Reviews in Oncology Reviews in Oncology CIC Edizioni Internationali 2013 January-March; 1(1): 5–14. ISSN: 2282-6378

Beyond trastuzumab: what does it?

Paola Malaguti, Sabrina Vari, Domenica Pellegrini, Alessandra Fabi, and Francesco Cognetti

Division of Medical Oncology A, Regina Elena National Cancer Institute, Rome, Italy

HER-2 is amplified in 20 to 25% of BC and in these cases the encoded protein is present in abnormally high levels in the malignant cells (1, 2). Women with BC that overexpressing HER-2 have an aggressive form of disease with significantly shortened disease-free survival and overall survival (15). Laboratory studies indicate that amplification of HER-2 has a direct role in the pathogenesis of BC, (610) thereby providing investigators with an opportunity to target a therapeutic agent directly against the alteration. Several murine monoclonal antibodies against the extra-cellular domain of the HER-2 protein were found to inhibit the proliferation of human cancer cells that overexpressed HER-2, both in vitro and in vivo(1113). To minimize immunogenicity, the antigen-binding region of one of the more effective antibodies was fused to the framework region of human IgG and tested against BC cells that overexpressed HER-2 in vitro and in vivo(14, 15). This antibody, called trastuzumab, inhibited tumor growth when used alone but had synergistic effects (13, 1517) when used in combination with cisplatin and carboplatin, docetaxel, and ionizing radiation and additive effects when used with doxorubicin, cyclophosphamide, methotrexate, and paclitaxel (13, 18, 19). Phase 1 clinical trials showed that the antibody is safe and confined to the tumor. Subsequent phase 2 trials demonstrated that many women with HER-2-positive metastatic disease who had relapsed after chemotherapy had a response to trastuzumab (20, 21); as suggested by the preclinical data, the efficacy of trastuzumab when given with chemotherapy was superior to its effectiveness when used alone (2124). In HER-2-positive metastatic breast cancer (MBC), trastuzumab provides significant clinical benefit as a monotherapy and in combination with numerous chemotherapies. In the phase III trial of first-line trastuzumab plus chemotherapy (25), overall response rate (ORR; 50%, P < 0.001), overall survival (25.1 months vs 20.3 months, P = 0.046) and time to disease progression improved significantly compared with chemotherapy alone (7.4 vs 4.6 months, P < 0.001), and second-line trastuzumab use after prior trastuzumab has resulted in ORRs of up to 50%. Clinical success in the metastatic setting provided the rationale for assessing trastuzumab in early breast cancer (2630). Four large trials of adjuvant trastuzumab demonstrated significant improvements in disease-free survival (33–52%) and overall survival (34–41%) despite tumor size, nodal or hormone-receptor status, and age. New approaches to maximize the clinical benefit of trastuzumab-based therapy are under investigation and include novel combinations with other targeted therapies such as bevacizumab, pertuzumab, lapatinib, neratinib and T-DM1.

This is an analysis of current knowledge on trastuzumab beyond progression in HER-2 positive advanced BC and our opinion on the future developments of new anti HER-2 drugs.

This review focused on the mechanisms of resistance to HER-2-targeted therapy and the strategies for overcoming this resistance. The reference list was update in February 2013. All identified phase III and II studies evaluating trastuzumab and other monoclonal antibodies and TKI in MBC are included. Data of this systematic review, obtained from the PubMed database using the following terms: “trastuzumab resistance”, “lapatinib”, “neratinib”, “T-DM1”, “HER-2 target therapy”, “HER-2 family TKIS” and “novel drug HER-2 pathway”. In addiction, abstracts from annual meetings of the American Society of Clinical Oncology (ASCO) and the San Antonio Breast Cancer Symposium 2012–2013 were retrieved for relevant abstracts using the same search terms.

