Selective cytotoxic effects of a ricin A chain immunotoxin made with the monoclonal antibody SWA11 recognising a human small cell lung cancer antigen

Immunotoxins constructed with ribosome-inactivating proteins and their enhancers: a lethal cocktail with tumor specific efficacy

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.

The novel oncogene CD24 and its arising role in the carcinogenesis of the GI tract: from research to therapy

CD24 was first described in the early 1980s and only attributed to scattered publications, referred to as a cell surface molecule in hematopoiesis. Recently, studies are accumulating to show that CD24 conveys a function in cell-to-cell interaction and regulation of proliferation and adhesion. CD24 appears to be highly expressed in a large variety of human cancers and to contribute to the acceleration of tumor growth and metastases shedding by binding to platelet (P)-selectin, L1 and by evoking--to date unknown--intracellular signal pathways. Anti-CD24 monoclonal antibodies thus act as a promising cancer treatment as was shown in the setting of gastrointestinal cancers. Recent articles also correlate CD24 expression with the identification of 'tumor stem cells'.

Application of the primer in situ DNA synthesis (PRINS) technique to titer recombinant virus and evaluation of the efficiency of viral transduction

Titration is an important and critical step in dosing recombinant virus for gene therapy. We present a relatively fast, convenient, and sensitive method that allows for precise quantification of recombinant retrovirus. The method is based on PCR amplification of a foreign gene by the PRINS (primer in situ DNA synthesis) technique. The PRINS technique is based on the sequence-specific annealing of unlabeled oligonucleotide DNA in situ. This oligonucleotide operates as a primer for in situ chain elongation catalyzed by the Taq I polymerase. Using digoxygenin-labeled nucleotides as a substrate for chain elongation, the neo-synthetic DNA is labeled by an FITC-conjugated anti-digoxygenin antibody. To avoid the possibility of false positives, we amplified the puromycin-resistance gene, which is associated with the transgene in the same viral vector and is not normally present in mammalian cells. The retroviral titer was evaluated by counting fluorescein isothiocyanate-positive cells after PRINS labeling, while knowing the number of plated cells that were transduced with different amounts of viral supernatant. A comparable viral concentration of 1 x 10(7) infectious units/mL was found among the retroviruses.

Internalization with high targeting potential of mouse monoclonal antibody ONS-M21 recognizing human malignant glioma antigen

In order to evaluate the targeting potential of mouse monoclonal antibody ONS-M21 recognizing a human astrocytoma- and medulloblastoma-associated antigen, the internalization ability of this antibody and the selective cytotoxicity in the toxin-conjugated form were examined. Internalization assay with 125I-labeled ONS-M21 showed that about 20% of the total radioactivities was detected in the cellular fraction of human medulloblastoma cell line ONS-76 cells and that the reaction reached a plateau level in 30 min. To examine the selective delivery capacity of a high molecular substance in place of 125I, an immunotoxin was prepared with ricin A chain and ONS-M21 via disulfide bonds. A cytotoxic effect against ONS-76 cells was found with [3H]thymidine incorporation assay using the immunotoxin, but not against antigen-negative HuH-7 and SW480 cells. These results suggest that ONS-M21 could effectively deliver toxins, chemotherapeutic agents or radionuclei to malignant glioma specifically.

Immunotoxins for targeted cancer therapy

Immunotoxins constitute a new modality for the treatment of cancer, since they target cells displaying specific surface-receptors or antigens. Immunotoxins contain a ligand such as a growth factor, monoclonal antibody, or fragment of an antibody which is connected to a protein toxin. After the ligand subunit binds to the surface of the target cell, the molecule internalizes and the toxin kills the cell. Bacterial toxins which have been targeted to cancer cells include Pseudomonas exotoxin and diphtheria toxin, which are well suited to forming recombinant single-chain or double-chain fusion toxins. Plant toxins include ricin, abrin, pokeweed antiviral protein, saporin and gelonin, and have generally been connected to ligands by disulfide-bond chemistry. Immunotoxins have been produced to target hematologic malignancies and solid tumors via a wide variety of growth factor receptors and antigens. Challenges facing the clinical application of immunotoxins are discussed.

