Transforming growth factor alpha-Pseudomonas exotoxin fusion protein prolongs survival of nude mice bearing tumor xenografts

Engineering growth factor ligands and receptors for therapeutic innovation

Growth factors signal through engagement and activation of their respective cell surface receptors to choreograph an array of cellular functions, including proliferation, growth, repair, migration, differentiation, and survival. Because of their vital role in determining cell fate and maintaining homeostasis, dysregulation of growth factor pathways leads to the development and/or progression of disease, particularly in the context of cancer. Exciting advances in protein engineering technologies have enabled innovative strategies to redesign naturally occurring growth factor ligands and receptors as targeted therapeutics. We review growth factor protein engineering efforts, including affinity modulation, molecular fusion, the design of decoy receptors, dual specificity constructs, and vaccines. Collectively, these approaches are catapulting next-generation drugs to treat cancer and a host of other conditions.

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.

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.

Immunotoxin Therapies for the Treatment of Epidermal Growth Factor Receptor-Dependent Cancers

Many epithelial cancers rely on enhanced expression of the epidermal growth factor receptor (EGFR) to drive proliferation and survival pathways. Development of therapeutics to target EGFR signaling has been of high importance, and multiple examples have been approved for human use. However, many of the current small molecule or antibody-based therapeutics are of limited effectiveness due to the inevitable development of resistance and toxicity to normal tissues. Recombinant immunotoxins are therapeutic molecules consisting of an antibody or receptor ligand joined to a protein cytotoxin, combining the specific targeting of a cancer-expressed receptor with the potent cell killing of cytotoxic enzymes. Over the decades, many bacterial- or plant-based immunotoxins have been developed with the goal of targeting the broad range of cancers reliant upon EGFR overexpression. Many examples demonstrate excellent anti-cancer properties in preclinical development, and several EGFR-targeted immunotoxins have progressed to human trials. This review summarizes much of the past and current work in the development of immunotoxins for targeting EGFR-driven cancers.

Endocytic Relay as a Potential Means for Enhancing Ligand Transport through Cellular Tissue Matrices: Analysis and Possible Implications for Drug Delivery

The transport of peptide ligands, such as cytokines, through tissue is complicated by resistances due to cell multilayers and holdup in extracellular matrix. To determine whether it is possible for receptor-mediated endocytic trafficking to enhance ligand transport, we have developed a mathematical model of ligand flux through tissue containing cells possessing complementary receptors. Tissue is considered as two phases: the cell phase and the matrix phase; thus tissue is modeled as analogous to a packed bed reactor. This model allows calculation of steady-state flux of intact and degraded peptide through a one-dimensional cell/tissue matrix. Both environmental and molecular parameters were considered in this study. Results predict that three quantities should have a major influence on growth factor flux: the ratio of matrix diffusivity to intracellular "diffusivity" (D(m)/D(i)), the extracellular matrix proteolysis rate constant (k (prot)), and the fraction of internalized growth factor degraded (f(1)). For basal levels of intracellular degradation (0 < f(1) >/= 0.05) but no extracellular proteolysis, significant enhancement is possible only for D(m)/D(i) >/=1. f(1) increases, enhancement is only possible up to f(1)= 0.07 even for D(m)/D(i) < 1. For significant levels of extracellular proteolysis (k (prot) > 0), the requirements for D(m)/D(i) and f(1) to permit transport enhancement encompass a broader range with the exact values dependent on k (prot). These insights may be helpful for delivery of ligands generated from controlled-release devices or genetically modified autocrine cells, and may also provide better understanding of cytokine transport in embryonic development.

Physiological effects of TGF(alpha)-PE40 expression in recombinant Escherichia coli JM109

