Enzalutamide in castration-resistant prostate cancer patients progressing after docetaxel and abiraterone

Enzalutamide: targeting the androgen signalling pathway in metastatic castration-resistant prostate cancer

Significant progress has been made in the understanding of the underlying cancer biology of castration‐resistant prostate cancer (CRPC) with the androgen receptor (AR) signalling pathway remaining implicated throughout the prostate cancer disease continuum. Reactivation of the AR signalling pathway is considered to be a key driver of CRPC progression and, as such, the AR is a logical target for therapy in CRPC. The objective of this review was to understand the importance of AR signalling in the treatment of patients with metastatic CRPC (mCRPC) and to discuss the clinical benefits associated with inhibition of the AR signalling pathway. A search was conducted to identify articles relating to the role of AR signalling in CRPC and therapies that inhibit the AR signalling pathway. Current understanding of prostate cancer has identified the AR signalling pathway as a logical target for the treatment of CRPC. Available therapies that inhibit the AR signalling pathway include AR blockers, androgen biosynthesis inhibitors, and AR signalling inhibitors. Enzalutamide, the first approved AR signalling inhibitor, has a novel mode of action targeting AR signalling at three key stages. The direct mode of action of enzalutamide has been shown to translate into clinical responses in patients with mCRPC. In conclusion, the targeting of the AR signalling pathway in patients with mCRPC results in numerous clinical benefits. As the number of treatment options increase, more trials evaluating the sequencing and combination of treatments are required. This review highlights the continued importance of targeting a key driver in the progression of CRPC, AR signalling, and the clinical benefits associated with inhibition of the AR signalling pathway in the treatment of patients with CRPC.

[How to manage patients with CRPC?]

Despite castrate levels of androgens, the androgen receptor (AR) remains active and continues to drive prostate cancer progression. Since the approval of docetaxel, four additional agents that show a survival benefit have been registered on the basis of randomized phase 3 trials. These have included enzalutamide and abiraterone, two agents designed specifically to affect the androgen axis, sipuleucel-T, which stimulates the immune system and cabazitaxel, a chemotherapeutic agent. Denosumab was shown to significantly delay skeletal-related events. Clinicians are challenged with a multitude of treatment options and potential sequencing of these agents that, consequently, make clinical decision making more complex. The induction of constitutively-active AR splice variants (AR-Vs) driving clonal proliferation of AR-negative and AR-independent metastases may be one major potential mechanism of resistance to new hormone therapies.

AZD3514, an oral selective androgen receptor down-regulator in patients with castration-resistant prostate cancer - results of two parallel first-in-human phase I studies

BACKGROUND

AZD3514 is a first-in-class, orally bio-available, androgen-dependent and -independent androgen receptor inhibitor and selective androgen-receptor down-regulator (SARD).

METHODS

In study 1 and 2, castration-resistant prostate cancer (CRPC) patients (pts) were initially recruited into a once daily (QD) oral schedule (A). In study 1, pharmacokinetic assessments led to twice daily (BID) dosing (schedule B) to increase exposure. Study 2 explored a once daily schedule.

RESULTS

In study 1, 49 pts were treated with escalating doses of AZD3514 (A 35 pts, B 14 pts). Starting doses were 100 mg (A) and 1000 mg (B). The AZD3514 formulation was switched from capsules to tablets at 1000 mg QD. 2000 mg BID was considered non-tolerable due to grade (G) 2 toxicities (nausea [N], vomiting [V]). No adverse events (AEs) met the dose-limiting toxicity (DLT) definition. Thirteen pts received AZD3514 in study 2, with starting doses of 250 mg QD. The most frequent drug-related AEs were N: G1/2 in 55/70 pts (79 %); G3 in 1 pt (1.4 %); & V: G1/2 in 34/70 pts (49 %) & G3 in 1 pt (1.4 %). PSA declines (≥50 %) were documented in 9/70 patients (13 %). Objective soft tissue responses per RECIST1.1 were observed in 4/24 (17 %) pts in study 1.

CONCLUSION

AZD3514 has moderate anti-tumour activity in pts with advanced CRPC but with significant levels of nausea and vomiting. However, anti-tumour activity as judged by significant PSA declines, objective responses and durable disease stabilisations, provides the rationale for future development of SARD compounds.

Activity of enzalutamide in men with metastatic castration-resistant prostate cancer is affected by prior treatment with abiraterone and/or docetaxel

BACKGROUND

Enzalutamide and abiraterone are new androgen-axis disrupting treatments for metastatic castration-resistant prostate cancer (mCRPC). We examined the response and outcomes of enzalutamide-treated mCRPC patients in the real-world context of prior treatments of abiraterone and/or docetaxel.

METHODS

We conducted a seven-institution retrospective study of mCRPC patients treated with enzalutamide between January 2009 and February 2014. We compared the baseline characteristics, PSA declines, PSA progression-free survival (PSA-PFS), duration on enzalutamide and overall survival (OS) across subgroups defined by prior abiraterone and/or docetaxel.

RESULTS

Of 310 patients who received enzalutamide, 36 (12%) received neither prior abiraterone nor prior docetaxel, 79 (25%) received prior abiraterone, 30 (10%) received prior docetaxel and 165 (53%) received both prior abiraterone and prior docetaxel. Within these groups, respectively, ⩾30% PSA decline was achieved among 67, 28, 43 and 24% of patients; PSA-PFS was 5.5 (95% CI 4.2-9.1), 4.0 (3.2-4.8), 4.1 (2.9-5.4) and 2.8 (2.5-3.2) months; median duration of enzalutamide was 9.1 (7.3-not reached), 4.7 (3.7-7.7), 5.4 (3.8-8.4) and 3.9 (3.0-4.6) months. Median OS was reached only for the patients who received both prior abiraterone and docetaxel and was 12.2 months (95% CI 10.7-16.5). 12-month OS was 78% (59-100%), 64% (45-90%), 77% (61-97%) and 51% (41-62%). Of 70 patients who failed to achieve any PSA decline on prior abiraterone, 19 (27%) achieved ⩾30% PSA decline with subsequent enzalutamide.

