The targeted therapy revolution in neuroendocrine tumors: in search of biomarkers for patient selection and response evaluation

Promising targetable biomarkers in pancreatic neuroendocrine tumours

INTRODUCTION

In the treatment scenario of PanNETs-targeted therapies are desired but limited, as rarity and heterogeneity on PanNETs pose limitations to their development.

AREAS COVERED

We performed a literature review searching for promising druggable biomarkers and potential treatments to be implemented in the next future. We focused on treatments which have already reached clinical experimentation, although in early phases. Six targets were identified, namely Hsp90, HIFa, HDACs, CDKs, uPAR, and DDR. Even though biological rational is strong, so far reported efficacy outcomes are quite disappointing. The reason of that should be searched in the patients' heterogeneity, lack of biomarker selection, poor knowledge of interfering mechanisms as well as difficulties in patients accrual. Moreover, different ways to assess treatment efficacy should be considered, other than response rate, in light of the more indolent nature of NETs.

EXPERT OPINION

Development of targeted treatments in PanNETs is still an uncovered area, far behind other more frequent cancers. Rarity of NETs led to accrual of unselected populations, possibly jeopardizing the drug efficacy. Better patients' selection, both in terms of topography, grading and biomarkers is crucial and will help understanding which role targeted therapies can really play in these tumors.

Heterogeneity of Vascular Endothelial Cells, De Novo Arteriogenesis and Therapeutic Implications in Pancreatic Neuroendocrine Tumors

Arteriogenesis supplies oxygen and nutrients in the tumor microenvironment (TME), which may play an important role in tumor growth and metastasis. Pancreatic neuroendocrine tumors (pNETs) are the second most common pancreatic malignancy and are frequently metastatic on presentation. Nearly a third of pNETs secrete bioactive substances causing debilitating symptoms. Current treatment options for metastatic pNETs are limited. Importantly, these tumors are highly vascularized and heterogeneous neoplasms, in which the heterogeneity of vascular endothelial cells (ECs) and de novo arteriogenesis may be critical for their progression. Current anti-angiogenetic targeted treatments have not shown substantial clinical benefits, and they are poorly tolerated. This review article describes EC heterogeneity and heterogeneous tumor-associated ECs (TAECs) in the TME and emphasizes the concept of de novo arteriogenesis in the TME. The authors also emphasize the challenges of current antiangiogenic therapy in pNETs and discuss the potential of tumor arteriogenesis as a novel therapeutic target. Finally, the authors prospect the clinical potential of targeting the FoxO1-CD36-Notch pathway that is associated with both pNET progression and arteriogenesis and provide insights into the clinical implications of targeting plasticity of cancer stem cells (CSCs) and vascular niche, particularly the arteriolar niche within the TME in pNETs, which will also provide insights into other types of cancer, including breast cancer, lung cancer, and malignant melanoma.

Translational research in neuroendocrine tumors: pitfalls and opportunities

Interest in research on neuroendocrine tumors (NETs) has grown in the past 10 years, coinciding with improvements in our understanding of the molecular pathogenesis of NETs. In addition, NETs have become one of the most exciting settings for drug development. Two targeted agents for the management of advanced pancreatic NETs have been approved, but the development of targeted agents for NETs is limited by problems with both patient selection and demonstration of activity. In this review, we analyze these limitations and discuss ways to increase the predictive value of preclinical models for target discovery and drug development. The role of translational research and 'omics' methodologies is emphasized, with the final aim of developing personalized medicine. Because NETs usually grow slowly and metastatic tumors are found at easily accessible locations, and owing to improvements in techniques for liquid biopsies, NETs provide a unique opportunity to obtain tumor samples at all stages of the evolution of the disease and to adapt treatment to changes in tumor biology. Combining clinical and translational research is essential to achieve progress in the NET field. Slow growth and genetic stability limit and challenge both the availability and further development of preclinical models of NETs, one of the most crucial unmet research needs in the field. Finally, we suggest some useful approaches for improving clinical drug development for NETs: moving from classical RECIST-based response end points to survival parameters; searching for different criteria to define response rates (for example, antiangiogenic effects and metabolic responses); implementing randomized phase II studies to avoid single-arm phase II studies that produce limited data on drug efficacy; and using predictive biomarkers for patient selection.

CXCR4/CXCL12/CXCR7 axis is functional in neuroendocrine tumors and signals on mTOR

Objective

To evaluate the possible crosstalk between C-X-C chemokine receptor 4 (CXCR4)/C-X-C motif chemokine 12 (CXCL12)/C-X-C chemokine receptor 7 (CXCR7) axis with the mammalian target of rapamycin (mTOR) pathway in neuroendocrine tumors (NETs).

Methods

Sixty-one human NETs were included into the study. CXCR4/CXCL12/CXCR7 axis and mTOR pathway were assessed by qRT-PCR and immunohistochemistry (IHC). The effect of mTOR inhibitor, RAD001, was evaluated on CXCR4 pathway through proliferation and p-Erk and p-AKT induction. Results: CXCR4/CXCL12/CXCR7 axis and p-mTOR were found to be active and correlated with grading, Ki67 index and tumor stage. mTOR pathway activation significantly correlated with poor prognosis. In human NET cells, CXCL12 induced mTOR signalling while AMD3100 (CXCR4-antagonist) impaired it. The mTOR-antagonist, RAD001, impaired the CXCL12-dependent induction of CXCR4 downstream effectors. Combination of AMD3100 and RAD001 potentiate cell growth inhibition.

