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Review

Somatostatin analogs: does pharmacology impact antitumor efficacy? Mounira Chalabi1,2, Camille Duluc1,2, Philippe Caron1,2,3, Delphine Vezzosi1,2,3, Julie Guillermet-Guibert1,2, Ste´phane Pyronnet1,2, and Corinne Bousquet1,2 1

Institut National de la Sante´ et de la Recherche Me´dicale (INSERM), Unite´ Mixte de Recherche (UMR) 1037, Centre de Recherche en Cance´rologie de Toulouse (CRCT), Equipe labellise´e Ligue Contre le Cancer and Laboratoire d’Excellence Toulouse Cancer (TOUCAN), 31432 Toulouse, France 2 Universite´ Toulouse III Paul Sabatier, 31062 Toulouse, France 3 Service d’Endocrinologie et Maladies Me´taboliques, Poˆle Cardio-Vasculaire et Me´tabolique, Centre Hospitalier Universitaire (CHU) Larrey, 31059 Toulouse, France

Somatostatin is an endogenous inhibitor of secretion and cell proliferation. These features render somatostatin a logical candidate for the management of neuroendocrine tumors that express somatostatin receptors. Synthetic somatostatin analogs (SSAs) have longer half-lives than somatostatin, but have similar activities, and are used for the treatment of these types of disorders. Interest has focused on novel multireceptor analogs with broader affinity to several of the five somatostatin receptors, thereby presenting putatively higher antitumor activities. Recent evidence indicates that SSAs cannot be considered mimics of native somatostatin in regulating signaling pathways downstream of receptors. Here we review this knowledge, discuss the concept of biased agonism, and highlight what considerations need to be taken into account for the optimal clinical use of SSAs. Somatostatin, somatostatin receptors, and somatostatin analogs Discovered in 1973, somatostatin is a native inhibitory peptide hormone distributed throughout the central nervous system and peripheral tissues [1]. Somatostatin inhibits endocrine and exocrine secretions, gastric and intestinal motility, and gallbladder contraction. It also inhibits tumor growth through the inhibition of cell proliferation and angiogenesis, and by the induction of apoptosis [2,3]. Synthesized as a large precursor peptide, somatostatin undergoes tissuespecific degradation to produce two mature forms of 14 and 28 amino acids (somatostatin 14 and somatostatin 28), respectively [2]. Five G protein-coupled receptor (GPCR) Corresponding author: Bousquet, C. ([email protected]). Keywords: somatostatin analogs; antitumor efficacy; pharmacotherapy; biased agonism; neuroendocrine tumors. 1043-2760/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tem.2013.11.003

subtypes of somatostatin receptors (ssts) subtypes have been identified (sst1–5) that bind to native somatostatin. The hormone activates receptors located primarily in the central and peripheral nervous systems, gastrointestinal (GI) tract, and endocrine organs. Upon activation, each receptor recruits specific G proteins, enzymes, and/or scaffold proteins, resulting in the activation or inhibition of several transduction pathways specific to each receptor subtype and to the cell type in which the receptor is expressed [1,3]. All five ssts are expressed in a variety of normal human tissues, although sst2 and sst5 are the predominant subtypes found in endocrine organs (including the pancreas, pituitary, thyroid, and parathyroid) [4]. In rodents Glossary Biased agonism: also known as functionally selective agonism, the term describes the concept that different agonists, acting through a single sst, can selectively activate different signaling pathways. It is theorized that different agonists exhibit different affinities for a receptor and that they stabilize different active receptor conformations; the latter determines the affinity of the sst for a variety of intracellular proteins that mediate signaling, growth regulation, and receptor internalization, recycling, or degradation. Thus, based on pharmacologic properties, the relative potency, therapeutic efficacy, or both, of two different agonists may differ in any given cell type, despite triggering a single sst. Gastroenteropancreatic (GEP) NET: NET arising from the GI tract. Most GEP NET express multiple sst subtypes, with sst2 being the most prominent. Neuroendocrine tumors (NET): originate from neuroendocrine cells in neuronal and endocrine tissue throughout the body. Octreotide and lanreotide: synthetic somatostatin analogs (SSAs) with longer half-lives than native somatostatin. These analogs primarily target sst2, and bind to sst5 with lower affinity. SSAs improve the symptoms of severe diarrhea and flushing associated with carcinoid syndrome. SSAs are being investigated for their antitumor activity. Octreotide and Lanreotide are SSAs with documented Phase III trial data, substantiating their antitumor effect in the treatment of patients with NET (PROMID and CLARINET, respectively). Pasireotide: a multireceptor analog with high affinity for sst1–3 and sst5. Somatostatin: a native, mostly inhibitory, peptide hormone that inhibits both endocrine and exocrine secretions, gastric and intestinal motility, and gallbladder contraction. It also inhibits tumor growth through the inhibition of cell proliferation and angiogenesis, as well as via the induction of apoptosis. The short plasma half-life of natural somatostatin has prohibited its therapeutic development. Somatostatin receptors (ssts): G protein-coupled receptors that bind to somatostatin. Somatostatin receptors are seven transmembrane receptors, and are expressed in a tissue-specific manner. Five subtypes, sst1 to sst5, have been identified to date.

