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The HER-2 Receptor and Breast Cancer: Ten Years of Targeted Anti–HER-2 Therapy and Personalized Medicine

Writer and Curator:  Larry H Bernstein, MD, FCAP

LPBI

3.3 The HER-2 Receptor and Breast Cancer: Ten Years of Targeted Anti–HER-2 Therapy and Personalized Medicine

Jeffrey S. Ross,  Elzbieta A. Slodkowska,  W. Fraser Symmans, et al.
The Oncologist 2009; 14:320 –368
http://cme.theoncologist.com

Objectives:

  1. Contrast the current strengths and limitations of the three main slide-based techniques (IHC, FISH, and CISH) currently in clinical use for testing breast cancer tissues for HER-2 status.
  2. Compare the efficacy of trastuzumab- and lapatinib-based regimens in the adjuvant and metastatic settings as reported in published clinical trials and regulatory approval databases.
  3. Contrast the list of biomarkers that have been associated with clinical resistance to trastuzumab and lapatinib and describe their current level of validation.

The human epidermal growth factor receptor (HER-2) oncogene encodes a transmembrane tyrosine kinase receptor that has evolved as a major classifier of invasive breast cancer and target of therapy for the disease. The validation of the general prognostic significance of HER-2 gene amplification and protein overexpression in the absence of anti–HER-2 targeted therapy is discussed in a study of 107 published studies involving 39,730 patients, which produced an overall HER-2– positive rate of 22.2% and a mean relative risk for overall survival (OS) of 2.74. The issue of HER-2 status in primary versus metastatic breast cancer is considered along with a section on the features of metastatic HER- 2–positive disease. The major marketed slide-based HER-2 testing approaches, immunohistochemistry, fluorescence in situ hybridization, and chromogenic in situ hybridization, are presented and contrasted in detail against the background of the published American Society of Clinical Oncology–College of American Pathologists guidelines for HER-2 testing. Testing issues, such as the impact of chromosome 17 polysomy and local versus central HER-2 testing, are also discussed. Emerging novel HER-2 testing techniques, including mRNA-based testing by real-time polymerase chain reaction and DNA microarray methods, HER-2 receptor dimerization, phosphorylated HER-2 receptors, and HER-2 status in circulating tumor cells, are also considered. A series of biomarkers potentially associated with resistance to trastuzumab is discussed with emphasis on the phosphatase and tensin homologue deleted on chromosome ten/Akt and insulin-like growth factor receptor pathways. The efficacy results for the more recently approved small molecule HER- 1/HER-2 kinase inhibitor lapatinib are also presented along with a more limited review of markers of resistance for this agent. Additional topics in this section include combinations of both anti–HER-2 targeted therapies together as well as with novel agents including bevacizumab, everolimus, and tenespimycin. A series of novel HER-2–targeting agents is also presented, including pertuzumab, ertumaxomab, HER-2 vaccines, and recently discovered tyrosine kinase inhibitors. Biomarkers predictive of HER-2 targeted therapy toxicity are included, and the review concludes with a consideration of HER-2 status in the prediction of response to non–HER-2 targeted treatments including hormonal therapy, anthracyclines, and taxanes.

Biology, Pathology, Diagnosis, And Clinical Significance Of Her-2–Positive Breast Cancer