Physiological effects of isopropyl-thiogalactopyranoside (IPTG) induction were examined in Escherichia coli strain JM109 expressing a fusion protein composed of transforming growth factor alpha and a 40-kD portion of Pseudomonas aeruginosa exotoxin A (TGF(alpha)-PE40) under control of the tac promoter. Fermentations at the 15-L scale in complex medium compared growth and metabolite profiles of the untransformed JM109 host strain, the strain transformed with the vector lacking the TGF(alpha)-PE40 open reading frame (JM109[pKK2.7]), and the strain with the complete plasmid for TGF(alpha)-PE40 expression (JM109[pTAC-TGF57-PE40]). Metabolite and growth profiles of JM109 (pTAC-TGF57-PE40) cultures changed significantly in IPTG-induced versus uninduced cultures. Prior to induction, glucose was metabolized to acetate or completely oxidized to CO(2). Following induction, pyruvate was also excreted in addition to acetate. In the absence of inducer, pyruvate was excreted by JM109 (pTAC-TGF57-PE40) only when dissolved oxygen levels fell to less than 10% of saturation (microaerobic rather than anaerobic conditions). The untransformed JM109 host strain or JM109 (pKK2.7) did not excrete pyruvate in the presence or absence of inducer, although JM109 (pKK2.7) exhibited a pattern of growth following addition of IPTG that closely resembled JM109 (pTAC-TFG57-PE40). Fermentations of JM109 (pTAC-TFG57-PE40) in a synthetic medium supported lower expression levels, but resulted in similar alterations in metabolite profiles. Induction in synthetic medium resulted in pyruvate excretion without further acetate accumulation. Taken together, these data suggest that one consequence of TGF(alpha)-PE40 expression in JM109 is altered patterns of pyruvate oxidation.

A bispecific enediyne-energized fusion protein containing ligand-based and antibody-based oligopeptides against epidermal growth factor receptor and human epidermal growth factor receptor 2 shows potent antitumor activity

PURPOSE

The cooverexpression of epidermal growth factor receptor (EGFR) and human epidermal growth factor receptor 2 (HER2) observed in many human tumors and their synergistic interaction in the transformation of cells make these receptors important targets for the development of new targeted therapeutics. Targeting of EGFR and HER2 simultaneously has been pursued as a strategy with which to potentially increase efficiency and selectivity in therapy of certain cancers. This study was set to construct a bispecific energized fusion protein (Ec-LDP-Hr-AE) consisting of two oligopeptides against EGFR and HER2, and lidamycin, and investigate its antitumor efficacy.

EXPERIMENTAL DESIGN

In vitro experiments measured the binding and internalization of bispecific Ec-LDP-Hr fusion protein. The potency of energized fusion proteins was also done in which the bispecific Ec-LDP-Hr-AE was compared with lidamycin (LDM) and its monospecific counterparts, Ec-LDP-AE and LDP-Hr-AE. In vivo, Ec-LDP-Hr-AE was given i.v. to nude mice bearing human ovarian carcinoma SK-OV-3 xenografts.

RESULTS

Binding and internalization studies showed that bispecific fusion protein Ec-LDP-Hr bound to carcinoma cells specifically and then were internalized into the cytoplasm. Bispecific Ec-LDP-Hr-AE was more potent and selective in its cytotoxicity against different carcinoma cell lines than corresponding momospecific agents and LDM in vitro. In addition, Ec-LDP-Hr-AE significantly inhibited the growth of SK-OV-3 xenografts in nude mouse model. In vivo imaging study showed that FITC-labeled Ec-LDP-Hr was targeted and accumulated in the tumors.

CONCLUSION

A ligand-based and an antibody-based oligopeptide fused to the enediyne antibiotic LDM created a new bispecific fusion protein with low molecular weight and more potent in vitro and in vivo antitumor activity (than momospecific fusion proteins).

Enhancement of cell type specificity by quantitative modulation of a chimeric ligand

Evolution modulates the quantitative characteristics of protein interactions and often uses combinations of weak interactions to achieve a particular specificity. We addressed how quantitative optimization might be used in the design of multidomain proteins, using a chimera containing epidermal growth factor (EGF) as a cell targeting element and interferon-alpha-2a (IFNalpha-2a) to initiate signal transduction. We first connected EGF and IFNalpha-2a via a linker that allows both ligands to bind to their receptors on a cell surface and then incorporated a series of mutations into the IFNalpha-2a portion that progressively decrease both the on rate and the dissociation constant of the IFNalpha-2a-IFNalpha receptor 2 (IFNAR2) interaction. Using this strategy, we designed chimeric proteins in which the activation of the IFNalpha receptor in HeLa, A431, and engineered Daudi cells depends on the presence of EGF receptor on the same cell. The mutant chimeric proteins also inhibited proliferation of IFNalpha-sensitive cells in an EGF receptor-dependent manner. These results provide insights into the quantitative requirements for specific binding to multisubunit receptors and illustrate the value of a quantitative approach in the design of synthetic-biological constructs.

