A protease-resistant immunotoxin against CD22 with greatly increased activity against CLL and diminished animal toxicity

T and B cell epitope analysis for the immunogenicity evaluation and mitigation of antibody-based therapeutics

The surge in the clinical use of therapeutic antibodies has reshaped the landscape of pharmaceutical therapy for many diseases, including rare and challenging conditions. However, the administration of exogenous biologics could potentially trigger unwanted immune responses such as generation of anti-drug antibodies (ADAs). Real-world experiences have illuminated the clear correlation between the ADA occurrence and unsatisfactory therapeutic outcomes as well as immune-related adverse events. By retrospectively examining research involving immunogenicity analysis, we noticed the growing emphasis on elucidating the immunogenic epitope profiles of antibody-based therapeutics aiming for mechanistic understanding the immunogenicity generation and, ideally, mitigating the risks. As such, we have comprehensively summarized here the progress in both experimental and computational methodologies for the characterization of T and B cell epitopes of therapeutics. Furthermore, the successful practice of epitope-driven deimmunization of biotherapeutics is exceptionally highlighted in this article.

Hosts and Heterologous Expression Strategies of Recombinant Toxins for Therapeutic Purposes

The production of therapeutic recombinant toxins requires careful host cell selection. Bacteria, yeast, and mammalian cells are common choices, but no universal solution exists. Achieving the delicate balance in toxin production is crucial due to potential self-intoxication. Recombinant toxins from various sources find applications in antimicrobials, biotechnology, cancer drugs, and vaccines. "Toxin-based therapy" targets diseased cells using three strategies. Targeted cancer therapy, like antibody-toxin conjugates, fusion toxins, or "suicide gene therapy", can selectively eliminate cancer cells, leaving healthy cells unharmed. Notable toxins from various biological sources may be used as full-length toxins, as plant (saporin) or animal (melittin) toxins, or as isolated domains that are typical of bacterial toxins, including Pseudomonas Exotoxin A (PE) and diphtheria toxin (DT). This paper outlines toxin expression methods and system advantages and disadvantages, emphasizing host cell selection's critical role.

Facile preparation of PEGylated polyethylenimine polymers as vaccine carriers with reduced cytotoxicity and enhanced Interleukin-2 (IL-2) production

Developing low-cost, easy-to-prepare, biocompatible and highly efficient vaccine carriers is a promising approach to realize practical cancer immunotherapy. In this study, through facile modification of mPEG5k-4-toluenesulfonate (mPEG5k-OTs) on PEI25k under mild conditions, a series of "stealth" mPEG5k-PEI25k polymers (PP1, PP2 and PP3) were prepared, their structures and physicochemical properties were characterized and theoretically analyzed. The polymers could bind/load ovalbumin (OVA) to form mPEG5k-PEI25k/OVA complexes as negatively charged nanoparticles with small hydrodynamic particle size (80-210 nm) and narrow size distribution. Compared to PEI25k/OVA, lower cytotoxicity could be achieved on mPEG5k-PEI25k/OVA complexes in dendritic cells (DCs). In DCs-RF 33.70 T-cells co-culture system, the mPEG5k-PEI25k/OVA complexes could bring about higher IL-2 production /secretion than that of PEI25k/OVA, notably, the optimum IL-2 secretion could reach 9.3-folds of the PEI25k/OVA under serum condition (10% FBS). Moreover, the cell biological features could be optimized by selecting suitable mPEG5k-grafting ratios and/or mPEG5k-PEI25k/OVA weight ratios. Intracellular imaging results showed that the mPEG5k-PEI25k(PP3)/Rhodamine-OVA complexes mainly localized inside lysosomes. Taken together, this work provided a facile method to prepare "stealth" PEGylated-PEI25k polymers with reduced cytotoxicity, promoted OVA cross-presentation efficiency and improved serum compatibility towards cancer immunotherapy.

Improving the cytotoxicity of immunotoxins by reducing the affinity of the antibody in acidic pH

BACKGROUND

Immunotoxins are antibody-toxin conjugates that bind to surface antigens and exert effective cytotoxic activity after internalization into tumor cells. Immunotoxins exhibit effective cytotoxicity and have been approved by the FDA to treat multiple hematological malignancies, such as hairy cell leukemia and cutaneous T-cell lymphoma. However, most of the internalized immunotoxin is degraded in lysosomes, and only approximately 5% of free toxin escapes into the cytosol to exert cytotoxicity. Many studies have improved immunotoxins by engineering the toxin fragment to reduce immunogenicity or increase stability, but how the antibody fragment contributes to the activity of immunotoxins has not been well demonstrated.

METHODS

In the current study, we used 32A9 and 42A1, two anti-GPC3 antibodies with similar antigen-binding capabilities and internalization rates, to construct scFv-mPE24 immunotoxins and evaluated their in vitro and in vivo antitumor activities. Next, the antigen-binding capacity, trafficking, intracellular protein stability and release of free toxin of 32A9 scFv-mPE24 and 42A1 scFv-mPE24 were compared to elucidate their different antitumor activities. Furthermore, we used a lysosome inhibitor to evaluate the degradation behavior of 32A9 scFv-mPE24 and 42A1 scFv-mPE24. Finally, the antigen-binding patterns of 32A9 and 42A1 were compared under neutral and acidic pH conditions.

RESULTS

Although 32A9 and 42A1 had similar antigen binding capacities and internalization rates, 32A9 scFv-mPE24 had superior antitumor activity compared to 42A1 scFv-mPE24. We found that 32A9 scFv-mPE24 exhibited faster degradation and drove efficient free toxin release compared to 42A1 scFv-mPE24. These phenomena were determined by the different degradation behaviors of 32A9 scFv-mPE24 and 42A1 scFv-mPE24 in lysosomes. Moreover, 32A9 was sensitive to the low-pH environment, which made the 32A9 conjugate easily lose antigen binding and undergo degradation in lysosomes, and the free toxin was then efficiently produced to exert cytotoxicity, whereas 42A1 was resistant to the acidic environment, which kept the 42A1 conjugate relatively stable in lysosomes and delayed the release of free toxin.

CONCLUSIONS

These results showed that a low pH-sensitive antibody-based immunotoxin degraded faster in lysosomes, caused effective free toxin release, and led to improved cytotoxicity compared to an immunotoxin based on a normal antibody. Our findings suggested that a low pH-sensitive antibody might have an advantage in the design of immunotoxins and other lysosomal degradation-dependent antibody conjugate drugs.

Synthesis of an Anti-CD7 Recombinant Immunotoxin Based on PE24 in CHO and E. coli Cell-Free Systems

Recombinant immunotoxins (RITs) are an effective class of agents for targeted therapy in cancer treatment. In this article, we demonstrate the straight-forward production and testing of an anti-CD7 RIT based on PE24 in a prokaryotic and a eukaryotic cell-free system. The prokaryotic cell-free system was derived from Escherichia coli BL21 StarTM (DE3) cells transformed with a plasmid encoding the chaperones groEL/groES. The eukaryotic cell-free system was prepared from Chinese hamster ovary (CHO) cells that leave intact endoplasmic reticulum-derived microsomes in the cell-free reaction mix from which the RIT was extracted. The investigated RIT was built by fusing an anti-CD7 single-chain variable fragment (scFv) with the toxin domain PE24, a shortened variant of Pseudomonas Exotoxin A. The RIT was produced in both cell-free systems and tested for antigen binding against CD7 and cell killing on CD7-positive Jurkat, HSB-2, and ALL-SIL cells. CD7-positive cells were effectively killed by the anti-CD7 scFv-PE24 RIT with an IC50 value of 15 pM to 40 pM for CHO and 42 pM to 156 pM for E. coli cell-free-produced RIT. CD7-negative Raji cells were unaffected by the RIT. Toxin and antibody domain alone did not show cytotoxic effects on either CD7-positive or CD7-negative cells. To our knowledge, this report describes the production of an active RIT in E. coli and CHO cell-free systems for the first time. We provide the proof-of-concept that cell-free protein synthesis allows for on-demand testing of antibody−toxin conjugate activity in a time-efficient workflow without cell lysis or purification required.

