MetMAb, the one-armed 5D5 anti-c-Met antibody, inhibits orthotopic pancreatic tumor growth and improves survival

RON (MST1R) is a novel prognostic marker and therapeutic target for gastroesophageal adenocarcinoma

RON (MST1R) is one of two members of the MET receptor tyrosine kinase family, along with parent receptor MET. RON has a putative role in several cancers, but its expression and function is poorly characterized in gastroesophageal adenocarcinoma. A recognized functional role of MET tyrosine kinase in gastroesophageal cancer has led to early phase clinical trials using MET inhibitors, with unimpressive results. Therefore, the role of RON in gastroesophageal cancer, as well as its role in cooperative signaling with MET and as a mechanism of resistance to MET inhibition, was studied in gastroesophageal tissues and cell lines. By IHC, RON was highly over-expressed in 74% of gastroesophageal samples (n=94), and over-expression was prognostic of poor survival (p=0.008); RON and MET co-expression occurred in 43% of samples and was prognostic of worst survival (p=0.03). High MST1R gene copy number by quantitative polymerase chain reaction, and confirmed by fluorescence in situ hybridization and/or array comparative genomic hybridization, was seen in 35.5% (16/45) of cases. High MST1R gene copy number correlated with poor survival (p=0.01), and was associated with high MET and ERBB2 gene copy number. A novel somatic MST1R juxtamembrane mutation R1018G was found in 11% of samples. RON signaling was functional in cell lines, activating downstream effector STAT3, and resulted in increased viability over controls. RON and MET co-stimulation assays led to enhanced malignant phenotypes over stimulation of either receptor alone. Growth inhibition as evidenced by viability and apoptosis assays was optimal using novel blocking monoclonal antibodies to both RON and MET, versus either alone. SU11274, a classic MET small molecule tyrosine kinase inhibitor, blocked signaling of both receptors, and proved synergistic when combined with STAT3 inhibition (combination index < 1). These preclinical studies define RON as an important novel prognostic marker and therapeutic target for gastroesophageal cancer warranting further investigation.

Therapeutic potential and challenges of targeting receptor tyrosine kinase ROR1 with monoclonal antibodies in B-cell malignancies

BACKGROUND

Based on its selective cell surface expression in chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL), receptor tyrosine kinase ROR1 has recently emerged as a promising target for therapeutic monoclonal antibodies (mAbs). To further assess the suitability of ROR1 for targeted therapy of CLL and MCL, a panel of mAbs was generated and its therapeutic utility was investigated.

METHODOLOGY AND PRINCIPAL FINDINGS

A chimeric rabbit/human Fab library was generated from immunized rabbits and selected by phage display. Chimeric rabbit/human Fab and IgG1 were investigated for their capability to bind to human and mouse ROR1, to mediate antibody-dependent cellular cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), and internalization, and to agonize or antagonize apoptosis using primary CLL cells from untreated patients as well as MCL cell lines. A panel of mAbs demonstrated high affinity and specificity for a diverse set of epitopes that involve all three extracellular domains of ROR1, are accessible on the cell surface, and mediate internalization. The mAb with the highest affinity and slowest rate of internalization was found to be the only mAb that mediated significant, albeit weak, ADCC. None of the mAbs mediated CDC. Alone, they did not enhance or inhibit apoptosis.

CONCLUSIONS AND SIGNIFICANCE

Owing to its relatively low cell surface density, ROR1 may be a preferred target for armed rather than naked mAbs. Provided is a panel of fully sequenced and thoroughly characterized anti-ROR1 mAbs suitable for conversion to antibody-drug conjugates, immunotoxins, chimeric antigen receptors, and other armed mAb entities for preclinical and clinical studies.

Receptor-tyrosine-kinase-targeted therapies for head and neck cancer

Molecular therapeutics for treating epidermal growth factor receptor-(EGFR-) expressing cancers are a specific method for treating cancers compared to general cell loss with standard cytotoxic therapeutics. However, the finding that resistance to such therapy is common in clinical trials now dampens the initial enthusiasm over this targeted treatment. Yet an improved molecular understanding of other receptor tyrosine kinases known to be active in cancer has revealed a rich network of cross-talk between receptor pathways with a key finding of common downstream signaling pathways. Such cross talk may represent a key mechanism for resistance to EGFR-directed therapy. Here we review the interplay between EGFR and Met and the type 1 insulin-like growth factor receptor (IGF-1R) tyrosine kinases, as well as their contribution to anti-EGFR therapeutic resistance in the context of squamous cell cancer of the head and neck, a tumor known to be primarily driven by EGFR-related oncogenic signals.

