Analysis Of Prevalence And Prognostic Implications Of Early Versus Delayed Psa Declines During Radium-223 (Ra223) Treatment Among Men With Metastatic Castration-Resistant Prostate Cancer (Mcrpc)

117 Background: Ra233 is a well-tolerated, bone-seeking alpha emitter that prolongs the survival of men with mCRPC. Since there is a lack of validated biomarkers useful for disease monitoring during Ra223 therapy, we sought to characterize prostate-specific antigen (PSA) response patterns within a retrospective cohort study of real-world Ra223 treatment in four cancer centres across Ontario. Methods: Of 198 patients (pts) overall, 64 were not included due to incomplete data. The remaining 134 pts were grouped into (i) early PSA responders with a ≥30% PSA decrease (PSA30) within 12 weeks of treatment start, (ii) pts with an initial PSA flare followed by decreasing PSA, and (iii) non-responders. We analyzed the overall survival (OS) of these three cohorts using the Kaplan-Meier method and log-rank testing. Results: PSA30 responses were rare (n = 11; 8.2%), PSA flares were seen in 20.2% of pts (n = 27), but the majority of pts did not achieve a PSA response (n = 96; 71.7%). Of the pts identified as having a PSA flare, 11/27 (40.7%) presented with a post-flare PSA decrease below baseline, whereas in 16/27 (59.3%) the PSA decreased below the flare peak but not below baseline. 5/27 (18.5%) pts with a PSA flare achieved a PSA30. The median OS was not reached, 15.8 months and 14.7 months in responders, flare pts and non-responders, respectively (p > 0.05). Similarly, one year OS was 90.9%, 67.7% and 63.9% in responders, flare pts and non-responders. Conclusions: PSA flares are seen in one fifth of men undergoing Ra223 therapy for mCRPC. Despite delayed PSA declines of varying degrees, the survival of PSA flare pts is comparable to men without any PSA decline.