Targeted anti-cancer therapy using rituximab, a chimaeric anti-CD20 antibody (IDEC-C2B8) in the treatment of non-Hodgkin's B-cell lymphoma

Bispecific anti-CD20/22 antibodies inhibit B-cell lymphoma proliferation by a unique mechanism of action

Combination immunotherapy with anti-CD20 and anti-CD22 mAbs shows promising activity in non-Hodgkin lymphoma. Therefore, bispecific mAbs (bsAbs) were recombinantly constructed from veltuzumab (humanized anti-CD20) and epratuzumab (humanized anti-CD22) and evaluated in vitro and in vivo. While none of the parental mAbs alone or mixed had notable antiproliferative activity against Burkitt lymphoma cells when not cross-linked, the bsAbs [eg, anti-CD20 IgG-anti-CD22 (scFv)(2)] were inhibitory without cross-linking and synergistic with B-cell antigen (BCR)-mediated inhibition. The bsAbs demonstrated higher antibody-dependent cellulary cytoxicity (ADCC) activity than the parental mAbs, but not complement-dependent cytoxicity (CDC) of the parental CD20 mAb. Cross-linking both CD20 and CD22 with the bsAbs resulted in the prominent redistribution of not only CD20 but also CD22 and BCR into lipid rafts. Surprisingly, appreciable translocation of CD22 into lipid rafts was also observed after treatment with epratuzumab. Finally, the bsAbs inhibited Daudi lymphoma transplant growth, but showed a significant advantage over the parental anti-CD20 mAb only at the highest dose tested. These results suggest that recombinantly fused, complementary, bispecific, anti-CD20/22 antibodies exhibit functional features distinct from their parental antibodies, perhaps representing new candidate therapeutic molecules.

Rituximab pharmacokinetics in patients with rheumatoid arthritis: B-cell levels do not correlate with clinical response

This study characterized the relationship between clinical response, serum rituximab concentrations, and peripheral B-cell levels in patients with rheumatoid arthritis treated with rituximab. Data were analyzed from a double-blind, phase IIa trial in which 161 patients with active rheumatoid arthritis despite continuing methotrexate were randomized to methotrexate alone (10-25 mg/wk), rituximab alone (single course: 1000 mg administered intravenously on days 1 and 15), rituximab plus cyclophosphamide (750 mg administered intravenously on days 3 and 17), or rituximab plus methotrexate. Serum samples for pharmacokinetic analysis were collected through 24 weeks, and peripheral circulating CD19+ B-cell levels were measured through 48 weeks. All treatments were generally well tolerated, with no clinically relevant excess of adverse events leading to withdrawal among patients who received rituximab compared with those who received methotrexate alone. The proportions of patients who achieved an American College of Rheumatology score of 50 at week 24 were 13% (methotrexate alone), 33% (rituximab alone), 41% (rituximab plus cyclophosphamide), and 43% (rituximab plus methotrexate). Peripheral B-cell depletion occurred by day 15 in all patients treated with rituximab. There was no relationship between B-cell depletion and clinical response. Recovery of peripheral B cells was variable and showed no relationship with return of disease activity in patients who responded to rituximab. The mean terminal half-life of rituximab was 19 to 22 days; pharmacokinetic parameters were similar whether rituximab was administered alone or with methotrexate or cyclophosphamide. Because the level of peripherally circulating B cells does not appear to correlate with a maintained clinical response in patients with rheumatoid arthritis, the timing of rituximab retreatment should be based on clinical symptoms rather than peripheral B-cell levels.

FCGR2A and FCGR3A polymorphisms associated with clinical outcome of epidermal growth factor receptor expressing metastatic colorectal cancer patients treated with single-agent cetuximab

PURPOSE

Cetuximab, a chimeric immunoglobulin G 1 (IgG1) anti-epidermal growth factor receptor (EGFR) monoclonal antibody (mAb), has shown efficacy in 10% of patients with metastatic colorectal cancer (CRC). Recent studies demonstrate antibody-dependent cell-mediated cytotoxicity (ADCC) is one of the modes of action for rituximab and trastuzumab. Fragment c (Fc) portion of IgG1 mAb has shown to induce ADCC. Fragment c gamma receptors (FcgammaR) play an important role in initiating ADCC. Studies have shown that two IgG FcgammaR polymorphisms (FCGR2A-H131R and FCGR3A-V158F) independently predict response to rituximab in patients with follicular lymphoma. We tested the hypothesis of whether these two polymorphisms are associated with clinical outcome in metastatic CRC patients treated with single-agent cetuximab.

PATIENTS AND METHODS

Thirty-nine metastatic CRC patients were enrolled onto the ImClone0144 trial. Using an allele-specific polymerase chain reaction (PCR) -based method, gene polymorphisms of FCGA2A-H131R and FCGA3A-V158F were assessed from genomic DNA extracted from peripheral blood samples.

RESULTS

FCGR2A-H131R and FCGR3A-V158F polymorphisms were independently associated with progression-free survival (PFS; P = .037 and .055, respectively; log-rank test). Combined analysis of these two polymorphisms showed that patients with the favorable genotypes (FCGR2A, any histidine allele, and FCGR3A, any phenylalanine allele) showed a median PFS of 3.7 months (95% CI, 2.4 to 4.4 months), whereas patients with any two unfavorable genotypes (FCGR2A arginine/arginine or valine/valine) had a PFS of 1.1 months (95% CI, 1.0 to 1.4 months; P = .004; log-rank test).

CONCLUSION

Our preliminary data suggest that these two polymorphisms may be useful molecular markers to predict clinical outcome in metastatic CRC patients treated with cetuximab and that they may indicate a role of ADCC of cetuximab.

Fragments of protein A eluted during protein A affinity chromatography

Protein A affinity chromatography is a common method for process scale purification of monoclonal antibodies. During protein A affinity chromatography, protein A ligand co-elutes with the antibody (commonly called leaching), which is a potential disadvantage since the leached protein A may need to be cleared for pharmaceutical antibodies. To determine the mechanism of protein A leaching and characterize the leached protein A, we fluorescently labeled the protein A ligand in situ on protein A affinity chromatography media. We found that intact protein A leaches when loading either purified antibody or unpurified antibody in harvested cell culture fluid (HCCF), and that additionally fragments of protein A leach when loading HCCF. The leaching of protein A fragments can be reduced by EDTA, suggesting that proteinases contribute to the generation of protein A fragments. We found that protein A fragments larger than about 6000 Da can be measured by enzyme linked immunosorbent assay, and that they can be more difficult to clear than whole protein A by cation-exchange chromatography.

Recombinant therapeutic monoclonal antibodies: mechanisms of action in relation to structural and functional duality

Naked therapeutic recombinant monoclonal antibodies (mAbs) are bifunctional molecules. On the one hand, they recognize their antigen through the variable regions of the antigen binding portion (Fab). The recombinant mAb binding to a soluble or a membrane antigen may interfere with one or several functions of this antigen, leading to the therapeutic effect. On the other hand, since their crystalisable portion (Fc) is humanized (usually IgG1), they interact efficiently with human Fc-binding molecules, such as C1q and receptors for the Fc portion of IgG (FcgammaR). Thus, they initiate the classical pathway of complement and activate FcgammaR-expressing cells. The recruitment of these patient immune effector functions is essential in the therapeutic effect of several recombinant mAbs used in oncology. The aim of this review is to describe the main mechanisms of action of recombinant mAbs in relation to this structural and functional duality.

