Augmenting the Efficacy of Immunotoxins and Other Targeted Protein Toxins by Endosomal Escape Enhancers

Mutational Analysis of RIP Type I Dianthin-30 Suggests a Role for Arg24 in Endocytosis

Saponin-mediated endosomal escape is a mechanism that increases the cytotoxicity of type I ribosome-inactivating proteins (type I RIPs). In order to actualize their cytotoxicity, type I RIPs must be released into the cytosol after endocytosis. Without release from the endosomes, type I RIPs are largely degraded and cannot exert their cytotoxic effects. Certain triterpene saponins are able to induce the endosomal escape of these type I RIPs, thus increasing their cytotoxicity. However, the molecular mechanism underlying the endosomal escape enhancement of type I RIPs by triterpene saponins has not been fully elucidated. In this report, we investigate the involvement of the basic amino acid residues of dianthin-30, a type I RIP isolated from the plant Dianthus caryophyllus L., in endosomal escape enhancement using alanine scanning. Therefore, we designed 19 alanine mutants of dianthin-30. Each mutant was combined with SO1861, a triterpene saponin isolated from the roots of Saponaria officinalis L., and subjected to a cytotoxicity screening in Neuro-2A cells. Cytotoxic screening revealed that dianthin-30 mutants with lysine substitutions did not impair the endosomal escape enhancement. There was one particular mutant dianthin, Arg24Ala, that exhibited significantly reduced synergistic cytotoxicity in three mammalian cell lines. However, this reduction was not based on an altered interaction with SO1861. It was, rather, due to the impaired endocytosis of dianthin Arg24Ala into the cells.

Enhanced cytotoxicity of a Pseudomonas Exotoxin A based immunotoxin against prostate cancer by addition of the endosomal escape enhancer SO1861

Immunotoxins consist of an antibody or antibody fragment that binds to a specific cell surface structure and a cytotoxic domain that kills the cell after cytosolic uptake. Pseudomonas Exotoxin A (PE) based immunotoxins directed against a variety of tumor entities have successfully entered the clinic. PE possesses a KDEL-like motif (REDLK) that enables the toxin to travel from sorting endosomes via the KDEL-receptor pathway to the endoplasmic reticulum (ER), from where it is transported into the cytosol. There, it ADP-ribosylates the eukaryotic elongation factor 2, resulting in ribosome inhibition and finally apoptosis. One major problem of immunotoxins is their lysosomal degradation causing the need for much more immunotoxin molecules than finally required for induction of cell death. The resulting dose limitations and substantially increased side effects require new strategies to achieve improved cytosolic uptake. Here we generated an immunotoxin consisting of a humanized single chain variable fragment (scFv) targeting the prostate specific membrane antigen (PSMA) and the de-immunized PE variant PE24mut. This immunotoxin, hD7-1(VL-VH)-PE24mut, showed high and specific cytotoxicity in PSMA-expressing prostate cancer cells. We deleted the REDLK sequence to prevent transport to the ER and achieve endosomal entrapment. The cytotoxicity of this immunotoxin, hD7-1(VL-VH)-PE24mutΔREDLK, was greatly reduced. To restore activity, we added the endosomal escape enhancer SO1861 and observed an up to 190,000-fold enhanced cytotoxicity corresponding to a 57-fold enhancement compared to the initial immunotoxin with the REDLK sequence. A biodistribution study with different routes of administration clearly showed that the subcutaneous injection of hD7-1(VL-VH)-PE24mutΔREDLK in mice resulted in the highest tumor uptake. Treatment of mice bearing prostate tumors with a combination of hD7-1(VL-VH)-PE24mutΔREDLK plus SO1861 resulted in inhibition of tumor growth and enhanced overall survival compared to the monotherapies. The endosomal entrapment of non-toxic anti-PSMA immunotoxins followed by enhanced endosomal escape by SO1861 provides new therapeutic options in the future management of prostate cancer.