Hepatocyte growth factor inhibits anoikis of pancreatic carcinoma cells through phosphatidylinositol 3-kinase pathway

OBJECTIVES

Escape from anoikis, apoptosis induced by loss of cell-cell or cell-extracellular matrix interactions, is important in tumor invasion and metastasis. Hepatocyte growth factor (HGF) is known to play a pivotal role in pancreatic carcinomas. This study aimed to determine the antianoikis effect of HGF in pancreatic carcinoma cells.

METHODS

Antianoikis effect of HGF was evaluated in human pancreatic carcinoma cells in nonadherent culture with or without anti-E-cadherin antibody. Signal pathways were investigated by Western blot analysis and inhibition assay using inhibitors for phosphatidylinositol 3-kinase and p38.

RESULTS

Pancreatic carcinoma cells underwent anoikis in nonadherent culture. However, some of the carcinoma cells survived by forming aggregations in suspension. Anti-E-cadherin antibody dissociated the aggregations, and the separated cells underwent additional anoikis. Hepatocyte growth factor inhibited anoikis irrespective of E-cadherin-mediated cell-cell contact. Inhibition of the phosphatidylinositol 3-kinase/Akt pathway abolished the antianoikis effect of HGF. Phosphorylation of Akt was induced by HGF, and the phosphorylated Akt persisted even when E-cadherin was inhibited.

CONCLUSIONS

Hepatocyte growth factor inhibits anoikis of pancreatic carcinoma cells through phosphatidylinositol 3-kinase pathway in which activation of Akt may be involved. It is thus supposed that HGF may have a potent role in invasion and metastasis of pancreatic carcinoma cells by exerting its antianoikis effect.

Rational engineering of antibody therapeutics targeting multiple oncogene pathways

Monoclonal antibodies have significantly advanced our ability to treat cancer, yet clinical studies have shown that many patients do not adequately respond to monospecific therapy. This is in part due to the multifactorial nature of the disease, where tumors rely on multiple and often redundant pathways for proliferation. Bi- or multi- specific antibodies capable of blocking multiple growth and survival pathways at once have a potential to better meet the challenge of blocking cancer growth, and indeed many of them are advancing in clinical development. ( 1) However, bispecific antibodies present significant design challenges mostly due to the increased number of variables to consider. In this perspective we describe an innovative integrated approach to the discovery of bispecific antibodies with optimal molecular properties, such as affinity, avidity, molecular format and stability. This approach combines simulations of potential inhibitors using mechanistic models of the disease-relevant biological system to reveal optimal inhibitor characteristics with antibody engineering techniques that yield manufacturable therapeutics with robust pharmaceutical properties. We illustrate how challenges of meeting the optimal design criteria and chemistry, manufacturing and control concerns can be addressed simultaneously in the context of an accelerated therapeutic design cycle. Finally, to demonstrate how this rational approach can be applied, we present a case study where the insights from mechanistic modeling were used to guide the engineering of an IgG-like bispecific antibody.

Bi-specific aptamers mediating tumor cell lysis

Antibody-dependent cellular cytotoxicity plays a pivotal role in antibody-based tumor therapies and is based on the recruitment of natural killer cells to antibody-bound tumor cells via binding of the Fcγ receptor III (CD16). Here we describe the generation of chimeric DNA aptamers that simultaneously bind to CD16α and c-Met, a receptor that is overexpressed in many tumors. By application of the systematic evolution of ligands by exponential enrichment (SELEX) method, CD16α specific DNA aptamers were isolated that bound with high specificity and affinity (91 pm-195 nm) to their respective recombinant and cellularly expressed target proteins. Two optimized CD16α specific aptamers were coupled to each of two c-Met specific aptamers using different linkers. Bi-specific aptamers retained suitable binding properties and displayed simultaneous binding to both antigens. Moreover, they mediated cellular cytotoxicity dependent on aptamer and effector cell concentration. Displacement of a bi-specific aptamer from CD16α by competing antibody 3G8 reduced cytotoxicity and confirmed the proposed mode of action. These results represent the first gain of a tumor-effective function of two distinct oligonucleotides by linkage into a bi-specific aptamer mediating cellular cytotoxicity.

Antibodies in oncology

Monoclonal antibodies (mAbs) have become one of the largest classes of new therapeutic agents approved for use in oncology, and have revolutionised the treatment of many human malignancies. Clinically useful mAbs can function through several different mechanisms, including inhibition of tumour-related signalling, induction of apoptosis, inhibition of angiogenesis, enhancing host immune response against cancer and targeted delivery of payloads (such as toxins, cytotoxic agents or radioisotopes) to the tumour site. The increasing knowledge of key molecules and cellular pathways involved in tumour induction and progression has led to a rise in the proportion of therapeutic mAbs entering clinical trials. These mAbs consist of various conventional or recombinant, murine, humanised, chimeric or fully human and fusion constructs. In this review, we provide an overview of mAbs approved for use in clinical oncology and those currently in clinical development. We also discuss the mechanisms of action of anti-cancer mAbs, as well as the antigen targets recognised by these antibodies.