CD3e-immunotoxin spares CD62Llo Tregs and reshapes organ-specific T-cell composition by preferentially depleting CD3ehi T cells

CD3-epsilon(CD3e) immunotoxins (IT), a promising precision reagent for various clinical conditions requiring effective depletion of T cells, often shows limited treatment efficacy for largely unknown reasons. Tissue-resident T cells that persist in peripheral tissues have been shown to play pivotal roles in local and systemic immunity, as well as transplant rejection, autoimmunity and cancers. The impact of CD3e-IT treatment on these local cells, however, remains poorly understood. Here, using a new murine testing model, we demonstrate a substantial enrichment of tissue-resident Foxp3+ Tregs following CD3e-IT treatment. Differential surface expression of CD3e among T-cell subsets appears to be a main driver of Treg enrichment in CD3e-IT treatment. The surviving Tregs in CD3e-IT-treated mice were mostly the CD3e^dimCD62L^lo effector phenotype, but the levels of this phenotype markedly varied among different lymphoid and nonlymphoid organs. We also found notable variations in surface CD3e levels among tissue-resident T cells of different organs, and these variations drive CD3e-IT to uniquely reshape T-cell compositions in local organs. The functions of organs and anatomic locations (lymph nodes) also affected the efficacy of CD3e-IT. The multi-organ pharmacodynamics of CD3e-IT and potential treatment resistance mechanisms identified in this study may generate new opportunities to further improve this promising treatment.

EGFRvⅢ-targeted immunotoxin combined with temozolomide and bispecific antibody for the eradication of established glioblastoma

EGFRvⅢ is an established target for immunotherapy of glioblastoma (GBM). Current study aims to explore the efficacy of EGFRvⅢ-targeted immunotoxin combined with temozolomide (TMZ) or T cell-engaged bispecific antibody for the treatment of GBM. We generated three rabbit monoclonal antibodies (R1, R2, and R6) that specifically bound to EGFRvⅢ, but not EGFR, with high affinity. Immunotoxins were made by fusing the scFv of these antibodies with engineered Pseudomonas exotoxin PE24. The in vitro cytotoxicity and specificity of the immunotoxins was rigorously validated by EGFRvⅢ and EGFR-expressed cell lines. The in vivo efficacy of immunotoxin monotherapy and in combination with TMZ or EGFRvⅢ-targeted bispecific antibody was evaluated in orthotopic and subcutaneous xenograft mouse models. EGFRvⅢ immunotoxins potently killed U87, U251 and GL261 cells that were forcefully expressing EGFRvⅢ, with IC₅₀ values bellow 1.2 ng/ml. In a subcutaneous model, multiple intratumoral injections of immunotoxin at a dose of 2 mg/kg resulted in complete tumor regression in 3/5 of mice. In a C57BL/6 orthotopic glioblastoma model transplanted with GL261 cells that expressed a mouse version of EGFRvⅢ, two injections of 10 micrograms of immunotoxin in the lateral ventricles significantly improved the survival, with 2/5 mice being completely cured. Furthermore, in a subcutaneous xenograft model transplanted with EGFRvⅢ-expressed U87 cells, a single intratumoral injection of immuntoxin followed by i.v. injections of TMZ or EGFRvⅢ-targeted bispecific antibody achieved complete regression in mice. Taken together, EGFRvⅢ immunotoxin combined with TMZ or T cell-engaged bispecific antibody offers promise for curative treatment of GBM.

Strategies to mitigate the on- and off-target toxicities of recombinant immunotoxins: an antibody engineering perspective

Targeted cancer therapies using immunotoxins has achieved remarkable efficacies in hematological malignancies. However, the clinical development of immunotoxins is also faced with many challenges like anti-drug antibodies and dose-limiting toxicity issues. Such a poor efficacy/safety ratio is also the major hurdle in the research and development of antibody-drug conjugates. From an antibody engineering perspective, various strategies were summarized/proposed to tackle the notorious on target off tumor toxicity issues, including passive strategy (XTENylation of immunotoxins) and active strategies (modulating the affinity and valency of the targeting moiety of immunotoxins, conditionally activating immunotoxins in the tumor microenvironments and reconstituting split toxin to reduce systemic toxicity etc.). By modulating the functional characteristics of the targeting moiety and the toxic moiety of immunotoxins, selective tumor targeting can be augmented while sparing the healthy cells in normal tissues expressing the same target of interest. If successful, the improved therapeutic index will likely help to address the dose-limiting toxicities commonly observed in the clinical trials of various immunotoxins.

GPC1-Targeted Immunotoxins Inhibit Pancreatic Tumor Growth in Mice via Depletion of Short-lived GPC1 and Downregulation of Wnt Signaling

Glypican-1 (GPC1) is a cell surface proteoglycan that is upregulated in multiple types of human cancers including pancreatic cancer. Here, we investigated whether GPC1 could be a target of antibody-toxin fusion proteins (i.e., immunotoxins) for treating pancreatic cancer. We constructed a panel of GPC1-targeted immunotoxins derived from a functional domain of Pseudomonas exotoxin A. An albumin-binding domain was also introduced into the anti-GPC1 immunotoxin to improve serum half-life. Small-molecule screening was performed to identify irinotecan that shows synergistic efficacy with the immunotoxin. We showed that GPC1 was internalized upon antibody binding. Anti-GPC1 immunotoxins alone inhibited tumor growth in a pancreatic cancer xenograft model. The immunotoxin treatment reduced active β-catenin expression in tumor cells. Furthermore, immunotoxins containing an albumin-binding domain in combination with irinotecan caused pancreatic tumor regression. GPC1 expression was reduced by the immunotoxin treatment due to the degradation of the internalized GPC1 and its short cellular turnover rate. Our data indicate that the GPC1-targeted immunotoxin inhibits pancreatic tumor growth via degradation of internalized GPC1, downregulation of Wnt signaling, and inhibition of protein synthesis. The anti-GPC1 immunotoxin in combination with irinotecan thus provides a potential new treatment strategy for patients with pancreatic tumors.

Comparison of CD3e Antibody and CD3e-sZAP Immunotoxin Treatment in Mice Identifies sZAP as the Main Driver of Vascular Leakage

Anti-CD3-epsilon (CD3e) monoclonal antibodies (mAbs) and CD3e immunotoxins (ITs) are promising targeted therapy options for various T-cell disorders. Despite significant advances in mAb and IT engineering, vascular leakage syndrome (VLS) remains a major dose-limiting toxicity for ITs and has been poorly characterized for recent "engineered" mAbs. This study undertakes a direct comparison of non-mitogenic CD3e-mAb (145-2C11 with Fc-silentTM murine IgG1: S-CD3e-mAb) and a new murine-version CD3e-IT (saporin-streptavidin (sZAP) conjugated with S-CD3e-mAb: S-CD3e-IT) and identifies their distinct toxicity profiles in mice. As expected, the two agents showed different modes of action on T cells, with S-CD3e-mAb inducing nearly complete modulation of CD3e on the cell surface, while S-CD3e-IT depleted the cells. S-CD3e-IT significantly increased the infiltration of polymorphonuclear leukocytes (PMNs) into the tissue parenchyma of the spleen and lungs, a sign of increased vascular permeability. By contrast, S-CD3e-mAbs-treated mice showed no notable signs of vascular leakage. Treatment with control ITs (sZAP conjugated with Fc-silent isotype antibodies) induced significant vascular leakage without causing T-cell deaths. These results demonstrate that the toxin portion of S-CD3e-IT, not the CD3e-binding portion (S-CD3e-mAb), is the main driver of vascular leakage, thus clarifying the molecular target for improving safety profiles in CD3e-IT therapy.

