ESMO 2021: SAABR: Single Arm Phase II Study of Abiraterone + Atezolizumab + GnRH Analog and Stereotactic Body Radiotherapy (SBRT) to the Prostate in Men with Newly Diagnosed Hormone-Sensitive Metastatic Prostate Cancer (mHSPC)

( Since 2015, several phase 3 randomized controlled trials have demonstrated that adding either androgen pathway inhibitors or chemotherapy to testosterone suppression can prolong overall survival for men diagnosed with metastatic hormone-sensitive metastatic prostate cancer (mHSPC). The STAMPEDE study has specifically demonstrated a survival advantage in mHSPC with the addition of the androgen synthesis inhibitor abiraterone acetate. Based on observations from STAMPEDE that radiation therapy may reduce treatment failure rates in locally advanced prostate cancers as well as preclinical data suggesting that stereotactic body radiotherapy (SBRT) can induce expression of PD-L1 on tumor and immune cells, the authors of this abstract tested the hypothesis that the combination of testosterone suppression, abiraterone acetate, SBRT and the anti-PD-L1 drug atezolizumab may together exert synergistic effects to improve outcomes in men with mHSPC.

In this study that is ongoing, the primary outcome is the failure-free rate at 2 years, which is a composite endpoint including biochemical failure, radiographic progression or death from any cause. Treatment is administered with atezolizumab 1200 mg every three weeks, abiraterone acetate, testosterone suppression with a GnRH analog, and SBRT at 7.25-7.50 Gy for five fractions to the prostate and seminal vesicles every other day at Cycle 5. Systemic treatment will be continued until unacceptable toxicity, progression, or two years of treatment. Additional systemic therapy beyond two years with abiraterone and testosterone suppression will be at the discretion of the treating physician. Biopsies will be done in cohorts of patients after starting atezolizumab but before abiraterone, and after initiation of abiraterone and atezolizumab but before SBRT. A total of 42 patients are expected to be enrolled on this study.

Presented by: Dana E. Rathkopf, MD, Memorial Sloan Kettering Cancer Center, New York, New York

Written by: Alok Tewari, MD, PhD – Genitourinary Medical Oncologist, Instructor in Medicine, Dana-Farber Cancer Institute, Harvard Medical School, Twitter: @aloktewar during the 2021 European Society for Medical Oncology (ESMO) Annual Congress 2021, Thursday, Sep 16, 2021 – Tuesday, Sep 21, 2021.