The human epidermal growth factor receptor 2 (HER-2, HER-2/neu, c-erbB-2) gene, first discovered in 1984 by Weinberg and associates [1], is localized to chromosome 17q and encodes a transmembrane tyrosine kinase receptor protein that is a member of the epidermal growth factor receptor (EGFR) or HER family (Fig. 1) [2]. This family of receptors is involved in cell– cell and cell–stroma communication primarily through a process known as signal transduction, in which external growth factors, or ligands, affect the transcription of various genes, by phosphorylating or dephosphorylating a series of transmembrane proteins and intracellular signaling intermediates, many of which possess enzymatic activity. Signal propagation occurs as the enzymatic activity of one protein turns on the enzymatic activity of the next protein in the pathway [3]. Major pathways involved in signal transduction, including the Ras/mitogen-activated protein kinase pathway, the phosphatidylinositol 3 kinase (PI3K)/Akt pathway, the Janus kinase/signal transducer and activator of transcription pathway, and the phospholipase C pathway, ultimately affect cell proliferation, survival, motility, and adhesion. Receptor activation requires three variables, a ligand, a receptor, and a dimerization partner [4]. After a ligand binds to a receptor, that receptor must interact with another receptor of identical or related structure in a process known as dimerization in order to trigger phosphorylation and activate signaling cascades. Therefore, after ligand binding to an EGFR family member, the receptor can dimerize with various members of the family (EGFR, HER-2, HER-3, or HER-4). It may dimerize with a like member of the family (homodimerization) or it may dimerize with a different member of the family (heterodimerization). The specific tyrosine residues on the intracellular portion of the HER-2/neu receptor that are phosphorylated, and hence the signaling pathways that are activated, depend on the ligand and dimerization partner. The wide variety of ligands and intracellular crosstalk with other pathways allow for significant diversity in signaling. Although no known ligand for the HER-2 receptor has been identified, it is the preferred dimerization partner of the other family members. HER-2 heterodimers are more stable [5, 6] and their signaling is more potent [7] than receptor combinations without HER-2. HER-2 gene amplification and/or protein overexpression has been identified in 10%–34% of invasive breast cancers [1]. Unlike a variety of other epithelial malignancies, in breast cancer, HER-2 gene amplification is uniformly associated with HER-2 (p185neu) protein overexpression and the incidence of single copy overexpression is exceedingly rare [8]. HER-2 gene amplification in breast cancer has been associated with increased cell proliferation, cell motility, tumor invasiveness, progressive regional and distant metastases, accelerated angiogenesis, and reduced apoptosis [9].When classified by routine clinicopathologic parameters and compared with HER-2– negative tumors, HER-2–positive breast cancer is more often of intermediate or high histologic grade, more often lacking estrogen receptors (ERs) and progesterone receptors (PgRs) (ER and PgR negative), and featuring positive lymph node metastases at presentation [1]. In the recent molecular classification of breast cancer, positive HER-2 status does not constitute a unique molecular category and is identified in both the “HER-2” and “luminal” tumor classes [10].

Figure 1 (not shown)

Figure 1. The human epidermal growth factor receptor (HER) gene family. This image depicts the complex crosstalk between members of the HER family of receptor tyrosine kinases and intracellular signaling. Activated HER receptors can function to both stimulate and inhibit downstream signaling of members of other biologic pathways. Note that HER-2 has no activating ligands and HER-3 lacks a tyrosine kinase domain. HER-2–mediated signaling is associated with cell proliferation, motility, resistance to apoptosis, invasiveness, and angiogenesis. The figure shows the complexity of signaling pathways initiated by, and influenced by, HER family protein receptors at the cell surface.

HER-2 Status and Prognosis in Breast Cancer Both morphology-based and molecular-based techniques have been used to measure HER-2/neu status in breast cancer clinical samples [11–117]. By a substantial majority, abnormalities in HER-2 expression at the gene, message, or protein level have been associated with adverse prognosis in both lymph node–negative and lymph node–positive breast cancer. Of the 107 studies considering 39,730 patients listed in Table 1, 95 (88%) of the studies determined that either HER-2 gene amplification or HER-2 (p185 neu) protein overexpression predicted breast cancer outcome on either univariate or multivariate analysis. In 68 (73%) of the 93 studies that featured multivariate analysis of outcome data, the adverse prognostic significance of HER-2 gene, message, or protein overexpression was independent of all other prognostic variables. In only 13 (12%) of the studies, no correlation between HER-2 status and clinical outcome was identified. Of these 13 noncorrelating studies, eight (62%) used immunohistochemistry (IHC) on paraffin-embedded tissues as the HER-2/protein detection technique, two (15%) used fluorescence in situ hybridization (FISH), two (15%) used Southern analysis, and one (7%) used a real-time polymerase chain reaction (RT-PCR) technique. Of the 15 studies that used the FISH technique, 13 (87%) showed univariate prognostic significance of gene amplification, and 11 of these (85%) showed prognostic significance on multivariate analysis as well. The two studies that used chromogenic in situ hybridization (CISH) HER-2 gene amplification detection techniques both found that HER-2 amplification was an independent predictor of outcome on multivariate analysis [100, 112]. However, interpretation of these studies is complicated by the fact that most studies included patients who received variable types of systemic adjuvant therapy; therefore, the pure prognostic value of HER-2 overexpression in the absence of any systemic adjuvant therapy is incompletely understood.