GD₃ synthase expression enhances proliferation and tumor growth of MDA-MB-231 breast cancer cells through c-Met activation

The disialoganglioside G(D3) is overexpressed in ∼50% of invasive ductal breast carcinoma, and the G(D3) synthase gene (ST8SIA1) displays higher expression among estrogen receptor-negative breast cancer tumors, associated with a decreased overall survival of breast cancer patients. However, no relationship between ganglioside expression and breast cancer development and aggressiveness has been reported. We have previously shown that overexpression of G(D3) synthase induces the accumulation of b- and c-series gangliosides (G(D3), G(D2), and G(T3)) at the cell surface of MDA-MB-231 breast cancer cells together with the acquisition of a proliferative phenotype in the absence of serum. Here, we show that phosphoinositide 3-kinase/Akt and mitogen-activated protein kinase/extracellular signal-regulated kinase pathways are constitutively activated in G(D3) synthase-expressing cells. Analysis of phosphorylation of tyrosine kinase receptors shows a specific c-Met constitutive activation in G(D3) synthase-expressing cells, in the absence of its ligand, hepatocyte growth factor/scatter factor. In addition, inhibition of c-Met or downstream signaling pathways reverses the proliferative phenotype. We also show that G(D3) synthase expression enhances tumor growth in severe combined immunodeficient mice. Finally, a higher expression of ST8SIA1 and MET in the basal subtype of human breast tumors are observed. Altogether, our results show that G(D3) synthase expression is sufficient to enhance the tumorigenicity of MDA-MB-231 breast cancer cells through a ganglioside-dependent activation of the c-Met receptor.

Potential therapeutics specific to c-MET/RON receptor tyrosine kinases for molecular targeting in cancer therapy

Products of proto-oncogenes c-MET and RON belong to a subfamily of receptor tyrosine kinases that contribute significantly to tumorigenic progression. In primary tumors, altered c-MET/RON expression transduces signals regulating invasive growth that is characterized by cell migration and matrix invasion. These pathogenic features provide the basis for targeting c-MET/RON in cancer therapy. In the last decade, various approaches have been investigated to suppress c-MET/RON-transduced oncogenesis. Among the therapeutics developed, monoclonal antibodies (mAbs) and small-molecule inhibitors (SMIs) have emerged as promising candidates. The mechanism of these therapeutic candidates is the disruption of tumor dependency on c-MET/RON signals for survival. The mAbs specific to hepatocyte growth factor (AMG102) and c-MET (MetMAb) are both humanized and able to block c-MET signaling, leading to inhibition of tumor cell proliferation in vitro and inhibition of tumor growth in xenograft models. The mAb AMG102 neutralizes hepatocyte growth factor and enhances the cytotoxicity of various chemotherapeutics to tumors in vivo. AMG102 is currently in phase II clinical trials for patients with advanced solid tumors. IMC-41A40 and Zt/f2 are RON-specific mAbs that down-regulate RON expression and inhibit ligand-induced phosphorylation. Both mAbs inhibit tumor growth in mice mediated by colon and pancreatic cancer cells. SMIs specific to c-MET (ARQ107 and PF-02341066) are in various phases of clinical trials. Therapeutic efficacy has also been observed with dual inhibitors such as Compound I, which is specific to c-MET/RON. However, a potential issue is the emergence of acquired resistance to these inhibitors. Clearly, development of c-MET/RON therapeutics provides opportunities and challenges for combating cancer in the future.

Personalized therapies in the cancer "omics" era

A molecular hallmark of cancer is the presence of genetic alterations in the tumoral DNA. Understanding how these alterations translate into the malignant phenotype is critical for the adequate treatment of oncologic diseases. Several cancer genome sequencing reports have uncovered the number and identity of proteins and pathways frequently altered in cancer. In this article we discuss how integration of these genomic data with other biological and proteomic studies may help in designing anticancer therapies "a la carte". An important conclusion is that next generation treatment of neoplasias must be based on rational drug combinations that target various pathways and cellular entities that sustain the survival of cancer cells.

