Signalling Pathways Passing Src in Pancreatic Endocrine Tumours: Relevance for Possible Combined Targeted Therapies

he most frequent molecular abnormalities in pancreatic endocrine tumours (PETs) are mutations of the MEN1 gene, deregulation of the PI3K/AKT/mTOR signalling pathway and overactivation of growth factors and their receptors, such as the VEGF. On this basis, everolimus (Afinitor®; Novartis) and sunitinib (Sutent®; Pfizer) have both been approved by the FDA for the treatment of progressive, unresectable, locally advanced or metastatic PETs. However, molecular or surrogate markers able to predict the response of PET patients to treatment with these drugs are not available, and cancer cells treated with targeted therapies might develop escape pathways that evoke pro-survival feedback responses. The existence of cross-talk between different molecular pathways in PETs has been poorly investigated. In the present review, we present data supporting an important role for Src family kinases (SFKs) in PETs, together with the recent observation of a novel role for SFK in modulating the mTOR pathway activity. Of note, while treatment with everolimus triggered the activation of a survival response dependent on PI3K/AKT signalling in vitro, the simultaneous inhibition of SFKs blocked the activation of this unwanted escape signal. These studies might set the ground for the investigation of combined treatment of PETs with SFK and mTOR inhibitors.

Copyright © 2012 S. Karger AG, Basel

Targeted Therapies for Pancreatic Endocrine Tumours: Current Status and Possible Limitations

The therapeutic strategies for the treatment of pancreatic endocrine tumours (PETs) have seen important changes in the past few years, largely due to a better knowledge of the molecular changes occurring in such tumours and to the development of the first well-designed placebo-controlled randomized clinical trials.

The molecular pathogenesis of PETs seems largely different from that of pancreatic adenocarcinoma, configuring PETs as a distinct entity with a peculiar molecular background. Indeed, the most typical and frequent molecular abnormalities in PETs regard the MEN1 gene, even in sporadic tumours [1], and the deregulation of the PI3K/AKT/mTOR signalling pathway [2]. Mutations or altered activity of the components of this pathway, such as the tumour suppressor genes PTEN or TSC, or overexpression of other upstream genes, appears to be a typical molecular signature of PETs, which also displays prognostic relevance [2]. These latter findings are in line with the results of a recent large-scale mutational analysis, where somatic mutations in MEN-1 and the mTOR pathway genes emerged as frequent molecular events in PETs [3]. Furthermore, the role of growth factors and their receptors (PDGFRs, c-kit, VEGFRs and cognate ligands) in PETs is suggested by data from animal models and confirmed by other lines of research [4]. The importance of VEGF in PETs is also suggested by the common clinical experience of the extraordinary vascularization of most PETs, which represents another striking difference with respect to ductal adenocarcinoma.

On the basis of these findings, the development of different novel agents aimed at targeting specific molecules of a complex pathway seems a very attractive strategy. In February 2011 the reports of two large phase 3, multicentre, double-blind, randomized, placebo-controlled trials testing the mTOR inhibitor everolimus and the multi-tyrosine kinase inhibitor sunitinib in malignant PETs were published, showing positive significant results in terms of tumour response, overall survival (sunitinib) and progression-free survival (everolimus) [5,6]. Subsequently, everolimus (Afinitor®; Novartis) and sunitinib (Sutent®; Pfizer) have both been approved by the FDA for the treatment of progressive, unresectable, locally advanced or metastatic PETs.

While these findings have raised enthusiasm in clinicians dealing with PETs, a number of important issues have not been addressed yet. For instance, some questions regarding the rationale for such treatments, as well as the molecular changes induced by these treatments in the tumour cells, remain unanswered. First of all, it is still not clear to what extent tumour proliferation is dependent on the effect exerted directly on the targeted molecules and the related overexpressed pathways, and whether these molecules represent real oncogenic targets or just an easily replaceable phenotype maintainer in cancer cells. Notably, while the choice of targeted therapies in other tumour types is driven by the findings of precise molecular alterations (i.e. c-kit mutations in GIST, k-ras status in colorectal cancer, HER-2 status in gastric cancer), molecular or surrogate markers able to predict the response of PET patients to treatment with either Afinitor or Sutent are not available. Moreover, the capacity of PET cells to develop escape pathways that evoke pro-survival feedback responses, and the existence of cross-talk between different molecular pathways in this cancer type, have been poorly investigated in this setting. However, it has been demonstrated that: (a) mTOR inhibition leads to increased IGF production, which in turn would stimulate tumour proliferation [7]; (b) mTOR inhibition leads to feedback survival responses that trigger activation of PI3K and AKT [8], thereby limiting their clinical application [9], and (c) antiangiogenic treatment results in adaptive-evasive responses involving an augmented invasive phenotype and increased distant metastasis in a PET mouse model (RIP-Tag2) [10].

The activation of escape pathways in tumour cells exposed to targeted therapies has raised interest in the development of combined treatments. Combined molecular-targeted treatments in PET cells may imply either a 'vertical blockage', targeting several steps in the same pathways (i.e. PI3K and mTOR, or VEGF and HIF), or a 'horizontal blockage' targeting distinct molecular pathways at the same time, which may converge on the same player in the cell. This second approach requires a preclinical demonstration of a cross-talk between given pathways, and of the benefit of a combined therapy targeting these pathways. There are few preclinical studies evaluating this approach in PETs. Interestingly, in one of them, the combined targeting of the mTOR and EGFR signalling pathways, yielded interesting results on tumour growth and produced remarkable survival benefit in the previously mentioned PET animal model [11].

In the present review, observations supporting an important role for Src family kinases (SFKs) in PETs will be presented, together with the recent observation of a role for SFK in modulating the mTOR pathway activity. These studies might set the ground for the investigation of combined treatment of PET cells with SFK and mTOR inhibitors. A possible clinical role of SFK