Targeting SRC family kinases inhibits growth and lymph node metastases of prostate cancer in an orthotopic nude mouse model

Aberrant expression and/or activity of members of the Src family of nonreceptor protein tyrosine kinases (SFK) are commonly observed in progressive stages of human tumors. In prostate cancer, two SFKs (Src and Lyn) have been specifically implicated in tumor growth and progression. However, there are no data in preclinical models demonstrating potential efficacy of Src inhibitors against prostate cancer growth and/or metastasis. In this study, we used the small molecule SFK/Abl kinase inhibitor dasatinib, currently in clinical trials for solid tumors, to examine in vitro and in vivo effects of inhibiting SFKs in prostate tumor cells. In vitro, dasatinib inhibits both Src and Lyn activity, resulting in decreased cellular proliferation, migration, and invasion. In orthotopic nude mouse models, dasatinib treatment effectively inhibits expression of activated SFKs, resulting in inhibition of both tumor growth and development of lymph node metastases in both androgen-sensitive and androgen-resistant tumors. In primary tumors, SFK inhibition leads to decreased cellular proliferation (determined by immunohistochemistry for proliferating cell nuclear antigen). In vitro, small interfering RNA (siRNA)-mediated inhibition of Lyn affects cellular proliferation; siRNA inhibition of Src affects primarily cellular migration. Therefore, we conclude that SFKs are promising therapeutic targets for treatment of human prostate cancer and that Src and Lyn activities affect different cellular functions required for prostate tumor growth and progression.     

Combined inhibition of c-Src and epidermal growth factor receptor abrogates growth and invasion of head and neck squamous cell carcinoma

PURPOSE: Increased expression and/or activation of epidermal growth factor receptor (EGFR) is associated with tumor progression and poor prognosis in many cancers, including head and neck squamous cell carcinoma (HNSCC). Src family kinases, including c-Src, mediate a variety of intracellular or extracellular signals that contribute to tumor formation and progression. This study was undertaken to elucidate the role of c-Src in the growth and invasion of HNSCC and to determine the effects of combined targeting of EGFR and Src kinases in HNSCC cell lines. EXPERIMENTAL DESIGN: HNSCC cells were engineered to stably express a dominant-active form of c-Src and investigated in cell growth and invasion assays. The biochemical effects of combined treatment with the Src inhibitor AZD0530, a potent, orally active Src inhibitor with Bcr/Abl activity, and the EGFR kinase inhibitor gefitinib were examined, as well as the consequences of dual Src/EGFR targeting on the growth and invasion of a panel of HNSCC cell lines. RESULTS: HNSCC cells expressing dominant-active c-Src showed increased growth and invasion compared with vector-transfected controls. Combined treatment with AZD0530 and gefitinib resulted in greater inhibition of HNSCC cell growth and invasion compared with either agent alone. CONCLUSIONS: These results suggest that increased expression and activation of c-Src promotes HNSCC progression where combined targeting of EGFR and c-Src may be an efficacious treatment approach.     

Targeting epidermal growth factor receptor and SRC pathways in head and neck cancer

Epidermal growth factor receptor (EGFR), a member of the ErbB family of receptor tyrosine kinases (RTKs), is highly expressed in head and neck squamous cell carcinoma (HNSCC) where increased EGFR expression levels in tumors are associated with decreased survival. HNSCC patient responses to EGFR-targeted monotherapies in clinical trials, though significant, have been limited. Tumor signaling pathway components that work in cooperation with EGFR or provide compensation for the loss of EGFR-initiated signaling will be ideal targets for therapies to be used in combination with EGFR-targeted agents. Based on the current understanding of molecular signaling pathways and available agents, ErbB family-targeted and Src family-targeted agents represent strategies for further exploration. Here, we discuss agents targeting ErbB and Src family kinases in clinical development, provide an overview of completed and ongoing clinical trials, and outline a molecular rationale for combining ErbB- and Src-targeted therapeutics.     

