Do Patients With AR-V7-Positive Prostate Cancer Benefit from Novel Hormonal Therapies? It All Depends on Definitions

The androgen receptor

The Androgen Receptor (AR) is a ligand (androgen) activated transcription factor and a member of the nuclear receptor (NR) superfamily. It is required for male sex hormone function. AR-FL (full-length) has the domain structure of NRs, an N-terminal domain (NTD) required for transactivation, a DNA-binding domain (DBD), a nuclear localization signal (NLS) and a ligand-binding domain (LBD). Paradoxes exist in that endogenous ligands testosterone (T) and 5α-dihydrotestosterone (DHT) have differential effects on male sexual development while binding to the same receptor and transcriptional specificity is achieved even though the androgen response elements (AREs) are identical to those seen for the progesterone, glucocorticoid and mineralocorticoid receptors. A high resolution 3-dimensional structure of AR-FL by either cryo-EM or X-ray crystallography has remained elusive largely due to the intrinsic disorder of the NTD. AR function is regulated by post-translational modification leading to a large number of proteoforms. The interaction of these proteoforms in multiprotein complexes with co-activators and co-repressors driven by interdomain coupling mediates the AR transcriptional output. The AR is a drug target for selective androgen receptor modulators (SARMS) that either have anabolic or androgenic effects. Protstate cancer is treated with androgen deprivation therapy or by the use of AR antagonists that bind to the LBD. Drug resistance occurs due to adaptive AR upregulation and the appearance of splice variants that lack the LBD and become constitutively active. Bipolar T treatment and NTD-antagonists could surmount these resistance mechanisms, respectively. These recent advances in AR signaling are described.

Using biochemistry and biophysics to extinguish androgen receptor signaling in prostate cancer

Castration resistant prostate cancer (CRPC) continues to be androgen receptor (AR) driven. Inhibition of AR signaling in CRPC could be advanced using state-of-the-art biophysical and biochemical techniques. Structural characterization of AR and its complexes by cryo-electron microscopy would advance the development of N-terminal domain (NTD) and ligand-binding domain (LBD) antagonists. The structural basis of AR function is unlikely to be determined by any single structure due to the intrinsic disorder of its NTD, which not only interacts with coregulators but likely accounts for the constitutive activity of AR-splice variants (SV), which lack the LBD and emerge in CRPC. Using different AR constructs lacking the LBD, their effects on protein folding, DNA binding, and transcriptional activity could reveal how interdomain coupling explains the activity of AR-SVs. The AR also interacts with coregulators that promote chromatin looping. Elucidating the mechanisms involved can identify vulnerabilities to treat CRPC, which do not involve targeting the AR. Phosphorylation of the AR coactivator MED-1 by CDK7 is one mechanism that can be blocked by the use of CDK7 inhibitors. CRPC gains resistance to AR signaling inhibitors (ARSI). Drug resistance may involve AR-SVs, but their role requires their reliable quantification by SILAC-mass spectrometry during disease progression. ARSI drug resistance also occurs by intratumoral androgen biosynthesis catalyzed by AKR1C3 (type 5 17β-hydroxysteroid dehydrogenase), which is unique in that its acts as a coactivator of AR. Novel bifunctional inhibitors that competitively inhibit AKR1C3 and block its coactivator function could be developed using reverse-micelle NMR and fragment-based drug discovery.

The Prognostic Value of Androgen Receptor Splice Variant 7 in Castration-Resistant Prostate Cancer Treated With Novel Hormonal Therapy or Chemotherapy: A Systematic Review and Meta-analysis

Purpose

This study aimed to evaluate the prognostic role of AR-V7 in terms of prostate-specific antigen (PSA) response, progression-free survival (PFS), and overall survival (OS) in CRPC patients treated with novel hormonal therapy (NHT) (Abiraterone and Enzalutamide) or taxane-based chemotherapy (Docetaxel and Cabazitaxel).

Methods

A comprehensive literature search was conducted on PubMed, Embase, and the Web of Science from inception to February 2020. Studies focusing on the prognostic values of AR-V7 in CRPC patients treated with NHT or chemotherapy were included in our meta-analysis. The OS and PFS were analyzed based on Hazard ratios (HRs) and 95% confidence intervals (CIs). Furthermore, Odds ratios (ORs) and 95% CIs were summarized for the AR-V7 conversion after treatment and the PSA response.