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1

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Review there are two splice variants of sst2 (2a and 2b), whereas only subtype 2a is expressed in humans [4]. Most neuroendocrine neoplasms, including pituitary and gastroenteropancreatic (GEP) neuroendocrine tumors (NET) (see Glossary), express different sst isoforms [3]. The inhibitory actions of somatostatin on hormone secretion and tumor growth render it a logical candidate for treating patients with these disorders. However, the short plasma half-life of native somatostatin (i.e., less than 3 min) precludes its development as a therapeutic agent [2]. Synthetic derivatives of somatostatin that have longer halflives but similar actions have been developed. Three SSAs with both antisecretory and antitumor effects – octreotide, lanreotide, and pasireotide – are currently in clinical use and/or testing. Octreotide (Sandostatin1) has been available in both immediate-release and long-acting formulations (long-acting release – LAR) for injection since the mid-1980s and the mid-1990s, respectively [5]. Octreotide is approved to manage symptoms in patients with metastatic carcinoid tumors (diarrhea and flushing), diarrhea associated with vasoactive intestinal peptide tumors, glucagonomas, gastrinomas (Zollinger–Ellison syndrome), insulinomas, GRFomas (growth hormone-releasing factor-secreting tumor), and acromegaly [6]. Lanreotide (Somatuline1) is approved to manage symptoms associated with NET (in Europe only) and for the long-term management of acromegaly [7]. The sustained-release formulation of lanreotide became available in the mid-1990s, followed by the Autogel1 formulation in the early to mid2000s [5]. Recently, pasireotide (Signifor1) was approved in the EU and the USA (December 2012) as first-line treatment for patients with Cushing’s disease [8]. Octreotide and lanreotide primarily target sst2 and bind sst5 with lower affinity; by contrast, pasireotide is a multireceptor analog with high affinity for sst1–3 and sst5 [5]. In addition to their approved use for indications based on their antisecretory effects, these SSAs are also being investigated for their antitumor activity in patients with acromegaly [growth hormone (GH)-secreting pituitary adenomas], NET, and polycystic liver disease, as well as in patients with cancers of the thyroid, prostate, breast, ovary, GI tract and other solid tumors [3,5,9,10]. Recent evidence indicates that SSAs cannot be considered simple mimetics of native somatostatin in regulating signaling pathways downstream of sst because of the functional selectivity of each of these analogs (biased agonism). Differences in mechanism of action among SSAs must be taken into account to optimize their effective use in the clinic. Based on which ssts are expressed in a specific tumor, and the pharmacology of each SSA binding to a specific sst, personalized pharmacotherapies using SSAs will become available in the future. The choice of a personalized SSA therapy will depend on the answers to the following: (i) sst expression in a specific tumor type; (ii) detailed knowledge regarding the intracellular signal transduction pathway for each sst; (iii) the receptor subtype interactions (i.e., dimerization, internalization, constitutive activity) utilized in modulating signaling; (iv) knowledge of sst monomer/homodimer/heterodimers differential effects on SSA signaling and how this may translate to improved patient treatment and outcomes. 2

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Although we are, as yet, unable to answer these questions fully, we review herein the current state of knowledge regarding mechanisms of action for the antitumor role of SSAs with the goal of attaining a clearer understanding of their clinical efficacy, focused primarily on neuroendocrine neoplasms. Expression of ssts on tumor cells Receptor expression patterns are related directly to the choice of SSA therapy. Different NET types express varying sst subtypes. Heterogeneous receptor subtype expression is not well understood, although it appears to depend more on individual characteristics of the tumor than on universal features of NET [11]. Further studies are required to define sst profiles for specific NET subtypes and the factors that may regulate their expression. Tumors from somatostatin-target tissues with a high density of ssts include pituitary adenomas, GEP NET, paragangliomas, pheochromocytomas, carcinoids, smallcell lung cancers, medullary thyroid carcinomas, malignant lymphoma, and breast cancers [12,13]. Pituitary tumors are generally benign, slow-growing tumors that display different patterns of sst expression, based on the hormone-secreting cells from which they originate. GHsecreting pituitary adenomas predominantly express sst2 and, to a lesser extent, sst5. Adrenocorticotropic hormone (ACTH)-secreting adenomas predominantly coexpress sst5 and sst2, and prolactinomas predominantly express sst1 and sst5. In clinically nonfunctioning pituitary adenomas, sst3 is highly expressed, followed by sst2 and then sst5. In thyroid-stimulating hormone (TSH)-secreting tumors, sst2 is mainly coexpressed with sst3 and sst5. More than three-quarters of GEP NET express multiple sst subtypes, although sst2 generally predominates, followed by sst5; in fact, simultaneous expression of these two subtypes is associated with better prognosis, and only weak expression of subtypes sst1, sst3, and sst4 is observed in these tumors. Considerable variation exists, however, in sst subtype expression between different tumor types and among tumors of the same type [1,11,14]. Somatostatin receptors are expressed to varying degrees in solid organ tumors. sst1 dominates in prostate cancer tissue, although all five receptors are present. Other cancers that express all five receptors include breast (primarily sst1–3), thyroid, melanoma, and GI tumor tissue. sst1–3 and sst5 are expressed in ovarian tissue (both benign and malignant). sst5 is the predominant receptor expressed in hepatocellular carcinoma, although sst1–3 are also present [5,10]. Some studies have demonstrated that octreotide and, to a lesser extent, lanreotide are associated with positive outcomes in patients with solid tumors in which sst2 and/or sst3 predominate, such as prostate and gastric cancers [5]. Recently, studies have successfully characterized the expression of sst2, sst3, and sst5 in human normal and neoplastic tissues using rabbit monoclonal antibodies (UMB-1, UMB-4, and UMB-5). Investigators have concluded that use of these monoclonal antibodies may be of value in the assessment of receptor subtype status in NET during routine histopathologic examination [13,15,16], and this could represent a first step for personalized

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pharmacotherapy using SSAs, at least in patients who have undergone tumor biopsy or surgical resection. Molecular biology of antisecretory and antitumor activities of somatostatin and SSAs Somatostatin and SSAs have antisecretory and antitumor effects, and these have recently been dissociated in an attempt to understand better the molecular and therapeutic actions of these agents. The antisecretory effects result in reduced secretion of many hormones and growth factors [17,18], making somatostatin and SSAs suitable candidates for the treatment of patients with acromegaly and NET [3] (Box 1). Somatostatin and its analogs also exert antitumor activity through direct and indirect mechanisms, acting through sst1–5, which are found on tumor cells and cells in the tumor microenvironment (Figure 1; Box 2). Direct SSA actions at ssts on tumor cells significantly impact tumor cell biology. ssts trigger a variety of pertussis toxin-sensitive, GPCR intracellular signals, with each receptor subtype being coupled to several signal transduction pathways [19] (Figure 2). SSA-induced activation of intracellular signaling pathways may also involve regulatory effects that are independent of G proteins [20]. ssts regulate specific signaling pathways activated by growth or angiogenic factors either through tyrosine dephosphorylation of tyrosine kinase receptors expressed by tumor cells, or via inhibition of intracellular effectors and downstream substrates [3]. Indirect antitumor effects are exerted through inhibition of hormone, growth factor, and angiogenic factor secretion by tumor cells that are involved in autocrine and paracrine stimulatory pathways [3], or by blockade of ssts expressed on endothelial and monocytic cells. Coexpression of different subtypes of somatostatin receptors also affects their pharmacology and functional properties, especially in NET, which often coexpress high levels of different ssts. Signal transduction via ssts is significantly altered by receptor homo- and/or heterodimerization, and each sst displays a specific pattern of dimerization, which is ligand-regulated (Table 1). Dimerization of ssts has been demonstrated to occur early after receptor synthesis and appears to be a prerequisite for correct targeting and specific subcellular localization. sst heterodimerization may represent a mechanism by which

Box 1. Mechanism of the antisecretory effect of somatostatin and analogs The antisecretory effects of somatostatin and its analogs are mediated through all five ssts and result from the inhibition of voltage-dependent calcium channels and the stimulation of potassium channels in a G protein-dependent manner. Ssts induce the lowering of intracellular calcium concentrations and the inhibition of adenylyl cyclase, leading ultimately to decreased cAMP-levels. Hormones whose expression is inhibited by somatostatin and SSAs include pituitary GH, thyroid-stimulating hormone (TSH), ACTH, insulin, glucagon, gastrin, secretin, cholecystokinin (CCK), and vasoactive intestinal polypeptide (VIP). Secretion of growth factors such as vascular endothelial growth factor (VEGF), IGF-1, epidermal growth factor (EGF), and fibroblast growth factor (FGF) is also regulated by SSAs.