Evaluation of the off-label usage of rituximab in a large teaching hospital in New South Wales

A retrospective review of patients receiving rituximab off label in a large teaching hospital was conducted between July 2002 and January 2006. The indication, dosing regimen, efficacy and cost of rituximab were evaluated. Rituximab was prescribed for three clinical indications; acute organ transplant rejection, post-transplant lymphoproliferative disease and autoimmune disease. On average, 600 mg of rituximab was prescribed weekly for 4 weeks, costing the hospital $108,739.37. We suggest an initial approval for a limited number of doses with subsequent approval dependent on improvement in predefined clinical or biochemical end-points. Furthermore, we suggest an Australia-wide central database be established to enable delineation of the optimal dosing schedule, as well as monitoring of clinical outcome.

Cored anion-exchange chromatography media for antibody flow-through purification

Agarose-based anion-exchangers (e.g. quaternary amine, Q) have been widely used in monoclonal antibody flow-through purification to remove trace levels of impurities. Such media are often packed in a large column and the operation is usually robust but with limited throughput due to the compressibility of agarose and consequentially low bed permeability. In order to address this limitation, cored Q beads consisting of a rigid core and a thin agarose gel coating were developed and evaluated for protein flow-through chromatography. Using laboratory-scale columns it was found that, the cored beads indeed provide significantly enhanced rigidity and flow permeability relative to conventional homogeneous agarose resins. Depending on the structure and size of the cored beads, the permeability was 2-4-fold higher than that of a commonly used commercial agarose resin. Good virus and host cell protein clearance was achieved with the cored Q beads even at increased flow velocities. In addition, the impermeable core allows for more efficient use of buffers without loss of useful capacity in polishing applications. Process analyses based upon the experimental data demonstrated that the enhanced permeability achieved with the cored beads can significantly improve process throughput and economics.

Complement as effector system in cancer immunotherapy

The contribution of the complement system to the control of tumour growth has been neglected for a long time as the major emphasis has been put mainly on cell-mediated immune response against cancer. With the introduction of monoclonal antibodies in cancer immunotherapy complement has come into play with a great potential as effector system. Complement has a number of advantages over other effector systems in that it is made of molecules that can easily penetrate the tumour tissue and a large majority, if not all, of the components of this system can be supplied locally by many cells at tissue site. Further advances are being made to increase the anti-tumour efficiency of the complements system using C-fixing antibodies that are modified in the Fc portion to be more active in complement activation. Another strategy currently investigated is essentially based on the use of a combination of two antibodies directed against different molecules or different epitopes of the same molecule expressed on the cell surface in order to increase the number of the binding sites for the antibodies on the tumor cells and the chance for them to activate complement more efficiently. One of the problems to solve in exploiting complement as an effector system in cancer immunotherapy is to neutralize the inhibitory effect of complement regulatory proteins which are often over-expressed on tumour cells and represent a mechanism of evasion of these cells from complement attack. This situation can be overcome using neutralizing antibodies to target onto tumour cells together with the specific antibodies directed against tumor specific antigens. This is an area of active investigation and the initial data that start to be available from animal models seem to be promising.

Tumor specific phage particles promote tumor regression in a mouse melanoma model

Within cancer research, phage display libraries have been widely used for the identification of tumor targeting peptides and antibodies. Additionally, phages are known to be highly immunogenic; therefore we evaluated the immunotherapeutic potential of tumor specific phages to treat established solid tumors in a mouse model of melanoma. We developed two tumor specific phages, one derived from a peptide phage display library and one Fab expressing phage with known specificity, for the treatment of mice bearing palpable B16-F10 or B16/A2K(b) tumors. Therapy in B16-F10 tumor bearing mice with tumor specific phages was superior to treatment with non-tumor specific phages and lead to delayed tumor growth and increased survival. In B16/A2K(b )tumor bearing mice, therapy with tumor specific phages resulted in complete tumor regression and long-term survival in 50% of the mice. Histological analysis of tumors undergoing treatment with tumor specific phages revealed that phage administration induced a massive infiltration of polymorphonuclear neutrophils. Furthermore, phages induced secretion of IL-12 (p70) and IFN-gamma as measured in mouse splenocyte culture supernatants. These results demonstrate a novel, immunotherapeutic cancer treatment showing that tumor specific phages can promote regression of established tumors by recruitment of inflammatory cells and induction of Th1 cytokines.

What is the role of rituximab in the treatment of rheumatoid arthritis?

Rituximab is a monoclonal antibody against CD20 that was developed for the treatment of relapsed or refractory B-cell non-Hodgkin's lymphoma (NHL). Recent controlled trials have shown that B cell-targeted therapy with rituximab is effective in RA (which suggest that B lymphocytes may be critical in its pathogenesis of RA) and early exposure data suggest that the tolerability and safety profile of rituximab may be even better in RA than in NHL patients. Rituximab is generally well tolerated, with a low incidence of serious adverse events, including serious infections. Available evidence suggests that its clinical benefits depend on effective B cell depletion, and the fact that its novel mode of action leads to the depletion of B cells makes it distinct from other biological therapies for RA that target T cells and their related cytokines. Although complete peripheral B cell depletion is regularly seen in RA and other autoimmune diseases, especially systemic lupus erythematosus (SLE), incomplete depletion has been reported in a subset of patients, even after full dosing with rituximab.

Immunosuppressive preconditioning or induction regimens : evidence to date

The success of solid organ transplantation has been directly related to the development of immunosuppressive drug therapies. Preconditioning or induction therapy was developed to reduce early immunological and nonimmunological renal injury, with the goal of increasing long-term graft survival. However, the routine induction of immunological tolerance to solid organ allograft is currently not achievable because of the morbidity and mortality related to the immunosuppressive regimens themselves. The different therapeutic preconditioning or induction agents and their associated effects on cellular rejection, graft survival outcomes and the need for multiagent post-transplant maintenance therapy are reviewed.

Fcgamma receptor-dependent effector mechanisms regulate CD19 and CD20 antibody immunotherapies for B lymphocyte malignancies and autoimmunity

Immunotherapy using Rituximab, an unconjugated CD20 monoclonal antibody, has proven effective for treating non-Hodgkin's lymphoma and autoimmune disease. CD19 antibody immunotherapy is also effective in mouse models of lymphoma and autoimmunity. In both cases, mouse models have demonstrated that effector cell networks effectively deplete the vast majority of circulating and tissue B lymphocytes through Fcgamma receptor-dependent pathways. In mice, B cell depletion is predominantly, if not exclusively, mediated by monocytes. CD20 mAbs rapidly deplete circulating and tissue B cells in an antibody isotype-restricted manner with a hierarchy of antibody effectiveness: IgG2a/c > IgG1 > IgG2b >> IgG3. Depending on antibody isotype, mouse B cell depletion is regulated by FcgammaRI-, FcgammaRII-, FcgammaRIII-, and FcgammaRIV-dependent pathways. The potency of IgG2a/c mAbs for B cell depletion in vivo results from FcgammaRIV interactions, with likely contributions from high-affinity FcgammaRI. IgG1 mAbs induce B cell depletion through preferential, if not exclusive, interactions with low-affinity FcgammaRIII, while IgG2b mAbs interact preferentially with intermediate-affinity FcgammaRIV. By contrast, inhibitory FcgammaRIIB-deficiency significantly increases CD20 mAb-induced B cell depletion at low mAb doses by enhancing monocyte function. Thus, isotype-specific mAb interactions with distinct FcgammaRs contribute significantly to the effectiveness of CD20 mAbs in vivo. These results provide a molecular basis for earlier observations that human FcgammaRII and FcgammaRIII polymorphisms correlate with the in vivo effectiveness of CD20 antibody therapy. That the innate monocyte network depletes B cells through FcgammaR-dependent pathways during immunotherapy has important clinical implications for CD19, CD20, and other antibody-based therapies for the treatment of diverse B cell malignancies and autoimmune disease.

