Immunotoxin therapy of small-cell lung cancer: a phase I study of N901-blocked ricin

Directing ricin-based immunotoxins with targeting affibodies and KDEL signal peptide to cancer cells effectively induces apoptosis and tumor suppression

The plant toxin ricin, especially its cytotoxic A chain (RTA), can be genetically engineered with targeting ligands to develop specific anti-cancer recombinant immunotoxins (RITs). Here, we used affibody molecules targeting two cancer biomarkers, the receptors HER2 and EGFR, along with the KDEL signal peptide to construct two cancer-specific ricin-based RITs, HER2Afb-RTA-KDEL and EGFRAfb-RTA-KDEL. The affibodies successfully provided target-specificity and subsequent receptor-mediated endocytosis and the KDEL signal peptide routed the RITs through the retrograde transport pathway, effectively delivering RTA to the cytosol as well as avoiding the alternate recycling pathway that typical cancer cells frequently have. The in vivo efficacy of RITs was enhanced by introducing the albumin binding domain (AlBD) to construct AlBD/HER2Afb/RTA-KDEL. Systemic administration of AlBD-containing RITs to tumor-bearing mice significantly suppressed tumor growth without any noticeable side-effects. Collectively, combining target-selective affibody molecules, a cytotoxic RTA, and an intracellularly designating peptide, we successfully developed cancer-specific and efficacious ricin-based RITs. This approach can be applied to develop novel protein-based “magic bullets” to effectively suppress tumors that are resistant to conventional anti-cancer drugs.

Plant-made immunotoxin building blocks: A roadmap for producing therapeutic antibody-toxin fusions

Molecular farming in plants is an emerging platform for the production of pharmaceutical proteins, and host species such as tobacco are now becoming competitive with commercially established production hosts based on bacteria and mammalian cell lines. The range of recombinant therapeutic proteins produced in plants includes replacement enzymes, vaccines and monoclonal antibodies (mAbs). But plants can also be used to manufacture toxins, such as the mistletoe lectin viscumin, providing an opportunity to express active antibody-toxin fusion proteins, so-called recombinant immunotoxins (RITs). Mammalian production systems are currently used to produce antibody-drug conjugates (ADCs), which require the separate expression and purification of each component followed by a complex and hazardous coupling procedure. In contrast, RITs made in plants are expressed in a single step and could therefore reduce production and purification costs. The costs can be reduced further if subcellular compartments that accumulate large quantities of the stable protein are identified and optimal plant growth conditions are selected. In this review, we first provide an overview of the current state of RIT production in plants before discussing the three key components of RITs in detail. The specificity-defining domain (often an antibody) binds cancer cells, including solid tumors and hematological malignancies. The toxin provides the means to kill target cells. Toxins from different species with different modes of action can be used for this purpose. Finally, the linker spaces the two other components to ensure they adopt a stable, functional conformation, and may also promote toxin release inside the cell. Given the diversity of these components, we extract broad principles that can be used as recommendations for the development of effective RITs. Future research should focus on such proteins to exploit the advantages of plants as efficient production platforms for targeted anti-cancer therapeutics.

Ricin and Ricinus communis in pharmacology and toxicology-from ancient use and "Papyrus Ebers" to modern perspectives and "poisonous plant of the year 2018"

While probably originating from Africa, the plant Ricinus communis is found nowadays around the world, grown for industrial use as a source of castor oil production, wildly sprouting in many regions, or used as ornamental plant. As regards its pharmacological utility, a variety of medical purposes of selected parts of the plant, e.g., as a laxative, an anti-infective, or an anti-inflammatory drug, have been described already in the sixteenth century BC in the famous Papyrus Ebers (treasured in the Library of the University of Leipzig). Quite in contrast, on the toxicological side, the native plant has become the "poisonous plant 2018" in Germany. As of today, a number of isolated components of the plant/seeds have been characterized, including, e.g., castor oil, ricin, Ricinus communis agglutinin, ricinin, nudiflorin, and several allergenic compounds. This review mainly focuses on the most toxic protein, ricin D, classified as a type 2 ribosome-inactivating protein (RIP2). Ricin is one of the most potent and lethal substances known. It has been considered as an important bioweapon (categorized as a Category B agent (second-highest priority)) and an attractive agent for bioterroristic activities. On the other hand, ricin presents great potential, e.g., as an anti-cancer agent or in cell-based research, and is even explored in the context of nanoparticle formulations in tumor therapy. This review provides a comprehensive overview of the pharmacology and toxicology-related body of knowledge on ricin. Toxicokinetic/toxicodynamic aspects of ricin poisoning and possibilities for analytical detection and therapeutic use are summarized as well.

Targeted toxins

Objective. Current modalities used in the treatment of cancer often cause unacceptable damage to normal tissue. Toxins targeted toward tumor cells by antibodies or growth factors have the potential to selectively kill tumor cells while leaving normal tissue intact. The purpose of this review is to provide background information on targeted toxins and current clinical studies for this new class of anti-cancer compounds.

