New therapies in HER2-positive breast cancer: A major step towards a cure of the disease?

Summary

Overexpression of the human epidermal growth factor receptor 2 (HER2) predicts a poor prognosis in metastatic breast cancer. While the introduction of HER2-targeted therapies, such as the monoclonal antibody trastuzumab and the small-molecule tyrosine kinase inhibitor lapatinib, has significantly improved outcomes in HER2+ breast cancer compared with previously available therapies, use of these targeted therapies is often limited by the development of drug resistance and tolerability issues. These limitations create the need for further development and investigation of new targeted therapies that show potent and selective inhibition of these targets or closely connected molecular pathways. Recently, several agents have demonstrated promising activity in HER2+ metastatic breast cancer, either as monotherapy or in combination therapy, including the tyrosine-kinase inhibitors neratinib (HKI-272) and afatinib (BIBW-2992) and the anti-HER2 monoclonal antibodies pertuzumab and trastuzumab-DM1 (T-DM1). Agents that target other molecular pathways, such as the vascular endothelial growth factor receptor, mammalian target of rapamycin, PI3-kinases, insulin-like growth factor (IGFR), HSP-90, and other kinases also have potential, in combination with anti-HER2 and/or other systemic therapies, to be active in this subtype of breast cancer. Innovative clinical studies are required in well-characterized patient populations to define the true clinical value of these emerging new approaches.

Introduction

Breast cancer patients diagnosed with distant metastatic disease still face a dismal 5-year survival rate of only 27%, while those initially diagnosed with localized and regional breast cancer have shown 5-year survival rates of 99% and 84%, respectively.¹ Currently, the treatment of patients with metastatic breast cancer (MBC) involves the use of multiple agents, including endocrine therapies for hormone receptor-positive (HR+) disease, cytotoxic chemotherapy (e.g., anthracycline-, taxane-, or antimetabolite-based regimens), therapies targeting HER2 and vascular endothelial growth factor (VEGF) pathways, and more recently, though not yet an approved treatment, the poly-ADP-ribose polymerase (PARP) inhibitors in breast cancer gene 1 or 2 (BRCA1/2)-mutated tumors as well as triple negative subtypes.²

In particular, human epidermal growth factor (EGF) receptor 2 (HER2; ErbB2) has been identified as an important target for breast cancer.³ HER2 is amplified or overexpressed in approximately 20% of breast cancers, and increased HER2 expression correlates with more aggressive breast tumors and, prior to the introduction of trastuzumab, a poorer prognosis than tumors with normal HER2 expression. This review examines the current state of HER2-targeted therapies for MBC, identifies unmet needs for these patients, particularly for those who have already received treatment with trastuzumab, and examines emerging data with investigational agents and combination therapies that might help improve outcomes for patients with MBC. Ultimately, these new agents or approaches that exhibit significant efficacy in the metastatic setting will be moved quickly to the adjuvant setting, where cure is the main objective.

Trastuzumab, a humanized anti-HER2 monoclonal antibody (MAb), is currently recommended as first-line treatment for patients with metastatic HER2+ tumors, either as a single agent (limited group of patients) or in combination with endocrine therapy or chemotherapy, as well as in the adjuvant setting.² Although the mechanisms of action of trastuzumab are not yet completely understood, trastuzumab is likely to exerts its antitumor activity through a combination of antibody-dependent cellular cytotoxicity, antiangiogenic effects, downregulation of HER2 receptors, disruption of downstream proliferative pathways, and inhibition of cell-cycle progression (Fig. 1).⁴ Overall, trastuzumab remains the backbone of any treatment for HER2+ tumors.

Single-agent trastuzumab was shown to be active and well tolerated as first-,⁵ second-, or third-line treatment.⁶ The pivotal phase 3 trial compared first-line treatment with trastuzumab plus chemotherapy (doxorubicin plus cyclophosphamide or paclitaxel) vs. chemotherapy alone in 469 women with HER2+ MBC (Table 1).⁷ The addition of trastuzumab to chemotherapy prolonged the median time to disease progression (7.4 vs. 4.6 months with chemotherapy alone; P < 0.001), improved the objective response rate (ORR; 50% vs. 32%, respectively; P < 0.001), increased the median duration of response (9.1 vs. 6.1 months, respectively; P < 0.001), reduced the 1-year mortality rate (22% vs. 33%, respectively; P = 0.008), and more interestingly, improved median survival (25.1 vs. 20.3 months, respectively; P = 0.01). Cardiac dysfunction was reported in 27% of patients who received trastuzumab plus doxorubicin and cyclophosphamide compared with 8% of those receiving chemotherapy alone, and in 13% vs. 1% of patients receiving paclitaxel with or without trastuzumab, respectively.⁷

Subsequent trials have evaluated trastuzumab in combination with a variety of chemotherapy agents in HER2+ MBC (Table 1), including paclitaxel with or without carboplatin or docetaxel with or without capecitabine.7, 8, 9, 10 The ORRs reported for trastuzumab combined with chemotherapy ranged between 50% and 73% in these studies, and the combinations were generally associated with acceptable toxicity profiles. Of note, the addition of carboplatin or docetaxel to trastuzumab was associated with neutropenia and thrombocytopenia, and the incidence of grade 3/4 hand-foot syndrome was more common among patients receiving capecitabine.7, 8, 9, 10 It was clear that, based on these positive results in the metastatic setting, trastuzumab was moved quickly into the (neo)adjuvant setting with very positive results; however, this discussion is beyond the scope of this review.