CONCLUSIONS

The activity of enzalutamide is blunted after abiraterone, after docetaxel, and still more after both, suggesting subsets of overlapping and distinct mechanisms of resistance.

Sequencing of agents in castration-resistant prostate cancer

Until 2010, docetaxel was the only agent with proven survival benefit for castration-resistant prostate cancer. The development of cabazitaxel, abiraterone acetate, enzalutamide, radium-223, and sipuleucel-T has increased the number of treatment options. Because these agents were developed concurrently within a short period of time, prospective data on their sequential use efficacy are scarce. The challenge now is to reach a consensus on the best way to sequence effective treatments, ideally by the use of an approach specific to patient subgroups. However, the absence of robust surrogates of survival and the lack of predictive biomarkers makes data for the sequential use of these agents difficult to obtain and interpret.

The third line of treatment for metastatic prostate cancer patients: Option or strategy?

New agents for metastatic castration-resistant prostate cancer (CRPC) developed in the past 3 years include cabaziataxel (Cbz), abiraterone acetate (AA) and enzalutamide (E). In this review, the results of clinical studies in which one of these drugs is included as the third line of treatment are discussed. Our review suggests that AA and E have limited activity, while Cbz seems to retain its efficacy. Prospective studies that further examine sequential treatments are warranted.

Circulating cell-free AR and CYP17A1 copy number variations may associate with outcome of metastatic castration-resistant prostate cancer patients treated with abiraterone

BACKGROUND

This study aimed to investigate copy number variations (CNVs) of CYP17A1 and androgen receptor (AR) genes in serum cell-free DNA collected before starting abiraterone in 53 consecutive patients with castration-resistant prostate cancer (CRPC).

METHODS

Serum DNA was isolated and CNVs were analysed for AR and CYP17A1 genes using Taqman copy number assays. The association between CNVs and progression-free/overall survival (PFS/OS) was evaluated by the Kaplan-Meier method and log-rank test.

RESULTS

Median PFS of patients with AR gene gain was 2.8 vs 9.5 months of non-gained cases (P < 0.0001). Patients with CYP17A1 gene gain had a median PFS of 2.8 months vs 9.2 months in the non-gained patients (P = 0.0014). A lower OS was reported in both cases (AR: P < 0.0001; CYP17A1: P = 0.0085). Multivariate analysis revealed that PSA decline ⩾ 50%, AR and CYP17A1 CNVs were associated with shorter PFS (P < 0.0001, P = 0.0004 and P = 0.0450, respectively), while performance status, PSA decline ⩾ 50%, AR CNV and DNA concentration were associated with OS (P = 0.0021, P = 0.0014, P = 0.0026 and P = 0.0129, respectively).

CONCLUSIONS

CNVs of AR and CYP17A1 genes would appear to be associated with outcome of CRPC patients treated with abiraterone.

Targeting the androgen receptor pathway in castration-resistant prostate cancer: progresses and prospects

Androgen receptor (AR) signaling is a critical pathway for prostate cancer cells, and androgen-deprivation therapy (ADT) remains the principal treatment for patients with locally advanced and metastatic disease. However, over time, most tumors become resistant to ADT. The view of castration-resistant prostate cancer (CRPC) has changed dramatically in the last several years. Progress in understanding the disease biology and mechanisms of castration resistance led to significant advancements and to paradigm shift in the treatment. Accumulating evidence showed that prostate cancers develop adaptive mechanisms for maintaining AR signaling to allow for survival and further evolution. The aim of this review is to summarize molecular mechanisms of castration resistance and provide an update in the development of novel agents and strategies to more effectively target the AR signaling pathway.

Metastatic castration-resistant prostate cancer (CRPC): preclinical and clinical evidence for the sequential use of novel therapeutics

With five novel therapies shown to improve survival in metastatic castration-resistant prostate cancer (CRPC) in the last 3 years, patients are now living longer and experiencing better quality of life. Since docetaxel became standard of care for men with symptomatic metastatic CRPC, three artificial treatment "spaces" have emerged for prostate cancer drug development: pre-docetaxel, docetaxel combinations, and following docetaxel. Multiple therapies are currently under development in both early and late stage CRPC. Additionally, the novel agents abiraterone, radium-223, cabazitaxel, and enzalutamide have all been approved in the post-docetaxel setting. Strategies for patient selection and treatment sequencing are therefore urgently required. In this comprehensive review, we will summarize the preclinical and clinical data available with regards to sequencing of the novel treatments for CRPC.

Sequencing current therapies in the treatment of metastatic prostate cancer

The standard treatment for metastatic prostate cancer is androgen deprivation therapy. However, progressive, metastatic disease usually develops, giving rise to metastatic castration-resistant prostate cancer (mCRPC). Great improvements have been made recently in the management of mCRPC, with current approved treatments including chemotherapy, androgen receptor-targeted agents, immunotherapies and radiopharmaceuticals. While the emergence of multiple effective therapies is encouraging, devising a treatment strategy can be difficult and it is becoming increasingly important, and challenging, to identify factors that influence the ideal timing of specific therapies. Considering where to place these agents in the treatment schedule of mCRPC, or whether these agents should be sequenced or combined to derive the optimal benefit for the patient, is not yet clear. Furthermore, cross-resistance may exist between these agents, which may ultimately influence treatment decisions and sequence choices. Preliminary data are emerging regarding the safety and activity for sequential treatment regimens, but there are currently no prospective studies. As prostate cancer is highly heterogeneous clinically, it is likely that no single treatment sequence will be optimal for all patients. However, at present, there are no validated biomarkers to guide individualized treatment for mCRPC. Here we review available data for the different mCRPC treatments, discussing potential sequencing of agents and possible cross-resistance or synergy among the recently approved and emerging therapies.

