Trastuzumab in Breast Cancer

ABSTRACT: Trastuzumab (Herceptin) is a therapeutic monoclonal antibody specific for the human epidermal growth factor receptor type 2 (HER2), a cell-surface tyrosine kinase receptor overexpressed by 25% to 30% of breast cancers. The drug is now regarded as one option for standard therapy in HER2-overexpressing metastatic breast cancers. It is associated with a moderate response rate as a single agent, and in combination with standard chemotherapy, can produce greater response rates and prolong the survival of women with advanced breast cancer. Its activity in metastatic breast cancer has led to active clinical trials examining its potential role in the neoadjuvant and adjuvant settings. The successful clinical development of trastuzumab provides further proof of principle that biologically targeted therapies can have a profound impact on the management of breast cancer. Here we review the clinical development of this novel agent, emphasizing the potential for therapeutic synergy when trastuzumab is combined with both standard chemotherapy and innovative molecularly targeted and biologic agents.

Since the early 1980s, concerted international efforts to optimize the use of traditional cancer treatment modalities have improved the outlook for women with breast cancer. The morbidity associated with surgery, radiation therapy, and chemotherapy has declined, improving quality of life both for women undergoing therapy and for breast cancer survivors. These improvements have also resulted in a decline in breast cancer mortality.[1] Despite this success, 37% of women diagnosed with breast cancer will still relapse and ultimately die of their disease.[1] Therapeutic failure is thought to result from the outgrowth of dormant breast tumor cells that are inherently resistant to standard treatments, leading to an increasing focus on developing novel therapeutics that target specific molecular pathways essential for the initiation and maintenance of the transformed phenotype.

Trastuzumab (Herceptin), a humanized monoclonal antibody specific for the human epidermal growth factor receptor type 2 (HER2), is a novel therapeutic agent that targets signaling pathways essential for transformation in a subset of breast cancers. Its efficacy in the management of HER2-overexpressing breast cancers illustrates the immense potential of targeted therapies for further improving the outcome of breast cancer therapy. Here we review the clinical development of trastuzumab, summarize its role in the management of HER2-overexpressing metastatic breast cancer, and outline ongoing trials evaluating its use in the neoadjuvant and adjuvant settings. Finally, we highlight opportunities for combining trastuzumab with other molecularly targeted biologic agents to achieve a synergistic antitumor effect.

HER2: A Therapeutic Target in Breast Cancer

FIGURE 1

Structure and Combinatorial Associations of the erbB Receptor Family

HER2 is a member of the erbB family of transmembrane tyrosine kinase receptors that also includes the epidermal growth factor receptor (EGFR or HER1), HER3, and HER4.[2] Although HER2 has no ligand of its own (Figure 1), it is preferentially recruited into ligand-activated complexes with other erbB family members, thereby potentiating signaling. At high density in transformed mammary carcinoma cells, HER2 is constitutively activated by phosphorylation, which is thought to promote dimerization and the subsequent promotion of tumor cell growth in a ligand-independent fashion. 

HER2 is overexpressed due to gene amplification in 25% to 30% of breast tumors,[3] resulting in more aggressive disease and shorter survival in both node-negative and node-positive patients.[4] Overexpression of HER2 often occurs in the context of other adverse prognostic features, including large tumor size, high histopathologic grade, high S-phase fraction, aneuploidy, and lack of estrogen-receptor and progesterone-receptor expression.[ 3] HER2 is thought to be essential for initiating and maintaining tumor growth and progression. HER2 gene amplification occurs early in the development of invasive breast tumors, and is also frequently observed in ductal carcinoma in situ.[5] These features, together with relatively low levels of HER2 expression in normal adult tissues, identify HER2 as an attractive target for breast cancer therapy.

Trastuzumab

Mechanisms of Action and Preclinical Pharmacology
Trastuzumab is a recombinant, humanized immunoglobulin G (IgG₁) monoclonal antibody that is specific for the extracellular domain of HER2. It is thought to inhibit the proliferation of mammary tumor cells by multiple mechanisms.[6] The antibody blocks cell-cycle progression by upregulating p27^kip1, an intracellular inhibitor of cyclin-dependent kinase 2 (CDK2), thereby promoting the formation of inactive p27/CDK2 complexes.[7]

Trastuzumab inhibits the recruitment of HER2 into heterodimers with HER1, HER3, and HER4 by accelerating the rate of endocytosis and subsequent degradation, thereby preventing HER2-mediated potentiation of erbB signaling through the ras/Raf/ mitogen-activated protein kinase (MAPK) and phosphoinositide 3- kinase (PI3K)/serine/threonine kinase (Akt) pathways.[6] It has also been shown to inhibit the PI3K/Akt pathway directly.[6] The antibody additionally blocks the metalloproteinase-induced cleavage of HER2, preventing the shedding of the extracellular domain and the concomitant constitutive activation of the membrane-associated cytoplasmic domain.[6]

