Use of the DAF assay to assess the functional properties of polyclonal and monoclonal RhD antibodies

Effect of zinc on human IgG1 and its FcγR interactions

In the present study, we show that histidines 310 and 435 at the CH2-CH3 interface of the Fc portion of human IgG1 can coordinate a Zn2+ and participate in the control of the CH2-CH2 interdomain opening. Structures obtained in the absence of Zn2+ have a reduced interdomain gap that likely hamper FcγR binding. This closed conformation of the Fc is stabilized by inter-CH2 domain sugar contacts. Zinc appears to counteract the sugar mediated constriction, suggesting that zinc could be an important control factor in IgG1/FcγR interactions. The results of binding studies performed in the presence of EDTA on FcγR expressing cells supports this hypothesis. When a mutated Fc fragment, in which histidines 310 and 435 have been substituted by lysines (Fc H/K), was compared with the wild-type Fc in crystallographic studies, we found that the mutations leave the interface unaltered but have a long-range effect on the CH2 interdomain separation. Moreover, these substitutions have a differential effect on the binding of IgG1 to Fcγ receptors and their functions. Interaction with the inhibitory FcγRIIB is strongly perturbed by the mutations and mutant IgG1 H/K only weakly engages this receptor. By contrast, higher affinity FcγR are mostly unaffected.

Impact on N-glycosylation profile of monoclonal anti-D antibodies as a way to control their immunoregulatory and cytotoxic properties

Prophylaxis of hemolytic disease of newborns is based on the ability of polyclonal anti-D antibodies for suppressing maternal immune response against D-positive fetal red blood cells. The immunosuppressive effect of anti-D antibody is mediated by interaction between its Fc-fragment and low-affinity IgG Fc-receptor (FcγR) on the immune cell. No clinically effective monoclonal anti-D antibody (mAb) that can replace polyclonal anti-D immunoglobulin has been developed yet. The goals of this study were comparison of structural and functional properties of human anti-D polyclonal and monoclonal Abs and assessment of the possibility to manipulate the effector properties of the mAb. N-Glycosylation and particularly the content of nonfucosylated glycans are crucial for affinity of mAb to FcγRIIIA, which plays the key role in the clearance of sensitized cells. We studied and compared glycoprofiles and FcγRIIIA-mediated hemolytic ability of human polyclonal antibodies and anti-D mAbs produced by human B-cell lines, human-rodent heterohybridomas, and a human non-lymphoid cell line PER.C6. Replacement of producing cell line and use of glycosylation modulators can convert an inert mAb into an active one. Nevertheless, rodent cell lines, as well as human non-lymphoid cells, distort natural glycosylation of human IgG and could lead to the loss of immunosuppressive properties. All of the anti-D mAbs secreted by human B-cell lines have a glycoprofile close to human serum IgG. Hence, the constant ratio of IgG glycoforms in human serum is predetermined by glycosylation at the level of the individual antibody-producing cell. The anti-D fraction of polyclonal anti-D immunoglobulin compared to the total human IgG contains more nonfucosylated glycans. Thus, only human transformed B-cells are an appropriate source for efficient anti-D mAbs that can imitate the action of polyclonal anti-D IgG.

Effect of producer cell line on functional activity of anti-D monoclonal antibodies destined for prevention of rhesus sensitization

The ability of anti-D antibodies to cause antigen-specific immunosuppression depends on their interaction with low-affinity Fcgamma-receptors. Human monoclonal antibodies to D antigen of the rhesus system were investigated by antibody-dependent cytotoxicity assay in order to estimate their ability to induce hemolysis mediated by low-affinity Fcgamma receptors. We demonstrate that affinity of monoclonal antibodies to receptors of this type does not depend on primary structure of Fc-fragment, but depends on the producer cell line which expresses the antibodies. Monoclonal IgG1 antibodies interacting with FcgammaRIIa and FcgammaRIII lost this property, if they were secreted by human-mouse heterohybridoma, but not by human B-cell line. On the opposite, monoclonal antibodies that could not activate low-affinity Fcgamma receptors were highly active after human cells fusion with rat myeloma YB2/0. Hemolytic activity of IgG3 remained unchanged after fusion of human cells with rodent cells.

Use of pharmacokinetic/ pharmacodynamic modelling for starting dose selection in first-in-human trials of high-risk biologics

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

Recent regulatory guidance has highlighted the importance of using pharmacokinetic-pharmacodynamic (PK-PD) modelling in the selection of starting doses in first-in-human trials of high-risk biologics. However, limited examples exist in literature illustrating this procedure.

WHAT THIS STUDY ADDS

An interpretation of the recommended dose-selection methodology and the minimum anticipated biological effect level (MABEL) principle, contained in the updated European Medicines Agency guidance on risk-mitigation strategies for first-in-human studies, is presented. Some literature and simulation-based examples of the application of PK-PD modelling principles to starting dose selection using in vitro and in vivo data under the MABEL paradigm are highlighted, along with the advantages and limitations of this approach.