Reduced cytotoxicity by mutation of Lysine 590 of Pseudomonas exotoxin can be restored in an optimized, Lysine-free immunotoxin

Immunotoxins, which are fusion proteins of an antibody fragment and a fragment of a bacterial or a plant toxin, induce apoptosis in target cells by inhibition of protein synthesis. ADP-ribosylating toxins often have few lysine residues in their catalytic domain. As they are the target for ubiquitination, the low number of lysines possibly prevents ubiquitin-dependent degradation of the toxin in the cytosol. To reduce this potential degradation, we aimed to generate a lysine-free (noK), Pseudomonas exotoxin (PE)-based immunotoxin. The new generation 24 kDa PE, which lacks all but the furin-cleavage site of domain II, was mutated at lysine 590 (K590) and at K606 in a CD22-targeting immunotoxin and activity was determined against various B cell malignancies in vitro and in vivo. On average, K590 mutated to arginine (R) reduced cytotoxicity by 1.3-fold and K606R enhanced cytotoxicity by 1.3-fold compared to wild type (wt). Mutating K590 to histidine or deleting K590 did not prevent this loss in cytotoxicity. Neither stability nor internalization rate of K590R could explain reduced cytotoxicity. These results highlight the relevance of lysine 590 for PE intoxication. In line with in vitro results, the K606R mutant was more than 1.8-fold more active than the other variants in vivo suggesting that this single mutation may be beneficial when targeting CD22-positive malignancies. Finally, reduced cytotoxicity by K590R was compensated for by K606R and the resulting lysine-free variant achieved wt-like activity in vitro and in vivo. Thus, PE24-noK may represent a promising candidate for down-stream applications that would interfere with lysines.

Generation of Single-Domain Antibody-Based Recombinant Immunotoxins

The discovery of single-domain antibodies has opened new avenues for drug development. Single-domain antibodies, also known as nanobodies, can access buried epitopes that are inaccessible to conventional antibodies. These antigen-binding domains have a high level of solubility and stability, which makes them well suited for therapeutic development. This chapter will discuss the design, production, and testing of single-domain antibody-based recombinant immunotoxins. Recombinant immunotoxins are chimeric proteins that combine the specificity of an antibody with the ribosomal-inhibitory domain of a bacterial toxin. Immunotoxins using the Pseudomonas exotoxin domain have been well studied in clinical trials. Recently, an anti-CD22 immunotoxin was granted marketing approval for use in patients with relapsed or refractory hairy cell leukemia. This supports the idea that treatment with recombinant immunotoxins can be explored for cancers that have not responded to standard therapies.

Immunotoxins and nanobody-based immunotoxins: review and update

Immunotoxins (ITs) are protein-based drugs that compose of targeting and cytotoxic moieties. After binding the IT to the specific cell-surface antigen, the IT internalises into the target cell and kills it. Targeting and cytotoxic moieties usually include monoclonal antibodies and protein toxins with bacterial or plant origin, respectively. ITs have been successful in haematologic malignancies treatment. However, ITs penetrate poorly into solid tumours because of their large size. Use of camelid antibody fragments known as nanobodies (Nbs) as a targeting moiety may overcome this problem. Nbs are the smallest fragment of antibodies with excellent tumour tissue penetration. The ability to recognise cryptic (immuno-evasive) target antigens, low immunogenicity, and high-affinity are other fundamental characteristics of Nbs that make them suitable candidates in targeted therapy. Here, we reviewed and discussed the structure and function of ITs, Nbs, and nanobody-based ITs. To gain sound insight into the issue at hand, we focussed on nanobody-based ITs.

Targeted Toxins for the Treatment of Prostate Cancer

Prostate cancer is the second most common cancer and the fifth leading cause of cancer deaths worldwide. Despite improvements in diagnosis and treatment, new treatment options are urgently needed for advanced stages of the disease. Targeted toxins are chemical conjugates or fully recombinant proteins consisting of a binding domain directed against a target antigen on the surface of cancer cells and a toxin domain, which is transported into the cell for the induction of apoptosis. In the last decades, targeted toxins against prostate cancer have been developed. Several challenges, however, became apparent that prevented their direct clinical use. They comprise immunogenicity, low target antigen binding, endosomal entrapment, and lysosomal/proteasomal degradation of the targeted toxins. Moreover, their efficacy is impaired by prostate tumors, which are marked by a dense microenvironment, low target antigen expression, and apoptosis resistance. In this review, current findings in the development of targeted toxins against prostate cancer in view of effective targeting, reduction of immunogenicity, improvement of intracellular trafficking, and overcoming apoptosis resistance are discussed. There are promising approaches that should lead to the clinical use of targeted toxins as therapeutic alternatives for advanced prostate cancer in the future.

Soluble Cytoplasmic Expression and Purification of Immunotoxin HER2(scFv)-PE24B as a Maltose Binding Protein Fusion

Human epidermal growth factor receptor 2 (HER-2) is overexpressed in many malignant tumors. The anti-HER2 antibody trastuzumab has been approved for treating HER2-positive early and metastatic breast cancers. Pseudomonas exotoxin A (PE), a bacterial toxin of Pseudomonas aeruginosa, consists of an A-domain with enzymatic activity and a B-domain with cell binding activity. Recombinant immunotoxins comprising the HER2(scFv) single-chain Fv from trastuzumab and the PE24B catalytic fragment of PE display promising cytotoxic effects, but immunotoxins are typically insoluble when expressed in the cytoplasm of Escherichia coli, and thus they require solubilization and refolding. Herein, a recombinant immunotoxin gene was fused with maltose binding protein (MBP) and overexpressed in a soluble form in E. coli. Removal of the MBP yielded stable HER2(scFv)-PE24B at 91% purity; 0.25 mg of pure HER2(scFv)-PE24B was obtained from a 500 mL flask culture. Purified HER2(scFv)-PE24B was tested against four breast cancer cell lines differing in their surface HER2 level. The immunotoxin showed stronger cytotoxicity than HER2(scFv) or PE24B alone. The IC50 values for HER2(scFv)-PE24B were 28.1 ± 2.5 pM (n = 9) and 19 ± 1.4 pM (n = 9) for high HER2-positive cell lines SKBR3 and BT-474, respectively, but its cytotoxicity was lower against MDA-MB-231 and MCF7. Thus, fusion with MBP can facilitate the soluble expression and purification of scFv immunotoxins.

