Translational PK/PD and the first-in-human dose selection of a PD1/IL15: an engineered recombinant targeted cytokine for cancer immunotherapy.

Introduction: Interleukin 15 (IL-15) is a potential anticancer agent and numerous engineered IL-15 agonists are currently under clinical investigation. Selective targeting of IL-15 to specific lymphocytes may enhance therapeutic effects while helping to minimize toxicities. Methods: We designed and built a heterodimeric targeted cytokine (TaCk) that consists of an anti-programmed cell death 1 receptor antibody (anti-PD-1) and an engineered IL-15. This "PD1/IL15" selectively delivers IL-15 signaling to lymphocytes expressing PD-1. We then investigated the pharmacokinetic (PK) and pharmacodynamic (PD) effects of PD1/IL15 TaCk on immune cell subsets in cynomolgus monkeys after single and repeat intravenous dose administrations. We used these results to determine the first-in-human (FIH) dose and dosing frequency for early clinical trials. Results: The PD1/IL15 TaCk exhibited a nonlinear multiphasic PK profile, while the untargeted isotype control TaCk, containing an anti-respiratory syncytial virus antibody (RSV/IL15), showed linear and dose proportional PK. The PD1/IL15 TaCk also displayed a considerably prolonged PK (half-life range ∼1.0-4.1 days) compared to wild-type IL-15 (half-life ∼1.1 h), which led to an enhanced cell expansion PD response. The PD was dose-dependent, durable, and selective for PD-1+ lymphocytes. Notably, the dose- and time-dependent PK was attributed to dynamic TMDD resulting from test article-induced lymphocyte expansion upon repeat administration. The recommended first-in-human (FIH) dose of PD1/IL15 TaCk is 0.003 mg/kg, determined based on a minimum anticipated biological effect level (MABEL) approach utilizing a combination of in vitro and preclinical in vivo data. Conclusion: This work provides insight into the complex PK/PD relationship of PD1/IL15 TaCk in monkeys and informs the recommended starting dose and dosing frequency selection to support clinical evaluation of this novel targeted cytokine.

AMG 509 (Xaluritamig), an Anti-STEAP1 XmAb 2+1 T-cell Redirecting Immune Therapy with Avidity-Dependent Activity against Prostate Cancer.

The tumor-associated antigen STEAP1 is a potential therapeutic target that is expressed in most prostate tumors and at increased levels in metastatic castration-resistant prostate cancer (mCRPC). We developed a STEAP1-targeted XmAb 2+1 T-cell engager (TCE) molecule, AMG 509 (also designated xaluritamig), that is designed to redirect T cells to kill prostate cancer cells that express STEAP1. AMG 509 mediates potent T cell-dependent cytotoxicity of prostate cancer cell lines in vitro and promotes tumor regression in xenograft and syngeneic mouse models of prostate cancer in vivo. The avidity-driven activity of AMG 509 enables selectivity for tumor cells with high STEAP1 expression compared with normal cells. AMG 509 is the first STEAP1 TCE to advance to clinical testing, and we report a case study of a patient with mCRPC who achieved an objective response on AMG 509 treatment. SIGNIFICANCE: Immunotherapy in prostate cancer has met with limited success due to the immunosuppressive microenvironment and lack of tumor-specific targets. AMG 509 provides a targeted immunotherapy approach to engage a patient's T cells to kill STEAP1-expressing tumor cells and represents a new treatment option for mCRPC and potentially more broadly for prostate cancer. See related commentary by Hage Chehade et al., p. 20. See related article by Kelly et al., p. 76. This article is featured in Selected Articles from This Issue, p. 5.

Complex PK-PD of an engineered IL-15/IL-15Rα-Fc fusion protein in cynomolgus monkeys: QSP modeling of lymphocyte dynamics.

