Comparative biochemical, cytotoxic and pharmacokinetic properties of immunotoxins made with native ricin A chain, ricin A1 chain and recombinant ricin A chain

Sepharose-unbinding ricin E as a source for ricin A chain immunotoxin

To evaluate the Sepharose-unbinding ricin E as a preference source material for ricin A chain (RTA) in immunotoxin studies, RTA of ricin E (RTA(E)) was characterized and compared with RTA of the Sepharose-binding ricin D (RTA(D)). RTA(E) and RTA(D) were separated into two subunits of A(1) and A(2) by capillary electrophoresis. The isoelectric points of A(1) and A(2) subunits were determined to be 7.6 and 7.4, respectively, for RTA(E), while they were 7.4 and 7.3, respectively, for RTA(D). The molecular masses of A(1) and A(2) isomers determined by the matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry were 31059 and 32266 Da, respectively, for RTA(E), while they were 30892 and 32179 Da, respectively, for RTA(D). There were no significant differences in the cell surface affinity and cytotoxicity between RTA(E) and RTA(D). Anti-CD4-RTA(E) immunotoxin was prepared by conjugating RTA(E) with anti-CD4 monoclonal antibody using a heterobifunctional crosslinker, 4-succinimidyl-oxycarbonyl-alpha-methyl-alpha-(2-pyridyldithio) toluene. Anti-CD4-RTA(E) immunotoxin showed comparable cytotoxic effects to anti-CD4-RTA(D) immunotoxin to antigen-positive CEM cells in vitro. It is concluded that RTA(E) from ricin E is one of different variants of RTA(D) and may be used as a preference source material of RTA in immunotoxin studies.

Stability and cytotoxicity of Fab-ricin A immunotoxins prepared with water soluble long chain heterobifunctional crosslinking agents

The effects of the hindered and non-hindered water soluble long-chain disulfide bonds on the stability and cytotoxicity of the ricin A chain (RTA) immunotoxin were examined. The RTA immunotoxins were prepared with the Fab fragments of anti-common acute lymphoblastic leukemia antigen (CALLA) monoclonal antibody (Fab-RTA) using sulfosuccinimidyl-6-[(-methyl-(-(2-pyridyldithio)toluamido]hexanoate (S-LC-SMPT) and sulfosuccinimidyl-6-[3-(2-pyridyldithio)-propionamido]hexanoate (S-LC-SPDP). The prepared Fab-RTA immunotoxins were evaluated for their conjugation yield, immunoreactivity, thermal and disulfide bond stability and cytotoxicity. The conjugation yield of the Fab-RTA immunotoxin from the water soluble long chain crosslinking agents, S-LC-SMPT and S-LC-SPDP, were comparable. Both Fab-RTA immunotoxins exhibited a similar immunoreactivity and thermal stability in aqueous solution. However, S-LC-SMPT -mediated Fab-RTA, sterically hindered, showed an enhanced disulfide bond stability in vitro over S-LC-SPDP mediated one. In the cytotoxicity against antigenic cell Daudi, the S-LC-SMPT -mediated RTA immunotoxin maintained a comparable cytotoxicity, compared with S-LC-SPDP mediated Fab-RTA immunotoxin.

Use of monoclonal antibodies for the diagnosis and treatment of bladder cancer

Although the management of cancer by exploiting properties distinguishing neoplastic and normal cells has always been an attractive concept, it was the development of hybridoma technology and the resulting tumor-associated monoclonal antibodies (MAbs) that offered new prospects for this strategy. Twenty years later, some of the applications of MAbs in oncology are now part of the everyday diagnosis and treatment, while others are the subject of intensive investigation. We reviewed the current applications of MAbs in the diagnosis and treatment of bladder cancer and attempted to put the issue into perspective, with particular presentation of their therapeutic potential.

