Prospects for immunotoxin therapy in cancer

Targeted Delivery of Endosomal Escape Peptides to Enhance Immunotoxin Potency and Anti-cancer Efficacy

This work describes use of anti-carcinoembryonic antigen antibodies (10H6, T84.66) for targeted delivery of an endosomal escape peptide (H6CM18) and gelonin, a type I ribosome inactivating protein. The viability of colorectal cancer cells (LS174T, LoVo) was assessed following treatment with gelonin or gelonin immunotoxins, with or without co-treatment with T84.66-H6CM18. Fluorescent microscopy was used to visualize the escape of immunoconjugates from endosomes of treated cells, and efficacy and toxicity were assessed in vivo in xenograft tumor-bearing mice following single- and multiple-dose regimens. Application of 25 pM T84.66-H6CM18 combined with T84.66-gelonin increased gelonin potency by ~ 1,000-fold and by ~ 6,000-fold in LS174T and LoVo cells. Intravenous 10H6-gelonin at 1.0 mg/kg was well tolerated by LS174T tumor-bearing mice, while 10 and 25 mg/kg doses led to signs of toxicity. Single-dose administration of PBS, gelonin conjugated to T84.66 or 10H6, T84.66-H6CM18, or gelonin immunotoxins co-administered with T84.66-H6CM18 were evaluated. The combinations of T84.66-gelonin + 1.0 mg/kg T84.66-H6CM18 and 10H6-gelonin + 0.1 mg/kg T84.66-H6CM18 led to significant delays in LS174T growth. Use of a multiple-dose regimen allowed further anti-tumor effects, significantly extending median survival time by 33% and by 69%, for mice receiving 1 mg/kg 10H6-gelonin + 0.1 mg/kg T84.66-H6CM18 (p = 0.0072) and 1 mg/kg 10H6-gelonin + 1 mg/kg T84.66-H6CM18 (p = 0.0017). Combined administration of gelonin immunoconjugates with antibody-targeted endosomal escape peptides increased the delivery of gelonin to the cytoplasm of targeted cells, increased gelonin cell killing in vitro by 1,000-6,000 fold, and significantly increased in vivo efficacy.

Targeting a Cancer-Specific Epitope of the Epidermal Growth Factor Receptor in Triple-Negative Breast Cancer

BACKGROUND

Triple-negative breast cancers (TNBCs) are typically more aggressive and result in poorer outcomes than other breast cancers because treatment options are limited due to lack of hormone receptors or amplified human epidermal growth factor receptor 2 (HER2). Many TNBCs overexpress the epidermal growth factor receptor (EGFR) or manifest amplification of theEGFRgene, supporting EGFR as a therapeutic target. While EGFR-directed small molecule inhibitors have shown limited effectiveness in clinical settings, use of EGFR as a mechanism of delivering enzymatic cytotoxins to TNBC has not been demonstrated.

METHODS

Using the single-chain variable fragment (scFv) of the 806 antibody that binds only cells with overexpressed, misfolded, or mutant variants of the EGFR, a recombinant immunotoxin was engineered through gene fusion withPseudomonas aeruginosaExotoxin A (806-PE38). The potency of 806-PE38 on reducing TNBC cell growth in vitro and in xenograft models (n ≥ 6) was examined for six TNBC cell lines. All statistical tests were two-sided.

RESULTS

806-PE38 statistically significantly reduced the viability of all tested TNBC lines, with IC50values below 10 ng/mL for three of six cell lines, while not affecting cells with wild-type EGFR (IC50>300 ng/mL). Systemic treatments with 806-PE38 vs vehicle resulted in statistically significantly reduced tumor burdens (806-PE38 mean = 128 mm(3)[SD = 46 mm(3)] vs vehicle mean = 749 mm(3)[SD = 395 mm(3)], P = .001) and increased median survival (806-PE38 median = 82 days vs vehicle median = 50 days,P= .01) in a MDA-MB-468 TNBC mouse xenograft. Deletion of the catalytic residue eliminated both cytotoxic activity in vitro and the reduction in tumor burden and survival (P= .52).

