Recent advances in the therapy of castration-resistant prostate cancer: the price of progress

Within the past two years, three agents have garnered approval from the US FDA for the specific treatment of metastatic castration resistant prostate cancer (mCRPC) – (1) abiraterone, (2) cabazitaxel and (3) sipuleucel-T. In separate phase III studies, each agent led to an improvement in overall survival (OS) of 2–4 months over a suitable comparator. With these costly therapies all having potential application in the patient with mCRPC, multiple entities (industry, government, and the general public) must strategize to determine how the cost burden of these agents can be balanced with the potential gains for the individual patient. Herein, we provide a framework with which to approach this dilemma.     

New therapies for castration-resistant prostate cancer: efficacy and safety

Context

Prostate cancer (PCa) is the most common noncutaneous malignancy and the second leading cause of cancer mortality amongst men in the Western world. Up to 40% of men diagnosed with PCa will eventually develop metastatic disease, and although most respond to initial medical or surgical castration, progression to castration resistance is universal. The average survival for patients with castration-resistant prostate cancer (CRPC) is 2-3 yr.

Objective

To discuss the biologic rationale and evidence supporting current management of patients with CRPC and to review promising novel agents.

Evidence acquisition

Electronic databases (PubMed, ClinicalTrials.gov), relevant journals, and conference proceedings were searched manually for preclinical studies, clinical trials, and biomarker analyses focused on the treatment of CRPC. Keywords included castrate resistant prostate cancer and: targeted therapy, novel therapy, immunotherapy, androgen therapy, bone therapy, mechanisms, biomarkers, and trial endpoints; no time range was specified. Information pertaining to current studies was discussed with key opinion leaders.

Evidence synthesis

We focus on the efficacy and safety of approved agents, promising therapies that have proceeded to phase 3 evaluation, and those that have enhanced our understanding of the biology of CRPC. Biomarkers are considered in the context of novel targeted agents and immunotherapy.

Conclusions

CRPC has many targets. Four new agents with different mechanisms of action have recently been shown to have positive results in large phase 3 randomized trials, and have already been approved in the United States for CRPC: cabazitaxel, sipuleucel-T, denosumab, and abiraterone acetate. With our improved understanding of tumor biology and the incorporation of new prognostic and molecular biomarkers into clinical trials, we are making progress in the management of patients with CRPC.     

Optimal management of metastatic castration-resistant prostate cancer: Highlights from a European Expert Consensus Panel

The exponential growth of novel therapies for the treatment of metastatic castration-resistant prostate cancer (mCRPC) over the last decade has created an acute need for education and guidance of clinicians regarding optimal strategies for patient management. A multidisciplinary panel of 21 European experts in mCRPC assembled for comprehensive discussion and consensus development, seeking to move the field forward and provide guidance and perspectives on optimal selection and sequencing of therapeutic agents and monitoring of response to treatment and disease progression. A total of 110 clinically-relevant questions were addressed and a modified Delphi method was utilised to obtain a consensus. The panel reached a consensus on several important issues, providing recommendations on appropriate phase III clinical trial end-points and optimal strategies for imaging and monitoring of bone metastases. Guidance regarding selection and sequencing of therapy in patients with newly diagnosed or progressive mCRPC is emphasised, including the use of novel bone-targeted agents, chemotherapy, androgen receptor pathway-targeted agents and immunotherapy. The impact of drug resistance and prostate-specific antigen flare on treatment decisions was also addressed. Ultimately, individualised therapy for patients with mCRPC is dependent on continued refinement of clinical decision-making based on patient and disease characteristics. This consensus statement offers clinicians expert guidance on the implementation of recent advances to improve patient outcome, focusing on the future of prostate cancer care.     

Cellular immunotherapies for prostate cancer.