Bile acid-induced proliferation of a human colon cancer cell line is mediated by transactivation of epidermal growth factor receptors

Although epidemiological studies indicate an association between elevations in fecal bile acids and the development of colorectal cancer, the cellular mechanism for the proliferative actions of bile acids is not clear. Studies from other laboratories indicate a paradoxical pro-apoptotic action of bile acids on cell culture lines. Our previous studies indicate that cholinergic agonist-induced proliferation of colon cancer cells that express M3 muscarinic receptors (M3R) is mediated by transactivation of the epidermal growth factor receptor (EGFR) and that bile acids stimulate proliferation of colon cancer cells that express M3R. In the present study, we investigated the effects of bile acids on cell signaling and proliferation of a human colon cancer cell line (H508 cells) that abundantly expresses M3R and EGFR. Treatment with taurine and glycine conjugates of lithocholic and deoxycholic acids stimulated reversible activation of the p44/42 MAP kinase signaling cascade and proliferation of H508 cells. Bile acids did not stimulate proliferation of SNU-C4 colon cancer cells that express EGFR but not muscarinic receptors. Atropine, a muscarinic receptor inverse agonist, blocked bile acid-induced H508 cell proliferation. At concentrations that stimulate cell proliferation, conjugated bile acids did not activate caspase-3, a key mediator of apoptosis. Conjugated bile acids stimulated phosphorylation of EGFR Tyr992, thereby implicating EGFR transactivation in the cellular mechanism underlying their proliferative actions. This was confirmed by observing that inhibitors of EGFR activation and antibodies to the ligand-binding domain of EGFR blocked both the signaling and proliferative actions of bile acids. Collectively, these results suggest that in this colon cancer cell line, bile acid-induced colon cancer cell proliferation is M3R-dependent and is mediated by transactivation of EGFR.

Recombinant toxins in haematologic malignancies and solid tumours

Recombinant toxins constitute a new modality for the treatment of cancer, since they target cells displaying specific surface-receptors or antigens. They are fusion proteins, which contain toxin and ligand regions, and are produced in Escherichia coli. The ligand may be a growth factor or a fragment of an antibody, and the toxin is usually one of the two bacterial toxins: Pseudomonas exotoxin and diphtheria toxin. Compared to the earlier generation chemical conjugates of ligands and toxins, recombinant toxins have many advantages, including homogeneity with respect to the connection between the ligand and toxin, ease and yield of production and small size. A variety of chemotherapy-resistant haematologic and solid tumours have been targeted with recombinant toxins, and clinical trials with many of them have recently demonstrated their effectiveness. Moreover, their unwanted toxic effects are different from those of most chemotherapeutic agents, supporting the expectation that they can be combined with existing modalities to improve the clinical resources available to treat cancer in humans.

Recombinant immunotoxins for treating cancer

Recombinant immunotoxins are antibody-toxin chimeric molecules that kill cancer cells via binding to a surface antigen, internalization and delivery of the toxin moiety to the cell cytosol. In the cytosol, toxins catalytically inhibit a critical cell function and cause cell death. The antibody portion of the chimera targets antigens that are expressed preferentially on the surface of cancer cells. Truncated versions of either diphtheria toxin (DT) or Pseudomonas exotoxin (PE) can be used to construct fusions with cDNAs encoding antibody fragments or cell-binding ligands. Recombinant immunotoxins are routinely produced in E. coli and purified using standard chromatographic methods. Before they can be evaluated for anticancer activity in humans, recombinant immunotoxins undergo extensive preclinical testing. Immunotoxins must demonstrate cell-killing activity in tissue culture, antitumor activity in an animal model and have favorable pharmacokinetic and toxicity profiles. Candidate molecules with favorable characteristics are then evaluated in clinical trials. Here we report on the initial evaluation of BL22, a recombinant immunotoxin targeted to CD22 expressed on the surface of B-cell malignancies.

A phase I study of 99mTc-hR3 (DiaCIM), a humanized immunoconjugate directed towards the epidermal growth factor receptor

