The anti-HER2 drugs trastuzumab and lapatinib are increasingly changing the natural history of early and metastatic HER2-overexpressing breast cancer. Many other agents targeted against the HER2 signaling network are in clinical development, and these are or will soon be combined with the currently approved anti-HER2 therapies. We review herein recent data in support of the early use of combinations of agents targeted to the HER2 network as the most rational approach against this subtype of breast cancer. We propose that the optimal combination or combinations of anti-HER2 agents delivered early in the natural history of HER2+ breast cancer should close to eliminate acquired drug resistance, shorten the duration of therapy, and potentially dispense with the need of concurrent chemotherapy. Clin Cancer Res; 17(5); 952–8. ©2011 AACR.

The antibody trastuzumab and the tyrosine kinase inhibitor (TKI) lapatinib are approved by the U.S. Food and Drug Administration (FDA) for the treatment of HER2-overexpressing (HER2+) metastatic breast cancer (MBC). Trastuzumab binds to an epitope in the juxtamembrane region of the HER2 receptor. This binding induces uncoupling of ligand-independent HER2-HER3 heterodimers, inhibition of downstream signaling (1), and antibody-dependent cell-mediated cytotoxicity (ADCC; ref. 2). Several randomized adjuvant trials (NCCTG N9831, NSABP B-31, BCIRG 006, and HERA) have shown that the addition of trastuzumab to standard chemotherapy reduces disease recurrence and the risk of death compared with chemotherapy alone in patients with surgically resected tumors (3–5). In N9831, a recent interim analysis showed that the benefit of concurrent trastuzumab and chemotherapy was more pronounced than that of chemotherapy followed by trastuzumab (6). On the basis of these data, the addition of trastuzumab to adjuvant chemotherapy has become standard of care in women with HER2+ early breast cancer.

The trastuzumab adjuvant trials focused on high-risk, lymph node–positive HER2+ tumors. Thus, data are limited or nonexistent on small tumors (≤1 cm) with negative nodes and on patient outcome. However, 2 recent studies found a significantly higher rate of recurrence among T1abN0 HER2+ compared with HER2-negative (HER2−) tumors regardless of estrogen receptor (ER) status (7, 8), suggesting adjuvant trastuzumab should be considered for these patients. However, the amount and type of chemotherapy to combine with the antibody in this setting is undetermined. Most of the adjuvant trials used 1 year of trastuzumab. One study delivered only 9 weeks of the antibody, whereas the HERA trial included an arm in which it was given for 2 years. In the first study, patients in the trastuzumab arm exhibited fewer overall recurrences and improved overall survival compared with patients treated with chemotherapy alone (9). Results in the 2-year arm in HERA are pending.

The dual epidermal growth factor receptor (EGFR)/HER2 TKI lapatinib is active as first-line monotherapy in patients with HER2+ MBC and, in combination with capecitabine, improves progression-free survival (PFS) compared with capecitabine alone (10, 11). In the latter registration trial, fewer brain metastases occurred in women in the combination arm than in the monotherapy arm, suggesting a potential difference between lapatinib and trastuzumab as it applies to recurrences in the central nervous system (CNS; 11). In the registration study and in a second randomized trial of paclitaxel ± lapatinib in patients with MBC, the clinical benefit of lapatinib was limited to patients with HER2 overexpression by immunohistochemistry (IHC) and/or FISH (12).

HER2 testing, discordance, and conversion

The clinical activity of anti-HER2 agents has been limited to patients with HER2+ tumors as defined by intense membrane staining with HER2 antibodies in the majority of tumor cells (3 + by IHC) or ≥2 copies of the HER2 gene determined by FISH. In general, HER2 IHC and FISH correlate with each other (13–15). FISH seems superior to IHC to reproducibly assess tumors for HER2 overexpression at outside and/or local laboratories for entry into clinical trials (16). Intrinsic limitations of IHC are the variability in fixation methods and the impact of fixation of antigenicity of the HER2 protein. Conversely, the more stable DNA, of which the loci are measured by FISH, is less susceptible to tissue fixation. For these reasons, excess copies of the HER2 gene (so-called HER2 positivity) defined by FISH have gained ground as the standard to define odds of tumor dependence on HER2 and, therefore, response to HER2 antagonists (17).

A reanalysis in a central laboratory of NSABP B-31 showed that 9.7% of patients enrolled on the basis of a test done in a local laboratory had tumors that did not meet criteria for HER2 amplification by FISH or IHC (18). Notably, these patients also benefited from trastuzumab. This finding suggests that the local laboratory was correct and/or there is discordance in the levels of HER2 expression between micrometastases, of which the clinical recurrence defines the endpoint of adjuvant trials, and the primary tumor, in which the HER2 alteration was measured. This possibility is further suggested by a study in which 9 out of 24 patients with breast cancer whose primary tumor was HER2− acquired HER2 amplification in their circulating tumor cells (CTC) during cancer progression (19). In another study, 10% of patients who recurred on adjuvant tamoxifen converted from HER2− to HER2+ in the relapsing tumor (20). Of note, however, the HER2 status of CTCs has yet to be linked to clinical outcome. On the basis of these data, the NSABP is initiating a phase III trial in which patients with 1+ or 2+ HER2 by IHC and no HER2 amplification by FISH will be randomized to adjuvant chemotherapy followed by 1 year of trastuzumab versus placebo.

Several studies have shown changes in HER2 status in patients treated with trastuzumab. Pectasides and colleagues showed that 37% of patients with HER2+ primary tumors no longer exhibited HER2 amplification in their metastatic lesions. Further, these patients exhibited a shorter time to progression (TTP) than the group that remained HER2+ (21). Hurley and colleagues reported that following treatment with neoadjuvant trastuzumab and chemotherapy, 43% of HER2+ tumors became HER2− as measured by FISH (22). Finally, Mittendorf and colleagues also reported that 32% of HER2+ tumors treated with neoadjuvant chemotherapy and trastuzumab “converted” to HER2− by FISH. Notably, at 37 months, relapse-free survival (RFS) was statistically superior in patients whose residual tumors retained HER2 amplification (23). These results have several implications. First, the change in HER2 status may reflect heterogeneity in HER2 expression in the primary tumor; the antioncogene therapy eliminates the HER2+ compartment and enriches for HER2− clones. Second, patients with HER2+ breast cancer who relapse after adjuvant or neoadjuvant anti-HER2 therapy should considering having their recurrent disease biopsied for reassessment of the HER2 status. Third, patients with ER−/HER2+ tumors at diagnosis that “convert” to HER2− after treatment are at high risk of early recurrence. Further, there are no clear adjuvant (targeted) therapy standards for these patients who, as a result, may exhibit a poor outcome.

