The androgen receptor splice variant AR-V7 has recently been discussed as a predictive biomarker for nonresponse to next-generation androgen deprivation therapy (ADT) in patients with castration-resistant prostate cancer. However, we recently identified one patient showing a response from abiraterone despite expression of AR-V7 in his circulating tumour cells (CTC).
Therefore, we precisely assessed the response in a cohort of 21 AR-V7 positive castration-resistant prostate cancer patients who had received therapy with abiraterone or enzalutamide. We detected a subgroup of six AR-V7 positive patients showing benefit from either abiraterone or enzalutamide. Their progression free survival was 26 d (censored) to 188 d. Four patients displayed a prostate-specific antigen decrease of >50%. When analysing prior therapies, we noticed that only one of the six patients had received next-generation ADT prior to CTC collection.
As a result, we conclude that AR-V7 status in CTC cannot entirely predict nonresponse to next generation ADT and AR-V7-positive patients should not be systematically denied abiraterone or enzalutamide treatment, especially as effective alternative treatment options are still limited.
A subgroup of patients can benefit from abiraterone and/or enzalutamide despite detection of AR-V7 splice variants in their circulating tumour cells.
Keywords: Abiraterone, Enzalutamide, Androgen receptor splice variant, AR-V7, Castration resistant prostate cancer.
One of the major mechanisms conferring resistance to androgen deprivation therapy (ADT) is the expression of androgen receptor (AR) splice variants. The most abundant splice variant, AR-V7, encodes a constitutively active AR variant, in which the ligand-binding domain is missing, making it potentially invulnerable to next generation ADT  and .
Recently, several publications showed the feasibility to detect AR-V7 in circulating tumour cells (CTC) of castration-resistant prostate cancer (CRPC) patients. Indeed, positivity for AR-V7 greatly predicted resistance to both abiraterone and enzalutamide , but not to taxane-based chemotherapy  and . Thus, AR-V7 has recently been discussed as a predictive biomarker for nonresponse to next-generation ADT.
However, we recently identified one AR-V7-positive patient, who showed response to abiraterone with a prostate-specific antigen (PSA) decrease of more than 50% . Likewise, Antonarakis et al.  detected one patient showing a PSA decrease of approximately 30% upon enzalutamide treatment despite AR-V7 expression. In both studies, biochemical response was defined as a PSA decline of more than 50%. However, in clinical routine a PSA decrease of less than 50% and even an increase of less than 25% of the PSA nadir would not argue for a therapy switch according to current clinical prostate cancer guidelines . Thus, we hypothesized that a subgroup of patients might benefit from next-generation ADT in terms of stable disease despite expression of AR-V7 in CTCs.
Therefore, we retrospectively analysed AR-V7-positive CRPC patients who had received next-generation ADT. A CTC collection had been performed right before the start of therapy with either abiraterone or enzalutamide. CTC analyses were performed using immuno-magnetic bead-based tumour cell isolation (ProstateCancer Select and ProstateCancer Detect; Qiagen, Hannover, Germany) followed by reverse transcription and quantitative real-time polymerase chain reaction (see Steinestel et al. ). We defined substantial benefit as either stable disease (< 50% PSA decrease to < 25% PSA increase from PSA nadir), or response (PSA decrease > 50%).
Six out of 21 AR-V7-positive patients experienced benefit from next-generation ADT. Four patients even displayed a PSA decrease of more than 50%, which is in strong contrast to other reports describing a strict correlation between expression of AR-V7 and resistance to next-generation ADT  and . The six AR-V7-positive patients responding to abiraterone or enzalutamide showed a median progression-free survival (PFS) of 2.9 mo. The median PFS for the four patients with more than a 50% PSA decrease was 3.4 mo. Additionally, one of these four patients had a 73% PSA decrease and died after 26 d (0.9 mo) from a cause other than prostate cancer. Therefore, the true median PFS might even be higher than 2.9 mo or 3.4 mo.
However, the median PFS for AR-V7-negative patients receiving abiraterone or enzalutamide in the cohort of Antonarakis et al.  was approximately 6 mo. Thus, the duration of response might still be longer in AR-V7-negative patients.