Drug resistence remains a major clinical problem that hinders the successful management of BC patients. Several mechanisms of resistance to HER-2-targeted therapy have been proposed. These include impediments to HER-2-trastuzumab binding, signaling through alternative pathways, up regulation of signaling pathways downstream of HER-2, and failure to elicit an appropriate immune response (31, 32). Specific factors implicated in resistance include the HER-2 copy number; HER-2 dimerization status; presence of truncated HER-2 (p95HER-2); Fc receptor status; loss of phosphatase and tensin homolog (PTEN) and p27Kip1 expression; activation of mutations of PI3KCA; amplification of the epidermal growth factor receptor (EGFR) gene or overexpression of the EGFR protein; overexpression of insulin-like growth factor receptor 1 (IGF1R), vascular endothelial growth factor receptor, and heat shock protein 90 (HSP90); activation of the cytoplasmic tyrosine kinase SRC; and mucin 4 glycopeptide expression (3134).

Switching chemotherapeutic agent
In the clinical practice, various treatment strategies are used in attempts to overcome trastuzumab resistance (
35). One strategy is to continue trastuzumab therapy beyond progression and combined it with an alternative chemotherapy regimen (3640). Although the optimal duration of trastuzumab therapy remains controversial, data from preclinical studies indicating additive or synergistic effects of trastuzumab with several cytotoxic agents has led many clinicians to continue to administer trastuzumab beyond progression, in combination with second- or third-line chemotherapeutic agents (3638). In one small, retrospective study, some patients experienced benefit with as many as four consecutive trastuzumab-containing regimens (37). Also, a randomized clinical trial of women with HER-2-positive BC that progressed after previous trastuzumab therapy indicated that combination therapy with trastuzumab plus capecitabine provided significant benefit compared with capecitabine alone (39). Although this phenomenon may be explained by an additive action of trastuzumab and chemotherapy, further prospective randomized studies are needed to evaluate the clinical benefits of continuing trastuzumab beyond progression vs switching to a non-trastuzumab-containing regimen.

Switching HER-2-targeted therapy
Another strategy for patients with trastuzumab-refractory disease is to switch to a different HER-2-targeted therapy. In a phase III study of women with HER-2-positive metastatic breast cancer that progressed on trastuzumab, combination therapy with capecitabine and lapatinib, substantially extended the time to progression by 4 months over capecitabine alone (8.4 months vs 4.4 months; P < 0.001) (41). Of note, multityrosine kinase inhibitors may be effective for women who overexpress p95HER-2. Expression of p95 HER-2 leads to de novo trastuzumab resistance (4244) and, thus, poorer clinical outcomes (45), because the truncated protein lacks the extracellular domain required for trastuzumab binding. The binding of lapatinib to the intracellular domain of HER-2 (46) allows it to inhibit both full-length HER-2 and truncated p95HER-2 (44). Indeed, in a pooled analysis, lapatinib as monotherapy or in combination with capecitabine provided the same clinical benefit in patients who did and did not express p95HER-2 (47).

