Neuroendocrine tumours (NETs) arise from the diffuse neuroendocrine cell system and, although considered rare, their prevalence has apparently increased over the last 2 decades . Most NETs originate in the lung and the gastrointestinal tract, and the latter also referred to as gastroenteropancreatic NETs (GEP NET). NETs can present with distinct clinical syndromes (functional tumours) related to secreted bioactive molecules emanating from the NET cells or may be nonfunctional [1–6].
The vast majority of NETs occur sporadically, but they are also observed as part of hereditary syndromes, including multiple endocrine neoplasia type 1 (MEN-1), von Hippel–Lindau (VHL) disease, neurofibromatosis I (NF-1) and tuberous sclerosis (TSC) [1,3,4]. The pathological classification of NETs has been quite confusing over the years, related mainly to various sites of NET origin and their heterogeneous tumour biology, which did not provide an overarching system of nomenclature, grading and staging. Most recently, the European Neuroendocrine Tumor Society (ENETS) and WHO 2010 guidelines have been adopted for GEP NET classification with grading and staging. In general, well differentiated and poorly differentiated NETs would include low-grade and intermediate grade and high-grade tumours, respectively, and low-grade tumours would follow an indolent clinical course, whereas high-grade NETs are very aggressive tumours reflecting their significant differences in tumour biology [7–20].
Consequently, treatment options for NETs may vary greatly and include surgery, systemic chemotherapy, targeted radiotherapy and regional strategies such as ablative therapies and chemoembolization. However, owing to the overall rarity of NETs and their clinical–pathological heterogeneity, until recently, no well designed randomized clinical phase III trials have been conducted to assess medical treatment options in larger patient cohorts [1–6,21].
SOMATOSTATIN ANALOGUES IN NEUROENDOCRINE TUMOURS
Certain NETs have been treated in the past with somatostatin analogues or acid-reducing agents such as proton pump inhibitors to control clinical symptoms due to overt overproduction of certain bioactive molecules, for instance carcinoid syndrome owing to serotonin and metabolites or Zollinger-Ellison syndrome due to hypergastrinemia. Somatostatin analogues such as octreotide and lanreotide would bind to their high-affinity somatostatin receptors (predominantly subtypes 2 and 5) that are frequently expressed on low-grade and intermediate-grade NETs and improve or alleviate clinical symptoms of hormone excess syndromes. However, albeit antiproliferative properties of somatostatin analogues against NETs have been reported either alone or in combination with interferon alpha, it remained controversial whether these treatment modalities would truly result in prolonged survival and control of tumour growth [1–6].
For the first time, results of an interim analysis of a placebo-controlled, double-blind, prospective randomized study were published in 2009 to demonstrate that octreotide long-acting repeatable (LAR) significantly lengthened time to tumour progression compared with placebo in patients with functionally active and inactive metastatic well differentiated midgut NETs . Eighty-five patients with locally inoperable or metastatic NET of midgut or unknown origin with putative midgut origin were assigned randomly to either placebo (N = 43) or monthly octreotide LAR 30-mg intramuscular injections (N = 42) until tumour progression or death. The primary endpoint was time to tumour progression as determined by computed tomography (CT) or MRI. Data represented results from the interim analysis on the basis of 67 tumour progressions and 16 observed deaths. Carcinoid syndrome was present in 40.5% of patients in the octreotide group and 37.2% in the placebo group.
Time to tumour progression in the octreotide group was significantly longer with 14.3 months than with 6 months in the placebo group (P < 0.0001), and stable disease after 6 months treatment was observed in 66.7% of patients on octreotide therapy, whereas only 37.2% in the placebo group. Treatment effects were similar in both functionally active and nonfunctional tumours. Further bivariate and multivariate analyses suggested that the extent of hepatic tumour burden, resection of the primary tumour and the time since diagnosis may also affect the time to tumour progression. Overall, in this treatment trial of therapy-naive patients, high tumour burden and resection of the primary tumour suggested a better prognosis .
