A bivalent single-chain antibody-toxin specific for ErbB-2 and the EGF receptor

Bispecific Targeting of EGFR and Urokinase Receptor (uPAR) Using Ligand-Targeted Toxins in Solid Tumors

Ligand-targeted toxins (LTTs) are bioengineered molecules which are composed of a targeting component linked to a toxin that induces cell death once the LTT binds its target. Bispecific targeting allows for the simultaneous targeting of two receptors. In this review, we mostly focus on the epidermal growth factor receptor (EGFR) as a target. We discuss the development and testing of a bispecific LTT targeting EGFR and urokinase-type plasminogen activator receptor (uPAR) as two attractive targets implicated in tumor growth and in the regulation of the tumor microvasculature in solid tumors. In vitro and mouse xenograft studies have shown that EGFR-targeted bispecific angiotoxin (eBAT) is effective against human solid tumors. Canine studies have shown that eBAT is both safe and effective against canine hemangiosarcoma, which is physiologically similar to human angiosarcoma. Finding the appropriate dosing strategy and sequencing of eBAT administration, in combination with other therapeutics, are among important factors for future directions. Together, the data indicate that eBAT targets cancer stem cells, it may have a role in inhibiting human tumor vasculature, and its bispecific conformation may have a role in reducing toxicity in comparative oncologic trials in dogs.

Quantitative molecular phenotyping with topically applied SERS nanoparticles for intraoperative guidance of breast cancer lumpectomy

There is a need to image excised tissues during tumor-resection procedures in order to identify residual tumors at the margins and to guide their complete removal. The imaging of dysregulated cell-surface receptors is a potential means of identifying the presence of diseases with high sensitivity and specificity. However, due to heterogeneities in the expression of protein biomarkers in tumors, molecular-imaging technologies should ideally be capable of visualizing a multiplexed panel of cancer biomarkers. Here, we demonstrate that the topical application and quantification of a multiplexed cocktail of receptor-targeted surface-enhanced Raman scattering (SERS) nanoparticles (NPs) enables rapid quantitative molecular phenotyping (QMP) of the surface of freshly excised tissues to determine the presence of disease. In order to mitigate the ambiguity due to nonspecific sources of contrast such as off-target binding or uneven delivery, a ratiometric method is employed to quantify the specific vs. nonspecific binding of the multiplexed NPs. Validation experiments with human tumor cell lines, fresh human tumor xenografts in mice, and fresh human breast specimens demonstrate that QMP imaging of excised tissues agrees with flow cytometry and immunohistochemistry, and that this technique may be achieved in less than 15 minutes for potential intraoperative use in guiding breast-conserving surgeries.

In vivo multiplexed molecular imaging of esophageal cancer via spectral endoscopy of topically applied SERS nanoparticles

The biological investigation and detection of esophageal cancers could be facilitated with an endoscopic technology to screen for the molecular changes that precede and accompany the onset of cancer. Surface-enhanced Raman scattering (SERS) nanoparticles (NPs) have the potential to improve cancer detection and investigation through the sensitive and multiplexed detection of cell-surface biomarkers. Here, we demonstrate that the topical application and endoscopic imaging of a multiplexed cocktail of receptor-targeted SERS NPs enables the rapid detection of tumors in an orthotopic rat model of esophageal cancer. Antibody-conjugated SERS NPs were topically applied on the lumenal surface of the rat esophagus to target EGFR and HER2, and a miniature spectral endoscope featuring rotational scanning and axial pull-back was employed to comprehensively image the NPs bound on the lumen of the esophagus. Ratiometric analyses of specific vs. nonspecific binding enabled the visualization of tumor locations and the quantification of biomarker expression in agreement with immunohistochemistry and flow cytometry validation data.

Rapid ratiometric biomarker detection with topically applied SERS nanoparticles

Multiplexed surface-enhanced Raman scattering (SERS) nanoparticles (NPs) offer the potential for rapid molecular phenotyping of tissues, thereby enabling accurate disease detection as well as patient stratification to guide personalized therapies or to monitor treatment outcomes. The clinical success of molecular diagnostics based on SERS NPs would be facilitated by the ability to accurately identify tissue biomarkers under time-constrained staining and detection conditions with a portable device. In vitro, ex vivo and in vivo experiments were performed to optimize the technology and protocols for the rapid detection (0.1-s integration time) of multiple cell-surface biomarkers with a miniature fiber-optic spectral-detection probe following a brief (5 min) topical application of SERS NPs on tissues. Furthermore, we demonstrate that the simultaneous detection and ratiometric quantification of targeted and nontargeted NPs allows for an unambiguous assessment of molecular expression that is insensitive to nonspecific variations in NP concentrations.

Fluorescent IgG fusion proteins made in E. coli

Antibodies are among the most powerful tools in biological and biomedical research and are presently the fastest growing category of new bio-pharmaceutics. The most common format of antibody applied for therapeutic, diagnostic and analytical purposes is the IgG format. For medical applications, recombinant IgGs are made in cultured mammalian cells in a process that is too expensive to be considered for producing antibodies for diagnostic and analytical purposes. Therefore, for such purposes, mouse monoclonal antibodies or polyclonal sera from immunized animals are used. While looking for an easier and more rapid way to prepare full-length IgGs for therapeutic purposes, we recently developed and reported an expression and purification protocol for full-length IgGs, and IgG-based fusion proteins in E. coli, called "Inclonals." By applying the Inclonals technology, we could generate full-length IgGs that are genetically fused to toxins. The aim of the study described herein was to evaluate the possibility of applying the "Inclonals" technology for preparing IgG-fluorophore fusion proteins. We found that IgG fused to the green fluorescent proteins enhanced GFP (EGFP) while maintaining functionality in binding, lost most of its fluorescence during the refolding process. In contrast, we found that green fluorescent Superfolder GFP (SFGFP)-fused IgG and red fluorescent mCherry-fused IgG were functional in antigen binding and maintained fluorescence intensity. In addition, we found that we can link several SFGFPs in tandem to each IgG, with fluorescence intensity increasing accordingly. Fluorescent IgGs made in E. coli may become attractive alternatives to monoclonal or polyclonal fluorescent antibodies derived from animals.

