Phase 1 study of an anti-CD33 immunotoxin, humanized monoclonal antibody M195 conjugated to recombinant gelonin (HUM-195/rGEL), in patients with advanced myeloid malignancies

A cleavable peptide adapter augments the activity of targeted toxins in combination with the glycosidic endosomal escape enhancer SO1861

BACKGROUND

Treatment with tumor-targeted toxins attempts to overcome the disadvantages of conventional cancer therapies by directing a drug's cytotoxic effect specifically towards cancer cells. However, success with targeted toxins has been hampered as the constructs commonly remain bound to the outside of the cell or, after receptor-mediated endocytosis, are either transported back to the cell surface or undergo degradation in lysosomes. Hence, solutions to ensure endosomal escape are an urgent need in treatment with targeted toxins. In this work, a molecular adapter that consists of a cell penetrating peptide and two cleavable peptides was inserted into a targeted toxin between the ribosome-inactivating protein dianthin and the epidermal growth factor. Applying cell viability assays, this study examined whether the addition of the adapter further augments the endosomal escape enhancement of the glycosylated triterpenoid SO1861, which has shown up to more than 1000-fold enhancement in the past.

RESULTS

Introducing the peptide adapter into the targeted toxin led to an about 12-fold enhancement in the cytotoxicity on target cells while SO1861 caused a 430-fold increase. However, the combination of adapter and glycosylated triterpenoid resulted in a more than 4300-fold enhancement and in addition to a 51-fold gain in specificity.

CONCLUSIONS

Our results demonstrated that the cleavable peptide augments the endosomal escape mediated by glycosylated triterpenoids while maintaining specificity. Thus, the adapter is a promising addition to glycosylated triterpenoids to further increase the efficacy and therapeutic window of targeted toxins.

Immunotherapies of acute myeloid leukemia: Rationale, clinical evidence and perspective

Acute myeloid leukemia (AML) is a prevalent hematological malignancy that exhibits a wide array of molecular abnormalities. Although traditional treatment modalities such as chemotherapy and allogeneic stem cell transplantation (HSCT) have become standard therapeutic approaches, a considerable number of patients continue to face relapse and encounter a bleak prognosis. The emergence of immune escape, immunosuppression, minimal residual disease (MRD), and other contributing factors collectively contribute to this challenge. Recent research has increasingly highlighted the notable distinctions between AML tumor microenvironments and those of healthy individuals. In order to investigate the potential therapeutic mechanisms, this study examines the intricate transformations occurring between leukemic cells and their surrounding cells within the tumor microenvironment (TME) of AML. This review classifies immunotherapies into four distinct categories: cancer vaccines, immune checkpoint inhibitors (ICIs), antibody-based immunotherapies, and adoptive T-cell therapies. The results of numerous clinical trials strongly indicate that the identification of optimal combinations of novel agents, either in conjunction with each other or with chemotherapy, represents a crucial advancement in this field. In this review, we aim to explore the current and emerging immunotherapeutic methodologies applicable to AML patients, identify promising targets, and emphasize the crucial requirement to augment patient outcomes. The application of these strategies presents substantial therapeutic prospects within the realm of precision medicine for AML, encompassing the potential to ameliorate patient outcomes.

Crotamine-based recombinant immunotoxin targeting HER2 for enhanced cancer cell specificity and cytotoxicity

Crotamine, one of the major toxins present in the venom of the South American rattlesnake Crotalus durissus terrificus, exhibits potent cytotoxic properties and has been suggested for cancer therapy applications. However, its selectivity for cancer cells needs to be improved. This study designed and produced a novel recombinant immunotoxin, HER2(scFv)-CRT, composed of crotamine and single-chain Fv (scFv) derived from trastuzumab targeting human epidermal growth factor receptor 2 (HER2). The recombinant immunotoxin was expressed in Escherichia coli and purified using various chromatographic techniques. The cytotoxicity of HER2(scFv)-CRT was assessed in three breast cancer cell lines, demonstrating enhanced specificity and toxicity in HER2-expressing cells. These findings suggest that the crotamine-based recombinant immunotoxin has the potential to expand the repertoire of recombinant immunotoxin applications in cancer therapy.

Tumor-specific intracellular delivery: peptide-guided transport of a catalytic toxin

There continues to be a need for cancer-specific ligands that can deliver a wide variety of therapeutic cargos. Ligands demonstrating both tumor-specificity and the ability to mediate efficient cellular uptake of a therapeutic are critical to expand targeted therapies. We previously reported the selection of a peptide from a peptide library using a non-small cell lung cancer (NSCLC) cell line as the target. Here we optimize our lead peptide by a series of chemical modifications including truncations, N-terminal capping, and changes in valency. The resultant 10 amino acid peptide has an affinity of <40 nM on four different NSCLC cell lines as a monomer and is stable in human serum for >48 h. The peptide rapidly internalizes upon cell binding and traffics to the lysosome. The peptide homes to a tumor in an animal model and is retained up to 72 h. Importantly, we demonstrate that the peptide can deliver the cytotoxic protein saporin specifically to cancer cells in vitro and in vivo, resulting in an effective anticancer agent.

Plant-Derived Type I Ribosome Inactivating Protein-Based Targeted Toxins: A Review of the Clinical Experience

Targeted toxins (TT) for cancer treatment are a class of hybrid biologic comprised of a targeting domain coupled chemically or genetically to a proteinaceous toxin payload. The targeting domain of the TT recognises and binds to a defined target molecule on the cancer cell surface, thereby delivering the toxin that is then required to internalise to an appropriate intracellular compartment in order to kill the target cancer cell. Toxins from several different sources have been investigated over the years, and the two TTs that have so far been licensed for clinical use in humans; both utilise bacterial toxins. Relatively few clinical studies have, however, been undertaken with TTs that utilise single-chain type I ribosome inactivating proteins (RIPs). This paper reviews the clinical experience that has so far been obtained for a range of TTs based on five different type I RIPs and concludes that the majority studied in early phase trials show significant clinical activity that justifies further clinical investigation. A range of practical issues relating to the further clinical development of TT’s are also covered briefly together with some suggested solutions to outstanding problems.

