High specific cytotoxicity of antibody-toxin hybrid molecules (immunotoxins) for target cells

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.

Immunotoxins: an update

The use of immunotoxins (ITs) in the therapy of cancer, graft-vs-host disease (GvHD), autoimmune diseases, and AIDS has been ongoing for the past two decades. ITs contain a targeting moiety for delivery and a toxic moiety for cytotoxicity. Theoretically, one molecule of a toxin, routed to the appropriate cellular compartment, will be lethal to a cell. Newly developed MoAbs, toxins, and molecular biological technologies have enabled researchers to construct ITs that can effectively kill many different cell types. In fact, phase I/II clinical trials have given promising results. Although nonspecific toxicity and immunogenicity still limit the use of IT therapy, these agents hold enormous promise in an optimal setting to treat minimal disease.

An immunotoxin, anti-VIP antibody-ricin A chain conjugate eliminates neurons in the hypothalamic suprachiasmatic nucleus selectively and abolishes the circadian rhythm of water intake

In mammals, a master circadian oscillator is known to be located in the suprachiasmatic nucleus (SCN) of the hypothalamus. We examined the function of SCN neurons involved in the mechanism of circadian rhythm of water intake by lesioning them with an immunotoxin, anti-vasoactive intestinal polypeptide (VIP) antibody-ricin A conjugate. We found that the immunotoxin had a specific lethal effect on cultured PC12h cells when VIP was added to the medium. When the conjugate was infused into the third cerebral ventricle of rats above the SCN, two specific types of selective lesions of neurons were observed in the SCN: selective lesions of neurons containing arginine vasopressin (AVP) (AVP-neurons), and selective lesions of neurons containing VIP (VIP neurons). The former lesions caused disappearance of the circadian rhythm of drinking behavior, whereas the latter lesions did not affect the rhythm of water intake under constant dim lighting. Lesions that did not selectively affect one of these neurochemically identified SCN cell populations were also observed after the infusion of the conjugate or normal rabbit serum immunoglobulin G-ricin A chain conjugate. If these nonspecific lesions included entire region of the SCN, the circadian rhythm of water intake was abolished. These findings suggest that SCN neurons bearing VIP receptors such as AVP neurons, but not VIP neurons, may be involved in the mechanism of the circadian rhythm of water intake.

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.

Redesigning nature's poisons to create anti-tumor reagents

Immunotoxins are conjugates of cell-reactive antibodies and toxins or their subunits. In this report, the chemistry, biology, pharmacokinetics, and anti-tumor effects of first generation immunotoxins; the preparation of improved second generation immunotoxins that display greater anti-tumor efficacy; and the role of genetic engineering in creating third-generation immunotoxins are discussed.

IgE-immunotoxins. I. IgE-intact ricin

An IgE immunotoxin consisting of rat IgE myeloma protein, IR 162, conjugated via the heterobifunctional linking agent N-succinimidyl-3-(2-pyridyldithio)propionate to intact ricin was synthesized and evaluated. The capacity of this IgE-immunotoxin to bind to rat basophilic leukemia cells (RBL cells) and to inhibit RBL cell incorporation of [3H]leucine was assessed. The IgE-intact ricin conjugate sensitized RBL cells for histamine release after treatment with anti-IgE with a time-course of sensitization and dose-response equivalent to native IgE. Intact ricin and IgE-intact ricin were both cytotoxic to RBL cells as assessed by [3H]leucine incorporation. Lactose (50 mM) competed with intact ricin binding and toxicity such that more than 100 ng/ml ricin (8 times its IC50 in the absence of lactose) was required for ricin to kill RBL cells in the presence of lactose. Lactose (50 mM) was not able to fully inhibit 1-100 ng/ml IgE-ricin immunotoxin killing of RBL cells. Saturation of RBL cell IgE receptors by preincubation with IgE totally inhibited IgE-intact ricin-induced toxicity, in the presence of lactose, indicating that toxicity required IgE Fc receptor binding.

The pharmacokinetics and toxicity of murine monoclonal antibodies and of gelonin conjugates of these antibodies