A phase I trial was conducted to evaluate the safety, tumour and normal tissue localization, pharmacokinetics and radiation dosimetry of Tc-hR3, a humanized monoclonal antibody directed towards the epidermal growth factor receptor, in 12 patients with recurrent or metastatic epithelial malignancies. Patients were injected intravenously with 3.0 mg or 6.0 mg (1010 MBq) of Tc-hR3. Blood and plasma concentrations of radioactivity were measured and a complete 24 h urine collection was obtained. Whole-body images were acquired up to 24 h post-injection and normal organ uptake quantified. Radiation dosimetry was estimated using MIRDose. Safety was evaluated by clinical observation, biochemical/haematological testing and by measuring immune response to Tc-hR3. There were no adverse effects, no changes in biochemical/haematological indices and no immune response to Tc-hR3. Tc-hR3 was rapidly cleared from the blood with a distribution half-life of 10.8+/-3.8 min. The volume of distribution, and clearance, were 180+/-37 ml.kg and 14+/-3 ml.kg.min, respectively. The elimination phase could not be discerned due to increasing blood radioactivity at later times. About 19-24% was excreted in the urine. Normal tissue uptake was mainly in the liver (44-50%), spleen (3-4%) and kidneys (3%). Imaging was positive in one patient with squamous cell carcinoma of the mouth and an involved cervical lymph node. The whole-body radiation dose from Tc-hR3 was 1.34+/-0.02x10 mSv.Bq. We conclude that Tc-hR3 exhibited an excellent safety profile. Future studies to determine the sensitivity and specificity of imaging with Tc-hR3 in a larger group of patients with pre-selection for epidermal growth factor receptor positivity are planned.

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.

Effect of fibroblast growth factor saporin mitotoxins on human bladder cell lines

Mitotoxins targeted via high-affinity growth factor receptors on the cell surface are a potential means of anticancer therapy. We have evaluated the effect of a chemically conjugated (FGF2-SAP) and a fusion protein (rFGF2-SAP) mitotoxin containing FGF-2 and saporin on normal (FHs 738B1) and malignant bladder cell lines (HT1197, TCCSUP, EJ-6, and RT4). The FGF-saporins demonstrated potent cytotoxicity in malignant bladder cell lines with an ID50 range of 0.13-13.6 nM, whereas cells derived from normal fetal bladder (FHs 738B1) were less sensitive to FGF2-saporins (ID50 > 100 nM). Greater than a 100-fold difference in cytotoxicity between FGF-saporins and unconjugated saporin was observed. Assessment of cellular FGF-2 content and secretion showed that FHs 738B1 and TCCSUP contained and secreted significantly more FGF-2 compared to other cell lines tested. (125)I-FGF-2 receptor binding studies showed the presence of high-affinity (pM) FGF receptors on all bladder cell lines. Cross-linking studies revealed the presence of a major receptor-ligand complex of 90 kDa on FHs 738B1 and 160-170 kDa on the other bladder cell lines. All cell lines studied, except RT4, expressed solely FGFR-1. These studies demonstrate that FGF2-saporins have antiproliferative activity on human bladder cancer cell lines. However, the number of high-affinity FGF receptors, and FGF-2 cellular content and secretion are not absolute determinants of cellular sensitivity to FGF2-saporins.

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.

Targeted inhibition of tumor-cell growth by recombinant heregulin-toxin fusion proteins

Fusion of functional domains of proteins by in vitro recombination of gene fragments can be used to generate novel anti-tumor agents. The combination of tumor-cell-recognition functions and toxic functions results in cytotoxic molecules with a high specificity for tumor cells. Human adenocarcinomas are frequently characterized by over-expression of members of the epidermal-growth-factor (EGF) receptor family (ErbB-1, 2, 3 and 4), when compared with normal cells. These tumors are particularly suited to treatment with recombinant toxins. The human heregulins (HRG) and their rat counterparts (neu differentiation factor, NDF) have been identified as ligands for these receptors. Two chimeric heregulin-toxin fusions consisting of the EGF-like receptor recognition domain of the heregulin isoforms HRG alpha and HRG beta I, and the domains II, Ib and III of the Pseudomonas exotoxin A (ETA) were constructed. HRG beta I-ETA is highly cytotoxic for the mammary carcinoma cell lines SK-BR-3 and MDA-MB-453. HRG alpha-ETA was less active than HRG beta I-ETA. The killing activity of the recombinant toxins correlated with the expression levels of ErbB-3 and/or ErbB-4 in the cell lines studied. High expression of ErbB-2 is not sufficient to confer sensitivity towards the HRG-ETA. Treatment of mice with 0.4 mg/kg/day of HRG beta I-ETA caused growth retardation of transplanted human breast tumor cells. Higher levels of HRG beta I-ETA administration resulted in acute hemorrhagic necrosis of the liver.

