Protein engineering of diphtheria-toxin-related interleukin-2 fusion toxins to increase cytotoxic potency for high-affinity IL-2-receptor-bearing target cells

Targeted Diphtheria Toxin-Based Therapy: A Review Article

Cancer remains one of the leading causes of death worldwide. Conventional therapeutic strategies usually offer limited specificity, resulting in severe side effects and toxicity to normal tissues. Targeted cancer therapy, on the other hand, can improve the therapeutic potential of anti-cancer agents and decrease unwanted side effects. Targeted applications of cytolethal bacterial toxins have been found to be especially useful for the specific eradication of cancer cells. Targeting is either mediated by peptides or by protein-targeting moieties, such as antibodies, antibody fragments, cell-penetrating peptides (CPPs), growth factors, or cytokines. Together with a toxin domain, these molecules are more commonly referred to as immunotoxins. Targeting can also be achieved through gene delivery and cell-specific expression of a toxin. Of the available cytolethal toxins, diphtheria toxin (DT) is one of the most frequently used for these strategies. Of the many DT-based therapeutic strategies investigated to date, two immunotoxins, Ontak^TM and Tagraxofusp^TM, have gained FDA approval for clinical application. Despite some success with immunotoxins, suicide-gene therapy strategies, whereby controlled tumor-specific expression of DT is used for the eradication of malignant cells, are gaining prominence. The first part of this review focuses on DT-based immunotoxins, and it then discusses recent developments in tumor-specific expression of DT.

Principles of Immunotherapy: Implications for Treatment Strategies in Cancer and Infectious Diseases

The advances in cancer biology and pathogenesis during the past two decades, have resulted in immunotherapeutic strategies that have revolutionized the treatment of malignancies, from relatively non-selective toxic agents to specific, mechanism-based therapies. Despite extensive global efforts, infectious diseases remain a leading cause of morbidity and mortality worldwide, necessitating novel, innovative therapeutics that address the current challenges of increasing antimicrobial resistance. Similar to cancer pathogenesis, infectious pathogens successfully fashion a hospitable environment within the host and modulate host metabolic functions to support their nutritional requirements, while suppressing host defenses by altering regulatory mechanisms. These parallels, and the advances made in targeted therapy in cancer, may inform the rational development of therapeutic interventions for infectious diseases. Although "immunotherapy" is habitually associated with the treatment of cancer, this review accentuates the evolving role of key targeted immune interventions that are approved, as well as those in development, for various cancers and infectious diseases. The general features of adoptive therapies, those that enhance T cell effector function, and ligand-based therapies, that neutralize or eliminate diseased cells, are discussed in the context of specific diseases that, to date, lack appropriate remedial treatment; cancer, HIV, TB, and drug-resistant bacterial and fungal infections. The remarkable diversity and versatility that distinguishes immunotherapy is emphasized, consequently establishing this approach within the armory of curative therapeutics, applicable across the disease spectrum.

In Silico Design of Fusion Toxin DT389GCSF and a Comparative Study

BACKGROUND

Chemotherapy and radiotherapy have negative effects on normal tissues and they are very expensive and lengthy treatments. These disadvantages have recently attracted researchers to the new methods that specifically affect cancerous tissues and have lower damage to normal tissues. One of these methods is the use of intelligent recombinant fusion toxin. The fusion toxin DTGCSF, which consists of linked Diphtheria Toxin (DT) and Granulocyte Colony Stimulate Factor (GCSF), was first studied by Chadwick et al. in 1993 where HATPL linker provided the linking sequence between GCSF and the 486 amino acid sequences of DT.

METHODS

In this study, the fusion toxin DT389GCSF is evaluated for functional structure in silico. With the idea of the commercial fusion toxin of Ontak, the DT in this fusion protein is designed incomplete for 389 amino acids and is linked to the beginning of the GCSF cytokine via the SG4SM linker (DT389GCSF). The affinity of the DT389GCSF as a ligand with GCSF-R as receptor was compared with DT486GCSF as a ligand with GCSF-R as receptor. Both DT486GCSF and its receptor GCSF-R have been modeled by Easy Modeler2 software. Our fusion protein (DT389GCSF) and GCSF-R are modeled through Modeller software; all of the structures were confirmed by server MDWEB and VMD software. Then, the interaction studies between two proteins are done using protein-protein docking (HADDOCK 2.2 web server) for both the fusion protein in this study and DT486GCSF.

RESULTS

The HADDOCK results demonstrate that the interaction of DT389GCSF with GCSF-R is very different and has a more powerful interaction than DT486GCSF with GCSF-R.

CONCLUSION

HADDOCK web server is operative tools for evaluation of protein-protein interactions, therefore, in silico study of DT389GCSF will help with studying the function and the structure of these molecules. Moreover, DT389GCSF may have important new therapeutic applications.

