Molecular mechanisms underlying IGF-I-induced attenuation of the growth-inhibitory activity of trastuzumab (Herceptin) on SKBR3 breast cancer cells

Dissecting therapeutic resistance to RAF inhibition in melanoma by tumor genomic profiling

A detailed understanding of the mechanisms by which tumors acquire resistance to targeted anticancer agents should speed the development of treatment strategies with lasting clinical efficacy. RAF inhibition in BRAF-mutant melanoma exemplifies the promise and challenge of many targeted drugs; although response rates are high, resistance invariably develops. Here, we articulate overarching principles of resistance to kinase inhibitors, as well as a translational approach to characterize resistance in the clinical setting through tumor mutation profiling. As a proof of principle, we performed targeted, massively parallel sequencing of 138 cancer genes in a tumor obtained from a patient with melanoma who developed resistance to PLX4032 after an initial dramatic response. The resulting profile identified an activating mutation at codon 121 in the downstream kinase MEK1 that was absent in the corresponding pretreatment tumor. The MEK1(C121S) mutation was shown to increase kinase activity and confer robust resistance to both RAF and MEK inhibition in vitro. Thus, MEK1(C121S) or functionally similar mutations are predicted to confer resistance to combined MEK/RAF inhibition. These results provide an instructive framework for assessing mechanisms of acquired resistance to kinase inhibition and illustrate the use of emerging technologies in a manner that may accelerate personalized cancer medicine.

Discovery of OSI-906: a selective and orally efficacious dual inhibitor of the IGF-1 receptor and insulin receptor

BACKGROUND

The IGF-1 receptor (IGF-1R) has been implicated in the promotion of tumorigenesis, metastasis and resistance to cancer therapies. Therefore, this receptor has become a major focus for the development of anticancer agents.

RESULTS

Our lead optimization efforts that blended structure-based design and empirical medicinal chemistry led to the discovery of OSI-906, a novel small-molecule dual IGF-1R/insulin receptor (IR) kinase inhibitor. OSI-906 potently and selectively inhibits autophosphorylation of both human IGF-1R and IR, displays in vitro antiproliferative effects in a variety of tumor cell lines and shows robust in vivo anti-tumor efficacy in an IGF-1R-driven xenograft model when administered orally once daily.

CONCLUSION

OSI-906 is a novel, potent, selective and orally bioavailable dual IGF-1R/IR kinase inhibitor with favorable preclinical drug-like properties, which has demonstrated in vivo efficacy in tumor models and is currently in clinical testing.

Targeted inhibition of kinases in cancer therapy

With an understanding of the molecular changes that accompany cell transformation, cancer drug discovery has undergone a dramatic change in the past few years. Whereas most of the emphasis in the past has been placed on developing drugs that induce cell death based on mechanisms that do not discriminate between normal and tumor cells, recent strategies have emphasized targeting specific mechanisms that have gone awry in tumor cells. However, the identification of cancer-associated mutations in oncogenes and their amplification in tumors has suggested that inhibitors against such proteins might represent attractive substrates for targeted therapy. In the clinic, the success of imatinib (Gleevec®, STI571) and trastuzumab (Herceptin®), both firsts of their kind, spurred further development of new, second-generation drugs that target kinases in cancer. This review highlights a few important examples each of these types of therapies, along with some newer agents that are in various stages of development. Second-generation kinase inhibitors aimed at overriding emerging resistance to these therapies are also discussed.

Identifying responders to trastuzumab therapy in breast cancer

In breast cancer, HER2-targeted therapy with trastuzumab has gained significant attention, owing to the dramatic response observed in a subset of HER2-positive patients. The mechanisms of action are complex and not fully understood, and much effort has been spent in order to identify responders. Good patient management, side effects of the humanized monoclonal antibody and socioeconomics all demand that the drug should be administered only to the patients who will benefit from it. This has been a difficult task and contributions to solve it have been proposed from a variety of research. In this article we describe some of these contributions based on the literature and provide our viewpoint as to which identifiers will emerge in the following decade.

Immune effects of trastuzumab

Trastuzumab's targeted therapy has become a stronghold for human epidermal growth factor receptor 2 positive breast cancer patients. This humanized monoclonal antibody binds to the extracellular juxta-membrane domain of HER2 and inhibits the proliferation and survival of HER2 dependent cancer cells. The ways by which this molecule exerts its action have been partially elucidated but several new mechanisms are being constantly identified. Several new agents are being introduced that interfere with HER2. Several new immunotherapy strategies are being introduced in order to direct the immune system against cells and tissues that aberrantly overexpressed HER2. We review the strategies currently adopted and those suggested against HER2 expressing tumors.

E-cadherin expression on human carcinoma cell affects trastuzumab-mediated antibody-dependent cellular cytotoxicity through killer cell lectin-like receptor G1 on natural killer cells

Trastuzumab is a recombinant antibody drug that is widely used for the treatment of HER2-overexpressing breast carcinoma. Despite encouraging clinical results, many HER2-overexpressing carcinomas are primarily resistant to trastuzumab. We attempted to explain trastuzumab resistance and search for solutions. Since the killer cell lectin-like receptor G1 (KLRG1), an inhibitory receptor expressed on subsets of natural killer (NK) cells recognizes E-cadherin as ligands and may inhibit immune responses by regulating the effector function of NK cells, we used HER2-overexpressing carcinoma cells which were expressing E-cadherin to investigate the role of antibody-dependent cellular cytotoxicity (ADCC) through KLRG1 on NK cells in vitro and vivo. The results indicated that HER2-overexpressing carcinoma cells were killed by trastuzumab-mediated ADCC and the ADCC activity was reflected the degree of E-cadherin expression on carcinoma cells. We found that expression of E-cadherin was shown to be a predictor of response to trastuzumab-based treatment for HER2-overexpressing carcinomas, furthermore, trastuzumab-mediated ADCC was markedly enhanced by KLRG1-negative peripheral blood mononuclear cells (PBMCs(KLRG1(-))).

Trastuzumab regulates IGFBP-2 and IGFBP-3 to mediate growth inhibition: implications for the development of predictive biomarkers for trastuzumab resistance

Activation of insulin-like growth factor-I receptor (IGF-IR) signaling is an important mechanism for trastuzumab resistance. IGF-binding proteins (IGFBP) modulate IGF-IR signaling and play important roles in the control of breast cancer progression. In this article, we report that trastuzumab treatment enhances the expression and secretion of IGFBP-3 in SKBR3 cells, a trastuzumab-sensitive breast cancer cell line, and that this upregulation of IGFBP-3 induced by trastuzumab correlates with trastuzumab-mediated growth inhibition. We describe a new role for IGFBP-3 in the regulation of IGF-I-mediated cross-talk between IGF-IR and ErbB2 signaling pathways. In particular, treatment of SKBR3 cells with recombinant IGFBP-3 blocks IGF-I-induced activation of IGF-IR and ErbB2, and stable expression of IGFBP-3 inhibits SKBR3 cell growth. We find an inverse relationship in the levels of secreted IGFBP-3 such that high levels of IGFBP-3 are associated with trastuzumab-sensitive breast cancer cells (SKBR3 and BT-474), whereas low levels of IGFBP-3 are found in trastuzumab-resistant cells (clone 3 and JIMT-1). In contrast to IGFBP-3, the secretion and expression of IGFBP-2 are upregulated in trastuzumab-resistant SKBR3 cells. Furthermore, we show that IGFBP-2 stimulates activation of ErbB2 and that trastuzumab reduces IGFBP-2-stimulated ErbB2 activation. Based on our data, we propose a novel mechanism of action whereby trastuzumab enhances the expression and secretion of IGFBP-3, which interferes with IGF-I-mediated mitogenic signaling via autocrine and paracrine mechanisms and reduces IGFBP-2-induced ErbB2 activation to mediate growth inhibition. Changes in secretion profiles of IGFBP-2 and IGFBP-3 in trastuzumab-sensitive and trastuzumab-resistant cells may promote the development of IGFBP-2 and IGFBP-3 as predictive biomarkers for trastuzumab resistance.