Theories on the metastatic process and possible therapeutic options

A sequence of steps are prerequisite for cancer cells before metastases are established. Metastasis has been shown to be an inefficient process limited by both random and selective events. By differentiating invasion from metastasis, sequential steps in the metastatic cascade have been defined and studied separately. This approach has yielded a variety of new potential therapeutic strategies. However, increasing knowledge of the mechanisms relating to metastasis has also revealed the complexity of each step. In spite of difficulties in translating results obtained in preclinical models into the clinical setting, continued development of such model systems and further research into the genetic control of metastatic dissemination will lead to improved strategies for prevention of metastasis formation and for treatment of metastatic tumor cells.

Targeted toxins as anticancer agents

Transformed cells, such as those found in breast cancer, often overexpress a variety of cell surface receptors and antigens. Antibodies or growth factors that specifically recognize these membrane-bound structures can be linked with protein toxins, resulting in cell-specific cytotoxic reagents. Many of these cytotoxic molecules have been produced and are referred to as oncotoxins, mitotoxins, or immunotoxins, depending on the components of the chimeric molecule. These bifunctional reagents are constructed as either chemical conjugates or fusion proteins between a ligand/antibody and a toxin. This report focuses on the use of cytotoxic proteins targeted to epidermal growth factor receptors, fibroblast growth factor receptors, erbB-2/HER-2, and tumor-associated carbohydrate antigens. Using immunotoxin therapy, total regression of established tumors in animal xenograft models have been demonstrated. These results suggest that immunotoxin molecules offer exciting opportunities for the treatment of human cancer.

Cytotoxic effect of a fusion protein from transforming growth factor alpha and Pseudomonas exotoxin on rat and human bladder carcinoma cells in vitro

A protein formed by fusion of transforming growth factor alpha with Pseudomonas exotoxin (TGF alpha-PE40) has been shown to have the ability to kill or inhibit the growth of several carcinoma cell lines. This study was designed to evaluate the in vitro cytotoxic effects of TGF alpha-PE40 on rat and human bladder carcinoma cell lines with different biological potential, and normal rat urothelial cells. The rat cell lines used were D44c, LMC19, and MYU3L, which were established in our laboratory. Human cell lines used were RT4, T24, and 253J. As a normal control, we used the first-passage culture of normal rat bladder urothelium (RU-P1). We examined the number and affinity of epidermal growth factor receptors (EGFR) in these cells, the ability of TGF alpha-PE40 to bind EGFR, and the cytotoxic effect of TGF alpha-PE40 and PE40. Rat cell lines, D44c, LMC19, and MYU3L (EGFR = 4.9 x 10(3)-11.4 x 10(3)/cell) had ED50 values (the concentration of TGF alpha-PE40 needed to reduce the viable cell population by 50%) of 180 pM, 540 pM and 6000 pM respectively; for cI (the concentration required to achieve complete inhibition of growth under continuous serum stimulation) TGF alpha-PE40 concentrations of 10(4) pM, 10(4) pM and higher than 10(4) pM respectively were required. Human cell lines, RT4, T24, and 253J (EGFR = 32 x 10(3)-126 x 10(3)/cell) had ED50 values of 20 pM, 66 pM, and 330 pM respectively and T24 showed cI values of 10(3) pM. RU-P1 (EGFR = 92.6 x 10(3)/cell) had the highest ED50 value of 8000 pM. These data indicate that the susceptibility to TGF alpha-PE40 does not always depend on the number of EGFR, that cells having a relatively small number of EGFR respond well to TGF alpha-PE40, and that normal urothelial cells are more resistant to TGF alpha-PE40 than are cancer cells. The differential effect of TGF alpha-PE40 on normal and neoplastic cells provides a rational basis for its use in vivo to control tumor growth.

In vitro sensitivity testing of human bladder cancers and cell lines to TP-40, a hybrid protein with selective targeting and cytotoxicity

TP-40 is a hybrid fusion protein produced by recombinant technology and consists of a molecule of transforming growth factor-alpha (TGF-alpha) fused to the Pseudomonas exotoxin PE-40. A panel of human and murine bladder cancer cell lines was found to be universally sensitive in vitro to TP-40 in a clonogenic assay. All lines expressed receptor for epidermal growth factor (EGF), though none demonstrated gene amplification for the EGF receptor. The sensitivity to TP-40 may be blocked by preexposure to EGF. Six human bladder tumors taken directly from patients were all sensitive in vitro to TP-40; these included well-differentiated tumors. TP-40 may prove to be effective as an intravesical agent in bladder cancer via selective targeting to cells that express EGF receptors, as do the majority of human bladder cancers.

