The immunogenicity of the 7E3 murine monoclonal Fab antibody fragment variable region is dramatically reduced in humans by substitution of human for murine constant regions

A novel Shiga toxin 2a neutralizing antibody therapeutic with low immunogenicity and high efficacy

Shiga toxin-producing Escherichia coli infections are difficult to treat due to the risk of antibiotic-induced stress upregulating the production of toxins, medical treatment is consequently limited to supportive care to prevent the development of hemolytic uremic syndrome (HUS). Here, we introduce a potentially therapeutic humanized mouse monoclonal antibody (Hu-mAb 2-5) targeting Stx2a, the most common Shiga toxin subtype identified from outbreaks. We demonstrate that Hu-mAb 2-5 has low immunogenicity in healthy adults ex vivo and high neutralizing efficacy in vivo, protecting mice from mortality and HUS-related tissue damage.

Review on Monoclonal Antibodies (mAbs) as a Therapeutic Approach for Type 1 Diabetes

Monoclonal antibodies have been successfully utilized in a variety of animal models to treat auto-immune illnesses for a long time. Immune system responses will either be less active or more active depending on how the immune system is operating abnormally. Immune system hypoactivity reduces the body's capacity to fight off various invading pathogens, whereas immune system hyperactivity causes the body to attack and kill its own tissues and cells. For maximal patient compliance, we will concentrate on a variety of antibody therapies in this study to treat Type 1 diabetes (an autoimmune condition). T-cells are responsible for the auto-immune condition known as T1D, which causes irregularities in the function of β-cells in the pancreas. As a result, for the treatment and prevention of T1D, immunotherapies that selectively restore continuous beta cellspecific self-tolerance are needed. Utilizing monoclonal antibodies is one way to specifically target immune cell populations responsible for the auto-immune-driven disease (mAb). Numerous mAbs have demonstrated clinical safety and varied degrees of success in modulating autoimmunity, including T1D. A targeted cell population is exhausted by mAb treatments, regardless of antigenic specificity. One drawback of this treatment is the loss of obtained protective immunity. Immune effector cell function is regulated by nondepleting monoclonal antibodies (mAb). The antigenfocused new drug delivery system is made possible by the adaptability of mAbs. For the treatment of T1D and T-cell-mediated autoimmunity, different existing and potential mAb therapy methods are described in this article.

Critical Analysis of Thrombocytopenia Associated With Glycoprotein IIb/IIIa Inhibitors and Potential Role of Zalunfiban, a Novel Small Molecule Glycoprotein Inhibitor, in Understanding the Mechanism(s)

Thrombocytopenia is a rare but serious complication of the intravenous glycoprotein IIb/IIIa (GPIIb/IIIa; integrin αIIbβ3) receptor inhibitors (GPIs), abciximab, eptifibatide, and tirofiban. The thrombocytopenia ranges from mild (50 000-100 000 platelets/μL), to severe (20 000 to <50 000/μL), to profound (<20 000/μL). Profound thrombocytopenia appears to occur in <1% of patients receiving their first course of therapy. Thrombocytopenia can be either acute (<24 hours) or delayed (up to ~14 days). Both hemorrhagic and thrombotic complications have been reported in association with thrombocytopenia. Diagnosis requires exclusion of pseudothrombocytopenia and heparin-induced thrombocytopenia. Therapy based on the severity of thrombocytopenia and symptoms may include drug withdrawals and treatment with steroids, intravenous IgG, and platelet transfusions. Abciximab-associated thrombocytopenia is most common and due to either preformed antibodies or antibodies induced in response to abciximab (delayed). Readministration of abciximab is associated with increased risk of thrombocytopenia. Evidence also supports an immune basis for thrombocytopenia associated with the 2 small molecule GPIs. The latter bind αIIbβ3 like the natural ligands and thus induce the receptor to undergo major conformational changes that potentially create neoepitopes. Thrombocytopenia associated with these drugs is also immune-mediated, with antibodies recognizing the αIIbβ3 receptor only in the presence of the drug. It is unclear whether the antibody binding depends on the conformational change and whether the drug contributes directly to the epitope. Zalunfiban, a second-generation subcutaneous small molecule GPI, does not induce the conformational changes; therefore, data from studies of zalunfiban will provide information on the contribution of the conformational changes to the development of GPI-associated thrombocytopenia.

The mosaic puzzle of the therapeutic monoclonal antibodies and antibody fragments - A modular transition from full-length immunoglobulins to antibody mimetics

The use of monoclonal antibodies represents an important and efficient diagnostic and therapeutic tool in disease management and modern science but remains limited by several factors including the uneven distribution in diseased tissues as well as undesired activation of side immune reactions. Major scientific advancements including Recombinant DNA Technology, Hybridoma Technology, and Polymerase Chain Reaction have considerably impacted the use of monoclonal antibodies providing technical and effective solutions to overcome the shortcomings encountered with conventional antibodies. Initially, the introduction of antibody fragments allowed a more uniform and deeper penetration of the targeted tissue and reduced unwanted activation of Fc-mediated immune reactions. On another level, the immunogenicity of murine-derived antibodies was overcome by humanizing their encoding genes with specific sequences of human origin andtransgenic mice able to synthesize fully human antibodies were successfully created. Moreover, the advancement of genetic engineering techniques supported by the modular structure of antibody coding genes paved the way for the development of a new generation of antibody fragments with a wide spectrum of monospecific and bispecific agents. These later could be monovalent, bivalent, or multivalent, and either expressed as a single chain, assembled in multimeric forms or stringed in tandem. This has conferred improved affinity, stability, and solubility to antibody targetting. Lately, a new array of monoclonal antibody fragments was introduced with the engineering of nanobody and antibody mimetics as non-immunoglobulin-derived fragments with promising diagnostic and therapeutic applications. In this review, we decipher the molecular basis of monoclonal antibody engineering with a detailed screening of the antibody derivatives that provides new perspectives to expand the use of monoclonal fragments into previously unexplored fields.

Investigation of pre-existing reactivity to biotherapeutics can uncover potential immunogenic epitopes and predict immunogenicity risk

Despite recent advances in the development of tools to predict immunogenicity risk of biotherapeutic molecules, the ability of a protein to elicit the formation of anti-drug antibodies (ADA) remains one of the most common causes for termination of clinical development programs. In this study, we use ADA assays to detect and measure pre-existing reactivity or the ability of a molecule to produce an ADA-like response in serum from treatment-naïve, healthy donors. We report herein that the magnitude of pre-existing reactivity evaluated pre-clinically and expressed as the 90th percentile of Tier 2 inhibition correlates with the subsequent rate of ADA emergence in the clinic. Furthermore, a multi-domain biotherapeutic (IgG-scFv bispecific antibody) showed the highest pre-existing reactivity and incidence of treatment-emergent ADA (TE-ADA) (57% and 93%, respectively). Using the components of the multidomain molecule in the Tier 2 step of the ADA assay, we were able to identify the scFv as the target of the serum pre-existing reactivity. Most importantly, the domain specificity of pre-existing ADA was the same as that of the TE-ADA from patients treated with the molecule. Based on these data, we propose the evaluation of the magnitude and of the domain specificity of pre-existing reactivity as a powerful tool to understand the immunogenic potential of novel biotherapeutics.