SMALL MOLECULE TYROSINE KINASE AND MAMMALIAN TARGET OF RAPAMYCIN INHIBITORS IN NEUROENDOCRINE TUMOURS
Inhibition of growth factor receptors, specifically related to angiogenesis, which are frequently expressed in NETs and, similarly, inhibition of the mammalian target of rapamycin (mTOR) pathway had been identified as promising targets for the treatment of NETs [3–5,21,23–26].
Accordingly, the treatment of advanced, well or moderately differentiated NETs associated with carcinoid syndrome was assessed in a randomized, placebo-controlled phase-III b study (RAD001 in Advanced Neuroendocrine Tumors 2) comparing the oral mTOR inhibitor everolimus at 10 mg daily with placebo, both in conjunction with intramuscular octreotide LAR 30 mg every 28 days [27▪]. The primary endpoint of this study was progression-free survival defined as the time to first recorded disease progression or death from any cause. A total of 429 patients with NETs of various tissue origins including the small intestine, lung, pancreas and others were enrolled, but only 37 patients in the everolimus arm and 34 patients in the placebo arm completed the trial. Both treatment groups showed differences in baseline demographics and primary tumour sites, which warrant confirmation of the results in future trials without these shortcomings. The median progression-free survival in the everolimus group and in the placebo group was 16.4 and 11.3 months, respectively (P = 0.026), and everolimus treatment was associated with an estimated 23% risk reduction of disease progression [27▪]. The NET biomarkers chromogranin A and 24-h urinary hydroxyindoleacetic acid concentrations were reduced significantly more in patients treated with everolimus along with octreotide than in those in the placebo group. Therefore, treatment of advanced NETs associated with carcinoid syndrome with the mTOR inhibitor everolimus in combination with the somatostatin analogue octreotide significantly improved outcomes of disease progression and neuroendocrine biomarkers in this study [27▪]. Owing to the inclusion of patients with a multitude of primary tumour sites in this study and the heterogeneous tumour biology of NETs [18,28], further studies will need to assess the efficacy of this targeted treatment in more homogenous groups of NETs of specific tissue origin.
Two recent well designed studies addressed this issue [29▪,30▪]. In one randomized, prospective phase 3 study, the oral mTOR inhibitor everolimus was tested in advanced, low-grade or intermediate-grade pancreatic NETs (PNETs) at a dose of 10 mg against placebo [29▪]. Overall, 410 patients were enrolled and randomly assigned to everolimus (N = 207) or placebo (N = 203). Tumour characteristics and patient demographics were similar in both groups. Most patients had metastatic disease to the liver (>90%) and well differentiated PNETs (>80%). Median progression-free survival, the primary endpoint, was significantly (P < 0.001) superior (11.0 months) in the everolimus group as compared with placebo (4.6 months), and everolimus therapy reduced the relative risk of progression by 65% (P < 0.001) [29▪].
In the other randomized, double-blind, placebo-controlled study [30▪], the efficacy of the multitargeted tyrosine kinase inhibitor sunitinib was assessed in patients with advanced well differentiated PNETs. One hundred and seventy-one patients were randomly assigned to either sunitinib 37.5 mg per day or placebo with the primary endpoint of progression-free survival. Similar to the results with mTOR inhibitors, the sunitinib group showed a significantly (P < 0.001) prolonged median progression-free survival of 11.4 months as compared with 5.5 months in the placebo group [30▪].
Data from these randomized clinical trials therefore provide now substantial evidence that rationally used small molecule inhibitors of certain tyrosine kinases and the mTOR pathway [1,3–5,23] resulted in meaningful clinical improvements of advanced NETs of various tissue origins and also specifically PNETs even in patients with previous treatment failures [21,27▪,29▪,30▪].
PUTATIVE MOLECULAR TARGETS IN THE THERAPY OF NEUROENDOCRINE TUMOURS
It has been documented in various reports and reviewed recently [1–5,23,31,32] that targeted therapies in NETs should evaluate compounds inhibiting the mTOR pathway, growth factor receptors and their intracellular signalling, and angiogenesis. The above-detailed clinical trials demonstrated first clinical proof of this concept.