The Fc Portion of UV3, an Anti-CD54 Monoclonal Antibody, Is Critical for its Antitumor Activity in SCID Mice With Human Multiple Myeloma or Lymphoma Cell Lines

UV3 is a monoclonal antibody that recognizes human CD54 (intercellular adhesion molecule-1), and it was generated for the therapy of human multiple myeloma. In a severe combined immunodeficient (SCID) xenograft model of human multiple myeloma, UV3 significantly prolonged the survival of mice with either early or advanced stages of disease. However, the mechanism by which UV3 exerted its antitumor effect remained unknown. As reported previously UV3 could mediate antibody-dependent cell-mediated cytotoxicity or complement-dependent cytotoxicity in vitro. F(ab)'2 fragments of UV3 had therapeutic efficacy in vivo, suggesting that effector functions were not critical. The purpose of this study was to further define the importance of the Fc portion of UV3 for its antitumor activity in vivo. To this end, we examined the effect of an "ultrapure" preparation of UV3 F(ab)'2 to treat SCID mice xenografted with either ARH-77 cells, a human multiple myeloma cell line, or Daudi cells, a human Burkitt's lymphoma cell line. In addition, we evaluated different doses of UV3 immunoglobulin G (IgG) in these mice to determine the minimum amount of IgG that would produce a therapeutic effect. Data obtained from this study suggest that (1) the Fc portion of UV3 is critical for its antitumor activity in vivo, (2) low levels of UV3 IgG in a preparation of F(ab)'2 fragments account for all of its in vivo activity in multiple myeloma and most of its activity in lymphoma, and (3) UV3 IgG significantly prolongs the survival of SCID/ARH-77 mice as well as SCID/Daudi mice.

Modulation of the effector functions of a human IgG1 through engineering of its hinge region

We report here the engineering of a humanized anti-human EphA2 mAb (mAb 12G3H11) in an effort to explore the relationship between the hinge of a human IgG1 and its effector functions. mAb 12G3H11, used here as a model, is directed against the human receptor tyrosine kinase EphA2, which is an actively investigated target for cancer therapy due to its up-regulation in many cancer cells. Various rational modifications were introduced into the hinge region of mAb 12G3H11. These mutations were predicted to modulate the hinge's length, flexibility, and/or biochemical properties. We show that the upper and middle hinge both play important, although functionally distinct roles. In particular, middle hinge modifications predicted to decrease its rigidity or length as well as eliminating either one of its two cysteine residues had a strong negative impact on C1q binding and complement-dependent cytotoxicity. Disruption of covalent bonds between both H chains may account in part for these effects. We also describe middle hinge mutants with a significantly decreased ability to bind FcgammaRIIIA and trigger Ab-dependent cell-mediated cytotoxicity. Conversely, we also generated upper hinge mutants exhibiting an increase in C1q binding and complement-dependent cytotoxicity activity. Therefore, this approach represents a novel strategy to fine-tune the biological activity of a given human IgG1. We also define, for the first time in such a systematic fashion, the relationship between various characteristics of the middle and upper hinge and the corresponding effector functions.

Engineering of therapeutic antibodies to minimize immunogenicity and optimize function

One of the first difficulties in developing monoclonal antibody therapeutics was the recognition that human anti-mouse antibody (HAMA) response limited the administration of murine antibodies. Creative science has lead to a number of ways to counter the immunogenicity of non-human antibodies, primarily through chimeric, humanized, de-immunized, and most recently, human-sequence therapeutic antibodies. Once therapeutic antibodies of low or no immunogenicity were available, the creativity then turned to engineering both the antigen-binding domains (e.g., affinity maturation, stability) and altering the effector functions (e.g. antibody-dependent cellular cytotoxicity, complement-dependent cellular cytotoxicity, and clearance rate).

The efficacy and safety of rituximab in patients with active rheumatoid arthritis despite methotrexate treatment: results of a phase IIB randomized, double-blind, placebo-controlled, dose-ranging trial

OBJECTIVE

To examine the efficacy and safety of different rituximab doses plus methotrexate (MTX), with or without glucocorticoids, in patients with active rheumatoid arthritis (RA) resistant to disease-modifying antirheumatic drugs (DMARDs), including biologic agents.

METHODS

A total of 465 patients were randomized into 9 treatment groups: 3 rituximab groups (placebo [n = 149], 500 mg [n = 124], or 1,000 mg [n = 192] on days 1 and 15) each also taking either placebo glucocorticoids, intravenous methylprednisolone premedication, or intravenous methylprednisolone premedication plus oral prednisone for 2 weeks. All patients received MTX (10-25 mg/week); no other DMARDs were permitted.

RESULTS

Significantly more patients who received 2 500-mg or 2 1,000-mg infusions of rituximab met the American College of Rheumatology 20% improvement criteria (achieved an ACR20 response) at week 24 (55% and 54%, respectively) compared with placebo (28%; P < 0.0001). ACR50 responses were achieved by 33%, 34%, and 13% of patients, respectively (P < 0.001), and ACR70 responses were achieved by 13%, 20%, and 5% of patients (P < 0.05). Changes in the Disease Activity Score in 28 joints (-1.79, -2.05, -0.67; P < 0.0001) and moderate to good responses on the European League Against Rheumatism criteria (P < 0.0001) reflected the ACR criteria responses. Glucocorticoids did not contribute significantly to the primary efficacy end point, ACR20 response at 24 weeks. Intravenous glucocorticoid premedication reduced the frequency and intensity of first infusion-associated events; oral glucocorticoids conferred no additional safety benefit. Rituximab was well tolerated; the type and severity of infections was similar to those for placebo.

CONCLUSION

Both rituximab doses were effective and well tolerated when added to MTX therapy in patients with active RA. The primary end point (ACR20 response) was independent of glucocorticoids, although intravenous glucocorticoid premedication improved tolerability during the first rituximab infusion.

Properties of human IgG1s engineered for enhanced binding to the neonatal Fc receptor (FcRn)

We describe here the functional implications of an increase in IgG binding to the neonatal Fc receptor. We have defined in a systematic fashion the relationship between enhanced FcRn binding of a humanized anti-respiratory syncytial virus (RSV) monoclonal antibody (MEDI-524) and the corresponding biological consequences in cynomolgus monkeys. The triple mutation M252Y/S254T/T256E (YTE) was introduced into the Fc portion of MEDI-524. Whereas these substitutions did not affect the ability of MEDI-524 to bind to its cognate antigen and inhibit RSV replication, they resulted in a 10-fold increase in its binding to both cynomolgus monkey and human FcRn at pH 6.0. MEDI-524-YTE was efficiently released from FcRn at pH 7.4 in both cases. We show that MEDI-524-YTE consistently exhibited a nearly 4-fold increase in serum half-life in cynomolgus monkeys when compared with MEDI-524. This constituted the largest half-life improvement described to date for an IgG in a primate. For the first time, we demonstrate that these sustained serum levels resulted in an up to 4-fold increase in lung bioavailability. Importantly, we also establish that our non-human primate model is relevant to human. Finally, we report that the YTE triple substitution provided a means to modulate the antibody-dependent cell-mediated cytotoxicity (ADCC) activity of a humanized IgG1 directed against the human integrin alpha(v)beta3. Therefore, the YTE substitutions allow the simultaneous modulation of serum half-life, tissue distribution and activity of a given human IgG1.

