Androgen receptor (AR) gene dynamics in ctDNA post-abiraterone/enzalutamide: Longitudinal study in mCRPC with germline results.

5076 Background: Approximately 10–15% of men with metastatic prostate cancer harbor germline mutations. Circulating tumor DNA (ctDNA) provides a non-invasive approach to identifying AR mutations as resistance mechanism after abiraterone and enzalutamide initiation. In this study, we report longitudinal ctDNA profiling in patients with metastatic castration-resistant prostate cancer (mCRPC) who had germline testing. Methods: A retrospective review was conducted at Tulane Cancer Center to identify mCRPC patients with serial ctDNA after starting abiraterone and/or enzalutamide and germline test results. Germline testing was performed using a multi-gene cancer panel from Invitae, covering 50–86 genes, while somatich alterations in ctDNA were assessed through Guardant360, which included 70–83 genes. Linear mixed-effects models (LMMs) with random intercepts were used to model AR gene allele fraction and AR amplification copy number as continuous outcomes. Fixed effects included serial ctdna timepoints and germline gene pathogenicity, and random effects accounted for repeated measure using patient-specific intercepts. Cox Proportional Model was used to analyze the time from initiation of ARPi to AR amp and/or AR mutation in ctDNA. Analyses were performed using the R version 4.4.2. Results: From 2015-2025, 341 mCRPC patients with germline test results received an average of 2.96 ctDNA tests post abiraterone or enzalutamide. In this cohort, 47 patients had germline pathogenic mutations (14%) and 294 patients had germline non-pathogenic mutations (86%). Among those with pathogenic germline findings, there were 29 patients with mutation in homologous recombination repair genes (62%) and 18 patients with mutation in other genes. Overall, 55.7% patient in this cohort developed either AR amplification or AR pathogenic mutation in ctDNA. In linear mixed models, germline pathogenicity and pathogenic HRR mutations were not associated with AR copy number, AR allele frequency, or occurrence of somatic AR alterations. Serial ctDNA timepoints following baseline assessment are associated with a significant increase in AR amplifications copy numbers in ctDNA(p=0.003). AR amplification copy number increases by 0.12 for each subsequent timepoint from baseline. Only 318 patients were included in survival analysis due to missing ARPi start date. Time to ctDNA AR amplification or mutation after 1 st ARPi initiation was shorter in mCRPC patients without pathogenic HRR germline mutations compared to those with pathogenic HRR mutations (median: 38 vs 58 months, p = 0.049). Conclusions: Later serial timepoints correlated with a significant increase in AR amplifications copy numbers but not correlated with highest AR mutant allele frequency. mCRPC patients without germline HRR mutations will find ctDNA AR amplification or mutation sooner than those with germline HRR mutations.