AIMS

To illustrate the use of pharmacokinetic-pharmacodynamic (PK-PD) models to select rational starting doses in clinical trials within the minimum anticipated biological effect level (MABEL) principle using literature data and through simulations.

METHODS

The new European Medicines Agency guidance on starting dose selection of high-risk biologics was analysed considering the basic pharmacological properties and preclinical testing limitations of many biologics. The MABEL approach to dose selection was illustrated through simulations and through literature-reported examples on the selection of starting doses for biologics such as antibodies based on in vitro biomarker data, in vivo PK and PK-PD data.

RESULTS

Literature reports indicating the use of preclinical pharmacological and toxicological data to select successfully safe starting doses in line with the MABEL principle are summarized. PK-PD model-based simulations of receptor occupancy for an anti-IgE antibody system indicate that the relative abundance of IgE in animal models and patients and the turnover rate of the IgE-antibody complex relative to the off-rate of the antibody from IgE are important determinants of in vivo receptor occupancy.

CONCLUSIONS

Mechanistic PK-PD models are capable of integrating preclinical in vitro and in vivo data to select starting doses rationally in first-in-human trials. Biological drug-receptor interaction dynamics is complex and multiple factors affect the dose-receptor occupancy relationship. Thus, these factors should be taken into account when selecting starting doses.

Functional in vitro studies of recombinant human immunoglobulin G and immunoglobulin A anti-D

BACKGROUND

The use of anti-D purified from human serum to prevent hemolytic disease of the fetus and newborn due to D is well established. Owing to supply and safety reasons, however, an unlimited and non-plasma-derived source of antibodies for Rhesus prophylaxis is needed.

STUDY DESIGN AND METHODS

Recombinant human immunoglobulin G (IgG)1, IgG2, IgG3, IgG4, IgA1, and IgA2 anti-D with the same variable region were expressed in Chinese hamster ovary cells. The effector functions of these antibodies were assessed by an antibody-dependent cell-mediated cytotoxicity (ADCC) assay and a chemiluminescence (CL) method for detection of respiratory burst.

RESULTS

In the ADCC assay, IgG1, IgG3, and IgA1 did the best and were as active as a currently used prophylactic polyclonal anti-D. IgG4 and IgA2 were moderately active, whereas IgG2 was not active. In the CL assay, IgG1 and IgG3 were active but much less so than a currently used prophylactic polyclonal anti-D. For some effector cell preparations, IgG4 was active in the CL assay, whereas IgG2, IgA1, and IgA2 were not. A mixture of IgG1 and IgG3 showed a synergistic effect in the CL assay and did as well as the prophylactic polyclonal anti-D in ADCC and CL. Mixtures of IgA1 and either IgG1 or IgG3 showed no synergistic effect.

CONCLUSION

A mixture of recombinant human IgG1 and IgG3 anti-D could be of value in future Rhesus prophylaxis.

Lymphocyte antibody-dependent cytotoxicity test for evaluation of clinical role of monoclonal anti-D-antibodies for prevention of rhesus sensitization

Monoclonal antibodies to D antigen were studied in the reaction of antibody-dependent cytotoxicity for evaluation of the possibility of using these antibodies for preventing rhesus sensitization. High hemolytic activity of four anti-D-monoclonal antibodies in the antibody-dependent cytotoxicity test, mediated by their interaction with FcgammaRI, and the capacity to accelerate elimination of D+ erythrocytes from circulation did not provide the immunosuppressive effect. It was hypothesized that monoclonal antibodies for prevention of rhesus sensitization should interact with FcgammaRIII on lymphocytes. These monoclonal antibodies are extremely rare: only 4 of 125 studied antibodies mediated hemolysis in the antibody-dependent cytotoxicity test with lymphocytes, while all polyclonal anti-D-preparations exhibited this activity.

Selection of a human anti-RhD monoclonal antibody for therapeutic use: impact of IgG glycosylation on activating and inhibitory Fc gamma R functions

The substitution of plasmatic anti-RhD polyclonal antibodies by a monoclonal antibody (mAb) for preventing the hemolytic disease of the newborn (HDN) is an important issue due to supply and safety concerns. Since it has been suggested that FcgammaR are involved in the prevention of HDN, the in vitro functional properties of two anti-RhD mAbs differing through their glycosylation profiles were compared using FcgammaR-based assays to select a candidate mAb. T125(YB2/0), a low fucosylated antibody, bound strongly to both activating FcgammaRIII and inhibitory FcgammaRII, as opposed to its highly fucosylated counterpart. It also exerted a strong ADCC against RhD+ RBCs and a potent FcgammaRIIB-mediated inhibition of cytokine release. Moreover, an in vivo RhD+ red blood cells (RBCs) clearance assay showed that this antibody exhibits a RhD+ RBCs clearance as potent as polyclonal anti-RhD antibodies in NOD-SCID mice. Thus, T125(YB2/O) has been selected to be tested for the prevention of anti-RhD allo-immunization.