Androgen receptor splice variant 7 (AR-V7) and drug efficacy in castration-resistant prostate cancer: Biomarker for treatment selection exclusion or inclusion?

ABSTRACT

Currently there are no molecular biomarkers used to help guide treatment selection for those patients with castration-resistant prostate cancer. A recent study published in JAMA Oncology (Antonarakis et al.) presents evidence supporting the potential use of androgen receptor splice variant 7 as a biomarker for optimal treatment selection in this population.

Keywords:

• Androgen receptor; prostate cancer; splice variants

Prostate cancer is the second leading cause of cancer-related death in American men. While many patients initially respond well to androgen deprivation therapy (ADT), the disease usually progresses to castration-resistant prostate cancer (CRPC), defined as the progression of disease despite castrate levels of testosterone. Unfortunately, treatment for CRPC is limited and the disease is not curable. It is critical to understand the underlying mechanisms of CRPC.

The androgen receptor (AR) is a key transcription regulator that is overly expressed and/or activated in CRPC. AR has an N-terminal domain responsible for transactivation, a C-terminal ligand-binding domain, and a DNA-binding domain. The ligand-binding domain is responsible for binding to androgen, and when absent creates an androgen-independent constitutively active AR splice variant with continued AR transcriptional activity. AR splice variants are truncated versions of wild-type AR full-length (AR-Fl) and may lack the C-terminal ligand-binding domain or, less commonly, the N-terminal domain which results in loss of the DNA-binding domain. AR splice variants modulate the AR signaling pathway, can resist ADT, and can potentially contribute to the progression of CRPC.

AR splice variant 7 (AR-V7), which lacks the ligand-binding domain, expressed in circulating tumor cells has been implicated in drug resistance in CRPC patients. In 2014, Antonarakis et al. found that detection of AR-V7 in CRPC patients was associated with resistance to enzalutamide and abiraterone therapy.¹ A recent study published in June 2015 in JAMA Oncology (Antonarakis et al.) found that AR-V7 status had no negative effect on taxane efficacy.² In the previous study, AR-V7 positive and in AR-V7 negative patients treated with enzalutamide and abiraterone treatment resulted in 0% and 53% decreases (P = 0.004) in prostate-specific antigen (PSA), while in this study docetaxel or cabazitaxel treatment resulted in 41% and 65% decreases (P = 0.19) respectively. The sample size in both studies was small, and further validation of AR-V7 in drug efficacy must be demonstrated before its clinical use as a treatment biomarker. If AR-V7 can be validated, then it may be useful to detect the presence of this variant in patients with prostate cancer prior to initiating ADT.

These most recent findings, that taxane response is not affected by AR-V7 status in CRPC patients, contrast recently published research on AR-V7 and taxane efficacy. While taxanes are known to stabilize microtubules within cells to inhibit cell division, studies have also shown that taxanes may disrupt cytoplasmic-to-nuclear trafficking of AR along the microtubule network.³ Thadani-Mulero et al., using a mouse model, found that AR-V7 was not sensitive to taxane therapy.⁴ In a 2015 cell-based study, Zhang et al. found that AR splice variants played a role in taxane resistance.⁵ Specifically they discovered that AR-Fl transcription activity, but not AR splice variant transcription activity, was inhibited by taxanes. Enzalutamide and abiraterone both target the AR ligand-binding domain, which explains their lack of efficacy in AR-V7 positive CRPC patients. In light of their findings, Antonarakis et al. present an argument for the use of AR-V7 as a treatment selection biomarker.

While these studies provide clues as to why enzalutamide and abiraterone eventually fail in many CRPC patients, and thus suggesting that AR-V7 may be a valid treatment biomarker, there are issues with using this as a biomarker for treatment selection of taxanes. Pastina et al. found that polymorphisms in CYP1B1 were related to docetaxel therapy efficacy, specifically that those CRPC patients carrying the CYP1B1*3 genotype responded significantly less to docetaxel than those without this genotype.⁶ The primary estrogen metabolite of CYP1B1, 4-hydroxyestradiol, when oxidized inhibits tubulin polymerization, and this metabolite is formed more readily by the CYP1B1*3 allele.⁷ OATP1B3 is also being explored as a potential biomarker. Testosterone is a substrate for OATP1B3, and OATP1B3 expression has been shown to be associated with Gleason score.⁸ While AR-V7 may be useful for predicting treatment response with enzalutamide and abiraterone, other potential biomarkers, like CYP1B1*3 and OATP1B3, may be better predictors of taxane efficacy in CRPC patients.

