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Robin L Jones
BSc MB BS MRCP
Clinical Research Fellow
Royal Marsden Hospital
There are numerous complex interrelated signalling pathways involved in the regulation of cell proliferation and growth.(1) The relationship between these signalling networks and the pathogenesis of cancer has been of considerable interest over the last few years.
The hallmark characteristics of neoplastic disease include unrestrained proliferation involving activation of growth signals, loss of negative regulation and dysfunctional apoptotic pathways. Targeting abnormal cell signalling pathways should provide a more selective approach to cancer treatment than conventional cytotoxic chemotherapy.
Tyrosine kinases play an essential role in the signalling pathways involved in the control of cellular proliferation and growth, by facilitating the transmission of extracellular signals to the nucleus and facilitating communication between cells. Following the binding of growth factors, complex signalling pathways are activated, resulting in proliferative and survival signals.
Deregulation of the activity of protein tyrosine kinases is found in many malignant conditions; for example, overexpression of the epidermal growth factor receptor (EGFR) is found in glioblastoma, head and neck, prostatic and nonsmall-cell lung malignancies.(2) These enzymes can be divided into subgroups depending on their sequence within the kinase domain.
HER2 (human epidermal growth factor receptor 2) is a growth factor receptor overexpressed in 20–25% of breast cancers.(3,4) HER2 is a member of the erbB epidermal growth factor receptor tyrosine kinase family, which also consists of three other members, HER1, HER3 and HER4. These four receptors share an overall membrane-spanning structure composed of extracellular and transmembrane components, together with an intracellular region containing a kinase domain flanked by tyrosine autophosphorylation sites. Activation of HER2 results in downstream effects on cell growth, division, differentiation, migration and adhesion.(5)
HER2 overexpression is an independent negative prognostic indicator in both node-negative and -positive primary breast cancer,(6–8) and this led to the development of murine monoclonal antibodies (MAbs) to the extracellular domain of HER2. Some of these antibodies were capable of inhibiting the growth of cell lines that overexpressed the HER2 receptor.(9,10) Pietras and colleagues demonstrated synergistic effects between antibody and chemotherapeutic drugs such as cisplatin in human breast xenografts.(11) Researchers developed a potent murine MAb that was found to inhibit the proliferation of cell lines overexpressing HER2+ markedly but had little or no effect on cells without elevated levels of HER2.(12) Following this, a chimeric MAb called trastuzumab (Herceptin) was developed.
Cobleigh and colleagues explored the use of single- agent trastuzumab in 222 women with HER2-overexpressing metastatic breast cancer in the context of a phase II trial.(13) All patients had developed progressive disease after at most two prior chemotherapy regimens, and HER2 status had been determined either at initial diagnosis or at recurrence by immunohistochemistry, with 2+ or 3+ overexpression required for trial entry. Trastuzumab was given at a loading dose of 4mg/kg and subsequently administered at 2mg/kg. In the intent-to-treat population, there were eight (4%) complete responses (CRs) and 26 (11%) partial responses (PRs), which resulted in an objective response rate (ORR) of 15%. In the treated group, the investigators identified nine CRs (4%) and 37 PRs (17%), translating into an ORR of 22%. Interestingly, patients with HER2 3+ overexpression tended to have higher response rates (RRs) than those with 2+ overexpression (18% vs 6%, p=0.06). Adverse events were uncommon and occurred mainly with first administration of trastuzumab. These included pain, chills and dyspnoea; one woman developed an anaphylactic reaction during the initial administration. An unexpected event in this trial was the development of significant cardiotoxicity in 10 women (4.7%), with nine of these having received prior anthracycline-based chemotherapy. As a result, a Cardiac Review and Evaluation Committee (CREC) was established and is discussed below. This important trial demonstrated a point of principle: single-agent trastuzumab is clinically active against HER2-overexpressing metastatic breast cancer, with durable objective responses and low toxicity.
Another pivotal study assessed the efficacy and safety of trastuzumab as first-line monotherapy in 114 women with HER2 2+ and 3+ metastatic breast cancer before chemotherapy.(14) One arm received a loading dose of 4mg/kg followed by weekly doses of 2mg/kg. The patients in the other group were treated with a loading dose of 8mg/kg followed by 4mg/kg weekly. In the intent-to-treat population, seven women had a CR and 23 a PR, resulting in an ORR of 26%.
In addition, 13 women had a minor response or stable disease for longer than six months;(13) thus, in total, a clinical benefit rate of 38% was achieved. RRs were similar in both dose groups of trastuzumab. Similar findings were observed between trastuzumab 4mg/kg and 2mg/kg for median duration of survival (25.8 and 22.9 months, respectively) and median time to progression (TTP; 3.8 and 3.5 months, respectively). Again, therapy-related adverse events were uncommon and usually occurred with the first administration. There was no statistically significant association between adverse events and the two dose levels of trastuzumab administered. However, the 4mg/kg group had a numerically higher incidence of adverse events, suggesting the possibility of a dose effect on the incidence of toxicity.
