Abstract 968: Evaluation of Her2 RNA expression as a potential predictive biomarker for anti-Her2 therapy
Authors: Lamb Laura E. , Khazanov Nickolay A. , Hovelson Daniel H. , Kwiatkowski Kat , Johnson D. Bryan , Rhodes Daniel R. , Tomlins Scott A. 
Published: April 2023

Abstract
Trastuzumab deruxtecan (Enhertu) is effective in "HER2 Low" breast cancer, defined by 1+ or 2+ expression by immunohistochemistry (IHC). Interest has now turned toward defining a sub-population of IHC 0+ tumors that may have HER2 expression below the limit of IHC detection/quantification and may thus also be responsive. We previously validated a high dynamic range HER2 RNA expression assay run as part of our comprehensive genomic profiling test, StrataNGS.
Herein, we evaluated the HER2 RNA expression data together with copy number and clinical outcome data from the Strata Clinical Molecular Database (SCMD) in advanced breast cancer (n = 3,063) and other advanced solid tumors (n = 26,715). As expected, HER2 gene expression was significantly higher in tumors with DNA amplification (&gt;=6 copies; median: 13.9 vs. 10.0 in log2 units; p &lt; 1e-100). Despite similar copy number levels in amplified breast vs. other cancers (median: 21.8 vs. 19.8 copies), HER2 expression levels were ~2-fold higher (median: 14.5 vs. 13.5; p = 1.3e-10). Similarly, HER2 expression levels were higher in non-amplified breast vs. other cancers (median: 10.7 vs. 9.9; p&lt;1e-100), suggesting that DNA amplification and cell lineage affect HER2 expression. Using our previously validated HER2 threshold, among 75 eligible SCMD breast cancer patients treated with 1st or 2nd line systemic trastuzumab or pertuzumab containing therapy, HER2 RNA High patients (n=46, 59%) had significantly longer time to next therapy (TTNT) compared to HER2 RNA Not High patients (median TTNT 26.9 vs. 5.6 months, adjusted hazard ratio 0.31, p=0.005 when adjusted for 1st vs. 2nd line, pertuzumab inclusion, and inclusion of chemotherapy or hormonal therapy).
In patients with available IHC data (n = 388), HER2 RNA expression trended with IHC across the 0-3+ range, however, while 3+ tumors had distinctly high RNA expression (median: 14.4), 0-2+ tumors had lower expression with overlapping distributions (median: 10.5, 10.9, 11.5, respectively), suggesting that 0-2+ tumors do not represent distinct biological groups, but rather a continuum of low expression. We defined a HER2 RNA Low threshold (&gt;10.6), corresponding to the top 75% of IHC 1-2+ breast cancers. Importantly, at this threshold, nearly half (44.1%) of 0+ breast cancers were also classified as HER2 RNA Low. Additionally, 25.8% of all non-breast solid tumors were classified as HER2 RNA Low. Given that HER2 RNA High predicted benefit from 1st generation anti-HER2 therapies, future studies should consider HER2 RNA Low as an alternative biomarker to Her2 IHC Low, with the opportunity to further expand trastuzumab deruxtecan use into the IHC 0+ breast cancer population and potentially to additional solid tumors.
Citation Format: Laura E. Lamb, Nickolay A. Khazanov, Daniel H. Hovelson, Kat Kwiatkowski, D. Bryan Johnson, Daniel R. Rhodes, Scott A. Tomlins. Evaluation of Her2 RNA expression as a potential predictive biomarker for anti-Her2 therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 968.