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The HER receptor family: a rich target for therapeutic development
Int. J. Radiation Oncology Biol. Phys., Vol. 58, No. 3, pp. 932–940, 2004 Copyright © 2004 Elsevier Inc. Printed in the USA. All rights reserved 0360-3016/04/$–see front matter
doi:10.1016/j.ijrobp.2003.09.093
EGFR INHIBITORS
THE HER RECEPTOR FAMILY: A RICH TARGET FOR THERAPEUTIC DEVELOPMENT ROBERT D. MASS, M.D. Genentech BioOncology, Inc., South San Francisco, CA Purpose: The key role of the HER family of receptors in cancer has been widely acknowledged. HER receptor activation occurs via ligand binding or nonligand-dependent receptor dimerization, initiating signals that promote tumorigenesis via cell proliferation, survival, migration, adhesion, and differentiation. Therapeutic strategies designed to target and inhibit HER activation that are in clinical development are reviewed, including examples of both small-molecule tyrosine kinase inhibitors and monoclonal antibodies. Materials and Methods: A literature review. Results: Tarceva is a potent, highly selective, reversible inhibitor of HER1/epidermal growth factor receptor tyrosine kinase with inhibitory activity against various in vitro and in vivo models of solid human tumors. Phase II trials in refractory non–small-cell lung, head-and-neck, and ovarian cancer have demonstrated clinical activity, including objective responses and prolonged, stable disease. Four Phase III trials are ongoing evaluating primarily the effect on survival of Tarceva in combination with chemotherapy. 2C4 is a humanized anti-HER2 monoclonal antibody that binds to a broad, extracellular epitope, resulting in steric inhibition of HER–receptor complex formation that involves HER2. 2C4 has shown significant activity in xenograft models of prostate, lung, and breast cancer. 2C4’s activity, unlike Herceptin’s, is not dependent on HER2 amplification. This antibody is in early clinical development. Conclusion: The strategy of targeting the HER system has been further validated by early experience with Tarceva and 2C4. The optimal clinical benefit of these agents will likely involve combinations of biologic agents, with or without traditional chemotherapy, and will be guided by critical predictive diagnostic information. © 2004 Elsevier Inc. Tarceva, 2C4, Cancer, HER, Targeted therapy.
addition, there are 3 mutant HER1/EGFR receptors, of which EGFRvIII is the most commonly detected in human solid tumors (3), where it is also frequently overexpressed (4). Wild-type HER receptors are each divided into 3 regions: an extracellular ligand binding region, an intracellular region containing the locus responsible for tyrosine kinase activity and regulatory functions, and a transmembrane region that anchors the receptor to the cell (2). Each HER receptor is fundamentally an inactive monomer that dimerizes in response to ligand binding. This process takes the form of homodimerization with a receptor of the same type or heterodimerization with another member of the HER family. This dimerization process results in receptor activation through tyrosine kinase–mediated phosphorylation that produces a biochemical “trigger” that starts a cascade of metabolic events constituting a complex downstream signaling network, leading to various effects on specific aspects of cell function (2).
INTRODUCTION Since the early 1970s, research has provided an exponential growth in our understanding of the intricate and complex interplay between the vast range of genetic, molecular, and biochemical mechanisms responsible for the regulation of both normal and abnormal cellular function. Notably, the role of cell membrane receptors in maintaining the delicate balance between cell proliferation and programmed cell death (apoptosis) has been widely investigated. The involvement of cell membrane receptors in angiogenesis and cell migration—processes integral to cancer spread— has also been studied. The resulting body of knowledge has led to novel approaches for targeted cancer therapy. Of the many receptors and their associated signaling proteins identified, the epidermal growth factor receptor (EGFR), or HER family, has been widely acknowledged as playing a crucial role in tumorigenesis and disease progression. HER receptors comprise 4 distinct transmembrane receptors: HER1/EGFR, HER2, HER3, and HER4 (1, 2). In Reprint requests to: Robert Mass, M.D., Genentech BioOncology, Inc., 1 DNA Way, South San Francisco, CA 94080-4990. Tel: (650) 225-7223; Fax: (650) 225-4538; E-mail: bobmass@ gene.com
Received May 21, 2003, and in revised form Aug 29, 2003. Accepted for publication Sep 8, 2003.
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Fig. 1. The HER signaling network. Reprinted by permission from Nature Reviews Molecular Cell Biology (www. nature.com/reviews) Yarden and Sliwkowski 2001;2:127–137 (5).
The HER family cell signaling system uses at least 11 epidermal growth factor–like peptide mediators, or ligands (5). HER2 shows substantial structural homology to HER1, but has no known ligand (6, 7), though it is frequently activated because of its common coreceptor function (8, 9). HER3 and HER4 are also structurally related to HER1; however, HER3 lacks intrinsic tyrosine kinase activity (10) and therefore can initiate signaling only via association with another HER-family receptor, usually HER2 (9, 11). The mutant HER receptor subtype EGFRvIII has a truncated extracellular-binding domain and cannot bind ligand. However, it does have an intact tyrosine kinase domain that is constitutively phosphorylated and can, therefore, activate downstream signaling cascades independent of ligand binding (4, 12, 13). In general, dysregulated HER-receptor activation initiates a signaling cascade that promotes tumorigenesis via cell proliferation, survival, migration, adhesion, and differentiation (2, 14). The HER signaling system comprises a highly complex and interactive “multilayered” network (Fig. 1). Signals are initiated at the cell surface, or “input layer,” where ligand– receptor interactions take place, and the resulting receptor dimerization and activation relays and amplifies the signal through a cytoplasmic “signal-processing layer” via an intricate system of enzymes, proteins, and small-molecule secondary messengers (5). This process of signal transduction culminates in the nucleus, or “output layer,” where
gene control and protein transcription are modified, producing effects on key cellular regulatory processes, such as differentiation, adhesion, growth, migration, and apoptosis. Dysregulated receptor function and disruptions in any or all downstream processes may result in cell transformation and malignancy. The Ras and Shc-activated mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3-OH kinase (PI3K)/Akt pathways are the two most studied and, so far, best understood components of the HER signaling cascade. These pathways are involved in the regulatory mechanisms of several cellular processes, including apoptosis, migration, growth, adhesion, and differentiation. The Ras/MAPK pathway is involved in all HER-receptor activations, and the PI3-K/Akt pathway is triggered downstream of many, but not all, active HER dimers. For example, whereas PI3-K couples directly with HER3 and HER4, it does so indirectly with HER1/EGFR and HER2, via an intermediary adaptor protein known as Cbl. Thus, the potency and kinetics of PI3-K vary depending on the HER dimer and any dysregulation, and so, therefore, do its net effects (5). The most widely studied and best understood HER receptors are HER1/EGFR and HER2. Both display abnormal or enhanced expression in many types of cancer, suggesting their involvement in tumorigenesis (14 –16). Moreover, overexpression has been shown to correlate with disease progression, survival, stage, and response to therapy (15,
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Fig. 3. The HER receptor signaling system can be targeted at a range of different loci inside and outside the cell membrane. Fig. 2. HER2 tumors can show ligand-independent or liganddependent receptor activation and signal induction.