XmAb24306 is a lymphoproliferative interleukin (IL)-15/IL-15 receptor α (IL-15Rα) Fc-fusion protein currently under clinical investigation as an immunotherapeutic agent for cancer treatment. XmAb24306 contains mutations in IL-15 that attenuate its affinity to the heterodimeric IL-15 receptor ßγ (IL-15R). We observe substantially prolonged pharmacokinetics (PK) (half-life ∼ 2.5 to 4.5 days) in single- and repeat-dose cynomolgus monkey (cyno) studies compared to wild-type IL-15 (half-life ∼ 1 hour), leading to increased exposure and enhanced and durable expansion of NK cells, CD8+ T cells and CD4-CD8- (double negative [DN]) T cells. Drug clearance varied with dose level and time post-dose, and PK exposure decreased upon repeated dosing, which we attribute to increased target-mediated drug disposition (TMDD) resulting from drug-induced lymphocyte expansion (i.e., pharmacodynamic (PD)-enhanced TMDD). We developed a quantitative systems pharmacology (QSP) model to quantify the complex PKPD behaviors due to the interactions of XmAb24306 with multiple cell types (CD8+, CD4+, DN T cells, and NK cells) in the peripheral blood (PB) and lymphoid tissues. The model, which includes nonspecific drug clearance, binding to and TMDD by IL15R differentially expressed on lymphocyte subsets, and resultant lymphocyte margination/migration out of PB, expansion in lymphoid tissues, and redistribution to the blood, successfully describes the systemic PK and lymphocyte kinetics observed in the cyno studies. Results suggest that after 3 doses of every-two-week (Q2W) doses up to 70 days, the relative contributions of each elimination pathway to XmAb24306 clearance are: DN T cells > NK cells > CD8+ T cells > nonspecific clearance > CD4+ T cells. Modeling suggests that observed cellular expansion in blood results from the influx of cells expanded by the drug in lymphoid tissues. The model is used to predict lymphoid tissue expansion and to simulate PK-PD for different dose regimens. Thus, the model provides insight into the mechanisms underlying the observed PK-PD behavior of an engineered cytokine and can serve as a framework for the rapid integration and analysis of data that emerges from ongoing clinical studies in cancer patients as single-agent or given in combination.

Dissecting strategies to tune the therapeutic potential of SARS-CoV-2-specific monoclonal antibody CR3022.

The rapid spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), coupled with a lack of therapeutics, has paralyzed the globe. Although significant effort has been invested in identifying antibodies that block infection, the ability of antibodies to target infected cells through Fc interactions may be vital to eliminate the virus. To explore the role of Fc activity in SARS-CoV-2 immunity, the functional potential of a cross-SARS-reactive antibody, CR3022, was assessed. CR3022 was able to broadly drive antibody effector functions, providing critical immune clearance at entry and upon egress. Using selectively engineered Fc variants, no protection was observed after administration of WT IgG1 in mice or hamsters. Conversely, the functionally enhanced Fc variant resulted in increased pathology in both the mouse and hamster models, causing weight loss in mice and enhanced viral replication and weight loss in the more susceptible hamster model, highlighting the pathological functions of Fc-enhancing mutations. These data point to the critical need for strategic Fc engineering for the treatment of SARS-CoV-2 infection.

Targeting Multiple Myeloma with AMG 424, a Novel Anti-CD38/CD3 Bispecific T-cell-recruiting Antibody Optimized for Cytotoxicity and Cytokine Release.

PURPOSE: Despite advances in the treatment of multiple myeloma, new therapies are needed to induce more profound clinical responses. T-cell-redirected lysis triggered by bispecific antibodies recruiting T cells to cancer cells is a clinically validated mechanism of action against hematologic malignancies and CD38 is a tumor-associated antigen with near-universal expression in multiple myeloma. Thus, an anti-CD38/CD3 bispecific T-cell-recruiting antibody has the potential to be an effective new therapeutic for multiple myeloma. EXPERIMENTAL DESIGN: Anti-CD38/CD3 XmAb T-cell-recruiting antibodies with different affinities for CD38 and CD3 were assessed in vitro and in vivo for their redirected T-cell lysis activity against cancer cell lines, their lower levels of cytokine release, and their potency in the presence of high levels of soluble CD38. Select candidates were further tested in cynomolgus monkeys for B-cell depletion and cytokine release properties. RESULTS: AMG 424 was selected on the basis of its ability to kill cancer cells expressing high and low levels of CD38 in vitro and trigger T-cell proliferation, but with attenuated cytokine release. In vivo, AMG 424 induces tumor growth inhibition in bone marrow-invasive mouse cancer models and the depletion of peripheral B cells in cynomolgus monkeys, without triggering excessive cytokine release. The activity of AMG 424 against normal immune cells expressing CD38 is also presented. CONCLUSIONS: These findings support the clinical development of AMG 424, an affinity-optimized T-cell-recruiting antibody with the potential to elicit significant clinical activity in patients with multiple myeloma.

