Differential effects of octreotide and pasireotide on somatostatin receptor internalization and trafficking in vitro

Integrative clinical, hormonal, and molecular data associate with invasiveness in acromegaly: REMAH study

INTRODUCTION

Growth hormone (GH)-secreting pituitary tumors (GHomas) are the most common acromegaly cause. At diagnosis, most of them are macroadenomas, and up to 56% display cavernous sinus invasion. Biomarker assessment associated with tumor growth and invasion is important to optimize their management.

OBJECTIVES

The study aims to identify clinical/hormonal/molecular biomarkers associated with tumor size and invasiveness in GHomas and to analyze the influence of pre-treatment with somatostatin analogs (SSAs) or dopamine agonists (DAs) in key molecular biomarker expression.

METHODS

Clinical/analytical/radiological variables were evaluated in 192 patients from the REMAH study (ambispective multicenter post-surgery study of the Spanish Society of Endocrinology and Nutrition). The expression of somatostatin/ghrelin/dopamine system components and key pituitary/proliferation markers was evaluated in GHomas after the first surgery. Univariate/multivariate regression studies were performed to identify association between variables.

RESULTS

Eighty percent of patients harbor macroadenomas (63.8% with extrasellar growth). Associations between larger and more invasive GHomas with younger age, visual abnormalities, higher IGF1 levels, extrasellar/suprasellar growth, and/or cavernous sinus invasion were found. Higher GH1 and lower PRL/POMC/CGA/AVPR1B/DRD2T/DRD2L expression levels (P < .05) were associated with tumor invasiveness. Least Absolute Shrinkage and Selection Operator's penalized regression identified combinations of clinical and molecular features with areas under the curve between 0.67 and 0.82. Pre-operative therapy with DA or SSAs did not alter the expression of any of the markers analyzed except for DRD1/AVPR1B (up-regulated with DA) and FSHB/CRHR1 (down-regulated with SSAs).

CONCLUSIONS

A specific combination of clinical/analytical/molecular variables was found to be associated with tumor invasiveness and growth capacity in GHomas. Pre-treatment with first-line drugs for acromegaly did not significantly modify the expression of the most relevant biomarkers in our association model. These findings provide valuable insights for risk stratification and personalized management of GHomas.

Management of pasireotide-induced hyperglycemia in patients with acromegaly: An experts' consensus statement

Pasireotide is a somatostatin analogue for the treatment of acromegaly, a chronic condition caused by excess growth hormone. Despite the therapeutic benefits of pasireotide as a second-line treatment for inadequately controlled acromegaly, a major concern is its hyperglycemic side-effect. Here, we provide guidance on how to select appropriate patients with acromegaly for treatment with pasireotide. We summarize baseline characteristics of patients at high risk for pasireotide-associated hyperglycemia and recommend a monitoring strategy based on the risk profile. Self-monitoring of blood glucose levels (SMBG), measurements of fasting plasma glucose (FPG), postprandial plasma glucose (PPG) and regular HbA1c measurements are the foundation of our proposed monitoring approach. The pathophysiology of pasireotide-induced hyperglycemia involves decreased secretion of the incretin hormones GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). Our expert recommendations address the specific pathophysiology of pasireotide-induced hyperglycemia by recommending the incretin-based therapeutics dipeptidyl peptidase-4 inhibitors (DPP-4i) and glucagon-like peptide-1 receptor agonists (GLP-1 RA) in all appropriate patients as an alternative to first-line monotherapy with metformin. Furthermore, we emphasize the importance of adequate control of acromegaly, excellent diabetes education, nutrition and lifestyle guidance and advise to consult expert diabetologists in case of uncertainty in the management of patients with hyperglycemia under pasireotide.

Current role of pasireotide in the treatment of acromegaly

"First-generation" somatostatin receptor agonists (SSTRAs) octreotide and lanreotide are the most commonly used first-line pharmacological therapy for patients with acromegaly. A subset of patients respond only partially or not at all to the first-generation SSTRA, necessitating the use of additional pharmacological agents or other modes of therapy. Pasireotide is a "second-generation" SSTRA that has multi-receptor activity. Prospective studies have shown promise in the use of pasireotide in patients with poor response to first-generation SSTRA. Here we elucidate the molecular pathways of resistance to first-generation SSTRA, the mechanism of action, pre-clinical and clinical evidence of the use of pasireotide in patients having incomplete / lack of response to first-generation SSTRA. We also discuss the clinical, pathological, and radiological markers predicting response to pasireotide, and the difference in side-effect profiles of pasireotide, compared to first-generation SSTRA.

The Wnt pathway protein Dvl1 targets somatostatin receptor 2 for lysosome-dependent degradation

The Somatostatin receptor 2 (Sstr2) is a heterotrimeric G protein-coupled receptor that is highly expressed in neuroendocrine tumors and is a common pharmacological target for intervention. Unfortunately, not all neuroendocrine tumors express Sstr2, and Sstr2 expression can be downregulated with prolonged agonist use. Sstr2 is rapidly internalized following agonist stimulation and, in the short term, is quantitatively recycled back to the plasma membrane. However, mechanisms controlling steady state expression of Sstr2 in the absence of agonist are less well described. Here, we show that Sstr2 interacts with the Wnt pathway protein Dvl1 in a ligand-independent manner to target Sstr2 for lysosomal degradation. Interaction of Sstr2 with Dvl1 does not affect receptor internalization, recycling, or signaling to adenylyl cyclase but does suppress agonist-stimulated ERK1/2 activation. Importantly, Dvl1-dependent degradation of Sstr2 can be stimulated by overexpression of Wnts and treatment of cells with Wnt pathway inhibitors can boost Sstr2 expression in neuroendocrine tumor cells. Taken together, this study identifies for the first time a mechanism that targets Sstr2 for lysosomal degradation that is independent of Sstr2 agonist and can be potentiated by Wnt ligand. Intervention in this signaling mechanism has the potential to elevate Sstr2 expression in neuroendocrine tumors and enhance Sstr2-directed therapies.

Comparison of somatostatin receptor expression in patients with neuroendocrine tumours with and without somatostatin analogue treatment imaged with [18F]SiTATE

Purpose

Somatostatin analogues (SSA) are frequently used in the treatment of neuroendocrine tumours. Recently, [18F]SiTATE entered the field of somatostatin receptor (SSR) positron emission tomography (PET)/computed tomography (CT) imaging. The purpose of this study was to compare the SSR-expression of differentiated gastroentero-pancreatic neuroendocrine tumours (GEP-NET) measured by [18F]SiTATE-PET/CT in patients with and without previous treatment with long-acting SSAs to evaluate if SSA treatment needs to be paused prior to [18F]SiTATE-PET/CT.

Methods

77 patients were examined with standardised [18F]SiTATE-PET/CT within clinical routine: 40 patients with long-acting SSAs up to 28 days prior to PET/CT examination and 37 patients without pre-treatment with SSAs. Maximum and mean standardized uptake values (SUVmax and SUVmean) of tumours and metastases (liver, lymphnode, mesenteric/peritoneal and bones) as well as representative background tissues (liver, spleen, adrenal gland, blood pool, small intestine, lung, bone) were measured, SUV ratios (SUVR) were calculated between tumours/metastases and liver, likewise between tumours/metastases and corresponding specific background, and compared between the two groups.

Results

SUVmean of liver (5.4 ± 1.5 vs. 6.8 ± 1.8) and spleen (17.5 ± 6.8 vs. 36.7 ± 10.3) were significantly lower (p < 0.001) and SUVmean of blood pool (1.7 ± 0.6 vs. 1.3 ± 0.3) was significantly higher (p < 0.001) in patients with SSA pre-treatment compared to patients without. No significant differences between tumour-to-liver and specific tumour-to-background SUVRs were observed between both groups (all p > 0.05).

Conclusion

In patients previously treated with SSAs, a significantly lower SSR expression ([18F]SiTATE uptake) in normal liver and spleen tissue was observed, as previously reported for 68Ga-labelled SSAs, without significant reduction of tumour-to-background contrast. Therefore, there is no evidence that SSA treatment needs to be paused prior to [18F]SiTATE-PET/CT.

Resistance of neuroendocrine tumours to somatostatin analogs

INTRODUCTION

A common feature shared by most neuroendocrine tumors (NETs) is the expression on their surface of somatostatin receptors (SSTRs) that are essential for their pathophysiological regulation, diagnosis, and management. The first-generation synthetic somatostatin analogs (SSAs), octreotide and lanreotide, constitute the cornerstone of treatment for growth hormone secreting pituitary adenomas and functioning, progressive functioning, and non-functioning gastro-entero-pancreatic (GEP-NETs). SSAs exert their mechanism of action through binding to the SSTRs; however, their therapeutic response is frequently attenuated or diminished by the development of resistance. The phenomenon of resistance is complex implicating the presence of additional epigenetic and genetic mechanisms.

AREAS COVERED

We aim to analyze the molecular, genetic, and epigenetic mechanisms of resistance to SSA treatment. We also summarize recent clinical data related to the development of resistance on conventional and non-conventional modes of administration of the first-generation SSAs and the second-generation SSA pasireotide. We explore mechanisms used to counteract the resistance to SSAs using higher doses or more frequent mode of administration of SSAs and/or combination treatments.

EXPERT OPINION

There is considerable heterogeneity in the development of resistance to SSAs that is tumor-specific necessitating the delineation of the underlying pathophysiological processes to further expand their therapeutic applications.