16). Dysregulation of signaling from HER1 and HER2, and perhaps from HER3 and HER4, also plays a part in disease development and progression (15). The exact mechanisms that are responsible for tumorigenic activity arising from these receptors in different types of cancer are not fully understood. For simplicity, tumors can be considered as either ligand independent or ligand dependent (Fig. 2). Because there are different underlying functional abnormalities at different points of the signaling network involved in tumorigenesis, various therapeutic strategies can be employed to interrupt these processes. This observation is both challenging and exciting, because evidence of significant involvement of, HER1/EGFR and HER2 in cancer pathogenesis provides a strong rationale for developing agents that target the HER-family signaling system. Unsurprisingly, therefore, a range of therapeutic approaches has evolved. Broadly speaking, signal inhibition can be achieved by the following: ●
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modulating downstream signaling from the receptor by inhibiting or attenuating the activity of secondary messenger proteins, inhibiting receptor expression or activity, reducing the amount of available ligand.
Most progress has been made with the first two options. Small-molecule tyrosine kinase inhibitors comprise the largest and most promising class of agents in development. These compounds act directly at one or more receptor subtypes to inhibit intracellular receptor autophosphorylation, thus preventing downstream signaling by blocking signal initiation. Examples include Tarceva (erlotinib HCl) (Fig. 3) and Iressa (gefitinib), both HER1/EGFR-targeted reversible tyrosine kinase inhibitors currently being evaluated in late-phase clinical development (Table 1). With respect to inhibition of receptor activity, monoclonal antibodies (MAbs) targeting extracellular receptor loci were one of the first methods used, and several agents have already been approved for cancer therapy or are undergoing
advanced preapproval trials. Erbitux (cetuximab) is a HER1/EGFR-targeted MAb now advanced in clinical development, whereas the HER2-specific Herceptin (trastuzumab) was the first such agents to be licensed and is now an established mainstay of clinical oncologic practice (Table 1). 2C4 is a MAb that binds to a broad, extracellular HER2 epitope, resulting in steric inhibition of HER-receptor complex formation that involves HER2. This agent is currently undergoing early clinical evaluation (Fig. 3). Other ways of attenuating defective signaling by interfering with receptor expression or function at a nuclear level include the use of antisense oligonucleotides and ribozymemediated DNA inhibition, but these are still in early clinical and preclinical development, respectively. However, some gene therapy strategies have already shown promising progress. For instance, the adenovirus type 5 E1A gene known to downregulate HER2 expression can inhibit human ovarian and breast cancer cells in vitro and suppress the production of HER2 protein in mice bearing human tumor xenografts, greatly improving survival (18, 19). Phase I trial results in patients with advanced breast and ovarian cancers show HER2 downregulation, increased apoptosis, and reduced proliferation (20). Interestingly, adenovirus type 5 E1A can also enhance the sensitivity of paclitaxel-resistant, HER2-overexpressing human ovarian cancer cells in vitro, and, in an associated xenograft model, mice receiving both gene therapy and paclitaxel survived significantly longer than those receiving either treatment alone (21). Studies using this approach are continuing. Tarceva Preclinical studies. Tarceva is an orally available, potent, highly selective inhibitor of HER1/EGFR, requiring very low concentrations to inhibit activity against isolated tyrosine kinase (concentration required to produce 50% inhibition, IC50 2 nM) and to reduce HER1/EGFR autophosphorylation in intact human tumor cells in vitro (IC50 20 nM) (22). Its selectivity for HER1/EGFR tyrosine kinase over other similar enzymes essential for normal cellular function is important, because poor selectivity could potentially reduce tolerability in a clinical setting. Studies have
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shown that Tarceva inhibits not only target receptor autophosphorylation, but also various downstream signaling intermediaries (23, 24). However, the concentration required to inhibit receptor-mediated downstream signaling is much higher than that required to inhibit receptor phosphorylation (23). Therefore, it would seem appropriate to use Tarceva at the highest dose possible in the clinical setting, i.e., at the maximum tolerated dose (MTD). In vivo, Tarceva dose dependently inhibits tumor growth in a xenograft model of human head-and-neck carcinoma that overexpresses HER1/EGFR (25). Tarceva also shows significant in vitro antitumor activity against cell cultures prepared from a range of human tumors taken directly from patients (26). Responses were also seen in samples found to be resistant to a range of chemotherapeutic agents. Notable responses occurred in breast, non–small-cell lung, and ovarian cancers. There is additional preclinical evidence of at least additive effects of Tarceva with several chemotherapeutic agents with no exacerbation of the toxicity of either agent (25). The antitumor effects of Tarceva are mediated by several mechanisms, particularly inhibition of proliferation (via cell cycle arrest) and induction of apoptosis (22). Because blocking HER1/EGFR can also inhibit angiogenesis, motility, invasion, and tumor cells’ ability to overcome damage from chemotherapy or radiation (14, 27, 28), interference with these processes is also likely to contribute to Tarceva’s antitumor effects. Furthermore, recent data demonstrate that Tarceva can inhibit EGFRvIII (29); this may be clinically significant, because the expression of this mutant receptor is believed to confer a more aggressive phenotype (12, 30, 31). Taken together, these data provide a strong rationale for examining Tarceva in the clinical setting. Clinical studies. There is an extensive, ongoing clinical development program under way for Tarceva, with 4 Phase I trials completed, 2 of which used healthy volunteers, an approach made possible by the compound’s good tolerability profile. Of the 2 studies conducted in patients with advanced or refractory solid tumors, a study evaluating escalating doses (25–200 mg)— given orally once daily for 3 days each week for 3 out of 4 weeks, every day for 3 out of 4 weeks, or continuously—showed dose-independent pharmacokinetics with no drug accumulation (32). The majority of patients had serum concentrations sufficient to provide effective HER1/EGFR inhibition, as estimated from preclinical studies, at therapeutic doses. Tarceva was well tolerated with an MTD of 150 mg/day (uninterrupted schedule), above which principal dose-limiting toxicities were diarrhea and acneiform skin rash. Several patients showed either tumor regression or relatively long periods of disease stability. The recommended dosing schedule arrived at from this study was 150 mg/day on a daily, continuous uninterrupted schedule, equivalent to the MTD. This contrasts with the approach for Iressa, for which recommended doses are 250 and 500 mg/day (33, 34), below the MTD of 52531,000 mg/day (35–37). These different dosing strategies result in