Antibody-chemotherapy conjugates

Trastuzumab (T)–derivative of maytansine 1 (DM1) is an antibody–drug conjugate in which 1 molecule of trastuzumab is covalently bonded via a noncleavable linker to 3 molecules of the microtubule polymerization inhibitor DM1 (24). T-DM1 binds to HER2 with similar affinity as trastuzumab. It is postulated that after binding, the T-DM1/HER2 complex is internalized followed by degradation in the lysosome, release of DM1, and subsequent cell lysis. Although used at lower doses and frequency than trastuzumab, T-DM1 retains the ability to inhibit signaling and engaging of immune effectors that mediate ADCC, and it is active against lapatinib-resistant xenografts (25). Phase I-II studies of T-DM1 showed mild, reversible toxicity and a remarkable clinical response rate in excess of 25% in patients with heavily pretreated HER2+ MBC who had progressed after trastuzumab and lapatinib (26, 27). T-DM1 is being further evaluated in 2 large phase III randomized studies. The first trial compares T-DM1 versus T-DM1 plus pertuzumab versus the standard of trastuzumab plus a taxane in patients with HER2+ MBC previously untreated in the metastatic setting. The second trial compares T-DM1 versus the standard of lapatinib and capecitabine in similar patients but who have previously received trastuzumab (Table 1).

Table 1.

Planned and ongoing high-impact randomized phase III trials in patients with HER2-overexpressing breast cancer

TrialDesignPopulationNo. of patientsTreatmentTrial endpointsData expected
TEACH Phase III, adjuvant Stage I-IIIC, s/p neoadjuvant chemotherapy with an anthracycline, a taxane, and CMF 3,000 Placebo vs. L 1,500 mg/d for 12 months DFS, RFS, DRFS, OS, rate of CNS recurrence, QoL 2012 
ALTTO, BIG2–06/N063D Phase III, adjuvant Stage I-IIIC, s/p neoadjuvant anthracycline-based chemotherapy (≥4 cycles) 8,000 Standard chemotherapy with (a) H, (b) L, (c) H × 12 weeks → L after 6-week washout, (d) H + L (all regimens for up to 1 year) DFS, OS, TTDR, TTR, CNS recurrence rate 2013 
ExteNET Phase III, adjuvant Stage II-IIIC after adjuvant H for 1 year 120 Placebo vs. neratinib DFS 2017 
NEFERTT Phase III, MBC HER2+, no prior therapy for MBC 1,200 P + H vs. P + neratinib PFS, OS 2012 
Neo-ALTTO Phase III, neoadjuvant HER2+ >2 cm 450 (a) L × 6 weeks → L + P × 12 weeks vs. (b) H × 6 weeks → H + P × 12 weeks vs. (c) L + H × 6 weeks → H + L + P × 12 weeks; surgery; FEC × 3 → (a) L vs. (b) H vs. (c) H + L to complete 1 year pCR rate, ORR, DFS, OS 2011 
NSABP B-41 Phase III, neoadjuvant HER2+ >2 cm 522 AC × 4 → (a) P + H × 12 weeks vs. (b) P + L × 12 weeks vs. (c) P + H + L × 12 weeks pCR rate, ORR, RFS, OS 2010 
CALGB 40601 Phase III, neoadjuvant HER2+ stage II-III  H + P vs. L + P vs. H + L + P → surgery → adjuvant chemotherapy pCR rate, ORR, DFS, OS 2011 
Gepar-Quinto Phase III, neoadjuvant HER2+ requiring neoadjuvant chemotherapy 2,547 (whole trial) EC → D → H vs. EC → D → L pCR rate 2010 
BETH, NSABP B-44 Phase III, adjuvant HER2+, LN+, or high-risk LN− 3,500 TCH→ H (up to 1 year) vs. TCH → H + bevacizumab (up to 1 year) DFS, OS, RFS 2012 
BO22589 Phase III, MBC HER2+, no prior therapy for MBC 1,092 H + taxane vs. T-DM1 vs. T-DM1 + pertuzumab PFS 2012 
EMILIA Phase III, MBC HER2+ LABC or MBC 580 T-DM1 vs. capecitabine + L PFS, OS, ORR 2013 
CLEOPATRA Phase III, MBC HER2+, no prior therapy for MBC 800 D + H + placebo vs. D + H + pertuzumab PFS 2011 
PHEREXA Phase II, MBC HER2+ MBC, second line after H 450 Capecitabine + H vs. capecitabine + H + pertuzumab PFS, TTP, TTF, ORR, CBR 2015 
BOLERO-1 Phase III, MBC HER2+, no prior therapy for MBC 717 P + H vs. P + H + everolimus PFS, OS, ORR, CBR, TTR 2012 
BOLERO3 Phase III, MBC HER2+, resistant to H 572 Vinorelbine + H vs. vinorelbine + H + everolimus PFS, OS, ORR, CBR 2012 
TrialDesignPopulationNo. of patientsTreatmentTrial endpointsData expected
TEACH Phase III, adjuvant Stage I-IIIC, s/p neoadjuvant chemotherapy with an anthracycline, a taxane, and CMF 3,000 Placebo vs. L 1,500 mg/d for 12 months DFS, RFS, DRFS, OS, rate of CNS recurrence, QoL 2012 
ALTTO, BIG2–06/N063D Phase III, adjuvant Stage I-IIIC, s/p neoadjuvant anthracycline-based chemotherapy (≥4 cycles) 8,000 Standard chemotherapy with (a) H, (b) L, (c) H × 12 weeks → L after 6-week washout, (d) H + L (all regimens for up to 1 year) DFS, OS, TTDR, TTR, CNS recurrence rate 2013 
ExteNET Phase III, adjuvant Stage II-IIIC after adjuvant H for 1 year 120 Placebo vs. neratinib DFS 2017 
NEFERTT Phase III, MBC HER2+, no prior therapy for MBC 1,200 P + H vs. P + neratinib PFS, OS 2012 
Neo-ALTTO Phase III, neoadjuvant HER2+ >2 cm 450 (a) L × 6 weeks → L + P × 12 weeks vs. (b) H × 6 weeks → H + P × 12 weeks vs. (c) L + H × 6 weeks → H + L + P × 12 weeks; surgery; FEC × 3 → (a) L vs. (b) H vs. (c) H + L to complete 1 year pCR rate, ORR, DFS, OS 2011 
NSABP B-41 Phase III, neoadjuvant HER2+ >2 cm 522 AC × 4 → (a) P + H × 12 weeks vs. (b) P + L × 12 weeks vs. (c) P + H + L × 12 weeks pCR rate, ORR, RFS, OS 2010 
CALGB 40601 Phase III, neoadjuvant HER2+ stage II-III  H + P vs. L + P vs. H + L + P → surgery → adjuvant chemotherapy pCR rate, ORR, DFS, OS 2011 
Gepar-Quinto Phase III, neoadjuvant HER2+ requiring neoadjuvant chemotherapy 2,547 (whole trial) EC → D → H vs. EC → D → L pCR rate 2010 
BETH, NSABP B-44 Phase III, adjuvant HER2+, LN+, or high-risk LN− 3,500 TCH→ H (up to 1 year) vs. TCH → H + bevacizumab (up to 1 year) DFS, OS, RFS 2012 
BO22589 Phase III, MBC HER2+, no prior therapy for MBC 1,092 H + taxane vs. T-DM1 vs. T-DM1 + pertuzumab PFS 2012 
EMILIA Phase III, MBC HER2+ LABC or MBC 580 T-DM1 vs. capecitabine + L PFS, OS, ORR 2013 
CLEOPATRA Phase III, MBC HER2+, no prior therapy for MBC 800 D + H + placebo vs. D + H + pertuzumab PFS 2011 
PHEREXA Phase II, MBC HER2+ MBC, second line after H 450 Capecitabine + H vs. capecitabine + H + pertuzumab PFS, TTP, TTF, ORR, CBR 2015 
BOLERO-1 Phase III, MBC HER2+, no prior therapy for MBC 717 P + H vs. P + H + everolimus PFS, OS, ORR, CBR, TTR 2012 
BOLERO3 Phase III, MBC HER2+, resistant to H 572 Vinorelbine + H vs. vinorelbine + H + everolimus PFS, OS, ORR, CBR 2012 