In addition to its endosomal escape effect, platycodin D also synergizes with ribosomal inactivation protein to induce apoptosis in hepatoma cells through AKT and MAPK signaling pathways

Efficient endosomal escape after cellular uptake is a major challenge for the clinical application of therapeutic proteins. To overcome this obstacle, several strategies have been used to help protein drugs escape from endosomes without affecting the integrity of the cell membrane. Among them, some triterpenoid saponins with special structures were used to greatly enhance the anti-tumor therapeutic effect of protein toxins. Herein, we demonstrated that platycodin D (PD), polygalacin D (PGD) and platycodin D2 (PD2) from Platycodonis Radix significantly enhanced the ability of MHBP (a type I ribosome-inactivating protein toxin MAP30 fused with a cell-penetrating peptide HBP) to induce apoptosis in hepatoma cells. Based on the results of co-localization of endocytosed EGFP-HBP with a lysosomal probe and Galectin-9 vesicle membrane damage sensor, we demonstrated that PD, PGD and PD2 have the ability to promote endosomal escape of endocytic proteins without affecting the integrity of the plasma membrane. Meanwhile, we observed that cholesterol metabolism plays an important role in the activity of PD by RNA-seq analysis and KEGG pathway enrichment analysis, and confirm that PD, PGD and PD2 enhance the anti-tumor activity of MHBP by inducing the redistribution of free cholesterol and inhibiting the activity of cathepsin B and cathepsin D. Finally, we found that PD synergized with MHBP to induce caspase-dependent apoptosis through inhibiting Akt and ERK1/2 signaling pathways and activating JNK and p38 MAPK signaling pathways. This study provides new insights into the application of PD in cancer therapy and provides efficient and promising strategies for the cytosolic delivery of therapeutic proteins.

Improved Therapy of B-Cell Non-Hodgkin Lymphoma by Obinutuzumab-Dianthin Conjugates in Combination with the Endosomal Escape Enhancer SO1861

Immunotoxins do not only bind to cancer-specific receptors to mediate the elimination of tumor cells through the innate immune system, but also increase target cytotoxicity by the intrinsic toxin activity. The plant glycoside SO1861 was previously reported to enhance the endolysosomal escape of antibody-toxin conjugates in non-hematopoietic cells, thus increasing their cytotoxicity manifold. Here we tested this technology for the first time in a lymphoma in vivo model. First, the therapeutic CD20 antibody obinutuzumab was chemically conjugated to the ribosome-inactivating protein dianthin. The cytotoxicity of obinutuzumab-dianthin (ObiDi) was evaluated on human B-lymphocyte Burkitt’s lymphoma Raji cells and compared to human T-cell leukemia off-target Jurkat cells. When tested in combination with SO1861, the cytotoxicity for target cells was 131-fold greater than for off-target cells. In vivo imaging in a xenograft model of B-cell lymphoma in mice revealed that ObiDi/SO1861 efficiently prevents tumor growth (51.4% response rate) compared to the monotherapy with ObiDi (25.9%) and non-conjugated obinutuzumab (20.7%). The reduction of tumor volume and overall survival was also improved. Taken together, our results substantially contribute to the development of a combination therapy with SO1861 as a platform technology to enhance the efficacy of therapeutic antibody-toxin conjugates in lymphoma and leukemia.

Emerging trends in immunotoxin targeting cancer stem cells

Cancer stem cells (CSCs) are self-renewing multipotent cells that play a vital role in the development of cancer drug resistance conditions. Various therapies like conventional, targeted, and radiotherapies have been broadly used in targeting and killing these CSCs. Among these, targeted therapy selectively targets CSCs and leads to overcoming disease recurrence conditions in cancer patients. Immunotoxins (ITs) are protein-based therapeutics with selective targeting capabilities. These chimeric molecules are composed of two functional moieties, i.e., a targeting moiety for cell surface binding and a toxin moiety that induces the programmed cell death upon internalization. Several ITs have been constructed recently, and their preclinical and clinical efficacies have been evaluated. In this review, we comprehensively discussed the recent preclinical and clinical advances as well as significant challenges in ITs targeting CSCs, which might reduce the burden of drug resistance conditions in cancer patients from bench to bedside.

Vector engineering, strategies and targets in cancer gene therapy

Understanding the molecular basis of disease and the design of rationally designed molecular therapies has been the holy grail in the management of human cancers. Gene-based therapies are an important avenue for achieving a possible cure. Focused research in the last three decades has provided significant clues to optimize the potential of cancer gene therapy. The development of gene therapies with a high potential to kill the target cells at the lowest effective dose possible, the development of vectors with significant ability to target cancer-associated antigen, the application of adjunct therapies to target dysregulated microRNA, and embracing a hybrid strategy with a combination of gene therapy and low-dose chemotherapy in a disease-specific manner will be pivotal. This article outlines the advances and challenges in the field with emphasis on the biology and scope of vectors used for gene transfer, newer targets identified, and their outcome in preclinical and clinical studies.

Targeted Delivery of Endosomal Escape Peptides to Enhance Immunotoxin Potency and Anti-cancer Efficacy

This work describes use of anti-carcinoembryonic antigen antibodies (10H6, T84.66) for targeted delivery of an endosomal escape peptide (H6CM18) and gelonin, a type I ribosome inactivating protein. The viability of colorectal cancer cells (LS174T, LoVo) was assessed following treatment with gelonin or gelonin immunotoxins, with or without co-treatment with T84.66-H6CM18. Fluorescent microscopy was used to visualize the escape of immunoconjugates from endosomes of treated cells, and efficacy and toxicity were assessed in vivo in xenograft tumor-bearing mice following single- and multiple-dose regimens. Application of 25 pM T84.66-H6CM18 combined with T84.66-gelonin increased gelonin potency by ~ 1,000-fold and by ~ 6,000-fold in LS174T and LoVo cells. Intravenous 10H6-gelonin at 1.0 mg/kg was well tolerated by LS174T tumor-bearing mice, while 10 and 25 mg/kg doses led to signs of toxicity. Single-dose administration of PBS, gelonin conjugated to T84.66 or 10H6, T84.66-H6CM18, or gelonin immunotoxins co-administered with T84.66-H6CM18 were evaluated. The combinations of T84.66-gelonin + 1.0 mg/kg T84.66-H6CM18 and 10H6-gelonin + 0.1 mg/kg T84.66-H6CM18 led to significant delays in LS174T growth. Use of a multiple-dose regimen allowed further anti-tumor effects, significantly extending median survival time by 33% and by 69%, for mice receiving 1 mg/kg 10H6-gelonin + 0.1 mg/kg T84.66-H6CM18 (p = 0.0072) and 1 mg/kg 10H6-gelonin + 1 mg/kg T84.66-H6CM18 (p = 0.0017). Combined administration of gelonin immunoconjugates with antibody-targeted endosomal escape peptides increased the delivery of gelonin to the cytoplasm of targeted cells, increased gelonin cell killing in vitro by 1,000-6,000 fold, and significantly increased in vivo efficacy.

In vivo solid tumor targeting with recombinant VEGF-diphtheria immunotoxin

Objectives

A variety of signaling molecules have been identified that play a role in angiogenesis, of prime importance, vascular endothelial growth factor (VEGF) and its resceptor (VEGFR), which is highly expressed in most human solid tumors. Targeting VEGF or/and VEGFR with immunotoxin may be a promising approach to directly affect cancer cells. Immunotoxins are for targeted treatment comprising two functional moieties, an antibody that binds to target cells along with toxin that kills molecules.

