368 Background: 177Lu-DOTATATE is an approved therapy for somatostatin receptor (sstr) positive gastroenteropancreatic neuroendocrine tumors (NETs). There are little data available on response and outcomes for well differentiated (WD) high grade (HG) NETs treated with 177Lu-DOTATATE. Methods: Pts with WD HGNETs treated with 177Lu-DOTATATE at MSK from 2018-2020 were identified. Demographics, response to treatment, and progression-free survival (PFS) were determined. In pts with archival tumor tissue, next-generation sequencing (NGS) was performed through an institutional platform (MSK-IMPACT). Results: 19 pts were identified (mean age 54, 63% female). Site of tumor origin included: pancreas (14/19, 74%), small bowel (2/19, 10.5%), rectal (2/19, 10.5%), lung (1/19, 5%). Average tumor Ki-67 was 34.8 (range 22-56). All tumors were sstr avid on pre-treatment Ga68-DOTATATE PET/CT; none of the patients had sstr negative lesions detected. Median number of prior treatments (systemic and/or liver-directed) was 4 (range 2-7). All pts had progressive disease prior to initiation of 177Lu-DOTATATE. 13 pts (68%) completed all four treatment cycles; treatment was incomplete in 6 pts due to treatment-related toxicities (n = 3) and clinical progression (n = 3). Best response by radiographic report was available in 16 patients (84%):10/16 (63%) with partial response, 1/16 (6%) with stable disease, 5/16 (31%) with disease progression. One pt with stable disease as best response received two additional cycles of 177Lu-DOTATATE at progression. Median PFS (from date of first treatment with 177Lu-DOTATATE until progression/death) was 11.1 months (95% CI 10.6 to NA). Five pts (26%) experienced dose modifying toxicity with 177Lu-DOTATATE. The most common treatment-related toxicities were thrombocytopenia (9 pts, 47%; G3/4 in 1 pt, 5%), anemia (7 pts, 37%; G3/4 in 2 pts, 10.5%), leukopenia (6 pts, 32%; G3/4 in 0 pts), and AST/ALT elevation (4 pts, 21%; G3/4 in 0 pts). NGS results were available in the tumor tissue of 13 pts (68%). The most commonly observed alterations were in MEN1 (6/13, 46%) and DAXX (4/13, 31%). No RB1 alterations were identified. Conclusions: We observed a meaningful disease control rate of 69% during treatment of WD HGNETs with 177Lu-DOTATATE. In this heavily pre-treated population, more than half of pts received all four treatment cycles with treatment-related toxicities largely bone-marrow related, as expected, based on historical data. As would be expected in sstr avid tumors, the vast majority had alterations in chromatin remodeling genes (MEN1, DAXX) consistent with WD NETs, with no RB1 alterations identified.
Somatostatin Receptor (SSTR) Expression and Proliferative Index (KI 67) in 100 Patients (PTS) with Gastroenteropancreatic Neuroendocrine Tumors (Gep-Nets). Clinical-Pathological Correlation