Recombinant toxins as novel therapeutic agents

Pharmacological agents targeting drug-tolerant persister cells in cancer

Current cancer therapy can be effective, but the development of drug resistant disease is the usual outcome. These drugs can eliminate most of the tumor burden but often fail to eliminate the rare, "Drug Tolerant Persister" (DTP) cell subpopulations in residual tumors, which can be referred to as "Persister" cells. Therefore, novel therapeutic agents specifically targeting or preventing the development of drug-resistant tumors mediated by the remaining persister cells subpopulations are needed. Since approximately ninety percent of cancer-related deaths occur because of the eventual development of drug resistance, identifying, and dissecting the biology of the persister cells is essential for the creation of drugs to target them. While there remains uncertainty surrounding all the markers identifying DTP cells in the literature, this review summarizes the drugs and therapeutic approaches that are available to target the persister cell subpopulations expressing the cellular markers ATP-binding cassette sub-family B member 5 (ABCB5), CD133, CD271, Lysine-specific histone demethylase 5 (KDM5), and aldehyde dehydrogenase (ALDH). Persister cells expressing these markers were selected as the focus of this review because they have been found on cells surviving following drug treatments that promote recurrent drug resistant cancer and are associated with stem cell-like properties, including self-renewal, differentiation, and resistance to therapy. The limitations and obstacles facing the development of agents targeting these DTP cell subpopulations are detailed, with discussion of potential solutions and current research areas needing further exploration.

Targeting EphA2 in cancer

Eph receptors and the corresponding Eph receptor-interacting (ephrin) ligands jointly constitute a critical cell signaling network that has multiple functions. The tyrosine kinase EphA2, which belongs to the family of Eph receptors, is highly produced in tumor tissues, while found at relatively low levels in most normal adult tissues, indicating its potential application in cancer treatment. After 30 years of investigation, a large amount of data regarding EphA2 functions have been compiled. Meanwhile, several compounds targeting EphA2 have been evaluated and tested in clinical studies, albeit with limited clinical success. The present review briefly describes the contribution of EphA2-ephrin A1 signaling axis to carcinogenesis. In addition, the roles of EphA2 in resistance to molecular-targeted agents were examined. In particular, we focused on EphA2's potential as a target for cancer treatment to provide insights into the application of EphA2 targeting in anticancer strategies. Overall, EphA2 represents a potential target for treating malignant tumors.

Targeting IL-13Rα2 for effective treatment of malignant peripheral nerve sheath tumors in mouse models

OBJECTIVE

Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive soft tissue sarcomas that harbor a high potential for metastasis and have a devastating prognosis. Combination chemoradiation aids in tumor control and decreases tumor recurrence but causes deleterious side effects and does not extend long-term survival. An effective treatment with limited toxicity and enhanced efficacy is critical for patients suffering from MPNSTs.

METHODS

The authors recently identified that interleukin-13 receptor alpha 2 (IL-13Rα2) is overexpressed on MPNSTs and could serve as a precision-based target for delivery of chemotherapeutic agents. In the work reported here, a recombinant fusion molecule consisting of a mutant human IL-13 targeting moiety and a point mutant variant of Pseudomonas exotoxin A (IL-13.E13 K-PE4E) was utilized to treat MPNST in vitro in cell culture and in an in vivo murine model.

RESULTS

IL-13.E13 K-PE4E had a potent cytotoxic effect on MPNST cells in vitro. Furthermore, intratumoral administration of IL-13.E13 K-PE4E to orthotopically implanted MPNSTs decreased tumor burden 6-fold and 11-fold in late-stage and early-stage MPNST models, respectively. IL-13.E13 K-PE4E treatment also increased survival by 23 days in the early-stage MPNST model.

CONCLUSIONS

The current MPNST treatment paradigm consists of 3 prongs: surgery, chemotherapy, and radiation, none of which, either singly or in combination, are curative or extend survival to a clinically meaningful degree. The results presented here provide the possibility of intratumoral therapy with a potent and highly tumor-specific cytotoxin as a fourth treatment prong with the potential to yield improved outcomes in patients with MPNSTs.

Setting the target for pemphigus vulgaris therapy

Despite the rising incidence of autoimmunity, therapeutic options for patients with autoimmune disease still rely on decades-old immunosuppressive strategies that risk severe and potentially fatal complications. Thus, novel therapeutic approaches for autoimmune diseases are greatly needed in order to minimize treatment-related toxicity. Such strategies would ideally target only the autoreactive immune components to preserve beneficial immunity. Here, we review how several decades of basic, translational, and clinical research on the immunology of pemphigus vulgaris (PV), an autoantibody-mediated skin disease, have enabled the development of targeted immunotherapeutic strategies. We discuss research to elucidate the pathophysiology of PV and how the knowledge afforded by these studies has led to the preclinical and clinical testing of targeted approaches to neutralize autoantibodies, to induce antigen-specific tolerance, and to specifically eliminate autoreactive B cells in PV.

Ribosome-Inactivating Proteins from Plants: A Historical Overview

This review provides a historical overview of the research on plant ribosome-inactivating proteins (RIPs), starting from the first studies at the end of eighteenth century involving the purification of abrin and ricin, as well as the immunological experiments of Paul Erlich. Interest in these plant toxins was revived in 1970 by the observation of their anticancer activity, which has given rise to a large amount of research contributing to the development of various scientific fields. Biochemistry analyses succeeded in identifying the enzymatic activity of RIPs and allowed for a better understanding of the ribosomal machinery. Studies on RIP/cell interactions were able to detail the endocytosis and intracellular routing of ricin, thus increasing our knowledge of how cells handle exogenous proteins. The identification of new RIPs and the finding that most RIPs are single-chain polypeptides, together with their genetic sequencing, has aided in the development of new phylogenetic theories. Overall, the biological properties of these proteins, including their abortifacient, anticancer, antiviral and neurotoxic activities, suggest that RIPs could be utilized in agriculture and in many biomedical fields, including clinical drug development.

Targeted toxins

Objective. Current modalities used in the treatment of cancer often cause unacceptable damage to normal tissue. Toxins targeted toward tumor cells by antibodies or growth factors have the potential to selectively kill tumor cells while leaving normal tissue intact. The purpose of this review is to provide background information on targeted toxins and current clinical studies for this new class of anti-cancer compounds.

Data sources. A MEDLINE search was conducted using the term “immunotoxins.” Relevant articles were also obtained by the systematic examination of article references.