Synergy between inhibitors of androgen receptor and MEK has therapeutic implications in estrogen receptor-negative breast cancer

INTRODUCTION

Estrogen receptor-negative (ER-) breast cancer is a heterogeneous disease with limited therapeutic options. The molecular apocrine subtype constitutes 50% of ER-tumors and is characterized by overexpression of steroid response genes including androgen receptor (AR). We have recently identified a positive feedback loop between the AR and extracellular signal-regulated kinase (ERK) signaling pathways in the molecular apocrine subtype. In this feedback loop, AR regulates ERK phosphorylation through the mediation of ErbB2 and, in turn, ERK-CREB1 signaling regulates the transcription of AR in molecular apocrine cells. In this study, we investigated the therapeutic implications of the AR-ERK feedback loop in molecular apocrine breast cancer.

METHODS

We examined a synergy between the AR inhibitor flutamide and the MEK inhibitor CI-1040 in the molecular apocrine cell lines MDA-MB-453, HCC-1954 and HCC-202 using MTT cell viability and annexin V apoptosis assays. Synergy was measured using the combination index (CI) method. Furthermore, we examined in vivo synergy between flutamide and the MEK inhibitor PD0325901 in a xenograft model of the molecular apocrine subtype. The effects of in vivo therapies on tumor growth, cell proliferation and angiogenesis were assessed.

RESULTS

We demonstrate synergistic CI values for combination therapy with flutamide and CI-1040 across three molecular apocrine cell lines at four dose combinations using both cell viability and apoptosis assays. Furthermore, we show in vivo that combination therapy with flutamide and MEK inhibitor PD0325901 has a significantly higher therapeutic efficacy in reducing tumor growth, cellular proliferation and angiogenesis than monotherapy with these agents. Moreover, our data suggested that flutamide and CI-1040 have synergy in trastuzumab resistance models of the molecular apocrine subtype. Notably, the therapeutic effect of combination therapy in trastuzumab-resistant cells was associated with the abrogation of an increased level of ERK phosphorylation that was developed in the process of trastuzumab resistance.

CONCLUSIONS

In this study, we demonstrate in vitro and in vivo synergies between AR and MEK inhibitors in molecular apocrine breast cancer. Furthermore, we show that combination therapy with these inhibitors can overcome trastuzumab resistance in molecular apocrine cells. Therefore, a combination therapy strategy with AR and MEK inhibitors may provide an attractive therapeutic option for the ER-/AR+ subtype of breast cancer.

Small molecule inhibitors of the IGF-1R/IR axis for the treatment of cancer

INTRODUCTION

The IGF-1 receptor (IGF-1R) is a receptor tyrosine kinase and is well established as a key regulator of tumor cell growth and survival. There is also a growing body of data to support a role for the structurally and functionally related insulin receptor (IR) in human cancer. Bidirectional crosstalk between IGF-1R and IR is observed, where specific inhibition of either receptor confers a compensatory increase in the activity for the reciprocal receptor, therefore dual inhibition of both IGF-1R and IR may be important for optimal efficacy. The importance of IGF-1R and IR as targets in cancer is further underscored by their contribution to resistance against both cytotoxic and molecularly targeted anti-cancer therapeutics. Currently, both IGF-1R-neutralizing antibodies and small-molecule tyrosine kinase inhibitors of IGF-1R/IR are in clinical development.

AREAS COVERED

The importance of IGF-1R and IR as cancer targets and how IGF-1R/IR inhibitors may sensitize tumor cells to the anti-proliferative and pro-apoptotic effects of other anti-tumor agents. The potential advantages of small molecule IGF-1R/IR inhibitors compared with IGF-1R-specific neutralizing antibodies, and the characteristics of small-molecule IGF-1R inhibitors that have entered clinical development.

EXPERT OPINION

Because of compensatory crosstalk between IGF-1R and IR, dual IGF-1R and IR tyrosine kinase inhibitors may have superior anti-tumor activity compared to anti-IGF-1R specific antibodies. The clinical success for IGF-1R/IR inhibitors may ultimately be dependent upon our ability to correctly administer these agents to the right niche patient subpopulation using single agent therapy, when appropriate, or using the right combination therapy.

Evasion mechanisms to Igf1r inhibition in rhabdomyosarcoma

Inhibition of the insulin-like growth factor 1 receptor (Igf1r) is an approach being taken in clinical trials to overcome the dismal outcome for metastatic alveolar rhabdomyosarcoma (ARMS), an aggressive muscle cancer of children and young adults. In our study, we address the potential mechanism(s) of Igf1r inhibitor resistance that might be anticipated for patients. Using a genetically engineered mouse model of ARMS, validated for active Igf1r signaling, we show that the prototypic Igf1r inhibitor NVP-AEW541 can inhibit cell growth and induce apoptosis in vitro in association with decreased Akt and Mapk phosphorylation. However, drug resistance in vivo is more common and is accompanied by Igf1r overexpression, Mapk reactivation, and Her2 overexpression. Her2 is found to form heterodimers with Igf1r in resistant primary tumor cell cultures, and stimulation with Igf2 leads to Her2 phosphorylation. The Her2 inhibitor lapatinib cooperates with NVP-AEW541 to reduce Igf1r phosphorylation and to inhibit cell growth even though lapatinib alone has little effect on growth. These results point to the potential therapeutic importance of simultaneous targeting of Igf1r and Her2 to abrogate resistance.

Insulin-like growth factor-1 receptor as a novel prognostic marker and its implication as a cotarget in the treatment of human adenocarcinoma of the esophagus

Insulin-like growth factor-1 receptor (IGF-1R) and human epidermal growth factor receptor-2 (HER2) receptor expression has been found to be a key regulator of tumorigenesis. The purpose of our study was to establish the prognostic significance of IGF-1R in esophageal cancer and to determine the effect of IGF-1R and HER2 targeting with alpha-IR3 and Herceptin antibodies on the proliferation of esophageal cancer cells in vitro. IGF-1R expression and clinicopathological correlations were analyzed with a tissue microarray containing 234 esophageal cancer specimens (133 adenocarcinomas and 101 squamous cell carcinomas). Proliferation changes associated with Herceptin and alpha-IR3 blockage were evaluated with the unique human esophageal cancer cell lines Pt1590 and LN1590. IGF-1R and HER2 expression levels, activation and phosphorylation status of downstream signaling proteins involved in the activation pathways were analyzed by Western blotting. IGF-1R overexpression was detected in 121 (52%) of the 234 esophageal tumors examined. In the subgroup of 87 HER2-positive tumors, 93.1% showed concordant overexpression for IGF-1R. IGF-1R was identified as a variable associated with reduced overall survival for adenocarcinoma (p = 0.05), but not for squamous cell carcinoma. The combination of Herceptin and alpha-IR3 was more effective in inhibiting in vitro proliferation than treatment with either agent alone (p < 0.01). This was associated with a decrease in HER2 and IGF-1R protein levels and suppression of Akt- and MAP kinase phosphorylation. IGF-1R expression can be used as a novel prognostic marker for adenocarcinomas of the esophagus. Cotreatment with IGF-1R and HER2 antibodies might become a valuable and effective treatment option in esophageal adenocarcinoma.

