Attainment of early, deep prostate-specific antigen response in metastatic castration-sensitive prostate cancer: A comparison of patients initiated on apalutamide or enzalutamide

43 Background: Deep prostate-specific antigen (PSA) response, defined as a ≥90% decline in PSA (PSA90), is an important early prognostic factor for achieving radiographic progression free survival and overall survival in patients with metastatic castration-sensitive prostate cancer (mCSPC) treated with a next generation androgen receptor inhibitor (ARI) or androgen biosynthesis inhibitor. This study compared PSA90 responses among patients with mCSPC at first use of a next generation ARI. Methods: Clinical data from 69 community urology practices in the United States were evaluated. Patients with mCSPC were classified into treatment cohorts based on their first filled prescription (index date) for apalutamide (APA) or enzalutamide (ENZ) after 12/16/2019. Patients were followed from index date until the earliest of index regimen discontinuation, treatment switch, end of clinical activity or end of data availability (03/05/2021). Included patients had ≥12 months of clinical activity to assess baseline characteristics. Inverse probability of treatment weighting (IPTW) was used to reduce baseline confounding by controlling for age, race, index year, androgen deprivation therapy (ADT) use ≥6 months, first-generation antiandrogen use, most recent PSA level, most recent testosterone level, Gleason score, and time between metastasis and the index date. PSA90 was defined as the earliest attainment of ≥90% decline in PSA relative to baseline PSA (most recent value within 13 weeks pre-index). The proportion of patients achieving a PSA90 and the time to PSA90 was compared using a weighted Kaplan-Meier (KM) analysis and weighted Cox proportional hazards models. Results: A total of 186 APA patients and 165 ENZ patients met the study criteria. Patients’ characteristics were generally well balanced after IPTW (Table). By 6 months, 73.6% of APA and 69.5% of ENZ patients had a post index PSA measurement. At 6 months, significantly more mCSPC patients initiated on APA attained PSA90 response than patients initiated on ENZ (p=0.014; Table). This result continued to remain significant at 9 months and at the end of the follow-up (Table). The median time to PSA90 was 3.1 months for APA patients and 5.2 months for ENZ patients. Conclusions: This real-world study of mCSPC cohorts demonstrates that initiation of APA leads to significantly more patients achieving a deep PSA response more rapidly than patients initiating ENZ. [Table: see text]