Diphtheria toxin-based anti-human CD19 immunotoxin for targeting human CD19+ tumors

Advances in immunotoxin engineering: precision therapeutic strategies in modern oncology

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

Solving the mystery of the FMC63-CD19 affinity

The majority of approved CAR T cell products are based on the FMC63-scFv directed against CD19. Surprisingly, although antigen binding affinity is a major determinant for CAR function, the affinity of the benchmark FMC63-scFv has not been unambiguously determined. That is, a wide range of affinities have been reported in literature, differing by more than 100-fold. Using a range of techniques, we demonstrate that suboptimal experimental designs can cause artefacts that lead to over- or underestimation of the affinity. To minimize these artefacts, we performed SPR with strictly monomeric and correctly folded soluble CD19, yielding an FMC63-scFv affinity of 2-6 nM. Together, apart from analyzing the FMC63-scFv affinity under optimized conditions, we also provide potential explanations for the wide range of published affinities. We expect that this study will be highly valuable for interpretations of CAR affinity-function relationships, as well as for the design of future CAR T cell generations.

Ginsenoside 24-OH-PD from red ginseng inhibits acute T-lymphocytic leukaemia by activating the mitochondrial pathway

Ginsenoside 24-hydroxy-ginsengdiol (24-OH-PD), extracted from red ginseng, is a novel diol-type ginsenoside, strongly inhibits the growth of human T-cell acute lymphoblastic leukaemia (T-ALL) CCRF-CEM cells. Our research aimed at investigating the mechanism underlying this inhibition. Cell viability was determined using the cell counting kit-8 (CCK-8) assay, and NOD/SCID mice bearing CCRF-CEM cells were used to verify the therapeutic effect of 24-OH-PD on T-ALL in vivo. We equally analysed pathways related to 24-OH-PD in CCRF-CEM cells using RNA-Seq analysis. Cell apoptosis, reactive oxygen species (ROS), mitochondrial membrane potential (ΔΨm), and mitochondrial permeability transition pore (mPTP) levels were detected by flow cytometry. The activity of caspase3 and caspase9 was detected by enzyme activity detection kits. The expression levels of apoptosis-related proteins and mRNA were determined through western blotting and quantitative reverse-transcription PCR assays (qRT-PCR). CCK-8 assay and animal xenograft experiments confirmed that 24-OH-PD significantly inhibited T-ALL in a dose-dependent manner, both in vivo and in vitro. RNA-Seq results suggest that mitochondria-mediated apoptosis pathway plays an important role in this process. Furthermore, intracellular ROS levels increased, mPTP opened, and ΔΨm decreased following 24-OH-PD treatment. Pretreatment with the antioxidant, NAC, reversed the effects of 24-OH-PD on apoptosis and ROS generation. Moreover, 24-OH-PD treatment increased the expression of Bax and caspase family members, thereby releasing cytochrome c (Cytc) and inducing apoptosis. Our findings showed that, 24-OH-PD induces apoptosis in CCRF-CEM cells by activating the mitochondrial-dependent apoptosis pathway through ROS accumulation. This inhibitory effect implies that 24-OH-PD could be further developed as treatment of T-ALL.

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

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

Fundamentals of utilizing microbes in advanced cancer therapeutics: Current understanding and potential applications

One of the biggest health related issues in the twenty-first century is cancer. The current therapeutic platforms have not advanced enough to keep up with the number of rising cases. The traditional therapeutic approaches frequently fail to produce the desired outcomes. Therefore, developing new and more potent remedies is crucial. Recently, investigating microorganisms as potential anti-cancer treatments have garnered a lot of attention. Tumor-targeting microorganisms are more versatile at inhibiting cancer than the majority of standard therapies. Bacteria preferentially gather and thrive inside tumors, where they can trigger anti-cancer immune responses. They can be further trained to generate and distribute anticancer drugs based on clinical requirements using straightforward genetic engineering approaches. To improve clinical outcomes, therapeutic strategies utilizing live tumor-targeting bacteria can be used either alone or in combination with existing anticancer treatments. On the other hand, oncolytic viruses that target cancer cells, gene therapy via viral vectors, and viral immunotherapy are other popular areas of biotechnological investigation. Therefore, viruses serve as a unique candidate for anti-tumor therapy. This chapter describes the role of microbes, primarily bacteria and viruses in anti-cancer therapeutics. The various approaches to utilizing microbes in cancer therapy are discussed and examples of microorganisms that are now in use or that are undergoing experimental research are briefly discussed. We further point out the hurdles and the prospects of microbes-based remedies for cancer treatment.