Mechanisms of resistance to anti-human epidermal growth factor receptor 2 agents in breast cancer

Approximately 20% of breast cancers are characterized by overexpression of human epidermal growth factor receptor 2 (HER2) protein and associated gene amplification, and the receptor tyrosine kinase is believed to play a critical role in the pathogenesis of these tumors. The development and implementation of trastuzumab, a humanized monoclonal antibody against the extracellular domain of HER2 protein, has significantly improved treatment outcomes in patients with HER2-overexpressing breast cancer. However, despite this clinical usefulness, unmet needs for better prediction of trastuzumab's response and overcoming primary and acquired resistance remain. In this review, we discuss several potential mechanisms of resistance to trastuzumab that have been closely studied over the last decade. Briefly, these mechanisms include: impaired access of trastuzumab to HER2 by expression of extracellular domain-truncated HER2 (p95 HER2) or overexpression of MUC4; alternative signaling from insulin-like growth factor-1 receptor, other epidermal growth factor receptor family members, or MET; aberrant downstream signaling caused by loss of phosphatase and tensin homologs deleted from chromosome 10 (PTEN), PIK3CA mutation, or downregulation of p27; or FCGR3A polymorphisms. In addition, we discuss potential strategies for overcoming resistance to trastuzumab. Specifically, the epidermal growth factor receptor/HER2 tyrosine kinase inhibitor lapatinib partially overcame trastuzumab resistance in a clinical setting, so its efficacy results and limited data regarding potential mechanisms of resistance to the drug are also discussed.

Use of molecular markers for predicting therapy response in cancer patients

Predictive markers are factors that are associated with upfront response or resistance to a particular therapy. Predictive markers are important in oncology as tumors of the same tissue of origin vary widely in their response to most available systemic therapies. Currently recommended oncological predictive markers include both estrogen and progesterone receptors for identifying patients with breast cancers likely to benefit from hormone therapy, HER-2 for the identification of breast cancer patients likely to benefit from trastuzumab, specific K-RAS mutations for the identification of patients with advanced colorectal cancer unlikely to benefit from either cetuximab or panitumumab and specific EGFR mutations for selecting patients with advanced non-small-cell lung cancer for treatment with tyrosine kinase inhibitors such as gefitinib and erlotinib. The availability of predictive markers should increase drug efficacy and decrease toxicity, thus leading to a more personalized approach to cancer treatment.

Current standards in the treatment of metastatic breast cancer with focus on Lapatinib: a review by a Central European Consensus Panel

In breast cancer, early detection as well as new developments in therapeutic options has resulted in less patients presenting with metastatic disease. However, about one-third of women with early stage breast cancer will eventually develop metastatic disease. Furthermore, approximately 20-30% of patients with breast cancer have tumors that overexpress human epidermal growth factor receptor (HER-2), which is associated with an aggressive tumor phenotype and poor prognosis. The identification of the HER-2 protein led to the development of highly effective therapeutics directed at this receptor. Trastuzumab, a recombinant, humanized, monoclonal antibody that binds to the extracellular domain of the HER-2 protein, has shown significant clinical benefit in metastatic and early-stage HER-2-positive breast cancer. Since the cancer recurs after adjuvant therapy in some women, and metastatic breast cancer eventually develops resistance to trastuzumab, there is a need for alternative treatment modalities to block HER-2 signaling. One of these treatment options is lapatinib, an orally active small molecule that inhibits the tyrosine kinases of HER-2 and the epidermal growth factor receptor type 1 (EGFR). In this consensus statement current treatment options in metastatic and locally advanced disease are discussed with a special focus on lapatinib.

Targeting the insulin-like growth factor receptor-1R pathway for cancer therapy

Signaling through the insulin-like growth factor receptor (IGF-1R) is required for neoplastic transformation by a number of oncogenes, and preclinical validation studies have suggested IGF-1R is an attractive target for anticancer therapy. A number of small molecules and antibodies targeting IGF-1R have entered clinical development, and early results have suggested that these agents have generally acceptable safety profiles as single agents. Some evidence of antitumor activity has also been reported. This review highlights key aspects of the IGF-1R signaling pathway that implicate it as an attractive therapeutic target in the management of cancer, as well as some key lessons that have emerged from early clinical development of anti-IGF-1R targeting agents. In addition, we consider the importance of selecting indications characterized by pathological alterations in the signaling pathway, rational selection of combinations based on signaling pathway interactions, and strategies for patient selection based on analysis of predictive biomarkers.

Molecular mechanisms of acquired resistance to tyrosine kinase targeted therapy

In recent years, tyrosine kinases (TKs) have been recognized as central players and regulators of cancer cell proliferation, apoptosis, and angiogenesis, and are therefore considered suitable potential targets for anti-cancer therapies. Several strategies for targeting TKs have been developed, the most successful being monoclonal antibodies and small molecule tyrosine kinase inhibitors. However, increasing evidence of acquired resistance to these drugs has been documented, and extensive preclinical studies are ongoing to try to understand the molecular mechanisms by which cancer cells are able to bypass their inhibitory activity.This review intends to present the most recently identified molecular mechanisms that mediate acquired resistance to tyrosine kinase inhibitors, identified through the use of in vitro models or the analysis of patient samples. The knowledge obtained from these studies will help to design better therapies that prevent and overcome resistance to treatment in cancer patients.

IGF activation in a molecular subclass of hepatocellular carcinoma and pre-clinical efficacy of IGF-1R blockage

BACKGROUND & AIMS

IGF signaling has a relevant role in a variety of human malignancies. We analyzed the underlying molecular mechanisms of IGF signaling activation in early hepatocellular carcinoma (HCC; BCLC class 0 or A) and assessed novel targeted therapies blocking this pathway.

METHODS

An integrative molecular dissection of the axis was conducted in a cohort of 104 HCCs analyzing gene and miRNA expression, structural aberrations, and protein activation. The therapeutic potential of a selective IGF-1R inhibitor, the monoclonal antibody A12, was assessed in vitro and in a xenograft model of HCC.

RESULTS

Activation of the IGF axis was observed in 21% of early HCCs. Several molecular aberrations were identified, such as overexpression of IGF2 -resulting from reactivation of fetal promoters P3 and P4-, IGFBP3 downregulation and allelic losses of IGF2R (25% of cases). A gene signature defining IGF-1R activation was developed. Overall, activation of IGF signaling in HCC was significantly associated with mTOR signaling (p=0.035) and was clearly enriched in the Proliferation subclass of the molecular classification of HCC (p=0.001). We also found an inverse correlation between IGF activation and miR-100/miR-216 levels (FDR<0.05). In vitro studies showed that A12-induced abrogation of IGF-1R activation and downstream signaling significantly decreased cell viability and proliferation. In vivo, A12 delayed tumor growth and prolonged survival, reducing proliferation rates and inducing apoptosis.

CONCLUSIONS

Integrative genomic analysis showed enrichment of activation of IGF signaling in the Proliferation subclass of HCC. Effective blockage of IGF signaling with A12 provides the rationale for testing this therapy in clinical trials.