Suppressor Cell-Depleting Immunotherapy With Denileukin Diftitox is an Effective Host-Directed Therapy for Tuberculosis

Host-directed therapies that augment host immune effector mechanisms may serve as important adjunctive therapies for tuberculosis treatment. We evaluated the activity of denileukin diftitox in an acute mouse model of tuberculosis (TB) infection and analyzed the cellular composition and bacterial burden in lungs and spleens. These in vivo studies show that denileukin diftitox potentiates standard TB treatment in the mouse model, an effect which may be due to depletion of T-regulatory and myeloid-derived suppressor cells during TB infection. Our results indicate that denileukin diftitox and other suppressor cell-depleting therapies may be useful adjunctive, host-directed therapies for TB.

Antibody-Based Cancer Therapy: Successful Agents and Novel Approaches

Since their discovery, antibodies have been viewed as ideal candidates or "magic bullets" for use in targeted therapy in the fields of cancer, autoimmunity, and chronic inflammatory disorders. A wave of antibody-dedicated research followed, which resulted in the clinical approval of a first generation of monoclonal antibodies for cancer therapy such as rituximab (1997) and cetuximab (2004), and infliximab (2002) for the treatment of autoimmune diseases. More recently, the development of antibodies that prevent checkpoint-mediated inhibition of T cell responses invigorated the field of cancer immunotherapy. Such antibodies induced unprecedented long-term remissions in patients with advanced stage malignancies, most notably melanoma and lung cancer, that do not respond to conventional therapies. In this review, we will recapitulate the development of antibody-based therapy, and detail recent advances and new functions, particularly in the field of cancer immunotherapy. With the advent of recombinant DNA engineering, a number of rationally designed molecular formats of antibodies and antibody-derived agents have become available, and we will discuss various molecular formats including antibodies with improved effector functions, bispecific antibodies, antibody-drug conjugates, antibody-cytokine fusion proteins, and T cells genetically modified with chimeric antigen receptors. With these exciting advances, new antibody-based treatment options will likely enter clinical practice and pave the way toward more successful control of malignant diseases.

Differential effects of denileukin diftitox IL-2 immunotoxin on NK and regulatory T cells in nonhuman primates

Denileukin diftitox (DD), a fusion protein comprising IL-2 and diphtheria toxin, was initially expected to enhance antitumor immunity by selectively eliminating regulatory T cells (Tregs) displaying the high-affinity IL-2R (α-β-γ trimers). Although DD was shown to deplete some Tregs in primates, its effects on NK cells (CD16(+)CD8(+)NKG2A(+)CD3(-)), which constitutively express the intermediate-affinity IL-2R (β-γ dimers) and play a critical role in antitumor immunity, are still unknown. To address this question, cynomolgus monkeys were injected i.v. with two doses of DD (8 or 18 μg/kg). This treatment resulted in a rapid, but short-term, reduction in detectable peripheral blood resting Tregs (CD4(+)CD45RA(+)Foxp3(+)) and a transient increase in the number of activated Tregs (CD4(+)CD45RA(-)Foxp3(high)), followed by their partial depletion (50-60%). In contrast, all NK cells were deleted immediately and durably after DD administration. This difference was not due to a higher binding or internalization of DD by NK cells compared with Tregs. Coadministration of DD with IL-15, which binds to IL-2Rβ-γ, abrogated DD-induced NK cell deletion in vitro and in vivo, whereas it did not affect Treg elimination. Taken together, these results show that DD exerts a potent cytotoxic effect on NK cells, a phenomenon that might impair its antitumoral properties. However, coadministration of IL-15 with DD could alleviate this problem by selectively protecting potentially oncolytic NK cells, while allowing the depletion of immunosuppressive Tregs in cancer patients.

A Phase II trial of Denileukin Diftitox in patients with previously treated advanced non-small cell lung cancer

INTRODUCTION

Denileukin diftitox, a chimeric protein, uses the cytocidal properties of diphtheria toxin to cells expressing interleukin-2 receptors. The aim of this study was to evaluate the efficacy and safety of denileukin diftitox in the treatment of advanced relapsed nonsmall cell lung cancer (NSCLC).

PATIENTS AND METHODS

Multicenter phase II trial in patients with NSCLC with Eastern Cooperative Oncology Group PS 0-2, stage IIIB/IV at diagnosis, who had failed at least 1 previous chemotherapy regimen. Denileukin diftitox was infused at 18 microg/kg/d x 5 days, every 21 days for 6 cycles.

