Revisiting HER2: Recent Research Shakes Views on Famed Marker

HER Receptors in Action

Genomic analyses have shed new light on the molecular drivers of HER activity that can activate cancer-causing pathways. Examples are illustrated above.

AR indicates amphiregulin; EGF, epidermal growth factor; HER, human epidermal growth factor receptor; HRG, heregulin; TGFα, transforming growth factor alpha; TK, tyrosine kinase.

Source: Recreated from Chang JC. Dual HER2 blockade: with and without chemotherapy. Presented at: 29th Annual Miami Breast Cancer Conference; March 14-17, 2012; Miami Beach, FL.

At least 20% of patients with breast cancer have tumors that demonstrate amplification of the gene encoding the human epidermal growth factor receptor (HER)-2 protein, a receptor tyrosine kinase. The development of a monoclonal antibody targeting HER2, trastuzumab (Herceptin), has had a huge impact on the survival of this population of patients.

HER2 and Breast Cancer

Currently, HER2 gene amplification guides clinical decision-making in breast cancer, and HER2-targeted drugs are only approved for use in patients designated HER2-positive. Yet new research suggests that these drugs may actually have much broader applications, benefiting patients who are not designated HER2-positive by routine testing, with far-reaching implications not just for the diagnosis and treatment of breast cancer, but for the understanding of the molecular drivers of cancers as a whole and the development of targeted therapies in general.In 1998, trastuzumab became the first HER2-targeted therapy to be approved by the FDA for the treatment of advanced, metastatic, HER2-overexpressing breast cancer in combination with chemotherapy or as a single agent, based on pivotal clinical trials showing overall response rates of 45% and 14%, respectively. This was followed in 2006 by approval to treat early-stage HER2-positive breast cancer in combination with a chemotherapeutic regimen containing doxorubicin, cyclophosphamide, and paclitaxel, following studies demonstrating greater than 50% reduction in the risk of recurrence, second primary cancer, or death among patients treated with trastuzumab plus chemotherapy, compared with chemotherapy alone.

Table 1. Approved HER2-Targeting Therapies for Breast Cancer

Agent

Breast Cancer Indications^a

(all approved for HER+ patients)

Company

Initial FDA Approval

Trastuzumab (Herceptin)

• Adjuvant treatment with chemotherapy or as single agent after multimodality therapy

• MBC with chemotherapy in first line or as single agent after chemotherapy

Genentech

1998

Lapatinib (Tykerb)

• Advanced BC or MBC with capecitabine after prior therapy including trastuzumab

• Postmenopausal women with HR+ MBC in combination with letrozole

GlaxoSmith- Kline

2007

Pertuzumab (Perjeta)

• MBC in combination with trastuzumab and docetaxel in patients with no prior anti-HER2 or chemotherapy for metastatic disease

Genentech

2012

Ado-trastuzumab emtansine, or T-DM1 (Kadcyla)

• MBC as single agent after prior trastuzumab and taxane-based therapy, or disease recurrence within 6 months of adjuvant therapy

Genentech

2013

^aHerceptin also is indicated for HER2+ gastric cancer.

BC indicates breast cancer; HER2+, patients overexpressing HER2 based on FDA-approved testing; HR+, hormone receptor-positive; MBC, metastatic breast cancer.

In the years that followed its approval, trastuzumab has been joined by several other FDA-approved HER2-targeted agents (Table 1). The clinical trials that were instrumental in gaining approval for these HER2-targeted agents indicated that their benefits were restricted to patients whose tumors exhibited overexpression of the HER2 protein or amplification of the HER2 gene. As such, administration of these drugs is dependent on the demonstration of a patient’s HER2-positive status, typically using the standard FDA-approved assays⎯fluorescence in situ hybridization, to evaluate the number of HER2 genes in a cancer cell (gene amplification), and immunohistochemistry, to examine the level of HER2 protein present on the cancer cell surface (protein overexpression) (Table 2). A third method, chromogenic in situ hybridization, also has been approved to evaluate gene expression.

Redefining HER2 Status

A hint of controversy surrounding the use of HER2-targeted agents solely in HER2-positive patients first emerged in 2008 (Paik et al, 2008), when re-evaluation of tumors previously thought to be HER2-positive showed that many were in fact HER2-negative. This finding posed a significant conundrum, since patients who were posited to be unresponsive to HER2 treatment did, in fact, appear to benefit from trastuzumab treatment in the same way as their HER2-positive counterparts.A number of studies are now being conducted to confirm the findings of these initial reports, challenging the notion that HER2-targeted therapies only work in HER2-positive breast cancer patients. These studies indicate that some patients whose tumors are HER2-negative, based on the standard FDA-approved assays, and who would therefore be ineligible for HER2-targeted therapy, may in fact be highly sensitive to this type of therapy. While the approved indications for HER2 therapy currently remain unchanged, one study quantifies the potential future benefit to expanding treatment to certain HER2-negative patients, suggesting that we may be able to help significantly more patients with breast cancer. Importantly, researchers are also providing possible molecular explanations for these findings.