The novel antibody-drug conjugate T-DM1, may also provide some promise in patients who have developed resistance to trastuzumab alone. Recently, results from a randomised phase-II trial of T-DM1 vs trastuzumab/docetaxel in first-line, HER-2-positive MBC were presented by Perez et al. (48). In 137 patients with a median follow-up of 6 months, single-agent T-DM1 achieved an objective RR of 47.8%, as compared with 41.4% for the trastuzumab/docetaxel arm. A recent update of these data also demonstrated a significant increase in investigator-reported PFS with T-DM1 compared with the control arm (14.2 vs 9.2 months, respectively). Preliminary data from a single-arm phase-Ib/II trial evaluating the combination of pertuzumab and T-DM1 in patients with previously untreated (n=21) and relapsed (n=46) HER-2-positive MBC showed a RR of 57.1% in previously untreated patients [majority had received trastuzumab (86%), taxanes (71%) and anthracyclines (62%) in the adjuvant setting] and a RR of 34.8% in patients with relapsed disease (49). T-DM1 plus pertuzumab appeared to be well-tolerated overall, although cardiotoxicity was observed with LVEF declines in two patients. The results of large global phase-III trials of T-DM1 (EMILIA; NCT00829166), including T-DM1 vs lapatinib plus capecitabine in 991 patients previously treated with a taxane and trastuzumab, showed a median PFS of 9.6 months with T-DM1 vs 6.4 months with lapatinib plus capecitabine [hazard ratio for progression or death from any cause, 0.65; 95% confidence interval (CI), 0.55 to 0.77; P<0.001], and median OS at the second interim analysis crossed the stopping boundary for efficacy (30.9 months vs 25.1 months; hazard ratio for death from any cause, 0.68; 95% CI, 0.55 to 0.85; P<0.001). The ORR was higher with T-DM1 (43.6%, vs 30.8% with lapatinib plus capecitabine; P<0.001); results for all additional secondary end points favored T-DM1 (50). It is believed that T-DM1 will play a role in the management of patients with advanced and early stage HER-2-positive breast cancer, but this awaits further study. Especially the ongoing phase III trials, MARIANNE (a three-arm trial evaluating T-DM1 vs T-DM1/pertuzumab vs trastuzumab/taxane in the first-line setting, NCT011 20184) (51) and THERESA (T-DM1 in comparison with treatment of physician’s choice in patients with HER-2-positive MBC who have received two prior regimens of HER-2-directed therapy, NCT01419197) (52) will further give information the place of T-DM1 in the tretament algorithms for HER-2-positive disease. The trials of T-DM1 as a single agent and in combination with other chemotherapies have shown clinical activity and a favorable safety profile in patients with HER-2-positive metastatic breast cancer. There are ongoing studies with T-DM1 increasing tendency towards moving the study of these agents to earlier stages of the HER-2-positive breast cancer (Tab.1) (53).

Table 1 Table 1
Featured Ongoing Trials of T-DM1.

Combining HER-2 inhibitors
Recent data suggest that dual HER-2 inhibition provides the clinical benefit in trastuzumab-resistant disease. In a phase III study of patients with metastatic breast cancer exposed to a median of three previous trastuzumab-containing regimens, treatment with trastuzumab plus lapatinib significantly improved median PFS compared with lapatinib alone (12.0 weeks vs 8.1 weeks; P = 0.008) without compromising safety (
54). Importantly, data thus far only suggest a trend to overall survival improvement (51.6 vs 39.0 weeks; HR=0.75; P=0.106). Interim results of a phase-II trial evaluating the combination of lapatinib and trastuzumab in patients with HER-2-positive MBC [cohort 1 (n=40): no prior lapatinib, trastuzumab or chemotherapy for metastatic disease and >1 year since adjuvant trastuzumab, if received; cohort 2 (n=47): one to two prior lines of chemotherapy, including trastuzumab, or relapse within 1 year of adjuvant trastuzumab] were recently presented and showed objective RRs of 41.7% and 25% in cohorts 1 and 2, respectively (55). In phase II and III studies, the developmental humanized monoclonal antibody pertuzumab, shows promising efficacy when added to trastuzumab in several different settings (5659). As shown in cohort 3 of the BO17929 study, combination therapy with trastuzumab and pertuzumab provided the clinical benefit to patients who progressed after sequential trastuzumab and pertuzumab monotherapy (59). In the phase III Clinical Evaluation of Pertuzumab and Trastuzumab study of patients with HER-2-positive metastatic breast cancer, first-line combination therapy with trastuzumab plus pertuzumab and docetaxel prolonged median PFS by 6.1 months compared with trastuzumab and docetaxel alone (18.5 months vs 12.4 months; P < 0.001) (56). Ongoing trials are currently evaluating combinations of pertuzumab with trastuzumab and chemotherapy in HER-2-positive MBC, including a phase-II trial of trastuzumab/capecitabine±pertuzumab in the second-line setting (PHEREXA; NCT01026142) (60) and a large global phase-III trial of trastuzumab/docetaxel±pertuzumab in the first-line setting (CLEOPATRA; NCT 00567190) where the preclinical data are very encouraging; the combination of the anti-HER-2 monoclonal antibodies pertuzumab and trastuzumab with docetaxel as first-line therapy prolonged PFS in patients with HER-2-positive metastatic breast cancer. The increase of 6 months in PFS is striking. Overall survival data are event driven and are estimated to be released in 2013 (61, 62). Combinations of pertuzumab with T-DM1 (discussed before) are also under investigation in multiple phase-I–III studies (49).

