PURPOSE: Novel therapeutic approaches are needed to improve the postoperative management of residual nonfunctioning pituitary adenomas (NFPAs), given their high relapse rate. Here, we evaluated the antitumor efficacy of the dual PI3K/mTOR inhibitor NVP-BEZ235 in the only available model of spontaneous NFPAs (MENX rats). EXPERIMENTAL DESIGN: Organotypic cultures of rat primary NFPAs were incubated with NVP-BEZ235 and assessed for cell viability, proliferation, apoptosis, PI3K/mTOR inhibition. NVP-BEZ235, or placebo, was administered to MENX rats and tumor response was monitored non-invasively by diffusion weighted-magnetic resonance imaging (DW-MRI). Following treatment, tumor tissues were investigated for cell proliferation, apoptosis, PI3K/mTOR inhibition. Genes mediating the cytotoxic activity of NVP-BEZ235 were identified by gene expression profiling. Among them, Defb1, encoding beta-defensin 1, was further studied for its role in pituitary cells and in human pancreatic neuroendocrine tumor (NET) cells. RESULTS: NVP-BEZ235 showed anti-proliferative and pro-cell death activities against NFPAs both in vitro and in vivo, and the response to the drug correlated with inhibition of the PI3K pathway. DW-MRI identified early functional changes (decreased cellularity) in the adenomas before their size was affected and emerged as a useful modality to assess therapy response. The cytotoxic effect of PI3K/mTOR blockade in NFPA was mediated by several genes, including Defb1. NVP-BEZ235 treatment induced Defb1 expression in NFPAs in vitro and in vivo, and in pancreatic NET cells. High Defb1 levels sensitized NET cells to PI3K/mTOR inhibition. CONCLUSIONS: Our findings provide rationale for clinical investigation of PI3K/mTOR inhibition in NFPAs and identify novel effectors of PI3K-mediated neuroendocrine cell survival.