Redirected T Cell Cytotoxicity in Cancer Therapy.

Bispecific antibodies that recruit and redirect T cells to attack tumor cells have tremendous potential for the treatment of various malignancies. In general, this class of therapeutics, known as CD3 bispecifics, promotes tumor cell killing by cross-linking a CD3 component of the T cell receptor complex with a tumor-associated antigen on the surface of the target cell. Importantly, this mechanism does not rely on a cognate interaction between the T cell receptor and a peptide:HLA complex, thereby circumventing HLA (human leukocyte antigen) restriction. Hence, CD3 bispecifics may find a key role in addressing tumors with low neoantigen content and/or low inflammation, and this class of therapeutics may productively combine with checkpoint blockade. A wide array of formats and optimization approaches has been developed, and a wave of CD3 bispecifics is proceeding into human clinical trials for a range of indications, with promising signs of therapeutic activity.

A robust heterodimeric Fc platform engineered for efficient development of bispecific antibodies of multiple formats.

Bispecific monoclonal antibodies can bind two protein targets simultaneously and enable therapeutic modalities inaccessible by traditional mAbs. Bispecific formats containing a heterodimeric Fc region are of particular interest, as a heterodimeric Fc empowers both bispecificity and altered valencies while retaining the developability and druggability of a monoclonal antibody. We present a robust heterodimeric Fc platform, called the XmAb® bispecific platform, engineered for efficient development of bispecific antibodies and Fc fusions of multiple formats. First, we engineer a purification solution for proteins containing a heterodimeric Fc using engineered isoelectric point differences in the Fc region that enable straightforward purification of the heterodimeric species. Then, we combine this purification solution with a novel set of Fc substitutions capable of achieving heterodimer yields over 95% with little change in thermostability. Next, we illustrate the flexibility of our heterodimeric Fc with a case study in which a wide range of tumor-associated antigen × CD3 bispecifics are generated, differing in choice of tumor antigen, affinities for both tumor antigen and CD3, and tumor antigen valency. Finally, we present manufacturing data reinforcing the robustness of the heterodimeric Fc platform at scale.

Suppression of rheumatoid arthritis B cells by XmAb5871, an anti-CD19 antibody that coengages B cell antigen receptor complex and Fcγ receptor IIb inhibitory receptor.

OBJECTIVE: Engagement of Fcγ receptor IIb (FcγRIIb) suppresses B cell activation and represents a promising target for therapy in autoimmunity. The aim of this study was to characterize B cell immunosuppression mediated by the Fc-engineered antibody, XmAb5871, which coengages FcγRIIb with the B cell antigen receptor (BCR) complex and that is currently in clinical development for the treatment of rheumatoid arthritis (RA). Because rheumatoid factor (RF) might interfere with the binding of XmAb5871 to FcγRIIb, we correlated RF titers with the potency of XmAb5871. METHODS: We analyzed the expression of CD19, FcγRIIb, and CD86 on naive and memory B cells from 50 patients with RA and 66 healthy donors, quantified XmAb5871-induced promotion of FcγRIIb phosphorylation and suppression of calcium flux in activated B cells, measured CD86 inhibition in whole blood, and correlated RF and anti-citrullinated protein antibody (ACPA) levels with drug potency. We engrafted RA peripheral blood mononuclear cells (PBMCs) into SCID mice, treated them with XmAb5871, and quantified human total IgG, total IgM, and anti-tetanus IgG antibody levels in vivo. RESULTS: B cells from all donors expressed CD19 and FcγRIIb, and the expression of FcγRIIb was higher on naive, but not memory, B cells from donors with RA compared with healthy donors. BCR-mediated calcium flux was suppressed by XmAb5871 and was associated with FcγRIIb phosphorylation. XmAb5871 inhibited CD86 induction, and the levels of RF and ACPAs did not affect efficacy. XmAb5871 suppressed B cell activation regardless of disease severity. In SCID mice engrafted with PBMCs from a patient with RA, XmAb5871 suppressed humoral responses. CONCLUSION: Coengagement of the BCR complex and FcγRIIb by XmAb5871 inhibits B cell activation and function. The similar potency in patients with RA and healthy donors and the absence of autoantibody interference suggest that XmAb5871 may represent a new therapeutic strategy to suppress autoreactive B cells in RA.

