In vivo administration of lymphocyte-specific monoclonal antibodies in nonhuman primates. IV. Cytotoxic effect of an anti-T11-gelonin immunotoxin

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

The term ribosome-inactivating protein (RIP) is used to denominate proteins mostly of plant origin, which have N-glycosidase enzymatic activity leading to a complete destruction of the ribosomal function. The discovery of the RIPs was almost a century ago, but their usage has seen transition only in the last four decades. With the advent of antibody therapy, the RIPs have been a subject of extensive research especially in targeted tumor therapies, which is the primary focus of this review. In the present work we enumerate 250 RIPs, which have been identified so far. An attempt has been made to identify all the RIPs that have been used for the construction of immunotoxins, which are conjugates or fusion proteins of an antibody or ligand with a toxin. The data from 1960 onwards is reviewed in this paper and an extensive list of more than 450 immunotoxins is reported. The clinical reach of tumor-targeted toxins has been identified and detailed in the work as well. While there is a lot of potential that RIPs embrace for targeted tumor therapies, the success in preclinical and clinical evaluations has been limited mainly because of their inability to escape the endo/lysosomal degradation. Various strategies that can increase the efficacy and lower the required dose for targeted toxins have been compiled in this article. It is plausible that with the advancements in platform technologies or improved endosomal escape the usage of tumor targeted RIPs would see the daylight of clinical success.

The effect of immunotoxins directed against CD80 and CD86 on primary T-cell alloresponses

Activation of primary resting T cells requires costimulation which can be delivered by the B7 molecules (CD80 and CD86) expressed on activated antigen-presenting cells (APC). In the present study, we examined in vitro effects of immunotoxins (ITs) composed of gelonin conjugated to mAbs against CD80 or CD86 (alphaCD80-IT and alphaCD86-IT). The specificity of both ITs was demonstrated using CD80 and CD86 transfected cell lines. In primary mixed lymphocyte cultures (MLCs), it was found that the average inhibitory capacity of alphaCD86-IT (72%) and alphaCD80-IT (30%) was significantly higher than alphaCD86 (54%) and alphaCD80 (11%). In reculture MLC experiments it was found that peripheral blood mononuclear cells pretreated with alphaCD86/alphaCD80 regained full stimulatory capacity whereas alphaCD86-IT/alphaCD80-IT pretreatment induced >95% loss of stimulatory capacity. Our results therefore demonstrate that these alphaB7-ITs functionally block B7-CD28 costimulatory signaling and eliminate activated APC.

Pharmacokinetics, tissue distribution, and in vivo antitumor effects of the antimelanoma immunotoxin ZME-gelonin

Antibody ZME-018 is directed against the gp240 glycoprotein on the surface of more than 80% of human melanoma cell lines and fresh biopsy specimens. Previous studies in our laboratory described the in vitro cytotoxicity and specificity of an immunoconjugate composed of mAb ZME-018 and the plant toxin gelonin. The present study described the in vivo pharmacokinetics and therapeutic effects of ZME-gelonin in human xenograft/nude mouse models. Pharmacokinetic studies of 125I-labeled ZME-018 and ZME-gelonin demonstrated a shorter terminal-phase plasma half-life of the immunoconjugate than native ZME (20.6 h compared to 41.3 h). The initial volume of distribution of the ZME-gelonin was also higher compared to that of ZME alone (2.85 ml compared to 1.94 ml) suggesting an enhanced distribution of the conjugate outside the vasculature. The corresponding area under the concentration/time curve for the ZME-gelonin conjugate was 40% lower than that of ZME alone (80.8 compared to 139.6 microCi.ml-1 x min). In nude mice bearing well-developed human tumor A375 melanoma xenografts, administration of 125I-labeled ZME and ZME-gelonin resulted in tumor-to-blood ratios of 1.9 +/- 0.5 and 1.5 +/- 0.6 respectively by 72 h. Compared with ZME, ZME-gelonin conjugate caused an increase in the content of radiolabel in kidney, spleen and liver. Treatment of nude mice bearing well-developed (150 mm3) s.c. A375-M xenografts with divided doses of ZME-gelonin, ZME, gelonin, or saline resulted in suppression of tumor growth in the immunotoxin group but virtually no retardation of tumor growth in the control groups. Using a murine model for a rapidly growing lethal metastatic human melanoma, treatment with ZME-gelonin resulted in a mean survival of 44 days, 213% increase in mean survival time compared with the saline treatment (14.2 +/- 2 day survival). Given these encouraging results, we are proceeding with further preclinical development of this immunotoxin.

