Castration-Resistant Prostate Cancer: Mechanisms, Targets and Treatment

Prostate cancer is the most common malignancy in men, and remains the second leading cause of cancer-related death in this gender [1]. Data suggests that 10-20% of patients with prostate cancer metastasis develop castration-resistant prostate cancer (CRPC) within 5 years of follow-up, and that the median survival since development of castration resistance is approximately 14 months (range 9-30) [2]. Additionally, patients with non-metastatic CRPC are at higher risk of disease progression. Approximately 15-33% of patients develop metastasis within 2 years, increasing the mortality burden in this population [3, 4].

Patient characteristics and overall survival in patients with post-docetaxel metastatic castration-resistant prostate cancer in the community setting

It is unclear how treatment sequencing for metastatic castration-resistant prostate cancer (mCRPC) affects real-world patient outcomes. We assessed treatment sequences, patient characteristics and overall survival (OS) in post-docetaxel mCRPC patients. mCRPC patients receiving second-line cabazitaxel or androgen receptor-targeted therapy (ART; abiraterone/enzalutamide) post-docetaxel were identified using electronic medical records. OS was assessed from second-line therapy initiation using Cox regressions adjusting for: metastases; prostate-specific antigen (PSA); hemoglobin; alkaline phosphatase (ALP); albumin; second-line therapy initiation year. Following docetaxel (n = 629), 123 (19.6%) and 506 (80.4%) patients received cabazitaxel and ART, respectively. One hundred and ninety-five patients received additional treatments thereafter (54 following cabazitaxel; 141 following ART). Although patients receiving second-line cabazitaxel versus ART had similar disease characteristics at first-line therapy initiation, at second-line therapy initiation they had higher mean PSA (386.6 vs. 233.9 ng/mL) and ALP (182.0 vs. 167.3 u/L), lower mean hemoglobin (10.8 vs. 11.5 g/dL), and more frequently had intermediate/high-risk Halabi scores (61.8 vs. 48.4%); all p < 0.05. Overall, crude survival was not significantly different. Among Halabi high-risk patients, adjusted median OS was significantly longer in patients receiving cabazitaxel versus ART (HR 0.48; 95% CI 0.24–0.93; p = 0.030). Low albumin and hemoglobin led to similar findings (HR 0.43; 95% CI 0.23–0.80; p = 0.0077; HR 0.60; 95% CI 0.40–0.90; p = 0.014). Most post-docetaxel patients received second-line ART. Patients receiving second-line cabazitaxel had more high-risk features; however, second-line cabazitaxel administered after docetaxel may improve OS in patients with Halabi high-risk scores or low albumin/hemoglobin.

The predictive value of ERG protein expression for development of castration-resistant prostate cancer in hormone-naïve advanced prostate cancer treated with primary androgen deprivation therapy

BACKGROUND

Biomarkers predicting response to primary androgen deprivation therapy (ADT) and risk of castration-resistant prostate cancer (CRPC) is lacking. We aimed to analyse the predictive value of ERG expression for development of CRPC.

METHODS

In total, 194 patients with advanced and/or metastatic prostate cancer (PCa) treated with first-line castration-based ADT were included. ERG protein expression was analysed in diagnostic specimens using immunohistochemistry (anti-ERG, EPR3864). Time to CRPC was compared between ERG subgroups using multiple cause-specific Cox regression stratified on ERG-status. Risk reclassification and time-dependent area under the ROC curves were used to assess the discriminative ability of ERG-status. Time to PSA-nadir, proportion achieving PSA-nadir ≤0.2 ng/ml, and risk of PCa-specific death were secondary endpoints.

RESULTS

Median follow-up was 6.8 years (IQR: 4.9-7.3). In total, 105 patients (54.1%) were ERG-positive and 89 (45.9%) were ERG-negative. No difference in risk of CRPC was observed between ERG subgroups (P = 0.51). Median time to CRPC was 3.9 years (95%CI: 3.2-5.1) and 4.5 years (95%CI: 2.3-not reached) in the ERG-positive and ERG-negative group, respectively. Compared to a model omitting ERG-status, the ERG-stratified model showed comparable AUC values 1 year (77.6% vs. 78.0%, P = 0.82), 2 years (71.7% vs. 71.8%, P = 0.85), 5 years (68.5% vs. 69.9%, P = 0.32), and 8 years (67.9% vs. 71.4%, P = 0.21) from ADT initiation. No differences in secondary endpoints were observed.

CONCLUSIONS

ERG expression was not associated with risk of CRPC suggesting that ERG is not a candidate biomarker for predicting response to primary ADT in patients diagnosed with advanced and/or metastatic PCa.

Drug therapies for metastatic castration-resistant prostate cancer

Management of metastatic castration-resistant prostate cancer has changed markedly over the last decade with major shifts in the treatment paradigm, although ultimately still will progress despite currently available therapies. The sequencing or combination of these agents is an area of active investigation, since definitive prospective randomized trials to define the optimal choice of drug sequence have yet to be done or resulted. This article will highlight pivotal trials for currently approved therapies for metastatic castration-resistant prostate cancer and a suggestion for sequencing of these agents, as well as highlighting investigations using novel therapies for advanced prostate cancer.

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.

Docetaxel rechallenge after an initial good response in patients with metastatic castration-resistant prostate cancer

OBJECTIVE

To evaluate the benefit of docetaxel rechallenge in patients with metastatic castration-resistant prostate cancer (mCRPC) relapsing after an initial good response to first-line docetaxel.