Targeting the Inside of Cells with Biologicals: Toxin Routes in a Therapeutic Context

Numerous toxins translocate to the cytosol in order to fulfil their function. This demonstrates the existence of routes for proteins from the extracellular space to the cytosol. Understanding these routes is relevant to multiple aspects related to therapeutic applications. These include the development of anti-toxin treatments, the potential use of toxins as shuttles for delivering macromolecular cargo to the cytosol or the use of drugs based on toxins. Compared with other strategies for delivery, such as chemicals as carriers for macromolecular delivery or physical methods like electroporation, toxin routes present paths into the cell that potentially cause less damage and can be specifically targeted. The efficiency of delivery via toxin routes is limited. However, low-delivery efficiencies can be entirely sufficient, if delivered cargoes possess an amplification effect or if very few molecules are sufficient for inducing the desired effects. This is known for example from RNA-based vaccines that have been developed during the coronavirus disease 2019 pandemic as well as for other approved RNA-based drugs, which elicited the desired effect despite their typically low delivery efficiencies. The different mechanisms by which toxins enter cells may have implications for their technological utility. We review the mechanistic principles of the translocation pathway of toxins from the extracellular space to the cytosol, the delivery efficiencies, and therapeutic strategies or applications that exploit toxin routes for intracellular delivery.

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.

Raddeanin A synergistically enhances the anti-tumor effect of MAP30 in multiple ways, more than promoting endosomal escape

Biomacromolecules such as proteins and nucleic acids are very attractive due to their high efficiency and specificity as cancer therapeutics. In fact, the endocytosed macromolecules are often trapped in the endosomes and cannot exhibit pharmacological effects well. Many strategies have been used to address this bottleneck, and one promising approach is to exploit the endosomal escape-promoting effect of triterpenoid saponins to aid in the release of biomacromolecules. Here, Raddeanin A (RA, an oleanane-type triterpenoid saponin) was proved to significantly promote endosomal escape as it recruited Galectin-9, an endosomal escape event reporter. As expected, RA effectively enhanced the anti-tumor effect of MAP30 (a type I ribosome-inactivating protein derived from Momordica charantia). However, based on the results of fluorescent colocalization, RA did not significantly promote MAP30 release from endosomes, suggesting that RA enhances MAP30 activity not only by promoting endosomal escape. Furthermore, it was found that the inhibitors of micropinocytosis and caveolae could almost completely inhibit the cytotoxicity of MAP30 combined with RA without affecting the cytotoxicity of MAP30 alone, indicating that RA may regulate the endocytic pathway of MAP30. Meanwhile, the effect of RA is related to the intra vesicular pH and cholesterol content on cell membrane, and is also cell-type dependent. Therefore, RA enhanced the anti-tumor effect of MAP30 in multiple ways, not just by promoting endosomal escape. Our findings will help to further decipher the possible mechanisms by which triterpenoid saponins enhance drug activity, and provide a new perspective for improving the activity of endocytosed drugs.

Antibody-Based Immunotoxins for Colorectal Cancer Therapy

Monoclonal antibodies (mAbs) are included among the treatment options for advanced colorectal cancer (CRC). However, while these mAbs effectively target cancer cells, they may have limited clinical activity. A strategy to improve their therapeutic potential is arming them with a toxic payload. Immunotoxins (ITX) combining the cell-killing ability of a toxin with the specificity of a mAb constitute a promising strategy for CRC therapy. However, several important challenges in optimizing ITX remain, including suboptimal pharmacokinetics and especially the immunogenicity of the toxin moiety. Nonetheless, ongoing research is working to solve these limitations and expand CRC patients' therapeutic armory. In this review, we provide a comprehensive overview of targets and toxins employed in the design of ITX for CRC and highlight a wide selection of ITX tested in CRC patients as well as preclinical candidates.

Immunotoxins and nanobody-based immunotoxins: review and update

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

Targeted Toxins for the Treatment of Prostate Cancer

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

Cytotoxicity of Quillaja saponaria Saponins towards Lung Cells Is Higher for Cholesterol-Rich Cells

The purpose of the study was to compare cytotoxicity of two Quillaja saponaria bark saponin (QBS) mixtures against two lung cell lines: normal MRC-5 fibroblast cell line and tumor A-549 epithelial cells of lungs’ alveoli. The study, performed both at a macro-scale and in a dedicated microfluidic device, showed that QBS was more toxic to the cell line more abundant in cholesterol (MRC-5). The QBS mixture with higher saponin fraction was found to be more cytotoxic towards both cell lines. The results may help to better understand the cytotoxicity of saponin-rich herbal medicines towards normal and tumor cells depending on their cholesterol content.