Data sources. A MEDLINE search was conducted using the term “immunotoxins.” Relevant articles were also obtained by the systematic examination of article references.

Data synthesis. The toxins Pseudomonas exotoxin, diphtheria toxin, and ricin toxin are often used as targeted toxins. Deletion or mutation of the binding domains of these toxins decreased binding of the toxins to normal tissues. Antibodies or growth factors can be used as targeting moiety, and the resulting agents are called immunotoxins or fusion proteins, respectively. DNA technology and chemical modifications of the toxin as well as the antibody moiety led to smaller and less immunogenic targeted toxins. Smaller targeted toxins are less toxic and penetrate further into the tumor. The summary of several targeted toxins elicited during clinical trials in this review makes it clear that several targeted toxins are potential agents for the treatment of various cancers, although some problems still need to be overcome. These problems include toxicity, immunogenicity, cross-reactivity of the targeted toxin with life-sustaining tissue, heterogenicity of tumor cells, and limited tumor penetration.

Plant Toxin-Based Immunotoxins for Cancer Therapy: A Short Overview

Immunotoxins are chimeric proteins obtained by linking a toxin to either an intact antibody or an antibody fragment. Conjugation can be obtained by chemical or genetic engineering, where the latter yields recombinant conjugates. An essential requirement is that the target molecule recognized by the antibody is confined to the cell population to be deleted, or at least that it is not present on stem cells or other cell types essential for the organism’s survival. Hundreds of different studies have demonstrated the potential for applying immunotoxins to many models in pre-clinical studies and in clinical trials. Immunotoxins can be theoretically used to eliminate any unwanted cell responsible for a pathological condition. The best results have been obtained in cancer therapy, especially in hematological malignancies. Among plant toxins, the most frequently employed to generate immunotoxins are ribosome-inactivating proteins, the most common being ricin. This review summarizes the various approaches and results obtained in the last four decades by researchers in the field of plant toxin-based immunotoxins for cancer therapy.

Immunotherapy for non-small cell lung cancer: current concepts and clinical trials

Recent successes in immunotherapeutic strategies are being investigated to combat cancers that have less than ideal responses to standard of care treatment, such as non-small-cell lung cancer. In this paper, we summarize concepts and the current status of immunotherapy for non-small cell lung cancer, including salient features of the major categories of immunotherapy-monoclonal antibody therapy, immune checkpoint blockade, immunotoxins, anticancer vaccines, and adoptive cell therapy.

The future of antiviral immunotoxins

There is a constant need for new therapeutic interventions in a wide range of infectious diseases. Over the past few years, the immunotoxins have entered the stage as promising antiviral treatments. Immunotoxins have been extensively explored in cancer treatment and have achieved FDA approval in several cases. Indeed, the design of new anticancer immunotoxins is a rapidly developing field. However, at present, several immunotoxins have been developed targeting a variety of different viruses with high specificity and efficacy. Rather than blocking a viral or cellular pathway needed for virus replication and dissemination, immunotoxins exert their effect by killing and eradicating the pool of infected cells. By targeting a virus-encoded target molecule, it is possible to obtain superior selectivity and drastically limit the side effects, which is an immunotoxin-related challenge that has hindered the success of immunotoxins in cancer treatment. Therefore, it seems beneficial to use immunotoxins for the treatment of virus infections. One recent example showed that targeting of virus-encoded 7 transmembrane (7TM) receptors by immunotoxins could be a future strategy for designing ultraspecific antiviral treatment, ensuring efficient internalization and hence efficient eradication of the pool of infected cells, both in vitro and in vivo. In this review, we provide an overview of the mechanisms of action of immunotoxins and highlight the advantages of immunotoxins as future anti-viral therapies.

Advances in anticancer immunotoxin therapy

Immunotoxins are a novel class of antibody-conjugated therapeutics currently in clinical development for a variety of malignancies. They consist of an antibody-based targeting domain fused to a bacterial toxin payload for cell killing. Immunotoxins kill cells by inhibiting protein synthesis, a unique mechanism of action that is toxic to both dividing and nondividing cells. Recent advances in the design and administration of immunotoxins are overcoming historical challenges in the field, leading to renewed interest in these therapeutics.

Immunotoxins: the role of the toxin

Immunotoxins are antibody-toxin bifunctional molecules that rely on intracellular toxin action to kill target cells. Target specificity is determined via the binding attributes of the chosen antibody. Mostly, but not exclusively, immunotoxins are purpose-built to kill cancer cells as part of novel treatment approaches. Other applications for immunotoxins include immune regulation and the treatment of viral or parasitic diseases. Here we discuss the utility of protein toxins, of both bacterial and plant origin, joined to antibodies for targeting cancer cells. Finally, while clinical goals are focused on the development of novel cancer treatments, much has been learned about toxin action and intracellular pathways. Thus toxins are considered both medicines for treating human disease and probes of cellular function.

