Efficacy of immune checkpoint inhibitors in patients with advanced pancreatic NETs displaying high TMB and MMR alterations following treatment with alkylating agents.

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.