Antonarakis et al. mentions in both their 2014 study and their most recent study a phenomenon of AR-V7 status conversion. In the 2014 study, the investigators found that some patients who were AR-V7 negative status at baseline converted to AR-V7 positive status while on enzalutamide and abiraterone therapy; no patients who were AR-V7 positive status at baseline converted to AR-V7 negative status while on enzalutamide and abiraterone therapy. In this most recent study, one patient who was AR-V7 negative status converted to AR-V7 positive status while on taxane therapy; just over half of those in the study who were AR-V7 positive status converted to AR-V7 negative status while on taxane therapy. Similarly, Nakazawa et al. found that both AR-directed therapies and taxanes can cause conversions to AR-V7 position status, while reversions to AR-V7 negative status only occurred with taxanes.⁹ This may be evidence that these AR-V7 status conversions lead to drug resistance. AR-directed therapies may be selecting for AR-V7 and potentially other AR splice variants in circulating tumor cells, allows these resistant cells to proliferate. Taxanes may convert AR-V7 positive status patients to negative status, which would allow the use of enzalutamide and abiraterone therapy once taxane efficacy ceases. It is not clear however how taxanes may cause this AR-V7 status conversion. Future research may be able to elucidate potential mechanisms.

While the investigators conclude that AR-V7 positive status should exclude enzalutamide and abiraterone therapy and instead select for taxane therapy, alternatively these recent findings provoke the question of whether targeting both AR and AR-V7 may be a more useful drug therapy strategy. Other investigators have suggested that targeting both AR-V7 and the AR ligand-binding domain may be useful in early therapies.^10,11 AR-V7 appears to be induced after ADT, potentially selecting for circulating tumor cells which exhibit this splice variant.¹¹ A recent study by Cao et al. found that AR-Vs can cause AR-FL to induce gene expression in the absence of androgen, in addition to AR-V ability to be active in the absence of androgen on their own.¹⁰ Targeting both AR-V7 and the AR-ligand binding domain may help prevent resistance mechanisms from emerging.

Recent studies have started testing compounds targeting AR splice variants, though these studies have been limited to cell models and not specifically with AR-V7. Some compounds, like 3,30 -diindolylmethane (BR-DIM), are being used to repress AR splice variants, which may prove useful in preventing CRPC.¹² Another compound, niclosamide, has also shown promise in inhibiting AR splice variant expression.^13,14 Multiple pathways have been implicated as potential targets for AR splice variants. Inhibition of insulin-like growth factor 1 receptor (IGF-1R) has been shown to downregulate AR splice variant signaling and provides a rationale for CRPC therapies targeting growth factor receptors.¹⁵ Downregulation of heterogeneous nuclear RNA-binding protein A1 (hnRNPA1) resensitizes enzalutamide-resistant cells to enzalutamide, indicating that this may inhibit the generation of AR splice variants.¹⁶ Stockley et. al. found that the RNA-binding protein Sam68, encoded by the KHDRBS1 gene, may play a role in regulating AR-V7 expression and transcriptional activity, suggesting Sam68 may be another target for AR splice variant downregulation.¹⁷ These new findings may provide a drug design pathway which can target AR-V7 and other splice variants.

Other AR splice variants lacking the ligand-binding domain, such as ARv567es, have also been studied, though not in human drug efficacy studies as have been conducted with AR-V7.^5,10,18,19 AR splice variants lacking the ligand-binding domain would likely exhibit similar effects with drug efficacy as AR-V7. Future research should determine the prevalence of similar AR splice variants in CRPC patients, whether they also correlate with decreased AR-directed therapy efficacy, and if an assay can reliably detect these splice variants in patients. It may be possible that targeting AR-V7, as well as other AR splice variants, like ARv567es, may be beneficial in conferring better therapy efficacy.

The investigators suggest that AR-V7 status can be used to help guide optimal treatment choices by eliminating abiraterone and enzalutamide as potential therapies in AR-V7 positive status patients. Another viewpoint is that if AR-V7-targeting therapies become available then AR-V7 as a biomarker should not exclude abiraterone and enzalutamide treatment in AR-V7 positive patients, rather it should include these treatments along with an AR-V7-targeting therapy. AR-V7 detection could then help prevent treatment failure seen with abiraterone and enzalutamide. Further studies are needed to validate AR-V7 and other potential treatment biomarkers, such as CYP1B1 and OATP1B3, for patients with prostate cancer before utilizing them to guide treatment.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.