Combination treatments with ABT-263 and an immunotoxin produce synergistic killing of ABT-263-resistant small cell lung cancer cell lines

Synergistic killing was achieved when Small Cell Lung Cancer (SCLC) cell lines were incubated with ABT-263 and an immunotoxin directed to the transferrin receptor. SCLC lines are variably sensitive to the BH-3 only peptide mimetic, ABT-263. To determine their sensitivity to toxin-based reagents, we incubated four representative SCLC lines with a model Pseudomonas exotoxin-based immunotoxin directed to the transferrin receptor. Remarkably in 4-of-4 lines, there was little evidence of immunotoxin-mediated cytotoxicity despite near complete inhibition of protein synthesis. However, when combinations of ABT-263 and immunotoxin were added to the ABT-263-resistant cell lines (H196 and H69AR), there was synergistic killing as evidenced by increased activation of caspase 3/7, annexin V staining, and loss of cell integrity. Synergistic killing was evident at 6 hr and correlated with loss of Mcl-1. This synergy was also noted when the closely related compound ABT-737 was combined with the same immunotoxin. To establish that the synergy seen in tissue culture could be achieved in vivo, H69AR cells were grown as tumors in nude mice and shown to be susceptible to the killing action of an immunotoxin-ABT-737 combination but not to either agent alone. When immunotoxin-ABT combinations were added to ABT-263-sensitive lines (H146 and H1417), killing was additive. Our data support combination approaches for treating ABT-263-resistant SCLC with ABT-263 and a second agent that provides synergistic killing action.

Immunotoxins constructed with chimeric, short-lived anti-CD22 monoclonal antibodies induce less vascular leak without loss of cytotoxicity

An immunotoxin (IT) constructed with RFB4, a murine anti-CD22 monoclonal antibody, and the "deglycosylated" A chain of ricin has shown activity at safe doses in patients with non-Hodgkin lymphoma and in children with acute lymphoblastic leukemia. The dose limiting toxicity is vascular leak syndrome (VLS), which appears to be due to a unique amino acid motif in the ricin toxin A (RTA) chain that damages vascular endothelial cells. We mutated recombinant (r) RTA to disable this site, but await testing of the IT prepared with this mutant RTA in humans. Another possible approach to reducing IT-induced VLS is to shorten the half-life of the IT in vivo. We previously constructed a mouse-human chimeric RFB4 by grafting the variable genes of RFB4 onto the human IgG1k constant regions. Here, we report the expansion of our panel of mutant chimeric RFB4s (mcRFB4s) that lack the ability to bind to the neonatal Fc receptor (FcRn). In comparison with cRFB4, which had a T1/2 of 263 h, the mcRFB4s had T1/2s ranging from 39 to 106 h. ITs were constructed with these mcRFB4s and rRTA. The mcRFB4-RTA ITs retained their cytotoxicity in vitro and had shorter half lives than the parental cRFB4-RTA IT. In addition, the mcRFB4 IT with the shortest T1/2 induced less pulmonary vascular leak in mice, which we have postulated is a surrogate marker for VLS in humans.

Toxin-based therapeutic approaches

Protein toxins confer a defense against predation/grazing or a superior pathogenic competence upon the producing organism. Such toxins have been perfected through evolution in poisonous animals/plants and pathogenic bacteria. Over the past five decades, a lot of effort has been invested in studying their mechanism of action, the way they contribute to pathogenicity and in the development of antidotes that neutralize their action. In parallel, many research groups turned to explore the pharmaceutical potential of such toxins when they are used to efficiently impair essential cellular processes and/or damage the integrity of their target cells. The following review summarizes major advances in the field of toxin based therapeutics and offers a comprehensive description of the mode of action of each applied toxin.

Targeted therapies in small cell lung cancer: a review

Small cell lung cancer (SCLC) is an aggressive form of lung cancer that is characterized by a rapid doubling time, early onset of dissemination and high sensitivity to chemotherapy. Despite the potential for cure in patients with limited disease with concurrent chemoradiation and an initial good response to chemotherapy in extensive disease, there is a high chance of disease relapse with an overall poor median survival for both stages. With increasing translational research and a better understanding of the molecular basis of cancer, a number of molecular targets have been identified in various preclinical studies. This review summarizes potentially viable targets and new agents that have been developed and employed in recent, ongoing and future clinical trials to attempt to improve clinical outcomes in this disease.

The value of gamma camera and computed tomography data set coregistration to assess Lewis Y antigen targeting in small cell lung cancer by (111)Indium-labeled humanized monoclonal antibody 3S193

AIM

To assess the value of data set coregistration of gamma camera and computed tomography (CT) in the assessment of targeting of humanized monoclonal antibody 3S193 labeled with indium-111 ((111)In-hu3S193) to small cell lung cancer (SCLC).

