External Validation of a Prognostic Model Predicting Metastatic Castration-Resistant Prostate Cancer Survival in Patients Receiving Post-Docetaxel Second-Line Chemotherapy.

Introduction: The Halabi model predicts the overall survival (OS) of patients with metastatic castration-resistant prostate cancer (mCRPC) treated with second-line therapy after docetaxel. We aimed to validate this model externally with an independent cohort, outside the setting of a clinical trial.

Methods: In a multi-institutional study, we included 66 patients treated with cabazitaxel after docetaxel for mCRPC. Patients were stratified according to the two- and three-risk groups of the Halabi nomogram. Kaplan-Meier and Cox proportional hazard analyses were performed to estimate survival and hazard ratios (HRs). The model performance was assessed using receiver operating characteristic curves, and the associated c-index (area under the curve [AUC]).

Results: The median OS in the two-risk groups was 5.06 months in the high-risk group (n=22) and 12.9 months in the low-risk group (n=44, p<0.001). High-risk patients had an HR of 9.50 (95% confidence interval (CI) 4.12-21.6, p<0.001) compared to low-risk patients. For the three-risk groups, the median OS was 6.44 months in the high-risk group (n=15), 5.75 months in the intermediate-risk group (n=11), and 13.7 months in the low-risk group (n=40, p=0.84). Compared to low-risk patients, intermediate-risk patients had an HR of 7.49 (95% CI 3.08-20.4, p<0.001), and high-risk patients had an HR of 8.48 (95% CI, 3.39-21.7, p<0.001). The AUC was 0.72 (95% CI 0.64-0.76) for the two-risk stratification. When comparing different risks, the AUCs were 0.48 (high vs intermediate), 0.66 (high vs low), and 0.65 (intermediate vs low).

Conclusions: The two-risk stratification version but not the three-risk group analysis confirmed the ability of the model to predict survival. These results support the value of the Halabi nomogram in men receiving post-docetaxel second-line chemotherapy for mCRPC.