Increasing FcγRIIa affinity of an FcγRIII-optimized anti-EGFR antibody restores neutrophil-mediated cytotoxicity.

Antibody-dependent cell-mediated cytotoxicity (ADCC) has been suggested as an essential mechanism for the in vivo activity of cetuximab, an epidermal growth factor receptor (EGFR)-targeting therapeutic antibody. Thus, enhancing the affinity of human IgG1 antibodies to natural killer (NK) cell-expressed FcγRIIIa by glyco- or protein-engineering of their Fc portion has been demonstrated to improve NK cell-mediated ADCC and to represent a promising strategy to improve antibody therapy. However, human polymorphonuclear (PMN) effector cells express the highly homologous FcγRIIIb isoform, which is described to be ineffective in triggering ADCC. Here, non-fucosylated or protein-engineered anti-EGFR antibodies with optimized FcγRIIIa affinities demonstrated the expected benefit in NK cell-mediated ADCC, but did not mediate ADCC by PMN, which could be restored by FcγRIIIb blockade. Furthermore, eosinophils and PMN from paroxysmal nocturnal hemoglobinuria patients that expressed no or low levels of FcγRIIIb mediated effective ADCC with FcγRIII-optimized anti-EGFR antibody. Additional experiments with double FcγRIIa/FcγRIII-optimized constructs demonstrated enhanced PMN-mediated ADCC compared with single FcγRIII-optimized antibody. In conclusion, our data demonstrate that FcγRIIIb engagement impairs PMN-mediated ADCC activity of FcγRIII-optimized anti-EGFR antibodies, while further optimization of FcγRIIa binding significantly restores PMN recruitment.

Immune suppression in cynomolgus monkeys by XPro9523: an improved CTLA4-Ig fusion with enhanced binding to CD80, CD86 and neonatal Fc receptor FcRn.

The CTLA4-Ig fusion proteins abatacept and belatacept are clinically proven immunosuppressants used for rheumatoid arthritis and renal transplant, respectively. Given that both biologics are typically administered chronically by infusion, a need exists for a next-generation CTLA4-Ig with more convenient dosing. We used structure-based protein engineering to optimize the affinity of existing CTLA4-Ig therapeutics for the ligands CD80 and CD86, and for the neonatal Fc receptor, FcRn. From a rationally designed library, we identified four substitutions that enhanced binding to human CD80 and CD86. Coupled with two IgG1 Fc substitutions that enhanced binding to human FcRn, these changes comprise the novel CTLA4-Ig fusion protein, XPro9523. Compared with abatacept, XPro9523 demonstrated 5.9-fold, 23-fold, and 12-fold increased binding to CD80, CD86, and FcRn, respectively; compared with belatacept, CD80, CD86, and FcRn binding increased 1.5-fold, 7.7-fold, and 11-fold, respectively. XPro9523 and belatacept suppressed human T cell proliferation and IL-2 production more potently than abatacept. XPro9523 also suppressed inflammation in the mouse collagen-induced arthritis model. In cynomolgus monkeys, XPro9523 saturated CD80 and CD86 more effectively than abatacept and belatacept, potently inhibited IgM and IgG immunization responses, and demonstrated longer half-life. Pharmacokinetic modeling of its increased potency and persistence suggests that, in humans, XPro9523 may demonstrate superior efficacy and dosing convenience compared with abatacept and belatacept.

Suppression of mast cell degranulation through a dual-targeting tandem IgE-IgG Fc domain biologic engineered to bind with high affinity to FcγRIIb.