Data synthesis. The toxins Pseudomonas exotoxin, diphtheria toxin, and ricin toxin are often used as targeted toxins. Deletion or mutation of the binding domains of these toxins decreased binding of the toxins to normal tissues. Antibodies or growth factors can be used as targeting moiety, and the resulting agents are called immunotoxins or fusion proteins, respectively. DNA technology and chemical modifications of the toxin as well as the antibody moiety led to smaller and less immunogenic targeted toxins. Smaller targeted toxins are less toxic and penetrate further into the tumor. The summary of several targeted toxins elicited during clinical trials in this review makes it clear that several targeted toxins are potential agents for the treatment of various cancers, although some problems still need to be overcome. These problems include toxicity, immunogenicity, cross-reactivity of the targeted toxin with life-sustaining tissue, heterogenicity of tumor cells, and limited tumor penetration.

Plant Toxin-Based Immunotoxins for Cancer Therapy: A Short Overview

Immunotoxins are chimeric proteins obtained by linking a toxin to either an intact antibody or an antibody fragment. Conjugation can be obtained by chemical or genetic engineering, where the latter yields recombinant conjugates. An essential requirement is that the target molecule recognized by the antibody is confined to the cell population to be deleted, or at least that it is not present on stem cells or other cell types essential for the organism’s survival. Hundreds of different studies have demonstrated the potential for applying immunotoxins to many models in pre-clinical studies and in clinical trials. Immunotoxins can be theoretically used to eliminate any unwanted cell responsible for a pathological condition. The best results have been obtained in cancer therapy, especially in hematological malignancies. Among plant toxins, the most frequently employed to generate immunotoxins are ribosome-inactivating proteins, the most common being ricin. This review summarizes the various approaches and results obtained in the last four decades by researchers in the field of plant toxin-based immunotoxins for cancer therapy.

CD133, Selectively Targeting the Root of Cancer

Cancer stem cells (CSC) are capable of promoting tumor initiation and self-renewal, two important hallmarks of carcinoma formation. This population comprises a small percentage of the tumor mass and is highly resistant to chemotherapy, causing the most difficult problem in the field of cancer research, drug refractory relapse. Many CSC markers have been reported. One of the most promising and perhaps least ubiquitous is CD133, a membrane-bound pentaspan glycoprotein that is frequently expressed on CSC. There is evidence that directly targeting CD133 with biological drugs might be the most effective way to eliminate CSC. We have investigated two entirely unrelated, but highly effective approaches for selectively targeting CD133. The first involves using a special anti-CD133 single chain variable fragment (scFv) to deliver a catalytic toxin. The second utilizes this same scFv to deliver components of the immune system. In this review, we discuss the development and current status of these CD133 associated biological agents. Together, they show exceptional promise by specific and efficient CSC elimination.

Interleukin-4 receptor signaling and its binding mechanism: A therapeutic insight from inhibitors tool box

Studies on Interlukin-4 (IL-4) disclosed great deal of information about its various physiological and pathological roles. All these roles depend upon its interaction and signaling through either type-I (IL-4Rα/common γ-chain) or type-II (IL-4Rα/IL-13Rα) receptors. Another cytokine, IL-13, shares some of the functions of IL-4, because both cytokines use a common receptor subunit, IL-4Rα. Here in this review, we discuss the structural details of IL-4 and IL-4Rα subunit and the structural similarities between IL-4 and IL-13. We also describe detailed chemistry of type-I and type-II receptor complexes and their signaling pathways. Furthermore, we elaborate the strength of type-II hetero dimer signals in response to IL-4 and IL-13. These cytokines are prime players in pathogenesis of allergic asthma, allergic hypersensitivity, different cancers, and HIV infection. Recent advances in the structural and binding chemistry of these cytokines various types of inhibitors were designed to block the interaction of IL-4 and IL-13 with their receptor, including several IL-4 mutant analogs and IL-4 antagonistic antibodies. Moreover, different targeted immunotoxins, which is a fusion of cytokine protein with a toxin or suicidal gene, are the new class of inhibitors to prevent cancer progression. In addition few small molecular inhibitors such as flavonoids have also been developed which are capable of binding with high affinity to IL-4Rα and, therefore, can be very effective in blocking IL-4-mediated responses.

Mutagenic Deimmunization of Diphtheria Toxin for Use in Biologic Drug Development

BACKGROUND

Targeted toxins require multiple treatments and therefore must be deimmunized. We report a method of protein deimmunization based on the point mutation of highly hydrophilic R, K, D, E, and Q amino acids on the molecular surface of truncated diphtheria-toxin (DT390).

METHODS

Based on their surface position derived from an X-ray-crystallographic model, residues were chosen for point mutation that were located in prominent positions on the molecular surface and away from the catalytic site. Mice were immunized with a targeted toxin containing either a mutated DT390 containing seven critical point mutations or the non-mutated parental toxin form.

RESULTS

Serum analysis revealed a significant 90% reduction in anti-toxin antibodies in mice immunized with the mutant, but not the parental drug form despite multiple immunizations. The experiment was repeated in a second strain of mice with a different MHC-haplotype to address whether point mutation removed T or B cell epitopes. Findings were identical indicating that B cell epitopes were eliminated from DT. The mutant drug form lost only minimal activity in vitro as well as in vivo.

CONCLUSION

These findings indicate that this method may be effective for deimmunizing of other proteins and that discovery of a deimmunized form of DT may lead to the development of more effective targeted toxin.

In vitro effects and ex vivo binding of an EGFR-specific immunotoxin on rhabdomyosarcoma cells

PURPOSE

Rhabdomyosarcoma (RMS) is a rare and aggressive soft tissue sarcoma with limited treatment options and a high failure rate during standard therapy. New therapeutic strategies based on targeted immunotherapy are therefore much in demand. The epidermal growth factor receptor (EGFR) has all the characteristics of an ideal target. It is overexpressed in up to 80 % of embryonal RMS and up to 50 % of alveolar RMS tumors. We therefore tested the activity of the EGFR-specific recombinant immunotoxin (IT) 425(scFv)-ETA' against EGFR(+) RMS cells in vitro and ex vivo.

METHODS

We tested the specific binding and internalization behavior of 425(scFv)-ETA' in RMS cell lines in vitro by flow cytometry, compared to the corresponding imaging probe 425(scFv)-SNAP monitored by live cell imaging. The cytotoxic activity of 425(scFv)-ETA' was tested using cell viability and apoptosis assays. Specific binding of the IT was confirmed on formalin-fixed paraffin-embedded tissue samples from two RMS patients.

RESULTS

We confirmed the specific binding of 425(scFv)-ETA' to RMS cells in vitro and ex vivo. Both the IT and the corresponding imaging probe were rapidly internalized. The IT killed EGFR(+) RMS cells in a dose-dependent manner, while showing no effect against control cells. It showed specific apoptotic activity against one selected RMS cell line.

CONCLUSIONS

This is the first study showing the promising therapeutic potential of a recombinant, EGFR-targeting, ETA'-based IT on RMS cells. We confirmed the selective killing with IC50 values of up to 50 pM, and immunohistochemical staining confirmed the specific ex vivo binding to primary RMS material.

