Reduction of non-specific toxicity of immunotoxin by intein mediated reconstitution on target cells

Hypoxia-Inducible Factor-1α-Activated Protein Switch Based on Allosteric Self-Splicing Reduces Nonspecific Cytotoxicity of Pharmaceutical Drugs

Protein-based therapeutic agents currently used for targeted tumor therapy exhibit limited penetrability, nonspecific toxicity, and a short circulation half-life. Although targeting cell surface receptors improves cancer selectivity, the receptors are also slightly expressed in normal cells; consequently, the nonspecific toxicity of recombinant protein-based therapeutic agents has not been eliminated. In this study, an allosteric-regulated protein switch was designed that achieved cytoplasmic reorganization of engineered immunotoxins in tumor cells via interactions between allosteric self-splicing elements and cancer markers. It can target the accumulated HIF-1α in hypoxic cancer cells and undergo allosteric activation, and the splicing products were present in hypoxic cancer cells but were absent in normoxic cells, selectively killing tumor cells and reducing nonspecific toxicity to normal cells. The engineered pro-protein provides a platform for targeted therapy of tumors while offering a novel universal strategy for combining the activation of therapeutic functions with specific cancer markers. The allosteric self-splicing element is a powerful tool that significantly reduces the nonspecific cytotoxicity of therapeutic proteins.

A novel bispecific antibody drug conjugate targeting HER2 and HER3 with potent therapeutic efficacy against breast cancer

Antibody drug conjugate (ADC) therapy has become one of the most promising approaches in cancer immunotherapy. Bispecific targeting could enhance the efficacy and safety of ADC by improving its specificity, affinity and internalization. In this study we constructed a HER2/HER3-targeting bispecific ADC (BsADC) and characterized its physiochemical properties, target specificity and internalization in vitro, and assessed its anti-tumor activities in breast cancer cell lines and in animal models. The HER2/HER3-targeting BsADC had a drug to antibody ratio (DAR) of 2.89, displayed a high selectivity against the target JIMT-1 breast cancer cells in vitro, as well as a slightly higher level of internalization than HER2- or HER3-monospecific ADCs. More importantly, the bispecific ADC potently inhibited the viability of MCF7, JIMT-1, BT474, BxPC-3 and SKOV-3 cancer cells in vitro. In JIMT-1 breast cancer xenograft mice, a single injection of bispecific ADC (3 mg/kg, i.v.) significantly inhibited the tumor growth with an efficacy comparable to that caused by combined injection of HER2 and HER3-monospecific ADCs (3 mg/kg for each). Our study demonstrates that the bispecific ADC concept can be applied to development of more potent new cancer therapeutics than the monospecific ADCs.

The immunotoxin targeting PRLR increases tamoxifen sensitivity and enhances the efficacy of chemotherapy in breast cancer

BACKGROUND

Though tamoxifen achieves success in treating estrogen receptor α (ERα)-positive breast cancer, the followed development of tamoxifen resistance is a common challenge in clinic. Signals downstream of prolactin receptor (PRLR) could synergize with ERα in breast cancer progression. However, the potential effect of targeting PRL-PRLR axis combined with tamoxifen has not been thoroughly investigated.

METHODS

High-throughput RNA-seq data obtained from TCGA, Metabric and GEO datasets were analyzed to explore PRLR expression in breast cancer cell and the association of PRLR expression with tamoxifen treatment. Exogenous or PRL overexpression cell models were employed to investigate the role of activated PRLR pathway in mediating tamoxifen insensitivity. Immunotoxin targeting PRLR (N8-PE24) was constructed with splicing-intein technique, and the efficacy of N8-PE24 against breast cancer was evaluated using in vitro and in vivo methods, including analysis of cells growth or apoptosis, 3D spheroids culture, and animal xenografts.

