Targeted therapy of cancer using diphtheria toxin-derived immunotoxins

Nano- and Micro-Platforms in Therapeutic Proteins Delivery for Cancer Therapy: Materials and Strategies

Proteins have emerged as promising therapeutics in oncology due to their great specificity. Many treatment strategies are developed based on protein biologics, such as immunotherapy, starvation therapy, and pro-apoptosis therapy, while some protein biologics have entered the clinics. However, clinical translation is severely impeded by instability, short circulation time, poor transmembrane transportation, and immunogenicity. Micro- and nano-particles-based drug delivery platforms are designed to solve those problems and enhance protein therapeutic efficacy. This review first summarizes the different types of therapeutic proteins in clinical and research stages, highlighting their administration limitations. Next, various types of micro- and nano-particles are described to demonstrate how they can overcome those limitations. The potential of micro- and nano-particles are then explored to enhance the therapeutic efficacy of proteins by combinational therapies. Finally, the challenges and future directions of protein biologics carriers are discussed for optimized protein delivery.

Advances in immunotoxin engineering: precision therapeutic strategies in modern oncology

Immunotoxins (ITs) are specialized therapeutic agents designed for targeted treatment, particularly in cancer therapy. They consist of a monoclonal antibody or antibody fragment linked to a potent cytotoxic agent, such as bacterial- or plant-derived toxins like diphtheria toxin, ricin, or pseudomonas exotoxin. The monoclonal antibody component specifically binds to antigens expressed on the surface of target cells, facilitating the internalization of the IT. Once inside the cell, the cytotoxic agent is released, disrupting essential cellular processes and leading to cell death. This targeted approach minimizes damage to healthy tissues while effectively eliminating diseased cells. The production of ITs involves two primary methods: recombinant fusion and chemical conjugation. In recombinant fusion, genetic engineering is used to create a fusion protein that combines the antibody and toxin, ensuring precise control over their ratio and functionality. In chemical conjugation, pre-existing antibodies are chemically linked to toxins, allowing for greater flexibility in combining different antibodies and cytotoxic agents. Each method has its advantages and challenges, influencing the specificity, production complexity, and therapeutic potential of the resulting ITs. As research advances, ITs continue to show promise not only in oncology but also in treating other diseases, including inflammatory conditions and atherosclerosis. The precise targeting and potent effects of ITs make them a valuable tool in the development of new therapeutic strategies.

Novel Oxygen-Dependent Degradable Immunotoxin Regulated by the Ubiquitin-Proteasome System Reduces Nonspecific Cytotoxicity

The use of bacterial toxins as antitumor agents has received considerable attention. Immunotoxins based on antigen recognition of single-chain antibodies have been widely explored for cancer therapy. Despite their impressive killing effect on tumor cells, immunotoxins still display unspecific toxicity with undesired side effects. High levels of hypoxia-inducible factor 1α (HIF-1α) are well-known indicators of hypoxia in cancer cells. In this study, different linkers were employed to fuse the immunotoxin DAB389-4D5 scFv (DS) with the oxygen-dependent degradation domain (ODDD) of HIF-1α, a domain selectively facilitating the accumulation of HIF-1α under hypoxia, to construct the oxygen-dependent degradable immunotoxin DS-ODDD (DSO). The engineered fusion protein DSO-2 containing a linker (G₄S)₃ possesses the best killing effect on cancer cells under hypoxia and displayed considerably reduced nonspecific toxicity to normal cells under normoxic conditions. Flow cytometry, immunofluorescence, and immunoblot analyses demonstrated that DSO-2 was degraded via the ubiquitin-proteasome pathway regulated by the oxygen-sensitive mechanism. Western blot analysis indicated that the degradation of DSO-2 significantly decreased the activation of apoptosis-related molecules in normal cells. The engineered immunotoxin with oxygen-sensing properties developed herein is a potential therapeutic agent for cancer treatment.

A New Combination: Anti Glypican-3 scFv and Diphtheria Toxin with the Best Flexible Linker

Along with all cancer treatments, including chemotherapy, radiotherapy, and surgery, targeting therapy is a new treatment manner. Immunotoxins are new recombinant structures that kill cancer cells by targeting specific antigens. Immunotoxins are composed of two parts: toxin moiety, which disrupts protein synthesis process, and antigen binding moiety that bind to antigens on the surface of cancer cells. Glypican 3 (GPC3) is an oncofetal antigen on the surface of Hepatocellular carcinoma (HCC) cells. In this study, truncated Diphtheria toxin (DT389) was fused to humanized scFv YP7 by one, two and three repeats of GGGGS linkers (DT389-(GGGGS)_1-3YP7). In-silico and experimental investigation were performed to find out how many repeats of linker between toxin and scFv moieties are sufficient. Results of in-silico investigations revealed that the difference in the number of linkers does not have a significant effect on the main structures of the immunotoxin; however, the three-dimensional structure of two repeats of linker had a more appropriate structure compared to others with one and three linker replications. In addition, with enhancing the number of linkers, the probability of protein solubility has increased. Generally, the bioinformatics results of DT389-(GGGGS)₂-YP7 structure showed that expression and folding is suitable; and YP7 scFv has appropriate orientation to bind GPC3. The experimental investigations indicated that the fusion protein was expressed as near to 50% soluble. Due to the high binding affinity of YP7 scFv and the proven potency of diphtheria in inhibiting protein synthesis, the proposed DT389-(GGGGS)₂-YP7 immunotoxin is expected to function well in inhibiting HCC.