Lapatinib is a reversible dual EGFR/HER1 and HER2 tyrosine kinase inhibitor (TKI) that is approved in combination with capecitabine for the treatment of advanced or HER2+ breast cancer after failure of chemotherapy and trastuzumab. Binding of lapatinib to the intracellular TK domains of HER1 and HER2 results in inhibition of receptor phosphorylation, which then leads to inhibition of downstream pathways that control cell proliferation and survival (Fig. 1).¹¹ Thus, lapatinib selectively inhibits human tumor cells that overexpress HER1 or HER2.

Lapatinib, as a single agent or in combination with other therapies, has shown clinical efficacy in patients with HER2+ MBC. A phase 2 study of single-agent lapatinib in chemotherapy-refractory MBC showed limited clinical activity in HER2+ disease, with an ORR of 4.3% and a clinical benefit rate (CBR; complete response [CR], partial response [PR], or stable disease [SD] ⩾6 months) of 5.7%, compared with a 0% ORR and CBR among patients with HER2-negative disease.¹² Another phase 2 study of lapatinib monotherapy in patients with HER2+ MBC who had relapsed after trastuzumab therapy showed slightly better results, with an ORR of 19.0% and a CBR of 25%.¹³ Also, lapatinib was shown to be potentially effective as a single-agent therapy in patients with relapsed or refractory locally inflammatory HER2+ breast cancer; the PR was 39%, and no patients had a CR.¹⁴ Approval for lapatinib in combination with capecitabine in the treatment of MBC was based on interim results of a pivotal phase 3 study, in which 324 women with HER2+ disease that had progressed after treatment with trastuzumab, anthracyclines, and taxanes were randomized to treatment with the combination or capecitabine alone.¹⁵ Combination treatment was associated with a significantly longer time to progression (8.4 months) compared with capecitabine monotherapy (4.4 months; P < 0.001). Updated efficacy analysis of data from 399 patients confirmed superior time to progression with lapatinib plus capecitabine (P < 0.001) and showed an improved response rate with the combination (P = 0.017; Table 1). Cumbersome adverse events (AEs) reported more commonly in the lapatinib group included diarrhea and rash.¹⁶ Quality of life assessments showed no significant deleterious effect on patients’ well being and functioning from adding lapatinib to capecitabine therapy.¹⁷ Additionally, preclinical and early clinical results suggest that the combination of lapatinib and capecitabine may be effective for the treatment of brain metastases that result from HER2+ MBC.18, 19

Recently, the combination of lapatinib and letrozole was approved by the US Food and Drug Administration (FDA) for use in HER2+ MBC for post-menopausal women who have an indication for hormone therapy.²⁰ This approval was based on results from a randomized, double-blind, multi-center, phase 3 study²¹ that showed the combination significantly increased progression-free survival (PFS; 3.0 months vs. 8.2 months, respectively; P = 0.019), ORR (15% vs. 28%; P = 0.021), and CBR (29% vs. 48%;P = 0.003) compared with letrozole alone. Similar to previous studies of lapatinib combination therapy, diarrhea and rash were more commonly reported in the combination arm; no new side effects were identified for either drug.

In another phase 3 study of 296 women with HER2+ MBC that had progressed on trastuzumab-containing regimens, the addition of lapatinib to trastuzumab provided longer PFS (P = 0.008) and a superior CBR (P = 0.01) compared with switching to lapatinib alone. In this study, there was a meaningful difference in survival (52 weeks vs. 39 weeks); however, response rates were not significantly different between groups (Table 1).²² Recent data of the neo ALTTO trial reported at the 2010 San Antonio Breast Cancer Symposium (SABCS) confirmed the clinical value of adding lapatinib to trastuzumab (plus chemotherapy) and the importance of dual inhibition of the extracellular and intracellular domain of HER2.²³

Although trastuzumab-based regimens have become the standard of care in HER2+ MBC, up to 40% of patients with this type of cancer do not respond to these regimens.²⁴ In addition, even in patients who respond, the median time to progression is approximately 1 year for patients treated with trastuzumab and chemotherapy.7, 24, 25 Resistance to commonly used HER2 therapies is a serious concern. Development of drug resistance ultimately results in disease progression, as shown by times to tumor progression and limited overall survival in studies with trastuzumab7, 10, 26, 27 and lapatinib.15, 16, 22, 28

