First results of the PROMPT trial: Precision oncology allocation in patients with early castration-resistant prostate cancer following routine molecular profiling

40 Background: Castration-resistant prostate cancer (CRPC) comprises distinct molecular actionable subtypes. Radboudumc, a tertiary referral center for PCa, hosts a Molecular Tumor Board (MTB), and frequently sees CRPC patients (pts) for referral for genetic testing. In the period 2017-2020, almost 50% of heavily pre-treated CRPC pts received a recommendation for a genetically matched therapy (GMT). In 2020 we initiated the PROMPT trial (NCT04746300) to prospectively study the impact of routine early molecular characterization in CRPC. Increased GMT utilization of targeted- and immunotherapy may improve outcome and quality of life of pts with metastatic CRPC, and lead to lower medical resource utilization (MRU). Here we report on first results from our observational trial. Methods: Within the PROMPT trial, prior to receiving first- or second line standard of care CRPC therapy, up to 300 pts were offered molecular tumor characterization with the TruSightOncology 500 panel (TSO500). Formalin-fixed paraffin-embedded prostate or metastatic tissue biopsies were used, preferably newly obtained. To assess both mutations and copy number alterations, the TSO500 required a minimal tumor cell percentage (TC%) of 30%. Results were discussed within the Radboudumc MTB for GMT recommendation. Actionable targets were defined per Precision Medicine Working group criteria and, when druggable, within the Drug Rediscovery Protocol trial (NCT02925234). All pts with tumor mutations carrying with a risk for cancer predisposition were referred for genetic counselling. Follow-up with patient-reported outcomes (EORTC QLQ-C30, EQ5D, BPI & EPIC-26 questionnaires) and MRU was conducted every 3 months until withdrawal or death. Results: From February 2020 until October 2021 we included 284 consecutive CRPC pts with a median age of 70 years (range 46-86) with a median follow-up of 6.8 months. Newly obtained biopsies and archival material was used in 131 and 126 pts, respectively. Median TC% was 60% (range: 20-90%). TSO500 results could be reported in 254 (89%) cases, with at least one putative clinically relevant aberration in 188 evaluable pts (74%). In 100 pts (39,4%) ≥ 1 druggable target was found. Most common actionable alterations were in PTEN (19%), BRCA2 (9%) or in mismatch-repair genes or resulting in high tumor mutational burden (5%). Out of the 100 druggable pts, 31 pts (31%) initiated a form of GMT, 4 pts (4%; 2 BRCA2, 2 PTEN) died prior to receiving GMT, in 65 pts (65%) GMT is pending as they receive standard of care. Conclusions: Routine molecular profiling early in the CRPC setting is feasible in a tertiary referral center with a MTB and high volume of CRPC pts. Almost 40% CRPC pts harbored an actionable target with 31% of these pts already allocated to a GMT. Failure rates of NGS were low at 11%.