Pilot study of the clinical correlation of CTC counts and AR-V7 expression with resistance to androgen receptor inhibitors in metastatic prostate cancer

Introduction Androgen receptor splice variant 7 (AR-V7) is a constitutively active isoform of the androgen receptor and has been associated with resistance to AR-targeting therapies and progression to metastatic prostate cancer (mPCa). However, tissue-based detection of AR-V7 is limited by the poor accessibility of metastatic lesions, particularly before and after treatment. To address this limitation, we developed a CTC-based AR-V7 detection system using CytoGen's Smart Biopsy™ CTC platform and investigated the clinical association of CTC counts and AR-V7 expression androgen receptor inhibitor (ARI). Method Peripheral blood samples (20 mL) from 12 mPCa patients were processed using the Smart Biopsy™ Cell Isolator for CTC enrichment. 5 samples from healthy donor were used as negative control of CTC counts by immunofluorescence and the cut-off of AR-V7 expression. CTCs were enumerated by immunofluorescence (IF), and AR-V7 transcripts were analyzed using droplet digital PCR (ddPCR) Results To investigate the clinical correlation of CTC counts and AR-V7 expression to the resistance of ARI, blood samples of mPCa patients were collected and CTC enrichment was proceeded with Smart Biopsy™ Cell Isolator. Number of CTC in all of 12 mPCa patients' samples were successfully counted by IF assay. 10 patients showed the high CTC burden (≥10 CTCs per 5mL of blood) and it was statistically significant correlation with the resistance of ARI. AR-V7 expression in CTC were also tested by ddPCR. 5 out of 12 mPCa patients showed AR-V7 positive CTC and 4 out of 5 patients were associated with ARI's resistance. Conclusions In this pilot study, CTC counts and/or AR-V7 detection in CTC showed the meaningful correlation to the resistance of ARI, suggesting that they may serve as potential diagnostic tools for predicting the treatment response of ARI.