Clinical implications of MET gene copy number in lung cancer

MET, the receptor for HGF, has recently been identified as a novel promising target in several human malignancies, including non-small-cell lung cancer (NSCLC). Deregulation of the HGF/MET signaling pathway can occur via different mechanisms, including HGF and/or MET overexpression, MET gene amplification, mutations or rearrangements. While the role of MET mutations in NSCLC is not yet fully understood, MET amplification emerged as a critical event in driving cell survival, with preclinical data suggesting that MET-amplified cell lines are exquisitely sensitive to MET inhibition. True MET amplification, which has been associated with poor prognosis in different retrospective series, is a relatively uncommon event in NSCLC, occurring in 1-7% of unselected cases. Nevertheless, in highly selected cohorts of patients, such as those harboring somatic mutations of the EGF receptor (EGFR) with acquired resistance to EGFR tyrosine kinase inhibitors (TKIs), MET amplification can be observed in up to 20% of cases. Preclinical data suggested that a treatment approach including a combination of EGFR and MET TKIs could be an effective strategy in this setting and led to the clinical investigation of multiple MET TKIs in combination with erlotinib. Results from ongoing and future trials will clarify the role of MET TKIs for the treatment of NSCLC and will provide insights into the most appropriate timing for their use.

An orally available small-molecule inhibitor of c-Met, PF-2341066, reduces tumor burden and metastasis in a preclinical model of ovarian cancer metastasis

Deregulated expression of the hepatocyte growth factor (HGF) receptor, c-Met, in cancer contributes to tumor progression and metastasis. The objective of this study was to determine whether blocking c-Met with an orally available c-Met inhibitor, PF-2341066, reduces tumor burden and increases survival in a xenograft model of ovarian cancer metastasis. Treatment of mice injected interperitoneally with SKOV3ip1 cells showed reduced overall tumor burden. Tumor weight and the number of metastases were reduced by 55% (P < .0005) and 62% (P < .0001), respectively. Treatment also increased median survival from 45 to 62 days (P = .0003). In vitro, PF-2341066 reduced HGF-stimulated phosphorylation of c-Met in the tyrosine kinase domain as well as phosphorylation of the downstream signaling effectors, Akt and Erk. It was apparent that inhibition of the pathways was functionally important because HGF-induced branching morphogenesis was also inhibited. In addition, proliferation and adhesion to various extracellular matrices were inhibited by treatment with PF-2341066, and the activity of matrix metalloproteinases was decreased in tumor tissue from treated mice compared with those receiving vehicle. Overall, these data indicate that PF-2341066 effectively reduces tumor burden in an in vivo model of ovarian cancer metastasis and may be a good therapeutic candidate in the treatment of patients with ovarian cancer.

Enhancing antibody Fc heterodimer formation through electrostatic steering effects: applications to bispecific molecules and monovalent IgG

Naturally occurring IgG antibodies are bivalent and monospecific. Bispecific antibodies having binding specificities for two different antigens can be produced using recombinant technologies and are projected to have broad clinical applications. However, co-expression of multiple light and heavy chains often leads to contaminants and pose purification challenges. In this work, we have modified the CH3 domain interface of the antibody Fc region with selected mutations so that the engineered Fc proteins preferentially form heterodimers. These novel mutations create altered charge polarity across the Fc dimer interface such that coexpression of electrostatically matched Fc chains support favorable attractive interactions thereby promoting desired Fc heterodimer formation, whereas unfavorable repulsive charge interactions suppress unwanted Fc homodimer formation. This new Fc heterodimer format was used to produce bispecific single chain antibody fusions and monovalent IgGs with minimal homodimer contaminants. The strategy proposed here demonstrates the feasibility of robust production of novel Fc-based heterodimeric molecules and hence broadens the scope of bispecific molecules for therapeutic applications.

Cyr61 mediates hepatocyte growth factor-dependent tumor cell growth, migration, and Akt activation

Certain tumor cell responses to the growth factor-inducible early response gene product CCN1/Cyr61 overlap with those induced by the hepatocyte growth factor (HGF)/c-Met signaling pathway. In this study, we investigate if Cyr61 is a downstream effector of HGF/c-Met pathway activation in human glioma cells. A semiquantitative immunohistochemical analysis of 112 human glioma and normal brain specimens showed that levels of tumor-associated Cyr61 protein correlate with tumor grade (P < 0.001) and with c-Met protein expression (r(2) = 0.4791, P < 0.0001). Purified HGF rapidly upregulated Cyr61 mRNA (peak at 30 minutes) and protein expression (peak at 2 hours) in HGF(-)/c-Met(+) human glioma cell lines via a transcription- and translation-dependent mechanism. Conversely, HGF/c-Met pathway inhibitors reduced Cyr61 expression in HGF(+)/c-Met(+) human glioma cell lines in vitro and in HGF(+)/c-Met(+) glioma xenografts. Targeting Cyr61 expression with small interfering RNA (siRNA) inhibited HGF-induced cell migration (P < 0.01) and cell growth (P < 0.001) in vitro. The effect of Cyr61 on HGF-induced Akt pathway activation was also examined. Cyr61 siRNA had no effect on the early phase of HGF-induced Akt phosphorylation (Ser(473)) 30 minutes after stimulation with HGF. Cyr61 siRNA inhibited a second phase of Akt phosphorylation measured 12 hours after cell stimulation with HGF and also inhibited HGF-induced phosphorylation of the Akt target glycogen synthase kinase 3alpha. We treated preestablished subcutaneous glioma xenografts with Cyr61 siRNA or control siRNA by direct intratumoral delivery. Cyr61 siRNA inhibited Cyr61 expression and glioma xenograft growth by up to 40% in a dose-dependent manner (P < 0.05). These results identify a Cyr61-dependent pathway by which c-Met activation mediates cell growth, cell migration, and long-lasting signaling events in glioma cell lines and possibly astroglial malignancies.