Pseudomonas Exotoxin A Based Toxins Targeting Epidermal Growth Factor Receptor for the Treatment of Prostate Cancer

The epidermal growth factor receptor (EGFR) was found to be a valuable target on prostate cancer (PCa) cells. However, EGFR inhibitors mostly failed in clinical studies with patients suffering from PCa. We therefore tested the targeted toxins EGF-PE40 and EGF-PE24mut consisting of the natural ligand EGF as binding domain and PE40, the natural toxin domain of Pseudomonas Exotoxin A, or PE24mut, the de-immunized variant thereof, as toxin domains. Both targeted toxins were expressed in the periplasm of E.coli and evoked an inhibition of protein biosynthesis in EGFR-expressing PCa cells. Concentration- and time-dependent killing of PCa cells was found with IC50 values after 48 and 72 h in the low nanomolar or picomolar range based on the induction of apoptosis. EGF-PE24mut was found to be about 11- to 120-fold less toxic than EGF-PE40. Both targeted toxins were more than 600 to 140,000-fold more cytotoxic than the EGFR inhibitor erlotinib. Due to their high and specific cytotoxicity, the EGF-based targeted toxins EGF-PE40 and EGF-PE24mut represent promising candidates for the future treatment of PCa.

Desirable drug-drug interactions or when a matter of concern becomes a renewed therapeutic strategy

Drug-drug interactions are sometimes considered to be detrimental and responsible for adverse effects. In some cases, however, some are stakeholders of the efficiency of the treatment and this combinatorial strategy is exploited by some drug associations, including levodopa (L-Dopa) and dopadecarboxylase inhibitors, β-lactam antibiotics and clavulanic acid, 5-fluorouracil (5-FU) and folinic acid, and penicillin and probenecid. More recently, some drug-drug combinations have been integrated in modern drug design strategies, aiming to enhance the efficiency of already marketed drugs with new compounds acting not only as synergistic associations, but also as real boosters of activity. In this review, we provide an update of examples of such strategies, with a special focus on microbiology and oncology.

A chemical conjugate between HER2-targeting antibody fragment and Pseudomonas exotoxin A fragment demonstrates cytotoxic effects on HER2-expressing breast cancer cells

Conventionally, immunotoxins have been produced as a single polypeptide from fused genes of an antibody fragment and a toxin. In this study, we adopted a unique approach of chemical conjugation of a toxin protein and an antibody fragment. The two genes were separately expressed in Escherichia coli and purified to high levels of purity. The two purified proteins were conjugated using a chemical linker. The advantage of this approach is its ability to overcome the problem of low recombinant immunotoxin production observed in some immunotoxins. Another advantage is that various combinations of immunotoxins can be prepared with fewer efforts, because the chemical conjugation of components is relatively simpler than the processes involved in cloning, expression, and purification of multiple immunotoxins. As a proof of concept, the scFv of trastuzumab and the PE24 fragment of Pseudomonas exotoxin A were separately produced using E. coli and then chemically crosslinked. The new immunotoxin was tested on four breast cancer cell lines variably expressing HER2. The chemically crosslinked immunotoxin exhibited cytotoxicity in proportion to the expression level of HER2. In conclusion, the present study revealed an alternative method of generating an immunotoxin that could effectively reduce the viability of HER2-expressing breast cancer cells. These results suggest the effectiveness of this method of immunotoxin crosslinking as a suitable alternative for producing immunotoxins. [BMB

Analysis of Triterpenoid Saponins Reveals Insights into Structural Features Associated with Potent Protein Drug Enhancement Effects

Plant-based saponins are amphipathic glycosides composed of a hydrophobic aglycone backbone covalently bound to one or more hydrophilic sugar moieties. Recently, the endosomal escape activity of triterpenoid saponins has been investigated as a potentially powerful tool for improved cytosolic penetration of protein drugs internalized by endocytic uptake, thereby greatly enhancing their pharmacological effects. However, only a few saponins have been studied, and the paucity in understanding the structure-activity relationship of saponins imposes significant limitations on their applications. To address this knowledge gap, 12 triterpenoid saponins with diverse structural side chains were screened for their utility as endosomolytic agents. These compounds were used in combination with a toxin (MAP30-HBP) comprising a type I ribosome-inactivating protein fused to a cell-penetrating peptide. Suitability of saponins as endosomolytic agents was assessed on the basis of cytotoxicity, endosomal escape promotion, and synergistic effects on toxins. Five saponins showed strong endosomal escape activity, enhancing MAP30-HBP cytotoxicity by more than 10⁶ to 10⁹ folds. These saponins also enhanced the apoptotic effect of MAP30-HBP in a pH-dependent manner. Additionally, growth inhibition of MAP30-HBP-treated SMMC-7721 cells was greater than that of similarly treated HeLa cells, suggesting that saponin-mediated endosomolytic effect is likely to be cell-specific. Furthermore, the structural features and hydrophobicity of the sugar side chains were analyzed to draw correlations with endosomal escape activity and derive predictive rules, thus providing new insights into structure-activity relationships of saponins. This study revealed new saponins that can potentially be exploited as efficient cytosolic delivery reagents for improved therapeutic drug effects.

