Disease Characteristics and Completion of Treatment in Patients With Metastatic Castration-Resistant Prostate Cancer Treated With Radium-223 in an International Early Access Program

In this post hoc analysis we investigated associations between baseline characteristics and number of radium-223 injections in a phase IIIb, single-arm study in the setting of an international early access program. Patients with less advanced metastatic castration-resistant prostate cancer described by more favorable baseline characteristics were more likely to complete 5 to 6 than 1 to 4 injections and had longer overall survival. Background: Radium-223 is approved by the US Food and Drug Administration and European Medicines Agency for the treatment of metastatic castration-resistant prostate cancer (mCRPC). There are currently no markers for selecting patients most likely to complete radium-223 treatment. Patients and Methods: In this phase IIIb, international, single-arm study, patients received radium-223, 55 kBq/kg, every 4 weeks for <= 6 cycles. Primary end points were safety and overall survival. In post hoc analyses patients were grouped according to number of radium-223 injections received (1-4 or 5-6). Associations between baseline covariates and number of injections were investigated. Results: Of 696 eligible patients, 473 (68%) had received 5 to 6 radium-223 injections and 223 (32%) 1 to 4 injections. Patients with less pain (moderate-severe vs. none-mild, odds ratio [OR], 0.41; P < .0001), lower Eastern Cooperative Oncology Group performance status (>= 2 vs. 0-1, OR, 0.51; P = .0074), lower prostate-specific antigen level (>141 mu g/L vs. <= 141 mu g/L, OR, 0.40; P < .0001), and higher hemoglobin level (<10 g/dL vs. >= 10 g/dL, OR, 0.50; P = .0206) were more likely to receive 5 to 6 than 1 to 4 injections. Median overall survival was not reached and was 6.3 months (95% confidence interval, 5.4-7.4) in patients who had received 5 to 6 and 1 to 4 radium-223 injections, respectively. Adverse events were less common in patients who received 5 to 6 than 1 to 4 injections; anemia was reported in 87 (18%) and 64 (29%) patients, respectively. Conclusion: Patients with less advanced mCRPC are more likely to receive 5 to 6 radium-223 injections and to achieve better overall survival. Consideration of baseline and disease characteristics is recommended before initiation of radium-223 treatment.