Cancer Metastasis Rev (2014) 33:367–372 DOI 10.1007/s10555-013-9467-z
Systemic therapeutic strategies for GEP-NETS: what can we expect in the future? E. Raymond & R. García-Carbonero & B. Wiedenmann & E. Grande & M. Pavel
Published online: 28 December 2013 # Springer Science+Business Media New York 2013
Abstract Over the last few years, there have been important advances in the understanding of the molecular biology of neuroendocrine tumors (NETs) that have already translated into relevant advances in the clinic. Several studies have extensively assessed the mutational profile of NETs, and have shown the key roles that angiogenesis and the PI3K-AKTmTOR pathway play in the pathogenesis of these tumors. Recent data has also revealed the potential relevance of transcription factors such as death domain-associated protein, xlinked mental retardation, and α-thalassemia syndrome protein or ataxia telangiectasia-mutated in NETs of pancreatic origin. This fast progress is leading to a rapidly increasing number of new agents being explored in the field of NETs. However, and despite some unquestionable success, objective remission rates remain low, and evidence of a substantial
E. Raymond (*) Department of Medical Oncology, Beaujon University Hospital, Clichy, France e-mail: [email protected] R. García-Carbonero Department of Medical Oncology, Hospital Universitario Virgen del Rocío, Instituto de Biomedicina de Sevilla (IBIS) [Universidad de Sevilla, CSIC, HUVR], Sevilla, Spain e-mail: [email protected] B. Wiedenmann : M. Pavel Department of Hepatology, Gastroenterology, Endocrinology and Metabolic Diseases, Charité University Medicine, Humboldt University, Berlin, Germany B. Wiedenmann e-mail: [email protected] M. Pavel e-mail: [email protected] E. Grande Department of Medical Oncology, Hospital Universitario Ramón y Cajal, Madrid, Spain e-mail: [email protected]
survival impact is lacking. Thus, there is an important need to improve our ability to identify patients most likely to benefit from specific therapies, and to incorporate biomarkers in the management of NETs. In addition, further efforts to understand mechanisms of escape and acquired resistance to the different available agents is of utmost importance, and will likely require performing paired tumor biopsies (prior and after treatment) or sequential sampling of surrogate tissues. Combinations of approved agents with new agents, either in a rational or biomarker-driven manner, are certainly warranted in this field. Likewise, sequential strategies to modulate and compensate for escape phenomenons are also of great interest. It should also be noted, however, that targeted agents are not innocuous and frequently yield toxicities that need to be adequately addressed by experienced specialists, particularly when drug combinations are considered. This review summarizes the salient data on biomarker and new agent development for the treatment of NETs. Keywords Everolimus . Temsirolimus . Rapamycin . Sunitinib . Temozolomide . Somatostatin analogues . Pancreatic neuroendocrine tumors . Carcinoid . mTOR . VEGFR . PDGFR . Angiogenesis . Drug combination . Sequencial therapy . Drug resistance . Personalized medicine
1 Patient selection and sequencing strategies in the future management of NETs During the last few years, several treatment options have been made available for patients with advanced neuroendocrine tumors (NETs), including cytotoxic therapy and noveltargeted agents. Although each novel treatment has had a preclinical rational development, little translational data are yet available to help select the right treatment for the individual patient. Nevertheless, as options remain limited and lifespan of patients with NETs is long, they are likely to
368
eventually receive all available treatments one after the other. Current guidelines are based on available clinical evidence and do not incorporate biomarker analysis (Fig. 1). Therefore, new algorithms are needed to guide treatment selection. The key question is whether we can select patients for specific treatments based on the clinical and biological information available at the time of presentation. Baseline available information usually includes, besides extent of disease, the degree of tumor differentiation (well vs poorly differentiated), the proliferation index (20 % cutoff values discriminate low, intermediate, and highly proliferative neoplasms), the results of somatostatin receptor imaging (octreoscan or 68Ga-DOTATOC-PET/CT to determine SSR expression status) and, occasionally, 18FDG-PET-scan (glucose uptake in cancer cells proliferating in aerobic conditions), which all provide direct or indirect assessments of tumor biology and cell proliferation. Information that is not readily available but could potentially be relevant for clinical management is the differential somatostatin receptor (sstr) expression, the degree of vascular endothelial growth factor (VEGF)/vascular endothelial growth factor receptor (VEGFR), and mammalian target of rapamycin (mTOR) activation, the mutational status of NET-related genes including those involved in the apoptosis/repair machinery (P53, BCL2, O6-methylguanine-DNA methyltransferase (MGMT), MMR, NER). It should also be noted that the information, when available, often comes from the evaluation of the primary tumor which may not properly reflect the biology of metastatic disease at the time therapeutic decisions are being made. Thus, studies incorporating predictive biomarkers for systemic therapies shall consider to rebiopsy the tumor before initiation of therapy to more accurately characterize tumor biology. Table 1 summarizes the key mutational activation pathways in pancreatic NETs (pNET) [1–3]. These mutations can be determined in formalin-fixed paraffin-embedded
Fig. 1 National Comprehensive Cancer Network (NCCN) pancreatic NET guidelines
Cancer Metastasis Rev (2014) 33:367–372 Table 1 Genomic alterations in pNETs Gene
Frequency (%)
Pathway
MEN1
30–44
DAXX/ATRX TSC2 PTEN
43 9 7
PI3K/Akt/mTOR Chromatin remodeling Chromatin remodeling PI3K/Akt/mTOR
PI3K ATM
1 6
ATM/p53 pathway
DAXX death domain-associated protein; ATRX alpha thalassemia/mental retardation syndrome x-linked; ATM ataxia telangiectasia-mutated
(FFPE) tissues at a low cost and have proved to be very informative. For example, decreased expression of phosphatase and tensin homolog (PTEN) and tuberous sclerosis 2 (TSC2) determined in FFPE tissues has been linked to poor prognosis [4]. Patients with PTEN-deficient tumors, or patients with tumors bearing mutations leading to mTOR activation might presumably be better candidates for mTOR-targeted, although this remains to be properly assessed in the clinic. Another salient feature of NETs is that they are highly angiogenic. NETs are highly vascularized tumors, and VEGF and VEGFR are overexpressed in 60–84 % of carcinoids and pancreatic islet cells NETs. Other pro-angiogenic factors like the platelet-derived growth factor and the fibroblast growth factor (FGF) have also been involved in NET development and progression. Some authors have correlated VEGF expression with increased angiogenesis, metastases, and decreased progression-free survival among patients with gastroenteropancreatic (GEP)-NETs. Moreover, activation of the hypoxia-inducible factor (HIF)-pathway has been correlated with a shortened disease-free survival in pancreatic endocrine tumors [5, 6].