Evaluation of a bispecific biological drug designed to simultaneously target glioblastoma and its neovasculature in the brain

OBJECT

The authors of this study aimed to genetically design a bispecific targeted toxin that would simultaneously target overexpressed markers on glioma as well as the tumor vasculature, to mutate certain amino acids to reduce the immunogenicity of this new drug, and to determine whether the drug was able to effectively reduce aggressive human brain tumors in a rat xenograft model via a novel hollow fiber (HF) catheter delivery system.

METHODS

A new bispecific ligand-directed toxin (BLT) was created in which 2 human cytokines-epidermal growth factor ([EGF], targeting overexpressed EGF receptor) and amino acid terminal fragment ([ATF], targeting urokinase plasminogen activator receptor)-were cloned onto the same single-chain molecule with truncated Pseudomonas exotoxin with a terminal lysyl-aspartyl-glutamyl-leucine (KDEL) sequence. Site-specific mutagenesis was used to mutate amino acids in 7 key epitopic toxin regions that dictate the B cell generation of neutralizing antitoxin antibodies to deimmunize the drug, now called "EGFATFKDEL 7mut." Bioassays were used to determine whether mutation reduced the drug's potency, and enzyme-linked immunosorbent assay studies were performed to determine whether antitoxin antibodies were decreased. Aggressive brain tumors were intracranially established in nude rats by using human U87 glioma genetically marked with a firefly luciferase reporter gene (U87-luc), and the rats were stereotactically treated with 2 intracranial injections of deimmunized EGFATFKDEL via convection-enhanced delivery (CED). Drug was administered through a novel HF catheter to reduce drug backflow upon delivery.

RESULTS

In vitro, EGFATFKDEL 7mut selectively killed the human glioblastoma cell line U87-luc as well as cultured human endothelial cells in the form of the human umbilical vein endothelial cells. Deimmunization did not reduce drug activity. In vivo, when rats with brain tumors were intracranially treated with drug via CED and a novel HF catheter to reduce backflow, there were significant tumor reductions in 2 experiments (p < 0.01). Some rats survived with a tumor-free status until 130 days post-tumor inoculation. An irrelevant BLT control did not protect establishing specificity. The maximal tolerated dose of EGFATFKDEL 7mut was established at 2 μg/injection or 8.0 μg/kg, and data indicated that this dose was nontoxic. Antitoxin antibodies were reduced by at least 90%.

CONCLUSIONS

First, data indicated that the BLT framework is effective for simultaneously targeting glioma and its neovasculature. Second, in the rodent CED studies, newly developed HF catheters that limit backflow are effective for drug delivery. Third, by mutating critical amino acids, the authors reduced the threat of the interference of neutralizing antibodies that are generated against the drug. The authors' experiments addressed some of the most urgent limitations in the targeted toxin field.

A new drug delivery method of bispecific ligand-directed toxins, which reduces toxicity and promotes efficacy in a model of orthotopic pancreatic cancer

OBJECTIVE

Biologicals targeting epidermal growth factor (EGF) and interleukin 13 receptors not only react with overexpressed markers on cancer cells but also react with receptors on normal cells. Because we developed novel bispecific ligand-directed toxins synthesized by cloning EGF and interleukin 13 on the same molecule with toxin, our objective was to determine whether we could block normal receptors while still targeting receptors overexpressed on cancer cells, thereby decreasing toxicity while maintaining efficacy.

METHODS

A method, toxicity blocking (ToxBloc), was developed in which a bolus intraperitoneal dose of recombinant EGF13 (without toxin) was given to mice approximately 15 to 20 minutes before DTEGF13. Experiments were then performed to determine whether the maximal tolerated dose (MTD) was reduced and whether we were still able to eliminate progression of aggressive human, metastatic, pancreatic cancer induced by orthotopic injection (OT) in nude mice.

RESULTS

ToxBloc permitted us to safely exceed the DTEGF13 maximal tolerated dose by 15-fold. This approach permitted repetitive high dosing with the bispecific ligand-directed toxin resulting in tumor regression (P < 0.01). Tumor effects were documented using a tumor imaging model in which OT tumor growth was monitored noninvasively in real time. ToxBloc was selective because other bispecific peptides did not block.

CONCLUSIONS

ToxBloc represents a new method of drug delivery and a potential solution to the problem of toxicity.

Targeted immunotherapy for colorectal cancer: monoclonal antibodies and immunotoxins

Colorectal cancer (CRC) is a major health concern worldwide. It is the third most frequently diagnosed cancer and the second leading cause of cancer death. There currently are a number of treatment options for CRC, however many of them have failed to demonstrate desired therapeutic benefit. Therefore, significant efforts are being directed towards the development of new biological therapies with improved efficacy. Immunotherapy is an emerging treatment modality for a variety of cancers. Several promising treatments have already been approved by the US FDA and are being tested in clinical trials. Antibodies have been proved to be useful in cancer therapy due to their ability to recognize tumor-associated antigens expressed at higher density on malignant cells in comparison with those that are normal. Antibodies can be used as a single therapy or in combination with other therapies. A large variety of monoclonal antibodies have been developed. However, only a very few are able to kill a sufficient number of malignant cells and cause tumor regression. Hence, it is often necessary to arm the antibody with a cytotoxic agent to enhance the efficacy of the anti-tumor activity. This review provides a brief overview of some of the current agents being employed in targeted immunotherapy for CRC.

A bispecific enediyne-energized fusion protein containing ligand-based and antibody-based oligopeptides against epidermal growth factor receptor and human epidermal growth factor receptor 2 shows potent antitumor activity

PURPOSE

The cooverexpression of epidermal growth factor receptor (EGFR) and human epidermal growth factor receptor 2 (HER2) observed in many human tumors and their synergistic interaction in the transformation of cells make these receptors important targets for the development of new targeted therapeutics. Targeting of EGFR and HER2 simultaneously has been pursued as a strategy with which to potentially increase efficiency and selectivity in therapy of certain cancers. This study was set to construct a bispecific energized fusion protein (Ec-LDP-Hr-AE) consisting of two oligopeptides against EGFR and HER2, and lidamycin, and investigate its antitumor efficacy.