Proteinase-nicked IgGs: an unanticipated target for tumor immunotherapy

The host immune system adopts multiple mechanisms involving antibodies to confront cancer cells. Accordingly, anti-tumor mAbs have become mainstays in cancer treatment. However, neither host immunity nor mAb therapies appear capable of controlling tumor growth in all cases. Structural instability of IgG was overlooked as a factor contributing to immunosuppression in the tumor microenvironment. Recently, physiological proteinases were identified that disable IgG immune effector functions. Evidence shows that these proteinases cause localized IgG impairment by selective cleavage of a single IgG peptide bond in the hinge-region. The recognition of IgG cleavage in the tumor microenvironment provides alternatives for tumor immunotherapy.

GPIIb/IIIa inhibitors: From bench to bedside and back to bench again

From the discovery of the platelet glycoprotein (GP) IIb/IIIa and identification of its central role in haemostasis, the integrin GPIIb/IIIa (αIIbβ3, CD41/CD61) was destined to be an anti-thrombotic target. The subsequent successful development of intravenous ligand-mimetic inhibitors occurred during a time of limited understanding of integrin physiology. Although efficient inhibitors of ligand binding, they also mimic ligand function. In the case of GPIIb/IIIa inhibitors, despite strongly inhibiting platelet aggregation, paradoxical fibrinogen binding and platelet activation can occur. The quick progression to development of small-molecule orally available inhibitors meant that this approach inherited many potential flaws, which together with a short half-life resulted in an increase in mortality and a halt to the numerous pharmaceutical development programs. Limited clinical benefits, together with the success of other anti-thrombotic drugs, in particular P2Y12 ADP receptor blockers, have also led to a restrictive use of intravenous GPIIb/ IIIa inhibitors. However, with a greater understanding of this key platelet-specific integrin, GPIIb/IIIa remains a potentially attractive target and future drug developments will be better informed by the lessons learnt from taking the current inhibitors back to the bench. This overview will review the physiology behind the inherent problems of a ligand-based integrin inhibitor design and discuss novel promising approaches for GPIIb/IIIa inhibition.

Cross-reactive and pre-existing antibodies to therapeutic antibodies--Effects on treatment and immunogenicity

The potential for immunogenicity is an ever-present concern during the development of biopharmaceuticals. Therapeutic antibodies occasionally elicit an antibody response in patients, which can result in loss of response or adverse effects. However, antibodies that bind a drug are sometimes found in pre-treatment serum samples, with the amount depending on drug, assay, and patient population. This review summarizes published data on pre-existing antibodies to therapeutic antibodies, including rheumatoid factors, anti-allotype antibodies, anti-hinge antibodies, and anti-glycan antibodies. Unlike anti-idiotype antibodies elicited by the drug, pre-formed antibodies in general appear to have little consequences during treatment. In the few cases where (potential) clinical consequences were encountered, antibodies were characterized and found to bind a distinct, unusual epitope of the therapeutic. Immunogenicity testing strategies should therefore always include a proper level of antibody characterization, especially when pre-formed antibodies are present. This minimizes false-positives, particularly due to rheumatoid factors, and helps to judge the potential threat in case a genuine pre-dose antibody reactivity is identified.

A peptide immunization approach to counteract a Staphylococcus aureus protease defense against host immunity

Pathogens that induce acute and chronic infections, as well as certain cancers, employ numerous strategies to thwart host cellular and humoral immune defenses. One proposed evasion mechanism against humoral immunity is a localized expression of extracellular proteases that cleave the IgG hinge and disable host IgG functions. Host immunity appears to be prepared to counter such a proteolytic tactic by providing a group of autoantibodies, denoted anti-hinge antibodies that specifically bind to cleaved IgGs and provide compensating functional restoration in vitro. These respective counter-measures highlight the complex interrelationships among pathogens and host immunity and suggested to us a possible means for therapeutic intervention. In this study, we combined an investigation of pathogen-mediated proteolysis of host IgGs with an immunization strategy to boost host anti-hinge antibodies. In a Staphylococcus aureus infection model using an artificial tissue cage (wiffle ball) implanted into rabbits, cleaved rabbit IgGs were detected in abundance in the abscesses of untreated animals early after infection. However, in animals previously immunized with peptide analogs of the cleaved IgG hinge to generate substantial anti-hinge antibody titers, S. aureus colony formation was markedly reduced compared to control animals or those similarly immunized with a scrambled peptide sequence. The results of this study demonstrate that extensive local proteolysis of IgGs occurs in a test abscess setting and that immunization to increase host anti-hinge antibodies provided substantial acute protection against bacterial growth.

Tumour necrosis factor α antagonists in the treatment of rheumatoid arthritis: an immunological perspective

Rheumatoid arthritis (RA) is one of the most prevalent autoimmune conditions, affecting approximately 1% of the adult population. It is associated with decreased quality of life and considerable morbidity and mortality. Various inflammatory cells, including macrophages, neutrophils, mast cells, natural killer cells, B and T cells and stromal cells play key pathophysiological roles in joint inflammation and RA progression. Several cytokines, including interleukin (IL)-1α and/or IL-1β, and tumour necrosis factor (TNF)-α, are involved at each stage of RA pathogenesis; namely, by augmenting autoimmunity, sustaining long-term inflammatory synovitis and promoting joint damage. Different cell types are involved in RA pathogenesis through upregulation of several cytokine and soluble pro-inflammatory mediators. As early as the late 1980s, TNF had been identified as a potential target in RA. Five anti-TNF drugs, infliximab, adalimumab, certolizumab pegol, etanercept and golimumab, are now approved for the treatment of RA in various countries. All are bivalent monoclonal antibodies, with the exception of the monovalent certolizumab and etanercept, which is an engineered dimeric receptor. Although all react with and neutralise soluble TNF in vitro, structural differences in the molecules may contribute to differences in their therapeutic effects and the occurrence of side effects. Pegylated certolizumab permits once-monthly dosing. Other mechanisms of action proposed to be important for the efficacy of anti-TNF agents are as follows: induction of apoptosis of both monocytes and T cells; neutralization of membrane TNF; antibody-dependent cell-mediated and complement-dependent cytotoxicity; and reverse signaling via membrane TNF.