Molecular targets in acromegaly

Molecular therapeutic targets in growth hormone (GH)-secreting adenomas range from well-characterized surface receptors that recognize approved drugs, to surface and intracellular markers that are potential candidates for new drug development. Currently available medical therapies for patients with acromegaly bind to somatostatin receptors, GH receptor, or dopamine receptors, and lead to attainment of disease control in most patients. The degree of control is variable: however, correlates with both disease aggressiveness and tumor factors that predict treatment response including somatostatin receptor subtype expression, granulation pattern, kinases and their receptors, and other markers of proliferation. A better understanding of the mechanisms underlying these molecular markers and their relationship to outcomes holds promise for expanding treatment options as well as a more personalized approach to treating patients with acromegaly.

Clinical Management of Acromegaly: Therapeutic Frontiers and New Perspectives for Somatostatin Receptor Ligands (SRLs)

Somatostatin receptor ligands (SRLs) represent a true milestone in the medical therapy for acromegaly. The first-generation SRLs (FG-SRLs), octreotide and lanreotide, have demonstrated good efficacy in disease control and tumor shrinkage, and are still considered first-line medical therapies. The development of long-acting release (LAR) formulations has certainly improved the therapeutic tolerability of these drugs, although many patients still experience therapy-related burden. As such, new formulations have recently been developed to improve adherence and therapeutic efficacy and more solutions are on the way. In the case of FG-SRL-resistant disease, pasireotide, the only second generation SRL currently available, demonstrated superiority in disease control and tumor shrinkage compared to FG-SRLs. However, its use in clinical practice is still limited due to concern for impairment in glucose homeostasis. In this review, we discuss the news about the present and future role of SRLs in acromegaly, exploring the therapeutical frontiers of this drug class. Moreover, we provide practical guidance on the use of pasireotide, based on the data in the literature and our clinical experience.

Pasireotide-a novel somatostatin receptor ligand after 20 years of use

Pasireotide, a novel multireceptor-targeted somatostatin receptor ligand (SRL) is characterized by a higher affinity to somatostatin receptor type 5 than type 2, unlike first-generation SRLs. Because of the broader binding profile, pasireotide has been suggested to have a greater clinical efficacy in acromegaly than first-generation SRLs and to be efficacious in Cushing's disease. The consequence of this binding profile is the increased blood glucose level in some patients. This results from the inhibition of both insulin secretion and the incretin effect and only a modest suppression of glucagon. A monthly intramuscular formulation of long-acting release pasireotide has been approved for both acromegaly and Cushing's disease treatment. This review presents data on the efficacy and safety of pasireotide treatment mostly in patients with acromegaly and Cushing's disease. Moreover, other possible therapeutic applications of pasireotide are mentioned.

Somatostatin analogues for the prevention of pancreatic fistula after open pancreatoduodenectomy: A nationwide analysis

BACKGROUND

Somatostatin analogues (SA) are currently used to prevent postoperative pancreatic fistula (POPF) development. However, its use is controversial. This study investigated the effect of different SA protocols on the incidence of POPF after pancreatoduodenectomy in a nationwide population.

METHODS

All patients undergoing elective open pancreatoduodenectomy were included from the Dutch Pancreatic Cancer Audit (2014-2017). Patients were divided into six groups: no SA, octreotide, lanreotide, pasireotide, octreotide only in high-risk (HR) patients and lanreotide only in HR patients. Primary endpoint was POPF grade B/C. The updated alternative Fistula Risk Score was used to compare POPF rates across various risk scenarios.

RESULTS

1992 patients were included. Overall POPF rate was 13.1%. Lanreotide (10.0%), octreotide-HR (9.4%) and no protocol (12.7%) POPF rates were lower compared to the other protocols (varying from 15.1 to 19.1%, p = 0.001) in crude analysis. Sub-analysis in patients with HR of POPF showed a significantly lower rate of POPF when treated with lanreotide (10.0%) compared to no protocol, octreotide and pasireotide protocol (21.6-26.9%, p = 0.006). Octreotide-HR and lanreotide-HR protocol POPF rates were comparable to lanreotide protocol, however not significantly different from the other protocols. Multivariable regression analysis demonstrated lanreotide protocol to be positively associated with a low odds-ratio (OR) for POPF (OR 0.387, 95% CI 0.180-0.834, p = 0.015). In-hospital mortality rates were not affected.

CONCLUSION

Use of lanreotide in all patients undergoing pancreatoduodenectomy has a potential protective effect on POPF development. Protocols for HR patients only might be favorable too. However, future studies are warranted to confirm these findings.

Hyperfractionated Treatment with 177Lu-Octreotate Increases Tumor Response in Human Small-Intestine Neuroendocrine GOT1 Tumor Model

Simple Summary

Neuroendocrine tumors are slow growing and initially associated with vague symptoms and, therefore, often spread in the patient’s body at diagnosis, leading to a poor prognosis without means of curation through surgery. Although tumor-targeting treatments exist and are used in clinics, they are not fully optimized. The aim of this study was to test different dosages and time intervals of the radioactive pharmaceutical ¹⁷⁷Lu-octreotate. We found that dividing a dosage into several portions and administering it at short time intervals resulted in a stronger tumor reduction and/or prolonged time for regrowth in mice than if given as a single dose. The biggest differences were seen in the lower dosage levels of the study. The findings indicate that there is clear room for improvements in the treatment of neuroendocrine tumors with ¹⁷⁷Lu-octreotate.

Abstract

Radionuclide treatment of patients with neuroendocrine tumors has advanced in the last decades with favorable results using ¹⁷⁷Lu-octreotate. However, the gap between the high cure rate in animal studies vs. patient studies indicates a potential to increase the curation of patients. The aim of this study was to investigate the tumor response for different fractionation schemes with ¹⁷⁷Lu-octreotate. BALB/c mice bearing a human small-intestine neuroendocrine GOT1 tumor were either mock treated with saline or injected intravenously with a total of 30–120 MBq of ¹⁷⁷Lu-octreotate: 1 × 30, 2 × 15, 1 × 60, 2 × 30, 1 × 120, 2 × 60, or 3 × 40 MBq. The tumor volume was measured twice per week until the end of the experiment. The mean tumor volume for mice that received 2 × 15 = 30 and 1 × 30 MBq ¹⁷⁷Lu-octreotate was reduced by 61% and 52%, respectively. The mean tumor volume was reduced by 91% and 44% for mice that received 2 × 30 = 60 and 1 × 60 MBq ¹⁷⁷Lu-octreotate, respectively. After 120 MBq ¹⁷⁷Lu-octreotate, given as 1–3 fractions, the mean tumor volume was reduced by 91–97%. Multiple fractions resulted in delayed regrowth and prolonged overall survival by 20–25% for the 120 MBq groups and by 45% for lower total activities, relative to one fraction. The results indicate that fractionation and hyperfractionation of ¹⁷⁷Lu-octreotate are beneficial for tumor reduction and prolongs the time to regrowth.

Long-term effects of somatostatin analogues in rat GH-secreting pituitary tumor cell lines

PURPOSE

First-generation somatostatin analogs, octreotide (OCT) and lanreotide, are the cornerstone for the medical treatment of growth hormone (GH)-secreting pituitary tumors. A new multireceptor analog, such as pasireotide (PAS), showed better activity than OCT in long-term treatment of patients with acromegaly, but modulation of intracellular key processes is still unclear in vitro. In this study, we evaluated the antitumor activity of OCT and PAS in two GH-secreting pituitary tumor cell lines, GH3 and GH4C1, after a long-term incubation.

METHODS

The effects of PAS and OCT on the cell viability, cell cycle, apoptosis, GH secretion, and tumor-induced angiogenesis have been evaluated through a colorimetric method (MTS Assay), DNA flow cytometry with propidium iodide, and Annexin V-FITC/propidium iodide staining, ELISA assay and zebrafish platform, respectively.

RESULTS

PAS showed a more potent antitumor activity compared to OCT in GH3 cell line exerted through inhibition of cell viability, perturbation of cell cycle progression, and induction of apoptosis after 6 days of incubation. A concomitant decrease in GH secretion has been observed after 2 days of incubation only with PAS. No effect on tumor-induced angiogenesis has been reported after treatment with OCT or PAS in zebrafish/tumor xenograft model.

CONCLUSION

Long-term incubation with PAS showed a more potent antitumor activity than that reported after OCT in GH3 cells, mainly modulated by a cell cycle perturbation and a relevant induction in apoptosis.

Pharmacological Characterization of Veldoreotide as a Somatostatin Receptor 4 Agonist

Veldoreotide, a somatostatin analogue, binds to the somatostatin receptors (SSTR) 2, 4, and 5. The current aim was to assess its pharmacological activity as an SSTR4 agonist. G-protein signaling was assessed using a fluorescence-based membrane potential assay in human embryonic kidney 293 (HEK293) cells stably co-expressing G-protein-coupled inwardly rectifying potassium 2 channels and the individual SSTR2, SSTR4, and SSTR5, and in human BON-1 cells stably expressing these SSTRs. Veldoreotide effects on chromogranin A (CgA) secretion and cell proliferation were examined in BON-1 cells. In HEK293 transfected cells, veldoreotide showed a high efficacy for activating the SSTR4; octreotide and pasireotide had little activity (E_max, 99.5% vs. 27.4% and 52.0%, respectively). Veldoreotide also activated SSTR2 and SSTR5 (E_max, 98.4% and 96.9%, respectively). In BON-1 cells, veldoreotide activated SSTR2, SSTR4, and SSTR5 with high potency and efficacy. CgA secretion was decreased to a greater degree in the BON-1 cells expressing SSTR4 versus the cells expressing SSTR2 and SSTR5 (65.3% vs. 80.3% and 77.6%, respectively). In the BON-1 cells expressing SSTR4, veldoreotide inhibited cell proliferation more than somatostatin SS-14 (71.2% vs. 79.7%) and to a similar extent as the SSTR4 agonist J-2156 in the presence of SSTR2 and SSTR5 antagonists. Veldoreotide is a full agonist of SSTR2, SSTR4, and SSTR5.