Trastuzumab also inhibits tumor neovascularization, in part by preventing the induction of proangiogenic factors by HER2 signaling.[8] It further impairs the ability of cells to repair DNA damage by disrupting the activity of the cyclin-dependent kinase inhibitor p21WAF1,[9] and may itself promote the formation of DNA strand breaks.[10]

Finally, trastuzumab has inherent immunomodulatory activity. In a HER2-overexpressing xenograft model, abrogation of the antibody's therapeutic effect in the absence of Fc gamma RIII suggests that much of its therapeutic effect may be due to the recruitment of innate immune effector cells that mediate antibody-dependent cellular cytotoxicity.[11] Trastuzumab may also potentiate the adaptive immune response. It has been shown to augment the activity of HER2-specific CD8+ cytotoxic T lymphocytes in vitro,[ 12] and preclinical data suggest that similar murine antibodies can induce HER2-specific CD8+ cytotoxic T lymphocytes in vivo.[13]

Potential interactions between trastuzumab and radiation or cytotoxic therapy have also been evaluated in HER2-overexpressing xenograft models.[ 14,15] Consistent with its ability to promote the induction and maintenance of DNA strand breaks, the antibody was found to enhance the radiosensitivity of HER2-overexpressing breast tumor cells.[9] In a systematic screen of potential interactions with standard chemotherapeutic agents, trastuzumab synergized with cisplatin, docetaxel (Taxotere), thiotepa (Thioplex), cyclophosphamide (Cytoxan, Neosar), vinorelbine (Navelbine), and etoposide. Additive antitumor activity resulted from the combination of trastuzumab with doxorubicin, paclitaxel, vinblastine, or methotrexate. Interestingly, the combination of fluorouracil (5-FU) and trastuzumab was less effective than either drug alone, suggesting therapeutic antagonism. These observations underlie the rationale for testing trastuzumab in combination with chemotherapy in the clinical setting.

TABLE 1

Pivotal Clinical Trials of Trastuzumab in Metastatic Breast Cancer

Pivotal Trials in Metastatic Breast Cancer

Early Trials-Early phase I and II clinical trials tested trastuzumab as a single agent, or in combination with cisplatin, in heavily pretreated patients with advanced breast cancer.[16] These studies provided preliminary data regarding the safety and clinical activity of trastuzumab in patients with HER2-positive metastastic breast cancer. Based on these studies, three pivotal trials were conducted to further evaluate the safety and efficacy of trastuzumab in women with advanced HER2-overexpressing breast cancer (Table 1).[17-19] All three studies administered trastuzumab at a loading dose of 4 mg/kg intravenously (IV), followed by weekly doses of 2 mg/kg IV until disease progression. This dosing scheme was designed to maintain the serum trough concentrations of 10 to 20 μg/mL shown to be effective in animal models and early clinical trials.

Single-Agent Trastuzumab-Cobleigh and colleagues conducted a large, multinational phase II study of trastuzumab as a single agent in 222 women with HER2-overexpressing (immunohistochemistry [IHC] 2+ or 3+ in a central reference laboratory) stage IV breast cancer that had progressed after one or two chemotherapy regimens for metastatic disease.[17] The primary end point was objective tumor response, and the secondary end points were duration of response, time to disease progression, time to treatment failure, and survival. Pharmacokinetic analysis revealed that about 90% of patients achieved the target trough concentration of 20 μg/mL. When analyzed on an intent- to-treat basis, the overall response rate was 15% (95% confidence interval [CI] = 11%-21%), with 8 complete responses and 26 partial responses. The median response duration was 9.1 months (95% CI = 6.5- 10.5 months), with a median survival of 13 months, a median time to progression of 3.1 months, and a median time to treatment failure of 11 months.

Trastuzumab Plus Chemotherapy-Based on preclinical data demonstrating synergy between trastuzumab and cytotoxic agents, Slamon and colleagues simultaneously conducted a multinational, phase III study that randomized 469 study subjects with HER2-overexpressing (IHC 2+ or 3+ in a central reference laboratory) metastatic stage IV breast cancer to receive chemotherapy alone or chemotherapy plus trastuzumab (at the dosing schedule described above) as first-line therapy.[18] Individuals who received an anthracycline in the adjuvant setting received paclitaxel at 175 mg/m² every 3 weeks alone or with trastuzumab; the remaining patients received an anthracycline (usu- ally doxorubicin, 60 mg/m²) with cyclophosphamide at 600 mg/m² every 3 weeks alone or with trastuzumab. Chemotherapy was given for six cycles, and then continued at investigator discretion; weekly trastuzumab at 2 mg/kg was continued until disease progression. The primary end point was time to progression, with secondary end points of overall response rate, duration of response, time to treatment failure, and 1-year survival.

