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

person Anna Gasior APIS Assay Technologies Ltd., Manchester, United Kingdom info_outline Anna Gasior, Anne-Sophie Wegscheider, Jack Paveley, Kimberly Jane Howard, Mathew Harrison, Leanne Gough, Joanna Gorniak, Sara Rollinson, Zoe Pounce, Axel Niendorf

Authors person Anna Gasior APIS Assay Technologies Ltd., Manchester, United Kingdom info_outline Anna Gasior, Anne-Sophie Wegscheider, Jack Paveley, Kimberly Jane Howard, Mathew Harrison, Leanne Gough, Joanna Gorniak, Sara Rollinson, Zoe Pounce, Axel Niendorf Organizations APIS Assay Technologies Ltd., Manchester, United Kingdom, MVZ Prof. Dr. Med. A. Niendorf Pathologie Hamburg-West GmbH, Hamburg, Germany Abstract Disclosures Research Funding No funding sources reported 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.