Current Treatment Options in Oncology (2014) 15:365–379
`DOI 10.1007/s11864-014-0294-4
`
`Upper Gastrointestinal Cancers (L Rajdev, Section Editor)
`
`Targeting the mTOR Signaling
`Pathway in Neuroendocrine
`Tumors
`
`Jennifer Chan, MD, MPH
`Matthew Kulke, MD, MMSc*
`
`Address
`*Department of Medical Oncology, Dana-Farber Cancer Institute, 450 Brookline
`Avenue, Boston, MA 02215, USA
`Email: matthew_kulke@dfci.harvard.edu
`Email: jang@partners.org
`
`Published online: 5 August 2014
`* The Author(s) 2014. This article is published with open access at Springerlink.com
`
`Keywords Neuroendocrine tumor I Carcinoid I Pancreatic neuroendocrine tumor I mTOR inhibitor I Everolimus
`
`Opinion statement
`
`Neuroendocrine tumors (NETs) are a heterogeneous group of malignancies character-
`ized by variable but most often indolent biologic behavior. Well-differentiated NETs
`can be broadly classified as either carcinoid or pancreatic NET. Although they have sim-
`ilar characteristics on routine histologic evaluation, the 2 tumor subtypes have differ-
`ent biology and respond differently to treatment, with most therapeutic agents
`demonstrating higher response rates in pancreatic NETs compared with carcinoid. Until
`recently, systemic treatment options for patients with advanced NETs were limited.
`However, improvements in our understanding of signaling pathways involved in the
`pathogenesis, growth, and spread of NETs have translated into an expansion of treat-
`ment options. Aberrant signaling through the mechanistic pathway of rapamycin
`(mTOR) pathway has been implicated in neuroendocrine tumorigenesis. Additionally,
`altered expression of mTOR pathway components has been observed in NETs and has
`been associated with clinical outcomes. Targeting the mTOR pathway has emerged as
`an effective treatment strategy in the management of advanced NETs. In a randomized,
`placebo-controlled study of patients with advanced pancreatic NET, treatment with the
`mTOR inhibitor everolimus was associated with improved progression-free survival
`(PFS). Largely based upon these data, everolimus has been approved in the United
`States and Europe for the treatment of patients with advanced pancreatic NET. The ac-
`tivity of everolimus remains under investigation in patients with carcinoid tumors. In a
`randomized study of patients with advanced carcinoid tumors associated with carcinoid
`syndrome, the addition of everolimus to octreotide was associated with improved PFS
`compared with octreotide. However, the results did not meet the prespecified level of
`statistical significance based on central review of radiographic imaging. Results from a
`randomized study examining the efficacy of everolimus in patients with nonfunctional
`gastrointestinal and lung NETs are awaited. In addition, further investigation is needed
`to determine whether primary tumor site or other clinical and molecular factors can im-
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`pact response to mTOR inhibition. Although everolimus can slow tumor progression,
`significant tumor reduction is rarely obtained. Targeting multiple signaling pathways
`is a treatment strategy that may provide better tumor control and overcome resistance
`mechanisms involved with targeting a single pathway. Results of ongoing and future
`studies will provide important information regarding the added benefit of combining
`mTOR inhibitors with other targeted agents, such as VEGF pathway inhibitors, and cy-
`totoxic chemotherapy in the treatment of advanced NETs.
`
`Introduction
`Neuroendocrine tumors (NET) are a rare and het-
`erogeneous group of neoplasms that arise from
`neuroendocrine cells located throughout the body.
`These tumors are characterized by their ability to
`secrete peptides resulting in distinctive hormonal
`syndromes. NETs consist of a spectrum of disease
`ranging from well-differentiated, low-grade tumors
`to poorly differentiated, high-grade carcinomas
`[1•, 2]. In general, poorly differentiated, high-
`grade carcinomas represent aggressive cancers that
`have a different natural history and response to
`treatment compared with well-differentiated, low-
`grade NETs.