Glycans in pathogenic bacteria--potential for targeted covalent therapeutics and imaging agents

A substantial obstacle to the existing treatment of bacterial diseases is the lack of specific probes that can be used to diagnose and treat pathogenic bacteria in a selective manner while leaving the microbiome largely intact. To tackle this problem, there is an urgent need to develop pathogen-specific therapeutics and diagnostics. Here, we describe recent evidence that indicates distinctive glycans found exclusively on pathogenic bacteria could form the basis of targeted therapeutic and diagnostic strategies. In particular, we highlight the use of metabolic oligosaccharide engineering to covalently deliver therapeutics and imaging agents to bacterial glycans.

Advances in strategies to improve drug delivery to brain tumors

Brain tumors remain a significant health problem. Advances in the biology of the blood-brain barrier are improving the ability of researchers to target therapeutic peptides, small molecules and other drugs to brain tumors. Simple methods to improve blood-brain barrier penetration include chemical modification, glycosylation and pegylation. Drug-delivery vehicles, such as nanoparticles and liposomes, are also under study. Targeting vectors include natural ligands (e.g., epidermal growth factor) or monoclonal antibodies to receptors (e.g., transferrin or insulin). Other vector-mediated delivery approaches involve the conjugation of a therapeutic peptide or protein with a targeting molecule that can induce transcytosis across blood-brain barrier endothelial cells. The most commonly used vectors are peptidomimetic antibodies to endothelial receptors, such as the transferrin and insulin receptors.

Immunotoxins: the role of the toxin

Immunotoxins are antibody-toxin bifunctional molecules that rely on intracellular toxin action to kill target cells. Target specificity is determined via the binding attributes of the chosen antibody. Mostly, but not exclusively, immunotoxins are purpose-built to kill cancer cells as part of novel treatment approaches. Other applications for immunotoxins include immune regulation and the treatment of viral or parasitic diseases. Here we discuss the utility of protein toxins, of both bacterial and plant origin, joined to antibodies for targeting cancer cells. Finally, while clinical goals are focused on the development of novel cancer treatments, much has been learned about toxin action and intracellular pathways. Thus toxins are considered both medicines for treating human disease and probes of cellular function.

Expression of the h19 oncofetal gene in premalignant lesions of cervical cancer: a potential targeting approach for development of nonsurgical treatment of high-risk lesions. ISRN Obstet Gynecol. 2013;2013:137509. [PMID: 23984081 DOI: 10.1155/2013/137509]

Background. Recent data suggest a role for H19 gene in promoting cancer transformation and progression. Cervical cancer, progresses from high-grade lesions (CIN3). At present, it is unclear if CIN lesions express H19. Objectives. To determine H19 expression in patient samples of CIN3 as well as the ability of a construct in which the promoter from the H19 gene drives expression of the diphtheria toxin A chain (DTA) to inhibit cervical cancer cell growth in vitro. Methods. H19 transcript levels were evaluated on 10 biopsies of CIN3 using in situ hybridization. PCR was used to examine H19 expression in cervical cancer cell lines and in two samples from a patient with cervical carcinoma. Cell lines were transfected with H19-DTA to determine its impact on cell number. Results. H19 gene was expressed in the area of CIN3 in 9 out of 10 samples. RT-PCR indicated expression of H19 in cervical cancer samples and in one of the three cell lines examined. Transfection of all cell lines with H19-DTA vector resulted in inhibited cell growth. Conclusions. H19 is expressed in the majority of CIN3 samples. These results suggest that most CIN3 lesions could be targeted by H19-DTA. Further in vivo preclinical studies are thus warranted.

Current status of local therapy in malignant gliomas--a clinical review of three selected approaches

Malignant gliomas are the most frequently occurring, devastating primary brain tumors, and are coupled with a poor survival rate. Despite the fact that complete neurosurgical resection of these tumors is impossible in consideration of their infiltrating nature, surgical resection followed by adjuvant therapeutics, including radiation therapy and chemotherapy, is still the current standard therapy. Systemic chemotherapy is restricted by the blood-brain barrier, while methods of local delivery, such as with drug-impregnated wafers, convection-enhanced drug delivery, or direct perilesional injections, present attractive ways to circumvent these barriers. These methods are promising ways for direct delivery of either standard chemotherapeutic or new anti-cancer agents. Several clinical trials showed controversial results relating to the influence of a local delivery of chemotherapy on the survival of patients with both recurrent and newly diagnosed malignant gliomas. Our article will review the development of the drug-impregnated release, as well as convection-enhanced delivery and the direct injection into brain tissue, which has been used predominantly in gene-therapy trials. Further, it will focus on the use of convection-enhanced delivery in the treatment of patients with malignant gliomas, placing special emphasis on potential shortcomings in past clinical trials. Although there is a strong need for new or additional therapeutic strategies in the treatment of malignant gliomas, and although local delivery of chemotherapy in those tumors might be a powerful tool, local therapy is used only sporadically nowadays. Thus, we have to learn from our mistakes in the past and we strongly encourage future developments in this field.

Hemangiosarcoma and its cancer stem cell subpopulation are effectively killed by a toxin targeted through epidermal growth factor and urokinase receptors

Targeted toxins have the potential to overcome intrinsic or acquired resistance of cancer cells to conventional cytotoxic agents. Here, we hypothesized that EGFuPA-toxin, a bispecific ligand-targeted toxin (BLT) consisting of a deimmunized Pseudomonas exotoxin (PE) conjugated to epidermal growth factor and urokinase, would efficiently target and kill cells derived from canine hemangiosarcoma (HSA), a highly chemotherapy resistant tumor, as well as cultured hemangiospheres, used as a surrogate for cancer stem cells (CSC). EGFuPA-toxin showed cytotoxicity in four HSA cell lines (Emma, Frog, DD-1 and SB) at a concentration of ≤100 nM, and the cytotoxicity was dependent on specific ligand-receptor interactions. Monospecific targeted toxins also killed these chemoresistant cells; in this case, a "threshold" level of EGFR expression appeared to be required to make cells sensitive to the monospecific EGF-toxin, but not to the monospecific uPA-toxin. The IC₅₀ of CSCs was higher by approximately two orders of magnitude as compared to non-CSCs, but these cells were still sensitive to EGFuPA-toxin at nanomolar (i.e., pharmacologically relevant) concentrations, and when targeted by EGFuPA-toxin, resulted in death of the entire cell population. Taken together, our results support the use of these toxins to treat chemoresistant tumors such as sarcomas, including those that conform to the CSC model. Our results also support the use of companion animals with cancer for further translational development of these cytotoxic molecules.

