ARN-509 in Men with Metastatic Castration-Resistant Prostate Cancer (CRPC)

New agents for prostate cancer

The therapeutic landscape of metastatic castration-resistant prostate cancer (mCRPC) has been revolutionized by the arrival of multiple novel agents in the past 2 years. Immunotherapy in the form of sipuleucel-T, androgen axis inhibitors, including abiraterone acetate and enzalutamide, a chemotherapeutic agent, cabazitaxel, and a radiopharmaceutical, radium-223, have all yielded incremental extensions of survival and have been recently approved. A number of other agents appear promising in early studies, suggesting that the armamentarium against castrate-resistant prostate cancer is likely to continue to expand. Emerging androgen pathway inhibitors include androgen synthesis inhibitors (TAK700), androgen receptor inhibitors (ARN-509, ODM-201), AR DNA binding domain inhibitors (EPI-001), selective AR downregulators or SARDs (AZD-3514), and agents that inhibit both androgen synthesis and receptor binding (TOK-001/galeterone). Promising immunotherapeutic agents include poxvirus vaccines and CTLA-4 inhibitor (ipilimumab). Biologic agents targeting the molecular drivers of disease are also being investigated as single agents, including cabozantinib (Met and VEGFR2 inhibitor) and tasquinimod (angiogenesis and immune modulatory agent). Despite the disappointing results seen from studies evaluating docetaxel in combination with other agents, including GVAX, anti-angiogentic agents (bevacizumab, aflibercept, lenalinomide), a SRC kinase inhibitor (dasatinib), endothelin receptor antagonists (atrasentan, zibotentan), and high-dose calcitriol (DN-101), the results from the trial evaluating docetaxel in combination with the clusterin antagonist, custirsen, are eagerly awaited. New therapeutic hurdles consist of discovering new targets, understanding resistance mechanisms, the optimal sequencing and combinations of available agents, as well as biomarkers predictive for benefit. Novel agents targeting bone metastases are being developed following the success of zoledronic acid and denosumab. Finally, all of these modalities do not appear curative, suggesting that clinical trial enrollment and a better understanding of biology remain of paramount importance.

Castration-resistant prostate cancer: latest evidence and therapeutic implications

Medical oncologists who treat men with castration-resistant prostate cancer (CRPC) have seen an abundance of new agents approved by the United States Food and Drug Administration in the last decade for a disease that was previously difficult to treat after becoming resistant to androgen-deprivation therapy. Advances in understanding of the mechanisms of castration-resistance and prostate cancer progression have highlighted several pathways and targets that appear promising to better treat CRPC. As the majority of CRPC appears to continue to rely on the androgen receptor for growth and progression, several of these agents directly or indirectly target the androgen receptor. A novel microtubule-targeted agent, cabazitaxel, has demonstrated an overall survival benefit following progression on docetaxel. Other agents target tumor immunogenicity and immune checkpoint pathways to attempt to harness the host immune system. The recently approved radiopharmaceutical, radium-223 dichloride, has demonstrated impressive results in patients with extensive bony metastases with minimal toxicity. Lastly, further understanding of the pathways underlying CRPC progression has led to late-phase clinical trials with the novel agents: custirsen, tasquinimod and cabozantinib. This article reviews the approved therapies for CRPC, the agents currently in late-phase clinical trials, and notable early-phase trials of novel therapies and their combinations, with particular attention to trials incorporating novel biomarkers and intermediate endpoints to better identify those men who may or may not benefit from specific therapies.

Enzalutamide, a second generation androgen receptor antagonist: development and clinical applications in prostate cancer

Enzalutamide, formerly known as MDV3100, is an oral second generation androgen receptor (AR) inhibitor that was chosen from a screen of agents and shown in preclinical studies to have greater affinity for the AR than its predecessors without any agonistic effects. The pre-clinical work that led to the interest in studying this agent and the history of the clinical development of enzalutamide from first in man phase 1 through phase 3 and regulatory approval are reviewed. Information about the toxicity profile and prescribing enzalutamide are discussed in detail. The availability of enzalutamide is put into context with the five other agents that modify survival outcomes in metastatic castration resistant prostate cancer. Some of the new challenges confronting the field regarding sequencing and combinations of these agents and the potential for a change in the natural history of the disease, are also discussed.