Results

The AR-V7 positive proportion increased significantly after NHT treatment (OR 2.56, 95% CI 1.51–4.32, P<0.001), however, it declined after chemotherapy (OR 0.51, 95% CI 0.28–0.93, P=0.003). AR-V7-positive patients showed a significantly decreased PSA response rate after NHT (OR 0.13, 95% CI 0.09–0.19, P<0.001) but not statistically significant for chemotherapy (OR 0.63, 95% CI 0.40-1.01, P=0.06). Notably, PFS (HR 3.56, 95% CI 2.53–5.01, P<0.001) and OS (HR 4.47, 95% CI 3.03–6.59, P<0.001) were worse in AR-V7-positive ttreated with NHT. Similarly, AR-V7 positivity correlated with poor prognosis after chemotherapy as evidenced by shorter OS (HR 1.98, 95% CI 1.48-2.66, P<0.001) and a significantly shorter PFS (HR 1.35, 95% CI 0.97-1.87, P=0.07).

Conclusion

NHT treatment increased AR-V7 positive proportion whereas chemotherapy decreased it. Moreover, AR-V7 positivity correlated with lower PSA response, poorer PFS, and OS in CRPC treated with NHT, and shorter OS in patients receiving chemotherapy.

Molecular analysis of circulating tumor cells of metastatic castration-resistant Prostate Cancer Patients receiving 177Lu-PSMA-617 Radioligand Therapy

Rationale: Lu-177-PSMA-617 radioligand therapy (RLT) is currently under approval for treatment of metastatic castration resistant prostate cancer (mCRPC) patients with late stage disease. However, previous studies demonstrated both heterogeneity of prostate specific membrane antigen (PSMA) expression, as well as response to PSMA treatment among mCRPC patients. Thus, there is an unmet need for identifying predictive parametres prior or under PSMA-RLT treatment. We therefore aimed to correlate several clinical and molecular parameters with response to PSMA treatment in a cohort of mCRPC patients undergoing PSMA RLT followed by a detailed analysis of promising candidates.

Methods: Nineteen patients, median age 68.8 years (range: 56.9 - 83.3) with mCRPC were included in this study. We performed baseline analysis of clinical parameters based on PSMA PET/CT, (metabolic tumor volume (MTV), total tumor volume (TTV)), serum PSA, ALP, LDH and gene expression analysis of circulating tumor cells (expression of AR full length (AR-FL), AR splice variant 7 (AR-V7), PSA and PSMA) as well as common markers for neuroendocrine differentiation (NED).

Results: Patients presented with bone, lymph node, and visceral metastases (89%, 68%, and 21%, respectively). All patients were pretreated with docetaxel, either abiraterone or enzalutamide, or both. Biochemical response in terms of PSA decline ≥50 or ≥30% was observed in 42% and 63%, respectively. There were significant correlations between PSA and PSMA mRNA expression, as well as tumor volumes (both MTV and TTV), AR-FL and AR-V7 mRNA expression. However, there was no correlation with response to PSMA treatment. Furthermore, none of these parameters was significantly correlated with baseline serum PSA values. Common NED markers were shown to be specifically high expressed and revealed impact on OS independent from AR-V7 gene expression.

Conclusion: We demonstrate that AR-FL and its splice variant AR-V7 might serve as prognostic biomarkers displaying high tumor burden in mCRPC patient prior to PSMA-RLT. Contrary, PSMA, which has been discussed as a biomarker for PSMA targeted treatment, does not display strong prognostic ability - at least on the mRNA level. Surprisingly, none of these parameters correlates to response to PSMA treatment. In contrast, commom NED markers such as SYP and ENO2 as well as FOXA1 expression level seem to predict OS, but not PFS, more reliably. We admit that a limitation of our study is the focus on mRNA expression of potential biomarkers only. Further investigations analyzing the potential role of protein expression of these markers are therefore warranted.

AR-V7 Protein Expression in Circulating Tumour Cells Is Not Predictive of Treatment Response in mCRPC

INTRODUCTION

Androgen receptor variant 7 (AR-V7) plays an important role in the progression of castration-resistant prostate cancer (CRPC) and has shown potential as a predictive biomarker in circulating tumour cells (CTCs) isolated from the bloodstream in terms of a liquid biopsy. Studies have shown that AR-V7 is a potential surrogate for selecting drug classes for systemic treatment by detecting nuclear AR-V7 by immunofluorescence or measuring AR-V7 messenger RNA by quantitative PCR. Here, we assessed the predictive value of AR-V7 detected by classical immunohistochemistry (IHC) for treatment response.