Magnetic nanoparticles for tumor imaging and therapy: a so-called theranostic system

In this review, we discussed the establishment of a so-called "theranostic" system by instituting the basic principles including the use of: [1] magnetic iron oxide nanoparticles (MION)-based drug carrier; [2] intra-arterial (I.A.) magnetic targeting; [3] macromolecular drugs with unmatched therapeutic potency and a repetitive reaction mechanism; [4] cell-penetrating peptide-mediated cellular drug uptake; and [5] heparin/protamine-regulated prodrug protection and tumor-specific drug re-activation into one single drug delivery system to overcome all possible obstacles, thereby achieving a potentially non-invasive, magnetic resonance imaging-guided, clinically enabled yet minimally toxic brain tumor drug therapy. By applying a topography-optimized I.A. magnetic targeting to dodge rapid organ clearance of the carrier during its first passage into the circulation, tumor capture of MION was enriched by >350 folds over that by conventional passive enhanced permeability and retention targeting. By adopting the prodrug strategy, we observed by far the first experimental success in a rat model of delivering micro-gram quantity of the large β-galactosidase model protein selectively into a brain tumor but not to the ipsi- or contra-lateral normal brain regions. With the therapeutic regimens of most toxin/siRNA drugs to fully (>99.9%) eradicate a tumor being in the nano-molar range, the prospects of reaching this threshold become practically accomplishable.

A novel recombinant immunocasp-6 fusion gene specifically and efficiently suppresses HER2-overexpressing osteosarcoma

Osteosarcoma is the most common primary malignant tumor of bone for adolescent or children. The poor prognosis of patients, due to its remote metastasis, has led to the exploration of more effective and less toxic treatments. Immunotherapy is a promising strategy for the treatment of human epidermal growth factor receptor 2 (HER2)-overexpressing tumors. Herein, we describe experiments conducted with a fusion gene, immunocasp-6, which was generated by fusing a HER2-specific single-chain Ab, a single-chain Pseudomonas exotoxin A and an active caspase-6 which can directly cleave lamin A leading to nucleus damage inducing programmed cell death. We demonstrated that immunocasp-6 can specifically and efficiently recognize and induce apoptosis in HER2-overexpressing osteosarcoma cells in vitro. The immunocasp-6 was transferred into BALB/c athymic mice bearing human osteosarcoma by i.m. injection of liposome-encapsulated pCMV-immunocap-6. Expression of immunocasp-6 not only strongly inhibited tumor growth and significantly prolonged animal survival, but also greatly prevented tumor metastasis. Our data showed that the immuno-casp-6 can specifically recognize HER2-overexpressing osteosarcoma cells, can also promptly attack their nucleus and induce apoptotic death, suggesting the potential of this strategy for the treatment of human HER2-overexpressing tumors.

Toward intracellular targeted delivery of cancer therapeutics: progress and clinical outlook for brain tumor therapy

A number of anti-cancer drugs have their targets localized to particular intracellular compartments. These drugs reach the targets mainly through diffusion, dependent on biophysical and biochemical forces that allow cell penetration. This means that both cancer cells and normal cells will be subjected to such diffusion; hence many of these drugs, like chemotherapeutics, are potentially toxic and the concentration achieved at the site of their action is often suboptimal. The same relates to radiation that indiscriminately affects normal and diseased cells. However, nature-designed systems enable compounds present in the extracellular environment to end up inside the cell and even travel to more specific intracellular compartments. For example, viruses and bacterial toxins can more or less specifically recognize eukaryotic cells, enter these cells, and direct some protein portions to designated intracellular areas. These phenomena have led to creative thinking, such as employing viruses or bacterial toxins for cargo delivery to cells and, more specifically, to cancer cells. Proteins can be genetically engineered in order to not only mimic what viruses and bacterial toxins can do, but also to add new functions, extending or changing the intracellular routes. It is possible to make conjugates or, more preferably, single-chain proteins that recognize cancer cells and deliver cargo inside the cells, even to the desired subcellular compartment. These findings offer new opportunities to deliver drugs/labels only to cancer cells and only to their site of action within the cells. The development of such dual-specificity vectors for targeting cancer cells is an attractive and potentially safer and more efficacious way of delivering drugs. We provide examples of this approach for delivering brain cancer therapeutics, using a specific biomarker on glioblastoma tumor cells.

Targeted toxins for glioblastoma multiforme: pre-clinical studies and clinical implementation

Glioblastoma multiforme (GBM) is the most common primary brain tumor in adults. GBM is very aggressive due to its poor cellular differentiation and invasiveness, which makes complete surgical resection virtually impossible. Therefore, GBM's invasive nature as well as its intrinsic resistance to current treatment modalities makes it a unique therapeutic challenge. Extensive examination of human GBM specimens has uncovered that these tumors overexpress a variety of receptors that are virtually absent in the surrounding non-neoplastic brain. Human GBMs overexpress receptors for cytokines, growth factors, ephrins, urokinase-type plasminogen activator (uPA), and transferrin, which can be targeted with high specificity by linking their ligands with highly cytotoxic molecules, such as Diptheria toxin and Pseudomonas exotoxin A. We review the preclinical development and clinical translation of targeted toxins for GBM. In view of the clinical experience, we conclude that although these are very promising therapeutic modalities for GBM patients, efforts should be focused on improving the delivery systems utilized in order to achieve better distribution of the immuno-toxins in the tumor/resection cavity. Delivery of targeted toxins using viral vectors would also benefit enormously from improved strategies for local delivery.

Production and characterization of a colon cancer-specific immunotoxin based on the fungal ribotoxin α-sarcin

A single-chain fusion protein that directed the cytolytic activity of α-sarcin to A33 tumor antigen expressing cells was constructed and shown to effectively kill targeted cells. Glycoprotein A33 (GPA33) is a well-known colon cancer marker and a humanized antibody against it was used to target the α-sarcin. The fungal ribotoxin α-sarcin is one of the most potent and specific toxins known. It is small, protease resistant, thermostable and highly efficient towards the inactivation of ribosomes. This work describes the production and characterization of an immunotoxin resulting from fusing the single-chain variable fragment (scFv) of the monoclonal antibody that targets GPA33 to fungal α-sarcin. This chimeric protein (scFvA33αsarcin), produced in Pichia pastoris and purified in high yield was proven to be properly folded, active, specific and stable. It showed high specific toxicity against GPA33-positive tumoral cell lines providing scientific evidence to sustain that scFvA33αsarcin is a good immunotherapeutic candidate against GPA33-positive colon carcinomas.

