Survival outcomes and risk group validation from SWOG S0925, a randomized phase II study of cixutumumab in new metastatic hormone-sensitive prostate cancer

BACKGROUND: Cixutumumab, a monoclonal antibody targeting insulin-like growth factor I receptor, did not improve undetectable PSA rate at 28 weeks when combined with androgen deprivation in the randomized phase II SWOG S0925 trial for patients with new metastatic hormone-sensitive prostate cancer. We now present mature survival analyses, along with pre-specified secondary and exploratory endpoints. METHODS: We randomized 210 patients to androgen deprivation with or without cixutumumab, 105 per treatment arm. We used Kaplan-Meier curves to analyze overall survival, radiographic progression-free survival, and castration resistance-free survival by treatment arm, disease volume, and risk group. We explored differences in survival by treatment arm via covariate-adjusted Cox proportional hazards models adjusted for disease volume and risk. RESULTS: No difference was seen between treatment arms in overall survival (HR 1.01 [0.70-1.45]; p=0.97), radiographic progression-free survival (HR 1.17 [0.85-1.60]; p=0.35), or castration resistance-free survival (HR 1.02 [0.75-1.41]; p=0.88). At baseline, 105/198 (53.0%) patients had high risk features and 119/210 (56.7%) had high volume disease; 16.7% of patients had discordant classifications of high or low category for risk and volume. Adjusting for risk or volume yielded no differences in overall survival between arms. Inferior survival was observed in high risk (HR 1.89 [1.29-2.80]; p=0.001) and high volume (HR 2.75 [1.84-4.10]; p<0.0001) disease. Disease volume was a better fit to survival data than risk group (AIC 878.3 vs. 889.2). Compared to patients achieving undetectable PSA at 28 weeks, inferior survival was observed in patients whose PSA was >0.2 to ≤4.0 ng/mL (HR 3.72 [1.99-6.95]; p<0.0001) or >4.0 ng/mL (HR 7.13 [4.24-11.9]; p<0.0001). CONCLUSIONS: In new metastatic hormone-sensitive prostate cancer, addition of cixutumumab to androgen deprivation did not improve survival. Baseline risk and disease volume carried prognostic value for this distinct trial population, although disease volume added more prognostic information. PSA treatment response was a strong intermediate endpoint for survival.