Discovery of novel antagonists of glycoprotein IIb/IIIa-mediated platelet aggregation through virtual screening

The glycoprotein IIb/IIIa receptor is the final common pathway of platelet aggregation, regardless of the agonist, and thus represents an ideal therapeutic target for blocking thrombus formation. RUC-2 is a novel glycoprotein IIb/IIIa inhibitor of adenosine-5'-diphosphate (ADP)-induced platelet aggregation, importantly which exhibits a unique mode of binding with respect to classical Arg-Gly-Asp (RGD)-based glycoprotein IIb/IIIa antagonists. To identify new chemotypes that inhibit glycoprotein IIb/IIIa-mediated platelet aggregation like RUC-2, we performed a combination of structure-based pharmacophore screening and structure-based virtual screening approach to screen over 7.3 million small molecules based on the RUC-2-glycoprotein IIb/IIIa crystal structure. Three of the eleven hit compounds identified by virtual screening showed promising activity with IC50 values between 16.9 and 90.6μmolL(-1) in a human platelet aggregation assay induced by ADP and thrombin. The binding conformations of these three were analyzed to provide a rationalization of their activity profile. These compounds may serve as potential novel scaffolds for further development of glycoprotein IIb/IIIa antagonists.

A monoclonal antibody against hinge-cleaved IgG restores effector function to proteolytically-inactivated IgGs in vitro and in vivo

We report a chimeric monoclonal antibody (mAb) directed to a neo-epitope that is exposed in the IgG lower hinge following proteolytic cleavage. The mAb, designated 2095-2, displays specificity for IdeS-generated F(ab')₂ fragments, but not for full-length IgG or for closely-related F(ab')₂ fragments generated with other proteases. A critical component of the specificity is provided by the C-terminal amino acid of the epitope corresponding to gly-236 in the IgG1 (also IgG4) hinge. By its ability to bind to IdeS-cleaved anti-CD20 mAb, mAb 2095-2 fully restored antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) against WIL2-S cells to the otherwise inactive anti-CD20 IgG1 F(ab')₂ fragment. Similarly, 2095-2 reinstated ADCC against MDA-MB-231 cells to an anti-CD142 IgG1 F(ab')₂ fragment. mAb 2095-2 was also capable of eliciting both CDC and ADCC to IgG4 F(ab')₂ fragments, an IgG subclass that has weaker ADCC and CDC when intact relative to intact IgG1. The in vitro cell-based efficacy of 2095-2 was extended to the in vivo setting using platelets as a cell clearance surrogate. In a canine model, the co-administration of 2095-2 together with IdeS-generated, platelet-targeting anti-CD41/61 F(ab')₂ fragment not only restored platelet clearance, but did so at a rate and extent of clearance that exceeded that of intact anti-CD41/61 IgG at comparable concentrations. To further explore this unexpected amplification effect, we conducted a rat study in which 2095-2 was administered at a series of doses in combination with a fixed dose of anti-CD41/61 F(ab')₂ fragments. Again, the combination, at ratios as low as 1:10 (w/w) 2095-2 to F(ab')₂, proved more effective than the anti-CD41/61 IgG1 alone. These findings suggest a novel mechanism for enhancing antibody-mediated cell-killing effector functions with potential applications in pathologic settings such as tumors and acute infections where protease activity is abundant.

Harnessing gene conversion in chicken B cells to create a human antibody sequence repertoire

Transgenic chickens expressing human sequence antibodies would be a powerful tool to access human targets and epitopes that have been intractable in mammalian hosts because of tolerance to conserved proteins. To foster the development of the chicken platform, it is beneficial to validate transgene constructs using a rapid, cell culture-based method prior to generating fully transgenic birds. We describe a method for the expression of human immunoglobulin variable regions in the chicken DT40 B cell line and the further diversification of these genes by gene conversion. Chicken V_L and V_H loci were knocked out in DT40 cells and replaced with human V_K and V_H genes. To achieve gene conversion of human genes in chicken B cells, synthetic human pseudogene arrays were inserted upstream of the functional human V_K and V_H regions. Proper expression of chimeric IgM comprised of human variable regions and chicken constant regions is shown. Most importantly, sequencing of DT40 genetic variants confirmed that the human pseudogene arrays contributed to the generation of diversity through gene conversion at both the Igl and Igh loci. These data show that engineered pseudogene arrays produce a diverse pool of human antibody sequences in chicken B cells, and suggest that these constructs will express a functional repertoire of chimeric antibodies in transgenic chickens.

The birth pangs of monoclonal antibody therapeutics: the failure and legacy of Centoxin

This paper examines the development and termination of nebacumab (Centoxin®), a human IgM monoclonal antibody (mAb) drug frequently cited as one of the notable failures of the early biopharmaceutical industry. The non-approval of Centoxin in the United States in 1992 generated major concerns at the time about the future viability of any mAb therapeutics. For Centocor, the biotechnology company that developed Centoxin, the drug posed formidable challenges in terms of safety, clinical efficacy, patient selection, the overall economic costs of health care, as well as financial backing. Indeed, Centocor's development of the drug brought it to the brink of bankruptcy. This article shows how many of the experiences learned with Centoxin paved the way for the current successes in therapeutic mAb development.

Developing the next generation of monoclonal antibodies for the treatment of rheumatoid arthritis

Rheumatoid arthritis is one of the commonest autoimmune diseases affecting 0.8% of the population. Over the last decade the treatment of this chronic disease has been revolutionized by the use of monoclonal antibodies and fusion proteins, targeting molecules like tumour necrosis factor alpha. Nevertheless, approximately one-third of subjects fail to respond to these therapies and therefore significant unmet medical need remains. Following a decade of use, clinical, government and regulatory agency expectations have changed for new antibodies therapies entering this highly competitive area. In this review, we discuss the current advances being made in antibody engineering and how they are being considered and used in the development of the next generation of antibodies to meet future expectations of healthcare providers, physicians and patients. Moreover, we discuss how pattern recognition receptors may provide new antibody tractable targets that may break the cycle of autoimmunity in rheumatoid arthritis.

Cleavage of IgGs by proteases associated with invasive diseases: an evasion tactic against host immunity?