RESULTS

For the 41 patients enrolled, the median age was 56 years (range, 21-80), 25 were men, and the median number of previous chemotherapy regimens was 2 (range, 1-5). The median number of treatment cycles was 2 (range, 1-6). By RECIST criteria, 18 (44%) had stable disease, 10 (24%) progressive disease, and 13 (32%) were not evaluable for response as they received less than 2 treatment cycles. The median time to disease progression was 1.8 months [range, 0.3-11.3; 95% confidence interval (CI) 1.3-2.6]. Median survival was 5.8 months (range, 0.3-33.6; 95% CI 3.4-11.4). The median follow-up time was 16.1 month. One death from myocarditis verified at autopsy was attributed to treatment. One grade 4 toxicity (vascular leak syndrome) was encountered, and 18 grade 3 toxicities, primarily gastro-intestinal, vascular leak syndrome, and constitutional symptoms.

CONCLUSION

Denileukin diftitox at current dose schedule has limited activity in patients with previously treated NSCLC, manifested by disease control without impact on survival.

Effect of DAB(389)IL-2 immunotoxin on the course of experimental autoimmune encephalomyelitis in Lewis rats

Activated T cells express the high affinity interleukin 2 receptor (IL-2R also CD25) that binds interleukin 2 (IL-2) and transduces signals important for the proliferation and survival of these cells. We investigated the effect of the genetically engineered immunotoxin DAB(389)IL-2 on experimental autoimmune encephalomyelitis (EAE), an autoimmune disease of the central nervous system (CNS) mediated by activated myelin-reactive T cells. EAE is the most commonly used animal model of the human disease multiple sclerosis (MS). DAB(389)IL-2 is a recombinant fusion product made of a portion of diphtheria toxin, which contains binding and translocation components of the toxin linked to IL-2. The diphtheria toxin targets and kills cells expressing the high affinity IL-2 receptor and has been successfully used in several autoimmune and neoplastic conditions. We observed a significant suppression of guinea-pig spinal cord homogenate (gpSCH)-MBP induced active EAE in Lewis rats at 2 x 1,600 kU of DAB(389)IL-2 given on days 7 and 9 post-immunization and complete suppression with the same dose on days 7, 8 and 9 or 7, 8, 9 and 10 after immunization during the active disease period. There were reduced mononuclear cell infiltrates of CD4(+), CD8(+), CD25(+) and alphabetaTCR(+) T cells in the spinal cord of treated rats. However, treatment at day 11 or 12 post-immunization led to severe, fatal disease. The toxin added to cultures in vitro or injected in vivo suppressed antigen- and mitogen-induced T cell proliferation. DAB(389)IL-2 treatment in vivo or exposure of encephalitogenic T cells in vitro prior to transfer did have a significant inhibitory effect on adoptive transfer EAE. Our data demonstrate that DAB(389)IL-2 immunotoxin can suppress active and passive EAE if applied at specific, early time points, but can have negative consequences at later time points.

The rGel/BLyS fusion toxin specifically targets malignant B cells expressing the BLyS receptors BAFF-R, TACI, and BCMA

B lymphocyte stimulator (BLyS) is crucial for B-cell survival, and the biological effects of BLyS are mediated by three cell surface receptors designated B cell-activating factor receptor (BAFF-R), transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI), and B-cell maturation antibody (BCMA). Increased expression of BLyS and its receptors has been identified in numerous B-cell malignancies. We generated a fusion toxin designated rGel/BLyS for receptor-mediated delivery of the recombinant gelonin (rGel) toxin to neoplastic B cells, and we characterized its activity against various B-cell tumor lines. Three mantle cell lymphoma (MCL) cell lines (JeKo-1, Mino, and SP53) and two diffuse large B-cell lymphoma (DLBCL) cell lines (SUDHL-6 and OCI-Ly3) expressing all three distinct BLyS receptors were found to be the most sensitive to the fusion toxin (IC(50) = 2-5 pmol/L and 0.001-5 nmol/L for MCL and DLBCL, respectively). The rGel/BLyS fusion toxin showed specific binding to cells expressing BLyS receptors and rapid internalization of the rGel component into target cells. The cytotoxic effects of rGel/BLyS were inhibited by pretreatment with free BLyS or with soluble BAFF-R, TACI, and BCMA decoy receptors. This suggests that the cytotoxic effects of the fusion toxin are mediated through BLyS receptors. The rGel/BLyS fusion toxin inhibited MCL cell growth through induction of apoptosis associated with caspase-3 activation and poly (ADP-ribose) polymerase cleavage. Our results suggest that BLyS has the potential to serve as an excellent targeting ligand for the specific delivery of cytotoxic molecules to neoplastic B cells expressing the BLyS receptors, and that the rGel/BLyS fusion toxin may be an excellent candidate for the treatment of B-cell malignancies especially MCL and DLBCL.

