Blockade of the galactose-binding sites of ricin by its linkage to antibody. Specific cytotoxic effects of the conjugates

Targeted toxins

Objective. Current modalities used in the treatment of cancer often cause unacceptable damage to normal tissue. Toxins targeted toward tumor cells by antibodies or growth factors have the potential to selectively kill tumor cells while leaving normal tissue intact. The purpose of this review is to provide background information on targeted toxins and current clinical studies for this new class of anti-cancer compounds.

Data sources. A MEDLINE search was conducted using the term “immunotoxins.” Relevant articles were also obtained by the systematic examination of article references.

Data synthesis. The toxins Pseudomonas exotoxin, diphtheria toxin, and ricin toxin are often used as targeted toxins. Deletion or mutation of the binding domains of these toxins decreased binding of the toxins to normal tissues. Antibodies or growth factors can be used as targeting moiety, and the resulting agents are called immunotoxins or fusion proteins, respectively. DNA technology and chemical modifications of the toxin as well as the antibody moiety led to smaller and less immunogenic targeted toxins. Smaller targeted toxins are less toxic and penetrate further into the tumor. The summary of several targeted toxins elicited during clinical trials in this review makes it clear that several targeted toxins are potential agents for the treatment of various cancers, although some problems still need to be overcome. These problems include toxicity, immunogenicity, cross-reactivity of the targeted toxin with life-sustaining tissue, heterogenicity of tumor cells, and limited tumor penetration.

Immunotoxins and anticancer drug conjugate assemblies: the role of the linkage between components

Immunotoxins and antibody-drug conjugates are protein-based drugs combining a target-specific binding domain with a cytotoxic domain. Such compounds are potentially therapeutic against diseases including cancer, and several clinical trials have shown encouraging results. Although the targeted elimination of malignant cells is an elegant concept, there are numerous practical challenges that limit conjugates’ therapeutic use, including inefficient cellular uptake, low cytotoxicity, and off-target effects. During the preparation of immunoconjugates by chemical synthesis, the choice of the hinge component joining the two building blocks is of paramount importance: the conjugate must remain stable in vivo but must afford efficient release of the toxic moiety when the target is reached. Vast efforts have been made, and the present article reviews strategies employed in developing immunoconjugates, focusing on the evolution of chemical linkers.

Development of sulfhydryl-reactive silica for protein immobilization in high-performance affinity chromatography

Two techniques were developed for the immobilization of proteins and other ligands to silica through sulfhydryl groups. These methods made use of maleimide-activated silica (the SMCC method) or iodoacetyl-activated silica (the SIA method). The resulting supports were tested for use in high-performance affinity chromatography by employing human serum albumin (HSA) as a model protein. Studies with normal and iodoacetamide-modified HSA indicated that these methods had a high selectivity for sulfhydryl groups on this protein, which accounted for the coupling of 77-81% of this protein to maleimide- or iodoacetyl-activated silica. These supports were also evaluated in terms of their total protein content, binding capacity, specific activity, nonspecific binding, stability, and chiral selectivity for several test solutes. HSA columns prepared using maleimide-activated silica gave the best overall results for these properties when compared to HSA that had been immobilized to silica through the Schiff base method (i.e., an amine-based coupling technique). A key advantage of the supports developed in this work is that they offer the potential of giving greater site-selective immobilization and ligand activity than amine-based coupling methods. These features make these supports attractive in the development of protein columns for such applications as the study of biological interactions and chiral separations.

Effect of therapeutic macromolecules in spheroids

Recent advances in biotechnology have allowed the production of new types of macromolecular therapeutic agents (antibodies, immunotoxins, cytokines, extracellular matrix molecule (ECM) proteins, vectors) that may eventually find broad clinical applications in the treatment of human tumors and other diseases. The model of the Multicellular Tumor Spheroids (MTS) represents a valuable tool to test the therapeutic potential of these new pharmacologic agents in a 3-D context. Specific questions pertaining to the behaviour in a 3-D setting of some of the macromolecules under evaluation for in vivo applications can also be addressed in the MTS model (e.g. 'binding site barrier', role of cell-cell and cell-ECM interactions). This paper reviews the most significant contributions regarding the delivery of macromolecules to MTS, the penetration and therapeutic effects of antibodies, radiolabelled antibodies, immunotoxins and other macromolecular compounds.

Characterization of the Asialofetuin microtitre plate-binding assay for evaluating inhibitors of ricin lectin activity

Optimum conditions for the binding of ricin to the glycoprotein asialofetuin immobilized on microtitre plates were investigated for the purpose of evaluating inhibitors of ricin B-chain lectin activity. Such inhibitors are of potential value in the use of immunotoxins based on ricin. This assay was first reported in 1986, but has not been characterized fully. Maximum binding of asialofetuin to the plate was observed at a concentration of ca. 4 microg ml(-1). Binding increased with time of incubation (1-24 h), pH (7.4-9.9) and temperature (2-37 degrees C). The pH effects were more marked at lower temperatures. Saturable binding of ricin to immobilized asialofetuin was observed, and at least 80% of maximum binding was observed by 10 min of incubation time. The binding was found to be very tight, such that an appreciable proportion of ricin added to the wells was bound at low concentrations, and binding was only partially reversible by addition of free galactose. Consequently, only estimates of the ricin-asialofetuin and ricin-galactose dissociation constants could be determined: 1.9 nM and 83 microM, respectively. Binding of ricin A- and B-chains was found to be 47% (at a 200-fold higher concentration) and 26% (at a twofold higher concentration) of that of the whole ricin molecule, respectively. The assay permits qualitative comparison of inhibitors of ricin B-chain lectin activity.

