Association of androgen receptor (AR) copy number gain with ARV7 expression and response to chemotherapy.

Abstract

180 Background: Emerging data suggests that ARV7 mRNA (ARV7+) expression and AR copy number gain/amplification (AR+) are associated with resistance to androgen deprivation therapy. Most studies thus far have evaluated either AR copy number or ARV7 mRNA, but not both markers. It is likely that both markers are indicators of genomic changes that are seen in the prostate cancer cells in response to tumour evolution or treatment. Here we developed a digital droplet PCR (ddPCR) based assay to evaluate plasma ARV7 mRNA expression in the same samples used for AR copy number analysis in castrate resistant prostate cancer (CRPC). This assay was use to evaluate the role of AR+ and ARV7+ in CRPC using samples collected from multiple centers and correlated with outcome. Methods: We collected plasma samples from patients with CRPC treated with chemotherapy (C) or next generation androgen targeted agents (NAR) to evaluate circulating tumour DNA (ctDNA) for AR and plasma mRNA for ARV7. Some patients had sequential samples and we correlated PSA response (reduction PSA≥50 confirmed 3wks later (RR)) with AR and ARV7 status and clinical features. Results: A total of 52 samples were included from 26 patients, with matched samples from some patients at different time points. Of the CRPC patients, AR+ was seen in 70% (18/26), AR- in 30% (8/26), ARV7+ in 67% (12/18) and ARV7- in 33% (6/18). 75% (9/12) patients with AR+ also had ARV7+. In matched samples: 3/7 had change in AR status, 2/7 had change in ARV7 status, including 1 patient where both changed; and interestingly there was a group with no AR or ARV7 detected. Number of patients expressing both markers AR+/ARV7+ in 50% (9/18), AR+/ARV7- in 17% (3/18), AR-/ARV7+ 17% (3/18) and AR-/ARV7- in 11% (2/18). RR to C or NAR was 63% (12/19) for AR+ and 75% (9/12) for ARV7+ patients. In AR+ patients, response to NAR was 58% (12/16) but response to chemotherapy was 100% (5/5). Conclusions: AR gain/amplification in CRPC was associated with ARV7+ expression and leads to lower response to NAR compared to C. This approach can be used to undertake integrated DNA and mRNA analysis, has wide-scale utility and may be used for analysis of a larger number of circulating biomarkers.