The effective functioning of immunoglobulins and IgG mAbs in removing pathological cells requires that the antigen binding regions and the Fc (effector) domain act in concert. The hinge region that connects these domains itself presents motifs that engage Fc receptors on immune effector cells to achieve cell lysis. In addition, sequences in the lower hinge/CH2 and further down the CH2 region are involved in C1q binding and complement-mediated cell killing. Proteolytic enzymes of little relevance to human physiology were successfully used for decades to generate fragments of IgGs for reagent and therapeutic use. It was subsequently noted that tumor-related and microbial proteases also cleaved human IgG specifically in the hinge region. We have shown previously that the "nick" of just one of the lower hinge heavy chains of IgG unexpectedly prevented many effector functions without impacting antigen binding. Of interest, related single-cleaved IgG breakdown products were detected in breast carcinoma extracts. This suggested a pathway by which tumors might avoid host immune surveillance under a cloak of proteolytically-generated, dysfunctional antibodies that block competent IgG binding. The host immune system cannot be blind to this pathway since there exists a widespread, low-titer incidence of anti-hinge (cleavage-site) antibodies in the healthy population. The prevalence of anti-hinge reactivity may reflect an ongoing immune recognition of normal IgG catabolism. Tumor growth and bacterial infections potentially generate hostile proteolytic environments that may pose harsh challenges to host immunity. Recent findings involving physiologically-relevant proteases suggest that the potential loss of key effector functions of host IgGs may result from subtle and limited proteolytic cleavage of IgGs and that such events may facilitate the incursion of invasive cells in local proteolytic settings.

Pharmacodynamic enhancement of the anti-platelet antibody Fab abciximab by site-specific pegylation

Monoclonal antibodies have been firmly established as human therapeutics. Their high affinity and specificity for target antigens minimize adverse reactions and their molecular size results in extended circulation times relative to small molecule pharmaceuticals. The ability to customize the pharmacokinetics in a rational manner can enhance the potential for these and other classes of biologicals. We have systematically studied the effect of site-specific pegylation of the Fab' fragment of the anti-GPIIb/IIIa, alphavbeta3 antibody c7E3. Regardless of the molecular weight of the PEG molecules, the intrinsic affinity of the resulting constructs remained unchanged. However, in functional assays measuring inhibition of platelet aggregation, the calculated IC50 values of the conjugates decreased with increasing molecular weight of the conjugated PEG. It was determined that the molecular size of the conjugates affects antigen accessibility and whereas high levels of binding to antigen molecules on cells with high antigen density can be demonstrated with the Fab fragment, comparable levels are not achievable with large molecular weight conjugates. In spite of the inability of the larger PEG constructs to achieve saturation binding, functional inhibition of platelet aggregation consistent with saturation binding was demonstrated and the increased molecular size of the conjugates led to predictably prolonged inhibition of platelet aggregation.

Immunomodulation in the treatment of haematological malignancies

Despite the continuous advances in immunology and cancer biology, haematological malignancies are often incurable. Conventional chemotherapy and radiation are efficacious for some lymphoma and leukaemia, however relapse and progressive disease often occurs. The evidence that the immune system can play an essential role in controlling cancer progression provide a basis for the development of active therapies, such as immunization, aimed to evoke or amplify a tumour-specific immune response. However, the inability of the patient's own immune system to mount effective responses against tumour antigens is a major limit of vaccination approaches. The adoptive transfer of effectors of the adaptive immune system is an attractive strategy to circumvent the limitations of autologous immune responses. Donor lymphocyte infusion and the transfer of monoclonal antibodies (MoAbs) have been the first forms of adoptive therapy approved for clinical use and are still fundamental components of immunotherapy of haematological malignancies. Due to the continuous characterization of tumour-specific antigen, the development of tumour-tailored therapies that exploit the specificity of antibodies and T cell receptors (TCRs) is progressing rapidly. This review highlights the current advances in the field of adoptive immunotherapy of haematological malignancies, starting by elucidating the ongoing progress in passive transfer of MoAbs. We will also discuss recent advances in the adoptive transfer with tumour-specific high avidity T cells, which can be generated ex vivo by the transfer of gene constructs encoding single chain antibodies or TCRs, thus redirecting T cell specificity to selected tumour antigens. The ability to produce gene-modified T cells of desired specificity and defined functional activity may improve in the future T cell based immunotherapy of cancer.

Fully human antibodies from transgenic mouse and phage display platforms

Over the past two decades, technologies have emerged for generating monoclonal antibodies (MAbs) derived from human immunoglobulin gene sequences. These fully human MAbs provide an alternative to re-engineered, or de-immunized, rodent MAbs as a source of low immunogenicity therapeutic antibodies. There are now two marketed fully human therapeutic MAbs, adalimumab and panitumumab, and several dozen more in various stages of human clinical testing. Most of the drugs, including adalimumab and panitumumab, were generated using either phage display or transgenic mouse platforms. The reported clinical experience with fully human MAbs demonstrates that these two platforms are, and should continue to be, a significant source of active and well tolerated experimental therapeutics. While this body of reported clinical data does not yet provide a clear distinction between the platforms, the available descriptions of the drug discovery processes used to identify the clinical candidates highlight one difference. It appears that lead optimization is more commonly applied to phage display derived leads than transgenic mouse derived leads.

Anaphylactic reaction after initial exposure of Basiliximab: case reports

Basiliximab is potent, relatively safe immunosuppressive induction agent used in transplantation. Prophylactic use at the time of transplantation has been advocated to improve allograft outcomes. We report two cases of kidney transplant recipients with anaphylactic reactions after initial exposure to Basiliximab.

Anti-TNF antibodies: lessons from the past, roadmap for the future

Tumor necrosis factor alpha (TNF) is an important cell-signaling component of the immune system. Since its discovery over 20 years ago, much has been learned about its functions under normal and disease conditions. Nonclinical studies suggested a role for TNF in chronic immune-mediated inflammatory diseases, such as rheumatoid arthritis, Crohn's disease, and psoriasis, and therefore neutralizing monoclonal antibodies specific to human TNF were developed for clinical evaluation. Treatment with anti-TNF monoclonal antibodies (infliximab, adalimumab, and certolizumab pegol) has been shown to provide substantial benefit to patients through reductions in both localized and systemic expression of markers associated with inflammation. In addition, there are beneficial effects of anti-TNF treatment on markers of bone and cartilage turnover. Further exploration of changes in these markers and their correlation with clinical measures of efficacy will be required to allow accurate prediction of those patients most in need of these treatments. Both the clinical and commercial experience with these anti-TNF antibodies provide a wealth of information regarding their pharmacological effects in humans.