Response to pemetrexed chemotherapy in lung adenocarcinoma-bronchioloalveolar carcinoma insensitive to erlotinib

Targeted therapy against epidermal growth factor receptor (EGFR) in non-small-cell lung cancer has heralded an era of mutationally targeted inhibition of this receptor and its oncogenic signal transduction using the tyrosine kinase inhibitors (TKIs) gefitinib and erlotinib. EGFR TKIs have helped facilitate the concept of "personalized" cancer therapy into a reality. A majority of unselected patients remain as nonresponders with primary resistance to EGFR TKIs. Initial responders to EGFR TKIs all invariably relapse later with resistant disease. The optimal alternative therapeutic approach after a failed therapeutic trial of treatment with EGFR TKI remains to be better defined. Herein, we report a case of a patient with recurrent metastatic lung adenocarcinoma-bronchioloalveolar carcinoma that showed primary insensitivity to erlotinib therapy who later demonstrated substantial durable response to single-agent pemetrexed. We also present discussion on the evolving paradigm of the use of erlotinib in lung cancer and the current status of determinants of sensitivity in pemetrexed chemotherapy.

Expression of hepatocyte growth factor and its receptor c-Met in human pituitary adenomas

Hepatocyte growth factor (HGF) and its receptor c-Met have been known as key determinants of growth and angiogenesis in some brain tumors like gliomas, meningiomas, and schwannomas. But little is known about their expression in pituitary adenomas. In this study, the expression of HGF and c-Met in pituitary adenomas of different histology types was investigated by immunohistochemistry, and correlative analysis of their expression with microvessel density (MVD), Ki-67 expression, and other clinicopathologic factors was made. The results showed that the expression of HGF and c-Met exists in 98% (64 of 65) and 92% (60 of 65) pituitary adenomas, respectively, and co-expression of them existed in 91% (59 of 65) adenomas. HGF had significant correlation with MVD (Spearman's correlation coefficient, r = .31, P = .01) and Ki-67 (r = .32, P = .01). c-Met had significant correlation with MVD (r = .30, P = .02) and Ki-67 (r = .38, P = .00). HGF and c-Met expression had no significant correlation with age or extrasellar extension. There were no significant differences in HGF and c-Met expression between pituitary adenomas of different histology types. The results indicate that HGF and c-Met are widely expressed in pituitary adenomas, and their expression correlates with MVD and Ki-67 expression.

Selection criteria for c-Met-targeted therapies: emerging evidence for biomarkers

Extensive development of targeted therapies emphasize the critical need for biomarkers and major efforts have been engaged to identify screening, prognostic, stratification and therapy-monitoring markers. One of the challenges in translating preclinical studies into effective clinical therapies remains the accurate identification of a responsive subsets of patients. Studies on trastuzumab demonstrated that patient response could be specifically correlated with the amplification of the Her2 gene. However, for the EGF receptor, it has been more difficult to find the right stratification biomarker and recent data demonstrate that genetic alterations for the EGF receptor have to be considered. Taken together, these data underline the need for a deeper understanding of both targeted receptor and human disease to determine pathways that might be investigated during early clinical trials in order to define relevant biomarkers for patient selection. This article, dealing with the c-Met tyrosine kinase receptor, provides an overview of c-Met alterations observed in cancer and proposes approaches for stratification biomarker selection.