Androgen Receptor Gene Aberrations in Circulating Cell-Free DNA: Biomarkers of Therapeutic Resistance in Castration-Resistant Prostate Cancer

PURPOSE

Although novel agents targeting the androgen-androgen receptor (AR) axis have altered the treatment paradigm of metastatic castration-resistant prostate cancer (mCRPC), development of therapeutic resistance is inevitable. In this study, we examined whether AR gene aberrations detectable in circulating cell-free DNA (cfDNA) are associated with resistance to abiraterone acetate and enzalutamide in mCRPC patients.

EXPERIMENTAL DESIGN

Plasma was collected from 62 mCRPC patients ceasing abiraterone acetate (n = 29), enzalutamide (n = 19), or other agents (n = 14) due to disease progression. DNA was extracted and subjected to array comparative genomic hybridization (aCGH) for chromosome copy number analysis, and Roche 454 targeted next-generation sequencing of exon 8 in the AR.

RESULTS

On aCGH, AR amplification was significantly more common in patients progressing on enzalutamide than on abiraterone or other agents (53% vs. 17% vs. 21%, P = 0.02, χ(2)). Missense AR exon 8 mutations were detected in 11 of 62 patients (18%), including the first reported case of an F876L mutation in an enzalutamide-resistant patient and H874Y and T877A mutations in 7 abiraterone-resistant patients. In patients switched onto enzalutamide after cfDNA collection (n = 39), an AR gene aberration (copy number increase and/or an exon 8 mutation) in pretreatment cfDNA was associated with adverse outcomes, including lower rates of PSA decline ≥ 30% (P = 0.013, χ(2)) and shorter time to radiographic/clinical progression (P = 0.010, Cox proportional hazards regression).

CONCLUSIONS

AR gene aberrations in cfDNA are associated with resistance to enzalutamide and abiraterone in mCRPC. Our data illustrate that genomic analysis of cfDNA is a minimally invasive method for interrogating mechanisms of therapeutic resistance in mCRPC.

An update on enzalutamide in the treatment of prostate cancer

Enzalutamide is an oral androgen receptor inhibitor that targets multiple steps in the androgen receptor signaling pathway. In the randomized phase III AFFIRM study, significant improvements in survival versus placebo were observed when enzalutamide was used as a treatment for patients with metastatic castration-resistant prostate cancer (mCRPC) following prior treatment with docetaxel. Additional benefits included significant delay in time to first skeletal-related event, and improvement in several measures of pain and health-related quality of life. Treatment effects were consistent across all prespecified subgroups. The phase III PREVAIL study evaluated enzalutamide versus placebo in patients with mCRPC who had not received chemotherapy. Enzalutamide significantly decreased the risk of radiographic progression and death. There were also significant improvements in all secondary and prespecified exploratory endpoints, including delayed initiation of chemotherapy, reduction in risk of first skeletal-related event and a high percentage of patients with objective response compared with placebo. Enzalutamide was also studied in hormone naïve patients (as monotherapy) in a small, open-label phase II study in patients with prostate cancer who were eligible for androgen-deprivation therapy. A prostate-specific antigen (PSA) response, defined as ⩾80% decline in PSA level from baseline at week 25, was achieved in 92.5% of patients. Long-term follow up is ongoing. Despite differences between these three trials, enzalutamide displayed a favorable safety profile in all three patient populations. Similar rates of adverse events between the enzalutamide and placebo groups were observed in AFFIRM and PREVAIL, with fatigue, diarrhea, back pain and hot flashes being more common with enzalutamide than with placebo. Hypertension was reported at a higher rate in the enzalutamide group than in the placebo group in PREVAIL. Breast-related disorders associated with enzalutamide treatment were also reported in the Monotherapy trial. Few seizures were reported in any trial. Enzalutamide is being studied in several early disease state populations.

Exploring the Clinical Benefit of Docetaxel or Enzalutamide After Disease Progression During Abiraterone Acetate and Prednisone Treatment in Men With Metastatic Castration-Resistant Prostate Cancer

BACKGROUND

Abiraterone acetate (AA) has demonstrated improved outcomes in men with metastatic castration-resistant prostate cancer (mCRPC). However, data are lacking on the effect of AA on subsequent efficacy of enzalutamide or docetaxel.

PATIENTS AND METHODS

We included men with mCRPC who received AA and subsequent enzalutamide or docetaxel by August 12, 2013. Patients were separated into 3 groups: group A, treated with AA then enzalutamide before chemotherapy; group B, treated with AA then docetaxel; and group C, treated with AA and enzalutamide after chemotherapy. The primary objective was to describe the response and overall survival with subsequent therapy.

RESULTS

There were 28 evaluable patients who received enzalutamide after AA (9 in group A and 19 in group C) and 13 patients who received docetaxel after AA (group B). Group A patients had more visceral disease and higher baseline prostate-specific antigen (PSA) levels, and group C men had a higher level of pain and multiple poor prognostic features. Median progression-free survival was 3.6, 5.1, and 2.8 months, respectively, and median overall survival was 8.5, not reached, and 9.6 months, respectively. A ≥ 50% PSA decline was achieved in 11%, 63%, and 5% of group A, B, and C patients, respectively. Radiographic or clinical progression as best response was noted in 55.5%, 30.8%, and 68.4% in each respective group.

CONCLUSION

In this chart review of consecutive men with progressive mCRPC after AA, we found modest activity for enzalutamide and docetaxel, with clear cross-resistance for AA and enzalutamide. These data might inform the complex treatment decisions after AA treatment.