Eukaryotic expression vectors bearing genes encoding cytotoxic proteins for cancer gene therapy

Cancer gene therapy is a promising direction for the treatment of cancer patients. A primary goal of all cancer therapies is to selectively target and kill tumour cells. Such therapies are administered via different approaches, including both viral and non-viral delivery; however, both methods have advantages and disadvantages. Transcriptional targeting enables genes encoding toxic proteins to be expressed directly in cancer cells. Numerous vectors have been created with the purpose of killing cancer cells, and some have successfully suppressed malignant tumours. Data concerning the function of vectors bearing genes that encode cytotoxic proteins under the control of different promoters, including tissue/tumour specific and constitutive promoters, is summarised here. This review focuses on vectors that bear genes encoding diphtheria toxin, Pseudomonas exotoxin A, caspases, gef, streptolysin, and melittin. Data describing the efficacy of such vectors have been summarised. Notably, there are vectors that killed cancer cell lines originating from the same type of cancer with differential efficiency. Thus, there is differential inhibition of cancer cell growth dependent on the cell line. In this review, the constructs employing genes whose expression induces cell death and the efficiency with which they suppress cancer cell growth will be summarised.

Convection-enhanced delivery of therapeutic agents into the brain

CED of therapeutic agents remains a promising strategy for treating malignant gliomas and non-neoplastic neurological diseases. Although initial clinical trials have failed to show survival benefit for new agents delivered via this approach, multiple earlier stage trials have addressed only a fraction of the myriad of technical and technological issues that surround this novel approach. Development of CED has been limited by the fact that both new technologies and novel therapeutic agents are being developed simultaneously.New trials are being planned to investigate agents that can be coinfused with radiographic tracers, as well as novel catheters that avoid problems with backflow and potentially will provide more reliable drug distribution.

Orexin 2 receptor as a potential target for immunotoxin and antibody-drug conjugate cancer therapy

Targeting tumor-specific receptors is a promising approach for cytotoxic agents. The orexin 2 receptor (OX2R) has reportedly been expressed in a few types of cancer, but not in normal, cells. This study aimed to explore and assess the expression levels of OX2R in a wide range of cancer cell lines and clinical samples to identify its localization. To analyze OX2R expression, we developed a polyclonal antibody specific to OX2R by immunizing two rabbits with a peptide cocktail. A total of 36 cancer cell lines were employed for reverse transcription polymerase chain reaction (RT-PCR) and western blot analysis, and 221 samples from various tissue arrays were used for the immunohistochemistry of OX2R expression. OX2R was identified in three cancerous cell lines, from the gallbladder, squamous cell carcinoma of the head and neck (SCCHN) and glioblastoma. With clinical samples of tissue arrays, 69/221 (31.2%) samples reacted positively with the OX2R antibody. We confirmed its presence on the cell membrane. In conclusion, OX2R was identified on several cancer cells as well as clinical samples. Further studies with larger numbers of clinical samples are required to confirm the statistical significance of the presence and relationships of OX2R with tumor histology. Results of the current study suggested that OX2R is a potent target for immunotoxin or antibody-drug conjugate (ADC) cancer therapy on OX2R-positive cancer cells.

Increasing stability and toxicity of Pseudomonas exotoxin by attaching an antiproteasic Peptide

Trypsin-like activities are present within the endocytic pathway and allow cells to inactivate a fraction of incoming toxins, such as Pseudomonas exotoxin (PE), that require endocytic uptake before reaching the cytosol to inactivate protein synthesis. PE is a favorite toxin for building immunotoxins. The latter are promising molecules to fight cancer or transplant rejection, and producing more active toxins is a key challenge. More broadly, increasing protein stability is a potentially useful approach to improve the efficiency of therapeutic proteins. We report here that fusing an antiproteasic peptide (bovine pancreatic trypsin inhibitor, BPTI) to PE increases its toxicity to human cancer cell lines by 20-40-fold. Confocal microscopic examination of toxin endocytosis, digestion, and immunoprecipitation experiments showed that the fused antiproteasic peptide specifically protects PE from trypsin-like activities. Hence, the attached BPTI acts as a bodyguard for the toxin within the endocytic pathway. Moreover, it increased the PE elimination half-time in mice by 70%, indicating that the fused BPTI stabilizes the toxin in vivo. This BPTI-fusion approach may be useful for protecting other circulating or internalized proteins of therapeutic interest from premature degradation.

Gene therapy and targeted toxins for glioma

The most common primary brain tumor in adults is glioblastoma. These tumors are highly invasive and aggressive with a mean survival time of 15-18 months from diagnosis to death. Current treatment modalities are unable to significantly prolong survival in patients diagnosed with glioblastoma. As such, glioma is an attractive target for developing novel therapeutic approaches utilizing gene therapy. This review will examine the available preclinical models for glioma including xenographs, syngeneic and genetic models. Several promising therapeutic targets are currently being pursued in pre-clinical investigations. These targets will be reviewed by mechanism of action, i.e., conditional cytotoxic, targeted toxins, oncolytic viruses, tumor suppressors/oncogenes, and immune stimulatory approaches. Preclinical gene therapy paradigms aim to determine which strategies will provide rapid tumor regression and long-term protection from recurrence. While a wide range of potential targets are being investigated preclinically, only the most efficacious are further transitioned into clinical trial paradigms. Clinical trials reported to date are summarized including results from conditionally cytotoxic, targeted toxins, oncolytic viruses and oncogene targeting approaches. Clinical trial results have not been as robust as preclinical models predicted; this could be due to the limitations of the GBM models employed. Once this is addressed, and we develop effective gene therapies in models that better replicate the clinical scenario, gene therapy will provide a powerful approach to treat and manage brain tumors.

A novel bispecific ligand-directed toxin designed to simultaneously target EGFR on human glioblastoma cells and uPAR on tumor neovasculature

A bispecific ligand-directed toxin (BLT), called EGFATFKDEL, consisting of human epidermal growth factor, a fragment of urokinase, and truncated pseudomonas exotoxin (PE38) was assembled in order to target human glioblastoma. Immunogenicity was reduced by mutating seven immunodominant B-cell epitopes on the PE38 molecule to create a new agent, EGFATFKDEL 7mut. In vitro, the drug selectively killed several human glioblastoma cell lines. EGFATFKDEL is our first BLT designed to simultaneously target EGFR on solid tumors and uPAR on the tumor neovasculature. In vitro assays revealed that the agent is effective against glioblastoma cell lines as well as human umbilical vein endothelial cells (HUVEC). Additionally, the bispecific drug displayed enhanced binding to overexpressed epidermal growth factor receptor and urokinase receptor when compared to similar monospecific drugs, EGFKDEL and ATFKDEL. In vivo, an aggressive human glioblastoma cell line was genetically marked with a firefly luciferase reporter gene and administered to the flanks of nude mice. Treatment with intratumoral injections of EGFATFKDEL 7mut eradicated small tumors in over half of the treated mice, which survived with tumor free status at least 100 days post tumor inoculation. ATFKDEL, which primarily targets the tumor neovasculature, prevented tumor growth but did not result in tumor-free mice in most cases. Specificity was shown by treating with an irrelevant BLT control which did not protect mice. Finally, immunization experiments in immunocompetent mice revealed significantly reduced anti-toxin production in EGFATFKDEL 7mut treated groups. Thus, EGFATFKDEL 7mut is an effective drug for glioblastoma therapy in this murine model and warrants further study.