METHODS AND MATERIALS

Ten patients (6 male and 4 female; mean age+/-S.D., 60+/-4 years), from an overall population of 20 patients with SCLCs expressing Lewis Y antigen at immunohistochemical analysis, completed a four weekly injections of (111)In-hu3S193 and underwent gamma camera imaging. All had had, as part of their baseline evaluation, Fluorine18 fluoro-2-deoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT). Two readers in consensus retrospectively coregistered the gamma camera images with the CT component of the FDG PET/CT by automatic or manual alignment. The resulting image sets were visually examined and SCLC lesions targeting at coregistered gamma camera and CT was correlated side-by-side with the (18)F-FDG uptake.

RESULTS

A total number of 31 lesions from SCLC with a thoracic (n=13) or extrathoracic location (n=18) were all positive on FDG PET/CT. Coregistration of the gamma camera to the CT demonstrated targeting of antibody to all lesions >2 cm (n=20) and in a few lesions < or =2 cm (n=2), with no visualization of most lesions < or = 2 cm (n=9). No (111)In-hu3S193 uptake in normal tissues was observed.

CONCLUSION

Coregistration of antibody gamma camera imaging to FDG PET/CT is feasible and allows valuable assessment of (111)In-hu3S193 antibody targeting to SCLC lesions >2cm, while lesions < or =2 cm reveal a limited targeting.

Immunotoxin treatment of cancer

Immunotoxins are proteins used to treat cancer that are composed of an antibody fragment linked to a toxin. The immunotoxin binds to a surface antigen on a cancer cell, enters the cell by endocytosis, and kills it. The most potent immunotoxins are made from bacterial and plant toxins. Refinements over many years have produced recombinant immunotoxins; these therapeutic proteins are made using protein engineering. Individual immunotoxins are designed to treat specific cancers. To date, most success has been achieved treating hematologic tumors. Obstacles to successful treatment of solid tumors include poor penetration into tumor masses and the immune response to the toxin component of the immunotoxin, which limits the number of cycles that can be given. Strategies to overcome these limitations are being pursued.

Small cell lung cancer and targeted therapies

PURPOSE OF REVIEW

Small cell lung cancer is a chemosensitive malignancy, yet long-term survival remains elusive for the majority of patients. Here, we report on progress in evaluating novel targeted therapies for the treatment of this disease.

RECENT FINDINGS

Interferons, matrix metalloproteinase inhibitors, thalidomide, bevacizumab, ZD6474, imatinib, gefitinib, oblimersen and aplidine have all entered clinical trial in patients with small cell lung cancer. Immunotherapy approaches targeting cell surface antigens such as CD-56 (BB10901) and GD3 ganglioside are also being evaluated. Interferons, matrix metalloproteinase inhibitors, imatinib and gefitinib have failed to demonstrate efficacy for this disease. Preliminary data for thalidomide are promising and so results from trials of other antiangiogenics such as bevacizumab and ZD6474 are awaited with interest.

SUMMARY

Although the promise of targeted therapy has yet to be realized in patients with small cell lung cancer, the number of agents available for evaluation provides new optimism that progress will be made over the next decades.

Immunotoxins for targeted cancer therapy

Immunotoxins are proteins that contain a toxin along with an antibody or growth factor that binds specifically to target cells. Nearly all protein toxins work by enzymatically inhibiting protein synthesis. For the immunotoxin to work, it must bind to and be internalized by the target cells, and the enzymatic fragment of the toxin must translocate to the cytosol. Once in the cytosol, 1 molecule is capable of killing a cell, making immunotoxins some of the most potent killing agents. Various plant and bacterial toxins have been genetically fused or chemically conjugated to ligands that bind to cancer cells. Among the most active clinically are those that bind to hematologic tumors. At present, only 1 agent, which contains human interleukin-2 and truncated diphtheria toxin, is approved for use in cutaneous T-cell lymphoma. Another, containing an anti-CD22 Fv and truncated Pseudomonas exotoxin, has induced complete remissions in a high proportion of cases of hairy-cell leukemia. Refinement of existing immunotoxins and development of new immunotoxins are underway to improve the treatment of cancer.

Monoclonal antibodies in the treatment of lung cancer

BACKGROUND

Lung cancer is an aggressive disease and its conventional therapy is far from success. There is a strong need for new, better approaches to improve survival, symptom control and quality of life.

METHODS

The authors searched the literature for indexed articles published over the past 30 years from Pubmed concentrating on all possibilities of monoclonal antibodies in the therapy of tumours and especially of lung cancer.

RESULTS

The search resulted in more then 200 published articles. Important major reports of the pre-clinical/clinical investigations of monoclonal antibodies in the therapy of tumours, with an emphasis on lung cancer were reviewed, screened and tracked for other relevant publications and the yielded data were summarized and systematized.