Table 1 HER-2 status and prognosis in breast cancer (not shown)

HER-2 Positivity Rates The frequency of HER-2 positivity in all of the studies presented in Table 1 was 22.2%, with a range of 9%–74%. The HER-2–positive rate was similar for IHC, at 22% (range, 10%–74%), and FISH, at 23.9% (range, 14.7%– 68%). In current practice, HER-2–positive rates have trended below 20%, with most investigators currently reporting that the true positive rate is in the range of 15%–20%. The HER-2– positive rate may be higher when metastatic lesions are tested, and tertiary hospitals and cancer centers report slightly higher rates than community hospitals and national reference laboratories. Relative Risk and Hazard Ratio In Table 1, a number of studies provided data as to the relative risk (RR) of untreated HER-2–positive breast cancer being associated with an adverse clinical outcome. For OS, the mean RR was 2.74 (range, 1.39 – 6.93) and the median was 2.33; for disease-free survival (DFS), the mean RR was 2.04 (range, 1.30 –3.01) and the median was 1.8. In several studies, the RR was estimated with a hazard ratio (HR) model. The mean HR was 2.12 (range, 1.6 –2.7) and the median was 2.08. HER-2 Expression and Breast Pathology The association of HER-2–positive status with specific pathologic conditions of the breast is summarized in Table 2. HER-2 overexpression has been consistently associated with higher grades and extensive forms of ductal carcinoma in situ (DCIS) and DCIS featuring comedo-type necrosis [118 –121]. The incidence of HER-2 positivity in DCIS has varied in the range of 24%–38% in the published literature, which appears to be slightly higher than that for invasive breast cancer [118 –121]. Routine testing for HER-2 status in DCIS is not widely performed. However, should anti– HER-2 targeted therapies directed at HER-2–positive DCIS result in a reduction in the development of invasive disease, the widespread use of HER-2 testing in DCIS would be adopted. Finally, the invasive carcinoma that develops in association with HER-2–positive DCIS may, on occasion, not feature a HER-2–positive status, a finding that has led investigators to believe that HER-2 gene amplification may not be required for the local progression of breast cancer [122]. Compared with invasive ductal carcinoma (IDC), HER-2 gene amplification occurs at a significantly lower rate in invasive lobular carcinoma (ILC) (10%), but has also been linked to an adverse outcome [85]. HER-2 positivity is linked exclusively to the pleomorphic variant of ILC and is not encountered in classic ILC [123]. HER-2 amplification is strongly correlated with tumor grade in both IDC and ILC. For example, in one study, only one of 73 grade I IDC cases and one of 67 low-grade classic ILC cases showed HER-2 amplification detected by FISH [86]. HER-2 overexpression and HER-2 amplification have been a consistent feature of both mammary and extramammary Paget’s disease [124, 125] (Fig. 2). HER-2 amplification and HER-2 overexpression have been associated with adverse outcome in some studies of male breast carcinoma [126 –129], but not in others [130 –132]. The incidence of HER-2 positivity appears to be lower in male breast cancer than in female breast cancer [126 –132]. Documented responses in male breast cancer to HER-2–targeting agents have been described, and therefore treatment with trastuzumab is an acceptable option for these patients, but the true activity rate remains uncertain [133]. The rate of HER-2 overexpression in mucinous (colloid) breast cancers is extremely low, although, on occasion, it has been associated with aggressive disease [134 –136]. In medullary breast carcinoma, HER-2 testing has consistently found negative results [137]. Similarly, HER-2 positivity is extremely rare in cases of tubular carcinoma [138]. HER-2 status has not been consistently linked to the presence of inflammatory breast cancer [139, 140]. Molecular studies of hereditary breast cancer including cases with either BRCA1 or BRCA2 germline mutations have found a consistently lower incidence of HER-2–positive status for these tumors [141].