Materials and Methods

In this study, an immunotoxin comprising domain of diphtheria toxin subunit A (DT386) genetically fused to mouse VEGF (mVEGF-DT) was developed. The second construct, which contains the DT386 domain, was made to investigate the action of the DT386 domain on tumor cells. Both gene constructs were cloned, expressed, and were further purified. The biological activity of mVEGF-DT and DT386 proteins was assessed on the TC1 cell line bearing mouse model. Proteins were injected intra-tumoral in mice, in separate groups.

Results

Tumors in the mVEGF-DT group started to dwindle after six injections, but tumor size in both control groups (DT386 and PBS), continued to grow.

Conclusion

Successful targeting of solid tumor cells by mVEGF-DT immunotoxin demonstrates the therapeutic potential utility of these conjugates for tumor targeting.

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.

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.

Engineering of Ribosome-inactivating Proteins for Improving Pharmacological Properties

Ribosome-inactivating proteins (RIPs) are N-glycosidases, which depurinate a specific adenine residue in the conserved α-sarcin/ricin loop (α-SRL) of rRNA. This loop is important for anchoring elongation factor (EF-G for prokaryote or eEF2 for eukaryote) in mRNA translocation. Translation is inhibited after the attack. RIPs therefore may have been applied for anti-cancer, and anti-virus and other therapeutic applications. The main obstacles of treatment with RIPs include short plasma half-life, non-selective cytotoxicity and antigenicity. This review focuses on the strategies used to improve the pharmacological properties of RIPs on human immunodeficiency virus (HIV) and cancers. Coupling with polyethylene glycol (PEG) increases plasma time and reduces antigenicity. RIPs conjugated with antibodies to form immunotoxins increase the selective toxicity to target cells. The prospects for future development on the engineering of RIPs for improving their pharmacological properties are also discussed.

Reinforcing the primary immunotherapy modulators against acute leukemia; monoclonal antibodies in AML

Recent therapeutic advances in cancer treatment recruit immune system potentiation against malignant cells. Numerous ongoing clinical trials on immunotherapy methods, either monotherapy or combination therapy, are investigating the impeding factors on the way of acute myeloid leukemia (AML) treatment. Due to the genetic diversity in AML progenitors, combining various strategies is more likely to be useful for improving patient outcomes. This review describes the details of applying monoclonal antibodies against AML, focusing on CD33, CD123, FLT3, CD45 and CD66 targeting. Furthermore, it clarifies the importance of immunotoxins, bispecific antibodies, chimeric antigen receptor (CAR)-T cells and T cell receptor-modified cells as reinforcing agents for monoclonal antibodies.

Improved Protein Toxin Delivery Based on ATTEMPTS Systems

BACKGROUND

Ribosome-inactivating proteins (RIPs) are wildly found in multiple species of plants, bacteria and fungi. As a special family of protein toxins, RIPs can inhibit protein synthesis and induce cell death via inactivating ribosome in eukaryotic cells. Thus, RIPs have been applied for anti-tumor therapy in the past two decades. However, because of poor cell permeability, nonselective mode of action for tumor cells, poor pharmacokinetic profiles and immunogenicity, their clinical application has been severely constrained. As an effort to overcome these obstacles, tumor-specific monoclonal antibodies (mAb) have been conjugated to RIPs (forming so called "immunotoxins") specifically to increase their cytotoxicity and provide tumor targeting. Nevertheless, immunotoxins yet have not fully resolved all the issues and critical challenges still remain, such as immunogenicity and inability to penetrate into the deep site of tumor.

OBJECTIVE

To overcome the constrain of immunotoxins, the novel cell-penetrating peptide (CPP)- modified ATTEMPTS systems based on combination of CPP-mediated penetration and antibodymediated tumor targeting, with triggerable drug release function, were developed to achieve effective and safe delivery of protein toxin.

RESULTS

The CPP-modified ATTEMPTS systems showed effective protamine-triggered CPP-toxin release and thus enhanced CPP-mediated cellular uptake and cytotoxicity. It also showed antibodymediated in vivo tumor targeting and significantly increased in vivo tumor growth suppression with limited systematic toxicity.

CONCLUSION

The CPP-modified ATTEMPTS systems were developed and demonstrated as a proof-ofconcept for CPP-based protein toxin delivery with triggerable antibody targeting to improve the druggability of protein toxin drugs. The systems showed the potential application of protein toxin clinical translation in anticancer treatment.

Novel monoclonal antibody-based therapies for acute myeloid leukemia

There has been long-standing interest in using monoclonal antibodies to improve outcomes of people with acute myeloid leukemia (AML). While several candidate therapeutics have failed at various stages of clinical testing, improved survival of some patients receiving the CD33 antibody-drug conjugate gemtuzumab ozogamicin has provided first evidence that monoclonal antibodies have a role in the armamentarium against AML. Over the last several years, work to improve the success of monoclonal antibody-based therapies in AML has focused on the identification and exploration of new antigen targets as much as on the development of novel treatment formats such as use of unconjugated engineered monoclonal antibodies and conjugated antibodies, delivering highly potent small molecule drugs or radionuclides to AML cells. Here, we will provide a brief overview of current efforts with such investigational monoclonal antibody-based therapeutics.

Targeting cell-bound MUC1 on myelomonocytic, monocytic leukemias and phenotypically defined leukemic stem cells with anti-SEA module antibodies

Cell surface molecules aberrantly expressed or overexpressed by myeloid leukemic cells represent potential disease-specific therapeutic targets for antibodies. MUC1 is a polymorphic glycoprotein, the cleavage of which yields two unequal chains: a large extracellular α subunit containing a tandem repeat array bound in a strong noncovalent interaction to a smaller β subunit containing the transmembrane and cytoplasmic domains. Because the α-chain can be released from the cell-bound domains of MUC1, agents directed against the α-chain will not effectively target MUC1⁺ cells. The MUC1 SEA (a highly conserved protein module so called from its initial identification in a sea urchin sperm protein, in enterokinase, and in agrin) domain formed by the binding of the α and β chains  represents a stable structure fixed to the cell surface at all times. DMB-5F3, a partially humanized murine anti-MUC1 SEA domain monoclonal antibody, was used to examine MUC1 expression in acute myeloid leukemia (AML) and was found to bind acute myelomonocytic and monocytic leukemia (AML-M4 and AML-M5) cell lines. We also examined monocytic neoplasms freshly obtained from patients including chronic myelomonocytic leukemia and juvenile myelomonocytic leukemia, which were found to uniformly express MUC1. CD34⁺/lin^-/CD38^- or CD38⁺ presumed leukemic stem cell populations from CD34⁺ AML and CD34^-CD38^- or CD38⁺ populations from CD34^- AML were also found to express MUC1, although at low percentages. Based on these studies, we generated an anti-MUC1 immunotoxin to directly gauge the cytotoxic efficacy of targeting AML-bound MUC1. Using single-chain DMB-5F3 fused to recombinant gelonin toxin, the degree of AML cytotoxicity was found to correlate with MUC1 expression. Our data support the use of an anti-MUC1 SEA module-drug conjugates to selectively target and inhibit MUC1-expressing myelomonocytic leukemic cells.