METHODS

CTCs were isolated by cell separation by density gradient centrifugation from patients with metastatic CRPC (n = 26) before, while, and after undergoing a new therapy with chemotherapy (cabazitaxel or docetaxel) or antiandrogen (enzalutamide or abiraterone). CTCs were sequentially cytospun on object slides, and AR-V7 status was then detected by IHC based on a staining regime established on a 22Rv1 cell line with antibodies against CK8/18 und AR-V7.

RESULTS

AR-V7 status detected by IHC showed no predictive value for progression-free survival (PFS). Kaplan-Meier analysis revealed that there was no difference in PFS between patients found positive or negative for AR-V7.

DISCUSSION/CONCLUSION

AR-V7 detected by classical IHC has no predictive value for treatment response in the described setting. The future role of AR-V7 in CTCs as a biomarker in clinical routine remains elusive.

Assessment of the Validity of Nuclear-Localized Androgen Receptor Splice Variant 7 in Circulating Tumor Cells as a Predictive Biomarker for Castration-Resistant Prostate Cancer

Importance

A blood test to determine whether to treat patients with metastatic castration-resistant prostate cancer (mCRPC) with an androgen receptor signaling (ARS) inhibitor or taxane is an unmet medical need.

Objective

To determine whether a validated assay for the nuclear-localized androgen receptor splice variant 7 (AR-V7) protein in circulating tumor cells can determine differential overall survival among patients with mCRPC treated with taxanes vs ARS inhibitors.

Design, Setting, and Participants

This blinded correlative study conducted from December 31, 2012, to September 1, 2016, included 142 patients with histologically confirmed mCRPC and who were treated at Memorial Sloan Kettering Cancer Center, The Royal Marsden, or the London Health Sciences Centre. Blood samples were obtained prior to administration of ARS inhibitors or taxanes as a second-line or greater systemic therapy for progressing mCRPC.

Main Outcomes and Measures

Overall survival after treatment with an ARS inhibitor or taxane in relation to pretherapy AR-V7 status.

Results

Among the 142 patients in the study (mean [SD] age, 69.5 [9.6] years), 70 were designated as high risk by conventional prognostic factors. In this high-risk group, patients positive for AR-V7 who were treated with taxanes had superior overall survival relative to those treated with ARS inhibitors (median overall survival, 14.3 vs 7.3 months; hazard ratio, 0.62; 95% CI, 0.28-1.39; P = .25). Patients negative for AR-V7 who were treated with ARS inhibitors had superior overall survival relative to those treated with taxanes (median overall survival, 19.8 vs 12.8 months; hazard ratio, 1.67; 95% CI, 1.00-2.81; P = .05).

Conclusions and Relevance

This study suggests that nuclear-localized AR-V7 protein in circulating tumor cells can identify patients who may live longer with taxane chemotherapy vs ARS inhibitor treatment.

Performance comparison of two androgen receptor splice variant 7 (AR-V7) detection methods

OBJECTIVES

To compare the performance of two established androgen receptor splice variant 7 (AR-V7) mRNA detection systems, as paradoxical responses to next-generation androgen-deprivation therapy in AR-V7 mRNA-positive circulating tumour cells (CTC) of patients with castration-resistant prostate cancer (CRPC) could be related to false-positive classification using detection systems with different sensitivities.

MATERIALS AND METHODS

We compared the performance of two established mRNA-based AR-V7 detection technologies using either SYBR Green or TaqMan chemistries. We assessed in vitro performance using eight genitourinary cancer cell lines and serial dilutions in three AR-V7-positive prostate cancer cell lines, as well as in 32 blood samples from patients with CRPC.

RESULTS

Both assays performed identically in the cell lines and serial dilutions showed identical diagnostic thresholds. Performance comparison in 32 clinical patient samples showed perfect concordance between the assays. In particular, both assays determined AR-V7 mRNA-positive CTCs in three patients with unexpected responses to next-generation anti-androgen therapy. Thus, technical differences between the assays can be excluded as the underlying reason for the unexpected responses to next-generation anti-androgen therapy in a subset of AR-V7 patients.