Cancer Metastasis Rev (2014) 33:367–372
Currently, there are four targeted approaches available for NET tumors, including the sstr which can be targeted with somatostatin analogs (SSA) or radiopeptides (PRRT), interferon (IFN) receptors, and VEGF/VEGFR or mTOR pathways that can be inactivated using either antiangiogenic agents or mTOR inhibitors [5]. The SSA octreotide has been shown to improve progression-free survival (PFS) in lowgrade midgut NETs with low hepatic tumor burden, both in functioning and non-functioning [7]. On the other hand, sunitinib and everolimus improved PFS in a well-differentiated pNETs [8, 9]. Treatment options are therefore increasing. However, there is still no validated biomarker to guide the selection of the most appropriate strategy for each individual patient. With regard to more conventional chemotherapy, several non-controlled studies have suggested a potential role for MGMT expression, an enzyme that repairs DNA damage induced by alkylating agents, as a predictive marker of resistance to this class of agents. Indeed, temozolomide, a cytotoxic, DNA-methylating agent, with promising activity in the field of NETs appeared to be particularly effective in tumors lacking the MGMT repair enzyme, with response rates of 0 vs 80 %, and PFS of 9.3 vs 19 months in MGMT positive vs negative tumors, respectively [10–12]. This lack of expression has been documented in approximately half of pancreatic NETs and is rarely observed in tumors of intestinal origin. However, data are still controversial and limited; thus, determination of MGMT expression does not allow preselection for temozolomide therapy yet. Another critical issue, particularly in pNETs, is that tumor biology may change over time, either spontaneously or as a consequence of treatment. Indeed, because these patients often have a good prognosis and receive, in general, multiple lines of therapy over prolonged periods of time, the question that then arises is to what extent and how each treatment impacts on tumor biology over time and how the sequential administration of different agents could potentially modify or revert molecular events driving drug resistance and/or tumor progression. NETs, like renal cell cancer and liver cancer, among others, are HIF1α-addicted tumors. HIF1α is regulated by oxygen in a VHL-dependent manner. Under normoxic conditions, VHL induces the degradation of HIF1α. In tumors lacking Von Hippel–Lindau (VHL) and in hypoxic conditions, HIF1α accumulates and triggers the transcription of multiple survival pathways including angiogenesis. HIF1α can also be activated by mTOR under either normoxic or hypoxic conditions. mTOR can be activated by membrane receptor-mediated pathways such as epidermal growth factor receptor (EGFR) or insulin growth factor receptor (IGF1R) through activation of intracellular signaling pathways such as RAS, MEK, and PI3K and also by nutrient and oxygen supply. HIF1α activates multiple pathways such as the Glut transporters, TGFβ,
369
chemokine (C-X-C motif) receptor 4 (CXCR4)/SDF1, VEGF/VEGFR, and carbonic anhydrase that mediate nutrient uptake, growth, survival, chemotaxis, and angiogenesis among others. HIF1α is also involved in apoptosis, which may be linked to the response to chemotherapy. HIF1α-addicted tumors are likely to respond well to targeted agents. However, resistance invariably emerges in most patients following drug exposure [13]. The pattern of tumor progression in the clinic appears to be very similar from one tumor type to another. HIF1α can be activated in NETs by genetic mechanisms such as mutations in VHL that occur in up to 30 % of sporadic pNETs or in familial cases such as Von Hippel−Lindau disease. In addition, HIF1α can be also activated as a consequence of hypoxia induced by agents such as angiogenesis inhibitors. Activation of HIF1α results in activation of canonical pathways such as mTOR and VEGF, which leads to increased sensitivity to drugs targeting these pathways. However, other routes such as those mediated by FGF, ephrins, and angiopoietin may be activated and ultimately lead to EMT and evasion or resistance to VEGF and mTOR inhibition [13]. Thus, tumor plasticity in response to chemotherapy or targeted agents can substantially modify these equilibriums and may substantially impact on the tumor biology and natural history of NETs. In this sense, the SEQTOR ENETS-GETNE trial is of great interest as it will compare conventional chemotherapy with STZ + 5FU followed by everolimus versus the opposite sequence with composite PFS as the primary endpoint. Preclinical cancer models may help understand some of these relevant biological shifts that occur as a consequence of drug exposure. In a study of renal cell cancer xenografts, in which it was demonstrated that mice treated with sunitinib had a significant improvement in time to progression as compared to those treated with placebo, the analysis of tumors upon disease progression showed an increased proportion of cancer stem cells and higher levels of mesenchymal markers, as well as of other pro-angiogenic and invasive markers such as CXCR4 and SDF1. At the time of progression, G proteincoupled receptors and chemokine receptors such as CXCR12SDF1 and CXCR4 were involved in switching from VEGFmediated angiogenesis eventually leading to resistance to VEGF inhibitors. However, these new emerging receptors and pathways may still signal through PI3K/Akt/mTOR suggesting there could be susceptibility to mTOR inhibitors [13]. Second line treatment with everolimus was indeed effective in these tumors.
2 New drugs and strategies in nets There have been important advances in the management of NET. However, a significant number of issues remain unsolved. Complete or remarkable partial tumor remissions are
370
Cancer Metastasis Rev (2014) 33:367–372
pathway involved in pancreatic cancer tumorigenesis, and the gastric inhibitory polypeptide receptors that can be a target for PRRT [16, 17].
rare with targeted drugs including SSA, IFN, everolimus, sunitinib, and PRRT strategies. The added value is prolonged PFS or TTP; if this translates into an improvement of overall survival is still unknown. Systemic chemotherapy is effective in poorly differentiated neoplasms or pNET but efficacy remains transient. Overall 5-year survival in stage IV disease is still limited to 40–50 % in pNETs and 70–90 % in midgut NETs. There is a number of relevant questions unanswered as follows: (a) Which drug to use in which line of treatment?; (b) Is sequential monotherapy or combination therapy the best treatment strategy?; (c) What is the optimal combination of agents considering the balance between efficacy and toxicity as well as the crosstalk of the pathways they target? Other key questions are related to the onset and duration of drug treatment: should treatment be started at the time of disease progression or at diagnosis to prevent tumor progression; for how long should the treatment in responding patients be continued; and is there a role for maintenance therapy?
LX1606 is an inhibitor of tryptophan hydroxylase, the rate-limiting step in 5-hydroxytryptamin synthesis, which is currently undergoing clinical development. Two phaseII studies have evaluated the safety and efficacy of this drug in subjects with symptomatic carcinoid syndrome refractory to stable-dose long-acting SSA. In one of these studies, 56 % of patients experienced a biochemical response (>50 % reduction in urinary 5-HIAA) and 28 % a clinical response (>30 % reduction in bowel movements), whereas no placebo subjects achieved either biochemical or clinical response [18]. Based on the results of both studies, a large placebo-controlled phase-III study is under development.