A potent tetravalent T-cell-engaging bispecific antibody against CD33 in acute myeloid leukemia

Acute myeloid leukemia (AML), the most common acute leukemia in adults and the second most common cancer in children, is still a lethal disease in the majority of patients, but immunologic approaches have improved outcome. Bispecific antibodies (BsAbs) are novel immunotherapeutics that can redirect immune cells against AML. We now report a tetravalent (2+2) humanized BsAb in the immunoglobulin G light chain single chain fragment variable [IgG(L)-scFv] format to engage polyclonal T cells to kill CD33+ AML targets. In vitro, this BsAb demonstrated strong antigen-specific T-cell-dependent cell-mediated cytotoxicity (TDCC) with an 50% effective concentration (EC50) in the femtomolar range that translated into treatment of established human AML IV xenografts in vivo. Importantly, it could redirect intraperitoneally injected T cells to ablate established and rapidly growing extramedullary subcutaneous AML xenografts in vivo. Furthermore, internalization of CD33 upon BsAb binding was identical to that of a bivalent (1+1) heterodimer, both being substantially less than anti-CD33 IgG. In contrast to the heterodimer, the tetravalent IgG-scFv BsAb was >10-fold more efficient in TDCC of AML cells in vitro and in vivo. This BsAb did not react with and did not kill CD38-CD34+ hematopoietic stem cells from cord blood. We conclude that the novel anti-CD33 IgG(L)-scFv BsAb construct reported here is a potential candidate for clinical development.

Investigational CD33-targeted therapeutics for acute myeloid leukemia

INTRODUCTION

There is long-standing interest in drugs targeting the myeloid differentiation antigen CD33 in acute myeloid leukemia (AML). Positive results from randomized trials with the antibody-drug conjugate (ADC) gemtuzumab ozogamicin (GO) validate this approach. Partly stimulated by the success of GO, several CD33-targeted therapeutics are currently in early phase testing.

AREAS COVERED

CD33-targeted therapeutics in clinical development include Fc-engineered unconjugated antibodies (BI 836858 [mAb 33.1]), ADCs (SGN-CD33A [vadastuximab talirine], IMGN779), radioimmunoconjugates (²²⁵Ac-lintuzumab), bi- and trispecific antibodies (AMG 330, AMG 673, AMV564, 161533 TriKE fusion protein), and chimeric antigen receptor (CAR)-modified immune effector cells. Besides limited data on ²²⁵Ac-lintuzumab showing modest single-agent activity, clinical data are so far primarily available for SGN-CD33A. SGN-CD33A has single-agent activity and has shown encouraging results when combined with an azanucleoside or standard chemotherapeutics. However, concerns about toxicity to the liver and normal hematopoietic cells - the latter leading to early termination of a phase 3 trial - have derailed the development of SGN-CD33A, and its future is uncertain.

EXPERT OPINION

Early results from a new generation of CD33-targeted therapeutics are anticipated in the next 2-3 years. Undoubtedly, re-approval of GO in 2017 has changed the landscape and rendered clinical development for these agents more challenging.

Identifying biomarkers of breast cancer micrometastatic disease in bone marrow using a patient-derived xenograft mouse model

BACKGROUND

Disseminated tumor cells (DTCs) found in the bone marrow (BM) of patients with breast cancer portend a poor prognosis and are thought to be intermediaries in the metastatic process. To assess the clinical relevance of a mouse model for identifying possible prognostic and predictive biomarkers of these cells, we have employed patient-derived xenografts (PDX) for propagating and molecularly profiling human DTCs.

METHODS

Previously developed mouse xenografts from five breast cancer patients were further passaged by implantation into NOD/SCID mouse mammary fat pads. BM was collected from long bones at early, serial passages and analyzed for human-specific gene expression by qRT-PCR as a surrogate biomarker for the detection of DTCs. Microarray-based gene expression analyses were performed to compare expression profiles between primary xenografts, solid metastasis, and populations of BM DTCs. Differential patterns of gene expression were then compared to previously generated microarray data from primary human BM aspirates from patients with breast cancer and healthy volunteers.

RESULTS

Human-specific gene expression of SNAI1, GSC, FOXC2, KRT19, and STAM2, presumably originating from DTCs, was detected in the BM of all xenograft mice that also developed metastatic tumors. Human-specific gene expression was undetectable in the BM of those xenograft lines with no evidence of distant metastases and in non-transplanted control mice. Comparative gene expression analysis of BM DTCs versus the primary tumor of one mouse line identified multiple gene transcripts associated with epithelial-mesenchymal transition, aggressive clinical phenotype, and metastatic disease development. Sixteen of the PDX BM associated genes also demonstrated a statistically significant difference in expression in the BM of healthy volunteers versus the BM of breast cancer patients with distant metastatic disease.

CONCLUSION

Unique and reproducible patterns of differential gene expression can be identified that presumably originate from BM DTCs in mouse PDX lines. Several of these identified genes are also detected in the BM of patients with breast cancer who develop early metastases, which suggests that they may be clinically relevant biomarkers. The PDX model may also provide a clinically relevant system for analyzing and targeting these intermediaries of metastases.

The progress and current status of immunotherapy in acute myeloid leukemia

Recently, there has been remarkable progress in basic and preclinical studies of acute myeloid leukemia (AML). The improved outcomes of AML can largely be attributed to advances in supportive care and hematopoietic cell transplantation as opposed to conventional chemotherapy. However, as the 5-year survival rate remains low due to a high incidence of relapse, novel and effective treatments are urgently needed. Increasing attention is focusing on identifying suitable immunotherapeutic strategies for AML. Here, we describe the immunological features, mechanisms of immune escape, and recent progress in immunotherapy for AML. Problems encountered in the clinic will also be discussed. Although current outcomes may be limited, ongoing preclinical or clinical efforts are aimed at improving immunotherapy modalities and designing novel therapies, such as vaccines, monoclonal antibody therapy, chimeric antibody receptor-engineered T cells (CAR-T), TCR-engineered T cells (TCR-T), and checkpoint inhibitors, which may provide promising and effective therapies with higher specificity and efficacy for AML.

