<b><i>Introduction:</i></b> Dysregulation of the mechanistic target of rapamycin complex 1 (mTORC1)-dependent pathways in pancreatic neuroendocrine neoplasms (PanNENs) underlies the introduction of the mTORC1 inhibitor everolimus as treatment of advanced progressive PanNENs. Although everolimus significantly increases progression-free survival, most patients acquire secondary resistance to the drug. This study aimed at identifying mechanisms involved in acquisition of resistance to everolimus. <b><i>Methods:</i></b> BON-1 and everolimus-resistant (ER) BON-1 cells were used as in vitro system of sensitivity and acquired resistance. Transcriptome changes occurring in BON-1 and ER-BON-1 were investigated by RNA sequencing and validated by quantitative PCR analysis. RNA extracted from patients’ biopsies was used to validate MYC upregulation. Drug screening and functional assays were performed using ER-BON-1 cells. Cell cycle progression was evaluated by FACS analysis. <b><i>Results:</i></b> Our results show that MYC overexpression is a key event in the development of secondary resistance to everolimus in PanNEN cell lines and in metastatic lesions from neuroendocrine neoplasm patients. MYC knockdown restored ER-BON-1 sensitivity to everolimus. Pharmacological inhibition of MYC mediated by the cyclin-dependent kinase inhibitor dinaciclib strongly reduced viability of ER-BON-1. Dinaciclib synergized with everolimus and inhibited ER-BON-1 cell cycle progression. <b><i>Discussion:</i></b> Our findings suggest that MYC upregulation drives the development of secondary resistance to everolimus in PanNENs and that its inhibition is an exploitable vulnerability. Indeed, our results indicate that combined treatments with cyclin-dependent kinase and mTOR inhibitors may counteract secondary resistance to everolimus in PanNENs and may pave the ground for new therapeutic regimens for these tumors.

Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive cancer and current treatments exert small effects on life expectancy. The most common adjuvant treatment for PDAC is gemcitabine. However, relapse almost invariably occurs and most patients develop metastatic, incurable disease. The aim of the present study was to assess the activity of nanoparticle albumin-bound paclitaxel (nab-paclitaxel) alone or in combination with gemcitabine in PDAC cell lines displaying different degrees of sensitivity to gemcitabine treatment. We evaluated the effects of gemcitabine and nab-paclitaxel and their combination on cell proliferation, death, apoptosis and cell cycle distribution in PDAC cell lines either sensitive to gemcitabine, or with primary or secondary resistance to gemcitabine. Our results indicated that the dose‑response of PDAC cell lines to nab-paclitaxel was similar, regardless of their sensitivity to gemcitabine. In addition, nab-paclitaxel elicited similar cytotoxic effects on a PDAC cell line highly resistant to gemcitabine that was selected after prolonged exposure to the drug. Notably, we found that combined treatment with gemcitabine and nab-paclitaxel exerted additive effects on cell death, even at lower doses of the drugs. The combined treatment caused an increase in cell death by apoptosis and in cell cycle blockage in S phase, as assessed by flow cytometry and western blot analysis of the PARP-1 cleavage. These results revealed that a combined treatment with nab-paclitaxel may overcome resistance to gemcitabine and may represent a valuable therapeutic approach for PDAC.