Advances in the treatment of small vessel vasculitis

The systemic small vessel vasculitides encompass Wegener's granulomatosis, microscopic polyangiitis, and Churg-Strauss syndrome. The traditional approach to therapy, which incorporates cytotoxic drugs and glucocorticoids, is associated with a significant risk of relapse and treatment-related toxicity. The formation of multicenter collaborative groups has facilitated the execution of randomized controlled trials, and has led to the identification of several effective therapeutic options. This article reviews the existing data and refers to studies in progress that may answer some of the questions that remain regarding the treatment of the small vessel vasculitides.

MONOCLONAL ANTIBODIES AS TARGETING AND THERAPEUTIC AGENTS: PROSPECTS FOR LIVER TRANSPLANTATION, HEPATITIS AND HEPATOCELLULAR CARCINOMA

1. Monoclonal antibodies (mAbs) of high specificity and stability have become key resources in the therapeutic, diagnostic and drug discovery fields to treat various immunological disorders and malignancies of different organs. 2. The latest genetic engineering technology applied in antibody design and production, such as phage display technology and genetically modified mouse, have revolutionized the clinical applicability and feasibility of the use of mAbs in humans. 3. Innovative antibody products in the forms of single-chain or super-humanized antibody therapeutics having a higher affinity for target antigens and minimal antigenicity in hosts have been introduced for experimental purposes and/or clinical trials. 4. Although there are successful examples of antibody therapeutics in the market, the use of mAbs in treating hepatitis-related disease and hepatocellular carcinoma is rare and remains to be exploited.

Antibody isotype-specific engagement of Fcgamma receptors regulates B lymphocyte depletion during CD20 immunotherapy

CD20 monoclonal antibody (mAb) immunotherapy is effective for lymphoma and autoimmune disease. In a mouse model of immunotherapy using mouse anti–mouse CD20 mAbs, the innate monocyte network depletes B cells through immunoglobulin (Ig)G Fc receptor (FcγR)-dependent pathways with a hierarchy of IgG2a/c>IgG1/IgG2b>IgG3. To understand the molecular basis for these CD20 mAb subclass differences, B cell depletion was assessed in mice deficient or blocked for stimulatory FcγRI, FcγRIII, FcγRIV, or FcR common γ chain, or inhibitory FcγRIIB. IgG1 CD20 mAbs induced B cell depletion through preferential, if not exclusive, interactions with low-affinity FcγRIII. IgG2b CD20 mAbs interacted preferentially with intermediate affinity FcγRIV. The potency of IgG2a/c CD20 mAbs resulted from FcγRIV interactions, with potential contributions from high-affinity FcγRI. Regardless, FcγRIV could mediate IgG2a/b/c CD20 mAb–induced depletion in the absence of FcγRI and FcγRIII. In contrast, inhibitory FcγRIIB deficiency significantly increased CD20 mAb–induced B cell depletion by enhancing monocyte function. Although FcγR-dependent pathways regulated B cell depletion from lymphoid tissues, both FcγR-dependent and -independent pathways contributed to mature bone marrow and circulating B cell clearance by CD20 mAbs. Thus, isotype-specific mAb interactions with distinct FcγRs contribute significantly to the effectiveness of CD20 mAbs in vivo, which may have important clinical implications for CD20 and other mAb-based therapies.

Rituximab for rheumatoid arthritis refractory to anti–tumor necrosis factor therapy: Results of a multicenter, randomized, double-blind, placebo-controlled, phase III trial evaluating primary efficacy and safety at twenty-four weeks

OBJECTIVE

To determine the efficacy and safety of treatment with rituximab plus methotrexate (MTX) in patients with active rheumatoid arthritis (RA) who had an inadequate response to anti-tumor necrosis factor (anti-TNF) therapies and to explore the pharmacokinetics and pharmacodynamics of rituximab in this population.

METHODS

We evaluated primary efficacy and safety at 24 weeks in patients enrolled in the Randomized Evaluation of Long-Term Efficacy of Rituximab in RA (REFLEX) Trial, a 2-year, multicenter, randomized, double-blind, placebo-controlled, phase III study of rituximab therapy. Patients with active RA and an inadequate response to 1 or more anti-TNF agents were randomized to receive intravenous rituximab (1 course, consisting of 2 infusions of 1,000 mg each) or placebo, both with background MTX. The primary efficacy end point was a response on the American College of Rheumatology 20% improvement criteria (ACR20) at 24 weeks. Secondary end points were responses on the ACR50 and ACR70 improvement criteria, the Disease Activity Score in 28 joints, and the European League against Rheumatism (EULAR) response criteria at 24 weeks. Additional end points included scores on the Functional Assessment of Chronic Illness Therapy-Fatigue (FACIT-F), Health Assessment Questionnaire (HAQ) Disability Index (DI), and Short Form 36 (SF-36) instruments, as well as Genant-modified Sharp radiographic scores at 24 weeks.

RESULTS

Patients assigned to placebo (n = 209) and rituximab (n = 311) had active, longstanding RA. At week 24, significantly more (P < 0.0001) rituximab-treated patients than placebo-treated patients demonstrated ACR20 (51% versus 18%), ACR50 (27% versus 5%), and ACR70 (12% versus 1%) responses and moderate-to-good EULAR responses (65% versus 22%). All ACR response parameters were significantly improved in rituximab-treated patients, who also had clinically meaningful improvements in fatigue, disability, and health-related quality of life (demonstrated by FACIT-F, HAQ DI, and SF-36 scores, respectively) and showed a trend toward less progression in radiographic end points. Rituximab depleted peripheral CD20+ B cells, but the mean immunoglobulin levels (IgG, IgM, and IgA) remained within normal ranges. Most adverse events occurred with the first rituximab infusion and were of mild-to-moderate severity. The rate of serious infections was 5.2 per 100 patient-years in the rituximab group and 3.7 per 100 patient-years in the placebo group.

CONCLUSION

At 24 weeks, a single course of rituximab with concomitant MTX therapy provided significant and clinically meaningful improvements in disease activity in patients with active, longstanding RA who had an inadequate response to 1 or more anti-TNF therapies.

Designing antibodies for oncology

The past five years have witnessed the emergence of monoclonal antibodies as important therapeutics for cancer treatment. Lower toxicity for antibodies versus small molecules, the potential for increased efficacy by conjugation to radioisotopes and cellular toxins, or the ability to exploit immune cell functions have led to clinical performances on par or superior to conventional drug therapies. This review outlines the various immunoglobulin design strategies currently available, techniques used to reduce Ig antigenicity and toxicity, and points to consider during the manufacture of antibodies for use in clinical oncology.