Somatostan analogs

Binding on the somatostan receptor of tumor cells

Inhibion of growth factor signaling

Inhibion of cell cycle

Proapoptoc effect

Inhibion of growth factor effects

Indirect antumor effect

Inhibion of angiogenesis

Modulaon of immune system

Inhibion of the release of growth factor and trophic hormones TRENDS in Endocrinology & Metabolism

Figure 1. Antitumor effects of somatostatin analogs. Somatostatin analogs exert antitumor activity through both direct and indirect mechanisms, acting through sst1–5, which are found on tumor cells and on cells in the tumor microenvironment.

the cell generates a new receptor that mediates distinct cellular functions. Dimerization affects ligand binding, receptor expression (trafficking and desensitization), and signal transduction. For example, coexpression of sst5 with sst2 has been shown to reduce sst2 internalization and desensitization [21]. sst5 may facilitate sst2 recycling, thereby affecting long-term responsiveness to octreotide therapy for tumors, such as NET and pituitary tumors, which coexpress both receptors. The notion that heterodimerization of these receptors influences the response of NET to SSAs has been reinforced by the recent finding that expression in pituitary tumors of a naturally occurring sst5-truncated variant, which is able to heterodimerize with sst2 but which reduces its access to the plasma membrane, is negatively correlated with the pituitary tumor response to SSA treatment [22,23]. Importantly, in addition to heterodimerization with other ssts, somatostatin receptor subtypes have been shown to form heterodimers with other GPCRs such as the type 2 dopamine receptor and the m-opioid 1 receptor. Distinct pharmacologic and functional properties are conferred when GPCR for such diverse ligands form heterodimers [24]. Further characterization of sst homodimers and heterodimers may result in a clear understanding of the relationships between the molecular biology, receptor specificity, and therapeutic efficacy of different SSAs [25]. Another consideration is that high expression of ssts on tumor cells may facilitate receptor homodimerization or heterodimerization in specific constitutively activated conformations, rendering the cells insensitive to a specific SSA. Constitutive receptor activation has been observed in cells with high expression of pituitary sst, and it has been postulated that this receptor conformation may play a role in the maintenance of cell secretory function and growth homeostasis [26]. Exogenous expression of sst2 3

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Review Box 2. Mechanisms of antitumor effect of somatostatin and analogs Direct antitumor effects of somatostatin and its analogs are initiated by induction of cell cycle arrest and apoptosis, and by inhibition of cell migration and invasion. All five receptors induce cell cycle arrest through the activation of protein tyrosine phosphatases (PTPs), leading to the regulation of downstream signaling pathways, including ERK1/2, phosphoinositide 3-kinase (PI3K), AKT, nitric oxide synthase (NOS), and cGMP-dependent protein kinase, with subsequent induction of a cyclin-dependent kinase inhibitor [19]. Only sst2 and sst3 are responsible for stimulating apoptosis in normal and tumor cells through regulation of the two main signaling pathways: the extrinsic pathway (triggered by death receptors) and the intrinsic (mitochondrial) pathway [62–65]. These mechanisms are protein 53 (p53)-dependent (sst3) or -independent (sst2) [3,19]. sst1 and sst2 suppress cell migration/invasion through inhibition of PI3K and/or mitogen-activated protein kinase (MAPK) downstream of platelet-derived growth factor (PDGF) or activation of the integrin receptor [66–69]. A novel G protein-independent mechanism has been described for the inhibition of PI3K activity by sst2 [27,70]. It results in negative regulation of protein translation/ synthesis through inactivation of mechanistic target of rapamycin (mTOR) and in subsequent dephosphorylation and activation of eIF4E-binding protein-1 (4E-BP1), a negative regulator of protein translation [3,71]. Another mechanism important to the antiproliferative effects of SSAs involves restoration of functional gap junctions. These junctions are essential for maintaining the differentiated state and restoring cell contact inhibition [72]. Indirect antitumor effects of SSAs result from suppression of the secretion of growth or angiogenic factors. sst2 and sst5 are primarily involved in central suppression of the GH/IGF-1 axis and inhibition of SSA-mediated pituitary GH release. SSAs also peripherally suppress hepatic GH-induced IGF-1 production through activation of a tyrosine phosphatase, dephosphorylation of signal transducer and activator of transcription 5b (STAT5b), and, ultimately, a decrease in IGF-1 gene transcription mediated through sst2 and/or sst3 receptor subtypes [3,73]. Angioinhibitory action of sst2 in tumors (such as pancreatic cancer) involves the upregulation of the expression of thrombospondin-1 (TSP-1), a potent antiangiogenic factor. TSP-1 inactivates the angiogenic effects of VEGF and therefore plays a crucial role in sst2 tumor-suppressive activity on pancreatic tumor growth [18]. Finally, tumor growth and metastatic development are indirectly inhibited through suppression of the proliferation and migration of endothelial cells and monocytes (which also secrete proangiogenic factors) via activation of sst on these cells. Receptor subtypes sst2, sst3, and sst5, expressed on endothelial cells, mediate these effects by inhibiting the activity of endothelial NOS and ERK1/2 [74,75].

in cells that do not express it has also been described to confer constitutive activity to this receptor that is dependent on the ability of the re-expressed sst2 to induce the expression and secretion of its own ligand [27–31]. Differentiating for NET: molecular basis for differing actions of available SSAs Choosing between available SSAs in the NET clinical treatment setting necessitates a thorough understanding of the different actions of SSAs and of how those actions may relate to therapeutic efficacy. Furthermore, individual SSAs may display different antitumor activity despite similarities in efficacy for antisecretory activities. Several molecular factors may assist in explaining the variable efficacy of SSAs in NET treatment. As discussed above, one explanation is that the therapeutic efficacy of different SSAs is dependent on the particular sst affinity for the SSA as well as on the distribution of sst subtypes in the tumor tissue and 4

