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Prostate specific antigen (PSA) is useful for screening prostate cancer, while it has no reliable correlation with pathological malignancy of prostate cancer and is not a predictor for castration resistant prostate cancer (CRPC). In order to explore novel biomarkers to predict the development of CRPC, we performed proteomics from secreted proteins from human prostate cancer cells and identified the candidates of biomarker for CRPC. Materials and Methods: The proteins secreted from 6 prostate cancers in culture medium were analyzed and compared with 8 other cancer cells including renal and urothelial cancers using LTQ Orbitrap mass spectrometer. With the focus on high tissue specificity, the candidate biomarker proteins were then identified through analysis of gene expressions in proteins common to human prostate cancers by real time qPCR. Next, a system to measure the identified mouse monoclonal antibodies against the focused proteins was established. Finally, serum levels of these proteins from 33 patients with benign prostate hyperplasia (BPH), 31 with untreated prostate cancer (PCa) and 35 with CRPC, were measured. Results: The proteome analysis identified 12 candidates of secreted cell membrane proteins as new biomarkers. The proteome analysis indicated that not only matured GDF15, but pro-peptide as well as fragments (GDPP) are released from prostate cancer cells. Patients’ serum was analyzed for matured and pro-peptide GDF15 using ELISA and immunoprecipitation-MRM mass spectrometry. The results showed that the serum level of GDDP-1, one of the processing forms of GDPP, was significantly higher in CRPC than those in BPH and untreated PCa (P Conclusion: GDPP-1 shows potential as a novel biomarker for CRPC. Citation Format: Hiroji Uemura, Noriaki Arakawa, Yusuke Itoh, Takashi Kawahara, Yasuhide Miyoshi, Masahiro Yao, Shouhei Akiba, Norihisa Ohtake. Identification of a novel biomarker to predict castration-resistant prostate cancer (CRPC) by the proteome analysis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 228. doi:10.1158/1538-7445.AM2017-228

247 Background: It is well known that prostate specific antigen (PSA) level has no reliable correlation with pathological malignancy of prostate cancer and is not a predictor for the development of castration resistant prostate cancer (CRPC). The aim of this study is to explore novel biomarkers to predict the development of CRPC by using proteomics from secreted proteins from human prostate cancer cells. Methods: The proteins secreted from 6 prostate cancers in culture medium were analyzed and compared with 8 other cancer cells including renal and urothelial cancers using LTQ Orbitrap mass spectrometer. With the focus on high tissue specificity, the candidate biomarker proteins were then identified through analysis of gene expressions in proteins common to human prostate cancers by real time qPCR. Next, a system to measure the identified mouse monoclonal antibodies against the focused proteins was established. Finally, serum levels of these proteins from 33 patients with benign prostate hyperplasia (BPH), 31 with untreated prostate cancer (PCa) and 35 with CRPC, were measured. Results: The proteome analysis identified 12 candidates of secreted cell membrane proteins as new biomarkers. The proteome analysis indicated that not only matured GDF15, but pro-peptide as well as fragments (GDDP) are released from prostate cancer cells. Patients’ serum was analyzed for matured and pro-peptide GDF15 using ELISA and immunoprecipitation-MRM mass spectrometry. The results showed that the serum level of GDDP-1, one of the processing forms of GDDP, was significantly higher in CRPC than those in BPH and untreated PCa (P < 0.01). ROC analysis also showed that the AUC of GDDP-1(0.86) was higher than that of matured GDF15 (0.76). When the cutoff value of GDDP-1 was set at 4.0 ng/mL, there was a significant difference of overall survival (OS) in CRPC patients between those with more than 4.0 ng/mL compared to less than 4.0 ng/mL of GDDP-1, whereas there was no significant difference of OS measurable by PSA in CRPC patients. These data suggest that GDDP-1 may be a novel biomarker for CRPC. Conclusions: GDDP-1 shows potential as a novel biomarker for CRPC.