Ligand release-independent transactivation of epidermal growth factor receptor by transforming growth factor-beta involves multiple signaling pathways

Many of the signaling responses induced by transforming growth factor-beta (TGF-beta) are mediated by Smad proteins, but there is evidence that it can also signal independently of Smads. Here, we provide evidence that multiple signal pathways induced by TGF-beta1-including Src family tyrosine kinases (SFKs), generation of reactive oxygen species (ROS), de novo protein synthesis and E-cadherin-dependent cell-cell interactions-transactivate the epidermal growth factor receptor (EGFR), which in turn regulates expression of c-Fos and c-Jun. Immunoprecipitation and immunofluorescence staining showed that EGFR was phosphorylated on tyrosine in response to TGF-beta1. EGFR transactivation required the activation of SFKs and the production of ROS via NADPH oxidase, but was not dependent on metalloproteases or the release of EGF-like ligands. In addition, the production of ROS was dependent on signaling by specific SFKs as well as de novo protein synthesis. Stable transfection of E-cadherin into MDA-MB-231 cells as well as E-cadherin-blocking assays revealed that E-cadherin-mediated cell-cell interactions were also essential for EGFR transactivation. Finally, EGFR transactivation was involved in the expression of c-Fos and c-Jun via the extracellular signal-regulated kinase signaling cascade. Taken together our data suggest that ligand release-independent transactivation of EGFR may diversify early TGF-beta signaling and represent a novel pathway leading to TGF-beta-mediated gene expression.     

Epithelial to mesenchymal transition in head and neck squamous carcinoma: association of Src activation with E-cadherin down-regulation, vimentin expression, and aggressive tumor features

BACKGROUND: Epithelial-mesenchymal transformations (EMT) are critical for the invasion, progression, and metastasis of epithelial carcinogenesis. The role of EMT in head and neck squamous carcinoma (HNSC) tumorigenesis remains unexplored. In the current study, the expressions of several factors associated with the induction of EMT in HNSC cell lines and tumor specimens were investigated to define their functional and pathologic role in HNSC. METHODS: Eleven HNSC cell lines and 50 primary tumor tissue specimens formed the materials of this study. Western blot analysis as well as immunohistochemical, and functional techniques were used to assess the status of activated Src (p-Src), E-cadherin, and vimentin in both cell lines and tumor tissues and the results were correlated with patients' clinicopathologic parameters. RESULTS: The results demonstrated the inverse expression of p-Src and E-cadherin in the majority of cell lines and in primary tumor tissues compared with normal squamous mucosa. Elevated levels of p-Src were accompanied by down-regulation of E-cadherin and the expression of vimentin in epithelial tumor cells. In vitro inhibition of Src led to E-cadherin reexpression and increased cell contact in squamous carcinoma cell lines. Immunophenotypic analysis of these markers in primary tumor tissues demonstrated a significant correlation between increased p-Src, decreased E-cadherin, and vimentin expression and aggressive tumor features including penetrating invasive fronts, high-grade sarcomatoid transformation, and lymph node metastasis. CONCLUSIONS: The results of the current study indicate that Src and E-cadherin may play an important role in EMT, invasion, and aggressive clinicopathologic features of HNSC. These proteins may be targeted for the therapeutic intervention of patients with HNSC.     

Src family kinases and paclitaxel sensitivity

Src-family Kinases (SFKs) participate in the regulation of proliferation, differentiation, apoptosis, autophagy, adhesion, migration, invasion and angiogenesis in normal and cancer cells. Abnormal expression of SFKs has been documented in cancers that arise in breast, colon, ovary, melanocyte, gastric mucosa, head and neck, pancreas, lung, and brain. Targeting SFKs in cancer cells has been shown to be a promising therapeutic strategy in solid tumors, particularly in ovarian, colon and breast cancers. Paclitaxel is one of most widely used chemotherapeutic agents for the management of ovarian, breast, lung and head/neck cancers. As a microtubule-stabilizing agent, paclitaxel possesses both mitosis-dependent and mitosis-independent activities against cancer cells. A variety of mechanisms such as deregulation of P-glycoprotein, alteration of tubulin isotypes, alteration of microtubule-regulatory proteins, deregulation of apoptotic signaling pathways, mutation of tubulins and overexpression of copper transporters have been implicated in the development of primary or secondary resistance to paclitaxel. By affecting cancer cell survival, proliferation, autophagy, microtubule stability, motility, and/or angiogenesis, SFKs interact with mechanisms that regulate paclitaxel sensitivity. Inhibition of SFKs can potentiate the anti-tumor activity of paclitaxel by enhancing apoptosis, autophagy and microtubule stability. Based on pre-clinical observations, administration of SFK inhibitors in combination with paclitaxel could improve treatment for ovarian, breast, lung and head/neck cancers. Identification and validation of predictive biomarkers could also permit personalization of the therapy.     

