Immunotoxins

To date, generation of single-chain fragment variable (scFv) has become an established technique used to produce a completely functional antigen-binding fragment in bacterial systems. The advances in antibody engineering have now facilitated a more efficient and generally applicable method to produce Fv fragments. Basically, scFv antibodies produced from phage display can be genetically fused to the marker proteins, such as fluorescent proteins or alkaline phosphatase. These bifunctional proteins having both antigen-binding capacity and marker activity can be obtained from transformed bacteria and used for one-step immunodetection of biological agents. Alternatively, antibody fragments could also be applied in the construction of immunotoxins, therapeutic gene delivery, and anticancer intrabodies for therapeutic purposes. This paper provides an overview of the current studies on the principle, generation, and application of scFv. The potential of scFv in breast cancer research is also discussed in this paper.

Over the past couple of decades, antibody-drug conjugates (ADCs) have revolutionized the field of cancer chemotherapy. Unlike conventional treatments that damage healthy tissues upon dose escalation, ADCs utilize monoclonal antibodies (mAbs) to specifically bind tumour-associated target antigens and deliver a highly potent cytotoxic agent. The synergistic combination of mAbs conjugated to small-molecule chemotherapeutics, via a stable linker, has given rise to an extremely efficacious class of anti-cancer drugs with an already large and rapidly growing clinical pipeline. The primary objective of this paper is to review current knowledge and latest developments in the field of ADCs. Upon intravenous administration, ADCs bind to their target antigens and are internalized through receptor-mediated endocytosis. This facilitates the subsequent release of the cytotoxin, which eventually leads to apoptotic cell death of the cancer cell. The three components of ADCs (mAb, linker and cytotoxin) affect the efficacy and toxicity of the conjugate. Optimizing each one, while enhancing the functionality of the ADC as a whole, has been one of the major considerations of ADC design and development. In addition to these, the choice of clinically relevant targets and the position and number of linkages have also been the key determinants of ADC efficacy. The only marketed ADCs, brentuximab vedotin and trastuzumab emtansine (T-DM1), have demonstrated their use against both haematological and solid malignancies respectively. The success of future ADCs relies on improving target selection, increasing cytotoxin potency, developing innovative linkers and overcoming drug resistance. As more research is conducted to tackle these issues, ADCs are likely to become part of the future of targeted cancer therapeutics.

Synthetic pyrrolobenzodiazepine (PBD) dimers, where two PBD monomers are linked through their aromatic A-ring phenolic C8-positions via a flexible propyldioxy tether, are highly efficient DNA minor groove cross-linking agents with potent cytotoxicity. PBD dimer SG3199 is the released warhead component of the antibody-drug conjugate (ADC) payload tesirine (SG3249), currently being evaluated in several ADC clinical trials. SG3199 was potently cytotoxic against a panel of human solid tumour and haematological cancer cell lines with a mean GI50 of 151.5 pM. Cells defective in DNA repair protein ERCC1 or homologous recombination repair showed increased sensitivity to SG3199 and the drug was only moderately susceptible to multidrug resistance mechanisms. SG3199 was highly efficient at producing DNA interstrand cross-links in naked linear plasmid DNA and dose-dependent cross-linking was observed in cells. Cross-links formed rapidly in cells and persisted over 36 hours. Following intravenous (iv) administration to rats SG3199 showed a very rapid clearance with a half life as short as 8 minutes. These combined properties of cytotoxic potency, rapid formation and persistence of DNA interstrand cross-links and very short half-life contribute to the emerging success of SG3199 as a warhead in clinical stage ADCs.

Background and purposeKaposi's sarcoma-associated herpesvirus (KSHV) is a common virus with severe outcome and no effective antiviral treatment. KSHV encodes the constitutive active chemokine receptor ORF74 with broad-spectrum CXC-chemokine binding. Here, we leverage ORF74's mimic of endogenous receptors to design chemokine-based immunotoxins for selective killing of KSHV-infected cells.Experimental approachFour CXC-chemokines with high affinity to ORF74 were fused to domain II, IB, and III of Pseudomonas exotoxin A to generate fusion toxin proteins (FTPs). FTP-induced cell killing was tested in cells expressing ORF74 or one of four chemokine receptors (CXCR1-4). Internalization of all receptors was probed using SNAP-tagged receptors. Second-generation FTPs were designed from receptor structures and molecular modelling to increase selectivity for ORF74 over CXCR1-4. Finally, antiviral activity of FTPs was tested using genetically engineered KSHV.Key resultsFTPs, based on the agonists (CXCL1, and -8) and inverse agonists (CXCL10 and -12) of ORF74 potently killed ORF74-expressing cells. The inverse agonist based FTPs leveraged constitutive internalization for efficient toxin delivery via ORF74, whereas agonists increased internalization further. CXCL10-FTP had the strongest cell-killing and, as the only FTP, selectivity for ORF74 over its endogenous receptor, CXCR3. Second-generation FTPs improved this selectivity from 25-fold to 126-fold by the mutation (R8D) in CXCL10-FTP, designed to lose ionic interaction within CXCR3's main binding pocket. Both inverse agonist-based FTPs effectively prevented KSHV-reactivation.Conclusion and implicationsOur findings highlight the versatility of FTPs in precise delivery of toxin payloads and provide a foundation for potential applications in antiviral and anticancer therapies targeting KSHV-associated diseases.