Immunotoxins are proteins that include a toxin along with an antibody or growth factor that binds specifically to target. An immunotoxin consists of a specific cell-targeting constituent, usually a monoclonal antibody (mAb) linked to a cell-killing toxin. Chemical conjugation of antibody to whole protein toxin creates immunotoxins, lacking of its natural binding domain. Immunotoxins are created in Escherichia coli that are distorted with a plasmid encoding the recombinant toxin. Harvesting recombinant protein is the most common method for manufacturing a substance for clinical trial. The insoluble protein is washed thoroughly with detergent to remove solublized, endotoxin, denatured, and condensed in guanidine-dithioerythritol solution. Then the recombinant protein is restored as a denatured protein or also known as renatured protein by rapid intensity into redox buffer containing glutathione and arginine, and the dialyzed renatured protein purified by sizing chromatography and anion exchange.
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Toxins used in immunotoxin structures are derived from fungi, bacteria and plants, while most function by inhibiting protein synthesis. The bacterial toxins that are commonly used in immunotoxins include the toxin from Pseudomonas exotoxin (PE) and Diphtheria toxin (DT). Ribosomes inactivating proteins (RIPs) gelonin, the A chain of ricin (RTA), pokeweed antiviral protein and dodecandron are included in plant toxins that are utilized in immunotoxins. As it is an enzyme, one toxin molecule can work on many substrate molecules, having a lethal effect on the cell. Immunotoxin’s use in haematological malignancies is considered to be very important, which are characterized by a high percentage of malignant cells that express the target antigen in contrast to solid tumours, which are characterized by a mixed cell population, and cells that are often not easily accessible for immunotoxin.
According to a research, cancer was observed as the most frequent cause of death in most of the developed countries. In the U.S., the estimated number of new cancer cases was above 1.5 million in 2010, with a mortality rate accounting for 23% of total deaths. Thus, cancer is observed to be the major application of immunotoxin, to use highly potent agents as a cancer therapy, it is necessary to target the killing agent selectively to the cancer cells and immunotoxin therapy is also considered to be an important and effective molecular cancer treatment strategy. The major driver of immunotoxins market is increasing global prevalence of cancer. For instance, according to World Health Organization (WHO) in 2012, Cancers figure among the leading causes of death worldwide, accounting for 8.2 million deaths while lung, liver, stomach, colorectal and breast cancers cause the most cancer deaths each year. It is also expected that annual cancer cases will rise from 14 million in 2012 to 2022 within the next two decades.
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Major advantages of immunotoxins are minimum toxicity in normal tissues and discerning cytotoxicity towards tumor cells. The immunotoxin’s clinical development in the treatment of solid tumors has been impeded in part, by the initiation of an immune response directed mainly against the toxin moiety, and in part by loss of activity of the fusion protein, e.g. by sterical barrier in the case of large toxin moieties. Strategies to overcome these limitations and develop the clinical performance of immunotoxins may reduce immunogenicity of it by removing the T-cell epitopes and use bispecific or bivalent constructs as next generation molecules.