It's all in for the HER family in tumorigenesis

The EGF receptor family is a group of receptor tyrosine kinases that have been implicated in the development of a variety of malignancies. As such, they have been targeted in the generation of pharmacologic agents, several of which have been approved as anti-tumor therapeutics. The lone exception is ERBB4, for which the function and relationship to cancer are not yet clear and no targeted therapies exist. The paper under evaluation demonstrates a role for ERBB4 mutations in the development of melanoma. It identifies ERBB4 mutations present in melanomas that augment proliferation and cell survival and thus contribute to dysregulated growth. Furthermore, it shows that agents targeting the EGF receptor family can reduce the proliferation of melanoma cells harboring these mutations. These findings further emphasize the role of the ERBB subfamily in tumorigenesis and establish ERBB4 as a new target in the development of anti-tumor strategies.

Heterotrimerization of the growth factor receptors erbB2, erbB3, and insulin-like growth factor-i receptor in breast cancer cells resistant to herceptin

Primary and acquired resistance to the breast cancer drug trastuzumab (Herceptin) is a significant clinical problem. Here, we report enhanced activation of downstream signaling pathways emanating from the growth factor receptors erbB2, erbB3, and insulin-like growth factor-I receptor (IGF-IR) in trastuzumab-resistant breast cancer cells. Interactions between IGF-IR and erbB2 or erbB3 occur exclusively in trastuzumab-resistant cells, where enhanced erbB2-erbB3 interactions are also observed. Moreover, these three receptors form a heterotrimeric complex in resistant cells. erbB3 or IGF-IR knockdown by short hairpin RNA-mediated strategies upregulates p27(kip1), inactivates downstream receptor signaling, and resensitizes resistant cells to trastuzumab. Our findings reveal a heterotrimer complex with a key role in trastuzumab resistance. On the basis of our results, we propose that trastuzumab resistance in breast cancer might be overcome by therapeutic strategies that jointly target erbB3, erbB2, and IGF-IR.

Lapatinib in metastatic breast cancer

Lapatinib is an oral, small-molecule, dual kinase inhibitor that targets both HER2 and the EGF receptor. Lapatinib was approved in June 2008 in Europe for the treatment of advanced HER2-positive breast cancer. Promising results in trastuzumab-refractory metastatic breast cancer were obtained from Phase I, II and III studies in combination with chemotherapy. Diarrhea and rash are the most common side-effects and are mostly moderate and treatable. Cardiac toxicity occurs rarely and mostly as an asymptomatic and reversible decrease of left ventricular ejection fraction. Unlike trastuzumab, some data show that lapatinib could cross the blood-brain barrier, with some evidence of activity in treating or preventing brain metastases. Its evaluation is actively ongoing, in combination with trastuzumab and in the adjuvant setting.

Mechanisms of resistance to HER family targeting antibodies

The epidermal growth factor (EGF) family of receptor tyrosine kinases consists of four members: EGFR (HER1/ErbB1), HER2/neu (ErbB2), HER3 (ErbB3) and HER4 (ErbB4). Receptor activation via ligand binding leads to downstream signaling that influence cell proliferation, angiogenesis, invasion and metastasis. Aberrant expression or activity of EGFR and HER2 have been strongly linked to the etiology of several human epithelial cancers including but not limited to head and neck squamous cell carcinoma (HNSCC), non-small cell lung cancer (NSCLC), colorectal cancer (CRC), and breast cancer. With this, intense efforts have been made to inhibit the activity of the EGFR and HER2 by designing antibodies against the ligand binding domains (cetuximab, panitumumab and trastuzumab) or small molecules against the tyrosine kinase domains (erlotinib, gefitinib, and lapatinib). Both approaches have shown considerable clinical promise. However, increasing evidence suggests that the majority of patients do not respond to these therapies, and those who show initial response ultimately become refractory to treatment. While mechanisms of resistance to tyrosine kinase inhibitors have been extensively studied, resistance to monoclonal antibodies is less well understood, both in the laboratory and in the clinical setting. In this review, we discuss resistance to antibody-based therapies against the EGFR and HER2, similarities between these resistance profiles, and strategies to overcome resistance to HER family targeting monoclonal antibody therapy.

Resistance to Trastuzumab in Breast Cancer

HER2 is a transmembrane oncoprotein encoded by the HER2/neu gene and is overexpressed in approximately 20 to 25% of invasive breast cancers. It can be therapeutically targeted by trastuzumab, a humanized IgG1 kappa light chain monoclonal antibody. Although trastuzumab is currently considered one of the most effective treatments in oncology, a significant number of patients with HER2-overexpressing breast cancer do not benefit from it. Understanding the mechanisms of action and resistance to trastuzumab is therefore crucial for the development of new therapeutic strategies. This review discusses proposed trastuzumab mode of action as well as proposed mechanisms for resistance. Mechanisms for resistance are grouped into four main categories: (1) obstacles preventing trastuzumab binding to HER2; (2) upregulation of HER2 downstream signaling pathways; (3) signaling through alternate pathways; and (4) failure to trigger an immune-mediated mechanism to destroy tumor cells. These potential mechanisms through which trastuzumab resistance may arise have been used as a guide to develop drugs, presently in clinical trials, to overcome resistance. The mechanisms conferring trastuzumab resistance, when completely understood, will provide insight on how best to treat HER2-overexpressing breast cancer. The understanding of each mechanism of resistance is therefore critical for the educated development of strategies to overcome it, as well as for the development of tools that would allow definitive and efficient patient selection for each therapy. (Clin Cancer Res 2009;15(24):7479-91).

Beyond trastuzumab: small molecule tyrosine kinase inhibitors in HER-2-positive breast cancer

HER-2 is a transmembrane, tyrosine kinase (TK) receptor whose overexpression is associated with adverse prognosis in breast cancer. The biological effects of HER-2 are mediated by kinase activity causing phosphorylation of tyrosine residues in the cytoplasmic domain of the receptor molecule, leading to activation of downstream growth-promoting pathways. Antibody-mediated inhibition by trastuzumab as well as TK inhibition are clinically effective anti-HER-2 strategies. Kinase inhibitors offer some potential therapeutic advantages over antibody-based therapies. Being small molecules, TK inhibitors (TKIs) have oral bioavailability and ability to cross the blood-brain barrier. Because of their different mode of action, TKIs may be able to overcome some of the mechanisms of trastuzumab resistance. Preclinical, and limited clinical data also suggest that TKIs and trastuzumab have synergistic activity. Lapatinib is the only TKI available for clinical use at present, but several molecules with anti-HER-2 activity have been identified and are undergoing evaluation. These differ in the spectrum of kinases that they inhibit, potency of HER-2 inhibition, pharmacokinetic properties, and toxicity profiles, and are at various stages of clinical development. In this article we summarize selected HER-2 TKIs approved for clinical use or in development for which clinical data are available.

Understanding the mechanisms behind trastuzumab therapy for human epidermal growth factor receptor 2-positive breast cancer

PURPOSE

Targeted therapy with the humanized monoclonal antibody trastuzumab has become a mainstay for human epidermal growth factor receptor 2 (HER2) -positive breast cancer (BC). The mechanisms of action of trastuzumab have not been fully elucidated, and data available to date are reviewed here. The impact of the mechanisms of action on clinical benefit also is discussed.

METHODS

An extensive literature review of trastuzumab and proposed mechanisms of action was performed.