Abbreviations: H, herceptin; L, lapatinib; P, paclitaxel; D, docetaxel; TC, docetaxel, carboplatin; TCH, docetaxel, carboplatin, herceptin; DFS, disease-free survival; DRFS, distant relapse-free survival; OS, overall survival; QoL, quality of life; TTR, time to recurrence; TTDR, time to distant recurrence; DFS, disease-free survival; ORR, overall response rate; TTF, time to treatment failure; CBR, clinical benefit rate; AC, adriamycin/taxol; LABC, locally advanced breast cancer; CMF, cyclophosphamide, methotrexate, 5-fluoruracil; LN, lymph node; EC, epirubicin and cyclophosphamide

Combination of anti-HER2 therapies and abrogation of drug resistance

Dual HER2 blockade

Many HER2-amplified breast cancers do not respond to or eventually escape trastuzumab, suggesting both de novo and acquired mechanisms of resistance. A possible mechanism of de novo resistance is expression of the HER2 receptor as a kinase-active 95-kDa cytosolic fragment that lacks the trastuzumab-binding epitope (28). Analysis of a cohort of patients with HER2+ MBC treated with trastuzumab and chemotherapy showed a very low response rate in tumors with cytosolic p95HER2 compared with those without (29). Lapatinib has been shown to inhibit the catalytic activity of p95HER2. Therefore, patients with p95HER2-positive breast cancers treated with lapatinib alone or in combination with capecitabine exhibited a similar PFS and overall response rate compared with p95HER2-negative tumors (30), suggesting a clinical setting in which a HER2 TKI might be advantageous.

Like lapatinib, the HER2/EGFR dual TKI neratinib (31) has shown clinical activity in patients with HER2+ MBC who have progressed on trastuzumab. These data suggest that trastuzumab-resistant tumors continue to be dependent on the HER2 tyrosine kinase. However, the response to each single-agent TKI tends to be short-lived (10, 11). Further, these patients may still need trastuzumab beyond progression as suggested by a recent study in which the combination of lapatinib and trastuzumab was superior to lapatinib alone at improving PFS, clinical response, and overall survival of patients with HER2+ MBC who had progressed on trastuzumab (32). Taken together, these data imply that even in advanced stages, HER2+ breast cancers remain dependent on HER2 and that single-agent trastuzumab and lapatinib are not adequate to inhibit the HER2 network completely. They also imply that using combinations of HER2-targeted agents delivered early against HER2+ breast cancer is the rational approach to take. Along these lines, Adjuvant Lapatinib and/or Trastuzumab Treatment Optimization (ALTTO) is an ongoing large international adjuvant study comparing trastuzumab versus lapatinib versus dual HER2 blockade using both drugs. The activity of trastuzumab beyond progression is not limited to combinations with TKIs, as has also been shown in a study in which the combination of trastuzumab plus capecitabine was clearly superior to capecitabine alone (33).

Inhibition of HER3 and HER2/HER3 dimers

The HER2 coreceptor HER3 is the key adaptor that, once dimerized with and phosphorylated by HER2, engages and activates the phosphoinositide 3-kinase (PI3K)/Akt pathway (1, 34, 35). The association of HER2/HER3 dimers with PI3K is essential for the viability of HER2-dependent cells (36, 37). Indeed, HER2+ breast cancer cells are particularly sensitive to apoptosis induced by PI3K inhibitors (38), further underscoring the importance of HER3-mediated signaling in HER2-dependent cells. It is generally accepted that sustained inhibition of the output of HER2/HER3 to PI3K/Akt is required for the antitumor effect of HER2 inhibitors. Interestingly, HER2+ breast cancer cells upregulate HER3 expression upon inhibition of HER2 with lapatinib (39). This upregulation is problematic because several kinases such as MET, EGFR, Src, and fibroblast growth factor receptor 2 (FGFR2) can phosphorylate HER3 (40–42), thus partially maintaining PI3K activity and limiting the antitumor effect of the HER2 inhibitor (Fig. 1).