`A number of different complex classification sys-
`tems exist for grading NET pathology [1•]. In the
`2010 World Health Organization (WHO) classifica-
`tion, neuroendocrine neoplasms of the digestive sys-
`tem are categorized as low-grade (G1), intermediate-
`grade (G2), and high-grade (G3) based upon mitotic
`count and proliferative index (Ki-67) [3]. High grade
`carcinomas are those with a mitotic count of 920 per
`10 high powered fields (HPF) or a Ki-67 proliferation
`index of 920 %. High grade carcinomas have a more
`aggressive biology and are generally treated with plat-
`inum-based chemotherapy regimens used to treat
`small cell lung cancer. In contrast, well-differentiated,
`low- and intermediate-grade NETs have a more indo-
`lent biology and lower measures of cell proliferation.
`W e l l - d i f f e r e n t i a t e d N E T s c a n b e b r o a d l y
`subclassified as either carcinoid or pancreatic NETs.
`Carcinoid tumors may arise from multiple different
`organs and historically have been classified according
`to site of embryonic origin, namely foregut (gastric,
`bronchial), midgut (small intestine, appendix, proxi-
`mal large bowel), and hindgut (distal colon, rectum,
`genitourinary) [4]. While carcinoid and pancreatic
`NETs may have similar histologic characteristics, these
`
`2 tumor subtypes have different biology and respond
`differently to therapy, with most agents demonstrating
`higher response rates in pancreatic NET patients com-
`pared with carcinoid.
`When NETs are diagnosed at an early stage, surgical
`resection is often curative. Unfortunately, curative sur-
`gery is rarely an option for patients with metastatic dis-
`ease. Recent studies have demonstrated that in
`addition to improving symptoms related to hormone
`hypersecretion, somatostatin analogs slow disease pro-
`gression in patients with small bowel carcinoid tumors
`and gastrointestinal neuroendocrine tumors, including
`pancreatic NET [5, 6]. Treatment approaches with
`targeted therapy, including the use of agents inhibiting
`the vascular endothelial growth factor (VEGF) and
`mTOR signaling pathways and other pathways involved
`in neuroendocrine tumorigenesis, also provide new ther-
`apeutic options for patients with NET [7, 8••].
`Notably, there are a subset of patients with NETs that
`appear histologically well- or moderately differentiated
`but are associated with Ki-67 proliferation indices
`920 % that fall into the high-grade range. The most ap-
`propriate therapy for this subgroup of patients has not
`been well established. A recent series of patients with
`high-grade gastrointestinal neuroendocrine carcinomas
`demonstrated that response rates to platinum-based che-
`motherapy were lower in patients with a Ki-67 G55 %
`[9]. Because sensitivity to platinum-based chemotherapy
`appears to be associated with higher Ki-67 proliferation
`rates, other cytotoxic agents, such as temozolomide, or
`targeted agents, such as mTOR inhibitors or angiogenesis
`inhibitors, may play a role in the management of the
`management of these patients.
`The aim of this review is to provide an overview of
`the role of the mTOR pathway in the pathogenesis of
`neuroendocrine tumors and to review the role of
`mTOR inhibitors in the treatment of this disease.
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`The mTOR pathway
`
`The mechanistic target (originally referred to as “mammalian target”) of
`rapamycin (mTOR) is an intracellular serine/threonine kinase that regulates
`key cell functions involved in cell survival, proliferation, and metabolism.
`mTOR interacts with several proteins to form 2 multiprotein complexes re-
`ferred to as mTOR complex 1 (mTORC1) and 2 (mTORC2) [10]. By inte-
`grating signals from growth factors and nutrients, mTOR regulates various
`anabolic and catabolic cellular processes [11, 12•, 13].
`mTORC1, which is the better characterized of the 2 complexes, is activat-
`ed by extracellular growth factors and nutrients (Fig. 1). When active,
`mTORC1 phosphorylates the translational regulators eukaryotic initiation
`factor 4E (eIF4E) binding protein 1 (4E-BP1) and S6 kinase 1 (S6K1). These
`events lead to cell proliferation by promoting translation of specific mRNAs
`encoding proteins regulating cell-cycle progression, angiogenesis, energy
`metabolism, and metastasis [14]. mTORC1 also promotes lipid biosynthesis
`and suppresses autophagy through phosphorylation of other key cellular
`effectors [12•].