Enzalutamide: a new prostate cancer targeted therapy against the androgen receptor

Enzalutamide (MDV3100), an androgen receptor-signalling inhibitor, represents the most recent compound added to the therapeutic armamentarium for the treatment of metastatic castration-resistant prostate cancer (mCRPC) who progressed to docetaxel. The anti-tumour activity and safety of enzalutamide has been demonstrated in a phase III clinical trial, showing a benefit in overall survival, which was the primary endpoint. There are no head-to-head studies comparing the different treatment options in this subset of patients. In this article, most relevant data published in the literature have been reviewed, with special attention to the therapeutic alternatives currently available for postdocexatel mCRPC patients, emphasising the mechanisms of action of the different drugs, efficacy and quality of life-related aspects.

Effect of bipolar androgen therapy for asymptomatic men with castration-resistant prostate cancer: results from a pilot clinical study

Targeting androgen receptor (AR) axis signaling by disrupting androgen-AR interactions remains the primary treatment for metastatic prostate cancer. Unfortunately, all men develop resistance to primary castrating therapy and secondary androgen deprivation therapies (ADTs). Resistance develops in part because castration-resistant prostate cancer (CRPC) cells adaptively up-regulate AR levels through overexpression, amplification, and expression of ligand-independent variants in response to chronic exposure to a low-testosterone environment. However, preclinical models suggest that AR overexpression represents a therapeutic liability that can be exploited via exposure to supraphysiologic testosterone to promote CRPC cell death. Preclinical data supported a pilot study in which 16 asymptomatic CRPC patients with low to moderate metastatic burden were treated with testosterone cypionate (400 mg intramuscular; day 1 of 28) and etoposide (100 mg oral daily; days 1 to 14 of 28). After three cycles, those with a declining prostate-specific antigen (PSA) continued on intermittent testosterone therapy monotherapy. Castrating therapy was continued to suppress endogenous testosterone production, allowing for rapid cycling from supraphysiologic to near-castrate serum testosterone levels, a strategy termed bipolar androgen therapy (BAT). BAT was well tolerated and resulted in high rates of PSA (7 of 14 evaluable patients) and radiographic responses (5 of 10 evaluable patients). Although all men showed eventual PSA progression, four men remained on BAT for ≥1 year. All patients (10 of 10) demonstrated PSA reductions upon receiving androgen-ablative therapies after BAT, suggesting that BAT may also restore sensitivity to ADTs. BAT shows promise as treatment for CRPC and should be further evaluated in larger trials.

Targeting extra-gonadal androgens in castration-resistant prostate cancer

Metastatic castration resistant prostate cancer (CRPC) is associated with a rise in PSA, suggesting an increase in transcription of steroid receptor regulated genes. The efficacy of the new anti-androgen therapies abiraterone and enzalutamide, that target extra-gonadal activation of androgen signaling, confirm CRPC's addiction to genes regulated by the androgen receptor (AR). However, patients invariably progress and develop resistance. This review focuses on mechanisms of drug resistance associated with the AR and steroidogenesis in CRPC. Understanding this persistent dependency and adaptation to the androgen axis in CRPC will lead to an understanding of resistance to new licensed therapies and to novel drug discovery, ultimately improving clinical outcome in CRPC. This article is part of a Special Issue entitled 'Essential role of DHEA'.

Treatment sequencing in metastatic castrate-resistant prostate cancer

Six different treatments have demonstrated improved survival in phase III trials targeted to patients with metastatic castration-resistant prostate cancer (mCRPC). Front-line therapeutic options for mCRPC include docetaxel, sipuleucel-T, abiraterone and radium-223. Post-docetaxel options include cabazitaxel, abiraterone, enzalutamide and radium-223. Despite much progress in recent years, much is yet unknown and debates occur over optimal treatment choices and sequences. None of the new agents have been compared to one another, thus physicians in practice today must make choices based on non-randomized comparisons, toxicity considerations and various assumptions. Abiraterone is now moving into the front line mCRPC space given recent regulatory approvals and enzalutamide will follow soon. Both of the hormonal agents have less toxicity when compared to chemotherapeutic options and both of these hormonal agents are expected to be used in a considerable number of mCRPC patients in the years ahead. Little data are available for the post-abiraterone or post-enzalutamide setting. In this review the currently available sequencing data are summarized and interpreted. It is now clear that cross resistance is a potential issue between various treatments, especially those agents that target the androgen axis. This review highlights the need for additional studies to optimize the current treatments for these patients.

Clinical activity of enzalutamide in Docetaxel-naïve and Docetaxel-pretreated patients with metastatic castration-resistant prostate cancer

BACKGROUND

Two randomized clinical trials have demonstrated a survival advantage with enzalutamide over placebo in both docetaxel (D)-pretreated and D-naïve metastatic castration-resistant prostate cancer (mCRPC) patients. Cross-resistance between androgen receptor-directed therapies and taxanes has been suggested, possibly leading to lower efficacy of enzalutamide in the post-D setting.

METHODS

We aimed to examine the impact of prior D treatment on the clinical activity of enzalutamide in patients with mCRPC. We retrospectively reviewed an institutional database to identify men with mCRPC treated with standard-of-care enzalutamide. Patients were classified as D-naïve or D-pretreated. The efficacy end points were prostate-specific antigen (PSA) response rates (≥ 50% PSA decline), time to PSA progression (TTPP) and clinical/radiographic progression-free survival (PFS) in response to enzalutamide. Differences between groups (D-naïve and D-pretreated) were assessed by univariate and multivariable analyses using logistic and Cox regression models.

RESULTS

One-hundred-seven (107) consecutive patients were included: 60 were D-pretreated and 47 were D-naïve. PSA responses were 43.2% in D-naïve patients and 25.4% in D-pretreated patients (P = 0.089). Median TTPP was 7.2 months (95% CI = 4.5 - 17.2) in the D-naïve group versus 2.6 mo (95% CI = 1.9 - 3.5) in the D-pretreated group (P < 0.0001). Median PFS was not reached for D-naïve men and was 3.3 mo (95% CI = 2.5 - 4.8) for D-pretreated men (P < 0.0001). After adjusting for potential confounders including prior abiraterone use, differences remained statistically significant for TTPP (HR = 2.32; 95% CI = 1.19 - 4.50; P = 0.013) and marginally significant for PFS (HR = 1.90; 95% CI = 0.94 - 3.84; P = 0.073) in multivariable analyses. Among patients who achieved a PSA response to enzalutamide (n = 34), results suggested a trend towards shorter duration of response in D-pretreated patients.