Figure 1.

Upregulation of HER3 and resistance to anti-HER2 therapies. The kinase-dead HER3 receptor, but not EGFR or HER2, can couple directly to p85, the regulatory subunit to PI3K. HER2/HER3 heterodimers are the most potent signaling complexes of the HER receptor network. Upon inhibition of HER2 and PI3K (as with lapatinib in the figure), tumor cells upregulate expression of the HER3 protein. Other tyrosine kinases such as MET, FGFR2, Src, and EGFR present in HER2+ cancer cells can then phosphorylate tyrosines in the C terminus of HER3, which, in turn, couples to p85 to partially maintain PI3K active and, thus, limits the antitumor effect of HER2 inhibitors. Alternatively, the cell can use other receptor networks, that is, IGF-IR, to activate the PI3K/Akt survival pathway.

Figure 1.

Upregulation of HER3 and resistance to anti-HER2 therapies. The kinase-dead HER3 receptor, but not EGFR or HER2, can couple directly to p85, the regulatory subunit to PI3K. HER2/HER3 heterodimers are the most potent signaling complexes of the HER receptor network. Upon inhibition of HER2 and PI3K (as with lapatinib in the figure), tumor cells upregulate expression of the HER3 protein. Other tyrosine kinases such as MET, FGFR2, Src, and EGFR present in HER2+ cancer cells can then phosphorylate tyrosines in the C terminus of HER3, which, in turn, couples to p85 to partially maintain PI3K active and, thus, limits the antitumor effect of HER2 inhibitors. Alternatively, the cell can use other receptor networks, that is, IGF-IR, to activate the PI3K/Akt survival pathway.

Close modal

Trastuzumab has been shown to block ligand-independent association between HER2 and HER3, whereas pertuzumab, an antibody that recognizes an epitope in heterodimerization domain II of HER2, blocks ligand-induced HER2-HER3 dimerization (43). Recent data using Clinical Laboratory Improvement Amendments (CLIA)–certified dimer assays suggest that levels of HER2-containing homo- and heterodimers (with EGFR and HER3) can be measured in situ and are highly variable among HER2+ tumors (44, 45). We hypothesize, for example, that a tumor with high levels of HER2/HER3 heterodimers would be relatively unresponsive to trastuzumab and, thus, may be a candidate for lapatinib or pertuzumab, each in combination with trastuzumab. These speculations and the question of whether the addition of quantitative dimer assays to FISH and IHC for HER2 will refine the selection of the type of anti-HER2 therapy remain to be investigated.

In trastuzumab-resistant xenografts and in patients with HER2+ breast cancer who have progressed on trastuzumab, only the combination of pertuzumab and trastuzumab, but not each antibody alone, exhibited clinical activity (46, 47). These data suggest that both HER2 antibodies might be required to completely inhibit HER2-HER3 dimerization in situ, potentially explaining their clinical activity in combination. To test this hypothesis, the phase III Cleopatra study (Table 1) is currently randomizing patients with HER2+ MBC to trastuzumab and docetaxel ± pertuzumab as first-line therapy in the metastatic setting using PFS as a primary endpoint. Of note, in the recently reported NeoSphere trial in patients with HER2+ primary breast cancer (Table 1), the pathologic complete response (pCR) rate was 45.8% versus 29% (P = 0.01) in patients treated with neoadjuvant docetaxel-trastuzumab-pertuzumab versus docetaxel-trastuzumab, respectively (48). Currently, the HER3 monoclonal antibodies AMG-888 (49) and MM-121 (50) are completing phase I testing. We anticipate that, like pertuzumab, they may also exert a synergistic effect in combination with trastuzumab or lapatinib in patients with HER2+ MBC.

PI3K and drug resistance

Amplification of PI3K signaling as a result of coexpression of PIK3CA-activating mutations or loss of the lipid phosphatase PTEN in HER2+ breast cancer cells and primary tumors is associated with a lower response to trastuzumab and lapatinib (51–55). Several PI3K pathway antagonists are in clinical development and are the subject of recent reviews (56, 57). In preclinical studies, the addition of some of these inhibitors to trastuzumab or lapatinib has inhibited growth of HER2+ tumors resistant to anti-HER2 therapy (53, 54). Interestingly, inhibitors of mTOR, a serine-threonine kinase downstream of PI3K, have shown activity after progression on trastuzumab. Dalenc and colleagues recently reported a multicenter phase II study of 55 women with HER2+ MBC whose tumors were resistant to trastuzumab and taxanes. Patients were treated with the TOR inhibitor everolimus, paclitaxel, and trastuzumab, exhibiting an impressive partial response rate of 19% and an overall clinical benefit rate of 81% (58).

Neoadjuvant therapy as a platform for clinical research

Trastuzumab has been administered with chemotherapy in the neoadjuvant setting with rates of pCR as high as 65% (22, 59–62). Achievement of pCR after neoadjuvant chemotherapy has been widely associated with improved long-term outcome. Although not yet clear, recent data suggest similar conclusions may eventually also apply to patients treated with neoadjuvant anti-HER2 therapy. The neoadjuvant herceptin (NOAH) trial tested the efficacy of chemotherapy ± trastuzumab in patients with HER2+ locally advanced or inflammatory breast cancer. pCR rate was 38% versus 19% in the trastuzumab versus the control group. There was a 71% versus 56% 3-year event-free survival in the trastuzumab versus control arms in all subgroups tested. Overall survival was not different between both arms, but this finding is qualified by the fact that a significant proportion of patients “crossed-over” to adjuvant trastuzumab (62). Nonetheless, NOAH is the first trial in patients with HER2+ tumors in which pCR mirrors longer term event-free survival, suggesting use of the neoadjuvant therapy space as a platform for clinical investigation, which we discuss below.