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surrounding cells within the tumor microenvironment. Additionally, therapeutic efficacy may also depend on the ability of individual SSAs to stabilize particular receptor conformations that then can induce the activation of specific signaling pathways. The terms ‘biased agonism’ and ‘functionally selective agonism’ have been coined to describe the concept that different agonists can selectively activate different signaling pathways through a single sst (Box 3) [32–34]. In this model it is theorized that different agonists exhibit different affinities for a receptor and stabilize different active receptor conformations; the latter determines the affinity of the sst for a variety of intracellular proteins that mediate signaling and growth regulation as well as receptor internalization, recycling, and degradation. Thus, based on pharmacologic properties, the relative potency or therapeutic efficacy, or both, of two agonists may differ in any given tumor cell type, despite binding the same sst subtype(s) [34]. After one year of treatment with octreotide or lanreotide, one-third of NET patients show a loss of response that may be attributed to sst2 internalization and desensitization after agonist-mediated phosphorylation [35,36]. Such tachyphylaxis is not observed for patients with acromegaly treated long-term with these SSAs [37–39], even though both types of tumors (NET and GH-secreting pituitary adenomas) coexpress sst2 and sst5. Furthermore, pasireotide fails to trigger sst2 internalization and induces fast recycling to the plasma membrane, and this may counteract desensitization and result in a longer-lasting response [40,41]. How are these observations reconciled at a molecular level? In contrast to sst3, which predominantly undergoes downregulation, and to sst1, which does not internalize, almost all activated sst2 and sst5 enter the recycling pathway [35–39,41,42] (Table 1). However, major differences in sst2 and sst5 recycling are observed. They result from differences in the phosphorylation/dephosphorylation status at specific serine (Ser) and threonine (Thr) residues within the sst2 and sst5 C-terminal tail, determining the stability of sst complexes with b-arrestin. Although sst2 forms a stable complex with b-arrestin, resulting in internalization and slow recycling to the plasma membrane, sst5 is rapidly recycled and resensitized after somatostatin-14 activation owing to its rapid dephosphorylation, a prerequisite for b-arrestin detachment [43]. Because octreotide and pasireotide differentially affect phosphorylation at these specific residues on sst2 or sst5, they exert biased agonism at the same receptor. In heterologous systems, sst2 or sst5 have been separately introduced into cells in vitro, but data regarding the differential effects of octreotide and pasireotide on the sst2/ sst5 heterodimer are not yet available. At least six residues in the C terminus of the sst2 receptor are phosphorylated shortly after ligand binding (native somatostatin-14), including two Ser and four Thr residues. Although octreotide facilitates both human and rat sst2 phosphorylation at all six residues and rapid sst2 internalization, pasireotide phosphorylates only two Ser residues and induces partial internalization of the human, but not the rat, sst2. This observation suggests species differences in biased agonism at sst2 and highlights a specific role of Ser phosphorylation in sst2 internalization [44–46]. Agonist-induced

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SSAs: Antumor effects in NET

SSAs

IGF-1

K+

sst1−5

IGF-1R Ca2+ Signaling cAMP cGMP

Pl3K

PTP Caspase 8 p53 Bax

Apoptosis

MAPK NF-κB

Secreon

mTOR

Ligands

4E-BP1

Survival Protein synthesis Angiogenesis

Growth and proliferaon

Growth and proliferaon

Transcripon

TRENDS in Endocrinology & Metabolism

Figure 2. Mechanisms of antitumor activity of synthetic somatostatin agonists (SSAs) in neuroendocrine tumors (NET). SSAs bind to somatostatin receptors (ssts) on the cell surface, leading to reduced hormone secretion by inhibiting cyclic adenosine monophosphate and intracellular calcium (sst1–sst5), to increased apoptosis (sst2–sst3) and/or to reduced cell proliferation (sst1–sst5) by activating PTPs (SHP-1, SHP-2, PTP-h) and subsequent regulation of different intracellular second messengers and pathways including cGMP (sst2 and sst5), MAPK (sst1, sst2, sst4, sst5) and/or PI3K–mTOR or PI3K–NF-kB (sst2). It results in both reduced transcription and/or translation, inhibiting the expression of genes involved in cell survival (apoptosis is subsequently increased), in cell proliferation and tumor growth, and/or in angiogenesis. Activation of PTPs also results in inhibition of IGF-1 receptor signaling (sst1–sst5) (and probably also of other receptors such as insulin receptor) through post-translational regulations involving dephosphorylation of receptors and/or their intracellular effectors. Abbreviations: IGF, insulin-like growth factor; IGF-1R, IGF receptor 1; MAPK, mitogen-activated protein kinase; mTOR, mechanistic target of rapamycin; NET, neuroendocrine tumor; NF-kB, nuclear factor-kB; PI3K, phosphoinositide 3-kinase; PTP, protein tyrosine phosphatase; SHP, src homology domain-containing phosphatase.

phosphorylation of the four Thr residues on sst2 (with somatostatin-14 or octreotide) allows recruitment of barrestin and subsequent activation of extracellular regulated kinases 1/2 (ERK1/2) [47]. This may explain why pasireotide couples only with Gi-dependent but not barrestin-dependent signals, and does not regulate ERK or calcium, in contrast to octreotide and native somatostatin, which are similar in terms of downstream signaling pathways [20]. Whereas octreotide is a potent agonist at sst2, pasireotide is only a partial agonist at sst2 but is a full agonist at sst3 and sst5 (and presumably sst1) [48]. High potency and efficacy of a synthetic ligand to all known ssts (except sst4), such as pasireotide, may not reproduce the effects of native somatostatin. Efficacy of octreotide and pasireotide to activate each sst heterodimer needs to be investigated to predict accurately which SSAs may work best in tumors which have a specific sst expression profile. Based on previous in vitro preclinical studies of different SSAs, possible recommendations are presented in Box 4. Current clinical development of SSAs in NET Recommendations for which SSA to use in which tumors from in vitro data should be carefully interpreted in context with results from clinical trials. Based on clinical trial data and on the expression/pharmacology/function of sst

subtypes discussed previously herein, one can speculate about which alternative SSA should be used when faced with therapeutic pitfalls encountered using standard FDAapproved SSA treatment (Table 2). In GH-secreting pituitary adenomas, octreotide and lanreotide act primarily on sst2 and are slightly less effectively on sst5, inhibit GH secretion, normalize insulin-like growth factor 1 (IGF-1) serum levels, and may reduce tumor sizes in some cases [9,49]. Transphenoidal surgery is the treatment of choice for intrasellar microadenomas and non-invasive macroadenomas, or for when the GH-secreting pituitary tumors are causing compression symptoms. By contrast, primary therapy with somatostatin analogs can be recommended for patients with macroadenomas when a surgical cure is unlikely or if surgery is refused or contraindicated (37). Control of tumor growth in nearly all patients receiving SSAs, with tumor shrinkage in about 50–70% of patients, is an important consideration in selecting an SSA as a medical therapy for acromegaly caused by GH-secreting macroadenomas, especially for patients receiving an SSA as the primary pharmacotherapy [9]. Functional interactions between sst2 and sst5 in these tumors provide the rationale for studies using pasireotide (displaying a binding affinity sst5 > sst2). A 16 week Phase II trial has shown that subcutaneous 5