Trastuzumab and chemotherapy
A pivotal phase III trial evaluated the efficacy and safety of trastuzumab combined with chemotherapy.(15) Four hundred and sixty-nine women with metastatic breast cancer overexpressing HER2 were randomly assigned to standard chemotherapy with or without trastuzumab. Those who had received an anthracycline-based adjuvant regimen were treated with paclitaxel 175mg/m(2) three-weekly. Patients who had not been given adjuvant anthracycline- based therapies received cyclophosphamide 600mg/m(2) and doxorubicin 60mg/m(2) (or epirubicin 75mg/m(2)) three-weekly.
There was significantly improved survival in the trastuzumab-plus-chemotherapy group (25.1 months) compared with the chemotherapy-only group (20.3 months) at a median follow-up of 30 months. Such a treatment- related survival difference has until now been unusual in metastatic breast cancer. Likewise, TTP in the combination trastuzumab/chemotherapy arms was 7.4 months, compared with 4.6 months in the chemotherapy- alone arms (p<0.001). The subgroup that was treated with trastuzumab and an anthracycline-based regimen had a median TTP of 7.8 months, compared with 6.1 months in those given an anthracycline and cyclophosphamide only (p<0.001). A similar statistically significant result was observed in the two subgroups given paclitaxel, with or without trastuzumab. Those treated with chemotherapy in combination with trastuzumab also had a statistically higher rate of overall response, longer duration of response and longer time to treatment failure compared with those given chemotherapy alone. Those with HER2 overexpression 3+ benefited more from trastuzumab combination therapy than women with overexpression at the 2+ level. The combination of trastuzumab and chemotherapy was well tolerated, apart from an unexpected 27% incidence of cardiotoxicity in the trastuzumab and anthracycline arm. This is further discussed below.
Marty and colleagues have reported on a randomised phase II trial comparing docetaxel alone or in combination with trastuzumab in patients with HER2-positive metastatic breast cancer.(16) In both groups, women received docetaxel (100mg/m(2)) every three weeks with or without trastuzumab (loading dose of 4mg/kg followed by 2mg/kg) until disease progression. The ORR of 61% in the trastuzumab-plus- docetaxel arm was significantly better than 36% in the group treated with docetaxel alone (p=0.001). Also, median TTP was 10.6 months in the combination arm, compared with 6.1 months in the docetaxel arm (p=0.0001). Overall survival was 24.1 months in the combination arm, which again was significantly better than 13.2 months in the docetaxel group (p=0.0001). Of note, fewer women in the docetaxel and trastuzumab group discontinued treatment due to toxicity than those treated with docetaxel alone. However, there was a slight increase in haematological toxicity in the combination group compared with the docetaxel-only arm, but this was manageable (21% vs 16% episodes of febrile neutropenia, respectively). There was a low incidence of cardiac failure in the trastuzumab group (2%).
Another randomised phase III study analysed the use of paclitaxel/trastuzumab with or without carboplatin as first-line therapy for metastatic breast cancer in 196 women.(17) The RR was 52% in those treated with the carboplatin combination, versus 36% in the group with paclitaxel and trastuzumab alone (p=0.04). Those who overexpressed HER2 at the 3+ level had a response rate of 57% in the carboplatin combination group, versus 37% in the other group (p=0.03). TTP was also statistically significantly better in the carboplatin-based group (11.9 months vs 6.8 months), and again those with 3+ overexpression derived more benefit. Grade 3 and 4 haematological toxicity was more common in the carboplatin group. There was a trend favouring the carboplatin combination in terms of overall survival (42.1 vs 33.3 months).
Trastuzumab and endocrine therapy
Preclinical and clinical work suggest that HER2 overexpression may be associated with resistance to endocrine treatment, particularly tamoxifen.(18–20) This may occur through crosstalk between the ER (oestrogen receptor) and HER2 signalling pathways. A prospective randomised study of 324 women with primary breast cancer treated with neoadjuvant tamoxifen or the aromatase inhibitor letrozole demonstrated that those with ER- and HER2-positive breast cancer had a significantly better response to letrozole (88% vs 21%).(21)
A recent study investigated the use of letrozole compared with tamoxifen in women with both normal and overexpression of HER 1/2 metastatic breast cancer.(22) Patients with normal HER2 expression had a significantly better overall response rate and clinical benefit if treated with letrozole. However, those with HER2 overexpression displayed no difference in overall ORR or clinical benefit whether treated with letrozole or tamoxifen. There was a trend favouring letrozole in terms of longer TTP and time to treatment failure. Osborne and colleagues have investigated the role of the ER coactivator AIB1 (SRC-3) and HER2 expression in tamoxifen resistance in breast cancer.(23) In their study, tumours that overexpressed HER2 and also had high expression of AIB1 were relatively resistant to tamoxifen therapy. These data support the use of trastuzumab in conjunction with aromatase inhibitors, and trials are in progress to investigate this combination of agents.