Additional experimental evidence of these putative treatment targets was recently demonstrated in a detailed molecular study of 68 PNETs [33▪▪], whereby sporadic PNET tissue samples were investigated by exomic sequencing of approximately 18 000 protein-coding genes. An initial analysis of a discovery set of 10 tumours revealed 157 somatic mutations in 149 genes. Further examination of all 68 tumours then demonstrated that most commonly mutated genes affected proteins relevant in chromatin remodelling. The MEN-1 gene had somatic inactivating mutations in 44% of tumours, 43% had mutations in genes encoding either of the two subunits of a transcription/chromatin remodelling complex consisting of death-domain associated protein (DAXX) and alpha thalassemia/mental retardation syndrome X-linked (ATRX), and 14% tumours had mTOR pathway gene mutations [33▪▪]. Furthermore, in direct comparison to the distinct pancreatic ductal adenocarcinoma (PDAC), the mutation profile for PNET was entirely different with almost perfect mutual exclusivity of the observed gene mutations. For instance, v-Ki-ras2 Kirsten rat sarcoma viral oncogene homologue (KRAS) gene mutations were detected in all (100%) PDAC samples, but in none of the PNETs. Conversely, MEN-1 and DAXX/ATRX gene mutations were only detectable in PNETs but not in PDAC [33▪▪]. These findings support the significant differences of tumour biology of both types of pancreatic neoplasia and could also possibly provide a rationale for novel targeted therapies in NET.
Proteins encoded by ATRX and DAXX genes participate in chromatin remodelling at telomeres, and mutations were frequently detected in PNETs [33▪▪]. Therefore, the telomere status of PNETs was examined and 61% of PNETs displayed abnormal telomeres that are characteristic of a telomerase-independent telomere maintenance mechanism termed alternative lengthening of telomeres (ALT). All of the PNETs exhibiting these abnormal telomeres had ATRX or DAXX mutations or loss of nuclear ATRX or DAXX protein . Furthermore, ATRX and DAXX protein expression was determined by immunohistochemistry and telomere status by telomere-specific fluorescence in-situ hybridization in 109 well differentiated pancreatic neuroendocrine lesions from 28 MEN-1 syndrome patients. ATRX and/or DAXX expression was lost in three of 50 (6%) PNETs demonstrating the existence of ATRX and DAXX defects and the ALT phenotype in PNETs of greater than 3 cm size in the context of MEN-1 syndrome .
A very recent study  of 71 patients with PNETs examined the prognostic value of a single nucleotide polymorphism whereby arginine (R) was replaced for glycine (G) in codon 388 of the fibroblast growth factor receptor (FGFR)-4 transmembrane domain and correlated with biological behaviour. The PNET BON1 cells were transfected with either FGFR4-G388 or FGFR4-R388 to determine response to the mTOR inhibitor everolimus. The results of the experimental studies in vitro were then examined in a group of patients treated with everolimus. The presence of FGFR4-R388 was associated with more aggressive clinical behaviour in patients with PETs with a statistically significant higher risk of advanced tumour stage and liver metastasis. In a mouse model, FGFR4-R388 promoted tumour progression by increasing intraperitoneal spread and metastatic growth within the liver. Unlike FGFR4-G388, FGFR4-R388 BON1 tumours exhibited diminished responsiveness to everolimus .
NETs comprise a large and heterogeneous group of neoplasia of neuroendocrine cells mainly occurring in the lung and the gastrointestinal tract. Molecular studies have identified kinases and tyrosine kinase receptors relevant to the growth of these tumours. Recent use of small molecule inhibitors of tyrosine kinases and the mTOR pathway as well as somatostatin analogues in randomized clinical trials in patients with advanced NETs has demonstrated their efficacy in extending progression-free survival and related clinical hormone-induced syndromes.
Conflicts of interest
There are no conflicts of interest.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
- ▪▪ of outstanding interest
Additional references related to this topic can also be found in the Current World Literature section in this issue (pp. 75–76).
1. Modlin IM, Oberg K, Chung DC, et al. Gastroenteropancreatic neuroendocrine tumours
. Lancet Oncol 2008; 9:61–72.