Neo-ALTTO is a 450-patient study in which HER2+ tumors >2 cm were randomized to trastuzumab, lapatinib, or the combination for 6 weeks, at which time paclitaxel is added to each of the arms for an additional 12 weeks. After surgery, all 3 arms will receive adjuvant chemotherapy with 5-fluorouracil (5-FU), epirubicin, and cyclophosphamide (FEC) followed by the respective HER2 inhibitor either alone or in combination for 34 weeks. About half of the patients enrolled had ER+ tumors. There was increased but manageable toxicity in the lapatinib arms (diarrhea, transaminitis). pCR, defined as no invasive cancer in the breast or only ductal carcinoma in situ (DCIS) in the breast specimen, was significantly higher in the combination arm (51.3%) versus 29.5% and 24.7% in the trastuzumab and lapatinib arms, respectively. In all 3 arms, the pCR rate was lower in the ER+ versus the ER− tumors (63). Whether pCR correlates with disease-free and overall survival is pending further follow-up.

Other combinations

The data summarized above suggest that, in addition to the combination of trastuzumab and lapatinib, many other rational combinations are or will soon be available for clinical testing. Examples of 2-drug combinations that will inhibit the HER2 network and its output to PI3K/Akt more comprehensively are trastuzumab plus pertuzumab, trastuzumab or lapatinib plus a HER3-neutralizing antibody, trastuzumab or lapatinib plus a PI3K or an AKT inhibitor, a PI3K pathway inhibitor plus a HER3 antibody, T-DM1 plus pertuzumab or a PI3K inhibitor, a combination of HER2 and insulin like growth factor-IR (IGF-IR) pathway antagonists, and others. Some of these combinations are shown in Table 1. We speculate that many or all of these will be well tolerated and effective against HER2+ MBC, but that gain will likely only be incremental. With the establishment of current anti-HER2 agents as adjuvant therapy for HER2+ early disease and the increasing limitation in patient resources, it will be difficult to move these combinations to the adjuvant setting to test their true anti-(micro)metastatic potency using survival as an endpoint.

However, the increasing use of preoperative therapy should provide a clinical research platform in which these combinations can be compared and triaged using pCR as a clinical endpoint predictive of long-term outcome. Another benefit of a preoperative platform is that residual tumor tissue is available at the time of surgery. These “drug-resistant” residual cancers may well reflect the molecular profile of drug-resistant micrometastases and can be interrogated with open-ended molecular approaches to identify biomarkers and/or effectors of resistance to anti-HER2 therapies. It would not be too surprising if the clinical activity of these combinations turns out to be equivalent. However, toxicity and cost may turn out to be important differentiating factors. If so, just like options for endocrine therapy in ER+ breast cancer, this scenario would provide a plethora of treatment choices for patients with HER2-overexpressing breast cancer who, in the end, will be the winners.

No potential conflicts of interest were disclosed.

Supported in part by National Cancer Institute (NCI) R01 grant CA80195, American Cancer Society (ACS) Clinical Research Professorship Grant CRP-07–234, the Lee Jeans Translational Breast Cancer Research Program, Breast Cancer Specialized Program of Research Excellence (SPORE) P50 CA98131, and Vanderbilt-Ingram Cancer Center Support Grant P30 CA68485.