Cytotoxicity and antitumor effects of growth factor-toxin fusion proteins on human glioblastoma multiforme cells

The prognosis of glioblastoma multiforme remains poor despite advances in treatment by surgery, irradiation, and chemotherapy. Many malignant gliomas overexpress growth factor receptors. The possibility of targeting these receptors with selective cytotoxic molecules constructed by fusing deoxyribonucleic acid (DNA)-encoding mutant forms of Pseudomonas exotoxin A (PE) with complementary DNA-encoding growth factors was investigated. Several recombinant toxins have been produced, including those in which transforming growth factor (TGF)-alpha, insulin-like growth factor (IGF)-I, and acidic fibroblast growth factor (FGF) were fused to mutant forms of PE lacking the native cell-binding domain. These recombinant proteins are cytotoxic to cells that express specific cell-surface receptors. The cytotoxic activity of TGF-alpha, IGF-I, and acidic FGF chimeric toxins was tested in vitro against human glioblastoma cell lines. Each recombinant toxin exhibited potent and specific killing of cells. The TGF-alpha-PE40 construct was cytotoxic to seven of the eight cell lines and was active at concentrations as low as 0.5 ng/ml (1.1 x 10(-11) M). The acidic FGF-PE40 toxin was also active on seven of the eight cell lines but was 50-fold less active than the TGF-alpha-PE40. The IGF-I-PE40 construct was active on only two cell lines. To determine the possible therapeutic effect in animals, TGF-alpha-PE40 was administered to nude mice bearing subcutaneous human glioblastoma xenografts. The animals were treated for 7 days via a continuous infusion pump placed in the peritoneal cavity. A constant serum level of TGF-alpha-PE40 was achieved that was nontoxic to the mice yet caused a reduction in tumor volume and retarded growth beyond the treatment period. The overexpression of the epidermal growth factor receptor in glioblastomas multiforme and the potency and specificity of the TGF-alpha-PE40 construct designed to target this receptor suggests that TGF-alpha-PE40 has the potential to be an effective antitumor agent for the adjuvant therapy of these carcinomas.

Pseudomonas exotoxin and recombinant immunotoxins derived from it

Pseudomonas exotoxin (PE) is a bacterial toxin that kills mammalian cells by gaining entry to the cytosol and inactivating protein synthesis. The toxin binds and enters cells via the alpha 2-macroglobulin receptors. Within cells, the toxin is processed in several steps to produce an enzymatically active 37-kDa C-terminal fragment which translocates to the cytosol and ADP-ribosylates elongation factor 2. Because PE is a very potent toxin, derivatives of it have been produced which, when joined to various binding ligands, are capable of killing specific target cells. It is hoped that this strategy will lead to the development of effective therapeutic agents for the treatment of human diseases such as cancer, AIDS, and various immunologic disorders.

A recombinant form of Pseudomonas exotoxin A containing transforming growth factor alpha near its carboxyl terminus for the treatment of bladder cancer

The epidermal growth factor receptor (EGFR) is overexpressed on the superficial layers of malignant urothelium and is suspected of playing a role in tumor progression. TP40 is a chimeric protein composed of transforming growth factor-alpha (TGF alpha) fused to a modified form of Pseudomonas exotoxin A (PE) that is selectively cytotoxic to EGFR-bearing cells and is currently undergoing clinical study for the intravesical therapy of bladder cancer. We constructed a recombinant toxin PE35/TGF alpha-KDEL as an improved agent for the local therapy of EGFR-bearing bladder cancer. PE35/TGF alpha-KDEL does not require intracellular proteolysis to generate a carboxyl-terminal fragment capable of reaching the target cell cytosol and contains a modified carboxyl-terminal sequence KDEL, that increases toxin activity. These features make PE35/TGF alpha-KDEL from 10- to 700-fold more potent than TP40 on four human bladder cancer cell lines. PE35/TGF alpha-KDEL may be a useful agent for treatment of EGFR-bearing cancers.

The intriguing link between modulation of both multidrug resistance and ligand-toxin conjugate cytotoxicity

Pharmacological agents which possess a chemosensitizing activity (i.e. the ability to modulate the multidrug resistance phenotype) can equally enhance ligand-toxin conjugate cytotoxicity. By confronting results obtained in both fields of research it appears that quite a number of agents, which are structurally unrelated, possess this bilateral effect. We have therefore attempted to provide a brief review of the literature and to discuss a hypothesis by which a common mechanism such as modifications in intracellular vesicle sorting and/or lipid metabolism may be implicated. We believe that these observations may provide clues for future research.