Immunotoxins recognising a new epitope on the neural cell adhesion molecule have potent cytotoxic effects against small cell lung cancer

The present study describes a comparison of two potent immunotoxins which utilise an identical targeting component, a monoclonal antibody (SEN7) specific for small cell lung cancer (SCLC), conjugated to two different effector components, blocked ricin (bR) and Pseudomonas exotoxin A (PE). SEN7 recognises a novel epitope on the neural cell adhesion molecule (NCAM) which is highly associated with SCLC. The immunotoxins SEN7-PE and SEN7-bR were selectively and potently active against a number of SCLC cell lines, of both classic and variant morphologies, inhibiting the incorporation of [3H]leucine with IC50 values ranging between 22 pM and 85 pM and between 7 pM and 62 pM for SEN7-PE and SEN7-bR respectively. Intoxication by both immunotoxins proceeded rapidly following short 2 h lag phases; the initial rates of protein synthesis inhibition occurred with t50 values of 6.5 h for SEN7-PE and 5.5 h for SEN7-bR. Monensin drastically enhanced the cytotoxic activity of the weakly active SEN7-ricin A-chain by 2,100-fold and of SEN7-bR by 80-fold but had no effect on SEN7-PE. In limiting dilution assays, four and more than 4.5 logs of clonogenic SW2 tumour cells were selectively eliminated from the cultures during continuous exposure to the immunotoxins SEN7-PE and SEN7-bR respectively, while antigen-negative cells required up to 1,000-fold more drug for a similar cell kill. SW2 cells surviving SEN7-bR treatment in the cultures did not express NCAM and consequently were not selectively killed by SEN7 immunotoxins. SW2 cells surviving continuous exposure to SEN7-PE showed no alteration in NCAM expression but were more resistant to intoxication mediated by PE. These cells were still sensitive to SEN7-bR.

A comparison of 67Cu- and 131I-labelled forms of monoclonal antibodies SEN7 and SWA20 directed against small-cell lung cancer

The intact anti-SCLC monoclonal antibody (MAb) SEN7 and its F(ab')2 were labelled with the beta-emitting isotope 67Cu. Both materials retained their biological activity in vitro as determined by the Lindmo assay. In a direct comparison of in vivo distribution in a xenograph model, 131I- and 67Cu-labelled intact SEN7 showed similar absolute tumour accumulation. Blood levels were markedly lower in the case of the 67Cu-labelled antibody, resulting in improved tumour:blood ratios which reached a maximum of 13:1 compared with only 4.5:1 for 131I-SEN7. In the case of the 67Cu-labelled F(ab')2, very high accumulation of the nuclide was observed in the kidney. Levels of radio copper in liver and spleen were also found to be significantly raised when compared with radio iodine. SWA20, a MAb which had previously failed to show any selective in vivo accumulation in tumour xenografts when labelled with radio iodine showed higher and more stable tumour accumulation when labelled with 67Cu.

Antibodies to the protein core of the small cell lung cancer workshop antigen cluster-w4 and to the leucocyte workshop antigen CD24 recognize the same short protein sequence leucine-alanine-proline

We recently described the identity of the small cell lung cancer (SCLC) cluster-w4 antigen and the human B cell differentiation marker CD24, a glycosylphosphatidylinositol (GPI)-anchored, highly glycosylated surface molecule of only 31-35 amino acids [15]. The specificities of three anti-cluster-w4 and of eleven anti-CD24 MoAbs have been investigated with respect to their binding capacity to the protein core of cluster-w4/CD24 antigen. Four overlapping peptides spanning this protein core were synthesized. MoAbs shown to bind to two overlapping peptides by antibody binding inhibition using the cluster-w4/CD24-positive SCLC cell line SW2 and by direct peptide binding detected in an ELISA were investigated in more detail. To determine the exact epitopes recognized by these MoAbs, an epitope mapping assay using peptides synthesized onto polyethylene pins was established. The three anti-cluster-w4 MoAbs SWA11, SWA21 and SWA22 and the anti-CD24 MoAbs OKB2 and ALB9 recognized the same short leucine-alanine-proline (LAP) sequence in an area without potential glycosylation sites close to the GPI anchor of the protein core of the cluster-w4/CD24 antigen.