CONCLUSIONS

These data support the further development of the 806-PE38 immunotoxin as a therapeutic agent for the treatment of patients with EGFR-positive TNBC. Follow-up experiments with combination therapies will be attempted to achieve full remissions.

Targeted delivery of saporin toxin by monoclonal antibody to the transcobalamin receptor, TCblR/CD320

Cellular uptake of cobalamin (Cbl) occurs by endocytosis of transcobalamin saturated with Cbl by the transcobalamin receptor (TCblR/CD320). The cell cycle-associated overexpression of this receptor in many cancer cells provides a suitable target for delivering chemotherapeutic drugs and cytotoxic molecules to these cells while minimizing the effect on the normal cell population. We have used monoclonal antibodies to the extracellular domain of TCblR to deliver saporin-conjugated secondary antibody to various cell lines propagating in culture. A molar ratio of 2.5:10 nmol/L of primary:secondary antibody concentration was identified as the lowest concentration needed to produce the optimum cytotoxic effect. The effect was more pronounced when cells were seeded at lower density, suggesting lack of cell division in a fraction of the cells at higher density as the likely explanation. Cells in suspension culture, such as K562 and U266 cells, were more severely affected than adherent cultures, such as SW48 and KB cells. This differential effect of the anti-TCblR-saporin antibody conjugate and the ability of an anti-TCblR antibody to target proliferating cells were further evident by the virtual lack of any effect on primary skin fibroblasts and minimal effect on bone marrow cells. These results indicate that preferential targeting of some cancer cells could be accomplished through the TCblR.

Targeted brain delivery of AZT via transferrin anchored pegylated albumin nanoparticles

Hydrophilic drugs/peptides have poor cross Blood-brain permeability. Various drug delivery systems with diverse surfacial characteristics have been reported for effective translocation of drugs across Blood-brain barrier. In present investigation, the potential of engineered albumin nanoparticles was evaluated for brain specific delivery after intravenous administration. Long circulatory PEGylated albumin nanoparticles encapsulating water-soluble antiviral drug azidothymidine (AZT) were prepared by ultra-emulsification method using chemical cross-linking by glutaraldehyde. Surface of the PEGylated nanoparticles was modified by anchoring transferrin as a ligand for brain targeting. Nanoparticles were characterized for their size, polydispersity, surfacial charge, drug loading and in vitro drug release. Fluorescence studies revealed the enhanced uptake of transferrin-anchored nanoparticles in the brain tissues when compared with unmodified nanoparticles. In vivo evaluation was carried out on albino rats to evaluate tissue distribution of engineered nanoparticles after intravenous administration. A significant ((*)P < 0.01) enhancement of brain localization of AZT was observed for transferrin anchored pegylated albumin nanopariticles (Tf-PEG-NPs). Hence, the specific role of transferrin ligand on nanoparticles for brain targeting was confirmed.

Advances in monoclonal antibody therapy of cancer

Tumor-associated antigens can be seen as unique targets for the delivery of anti-cancer therapy. Monoclonal antibodies directed at such antigens are increasingly being seen as important biologic reagents that will complement the group of existing cytotoxic drugs. This report briefly overviews recent advances in the field of monoclonal antibody therapy of cancer and provides insight regarding the promises and limitations of this novel therapeutic approach.

