Genetic construction, expression, and melanoma-selective cytotoxicity of a diphtheria toxin-related alpha-melanocyte-stimulating hormone fusion protein

Targeting Receptors on Cancer Cells with Protein Toxins

Cancer cells frequently upregulate surface receptors that promote growth and survival. These receptors constitute valid targets for intervention. One strategy involves the delivery of toxic payloads with the goal of killing those cancer cells with high receptor levels. Delivery can be accomplished by attaching a toxic payload to either a receptor-binding antibody or a receptor-binding ligand. Generally, the cell-binding domain of the toxin is replaced with a ligand or antibody that dictates a new binding specificity. The advantage of this “immunotoxin” approach lies in the potency of these chimeric molecules for killing cancer cells. However, receptor expression on normal tissue represents a significant obstacle to therapeutic intervention.

Corynebacterium diphtheriae: Diphtheria Toxin, the tox Operon, and Its Regulation by Fe2+ Activation of apo-DtxR

Diphtheria is one of the most well studied of all the bacterial infectious diseases. These milestone studies of toxigenic Corynebacterium diphtheriae along with its primary virulence determinant, diphtheria toxin, have established the paradigm for the study of other related bacterial protein toxins. This review highlights those studies that have contributed to our current understanding of the structure-function relationships of diphtheria toxin, the molecular mechanism of its entry into the eukaryotic cell cytosol, the regulation of diphtheria tox expression by holo-DtxR, and the molecular basis of transition metal ion activation of apo-DtxR itself. These seminal studies have laid the foundation for the protein engineering of diphtheria toxin and the development of highly potent eukaryotic cell-surface receptor-targeted fusion protein toxins for the treatment of human diseases that range from T cell malignancies to steroid-resistant graft-versus-host disease to metastatic melanoma. This deeper scientific understanding of diphtheria toxin and the regulation of its expression have metamorphosed the third-most-potent bacterial toxin known into a life-saving targeted protein therapeutic, thereby at least partially fulfilling Paul Erlich's concept of a magic bullet-"a chemical that binds to and specifically kills microbes or tumor cells."

A quantitative comparison of cytosolic delivery via different protein uptake systems

Over many years, a variety of delivery systems have been investigated that have the capacity to shuttle macromolecular cargoes, especially proteins, into the cytosol. Due to the lack of an objective way to quantify cytosolic delivery, relative delivery efficiencies of the various transport systems have remained unclear. Here, we demonstrate the use of the biotin ligase assay for a quantitative comparison of protein transport to the cytosol via cell-penetrating peptides, supercharged proteins and bacterial toxins in four different cell lines. The data illustrate large differences in both the total cellular internalization, which denotes any intracellular location including endosomes, and in the cytosolic uptake of the transport systems, with little correlation between the two. Also, we found significant differences between the cell lines. In general, protein transport systems based on cell-penetrating peptides show a modest total uptake, and mostly do not deliver cargo to the cytosol. Systems based on bacterial toxins show a modest receptor-mediated internalization but an efficient delivery to the cytosol. Supercharged proteins, on the contrary, are not receptor-specific and lead to massive total internalization into endosomes, but only low amounts end up in the cytosol.

Cellular Entry of the Diphtheria Toxin Does Not Require the Formation of the Open-Channel State by Its Translocation Domain

Cellular entry of diphtheria toxin is a multistage process involving receptor targeting, endocytosis, and translocation of the catalytic domain across the endosomal membrane into the cytosol. The latter is ensured by the translocation (T) domain of the toxin, capable of undergoing conformational refolding and membrane insertion in response to the acidification of the endosomal environment. While numerous now classical studies have demonstrated the formation of an ion-conducting conformation-the Open-Channel State (OCS)-as the final step of the refolding pathway, it remains unclear whether this channel constitutes an in vivo translocation pathway or is a byproduct of the translocation. To address this question, we measure functional activity of known OCS-blocking mutants with H-to-Q replacements of C-terminal histidines of the T-domain. We also test the ability of these mutants to translocate their own N-terminus across lipid bilayers of model vesicles. The results of both experiments indicate that translocation activity does not correlate with previously published OCS activity. Finally, we determined the topology of TH5 helix in membrane-inserted T-domain using W281 fluorescence and its depth-dependent quenching by brominated lipids. Our results indicate that while TH5 becomes a transbilayer helix in a wild-type protein, it fails to insert in the case of the OCS-blocking mutant H322Q. We conclude that the formation of the OCS is not necessary for the functional translocation by the T-domain, at least in the histidine-replacement mutants, suggesting that the OCS is unlikely to constitute a translocation pathway for the cellular entry of diphtheria toxin in vivo.