Advances on Delivery of Cytotoxic Enzymes as Anticancer Agents

Cancer is one of the most serious human diseases, causing millions of deaths worldwide annually, and, therefore, it is one of the most investigated research disciplines. Developing efficient anticancer tools includes studying the effects of different natural enzymes of plant and microbial origin on tumor cells. The development of various smart delivery systems based on enzyme drugs has been conducted for more than two decades. Some of these delivery systems have been developed to the point that they have reached clinical stages, and a few have even found application in selected cancer treatments. Various biological, chemical, and physical approaches have been utilized to enhance their efficiencies by improving their delivery and targeting. In this paper, we review advanced delivery systems for enzyme drugs for use in cancer therapy. Their structure-based functions, mechanisms of action, fused forms with other peptides in terms of targeting and penetration, and other main results from in vivo and clinical studies of these advanced delivery systems are highlighted.

Refined immunoRNases for the efficient targeting and selective killing of tumour cells: A novel strategy

In order to overcome limitations of conventional cancer therapy methods, immunotoxins with the capability of target-specific action have been designed and evaluated pre-clinically, and some of them are in clinical studies. Targeting cancer cells via antibodies specific for tumour-associated surface proteins is a new biomedical approach that could provide the selectivity that is lacking in conventional cancer therapy methods such as radiotherapy and chemotherapy. A successful example of an approved immunotoxin is represented by immunoRNases. ImmunoRNases are fusion proteins in which the toxin has been replaced by a ribonuclease. Conjugation of RNase molecule to monoclonal antibody or antibody fragment was shown to enhance specific cell-killing by several orders of magnitude, both in vitro and in animal models. There are several RNases obtained from different mammalian cells that are expected to be less immunogenic and systemically toxic. In fact, RNases are pro-toxins which become toxic only upon their internalization in target cells mediated by the antibody moiety. The structure and large size of the antibody molecules assembled with the immunoRNases have always been a challenge in the application of immunoRNases as an antitoxin. To overcome this obstacle, we have offered a new strategy for the application of immunoRNases as a promising approach for upgrading immunoRNAses with maximum affinity and high stability in the cell, which can ultimately act as an effective large-scale cancer treatment. In this review, we introduce the optimized antibody-like molecules with small size, approximately 10 kD, which are presumed to significantly enhance RNase activity and be a suitable agent with the potential for anti-cancer functionality. In addition, we also discuss new molecular entities such as monobody, anticalin, nonobody and affilin as refined versions in the development of immunoRNases. These small molecules express their functionality with the suitable small size as well as with low immunogenicity in the cell, as a part of immunoRNases.

Dual Regioselective Targeting the Same Receptor in Nanoparticle-Mediated Combination Immuno/Chemotherapy for Enhanced Image-Guided Cancer Treatment

When combined with immunotherapy, image-guided targeted delivery of chemotherapeutic agents is a promising direction for combination cancer theranostics, but this approach has so far produced only limited success due to a lack of molecular targets on the cell surface and low therapeutic index of conventional chemotherapy drugs. Here, we demonstrate a synergistic strategy of combination immuno/chemotherapy in conditions of dual regioselective targeting, implying vectoring of two distinct binding sites of a single oncomarker (here, HER2) with theranostic compounds having a different mechanism of action. We use: (i) PLGA nanoformulation, loaded with an imaging diagnostic fluorescent dye (Nile Red) and a chemotherapeutic drug (doxorubicin), and functionalized with affibody Z_HER2:342 (8 kDa); (ii) bifunctional genetically engineered DARP-LoPE (42 kDa) immunotoxin comprising of a low-immunogenic modification of therapeutic Pseudomonas exotoxin A (LoPE) and a scaffold targeting protein, DARPin9.29 (14 kDa). According to the proposed strategy, the first chemotherapeutic nanoagent is targeted by the affibody to subdomain III and IV of HER2 with 60-fold specificity compared with nontargeted particles, while the second immunotoxin is effectively targeted by DARPin molecule to subdomain I of HER2. We demonstrate that this dual targeting strategy can enhance anticancer therapy of HER2-positive cells with a very strong synergy, which made possible 1000-fold decrease of effective drug concentration in vitro and a significant enhancement of HER2 cancer therapy compared to monotherapy in vivo. Moreover, this therapeutic combination prevented the appearance of secondary tumor nodes. Thus, the suggested synergistic strategy utilizing dual targeting of the same oncomarker could give rise to efficient methods for aggressive tumors treatment.