The conjugation of an AQP1-directed immunotoxin in the study of site-directed therapy within the CNS

PURPOSE

The water channel, aquaporin (AQP)1, is highly specific to the choroid plexus (CP) epithelium within the brain. It is therefore a potential target through which therapeutic agents could be selectively directed to the CP. Here we describe the conjugation of a monoclonal antibody (mAb), raised against an extra-cellular domain of AQP1, to the A chain of ricin (RCA). This reagent should allow study of a highly specific chemical lesion of the CP.

METHODS

A published method was used to couple the anti-AQP1 mAb to the A chain of ricin using an SPDP cross-linker. The conjugate was purified by Superdex S-200 chromatography. Column fractions were analysed by SDS-PAGE and Western blotting. Fractions containing conjugate were assayed for cytotoxicity on rat CP cells in culture, using a WST-1 viability assay to assess cell death.

RESULTS

SDS-PAGE and Western blotting demonstrated separation of conjugated anti-AQP1 mAb from unconjugated ricin by the Superdex column. Cultured rat CP cells were killed with an IC(50) of 1.81 μg/ml when exposed to the anti-AQP1 mAb-RCA conjugation reaction products. Selectivity of this reagent was demonstrated by the higher IC(50) exhibited by non-AQP1-expressing primary fibroblast cultures and cultures exposed to the reaction products from the non-specific IgG-RCA conjugation attempt.

CONCLUSION

We have been able to demonstrate selective cytotoxicity in vitro of an anti-AQP1 mAb-RCA conjugate. We hope that this work will generate further interest in the use of this site-specific conjugate to target the CP in conditions such as hydrocephalus, tumours that arise from here, and other AQP1-expressing tumours.

From toxins targeting ligand gated ion channels to therapeutic molecules

Ligand-gated ion channels (LGIC) play a central role in inter-cellular communication. This key function has two consequences: (i) these receptor channels are major targets for drug discovery because of their potential involvement in numerous human brain diseases; (ii) they are often found to be the target of plant and animal toxins. Together this makes toxin/receptor interactions important to drug discovery projects. Therefore, toxins acting on LGIC are presented and their current/potential therapeutic uses highlighted.

Genetically-Engineered Protease-Activated Triggers in a Pore-Forming Protein

Protease-activated triggers have been introduced Into a pore-forming protein, staphylococcal a-hemolysin (αHL). The hemolysin was remodeled by genetic engineering to form two-chain constructs with redundant polypeptide sequences at the central loop, the Integrity of which Is crucial for efficient pore formation. The new hemolysins are activated when the polypeptide extensions are removed by proteases. By alterating the protease recognition sequence in the loop, selective activation by specified proteases can be obtained. Protease-triggered pore-forming proteins might be used for the selective destruction of cancer cells that bear tumor-associated proteases. When certain two-chain constructs are treated with proteases, a full-length polypeptide chain forms as the result of a protease-mediated transpeptidation reaction. This reaction might be used to produce chimeric hemolysins that are Inaccessible by conventional routes.

Investigation of a plasmid containing a novel immunotoxin VEGF165-PE38 gene for antiangiogenic therapy in a malignant glioma model

Inhibition of tumor neovascularization has profound effects on the growth of solid tumors. Our previous studies have shown the effect of VEGF165-PE38 recombinant immunotoxin on proliferation and apoptosis in human umbilical vein endothelial cells in vitro. In this study, we explored the direct inhibition of angiogenesis in chick chorioallantoic membrane and antiangiogenic therapy in a malignant glioma model. HEK293 cells were transfected with the pVEGF165PE38-IRES2-EGFP plasmid. ELISA was used to confirm the expression of VEGF165-PE38 in the transfected cells. These cells released 1396 + or - 131.9 pg VEGF165-PE38/1x10(4) cells/48 h into the culture medium and the supernatant was capable of inhibiting the growth of capillary-like structures in chick chorioallantoic membrane assay. In a murine malignant glioma model, plasmid was directly administered via multiple local intratumoral delivery. After day 16 the tumor volume in mice treated with pVEGF165PE38-IRES2-EGFP was significantly lower than that in mice in the control groups. Immunohistochemistry studies showed that the treated group had decreased expression of CD31. Quantitative analysis of microvessel density in the treated group was 1.99 + or - 0.69/0.74 mm(2), and was significantly lower than that in the control groups (9.33 + or - 1.99/0.74 mm(2), 8.09 + or - 1.39/0.74 mm(2) and 8.49 + or - 1.69/0.74 mm(2)). Immunohistochemistry analysis indicated that immunotoxin VEGF165-PE38 was distributed in the treated group in malignant glioma tissue. Our findings provide evidence that the in vivo production of VEGF165-PE38 through gene therapy using a eukaryotic expression plasmid had potential antiangiogenic activity in malignant glioma in vivo.

Proteolysis of Pseudomonas exotoxin A within hepatic endosomes by cathepsins B and D produces fragments displaying in vitro ADP-ribosylating and apoptotic effects

To assess Pseudomonas exotoxin A (ETA) compartmentalization, processing and cytotoxicity in vivo, we have studied the fate of internalized ETA with the use of the in vivo rodent liver model following toxin administration, cell-free hepatic endosomes, and pure in vitro protease assays. ETA taken up into rat liver in vivo was rapidly associated with plasma membranes (5-30 min), internalized within endosomes (15-60 min), and later translocated into the cytosolic compartment (30-90 min). Coincident with endocytosis of intact ETA, in vivo association of the catalytic ETA-A subunit and low molecular mass ETA-A fragments was observed in the endosomal apparatus. After an in vitro proteolytic assay with an endosomal lysate and pure proteases, the ETA-degrading activity was attributed to the luminal species of endosomal acidic cathepsins B and D, with the major cleavages generated in vitro occurring mainly within domain III of ETA-A. Cell-free endosomes preloaded in vivo with ETA intraluminally processed and extraluminally released intact ETA and ETA-A in vitro in a pH-dependent and ATP-dependent manner. Rat hepatic cells underwent in vivo intrinsic apoptosis at a late stage of ETA infection, as assessed by the mitochondrial release of cytochrome c, caspase-9 and caspase-3 activation, and DNA fragmentation. In an in vitro assay, intact ETA induced ADP-ribosylation of EF-2 and mitochondrial release of cytochrome c, with the former effect being efficiently increased by a cathepsin B/cathepsin D pretreatment. The data show a novel processing pathway for internalized ETA, involving cathepsins B and D, resulting in the production of ETA fragments that may participate in cytotoxicity and mitochondrial dysfunction.