Matrix metalloproteinase 9 is a mediator of epidermal growth factor-dependent e-cadherin loss in ovarian carcinoma cells

Epidermal growth factor (EGF) receptor (EGFR) is frequently elevated in epithelial ovarian cancer, and E-cadherin expression is often reduced in advanced disease. In this study, we investigated a mechanism by which EGFR activation promotes disruption of adherens junctions through induction of matrix metalloproteinase 9 (MMP-9). We show that EGFR activation down-modulates E-cadherin, and broad spectrum MMP inhibition ameliorates EGF-stimulated junctional disruption and loss of E-cadherin protein. MMP-9 involvement in EGF-dependent down-regulation of E-cadherin was determined by siRNA specifically directed against MMP-9. Furthermore, treatment with recombinant MMP-9 or transient expression of MMP-9 is sufficient to reduce E-cadherin levels in differentiated ovarian tumor cells. Stable overexpression of MMP-9 led to a loss of E-cadherin and junctional integrity, and promoted a migratory and invasive phenotype. Thus, elevated MMP-9 protein expression is sufficient for junctional disruption and loss of E-cadherin in these cells. The associations between EGFR activation, MMP-9 expression, and E-cadherin were investigated in human ovarian tumors and paired peritoneal metastases wherein immunohistochemical staining for activated (phospho) EGFR and MMP-9 colocalized with regions of reduced E-cadherin. These data suggest that regulation of MMP-9 by EGFR may represent a novel mechanism for down-modulation of E-cadherin in ovarian cancer.     

MicroRNA-205 inhibits Src-mediated oncogenic pathways in renal cancer

The Src family of protein kinases (SFK) plays key roles in regulating fundamental cellular processes, including cell growth, differentiation, cell shape, migration, and survival, and specialized cell signals in various malignancies. The pleiotropic functions of SFKs in cancer make them promising targets for intervention. Here, we sought to investigate the role of microRNA-205 (miR-205) in inhibition of Src-mediated oncogenic pathways in renal cancer. We report that expression of miR-205 was significantly suppressed in renal cancer cell lines and tumors when compared with normal tissues and a nonmalignant cell line and is correlated inversely with the expression of SFKs. miR-205 significantly suppressed the luciferase activity of reporter plasmids containing the 3'-UTR (untranslated region) sequences complementary to either Src, Lyn, or Yes, which was abolished by mutations in these 3'-UTR regions. Overexpression of miR-205 in A498 cells reduced Src, Lyn, and Yes expression, both at mRNA and protein levels. Proliferation of renal cancer cells was suppressed by miR-205, mediated by the phospho-Src-regulated ERK1/2 pathway. Cell motility factor FAK (focal adhesion kinase) and STAT3 activation were also inhibited by miR-205. Transient and stable overexpression of miR-205 in A498 cells resulted in induction of G?/G? cell-cycle arrest and apoptosis, as indicated by decreased levels of cyclin D1 and c-Myc, suppressed cell proliferation, colony formation, migration, and invasion in renal cancer cells. miR-205 also inhibited tumor cell growth in vivo. This is the first study showing that miR-205 inhibits proto-oncogenic SFKs, indicating a therapeutic potential of miR-205 in the treatment of renal cancer.     

Fyn is induced by Ras/PI3K/Akt signaling and is required for enhanced invasion/migration

Src family kinases (SFKs) are frequently over‐expressed and/or activated in human cancers, and play key roles in cancer cell invasion, metastasis, proliferation, survival, and angiogenesis. Allosteric activation of SFKs occurs through well‐defined post‐translational mechanisms, however the SFK member Fyn is over‐expressed in multiple human cancers (prostate, melanoma, pancreatic, glioma, chronic myelogenous leukemia) and the mechanism of increased Fyn expression is unclear. Since activation of Ras oncogenes is a common oncogenic event leading to the activation of multiple effector pathways, we explored if Ras could induce Fyn expression. Retroviral transduction of the human keratinocyte cell line HaCaT with oncogenic H‐Ras dramatically up‐regulated Fyn mRNA (>100‐fold, P < 0.001), protein, and kinase activity without affecting Src levels or activity. Activation of Akt, but not MAPK or EGFR, was necessary and sufficient for induction of Fyn by H‐Ras. Expression of active Fyn was sufficient to increase HaCaT cell migration and invasion, and the enhanced migration and invasion induced by H‐Ras could be significantly blocked (70% reduction, P < 0.01) by knockdown of Fyn with a specific siRNA or inhibition of SFKs with PP2. In addition, expression of Fyn in MDA‐MB‐231 breast cancer cells was dependent on PI3K activity and was involved in their invasive phenotype. Thus, the Ras/PI3K/Akt pathway can account for Fyn over‐expression in cancers, and Fyn is a critical mediator of the Ras‐stimulated invasive cell phenotype. These results support the development of therapeutic strategies targeting Akt/Fyn pathway to block migration and invasion of tumor cells. Mol. Carcinog. © 2010 Wiley‐Liss, Inc.     