Safe and Efficacious Diphtheria Toxin-Based Treatment for Melanoma: Combination of a Light-On Gene-Expression System and Nanotechnology

The controversy surrounding the use of diphtheria toxin (DT) as a therapeutic agent against tumor cells arises mainly from its unexpected harmfulness to healthy tissues. We encoded the cytotoxic fragment A of DT (DTA) as an objective gene in the Light-On gene-expression system to construct plasmids pGAVPO (pG) and pU5-DTA (pDTA). Meanwhile, a cRGD-modified ternary complex comprising plasmids, chitosan, and liposome (pG&pDTA@cRGD-CL) was prepared as a nanocarrier to ensure transfection efficiency. Benefiting from spatiotemporal control of this light-switchable transgene system and the superior tumor targeting of the carrier, toxins were designed to be expressed selectively in illuminated lesions. In vitro studies suggested that pG&pDTA@cRGD-CL exerted arrest of the S phase in B16F10 cells upon blue light irradiation and, ultimately, induced the apoptosis and necrosis of tumor cells. Such DTA-based treatment exerted enhanced antitumor activity in mice bearing B16F10 xenografts and displayed prolonged survival time with minimal side effects. Hence, we described novel DTA-based therapy combined with nanotechnology and the Light-On gene-expression system: such treatment could be a promising strategy against melanoma.

Preparation of Diphtheria and Pseudomonas Exotoxin A Immunotoxins and Evaluation of Their Cytotoxicity Effect on SK-BR-3, BT-474, and MDA-MB-231 Breast Cancer Cell Lines

Immunotoxin targeted therapy is a promising way of cancer therapy that is made from a toxin attached to an antibody which target a specific protein presented on cancer cells. In this study, we introduce immunotoxins comprising of truncated pseudomonas exotoxin A (PEA) and diphtheria toxin (DT) conjugated to trastuzumab. The effectiveness of 20 and 30 μg/ml immunotoxins and trastuzumab were studied on SK-BR-3 and BT-474 HER2/neu positive breast cancer cell lines by a cell death assay test. The produced immunotoxins have the potential to reduce the therapeutic dose of the trastuzumab and in the same time achieve higher efficiency.

Design of a recombinant immunotoxin against the human granulocyte-colony stimulating factor receptor

Immunotoxin is a new strategy for protein therapy of cancer. This engineered protein contains two parts, the immune part which is an antibody or cytokine, directed against the cancer cell receptor, and the toxin part consisting of a plant or bacterial toxin leading to apoptosis by protein synthesis inhibition. The knowledge of cell-surface receptor overexpression in cancer cells can help scientists to construct new anti-cancer agents. The granulocyte colony stimulating factor (G-CSF) receptor is expressed on the cell surface of some blood cancers such as acute myeloid leukemia (AML). Therefore, this receptor can be used as an immunotoxin for treatment of some cancers. The aim of this work was to design and produce DT-GCSF immunotoxin using truncated DT fused to G-CSF. For fusion protein construction, DT389 and G-CSF fragments, were amplified by PCR using specific primers. A flexible linker SerGly4SerMet (SG4SM) was used to fuse the PCR products by SOEing PCR procedure to achieve an appropriate fusion protein, and the fused fragment was subcloned into pET21b. The new construction (pET-DT₃₈₉GCSF) was transformed into E. coli strain BL21 (DE3) and the expression of the construction was confirmed by SDS-PAGE and Western blotting techniques. The data demonstrated the expression and purity rates of DT₃₈₉GCSF about 25% and 90%, respectively. This chimeric protein construction can be used as a new anti-AML drug, but its in vitro and in vivo biological activity should be analyzed.

Targeting oncogenic Ras by the Clostridium perfringens toxin TpeL

Clostridium perfringens toxin TpeL belongs to the family of large clostridial glycosylating toxins. The toxin causes N-acetylglucosaminylation of Ras proteins at threonine35 thereby inactivating the small GTPases. Here, we show that all main types of oncogenic Ras proteins (H-Ras, K-Ras and N-Ras) are modified by the toxin in vitro and in vivo. Toxin-catalyzed modification of Ras was accompanied by inhibition of the MAP kinase pathway. Importantly, TpeL inhibited the paradoxical activation of the MAP kinase pathway induced by the BRAF inhibitor Vemurafenib in the human melanoma cell line SBCL2. The toxin also blocked Ras signaling in a zebrafish embryo model expressing oncogenic H-Ras^G12V, resulting in a reduction of melanocyte number. By using the binding and translocation component of anthrax toxin (protective antigen), the glucosyltransferase domain of TpeL was effectively introduced into target cells that were not sensitive to native TpeL toxin. To reach a higher specificity towards cancer cells, a chimeric TpeL toxin was engineered that possessed the knob region of adenovirus serotype 35 fiber, which interacts with CD46 of target cells frequently overexpressed in cancer cells. The chimeric TpeL fusion toxin efficiently inhibited Ras and MAP kinases in human pancreatic cancer Capan-2 cells, which were insensitive to the wild-type toxin. The data reveal that TpeL and TpeL-related immunotoxins provide a new toolset as Ras-inactivating agents.

