Randomized Salvage Radiation Therapy Plus Enzalutamide Post Prostatectomy

Enzalutamide is a second-generation androgen receptor signaling inhibitor that significantly prolongs survival in patients with metastatic castration-resistant prostate cancer who have received prior docetaxel chemotherapy 35,36. Enzalutamide has demonstrated activity in cells that overexpress the androgen receptor. Unlike previous androgen receptor blocker (ARB) agents, Enzalutamide does not display any agonist properties and blocks translocation of the ligand-receptor complex into the nucleus preventing DNA binding 33. Enzalutamide is an oral agent that is generally well tolerated and does not require concurrent steroid administration, which makes it an ideal candidate for combination with salvage radiation therapy (SRT). Finally, provocative preliminary Phase II data presented at the American Society of Clinical Oncology (ASCO) 2013 by M. Smith and colleagues assessed the efficacy and safety of 25-weeks (\~6-mos) of enzalutamide alone in prostate cancer of all stages who had never received hormone therapy; presenting with non-castrate testosterone levels ( 230 ng/dL). Enzalutamide alone for 6-mos achieved a high PSA response rate with efficacy similar to castration, but .in contrast to castration, bone mineral density (BMD) remained stable and metabolic variables were not substantially impacted. The trial described here differs from Radiation Therapy Oncology Group (RTOG) 96-01, RTOG 05-34 and RADICALS in several ways. First, the eligibility criteria are stricter; less favorable patients have been selected. Second, short-term ARB is being tested, while in RTOG 96-01 and RADICALS long-term ARB of 2-years was examined. Finally, and most importantly, we are testing the second generation ARB agent, enzalutamide, alone in combination with SRT as opposed to RTOG 05-34 and RADICALS which use androgen deprivation (AD). This trial is not intended to address the efficacy of SRT alone over observation. The complete response rate (a drop in PSA to undetectable levels) after SRT is 70%-80% and durable responses are observed in 30%-40% of patients. For these reasons, it is not feasible or appropriate to randomize men between observation and SRT. The more important issue is whether the proportion of durable responses is increased by altering the therapeutic approach, such as the use of enhanced ARB using enzalutamide.