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 describes the in vivo pharmacokinetics and therapeutic effects of ZME-gelonin in human xenograft/nude mouse models. Pharmacokinetic studies of¹²⁵I-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 1.91 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 μCi·ml^?1 x min). In nude mice bearing well-developed human tumor A375 melanoma xenografts, administration of¹²⁵I-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, a 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.     

Cellular resistance to the antimelanoma immunotoxin ZME-gelonin and strategies to target resistant cells

 The development of cellular resistance to immunotoxins has been demonstrated in a variety of models and can involve a number of mechanisms. For the present study, an immunotoxin was utilized composed of an antimelanoma antibody ZME-018 recognizing a 240-kDa surface glycoprotein (gp 240) and the plant toxin gelonin. Human melanoma cells (A375-M) were grown in the presence of increasing amounts of ZME-gelonin and a clonal variant (A-375-ZR) was developed that was 100-fold resistant to ZME-gelonin compared to parental cells. Scatchard analysis showed that the A375-M parental cells had 260×10³ ZME-gelonin-binding sites/cell with relatively low affinity (5 nM). In contrast, resistant A375-ZR cells demonstrated a reduced number of low-affinity sites (160×10³/cell), but showed a small number (47×10³) of higher-affinity sites (0.8 nM). Internalization rates and degradation rates of ¹²⁵I-labeled ZME-gelonin were identical in both the parental and resistant cells. A375-ZR cells were found to be more resistant to vincristine and doxorubicin than were parental cells. Both cell lines were almost equally sensitive to native gelonin, 5-fluorouracil (5-FU), cisplatin, melphalan, carmustine, interferon γ (IFNγ) and IFNα. In addition, both cell lines were equally sensitive to another gelonin-antibody conjugate that binds to cell-surface, GD₂ (antibody 14G₂A). However, resistant cells were twice as sensitive to the cytotoxic effects of etoposide than were parental cells. Finally, a variety of agents were tested in combination with ZME-gelonin against A375-ZR cells in an attempt to identify agents to augment immunotoxin cytotoxic effects against resistant cells. The agents 5-FU, cisplatin, IFNγ, IFNα, and etoposide were the most effective in augmenting the cytotoxicity of ZME-gelonin against resistant cells. These studies suggest that development of resistance to one immunotoxin does not cause development of cross-resistance to other gelonin immunotoxins. Further, specific biological response modifiers and chemotherapeutic agents may be effective in augmenting the effectiveness of immunotoxins and specifically targeting or reducing the emergence of immunotoxin-resistant cells. Received: 15 March 1995 / Accepted: 28 November 1995     

Cell Internalization Studies of Gadofullerene-(ZME-018) Immunoconjugates into A375m Melanoma Cells

Fullerene (C(60))-monoclonal antibody (mAb) immunoconjugates have been determined to internalize into target cells using water-soluble Gd(3+) ion-filled metallofullerenes (Gd@C(60)[OH](x)). Two separate conjugations of Gd@C(60)(OH)(x) with the antibody ZME-018 and a murine antibody mixture (MuIgG) were performed in a 1:5 mAb/Gd@C(60) ratio. Characterization of the immunoconjugates was established using inductively coupled plasma mass spectrometry (ICP-MS) for Gd(3+) and UV-Vis spectrometry (for Gd@C(60) + C(60)). Once conjugated, enzyme-linked immunosorbent assays showed little change in the specific binding of ZME-018. Each immunoconjugate was exposed to two cancer cell lines, A375m (antigen positive), and T24, bladder carcinoma (antigen negative). Internalization levels of the immunoconjugate were determined at various time points during 24 hours by harvesting and digesting the cells with 70% HNO(3) for Gd(3+) ion analysis by ICP-MS. These results are the first to demonstrate the practicality of a targeted cancer therapy based on fullerene immunotherapy.     