Recent development and optimization of pseudomonas aeruginosa exotoxin immunotoxins in cancer therapeutic applications

Recombinant immunotoxins are fusion proteins composed of a peptide toxin and a specific targeting domain through genetic recombination. They are engineered to recognize disease-specific target receptors and kill the cell upon internalization. Full-sized monoclonal antibodies, smaller antibody fragments and ligands, such as a cytokine or a growth factor, have been commonly used as the targeting domain, while bacterial Pseudomonas aeruginosa exotoxin (PE) is the usual toxin fusion partner, due to its natural cytotoxicity and other unique advantages. PE-based recombinant immunotoxins have shown remarkable efficacy in the treatment of tumors and autoimmune diseases. At the same time, efforts are underway to address major challenges, including immunogenicity, nonspecific cytotoxicity and poor penetration, which limit their clinical applications. Recent strategies for structural optimization of PE-based immunotoxins, combined with mutagenesis approaches, have reduced the immunogenicity and non-specific cytotoxicity, thus increasing both their safety and efficacy. This review highlights novel insights and design concepts that were used to advance immunotoxins for the treatment of hematological and solid tumors and also presents future development prospect of PE-based recombinant immunotoxins that are expected to play an important role in cancer therapy.

Natural and Designed Toxins for Precise Therapy: Modern Approaches in Experimental Oncology

Cancer cells frequently overexpress specific surface receptors providing tumor growth and survival which can be used for precise therapy. Targeting cancer cell receptors with protein toxins is an attractive approach widely used in contemporary experimental oncology and preclinical studies. Methods of targeted delivery of toxins to cancer cells, different drug carriers based on nanosized materials (liposomes, nanoparticles, polymers), the most promising designed light-activated toxins, as well as mechanisms of the cytotoxic action of the main natural toxins used in modern experimental oncology, are discussed in this review. The prospects of the combined therapy of tumors based on multimodal nanostructures are also discussed.

Anti-cancer Immunotoxins, Challenges, and Approaches

The development of recombinant immunotoxins (RITs) as a novel therapeutic strategy has made a revolution in the treatment of cancer. RITs result from the fusion of antibodies to toxin proteins for targeting and eliminating cancerous cells by inhibiting protein synthesis. Despite indisputable outcomes of RITs regarding inhibition of multiple cancer types, high immunogenicity has been known as the main obstacle in the clinical use of RITs. Various strategies have been proposed to overcome these limitations, including immunosuppressive therapy, humanization of the antibody fragment moiety, generation of immunotoxins originated from endogenous human cytotoxic enzymes, and modification of the toxin moiety to escape the immune system. This paper is devoted to review recent advances in the design of immunotoxins with lower immunogenicity.

Triton X-114 and Amine-Based Wash Strategy Reduces Lipopolysaccharides to FDA Limit and Achieves Purer, More Potent Recombinant Immunotoxin

Many proteins are still routinely expressed prokaryotically in Escherichia coli, some because they are toxic to eukaryotes. Immunotoxins, which are fusion proteins of a targeting moiety and a truncated Pseudomonas exotoxin A, kill target cells by arresting protein synthesis. Thus, immunotoxins must be expressed in E. coli. Proteins expressed in E. coli are contaminated by endotoxin (also called lipopolysaccharides (LPS)). LPS binds to toll-like receptors, inducing up to life-threatening systemic inflammation in mammals. Therefore, accepted LPS limits for therapeutics as well as for substances used in immunological studies in animals are very low. Here, we report the use of Triton X-114 and polyamine-based wash strategies, which only in combination achieved LPS-contamination well below FDA limits. Resulting LPS-reduced immunotoxins were purer and up to 2.4-fold more active in vitro. Increased activity was associated with a 2.4-fold increase in affinity on cell surface expressed target antigen. The combination method maintained enzymatic function, protein stability, and in vivo efficacy and was effective for Fab as well as dsFv formats. With some modifications, the principle of this novel combination may be applied to any chromatography-based purification process.

Plant-made immunotoxin building blocks: A roadmap for producing therapeutic antibody-toxin fusions

Molecular farming in plants is an emerging platform for the production of pharmaceutical proteins, and host species such as tobacco are now becoming competitive with commercially established production hosts based on bacteria and mammalian cell lines. The range of recombinant therapeutic proteins produced in plants includes replacement enzymes, vaccines and monoclonal antibodies (mAbs). But plants can also be used to manufacture toxins, such as the mistletoe lectin viscumin, providing an opportunity to express active antibody-toxin fusion proteins, so-called recombinant immunotoxins (RITs). Mammalian production systems are currently used to produce antibody-drug conjugates (ADCs), which require the separate expression and purification of each component followed by a complex and hazardous coupling procedure. In contrast, RITs made in plants are expressed in a single step and could therefore reduce production and purification costs. The costs can be reduced further if subcellular compartments that accumulate large quantities of the stable protein are identified and optimal plant growth conditions are selected. In this review, we first provide an overview of the current state of RIT production in plants before discussing the three key components of RITs in detail. The specificity-defining domain (often an antibody) binds cancer cells, including solid tumors and hematological malignancies. The toxin provides the means to kill target cells. Toxins from different species with different modes of action can be used for this purpose. Finally, the linker spaces the two other components to ensure they adopt a stable, functional conformation, and may also promote toxin release inside the cell. Given the diversity of these components, we extract broad principles that can be used as recommendations for the development of effective RITs. Future research should focus on such proteins to exploit the advantages of plants as efficient production platforms for targeted anti-cancer therapeutics.

Glypican-3: A molecular marker for the detection and treatment of hepatocellular carcinoma☆

Hepatocellular carcinoma (HCC) is a malignant tumor with a fairly poor prognosis (5-year survival of less than 50%). Using sorafenib, the only food and drug administration (FDA)-approved drug, HCC cannot be effectively treated; it can only be controlled at most for a couple of months. There is a great need to develop efficacious treatment against this debilitating disease. Glypican-3 (GPC3), a member of the glypican family that attaches to the cell surface by a glycosylphosphatidylinositol anchor, is overexpressed in HCC cases and is elevated in the serum of a large proportion of patients with HCC. GPC3 expression contributes to HCC growth and metastasis. Furthermore, several different types of antibodies targeting GPC3 have been developed. The aim of this review is to summarize the current literatures on the GPC3 expression in human HCC, molecular mechanisms of GPC3 regulation and antibodies targeting GPC3.

Advances in epidermal growth factor receptor specific immunotherapy: lessons to be learned from armed antibodies

The epidermal growth factor receptor (EGFR) has been recognized as an important therapeutic target in oncology. It is commonly overexpressed in a variety of solid tumors and is critically involved in cell survival, proliferation, metastasis, and angiogenesis. This multi-dimensional role of EGFR in the progression and aggressiveness of cancer, has evolved from conventional to more targeted therapeutic approaches. With the advent of hybridoma technology and phage display techniques, the first anti-EGFR monoclonal antibodies (mAbs) (Cetuximab and Panitumumab) were developed. Due to major limitations including host immune reactions and poor tumor penetration, these antibodies were modified and used as guiding mechanisms for the specific delivery of readily available chemotherapeutic agents or plants/bacterial toxins, giving rise to antibody-drug conjugates (ADCs) and immunotoxins (ITs), respectively. Continued refinement of ITs led to deimmunization strategies based on depletion of B and T-cell epitopes or substitution of non-human toxins leading to a growing repertoire of human enzymes capable of inducing cell death. Similarly, the modification of classical ADCs has resulted in the first, fully recombinant versions. In this review, we discuss significant advancements in EGFR-targeting immunoconjugates, including ITs and recombinant photoactivable ADCs, which serve as a blueprint for further developments in the evolving domain of cancer immunotherapy.