There are several mechanisms by which tumor cells may develop resistance to HER2-directed therapies.29, 30, 31 Redundancy in growth factors and cross-talk between intracellular signaling pathways are thought to facilitate the development of resistance in most breast cancer patients (Fig. 1).29, 30, 31 For example, inhibition of HER2-mediated activation of PI3K by trastuzumab may be bypassed by lateral activation of the pathway by other HER2 family members (i.e., HER1 and HER3), thus promoting cellular proliferation.29, 32, 33 Resistance may be mediated through an altered interaction between the receptor and antibody, either through mutations in HER2 that disrupt binding, the masking of antigens on the surface of tumor cells through glycoproteins such as MUC-4, or the overexpression of p95,³⁴ a truncated HER2 that lacks its extracellular domain and thus does not have a trastuzumab binding site. In addition, loss-of-function mutations in the phosphatase and tensin homolog protein (PTEN) or activating mutations in PI3K lead to enhanced Akt phosphorylation and signaling, which is downstream of HER2 and would effectively bypass any HER2-directed therapy, resulting in cellular proliferation.³² One of the ultimate effects of trastuzumab inhibition is thought to be phosphorylation of the tumor suppressor p27, which prevents its degradation and leads to cell-cycle arrest; therefore, any conditions that lead to a loss of p27 may also contribute to trastuzumab resistance.²⁹ Finally, amplification or overexpression of cyclin E may result in resistance and decreased sensitivity to trastuzumab. Scaltriti et al. demonstrated that HER2-amplified breast cancer cell lines selected for trastuzumab resistance in vitro or in vivo carried a focal amplification of a region of genomic DNA containing the cyclin E gene.³⁵ Blocking cyclin E in trastuzumab-resistant cells with either siRNA-mediated knockdown of cyclin E expression or treatment with cyclin-dependent kinase 2 (CDK2 inhibitors) resulted in decreased proliferation in vitro and reduction of tumor growth in vivo.³⁵ This model has clinical relevance, as cyclin E amplification or overexpression in a cohort of 34 HER2+ patients was associated with significantly worse clinical benefit rate (33.3% vs. 87.5%, respectively; P < 0.02) and PFS (6 months vs. 14 months; P < 0.002).³⁵

The use of trastuzumab may also be limited by the development of drug intolerance, such as cardiac dysfunction.36, 37, 38, 39 Trastuzumab-containing regimens have been associated with a 0.4–3.9% incidence of severe congestive heart failure and a 3–18.1% decrease in left ventricular ejection fraction (LVEF) in the adjuvant setting. Notably, the highest cardiac AE rates were associated with trastuzumab-containing regimens in patients concomitantly on or previously treated with anthracycline-based chemotherapy.36, 37, 38, 39 Better cardiac profiles of liposomal anthracyclines over that of conventional doxorubicin have been well demonstrated.⁴⁰ In a phase 1 trial of patients with MBC, liposomal doxorubicin plus trastuzumab demonstrated anti-tumor efficacy and was associated with less cardiac toxicity than conventional doxorubicin plus trastuzumab.⁴¹ Moreover, trastuzumab in combination with liposomal doxorubicin and either cyclophosphamide⁴² or docetaxel⁴³ was associated with low rates (0–3%) of congestive heart failure in two recent phase 2 trials. Thus liposomal doxorubicin might provide a safer alternative to conventional doxorubicin in combination with trastuzumab.

Another fairly debilitating finding for breast cancer patients is the development of brain metastases. Approximately 50% of women with metastatic HER2+ breast cancer develop central nervous system (CNS) metastases.⁴⁴ The higher incidence of brain metastases in trastuzumab-treated patients may be a consequence of prolonged survival in these patients and/or a failure of trastuzumab to penetrate the blood–brain barrier.45, 46, 47, 48, 49 Therapeutic options (besides surgery and/or radiation) are very limited for the treatment of brain metastases, and prognosis is poor. Novel targeted treatment options are, therefore, urgently required to improve outcomes in these patients.

