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

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.     

A novel bispecific antibody format enables simultaneous bivalent and monovalent co-engagement of distinct target antigens

Bispecific antibodies based on full-length antibody structures are more optimal than fragment-based formats because they benefit from the favorable properties of the Fc region. However, the homodimeric nature of Fc effectively imposes bivalent binding on all current full-length bispecific antibodies, an attribute that can result in nonspecific activation of cross-linked receptors. We engineered a novel bispecific format, referred to as mAb-Fv, that utilizes a heterodimeric Fc region to enable monovalent co-engagement of a second target antigen in a full-length context. mAb-Fv constructs co-targeting CD16 and CD3 were expressed and purified as heterodimeric species, bound selectively to their co-target antigens and mediated potent cytotoxic activity by NK cells and T cells, respectively. The capacity to co-engage distinct target antigens simultaneously with different valencies is an improved feature for bispecific antibodies with promising therapeutic implications.Key words: bispecific, mAb-Fv, Fc, heterodimer, CD16, CD3, HER2, HM1.24, anti-tumor, cancerDespite the enormous success of antibody-based therapeutics for the treatment of a variety of diseases, research efforts to improve their clinical efficacy continue. One avenue being explored is the engineering of new antigen binding sites to permit co-engagement of two distinct targets. Such engineered antibodies are commonly referred to as bispecifics, and a wide variety of formats have been described in references ¹ and ². Co-target antigens can include two targets believed to be causal in the pathology of a particular disease, e.g., two cytokines or growth factors.^3–5 Alternatively, the co-target pair may be a cell surface antigen and an immune receptor such that a novel “effector” mechanism can be built into the antibody, beyond those mediated naturally by the Fc region.²In the 1980s, bispecific antibodies were made by fusing two cell lines that each produced a single monoclonal antibody (mAb).⁶ Although the resulting hybrid hybridoma or quadroma did produce bispecifics, they were only a minor population and extensive purification was required to isolate the desired antibody. Antibody fragments provided an engineering solution to this problem; because they lack the complex quaternary structure of a full-length antibody, multiple variable regions can be linked in single genetic constructs. Antibody fragments of many different forms have been generated, including diabodies, single chain diabodies, tandem scFvs and F(ab'')₂ bispecifics.^2,7 While these formats can be expressed at high levels in bacteria and, arguably, may have benefits due to their small size, they suffer from poor half-life in vivo and can present manufacturing challenges related to their production and stability. For example, the rapid clearance of some fragment-based bispecifics requires that they be infused continuously via a portable pump over one to two months.⁸ The principal source of these limitations for fragment formats is the lack of an antibody Fc region with its associated structural and functional benefits, including large size that precludes renal filtration; high stability; binding to various Fc ligands, one of which maintains serum persistence (the neonatal Fc receptor FcRn) and binding to proteins A and G, which facilitates large scale purification.Recent work has attempted to address the shortcomings of fragment-based bispecifics by engineering a second antigen binding site into full-length antibody-like formats.^5,9–12 The presence of an Fc region in theory provides these formats with the developability and pharmacokinetic properties of standard IgG mAbs. However, because these constructs build new antigen binding sites on top of a homodimeric constant chain, binding to the new antigen is always bivalent. This consequence may pose a constraint depending on the co-targeting goal.For many immune receptors, cellular activation is accomplished by cross-linking of a monovalent binding interaction. The mechanism of cross-linking is typically mediated by antibody/antigen immune complexes, or via effector cell to target cell engagement. For example, the low affinity activating Fc gamma receptors (FcγRs) such as CD16 (FcγRIIIa) and CD32a (FcγRIIa) that mediate cellular killing bind monovalently to the antibody Fc region. While monovalent binding does not result in cellular signaling, upon effector cell engagement with the target cell, receptors are cross-linked and clustered on the cell surface, leading to activation.¹³ On T cells, CD3 activation occurs when its associated T-cell receptor (TCR) engages antigen-loaded major histocompatibility complex (MHC) on antigen-presenting cells in an avid cell-to-cell synapse.¹⁴ Bivalent antibodies targeting CD3 can elicit massive cytokine release, and the consequent toxicity has presented challenges for the development of anti-CD3 antibodies as drugs;^15,16 in contrast, monovalent binding of CD3 in Fab^17,18 and bispecific¹⁹ formats generates much lower levels of T-cell activation. For bispecifics, a consequence of this biology is that bivalent cross-linking of receptors can lead to non-specific activation of an effector cell in the absence of target cell.Thus, when the therapeutic goal is the co-engagement of an immune receptor, the desired binding may be monovalent rather than bivalent. This mode is incompatible with the majority of current full-length bispecifics. We describe an engineering solution to this problem that utilizes a heterodimeric Fc region to enable a single additional variable region to be built monomerically onto an antibody. Our new bispecific format, which we refer to as mAb-Fv, enables the simultaneous bivalent and monovalent co-engagement of distinct target antigens.