Conjugation to preactivated proteins using divinylsulfone and iodoacetic acid

Two methods for the preactivation of proteins and conjugation of peptides to proteins under mild conditions are presented. Preactivation of proteins with divinylsulfone (DVS) permits peptide conjugation through either amino, hydroxyl or sulphydryl groups depending on the coupling pH used, while preactivation with iodoacetic acid (IAA) N-hydroxy-succinimide ester permits selective conjugation through sulphydryl groups. In addition, the latter method allows quantitation of the conjugation ratio through determination of carboxymethyl cysteine after acid hydrolysis. The divinylsulfone activated proteins can be stored for extended periods of time at -20 degrees C until required for conjugation, while the iodoacetic acid activated protein can be stored for a few days at -20 degrees C. These conjugation methods were investigated with respect to obtaining peptide/protein conjugates for immunization purposes and for use as reagents in immunoassays. The DVS activated proteins permitted direct conjugation of luteinizing releasing hormone (LHRH) through its tyrosine side chain and allowed synthesis of well defined conjugates. The DVS derivatives of bovine serum albumin (BSA), reduced and carboxymethylated BSA and purified protein derivative (PPD) were compared with respect to their potential value as carriers for obtaining antibodies to LHRH (M(r) 1000) and epidermal growth factor (EGF, M(r) 5000). IAA-PPD was evaluated as a carrier for the conjugation of glutathione specifically through its cysteine side chain and for obtaining antibodies to glutathione. The antisera obtained were specific and of high titer, and the methods described here will thus allow the convenient synthesis of carrier conjugates with well defined characteristics.

Cytotoxic effects of ricin without an interchain disulfide bond: genetic modification and chemical crosslinking studies

Ricin is a toxic glycoprotein made of two polypeptide chains (A and B) linked by a disulfide bond. Ricin binds to cells by the B chain and is then internalized. The interchain disulfide bond is believed to be reduced in endosomes, and the A chain is then subsequently translocated to cytoplasm where it inactivates ribosomes. To understand the role of the disulfide bond in ricin toxicity, we prepared two types of ricin molecules. First, cysteine 259 of the A chain was mutated to an alanine residue. The mutant A chain was then reassociated with the native B chain to determine whether ricin is biologically active in the absence of an interchain disulfide bond. Reassociated mutant ricin showed a 40-fold reduction in biological activity. Binding studies using a hydrophobic fluorescence probe indicated that the associated complex was stable only at neutral pH and became highly unstable at a lower pH characteristic of the endosomal milieu. In the second construct, the interchain disulfide bond was replaced with a non-reducible bond by chemical derivatization. Interestingly, the non-reducible ricin molecule was equally cytotoxic as native ricin. These results show: (i) that the interchain disulfide bond is necessary to hold the A chain and the B chain together at endosomal pH, and (ii) that intact ricin may be transported to the cytoplasm where proteolysis or hydrolysis may occur to release the biologically active moiety.

Pharmacokinetics of an antibody-ricin conjugate administered intraperitoneally to mice

Immunotoxins have been extensively studied for the treatment of neoplasias; their intracavitary administration could be useful for the therapy of tumors confined to the pleural or peritoneum spaces. To study the feasibility of this "locoregional" treatment, a pharmacokinetic study of immunotoxins delivery is necessary. Ricin, a plant toxin extracted from the seeds of Ricinus communis, has often been used in immunoconjugates for its high activity; nevertheless, appropriate strategies have been necessary to limit the aspecific toxicity. We previously prepared a AR-3-ricin immunotoxin lacking the ability to bind galactosidic cell surface residues, a so-called sterically blocked immunotoxin. The monoclonal antibody AR-3, an IgG1 specific to the CAR-3 antigen, was able to recognize human colorectal adenocarcinomas. Preclinical trials in nude mice, intraperitoneally grafted with the target neoplasia, showed that this immunotoxin suppressed tumor growth without showing any undesirable ricin toxicity. In the present work we report the pharmacokinetic properties of this immunotoxin, showing the in vivo stability and a relatively long blood survival. With a biodistribution study in tumor-bearing mice, we demonstrate that in tumor-invaded tissues, the concentration of the specific AR-3-ricin immunotoxin was higher and progressively increased in a multiple-dose regimen. In contrast, an irrelevant immunotoxin behaved differently because it did not show specific tumor uptake. Moreover the pharmacokinetic data reported in this work improve the potential for "locoregional" treatment of malignancy with blocked immunotoxins.

Toxin-targeted design for anticancer therapy. I: Synthesis and biological evaluation of new thioimidate heterobifunctional reagents

In an effort to obtain a more potent and specific immunotoxin for cancer therapy, we designed a series of heterobifunctional linkers characterized by a thioimidate group linked to a S-acetyl thiol (4, 5) or substituted aryldithio group (6-10). These ligands were synthesized by a Pinner-type process from the corresponding nitrile derivatives obtained by thiol-disulphide exchange reaction, reaction with substituted benzene-sulphenyl chloride, or other known procedures. To check the reagent of choice for immunoconjugate preparation, we studied thioldisulphide exchange kinetics between the intermediate nitrile derivatives and cysteine. Among the tested aryldithio derivatives (6-10), we selected ethyl 3-(4-carboxamido-phenyldithio)propionthioimidate (CDPT, 9) for further studies. By analyzing the rate of incorporation of the linkers 4, 5, and 9 in a model immunoglobulin G protein, we found similar results with CDPT 9 and ethyl S-acetyl 3-mercaptopropionthioimidate ester hydrochloride (AMPT, 5) because both reagents showed a linear correlation between the number of introduced thiol groups and factors such as time and protein and reagent concentrations. Comparison of the two acetylthio-derivative ligands 4 and 5 showed that AMPT 5 was more stable toward deacetylation than ethyl S-acetyl 2-mercaptopropionthioimidate ester hydrochloride (AMAT, 4). By comparing the kinetic and biological parameters of seven new thioimidate linkers, we found that two of these (CDPT and AMPT) could be superior ligands for protein-protein conjugation. They offer advantages over the commercially available compounds, such as minimal perturbation of the protein structure, controlled reactivity, and good stability.