The Use of Plant-Derived Ribosome Inactivating Proteins in Immunotoxin Development: Past, Present and Future Generations

Ribosome inactivating proteins (RIPs) form a class of toxins that was identified over a century ago. They continue to fascinate scientists and the public due to their very high activity and long-term stability which might find useful applications in the therapeutic killing of unwanted cells but can also be used in acts of terror. We will focus our review on the canonical plant-derived RIPs which display ribosomal RNA N-glycosidase activity and irreversibly inhibit protein synthesis by cleaving the 28S ribosomal RNA of the large 60S subunit of eukaryotic ribosomes. We will place particular emphasis on therapeutic applications and the generation of immunotoxins by coupling antibodies to RIPs in an attempt to target specific cells. Several generations of immunotoxins have been developed and we will review their optimisation as well as their use and limitations in pre-clinical and clinical trials. Finally, we endeavour to provide a perspective on potential future developments for the therapeutic use of immunotoxins.

Recent developments in immunotherapy of acute myeloid leukemia

The advent of new immunotherapeutic agents in clinical practice has revolutionized cancer treatment in the past decade, both in oncology and hematology. The transfer of the immunotherapeutic concepts to the treatment of acute myeloid leukemia (AML) is hampered by various characteristics of the disease, including non-leukemia-restricted target antigen expression profile, low endogenous immune responses, and intrinsic resistance mechanisms of the leukemic blasts against immune responses. However, considerable progress has been made in this field in the past few years.Within this manuscript, we review the recent developments and the current status of the five currently most prominent immunotherapeutic concepts: (1) antibody-drug conjugates, (2) T cell-recruiting antibody constructs, (3) chimeric antigen receptor (CAR) T cells, (4) checkpoint inhibitors, and (5) dendritic cell vaccination. We focus on the clinical data that has been published so far, both for newly diagnosed and refractory/relapsed AML, but omitting immunotherapeutic concepts in conjunction with hematopoietic stem cell transplantation. Besides, we have included important clinical trials that are currently running or have recently been completed but are still lacking full publication of their results.While each of the concepts has its particular merits and inherent problems, the field of immunotherapy of AML seems to have taken some significant steps forward. Results of currently running trials will reveal the direction of further development including approaches combining two or more of these concepts.

Clinical targeting recombinant immunotoxins for cancer therapy

Recombinant immunotoxins (RITs) are proteins that contain a toxin fused to an antibody or small molecules and are constructed by the genetic engineering technique. RITs can bind to and be internalized by cells and kill cancerous or non-cancerous cells by inhibiting protein synthesis. A wide variety of RITs have been tested against different cancers in cell culture, xenograft models, and human patients during the past several decades. RITs have shown activity in therapy of several kinds of cancers, but different levels of side effects, mainly related to vascular leak syndrome, were also observed in the treated patients. High immunogenicity of RITs limited their long-term or repeat applications in clinical cases. Recent advances in the design of immunotoxins, such as humanization of antibody fragment, PEGylation, and modification of human B- and T-cell epitopes, are overcoming the above mentioned problems, which predict the use of these immunotoxins as a potential therapeutic method to treat cancer patients.

Gemtuzumab ozogamicin in acute myeloid leukemia

CD33 is variably expressed on leukemia blasts in almost all patients with acute myeloid leukemia (AML) and possibly leukemia stem cells in some. Efforts to target CD33 therapeutically have focused on gemtuzumab ozogamicin (GO; Mylotarg), an antibody-drug conjugate delivering a DNA-damaging calicheamicin derivative. GO is most effective in acute promyelocytic leukemia but induces remissions in other AML types and received accelerated approval in the US in 2000. However, because a large follow-up study showed no survival improvement and increased early deaths the drug manufacturer voluntarily withdrew the US New Drug Application in 2010. More recently, a meta-analysis of data from several trials reported better survival in adults with favorable- and intermediate-risk cytogenetics but not adverse-risk AML randomized to receive GO along with intensive induction chemotherapy. As a result, GO is being re-evaluated by regulatory agencies. Responses to GO are diverse and predictive biological response markers are needed. Besides cytogenetic risk, ATP-binding cassette transporter activity and possibly CD33 display on AML blasts may predict response, but established clinical assays and prospective validation are lacking. Single-nucleotide polymorphisms in CD33 may also be predictive, most notably rs12459419 where the minor T-allele leads to decreased display of full-length CD33 and preferential translation of a splice variant not recognized by GO. Data from retrospective analyses suggest only patients with the rs12459419 CC genotype may benefit from GO therapy but confirmation is needed. Most important may be markers for AML cell sensitivity to calicheamicin, which varies over 100 000-fold, but useful assays are unavailable. Novel CD33-targeted drugs may overcome some of GO's limitations but it is currently unknown whether such drugs will be more effective in patients benefitting from GO and/or improve outcomes in patients not benefitting from GO, and what the supportive care requirements will be to enable their safe use.

Development and validation of an enzyme-linked immunosorbent assay for the quantification of gelonin in mouse plasma

This article details the development and validation of an enzyme-linked immunosorbent assay (ELISA) for the quantification of gelonin in mouse plasma. The ELISA was validated for intra- and inter-day variability and for accuracy over a standard curve range of 7.5-100 ng/mL. The assay was then applied to assess gelonin pharmacokinetics in mice. Results from the ELISA were compared to data obtained from a parallel study conducted with (125)Iodine-labeled gelonin, with quantification via gamma counting. The ELISA demonstrated good precision, as the percent coefficient of variation of quality control samples in intra-day and inter-day validation ranged from 5.4-9.3% and 2.9-7.3%, respectively. Sample recoveries ranged from 98.3-105% of nominal values. The ELISA method yielded lower plasma concentrations of gelonin than found from the less-specific gamma counting method. Consequently, pharmacokinetic analyses yielded significantly higher estimates for volume of distribution (106 ± 31 vs. 55.8 ± 13 mL/kg) and plasma clearance (34.7 ± 6.6 vs. 10.9 ± 2.1 mL/min/kg) for data determined by ELISA vs. by gamma counting.

Design and In Vivo Characterization of Immunoconjugates Targeting HIV gp160

The envelope (Env) glycoprotein of HIV is expressed on the surface of productively infected cells and can be used as a target for cytotoxic immunoconjugates (ICs), in which cell-killing moieties, including toxins, drugs, or radionuclides, are chemically or genetically linked to monoclonal antibodies (MAbs) or other targeting ligands. Such ICs could be used to eliminate persistent reservoirs of HIV infection. We have found that MAbs which bind to the external loop of gp41, e.g., MAb 7B2, make highly effective ICs, particularly when used in combination with soluble CD4. We evaluated the toxicity, immunogenicity, and efficacy of the ICs targeted with 7B2 in mice and in simian-human immunodeficiency virus-infected macaques. In the macaques, we tested immunotoxins (ITs), consisting of protein toxins bound to the targeting agent. ITs were well tolerated and initially efficacious but were ultimately limited by their immunogenicity. In an effort to decrease immunogenicity, we tested different toxic moieties, including recombinant toxins, cytotoxic drugs, and tubulin inhibitors. ICs containing deglycosylated ricin A chain prepared from ricin toxin extracted from castor beans were the most effective in killing HIV-infected cells. Having identified immunogenicity as a major concern, we show that conjugation of IT to polyethylene glycol limits immunogenicity. These studies demonstrate that cytotoxic ICs can target virus-infected cells in vivo but also highlight potential problems to be addressed.

