Human FcRn transgenic mice for pharmacokinetic evaluation of therapeutic antibodies

Cellular neonatal Fc receptor recycling efficiencies can differentiate target-independent clearance mechanisms of monoclonal antibodies

In vivo clearance mechanisms of therapeutic monoclonal antibodies (mAbs) encompass both target-mediated and target-independent processes. Two distinct determinants of overall mAb clearance largely separate of target-mediated influences are non-specific cellular endocytosis and subsequent pH-dependent mAb recycling mediated by the neonatal Fc receptor (FcRn), where inter-mAb variability in the efficiency of both processes is observed. Here, we implemented a functional cell-based FcRn recycling assay via Madin-Darby canine kidney type II cells stably co-transfected with human FcRn and its light chain β2-microglobulin. A series of pH-dependent internalization studies using a model antibody demonstrated proper function of the human FcRn complex. We then applied our cellular assays to assess the contribution of FcRn and non-specific interactions in the cellular turnover for a panel of 8 clinically relevant mAbs exhibiting variable human pharmacokinetic behavior. Our results demonstrate that non-specific endocytosis rates, pH-dependent non-specific interactions, and engagement with FcRn all contribute to the overall recycling efficiency of therapeutic monoclonal antibodies. The predictive capacity of our assay approach was highlighted by successful identification of all mAbs within our panel possessing clearance in humans greater than 5 mL/day/kg. These results demonstrate that a combination of cell-based in vitro assays can properly resolve individual mechanisms underlying the overall in vivo recycling efficiency and non-target mediated clearance of therapeutic mAbs.

Daily intranasal palivizumab to prevent respiratory syncytial virus infection in healthy preterm infants: a phase 1/2b randomized placebo-controlled trial

Background

Mucosal administration of monoclonal antibodies (mAbs) against respiratory pathogens is a promising alternative for systemic administration because lower doses are required for protection. Clinical development of mucosal mAbs is a highly active field yet clinical proof-of-concept is lacking.

Methods

In this investigator-initiated, double-blind, randomized placebo-controlled trial, we evaluated intranasal palivizumab for the prevention of RSV infection in preterm infants (Dutch Trial Register NTR7378 and NTR7403). We randomized infants 1:1 to receive intranasal palivizumab (1 mg/mL) or placebo once daily during the RSV season. Any RSV infection was the primary outcome and RSV hospitalization was the key secondary outcome. The primary outcome was analyzed with a mixed effect logistic regression on the modified intention-to-treat population.

Findings

We recruited 268 infants between Jan 14, 2019 and Jan 28, 2021, after which the trial was stopped for futility following the planned interim analysis. Adverse events were similar in both groups (22/134 (16.4%) palivizumab arm versus 26/134 (19.4%) placebo arm). There were 6 dropouts and 168 infants were excluded from the efficacy analyses due to absent RSV circulation during the SARS-CoV-2 pandemic. Any RSV infection was similar in infants in both groups (18/47 (38.3%) palivizumab arm versus 11/47 (23.4%) placebo arm; aOR 2.2, 95% CI 0.7-6.5).

Interpretation

Daily intranasal palivizumab did not prevent RSV infection in late preterm infants. Our findings have important implications for the clinical development of mucosal mAbs, namely the necessity of timely interim analyses and further research to understand mucosal antibody half-life.

Funding

Funded by the Department of Pediatrics, University Medical Centre Utrecht, the Netherlands.

A precisely humanized FCRN transgenic mouse for preclinical pharmacokinetics studies

Monoclonal antibodies (mAbs) are one of the fastest-growing classes of drugs and have been approved to treat several diseases, including cancers and autoimmune disorders. Preclinical pharmacokinetics studies are performed to determine the therapeutically meaningful dosages and efficacy of candidate drugs. These studies are typically performed in non-human primates; however, using primates is costly and raises ethical considerations. As a result, rodent models that better mimic human-like pharmacokinetics have been generated and remain an area of active investigation. Pharmacokinetic characteristics of a candidate drug, such as half-life, are partly controlled by antibody binding to the human neonatal receptor hFCRN. Due to the abnormally high binding of human antibodies to mouse FCRN, traditional laboratory rodents do not accurately model the pharmacokinetics of human mAbs. In response, humanized rodents expressing hFCRN have been generated. However, these models generally use large inserts randomly integrated into the mouse genome. Here, we report the production and characterization of a CRISPR/Cas9-mediated hFCRN transgenic mouse termed SYNB-hFCRN. Using CRISPR/Cas9-assisted gene targeting, we prepared a strain with a simultaneous knockout of mFcrn and insertion of a hFCRN mini-gene under the control of the endogenous mouse promoter. These mice are healthy and express hFCRN in the appropriate tissues and immune cell subtypes. Pharmacokinetic evaluation of human IgG and adalimumab (Humira®) demonstrate hFCRN-mediated protection. These newly generated SYNB-hFCRN mice provide another valuable animal model for use in preclinical pharmacokinetics studies during early drug development.

