Al‐Toubah, Taymeyah

662 Background: Alkylating-based chemotherapy (ALK) is one of the main treatments used in patients with advanced pancreatic neuroendocrine tumors (PanNETs). Its use has been associated with an increased risk of grade progression and acquisition of a hypermutator phenotype, associated with frequent alterations of the mismatch repair (MMR) genes. This suggests a potential benefit of immune checkpoint inhibitors (ICI) that are otherwise ineffective in ALK-naïve PanNETs, which are mostly hypomutated. We aimed to describe the efficacy of ICI in patients with advanced PanNET pretreated with ALK. Methods: We assembled an international retrospective cohort of patients with advanced PanNET who received at least one cycle of ICI, and who had previously received at least 3 months of ALK. Patients with poorly-differentiated neuroendocrine carcinoma were excluded. The primary endpoint was progression-free survival (PFS) and the secondary endpoint was the objective response rate (ORR). The impact of tumor mutational burden (TMB, high being defined by >10 mut/Mb) and MMR alterations (mutations or IHC) was explored. Results: We included 39 patients (median age 58.7, male sex 62%). All PanNETs were progressive at baseline and presented a median of 3 (IQR, 2-4) metastatic sites. Tumors were predominantly classified as G2 (47%) or G3 (42%), with a median Ki-67 of 18.5% (IQR, 12-35). 23/27 patients with tumor tissue or ctDNA NGS testing had high TMB (median, 35 mut/Mb; IQR, 24-106) and 12/22 had MMR alterations. Patients had previously received an average of 14 cycles (IQR, 7-25) of ALK (temozolomide 78%, streptozotocin 18%, dacarbazine 4%), which had yielded objective response in 72% of cases. In addition, patients had received a median of 4 (IQR, 3-5) other treatments (including platinum drugs in 80% and PRRT in 51%). ICI mainly consisted in an anti-PDL1/PD1 + anti-CTLA4 combination (85%) or a monotherapy (15%), with a median of 3 (IQR, 2-8) cycles. The ORR was 18% and the disease control rate was 46%. Median PFS was 2.8 months (95% CI, 0.62-4.96) and 6-month PFS rate was 31%. 19% of patients had grade 3-4 toxicity. Patients with high TMB had higher ORR (30% vs. 0%, p=0.03) and longer median PFS (4.9 vs. 2.1 months, p=0.004) compared with patients with low/unknown TMB. Patients with altered MMR had higher ORR (42% vs. 7%, p=0.02) and longer median PFS (8.9 vs. 2.5 months, p=0.014) compared with patients with no/unknown MMR alterations. Conclusions: While ICI shows limited efficacy in unselected patients with advanced PanNETs pretreated with ALK, it may be an effective treatment option in case of high TMB and MMR alterations.

370 Background: Dual checkpoint inhibitor therapy with anti-PD-1 and ant-CTLA-4 therapy has shown promising results in patients with high-grade NENs, demonstrating varying response rates of 9 – 44%. More data are needed to evaluate the true response in a real-world cohort of patients. Methods: Retrospective study of all patients with high-grade neuroendocrine neoplasms treated at the Moffitt Cancer Center and Mayo Clinic between 9/2017 and 7/2020 who received combination therapy with ipilimumab and nivolumab. Primary endpoint was objective response rate. Results: 34 patients met eligibility criteria for evaluation. Patients had received an average of 2 lines of therapy prior to treatment with ipilimumab/nivolumab, including at least one cytotoxic chemotherapy regimen. 27 (79.4%) of patients had poorly differentiated NECs and 7 (20.6%) had well-differentiated high grade NETs. The most common primary site (10, 29.4%) was pancreas; other primary sites of disease included unknown primary (n = 9), colon (n = 5), uterus (n = 3), anorectum (n = 2), esophagus (n = 2), cervix (n = 1), stomach (n = 1), and small intestine (n = 1). 5 patients (14.7%) exhibited a best response of PR per RECIST 1.1 criteria, 9 (26.5%) SD, and 17 (50%) PD: 3 patients did not have a follow-up scan and discontinued treatment shortly after initiation due to clinical progression. ORR was 14.7% and DCR was 41.2%. Median PFS was 1 month (95% CI, 0.45 – 1.55); median OS from time of treatment initiation was 5.0 months (95% CI, 3.42 – 6.59) and median OS from diagnosis was 14.0 months (95% CI, 1.49 – 26.51). Median duration on treatment was 1 month (range 0 – 10 months). 26 patients discontinued treatment for progression, 4 patients for toxicity, and 4 remain on treatment at the time of data cut off. 12 patients (35%) experienced grade 3 and 4 treatment-emergent toxicities: liver transaminitis, elevated blood bilirubin, arthralgia, myalgia, peripheral sensory neuropathy, lower extremity edema, confusion, diarrhea, encephalopathy, acute kidney injury, rhabdomyolysis, myocarditis, and colitis. Conclusions: The ipilimumab and nivolumab regimen has modest activity in aggressive and heavily pretreated high-grade NENs who have progressed on prior cytotoxic chemotherapy. Use of this regimen may be considered in patients with particularly aggressive, refractory high-grade NENs.

398 Background: The PI3K/mTOR pathway has emerged as a potential target for anticancer therapy. Considerable evidence suggests that targeting a single isoform of PI3K (p110α) would have sufficient antitumor activity and improved therapeutic window. Further, PI3KCA mutations, gene encoding isoform p110α, are described in pancreatic adenocarcinoma (PAC). BYL719 is an oral class I α-specific PI3K inhibitor that showed preclinical anti-tumor activity. The first in human phase 1 trial of BYL719 defined the maximum tolerated dose (MTD) at 400mg QD. Methods: This was a phase I, single center study (standard 3+3 design). The primary objective was to determine the MTD of BYL719 in combination with gemcitabine (G) and nab-paclitaxel (nabP) as frontline therapy in locally advanced or metastatic PAC. BYL719 was given orally daily (Table). Patients (pts) were restaged q2 cycles. The study was closed prematurely due to slow accrual. Results: Fifteen pts were enrolled (median age was 58 years). Three pts each participated in cohorts 1 and 2. Nine pts were enrolled in cohort 3, but 4 were replaced (3 pts withdrew consent prior to evaluation and 1 missed > 10 days of treatment). One pt in cohort 3 had DLT related to grade 3 nausea and vomiting. A total of 19 grade 3 and 4 adverse events were records as probably or possibly associated with BYL719. The most common ones were hyperglycemia, anemia, and neutrophil count decreased. One pt developed Posterior reversible encephalopathy syndrome (PRES) during cycle 7. Although we could not completely exclude BYL719 as a cause, PRES was attributed to G. One pt had sudden death during cycle 4 that was attributed to progression. Only 8 pts were evaluable for response. Two had stable disease, 5 had partial responses and 1 had progression. The median progression-free survival and overall survival were 5.36 months (1.6 to 10 months) and 8.74 months (3.8 to 21.2 months) respectively. Conclusions: The combination of full doses of G + nabP and BYL719 can be safely administered up to BYL dose of 250 mg/day. Clinical trial information: NCT02155088. [Table: see text]