Figure 2 not shown

Figure 2. Human epidermal growth factor receptor (HER)-2–positive Paget’s disease of the nipple. In this patient, who presented with HER-2–positive invasive duct carcinoma, classic clinical features of Paget’s disease of the nipple were present. A section of the nipple from the mastectomy specimen shows 3+ continuous cell membrane immunoreactivity for HER-2 protein. Nearly 100% of Paget’s disease of the breast cases are HER-2 positive (see text).

Breast sarcomas and phyllodes tumors have consistently been HER-2 negative [142]. Finally, low-level HER-2/neu overexpression has been identified in benign breast disease biopsies and is associated with a greater risk for subsequent invasive breast cancer [143].

HER-2 Status in Primary Versus Metastatic Breast Cancer The majority of studies that have compared the HER-2 status in paired primary and metastatic tumor tissues have found an overwhelming consistency in the patient’s status regardless of the method of testing (IHC versus FISH) [144 –151]. However, several recent studies indicated 20%–30% discordance rates between the HER-2 status of primary and metastatic lesions. Some of these studies have featured relatively high HER-2–positive rates on both paired specimens (> 35% positive), which has created concern about the conclusions of these reports [152]. Also, considering that 10%–30% discordance rates have been reported even when the same tumor is tested repeatedly, it remains uncertain if the discordance rates seen between primary and metastatic sites is higher than expected by the less than perfect reproducibility of the various HER-2 assays. Increasingly, emerging data suggest that there are changes in HER-2 expression between primary and metastatic disease. This is particularly true after intervening HER-2– directed therapy, but also happens in the absence of such treatment. In cases where the original primary HER-2 test result is questioned because of technical or interpretive issues and in patients where there has been an unusually long (i.e., > 5-year) interval between the primary occurrence and the detection of metastatic disease, retesting of a metastatic lesion may be warranted. Thus, although routine HER-2 testing of metastatic disease is advocated by some investigators, the preponderance of data indicates that the HER-2 status remains stable and that routine retesting of HER-2 may not be needed for most patients with metastatic disease.

Features of Metastatic HER-2–Positive Breast Cancer Metastatic HER-2–positive breast cancer retains the phenotype of the primary tumor not only in HER-2 status, but also is typically ER/PgR negative, moderate to high tumor grade, DNA aneuploid with high S phase fraction, and featuring ductal rather than lobular histology. In the era prior to the initiation of HER-2–targeted therapy, HER-2–positive breast cancer was more likely to spread early to major visceral sites including the axillary lymph nodes, bone marrow, lungs, liver, adrenal glands, and ovaries [153]. In the post–HER-2 targeted therapy era, the incidence of progressive visceral metastatic disease in HER-2–positive tumors has diminished and has frequently been superseded by the development of clinically significant central nervous system (CNS) metastatic disease [154 –157]. It is widely held that the success in the control of visceral disease with trastuzumab has unmasked previously occult CNS disease and, because of the inability of the therapeutic antibody to cross the blood– brain barrier, allowed brain metastases to progress during the extended OS duration of treated patients [154, 155]. The small-molecule drug lapatinib has shown some promise for targeting HER-2–positive CNS metastases that are resistant to trastuzumab-based therapies in initial studies [158].