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greater exposure to Tarceva than Iressa at therapeutic doses (Tarceva 150 mg/day [32], Iressa 225 and 525 mg/day [35]), but it is currently unclear whether this will translate into a difference in efficacy. Phase Ib studies evaluating Tarceva in combination with various cytotoxic agents are in progress. Data obtained so far suggest that such combined use shows no adverse interactions and has little or no effect on either tolerability or pharmacokinetics of either agent (38 – 40). Results from 3 Phase II trials of Tarceva monotherapy in patients with advanced, refractory disease show encouraging response rates and tolerability. Of 57 patients with non–small-cell lung cancer, 12% had a complete or partial response to treatment with 150 mg/day, whereas in separate studies, 6% of 34 patients with ovarian carcinoma and 6% of 124 patients with squamous carcinoma of the head and neck also responded to this regimen (41– 43). Almost 50% of patients in all 3 studies had either partial response or a period of prolonged stable disease. The most frequently encountered side effects were cutaneous. Two Phase III trials of Tarceva in combination with chemotherapy are ongoing. TRIBUTE is a multicenter, randomized double-blind trial based in the United States that is comparing first-line treatment with Tarceva, plus a standard regimen comprising carboplatin and paclitaxel, with chemotherapy alone, in 1,050 patients with advanced non– small-cell lung cancer. The primary objective is to establish whether addition of Tarceva improves overall survival, with secondary measures of time to disease progression, response rate, duration of response, and quality of life. Safety and pharmacokinetic outcomes will also be evaluated. TALENT is a similar trial based primarily in Europe and Asia, but it compares gemcitabine with cisplatin with and without the addition of Tarceva. Other studies include a placebo-controlled Phase III trial of Tarceva monotherapy in approximately 700 patients with advanced, refractory non–smallcell lung cancer (Fig. 4), another examining the combination of Tarceva with gemcitabine for advanced pancreatic cancer (PA.3), and a range of Phase II and Phase Ib trials in other indications looking at Tarceva as monotherapy, and with radiotherapy or chemotherapy. A plethora of genes and their associated receptors and signaling pathways are involved in tumor development and growth. Consequently, the concept of using combinations of biologically targeted agents for cancer therapy is particularly exciting. Tumors are likely to be driven by numerous processes; therefore, inhibiting 1, 2, or even 3 pathways has the potential for at least an additive effect on the activity of the regimen. Moreover, one may also expect that the likelihood of tumors becoming resistant to regimens that inhibit multiple biologic targets is low. Based on the specificity of these biologically targeted agents, these regimens will probably be well tolerated. Preclinical data with various combinations of biologically targeted agents support this approach. For example, in a recent study Tarceva and 2C4 were shown to have an additive effect on apoptosis when used in combination (44).
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Table 1. HER-targeted anticancer agents Type of agent Tyrosine kinase inhibitors
Monoclonal antibodies
Target
Agent
HER1
Tarceva
HER1
Iressa (approved by the FDA for pretreated patients with advanced NSCLC)
HER1/HER2
GW572016
HER1/HER2
EKB-569*
HER1/HER2 Pan HER
PD153035 CI-1033‡
HER2
Herceptin (licensed as: second-/ third-line monotherapy and second-/third-line monotherapy and first line with paclitaxel in HER2-overexpressing metastatic breast cancer)
HER2
2C4
HER1
Erbitux
Highest development phase* Phase III Advanced NSCLC First line with carboplatin plus paclitaxel (TRIBUTE) First line with cisplatin plus gemcitabine (TALENT) Second-/third-line monotherapy (BR.21) Advanced pancreatic cancer First line with gemcitabine (PA.3) Advanced ovarian cancer First line with paclitaxel and carboplatin Phase II trials examining various tumor types, including glioma† Phase III Advanced NSCLC First line with chemotherapy (INTACT 1 and 2) (Complete) Second-/third-line monotherapy (IDEAL 1 and 2) (Complete) Phase II trials examining various tumor types† Phase II Metastatic colorectal cancer First line with irinotecan and/or oxaliplatin Second-/third-line monotherapy Metastatic breast cancer Second-/third-line with Herceptin威 Phase I/II Advanced colorectal cancer First line with FOLFIRI Preclinical Phase II Advanced NSCLC Second-/third-line monotherapy Metastatic breast cancer Second-/third-line monotherapy Phase IV Metastatic breast cancer Patients selected by FISH for treatment with docetaxel or paclitaxel (HER first) Phase III Primary breast cancer Four trials in combination with chemotherapy and/or radiotherapy as adjuvant therapy Phase II Advanced ovarian cancer Second-/third-line monotherapy Hormone-resistant prostate cancer Second-/third-line monotherapy Advanced NSCLC Second-/third-line monotherapy Phase III Advanced colorectal Second line with irinotecan or as monotherapy Advanced HNSCC First line with radiotherapy First line with cisplatin Phase II Advanced NSCLC Second line with docetaxel (Complete) (Continued)
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Table 1. HER-targeted anticancer agents (continued) Type of agent
Target
Agent
HER1
ABX-EGF
HER1
h-R3
HER1
EMD-72000
Highest development phase* Phase II Metastatic colorectal cancer Second-/third-line monotherapy First line with chemotherapy Advanced renal cancer Second-/third-line monotherapy Hormone-resistant prostate cancer Second-/third-line monotherapy Advanced NSCLC First line with chemotherapy Phase II Advanced HNSCC First line in combination with radiotherapy Phase I
* Selected studies only. Trials ongoing as monotherapy, in combination with chemotherapy and/or radiotherapy, and other targeted agents. ‡ Irreversible inhibitors. Abbreviations: NSCLC ⫽ non–small-cell lung cancer. †
Against this background, a number of clinical trials are in progress examining Tarceva in combination with other biologically targeted agents in patients with advanced disease. Of these ongoing trials, the most advanced are 2 trials examining Tarceva in combination with Herceptin and Avastin (bevacizumab), respectively. Using Tarceva and Herceptin in combination is particularly interesting, because it will address the clinical effectiveness of inhibiting a large part of the HER signaling network. In this Phase I/II trial, a standard dose of Herceptin with escalating doses of Tarceva are being investigated in patients with metastatic HER2positive breast cancer. It will evaluate the optimal dosage, tolerability, response, and pharmacokinetic parameters of this regimen. Avastin is an antivascular endothelial growth factor MAb that inhibits the development of new blood vessels (angiogenesis) and damages existing tumor vasculature. Using Avastin with Tarceva provides the opportunity to inhibit 2 pathways that are known to be important for tumor growth. In this trial, patients with Stage IIIb or IV recurrent non–small-cell lung cancer will receive escalating doses of both agents in three stages. Preliminary data from both trials should be available in 2003.
Fig. 4. Trial design for Phase III study of Tarceva in refractory non–small-cell lung cancer (NSCLC) (BR.21).