`Compared with mTORC1, less is known about mTORC2. It also responds
`to growth factor signals, and when active, mTORC2 regulates cell survival,
`cytoskeletal remodeling, and cell migration [15, 16]. It also serves to regulate
`
`Fig. 1. The mTOR signaling pathway. Simplified representation of key components of the mTOR signaling network. The mTOR
`pathway plays an important role in mediating growth factor signals that stimulate cell growth and proliferation and regulate
`angiogenesis and cell metabolism. Arrows represent activation; bars represent inhibition. Adapted from Yao et al, 2013 [66].
`
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`the PI3K/AKT pathway via phosphorylation and activation of Akt [17].
`Whereas mTORC1 is sensitive to inhibition by rapamycin, mTORC2 is
`considered insensitive to rapamycin [12•].
`In addition to regulation by energy and nutrient status, the mTOR path-
`way responds to growth factors through signaling involving the phos-
`phatidylinositol 3-kinase (PI3K) pathway (Fig. 1) [13]. Binding of insulin or
`insulin-like growth factors to their receptors leads to phosphorylation of
`insulin receptor substrate (IRS). PI3K is subsequently recruited to the cell
`membrane, leading to phosphorylation of phosphatidylinositiol-4,5-
`bisphospate (PIP2) to phosphatidylinositiol-3,4,5-bisphospate (PIP3), and
`ultimately activation of Akt. The phosphatase PTEN is an inhibitory regulator
`of the PI3K-Akt-mTOR pathway that antagonizes the action of PI3K by de-
`phosphorylating PIP3 to PIP2, causing suppression of PI3K-dependent cell
`signaling.
`mTOR is linked to the PI3K-Akt pathway by the tuberous sclerosis pro-
`teins TSC1 and TSC2, which act as a heterodimer that negatively regulates
`mTOR signaling. In response to insulin and other growth factors, TSC2 is
`phosphorylated and inactivated by Akt, which then leads to mTOR activation
`[18–20].
`The mTOR pathway and pathogenesis of NET
`
`Several observations support the importance of the mTOR pathway in the
`pathogenesis of NET. First, although most NETs arise sporadically, NETs can
`arise within the context of several familial cancer syndromes that are due to
`mutations in genes encoding proteins that lie upstream from mTOR. Neu-
`rofibromatosis type 1 (NF-1) and tuberous sclerosis (TS) are autosomal
`dominant tumor susceptibility syndromes caused by inactivating mutations
`in the tumor suppressor genes NF1 and TSC1 and TSC2, respectively [21].
`NF1 encodes the protein neurofibromin, which regulates TSC1 and TSC2
`[22]. Loss of NF1 in neurofibromatosis leads to constitutive activation of
`mTOR and is associated with NETs involving the ampulla of Vater, duode-
`num, and mediastinum. Loss of function of TSC1 and TSC2 leads to mTOR
`activation in patients with tuberous sclerosis, which has been associated with
`pancreatic NETs [23].
`Second, whole exome sequencing analysis of sporadic pancreatic NETs
`has identified somatic mutations in genes involved in the mTOR pathway,
`including PTEN, TS2, and PIK3CA, in 15 % of cases [24•]. Additionally,
`chromosomal changes, including loss of 16p, the region containing TSC2,
`and loss of 10q, which contains PTEN, have been reported in pancreatic NET
`[25, 26].