Effectiveness of combinations of bispecific antibodies for delivering saporin to human acute T-cell lymphoblastic leukaemia cell lines via CD7 and CD38 as cellular target molecules

We have investigated the effectiveness of three different F(ab' gamma)2 bispecific antibodies (BsAb) for delivering the ribosome inactivating protein (RIP) saporin via the CD7 or CD38 cell surface molecules to the human T-ALL cell lines HSB-2 and HPB-ALL. Inhibition of 3H-leucine uptake by target cells was used as the parameter of cellular cytotoxicity. Used singly against HSB-2 cells in the presence of varied concentrations of saporin, an anti-CD7 BsAb, (HB2 x DB7-18) and an anti-CD38 BsAb (OKT10 x RabSap), gave 435- and 286-fold increases in saporin toxicity, respectively. For HPB-ALL cells the anti-CD7 BsAb performed poorly giving only an eight-fold increase in toxicity whilst on the same cell line the anti-CD38 BsAb was highly potent giving an 80,000-fold increase in saporin toxicity. A combination of both BsAb used together against HSB-2 cells was ten times more effective, than the best single BsAb HB2 x DB7-18 used alone. Kinetic studies conducted with HSB-2 cells revealed that the BsAb combination also gave an increased rate of protein synthesis inactivation in comparison to either BsAb used alone. These investigations clearly demonstrate a synergistic action when both BsAb are used in combination to target saporin against CD7 and CD38 expressed on the surface of the HSB-2 cell line.

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.

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.

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.

Purification and properties of new ribosome-inactivating proteins with RNA N-glycosidase activity

Ribosome-inactivating proteins (RIPs) similar to those already known (Stirpe & Barbieri (1986) FEBS Lett. 195, 1-8) were purified from the seeds of Asparagus officinalis (two proteins, asparin 1 and 2), of Citrullus colocynthis (two proteins, colocin 1 and 2), of Lychnis chalcedonica (lychnin) and of Manihot palmata (mapalmin), from the roots of Phytolacca americana (pokeweed antiviral protein from roots, PAP-R) and from the leaves of Bryonia dioica (bryodin-L). The two latter proteins can be considered as isoforms, respectively, of previously purified PAP, from the leaves of P. americana, and of bryodin-R, from the roots of B. dioica. All proteins have an Mr at approx, 30,000, and an alkaline isoelectric point. Bryodin-L, colocins, lychnin and mapalmin are glycoproteins. All RIPs inhibit protein synthesis by a rabbit reticulocyte lysate and phenylalanine polymerization by isolated ribosomes and alter rRNA in a similar manner as the A-chain of ricin and related toxins (Endo et al. (1987) J. Biol. Chem. 262, 5908-5912).

Toxicity of, and histological lesions caused by, ribosome-inactivating proteins, their IgG-conjugates, and their homopolymers

The toxicity of, and the lesions brought about by, several ribosome-inactivating proteins (bryodin, gelonin, momordin, pokeweed antiviral protein from seeds, saporin 6, trichokirin and momorcochin-S), either native, or conjugated to bovine IgG, or polymerized, were studied in the mouse. Severe necrotic liver damage was the main lesion present in animals receiving lethal doses of the proteins. The toxicity of ribosome-inactivating proteins increased after conjugation to IgG or homopolymerization. The toxicity of conjugates to mouse was not predictable from the inhibitory activity on cell-free protein synthesis.

Pharmacokinetics of a mouse/human chimeric monoclonal antibody (C-17-1A) in metastatic adenocarcinoma patients

The pharmacokinetic characteristics of a mouse/human chimeric monoclonal antibody (C-17-1A) were determined in 10 patients with metastatic adenocarcinoma following the administration of either 10-mg or 40-mg infusions as a single or multiple dose. The administration of single 10-mg (n = 5) and 40-mg (n = 5) doses infused over 1 hr resulted in mean apparent steady-state distribution volumes of 4.13 +/- 0.97 and 5.16 +/- 1.92 liters, respectively, indicating that C-17-1A appears to distribute throughout the vascular compartment and into limited extracellular fluid volume. The disposition of C-17-1A was adequately characterized using a two-compartment open model with mean distribution half-lives of 15.8 and 18.5 hr and mean elimination half-lives of 90.0 and 97.6 hr for the 10- and 40-mg groups, respectively. A linear relationship was observed between AUC and dose (micrograms/kg). The clearance of C-17-1A was correlated linearly with total Ig, IgG, and tumor size. Multiple administration of either 10-mg (n = 3) or 40-mg (n = 3) doses of C-17-1A infused over 1 hr every 14 days for a total of three doses resulted in superimposable mean serum concentration versus time data and consistent mean pharmacokinetic characteristics. These data indicate that C-17-1A exhibits linear, nonsaturable distribution and elimination characteristics in man over the dose range studied (i.e., 130 to 880 micrograms/kg). The multiple-dose pharmacokinetics of C-17-1A were predictable, indicating a lack of an antibody response to C-17-1A over a period of 42 days.(ABSTRACT TRUNCATED AT 250 WORDS)