32A9, a novel human antibody for designing an immunotoxin and CAR-T cells against glypican-3 in hepatocellular carcinoma

Background

Treatment of hepatocellular carcinoma (HCC) using antibody-based targeted therapies, such as antibody conjugates and chimeric antigen receptor T (CAR-T) cell therapy, shows potent antitumor efficacy. Glypican-3 (GPC3) is an emerging HCC therapeutic target; therefore, antibodies against GPC3 would be useful tools for developing immunotherapies for HCC.

Methods

We isolated a novel human monoclonal antibody, 32A9, by phage display technology. We determined specificity, affinity, epitope and anti-tumor activity of 32A9, and developed 32A9-based immunotherapy technologies for evaluating the potency of HCC treatment in vitro or in vivo.

Results

32A9 recognized human GPC3 with potent affinity and specificity. The epitope of 32A9 was located in the region of the GPC3 protein core close to the modification sites of the HS chain and outside of the Wnt-binding site of GPC3. The 32A9 antibody significantly inhibited HCC xenograft tumor growth in vivo. We then pursued two 32A9-based immunotherapeutic strategies by constructing an immunotoxin and CAR-T cells. The 32A9 immunotoxin exhibited specific cytotoxicity to GPC3-positive cancer cells, while 32A9 CAR-T cells efficiently eliminated GPC3-positive HCC cells in vitro and caused HCC xenograft tumor regressions in vivo.

Conclusions

Our study provides a rationale for 32A9 as a promising GPC3-specific antibody candidate for HCC immunotherapy.

Mechanisms of Resistance to Immunotoxins Containing Pseudomonas Exotoxin A in Cancer Therapy

Immunotoxins are a class of targeted cancer therapeutics in which a toxin such as Pseudomonas exotoxin A (PE) is linked to an antibody or cytokine to direct the toxin to a target on cancer cells. While a variety of PE-based immunotoxins have been developed and a few have demonstrated promising clinical and preclinical results, cancer cells frequently have or develop resistance to these immunotoxins. This review presents our current understanding of the mechanism of action of PE-based immunotoxins and discusses cellular mechanisms of resistance that interfere with various steps of the pathway. These steps include binding of the immunotoxin to the target antigen, internalization, intracellular processing and trafficking to reach the cytosol, inhibition of protein synthesis through ADP-ribosylation of elongation factor 2 (EF2), and induction of apoptosis. Combination therapies that increase immunotoxin action and overcome specific mechanisms of resistance are also reviewed.

Immunogenicity of Immunotoxins Containing Pseudomonas Exotoxin A: Causes, Consequences, and Mitigation

Immunotoxins are cytolytic fusion proteins developed for cancer therapy, composed of an antibody fragment that binds to a cancer cell and a protein toxin fragment that kills the cell. Pseudomonas exotoxin A (PE) is a potent toxin that is used for the killing moiety in many immunotoxins. Moxetumomab Pasudotox (Lumoxiti) contains an anti-CD22 Fv and a 38 kDa portion of PE. Lumoxiti was discovered in the Laboratory of Molecular Biology at the U.S. National Cancer Institute and co-developed with Medimmune/AstraZeneca to treat hairy cell leukemia. In 2018 Lumoxiti was approved by the US Food and Drug Administration for the treatment of drug-resistant Hairy Cell Leukemia. Due to the bacterial origin of the killing moiety, immunotoxins containing PE are highly immunogenic in patients with normal immune systems, but less immunogenic in patients with hematologic malignancies, whose immune systems are often compromised. LMB-100 is a de-immunized variant of the toxin with a humanized antibody that targets mesothelin and a PE toxin that was rationally designed for diminished reactivity with antibodies and B cell receptors. It is now being evaluated in clinical trials for the treatment of mesothelioma and pancreatic cancer and is showing somewhat diminished immunogenicity compared to its un modified parental counterpart. Here we review the immunogenicity of the original and de-immunized PE immunotoxins in mice and patients, the development of anti-drug antibodies (ADAs), their impact on drug availability and their effect on clinical efficacy. Efforts to mitigate the immunogenicity of immunotoxins and its impact on immunogenicity will be described including rational design to identify, remove, or suppress B cell or T cell epitopes, and combination of immunotoxins with immune modulating drugs.

Development of Glypican-3 Targeting Immunotoxins for the Treatment of Liver Cancer: An Update

Hepatocellular carcinoma (HCC) accounts for most liver cancers and represents one of the deadliest cancers in the world. Despite the global demand for liver cancer treatments, there remain few options available. The U.S. Food and Drug Administration (FDA) recently approved Lumoxiti, a CD22-targeting immunotoxin, as a treatment for patients with hairy cell leukemia. This approval helps to demonstrate the potential role that immunotoxins can play in the cancer therapeutics pipeline. However, concerns have been raised about the use of immunotoxins, including their high immunogenicity and short half-life, in particular for treating solid tumors such as liver cancer. This review provides an overview of recent efforts to develop a glypican-3 (GPC3) targeting immunotoxin for treating HCC, including strategies to deimmunize immunotoxins by removing B- or T-cell epitopes on the bacterial toxin and to improve the serum half-life of immunotoxins by incorporating an albumin binding domain.

Targeted Therapy With Immunoconjugates for Multiple Myeloma

The introduction of proteasome inhibitors (PI) and immunomodulatory drugs (IMiD) has markedly increased the survival of multiple myeloma (MM) patients. Also, the unconjugated monoclonal antibodies (mAb) daratumumab (anti-CD38) and elotuzumab (anti-SLAMF7) have revolutionized MM treatment given their clinical efficacy and safety, illustrating the potential of targeted immunotherapy as a powerful treatment strategy for MM. Nonetheless, most patients eventually develop PI-, IMiD-, and mAb-refractory disease because of the selection of resistant MM clones, which associates with a poor prognosis. Accordingly, these patients remain in urgent need of new therapies with novel mechanisms of action. In this respect, mAbs or mAb fragments can also be utilized as carriers of potent effector moieties to specifically target surface antigens on cells of interest. Such immunoconjugates have the potential to exert anti-MM activity in heavily pretreated patients due to their distinct and pleiotropic mechanisms of action. In addition, the fusion of highly cytotoxic compounds to mAbs decreases the off-target toxicity, thereby improving the therapeutic window. According to the effector moiety, immunoconjugates are classified into antibody-drug conjugates, immunotoxins, immunocytokines, or radioimmunoconjugates. This review will focus on the mechanisms of action, safety and efficacy of several promising immunoconjugates that are under investigation in preclinical and/or clinical MM studies. We will also include a discussion on combination therapy with immunoconjugates, resistance mechanisms, and future developments.

Breast cancer targeted/ therapeutic with double and triple fusion Immunotoxins

Target-specific transport of therapeutic agents holds promise to increase the efficacy of cancer treatment by decreasing injury to normal tissues and post treatment problems. HER2 is a tumor cell surface marker that is expressed in 25-30 % of breast cancer patients. The significant role of HER2 in cancer development and its biological feature makes it a highly appealing goal for the therapeutic treatment of cancer targeted therapy using HER2 monoclonal antibody. This approach is currently used as a special treatment against breast cancer in some research. In the present study, HER2 monoclonal antibody (mAb), (Herceptin) fused to PE38 by recombinant DNA technology and a new recombinant IT was developed. The scFv(Herceptin)-PE-STXA and scFv(Herceptin)-PE fusions cloned in pET28a and recombinant protein expression was carried out and then the proteins were purified. MCF-7 and SKBR-3 cells were used as HER2-negative and HER2-positive breast cancer cells, respectively. The cytotoxicity of its evaluated using MTT assay. The cell ELISA was used to determine the binding ability of immunotoxins (ITs) to the cell receptor and internalization and apoptosis were also assessed. The results revealed that cell cytotoxicity occurred in SKBR-3 cells in a dose-dependent manner but not in MCF-7 cells. It is possible that this ITs can attach to HER2-positive breast cancer cells and then, internalize and eradicate cancer cells by apoptosis. Here, we concluded that the recombinant ITs have therapeutic potential against HER2-positive breast cancer.