Exploring Cytotoxic mRNAs as a Novel Class of Anti-cancer Biotherapeutics

New treatments to overcome the obstacles of conventional anti-cancer therapy are a permanent subject of investigation. One promising approach is the application of toxins linked to cell-specific ligands, so-called immunotoxins. Another attractive option is the employment of toxin-encoding plasmids. However, immunotoxins cause hepatoxicity, and DNA therapeutics, among other disadvantages, bear the risk of insertional mutagenesis. As an alternative, this study examined chemically modified mRNAs coding for diphtheria toxin, subtilase cytotoxin, and abrin-a for their ability to reduce cancer cell growth both in vitro and in vivo. The plant toxin abrin-a was the most promising candidate among the three tested toxins and was further investigated. Its expression was demonstrated by western blot. Experiments with firefly luciferase in reticulocyte lysates and co-transfection experiments with EGFP demonstrated the capability of abrin-a to inhibit protein synthesis. Its cytotoxic effect was quantified employing viability assays and propidium iodide staining. By studying caspase-3/7 activation, Annexin V-binding, and chromatin condensation with Hoechst33258 staining, apoptotic cell death could be confirmed. In mice, repeated intratumoral injections of complexed abrin-a mRNA resulted in a significant reduction (89%) of KB tumor size compared to a non-translatable control mRNA.

A novel anti-CD22 scFv-apoptin fusion protein induces apoptosis in malignant B-cells

CD22 marker is a highly internalizing antigen which is located on the surface of B-cells and is being used as a promising target for treatment of B cell malignancies. Monoclonal antibodies targeting CD22 have been introduced and some are currently under investigation in clinical trials. Building on the success of antibody drug conjugates, we developed a fusion protein consisting of a novel anti-CD22 scFv and apoptin and tested binding and therapeutic effects in lymphoma cells. The recombinant protein was expressed in E. coli and successfully purified and refolded. In vitro binding analysis by immunofluorescence and flow cytometry demonstrated that the recombinant protein specifically binds to CD22 positive Raji cells but not to CD22 negative Jurkat cells. The cytotoxic properties of scFv–apoptin were assessed by an MTT assay and Annexin V/PI flow cytometry analysis and showed that the recombinant protein induced apoptosis preferentially in Raji cells with no detectable effects in Jurkat cells. Our findings indicated that the recombinant anti-CD22 scFv–apoptin fusion protein could successfully cross the cell membrane and induce apoptosis with high specificity, make it as a promising molecule for immunotherapy of B-cell malignancies.

The future of antiviral immunotoxins

There is a constant need for new therapeutic interventions in a wide range of infectious diseases. Over the past few years, the immunotoxins have entered the stage as promising antiviral treatments. Immunotoxins have been extensively explored in cancer treatment and have achieved FDA approval in several cases. Indeed, the design of new anticancer immunotoxins is a rapidly developing field. However, at present, several immunotoxins have been developed targeting a variety of different viruses with high specificity and efficacy. Rather than blocking a viral or cellular pathway needed for virus replication and dissemination, immunotoxins exert their effect by killing and eradicating the pool of infected cells. By targeting a virus-encoded target molecule, it is possible to obtain superior selectivity and drastically limit the side effects, which is an immunotoxin-related challenge that has hindered the success of immunotoxins in cancer treatment. Therefore, it seems beneficial to use immunotoxins for the treatment of virus infections. One recent example showed that targeting of virus-encoded 7 transmembrane (7TM) receptors by immunotoxins could be a future strategy for designing ultraspecific antiviral treatment, ensuring efficient internalization and hence efficient eradication of the pool of infected cells, both in vitro and in vivo. In this review, we provide an overview of the mechanisms of action of immunotoxins and highlight the advantages of immunotoxins as future anti-viral therapies.

Expression and purification of recombinant antibody formats and antibody fusion proteins

In the laboratory-scale production of antibody fragments or antibody fusion proteins, it is often difficult to keep track on the most suitable affinity tags for protein purification from either prokaryotic or eukaryotic host systems. Here, we describe how such recombinant proteins derived from Escherichia coli lysates as well as HEK293 cell culture supernatants are purified by IMAC and by different affinity chromatography methods based on fusions to FLAG-tag, Strep-tag, and Fc domains.

Ablation of breast cancer cells using trastuzumab-functionalized multi-walled carbon nanotubes and trastuzumab-diphtheria toxin conjugate

Trastuzumab (Herceptin(®) ) is a monoclonal antibody (mAb) for specific ablation of HER2-overexpressing malignant breast cancer cells. Intensification of antiproliferative activity of trastuzumab through construction of immunotoxins and nano-immunoconjugates is a promising approach for treatment of cancer. In this study, trastuzumab was directly conjugated to diphtheria toxin (DT). Also, conjugates of trastuzumab and multiwalled carbon nanotubes (MWCNT) were constructed by covalent immobilization of trastuzumab onto MWCNTs. Then, antiproliferative activity of the fusion constructs against HER2-overexpressing SK-BR-3 and also HER2-negative MCF-7 cancer cell lines were examined. Cells treated with trastuzumab-MWCNT conjugates were irradiated with near-infrared (NIR) light. Efficient absorption of NIR radiation and its conversion to heat by MWCNTs can be resulted to thermal ablation of cancerous cells. Our results strongly showed that both trastuzumab-MWCNT and trastuzumab-DT conjugates were significantly efficient in the specific killing of SK-BR-3 cells. Targeting of MWCNTs to cancerous cells using trastuzumab followed by exposure of cells to NIR radiation was more efficient in repression of cell proliferation than treatment for cancer cells with trastuzumab-DT. Our results also showed that conjugation linkers can significantly affect the cytotoxicity of MWCNT-immunoconjugates. In conclusion, our data demonstrated that trastuzumab-MWCNT is a promising nano-immunoconjugate for killing of HER2-overexpressing cancerous cells.

