Up to 25% of patients diagnosed with breast cancer have tumors that overexpress HER2. HER2-positive breast cancer is highly proliferative, difficult to treat, and confers a poor prognosis. The advent of the anti-HER2 monoclonal antibody trastuzumab (Herceptin) has markedly altered the clinical course of both early and advanced HER2-driven breast cancer. Despite the use of trastuzumab, however, patients with HER2-positive breast cancer still experience disease progression. Overcoming that resistance to therapy is our next challenge. This review examines the current understanding of HER2 biology, the mechanisms of action of and resistance to trastuzumab, as well as new therapies on the horizon.
Laboratory scientists have confirmed what oncologists have known for years—breast cancer is a heterogeneous disease with multiple potential biologic abnormalities driving tumor growth and response to therapy. One of the most clearly defined biologic abnormalities with clinical relevance is HER2 overexpression. The development of trastuzumab (Herceptin), an amazingly active HER2-targeted drug with effectiveness in both the metastatic and adjuvant settings, has altered the course of HER2-positive breast cancer. Our challenge now is to better understand the heterogeneity of HER2-driven breast cancer, the mechanism of action of trastuzumab, and the mechanisms of resistance to HER2-targeted therapy, all of which will enable us to develop new therapies to treat patients with HER2-positive breast cancer.
HER2 is a tyrosine kinase receptor protein expressed on the surface of epithelial cells in a variety of normal tissues including the breast. It is a member of a family of tyrosine kinase receptors known as the ErbB tyrosine kinase receptors. The family comprises four homologous receptors: HER1 (ErbB1, the epidermal growth factor receptor, or EGFR), HER2 (ErbB2), HER3 (ErbB3) and HER4 (ErbB4). The biology of these receptors has been extensively reviewed by several authors, and the reader is referred to their papers for detailed information.[2-4] In general, each receptor protein is composed of an extracellular binding domain, a transmembrane lipophilic segment and an intracellular tyrosine kinase domain with a regulatory carboxyl terminal segment. While HER1, HER2, and HER4 are all intact receptors, HER3 has an inactive tyrosine kinase domain. A soluble ligand has been identified for all members of the HER family except for HER2.
All of these receptors are activated by dimerization, either with an identical receptor (homodimerization) or with a different receptor of the same family (heterodimerization). Normally, HER2 activation occurs when a ligand binds to another HER family member heterodimerized with HER2. Ligand-binding activates the intracellular tyrosine kinase domain, which results in activation of downstream targets through phosphorylation. As illustrated in Figure 1, these targets include pathways involved in cell proliferation (via Ras/Raf/MAP-kinase) and survival (via PI3-Kinase/Akt).
In approximately 25% of breast cancers, up to 100-fold overexpression of the HER2 receptor occurs, primarily due to excess gene copy number.[5,6] In these HER2-overexpressing breast cancers, increased receptor density on the cellular surface probably results in HER2 homodimer formation leading to autoactivation, unregulated proliferation, escape from apoptosis, and transformation from benign to malignant cells.
Clinical Implications of HER2-Positive Breast Cancer
Although much is made of the breast cancer "intrinsic" subtypes identified by comprehensive gene-expression profiling,[7-11] increased HER2 expression is seen in at least two of the intrinsic subtypes. One, the HER2-positive/ER-negative subtype, is characterized by high expression of HER2-related genes and low expression of estrogen receptor-related genes. High HER2 expression can also be seen in the luminal (ER-positive) subtypes, although less frequently. It remains unclear whether these two groups, HER2-positive/hormone receptor-negative and HER2-positive/hormone receptor-positive, respond differently to HER2-directed therapy. Regardless of subtype, HER2 overexpression confers strong proliferative and survival impulses and is associated with larger tumors, higher likelihood of nodal involvement, high histologic tumor grade, aneuploidy, and poor outcome.[12-16]
HER2-Targeted Therapy With Trastuzumab
Trastuzumab—a humanized mouse monoclonal antibody targeted to an epitope on the extracellular domain of HER2—was the first US Food and Drug Administration (FDA)-approved drug to target the HER2 cell-signaling pathway. Clear evidence of the drug's clinical efficacy was first established in a phase III trial, in which previously untreated metastatic breast cancer patients with HER2-overexpressing tumors (2+ or 3+ by immunohistochemical [IHC] staining) were treated with standard chemotherapy with or without trastuzumab. Standard chemotherapy was defined as an anthracycline plus cyclophosphamide (AC) for chemotherapy-naive patients, or paclitaxel given once every 3 weeks for patients who had received adjuvant anthracycline. Chemotherapy was given simultaneously with trastuzumab, which was administered weekly until disease progression.
The addition of trastuzumab to chemotherapy resulted in a longer time to progression, a higher rate of objective response, longer survival and a lower rate of death at 1 year compared to chemotherapy alone. In addition, the duration of response was increased from 6 to 9 months, which speaks to both the effectiveness of the drug and the inevitability of acquired resistance. In general, the combination of chemotherapy plus trastuzumab was well tolerated. However, a surprisingly high level of cardiac-dysfunction (27%) was seen in the AC-plus-trastuzumab arm, compared to 8% in the AC-alone arm and 13% in the paclitaxel-plus-trastuzumab arm. This had not been predicted by preclinical or early clinical studies. For this reason, avoidance of cardio-toxicity is a major theme of research aimed at improving HER2-directed therapy.