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Table 1. sst dimerization on ligand binding and function of receptors – recommended SSAs Monomer/dimer Effect of agonista,b,c,d,e Monomer Basal and induced upon agonist binding Homodimer Dissociation in monomer induced upon agonist binding Homodimer Dissociation in monomer induced upon agonist binding Homodimer Increased dimerization induced upon agonist binding Monomer Dimerization induced upon agonist binding Increased dimerization induced upon sst1/sst2 bispecific agonist

Internalization upon ligand-binding f No

Signaling f # cAMP

# Proliferation

[76,77]

Yes Recycling

# cAMP

# Proliferation " Apoptosis

[78]

Octreotide Lanreotide

Yes Degradation

# cAMP

# Proliferation " Apoptosis

[79,80]

Pasireotide

Yes Recycling

# cAMP

# Proliferation

[81]

Somatostatin-14

Yes Recycling

# cAMP

# Proliferation

[82,83]

Pasireotide

ND g

ND g

[84]

Pasireotide > octreotide or lanreotide

sst1/sst5

Induced by sst5 selective agonist

"/sst1 monomer #/sst5 homodimer

[78,82]

Pasireotide

sst2/sst3

Basal Decreased dimerization Induced upon agonist binding

#/sst2 homodimer with sst2 agonist No effect of sst3 agonist

Octreotide or lanreotide

Basal Increased dimerization induced upon sst2- (but not sst5)-selective agonistbinding Induced by somatostatin14 and by sst4- (but not sst5)-specific agonists

#/sst2 homodimer with sst2 agonist and sst2–sst5 bi-specific agonist

" ERK sst2 homodimer < sst2–sst3 heterodimer with somatostatin-14 or sst2 specific agonist (but not sst3 agonist) # Proliferation sst2 < sst2–sst5 bi-specific agonist

[80]

sst2/sst5

# cAMP sst5 homodimer < sst1–sst5 heterodimer with somatostatin-14 or sst2–sst5 bi-specific agonist (but not sst1 agonist) # cAMP sst2 homodimer < sst2–sst3 heterodimer with SST14 or sst2 specific agonist (but not sst3 agonist) # cAMP sst2 < sst2–sst5 bi-specific agonist

# Proliferation sst1 agonist > sst1–sst2 bi-specific agonist ND g

[21,76,84,85]

Octreotide or lanreotide if sst2 >> sst5 and pasireotide if sst5 >> sst2 Somatostatin-14 or pasireotide if sst5 >> sst4

sst sst1

sst2

sst3

sst4

sst5

sst1/sst2

sst4/sst5

Yes Recycling

Refs

# cAMP sst4 homodimer < sst4–sst5 heterodimer

ND g

[81]

Preferred SSA to use Pasireotide

a

An sst1-specific agonist is able to activate sst1 monomers only, whereas bi-specific agonists induces sst1/sst2 heterodimers (sst1–sst2 bi-specific agonist) or sst1/sst5 heterodimers (sst2–sst5 bi-specific agonist). When sst1 is coexpressed with sst2, an sst1-specific agonist more efficiently inhibits proliferation than a bi-specific sst1–sst2 agonist, suggesting that sst1 monomers more efficiently transduce this effect than sst1/sst2 heterodimers. When sst1 is coexpressed with sst5, it induces its internalization, decreasing sst1 stability at the membrane.

b

An sst2-specific agonist is able to activate sst2 homodimers, sst1/sst2 heterodimers, sst2/sst3 heterodimers, and sst2/sst5 heterodimers. When sst2 is coexpressed with sst3, and/or with sst5, it is stabilized at the membrane (the internalization of the heterodimer is decreased as compared to the sst2 homodimers), probably explaining why sst2/sst3 heterodimers are more efficient in transducing inhibition of cAMP or of cell proliferation than sst2 homodimers, and why sst2–sst5 bi-specific agonists are more efficient than sst2 agonists.

c

An sst3-specific agonist is able to activate sst3 homodimers but not sst2/sst3 heterodimers.

d

An sst4-specific agonist is able to activate sst4 homodimer and sst4/sst5 heterodimers.

e

An sst5-specific agonist is able to activate dimerization of sst5 monomers into homodimers and sst2/sst5 heterodimers, but not sst4/sst5 heterodimers.

f

Symbols: ", increase; #, decrease.

g

Not determined.

pasireotide may be an effective medical therapy for acromegaly [50]. An extension to this trial assessing the longterm efficacy and safety of pasireotide (up to 24 months) has demonstrated that pasireotide has the potential to be an effective, long-term medical treatment for patients with acromegaly, producing sustained biochemical control and significant reductions in tumor volume, even for some patients whose tumor volume is not normalized during 6

the shorter study [51]. Results from a large, randomized, double-blind Phase III study which compared octreotide and pasireotide (LAR forms) in patients with acromegaly have been recently presented [52]. Significantly more patients with acromegaly were biochemically cured after 12 months treatment with pasireotide than with octreotide (31.3% vs 19.2%, P = 0.007). Data from the 6 month extension study in which patients who did not achieve full biochemical control