Removal of the Translocation Domain and the Furin Cleavage Site Decreases the Relative Hepatotoxicity of the Targeted Antitumor Toxins

Targeted toxins are promising anticancer agents that allow selectively destroying cancer cells due to the increased content of onco-specific markers on their surface. The use of such anti-cancer toxins in medicine is mainly hampered by their high non-specific toxicity, in particular, hepatotoxicity. In our work on human cell line, we have shown that the removal of the DARPin-PE40 translocation toxin domain leads to a decrease in hepatotoxicity. The same effect is also observed when inactivation of the furin cleavage site in the DARPin-PE40 molecule was done. Simultaneous removal of both the translocation domain and the furin cleavage site showed the best results. This toxin modification can be used to create more selective anti-cancer toxins.

A Spectroscopic Study on Secondary Structure and Thermal Unfolding of the Plant Toxin Gelonin Confirms Some Typical Structural Characteristics and Unravels the Sequence of Thermal Unfolding Events

Gelonin from the Indian plant Gelonium multiflorum belongs to the type I ribosome-inactivating proteins (RIPs). Like other members of RIPs, this toxin glycoprotein inhibits protein synthesis of eukaryotic cells; hence, it is largely used in the construction of immunotoxins composed of cell-targeted antibodies. Lysosomal degradation is one of the main issues in targeted tumor therapies, especially for type I RIP-based toxins, as they lack the translocation domains. The result is an attenuated cytosolic delivery and a decrease of the antitumor efficacy of these plant-derived toxins; therefore, strategies to permit their release from endosomal vesicles or modifications of the toxins to make them resistant to degradation are necessary to improve their efficacy. Using infrared spectroscopy, we thoroughly analyzed both the secondary structure and the thermal unfolding of gelonin. Moreover, by the combination of two-dimensional correlation spectroscopy and phase diagram method, it was possible to deduce the sequence of events during the unfolding, confirming the typical characteristic of the RIP members to denature in two steps, as a sequential loss of tertiary and secondary structure was detected at 58 °C and at 65 °C, respectively. Additionally, some discrepancies in the unfolding process between gelonin and saporin-S6, another type I RIP protein, were detected.

The immunotoxin activity of exotoxin A is sensitive to domain modifications

Immunotoxins are a class of recombinant proteins which consist of an antibody and a part of a bacterial or herbal toxin. Immunotoxins containing Pseudomonas aeruginosa exotoxin A (PEA) have been found to be very applicable in clinical trials. Many obstacles such as solubility and absorbency reduce their usability in solid tumors. The current study aims to overcome the mentioned barriers by addition and removal of functional and non-functional domains with a structural approach. In the experimental section, we took advantage of molecular dynamics simulations to predict the functionality of candidate immunotoxins which target human HER2 receptors and confirmed our findings with in vitro experiments. We found out when no changes were made to domain II of PEA, addition of solubilizing domains to immunotoxins would not reduce their targeting and anti-tumor activity, while increasing the yield of expression and stability. On the other side, when we replaced domain II with eleven amino acids of furin cleavage site (FCS), the activity of the immunotoxin was mainly affected by the FCS neighboring domains and linkers. A combination of seven beneficial point mutations in domain III was also assessed and reconfirmed that the toxicity of the immunotoxin would be reduced dramatically. The obtained results indicate that the addition or removal of domains cannot depict the activity of immunotoxins and the matter should be assessed structurally in advance.

Construction of an immunotoxin via site-specific conjugation of anti-Her2 IgG and engineered Pseudomonas exotoxin A

BACKGROUND

Immunotoxins consisting of a toxin from bacteria or plants and a targeting module have been developed as potent anti-cancer therapeutics. The majority of them, especially those in preclinical or clinical testing stages, are fusion proteins of a toxin and antibody fragment. Immunotoxins based on full-length antibodies are less studied, even though the fragment crystallizable (Fc) domain plays an important role in regulating the concentration of immunoglobulin G (IgG) in the serum and in antibody-mediated immune responses against pathogens.

RESULTS

We devised a method to site-specifically conjugate IgG and another protein using a cysteine residue introduced into the IgG and a bio-orthogonally reactive unnatural amino acid incorporated into the other protein. The human epidermal growth factor receptor 2 (Her2)-targeting IgG, trastuzumab, was engineered to have an unpaired cysteine in the heavy chain, and an unnatural amino acid with the azido group was incorporated into an engineered Pseudomonas exotoxin A (PE24). The two protein molecules were conjugated site-specifically using a bifunctional linker having dibenzocyclooctyne and maleimide groups. Binding to Her2 and interaction with various Fc receptors of trastuzumab were not affected by the conjugation with PE24. The trastuzumab-PE24 conjugate was cytotoxic to Her2-overexpressing cell lines, which involved the inhibition of cellular protein synthesis due to the modification of elongation factor-2.

CONCLUSIONS

We constructed the site-specifically conjugated immunotoxin based on IgG and PE24, which induced target-specific cytotoxicity. To evaluate the molecule as a cancer therapeutic, animal studies are planned to assess tumor regression, half-life in blood, and in vivo immunogenicity. In addition, we expect that the site-specific conjugation method can be used to develop other antibody-protein conjugates for applications in therapeutics and diagnostics.

Evaluating the influence of common antibiotics on the efficacy of a recombinant immunotoxin in tissue culture

OBJECTIVE

Recombinant immunotoxins (RITs) are antibody-toxin fusion proteins that can selectively eliminate populations of cells expressing specific surface receptors. They are in evaluation as therapeutic agents for cancer. RITs based on Pseudomonas exotoxin A (PE) are in use clinically for the treatment of hairy cell leukemia, and under trial for the treatment of other cancers. In an effort to improve the efficacy of PE-based RITs, we evaluated the potential of combination therapy with several common antibiotics (tetracycline, chloramphenicol, streptomycin, linezolid, fusidic acid, and kanamycin) on human cell lines HEK293, OVCAR8, and CA46. Antibiotics were selected based on their potential to inhibit mitochondrial protein synthesis and disrupt energy metabolism in cancer cells.

RESULTS

Tetracycline, chloramphenicol, linezolid, and fusidic acid alone killed cultured human cells at high concentrations. At high but nontoxic concentrations of each antibiotic, only chloramphenicol treatment of the Burkitt's lymphoma cell line CA46 showed enhanced cytotoxicity when paired with an anti-transferrin receptor/PE RIT. This result, however, could not be replicated in additional Burkitt's lymphoma cell lines Ramos and Raji. Although the six antibiotics we tested are not promising candidates for RIT combination therapy, we suggest that fusidic acid could be considered independently as a potential cancer therapeutic.