Drug resistance in pituitary tumours: from cell membrane to intracellular signalling

The pharmacological treatment of pituitary tumours is based on the use of stable analogues of somatostatin and dopamine. The analogues bind to somatostatin receptor types 2 and 5 (SST2 and SST5) and dopamine receptor type 2 (DRD2), respectively, and generate signal transduction cascades in cancerous pituitary cells that culminate in the inhibition of hormone secretion, cell growth and invasion. Drug resistance occurs in a subset of patients and can involve different steps at different stages, such as following receptor activation by the agonist or during the final biological responses. Although the expression of somatostatin and dopamine receptors in cancer cells is a prerequisite for these drugs to reach a biological effect, their presence does not guarantee the success of the therapy. Successful therapy also requires the proper functioning of the machinery of signal transduction and the finely tuned regulation of receptor desensitization, internalization and intracellular trafficking. The present Review provides an updated overview of the molecular factors underlying the pharmacological resistance of pituitary tumours. The Review discusses the experimental evidence that supports a role for receptors and intracellular proteins in the function of SSTs and DRD2 and their clinical importance.

Octreotide and Pasireotide Combination Treatment in Somatotroph Tumor Cells: Predominant Role of SST2 in Mediating Ligand Effects

Simple Summary

First-generation somatostatin receptor ligands, such as octreotide, are the first-line medical therapy in acromegaly. Octreotide shows preferential binding for somatostatin receptor subtype 2 (SST₂), while the second-generation ligand, pasireotide, has high affinity for multiple SSTs. We aimed to elucidate whether pasireotide acts via other receptors than SST₂ in somatotroph tumors, and to investigate the potential role of the combination therapy octreotide plus pasireotide. We found that octreotide and pasireotide are superimposable in reducing GH secretion in cultured somatotroph tumor cells, as well as in inhibiting cell proliferation and intracellular pathway activity in rat GH4C1 cells (a model of somatotroph tumors). We did not find any additive/synergistic effect for the combination treatment. Furthermore, we observed that co-incubation with a SST₂-selective antagonist reversed the inhibitory effect of both compounds. Therefore, the two drugs act mainly via SST₂ in somatotroph tumor cells, and their combination is not superior to single agent treatment.

Abstract

First-generation somatostatin receptor ligands (fg-SRLs), such as octreotide (OCT), represent the first-line medical therapy in acromegaly. Fg-SRLs show a preferential binding affinity for somatostatin receptor subtype-2 (SST₂), while the second-generation ligand, pasireotide (PAS), has high affinity for multiple SSTs (SST₅ > SST₂ > SST₃ > SST₁). Whether PAS acts via SST₂ in somatotroph tumors, or through other SSTs (e.g., SST₅), is a matter of debate. In this light, the combined treatment OCT+PAS could result in additive/synergistic effects. We evaluated the efficacy of OCT and PAS (alone and in combination) on growth hormone (GH) secretion in primary cultures from human somatotroph tumors, as well as on cell proliferation, intracellular signaling and receptor trafficking in the rat GH4C1 cell line. The results confirmed the superimposable efficacy of OCT and PAS in reducing GH secretion (primary cultures), cell proliferation, cAMP accumulation and intracellular [Ca²⁺] increase (GH4C1 cells), without any additive effect observed for OCT+PAS. In GH4C1 cells, co-incubation with a SST₂-selective antagonist reversed the inhibitory effect of OCT and PAS on cell proliferation and cAMP accumulation, while both compounds resulted in a robust internalization of SST₂ (but not SST₅). In conclusion, OCT and PAS seem to act mainly through SST₂ in somatotroph tumor cells in vitro, without inducing any additive/synergistic effect when tested in combination.

The PDZ Domain Protein SYNJ2BP Regulates GRK-Dependent Sst2A Phosphorylation and Downstream MAPK Signaling

The somatostatin receptor 2A (SST2) is a G-protein-coupled receptor (GPCR) that is expressed in neuroendocrine tissues within the gastrointestinal tract and brain, and is commonly overexpressed in many neuroendocrine tumors. Moreover, SST2 agonists are used clinically as the primary pharmacological treatment to suppress excess hormone secretion in a variety of neuroendocrine tumors. Despite its wide clinical use, mechanisms controlling the trafficking and signaling of SST2 are not fully understood. SST2 contains a C-terminal post-synaptic density 95, Drosophila discs large, zona-occludens 1 (PDZ) domain-binding motif that has been shown to interact with 3 different PDZ domain-containing proteins. However, the consequences of these interactions are not well understood, nor is it known whether additional PDZ domain proteins interact with SST2. Through unbiased screening we have identified 10 additional PDZ domain proteins that interact with SST2. We chose one of these, SYNJ2BP, for further study. We observed that SYNJ2BP interacted with SST2 in an agonist-dependent manner, and that this required the PDZ binding site of SST2. Importantly, overexpression of SYNJ2BP enhanced ligand-stimulated receptor internalization. Mechanistically, SYNJ2BP interacted with G-protein-coupled receptor kinase 2 (GRK2) and promoted GRK-dependent phosphorylation of the receptor after somatostatin stimulation. Interaction with GRK2 required the C-terminus of SYNJ2BP. Binding to SYNJ2BP did not affect the ability of SST2 to suppress 3',5'-cyclic adenosine 5'-monophosphate production, but was required for optimal agonist-stimulated extracellularly regulated kinase 1/2 activation. These data indicated that SYNJ2BP is an SST2-interacting protein that modulates agonist-stimulated receptor regulation and downstream signaling.

Somatostatin receptor expression and patients' response to targeted medical treatment in pituitary tumors: evidences and controversies

BACKGROUND

Somatostatin receptors (SSTs) are widely co-expressed in pituitary tumors. SST₂ and SST₅ are the most represented SST subtypes. First-generation somatostatin receptor ligands (SRLs) mainly target SST₂, while pasireotide, a multi-receptor ligand, shows high binding affinity for both SST₅ and SST₂. Therefore, SRLs are routinely used as medical treatment for GH-, TSH-, and ACTH-secreting pituitary tumors.

METHODS

Critical revision of literature data correlating SST expression with patients' response to SRLs.

RESULTS

SST₂ expression in somatroph tumors directly correlates with GH and IGF-1 decrease after first-generation SRL treatment. SST₂ immunohistochemistry represents a valuable tool to predict biochemical response to first-generation SRLs in acromegalic patients. Pasireotide seems to exert its biological effects via SST₂ in unselected patients. However, in those subjects resistant to first-generation SRLs, harbouring tumors with negligible SST₂ expression, pasireotide can act throughout SST₅. More than somatotroph tumors, TSH-omas represent the paradigm of tumors showing a satisfactory response to SRLs. This is probably due to the high SST₂ expression observed in nearly 100% of cases, as well as to the balanced amount of SST₅. In corticotroph tumors, pasireotide mainly act via SST₅, although there is a need for translational studies correlating its efficacy with SST expression in this peculiar tumor histotype.

CONCLUSIONS

The assumption "more target receptor, more drug efficacy" is not straightforward for SRLs. The complex pathophysiology of SSTs, and the technical challenges faced to translate research findings into clinical practice, still need our full commitment to make receptor evaluation a worthwhile procedure for individualizing treatment decisions.

Characterization of agonist-dependent somatostatin receptor subtype 2 trafficking in neuroendocrine cells

BACKGROUND

Somatostatin (SOM) receptor subtype 2 (SSTR2) is the major receptor subtype mediating SOM effects throughout the neuraxis. We previously demonstrated that the non-selective agonist [D-Trp⁸]-SOM induces intracellular sequestration of SSTR2, whereas this receptor is maintained at the cell surface after treatment with the SSTR2-selective agonist L-779,976 in cells co-expressing SSTR2 and SSTR5.

METHODS AND RESULTS

In this study, we knocked-out SSTR5 in AtT20 cells endogenously expressing both SSTR2 and SSTR5 and used immuno-labeling and confocal microscopy to investigate the effect of SSTR5 on regulation of SSTR2 trafficking. Our results indicate that unlike [D-Trp⁸]-SOM-induced intracellular sequestration, L-779,976 stimulation results in the maintenance of SSTR2 at the cell surface regardless of whether SSTR5 is present or not. We then examined the trafficking pathways of SSTR2 upon stimulation by either agonist. We found that both [D-Trp⁸]-SOM and L-779,976 induce SSTR2 internalization via transferrin-positive vesicles. However, SSTR2 internalized upon L-779,976 treatment undergoes rapid recycling to the plasma membrane, whereas receptors internalized by [D-Trp⁸]-SOM recycle slowly after washout of the agonist. Furthermore, [D-Trp⁸]-SOM stimulation induces degradation of a fraction of internalized SSTR2 whereas L-779,976-dependent, rapid SSTR2 recycling appears to protect internalized SSTR2 from degradation. In addition, Octreotide which has preferential SSTR2 affinity, induced differential effects on both SSTR2 trafficking and degradation.

