ABSTRACT: The diagnosed incidence rate of neuroendocrine tumors (NETs) is on the rise. Prevalence calculations show NETs to be more common then previously thought. Although generally thought to be indolent, advanced NETs remain incurable and are resistant to most cytotoxic agents. The available biologic agents have limited activity against these tumors. Novel and active agents are clearly needed. The recent emergence of molecularly targeted therapy in oncology has brought renewed interest in the development of novel agents for this group of diseases. This paper will review the molecular biology of NETs, promising novel targeted therapy approaches including agents targeting angiogenesis and mammalian target of rapamycin (mTOR) pathways, as well as pivotal phase III studies that may set new standards of care for this disease.
One hundred years after Oberndorfer coined the word “carcinoid,” neuroendocrine tumors (NETs) are thought to be rare tumors characterized by the capacity for hormone production and often an indolent course. Recent data from population-based registries have shown a significant rise in the diagnosed incidence of NETs over the past 3 decades. The possible explanation for this increasing incidence is multifactorial, including better diagnostics, improved awareness, and other undetermined environmental and genetic factors. Our analysis of the Surveillance, Epidemiology, and End Results (SEER) program database showed an age-adjusted incidence of 5.25 cases per 100,000 in 2004, accounting for just over 1% of diagnosed malignancies. This, together with a generally longer associated survival, leads to a prevalence of 35 per 100,000, which exceeds that of esophageal, gastric, pancreatic, or hepatobiliary cancer.
The management of NETs is generally guided by histology. High-grade or poorly differentiated NETs have an aggressive course, and their management parallels that of small-cell carcinoma of the lung.
In this manuscript, we will focus on the well- to moderately differentiated groups of neuroendocrine tumors that are more indolent but more resistant to cytotoxic chemotherapy. Today, “carcinoid” is typically used to describe a well- to moderately differentiated NET arising outside the pancreas. Those arising from the pancreas—pancreatic NET (PNET), or islet cell carcinoma—are recognized to have a different genetic profile, more aggressive clinical course, and different pattern of response to cytotoxic chemotherapy. These more differentiated tumors are also more likely to produce hormonal syndromes such as carcinoid syndrome (manifesting as flushing and diarrhea) and Verner-Morrison syndrome (watery diarrhea, hypokalemia, and achlorhydria, or WDHA syndrome). Advanced NETs are incurable. The median overall survival duration among patients with advanced well- to moderately differentiated NETs of the small bowel, cecum, appendix, pancreas, and rectum are 65, 55, 31, 27, and 26 months, respectively.
Current Systemic Therapy Options
Therapeutic objectives for patients with NETs include control of the paraneoplastic hormonal syndrome and tumor growth. Treatment often includes the use of somatostatin analogs (SSAs), interferon, chemotherapy, liver-directed therapy, surgical resection, or ablation of hepatic metastases, and radiation therapy for palliative benefit. Peptide receptor radiotherapy with [177Lu-DOTA0,Tyr3] octreotate or [90Y-DOTA0,Tyr3] octreotide represents an additional option in Europe.
Somatostatin receptors (SSTRs) are expressed on the majority of NETs. Somatostatin is a peptide hormone that decreases hormone leading to inactivation of the gene encoding menin located on chromosome 11 (11q13). The classic syndrome includes neoplasia of the parathyroid glands, anterior pituitary, endocrine pancreas, and endocrine duodenum, but rarely adrenal neoplasms and neuroendocrine tumors of the lung, thymus, and stomach. The MEN1 gene may also be involved in the tumorigenesis of sporadic carcinoid tumors. The loss of heterozygosity (LOH) on chromosome 11, the site of the MEN1 gene, is frequently found in pulmonary carcinoids. Menin appears to be involved in the regulation of gene transcription through its interaction as part of a histone methyltransferase complex. Menin also appears to be involved in the control of cell-cycle regulation through control of p27 expression.
Neurofibromatosis (NF) and tuberous sclerosis (TSC) can manifest with benign lesions, such as hamartomas and astrocytomas, as well as together with well-differentiated tumors in the brain, heart, skin, kidney, lung, and endocrine pancreas. The genes associated with TSC are TSC-1 (located on 9q34) and TSC-2 (located on 16p13.3), and they encode the proteins hamartin and tuberin, respectively. The development of PNETs in patients with TSC is thought to be particularly related to mutations in TSC2. TSC-1/2 complex functions as our endogenous inhibitor of mTOR, and is normally expressed in neuroendocrine cells. The loss of TSC function leads to constitutive activation of the mTOR pathway.
