Tandem duplicator phenotype: A novel targetable subgroup in pancreatic cancer?

4161 Background: Validated biomarkers are urgently needed to provide a more precise approach in pancreatic cancer (PDAC). Homologous recombination deficiency (HRD) is a heterogenous cohort in PDAC and predicts platinum and PARP inhibitor sensitivity. The BRCA-1 genotype associates with distinct tandem duplications (TDs), however non-BRCA 1 TD phenotypes (TDP) have been observed across tumour types and may be associated with perturbations in CDK12, FBXW17, and CCNE1. Here, we characterize the prevalence and outcomes of TDP in a large series of prospectively sequenced PDAC. Methods: Whole-genome sequencing was performed in 191 early stage resected and 275 advanced PDAC cases in the PanCuRx initiative and the COMPASS trial at the Ontario Institute for Cancer Research and Princess Margaret Cancer Centre. Tumors underwent laser capture enrichment prior to sequencing. TD scores were calculated as previously described by Menghi et al, and HRDetect scores assigned. Samples were classified as classical or basal-like by Moffitt. Outcomes of patients with TDP tumors were evaluated. Tumors that are nonHRD and nonTDP are referred to as typicals. Results: Of 466 cases, 46 were identified as TDP (9.9%; 17 resected, 29 advanced). Subgroups of TDP by etiology included BRCA1 (n=8; 6 germline, 2 somatic), somatic CCNE1 (n=1), and unknown (n=37; no identified alterations in TDP-related genes). Pathogenic germline variants were not found in unique genes other than BRCA1. HRD TDP was associated with a significantly smaller TD size compared to nonHRD TDP (median 12 kb vs. 125 kb, p<0.0001) and median TD load of 114. NonHRD TDP genomes exhibited elevated TD load (median 70 vs. 13, p<0.0001) and HRDetect scores (p<0.0001) compared with typicals. NonHRD TDP was not prognostic in resected PDAC disease (p=0.6361) compared with typicals. We observed a trend towards improved response to 1st-line platinum therapy in nonHRD TDP vs. typicals (p=0.066) for patients with advanced disease. When stratified for only ‘classical’ RNA subtype cases as per Moffitt classification (HRD TDP:2 basal-like, 8 classical, 1 unknown; nonHRD TDP: 9 basal-like, 25 classical, 1 unknown), platinum therapy was correlated with better response in non-HRD TDP vs. typicals (n= 10, 70% ORR, p=0.0108) highlighting the chemoresistance of the basal-like subgroup. Superior survival was not observed in this small cohort. Conclusions: In PDAC, the etiology of nonHRD TDP is unclear but may represent a potential marker for platinum and DNA damage response agents. Further investigation is warranted with a larger sample size.