RTOG 3506 (STEEL): A study of salvage radiotherapy with or without enzalutamide in recurrent prostate cancer following surgery.

Abstract

TPS5601 Background: Salvage radiotherapy (SRT) is an important intervention for men with prostate cancer (PCa) who experience biochemical recurrence (BCR) after radical prostatectomy (RP). These patients are in need of cure or else they will develop metastatic disease. NRG/RTOG 9601 (WU Shipley, N Eng J Med 2017) identified a survival benefit from the addition of androgen receptor (AR) inhibition to SRT that was most prominent in men with high-risk features. Enzalutamide (Enza) is a non-steroidal anti-androgen that improves survival in castration-resistant and -sensitive PCa. We hypothesized that enhanced AR suppression with Enza would augment the benefit of SRT + androgen deprivation therapy (ADT) in BCR with high risk features. Methods: RTOG 3506 (STEEL, NCT03809000) is a randomized phase II study of SRT in BCR after RP with a serum PSA ≥ 0.2 ng/mL active in the USA and Canada. Patients are stratified by number of high-risk features including Gleason score (8-10), locoregional node involvement at RP, seminal vesicle invasion, persistently elevated PSA after RP, and PSA > 0.7 ng/mL. All patients receive SRT with 2 years of ADT. The experimental arm also receives Enza 160 mg daily for 2 years. Patients are followed by PSA every 3 months. SRT can be highly individualized per treating physician beyond the mandatory treatment of the prostatic fossa. Treatment of the pelvis and/or para-aortic nodes, as well as sequential or concurrent boosts to a prostatic fossa mass and/or suspicious lymph nodes, are allowed options. This permits individualization of radiotherapy guided by CT, MRI, PET, and/or biopsy findings. The primary goal of this study is to determine whether SRT enhanced ADT with Enza, will improve progression-free survival (PFS) compared to SRT with standard ADT. PFS defined as the first occurrence of biochemical failure, clinical failure, or initiation of new anticancer treatment. STEEL is designed to demonstrate a 35% reduction in the risk of progression at 5 years. An accrual goal of 242 patients will provide 80% power with a one-sided alpha = 0.10. Secondary endpoints include disease control rates, acute and late physician- and patient-reported toxicity, and quality of life. This study was activated in February 2019. Site recruitment and activation are underway. Conclusions: There is an unmet and urgent need for individualized strategies to optimize systemic therapy used with SRT for men with BCR. Outcomes from this study will further clarify the approach to systemic therapy for SRT in high-risk BCR patients. Support: Provided by Pfizer. Clinical trial information: NCT03809000 .