Non-invasive diagnosis and tracking of tumor evolution by targeted sequencing of circulating tumor DNA in metastatic pancreatic cancer patients

e15769 Background: Treatment of stage IV pancreatic ductal adenocarcinoma (PDAC) has made substantial progress over the last years, therapy monitoring still is at an early stage. This could be substantially supported by tools that allow to establish and monitor the molecular setup of the tumor even during treatment. In particular, non-invasive approaches are desirable. Characterization of circulating tumor DNA (ctDNA) may help to achieve this goal. Methods: We analyzed a cohort of 20 patients with histologically confirmed metastatic PDAC (mPDAC) prior to and during palliative treatment including disease progression. ctDNA and corresponding tumor tissue were analyzed by targeted NGS and droplet digital PCR for the 7 most frequently mutated genes in PDAC ( TP53, SMAD4, CDKN2A, KRAS, APC, ATM, FBXW7). Findings were correlated with clinical and imaging data to establish its prognostic and predictive value. Results: ctDNA was analyzed at baseline prior to initiation of the respective line of treatment. Mutations in either of the genes examined were detectable in 15/20 patients (75%). Tissue-blood concordance was 80% in therapy naïve patients. 96% of mutations in ctDNA of therapy naïve patients were in KRAS and/or TP53. The combined mutated allele frequencies (CMAF) of theese 2 genes significantly decreased (p = 0.0173) during therapy and increased at progression (p = 0.0145) across all treatment lines. By sequential ctDNA analyses we detected a change in the mutational landscape compared to the respective baseline ctDNA status in 7/11 patients during 1st line, in 3/7 patients during 2nd line and 2/2 patients during 3rdline treatment. In therapy naïve patients, the decline of the CMAF during therapy significantly correlated with progression-free survival (p = 0.0013). Conclusions: Molecular genotyping of ctDNA in mPDAC patients proved to be feasible and there was a high concordance between tumor tissue and ctDNA. The molecular genotype changed significantly during treatment and changes correlated with outcome. Monitoring of ctDNA may enable to adapt therapeutic strategies to the specific molecular changes present at a certain time during treatment of mPDAC.