Interaction of HER-2 Expression with Other Prognosis Variables HER-2 gene amplification and protein overexpression have been associated consistently with high tumor grade, DNA aneuploidy, high cell proliferation rate, negative assays for nuclear protein receptors for estrogen and progesterone, p53 mutation, topoisomerase IIa amplification, and alterations in a variety of other molecular biomarkers of breast cancer invasiveness and metastasis [159 –161].

Figure 3. Human epidermal growth factor receptor (HER)-2 testing.
(not shown)  (A): Immunohistochemistry (IHC). This panel depicts the four categories of HER-2 IHC staining including 0 and 1+ (negative), 2+ (equivocal), and 3+ (positive) using the American Society of Clinical Oncology–College of American Pathologists guidelines for HER-2 IHC scoring. (B): Fluorescence in situ hybridization (FISH). This panel demonstrates a case of invasive duct carcinoma, on the left, negative for HER-2 gene amplification (gene copy number < 4) and a case of HER-2 gene–amplified breast cancer (gene copy number > 6),

FISH. The FISH technique (Fig. 3B), like IHC, is a morphology-driven slide-based DNA hybridization assay using fluorescent-labeled probes. Both the hybridization steps and the slide scoring can be automated. FISH has the advantages of a more objective scoring system and the presence of a built-in internal control consisting of the two HER-2 gene signals present both in benign cells and in malignant cells that do not feature HER-2 gene amplification.

IHC Versus FISH. Although the FISH method is more expensive and time-consuming than IHC, numerous studies have concluded that this cost is well borne by the greater accuracy and more precise use of anti–HER-2 targeted therapies [179 –180, 182–183]. FISH is considered to be more objective and reproducible in a number of systematic reviews [165, 180, 183–186]. In one study, the concordance rates between IHC and FISH were highest in tumors scored by IHC as 0 and 1+ and lowest for 2+ and 3+ cases [183]. Currently, the majority (approximately 80%) of HER-2 testing in the U.S. commences with a screen by IHC, with results of 0 and 1+ considered negative, 2+ considered equivocal and referred for FISH testing, and 3+ considered positive. In a pharmacoeconomic study of patients being considered for trastuzumab-based treatment for HER-2– positive tumors, FISH was found to be a cost-effective diagnostic approach “from a societal perspective” [187].

CISH and Silver In Situ Hybridization. The CISH method (Fig. 3E) and silver in situ hybridization (SISH) method feature the advantages of both IHC (routine microscope, lower cost, familiarity) and FISH (built-in internal control, subjective scoring, the more robust DNA target) [190, 191]. The CISH technique uses a single HER-2 probe, detects HER-2 gene copy number only, and was recently approved by the FDA to define patient eligibility for trastuzumab treatment. The SISH method employs both HER-2 and chromosome 17 centromere probes hybridized on separate slides and is currently under review by the FDA. Numerous studies have confirmed a very high concordance between CISH and FISH, typically in the 97%–99% range [191–203]. Similar to FISH, CISH has its highest correlation with IHC 0, 1+, and 3+ results and lowest correlation with IHC 2+ staining.

Chromosome 17 Polysomy. The incidence of chromosome 17 polysomy has varied from as low as 4% to as high as 30% in studies of invasive breast cancer [204 –208]. This may reflect differences in the definition of polysomy ranging from a low-level definition of more than two copies per cell to a high of more than four copies per cell of the chromosome. Most studies have linked chromosome 17 polysomy with greater HER-2 protein overexpression [204 –207], but some have found that protein overexpression only occurs in the presence of selective HER-2 gene amplification [204].

The 2007 ASCO-CAP Guidelines. In early 2007, a combined task force from ASCO and the CAP issued a series of recommendations designed to improve the accuracy of tissue-based HER-2 testing in breast cancer [212]. A summary of the ASCO-CAP guidelines is provided in Table 4. Highlights of these recommendations include (a) standardizing fixation in neutral-buffered formalin for no less than 6 hours and no more than 48 hours, (b) unlike their respective FDA-approval specifications, defining equivocal zones for the IHC, FISH, and CISH tests, (c) establishing a standardized quality assurance program for testing laboratories, and (d) requiring the participation of these laboratories in a proficiency testing program [212]. The published guidelines were designed to improve the overall precision and reliability of all types of slide-based HER-2 tests and remained neutral as to the relative superiority of one test over the others.