Malignant melanoma and melanocortin 1 receptor

The conventional chemotherapeutic treatment of malignant melanoma still remains poorly efficient in most cases. Thus the use of specific features of these tumors for development of new therapeutic modalities is highly needed. Melanocortin 1 receptor (MC1R) overexpression on the cell surface of the vast majority of human melanomas, making MC1R a valuable marker of these tumors, is one of these features. Naturally, MC1R plays a key role in skin protection against damaging ultraviolet radiation by regulating eumelanin production. MC1R activation is involved in regulation of melanocyte cell division. This article reviews the peculiarities of regulation and expression of MC1R, melanocytes, and melanoma cells, along with the possible connection of MC1R with signaling pathways regulating proliferation of tumor cells. MC1R is a cell surface endocytic receptor, thus considered perspective for diagnostics and targeted drug delivery. A number of new therapeutic approaches that utilize MC1R, including endoradiotherapy with Auger electron and α- and β-particle emitters, photodynamic therapy, and gene therapy are now being developed.

pH-triggered conformational switching along the membrane insertion pathway of the diphtheria toxin T-domain

The translocation (T)-domain plays a key role in the action of diphtheria toxin and is responsible for transferring the catalytic domain across the endosomal membrane into the cytosol in response to acidification. Deciphering the molecular mechanism of pH-dependent refolding and membrane insertion of the T-domain, which is considered to be a paradigm for cell entry of other bacterial toxins, reveals general physicochemical principles underlying membrane protein assembly and signaling on membrane interfaces. Structure-function studies along the T-domain insertion pathway have been affected by the presence of multiple conformations at the same time, which hinders the application of high-resolution structural techniques. Here, we review recent progress in structural, functional and thermodynamic studies of the T-domain archived using a combination of site-selective fluorescence labeling with an array of spectroscopic techniques and computer simulations. We also discuss the principles of conformational switching along the insertion pathway revealed by studies of a series of T-domain mutants with substitutions of histidine residues.

Macrophage-targeted therapy: CD64-based immunotoxins for treatment of chronic inflammatory diseases

Diseases caused by chronic inflammation (e.g., arthritis, multiple sclerosis and diabetic ulcers) are multicausal, thus making treatment difficult and inefficient. Due to the age-associated nature of most of these disorders and the demographic transition towards an overall older population, efficient therapeutic intervention strategies will need to be developed in the near future. Over the past decades, elimination of activated macrophages using CD64-targeting immunotoxins has proven to be a promising way of resolving inflammation in animal models. More recent data have shown that the M1-polarized population of activated macrophages in particular is critically involved in the chronic phase. We recapitulate the latest progress in the development of IT. These have advanced from full-length antibodies, chemically coupled to bacterial toxins, into single chain variants of antibodies, genetically fused with fully human enzymes. These improvements have increased the range of possible target diseases, which now include chronic inflammatory diseases. At present there are no therapeutic strategies focusing on macrophages to treat chronic disorders. In this review, we focus on the role of different polarized macrophages and the potential of CD64-based IT to intervene in the process of chronic inflammation.

Mechanism of diphtheria toxin catalytic domain delivery to the eukaryotic cell cytosol and the cellular factors that directly participate in the process

Research on diphtheria and anthrax toxins over the past three decades has culminated in a detailed understanding of their structure function relationships (e.g., catalytic (C), transmembrane (T), and receptor binding (R) domains), as well as the identification of their eukaryotic cell surface receptor, an understanding of the molecular events leading to the receptor-mediated internalization of the toxin into an endosomal compartment, and the pH triggered conformational changes required for pore formation in the vesicle membrane. Recently, a major research effort has been focused on the development of a detailed understanding of the molecular interactions between each of these toxins and eukaryotic cell factors that play an essential role in the efficient translocation of their respective catalytic domains through the trans-endosomal vesicle membrane pore and delivery into the cell cytosol. In this review, I shall focus on recent findings that have led to a more detailed understanding of the mechanism by which the diphtheria toxin catalytic domain is delivered to the eukaryotic cell cytosol. While much work remains, it is becoming increasingly clear that the entry process is facilitated by specific interactions with a number of cellular factors in an ordered sequential fashion. In addition, since diphtheria, anthrax lethal factor and anthrax edema factor all carry multiple coatomer I complex binding motifs and COPI complex has been shown to play an essential role in entry process, it is likely that the initial steps in catalytic domain entry of these divergent toxins follow a common mechanism.