In vivo molecular targeted radiotherapy

Unsealed radionuclides have been in clinical therapeutic use for well over half a century. Following the early inappropriate clinical administrations of radium salts in the early 20th century, the first real clinical benefits became evident with the use of ¹³¹I-sodium iodide for the treatment of hypothyroidism and differentiated thyroid carcinoma and ³²P-sodium phosphate for the treatment of polycythaemia vera. In recent years the use of bone seeking agents ⁸⁹Sr, ¹⁵³Sm and ¹⁸⁶Re for the palliation of bone pain have become widespread and considerable progress has been evident with the use of ¹³¹I-MIBG and ⁹⁰Y-somatostatin receptor binding agents. Although the use of monoclonal antibody based therapeutic products has been slow to evolve, the start of the 21st century has witnessed the first licensed therapeutic antibody conjugates based on ⁹⁰Y and ¹³¹I for the treatment of non-Hodgkin's lymphoma. The future clinical utility of this form of therapy will depend upon the development of radiopharmaceutical conjugates capable of selective binding to molecular targets. The availability of some therapeutic radionuclides such as ¹⁸⁸Re produced from the tungsten generator system which can produce activity as required over many months, may make this type of therapy more widely available in some remote and developing countries.

Future products will involve cytotoxic radionuclides with appropriate potency, but with physical characteristics that will enable the administration of therapeutic doses with the minimal need for patient isolation. Further developments are likely to involve molecular constructs such as aptamers arising from new developments in biotechnology.

Patient trials are still underway and are now examining new methods of administration, dose fractionation and the clinical introduction of alpha emitting radiopharmaceutical conjugates. This review outlines the history, development and future potential of these forms of therapy.

Cholesterol depletion inhibits src family kinase-dependent calcium mobilization and apoptosis induced by rituximab crosslinking

The monoclonal antibody (mAb) rituximab produces objective clinical responses in patients with B-cell non-Hodgkin's lymphoma and antibody-based autoimmune diseases. Mechanisms mediating B-cell depletion by rituximab are not completely understood and may include direct effects of signalling via the target antigen CD20. Like most but not all CD20 mAbs, rituximab induces a sharp change in the solubility of the CD20 protein in the non-ionic detergent Triton-X-100, reflecting a dramatic increase in the innate affinity of CD20 for membrane raft signalling domains. Apoptosis induced by rituximab hypercrosslinking has been shown to require src family kinases (SFK), which are enriched in rafts. In this report we provide experimental evidence that SFK-dependent apoptotic signals induced by rituximab are raft dependent. Cholesterol depletion prevented the association of hypercrosslinked CD20 with detergent-insoluble rafts, and attenuated both calcium mobilization and apoptosis induced with rituximab. CD20 cocapped with the raft-associated transmembrane adaptor LAB/NTAL after hypercrosslinking with CD20 mAbs, regardless of their ability to induce a change in the affinity of CD20 for rafts. Taken together, the data demonstrate that CD20 hypercrosslinking via rituximab activates SFKs and downstream signalling events by clustering membrane rafts in which antibody-bound CD20 is localized in a high-affinity configuration.

Systemic eight-cycle anti-CD20 monoclonal antibody (rituximab) therapy in primary cutaneous B-cell lymphomas - an applicational observation

BACKGROUND

Primary cutaneous B-cell lymphomas (PCBCLs) are characterized by restriction to the skin and a variable but mostly favourable prognosis. Since 1997 the recombinant, chimeric anti-CD20 antibody rituximab has been used in patients suffering from non-Hodgkin's B-cell lymphomas. Different studies have shown that the effectiveness and safety in the treatment of patients with low-grade follicular lymphoma is comparable to or even higher than the standard CHOP chemotherapy. So far it has been unclear whether an extended duration of therapy leads to a benefit for the patients with PCBCL.

OBJECTIVES

To evaluate the objective response rate, time to progression, remission quality and histological changes and to compare our data with the literature.

PATIENTS/METHODS

Ten patients with PCBCL [eight with follicle centre cell lymphoma (FCCL), one with marginal zone lymphoma (MZL) and one with diffuse large B-cell lymphoma of the leg (DLBCL)] were treated by intravenous application of a chimeric antibody against the CD20 transmembrane antigen (rituximab) with a dosage of eight cycles, 375 mg m(-2) body surface, weekly.

RESULTS

The treatment regimen resulted in clinical overall response in 9 of 10 patients, in particular there were seven complete responses (70%) plus two partial responses (20%). The median duration of remission (durable remission, DR) is 23 months (4-30 months) to date. Histological assessment of responses in four patients showed no tumour-specific infiltration. In two patients histology revealed a residual infiltration and in one patient an increasing infiltration. In two patients no histology was taken after treatment; one patient developed a new lesion. No severe side-effects occurred. Observed side-effects were two bacterial infections, two patients with shivering during infusion, one patient with sweating for months and one patient with persisting itching. As expected the B-cell count in peripheral blood was depressed in all patients after infusion.

CONCLUSIONS

Intravenous therapy with eight cycles of the anti-CD20 antibody rituximab is a non-toxic and effective treatment for a subset of patients with PCBCL (relapsed, aggressive entity, old patients, multiple lesions) with a long DR.

Introduction of combined CHOP plus rituximab therapy dramatically improved outcome of diffuse large B-cell lymphoma in British Columbia

PURPOSE

For more than two decades, cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) has been the standard therapy for diffuse large B-cell lymphoma (DLBCL). The addition of rituximab to CHOP has been shown to improve outcome in elderly patients with DLBCL. We conducted a population-based analysis to assess the impact of this combination therapy on adult patients with DLBCL in the province of British Columbia (BC).

METHODS

We compared outcomes during a 3-year period; 18 months before (prerituximab) and 18 months after (postrituximab) institution of a policy recommending the combination of CHOP and rituximab for all patients with newly diagnosed advanced-stage (stage III or IV or stage I or II with "B" symptoms or bulky [> 10 cm] disease) DLBCL.

RESULTS

A total of 292 patients were evaluated; 140 in the prerituximab group (median follow-up, 42 months) and 152 in the postrituximab group (median follow-up, 24 months). Both progression-free survival (risk ratio, 0.56; 95% CI, 0.39 to 0.81; P = .002) and overall survival (risk ratio, 0.40; 95% CI, 0.27 to 0.61, P < .0001) were significantly improved in the postrituximab group. After controlling for age and International Prognostic Index score, era of treatment remained a strong independent predictor of progression-free survival (risk ratio, 0.59; 95% CI, 0.41 to 0.85; P = .005) and overall survival (risk ratio, 0.43; 95% CI, 0.29 to 0.66; P < .001). The benefit of treatment in the postrituximab era was present regardless of age.

CONCLUSION

The addition of rituximab to CHOP chemotherapy has resulted in a dramatic improvement in outcome for DLBCL patients of all ages in the province of BC.

Successful treatment of mediastinal lymphomatoid granulomatosis with rituximab monotherapy

Lymphomatoid granulomatosis is a rare Epstein-Barr virus (EBV)-positive-B-cell lymphoproliferative disorder. Treatment options include corticosteroids, antiviral therapy, interferon-alpha and chemotherapy. However, long-term prognosis is poor and no therapeutic standard has been established yet. In a 21-year-old woman, a biopsy of mediastinal mass revealed lymphomatoid granulomatosis. Combined therapy with valganciclovir and interferon-alpha proved ineffective. In view of the CD20 expression of the tumor cells, monotherapy with rituximab was intiated. After 3 months a complete remission was achieved. Rituximab was continued for another 6 months with subsequent consolidation radiotherapy. This is the first report of an enduring complete remission (20 months) of a non-CNS lymphomatoid granulomatosis treated with rituximab.