CONCLUSIONS

The clinical activity of enzalutamide appears to be blunted in patients who have previously received docetaxel chemotherapy. These results support the concept of cross-resistance between these two agents.

Further analysis of PREVAIL: enzalutamide use in chemotherapy-naïve men with metastatic castration-resistant prostate cancer

PREVAIL was a phase III multinational, double-blind, placebo-controlled trial that enrolled chemotherapy-naïve men with metastatic castration-resistant prostate cancer (mCRPC), which showed remarkable improvement in co-primary endpoints with an overall 81% reduction in the risk of radiographic progression, as well as 29% reduction in the risk of death in favor of the enzalutamide arm over placebo. All secondary endpoints including time to subsequent chemotherapy initiation and prostate specific antigen (PSA) progression were in favor of the enzalutamide arm. The results of PREVAIL shows the utility of enzalutamide that would likely soon expand the indication to asymptomatic or minimally symptomatic men with mCRPC not previously treated with chemotherapy.

Mechanisms of acquired resistance to androgen receptor targeting drugs in castration-resistant prostate cancer

After initial response to androgen receptor (AR) targeting drugs abiraterone or enzalutamide, most patients develop progressive disease and therefore, castration resistant prostate cancer remains a terminal disease. Multiple mechanisms underlying acquired resistance have been postulated. Intratumoral androgen synthesis may resume after abiraterone treatment. A point mutation in the ligand-binding domain of AR may confer resistance to enzalutamide. Emergence of AR splice variants lacking the ligand-binding domain may mediate resistance to abiraterone and enzalutamide. Steroid receptors such as glucocorticoid receptor may substitute for AR. Drugs with novel mechanisms of action or combination therapy, along with biomarkers for patient selection, may be needed to improve the therapy of castration resistant prostate cancer.

Enzalutamide for the treatment of prostate cancer: results and implications of the AFFIRM trial

Enzalutamide is a second-generation androgen receptor signaling inhibitor that was approved by the US FDA in 2012 for the treatment of metastatic docetaxel-pretreated castrate-resistant prostate cancer. In preclinical studies, enzalutamide demonstrated higher affinity to the androgen receptor compared with the first-generation androgen receptor inhibitors. In the well-designed Phase III AFFIRM study, enzalutamide treatment showed improved overall survival compared with placebo in addition to improvement of all preplanned secondary parameters. Overall, enzalutamide seemed to be very well tolerated with a favorable side-effect profile, with a lower incidence of grade 3-4 adverse events. A potentially concerning adverse effect was the occurrence of seizures that were reported in approximately 1% of the patients receiving enzalutamide (compared with 0% in the placebo arm). This review will summarize the mechanism of action of enzalutamide, the preclinical and clinical development that led to its approval focusing on the AFFIRM trial results, its safety and efficacy and the ongoing trials, as well as patterns of resistance to this drug in the context of five new drugs approved for the treatment of metastatic castration-resistant prostate cancer. With a changing landscape for these patients, treatment sequencing and best treatment for individual patients remains challenging.

Clinical Outcomes of Castration-resistant Prostate Cancer Treatments Administered as Third or Fourth Line Following Failure of Docetaxel and Other Second-line Treatment: Results of an Italian Multicentre Study

BACKGROUND

The availability of new agents (NAs) active in patients with metastatic castration-resistant prostate cancer (mCRPC) progressing after docetaxel treatment (abiraterone acetate, cabazitaxel, and enzalutamide) has led to the possibility of using them sequentially to obtain a cumulative survival benefit.

OBJECTIVE

To provide clinical outcome data relating to a large cohort of mCRPC patients who received a third-line NA after the failure of docetaxel and another NA.

DESIGN, SETTING, AND PARTICIPANTS

We retrospectively reviewed the clinical records of patients who had received at least two successive NAs after the failure of docetaxel.

OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS

The independent prognostic value of a series of pretreatment covariates on the primary outcome measure of overall survival was assessed using Cox regression analysis.

RESULTS AND LIMITATIONS

We assessed 260 patients who received one third-line NA between January 2012 and December 2013, including 38 who received a further NA as fourth-line therapy. The median progression-free and overall survival from the start of third-line therapy was, respectively, 4 mo and 11 mo, with no significant differences between the NAs. Performance status, and haemoglobin and alkaline phosphatase levels were the only independent prognostic factors. The limitations of the study are mainly due its retrospective nature and the small number of patients treated with some of the sequences.

CONCLUSIONS

We were unable to demonstrate a difference in the clinical outcomes of third-line NAs regardless of previous NA therapy.

PATIENT SUMMARY

It is debated which sequence of treatments to adopt after docetaxel. Our data do not support the superiority of any of the three new agents in third-line treatment, regardless of the previously administered new agent.

Cytotoxic chemotherapy in the contemporary management of metastatic castration-resistant prostate cancer (mCRPC)

For several years, docetaxel was the only treatment shown to improve survival of patients with metastatic castration-resistant prostate cancer (mCRPC). There are now several novel agents available, although chemotherapy with docetaxel and cabazitaxel continues to play an important role. However, the increasing number of available agents will inevitably affect the timing of chemotherapy and therefore it may be important to offer this approach before declining performance status renders patients ineligible for chemotherapy. Patient selection is also important to optimise treatment benefit. The role of predictive biomarkers has assumed greater importance due to the development of multiple agents and resistance to available agents. In addition, the optimal sequence of treatments remains undefined and requires further study in order to maximize long-term outcomes. We provide an overview of the clinical data supporting the role of chemotherapy in the treatment of mCRPC and the emerging role in metastatic castration-sensitive prostate cancer. We review the key issues in the management of patients including selection of patients for chemotherapy, when to start chemotherapy, and how best to sequence treatments to maximise outcomes. In addition, we briefly summarise the promising new chemotherapeutic agents in development in the context of emerging therapies.