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.

A chemical conjugate between HER2-targeting antibody fragment and Pseudomonas exotoxin A fragment demonstrates cytotoxic effects on HER2-expressing breast cancer cells

Conventionally, immunotoxins have been produced as a single polypeptide from fused genes of an antibody fragment and a toxin. In this study, we adopted a unique approach of chemical conjugation of a toxin protein and an antibody fragment. The two genes were separately expressed in Escherichia coli and purified to high levels of purity. The two purified proteins were conjugated using a chemical linker. The advantage of this approach is its ability to overcome the problem of low recombinant immunotoxin production observed in some immunotoxins. Another advantage is that various combinations of immunotoxins can be prepared with fewer efforts, because the chemical conjugation of components is relatively simpler than the processes involved in cloning, expression, and purification of multiple immunotoxins. As a proof of concept, the scFv of trastuzumab and the PE24 fragment of Pseudomonas exotoxin A were separately produced using E. coli and then chemically crosslinked. The new immunotoxin was tested on four breast cancer cell lines variably expressing HER2. The chemically crosslinked immunotoxin exhibited cytotoxicity in proportion to the expression level of HER2. In conclusion, the present study revealed an alternative method of generating an immunotoxin that could effectively reduce the viability of HER2-expressing breast cancer cells. These results suggest the effectiveness of this method of immunotoxin crosslinking as a suitable alternative for producing immunotoxins. [BMB Reports 2019; 52(8): 496-501].

Targeted human cytolytic fusion proteins at the cutting edge: harnessing the apoptosis-inducing properties of human enzymes for the selective elimination of tumor cells

Patient-specific targeted therapy represents the holy grail of anti-cancer therapeutics, allowing potent tumor depletion without detrimental off-target toxicities. Disease-specific monoclonal antibodies have been employed to bind to oncogenic cell-surface receptors, representing the earliest form of immunotherapy. Targeted drug delivery was first achieved by means of antibody-drug conjugates, which exploit the differential expression of tumor-associated antigens as a guiding mechanism for the specific delivery of chemically-conjugated chemotherapeutic agents to diseased target cells. Biotechnological advances have expanded the repertoire of immunology-based tumor-targeting strategies, also paving the way for the next intuitive step in targeted drug delivery: the construction of recombinant protein drugs consisting of an antibody-based targeting domain genetically fused with a cytotoxic peptide, known as an immunotoxin. However, the most potent protein toxins have typically been derived from bacterial or plant virulence factors and commonly feature both off-target toxicity and immunogenicity in human patients. Further refinement of immunotoxin technology thus led to the replacement of monoclonal antibodies with humanized antibody derivatives, including the substitution of non-human toxic peptides with human cytolytic proteins. Preclinically tested human cytolytic fusion proteins (hCFPs) have proven promising as non-immunogenic combinatory anti-cancer agents, however they still require further enhancement to achieve convincing candidacy as a single-mode therapeutic. To date, a portfolio of highly potent human toxins has been established; ranging from microtubule-associated protein tau (MAP tau), RNases, granzyme B (GrB) and death-associated protein kinase (DAPk). In this review, we discuss the most recent findings on the use of these apoptosis-inducing hCFPs for the treatment of various cancers.

Membrane reorganization after photochemical internalization to release transferrin-biofunctionalized polystyrene microparticles

Therapeutic drug carriers can drive their cargo to their target cells. However, an obstacle is usually the entrapment of the drug inside the endolysosomal compartment, which physically impedes its actuation by the impossibility of reaching its molecular site of action. To overcome this hurdle, photochemical internalization (PCI) has been proposed, but the extent of PCI-induced membrane disruption and its capability to allow the release of microparticles is unknown. The aim of the present study was to determine if PCI allows the release of microparticles from the endolysosomal compartment to the cytosol and to analyze at the ultrastructural level the effect of PCI on the membrane surrounding the particles. Confocal microscope allowed us to detect that endolysosomal membranes suffered some disruption after PCI, evidenced by the diffusion of soluble transferrin from the endolysosomes to the cytosol and by a decrease of LAMP1-microparticles co-localization. Transmission electron microscopy (TEM) showed a decrease in the number of well-defined membranes around microparticles after PCI, and scanning TEM combined with energy dispersive x-ray revealed an increase in the width of endolysosomal membranes after treatment. These results suggest that endolysosomal membranes suffered an ultrastructure alteration after PCI, enough to liberate soluble transferrin but not the entire microparticles.

