Cancer cells frequently overexpress specific surface receptors providing tumor growth and survival which can be used for precise therapy. Targeting cancer cell receptors with protein toxins is an attractive approach widely used in contemporary experimental oncology and preclinical studies.
Cancer treatment has traditionally been based on surgery, radiation, and chemotherapy, which have shown limited therapeutic benefits in patients with metastatic disease. Despite significant advances in the development of systemic treatment, traditional chemotherapeutic agents cause serious side toxicity, restricting treatment to certain therapeutic dosages. In light of this, new approaches to selective treatment are urgently needed.
Protein toxins possessing such features as high cytotoxicity and efficiency have become promising components for anticancer therapy. Cancer cells frequently upregulate surface receptors that promote growth and survival, that is why various antigen-specific proteins including antibodies, antibody fragments (e.g., Fab and scFv), and other protein scaffolds (e.g., affibody and DARPin) have been developed as a moiety to target cancer cells . Being genetically encoded, toxins can be expressed as fusion proteins with targeting moieties mentioned above and can have a wide range of modifications to prolong circulation in the bloodstream and increase tumor retention. Complete biodegradation within an organism is also an important advantage of protein toxins as anticancer agents .
In addition to natural protein toxins, designed toxins are also used in experimental oncology, for example, as an alternative to chemical photosensitizers . The main advantage of protein photosensitizers is the opportunity to use a genetic engineering approach to combine cytotoxic and targeting moieties, avoiding chemical conjugation.
The history of targeted toxins began with the chemical conjugation of natural diphtheria toxin (DT) with anti-lymphocyte antibodies or their F(ab)2 fragments to produce agents for killing lymphoblastoid tumor cells . This strategy helped to couple cell-specific delivery of antibodies with extremely high toxicity of DT, previously shown for mammalian cells . The first generation of immunotoxins used chemical conjugation to couple natural toxins with full-length antibodies . The introduction of hybridoma technology  enabled the production of precisely characterized bifunctional agents with a certain specificity. The second generation of immunotoxins arise due to the use of truncated fragments of protein toxins, lacking natural tropism, which helped to reduce in vivo side toxicity . Over time, the variety of toxins used in the design of targeted therapy has grown , but the next breakthrough was made due to molecular cloning, which allowed for the production of the third-generation immunotoxins: fusion proteins consisting of antibody fragments linked to enzymatically active toxin domains .
Soluble targeted toxins are thought to be the embodiment of a “magic bullet” idea. Being applied systemically, these agents can reach disseminated, metastatic, or inoperable tumors and kill cancer cells. Still, there are several factors affecting the efficiency of targeted toxins (summarized in Figure 1).
Figure 1. The main factors affecting the efficiency of targeted toxin. Green up arrows—factors enhancing circulation time and tumor cell targeting. Red down arrow—reducing factor. FcRn is the neonatal Fc receptor.