RESULTS

PRLR pathway activated by PRL could significantly decrease sensitivity of ERα-positive breast cancer cells to tamoxifen. Tamoxifen treatment upregulated transcription of PRLR and could induce significant accumulation of PRLR protein in breast cancer cells by alkalizing lysosomes. Meanwhile, tamoxifen-resistant MCF7 achieved by long-term tamoxifen pressure exhibited both upregulated transcription and protein level of PRLR. Immunotoxin N8-PE24 enhanced sensitivity of breast cancer cells to tamoxifen both in vitro and in vivo. In xenograft models, N8-PE24 significantly enhanced the efficacy of tamoxifen and paclitaxel when treating PRLR-positive triple-negative breast cancer.

CONCLUSIONS

PRL-PRLR axis potentially associates with tamoxifen insensitivity in ERα-positive breast cancer cells. N8-PE24 could inhibit cell growth of the breast cancers and promote drug sensitivity of PRLR-positive breast cancer cells to tamoxifen and paclitaxel. Our study provides a new perspective for targeting PRLR to treat breast cancer.

Int&in: A machine learning-based web server for active split site identification in inteins

Inteins are proteins that excise themselves out of host proteins and ligate the flanking polypeptides in an auto-catalytic process called protein splicing. In nature, inteins are either contiguous or split. In the case of split inteins, the two fragments must first form a complex for the splicing to occur. Contiguous inteins have previously been artificially split in two fragments because split inteins allow for distinct applications than contiguous ones. Even naturally split inteins have been split at unnatural split sites to obtain fragments with reduced affinity for one another, which are useful to create conditional inteins or to study protein-protein interactions. So far, split sites in inteins have been heuristically identified. We developed Int&in, a web server freely available for academic research (https://intein.biologie.uni-freiburg.de) that runs a machine learning model using logistic regression to predict active and inactive split sites in inteins with high accuracy. The model was trained on a dataset of 126 split sites generated using the gp41-1, Npu DnaE and CL inteins and validated using 97 split sites extracted from the literature. Despite the limited data size, the model, which uses various protein structural features, as well as sequence conservation information, achieves an accuracy of 0.79 and 0.78 for the training and testing sets, respectively. We envision Int&in will facilitate the engineering of novel split inteins for applications in synthetic and cell biology.

Hosts and Heterologous Expression Strategies of Recombinant Toxins for Therapeutic Purposes

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

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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.

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

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

Generating a Bispecific Antibody Drug Conjugate Targeting PRLR and HER2 with Improving the Internalization

Antibody drug conjugate (ADC) therapy has become one of the most promising approaches in cancer immunotherapy. The bispecific targeting could improve the specificity, affinity, and internalization of the ADC molecules. Prolactin preceptor (PRLR) and HER2 have crosstalk signaling in breast cancer, and PRLR undergoes a rapid internalization compared with HER2. To improve the efficacy of HER2 ADCs with enhancing the target specificity and internalization, we constructed a PRLR/HER2-targeting bispecific ADC (BsADC). We evaluated the characterization of PRLR × HER2 BsADC from the affinity and internalization, and further assessed its in vitro cytotoxicity in human breast-cancer cell lines (BT474, T47D, and MDA-MB-231) using Cell Count Kit-8 analysis. Our data demonstrated that PRLR × HER2 BsADC kept the affinity to two targeting antigens after conjugating drugs and exhibited higher internalization efficiency in comparison to HER2 ADC. Furthermore, PRLR × HER2 BsADC demonstrated to have superior antitumor activity in human breast cancer in vitro. In conclusion, our findings indicate that it is feasible through increasing the internalization of target antibody to enhance the antitumor activity and therapeutic potential that could be further evaluated in in vivo animal model.