An antimelanoma immunotoxin containing recombinant human tumor necrosis factor: tissue disposition,pharmacokinetic, and therapeutic studies in xenograft models

The ability of monoclonal antibody conjugates to re-direct plant or bacterial toxins, chemotherapeutic agents and radionuclides to selected target cells has been well-documented. Recombinant human tumor necrosis factor (TNF) is a macrophage-derived, non-glycosylated (17 kDa) peptide with a broad range of biological and immunological effects including antiviral activity, cytotoxic and cytostatic effects. A conjugate of the antimelanoma antibody ZME-018 and TNF in previous studies has shown melanoma-selective cytotoxic effects in vitro. Pharmacokinetic studies of the ZME-TNF immunotoxin showed that the agent cleared from plasma biphasically with -and -phase half-lives similar to that of ZME itself (72 min and 36 h compared to 84 min and 41 h respectively). In contrast, TNF itself was cleared rapidly from plasma with a terminalphase half-life of only 2.7 h. The clearance rate of ZME-TNF from plasma (Cl_p) was almost tenfold more rapid than for ZME (1.1 versus 0.16 ml/kg x min) but was threefold slower than the clearance for TNF itself (3.4 ml/kg x min). Tissue distribution studies in nude mice bearing human melanoma xenografts showed similar tumor localization of the immunotoxin compared to the free antibody and slightly higher concentrations in liver and kidney compared to ZME itself. Treatment of nude mice bearing well-developed A375 tumors with the immunotoxin resulted in a statistically significant (P<0.002) suppression in tumor growth rate (fivefold increase) compared to saline-treated controls, which increased 20-fold over the same period. These studies demonstrate the feasibility of this approach and suggest that TNF may represent a non-antigenic alternative to immunotoxins containing plant and bacterial toxins.Research conducted, in part, by the Clayton Foundation for Research     

Antitumor properties of vindesine-monoclonal antibody conjugates

Summary The anticancer alkaloid vindesine (VDS) was conjugated to four mouse monoclonal antibodies recognizing human tumor-associated antigens. The antibodies were 96.5 (antimelanoma, IgG2a); 791T/36 (antiosteogenic sarcoma, IgG2b); 11.285.14, and 14.95.55 (anticarcinoembryonic antigen, IgG1 and IgG2a respectively). Conjugates VDS-96.5 and VDS-791T/36 were tested in vitro and shown to be specifically cytotoxic for target cells expressing the appropriate antigen. The in vivo effects of the antibodies and conjugates were tested against human tumor xenografts in athymic or immunodeprived mice using multiple treatments. Conjugate VDS-96.5 retarded the initial growth of a melanoma xenograft, whereas free antibody was without effect. Similarly, VDS-791T/36 but not free antibody retarded the growth of osteogenic sarcoma 791T. The most marked antitumor effects observed were those obtained with VDS conjugates of the anti-CEA antibodies against a colorectal tumor xenograft. Antibody 14.95.55 suppressed tumor growth both alone and as a VDS conjugate, whereas 11.285.14 produced only a slight effect alone but an almost complete and lasting suppression of tumor growth as a VDS conjugate. Free VDS had little effect at nontoxic levels. Acute studies showed that VDS-11.285.14 conjugate was considerably less toxic than free VDS in Balb/c mice.     

Selective killing of target cells by antibody-ricin A chain or antibody-gelonin hybrid molecules: comparison of cytotoxic potency and use in immunoselection procedures

Ricin A chain and gelonin, two plant proteins that can powerfully reduce the protein synthetic capacity of ribosome preparations, were covalently coupled to a monoclonal anti-Thy-1.2 antibody. Two conjugates were prepared by using N-succinimidyl-3-2(-pyridyldithio)propionate, which generates a disulfide linkage between the component molecules. Both conjugates specifically killed Thy-1.2 antigen-bearing EL-4 cells, but had no effect on Thy-1.2- BW5147 cells. The cytotoxic potency of both reagents was determined by comparing the cloning efficiency of E1-4 target cells after treatment with the conjugate. The frequency of cells surviving treatment with 45 micrograms/10(6) cells of the gelonin conjugate was 1/72, whereas this frequency was 1/836 after exposure to 7 micrograms of the Ricin A chain conjugate. Both reagents could be successfully used to select Thy-1.2- cells from cell mixtures consisting of Thy-1.2+ and Thy-1.2- cells.     