Shear stress induces internalization of E-cadherin and invasiveness in metastatic oesophageal cancer cells by a Src-dependent pathway

Metastatic disease is dependent on tumor cell migration through the venous and lymphatic systems and requires dynamic rearrangement of adherens junctions. Endocytosis of cadherins is a key mechanism to dynamically arrange adherens junctions, signaling, and motility in tumor cells; however, the role of shear in regulating this process in metastatic cells is unknown. In this study, the role of shear in regulating cell surface expression of E-cadherin was investigated. We found that exposure to venous shear (shear rate, 200/s) induced internalization of E-cadherin in adherent metastatic oesophageal tumor cells (OC-1 tumor cell line). Internalized E-cadherin was found localized to Rab5-positive endosomes and was not present in lysosomes. As the Src family of tyrosine kinase have been implicated in regulating cadherin expression, we investigated the role of shear in regulating E-cadherin through Src activity. Pretreatment of OC-1 cells with the specific Src kinase inhibitor 4-amino-5- (4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP1) prevented shear-induced internalization of E-cadherin. Direct measurement of Src activity (phosphorylation on Y416) showed that Src is activated in sheared OC-1 cells and that the shear-induced increase in phospho-Src is inhibited by the presence of PP1. Moreover, we show that shear stress significantly increased the invasive capacity of OC-1 cells ( P  < 0.001), a process inhibited by the presence of PP1. These results indicate a novel role for shear in regulating the endocytosis of E-cadherin and invasiveness in metastatic cells. ( Cancer Sci 2009; 100: 1082–1087)     

Identification of an Exon 4-Deletion Variant of Epidermal Growth Factor Receptor with Increased Metastasis-Promoting Capacity

Several types of epidermal growth factor receptor (EGFR) gene alternations have been observed in human tumors. Here we present a novel EGFR variant with aberrant splicing of exon 4 (named as de4 EGFR). Variant-specific polymerase chain reaction showed that de4 EGFR was expressed in some glioma (4/40), prostate cancer (3/11), and ovarian cancer (3/9) tissues but not in tissues adjacent to tumors or normal tissues. de4 EGFR displayed an enhanced transformation and a higher metastasis-promoting capacity in comparison to wild-type EGFR. With minimal EGF-binding activity, de4 EGFR underwent ligand-independent autophosphorylation and self-dimerization. Moreover, in serum-starved condition, de4 EGFR expression in U87 MG cells significantly upregulated the extracellular signal-regulated kinase and AKT phosphorylation and expression of JUN and Src. Importantly, E-cadherin expression was barely detectable in the U87 MG cells expressing de4 EGFR and restored expression of E-cadherin in these cells inhibited their metastatic behaviors. Taken together, we identified a novel EGFR variant with increased metastasis-promoting activity that may become a promising new target for cancer therapy.     

Prognostic significance of cortactin levels in head and neck squamous cell carcinoma: comparison with epidermal growth factor receptor status

Cortactin is an actin-binding Src substrate involved in cell motility and invasion. In this study, we sought to examine the prognostic importance of cortactin protein expression in head and neck squamous cell carcinoma (HNSCC). To do so, cortactin and EGF receptor (EGFR) expression was retrospectively evaluated by immunohistochemistry in a tissue microarray composed of 176 HNSCCs with a mean follow-up time of 5 years. Cortactin immunoreactivity was weak to absent in normal epithelial tissue. Overexpression of the protein in 77 out of 176 tumours (44%) was associated with more advanced tumour-node-metastasis stage and higher histologic grade. Cortactin overexpression was associated with significantly increased local recurrence rates (49 vs 28% for high and low expressing carcinomas, respectively), decreased disease-free survival (17 vs 61%), and decreased the 5-year overall survival of (21 vs 58%), independently of the EGFR status. In multivariate analysis, cortactin expression status remained an independent prognostic factor for local recurrence, disease-free survival, and overall survival. Importantly, we identified a subset of patients with cortactin-overexpressing tumours that displayed low EGFR levels and a survival rate that equalled that of patients with tumoral overexpression of both EGFR and cortactin. These findings identify cortactin as a relevant prognostic marker and may have implications for targeted therapies in patients with HNSCC.     