Activation of RhoA,B,C by Yersinia Cytotoxic Necrotizing Factor (CNFy) induces apoptosis in LNCaP prostate cancer cells

Prostate cancer is the most common malignancy, accounting for about 25% of all incident cases among men in industrialized countries. The human androgen-dependent prostate cancer cell line LNCaP, which is derived from a metastatic lesion of human prostatic adenocarcinoma, is frequently used to study prostate cancer associated signaling pathways in vitro. Recently it was described that Rho GTPase activation in these cells leads to apoptotic responses. We used the bacterial toxins CNFy and CNF1, which specifically and directly activate Rho GTPases by deamidation of a single glutamine. We asked whether these Rho activators could induce apoptosis in LNCaP cells. Our results indicate that RhoA activation, induced by CNFy, does lead to intrinsic apoptosis of the cells. Analysis of the underlying signaling pathway reveals that apoptosis induction requires the activity of Rho kinase (ROCK) and myosin activation, an apoptotic pathway previously identified in cancer stem cells.

Acquired bone marrow failure after diphtheria

Acquired bone marrow failure can result after infections caused by viruses, exposure to toxins, or chemicals. Majority of acquired cases of marrow failure labeled as "idiopathic" are probably immune mediated through activated T-cells and cytokines. We observed pancytopenia in the second week, after pharyngeal diphtheria in a 14-year-old child. Bone marrow examination showed reduced marrow precursors affecting all 3 cell lines. There are no clinical cases of myelosuppression reported in diphtheria infection and this is probably the first reported case of bone marrow failure after diphtheria infection. However, such observations merit further evaluation and confirmation by experimental studies.

Semaphorin 3A lytic hybrid peptide binding to neuropilin-1 as a novel anti-cancer agent in pancreatic cancer

We previously reported that novel targeted "hybrid peptide" in which epidermal growth factor receptor (EGFR) binding peptide was conjugated with lytic-type peptide had selective cytotoxic activity to EGFR expressing cancer cells. In this study, we have generated a novel type hybrid peptide, semaphorin 3A lytic (Sema3A-lytic), which is composed of two functional amino acid domains: a sequence derived from Sema3A that binds to neuropilin-1 (NRP1) and a cytotoxic lytic peptide. We found that this hybrid peptide had cytotoxic activity against NRP1-positive pancreatic cancer cell lines such as BxPC-3 and Panc-1, whereas the peptide did not affect the viability of normal cells in vitro. It was also found by affinity analysis that Sema3A peptide binds to NRP1, and two arginines (372R and 377R) in Sema3A peptide are involved in the interaction with NRP1 protein. In addition, confocal microscopy analysis revealed that Sema3A-lytic peptide could not penetrate normal cells regardless of the presence of NRP1 mRNA, suggesting that the ability of Sema3A-lytic peptide to concentrate adjacent to the cell membrane by binding to NRP1 with the target-binding moiety contributes to its selective cytotoxic activity. These results indicate that Sema3A-lytic hybrid peptide would be a possible anti-cancer agent for treatment of human pancreatic cancer.

Molecular targeting of malignant glioma cells with an EphA2-specific immunotoxin delivered by human bone marrow-derived mesenchymal stem cells

Immunotoxins have shown great promise as an alternative treatment for brain malignancies such as gliomas, but their failure to penetrate into the tumor mass remains a major problem. Mesenchymal stem cells exhibit tropism to tumor tissue and may serve as a cellular vehicle for the delivery and local production of antitumor agents. In this study, we used human bone marrow-derived mesenchymal stem cells (hMSCs) as a vehicle for the targeted delivery of EphrinA1-PE38, a very specific immunotoxin against the EphA2 receptor that is overexpressed in gliomas. hMSCs were transduced with adenovirus to express secretable EphrinA1-PE38. Our invitro assays confirmed the expression, release and selective killing effect of the immunotoxin produced by hMSCs. Furthermore, the intratumoral injection of engineered hMSCs was effective at inhibiting tumor growth in a malignant glioma tumor model. These results indicate that gene therapy utilizing EphrinA1-PE38-secreting hMSCs may provide a novel approach for the local treatment of malignant gliomas.

Antibody-enzyme fusion proteins for cancer therapy

Advances in biomolecular technology have allowed the development of genetically fused antibody-enzymes. Antibody-enzyme fusion proteins have been used to target tumors for cancer therapy in two ways. In one system, an antibody-enzyme is pretargeted to the tumor followed by administration of an inactive prodrug that is converted to its active form by the pretargeted enzyme. This system has been described as antibody-directed enzyme prodrug therapy. The other system uses antibody-enzyme fusion proteins as direct therapeutics, where the enzyme is toxic in its own right. The key feature in this approach is that the antibody is used to internalize the toxic enzyme into the tumor cell, which activates cell-death processes. This antibody-enzyme system has been largely applied to deliver ribonucleases. This article addresses these two antibody-enzyme targeting strategies for cancer therapy from concept to (pre)clinical trials.