EXPERIMENTAL DESIGN

In vitro experiments measured the binding and internalization of bispecific Ec-LDP-Hr fusion protein. The potency of energized fusion proteins was also done in which the bispecific Ec-LDP-Hr-AE was compared with lidamycin (LDM) and its monospecific counterparts, Ec-LDP-AE and LDP-Hr-AE. In vivo, Ec-LDP-Hr-AE was given i.v. to nude mice bearing human ovarian carcinoma SK-OV-3 xenografts.

RESULTS

Binding and internalization studies showed that bispecific fusion protein Ec-LDP-Hr bound to carcinoma cells specifically and then were internalized into the cytoplasm. Bispecific Ec-LDP-Hr-AE was more potent and selective in its cytotoxicity against different carcinoma cell lines than corresponding momospecific agents and LDM in vitro. In addition, Ec-LDP-Hr-AE significantly inhibited the growth of SK-OV-3 xenografts in nude mouse model. In vivo imaging study showed that FITC-labeled Ec-LDP-Hr was targeted and accumulated in the tumors.

CONCLUSION

A ligand-based and an antibody-based oligopeptide fused to the enediyne antibiotic LDM created a new bispecific fusion protein with low molecular weight and more potent in vitro and in vivo antitumor activity (than momospecific fusion proteins).

Intracranial elimination of human glioblastoma brain tumors in nude rats using the bispecific ligand-directed toxin, DTEGF13 and convection enhanced delivery

A bispecific ligand-directed toxin (BLT) consisting of human interleukin-13, epithelial growth factor, and the first 389 amino acids of diphtheria toxin was assembled in order to target human glioblastoma. In vitro, DTEGF13 selectively killed the human glioblastoma cell line U87-luc as well as other human glioblastomas. DTEGF13 fulfilled the requirement of a successful BLT by having greater activity than either of its monospecific counterparts or their mixture proving it necessary to have both ligands on the same single chain molecule. Aggressive brain tumors established intracranially (IC) in nude rats with U87 glioma genetically marked with a firefly luciferase reporter gene were treated with two injections of DTEGF13 using convection enhanced delivery resulting in tumor eradication in 50% of the rats which survived with tumor free status at least 110 days post tumor inoculation. An irrelevant BLT control did not protect establishing specificity. The bispecific DTEGF13 MTD dose was measured at 2 microg/injection or 0.5 microg/kg and toxicity studies indicated safety in this dose. Combination of monospecific DTEGF and DTIL13 did not inhibit tumor growth. ELISA assay indicated that anti-DT antibodies were not generated in normal immunocompetent rats given identical intracranial DTEGF13 therapy. Thus, DTEGF13 is safe and efficacious as an alternative drug for glioblastoma therapy and warrants further study.

Single-chain antibody/activated BID chimeric protein effectively suppresses HER2-positive tumor growth

BH3-interacting domain death agonist (BID) is a crucial element in death signaling pathways and is recognized as an intracellular link connecting the intrinsic mitochondrial apoptotic and extrinsic death receptor-mediated apoptotic pathways. Herein, we describe experiments conducted with a fusion protein, which was generated by fusing a human epidermal growth factor receptor-2 (HER2)-specific single-chain antibody with domain II of Pseudomonas exotoxin A and the truncated active BID (tBID). These experiments extend our previous work on several other immuno-proapoptotic proteins. Specifically, by excluding cells with undetectable HER2, we showed that the secreted immuno-tBID molecule selectively recognized and killed HER2-overexpressing tumor cells in vitro by attacking their mitochondria and inducing their apoptotic death. This apoptosis could only be inhibited partially by caspase pan-inhibitor zVAD and mitochondrial protector TAT-BH4. Subsequently, we transferred the immuno-tbid gene into BALB/c athymic mice bearing HER2-positive tumors together with other immuno-proapoptotic proteins using i.m. injections of liposome-encapsulated vectors. The expression of the immuno-tbid gene suppressed tumor growth and prolonged animal survival significantly. We also shortened the translocation domain of Pseudomonas exotoxin A II to only 10-amino acid sequence, which were crucial for furin cleavage. The new recombinant molecule retained the translocation efficiency and the ability of specific killing HER2-positive tumor cells. Our data showed that, compared with the toxins employed before, the chimeric immuno-tBID molecule can not only specifically recognize HER2-positive tumor cells but also certainly induce apoptosis even in the presence of zVAD and TAT-BH4, thereby suggesting an alternative approach to treating HER2/neu-positive tumors.

Antibody internalization studied using a novel IgG binding toxin fusion

Targeted therapy encompasses a wide variety of different strategies, which can be divided into direct or indirect approaches. Direct approaches target tumor-associated antigens by monoclonal antibodies (mAbs) binding to the relevant antigens or by small-molecule drugs that interfere with these proteins. Indirect approaches rely on tumor-associated antigens expressed on the cell surface with antibody-drug conjugates or antibody-based fusion proteins containing different kinds of effector molecules. To deliver a lethal cargo into tumor cells, the targeting antibodies should efficiently internalize into the cells. Similarly, to qualify as targets for such drugs newly-discovered cell-surface molecules should facilitate the internalization of antibodies that bind to them. Internalization can be studied be several biochemical and microscopy approaches. An undisputed proof of internalization can be provided by the ability of an antibody to specifically deliver a drug into the target cells and kill it. We present a novel IgG binding toxin fusion, ZZ-PE38, in which the Fc-binding ZZ domain, derived from Streptococcal protein A, is linked to a truncated Pseudomonas exotoxin A, the preparation of complexes between ZZ-PE38 and IgGs that bind tumor cells and the specific cytotoxicity of such immunocomplexes is reported. Our results suggest that ZZ-PE38 could prove to be an invaluable tool for the evaluation of the suitability potential of antibodies and their cognate cell-surface antigens to be targeted by immunotherapeutics based on armed antibodies that require internalization.