A transforming growth factor-alpha-Pseudomonas exotoxin hybrid protein undergoes pH-dependent conformational changes conducive to membrane interaction

TP40 is a chimeric protein containing transforming growth factor alpha (TGF-alpha) at the N-terminus and a derivative of a 40,000-Da segment (PE40 delta cys) of Pseudomonas exotoxin (PE). PE40 delta cys contains domains Ib, II, and III of PE in which the cysteines are mutated to alanines. The rationale for inclusion of TGF-alpha is to provide TP40 with selective targeting toward cells expressing the epidermal growth factor receptor (EGFr) on their surface [Pastan, I., & FitzGerald, D. (1989) J. Biol. Chem. 264, 15157-15160]. Translocation across endosomal membranes is thought to be a required step for cytotoxic activity of PE. This step is presumably facilitated by the low pH in endosomes which induces exposure of a hydrophobic surface of the protein, which in turn becomes available to interact with and translocate across the membrane. We have employed the hydrophobic fluorescence probe 2-p-toludinylnaphthalene-6-sulfonate (TNS) and the intrinsic tryptophan fluorophores of TP40 to investigate pH-induced changes in the tertiary structure of this protein. The pH dependence of TP40 interaction with liposomes also provided a model for studying protein-membrane interactions. TNS fluorescence was markedly enhanced in the presence of TP40 below pH 4 and to a lesser degree between pH 7 and 5. A progressive red shift of tryptophan fluorescence with decreasing pH was also seen with the approximate midpoint for this transition occurring around pH 3. Both observations suggest that acidic pH induces exposure of hydrophobic regions of TP40, making them accessible to solvent and TNS. No major alteration of the secondary structure was manifested in the far-UV CD spectrum of TP40 upon a reduction in pH from 7 to 2. Thus, the low-pH-induced structural change of TP40 appears to involve a subtle exposure of one or more hydrophobic surfaces without an extensive unfolding of the protein's secondary structure. In the presence of anionic liposomes, a low-pH-induced blue shift of the TP40 tryptophan fluorescence was observed, suggesting that interaction with liposomes also required the low-pH conformation of the protein. However, the midpoint of this fluorescence blue shift occurred at approximately pH 5, which is presumably closer to the physiological pH within endosomes. Neutral liposomes failed to induce these spectral changes in TP40, implying a lack of interaction with these lipids. At acidic pH values between 2 and 4, self-association of TP40 in solution was detected by equilibrium sedimentation and quasielastic light scattering measurements. This probably results from intermolecular interaction between exposed hydrophobic surfaces.(ABSTRACT TRUNCATED AT 400 WORDS)

Prevention of smooth muscle cell outgrowth from human atherosclerotic plaque by a recombinant cytotoxin specific for the epidermal growth factor receptor

Smooth muscle cell proliferation in the intima of arteries is a principal event associated with vascular narrowing after balloon angioplasty and bypass surgery. Techniques for limiting smooth muscle cell proliferation, however, have not as yet yielded any therapeutic benefit for these conditions. This may reflect the present lack of sufficiently potent and specific inhibitors of smooth muscle cell proliferation. DAB389 EGF is a genetically engineered fusion protein in which the receptor-binding domain of diphtheria toxin has been replaced by human epidermal growth factor. We evaluated the effect of this fusion toxin on human vascular smooth muscle cells in culture. Incubation of proliferating cells with DAB389 EGF yielded a dose-dependent inhibition of protein synthesis, as assessed by uptake of [3H]leucine, with an IC50 of 40 pM. The cytotoxic effect was inhibited in the presence of excess EGF or with monoclonal antibody to the EGF receptor. We further studied the effect of the fusion toxin on smooth muscle cell outgrowth from human atherosclerotic plaque. Outgrowth was markedly inhibited after as little as 1 h of exposure to the fusion protein. Furthermore, complete inhibition of proliferation of cells within the plaque could be attained. These results demonstrate that DAB389 EGF is highly cytotoxic to human smooth muscle cells proliferating in culture and can prevent smooth muscle cell outgrowth from "growth-stimulated" human atherosclerotic plaque. DAB389 EGF may therefore be of therapeutic value in vascular diseases characterized by smooth muscle cell accumulation.