Prostate cancer is currently the most commonly diagnosed malignancy, and the second leading cause of cancer-related death, in men in the United States. Most deaths from prostate cancer are due to metastatic disease that no longer responds to androgen deprivation. There have been several advances within the last decade leading to the approval of new agents for treating patients with castration-resistant prostate cancer. However, these agents can have significant side effects, and the average survival benefit from chemotherapy has been modest. Results from several clinical trials have suggested that immune-based therapies may have clinical benefits in patients with prostate cancer with significantly fewer adverse events. These new therapies, however, pose new potential benefits and risks. We review here two cellular immunotherapies furthest in clinical development for prostate cancer, and discuss the potential challenges to the treating oncologist with the possible advent of these new therapies.     

Immunotherapy for metastatic prostate cancer: where are we at with sipuleucel-T?

Importance of the field: Prostate cancer is the leading malignancy in North American men and despite improvements in treatments 20 – 30% of patients will relapse. Immunotherapy using activated mononuclear cells is a way to harness the body's adaptive immune response to fight metastatic prostate cancer.

Areas covered in this review: In 2005, at least 10 therapeutic cancer vaccines, designed to confer active, specific immunotherapy against tumor-associated antigens, were in clinical trials. These covered potential fields of immunological strategy to overcome castration-resistant prostate cancer.

What the reader will gain: A literature review was performed using the search terms sipuleucel-T, Provenge and APC8015 or APC-8015, and restricted to English language articles from 2000 to 2010. The immunological design and development of sipuleucel-T are summarized. The efficacy and safety of sipuleucel-T are discussed based on current data from clinical trials. Ongoing clinical trials involving sipuleucel-T are summarized.

Take home message: Efficacy and safety with sipuleucel-T has been demonstrated in Phase I/II trials. The latest data from a Phase III trial shows that sipuleucel-T has met the primary endpoint of survival benefit. Further work is needed to understand the mechanisms behind cancer vaccine failure and elucidate the population for whom this vaccine will be suitable.     

New treatment options for patients with metastatic castration-resistant prostate cancer

Chemotherapy for men with metastatic castration-resistant prostate cancer (CRPC) conferred no survival advantage until 2004 when docetaxel was shown to improve survival when compared with mitoxantrone, which was approved for palliation of symptomatic disease in 1996. Since then, clinical trials have concentrated on three main populations of patients with metastatic CRPC: those who are chemotherapy naïve and are asymptomatic or minimally symptomatic, those who need docetaxel therapy, and those who have received docetaxel previously and/or those with symptomatic disease. Over the last year, four Phase III therapeutic trials have met their primary endpoint of improved overall survival: sipuleucel-T in the pre-chemotherapy setting, cabazitaxel and abiraterone in the post-docetaxel setting, and radium-223 for those with symptomatic bone metastases who have received or are not suitable to receive docetaxel. In addition to these therapeutic trials, a Phase III head-to-head trial of denosumab compared to zoledronic acid showed that denosumab was superior to zoledronic acid in delaying or preventing skeletal related events. As a result, the treatment paradigm for metastatic CRPC is changing rapidly. This paper reviews the data from these five completed Phase III trials and places these new agents, as well as those in ongoing Phase III trials, in the context of the old treatment paradigm for metastatic CRPC and discusses some of the challenges ahead for determining optimal timing and sequencing of treatments for metastatic CRPC.     

Current vaccination strategies for prostate cancer

Context

The first therapeutic cancer vaccine demonstrating effectiveness in a phase 3 study was approved by the US Food and Drug Administration on 29 April 2010. The pivotal trial demonstrated overall survival (OS) benefit in patients treated with antigen-loaded leukapheresis cells compared with a control infusion. Results of other prostate cancer (PCa) vaccination strategies are awaited, as this approach may herald a new era in the care for patients with advanced PCa.

Objective

Consider effectiveness and safety of vaccination strategies in the treatment of PCa.

Evidence acquisition

We searched three bibliographic databases (January 1995 through October 2010) for randomised phase 2 and 3 studies of vaccination strategies for PCa based on predetermined relevant Medical Subject Heading terms and free text terms.