Sensitivity of cancer cells to truncated diphtheria toxin

BACKGROUND

Diphtheria toxin (DT) has been utilized as a prospective anti-cancer agent for the targeted delivery of cytotoxic therapy to otherwise untreatable neoplasia. DT is an extremely potent toxin for which the entry of a single molecule into a cell can be lethal. DT has been targeted to cancer cells by deleting the cell receptor-binding domain and combining the remaining catalytic portion with targeting proteins that selectively bind to the surface of cancer cells. It has been assumed that "receptorless" DT cannot bind to and kill cells. In the present study, we report that "receptorless" recombinant DT385 is in fact cytotoxic to a variety of cancer cell lines.

METHODS

In vitro cytotoxicity of DT385 was measured by cell proliferation, cell staining and apoptosis assays. For in vivo studies, the chick chorioallantoic membrane (CAM) system was used to evaluate the effect of DT385 on angiogenesis. The CAM and mouse model system was used to evaluate the effect of DT385 on HEp3 and Lewis lung carcinoma (LLC) tumor growth, respectively.

RESULTS

Of 18 human cancer cell lines tested, 15 were affected by DT385 with IC(50) ranging from 0.12-2.8 microM. Furthermore, high concentrations of DT385 failed to affect growth arrested cells. The cellular toxicity of DT385 was due to the inhibition of protein synthesis and induction of apoptosis. In vivo, DT385 diminished angiogenesis and decreased tumor growth in the CAM system, and inhibited the subcutaneous growth of LLC tumors in mice.

CONCLUSION

DT385 possesses anti-angiogenic and anti-tumor activity and may have potential as a therapeutic agent.

Recombinant immunotoxins containing truncated bacterial toxins for the treatment of hematologic malignancies

Immunotoxins are molecules that contain a protein toxin and a ligand that is either an antibody or a growth factor. The ligand binds to a target cell antigen, and the target cell internalizes the immunotoxin, allowing the toxin to migrate to the cytoplasm where it can kill the cell. In the case of recombinant immunotoxins, the ligand and toxin are encoded in DNA that is then expressed in bacteria, and the purified immunotoxin contains the ligand and toxin fused together. Among the most active recombinant immunotoxins clinically tested are those that are targeted to hematologic malignancies. One agent, containing human interleukin-2 and truncated diphtheria toxin (denileukin diftitox), has been approved for use in cutaneous T-cell lymphoma, and has shown activity in other hematologic malignancies, including leukemias and lymphomas. Diphtheria toxin has also been targeted by other ligands, including granulocyte-macrophage colony-stimulating factor and interleukin-3, to target myelogenous leukemia cells. Single-chain antibodies containing variable heavy and light antibody domains have been fused to truncated Pseudomonas exotoxin to target lymphomas and lymphocytic leukemias. Recombinant immunotoxins anti-Tac(Fv)-PE38 (LMB-2), targeting CD25, and RFB4(dsFv)-PE38 (BL22, CAT-3888), targeting CD22, have each been tested in patients. Major responses have been observed after failure of standard chemotherapy. The most successful application of recombinant immunotoxins today is in hairy cell leukemia, where BL22 has induced complete remissions in most patients who were previously treated with optimal chemotherapy.

Recent developments in the use of adenoviruses and immunotoxins in cancer gene therapy

Despite setbacks in the past and apparent hurdles ahead, gene therapy is advancing toward reality. The past several years have witnessed this new field of biomedicine developing rapidly both in breadth and depth, especially for the treatment of cancer, thanks largely to the better understanding of molecular and genetic basis of oncogenesis and the development of new and improved vectors and technologies for gene delivery and targeting. This article is intended to provide a brief review of recent advances in cancer gene therapy using adenoviruses, both as vectors and as oncolytic agents, and some of the recent progress in the development of immunotoxins for use in cancer gene therapy.

Expression of interleukin-3 receptor subunits on defined subpopulations of acute myeloid leukemia blasts predicts the cytotoxicity of diphtheria toxin interleukin-3 fusion protein against malignant progenitors that engraft in immunodeficient mice

