Delivery of ricin and abrin A-chains to human carcinoma cells in culture following covalent linkage to monoclonal antibody LICR-LOND-Fib 75

Encapsulation and Controlled Release of Protein Guests by the Bacillus subtilis Lumazine Synthase Capsid

In Bacillus subtilis, the 60-subunit dodecahedral capsid formed by lumazine synthase (BsLS) acts as a container for trimeric riboflavin synthase (BsRS). To test whether the C-terminal sequence of BsRS is responsible for its encapsulation by BsLS, the green fluorescent protein (GFP) was fused to either the last 11 or the last 32 amino acids of BsRS, yielding variant GFP11 or GFP32, respectively. After purification, BsLS capsids that had been co-produced in bacteria with GFP11 and GFP32 are 15- and 6-fold more fluorescent, respectively, than BsLS co-produced with GFP lacking any BsRS fragment, indicating complex formation. Enzyme-linked immunosorbent assay experiments confirm that GFP11 is localized within the BsLS capsid. In addition, fusing the last 11 amino acids of BsRS to the C-terminus of the Abrin A chain also led to its encapsulation by BsLS at a level similar to that of GFP11. Together, these results demonstrate that the C-terminal tail of BsRS can act as an encapsulation tag capable of targeting other proteins to the BsLS capsid interior. As with the natural BsLS-BsRS complex, mild changes in pH and buffer identity trigger dissociation of the GFP11 guest, accompanied by a substantial expansion of the BsLS capsid. This system for protein encapsulation and release provides a novel tool for bionanotechnology.

Sensitive Bioassay for Detection of Biologically Active Ricin in Food

The potential use of ricin as an agent of biological warfare highlights the need to develop fast and effective methods to detect biologically active ricin. The current “gold standard” for ricin detection is an in vivo mouse bioassay; however, this method is not practical to test on a large number of samples and raises ethical concerns with regard to the use of experimental animals. In this work, we generated adenoviral vectors that express the green fluorescent protein gene and used the relative fluorescence units intensity inhibition by transduced cells for quantitative measurement of biologically active ricin. The detection limit of the assay was 200 pg/ml, which is over 500,000 times greater than the adult human lethal oral dose. The inhibition of fluorescence intensity between ricin treatment and control was higher in 72-h posttransduction Vero cells than 24-h human embryonic kidney cells. Therefore, to detect biologically active ricin in food matrices that might influence the assay, we used 72-h posttransduction Vero cells. This simple assay could be used for large-scale screening to detect biologically active ricin in food without added substrates or use of cell fixation methods.

Abrin-a A chain expressed as soluble form in Escherichia coli from a PCR-synthesized gene is catalytically and functionally active

Abrin-a A chain (ABRaA) is a potent plant toxin, which possesses N-glycosylase activity toward eukaryotic 28S rRNA, and may have potential use in cancer therapy. To improve levels of expression in Escherichia coli, the gene encoding ABRaA was optimized by replacing rare codons with high-frequency ones, and synthesized using two-step PCR. The optimized ABRaA was cloned into the pET-His vector, and highly expressed in cytoplasm of E. coli. The yield of the purified recombinant (r) ABRaA proteins was up to 80 mg/l of induced culture. The rABRaA was one-step purified to homogeneity and its RNA-N-glycosylase ability to inhibit protein biosynthesis in a cell-free system and to depurinate 28S rRNA in rat liver ribosomes was demonstrated in vitro. The MTT assay showed that it also had a killing effect on human hepatoma cell line SMMC-7721 and myeloma cell line Sp2/0. For the first time, ABRaA expressed as soluble form in E. coli from a PCR-synthesized gene is catalytically and functionally active.

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.

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.

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.

The effect of ricin B chain on the intracellular trafficking of an A chain immunotoxin

Covalent linkage of the A chain of ricin to the LICR-LOND-Fib75 monoclonal antibody produced an immunotoxin, Fib75-SS-ricin A, which demonstrated immunospecific toxicity to human bladder carcinoma cells in tissue culture (Forrester et al., 1984). The present studies have shown that ricin B chain potentiates the toxicity of the immunotoxin by two orders of magnitude and also significantly increases the rate of protein synthesis inhibition. Using immunoelectron microscopy, the receptor-mediated endocytosis and intracellular routing of the immunotoxin was studied with and without ricin B chain treatment after immunolocalisation of the conjugate. Fib75-SS-ricin A was internalised by the EJ cells predominantly in uncoated pits and vesicles and directed to the endosomes. Some degradation of the complex appeared to take place in multivesicular endosomes at early timepoints and 24 h after internalisation, most of the immunotoxin was found in lysosomes. Some ricin A chain epitopes were detected in Golgi vesicles. Cells treated with immunotoxin and ricin B chain endocytosed the complex predominantly in coated pits and coated vesicles. Using pre-embedding immunoperoxidase techniques, ricin chains were found in the whole Golgi complex and most of the conjugate escaped lysosomal degradation. Internalised immunotoxin was recycled back to the plasma membrane in an active form associated with vesicles which appeared to be derived predominantly from multivesicular endosomes. A similar mode of recycling has recently been reported (McIntosh et al., 1990) for ricin holotoxin in the same cell line. These observations may explain the potentiating effect of toxin B chains in the antibody-directed targeting of toxin A chains.