Tumour and patient characteristics of the six AR-V7-positive patients who experienced benefit from next-generation ADT did not differ significantly from the 15 AR-V7-positive patients showing PSA progression. The median age of benefiting patients (BP; n = 6) versus nonbenefiting patients (NBP; n = 15) was 77 yr versus 75 yr. They had presented with an initial Gleason score ≥8 in 100% versus 60% of cases. BP and NBP had been stable to first-generation ADT for a median of 12 mo and 13 mo, respectively. However, the BP might have been in a slightly earlier state in their course of disease compared with the NBP since their PSA level at the time of blood collection and start of next-generation ADT with either abiraterone or enzalutamide was 119 ml (range, 30–371 ng/ml) in BP and 532 ml (range, 33–1338 ng/ml) in NBP.
Although the numbers are too low to draw conclusions, we noted that only one of the six (16.7%) BP but nine of the 15 (60%) NBP had prior novel AR-targeted therapy (Fig. 1). None of the four AR-V7-positive patients with more than a 50% PSA reduction had prior therapy with abiraterone or enzalutamide. This raises the question whether the splice variant AR-V7 might not represent the mechanism of resistance itself, but is rather an epiphenomenon of significant AR targeting pretreatment. In this case, an (yet unknown) additional mechanism would underlie the development of resistance that develops increasingly with effective ADT.
Overview of prior and new therapies along with androgen receptor splice variant (AR-V7) to androgen receptor full length (AR-FL) ratio (%), prostate-specific antigen (PSA) response (%), and progression free survival (PFS; d) for the six AR-V7-positive patients experiencing benefit from next generation androgen deprivation therapy (ADT).
Further, we evaluated whether the AR-V7 to AR-full length ratio could distinguish BP from NBP but found no correlation in our cohort (median: 7.5% vs 8.0%, respectively; Fig. 1). This might be because CTCs represent a specialised tumour cell population and comparison of their biological profile with the main tumour from which they are derived is a field of ongoing research  and . While we are convinced that the presence of AR-V7 in the CTC population proves the existence of an AR-V7-expressing tumour, it would be rather hypothetical to extrapolate the absolute or relative levels of AR-V7 in these cells on the whole tumour mass. Accordingly, a recent publication analysing CTCs on the single cell level showed heterogeneity among the CTCs with respect to splice variant expression even within one patient. Whereas some cells expressed high amounts of AR-V7, other CTCs of the same patient from the same blood sample expressed only low levels or were negative for AR-V7  and . It is impossible to differentiate whether only a few CTCs express high levels of AR-V7 or if a large number of CTCs are characterised by weak AR-V7 expression, which is a limitation of the technique used in our study. However, we feel that our observations are of crucial relevance for the discussion regarding the usability of detectable AR-V7 transcripts in CTCs as a predictive biomarker in CRPC, all the more since we employed the identical technique used by Antonarakis et al.  in their ground-breaking 2014 publication.
Our findings are of crucial clinical relevance as they demonstrate that AR-V7 expression cannot entirely predict nonresponse to next-generation ADT. Therefore, AR-V7-positive patients should not be denied abiraterone or enzalutamide treatment systematically, even more since effective alternative treatment options are still limited. Additional studies will have to focus on specifically delineating the differences in benefit from abiraterone and enzalutamide in AR-V7-positive versus -negative patients.
Author contributions: Julie Steinestel had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Study concept and design: Bernemann, Steinestel J.
Acquisition of data: Schnoeller.
Analysis and interpretation of data: Steinestel J, Schrader.
Drafting of the manuscript: Bernemann, Steinestel J, Steinestel K.
Critical revision of the manuscript for important intellectual content: Schrader, Steinestel J, Steinestel K.
Statistical analysis: Luedecke, Steinestel J.
Obtaining funding: Schrader.
Administrative, technical, or material support: Boegemann.
Supervision: Steinestel J, Schrader.
Financial disclosures: Julie Steinestel certifies that all conflicts of interest, including specific financial interests and relationships and affiliations relevant to the subject matter or materials discussed in the manuscript (eg, employment/affiliation, grants or funding, consultancies, honoraria, stock ownership or options, expert testimony, royalties, or patents filed, received, or pending), are the following: None.
Funding/Support and role of the sponsor: Anneliese Pohl Foundation played a part in the design and conduct of the study.
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a Clinic of Urology, University Hospital Muenster, Muenster, Germany
b Clinic of Urology, University Hospital Ulm, Ulm, Germany
c Gerhard-Domagk-Institute of Pathology, University Hospital Muenster, Muenster, Germany
Corresponding author. Department of Urology, Muenster University Medical Centre, Albert-Schweitzer-Campus 1, Building A 1, Muenster D-48149, Germany. Tel. +49 (0) 2 51/83-44609; Fax: +49 (0) 2 51/83-44619.
© 2016 European Association of Urology, Published by Elsevier B.V.