Human Granzyme B Based Targeted Cytolytic Fusion Proteins

Cancer immunotherapy aims to selectively target and kill tumor cells whilst limiting the damage to healthy tissues. Controlled delivery of plant, bacterial and human toxins or enzymes has been shown to promote the induction of apoptosis in cancerous cells. The 4th generation of targeted effectors are being designed to be as humanized as possible—a solution to the problem of immunogenicity encountered with existing generations. Granzymes are serine proteases which naturally function in humans as integral cytolytic effectors during the programmed cell death of cancerous and pathogen-infected cells. Secreted predominantly by cytotoxic T lymphocytes and natural killer cells, granzymes function mechanistically by caspase-dependent or caspase-independent pathways. These natural characteristics make granzymes one of the most promising human enzymes for use in the development of fusion protein-based targeted therapeutic strategies for various cancers. In this review, we explore research involving the use of granzymes as cytolytic effectors fused to antibody fragments as selective binding domains.

Initial Evaluation of Antibody-conjugates Modified with Viral-derived Peptides for Increasing Cellular Accumulation and Improving Tumor Targeting

Antibody-conjugates (ACs) modified with virus-derived peptides are a potentially powerful class of tumor cell delivery agents for molecular payloads used in cancer treatment and imaging due to increased cellular accumulation over current ACs. During early AC in vitro development, fluorescence techniques and radioimmunoassays are sufficient for determining intracellular localization, accumulation efficiency, and target cell specificity. Currently, there is no consensus on standardized methods for preparing cells for evaluating AC intracellular accumulation and localization. The initial testing of ACs modified with virus-derived peptides is critical especially if several candidates have been constructed. Determining intracellular accumulation by fluorescence can be affected by background signal from ACs at the cell surface and complicate the interpretation of accumulation. For radioimmunoassays, typically treated cells are fractionated and the radioactivity in different cell compartments measured. However, cell lysis varies from cell to cell and often nuclear and cytoplasmic compartments are not adequately isolated. This can produce misleading data on payload delivery properties. The intravenous injection of radiolabeled virus-derived peptide-modified ACs in tumor bearing mice followed by radionuclide imaging is a powerful method for determining tumor targeting and payload delivery properties at the in vivo phase of development. However, this is a relatively recent advancement and few groups have evaluated virus-derived peptide-modified ACs in this manner. We describe the processing of treated cells to more accurately evaluate virus-derived peptide-modified AC accumulation when using confocal microscopy and radioimmunoassays. Specifically, a method for trypsinizing cells to remove cell surface bound ACs. We also provide a method for improving cellular fractionation. Lastly, this protocol provides an in vivo method using positron emission tomography (PET) for evaluating initial tumor targeting properties in tumor-bearing mice. We use the radioisotope ⁶⁴Cu (t1/2 = 12.7 h) as an example payload in this protocol.

NLS-Cholic Acid Conjugation to IL-5Rα-Specific Antibody Improves Cellular Accumulation and In Vivo Tumor-Targeting Properties in a Bladder Cancer Model

