The aim of this study was to determine the suitability of a ribosome-inactivating protein (RIP) from barley endosperm for use as an immunotoxin. This barley RIP is identical with the 30-kDa protein first reported by Coleman and Roberts [(1982) Biochim. Biophys. Acta 696, 239] and sequenced by Asano and co-workers [(1986) Carlsberg Res. Commun. 51, 75]. Use of the terms barley toxin I, II, and III is proposed to describe the three isoforms resolved by cation-exchange chromatography. An improved procedure for isolating the protein involving the steps of aqueous extraction, ammonium sulfate precipitation, and cation-exchange HPLC is described. Barley toxin II retained activity after exposure to ca. 40% acetonitrile and 0.1% trifluoroacetic acid or lyophilization. In a comparative study using the rabbit reticulocyte lysate assay, the protein was about 68% and 30% as potent as gelonin and ricin A-chain (RTA), respectively. Introduction of SH groups with 2-iminothiolane resulted in a substantial loss of activity as the number of thiol groups approached four. Therefore, it was necessary to limit thiolation to an average of one to two SH groups per toxin molecule. Anti-transferrin receptor-based immunotoxins constructed with RTA, gelonin, and barley toxin II exhibited comparable cytotoxicity against a human colon tumor cell line. We conclude that the availability of raw material, ease of purification, and stability of barley toxin II to lyophilization and denaturing conditions render it a suitable protein for the construction of immunotoxins.
[Show abstract][Hide abstract] ABSTRACT: The term ribosome-inactivating protein (RIP) is used to denominate proteins mostly of plant origin, which have N-glycosidase enzymatic activity leading to a complete destruction of the ribosomal function. The discovery of the RIPs was almost a century ago, but their usage has seen transition only in the last four decades. With the advent of antibody therapy, the RIPs have been a subject of extensive research especially in targeted tumor therapies, which is the primary focus of this review. In the present work we enumerate 250 RIPs, which have been identified so far. An attempt has been made to identify all the RIPs that have been used for the construction of immunotoxins, which are conjugates or fusion proteins of an antibody or ligand with a toxin. The data from 1960 onwards is reviewed in this paper and an extensive list of more than 450 immunotoxins is reported. The clinical reach of tumor-targeted toxins has been identified and detailed in the work as well. While there is a lot of potential that RIPs embrace for targeted tumor therapies, the success in preclinical and clinical evaluations has been limited mainly because of their inability to escape the endo/lysosomal degradation. Various strategies that can increase the efficacy and lower the required dose for targeted toxins have been compiled in this article. It is plausible that with the advancements in platform technologies or improved endosomal escape the usage of tumor targeted RIPs would see the daylight of clinical success.
Current Pharmaceutical Design 10/2014; 20(42):6584-643. DOI:10.2174/1381612820666140826153913 · 3.45 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Many types of tumour cell express high levels of surface antigens which are largely absent from normal cells. Monoclonal antibodies against these surface antigens can be coupled to potent toxins to form conjugates (immunotoxins) which selectively kill the antigen-bearing cells. This article describes the components of immunotoxins, their modes of action and their clinical applications.
Trends in Biotechnology 01/1986; 3(7):175-179. DOI:10.1016/0167-7799(85)90118-0 · 11.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To obtain more potent immunotoxins for anticancer therapy a gelonin-AR3 antibody immunoconjugate was prepared with different new linkers and coupling procedures. The gelonin was derivatized with the heterobifunctional thioimidate linkers ethyl-acetyl-3-mercaptopropionthioimidate (AMPT) and 3-(4-carboxamidophenyldithio)propionthioimidate (CDPT), and with the succinimidyl type reagents N-succinimidyl-3-(4-carboxamidophenyldithio)propionate (SCDP) and N-succinimidyl-S-acetyl thiolacetate (SATA). The biological activity of gelonin modified with different linkers (AMPT, CDPT, SCDP, SATA) was determined by a rabbit reticulocyte assay. We found that AMPT was the molecule of choice to derivatize the toxin, confirming the preferability of thioimidate linkers. The monoclonal antibody Mab was derivatized with CDPT and SCDP. Then the following immunoconjugates were prepared with different procedures: Mab-CDPT with gelonin-AMPT; Mab-CDPT with gelonin-CDPT; Mab-SCDP with gelonin-SATA. To verify whether selection of the most suitable coupling procedure could affect the antitumoral activity of the gelonin-AR3 immunoconjugate, the three immunotoxins were tested on target HT-29 human colon carcinoma cells versus nontarget MeWo cells. The gelonin immunoconjugate linked via the AMPT-CDPT thioimidate reagents showed highest antitumoral activity as well as best selectivity for the target cells.
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