RESULTS

At least five potential extracellular and intracellular antitumor mechanisms of trastuzumab have been identified in the preclinical setting. These include activation of antibody-dependent cellular cytotoxicity, inhibition of extracellular domain cleavage, abrogation of intracellular signaling, reduction of angiogenesis, and decreased DNA repair. These effects lead to tumor cell stasis and/or death. Clinical benefit from trastuzumab-based therapy in both early and advanced BC has been demonstrated. The benefit of trastuzumab use beyond progression has also been shown, which indicates the need for continuous suppression of the HER2 pathway. Targeting both HER2, with various approaches, and other pathways may enhance the clinical benefit observed with trastuzumab and overcome potential resistance. Novel combinations include pertuzumab (a HER2 dimerization inhibitor), lapatinib (a HER1/HER2 tyrosine kinase inhibitor), bevacizumab (an antiangiogenic agent), tanespimycin (a heat shock protein inhibitor), antiestrogen therapies, and an antibody-drug conjugate (trastuzumab-DM1).

CONCLUSION

Trastuzumab is the foundation of care for patients with HER2-positive BC. Emerging data from studies of other targeted agents may provide alternative treatment combinations to maximize the clinical benefit from trastuzumab and prevent or delay resistance. The continued development of trastuzumab highlights promising treatment approaches for the future.

Overcoming drug resistance in patients with metastatic breast cancer

Metastatic breast cancer is generally considered to be incurable, with response rates and duration of response progressively declining with subsequent lines of treatment. Tumors are either intrinsically resistant to systemic therapy or acquire resistance at some point during multiple courses of therapy. Mechanisms of drug resistance are numerous and include accelerated drug efflux, drug activation and inactivation, alterations in drug target, processing of drug-induced damage, and evasion of apoptosis. Targeted anticancer agents for the treatment of breast cancer, such as hormonal agents or the more recently approved epidermal growth factor receptor inhibitors, are also associated with intrinsic and acquired resistance. A variety of strategies have been devised to prevent or overcome resistance to systemic anticancer therapy, including drug combinations and sequential regimens. However, it appears that resistance to established cytotoxic and targeted agents is inevitable. Novel agents with reduced susceptibility to resistance may prevent or delay the emergence of resistance and improve survival in patients with common solid tumors, including metastatic breast cancer. We are hopeful that further elucidation of the cellular and molecular processes that allow tumor cells to develop resistance and the use of new agents to combat these mechanisms will improve outcomes for patients with metastatic breast cancer.

Reversal of oncogene transformation and suppression of tumor growth by the novel IGF1R kinase inhibitor A-928605

Background

The insulin-like growth factor (IGF) axis is an important signaling pathway in the growth and survival of many cell and tissue types. This pathway has also been implicated in many aspects of cancer progression from tumorigenesis to metastasis. The multiple roles of IGF signaling in cancer suggest that inhibition of the pathway might yield clinically effective therapeutics.

Methods

We describe A-928605, a novel pyrazolo [3,4-d]pyrimidine small molecule inhibitor of the receptor tyrosine kinases (IGF1R and IR) responsible for IGF signal transduction. This compound was first tested for its activity and selectivity via conventional in vitro kinome profiling and cellular IGF1R autophosphorylation. Additionally, cellular selectivity and efficacy of A-928605 were analyzed in an IGF1R oncogene-addicted cell line by proliferation, signaling and microarray studies. Finally, in vivo efficacy of A-928605 was assessed in the oncogene-addicted cell line and in a neuroblastoma model as a single agent as well as in combination with clinically approved therapeutics targeting EGFR in models of pancreatic and non-small cell lung cancers.

Results

A-928605 is a selective IGF1R inhibitor that is able to abrogate activation of the pathway both in vitro and in vivo. This novel compound dosed as a single agent is able to produce significant growth inhibition of neuroblastoma xenografts in vivo. A-928605 is also able to provide additive effects when used in combination with clinically approved agents directed against EGFR in non-small cell lung and human pancreatic tumor models.

Conclusion

These results suggest that a selective IGF1R inhibitor such as A-928605 may provide a useful clinical therapeutic for IGF pathway affected tumors and warrants further investigation.

Autophagy facilitates the development of breast cancer resistance to the anti-HER2 monoclonal antibody trastuzumab

Autophagy has been emerging as a novel cytoprotective mechanism to increase tumor cell survival under conditions of metabolic stress and hypoxia as well as to escape chemotherapy-induced cell death. To elucidate whether autophagy might also protect cancer cells from the growth inhibitory effects of targeted therapies, we evaluated the autophagic status of preclinical breast cancer models exhibiting auto-acquired resistance to the anti-HER2 monoclonal antibody trastuzumab (Tzb). We first examined the basal autophagic levels in Tzb-naive SKBR3 cells and in two pools of Tzb-conditioned SKBR3 cells (TzbR), which optimally grow in the presence of Tzb doses as high as 200 µg/ml Tzb. Fluorescence microscopic analyses revealed that the number of punctate LC3 structures -a hallmark of autophagy- was drastically higher in Tzb-refractory cells than in Tzb-sensitive SKBR3 parental cells. Immunoblotting analyses confirmed that the lipidation product of the autophagic conversion of LC3 was accumulated to high levels in TzbR cells. High levels of the LC3 lipidated form in Tzb-refractory cells were accompanied by decreased p62/sequestosome-1 protein expression, a phenomenon characterizing the occurrence of increased autophagic flux. Moreover, increased autophagy was actively used to survive Tzb therapy as TzbR pools were exquisitely sensitive to chemical inhibitors of autophagosomal formation/function. Knockdown of LC3 expression via siRNA similarly resulted in reduced TzbR cell proliferation and supra-additively interacted with Tzb to re-sensitize TzbR cells. Sub-groups of Tzb-naive SKBR3 parental cells accumulated LC3 punctate structures and decreased p62 expression after treatment with high-dose Tzb, likely promoting their own resistance. This is the first report showing that HER2-overexpressing breast cancer cells chronically exposed to Tzb exhibit a bona fide up-regulation of the autophagic activity that efficiently works to protect breast cancer cells from the growth-inhibitory effects of Tzb. Therapeutic targeting autophagosome formation/function might represent a novel molecular avenue to reduce the emergence of Tzb resistance in HER2-dependent breast carcinomas.

Treatment of HER2-positive metastatic breast cancer following initial progression

Significant advances in molecular-targeted therapies have provided more effective and less aggressive forms of therapy for patients with HER2-overexpressing metastatic breast cancers. Despite the initial encouraging results of many therapeutic randomized trials that have been undertaken in this setting, de novo and acquired resistance to trastuzumab, the first anti-HER2 monoclonal antibody to demonstrate significant activity in this setting, can occur. Because recent studies strongly support a role for trastuzumab in not only the management of metastatic disease but also the adjuvant setting for HER2-overexpressing breast cancers, the clinical problem of trastuzumab resistance is becoming increasingly important. Specific recommendations for the optimal treatment of HER2-overexpressing metastatic disease are challenging because considerable advances in the field have been made. This article will review some of the main points to be considered for decision-making in anti-HER2 treatment in the metastatic setting: (1) the benefit of continued trastuzumab after progression on a first-line trastuzumab-containing regimen, (2) novel agents that have been recently added to the plethora of drugs available to treat HER2-overexpressing breast cancers, and (3) molecular mechanisms that contribute to trastuzumab resistance. These issues are imperative in identifying novel therapeutic targets with the goal of increasing the magnitude and duration of response to trastuzumab-based treatment.

Sequencing of type I insulin-like growth factor receptor inhibition affects chemotherapy response in vitro and in vivo

PURPOSE

The aim of this study was to determine the optimal sequence of combining anti-type I insulin-like growth factor receptor (IGF1R) antibodies with chemotherapeutic drugs in cancer cells in vitro and in vivo.