CONCLUSION

It is concluded, that immunotherapy and the reviewed use of monoclonal antibodies in the therapy of tumours (including lung cancer) certainly carries a hope. However, studies of this topic are in a wide range of phases, from experiments to clinical trials, thereby their results are not comparable with each other. Based on the data available though the authors feel that active immunization with monoclonal antibodies as anti-idiotype vaccines, and antibody targeting with immunoconjugates (immunotoxins, radioimmunoconjugates and chemoimmunoconjugates) are the most promising methods. Radioimmunoguided surgery and immunoguided focal ablation are also valuable. Anti-growth factor monoclonal antibodies are the most evaluated agents so far. They certainly have an objective effect, though they are still not the 'magic bullets', waited for by many clinicians. The use of monoclonal antibodies against the escape mechanisms of tumours can be a good auxiliary method. There are too little data on the value of antibodies directly targeting tumour cells and on combined passive immunotherapy. Due to constant research, other modalities, such as prodrug activation, T cell activation, the use of intrabodies, T bodies, and conjugated antibody fragments might also prove to be valuable.

Structural Characterization of a Recombinant Monoclonal Antibody by Electrospray Time-of-Flight Mass Spectrometry

PURPOSE

The aim of this study was to perform structural characterization of a recombinant monoclonal antibody (MAb), huN901, by electrospray time-of-flight mass spectrometry (ESI-TOFMS) using both "top-down" and "bottom-up" approaches.

METHODS

In the top-down approach, the molecular masses of the deglycosylated huN901 and the light and heavy chains of the antibody were measured by direct infusion MS and liquid chromatography-mass spectrometry (LC-MS). In the bottom-up approach, trypsin and Asp-N protease were used to digest the separated, reduced and alkylated light and heavy chains followed by LC-MS analysis of the digests.

RESULTS

The primary structure and post-translational modifications of huN901 were characterized by both top-down and bottom-up MS approaches. Modifications of N-terminal pyroglutamate formation, cleavage of C-terminal lysine, glycosylation, and deamidation were identified in the antibody heavy chain by both protein mass measurement and peptide mapping. No modifications were found in the complementarity determining regions (CDRs) of both chains. Both trypsin and Asp-N protease digestion had an average sequence recovery of 97%, and generated complimentary mapping results with complete sequence recovery.

CONCLUSIONS

ESI-TOFMS is a superior tool to characterize MAb and other complex protein pharmaceuticals.

Novel therapies in lung cancer

This article reviews novel therapies that are under development for non-small cell lung cancer and small cell lung cancer.

Chemotherapy strategies in the treatment of small cell lung cancer

Lung cancer is the most prevalent, yet most preventable malignancy worldwide. Given the tendency of small cell lung cancer (SCLC) to early relapse and its subsequent resistance to treatment, there is an urgent need to optimize standard treatment strategies and develop new treatments. Over the last decade, several strategies have been adopted and advances in the molecular biology of lung cancer have identified a number of targets for future therapy. In this article, we review chemotherapy strategies that have been evaluated in the management of patients with SCLC.

In vitro and in vivo activity of the maytansinoid immunoconjugate huN901-N2'-deacetyl-N2'-(3-mercapto-1-oxopropyl)-maytansine against CD56+ multiple myeloma cells

HuN901 is a humanized monoclonal antibody that binds with high affinity to CD56, the neuronal cell adhesion molecule. HuN901 conjugated with the maytansinoid N(2')-deacetyl-N(2')-(3-mercapto-1-oxopropyl)-maytansine (DM1), a potent antimicrotubular cytotoxic agent, may provide targeted delivery of the drug to CD56 expressing tumors. Based on gene expression profiles of primary multiple myeloma (MM) cells showing expression of CD56 in 10 out of 15 patients (66.6%) and flow cytometric profiles of MM (CD38(bright)CD45(lo)) cells showing CD56 expression in 22 out of 28 patients (79%), we assessed the efficacy of huN901-DM1 for the treatment of MM. We first examined the in vitro cytotoxicity and specificity of huN901-DM1 on a panel of CD56(+) and CD56(-) MM cell lines, as well as a CD56(-) Waldenstrom's macroglobulinemia cell line. HuN901-DM1 treatment selectively decreased survival of CD56(+) MM cell lines and depleted CD56(+) MM cells from mixed cultures with a CD56(-) cell line or adherent bone marrow stromal cells. In vivo antitumor activity of huN901-DM1 was then studied in a tumor xenograft model using a CD56(+) OPM2 human MM cell line in SCID mice. We observed inhibition of serum paraprotein secretion, inhibition of tumor growth, and increase in survival of mice treated with huN901-DM1. Our data therefore demonstrate that huN901-DM1 has significant in vitro and in vivo antimyeloma activity at doses that are well tolerated in a murine model. Taken together, these data provide the framework for clinical trials of this agent to improve patient outcome in MM.

