Uptake of native and deglycosylated ricin A-chain immunotoxins by mouse liver parenchymal and non-parenchymal cells in vitro and in vivo

Protein deglycosylation can drastically affect the cellular uptake

Targeted drug delivery mediated by nanocarriers is a major issue in modern-day medicine. Upon coming in contact with biological fluids (e.g. blood), nanocarriers are rapidly covered by biomolecules (proteins, lipids, etc.) which results in the formation of a surface layer, widely known as the biomolecular corona. The biomolecular corona subsequently confers a certain biological identity to the corona-covered nanocarriers which can be crucial during their subsequent interactions with cells or other biological entities. In contrast to the proteins of the corona, little is known about the impact of the non-protein constituents of the corona, such as sugars. Here, we investigate the role of protein glycosylation of the corona in cellular uptake. We show that deglycosylation of clusterin (CLU) and apolipoprotein AI (Apo AI) significantly changes (increases and decreases, respectively) the cellular uptake of nanocarriers covered with these proteins.

Bacterial Toxins for Cancer Therapy

Several pathogenic bacteria secrete toxins to inhibit the immune system of the infected organism. Frequently, they catalyze a covalent modification of specific proteins. Thereby, they block production and/or secretion of antibodies or cytokines. Moreover, they disable migration of macrophages and disturb the barrier function of epithelia. In most cases, these toxins are extremely effective enzymes with high specificity towards their cellular substrates, which are often central signaling molecules. Moreover, they encompass the capacity to enter mammalian cells and to modify their substrates in the cytosol. A few molecules, at least of some toxins, are sufficient to change the cellular morphology and function of a cell or even kill a cell. Since many of those toxins are well studied concerning molecular mechanisms, cellular receptors, uptake routes, and structures, they are now widely used to analyze or to influence specific signaling pathways of mammalian cells. Here, we review the development of immunotoxins and targeted toxins for the treatment of a disease that is still hard to treat: cancer.

Clinical targeting recombinant immunotoxins for cancer therapy

Recombinant immunotoxins (RITs) are proteins that contain a toxin fused to an antibody or small molecules and are constructed by the genetic engineering technique. RITs can bind to and be internalized by cells and kill cancerous or non-cancerous cells by inhibiting protein synthesis. A wide variety of RITs have been tested against different cancers in cell culture, xenograft models, and human patients during the past several decades. RITs have shown activity in therapy of several kinds of cancers, but different levels of side effects, mainly related to vascular leak syndrome, were also observed in the treated patients. High immunogenicity of RITs limited their long-term or repeat applications in clinical cases. Recent advances in the design of immunotoxins, such as humanization of antibody fragment, PEGylation, and modification of human B- and T-cell epitopes, are overcoming the above mentioned problems, which predict the use of these immunotoxins as a potential therapeutic method to treat cancer patients.

Targeted toxins

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

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

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

Toxicity of ricin toxin A chain in rats

Ricin toxin A chain (RTA) is the cytotoxic component of the dimeric protein, ricin, one of the most potent and deadly plant toxins extracted from the seeds of Ricinus communis. RTA has been investigated as a potential candidate for cancer chemotherapy, in the form of immunotoxins, and as a method for depleting macrophages in vivo. The toxicity of RTA immunotoxins is mostly characterized by inflammation and necrosis and has been attributed to the RTA moiety of the conjugate. The present study was carried out to investigate the toxicity of intravenously (i.v.) administered RTA alone and to assess whether the observed tissue injuries are associated with increases in oxidative stress (OS) and inflammation. RTA (10 or 90 µg/kg body weight) was administered to animals i.v., and 5 or 24 hours later, liver, lungs, kidneys, and hearts were examined. RTA, at a dose of 90 µg/kg (i.v.), resulted in significant increases (P < 0.05) in an inflammatory response (i.e., increases in hepatic and lung myeloperoxidase activity) and increases in oxidant response (increases in lipid peroxidation and decreases in glutathione levels in hepatic and lung homogenates). These data suggest that i.v. administration of RTA resulted in organ injuries that were associated with inflammation and OS.