Immunotoxins for targeted cancer therapy

Immunotoxins are proteins that contain a toxin along with an antibody or growth factor that binds specifically to target cells. Nearly all protein toxins work by enzymatically inhibiting protein synthesis. For the immunotoxin to work, it must bind to and be internalized by the target cells, and the enzymatic fragment of the toxin must translocate to the cytosol. Once in the cytosol, 1 molecule is capable of killing a cell, making immunotoxins some of the most potent killing agents. Various plant and bacterial toxins have been genetically fused or chemically conjugated to ligands that bind to cancer cells. Among the most active clinically are those that bind to hematologic tumors. At present, only 1 agent, which contains human interleukin-2 and truncated diphtheria toxin, is approved for use in cutaneous T-cell lymphoma. Another, containing an anti-CD22 Fv and truncated Pseudomonas exotoxin, has induced complete remissions in a high proportion of cases of hairy-cell leukemia. Refinement of existing immunotoxins and development of new immunotoxins are underway to improve the treatment of cancer.

A universal strategy for stable intracellular antibodies

The expression of intracellular antibodies (intrabodies) in mammalian cells has provided a powerful tool to manipulate microbial and cellular signalling pathways in a highly precise manner. However, several technical hurdles have thus far restricted their more widespread use. In particular, single-chain antibodies (scFvs) have been reported to fold poorly in the reducing environment of the cytoplasm and as such there has been a reluctance to use scFv-phage libraries as a source of intrabodies unless a preselection step was applied to identify these rare scFvs that could fold properly in the absence of disulfide bonds. Recently, we reported that scFvs can be efficiently expressed within the cytoplasm of bacteria when fused at the C-terminus of the Escherichia coli maltose-binding protein (MBP). Here, we demonstrate that such MBP-scFvs are similarly stabilized when expressed in the mammalian cell cytoplasm as well as other compartments. This was demonstrated by comparing MBP-scFv fusions to the corresponding unfused scFvs that activate a defective beta-galactosidase enzyme, others that neutralize the wild-type beta-galactosidase enzyme, and an antibody that blocks the epidermal growth factor receptor. In all cases, the MBP-scFvs significantly outperformed their unfused counterparts. Our results suggest that fusion of scFvs to MBP, and possibly to other "chaperones in the context of a fusion protein", may provide a universal approach for efficient expression of intrabodies in the mammalian cell cytoplasm. This strategy should allow investigators to bypass much of the in vitro scFv characterization that is often not predictive of in vivo intrabody function and provide a more efficient use of large native and synthetic scFv-phage libraries already in existence to identify intrabodies that will be active in vivo.

Recombinant toxins in haematologic malignancies and solid tumours

Recombinant toxins constitute a new modality for the treatment of cancer, since they target cells displaying specific surface-receptors or antigens. They are fusion proteins, which contain toxin and ligand regions, and are produced in Escherichia coli. The ligand may be a growth factor or a fragment of an antibody, and the toxin is usually one of the two bacterial toxins: Pseudomonas exotoxin and diphtheria toxin. Compared to the earlier generation chemical conjugates of ligands and toxins, recombinant toxins have many advantages, including homogeneity with respect to the connection between the ligand and toxin, ease and yield of production and small size. A variety of chemotherapy-resistant haematologic and solid tumours have been targeted with recombinant toxins, and clinical trials with many of them have recently demonstrated their effectiveness. Moreover, their unwanted toxic effects are different from those of most chemotherapeutic agents, supporting the expectation that they can be combined with existing modalities to improve the clinical resources available to treat cancer in humans.

A diabody that dissociates to monomer forms at low concentration: effects on binding activity and tumor targeting

A human scFv, 15-9, was selected from a phage display library for binding to murine laminin-1. A diabody was made from the scFv by shortening the linker from 15 to 5 amino acids between the VH and VL sequence. Radioiodinated scFv and diabody were analyzed for size, binding to laminin, and biodistribution in tumor bearing mice. Diabody preparations at concentrations greater than 10 nM were largely dimer forms (approximately 60 kDa) as judged by gel filtration, but diluted diabody was eluted as a monomer (approximately 30 kDa). At low concentrations the radiolabeled diabody did not bind well to laminin. The (125)I diabody had significantly lower accumulation in tumors than did the scFv when injected at lower concentrations. These data indicate that the diabody dimer dissociates at concentrations of about 10nM resulting in monomers with no binding activity for laminin and poor tumor homing properties.

Immunoscreening protocols for the identification of clinically useful antibodies and antigens

The antigen-antibody interaction is a powerful tool for the immuno-screening of several diseases, including cancer and genetic disorders. The high specificity of monoclonal antibodies (mAbs) enables them to target antigens and form complexes that can be detected with enzymes, radionuclides, fluorescent dyes or other markers. The antibody molecule, which has an antigen binding site, can be used as an intact molecule or as a fragment, for example, F(ab)(2), Fab, Fv or scFv. Similarly, the antigen can also be varied. In this review, immuno-screening techniques that can be used to detect clinically relevant antibody-antigen interactions will be discussed.

Antibody-directed therapy for human hepatocellular carcinoma

The goals of our research are to develop high-affinity and high-stability antibodies and fragments thereof for targeting tumor-specific antigens in an attempt to develop new therapeutic agents for human hepatocellular carcinoma (HCC). Tumor-associated antigens are excellent targets for drug and gene delivery, and offer the advantage of high cellular specificity. We have explored the use of a monoclonal antibody (Mab) AF-20 raised against a human hepatoma cell line (FOCUS) as a model system. This antibody binds to a 180-kDa homodimeric cell surface glycoprotein with high affinity. The antigen is uniformly expressed in HCC-derived cell lines and human tumors, including those with distant metastasis. There is minimal expression in nontumor tissues, and none detectable in normal liver. Because the AF-20 antigen antibody interactions on the cell surface is rapidly internalized at 37 degrees C, there is an opportunity to deliver cytotoxic payloads to tumor cells. In addition, high-affinity single-chain monoclonal antibody fragments (scFv) have been created using a novel yeast display system. Drug conjugates with AF-20 monoclonal antibodies have been prepared for gene targeting of HCC both in vitro and in vivo using preclinical animal model systems. These studies show that it is possible to generate high-affinity intact scFv antibody fragments that will allow specific tumor targeting of adenoviruses containing suicide genes, chemotherapeutic agents such as methotrexate, and cytotoxic peptides to produce antitumor effects. Therefore, specific antibody targeting of antitumor agents to HCC cells has the potential for therapeutic application in this devastating disease.