`Altered expression of mTOR pathway components also has been observed
`in NETs and has been associated with clinical outcomes in several studies. In
`an analysis of gene expression profiles of 72 primary pancreatic NETs, TSC2
`and PTEN were found to be downregulated in most of the primary tumors
`[27•]. In this study, 85 % of primary tumors showed altered protein levels of
`TSC2, PTEN, or both. Low levels of expression of TSC2 and PTEN were as-
`sociated with shorter disease-free and overall survival. Moreover, 8/25
`(32 %) patients with low levels of TSC2 and PTEN developed liver metas-
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`tases and progression of disease compared with none of 20 patients with
`normal levels of both TSC2 and PTEN. Studies have also demonstrated that
`expression of mTOR and its downstream targets are associated with clinical
`outcome [28, 29•]. In an analysis of tumor from 195 patients with NETs
`arising in various sites, primarily small intestine, expression of mTOR or its
`activated downstream target p-EIF4EBP1 was associated with a higher pro-
`liferative index. Furthermore, high expression of mTOR or its activated
`downstream products were associated with shorter survival [29•].
`Interestingly, there appears to be differential expression of mTOR depend-
`ing on the primary tumor site. Expression levels of mTOR and activation of
`its downstream targets have been found to be higher in foregut tumors
`compared with midgut tumors [28]. Additionally, although low expression
`of PTEN, TSC1, and TSC2 have been found in pancreatic NETs, TSC1 and
`TSC2 expression appear preserved in small intestinal NET [29•]. This suggests
`that there may be potential differences in the mechanisms of mTOR activa-
`tion in different subgroups of NETs.
`
`Treatment
`Targeting the mTOR pathway
`
`& The mTOR inhibitor rapamycin and its analogs bind FK506 binding
`protein, and this complex binds to mTORC1, inhibiting downstream
`signaling [30]. Everolimus and temsirolimus are rapamycin deriva-
`tives that have been evaluated in the treatment NET (Tables 1 and 2).
`
`Everolimus
`Pancreatic NET
`
`& The activity of everolimus in pancreatic NET was explored in the
`RADIANT-1 trial, an international multicenter phase II trial of 160
`
`Table 1. Clinical trials of mTOR inhibitors in carcinoid tumors
`
`Agent
`
`No. patients
`
`Tumor
`response
`rate (%)
`
`Median
`TTP or
`PFS
`
`Reference
`
`Phase II studies
`Everolimus a
`Temsirolimus a
`Phase III studies
`RADIANT-
`Everolimus + octreotide LAR vs.
`2
`Placebo + octreotide LAR
`RADIANT-
`Everolimus vs.
`4
`Placebo
`PFS progression-free survival, TTP time to progression
`a Data from the subset of patients with carcinoid tumors in these phase II studies of unselected patients with NET are presented
`
`Yao et al. 2008 [37]
`Duran et al. 2006 [41]
`
`Pavel et al. 2011[38••]
`
`30
`21
`
`216
`214
`Ongoing
`
`17
`5
`
`2
`2
`
`63 wk
`6.0 mo
`
`16.4 mo
`11.3 mo
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`Table 2. Clinical trials of mTOR inhibitors in Pancreatic NET tumors
`
`Agent
`
`No. patients
`
`Phase II studies
`
`RADIANT-1
`
`Everolimusa
`Everolimus
`Everolimus + octreotide
`
`Temsirolimusa
`
`30
`115
`45
`
`15
`
`Tumor
`response
`rate (%)
`
`27
`9
`4
`
`7
`
`Median
`TTP or
`PFS
`
`50 wk
`9.7 mo
`16.7 mo
`
`Reference
`
`Yao et al. 2008 [37]
`Yao et al. 2010 [31]
`
`10.6 mo
`
`Duran et al. 2006 [41]
`
`Phase III studies
`RADIANT-3
`
`CALGB 80701
`
`207
`203
`Ongoing
`
`Everolimus vs.
`Placebo
`Everolimus + octreotide vs.
`Everolimus + bevacizumab
`+ octreotide
`PFS progression-free survival, TTP time to progression
`a Data from the subset of patients with pancreatic NET in this phase II study of unselected patients with NET are presented
`
`5
`2
`
`11 mo
`4.6 mo
`
`Yao et al. 2011[8••]
`
`patients, 45 of whom also received concurrent treatment with octreotide
`at the discretion of investigators [31]. Upstream regulation of the IGF
`pathway is thought to be a potential resistance mechanism for everolimus
`[32, 33]. Because octreotide has been shown to reduce serum IGF-1 levels
`in patients with advanced solid tumors, the use of everolimus plus a so-
`matostatin analog to target both upstream and downstream components
`of the mTOR pathway has been postulated to potentially have greater
`efficacy than single agent therapy. Among patients receiving octreotide
`plus everolimus, median PFS was longer compared with those receiving
`everolimus alone (17 vs 9.7 months). However, whether the addition
`octreotide to everolimus contributed to higher PFS is uncertain since the
`study was not randomized or designed to make this comparison.