Mast cells and basophils play a central role in allergy, asthma, and anaphylaxis, as well as in non-allergic inflammatory, neurological and autoimmune diseases. Allergen-mediated cross-linking of IgE bound to FcεRI leads to cellular activation, and the low-affinity Fc receptor FcγRIIb is a key inhibitor of subsequent degranulation. FcγRIIb, when coengaged with FcεRI via allergen bound to IgE, stimulates ITIM domain-mediated inhibitory signaling that efficiently suppresses mast cell and basophil activation. To assess the therapeutic potential of directed coengagement of FcεRI and FcγRIIb in the absence of FcεRI crosslinking, we developed a fusion protein comprising the coupled Fc domains of murine IgE and human IgG1. As a key functional component of this tandem Fcε-Fcγ biologic, we engineered its IgG1 Fc domain to bind to human FcγRIIb with 100-fold enhanced affinity relative to native IgG1 Fc. Using mast cells from mice transgenic for human FcγRIIb, we show that this tandem Fc binds with high affinity to murine FcεRI and human FcγRIIb on mast cells, triggers phosphorylation of FcγRIIb, and inhibits FcεRI-dependent calcium mobilization. Control tandem Fc biologics containing a native IgG1 Fc domain or lacking binding to Fcγ receptors were markedly less active, demonstrating that the affinity-optimized tandem Fc can inhibit degranulation through stimulation of FcγRIIb signaling as well as through competition with allergen-IgE immune complex for FcεRI binding. We propose that in the context of a fully human tandem Fc biologic, high-affinity coengagement of FcεRI and FcγRIIb has potential as a novel therapy for allergy and other mast cell and basophil-mediated pathologies.

Reduction of total IgE by targeted coengagement of IgE B-cell receptor and FcγRIIb with Fc-engineered antibody.

BACKGROUND: Sequestration of IgE to prevent its binding to high-affinity IgE receptor FcεRI on basophils and mast cells is an effective therapy for allergic asthma. IgE production requires differentiation of activated IgE(+) B cells into plasma cells upon allergen sensitization. B-cell receptor signaling is suppressed by the inhibitory IgG Fc receptor FcγRIIb; therefore, we reasoned that a therapeutic antibody that coengages FcγRIIb and IgE B-cell receptor would not only sequester IgE but also suppress its production by blocking IgE(+) B-cell activation and differentiation to IgE-secreting plasma cells. OBJECTIVE: To explore the effects of IgE sequestration versus IgE suppression by comparing omalizumab to FcγRIIb-optimized anti-IgE antibodies in humanized mouse models of immunoglobulin production. METHODS: By using a murine anti-IgE antibody as a template, we humanized, increased IgE binding, and modified its Fc domain to increase affinity for FcγRIIb. We next compared effects of this antibody (XmAb7195) versus omalizumab on the secretion of IgE and other isotypes in human PBMC cultures and in PBMC-engrafted severe combined immunodeficiency mice. RESULTS: Relative to omalizumab, XmAb7195 has a 5-fold higher affinity for human IgE and more than 400-fold higher affinity for FcγRIIb. In addition to sequestering soluble IgE, XmAb7195 inhibited plasma cell differentiation and consequent human IgE production through coengagement of IgE B-cell receptor with FcγRIIb. In PBMC-engrafted mice, XmAb7195 reduced total human IgE (but not IgG or IgM) levels by up to 40-fold relative to omalizumab. CONCLUSION: XmAb7195 acts by IgE sequestration coupled with an FcγRIIb-mediated inhibitory mechanism to suppress the formation of IgE-secreting plasma cells and reduce both free and total IgE levels.

Potent in vitro and in vivo activity of an Fc-engineered humanized anti-HM1.24 antibody against multiple myeloma via augmented effector function.

HM1.24, an immunologic target for multiple myeloma (MM) cells, has not been effectively targeted with therapeutic monoclonal antibodies (mAbs). In this study, we investigated in vitro and in vivo anti-MM activities of XmAb5592, a humanized anti-HM1.24 mAb with Fc-domain engineered to significantly enhance FcγR binding and associated immune effector functions. XmAb5592 increased antibody-dependent cellular cytotoxicity (ADCC) several fold relative to the anti-HM1.24 IgG1 analog against both MM cell lines and primary patient myeloma cells. XmAb5592 also augmented antibody dependent cellular phagocytosis (ADCP) by macrophages. Natural killer (NK) cells became more activated by XmAb5592 than the IgG1 analog, evidenced by increased cell surface expression of granzyme B-dependent CD107a and MM cell lysis, even in the presence of bone marrow stromal cells. XmAb5592 potently inhibited tumor growth in mice bearing human MM xenografts via FcγR-dependent mechanisms, and was significantly more effective than the IgG1 analog. Lenalidomide synergistically enhanced in vitro ADCC against MM cells and in vivo tumor inhibition induced by XmAb5592. A single dose of 20 mg/kg XmAb5592 effectively depleted both blood and bone marrow plasma cells in cynomolgus monkeys. These results support clinical development of XmAb5592, both as a monotherapy and in combination with lenalidomide, to improve patient outcome of MM.