2.1 Novel target inhibition
2.2 c-Met inhibition in VEGFR-resistant tumors
Despite the undeniable therapeutic progress made over the last years in the field of NETs, the discovery of novel effective drugs is still of utmost importance since all medical treatments do not lead to cure but just to transient disease control. Over the years, a number of agents such as proteasome inhibitors or STAT3 inhibitors have been developed but abandoned for either excess toxicity or lack of efficacy. Table 2 summarizes some of the most promising agents currently in clinical development. Several new targets are emerging with potential in NETs. The NOTCH-1 genes, which function as tumor suppressor genes, are differentially expressed in NETs with higher expression in tumors of rectal vs pancreatic vs intestinal origin [14]. Another interesting target involved in cell cycle control is aurora kinase. Agents targeting this enzyme, such as Danusertib, have shown efficacy in an orthotopic xenograft model of NETs [15]. Inhibitors of the Src kinase family member SFK, that coordinate cell adhesion and spreading, has shown synergic inhibition in combination with mTOR inhibitors in preclinical pNET models. Another two potential targets are the hedgehog signaling inhibitors, a
Another critical issue when treating NETs with angiogenesis inhibitors, as demonstrated in some preclinical models, is that invasion and metastasis may be induced after inhibition of VEGF signaling. Indeed, in the RIP-Tag2 mice model, treatment with anti-VEGF antibody reduced tumor burden but increased tumor hypoxia, HIF-1α, and c-Met activation leading to increased invasion and metastasis [19]. Based on these observations, Cabozantinib (XL184), a drug that simultaneously blocks VEGF and c-MET, is currently being assessed in NETs in phase II clinical trials.
2.1.1 Serotonin synthesis inhibitors
2.3 PI3K/Akt/mTOR pathway inhibition The PI3K/Akt/mTOR pathway is now well established as a critical pathway to target in NETs [15]. Beyond everolimus, there are a number of specific inhibitors of PI3K and mTOR kinase, and Akt and S6 kinase in development. Furthermore, some of these agents have been combined with novel SSA, such as pasireotide, as this last class of drugs, by reducing levels of IGF1, also contributes to inhibition of this pathway.
Table 2 Selected new agents undergoing current clinical investigation in NETs Drug
Mechanism of action
Application
Clinical trials
Telotristat etiprate (LX 1606)
Serotonin synthesis inhibitor; (tryptophan hydroxylase) VEGFR-1,2,3 inhibitor Met inhibitor Dual PI3K/mTOR inhibitor Akt inhibitor
Carcinoid syndrome
Placebo-controlled phase III (TELESTAR)
Angiogenesis inhibition Tumor growth inhibition Tumor growth inhibition, pNET Tumor growth inhibition
Placebo-controlled phase II randomized Phase II Phase II Phase II
Axitinib Cabozantinib BEZ235 MK2206
Cancer Metastasis Rev (2014) 33:367–372
There are several PI3K inhibitors currently undergoing clinical assessment such as BEZ235 (dual PI3K/mTOR inhibitor), CC-223 (dual mTOR1 and mTOR2 inhibitor), BKM120 (pan-PI3K inhibitor) and BYL719 (selective PI3Kα inhibitor) which result in cell growth inhibition, induction of apoptosis in various cell lines/animal models, and antiangiogenic effects [14, 20–22]. BEZ235 is currently under investigation in two phase-II studies in pNETs. The BEZ235F2201 trial is evaluating BEZ235 in patients with advanced G1-2 pNETs, refractory to mTOR inhibitors. Measurable disease is required and no more than three prior systemic lines and no prior PI3Ktargeted therapy is allowed. The CBEZ235Z2401 trial is a randomized phase-II study in which patients with advanced pNETs naive of mTOR-targeted therapy are randomly assigned to receive either BEZ235 or everolimus. The primary endpoint of these studies is PFS (www.clinicaltrials.gov). 2.4 Combination strategies As single-agent activity is still limited, one area of intense development is combination therapy. So far, there are several well-established systemic treatment strategies for these patients including SSA, IFN-alpha, angiogenesis inhibitors, mTOR inhibitors, and chemotherapy [23]. Multiple small and heterogeneous studies have evaluated different angiogenesis inhibitors in combination with other angiogenesis inhibitors or chemotherapy. The results from these studies are difficult to interpret due to small sample size and heterogeneity of patient population, which often includes carcinoids of different sites of origin and pancreatic NETs. So far, there is only one large randomized study testing bevacizumab plus octreotide vs IFN plus octreotide in patients with metastatic or locally advanced non-pancreatic NETs, the results of which shall be soon available and are awaited with great interest [23]. A consecutive study, the CALGB Trial (80701) is addressing the efficacy of adding bevacitumab to everolimus monotherapy. To date, however, the real add-on value of antiangiogenic drugs remains uncertain until results from these comparative ongoing trials become available. Meanwhile, preclinical research is of great importance to help prioritize strategies to be tested in clinical trials. For example, combination of EGFR and mTOR inhibitors led to improved survival in the RIP Tag2 preclinical model, consistent with the observation of progressive activation of the EGFR and mTOR pathways as tumors develop in these mice [24]. To date, there is a large number of ongoing studies including those combining everolimus with pasireotide (COOPERATE-1/COOPERATE-2); IGF1R Inhibitor (cixutumumab) + octreotide; and EGFR inhibitor (erlotinib). Regarding angiogenesis inhibitors, there are studies assessing the combination of bevacizumab plus everolimus (NCI CALGB80701) or bevacizumab plus STZ/5-FU in pNETs, bevacizumab with octreotide and pertuzumab; or octreotide
371
and capecitabine in advanced GI NETs. In addition, there are several studies evaluating different combinations with other tyrosine kinase inhibitors with antiangiogenic activity such as Axitinib and octreotide (GETNE AXI-IIG-02, EUDRACT: 2011-001550-29) in GI NETs, or sorafenib plus cyclophosphamide or everolimus. Preliminary results of a recent study assessing the combination of temsirolimus (25-mg iv days 1, 8, 15, 22) and bevacizumab (10 mg/kg iv days 1, 15) in 55 patients with progressive G1-2 pNETs indicate promising activity with an acceptable toxicity profile. An objective response was documented in 37 % of treated patients, and PFS rate was 80 and 49 % at 6 and 12 months. As the number of potential combination therapies increases, however, it should be kept in mind that treatment with targeted agents is not innocuous [25, 26]. In this regard, a recent analysis of drug approvals for cancer along the period 2000–2010 that included 38 new agents showed that the risk of death, treatment discontinuation, and grades 3–4 adverse events increases with targeted agents, especially with multiple tyrosine kinase inhibitors, with a HR in the 1.2 to 1.4 range [21]. Likewise, a meta-analysis of 4,679 patients enrolled in 10 randomized controlled trials (2,856 patients from sorafenib, 1,388 from sunitinib, and 435 from pazopanib trials) showed that the incidence of fatal adverse events related to VEGFR TKIs was 1.5 % (95 % CI, 0.8 to 2.4 %), with a RR of 2.23 (95 % CI, 1.12 to 4.44 %; P=0.023) compared with control patients [22]. Careful patient surveillance by experienced specialists is therefore mandatory, and the use of noveltargeted agents should be clearly justified in the clinical setting.