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Review Box 3. Mechanism of sst signaling via GPCR proteins Agonist-bound receptors initiate signaling by activating heterotrimeric G proteins at the plasma membrane, and then rapidly undergo phosphorylation by GPCR kinases (GRKs) that selectively phosphorylate agonist-activated receptors. Specifically, following activation by octreotide, but not by pasireotide, GRK2 and GRK3 phosphorylate the rat sst2 (2a) [46]. Phosphorylation of the receptor, and subsequent binding of b-arrestin, prevents subsequent interaction of receptors with G proteins, terminating the G proteinmediated signal. Conversely, arrestins represent scaffold platforms for binding with intracellular proteins, resulting in the activation of specific signal pathways independent of G proteins. Arrestindependent activation of ERK has been observed following sst2 activation by octreotide, but not by pasireotide [44,47]. Dephosphorylation of sst2 by a protein phosphatase 1b is necessary to stop arrestin-mediated ERK activation through sst2 [47]. Arrestins can also bind to the coat structure of clathrin-coated pits, thereby promoting endocytosis. Whereas sst1 becomes phosphorylated and desensitized, it does not internalize or eventually is subjected to fast internalization/recycling to the plasma membrane [24]. Endocytic sorting plays a crucial role in dictating the receptor signaling response after the initial ‘desensitization’ event. Internalized receptors are sorted either to lysosomal degradation, a major mechanism by which many GPCRs are downregulated after ligand-induced activation, or to recycling pathways that allow more or less rapid GPCR resensitization [86]. Whereas sst3 is mostly directed to a lysosomal/proteasomal degradation after ubiquitination, sst2 and sst5 are mostly recycled to the plasma membrane at a moderate or rapid recycling rate, respectively. Accordingly, the lack of a cluster of phosphorylation sites in the C terminus of sst5 seems to be responsible for the low stability of the interaction between sst5 and b-arrestins, favoring rapid recycling [24,35]. Alternatively, some GPCRs continue to signal, or to initiate new signaling pathways, from the endosomal membrane, but this has not yet been described for ssts. In addition, GPCR sorting may differ after short-term or prolonged/repeated activation by agonists. Such plasticity in receptor regulation is thought to be physiologically important and may contribute to the loss of drug potency or effectiveness (i.e., tachyphylaxis) observed over time in clinical settings such as longterm treatment of NET patients with somatostatin analogs. Such tachyphylaxis may vary depending on which homo-/heterodimers of sst are expressed and on the SSA used. On sst2, tachyphylaxis may be more potently induced by octreotide than by pasireotide [40,41]. Coexpression of sst5 with sst2 may reduce such tachyphylaxis [21].

were offered the other treatment showed that 21% of the 81 patients who switched to pasireotide achieved biochemical control of their disease compared with 2.6% of 38 patients who switched to octreotide. It is of interest that the control of GH levels with pasireotide was similar to that achieved with octreotide, except that IGF-1 levels were significantly lower with pasireotide. It is therefore possible that the differential effects of the analogs are more dependent on pasireotide selectively inhibiting insulin release, which is a codeterminant with GH of IGF-1 levels. These results indicate that pasireotide offers a promising therapeutic option for patients with acromegaly who are resistant or refractory to currently available somatostatin analogs. Based on pharmacological in vitro studies, pasireotide should be more effective than octreotide, and thus should be recommended for tumor types in which sst2 is poorly or not expressed and sst5 is highly expressed (Table 2). In addition, pasireotide should present longer-term efficacy than octreotide, considering that activation of sst5 in the sst2/sst5 heterodimer stabilizes sst2 expression at the plasma membrane.

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Box 4. Recommendations for which SSA to use according to sst expression pattern and ssts dimerization/ pharmacology Expression of sst1: in the absence of clinically approved SSAs specific for sst1, we suggest the use of an sst1 agonist when sst1 is expressed alone (pasireotide), an sst1 or sst2 agonist when sst1 and sst2 are coexpressed (prefer octreotide or lanreotide if sst2 >> sst1, and pasireotide if sst1 >> sst2), and an sst5 agonist when sst1 is expressed with sst5 (pasireotide). Expression of sst2: we suggest the use of an sst2 agonist when sst2 is expressed alone (octreotide or lanreotide), an sst1 or sst2 agonist when sst1 and sst2 are coexpressed (prefer octreotide or lanreotide if sst2 >> sst1 and pasireotide if sst1 >> sst2), an sst2 agonist when sst2 is expressed with sst3 (octreotide or lanreotide), and an sst2–sst5 bi-specific agonist when sst2 is coexpressed with sst5 (prefer octreotide or lanreotide if sst2 >> sst5 and pasireotide if sst5 >> sst2). Expression of sst3: we suggest the use of an sst3 agonist when sst3 is expressed alone (pasireotide), and an sst2 agonist when sst3 is coexpressed with sst2 (octreotide or lanreotide). Expression of sst4: in the absence of a clinically-approved SSA targeting sst4, somatostatin-14 is recommended when sst4 is expressed alone or if sst4 >> sst5, or pasireotide when sst5 >> sst4 to favor sst5 homodimers. Expression of sst5: we suggest using an sst5 agonist (pasireotide) when sst5 is expressed alone, or coexpressed with sst1, or coexpressed with sst2 (if sst5 >> sst2), or with sst4. Prefer using octreotide or lanreotide if sst2 >> sst5. In cases of coexpression of sst2 and sst5, where sst2  sst5, sst2– sst5 bi-specific agonists are recommended. Considering that (i) octreotide is a potent agonist for sst2 but a very poor agonist for sst5 [48]; (ii) pasireotide is a potent agonist for sst5 and partial agonist for sst2 [48]; (iii) activation of sst5 facilitates the stabilization of sst2 at the plasma membrane when present in the sst2/sst5 heterodimer (Table 1); (iv) pasireotide induces much less sst2 internalization than octreotide [40], one is tempted to recommend pasireotide rather that octreotide (and lanreotide), especially if sustained long-term efficacy of SSA treatment is necessary.

Because ACTH-secreting tumors markedly express sst5 and, to a lesser extent, sst1, sst2, and sst3, pasireotide, which has a higher affinity than octreotide or lanreotide for sst5, and which is a potent agonist of sst5 in contrast to octreotide [48], efficiently inhibited ACTH secretion from primary cultures of human ACTH-secreting pituitary adenomas [53] and reduced urinary free cortisol levels in patients with Cushing’s disease [54]. In this light, pasireotide was recently approved for the treatment of patients with ACTH-secreting pituitary adenomas [55]. Up to 50% of patients with GEP NET have advanced metastatic disease at the time of diagnosis, and the presence of metastasis is associated with a significantly worse prognosis [56]. Clearly, further advances in therapy for GEP NET are necessary for the optimization of patient outcomes. SSAs have previously been evaluated for the treatment of symptoms of carcinoid disease. Although several pituitary NET are non-secreting, many GI NET (also termed ‘carcinoid tumors’) are associated with a constellation of symptoms, including flushing, diarrhea, and wheezing, referred to collectively as the carcinoid syndrome. These symptoms are the result of tumor secretion of specific hormones or vasoactive peptides into the systemic circulation [57]. A quarter century of evidence indicates that SSAs (octreotide, lanreotide) attenuate the symptoms of severe diarrhea and flushing associated with carcinoid syndrome. Recently, a Phase II, open-label, 7

Disease (type of tumor)

Most patients sst isoforms expressed

Resistance to preferred SSA treatment Preferred SSA treatment

Preferred SSA treatment target receptor

Alternative treatments that should increase SSA efficacy

Homodimer

Heterodimer sst2/sst5 sst2/sst3

Sst isoforms expressed If sst5 > sst2,sst1, sst3

sst1, 2, 3, 5 (sst2 > sst5, sst1, sst3)

