Quantitative measurements of p95HER2 (p95) and total HER2 (H2T) protein expression in patients with trastuzumab-treated, metastatic breast cancer (MBC): Independent confirmation of clinical cutoffs.

Abstract

586 Background: Expression of p95 in HER2-positive breast cancer patients is potentially a major determinant of trastuzumab resistance because p95 lacks the trastuzumab binding site while retaining kinase activity, and is associated with poor prognosis. Previously, optimal cutoffs for p95 (Clin Cancer Res, 16:4226, 2010) and H2T (Cancer, 116:5168, 2010) were defined in a training set of trastuzumab-treated MBC patients. Methods: Quantitative H2T and p95 assays (VeraTag, Monogram Biosciences) were retrospectively performed on formalin-fixed, paraffin-embedded samples from an independent series of 142 trastuzumab-treated MBC patients. Previously-defined cutoffs for H2T and p95 were tested to determine whether H2T above the cutoff correlated with better outcomes and whether p95 above the cutoff in the HER2-positive subset correlated with worse outcomes, as had been observed in the training dataset. Results: H2T above the pre-defined cutoff was found to correlate with longer progression-free survival (HR=0.61; p=0.044; stratified by hormone receptor status and tumor grade). Additionally, in the subset of patients assessed as H2T-positive, p95 values above the pre-defined cutoff correlated with shorter progression-free survival (HR=1.9; p=0.018; stratified by hormone receptor status). Conclusions: Optimal H2T and p95 cutoffs derived from a previous training dataset were confirmed in a second independent clinical series. The observed consistency in H2T and p95 cutoffs as measured by VeraTag (HERmark) across different cohorts of trastuzumab-treated MBC in two independent exploratory analyses justifies additional studies employing blinded analyses in larger series of patients, and are currently planned in order to formally validate these cutoffs.