Primary Pancreatic Adenocarcinoma (PPDA) and Metastatic Pancreatic Adenocarcinoma (MPDA): Are They Genomically Distinct?

ASCO Annual Meeting 2021 Abstract
Authors Sunny R. K. Singh, Shravan Leonard-Murali, Ruicong She, Chun-Hui Lin, Jonathan Freaney, Laila Poisson, Gazala Khan

Background: PDA is an aggressive disease with a dismal prognosis. Advances in next-generation sequencing (NGS) have enabled targeted therapies, revolutionizing treatments of several solid tumors. Their role in the management of PDA is limited, in part due to the paucity in our understanding of targetable genomic events. We sought to evaluate genomic differences between PPDA and MPDA in the tumor and tumor immune microenvironment (TIME). The genomic changes after chemotherapy (CTX) administration were also evaluated.

Methods: NGS data derived from tumor samples of 150 unique patients was analyzed. Targeted 648 gene DNA sequencing was performed using the Tempus xT and xO assays. Patients were allocated into 2 cohorts: PPDA (n = 75) and MPDA (n = 75), which were overall similar in terms of their demographics. The frequencies of somatic mutations were compared. The immune infiltrate was imputed from RNAseq. Proportions of immune cells (IC), and the tumor mutational burden (TMB) relative to the proportion of IC in the TIME, were also analyzed. Kaplan Meier Survival estimates for most frequent mutations and TMB were calculated.

Results: The most frequently mutated genes amongst the 150 study subjects were: KRAS (92.7%), TP53 (76.7%), CDKN2A (45.3%), SMAD4 (31.3%), ATM (12.7%) and ARID1A (10.7%). Patients in the MPDA cohort had a higher rate of mutation in several genes when compared to PPDA, most notably in TP53 (85.3% vs 68.0%, p = 0.010), ARID1A (16.0 vs 5.3%, p = 0.037) CDKN2A (49.3 vs. 41.3%, p = 0.3) and SMAD4 (33.3 vs. 29.3 p = 0.7). We also evaluated if the genetic changes between PPDA and MPDA are associated with alterations in the TMB and differences in the TIME. A higher TMB was seen amongst patients in the MPDA vs PPDA cohort (2.73 vs 1.73 Mut/Mb, p = 0.008). TMB was also significantly increased after CTX (2.22 vs 1.63 Mut/Mb p = 0.049). TMB ≥ 3 was associated with decreased odds of progression free survival ≥ 12 months (OR 0.26 95% CI 0.078-0.822, p = 0.023). Regarding immune infiltrate, the proportion of CD4 and CD8 T cells were higher in the PPDA cohort. Macrophages and NK cells were more prevalent the MPDA cohort. TMB had a positive correlation with the degree of macrophage infiltration in the TIME (Multivariate estimate: 1.70, p value 0.01).

Conclusions: PPDA and MPDA are biologically dissimilar entities with genomic disparity. Associated differences were observed in TMB and TIME. There is a differential increase in the spectrum of mutations in MPDA as compared to PPDA, specifically in p53, ARID1A, CDKN2A and SMAD4. The burden of increased mutations in MPDA is associated with an increase in the TMB and tumor associated macrophages. The role of serial NGS in the management of PDA both in early and late disease should be investigated further to identify evolving genomic changes that correlate with outcome.