Rationale for the use of immunotoxins in the treatment of HIV-infected humans

The first step in the replication of human immunodeficiency virus (HIV) is selective binding of the envelope glycoprotein (gp120) to CD4 receptors on T cells or macrophages. After penetration in these cells, the genome of the virus is integrated in the human genome. HIV-infection causes depletion of CD4-positive cells resulting in a severe immunosuppression. It is believed that eliminating HIV-infected cells is crucial in limiting further reduction of CD4-positive cells and thus, preventing disease progression. The most commonly used drugs, such as zidovudine (AZT), appeared to be not completely effective. Therefore many investigators are searching for alternative treatment modalities. The use of immunotoxins (ITs) to eliminate HIV-infected cells is discussed. ITs are chimeric molecules in which cell-binding ligands are coupled to toxins and can specifically eliminate undesired cells. The cell-binding carriers of anti-HIV ITs have been directed against different regions of the HIV envelope glycoprotein (gp120 and gp41) and surface antigens (e.g CD4, CD25). The ITs have been composed of different ribosome-inactivating proteins (RIPs) like pokeweed antiviral protein (PAP), Pseudomonas exotoxin (PE), Diphtheria toxin (DT), or ricin. In in vitro studies, several of these ITs have been shown to be effective and specific in killing acute and persistently HIV-infected cells. The ITs were effective at concentrations (ID50 range from 10(-9) M to 10(-12) M) that were not toxic to uninfected cells or cells without the antigen. The IT CD4(178)PE40, a fusion protein directed against the CD4 binding site of gp120, has been investigated in two in vivo trials. The results were disappointing considering the antiviral activity in vitro. This was thought to be due to the rapid clearance of the IT and the differential resistance of clinical HIV isolates. Use of a panel of ITs is likely to be more effective because multiple approaches cover the intrinsic variability of HIV and the presence of IT-resistant or latently infected cells, as well as the blocking presence of neutralizing anti-HIV antibodies and the immunogenicity of most ITs. It may be possible to control the virus completely with a panel of ITs in combination with other antiviral or immunosuppressive agents such as RT inhibitors (e.g AZT), interferon alpha, or cyclosporine. More research will be necessary to develop such a combined therapy.

Isolation and partial characterisation of highly toxic lectins from Abrus pulchellus seeds

The seeds of Abrus pulchellus, sub-specie tenuiflorus, belonging to the Leguminosae, subfamily Papilionoideae contain highly toxic lectins exhibiting specificity for galactose and galactose-containing structures. The toxins which agglutinate rabbit erythrocytes, present a highly toxic activity in vivo when injected in the peritoneal cavity of mice (LD50=31 microg x kg(-1)) or when tested with the microcrustacean Arthemia salina (LD50=3.5 microg x ml(-1)). The active fraction was purified in a single step, by affinity chromatography on a Sepharose-4B column. The purified toxins migrated as two single bands of Mr 63000 and 61500 Da (SDS-PAGE) and Mr 31500 and 29000 Da (SDS-PAGE with 2-mercaptoethanol), respectively, suggesting the presence of disulphide-bridge interchains as occurs in other plant toxins. The antibodies anti-A. pulchellus toxins did not recognize ricin preparation and only partial identity was observed to A. precatorius toxic lectins prepared in a similar way to ricin and A. pulchellus toxins.

Mannose receptor dependent uptake of ricin A1 and A2 chains by macrophages

The ricin A chain, the toxic subunit of ricin, consists of two forms which differ in sugar content. The major component A1 contains one high mannose chain while the minor component A2 contains an additional high mannose chain. Endocytosis of this toxin occurs in macrophages via the mannose receptor. To study the role of the sugar residues in ricin A chain cytotoxicity, we have purified the two forms by ion-exchange chromatography. The uptake of A1 and A2 by a macrophage cell line was concentration and time dependent. The total amount of A2 internalized was approximately twice the amount of A1, indicating a higher affinity of A2 for the mannose receptor. Ricin A2 was four times more toxic to macrophages than A1, in agreement with the higher affinity of A2 compared to the A1. These experiments suggest that the high mannose chains on the A chain promote mannose-receptor-mediated endocytosis by providing the initial binding to the cell surface. Once the toxin is accumulated inside the cell however, the carbohydrates do not seem to influence intracellular transport and/or translocation of the ricin A chain into the cytoplasm.

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.