Monoclonal Antibodies
Pertuzumab (Perjeta) is a monoclonal antibody, similar to trastuzumab. It is the latest drug to be approved for the treatment of HER-2 positive MBC and receveid its approval from FDA on June 2012. HER-2 is the preferred binding partner for dimerization for all the EGFR/ErbB receptor family members (EGFR/HER1, HER3 and HER4). By binding to and inhibiting HER-2, pertuzumab inhibits the potential dimerization of HER 2 with all other family receptors, resulting in the inhibition of growth (
6366). Preclinical studies in HER-2-positive breast cancer have demonstrated promising antitumors efficacy with associated down regulation of PI3K/Akt and MAPK signaling pathways both as a single-agent and synergistically with trastuzumab (67, 68).

T-DM1: the monoclonal antibody trastuzumab is conjugated to an anti-microtubule agent (emtansine). In addition to potently inhibiting microtubule assembly, T-DM 1 also appears to flag HER-2-positive cells for cytotoxic destruction by antibodies (69, 70). In a phase II study, intravenous T-DM 1 showed robust activity in 112 patients with heavily pretreated, trastuzumab resistant HER-2-positive breast cancer, recording an objective response rate of 25.9%, and median progression-free survival (PFS) time of 4.6 months. T-DM1 was well tolerated, with grade 3 or 4 hypokalemia (8.9%) and thrombocytopenia (8%) being the most common (72). In a second phase II study of 110 extensively pretreated patients, T-DM1 treatment provided an overall response rate of 33%, and a clinical benefit rate of 48%. The median PFS time was 6.9 months (71). These data suggest that an anti-HER-2 antibody may retain the capacity to interact with HER-2 in a clinically meaningful way, even after the development of resistance, and indicate that conjugated agents such as T-DM 1 may provide a new treatment alternative for those who have previously progressed on native trastuzumab.

HER-2 family TKIS
Lapatinib (Tyverb): similar to trastuzumab is also a drug that targets HER-2; however, it not an antibody. Chemically, it is an oral small molecule derivative of 4-anilinoquinazolin. Also it does not target HER-2 alone but is rather known to inhibit to tyrosine kinase activity of HER-2 as well as EGFR (73, 74). It was approved by FDA in 2007 for clinical use with capecitabine in combination therapy for MBC patients (7577). In 2010, it was approved for treatment of postmenopausal women with hormone receptor positive, HER-2 over exespressing MBC (78). The mode of action of lapatinib involves targeting of C-terminus tyrosine kinase domain of target HER-2/EGFR where it binds to ATP binding site, resulting in the inhibition of phosphorylation and subsequent activation of downstream intracellular signalling pathway. Lapatinib has shown promise against trastuzumab-resistance cell (79, 80), which is of interest to clinicians dealing with the drug resistance associated with trastuzumab.

VEGFR TKIs
Sunitinib (Sutent; Pfizer) is an oral, multitargeted TKI against VEGFR, platelet-derived growth factor receptor and stem cell factor receptor (c-kit). In an open-label phase-II study of sunitinib monotherapy in patients with MBC previously treated with taxanes and anthracyclines, an overall RR of 11% was observed (81). However, no correlation between clinical response and ER or HER-2 status was found. At present, two early-phase-clinical trials are evaluating sunitinib/trastuzumab combinations in HER-2-positive breast cancer, including a phase-I trial of sunitinib plus trastuzumab and docetaxel in the first-line setting (82) and a phase-II trial of sunitinib plus trastuzumab in the second-line setting (83).