Octreotide-LAR Lanreotide-SR Lanreotide-Autogel

sst2/sst2

Pancreatic NET: gastrinoma Pancreatic NET: insulinoma

sst2

sst2/sst2

Pancreatic NET: glucagonoma

sst2

Acromegaly: GH-secretory pituitary tumors Cushing’s: ACTH-secretory pituitary tumors Non-functioning pituitary tumors

sst2 and sst5 (sst2 > sst5) or sst2 only sst5 and sst2 (sst5 > sst2)

Octreotide Lanreotide-SR Octreotide Octreotide-LAR Lanreotide-SR Lanreotide-Autogel Octreotide Octreotide-LAR Lanreotide-SR Lanreotide-Autogel Octreotide-LAR Lanreotide-SR Lanreotide-Autogel Pasireotide

Prolactinomas

sst1 and sst5

TSH-secreting adenomas

sst2, 3, 5

Octreotide-LAR Lanreotide-SR Lanreotide- Autogel Octreotide-LAR Lanreotide-SR Lanreotide Autogel Octreotide-LAR Lanreotide-SR Lanreotide-Autogel

sst2, 4, 5

sst3 > sst2 > sst5

a

sst2/sst2

sst2/sst5 sst4/sst5

If sst5 > sst2, sst4

Target receptor

Pasireotide a

sst5/sst5 sst2/sst5 sst1/sst2 sst1

Pasireotide a

sst5/sst5 sst2/sst5 sst4/sst5

sst2/sst2

sst2/sst2

sst2/sst5

If sst5 > sst2

Pasireotide a

sst5/sst5 sst2/sst5

sst5/sst5

sst2/sst5

If sst2 > sst5

sst2/sst2 sst2/sst5

sst2/sst2

sst2/sst3 sst2/sst5

If sst5 > sst2,sst3

Octreotide-LARa,b Lanreotide-SRa,b Lanreotide-Autogel a Pasireotide a

sst5/sst5

sst1/sst5

sst1 and sst5

Pasireotide a

sst2/sst2

sst2/sst3 sst2/sst5

If sst5 > sst2,sst3

Pasireotide a

sst5/sst5 sst2/sst5 sst3/sst3 sst1 sst1/sst5 sst5/sst5 sst2/sst5 sst5/sst5 sst3/sst3

Refs

Surgery, localized therapy for liver metastases (e.g., resection, radio-frequency ablation, hepatic artery chemo-embolization), systemic management with cytotoxic chemotherapy and biologic therapies (e.g., interferon-a, antiangiogenic drugs, mTOR inhibitors, multikinase inhibitors and peptide receptor radiotherapy)

[58,87–89]

Surgery, radiotherapy, pegvisomant, dopamine agonists Surgery, ketoconazole, mitotane, radiotherapy, dopamine agonists Surgery, radiotherapy, dopamine agonists

[93–96]

Dopamine agonists

[93,98,99]

Surgery, radiotherapy

[93,100]

[89,90] [91,92]

[89]

[55,93]

[93,94,97]

Suggestions of an alternative treatment if resistance to the preferred SSA treatment is observed are based on receptor expression pattern. The alternative types of treatment that can be performed are also mentioned.

b

LAR, long-acting release; SR, sustained release.

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Midgut tumors

SSA

Other types of treatment

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Review

8

Table 2. Expression pattern of sst receptor subtypes in NETs and description of which subtypes are targeted by the preferred SSA treatment

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Table 3. Prospective clinical trials of SSAs in the treatment of patients with NET NET type (number of patients)a,b

Treatment c

Duration of drug administration d

Tumor response, % of patients e

Metastatic carcinoid or pNET (n = 10) Metastatic midgut carcinoids (n = 22) Metastatic NET (n = 14) Progressive, advanced carcinoid or pNET (n = 34) Moderate or well-differentiated advanced carcinoid or pNET (n = 103) Progressive, advanced carcinoid or pNET (n = 58) Metastatic GEP NET or pNET (n = 10) Progressive, advanced carcinoid or pNET (n = 15) Advanced carcinoid or pNET (n = 16) Progressive, advanced carcinoid or pNET (n = 17) Gastrinoma (n = 13) Progressive, advanced carcinoid or pNET (n = 48) Advanced, well-differentiated NET (n = 31) Well-differentiated, progressive, advanced pNET (n = 21) Well-differentiated, nonfunctioning advanced pNET (n = 21) Nonfunctioning pNET or bronchial tract NET (n = 19) Well-differentiated midgut NET (n = 42, octreotide; n = 43, placebo) Metastatic NET (n = 13) GI NET (n = 18) Progressive GI NET (n = 31) Progressive, metastatic GEP NET (n = 30) GI NET (n = 39) Metastatic NET (n = 25) Metastatic NET (n = 11) Progressive GEP NET (n = 25) Well-differentiated NET (n = 28 for each)

Octreotide SC 100–250 mg qd 6–12 h

6–24 months

PR 20

SD 50

[101]

Octreotide SC 50 mg bid up to median of 200 mg 2–3 times/day Octreotide SC 500–2000 mg qd 8 h

1–30 months; median 12 months

28

36

[102]

ND

31

15

[103]

Octreotide SC 50 mg bid to 150–250 mg tid

1–47 months; median 29 months

0

50

[104]

Octreotide SC 200–500 mg tid

>6 months; follow-up to 36 months in some patients

0

48

[105]

Octreotide SC 500 (n = 23)–1000 (n = 35) tid

2–>32 months; median 5 months

3

47

[106]

Octreotide SC 500 mg qd

12 months

11

67

[107]

Octreotide LAR 20 mg qd 4 weeks

3–>12; median 7 months

7

40

[108]

Octreotide LAR 20 mg qd 28 days

6–15 months; mean 11 months

0

88

[109]

Octreotide SC 100 mg tid

1–48 months; median 7 months f

ND d

59

[110]

Octreotide SC 100–200 mg/12 h or octreotide LAR 20–30 mg/months Octreotide SC 200 mg tid

3–54 months

8

54

[90]

3 months

2

46

[111]

Octreotide LAR 30 mg qd 28 days

1–49; median 18 months

6

52

[112]

Octreotide LAR 30 mg qd 28 days

6–60 months; median 18 months f

0

48

[113]

Octreotide LAR 20 mg qd 28 days [59]

>74 months; median 49.5 months

ND

38

[114]

Octreotide LAR 30 mg/months

6 months

11

26

[115]

Octreotide LAR 30 mg; months or placebo

6 months

2

67

[29]

Lanreotide 750–3000 mg qd 8 h

ND d

31

8

[103]