Concepts in radiotherapy and immunotherapy: Anti-CD20 mechanisms of action and targets

Immunotherapy became a feasible therapeutic approach following the development of monoclonal antibody technology. Despite many small clinical trials using a wide variety of antibodies, in hematologic malignancies, success has largely been restricted to a few antibodies generated against antigens expressed on the tumor cell surface: rituximab (anti-CD20) and alemtuzumab (anti-CD52) are the most widely used monoclonal antibodies. CD20 is expressed on B cells, and rituximab has been widely used in the treatment of various histologies of B-cell non-Hodgkin's lymphoma. The mode of action of rituximab is discussed in this article. Despite extensive empiric clinical trial experience, the critical factors important in the mechanism of tumor cell kill by monoclonal antibodies continue to be elusive and the subject of debate. The major immune mechanisms of action of rituximab include complement-dependent cytotoxicity and antibody-dependent cellular cytotoxicity. While some investigations have suggested that the complement system is essential to tumor cell kill with rituximab; most lines of evidence point to the importance of antibody-dependent cellular cytotoxicity. Fc receptor binding appears to be critical in determining efficacy. Observations from several studies have shown that the response rate to single-agent rituximab is better in patients who have higher affinity polymorphisms in their Fc receptors. The other mechanisms that may play a role in rituximab therapy include direct anti-tumor effects mediated by the antibody binding to cell-surface CD20 antigen. Although this is more controversial, observations in tumor cell lines following CD20 ligation suggest that direct effects may be important.

The Evolution of Antibodies into Versatile Tumor-Targeting Agents

In recent years, monoclonal antibodies have become important weapons in the arsenal of anticancer drugs, and in select cases are now the drugs of choice due to their favorable toxicity profiles. Originally developed to confer passive immunity against tumor-specific antigens, clinical uses of monoclonal antibodies are expanding to include growth factor sequestration, signal transduction modulation, and tumor-specific drug delivery. In this review, we shall present the origins of antibody therapeutics within the field of immunotherapy and their evolution into effective anticancer agents, then discuss their multiple mechanisms of action, the basis of their tumor selectivity, and their therapeutic properties compared with traditional therapies. Antibodies are complex molecules whose efficacy and toxicity depend on the antigen, the antibody, any conjugated groups, and even the patient. Finally, we shall present new technologies being developed to increase the efficacy and selectivity of antibody-based therapeutics. Interestingly, many of the new approaches straddle the middle ground between immunotherapy and the traditional modalities of chemotherapy and radiotherapy, and can be seen as ways of combining the selectivity of the former with the efficacy of the latter.

From the bench to the bedside: ways to improve rituximab efficacy

Rituximab (MabThera, Rituxan) is a chimeric IgG1 monoclonal antibody that specifically targets the CD20 surface antigen expressed on normal and neoplastic B-lymphoid cells. Rituximab is currently used in the treatment of both follicular and aggressive B-cell non-Hodgkin lymphomas. Despite its demonstrated clinical effectiveness, its in vivo mechanisms of action remain unknown and could differ by subtype of lymphoma. Rituximab has been shown to induce apoptosis, complement-mediated lysis, and antibody-dependent cellular cytotoxicity in vitro, and some evidence points toward an involvement of these mechanisms in vivo. Rituximab also has a delayed therapeutic effect as well as a potential “vaccinal” effect. Here, we review the current understanding of the mechanism of action of rituximab and discuss approaches that could increase its clinical activity. A better understanding of how rituximab acts in vivo should make it possible to develop new and more effective therapeutic strategies. (Blood. 2004;104:2635-2642)

Treatment of refractory autoimmune diseases with ablative immunotherapy

Immunological manipulations are the basis of modern therapy for refractory autoimmune diseases (AID). Ablative chemotherapy with stem cell support (autotransplant) as well as targeted immunotherapy using specific monoclonal antibodies, such as rituximab and campath 1-H have become acceptable second line therapy for severe refractory AID. Until now, more than 500 autotransplants have been performed worldwide for various autoimmune disorders, including multiple sclerosis (MS), systemic sclerosis (SSc), systemic lupus erythematosis (SLE) and rheumatoid arthritis (RA) with encouraging results, although transplant related mortality (TRM) in the range between 2 and 17% still remains one of the major limitations of the procedure. Immunotherapy is a relatively safe approach associating with sustained remissions in a considerable proportion of treated patients. Better selection of patients and earlier immunotherapy, preceded an irreversible organ damage might further improve the clinical outcome of patients with AID.

The innate mononuclear phagocyte network depletes B lymphocytes through Fc receptor-dependent mechanisms during anti-CD20 antibody immunotherapy

Anti-CD20 antibody immunotherapy effectively treats non-Hodgkin's lymphoma and autoimmune disease. However, the cellular and molecular pathways for B cell depletion remain undefined because human mechanistic studies are limited. Proposed mechanisms include antibody-, effector cell-, and complement-dependent cytotoxicity, the disruption of CD20 signaling pathways, and the induction of apoptosis. To identify the mechanisms for B cell depletion in vivo, a new mouse model for anti-CD20 immunotherapy was developed using a panel of twelve mouse anti-mouse CD20 monoclonal antibodies representing all four immunoglobulin G isotypes. Anti-CD20 antibodies rapidly depleted the vast majority of circulating and tissue B cells in an isotype-restricted manner that was completely dependent on effector cell Fc receptor expression. B cell depletion used both FcgammaRI- and FcgammaRIII-dependent pathways, whereas B cells were not eliminated in FcR common gamma chain-deficient mice. Monocytes were the dominant effector cells for B cell depletion, with no demonstrable role for T or natural killer cells. Although most anti-CD20 antibodies activated complement in vitro, B cell depletion was completely effective in mice with genetic deficiencies in C3, C4, or C1q complement components. That the innate monocyte network depletes B cells through FcgammaR-dependent pathways during anti-CD20 immunotherapy has important clinical implications for anti-CD20 and other antibody-based therapies.

Rituximab in autoimmune diseases

Modern treatments of autoimmune diseases are based on immunological therapies. Rituximab induces a targeted B-cell depletion in the aim of eradicating autoreactive clones in various autoimmune disorders. Several studies are being undertaken and preliminary reports are very encouraging. The mechanism of action is not evident, but appears to be connected with the lowering of autoantibody levels, in the diseases where relevant antibody titres are relievable. Most of the patients treated were affected by idiopathic thrombocytopenic purpura (ITP) and autoimmune haemolytic anaemia, but also very rare diseases like acquired haemophilia are reported. Best results are described in autoimmune haemolytic anaemia, in many others there is clear evidence for efficacy; in all the diseases the number of complete or partial remission, though temporary, is much greater than 50%. Side effects are rarely reported, and immunosuppression is not a great problem. The persistence of clinical improvement for more than 1 year after B-lymphocyte repopulation supports the hypothesis of a stochastic generation of pathogenic B-cell subsets. Other studies and controlled trials are required to establish when and which patients are to be treated, and find the opportunity of the association of others drugs.

Phase I studies of interleukin (IL)-2 and rituximab in B-cell non-hodgkin's lymphoma: IL-2 mediated natural killer cell expansion correlations with clinical response

PURPOSE

Expansion and activation of natural killer (NK) cells with interleukin-2 (IL-2) may enhance antibody-dependent cellular cytotoxicity (ADCC), an important mechanism of rituximab activity. Two parallel Phase I studies evaluated combination therapy with rituximab and IL-2 in relapsed or refractory B-cell non-Hodgkin's lymphoma (NHL).

EXPERIMENTAL DESIGN

Thirty-four patients with advanced NHL received rituximab (375 mg/m(2) i.v. weekly, weeks 1-4) and escalating doses of s.c. IL-2 [2-7.5 MIU daily (n = 19) or 4.5-14 million international units three times weekly (n = 15), weeks 2-5]. Safety, tolerability, clinical responses, NK cell counts, and ADCC activity were evaluated.