Subsequent work has revealed that normal cardiac myocyte function depends upon HER2 expression. Interestingly, trastuzumab-related cardiotoxicity appears to differ from anthracycline-induced cardiotoxicity. Anthracycline-mediated cardiac damage is dose-related, infrequently reversible, and associated with pathologic changes in the myocardial muscle, whereas trastuzumab cardiotoxicity may be reversible, not dose-related, and not associated with specific pathologic changes in the myocardium.[20,21]
Trastuzumab was also found to be effective when used as a single agent in metastatic breast cancer patients with HER2-overexpressing tumors (2+ or 3+ by IHC), demonstrating a 26% objective response rate in the first line setting and 12% in pretreated patients. During these early years of HER2 targeting, the testing methods for HER2 expression improved, and the ambiguity of moderate (2+) IHC staining became clearer. An even greater clinical impact of the drug was seen when more stringent HER2 criteria were used; for example, when the first-line single-agent data were stratified by HER2 categories, the objective response rate was 35% in 3+ HER2 tumors vs 0% in 2+ tumors, and 34% in fluorescence in situ hybridization (FISH)-positive tumors (gene-amplified) vs 7% in FISH-negative tumors. These observations highlight the fact that accurate HER2 testing is crucial and that only patients with HER2-positivity defined as 3+ overexpression by IHC or gene amplification (FISH-positive) benefit from trastuzumab therapy.
Since these landmark trials, several other investigators have combined trastuzumab with various chemotherapy regimens in the metastatic setting, including vinorelbine, docetaxel (Taxotere), paclitaxel plus carboplatin, and even pegylated liposomal doxorubicin (Doxil).[25-27] In all these trials, the combination of trastuzumab plus chemotherapy was more effective than chemotherapy alone. The cardiac toxicity (defined as asymptomatic left-ventricular ejection fraction decline or symptomatic congestive heart failure) from trastuzumab plus chemotherapy ranged from 2% to 17%.
The benefit of trastuzumab in HER2-overexpressing breast cancer patients outside of the metastatic setting was first suggested in a randomized neoadjuvant trial of paclitaxel followed by FEC75 (fluorouracil, epirubicin [Ellence] at 75mg/m2, and cyclophosphamide) with or without trastuzumab. The addition of trastuzumab more than doubled the pathologic complete response rate (67% vs 25%) when compared to chemotherapy alone. However, resistance exists even in the most active trastuzumab-containing regimens given to the most treatment-naive patients; 3 of 23 patients had residual nodal disease even after 24 weeks of combined chemobiotherapy.
The proof of effectiveness in the adjuvant setting came with the reports from the joint analysis of the National Surgical Adjuvant Breast and Bowel Project (NSABP) B31 and North Central Cancer Treatment Group (NCCTG) N9831 trials,[29,30] the Breast Cancer International Research Group (BCIRG) 006 trial,[31,32] and the Herceptin Adjuvant (HERA) trials.[29,30] The B31/N9831 analysis and BCIRG 006 trial revealed that 1 year of trastuzumab added to AC followed by a taxane (AC-TH) is remarkably effective, producing an approximately 10% absolute improvement in disease-free survival even by 3 years; this benefit was seen equally across all clinically identifiable subgroups. BCIRG 006 found a slightly lower but statistically similar benefit with a nonanthracycline regimen (docetaxel, carboplatin, and trastuzumab, or TCH), which improved disease-free survival by 3%. HERA, in which HER2-positive patients received 1 year of adjuvant trastuzumab therapy vs observation after at least four cycles of neoadjuvant or adjuvant chemotherapy, demonstrated an absolute benefit of 9% at 2 years.
By avoiding concurrent anthracycline/trastuzumab therapy, as in the TCH arm of BCIRG 006, or giving the trastuzumab after chemotherapy, as in HERA, the incidence of clinical cardiotoxicity improves (approximately 2% vs 4%), but at the risk of potentially poorer efficacy. In all of these studies, 10% to 15% of patients relapsed in spite of highly effective regimens that involved more than 1 year of infusional trastuzumab therapy and a risk of clinical cardiotoxicity.
A substudy within BCIRG 006 suggests that the benefit of an anthracycline-based regimen such as AC-TH over a non-anthracycline-based regimen such as TCH may be primarily limited to HER2-positive tumors that also demonstrate topoisomerase II-alpha gene amplification. This intriguing but preliminary finding is consistent with earlier observations regarding topoisomerase II-alpha gene amplification and anthracycline sensitivity[33-35] and may identify a group in which the less cardiotoxic trastuzumab regimen (TCH) may be used without loss of clinical efficacy. None of the adjuvant trastuzumab trials identified a subset of patients more or less likely to benefit from trastuzumab therapy. Thus, unlike the emerging evidence that the benefit of chemotherapy, in part, can be predicted by hormone-receptor status, we do not yet have a predictive factor for trastuzumab efficacy in patients with HER2-positive breast cancer.
Dr. Carey has received research support from Bristol-Myers Squibb, Genentech, Boehringer-Ingelheim, and GlaxoSmithKline; serves on the speakers bureaus (unpaid) for Genentech, Pfizer, and Genomic Health; and is an advisory board member for GlaxoSmithKline, Bristol-Myers Squibb, and Genentech. Dr. Morris has received fellowship salary support from GlaxoSmithKline.
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