Preproabrin: genomic cloning, characterisation and the expression of the A-chain in Escherichia coli

Synthetic oligonucleotides representing all possible sequences of an N-terminal and an internal region of the A-chain of abrin C were used to generate a probe specific for abrin-related sequences using the polymerase chain reaction on Abrus precatorius genomic DNA. A lambda phage library constructed from genomic DNA isolated from leaf tissue of A. precatorius was screened and positive hybridising clones were characterised by restriction enzyme analysis. The coding regions of unique clones were characterised by DNA sequencing. One clone encodes a preproprotein closely related to abrin C with 83% similarity between the A-chain sequences. Based on similarity with the ricin toxins and Ricinus communis agglutinin, the preproabrin consists of an A-chain of 251 amino acids preceded by 34 amino acids containing an N-terminal signal peptide, followed by a 14-amino-acid linker and a B-chain of 263 amino acids. The mature A-chain of the preproabrin has been expressed cytoplasmically in Escherichia coli and the soluble recombinant protein was produced at levels exceeding 6% of total cell protein. The recombinant A-chain has been purified to homogeneity and its ability to depurinate 28S rRNA in rat liver ribosomes has been demonstrated in vitro.

Biochemical, cytotoxic and pharmacokinetic properties of an immunotoxin composed of a mouse monoclonal antibody Fib75 and the ribosome-inactivating protein alpha-sarcin from Aspergillus giganteus

An immunotoxin was synthesized by the attachment of alpha-sarcin, the ribosome-inactivating protein derived from the mould Aspergillus giganteus, to a monoclonal mouse IgG2 antibody Fib75. The alpha-sarcin immunotoxin exerted toxic effects in tissue culture against the EJ human bladder carcinoma cell line, expressing the antigen recognised by the Fib75 antibody, inhibiting the incorporation of [3H]leucine by 50% at a concentration of 0.46 nM. The cytotoxic effects of the alpha-sarcin immunotoxin were indistinguishable from those of a Fib75 immunotoxin made with ricin A chain. Fib75-alpha-sarcin was cleared from the circulation of the rat with biphasic kinetics following intravenous administration. The alpha- and beta-phase half-lives were 0.8 h and 6 h, respectively, similar to the serum half-lives of analogous Fib75 immunotoxins made with ribosome-inactivating proteins derived from plants. alpha-Sarcin was completely stable in physiological saline buffer at 37 degrees C, whereas the ribosome-inactivating activity of ricin A chain was gradually lost under identical conditions. alpha-Sarcin may be a valuable alternative to ricin A chain for the construction of therapeutic immunotoxins because of its smaller size and greater thermostability.

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

Immunotoxins were constructed by attaching native ricin A chain, ricin A1 chain and recombinant ricin A chain to the mouse monoclonal IgG2a antibody Fib75 by means of a disulphide linkage using the hetero-bifunctional cross-linker SPDP. The Fib75 immunotoxins were of similar composition and exerted identical cytotoxic effects against the EJ human bladder carcinoma cell line in tissue culture. All 3 immunotoxins broke down to the same extent upon incubation with glutathione in vitro. The clearance of the immunotoxins from the circulation of normal Wistar rats was determined following i.v. administration. The concentration of intact immunotoxin in serum samples taken at various intervals up to 48hr after injection was measured by a ricin A chain-specific ELISA. The Fib75 immunotoxin made with native ricin A chain was removed from the circulation most rapidly. Fib75-recombinant ricin A chain persisted in the circulation at a higher level than Fib75-ricin A1 chain. A higher proportion of the ricin A1 chain immunotoxin was lost from the bloodstream during the alpha-phase. The beta-phase half-lives of Fib75-recombinant ricin A chain and Fib75-ricin A1 chain were similar, consistent with the identical susceptibility of the immunotoxins to cleavage by glutathione. The presence of the complex-type oligosaccharide side-chain on the A1 chain may have accelerated the clearance of the A1 chain immunotoxin in relation to that of the immunotoxin made with the aglycosyl recombinant A chain.

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.

Purification of immunotoxins containing the ribosome-inactivating proteins gelonin and momordin using high performance liquid immunoaffinity chromatography compared with blue sepharose CL-6B affinity chromatography

Comparable amounts of the ribosome-inactivating proteins (RIPs) ricin A chain, gelonin and momordin were allowed to bind to Blue Sepharose CL-6B (immobilised Cibacron Blue F3GA) in phosphate buffer, pH 7.5, and were then eluted quantitatively with buffer containing 0.5 M NaCl. Differences in the elution profiles indicated that the RIPs possessed different affinities for the Cibacron Blue F3GA dye. Conjugation of the RIPs to the monoclonal antibody Fib75 resulted in decreased affinity for Blue Sepharose. Under conditions allowing the complete separation of the Fib75-ricin A chain immunotoxin from unconjugated Fib75, the Fib75 immunotoxins made with gelonin and momordin failed to bind completely to the Blue Sepharose column. The Fib75-gelonin and Fib75-momordin fractions that eluted from the column with 0.5 M NaCl were free of unconjugated Fib75 but were enriched in multiply substituted conjugate molecules. A high performance liquid immunoaffinity chromatography procedure based on the selective binding of conjugated RIP to immobilised affinity-purified anti-RIP antibody permitted the complete separation of the gelonin and momordin immunotoxins from unconjugated Fib75 without altering the composition, molecular integrity or cytotoxic activity of the immunotoxins.