Characterization of the increased cytotoxicity of gelonin anti-T cell immunoconjugates compared with ricin A chain immunoconjugates

Ribosomal inactivating proteins such as gelonin (Gel) and ricin A chain (RTA) conjugated to MoAbs bind to specific target cells, and upon internalization inhibit protein synthesis, ultimately resulting in cell death. We report here that Gel anti-T cell MoAb conjugates are more cytotoxic than RTA conjugates when tested against human peripheral blood mononuclear cells (PBMC). This increased cytotoxicity is observed whether Gel is conjugated to the anti-T cell MoAb or to an anti-mouse immunoglobulin Fab' fragment which then binds to the murine anti-human T cell MoAb. Gel conjugates are not only effective at lower concentrations, but also produce a greater extent of inhibition of cellular proliferation. Moreover, a 10 min exposure to a Gel conjugate is as effective as a 90 h exposure to an RTA conjugate. When part of anti-T cell F(ab')2 or Fab' conjugates, Gel affects the early steps in cellular intoxication more than RTA; Gel conjugates bind more avidly and accelerate the modulation of antigen. In contrast, when part of whole IgG conjugates, Gel does not affect the binding to or modulation of surface antigen compared with RTA, while it does increase conjugate cytotoxicity. These observations suggest that Gel may be delivered more efficiently into the cytosol than RTA. A divergent intracellular pathway for Gel is also supported by the inability of chemical potentiators, which strongly enhance RTA potency, to affect Gel potency. These properties of Gel might also be advantageous for immunoconjugates made with other MoAbs or receptor-binding molecules.

Use of blue-sepharose for purification of immunotoxin containing type 1 ribosome-inactivating protein, gelonin

This paper describes a method suitable for purifying immunotoxin containing type 1 ribosome-inactivating protein, gelonin. The separation of free (unreacted) 80G, a monoclonal antibody against alpha-fetoprotein (AFP), from semipurified 80G-gelonin conjugate was unsuccessful by conventional CM-Sepharose ion-exchange chromatography because the isoelectric point of the conjugate did not increase enough to reach that of gelonin alone. In contrast, Blue Sepharose affinity chromatography could efficiently separate free 80G from the semipurified conjugate because the conjugate was bound to the column by its gelonin moiety while free 80G was not in buffer containing NaCl of a particular concentration range. However, a small amount of conjugate containing gelonin modified with N-succinimidyl 3-(2-pyridyldithio)propionate, but not with 2-iminothiolane, could not bind to the column. The conjugate purified by the use of Blue Sepharose showed selective cytotoxicity against AFP-producing human hepatoma cells.

Immunotoxins composed of monoclonal antibody to alpha-fetoprotein and gelonin as a potent hepatoma-targeted drug delivery system

