Novel antibody-drug conjugates against HER2 are showing high activity in HER2-negative breast cancer (BC) with low HER2 expression (i.e., 1+ or 2+ and lack of ERBB2 amplification). However, the clinical and molecular features of HER2-low BC are yet to be elucidated. Here, we collected retrospective clinicopathological and PAM50 data from 3,689 patients with HER2-negative disease and made the following observations. First, the proportion of HER2-low was higher in HR-positive disease (65.4%) than triple-negative BC (TNBC, 36.6%). Second, within HR-positive disease, ERBB2 and luminal-related genes were more expressed in HER2-low than HER2 0. In contrast, no gene was found differentially expressed in TNBC according to HER2 expression. Third, within HER2-low, ERBB2 levels were higher in HR-positive disease than TNBC. Fourth, HER2-low was not associated with overall survival in HR-positive disease and TNBC. Finally, the reproducibility of HER2-low among pathologists was suboptimal. This study emphasizes the large biological heterogeneity of HER2-low BC, and the need to implement reproducible and sensitive assays to measure low HER2 expression.

Purpose: The therascreenPIK3CAmutation assay and the alpha-specific PI3K inhibitor alpelisib are FDA-approved for identifying and treating patients with advancedPIK3CA-mutated (PIK3CAmut) breast cancer (BC). However, it is currently unknown to what extend this assay detects mostPIK3CAmutations in BC. This information is critical as patients and clinicians are using this and other genomic assays to indicate alpelisib.Methods: Data from 6338 patients with BC was explored across 10 publicly available studies. The primary objective was to evaluate the proportion and distribution ofPIK3CAmutations in BC. Secondary objectives were (1) to evaluate in silico the spectrum ofPIK3CAmutations in BC that would be captured by the therascreen panel; (2) to evaluate the proportion and distribution ofPIK3CAmutations in hormone receptor-positive/HER2-negative (HR+/HER2−), HER2+, and triple-negative BC (TNBC); and (3) to explore the identification ofPIK3CAmutations in a cohort of 48 HR+/HER2− advanced BC patients by the Guardant B360 circulating tumor DNA (ctDNA) assay.Results: Patients withPIK3CAmut tumors represented 35.7% (2261/6338). FivePIK3CAmutations comprised 73% of allPIK3CAmutations: H1047R (35%), E545K (17%), E542K (11%), N345K (6%), and H1047L (4%). Therascreen gene list would capture 72% of allPIK3CAmutations and 80% of patients with a knownPIK3CAmut BC. Among patients with doublePIK3CAmut tumors (12% of allPIK3CAmut), the therascreen panel would capture 78% as harboring 1 singlePIK3CAmutation, 17% asPIK3CAmut undetected, and 5% asPIK3CAdouble-mut.PIK3CAmutation rates were lower in TNBC (16%) compared to HR+/HER2 (42%) and HER2+ (31%) BC; however, the distribution of the 4 mainPIK3CAmutations across subtypes was similar. Finally, 28% ofPIK3CAmutations identified in ctDNA in 48 patients with advanced HR+/HER2− BC were not part of the therascreen panel.Conclusion: PIK3CAmutations in BC are heterogenous and ~ 20% of patients with a knownPIK3CAmutation, and 95% with a known doublePIK3CAmut tumor, would not be captured by the therascreen panel. Finally, the clinical utility ofPIK3CAmutations not present in the therascreen companion diagnostic assay or identified by other sequencing-based assays needs further investigation.

Poly(ADP‐ribose) polymerase (PARP) inhibitors (PARPi) are effective in cancers with defective homologous recombination DNA repair (HRR), including BRCA1/2‐related cancers. A test to identify additional HRR‐deficient tumors will help to extend their use in new indications. We evaluated the activity of the PARPi olaparib in patient‐derived tumor xenografts (PDXs) from breast cancer (BC) patients and investigated mechanisms of sensitivity through exome sequencing, BRCA1 promoter methylation analysis, and immunostaining of HRR proteins, including RAD51 nuclear foci. In an independent BC PDX panel, the predictive capacity of the RAD51 score and the homologous recombination deficiency (HRD) score were compared. To examine the clinical feasibility of the RAD51 assay, we scored archival breast tumor samples, including PALB2‐related hereditary cancers. The RAD51 score was highly discriminative of PARPi sensitivity versus PARPi resistance in BC PDXs and outperformed the genomic test. In clinical samples, all PALB2‐related tumors were classified as HRR‐deficient by the RAD51 score. The functional biomarker RAD51 enables the identification of PARPi‐sensitive BC and broadens the population who may benefit from this therapy beyond BRCA1/2‐related cancers. Synopsis Sensitive and highly specific biomarkers usable in archived formalin fixed parafin embedded (FFPE) tumour samples are needed to extend the use of PARP inhibitors beyond BRCA1/2‐related cancers. The RAD51 score may satisfy this clinical unmet need. The RAD51 score shows complete discriminative capacity in predicting PARP inhibitor response. The RAD51 score is feasible in routine breast tumor samples without prior exposure to DNA damaging agents. Carrying a PALB2 mutation is associated with a low RAD51score.