Anti-tumor activity of a blocked ricin immunotoxin with specificity against the cluster-5A antigen associated with human small-cell lung cancer

The monoclonal antibody (MAb SEN31, a mouse IgG1 which recognizes the cluster-5a antigen on small-cell lung cancer (SCLC) cells, was used to prepare a selective and potent blocked ricin immunotoxin. In a series of experiments in vitro and in a SCLC xenograft model in nude mice, the tumor localization potential of the radiolabeled antibody SEN31 and the anti-tumor activity of the immunotoxin SEN31-bR, the non-specific binding activity of which had been greatly reduced by blocking of the galactose binding domains of the B-chain, was determined. Radiolabeling of SEN31 was performed by linking a 67Ga-labeled desferrioxamine moiety to the oligosaccharide side chains of the antibody in order to preserve the specific cell-binding activity. 67Ga-SEN31 bound to the antigenic sites on cells of the SW2 SCLC cell line, with a dissociation constant of 3.5 nM and, when injected i.v., selectively localized at the site of s.c.-growing SW2 tumor xenografts in nude mice, with a tumor-to-blood ratio of 3.5. The immunotoxin SEN31-bR was potently and selectively active against SCLC cell lines both of classic and of variant morphologies. At a concentration of 300 pM the immunotoxin selectively eliminated 4.5 logs of clonogenic tumor cells. In nude mice, SEN31-bR was cleared from the blood with biphasic kinetics following i.v. injection and maintained a stable serum level during continuous i.p. infusion. The growth of s.c. SW2 solid-tumor xenografts was delayed following a single i.v. injection or a continuous i.p. infusion, each at a non-toxic dose.

Refinement of an indirect immunotoxin assay of monoclonal antibodies recognising the human small cell lung cancer cluster 2 antigen

Monoclonal antibodies (Mabs) from the Second International Workshop on Small Cell Lung Cancer (SCLC) Antigens that recognise the cluster 2 SCLC-associated antigen mediated potent and selective cytotoxic effects in an indirect assay of immunotoxin cytotoxicity. In this assay, the NCI-H69 cell line was treated with each Mab at 4 degrees C, washed to remove unbound Mab, and then incubated at 37 degrees C in the presence of a fixed concentration, 1 x 10(-8) M, of the screening agent, sheep anti-mouse IgG-ricin A chain. The use of a fixed high concentration of screening agent led to a 300-fold overestimate of the potency of a cluster 2-directed immunotoxin, MOC-31-ricin A chain. In contrast, when the concentration of the screening agent was identical to the Mab concentration, a precise match to immunotoxin potency was obtained. MOC-31-ricin A chain selectivity inhibited the incorporation of [3H]leucine by the NCI-H69, SW2 and GLC-8 SCLC cell lines by 50% at a concentration between 3 x 10(-11) M and 3 x 10(-10) M, and by the NCI-H125 lung adenocarcinoma cell line at 7 x 10(-11) M, but exerted no selective toxic effects upon human lung and non-lung tumour cell lines lacking surface expression of the cluster 2 antigen.

Action of a CD24-specific deglycosylated ricin-A-chain immunotoxin in conventional and novel models of small-cell-lung-cancer xenograft

The therapeutic efficacy of an immunotoxin, SWAII-SPDB-dg.ricin A chain, recognizing the leukocyte-differentiation antigen CD24, was evaluated against SCLC cell lines in tissue culture and in 2 nude-mouse models. The first model used conventional s.c. solid-tumor xenografts. The second used small tumor-cell deposits established in s.c. implanted sponge matrices and allowed us to directly estimate the killing efficiency of the immunotoxin under experimentally defined conditions in vivo. It also mimics the clinical setting of disseminated tumor cells which form the basis of residual disease in SCLC. The cytotoxic potency of SWAII-SPDB-dg.ricin A chain was demonstrated in tissue culture by the inhibition of 3H-leucine incorporation and by the selective elimination of CD24-positive tumor cells in clonogenic assays. In nude mice, SWAII-SPDB-dg.ricin A chain was cleared from the blood circulation with biphasic kinetics: an initial alpha phase of 1 hr and a second beta phase of 20.5 hr. Following i.v. injection of a dose equivalent to 30% of the LD50, the immunotoxin delayed the growth of SW2 solid-tumor xenografts by 16 days. The therapeutic efficacy of SWAII-SPDB-dg.ricin A chain was further demonstrated by the selective elimination of clonogenic SW2 cells from small tumor-cell deposits established in sponge matrices. Regrowth of the solid tumors after the initial response and the clonogenic activity in the sponge-derived cell population were mediated by CD24-positive cells, excluding the selection of CD24-negative mutants during immunotoxin therapy.