Reference Guide

Therapeutic Agents
Mentioned in This Article

Anastrozole (Arimidex)
Carboplatin (Paraplatin)
Cetuximab (Erbitux)
Cisplatin
Cyclophosphamide
    (Cytoxan, Neosar)
Docetaxel (Taxotere)
Doxorubicin
Doxorubicin, liposomal (Doxil)
Epirubicin (Ellence)
Fluorouracil (5-FU)
Gefitinib (Iressa)
Gemcitabine (Gemzar)
Interleukin-2 (Proleukin)
Interleukin-12
Letrozole (Femara)
Methotrexate
Paclitaxel
Tamoxifen
Thiotepa (Thioplex)
Trastuzumab (Herceptin)
Vinblastine
Vinorelbine (Navelbine)
Brand names are listed in parentheses only if a drug is not available generically and is marketed as no more than two trademarked or registered products. More familiar alternative generic designations may also be included parenthetically.

At a median follow-up of 30 months, the addition of trastuzumab to chemotherapy was associated with a longer time to disease progression (7.4 vs 4.6 months, P < .001), a higher overall response rate (50% vs 32%, P < .001), and a longer duration of median response (9.1 vs 6.1 months, P < .001). Combination therapy was associated with a distinct survival advantage, with a longer median survival (25.1 vs 20.3 months, P = .046), a lower rate of death at 1 year (22% vs 33%, P = .008), and a 20% overall reduction in the risk of death. The addition of trastuzumab to paclitaxel increased the overall response rate from 17% to 41% (P < .001), and the median response duration from 4.5 to 10.5 months (P < .01). The addition of trastuzumab to the anthracyclinecontaining regimens increased the overall response rate from 42% to 56% (P = .02), and the median response duration from 6.7 to 9.1 months (P = .005). Importantly, 72% of patients randomized to receive chemotherapy alone subsequently received trastuzumab. This crossover is likely to underestimate the survival benefit associated with the addition of trastuzumab to chemotherapy in this patient population.

Evaluating Dose Levels-Subsequently, Vogel and colleagues reported the results of a randomized, multicenter phase II study that evaluated two dose levels of trastuzumab as first-line therapy for HER2-overexpressing metastatic breast cancer (IHC 2+ or 3+ by a central core laboratory).[ 19] In this trial, 114 women were randomized to receive trastuzumab at either a loading dose of 4 mg/kg followed by 2 mg/kg weekly, or a loading dose of 8 mg/kg followed by 4 mg/kg weekly. The primary end point was overall response rate, and secondary end points were duration of response, time to progression, survival, and quality of life. In an intent-to-treat analysis, the overall response rate was 26% (95% CI = 8.2%-34.4%), and the overall clinical benefit rate (complete response, partial response, minor response, plus stable disease ≥ 6 months) was 38% (95% CI = 28.8%-46.9%). The clinical benefit rate increased to 48% for tumors that scored IHC 3+ for HER2 expression.

Patients with an objective response (57%) or a clinical benefit (51%) had not experienced disease progression at a follow-up of 1 year or more. Importantly, the median survival of 24.4 months observed for trastuzumab monotherapy in this trial compares favorably with the median survival of 25.1 months seen with trastuzumab in combination with chemotherapy, suggesting that patients may not sustain a survival disadvantage if they are managed with trastuzumab alone as firstline therapy for metastatic disease. No relationship between trastuzumab dose and response, survival, or adverse events emerged in this study.

Resulting FDA Approval-Together, these trials established trastuzumab as an effective therapy for HER2-overexpressing metastatic breast cancer. In 1998, the US Food and Drug Administration (FDA) approved trastuzumab for the treatment of HER2-overexpressing metastatic breast cancer in combination with paclitaxel as first-line therapy, and as second- and third-line monotherapy in patients with stage IV disease previously treated with chemotherapy.

Cardiac Toxicity: An Unpredicted Adverse Effect
These pivotal trials were remarkable for the observation of unexpected cardiac toxicity with trastuzumab therapy. An independent cardiac review and evaluation committee conducted a retrospective analysis of each trial.[20] Cardiac dysfunction, defined as congestive heart failure, cardiomyopathy, and/or a decrease in cardiac ejection fraction of 10% or more, occurred in 4.7% of study subjects (10 patients) in the Cobleigh study. All but one of these patients had received prior anthracycline therapy. Two patients remained asymptomatic with a diminished ejection fraction, and the remaining eight responded to standard medical therapy for congestive heart failure.