CONCLUSION

Our results indicate that the biased agonistic property of L-779,976 protects against SSTR2 surface depletion by rapidly initiating SSTR2 recycling while SSTR5 does not regulate L-779-976-dependent SSTR2 trafficking.

Acromegaly: Update on Management and Long-Term Morbidities

Acromegaly is a systemic disease associated with great morbidity and increased mortality if not adequately treated. In the past decades much improvement has been achieved in its treatment and in the knowledge of its comorbidities. We provide an update of acromegaly management with current recommendations. We also address long-term comorbidities emphasizing the changing face of the disease in more recent series, with a decrease of cardiovascular disease severity and an increased awareness of comorbidities like bone disease, manifested mainly as vertebral fractures and the change in the main cause of death (from cardiovascular disease to cancer in more recent series).

Polycystic liver disease: Classification, diagnosis, treatment process, and clinical management

Polycystic liver disease (PLD) is a rare hereditary disease that independently exists in isolated PLD, or as an accompanying symptom of autosomal dominant polycystic kidney disease and autosomal recessive polycystic kidney disease with complicated mechanisms. PLD currently lacks a unified diagnostic standard. The diagnosis of PLD is usually made when the number of hepatic cysts is more than 20. Gigot classification and Schnelldorfer classification are now commonly used to define severity in PLD. Most PLD patients have no clinical symptoms, and minority with severe complications need treatments. Somatostatin analogues, mammalian target of rapamycin inhibitor, ursodeoxycholic acid and vasopressin-2 receptor antagonist are the potentially effective medical therapies, while cyst aspiration and sclerosis, transcatheter arterial embolization, fenestration, hepatic resection and liver transplantation are the options of invasion therapies. However, the effectiveness of these therapies except liver transplantation are still uncertain. Furthermore, there is no unified strategy to treat PLD between medical centers at present. In order to better understand recent study progresses on PLD for clinical practice and obtain potential directions for future researches, this review mainly focuses on the recent progress in PLD classification, clinical manifestation, diagnosis and treatment. For information, we also provided medical treatment processes of PLD in our medical center.

SST5 expression and USP8 mutation in functioning and silent corticotroph pituitary tumors

OBJECTIVE

Somatostatin receptor type 5 (SST5) is inconsistently expressed by corticotroph tumors, with higher expression found in corticotropinomas having ubiquitin-specific protease 8 (USP8) mutations. Aims were to study the correlation between characteristics of corticotropinomas and SST5 expression/USP8 mutation status and to describe the response to pasireotide in 5 patients.

DESIGN

Retrospective cohort study.

METHODS

Clinico-biochemical, radiological and pathological data of 62 patients, operated for a functioning or silent corticotropinoma between 2013 and 2017, were collected. SST5 expression was measured by immunohistochemistry (clone UMB-4, Abcam, IRS>1 being considered positive) and Sanger sequencing was performed on 50 tumors to screen for USP8 mutations.

RESULTS

SST5 expression was positive in 26/62 pituitary tumors. A moderate or strong IRS was found in 15/58 corticotropinomas and in 13/35 functioning corticotropinomas. Among functioning tumors, those expressing SST5 were more frequent in women (22/24 vs 9/15, P=0.04) and had a lower grade (P=0.04) compared to others. USP8 mutations were identified in 13/50 pituitary tumors and were more frequent in functioning compared to silent tumors (11/30 vs 2/20, P=0.05). SST5 expression was more frequent in USP8mut vs USP8wt tumors (10/11 vs 7/19, P=0.007). Among treated patients, normal urinary free cortisol levels were obtained in 3 patients (IRS 0, 2, 6) while a 4-fold decrease was observed in one patient (IRS 4).

CONCLUSION

SST5 expression appears to be associated with functioning, USP8mut and lower grade corticotropinomas. A correlation between SST5 expression or USP8mut and response to pasireotide remains to be confirmed.

Cytoskeleton Protein Filamin A Is Required for Efficient Somatostatin Receptor Type 2 Internalization and Recycling through Rab5 and Rab4 Sorting Endosomes in Tumor Somatotroph Cells

The high expression of somatostatin receptor 2 (SST2) in growth hormone (GH)-secreting tumors represents the rationale for the clinical use of somatostatin analogs (SSAs) in acromegaly. Recently, the cytoskeletal protein Filamin A (FLNA) has emerged as key modulator of the responsiveness of GH-secreting pituitary tumors to SSAs by regulating SST2 signaling and expression. The aim of this study was to explore FLNA involvement in SST2 intracellular trafficking in tumor somatotroph cells. By biotinylation assay, we found that FLNA silencing abolished octreotide-mediated SST2 internalization in rat GH3 cell line (28.0 ± 2.7 vs. 4 ± 4.3% SST2 internalization, control versus FLNA small interfering RNAs (siRNA) cells, respectively, p < 0.001) and human GH-secreting primary cultured cells (70.3 ± 21.1 vs. 24 ± 19.2% SST2 internalization, control versus FLNA siRNA cells, respectively, p < 0.05). In addition, confocal imaging revealed impaired SST2 recycling to the plasma membrane in FLNA silenced GH3 cells. Coimmunoprecipitation and immunofluorescence experiments showed that FLNA, as well as β-arrestin2, is timely dependent recruited to octreotide-stimulated SST2 receptors both in rat and human tumor somatotroph cells. Although FLNA expression knock down did not prevent the formation of β-arrestin2-SST2 complex in GH3 cells, it significantly impaired efficient SST2 loading into cytosolic vesicles positive for the early endocytic and recycling markers Rab5 and 4, respectively (33.7 ± 8.9% down to 25.9 ± 6.9%, p < 0.05, and 28.4 ± 7.4% down to 17.6 ± 5.7%, p < 0.01, for SST2-Rab5 and SST2-Rab4 colocalization, respectively, in control versus FLNA siRNA cells). Altogether these data support an important role for FLNA in the mediation of octreotide-induced SST2 trafficking in GH-secreting pituitary tumor cells through Rab5 and 4 sorting endosomes.

Biological and Biochemical Basis of the Differential Efficacy of First and Second Generation Somatostatin Receptor Ligands in Neuroendocrine Neoplasms

Endogenous somatostatin shows anti-secretory effects in both physiological and pathological settings, as well as inhibitory activity on cell growth. Since somatostatin is not suitable for clinical practice, researchers developed synthetic somatostatin receptor ligands (SRLs) to overcome this limitation. Currently, SRLs represent pivotal tools in the treatment algorithm of neuroendocrine tumors (NETs). Octreotide and lanreotide are the first-generation SRLs developed and show a preferential binding affinity to somatostatin receptor (SST) subtype 2, while pasireotide, which is a second-generation SRL, has high affinity for multiple SSTs (SST₅ > SST₂ > SST₃ > SST₁). A number of studies demonstrated that first-generation and second-generation SRLs show distinct functional properties, besides the mere receptor affinity. Therefore, the aim of the present review is to critically review the current evidence on the biological effects of SRLs in pituitary adenomas and neuroendocrine tumors, by mainly focusing on the differences between first-generation and second-generation ligands.

Lanreotide Reduces Liver Growth In Patients With Autosomal Dominant Polycystic Liver and Kidney Disease

BACKGROUND & AIMS

Polycystic liver disease is the most common extrarenal manifestation of autosomal dominant polycystic kidney disease (ADPKD). There is need for robust long-term evidence for the volume-reducing effect of somatostatin analogues. We made use of data from an open-label, randomized trial to determine the effects of lanreotide on height-adjusted liver volume (hTLV) and combined height-adjusted liver and kidney volume (hTLKV) in patients with ADPKD.

METHODS

We performed a 120-week study comparing the reno-protective effects of lanreotide vs standard care in 305 patients with ADPKD (the DIPAK-1 study). For this analysis, we studied the 175 patients with polycystic liver disease with hepatic cysts identified by magnetic resonance imaging and liver volume ≥2000 mL. Of these, 93 patients were assigned to a group that received lanreotide (120 mg subcutaneously every 4 weeks) and 82 to a group that received standard care (blood pressure control, a sodium-restricted diet, and antihypertensive agents). The primary endpoint was percent change in hTLV between baseline and end of treatment (week 120). A secondary endpoint was change in hTLKV.

RESULTS

At 120 weeks, hTLV decreased by 1.99% in the lanreotide group (95% confidence interval [CI], -4.21 to 0.24) and increased by 3.92% in the control group (95% CI, 1.56-6.28). Compared with the control group, lanreotide reduced the growth of hTLV by 5.91% (95% CI, -9.18 to -2.63; P < .001). Growth of hTLV was still reduced by 3.87% at 4 months after the last injection of lanreotide compared with baseline (95% CI, -7.55 to -0.18; P = .04). Lanreotide reduced growth of hTLKV by 7.18% compared with the control group (95% CI, -10.25 to -4.12; P < .001).

CONCLUSIONS

In this subanalysis of a randomized trial of patients with polycystic liver disease due to ADPKD, lanreotide for 120 weeks reduced the growth of liver and combined liver and kidney volume. This effect was still present 4 months after the last injection of lanreotide. ClinicalTrials.gov, Number: NCT01616927.