Loss of E-cadherin and p27 expression is associated with head and neck squamous tumorigenesis

BACKGROUND: In neoplastic head and neck lesions, it has been found that the loss or reduction in E-cadherin expression is a late event and is associated with invasion. Low p27 levels have been associated with a poor prognosis in many different tumors, including laryngeal carcinoma. The authors investigated p27 and E-cadherin protein expression in the early stages of head and neck tumorigenesis and evaluated their predictive roles individually and in association with carcinogenesis. METHODS: Tissue biopsies from 46 patients who were participants in 3 chemoprevention trials were analyzed for E-cadherin expression, and 40 samples were analyzed for p27 expression using immunohistochemistry. RESULTS: The data suggested that loss of both E-cadherin expression and p27 expression occurred early during the preneoplastic steps of head and neck carcinogenesis, and loss of p27 protein expression alone (P=0.02) and in combination with loss of E-cadherin expression (P=0.04) was a significant predictor of the risk for head and neck carcinoma. CONCLUSIONS: The loss of p27 expression may be useful in the construction of a risk model for head and neck carcinogenesis and may represent a potential target for chemopreventive interventions. Longer follow-up of the high percentage of low-risk preneoplastic lesions in the current study and validation in a larger sample size may be required to establish the predictive role of these abnormalities.     

Src酪氨酸激酶在人胃癌细胞转移中的作用和机制

背景与目的:Src酪氨酸激酶的活化在胃癌浸润和转移中发挥了重要的作用。本研究旨在探讨Src酪氨酸激酶对人胃癌细胞E-钙黏着蛋白(E-cadherin)和基质金属蛋白酶(matrix metalloproteinase,MMP)-2、9表达、血管内皮生长因子(vascular endothelial growth factor,VEGF)分泌、转录因子NF-κB和AP-1表达以及胃癌细胞体外侵袭能力的影响。方法:使用Src酪氨酸激酶抑制剂4-苯胺喹唑啉(3和10μmol/L)后,应用Western blot法检测人胃癌SGC7901细胞中E-cadherin、MMP-2和MMP-9蛋白表达变化;应用实时PCR(real-time PCR)检测细胞中E-cadherin、MMP-2和MMP-9的mRNA表达变化;应用ELISA法检测细胞培养上清液中VEGF表达变化;应用双荧光报告基因法检测NF-κB和AP-1转录活性;应用Transwell法检测肿瘤细胞侵袭能力变化。结果:当Src酪氨酸激酶抑制剂4-苯胺喹唑啉浓度为3和10μmol/L时,SGC7901细胞中E-cadherin蛋白表达量显著增加,E-cadherin的mRNA表达是对照组的263.8%(P<0.05)和403.3%(P<0.05);MMP-2和MMP-9的蛋白表达显著降低,MMP-2的mRNA表达是对照组的28.3%(P<0.05)和16.7%(P<0.05),MMP-9的mRNA表达是对照组的23.6%(P<0.05)和13.3%(P<0.05);VEGF含量是对照组的62.6%(P<0.05)和38.7%(P<0.05);AP-1转录活性是对照组的54.4%(P<0.05)和18%(P<0.05),NF-κB转录活性是对照组的38.3%(P<0.05)和11.5%(P<0.05);胃癌细胞侵袭能力是对照组的43.3%(P<0.05)和28.2%(P<0.05),呈现明显的剂量依赖性抑制作用。结论:Src酪氨酸激酶活化与人胃癌SGC7901细胞体外转移能力密切相关,其机制可能与肿瘤转移相关基因表达有关。     

Degradation of Epidermal Growth Factor Receptor Mediates Dasatinib-Induced Apoptosis in Head and Neck Squamous Cell Carcinoma Cells