A urokinase-activated recombinant diphtheria toxin targeting the granulocyte-macrophage colony-stimulating factor receptor is selectively cytotoxic to human acute myeloid leukemia blasts

Novel agents to treat acute myeloid leukemia (AML) are needed with increased efficacy and specificity. We have synthesized a dual-specificity fusion toxin DTU2GMCSF composed of the catalytic and translocation domains of diphtheria toxin (DT) fused to the granulocyte-macrophage colony-stimulating factor (GM-CSF) in which the DT furin cleavage site 163RVRRSV170 is modified to a urokinase plasminogen activator (uPA) cleavage site 163GSGRSA170, termed U2. DTU2GMCSF was highly toxic to the TF1-vRaf AML cell line (proliferation inhibition assay; IC50 = 3.14 pM), and this toxicity was greatly inhibited following pretreatment with anti-uPA and anti-GM-CSF antibodies. The activity of this toxin was then tested on a larger group of 13 human AML cell lines; 5 of the 13 cell lines were sensitive to DTU2GMCSF. An additional 5 of the 13 cell lines became sensitive when exogenous pro-uPA was added. Sensitivity to DTU2GMCSF strongly correlated with the expression levels of uPA receptors (uPARs) and GM-CSF receptors (GM-CSFRs) as well as with total uPA levels. DTU2GMCSF was less toxic to normal cells expressing uPAR or GMCSFR alone, that is, human umbilical vein endothelial cells and peripheral macrophages, respectively. These results indicate that DTU2GMCSF may be a selective and potent agent for the treatment of patients with AML. (Blood. 2004;104:2143-2148)

Recombinant single-chain antibody fusion construct targeting human melanoma cells and containing tumor necrosis factor

Fusion constructs targeting tumor cells have significant potential applications against both solid tumors and hematologic malignancies. We developed a fusion construct of tumor necrosis factor (TNF) and a single-chain antibody (scFvMEL) recognizing the gp240 antigen on human melanoma cells. The scFvMEL/TNF construct, like TNF itself, was found to exist in solution primarily as a trimer of 45 kDa monomers (trimeric molecular weight = 135 kDa). The fusion construct bound specifically to gp240 antigen-positive but not to antigen-negative cells. The TNF component of the construct was biologically active (specific activity = 1 x 10(7) U/mg) compared with free TNF (specific activity = 2.6 x 10(7) U/mg) and was more cytotoxic to antigen-positive A375-M melanoma cells (IC(50) = 100 pM) than TNF alone (IC(50) = 1,000 pM) and, additionally, was active against AAB-527 melanoma cells (IC(50) = 20 nM) resistant to TNF itself (IC(50) > 1,000 nM). The augmented cytotoxicity was mediated by antibody-specific binding to the cell surface. Both A375-M and AAB-527 cells were shown to express TNFR1 and TNFR2 on the cell surface. The TNF moiety of the fusion construct was efficiently delivered into cells in time-dependent increase in cytosol as assessed by immunofluorescent staining of human melanoma cells. Radiolabeled scFvMEL/TNF localized effectively in human melanoma xenografts in nude (nu/nu) mice with a tumor:blood ratio of approximately 8 at 72 hr after administration. Our studies suggest that because of its unique biologic activity and low antigenic potential, scFvMEL/TNF makes an excellent cytotoxic protein for potential clinical treatment of human melanoma.

HER-2 in breast cancer--methods of detection, clinical significance and future prospects for treatment

The Human Epidermal Growth Factor (HER-2) oncogene encodes a transmembrane tyrosine kinase receptor with extensive homology to the Epidermal Growth Factor Receptor (EGFR) which is the prototypal member of this family of receptor tyrosine kinases. HER-2 gene amplification is found in 20-30% of breast cancers. Various methods such as immunohistochemistry, southern and slot blotting, enzyme immunoassays and fluorescence in situ hybridization have all been employed to evaluate HER-2 gene and protein abnormalities. Of these immunohistochemistry is the most frequently employed but there are valid indications for the other avaliable methods. However, it is prudent that whichever methods employed are standardized, especially those that possess may have a degree of subjectivity in their assesment.

Cell-type specific targeting and gene expression using a variant of polyoma VP1 virus-like particles

The variant VP1-Z of the polyomavirus coat protein VP1 has been recently described as an engineered fusion protein of VP1 and the antibody binding domain protein Z. This construct is able to specifically bind and functionally present antibodies on the surface of virus-like particles of VP1-Z. Here we demonstrate that with the binding of Herceptin, an antibody directed against the receptor tyrosine kinase ErbB2, a cell type-specific targeting was established. ErbB2-positive cell lines were transduced with different plasmids encoding eGFP or beta-galactosidase. With both reporter systems functional gene expression in transduced cells could be observed. The transduction was strictly dependent on the use of a ternary complex formed of VLPs of VP1-Z, Herceptin, and the reporter plasmid DNA. The use of single components or ErbB2-negative cell lines did not result in functional gene transfer. The transduction was also completely dependent on the use of chloroquine, a lysosomotropic reagent. This indicates that the complex is internalized by ErbB2-mediated endocytosis.