Physiopathological Premises to Nuclear Medicine Imaging of Pancreatic Neuroendocrine Tumours

Background:

Pancreatic Neuroendocrine Tumors (P-NETs) are a challenge in terms of both diagnosis and therapy; morphological studies need to be frequently implemented with nonstandard techniques such as Endoscopic Ultrasounds, Dynamic CT, and functional Magnetic Resonance.

Discussion:

The role of nuclear medicine, being scarcely sensitive F-18 Fluorodeoxyglucose, is mainly based on the over-expression of Somatostatin Receptors (SSTR) on neuroendocrine tumor cells surface. Therefore, SSTR can be used as a target for both diagnosis, using radiotracers labeled with gamma or positron emitters, and therapy. SSTRs subtypes are capable of homo and heterodimerization in specific combinations that alter both the response to ligand activation and receptor internalization.

Conclusion:

Although agonists usually provide efficient internalization, also somatostatin antagonists (SS-ANTs) could be used for imaging and therapy. Peptide Receptor Radionuclide Therapy (PRRT) represents the most successful option for targeted therapy. The theranostic model based on SSTR does not work in insulinoma, in which different radiotracers such as F-18 FluoroDOPA or tracers for the glucagon-like peptide-1 receptor have to be preferred.

A Practical and Total Synthesis of Pasireotide: Synthesis of Cyclic Hexapeptide via a Three-Component Condensation

Pasireotide is a multi-receptor ligand somatostatin analogue approved for medical treatment of Cushing’s disease and acromegaly. The liquid-phase total synthesis of pasireotide-a 18-membered cyclic hexapeptide-was achieved by the 3 + 2 + 1 strategy, and the Pro¹-Phe⁶ peptide bond was selected as the final cyclization position. Two key fragments were simply synthesized using N,O-bis(trimethylsilyl)acetamide/N-hydroxysuccinimide ester (BSA/NHS) as coupling agents, and processes of the two key fragments were simple without any chromatographic purification. The current synthesis method is easily scalable and produces the target peptide with an overall yield of 15%.

Molecular determinants of the response to medical treatment of growth hormone secreting pituitary neuroendocrine tumors

Acromegaly is a chronic systemic disease mainly caused by a growth hormone (GH)-secreting pituitary neuroendocrine tumor (PitNETs), which is associated with many health complications and increased mortality when not adequately treated. Transsphenoidal surgery is considered the treatment of choice in GH-secreting PitNETs, but patients in whom surgery cannot be considered or with persistent disease after surgery require medical therapy. Treatment with available synthetic somatostatin analogues (SSAs) is considered the mainstay in the medical management of acromegaly which exert their beneficial effects through the binding to a family of G-protein coupled receptors encoded by 5 genes (SSTR1-5). However, although it has been demonstrated that the SST1-5 receptors are physically present in tumor cells, SSAs are in many cases ineffective (i.e. approximately 10-30% of patients with GH-secreting PitNET are unresponsive to SSAs), suggesting that other cellular/molecular determinants could be essential for the response to the pharmacological treatment in patients with GH-secreting PitNETs. Therefore, the scrutiny of these determinants might be used for the identification of subgroups of patients in whom an appropriate pharmacological treatment can be successfully employed (responders vs. non-responders). In this review, we will describe some of the existing, classical and novel, genetic and molecular determinants involved in the response of patients with GH-secreting PitNETs to the available therapeutic treatments, as well as new molecular/therapeutic approaches that could be potentially useful for the treatment of GH-secreting PitNETs.

Pasireotide Responsiveness in Acromegaly Is Mainly Driven by Somatostatin Receptor Subtype 2 Expression

BACKGROUND

The response to first-generation somatostatin receptor ligands (SRLs) treatment in acromegaly correlates with expression of somatostatin receptor subtype 2 (SSTR2). However, pasireotide shows the highest binding affinity for SSTR subtype 5 (SSTR5). It has been suggested that in acromegaly, SSTR5 expression is better at predicting the response to pasireotide long-acting release (PAS-LAR) treatment than SSTR2 expression.

AIM

To investigate in patients with active acromegaly whether response to SRL treatment correlates to PAS-LAR treatment and to what extent SSTR2 and SSTR5 expression are correlated to the response to PAS-LAR treatment.

METHODS

We included 52 patients from a cohort that initially received SRL treatment, followed by SRL and pegvisomant combination treatment, and finally PAS-LAR treatment. The long-term response to PAS-LAR was evaluated using a PAS-LAR score. In 14 out of 52 patients, somatotroph adenoma tissue samples were available to evaluate SSTR2 and SSTR5 expression using a previously validated immunoreactivity score (IRS).

RESULTS

The percentage IGF-I (times the upper limit of normal) reduction, which was observed after SRL treatment, correlated with PAS-LAR response score during follow-up (r = 0.40; P = 0.003; n = 52). After exclusion of SRL-pretreated patients, SSTR2 IRS was positively correlated to PAS-LAR score (r = 0.58; P = 0.039; n = 9), whereas SSTR5 IRS showed no relation (r = 0.35; P = 0.36; n = 9).

CONCLUSIONS

In a cohort of patients partially responsive to SRLs, the IGF-I-lowering effects of PAS-LAR treatment correlated with the effect of SRL treatment and seemed to be mainly driven by SSTR2 expression instead of SSTR5.

Evaluation of expression of somatostatin receptor 1, 2, 3, 5 and dopamine D2 receptor in spindle cell oncocytomas of posterior pituitary

PURPOSE

Spindle cell oncocytomas (SCOs) are very rare tumors of the posterior pituitary with potential for locally aggressive behaviour. Their treatment includes surgery and possibly radiotherapy, however other options are lacking. Somatostatin receptors (SSTs) are a possible therapeutic target for somatostatin analogues and their expression has been demonstrated recently in closely related pituicytomas, but there are no data about their presence in SCOs.

METHODS

We collected five cases of SCO from four patients including one recurrent case. Immunohistochemical detection of TTF1, GFAP, CD68, SST₁, SST₂, SST₃, SST₅ and D2 dopamine receptor (D2DR) was performed. Intensity, percentage of positive cells and pattern of expression was evaluated in semiquantitative fashion. Protein expression of SST_1-5 and D2DR was further evaluated by western blot.

RESULTS

Mean patient age was 61.8 years (range 47-71 years) with male to female ratio 1:1. In one patient, samples from the original tumor and its recurrence 16 years later were assessed. TTF1 was positive in all five cases, no expression of GFAP and CD68 was seen. Immunohistochemical expression of SST₁ was noted in 1/5 cases, SST₂ in 2/5 cases, including recurrent case but not the original case. SST₃ was expressed in 3/5 tumors and D2 dopamine receptor in 4/5 cases. Western blot was successfully performed in four samples. SST₂, SST₃ and D2DR expression was identified in all the samples, including two cases originally negative for SST₂ and one case negative for SST₃ by immunohistochemistry. The number of positive cells and level of expression varied among different areas of the same tumors. No expression of SST₅ was observed. In the patient with the recurrent tumor, intensity of SST₂, SST₃ and D2DR expression varied between original tumor and its recurrence.

CONCLUSIONS

We demonstrated presence of different SST subtypes and D2DR in spindle cell oncocytomas. The most commonly expressed subtype was SST₂ and SST₃, while no expression of SST₅ was observed. Expression showed spatial heterogeneity and temporal changes as seen in the recurrent case. The biological meaning of SSTs expression in SCOs is unclear as well as whether it may be exploited in treatment of selected cases.

Cell specific interaction of pasireotide: review of preclinical studies in somatotroph and corticotroph pituitary cells

BACKGROUND

Pasireotide is a second-generation somatostatin (SRIF) receptor ligand (SRL), approved for medical treatment of acromegaly and Cushing's disease (CD). The molecule is a stable cyclohexapeptide synthetized based on SRIF structure. Differently from first-generation SRLs (e.g. octreotide), preferentially binding somatostatin receptor (SST) subtype 2 (SST₂), pasireotide has high affinity for multiple SSTs (SST₅ > SST₂ > SST₃ > SST₁). Interestingly, early preclinical studies demonstrated that pasireotide shows distinct functional properties compared to SRIF and first-generation SRLs when binding SSTs.

METHODS

We aimed to highlight the differential receptor-targeted action of pasireotide in the treatment of somatotroph and corticotroph adenomas, throughout the critical revision of preclinical studies carried out on acromegaly and CD models.

RESULTS

Different authors demonstrated that the antisecretory effect of pasireotide in somatotroph adenoma cell cultures is comparable to that of the SST₂-preferential agonist octreotide. Some reports even show a direct correlation between SST₂ mRNA expression and GH reduction after pasireotide treatment, thus laying for a predominant role of SST₂ in driving pasireotide efficacy in somatotropinomas in vitro. On the other hand, the inhibitory effect of pasireotide on ACTH secretion in corticotropinoma cells seems to be mainly mediated by SST₅. Indeed, most reports show a higher potency and efficacy of pasireotide compared to SST₂ preferential agonists, while functional studies confirm the pivotal role of SST₅ targeting in corticotroph cells.

CONCLUSIONS

The analysis of preclinical studies carried out in somatotroph and corticoph adenomas points out that pasireotide shows a cell-specific activity, exerting its biological effects via different SSTs in the different adenoma histotypes.