2C4 2C4, a novel anticancer agent currently in Phase I trials, is a humanized anti-HER2 MAb that binds to a different portion of the receptor’s extracellular domain from Herceptin and stops the receptor from associating with other HER family members; i.e., it acts via ligand-dependent processes (45) (Fig. 5). In contrast to Herceptin, the activity of 2C4 is not dependent on HER2 overexpression. 2C4 blocks the association of HER2 with other HER receptor subtypes, thus preventing ligand-dependent HER2 signaling in both HER2-negative and HER2-positive tumor cell lines (46). Therefore, 2C4 is expected to show activity against tumors that would not be targets for treatment with Herceptin. 2C4 inhibits ligand-initiated cell signaling through two crucial signal transduction pathways: MAPK, a major proliferative pathway determining cell growth; and Akt, a major antiapoptotic pathway determining cell survival (45). This suggests that it may have a high level of antitumor activity. 2C4 has shown significant growth inhibition of several breast and prostate tumor models in vitro and in vivo, including both hormone-dependent and hormone-independent cell lines (45). In the same study, 2C4, but not Herceptin, inhibited heregulin-dependent Akt activation. Because this ligand is selective for HER3, 2C4 is preventing HER2/HER3 heterodimerization by effectively blocking HER2 coreceptor function. In mouse xenograft studies, chronic dosing with i.p. 2C4 inhibited the growth of low HER2-expressing human non–small-cell lung cancer cell lines, and of both HER2-negative and HER2-positive human breast cancer cell lines. In low HER2-expressing human prostate xenografts, the tumor inhibitory activity of 2C4 alone was equivalent to that of paclitaxel alone, whereas when given in combination, the effect was magnified. The effect of Herceptin, however, was not significantly different from control, as would be expected, because this
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Fig. 5. 2C4 prevents the HER2 from heterodimerizing with other HER family members, via a ligand-dependent mechanism. Reproduced with permission from R. Akita, Genentech, Inc.
tumor type is generated by a ligand-dependent process involving receptors other than HER2 alone. Studies indicate that 2C4 may be active against tumors that have acquired resistance to other HER-targeted agents, such as Iressa (47). The Phase I trial of 2C4 is now fully recruited. The highest planned doses of 2C4 have been achieved with no unexpected toxicity. Phase II plans are to focus on non–small-cell lung cancer, metastatic HER2negative breast cancer, hormone-refractory prostate cancer, and ovarian cancer. USING PREDICTIVE DIAGNOSTICS TO OPTIMIZE TARGETED ANTICANCER AGENTS Before treatment with Herceptin, patients are screened for expression of HER2, the target receptor; only patients with a high level of HER2 expression receive Herceptin; theoretically, only the patients with the biologic capability of responding to treatment are treated. In view of this, it is reasonable to assume that most targeted agents should be used only in patients who, based on phenotype or genotype, are likely to benefit. However, as our clinical experience with agents other than Herceptin builds, it is evident that identifying molecules that predict for response to therapy, although necessary, is problematic. As discussed, several HER1/EGFR-targeted agents are advanced in clinical development, and, based on the response data, it seems that only a small population of patients is responding to therapy. Moreover, two recent Phase III trials with Iressa showed no additional benefit when Iressa was added to standard chemotherapy in patients with advanced non–small-cell lung cancer (48, 49). Therefore, there is increasing impetus to try and find a marker to identify those responsive patients. Various routes of investigation to answer this question are being explored. One
route is to use tissue samples from treated patients to correlate marker expression or gene profile with response. The power of this approach has been increased by the development of tissue microarray and gene-chip-array technology, which allow faster and more comprehensive analysis of tissue samples. Tissue samples are being collected from many large Phase III trials, including TALENT and TRIBUTE, and it is hoped this area of research will prove fruitful. Another route of investigation is identifying the key regulators in tumorigenic development in different types of cancer. One way to achieve this is by comparing the gene expression profiles of normal and tumor tissue. Ultimately, this may enable us to identify subtypes of tumors that are more dependent on certain tumorigenic pathways than others, thus, making these tumors good candidates for therapy with specific targeted agents. Identifying markers that predict for response to targeted therapies is likely to be essential for the optimal clinical use of these agents, particularly in view of the range of targeted therapies that is likely to be available in the future. CONCLUSION Our growing understanding of both the complexities and the significance of HER receptor involvement in cancer underpins a promising range of potential new therapies. This gives new hope, particularly for the largely underserved population of patients whose cancers are of the aggressive variety: advanced and refractory to conventional treatment. In these cases, Tarceva is rapidly proving itself to be effective, whereas 2C4 is already yielding promising results predictive of good clinical outcome. Subtle differences in mechanism and site of action give ample scope for optimizing treatment regimens per cancer. Approaches may include combination of these agents with conventional ther-
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apies, such as chemotherapy and radiotherapy, or with other biologically targeted agents. Considerable data already exist in support of using biologic agents in combination. Another key area of investigation that holds great promise for increasing the activity of these biologically targeted approaches is identifying markers that predict for response. This may enable us to select those patients who are most likely to benefit, thereby improving efficacy. Moreover, in the future we
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may be able to use phenotypic or genotypic information to tailor therapeutic regimens to individual patients. In summary, it seems only a matter of time before new, effective strategies are made available to the oncologist’s armamentarium. Given the center-stage role of the HER system in oncogenesis, future investigations should focus on the use of these agents in early-stage disease, i.e., in the adjuvant/neoadjuvant setting, or as preventative agents.
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non-small cell lung cancer (IDEAL 1) (Abstract). Proc Am Soc Clin Oncol 2002;21:298a. Kris MG, Natale RB, Herbst RS, et al. A Phase II trial of ZD1839 (‘Iressa’) in advanced non-small cell lung cancer (NSCLC) patients who had failed platinum- and docetaxelbased regimens (IDEAL 2) (Abstract). Proc Am Soc Clin Oncol 2002;21:292a. Ranson M, Hammond LA, Ferry D, et al. ZD1839, a selective oral epidermal growth factor receptor-tyrosine kinase inhibitor, is well tolerated and active in patients with solid, malignant tumors: Results of a Phase I trial. J Clin Oncol 2002;20: 2240–2250. Herbst RS, Maddox A-M, Rothenberg ML, et al. Selective oral epidermal growth factor receptor tyrosine kinase inhibitor ZD1839 is generally well-tolerated and has activity in nonsmall cell lung cancer and other solid tumors: Results of a Phase I trial. J Clin Oncol 2002;20:3815–3825. Baselga J, Rischin D, Ranson M, et al. Phase I safety, pharmacokinetic, and pharmacodynamic trial of ZD1839, a selective oral epidermal growth factor receptor tyrosine kinase inhibitor, in patients with five selected solid tumor types. J Clin Oncol 2002;20:4292–4302. Patnaik A, Goetz A, Hammond LA, et al. Phase I, pharmacokinetic and biologic study of OSI-774 (Tarceva™), a selective epidermal growth factor receptor tyrosine kinase inhibitor in combination with paclitaxel and carboplatin in patients with advanced solid malignancies. Presented at EORTC-NCIAACR 2002; Abstract 169. Mita A, Forouzesh B, Hidalgo M, et al. Phase I, pharmacokinetic (PK) and biological studies of the epidermal growth factor receptor-tyrosine kinase (HER1/EGFR-TK) inhibitor erlotinib (OSI-774; Tarceva™) in combination with docetaxel. Presented at EORTC-NCI-AACR 2002; Abstract 168. Ratain MJ, George CM, Janisch L, et al. Phase I trial of erlotinib (OSI-774) in combination with gemcitabine (G) and cisplatin (P) in patients with advanced solid tumors (Abstract). Proc Am Soc Clin Oncol 2002;21:76b. Perez-Soler R, Chachoua A, Huberman M, et al. Determinants
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45. 46. 47.