The growth factor fusion construct containing B-lymphocyte stimulator (BLyS) and the toxin rGel induces apoptosis specifically in BAFF-R-positive CLL cells

The cytokine B lymphocyte stimulator (BLyS) mediates its effect through cell-surface receptors BAFF-R, TACI, and BCMA. BLyS receptors are expressed only on B cells and not present in other normal cells including normal T lymphocytes. Chronic lymphocytic leukemia (CLL) is a B-cell disease and CLL lymphocytes express BLyS receptors. Gelonin, a type 1 ribosome-inactivating toxin, lacks cell membrane binding domain and hence is nontoxic to intact cells. We generated a construct of recombinant gelonin (rGel) fused to BLyS to specifically target quiescent B-CLL lymphocytes. The construct rGel/BLyS specifically binds and internalizes through BAFF-R into CD19(+) B-CLL lymphocytes and induces apoptosis at nanomolar concentrations. In contrast, rGel alone was not able to internalize into these leukemic lymphocytes. Mechanistically, the rGel/BLyS construct inhibits protein synthesis with an IC(50) of less than 3 nM compared with more than 5000 nM for rGel toxin alone. This rGel/BLyS-mediated decrease in protein synthesis was associated with a decline in short-lived proteins such as MCL-1 and XIAP, the 2 survival proteins in B-CLL. There was a strong relationship between a decrease in these proteins and the cleavage of PARP, a hallmark feature of apoptosis. Taken together, these data suggest that the rGel/BLyS fusion toxin may have potential therapeutic efficacy for B-CLL patients.

Novel approaches in the treatment of systemic mastocytosis

In the absence of curative options, therapy for aggressive forms of systemic mastocytosis (SM) has relied in the use of cytoreductive agents, mainly interferon-alpha (IFN-alpha) and cladribine. However, responses are transient and only occur in a subset of patients. Gain-of-function mutations at codon 816 of the KIT protooncogene lead to constitutively active Kit receptor molecules, which are central to the pathogenesis of SM. Recent advances in the understanding of the molecular underpinnings of SM have led to the development of small molecules targeting mutant Kit tyrosine kinase isoforms that significantly have widened the range of therapeutic options for patients with SM. Some of these promising agents, such as dasatinib, AMN107, and PKC412, currently are under investigation in clinical trials whereas, others are at different stages of preclinical development. In addition, monoclonal antibodies directed to neoplastic mast cell-restricted surface antigens constitute a viable option for the treatment of SM that warrants further investigation.

High efficient mammalian expression and secretion of a functional humanized single-chain Fv/human interleukin-2 molecules

AIM: To construct and produce a recombinant bispecific humanized single-chain Fv (sFv) /Interleukin-2 (IL-2) fusion protein by using mammalian cells.

METHODS: The sFv/IL-2 protein was genetically engineered, and transfected to mammalian cells to determine whether the mammalian protein folding machinery can produce and secrete active sFv/IL-2 with high efficiency.

RESULTS: The fusion protein was constructed and high efficiently expressed with yields up to 102 ± 4.2 mg/L in culture supernatant of the stably transfected 293 cell line. This recombinant fusion protein consisted of humanized variable heavy (V_H) and light (V_L) domains of monoclonal antibody (mAb) 520C9 directed against the human HER-2/neu (c-erbB2) proto-oncogene product p185, and human IL-2 connected by polypeptide linker. The fusion protein was shown to retain the immunostimulatory activities of IL-2 as measured by IL-2-dependent cell proliferation and cytotoxicity assays. In addition to its IL-2 activities, this fusion protein also possessed antigen-binding specificity against p185, as determined by indirect ELISA using p185 positive SKOV 3ip1 cells.

CONCLUSION: The large-scale preparation of the recombinant humanized sFv antibody/IL-2 fusion protein is performed with 293 cells. The recombinant humanized sFv antibody/IL-2 fusion protein may provide an effective means of targeting therapeutic doses of IL-2 to p185 positive tumors without increasing systemic toxicity or immunogenicity.

Phase II study of denileukin diftitox for relapsed/refractory B-Cell non-Hodgkin's lymphoma

PURPOSE

Denileukin diftitox is a fusion protein combining diphtheria toxin and interleukin-2 (IL-2) that targets tumor cells expressing the IL-2 receptor. Its efficacy has been shown in CD25+ cutaneous T-cell lymphoma, but not in B-cell non-Hodgkin's lymphoma (NHL). A phase II study was performed to evaluate the efficacy and tolerability of denileukin diftitox for relapsed or refractory B-cell NHL.

PATIENTS AND METHODS

Patients with relapsed or refractory B-cell NHL were eligible. Tumor CD25 expression was determined by immunohistochemistry or flow cytometry. Denileukin diftitox was administered intravenously at a dose of 18 microg/kg once daily for 5 days every 3 weeks, up to eight cycles.

