Abstract P2-10-31: Correlation of quantitative p95HER2 and total HER2 levels with clinical outcomes in a combined analysis of two cohorts of trastuzumab-treated metastatic breast cancer patients

Abstract

Abstract Background: Expression of p95HER2 (p95), a truncated form of HER2 also known as p110 or M611-CTF, is a possible trastuzumab resistance mechanism and has been associated with poor prognosis in trastuzumab-treated HER2-positive metastatic breast cancer (MBC). Previously we reported on optimal clinical cutoffs for quantitative p95 (Clin Cancer Res, 16:4226, 2010) and quantitative HER2 protein expression (H2T) by HERmark® (Cancer, 116:5168, 2010) that defined patient subsets with different progression-free survival (PFS). These cutoffs were confirmed in an independent trastuzumab-treated MBC cohort (ASCO 2011, #586). Here, using individual patient data, we performed an analysis on the combined data set of 243 cases from the discovery and validation cohorts to derive optimal cutoffs for quantitative p95 and H2T. Methods: Both quantitative H2T (HERmark, Monogram Biosciences) and p95 assays employed the VeraTag® method to quantify protein expression in formalin-fixed, paraffin-embedded tumor samples from two cohorts of 101 and 142 cases of trastuzumab-treated MBC with 7.4 and 9.2 months median PFS, respectively. All analyses were stratified by hormone receptor status, tumor grade (3 vs. 1+2) and cohort. H2T measurements were compared to pre-specified cutoffs for HERmark negative (H2T<10.5 Relative Fluorescence/mm2 tumor [RF/mm2]) and HERmark positive (H2T>17.8 RF/mm2), derived from the <5th percentile of centrally determined HER2-positives and the >95th percentile of centrally determined HER2-negatives, respectively, within a reference database of 1,090 breast cancer patient samples. Results: Patients classified as HERmark-positive had longer PFS than those classified as HERmark-negative (HR = 0.52; p = 0.0006; medians 10.0 and 5.9 months). The previously determined optimal H2T cutoff of 13.8 RF/mm2 in the center of the HERmark-equivocal zone, gave a similar result (HR = 0.54; p = 0.0005). This was close to the optimal cutoff of 12.75 RF/mm2 (HR = 0.48; p < 0.0001, unadjusted) for the combined data set. The PFS for the small group of patients in the HERmark-equivocal zone (n = 20) was more similar to the HERmark-negatives (equivocal vs. negative: HR=0.98; p = 0.9) than the HERmark-positives (positive vs. equivocal: HR=0.57; p = 0.057). The pre-specified p95 cutoff at 2.8 RF/mm2 separated the 174 HERmark-positive cases into two groups of longer (p95<2.8 RF/mm2) vs. shorter PFS (HR = 1.9; p = 0.0014; medians 13.1 and 7.4 months). Increasing continuous p95 also correlated with shorter PFS (HR = 1.9/log; p = 0.022) in the HERmark-positive subset. An optimal p95 cutoff was identified at 2.7 RF/mm2 (HR = 2.0; p = 0.0009, unadjusted), although a slightly higher local HR maximum was found at 1.55 RF/mm2 (HR = 2.3; p = 0.0004, unadjusted). Conclusions: HERmark positive and negative categories, defined by analytical comparison with centrally determined HER2 status, were confirmed to have significantly different PFS in trastuzumab-treated MBC patients. The optimal H2T clinical cutoff for this combined analysis was centered in the HERmark analytical equivocal zone. An optimal p95 clinical cutoff of 2.7 RF/mm2 derived from this combined analysis was nearly identical to the previously established cutoff of 2.8 RF/mm2. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P2-10-31.