EGF‑IL2 bispecific and bivalent EGF fusion toxin efficacy against syngeneic head and neck cancer mouse models

The epidermal growth factor receptor (EGFR) remains one of the best molecules for developing targeted therapy for multiple human malignancies, including head and neck squamous cell carcinoma (HNSCC). Small molecule inhibitors or antibodies targeting EGFR have been extensively developed in recent decades. Immunotoxin (IT)‑based therapy, which combines cell surface binding ligands or antibodies with a peptide toxin, represents another cancer treatment option. A total of 3 diphtheria toxin (DT)‑based fusion toxins that target human EGFR‑monovalent EGFR IT (mono‑EGF‑IT), bivalent EGFR IT (bi‑EGF‑IT), and a bispecific IT targeting both EGFR and interleukin‑2 receptor (bis‑EGF/IL2‑IT) were recently generated by the authors. Improved efficacy and reduced toxicity of bi‑EGF‑IT compared with mono‑EGF‑IT in immunocompromised HNSCC mouse models was reported. In the present study, bis‑EGF/IL2‑IT were generated using a unique DT‑resistant yeast expression system and evaluated the in vitro and in vivo efficacy and toxicity of the 3 EGF‑ITs in immunocompetent mice. The results demonstrated that while the three EGF‑ITs had different efficacies in vitro and in vivo against HNSCC, bi‑EGF‑IT and bis‑EGF/IL2‑IT had significantly improved in vivo efficacy and remarkably less off‑target toxicity compared with mono‑EGF‑IT. In addition, bis‑EGF/IL2‑IT was superior to bi‑EGF‑IT in reducing tumor size and prolonging survival in the metastatic model. These data suggested that targeting either the tumor immune microenvironment or enhancing the binding affinity could improve the efficacy of IT‑based therapy. Bi‑EGF‑IT and bis‑EGF/IL2‑IT represent improved candidates for IT‑based therapy for future clinical development.

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

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

Bacterial-Based Cancer Therapy (BBCT): Recent Advances, Current Challenges, and Future Prospects for Cancer Immunotherapy

Currently approximately 10 million people die each year due to cancer, and cancer is the cause of every sixth death worldwide. Tremendous efforts and progress have been made towards finding a cure for cancer. However, numerous challenges have been faced due to adverse effects of chemotherapy, radiotherapy, and alternative cancer therapies, including toxicity to non-cancerous cells, the inability of drugs to reach deep tumor tissue, and the persistent problem of increasing drug resistance in tumor cells. These challenges have increased the demand for the development of alternative approaches with greater selectivity and effectiveness against tumor cells. Cancer immunotherapy has made significant advancements towards eliminating cancer. Our understanding of cancer-directed immune responses and the mechanisms through which immune cells invade tumors have extensively helped us in the development of new therapies. Among immunotherapies, the application of bacteria and bacterial-based products has promising potential to be used as treatments that combat cancer. Bacterial targeting of tumors has been developed as a unique therapeutic option that meets the ongoing challenges of cancer treatment. In comparison with other cancer therapeutics, bacterial-based therapies have capabilities for suppressing cancer. Bacteria are known to accumulate and proliferate in the tumor microenvironment and initiate antitumor immune responses. We are currently well-informed regarding various methods by which bacteria can be manipulated by simple genetic engineering or synthetic bioengineering to induce the production of anti-cancer drugs. Further, bacterial-based cancer therapy (BBCT) can be either used as a monotherapy or in combination with other anticancer therapies for better clinical outcomes. Here, we review recent advances, current challenges, and prospects of bacteria and bacterial products in the development of BBCTs.