Pharmacokinetic studies in mice with ICI D0490, a novel recombinant ricin A-chain immunotoxin

A colorectal tumour-directed immunotoxin, ICI D0490, has been constructed by linking recombinant ricin A-chain to C242, a mouse monoclonal antibody, by means of a methyl-hindered disulphide bond. Recombinant ricin A-chain and a hindered disulphide linker were anticipated to confer favourable pharmacokinetic properties on the immunotoxin. The pharmacokinetics of ICI D0490 have been studied in mice following single and repeated i.v. administration. The concentrations of intact immunotoxin in mouse plasma at various time intervals after injection for up to 96 h were measured by a solid-phase enzyme-linked immunosorbent assay (ELISA) and the data analysed by both model-dependent (two compartment) and model-independent methods. Following a single i.v. bolus dose of 2.5 mg kg-1 (50% of the LD10 in mice), the clearance of ICI D0490 from the plasma was extremely slow; 34 microliters min-1 kg-1, t1/2 beta = 33 h. Model-dependent and model-independent analyses gave comparable results with steady state volumes of distribution of 93 and 69 ml kg-1, respectively. The two compartment analysis gave an initial volume of distribution (63 ml kg-1) which is consistent with the predicted plasma volume. Over the dose range 0.05-5 mg ICI D0490 kg-1, plasma levels at 2 and 24 h were linearly related to dose (r > or = 0.98) indicating that at doses up to 5 mg ICI D0490 kg-1 clearance does not appear to have a saturable component. Repeated doses of ICI D0490 (1 mg kg-1 day x 5) did not lead to drug accumulation. These studies demonstrate that ICI D0490 has excellent in vivo stability and persistence which, in conjunction with activity and toxicity data, identify ICI D0490 as a promising candidate for clinical evaluation in the treatment of colorectal cancer.

Rationale for the clinical use of immunotoxins: monoclonal antibodies conjugated to ribosome-inactivating proteins

The use of chemotherapeutic drugs in combination with bone marrow transplantation to treat cancer patients has markedly improved the disease-free survival and cure rate. Part of the tumor cells, however, can escape from therapy due to resistance. Tumor-specific delivery of toxins that do not interfere with conventional drugs and are not cell cycle dependent seems to be a reasonable approach to overcome this problem. Natural ribosome-inhibiting-proteins (RIPs) from plants, bacteria and fungi which are extremely toxic inhibitors of protein synthesis are isolated and coupled to monoclonal antibodies (MoAbs) and receptor-specific ligands, immunotoxins (ITs), to fulfil this purpose. ITs are very suitable to eliminate malignant cells in vitro and in vivo. RIPs contain two or three active sites: a binding site which can be absent in a part of the RIPs and can be replaced by the MoAb; a translocation site that facilitates transport into the cytosol after internalization, and a cytotoxic site that enzymatically inhibits protein synthesis. Binding site containing toxins induce strong nonspecific cytotoxicity when coupled to MoAbs. Recent developments in recombinant DNA techniques enable genetic elimination of the binding site to reduce nonspecific cytotoxicity of these toxins. In this review the structures and mechanisms of action of RIPs as well as factors that influence cytotoxicity of immunotoxins are discussed. Moreover the problems dealing with in vivo application of ITs such as blood clearance by instability of the IT and hepatic entrapment, and production of antibodies directed against MoAb and toxin are reviewed.

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)

Molecular and biological properties of an abrin A chain immunotoxin designed for therapy of human small cell lung cancer

An immunotoxin (IT) comprising abrin A chain attached to the mouse monoclonal antibody SWA11, recognising a cell surface antigen highly associated with human small cell lung cancer (SCLC), was synthesised using a hindered disulphide crosslinker, N-succinimidyl 3-(2-pyridyldithio) butyrate (SPDB), and purified by Blue Sepharose CL-6B affinity chromatography. The IT preparation contained monomeric conjugate, composed of one abrin A chain molecule linked to one SWA11 molecule, and was free from unconjugated A chain or antibody. The IT fully retained the cell-binding capacity of the antibody component and the ribosome-inactivating activity of the A chain. In cytotoxicity assays using the SW2 SCLC cell line in tissue culture, SWA11-SPDB-abrin A chain inhibited the incorporation of 3H-leucine by 50% at a concentration of 10 pM and by 99% at a concentration of 1 nM. The anti-tumour efficacy of the IT was tested in nude mice bearing established s.c. solid SW2 tumour xenografts. A single i.v. injection of SWA11-SPDB-abrin A chain at a non-toxic dose induced a significant 7 to 10 day growth delay that could not be matched by administration of equivalent doses of either unconjugated SWA11 or abrin A chain alone. The results of this study indicate that the antigen recognised by SWA11 is an effective target for therapy of SCLC with A chain ITs in vivo.

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.