Comparison of blocked and non-blocked ricin-antibody immunotoxins against human gastric carcinoma and colorectal adenocarcinoma cell lines

Effects of Scrophularia ningpoensis Hemsl. on Inhibition of Proliferation, Apoptosis Induction and NF-κB Signaling of Immortalized and Cancer Cell Lines

Scrophularia ningpoensis has been used in China for centuries as a herbal tea to treat various diseases. Based on the numerous animal studies on its pharmaceutical effects and the long time clinical experiences, we studied the molecular and cellular mechanism underlying the bioactivity of aqueous extract of Scrophularia and its isolated compounds. Seven isolated compounds, unlike Scrophularia extract, failed to induce cytotoxicity on HaCaT cells, but their combination improved the effect of extract. Tumor cell line selectivity was not observed, when we studied its cytotoxic effect on melanoma cell lines. The apoptotic and anti-inflammatory effects of Scrophularia extract have been demonstrated on HaCaT cells. The extract induced those effects potentially through affecting the MAPK pathway and inhibition of the NF-κB pathway, Microarray-based bioinformatical analyses on the compound acetoside from Scrophularia revealed a gene expression profile which confirmed our findings with the extract on proliferation inhibition, anti-inflammation and apoptosis. With DNA alkylation as major proposed mechanism of action, we assume acetoside as one of the active compounds in Scrophularia.

Immunotoxins and anticancer drug conjugate assemblies: the role of the linkage between components

Immunotoxins and antibody-drug conjugates are protein-based drugs combining a target-specific binding domain with a cytotoxic domain. Such compounds are potentially therapeutic against diseases including cancer, and several clinical trials have shown encouraging results. Although the targeted elimination of malignant cells is an elegant concept, there are numerous practical challenges that limit conjugates’ therapeutic use, including inefficient cellular uptake, low cytotoxicity, and off-target effects. During the preparation of immunoconjugates by chemical synthesis, the choice of the hinge component joining the two building blocks is of paramount importance: the conjugate must remain stable in vivo but must afford efficient release of the toxic moiety when the target is reached. Vast efforts have been made, and the present article reviews strategies employed in developing immunoconjugates, focusing on the evolution of chemical linkers.

Use of antibodies and immunoconjugates for the therapy of more accessible cancers

There are currently 6 unconjugated antibodies and 3 immunoconjugates approved for use in the United States in a variety of cancers, with a considerable number of new agents in clinical testing and preclinical development. Unconjugated antibodies alone can be effective, but more often, antibodies need to be combined with chemotherapy, which enhances the efficacy of the standard treatment. Immunoconjugates tend to be more effective than their unconjugated counterparts, but their increased toxicity often restricts when and how they are used. In order to improve efficacy, a number of immunoconjugates are being examined in settings where the disease is more easily accessible, such as leukemias, or within compartments that allow easier and more direct access to the tumor, such as in the peritoneal cavity or brain, or both locally and systemically, in adjuvant situations, where the disease burden has been reduced by some other means, and with the main goal of these treatments being to kill residual disease.

Tumor galectinology: insights into the complex network of a family of endogenous lectins

Beta-Galactosides of cell surface glycoconjugates are docking sites for endogenous lectins of the galectin family. In cancer cells, primarily galectins-1 and -3 have been studied to date. With the emergence of insights into their role in growth control, resistance to or induction of apoptosis and invasive behavior the notion is supported that they can be considered as functional tumor markers. In principle, the same might hold true for the other members of the galectin family. But their expression in tumors has hitherto been a subject of attention only to a very limited extent. Pursuing our concept to define the complexity of the galectin network in cancer cells and the degree of functional overlap/divergence with diagnostic/therapeutic implications, we have introduced comprehensive RT-PCR monitoring to map their galectin gene expression. The data on so far less appreciated galectins in this context such as galectins-4 and -8 vindicate this approach. They, too, attach value to extend the immunohistochemical panel accordingly. Our initial histopathological and cell biological studies, for example on colon cancer progression, prove the merit of this procedure. Aside from the detection of gene expression profiles by RT-PCR, the detailed molecular biological monitoring yielded further important information. We describe different levels of regulation of galectin production in colon cancer cells in the cases of the tandem-repeat-type galectins-8 and -9. Isoforms for them are present with insertions into the peptide linker sequence attributed to alternative splicing. Furthermore, variants with distinct amino acid substitutions (galectin-8, Po66-CBP, PCTA-1, CocaI/II and galectin-9/ecalectin) and generation of multiple mRNA species, notably those coding for truncated galectin-8 and -9 versions with only one lectin site, justify to portray these two family members not as distinct individuals but as groups. In aggregate, the ongoing work to thoroughly chart the galectin network and to disentangle the individual functional contributions is expected to make its mark on our understanding of the malignant phenotype in certain tumor types.