The interleukin-3 receptor (IL-3R) subunits are overexpressed on acute myeloid leukemia (AML) blasts compared with normal hematopoietic cells and are thus potential targets for novel therapeutic agents. Both fluorescence-activated cell sorter (FACS) analysis and quantitative real-time reverse transcription-polymerase chain reaction (QRT-PCR) were used to quantify expression of the IL-3Rα and βc subunits on AML cells. QRT-PCR for both subunits was most predictive of killing of AML colony-forming cells (AML-CFCs) by diphtheria toxin-IL-3 fusion protein (DT388IL3). Among 19 patient samples, the relative level of the IL-3Rα was higher than the IL-3Rβc and highest in CD34+CD38-CD71- cells, enriched for candidate leukemia stem cells, compared with cell fractions depleted of such progenitors. Overall, the amount of IL-3Rβc subunit did not vary among sorted subpopulations. However, expression of both subunits varied by more than 10-fold among different AML samples for all subpopulations studied. The level of IL-3Rβc expression versus glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (set at 1000) ranged from 0.14 to 13.56 in CD34+CD38-CD71- cells from different samples; this value was correlated (r = .76, P = .05) with the ability of DT388IL3 to kill AML progenitors that engraft in β2-microglobin-deficient nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice (n = 7). Thus, quantification of IL-3R subunit expression on AML blasts predicts the effectiveness IL-3R-targeted therapy in killing primitive leukemic progenitors.

Enhancement of MSH receptor- and GAL4-mediated gene transfer by switching the nuclear import pathway

Efficient nuclear delivery of plasmid DNA represents a major barrier in nonviral gene transfer. One approach has been to use DNA-binding proteins such as GAL4 from yeast as DNA carriers with nuclear targeting properties. We recently showed, however, that GAL4 is inefficient in targeting DNA to the nucleus because its DNA-binding and nuclear targeting activities are mutually exclusive, which relates to the fact that GAL4 nuclear import occurs via a novel pathway. Here, we 'switch' this pathway to a more conventional one by adding a modified poly-lysine to which an optimized nuclear targeting signal, based on that of the SV40 large T-antigen, is linked. We also use a chimeric GAL4-alpha-melanocyte stimulating hormone (MSH) fusion protein to enable gene transfer to cells expressing the MSH receptor. Switching the nuclear import pathway of the transfecting complex significantly enhances receptor-mediated gene transfer through enabling interaction with desired components of the cellular nuclear import machinery. The present study represents the first demonstration that nuclear targeting signals can enhance receptor-mediated gene delivery, the approaches having important relevance to research and clinical applications, such as in generating transgenic or knock-out animals, or in gene therapy.

NF-kappaB activation is inhibited in human pulmonary epithelial cells transfected with alpha-melanocyte-stimulating hormone vector

alpha-Melanocyte-stimulating hormone (alpha-MSH) modulates inflammation. We investigated the influence of alpha-MSH on NF-kappaB activation in human pulmonary epithelial cells (A549) using a plasmid vector encoding alpha-MSH (pCMV-ssMSH). Electrophoretic mobility shift assays demonstrated that NF-kappaB activation induced by lipopolysaccharide was inhibited in A549 cells transfected with pCMV-ssMSH. Western blot analysis revealed that this inhibition was linked to preservation of expression of IkappaBalpha protein. Chloramphenicol acetyltransferase assay indicated that NF-kappaB-dependent reporter gene expression was suppressed in A549 cells transfected with pCMV-ssMSH. The findings indicate that anti-inflammatory actions are exerted via modulation of NF-kappaB activation by preservation of IkappaBalpha protein in human pulmonary epithelial cells transfected with alpha-MSH vector. We showed a possibility of gene therapy for chronic inflammatory lung diseases.

Enhancement of diphtheria toxin potency by replacement of the receptor binding domain with tetanus toxin C-fragment: a potential vector for delivering heterologous proteins to neurons

This study describes the expression, purification, and characterization of a recombinant fusion toxin, DAB(389)TTC, composed of the catalytic and membrane translocation domains of diphtheria toxin (DAB(389)) linked to the receptor binding fragment of tetanus toxin (C-fragment). As determined by its ability to inhibit cellular protein synthesis in primary neuron cultures, DAB(389)TTC was approximately 1,000-fold more cytotoxic than native diphtheria toxin or the previously described fusion toxin, DAB(389)MSH. The cytotoxic effect of DAB(389)TTC on cultured cells was specific toward neuronal-type cells and was blocked by coincubation of the chimeric toxin with tetanus antitoxin. The toxicity of DAB(389)TTC, like that of diphtheria toxin, was dependent on passage through an acidic compartment and ADP-ribosyltransferase activity of the DAB(389) catalytic fragment. These results suggest that a catalytically inactive form of DAB(389)TTC may be useful as a nonviral vehicle to deliver exogenous proteins to the cytosolic compartment of neurons.