Activity of a recombinant transforming growth factor-alpha-Pseudomonas exotoxin hybrid protein against primary human tumor colony-forming units

Transforming growth factor-alpha-Pseudomonas exotoxin-40 (TP40) is a recombinant fusion protein. TP40 consists of the entire human transforming growth factor-alpha (TGF alpha) protein fused to a 40,000 Da. segment of the Pseudomonas exotoxin A protein. TP40 is a bifunctional molecule that possesses the epidermal growth factor (EGF) receptor binding properties of TGF alpha and the cell killing properties of Pseudomonas exotoxin A. These properties make TP40 a selective cytotoxic agent that kills EGF receptor bearing cells. TP40 has been shown to effectively kill human tumor cell lines that possess EGF receptors in vitro and in nude mice. In the present study, TP40 was tested against tumors taken directly from patients and grown in a soft agar human tumor cloning system. A total of 107 patients' tumors (taken from patients with tumors refractory to chemotherapy) were tested with a continuous exposure to 0.5-50 nM concentrations of the agent. TP40 exhibited a clear dose response effect against a wide variety of human solid tumor colony-forming units with greater than or equal to 84% of evaluable tumors responding at a drug concentration greater than or equal to 24 nM. When used as a continuous exposure, concentrations of TP40 as low as 5 nM demonstrated substantial in vitro activity. This activity included cytotoxicity against breast, colorectal, endometrial, head and neck, non small-cell lung, gastric, sarcoma, and pancreatic cancer tumor colony-forming units. Additional in vivo testing of this compound is warranted.

Immunotoxins and central nervous system neoplasia

The poor prognosis associated with central nervous system (CNS) malignancy has led investigators to seek new, innovative treatment modalities. Immunotoxins, carrier molecules linked to toxic agents, combine high specificity for tumor-associated antigens with extreme potency. The rationale for both the development of these compounds and for their application to CNS neoplasia is explained. This report discusses the design and construction of immunoconjugates, using toxins that differ in their mechanism of action bound to ligands directed against various antigens. A comparison is made between the in vitro efficacy of standard chemotherapy and immunotoxins in glioblastoma- and medulloblastoma-derived cell lines. A review is included of the results of experiments in animals with leptomeningeal neoplasia, where prolongation of survival following intrathecal administration of immunotoxins has been reported. The obstacles encountered in clinical trials with other types of cancer are addressed and approaches to optimize the use of these novel agents in the context of treating malignant disease of the CNS are suggested.

Recombinant toxins for cancer treatment

Recombinant toxins target cell surface receptors and antigens on tumor cells. They kill by mechanisms different from conventional chemotherapy, so that cross resistance to conventional chemotherapeutic agents should not be a problem. Furthermore, they are not mutagens and should not induce secondary malignancies or accelerate progression of benign malignancies. They can be mass-produced cheaply in bacteria as homogeneous proteins. Either growth factor-toxin fusions or antibody-toxin fusions can be chosen, depending on the cellular target.

Influence of chloroquine and lidocaine on retention and cytotoxic effects of [131I]EGF: studies on cultured glioma cells

Targeting of toxic substances to the epidermal growth factor, EGF, receptor might be an attractive therapeutic approach because of the increased receptor-expression in some human tumours such as, for example, malignant gliomas and squamous lung carcinomas. Radiation effects of [131I]EGF on human malignant glioma cells growing as monolayers were analysed in this study. The cells were, in all cases, incubated for 25 min with about 350 kBq/ml [131I]EGF, which gave a total binding of 3.2-3.5 kBq/10(5) cells. The rapid release of activity from the cells caused by the normal degradation of EGF was inhibited by incubation with 30 microM chloroquine or 5 mM lidocaine added to the cell culture medium. These substances are, at these concentrations, known to inhibit proteolytic processes in lysosomes. No effects of the inhibitors alone were observed on cell growth and clonogenic survival. Inhibition of EGF degradation by chloroquine or lidocaine resulted in a significantly prolonged association of 131I with the test cells. About 70% of the initially bound radioactivity remained in the cells giving, after 6 h, a binding of 2.1-2.5 kBq/10(5) cells. A 6 h exposure to the radiation from 131I decays, mediated mainly by specifically bound EGF, gave a survival value of about 50%. Such an effect corresponds to a treatment of 2.5 Gy 60Co gamma-radiation. This is promising considering that, when monolayers are applied, only a very small fraction of the released energy from the 131I decays is deposited in the cells. Effects from non-receptor bound [131I]EGF were analysed after presaturation of the receptors with non-radioactive EGF, and gave no or very small changes in survival.