3 Conclusions The molecular biology of NETs is still poorly understood. The treatment of NETs has advanced, nevertheless, significantly over the past years. More preclinical models are warranted to select “best combinations” (consider, e.g., crosstalk of receptors). Major challenges represent identification of valid targets and tools that allow selection of the population most likely to benefit from a specific drug. With this purpose, and as the tumor biology is likely to change over time, collecting paired tumor biopsies may help guide physicians towards optimal sequences of drug administration. Several classes of novel drugs are under clinical investigation in phase-II trials though overall response rates are still low. Interesting new agents with novel and rational mechanism of action include the tryptophan hydroxylase inhibitor LX1606 in serotonin-producing tumors; combined PI3K/mTOR inhibitor (BEZ235) after failure to mTOR inhibitors and simultaneous inhibition of c-Met and VEGF after anti-VEGF therapy (e.g., Cabozantinib). Preclinical data support the hypothesis that novel drugs targeting Notch signaling,
372
Aurora kinases and Src kinases deserve further evaluation in NETs. SSA, mTOR inhibitors, antiangiogenic agents, and cytotoxic drugs are currently being assessed in combination therapy trials. There is, however, limited evidence for some of the combination therapies and optimal combinations still need to be identified. The use of novel-targeted agents may also be associated with increased morbidity and treatment-related mortality, and thus requires careful surveillance of patients for appropriate early management of side effects. Finally, the optimal sequencing of therapies is still unclear (further evidence will be provided by future trials, e.g., SEQTOR trial in pNET).
References 1. Corbo, V., et al. (2012). Pancreatic endocrine tumours: mutational and immunohistochemical survey of protein kinases reveals alterations in targetable kinases in cancer cell lines and rare primaries. Annals of Oncology, 23, 127–134. 2. Corbo, V., et al. (2010). MEN1 in pancreatic endocrine tumors: analysis of gene and protein status in 169 sporadic neoplasms reveals alterations in the vast majority of cases. Endocrine-Related Cancer, 17, 771–783. 3. Jiao, Y., et al. (2011). DAXX/ATRX, MEN1, and mTOR pathway genes are frequently altered in pancreatic neuroendocrine tumors. Science, 331, 1199–1203. 4. Missiaglia, E., et al. (2010). Pancreatic endocrine tumors: expression profiling evidences a role for AKT-mTOR pathway. Journal of Clinical Oncology, 28, 245–255. 5. Faivre, S., Sablin, M. P., Dreyer, C., & Raymond, E. (2010). Novel anticancer agents in clinical trials for well-differentiated neuroendocrine tumors. Endocrinology and Metabolism Clinics of North America, 39, 811–826. 6. Couvelard, A., et al. (2005). Microvascular density and hypoxiainducible factor pathway in pancreatic endocrine tumours: negative correlation of microvascular density and VEGF expression with tumour progression. British Journal of Cancer, 92, 94–101. 7. Rinke, A., et al. (2009). 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. Journal of Clinical Oncology, 27, 4656–4663. 8. Raymond, E., et al. (2011). Sunitinib malate for the treatment of pancreatic neuroendocrine tumors. The New England Journal of Medicine, 364, 501–513. 9. Yao, J. C., et al. (2011). Everolimus for advanced pancreatic neuroendocrine tumors. The New England Journal of Medicine, 364, 514–523. 10. Ekeblad, S., et al. (2007). Temozolomide as monotherapy is effective in treatment of advanced malignant neuroendocrine tumors. Clinical Cancer Research, 13, 2986–2991.
Cancer Metastasis Rev (2014) 33:367–372 11. Kulke, M. H., et al. (2009). O6-methylguanine DNA methyltransferase deficiency and response to temozolomide-based therapy in patients with neuroendocrine tumors. Clinical Cancer Research, 15, 338–345. 12. Strosberg, J. R., et al. (2011). First-line chemotherapy with capecitabine and temozolomide in patients with metastatic pancreatic endocrine carcinomas. Cancer, 117, 268–275. 13. Tijeras-Raballand, A., et al. (2012). Resistance to targeted therapies in pancreatic neuroendocrine tumors (PNETs): molecular basis, preclinical data, and counteracting strategies. Targeted Oncology, 7, 173–181. 14. Maira, S. M., et al. (2008). Identification and characterization of NVP-BEZ235, a new orally available dual phosphatidylinositol 3kinase/mammalian target of rapamycin inhibitor with potent in vivo antitumor activity. Molecular Cancer Therapeutics, 7, 1851–1863. 15. Fraedrich, K., et al. (2012). Targeting aurora kinases with danusertib (PHA-739358) inhibits growth of liver metastases from gastroenteropancreatic neuroendocrine tumors in an orthotopic xenograft model. Clinical Cancer Research, 18, 4621–4632. 16. Fendrich, V., et al. (2011). Hedgehog inhibition with cyclopamine represses tumor growth and prolongs survival in a transgenic mouse model of islet cell tumors. Annals of Surgery, 253, 546–552. 17. Waser, B., et al. (2012). Glucose- dependent insulinotropic polypeptide receptors in most gastroenteropancreatic and bronchial neuroendocrine tumors. Journal of Clinical Endocrinology and Metabolism, 97, 482–488. 18. O’Dorisio, B., et al. (2012) Relief of bowel-related symptoms with telotristat etiprate in octreotide refractory carcinoid syndrome: Preliminary results of a double-blind, placebo-controlled multicenter study. Journal Clinical Oncology, 30, suppl; abstr 4085. 19. Sennino, B., et al. (2012). Suppression of tumor invasion and metastasis by concurrent inhibition of c-Met and VEGF signaling in pancreatic neuroendocrine tumors. Cancer Discovery, 2, 270–287. 20. Yap, T. A., et al. (2008). Targeting the PI3K-AKT-mTOR pathway: progress, pitfalls, and promises. Current Opinion in Pharmacology, 8, 393–412. 21. Serra, V., et al. (2008). NVP-BEZ235, a dual PI3K/mTOR inhibitor, prevents PI3K signaling and inhibits the growth of cancer cells with activating PI3K mutations. Cancer Research, 68, 8022–8030. 22. Schnell, C. R., et al. (2008). Effects of the dual phosphatidylinositol 3-kinase/mammalian target of rapamycin inhibitor NVP-BEZ235 on the tumor vasculature: implications for clinical imaging. Cancer Research, 68, 6598–6607. 23. Pavel, M. (2013). Translation of molecular pathways into clinical trials of neuroendocrine tumors. Neuroendocrinology, 97(1), 99–112. 24. Chiu, C. W., Nozawa, H., & Hanahan, D. (2010). Survival benefit with proapoptotic molecular and pathologic responses from dual targeting of mammalian target of rapamycin and epidermal growth factor receptor in a preclinical model of pancreatic neuroendocrine carcinogenesis. Journal of Clinical Oncology, 28, 4425–4433. 25. Niraula, S., et al. (2012). The price we pay for progress: a metaanalysis of harms of newly approved anticancer drugs. Journal of Clinical Oncology, 30, 3012–3019. 26. Schutz, F. A., Je, Y., Richards, C. J., & Choueiri, T. K. (2012). Metaanalysis of randomized controlled trials for the incidence and risk of treatment-related mortality in patients with cancer treated with vascular endothelial growth factor tyrosine kinase inhibitors. Journal of Clinical Oncology, 30, 871–877.