Modulation of antibody effector function.

Several novel technologies have evolved over the last decade for the modification of antibodies to enhance their inherent effector functions. All focus on the constant Fc domain and utilize either amino acid substitutions or glycoform perturbations to modulate their interaction with Fc receptors and the effector cells that bear them. We review these technologies with an emphasis on their validation with animal models and human clinical data.

Antibody-mediated coengagement of FcγRIIb and B cell receptor complex suppresses humoral immunity in systemic lupus erythematosus.

Engagement of the low-affinity Ab receptor FcγRIIb downregulates B cell activation, and its dysfunction is associated with autoimmunity in mice and humans. We engineered the Fc domain of an anti-human CD19 Ab to bind FcγRIIb with high affinity, promoting the coengagement of FcγRIIb with the BCR complex. This Ab (XmAb5871) stimulated phosphorylation of the ITIM of FcγRIIb and suppressed BCR-induced calcium mobilization, proliferation, and costimulatory molecule expression of human B cells from healthy volunteers and systemic lupus erythematosus (SLE) patients, as well as B cell proliferation induced by LPS, IL-4, or BAFF. XmAb5871 suppressed humoral immunity against tetanus toxoid and reduced serum IgM, IgG, and IgE levels in SCID mice engrafted with SLE or healthy human PBMC. XmAb5871 treatment also increased survival of mice engrafted with PBMC from a unique SLE patient. Unlike anti-CD20 Ab, coengagement of FcγRIIb and BCR complex did not promote B cell depletion in human PBMC cultures or in mice. Thus, amplification of the FcγRIIb inhibitory pathway in activated B cells may represent a novel B cell-targeted immunosuppressive therapeutic approach for SLE and other autoimmune diseases that should avoid the complications associated with B cell depletion.

Fc-engineered anti-CD40 antibody enhances multiple effector functions and exhibits potent in vitro and in vivo antitumor activity against hematologic malignancies.

CD40 is highly expressed on various B-lineage malignancies and represents an attractive immunotherapy target for neoplastic disease. Previous work showed that engineering the Fc domain of an antibody for increased binding to Fcγ receptors (FcγRs) significantly enhanced Fc-mediated immune effector function and antitumor activity in vitro and in vivo. We developed a humanized anti-CD40 antibody similarly Fc-engineered for increased FcγR binding (XmAbCD40) and compared its efficacy with that of an anti-CD40 native IgG1 analog and the anti-CD20 antibody rituximab. XmAbCD40 increased antibody-dependent cell-mediated cytotoxicity (ADCC) up to 150-fold relative to anti-CD40 IgG1 against B-lymphoma, leukemia, and multiple myeloma cell lines, and significantly enhanced ADCC against primary tumors. XmAbCD40 was also superior to rituximab in enhancing ADCC (both in cell lines and primary tumors) and in augmenting antibody-dependent cellular phagocytosis. XmAbCD40 significantly inhibited lymphoma growth in disseminated and established mouse xenografts and was more effective than the IgG1 analog or rituximab. An anti-CD40 antibody constructed to abrogate FcγR binding showed no reduction of tumor growth, indicating that the in vivo antitumor activity of XmAbCD40 is primarily mediated via FcγR-dependent mechanisms. These data demonstrate that XmAbCD40 displays potent antitumor efficacy and merits further evaluation for the treatment of CD40(+) malignancies.

Engineering fully human monoclonal antibodies from murine variable regions.

Fully human monoclonal antibodies (mAbs) derived from transgenic mice or human antibody libraries are the current state of the art for reducing the immunogenicity risk of antibody drugs. Here, we describe a novel method for generating fully human mAbs from nonhuman variable regions using information from the human germline repertoire. Central to our strategy is the rational engineering of residues within and proximal to CDRs and the V(H)/V(L) interface by iteratively exploring substitutions to the closest human germline sequences using semi-automated computational methods. Starting from the parent murine variable regions of three currently marketed mAbs targeting CD25, vascular endothelial growth factor, and tumor necrosis factor alpha, we have generated fully human antibodies with 59, 46, and 45 substitutions, respectively, compared to the parent murine sequences. A large number of these substitutions were in the CDRs, which are typically avoided in humanization methods. Antigen affinities of the fully human variants were comparable to the chimeric mAbs in each case. Furthermore, in vitro functional characterization indicated that all retain potency of the chimeric mAbs and have comparable activity to their respective marketed drugs daclizumab, bevacizumab, and infliximab. Based on local and global sequence identity, the sequences of our engineered mAbs are indistinguishable from those of fully human mAbs isolated from transgenic mice or human antibody libraries. This work establishes a simple rational engineering methodology for generating fully human antibody therapeutics from murine mAbs produced from standard hybridoma technology.