A phase II study of the immunotoxin N901-blocked ricin in small-cell lung cancer

This study was designed to evaluate the efficacy and toxicity of the immunotoxin N901-blocked ricin (bR) in patients with small-cell lung cancer (SCLC) who achieved a complete or near-complete response following chemotherapy and/or radiation. Treatment consisted of a 7-day continuous infusion of N901-bR at a dose of 30 mg/kg/day followed by patient evaluation with repeat scans. Serum immunotoxin levels, human antimurine antibodies, and antiricin antibodies were determined during the course of the infusion. Nine patients enrolled in the study before it closed following a treatment-related death. Seven patients had extensive-stage disease and entered the study with a more than 90% reduction of their original tumor. Two patients with limited-stage SCLC had no evidence of disease at study entry. Maximum plasma levels of N901-bR ranged from 72-371 ng/mL. Laboratory toxicity consisted of transient transaminitis in 8 patients and creatine kinase elevation in 3 patients, 1 of whom developed premature ventricular contractions. One patient experienced progressive capillary leak syndrome following immunotoxin infusion, which proved fatal. All patients developed antibodies to the infused murine antibody as well as to the toxin. Six patients died of progressive SCLC and 1 patient died of presumed radiation pneumonitis. One patient with limited-stage disease is still alive more than 6 years after therapy. N901-bR therapy was associated with a fatal incident of capillary leak syndrome and a nearly universal development of human antimouse and antiricin antibodies, which limited its further clinical development.

A Phase I Study of Pegylated Liposomal Doxorubicin Hydrochloride (Caelyx™) in Combination with Cyclophosphamide and Vincristine as Second-Line Treatment of Patients with Small-Cell Lung Cancer

The purpose of this study was to determine the recommended phase II dose of liposomal doxorubicin (Caelyx ; Doxil in the United States) in combination with cyclophosphamide and vincristine for previously treated patients with good performance status with relapsed or refractory small-cell lung cancer. Twenty-one eligible patients were enrolled between November 1999 and September 2001 and received liposomal doxorubicin 25-40 mg/m2, cyclophosphamide 750-1000 mg/m2, and vincristine 1.2 mg/m2 intravenously (I.V.) every 21 days. At doses of liposomal doxorubicin 40 mg/m2, cyclophosphamide 750 mg/m2, and vincristine 1.2 mg/m2 I.V., 1 of 6 patients had dose-limiting neutropenia and fever in cycle 2 and 2 of 6 developed grade 3 hand-foot syndrome during cycle 3. Therefore, the recommended phase II doses are liposomal doxorubicin 35 mg/m2, cyclophosphamide 750 mg/m2, and vincristine 1.2 mg/m2 I.V. every 21 days. Antitumor activity was seen at all dose levels. This combination is well tolerated and has evidence of antitumor activity. A phase II evaluation is ongoing.

Recurrent small cell lung cancer: update

Small cell lung cancer continues to recur and claim the lives of most of its victims. Thus, management of the patient with recurrent disease remains an active area of research. This review provides an update of clinical research experience over the past decade with relatively new conventional cytotoxins in this setting, such as the topoisomerase I inhibitors and the taxanes. Additionally, novel molecular targeted approaches with specific relevance for small cell lung cancer are discussed.

Molecular and cellular biology of small cell lung cancer

For any tumor to become cancerous, various genetic mutations and biologic alterations must occur in the cell that in combination render it a malignant neoplasm. Small cell lung cancer (SCLC) is a neoplasm associated with several molecular and cellular abnormalities. SCLC is associated with early and frequent metastasis as well as a poor ultimate response to chemotherapy. New and novel therapies based on understanding the mechanisms of transformation are needed. SCLC is associated with multiple chromosomal abnormalities, the most common of which is chromosome 3p deletion, as well as with abnormal oncogenes and tumor-suppressor genes. Along with the genetic alterations, SCLC has been shown to overexpress various cell surface receptors, including receptor tyrosine kinases (RTKs), G-protein-coupled receptors, integrins, and others. Some downstream molecules are also activated, such as phosphatidylinositol 3'-kinase, and would serve as good candidates for therapeutic strategies.

Production of anti-CD3 and anti-CD7 ricin A-immunotoxins for a clinical pilot study

This report describes the preparation of an immunotoxin-combination, consisting of an anti-CD3 and anti-CD7 monoclonal antibody (MoAb) both conjugated to the A-chain of plant toxin ricin, for the experimental treatment of graft-versus-host disease. MoAbs and toxin were conjugated by conventional biochemical and chromatographic techniques. Raw materials, intermediate and final products were evaluated in accordance with the relevant 'points to consider' of the FDA. Yields, purity and sterility of the two final products were all satisfactory. Preservation of MoAb-affinity and toxin-activity were confirmed in biological assays. The LD50, 25-45 mg immunotoxin-combination/kg mouse, equalled that of similar immunotoxins already in clinical use. Because in vitro cross-reactivity screening revealed an unexpected binding of the CD3-MoAb to the esophagus epithelium, human doses of immunotoxin-combination were administered to two cynomolgus monkeys. Clinically relevant serum concentrations were obtained without irreversible toxicities occurring. The T(1/2) varied between approximately 6 and 9 h and the C(max) ranged from 1.8 to 3.9 microg/ml. The main side effect was a transient rise of serum creatine kinase. Importantly, neither damage nor binding of the CD3-immunotoxin to the monkey esophagus epithelium could be demonstrated. It was concluded that sufficient material of proper quality and with an acceptable toxicity profile was produced, warranting the evaluation in a clinical pilot-study.