CONCLUSIONS

Irrespective of the method used, patients with AR-V7 mRNA-positive CRPC should not be systematically precluded from an otherwise safe treatment option.

CTC-derived AR-V7 detection as a prognostic and predictive biomarker in advanced prostate cancer

INTRODUCTION

Prostate cancer is a highly heterogeneous disease, with remarkably different prognosis across all stages. Increased circulating tumor cell (CTC) count (≥ 5) using the CellSearch assay has been identified as one of the markers that can be used to predict survival, with added value beyond currently available prognostic factors. Recently, androgen receptor splice variant 7 (AR-V7) detection has been associated with worse outcomes for patients with castration-resistant prostate cancer (CRPC) treated with novel androgen receptor-signaling (ARS) inhibitors such as abiraterone and enzalutamide but not taxane chemotherapies. Areas covered: In this manuscript, the authors review the available biomarkers in CRPC and discuss emerging data on the value of CTC-derived AR-V7 status to assess prognosis and its potential role to guide treatment selection for patients with advanced prostate cancer. Expert commentary: Current evidence supports AR-V7 status as a prognostic biomarker and also as a potential predictive biomarker for patients with mCRPC. The authors expect that the incorporation of AR-V7 status and other biomarkers (e.g. AR mutations) in the sequential assessment of patients with advanced prostate cancer will lead to a more rational use of available and future therapies, with significant improvements in outcomes for our patients.

Nuclear-specific AR-V7 Protein Localization is Necessary to Guide Treatment Selection in Metastatic Castration-resistant Prostate Cancer

BACKGROUND

Circulating tumor cells (CTCs) expressing AR-V7 protein localized to the nucleus (nuclear-specific) identify metastatic castration-resistant prostate cancer (mCRPC) patients with improved overall survival (OS) on taxane therapy relative to the androgen receptor signaling inhibitors (ARSi) abiraterone acetate, enzalutamide, and apalutamide.

OBJECTIVE

To evaluate if expanding the positivity criteria to include both nuclear and cytoplasmic AR-V7 localization ("nuclear-agnostic") identifies more patients who would benefit from a taxane over an ARSi.

DESIGN, SETTING, AND PARTICIPANTS

The study used a cross-sectional cohort. Between December 2012 and March 2015, 193 pretherapy blood samples, 191 of which were evaluable, were collected and processed from 161 unique mCRPC patients before starting a new line of systemic therapy for disease progression at the Memorial Sloan Kettering Cancer Center. The association between two AR-V7 scoring criteria, post-therapy prostate-specific antigen (PSA) change (PTPC) and OS following ARSi or taxane treatment, was explored. One criterion required nuclear-specific AR-V7 localization, and the other required an AR-V7 signal but was agnostic to protein localization in CTCs.

OUTCOME MEASUREMENTS AND STATISTICAL ANALYSES

Correlation of AR-V7 status to PTPC and OS was investigated. Relationships with survival were analyzed using multivariable Cox regression and log-rank analyses.

RESULTS AND LIMITATIONS

A total of 34 (18%) samples were AR-V7-positive using nuclear-specific criteria, and 56 (29%) were AR-V7-positive using nuclear-agnostic criteria. Following ARSi treatment, none of the 16 nuclear-specific AR-V7-positive samples and six of the 32 (19%) nuclear-agnostic AR-V7-positive samples had ≥50% PTPC at 12 weeks. The strongest baseline factor influencing OS was the interaction between the presence of nuclear-specific AR-V7-positive CTCs and treatment with a taxane (hazard ratio 0.24, 95% confidence interval 0.078-0.79; p=0.019). This interaction was not significant when nuclear-agnostic criteria were used.

CONCLUSIONS

To reliably inform treatment selection using an AR-V7 protein biomarker in CTCs, nuclear-specific localization is required.

PATIENT SUMMARY

We analyzed outcomes for patients with metastatic castration-resistant prostate cancer on androgen receptor signaling inhibitors and standard chemotherapy. Patients with circulating tumor cells that had AR-V7 protein in the cellular nuclei were very likely to survive longer on taxane-based chemotherapy, and tests unable to distinguish where the protein is located in the cell are not as predictive of benefit.