Lanreotide 30 mg IM qd 10 days

5–18 months; mean 12 months

0

78

[116]

Lanreotide ATG 30 mg qd 2 weeks, escalated to weekly in 27% of subjects Lanreotide 5 mg SC tid

6 months; mean 21 weeks

6

81

[117]

1–>12 months

4

46

[118]

Lanreotide 30 mg IM qd 1–140 days

12 months or until TP

5

49

[119]

Lanreotide 30 mg IM qd 2 weeks

2–>30 months; median 10 months 1–48 months; median 7 months f

8

40

[120]

0

55

[110]

12 months

4

28

[121]

18 weeks

Lanreotide ATG: 0 Lanreotide LAR: 4

Lanreotide ATG: 68 Lanreotide LAR: 64

[122]

Lanreotide 30 mg IM qd 2 weeks; could be increased to qd 10 days Lanreotide 1 mg SC tid Lanreotide LAR 60 mg qd 3 weeks or Lanreotide ATG 120 mg qd 6 weeks (1:1)

Refs

9

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Table 3 (Continued ) NET type (number of patients)a,b

Treatment c

Duration of drug administration d

Well-differentiated, progressive, advanced pNET (n = 10) Advanced and/or metastatic, well-differentiated NET (n = 30) Metastatic GI NET (n = 100) (NCT00690430) g

Lanreotide SR 60 mg qd 28 days

6–60 months; median 18 months f

Lanreotide ATG 120 mg qd 28 days

Pasireotide LAR 60 mg IM or Octreotide LAR 40 mg IM

Tumor response, % of patients e

Refs

PR 0

SD 40

[113]

Until TP

4

89

[123]

–

–

–

a

GEP, gastroenteropancreatic; NET, neuroendocrine tumor; pNET, pancreatic neuroendocrine tumor.

b

n, the number of patients whose tumor response was monitored.

c

Dosage: bid, twice daily; qd, once daily; tid, three times daily. Route: IM, intramuscular; SC, subcutaneous. Formulation: ATG, autogel; LAR, long-acting release.

d

Abbreviations: ND, not disclosed; TP, tumor progression.

e

PR, partial response; SD, stable disease.

f

Median duration of treatment includes all drug regimens.

g

Trial halted.

multicenter study of pasireotide in patients with advanced NET, whose symptoms of carcinoid syndrome were inadequately controlled by octreotide, demonstrated that pasireotide was effective in controlling those symptoms in 27% of refractory or resistant patients [58]. One can hypothesize that sst5 rather than sst2 was expressed in these octreotide-resistant carcinoids, or eventually sst1 and/or sst3. Before the availability of SSAs, the mortality rate associated with endocrine secretion syndromes in patients with NET was high. By contrast, today the highest mortality rate is associated with high tumor burden [59,60]. The prospective clinical trials of SSAs in the treatment of patients with NET are listed in Table 3. Octreotide was until very recently the only SSA with documented data substantiating its antitumor effect in the treatment of NET patients. Results from the randomized, double-blind, placebo-controlled, Phase III (PROMID) trial of octreotide (LAR form) provided evidence of the antitumor activity of octreotide in 85 patients with treatment-naive, welldifferentiated, functioning carcinoid syndrome or nonfunctioning advanced NET of the midgut or unknown primary location [29]. Octreotide (LAR form) 30 mg more than doubled the time to progression of disease compared with placebo (14.3 months vs 6.0 months; hazard ratio, 0.34; 95% CI, 0.20–0.59; P < 0.00007). Stable disease was observed in 66.7% and 37.2% of patients in the octreotide and control groups, respectively. Antitumor activity was similar in patients regardless of the presence or absence of carcinoid syndrome. In addition, patients treated with octreotide experienced greater control of flushing and diarrhea than those treated with placebo [29]. Patients included in PROMID had midgut tumors and limited liver tumor burden. Results of a large Phase III prospective trial (CLARINET) evaluating the antiproliferative effects of lanreotide recently confirmed the PROMID results. CLARINET enrolled 204 patients with non-functioning GEPNET that were well- or moderately differentiated and stable, including pancreatic and GI tumors, defined as having less than 10% of proliferation marker Ki67, but 33% of patients had a hepatic tumor load greater than 25%. Results from the CLARINET trial indicate that, at a 10

time-point of 2 years following initiation of treatment, median progression-free survival was not reached with lanreotide compared to 18 months with placebo (hazard ratio 0.47; 95% confidence interval 0.30, 0.73; P = 0.0002). Neither disease progression nor death occurred in 62% of lanreotide patients compared to 22% of placebo patients [61]. The long-term safety of lanreotide is under evaluation in an open-label extension study (NCT00842348). Concluding remarks and future perspectives SSAs provide significant clinical benefit for patients with excess pituitary GH or ACTH secretion or with symptoms of diarrhea or flushing associated with carcinoid disease. Although these agents may have similar broad efficacy in terms of managing secretory symptoms, their individual antitumor efficacy may differ. It is tempting to speculate that the multireceptor analog pasireotide may have enhanced antitumor efficacy on NET that express different sst subtypes (emerging questions and trends are summarized in Box 5). However, functional differences resulting from the activation of different signaling pathways by different SSAs acting on the same receptor are still

Box 5. Emerging questions and trends Questions  How do differences in the pharmacology of multireceptor somatostatin analogs, compared with sst2-only agonists, affect their antitumor efficacy?  How do differences between tumors in the expression patterns of somatostatin receptors affect the antitumor efficacy of novel multireceptor somatostatin analogs?  Should we personalize pharmacotherapy using somatostatin analogs based on somatostatin receptor expression pattern in tumor tissues? Trends  Sst heterodimerization creates new receptors with specific ligand affinity and functionality.  Pharmacology of novel multireceptor SSAs should be tested on each sst heterodimer.  Specific heterodimers present in tumors may favor longer-lasting and more potent therapeutic efficacy of SSAs.  Novel multireceptor analogs should be designed on the basis of this knowledge.

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Review unknown, but may have to be accounted for if an understanding of how patients can benefit from these specificities in mechanisms of action is to be elucidated. To date, octreotide and lanreotide are the only available SSAs clinically shown to have antitumor efficacy in a Phase III trial setting (Table 3). Caution should be exercised when assessing the equivalence of available SSAs for treating patients with NET because specific tumor types may be more suited to particular SSAs. The antitumor activities of SSAs are under clinical investigation and results are eagerly awaited. Acknowledgments Editorial and writing assistance were provided by David Gibson and Jennifer M. Kulak (ApotheCom, Yardley, Pennsylvania, USA), with financial assistance from Novartis Pharmaceuticals, in compliance with international guidelines on Good Publication Practice.

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