Shortcomings with current treatments such as trastuzumab and lapatinib mean that a need exists for novel and improved therapies targeted to HER2+ MBC. Ideal targeted therapies would show certain desired characteristics, including potent inhibition of a commonly overexpressed molecule(s) in MBC, such as the whole HER family of receptors, and good selectivity for the chosen therapeutic target(s). The ideal agent would also display irreversible binding to its molecular target, thus producing longer-lasting effects. Any new therapy should demonstrate significant antitumor activity and a good tolerability profile in advanced-stage, drug-resistant breast cancer, either as a single agent or eventually in combination with other therapies, with a limited propensity for development of drug resistance. Accurate tests for screening suitable patients and monitoring of disease response or progression with new therapies are also necessary. A variety of novel targeted agents are being investigated for the treatment of MBC; many of these agents target the HER2 pathway (Table 2, Table 3) and may have the potential to address, at least in part, these unmet needs.⁵⁰

Neratinib (HKI 272) is an orally administered, irreversible pan-HER inhibitor with activity against HER1, HER2, and HER4.⁵¹ Neratinib irreversibly inhibits TK activity by binding with a cysteine in the ATP-binding pocket of the enzyme and preventing autophosphorylation. Preclinical experiments demonstrated that neratinib inhibits the proliferation of HER2-overexpressing breast cancer cell lines and HER1-overexpressing epidermal carcinoma cell lines.⁵²

In a phase 2 trial in advanced HER2+ breast cancer, 66 patients that were previously treated with trastuzumab and 70 trastuzumab-naive patients were treated with neratinib.⁵¹ Patients in this study could have received up to four prior cytotoxic chemotherapy regimens. The 16-week PFS rate and ORRs, respectively, were 59% and 24% for patients with prior trastuzumab treatment and 78% and 56% for patients with no prior trastuzumab treatment (Table 2). These early data of neratinib as single agent therapy are very promising. The most common AEs were diarrhea, nausea, vomiting, and fatigue. Diarrhea was the most frequent grade 3/4 AE, occurring in 30% of patients with previous trastuzumab treatment and 13% of trastuzumab-naive patients. Interestingly enough, serial LVEF measurements taken during the study revealed little variation in LVEF from baseline in most patients, regardless of prior trastuzumab therapy. No drug-related, grade 3/4 cardiotoxicity was reported.

Several ongoing trials are investigating the combination of neratinib with chemotherapy in patients with MBC. A phase 1/2 study investigated the combination of neratinib plus paclitaxel, which enrolled patients with any solid tumor in part 1 and with HER2+ MBC in part 2; patients had received up to three prior chemotherapy regimens for metastatic disease.53, 54 Data for 99 evaluable patients in part 2 showed an ORR of 73%, SD for ⩾24 weeks for 9% of patients, and a median PFS of 57.0 weeks (Table 2). The ORR was 71% among patients who had received prior trastuzumab (n = 20/28) or lapatinib therapy (n = 10/14). Diarrhea was the most common AE, with 93 (91%) patients experiencing diarrhea of any grade and 29 (28%) patients with grade 3/4 diarrhea. AEs led to neratinib dose reductions in 15% of patients and paclitaxel dose reductions in 36% of patients. One patient discontinued the study due to LVEF reduction, but there were no other cardiac AEs reported.

Another phase 1/2 study evaluated neratinib plus vinorelbine in patients with solid tumors in part 1 and with HER2+ MBC in part 2 (all patients had received at least one prior trastuzumab-containing regimen for metastatic disease).55, 56 Preliminary data for the 68 evaluable patients treated in part 2, 12 of whom had received prior lapatinib therapy, showed an ORR of 57% for lapatinib-naive patients and 50% for patients treated with prior lapatinib (Table 2). The most common grade 3/4 AEs were neutropenia (45% and 40% of lapatinib-naive and lapatinib pre-treated patients, respectively) and diarrhea, which occurred in 34% of lapatinib-naive patients and in 20% of patients who received prior lapatinib. Another similarly designed phase 2 study is currently investigating the combination of neratinib and capecitabine in HER2+ MBC that has progressed on a prior trastuzumab-containing regimen; preliminary results from part 1 of the study, involving patients with advanced solid tumors, have indicated acceptable tolerability of the regimen.57, 58

Neratinib is also being studied in combination with temsirolimus in HER2+ or triple-negative MBC (ClinicalTrials.gov, NCT01111825) and in solid tumors (NCT00838539); as monotherapy vs. lapatinib plus capecitabine in trastuzumab pre-treated HER2+ MBC (NCT00777101); and combined with paclitaxel vs. paclitaxel plus trastuzumab in the first-line treatment of HER2+ MBC (NCT00915018) and in the adjuvant setting upon completion of trastuzumab-based therapy as well as neoadjuvant treatment in locally advanced HER2+ breast cancer (NCT01008150). Table 4 summarizes these clinical trials.

In conclusion, the clinical results of neratinib reported to date, as either a single agent or as combination therapy, have been very promising. Diarrhea is the main side effect of neratinib and should be managed early and as effectively as possible in order to keep the drug at optimal dose intensity. Finally, it is noteworthy that neratinib shows no evidence of skin toxicity, suggesting that its EGFR effect may be weaker than other drugs, at least at the clinical level.