RESULTS

Of the 45 patients assessable for response, 32 (71%) were refractory to the last chemotherapy treatment, and all were previously treated with rituximab. Three complete responses (6.7%) and eight partial responses (17.8%) were observed, for an overall response rate of 24.5%. Nine patients (20%) had stable disease. Objective response rates were similar in CD25+ (22%) and CD25- histologies (29%), as were stable disease rates (22% and 18%, respectively). For responding patients, the median time to treatment failure was 7 months, with a median follow-up in survivors of 18 months (range, 9 to 28 months), and the projected progression-free survival at 20 months was 24% (95% CI, 0% to 60%). Most toxicities were low-grade and transient.

CONCLUSION

Denileukin diftitox seems to be effective in relapsed or refractory, CD25+ and CD25- B-cell NHL and is well-tolerated at the dosage evaluated. Evaluation of denileukin diftitox in combination with other agents may be warranted.

Treatment of cutaneous T cell lymphoma: current status and future directions

The treatment of cutaneous T cell lymphoma (CTCL), which includes mycosis fungoides and Sezary syndrome, has been in a state of continual change over recent decades, as new therapies are constantly emerging in the search for more effective treatments for the disease. However, prognosis and survival of patients with CTCL remains dependent upon overall clinical stage (stage IA-IVB) at presentation, as well as response to therapy. Past therapies have been limited by toxicity or the lack of consistently durable responses, and few treatments have been shown to actually alter survival, especially in the late stages of disease. Even aggressive chemotherapy has not been shown to improve overall survival compared to conservative sequential therapy in advanced disease, and adds the risk of immunosuppressive complications. Over the last decade, extracorporeal photopheresis has been the only single treatment that has been shown to improve survival in patients with Sezary syndrome, although its true efficacy and place in combination therapy remain unclear. Much of the focus of current research has been on combinations of skin-directed therapies and biological response modifiers, which improve response rates. The results of various trials over the years have also brought into favor the use of post-remission maintenance therapy with topical corticosteroids, topical mechlorethamine (nitrogen mustard), interferon-alpha, or phototherapy to prevent disease relapse. Recent novel developments in CTCL therapy include oral bexarotene, a retinoid X receptor-selective retinoid that has activity in all stages of CTCL, and the topical gel formulation of bexarotene, which plays a role in treating localized lesions. US Food and Drug Administration (FDA)-approved, oral systemic bexarotene has the advantage of a 48% overall response rate at a dosage of 300 mg/m(2)/day, and avoids immunosuppression and risk of central line and catheter-related infectious complications that are associated with other systemic therapies. Monitoring of triglycerides and use of concomitant lipid-lowering agents and thyroid replacement is required in most patients. Also recently FDA-approved, denileukin diftitox is the first of a novel class of fusion toxin proteins and is selective for interleukin-2R (CD25+) T cells, targeting the malignant T cell clones in CTCL. Denileukin diftitox is associated with capillary leak syndrome in 20 to 30% of patients, which may be ameliorated by hydration and corticosteroids. Higher response rates are possible by combining bexarotene with "statin" drugs and active CTCL therapies. Studies are being conducted on combining bexarotene and denileukin diftitox with other modalities. Biological response modifier therapies that are in current or future investigational trials include topical tazarotene, pegylated interferon, interleukin-2, and interleukin-12. At the forefront of systemic chemotherapy development, pegylated liposomal doxorubicin, gemcitabine, and pentostatin appear to have the greatest potential for success in CTCL therapy. Bone marrow transplantation, which is currently limited by the risk of graft-versus-host disease, offers the greatest potential for disease cure. Further developments for CTCL may include more selective immunomodulatory agents, vaccines, and monoclonal antibodies.

Quality-of-life improvements in cutaneous T-cell lymphoma patients treated with denileukin diftitox (ONTAK)

Cutaneous T-cell lymphoma (CTCL) can be associated with painful, pruritic, disfiguring lesions. As part of a multicenter, randomized phase III trial in patients with heavily pretreated advanced and/or recurrent CTCL, the effects of an interleukin-2 receptor-targeted fusion protein, denileukin diftitox (DAB389IL-2, ONTAK), on patient-rated overall quality of life (QOL), skin appearance, and pruritus severity were evaluated. A total of 71 patients with stage IB-IVA CTCL received intravenous denileukin diftitox 9 microg/kg/day or 18 microg/kg/day over 15-60 minutes for 5 consecutive days on an outpatient basis; cycles were planned for every 21 days for a total of 8 cycles over 6 months. Prior to each treatment cycle, patients were evaluated for disease response and were asked to self-rate their overall QOL via the Functional Assessment of Cancer Therapy-General (FACT-G) questionnaire, skin appearance (7-point scale), and pruritus severity (10-cm visual analogue scale). Composite FACT-G and most individual subscale scores (physical, social/family, emotional, and functional well being) in documented responders (n = 21) gradually increased during the study period, generally reaching statistical significance (P < 0.05) by cycle 3, and were significantly (P < or = 0.041) higher than the scores of nonresponders at endpoint. Additionally for responders, assessments of skin severity and pruritus severity showed significant (P < or = 0.05) improvements at study endpoint compared with baseline. Adverse transfusion-related events (eg, hypersensitivity reactions, flu-like syndrome) were common during cycles 1 and 2, and vascular-leak syndrome occurred in 25% of patients. Denileukin diftitox was not associated with any clinically significant myelosuppression. Heavily pretreated patients with advanced and/or recurrent CTCL who responded to denileukin diftitox therapy showed significant improvements in self-rated overall QOL, skin appearance, and pruritus severity.

