In this study, we prospectively evaluated how intratumor heterogeneity of HER2 affects response to T-DM1 plus pertuzumab. driver of therapeutic resistance. These data suggest HER2 heterogeneity is usually associated with resistance to HER2-targeted therapy and should be considered in efforts to optimize treatment strategies. (encoding human epidermal growth factor receptor 2 [HER2]) defined a subset of breast cancers with aggressive clinical features and poor outcomes (2,3). However, the development of the HER2-specific Rabbit Polyclonal to MRPL20 monoclonal antibodies trastuzumab and pertuzumab and the antibody-drug conjugate trastuzumab emtansine (T-DM1), which consists of trastuzumab covalently linked to an anti-microtubule cytoxic agent, have markedly decreased recurrence rates of patients with early-stage HER2-positive breast malignancy (4C6). The relevance of HER2 as a therapeutic target underscores the importance of accurate HER2 screening. The routine use of HER2 immunohistochemistry (IHC) and hybridization (ISH) assays followed a seminal study demonstrating that the benefit of trastuzumab was restricted to patients diagnosed with HER2-positive tumors (7,8). Over the years, published guidelines from your American Society of Clinical Oncology (ASCO) and the College of American Pathologists (CAP) have optimized thresholds and recommendations to define HER2 positivity (9C11). With the widespread use of HER2 screening, retrospective studies have reported different patterns of HER2 expression, coining the term HER2 heterogeneity (12). In parallel, diagnostic guidelines proposed definitions for HER2 heterogeneity (13C15), but its relevance in clinical practice has not been prospectively evaluated. Defining the impact of HER2 heterogeneity on responses to targeted anti-HER2 therapies is usually of particular importance as we endeavor to de-escalate standard therapeutic regimens and rely more on targeted anti-HER2 therapies for patients diagnosed with early-stage HER2-positive breast malignancy (16,17). In this Fusidate Sodium study, we aimed to determine the effect of HER2 heterogeneity on response to therapy. We hypothesized that tumors with heterogeneity for HER2 amplification would have lower rates of pathologic total response (pCR) when treated with a HER2-targeted regimen in the absence of standard chemotherapy. To test this hypothesis, we conducted a prospective study in which patients diagnosed Fusidate Sodium with HER2-positive breast malignancy were treated with T-DM1 in combination with pertuzumab prior to surgery. The specificity and potency of T-DM1 and pertuzumab against HER2-amplified cells were crucial components to the study design. Image-guided research biopsies performed prior to treatment initiation allowed a central pathology evaluation of HER2 heterogeneity. The study was powered to assess the impact of HER2 heterogeneity on the probability of achieving a pCR after a course of targeted anti-HER2 therapy. RESULTS Patients and Treatment A total of 164 patients were enrolled in the study from January 2015 to January 2018 (Fig. 1A and (Supplementary Table S1). Patients received 6 cycles of T-DM1 and pertuzumab (Fig. 1B). The baseline demographic and clinical characteristics of the enrolled patients are outlined in Table 1. Of all patients, 163 were treated with at least one dose of T-DM1 and pertuzumab. Central confirmation of HER2 status to define eligibility classified 74% (121/163) of cases as HER2 3+ by IHC and 25% (40/163) as HER2 2+. HER2 2+ cases were confirmed to be HER2-positive by fluorescence in situ hybridization (FISH) prior to study enrollment. HER2 positivity was defined by FISH without IHC information in two cases (1%, 2/163). All but one patient (99.4%) had either stage II or III malignancy at presentation. Two-thirds (68.7%) of tumors were classified as hormone receptor (HR)-positive and the remaining tumors as HR-negative. Open in a separate window Physique 1. Study Design.A, CONSORT diagram. B, Study design. Centrally confirmed HER2-positive breast malignancy Fusidate Sodium patients were treated in a single arm study. Treatment consisted of six infusions of T-DM1 given in combination with pertuzumab. C, Example of central pathology evaluation of HER2 heterogeneity assessed by FISH, with CEP17 probe in green and in reddish. copy number counting was performed in three different areas per core biopsy site counting approximately 50 cells in each area. Scale bar corresponds to 10 m. D, A.