Table 2. Algorithm for HER2 Testing

3 Categories of Results for Each Test

1. Positive

• IHC HER2 protein expression of 3+
• FISH HER2 gene/CEP17 ratio ≥2.2
• FISH HER2 gene copy number of >6.0

2. Equivocal

• IHC HER2 protein expression of 2+
• FISH HER2 gene/CEP17 ratio of 1.8-2.2
• FISH HER2 gene copy number of 4.0-6.0

3. Negative

• IHC HER2 protein expression of 1+ or 0
• FISH HER2 gene/CEP17 <1.8
• FISH HER2 gene copy number of <4.0

CEP indicates chromosome enumeration probe; FISH, fluorescence in situ hybridization; HER2, human epidermal growth factor receptor 2; IHC, immunohistochemistry.

Source: Hammond ME. Pitfalls and solutions in HER2/ER/PR testing. Presented at: 11th International Congress on the Future of Breast Cancer; July 26-28, 2012; Coronado, CA.

Molecular Mechanism #1: HER2-Activating Mutations

Researchers from the Siteman Cancer Center and The Genome Institute at Washington University in St. Louis, Missouri, analyzed data from eight breast cancer genome-sequencing studies (a total of 1500 patients) to further examine HER2 status. They identified a group of patients who did not have HER2 gene amplification (thus giving them a HER2-negative designation, meaning they would not receive HER2-targeted therapy according to current guidelines) but whose tumors displayed mutations in the HER2 gene that resulted in excess activity of the HER2 protein. They then went on to show that these activating mutations (G309A, D769H, D769Y, V777L, P780ins, V842I, and R896C) rendered the tumors susceptible to the approved HER2 agents trastuzumab and lapatinib (Tykerb), as well as neratinib, an inhibitor of HER2 and the epidermal growth factor receptor (EGFR).

All of the activating mutations described in this study were sensitive to neratinib. Importantly, however, a nonactivating mutation, L755S, was discovered in 25% of patients with somatic mutations; it likely drives resistance to lapatinib, and was also sensitive to neratinib. This suggests that neratinib could be an important agent in helping to overcome resistance to other HER2-targeted agents.

Lead author, Ron Bose, MD, PhD, said in an email interview that the findings about the L755S mutation prompted investigators to make changes to a phase II trial of lapatinib with trastuzumab in HER2-negative patients with stage IV breast cancer that had been launched to examine their HER2 genes for mutations and assess their response to anti-HER2 therapy. Now, neratinib has replaced lapatinib (NCT01670877). Bose and colleagues concluded that these newly identified mutations represent “an alternative mechanism to activate HER2 in breast cancer” and valid drug targets for breast cancer treatment. It is estimated that these mutations could be responsible for tumor growth in approximately 1.6% of cases, translating into some 4000 cases of breast cancer per year in the United States alone.

Molecular Mechanism #2: HER2-Positive Stem Cells

The cancer stem cell hypothesis is based on a growing body of evidence that suggests that tumor growth is initiated and maintained by a group of cells with similar properties to stem cells. Cancer stem cells (CSCs) are thought to be important drivers of metastasis, recurrence, and resistance to traditional cancer therapies such as chemotherapy.

Researchers at the University of Michigan Comprehensive Cancer Center in Ann Arbor and others have previously shown that the HER2 protein is an important regulator of CSCs in patients with HER2-positive breast cancer, driving self-renewal. More recently, the same research team used breast cancer cell lines, mouse xenograft models, and human breast cancer tissue samples to show that the same was also true in HER2-negative breast cancers. Since the proportion of CSCs within the entire tumor cell population is small (between 1% and 5%), the level of HER2 expression still falls below the threshold of assays used to determine a cancer patient’s HER2 status; however, because the CSCs overexpress HER2, they are still effectively targeted by HER2-directed therapies. This provides another explanation for the finding that HER2-negative patients respond to HER2-directed therapy; indeed, some of the clinical efficacy observed with trastuzumab may in fact relate to its ability to target the CSC population.