Receptor-mediated internalization followed by trafficking and degradation of antibody-conjugates (ACs) via the endosomal-lysosomal pathway is the major mechanism for delivering molecular payloads inside target tumor cells. Although a mainstay for delivering payloads with clinically approved ACs in cancer treatment and imaging, tumor cells are often able to decrease intracellular payload concentrations and thereby reduce the effectiveness of the desired application. Thus, increasing payload intracellular accumulation has become a focus of attention for designing next-generation ACs. We developed a composite compound (ChAcNLS) that enables ACs to escape endosome entrapment and route to the nucleus resulting in the increased intracellular accumulation as an interleukin-5 receptor α-subunit (IL-5Rα)-targeted agent for muscle invasive bladder cancer (MIBC). We constructed ⁶⁴Cu-A14-ChAcNLS, ⁶⁴Cu-A14-NLS, and ⁶⁴Cu-A14 and evaluated their performance by employing mechanistic studies for endosome escape coupled to nuclear routing and determining whether this delivery system results in improved ⁶⁴Cu cellular accumulation. ACs consisting of ∼20 ChAcNLS or NLS moieties per ⁶⁴Cu-A14 were prepared in good yield, high monomer content, and maintaining high affinity for IL-5Rα. Confocal microscopy analysis demonstrated ChAcNLS mediated efficient endosome escape and nuclear localization. ⁶⁴Cu-A14-ChAcNLS increased ⁶⁴Cu cellular accumulation in HT-1376 and HT-B9 cells relative to ⁶⁴Cu-A14 and ⁶⁴Cu-A14-NLS. In addition, we tested ⁶⁴Cu-A14-ChAcNLS in vivo to evaluate its tissue distribution properties and, ultimately, tumor uptake and targeting. A model of human IL-5Rα MIBC was developed by implanting NOD/SCID mice with subcutaneous HT-1376 or HT-B9MIBC tumors, which grow containing high and low IL-5Rα-positive tumor cell densities, respectively. ACs were intravenously injected, and daily blood sampling, biodistribution at 48 and 96 h, and positron emission tomography (PET) at 24 and 48 h were performed. Region of interest (ROI) analysis was also performed on reconstructed PET images. Pharmacokinetic analysis and biodistribution studies showed that ⁶⁴Cu-A14-ChAcNLS had faster clearance rates from the blood and healthy organs relative to ⁶⁴Cu-A14. However, ⁶⁴Cu-A14-ChAcNLS maintained comparable tumor accumulation relative to ⁶⁴Cu-A14. This resulted in ⁶⁴Cu-A14-ChAcNLS having superior tumor/normal tissue ratios at both 48 and 96 h biodistribution time points. Visualization of AC distribution by PET and ROI analysis confirmed that ⁶⁴Cu-A14-ChAcNLS had improved targeting of MIBC tumor relative to ⁶⁴Cu-A14. In addition, ⁶⁴Cu-A14 modified with only NLS had poor tumor targeting. This was a result of poor tumor uptake due to extremely rapid clearance. Thus, the overall findings in this model of human IL-5Rα-positive MIBC describe an endosome escape-nuclear localization cholic-acid-linked peptide that substantially enhances AC cellular accumulation and tumor targeting.

Targeted dianthin is a powerful toxin to treat pancreatic carcinoma when applied in combination with the glycosylated triterpene SO1861

Targeted cancer therapy provides the basis for the arrest of tumor growth in aggressive pancreatic carcinoma; however, a number of protein-based targeted toxins lack efficacy due to insufficient endosomal escape after being endocytosed. Therefore, we tested a fusion protein of the ribosome-inactivating protein dianthin and human epidermal growth factor in combination with a glycosylated triterpene (SO1861) that serves as an endosomal escape enhancer. In vitro investigations with the pancreatic carcinoma cell lines BxPC-3 and MIA PaCa-2 revealed no significant differences to off-target cells in the half maximal inhibitory concentration (IC50 ) for the fusion protein. In contrast, combination with SO1861 decreased the IC50 for BxPC-3 cells from 100 to 0.17 nm, whereas control cells remained unaffected. Monotherapy of BxPC-3 xenografts in CD-1 nude mice led to a 51.7% average reduction in tumor size (40.8 mm3 ) when compared to placebo; however, combined treatment with SO1861 resulted in a more than 13-fold better efficacy (3.0 mm3 average tumor size) with complete regression in 80% of cases. Immunohistochemical analyses showed that tumor cells with lower target receptor expression are, in contrast to the combination therapy, able to escape from the monotherapy, which finally results in tumor growth. At the effective concentration, we did not observe liver toxicity and saw no other side effects with the exception of a reversible skin hardening at the SO1861 injection site, alongside an increase in platelet counts, plateletcrit, and platelet distribution width. In conclusion, combining a targeted toxin with SO1861 is proven to be a very promising approach for pancreatic cancer treatment.