RESULTS

Maximally tolerated doses (MTD) of IL-2 were 6 MIU daily and 14 million international units thrice weekly. The most common adverse events were fever, chills, and injection site reactions. Dose-limiting toxicities were fatigue and reversible liver enzyme test elevations. Of the 9 patients enrolled at the daily schedule MTD, 5 showed clinical response. On the thrice-weekly schedule at the MTD, 4 of 5 patients responded. Responders showed median time to progression of 14.9 and 16.1 months, respectively, for the two studies. For the same total weekly dose, thrice-weekly IL-2 administration induced greater increases in NK cell counts than daily dosing, and NK cells correlated with clinical response on the thrice-weekly regimen. ADCC activity was increased and maintained after IL-2 therapy in responding and stable disease patients.

CONCLUSIONS

Addition of IL-2 to rituximab therapy is safe and, using thrice-weekly IL-2 dosing, results in NK cell expansion that correlates with response. This combination treatment regimen merits additional evaluation in a randomized clinical trial.

CD64 Surface Expression on Neutrophils and Monocytes Is Significantly Up-Regulated after Stimulation with Granulocyte Colony-Stimulating Factor during CHOP Chemotherapy for Patients with Non-Hodgkin’s Lymphoma

The present study was performed to examine whether the expression of CD64 Fc gamma receptor type I (FcgammaRI) on both neutrophils and monocytes can be modulated by multiple daily administrations of granulocyte colony-stimulating factor (G-CSF) to patients with non-Hodgkin's lymphoma in neutropenia caused by CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisone) chemotherapy. The expression of CD64 was determined by flow cytometric analysis at the following time points: before chemotherapy, at the nadir of the neutrophil count, at the fifth day after the start of G-CSF administration, and at more than 8 days after the start of G-CSF administration. CD64 expression was enhanced in patients given G-CSF during CHOP treatment, whereas CD64 expression remained unchanged in patients not given G-CSF CD64 expression levels on both neutrophils and monocytes were significantly up-regulated by the daily administration of G-CSF and reached peak levels at day 5 (P = .0007). Thereafter, expression on both cell types remained at almost the same levels as on day 5 for the rest of the treatment course, even though G-CSF therapy continued for 3 to 5 more days. Interestingly, CD64 expression on monocytes was already increased significantly (P = .0001) at the nadir of the neutrophil count relative to the baseline before chemotherapy and then was additionally up-regulated by day 5 after the start of G-CSF injections (P = .019). In antibody-dependent cellular cytotoxicity assays, we found that rituximab-mediated cell lysis was significantly enhanced at day 5 after the start of G-CSF treatment (P = .01). In conclusion, this study shows that multiple doses of G-CSF administered to lymphoma patients with neutropenia due to CHOP chemotherapy can enhance CD64 expression on both neutrophils and monocytes. Peak CD64 levels are reached at day 5 of G-CSF treatment, resulting in an activation of the rituximab-mediated antitumor ability of these effector cells. This finding may be useful in determining the optimal timing of administration for an antibody such as rituximab in a chemotherapeutic strategy designed to exert a maximal effect against tumor cells.

Factorial screening of antibody purification processes using three chromatography steps without protein A

Protein A affinity chromatography is often employed as a capture step to meet the purity, yield, and throughput requirements for pharmaceutical antibody purification. However, a trade-off exists between step performance and price. Protein A resin removes 99.9% of feed stream impurities; however, its price is significantly greater than those of non-affinity media. With many therapeutic indications for antibodies requiring high doses and/or chronic administration, the consideration of process economics is critical. We have systematically evaluated the purification performance of cation-exchange, anion-exchange, hydroxyapatite, hydrophobic interaction, hydrophobic charge induction, and small-molecule ligand resins in each step of a three-step chromatographic purification process for a CHO-derived monoclonal antibody. Host cell proteins were removed to less-than-detectable for three processes (cation-exchange-anion-exchange-hydrophobic interaction chromatography, cation-exchange-anion-exchange-mixed cation-exchange chromatography, and cation-exchange-mixed cation-exchange-anion-exchange chromatography). The order of the process steps affected purification performance significantly.

Two immunoglobulin G fragment C receptor polymorphisms independently predict response to rituximab in patients with follicular lymphoma

PURPOSE

Although rituximab is now routinely used in the treatment of B-cell non-Hodgkin's lymphoma, the mechanism of its antitumor effect is not clear. One potential mechanism of action involves antibody-dependent cellular cytotoxicity (ADCC). Two aspects of ADCC influence the effectiveness of this process: the susceptibility of tumor cells and the activation of effector cells via their immunoglobulin G fragment C receptors (Fc gamma Rs). Several Fc gamma R polymorphisms have been identified that may affect the killing function of natural killer cells and macrophages.

PATIENTS AND METHODS

The pretreatment tumor cells from 43 patients with follicular lymphoma were tested for their intrinsic susceptibility to rituximab-mediated ADCC. In addition, the Fc gamma RIIIa (CD16) and Fc gamma RIIa (CD32) polymorphisms were determined in an expanded group of 87 patients. The results were then correlated with clinical outcome of these patients.

RESULTS

No difference was found between the susceptibility of tumors from patients who clinically responded to rituximab versus those who did not respond. Conversely, both the Fc gamma RIIIa 158 valine/valine and the Fc gamma RIIa 131 histidine/histidine genotypes were found to be independently associated with the response rate and freedom from progression.

CONCLUSION

These data support the hypothesis that ADCC plays an important role in the clinical effect of rituximab at the level of the effector cell. It will be important to include information on Fc receptor polymorphisms in future trials of rituximab therapy.

Antibody specificity controls in vivo effector mechanisms of anti-CD20 reagents

Despite the success of anti-CD20 monoclonal antibody (mAb) in the treatment of lymphoma, there remains considerable uncertainty about their mechanism(s) of action. Here, we show that certain of these reagents (rituximab and 1F5), which redistribute CD20 into membrane rafts, are bound efficiently by C1q, deposit C3b, and result in complement-dependent cytotoxicity (CDC). This activity is important in vivo, because complement depletion using cobra venom factor (CVF) markedly reduced the efficacy of rituximab and 1F5 in 2 lymphoma xenograft models. However, complement depletion had no effect on the potent therapeutic activity of B1, a mAb that does not redistribute CD20 into membrane rafts, bind C1q, or cause efficient CDC. Equivalent immunotherapy also occurred in the presence or absence of natural killer (NK) cells. Perhaps most surprising was the observation that F(ab')2 fragments of B1 but not 1F5 were able to provide substantial immunotherapy, indicating that non-Fc-dependent mechanisms are involved with B1. In accordance with this, B1 was shown to induce much higher levels of apoptosis than rituximab and 1F5. Thus, although complement is important for the action of rituximab and 1F5, this is not so for B1, which more likely functions through its ability to signal apoptosis.

Mechanism of action and resistance to monoclonal antibody therapy

Monoclonal antibodies (MoAbs) are increasingly used in the treatment of patients with hematological malignancies and autoimmune diseases. The most commonly employed humanized and chimeric MoAbs are rituximab, alemtuzumab (Campath-1H, Ilex Pharmaceuticals, San Antonio, TX), and gemtuzumab-ozogamicin (Mylotarg, Wyeth-Ayerst Laboratories, St Davids, PA). The mechanism of action of these antibodies, and host and cellular factors influencing the response, are not completely known. Induction of apoptosis, antibody-dependent cell cytotoxicity (ADCC), and complement-mediated cell death (CDC) is the proposed mechanism of action of these antibodies. We review the current understanding of the mechanism of action of and resistance to these MoAbs.