Selective killing of carcinoembryonic-antigen (CEA)-producing cells in vitro by the immunoconjugate cytorhodin-S and CEA-reactive cytorhodin-S antibody CA208

Summary Cytorhodin-S, an anthracycline derivative, was covalently coupled to a monoclonal antibody (mAb) CA208, against carcinoembryonic antigen (CEA) in order to achieve selective killing of a CEA-producing colon carcinoma cell line, COLO 205. The conjugate (15 molecules of drugs/antibody) retained substantial antibody activity as well as drug activity as assessed by enzyme-linked immunosorbent assay and 24-h L1210 proliferation assay, respectively. Furthermore, the conjugate inhibited the growth of COLO 205 cells in a short-term cytostatic assay. This cytostatic effect of the immunoconjugate on COLO 205 cells was inhibited in a dose-dependent manner by pretreatment of the cells with unconjugated CA208 mAb. In addition, chloroquine, a lysosomotropic agent, inhibited the cytostatic effect of the immunoconjugate, indicating the involvement of lysosomal enzymes in releasing drugs from the immunoconjugate. The antibody (CA208) was significantly incorporated into the cytoplasm of COLO 205 cells as demonstrated by immuno-electron microscopy. These in vitro results indicate that cytorhodin-S may be a good partner in immunoconjugates. However, in vivo animal experiments with the immunoconjugate revealed that the immunoconjugate was not so effective in prolonging survival. Thus, in vivo efficacy of this immunoconjugate remains to be further improved in application to cancer immunotherapy.     

急非淋M_5型白血病单克隆抗体与柔红霉素偶合物的制备及体外细胞毒作用

用ＰＥＧ沉淀法及ＤＥＡＥ─Ｓｅｐｈａｃｅｌ离子交换层析和Ｓｅｐｈａｃｒｙｌｓ─３００凝胶层析提纯的抗急非淋Ｍ_５型白血病单克隆抗体ＩＡ_１（Ｍ_ｃＡｂ１Ａ_１）,纯度为９９．９％。用葡聚糖作中间体将Ｍ_ｃＡｂ_１Ａ_１与柔红霉素（ＤＮＲ）偶联,制备的免疫偶合物仍保持较好的抗体活性及药物活性。在４８小时杀伤实验中,此偶合物对Ｕ_（９３７）靶细胞的杀伤作用与游离ＤＮＲ相近（Ｐ＞０．０５）,明显大于对照偶合物（Ｐ＜０．０１）;在１小时预杀伤实验中,此偶合物对靶细胞的杀伤作用明显大于游离ＤＮＲ及对照偶合物（Ｐ＜０．０１）。在两个实验中,此偶合物对Ｗｉｓｈ非靶细胞的毒性都明显小于游离ＤＮＲ（Ｐ＜０．０１）;表明此偶合物在体外具有较好的选择性细胞毒作用。     

A potent and specific immunotoxin for tumor cells expressing disialoganglioside GD2

Summary Monoclonal antibody 14G2a (anti-GD₂) reacts with cell lines and tumor tissues of neuroectodermal origin that express disialoganglioside GD₂. mAb 14G2a was coupled to the ribosome-inactivating plant toxin gelonin with the heterobifunctional cross-linking reagentN-succinimidyl-3(2-pyridyldithio)propionate. The activity of the immunotoxin was assessed by a cell-free translation assay that confirmed the presence of active gelonin coupled to 14G2a. Data from an enzyme-linked immunosorbent assay demonstrated the specificity and immunoreactivity of the 14G2a-gelonin immunotoxin, which was identical to that of native 14G2a. Assays for complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC) revealed that these functional properties of the native 14G2a antibody were also preserved in the 14G2a-gelonin immunotoxin. The gelonin-14G2a immunotoxin was directly cytotoxic to human melanoma (A375-M and AAB-527) cells and was 1000-fold more active than native gelonin in inhibiting the growth of human melanoma cells in vitro. The augmentation of tumor cell killing of 14G2a-gelonin immunotoxin was examined with several lysosomotropic compounds. Chloroquine and monensin, when combined with 14G2a-gelonin immunotoxin, augmented its cytotoxicity more than 10-fold. Biological response modifiers such as tumor necrosis factor and interferon and chemotherapeutic agents such as cisplatinum andN,N-bis(2-chloroethyl)-N-nitrosourea (carmustine) augmented the cytotoxicity of 14G2a-gelonin 4- to 5-fold. The results of these studies suggest that 14G2a-gelonin may operate directly by both cytotoxic efforts and indirectly by mediating both ADCC and CDC activity against tumor cells; thus it may prove useful in the future for therapy of human neuroectodermal tumors.Research conducted, in part, by the Clayton Foundation for Research     