Pazopanib (Votrient; Glaxo Smith Kline) is an oral multitargeted TKI against VEGFR-1/2/3, platelet-derived growth factor receptor and c-kit. In a randomised phase-II study of pazopanib (400 mg per day) plus lapatinib (1000 mg per day) vs lapatinib alone (1500 mg per day) in HER-2-positive, locally advanced or MBC in the first-line setting, an interim analysis of 114 evaluable patients (total n ¼ 141) demonstrated modest efficacy with the dual TKI approach (84). Pazopanib plus lapatinib yielded a 12-week progressive disease rate of 15.9% vs 36.8% for lapatinib monotherapy (by investigator assessment). A secondary endpoint of 12-week RR also favoured the combination arm at 44.9% vs 27.8% (by investigator assessment; 36.2% vs 22.2% by independent assessment). Notably, four patients experienced declines in LVEF (three asymptomatic and one symptomatic) with the combined anti-HER-2/VEGF strategy.

Being tested HER-2 targeting drugs:
Neratinib (HKI-272; Pfizer, New York, NY, USA) is an irreversible, oral small-molecule TKI of EGFR/HER1, HER-2 and HER4 (85). Early clinical data for use of the multityrosine kinase inhibitor neratinib in trastuzumab-refractory disease are also promising. An open-label, phase-II multicenter trial of single-agent neratinib in advanced HER-2-positive breast cancer, which enrolled both trastuzumab-refractory (n=66) and trastuzumab-naive (n=70) patients, demonstrated modest clinical activity in both cohorts (42). Objective RRs of 24% and 56% were seen in the trastuzumab-refractory and trastuzumab-naive groups, respectively, with a median PFS of 22.3 and 39.6 weeks. Currently, studies of single-agent neratinib (neratinib vs lapatinib/capecitabine, NCT00777101) and neratinib combinations (with capecitabine, NCT00741260; trastuzumab, NCT00398567; paclitaxel, NCT00445458; vinorelbine, NCT00706030; and neratinib/paclitaxel vs trastuzumab/paclitaxel, NCT00915018) are under evaluation in HER-2-positive MBC (8691).

Afatinib: (BIBW 2992; Boehringer Ingelheim, Ingelheim, Germany), an anilino-quinazoline-derived irreversible, oral small-molecule ErbB family TKI (EGFR/HER1, HER-2 and HER4), has also demonstrated activity in early-phase trials of advanced solid tumours and trastuzumab-refractory HER-2-positive breast cancer (9294). An open-label, single-arm phase-II study of afatinib in 41 patients with HER-2-positive MBC following trastuzumab failure demonstrated partial responses in 4 patients and stable disease in 8 patients (maintained for at least four cycles) (92). A global phase-III trial of afatinib in HER-2-positive MBC was initiated (LUX-Breast 1; NCT01125566), which is evaluating vinorelbine/afatinib vs vinorelbine/trastuzumab in patients with prior trastuzumab therapy (95).

Future strategies for overcoming HER-2 resistance
There are several agents targeted against other pathways and molecules implicated in HER-2 resistance that are in various stages of clinical development. Aside from PI3K/Akt/mTOR inhibitors, other agents currently being investigated in clinical trials of HER-2-resistant breast cancer include inhibitors of IGFR1, HSP90, and telomerase.

IGF-1R Inhibitors: the rationale for assessing IGF1R inhibition in HER-2-resistant breast cancer is the hypothesis that cross-talk between IGF1R and HER-2 may occur in breast cancer cells, leading to receptor heterodimerization, which may, in turn, allow cells to escape trastuzumab cytotoxicity (96). In preclinical studies, overexpression of IGF1R led to trastuzumab resistance (97), and the addition of an IGF1R inhibitor to trastuzumab resulted in greater cell death than treatment with trastuzumab alone. Currently, there are no clinical data on the efficacy and safety of IGF1R inhibition in HER-2-resistant breast cancer. There are, however, ongoing trials of the IGF1R inhibitors BMS-754 807, cixutumumab, and OSI-9 06.