EXPERIMENTAL DESIGN

MCF-7 and LCC6 cells were treated with subcytotoxic concentrations of doxorubicin with or without anti-IGF1R antibodies (scFv-Fc or EM164 and its humanized version AVE1642). Treatments were given simultaneously, doxorubicin followed by anti-IGF1R antibody, or anti-IGF1R antibody followed by doxorubicin, with measurement of in vitro proliferation, apoptosis, and anchorage-independent growth. The effects of sequencing on LCC6 xenograft growth and metastasis were studied.

RESULTS

Doxorubicin followed by anti-IGF1R antibody (scFv-Fc or EM164) was the most effective combination strategy to inhibit cell monolayer growth and anchorage-independent growth. This sequential combination triggered increased poly (ADP-ribose) polymerase cleavage compared with other treatment sequences. The reverse sequence, antibody followed by doxorubicin treatment, protected cells from chemotherapy by decreasing apoptosis, arresting cells in S phase, and inhibiting the level and activity of topoisomerase IIalpha. Finally, our in vivo data show that recovery of IGF1R prior to doxorubicin therapy resulted in the best therapeutic responses. Low doses of AVE1642 that allowed IGF1R expression to recover at one week were more effective in combination with doxorubicin than higher antibody doses.

CONCLUSION

The timing of IGF1R inhibition affects responses to chemotherapy. The optimal sequence was doxorubicin followed by anti-IGF1R antibody, whereas the opposite sequence inhibited doxorubicin effects. Thus, the dose and sequencing of anti-IGF1R therapies should be considered in the design of future clinical trials.

ErbB receptors and signaling pathways in cancer

The ErbB receptor tyrosine kinases play important roles in normal physiology and in cancer. Epidermal growth factor receptor (EGFR) and ErbB2 in particular are mutated in many epithelial tumors, and clinical studies suggest that they play roles in cancer development and progression. These receptors have been intensely studied, not only to understand the mechanisms underlying their oncogenic potential, but also to exploit them as therapeutic targets. ErbB receptors activate a multiplicity of intracellular pathways via their ability to interact with numerous signal transducers. Furthermore, there are now many ErbB-targeted inhibitors used in the clinic. In this review we will concentrate on breast tumors with ERBB2 gene amplification/receptor overexpression and non-small cell lung cancer (NSCLC) with activating EGFR mutations. We will discuss data showing the important role that the PI3K/Akt pathway plays, not only in cancer development, but also in response to targeted therapies. Finally, mechanisms contributing to resistance to ErbB-targeted therapeutics will also be discussed.

Heterogeneity of receptor function in colon carcinoma cells determined by cross-talk between type I insulin-like growth factor receptor and epidermal growth factor receptor

This study identifies a novel cross-talk paradigm between the type I insulin-like growth factor receptor (IGF1R) and epidermal growth factor receptor (EGFR) in colon cancer cells. IGF1R activation by ligand exposure in growth factor-deprived cells induces Akt activation in the FET, CBS, and GEO colon cancer cell lines. Investigation of IGF1R-mediated signaling pathways using small interfering RNA approaches indicated that, as expected, phosphatidylinositol 3'-kinase (PI3K) was activated by IGF1R. Mitogen-activated protein kinase (MAPK) activity as reflected by phospho-extracellular signal-regulated kinase (ERK) induction was not significantly activated until later times following release of these cells from growth factor deprivation stress. The appearance of phospho-ERK was proximal to EGFR activation. Treatment of cells with the PI3K inhibitor LY294002 before release from stress resulted in a concentration-dependent loss of EGFR activation, whereas treatment with the MAPK inhibitor PD98059 did not block EGFR activation, indicating that EGFR activation was downstream of the IGF1R/PI3K pathway. PD98059 inhibition of MAPK was associated with a concentration-dependent reduction in EGFR-mediated phospho-ERK. EGFR inhibitor blocked induction of phospho-ERK, showing that MAPK activity was a consequence of EGFR-mediated signaling. On the other hand, a small-molecule IGF1R inhibitor, PQIP, blocked Akt phosphorylation. The divergent signaling functions of IGF1R and EGFR suggested the potential for synergism by a combination of therapy directed at the two receptors. Combination treatment with PQIP and EGFR inhibitor Tarceva resulted in synergistic effects as indicated by combination index analysis in all three cell lines tested.

HER2-targeted therapy in breast cancer. Monoclonal antibodies and tyrosine kinase inhibitors

There is strong clinical evidence that trastuzumab, a monoclonal antibody targeting the human epidermal growth factor receptor (HER) two tyrosine kinase receptor, is an important component of first-line treatment of patients with HER2-positive metastatic breast cancer. In particular the combination with taxanes and vinorelbine has been established. In the preoperative setting inclusion of trastuzumab has significantly increased the pathological complete response rate. Results from large phase III trials evaluating adjuvant therapy in HER2-positive early breast cancer indicate that the addition of trastuzumab to chemotherapy improves disease-free and overall survival. The use of lapatinib, a dual tyrosine kinase inhibitor of both HER1 and HER2, in combination with capecitabine in the second-line treatment of HER2-positive patients with metastatic breast cancer previously treated with trastuzumab has been established. There is modest, but still insufficient, support that the compound passes the blood-brain barrier. Several trials are ongoing both in the adjuvant and metastatic settings and we have to await the results of these to clarify the role of trastuzumab and lapatinib. The clinical problem of tumours developing resistance to HER2-directed therapy is becoming increasingly important. Several issues about optimal selection of patients, prevention of resistance and use of different treatment options are still unresolved. In this article, we summarise the current knowledge on clinical evidence of HER2-directed therapy and the potential mechanisms of underlying resistance, including the possible clinical implications and review new therapeutic options.

HER receptor signaling confers resistance to the insulin-like growth factor-I receptor inhibitor, BMS-536924

We have reported previously the activity of the insulin-like growth factor-I (IGF-IR)/insulin receptor (InsR) inhibitor, BMS-554417, in breast and ovarian cancer cell lines. Further studies indicated treatment of OV202 ovarian cancer cells with BMS-554417 increased phosphorylation of HER-2. In addition, treatment with the pan-HER inhibitor, BMS-599626, resulted in increased phosphorylation of IGF-IR, suggesting a reciprocal cross-talk mechanism. In a panel of five ovarian cancer cell lines, simultaneous treatment with the IGF-IR/InsR inhibitor, BMS-536924 and BMS-599626, resulted in a synergistic antiproliferative effect. Furthermore, combination therapy decreased AKT and extracellular signal-regulated kinase activation and increased biochemical and nuclear morphologic changes consistent with apoptosis compared with either agent alone. In response to treatment with BMS-536924, increased expression and activation of various members of the HER family of receptors were seen in all five ovarian cancer cell lines, suggesting that inhibition of IGF-IR/InsR results in adaptive up-regulation of the HER pathway. Using MCF-7 breast cancer cell variants that overexpressed HER-1 or HER-2, we then tested the hypothesis that HER receptor expression is sufficient to confer resistance to IGF-IR-targeted therapy. In the presence of activating ligands epidermal growth factor or heregulin, respectively, MCF-7 cells expressing HER-1 or HER-2 were resistant to BMS-536924 as determined in a proliferation and clonogenic assay. These data suggested that simultaneous treatment with inhibitors of the IGF-I and HER family of receptors may be an effective strategy for clinical investigations of IGF-IR inhibitors in breast and ovarian cancer and that targeting HER-1 and HER-2 may overcome clinical resistance to IGF-IR inhibitors.