Exchanging murine and human immunoglobulin constant chains affects the kinetics and thermodynamics of antigen binding and chimeric antibody autoreactivity

Mouse-human chimeric antibodies composed of murine variable (V) and human (C) chains are useful therapeutic reagents. Consequently, we investigated whether heterologous C-regions from mice and humans affected specificity and affinity, and determined the contribution of C(H) glycosylation to antigen binding. The interaction of a 12-mer peptide mimetic with monoclonal antibody (mAb) 18B7 to Cryptococcus neoformans glucuronoxylomannan, and its chimeric (ch) and deglycosylated forms were studied by surface plasmon resonance. The equilibrium and rate association constants for the chAb were higher than for mAb 18B7. V region affinity was not affected by C(H) region glycosylation whereas heterologous C region of the same isotype altered the Ab binding affinity and the specificity for self-antigens. Structural models displayed local differences that implied changes on the connectivity of residues. These findings suggest that V region conformational changes can be dictated by the C(H) domains through an allosteric effect involving networks of highly connected amino acids.

Potential of active and passive immunizations for the prevention and therapy of transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies are fatal neurodegenerative disorders that affect humans and certain animals and are caused by prions. In most cases, infection occurs by ingestion of prions. Their long-time persistence in the environment creates a reservoir of potentially infectious matter that renders the eradication of the disease problematic. Unfortunately, no cure is available to date. Yet, for both the treatment of infected and the protection of uninfected individuals, active and passive immunizations have been shown to have a beneficial effect on the course of the disease. The current review provides an overview of such antibody-based approaches and assesses their feasibility and potential in prophylaxis and therapy of transmissible spongiform encephalopathies.

Role of abciximab in the treatment of coronary artery disease

Glycoprotein IIb/IIIa complex is a crucial membrane receptor for platelet aggregation, binding platelets to fibrinogen and establishing interplatelet bridges. This receptor is the common end point of the multiple activation pathways of a platelet. Antiplatelet agents, such as aspirin or thienopyridines, including ticlopidine and clopidogrel, inhibit one or more but not all, of these pathways. Inhibitors of the receptor are powerful platelet antiaggregants and include two groups of agents: non-competitive receptor blockers, such as abciximab, and competitive antagonists, such as tirofiban and eptifibatide. Abciximab is a monoclonal antibody that binds to the glycoprotein IIb/IIIa complex, thus blocking the interaction with fibrinogen. It is used for treatment of coronary artery disease, being well-studied in the setting of acute coronary syndromes and percutaneous coronary intervention, in which a rapid and effective antiaggregation is clinically important.

Mycobacterium abscessus infection after use of tumor necrosis factor α inhibitor therapy: case report and review of infectious complications associated with tumor necrosis factor α inhibitor use

Tumor necrosis factor alpha (TNF-alpha) inhibitors, such as infliximab and etanercept, are now frequently used in the treatment of inflammatory diseases including rheumatoid arthritis (RA) and Crohn's disease. As an apparent result of their immune modulating activity, there has been an observed association between the use of these agents and the development of a wide range of infections, most notably Mycobacterium tuberculosis. We describe a case of infection with Mycobacterium abscessus in a 67-year-old woman receiving infliximab as a component of her therapy for RA. This case, along with extensive reports in the medical literature, illustrate how treatment with inhibitors of TNF-alpha has the potential to result in a wide range of infectious complications, including rapid growing Mycobacterium.

Tiny Solutions for Giant Cardiac Problems

Health care nanotechnology research has "domesticated" molecular and cellular processes to serve our needs. This paper introduces current cardiovascular therapies for nanotechnologists unfamiliar with the field or current developments. Although early in its development, cell-based disease therapy capitalizes on existing biologic systems to implement nanoscale functionality. We propose that the most efficient development pathway for nanomedicine is to merge biomolecular and cellular techniques with the nanotechnology knowledge base.

The role of antiplatelet therapy in the management of acute coronary syndromes

The benefit of aspirin use in the emergent care of acute coronary syndromes (ACS) has been well-established. Recent studies have further demonstrated the importance of antiplatelet therapy in the acute setting, primarily with the use of intravenous glycoprotein IIb/IIIa receptor inhibitors. Aspirin and the thienopyridines (ticlopidine and clopidogrel) are oral antiplatelet agents that interfere with platelet activation in complementary, but separate pathways. Combination therapy of aspirin with other antiplatelet agents has demonstrated a benefit for the management of ACS. This article reviews the pathophysiology of platelet activation in ACS, landmark trials regarding antiplatelet agents, and the current recommendations for the use of both intravenous and oral antiplatelet agents in the management of patients with ACS.

Immune thrombocytopenia caused by glycoprotein IIb/IIIa inhibitors

Agents that react with the platelet glycoprotein (GP) IIb/IIIa complex (alphaIIb/beta3 integrin) to block fibrinogen binding and platelet-platelet aggregation have been proved to be effective in reducing the incidence of complications following coronary angioplasty and are now widely used for this purpose. Acute thrombocytopenia, which is sometimes severe and life-threatening, is a recognized side effect of this class of drugs. In contrast to other types of drug-induced thrombocytopenia, this complication can occur within a few hours of a patient's first exposure to the medication. Accumulating evidence has indicated that drug-dependent antibodies, which can be naturally occurring, are the cause of platelet destruction in such individuals. In this review, we will consider the clinical aspects of thrombocytopenia resulting from sensitivity to GPIIb/IIIa inhibitors and will review evidence that the platelet destruction is antibody-mediated.

Delayed thrombocytopenia after treatment with abciximab: a distinct clinical entity associated with the immune response to the drug

BACKGROUND

Acute thrombocytopenia is a recognized side-effect of treatment with the fibrinogen receptor antagonist, abciximab, a chimeric (human/mouse) Fab fragment. The etiology of this complication is not fully understood. Generally, abciximab-induced thrombocytopenia occurs within a few hours of starting treatment with the drug. We have characterized a group of 13 patients who first developed thrombocytopenia 3-6 days after abciximab was discontinued.

OBJECTIVE

To characterize clinical and serological aspects of this newly recognized clinical entity.

PATIENTS AND METHODS

Clinical information was obtained from attending physicians and review of hospital records. Antibodies reactive with abciximab-coated platelets were characterized by flow cytometry.

RESULTS

In each patient, IgG and/or IgM antibodies reactive with abciximab-coated platelets were identified. These antibodies could be distinguished from similar antibodies present in many normal persons by two criteria-they were relatively resistant to inhibition by normal Fab fragments, and they reacted preferentially with platelets coated with 7E3, the murine monoclonal antibody from which peptide sequences in abciximab are derived. Antibodies with these characteristics were not found in pretreatment serum from three of the thrombocytopenic patients or in patients given abciximab who did not develop thrombocytopenia.

