Circulating tumour DNA abundance and potential utility in de novo metastatic prostate cancer

Background: Several systemic therapeutic options exist for metastatic castratesensitive prostate cancer (mCSPC). Circulating tumour DNA (ctDNA) can molecularly profile metastatic castration-resistant prostate cancer (mCRPC) and can influence decision-making, but remains untested in mCSPC. Objective: To determine ctDNA abundance at de novo mCSPC diagnosis and whether ctDNA provides complementary clinically-relevant information to a prostate biopsy. Design, Setting, and Participants: We collected plasma cell-free DNA (cfDNA) from 53 newly diagnosed patients with mCSPC and, where possible, during treatment. Targeted sequencing was performed on cfDNA and DNA from diagnostic prostate tissue. Results and Limitations: Median ctDNA fraction was 11% (range 0-84) among untreated patients but lower (1.0%, range 0-51) in patients after short term (median 22 days) androgen deprivation therapy (ADT). TP53 mutations and DNA repair defects were identified in 47% and 21% of the cohort, respectively. Concordance for mutation detection in matched samples was 80%. Combined ctDNA and tissue analysis identified potential driver alterations in 94% of patients, whereas ctDNA or prostate biopsy alone was insufficient in 19 cases (36%). Limitations include the use of a narrow gene panel and undersampling of primary disease by prostate biopsy. Conclusions: ctDNA provides additional information to a prostate biopsy in men with de novo mCSPC, but ADT rapidly reduces ctDNA availability. Primary tissue and ctDNA share relevant somatic alterations, suggesting that either are suitable for molecular subtyping in de novo mCSPC. The optimal approach for biomarker development should