Targeted tumor gene therapy based on loss of IGF2 imprinting

Loss of imprinting (LOI) of the insulin-like growth factor 2 gene (IGF2) is one of the most common epigenetic abnormalities seen in human neoplasms. LOI may be associated with the lack of Zinc-finger DNA binding protein CTCF-mediated enhancer insulation, presumably due to the gain of methylation on the maternal allele of the differentially methylated domain (DMD) of the imprinting control region. This results in an interaction between the IGF2 promoters and enhancers; and IGF2 is produced from both alleles. In this study we investigated the feasibility of a novel anti-cancer adenovirus (AdDC312-DT-A) driven by H19 enhancer DMD-H19 promoter complex. Cell lines with IGF2 LOI (HCT-8, HT-29 and H-522) that were infected with AdDC312-EGFP produced the EGFP protein. However, in cells in which imprinting was maintained (MOI) (MCF-7 and GES-1), no EGFP protein was produced. The AdDC312-DT-A significantly decreased cell viability and induced apoptosis only in LOI cells in vitro, and suppressed tumour development in HCT-8 xenografts in nude mice. In conclusion, the toxin gene therapy proves effective in inhibiting LOI cell growth in vitro and in vivo and provides a novel option for targeted gene therapy based on loss of IGF2 imprinting.

Treatment of ovarian cancer ascites by intra-peritoneal injection of diphtheria toxin A chain-H19 vector: a case report

Introduction

Ovarian cancer ascitic fluid, which contains malignant cells, is usually present in women with an advanced stage disease. There are currently no effective therapies for the treatment of ovarian cancer ascitic fluid. We developed a new therapeutic strategy to target expression of the diphtheria toxin fragment A gene in ovarian tumor cells under the control of H19 regulatory sequences.

Case presentation

A 64-year-old Caucasian woman was diagnosed with a stage IIIc epithelial ovarian cancer. She suffered from progressive disease, accumulation of malignant ascites that needed to be drained weekly, abdominal pain, vomiting, anorexia and severe weakness. Infusion of the diphtheria toxin A chain-H19 plasmid into the peritoneum of our patient resulted in complete resolution of the ascites with minimum adverse events.

Conclusion

On the basis of this preliminary experience, we are currently conducting an extensive Phase I study on a larger number of patients in order to assess the safety and preliminary efficacy of this novel patient-oriented treatment approach.

A novel composite immunotoxin that suppresses rabies virus production by the infected cells

Using strepavidin as a scaffold, we have assembled a composite immunotoxin that consists of recombinant Pseudomonas exotoxin A subunit (PE38) and recombinant 25-D1.16 Fab fragment which recognizes the SIINFEKL (pOV8) peptide from ovalbumin in association with H-2K(b) MHC class I protein. The composite immunotoxin exercises cytotoxicity against H-2K(b+) cells sensitized with pOV8 peptide but not with irrelevant peptide. Specific binding of the immunotoxin to H-2K(b+) cells infected with recombinant rabies virus (RV) expressing pOV8 epitope (RV-pOV8) resulted in the suppression of the production of virus particles by the infected cells. This strategy allows readily produce different immunotoxins with desired specificity by combining various targeting and toxin molecules. The results provide a proof of concept that composite immunotoxins can be utilized as novel immunotherapeutics to stop virus spread in the acute phase of the infection allowing winning time for the development of protective immune response.

Depleting syndecan-4+ T lymphocytes using toxin-bearing dendritic cell-associated heparan sulfate proteoglycan-dependent integrin ligand: a new opportunity for treating activated T cell-driven disease

Because syndecan-4 (SD-4) is expressed by some (but not all) T cells following activation and serves as the exclusive ligand of dendritic cell-associated heparan sulfate proteoglycan-dependent integrin ligand (DC-HIL), we envisioned the DC-HIL/SD-4 pathway to be a therapeutic target for conditions mediated by selectively activated T cells. We conjugated soluble DC-HIL receptor with the toxin saporin (SAP; DC-HIL-SAP) and showed it to bind activated (but not resting) T cells and become internalized by and deplete SD-4(+) T cells. In hapten-sensitized mice, DC-HIL-SAP injected i.v. prior to hapten challenge led to markedly suppressed contact hypersensitivity responses that lasted 3 wk and were restricted to the hapten to which the mice were originally sensitized. Such suppression was not observed when DC-HIL-SAP was applied during sensitization. Moreover, the same infusion of DC-HIL-SAP produced almost complete disappearance of SD-4(+) cells in haptenated skin and a 40% reduction of such cells within draining lymph nodes. Our results provide a strong rationale for exploring use of toxin-conjugated DC-HIL to treat activated T cell-driven disease in humans.

Convection-enhanced delivery for the treatment of brain tumors

The brain is highly accessible for nutrients and oxygen, however delivery of drugs to malignant brain tumors is a very challenging task. Convection-enhanced delivery (CED) has been designed to overcome some of the difficulties so that pharmacological agents that would not normally cross the BBB can be used for treatment. Drugs are delivered through one to several catheters placed stereotactically directly within the tumor mass or around the tumor or the resection cavity. Several classes of drugs are amenable to this technology including standard chemotherapeutics or novel experimental targeted drugs. The first Phase III trial for CED-delivered, molecularly targeted cytotoxin in the treatment of recurrent glioblastoma multiforme has been accomplished and demonstrated objective clinical efficacy. The lessons learned from more than a decade of attempts at exploiting CED for brain cancer treatment weigh critically for its future clinical applications. The main issues center around the type of catheters used, number of catheters and their exact placement; pharmacological formulation of drugs, prescreening patients undergoing treatment and monitoring the distribution of drugs in tumors and the tumor-infiltrated brain. It is expected that optimizing CED will make this technology a permanent addition to clinical management of brain malignancies.

Convection-enhanced delivery of free gadolinium with the recombinant immunotoxin MR1-1

A major obstacle in glioblastoma (GBM) therapy is the restrictive nature of the blood-brain barrier (BBB). Convection-enhanced delivery (CED) is a novel method of drug administration which allows direct parenchymal infusion of therapeutics, bypassing the BBB. MR1-1 is a novel recombinant immunotoxin that targets the GBM tumor-specific antigen EGFRvIII and can be delivered via CED infusion. However, drug distribution via CED varies dramatically, which necessitates active monitoring. Gadolinium conjugated to diethylenetriamine penta-acetic acid (Gd-DTPA) is a commonly used MRI contrast agent which can be co-infused with therapies using CED and may be useful in monitoring infusion leak and early distribution. Forty immunocompetent rats were implanted with intracerebral cannulas that were connected to osmotic pumps and subsequently randomized into four groups that each received 0.2% human serum albumin (HSA) mixed with a different experimental infusion: (1) 25 ng/ml MR1-1; (2) 0.1 micromol/ml Gd-DTPA; (3) 25 ng/ml MR1-1 and 0.1 micromol/ml Gd-DTPA; (4) 250 ng/ml MR1-1 and 0.1 micromol/ml Gd-DTPA. The rats were monitored clinically for 6 weeks then necropsied and histologically assessed for CNS toxicity. All rats survived the entirety of the study without clinical or histological toxicity attributable to the study drugs. There was no statistically significant difference in weight change over time among groups (P > 0.999). MR1-1 co-infused with Gd-DTPA via CED is safe in the long-term setting in a pre-clinical animal model. Our data supports the use of Gd-DTPA, as a surrogate tracer, co-infused with MR1-1 for drug distribution monitoring in patients with GBM.