Optimisation of small-scale coupling of A5B7 monoclonal antibody to carboxypeptidase G2

Conjugates of F(ab')2 fragment of the monoclonal antibody A5B7 coupled to carboxypeptidase G2 (CPG2) have been produced using the heterobifunctional reagents 2-mercapto-[S-acetyl]acetic acid, N-hydroxysuccinimide ester (SATA) and m-maleimidobenzoyl-N-hydroxysuccinimide ester (SMPB). The effect of various levels of modifying reagent on enzyme activity and antigen binding activity were determined, and it was shown that whilst CPG2 is relatively sensitive to modification, insertion of three maleimide groups per CPG2 resulted in the loss of 30% of enzyme activity; A5B7 F(ab')2 was insensitive to modification, little or no activity being lost. The coupling efficiency of the reaction was shown to be fairly constant over a wide range of substitution levels. There was thus no advantage to be gained in using high substitution levels, which may result in loss of enzyme activity. The formation of undesired high molecular weight aggregates could be controlled by adjustment of the protein concentration during the final coupling step.

Addition of an ER retention signal to the ricin A chain increases the cytotoxicity of the holotoxin

With the exception of diphtheria toxin, which translocates from acidified endosomes, the intracellular organelle from which the catalytic moieties of several plant and bacterial toxins enter the target cell during endocytic uptake has not been identified. We have recently proposed that some toxins may travel the entire secretory pathway in reverse, moving from the cell surface to the lumen of the ER, before entering the cytosol. Several bacterial toxins have the ER retention sequence KDEL or a related analogue at their carboxyl termini, suggesting that the KDEL receptor may play a role in delivering these toxins to the ER. Here we provide further support for this possibility since the cytotoxicity of ricin, which lacks a KDEL sequence, can be significantly increased by adding KDEL to the C-terminus of its A chain.

High-level expression and simplified purification of recombinant ricin A chain

Ricin toxin is a glycoprotein which catalytically inactivates eukaryotic ribosomes by depurination of a single adenosine residue from the 28S ribosomal RNA. The enzymatic activity is present in the A chain of the toxin molecule, whereas the B chain contains two binding sites for galactose. Since it is highly potent in inhibiting protein synthesis, the A chain is used to prepare cytotoxic conjugates effective against tumor cells. Such chimeric proteins are highly selective and have a wide range of clinical applications. Extensive preclinical studies on these conjugates require large amounts of purified A chain. Native ricin A chain is heterogeneous, since plants produce a number of isoforms of ricin toxin. Purified, native preparations often contain two types of ricin A chain which differ in the extent of glycosylation. By cloning and expressing the gene of A chain, one could obtain homogeneous toxin molecules devoid of carbohydrates. In addition, structural changes in the toxin polypeptide could be introduced by in vitro mutagenesis, which can improve the pharmacological properties and antitumor activity. Earlier methods of expression strategies using Escherichia coli have yielded only moderate levels of expression. In the present study, the coding region of ricin A chain was cloned into pET3b, a high-level expression vector under the control of the T7 promoter. Recombinant ricin A chain produced by this construct has an additional 14 amino acid residues at the NH2 terminus. Subsequently, a NdeI site was created at the 5' end of the gene by oligonucleotide-directed mutagenesis. The modified fragment was then introduced into pET3b vector to produce toxin polypeptide identical to the native sequence.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro efficacy of transferrin-toxin conjugates against glioblastoma multiforme

The cytotoxic activity of immunotoxins constructed with human diferric transferrin (Tfn) as the carrier ligand and an abrin variant Pseudomonas exotoxin A (PE) and the diphtheria toxin mutant cross-reacting material (CRM) 107 as the toxin moieties were studied in vitro. Three malignant human cell lines, the glioblastomas multiforme SNB19 and SF295 and the LOX melanoma, and a nonhuman control murine melanoma cell line B16 were assessed. The presence of transferrin receptors on the cell lines was confirmed by direct 125I-Tfn binding assays. The 50% protein synthesis inhibitory concentration (IC50) values for all cell lines demonstrated that Tfn-abrin variant and Tfn-PE had comparable potency and were both more effective than Tfn-CRM 107. Monensin, a carboxylic ionophore, potentiated the effect of Tfn-abrin variant against glioma cells approximately 35-fold with IC50 values of 4.0 x 10(-13) M and 4.7 x 10(-12) M for SNB19 and SF295, respectively. Cytotoxic activity of Tfn-abrin variant (with or without monensin) and Tfn-PE was correlated with the degree of Tfn receptor expression measured on the cell lines. The exquisite in vitro cytotoxicity of Tfn-abrin variant and Tfn-PE immunotoxins against glioma and melanoma cells warrants further in vivo evaluation and future consideration of these agents for potential clinical application against glioblastoma multiforme and leptomeningeal neoplasia.

Recombinant ricin B chain fragments containing a single galactose binding site retain lectin activity

Ricin B chain is an N-glycosylated galactose-specific lectin. Examination of the amino acid sequence of the protein has shown it to be the product of a series of gene duplication events based on an original galactose binding peptide. The X-ray crystallographic structure of the protein reveals that it consists of two globular domains, each composed of three smaller subdomains. In each globular domain only one of the three subdomains has retained its ability to bind galactose. Through DNA manipulation we have created a series of fusions of portions of ricin B chain, each carrying only one galactose binding site, to the ricin signal sequence. Transcripts synthesized in vitro using SP6 RNA polymerase were injected into Xenopus oocytes where the recombinant proteins were produced in a mature form. The products were shown to be N-glycosylated and produced in a soluble stable form. Also, they retained the ability to bind galactose. Preliminary experiments on the reassociation of these ricin B chain fragments with ricin A chain to create a modified holotoxin were also carried out.

The specific cytotoxicity of immunoconjugates containing blocked ricin is dependent on the residual binding capacity of blocked ricin: evidence that the membrane binding and A-chain translocation activities of ricin cannot be separated

Recently we have developed blocked ricin, a derivative of native ricin in which the galactose-binding sites of the B-chain are blocked by covalent modification with affinity ligands. This modification impedes the binding function of the B-chain, while sparing its ability to facilitate the entry of the toxic subunit of ricin, the A-chain, into the cytoplasm. Immunotoxins prepared with blocked ricin approach the cytotoxic potency of native ricin with antibody-dependent specificity. Here we report that the high cytotoxic potency of these immunoconjugates, which is attributed to the preserved translocation function of the ricin B-chain, is dependent on the minimal residual lectin activity of blocked ricin. Our findings support the notion that two functions of ricin, membrane binding and translocation, cannot be separated.

