2. UroToday Home
  3. Recent Abstracts
  4. Urologic Oncology
  5. Prostate Cancer

Urine-based Liquid Biopsy: Non-invasive and Sensitive AR-V7 Detection in Urinary EVs from Patients with Prostate Cancer - Beyond the Abstract

After the initial response to androgen deprivation therapy (ADT), almost all prostate cancer (PCa) patients will eventually progress despite the low levels of testosterone in the systemic circulation following a median 18-24 months of ADT.1  Disease at this stage is termed castration-resistant PCa (CRPC). Despite significant therapeutic advances in recent years, CRPC remains a lethal disease with a median survival of 15-36 months.2, 3   Among mechanisms of therapeutic resistance, several alternatively spliced androgen receptor variants (AR-Vs), lacking the C-terminal ligand binding domain, but retaining the transactivating N-terminal domain, can lead to constitutive activation in the absence of ligands.4 AR-V7 is one of the most common variants found in CRPC patients. The expression level of AR-V7 is higher in mCPRC patients (15%) compared to hormone naïve ones, and it can increase under the use of either abiraterone (55%) or enzalutamide (50%), which implies that both primary and acquired resistance to these agents could be associated with AR-V7.5-7  Antonarakis et al. first reported the analysis of AR-V7 with a circulating tumor cell (CTC) assay to predict the response of abiraterone or enzalutamide.8 Patients who harbored AR-V7 in their CTCs have a significantly worse response and treatment outcome. Later, it has been shown that it is feasible to detect AR-V7 mRNA transcript in whole blood without the CTC enrichment and AR-V7 protein level by immunohistochemistry in patient biopsy samples and CTCs.9-11 Positive AR-V7 status or high expression level was associated with worse treatment outcomes in all these studies.

Recently, extracellular vesicles (EVs) have emerged as novel biomarkers for liquid biopsies. Notably, plasma-derived EVs were investigated as carriers of AR-V7 for use as predictive biomarkers of resistance to hormonal therapy. [3] Since EVs are prevalently found in most body fluids and carry genetic information originated from the cell, the authors hypothesized that urinary EVs are a reliable source for AR-V7 mRNA detection. They established a practical liquid biopsy method for absolute quantification of AR-V7 and AR-FL by enriching urine-derived exosomes using size-filtration on a centrifugal microfluidic device and droplet digital polymerase chain reaction (ddPCR). Urine-derived exosomes were used to detect AR-V7 and AR-FL mRNA expression in CRPC and hormone-sensitive prostate cancer (HSPC) patients.

They enrolled 36 patients, 22 of which had HSPC and 14 of which had CRPC. AR-FL expression was higher in patients with HSPC (6.2–9053.6 copies/mL, median: 760.7 copies/mL) than in those with CRPC (12.9–562.5 copies/mL, median: 50.9 copies/mL; p < 0.001, Mann–Whitney test). In contrast, AR-V7 expression was higher in patients with CRPC (0–217.0 copies/mL, median: 8.8 copies/mL) than in those with HSPC (0–24.1 copies/mL, median: 3.2 copies/mL; p < 0.01). To provide background levels, urine samples from 11 HD were tested as well. Both AR-FL (2.4-316.1 copies/mL, median: 15.8 copies/mL) and AR-V7 (0-3.8 copies/mL, median: 0 copies/mL) expression in urinary EVs from HD were significantly lower than that those from patients with prostate cancer. Next, the authors questioned whether the ratios of AR-V7 to AR-FL in patients with HSPC and CRPC were different. The AR-V7/AR-FL ratio was significantly higher in patients with CRPC (0–4.3, median: 0.3) than in those with HSPC (0–0.5, median: 0.003; p < 0.001, Mann–Whitney test), while HD samples showed the lowest (0-0.1, median: 0). Furthermore, receiver operating characteristic (ROC) curves are indicating that AR-V7 is a great classifier for differentiating prostate cancer (including HSPC and CRPC) from HD (area under the curve, AUC, 0.81-0.94;). Also, AR-V7/AR-FL ratio gives a great distinction for classifying CRPC from HSPC (AUC, 0.87). There were no significant differences between the baseline characteristics of race and age in the patients with HSPC (54–81, median: 70) and CRPC (60–82, median: 71.5; p > 0.05).

Although promising, the clinical role of urinary EV RNA on OS or on the choice of therapy requires further investigation using larger patient numbers. In conclusion, they established an efficient and non-invasive method that detects genetic changes in the AR gene using urine samples to predict CRPC development. To the best of my knowledge, this is the first report that urine-derived EV RNA is a reliable source for analyzing AR-V7 expression. Taken together, they postulate that urinary EVs can be a sample source for non-invasive, simple, and frequent monitoring of AR-V7 expression in prostate cancer patients. Additionally, they found that AR-V7 transcript levels and the AR-V7/AR-FL ratio in urinary exosomes were higher in patients with advanced prostate cancer.

Written by: Liang Dong, The James Buchanan Brady Urological Institute, Johns Hopkins University School of Medicine, Baltimore, Maryland

References:
1. Taylor, B.S., et al., Integrative genomic profiling of human prostate cancer. Cancer Cell, 2010. 18(1): p. 11-22.
2. Higano, C.S. and E.D. Crawford, New and emerging agents for the treatment of castration-resistant prostate cancer. Urol Oncol, 2011. 29(6 Suppl): p. S1-8.
3. Kirby, M., C. Hirst, and E.D. Crawford, Characterizing the castration-resistant prostate cancer population: a systematic review. Int J Clin Pract, 2011. 65(11): p. 1180-92.
4. Dehm, S.M. and D.J. Tindall, Alternatively spliced androgen receptor variants. Endocr Relat Cancer, 2011. 18(5): p. R183-96.
5. Sun, S., et al., Castration resistance in human prostate cancer is conferred by a frequently occurring androgen receptor splice variant. J Clin Invest, 2010. 120(8): p. 2715-30.
6. Antonarakis, E.S., et al., Androgen Receptor Splice Variant 7 and Efficacy of Taxane Chemotherapy in Patients With Metastatic Castration-Resistant Prostate Cancer. JAMA Oncol, 2015. 1(5): p. 582-91.
7. Luo, J., et al., Role of Androgen Receptor Variants in Prostate Cancer: Report from the 2017 Mission Androgen Receptor Variants Meeting. Eur Urol, 2018. 73(5): p. 715-723.
8. Antonarakis, E.S., et al., AR-V7 and resistance to enzalutamide and abiraterone in prostate cancer. N Engl J Med, 2014. 371(11): p. 1028-38.
9. Hu, R., W.B. Isaacs, and J. Luo, A snapshot of the expression signature of androgen receptor splicing variants and their distinctive transcriptional activities. Prostate, 2011. 71(15): p. 1656-67.
10. Luo, J., Development of AR-V7 as a putative treatment selection marker for metastatic castration-resistant prostate cancer. Asian J Androl, 2016. 18(4): p. 580-5.
11. Scher, H.I., et al., Association of AR-V7 on Circulating Tumor Cells as a Treatment-Specific Biomarker With Outcomes and Survival in Castration-Resistant Prostate Cancer. JAMA Oncol, 2016. 2(11): p. 1441-1449.
12. M. Del Re, E. Biasco, S. Crucitta, L. Derosa, E. Rofi, C. Orlandini, M. Miccoli, L. Galli, A. Falcone and G. W. Jenster, Eur. Urol., 2017, 71, 680–687.
13. T. Takeuchi, Y. Okuno, M. Hattori-Kato, M. Zaitsu and K. Mikami, Res. Rep. Urol., 2016, 8, 21–25.

Read the Abstract