Combined Targeting of Pathogenetic Mechanisms in Pancreatic Neuroendocrine Tumors Elicits Synergistic Antitumor Effects

SIMPLE SUMMARY: Pancreatic neuroendocrine tumors (PanNETs) are often diagnosed when advanced or metastatic, and at this stage curative surgery in no longer an option. Given that available treatments for advanced disease have shown limited efficacy, novel therapies are urgently needed. In this scenario, we selected two drugs, inhibiting pathways known to be activated in PanNETs, and evaluated their efficacy in various preclinical tumor models. We chose a PI3K inhibitor (buparlisib) and a CDK4/6 inhibitor (ribociclib). We first tested these drugs, alone or in combination, on established cell lines representing distinct PanNET differentiation states. The combination buparlisib plus ribociclib reduced the proliferation of the cell lines more effectively than the single drugs. Inhibition of downstream target genes and/or proteins explained the drugs’ anti-proliferative activity. Buparlisib, but not ribociclib, promoted cell death. We then demonstrated that the combination treatment with buparlisib and ribociclib inhibits the viability of primary islets from a genetic animal model of PanNETs (Men1-deficient mice), without significantly affecting viability and function of primary islets from wild-type mice. Noteworthy, treatment of primary patient-derived PanNET cultures supported the efficacy of the combination treatment. Our findings indicate that the combined inhibition of PI3K and CDK4/6 pathways is a potentially effective therapeutic option for PanNETs. ABSTRACT: Pancreatic neuroendocrine neoplasms (PanNENs) are the second most common malignancy of the pancreas. Surgery remains the only curative treatment for localized disease. For patients with inoperable advanced or metastatic disease, few targeted therapies are available, but their efficacy is unpredictable and variable. Exploiting prior knowledge on pathogenetic processes involved in PanNEN tumorigenesis, we tested buparlisib (PI3K inhibitor) and ribociclib (CDK4/6 inhibitor), as single agents or in combination, in different preclinical models. First, we used cell lines representative of well-differentiated (INS-1E, NT-3) and poorly differentiated (BON-1) PanNENs. The combination of buparlisib with ribociclib reduced the proliferation of 2D and 3D spheroid cultures more potently than the individual drugs. Buparlisib, but not ribociclib, induced apoptosis. The anti-proliferative activity of the drugs correlated with downstream target inhibition at mRNA and protein levels. We then tested the drugs on primary islet microtissues from a genetic PanNET animal model (Men1-defective mice) and from wild-type mice: the drug combination was effective against the former without altering islet cell physiology. Finally, we treated PanNET patient-derived islet-like 3D tumoroids: the combination of buparlisib with ribociclib was effective in three out of four samples. Combined targeting of PI3K and CDK4/6 is a promising strategy for PanNENs spanning various molecular and histo-pathological features.