Multidimensional role of bacteria in cancer: Mechanisms insight, diagnostic, preventive and therapeutic potential

The active role of bacteria in oncogenesis has long been a topic of debate. Although, it was speculated to be a transmissible cause of cancer as early as the 16th-century, yet the idea about the direct involvement of bacteria in cancer development has only been explored in recent decades. More recently, several studies have uncovered the mechanisms behind the carcinogenic potential of bacteria which are inflammation, immune evasion, pro-carcinogenic metabolite production, DNA damage and genomic instability. On the other side, the recent development on the understanding of tumor microenvironment and technological advancements has turned this enemy into an ally. Studies using bacteria for cancer treatment and detection have shown noticeable effects. Therapeutic abilities of bioengineered live bacteria such as high specificity, selective cytotoxicity to cancer cells, responsiveness to external signals and control after ingestion have helped to overcome the challenges faced by conventional cancer therapies and highlighted the bacterial based therapy as an ideal approach for cancer treatment. In this review, we have made an effort to compile substantial evidence to support the multidimensional role of bacteria in cancer. We have discussed the multifaceted role of bacteria in cancer by highlighting the wide impact of bacteria on different cancer types, their mechanisms of actions in inducing carcinogenicity, followed by the diagnostic and therapeutic potential of bacteria in cancers. Moreover, we have also highlighted the existing gaps in the knowledge of the association between bacteria and cancer as well as the limitation and advantage of bacteria-based therapies in cancer. A better understanding of these multidimensional roles of bacteria in cancer can open up the new doorways to develop early detection strategies, prevent cancer, and develop therapeutic tactics to cure this devastating disease.

Directed Evolution of Stabilized Monomeric CD19 for Monovalent CAR Interaction Studies and Monitoring of CAR-T Cell Patients

CD19 is among the most relevant targets in cancer immunotherapy. However, its extracellular domain (ECD) is prone to aggregation and misfolding, representing a major obstacle for the development and analysis of CD19-targeted therapeutics. Here, we engineered stabilized CD19-ECD (termed SuperFolder) variants, which also showed improved expression rates and, in contrast to the wild type protein, they could be efficiently purified in their monomeric forms. Despite being considerably more stable, these engineered mutants largely preserved the wild type sequence (>98.8%). We demonstrate that the variant SF05 enabled the determination of the monovalent affinity between CD19 and a clinically approved FMC63-based CAR, as well as monitoring and phenotypic characterization of CD19-directed CAR-T cells in the blood of lymphoma patients. We anticipate that the SuperFolder mutants generated in this study will be highly valuable tools for a range of applications in basic immunology and CD19-targeted cancer immunotherapy.

A novel diphtheria toxin-based bivalent human EGF fusion toxin for treatment of head and neck squamous cell carcinoma

Epidermal growth factor receptor (EGFR) is often overexpressed in head and neck squamous cell carcinoma (HNSCC) and represents a top candidate for targeted HNSCC therapy. However, the clinical effectiveness of current Food and Drug Administration (FDA)-approved drugs targeting EGFR is moderate, and the overall survival rate for HNSCC patients remains low. Therefore, more effective treatments are urgently needed. In this study, we generated a novel diphtheria toxin-based bivalent human epidermal growth factor fusion toxin (bi-EGF-IT) to treat EGFR-expressing HNSCC. Bi-EGF-IT was tested for in vitro binding affinity, cytotoxicity, and specificity using 14 human EGFR-expressing HNSCC cell lines and three human EGFR-negative cancer cell lines. Bi-EGF-IT had increased binding affinity for EGFR-expressing HNSCC compared with the monovalent version (mono-EGF-IT), and both versions specifically depleted EGFR-positive HNSCC, but not EGFR-negative cell lines, in vitro. Bi-EGF-IT exhibited a comparable potency to that of the FDA-approved EGFR inhibitor, erlotinib, for inhibiting HNSCC tumor growth in vivo using both subcutaneous and orthotopic HNSCC xenograft mouse models. When tested in an experimental metastasis model, survival was significantly longer in the bi-EGF-IT treatment group than the erlotinib treatment group, with a significantly reduced number of metastases compared with mono-EGF-IT. In addition, in vivo off-target toxicities were significantly reduced in the bi-EGF-IT treatment group compared with the mono-EGF-IT group. These results demonstrate that bi-EGF-IT is more effective and markedly less toxic at inhibiting primary HNSCC tumor growth and metastasis than mono-EGF-IT and erlotinib. Thus, the novel bi-EGF-IT is a promising drug candidate for further development.