Trastuzumab-resistant breast cancer cells remain sensitive to the auger electron-emitting radiotherapeutic agent 111In-NLS-trastuzumab and are radiosensitized by methotrexate

Our goals in this study were to determine whether (111)In-trastuzumab coupled to peptides harboring nuclear localizing sequences (NLSs) could kill trastuzumab-resistant breast cancer cell lines through the emission of Auger electrons and whether the combination of radiosensitization with methotrexate (MTX) would augment the cytotoxicity of this radiopharmaceutical.

METHODS

Trastuzumab was derivatized with sulfosuccinimidyl-4-(N-maleimidomethyl)cyclohexane-1-carboxylate for reaction with NLS peptides and then conjugated with diethylenetriaminepentaacetic acid for labeling with (111)In. HER2 expression was determined by Western blot and by radioligand binding assay using (111)In-trastuzumab in a panel of breast cancer cell lines, including SK-BR-3, MDA-MB-231 and its HER2-transfected subclone (231-H2N), and 2 trastuzumab-resistant variants (TrR1 and TrR2). Nuclear importation of (111)In-NLS-trastuzumab and (111)In-trastuzumab in breast cancer cells was measured by subcellular fractionation, and the clonogenic survival of these cells was determined after incubation with (111)In-NLS-trastuzumab, (111)In-trastuzumab, or trastuzumab (combined with or without MTX). Survival curves were analyzed according to the dose-response model, and the radiation-enhancement ratio was calculated from the survival curve parameters.

RESULTS

The expression of HER2 was highest in SK-BR-3 cells (12.6 x 10(5) receptors/cell), compared with 231-H2N and TrR1 cells (6.1 x 10(5) and 5.1 x 10(5) receptors/cell, respectively), and lowest in MDA-MB-231 and TrR2 cells (0.4 x 10(5) and 0.6 x 10(5) receptors/cell, respectively). NLS peptides increased the nuclear uptake of (111)In-trastuzumab in MDA-MB-231, 231-H2N, TrR1, and TrR2 cells from 0.1%+/-0.01%, 2.5%+/-0.2%, 2.8%+/-0.7%, and 0.5%+/-0.1% to 0.5%+/-0.1%, 4.6%+/-0.1%, 5.2%+/-0.6%, and 1.5%+/-0.2%, respectively. The cytotoxicity of (111)In-NLS-trastuzumab on breast cancer cells was directly correlated with the HER2 expression densities of the cells. On a molar concentration basis, the effective concentration required to kill 50% of 231-H2N and TrR1 cells for (111)In-NLS-trastuzumab was 9- to 12-fold lower than for (111)In-trastuzumab and 16- to 77-fold lower than for trastuzumab. MDA-MB-231 and TrR2 cells were less sensitive to (111)In-NLS-trastuzumab or (111)In-trastuzumab, and both cell lines were completely insensitive to trastuzumab. The radiation-enhancement ratio induced by MTX for 231-H2N and TrR1 cells after exposure to (111)In-NLS-trastuzumab was 1.42 and 1.68, respectively.

CONCLUSION

Targeted Auger electron radioimmunotherapy with (111)In-NLS-trastuzumab can overcome resistance to trastuzumab, and MTX can potently enhance the sensitivity of HER2-overexpressing breast cancer cells to the lethal Auger electrons emitted by this radiopharmaceutical.

Expression and clinical significance of epidermal growth factor receptor and type 1 insulin-like growth factor receptor in nasopharyngeal carcinoma

OBJECTIVES

Epidermal growth factor (EGF) and type 1 insulin-like growth factor (IGF-1) receptors play an important role in the growth and apoptosis of nasopharyngeal carcinoma (NPC). They were separately found to be associated with prognosis in patients with NPC. To date, their expression correlation and clinicopathologic significance have never been specifically addressed in NPC.

METHODS

Seventy-five patients with NPC and 21 noncancerous nasopharyngeal epithelial samples were accrued between 1998 and 2006 in a single hospital. The expressions of EGF and IGF-1 receptors were detected by immunohistochemical staining in the 75 NPC samples and the 21 noncancerous samples. Furthermore, the messenger RNA and protein expressions were assessed by real-time quantitative polymerase chain reaction and the Western blot technique, respectively, in NPC cell lines and normal nasopharyngeal epithelial cells.

RESULTS

The 5-year survival rates, assessed by the Kaplan-Meier method, were 71.4% and 66.6% in the EGF and IGF-1 receptor protein-negative groups, respectively, whereas they were only 28.6% and 33.3% in the receptor protein-positive groups. The levels of these two proteins significantly correlated with each other, and the overexpression rates of EGF and IGF-1 receptors were 65.3% and 56% in nasopharyngeal samples, respectively. Furthermore, both protein expressions were significantly higher in NPC patients with cervical lymph node or distant metastasis than in NPC patients without lymph node or distant metastasis. Recurrence more often appears in cases positive for both proteins than in cases negative for both proteins. The expression levels of the receptor messenger RNA and proteins were higher in several NPC cell lines than in normal nasopharyngeal epithelial cells.

CONCLUSIONS

These findings demonstrate that both receptor proteins may play an important role in the invasion, metastasis, and recurrence of NPC. Both receptors are valuable markers for assessing the prognosis of NPC. Their expression at such high frequencies provides the basis of combined targeted therapy with specific pharmacologic inhibitors to enhance the effects of radiotherapy and chemotherapy.

ERBB3/HER3 and ERBB2/HER2 duet in mammary development and breast cancer

ERBB3/HER3 is one of the four members of the epidermal growth factor receptor (ERBB) family. It is activated by binding to ligands Neuregulin-1 and Neuregulin-2. Since ERBB3 lacks intrinsic kinase activity, signal transduction occurs through formation of heterodimers with EGFR, ERBB2, and ERBB4. ERBB3 is a signaling specialist since it has six binding sites for the p85 SH2 adapter subunit of phosphoinositide 3' kinases. These lipid kinases coordinate regulation of metabolism, cell size, proliferation, survival, and angiogenesis. Not surprisingly, ERBB3 signaling has been linked to cancer etiology and progression. In breast cancer, the partnership of ERBB2 and ERBB3 may be crucial for the aggressive properties of cancers with ERBB2 amplification, and may contribute to pre-existing and acquired resistance to therapy. This partnership creates opportunities for improving efficacy of ERBB-targeted pharmaceuticals, by interfering with coupling of ERBB2 to ERBB3 through dimerization inhibitors, and by use of therapeutic compounds that target AKT-dependent pathways activated through ERBB3. Additional therapeutic opportunities may be identified through better understanding of how ERBBs are regulated and deployed in normal mammary gland processes. Work using mouse models has identified the main processes regulated by each of the four ERBBs, which has practical implications in understanding breast cancer etiology, and eventual development of better prognostic, predictive, and therapeutic tools.