Epidermal growth factor receptor (EGFR) is an important oncoprotein that promotes cell growth and proliferation. Dasatinib, a bcr-abl inhibitor, has been approved clinically for the treatment of chronic myeloid leukemia and demonstrated to be effective against solid tumors in vitro through Src inhibition. Here, we disclose that EGFR degradation mediated dasatinib-induced apoptosis in head and neck squamous cell carcinoma (HNSCC) cells. HNSCC cells, including Ca9-22, FaDu, HSC3, SAS, SCC-25, and UMSCC1, were treated with dasatinib, and cell viability, apoptosis, and underlying signal transduction were evaluated. Dasatinib exhibited differential sensitivities against HNSCC cells. Growth inhibition and apoptosis were correlated with its inhibition on Akt, Erk, and Bcl-2, irrespective of Src inhibition. Accordingly, we found that down-regulation of EGFR was a determinant of dasatinib sensitivity. Lysosome inhibitor reversed dasatinib-induced EGFR down-regulation, and c-cbl activity was increased by dasatinib, indicating that dasatinib-induced EGFR down-regulation might be through c-cbl-mediated lysosome degradation. Increased EGFR activation by ligand administration rescued cells from dasatinib-induced apoptosis, whereas inhibition of EGFR enhanced its apoptotic effect. Estrogen receptor α (ERα) was demonstrated to play a role in Bcl-2 expression, and dasatinib inhibited ERα at the pretranslational level. ERα was associated with EGFR in dasatinib-treated HNSCC cells. Furthermore, the xenograft model showed that dasatinib inhibited HSC3 tumor growth through in vivo down-regulation of EGFR and ERα. In conclusion, degradation of EGFR is a novel mechanism responsible for dasatinib-induced apoptosis in HNSCC cells.     

The role of Src in prostate cancer.

The Src family kinases (SFKs) are the largest family of nonreceptor protein tyrosine kinases and are responsible for signal transduction during many cellular activities, including differentiation, adhesion, and migration. Aberrant Src/SFK activity has been widely implicated in cancer development. Several lines of evidence indicate a role for SFKs in the development of prostate cancer, e.g. SFK overexpression in prostate cancer cell lines and tissues and reduced cancer cell proliferation, invasion, and migration following Src inhibition. In particular, Src may be involved in androgen-independent growth during advanced stages of disease. Src signaling is also a key pathway during normal and dysregulated bone functioning, and bone metastases are responsible for substantial morbidity in advanced prostate cancer. Src/SFK inhibition therefore represents a potentially useful therapeutic strategy for patients with various stages of prostate cancer. To date, four Src inhibitors have reached clinical trials. Of these, the broadest range of in vitro prostate cancer data are available for dasatinib, which inhibits several SFKs as well as other tyrosine kinases. Src inhibitors may be specifically evaluated in prostate cancer clinical trials in the near future.     

Recombinant antibody toxins specific for ErbB2 and EGF receptor inhibit the in vitro growth of human head and neck cancer cells and cause rapid tumor regression in vivo

Overexpression of the ErbB2 and epidermal growth factor receptor (EGFR) tyrosine kinases is frequently observed in squamous cell carcinomas of the head and neck, and has been correlated with shorter overall survival. By immunoblot analysis, we have found EGFR and ErbB2 expression in 6 out of 6 established head and neck cancer cell lines. Elevated EGFR protein levels were noted in 3 and elevated ErbB2 levels in 5 of them. Significant expression of EGFR and ErbB2 was also detected in 17 of 47 and 26 of 45 primary tumor samples. Due to their enhanced expression on the tumor cell surface, these receptors can be regarded as suitable targets for directed cancer therapy. We have analyzed the antitumoral activity of recombinant single-chain antibody toxins specific for ErbB2 and EGFR against head and neck cancer cells in vitro and in vivo. The recombinant toxins consist of the variable domains of the heavy and light chains of monoclonal antibodies (MAbs) genetically fused to a truncated Pseudomonas exotoxin A (ETA). At low concentrations, the ErbB2-specific single-chain antibody (scFv) toxin scFv(FRP5)-ETA and the EGFR-specific toxins scFv(225)-ETA and scFv(14E1)-ETA inhibited the in vitro growth of established head and neck cancer cell lines and primary tumor cells. In a nude mouse tumor model, intratumoral injection of the antibody toxins resulted in the rapid regression of subcutaneously growing CAL 27 tumor xenografts, with scFv(FRP5)-ETA and scFv(14E1)-ETA treatment being most effective and leading to the cure of up to 50% of the animals. Our results suggest that EGFR and ErbB2-specific antibody toxins may become valuable therapeutic reagents for the treatment of squamous cell carcinomas of the head and neck.