Use of ribosome-inactivating proteins from Sambucus for the construction of immunotoxins and conjugates for cancer therapy

The type 2 ribosome-inactivating proteins (RIPs) isolated from some species belonging to the Sambucus genus, have the characteristic that although being even more active than ricin inhibiting protein synthesis in cell-free extracts, they lack the high toxicity of ricin and related type 2 RIPs to intact cells and animals. This is due to the fact that after internalization, they follow a different intracellular pathway that does not allow them to reach the cytosolic ribosomes. The lack of toxicity of type 2 RIPs from Sambucus make them good candidates as toxic moieties in the construction of immunotoxins and conjugates directed against specific targets. Up to now they have been conjugated with either transferrin or anti-CD105 to target either transferrin receptor- or endoglin-overexpressing cells, respectively.

Targeting the unfolded protein response in cancer therapy

Rapid growth of tumor cells coupled with inadequate vascularization leads to shortage of oxygen and nutrients. The unfolded protein response (UPR), a defense cellular mechanism activated during such stress conditions, is a complex process that includes upregulation of the endoplasmic reticulum chaperones, such as glucose-regulated protein 78 (GRP78). Due to its central role in UPR, GRP78 is overexpressed in many cancers; it is implicated in cancer cell survival through supporting of drug- and radioresistance as well as metastatic dissemination, and is generally associated with poor outcome. This is the reason why selective destruction of GRP78 could become a novel anticancer strategy. GRP78 is the only known substrate of the proteolytic A subunit (SubA) of a bacterial AB(5) toxin, and the selective SubA-induced cleavage of GRP78 leads to massive cell death. Targeted delivery of SubA into cancer cells via specific receptor-mediated endocytosis could be a suitable strategy for assaulting tumor cells. We fused SubA to epidermal growth factor (EGF), whose receptor (EGFR) is frequently overexpressed in tumor cells, and demonstrated that the resulting EGF-SubA immunotoxin is an effective killer of EGFR-positive tumor cells. Furthermore, because of its unique mechanism of action, EGF-SubA synergizes with UPR-inducing drugs, which opens a possibility for the development of mechanism-based combination regimens for effective anticancer therapy. In this chapter, we provide experimental protocols for the assessment of the effects of EGF-SubA on EGFR-positive cancer cells, either alone or in combination with UPR-inducing drugs.

Ribosome-inactivating proteins: from plant defense to tumor attack

Ribosome-inactivating proteins (RIPs) are EC3.2.32.22 N-glycosidases that recognize a universally conserved stem-loop structure in 23S/25S/28S rRNA, depurinating a single adenine (A4324 in rat) and irreversibly blocking protein translation, leading finally to cell death of intoxicated mammalian cells. Ricin, the plant RIP prototype that comprises a catalytic A subunit linked to a galactose-binding lectin B subunit to allow cell surface binding and toxin entry in most mammalian cells, shows a potency in the picomolar range. The most promising way to exploit plant RIPs as weapons against cancer cells is either by designing molecules in which the toxic domains are linked to selective tumor targeting domains or directly delivered as suicide genes for cancer gene therapy. Here, we will provide a comprehensive picture of plant RIPs and discuss successful designs and features of chimeric molecules having therapeutic potential.

Toxin-based therapeutic approaches

Protein toxins confer a defense against predation/grazing or a superior pathogenic competence upon the producing organism. Such toxins have been perfected through evolution in poisonous animals/plants and pathogenic bacteria. Over the past five decades, a lot of effort has been invested in studying their mechanism of action, the way they contribute to pathogenicity and in the development of antidotes that neutralize their action. In parallel, many research groups turned to explore the pharmaceutical potential of such toxins when they are used to efficiently impair essential cellular processes and/or damage the integrity of their target cells. The following review summarizes major advances in the field of toxin based therapeutics and offers a comprehensive description of the mode of action of each applied toxin.

On the interaction of Clostridium perfringens enterotoxin with claudins

Clostridium perfringens causes one of the most common foodborne illnesses, which is largely mediated by the Clostridium perfringens enterotoxin (CPE). The toxin consists of two functional domains. The N-terminal region mediates the cytotoxic effect through pore formation in the plasma membrane of the mammalian host cell. The C-terminal region (cCPE) binds to the second extracellular loop of a subset of claudins. Claudin-3 and claudin-4 have been shown to be receptors for CPE with very high affinity. The toxin binds with weak affinity to claudin-1 and -2 but contribution of these weak binding claudins to CPE-mediated disease is questionable. cCPE is not cytotoxic, however, it is a potent modulator of tight junctions. This review describes recent progress in the molecular characterization of the cCPE-claudin interaction using mutagenesis, in vitro binding assays and permeation studies. The results promote the development of recombinant cCPE-proteins and CPE-based peptidomimetics to modulate tight junctions for improved drug delivery or to treat tumors overexpressing claudins.