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

The potential of mouse monoclonal antibodies for recognising different antigens associated with human small cell lung cancer (SCLC) to form active immunotoxins was assessed by an indirect in vitro screening assay. The screening agent used was a conjugate made by linking ricin A chain to a sheep anti-mouse IgG Fab' fragment via a disulphide bond. The monoclonal antibodies SWA11 and SWA20 both mediated the toxic effects of ricin A chain against the HC12 classic SCLC cell line in dose-dependent fashion. The SWA11 antibody was the more effective; in combination with the screening agent at a concentration of 1 x 10(-7) M, it inhibited the incorporation of [3H] leucine into HC12 cells by 94% compared with only 44% inhibition in the case of SWA20. An immunotoxin made by the direct chemical conjugation of ricin A chain to SWA11 exhibited selective toxic effects upon HC12 cells in tissue culture inhibiting the incorporation of [3H] leucine by 50% at a concentration (IC50) of 6.2 x 10(-10) M and by 98% at 1 x 10(-7) M. SWA11-ricin A chain had an IC50 of 4.4 x 10(-10) M against the NCI-H69 classic SCLC cell line but showed no cytotoxic activity against the human lung adenocarcinoma cell line NCI-H23 at a concentration of 1 x 10(-8) M.

The low uptake of an abrin A-chain immunotoxin by rat hepatic cells in vivo and in vitro

The therapeutic value of antibody-ricin A-chain conjugates (immunotoxins) as antineoplastic agents is limited by their rapid removal from the circulation, primarily by cells of the liver which take up the immunotoxin through receptor mediated recognition of mannose-containing oligosaccharides in the toxin A-chain. We have therefore examined the uptake by rat hepatic cells of a monoclonal antibody (LICR-LOND Fib 75) conjugate assembled with the ricin related, but carbohydrate free, A-chain of the plant toxin abrin. The abrin A-chain immunotoxin was very poorly taken up in vivo and in vitro by both hepatic parenchymal and non parenchymal cells whereas a comparable conjugate assembled with ricin A-chain was actively accumulated by liver cells particularly the hepatic non-parenchymal cells. Furthermore, the abrin A-chain immunotoxin uptake by non-parenchymal cells in vitro was unaffected by mannose and the immunotoxin bound less readily to liver cells than did the ricin A-chain conjugate, consistent with a proposal that its accumulation by hepatic cells is brought about by endocytosis following non-specific binding or by fluid phase pinocytosis. These results suggest abrin A-chain immunotoxins might be further explored as anti-cancer agents since in some cases they could have an improved therapeutic efficacy over immunotoxins constructed with ricin A-chains.

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.

Late intensification with high-dose melphalan and autologous bone marrow support in breast cancer patients responding to conventional chemotherapy

Fifteen patients with advanced breast cancer who had achieved either a good partial or a complete response to conventional chemotherapy were selected to receive intensification treatment with high-dose melphalan 140-200 mg/m2 (HDM). All patients received autologous bone marrow rescue. All patients experienced marked haematological toxicity, and most experienced moderate or mild gastrointestinal side effects. There were three treatment-related deaths. Of twelve assessable patients eleven have relapsed; median time to relapse after HDM is 7 months. Nine of these eleven have died from recurrent breast cancer. Of the three patients remaining alive, only one is disease-free, at 18 months after HDM. Analysis of the pattern of metastatic relapse suggests that recurrence was due to failure of HDM to eradicate residual disease in the patient, rather than reinfusion of viable tumour cells. Treatment intensification with HDM has not succeeded in prolonging survival in patients already in good remission.

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.

Fate of an antibody-ricin A chain conjugate administered to normal rats

The pharmacokinetics and catabolism of ricin A chain, a mouse monoclonal antibody (LICR-LOND-Fib 75) and a disulphide linked conjugate of the two have been studied following their intravenous administration to normal rats. Results indicate that the conjugate was removed from the circulation much more rapidly than the antibody but less quickly than the free ricin A chain. Disappearance of the conjugate from the circulation appeared to be biphasic with an early rapid initial phase followed by a much more rapidly than the antibody but less quickly than the free ricin A chain. Disappearance of the conjugate from the circulation appeared to be biphasic with an early rapid initial phase followed by a much slower phase. The fate of a conjugate with a 125I iodide label in the antibody component was compared with that of a conjugate similarly labelled but in the ricin A chain component. The results indicate that breakdown of the conjugate involves both cleavage of the disulphide linkage and complete catabolism of the whole conjugate molecule with the release of 125I iodide. Rapid cleavage of the disulphide bond in the vasculature does not appear to be responsible for the initial rapid disappearance of the conjugate from the circulation.