Targeting myeloid leukemia with a DT(390)-mIL-3 fusion immunotoxin: ex vivo and in vivo studies in mice

The IL-3 receptor was expressed on a high frequency of myeloid leukemia cells and also on hematopoietic and vascular cells. We previously showed that a recombinant IL-3 fusion immunotoxin (DT(390)IL-3) expressed by splicing the murine IL-3 gene to a truncated diphtheria toxin (DT(390)) gene selectively killed IL-3R(+) expressing cells and was not uniformly toxic to uncommitted BM progenitor cells (Chan,C.-H., Blazar,B.R., Greenfield,L., Kreitman,R.J. and Vallera,D.A., 1996, Blood, 88, 1445-1456). Thus, we explored the feasibility of using DT(390)IL-3 as an anti-leukemia agent. DT(390)IL-3 was toxic when administered to mice at doses as low as 0.1 microg/day. The dose limiting toxicity appeared to be related to platelet and bleeding effects of the fusion toxin. Because of these effects, DT(390)IL-3 was studied ex vivo as a means of purging contaminating leukemia cells from BM grafts in a murine autologous BM transplantation. In this setting, as few as 1000 IL-3R-expressing, bcr/abl transformed myeloid 32Dp210 leukemia cells were lethal. An optimal purging interval of 10 nM/l for 8 h eliminated leukemia cells from 32Dp210/BM mixtures given to lethally irradiated (8 Gy) C3H/HeJ syngeneic mice. Mice given treated grafts containing BM and a lethal dose of 32Dp210 cells survived over 100 days while mice given untreated grafts did not survive (P < 0.00001). DT(390)IL-3 may prove highly useful for ex vivo purging of lethal malignant leukemia cells from autologous BM grafts.

Secretory expression of recombinant diphtheria toxin mutants in B. Subtilis

Three diphtheria toxin (DT) mutants CRM-197, DT-del (148) and DT-E148S-K516A-F530A were cloned in B. Subtilis plasmid PSM604 under the subtilisin signal sequence. The expression was effective in both SMS300 and SMS118, but higher yield of 7.1 mg/L was observed in SMS300 compared with 2.1 mg/L in SMS118. Western blot showed that the recombinant protein could be effectively secreted into the culture medium as a 58 ku peptide, and could be degraded into two peptides of 37 ku and 21 ku.

Production of recombinant DTctGMCSF fusion toxin in a baculovirus expression vector system for biotherapy of GMCSF-receptor positive hematologic malignancies

The fusion toxin DTctGMCSF has been constructed by genetically replacing the native receptor-binding domain of diphtheria toxin (DT) with human granulocyte-macrophage colony stimulating factor (GMCSF). This recombinant fusion toxin preserves the catalytic (c) and membrane translocation (t) domains of DT and includes a sterically neutral peptide linker separating the toxin and growth factor domains. Previous work using DTctGMCSF produced in Escherichia coli has shown that this chimeric toxin is selectively cytotoxic to GMCSF receptor (R)-positive acute myeloid leukemia (AML) cells both in vitro and in vivo. Its clinical development has been hampered due to very low expression levels, requirements for solubilization with guanidine hydrochloride and subsequent refolding, and concerns about bacterial endotoxin contamination. These difficulties prompted us to investigate the utility of a baculovirus/insect cell expression system for the production of DTctGMCSF. Here, we report that a soluble form of DTctGMCSF can be produced in the baculovirus expression vector system (BEVS) and purified to homogeneity by column chromatography. The BEVS-derived DTctGMCSF fusion toxin caused apoptotic death in GMCSF-R-positive human AML cells at nanomolar concentrations. In contrast to the 100 microg/L yields of purified DTctGMCSF obtained from E. coli, the BEVS allows us to routinely generate 8-10 mg/L of purified DTctGMCSF. This increased capacity provided by the BEVS for the production of DTctGMCSF makes it now possible to obtain sufficient quantities to carry out preclinical and clinical trials. To our knowledge, this is the first report of the successful utilization of the BEVS for producing a therapeutic fusion toxin.

Immunotoxins for targeted cancer therapy

Immunotoxins constitute a new modality for the treatment of cancer, since they target cells displaying specific surface-receptors or antigens. Immunotoxins contain a ligand such as a growth factor, monoclonal antibody, or fragment of an antibody which is connected to a protein toxin. After the ligand subunit binds to the surface of the target cell, the molecule internalizes and the toxin kills the cell. Bacterial toxins which have been targeted to cancer cells include Pseudomonas exotoxin and diphtheria toxin, which are well suited to forming recombinant single-chain or double-chain fusion toxins. Plant toxins include ricin, abrin, pokeweed antiviral protein, saporin and gelonin, and have generally been connected to ligands by disulfide-bond chemistry. Immunotoxins have been produced to target hematologic malignancies and solid tumors via a wide variety of growth factor receptors and antigens. Challenges facing the clinical application of immunotoxins are discussed.