SEOM clinical guidelines for the treatment of metastatic prostate cancer

Androgen deprivation treatment is the current standard first-line treatment for metastatic prostate cancer. For several years, docetaxel was the only treatment with a proven survival benefit for castration-resistant prostate cancer (CRPC). Since docetaxel became standard of care for men with symptomatic metastatic castration-resistant prostate cancer (CRPC), three treatment virtual spaces, for treatment and drug development in CPRC, have emerged: pre-docetaxel, docetaxel combinations and post-docetaxel. Sipuleucel-T, cabazitaxel, abiraterone, enzalutamide and radium-223 have been approved in the pre- or post-docetaxel setting in metastatic CRPC during the last few years. Patients are now living longer and experiencing better quality of life. Strategies for patient selection and treatment sequencing are therefore urgently required.

PCSD1, a new patient-derived model of bone metastatic prostate cancer, is castrate-resistant in the bone-niche

Introduction

Prostate cancer bone metastasis occurs in 50-90% of men with advanced disease for which there is no cure. Bone metastasis leads to debilitating fractures and severe bone pain. It is associated with therapy resistance and rapid decline. Androgen deprivation therapy (ADT) is standard of care for advanced prostate cancer, however, bone metastatic prostate cancer (PCa) often becomes resistant to ADT. There are few pre-clinical models to understand the interaction between the bone microenvironment and prostate cancer. Here we report the castrate resistant growth in the bone niche of PCSD1, a patient-derived intra-femoral xenograft model of prostate bone metastatic cancer treated with the anti-androgen, bicalutamide.

Methods

PCSD1 bone-niche model was derived from a human prostate cancer femoral metastasis resected during hemiarthroplasty and serially transplanted into Rag2^−/−;γ_c^−/− mice intra-femorally (IF) or sub-cutaneously (SC). At 5 weeks post-transplantation mice received bicalutamide or vehicle control for 18 days. Tumor growth of PCSD1 was measured with calipers. PSA expression in PCSD1 xenograft tumors was determined using quantitative RT-PCR and immunohistochemistry. Expression of AR and PSMA, were also determined with qPCR.

Results

PCSD1 xenograft tumor growth capacity was 24 fold greater in the bone (intra-femoral, IF) than in the soft tissue (sub-cutaneous, SC) microenvironment. Treatment with the anti-androgen, bicalutamide, inhibited tumor growth in the sub-cutaneous transplantation site. However, bicalutamide was ineffective in suppressing PCSD1 tumor growth in the bone-niche. Nevertheless, bicalutamide treatment of intra-femoral tumors significantly reduced PSA expression (p < =0.008) and increased AR (p < =0.032) relative to control.

Conclusions

PCSD1 tumors were castrate resistant when growing in the bone-niche compared to soft tissue. Bicalutamide had little effect on reducing tumor burden in the bone yet still decreased tumor PSA expression and increased AR expression, thus, this model closely recapitulated castrate-resistant, human prostate cancer bone metastatic disease. PCSD1 is a new primary prostate cancer bone metastasis-derived xenograft model to study bone metastatic disease and for pre-clinical drug development of novel therapies for inhibiting therapy resistant prostate cancer growth in the bone-niche.

Next-generation steroidogenesis inhibitors, dutasteride and abiraterone, attenuate but still do not eliminate androgen biosynthesis in 22RV1 cells in vitro

Castration resistant prostate cancer (CRPC) is often lethal and inevitably develops after androgen ablation therapy. However, in the majority of cases it remains androgen dependent. CRPC tumors have the ability to synthesize their own androgens from cholesterol by engaging in de novo steroidogenesis. We investigated the potential of 22RV1 prostate cancer cells to convert the supplemented steroid precursors within this pathway under the effects of current clinical steroidogenesis inhibitors such as abiraterone and dutasteride, either alone or in combination. Under steroid starved conditions, enzymes responsible for de novo steroidogenesis were upregulated. Testosterone and dihydrotestosterone (DHT) were formed by using both dehydroepiandrosterone (DHEA) and progesterone as substrates. Formation of testosterone and DHT was higher following incubation with DHEA compared to progesterone. Progesterone decreased the mRNA expression of enzymes responsible for steroidogenesis. Abiraterone treatment decreased testosterone production but increased several precursor steroids in both classical and backdoor pathways in the presence of progesterone. In contrast, the DHT levels were elevated following treatment with abiraterone when progesterone was absent. Dutasteride decreased the formation of testosterone, DHT and precursor steroids in the backdoor pathway but increased steroid precursors in the classical steroidogenesis pathway. The combination of abiraterone and dutasteride decreased testosterone and DHT in the presence of progesterone but increased DHT in the absence of progesterone. Abiraterone inhibited androgen receptor (AR) activation but not to the same extent as MDV3100. However, abiraterone and dutasteride treatment, either alone or in combination, were more effective in decreasing prostate specific antigen secretion into the media than MDV3100. Thus, while interventions with these drugs alone or in combination fail to completely inhibit steroidogenesis in the 22RV1 cells, the combined inhibition of androgen production and blockade of AR can exceed the effect of MDV3100. Further characterization of bypass mechanisms that may develop as a response to these inhibitors is necessary to achieve optimal suppression of testosterone and DHT synthesis as a part of therapeutic regimens for the treatment of CRPC.

Increased expression of CYP17A1 indicates an effective targeting of the androgen receptor axis in castration resistant prostate cancer (CRPC)

Recent breakthrough therapies targeting androgen receptor signalling in castration resistant prostate cancer (CRPC) involve multifunctional androgen receptor (AR) blockade and exhaustive androgen deprivation. Nevertheless, limitations to an enduring effectiveness of new drugs are anticipated in resistance mechanisms occurring under such treatments.