Significance of antigen, drug, and tumor cell targets in the preclinical evaluation of doxorubicin, daunorubicin, methotrexate, and mitomycin-C monoclonal antibody immunoconjugates

We tested drug monoclonal antibody immunoconjugates in vitro in 72 h 3H-thymidine assays and in vivo in athymic mice bearing human tumor xenografts of the same target cells. Experimental arms included control, monoclonal antibody, drug, drug + antibody, the test immunoconjugate, and a negative control immunoconjugate with an equivalent molar amount of drug for in vitro experiments, and the amount of drug conjugated to 500 micrograms of antibody in the animal experiments. Monoclonal antibodies included T101, an IgG2a that reacts with a rapidly modulating antigen, 9.2.27, an IgG2a that reacts with a slowly modulating antigen, and ME7, an IgG1 that reacts with a slowly modulating antigen. Cells used in testing included MOLT-4 (T lymphoma), 8392 (B lymphoma), and M21 (melanoma). Drugs tested were doxorubicin, daunorubicin, methotrexate, and mitomycin-C. M21 cells were resistant to daunorubicin in vitro but were inhibited by the 9.2.27 daunorubicin immunoconjugate. T101, 9.2.27, and ME7 cis-aconitate anthracycline immunoconjugates and mitomycin-C-glutarate immunoconjugates were specifically cytotoxic only for antigen positive cells in vitro and were superior to free drug in vivo. These results confirm that antigen specific-cytotoxic drug immunoconjugates can be produced that are superior to the same dose of free drug. However, each monoclonal antibody drug target system is unique and must be well-characterized for appropriate interpretation of data.     

The antileukemic efficacy of an immunotoxin composed of a monoclonal anti-Thy-1 antibody disulfide linked to the ribosome-inactivating protein gelonin

Summary We prepared an immunoconjugate consisting of a monoclonal antibody recognizing the Thy-1 antigen and the ribosome-inactivating protein gelonin linked by a disulfide bond. This immunotoxin preparation was judged to contain less than 5% free antibody or gelonin. It was highly toxic in vitro in an antigen-specific fashion to the Thy-1 expressing RADA leukemia of A/J mice. The IC₅₀ of this preparation on RADA in vitro was 10^–12 M, while the IC₅₀ on the Thy-1 negative S1509a fibrosarcoma of A/J mice was 10^–7 M. The toxicity of this immunoconjugate was also measured in a direct proliferation assay and it was found that a 4-h exposure and a 24-h exposure of RADA cells to a 1 nM concentration of immunotoxin killed 90% and 99.9% of cells, respectively. Furthermore, efficacy in vitro was not due to the intrinsic susceptibility of RADA cells to tis type of immunotoxin, as one prepared with gelonin and an antibody recognizing the TL^a determinant on this leukemia had no efficacy in vitro. Clearance of the anti-Thy-1-gelonin immunoconjugate from the circulation of A/J mice after i.v. injection was rapid, especially during the first 8 h after injection, possibly because of binding to Thy-1 expressing tissue. Delivery of immunoconjugate to ascitic tumor in vivo was substantially better if the immunoconjugate was given by i.p. injection, rather than by the i.v. route. When given either i.v. or i.p. at the time of i.p. tumor inoculation in vivo, the anti-Thy-1-gelonin immunotoxin showed potency in an antigen-specific fashion; while this immunoconjugate prolonged survival and frequently cured RADA-inoculated mice, neither anti-Thy-1 antibody, gelonin, a combination of the two, nor immunotoxin of irrelevant specificity had any significant effect on survival. Anti-Thy-1-gelonin also had no effect on survival of A/J mice inoculated i.p. with S1509a. Furthermore, it was determined that a single i.p. dose of anti-Thy-1-gelonin killed 90% to 99% cells in vivo, and that the immunoconjugate was about as effective in this model as either adriamycin or cytoxan.This work was supported by ImmunoGen Inc. and in part by a grant from the National Institutes of Health, CA-14723     