`& Everolimus monotherapy subsequently was compared with best
`supportive care alone in the placebo-controlled RADIANT-3 trial,
`which included 410 patients with advanced pancreatic NET [8••].
`Approximately 40 % of patients also received somatostatin analog
`therapy. Everolimus was associated with a significant prolongation in
`median PFS (11.0 vs 4.6 months, hazard ratio [HR] for progression
`0.35, 95 % confidence interval [CI] 0.27–0.45). Confirmed objective
`partial radiographic responses were observed in 5 % of patients re-
`ceiving everolimus compared with 2 % of those receiving placebo.
`The rate of tumor stabilization was high, 73 % among patients re-
`ceiving everolimus vs 51 % in the placebo group.
`& Drug-related adverse among patients with pancreatic NET receiving
`everolimus included stomatitis, rash, diarrhea, and fatigue [8••]. The
`most common grade 3 or 4 drug-related adverse events were sto-
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`matitis (7 %), anemia (6 %), and hyperglycemia (5 %). Though rare,
`everolimus has been associated with serious, adverse events, in-
`cluding pneumonitis.
`& Everolimus causes hyperglycemia, particularly in those with pre-
`existing hyperglycemia. In the RADIANT-3 trial, the frequency of
`severe (grade 3 or 4) hyperglycemia was higher in those with pre-
`existing diabetes mellitus or baseline hyperglycemia (15 % vs 3 % in
`those without diabetes or baseline hyperglycemia) [34]. Because of
`this effect, everolimus may be of particular value in patients with
`hypoglycemia related to insulinoma [35, 36]. In 1 report, 4 patients
`with malignant insulinoma and refractory hypoglycemia experienced
`normalization of glucose levels while receiving everolimus [35]. Two
`of these patients had an objective radiographic antitumor response,
`which may have led to improvements in insulin secretion. Clinical
`improvement in the 2 patients with stable disease suggests a possible
`direct effect of everolimus on insulin production and/or release or an
`effect on peripheral insulin sensitivity.
`
`& Everolimus has been evaluated in combination with octreotide in a
`phase II study of patients with advanced NETs. Partial responses were
`observed in 5 of 30 (17 %) patients with carcinoid tumors, with a
`median PFS of 63 weeks in this group of patients [37].
`& The activity of everolimus was further investigated in patients with
`functional NET in the randomized, placebo-controlled RADIANT-2 trial
`[38••]. In this study, 429 patients with advanced NETs associated with
`carcinoid syndrome and radiographic disease progression in the pre-
`ceding 12 months were randomly assigned to octreotide LAR with either
`everolimus or placebo. Half of patients had a primary small bowel tu-
`mor; lung primary tumors were the second most common tumor type.
`The median PFS as assessed by central radiology review was 16.4 months
`for patients receiving everolimus and octreotide LAR compared with
`11.3 months for patients receiving placebo and octreotide LAR (HR 0.77,
`95 % CI 0.59–1.00; P=0.026). These results did not meet the prespecified
`level of statistical significance. However, based on local investigator ra-
`diology assessment, combined therapy was associated with a median PFS
`duration of 12.0 months compared with 8.6 months with placebo (HR
`0.78, 95 % CI 0.62–0.98; P=0.018). Additionally, imbalances in prog-
`nostic variables favoring the control group, including disease site and
`performance status, could have affected the primary outcome results. A
`subsequent analysis found a significant PFS benefit for everolimus after
`adjusting for randomization imbalances (HR for progression 0.62, 95 %
`CI 0.51–0.87, P=0.003) [39].