International Union of Basic and Clinical Pharmacology. CV. Somatostatin Receptors: Structure, Function, Ligands, and New Nomenclature

Somatostatin, also known as somatotropin-release inhibitory factor, is a cyclopeptide that exerts potent inhibitory actions on hormone secretion and neuronal excitability. Its physiologic functions are mediated by five G protein-coupled receptors (GPCRs) called somatostatin receptor (SST)1-5. These five receptors share common structural features and signaling mechanisms but differ in their cellular and subcellular localization and mode of regulation. SST₂ and SST₅ receptors have evolved as primary targets for pharmacological treatment of pituitary adenomas and neuroendocrine tumors. In addition, SST₂ is a prototypical GPCR for the development of peptide-based radiopharmaceuticals for diagnostic and therapeutic interventions. This review article summarizes findings published in the last 25 years on the physiology, pharmacology, and clinical applications related to SSTs. We also discuss potential future developments and propose a new nomenclature.

Is receptor profiling useful for predicting pituitary therapy?

Medical treatment of pituitary tumours may present important challenges in the presence of resistance to first line therapy. In this setting, the availability of specific markers of responsiveness/resistance could be helpful to provide tailored patients' treatment. Pituitary receptor profiling has emerged as a potentially useful tool for predicting the response to specific pituitary-directed medical therapy, mainly somatostatin analogues and dopamine agonists. However, its utility is not always straightforward. In fact, agonist-receptor coupling to the consequent biological response is complex and sometimes jeopardizes the understanding of the molecular basis of pharmacological resistance. Defective expression of pituitary receptors, genetic alterations, truncated variants, impaired signal transduction or involvement of other proteins, such as cytoskeleton proteins or the Aryl hydrocarbon receptor interacting protein amongst others, have been linked to differential tumour phenotype or treatment responsiveness with conflicting results, keeping the debate on the utility of pituitary receptor profiling open. Why does this occur? How can we overcome the difficulties? Is there a true role for pituitary receptor profiling in the near future? All authors of this debate article agree on the need of prospective studies using standardized methods in order to assess the efficacy of receptor profiling as a reliable clinical predictive factor.

Targeting Somatostatin Receptors By Functionalized Mesoporous Silica Nanoparticles - Are We Striking Home?

The concept of delivering nanoformulations to desired tissues by means of targeting membrane receptors of high local abundance by ligands anchored to the nanocarrier has gained a lot of attention over the last decade. Currently, there is no unanimous opinion on whether surface functionalization of nanocarriers by targeting ligands translates into any real benefit in terms of pharmacokinetics or treatment outcomes. Having examined the published nanocarriers designed to engage with somatostatin receptors, we realized that in the majority of cases targetability claims were not supported by solid evidence of targeting ligand-targeted receptor coupling, which is the very crux of a targetability concept. Here, we present an approach to characterize targetability of mesoporous silica-based nanocarriers functionalized with ligands of somatostatin receptors. The targetability proof in our case comes from a functional assay based on a genetically-encoded cAMP probe, which allows for real-time capture of receptor activation in living cells, triggered by targeting ligands on nanoparticles. We elaborate on the development and validation of the assay, highlighting the power of proper functional tests in the characterization pipeline of targeted nanoformulations.

Multisite phosphorylation is required for sustained interaction with GRKs and arrestins during rapid μ-opioid receptor desensitization

G protein receptor kinases (GRKs) and β-arrestins are key regulators of μ-opioid receptor (MOR) signaling and trafficking. We have previously shown that high-efficacy opioids such as DAMGO stimulate a GRK2/3-mediated multisite phosphorylation of conserved C-terminal tail serine and threonine residues, which facilitates internalization of the receptor. In contrast, morphine-induced phosphorylation of MOR is limited to Ser³⁷⁵ and is not sufficient to drive substantial receptor internalization. We report how specific multisite phosphorylation controlled the dynamics of GRK and β-arrestin interactions with MOR and show how such phosphorylation mediated receptor desensitization. We showed that GRK2/3 was recruited more quickly than was β-arrestin to a DAMGO-activated MOR. β-Arrestin recruitment required GRK2 activity and MOR phosphorylation, but GRK recruitment also depended on the phosphorylation sites in the C-terminal tail, specifically four serine and threonine residues within the ³⁷⁰TREHPSTANT³⁷⁹ motif. Our results also suggested that other residues outside this motif participated in the initial and transient recruitment of GRK and β-arrestins. We identified two components of high-efficacy agonist desensitization of MOR: a sustained component, which required GRK2-mediated phosphorylation and a potential soluble factor, and a rapid component, which was likely mediated by GRK2 but independent of receptor phosphorylation. Elucidating these complex receptor-effector interactions represents an important step toward a mechanistic understanding of MOR desensitization that leads to the development of tolerance and dependence.

Pasireotide treatment does not modify hyperglycemic and corticosterone acute restraint stress responses in rats

Pasireotide is a new-generation somatostatin analog that acts through binding to multiple somatostatin receptor subtypes. Studies have shown that pasireotide induces hyperglycemia, reduces glucocorticoid secretion, alters neurotransmission, and potentially affects stress responses typically manifested as hyperglycemia and increased corticosterone secretion. This study specifically aimed to evaluate whether pasireotide treatment modifies glucose and costicosterone secretion in response to acute restraint stress. Male Holtzman rats of 150-200 g were treated with pasireotide (10 µg/kg/day) twice-daily for two weeks or vehicle for the same period. Blood samples were collected at baseline and after 5, 10, 30, and 60 min of restraint stress. The three experimental groups comprised of vehicle + restraint (VEHR), pasireotide + restraint (PASR), and pasireotide + saline (PASNR). Following pasireotide treatment, no significant differences in baseline glucose and corticosterone levels were observed among the three groups. During restraint, hyperglycemia was observed at 10 min (p < .01 for both comparisons), peaked at 30 min (p < .01 for both comparisons) and showed higher 60 min areas under glucose curves in the VEHR and PASR stressed groups when compared to the non-stressed PASNR group (p < .05 for both comparisons). Restraint also increased corticosterone secretion in the VEHR and PASR stressed groups at 5 min (p < .01 for both comparisons), and peaked at 30 min (p < .01 for both comparisons) with corresponding higher 60 min areas under corticosterone curves when compared to the non-stressed PASNR group (p < .01 for both comparisons). In conclusion, pasireotide treatment does not modify hyperglycemic- and corticosterone-restraint stress responses, thus preserving acute stress regulation.

Real-Time Signaling Assays Demonstrate Somatostatin Agonist Bias for Ion Channel Regulation in Somatotroph Tumor Cells

Acromegaly is a neuroendocrine disorder caused by excess secretion of GH by somatotroph tumor cells. It is often treated with somatostatin receptor (SSTR) 2 agonists, which suppress GH secretion. SOM230 is a somatostatin analogue that targets multiple SSTRs and was recently approved for patients with treatment-resistant acromegaly. Previous reports indicate that SOM230 may function as a biased agonist, suggesting that its ability to selectively activate SSTR-dependent signaling events may contribute to its therapeutic efficacy. To better understand how SOM230 modulates Sstr2A function, which is the most commonly expressed SSTR in somatotrophs, we used real-time assays to study SOM230-dependent signaling in rat pituitary tumor cells. We observed that SOM230 suppressed cAMP production in a Gαi-dependent manner, similar to conventional Sstr2A agonists. However, it did not cause receptor internalization as would be expected for an Sstr2A agonist. Surprisingly, SOM230 did not cause membrane hyperpolarization, which is an important mechanism by which Sstr2a activation suppresses intracellular calcium (Ca²⁺) accumulation and GH secretion. In fact, SOM230 inhibited the ability of conventional somatostatin analogues to control membrane potential. However, SOM230 still inhibited intracellular Ca²⁺ accumulation in a novel, Gβγ-dependent manner. These studies show that SOM230 exhibits strong agonist bias in regulating signaling pathways downstream of Sstr2A that control GH secretion.

An evaluation in vitro of the efficacy of nutlin-3 and topotecan in combination with 177Lu-DOTATATE for the treatment of neuroblastoma

Targeted radiotherapy of metastatic neuroblastoma using the somatostatin receptor (SSTR)-targeted octreotide analogue DOTATATE radiolabelled with lutetium-177 (¹⁷⁷Lu-DOTATATE) is a promising strategy. This study evaluates whether its effectiveness may be enhanced by combination with radiosensitising drugs. The growth rate of multicellular tumour spheroids, derived from the neuroblastoma cell lines SK-N-BE(2c), CHLA-15 and CHLA-20, was evaluated following treatment with ¹⁷⁷Lu-DOTATATE, nutlin-3 and topotecan alone or in combination. Immunoblotting, immunostaining and flow cytometric analyses were used to determine activation of p53 signalling and cell death. Exposure to ¹⁷⁷Lu-DOTATATE resulted in a significant growth delay in CHLA-15 and CHLA-20 spheroids, but not in SK-N-BE(2c) spheroids. Nutlin-3 enhanced the spheroid growth delay induced by topotecan in CHLA-15 and CHLA-20 spheroids, but not in SK-N-BE(2c) spheroids. Importantly, the combination of nutlin-3 with topotecan enhanced the spheroid growth delay induced by X-irradiation or by exposure to ¹⁷⁷Lu-DOTATATE. The efficacy of the combination treatments was p53-dependent. These results indicate that targeted radiotherapy of high risk neuroblastoma with ¹⁷⁷Lu-DOTATATE may be improved by combination with the radiosensitising drugs nutlin-3 and topotecan.