Antitumoral activity of liposomes and immunoliposomes containing 5-fluorouridine prodrugs

Liposomes and immunoliposomes containing cytotoxic agents may be highly efficacious in intracavity therapy of malignancies confined principally to the peritoneal cavity. To assess the feasibility of this locoregional treatment, we prepared two derivatives of 5-fluorouridine (5-FUR), a highly cytotoxic metabolite of 5-fluorouracile, and incorporated them into REV liposomes, prepared with the reverse phase evaporation method. Encapsulation efficiency, drug leakage, and stability were determined, and size analysis and differential scanning calorimetry were carried out to evaluate the drug delivery potential of liposomes containing 5'-palmitoyl-5-FUR, 5'-succinyl-5-FUR, or the parent drug 5-FUR. The most suitable drug for encapsulation, in terms of minimum leakage and encapsulation efficiency, was 5'-palmitoyl-5-FUR, which differential scanning calorimetry indicated as being firmly anchored to the lipid bilayer. Thus, 5'-palmitoyl-5-FUR was chosen to prepare a chemotherapeutic liposome-monoclonal antibody conjugate (immunoliposome). The covalent linkage between antibody and liposome was realized by coupling the thiolated monoclonal antibody AR-3 with REV liposomes, containing N-[4-(p-maleimidophenyl)butyryl]phosphatidylethanolamine. The cytotoxic activity of drug-bearing liposomes and immunoliposomes was evaluated on the HT-29 human colon adenocarcinoma cell line; the immunoliposomes had higher cytotoxicity than liposomes or 5-FUR. To explore the potential of these drug formulations in anticancer therapy, we ip injected liposomes or immunoliposomes into athymic mice ip grafted with human HT-29 cell line. In this mouse model, the immunoliposome containing 5'-palmitoyl-5-FUR displayed the best antitumoral activity, since on day 27 postgraft only 5% of residual tumor mass was present, compared to control mice; there was a close relationship between exposure time of tumor tissue to the drug and antitumor potency.

Pharmacokinetics of an antibody-ricin conjugate administered intraperitoneally to mice

Immunotoxins have been extensively studied for the treatment of neoplasias; their intracavitary administration could be useful for the therapy of tumors confined to the pleural or peritoneum spaces. To study the feasibility of this "locoregional" treatment, a pharmacokinetic study of immunotoxins delivery is necessary. Ricin, a plant toxin extracted from the seeds of Ricinus communis, has often been used in immunoconjugates for its high activity; nevertheless, appropriate strategies have been necessary to limit the aspecific toxicity. We previously prepared a AR-3-ricin immunotoxin lacking the ability to bind galactosidic cell surface residues, a so-called sterically blocked immunotoxin. The monoclonal antibody AR-3, an IgG1 specific to the CAR-3 antigen, was able to recognize human colorectal adenocarcinomas. Preclinical trials in nude mice, intraperitoneally grafted with the target neoplasia, showed that this immunotoxin suppressed tumor growth without showing any undesirable ricin toxicity. In the present work we report the pharmacokinetic properties of this immunotoxin, showing the in vivo stability and a relatively long blood survival. With a biodistribution study in tumor-bearing mice, we demonstrate that in tumor-invaded tissues, the concentration of the specific AR-3-ricin immunotoxin was higher and progressively increased in a multiple-dose regimen. In contrast, an irrelevant immunotoxin behaved differently because it did not show specific tumor uptake. Moreover the pharmacokinetic data reported in this work improve the potential for "locoregional" treatment of malignancy with blocked immunotoxins.

A new approach in the synthesis of immunotoxins: ribosome inactivating protein noncovalently bound to monoclonal antibody

This study describes the synthesis of a new generation of immunotoxins made by a noncovalent interaction between a monoclonal antibody derivatized with a dichlorotriazinic dye and the ribosomal inhibitor protein gelonin. The scheme of preparation has several advantages with respect to the traditional methods, which used heterobifunctional cross-linkers, such as a higher overall yield of production and the homogeneity of the obtained conjugate. Moreover, because no chemical derivatization of the gelonin was required, the unconjugated ribosome inactivating protein was recovered unaltered and therefore can be reused in other synthetic processes. This immunoconjugate was stable when tested in mouse serum and showed an interesting slow elimination rate when administered intravenously in mice. Although a high dye derivatization degree induced a modification of the specificity of the monoclonal antibody, the native specificity was restored after conjugation with gelonin. Furthermore the noncovalent linkage did not affect the gelonin inhibitory activity; in fact, the specific cytotoxic activity seemed to be similar to that of other disulfide-linked immunotoxins previously prepared in our laboratories.