Neoadjuvant and adjuvant use of trastuzumab
Burstein et al have assessed the combination of neoadjuvant trastuzumab and paclitaxel in 40 women using a standard loading dose of 4mg/kg trastuzumab followed by 2mg/kg for 11 weeks with paclitaxel (175mg/m(2)) every three weeks for four cycles.(24) A pathological CR rate of 18% (n=7) was observed, and 30 women (75%) had a clinical response (CR or PR). Patients with HER2 2+ tumours had a lower clinical response rate than those with overexpression at the 3+ level, rates being 38% and 84%, respectively (p=0.01). No difference between complete pathological RRs in the 2+ and 3+ overexpressing groups was observed. There are currently four large prospective randomised trials exploring the adjuvant (or postoperative) use of trastuzumab in early breast cancer, the results of which are awaited eagerly.
Cardiotoxicity due to trastuzumab therapy first became evident during phase III clinical trials.(25) As this adverse event had not been predicted in preclinical work, baseline and regular cardiac monitoring were not predefined requirements for entry into clinical trials. Consequently, comparison between these early trials is difficult and the identification of risk factors complicated. An independent Cardiac Review and Evaluation Committee (CREC) was established to assess baseline risk factors, cumulative doses of anthracyclines and trastuzumab and document the severity of cardiotoxicity. Analysis of 1,219 patients by the CREC revealed that 75.5% of cardiac dysfunction was symptomatic.(26) However, 79% of these patients improved with standard medical therapy for cardiac failure. Increasing age and combination therapy with AC (doxorubicin and cyclophosphamide) plus trastuzumab were the only statistically significant predictive factors for trastuzumab cardiotoxicity in this analysis. Of note, cardiotoxicity was reported infrequently at doxorubicin doses of less than 300 mg/m(2), but the incidence of cardiac toxicity was similar between patients who had received doxorubicin and the less cardiotoxic anthracycline epirubicin.
Many oncologists avoid concurrent treatment with trastuzumab and an anthracycline following the results of the pivotal study that demonstrated a 27% incidence of cardiotoxicity when these drugs were used in combination.(15) Before commencing therapy with trastuzumab, patients should have baseline cardiac function documented. Any patient with a past history of coronary artery disease, hypertension or cardiac failure, if commenced on trastuzumab, should be monitored very carefully.
Lapatinib is a dual inhibitor that downregulates both HER2 and EGFR (HER1). In preclinical studies, this agent has demonstrated tumour growth arrest and/or apoptosis.(27) A study of different doses of lapatinib (500, 650, 900, 1,200 and 1,600mg once daily) in heavily pretreated metastatic cancer patients with tissue amenable for biopsy has demonstrated this agent to be both well tolerated and clinically active.(28) In those treated at the 1,200mg dose, three PRs were observed (10%), and there was one PR at the 900mg dose, all of which had advanced breast cancer. Stable disease (SD) was observed in 12 patients (36%). Of the different tumour types that achieved clinical benefit, seven had trastuzumab-refractory breast cancer. Dual inhibitors could potentially overcome trastuzumab resistance, and these drugs appear to have synergy with chemotherapy. There are a number of ongoing phase II and III trials investigating the use of lapatinib in advanced breast cancer.
Small-molecule tyrosine kinase inhibitors
These compounds bind to the ATP-binding site on the HER1 receptor, thus preventing activation of, and signal transduction from, the receptor. Two oral agents, gefitinib (Iressa) and erlotinib (Tarceva), have been studied in breast cancer. Reported RRs have been disappointing, ranging from 1.6% to 9%.(29–31) Adverse effects included rash, diarrhoea, nausea, asthenia and anorexia. The phase II study reported by Albain and colleagues enrolled 63 patients, with 31 women having received more than three previous chemotherapy regimens.(30) One PR was reported, with another two obtaining SD. The treatment was well tolerated, with 28 (44%) women reported as experiencing grade 3 or 4 skin toxicity. A recent phase II trial has investigated the combination of docetaxel (at an initial dose of 75mg/m(2) and then 100mg/m(2) in the absence of toxicity) and gefitinib in metastatic breast cancer.(32) The RR was 64%, with two CRs and seven PRs; in addition, the combination was well tolerated. However, further clinical trials are required to ascertain which patient subgroup would best benefit from these agents and their optimal use in breast cancer.