1.
Junttila
TT
,
Akita
RW
,
Parsons
K
,
Fields
C
,
Lewis
Phillips GD
,
Friedman
LS
, et al
Ligand-independent HER2/HER3/PI3K complex is disrupted by trastuzumab and is effectively inhibited by the PI3K inhibitor GDC-0941
.
Cancer Cell
2009
;
15
:
429
40
.
2.
Park
S
,
Jiang
Z
,
Mortenson
ED
,
Deng
L
,
Radkevich-Brown
O
,
Yang
X
, et al
The therapeutic effect of anti-HER2/neu antibody depends on both innate and adaptive immunity
.
Cancer Cell
2010
;
18
:
160
70
.
3.
Romond
EH
,
Perez
EA
,
Bryant
J
,
Suman
VJ
,
Geyer
CE
 Jr
,
Davidson
NE
, et al
Trastuzumab plus adjuvant chemotherapy for operable HER2-positive breast cancer
.
N Engl J Med
2005
;
353
:
1673
84
.
4.
Smith
I
,
Procter
M
,
Gelber
RD
,
Guillaume
S
,
Feyereislova
A
,
Dowsett
M
, et al
2-year follow-up of trastuzumab after adjuvant chemotherapy in HER2-positive breast cancer: a randomised controlled trial
.
Lancet
2007
;
369
:
29
36
.
5.
Robert
N
,
Leyland-Jones
B
,
Asmar
L
,
Belt
R
,
Ilegbodu
D
,
Loesch
D
, et al
Randomized phase III study of trastuzumab, paclitaxel, and carboplatin compared with trastuzumab and paclitaxel in women with HER-2-overexpressing metastatic breast cancer
.
J Clin Oncol
2006
;
24
:
2786
92
.
6.
Perez
EA
,
Suman
VJ
,
Davidson
NE
,
Gralow
J
,
Kaufman
PA
,
Ingle
JN
, et al
Results of chemotherapy alone, with sequential or concurrent addition of 52 weeks of trastuzumab in the NCCTG N9831 HER2-positive adjuvant breast cancer trial [abstract]
.
Cancer Res
2009
;
69
(
24 Suppl 3
):
80
.
7.
Curigliano
G
,
Viale
G
,
Bagnardi
V
,
Fumagalli
L
,
Locatelli
M
,
Rotmensz
N
, et al
Clinical relevance of HER2 overexpression/amplification in patients with small tumor size and node-negative breast cancer
.
J Clin Oncol
2009
;
27
:
5693
9
.
8.
Gonzalez-Angulo
AM
,
Litton
JK
,
Broglio
KR
,
Meric-Bernstam
F
,
Rakkhit
R
,
Cardoso
F
, et al
High risk of recurrence for patients with breast cancer who have human epidermal growth factor receptor 2-positive, node-negative tumors 1 cm or smaller
.
J Clin Oncol
2009
;
27
:
5700
6
.
9.
Joensuu
H
,
Kellokumpu-Lehtinen
PL
,
Bono
P
,
Alanko
T
,
Kataja
V
,
Asola
R
, et al
Adjuvant docetaxel or vinorelbine with or without trastuzumab for breast cancer
.
N Engl J Med
2006
;
354
:
809
20
.
10.
Gomez
HL
,
Doval
DC
,
Chavez
MA
,
Ang
PC
,
Aziz
Z
,
Nag
S
, et al
Efficacy and safety of lapatinib as first-line therapy for ErbB2-amplified locally advanced or metastatic breast cancer
.
J Clin Oncol
2008
;
26
:
2999
3005
.
11.
Geyer
CE
,
Forster
J
,
Lindquist
D
,
Chan
S
,
Romieu
CG
,
Pienkowski
T
, et al
Lapatinib plus capecitabine for HER2-positive advanced breast cancer
.
N Engl J Med
2006
;
355
:
2733
43
.
12.
Press
MF
,
Finn
RS
,
Cameron
D
,
Di Leo
A
,
Geyer
CE
,
Villalobos
IE
, et al
HER-2 gene amplification, HER-2 and epidermal growth factor receptor mRNA and protein expression, and lapatinib efficacy in women with metastatic breast cancer
.
Clin Cancer Res
2008
;
14
:
7861
70
.
13.
Press
MF
,
Slamon
DJ
,
Flom
KJ
,
Park
J
,
Zhou
JY
,
Bernstein
L
. 
Evaluation of HER-2/neu gene amplification and overexpression: comparison of frequently used assay methods in a molecularly characterized cohort of breast cancer specimens
.
J Clin Oncol
2002
;
20
:
3095
105
.
14.
Mass
RD
,
Press
MF
,
Anderson
S
,
Cobleigh
MA
,
Vogel
CL
,
Dybdal
N
, et al
Evaluation of clinical outcomes according to HER2 detection by fluorescence in situ hybridization in women with metastatic breast cancer treated with trastuzumab
.
Clin Breast Cancer
2005
;
6
:
240
6
.
15.
Dybdal
N
,
Leiberman
G
,
Anderson
S
,
McCune
B
,
Bajamonde
A
,
Cohen
RL
, et al
Determination of HER2 gene amplification by fluorescence in situ hybridization and concordance with the clinical trials immunohistochemical assay in women with metastatic breast cancer evaluated for treatment with trastuzumab
.
Breast Cancer Res Treat
2005
;
93
:
3
11
.
16.
Press
MF
,
Sauter
G
,
Bernstein
L
,
Villalobos
IE
,
Mirlacher
M
,
Zhou
JY
, et al
Diagnostic evaluation of HER-2 as a molecular target: an assessment of accuracy and reproducibility of laboratory testing in large, prospective, randomized clinical trials
.
Clin Cancer Res
2005
;
11
:
6598
607
.
17.
Sauter
G
,
Lee
J
,
Bartlett
JM
,
Slamon
DJ
,
Press
MF
. 
Guidelines for human epidermal growth factor receptor 2 testing: biologic and methodologic considerations
.
J Clin Oncol
2009
;
27
:
1323
33
.
18.
Paik
S
,
Kim
C
,
Wolmark
N
. 
HER2 status and benefit from adjuvant trastuzumab in breast cancer
.
N Engl J Med
2008
;
358
:
1409
11
.
19.
Meng
S
,
Tripathy
D
,
Shete
S
,
Ashfaq
R
,
Haley
B
,
Perkins
S
, et al
HER-2 gene amplification can be acquired as breast cancer progresses
.
Proc Natl Acad Sci U S A
2004
;
101
:
9393
8
.
20.
Gutierrez
MC
,
Detre
S
,
Johnston
S
,
Mohsin
SK
,
Shou
J
,
Allred
DC
, et al
Molecular changes in tamoxifen-resistant breast cancer: relationship between estrogen receptor, HER-2, and p38 mitogen-activated protein kinase
.
J Clin Oncol
2005
;
23
:
2469
76
.
21.
Pectasides
D
,
Gaglia
A
,
Arapantoni-Dadioti
P
,
Bobota
A
,
Valavanis
C
,
Kostopoulou
V
, et al
HER-2/neu status of primary breast cancer and corresponding metastatic sites in patients with advanced breast cancer treated with trastuzumab-based therapy
.
Anticancer Res
2006
;
26
:
647
53
.
22.
Hurley
J
,