IMPORTANCE

It is not yet possible to cure HIV infection. Even after years of fully effective antiviral therapy, a persistent reservoir of virus-infected cells remains. Here we propose that a targeted conjugate consisting of an anti-HIV antibody bound to a toxic moiety could function to kill the HIV-infected cells that constitute this reservoir. We tested this approach in HIV-infected cells grown in the lab and in animal infections. Our studies demonstrated that these immunoconjugates are effective both in vitro and in test animals. In particular, ITs constructed with the deglycosylated A chain prepared from native ricin were the most effective in killing cells, but their utility was blunted because they provoked immune reactions that interfered with the therapeutic effects. We then demonstrated that coating of the ITs with polyethylene glycol minimized the immunogenicity, as has been demonstrated with other protein therapies.

Aminopeptidase N (APN/CD13) as a target molecule for scirrhous gastric cancer

BACKGROUND

Scirrhous gastric cancer is associated with peritoneal dissemination and advanced lymph node metastasis from an early stage, and the prognosis is still poor. In this study, we aimed to analyze candidate molecules for targeted therapy of scirrhous gastric cancer. We searched for molecules/metabolic activity that might be predominantly expressed in a subpopulation of scirrhous gastric cancer cells and might function as cancer stem cell markers.

RESULTS

For this purpose, we investigated the expression of various cell surface markers and of aldehyde dehydrogenase (ALDH) activity. These analyses showed that the scirrhous gastric cancer cell lines HSC-58 and HSC-44PE heterogeneously expressed CD13, while CD44, CDCP1, EpCAM and ABCG2 were expressed uniformly. Moreover, 10% of the total HSC-58 cell population expressed ALDH enzyme activity. A subpopulation of cells strongly positive for ALDH also expressed high levels of CD13, both of which are known as cancer stem cell markers. HSC-58 cells expressing high levels of CD13 showed lower sensitivity to a cancer drug cisplatin than cells with low levels of CD13. In contrast, CD13(-high) subpopulation of HSC-58 was more sensitive to an aminopeptidase N inhibitor bestatin. In terms of antibody-drug therapy, anti-CD13-immunotoxin was highly cytotoxic towards HSC-58 cells and was more cytotoxic than anti-EpCAM-immunotoxin.

CONCLUSION

These data suggest that CD13 is a suitable cell surface candidate for targeted antibody-drug therapy of scirrhous gastric cancer.

A deimmunised form of the ribotoxin, α-sarcin, lacking CD4+ T cell epitopes and its use as an immunotoxin warhead

Fungal ribotoxins that block protein synthesis can be useful warheads in the context of a targeted immunotoxin. α-Sarcin is a small (17 kDa) fungal ribonuclease produced by Aspergillus giganteus that functions by catalytically cleaving a single phosphodiester bond in the sarcin–ricin loop of the large ribosomal subunit, thus making the ribosome unrecognisable to elongation factors and leading to inhibition of protein synthesis. Peptide mapping using an ex vivo human T cell assay determined that α-sarcin contained two T cell epitopes; one in the N-terminal 20 amino acids and the other in the C-terminal 20 amino acids. Various mutations were tested individually within each epitope and then in combination to isolate deimmunised α-sarcin variants that had the desired properties of silencing T cell epitopes and retention of the ability to inhibit protein synthesis (equivalent to wild-type, WT α-sarcin). A deimmunised variant (D9T/Q142T) demonstrated a complete lack of T cell activation in in vitro whole protein human T cell assays using peripheral blood mononuclear cells from donors with diverse HLA allotypes. Generation of an immunotoxin by fusion of the D9T/Q142T variant to a single-chain Fv targeting Her2 demonstrated potent cell killing equivalent to a fusion protein comprising the WT α-sarcin. These results represent the first fungal ribotoxin to be deimmunised with the potential to construct a new generation of deimmunised immunotoxin therapeutics.

In Vitro and In Vivo Efficacy of a Novel CD33-Targeted Thorium-227 Conjugate for the Treatment of Acute Myeloid Leukemia

The clinical efficacy of the first approved alpha pharmaceutical, Xofigo (radium-223 dichloride, ²²³RaCl₂), has stimulated significant interest in the development of new alpha-particle emitting drugs in oncology. Unlike radium-223 (²²³Ra), the parent radionuclide thorium-227 (²²⁷Th) is able to form highly stable chelator complexes and is therefore amenable to targeted radioimmunotherapy. We describe the preparation and use of a CD33-targeted thorium-227 conjugate (CD33-TTC), which binds to the sialic acid receptor CD33 for the treatment of acute myeloid leukemia (AML). A chelator was conjugated to the CD33-targeting antibody lintuzumab via amide bonds, enabling radiolabeling with the alpha-emitter ²²⁷Th. The CD33-TTC induced in vitro cytotoxicity on CD33-positive cells, independent of multiple drug resistance (MDR) phenotype. After exposure to CD33-TTC, cells accumulated DNA double-strand breaks and were arrested in the G₂ phase of the cell cycle. In vivo, the CD33-TTC demonstrated antitumor activity in a subcutaneous xenograft mouse model using HL-60 cells at a single dose regimen. Dose-dependent significant survival benefit was further demonstrated in a disseminated mouse tumor model after single dose injection or administered as a fractionated dose. The data presented support the further development of the CD33-TTC as a novel alpha pharmaceutical for the treatment of AML. Mol Cancer Ther; 15(10); 2422-31. ©2016 AACR.

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

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

Update on antigen-specific immunotherapy of acute myeloid leukemia

Among the few drugs that have shown a benefit for patients with acute myeloid leukemia (AML) in randomized clinical trials over the last several decades is the CD33 antibody-drug conjugate, gemtuzumab ozogamicin (GO). Undoubtedly, this experience has highlighted the value of antigen-specific immunotherapy in AML. A wide variety of therapeutics directed against several different antigens on AML cells are currently explored in preclinical and early clinical studies. On the one hand, these include passive strategies such as unconjugated antibodies targeting one or more antigens, antibodies armed with drugs, toxic proteins, or radionuclides, or adoptive immunotherapies, in particular utilizing T cells engineered to express chimeric antigen receptors (CARs) or modified T cell receptor (TCR) genes; on the other hand, these include active strategies such as vaccinations. With the documented benefit for GO and the emerging data with several classes of therapeutics in other leukemias, in particular small bispecific antibodies and CAR T cells, the future is bright. Nevertheless, a number of important questions related to the choice of target antigen(s), patient population, exact treatment modality, and supportive care needs remain open. Addressing such questions in upcoming studies will ultimately be required to optimize the clinical use of antigen-specific immunotherapies in AML and ensure that such treatments become an effective, versatile tool for this disease for which the outcomes have remained unsatisfactory in many patients.