Cutaneous T-cell lymphoma. New immunomodulators

During the most recent decades, much knowledge has been gained concerning the immunologic and pathologic mechanisms of CTCL. The development of immunomodulators aimed at correcting aberrations in immunology and cellular growth and differentiation reflects this increased understanding. This review of the currently available immune-response modifying drugs shows that recombinant forms of natural cytokines and retinoids can be developed with tolerable toxicity profiles and substantial efficacy. Although milestone drugs such as bexarotene have been approved by the FDA- for treatment of CTCL, other agents such as IL-12 may also have a place in treatment of the disease. Even though unapproved, IFN-alpha may be the most active single immunomodulating agent against CTCL. It seems that further delineation of CTCL cytokine profile changes and immunologic aberrations are key in developing effective immunomodulators that are able to reverse these alterations.

Interleukin-2 fusion toxin: targeted therapy for cutaneous T cell lymphoma

The interleukin (IL)-2 receptor has proved an attractive target for T cell-directed therapies. Agents including monoclonal antibodies, single-chain antibody immunoconjugates, radioimmunoconjugates, and, most recently, ligand fusion toxins have demonstrated activity in vitro and in clinical trials in both hematologic malignancies and diseases characterized by proliferation of activated T cells, such as graft-versus-host disease. DAB389IL-2 (ONTAK) is a ligand fusion toxin consisting of the full-length sequence of the IL-2 gene genetically fused to the enzymatically active and translocating domains of diphtheria toxin. DAB389IL-2 and its predecessor, DAB486IL-2, have demonstrated activity in a variety of diseases, including cutaneous T cell lymphoma (CTCL), psoriasis, rheumatoid arthritis, and HIV infection. Further clinical development of IL-2 fusion toxins in CTCL and other hematopoietic malignancies is predicated on identification of the high-affinity IL-2 receptor complex on the malignant cells and on a better understanding of the biological determinants of cytotoxicity of these molecules in vivo.

Biological correlates of acute hypersensitivity events with DAB(389)IL-2 (denileukin diftitox, ONTAK) in cutaneous T-cell lymphoma: decreased frequency and severity with steroid premedication

DAB(389)IL-2 (denileukin diftitox, ONTAK) is a cytokine-targeted fusion protein that delivers the catalytic domain of diphtheria toxin to lymphoma cells expressing the interleukin-2 receptor (IL-2R). In phase I and phase III studies of DAB(389)IL-2 in patients with cutaneous T-cell lymphoma (CTCL), non-Hodgkin's lymphoma, and Hodgkin's disease in which premedications were limited to diphenhydramine and acetaminophen, acute infusion-related hypersensitivity reactions occurred in 70% of patients and vascular leak syndrome (VLS) in 27%, resulting in discontinuation of therapy in 29% of patients. There was no correlation between the dose or half-life of DAB(389)IL-2 and the occurrence of hypersensitivity events or VLS. To explore whether steroid premedication would improve the tolerability of DAB(389)IL-2, we treated 15 patients with CTCL with either dexamethasone or prednisone prior to each dose of DAB(389)IL-2. The incidence of acute infusion events was significantly decreased, with only three patients experiencing acute infusion events (one grade 4) and only two patients developing clinically apparent VLS. Grade 3 skin rash occurred in two patients and moderately severe asthenia in nine patients. A significantly improved response rate of 60% was noted with the use of steroid premedication compared to prior studies in which steroids were prohibited. We conclude that steroid premedication significantly improves the tolerability of DAB(389)IL-2 without compromising the clinical response.