Silencing of RON receptor signaling promotes apoptosis and gemcitabine sensitivity in pancreatic cancers

The RON receptor tyrosine kinase is overexpressed in premalignant pancreatic intraepithelial neoplasia (PanIN) and in the majority of pancreatic cancers. In pancreatic cells, RON is an important K-Ras effector and RON ligand can enhance migration/invasion and apoptotic resistance. However, the pathobiological significance of RON overexpression in pancreatic cancers has yet to be fully established. In this study, we demonstrate that RON signaling mediates a unique transcriptional program that is conserved between cultured cells derived from murine PanIN or human pancreatic cancer cells grown as subcutaneous tumor xenografts. In both systems, RON signaling regulates expression of genes implicated in cancer-cell survival, including Bcl-2 and the transcription factors signal transducer and activator of transcription 3 (STAT 3) and c-Jun. shRNA-mediated silencing of RON in pancreatic cancer xenografts inhibited their growth, primarily by increasing susceptibility to apoptosis and by sensitizing them to gemcitabine treatment. Escape from RON silencing was associated with re-expression of RON and/or expression of phosphorylated forms of the related c-Met or epidermal growth factor receptors. These findings indicate that RON signaling mediates cell survival and in vivo resistance to gemcitabine in pancreatic cancer, and they reveal mechanisms through which pancreatic cancer cells may circumvent RON-directed therapies.

Developing c-MET pathway inhibitors for cancer therapy: progress and challenges

Successfully developed target-based therapies have significantly changed cancer treatment. Among many targets, the c-MET receptor tyrosine kinase and its ligand hepatocyte growth factor have recently gained considerable attention. The c-MET pathway is dysregulated in most human malignancies, and regulates tumor formation, progression and dissemination, and numerous c-MET pathway inhibitors are currently being evaluated in the clinic. Although some studies have shown impressive evidence of antitumor activity, the data should be interpreted with caution because of the distinct properties of these agents and diverse patient populations studied. Furthermore, in tumor types where patients might benefit from c-MET inhibition, rational combination treatments might ultimately provide maximal clinical benefit. Here, we review the evidence linking c-MET activation to cancer, and discuss the latest progress, opportunities and challenges in the clinical development of c-MET pathway inhibitors.

Pancreatic cancer: molecular pathogenesis and new therapeutic targets

Patients with pancreatic cancer normally present with advanced disease that is lethal and notoriously difficult to treat. Survival has not improved dramatically despite routine use of chemotherapy and radiotherapy; this situation signifies an urgent need for novel therapeutic approaches. Over the past decade, a large number of studies have been published that aimed to target the molecular abnormalities implicated in pancreatic tumor growth, invasion, metastasis, angiogenesis and resistance to apoptosis. This research is of particular importance, as data suggest that a large number of genetic alterations affect only a few major signaling pathways and processes involved in pancreatic tumorigenesis. Although laboratory results of targeted therapies have been impressive, until now only erlotinib, an epidermal growth factor receptor tyrosine kinase inhibitor, has demonstrated modest survival benefit in combination with gemcitabine in a phase III clinical trial. Whilst the failures of targeted therapies in the clinical setting are discouraging, lessons have been learnt and new therapeutic targets that hold promise for the future management of the disease are continuously emerging. This Review describes some of the important developments and targeted agents for pancreatic cancer that have been tested in clinical trials.

Targeting the Met signaling pathway in renal cancer

Renal cell carcinoma (RCC), the most common form of kidney cancer, accounts for 3% of all adult malignancies and its incidence has significantly increased over the last 20 years. RCC claims 13,000 lives annually in the USA and more than 100,000 worldwide. A better understanding of the molecular basis of RCC has facilitated the development of novel and more selective therapeutic approaches. An important role in RCC oncogenesis is played by the receptor for HGF, Met, which has attracted considerable attention, more recently as a molecular target for cancer therapy, and several drugs selectively targeting this pathway are now in clinical trials. This review will focus on efforts to understand the role of the Met signaling pathway in renal cancer and how this has contributed to the development of potent and selective drug candidates.

Discovery of fully human anti-MET monoclonal antibodies with antitumor activity against colon cancer tumor models in vivo

The receptor tyrosine kinase MET is a major component controlling the invasive growth program in embryonic development and in invasive malignancies. The discovery of therapeutic antibodies against MET has been difficult, and antibodies that compete with hepatocyte growth factor (HGF) act as agonists. By applying phage technology and cell-based panning strategies, we discovered two fully human antibodies against MET (R13 and R28), which synergistically inhibit HGF binding to MET and elicit antibody-dependent cellular cytotoxicity. Cell-based phosphorylation assays demonstrate that R13 and R28 abrogate HGF-induced activation of MET, AKT1, ERK1/2, and HGF-induced migration and proliferation. FACS experiments suggest that the inhibitory effect is mediated by "locking" MET receptor in a state with R13, which then increases avidity of R28 for the extracellular domain of MET, thus blocking HGF binding without activating the receptor. In vivo studies demonstrate that the combination of R13/28 significantly inhibited tumor growth in various colon tumor xenograft models. Inhibition of tumor growth was associated with induction of hypoxia. Global gene expression analysis shows that inhibition of HGF/MET pathway significantly upregulated the tumor suppressors KLF6, CEACAM1, and BMP2, the negative regulator of phosphatidylinositol-3-OH-kinase PIK3IP1, and significantly suppressed SCF and SERPINE2, both enhancers of proliferation and invasiveness. Moreover, in an experimental metastasis model, R13/28 increased survival by preventing the recurrence of otherwise lethal lung metastases. Taken together, these results underscore the utility of a dual-antibody approach for targeting MET and possibly other receptor tyrosine kinases. Our approach could be expanded to drug discovery efforts against other cell surface proteins.