`& The best overall radiographic response was a partial response in 2 %
`of both groups; stable disease was the best response in 84 % of the
`patients treated with everolimus and 81 % of patients receiving
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`placebo [38••]. Patients treated with everolimus had a higher rate of
`biochemical responses. Serum chromogranin A levels decreased in 46 %
`of patients receiving everolimus compared with 36 % of patients re-
`ceiving placebo, and 24 hour urinary 5-hydroxyindoleacetic acid (5-
`HIAA) excretion decreased in 61 % of patients receiving everolimus
`compared with 54 % of patients receiving placebo. Data on control of
`symptoms related to carcinoid syndrome were not reported.
`& Additional studies are also needed to determine whether primary tumor
`site of origin may impact response to everolimus. Patients with advanced
`colorectal NET have a particularly poor prognosis. In a post-hoc analysis
`of the RADIANT-2 study, patients with colorectal NETs receiving evero-
`limus plus octreotide LAR had significantly longer PFS (29.9 mo; n=19)
`compared with those receiving placebo plus octreotide LAR (6.6 mo; n=
`20) [40]. Furthermore, some degree of tumor shrinkage was more fre-
`quently noted in patients receiving everolimus plus octreotide LAR
`compared with those receiving placebo plus octreotide LAR (67 % vs
`37 %).
`& The RADIANT-4 trial, a phase III study in which patients with ad-
`vanced, nonfunctional lung or gastrointestinal NETs were random-
`ized to receive everolimus or placebo, recently completed accrual
`(clinical trials.gov, NCT01524783). The results of this study will
`provide important information regarding the activity of everolimus
`in the treatment of nonpancreatic NET.
`
`& The single-agent activity of temsirolimus was evaluated in a multicenter
`phase II study of 37 patients with advanced, progressive neuroendocrine
`tumors [41]. Although the intent-to-treat response rate for the cohort was
`low (6 %), 54 % of patients experienced stable disease while on treatment
`with a median time to progression (TTP) of 6 months. Higher baseline
`tumor levels of phosphorylated mTOR predicted for better outcomes. Fur-
`thermore, temsirolimus appeared more active in patients with pancreatic
`NET compared with carcinoid; median TTP in patients with pancreatic NET
`was 10.6 months compared with 6 months in the carcinoid subgroup.
`However, the small size of this study limits definite conclusions regarding
`the impact of primary tumor site on efficacy of temsirolimus.
`
`Temsirolimus
`
`Future directions with mTOR inhibitor therapy
`
`& Rapamycin and its derivatives are generally cytostatic rather than
`cytotoxic. One of the factors contributing to their limited clinical
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`success is the existence of multiple feedback loops regulating cell
`survival (Fig. 1). Under normal circumstances, mTORC-1 activation
`of S6K1 promotes degradation of insulin receptor substrate (IRS),
`leading to attenuation of PI3K signaling. Inhibition of mTORC1 can
`lead to increased PI3K signaling by relieving this negative feedback
`[33, 42]. In addition, mTORC1-mediated signaling can inhibit
`mTORC2 through phosphorylation of rictor, one of the components
`of mTORC2. By blocking this feedback loop, rapamycin can con-
`tribute to mTORC2-mediated AKT activation. Furthermore, studies
`have also demonstrated that inhibition of the PI3K/AKT/mTOR
`pathway can result in activation of other receptor tyrosine kinases,
`resulting in downstream signaling promoting cell growth [43].
`& Targeting multiple signaling pathways may provide better tumor
`control and overcome resistance mechanisms. Combining an mTOR
`inhibitor with somatostatin analogs, inhibitors of the VEGF pathway
`and cytotoxic chemotherapy have been evaluated as treatment
`strategies for NETs.