Comparative animal models for the study of lymphohematopoietic tumors: strengths and limitations of present approaches

The lymphomas probably represent the most complex and heterogenous set of malignancies known to cancer medicine. Underneath the single term lymphoma exist some of the fastest growing cancers known to science (i.e Burkitt's and lymphoblastic lymphoma), as well as some of the slowest growing (i.e. small lymphocytic lymphoma [SLL] and follicular lymphoma). It is this very biology that can dictate the selection of drugs and treatment approaches for managing these patients, strategies that can range from very aggressive combination chemotherapy administered in an intensive care unit (for example, patients with Burkitt's lymphoma), to watch and wait approaches that may go on for years in patients with SLL. This impressive spectrum of biology emerges from a relatively restricted number of molecular defects. The importance of these different molecular defects is of course greatly influenced by the intrinsic biology that defines the lymphocyte at its different stages of differentiation and maturation. It is precisely this molecular understanding that is beginning to form the basis for a new approach to thinking about lymphoma, and novel approaches to its management. Unfortunately, while our understanding of human lymphoma has blossomed, our ability to generate appropriate animal models reflective of this biology has not. Most preclinical models of these diseases still rely upon sub-cutaneous xenograft models of only the most aggressive lymphomas like Burkitt's lymphoma. While these models clearly serve an important role in understanding biology, and perhaps more importantly, in identifying promising new drugs for these diseases, they fall short in truly representing the broader, more heterogenous biology found in patients. Clearly, depending upon the questions being posed, or the types of drugs being studied, the best model to employ may vary from situation to situation. In this article, we will review the numerous complexities associated with various animal models of lymphoma, and will try to explore several alternative models which might serve as better in vivo.

Expression of ricin A chain and ricin A chain-KDEL in Escherichia coli

Ricin and its A chains can be used to conjugate with monoclonal antibodies to prepare immunotoxins. Ricin A chain (RTA) and its modification RTA-KDEL (ER-retrieval signal) were expressed with the pKK223.3 system in Escherichia coli under control of a tac promoter. The recombinant proteins can be purified by one-step affinity chromatography on a column of Blue-Sepharose 6B. The toxicities of RTA and its mutant RTA-KDEL were evaluated by the MTT assay in HeLa, MCF, and ECV-304 cells following fluid-phase endocytosis. RTA-KDEL was somewhat more cytotoxic than RTA itself in the different cell lines. The results suggest that rRTA-KDEL may be useful for the synthesis of more potent immunotoxins.

The emerging role of ricin A-chain immunotoxins in leukemia and lymphoma

Since MRD is the major cause for relapses of malignant diseases, strategies utilizing ITs to target tumor cells surviving conventional treatment have attracted scientific and clinical interest. Many different ITs against various blood-borne as well as solid malignancies have demonstrated specific potent anti-tumor effects in vitro and in animal models. Some of these have already undergone clinical phase I/II-trials. The dose-limiting toxicities of RTA ITs include manifestation of VLS presenting as decreased urinary sodium excretion, hypoalbuminemia, fatigue, hypotonia, myalgia, pulmonary edema, or rhabdomyolysis. Problems encountered clinically include the development of HAMA, HARA, and HACA and the selection of antigen-deficient malignant clones. Most clinical trials performed with ITs so far were conducted in heavily pretreated patients presenting with high tumor burdens. Thus, the responses observed with ITs in these trials are very encouraging and warrant further exploration.

The synovial membrane, liver, and tongue: target organs for a ricin A-chain immunotoxin (ZD0490)

ZD0490 is an immunotoxin consisting of a mouse monoclonal antibody conjugated to recombinant ricin A-chain (rRAC). It was developed at Zeneca Pharmaceuticals as a treatment for certain antigen-bearing tumors. During safety evaluation studies in rats, a number of reversible inflammatory changes were seen. The synovial membranes of articular joints showed a marked degeneration and necrosis with an associated inflammation. When of mild severity only the synovial membrane was involved, but when more severe many adjacent tissues including the surface of the articular cartilage were affected. Some nonspecific skeletal muscle toxicity occurred. However, tongues from the intravenously (tail) dosed rats consistently showed inflammation specifically located in the ventral subepithelial area with myocyte degeneration and necrosis. Also, hepatic peliosis primarily located in the subcapsular areas was induced. Studies with rRAC alone indicated that ricin A-chain (RAC) is the component responsible for these findings. It is suggested that cells of a macrophage type with the ability to specifically bind RAC may at least in part determine the location and nature of the changes seen.