Transgenic mouse models to study the physiological and pathophysiological roles of human Siglecs

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are important immunomodulatory receptors. Due to differences between human and mouse Siglecs, defining the in vivo roles for human Siglecs (hSiglecs) can be challenging. One solution is the development and use of hSiglec transgenic mice to assess the physiological roles of hSiglecs in health and disease. These transgenic mice can also serve as important models for the pre-clinical testing of immunomodulatory approaches that are based on targeting hSiglecs. Four general methods have been used to create hSiglec-expressing transgenic mice, each with associated advantages and disadvantages. To date, transgenic mouse models expressing hSiglec-2 (CD22), -3 (CD33), -7, -8, -9, -11, and -16 have been created. This review focuses on both the generation of these hSiglec transgenic mice, along with the important findings that have been made through their study. Cumulatively, hSiglec transgenic mouse models are providing a deeper understanding of the differences between human and mice orthologs/paralogs, mechanisms by which Siglecs regulate immune cell signaling, physiological roles of Siglecs in disease, and different paradigms where targeting Siglecs may be therapeutically advantageous.

Therapeutic Antibodies: An Overview

Polyclonal immunoglobulin (Ig) preparations have been used for several decades for treatment of primary and secondary immunodeficiencies and for treatment of some infections and intoxications. This has demonstrated the importance of Igs, also called antibodies (Abs) for prevention and elimination of infections. Moreover, elucidation of the structure and functions of Abs has suggested that they might be useful for targeted treatment of several diseases, including cancers and autoimmune diseases. The development of technologies for production of specific monoclonal Abs (MAbs) in large amounts has led to the production of highly effective therapeutic antibodies (TAbs), a collective term for MAbs (MAbs) with demonstrated clinical efficacy in one or more diseases. The number of approved TAbs is currently around hundred, and an even larger number is under development, including several engineered and modified Ab formats. The use of TAbs has provided new treatment options for many severe diseases, but prediction of clinical effect is difficult, and many patients eventually lose effect, possibly due to development of Abs to the TAbs or to other reasons. The therapeutic efficacy of TAbs can be ascribed to one or more effects, including binding and neutralization of targets, direct cytotoxicity, Ab-dependent complement-dependent cytotoxicity, Ab-dependent cellular cytotoxicity or others. The therapeutic options for TAbs have been expanded by development of several new formats of TAbs, including bispecific Abs, single domain Abs, TAb-drug conjugates, and the use of TAbs for targeted activation of immune cells. Most promisingly, current research and development can be expected to increase the number of clinical conditions, which may benefit from TAbs.

Dynamics of intracellular neonatal Fc receptor-ligand interactions in primary macrophages using biophysical fluorescence techniques

The neonatal Fc receptor (FcRn) is responsible for the recycling of endocytosed albumin and IgG, and contributes to their long plasma half-life. We recently identified an FcRn-dependent recycling pathway from macropinosomes in macrophages; however, little is known about the dynamics of intracellular FcRn–ligand interactions to promote recycling. Here we demonstrate a multiplexed biophysical fluorescent microscopy approach to resolve the spatiotemporal dynamics of albumin–FcRn interactions in living bone marrow–derived macrophages (BMDMs). We used the phasor approach to fluorescence lifetime imaging microscopy (FLIM) of Förster resonance energy transfer (FRET) to detect the interaction of a FcRn–mCherry fusion protein with endocytosed Alexa Fluor 488–labeled human serum albumin (HSA–AF488) in BMDMs, and raster image correlation spectroscopy (RICS) analysis of single fluorescent-labeled albumin molecules to monitor the diffusion kinetics of internalized albumin. Our data identified a major fraction of immobile HSA–AF488 molecules in endosomal structures of human FcRn-positive mouse macrophages and an increase in FLIM-FRET following endocytosis, including detection of FRET in tubular-like structures. A nonbinding mutant of albumin showed minimum FLIM-FRET and high mobility. These data reveal the kinetics of FcRn–ligand binding within endosomal structures for recruitment into transport carriers for recycling. These approaches have wide applicability for analyses of intracellular ligand–receptor interactions.