CONCLUSIONS

'Delayed thrombocytopenia' after treatment with abciximab is caused by antibodies produced in response to the drug. These antibodies may be specific for murine peptide sequences in abciximab but could recognize other target epitopes on abciximab-coated platelets. Physicians administering abciximab should be aware of this potential complication of treatment, which usually occurs after discharge from hospital.

Final results of the ReoPro readministration registry

Because of its potential for antigenicity, theoretical concerns related to readministration of abciximab have been raised. We conducted the ReoPro Readministration Registry to assess the efficacy and safety of abciximab readministration. A total of 1,342 patients who underwent percutaneous coronary intervention and who received abciximab for at least a second time were recruited. Safety end points were hypersensitivity reactions, major bleeding, and thrombocytopenia (TCP). Human antichimeric antibody (HACA) titers were determined before and after readministration. Procedural success was 98% and was not influenced by the number of courses of abciximab or the presence of HACA. There were no cases of anaphylaxis. There were 5 minor allergic reactions, none of which required termination of the infusion. Clinically significant bleeding occurred in 31 patients (2.3%), including 1 (0.07%) with intracranial hemorrhage. TCP (<100 x 10(9)/L) developed in 5% of patients; profound TCP (<20 x 10(9)/L) occurred in 2%. In patients who received abciximab within 1 month of a previous treatment (n = 115), the risk of developing TCP and profound TCP was 16.5% and 12.2%, respectively. Having a positive HACA before readministration was not correlated with adverse clinical outcomes or bleeding, but was associated with TCP (14.1% vs 4.4%, p = 0.002) and profound TCP (5.6% vs 1.6%, p = 0.036). Readministration of abciximab can be accomplished without severe allergic responses and with a bleeding and efficacy profile similar to first-time administration. However, the rate of severe and profound TCP is increased relative to first-time administration, particularly when the time between treatments is <30 days or when HACA is present.

Platelet glycoprotein IIb/IIIa inhibitors in percutaneous coronary intervention: focus on the pharmacokinetic-pharmacodynamic relationships of eptifibatide

Eptifibatide is a truncated derivative of the naturally occurring rattlesnake venom protein known as barbourin. It is a cyclic heptapeptide that mimics the tertiary structure found in the parent compound which allows it to bind receptors with the KGD (Lys-Gly-Asp) peptide recognition sequence. Specifically, eptifibatide is a competitive antagonist for the activated platelet glycoprotein IIb/IIIa receptor. Its mechanism of action involves preventing the binding and cross-linking of fibrinogen to the platelet surface. This binding site for fibrinogen is associated with five Ca2+ ions that help maintain the tertiary structure of the receptor and affect the affinity of other ligands such as eptifibatide. Arterial injury induced by percutaneous coronary interventions (PCI) such as balloon angioplasty and stenting, and the spontaneously occurring disease process known as the acute coronary syndrome (ACS), share a common underlying pathophysiology. In both situations, disruption of integrity of the arterial wall initiates a cascade of platelet activation, adhesion and aggregation. Ultimately, this process may proceed to arterial thrombosis unless controlled or modified. Advances in understanding how the platelet plays a pivotal role in this process have significantly enhanced therapy for patients with ACS and have resulted in important reductions in thrombotic complications from PCI procedures. Central to these advances has been evolving understanding of platelet-inhibiting pharmaceutical agents such as eptifibatide. The development of a rational administration regimen for eptifibatide parallels the growth in the understanding of the underlying mechanisms of platelet receptor functions. The binding of eptifibatide to the receptor involves displacement of receptor-associated Ca2+ from the activated binding site. Early in the clinical development of eptifibatide, this was poorly appreciated and resulted in an underestimation of the appropriate doses for this agent. Through a series of small clinical trials and laboratory studies, deficiencies in the early administration regimens were identified and a more effective dose schedule was determined. Modelling of the drug based on its two-compartment pharmacokinetics further defined the role of a newer double-bolus initiation of therapy verses the original single-bolus approach. In a large-scale clinical trial using this double-bolus followed by infusion regimen in PCI procedures, clinical efficacy was shown to be significantly improved over placebo and the earlier, low-dose regimens used in the original trials of eptifibatide.

Meta-analysis of survival with platelet glycoprotein IIb/IIIa antagonists for percutaneous coronary interventions

We performed a cumulative meta-analysis of available studies to evaluate the effect of intravenous platelet glycoprotein (GP) IIb/IIIa antagonists on survival at 30 days and 6 months after percutaneous coronary intervention (PCI). Compounds that block the GP IIb/IIIa receptor substantially reduce myocardial infarctions (MIs) and repeat revascularization. We included 12 trials, which enrolled 20,186 patients in all, in the analysis. Overall, 30-day mortality was significantly reduced with GP IIb/IIIa inhibition (odds ratio 0.73, 95% confidence interval 0.55 to 0.96, p = 0.024). Although 10 of the 12 trials showed a beneficial effect of GP IIb/IIIa inhibitor treatment on mortality, no individual trial detected a statistically significant mortality benefit. The 30-day mortality benefit became significant at the p <0.05 level with addition of the ADMIRAL trial and was further enhanced by the CADILLAC trial. No significant heterogeneity was detected in the collection of trials. At 6 months, the odds ratio was 0.84 (95% confidence interval 0.69 to 1.03, p = 0.087). This survival benefit amounts to preventing approximately 1 of every 3 deaths that occur within 30 days after PCI, saving 2.8 lives/1,000 patients treated (number needed to treat, 357). Thus, patients who undergo PCI can expect significantly lower 30-day mortality, in addition to known reductions in nonfatal MI and repeat procedures, with GP IIb/IIa inhibition. There also is increasing evidence that mortality reductions are preserved at 6 months.

Marginal transfer of ReoPro (Abciximab) compared with immunoglobulin G (F105), inulin and water in the perfused human placenta in vitro

ReoPro (Abciximab), a Fab fragment of a human-murine chimeric monoclonal antibody, binds to glycoprotein IIb/IIIa receptors on human platelets and inhibits platelet aggregation. Can ReoPro transit the human placenta since it does not have an Fc (domain) as does IgG? This question was addressed using an in vitro term human placental lobular dual perfusion model. ReoPro, along with 3H(2)O, inulin or 125I-F105 human IgG(1), were added to the maternal reservoir for 6 or >12 h, ReoPro was equivalent to, or exceeded, clinically relevant plasma concentrations (0.3-3 microg/ml). 3H(2)O rapidly appeared in the fetal circuit, while fetal 14C-inulin never equilibrated with the maternal inulin. After 6 h, 125I-F105 was present with fetal/maternal percentages-0.55 per cent. ReoPro was not detectable (<3.9 ng/ml) in the fetal circuit during or at the end of any perfusion. Using immunohistochemistry, ReoPro was only detected attached to maternal and fetal platelets, and to the trophoblastic surface of the placental villi. Only pharmaceutically insignificant amounts of ReoPro were detected in the fetal circuit, which demonstrates a barrier capacity of the human term placenta for this Fab fragment compared with IgG.