Enhanced antibody half-life improves in vivo activity.

Improved affinity for the neonatal Fc receptor (FcRn) is known to extend antibody half-life in vivo. However, this has never been linked with enhanced therapeutic efficacy. We tested whether antibodies with half-lives extended up to fivefold in human (h)FcRn transgenic mice and threefold in cynomolgus monkeys retain efficacy at longer dosing intervals. We observed that prolonged exposure due to FcRn-mediated enhancement of half-life improved antitumor activity of Fc-engineered antibodies in an hFcRn/Rag1(-/-) mouse model. This bridges the demand for dosing convenience with the clinical necessity of maintaining efficacy.

CD19 targeting of chronic lymphocytic leukemia with a novel Fc-domain-engineered monoclonal antibody.

CD19 is a B cell-specific antigen expressed on chronic lymphocytic leukemia (CLL) cells but to date has not been effectively targeted with therapeutic monoclonal antibodies. XmAb5574 is a novel engineered anti-CD19 monoclonal antibody with a modified constant fragment (Fc)-domain designed to enhance binding of FcgammaRIIIa. Herein, we demonstrate that XmAb5574 mediates potent antibody-dependent cellular cytotoxicity (ADCC), modest direct cytotoxicity, and antibody-dependent cellular phagocytosis but not complement-mediated cytotoxicity against CLL cells. Interestingly, XmAb5574 mediates significantly higher ADCC compared with both the humanized anti-CD19 nonengineered antibody it is derived from and also rituximab, a therapeutic antibody widely used in the treatment of CLL. The XmAb5574-dependent ADCC is mediated by natural killer (NK) cells through a granzyme B-dependent mechanism. The NK cell-mediated cytolytic and secretory function with XmAb5574 compared with the nonengineered antibody is associated with enhanced NK-cell activation, interferon production, extracellular signal-regulated kinase phosphorylation downstream of Fcgamma receptor, and no increased NK-cell apoptosis. Notably, enhanced NK cell-mediated ADCC with XmAb5574 was enhanced further by lenalidomide. These findings provide strong support for further clinical development of XmAb5574 as both a monotherapy and in combination with lenalidomide for the therapy of CLL and related CD19(+) B-cell malignancies.

The impact of Fc engineering on an anti-CD19 antibody: increased Fcgamma receptor affinity enhances B-cell clearing in nonhuman primates.

CD19, a B cell-restricted receptor critical for B-cell development, is expressed in most B-cell malignancies. The Fc-engineered anti-CD19 antibody, XmAb5574, has enhanced Fcgamma receptor (FcgammaR) binding affinity, leading to improved FcgammaR-dependent effector cell functions and antitumor activity in murine xenografts compared with the non-Fc-engineered anti-CD19 IgG1 analog. Here, we use XmAb5574 and anti-CD19 IgG1 to further dissect effector cell functions in an immune system closely homologous to that of humans, the cynomolgus monkey. XmAb5574 infusion caused an immediate and dose-related B-cell depletion in the blood (to <10% of baseline levels) concomitant with a sustained reduction of natural killer (NK) cells. NK cells had fully recovered by day 15, whereas B-cell recovery was underway by day 57. B cells in secondary lymphoid tissues were depleted (to 34%-61% of vehicle), with involuted germinal centers apparent in the spleen. Anti-CD19 IgG1 had comparable serum exposure to XmAb5574 but demonstrated no B-cell depletion and no sustained NK-cell reduction. Thus, increasing FcgammaR binding affinity dramatically increased B-cell clearing. We propose that effector cell functions, possibly those involving NK cells, mediate XmAb5574 potency in cynomolgus monkeys, and that enhancing these mechanisms should advance the treatment of B-cell malignancies in humans.