The future of antiviral immunotoxins

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

Antibody-mediated delivery of therapeutics for cancer therapy

INTRODUCTION

Antibody-conjugated therapies (ACTs) combine the specificity of monoclonal antibodies to target cancer cells directly with highly potent payloads, often resulting in superior efficacy and/or reduced toxicity. This represents a new approach to the treatment of cancer. There have been highly promising clinical trial results using this approach with improvements in linker and payload technology. The breadth of current trials examining ACTs in haematological malignancies and solid tumours indicate the potential for clinical impact.

AREAS COVERED

This review will provide an overview of ACTs currently in clinical development as well as the principles of antibody delivery and types of payloads used, including cytotoxic drugs, radiolabelled isotopes, nanoparticle-based siRNA particles and immunotoxins.

EXPERT OPINION

The focus of much of the clinical activity in ACTs has, understandably, been on their use as a monotherapy or in combination with standard of care drugs. This will continue, as will the search for better targets, linkers and payloads. Increasingly, as these drugs enter routine clinical care, important questions will arise regarding how to optimise ACT treatment approaches, including investigation of resistance mechanisms, biomarker and patient selection strategies, understanding of the unique toxicities of these drugs, and combinatorial approaches with standard therapies as well as emerging therapeutic agents like immunotherapy.

Biological activities of ribosome-inactivating proteins and their possible applications as antimicrobial, anticancer, and anti-pest agents and in neuroscience research

Ribosome-inactivating proteins (RIPs) are enzymes which depurinate ribosomal RNA (rRNA), thus impeding the process of translation resulting in inhibition of protein synthesis. They are produced by various organisms including plants, fungi and bacteria. RIPs from plants are linked to plant defense due to their antiviral, antifungal, antibacterial, and insecticidal activities in which they can be applied in agriculture to combat microbial pathogens and pests. Their anticancer, antiviral, embryotoxic, and abortifacient properties may find medicinal applications. Besides, conjugation of RIPs with antibodies or other carriers to form immunotoxins has been found useful to research in neuroscience and anticancer therapy.

Anti-tumor activities and apoptotic mechanism of ribosome-inactivating proteins

Ribosome-inactivating proteins (RIPs) belong to a family of enzymes that attack eukaryotic ribosomes and potently inhibit cellular protein synthesis. RIPs possess several biomedical properties, including anti-viral and anti-tumor activities. Multiple RIPs are known to inhibit tumor cell proliferation through inducing apoptosis in a variety of cancers, such as breast cancer, leukemia/lymphoma, and hepatoma. This review focuses on the anti-tumor activities of RIPs and their apoptotic effects through three closely related pathways: mitochondrial, death receptor, and endoplasmic reticulum pathways.

Immunotherapy for Acute Myeloid Leukemia

Despite longstanding efforts in basic research and clinical studies, the prognosis for patients with acute myeloid leukemia (AML) remains poor. About half of the patients are not medically fit for intensive induction therapy to induce a complete remission and are treated with palliative treatment concepts. The patients medically fit for intensive induction therapy have a high complete remission rate but the majority suffers from relapse due to chemo-refractory leukemic cells. Allogeneic stem cell transplantation as post-remission therapy can significantly reduce the likelihood of relapse, but it is associated with a high rate of morbidity and mortality. Novel therapeutic concepts are therefore urgently sought after. During recent years, the focus has shifted towards the development of novel immunotherapeutic strategies. Some of the most promising are drug-conjugated monoclonal antibodies, T-cell engaging antibody constructs, adoptive transfer with chimeric antigen receptor (CAR) T cells, and dendritic cell vaccination. Here, we review recent progress in these four fields and speculate about the optimal time points during the course of AML treatment for their application.

Antigen-specific immunotherapies for acute myeloid leukemia

Antigen-specific immunotherapies have emerged as important components of curative treatment algorithms for many cancers. In acute myeloid leukemia (AML), success has been less obvious. Nonetheless, among the few drugs shown to improve survival in recent randomized trials is the CD33 antibody-drug conjugate gemtuzumab ozogamicin. Significant antileukemic activity is also well documented for radioimmunoconjugates targeting CD33, CD45, or CD66. These therapeutics can intensify conditioning before hematopoietic cell transplantation, but their effect on patient outcomes needs clarification. Emerging data now suggest clinical antileukemic activity of several novel antibodies and perhaps some adoptive T-cell immunotherapies and vaccines. In parallel, numerous other agents targeting a wider variety of antigens are currently being explored. However, the antigenic heterogeneity characteristic of AML is a considerable limitation for all these therapeutics, and many important questions related to the ideal target antigen(s), disease situation in which to use these therapies, most suitable patient populations, exact treatment modalities, and details of supportive care needs remain open. Addressing such questions in upcoming studies will be required to ensure that antigen-directed therapies become an effective tool in AML, a disease for which outcomes with standard "3 + 7"-based chemotherapy have remained unsatisfactory in many patients.

Target Therapy in Hematological Malignances: New Monoclonal Antibodies

Apart from radio- and chemotherapy, monoclonal antibodies (MoAbs) represent a new, more selective tool in the treatment of hematological malignancies. MoAbs bind with the specific antigens of the tumors. This interaction is a basis for targeted therapies which exhibit few side effects and significant antitumor activity. This review provides an overview of the functional characteristics of MoAbs, with some examples of their clinical application. The promising results in the treatment of hematological malignancies have led to the more frequent usage of MoAbs in the therapy. Development of MoAbs is a subject of extensive research. They are a promising method of cancer treatment in the future.

Recombinant TAT-gelonin fusion toxin: synthesis and characterization of heparin/protamine-regulated cell transduction

Protein toxins, such as gelonin, are highly desirable anti-cancer drug candidates due to their unparalleled potency and repetitive reaction mechanism in inhibiting protein translation. However, for its potential application in cancer therapy, there remains the cell membrane barrier that allows permeation of only small molecules, which must be overcome. To address this challenge, we conjugated gelonin with a protein transduction domain (PTD), the TAT peptide, via genetic recombination. The chimeric TAT-gelonin fusion protein (TAT-Gel) retained equipotent N-glycosidase activity yet displayed greater cell uptake than unmodified recombinant gelonin (rGel), thereby yielding a significantly augmented cytotoxic activity. Remarkably, TAT-Gel displayed up to 177-fold lower IC₅₀ (avg. 54.3 nM) than rGel (avg. IC₅₀ : 3640 nM) in tested cell lines. This enhanced cytotoxicity, however, also raised potential toxicity concerns due to the non-selectivity of PTD in its mediated cell transduction. To solve this problem, we investigated the plausibility of regulating the cell transduction of TAT-Gel via a reversible masking using heparin and protamine. Here, we demonstrated, both in vitro and in vivo, that the cell transduction of TAT-Gel can be completely curbed with heparin and yet this heparin block can be efficiently reversed by the addition of protamine. This reversible tight regulation of the cell transduction of TAT-Gel by heparin and protamine sheds light of possible application of TAT-Gel in achieving a highly effective yet safe drug therapy for the treatment of tumors.