Conclusions

CXCR4/CXCL12/CXCR7 axis is active in NETs and signals on mTOR. CXCR4 might be considered a prognostic factor in NETs. Combined treatment with AMD3100 and RAD001 may provide clinical benefits in NET patients with drug-resistant.

Sequential treatment in disseminated well- and intermediate-differentiated pancreatic neuroendocrine tumors: Common sense or low rationale?

Fortunately, the landscape of the systemic treatment for grade 1 and 2 pancreatic neuroendocrine tumors has changed in the last decade with at least four different alternatives approved in the field. Chemotherapy, somatostatin analogues, sunitinib and everolimus remind valid options according to the most referenced international guidelines. However, and although this is something done in the routine practice, there is a lack of evidence for the use of any of these strategies after failure to the others. Moreover, further sequential alternatives in third or fourth line have never been tested prospectively. The need for a better understanding of the rationale to sequence different systemic options is even greater in non-pancreatic neuroendocrine tumors since available therapies are scarce. Sequential strategies in other solid tumors have led to a clear improvement in overall survival. This is also believed to occur in neuroendocrine tumors but no clear data on it has been delivered yet. We postulate that the different mode of action of the systemic options available for the treatment of neuroendocrine tumors may avoid the complete resistance of one option after the other and that sequential use of these agents will be translated into a longer overall survival of patients. Prospective and randomized trials that seek for the activity of drugs after failure to another systemic alternatives are highly needed in this field of neuroendocrine tumors.

Cabozantinib and Tivantinib, but Not INC280, Induce Antiproliferative and Antimigratory Effects in Human Neuroendocrine Tumor Cells in vitro: Evidence for 'Off-Target' Effects Not Mediated by c-Met Inhibition

BACKGROUND/AIMS

The hepatocyte growth factor/transmembrane tyrosine kinase receptor c-Met has been defined as a potential target in antitumoral treatment of various carcinomas. We aimed to investigate the direct effect of c-Met inhibition on neuroendocrine tumor cells in vitro.

METHODS

The effects of the multi-tyrosine kinase inhibitors cabozantinib and tivantinib and of the highly specific c-Met inhibitor INC280 were investigated in human pancreatic neuroendocrine BON1, bronchopulmonary NCI-H727 and midgut GOT1 cells in vitro.

RESULTS

INC280, cabozantinib and tivantinib inhibited c-Met phosphorylation, respectively. However, while equimolar concentrations (10 μM) of cabozantinib and tivantinib inhibited cell viability and cell migration, INC280 had no inhibitory effect. Knockdown experiments with c-Met siRNA also did not demonstrate effects on cell viability. Cabozantinib and tivantinib caused a G2 arrest in neuroendocrine tumor cells.

CONCLUSIONS

Our in vitro data suggest that c-Met inhibition alone is not sufficient to exert direct antitumoral or antimigratory effects in neuroendocrine tumor cells. The multi-tyrosine kinase inhibitors cabozantinib and tivantinib show promising antitumoral and antimigratory effects in neuroendocrine tumor cells, which are most probably 'off-target' effects, not mediated by c-Met.

Personalized medicine for gastroenteropancreatic neuroendocrine tumors: a distant dream?

Neuroendocrine tumors are heterogeneous cancers that can present with advanced disease. Treatment stratification is often based on limited characterization of tumor behavior from histological grade and imaging assessments. Personalized medicine strategies focus on tailoring therapy through characterization of cancer pathways and the development of biomarkers. This review article explores the current personalized medicine landscape in gastroenteropancreatic neuroendocrine tumors, from tissue and circulating biomarkers development through to tumor heterogeneity and reimbursement issues.

Essential role of Her3 in two signaling transduction patterns: Her2/Her3 and MET/Her3 in proliferation of human gastric cancer

Various receptor tyrosine kinase (RTK) pathways were verified in many cancers including gastric cancer (GC), We sought to investigate the expression of RTKs including Her2, Her3, and Met and their transduction patterns in human GC. Over-expression of Her2, Her3, and c-Met in human GC was verified by immunohistochemistry leading to constitutive activation of RTK signaling pathways. Combined RTKs expression was valuable indicators for poor prognosis of GC patients. Using ErbB2 specific inhibitor Lapatinib and c-Met specific inhibitor PHA-665752, we further demonstrated that this constitutive activation of RTK signaling is necessary for the survival of GC cells. However, various RTK pattern: Her3/Her2 and Met/Her3 were verified in the transduction growth stimulus from outside via both AKT and MAPK signaling. Moreover, the essential roles of Her3 in both two heterodimers were obtained which showed significantly attenuated growth effect due to Her3 knockdown both in vitro and in vivo. In conclusion, various molecular transduction patterns: Her2/Her3 and Met/Her3 were verified in human GC, and Her3 could serve as a potential target in GC treatment.