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330 Background: Patients (pts) with localized prostate cancer (LPC) or locally advanced prostate cancer (PC) have several treatment options, including external beam radiation therapy (EBRT) and radical prostatectomy, which can be curative in some pts. However, limited real-world evidence exists on the long-term clinical outcomes of these pts, particularly those with high-risk LPC (HRLPC). This study examines real-world clinical outcomes for HRLPC and low/intermediate risk LPC (LIRLPC) pts treated with EBRT in the US. Methods: A retrospective study using Surveillance, Epidemiology, and End Results (SEER)-Medicare linked registry-claims data from 2012−2019 included LPC pts aged ≥65 treated with EBRT as initial definitive therapy. Baseline demographic and clinical characteristics were summarized for HRLPC and LIRLPC pts. Clinical outcomes of interest (overall survival, metastasis-free survival [MFS] and time to initiation of advanced PC treatment) were compared using Kaplan-Meier (KM) and Cox proportional hazards (PH) models. Results: Of 11,127 LPC pts treated with EBRT within 6 months of LPC diagnosis, ~40% (n=4,414) were HRLPC and ~60% (N=6,713) were LIRLPC. Patient characteristics for both groups appeared similar, with mean age at EBRT initiation >70 years, 86% white, and mean follow-up time >40 months. ADT was used with EBRT in 78% HRLPC and 34% LIRLPC. Median (IQR) duration of ADT therapy (via KM analysis) was 9.9 (5.8, 21.5) months for HRLPC and 7.2 (5.3, 9.8) months for LIRLPC. A higher proportion of HRLPC vs LIRLPC pts experienced metastasis, death, and progression to advanced PC therapies (medians were not reached). Unadjusted Cox PH survival analyses showed higher risk of mortality, metastasis, and advanced PC therapy use for HRLPC vs LIRLPC pts. Conclusions: This real-world study of clinical outcomes in pts with HRLPC and LIRLPC treated with EBRT suggested substantial additional disease burden in pts with HRLPC and highlights the need for additional strategies and treatments to improve clinical outcomes in pts with HRLPC. [Table: see text]

69 Background: For patients with nmCRPC, APA with androgen deprivation therapy is an important therapeutic option with demonstrated efficacy in improving metastasis-free survival (MFS), and other clinical outcomes. However, there is limited real-world information on these outcomes among patients with nmCRPC treated with APA. This study describes PSA response and MFS among patients with nmCRPC treated with APA in the United States as compared to the SPARTAN registrational trial. Methods: Clinical data from 2/2017 to 4/2022 collected from 77 community-based urology practices were used to evaluate patients with nmCRPC who received ≥1 APA dispensation (index date being the date of first dispensation). Patients were excluded if they had previously used another next-generation antiandrogen or had

65 Background: CR is an important clinical indicator of disease progression and worsening overall survival among pts with mCSPC. This study describes real-world time-to-CR progression in an oncology setting among pts with mCSPC who initiated APA, ENZ, or ABI in the United States. Methods: Clinical data from the Flatiron Metastatic Prostate Cancer (PC) Core Registry Electronic DataMart (1/1/2013 – 07/31/2022) were evaluated. Metastatic pts without evidence of CR before initiating a next-generation androgen inhibitor (AAI) were classified into exclusive treatment cohorts based on the start date (index date) for APA, ENZ, or ABI. CR was identified by a Flatiron algorithm searching for: 1) mention of CR in patient charts, 2) rising prostate-specific antigen (PSA) levels documented from laboratory testing, or 3) mention of rising PSA while on hormonal therapy from unstructured data sources. Pts had ≥12 months (mo) of clinical data pre-index date and were followed from the index date until the end of clinical activity (including death) or data availability. The proportion of pts developing CR was described separately for each cohort using Kaplan-Meier analysis up to 24 mo post-index, with no adjustment for baseline confounders. CR progression rates in the current analysis were descriptively compared to those observed in Phase 3 trials where available. Results: A total of 155 APA, 385 ENZ and 711 ABI pts were evaluated. Mean age at index was ~73 years in each group. Mean follow-up was 12.8 mo (APA), 13.4 mo (ENZ) and 20.6 mo (ABI). Use of androgen deprivation therapy prior to initiation of AAI was observed in ~85% of pts. The median time between documented metastasis and the index date was 2.3 mo (APA), 2.8 mo (ENZ), and 2.3 mo (ABI). Progression rates to CR are shown. By 24 mo, the median time-to-CR was not reached with any treatment studied. By 24 mo, progression to CR was observed in 26% of APA pts in this analysis (32% in TITAN); 36% of ENZ pts in this analysis (25% in ARCHES); and 33% of ABI pts in this analysis (progression to CR not reported in LATITUDE). Conclusions: In this real-world database, the observed proportions of pts progressing to CRPC for the APA and ENZ cohorts were generally consistent with results of Phase 3 trials and include a small proportion of pts with early progression to CRPC. Increased physician awareness of the benefits of early initiation of an AAI along with identification of baseline biomarkers associated with early progression to CRPC may improve clinical outcomes. [Table: see text]