Overexpression of MACC1 leads to downstream activation of HGF/MET and potentiates metastasis and recurrence of colorectal cancer

Survival rates from colorectal cancer (CRC) differ dramatically according to the stage of the tumor at diagnosis, with survival rates of 90% for patients with stage I disease but only 49% for those with stage III cancer. Many serum and tumor markers have been identified but none has provided a significant improvement over tumor stage as a prognostic indicator for cancer recurrence for patients with stage II or III disease. Aberrant activation of the hepatocyte growth factor (HGF)/HGF receptor (MET) signaling pathway is associated with both malignant transformation and metastatic potential of CRC. MACC1 (metastasis-associated in colon cancer-1) is a newly discovered gene that regulates this signaling cascade. The significant correlation between overexpression of MACC1 in CRC and both malignant transformation and subsequent risk for metastases in stage II and III CRC indicates that MACC1 tumor typing may prove valuable for determining risk for CRC recurrence. MACC1 may also be an important therapeutic target for CRC treatment.

The MET receptor tyrosine kinase is a potential novel therapeutic target for head and neck squamous cell carcinoma

Recurrent/metastatic head and neck cancer remains a devastating disease with insufficient treatment options. We investigated the MET receptor tyrosine kinase as a novel target for the treatment of head and neck squamous cell carcinoma (HNSCC). MET/phosphorylated MET and HGF expression was analyzed in 121 tissues (HNSCC/normal) by immunohistochemistry, and in 20 HNSCC cell lines by immunoblotting. The effects of MET inhibition using small interfering RNA/two small-molecule inhibitors (SU11274/PF-2341066) on signaling, migration, viability, and angiogenesis were determined. The complete MET gene was sequenced in 66 head and neck cancer tissue samples and eight cell lines. MET gene copy number was determined in 14 cell lines and 23 tumor tissues. Drug combinations of SU11274 with cisplatin or erlotinib were tested in SCC35/HN5 cell lines. Eighty-four percent of the HNSCC samples showed MET overexpression, whereas 18 of 20 HNSCC cell lines (90%) expressed MET. HGF overexpression was present in 45% of HNSCC. MET inhibition with SU11274/PF-2341066 abrogated MET signaling, cell viability, motility/migration in vitro, and tumor angiogenesis in vivo. Mutational analysis of 66 tumor tissues and 8 cell lines identified novel mutations in the semaphorin (T230M/E168D/N375S), juxtamembrane (T1010I/R988C), and tyrosine kinase (T1275I/V1333I) domains (incidence: 13.5%). Increased MET gene copy number was present with >10 copies in 3 of 23 (13%) tumor tissues. A greater-than-additive inhibition of cell growth was observed when combining a MET inhibitor with cisplatin or erlotinib and synergy may be mediated via erbB3/AKT signaling. MET is functionally important in HNSCC with prominent overexpression, increased gene copy number, and mutations. MET inhibition abrogated MET functions, including proliferation, migration/motility, and angiogenesis. MET is a promising, novel target for HNSCC and combination approaches with cisplatin or EGFR inhibitors should be explored.

The MET axis as a therapeutic target

The MET receptor tyrosine kinase and its ligand hepatocyte growth factor (HGF) have been implicated in transformation of a variety of malignancies. Chronic or dysregulated activation of the MET/HGF pathway may lead to increased cell growth, invasion, angiogenesis, and metastasis, reduced apoptosis, altered cytoskeletal functions and other biological changes. It has been suggested that ligand activated MET stimulation can be sufficient for a transforming phenotype. In addition, amplification and activation mutations (germline and/or somatic) within the tyrosine kinase domain, juxtamembrane domain, or semaphorin domain have been identified for MET. MET gain-of-function mutations lead to either deregulated or prolonged tyrosine kinase activity, which are instrumental to its transforming activity. A number of therapeutic strategies targeting ligand-dependent activation or the kinase domain have been employed to inhibit MET. The different structural requirements for activation of signaling events and biological functions regulated by MET will be summarized. Therapeutic targets and current pre-clinical and clinical approaches will be described. Targeting the HGF/MET pathway, alone or in combination with standard therapies, is likely to improve present therapies in MET-dependent malignancies.