PATIENTS AND METHODS

We retrospectively reviewed the records of consecutive patients with mCRPC with a good response to first-line docetaxel [serum prostate specific antigen (PSA) decrease ≥50%; no clinical/radiological progression]. We analysed the impact of management at relapse (docetaxel rechallenge or non-taxane-based therapy) on PSA response, symptomatic response (performance status/pain/analgesic consumption), and overall survival (OS). We used multivariate stepwise logistic regression to analyse potential predictors of a favourable outcome.

RESULTS

We identified 270 good responders to first-line docetaxel. The median progression-free interval (PFI) was 6 months from the last docetaxel dose. At relapse, 223 patients were rechallenged with docetaxel (82.5%) and 47 received non-taxane-based therapy. There was no significant difference in median OS {18.2 [95% confidence interval (CI) 16.1-22.00] and 16.8 [95%CI 13.4-21.5] months, respectively, P = 0.35}. However, good PSA response and symptom relief/stable disease were more frequent on docetaxel rechallenge (40.4% vs 10.6%, P < 0.001 for PSA). A PFI of >6 months and added estramustine predicted a good PSA response and symptomatic response on docetaxel rechallenge but only a PFI of >6 months predicted longer OS. Haemoglobin (<13 g/dL) and pain were associated with reduced OS. Docetaxel rechallenge increased the incidence of grade ≥3 sensory neuropathy, nail disorders and asthenia/fatigue.

CONCLUSIONS

Docetaxel rechallenge is a management option for responders to docetaxel with a PFI of >6 months, but did not prolong survival. Potential benefits should be weighed against the risk of cumulative toxicity.

ERG induces taxane resistance in castration-resistant prostate cancer

Taxanes are the only chemotherapies used to treat patients with metastatic castration-resistant prostate cancer (CRPC). Despite the initial efficacy of taxanes in treating CRPC, all patients ultimately fail due to the development of drug resistance. In this study, we show that ERG overexpression in in vitro and in vivo models of CRPC is associated with decreased sensitivity to taxanes. ERG affects several parameters of microtubule dynamics and inhibits effective drug-target engagement of docetaxel or cabazitaxel with tubulin. Finally, analysis of a cohort of 34 men with metastatic CRPC treated with docetaxel chemotherapy reveals that ERG-overexpressing prostate cancers have twice the chance of docetaxel resistance than ERG-negative cancers. Our data suggest that ERG plays a role beyond regulating gene expression and functions outside the nucleus to cooperate with tubulin towards taxane insensitivity. Determining ERG rearrangement status may aid in patient selection for docetaxel or cabazitaxel therapy and/or influence co-targeting approaches.

Metastatic castration-resistant prostate cancer is treated with the microtubule-stabilizing drugs taxanes, but resistance ultimately develops. Here Galletti et al. show that ERG, a transcription factor commonly overexpressed in prostate cancers, confers taxane resistance by binding to soluble tubulin.

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.

Prognostic Factors for Survival in Noncastrate Metastatic Prostate Cancer: Validation of the Glass Model and Development of a Novel Simplified Prognostic Model

BACKGROUND

The Glass model developed in 2003 uses prognostic factors for noncastrate metastatic prostate cancer (NCMPC) to define subgroups with good, intermediate, and poor prognosis.

OBJECTIVE

To validate NCMPC risk groups in a more recently diagnosed population and to develop a more sensitive prognostic model.

DESIGN, SETTING, AND PARTICIPANTS

NCMPC patients were randomized to receive continuous androgen deprivation therapy (ADT) with or without docetaxel in the GETUG-15 phase 3 trial. Potential prognostic factors were recorded: age, performance status, Gleason score, hemoglobin (Hb), prostate-specific antigen, alkaline phosphatase (ALP), lactate dehydrogenase (LDH), metastatic localization, body mass index, and pain.

OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS

These factors were used to develop a new prognostic model using a recursive partitioning method. Before analysis, the data were split into learning and validation sets. The outcome was overall survival (OS).

RESULTS AND LIMITATIONS

For the 385 patients included, those with good (49%), intermediate (29%), and poor (22%) prognosis had median OS of 69.0, 46.5 and 36.6 mo (p=0.001), and 5-yr survival estimates of 60.7%, 39.4%, and 32.1%, respectively (p=0.001). The most discriminatory variables in univariate analysis were ALP, pain intensity, Hb, LDH, and bone metastases. ALP was the strongest prognostic factor in discriminating patients with good or poor prognosis. In the learning set, median OS in patients with normal and abnormal ALP was 69.1 and 33.6 mo, and 5-yr survival estimates were 62.1% and 23.2%, respectively. The hazard ratio for ALP was 3.11 and 3.13 in the learning and validation sets, respectively. The discriminatory ability of ALP (concordance [C] index 0.64, 95% confidence interval [CI] 0.58-0.71) was superior to that of the Glass risk model (C-index 0.59, 95% CI 0.52-0.66). The study limitations include the limited number of patients and low values for the C-index.

CONCLUSION

A new and simple prognostic model was developed for patients with NCMPC, underlying the role of normal or abnormal ALP.

PATIENT SUMMARY

We analyzed clinical and biological factors that could affect overall survival in noncastrate metastatic prostate cancer. We showed that normal or abnormal alkaline phosphatase at baseline might be useful in predicting survival.

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.

Prostate cancer: the best fit for enzalutamide in metastatic prostate cancer

The PREVAIL trial compared enzalutamide and placebo in patients with metastatic castration-resistant prostate cancer who had not received prior chemotherapy, demonstrating an improvement in overall survival and other clinical, radiographic, and biochemical outcomes. Herein, the implications of these data in the rapidly changing landscape of metastatic prostate cancer therapy are discussed.