63P Enhancing HER2 evaluation: Correlation between APIS breast cancer subtyping kit and IHC/ISH for accurate HER2 quantification

HER2 status is a crucial prognostic and predictive biomarker in invasive breast cancer (BC). The advent of novel-anti HER2 therapies (e.g. Trastuzumab Deruxtecan (T-DXd)) has cast doubt upon traditional HER2 detection methods, as emerging data indicates the effectiveness of T-DXd in individuals categorized as HER2 immunohistochemistry (IHC) 1+ or 0 scores. More sensitive methods detecting HER2-low expression are key to identify patients who could benefit from treatment with novel anti-HER2 therapeutics. As IHC methods were developed for identifying HER2 overexpression, rather than distinguishing between HER2-low and expression absence, the ability of IHC to detect HER2-low cases remains uncertain. APIS Breast Cancer Subtyping Kit (APIS BCSK) was developed to detect expression level of four BC biomarkers – HER2, ER, PR, Ki67. Here, we report HER2 mRNA expression levels detected by APIS BCSK, in correlation with IHC HER2 scoring. Formalin-fixed paraffin-embedded (FFPE) BC sections (n=642), from core needle biopsy or resection, underwent histological assessment observing the ASCO/CAP guidelines for IHC. HER2 status of specimens with a HER2 2+ IHC score was resolved via in situ hybridization (ISH) amplification. All specimens were tested with APIS BCSK. To evaluate diagnostic accuracy of the kit, IHC/ISH and APIS BCSK mRNA expression level concordance was reported as Overall Percent Agreement (OPA), Negative Percent Agreement (NPA), and Positive Percent Agreement (PPA). Strong correlation between IHC/ISH results and HER2 expression detected by APIC BCSK was observed (OPA of 94.2%, PPA of 89.2% and NPA of 94.9%). ERBB2 mRNA expression was detected by APIS BCSK in a subset of patients with 0 and 1+ IHC HER2 score, highlighting the continuous nature of ERBB2 expression and higher sensitivity of RT-qPCR-based detection approaches. APIS BCSK accurately detects HER2 expression. Results confirm that IHC stratification may not be an adequate method for predicting the response to novel anti-HER2 therapies, such as T-DXd. Implementation of additional cut offs could allow further stratification of ERBB2 mRNA expression, however, additional studies are required to validate this approach.