Patients with NF-1 may also develop ampullary of Vater carcinoids, as well as duodenal and pancreatic NETs. The NF-1 gene is a tumor-suppressor gene that is located on 17q11.2 and encodes a protein called neurofibromin. The latter is also linked with mTOR through regulation of TSC function. It has been shown that NF-1 acts as a negative regulator of mTOR—specifically, LOH of the NF-1 gene results in the loss of neurofibromin expression, constitutive mTOR activation, and, therefore, tumor development.
The main clinical features of VHL syndrome include retinal or central nervous system hemangioblastomas, clear cell renal carcinomas, pheocromocytomas, and pancreatic cystic tumors or PNETs. Pancreatic lesions can be seen in 20% to 75% of patients with VHL. In the largest series with 94 VHL families, it was reported that the majority of pancreatic lesions were true cysts (91%), whereas NETs represented 12% of cases. The VHL gene is a tumor-suppressor gene that is located on chromosome 3p25–26 and regulates hypoxia-induced cell proliferation and angiogenesis. Specifically, the VHL protein, acting mainly as E3 ubiquitin ligase, would target many proteins for degradation.
The most well known target of VHL protein is the hypoxia inducible factor 1α (HIF-1α). Under hypoxic conditions, HIF-1α is produced, translocates to the nucleus, and combines with HIF-1β, initiating the transcription of hypoxia-regulated genes such as vascular endothelial growth factor (VEGF). PNETs, are generally vascular but continue to express a large amount of HIF-1α, suggesting that aberrant regulation of HIF-1α expression may be involved in pathogenesis. Allelic deletion at chromosome 3p, the site of the VHL gene, has also been described as occurring frequently in sporadic NETs.
Molecular Genetics of Sporadic NETs
Outside of defined genetic syndromes, the genes responsible for the development of NETs are unknown. Recent studies using comparative genomic hybridization (CGH), or high-density single-nucleotide polymorphism arrays, suggest that the genetics of most sporadic NETs are far more complex than previously thought, and likely involve many genes in a multistep process in their development and progression. For example, on a genetic level, allelic deletion of chromosome 18 is frequently observed in midgut carcinoid tumors. On an epigenetic level, hypomethylation of LINE-1 elements is associated with chromosome 18 loss. The molecular genetics of PNETs is even more complex. Our most recent study, using a high-density single-nucleotide polymorphism array, showed a pattern of gain and loss across multiple chromosomes.
1. Yao JC, Hassan M, Phan A, et al: One hundred years after “carcinoid”: epidemiology of and prognostic factors for neuroendocrine tumors in 35,825 cases in the United States. J Clin Oncol 26:3063-3072, 2008.
2. Yao JC: Molecular targeted therapy for carcinoid and islet-cell carcinoma. Best Pract Res Clin Endocrinol Metab 21:163-172, 2007.
3. Kouvaraki MA, Ajani JA, Hoff P, et al: Fluorouracil, doxorubicin, and streptozocin in the treatment of patients with locally advanced and metastatic pancreatic endocrine carcinomas. J Clin Oncol 22:4762-4771, 2004.
4. Lamberts SW, Krenning EP, Reubi JC: The role of somatostatin and its analogs in the diagnosis and treatment of tumors. Endocr Rev 12:450-482, 1991.
5. Schnirer II, Yao JC, Ajani JA: Carcinoid: A comprehensive review. Acta Oncol 42:672-692, 2003.
6. Kvols L, Wiedenmann B, Oberg K, et al: Safety and efficacy of pasireotide (SOM230) in patients with metastatic carcinoid tumors refractory or resistant to octreotide LAR: Results of a phase II study (abstract 4082). J Clin Oncol 24(18S):198s, 2006.
7. Oberg K, Funa K, Alm G: Effects of leukocyte interferon on clinical symptoms and hormone levels in patients with mid-gut carcinoid tumors and carcinoid syndrome. N Engl J Med 309:129-133, 1983.