Bevacizumab is a recombinant humanised MAb to vascular endothelial growth factor (VEGF), one of the modulators involved in tumour angiogenesis.(33)
A clinical benefit rate of 17% has been demonstrated in a phase II trial in breast cancer patients.(34) A phase III trial randomly assigned 462 patients to receive either capecitabine (2,500mg/m(2)/day for 14 days of a 21-day cycle), alone or in combination with bevacizumab 15mg/kg/week.(35) The ORR in the combination arm was significantly better (30.2% vs 19.1%), but progression-free survival was not significantly different between the two groups (4.86 months and 4.17 months, respectively). However, the patients recruited into this trial were heavily pretreated, and a first-line trial of paclitaxel with or without bevacizumab in patients with metastatic breast cancer is ongoing.
Farnesyl transferase inhibitors
An understanding of the signal transduction cascade downstream of cell membrane growth factor receptors and tyrosine kinases has revealed several essential proteins involved in malignant transformation, including the 21Kd guanine nucleotide-binding proteins encoded by the Ras proto-oncogene.(36) Approximately 2% of breast cancers have mutations of the Ras gene.(37) With permanent upstream growth factor activation, continued activation of Ras pathways can occur.(38) Aberrant function of the Ras pathway has been reported to be common in breast cancer.(39) Farnesyl transferase catalyses the first step in Ras activation, and farnesyl transferase inhibitors block this first step.
A phase II trial examining the farnesyl transferase inhibitor R115777 in advanced breast cancer has been reported by Johnston and colleagues.(36) Patients who had failed at least two lines of endocrine therapy or who were ER-negative were eligible for entry. One prior chemotherapy regimen was permitted. Two sequential cohorts of patients were studied, the first 41 women being treated with a continuous dose schedule of 300 or 400mg twice a day. The subsequent 35 women were treated with a schedule of 300mg twice a day for 21 days of a 28-day cycle. For the patients treated continuously, the RR was 10%, and a further six patients (15%) achieved stable disease. For the other schedule, 14% (five patients) had a PR and 9% (three patients) had an SD. Of note, the first six patients treated with the continuous schedule of 400mg twice daily developed grade 3 or 4 neutropenia, and the subsequent patients received 300mg twice daily. There was a significantly lower incidence of haematological toxicity in the group treated with the intermittent schedule.
Inhibitors of mTOR
The mammalian target of rapamycin (mTOR) is a member of a protein kinase family that regulates cell cycle progression and response to DNA damage and is a downstream effector of the P13K/Akt pathway, which plays a role in cell survival.(39) A phase II trial of two dose levels of an analogue of rapamycin (CCI-779) in metastatic breast cancer has been reported in abstract form.(40) Patients who had been treated with up to two previous chemotherapy regimens were entered into this study and received CCI-779 at either 75mg or 250mg intravenously once a week. For the first 85 patients, clinical benefit was observed in seven patients treated with 75mg and 10 of those treated with 250mg. Baselga and colleagues have compared letrozole alone, letrozole with CCI-779 daily and letrozole and CCI-779 daily for five days every two weeks.(41) The initial results of patients treated with CCI-779 reveal three PRs and one CR, all in the higher dose group. Nine patients achieved SD, six on the higher dose treatment and three in the lower dose group. Patients treated with letrozole alone achieved two PRs and four SDs.
Over the last few years, several different agents have been developed as targeted therapies for breast cancer. The combination of trastuzumab and chemotherapy has been shown to have a significant beneficial effect on ORR, duration of response, median survival, time to treatment failure and TTP in patients with metastatic breast cancer, when compared with chemotherapy alone.(15,16) Responses are also observed when trastuzumab is used as monotherapy in metastatic breast patients who have failed to respond to cytotoxic agents.(13) Generally, therapy with this agent is well tolerated, the most sinister side-effect being cardiotoxicity, which, in combination with anthracycline-based therapy, has been reported to have an incidence of 27%.(15) Further ongoing trials will help ascertain the mechanisms underlying this adverse effect. There is increasing interest in crosstalk between the ER and HER2 pathways, which may lead to combination endocrine therapy with trastuzumab. The role of this agent in the adjuvant setting is being explored in four large prospective trials. The exact role of other agents such as lapatinib, small molecule tyrosine kinase inhibitors, bevacizumab, farnesyl transferase and mTOR inhibitors remains to be determined. As the results of ongoing and future trials become available, a better understanding of these agents and their role in combination therapies will be identified. The information obtained from tumours, such as HER2 status, will help the oncologist make an informed decision regarding optimal treatment for an individual.