Toxin-targeted design for anticancer therapy. II: Preparation and biological comparison of different chemically linked gelonin-antibody conjugates

To obtain more potent immunotoxins for anticancer therapy a gelonin-AR3 antibody immunoconjugate was prepared with different new linkers and coupling procedures. The gelonin was derivatized with the heterobifunctional thioimidate linkers ethyl-acetyl-3-mercaptopropionthioimidate (AMPT) and 3-(4-carboxamidophenyldithio)propionthioimidate (CDPT), and with the succinimidyl type reagents N-succinimidyl-3-(4-carboxamidophenyldithio)propionate (SCDP) and N-succinimidyl-S-acetyl thiolacetate (SATA). The biological activity of gelonin modified with different linkers (AMPT, CDPT, SCDP, SATA) was determined by a rabbit reticulocyte assay. We found that AMPT was the molecule of choice to derivatize the toxin, confirming the preferability of thioimidate linkers. The monoclonal antibody Mab was derivatized with CDPT and SCDP. Then the following immunoconjugates were prepared with different procedures: Mab-CDPT with gelonin-AMPT; Mab-CDPT with gelonin-CDPT; Mab-SCDP with gelonin-SATA. To verify whether selection of the most suitable coupling procedure could affect the antitumoral activity of the gelonin-AR3 immunoconjugate, the three immunotoxins were tested on target HT-29 human colon carcinoma cells versus nontarget MeWo cells. The gelonin immunoconjugate linked via the AMPT-CDPT thioimidate reagents showed highest antitumoral activity as well as best selectivity for the target cells.

Toxin-targeted design for anticancer therapy. I: Synthesis and biological evaluation of new thioimidate heterobifunctional reagents

In an effort to obtain a more potent and specific immunotoxin for cancer therapy, we designed a series of heterobifunctional linkers characterized by a thioimidate group linked to a S-acetyl thiol (4, 5) or substituted aryldithio group (6-10). These ligands were synthesized by a Pinner-type process from the corresponding nitrile derivatives obtained by thiol-disulphide exchange reaction, reaction with substituted benzene-sulphenyl chloride, or other known procedures. To check the reagent of choice for immunoconjugate preparation, we studied thioldisulphide exchange kinetics between the intermediate nitrile derivatives and cysteine. Among the tested aryldithio derivatives (6-10), we selected ethyl 3-(4-carboxamido-phenyldithio)propionthioimidate (CDPT, 9) for further studies. By analyzing the rate of incorporation of the linkers 4, 5, and 9 in a model immunoglobulin G protein, we found similar results with CDPT 9 and ethyl S-acetyl 3-mercaptopropionthioimidate ester hydrochloride (AMPT, 5) because both reagents showed a linear correlation between the number of introduced thiol groups and factors such as time and protein and reagent concentrations. Comparison of the two acetylthio-derivative ligands 4 and 5 showed that AMPT 5 was more stable toward deacetylation than ethyl S-acetyl 2-mercaptopropionthioimidate ester hydrochloride (AMAT, 4). By comparing the kinetic and biological parameters of seven new thioimidate linkers, we found that two of these (CDPT and AMPT) could be superior ligands for protein-protein conjugation. They offer advantages over the commercially available compounds, such as minimal perturbation of the protein structure, controlled reactivity, and good stability.

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

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 evalauted 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 specifcity of the immunoconjugate, since the analogous treatment with an irrelevant immunotoxin showed therapeutic failure.

Recombinant ricin toxin A chain cytotoxicity against carcinoembryonic antigen expressing tumour cells mediated by a bispecific monoclonal antibody and its potentiation by ricin toxin B chain

A bispecific monoclonal antibody recognising both carcinoembryonic antigen (CEA) and ricin toxin A chain (RTA) was tested for its ability to target recombinant RTA (r-RTA) to CEA-expressing tumour cells, alone and in combination with ricin B chain (RTB). The antibody, 636 (Robins et al., 1990), induced significant RTA cytotoxicity against MKN45 gastric carcinoma cells which express high levels of CEA, using the r-RTA at a concentration below that known to be intrinsically cytotoxic. The addition of ricin toxin B chain (RTB) also potentiated cytotoxicity of r-RTA, and there was an additive increase in potentiation against CEA-positive cells when both RTB and 636 were included. The bispecific antibody restored potentiation by RTB after blocking of its binding site with excess galactose, and also the cytotoxic activity of whole ricin which had been blocked with galactose. It was concluded that the 636 bispecific antibody was highly effective in targeting the toxic moiety of the molecule to CEA-expressing cells, and allowed exploitation of the additional ability of the B chain to facilitate cellular incorporation. The facilitating function of the B chain was equally effective whether or not its lectin site was active.