In the Slamon trial, the rates of cardiac dysfunction in individuals receiving anthracycline-based chemotherapy and trastuzumab compared to anthracycline-based chemotherapy alone were 27% and 8%, respectively. The rates of cardiac dysfunction in patients receiving paclitaxel and trastuzumab compared to paclitaxel alone were 13% and 1%, respectively. The incidence of significant cardiac dysfunction in patients who had never received anthracycline therapy was 4%; all of these individuals had independent risk factors for cardiac disease. Most patients with symptomatic heart failure continued to receive trastuzumab, and 75% improved with standard medical management. A very small number of patients (6%) who received an anthracycline combined with trastuzumab were left with marked symptomatic cardiac dysfunction at the end of treatment. Death related to cardiac dysfunction was rare (two patients) and not statistically different between the groups.

A similar independent analysis of the Vogel study identified three patients with manifestations of cardiac toxicity. Of these three patients, two had preexisting heart disease, and one had received adjuvant anthracycline therapy. After discontinuation of trastuzumab, no patient required medical intervention for cardiac events.

To further evaluate the potential relationship of cardiac toxicity to interactions between trastuzumab and anthracyclines, the Eastern Cooperative Oncology Group (ECOG) conducted a phase II trial in 234 patients with stage II, HER2-positive breast cancer, randomizing them to receive PT chemotherapy (paclitaxel at 175 mg/m² every 3 weeks plus concurrent weekly trastuzumab for 10 weeks), followed by standard AC chemotherapy (doxorubicin [Adriamycin], cyclophosphamide [Cytoxan, Neosar]) or the same treatment regimen plus an additional year of weekly trastuzumab therapy.[21] The overall median baseline left-ventricular ejection fraction (LVEF) was 63% in both groups. A decrease in LVEF of more than 10% from baseline occurred in 9.5% of patients after PT and in 12.5% of patients after AC. The incidence of congestive heart failure was 1.7%. The trial is now in long-term follow-up.

HER2 Overexpression in Predicting Response
Immunohistochemistry and fluorescence in situ hybridization (FISH) are currently the primary techniques used to characterize the HER2 status of breast tumors.[22] Immunohistochemistry assays such as the HercepTest (Dako Corporation, Carpinteria, Calif) report expression results as 0, 1+, 2+, or 3+, with 0 and 1+ indicating lack of HER2 expression, 2+ representing indeterminant HER2 expression, and 3+ identifying tumors that overexpress HER2. In contrast, FISH analysis measures the extent of gene amplification and overexpression. For example, the Path- Vysion (Vysis Incorporated, Downers Grove, Ill) FISH analysis is positive when the ratio of HER2 to chromosome enumeration probe (CEP17) control is ≥ 2.

Further studies comparing the evaluation of HER2 overexpression by these two assays show that almost all HER2-expressing tumors that score 3+ by IHC are also positive for gene amplification by FISH. In addition, FISH analysis confirms HER2 gene amplification in an additional 24% of tumors that score IHC 2+ for HER2 expression.

Use in Clinical Trials-The National Surgical Adjuvant Breast and Bowel Project (NSABP) protocol B-31 and the Breast Intergroup trial N9831 are two studies evaluating the addition of trastuzumab to adjuvant chemotherapy for early breast cancer. Both studies also examined the concordance between community-based assays of HER2 overexpression and similar assays conducted in a central reference laboratory. The NSABP reviewed the first 104 patients enrolled in B-31. They found that 18% of local laboratories reported results that were not confirmed by Herceptest or FISH analysis in a central testing facility.[ 23] Similarly, Roche and colleagues conducted a central review of the first 119 patients enrolled in N9831, and found that 26% of local HER2 assessments could not be centrally reproduced.[24] Notably, the central and local results diverged less frequently when the local laboratories conducted high-volume HER2 testing.[25] The concordance rate for IHC and FISH for central testing was 92% and 94% for N9831 and NSABP B-31, respectively.

Importantly, subgroup analyses of the pivotal trastuzumab trials established the importance of selecting patients with the highest levels of HER2 overexpression for trastuzumab therapy. In the Cobleigh trial, study subjects whose tumors had IHC 3+ HER2 staining tended to derive greater benefit from trastuzumab therapy than those with IHC 2+ HER2 staining, with response rates of 18% vs 6% (P = .06).[17] Individuals with HER2 3+ tumors had a median survival of 16.4 months compared to a median survival of 13 months for the overall group. A retrospective analysis of gene expression by FISH revealed the response rate to be 19% for tumors with HER2 gene amplification (FISHpositive), and 0% for tumors without gene amplification (FISH-negative).

The Slamon study confirmed these observations.[18,24] The clinical benefit of trastuzumab was greater for study participants whose tumors overexpressed HER2 by IHC at 3+ compared to 2+, with relative risks for disease progression of 0.42 (95% CI = 0.33-0.54) and 0.76 (95% CI = 0.21- 1.35), and mortality of 0.7 (95% CI = 0.51-0.90) and 1.26 (95% CI = 0.82- 1.94), respectively. A further reanalysis of tumor HER2 status by FISH revealed that the benefit of trastuzumab was limited to individuals with tumors containing HER2 gene amplification. Study subjects with HER2- amplified tumors had response rates to chemotherapy or chemotherapy plus trastuzumab of 31% and 54%, and median survivals of 20 and 26.2 months (P = .007). There was no ben- efit from the addition of trastuzumab to chemotherapy for FISH-negative tumors.