Systemic therapeutic strategies for GEP-NETS: what can we expect in the future? E. Raymond & R. García-Carbonero & B. Wiedenmann & E. Grande & M. Pavel
Published online: 28 December 2013 # Springer Science+Business Media New York 2013
Abstract Over the last few years, there have been important advances in the understanding of the molecular biology of neuroendocrine tumors (NETs) that have already translated into relevant advances in the clinic. Several studies have extensively assessed the mutational profile of NETs, and have shown the key roles that angiogenesis and the PI3K-AKTmTOR pathway play in the pathogenesis of these tumors. Recent data has also revealed the potential relevance of transcription factors such as death domain-associated protein, xlinked mental retardation, and α-thalassemia syndrome protein or ataxia telangiectasia-mutated in NETs of pancreatic origin. This fast progress is leading to a rapidly increasing number of new agents being explored in the field of NETs. However, and despite some unquestionable success, objective remission rates remain low, and evidence of a substantial
E. Raymond (*) Department of Medical Oncology, Beaujon University Hospital, Clichy, France e-mail: [email protected] R. García-Carbonero Department of Medical Oncology, Hospital Universitario Virgen del Rocío, Instituto de Biomedicina de Sevilla (IBIS) [Universidad de Sevilla, CSIC, HUVR], Sevilla, Spain e-mail: [email protected] B. Wiedenmann : M. Pavel Department of Hepatology, Gastroenterology, Endocrinology and Metabolic Diseases, Charité University Medicine, Humboldt University, Berlin, Germany B. Wiedenmann e-mail: [email protected] M. Pavel e-mail: [email protected] E. Grande Department of Medical Oncology, Hospital Universitario Ramón y Cajal, Madrid, Spain e-mail: [email protected]
survival impact is lacking. Thus, there is an important need to improve our ability to identify patients most likely to benefit from specific therapies, and to incorporate biomarkers in the management of NETs. In addition, further efforts to understand mechanisms of escape and acquired resistance to the different available agents is of utmost importance, and will likely require performing paired tumor biopsies (prior and after treatment) or sequential sampling of surrogate tissues. Combinations of approved agents with new agents, either in a rational or biomarker-driven manner, are certainly warranted in this field. Likewise, sequential strategies to modulate and compensate for escape phenomenons are also of great interest. It should also be noted, however, that targeted agents are not innocuous and frequently yield toxicities that need to be adequately addressed by experienced specialists, particularly when drug combinations are considered. This review summarizes the salient data on biomarker and new agent development for the treatment of NETs. Keywords Everolimus . Temsirolimus . Rapamycin . Sunitinib . Temozolomide . Somatostatin analogues . Pancreatic neuroendocrine tumors . Carcinoid . mTOR . VEGFR . PDGFR . Angiogenesis . Drug combination . Sequencial therapy . Drug resistance . Personalized medicine
1 Patient selection and sequencing strategies in the future management of NETs During the last few years, several treatment options have been made available for patients with advanced neuroendocrine tumors (NETs), including cytotoxic therapy and noveltargeted agents. Although each novel treatment has had a preclinical rational development, little translational data are yet available to help select the right treatment for the individual patient. Nevertheless, as options remain limited and lifespan of patients with NETs is long, they are likely to
368
eventually receive all available treatments one after the other. Current guidelines are based on available clinical evidence and do not incorporate biomarker analysis (Fig. 1). Therefore, new algorithms are needed to guide treatment selection. The key question is whether we can select patients for specific treatments based on the clinical and biological information available at the time of presentation. Baseline available information usually includes, besides extent of disease, the degree of tumor differentiation (well vs poorly differentiated), the proliferation index (20 % cutoff values discriminate low, intermediate, and highly proliferative neoplasms), the results of somatostatin receptor imaging (octreoscan or 68Ga-DOTATOC-PET/CT to determine SSR expression status) and, occasionally, 18FDG-PET-scan (glucose uptake in cancer cells proliferating in aerobic conditions), which all provide direct or indirect assessments of tumor biology and cell proliferation. Information that is not readily available but could potentially be relevant for clinical management is the differential somatostatin receptor (sstr) expression, the degree of vascular endothelial growth factor (VEGF)/vascular endothelial growth factor receptor (VEGFR), and mammalian target of rapamycin (mTOR) activation, the mutational status of NET-related genes including those involved in the apoptosis/repair machinery (P53, BCL2, O6-methylguanine-DNA methyltransferase (MGMT), MMR, NER). It should also be noted that the information, when available, often comes from the evaluation of the primary tumor which may not properly reflect the biology of metastatic disease at the time therapeutic decisions are being made. Thus, studies incorporating predictive biomarkers for systemic therapies shall consider to rebiopsy the tumor before initiation of therapy to more accurately characterize tumor biology. Table 1 summarizes the key mutational activation pathways in pancreatic NETs (pNET) [1–3]. These mutations can be determined in formalin-fixed paraffin-embedded
Fig. 1 National Comprehensive Cancer Network (NCCN) pancreatic NET guidelines
Cancer Metastasis Rev (2014) 33:367–372 Table 1 Genomic alterations in pNETs Gene
Frequency (%)
Pathway
MEN1
30–44
DAXX/ATRX TSC2 PTEN
43 9 7
PI3K/Akt/mTOR Chromatin remodeling Chromatin remodeling PI3K/Akt/mTOR
PI3K ATM
1 6
ATM/p53 pathway
DAXX death domain-associated protein; ATRX alpha thalassemia/mental retardation syndrome x-linked; ATM ataxia telangiectasia-mutated
(FFPE) tissues at a low cost and have proved to be very informative. For example, decreased expression of phosphatase and tensin homolog (PTEN) and tuberous sclerosis 2 (TSC2) determined in FFPE tissues has been linked to poor prognosis [4]. Patients with PTEN-deficient tumors, or patients with tumors bearing mutations leading to mTOR activation might presumably be better candidates for mTOR-targeted, although this remains to be properly assessed in the clinic. Another salient feature of NETs is that they are highly angiogenic. NETs are highly vascularized tumors, and VEGF and VEGFR are overexpressed in 60–84 % of carcinoids and pancreatic islet cells NETs. Other pro-angiogenic factors like the platelet-derived growth factor and the fibroblast growth factor (FGF) have also been involved in NET development and progression. Some authors have correlated VEGF expression with increased angiogenesis, metastases, and decreased progression-free survival among patients with gastroenteropancreatic (GEP)-NETs. Moreover, activation of the hypoxia-inducible factor (HIF)-pathway has been correlated with a shortened disease-free survival in pancreatic endocrine tumors [5, 6].