An anti-mucin immunotoxin BrE-3-ricin A-chain is potently and selectively toxic to human small-cell lung cancer

Monoclonal antibodies (MAbs) known to recognize epithelial mucin or defined carbohydrate structures present on mucin molecules were screened for their ability to form cytotoxic agents with ricin A-chain active against human small-cell lung cancer (SCLC) in an indirect assay of immunotoxin cytotoxicity. Anti-X hapten and anti-Y hapten antibodies binding to a high proportion of SCLC cells mediated only weak to moderate effects on 3H-leucine incorporation in combination with the screening agent, sheep anti-mouse IgG F'ab-ricin A-chain. In contrast, the mouse MAb BrE-3, recognizing the polypeptide core of the MUCI mucin gene product, exerted potent and selective cytotoxic effects in the assay. An immunotoxin made by the direct attachment of ricin A-chain to BrE-3 was selectively toxic to SCLC cell lines in tissue culture. The cytotoxic activity of BrE-3-ricin A-chain was enhanced 100-fold in the presence of monensin but not by lysosomotropic amines or calcium antagonists. Our findings suggest that anti-mucin immunotoxins may have a therapeutic role to play in the treatment of SCLC.

Potent cytotoxic action of the immunotoxin SWA11-ricin A chain against human small cell lung cancer cell lines

The cytotoxic activity profile of an immunotoxin, SWA11-ricin A chain, recognising a cell-surface antigen associated with human small cell lung cancer (SCLC), was examined in detail using a panel of SCLC, non-SCLC and non lung tumour cell lines in tissue culture. SWA11-ricin A chain was potently and selectively active against three SCLC cell lines of both classic and variant morphologies, inhibiting the incorporation of 3H-leucine with an IC50 of 5 x 10(-11) M. At a concentration of 1 x 10(-8) M, the SWA11 immunotoxin could selectively eliminate in excess of 99.9% of clonogenic tumour cells. Intoxication proceeded rapidly following a 4 h lag phase; the initial rate of protein synthesis inhibition occurred with a t50 of 2 h and a t10 of 7 h. The cytotoxic activity of SWA11-ricin A chain was potentiated by 100-fold in the presence of the carboxylic ionophore monensin at 1 x 10(-7) M. Kinetic studies revealed that monensin enhanced the rate of protein synthesis inhibition by two-fold and eliminated the lag phase suggesting a rapid effect on either the rate or route of internalisation. Studies with SWA11 could detect no influence of monensin on the rate of antibody internalisation and a transient delay in the delivery of internalised antibody to lysosomes was observed by immunoelectron microscopy.

Potentiation of a weakly active ricin A chain immunotoxin recognizing the neural cell adhesion molecule

A ricin A chain immunotoxin, SEN36-ricin A chain, directed against the neural cell adhesion molecule (N-CAM) had no selective cytotoxic activity against three different small cell lung cancer (SCLC) cell lines in tissue culture despite expression of the target antigen on more than 98% of cells in each line detected by indirect immunofluorescence. Treatment of the SW2 SCLC cell line with suramin and interferons alpha and gamma increased the level of N-CAM expression only slightly and had no significant effect on the cytotoxic activity of the SEN36 immunotoxin. In the presence of the carboxylic ionophore monensin at a concentration of 0.1 microM, the toxicity of SEN36-ricin A chain to the SW2 cell line was enhanced by 12,000-fold. In contrast, lysosomotropic amines showed little or no potentiation of activity, suggesting that lysosomal degradation was not the major factor limiting the action of the anti-N-CAM immunotoxin. The findings of this study indicate that ricin A chain immunotoxins directed against N-CAM on SCLC are unlikely to have sufficient activity to be useful therapeutic agents in the absence of potentiating agents such as monensin, which can interfere with the normal intracellular pathways of antigen routing.