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of tumor response and survival in patients with relapsing NSCLC treated with Tarceva™ (erlotinib HCL; OSI-774). Final report of a phase II study. (Abstract) Presented at the Am Soc Clin Oncol Mol Ther Symp. November 2002. Finkler N, Gordon A, Crozier M, et al. Phase 2 evaluation of OSI-774, a potent oral antagonist of the EGFR-TK in patients with advanced ovarian carcinoma (Abstract). Proc Am Soc Clin Oncol 2001;20:208a. Senzer NN, Soulieres D, Siu L, et al. Phase 2 evaluation of OSI-774, a potent oral antagonist of the EGFR-TK in patients with advanced squamous cell carcinoma of the head and neck (Abstr.). Proc Am Soc Clin Oncol 2001;20:2a. Totpal K, Lewis-Phillips GD, Balter I, et al. Augmentation of rhuMAb2C4 induced growth inhibition by TARCEVA™ the EGFR tyrosine kinase inhibitor on human breast cancer cell line (Abstr.). Proc Am Assoc Cancer Res 2002;43:789. Agus D, Akita R, Fox W, et al. Targeting ligand-activated ErbB2 signaling inhibits breast and prostate tumor growth. Cancer Cell 2002;2:127. Baselga J. A new anti-ErbB2 strategy in the treatment of cancer: Prevention of ligand-dependent ErbB2 receptor heterodimerization. Cancer Cell 2002;2:93–95. Geller J, Galkin A, Mullen L, et al. Effects of the epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI) ZD1839 (‘Iressa’) on androgen-dependent and androgen-independent human prostate tumor xenografts: Growth inhibition and characteristics of resistant tumors (Abstract). Proc Am Assoc Cancer Res 2002;43:1003. Giaccone G, Johnson DH, Manegold C, et al. A Phase III clinical trial of ZD1839 (‘Iressa’) in combination with gemcitabine and cisplatin in chemotherapy-naı¨ve patients with advanced non-small-cell lung cancer (INTACT 1) (Abstract). Ann Oncol 2002;13(Suppl. 5):2. Johnson DH, Herbst R, Giaccone G, et al. ZD1839 (“Iressa”) in combination with paclitaxel and carboplatin in chemotherapy-naı¨ve patients with advanced non-small-cell lung cancer (NSCLC): Results from a Phase III clinical trial (INTACT 2) (Abstract). Ann Oncol 2002;13(Suppl. 5):127.
doi:10.1016/j.ijrobp.2003.09.093
EGFR INHIBITORS
THE HER RECEPTOR FAMILY: A RICH TARGET FOR THERAPEUTIC DEVELOPMENT ROBERT D. MASS, M.D. Genentech BioOncology, Inc., South San Francisco, CA Purpose: The key role of the HER family of receptors in cancer has been widely acknowledged. HER receptor activation occurs via ligand binding or nonligand-dependent receptor dimerization, initiating signals that promote tumorigenesis via cell proliferation, survival, migration, adhesion, and differentiation. Therapeutic strategies designed to target and inhibit HER activation that are in clinical development are reviewed, including examples of both small-molecule tyrosine kinase inhibitors and monoclonal antibodies. Materials and Methods: A literature review. Results: Tarceva is a potent, highly selective, reversible inhibitor of HER1/epidermal growth factor receptor tyrosine kinase with inhibitory activity against various in vitro and in vivo models of solid human tumors. Phase II trials in refractory non–small-cell lung, head-and-neck, and ovarian cancer have demonstrated clinical activity, including objective responses and prolonged, stable disease. Four Phase III trials are ongoing evaluating primarily the effect on survival of Tarceva in combination with chemotherapy. 2C4 is a humanized anti-HER2 monoclonal antibody that binds to a broad, extracellular epitope, resulting in steric inhibition of HER–receptor complex formation that involves HER2. 2C4 has shown significant activity in xenograft models of prostate, lung, and breast cancer. 2C4’s activity, unlike Herceptin’s, is not dependent on HER2 amplification. This antibody is in early clinical development. Conclusion: The strategy of targeting the HER system has been further validated by early experience with Tarceva and 2C4. The optimal clinical benefit of these agents will likely involve combinations of biologic agents, with or without traditional chemotherapy, and will be guided by critical predictive diagnostic information. © 2004 Elsevier Inc. Tarceva, 2C4, Cancer, HER, Targeted therapy.
addition, there are 3 mutant HER1/EGFR receptors, of which EGFRvIII is the most commonly detected in human solid tumors (3), where it is also frequently overexpressed (4). Wild-type HER receptors are each divided into 3 regions: an extracellular ligand binding region, an intracellular region containing the locus responsible for tyrosine kinase activity and regulatory functions, and a transmembrane region that anchors the receptor to the cell (2). Each HER receptor is fundamentally an inactive monomer that dimerizes in response to ligand binding. This process takes the form of homodimerization with a receptor of the same type or heterodimerization with another member of the HER family. This dimerization process results in receptor activation through tyrosine kinase–mediated phosphorylation that produces a biochemical “trigger” that starts a cascade of metabolic events constituting a complex downstream signaling network, leading to various effects on specific aspects of cell function (2).
INTRODUCTION Since the early 1970s, research has provided an exponential growth in our understanding of the intricate and complex interplay between the vast range of genetic, molecular, and biochemical mechanisms responsible for the regulation of both normal and abnormal cellular function. Notably, the role of cell membrane receptors in maintaining the delicate balance between cell proliferation and programmed cell death (apoptosis) has been widely investigated. The involvement of cell membrane receptors in angiogenesis and cell migration—processes integral to cancer spread— has also been studied. The resulting body of knowledge has led to novel approaches for targeted cancer therapy. Of the many receptors and their associated signaling proteins identified, the epidermal growth factor receptor (EGFR), or HER family, has been widely acknowledged as playing a crucial role in tumorigenesis and disease progression. HER receptors comprise 4 distinct transmembrane receptors: HER1/EGFR, HER2, HER3, and HER4 (1, 2). In Reprint requests to: Robert Mass, M.D., Genentech BioOncology, Inc., 1 DNA Way, South San Francisco, CA 94080-4990. Tel: (650) 225-7223; Fax: (650) 225-4538; E-mail: bobmass@ gene.com
Received May 21, 2003, and in revised form Aug 29, 2003. Accepted for publication Sep 8, 2003.
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Fig. 1. The HER signaling network. Reprinted by permission from Nature Reviews Molecular Cell Biology (www. nature.com/reviews) Yarden and Sliwkowski 2001;2:127–137 (5).