Cellular effects of AP102, a somatostatin analog with balanced affinities for the hSSTR2 and hSSTR5 receptors

BACKGROUND

Somatostatin analogs (SSAs) are first-line medical therapy for the treatment of acromegaly and neuroendocrine tumors that express somatostatin receptors (SSTR). Somatostatin suppresses secretion of a large number of hormones through the stimulation of the five SSTR. However, unbalanced inhibition of secretion as observed with the highly potent SSAs pasireotide causes hyperglycaemia mainly by inhibiting insulin secretion. In contrast, AP102 a new SSAs has neutral effect on blood glucose while suppressing GH secretion. Our objective was to establish the cellular effects of AP102 on SSTR2 and SSTR5 that may explain the differences observed between AP102 and other SSAs.

METHODS

We compared the binding and agonist activity of AP102 with somatostatin-14, octreotide and pasireotide in HEK293 cells transfected with human SSTR2 and SSTR5 receptors. SSAs signal transduction effects (cAMP concentrations) were measured in forskolin-treated cells in the presence of SSAs. Proliferation and apoptotic effects were determined and binding assays were performed using ¹²⁵I- somatostatin-14.

RESULTS

AP102 has comparable affinity and agonist effect to octreotide at SSTR2 (IC50's of 112 pM and 244 pM, respectively; EC50's of 230 pM and 210 pM, respectively) in contrast to pasireotide that exhibits a 12-27 fold higher IC50 (3110 pM) and about 5-fold higher EC50 (1097 pM). At SSTR5, AP102 has much higher affinity and stimulating effect than octreotide (IC50's of 773 pM and 16,737 pM, respectively; EC50's of 8526 pM and 26,800 pM), and an intermediate affinity and agonist effect between octreotide and pasireotide. AP102, octreotide and pasireotide have variable anti-proliferative effects on HEK cells transfected with SSTR2 and SSTR5.

CONCLUSION

AP102 is a new SSA that better reduces signaling at SSTR2 than SSTR5 and prevents cell proliferation at both receptors. The euglycaemic effect of AP102 observed in preclinical studies may be related to this intermediate agonistic potency between pasireotide and octreotide at SSTR2 and SSTR5.

Molecular evidence and clinical importance of β-arrestins expression in patients with acromegaly

β-arrestins seem to have a role in endocytosis and desensitization of somatostatin receptor subtype 2 (sst2) and could be associated with the responsiveness to somatostatin receptor ligands (SRL) in patients with acromegaly. To investigate the in vivo correlation between β-arrestins 1 and 2 with sst2, sst5 and dopamine receptor subtype 2 (D2) expressions, and the association of β-arrestins with response to first-generation SRL and invasiveness in somatotropinomas. β-arrestins 1 and 2, sst2, sst5 and D2 mRNA expressions were evaluated by quantitative real-time RT-PCR on tumoral tissue of 96 patients. Moreover, sst2 and sst5 protein expressions were also evaluated in 40 somatotropinomas by immunohistochemistry. Response to SRL, defined as GH <1 μg/l and normal IGF-I levels, was assessed in 40 patients. The Knosp-Steiner criteria were used to define invasiveness. Median β-arrestin 1, β-arrestin 2, sst2, sst5 and D2 mRNA copy numbers were 478; 9375; 731; 156; and 3989, respectively. There was a positive correlation between β-arrestins 1 and 2 (R = 0.444, P < 0.001). However, no correlation between β-arrestins and sst2, sst5 (mRNA and protein levels) or D2 was found. No association was found between β-arrestins expression and SRL responsiveness or tumour invasiveness. Although previous data suggest a putative correlation between β-arrestins and sst2, our data clearly indicated that no association existed between β-arrestins and sst2, sst5 or D2 expression, nor with response to SRL or tumour invasiveness. Therefore, further studies are required to clarify whether β-arrestins have a role in the response to treatment with SRL in acromegaly.

Association between dopamine and somatostatin receptor expression and pharmacological response to somatostatin analogues in acromegaly

Acromegaly is a hormonal disorder resulting from excessive growth hormone (GH) secretion frequently produced by pituitary adenomas and consequent increase in insulin‐like growth factor 1 (IGF‐I). Elevated GH and IGF‐I levels result in a wide range of somatic, cardiovascular, endocrine, metabolic and gastrointestinal morbidities. Somatostatin analogues (SSAs) form the basis of medical therapy for acromegaly and are currently used as first‐line treatment or as second‐line therapy in patients undergoing unsuccessful surgery. However, a considerable percentage of patients do not respond to SSAs treatment. Somatostatin receptors (SSTR1‐5) and dopamine receptors (DRD1‐5) subtypes play critical roles in the regulation of hormone secretion. These receptors are considered important pharmacological targets to inhibit hormone oversecretion. It has been proposed that decreased expression of SSTRs may be associated with poor response to SSAs. Here, we systematically examine SSTRs and DRDs expression in human somatotroph adenomas by quantitative PCR. We observed an association between the response to SSAs treatment and DRD4, DRD5, SSTR1 and SSTR2 expression. We also examined SSTR expression by immunohistochemistry and found that the immunohistochemical detection of SSTR2 in particular might be a good predictor of response to SSAs.

Somatostatin analogs in the treatment of neuroendocrine tumors: current and emerging aspects

INTRODUCTION

Neuroendocrine tumors (NETs) harbor somatostatin receptors and there is a strong rationale for using somatostatin analogs (SSAs) for treatment of NETs. Areas covered: This article discusses i) pharmacology of somatostatin and its analogs; ii) antisecretory and anti-proliferative effects of SSAs in NETs; iii) efficacy and safety of emerging therapeutic regimens with first generation SSAs administered at either high doses or in combination with antineoplastic drugs; iv) efficacy and safety of pasireotide and chimeric molecules; v) efficacy of radionuclide therapy of NETs using SSAs. Expert opinion: SSAs are the first-line medical therapy for functioning and non-functioning well-differentiated NETs. In patients not responder to first generation SSAs, the increase of drug dose over the conventional regimens, the combination of SSAs with other biotherapies or molecular targeted therapies, the switch to pasireotide or the use of SSAs in radionuclide therapy may improve the therapeutic success.

Effect of AP102, a subtype 2 and 5 specific somatostatin analog, on glucose metabolism in rats

PURPOSE

Somatostatin analogs are widely used to treat conditions associated with hormonal hypersecretion such as acromegaly and metastatic neuroendocrine tumors. First generation somatostatin analogs, such as octreotide and lanreotide, have high affinity for somatostatin receptor subtype 2 (SSTR2), but have incomplete efficacy in many patients. Pasireotide targets multiple SSTRs, having the highest affinity for SSTR5, but causes hyperglycemia and diabetes mellitus in preclinical and clinical studies. AP102 is a new somatostatin analogs with high affinity at both SSTR2 and SSTR5. We aimed to characterize the effects of AP102 vs. pasireotide on random and dynamic glucose levels, glucoregulatory hormone concentrations and growth axis measures in healthy Sprague-Dawley rats.

METHODS

Three doses of each compound were evaluated under acute conditions (1, 10, and 30 µg/kg s.c.), and two doses during a chronic (4-week) infusion (3 and 10 µg/kg/h s.c.).

RESULTS

Neither acute nor chronic AP102 administration altered blood glucose concentrations or dynamic responses following an intraperitoneal glucose tolerance test. In contrast, acute and chronic pasireotide dosing increased random and post-intraperitoneal glucose tolerance test blood glucose measures, compared to vehicle-treated controls. Both AP102 and pasireotide acutely suppressed growth hormone levels, although insulin-like growth factor-1 and somatic growth was suppressed to a greater extent with pasireotide.

CONCLUSIONS

AP102 is a new dual SSTR2/SSTR5-specific somatostatin analog that acutely reduces growth hormone but does not cause hyperglycemia during acute or chronic administration in a healthy rat model. Further studies in diabetic animals and in humans are necessary to determine the potential utility of AP102 in the clinical setting.

Pasireotide is more effective than octreotide, alone or combined with everolimus on human meningioma in vitro

Pasireotide is a somatostatin analog (SSA) that targets somatostatin receptor subtype 1 (SST1), SST2, SST3, and SST5 with a high affinity. Pasireotide has a better antisecretory effect in acromegaly, Cushing's disease, and neuroendocrine tumors than octreotide. In this study, we compared the effects of pasireotide to those of octreotide in vitro on meningioma primary cell cultures, both alone and in combination with the mTOR inhibitor everolimus. Significant mRNA expression levels of SST1, SST2, and SST5 were observed in 40.5%, 100%, and 35% of meningioma samples, respectively. Pasireotide had a significantly stronger inhibitory effect on cell proliferation than octreotide. The effect of pasireotide, but not of octreotide, was significantly stronger in the group expressing the highest level of SST1 mRNA. Combined treatment with pasireotide and everolimus induced a higher reduction in cell viability than that with octreotide plus everolimus. Moreover, pasireotide decreased Akt phosphorylation and reversed everolimus-induced Akt hyperphosphorylation to a higher degree than octreotide. Using 4E-BP1 siRNA (si4E-BP), we demonstrated that 4E-BP1 protein silencing significantly reversed the response to everolimus, both alone and in combination with SSAs. Moreover, si4E-BP completely reversed the inhibition of cyclin D1 expression level and the increase in p27kip1 induced by SSAs, both alone and in combination with everolimus. Our results strongly support the need for further studies on the combination of pasireotide and everolimus in medical therapy for meningiomas.