Genetic construction and characterization of an anti-monkey CD3 single-chain immunotoxin with a truncated diphtheria toxin

We have previously developed a chemically conjugated anti-rhesus monkey CD3 immunotoxin FN18-CRM9 that can deplete in vivo T cells and induce long term tolerance of mismatched renal allograft in rhesus monkeys. This immunotoxin is a monkey analogue of anti-human CD3 immunotoxin UCHT1-CRM9. In this study, we cloned the light and heavy chain variable regions of anti-monkey CD3 monoclonal antibody FN18 and constructed a single-chain Fv (sFv) by linking variable light and variable heavy regions with a (Gly4Ser)3 linker. The single-chain immunotoxin DT390-FN18sFv was constructed by ligating the sFv to the carboxyl terminus of DT390, a truncated form of diphtheria toxin. The DT390-FN18sFv fusion protein was expressed in Escherichia coli and purified with Ni-RTA affinity and anion exchange columns. Similar to the chemically conjugated immunotoxin FN18-CRM9, DT390-FN18sFv can also specifically inhibit protein synthesis in primary monkey T cells in a dose-dependent manner. DT390-FN18sFv at 10(-7) mol/L or FN18-CRM9 at 10(-8) mol/L is sufficient to reduce protein synthesis of monkey primary T cells to less than 5% of the control. The 50% inhibition dosage (IC50) of FN18-CRM9 is 1 x 10(-10) mol/L, while the IC50 of DT390-FN18sFv is 1 x 10(-8) mol/ L, reflecting the lowered affinity of monovalent Fab' FN18 to its parental divalent antibody. The availability of functional FN18sFv will provide the basis for the construction of divalent anti-CD3 immunotoxins for preclinical studies on the induction of tolerance in organ transplantation and experimental autoimmune diseases.

Melanocortin receptors: identification and characterization by melanotropic peptide agonists and antagonists

Hormones are chemical messengers released from cells to act on and control the activity of other cells. Hormonal ligands initiate their actions by interacting with receptive substances (Langley, 1906) of the target cells. These receptors are proteins that are either integral components of the cell membrane or are localized cytoplasmically within cells. Ligand-receptor interaction results in either the stimulation or inhibition of cellular activity. Since most hormones bind rather specifically to receptors possessed by their target cells, labeling of hormonal ligands can be utilized to identify and localize cells within an animal. In this report we discuss what is presently known about melanocortin receptors (MCRs) as studied by the use of labeled melanotropic peptide ligands.

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.

Diphtheria toxin-related cytokine fusion proteins: elongation factor 2 as a target for the treatment of neoplastic disease

We have used protein engineering and recombinant DNA methodologies in order to construct a fusion protein in which human interleukin-2 (IL-2) is genetically linked to the catalytic and transmembrane domains of diphtheria toxin. The fusion toxin, DAB486IL-2, is highly cytotoxic for only those cells which display the high affinity interleukin-2 receptor (IL-2R) on their surface. In phase I/II clinical studies the intravenous administration of DAB486IL-2 has been found to be safe, well tolerated and may lead to the induction of durable remissions in patients presenting with a variety of IL-2R positive lymphomas.

A circularly permuted recombinant interleukin 4 toxin with increased activity

Fusion of ligands such as growth factors to other proteins often dramatically reduces the affinity of the ligand for its receptor. With recombinant DNA techniques, the attachment point between the two proteins has until now been restricted to either the amino or the carboxyl terminus of the ligand. However, binding may be greatly compromised if both ends are close to the site at which the ligand binds to its receptor. To construct a single-chain growth factor fusion protein with the connection at a new site on the growth factor, we constructed a DNA fragment encoding circularly permuted interleukin 4 (IL4), termed IL4(38-37). This was accomplished by placing a start codon before position 38, connecting codons 1 and 129 with a sequence encoding a peptide linker, and placing a stop codon after codon 37 of IL4. IL4(38-37) was fused via its new carboxyl terminus, Lys37, to a truncated form of Pseudomonas exotoxin. The purified circularly permuted IL4-toxin bound to the IL4 receptor with 10-fold higher affinity than an IL4-toxin in which the toxin was fused to the carboxyl terminus of IL4. Circular permuteins of growth factors can improve the effectiveness of recombinant fusion proteins, because the junction can be moved to a site on the growth factor which allows it to bind with higher affinity.