Pharmacokinetics of human-mouse chimeric anti-GD2 mAb ch14.18 in a phase I trial in neuroblastoma patients

A comprehensive analysis of the pharmacokinetics of human-mouse chimeric anti-ganglioside GD2 antibody mAb ch14.18 was performed during a phase I clinical trial of ten children with neuroblastoma and one adult with osteosarcoma. The patients received a total of 20 courses of ch14.18 at dose levels from 10 mg/m2 to 200 mg/m2. The plasma clearance of ch14.18 was biphasic. Following the first course of treatment t1/2,alpha was 3.4 +/- 3.1 h and t1/2,beta 66.6 +/- 27.4 h in 9/10 children. The t1/2,beta values were significantly less than those of 181 +/- 73 h previously reported in adult melanoma patients (P < or = 0.001), and 147.5 h in the adult osteosarcoma patient in our trial. The latter suggests different pharmacokinetics of mAb ch14.18 in children and adults. After a second course of treatment, administered to 5/10 children, t1/2,beta decreased significantly from 72.9 +/- 19.8 h to 31.7 +/- 18.4 h (P = 0.015). We therefore conclude that the elimination kinetics of mAbs ch14.18 in children and adults are different, and furthermore that repeated administration of mAb ch14.18 to children with neuroblastoma leads to accelerated antibody clearance.

Alpha 2-macroglobulin receptor mediates binding and cytotoxicity of plant ribosome-inactivating proteins

It has been proposed that unconjugated type I ribosome-inactivating proteins (RIP) enter cells through passive mechanisms such as fluid-phase pinocytosis. However, some observations, such as the difference in sensitivity to type I RIP among different cell types, and the organ-specific toxicity of type I RIP, indicate a specific mechanism for the entry of these proteins into target cells. The alpha 2-macroglobulin receptor (alpha 2MR) is responsible for the binding and endocytosis of several ligands, including alpha 2-macroglobulin/proteinase complexes, plasminogen-activator-inhibitor complexes, apoE-enriched beta-very low density lipoproteins, and lipoprotein lipase. Here we demonstrate that saporin, a potent type I RIP, binds specifically to purified alpha 2MR and the binding is prevented by some alpha 2MR ligands. Moreover, the occupancy of specific ligand-binding sites on cell surface alpha 2MR decreases the cytotoxicity of saporin. The A chain of ricin, a type II RIP, also interacts with alpha 2MR. This, and the fact that saporin and ricin A chain both interact also with alpha 2-macroglobulin, indicates a general mechanism of complex interactions between RIP and cellular membranes that is mediated by alpha 2-macroglobulin and the alpha 2MR system.

In vivo and in vitro uptake of an anti-CD30/saporin immunotoxin by rat liver parenchymal and nonparenchymal cells

A Ber-H2/saporin immunotoxin, consisting of the single-chain ribosome-inactivating protein saporin-S6 and the anti-CD30 monoclonal antibody Ber-H2, gave encouraging results in the treatment of refractory Hodgkin's disease but caused a transient hepatotoxicity. The accumulation of Ber-H2/saporin conjugate and of its components by rat liver parenchymal and nonparenchymal cells was studied. The in vivo concentration of intravenously injected Ber-H2/saporin, saporin or Ber-H2 in nonparenchymal cells was 4-, 25- and 11-fold higher, respectively, than that in parenchymal cells. Adherent in vitro cultured nonparenchymal cells, mostly Kupffer cells, accumulated the proteins approximately 10 times more than parenchymal cells; traces of free saporin were taken up by both types of cells. In vitro protein synthesis by both cell types was inhibited by 50% at nanomolar concentrations of saporin. Nonparenchymal cells were sensitive to Ber-H2/saporin at picomolar concentrations, whereas parenchymal cells were unaffected by the immunotoxin up to 100 pmol/L. The results of the uptake of, and the sensitivity to, the immunotoxin suggest that the sensitivity of liver cells is proportional to the uptake and that the in vivo damage to parenchymal cells is at least in part mediated by the toxicity to nonparenchymal liver cells.

Immunotoxins: is there a clinical value?