Transformation and actions of extracellular NADP+ in the rat liver

The effectiveness of gelonin to arrest protein synthesis, thereby limiting the growth of cancer cells was studied by encapsulating it into liposomes. The protein was extracted from the seeds of Indian plant Gelonium multiflorum by ammonium sulfate precipitation and purified using cation-exchange and gel-filtration chromatography. Biological activity of purified gelonin was determined using a rabbit reticulocyte lysate assay in the cell-free translational experiments. Gelonin was encapsulated in conventional liposomes prepared by the dry film method in order to retain biological activity of the entrapped protein. Carcinogenesis was induced in Swiss albino mice by intravenous administration of DBN (10 mg kg(-1) body weight) at weekly intervals. Marker enzyme assays (GGT, AChE, and GST), GSH levels, cell proliferation assay, hepatocyte DNA analysis, histological examination of micro sections of liver tissues were parameters used to monitor carcinogenesis induction, and regression in mice. From the in vitro experiments conducted, it was observed that gelonin upon its encapsulation into liposome, resulted in significant destruction of the transformed liver cells by its cytotoxic effects that arrest protein synthesis. Various parameters studied to monitor regression also suggested mass cell destruction to liver upon administration of liposomal gelonin in mice exposed to DBN.     

Antibody-targeted photolysis: in vitro immunological, photophysical, and cytotoxic properties of monoclonal antibody-dextran-Sn(IV) chlorin e6 immunoconjugates.

A set of anti-melanoma immunoconjugates were prepared which contained chlorin e6: antibody molar ratios of 18.9:1, 11.2:1, 6.8:1, and 1.7:1. All immunoconjugates retained antigen binding activity regardless of the chromophore:antibody substitution ratio that was attained. In contrast, the ground-state absorption spectra of the immunoconjugates showed features which appeared to be dependent on the chromophore:antibody molar ratio. In addition, the quantum yield of singlet oxygen generated by the conjugated chromophores was observed to be significantly less than that observed with the unbound dye. Time-resolved absorbance spectroscopy of the chromophore excited triplet state indicated that the loss of singlet oxygen quantum yield resulted from diminished chromophore triplet yield. Analysis of data obtained from in vitro photolysis of target melanoma cells, in combination with that obtained from the immunochemical and photochemical studies, indicates that the observed immunoconjugate phototoxicity can be reasonably quantitatively represented by (1) the ability of the immunoconjugate to bind SK-MEL-2 cell surface antigen, (2) the amount of chromophore localized at the target cells by immunoconjugate binding, (3) the delivered dose of light at 634 nm, and (4) the singlet oxygen quantum yield of the antibody-bound photosensitizer. Though these data argue strongly for photolysis by the cumulative dosage of singlet oxygen at the cell membrane, nonetheless, the concurrent photoinduced release of other cytotoxic agents should not be ruled out.     

Effect of thiolation of amino groups of gelonin on its protein-biosynthesis inhibitor activity.

Gelonin was purified from the dry seeds of Gelonium multiflorum by ammonium sulfate fractionation followed by cation-exchange and gel-filtration chromatography in order to minimize extraction of non-proteineous material. Gelonin was characterized for its purity, homogeneity and molecular weight determination by RP-HPLC and SDS-PAGE analysis respectively. The amino groups of pure gelonin were thiolated by a hererobifunctional cross-linking agent, SPDP which is used in the design of cytotoxic hybrid molecules. Therefore, an attempt has been made to study the effect of thiolation on the ribosome inactivating property of gelonin. Thiolation of one amino group resulted in the loss of about 90% protein synthesis inhibition activity. Further modification of 2-3 amino groups further hampered the bioactivity (greater than 95-99.5%) of gelonin, suggesting that a 1:1 molar ratio of carrier-toxin conjugate would be highly active against the target cells.     

Cytotoxic ribonuclease chimeras. Targeted tumoricidal activity in vitro and in vivo.