Long-Acting Somatostatin Analog Therapy Differentially Alters 68Ga-DOTATATE Uptake in Normal Tissues Compared with Primary Tumors and Metastatic Lesions

Synthetic somatostatin analogs have been posed as a potential source of error in somatostatin receptor imaging through interference with tumor detection; however, experimental models and clinical studies have shown a complex mechanism of the effect of octreotide on tumors. The aim of this study was to assess whether ⁶⁸Ga-DOTATATE uptake before treatment with long-acting somatostatin analogs differs from that after treatment. Methods: Thirty patients (15 men; age [mean ± SD], 64.6 ± 13.4 y) who had intermediately differentiated to well-differentiated neuroendocrine tumors and who underwent ⁶⁸Ga-DOTATATE PET/CT scanning before and after receiving long-acting repeatable octreotide (Sandostatin LAR) were included in the study. The SUV_max and SUV_mean of healthy target organs, residual primary tumor, and up to 5 lesions with the highest SUV_max in each organ were compared before and after octreotide treatment. Results: The mean time interval between the 2 ⁶⁸Ga-DOTATATE studies was 9.6 ± 7.2 mo, and the mean time gap between the last Sandostatin LAR injection and the second ⁶⁸Ga-DOTATATE study was 25.1 ± 14.8 d. The pretreatment mean SUV_max and SUV_mean were both significantly higher in the thyroid, liver, and spleen (P < 0.05) than the values measured after the administration of Sandostatin LAR. No significant differences were found among the uptake indices for residual primary tumor or any metastatic lesions in the liver, bone, lung, or lymph nodes before and after Sandostatin LAR administration (P > 0.05). Conclusion: Long-acting octreotide treatment diminished ⁶⁸Ga-DOTATATE uptake in the liver, spleen, and thyroid but did not compromise tracer uptake in residual primary tumor and metastatic lesions. These findings have a direct impact on the interpretation of ⁶⁸Ga-DOTATATE PET/CT scans.

β-arrestin signalling and bias in hormone-responsive GPCRs

G protein-coupled receptors (GPCRs) play crucial roles in the ability of target organs to respond to hormonal cues. GPCRs' activation mechanisms have long been considered as a two-state process connecting the agonist-bound receptor to heterotrimeric G proteins. This view is now challenged as mounting evidence point to GPCRs being connected to large arrays of transduction mechanisms involving heterotrimeric G proteins as well as other players. Amongst the G protein-independent transduction mechanisms, those elicited by β-arrestins upon their recruitment to the active receptors are by far the best characterized and apply to most GPCRs. These concepts, in conjunction with remarkable advances made in the field of GPCR structural biology and biophysics, have supported the notion of ligand-selective signalling also known as pharmacological bias. Interestingly, recent reports have opened intriguing prospects to the way β-arrestins control GPCR-mediated signalling in space and time within the cells. In the present paper, we review the existing evidence linking endocrine-related GPCRs to β-arrestin recruitement, signalling, pathophysiological implications and selective activation by biased ligands and/or receptor modifications. Emerging concepts surrounding β-arrestin-mediated transduction are discussed in the light of the peculiarities of endocrine systems.

In Vitro Head-to-Head Comparison Between Octreotide and Pasireotide in GH-Secreting Pituitary Adenomas

CONTEXT

First-generation somatostatin analogs (SSAs), such as octreotide (OCT), are the first line medical therapy for acromegaly. Pasireotide (PAS), a newly developed SSA, has shown promising results in the treatment of acromegaly.

OBJECTIVE

To compare the antisecretory effect of OCT and PAS in primary cultures of growth hormone (GH)-secreting pituitary adenomas (GH-omas). To correlate responses with the adenoma somatostatin receptor (SSTR) profile.

DESIGN

The effect of OCT and PAS on GH (and PRL) secretion was tested in 33 GH-oma cultures. SSTR expression was evaluated in adenoma samples.

SETTING AND PATIENTS

Patients with acromegaly referred to the Erasmus Medical Center (Rotterdam, The Netherlands).

INTERVENTIONS

OCT and PAS treatment for 72 hours (10 nM).

MAIN OUTCOME MEASURES

GH (and PRL) concentrations in cell culture media. SSTR expression in adenoma samples.

RESULTS

The overall effect of OCT (-36.8%) and PAS (-37.1%) on GH secretion was superimposable. We identified three adenoma groups: PAS+ (PAS more effective than OCT), n = 6; PAS = OCT, n = 22; and OCT+ (OCT more effective than PAS), n = 5. PAS+ adenomas showed lower somatostatin receptor subtype (sst)2 messenger RNA (mRNA) and sst2/sst5 mRNA ratio, compared with the other groups (P < 0.05). PAS inhibited PRL hypersecretion more than OCT (P < 0.01).

CONCLUSIONS

Overall, OCT and PAS equally reduced GH secretion in vitro. Adenomas with lower sst2 mRNA expression and lower sst2/sst5 mRNA ratio were better responders to PAS compared with OCT. SSTR evaluation in GH-omas may become a tool for tailored SSA treatment in acromegaly.

Somatostatin Receptor Type 2 (SSTR2) Internalization and Intracellular Trafficking in Pituitary GH-Secreting Adenomas: Role of Scaffold Proteins and Implications for Pharmacological Resistance

Somatostatin receptor type 2 (SSTR2), together with SSTR5, represents the main target of medical treatment for growth hormone (GH)-secreting pituitary tumors, since it is expressed in most of these tumors and exerts both antiproliferative and cytostatic effects, and reduces hormone secretion, as well. However, clinical practice indicates a great variability in the frequency and entity of favorable responses of acromegalic patients to long-acting somatostatin analogues (SSAs), but the molecular mechanisms regulating this pharmacological resistance are not completely understood. So far, several potentially implied mechanisms have been suggested, including impaired expression of SSTRs, or post-receptor signal transduction alterations. More recently, new studies exploited the molecular factors involved in SSTRs intracellular trafficking regulation, this being a critical point for the modulation of the available active G-coupled receptors (GPCRs) amount at the cell surface. In this respect, the role of the scaffold proteins such as β-arrestins, and the cytoskeleton protein Filamin A (FLNA), have become of relevant importance for GH-secreting pituitary tumors. In fact, β-arrestins are linked to SSTR2 desensitization and internalization, and FLNA is able to regulate SSTR2 trafficking and stability at the plasma membrane. Therefore, the present review will summarize emerging evidence highlighting the role of β-arrestins and FLNA, as possible novel players in the modulation of agonist activated-SSTR2 receptor trafficking and response in GH-secreting pituitary tumors.

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.

Somatostatin receptor ligands in the treatment of acromegaly

First-generation somatostatin receptors ligands (SRL) are the mainstay in the medical treatment of acromegaly, however the percentage of patients controlled with these drugs significantly varies in the different studies. Many factors are involved in the resistance to SRL. In this review, we update the physiology of somatostatin and its receptors (sst), the use of SRL in the treatment of acromegaly and the factors involved in the response to these drugs. The SRL act through interaction with the sst, which up to now have been characterized as five subtypes. The first-generation SRL, octreotide and lanreotide, are considered sst2 specific and have biochemical response rates varying from 20 to 70%. Tumor volume reduction can be found in 36-75% of patients. Several factors may determine the response to these drugs, such as sst, AIP, E-cadherin, ZAC1, filamin A and β-arrestin expression in the somatotropinomas. In patients resistant to first-generation SRL, alternative medical treatment options include: SRL high dose regimens, SRL in combination with cabergoline or pegvisomant, or the use of pasireotide. Pasireotide is a next-generation SRL with a broader pattern of interaction with sst. In the light of the recent increase of treatment options in acromegaly and the deeper knowledge of the determinants of response to the current first-line therapy, a shift from a trial-and-error treatment to a personalized one could be possible.

Expression of Somatostatin Receptor 2 in Somatotropinoma Correlated with the Short-Term Efficacy of Somatostatin Analogues

The expression of somatostatin receptor subtypes (SSTRs) in pituitary growth hormone- (GH-) secreting adenomas may predict the response to somatostatin analogues (SSA). Our aim was to evaluate the value of the immunohistochemical (IHC) scores of 2 subtypes, SSTR2 and SSTR5, in predicting the short-term efficacy of SSA therapy in patients with active acromegaly. Ninety-three newly diagnosed acromegalic patients were included in our study. These patients were categorized into either a SSA-pretreated group (SA, n = 63) or a direct-surgery group (DS, n = 30), depending on whether or not presurgical SSA treatment was received. IHC analysis, using a 12-grade scoring system, with rabbit monoclonal antibodies against SSTR2 and SSTR5, was performed on all adenoma tissues. The reduction of GH, IGF-1, and tumor size after treatment with SSA for 3 months was measured. Compared with that in the DS group, SSTR2 expression was lower in the SA group. Additionally, in the SA group, SSTR2 expression was positively correlated with the reduction of IGF-1 and tumor volume. However, there was no correlation between the SSTR5 score and the efficacy of SSA. In conclusion, the protein expression of SSTR2, but not of SSTR5, is a valuable indicator in predicting biochemical and tumor size response to short-term SSA treatment in acromegalic patients.