ARv567es as detected in circulating tumor cells: An innovative methodology for the detection of a prognostic and therapeutic biomarker.

112 Background: Increased androgen receptor (AR) expression and signaling is associated with the progression of prostate cancer. Constitutively active AR splice variants (ARVs) are thought to arise following castration and play a key role in progression. The most well studied variant, ARv567es, lacks portions of the ligand-binding domain, increasing expression of full-length AR and transcriptional activity of AR in human prostate cancer cell lines. Tumor-derived tissue can be difficult to obtain for molecular studies on patients (pts) with castration-resistant prostate cancer (CRPC), with circulating tumor cells (CTCs) representing a unique avenue to investigate changes at the molecular level. Methods: Forty-seven whole blood samples drawn from 36 pts with metastatic CRPC (mCRPC) were processed for CTC isolation followed by molecular characterization of cDNA using epithelial tumor markers and prostate cancer specific markers. Microfluidics devices used in this study for CTCs isolation and retrieval is a surface marker independent, label-free cell trap system developed to isolate CTCs based on cell size and deformability (Clearbridge Biomedics). The ARv567eswas detected by multiplex digital PCR System (BioMark HD, Fluidigm) and confirmed by Sanger sequencing. Results: Four out of 36 pts and 6 out of 47 samples had the ARv567es.. The ARv567esamplified from three pts was confirmed by Sanger sequencing in which the ligand binding domain of the receptor was absent. Metastatic disease on presentation was seen in three out of four pts with the ARVs. The ARVs were detected late in the disease course, with the survival after assessment being brief (49, 106, and 200 days) for three out of four pts. One patient is still at 343 days. Due to the nature of this analysis it is unclear when these ARVs developed since baselines samples were not available. Conclusions: This is the first report of the ARv567es found in CTC-enriched peripheral blood samples from CRPC pts. This methodology can also be employed to find other ARVs, which could serve as potential biomarkers in pts, allowing for more appropriate treatment selection based on tumor evolution. This noninvasive method could significantly impact the molecular characterization and potentially the treatment science of CRPC.