PDZ Domain in the Engineering and Production of a Saporin Chimeric Toxin as a Tool for targeting Cancer Cells

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.

Antibody-Drug Conjugates: The New Frontier of Chemotherapy

In recent years, antibody-drug conjugates (ADCs) have become promising antitumor agents to be used as one of the tools in personalized cancer medicine. ADCs are comprised of a drug with cytotoxic activity cross-linked to a monoclonal antibody, targeting antigens expressed at higher levels on tumor cells than on normal cells. By providing a selective targeting mechanism for cytotoxic drugs, ADCs improve the therapeutic index in clinical practice. In this review, the chemistry of ADC linker conjugation together with strategies adopted to improve antibody tolerability (by reducing antigenicity) are examined, with particular attention to ADCs approved by the regulatory agencies (the U.S. Food and Drug Administration (FDA) and the European Medicines Agency (EMA)) for treating cancer patients. Recent developments in engineering Immunoglobulin (Ig) genes and antibody humanization have greatly reduced some of the problems of the first generation of ADCs, beset by problems, such as random coupling of the payload and immunogenicity of the antibody. ADC development and clinical use is a fast, evolving area, and will likely prove an important modality for the treatment of cancer in the near future.

Targeted therapy of human glioblastoma via delivery of a toxin through a peptide directed to cell surface nucleolin

Targeted anticancer therapies demand discovery of new cellular targets to be exploited for the delivery of toxic molecules and drugs. In this perspective, in the last few years, nucleolin has been identified as an interesting surface marker to be used for the therapy of glioblastoma. In this study, we investigated whether a synthetic antagonist of cell-surface nucleolin known as N6L, previously reported to decrease both tumor growth and tumor angiogenesis in several cancer cell lines, including glioblastoma cells, as well as endothelial cells proliferation, could be exploited to deliver a protein toxin (saporin) to glioblastoma cells. The pseudopeptide N6L cross-linked to saporin-S6 induced internalization of the toxin inside glioblastoma cancer cells. Our results in vitro demonstrated the effectiveness of this conjugate in inducing cell death, with an ID50 four orders of magnitude lower than that observed for free N6L. Furthermore, the preliminary in vivo study demonstrated efficiency in reducing the tumor mass in an orthotopic mouse model of glioblastoma.

Strategies to Improve the Clinical Utility of Saporin-Based Targeted Toxins

Plant Ribosome-inactivating proteins (RIPs) including the type I RIP Saporin have been used for the construction of Immunotoxins (ITxs) obtained via chemical conjugation of the toxic domain to whole antibodies or by generating genetic fusions to antibody fragments/targeting domains able to direct the chimeric toxin against a desired sub-population of cancer cells. The high enzymatic activity, stability and resistance to conjugation procedures and especially the possibility to express recombinant fusions in yeast, make Saporin a well-suited tool for anti-cancer therapy approaches. Previous clinical work on RIPs-based Immunotoxins (including Saporin) has shown that several critical issues must be taken into deeper consideration to fully exploit their therapeutic potential. This review focuses on possible combinatorial strategies (chemical and genetic) to augment Saporin-targeted toxin efficacy. Combinatorial approaches may facilitate RIP escape into the cytosolic compartment (where target ribosomes are), while genetic manipulations may minimize potential adverse effects such as vascular-leak syndrome or may identify T/B cell epitopes in order to decrease the immunogenicity following similar strategies as those used in the case of bacterial toxins such as Pseudomonas Exotoxin A or as for Type I RIP Bouganin. This review will further focus on strategies to improve recombinant production of Saporin-based chimeric toxins.

pH-Sensitive Coiled-Coil Peptide-Cross-Linked Hyaluronic Acid Nanogels: Synthesis and Targeted Intracellular Protein Delivery to CD44 Positive Cancer Cells

The clinical translation of protein drugs that act intracellularly is limited by the absence of safe and efficient intracellular protein delivery vehicles. Here, pH-sensitive coiled-coil peptide-cross-linked hyaluronic acid nanogels (HA-cNGs) were designed and investigated for targeted intracellular protein delivery to CD44 overexpressing MCF-7 breast cancer cells. HA-cNGs were obtained with a small size of 176 nm from an equivalent mixture of hyaluronic acid conjugates with GY(EIAALEK)₃GC (E3) and GY(KIAALKE)₃GC (K3) peptides, respectively, at pH 7.4 by nanoprecipitation. Circular dichroism (CD) proved the formation of coiled-coil structures between E3 and K3 peptides at pH 7.4 while fast uncoiling at pH 5.0. HA-cNGs showed facile loading of cytochrome C (CC) and greatly accelerated CC release under mild acidic conditions (18.4%, 76.8%, and 91.4% protein release in 24 h at pH 7.4, 6.0, and 5.0, respectively). Confocal microscopy and flow cytometry displayed efficient internalization of CC-loaded HA-cNGs and effective endosomal escape of CC in MCF-7 cancer cells. Remarkably, HA-cNGs loaded with saporin, a ribosome inactivating protein, exhibited significantly enhanced apoptotic activity to MCF-7 cells with a low IC₅₀ of 12.2 nM. These coiled-coil peptide-cross-linked hyaluronic acid nanogels have appeared as a simple and multifunctional platform for efficient intracellular protein delivery.

EV20-Sap, a novel anti-HER-3 antibody-drug conjugate, displays promising antitumor activity in melanoma

Melanoma is the most biologically aggressive skin cancer of well established constitutive and induced resistance to pharmacological treatment. Despite the recent progresses in immunotherapies, many advanced metastatic melanoma patients still face a significant mortality risk. The aggressive nature of this disease sustains an urgent need for more successful, effective drugs. HER-3 - one of the four member of the tyrosin kinase epidermal growth factor receptors (EGFRs) family- is frequently overexpressed in solid tumors, including melanoma. Moreover, up-regulation of HER-3 and its ligand NRGβ-1 are associated with poor prognosis, thus suggesting this receptor as a suitable target for cancer therapy. Several monoclonal antibodies targeting HER-3 are currently available, but preliminary results from clinical testing of these agents reveal a modest efficacy. Thus, a substantial improvement over this immunotherapeutic approach could be offered by an anti-HER-3 based Antibody-Drug Conjugate (ADC). In the present paper, we describe the generation of an ADC obtained by coupling the HER-3 targeting antibody EV20 linked to the plant toxin Saporin (Sap). In vitro, this ADC displays a powerful, specific and target-dependent cytotoxic activity which correlates with the degree of expression and internalization of HER-3 on tumor cells. Furthermore, in a murine melanoma model, EV20-Sap treatment leads to a significant reduction of the number of pulmonary metastasis.

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.