8. Detjen KM, Welzel M, Farwig K, et al: Molecular mechanism of interferon alfa-mediated growth inhibition in human neuroendocrine tumor cells. Gastroenterology 118:735-748, 2000.
9. Yao JC, Phan A, Hoff PM, et al: Targeting vascular endothelial growth factor in advanced carcinoid tumor: A random assignment phase II study of depot octreotide with bevacizumab and pegylated interferon alpha-2b. J Clin Oncol 26:1316-1323, 2008.
10. Janson ET, Oberg K: Long-term management of the carcinoid syndrome. Treatment with octreotide alone and in combination with alpha-interferon. Acta Oncol 32:225-229, 1993.
11. Frank M, Klose KJ, Wied M, et al: Combination therapy with octreotide and alpha-interferon: effect on tumor growth in metastatic endocrine gastroenteropancreatic tumors. Am J Gastroenterol 94:1381-1387, 1999.
12. Faiss S, Pape UF, Bohmig M, et al: Prospective, randomized, multicenter trial on the antiproliferative effect of lanreotide, interferon alfa, and their combination for therapy of metastatic neuroendocrine gastroenteropancreatic tumors—the International Lanreotide and Interferon Alfa Study Group. J Clin Oncol 21:2689-2696, 2003.
13. Arnold R, Rinke A, Klose KJ, et al: Octreotide versus octreotide plus interferon-alpha in endocrine gastroenteropancreatic tumors: a randomized trial. Clin Gastroenterol Hepatol 3:761-771, 2005.
14. Yao JC, Rindi G, Evans DB: Pancreatic endocrine tumors, in DeVita VT, Lawrence TS, Rosenberg SA (eds): Cancer: Principles & Practice of Oncology, pp 1702-1721. Philadelphia, Wolters Kluwer/Lippincott Williams & Wilkins, 2008.
15. Kulke MH, Stuart K, Enzinger PC, et al: Phase II study of temozolomide and thalidomide in patients with metastatic neuroendocrine tumors. J Clin Oncol 24:401-406, 2006.
16. Nagano Y, Kim DH, Zhang L, et al: Allelic alterations in pancreatic endocrine tumors identified by genome-wide single nucleotide polymorphism analysis. Endocr Relat Cancer 14:483-492, 2007.
17. Hughes CM, Rozenblatt-Rosen O, Milne TA, et al: Menin associates with a trithorax family histone methyltransferase complex and with the hoxc8 locus. Mol Cell 13:587-597, 2004.
18. Karnik SK, Hughes CM, Gu X, et al: Menin regulates pancreatic islet growth by promoting histone methylation and expression of genes encoding p27Kip1 and p18INK4c. Proc Natl Acad Sci U S A 102:14659-14664, 2005.
19. Verhoef S, van Diemen-Steenvoorde R, Akkersdijk WL, et al: Malignant pancreatic tumour within the spectrum of tuberous sclerosis complex in childhood. Eur J Pediatr 158:284-287, 1999.
20. Plank TL, Logginidou H, Klein-Szanto A, et al: The expression of hamartin, the product of the TSC1 gene, in normal human tissues and in TSC1- and TSC2-linked angiomyolipomas. Mod Pathol 12:539-545, 1999.
21. Johannessen CM, Reczek EE, James MF, et al: The NF1 tumor suppressor critically regulates TSC2 and mTOR. Proc Natl Acad Sci U S A 102:8573-8578, 2005.
22. Iliopoulos O: von Hippel-Lindau disease: Genetic and clinical observations. Front Horm Res 28:131-166, 2001.
23. Hammel PR, Vilgrain V, Terris B, et al: Pancreatic involvement in von Hippel-Lindau disease. The Groupe Francophone d’Etude de la Maladie de von Hippel-Lindau. Gastroenterology 119:1087-1095, 2000.
24. Zhao J, Moch H, Scheidweiler AF, et al: Genomic imbalances in the progression of endocrine pancreatic tumors. Genes Chromosomes Cancer 32:364-372, 2001.
25. Kim do H, Nagano Y, Choi IS, et al: Allelic alterations in well-differentiated neuroendocrine tumors (carcinoid tumors) identified by genome-wide single nucleotide polymorphism analysis and comparison with pancreatic endocrine tumors. Genes Chromosomes Cancer 47:84-92, 2008.