2. Chan JA, Kulke MH. New treatment options for patients with advanced neuroendocrine tumors. Curr Treat Options Oncol 2011; 12:136–148.
3. Dong M, Phan AT, Yao JC. New strategies for advanced neuroendocrine tumors in the era of targeted therapy. Clin Cancer Res 2012; 18:1830–1836.
4. Rindi G, Wiedenmann B. Neuroendocrine neoplasms of the gut and pancreas: new insights. Nature reviews. Endocrinology 2012; 8:54–64.
5. Metz DC, Jensen RT. Gastrointestinal neuroendocrine tumors: pancreatic endocrine tumors. Gastroenterology 2008; 135:1469–1492.
6. Modlin IM, Kidd M, Latich I, et al. Current status of gastrointestinal carcinoids. Gastroenterology 2005; 128:1717–1751.
7. Salazar R, Wiedenmann B, Rindi G, Ruszniewski P. ENETS 2011 Consensus Guidelines for the Management of Patients with Digestive Neuroendocrine Tumors: an update. Neuroendocrinology 2012; 95:71–73.
8. Rindi G, Falconi M, Klersy C, et al. TNM staging of neoplasms of the endocrine pancreas: results from a large international cohort study. J Natl Cancer Inst 2012; 104:764–777.
9. Volante M, Righi L, Berruti A, et al. The pathological diagnosis of neuroendocrine tumors: common questions and tentative answers. Virchows Archiv 2011; 458:393–402.
10. Kloppel G. Classification and pathology of gastroenteropancreatic neuroendocrine neoplasms. Endocr Relat Cancer 2011; 18 (Suppl 1):S1–S16.
11. Rindi G. The ENETS guidelines: the new TNM classification system. Tumori 2010; 96:806–809.
12. Kloppel G, Rindi G, Perren A, et al. The ENETS and AJCC/UICC TNM classifications of the neuroendocrine tumors of the gastrointestinal tract and the pancreas: a statement. Virchows Archiv 2010; 456:595–597.
13. Klimstra DS, Modlin IR, Adsay NV, et al. Pathology reporting of neuroendocrine tumors: application of the Delphic consensus process to the development of a minimum pathology data set. Am J Surg Pathol 2010; 34:300–313.
14. Boudreaux JP, Klimstra DS, Hassan MM, et al. The NANETS consensus guideline for the diagnosis and management of neuroendocrine tumors: well differentiated neuroendocrine tumors of the jejunum, ileum, appendix, and cecum. Pancreas 2010; 39:753–766.
15. Kloppel G, Couvelard A, Perren A, et al. ENETS Consensus Guidelines for the Standards of Care in Neuroendocrine Tumors: towards a standardized approach to the diagnosis of gastroenteropancreatic neuroendocrine tumors and their prognostic stratification. Neuroendocrinology 2009; 90:162–166.
16. Rindi G, Kloppel G, Couvelard A, et al. TNM staging of midgut and hindgut (neuro) endocrine tumors: a consensus proposal including a grading system. Virchows Archiv 2007; 451:757–762.
17. Kloppel G, Rindi G, Anlauf M, et al. Site-specific biology and pathology of gastroenteropancreatic neuroendocrine tumors. Virchows Archiv 2007; 451 (Suppl 1):S9–S27.
18. Kloppel G. Tumour biology and histopathology of neuroendocrine tumours
. Best practice & research. Clin Endocrinol Metab 2007; 21:15–31.
19. Rindi G, Kloppel G, Alhman H, et al. TNM staging of foregut (neuro)endocrine tumors: a consensus proposal including a grading system. Virchows Archiv 2006; 449:395–401.
20. Rindi G, de Herder WW, O’Toole D, Wiedenmann B. Consensus guidelines for the management of patients with digestive neuroendocrine tumors: why such guidelines and how we went about It. Neuroendocrinology 2006; 84:155–157.
21. Jensen RT, Delle Fave G. Promising advances in the treatment of malignant pancreatic endocrine tumors. N Engl J Med 2011; 364:564–565.