BIBW-2992 (afatinib) is an irreversible dual EGFR/HER1 and HER2 TKI.⁵⁹ In vitro experiments have demonstrated the inhibition of kinase activity of HER in a variety of cell lines, in particular HER3, a receptor that is phosphorylated by HER1 and HER2 heterodimerization and the activation of which is often associated with resistance to other TKIs.⁵⁹ In vivo preclinical antitumor activity was demonstrated in mouse xenografts overexpressing HER1 and/or HER2, in which administration of BIBW-2992 reduced tumor size and produced complete tumor regression in some animals.⁵⁹

A phase 2 study evaluated BIBW-2992 monotherapy in 41 patients with HER2+ MBC after failure of trastuzumab treatment.⁶⁰ Preliminary data demonstrated PR in 4 (11%) patients, 1 of whom remained on treatment until disease progression at 63 weeks, and SD for ⩾4 cycles in 15 (43%) patients (Table 2). The most common AEs were grade 3 diarrhea and grade 3 rash; 20 patients required dose reductions due to AEs.

Other ongoing phase 2 studies, summarized in Table 4, are investigating BIBW-2992 in HER2+ MBC in combination with trastuzumab (ClinicalTrials.gov NCT00950742) or letrozole (NCT00708214), and in comparison with trastuzumab or lapatinib as first-line treatment of locally advanced disease (NCT00826267). A phase 3 study is comparing the addition of BIBW-2992 or trastuzumab to vinorelbine in HER2+ MBC that has progressed on trastuzumab (NCT01125566).

One approach in treating MBC is to inhibit the cross-talk among different HERs by inhibiting multiple receptors at once, as do neratinib (HER1, HER2, and HER4) and BIBW-2992 (HER1 and HER2); such drugs could overcome one of the bypass mechanisms of resistance in patients previously exposed to trastuzumab by inhibiting the lateral activation by other HER proteins.³⁸ The degree of activity seen with neratinib in both trastuzumab-naive and trastuzumab-treated patients and early data with afatinib compare favorably with prior reports of single-agent trastuzumab therapy.⁶¹ A firm conclusion is awaiting the results of recent or ongoing phase 2 and 3 trials of both small-molecule HER2 TKIs.

Pertuzumab is a humanized MAb that binds to the extracellular domain of HER2, blocking receptor dimerization.⁶² Pertuzumab binds to a different site of HER2 than trastuzumab, and in tumor xenograft studies the addition of pertuzumab to ongoing trastuzumab after progression has been shown to synergistically enhance tumor inhibition compared with continuing trastuzumab alone.⁶² The binding of pertuzumab to tumors was not impaired by trastuzumab pretreatment. The results suggest that pertuzumab and trastuzumab display complementary mechanisms of action and subsequent inhibition of downstream intracellular pathways.

A phase 1 study of pertuzumab monotherapy for solid tumors found limited evidence of antitumor activity⁶³; however, there is clinical evidence of a synergistic effect in combination with trastuzumab. In a phase 2 study, 66 patients with HER2+ MBC after progression on trastuzumab were treated with pertuzumab plus trastuzumab.⁶⁴ The ORR was 24.2%, while the CBR was 50% (Table 2). Most AEs were mild or moderate in intensity; grade 3/4 AEs included diarrhea, rash, asthenia, and pruritus. Cardiac dysfunction was considered minimal, and cardiac AEs did not result in any patient withdrawals from treatment. LVEF was assessed regularly through the study, and no major changes from baseline were noted. However, a separate phase 2 study reported asymptomatic cardiac toxicity with this regimen of pertuzumab and trastuzumab.⁶⁵ This study enrolled 11 women with HER2+ MBC, all of whom had LVEF of 55% or greater at baseline. A total of six patients experienced a reduction in LVEF: three patients had LVEF between 50% and 55%, two patients had LVEF between 40% and 50%, and one patient had LVEF between 20% and 40% and experienced symptomatic congestive heart failure. Efficacy was similar to that observed in the previous study (Table 2).

Ongoing studies of pertuzumab in HER2+ MBC include combinations with trastuzumab and docetaxel in first-line treatment (ClinicalTrials.gov NCT00567190) and trastuzumab and capecitabine after progression on trastuzumab (NCT01026142). Other studies are investigating pertuzumab plus trastuzumab in early-stage HER2+ breast cancer (NCT00976989 and NCT00545688). Gianni et al. recently reported the neosphere neoadjuvant trial results, which confirms the importance of combining pertuzumab with trastuzumab (pathological CR [pCR], 18%) or combining both HER2 agents with docetaxel (pCR, 46%).⁶⁶

T-DM1 is a conjugation of the anti-HER2 MAb trastuzumab and the anti-microtubule chemotherapy agent maytansine (DM1).⁶⁷ Combination of these agents allows targeted delivery of a potent cytotoxic agent to tumor cells overexpressing HER2.⁶⁷ T-DM1 is a typically targeted chemotherapy agent.