In the Vogel study, the overall response rate and clinical benefit rates for tumors with HER2 IHC 2+ expression were 0% and 7%; for tumors with HER2 IHC 3+ expression, they were 35% and 48%. Moreover, the overall survival rate and clinical benefit for FISH-negative and FISH-positive tumors were 7% and 34%, and 10% and 48%, respectively.[19]

In Summary-FISH testing is now considered the gold standard assay for HER2 overexpression and provides the best correlation with clinical response to trastuzumab. Since FISH is not widely available, it is common practice to first characterize tumor HER2 expression by IHC. Patients whose tumors score IHC 3+ for HER2 expression are considered candidates for trastuzumab. Patients whose tumors score IHC 2+ for HER2 expression are candidates for trastuzumab therapy only if further analysis reveals that the tumor is positive for HER2 gene amplification by FISH.

Safety Profile
Trastuzumab is generally well tolerated, with side effects limited primarily to mild-to-moderate first infusion reactions consisting of fever, chills, and nausea.[17-19] These hypersensitivity reactions affect about 40% of treated patients and are easily managed by acetaminophen, diphenhydramine, and/or meperidine. They typically do not recur with subsequent infusions. Other side effects include asthenia, pain, diarrhea, anemia, and leukopenia. The two most serious adverse effects of trastuzumab are cardiac toxicity and pulmonary toxicity.

Cardiac Toxicity-Significant cardiac toxicity occurs in about 4% of patients receiving single-agent therapy, and in 10% to 30% of patients receiving trastuzumab in combination with chemotherapy.[26] Cardiac toxicity can be potentially life-threatening but generally improves with standard medical management. The major risk factors for trastuzumabrelated cardiac toxicity are the simultaneous use of trastuzumab and anthracyclines, and age over 60 years. The data also suggest an increased risk with a prior anthracycline dose of ≥ 400 mg/m², prior chest wall irradiation, and preexisting cardiac disease.

The cardiac toxicities associated with anthracyclines and trastuzumab are distinct.[26] Unlike the cardiac toxicity related to anthracycline therapy, there is no evidence that the risk of cardiac toxicity related to trastuzumab increases with cumulative dose. In comparison to anthracyclinerelated heart failure, trastuzumabrelated cardiac dysfunction is typically less severe, and more readily reversible with discontinuation of therapy and/or standard medical management for heart failure. Moreover, in contrast to the pathognomonic myofibrillar disarray associated with anthracycline toxicity, no characteristic pathologic changes associated with trastuzumab-related cardiac dysfunction have been identified on myocardial biopsy.

The pathogenesis of trastuzumabassociated cardiac toxicity remains unclear, but increasing evidence suggests that the antibody may have a direct cardiotoxic effect mediated by interaction with the cardiac HER2 receptor.[ 26] HER2 plays a clear role in embryonic cardiogenesis, and is essential in the prevention of dilated cardiomyopathy in experimental models. It has also been hypothesized that trastuzumab may exert a synergistic or additive cardiotoxic effect when administered concurrently with or subsequent to anthracyclines or other cardiotoxic drugs. Supporting this possibility, trastuzumab increases myofibrillar disarray in the presence of doxorubicin compared to the myofibrillar disarray generated by exposure to doxorubicin alone.[27]

Cardiac toxicity related to trastuzumab manifests as tachycardia, asymptomatic drop in LVEF, and clinical congestive heart failure.[26] New tachycardia may be an early indicator of trastuzumab-associated cardiac toxicity, and should prompt a formal evaluation of cardiac function. The measurement of LVEF by multigated acquisition scanning should be considered for patients at risk for trastuzumab- related cardiac dysfunction, and is the test of choice for evaluating changes in cardiac function while on therapy. Algorithms for monitoring cardiac function on trastuzumab therapy have been proposed by Keefe (Table 2).[26]

TABLE 2

Proposed Surveillance Guidelines for Trastuzumab-Associated Cardiac Toxicity

On therapy, increases in the heart rate of 10 to 20 beats per minute, increases in body weight of ≥ 2 kg in 1 week, or new symptoms of congestive heart failure, especially fatigue and exercise intolerance, should prompt a formal reevaluation of cardiac function. Minor declines in LVEF can often be managed with medical therapy while trastuzumab is continued, whereas more significant drops or symptoms will require the discontinuation of the antibody. Guidelines for managing declines in LVEF and the development of clinically significant congestive heart failure while on trastuzumab therapy have been published.[26]

Pulmonary Toxicity-Significant trastuzumab-related pulmonary toxicity has emerged in postmarketing studies as a rare adverse event that sometimes follows significant infusion reactions to the antibody.[16] Predisposing factors include symptomatic intrinsic primary or metastatic lung disease. The clinical syndrome can include dyspnea, pulmonary infiltrates, pleural effusions, noncardiogenic pulmonary edema, and the adult respiratory distress syndrome.