Figure 4. Real-time polymerase chain reaction (RT-PCR). In this RT-PCR assay using the Taqman RT-PCR System (Applied Biosystems Inc., Foster City, CA), note the detection of increased human epidermal growth factor receptor(HER)-2 mRNA expression in green detected at lower numbers of amplification cycles compared with the two housekeeping genes shown in red and blue.

Figure 5. DNA microarray. In this image, increased expression of human epidermal growth factor receptor (HER)-2 mRNA has been detected using a proprietary DNA microarray system (Millennium Pharmaceuticals, Inc., Cambridge, MA). The microarray demonstrates the coexpression of seven genes (HER-2 is second from the bottom) related to the amplification of HER-2 DNA in this case of HER-2–positive breast cancer.

Her-2–Targeted Therapy and the Treatment of Her-2–Positive Breast Cancer

Trastuzumab: HER-2 Testing and the Prediction of Response to Trastuzumab Therapy Using recombinant technologies, trastuzumab (Herceptin; Genentech, South San Francisco, CA), a monoclonal IgG1 class humanized murine antibody, was developed by the Genentech Corporation to specifically bind the extracellular portion of the HER-2 transmembrane receptor. This antibody therapy was initially targeted specifically for patients with advanced relapsed breast cancer that overexpresses HER-2 protein [262]. Since its launch in 1998, trastuzumab has become an important therapeutic option for patients with HER-2–positive breast cancer and is widely used for its approved indications in both the adjuvant and metastatic settings (Fig. 6) [185, 263–265]. Although trastuzumab is approved as a single-agent regimen, most patients are treated with trastuzumab plus cytotoxic agents. Table 5 summarizes the significant clinical trials that contributed to the regulatory approvals of trastuzumab.

This topic is scheduled for another article.

Trastuzumab Combinations. Since the FDA approval in 1998 of two trastuzumab plus chemotherapy combinations, a number of additional approaches have gained favor in the clinical practice community. The National Comprehensive Cancer Network (NCCN) Clinical Practice Guidelines [284] currently recommend the following regimens for the first-line treatment of HER-2–positive MBC: trastuzumab plus single agents— either paclitaxel (every 3 weeks or weekly), docetaxel (every 3 weeks or weekly), or vinorelbine (weekly). For combination therapies, the NCCN recommends trastuzumab plus paclitaxel and carboplatin (every 3 weeks) or docetaxel plus carboplatin. Recently, carboplatin-based trastuzumab combinations have gained interest as a result of both the apparent boost in efficacy as measured by a higher overall response rate and longer progression-free survival time and the cardioprotective benefits of avoiding an anthracycline-containing regimen [285].

Neoadjuvant Setting The results of trastuzumab-based neoadjuvant studies (Table 5) have received significant recent interest in the oncology community [289]. Virtually all completed and in progress clinical trials have demonstrated a significant enhancement in the rate of pathologic complete response (pCR), the primary endpoint in these studies, in cases of patients with HER-2–positive breast cancer that received trastuzumab in the neoadjuvant setting [290 –297]. This benefit of the addition of trastuzumab in the neoadjuvant setting appears to be independent of, if not enhanced by, the coexistence of ER positivity [297]. Among the potential explanations for the apparent greater chemosensitivity of HER-2–positive tumors cotreated with trastuzumab in the neoadjuvant setting is the concept that HER-2 gene amplification is in some way related to the growth and survival of breast cancer stem cells [298, 299].