Antitumour activity of a sterically blocked ricin immunotoxin on a human colorectal adenocarcinoma grafted subcutaneously in nude mice

We prepared a ricin-antibody conjugate, lacking the ability to bind the galactosidic residues of Sepharose 6B, a so-called blocked immunotoxin. The monoclonal antibody AR-3 was cross-linked to ricin through a thioether bond. Further studies showed that the immunoconjugate suppressed the tumour growth of HT-29 cells in intraperitoneally grafted nude mice, without showing any undesirable ricin toxicity. In this work, to demonstrate the therapeutic activity of the AR-3-ricin conjugate injected into mice bearing subcutaneous tumour, we first evalauted its pharmacokinetic behaviour and biodistribution. The behaviour of the immunoconjugate injected intravenously was almost intermediate between that of the antibody and ricin. Moreover, when the immunotoxin was intravenously administered to nude mice bearing subcutaneous tumour, no therapeutic effects appeared, in accordance with the relatively low permeability of the immunotoxin from the blood to the skin. In contrast, peritumoral treatment produced a strong reduction of the neoplastic nodules without substantial regrowth of the malignant cells. This result was also achieved when the immunotoxin treatment was performed on a well-established tumour. This finding was strictly related to the specifcity of the immunoconjugate, since the analogous treatment with an irrelevant immunotoxin showed therapeutic failure.

Blocked ricin-conjugated T cell immunotoxins: effect of anti-CD6-blocked ricin on normal T cell function

The biological properties of an immunotoxin composed of an anti-CD6 monoclonal antibody conjugated to whole ricin, which had been modified so that the galactose-binding sites of the B chain were blocked ("blocked ricin"), were examined. Treatment of peripheral blood lymphocytes with anti-CD6-blocked ricin for a 24-h period prevented T cell proliferation induced by phytohemagglutinin in a dose-dependent manner with concentrations causing 50% inhibition (IC50) ranging from 5 pM to 30 pM. In contrast, treatment with either blocked ricin alone or with a control immunotoxin prepared with a B-cell-lineage-restricted monoclonal antibody gave IC50 values of approximately 2 nM. Although shortening the duration of the anti-CD6-blocked ricin treatment to as little as 3 h had little significant effect on the observed inhibition, T cell viability experiments demonstrated that the magnitude of immunotoxin-induced killing after a given time period is significantly higher when the target cells become activated. Thus, from the initial concentration of cells treated with anti-CD6-blocked ricin placed in culture, 40%-45% viable cells remained after 2 days yet only 3%-9% remained if phorbol ester and Ca2+ ionophore were added; activation of T cells after mock treatment using blocked ricin plus nonconjugated anti-CD6 demonstrated that this effect was not the result of activation alone. The toxicity of anti-CD6-blocked ricin was also measured by inhibition of PHA-induced clonogenic growth of normal T cells. Continuous treatment of the cells using anti-CD6-blocked ricin at 0.1 nM resulted in a surviving fraction of about 3.5 x 10(-3); when immunotoxin treatment was for 24 h or less, the surviving fraction was only about 10(-1). As an indication of the unique specificity of anti-CD6-blocked ricin, immunotoxin pretreatment of potential responder cells prevented the generation of allogeneic cytolytic T lymphocytes in mixed lymphocyte cultures yet had little effect on the generation of interleukin-2-induced lymphokine-activated killer cell activity. We conclude that anti-CD6-blocked ricin demonstrates a cellular specificity and potency that make it a highly promising anti-T cell reagent.

Recombinant toxins for cancer treatment

Recombinant toxins target cell surface receptors and antigens on tumor cells. They kill by mechanisms different from conventional chemotherapy, so that cross resistance to conventional chemotherapeutic agents should not be a problem. Furthermore, they are not mutagens and should not induce secondary malignancies or accelerate progression of benign malignancies. They can be mass-produced cheaply in bacteria as homogeneous proteins. Either growth factor-toxin fusions or antibody-toxin fusions can be chosen, depending on the cellular target.

The galactose-binding sites of the cytotoxic lectin ricin can be chemically blocked in high yield with reactive ligands prepared by chemical modification of glycopeptides containing triantennary N-linked oligosaccharides

A glycopeptide containing a triantennary N-linked oligosaccharide from fetuin was modified by a series of chemical and enzymic reactions to afford a reagent that contained a terminal residue of 6-(N-methylamino)-6-deoxy-D-galactose on one branch of the triantennary structure and terminal galactose residues on the other two branches. Binding assays and gel filtration experiments showed that this modified glycopeptide could bind to the sugar-binding sites of ricin. The ligand was activated at the 6-(N-methylamino)-6-deoxy-D-galactose residue by reaction with cyanuric chloride. The resulting dichlorotriazine derivative of the ligand reacts with ricin, forming a stable covalent linkage. The reaction was confined to the B-chain and was inhibited by lactose. Bovine serum albumin and ovalbumin were not modified by the activated ligand under similar conditions, and we conclude, therefore, that the reaction of the ligand with ricin B-chain was dependent upon specific binding to sugar-binding sites. Ricin that had its galactose-binding sites blocked by the covalent reaction with the activated ligand was purified by affinity chromatography. The major species in this fraction was found to contain 2 covalently linked ligands per ricin B-chain, while a minor species contained 3 ligands per B-chain. The cytotoxicity of blocked ricin was at least 1000-fold less than that of native ricin for cultured cells in vitro, even though the activity of the A-chain in a cell-free system was equal to that from native ricin. Modified ricin that contained only 1 covalently linked ligand was also purified. This fraction retained an ability to bind to galactose affinity columns, although with a lower affinity than ricin, and was only 5- to 20-fold less cytotoxic than native ricin.