The HER family cell signaling system uses at least 11 epidermal growth factor–like peptide mediators, or ligands (5). HER2 shows substantial structural homology to HER1, but has no known ligand (6, 7), though it is frequently activated because of its common coreceptor function (8, 9). HER3 and HER4 are also structurally related to HER1; however, HER3 lacks intrinsic tyrosine kinase activity (10) and therefore can initiate signaling only via association with another HER-family receptor, usually HER2 (9, 11). The mutant HER receptor subtype EGFRvIII has a truncated extracellular-binding domain and cannot bind ligand. However, it does have an intact tyrosine kinase domain that is constitutively phosphorylated and can, therefore, activate downstream signaling cascades independent of ligand binding (4, 12, 13). In general, dysregulated HER-receptor activation initiates a signaling cascade that promotes tumorigenesis via cell proliferation, survival, migration, adhesion, and differentiation (2, 14). The HER signaling system comprises a highly complex and interactive “multilayered” network (Fig. 1). Signals are initiated at the cell surface, or “input layer,” where ligand– receptor interactions take place, and the resulting receptor dimerization and activation relays and amplifies the signal through a cytoplasmic “signal-processing layer” via an intricate system of enzymes, proteins, and small-molecule secondary messengers (5). This process of signal transduction culminates in the nucleus, or “output layer,” where
gene control and protein transcription are modified, producing effects on key cellular regulatory processes, such as differentiation, adhesion, growth, migration, and apoptosis. Dysregulated receptor function and disruptions in any or all downstream processes may result in cell transformation and malignancy. The Ras and Shc-activated mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3-OH kinase (PI3K)/Akt pathways are the two most studied and, so far, best understood components of the HER signaling cascade. These pathways are involved in the regulatory mechanisms of several cellular processes, including apoptosis, migration, growth, adhesion, and differentiation. The Ras/MAPK pathway is involved in all HER-receptor activations, and the PI3-K/Akt pathway is triggered downstream of many, but not all, active HER dimers. For example, whereas PI3-K couples directly with HER3 and HER4, it does so indirectly with HER1/EGFR and HER2, via an intermediary adaptor protein known as Cbl. Thus, the potency and kinetics of PI3-K vary depending on the HER dimer and any dysregulation, and so, therefore, do its net effects (5). The most widely studied and best understood HER receptors are HER1/EGFR and HER2. Both display abnormal or enhanced expression in many types of cancer, suggesting their involvement in tumorigenesis (14 –16). Moreover, overexpression has been shown to correlate with disease progression, survival, stage, and response to therapy (15,
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Fig. 3. The HER receptor signaling system can be targeted at a range of different loci inside and outside the cell membrane. Fig. 2. HER2 tumors can show ligand-independent or liganddependent receptor activation and signal induction.
16). Dysregulation of signaling from HER1 and HER2, and perhaps from HER3 and HER4, also plays a part in disease development and progression (15). The exact mechanisms that are responsible for tumorigenic activity arising from these receptors in different types of cancer are not fully understood. For simplicity, tumors can be considered as either ligand independent or ligand dependent (Fig. 2). Because there are different underlying functional abnormalities at different points of the signaling network involved in tumorigenesis, various therapeutic strategies can be employed to interrupt these processes. This observation is both challenging and exciting, because evidence of significant involvement of, HER1/EGFR and HER2 in cancer pathogenesis provides a strong rationale for developing agents that target the HER-family signaling system. Unsurprisingly, therefore, a range of therapeutic approaches has evolved. Broadly speaking, signal inhibition can be achieved by the following: ●
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modulating downstream signaling from the receptor by inhibiting or attenuating the activity of secondary messenger proteins, inhibiting receptor expression or activity, reducing the amount of available ligand.
Most progress has been made with the first two options. Small-molecule tyrosine kinase inhibitors comprise the largest and most promising class of agents in development. These compounds act directly at one or more receptor subtypes to inhibit intracellular receptor autophosphorylation, thus preventing downstream signaling by blocking signal initiation. Examples include Tarceva (erlotinib HCl) (Fig. 3) and Iressa (gefitinib), both HER1/EGFR-targeted reversible tyrosine kinase inhibitors currently being evaluated in late-phase clinical development (Table 1). With respect to inhibition of receptor activity, monoclonal antibodies (MAbs) targeting extracellular receptor loci were one of the first methods used, and several agents have already been approved for cancer therapy or are undergoing
advanced preapproval trials. Erbitux (cetuximab) is a HER1/EGFR-targeted MAb now advanced in clinical development, whereas the HER2-specific Herceptin (trastuzumab) was the first such agents to be licensed and is now an established mainstay of clinical oncologic practice (Table 1). 2C4 is a MAb that binds to a broad, extracellular HER2 epitope, resulting in steric inhibition of HER-receptor complex formation that involves HER2. This agent is currently undergoing early clinical evaluation (Fig. 3). Other ways of attenuating defective signaling by interfering with receptor expression or function at a nuclear level include the use of antisense oligonucleotides and ribozymemediated DNA inhibition, but these are still in early clinical and preclinical development, respectively. However, some gene therapy strategies have already shown promising progress. For instance, the adenovirus type 5 E1A gene known to downregulate HER2 expression can inhibit human ovarian and breast cancer cells in vitro and suppress the production of HER2 protein in mice bearing human tumor xenografts, greatly improving survival (18, 19). Phase I trial results in patients with advanced breast and ovarian cancers show HER2 downregulation, increased apoptosis, and reduced proliferation (20). Interestingly, adenovirus type 5 E1A can also enhance the sensitivity of paclitaxel-resistant, HER2-overexpressing human ovarian cancer cells in vitro, and, in an associated xenograft model, mice receiving both gene therapy and paclitaxel survived significantly longer than those receiving either treatment alone (21). Studies using this approach are continuing. Tarceva Preclinical studies. Tarceva is an orally available, potent, highly selective inhibitor of HER1/EGFR, requiring very low concentrations to inhibit activity against isolated tyrosine kinase (concentration required to produce 50% inhibition, IC50 2 nM) and to reduce HER1/EGFR autophosphorylation in intact human tumor cells in vitro (IC50 20 nM) (22). Its selectivity for HER1/EGFR tyrosine kinase over other similar enzymes essential for normal cellular function is important, because poor selectivity could potentially reduce tolerability in a clinical setting. Studies have
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shown that Tarceva inhibits not only target receptor autophosphorylation, but also various downstream signaling intermediaries (23, 24). However, the concentration required to inhibit receptor-mediated downstream signaling is much higher than that required to inhibit receptor phosphorylation (23). Therefore, it would seem appropriate to use Tarceva at the highest dose possible in the clinical setting, i.e., at the maximum tolerated dose (MTD). In vivo, Tarceva dose dependently inhibits tumor growth in a xenograft model of human head-and-neck carcinoma that overexpresses HER1/EGFR (25). Tarceva also shows significant in vitro antitumor activity against cell cultures prepared from a range of human tumors taken directly from patients (26). Responses were also seen in samples found to be resistant to a range of chemotherapeutic agents. Notable responses occurred in breast, non–small-cell lung, and ovarian cancers. There is additional preclinical evidence of at least additive effects of Tarceva with several chemotherapeutic agents with no exacerbation of the toxicity of either agent (25). The antitumor effects of Tarceva are mediated by several mechanisms, particularly inhibition of proliferation (via cell cycle arrest) and induction of apoptosis (22). Because blocking HER1/EGFR can also inhibit angiogenesis, motility, invasion, and tumor cells’ ability to overcome damage from chemotherapy or radiation (14, 27, 28), interference with these processes