DAB(389)IL-2 (ONTAK): a novel fusion toxin therapy for lymphoma

Fusion proteins are recombinant molecules that combine a targeting mechanism to a cytocidal moiety. DAB(389)IL-2 (denileukin diftitox; ONTAK), with a unique mechanism of action, is the first genetically constructed fusion protein to reach the clinic. In this molecule, the interleukin-2 (IL-2) gene is genetically fused to the enzymatically active and translocating domains of diphtheria toxin. DAB(389)IL-2 is internalized into IL-2 receptor-bearing cells by endocytosis. The ADP-ribosyltransferase activity of diphtheria toxin is cleaved in the endosome and is translocated into the cytosol where it inhibits protein synthesis, leading to apoptosis. DAB(389)IL-2 and its predecessor, DAB(486)IL-2, have shown clinical activity in a variety of diseases, including B-cell non-Hodgkin's lymphoma, cutaneous T-cell lymphoma (CTCL), Hodgkin's disease, psoriasis, rheumatoid arthritis, and HIV infection. The highest response rates were observed in CTCL, and this became the focus of clinical trials leading to its subsequent approval by the United States Food and Drug Administration for this disease. The potential applications of DAB(389)IL-2 in lymphomas are reviewed.

Immunotherapy of experimental autoimmune myasthenia gravis: selective effects of CTLA4Ig and synergistic combination with an IL2-diphtheria toxin fusion protein

We examined the effects of CTLA4Ig treatment in an experimental model of myasthenia gravis (EAMG) induced by immunizing Lewis rats with purified Torpedo acetylcholine receptor (AChR). During a primary response, CTLA4Ig treatment inhibited AChR antibody production profoundly, and induced a shift of AChR antibody isotypes from the normally predominant IgG2 isotype pattern toward an IgG1 response. Challenge of rats previously treated with CTLA4Ig produced markedly lower AChR antibody responses compared to untreated controls, persistent inhibition of the IgG2b isotype, and no development of EAMG. Treatment of a secondary AChR response with CTLA4Ig or with DAB389IL2 (which kills lymphocytes expressing IL2 receptors) inhibited AChR antibody responses, and clinical EAMG moderately. In contrast, combined treatment with CTLA4Ig plus DAB389IL2 strikingly inhibited AChR antibody levels, and completely prevented EAMG. Our results suggest that the therapeutic benefit of CTLA4Ig may be due to overall inhibition of AChR antibody production as well as a shift in the antibody isotype repertoire.

Interleukin-1 alpha increases the preferential cytotoxicity of an interleukin-2-diphtheria toxin fusion protein against neoplastic lymphocytes from patients with the Sezary syndrome compared to normal lymphocytes

DAB389IL-2 is a recombinant fusion toxin composed of the diphtheria A chain and a protion of the translocating region of the diphtheria B chain, replacing the receptor binding domain with human IL-2. DAB389IL-2 can be safely administered to humans with mycosis fungoides (MF) or Sezary syndrome (SS), and antineoplastic effects occur. This agent binds optimally to the high-affinity IL-2R. The decreased efficiency of uptake by neoplastic cells which do not express the high-affinity IL-2R represents a potential limitation. Treatment of the HUT-78 cutaneous T-cell lymphoma with IL-1 alpha preceding exposure to DAB389IL-2 overcame their resistance to the toxin, IL-1 alpha inducing high-affinity IL-2R expression. Similarly, pretreatment with IL-1 alpha of SS patient lymphocytes demonstrated increased cytotoxicity compared to treatment with the fusion toxin alone. Normal lymphocytes and monocytes were not sensitive to DAB389IL-2 when pretreated with IL-1 alpha, suggesting a differential sensitivity which may be exploited clinically in the treatment of lymphomas.

Characterization and receptor specific toxicity of two diphtheria toxin-related interleukin-3 fusion proteins DAB389-mIL-3 and DAB389-(Gly4Ser)2-mIL-3

We have constructed two fusion proteins, DAB389-mIL-3 and DAB389-(Gly4Ser)2-mIL-3, in which the receptor-binding domain of diphtheria toxin is replaced by mouse interleukin-3 (IL-3). Cytotoxic activity of the fusion toxins was observed on three out of six cell lines assayed. This toxicity was mediated through binding to the IL-3 receptor as it was inhibited in a dose-dependent manner with murine IL-3 or anti-IL-3 neutralizing antibodies. DAB389-(Gly4Ser)2-mIL-3 was up to 5 times more toxic than DAB389-mIL-3, depending on the cell line (0.8 x 10(-10) M < IC50 < 3 x 10(-10) M). These proteins can be used for the detection of IL-3 receptors on mouse cells and should allow for the selective elimination of IL-3 receptor-positive pluripotent hematopoietic stem cells prior to bone marrow transplantation.