Modified DT-IL2 fusion toxin targeting uniquely IL2Ralpha expressing leukemia cell lines - Construction and characterization

Immunotoxins are fusion proteins of modified toxin conjugated to tumor cell selective ligand. Denileukin diftitox approved by FDA for treatment of CTCL is diphtheria toxin (DT)/IL2 fusion protein targeted to high affinity IL2R. Here, we have attempted to target the more uniquely expressed low affinity IL2R (IL2Ralpha). We designed four immunotoxins, SPRSV1 was designed to code for a single protein of DT (390) and IL2 (133) without any extra amino acids at the junction. SPRSV2 was designed to selectively target low affinity IL2R, it codes for DT (390) and IL2 (69). We also constructed SPRSV3 encoding for only DT (390) without any ligand, as negative control and SPRSV4 was designed similar to commercial equivalent denileukin diftitox, it codes for DT (387) and IL2 (133) with His at the junction. The cytotoxic activities of these immunotoxins were tested in various cell lines, cell lines lacking IL2R expression and healthy MNC were used as controls. The activities of SPRSV1 and SPRSV2 were comparable to that of SPRSV4. SPRSV2 exhibited potent cytotoxicity effectively targeted to alpha subunit of IL2R on various leukemia cell lines. Our studies also showed a negative correlation between CD25 expression and percentage cell viability after treatment with immunotoxins.

Development of a nonintegrating Rev-dependent lentiviral vector carrying diphtheria toxin A chain and human TRAF6 to target HIV reservoirs

Persistence of HIV despite highly active antiretroviral therapy (HAART) is a lasting challenge to virus eradication. To develop a strategy complementary to HAART, we constructed a series of Rev-dependent lentiviral vectors carrying diphtheria toxin A chain (DT-A) and its attenuated mutants, as well as human TRAF6. Expression of these suicide genes following delivery through viral particles is dependent on Rev, which exists only in infected cells. Among these toxins, DT-A has been known to trigger cell death with as little as a single molecule, whereas two of the attenuated mutants in this study, DT-A(176) and DT-A(ΔN), were well-tolerated by cells at low levels. TRAF6 induced apoptosis only with persistent overexpression. Thus, these suicide genes, which induce cell death at different expression levels, offer a balance between efficacy and safety. To minimize possible mutagenesis introduced by retroviral integration in non-target cells, we further developed a non-integrating Rev-dependent (NIRD) lentiviral vector to deliver these genes. In addition, we constructed a DT-A-resistant human cell line by introducing a human elongation factor 2 (EF-2) mutant into HEK293T cells. This allowed us to manufacture the first high-titer NIRD lentiviral particles carrying DT-A to target HIV-positive cells.

A tandem scFv-based fusion protein and its enediyne-energized analogue show intensified therapeutic efficacy against lung carcinoma xenograft in athymic mice

Gelatinases play important roles in tumor progression and are abundantly expressed in a variety of malignant tumors. Antibody targeting gelatinases is a possible avenue to fight against cancer. However, antibody alone can not achieve curative efficacy. Herein, we demonstrated the intensified targeting therapy of a tandem scFv-based fusion protein and its enediyne-energized analogue against gelatinases-overexpressed tumor. A fusion protein dFv-LDP, comprising a tandem scFv of anti-gelatinases linked to the apoprotein (LDP) of lidamycin, was generated and showed strong tumor targeting capability in three different tumor xenografts. In PG-BE1 lung carcinoma xenograft, the tumor inhibition rate was 77.5% by dFv-LDP versus 94.2% by dFv-LDP-AE, the product of dFv-LDP assembled with the active enediyne chromophore (AE) of lidamycin. Moreover, the combination of dFv-LDP with dFv-LDP-AE further augmented the therapeutic efficacy, producing initial tumor shrinkage in five of six mice. The microvessel density (P<0.05) and proliferation index (P<0.05) were also stepwise decreased in groups of dFv-LDP, dFv-LDP-AE and the combination. In conclusion, our results demonstrated that the antibody-based therapy against gelatinases was stepwise intensified in use of dFv-LDP, dFv-LDP-AE and dFv-LDP plus dFv-LDP-AE, and indicated that the combination of an antibody with its drug-armed analogue might be of interest as a new approach to augment antitumor efficacy.

Claudin-4-targeting of diphtheria toxin fragment A using a C-terminal fragment of Clostridium perfringens enterotoxin

Claudin (CL)-4, a tight junction protein, is overexpressed in some human neoplasias, including ovarian, breast, pancreatic and prostate cancers. The targeting of CL-4 is a novel strategy for tumor therapy. We previously found that the C-terminal fragment of Clostridium perfringens enterotoxin (C-CPE) binds to CL-4. In the present study, we genetically prepared a novel CL-4-targeting molecule (DTA-C-CPE) by fusion of C-CPE and diphtheria toxin fragment A (DTA). Although DTA is not toxic to CL-4-expressing L cells, even at 20 microg/ml, DTA-C-CPE is toxic to CL-4-expressing L cells at 1 microg/ml. DTA-C-CPE-induced cytotoxicity was attenuated by pretreatment of the cells with C-CPE but not bovine serum albumin, indicating that DTA-C-CPE may bind to CL-4-expressing L cells through its C-CPE domain. To evaluate the specificity of DTA-C-CPE, we examined its cytotoxic effects in L cells that express CL-1, -2, -4 or -5. We found that DTA-C-CPE was toxic to only CL-4-expressing L cells. Thus, C-CPE may be a promising ligand for the development of cancer-targeting systems.