An anti-CD5 immunotoxin for chronic lymphocytic leukemia: enhancement of cytotoxicity with human serum albumin-monensin

Five patients with B-cell chronic lymphocytic leukemia (B-CLL) were treated with 6 courses of the anti-CD5 immunotoxin T101-ricin A chain (T101-RTA). Each course consisted of 8 bi-weekly infusions of T101-RTA (7 or 14 mg/m2). The immunotoxin was well tolerated in all cases with no major toxicities. Though saturation of circulating leukemic cell-associated target antigen was demonstrated by FACS analysis in all patients, no intact immunotoxin was detected in bone-marrow or lymph-node aspirates. Pharmacokinetic studies revealed rapid clearance of T101-RTA, with a half-life of 43 min. None of the patients developed detectable titers of antibody against either T101 murine antibody or ricin A chain. Clinical response was limited to a rapid and transient fall in WBC count lasting less than 24 hr, most likely secondary to the antibody portion of the conjugate. In vitro, fresh B-CLL cells were resistant to T101-RTA at concentrations up to 10(-8)M, while fresh malignant T-cells with a 10-fold increase in expression of CD5 antigen were sensitive. In the presence of the enhancing agent human serum albumin-monensin, fresh B-CLL cells were sensitive to T101-RTA, with an ID50 more than 2 logs below the maximal concentration of immunotoxin achieved in vivo. We conclude that T101-RTA is a potentially useful agent in the treatment of T-cell leukemias. In the presence of HSA-monensin, this spectrum of activity may be extended to B-CLL.

Immunological aspects of toxicology: premises not promises

The immune system is gaining increasing attention as a target of toxicant action. Many compounds, including metals, drugs, and pesticides, are able to alter immune functions. Additionally, the ability of toxicants to experimentally alter resistance to a variety of bacterial, viral, and tumor challenges is suggestive for the possible involvement of toxicants in morbidity and mortality. However, the implications of immunotoxicology as a subject of study may not be fully realized in some laboratories due to unfamiliarities into the workings of immunity by toxicologists. In an attempt to bridge this toxicology-immunology gap, this paper presents the major concepts of the immune system by reviewing specific examples of immune responses and their environmental interactions. Data from selected toxins are then used to illustrate how these responses may be altered.

Selective killing of squamous carcinoma cells by an immunotoxin that recognizes the EGF receptor

We have conjugated a murine monoclonal antibody (B4G7) against the human epidermal growth factor (EGF) receptor to gelonin, a 60S ribosome inactivating protein, via N-succinimidyl-3-(2-pyridyldithio)propionate (SPDP) and 2-iminothiolane. The B4G7-gelonin conjugate bound to the cell surface in proportion to the number of EGF receptors and competed with B4G7 antibody for binding to EGF receptors. The conjugate killed EGF receptor-hyperproducing squamous carcinoma cells (A431, NA, Ca9-22, TE5), and to some extent, human fibroblasts (HFO). It did not kill EGF receptor-deficient small-cell lung cancer cells (H69) and mouse fibroblasts (Swiss/3T3). Free B4G7, gelonin or a mixture of B4G7 and gelonin did not kill A431 cells. The number of EGF receptors was correlated to cytotoxicity at 10(-8) M of the conjugate, and the data were fitted to the regression equation: y = -35.83 log x +233.4 (correlation coefficient = -0.9995). These results suggest that the B4G7-gelonin conjugate may be a useful weapon for targeting therapy to squamous-cell carcinomas.