Plant Ribosome-Inactivating Proteins: Progesses, Challenges and Biotechnological Applications (and a Few Digressions)

Plant ribosome-inactivating protein (RIP) toxins are EC3.2.2.22 N-glycosidases, found among most plant species encoded as small gene families, distributed in several tissues being endowed with defensive functions against fungal or viral infections. The two main plant RIP classes include type I (monomeric) and type II (dimeric) as the prototype ricin holotoxin from Ricinus communis that is composed of a catalytic active A chain linked via a disulphide bridge to a B-lectin domain that mediates efficient endocytosis in eukaryotic cells. Plant RIPs can recognize a universally conserved stem-loop, known as the α-sarcin/ ricin loop or SRL structure in 23S/25S/28S rRNA. By depurinating a single adenine (A4324 in 28S rat rRNA), they can irreversibly arrest protein translation and trigger cell death in the intoxicated mammalian cell. Besides their useful application as potential weapons against infected/tumor cells, ricin was also used in bio-terroristic attacks and, as such, constitutes a major concern. In this review, we aim to summarize past studies and more recent progresses made studying plant RIPs and discuss successful approaches that might help overcoming some of the bottlenecks encountered during the development of their biomedical applications.

PDMS-b-PMOXA polymersomes for hepatocyte targeting and assessment of toxicity

Nanoparticles, such as polymersomes, can be directed to the hepatic asialoglycoprotein receptor to achieve targeted drug delivery. In this study, we prepared asialofetuin conjugated polymersomes based on the amphiphilic di-block copolymer poly(dimethylsiloxane)-b-poly(2-methyloxazoline) (PDMS-b-PMOXA). They had an average diameter of 150nm and formed monodisperse vesicles. Drug encapsulation and sustained release was monitored using the hydrophilic model compound carboxyfluorescein. Asialoglycoprotein receptor specific uptake by HepG2 cells in vitro was energy dependent and could be competitively inhibited by the free targeting ligand. Mechanistic uptake studies revealed intracellular trafficking of asialofetuin conjugated polymersomes from early endosomes and to the lysosomal compartment. Polymersomes showed no toxicity in the MTT assay up to concentrations of 500μg/mL. In addition, acute toxicity and tolerability of our PDMS-b-PMOXA polymersome formulations was assessed in vivo using zebrafish embryos as a vertebrate screening model. In conclusion, a hepatocyte specific drug delivery system was designed, which is safe and biocompatible and which can be used to implement liver-specific targeting strategies.

CSPG4: A Target for Selective Delivery of Human Cytolytic Fusion Proteins and TRAIL

Chondroitin-sulfate proteoglycan 4 (CSPG4) is a transmembrane glycoprotein overexpressed on malignant cells in several cancer types with only limited expression on normal cells. CSPG4 is implicated in several signaling pathways believed to drive cancer progression, particularly proliferation, motility and metastatic spread. Expression may serve as a prognostic marker for survival and risk of relapse in treatment-resistant malignancies including melanoma, triple negative breast cancer, rhabdomyosarcoma and acute lymphoblastic leukemia. This tumor-associated overexpression of CSPG4 points towards a highly promising therapeutic target for antibody-guided cancer therapy. Monoclonal αCSPG4 antibodies have been shown to inhibit cancer progression by blocking ligand access to the CSPG4 extracellular binding sites. Moreover, CSPG4-directed antibody conjugates have been shown to be selectively internalized by CSPG4-expressing cancer cells via endocytosis. CSPG4-directed immunotherapy may be approached in several ways, including: (1) antibody-based fusion proteins for the selective delivery of a pro-apoptotic factors such as tumor necrosis factor-related apoptosis-inducing ligand to agonistic death receptors 4 and 5 on the cell surface; and (2) CSPG4-specific immunotoxins which bind selectively to diseased cells expressing CSPG4, are internalized by them and induce arrest of biosynthesis, closely followed by initiation of apoptotic signaling. Here we review various methods of exploiting tumor-associated CSPG4 expression to improve targeted cancer therapy.

Glycosylated Triterpenoids as Endosomal Escape Enhancers in Targeted Tumor Therapies

Protein-based targeted toxins play an increasingly important role in targeted tumor therapies. In spite of their high intrinsic toxicity, their efficacy in animal models is low. A major reason for this is the limited entry of the toxin into the cytosol of the target cell, which is required to mediate the fatal effect. Target receptor bound and internalized toxins are mostly either recycled back to the cell surface or lysosomally degraded. This might explain why no antibody-targeted protein toxin has been approved for tumor therapeutic applications by the authorities to date although more than 500 targeted toxins have been developed within the last decades. To overcome the problem of insufficient endosomal escape, a number of strategies that make use of diverse chemicals, cell-penetrating or fusogenic peptides, and light-induced techniques were designed to weaken the membrane integrity of endosomes. This review focuses on glycosylated triterpenoids as endosomal escape enhancers and throws light on their structure, the mechanism of action, and on their efficacy in cell culture and animal models. Obstacles, challenges, opportunities, and future prospects are discussed.