Drug targeting is an attractive new approach to killing malignant cells, thereby leaving normal tissue unharmed. A decisive breakthrough was the advent of hybridoma technology, making monoclonal antibodies (MoAb) available in limitless supply. To construct reagents with selectivity for certain tumor cells, MoAbs or Fab' fragments were chemically linked to ribosome-damaging toxins derived from plants or bacteria like ricin, abrin, saporin, Pseudomonas exotoxin (PE), and diphtheria toxin (DT) to form immunotoxins, which combined the selectivity of the carrier moiety with the potency of the toxin moiety. The first generation of these immunotoxins showed impressive results in vitro but in most cases disappointing antitumour effects in animals or humans. By contrast, the second generation of immunotoxins, consisting of either A chain immunotoxins with a greatly improved stability in vivo or so-called 'blocked' ricin immunotoxins, have been demonstrated to be extremely effective in several animal models. Preliminary results of the current clinical trials suggest a possible clinical use of immunotoxins in leukemia and lymphoma patients. Genetically engineered fusion toxins have become available, which consist of a growth factor or a cytokine fused to a toxin moiety. In this paper, we will review the features of the three groups of immunotoxins which are most frequently used, i.e., ricin A chain and similar immunotoxins, blocked ricin immunotoxins, and recombinant toxins constructed with Pseudomonas exotoxin or diphtheria toxin.

Endocytosis of ricin by rat liver cells in vivo and in vitro is mainly mediated by mannose receptors on sinusoidal endothelial cells

Upon intravenous injection into rats, the plant toxin ricin was rapidly cleared from the circulation by the liver. Among the different liver cell populations, most of the injected ricin associated with the sinusoidal endothelial cells (EC), whereas the liver parenchymal cells (PC) and Kupffer cells (KC) yielded minor contributions to the total liver uptake in vivo. Co-injection of mannan strongly inhibited ricin uptake by the EC, showing that it was mediated by mannose receptors. On the other hand, co-injection of lactose, which inhibits the galactose-specific association of ricin with cells, enhanced ricin uptake by the EC. The carbohydrate-dependency of the EC contribution to the uptake of ricin in vivo was reflected in the carbohydrate-dependency of the uptake in vivo by whole liver. In vitro, the EC also endocytosed ricin more efficiently than did the PC or KC. Whereas uptake in vitro in the EC was mainly mannose-specific, uptake in the two other cell types was mainly galactose-specific. Western blotting showed that the mannose receptors of liver non-parenchymal cells are identical with the mannose receptor previously isolated from alveolar macrophages. The mannose receptors are expressed at a higher level in EC than in KC. Ligand blotting showed that, in the presence of lactose, the mannose receptor is the only protein in the EC that binds ricin, and the binding is mannose-specific and Ca(2+)-dependent.

Monoclonal antibodies in cancer detection and therapy

Anticancer antibodies have had a long history in the management of cancer, with major applications having been shown in the immunohistochemistry and immunoassay of tumor-associated antigen markers. With the advent of hybridoma-derived monoclonal antibodies, attempts to use these more reproducible reagents in vivo for cancer detection and therapy have intensified. Radiolabeled monoclonal antibodies appear to be gaining a role in the management of cancer by means of imaging methods to detect sites of increased radioactivity, and several products have been developed and tested clinically. In the area of radioimmunotherapy, a number of problems still need to be solved, including low tumor uptake of the radioimmunoconjugate, dose-limiting myelotoxicity, and the induction of an immune response to repeated doses of murine (foreign) immunoglobulins. Similar problems exist for toxin and drug immunoconjugates, but these also fail to benefit from the "bystander" effect of the ionizing radiation delivered with radioimmunoconjugates, and plant and bacterial toxin molecules appear to have additional immunogenicity that restricts repeated injections. Despite these limitations, recombinant engineering and other chemical approaches are making progress in developing second-generation immunoconjugates that may be more efficacious and less immunogenic as cancer-selective therapeutics. Although nonconjugated, "naked", murine monoclonal antibodies have shown limited success in the therapy of human neoplasms, human and "humanized" forms may be more effective, particularly in lymphatic tumors. Some evidence also suggests that anti-idiotype antibodies (antiantibodies) may serve as surrogate antigens in cancer vaccines. Thus, a number of promising immunologic approaches for cancer diagnosis, detection, and therapy have made important progress in recent years.

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

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

Influence of barrier-crossing limitations on the amount of macromolecular drug taken up by its target

Macromolecules (substitutive enzymes, polymeric prodrugs, immunotoxins, radiolabeled antibodies, or peptide hormones) are of interest in the treatment of several diseases. To reach the tissues, these macromolecular drugs have to cross the capillary wall, which represents an important transfer limitation. While pharmacokinetics usually studies the changes in drug concentration in different body compartments, analyzing the amount of drug gaining access to its target may be more relevant for assessing the efficiency of macromolecules than for low molecular mass drugs. To determine the influence of different parameters on the fraction of the injected dose gaining access to the pharmacologic target, we constructed pharmacokinetic models where two uptakes, both linear or nonlinear, work either in the same compartment (no transport limitation), or in compartments separated by a transport barrier. Numerical applications were carried out with parameters obtained either experimentally or from the literature. We conclude that it is of little use to increase the affinity (K(uptake)) of a macromolecular drug for its target when a transport limitation and an undesired elimination from the plasma space are both present. Likewise, an increase of the uptake (rate of uptake or maximal velocity) by the target is not very productive because permeability of the capillary wall is the factor limiting access of macromolecules to tissues. Maximal efficiency of therapeutic macromolecules could be achieved by increasing, where feasible, the transport across the barrier between the plasma and the target, and by preventing the undesired eliminations as much as possible.