Resistance of myeloid leukaemia cell lines to ricin A-chain immunotoxins

Nineteen monoclonal antibodies that recognize antigens on myeloid leukaemia cells were screened upon HL60, KG1, U937 and K562 cells for their ability to form effective ricin A-chain immunotoxins. The screening was performed using an indirect assay in which the cells were treated firstly with the test antibody and then with a Fab' immunotoxin directed against mouse immunoglobulin. Only two antibodies, MEM75 and 120-2A3, both directed against the transferrin receptor (TfR) were predicted to form immunotoxins that would inhibit protein synthesis by the cells by 50% at a concentration (IC50) of 10(-8) M or less. This prediction was subsequently confirmed using several of the antibodies directly conjugated to ricin A-chain. By contrast, the same immunotoxins were highly toxic to non-myeloid cells which shared the target antigens. A comparison was made between the rates of endocytosis and degradation by HL60 cells of an anti-TfR immunotoxin 120-2A3.dgA, that was effective at killing myeloid cells, and a CD33 immunotoxin, p67-7.dgA, that bound to myeloid cells but did not kill them. The difference in potency of the two immunotoxins on HL60 cells was not due to deficient uptake of p67-7.dgA but was probably due to the more rapid intracellular degradation of p67-7.dgA. Fast and effective degradation in lysosomes, if a general finding, could explain the poor susceptibility of myeloid cells to ricin A-chain immunotoxins.

Blocked and non-blocked ricin immunotoxins against the CD4 antigen exhibit higher cytotoxic potency than a ricin A chain immunotoxin potentiated with ricin B chain or with a ricin B chain immunotoxin

An immunotoxin consisting of ricin A chain linked to the monoclonal antibody M-T151, recognising the CD4 antigen, was weakly toxic to the human T-lymphoblastoid cell line CEM in tissue culture. The incorporation of [3H]leucine by CEM cells was inhibited by 50% at an M-T151--ricin-A-chain concentration (IC50) of 4.6 nM compared with an IC50 of 1.0 pM for ricin. In contrast, immunotoxins made by linking intact ricin to M-T151 in such a way that the galactose-binding sites of the B chain subunit were either blocked sterically by the antibody component or were left unblocked, were both powerfully cytotoxic with IC50 values of 20-30 pM. The addition of ricin B chain to CEM cells treated with M-T151--ricin-A-chain enhanced cytotoxicity by only eight-fold indicating that isolated B chain potentiated the action of the A chain less effectively than it did as an integral component of an intact ricin immunotoxin. Ricin B chain linked to goat anti-(mouse immunoglobulin) also potentiated weakly. Lactose completely inhibited the ability of isolated ricin B chain to potentiate the cytotoxicity of M-T151--ricin-A-chain and partially (3- to 4-fold) inhibited the cytotoxicity of the blocked and non-blocked ricin immunotoxins. Thus, in this system, the galactose-binding sites of the B chain contributed to cell killing regardless of whether isolated B chain was associated with the A chain immunotoxin or was present in blocked or non-blocked form as part of an intact ricin immunotoxin. The findings suggest that the blocked ricin immunotoxin may become unblocked after binding to the target antigen to re-expose the cryptic galactose-binding sites. However, the unblocking cannot be complete because the maximal inhibition of [3H]leucine incorporation by the blocked immunotoxin was only 80% compared with greater than 99% inhibition by the non-blocked immunotoxin.

Distribution of endocytosed molecules to intracellular acidic environments correlates with immunotoxin activity

We have investigated the internalization to low pH intracellular compartments of transferrin (Tfn), diphtheria toxin (DT) and of anti-cell surface antibodies (MAb) by a cytofluorometric assay based on low pH quenching of fluorescein (FITC) emission. FITC-labelled Tfn, anti-CD3, anti-CD5 and anti-Thy 1.2 MAb internalization resulted in a progressively lower FITC quenching effect. Following internalization, a distinction could be made between molecules that enter low pH compartments without undergoing intracellular degradation (e.g., Tfn, anti-CD3 MAb) and molecules that are internalized through low pH organelles and are then degraded within the cell (e.g., DT). A strict correlation was observed between quenching of internalized FITC-protein fluorescent emission and the cytotoxic activity of DT-based immunotoxins (IT).

Expression and functional properties of genetically engineered ricin B chain lacking galactose-binding activity

Ricin is a potent plant toxin consisting of two disulfide-bonded subunits. The A chain of ricin is an N-glycosidase which inactivates 28 S RNA and inhibits protein synthesis. The B chain is a galactose-specific lectin with two galactose-binding sites. The genes encoding preproricin and its A and B chains have been cloned and expressed. In addition, X-ray crystallographic studies have identified the galactose-contact residues in both the high- and low-affinity galactose-binding sites of the B chain. In this study, the high-affinity galactose-contact residue of the B chain was changed from Asn-255 to Ala-255 by oligonucleotide-directed mutagenesis. The resulting mutant was sequenced to confirm the presence of a single mutation and was expressed in Cos-M6 cells. Both wild-type and mutant recombinant B chain could be immunoprecipitated with a heterologous anti-B chain antibody and both could form A-B heterodimers. However, as compared to the wild-type, the mutant B chain lacked more than 99% of its lectin activity and cytotoxicity as an A-B dimer. In conclusion, altering the contact residue of the high-affinity galactose-binding site of ricin B chain from Asn-255 to Ala-255 abrogates more than 99% of its lectin activity and the cytotoxicity of the A-B heterodimer to ricin-sensitive cells.

Immunotoxins of Pseudomonas exotoxin A (PE): effect of linkage on conjugate yield, potency, selectivity and toxicity

Conjugates of monoclonal antibodies and Pseudomonas exotoxin A (PE) were formed with disulfide or thioether bonds. Thioether conjugates which formed with succinimidyl 4-(N-maleimidomethyl)-cyclohexane-1-carboxylate (SMCC) modified PE and reduced antibody formed with an 80% yield of equimolar conjugate within 30 min with an offering of one to one (toxin:antibody). The efficiency and kinetics of thioether formation were much higher with SMCC than with other maleimide reagents as well as more efficient than disulfide linkers. Thioether linkage resulted in immunotoxin consistently more potent and more selective in vitro than disulfide bonded conjugate. Thioether bonded conjugates also proved to have other favorable in vivo properties compared to disulfide conjugates: (1) a longer half-life in serum; (2) increased tumor localization; and (3) reduced toxicity. Toxicity of thioether linked holotoxin conjugates was directed at the liver hepatocyte but was easily monitored by serum liver enzymes. The conjugates are currently undergoing clinical evaluation for treatment of ovarian cancer with intraperitoneal administration. Research is ongoing to further decrease residual toxicity without reducing the potency of the conjugate.