22. Rinke A, Muller HH, Schade-Brittinger C, et al. Placebo-controlled, double-blind, prospective, randomized study on the effect of octreotide LAR in the control of tumor growth in patients with metastatic neuroendocrine midgut tumors: a report from the PROMID Study Group. J Clin Oncol 2009; 27:4656–4663.
23. Grozinsky-Glasberg S, Pavel M. Inhibition of mTOR in carcinoid tumors. Target Oncol 2012; 7:189–195.
24. Yao JC, Phan A. Overcoming antiangiogenic resistance. Clin Cancer Res 2011; 17:5217–5219.
25. Rindi G, Caplin M. mTOR inhibitor therapy for patients with carcinoid. Lancet 2011; 378:1978–1980.
26. Raymond E, Hobday T, Castellano D, et al. Therapy innovations: tyrosine kinase inhibitors for the treatment of pancreatic neuroendocrine tumors. Cancer Metastasis Rev 2011; 30 (Suppl 1):19–26.
27▪. Pavel ME, Hainsworth JD, Baudin E, et al. Everolimus
plus octreotide long-acting repeatable for the treatment of advanced neuroendocrine tumours
associated with carcinoid syndrome (RADIANT-2): a randomised, placebo-controlled, phase 3 study. Lancet 2011; 378:2005–2012.
This phase 3 randomized, placebo-controlled clinical trial in advanced NETs with carcinoid syndrome showed prolonged progression-free survival in patients treated with mTOR inhibitor everolimus in combination with long-acting octreotide.
28. Kloppel G, Anlauf M. Epidemiology, tumour biology and histopathological classification of neuroendocrine tumours
of the gastrointestinal tract. Best practice & research. Clin Gastroenterol 2005; 19:507–517.
29▪. Yao JC, Shah MH, Ito T, et al. Everolimus
for advanced pancreatic neuroendocrine tumors. N Engl J Med 2011; 364:514–523.
This phase 3 randomized, placebo-controlled clinical trial in patients with advanced PNETs showed prolonged progression-free survival in patients treated with mTOR inhibitor everolimus as compared with placebo.
30▪. Raymond E, Dahan L, Raoul JL, et al. Sunitinib
malate for the treatment of pancreatic neuroendocrine tumors. N Engl J Med 2011; 364:501–513.
This phase 3 randomized, placebo-controlled clinical trial in patients with advanced PNETs showed prolonged progression-free survival in patients treated with tyrosine kinase inhibitor sunitinib as compared with placebo.
31. Fazio N, Cinieri S, Lorizzo K, et al. Biological targeted therapies in patients with advanced enteropancreatic neuroendocrine carcinomas. Cancer Treat Rev 2010; 36 (Suppl 3):S87–S94.
32. Yao JC. Neuroendocrine tumors. Molecular targeted therapy for carcinoid and islet-cell carcinoma. Best practice & research. Clin Endocrinol Metab 2007; 21:163–172.
33▪▪. Jiao Y, Shi C, Edil BH, et al. DAXX/ATRX, MEN1, and mTOR pathway genes are frequently altered in pancreatic neuroendocrine tumors. Science 2011; 331:1199–1203.
Exomic sequencing study of tissue samples from 68 sporadic PNETs that showed high frequency of mutations of MEN-1, DAXX/ATRX and mTOR pathway genes, which is highly distinct from an analogous examination of PDAC samples.
34. Heaphy CM, de Wilde RF, Jiao Y, et al. Altered telomeres in tumors with ATRX and DAXX mutations. Science 2011; 333:425.
35. de Wilde RF, Heaphy CM, Maitra A, et al. Loss of ATRX or DAXX expression and concomitant acquisition of the alternative lengthening of telomeres phenotype are late events in a small subset of MEN-1 syndrome pancreatic neuroendocrine tumors. Mod Pathol 2012; 25:1033–1039.
36. Serra S, Zheng L, Hassan M, et al
. The FGFR4-G388R single nucleotide polymorphism alters pancreatic neuroendocrine tumor progression and response to mTOR inhibition therapy. Cancer Res 2012. [Epub ahead of print]