Detection of AR-V7 in Liquid Biopsies of Castrate Resistant Prostate Cancer Patients: A Comparison of AR-V7 Analysis in Circulating Tumor Cells, Circulating Tumor RNA and Exosomes

Abstract

Detection of androgen receptor (AR) variant 7 (AR-V7) is emerging as a clinically important biomarker in castrate resistant prostate cancer (CRPC). Detection is possible from tumor tissue, which is often inaccessible in the advanced disease setting. With recent progress in detecting AR-V7 in circulating tumor cells (CTCs), circulating tumor RNA (ctRNA) and exosomes from prostate cancer patients, liquid biopsies have emerged as an alternative to tumor biopsy. Therefore, it is important to clarify whether these approaches differ in sensitivity in order to achieve the best possible biomarker characterization for the patient. In this study, blood samples from 44 prostate cancer patients were processed for CTCs and ctRNA with subsequent AR-V7 testing, while exosomal RNA was isolated from 16 samples and tested. Detection of AR and AR-V7 was performed using a highly sensitive droplet digital PCR-based assay. AR and AR-V7 RNA were detectable in CTCs, ctRNA and exosome samples. AR-V7 detection from CTCs showed higher sensitivity and has proven specificity compared to detection from ctRNA and exosomes. Considering that CTCs are almost always present in the advanced prostate cancer setting, CTC samples should be considered the liquid biopsy of choice for the detection of this clinically important biomarker.

Androgen Receptor and Its Splicing Variant 7 Expression in Peripheral Blood Mononuclear Cells and in Circulating Tumor Cells in Metastatic Castration-Resistant Prostate Cancer

“…This result agrees with the detection of ARV7 in PBMC of almost all samples analyzed by Qu et al [9] and with the finding by Nimir et al of ARV7 exosomal mRNA in one of five (20%) healthy volunteers, by using a droplet digital PCR-based approach [27]. We assume that the sensitivity between different works varies since we and Qu et al could detect ARFL expression in all the samples while Nimir et al could detect it in only three of five healthy subjects [27]. ARV7 expression in whole blood from non-cancer individuals was also described by Todenhöfer et al [8] and Takeuchi et al [28].…”

The Prospect of Identifying Resistance Mechanisms for Castrate-Resistant Prostate Cancer Using Circulating Tumor Cells: Is Epithelial-to-Mesenchymal Transition a Key Player?

“…We recently compared some of these strategies and found both fulllength AR and V7 RNA detection is more sensitive and specific if performed on CTC samples, as compared to ctRNA or exosomes. We also demonstrated that AR-V7 is detectable from CTC-RNA up to 48 h post blood draw into common EDTA vacutubes [189,190]. With improved AR-V7-specific antibody availability, CTC immunocytostaining more recently revealed that specific detection of AR-V7 in CTC nuclei is an even better predictor of OS and PFS in CRPC patients [191,192].…”

Detection of BRAF splicing variants in plasma-derived cell-free nucleic acids and extracellular vesicles of melanoma patients failing targeted therapy therapies

“…Splicing variants of the androgen receptor, in particular AR-V7, have been found to be significantly higher in hormone refractory prostate cancer and is associated with poor clinical outcomes [ 55 ]. Similar to BRAF-splicing variants, AR-V7 can be detected in plasma EV RNA [ 53 , 56 ]. Altogether, this supports an alternative liquid biopsy modality based on EV RNA or ctRNA.…”