Another interesting finding from this study was that an increased level of HER2 expression was observed in bone metastases (a common site of breast cancer metastasis) in both mouse xenograft models and breast cancer patients. This phenomenon was not associated with HER2 gene amplification, but instead seemed to be regulated by the bone microenvironment. Thus, patients with advanced metastatic cancer may appear to be HER2-negative by conventional testing methods, but may still respond to HER2-targeted therapies as a result of HER2 expression regulated by the bone microenvironment. Importantly, however, appropriate timing of trastuzumab administration was vital. When trastuzumab was administered to mice when bone metastases were small, a significant decrease in the growth of breast cancer cells was observed, while treatment that was initiated after palpable tumors had become established was limited in its efficacy to tumors with HER2 gene amplification. These results suggest that, if administered to breast cancer patients before bone metastases develop, trastuzumab could help to prevent the spread of tumors.

The role of HER2-directed therapies in targeting the CSC population is being further explored in a large, randomized clinical trial (NCT01424865) and could drive an entirely new paradigm for the development of adjuvant therapies focusing on the prevention of tumor recurrence by targeting CSCs.

Key Anti-HER2 Agents in Development

Afatinib

(BIBW 2992) Boehringer Ingelheim

Afatinib, formerly known as BIBW 2992, is an oral, irreversible, small-molecule inhibitor of the ErbB tyrosine kinase family, which includes the HER receptors. It has shown activity as monotherapy in a phase II study of patients with HER2- positive metastatic breast cancer who progressed on trastuzumab, with 46% of 41 participating patients (19 patients) achieving a clinical benefit rate. Afatinib is being evaluated in several breast cancer trials, particularly a pivotal phase III trial in combination with vinorelbine versus trastuzumab plus vinorelbine. The studies are part of the broader LUX clinical trial program involving afatinib, which is pending FDA approval in non-small cell lung cancer.

NCT01125566, NCT01271725, NCT01441596

Neratinib

(PB272) Puma Biotechnology

Neratinib, formerly known as HKI-272, is an oral, irreversible, receptor tyrosine kinase inhibitor that targets HER2 and HER1 (also known as the epidermal growth factor receptor [EGFR]). In an open-label, multicenter phase II trial of patients with HER2-positive metastatic breast cancer, response rates of >50% were observed with single-agent neratinib in trastuzumab-naïve patients and >20% in trastuzumab-pretreated patients. In a phase II study of patients previously treated with trastuzumab and a taxane, a combination of neratinib and capecitabine showed an overall response rate of 50%. Neratinib is also being evaluated in a phase II trial involving 29 patients whose tumors are HER2-negative in conventional testing but carry HER2 gene mutations.

NCT01670877, NCT00741260, NCT00398567, NCT00445458

Nelipepimut-S

(NeuVax) Galena Biopharma

NeuVax, previously called E75, is a peptide vaccine derived from the extracellular domain of the HER2 protein and combined with the granulocyte macrophage colony-stimulating factor sargramostim (Leukine) to boost the immune system so that HER2-expressing cells are destroyed. The vaccine is aimed at preventing or delaying breast cancer recurrence in women with low levels of HER2 overexpression who would be considered HER2-negative, and are disease-free after standard therapies that include surgery, chemotherapy, and radiation. After phase I/II trials indicated a lower rate of recurrence among women who received the vaccine, investigators have launched the phase III PRESENT trial. Researchers are seeking to enroll 700 women. NCT01479244

Jane de Lartigue, PhD, is a freelance medical writer and editor based in Davis, California.

Key Research

• Bose R, Kavuri SM, Searleman AC, et al. Activating HER2 mutations in HER2 gene amplification negative breast cancer [published online ahead of print December 7, 2012]. Cancer Discov. 2013;3(2):224-237. doi:10.1158/2159-8290.CD-12-0349.
• Duru N, Fan M, Candas D, et al. HER2-associated radioresistance of breast cancer stem cells isolated from HER2-negative breast cancer cells [published online ahead of print October 22, 2012]. Clin Cancer Res. 2012;18(24):6634-6647. doi:10.1158/1078-0432.CCR-12-1436.
• Ithimakin S, Day KC, Malik F, et al. HER2 drives luminal breast cancer stem cells in the absence of HER2 amplification: implications for efficacy of adjuvant trastuzumab [published online ahead of print March 3, 2012]. Cancer Res. 2013;73(5):1635-1645. doi:10.1158/0008-5472.CAN-12-3349.
• Lin NU, Winer EP, Wheatley D, et al. A phase II study of afatinib (BIBW 2992), an irreversible ErbB family blocker, in patients with HER2-positive metastatic breast cancer progressing after trastuzumab [published online ahead of print March 15, 2012]. Breast Cancer Res Treat. 2012;133(3):1057-1065. doi:10.1007/s10549-012-2003-y.
• Paik S, Kim C, Wolmark N. HER2 status and benefit from adjuvant trastuzumab in breast cancer [letter]. N Engl J Med. 2008;358(13):1409-1411.