Obligate and facultative anaerobic bacteria in targeted cancer therapy: Current strategies and clinical applications

Traditional methods for cancer therapy, including radiotherapy, chemotherapy, and immunotherapy are characterized by inherent limitations. Bacteria-mediated tumor therapy is becoming a promising approach in cancer treatment due to the ability of obligate or facultative anaerobic microorganisms to penetrate and proliferate in hypoxic regions of tumors. It is widely known that anaerobic bacteria cause the regression of tumors and inhibition of metastasis through a variety of mechanisms, including toxin production, anaerobic lifestyle and synergy with anti-cancer drugs. These features have the potential to be used as a supplement to conventional cancer treatment. To the best of our knowledge, no reports have been published regarding the most common tumor-targeting bacterial agents with special consideration of obligate anaerobes (such as Clostridium sp., Bifidobacterium sp.) and facultative anaerobes (including Salmonella sp., Listeria monocytogenes, Lactobacillus sp., Escherichia coli, Corynebacterium diphtheriae and Pseudomonas sp). In this review, we summarize the latest literature on the role of these bacteria in cancer treatment.

Bispecific human IL2-CCR4 immunotoxin targets human cutaneous T-cell lymphoma

The majority of clinically diagnosed cutaneous T-cell lymphomas (CTCL) highly express the cell-surface markers CC chemokine receptor 4 (CCR4) and/or CD25. Recently, we have developed diphtheria toxin-based recombinant Ontak®-like human IL2 fusion toxin (IL2 fusion toxin) and anti-human CCR4 immunotoxin (CCR4 IT). In this study, we first compared the efficacy of the CCR4 IT vs IL2 fusion toxin for targeting human CD25⁺ CCR4⁺ CTCL. We demonstrated that CCR4 IT was more effective than IL2 fusion toxin. We further constructed an IL2-CCR4 bispecific IT. The bispecific IT was significantly more effective than either IL2 fusion toxin or CCR4 IT alone. The bispecific IT is a promising novel targeted therapeutic drug candidate for the treatment of refractory and recurrent human CD25⁺ and/or CCR4⁺ CTCL.

Getting CD19 Into Shape: Expression of Natively Folded "Difficult-to- Express" CD19 for Staining and Stimulation of CAR-T Cells

The transmembrane protein CD19 is exclusively expressed on normal and malignant B cells and therefore constitutes the target of approved CAR-T cell-based cancer immunotherapies. Current efforts to assess CAR-T cell functionality in a quantitative fashion both in vitro and in vivo are hampered by the limited availability of the properly folded recombinant extracellular domain of CD19 (CD19-ECD) considered as "difficult-to-express" (DTE) protein. Here, we successfully expressed a novel fusion construct consisting of the full-length extracellular domain of CD19 and domain 2 of human serum albumin (CD19-AD2), which was integrated into the Rosa26 bacterial artificial chromosome vector backbone for generation of a recombinant CHO-K1 production cell line. Product titers could be further boosted using valproic acid as a chemical chaperone. Purified monomeric CD19-AD2 proved stable as shown by non-reduced SDS-PAGE and SEC-MALS measurements. Moreover, flow cytometric analysis revealed specific binding of CD19-AD2 to CD19-CAR-T cells. Finally, we demonstrate biological activity of our CD19-AD2 fusion construct as we succeeded in stimulating CD19-CAR-T cells effectively with the use of CD19-AD2-decorated planar supported lipid bilayers.