Two novel mutations in the alpha IIb calcium-binding domains identify hydrophobic regions essential for alpha IIbbeta 3 biogenesis

The recently published crystal structure of the external domains of alphaVbeta3 confirms the prediction that the aminoterminal portion of alphaV, which shares 40% homology with alphaIIb, folds into a beta-propeller structure and that the 4 calcium-binding domains are positioned on the bottom of the propeller. To gain insight into the role of the calcium-binding domains in alphaIIb biogenesis, we characterized mutations in the second and third calcium-binding domains of alphaIIb in 2 patients with Glanzmann thrombasthenia. One patient inherited a Val298Phe mutation in the second domain, and the other patient inherited an Ile374Thr mutation in the third domain. Mammalian cell expression studies were performed with normal and mutant alphaIIb and beta3 cDNA constructs. By flow cytometry, expression of alphaIIb Val298Phe/beta3 in transfected cells was 28% of control, and expression of alphaIIbIle374Thr/beta3 was 11% of control. Pulse-chase analyses showed that both mutant pro-alphaIIb subunits are retained in the endoplasmic reticulum and degraded. Mutagenesis studies of the Val298 and Ile374 residues showed that these highly conserved, branch-chained hydrophobic residues are essential at these positions and that biogenesis and expression of alphaIIbbeta3 is dramatically affected by structural variations in these regions of the calcium-binding domains. Energy calculations derived from a new model of the alphaIIb beta-propeller indicate that these mutations interfere with calcium binding. These data suggest that the alphaIIb calcium-binding domains play a key structural role in the beta-propeller, and that the structural integrity of the calcium-binding domains is critical for integrin biogenesis.

The use of adjunctive GPIIb/IIIa inhibitors in patients with unstable angina/non-Q-wave MI undergoing percutaneous coronary intervention

Glycoprotein IIb/IIIa receptor inhibitors represent a relatively new therapeutic approach in the field of antiplatelet therapy. Following the development of abciximab a number of small molecule GPIIb/IIIa inhibitors have been introduced such as tirofiban and eptifibatide. In this fast-moving field the interventional cardiologist needs a framework to guide decision-making for the individual patient. This review covers the efficacy and safety data from the clinical trials of GPIIb/IIIa inhibitors in the context of patients undergoing percutaneous coronary intervention for unstable angina/non-Q-wave myocardial infarction. There is an increasing body of evidence to support the efficacy of GPIIb/IIIa inhibitors in reducing the risk of adverse ischemic events in high and low risk patients undergoing percutaneous coronary intervention. A number of unresolved efficacy and safety issues remain, including the duration of treatment before and after intervention; whether a reduction in the heparin dose would further decrease the risk of hemorrhage without affecting the periprocedural thrombotic rate in patients undergoing PTCA with adjunctive GPIIb/IIIa inhibitors; and the cost-effectiveness of this therapy. When a thorough analysis of cost-effectiveness has been made, it will be easier to advocate the widespread use of these agents in all patients undergoing coronary intervention.

Expression of full-length immunoglobulins in Escherichia coli: rapid and efficient production of aglycosylated antibodies

Many research and clinical applications require large quantities of full-length antibodies with long circulating half-lives, and production of these complex multi-subunit proteins has in the past been restricted to eukaryotic hosts. In this report, we demonstrate that efficient secretion of heavy and light chains in a favorable ratio leads to the high-level expression and assembly of full-length IgGs in the Escherichia coli periplasm. The technology described offers a rapid, generally applicable and potentially inexpensive method for the production of full-length therapeutic antibodies, as verified by the expression of several humanized IgGs. One E. coli-derived antibody in particular, anti-tissue factor IgG1, has been thoroughly evaluated and has all of the expected properties of an aglycosylated antibody, including tight binding to antigen and the neonatal receptor. As predicted, the protein lacks binding to C1q and the FcgammaRI receptor, making it an ideal candidate for research purposes and therapeutic indications where effector functions are either not required or are actually detrimental. In addition, a limited chimpanzee study suggests that the E. coli-derived IgG1 retains the long circulating half-life of mammalian cell-derived antibodies.

Thrombocytopenia after second exposure to abciximab is caused by antibodies that recognize abciximab-coated platelets

Thrombocytopenia, often severe, occurs in 1% to 2% of patients given the fibrinogen receptor antagonist abciximab, a chimeric Fab fragment containing murine specificity-determining and human framework sequences. The cause of this complication has not yet been defined. Studies of 9 patients who developed profound thrombocytopenia (platelets <10 x 10(9)/L [10 000/microL]) within a few hours of being given abciximab a second time showed that each had a strong immunoglobulin G (IgG) antibody that recognized platelets sensitized with abciximab. Five patients also had IgM antibodies. IgG antibodies reactive with abciximab-coated platelets were also found in 77 (74%) of 104 healthy subjects. However, the patient antibodies could be distinguished from "normal" ones in 2 ways: (1) only the patient antibodies reacted preferentially with platelets sensitized with the intact monoclonal antibody 7E3 from which the murine sequences in abciximab are derived; and (2) the "normal" antibodies could be inhibited by Fab fragments derived from normal human IgG, whereas the patient antibodies were relatively resistant to this treatment. The findings suggest that antibodies from the patients are specific for murine sequences in abciximab and are capable of causing life-threatening thrombocytopenia upon injection of this drug. The antibodies commonly found in healthy subjects are specific for the papain cleavage site of any Fab fragments and, although they react with abciximab-coated platelets, appear not to cause significant thrombocytopenia. It may be possible to identify patients at risk for developing thrombocytopenia if given abciximab by screening for antibodies that recognize 7E3-coated platelets.

Antibody engineering

Antibodies are unique in their high affinity and specificity for a binding partner, a quality that has made them one of the most useful molecules for biotechnology and biomedical applications. The field of antibody engineering has changed rapidly in the past 10 years, fueled by novel technologies for the in vitro isolation of antibodies from combinatorial libraries and their functional expression in bacteria. This review presents an overview of the methods available for the de novo generation of human antibodies, for engineering antibodies with increased antigen affinity, and for the production of antibody fragments. Select applications of recombinant antibodies are also presented.

Abciximab readministration: results of the ReoPro Readministration Registry

BACKGROUND

Platelet glycoprotein IIb/IIIa blockade with abciximab (ReoPro) improves the clinical outcomes of percutaneous coronary intervention. This registry was conducted to characterize the effects of repeated administration of abciximab during intervention.

