SLFN11 expression (exp) in castration-resistant prostate cancer (CRPC) patients (pts) to predict response to platinum-based chemotherapy (PLT)

5065 Background: Schlafen family member 11 (SLFN11) sensitizes tumor cells to DNA-damaging agents and has been investigated as a potential predictive biomarker of response to PLT and PARP inhibitors, especially in small cell lung cancer (Lok, CCR 2017; Pietanza, JCO 2018). We aimed to explore the predictive/prognostic role of SLFN11 in PLT-treated CRPC. Methods: We assessed tumor exp of SLFN11 in PLT-treated, metastatic CRPC pts by RNAseq (N=27) and/or CTCs (N=20) (via the Epic Sciences platform). In addition, tumor morphology for neuroendocrine (NE) features and genomic status of select genes (ie, AR, TP53, RB1, BRCA2, BRCA1, ATM) by whole exome sequencing were evaluated. Statistical comparisons used Cox regression analysis and Kaplan Meier method for the association with overall/radiographic progression free survival (OS/rPFS). A dose response curve with PLT was performed in patient-derived organoids using Cell Title Glo according to the manufacturer’s protocol. Results: 41 CRPC (including 20 CRPC-NE) treated with PLT monotherapy (N=3) or PLT combination therapy (N=38) between August 2013 and December 2017 were evaluated. Median age was 67.1 years (range 51-91). Median number of prior therapies was 2 (range 1-7). A longer median rPFS was observed in all SLFN11+ pts treated with PLT (regardless of histology, RB1, TP53, PTEN, or DNA repair status) compared to SLFN11- [5.2 vs 2.3 months, HR 3.5, 95%CI 1.6-7.7, P