A novel fusion protein diphtheria toxin-stem cell factor (DT-SCF)-purification and characterization

Fusion toxins are an emerging class of targeted therapeutics for the treatment of cancer. Diphtheria toxin-stem cell factor (DT-SCF) is one such novel fusion toxin designed to target malignancies expressing c-kit. Since, c-kit overexpression has been reported on many types of cancers, it appeared to be a reasonably good molecule to target. In the present study, we report construction, expression, purification, and characterization of DT-SCF. DT-SCF gene coding for 1-387 amino acids of diphtheria toxin, His-Ala linker, 2-141 amino acids of SCF was cloned into expression vector with C terminal His tag. The induced DT-SCF protein was exclusively expressed in insoluble fraction. Purification of DT-SCF was achieved by inclusion body isolation and metal affinity chromatography under denaturing and reducing conditions. Purified DT-SCF was renatured partially on-column by gradually reducing denaturant concentration followed by complete refolding through rapid dilution technique. Cell viability assay provided the evidence that DT-SCF is a potent cytotoxic agent selective to cells expressing c-kit. The novelty of this study lies in employing SCF as a ligand in construction of fusion toxin to target wide range of malignancies expressing c-kit. Efficacy of DT-SCF fusion toxin was demonstrated over a range of malignancies such as chronic myeloid leukemia (K562), acute lymphoblastic leukemia (MOLT4), pancreatic carcinoma (PANC-1), and cervical carcinoma (HeLa 229). This is the first study reporting specificity and efficacy of DT-SCF against tumor cells expressing c-kit. There was significant correlation (P = 0.007) between c-kit expression on cells and their sensitivity to DT-SCF fusion toxin.

Humanized immunotoxins: a new generation of immunotoxins for targeted cancer therapy

Chemotherapy, radiation, and surgery are the conventional treatment modalities for cancer. The success achieved with these approaches has been limited due to several factors like chemoresistance to drugs, non-specificity leading to peripheral toxicity, and non-resectable tumors. To combat these problems, the concept of targeted therapy using immunotoxins was developed. Immunotoxins are chimeric proteins with a cell-selective ligand chemically linked or genetically fused to a toxin moiety and can target cancer cells overexpressing tumor-associated antigens, membrane receptors, or carbohydrate antigens. Ligands for these receptors or monoclonal antibodies or single chain variable fragments directed against these antigens are fused with bacterial or plant toxins and are made use of as immunotoxins. Pseudomonas exotoxin, anthrax toxin, and diphtheria toxin are the commonly used bacterial toxins. Ricin, saporin, gelonin, and poke weed antiviral protein are the plant toxins utilized in immunotoxin constructs. Several such fusion proteins are in clinical trials, and denileukin difitox is a FDA-approved fusion protein. In spite of the promise shown by bacterial- and plant toxin-based chimeric proteins, their clinical application is hampered by several factors like immunogenicity of the toxin moiety and non-specific toxicity leading to vascular leak syndrome. In order to overcome these problems, a novel generation of immunotoxins in which the cytotoxic moiety is an endogenous protein of human origin like proapoptotic protein or RNase has been developed. This review summarizes the advances in this new class of fusion protein and the future directions to be explored.

Guanylyl cyclase C as a biomarker for targeted imaging and therapy of metastatic colorectal cancer

The guanylyl cyclase C (GCC) receptor posseses several well-established properties ideal for use as a biomarker in gastrointestinal malignancies. The GCC receptor is constitutively expressed in the apical membranes of the intestine and its expression is universally preserved in primary colorectal tumors and their metastases. Moreover, receptor binding is retained by GCC’s cognate ligand, the bacterial enterotoxin ST, even after conjugation to functional moieties. Selective tumor, but not gastrointestinal, uptake of ST in mice bearing GCC-expressing colon cancer xenografts demonstrates the potential of exploiting ST–GCC interaction for diagnostic imaging and targeted therapy of metastatic colorectal cancer. We expect this specific targeting provided by ST–GCC interaction to improve diagnosis, staging and management of colorectal cancer metastases, and ultimately prolong patient survival in this disease.

Inhibition of vascular endothelial growth factor receptor signaling in angiogenic tumor vasculature

Neovascularization takes place in a large number of pathologies, including cancer. Significant effort has been invested in the development of agents that can inhibit this process, and an increasing number of such agents, known as antiangiogenic drugs, are entering clinical trials or being approved for clinical use. The key players involved in the development and maintenance of tumor neovasculature are vascular endothelial growth factor (VEGF) and its receptors (VEGFRs), and therefore VEGF/VEGFR signaling pathways have been a focus of anticancer therapies for several decades. This review focuses on two main approaches designed to selectively target VEGFRs, inhibiting VEGFR with small molecule inhibitors of receptor tyrosine kinase activity and inhibiting the binding of VEGF to VEGFRs with specific antibodies or soluble decoy VEGF receptors. The major problem with these strategies is that they appeared to be effective only in relatively small and unpredictable subsets of patients. An alternative approach would be to subvert VEGFR for intracellular delivery of cytotoxic molecules. We describe here one such molecule, SLT-VEGF, a fusion protein containing VEGF121 and the highly cytotoxic catalytic subunit of Shiga-like toxin.