METHODS AND RESULTS

We recruited 500 consecutive patients at 22 centers in the United States who were receiving abciximab for at least a second time during percutaneous coronary intervention. Safety was measured as the incidence of hypersensitivity reactions, major bleeding, and thrombocytopenia. Efficacy was assessed as event-free clinical success. Human antichimeric antibody (HACA) responses were also characterized. There were no cases of hypersensitivity (95% upper confidence bound, 0.3%), major bleeding, or death. Clinical success was 94.4%. Thrombocytopenia occurred in 23 patients (4.6%; 95% CI, 2.8% to 6.4%), including 12 (2.4%; 95% CI, 1.1% to 3.7%) who developed profound thrombocytopenia (<20x10(9) cells/L). In 2 patients (0.4%), profound thrombocytopenia did not develop until after hospital discharge; in 4 (0.8%), profound thrombocytopenia recurred despite platelet transfusion. Before a first readministration, a positive HACA titer was present in 22 of 454 patients (4.8%); after a first readministration, an additional 82 of 432 (19.0%) became HACA-positive. HACA did not neutralize the in vitro inhibition of platelet aggregation by abciximab or correlate with clinical events.

CONCLUSIONS

The results, including overall rates of thrombocytopenia, were consistent with randomized clinical trials of first abciximab treatment. However, there was a shift from mild to profound thrombocytopenia, and cases of delayed presentation and of recurrent thrombocytopenia were seen. These findings suggest that indications and guidelines for first-time use apply to retreatment, particularly the systematic monitoring for thrombocytopenia.

Platelet aggregation inhibition with glycoprotein IIb--IIIa inhibitors

Inhibitors of the platelet receptor glycoprotein (GP) IIb--IIIa are a novel and potent class of antithrombotic drugs for the management of patients with non-ST-segment elevation acute coronary syndromes (ACS) and those undergoing percutaneous coronary intervention (PCI). Pharmacodynamic studies with three currently approved agents in this class (abciximab [ReoPro, Centocor, Inc., Malvern, Pennsylvania, and Eli Lilly & Company, Indianapolis, Indiana]; eptifibatide [INTEGRILIN, COR Therapeutics, Inc., South San Francisco, California, and Key Pharmaceuticals, Inc., Kenilworth, New Jersey]; and tirofiban HCI [Aggrastat, Merck & Co., Inc., Whitehouse Station, New Jersey]) all sought to identify dosing regimens that would establish and maintain >80 % inhibition of ex vivo platelet aggregation throughout the duration of intravenous infusion. Direct comparison of these prior studies is difficult, however, because the assays used different anticoagulants (sodium citrate [abciximab, tirofiban HCI] or D-phenylalanyl-L-prolyl-L-arginine chloromethyl ketone [PPACK] [eptifibatide]) and different concentrations of the platelet agonist adenosine diphosphate (ADP) (5 micromol [tirofiban HCI] or 20 micromol [abciximab, eptifibatide]). More recent work has attempted to overcome these limitations by using similar assay conditions for all GP IIb--IIIa inhibitors. These studies have indicated that the concentrations of all three agents required to provide the targeted effect for platelet inhibition are considerably higher in the presence of an anticoagulant that does not chelate calcium ions (e.g., heparin or PPACK) than in the presence of calcium-chelating anticoagulant (i.e., sodium citrate). Of the three currently approved GP IIb--IIIa inhibitors, eptifibatide is the only agent whose approved dosing is based on an ex vivo platelet aggregation assay that uses such an anticoagulant. Additionally, Kereiakes et al. have recently reported that the high levels of platelet inhibition (>80 %), using PPACK as an anticoagulant and ADP (20 micromol) as an agonist, are more consistently achieved with the approved dosing regimen of eptifibatide. The antiplatelet effect of abciximab showed more interpatient variability, whereas the median inhibition of ex vivo platelet aggregation with the approved dosing regimen for tirofiban HCl was <80 % at almost all time points during drug infusion.

Removal of amphipathic epitopes from genetically engineered antibodies: production of modified immunoglobulins with reduced immunogenicity

Several approaches have been developed to reduce the human immune response to nonhuman antibodies. However, chimeric antibodies and humanized antibodies often have decreased binding affinity. We described a new approach for reducing the immunogenicity of chimeric antibodies while maintaining the affinity. This approach seeks to prevent the recognition of murine immunogenic peptides from the antibody variable region by human lymphocytes. Putative immunogenic epitopes in the variable region are identified and subjected to site directed mutagenesis to make them human and/or to break the amphipathic motifs. The R3 antibody, which blocks the epidermal growth factor (EGF) receptor, was used as a model system to test this approach. Four segments containing possible amphipathic epitopes were found in the heavy variable domain using the program AMPHI. Six amino acids within two of these segments were substituted by the corresponding residues from a homologous human sequence. No mutations were made in the murine light variable domain. Experiments in monkeys suggested that the "detope" R3 antibody was less immunogenic than its chimeric analogue. A search for possible amphipathic epitopes in the Kabat database revealed the presence of conserved patterns in the different families of variable region sequences, suggesting that the proposed method may be of general applicability.

Intracellular trafficking of the alphaIIbbeta3 receptor antagonist, abciximab, in normal and Glanzmann's disease megakaryocytes

The alphaIIbbeta3 platelet receptor antagonist abciximab (c7E3Fab, ReoPro(R)) has proved to be effective in preventing arterial thrombosis. However, its binding capacity to the platelet precursors, megakaryocytes (MKs), which also express alphaIIbbeta3, is not known. The purpose of this study was to establish whether abciximab is able to react with alphaIIbbeta3 located on human MKs, and to follow its subsequent intracellular trafficking. MKs were grown from CD34+ progenitors from normal subjects and from a patient with type I Glanzmann's thrombasthenia, and abciximab was added at day 10 of culture (4 microgram/ml). Cells were fixed at day 12, cryosectioned, and immunolabelled for abciximab. Labelling was prominent on the MK plasma membrane; it also lined the demarcation membration system. Interestingly, alpha-granule membranes were labelled showing that the antibody was internalized and further stored into MK secretory granules. Abciximab was also strongly detected on and in newly-formed platelets. Glanzmann's disease MKs (which completely lacked alphaIIbbeta3) were consistently negative, confirming that the antibody fragment was specifically interacting with alphaIIbbeta3. In conclusion, this study demonstrated that abciximab: (i) binds MK plasma membrane and demarcation membranes, (ii) trafficks into alpha-granules, and (iii) is expressed on and in nascent platelets. These findings could be taken in account when monitoring anti-alphaIIbbeta3 receptor therapy.