Cytotoxicity of a recombinant diphtheria toxin-granulocyte colony-stimulating factor fusion protein on human leukemic blast cells

Granulocyte colony-stimulating factor (G-CSF) is a potent stimulator of the growth of normal and malignant hematopoietic cells and synergizes with other factors such as interleukin-3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF). The action of G-CSF is mediated through a specific membrane receptor, however it is not clear if all of the effects of G-CSF are direct or indirect. As a step towards addressing this problem, a recombinant diphtheria toxin (DT)-related human G-CSF fusion protein has been constructed and purified from E. coli. The 70,000 dalton chimeric protein has immunologic determinants characteristic of both DT and G-CSF. At high concentrations, DAB486-G-CSF is cytotoxic towards G-CSF-dependent OCI/AML1 cells, but not factor independent OCI/AML3 cells; colony formation by G-CSF-responsive leukemic blasts from a patient with acute myeloblastic leukemia (AML) was also inhibited. The G-CSF fusion toxin displayed ADP-ribosyltransferase activity in a cell-free system. Genetic conjugation of G-CSF to an enzymatically inactive DT mutant, CRM197, resulted in a 200-fold reduction in the ability of G-CSF to stimulate normal bone marrow colony formation. These results suggest that fusion of G-CSF to DT sequences interferes with some of the activity but not the specificity of the ligand binding domain of the molecule. Nevertheless, DAB486-G-CSF may be included with the increasing number of other toxin-hormone fusion proteins whose toxicity is directed towards specific receptor-bearing cells, and may represent a novel approach towards the study and treatment of leukemia.

Characterization of the diphtheria toxin receptor-binding domain

The carboxyl-terminal region of diphtheria toxin (DT) has been analysed in order to determine regions of receptor recognition. Biochemical cleavage of the toxin with hydroxylamine (HA) was used to generate the peptides HA9DT (residues 454-535), HA6DT (residues 482-535), and HA3DT (residues 454-481). Characterization of HA6DT demonstrated that the final 54 amino acids of DT are sufficient to constitute the receptor-binding domain of the toxin. Within HA9DT, the region encompassing HA3DT and containing the highly cationic polyphosphate-binding site did not contribute to the binding ability of HA6DT. Consistent with this observation, HA3DT itself did not compete for binding of radiolabelled DT to Vero cells. A 30-amino-acid synthetic peptide composed of residues 506-535 did not block receptor binding of DT, indicating that residues toward the amino-terminus of HA6DT, or the entire HA6DT region, are required for receptor recognition.

Polymeric drug-carriers containing doxorubicin and melanocyte-stimulating hormone: in vitro and in vivo evaluation against murine melanoma

N-(2-Hydroxypropyl)methacrylamide (HPMA) copolymers containing doxorubicin (DOX, approximately 8% by weight) bound via the lysosomally degradable spacer Gly-Phe-Leu-Gly and, in certain cases, also melanocyte-stimulating hormone (MSH, 5-10% by weight) were synthesized with the aim of developing a drug conjugate for site-specific delivery to malignant melanoma. Polymer-bound MSH, like free MSH, was able to stimulate tyrosinase activity in B16F10 cells in vitro, confirming the ability of conjugated hormone to interact with the MSH receptor. Similarly, a 125I-labelled conjugate containing MSH was captured by B16F10 cells in vitro more rapidly than a similar polymer without the targeting moiety. HPMA copolymers containing DOX bound via the lysosomally degradable Gly-Phe-Leu-Gly linkage were cytotoxic to a mouse melanoma cell line (M3 S91) in vitro, the MSH-containing conjugate being more active than that without (although the difference in the ID50 was not significant). When administered intraperitoneally or intravenously to C57BL/6J mice bearing intraperitoneal B16F10 tumours, HPMA copolymers containing DOX linked via this biodegradable spacer (with or without MSH) significantly increased animal survival, the maximum ratio of the mean survival of the test group (T) to that of the untreated control (C) T/C observed (approximately 200) over the dose range 5-20 mg DOX/kg being similar to that seen for free DOX. In contrast, neither polymer conjugates containing DOX bound via a non-degradable linkage (Gly-Gly) nor free MSH showed antitumour activity. In mice bearing established subcutaneous B16F10 tumours, biodegradable polymer-bound DOX conjugates given intraperitoneally were more effective than free DOX (which was virtually inactive in this system); conjugates containing MSH were significantly more effective than those without, the maximum T/C being approximately 148 and 324 respectively. Preliminary pharmacokinetic experiments showed evidence of selective MSH targeting of polymer conjugates to subcutaneous B16F10.