Evidence synthesis

Data from 3 randomised phase 3 and 10 randomised phase 2 vaccination trials are discussed with respect to clinical outcome in terms of progression-free survival and OS, toxicity, prostate-specific antigen (PSA) response, and immunologic response. Three phase 3 trials (D9901, D9902A, and D9902B) that enrolled a total of 737 patients, all controlled and double-blinded, tested the efficacy of sipuleucel-T. The largest of these three trials, called Immunotherapy for Prostate Adenocarcinoma Treatment (IMPACT), has demonstrated safety and effectiveness of sipuleucel-T (now marketed as Provenge) as measured by prolonged survival of 512 asymptomatic patients with metastatic castration-resistant PCa (mCRPC). The study showed a 4.1-mo median survival benefit in the sipuleucel-T vaccine-treated group compared with the control group (25.8 vs 21.7 mo; hazard ratio [HR]: 0.78; 95% confidence interval [CI], 0.62-0.98; p = 0.032) and extended 3-yr survival (31.7% vs 23.0%).In contrast, two phase 3 vaccination trials with a whole-tumour-cell mixture of two PCa cell lines (GVAX) and testing GVAX either alone or in combination with chemotherapy versus chemotherapy alone (VITAL1 and 2) were terminated prematurely based on futility and increased deaths. Other phase 2 vaccination trials testing different types of vaccines in castration-resistant PCa patients have been reported with variable outcomes.Notably, a controlled, double-blind, randomised phase 2 vaccine trial of PROSTVAC-VF, a recombinant viral vector containing complementary DNA encoding PSA, in 125 patients with chemotherapy-naïve, minimally symptomatic mCRPC also demonstrated safety but no significant effect on the time to disease progression. In comparison with controls (n = 40), PROSTVAC-VF-treated patients (n = 82) experienced longer median survival of 8.5 mo (25.1 vs 16.6 mo; HR: 0.56; 95% CI, 0.37-0.85; p = 0.0061) and extended 3-yr survival (30% vs 17%).In general, PCa vaccines are perceived to have less toxicity compared with current cytotoxic or targeted therapies. Evaluation of clinical efficacy of different vaccination strategies (eg, protein-, peptide- and DNA-based vaccines) in the context of properly designed and controlled phase 3 studies is warranted.

Conclusions

Cancer vaccines represent a new paradigm in the treatment of PCa. The IMPACT trial showed improved survival but no difference in time to disease progression in mCRPC patients with minimal tumour burden. Observations in phase 2 and 3 trials pave the way for other vaccination approaches for this disease, raise questions regarding the most appropriate clinical trial designs, and underscore the importance of identifying biomarkers for antitumour effect to better implement such therapies.     

The role of sipuleucel-T in therapy for castration-resistant prostate cancer: a critical analysis of the literature

Context

Sipuleucel-T, an autologous antigen-presenting cell vaccine loaded with prostate acid phosphatase conjugated with granulocyte-macrophage colony-stimulating factor (GM-CSF), yielded a survival advantage in men with metastatic castration-resistant prostate cancer (mCRPC).

Objective

Critically analyze the role of sipuleucel-T in therapy for mCRPC.

Evidence acquisition

A systematic review of the literature was performed in June 2011 using Medline and an abstract search of major cancer conferences. The search strategy included the terms sipuleucel-T, APC-8015, castration-resistant, prostate cancer, and immunotherapy.

Evidence synthesis

The era of targeted immunotherapy was initiated with the regulatory approval in 2010 of sipuleucel-T for asymptomatic and minimally symptomatic mCRPC. The median survival was prolonged by 4.1 mo (25.8 vs 21.7 mo; hazard ratio: 0.78; 95% confidence interval, 0.61–0.98; p = 0.03), coupled with an improvement in 3-yr survival (31.7% vs 23.0%). Outcomes were characterized by the lack of tumor regression or delay in progression. Further development is proceeding in earlier stages of prostate cancer and in the context of a host of emerging agents.

Conclusions

The addition of sipuleucel-T, an immunotherapeutic agent, to the armamentarium represents a paradigm shift in therapy for mCRPC. The rational combination and proper sequencing of sipuleucel-T with other newly approved agents (abiraterone acetate, cabazitaxel) and emerging agents (MDV3100, TAK-700, ipilimumab) will be important to evaluate.