Monoclonal antibody therapy for solid tumors

Monoclonal antibody therapy for solid tumors has many theoretical attractions and a long history. Until recently, with the approval and widespread use of rituximab (Rituxan) and trastuzumab (Herceptin), monoclonal antibody therapy for tumors had not had significant success. This article reviews basic theories behind antibody development and their clinical implementation as treatment for solid tumors. Medline was searched for articles over the past 15 years dealing with laboratory and clinical applications of antibody therapy for solid tumors. In addition, American Society of Clinical Oncology (ASCO) abstracts from the past 3 years were reviewed to complement the Medline search. This article focuses on treatment for common solid tumors, including breast, colon and lung cancers.

Phase I trial of recombinant immunotoxin anti-Tac(Fv)-PE38 (LMB-2) in patients with hematologic malignancies

PURPOSE

To evaluate the toxicity, pharmacokinetics, immunogenicity, and antitumor activity of anti-Tac(Fv)-PE38 (LMB-2), an anti-CD25 recombinant immunotoxin that contains an antibody Fv fragment fused to truncated Pseudomonas exotoxin.

PATIENTS AND METHODS

Patients with CD25(+) hematologic malignancies for whom standard and salvage therapies failed were treated with LMB-2 at dose levels that ranged from 2 to 63 microg/kg administered intravenously over 30 minutes on alternate days for three doses (QOD x 3).

RESULTS

LMB-2 was administered to 35 patients for a total of 59 cycles. Dose-limiting toxicity at the 63 microg/kg level was reversible and included transaminase elevations in one patient and diarrhea and cardiomyopathy in another. LMB-2 was well tolerated in nine patients at the maximum-tolerated dose (40 microg/kg QOD x 3); toxicity was transient and most commonly included transaminase elevations (eight patients) and fever (seven patients). Only six of 35 patients developed significant neutralizing antibodies after the first cycle. The median half-life was 4 hours. One hairy cell leukemia (HCL) patient achieved a complete remission, which is ongoing at 20 months. Seven partial responses were observed in cutaneous T-cell lymphoma (one patient), HCL (three patients), chronic lymphocytic leukemia (one patient), Hodgkin's disease (one patient), and adult T-cell leukemia (one patient). Responding patients had 2 to 5 log reductions of circulating malignant cells, improvement in skin lesions, and regression of lymphomatous masses and splenomegaly. All four patients with HCL responded to treatment.

CONCLUSION

LMB-2 has clinical activity in CD25(+) hematologic malignancies and is relatively nonimmunogenic. It is the first recombinant immunotoxin to induce major responses in cancer. LMB-2 and similar agents that target other cancer antigens merit further clinical development.

Chemical-biological terrorism and its impact on children: a subject review. American Academy of Pediatrics. Committee on Environmental Health and Committee on Infectious Diseases

There is an increasing threat that chemical and biological weapons will be used on a civilian population in an act of domestic terrorism. Casualties among adults and children could be significant in such an event. Federal, state, and local authorities have begun extensive planning to meet a chemical-biological incident by developing methods of rapid identification of potential agents and protocols for management of victims without injury to health care personnel. Because children would be disproportionately affected by a chemical or biological weapons release, pediatricians must assist in planning for a domestic chemical-biological incident. Government agencies should seek input from pediatricians and pediatric subspecialists to ensure that the situations created by multiple pediatric casualties after a chemical-biological incident are considered. This statement reviews key aspects of chemical-biological agents, the consequences of their use, the potential impact of a chemical-biological attack on children, and issues to consider in disaster planning and management for pediatric patients.

Immunotoxins in cancer therapy

Immunotoxins are composed of a protein toxin connected to a binding ligand such as an antibody or growth factor. These molecules bind to surface antigens (which internalize) and kill cells by catalytic inhibition of protein synthesis within the cell cytosol. Immunotoxins have recently been tested clinically in hematologic malignancies and solid tumors and have demonstrated potent clinical efficacy in patients with malignant diseases that are refractory to surgery, radiation therapy and chemotherapy - the traditional modalities of cancer treatment. This therapy is thus evolving into a separate modality of cancer treatment, capable of rationally targeting cells on the basis of surface markers. Efforts are underway to obviate impediments to clinical efficacy, including immunogenicity and toxicity to normal tissues. Immunotoxins are now being developed to new antigens for the treatment of cancer.

