The survival outcomes for men with metastatic castration-resistant prostate cancer (mCRPC) with and without homologous recombination deficiencies (HRD) treated with radium-223: Princess Margaret Cancer Centre (PMCC) experience.

Abstract

179 Background: Radium-223 (223Ra) is a radiopharmaceutical that emits alpha particles and specifically targets bone metastases in mCRPC, where it has been shown to improve overall survival (OS). Since 223Ra induces double-strand DNA breaks, we hypothesized that patients with HRD may exhibit heightened sensitivity to 223Ra, resulting in improved survival outcomes compared to patients without HRD. Methods: This retrospective analysis was performed in men with mCRPC and bone metastases, with and without HRD, treated with 223Ra at PMCC. Demographics and disease characteristics were collected. Germline and/or somatic DNA sequencing data were identified. OS and progression free survival (PFS) were calculated using the Kaplan–Meier method; differences in outcomes were assessed using the log-rank test. Alkaline phosphatase (ALP) and prostate specific antigen (PSA) responses were calculated at 12 weeks post 223Ra treatment. Results: We identified 40 mCRPC patients who had germline and/or somatic DNA sequencing and received 223Ra between December 2015 and May 2022. The median age at the start of 223Ra was 76.5 (range: 66.5-80.8), and ECOG was 0/1 (75%). A total of 29 (72.5%) patients received 233 Ra following abiraterone or enzalutamide treatment while 10 (25%) received 233 Ra post docetaxel. Overall, 22 (54%) received ≥4 cycles of ²²³Ra. Median baseline PSA was 58.6 (range: 25.2-143) and median baseline ALP was 109 (range: 71.8-200). Germline/somatic HRD mutations were found in 9/40 (22.5%) patients (BRCA2 [n=6], CHEK2 [n=2], CDK12 [n=1]). Baseline characteristics were well balanced between HRD and non-HRD groups. With median follow up of 13.7 months, the median OS of HRD group vs. non-HRD group was 24 months (95%CI: 14-not evaluable [NE]) vs. 12 months (95%CI: 7-22); p=0.038). The median PFS of HRD group vs. non-HRD group was 5.7 months (95%CI: 3-NE) vs. 3.3 months (95%CI: 2.5-13.4); p=0.74). The median time to the next treatment of HRD group vs. non-HRD group was 4.2 months (95%CI: 3.1-NE) vs. 3.75 months (95%CI: 3.5-13.5); p=0.89. ALP response was 66.7% of HRD group vs. 58.1% of non-HRD group; p=0.72. PSA response was 33.3% of HRD group vs. 9.7% of non-HRD group; p=0.11. For all patients with ALP response, the three years survival probability of HRD group vs. non-HRD group was 33% vs. 11%; p=0.03. Conclusions: While the number of patients included in our review was small, our analysis suggested that patients with HRD may have a slight improvement in OS after 223Ra treatment. Validation in a prospective dataset is required, and whether HRD status has implications for other radiopharmaceuticals such as lutetium-177 remains to be seen.