The Aspergillus toxin restriction is a suitable cytotoxic agent for generation of immunoconjugates with monoclonal antibodies directed against human carcinoma cells

The protein toxin restriction, isolated from the mould Aspergillus restrictus, inactivates protein synthesis in eukaryotic cells by blocking the ribosome elongation cycle. This protein acts as a specific nuclease that cuts off a small fragment from the 28-S rRNA. Biochemical and biological characterization of this toxin indicated that it is a non-glycosylated polypeptide of Mr 16836, exhibiting in cell-free systems a protein synthesis inhibition capacity similar to that of the ricin A chain. This polypeptide seemed unable to penetrate most of the cancer cell lines tested, as measured by its low in vitro cytotoxicity. In addition in vivo studies in BALB/c mice demonstrated that restriction toxicity was very low and that in rabbits, after intravenous injection 15% of the toxin was still present in the blood stream 24 h later. After derivatization with N-succinimidyl 3-(2-pyridyldithio)propionate and reduction by dithiothreitol, the restrictocin maintained its protein synthesis inhibitory activity, as assayed in a cell-free system. This derivatized toxin was then coupled to monoclonal antibodies (MBr1, MLuC1, MLuC2, MOv17, MOv18, MOv19) which exhibited a restricted spectrum of reactivity against human carcinomas. The biochemical and biological characterization of the immunoconjugates indicated that (a) when restrictocin was coupled to monoclonal antibodies with an average molar ratio of about 2, the immunoconjugates maintained the binding activity of the antibody and protein synthesis inhibition activity of the toxin; (b) four immunoconjugates were tested for cytotoxicity and three of them obtained with the MBr1, MLuC1 and MOv17 monoclonal antibodies exhibited a good level of cytotoxicity for relevant target cells and low or no toxicity for the irrelevant cell lines. The MLuC2 monoclonal antibody which gave rise to a completely ineffective immunoconjugate, induced internalization of less than one tenth of the antigenic sites whereas the MBr1, MLuC1 and MOv17 monoclonal antibodies exhibited about one third of the antigenic sites interanalized. From these data it is concluded that, providing an appropriate target antigen and coupling procedure are selected, restrictocin can be considered a suitable toxin for immunoconjugate generation.

In vivo efficacy of monoclonal antibody-drug conjugates of three different subisotypes which bind the human tumor-associated antigen defined by the KS1/4 monoclonal antibody

A panel of three hybridomas has been isolated each of which secretes a single species of monoclonal antibody (MoAb) directed against the KS1/4 tumor-associated antigen originally described by Varki et al. (Cancer Res 44: 681, 1984). These MoAbs were designated L1-(IgG2b), L2-(IgG1), and L4-(IgG2a)KS. Binding specificity, immuno-precipitation, and competitive binding analyses indicated that these MoAbs each recognize the same epitope of the KS1/4 antigen. The immunoprecipitation studies indicated that the MoAbs recognized a major antigenic component of 42 kDa and a minor component of 35 kDa. The L-KS antibodies were evaluated as MoAb-drug conjugates against a variety of human tumor targets grown in vivo as nude mouse xenografts. The MoAb-drug conjugates were constructed using protein-A-purified MoAbs conjugated to 4-desacetyl-vinblastine-3-carbohydrazide. Efficacy was determined using various dosing protocols on 2-14 day established tumors of lung, pharynx, colon, and skin origin. Control experiments included the use of dual-flank antigen-positive and negative tumors, free MoAbs, free drug, and mixtures of MoAbs and drug. These studies indicated that significant tumor growth suppression and actual tumor regression could be achieved by the MoAb-vinca conjugates and that this activity was antigen-mediated. The drug conjugates were more efficacious than free drug or free MoAbs administered either singly or in combination with each other.