Immunotherapy in neuromuscular disorders: current and future strategies

Autoimmune mechanisms have recently been implicated in the pathogenesis of an increasing number of neuromuscular diseases. Many of these diseases can be treated with immunotherapeutic agents that are currently available--often with striking success. However, lack of specificity and adverse side effects impose limits on the effectiveness of these immunosuppressive treatments. This article reviews the basic principles of autoimmunity and immune tolerance, and outlines strategies that produce (a) generalized immunosuppression; (b) "selective" immunotherapy; and (c) "antigen-specific" immunotherapy. General immunosuppressive treatments, which are the ones most commonly used in current practice, down-regulate the immune system at multiple levels in "shotgun" fashion. The agents described here include: adrenal corticosteroids, azathioprine, cyclophosphamide, chlorambucil, methotrexate, total lymphoid irradiation, plasmapheresis, and intravenous immunoglobulin. "Selective" immunotherapeutic strategies are designed to interfere with mechanisms intrinsic to the immune system. Agents that are now being used clinically, or are in advanced stages of development include: cyclosporin A, which interferes with synthesis of the cytokine interleukin 2 (IL2); IL2 toxin, which binds to IL2 receptors on activated T cells, is endocytosed, and kills the cells; and CTLA4Ig, which blocks costimulatory molecules, thus preventing full activation of T cells. We have found that combinations of the selective agents may enhance their effectiveness. "Specific" strategies are designed to inactivate or suppress antigen-specific T cells. Oral administration of autoantigens has been shown to prevent experimental autoimmune diseases specifically, but the conditions required to suppress ongoing autoimmune diseases are capricious, and depend on many factors. Finally, we describe a method that is still in the experimental stage, which is designed to modify the individual's own antigen-presenting cells so that they will target and inactivate antigen-specific T cells, and thereby turn off the specific autoimmune response. Currently available immunosuppressive methods can now be used successfully to treat many autoimmune neuromuscular diseases, and the application of selective and specific immunotherapeutic strategies promises more precise and effective treatment in the future.

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 recombinant single chain antibody interleukin-2 fusion protein

Recombinant interleukin-2 (rIL-2) therapy has been shown to be of value in the treatment of some cases of melanoma and renal cell carcinoma. However its use can be limited by severe systemic toxicity. Targeting rIL-2 to the tumour should improve the anti-tumour immune response and decrease the systemic toxicity. With this aim we have employed recombinant DNA techniques to construct a single chain antibody interleukin-2 fusion protein (SCA-IL-2). The protein used in this model system comprises the variable domains of the anti-lysozyme antibody D1.3 fused to human IL-2. It has been expressed by secretion from Escherichia coli and the purified product possesses antigen binding specificity and retains the immunostimulatory activities of rIL-2. This approach can be taken to generate SCA-IL-2 proteins that bind to appropriate cellular antigens. In vivo administration of a tumour binding SCA-IL-2 should result in a localised high concentration of IL-2 in tumour tissues, maximising the anti-tumour immune response, whilst keeping systemic side effects to a minimum.

A recombinant single-chain antibody interleukin-2 fusion protein

Recombinant interleukin-2 (rIL-2) therapy has been shown to be of value in the treatment of some cases of melanoma and renal cell carcinoma. However, its use can be limited by severe systemic toxicity. Targeting rIL-2 to the tumor should improve the antitumor immune response and decrease the systemic toxicity. With this aim, we have employed recombinant DNA techniques to construct a single-chain antibody interleukin-2 fusion protein (SCA-IL-2). The protein used in this model system consists of the variable domains of the antilysozyme antibody D1.3 fused to human IL-2 and is expressed in E. coli. It retains antigen-binding specificity and has the full biological activity of rIL-2. This approach can be taken to generate SCA-IL-2 proteins that bind to appropriate cellular antigens. In vivo administration of tumor-binding SCA-IL-2 should result in a localized high concentration of rIL-2 in the tumor tissues, maximizing the anti-tumor response while keeping systemic side effects to a minimum.

Fusion proteins in biotechnology

Gene fusion techniques allow the production of recombinant proteins featuring the combined characteristics of the parental products. Originally, these techniques were used to probe transcriptional and translational activity, to translocate proteins across cell membranes, and to facilitate the recovery of proteins. Recently, new applications have emerged in areas such as protein refolding, immunology, drug targeting and protein display. A slightly modified version of this review is also published in Current Opinion in Structural Biology 1992, 2:569-575.

The crystal structure of diphtheria toxin

The crystal structure of the diphtheria toxin dimer at 2.5 A resolution reveals a Y-shaped molecule of three domains. The catalytic domain, called fragment A, is of the alpha + beta type. Fragment B actually consists of two domains. The transmembrane domain consists of nine alpha-helices, two pairs of which are unusually apolar and may participate in pH-triggered membrane insertion and translocation. The receptor-binding domain is a flattened beta-barrel with a jelly-roll-like topology. Three distinct functions of the toxin, each carried out by a separate structural domain, can be useful in designing chimaeric proteins, such as immunotoxins, in which the receptor-binding domain is substituted with antibodies to target other cell types.