Combinations With Chemotherapy
Three observations led to the intensive investigation of trastuzumab in combination with a variety of cytotoxic drugs (summarized in Table 3). First, preclinical data clearly demonstrated synergism between multiple chemotherapeutic agents and trastuzumab.[ 14,15] Second, Slamon and colleagues clearly demonstrated the survival benefit associated with the addition of weekly trastuzumab to conventional chemotherapy.[18] Third, the survival benefit of trastuzumab in the Slamon trial emerged despite the fact that 67% of patients initially randomized to receive chemotherapy alone went on to receive trastuzumab, suggesting that utilizing trastuzumab early in the management of metastatic disease is clinically beneficial.

TABLE 3

Selected Recent Trials of Trastuzumab and Chemotherapy in Metastatic Breast Cancer

Taxanes and Platinums-Weekly trastuzumab has been tested in combination with single-agent paclitaxel or docetaxel given weekly or every 3 weeks in a number of studies, demonstrating an overall response rate of 41% to 73%.[16,28,29] Building on early data suggesting a benefit for combination therapy with platinum drugs,[30] the Breast Cancer International Research Group (BCIRG) conducted two phase II multicenter clinical trials testing either carboplatin (Paraplatin) or cisplatin with docetaxel and trastuzumab in patients with HER2-overexpressing IHC 2+ or 3+ metastatic breast cancer.[16,31] BCIRG 101 evaluated docetaxel (75 mg/m²) with cisplatin (75 mg/m²) every 3 weeks plus weekly trastuzumab, and BCIRG 102 evaluated docetaxel (75 mg/m²) with carboplatin (area under the concentration-time curve [AUC] of 6) every 3 weeks plus weekly trastuzumab; both studies enrolled 62 patients. Preliminary analysis demonstrated an overall response rate of 79% (95% CI = 66%-88%) and median time to disease progression of 9.9 months (95% CI = 8.3- 13.1 months) for the cisplatincontaining regimen, and an overall response rate of 56% (95% CI = 40%- 69%) and time to progression of 12.0 months (95% CI = 7.4-16.3 months) for the carboplatin-containing regimen.

Most recently, a multicenter, randomized phase III study registered 194 study subjects to compare the relative efficacy of weekly TP (trastuzumab plus paclitaxel, 175 mg/m², every 3 weeks) to weekly TPC (trastuzumab plus paclitaxel, 175 mg/m²) and carboplatin (AUC 6) every 3 weeks as first-line therapy for HER2-overexpressing metastatic disease.[32] At first report, 160 patients were evaluable for response rate and time to progression. In HER2 IHC 3+ patients, the overall response rate was 67% and 37% for TPC and TP, respectively (P < .01); corresponding times to progression were 17 and 9 months (P = .004). Toxicities were acceptable, with no evidence of significant cardiac toxicity. Phase III studies are now in progress.[31]

Vinorelbine-Trastuzumab has also been tested in combination with newer chemotherapeutics. Three studies have evaluated weekly vinorelbine plus weekly trastuzumab for HER2- overexpressing metastatic breast cancer, demonstrating an overall response rate of 68% to 78%.[33-35] Therapy was well tolerated, with only one instance of symptomatic congestive heart failure in 134 patients treated on study.[34]

Gemcitabine-Trastuzumab combined with gemcitabine (Gemzar) alone or gemcitabine plus paclitaxel is also under active investigation. A phase II study of gemcitabine at 1,200 mg/m² on days 1 and 8 every 21 days plus weekly trastuzumab in heavily pretreated patients with metastatic disease demonstrated a 32% overall survival response in preliminary analysis of 38 patients.[36] A second phase II trial in 46 patients tested the activity of weekly trastuzumab plus gemcitabine at 1,200 mg/m² on days 1 and 8 plus paclitaxel at 175 mg/m² on day 1 every 21 days as first-line therapy for HER2 IHC 2+ and 3+ disease.[37] An intent-to-treat analysis demonstrated a response rate of 63%, and a median time to progression of 10 months. Therapy was well tolerated, with one cardiac event (congestive heart failure).