In a single-arm phase 2 study in 112 patients with HER2+ MBC that had progressed on prior trastuzumab, T-DM1 treatment provided an ORR of 27% by independent review (Table 2).⁶⁸ The most common grade 3/4 AEs was thrombocytopenia in 7% of patients. An ongoing phase 2 study is evaluating T-DM1 monotherapy in 110 patients with HER2+ MBC previously treated with lapatinib, trastuzumab, and chemotherapy.69, 70 Preliminary results indicate an ORR of 33% and a CBR of 48% by independent review (Table 2).⁷⁰ The most common AEs (all grades) were fatigue (62%), nausea (37%), and thrombocytopenia (33%) and were primarily grade 1/2.⁷⁰ Of 107 patients with LVEF measurements, none showed LVEF values below 45% or reduction from baseline of 25% or more.⁶⁹ Encouraging data were presented at the 2011 meeting of the European Society of Medical Oncology from a randomized trial comparing T-DM1 to docetaxel plus trastuzumab in patients with HER2+ MBC who had no prior therapy for metastatic disease.⁷¹ Preliminary data showed comparable ORR in the T-DM1 and trastuzumab plus docetaxel arms (43% vs. 40%, respectively) with significant improvement in PFS for the T-DM1 arm (14.2 vs. 9.2 months). Consistent with previously reported results,⁷² grade 3/4 AEs were much lower in the T-DM1 arm (46.4% vs. 89.4%).⁷¹ Finally, the combination of T-DM1 plus pertuzumab is being evaluated in a phase 1b/2 study in 67 patients with HER2+ MBC as first-line therapy or after relapse from prior trastuzumab treatment. In patients receiving first-line therapy (n = 22), there were nine observed responses (confirmed, n = 2) after a median follow-up of three cycles. In relapsed patients (n = 45), after a median follow-up of eight cycles, there were 19 observed responses (confirmed, n = 15). Thrombocytopenia was the most common grade 3/4 AE and was reported for 11.9% of patients.⁷³

Other ongoing trials of T-DM1 in HER2+ MBC include first-line T-DM1 plus pertuzumab vs. trastuzumab plus paclitaxel (MARIANNE study; ClinicalTrials.gov NCT01120184), first-line T-DM1 compared with trastuzumab plus docetaxel (NCT00679341), T-DM1 vs. capecitabine plus lapatinib (EMILIA study; NCT00829166), and T-DM1 plus docetaxel (NCT00934856). A good quality HER2 test is a prerequisite for T-DM1 efficacy because in the absence of HER2 overexpression this drug has theoretically no efficacy.

MM-111 is a novel MAb that targets HER2 and HER3, preventing their dimerization.⁷⁴ Two phase 1/2 studies are investigating MM-111 as monotherapy and in combination with trastuzumab in HER2+ MBC that has progressed on previous therapies (ClinicalTrials.gov NCT00911898 and NCT01097460).

VEGF is often overexpressed in breast cancer and is associated with poor clinical outcomes.⁷⁵ Bevacizumab is a humanized anti-VEGF MAb that is approved for the first-line treatment of MBC in combination with paclitaxel. Approval was based on the results of a phase 3 study in which 722 women with MBC were randomized to receive paclitaxel plus bevacizumab or paclitaxel alone.⁷⁶ Only 8 (1%) patients in this study had HER2+ disease, all of whom had received prior trastuzumab; 91% (n = 614) of patients were HER2-negative and the remainder (8%; n = 51) were of unknown status. The combination significantly prolonged median PFS compared with paclitaxel alone (11.8 months vs. 5.9 months, respectively; P < 0.001) and increased ORR (37% vs. 21%, respectively; P < 0.001), although median overall survival rate was similar in the two groups (P = 0.16). Grade 3/4 AEs of hypertension (P < 0.0001), proteinuria (P < 0.0001), headache (P = 0.008), and cerebrovascular ischemia (P = 0.02) were reported more frequently with the combination vs. paclitaxel alone.⁷⁶ Another phase 3 study compared bevacizumab plus capecitabine vs. capecitabine alone in previously treated MBC, including 23% of patients with HER2+ disease who had progressed on prior trastuzumab.⁷⁷ The combination significantly improved ORR compared with capecitabine monotherapy (20% vs. 9%, respectively; P = 0.001), but not PFS or overall survival.

In HER2-negative MBC, the combination of bevacizumab with docetaxel has been investigated in the AVADO study⁷⁸ and bevacizumab plus anthracycline, taxane, or capecitabine were investigated in the RiBBON 1 and 2 studies.⁷⁹ The AVADO study suggested benefit with the combination vs. docetaxel alone,⁷⁸ and the possible benefits of bevacizumab in combination with docetaxel and trastuzumab in HER2+ disease are currently under investigation (NCT00428922). Interim results from a phase 2, multicenter study of bevacizumab and docetaxel with or without trastuzumab that evaluated 21 patients with HER2+ MBC showed an ORR of 81% (CR, 29%; PR, 52%) and a median PFS of 8.5 months; however, at the time of the data cutoff (April 2010) all patients had discontinued treatment, mostly because of disease progression (40%).⁸⁰ An additional study the in the adjuvant setting (BETH study; NCT00625898) is assessing bevacizumab in combination with trastuzumab and chemotherapy.

