Abstract P5-14-10: Targetable alterations and genomic signatures within breast cancer brain metastases: Data from comprehensive genomic profiling of 761 breast cancer brain metastases

Abstract Background: Breast cancer brain metastasis (BCBM) represents an area of unmet clinical need. We previously demonstrated the genomic differences between primary BC and BCBM. In this study we examined the genomic differences between BCBM by BC subtypes and the potential actionable targets that might be taken forward for each subtype in clinical trials. Material and Methods: A total of 761 BCBMs were analyzed by comprehensive genomic profiling (CGP) for alterations in up to 395 genes (Foundation Medicine, USA). The samples were classified by immunohistochemistry (IHC) of the BCBM to ER+ (ER+/HER2-, ER+/HER2+), ER-/HER2+ and ER-/HER2-. Homologous recombination deficiency (HRD-gLOH; cutoff 16%), tumour mutational burden (TMB; cutoff 10 mutations/Mb), microsatellite instability (MSI) and PD-L1 prevalence and expression by IHC using the VENTANA SP142 assay (Immune Cell Score [IC] cutoff 1%) were also investigated. Results: For all BC subtypes the most enriched gene alterations in BCBM (>20% prevalence) were: TP53, MYC and PIK3CA. ESR1 and ERBB2 were more prevalent in ER+ and HER2+ tumours respectively, with ESR1 alterations significantly enriched in ER+/HER2- BCBMs (p< 0.0001). Frequently altered genes by BCBM subtype were: ER+/HER2-: CDH1 (8%) and BRCA2 (7%); ER+/HER2+: PIK3C2B (11%), MDM4 (11%), TBX3 (9%) and AKT2 (8%); ER-/HER2+: LYN (9%). Significantly enriched genes (p< 0.01) by BCBM subtype were: HER2+: CDK12 (15%); ER-/HER2-: BRCA1 (14%), CCND3 (9%), VEGFA, JAK2 (8% each) and the immune checkpoint inhibition (ICPI) biomarkers PDCD1LG2 (PDL2), CD274 (PDL1) (7% each). HRD-gLOH was high in ER+/HER2-: (43%) and ER-/HER2-: (70%). TMB and MSI were present in 10%-21% and 1-3% of BCBM respectively, whereas PDL1 protein expression by subtypes was: ER+/HER2- 23%, ER+/HER2+ 27%, ER-/HER2+ 57% , ER-/HER2- 48%. Table 1 summarizes the proportion of BCBMs with actionable alterations and selected clinical trials in BCBM. Conclusion: Clinically-relevant genomic alterations were identified across all BCBM subtypes. High HRD-gLOH, TMB and MSI were observed across all the BCBMs subtypes whereas PDL1/PDL2 alterations were a distinctive feature of ER-/HER2- BCBM. These data highlight the need to assess the genomic landscape of BCBM to enable rational treatment decisions with targeted agents, as well as to enable enrichment of relevant populations within clinical trials. Table 1. Selected studies with targeted therapy and/or immunotherapy in breast cancer brain metastases. The percentage (%) of actionable alterations by ER and HER2 status is summarized. ND: Not Detectable. Citation Format: Athina Giannoudis, Ethan Sokol, Shakti Ramkissoon, Talvinder Bhogal, Jeff Ross, Kimberly McGregor, Evangelia Razis, Rupert Bartsch, Richard S. Huang, Carlo Palmieri. Targetable alterations and genomic signatures within breast cancer brain metastases: Data from comprehensive genomic profiling of 761 breast cancer brain metastases [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P5-14-10.