A novel therapy for pancreatic neuroendocrine tumors

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The incidence of the pancreatic neuroendocrine tumor (PNETs) has increased over the past two decades. The principal treatment for localized PNETs is surgical resection; however, there is no effective therapy for patients with advanced unresectable or metastatic disease. The progression-free survival rate is at the best 11 months with FDA approved agent Everolimus compared to 4.6 months with the placebo. However, the majority of the patients develop drug resistance and there is a void in our understanding of the mechanisms of resistance in PNETs. Frequent mutations in MEN1 (44%), DAXX/ATRX (43%), mTOR (15%) pathway genes and Von Hippel Lindau disease (VHL) alongside several other hereditary disorders are observed in PNETs. Loss of VHL has been linked to enhanced tumor aerobic glycolysis (Warburg effect). In this scenario, cancer cells rely more heavily on Nicotinamide Adenine Dinucleotide (NAD) pool that is a crucial co-factor in the redox reactions of metabolic pathways of cancer cells with high aerobic glycolysis. This over- dependence on NAD may provide actionable therapeutic avenues within the NAD salvage pathway. Our preliminary studies in PNET cell lines and patient derived tissue demonstrate activation of VHL regulated NAD biosynthesis rate-limiting enzyme Nicotinamide Phosphoribosyltransferase (NAMPT) alongside the over- expression of the mTOR promoter p21 activated kinase 4 (PAK4). PAK4 protein by virtue of its ability to engage multiple ligands has been shown to regulate a repertoire of signaling pathways including PNET resistance molecules. Significantly, PAK4-NAMPT dual RNAi suppressed proliferation in PNET cell lines. The CRISPR/Cas9 validated PAK4-NAMPT dual inhibitor KPT-9274 (a Phase I drug) shows antitumor activity in vitro, in PNET xenograft and could synergistically enhance the cytotoxicity of Everolimus. Molecular analysis of combination treatment showed down-regulation of known Everolimus resistance promoter suppression of ATP and NAD. Normal cells can utilize Nicotinate phosphoribosyltransferase (NAPRT1, an enzyme often absent in tumors due to promoter methylation) to generate NAD through nicotinic acid. Therefore, nicotinic acid co- treatment can allow KPT-9274 dose escalation without undue toxicity to normal tissue. We hypothesize that targeting of PAK4-NAMPT signaling could become a broad and clinically viable therapeutic strategy for PNET. Aim1: Profile the PAK4 and NAMPT aberrations in PNET patient tissue. Aim 2: Demonstrate the role of PAK4- NAMPT in PNET therapy resistance. Aim 3: Demonstrate the preclinical in vivo efficacy of PAK4-NAMPT dual inhibitors in PNET xenograft and define a biomarker of therapeutic response in biopsies from an ongoing Phase I trial. Clinical impact: Our studies will enhance the fundamental understanding of PAK signaling and NAD metabolism in PNET subsistence. This work will also uncover the ideal patient population based on their NAPRT status for best treatment response. Our preclinical studies will lead to the development of a novel tailored regimen for therapy-resistant and by far incurable PNETs.