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.

Cytotoxic properties of a ricin A chain immunotoxin recognising the cluster-5A antigen associated with human small-cell lung cancer

The cytotoxic properties of a ricin A chain immunotoxin made with the mouse monoclonal antibody SWA20, recognising a family of sialoglycoprotein antigens selectively expressed by human small-cell lung cancer (SCLC), were examined using a panel of tumour cell lines in tissue culture. SWA20-ricin-A-chain was selectively toxic to the SW2, NCI-H69 and GLC-8 SCLC cell lines, inhibiting the incorporation of [3H]leucine by 50% at a concentration of 0.2-2 nM, but had no selective activity against the NCI-H23 and NCI-H125 lung adenocarcinoma or the control CEM T-lymphoblastoid cell lines. The SWA20 immunotoxin intoxicated the SW2 cell line rapidly, inhibiting [3H]leucine incorporation by 50% within 2 h compared with 0.5 h for ricin. Analysis of the effects of SWA20-ricin-A-chain on the growth of SW2 cells using a limiting-dilution clonogenic assay revealed that the immunotoxin could eliminate 95% of clonogenic malignant cells. Although SWA20-ricin-A-chain was found to be rapidly active against the majority of tumour cells, its action was limited by the presence of insensitive cells expressing low levels of the target antigen.

The role of calcium ions for the expression of ricin toxicity in cultured macrophages

Ricin toxin, which consists of two distinct polypeptide moieties, A and B chains, is cytotoxic to the cultured macrophage cell line, J774A.1. Ricin is a protein synthesis inhibitor, and incubating macrophages for 4 hours with ricin (1 pM to 10 nM) in standard medium containing calcium and magnesium inhibited 3H-leucine incorporation into protein (97%, at 1 nM ricin). However, in Ca(2+)-free medium, protein synthesis was inhibited only 19%. EGTA pretreatment (to deplete intracellular calcium) also partly protected cells from protein synthesis inhibition, in spite of added calcium (2 mM) in the incubation medium. Decreased toxicity in the absence of extracellular calcium resulted from decreased toxin binding. Adding or deleting Mg2+ did not affect protein synthesis or binding of 125I-ricin in cultured macrophages. We conclude that calcium is required for ricin to exert its inhibitory effect on protein synthesis in cultured macrophages.

Monoclonal antibodies recognising the cluster 2 antigen associated with human small cell lung cancer mediate the toxic effects of ricin A chain in an indirect assay of immunotoxin cytotoxicity

Monoclonal antibodies (Mabs) submitted to the Second International Workshop on Small Cell Lung Cancer Antigens were screened for their ability to mediate the toxic effects of ricin A chain against the NCI-H69 cell line in an indirect assay of immunotoxin cytotoxicity. Cluster 1 Mabs, recognising the neural cell adhesion molecule, mediated little or no cytotoxic effect in combination with screening agent, ricin A chain linked to an antibody Fab' fragment recognising either mouse or rat Mabs. In contrast, cluster 2 Mabs, recognising an epithelial tumour-associated antigen, generally mediated potent cytotoxic effects with the screening agent, inhibiting the incorporation of 3H-leucine by NCI-H69 cells by between 90% and 99%. Measurements of Mab binding to the NCI-H69 cell line by indirect immunofluorescence and flow cytometry indicated that the cluster 2 Mabs generally bound in higher amounts than the cluster 1 Mabs suggesting that the cluster 1 Mabs were ineffective in the screen because they did not bind to the cells in sufficient amounts. However, Mabs recognising antigens other than cluster 1 bound to NCI-H69 cells in amounts similar to those of the cluster 2 Mabs yet did not mediate potent cytotoxic effects in the indirect assay suggesting that the cluster 2 antigen may be internalised in a fashion favouring the delivery of ricin A chain to the cytosol.