is also likely to contribute to Tarceva’s antitumor effects. Furthermore, recent data demonstrate that Tarceva can inhibit EGFRvIII (29); this may be clinically significant, because the expression of this mutant receptor is believed to confer a more aggressive phenotype (12, 30, 31). Taken together, these data provide a strong rationale for examining Tarceva in the clinical setting. Clinical studies. There is an extensive, ongoing clinical development program under way for Tarceva, with 4 Phase I trials completed, 2 of which used healthy volunteers, an approach made possible by the compound’s good tolerability profile. Of the 2 studies conducted in patients with advanced or refractory solid tumors, a study evaluating escalating doses (25–200 mg)— given orally once daily for 3 days each week for 3 out of 4 weeks, every day for 3 out of 4 weeks, or continuously—showed dose-independent pharmacokinetics with no drug accumulation (32). The majority of patients had serum concentrations sufficient to provide effective HER1/EGFR inhibition, as estimated from preclinical studies, at therapeutic doses. Tarceva was well tolerated with an MTD of 150 mg/day (uninterrupted schedule), above which principal dose-limiting toxicities were diarrhea and acneiform skin rash. Several patients showed either tumor regression or relatively long periods of disease stability. The recommended dosing schedule arrived at from this study was 150 mg/day on a daily, continuous uninterrupted schedule, equivalent to the MTD. This contrasts with the approach for Iressa, for which recommended doses are 250 and 500 mg/day (33, 34), below the MTD of 52531,000 mg/day (35–37). These different dosing strategies result in
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greater exposure to Tarceva than Iressa at therapeutic doses (Tarceva 150 mg/day [32], Iressa 225 and 525 mg/day [35]), but it is currently unclear whether this will translate into a difference in efficacy. Phase Ib studies evaluating Tarceva in combination with various cytotoxic agents are in progress. Data obtained so far suggest that such combined use shows no adverse interactions and has little or no effect on either tolerability or pharmacokinetics of either agent (38 – 40). Results from 3 Phase II trials of Tarceva monotherapy in patients with advanced, refractory disease show encouraging response rates and tolerability. Of 57 patients with non–small-cell lung cancer, 12% had a complete or partial response to treatment with 150 mg/day, whereas in separate studies, 6% of 34 patients with ovarian carcinoma and 6% of 124 patients with squamous carcinoma of the head and neck also responded to this regimen (41– 43). Almost 50% of patients in all 3 studies had either partial response or a period of prolonged stable disease. The most frequently encountered side effects were cutaneous. Two Phase III trials of Tarceva in combination with chemotherapy are ongoing. TRIBUTE is a multicenter, randomized double-blind trial based in the United States that is comparing first-line treatment with Tarceva, plus a standard regimen comprising carboplatin and paclitaxel, with chemotherapy alone, in 1,050 patients with advanced non– small-cell lung cancer. The primary objective is to establish whether addition of Tarceva improves overall survival, with secondary measures of time to disease progression, response rate, duration of response, and quality of life. Safety and pharmacokinetic outcomes will also be evaluated. TALENT is a similar trial based primarily in Europe and Asia, but it compares gemcitabine with cisplatin with and without the addition of Tarceva. Other studies include a placebo-controlled Phase III trial of Tarceva monotherapy in approximately 700 patients with advanced, refractory non–smallcell lung cancer (Fig. 4), another examining the combination of Tarceva with gemcitabine for advanced pancreatic cancer (PA.3), and a range of Phase II and Phase Ib trials in other indications looking at Tarceva as monotherapy, and with radiotherapy or chemotherapy. A plethora of genes and their associated receptors and signaling pathways are involved in tumor development and growth. Consequently, the concept of using combinations of biologically targeted agents for cancer therapy is particularly exciting. Tumors are likely to be driven by numerous processes; therefore, inhibiting 1, 2, or even 3 pathways has the potential for at least an additive effect on the activity of the regimen. Moreover, one may also expect that the likelihood of tumors becoming resistant to regimens that inhibit multiple biologic targets is low. Based on the specificity of these biologically targeted agents, these regimens will probably be well tolerated. Preclinical data with various combinations of biologically targeted agents support this approach. For example, in a recent study Tarceva and 2C4 were shown to have an additive effect on apoptosis when used in combination (44).
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Table 1. HER-targeted anticancer agents Type of agent Tyrosine kinase inhibitors
Monoclonal antibodies
Target
Agent
HER1
Tarceva
HER1
Iressa (approved by the FDA for pretreated patients with advanced NSCLC)
HER1/HER2
GW572016
HER1/HER2
EKB-569*
HER1/HER2 Pan HER
PD153035 CI-1033‡
HER2
Herceptin (licensed as: second-/ third-line monotherapy and second-/third-line monotherapy and first line with paclitaxel in HER2-overexpressing metastatic breast cancer)
HER2
2C4
HER1
Erbitux
Highest development phase* Phase III Advanced NSCLC First line with carboplatin plus paclitaxel (TRIBUTE) First line with cisplatin plus gemcitabine (TALENT) Second-/third-line monotherapy (BR.21) Advanced pancreatic cancer First line with gemcitabine (PA.3) Advanced ovarian cancer First line with paclitaxel and carboplatin Phase II trials examining various tumor types, including glioma† Phase III Advanced NSCLC First line with chemotherapy (INTACT 1 and 2) (Complete) Second-/third-line monotherapy (IDEAL 1 and 2) (Complete) Phase II trials examining various tumor types† Phase II Metastatic colorectal cancer First line with irinotecan and/or oxaliplatin Second-/third-line monotherapy Metastatic breast cancer Second-/third-line with Herceptin威 Phase I/II Advanced colorectal cancer First line with FOLFIRI Preclinical Phase II Advanced NSCLC Second-/third-line monotherapy Metastatic breast cancer Second-/third-line monotherapy Phase IV Metastatic breast cancer Patients selected by FISH for treatment with docetaxel or paclitaxel (HER first) Phase III Primary breast cancer Four trials in combination with chemotherapy and/or radiotherapy as adjuvant therapy Phase II Advanced ovarian cancer Second-/third-line monotherapy Hormone-resistant prostate cancer Second-/third-line monotherapy Advanced NSCLC Second-/third-line monotherapy Phase III Advanced colorectal Second line with irinotecan or as monotherapy Advanced HNSCC First line with radiotherapy First line with cisplatin Phase II Advanced NSCLC Second line with docetaxel (Complete) (Continued)
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Table 1. HER-targeted anticancer agents (continued) Type of agent
Target
Agent
HER1
ABX-EGF
HER1
h-R3
HER1
EMD-72000
Highest development phase* Phase II Metastatic colorectal cancer Second-/third-line monotherapy First line with chemotherapy Advanced renal cancer Second-/third-line monotherapy Hormone-resistant prostate cancer Second-/third-line monotherapy Advanced NSCLC First line with chemotherapy Phase II Advanced HNSCC First line in combination with radiotherapy Phase I
* Selected studies only. Trials ongoing as monotherapy, in combination with chemotherapy and/or radiotherapy, and other targeted agents. ‡ Irreversible inhibitors. Abbreviations: NSCLC ⫽ non–small-cell lung cancer. †
Against this background, a number of clinical trials are in progress examining Tarceva in combination with other biologically targeted agents in patients with advanced disease. Of these ongoing trials, the most advanced are 2 trials examining Tarceva in combination with Herceptin and Avastin (bevacizumab), respectively. Using Tarceva and Herceptin in combination is particularly interesting, because it will address the clinical effectiveness of inhibiting a large part of the HER signaling network. In this Phase I/II trial, a standard dose of Herceptin with escalating doses of Tarceva are being investigated in patients with metastatic HER2positive breast cancer. It will evaluate the optimal dosage, tolerability, response, and pharmacokinetic parameters of this regimen. Avastin is an antivascular endothelial growth factor MAb that inhibits the development of new blood vessels (angiogenesis) and damages existing tumor vasculature. Using Avastin with Tarceva provides the opportunity to inhibit 2 pathways that are known to be important for tumor growth. In this trial, patients with Stage IIIb or IV recurrent non–small-cell lung cancer will receive escalating doses of both agents in three stages. Preliminary data from both trials should be available in 2003.