ErbB-2 inhibition activates Notch-1 and sensitizes breast cancer cells to a gamma-secretase inhibitor

ErbB-2 overexpression in breast tumors is associated with poor survival. Expression of Notch-1 and its ligand, Jagged-1, is associated with the poorest survival, including ErbB-2-positive tumors. Trastuzumab plus chemotherapy is the standard of care for ErbB-2-positive breast cancer. A proportion of tumors are initially resistant to trastuzumab and acquired resistance to trastuzumab occurs in metastatic breast cancer and is associated with poor prognosis. Thus, we investigated whether Notch-1 contributes to trastuzumab resistance. ErbB-2-positive cells have low Notch transcriptional activity compared to non-overexpressing cells. Trastuzumab or a dual epidermal growth factor receptor (EGFR)/ErbB-2 tyrosine kinase inhibitor (TKI) increased Notch activity by 2- to 6-fold in SKBr3, BT474 and MCF-7/HER2-18 cells. The increase in activity was abrogated by a Notch inhibitor, gamma-secretase inhibitor (GSI) or Notch-1 small-interfering RNA (siRNA). Trastuzumab decreased Notch-1trade mark precursor, increased amount and nuclear accumulation of active Notch-1(IC) and increased expression of targets, Hey1 and Deltex1 mRNAs, and Hes5, Hey1, Hes1 proteins. Importantly, trastuzumab-resistant BT474 cells treated with trastuzumab for 6 months expressed twofold higher Notch-1, twofold higher Hey1, ninefold higher Deltex1 mRNAs and threefold higher Notch-1 and Hes5 proteins, compared to trastuzumab-sensitive BT474 cells. The increase in Hey1 and Deltex1 mRNAs in resistant cells was abrogated by a Notch-1 siRNA. Cell proliferation was inhibited more effectively by trastuzumab or TKI plus a GSI than either agent alone. Decreased Notch-1 by siRNA increased efficacy of trastuzumab in BT474 sensitive cells and restored sensitivity in resistant cells. Trastuzumab plus a GSI increased apoptosis in sensitive cells by 20-30%. A GSI alone was sufficient to increase apoptosis in trastuzumab-resistant BT474 cells by 20%, which increased to 30% with trastuzumab. Notch-1 siRNA alone decreased cell growth by 30% in sensitive and more than 50% in resistant BT474 cells. Furthermore, growth of both trastuzumab sensitive and resistant cells was completely inhibited by combining trastuzumab plus Notch-1 siRNA. More importantly, Notch-1 siRNA or a GSI resensitized trastuzumab-resistant BT474 cells to trastuzumab. These results demonstrate that ErbB-2 overexpression suppresses Notch-1 activity, which can be reversed by trastuzumab or TKI. These results suggest that Notch-1 might play a novel role in resistance to trastuzumab, which could be prevented or reversed by inhibiting Notch-1.

Her2 cross talk and therapeutic resistance in breast cancer

The HER2 receptor tyrosine kinase is amplified and/or overexpressed in approximately 30 percent of metastatic breast cancers. Interactions and cross signaling from the HER2 receptor to other growth factor receptors may potentially contribute to therapeutic resistance. In this review, we discuss HER2 receptor cross talk with the estrogen receptor and implications toward resistance to endocrine therapies. We also review mechanisms of resistance to the HER2-targeted antibody trastuzumab, including signaling from other members of the HER family, increased signaling through the PI3-kinase pathway, and cross talk from the insulin-like growth factor-I receptor to HER2. Finally, we will provide perspective on how HER2 receptor cross talk may provide critical information for developing novel therapeutic options for HER2-overexpressing breast cancers.

Met receptor contributes to trastuzumab resistance of Her2-overexpressing breast cancer cells

Her2 is overexpressed in 20% to 30% of breast tumors and correlates with reduced disease-free and overall patient survival. Trastuzumab, a humanized monoclonal antibody directed against Her2, represents the first Her2-targeted therapy, which decreases the risk of relapse and prolongs patient survival. Resistance to trastuzumab, both inherent and treatment-acquired, represents a significant barrier to the effective treatment of Her2 (+) breast cancer. The Met receptor tyrosine kinase is aberrantly expressed in breast cancer and predicts poor patient prognosis. In this study, we find that Met is frequently expressed in Her2-overexpressing breast cancer cells, as well as Her2 (+) breast cancer. Importantly, Met contributes to trastuzumab resistance, as inhibition of Met sensitizes cells to trastuzumab-mediated growth inhibition, whereas Met activation protects cells against trastuzumab by abrogating p27 induction. Remarkably, Her2-overexpressing breast cancer cells rapidly up-regulate Met expression after trastuzumab treatment, promoting their own resistance. Our study suggests that a subset of Her2 (+) patients may benefit from combined inhibition of Her2 and Met.

Predicting the efficacy of trastuzumab-based therapy in breast cancer: current standards and future strategies

Breast cancer is the most common female malignancy in many industrialized countries. Approximately one fourth of all women diagnosed with early breast cancer present with tumors that are characterized by erbB2 amplification. While the associated Her-2/neu receptor overexpression results in a high risk of relapse and poor prognosis, these tumors also represent a target for a selective monoclonal antibody therapy with trastuzumab (Herceptin). The combination of trastuzumab with chemotherapy has led to a considerable reduction of recurrences and to a significant reduction in breast cancer mortality both in the adjuvant and metastatic setting. Unfortunately, despite Her-2/neu overexpression, not all patients equally benefit from trastuzumab treatment, and almost all women with metastatic breast cancer eventually progress during antibody therapy. Moreover, trastuzumab is burdened with cardiotoxicity, thus increasing the risk of symptomatic congestive heart failure. In addition, the marginal costs for a 1 year therapy of trastuzumab-based therapy, which is currently considered to be the most effective treatment regimen in the adjuvant setting, may amount for up to US$ 40.000. Testing for erbB2 oncogene amplification by fluorescence in situ hybridization (FISH) and chromogenic in situ hybridization (CISH), respectively, and staining for Her-2/neu receptor overexpression by immunohistochemistry (IHC) represent the current standard for determining patient eligibility for trastuzumab-based therapy. However, while the negative predictive value of these assays for predicting the absence of benefit from trastuzumab-based therapy is sufficiently high, their positive predictive value remains insufficient, i.e. only a proportion of patients selected by these tests substantially benefit from trastuzumab-containing regimen. Accordingly, over the last years a number of biomarkers have been evaluated in their potential to predict response to trastuzumab-based therapies. These include markers auf activation of Her-2/neu (e.g., tyrosine phosphorylated Her-2/neu in tissue and cleaved Her-2/neu extracellular domain in serum) and its dimerization partners (e.g., EGFR), respectively, but also components of Her-2/neu-induced downstream signaling pathways that are crucial for the growth inhibitory effects of trastuzumab (e.g., PTEN and PI3K). Other parameters, such as topoisomerase-II alpha and c-myc co-amplifications, have also been identified as potentially useful predictors of response to trastuzumab-based chemotherapy regimen. While the benefit of these predictive biomarkers in the metastatic setting is currently explored, their usefulness in the adjuvant setting is still largely unknown. It is, however, undisputable that, within the group of Her-2/neu overexpressing tumors, further response predictors are needed in order to minimize trastuzumab-associated side effects, and to reduce the considerable societal costs that are associated with trastuzumab-based treatment regimen.

Drug resistance in cancer - searching for mechanisms, markers and therapeutic agents

Treatment resistance, whether inherent or acquired, is a major problem reducing the activity of conventional and newer, molecularly targeted, cancer drugs. A more complex picture of the causes and contributions of specific forms of resistance is now emerging through application of pharmacological, proteomic and gene expression technologies and we have entered an exciting time where new molecular research tools are being applied not only to characterise the causes of such resistance, but to identify rational new treatments and treatment combinations that are being rapidly translated to clinical evaluations with increasing success. This review outlines many of the contributing causes of resistance to established cytotoxics and to the new breed of molecularly targeted agents, both monoclonal antibodies and small molecules, and the research methods being used to wage war on resistant cancer.