Blockade of growth factor receptors in ductal carcinoma in situ inhibits epithelial proliferation

BACKGROUND

Ductal carcinoma in situ (DCIS) expresses c-erbB-2 receptor and epidermal growth factor receptor (EGFR). The aim of this study was to determine whether blocking of c-erbB-2 receptor with a humanized monoclonal antibody, 4D5 (HerceptinTM), or of EGFR with an epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI), ZD1839 (IressaTM), would decrease epithelial proliferation in DCIS.

METHODS

DCIS tissue from 18 women undergoing surgery was implanted into 16 to 20 athymic nude mice per experiment (eight xenografts per mouse). Treatment commenced 2 weeks after implantation and consisted either of twice-weekly intraperitoneal injections of 4D5 10 mg/kg or of daily gavage with ZD1839 at 100-200 mg/kg for 14 days; appropriate controls were included. Xenografts were removed on days 14, 21 and 28. Proliferation was assessed by counting 1000 epithelial cells after Ki67 immuno- staining.

RESULTS

ZD1839 inhibited proliferation compared with that in controls after 14 days (P < 0.01), whereas 4D5 did not.

CONCLUSION

Proliferation in DCIS was decreased by EGFR tyrosine kinase inhibition but not by c-erbB-2 receptor blockade. ZD1839, an orally active and selective EGFR-TKI, has potential as adjuvant therapy in DCIS.

Sensitivity of c-erbB positive cells to a ligand toxin and its utility in purging breast cancer cells from peripheral blood stem cell (PBSC) collections

Autografting following high-dose conditioning is being increasingly offered to breast cancer sufferers, without due regard to the reinfusion of malignant cells. We sought to determine if a breast cancer cell line could be successfully purged from peripheral blood stem cell (PBSC) harvests using a ligand-toxin molecule directed to heregulin-activated erbB receptors. Initial experiments demonstrated no reduction in hemopoietic colony-forming ability in the presence of ligand toxin (2 nM). Breast cancer cell lines which demonstrated differing sensitivities to the ligand toxin were subsequently seeded into stem cell collections and incubated with 2 nM ligand-toxin. One cell line, ZR-75-1, was completely sensitive to the ligand toxin in this mixture; a second, MDB-MA-361, was more profoundly sensitive to the ligand toxin in the presence of the PBSC, whereas a third was unaffected by the toxin. These results suggest purging may indeed be possible in the PBSC of breast cancer patients, but the parameters that define sensitivity are as yet unknown.

Tyrosine kinase inhibitors targeted to the epidermal growth factor receptor subfamily: role as anticancer agents

Abnormal cell signal transduction arising from protein tyrosine kinases has been implicated in the initiation and progression of a variety of human cancers. Over the past 2 decades pharmaceutical and university laboratories have been involved in a tremendous effort to develop compounds that can selectively modulate these abnormal signalling pathways. Targeting receptor tyrosine kinases, especially the epidermal growth factor receptor subfamily, has been at the forefront of this effort as a result of strong clinical data correlating over-expression of these receptors with more aggressive cancers. There are a variety of strategies under development for inhibiting the kinase activity of these receptors, targeting both the extracellular and intracellular domains. Antibody-based approaches, immunotoxins and ligand-binding cytotoxic agents use the extracellular domain for targeted tumour therapy. Small molecule inhibitors target the intracellular catalytic region by interfering with ATP binding, while nonphosphorylatable peptides are aimed at the intracellular substrate binding region. Compounds that inhibit subsequent downstream signals from the receptor by interrupting intracellular protein recognition sequences are also being investigated. In the past 5 years enormous progress has been made in developing tyrosine kinase inhibitor compounds with sufficient potency, bioavailability and selectivity against this subfamily of receptor tyrosine kinases. The anti-HER2 monoclonal antibody, trastuzumab, for patients with metastatic breast cancer is the first of these inhibitor compounds to gain FDA approval. However, preclinical and clinical trials are ongoing with a variety of other monoclonal antibodies, immunotoxins, and small molecule quinazoline and pyrimidine-based inhibitors. Although their cytotoxic and cytostatic potential has been proven, they are not likely to replace standard chemotherapy regimens as single-agent, first-line therapeutics. Instead, their promising additive and synergistic antitumour effects in combination with standard chemotherapeutics suggest that these novel agents will find their greatest utility and efficacy in conjunction with existing anticancer agents.

Clinical relevance of erbB-1 and -2 oncogenes in oral carcinomas

To gain a better understanding of molecular changes in oral squamous cell carcinomas, we tested fresh tumour specimens from 110 patients for erbB-1 and -2 oncogene aberrations using the competitive differential polymerase chain reaction. The significance of established tumour characteristics such as TNM stage, differentiation and oncogene aberrations for tumour progression were analyzed. ErbB-2 amplification with a gene copy number > 1.6 in tumour tissue and erbB-1 deletion with a gene copy number < 0.4 in tumour-surrounding mucosa are of clinical relevance and indicate an early tumour recurrence or metastasis (p < 0.05). In T1/T2 tumours an erbB-2 gene dosage study allows differentiation between tumours with high or low risk for early progression. In a multivariate statistical analysis T stage (p < 0.01) and erbB-2 amplification in tumour material (p < 0.05) were independent prognostic variables.

A tumor specific single chain antibody dependent gene expression system

The design of conditional gene expression systems restricted to given tissues or cellular types is an important issue of gene therapy. Systems based on the targeting of molecules characteristic of the pathological state of tissues would be of interest. We have developed a synthetic transcription factor by fusing a single chain antibody (scFv) directed against p53 with the bacterial tetracycline repressor as a DNA binding domain. This hybrid protein binds to p53 and can interact with a synthetic promoter containing tetracycline-operator sequences. Gene expression can now be specifically achieved in tumor cells harboring an endogenous mutant p53 but not in a wild-type p53 containing tumor cell line or in a non-transformed cell line. Thus, a functional transactivator centered on single chain antibodies can be expressed intracellularly and induce gene expression in a scFv-mediated specific manner. This novel class of transcriptional transactivators could be referred as 'trabodies' for transcription-activating-antibodies. The trabodies technology could be useful to any cell type in which a disease related protein could be the target of specific antibodies.