Pharmacokinetic studies of radiolabelled rat monoclonal antibodies recognising syngeneic sarcoma antigens. I. Comparison of IgG subclasses

The object of our current investigations is to explore the potential of antibodies for localisation and treatment of disseminated disease, using as a model rat monoclonal antibodies (mAbs) raised against syngeneic tumour-specific antigens. As part of this study, antibodies of differing isotypes with specificity for either HSN or MC24 sarcoma were labelled with 125iodine and injected intravenously into normal rats or those bearing paired tumours in contralateral flanks. The blood clearance rates of the radiolabelled antibodies were found to be influenced by immunoglobulin subclass (IgG2b greater than IgG2a greater than IgG1) and to be increased non-specifically by the presence of growing tumours. The tumour and normal tissue distributions of the antibodies tested were also found to vary according to their apparent degree of interaction with host Fc-receptor-bearing cells, to the extent that tumour specificity in vitro was not necessarily reflected in selectivity of localisation in vivo. Three IgG2b monoclonal antibodies showed preferential uptake in the spleens of syngeneic rats and non-specific accumulation in tumours. This effect was not observed with antibodies of IgG2a or IgG1 subclass, and was abolished by the use of IgG2b F(ab')2 preparations. In spite of the use of immunoglobulin fragments, varying the assay time and testing tumours of different sizes, specific tumour localisation was low with all seven monoclonal antibodies tested. The maximum uptake achieved was less than 1% of the injected dose of antibody per gram of tumour. Much higher levels of antibody localisation have been reported for human tumour xenografts growing in nude mice, but these are rarely achieved in other systems. We propose that the use of autologous monoclonal antibodies recognising tumour-associated antigens of relatively low epitope density in syngeneic hosts provides a valid alternative model in which to investigate the factors limiting more effective, specific immunolocalisation of malignant disease.

Ribosome-inactivating proteins from plants inhibit ribosome activity of Trypanosoma and Leishmania

Ribosomes from Trypanosoma brucei rhodesiense and from Leishmania infantum were isolated and optimal conditions for in vitro translation were established. The effect of ribosome-inactivating proteins extracted from several plants was then assessed in order to identify those suitable for the preparation of immunotoxins against these organisms. Ribosomes from both species were inactivated by some ribosome-inactivating proteins (dianthins, saporins, pokeweed antiviral proteins, and the ribosome-inactivating chain of abrin). The similarity of the effects on the ribosomes from the two species examined indicates that ribosome-inactivating proteins should also be effective in a similar way on ribosomes from other species of Trypanosoma and Leishmania.

Specific in vitro anti-tumour activity of methotrexate-monoclonal antibody conjugates prepared using human serum albumin as an intermediary

Methotrexate (MTX) is a folic acid antagonist widely used in the treatment of cancer but, like other such agents, has non-specific toxic side effects. With the aim of reducing these toxic effects, MTX was coupled to monoclonal antibodies (MoAb) in one of two ways; either (a) directly using an active ester derivative or (b) via human serum albumin (HSA), to act as an intermediary and so increase the amount of MTX bound. The MTX coupled directly to anti-Ly-2.1 antibody had approximately 10 molecules of MTX per antibody molecule, whereas the HSA coupled material had 24 molecules of MTX per antibody molecule. After coupling MTX directly to antibody there was a loss of potency of the MTX, and MTX-MoAb conjugates were 30-fold less potent than free MTX, although the antibody-coupled material was more specific than free MTX and bound only to the antibody-reactive target cells. By contrast, the MTX-HSA-MoAb conjugates were 3.5 times less potent than free MTX and were 8.5 times more potent than MTX-MoAb conjugates. Thus, by increasing the amount of drug bound to antibody, more toxic conjugates were made--an important principle for the use of such conjugates for the treatment of cancer in man.

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.