Synthesis and biological properties of a biotinylated derivative of ACTH1-17 for MSH receptor studies

A biotinylated derivative of [beta-Ala1,Lys17]-ACTH1-17-NH-(CH2)4-NH2 (ACTH1-17) was synthesized and biologically characterized. The heptadecapeptide with free N-terminus and blocked side-chains was prepared by the solid-phase method using TentaGel resin and a 4-aminobutylamide linker. Biotinyl-beta-Ala-OH was then coupled to the terminal amino group and the resulting [N alpha-(biotinyl-beta-alanyl)-beta-Ala1,Lys17]-ACTH1-17-NH-(CH2)4-N H2 (Bio-ACTH1-17) cleaved from the resin, purified and analyzed. Competition binding assays with mouse B16-F1 and human D10 and HBL melanoma cells using [125I]-alpha-MSH as radioligand gave dissociation constants for Bio-ACTH1-17 of 1.67 +/- 0.07 nM (B16-F1), 0.02 +/- 0.005 nM (D10) and 0.21 +/- 0.02 nM (HBL). The EC50 for Bio-ACTH1-17 in the B16 melanin assay was 4.15 +/- 1.0 nM. Analysis of the binding characteristics of [125I]-Bio-ACTH1-17 demonstrated that in human melanoma cells this radioligand was displaced by ACTH1-17 as well as alpha-MSH whereas in B16-F1 cells the tracer was only displaced from the binding site by ACTH1-17. Studies of Bio-ACTH1-17 with streptavidin showed that the peptide is to a large extent trapped specifically through reaction with biotin. These results demonstrate that (1) the biological characteristics of Bio-ACTH1-17 are almost identical to those of ACTH1-17, (2) Bio-ACTH1-17 is bound by avidin, and (3) Bio-ACTH1-17 may become a useful tool for MSH receptor targeting.

Molecular cloning and expression of the human melanocyte stimulating hormone receptor cDNA

Melanocytes and melanoma cells are known to possess receptors for melanocyte stimulating hormone (MSH). A cDNA clone, designated 11D, has been isolated from human melanoma cells and encodes a MSH receptor. The cloned cDNA encodes a 317 amino acid protein with transmembrane topography characteristics of a G-protein-coupled receptor, but it does not show striking similarity to already published sequences of other G-protein-coupled receptors. When 11D cDNA is expressed in COS-7 cells, it binds an 125I-labelled MSH analogue (NDP-MSH) in a specific manner. The bound ligand could be displaced by melanotropic peptides, alpha-MSH, beta-MSH, gamma-MSH and ACTH (adrenocorticotropic hormone), but not by the non-melanotropic peptide, beta-endorphin. This is the first report of the cloning of the receptor gene of the melanotropin receptor family.

Cytotoxic activity of a recombinant chimaeric protein between Pseudomonas aeruginosa exotoxin A and Corynebacterium diphtheriae diphtheria toxin

A segment of the exotoxin A gene of Pseudomonas aeruginosa, coding for the N-terminal end of domain I and domain II of the toxin (ETA), was genetically fused to the diphtheria toxin gene of Corynebacterium diphtheriae, coding for the N-terminal end of A fragment of diphtheria toxin (DT). The resulting hybrid protein (termed CED1) was produced in large amounts and exported to the periplasm in Escherichia coli. This chimaeric protein reacted with both anti-ETA and anti-DT antisera. Furthermore, the chimaeric protein displayed ADP-ribosylation activity and exhibited cytotoxicity to mouse 3T6 fibroblasts. These results demonstrated that the chimaeric protein is cytotoxic, and that the toxic potential of DTA can be selectively internalized and translocated via domains I and II of exotoxin A, which are thus sufficient to direct and translocate an enzymatically active heterologous polypeptide segment into the cytosol of sensitive cells.

Immunotoxins and cytokine toxin fusion proteins

Paul Ehrlich first suggested the simple and elegant concept of creating specific cell toxins or "magic bullets" through the fusion of cell-specific antibodies and toxins. In practice it has proven difficult to create safe and effective "magic bullets." In the past several years, several immunotoxins have been applied to clinical testing. These immunotoxins have been created by the biochemical coupling of cell- or lineage-specific monoclonal antibodies to plant toxins or fragments thereof. These immunotoxins have been used to treat bone marrow transplant recipients and patients with autoimmune disorders. In recent years, another strategy has also been pursued to create hybrid toxins. Rather than use antibodies as the targeting moiety, cytokines have been used to target a select population of cells bearing a high copy number of receptors for the specific cytokine. Rather than biochemically couple a cytokine to the toxin, the cytokine and toxin are fused by a peptide bond established via genetic engineering. A prototype IL-2 diphtheria toxin-related fusion protein is now being tested in the clinic for treatment of hematopoietic malignancies and autoimmune disorders.