Introduction
The mTOR pathway
The mTOR pathway and pathogenesis of NET
Treatment
Targeting the mTOR pathway
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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 derivatives that have been evaluated in the treatment NET (Tables 1 and 2).Table 1Clinical trials of mTOR inhibitors in carcinoid tumorsAgentNo. patientsTumor response rate (%)Median TTP or PFSReferencePhase II studiesEverolimus a301763 wkYao et al. 2008 [37]Temsirolimus a2156.0 moDuran et al. 2006 [41]Phase III studiesRADIANT-2Everolimus + octreotide LAR vs.216216.4 moPavel et al. 2011[38••]Placebo + octreotide LAR214211.3 moRADIANT-4Everolimus vs.OngoingPlaceboTable 2Clinical trials of mTOR inhibitors in Pancreatic NET tumorsAgentNo. patientsTumor response rate (%)Median TTP or PFSReferencePhase II studiesEverolimusa302750 wkYao et al. 2008 [37]RADIANT-1Everolimus11599.7 moYao et al. 2010 [31]Everolimus + octreotide45416.7 moTemsirolimusa15710.6 moDuran et al. 2006 [41]Phase III studiesRADIANT-3Everolimus vs.207511 moYao et al. 2011[8••]Placebo20324.6 moCALGB 80701Everolimus + octreotide vs.OngoingEverolimus + bevacizumab + octreotide
Everolimus
Pancreatic NET
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The activity of everolimus in pancreatic NET was explored in the RADIANT-1 trial, an international multicenter phase II trial of 160 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 somatostatin 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.
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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 receiving everolimus compared with 2 % of those receiving placebo. The rate of tumor stabilization was high, 73 % among patients receiving everolimus vs 51 % in the placebo group.
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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 stomatitis (7 %), anemia (6 %), and hyperglycemia (5 %). Though rare, everolimus has been associated with serious, adverse events, including pneumonitis.
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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.
Carcinoid tumors
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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].
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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 preceding 12 months were randomly assigned to octreotide LAR with either everolimus or placebo. Half of patients had a primary small bowel tumor; 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 radiology 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 prognostic 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].
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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 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 receiving 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.
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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 everolimus 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 frequently noted in patients receiving everolimus plus octreotide LAR compared with those receiving placebo plus octreotide LAR (67 % vs 37 %).
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The RADIANT-4 trial, a phase III study in which patients with advanced, nonfunctional lung or gastrointestinal NETs were randomized 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.
Temsirolimus
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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. Furthermore, 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.
Future directions with mTOR inhibitor therapy
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Rapamycin and its derivatives are generally cytostatic rather than cytotoxic. One of the factors contributing to their limited clinical 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 contribute 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].
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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
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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 patients 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 randomized 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 patients with advanced NETs who benefit most from the addition of everolimus to octreotide.
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Pasireotide is a novel somatostatin analog that binds to a broader range of somatostatin receptor subtypes (sst) than octreotide. Compared 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 growth inhibition in NET [44]. A phase I study has established the feasibility of combining pasireotide and everolimus [46]. Hyperglycemia was a commonly observed toxicity. A partial radiographic tumor 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 combination 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 additional information regarding the added efficacy of combining an mTOR inhibitor with a somatostatin analog.
Combining mTOR inhibitor and VEGF pathway inhibitor
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A key role for angiogenesis and VEGF pathway signaling in NET is suggested 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 carcinoid 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].
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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 (P < .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.
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Bevacizumab, a monoclonal antibody against VEGF, has been evaluated 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, 95 % of patients treated with octreotide plus bevacizumab remained progression-free compared with only 68 % of those receiving octreotide plus IFN-α.
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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 everolimus 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 confirmed 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
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Cytotoxic chemotherapy has minimal activity in patients with advanced 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].
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The combination of temozolomide and everolimus has been evaluated 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 reported PFS observed with everolimus alone in the randomized, placebo-controlled RADIANT-3 study. However, these results need to be interpreted with caution since this was a single-arm study. Furthermore, disease progression prior to study enrollment was not a requirement 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.