HSP90 Inhibitors: a novel therapeutic approach involves targeting the hsp90 molecular chaperone, whose function includes regulating the stability and maturation of various oncoproteins including HER (98). Preclinical data have shown that HER-2 is chaperoned by HSP90, suggesting that inhibition of HSP90 should be evaluated in HER-2-positive breast cancer patients. Tanespimycin (17-AAG, KOS-953; Bristol-Myers Squibb, New York, NY, USA), a first-generation geldanamycin derivative, has demonstrated robust antitumor activity in preclinical models of HER-2-positive breast cancer (99, 100). A phase-I study of tanespimycin plus trastuzumab was encouraging, and antitumor activity was observed in patients with HER-2-positive MBC (101). In a subsequent single-arm phase-II trial of tanespimycin (IV weekly) plus trastuzumab in patients with HER-2-positive MBC and disease progression following trastuzumab, an overall RR of 22% and CBR of 59% were reported (102). Tanespimycin was well-tolerated overall, with diarrhoea, fatigue, nausea and headache as the most common toxicities. Although further clinical development of tanespimycin has been halted, other hsp90 inhibitors, including retaspimycin (IPI-504; Infinity Pharmaceuticals, Cambridge, MA, USA) and AUY922 (Novartis, Cambridge, MA, USA) are currently under evaluation in early-phase clinical trials as single agents or in combination with trastuzumab (103105).

Telomerase expression is essential for cellular proliferation, and telomerase overexpression has been linked to tumorigenesis. Inhibition of telomerase ultimately results in apoptosis or cell senescence, making it a potential target for anticancer therapy. In a trastuzumab-resistant cell line, the telomerase inhibitor GRN163L was able to restore trastuzumab sensitivity, suggesting that telomerase inhibition may be a possible strategy for overcoming HER-2 resistance in breast cancer. Although no clinical data currently exist, a trial of GRN163L in women with trastuzumab-resistant breast cancer is ongoing (106).

Trastuzumab has provided significant clinical benefit in patients with HER-2-positive metastatic breast cancer, but de novo and acquired resistance is the major problem to be overcome and we needed to develop a novel treatment strategies or new combination drugs. All experimental drugs demonstrated modest clinical activity in this setting and highlights the importance of ongoing HER-2 blockade in trastuzumab-refractory states.

An alternative very interesting approach is the use of novel antibody-drug conjugates such as trastuzumab-emtansine, which recently demonstrated activity in MBC. T-DM1 is considered a “Miracle Drug” for advanced BC: almost half of the women in the trials (45 percent) had a positive response, which was defined essentially as a halt to tumor growth. Furthermore this was a population that had run out of options and whose tumors had not responded to previous drugs.

HER-2-positive patients with metastatic disease have another very powerful therapy that offers a real hope for prolonged disease control with less toxicity. We are still learning how to properly use this agent and the setting in which it might best be used.

Ongoing studies are anticipated to determine the efficacy of T-DM1 in combination with other HER-2-targeted agents, in combination with chemotherapy and in early-stage disease.

Especially the two ongoing phase III trials, MARIANNE, and THERESA, will give us further information about the place of T-DM1 in the treatment algorithms for HER-2-positive disease.

There are multiple effective therapies for patients with HER-2-positive breast cancer and it is quite likely that this new drug will be available in the near future.

Soon as it will be possible the use in clinical practice of the new drugs, the choice of anti-HER-2 targeted therapy in metastatic BC could be as follows:

  • - In first-line setting the association of trastuzumab plus pertuzumab and chemotherapy;
  • - In second-line the use of T-DM1;
  • - In third-line the association of lapatinib and capecitabine;
  • - In fourth-line also the use of trastuzumab plus chemotherapy.
The challenge now is how to define the better sequence of such treatments and just how much is “enough” for each patient. HER-2-positive BC research is a rapidly evolving field, and from now there is more treatment choices that have permissed a better outcome to the patients.