Immunotoxins up to the present day

Immunotoxins consist of monoclonal or polyclonal antibodies conjugated to bacterial or plant toxins. The toxins used are typically of the A-B type in which a toxic A chain is coupled to a B chain responsible for cell binding and facilitation of A chain entry into the cytosol. Two broad strategies have been followed: coupling intact toxins, or A chains alone, to antibodies. This review examines current progress in in vitro and in vivo research, including recent clinical studies, concentrating principally on ricin or ricin A chain conjugates. The future role of conjugates using membrane-acting toxins, immunolysins, is also discussed.

Intratumour therapy of solid tumours with ricin-antibody conjugates

Immunotoxin conjugates of whole ricin with monoclonal antibody were prepared with the galactose binding site on the ricin B chain blocked. These whole ricin-antibody conjugates were then injected directly into tumours (IT) in mice with established solid tumours. The conjugates were found to be effective, in vivo, in (C57BL/6XBALB/c)F1 mice carrying thymoma grafts and in nude mice bearing human tumour xenografts. Thymomas (Ly-1.1-, 2.1+) completely regressed following IT injection of either ricin-anti-Ly-2.1 or 'modified' (periodate treated to remove carbohydrate) ricin-anti-Ly-2.1 but did not regress when treated with the non-reactive ricin-anti-Ly-1.1. Similarly, established CEM (transferrin receptor+) or HT-29 (17.1/2+) tumours in nude mice completely regressed following IT injection of ricin-anti-transferrin receptor antibody or ricin-17.1/2 antibody conjugates. The tumours disappeared within 48 h, and in 80-100% of these there was no recurrence. Intact ricin-antibody conjugates did not require the presence of lactose to block the binding of native ricin and selective activity was entirely dependent on the reactivity of the monoclonal antibodies (MoAb). Further, the killing of target cells was specific because non-reactive ricin-antibody conjugates did not cause regression of tumours and MoAb alone did not inhibit tumour growth. In addition, there was no systemic toxicity evident in mice treated with reactive conjugates. By contrast, in mice treated with only ricin or with non-reactive ricin-antibody conjugates there was toxicity to liver and spleen due to diffusion from the tumour; thus the MoAb moiety of the immunotoxin serves to target the ricin to the tumour, to hold ricin in the tumour, and little escapes. It was found that ricin-antibody conjugate treatment (IT) with 1-2 micrograms of ricin-antibody conjugate was not harmful to mice, in contrast to ricin alone, which killed all treated tumour bearing mice at a dose of 0.5 micrograms. Thus whole ricin-antibody conjugates can be used successfully in vivo for local therapy, leading to the eradication of solid tumours by direct injection of the tumour.

Kinetics of uptake and degradation of an abrin immunotoxin by melanoma cells and studies of the rates of cellular intoxication

The mechanism of action of an abrin 9.2.27 antimelanoma antibody conjugate has been studied in 2 human melanoma cell lines, FEMX and LOX, which differ in sensitivity to the immunotoxin (IT) and to native abrin. The IT, which had been affinity purified before use to remove molecules with exposed gal-binding sites on the toxin B-chain, inhibited cellular protein synthesis at a faster rate in the LOX than in the FEMX cells despite the fact that the LOX cells express less specific antigen and bind less IT to the cell surface. Surface-bound abrin-IT, as well as surface-bound specific antibody, disappeared at equal rates from the cell surface of the 2 cell lines. After binding of labelled IT the disappearance of total cell-associated radioactivity, as well as the appearance of TCA-precipitable and TCA-soluble radioactivity in the incubation medium, occurred at faster rates in the LOX than in the FEMX cells. No free abrin or antibody B-chain complex could be detected in the medium or inside the cells. The results indicate that the different sensitivities of the melanoma cell lines reflect different abilities to process endocytosed IT and to translocate the active A-chain to the cytosol. Experiments carried out in the presence of lactose are interpreted to mean that the A-chain may be translocated to the cytosol by two mechanisms, one involving antigen-antibody interaction and one involving the B-chain, and that the lectin binding site contributes to the B-chain-facilitated mechanism.

Ricin B chain-containing immunotoxins prepared with heat-denatured B chain lacking galactose-binding ability potentiate the cytotoxicity of a cell-reactive ricin A chain immunotoxin

Ricin B chain incubated at 37 degrees C in the absence of lactose loses its ability to bind the galactose-containing protein, asialofetuin. Circular dichroism analysis of the B chain during thermal denaturation indicates that the loss of galactose-binding ability by the B chain correlates with limited unfolding of the molecule. As a result of this conformational change, disulfide bonds that are shielded from the solvent by the compact folded structure of the B chain become exposed and the chitobiosyl cores of both N-linked oligomannose chains become susceptible to cleavage by endoglycosidases. The heat-denatured B chain does not enhance the toxicity of a ricin A chain-containing rabbit anti-human immunoglobulin (RAHIg-A) to Daudi cells. However, when heat-denatured B chain is coupled to goat anti-rabbit immunoglobulin (GARIg), the resulting immunotoxin, GARIg-hdB, potentiates the killing of RAHIg-A-treated Daudi cells to an extent similar to that of an immunotoxin prepared with GARIg and native B chain. These results indicate that the native, galactose-binding structure of the B chain is not necessary to enhance the cytotoxicity of the cell-reactive A chain immunotoxin (IT-A) and suggests that regions of the B chain exposed by unfolding the molecule may mediate potentiation of cytotoxicity.