Bispecific antibodies as novel bioconjugates

Bispecific antibodies are unique macromolecular heterobifunctional cross-linkers with two different binding specificities within a single molecule. As ideal bioconjugates, they can specifically glue any two different molecules together without the need for chemical conjugation. With this unique feature, they have immense potential in biological and immunological fields. Their applications range from immunohistochemistry, immunoassays, radioimmunodiagnosis, radioimmunotherapy, and immunotherapy. Recently, a new second generation of bispecific molecules, bispecific single chain Fv and diabodies, has been produced by DNA recombinant technology. They can be considered as the ultimate magic bullets for in vivo applications. They may theoretically improve tumor or pathogen targeting and minimize side effects, eventually replacing the full-length bispecific antibodies. Emphasizing on developmental methodology and clinical applications of bispecific antibodies, this review gives a bird's-eye view of these unique bioconjugates.

ERBB2 oncogene in human breast cancer and its clinical significance

We reveiwed the relationships between ERBB2 amplification and/or overexpression in human breast cancer and the clinicopathological parameters described in the literature (97 studies involving 22,616 patients) in order to draw conclusions regarding its clinical interest. The mean of ERBB2 positivity (26%, ranging from 5 to 55%) is not dependent on the method used to evaluate ERBB2 amplification or overexpression. Despite the discrepancies observed between the different studies, several associations between ERBB2 positivity and the classical clinicopathological parameters were noted. There are clear relationships between ERBB2 positivity and the lack of steroid receptors, the histological subtypes of mammary tumours (ductal invasive and in situ), worse histological and nuclear grades, aneuploidy and high rate of proliferation. In univariate analyses, ERBB2 is strongly associated with poor prognosis. All these data indicate that ERBB2 is a marker of aggressiveness of the tumour. However, ERBB2 does not retain a clinical prognostic significance in multivariate analyses, since it is associated with several strong prognostic parameters. When considering the prognostic value of ERBB2 in relation to treatment, a significantly worse survival of the treated patients is noted in ERBB2 positive patients. This suggest that ERBB2 could be a marker of reduced response to chemotherapy and hormonal treatment. With respect to the tumour response to treatment, the results, provided as yet by pilot studies, remain controversial and further investigations are necessary to evaluate the predictive value of ERBB2. Finally, new therapeutic approaches targeting the cells overexpressing ERBB2 have been developed.

Immunotoxins for targeted cancer therapy

Immunotoxins constitute a new modality for the treatment of cancer, since they target cells displaying specific surface-receptors or antigens. Immunotoxins contain a ligand such as a growth factor, monoclonal antibody, or fragment of an antibody which is connected to a protein toxin. After the ligand subunit binds to the surface of the target cell, the molecule internalizes and the toxin kills the cell. Bacterial toxins which have been targeted to cancer cells include Pseudomonas exotoxin and diphtheria toxin, which are well suited to forming recombinant single-chain or double-chain fusion toxins. Plant toxins include ricin, abrin, pokeweed antiviral protein, saporin and gelonin, and have generally been connected to ligands by disulfide-bond chemistry. Immunotoxins have been produced to target hematologic malignancies and solid tumors via a wide variety of growth factor receptors and antigens. Challenges facing the clinical application of immunotoxins are discussed.

Expression of an oncogenic mutant EGF receptor markedly increases the sensitivity of cells to an EGF-receptor-specific antibody-toxin

EGFRvIII is a ligand-independent, constitutively active variant of the epidermal growth factor receptor (EGFR) that is specifically expressed in gliomas and various other human malignancies and has been proposed as a target for directed tumor therapy. We have recently constructed a highly potent single-chain antibody-toxin, scFv(14E1)-ETA, which consists of the variable domains of the antibody 14E1 specific for human full-length EGFR genetically fused to a truncated form of Pseudomonas exotoxin A. We demonstrate here binding of 14E1 antibody to both full-length and variant EGFR. In contrast to a recombinant toxin containing transforming growth factor-alpha (TGF-alpha) as a cell targeting domain, scFv(14E1)-ETA was highly active on cells expressing EGFRvIII. Surprisingly, scFv(14E1)-ETA displayed cell killing activity on EGFRvIII-expressing cells that was up to 100-fold higher than on control cells expressing full-length EGFR. No differences in the binding affinities of scFv(14E1)-ETA to full-length EGFR or EGFRvIII were observed, suggesting that events downstream of immunotoxin binding are responsible for the increased sensitivity of EGFRvIII-expressing cells. This might have implications for the development of therapeutic reagents simultaneously targeting different forms of the EGFR.

Novel therapeutic strategies to selectively kill cancer cells

Tumor invasion, metastasis, and resistance to chemotherapeutic drugs or radiation are major obstacles for the successful treatment of cancer. To overcome some of these limitations, therapeutic strategies that increase the specificity and efficacy and reduce the toxicity of the anti-cancer drugs or toxins are being explored. Cancer cells overexpress specific protein antigens and carbohydrate structures that may function as cell surface receptors. These cancer cell specific markers can be exploited while designing new cancer therapies. Monoclonal antibodies that have been humanized to reduce immunogenicity and targeted to specific antigens on cancer cells, enzyme-monoclonal antibody/prodrug conjugates that will selectively kill the target cells following drug activation, and recombinant toxins are some of the novel classes of agents in development. Another novel approach being investigated to treat cancers is the use of inactive pore-forming toxins with built-in biological "triggers" that will activate the toxin following a biological stimulus. These pore-forming cytolytic toxins can be rendered active by tumor-specific proteases, that are often overexpressed in cancer cells, thereby targeting the toxic effects. Such pore-forming or membrane-acting toxins may serve as novel cytolytic agents against solid tumors, which, to date, have proved to be more resistant to conventional toxins.