Inteins as Drug Targets and Therapeutic Tools

Multidrug-resistant pathogens are of significant concern in recent years. Hence new antifungal and anti-bacterial drug targets are urgently needed before the situation goes beyond control. Inteins are polypeptides that self-splice from exteins without the need for cofactors or external energy, resulting in joining of extein fragments. Inteins are present in many organisms, including human pathogens such as Mycobacterium tuberculosis, Cryptococcus neoformans, C. gattii, and Aspergillus fumigatus. Because intein elements are not present in human genes, they are attractive drug targets to develop antifungals and antibiotics. Thus far, a few inhibitors of intein splicing have been reported. Metal-ions such as Zn²⁺ and Cu²⁺, and platinum-containing compound cisplatin inhibit intein splicing in M. tuberculosis and C. neoformans by binding to the active site cysteines. A small-molecule inhibitor 6G-318S and its derivative 6G-319S are found to inhibit intein splicing in C. neoformans and C. gattii with a MIC in nanomolar concentrations. Inteins have also been used in many other applications. Intein can be used in activating a protein inside a cell using small molecules. Moreover, split intein can be used to deliver large genes in experimental gene therapy and to kill selected species in a mixed population of microbes by taking advantage of the toxin-antitoxin system. Furthermore, split inteins are used in synthesizing cyclic peptides and in developing cell culture model to study infectious viruses including SARS-CoV-2 in the biosafety level (BSL) 2 facility. This mini-review discusses the recent research developments of inteins in drug discovery and therapeutic research.

Kinetics study of the natural split Npu DnaE intein in the generation of bispecific IgG antibodies

Rapid and efficient bispecific antibody (BsAb) production for industrial applications is still facing many challenges. We reported a technology platform for generating bispecific IgG antibodies, "Bispecific Antibody by Protein Trans-splicing (BAPTS)." While the "BAPTS" method has shown potential in high-throughput screening of BsAbs, further understanding and optimizing the methodology is desirable. A large number of BsAbs were selected to illustrate the conversion efficiency and kinetics parameters. The temperature of reaction makes no significant influence in conversion efficiency, which can reach more than 70% within 2 h, and CD3 × HER2 BsAb can reach 90%. By fitting trans-splicing reaction to single-component exponential decay curves, the apparent first-order rate constants at a series of temperatures were determined. The rate constant ranges from 0.02 to 0.11 min^-1 at 37 °C, which is a high rate reported for the protein trans-splicing reaction (PTS). The reaction process is activated rapidly with activation energy of 8.9 kcal·mol^-1 (CD3 × HER2) and 5.2 kcal·mol^-1 (CD3 × EGFR). The BsAbs generated by "BAPTS" technology not only had the similar post-translation modifications to the parental antibodies, but also demonstrated excellent in vitro and in vivo bioactivity. The kinetics parameters and activation energy of the reaction illustrate feasible for high-throughput screening and industrial applications using the "BAPTS" approach. KEY POINTS: • The trans-splicing reaction of Npu DnaE intein in "BAPTS" platform is a rapid process with low reaction activation and high rate. • The BsAb generated by "BAPTS" remained effective in tumor cell killing. • The kinetics parameters and activation energy of the reaction illustrate feasible for high-throughput screening and industrial applications using the "BAPTS" approach.

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

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

Intein-mediated cytoplasmic reconstitution of a split toxin enables selective cell ablation in mixed populations and tumor xenografts

The application of proteinaceous toxins for cell ablation is limited by their high on- and off-target toxicity, severe side effects, and a narrow therapeutic window. The selectivity of targeting can be improved by intein-based toxin reconstitution from two dysfunctional fragments provided their cytoplasmic delivery via independent, selective pathways. While the reconstitution of proteins from genetically encoded elements has been explored, exploiting cell-surface receptors for boosting selectivity has not been attained. We designed a robust splitting algorithm and achieved reliable cytoplasmic reconstitution of functional diphtheria toxin from engineered intein-flanked fragments upon receptor-mediated delivery of one of them to the cells expressing the counterpart. Retargeting the delivery machinery toward different receptors overexpressed in cancer cells enables selective ablation of specific subpopulations in mixed cell cultures. In a mouse model, the transmembrane delivery of a split-toxin construct potently inhibits the growth of xenograft tumors expressing the split counterpart. Receptor-mediated delivery of engineered split proteins provides a platform for precise therapeutic and experimental ablation of tumors or desired cell populations while also greatly expanding the applicability of the intein-based protein transsplicing.