Biomarkers of Trastuzumab Resistance Since trastuzumab was introduced for the treatment of MBC in 1998, there has been growing interest in the discovery and potential clinical utility of biomarkers designed to predict resistance to the drug. Current approaches to HER-2 testing provide a negative predictor of drug response: the test does not predict which patients will respond to trastuzumab, it predicts which patients are unlikely to benefit.

Neoadjuvant Setting The Neo-ALTTO trial is a randomized, open-label, multicenter, phase III study comparing the efficacy of neoadjuvant lapatinib plus paclitaxel with that of trastuzumab plus paclitaxel and with concomitant lapatinib and trastuzumab plus paclitaxel given as neoadjuvant treatment in HER-2– positive primary breast cancer [337].

Biomarkers of Lapatinib Resistance In that lapatinib was approved 9 years after trastuzumab, considerably less information has been published concerning markers of efficacy or resistance to the drug [331, 341– 343].

Trastuzumab Since its introduction in the MBC setting and continuing throughout its advance into use in both the adjuvant and neoadjuvant settings, trastuzumab has been associated with the development of a variety of toxicities [384]. In the original registration trial for MBC, trastuzumab was associated with a variety of adverse events, including pain, gastrointestinal disturbances, minor hematologic deficiencies, pulmonary symptoms, and congestive heart failure (CHF) [265]. Cardiac toxicity has remained the most significant limiting factor for the use of trastuzumab [384 –389]. A major consideration in the development of cardiac toxicity in patients treated with trastuzumab has been their prior or concomitant exposure to anthracycline drugs, also associated with dose-dependent irreversible heart damage [384 – 389].

Lapatinib The most frequent adverse reactions in the lapatinib– capecitabine registration trial for MBC combination were diarrhea (65%), palmar–plantar erythrodysesthesia (53%), nausea (44%), rash (28%), vomiting (26%), and fatigue (23%) [332]. In a comprehensive analysis of the clinical trials featuring lapatinib in combination with various other agents, the overall incidence of LVEF decline was 1.6%, with 0.2% of patients experiencing symptomatic CHF [389].

HER-2 Status and the Prediction of Response to Non–HER-2 Targeted Therapy The use of HER-2 status to predict responsiveness or resistance to hormonal therapies, advocated by a number of oncologists, remains controversial. It has been reported that ER-positive/HER-2–positive patients are either less responsive or completely resistant to single-agent tamoxifen [391–393]. When measured as continuous variables, the expression of HER-2 appears to be inversely related to the expression of ER and PgR even in hormone receptor–positive tumors [394].

Anthracyclines HER-2 overexpression has also been associated with enhanced response rates to anthracycline-containing chemotherapy regimens in most, but not all, studies [42, 410 – 414].

Radiation Therapy Initially, in the era prior to the introduction of anti–HER-2 targeted therapy, HER-2–positive status was associated with a higher rate of local recurrence in some studies of breast cancer treated with surgery and radiation therapy alone, but not in others [427– 429]. However, although large-scale, randomized, prospective studies are lacking, HER-2–positive tumors treated with trastuzumab-based neoadjuvant chemotherapy combined with external-beam radiation have indicated a favorable response in locally advanced breast cancer [430].

Summary The history of the discovery of the HER-2 oncogene in an animal model in 1984, the translation of this finding to the clinical behavior of human breast cancer, and the introduction of the first anti-HER targeted therapy in 1998 is clearly a triumph of “bench to bedside” medicine. In the 10 years that have now passed since the regulatory approval of the first anti–HER-2 targeted therapy, trastuzumab, thousands of preclinical and clinical studies have considered HER-2 as a prognostic factor, its ability to predict response to hormonal and cytotoxic treatments, the best way to test for it in routine specimens, and the clinical efficacy of targeting it in a wide variety of clinical settings. Given the proven efficacy of trastuzumab and lapatinib for the treatment of MBC, and also in the adjuvant and neoadjuvant settings, the critical issue as to which test (IHC versus FISH versus CISH versus mRNA based) is the most accurate and reliable method to determine HER-2 status in breast cancer has continued to increase in importance.

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