ROR1 Potentiates FGFR Signaling in Basal-Like Breast Cancer

Among all breast cancer types, basal-like breast cancer (BLBC) represents an aggressive subtype that lacks targeted therapy. We and others have found that receptor tyrosine kinase-like orphan receptor 1 (ROR1) is overexpressed in BLBC and other types of cancer and that ROR1 is significantly correlated with patient prognosis. In addition, using primary patient-derived xenografts (PDXs) and ROR1-knockout BLBC cells, we found that ROR1⁺ cells form tumors in immunodeficient mice. We developed an anti-ROR1 immunotoxin and found that targeting ROR1 significantly kills ROR1⁺ cancer cells and slows down tumor growth in ROR1⁺ xenografts. Our bioinformatics analysis revealed that ROR1 expression is commonly associated with the activation of FGFR-mediated signaling pathway. Further biochemical analysis confirmed that ROR1 stabilized FGFR expression at the posttranslational level by preventing its degradation. CRISPR/Cas9-mediated ROR1 knockout significantly reduced cancer cell invasion at cellular levels by lowering FGFR protein and consequent inactivation of AKT. Our results identified a novel signaling regulation from ROR1 to FGFR and further confirm that ROR1 is a potential therapeutic target for ROR1⁺ BLBC cells.

HER2-Specific Targeted Toxin DARPin-LoPE: Immunogenicity and Antitumor Effect on Intraperitoneal Ovarian Cancer Xenograft Model

High immunogenicity and systemic toxicity are the main obstacles limiting the clinical use of the therapeutic agents based on Pseudomonas aeruginosa exotoxin A. In this work, we studied the immunogenicity, general toxicity and antitumor effect of the targeted toxin DARPin-LoPE composed of HER2-specific DARPin and a low immunogenic exotoxin A fragment lacking immunodominant human B lymphocyte epitopes. The targeted toxin has been shown to effectively inhibit the growth of HER2-positive human ovarian carcinoma xenografts, while exhibiting low non-specific toxicity and side effects, such as vascular leak syndrome and liver tissue degradation, as well as low immunogenicity, as was shown by specific antibody titer. This represents prospects for its use as an agent for targeted therapy of HER2-positive tumors.

Novel immunotherapeutic approaches for hepatocellular carcinoma treatment

INTRODUCTION

The introduction of immune checkpoint inhibitors has been lately proposed for the treatment of hepatocellular carcinoma (HCC) with respect to other cancer types. Several immunotherapeutic approaches are now under evaluation for HCC treatment including: i) antibodies acting as immune checkpoint inhibitors; ii) antibodies targeting specific tumor-associated antigens; iii) chimeric antigen receptor redirected T (CAR-T) cells targeting specific tumor-associated antigens; iv) vaccination strategies with tumor-specific epitopes. Areas covered: The review provides a wide description of the clinical trials investigating the efficacy of the main immunotherapeutic approaches proposed for the treatment of patients affected by HCC. Expert opinion: The balancing between immunostimulative and immunosuppressive factors in the context of HCC tumor microenvironment results in heterogeneous response rates to immunotherapeutic approaches such as checkpoint inhibitors, among HCC patients. In this context, it becomes crucial the identification of predictive factors determining the treatment response. A multiple approach using different biomarkers could be useful to identify the subgroup of HCC patients responsive to the treatment with a checkpoint inhibitor (as an example, nivolumab) as single agent, and to identify those patients in which other treatment regimens, such as the combination with sorafenib, or with locoregional therapies, could be more effective.

Co-Expression of a Chimeric Protease Inhibitor Secreted by a Tumor-Targeted Salmonella Protects Therapeutic Proteins from Proteolytic Degradation

Sunflower trypsin inhibitor (SFTI) is a 14-amino-acid bicyclic peptide that contains a single internal disulfide bond. We initially constructed chimeras of SFTI with N-terminal secretion signals from the Escherichia coli OmpA and Pseudomonas aeruginosa ToxA, but only detected small amounts of protease inhibition resulting from these constructs. A substantially higher degree of protease inhibition was detected from a C-terminal SFTI fusion with E. coli YebF, which radiated more than a centimeter from an individual colony of E. coli using a culture-based inhibitor assay. Inhibitory activity was further improved in YebF-SFTI fusions by the addition of a trypsin cleavage signal immediately upstream of SFTI, and resulted in production of a 14-amino-acid, disulfide-bonded SFTI free in the culture supernatant. To assess the potential of the secreted SFTI to protect the ability of a cytotoxic protein to kill tumor cells, we utilized a tumor-selective form of the Pseudomonas ToxA (OTG-PE38K) alone and expressed as a polycistronic construct with YebF-SFTI in the tumor-targeted Salmonella VNP20009. When we assessed the ability of toxin-containing culture supernatants to kill MDA-MB-468 breast cancer cells, the untreated OTG-PE38K was able to eliminate all detectable tumor cells, while pretreatment with trypsin resulted in the complete loss of anticancer cytotoxicity. However, when OTG-PE38K was co-expressed with YebF-SFTI, cytotoxicity was completely retained in the presence of trypsin. These data demonstrate SFTI chimeras are secreted in a functional form and that co-expression of protease inhibitors with therapeutic proteins by tumor-targeted bacteria has the potential to enhance the activity of therapeutic proteins by suppressing their degradation within a proteolytic environment.

Interactions Between Pseudomonas Immunotoxins and the Plasma Membrane: Implications for CAT-8015 Immunotoxin Therapy

Acute Lymphoblastic Leukemia (ALL) remains the most frequent cause of cancer-related mortality in children and novel therapies are needed for the treatment of relapsed/refractory childhood ALL. One approach is the targeting of ALL blasts with the Pseudomonas immunotoxin CAT-8015. Although CAT-8015 has potent anti-leukemia activity, with a 32% objective response rate in a phase 1 study of childhood ALL, haemolytic-uremic syndrome (HUS) and vascular leak syndrome (VLS), major dose-limiting toxicities, have limited the use of this therapeutic approach in children. Investigations into the pathogenesis of CAT-8015-induced HUS/VLS are hindered by the lack of an adequate model system that replicates clinical manifestations, but damage to vascular endothelial cells (ECs) and blood cells are believed to be major initiating factors in both syndromes. Since there is little evidence that murine models replicate human HUS/VLS, and CAT-8015-induced HUS/VLS predominantly affects children, we developed human models and used novel methodologies to investigate CAT-8015 interactions with red blood cells (RBCs) from pediatric ALL patients and ECs of excised human mesenteric arteries. We provide evidence that CAT-8015 directly interacts with RBCs, mediated by Pseudomonas toxin. We also show correlation between the electrical properties of the RBC membrane and RBC susceptibility to CAT-8015-induced lysis, which may have clinical implication. Finally, we provide evidence that CAT-8015 is directly cytototoxic to ECs of excised human mesenteric arteries. In conclusion, the human models we developed constitutes the first, and very important, step in understanding the origins of HUS/VLS in immunotoxin therapy and will allow further investigations of HUS/VLS pathogenesis.

Improving the In Vivo Efficacy of an Anti-Tac (CD25) Immunotoxin by Pseudomonas Exotoxin A Domain II Engineering

Tac (CD25) is expressed on multiple hematologic malignancies and is a target for cancer therapies. LMB-2 is an extremely active anti-Tac recombinant immunotoxin composed of an Fv that binds to Tac and a 38-kDa fragment of Pseudomonas exotoxin A (PE38). Although LMB-2 has shown high cytotoxicity toward Tac-expressing cancer cells in clinical trials, its efficacy was hampered by the formation of anti-drug antibodies against the immunogenic bacterial toxin and by dose-limiting off-target toxicity. To reduce toxin immunogenicity and nonspecific toxicity, we introduced six point mutations into domain III that were previously shown to reduce T-cell immunogenicity and deleted domain II from the toxin, leaving only the 11aa furin cleavage site, which is required for cytotoxic activity. Although this strategy has been successfully implemented for mesothelin and CD22-targeting immunotoxins, we found that removal of domain II significantly lowered the cytotoxic activity of anti-Tac immunotoxins. To restore cytotoxic activity in the absence of PE domain II, we implemented a combined rational design and screening approach to isolate highly active domain II-deleted toxin variants. The domain II-deleted variant with the highest activity contained an engineered disulfide-bridged furin cleavage site designed to mimic its native conformation within domain II. We found that this approach restored 5-fold of the cytotoxic activity and dramatically improved the MTD. Both of these improvements led to significantly increased antitumor efficacy in vivo We conclude that the next-generation anti-Tac immunotoxin is an improved candidate for targeting Tac-expressing malignancies. Mol Cancer Ther; 17(7); 1486-93. ©2018 AACR.