Monoclonal antibodies to the transferrin receptor or to the T cell antigen, CD5, were chemically linked to mammalian RNase A and found to specifically inhibit protein synthesis in antigen-positive cells. Antibody-mediated specificity of these cytotoxic ribonuclease chimeras (CRCs) was demonstrated in three ways. 1) Toxicity was due to the chemical linkage of RNase to antibody, as the individual components added separately or in combination did not inhibit protein synthesis; 2) the anti-transferrin receptor CRCs inhibited protein synthesis in those cells expressing the human transferrin receptor (K562, U251, Jurkat cells) but had no detectable toxicity to cells lacking the human transferrin receptor (Vero or NIH 3T3 cells); 3) free antibody to either the human transferrin receptor (454A12 or 5E-9) or to the T cell antigen, CD5 (T101), blocked the cytotoxicity of the respective CRC. Two CRC species, designated P1 and P2, that differed in size and stoichiometry of RNase A to antibody, were purified by size-exclusion high performance liquid chromatography. The higher molecular weight P1 conjugate had an IC50 of 20-30 nM, whereas the P2 conjugate had a higher IC50 of 300-500 nM. Bioactivity could be reversibly increased more than 10-fold by freezing. The cytotoxicity of the CRCs was examined in vivo in a solid tumor animal model. Intratumoral injections of an anti-transferrin receptor CRC into established U251 human glioblastoma tumors grown in the flanks of nude mice prevented tumor growth, whereas RNase A mixed with antibody was ineffective. CRCs, therefore, express cytotoxicity in vitro and in vivo. Mammalian nucleases coupled to antibodies may be utilized as cell type-selective cytotoxins and have potential as pharmacologic reagents. The systemic toxicity and immunogenicity observed with mammalian derived cytotoxins may be significantly less than that of the currently employed plant- and bacterial-derived immunotoxins.     

Selective toxicity of neocarzinostatin-monoclonal antibody conjugates to the antigen-bearing human melanoma cell line in vitro

Summary Monoclonal antibodies (IgG₁) against high molecular weight antigen A-1-43 on human melanoma cell line A-375 were successfully linked to the anti-tumour protein neocarzinostatin (NCS) using the heterobifunctional reagent N-succinimidyl 3-(2-pyridyldithio)-propionate (SPDP). The conjugate retained both the reactivity of the antibody and the toxicity of the drug. The antigen-bearing cell line A-375, antigen-lacking cell line MeWo and normal skin fibroblasts were exposed to NCS-monoclonal antibody conjugates. As negative control, cells were also treated with free NCS and NCS coupled to normal mouse IgG₁ antibodies. Inhibition of ³H-thymidine uptake after treatment was used to measure the biological activity of the cytotoxic drug complex or substance, respectively.Comparing the inhibition dose for 50% uptake (ID₅₀) it was found that the monoclonal antibody-drug complex is about 100 times more toxic for the antigen-bearing cell line than free NCS or normal mouse IgG₁-NCS. This high toxicity is due to a local increase of drug concentration on these cells. With the two cell lines lacking the appropriate antigen no significant differences in the ID₅₀ values were observed. A selectivity factor of 40–50 was obtained by comparing the cytotoxic effect of the monoclonal antibody-NCS conjugate upon the antigen-bearing as opposed to the antigen-lacking cell type. These data demonstrate, that the toxicity of NCS can be directed by monoclonal antibodies to human tumour cells carrying the corresponding surface antigen.     

Antitumour activity of a sterically blocked ricin immunotoxin on a human colorectal adenocarcinoma grafted subcutaneously in nude mice

Summary We prepared a ricin-antibody conjugate, lacking the ability to bind the galactosidic residues of Sepharose 6B, a so-called blocked immunotoxin. The monoclonal antibody AR-3 was cross-linked to ricin through a thioether bond. Further studies showed that the immunoconjugate suppressed the tumour growth of HT-29 cells in intraperitoneally grafted nude mice, without showing any undesirable ricin toxicity.In this work, to demonstrate the therapeutic activity of the AR-3—ricin conjugate injected into mice bearing subcutaneous tumour, we first evaluated its pharmacokinetic behaviour and biodistribution. The behaviour of the immunoconjugate injected intravenously was almost intermediate between that of the antibody and ricin. Moreover, when the immunotoxin was intravenously administered to nude mice bearing subcutaneous tumour, no therapeutic effects appeared, in accordance with the relatively low permeability of the immunotoxin from the blood to the skin. In contrast, peritumoral treatment produced a strong reduction of the neoplastic nodules without substantial regrowth of the malignant cells. This result was also achieved when the immunotoxin treatment was performed on a well-established tumour. This finding was strictly related to the specificity of the immunoconjugate, since the analogous treatment with an irrelevant immunotoxin showed therapeutic failure.     