Other Agents-The significant rate of cardiac toxicity observed when combining trastuzumab with anthracyline- containing regimens in the pivotal trial led the FDA to approve trastuzumab in combination with paclitaxel. Nevertheless, the substantial activity of trastuzumab in combination with anthracycline-based therapy has prompted the investigation of combination therapy with epirubicin (Ellence) or liposomal doxorubicin (Doxil), which are anthracyclines with potentially less cardiotoxicity. Although patient numbers are small, these two studies demonstrated overall response rates of 57% to 71%, with no evidence of increased cardiac toxicity related to the addition of trastuzumab in either study.[38,39]

Dosing Schedule
As discussed above, clinical studies of trastuzumab have utilized an IV loading dose of 4 mg/kg on day 1, followed by weekly infusions of 2 mg/kg in order to meet the target serum trough of 10 to 20 μg/mL established as therapeutic in preclinical and early clinical studies. This dose and schedule was based on the assumption that the antibody had dose-related nonlinear pharmacokinetics and an estimated elimination half-life of 8.3 days.[40] However, a reanalysis of population pharmacokinetics revealed that the elimination of trastuzumab fits a two-compartment model, and that the drug in reality has a half-life of about 28 days.[41] This long half-life should allow for longer dosing intervals that maintain target serum trough concentrations of 10 to 20 μg/mL.

Leyland-Jones and colleagues tested a dose and schedule of trastuzumab given as a loading dose of 8 mg/kg IV on day 1, with paclitaxel at 175 mg/m² IV on day 0, then trastuzumab at 6 mg/kg with paclitaxel at 175 mg/m² IV every 21 days for seven cycles in 32 women with HER2-overexpresing metastatic breast cancer.[42] Patients who responded were continued on single-agent trastuzumab therapy every 21 days until disease progression or patient withdrawal. Pharmacokinetic analysis revealed that trastuzumab trough levels were higher than 20 μg/mL by the end of cycle 1, and that the half-life of the drug was 18 to 27 days. No pharmacokinetic interaction between trastuzumab and paclitaxel was observed. Adverse events related to trastuzumab were infusion-related reactions and cardiac dysfunction.

Ten patients had a ≥ 15% decrease in ejection fraction, but only one developed symptomatic congestive heart failure. The overall response rate was 59%, with a median duration of response of 10.5 months and median time to progression of 12.2 months. Similar pharmacokinetic and toxicity profiles were observed in another study of trastuzumab monotherapy administered every 21 days.[43] These data strongly support further investigation of this new dose and schedule, which reduces costs and improves convenience and quality of life without apparently compromising efficacy.

Timing and Duration of Treatment
The optimal strategy for integrating trastuzumab into the management of metastatic disease remains unclear, and trials are ongoing to address this question. As discussed above, data from the pivotal trials argue that trastuzumab should be used early in the management of metastatic disease.[ 18,19] A rational approach would be to use trastuzumab monotherapy unless there was impending visceral crisis, in which case it would be reasonable to utilize trastuzumab in combination with chemotherapy. With the combination regimen, cytotoxic therapy would be continued to maximal response or patient tolerability, then discontinued while maintaining singleagent trastuzumab therapy.

The oncology community is divided on how long to continue trastuzumab, and whether to continue it at the time of disease progression. When a patient progresses on trastuzumab monotherapy or trastuzumab plus a cytotoxic agent, the practice of many oncologists has been to add a drug to the former treatment, and to change to a different class of drug in the latter. An ongoing clinical trial that randomizes patients who progress on trastuzumab plus a taxane to vinorelbine alone or vinorelbine plus trastuzumab should lend insight into these issues.

Trastuzumab in the Neoadjuvant Setting
Preoperative systemic therapy is widely used in the treatment of locally advanced and operable breast cancer. Neoadjuvant therapy is as effective as adjuvant chemotherapy in maximizing disease-free and overall survival, and increases the rate of breast-conserving therapy.[44] Multiple studies have shown that a complete pathologic response, defined as eradication of invasive disease in the surgical specimens, is a powerful predictor of outcome.[44]

Burstein and colleagues conducted a pilot study of preoperative therapy with weekly trastuzumab and paclitaxel (175 mg/m² every 21 days) for 12 weeks, followed by definitive surgery, and then adjuvant AC chemotherapy (doxorubicin, 60 mg/m², with cyclophosphamide, 600 mg/m², every 21 days for four cycles) in 40 women with stage II/III HER2-overexpressing primary breast cancer.[45] Preoperative trastuzumab plus paclitaxel resulted in a clinical response rate of 75%, and a complete pathologic response rate of 18%. This compares favorably with the complete pathologic response rates of other regimens, which range from 10% to 33%. Although numbers were small, HER2 IHC 3+ tumors were more likely to respond than HER2 IHC 2+ tumors (84% vs 38%, respectively). Four patients developed asymptomatic declines in LVEF consistent with grade 2 cardiac toxicity.

A second study is examining the activity of trastuzumab combined with docetaxel and cisplatin as neoadjuvant therapy for locally advanced and inflammatory breast cancer.[46] An interim report on 16 patients demonstrated a clinical response rate of 100%, and a complete pathologic response rate of 25%.

