1182O Temozolomide treatment induces an MMR-dependent hypermutator phenotype in well differentiated pancreatic neuroendocrine tumors

Temozolomide (TMZ), an alkylating agent, is an effective treatment. In glioblastomas, TMZ induces a hypermutator and hyperprogressor phenotype in a subset of patients through a specific MSH6-related MMR deficiency that does not seem to trigger an immune response. In contrast, in advanced colon cancer, studies suggest that TMZ priming increases the tumor mutational burden (TMB), favoring immunotherapy efficacy. TMZ treatment has been shown to be effective in patients with advanced pancreatic neuroendocrine tumors (PanNET). The aim of this study was to assess the potential of TMB-associated immunotherapy efficacy in TMZ-treated PanNET. We examined 57 patients with low grade G1/G2 PanNET treated by TMZ +/- capecitabine in one expert center between 2009 and 2020. Sequencing of 500 cancer genes was performed on 23 pre-TMZ and 23 matched post-TMZ samples, without intercurrent DNA damaging treatment. Results were validated in an independent cohort of 1079 PanNETs profiled by Foundation Medicine. TMB >30mut/Mb was considered TMB-high and only pathogenic variant with an allele frequency ≥10% were included in this analysis. After TMZ, 29/57 (51%) PanNET became high grade G3. They showed no difference in their preTMZ clinical or molecular profile nor in the TMZ dose received. In the profiled samples, 14/23 (61%) post-TMZ samples became G3 (median Ki-67 +20%) and 6/23 (26%; 5 G3) were TMB-high. In post-TMZ samples, TMB-high samples had more pathogenic alterations in MMR genes (88% vs. 26%, p=0.003). In the validation cohort, 25/1079 PanNETs were TMB-high. 67% of these displayed the predominant alkylating signature 11 and 44% had various MMR gene alterations; of note, MSH2 and MSH6 alterations were significantly more prevalent in TMB-high than TMB<10 samples (16-18% vs. <1%; p<0.01). In both cohorts, none of the TMB-high samples showed the glioblastoma-specific MSH6 mutation, suggesting a better immunogenic potential. These results suggest that TMZ induces an hyperprogressor and hypermutator phenotype in a subset of PanNETs, through a potentially immunogenic mechanism, thereby opening the path to immunotherapy, a treatment not otherwise effective in patients with treatment naive PanNET.