Although approved for the treatment of MBC, bevacizumab treatment is often suspect of having problems common to anti-angiogenic durgs. Both preclinical and clinical studies suggest that tumor growth is sometimes accelerated after termination of treatment with anti-angiogenic drugs.³⁰ Even with continuous treatment, tumor cells may become more aggressive and invasive over time. While the hypoxia created by reducing vascularization initially slows tumor growth, hypoxia also induces certain transcription factors, such as hypoxia inducible factor and other angiogenic factors, so that the surviving tumor cell populations express a more invasive or metastatic phenotype.³⁰ Such mechanisms may explain (at least in the clinical setting) why treatment with anti-angiogenic agents, particularly small molecules, did not result in significant overall survival advantage.76, 77, 81 To assess the clinical course of disease after discontinuation of bevacizumab therapy, Miles et al. performed a retrospective meta-analysis with five randomized, placebo-controlled trials comparing the combination of bevacizumab plus chemotherapy with chemotherapy alone, analyzing the time from discontinuation to disease progression or death.⁸² The results from the meta-analysis did not support decreased time to disease progression or increased mortality after cessation of treatment in the bevacizumab arm compared with the placebo control arm (however, their analysis did not address post-progression survival time). Bevacizumab and other anti-angiogenic agents have also been associated with poor tolerance, including vascular AEs, mainly in elderly or unfit patients.83, 84, 85

Mammalian target of rapamycin (mTOR) is a member of the phosphoinositide-kinase-related kinase family and plays a role in mediating cell growth and proliferation as a downstream activator along the PI3K/Akt signaling pathway.⁸⁶ Inhibitors of mTOR in development as antitumor agents include temsirolimus and everolimus. The efficacy and tolerability of temsirolimus in heavily pretreated MBC was investigated in a phase 2 study of 109 women, 35% of whom had HER2+ disease.⁸⁶ The ORR was 9% and median time to progression was 12.0 weeks. Grade 3/4 AEs included mucositis (9%), leucopenia (7%), and hyperglycemia (7%). Likewise, everolimus has demonstrated some activity in a phase 2 study involving 49 patients with pretreated MBC, 6 of whom had HER2+ tumors.⁸⁷ Out of 33 patients who received a daily schedule of treatment, 4 (12%) exhibited tumor response, while none of the 16 patients receiving a weekly schedule had responses. The most common grade 3/4 AE was fatigue, reported by five patients in each dosage group. A phase 1b dose-escalation study investigated the addition of everolimus to a combination therapy of paclitaxel and trastuzumab in patients with trastuzumab/paclitaxel-refractory, HER2-overexpressing MBC.⁸⁸ For 27 patients with measurable disease, the ORR was 44% and the median PFS was 34 weeks. The combination of everolimus (10 mg daily), weekly paclitaxel, and trastuzumab was generally well tolerated; neutropenia (52% of patients with grade 3/4) and stomatitis (81% of patients with grade 2/3) were the most common toxicities and were manageable with appropriate care.⁸⁸ In HER2+ disease refractory to trastuzumab and taxane therapy, preliminary results from a phase 2 study demonstrated antitumor activity with a combination of everolimus plus paclitaxel and trastuzumab (n = 37), with an ORR of 20% and SD achieved by 56% of patients,⁸⁹ and this combination is currently being investigated in a phase 3 study (BOLERO-3; NCT01007942). Everolimus is also being tested in combination with vinorelbine and trastuzumab, and in a phase 1b study (N = 47), ORR was 19.1% with a median PFS of 30.7 weeks. The combination was generally well tolerated, and the most common AEs included neutropenia (92%) and stomatitis (70%).⁹⁰ A phase 1 study (n = 18) has also demonstrated feasibility of a combination of everolimus plus letrozole in MBC that had not responded to first- or second-line endocrine therapy.⁹¹ Another study (phase 1/2 trial) evaluated safety and efficacy of the combination of everolimus and trastuzumab without chemotherapy in women with HER2+ MBC who progressed on trastuzumab-based therapy. The combination of trastuzumab and everolimus resulted in partial response in 7 of 47 (15%) patients, stable disease lasting for at least 6 months in 9 (19%) patients, a median PFS of 4.1 months, and a clinical benefit rate of 34%. Fatigue, infections, and mucositis (the majority of which were grade 2 events) were the most common non-hematologic toxicities.⁹² The inhibition of mTOR activity downstream of HER2 via temsirolimus or everolimus may reverse trastuzumab resistance in HER2-overexpressing MBC.

