Genomic classification of clinically advanced pancreatic ductal adenocarcinoma (PDAC) based on methylthioadenosine phosphorylase (MTAP) genomic loss (MTAP loss)

604 Background: MTAP loss is represented across a wide variety of cancer types including PDAC and is an emerging target for synthetic lethality-based cancer therapies. Preclinically, MTAP loss leads to the accumulation of 2-methylthioadenosine, reduced protein arginine N-methyltransferase 5 (PRMT5) methylation activity and increased vulnerability to targeting of the methionine adenosyltransferase IIα (MAT2A)/ PRMT5 axis. In addition, 9p21 loss, homozygous co-deletion of MTAP/CDKN2A or homozygous deletion of either gene have been associated with an immunologically “cold” tumor microenvironment, primary resistance to anti PD(L)1 immunotherapy (IO) and poor prognosis phenotype (Han G, Nat Commun 2021). We investigated concurrent mutations and immune biomarkers in clinical PDAC samples with MTAP-loss versus -intact status. Methods: From a series of 177705 consecutive cases, we performed comprehensive genomic profiling on 9423 cases of PDAC using an FDA-approved assay (F1CDx) to evaluate all classes of genomic alterations (GA). Tumor mutational burden (TMB) was determined on up to 1.1 Mbp of sequenced DNA and microsatellite instability (MSI) was determined on 114 loci. PD-L1 expression was determined by immunohistochemistry (Dako 22C3). Furthermore, we correlated pertinent findings within a database of 16558 cases of clinically advanced cancer with MTAP loss. Results: 2003 (21.3%) of 9423 PDAC demonstrated MTAP-loss. Similar gender, age and number of GA per tumor were observed between MTAP-loss and -intact groups. Frequencies of TP53, CDKN2A/B, SMAD4, PTEN and ARID1A were significantly higher in MTAP-loss PDAC. However, previously-described biomarkers of IO efficacy (MSI, TMB, CD274 amplification and PD-L1 expression) and resistance ( STK11, KEAP1 and MDM2) were infrequent and similar in both groups. The frequencies of other potentially targetable GA including BRCA1/2, ATM, KRAS G12C, ERBB2, BRAF, FGFR1, NF1 and PIK3CA were also infrequent and similar in both groups of PDAC patients. Amongst a database of 16558 cases of clinically advanced cancer with MTAP loss, 1538 (9.3%) featured co-alterations in MTAP and SMAD4. 52% of the MTAP/SMAD4 co-altered cases were PDAC. Conclusions: MTAP loss is associated with a distinctive concurrent genomic profile in PDAC and represents a potential new synthetic lethality-based opportunity for treatment with PRMT5 and MAT2A inhibitors. Furthermore, MTAP loss may represent an independent negative predictive biomarker for immune checkpoint inhibition in PDAC.[Table: see text]