Fig. 4. Trial design for Phase III study of Tarceva in refractory non–small-cell lung cancer (NSCLC) (BR.21).
2C4 2C4, a novel anticancer agent currently in Phase I trials, is a humanized anti-HER2 MAb that binds to a different portion of the receptor’s extracellular domain from Herceptin and stops the receptor from associating with other HER family members; i.e., it acts via ligand-dependent processes (45) (Fig. 5). In contrast to Herceptin, the activity of 2C4 is not dependent on HER2 overexpression. 2C4 blocks the association of HER2 with other HER receptor subtypes, thus preventing ligand-dependent HER2 signaling in both HER2-negative and HER2-positive tumor cell lines (46). Therefore, 2C4 is expected to show activity against tumors that would not be targets for treatment with Herceptin. 2C4 inhibits ligand-initiated cell signaling through two crucial signal transduction pathways: MAPK, a major proliferative pathway determining cell growth; and Akt, a major antiapoptotic pathway determining cell survival (45). This suggests that it may have a high level of antitumor activity. 2C4 has shown significant growth inhibition of several breast and prostate tumor models in vitro and in vivo, including both hormone-dependent and hormone-independent cell lines (45). In the same study, 2C4, but not Herceptin, inhibited heregulin-dependent Akt activation. Because this ligand is selective for HER3, 2C4 is preventing HER2/HER3 heterodimerization by effectively blocking HER2 coreceptor function. In mouse xenograft studies, chronic dosing with i.p. 2C4 inhibited the growth of low HER2-expressing human non–small-cell lung cancer cell lines, and of both HER2-negative and HER2-positive human breast cancer cell lines. In low HER2-expressing human prostate xenografts, the tumor inhibitory activity of 2C4 alone was equivalent to that of paclitaxel alone, whereas when given in combination, the effect was magnified. The effect of Herceptin, however, was not significantly different from control, as would be expected, because this
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Fig. 5. 2C4 prevents the HER2 from heterodimerizing with other HER family members, via a ligand-dependent mechanism. Reproduced with permission from R. Akita, Genentech, Inc.
tumor type is generated by a ligand-dependent process involving receptors other than HER2 alone. Studies indicate that 2C4 may be active against tumors that have acquired resistance to other HER-targeted agents, such as Iressa (47). The Phase I trial of 2C4 is now fully recruited. The highest planned doses of 2C4 have been achieved with no unexpected toxicity. Phase II plans are to focus on non–small-cell lung cancer, metastatic HER2negative breast cancer, hormone-refractory prostate cancer, and ovarian cancer. USING PREDICTIVE DIAGNOSTICS TO OPTIMIZE TARGETED ANTICANCER AGENTS Before treatment with Herceptin, patients are screened for expression of HER2, the target receptor; only patients with a high level of HER2 expression receive Herceptin; theoretically, only the patients with the biologic capability of responding to treatment are treated. In view of this, it is reasonable to assume that most targeted agents should be used only in patients who, based on phenotype or genotype, are likely to benefit. However, as our clinical experience with agents other than Herceptin builds, it is evident that identifying molecules that predict for response to therapy, although necessary, is problematic. As discussed, several HER1/EGFR-targeted agents are advanced in clinical development, and, based on the response data, it seems that only a small population of patients is responding to therapy. Moreover, two recent Phase III trials with Iressa showed no additional benefit when Iressa was added to standard chemotherapy in patients with advanced non–small-cell lung cancer (48, 49). Therefore, there is increasing impetus to try and find a marker to identify those responsive patients. Various routes of investigation to answer this question are being explored. One
route is to use tissue samples from treated patients to correlate marker expression or gene profile with response. The power of this approach has been increased by the development of tissue microarray and gene-chip-array technology, which allow faster and more comprehensive analysis of tissue samples. Tissue samples are being collected from many large Phase III trials, including TALENT and TRIBUTE, and it is hoped this area of research will prove fruitful. Another route of investigation is identifying the key regulators in tumorigenic development in different types of cancer. One way to achieve this is by comparing the gene expression profiles of normal and tumor tissue. Ultimately, this may enable us to identify subtypes of tumors that are more dependent on certain tumorigenic pathways than others, thus, making these tumors good candidates for therapy with specific targeted agents. Identifying markers that predict for response to targeted therapies is likely to be essential for the optimal clinical use of these agents, particularly in view of the range of targeted therapies that is likely to be available in the future. CONCLUSION Our growing understanding of both the complexities and the significance of HER receptor involvement in cancer underpins a promising range of potential new therapies. This gives new hope, particularly for the largely underserved population of patients whose cancers are of the aggressive variety: advanced and refractory to conventional treatment. In these cases, Tarceva is rapidly proving itself to be effective, whereas 2C4 is already yielding promising results predictive of good clinical outcome. Subtle differences in mechanism and site of action give ample scope for optimizing treatment regimens per cancer. Approaches may include combination of these agents with conventional ther-
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apies, such as chemotherapy and radiotherapy, or with other biologically targeted agents. Considerable data already exist in support of using biologic agents in combination. Another key area of investigation that holds great promise for increasing the activity of these biologically targeted approaches is identifying markers that predict for response. This may enable us to select those patients who are most likely to benefit, thereby improving efficacy. Moreover, in the future we
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may be able to use phenotypic or genotypic information to tailor therapeutic regimens to individual patients. In summary, it seems only a matter of time before new, effective strategies are made available to the oncologist’s armamentarium. Given the center-stage role of the HER system in oncogenesis, future investigations should focus on the use of these agents in early-stage disease, i.e., in the adjuvant/neoadjuvant setting, or as preventative agents.
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