We studied in mice the in vivo pharmacokinetics and toxicity of murine monoclonal antibodies (MCA) and of disulfide-linked MCA conjugates of gelonin, a ribosomal inhibitor prepared from the seeds of Gelonium multiflorum. Iodinated MCA with specificity for human determinants and of gamma 1 or gamma 2a isotype had a circulatory half life (T 1/2) in the mouse of 4 days, which is consistent with previously published estimates of the circulatory T 1/2 of heterogeneous murine IgG. Iodinated murine MCA with specificity for murine determinants had a much shorter T 1/2, probably reflecting antigen binding. This effect could be partially overcome by the simultaneous injection of unlabeled MCA of identical specificity. Clearance of MCA-gelonin conjugates was characterized by an initial rapid phase lasting 8-12 h with a T 1/2 or from 4 to 7 h, followed by a slower clearance phase with T 1/2 approaching that of MCA. Moreover, the presence of significant amounts of intact conjugate in the murine circulation was demonstrable, by SDS gel electrophoresis, for up to 48 h post injection. Intraperitoneal injection of MCA-gelonin conjugate resulted in circulating levels identical to those achieved after i.v. administration after an initial 4 h equilibration. The LD50 of MCA-gelonin conjugates was approximately 25 mg/kg (i.v.) while that of gelonin was approximately 75 mg/kg (i.v.) MCA alone showed no toxicity in doses in excess of 150 mg/kg. At doses below the LD50 immunoconjugates caused a dose-dependent reversible weight loss. The main site of toxicity of MCA-gelonin conjugates was the liver; histopathological examination revealed dose-dependent foci of necrosis and acute inflammation. No pathology was observed in lung, spleen, kidney, gut or brain. The relationship to previous work in this area is discussed.

Study of the plasma clearance of antibody--ricin-A-chain immunotoxins. Evidence for specific recognition sites on the A chain that mediate rapid clearance of the immunotoxin

In recent years, antibody--ricin-A-chain immunotoxins have been investigated as anti-neoplastic agents. To achieve in vivo therapy it is necessary that the immunotoxin remains in circulation at a sufficiently high level for a sufficiently long time to allow binding to tumor cells to occur. Therefore, examination of the pharmacology of immunotoxins may elucidate the reasons for the poor in vivo tumoricidal effect of immunotoxin described before. In this study the plasma clearance of antibody--ricin-A-chain immunotoxins, after intravenous injection in animals of different species, has been examined. Sensitive and reproducible techniques were developed to monitor the level of immunotoxin and its constituents in the blood. It is shown that immunotoxins are rapidly eliminated from the bloodstream. Neither the properties of the antibody moiety nor the nature of the linkage binding ricin A-chain to antibody account for the disappearance of immunotoxin from the plasma. On the other hand, we found that the rapid clearance of immunotoxin is due to the mannose residues on the ricin A-chain moiety which are specifically recognized by liver cells. When immunotoxin is administrated together with yeast mannan, which enhances the level of active immunotoxin in circulation by inhibition of liver uptake, the anti-cancer efficacy of immunotoxin in vivo is drastically improved.

Biochemical aspects of immunotoxin preparation

The preparation of immunotoxins, hybrid proteins formed by disulfide bonding an antibody and the A-chain of ricin, has been studied in detail. Optimal conditions, both for the modification of the antibody and the coupling reaction between the modified antibodies and the toxin subunit, have been determined. Conditions of time, temperature and stoichiometry studied suggested 2 protocols for each of the 2 steps of this preparation. Purification and analysis of the physicochemical and biochemical properties of the products yielded well-characterized agents, likely to be of value in clinical studies.

Autologous bone marrow transplantation in leukemia

In patients without HLA-identical donors autologous bone marrow transplantation should be considered as an alternative to conservative treatment of recurrent malignancies. Any patient in complete remission is suitable for an autologous bone marrow transplantation. Bone marrow is aspirated from the patient during complete remission, frozen, and stored at -196 degrees C, and it can be used for hematopoietic reconstitution after high-dose chemotherapy and radiotherapy of the patient. In patients with solid malignant tumors or leukemia the elimination of clonogenic tumor cells from the graft by means of physical separation techniques has not been successful. However, studies in mice and more recently in patients indicate that antibodies destroy residual leukemic cells of the bone marrow transplant without jeopardizing the capacity for regeneration. During our first study in children with acute lymphoblastic leukemia, bone marrow was taken from 50 patients in remission and cryopreserved. Preservation time ranged from 1 to 60 months and the mean number of aspirated nucleated bone marrow cells was 4 X 10(8)/kg body weight (1.9 - 7.4 X 10(8)/kg). After cryopreservation the stem cell viability of the standard samples was 80% - 100%. Autologous bone marrow was transplanted in five patients with common ALL (cALL). The transplant was prepared by incubation with specific antisera of high cytotoxic, selective activity against cALL cells. Our preliminary results show that bone marrow cells aspirated in remission and prepared with antileukemic antisera stimulate repopulation of the recipient's bone marrow and effect hematopoietic regeneration.

Anti-T-cell reagents for human bone marrow transplantation: ricin linked to three monoclonal antibodies

Three new reagents that react against human T cells were synthesized by covalently linking the toxin ricin to monoclonal antibodies recognizing differentiation antigens on the surface of T lymphocytes. Each of these immunotoxins selectively inhibited T-cell proliferation when the cells were incubated in the presence of lactose. Multipotent human stem cells were inhibited only at much higher concentrations. Mixtures of all three immunotoxins were more effective than any one alone. These reagents have the potential for preventing graft-versus-host disease in man.

