Abstract 687: A comparative study of p95-HER2 carboxy terminal fragment (CTF) detected by immunohistochemistry and VeraTag assays in human breast tumors (2012)

Abstract C-terminally truncated forms of HER2 (HER2-CTF) have been reported to be highly oncogenic and correlate with node-positive breast cancer, poor prognosis, and attenuated response to trastuzumab in patients with HER2+ metastatic breast cancer (MBC). A quantitative, sensitive and specific immunoassay using the VeraTag™ technology has been developed to detect the active Met611-HER2 CTF (p95) in FFPE tumor samples and has been used to identify HER2+ MBC patients who have a reduced response to trastuzumab in two independent cohorts. Recently, an IHC assay using an antibody directed to a similar p95 epitope was reported to detect a high percentage of HER2+ samples (79%) as p95 positive (#530, ASCO 2011). To verify the cellular specificity and accuracy of the VeraTag p95 assay, and evaluate the prevalence of p95 in breast tumors, an IHC assay was developed which utilizes the same p95 antibody as the VeraTag assay. The IHC and VeraTag assay formats were compared in 120 human breast tumor excisional and core biopsy samples submitted to central clinical reference lab testing, and having different levels of HER2 measured by HercepTest IHC: 0 (n=20); 1+ (n=20), 2+ (n=20), and 3+ (n=60). Replicate slides were run in both VeraTag and IHC assays to demonstrate reproducibility. Nearly all p95 IHC signal was associated with tumor cells only. Consistent with earlier VeraTag p95 data, a general correlation was observed between p95 and HER2 levels, with tumors expressing normal levels of HER2 rarely expressing high p95 levels. However, within the subset of HER2+ samples (IHC 3+ or FISH &amp;gt; 2.2; n=63), a ∼30-fold range of p95 VeraTag signal was observed, with 44% having p95 expression above the level of a previously determined clinical cutoff for trastuzumab response (Clin Cancer Res 16: 4226 2010). A similar relationship was observed for samples assayed by p95 IHC. All samples measured as p95 IHC 3+ (&amp;gt;10% cells with 3+ membranous staining) were HER2+ by central lab testing and represented nearly a third of the HER2+ samples. The VeraTag p95 assay generally agreed with p95 IHC scored by test category 0-3+ (r=0.54; p&amp;lt;0.0001) or histoscore (r=0.54; p&amp;lt;0.0001) with 79% of the p95 IHC 3+ samples falling above the p95 VeraTag assay clinical cutoff. Fewer p95 VeraTag positive samples were represented in p95 IHC 2+ and 1+ categories. These results indicate that the tumor-specific p95 signal detected by the p95 VeraTag assay is in general agreement with an IHC assay using the same antibody; however a number of cases with p95 VeraTag scores above the clinical cutoff do not fall into the p95 IHC 3+ category. Additionally, the prevalence of p95 positivity by VeraTag assay and by IHC 3+ scores is consistent with earlier peer-reviewed studies and lower than recently reported using a different IHC assay. The clinical significance of using the p95 IHC assay to complement the VeraTag p95 score is being evaluated. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 687. doi:1538-7445.AM2012-687