The role of CD33 as therapeutic target in acute myeloid leukemia

CD33 is a myeloid differentiation antigen that is displayed on acute myeloid leukemia (AML) blasts in most patients and, possibly, leukemic stem cells in some, and has thus served as target for antibody-based therapies for many years. Validation for this approach comes from the antibody-drug conjugate, gemtuzumab ozogamicin, which improves survival of some patients with AML when added to induction chemotherapy. Still, CD33 is a challenging target because of its low expression and slow internalization; these characteristics limit antibody-dependent cell-mediated cytotoxicity and intracellular drug accumulation and, consequently, the activity of unlabeled and toxin-carrying antibodies. Very promising preclinical data are now available from an improved antibody-drug conjugate and CD33-targeted strategies that redirect immune effector cells to eradicate the leukemia, most notably bispecific antibodies and chimeric antigen receptor T-cell immunotherapy. In parallel to their clinical testing, efforts will be needed to identify the patients that most likely benefit from such agents and the disease stage in which they are most efficacious. With enhanced activity of CD33-directed therapies, toxic effects on normal hematopoiesis will increase and require excellent supportive care measures, or even rescue with donor cells, to minimize morbidity and mortality from expected cytopenias and to optimize treatment outcomes with these therapeutics.

Immunotoxins for leukemia

Unconjugated monoclonal antibodies that target hematopoietic differentiation antigens have been developed to treat hematologic malignancies. Although some of these have activity against chronic lymphocytic leukemia and hairy cell leukemia, in general, monoclonal antibodies have limited efficacy as single agents in the treatment of leukemia. To increase their potency, the binding domains of monoclonal antibodies can be attached to protein toxins. Such compounds, termed immunotoxins, are delivered to the interior of leukemia cells based on antibody specificity for cell surface target antigens. Recombinant immunotoxins have been shown to be highly cytotoxic to leukemic blasts in vitro, in xenograft model systems, and in early-phase clinical trials in humans. These agents will likely play an increasing role in the treatment of leukemia.

Immunotoxins constructed with ribosome-inactivating proteins and their enhancers: a lethal cocktail with tumor specific efficacy

The term ribosome-inactivating protein (RIP) is used to denominate proteins mostly of plant origin, which have N-glycosidase enzymatic activity leading to a complete destruction of the ribosomal function. The discovery of the RIPs was almost a century ago, but their usage has seen transition only in the last four decades. With the advent of antibody therapy, the RIPs have been a subject of extensive research especially in targeted tumor therapies, which is the primary focus of this review. In the present work we enumerate 250 RIPs, which have been identified so far. An attempt has been made to identify all the RIPs that have been used for the construction of immunotoxins, which are conjugates or fusion proteins of an antibody or ligand with a toxin. The data from 1960 onwards is reviewed in this paper and an extensive list of more than 450 immunotoxins is reported. The clinical reach of tumor-targeted toxins has been identified and detailed in the work as well. While there is a lot of potential that RIPs embrace for targeted tumor therapies, the success in preclinical and clinical evaluations has been limited mainly because of their inability to escape the endo/lysosomal degradation. Various strategies that can increase the efficacy and lower the required dose for targeted toxins have been compiled in this article. It is plausible that with the advancements in platform technologies or improved endosomal escape the usage of tumor targeted RIPs would see the daylight of clinical success.

TWEAK/Fn14 Axis-Targeted Therapeutics: Moving Basic Science Discoveries to the Clinic

The TNF superfamily member TWEAK (TNFSF12) is a multifunctional cytokine implicated in physiological tissue regeneration and wound repair. TWEAK is initially synthesized as a membrane-anchored protein, but furin cleavage within the stalk region can generate a secreted TWEAK isoform. Both TWEAK isoforms bind to a small cell surface receptor named Fn14 (TNFRSF12A) and this interaction stimulates various cellular responses, including proliferation and migration. Fn14, like other members of the TNF receptor superfamily, is not a ligand-activated protein kinase. Instead, TWEAK:Fn14 engagement promotes Fn14 association with members of the TNFR associated factor family of adapter proteins, which triggers activation of various signaling pathways, including the classical and alternative NF-κB pathways. Numerous studies have revealed that Fn14 gene expression is significantly elevated in injured tissues and in most solid tumor types. Also, sustained Fn14 signaling has been implicated in the pathogenesis of cerebral ischemia, chronic inflammatory diseases, and cancer. Accordingly, several groups are developing TWEAK- or Fn14-targeted agents for possible therapeutic use in patients. These agents include monoclonal antibodies, fusion proteins, and immunotoxins. In this article, we provide an overview of some of the TWEAK/Fn14 axis-targeted agents currently in pre-clinical animal studies or in human clinical trials and discuss two other potential approaches to target this intriguing signaling node.

Sortase-catalyzed in vitro functionalization of a HER2-specific recombinant Fab for tumor targeting of the plant cytotoxin gelonin

We report on the preparation of a new type of immunotoxin via in vitro ligation of the αHer2 antigen binding fragment (Fab) of the clinically-validated antibody trastuzumab to the plant toxin gelonin, employing catalysis by the bacterial enzyme sortase A (SrtA). The αHer2 Fab was fused with the extended SrtA recognition motif LPET↓GLEH 6 at the C-terminus of its heavy chain, thereby preventing interference with antigen binding, while the toxin was equipped with a Gly 2 sequence at its N-terminus, distant to the catalytically active site in the C-terminal region. Site-specific in vitro transpeptidation led to a novel antibody-toxin conjugate wherein gelonin had effectively replaced the Fc region of a conventional (monomerized) immunoglobulin. After optimization of reaction conditions and incubation time, the resulting Fab-Gelonin ligation product was purified to homogeneity in a two-step procedure by means of Strep-Tactin affinity chromatography--utilizing the Strep-tag II appended to gelonin--and size exclusion chromatography. Binding activity of the immunotoxin for the Her2 ectodomain was indistinguishable from the unligated Fab as measured by real-time surface plasmon resonance spectroscopy. Specific cytotoxic potency of Fab-Gelonin was demonstrated against two Her2-positive cell lines, resulting in EC 50 values of ~1 nM or lower, indicating a 1000-fold enhanced cell-killing activity compared with gelonin itself. Thus, our strategy provides a convenient route to the modular construction of functional immunotoxins from Fabs of established tumor-specific antibodies with gelonin or related proteotoxins, also avoiding the elevated biosafety levels that would be mandatory for the direct biotechnological preparation of corresponding fusion proteins.