A more recent study assessed the activity of 3 weeks of single-agent weekly trastuzumab alone as neoadjuvant therapy; after 3 weeks of single-agent therapy, chemotherapy was to be added. There was a median decrease in tumor size of almost 20% (P = .0001) after only 3 weeks of single-agent antibody therapy.[47] Interestingly, analysis of surrogate markers of activity revealed an induction of apoptosis (as reflected by cleaved caspase-3) without change in cell proliferation (as reflected by Ki67). These data are consistent with the synergy between trastuzumab (proapoptotic) and chemotherapy (antiproliferative). The role of trastuzumab in neoadjuvant therapy clearly warrants further investigation.

Trastuzumab in the Adjuvant Setting
The improvement in survival with the use of trastuzumab in the management of metastatic breast cancer has prompted the development of multiple phase III trials testing the clinical benefit of trastuzumab as part of adjuvant therapy for high-risk, HER2-overexpressing early breast cancer.[48] Importantly, eligibility in these trials is restricted to patients who have tumors that are HER2 3+ by IHC or FISHpositive, based on observations that these two characteristics predict for response to trastuzumab in the metastatic setting. At least four trials (summarized in Table 4) will recruit more than 12,000 patients over the next several years and should define the role of trastuzumab in the adjuvant therapy of breast cancer. Currently, no data support the use of trastuzumab for adjuvant breast cancer therapy outside the context of a clinical trial.

TABLE 4

Current Trials of Trastuzumab and Chemotherapy in the Adjuvant Setting

Future Directions
Other opportunities for the rational combination of trastuzumab with both traditional and novel breast cancer therapeutics, such as tamoxifen, aromatase inhibitors, EGFR inhibitors, and cytokines have been suggested by preclinical and clinical data. Several lines of evidence suggest that HER2-overexpressing breast cancers are relatively resistant to tamoxifen therapy.[49-51] For example, Ellis and colleagues provided retrospective clinical data that the response rates for neoadjuvant letrozole (Femara) and tamoxifen given as neoadjuvant therapy for HER1/2-overexpressing tumors were 88% and 21%, respectively (P = .0004).[51]

Furthermore, a growing body of preclinical data supports cross talk between growth factor receptor- signaling pathways and hormonereceptor pathways as an important mechanism of endocrine resistance, suggesting that the inhibition of growth factor receptor signaling may restore endocrine responsiveness.[52] Thus, inhibiting HER2 signaling with trastuzumab in combination with tamoxifen or an aromatase inhibitor could reverse endocrine resistance. Studies comparing the efficacy of the aromatase inhibitor anastrozole (Arimidex) alone and in combination with trastuzumab for HER2-overexpressing metastatic disease are under way.

Preclinical data suggest that high levels of activated HER1 may mediate resistance to trastuzumab therapy.[6] Targeting HER1 with a monoclonal antibody (eg, cetuximab [Erbitux]) or small-molecule tyrosine kinase inhibitor specific for HER1 (gefitinib [Iressa]) synergizes with trastuzumab against HER2-overexpressing cells in vitro or in vivo. The ECOG is currently conducting a phase I/II clinical trial (E1100) testing the safety and efficacy of trastuzumab with gefitinib. Finally, combining trastuzumab with cytokines such as interleukin-2 (Proleukin) and interleukin-12 may augment the potential of the antibody to recruit innate immune effectors by expanding the pool of natural killer cells available. Early clinical trials exploring this concept have also begun.[53,54]

Conclusions

Trastuzumab represents the newest biologically targeted therapeutic to be incorporated into the standard management of breast cancer. Its successful clinical development heralds the era of biologically targeted, immune- based therapies for breast cancer, and defines a set of principles that can be applied to the development of other biologically targeted agents. First, both the definition of relevant predictive markers of therapeutic response and the optimization of their measurement are essential for selecting subgroups of patients that will benefit from therapy. Second, toxicity uniquely related to the molecular target may be unpredictable. Third, combining targeted therapeutics with conventional drugs may be synergistic, even if the new drug itself may have clinical activity that is modest.

Trastuzumab benefits breast cancer patients with tumors that overexpress HER2 as defined by IHC 3+ staining or gene amplification by FISH. It has a unique cardiac toxicity that is most likely related to HER2 expression in the heart. Trastuzumab clearly offers a survival benefit to women with HER2-overexpressing metastatic breast cancer, and will likely improve survival in the adjuvant setting as well. Trastuzumab can clearly synergize with a variety of chemotherapeutic agents, suggesting that other passive and active immunotherapies have the potential to synergize with standard therapies.[55-57] Since the rational combination of other novel therapeutics with trastuzumab has great potential for overcoming mechanisms of therapeutic resistance and augmenting its immunomodulatory activity, it is likely that the true impact of trastuzumab on breast cancer mortality has yet to be realized.

Financial Disclosure: The authors have no significant financial interest or other relationship with the manufacturers of any products or providers of any service mentioned in this article.

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