This study was carried out to evaluate our monoclonal antibody (MoAb) to alpha-fetoprotein (AFP), 80G, as a carrier for targeting AFP-producing hepatoma. Pharmacokinetic analysis showed that the MoAb 80G was actively incorporated into AFP-producing HuH-7N cells (xenograft of human hepatoma cell line, HuH-7) in nude mice. Four conjugates composed of MoAb 80G, and a type 1 ribosome-inactivating protein, gelonin, were prepared. They involve two disulfide-linked and two thioether-linked conjugates. The binding activity of conjugates against AFP remained as high as that of intact 80G according to enzyme-linked immunosorbent assay. The in vitro cytotoxic effects of all the conjugates were specific against AFP-producing HuH-7 cells. Of these conjugates, two containing gelonin modified with 2-iminothiolane were more potent than the others. They showed significant antitumor activity upon AFP-producing HuH-7N cells in nude mice. However, the disulfide conjugate was more toxic to mice than the thioether conjugate judging from the loss in body weight and the liver damage. These results suggest that our MoAb 80G is a suitable carrier for targeting AFP-producing hepatoma cells, and that the noncleavable thioether conjugate is promising as an AFP-producing hepatoma-targeted drug delivery system.

Cloning and expression of a gene encoding gelonin, a ribosome-inactivating protein from Gelonium multiflorum

A cDNA copy of the gel gene, encoding gelonin (Gel), has been cloned from the seeds of the Asian plant, Gelonium multiflorum. Gel is a type-I ribosome-inactivating protein which has been produced in Escherichia coli as a secreted protein under the transcriptional control of the Salmonella typhimurium araB promoter and linked to the pectate lyase (pelB) leader sequence from Erwinia carotovora. Recombinant, soluble Gel (re-Gel) can be recovered from the E. coli culture supernatant at a yield of greater than 1 mg/ml, and it inhibits protein synthesis in vitro to the same extent as the native protein isolated from plant seeds.

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.

Recombinant human soluble CD4 does not inhibit immune function in cynomolgus monkeys

Recombinant soluble CD4 (sT4) has been shown to inhibit infectivity of HIV. Because of the role CD4 plays in the interaction of T-helper lymphocytes and cells bearing MHC Class II antigens, a potential adverse effect of therapy with sT4 is interference with lymphocyte function. To address this issue, we studied the effects of sT4 on mitogen-mediated blastogenesis, mixed lymphocyte reactions, and delayed type hypersensitivity reactions (DTH) in cynomolgus monkeys. We found no evidence of sT4-mediated suppression on the in vitro response to concanavalin A, phytohemagglutinin or pokeweed mitogen in 2-way mixed lymphocyte reactions, either when sT4 was added to the cultures or when cells were obtained 3 hr after drug administration from animals that received up to 100 mg/kg as an intravenous bolus. Furthermore, we also found no effect of sT4 on lymphocyte subsets or on the ability of monkeys to respond to dinitrochlorobenzene (DNCB)-mediated DTH. Because of the high degree of conservation of CD4 and MHC Class II antigens across the macaque-human barrier, these data suggest that soluble CD4-like molecules are unlikely to be immunosuppressive in humans.

Immunotoxins containing single-chain ribosome-inactivating proteins

We have summarized what is currently known about the distribution, biological role, and the mechanism of action of the single chain ribosome-inactivating proteins and described the purification of one of them, gelonin, as an example. ITs have been made with several of these proteins and, depending upon the antibody used for conjugation, these immunoconjugates can show specific in vitro cytotoxicity which is similar to that shown by equivalent ITs prepared with ricin A chain. The most potent of these conjugates have shown antitumor efficacy in a variety of animal tumor models, including both syngeneic rodent tumors and xenografts in nude or immunosuppressed mice. An important point needs to be addressed, however, before concluding that ITs containing single chain toxins will be clinically useful. A major problem with this approach is that it is likely that both the antibody and the toxin components of these conjugates will be immunogenic. Both antitoxin and antixenogenic immunoglobulin responses have been shown to occur in animals after infusion of IT, although it has not yet been clearly demonstrated that such antibody responses adversely effect the pharmacokinetics or the efficacy of immunoconjugates. Thus, preliminary enthusiasm over the efficacy of these new reagents must be tempered with the knowledge that their use in the clinic may be limited by the host immune responses or other as yet undefined factors. The fact that there are many immunologically distinct single chain ribosome-inactivating proteins does suggest one way of evading the antitoxin response, by a sequential treatment with a panel of immunoconjugates, each containing a different single chain toxin.