Complement activation determines the therapeutic activity of rituximab in vivo

Rituximab is an anti-CD20 chimeric mAb effective for the treatment of B-NHL. It can lyse lymphoma cells in vitro through both C- and Ab-dependent cellular cytotoxicity. The mechanism of action of rituximab in vivo is however still unclear. We have set up a new in vivo model in nonimmunodeficient mice by stable transduction of the human CD20 cDNA in the murine lymphoma line EL4. Animals injected i.v. with the EL4-CD20(+) lymphoma cells died within 30 days with evident liver, spleen, and bone marrow involvement, confirmed by immunohistochemistry and PCR analysis. A single injection of rituximab or the murine anti-CD20 Ab 1F5, given i.p. 1 day after the tumor, cured 100% of the animals. Indeed, at week 4 after tumor cell inoculation, CD20(+) cells were undetectable in all organs analyzed in rituximab-treated animals, as determined by immunohistochemistry and PCR. Rituximab had no direct effect on tumor growth in vitro. Depletion of either NK cells or neutrophils or both in tumor-injected animals did not affect the therapeutic activity of the drug. Similarly, rituximab was able to eradicate tumor cells in athymic nude mice, suggesting that its activity is T cell independent. In contrast, the protective activity of rituximab or the 1F5 Ab was completely abolished in syngeneic knockout animals lacking C1q, the first component of the classical pathway of C (C1qa(-/-)). These data demonstrate that C activation is fundamental for rituximab therapeutic activity in vivo.

Immunotherapy with anti-CD20 compounds

The B-cell surface antigen CD20 is currently the prime target for near-selective treatment of mature B-cell malignancies and a range of reactive B-cell associated disorders (including virus-associated lymphoproliferation or autoimmune conditions). CD20 is strongly and homogeneously expressed on the majority of mature B-cell neoplasms except chronic lymphocytic leukaemia cells, and on all mature reactive B-cells. This review will summarise the modes of action of various reagents targeting CD20. Treatment results following their use in single and combination therapy for B-cell disorders are reviewed.

Design, construction, and in vitro analyses of multivalent antibodies

Some Abs are more efficacious after being cross-linked to form dimers or multimers, presumably as a result of binding to and clustering more surface target to either amplify or diversify cellular signaling. To improve the therapeutic potency of these types of Abs, we designed and generated Abs that express tandem Fab repeats with the aim of mimicking cross-linked Abs. The versatile design of the system enables the creation of a series of multivalent human IgG Ab forms including tetravalent IgG1, tetravalent F(ab')2, and linear Fab multimers with either three or four consecutively linked Fabs. The multimerized Abs target the cell surface receptors HER2, death receptor 5, and CD20, and are more efficacious than their parent mAbs in triggering antitumor cellular responses, indicating they could be useful both as reagents for study as well as novel therapeutics.

Radioimmunotherapy for Waldenstrom's macroglobulinemia

Radioimmunotherapy targeting CD20 is a promising novel treatment for lymphoma. Prior trials have established the safe dose of Zevalin ((90)Y-ibritumomab tiuxetan; IDEC Pharmaceuticals) for patients with no more than 25% bone marrow (BM) involvement. Zevalin is expected to be an effective treatment for WM; however, the safe dose has not been defined. A phase I clinical trial has been designed to define the maximum-tolerated dose (MTD) of Zevalin in patients with WM and BM involvement up to 50%. Eligible patients need to have adequate hematologic indices (absolute neutrophil count [ANC] > 1,500/microL, platelets > 100,000/microL). The starting dose of (90)Y-Zevalin is 0.08 mCi/kg. Dose escalation by 0.04 mCi/kg in cohorts of three to six patients will be performed. Patients will be re-treated at 12 weeks if there is no complete response, no progression, and no dose-limiting toxicity. If the degree of BM involvement remains in the 20% to 50% range, re-treatment will involve a similar dose of Zevalin; if it is <20%, patients will receive Zevalin to the maximum allowed cumulative dose of 0.4 mCi/kg (or 0.3 mCi/kg for mild thrombocytopenia). Despite the phase I design, the re-treatment provision is expected to result in significant clinical benefit.

An overview of the current clinical use of the anti-CD20 monoclonal antibody rituximab

The chimeric anti-CD20 monoclonal antibody rituximab has become part of the standard therapy for patients with non-Hodgkin's lymphoma (NHL). To date, more than 300 000 patients have been treated with rituximab worldwide, including patients with indolent and aggressive NHL, Hodgkin's disease and other B-cell malignancies. Combination of rituximab with cytotoxic agents or cytokines has been explored in a number of different studies. Rituximab is now also approved for patients with diffuse large B-cell lymphoma when combined with standard CHOP chemotherapy (cyclophosphamide, doxorubicin, vincristine and prednisone). The monoclonal antibody is generally well tolerated. Most adverse events are infusion-associated, including chills, fever and rigor related to the release of cytokines.

Complement-mediated lysis by anti-CD20 mAb correlates with segregation into lipid rafts

Despite the clinical success of anti-CD20 monoclonal antibody (mAb) in the treatment of lymphoma, there remains considerable uncertainty about its mechanism of action. Here we show that the ability of mAbs to translocate CD20 into low-density, detergent-insoluble membrane rafts appears to control how effectively they mediate complement lysis of lymphoma cells. In vitro studies using a panel of anti-B-cell mAbs revealed that the anti-CD20 mAbs, with one exception (B1), are unusually effective at recruiting human complement. Differences in complement recruitment could not be explained by the level of mAb binding or isotype but did correlate with the redistribution of CD20 in the cell membrane following mAb ligation. Membrane fractionation confirmed that B1, unlike 1F5 and rituximab, was unable to translocate CD20 into lipid rafts. In addition, we were able to drive B1 and a range of other anti-B-cell mAbs into a detergent-insoluble fraction of the cell by hyper-cross-linking with an F(ab')(2) anti-Ig Ab, a treatment that also conferred the ability to activate lytic complement. Thus, we have shown that an important mAb effector function appears to be controlled by movement of the target molecule into membrane rafts, either because a raft location favors complement activation by mAbs or because rafts are more sensitive to complement penetration.

The mechanisms of action of rituximab in the elimination of tumor cells

The anti-CD20 chimeric monoclonal antibody rituximab kills B cells by multiple mechanisms, including complement-dependent cytoxicity, antibody-dependent cellular cytotoxicity, and induction of apoptosis. Rituximab can also sensitize cells to the effects of chemotherapy. To optimize treatment of non-Hodgkin's lymphoma and improve response rates, a fuller understanding of these mechanisms and their relative contributions to clinical efficacy is required. Therefore, this has been an area of active research in recent years. Preclinical studies have established that the human Fc region of rituximab is important in mobilizing antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity effector mechanisms. In addition, molecular consequences of CD20 binding, including redistribution of membrane lipid "rafts," activation of src kinases, phospholipases and caspases, and down-regulation of interleukin-10, have been identified. Clinical studies have also identified differences in patients' responses to rituximab associated with Fc receptor polymorphisms, and increases in enzymes involved in apoptotic pathways have been seen in the lymphocytes of patients following rituximab treatment. This article reviews the current understanding of the mechanisms of rituximab cell killing in the light of the latest clinical and preclinical data.