Cancer Metastasis Rev (2014) 33:367–372
Currently, there are four targeted approaches available for NET tumors, including the sstr which can be targeted with somatostatin analogs (SSA) or radiopeptides (PRRT), interferon (IFN) receptors, and VEGF/VEGFR or mTOR pathways that can be inactivated using either antiangiogenic agents or mTOR inhibitors [5]. The SSA octreotide has been shown to improve progression-free survival (PFS) in lowgrade midgut NETs with low hepatic tumor burden, both in functioning and non-functioning [7]. On the other hand, sunitinib and everolimus improved PFS in a well-differentiated pNETs [8, 9]. Treatment options are therefore increasing. However, there is still no validated biomarker to guide the selection of the most appropriate strategy for each individual patient. With regard to more conventional chemotherapy, several non-controlled studies have suggested a potential role for MGMT expression, an enzyme that repairs DNA damage induced by alkylating agents, as a predictive marker of resistance to this class of agents. Indeed, temozolomide, a cytotoxic, DNA-methylating agent, with promising activity in the field of NETs appeared to be particularly effective in tumors lacking the MGMT repair enzyme, with response rates of 0 vs 80 %, and PFS of 9.3 vs 19 months in MGMT positive vs negative tumors, respectively [10–12]. This lack of expression has been documented in approximately half of pancreatic NETs and is rarely observed in tumors of intestinal origin. However, data are still controversial and limited; thus, determination of MGMT expression does not allow preselection for temozolomide therapy yet. Another critical issue, particularly in pNETs, is that tumor biology may change over time, either spontaneously or as a consequence of treatment. Indeed, because these patients often have a good prognosis and receive, in general, multiple lines of therapy over prolonged periods of time, the question that then arises is to what extent and how each treatment impacts on tumor biology over time and how the sequential administration of different agents could potentially modify or revert molecular events driving drug resistance and/or tumor progression. NETs, like renal cell cancer and liver cancer, among others, are HIF1α-addicted tumors. HIF1α is regulated by oxygen in a VHL-dependent manner. Under normoxic conditions, VHL induces the degradation of HIF1α. In tumors lacking Von Hippel–Lindau (VHL) and in hypoxic conditions, HIF1α accumulates and triggers the transcription of multiple survival pathways including angiogenesis. HIF1α can also be activated by mTOR under either normoxic or hypoxic conditions. mTOR can be activated by membrane receptor-mediated pathways such as epidermal growth factor receptor (EGFR) or insulin growth factor receptor (IGF1R) through activation of intracellular signaling pathways such as RAS, MEK, and PI3K and also by nutrient and oxygen supply. HIF1α activates multiple pathways such as the Glut transporters, TGFβ,
369
chemokine (C-X-C motif) receptor 4 (CXCR4)/SDF1, VEGF/VEGFR, and carbonic anhydrase that mediate nutrient uptake, growth, survival, chemotaxis, and angiogenesis among others. HIF1α is also involved in apoptosis, which may be linked to the response to chemotherapy. HIF1α-addicted tumors are likely to respond well to targeted agents. However, resistance invariably emerges in most patients following drug exposure [13]. The pattern of tumor progression in the clinic appears to be very similar from one tumor type to another. HIF1α can be activated in NETs by genetic mechanisms such as mutations in VHL that occur in up to 30 % of sporadic pNETs or in familial cases such as Von Hippel−Lindau disease. In addition, HIF1α can be also activated as a consequence of hypoxia induced by agents such as angiogenesis inhibitors. Activation of HIF1α results in activation of canonical pathways such as mTOR and VEGF, which leads to increased sensitivity to drugs targeting these pathways. However, other routes such as those mediated by FGF, ephrins, and angiopoietin may be activated and ultimately lead to EMT and evasion or resistance to VEGF and mTOR inhibition [13]. Thus, tumor plasticity in response to chemotherapy or targeted agents can substantially modify these equilibriums and may substantially impact on the tumor biology and natural history of NETs. In this sense, the SEQTOR ENETS-GETNE trial is of great interest as it will compare conventional chemotherapy with STZ + 5FU followed by everolimus versus the opposite sequence with composite PFS as the primary endpoint. Preclinical cancer models may help understand some of these relevant biological shifts that occur as a consequence of drug exposure. In a study of renal cell cancer xenografts, in which it was demonstrated that mice treated with sunitinib had a significant improvement in time to progression as compared to those treated with placebo, the analysis of tumors upon disease progression showed an increased proportion of cancer stem cells and higher levels of mesenchymal markers, as well as of other pro-angiogenic and invasive markers such as CXCR4 and SDF1. At the time of progression, G proteincoupled receptors and chemokine receptors such as CXCR12SDF1 and CXCR4 were involved in switching from VEGFmediated angiogenesis eventually leading to resistance to VEGF inhibitors. However, these new emerging receptors and pathways may still signal through PI3K/Akt/mTOR suggesting there could be susceptibility to mTOR inhibitors [13]. Second line treatment with everolimus was indeed effective in these tumors.
2 New drugs and strategies in nets There have been important advances in the management of NET. However, a significant number of issues remain unsolved. Complete or remarkable partial tumor remissions are
370
Cancer Metastasis Rev (2014) 33:367–372
pathway involved in pancreatic cancer tumorigenesis, and the gastric inhibitory polypeptide receptors that can be a target for PRRT [16, 17].
rare with targeted drugs including SSA, IFN, everolimus, sunitinib, and PRRT strategies. The added value is prolonged PFS or TTP; if this translates into an improvement of overall survival is still unknown. Systemic chemotherapy is effective in poorly differentiated neoplasms or pNET but efficacy remains transient. Overall 5-year survival in stage IV disease is still limited to 40–50 % in pNETs and 70–90 % in midgut NETs. There is a number of relevant questions unanswered as follows: (a) Which drug to use in which line of treatment?; (b) Is sequential monotherapy or combination therapy the best treatment strategy?; (c) What is the optimal combination of agents considering the balance between efficacy and toxicity as well as the crosstalk of the pathways they target? Other key questions are related to the onset and duration of drug treatment: should treatment be started at the time of disease progression or at diagnosis to prevent tumor progression; for how long should the treatment in responding patients be continued; and is there a role for maintenance therapy?
LX1606 is an inhibitor of tryptophan hydroxylase, the rate-limiting step in 5-hydroxytryptamin synthesis, which is currently undergoing clinical development. Two phaseII studies have evaluated the safety and efficacy of this drug in subjects with symptomatic carcinoid syndrome refractory to stable-dose long-acting SSA. In one of these studies, 56 % of patients experienced a biochemical response (>50 % reduction in urinary 5-HIAA) and 28 % a clinical response (>30 % reduction in bowel movements), whereas no placebo subjects achieved either biochemical or clinical response [18]. Based on the results of both studies, a large placebo-controlled phase-III study is under development.