Critical Issues in the Development of Immunotoxins for Anticancer Therapy

Immunotoxins (ITs) are attractive anticancer modalities aimed at cancer-specific delivery of highly potent cytotoxic protein toxins. An IT consists of a targeting domain (an antibody, cytokine, or another cell-binding protein) chemically conjugated or recombinantly fused to a highly cytotoxic payload (a bacterial and plant toxin or human cytotoxic protein). The mode of action of ITs is killing designated cancer cells through the effector function of toxins in the cytosol after cellular internalization via the targeted cell-specific receptor-mediated endocytosis. Although numerous ITs of diverse structures have been tested in the past decades, only 3 ITs-denileukin diftitox, tagraxofusp, and moxetumomab pasudotox-have been clinically approved for treating hematological cancers. No ITs against solid tumors have been approved for clinical use. In this review, we discuss critical research and development issues associated with ITs that limit their clinical success as well as strategies to overcome these obstacles. The issues include off-target and on-target toxicities, immunogenicity, human cytotoxic proteins, antigen target selection, cytosolic delivery efficacy, solid-tumor targeting, and developability. To realize the therapeutic promise of ITs, novel strategies for safe and effective cytosolic delivery into designated tumors, including solid tumors, are urgently needed.

Targeted Diphtheria Toxin-Based Therapy: A Review Article

Cancer remains one of the leading causes of death worldwide. Conventional therapeutic strategies usually offer limited specificity, resulting in severe side effects and toxicity to normal tissues. Targeted cancer therapy, on the other hand, can improve the therapeutic potential of anti-cancer agents and decrease unwanted side effects. Targeted applications of cytolethal bacterial toxins have been found to be especially useful for the specific eradication of cancer cells. Targeting is either mediated by peptides or by protein-targeting moieties, such as antibodies, antibody fragments, cell-penetrating peptides (CPPs), growth factors, or cytokines. Together with a toxin domain, these molecules are more commonly referred to as immunotoxins. Targeting can also be achieved through gene delivery and cell-specific expression of a toxin. Of the available cytolethal toxins, diphtheria toxin (DT) is one of the most frequently used for these strategies. Of the many DT-based therapeutic strategies investigated to date, two immunotoxins, Ontak^TM and Tagraxofusp^TM, have gained FDA approval for clinical application. Despite some success with immunotoxins, suicide-gene therapy strategies, whereby controlled tumor-specific expression of DT is used for the eradication of malignant cells, are gaining prominence. The first part of this review focuses on DT-based immunotoxins, and it then discusses recent developments in tumor-specific expression of DT.

Ontak-like human IL-2 fusion toxin

Ontak® is a FDA-approved diphtheria toxin-based recombinant fusion toxin for treatment of human CD25⁺ cutaneous T cell lymphoma (CTCL). However, it has been discontinued clinically due to the production issue related to the bacterial expression system with difficult purification. Recently we have developed monovalent and bivalent human IL-2 fusion toxins targeting human CD25⁺ cells using advanced unique diphtheria toxin resistant yeast Pichia Pastoris expression system. In vitro efficacy characterization using human CD25⁺ HUT102/6TG cells demonstrated that both monovalent and bivalent isoforms are potent and the bivalent isoform is approximately two logs more potent than the monovalent isoform. In this study, we further assessed the in vivo efficacy of the human IL-2 fusion toxins using human CD25⁺ HUT102/6TG tumor-bearing NSG mouse model. The data demonstrated that both monovalent and bivalent human IL-2 fusion toxins significantly prolonged the survival of the human CD25⁺ tumor-bearing NSG mice in a dose-dependent manner. Then we further assessed the residual tumor cells from the HUT102/6TG tumor-bearing NSG mice using the residual tumor cell bearing NSG mouse model. The results demonstrated that the residual tumor cells were still sensitive to the continual treatment with the human IL-2 fusion toxin. This yeast-expressed human IL-2 fusion toxin will be a promising candidate to replace the clinically discontinued Ontak®.