Targeted therapy of schwannoma cells in immunocompetent rats with an erbB2-specific antibody-toxin

Over-expression of the erbB2-receptor tyrosine kinase is frequently observed in many human tumors of epithelial origin. Due to its causal involvement in malignant transformation and its presence on the tumor cell surface erbB2 is an attractive target for directed tumor therapy. We earlier described the potent anti-tumoral activity of the recombinant single-chain antibody toxin scFv(FRP5)-ETA in vitro and in nude mouse tumor models in vivo. This molecule consists of the variable domains of the heavy and light chains of an erbB2-specific antibody genetically fused to a truncated Pseudomonas exotoxin A. Here we have investigated the in vivo effects of this immunotoxin on erbB2 expressing NV2Cd schwannoma cells growing as s.c. tumors in syngeneic BDIX rats. Established tumors were treated either locally by intra-tumoral injection of scFv(FRP5)-ETA or systemically by injection into the tail vein. Both routes of application resulted in pronounced inhibition of tumor growth with local treatment being more effective. Treatment with 25 micrograms/day of scFv(FRP5)-ETA for 10 days suppressed tumor growth almost completely. Antibodies directed mainly against the toxin domain of the fusion protein developed in all animals treated.

New protein engineering approaches to multivalent and bispecific antibody fragments

Multivalency is one of the hallmarks of antibodies, by which enormous gains in functional affinity, and thereby improved performance in vivo and in a variety of in vitro assays are achieved. Improved in vivo targeting and more selective localization are another consequence of multivalency. We summarize recent progress in engineering multivalency from recombinant antibody fragments by using miniantibodies (scFv fragments linked with hinges and oligomerization domains), spontaneous scFv dimers with short linkers (diabodies), or chemically crosslinked antibody fragments. Directly related to this are efforts of bringing different binding sites together to create bispecific antibodies. For this purpose, chemically linked fragments, diabodies, scFv-scFv tandems and bispecific miniantibodies have been investigated. Progress in E. coli expression technology makes the amounts necessary for clinical studies now available for suitably engineered fragments. We foresee therapeutic advances from a modular, systematic approach to optimizing pharmacokinetics, stability and functional affinity, which should prove possible with the new recombinant molecular designs.

Intra-tumoral application of a heregulin-exotoxin-a fusion protein causes rapid tumor regression without adverse systemic or local effects

Tumor toxins are recombinant, bifunctional proteins which comprise a tumor-cell-specific recognition domain and an enzymatic toxin domain. We have evaluated the in vivo effects of a tumor toxin that specifically recognizes the erbB-3 and erbB-4 receptors (HRG beta 1-ETA). High doses of HRG beta 1-ETA administered systemically (intracardially or intraperitoneally) caused acute liver necrosis and were lethal. The same dose of tumor toxins applied subcutaneously had no detectable histopathological effects. The anti-tumor activity of HRG beta 1-ETA was tested in nude mice with xenografts of a human breast tumor, MAXF1162. The MAXF1162 tumor grew rapidly upon s.c. implantation. Intra-tumoral application of HRG beta 1-ETA (7 times 5 micrograms over a period of 21 days) induced complete regression of tumors. At the time the treatment was terminated, no tumor cells were detectable microscopically. Evaluation of the liver of treated animals revealed no significant toxicity in the effective dose range. These experiments indicate that tumor toxins can become valuable for local tumor treatment and for reduction of tumor burden.

Construction and functional characterization of scFv(14E1)-ETA - a novel, highly potent antibody-toxin specific for the EGF receptor

Epidermal growth factor (EGF) receptor-overexpression is characteristic of many human tumours of epithelial origin and has been correlated with unfavourable patient prognosis. Its involvement in the malignant process, its elevated expression in tumours and its accessibility on the tumour cell surface make the EGF receptor a potential target for directed tumour therapy. We have previously characterized a recombinant antibody - Pseudomonas exotoxin A fusion protein, scFv(225)-ETA, which displayes antitumoral activity towards EGF receptor-overexpressing tumour cells but is less potent in tumour cell killing than TGF-alpha-ETA, a recombinant toxin using the natural EGF receptor ligand transforming growth factor alpha (TGF-alpha) as a targeting domain. Here, we describe the construction and functional characterization in vitro of a novel single-chain antibody-toxin, scFv(14E1)-ETA, based on the independently isolated EGF receptor-specific monoclonal antibody 14E1. ScFv(14E1)-ETA binds to an EGF receptor epitope that is very similar or identical to that of scFv(225)-ETA with nine times higher affinity than the latter and displays more than tenfold higher cytotoxic activity on EGF receptor-overexpressing tumour cells. ScFv(14E1)-ETA cell killing activity was very similar to that of TGF-alpha-ETA on receptor-overexpressing cells but, in contrast to the latter, scFv(14E1)-ETA was much more selective and did not display significant cytotoxic activity on cells expressing moderate EGF receptor levels.

Targeted inhibition of tumour cell growth by a bispecific single-chain toxin containing an antibody domain and TGF alpha

Overexpression of the epidermal growth factor receptor (EGFR) and ErbB-2 has been observed in a variety of human tumours, making these receptors promising targets for directed tumour therapy. Since many tumour cells express both ErbB-2 and EGFR and these receptors synergise in cellular transformation, therapeutic reagents simultaneously binding to ErbB-2 and EGFR might offer advantages for tumour therapy. We have previously described the potent anti-tumoral activity of a bispecific antibody toxin that contains ErbB-2- and EGFR-specific single-chain Fv (scFv) domains. Here we report the construction and functional characterisation of a novel bispecific recombinant toxin, scFv(FRP5)-TGF alpha-ETA. The fusion protein consists of the antigen-binding domain of the ErbB-2-specific MAb, FRP5, and the natural EGFR ligand, TGF alpha, inserted at different positions in truncated Pseudomonas exotoxin A. ScFv(FRP5)-TGF alpha-ETA protein displayed binding to EGFR and ErbB-2, thereby inducing activation of the receptors, which was dependent on the cellular context and the level of EGFR and ErbB-2 expression. The bispecific molecule was cytotoxic in vitro for tumour cells expressing various levels of the target receptors. In vivo scFv(FRP5)-TGF alpha-ETA potently inhibited the growth of established A431 tumour xenografts in nude mice.