Novel therapeutic inhibitors of the c-Met signaling pathway in cancer

A wide variety of human malignancies exhibit sustained c-Met stimulation, overexpression, or mutation, including carcinomas of the breast, liver, lung, ovary, kidney, and thyroid. Notably, activating mutations in c-Met have been positively identified in patients with a particular hereditary form of papillary renal cancer, directly implicating c-Met in human tumorigenesis. Aberrant signaling of the c-Met signaling pathway due to dysregulation of the c-Met receptor or overexpression of its ligand, hepatocyte growth factor (HGF), has been associated with an aggressive phenotype. Extensive evidence that c-Met signaling is involved in the progression and spread of several cancers and an enhanced understanding of its role in disease have generated considerable interest in c-Met and HGF as major targets in cancer drug development. This has led to the development of a variety of c-Met pathway antagonists with potential clinical applications. The three main approaches of pathway-selective anticancer drug development have included antagonism of ligand/receptor interaction, inhibition of the tyrosine kinase catalytic activity, and blockade of the receptor/effector interaction. Several c-Met antagonists are now under clinical investigation. Preliminary clinical results of several of these agents, including both monoclonal antibodies and small-molecule tyrosine kinase inhibitors, have been encouraging. Several multitargeted therapies have also been under investigation in the clinic and have demonstrated promise, particularly with regard to tyrosine kinase inhibition.

Antibody discovery via multiplexed single cell characterization

The secreted immunoglobulin footprint of single hybridoma cells, containing ~10 fg of antibody purified in situ, has been probed for 9 properties concurrently by use of detection labels comprising 280 nm combinatorially colored fluorescent latex beads functionalized with proteins. Specificity of each individual hybridoma cell's product has thereby been assessed in a primary screen. Varying the density of antigen on beads to modulate the avidity of the interaction between bead and secreted antibody footprint allowed rank ordering by affinity in the same primary screen. As more criteria were added to the selection process, the frequency of positive cells went down; in some cases, the favorable cell was present at <1/50,000. Recovery of the cell of interest was accomplished by plating the cells in a viscous medium on top of a membrane. After collecting the antibody footprint on a capture surface beneath the membrane, the immobilized cells were transferred to an incubator while the footprints were analyzed to locate the hybridoma cells of interest. The desired cells were then cloned by picking them from the corresponding locations on the membrane.

Systems pharmacology and genome medicine: a future perspective

Genome medicine uses genomic information in the diagnosis of disease and in prescribing treatment. This transdisciplinary field brings together knowledge on the relationships between genetics, pathophysiology and pharmacology. Systems pharmacology aims to understand the actions and adverse effects of drugs by considering targets in the context of the biological networks in which they exist. Genome medicine forms the base on which systems pharmacology can develop. Experimental and computational approaches enable systems pharmacology to obtain holistic, mechanistic information on disease networks and drug responses, and to identify new drug targets and specific drug combinations. Network analyses of interactions involved in pathophysiology and drug response across various scales of organization, from molecular to organismal, will allow the integration of the systems-level understanding of drug action with genome medicine. The interface of the two fields will enable drug discovery for personalized medicine. Here we provide a perspective on the questions and approaches that drive the development of these new interrelated fields.

Single-agent and combination therapeutic strategies to inhibit hepatocyte growth factor/MET signaling in cancer

Receptor tyrosine kinases are often aberrantly activated in human malignancies and contribute to cancer development and progression. Specific receptor tyrosine kinase inhibitors have been shown to be clinically effective therapies in subsets of cancer patients with either hematologic or solid tumors. Activation of the hepatocyte growth factor (HGF)/MET signaling pathway has been found to play a critical role in oncogenesis, cancer metastasis, and drug resistance. These observations have led to the development of agents that can effectively inhibit HGF/MET signaling through direct inhibition of the receptor (anti-MET antibodies), through inactivation of its ligand HGF (AMG102, L2G7), by interfering with HGF binding to MET (NK4), or by inhibiting MET kinase activity (PHA-665752 and SU11274). Moreover, the combination of anti-MET therapeutic agents with either signal transduction inhibitors (ERBB family or mTOR inhibitors) or with cytotoxic chemotherapy has been evaluated in preclinical models. These studies provide insight into the rational development of combination therapeutic strategies that can be evaluated in clinical trials. This review will discuss different strategies of MET inhibition with a specific focus on combination therapeutic approaches.