pancreatic neuroendocrine tumors

Pancreatic neuroendocrine tumors (pancreatic NET) are relatively rare, slowly growing tumors, although their incidence is increasing, and patients may survive for several years with metastatic disease. Apart from symptomatic relief, there have been few treatment options for these tumors in the past. More recently, investigators have explored the potential of molecularly targeted agents in treating pancreatic NET, with some success. In this review, we consider the data supporting exploitation of different targets in pancreatic NET, including peptide receptors, receptor tyrosine kinases (involved in tumor angiogenesis and more directly supporting tumor growth), and intracellular targets, such as the mammalian target of rapamycin (mTOR), which has a central role in regulating cell growth, metabolism, and apoptosis. Probably due to the paucity of pancreatic NET, many clinical trials to date have included heterogeneous NET populations, and there are few randomized studies of this specific patient population. Very recently, promising results have been achieved in placebo-controlled, phase III trials with the multitargeted tyrosine kinase inhibitor, sunitinib, and the mTOR inhibitor, everolimus. These agents have been approved or are currently being reviewed by authorities for use in patients with pancreatic NET. Here we review potential molecular targets in pancreatic NET and summarize the available data for targeted agents from phase II and III trials open to patients with this tumor.