`
`Combining mTOR inhibitor and somatostatin analog
`
`& Because octreotide has been shown to decrease IGF-1 levels and
`PI3K/Akt signaling in vitro, it has been postulated that combining
`an mTOR inhibitor with a somatostatin analog might result in
`enhanced antitumor activity [44]. Everolimus has been evaluated
`in combination with octreotide in several studies, including pa-
`tients with pancreatic NET in stratum 2 of the RADIANT-1 trial
`and patients with carcinoid tumors in the phase III RADIANT-2
`trial. In the RADIANT-1 trial, patients receiving octreotide and
`everolimus had longer PFS compared with patients receiving
`everolimus monotherapy [31]. However, the study was not ran-
`domized or designed to make this comparison. In the RADIANT-2
`trial, although combined therapy with everolimus and octreotide
`was associated with a significantly longer PFS duration compared
`with everolimus and placebo based on local investigator radiology
`review, the improvement in PFS was not statistically significant
`according to central radiology review [38••]. Further investigation
`is needed to determine whether there are specific subsets of pa-
`tients with advanced NETs who benefit most from the addition of
`everolimus to octreotide.
`& Pasireotide is a novel somatostatin analog that binds to a broader
`range of somatostatin receptor subtypes (sst) than octreotide. Com-
`pared with octreotide, pasireotide has a greater binding affinity to
`sst1, sst3, and sst5 and comparable affinity with sst2 [45]. Increased
`receptor binding may lead to additional antiproliferative activity and
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`growth inhibition in NET [44]. A phase I study has established the
`feasibility of combining pasireotide and everolimus [46]. Hypergly-
`cemia was a commonly observed toxicity. A partial radiographic tu-
`mor response was noted in 1/21 patients (5 %), and 17/21 (81 %)
`experienced at least some tumor regression as the best response to
`therapy. The COOPERATE-2 study, a multi-center randomized phase
`II study, recently completed accrual and has examined the combi-
`nation of everolimus alone or in combination with pasireotide LAR
`in patients with advanced, progressive pancreatic NET (clinical
`trials.gov, NCT01374451). The results of this study will provide ad-
`ditional information regarding the added efficacy of combining an
`mTOR inhibitor with a somatostatin analog.
`
`Combining mTOR inhibitor and VEGF pathway inhibitor
`
`& A key role for angiogenesis and VEGF pathway signaling in NET is sug-
`gested by clinical observations that neuroendocrine tumors are vascular
`tumors. Expression of VEGF has been demonstrated in carcinoid and
`pancreatic NETs [47, 48]. Increased expression of VEGF receptor-2
`(VEGFR-2) has been demonstrated in tissue from gastrointestinal carci-
`noid tumors and a carcinoid cell line [49, 50]. Additionally, pancreatic
`neuroendocrine tumors also show widespread expression of VEGFR-2
`and -3 in addition to platelet-derived growth factor receptors (PDGFRs) α
`and β, stem-cell factor receptor (c-kit) [51–53].
`& The tyrosine kinase inhibitor sunitinib has shown activity against a
`range of signaling pathways and growth factors/receptors, including
`VEGFR-1, -2 and -3, PDGFR-α and -β, KIT, RET, FMS-like tyrosine
`kinase-3 (FLT3), and colony-stimulating factor receptor (CSF-1R). In
`a randomized phase III study examining the activity of sunitinib in
`patients with progressive pancreatic NET, sunitinib was associated
`with a median progression-free survival (PFS) of 11.4 months, as
`compared with 5.5 months for placebo (PG.001) [7]. Two other
`small molecule tyrosine kinase inhibitors (TKIs), sorafenib, and
`pazopanib, have also been evaluated in phase II studies [54, 55].
`Although response rates to TKIs in carcinoid tumors have been low,
`all studies report a high rate of disease stabilization and potentially
`encouraging progression-free survival durations.
`& Bevacizumab, a monoclonal antibody against VEGF, has been eval-
`uated in a randomized phase II study of patients with advanced or
`metastatic carcinoid tumors on a stable dose of octreotide. Patients
`were randomly assigned to receive 18 weeks of bevacizumab or
`pegylated IFN-α 2b [56]. During the first 18 weeks of therapy, 18 %
`of the bevacizumab-treated patients experienced radiographic partial
`responses, and 77 % had stable disease. Furthermore, after 18 weeks,
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`Targeting the mTOR Signaling Pathway in Neuroendocrine Tumors
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`Chan and Kulke
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`95 % of patients treated with octreotide plus bevacizumab remained
`progression-free compared with only 68 % of those receiving
`octreotide plus IFN-α.
