Cell-Free Dna As A Biomarker For Taxane Treatment In Advanced Prostate Cancer.

5070 Background: Antitumor efficacy of chemotherapeutic drugs docetaxel and cabazitaxel against hormone-sensitive and castration-resistant prostate cancer (CRPC) is well recognized. The identification and validation of minimally invasive biomarkers of taxane response remain of paramount importance. Methods: Serial pre-, during and post-treatment plasma samples were collected prospectively from two large Phase III clinical trials, FIRSTANA (NCT01308567) and PROSELICA (NCT01308580). Chemotherapy-naïve patients (pts) were treated with docetaxel (75 mg/m2) or cabazitaxel (20 or 25 mg/m2) in FIRSTANA, and pts previously treated with docetaxel received cabazitaxel (20 or 25 mg/m2) as second-line chemotherapy in PROSELICA. Plasma cell-free DNA (cfDNA) was extracted, low-pass whole genome sequencing (lp-WGS) libraries were prepared using the QIAGEN QiaSeq FX DNA library kit, and samples sequenced on the Illumina NovaSeq 6000. Targeted next-generation sequencing (NGS) on a custom-made AmpliSeq panel of 30 genes, pre-selected for putative roles in taxane resistance, was also performed. lp-WGS copy number (CN) profiles were generated using ichorCNA (v0.1.0) and targeted NGS variant calls derived using IonReporter software. Results: Overall, lp-WGS data was generated from 265 samples (99 pts; 51 treated on the FIRSTANA study, 48 treated on the PROSELICA study), acquired at three time-points (pre-, during and at progression). Average cfDNA input was 10 ng and mean coverage achieved was ~2X (SD 1.2X). We observed changes in lp-WGS tumor purity over time as a result of therapy; samples from non-responding pts exhibited significantly higher tumor purity values post-treatment compared with samples from responding pts (p = 0.02). Targeted NGS results were available from 294 pts (153 from FIRSTANA, 141 from PROSELICA), and changes in tumor purity were also associated with treatment response (p = 0.04). CN frequency of key CRPC genes was similar to previously reported datasets; several aberrant loci associated with response to taxane therapy. Conclusions: cfDNA lp-WGS may have clinical utility in the management of lethal prostate cancer. Funding: Sanofi. Clinical trial information: NCT01308580, NCT01308567.