Immunotoxins against solid tumors

Antibody-toxin conjugates, termed immunotoxins, are currently being evaluated as potential new anticancer agents. The monoclonal antibodies that recognize antigens on the surface of tumor cells should deliver the toxins or the catalytic subunits of toxins to cancer cells. The catalytically active parts of the immunotoxins have to reach the cell cytoplasm where they inhibit protein synthesis. Immunotoxins against various solid tumors, including breast carcinoma and ovarian carcinoma, have been developed. In vitro, the activity of immunotoxins is affected by the number of target antigens on the cell surface, the internalization of the immunotoxins, the kind of toxin, the class of the antibody, the kind of linkage, and by other factors. Several problems arise with in vivo administration of immunotoxins. The short serum half-life of immunotoxins, due to their rapid hepatic uptake, decreases the number of immunotoxin molecules that reach the solid tumor. This, together with low tumor penetration by immunotoxins, could lead to low anti-tumor activity. Heterogeneity of tumors, immunogenicity of immunotoxins, and cross-reactivity of immunotoxins with normal tissues are other factors that might limit the clinical use of immunotoxins. It should be possible, however, to overcome these problems using methods that are already available or have yet to be developed.

Immunoconjugate generation between the ribosome inactivating protein restrictocin and an anti-human breast carcinoma MAB

In the perspective of therapeutic approaches the monoclonal antibody, MBrl, with a quite restricted spectrum of reactivity for human breast carcinoma, was coupled to restrictocin (Res), a ribosome inactivating protein produced by Aspergillus restrictus. In a cell-free system this toxin was found to have an activity comparable to that of other plant toxins, but its in vitro toxicity was shown to be low on different cell lines. Three batches of MBr1-Res conjugate were prepared and their specificity, efficiency, and maximum level of cytotoxicity were analyzed on the cell line MCF-7 expressing the relevant antigen, on several irrelevant tumor cell lines, and on normal cells. Conjugates were from 600 to 1500 times more efficient than the uncoupled derivatized Res towards MCF-7 cells and were completely ineffective on the other target cells. The antigen-driven cytotoxicity was confirmed by the nontoxicity of an irrelevant conjugate on MCF-7 cells. The cytotoxic efficiency of MBr1-Res was low when compared to the binding level of MBr1 at the same concentration and a portion of treated cells (from 10% to 30%) survived the treatment. The heterogeneity of expression of the relevant antigen, together with its only partial internalization, could account for these limitations. The lysosomotropic agent ammonium chloride and the carboxylic ionophore monensin were tested as potentiating agents but in both cases the cytotoxicity remained unmodified. A neutralization assay performed on a xenogenic model indicated that the MBr1-Res conjugate was capable of reducing the tumor take. These data indicate the possibility of using the Res to prepare a reproducible and highly selective breast cancer conjugate. However, there are still a number of problems which must first be solved before we can consider its clinical application.

Purification and biochemical characterization of immunotoxins

Written on a plastic bottle of liquid soap in one of our bathrooms (JML) is the phrase 'Absolute cleanliness is next to Godliness!'. Perhaps absolute purity for ITs does not rank so highly, but the availability of purified ITs that contain no nonconjugated antibody or toxin, and no material of very high Mr (aggregates) is essential for a proper comparison of the biological activities of the component proteins with their nonconjugated counterparts. Purified and biochemically well-defined ITs make it possible to perform quantitative binding assays, to evaluate receptor-mediated endocytosis and to do cytotoxicity tests in vitro and efficacy studies in vivo, without fear that competition by nonconjugated antibody is affecting the experimental result. This chapter illustrates some of the methods that in combination can be used to purify ITs, including affinity chromatography, gel filtration, and ion-exchange chromatography with buffers of carefully defined composition, using examples from our laboratory to illustrate the procedures. For further details concerning these methods, we would refer the reader to the excellent book by Scopes [40], Purification of Proteins: Principles and Practice, which contains much useful information of both a technical and a practical nature regarding methods for purification of proteins.

Comparison of blocked and non-blocked ricin-antibody immunotoxins against human gastric carcinoma and colorectal adenocarcinoma cell lines

To avoid non-specific binding of intact ricin-antibody conjugates, we prepared a new blocked thioether-linked ricin-antibody IT, in which the galactose binding site of ricin had lost the ability to bind to galactosidic residues of Sepharose 6B gel. As carrier agent, the monoclonal antibody AR-3, which defines the CAR-3 tumour-associated antigenic determinant expressed selectively on different human carcinoma cell lines, was used. Purification of the new conjugate was performed in three sequential steps: (1) by HPLC gel filtration on TSK G3000SW to remove the unconjugated ricin: (2) by affinity chromatography on Affi-Gel Blue to separate the free antibody from the conjugate and (3) by affinity chromatography on Sepharose 6B to separate the galactose-binding IT from the non-binding moiety. The cytotoxicity of the blocked and non-blocked thioether-linked IT was compared with that of classical ricin-antibody IT conjugated via SPDP and that of ricin A chain IT. The comparison was made on two different target cell lines (KATO III human gastric carcinoma and HT-29 human colorectal carcinoma) versus two control cell lines (HL-60 promyelocytic pre-leukaemic and COLO38 melanoma). The results showed that the blocked thioether IT displayed a more selective toxicity to target cells than the non-blocked IT and was much more potent than the ricin A chain conjugate.

Modification of the binding site(s) of lectins by an affinity column carrying an activated galactose-terminated ligand

An affinity column approach is described, aimed at the modification of the galactose binding site(s) of ricin in an effort to block the binding of ricin to cells. The affinity column was prepared by linking N-(2'-mercaptoethyl)lactamine to pyridyldithio-activated polyacrylamide heads. The linker between the ligand and the solid support thus contained a disulfide bond and an unmodified terminal galactose moiety. The amino group of the ligand was allowed to react with the bifunctional cross-linking reagent 2,4-dichloro-6-methoxytriazine. The lectin was then allowed to bind to the galactose functions on the activated column at pH 7.0, prior to raising the pH to 8.6 to initiate the cross-linking reaction between the ligand and the lectin. Lectin that was not covalently linked to the functionalized galactose residues on the column was eluted with galactose or lactose. Finally, the covalent ligand-lectin complexes were released from the solid support by reducing the disulfide bond between the ligand and the support. The affinity column was used in this way to modify the galactose binding site(s) of ricin. Upon release from the affinity column, blocked ricin was purified from unmodified ricin by affinity chromatography on columns of immobilized asialofetuin (a ligand to which ricin binds very tightly). The sulfhydryl group formed by cleavage of the ligand-ricin complex from the column was labeled with [3H]-N-ethylmaleimide to provide evidence that one blocking ligand was linked per ricin molecule. The blocked ricin and a conjugate of the blocked ricin with the monoclonal antibody J5 were toxic for cultures of Namalwa cells in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatotoxicity of immunotoxins made with saporin, a ribosome-inactivating protein from Saponaria officinalis