Design and modification of EGF4KDEL 7Mut, a novel bispecific ligand-directed toxin, with decreased immunogenicity and potent anti-mesothelioma activity

BACKGROUND

Potency, immunogenicity, and toxicity are three problems that limit the use of targeted toxins in solid tumour therapy.

METHODS

To address potency, we used genetic engineering to develop a novel bispecific ligand-directed toxin (BLT) called EGF4KDEL, a novel recombinant anti-mesothelioma agent created by linking human epidermal growth factor (EGF) and interleukin-4 (IL-4) to truncated pseudomonas exotoxin (PE38) on the same single-chain molecule. Immunogenicity was reduced by mutating seven immunodominant B-cell epitopes on the PE38 molecule to create a new agent, EGF4KDEL 7Mut.

RESULTS

In vitro, bispecific EGF4KDEL showed superior anti-mesothelioma activity compared with its monospecific counterparts. Toxicity in mice was diminished by having both ligands on the same molecule, allowing administration of a 10-fold greater dose of BLT than a mixture of monomeric IL4KDEL and EGFKDEL. EGF4KDEL 7Mut, retained all of its functional activity and induced about 87% fewer anti-toxin antibodies than mice given the parental, non-mutated form. In vivo, intraperitoneal (IP) injection of the BLT showed significant (P<0.01) and impressive effects against two aggressive, malignant IP mesothelioma models when treatment was begun 14-16 days post tumour innoculation.

CONCLUSION

These data show that EGF4KDEL 7Mut is a promising new anti-mesothelioma agent that was developed to specifically address the obstacles facing clinical utility of targeted toxins.

Development of targeted therapy for ovarian cancer mediated by a plasmid expressing diphtheria toxin under the control of H19 regulatory sequences

Background

Ovarian cancer ascites fluid (OCAF), contains malignant cells, is usually present in women with an advanced stage disease and currently has no effective therapy. Hence, we developed a new therapy strategy to target the expression of diphtheria toxin gene under the control of H19 regulatory sequences in ovarian tumor cells. H19 RNA is present at high levels in human cancer tissues (including ovarian cancer), while existing at a nearly undetectable level in the surrounding normal tissue.

Methods

H19 gene expression was tested in cells from OCAF by the in-situ hybridization technique (ISH) using an H19 RNA probe. The therapeutic potential of the toxin vector DTA-H19 was tested in ovarian carcinoma cell lines and in a heterotopic animal model for ovarian cancer.

Results

H19 RNA was detected in 90% of patients with OCAF as determined by ISH. Intratumoral injection of DTA-H19 into ectopically developed tumors caused 40% inhibition of tumor growth.

Conclusion

These observations may be the first step towards a major breakthrough in the treatment of human OCAF, while the effect in solid tumors required further investigation. It should enable us to identify likely non-responders in advance, and to treat patients who are resistant to all known therapies, thereby avoiding treatment failure.

Processing of Pseudomonas aeruginosa exotoxin A is dispensable for cell intoxication

Exotoxin A is a major virulence factor of Pseudomonas aeruginosa. This toxin binds to a specific receptor on animal cells, allowing endocytosis of the toxin. Once in endosomes, the exotoxin can be processed by furin to generate a C-terminal toxin fragment that lacks the receptor binding domain and is retrogradely transported to the endoplasmic reticulum for retrotranslocation to the cytosol through the Sec61 channel. The toxin then blocks protein synthesis by ADP ribosylation of elongation factor 2, thereby triggering cell death. A shorter intracellular route has also been described for this toxin. It involves direct translocation of the entire toxin from endosomes to the cytosol and therefore does not rely on furin-mediated cleavage. To examine the implications of endosomal translocation in the intoxication process, we investigated whether the toxin required furin-mediated processing in order to kill cells. We used three different approaches. We first fused to the N terminus of the toxin proteins with different unfolding abilities so that they inhibited or did not inhibit endosomal translocation of the chimera. We then assayed the amount of toxin fragments delivered to the cytosol during cell intoxication. Finally we used furin inhibitors and examined the fate and intracellular localization of the toxin and its receptor. The results showed that exotoxin cytotoxicity results largely from endosomal translocation of the entire toxin. We found that the C-terminal fragment was unstable in the cytosol.

Molecularly targeted therapies for malignant gliomas

This review critically evaluates current knowledge of molecularly targeted therapies of malignant gliomas.Various molecularly targeted single-agent therapies, including targeted therapies of growth and survival, have been evaluated in clinical trials but have failed to demonstrate a significant survival benefit compared with standard treatment regimens. The efficacy of multitargeted kinase inhibitors or combinations of single-targeted kinase inhibitors is a promising strategy, but requires additional clinical evaluation before definitive conclusions can be made. Important areas for further research include the assessment of serum or tissue biomarkers, the elucidation of prognostic molecular markers, and the determination of whether the mechanism of action of a drug is appropriate to the genetic alterations observed within individual tumors.

Immunotherapy of malignant brain tumors

Despite aggressive multi-modality therapy including surgery, radiation, and chemotherapy, the prognosis for patients with malignant primary brain tumors remains very poor. Moreover, the non-specific nature of conventional therapy for brain tumors often results in incapacitating damage to surrounding normal brain and systemic tissues. Thus, there is an urgent need for the development of therapeutic strategies that precisely target tumor cells while minimizing collateral damage to neighboring eloquent cerebral cortex. The rationale for using the immune system to target brain tumors is based on the premise that the inherent specificity of immunologic reactivity could meet the clear need for more specific and precise therapy. The success of this modality is dependent on our ability to understand the mechanisms of immune regulation within the central nervous system (CNS), as well as counter the broad defects in host cell-mediated immunity that malignant gliomas are known to elicit. Recent advances in our understanding of tumor-induced and host-mediated immunosuppressive mechanisms, the development of effective strategies to combat these suppressive effects, and a better understanding of how to deliver immunologic effector molecules more efficiently to CNS tumors have all facilitated significant progress toward the realization of true clinical benefit from immunotherapeutic treatment of malignant gliomas.