Doliny
P
,
Reis
I
,
Silva
O
,
Gomez-Fernandez
C
,
Velez
P
, et al
Docetaxel, cisplatin, and trastuzumab as primary systemic therapy for human epidermal growth factor receptor 2-positive locally advanced breast cancer
.
J Clin Oncol
2006
;
24
:
1831
8
.
23.
Mittendorf
EA
,
Wu
Y
,
Scaltriti
M
,
Meric-Bernstam
F
,
Hunt
KK
,
Dawood
S
, et al
Loss of HER2 amplification following trastuzumab-based neoadjuvant systemic therapy and survival outcomes
.
Clin Cancer Res
2009
;
15
:
7381
8
.
24.
Lewis
Phillips GD
,
Li
G
,
Dugger
DL
,
Crocker
LM
,
Parsons
KL
,
Mai
E
, et al
Targeting HER2-positive breast cancer with trastuzumab-DM1, an antibody-cytotoxic drug conjugate
.
Cancer Res
2008
;
68
:
9280
90
.
25.
Junttila
TT
,
Li
G
,
Parsons
K
,
Phillips
GL
,
Sliwkowski
MX
. 
Trastuzumab-DM1 (T-DM1) retains all the mechanisms of action of trastuzumab and efficiently inhibits growth of lapatinib insensitive breast cancer
.
Breast Cancer Res Treat
2010
.
Epub 21 Aug 2010
.
26.
Krop
IE
,
Beeram
M
,
Modi
S
,
Jones
SF
,
Holden
SN
,
Yu
W
, et al
Phase I study of trastuzumab-DM1, an HER2 antibody-drug conjugate, given every 3 weeks to patients with HER2-positive metastatic breast cancer
.
J Clin Oncol
2010
;
28
:
2698
704
.
27.
Burris
HA
 3rd
,
Rugo
HS
,
Vukelja
SJ
,
Vogel
CL
,
Borson
RA
,
Limentani
S
, et al
Phase II study of the antibody drug conjugate trastuzumab-dm1 for the treatment of human epidermal growth factor receptor 2 (HER2)-positive breast cancer after prior HER2-directed therapy
.
J Clin Oncol
2010
.
Epub 22 Dec 2010
.
28.
Anido
J
,
Scaltriti
M
,
Bech
Serra JJ
,
Santiago
Josefat B
,
Todo
FR
,
Baselga
J
, et al
Biosynthesis of tumorigenic HER2 C-terminal fragments by alternative initiation of translation
.
EMBO J
2006
;
25
:
3234
44
.
29.
Scaltriti
M
,
Rojo
F
,
Ocana
A
,
Anido
J
,
Guzman
M
,
Cortes
J
, et al
Expression of p95HER2, a truncated form of the HER2 receptor, and response to anti-HER2 therapies in breast cancer
.
J Natl Cancer Inst
2007
;
99
:
628
38
.
30.
Scaltriti
M
,
Chandarlapaty
S
,
Prudkin
L
,
Aura
C
,
Jimenez
J
,
Angelini
PD
, et al
Clinical benefit of lapatinib-based therapy in patients with human epidermal growth factor receptor 2-positive breast tumors coexpressing the truncated p95HER2 receptor
.
Clin Cancer Res
2010
;
16
:
2688
95
.
31.
Burstein
HJ
,
Sun
Y
,
Dirix
LY
,
Jiang
Z
,
Paridaens
R
,
Tan
AR
, et al
Neratinib, an irreversible ErbB receptor tyrosine kinase inhibitor, in patients with advanced ErbB2-positive breast cancer
.
J Clin Oncol
2010
;
28
:
1301
7
.
32.
Blackwell
KL
,
Burstein
HJ
,
Storniolo
AM
,
Rugo
H
,
Sledge
G
,
Koehler
M
, et al
Randomized study of Lapatinib alone or in combination with trastuzumab in women with ErbB2-positive, trastuzumab-refractory metastatic breast cancer
.
J Clin Oncol
2010
;
28
:
1124
30
.
33.
von Minckwitz
G
,
du Bois
A
,
Schmidt
M
,
Maass
N
,
Cufer
T
,
de Jongh
FE
, et al
Trastuzumab beyond progression in human epidermal growth factor receptor 2-positive advanced breast cancer: a german breast group 26/breast international group 03–05 study
.
J Clin Oncol
2009
;
27
:
1999
2006
.
34.
Yakes
FM
,
Chinratanalab
W
,
Ritter
CA
,
King
W
,
Seelig
S
,
Arteaga
CL
. 
Herceptin-induced inhibition of phosphatidylinositol-3 kinase and Akt Is required for antibody-mediated effects on p27, cyclin D1, and antitumor action
.
Cancer Res
2002
;
62
:
4132
41
.
35.
Hellyer
NJ
,
Kim
MS
,
Koland
JG
. 
Heregulin-dependent activation of phosphoinositide 3-kinase and Akt via the ErbB2/ErbB3 co-receptor
.
J Biol Chem
2001
;
276
:
42153
61
.
36.
Holbro
T
,
Beerli
RR
,
Maurer
F
,
Koziczak
M
,
Barbas
CF
 3rd
,
Hynes
NE
. 
The ErbB2/ErbB3 heterodimer functions as an oncogenic unit: ErbB2 requires ErbB3 to drive breast tumor cell proliferation
.
Proc Natl Acad Sci U S A
2003
;
100
:
8933
8
.
37.
Lee-Hoeflich
ST
,
Crocker
L
,
Yao
E
,
Pham
T
,
Munroe
X
,
Hoeflich
KP
, et al
A central role for HER3 in HER2-amplified breast cancer: implications for targeted therapy
.
Cancer Res
2008
;
68
:
5878
87
.
38.
Brachmann
SM
,
Hofmann
I
,
Schnell
C
,
Fritsch
C
,
Wee
S
,
Lane
H
, et al
Specific apoptosis induction by the dual PI3K/mTor inhibitor NVP-BEZ235 in HER2 amplified and PIK3CA mutant breast cancer cells
.
Proc Natl Acad Sci U S A
2009
;
106
:
22299
304
.
39.
Amin
DN
,
Sergina
N
,
Ahuja
D
,
McMahon
M
,
Blair
JA
,
Wang
D
, et al
Resiliency and vulnerability in the HER2-HER3 tumorigenic driver
.
Sci Transl Med
2010
;
2
:
16ra7
.
40.
Engelman
JA
,
Zejnullahu
K
,
Mitsudomi
T
,
Song
Y
,
Hyland
C
,
Park
JO
, et al
MET amplification leads to gefitinib resistance in lung cancer by activating ERBB3 signaling
.
Science
2007
;
316
:
1039
43
.
41.
Ishizawar
RC
,
Miyake
T
,
Parsons
SJ
. 
c-Src modulates ErbB2 and ErbB3 heterocomplex formation and function
.
Oncogene
2007
;
26
:
3503
10
.
42.
Kunii
K
,
Davis
L
,
Gorenstein
J
,
Hatch
H
,
Yashiro
M
,
Di Bacco
A
, et al
FGFR2-amplified gastric cancer cell lines require FGFR2 and Erbb3 signaling for growth and survival
.
Cancer Res
2008
;
68
:
2340
8
.
43.
Agus
DB
,
Akita
RW
,
Fox
WD
,
Lewis
GD
,
Higgins
B
,
Pisacane
PI
, et al
Targeting ligand-activated ErbB2 signaling inhibits breast and prostate tumor growth
.
Cancer Cell
2002
;
2
:
127
37
.
44.
Desmedt
C
,
Sperinde
J
,
Piette
F
,
Huang
W
,
Jin
X
,
Tan
Y
