<AbstractText>In gastroenteropancreatic (GEP) high-grade neuroendocrine neoplasms (H-NENs), Ki-67 threshold of 55%, instead of 20%, defines three prognosis subclasses: neuroendocrine tumor (NET) G3, neuroendocrine carcinoma (NEC) <55% and NEC ≥55%. We investigated whether the molecular profiling of H-NENs differ among these sub-categories and evaluated potential therapeutic targets, including PD-L1.</AbstractText><AbstractText>In consecutive GEP-NEN patients we evaluated: i) 55% threshold for Ki-67 labelling index for further stratifying NEC, ii) immunoreactivity and gene mutations by immunohistochemistry and Targeted Next Generation Sequencing (T-NGS).</AbstractText><AbstractText>15 NETs G3 and 39 NECs were identified. Ki-67 labelling index was <55% in 9 NECs and ≥55% in 30 NECs. Gene mutations by NGS (TP53, 32.9%, KRAS, 5.5%, BRAF, 4.1%) were detected in 46.6% NENs in more details significantly enriched in NEC ≥55% (76.7%) than NEC <55% (55.6%) or in NET (20.0%). PD-L1 was observed in NEC ≥55% (36.7%; p=0.03). Median OS was 4.3 years in NET G3, 1.8 years in NEC <55% and 0.7 years in NEC ≥55% (p<0.0001); it was 2.3 years with NGS Wild-Type, 0.7 years with ≥1 mutation (p<0.0001), 0.8 years in PD-L1 positive patients and 1.7 years in PD-L1 negative subjects (p=0.0004). At multivariable analysis, only our classification reached statistical significance (NEC <55% vs. NET G3, HR 14.1, 95% CI 2.2-89.8, p=0.005; NEC ≥55% vs. NET G3, HR 25.8, 95% CI 3.9-169, p=0.0007).</AbstractText><AbstractText>These findings identify NEC ≥55% as a biologically and prognostically distinct subtype besides they pave the way for personalized treatment.</AbstractText>