Cancer cells that have active PI3K/Akt signaling, even in the background of upstream HER2 inhibition, effectively bypass HER2-targeted therapies, either through ineffective HER2 inhibition (e.g., constitutively active PI3K) or by activation from additional kinases such as other HER family members. Because mTOR is a downstream effector of the PI3K pathway, drugs that can alter mTOR function may be able to overcome the drug resistance that results from the upstream cross-talk between the HER family members or other growth factor receptors.29, 86 The early positive results from the studies combining an mTOR inhibitor with paclitaxel or letrozole support this theory that combining two drugs to target multiple pathways may be an avenue to overcoming trastuzumab resistance. Clinical trials are ongoing to find optimum doses and treatment combinations (chemotherapy, hormone therapy, trastuzumab, or other anti-HER2 agent) for mTOR inhibitors.⁹³ For example, the phase 1/2 trial listed above that is evaluating the safety and benefit of temsirolimus in combination with neratinib for patient with HER2+ MBC.

Breast cancer cells exhibit simultaneous activation of various oncogenic pathways, some of which may have redundant activity and may therefore confer resistance to treatment.³¹ Because of this redundancy, targeting multiple pathways with combination therapy is a reasonable approach to improving treatment for MBC. As one example, evidence of cross-talk between the estrogen receptor and HER2 pathways provides a rationale for developing combinations of anti-HER2 agents with antihormonal therapies that may benefit a subset of HER2+ MBC patients. If successful, such strategies may delay the necessity of initiating chemotherapy.

Laboratory and retrospective clinical data established that HER2 amplification results in resistance to hormone therapy. Two prospective clinical studies provided evidence that HER2+ breast cancers are less responsive to hormonal therapy and demonstrated that addition of either trastuzumab or lapatinib to hormonal therapy with aromatase inhibitors resulted in significant improvement in PFS in patients with HER2+ and HR+ breast cancers.21, 94 The phase 3 TanDEM study investigated trastuzumab plus the aromatase inhibitor anastrozole vs. anastrozole alone in 207 postmenopausal women with HER2+, HR+ MBC.⁹⁴ The study achieved its primary endpoint with a significant difference in median PFS (5.6 months for the combination vs. 3.8 months for anastrozole monotherapy; P = 0.006). Overall, grade 3/4 AEs were more common with the combination regimen compared with anastrozole monotherapy, the most common being vomiting (3% vs. 1%), back pain (2% in both groups), bone pain (2% vs. 0%), and hypertension (4% vs. 2%). Another phase 3 study compared lapatinib plus letrozole vs. letrozole alone as first-line therapy in 1286 postmenopausal women with HR+ MBC.²¹ Among HER2+ patients (n = 219), the addition of lapatinib significantly prolonged median PFS (8.2 months vs. 3.0 months, respectively; P = 0.019). Again, grade 3/4 AEs were more common with the combination vs. aromatase inhibitor monotherapy (diarrhea, 10% vs. 1%; rash, 1% vs. 0%, respectively). Preliminary results from a phase 2 study recently showed that the addition of BIBW-2992 to letrozole in 28 patients with hormone-resistant MBC (19 of whom were HER2-negative) provided SD of 16 weeks or more for seven patients.⁹⁵

Combination therapy with HER2 inhibitors and hormone therapy has shown significant improvements in PFS, CBR, and time to progression. These results are reassuring, since aromatase inhibitors should be started in the adjuvant setting while adjuvant trastuzumab administration is still ongoing. Future studies should determine the quality of life gain associated with using this combination of biological agents, the optimal treatment sequence, and when to use chemotherapy. Studies should also focus on additional combination therapies for the appropriate selection of patients within the HER2+ MBC population. Chemotherapy plus HER2 therapy should be considered the standard of care for patients with HER2+ MBC, since it is an aggressive disease generally resistant to hormonal therapy and the benefits achieved with chemotherapy (i.e., docetaxel)²⁴ plus trastuzumab appear higher compared with aromatase inhibitor-based therapy.⁹⁶ Whether anti-HER2, hormonal therapy, or a combination of the two should be used instead of a combination of chemotherapy and anti-HER2 therapy remains a difficult question that needs to be addressed, especially in patients with a less aggressive course of disease.⁹⁶ Results from studies in individualized clinical settings suggest that anti-HER2 therapy be accompanied by endocrine therapy, while results from randomized clinical trials suggest that the combination of anthracycline- or taxane-based chemotherapy plus anti-HER2 therapy offers better antitumor activity.⁹⁶ Further classification between responders and non-responders among breast cancer patients of luminal B subtypes might be necessary to substantiate the difference in treatment response between endocrine therapy and chemotherapy and to unveil the drug resistance mechanism as well as the intricate signaling pathways leading to tumor progression. In this aspect, pharmacogenomics are expected to play a more crucial role in therapeutic decision making for patients with HER2+ MBC.