Monoclonal antibody to human 66,000 molecular weight plasminogen activator from melanoma cells. Specific enzyme inhibition and one-step affinity purification

Hybridomas producing a monoclonal IgG1 antibody to a human plasminogen-activating enzyme with an apparent mol. wt. of 66,000 (66 K, HPA66) from human melanoma cells were obtained by fusion of NSI-Ag 4/1 mouse myeloma cells with spleen cells from a mouse immunized with a partially purified preparation of the enzyme. Screening for clones of hybridomas producing antibodies to HPA66 was performed with the impure enzyme preparation. A preliminary screening included enzyme-linked immunosorbent assay and SDS-polyacrylamide gel electrophoresis (SDS-PAGE) followed by immunoblotting; the final identification was based on inhibition of the enzymatic activity of HPA66 which was complete at high antibody concentrations. No inhibition of three other human and murine plasminogen activators or of plasmin was observed. Employing a one-step affinity procedure with the antibody coupled to Sepharose, HPA66 was purified approximately 200-fold from conditioned medium from the melanoma cells with a yield of 79%. The purified HPA66 was homogeneous as evaluated by SDS-PAGE. Electrophoresis under reducing conditions indicated that it consisted of one polypeptide chain. The binding constant between the antibody and 125I-labelled HPA66 was approximately 2.5 x 10(9) l/mol. The antibody did not bind to a variety of other plasminogen activators, including 52-K and 36-K human enzymes and 48-K and 75-K murine enzymes. Previously, a monoclonal antibody against another enzyme was derived by the sole use of enzyme inhibition for screening. The present study represents a modification of this procedure that can be used when antibody-unrelated inhibitors of the enzyme are present in hybridoma culture fluid.     

HER2 as a Promising Target for Cytotoxicity T Cells in Human Melanoma Therapy

Anti-HER2/neu antibody therapy has been reported to mediate tumor regression of HER2/ neu⁺ tumors. Here we demonstrated the expression of HER2 in a wide range of human melanoma cells including a primary culture and seven cell lines, and we further investigated whether HER2 could be served as a target for T cell mediated immunotherapy of human melanoma. Specific cytolytic activity of activated T cells (ATC) armed with anti-CD3 x anti-HER2 bispecific antibody (HER2Bi-Ab) against Malme-3M-luc cells was evaluated by bioluminescent signal generated by luciferase reporter which did not alter HER2 expression or proliferation ability of Malme-3M cells. Contrast with unarmed ATC, increased cytotoxic activity of HER2Bi-armed ATC against Malme-3M-luc cells was observed at effector/target (E/T) ratios of 1:1, 5:1, and 20:1. Moreover, HER2Bi-armed ATC expressed higher level of activation marker CD69 and secreted significantly higher level of IFN-γ than unarmed ATC counterpart at the E/T ratio of 20:1. In addition, compared with anti-HER2 mAb (Herceptin®) or unarmed ATC, HER2Bi-armed ATC showed remarkable suppression effect on Malme-3M-luc tumor cells. Furthermore, in melanoma tumor cell xenograft mice, infusion of HER2Bi-armed ATC successfully inhibited the growth of melanoma tumors. The anti-tumor effect of HER2Bi-armed ATC may provide a promising immunotherapy for melanoma in the future.     

Somatic cell mutants resistant to ricin, diphtheria toxin, and to immunotoxins

The human B-cell line Namalwa expresses the common acute lymphoblastic leukemia antigen (CALLA). Frame-shift mutants in Namalwa cell cultures were generated with ICR-191, and mutants were then selected for resistance to ricin or resistance to a conjugate of ricin with the anti-CALLA antibody J5 in the presence of lactose. Three mutants were found that were resistant to ricin and were in addition shown to be resistant to diphtheria toxin, to a J5-ricin conjugate, and to a conjugate between ricin B-chain and gelonin. The mutants, however, were sensitive to a J5-gelonin conjugate. These mutants expressed high levels of CALLA and/or receptors for ricin, and their cell-free translation systems appeared to be as sensitive to the inhibitory action of ricin A-chain and of gelonin as the translation system of wild-type Namalwa cells. The behavior of these mutants was consistent with the hypothesis that these cells possess an alteration of their surface that impedes the passage of ricin and diphtheria toxin across the plasma membrane. A fourth mutant was found to bind reduced quantities of ricin and was resistant to ricin but was sensitive to J5-ricin. The properties of this cell line provide evidence that the binding of antibody-ricin conjugates to cells via the ricin moiety may be prevented without impeding the cytotoxicity of the conjugates.