In this study we used CRPC cell models VCaP and LNCaP as well as AR-negative PC-3- and non-neoplastic epithelial BPH-1-cells treated with 5, 10 or 25 μmol/L abiraterone hydrolyzed from abiraterone acetate (AA). The origin of CYP17A1 up-regulation under AA treatment was investigated in CRPC cell models by qRT-PCR and western-blot procedures.

AA treatments of AR positive CRPC cell models led to decreased expression of androgen regulated genes such as PSA. In these cells diminished expression of androgen regulated genes was accompanied by an up-regulation of CYP17A1 expression within short-term treatments. No such effects became evident in AR-negative PC-3 cells. AR directed siRNA (siAR) used in VCaP cells significantly reduced mRNA expression and AR protein abundance. Such interference with AR signalling in the absence of abiraterone acetate also caused a marked up-regulation of CYP17A1 expression. Down-regulation of androgen regulated genes occurs in spite of an elevated expression of CYP17A1, the very target enzyme for this drug. CYP17A1 up-regulation already takes place within such short treatments with AA and does not require adaptation events over several cell cycles. CYP17A1 is also up-regulated in the absence of AA when AR signalling is physically eliminated by siAR.

These results reveal an immediate counter-regulation of CYP17A1 expression whenever AR-signalling is inhibited adequately but not a persisting adaptation yielding drug resistance.

Novel anti-androgen receptor signaling agents: Understanding the mechanisms of resistance

Prostate cancer remains an intractable threat to the lives of men worldwide. Although deaths from prostate cancer (PCa) in the United States have declined in recent years, in other parts of the world Pca mortality is increasing. The introduction of 2nd generation anti-androgen receptor agents into the therapeutic armamentarium for metastatic castration-resistant prostate cancer (mCRPC) has resulted in modestly increased survival advantages as demonstrated by initial clinical trials. However, analysis of the molecular pathways affected by these agents may lead to new insight into mechanisms of resistance that drive mCRPC, including proliferation and survival signaling pathways that are derepressed by maximum repression of androgen signaling. Combination therapies that involve anti-AR signaling agents together with agents that target these pathways establish a paradigm for the development of more effective treatment of mCRPC. In this review, we briefly summarize the current clinical trial literature with regard to novel anti-AR signaling agents such as abiraterone acetate and enzalutamide. We discuss observational data that point to mechanisms of resistance that emerged from these studies. We further present and discuss recent experimental studies that address the mechanisms of resistance to these treatments. Finally, we discuss novel and rational therapeutic approaches, including combination therapy, for patients with mCRPC.

Safety of antiandrogen therapy for treating prostate cancer

INTRODUCTION

Antiandrogens are a treatment option in patients with prostate cancer, given either in combination with androgen deprivation or, in selected cases, as monotherapy. New-generation antiandrogens have been recently introduced in clinical practice (enzalutamide) or are under evaluation in clinical trials (ARN-509).

AREAS COVERED

This review elucidates the safety profile of antiandrogens, in particular focusing on the tolerability profile of each drug either when employed in combination with castration or as monotherapy, in hormone-naive or in castration-resistant patients.

EXPERT OPINION

Non-steroidal antiandrogens are widely used in the management of hormone-sensitive disease in combination with luteinizing hormone-releasing hormone agonists or in patients failing front-line treatment with androgen-deprivative maneuvers. In selected patients, non-steroidal antiandrogen monotherapy appears to yield comparable results as castration. Novel non-steroidal antiandrogens have been investigated with promising results in castration-resistant prostate cancer. Beyond the safety profile specific to any individual compound, increased testosterone and 17β-estradiol levels are commonly observed during antiandrogen monotherapy, leading to gynecomastia and breast pain. The safety profile of old and novel antiandrogens should be taken into account by clinicians in decision making and in selecting the most suitable patients. Beyond patient selection, full clinical evaluation of patient co-morbidities that might affect the drug tolerability and clinical monitoring are anyway required.

Quo vadis: advanced prostate cancer-clinical care and clinical research in the era of multiple androgen receptor-directed therapies

The novel androgen receptor-directed therapies abiraterone acetate and enzalutamide, having demonstrated improved survival in randomized phase 3 studies of men with metastatic castration-resistant prostate cancer, have ushered in a new era in the treatment of this disease. Additional novel androgen receptor-directed therapies, such as ARN-509 and orteronel, are in various phases of clinical trials and development. However, the emergence of therapeutic resistance and clinical disease progression is inevitable. Although advances in genomic technologies have led to unprecedented understanding of the biology of castration-resistant prostate cancer, efforts only now are underway to elucidate the mechanisms of resistance associated with abiraterone and enzalutamide. A tremendous challenge in the near future will be to determine the optimal sequence or combination of therapies to overcome resistance mechanisms. In this review, the current landscape of androgen receptor-directed therapies and future directions necessary to enhance their clinical efficacy for the maximal benefit of patients are discussed.

Emerging mechanisms of enzalutamide resistance in prostate cancer

The majority of prostate cancers are hormone-dependent at diagnosis highlighting the central role of androgen signalling in this disease. Surprisingly, most forms of castration-resistant prostate cancer (CRPC) are still dependent on the androgen receptor (AR) for survival. Therefore, the advent of new AR-targeting drugs, such as enzalutamide, is certainly beneficial for the many patients with metastatic CRPC. Indeed, this compound provides a substantial survival benefit-but it is not curative. This Perspectives article describes the different ways through which cancer cells can become resistant to enzalutamide, such as AR truncation and other mutations, as well as by-pass of the AR dependence of prostate cancer cells through expression of the glucocorticoid receptor. The clinical relevance of these mechanisms and emerging questions concerning new therapeutic regimens in the treatment of metastatic CRPC are being discussed.