Immunotoxins were prepared by conjugating saporin, a ribosome-inactivating protein from Saponaria officinalis, to a monoclonal antibody against the Thy1.1 antigen, or to its F(ab')2 fragment. The immunotoxins were eight- to 16-fold more toxic to mice than free saporin. Injection of the immunotoxins induced necrosis of the liver and spleen, whereas free saporin caused necrosis of the epithelium of the kidney tubules. The cytoplasm of the hepatic parenchymal cells was affected by the immunotoxins, lesions being apparent in the rough endoplasmic reticulum and, later, in the mitochondria. These changes were associated with a reduced capacity to synthesise proteins both in the intact liver and by isolated liver microsomes. Studies of the in vivo distribution showed that 90% of the free saporin was removed from the bloodstream, mainly by the kidneys, within 10 min of injection. By contrast, the immunotoxins persisted in the blood for several hours and the only organ in which they consistently accumulated was the liver. The hepatotoxic effect of the immunotoxins was not due to their binding to liver cells via the antigen-binding sites or the Fc-piece of the antibody moiety, nor was it due to hepatic recognition of carbohydrate in the immunotoxin. It is concluded that free saporin, although capable of entering liver cells, is filtered so rapidly by the kidney that liver damage does not occur to a significant extent. Filtered saporin, however, is reabsorbed by renal tubules, whose epithelial cells are damaged. The antibody-saporin conjugate is too large to filter at the glomerulus and so has greater opportunity to penetrate into and to damage the hepatic parenchymal cell.

Redesigning nature's poisons to create anti-tumor reagents

Immunotoxins are conjugates of cell-reactive antibodies and toxins or their subunits. In this report, the chemistry, biology, pharmacokinetics, and anti-tumor effects of first generation immunotoxins; the preparation of improved second generation immunotoxins that display greater anti-tumor efficacy; and the role of genetic engineering in creating third-generation immunotoxins are discussed.

Mutations in diphtheria toxin separate binding from entry and amplify immunotoxin selectivity

Monoclonal antibodies linked to toxic proteins (immunotoxins) can selectively kill some tumor cells in vitro and in vivo. However, reagents that combine the full potency of the native toxins with the high degree of cell type selectivity of monoclonal antibodies have not previously been designed. Two heretofore inseparable activities on one polypeptide chain of diphtheria toxin and ricin account for the failure to construct optimal reagents. The B chains (i) facilitate entry of the A chain to the cytosol, which allows immunotoxins to efficiently kill target cells, and (ii) bind to receptors present on most cells, which imparts to immunotoxins a large degree of non-target cell toxicity. This report identifies point mutations in the B polypeptide chain of diphtheria toxin that block binding but allow cytosol entry. Three mutants of diphtheria toxin have 1/1,000 to 1/10,000 the toxicity and 1/100 to 1/8,000 the binding activity of diphtheria toxin. Linking of either of two of the inactivated mutant toxins (CRM103, Phe508; CRM107, Phe390, Phe525) to a monoclonal antibody specific for human T cells reconstitutes full target-cell toxicity--indistinguishable from that of the native toxin linked to the same antibody--without restoring non-target cell toxicity. This separation of the entry function from the binding function generates a uniquely potent and cell type-specific immunotoxin that retains full diphtheria toxin toxicity, yet is four to five orders of magnitude less toxic than the native toxin is to nontarget cells.

Cloning and expression of recombinant, functional ricin B chain

The cDNA encoding the B chain of the plant toxin ricin has been cloned and expressed in monkey kidney COS-M6 cells. The recombinant B chain was detected by labeling the transfected cells with [35S]methionine and [35S]-cysteine and demonstrating the secretion of a protein with a Mr of 30,000-32,000 that was not present in the medium of mock-transfected COS-M6 cells. This protein was specifically immunoprecipitated by an anti-ricin or anti-B-chain antibody and the amount of recombinant B chain secreted by the COS-M6 cells was determined by a radioimmunoassay. Virtually all of the recombinant B chain formed active ricin when mixed with native A chain; it could also bind to the galactose-containing glycoprotein asialofetuin as effectively as native B chain. These results indicate that the vast majority of recombinant B chains secreted into the medium of the COS-M6 cells retain biological function.

Potent and specific killing of human malignant brain tumor cells by an anti-transferrin receptor antibody-ricin immunotoxin

Immunotoxins are hybrid molecules which combine the exquisite selectivity of monoclonal antibodies with the potent toxicity of protein toxins. An immunotoxin was constructed by linking a murine monoclonal antibody against the human transferrin receptor (TR) to the plant toxin, ricin. The cytotoxic activity of the anti-TR-ricin immunotoxin was tested in vitro and demonstrated highly potent and cell type-specific killing of cells derived from human glioblastoma, medulloblastoma, and leukemia. The anti-TR-ricin immunotoxin killed more than 50% of "target" cells at a concentration of 5.6 X 10(-13) M after an 18-hour incubation with the ionophore, monensin. This potency exceeds that of any other anti-TR immunotoxin reported in the literature. When the activity of the anti-TR-ricin immunotoxin against "target" tumor-derived cells was compared with the immunotoxin's activity against "non-target" cells, it could be predicted that a selective toxicity of anti-TR-ricin immunotoxin between tumor cells and normal brain was more than 150- to 1380-fold. Solid-phase indirect radioimmunoassay techniques were used to demonstrate significantly higher levels of TR in the glioblastoma- and medulloblastoma-derived cell lines, as well as in surgical tissue samples of medulloblastoma and glioblastoma, as compared to normal brain. Immunotoxins targeted to the TR may possess sufficient specificity to be of therapeutic importance, particularly to treat neoplastic disease of the central nervous system involving compartments (such as intrathecal, intraventricular, or cystic) where delivery of immunotoxins to tumor would not require transvascular transport.