2.1 Novel target inhibition
2.2 c-Met inhibition in VEGFR-resistant tumors
Despite the undeniable therapeutic progress made over the last years in the field of NETs, the discovery of novel effective drugs is still of utmost importance since all medical treatments do not lead to cure but just to transient disease control. Over the years, a number of agents such as proteasome inhibitors or STAT3 inhibitors have been developed but abandoned for either excess toxicity or lack of efficacy. Table 2 summarizes some of the most promising agents currently in clinical development. Several new targets are emerging with potential in NETs. The NOTCH-1 genes, which function as tumor suppressor genes, are differentially expressed in NETs with higher expression in tumors of rectal vs pancreatic vs intestinal origin [14]. Another interesting target involved in cell cycle control is aurora kinase. Agents targeting this enzyme, such as Danusertib, have shown efficacy in an orthotopic xenograft model of NETs [15]. Inhibitors of the Src kinase family member SFK, that coordinate cell adhesion and spreading, has shown synergic inhibition in combination with mTOR inhibitors in preclinical pNET models. Another two potential targets are the hedgehog signaling inhibitors, a
Another critical issue when treating NETs with angiogenesis inhibitors, as demonstrated in some preclinical models, is that invasion and metastasis may be induced after inhibition of VEGF signaling. Indeed, in the RIP-Tag2 mice model, treatment with anti-VEGF antibody reduced tumor burden but increased tumor hypoxia, HIF-1α, and c-Met activation leading to increased invasion and metastasis [19]. Based on these observations, Cabozantinib (XL184), a drug that simultaneously blocks VEGF and c-MET, is currently being assessed in NETs in phase II clinical trials.
2.1.1 Serotonin synthesis inhibitors
2.3 PI3K/Akt/mTOR pathway inhibition The PI3K/Akt/mTOR pathway is now well established as a critical pathway to target in NETs [15]. Beyond everolimus, there are a number of specific inhibitors of PI3K and mTOR kinase, and Akt and S6 kinase in development. Furthermore, some of these agents have been combined with novel SSA, such as pasireotide, as this last class of drugs, by reducing levels of IGF1, also contributes to inhibition of this pathway.
Table 2 Selected new agents undergoing current clinical investigation in NETs Drug
Mechanism of action
Application
Clinical trials
Telotristat etiprate (LX 1606)
Serotonin synthesis inhibitor; (tryptophan hydroxylase) VEGFR-1,2,3 inhibitor Met inhibitor Dual PI3K/mTOR inhibitor Akt inhibitor
Carcinoid syndrome
Placebo-controlled phase III (TELESTAR)
Angiogenesis inhibition Tumor growth inhibition Tumor growth inhibition, pNET Tumor growth inhibition
Placebo-controlled phase II randomized Phase II Phase II Phase II
Axitinib Cabozantinib BEZ235 MK2206
Cancer Metastasis Rev (2014) 33:367–372
There are several PI3K inhibitors currently undergoing clinical assessment such as BEZ235 (dual PI3K/mTOR inhibitor), CC-223 (dual mTOR1 and mTOR2 inhibitor), BKM120 (pan-PI3K inhibitor) and BYL719 (selective PI3Kα inhibitor) which result in cell growth inhibition, induction of apoptosis in various cell lines/animal models, and antiangiogenic effects [14, 20–22]. BEZ235 is currently under investigation in two phase-II studies in pNETs. The BEZ235F2201 trial is evaluating BEZ235 in patients with advanced G1-2 pNETs, refractory to mTOR inhibitors. Measurable disease is required and no more than three prior systemic lines and no prior PI3Ktargeted therapy is allowed. The CBEZ235Z2401 trial is a randomized phase-II study in which patients with advanced pNETs naive of mTOR-targeted therapy are randomly assigned to receive either BEZ235 or everolimus. The primary endpoint of these studies is PFS (www.clinicaltrials.gov). 2.4 Combination strategies As single-agent activity is still limited, one area of intense development is combination therapy. So far, there are several well-established systemic treatment strategies for these patients including SSA, IFN-alpha, angiogenesis inhibitors, mTOR inhibitors, and chemotherapy [23]. Multiple small and heterogeneous studies have evaluated different angiogenesis inhibitors in combination with other angiogenesis inhibitors or chemotherapy. The results from these studies are difficult to interpret due to small sample size and heterogeneity of patient population, which often includes carcinoids of different sites of origin and pancreatic NETs. So far, there is only one large randomized study testing bevacizumab plus octreotide vs IFN plus octreotide in patients with metastatic or locally advanced non-pancreatic NETs, the results of which shall be soon available and are awaited with great interest [23]. A consecutive study, the CALGB Trial (80701) is addressing the efficacy of adding bevacitumab to everolimus monotherapy. To date, however, the real add-on value of antiangiogenic drugs remains uncertain until results from these comparative ongoing trials become available. Meanwhile, preclinical research is of great importance to help prioritize strategies to be tested in clinical trials. For example, combination of EGFR and mTOR inhibitors led to improved survival in the RIP Tag2 preclinical model, consistent with the observation of progressive activation of the EGFR and mTOR pathways as tumors develop in these mice [24]. To date, there is a large number of ongoing studies including those combining everolimus with pasireotide (COOPERATE-1/COOPERATE-2); IGF1R Inhibitor (cixutumumab) + octreotide; and EGFR inhibitor (erlotinib). Regarding angiogenesis inhibitors, there are studies assessing the combination of bevacizumab plus everolimus (NCI CALGB80701) or bevacizumab plus STZ/5-FU in pNETs, bevacizumab with octreotide and pertuzumab; or octreotide
371
and capecitabine in advanced GI NETs. In addition, there are several studies evaluating different combinations with other tyrosine kinase inhibitors with antiangiogenic activity such as Axitinib and octreotide (GETNE AXI-IIG-02, EUDRACT: 2011-001550-29) in GI NETs, or sorafenib plus cyclophosphamide or everolimus. Preliminary results of a recent study assessing the combination of temsirolimus (25-mg iv days 1, 8, 15, 22) and bevacizumab (10 mg/kg iv days 1, 15) in 55 patients with progressive G1-2 pNETs indicate promising activity with an acceptable toxicity profile. An objective response was documented in 37 % of treated patients, and PFS rate was 80 and 49 % at 6 and 12 months. As the number of potential combination therapies increases, however, it should be kept in mind that treatment with targeted agents is not innocuous [25, 26]. In this regard, a recent analysis of drug approvals for cancer along the period 2000–2010 that included 38 new agents showed that the risk of death, treatment discontinuation, and grades 3–4 adverse events increases with targeted agents, especially with multiple tyrosine kinase inhibitors, with a HR in the 1.2 to 1.4 range [21]. Likewise, a meta-analysis of 4,679 patients enrolled in 10 randomized controlled trials (2,856 patients from sorafenib, 1,388 from sunitinib, and 435 from pazopanib trials) showed that the incidence of fatal adverse events related to VEGFR TKIs was 1.5 % (95 % CI, 0.8 to 2.4 %), with a RR of 2.23 (95 % CI, 1.12 to 4.44 %; P=0.023) compared with control patients [22]. Careful patient surveillance by experienced specialists is therefore mandatory, and the use of noveltargeted agents should be clearly justified in the clinical setting.