Clinical activity of enzalutamide versus docetaxel in men with castration-resistant prostate cancer progressing after abiraterone

BACKGROUND

The optimal sequencing of the multiple active agents now available for metastatic castration-resistant prostate cancer (mCRPC) is unclear. Prior reports have suggested diminished responses to sequential lines of androgen receptor (AR)-targeted therapies, but it is unknown whether subsequent taxane-based chemotherapy may be more effective than sequential AR-targeting treatment. We sought to evaluate the clinical activity of enzalutamide versus docetaxel in men with mCRPC who progressed on abiraterone.

METHODS

We performed a single-institution retrospective analysis of consecutive mCRPC patients who had progressed on abiraterone therapy and subsequently received either enzalutamide (n=30) or docetaxel (n=31). We evaluated clinical outcomes including prostate-specific antigen decline of >30% (PSA30) or >50% (PSA50), PSA-progression-free survival (PSA-PFS), and clinical/radiographic PFS. We performed multivariable modeling to control for baseline and on-treatment differences between groups.

RESULTS

Compared to subjects who received enzalutamide post-abiraterone, subjects who received docetaxel post-abiraterone had more bone metastases, more visceral metastases, higher baseline PSA, and had more frequent PSA tests while on-treatment. There were no significant differences in PSA30 (41% for enzalutamide vs. 53% for docetaxel) or PSA50 (34% vs. 40%) response rates between the two groups; there remained no difference after stratifying by presence/absence of prior response to abiraterone. Median PSA-PFS was 4.1 versus 4.1 months for the enzalutamide and docetaxel cohorts, respectively (HR 1.35, 95% CI, 0.53-3.66, P=0.502). Median PFS was 4.7 versus 4.4 months, respectively (HR 1.44, 95% CI, 0.77-2.71, P=0.257). PSA-PFS and PFS did not differ after stratifying by prior response to abiraterone. In multivariable analyses, there were no significant differences in PSA-PFS or PFS between the two groups.

CONCLUSIONS

Treatment with either enzalutamide or docetaxel produced modest PSA responses and PFS intervals in this abiraterone-pretreated mCRPC population. In this retrospective study with small sample size, no significant differences in outcomes were observed between groups. Therefore, either enzalutamide or docetaxel may be a reasonable option in men who have progressed on abiraterone.

[Therapeutic targeted approaches on androgen receptors in prostate cancer]

The treatment of metastatic prostate cancer since the 1940s is based on the consideration of oncogenic addiction to its androgen receptor (AR). The significant improvement in survival outcomes over the past decade depends not only on the development of effective cytotoxic chemotherapy but also new molecules targeting the AR or decreasing testosterone levels, even in case of castration-resistant cancer. In this review, we summarize the structure and function of the RA, the mechanisms of androgen suppression, the concept of resistance to castration, historical targeted treatment on the AR and those recently marketed as abiraterone acetate and enzalutamide.

Stilbene induced inhibition of androgen receptor dimerization: implications for AR and ARΔLBD-signalling in human prostate cancer cells

Background

Advanced castration resistant prostate cancer (CRPC) is often characterized by an increase of C-terminally truncated, constitutively active androgen receptor (AR) variants. Due to the absence of a ligand binding domain located in the AR-C-terminus, these receptor variants (also termed ARΔLBD) are unable to respond to all classical forms of endocrine treatments like surgical/chemical castration and/or application of anti-androgens.

Methodology

In this study we tested the effects of the naturally occurring stilbene resveratrol (RSV) and (E)-4-(2, 6-Difluorostyryl)-N, N-dimethylaniline, a fluorinated dialkylaminostilbene (FIDAS) on AR- and ARΔLBD in prostate cancer cells. The ability of the compounds to modulate transcriptional activity of AR and the ARΔLBD-variant Q640X was shown by reporter gene assays. Expression of endogenous AR and ARΔLBD mRNA and protein levels were determined by qRT-PCR and Western Blot. Nuclear translocation of AR-molecules was analyzed by fluorescence microscopy. AR and ARΔLBD/Q640X homo-/heterodimer formation was assessed by mammalian two hybrid assays. Biological activity of both compounds in vivo was demonstrated using a chick chorioallantoic membrane xenograft assay.

Results

The stilbenes RSV and FIDAS were able to significantly diminish AR and Q640X-signalling. Successful inhibition of the Q640X suggests that RSV and FIDAS are not interfering with the AR-ligand binding domain like all currently available anti-hormonal drugs. Repression of AR and Q640X-signalling by RSV and FIDAS in prostate cancer cells was caused by an inhibition of the AR and/or Q640X-dimerization. Although systemic bioavailability of both stilbenes is very low, both compounds were also able to downregulate tumor growth and AR-signalling in vivo.

Conclusion

RSV and FIDAS are able to inhibit the dimerization of AR and ARΔLBD molecules suggesting that stilbenes might serve as lead compounds for a novel generation of AR-inhibitors.

Targeting the androgen receptor in prostate cancer

INTRODUCTION

The androgen receptor (AR) is a ligand-activated transcription factor that is expressed in primary and metastatic prostate cancers. There are advances in endocrine therapy for prostate cancer that are based on improved understanding of AR function.

AREAS COVERED

PubMed has been used to include most important publications on targeting the AR in prostate cancer. AR expression may be downregulated by agents used for chemoprevention of prostate cancer or, in models of advanced prostate cancer, by antisense oligonucleotides. New drugs that inhibit the steroidogenic enzyme CYP17A1 (abiraterone acetate) or diminish nuclear translocation of the AR (enzalutamide) have been shown to improve patients' survival in prostate cancer. However, it is clear that there is a development of resistance to these novel therapies. They may include increased expression of truncated, constitutively active AR or activation of the signaling pathway of signal transducers and activators of transcription.

EXPERT OPINION

Although introduction of novel drugs have improved patients' survival, there is a need to investigate the mechanisms of resistance further. The role of truncated AR and compensatory activation of signaling pathways as well as the development of scientifically justified combination therapies seems to be issues of a high priority.