Migration of mesenchymal stem cells to tumor xenograft models and in vitro drug delivery by doxorubicin

Mesenchymal stem cells (MSCs) show therapeutic effects in various types of diseases. MSCs have been shown to migrate towards inflamed or cancerous tissues, and visualized after sacrificing the animal. MSCs are able to deliver drugs to target cells, and are an ideal candidate for cancer therapy. The purpose of this study was to track the migration of MSCs in tumor-bearing mice; MSCs were also used as drug delivery vehicles. Human breast cancer cells (MDA-MB-231) and anaplastic thyroid cancer cells (CAL62) were transduced with lentiviral particles, to express the Renilla luciferase and mCherry (mCherry-Rluc) reporter genes. Human bone marrow-derived MSCs were transduced with lentiviral particles, to express the firefly luciferase and enhanced green fluorescence protein (Fluc2-eGFP) reporter genes (MSC/Fluc). Luciferase activity of the transduced cells was measured by bioluminescence imaging (BLI). Further in vitro migration assays were performed to confirm cancer cells conditioned medium dependent MSC and doxorubicin (DOX) treated MSC migration. MSCs were loaded with DOX, and their therapeutic effects against the cancer cells were studied in vitro. In vivo MSC/Fluc migration in mice having thyroid or breast cancer xenografts was evaluated after systemic injection. Rluc activity of CAL62/Rluc (R2=0.911), MDA-MB-231/Rluc (R2=0.934) cells and Fluc activity of MSC/Fluc (R2=0.91) cells increased with increasing cell numbers, as seen by BLI. eGFP expression of MSC/Fluc was confirmed by confocal microscopy. Similar migration potential was observed between MSC/Fluc and naïve MSCs in migration assay. DOX treated MSCs migration was not decreased compared than MSCs. Migration of the systemically injected MSC/Fluc cells into tumor xenografts (thyroid and breast cancer) was visualized in animal models (p<0.05) and confirmed by ex vivo (p<0.05) BLI. Additionally, MSCs delivered DOX to CAL62/Rluc and MDA-MB-231/Rluc cells, thereby decreasing their Rluc activities. In this study, we confirmed the migration of MSCs to tumor sites in cancer xenograft models using both in vivo and ex vivo BLI imaging. DOX-pretreated MSCs showed enhanced cytotoxic effects. Therefore, this noninvasive reporter gene (Fluc2)-based BLI may be useful for visualizing in vivo tracking of MSCs, which can be used as a drug delivery vehicle for cancer therapy.

Strategies to Reduce the Immunogenicity of Recombinant Immunotoxins

Recombinant immunotoxins (RITs) are genetically engineered proteins being developed to treat cancer. They are composed of an Fv that targets a cancer antigen and a fragment of a bacterial toxin that kills tumor cells. Because the toxin is a foreign protein, it is immunogenic. The clinical success of RITs in patients with a normal immune system is limited by their immunogenicity. In this review, we discuss our progress in therapeutic protein deimmunization and the balancing act between immunogenicity and therapeutic potency. One approach is to prevent the activation of B cells by mapping and elimination of B-cell epitopes. A second approach is to prevent helper T-cell activation by interfering with major histocompatibility complex II presentation or T-cell recognition. Immunizing mice with RITs that were deimmunized by elimination of the murine B- or T-cell epitopes showed that both approaches are effective. Another approach to control immunogenicity is to modify the host immune system. Nanoparticles containing synthetic vaccine particles encapsulating rapamycin can induce immune tolerance and prevent anti-drug antibody formation. This treatment restores RIT anti-tumor activity that is otherwise neutralized because of immunogenicity.

Domain II of Pseudomonas Exotoxin Is Critical for Efficacy of Bolus Doses in a Xenograft Model of Acute Lymphoblastic Leukemia

Moxetumomab pasudotox is a fusion protein of a CD22-targeting antibody and Pseudomonas exotoxin. Minutes of exposure to Moxetumomab achieves similar cell killing than hours of exposure to a novel deimmunized variant against some acute lymphoblastic leukemia (ALL). Because blood levels fall quickly, Moxetumomab is more than 1000-fold more active than the deimmunized variant in vivo. We aimed to identify which part of Moxetumomab increases in vivo efficacy and generated five immunotoxins, tested time-dependent activity, and determined the efficacy in a KOPN-8 xenograft model. Full domain II shortened the time cells had to be exposed to die to only a few minutes for some ALL; deimmunized domain III consistently extended the time. Against KOPN-8, full domain II accelerated time to arrest protein synthesis by three-fold and tripled PARP-cleavage. In vivo efficacy was increased by more than 10-fold by domain II and increasing size, and therefore half-life enhanced efficacy two- to four-fold. In summary, in vivo efficacy is determined by the time cells have to be exposed to immunotoxin to die and serum half-life. Thus, domain II is most critical for activity against some ALL treated with bolus doses; however, immunotoxins lacking all but the furin-cleavage site of domain II may be advantageous when treating continuously.

Construction of an immunotoxin, HN3-mPE24, targeting glypican-3 for liver cancer therapy

Glypican-3 (GPC3) is overexpressed in hepatocellular carcinoma (HCC). We constructed a recombinant immunotoxin, HN3-mPE24, which contains a truncated form of Pseudomonas exotoxin A. The toxin portion lacks most of domain II and has seven point mutations in domain III to remove the B-cell epitopes thought to be responsible for causing off-target side effects and immunogenicity. We also fused a bivalent HN3 to mPE24. We tested these two molecules for GPC3 binding and cytotoxicity in HCC cell models. The K_D values of HN3-mPE24 and HN3-HN3-mPE24 for GPC3-expressing tumor cells were 12 nM and 1.4 nM, respectively. The IC₅₀ values of HN3-mPE24 and HN3-HN3-mPE24 for HCC cells were 0.2 nM and 0.4 nM, respectively. We also evaluated their toxicity and anti-tumor efficacy in mice. The maximum tolerated doses of HN3-mPE24 and HN3-HN3-mPE24 were 7 mg kg⁻¹ and 3.6 mg kg⁻¹, respectively. We treated mice with 5 mg kg⁻¹ of HN3-mPE24 intravenously every other day for ten injections. The alpha-fetoprotein level of HN3-mPE24 treated group was approximately 700 fold less than that of the untreated group (1.1 μg ml⁻¹ vs. 692.1 μg ml⁻¹). In addition, 25% of the mice treated with HN3-mPE24 survived to the end of this study, which was 105 days after HCC tumor implantation. In conclusion, the HN3-mPE24 immunotoxin caused liver tumor regressions and extended survival with no significant side effects in mice. It is a promising candidate for the treatment of liver cancer that may be readily translated to humans.