Abstract 1077: Genomic alterations in plasma cell-free DNA predict treatment response to abiraterone acetate and prednisone and survival in metastatic castration-resistant prostate cancer

Background: Abiraterone acetate/prednisone (AA/P) is used for treatment of metastatic castration-resistant prostate cancer (mCRPC), but response is variable and there are no known molecular predictive factors. Methods: We prospectively collected germline and plasma specimens from 72 mCRPC patients progressing on androgen-deprivation therapy (ADT) before initiating AA/P treatment (baseline) and serially after 12 weeks of treatment. After extracting cell-free DNA (cfDNA), we performed copy number analysis using low pass whole-genome sequencing approach. The primary endpoints were to determine association with response and survival. Cox regression and Kaplan-Meier survival analyses were used to associate the CNV with overall survival (OS) and progression-free survival (PFS) with statistical significance at p≤0.05. For determining primary resistance to AA/P, responders and nonresponders were defined as patients showing no progression or progression after 12 weeks treatment. T-test was used to evaluate the CNV with AA/P response at 12 weeks. Results: Whole-genome sequencing generated approximately 8.0 million mappable sequence reads per subject (total plasma samples sequenced, n=144). By comparing cfDNA to germline control DNA, our copy number analysis showed significant genomic abnormalities in 37 of 144 plasma samples. We observed the most frequent amplification of 1q, 8q, Xq and deletion of 2p, 6q,10q, 13q. Among these CNVs, amplification of MYC at 8q24.21, AR at Xq12, deletion of FOXP1 at 3p14, PTEN at 10q23.31, RB1 at 13q14.2, TP53 at 17P13.1, and gene fusions (TMPRSS2:ERG) were consistent with previous reports from tumor tissue-based studies. Survival analysis showed significant association of a fragment deletion at 13q14-32 with both OS and PFS (FDR= 0.0207/0.0141). The most significant fragment showing association with OS was the deletion of fragment at 2p22.1-p14 (FDR=0.00219). Genomic gains of MYC at 8q24.21 and NCOA2 at 8q13.3 were associated with PFS with FDR= 0.0085 and 0.0072, respectively. By comparing baseline CNVs between responder and nonresponders, we identified significant association of the deletion of PTEN at 10q23.31 (p= 0.0003), RB1 at 13q14.2 (p= 0.0008) and amplification of NCOA2 at 8q13.3 (p= 0.0030) with AA/P response/resistance at 12 weeks. Although statistical analysis showed the significant CNV change at AR locus between baseline and 12-week post-AA/P treatment (p=0.0002), the CNV change was not significantly different between responders and nonresponders (p=0.1697). Conclusions: cfDNA CNV may serve as a sensitive biomarker for prediction of treatment response to AA/P. Further detailed CNV and clinical association analyses are needed to fully validate the potential clinical utilities of cfDNA-based liquid biopsy for clinical management. Citation Format: Meijun Du, Manish Kohli, Chiang-Ching Huang, Liang Wang. Genomic alterations in plasma cell-free DNA predict treatment response to abiraterone acetate and prednisone and survival in metastatic castration-resistant prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1077.