Prognostic/Predictive Immunohistochemistry Assays for Estrogen Receptor–Positive Breast Cancer: Back to the Future?

Given its identity as the first common malignancy with a molecular target, it is not surprising that breast cancer is the battleground for the many controversies that surround the use of multiparametric biomarker assays to guide cancer therapy. Historically, one of the most important classifications for breast cancer is its division into estrogen receptor (ER) –positive and ER-negative disease, because this division has important prognostic and predictive implications for the use of endocrine therapy. During the last 10 years, breast cancer classification has been refined and extended through transcriptional profiling, whole genome and exome sequencing, as well as other sophisticated molecular assays with the hope that such knowledge will facilitate determination of the exact prognosis and correct treatment plan for an individual patient. No fewer than a dozen such assays are in various phases of development or application for early breast cancer. Their credentialing raises a number of critical questions. In the article that accompanies this editorial, Bartlett et al describe the performance of a multiprotein immunohistochemical (IHC) assay called Mammostrat in tumors from patients who were enrolled on the Tamoxifen versus Exemestane Adjuvant Multicenter (TEAM) trial. First reported by Ring et al, this five-marker panel (p53, NDRG1, CEACAM5, SLC7A5, and HTF9C) was developed to assess prognosis in patients with early-stage, ER-positive breast cancer and was tested further in several additional cohorts, largely in the setting of adjuvant tamoxifen. The markers used in the panel were purposely selected to be distinct from conventional breast cancer markers such as ER, progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), or Ki-67. They represent components of DNA damage and cell cycle (p53, HTF9C), stress response (NDRG1), nutritional (SLC7A5), and differentiation (CEACAM5) pathways that are linked to cell growth and differentiation. Their expression is qualitatively assessed and then combined into a quantitative risk index for recurrence using a defined mathematical algorithm to identify individuals with low (score 0), moderate (score 0 and 7), or high (score 7) risk of recurrence. This algorithm is similar to the more commonly used 21-gene test (Oncotype DX assay; Genomic Health, Redwood City, CA), and the less well-known sevengene test (bioTheranostics Breast Cancer Index; bioTheranostics, San Diego, CA). The study by Bartlett et al is the largest evaluation of the Mammostrat (Clarient, Aliso Viejo, CA) prognostic panel to date. Tissue microarrays with triplicate cores derived from 4,598 patients who were enrolled onto the TEAM trial (which randomly assigned postmenopausal patients with early-stage, hormone receptor–positive breast cancer to 5 years of exemestane or a sequence of tamoxifen followed by exemestane) were assessed in a central laboratory. In multivariate regression analyses that were corrected for conventional clinicopathologic parameters, the Mammostrat assay provided significant additional information on distant recurrence-free survival after endocrine therapy in patients with ER-positive, node-negative disease, as well as all patients not exposed to chemotherapy, and in the entire patient cohort (n 3,837). Disappointingly, but not surprisingly, no differences were seen between the two endocrine regimens. No markers that can reliably differentiate between the benefit of tamoxifen or the aromatase inhibitor have been identified to date. The utility of the assay to predict for response to chemotherapy, which is arguably the most important question for management of early-stage, hormoneresponsive breast cancer, could not be assessed. The strengths of this study include the use of tissues from a well-designed, well-conducted clinical trial of substantial size with clear outcomes and the performance of the assay in a central laboratory using predefined guidelines. Because Mammostrat is an IHC assay, it has a distinct advantage over nonmorphologic molecular assays in that it evaluates protein expression at the in situ level. The results are therefore not confounded by the admixture of normal or proliferative breast tissue, the presence of inflammatory cells, or biopsy site changes that are commonly present within the tumor tissue block and can complicate interpretation of assays such as Oncotype DX that use mRNA isolated from tissue sections. Like many such analyses, the study does suffer from the common problem of the vanishing denominator. A total of 9,778 patients were enrolled onto the TEAM trial and blocks were recovered from 4,598 patients to construct the tissue microarrays; Mammostrat JOURNAL OF CLINICAL ONCOLOGY E D I T O R I A L VOLUME 30 NUMBER 36 DECEMBER 2