, et al
Quantitation of HER2 expression or HER2:HER2 dimers and differential survival in a cohort of metastatic breast cancer patients carefully selected for trastuzumab treatment primarily by FISH
.
Diagn Mol Pathol
2009
;
18
:
22
9
.
45.
Williams
SJ
,
Mukherjee
A
,
Badal
Y
, et al
Profiling PI3K pathway activation in formalin-fixed, paraffin-embedded tumor cell lines and breast and ovarian tumors using novel proximity assays [abstract]
.
Proc Am Assoc Cancer Res
2009
:
5251
.
46.
Scheuer
W
,
Friess
T
,
Burtscher
H
,
Bossenmaier
B
,
Endl
J
,
Hasmann
M
. 
Strongly enhanced antitumor activity of trastuzumab and pertuzumab combination treatment on HER2-positive human xenograft tumor models
.
Cancer Res
2009
;
69
:
9330
6
.
47.
Baselga
J
,
Gelmon
KA
,
Verma
S
,
Wardley
A
,
Conte
P
,
Miles
D
, et al
Phase II trial of pertuzumab and trastuzumab in patients with human epidermal growth factor receptor 2-positive metastatic breast cancer that progressed during prior trastuzumab therapy
.
J Clin Oncol
2010
;
28
:
1138
44
.
48.
Gianni
L
,
Pienkowski
T
,
Im
Y-H
,
Roman
L
,
Tseng
L-M
,
Liu
M-C
, et al
Neoadjuvant pertuzumab (P) and trastuzumab (T): Antitumor and safety analysis of randomized phase II study (NeoSphere)
.
Cancer Res
2010
;
70
:
82s
.
49.
Freeman
D
,
Ogbagabriel
S
,
Rothe
M
, et al
Fully human anti-HER3 monoclonal antibodies (mAbs) have unique in vitro and in vivo functional and anti-tumor activities versus othe HER family inhibitors
.
Proc Am Assoc Cancer Res
2008
;
49
.
50.
Schoeberl
B
,
Faber
AC
,
Li
D
,
Liang
MC
,
Crosby
K
,
Onsum
M
, et al
An ErbB3 antibody, MM-121, is active in cancers with ligand-dependent activation
.
Cancer Res
2010
;
70
:
2485
94
.
51.
Berns
K
,
Horlings
HM
,
Hennessy
BT
,
Madiredjo
M
,
Hijmans
EM
,
Beelen
K
, et al
A functional genetic approach identifies the PI3K pathway as a major determinant of trastuzumab resistance in breast cancer
.
Cancer Cell
2007
;
12
:
395
402
.
52.
Nagata
Y
,
Lan
KH
,
Zhou
X
,
Tan
M
,
Esteva
FJ
,
Sahin
AA
, et al
PTEN activation contributes to tumor inhibition by trastuzumab, and loss of PTEN predicts trastuzumab resistance in patients
.
Cancer Cell
2004
;
6
:
117
27
.
53.
Serra
V
,
Markman
B
,
Scaltriti
M
,
Eichhorn
PJ
,
Valero
V
,
Guzman
M
, et al
NVP-BEZ235, a dual PI3K/mTOR inhibitor, prevents PI3K signaling and inhibits the growth of cancer cells with activating PI3K mutations
.
Cancer Res
2008
;
68
:
8022
30
.
54.
Eichhorn
PJ
,
Gili
M
,
Scaltriti
M
,
Serra
V
,
Guzman
M
,
Nijkamp
W
, et al
Phosphatidylinositol 3-kinase hyperactivation results in lapatinib resistance that is reversed by the mTOR/phosphatidylinositol 3-kinase inhibitor NVP-BEZ235
.
Cancer Res
2008
;
68
:
9221
30
.
55.
Esteva
FJ
,
Guo
H
,
Zhang
S
,
Santa-Maria
C
,
Stone
S
,
Lanchbury
JS
, et al
PTEN, PIK3CA, p-AKT, and p-p70S6K status: association with trastuzumab response and survival in patients with HER2-positive metastatic breast cancer
.
Am J Pathol
2010
;
177
:
1647
56
.
56.
Engelman
JA
. 
Targeting PI3K signalling in cancer: opportunities, challenges and limitations
.
Nat Rev Cancer
2009
;
9
:
550
62
.
57.
Brachmann
S
,
Fritsch
C
,
Maira
SM
,
Garcia-Echeverria
C
. 
PI3K and mTOR inhibitors: a new generation of targeted anticancer agents
.
Curr Opin Cell Biol
2009
;
21
:
194
8
.
58.
Dalenc
F
,
Campone
M
,
Hupperets
P
,
O'Regan
R
,
Manilus
C
,
Vittori
L
, et al
Everolimus in combination with weekly paclitaxel and trastuzumab in patients (pts) with HER2-overexpressing metastatic breast cancer (MBC) with prior resistance to trastuzumab and taxanes: A multicenter phase II clinical trial
.
J Clin Oncol
2010
;
28
(
Suppl
):
15s
.
59.
Burstein
HJ
,
Harris
LN
,
Gelman
R
,
Lester
SC
,
Nunes
RA
,
Kaelin
CM
, et al
Preoperative therapy with trastuzumab and paclitaxel followed by sequential adjuvant doxorubicin/cyclophosphamide for HER2 overexpressing stage II or III breast cancer: a pilot study
.
J Clin Oncol
2003
;
21
:
46
53
.
60.
Buzdar
AU
,
Ibrahim
NK
,
Francis
D
,
Booser
DJ
,
Thomas
ES
,
Theriault
RL
, et al
Significantly higher pathologic complete remission rate after neoadjuvant therapy with trastuzumab, paclitaxel, and epirubicin chemotherapy: results of a randomized trial in human epidermal growth factor receptor 2-positive operable breast cancer
.
J Clin Oncol
2005
;
23
:
3676
85
.
61.
Harris
LN
,
You
F
,
Schnitt
SJ
,
Witkiewicz
A
,
Lu
X
,
Sgroi
D
, et al
Predictors of resistance to preoperative trastuzumab and vinorelbine for HER2-positive early breast cancer
.
Clin Cancer Res
2007
;
13
:
1198
207
.
62.
Gianni
L
,
Eiermann
W
,
Semiglazov
V
,
Manikhas
A
,
Lluch
A
,
Tjulandin
S
, et al
Neoadjuvant chemotherapy with trastuzumab followed by adjuvant trastuzumab versus neoadjuvant chemotherapy alone, in patients with HER2-positive locally advanced breast cancer (the NOAH trial): a randomised controlled superiority trial with a parallel HER2-negative cohort
.
Lancet
2010
;
375
:
377
84
.
63.
Baselga
J
,
Bradbury
I
,
Eidtmann
H
,
Di Cosimo
S
,
Aura
C
,
de Azambuja
D
, et al
First results of the NeoALTTO trial (BIG 01–06/EGF 106903): A phase III, randomized, open label, neoadjuvant study of lapatinib, trastuzumab, and their combination plus paclitaxel in women with HER2-positive primary breast cancer
.
Cancer Res
2010
;
70
:
82s
.