Purification of Sepharose-unbinding ricin from castor beans (Ricinus communis) by hydroxyapatite chromatography

Sepharose-unbinding ricin was one-step separated and purified from a crude extract of castor beans (Ricinus communis) by affinity chromatography on hydroxyapatite. This purification method does not require the time-consuming and complicated steps, such as gel filtration and ion-exchange chromatography, that have been essential in the separation of Sepharose-unbinding toxins. Using this method, approximately 180 mg of ricin was obtained from 100 g of castor beans using a bed volume of 80 ml on a hydroxyapatite column. Weak affinity of the ricin on Sepharose was confirmed and compared with Sepharose-binding ricin (ricin D), using radioiodinated ricins. The molecular mass of the ricin was estimated to be 62 kDa by 10% SDS-PAGE under nonreducing conditions. Under reducing conditions, the purified ricin appeared to be two subunits, corresponding to the molecular masses of 30 and 32 kDa. The pI value was determined to be approximately 8.9 for the ricin. Uptake of the ricin by HeLa cells was measured as almost half of ricin D uptake. Similar results were observed on CEM cells as well. In vitro cytotoxicity of ricins on different cell lines was measured by the MTT method. When compared with ricin D, the purified ricin showed approximately 10-fold less cytotoxicity to HUT78 or K562 cells and 30-fold less toxicity to CEM cells. This lower cytotoxicity of the ricin may be due to its lower cell-binding properties as evidenced by its low affinity for the cell surfaces. From these results, the purified ricin was considered to be ricin E.

Role of cross-linking agents in determining the biochemical and pharmacokinetic properties of Mgr6-clavin immunotoxins

Several immunotoxins (ITs) were synthesized by the attachment of clavin, a recombinant toxic protein derived from Aspergillus clavatus, to the monoclonal antibody Mgr6 that recognizes an epitope of the gp185(HER-2) extracellular domain expressed on breast and ovarian carcinoma cells. Conjugation and purification parameters were analyzed in an effort to optimize the antitumor activity and stability of the ITs in vivo. To modulate the in vitro and in vivo properties of the immunotoxins, different coupling procedures were used and both disulfide and thioether linkages were obtained. Unhindered and hindered disulfide with a methyl group linkage ethyl S-acetyl 3-mercaptopropionthioimidate ester hydrochloride (AMPT) or ethyl S-acetyl 3-mercaptobutyrothioimidate ester hydrochloride (M-AMPT) were obtained by reaction with recombinant clavin, while the monoclonal antibody Mgr6 was derivatized with ethyl 3-[(4-carboxamidophenyl)dithio]propionthioimidate ester hydrochloride (CDPT). To achieve higher hindrance (a disulfide bond with a geminal dimethyl group), Mgr6 was derivatized with the N-hydroxysuccinimidyl 3-methyl-3-(acetylthio)butanoate (SAMBA) and clavin with CDPT. To evaluate the relevance of the disulfide bond in the potency and pharmacokinetic behavior of the ITs, a conjugate consisting of a stable thioether bond was also prepared by derivatizing Mgr6 with the N-hydroxysuccinimidyl ester of iodoacetic acid (SIA) and clavin with AMPT. The immunotoxins were purified and characterized using a single-step chromatographic procedure. Specificity and cytotoxicity were assayed on target and unrelated cell lines. The data indicate that the introduction of a hindered disulfide linkage into ITs has little or no effect on antitumor activity and suggest that disulfide cleavage is essential for activity; indeed, the intracellularly unbreakable thioether linkage produced an inactive IT. Analysis of IT stability in vitro showed that the release of mAb by incubation with glutathione is proportional to the presence of methyl groups and increases exponentially with the increase in steric hindrance. Analysis of the pharmacokinetic behavior of ITs in Balb/c mice given intravenous bolus injections indicated that ITs with higher in vitro stability were eliminated more slowly; i.e., the disulfide bearing a methyl group doubled the beta-phase half-life (from 3.5 to 7.1 h) compared with that of the unhindered, while a geminal dimethyl protection increased the elimination phase to 24 h. The thioether linkage showed its intrinsic stability with a beta-phase half-life of 46 h. The thioether linkage also increased the distribution phase from 17 to 32 min. The in vitro characteristics and in vivo stability of Mgr6-clavin conjugates composed of a methyl and dimethyl steric hindered disulfide suggest clinical usefulness.

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.

A single chain Fv fragment of P-glycoprotein-specific monoclonal antibody C219. Design, expression, and crystal structure at 2.4 A resolution

A construct encoding a single chain variable fragment of the anti-P-glycoprotein monoclonal antibody C219 was made by combining the coding sequences for the heavy and light chain variable domains with a sequence encoding the flexible linker (GGGGS)3, an OmpA signal sequence, a c-myc identification tag, and a five-histidine purification tag. The construct was expressed in Escherichia coli and purified from the periplasmic fraction using a nickel chelate column and ion exchange chromatography. Three-step Western blot analysis showed that the construct retains binding affinity for P-glycoprotein. Crystals of 1.0 x 0.2 x 0.2 mm were grown in 100 mM citrate, pH 4.5, 21% polyethylene glycol 6000 in the presence of low concentrations of subtilisin, resulting in proteolytic removal of the linker and purification tags. The structure was solved to a resolution of 2.4 A with an R factor of 20.6, an Rfree of 28.5, and good stereochemistry. This result could lead to a clinically useful product based on antibody C219 for the diagnosis of P-glycoprotein-mediated multidrug resistance. The molecule will also be useful in biophysical studies of functional domains of P-glycoprotein, as well as studies of the intact molecule.