Evaluation of PSA progression after initiation of enzalutamide or abiraterone: Real-world data on metastatic castration-resistant prostate cancer (mCRPC)

5024 Background: PSA value is widely used for the monitoring of treatment outcome in mCRPC in the clinical real-world setting. Early PSA changes are not considered in the definition of PSAProg due to the potential for spurious “flare” reactions. We aimed to evaluate the significance of an early PSA increase in mCRPC patients (pts) treated with enzalutamide or abiraterone (Enz/Abi). Methods: We retrospectively evaluated Enz/Abi-treated mCRPC pts from 11 hospitals between 2011-2020. Early PSAProg was defined as a 25% increase in PSA from baseline at 4 (PSAProg4) or 8 (PSAProg8) weeks after treatment initiation. PSA progression at 12 weeks (PSAProg12) was confirmed by a second reading. Uni- and multivariable (MV) Cox regression models were conducted to explore the association of PSAProg and overall survival (OS) in chemotherapy naïve patients treated with Abi or Enz. Interaction tests were conducted to explore differences in the impact of PSA progression on OS in Abi or Enz-treated pts. Results: We analyzed 511 chemotherapy-naïve mCRPC pts treated with Abi (N=391; 76.5%) or Enz (N=120; 23.5%). Median follow-up: 30.2 months. OS was longer in Enz-treated pts (38.1 vs 29m; HR 1.4; p=0.027). 59 (15.1%), 70 (17.9%) and 48 (12.3%) of Abi-treated and 9 (7.5%), 11 (9.2%) and 10 (8.3%) of Enz-treated pts experienced PSAProg4, PSAProg8 and PSAProg12, respectively, although differences were not statistically significant. PSAProg was associated with worse OS at all 3 timepoints only in Abi-treated pts. In Enz-treated pts, PSAProg4 had a large impact on OS, not observed in PSAProg8 or PSAProg12. We observed no significant interaction between agent (Enz/Abi) and PSA progression (Table). Conclusions: PSA progression at 4 weeks after Enz/Abi is significantly associated with shorter OS and may help identify pts not benefitting from Abi/Enz before clinical or radiographic progression. PSA pattern progression and its association with OS might differ depending on the drug used (Enz/Abi). Prospective validation studies are needed.[Table: see text]