Enhancing HER2 evaluation: Correlation of APIS Breast Cancer Subtyping Kit and IHC/ISH with accurate HER2 quantification.

e13028 Background: Human epidermal growth factor receptor 2 (HER2) status is a crucial prognostic and predictive biomarker in invasive breast cancer (BC). The advent of novel-anti HER2 therapies (such as Trastuzumab deruxtecan (T-DXd)) has cast doubt upon traditional HER2 detection methods, as emerging data indicates the effectiveness of T-DXd in individuals who were previously categorized as HER2 immunohistochemistry (IHC) 1+ or 0 scores. More sensitive and precise methods capable of detecting HER2-low expression are therefore crucial to correctly identify patients who could benefit from treatment with these novel anti-HER2 therapeutics. As IHC methods were primarily developed for the identification of tumors exhibiting HER2 overexpression, rather than for distinguishing between HER2-low and complete absence of expression, the ability of these assays to precisely 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 the correlation between HER2 mRNA expression levels detected by APIS BCSK, and IHC HER2 scoring. Methods: A total of n=642 formalin-fixed paraffin-embedded (FFPE) BC sections, obtained by core needle biopsy or resection, underwent histological assessment observing the ASCO/CAP guidelines for IHC. The HER2 status of specimens with a HER2 2+ IHC score was resolved via in situ hybridization (ISH) amplification. All specimens underwent testing with APIS BCSK. To evaluate the diagnostic accuracy of the kit, the concordance between IHC/ISH and APIS BCSK mRNA expression level was reported in terms of Overall Percent Agreement (OPA), Negative Percent Agreement (NPA), and Positive Percent Agreement (PPA). Results: 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. Conclusions: APIS BCSK accurately detects HER2 expression. The 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 correlating the expression level to anti-HER2 treatment response are required to validate this approach.