Humanized CD7 nanobody-based immunotoxins exhibit promising anti-T-cell acute lymphoblastic leukemia potential

BACKGROUND

Nanobodies, named as VHHs (variable domain of heavy chain of HCAb [heavy-chain antibodies]), are derived from heavy-chain-only antibodies that circulate in sera of camelids. Their exceptional physicochemical properties, possibility of humanization, and unique antigen recognition properties make them excellent candidates for targeted delivery of biologically active components, including immunotoxins. In our previous efforts, we have successfully generated the monovalent and bivalent CD7 nanobody-based immunotoxins, which can effectively trigger the apoptosis of CD7-positive malignant cells. To pursue the possibility of translating those immunotoxins into clinics, we humanized the nanobody sequences (designated as dhuVHH6) as well as further truncated the Pseudomonas exotoxin A (PE)-derived PE38 toxin to produce a more protease-resistant form, which is named as PE-LR, by deleting majority of PE domain II.

METHODS AND RESULTS

Three new types of immunotoxins, dhuVHH6-PE38, dVHH6-PE-LR, and dhuVHH6-PE-LR, were successfully constructed. These recombinant immunotoxins were expressed in Escherichia coli and showed that nanobody immunotoxins have the benefits of easy soluble expression in a prokaryotic expression system. Flow cytometry results revealed that all immunotoxins still maintained the ability to bind specifically to CD7-positive T lymphocyte strains without binding to CD7-negative control cells. Laser scanning confocal microscopy revealed that these proteins can be endocytosed into the cytoplasm after binding with CD7-positive cells and that this phenomenon was not observed in CD7-negative cells. WST-8 experiments showed that all immunotoxins retained the highly effective and specific growth inhibition activity in CD7-positive cell lines and primary T-cell acute lymphoblastic leukemia (T-ALL) cells. Further in vivo animal model experiments showed that humanized dhuVHH6-PE38 immunotoxin can tolerate higher doses and extend the survival of NOD-Prkdcem26Il2rgem26Nju (NCG) mice transplanted with CEM cells without any obvious decrease in body weight. Further studies on NCG mice model with patient-derived T-ALL cells, dhuVHH6-PE38 treatment, significantly prolonged mice survival with ~40% survival improvement. However, it was also noticed that although dhuVHH6-PE-LR showed strong antitumor effect in vitro, its in vivo antitumor efficacy was disappointing.

CONCLUSION

We have successfully constructed a targeted CD7 molecule-modified nanobody (CD7 molecule-improved nanobody) immunotoxin dhuVHH6-PE38 and demonstrated its potential for treating CD7-positive malignant tumors, especially T-cell acute lymphoblastic leukemia.

Designing the Sniper: Improving Targeted Human Cytolytic Fusion Proteins for Anti-Cancer Therapy via Molecular Simulation

Targeted human cytolytic fusion proteins (hCFPs) are humanized immunotoxins for selective treatment of different diseases including cancer. They are composed of a ligand specifically binding to target cells genetically linked to a human apoptosis-inducing enzyme. hCFPs target cancer cells via an antibody or derivative (scFv) specifically binding to e.g., tumor associated antigens (TAAs). After internalization and translocation of the enzyme from endocytosed endosomes, the human enzymes introduced into the cytosol are efficiently inducing apoptosis. Under in vivo conditions such enzymes are subject to tight regulation by native inhibitors in order to prevent inappropriate induction of cell death in healthy cells. Tumor cells are known to upregulate these inhibitors as a survival mechanism resulting in escape of malignant cells from elimination by immune effector cells. Cytosolic inhibitors of Granzyme B and Angiogenin (Serpin P9 and RNH1, respectively), reduce the efficacy of hCFPs with these enzymes as effector domains, requiring detrimentally high doses in order to saturate inhibitor binding and rescue cytolytic activity. Variants of Granzyme B and Angiogenin might feature reduced affinity for their respective inhibitors, while retaining or even enhancing their catalytic activity. A powerful tool to design hCFPs mutants with improved potency is given by in silico methods. These include molecular dynamics (MD) simulations and enhanced sampling methods (ESM). MD and ESM allow predicting the enzyme-protein inhibitor binding stability and the associated conformational changes, provided that structural information is available. Such "high-resolution" detailed description enables the elucidation of interaction domains and the identification of sites where particular point mutations may modify those interactions. This review discusses recent advances in the use of MD and ESM for hCFP development from the viewpoints of scientists involved in both fields.