`& A number of recently completed and ongoing studies have evaluated
`the combination of an mTOR inhibitor with inhibitors of the VEGF
`pathway. Combining everolimus with tyrosine kinase inhibitors of
`VEGFR and other growth factor receptors may be limited by toxicity.
`In a phase I study of everolimus in combination with sorafenib,
`dose-limiting toxicity precluding escalation to full doses of each
`agent was observed [57]. However, the combination of the everoli-
`mus and bevacizumab was shown to be well tolerated and associated
`with antitumor activity (overall response rate 26 %) in an initial
`phase II study enrolling patients with low or intermediate grade
`neuroendocrine tumors [58]. Furthermore, encouraging early results
`have been noted in a phase II trial of temsirolimus plus bevacizumab
`in 55 patients with progressive NET. In a preliminary report, a con-
`firmed partial response was documented in 20 patients (37 %), and
`44 (80 %) remained progression-free at 6 months [59]. Results of
`CALGB 80701, a phase II trial of patients with advanced pancreatic
`NETs randomized to receive either everolimus and octreotide or
`everolimus plus bevacizumab and octreotide, will provide additional
`information about the benefits of combined mTOR and VEGF
`pathway inhibition (clinical trials.gov, NCT01229943).
`
`Combining mTOR inhibitor and cytotoxic chemotherapy
`
`& Cytotoxic chemotherapy has minimal activity in patients with ad-
`vanced carcinoid tumors. In contrast, pancreatic NETs may respond
`well to treatment with streptozocin and other alkylating agents [60,
`61]. Recent prospective and retrospective studies have suggested that
`temozolomide-based regimens may be comparable in efficacy and
`more tolerable than streptozocin-based regimens. In retrospective
`series, temozolomide-based therapy has been associated with overall
`response rates of 8 %–70 % in patients with advanced pancreatic
`NET [62–64].
`& The combination of temozolomide and everolimus has been evalu-
`ated in a phase I/II study of patients with advanced pancreatic NET
`[65]. Treatment was associated with known side effects of each drug
`without evidence of synergistic toxicity. Encouraging evidence of
`antitumor activity with this combination was observed. Among 40
`evaluable patients, 16 (40 %) experienced a partial response. The
`median PFS duration was 15.4 months, which is superior to the re-
`ported PFS observed with everolimus alone in the randomized,
`placebo-controlled RADIANT-3 study. However, these results need to
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`Upper Gastrointestinal Cancers (L Rajdev, Section Editor)
`
`Conclusions
`
`be interpreted with caution since this was a single-arm study. Fur-
`thermore, disease progression prior to study enrollment was not a re-
`quirement in this study, as it was in the RADIANT-3 study. Future studies
`evaluating the relative efficacy of combining chemotherapy with an mTOR
`inhibitor compared with treatment with either agent alone are warranted.
`
`Recent improvements in our understanding of the molecular biology of NETs
`have led to an expansion of treatment options for patients with advanced
`disease. Studies indicate that the mTOR pathway plays an important role in
`the pathogenesis of NET. The mTOR inhibitor everolimus has been shown to
`significantly delay disease progression in patients with pancreatic NET. Fur-
`ther studies evaluating everolimus in advanced carcinoid are anticipated.
`Results of ongoing and future studies will provide important information
`regarding the added benefit of combining an mTOR inhibitor with cytotoxic
`chemotherapy and other targeted agents, such as VEGF pathway inhibitors,
`in the treatment of advanced NETs.
`
`Compliance with Ethics Guidelines
`
`Conflict of Interest
`Jennifer Chan reports grants from Novartis, Merck, and Bayer. Matthew Kulke reports grants from Novartis and
`has served as a consultant to Pfizer, Ipsen, Lexicon, and Novartis.
`
`Human and Animal Rights and Informed Consent
`This article does not contain any studies with human or animal subjects performed by any of the authors.
`
`Open Access This article is distributed under the