3 Conclusions The molecular biology of NETs is still poorly understood. The treatment of NETs has advanced, nevertheless, significantly over the past years. More preclinical models are warranted to select “best combinations” (consider, e.g., crosstalk of receptors). Major challenges represent identification of valid targets and tools that allow selection of the population most likely to benefit from a specific drug. With this purpose, and as the tumor biology is likely to change over time, collecting paired tumor biopsies may help guide physicians towards optimal sequences of drug administration. Several classes of novel drugs are under clinical investigation in phase-II trials though overall response rates are still low. Interesting new agents with novel and rational mechanism of action include the tryptophan hydroxylase inhibitor LX1606 in serotonin-producing tumors; combined PI3K/mTOR inhibitor (BEZ235) after failure to mTOR inhibitors and simultaneous inhibition of c-Met and VEGF after anti-VEGF therapy (e.g., Cabozantinib). Preclinical data support the hypothesis that novel drugs targeting Notch signaling,
372
Aurora kinases and Src kinases deserve further evaluation in NETs. SSA, mTOR inhibitors, antiangiogenic agents, and cytotoxic drugs are currently being assessed in combination therapy trials. There is, however, limited evidence for some of the combination therapies and optimal combinations still need to be identified. The use of novel-targeted agents may also be associated with increased morbidity and treatment-related mortality, and thus requires careful surveillance of patients for appropriate early management of side effects. Finally, the optimal sequencing of therapies is still unclear (further evidence will be provided by future trials, e.g., SEQTOR trial in pNET).
References 1. Corbo, V., et al. (2012). Pancreatic endocrine tumours: mutational and immunohistochemical survey of protein kinases reveals alterations in targetable kinases in cancer cell lines and rare primaries. Annals of Oncology, 23, 127–134. 2. Corbo, V., et al. (2010). MEN1 in pancreatic endocrine tumors: analysis of gene and protein status in 169 sporadic neoplasms reveals alterations in the vast majority of cases. Endocrine-Related Cancer, 17, 771–783. 3. Jiao, Y., et al. (2011). DAXX/ATRX, MEN1, and mTOR pathway genes are frequently altered in pancreatic neuroendocrine tumors. Science, 331, 1199–1203. 4. Missiaglia, E., et al. (2010). Pancreatic endocrine tumors: expression profiling evidences a role for AKT-mTOR pathway. Journal of Clinical Oncology, 28, 245–255. 5. Faivre, S., Sablin, M. P., Dreyer, C., & Raymond, E. (2010). Novel anticancer agents in clinical trials for well-differentiated neuroendocrine tumors. Endocrinology and Metabolism Clinics of North America, 39, 811–826. 6. Couvelard, A., et al. (2005). Microvascular density and hypoxiainducible factor pathway in pancreatic endocrine tumours: negative correlation of microvascular density and VEGF expression with tumour progression. British Journal of Cancer, 92, 94–101. 7. Rinke, A., et al. (2009). 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. Journal of Clinical Oncology, 27, 4656–4663. 8. Raymond, E., et al. (2011). Sunitinib malate for the treatment of pancreatic neuroendocrine tumors. The New England Journal of Medicine, 364, 501–513. 9. Yao, J. C., et al. (2011). Everolimus for advanced pancreatic neuroendocrine tumors. The New England Journal of Medicine, 364, 514–523. 10. Ekeblad, S., et al. (2007). Temozolomide as monotherapy is effective in treatment of advanced malignant neuroendocrine tumors. Clinical Cancer Research, 13, 2986–2991.
Cancer Metastasis Rev (2014) 33:367–372 11. Kulke, M. H., et al. (2009). O6-methylguanine DNA methyltransferase deficiency and response to temozolomide-based therapy in patients with neuroendocrine tumors. Clinical Cancer Research, 15, 338–345. 12. Strosberg, J. R., et al. (2011). First-line chemotherapy with capecitabine and temozolomide in patients with metastatic pancreatic endocrine carcinomas. Cancer, 117, 268–275. 13. Tijeras-Raballand, A., et al. (2012). Resistance to targeted therapies in pancreatic neuroendocrine tumors (PNETs): molecular basis, preclinical data, and counteracting strategies. Targeted Oncology, 7, 173–181. 14. Maira, S. M., et al. (2008). Identification and characterization of NVP-BEZ235, a new orally available dual phosphatidylinositol 3kinase/mammalian target of rapamycin inhibitor with potent in vivo antitumor activity. Molecular Cancer Therapeutics, 7, 1851–1863. 15. Fraedrich, K., et al. (2012). Targeting aurora kinases with danusertib (PHA-739358) inhibits growth of liver metastases from gastroenteropancreatic neuroendocrine tumors in an orthotopic xenograft model. Clinical Cancer Research, 18, 4621–4632. 16. Fendrich, V., et al. (2011). Hedgehog inhibition with cyclopamine represses tumor growth and prolongs survival in a transgenic mouse model of islet cell tumors. Annals of Surgery, 253, 546–552. 17. Waser, B., et al. (2012). Glucose- dependent insulinotropic polypeptide receptors in most gastroenteropancreatic and bronchial neuroendocrine tumors. Journal of Clinical Endocrinology and Metabolism, 97, 482–488. 18. O’Dorisio, B., et al. (2012) Relief of bowel-related symptoms with telotristat etiprate in octreotide refractory carcinoid syndrome: Preliminary results of a double-blind, placebo-controlled multicenter study. Journal Clinical Oncology, 30, suppl; abstr 4085. 19. Sennino, B., et al. (2012). Suppression of tumor invasion and metastasis by concurrent inhibition of c-Met and VEGF signaling in pancreatic neuroendocrine tumors. Cancer Discovery, 2, 270–287. 20. Yap, T. A., et al. (2008). Targeting the PI3K-AKT-mTOR pathway: progress, pitfalls, and promises. Current Opinion in Pharmacology, 8, 393–412. 21. Serra, V., et al. (2008). NVP-BEZ235, a dual PI3K/mTOR inhibitor, prevents PI3K signaling and inhibits the growth of cancer cells with activating PI3K mutations. Cancer Research, 68, 8022–8030. 22. Schnell, C. R., et al. (2008). Effects of the dual phosphatidylinositol 3-kinase/mammalian target of rapamycin inhibitor NVP-BEZ235 on the tumor vasculature: implications for clinical imaging. Cancer Research, 68, 6598–6607. 23. Pavel, M. (2013). Translation of molecular pathways into clinical trials of neuroendocrine tumors. Neuroendocrinology, 97(1), 99–112. 24. Chiu, C. W., Nozawa, H., & Hanahan, D. (2010). Survival benefit with proapoptotic molecular and pathologic responses from dual targeting of mammalian target of rapamycin and epidermal growth factor receptor in a preclinical model of pancreatic neuroendocrine carcinogenesis. Journal of Clinical Oncology, 28, 4425–4433. 25. Niraula, S., et al. (2012). The price we pay for progress: a metaanalysis of harms of newly approved anticancer drugs. Journal of Clinical Oncology, 30, 3012–3019. 26. Schutz, F. A., Je, Y., Richards, C. J., & Choueiri, T. K. (2012). Metaanalysis of randomized controlled trials for the incidence and risk of treatment-related mortality in patients with cancer treated with vascular endothelial growth factor tyrosine kinase inhibitors. Journal of Clinical Oncology, 30, 871–877.