Targeting potential of antibody conjugates

Although the use of conjugates of enzymes has been considered, their use has not been very actively pursued. Much more interest has focused on the possibilities offered by the use of toxins, their subunits or of ribosome inhibitors. Conjugates of holotoxin which were very active and specific in vitro have been prepared. High in vivo activity and some specificity together with reduced whole body toxicity has been described. When A chain subunits or ribosome inhibiting proteins are used, the results are more mixed. Some very active and specific conjugates are known but others have relatively low activity. The reasons for this may be associated with the particular antigen to which the antibody component is directed, the nature of the A chain or inhibitor and the type and physiological state of the target cell. Application to man seems likely in the first instance to involve the removal of undesirable cells from bone marrow transplants.

Human melanoma cells can be killed in vitro by an immunotoxin specific for melanoma-associated antigen p97

Conjugates (immunotoxins) comprising ricin A-chain and monoclonal antibody 96.5, which is specific for human melanoma-associated antigen p97, inhibited protein synthesis and colony formation of cultured human melanoma cells that expressed more than 80,000 molecules of p97 per cell. Cells expressing fewer than 5,000 molecules of p97 were not killed. The presence of 10 mM ammonium chloride significantly increased the efficiency of the immunotoxin, tumor cells expressing high levels of p97 being killed at immunotoxin concentrations as low as 10(-10) M.

Killing of cultured hepatocytes by conjugates of asialofetuin and EGF linked to the A chains of ricin or diphtheria toxin

A disulfide-linked conjugate between asialofetuin (ASF) and the toxic A chain (RTA) of ricin is as potent a toxin for cultured rat hepatocytes as our previously described conjugate between ASF and fragment A of diphtheria toxin (DTA). An RTA conjugate of epidermal growth factor (EGF) was a potent toxin for 3T3 cells. In contrast, EGF-DTA was essentially nontoxic for 3T3 cells. We have now examined the toxicity of EGF-RTA and EGF-DTA on cultured hepatocytes. The EGF-DTA conjugate, nontoxic to 3T3 cells, is also a potent toxin for hepatocytes. We also observed a decrease with time of culture in the sensitivity of hepatocytes to the ASF and EGF conjugates. This decrease is not a result of a decrease in EGF or asialoglycoprotein receptors.

Cytotoxicity acquired by conjugation of an anti-Thy1.1 monoclonal antibody and the ribosome-inactivating protein, gelonin

Gelonin, a plant protein which can powerfully reduce the protein-synthetic capacity of ribosome preparations, was covalently coupled to anti-Thy1.2 antibody. The conjugate was prepared using N-succinimidyl-3-(2-pyridyldithio)propionate which generates a disulphide linkage between the component molecules. Two conjugate fractions were obtained with Mr of 180 000 and greater than 200 000. After its linkage of the antibody, gelonin suppressed those Thy1.1-bearing T lymphocytes from AKR mice which will respond to phytohaemagglutinin and concanavalin A in tissue culture. The [3H]leucine incorporation with the T-cell mitogens was inhibited by 50% with the 180 000-Mr fraction at a concentration of 0.4 nM and with the greater than 200 000-Mr fraction of pM. Unconjugated gelonin induced comparable reductions in T-cell responsiveness but at concentrations of 30 nM. The conjugates exerted little or no effect upon B lymphocytes or T lymphocytes from CBA mice (Thy1.2 + ve). Thy1.1-expressing AKR lymphoma cell lines, AKR-A and BW5147, were found to be sensitive to the conjugates, albeit much less so than the normal T lymphocytes. The conjugates injected in vivo significantly prolonged the life of CBA mice bearing in an AKR-A lymphoma allograft. It is concluded that gelonin can, by its linkage to an antibody, be rendered cytotoxic with a potency to match or exceed those of the toxins abrin and ricin.

Immunotoxins: hybrid molecules of monoclonal antibodies and a toxin subunit specifically kill tumour cells

Several attempts to attack tumours in experimental systems have been made using conjugates of chemotherapeutic agents or potent toxins with antibodies (immunotoxins). In vitro studies have been highly successful, showing target specificity of a high order in some cases. However, so far, such conjugates have been inadequate in vivo, probably for two main reasons. First, conventional heteroclonal antibodies are perhaps inappropriate, because purification by biochemical methods leaves a large amount of non-antibody gamma-globulins. The use of monoclonal antibodies may overcome this problem. Second, when whole toxins have been conjugated to antibodies there has been a strong residual nonspecific cytotoxicity due to the binding capacity of a subunit, the B-piece of the toxin. (Diphtheria toxin or ricin consist of two polypeptide subunits. The A-piece is responsible for inhibition of protein synthesis on ribosomes, and the B-piece binds to galactose residues on the cell membrane and facilitates the transmembrane passage of the A-piece.) In the present work the problem of nonspecific binding by the B-piece has been circumvented by using the A-piece only as the toxin component of immunotoxins; these immunotoxins are active both in vitro and in vivo.