Experimental therapy in Hodgkin's disease

Hodgkin/Reed-Sternberg (H-RS) cells express lymphoid activation markers like CD25 and CD30 which are present only on a small minority of normal cells. Currently, most experimental approaches in Hodgkin's lymphoma are aimed at targeting H-RS cells via monoclonal antibodies against CD25 and CD30: immunotoxins constructed by linking the antibody moiety chemically to deglycosylated ricin A-chain destroy up to 60% of small H-RS tumors in mice. The most potent immunotoxin is currently being scaled up for clinical trials. Other experimental strategies use bispecific constructs that, after binding to the cell surface of H-RS cells, convert prodrugs into their toxic counterparts, or employ monoclonal antibodies for active immunotherapy.

Immunotoxins constructed with anti-CD25 monoclonal antibodies and deglycosylated ricin A-chain have potent anti-tumour effects against human Hodgkin cells in vitro and solid Hodgkin tumours in mice

Twenty-three monoclonal antibodies (MAbs) against the IL-2 receptor alpha-chain (CD25) were evaluated as ricin A-chain immunotoxins for the treatment of Hodgkin's disease. Primary screening used an indirect assay in which the cells were treated with the test antibody followed by a Fab' immunotoxin against mouse immunoglobulin. This screening identified 5 MAbs which inhibited protein synthesis in L540 Hodgkin cells by 50% at a concentration (IC50) of 6 x 10(-11) M or less: RFT5 gamma 1, RFT5 gamma 2a, B-B10, B-F2 and B-G3. These MAbs were then linked directly to deglycosylated ricin A-chain (dgA) and were confirmed to have potent and specific toxicity for L540 cells. The immunotoxins had the following potency order: RFT5 gamma 1 greater than RFT5 gamma 2a greater than B-B10 greater than B-F2 greater than B-G3. The most effective immunotoxin, RFT5 gamma 1.dgA, had an IC50 value of 7 x 10(-12) M, which is the same as that of whole ricin. In vivo, a single intravenous injection of 48 micrograms of RFT5 gamma 1.dgA, RFT5 gamma 2a.dgA, B-B10.dgA or B-F2 induced lasting complete remissions in 78, 66, 50 and 44%, respectively, of nude mice bearing subcutaneous solid L540 tumours of 0.7 cm diameter. Two tumours which regrew after B-B10.dgA treatment were re-established in tissue culture. Both had reduced sensitivity to B-B10.dgA in vitro but not to immunotoxins recognizing different antigens on Hodgkin cells. The MAbs that produced the most potent immunotoxins, RFT5 gamma 1, RFT5 gamma 2a and B-B10, had no significant cross-reactivity with normal human tissues outside the lymphoid system as judged from indirect immunoperoxidase staining of frozen sections. By contrast, B-F2 strongly stained normal human renal tubules.

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.

Biologically active A-chain of the plant toxin ricin expressed from a synthetic gene in Escherichia coli

To assess the biological activity and pharmacokinetic properties of nonglycosylated ricin A-chain (RA), we have obtained the polypeptide following expression of a synthetic 842-bp RA gene in Escherichia coli. Expression of the gene was carried out using the phage T5 PN25 promoter fused to the E. coli lac operator. The RA polypeptide was synthesized in a completely soluble form and was purified in one step by immunoabsorption. It was shown to be as cytotoxic for a human cell line as both native RA and chemically deglycosylated native RA. Reconstituted whole ricin and an immunotoxin containing the recombinant RA were also biologically active. Immunotoxins made with recombinant and deglycosylated RA had similar clearance rates in vivo showing, after a short period of rapid elimination, stabilities far higher than that of an immunotoxin made with native RA. Our results show that the complete elimination of sugar side chains from the RA is not sufficient to entirely eradicate the rapid initial in vivo clearance of RA-based biologicals.

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.