Monoclonal antibodies as magic bullets

Monoclonal antibodies have been used as experimental therapy in humans since the beginning of the 1980s (A.N. Houghton and D. A. Scheinberg. Semin. Oncol. 13:165-179, 1986). They have been hailed as the prototypical magic bullet drug because of their inherent capacity for specificity. Consequently, monoclonal antibodies have many possible therapeutic applications with varied potential for successful outcome. Current review articles discussing monoclonal antibody therapy deal with the application of monoclonal antibodies within specific areas of medicine. The aim of this review is to summarize the current reviews and provide a broader perspective on the medical applications of monoclonal antibodies along with some general principles by which their therapeutic success or failure might be understood.

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)

Effect of ribosome-inactivating proteins on ribosomes from Tetrahymena pyriformis and Acanthamoeba castellanii

The effect of ribosome-inactivating proteins type 1 (single-chain) and type 2 (two-chain, toxins) on polyphenylalanine polymerization by Tetrahymena pyriformis and Acanthamoeba castellanii ribosomes has been studied. The reaction catalysed by tetrahymena ribosomes was inhibited by two ribosome-inactivating proteins type 1 (dianthin 32 and, less effectively, momordin) whereas the reaction catalysed by amoeba ribosomes was inhibited, in a decreasing order of activity, by three ribosome-inactivating proteins type 1 (dianthin 32, saporin 6 and bryodin) and by two toxins (abrin and volkensin).

Tolerogenic polyethylene glycol derivatives of xenogeneic monoclonal immunoglobulins

It is becoming apparent that the effectiveness of xenogeneic monoclonal antibodies (XIg), which are increasingly used for diverse therapeutic purposes in man, may be counteracted by their inherent immunogenicity. Since conjugates of proteins with monomethoxypolyethylene glycol (mPEG) have proved to be effective tolerogens in other systems, we have used an experimental model in mice to explore the tolerogenicity of mPEG conjugates of a human monoclonal IgG (HIgG), i.e. a myeloma protein. Administration of these conjugates prior to immunization with heat aggregated HIgG (ha-HIgG) resulted in specific tolerance, as manifested by a marked reduction in the level of antibodies to HIgG, which was related to the degree of conjugation and the dose of conjugate administered. Thus, administration of HIgG(mPEG)20 6 to 43 days prior to immunization with ha-HIgG resulted in an inhibition of anti-HIgG antibody formation of the order of 85-90%, in relation to the titres of mice receiving PBS in lieu of HIgG(mPEG)20; these results hold the promise that mPEG conjugates of XIg may prove therapeutically useful in man in relation to organ transplantation, localization of tumours by immuno-imaging and tumour destruction by immunotoxins.

Humoral immune response to a ricin A chain immunotoxin in patients with metastatic melanoma

Immunotoxins, hybrid molecules consisting of a monoclonal antibody linked to a polypeptide toxin have shown anti-tumor activity in both animal models and early clinical trials. However, their potential value in the treatment of human cancer may be limited by the development of host antibodies against the conjugate. Such antibodies could potentially alter immunotoxin pharmacokinetics and pharmacodynamics as well as precipitate serum sickness or anaphylaxis. Using a radioimmunoassay we have measured serial anti-ricin A chain (anti-RTA) and anti-murine immunoglobulin (anti-MIG) titers in 22 patients who received the anti-melanoma immunotoxin XomaZymeR-Mel. Significant titers of anti-RTA and/or anti-MIG were detected in 17 of 21 evaluable patients. Of the four patients not developing antibodies, two were likely immunosuppressed secondary to dexamethasone, and CCNU and dexamethasone respectively. Both patients who received immunotoxin at a time when they had detectable anti-immunotoxin antibodies experienced infusion reactions consistent with immune mediated allergic responses. There was a decrease in peak immunotoxin level in the one patient who had serum immunotoxin levels measured at a time when anti-RTA was present. Strategies to suppress the human immune response to immunotoxins are required before repetitive courses of immunotoxin of this design may be administered.