26. Choi IS, Estecio MR, Nagano Y, et al: Hypomethylation of LINE-1 and Alu in well-differentiated neuroendocrine tumors (pancreatic endocrine tumors and carcinoid tumors). Mod Pathol 20:802-810, 2007.
27. Terris B, Scoazec JY, Rubbia L, et al: Expression of vascular endothelial growth factor in digestive neuroendocrine tumours. Histopathology 32:133-138, 1998.
28. Ambs S, Bennett WP, Merriam WG, et al: Vascular endothelial growth factor and nitric oxide synthase expression in human lung cancer and the relation to p53. Br J Cancer 78:233-239, 1998.
29. Hobday TJ, Rubin J, Goldberg R, et al: Molecular markers in metastatic gastrointestinal neuroendocrine tumors (abstract 1078). Proc Am Soc Clin Oncol 22:269, 2003.
30. Cohen T, Herzog Y, Brodzky A, et al: Neuropilin-2 is a novel marker expressed in pancreatic islet cells and endocrine pancreatic tumours. J Pathol 198:77-82, 2002.
31. Paraskevakou H, Saetta A, Skandalis K, et al: Morphological-histochemical study of intestinal carcinoids and K-ras mutation analysis in appendiceal carcinoids. Pathol Oncol Res 5:205-210, 1999.
32. Kulke MH, Lenz HJ, Meropol NJ, et al: Activity of sunitinib in patients with advanced neuroendocrine tumors. J Clin Oncol 26:3403-3410, 2008.
33. Hobday TJ, Rubin J, Holen K, et al: MC044h, a phase II trial of sorafenib in patients (pts) with metastatic neuroendocrine tumors (NET): A Phase II Consortium (P2C) study. J Clin Oncol 25:4504, 2007.
34. Duran I, Kortmansky J, Singh D, et al: A phase II clinical and pharmacodynamic study of temsirolimus in advanced neuroendocrine carcinomas. Br J Cancer 95:1148-1154, 2006.
35. Yao JC, Phan AT, Chang DZ, et al: Efficacy of RAD001 (everolimus) and octreotide LAR in advanced low- to intermediate-grade neuroendocrine tumors: Results of a phase II study. J Clin Oncol 26:4311-4318, 2008.
36. Yao JC, Lombard-Bohas C, Baudin E, et al: A phase II trial of daily oral RAD001 (everolimus) in patients with metastatic pancreatic neuroendocrine tumors (NET) after failure of cytotoxic chemotherapy (abstract 508PD). Ann Oncol 19:viii167, 2008.
37. Yao JC, Zhang JX, Rashid A, et al: Clinical and in vitro studies of imatinib in advanced carcinoid tumors. Clin Cancer Res 13:234-240, 2007.
38. Gross DJ, Munter G, Bitan M, et al: The role of imatinib mesylate (Glivec) for treatment of patients with malignant endocrine tumors positive for c-kit or PDGF-R. Endocr Relat Cancer 13:535-540, 2006.
39. Papouchado B, Erickson LA, Rohlinger AL, et al: Epidermal growth factor receptor and activated epidermal growth factor receptor expression in gastrointestinal carcinoids and pancreatic endocrine carcinomas. Mod Pathol 18:1329-1335, 2005.
40. Hobday T, Holen K, Donehower R, et al: A phase II trial of gefitinib in patients (pts) with progressive metastatic neuroendocrine tumors (NET): A Phase II Consortium (P2C) study (abstract 4043). J Clin Oncol 24(18S):189s, 2006.
41. Shah MH, Young D, Kindler HL, et al: Phase II study of the proteasome inhibitor bortezomib (PS-341) in patients with metastatic neuroendocrine tumors. Clin Cancer Res 10:6111-6118, 2004.
42. Shah MH, Binkley P, Chan K, et al: Cardiotoxicity of histone deacetylase inhibitor depsipeptide in patients with metastatic neuroendocrine tumors. Clin Cancer Res 12:3997-4003, 2006.
43. Anthony L, Chester M, Michael S, et al: Phase II open-label clinical trial of vatalanib (PTK/ZK) in patients with progressive neuroendocrine cancer (abstract 14624). J Clin Oncol 26(15S):637s, 2008.