A Single Animal Species-Based Prediction of Human Clearance and First-in-Human Dose of Monoclonal Antibodies: Beyond Monkey

These days, there is a lot of emphasis on the prediction of human clearance (CL) from a single species for monoclonal antibodies (mabs). Many studies indicate that monkey is the most suitable species for the prediction of human clearance for mabs. However, it is not well established if rodents (mouse or rat) can also be used to predict human CL for mabs. The objectives of this study were to predict and compare human CL as well as first-in-human dose of mabs from mouse or rat, ormonkey. Four methods were used for the prediction of human CL of mabs. These methods were: use of four allometric exponents (0.75, 0.80, 0.85, and 0.90), a minimal physiologically based pharmacokinetics method (mPBPK), lymph flow rate, and liver blood flow rate. Based on the predicted CL, first-in-human dose of mabs was projected using either exponent 1.0 (linear scaling) or exponent 0.85, and human-equivalent dose (HED) from each of these species. The results of the study indicated that rat or mouse could provide a reasonably accurate prediction of human CL as well as first-in-human dose of mabs. When exponent 0.85 was used for CL prediction, there were 78%, 95%, and 92% observations within a 2-fold prediction error for mouse, rat, and monkey, respectively. Predicted human dose fell within the observed human dose range (administered to humans) for 10 out of 13 mabs for mouse, 11 out of 12 mabs for rat, and 12 out of 15 mabs for monkey. Overall, the clearance and first-in-human dose of mabs were predicted reasonably well by all three species (a single species). On average, monkey may be the best species for the prediction of human clearance and human dose but mouse or rat especially; rat can be a very useful species for conducting the aforementioned studies.

In Translation: FcRn across the Therapeutic Spectrum

As an essential modulator of IgG disposition, the neonatal Fc receptor (FcRn) governs the pharmacokinetics and functions many therapeutic modalities. In this review, we thoroughly reexamine the hitherto elucidated biological and thermodynamic properties of FcRn to provide context for our assessment of more recent advances, which covers antigen-binding fragment (Fab) determinants of FcRn affinity, transgenic preclinical models, and FcRn targeting as an immune-complex (IC)-clearing strategy. We further comment on therapeutic antibodies authorized for treating SARS-CoV-2 (bamlanivimab, casirivimab, and imdevimab) and evaluate their potential to saturate FcRn-mediated recycling. Finally, we discuss modeling and simulation studies that probe the quantitative relationship between in vivo IgG persistence and in vitro FcRn binding, emphasizing the importance of endosomal transit parameters.

Quantitative modeling predicts competitive advantages of a next generation anti-NKG2A monoclonal antibody over monalizumab for the treatment of cancer

A semimechanistic pharmacokinetic (PK)/receptor occupancy (RO) model was constructed to differentiate a next generation anti-NKG2A monoclonal antibody (KSQ mAb) from monalizumab, an immune checkpoint inhibitor in multiple clinical trials for the treatment of solid tumors. A three-compartment model incorporating drug PK, biodistribution, and NKG2A receptor interactions was parameterized using monalizumab PK, in vitro affinity measurements for both monalizumab and KSQ mAb, and receptor burden estimates from the literature. Following calibration against monalizumab PK data in patients with rheumatoid arthritis, the model successfully predicted the published PK and RO observed in gynecological tumors and in patients with squamous cell carcinoma of the head and neck. Simulations predicted that the KSQ mAb requires a 10-fold lower dose than monalizumab to achieve a similar RO over a 3-week period following q3w intravenous (i.v.) infusion dosing. A global sensitivity analysis of the model indicated that the drug-target binding affinity greatly affects the tumor RO and that an optimal affinity is needed to balance RO with enhanced drug clearance due to target mediated drug disposition. The model predicted that the KSQ mAb can be dosed over a less frequent regimen or at lower dose levels than the current monalizumab clinical dosing regimen of 10 mg/kg q2w. Either dosing strategy represents a competitive advantage over the current therapy. The results of this study demonstrate a key role for mechanistic modeling in identifying optimal drug parameters to inform and accelerate progression of mAb to clinical trials.

Removal of variable domain N-linked glycosylation as a means to improve the homogeneity of HIV-1 broadly neutralizing antibodies

Broadly neutralizing antibodies are showing promise in the treatment and prevention of HIV-1, with several now being evaluated clinically. Some lead clinical candidates, including antibodies CAP256-VRC26.25, N6, PGT121, and VRC07-523, have one or more N-linked glycosylation sequons in their variable domains (Fvs) from somatic hypermutation, and these glycans increase chemical heterogeneity, complicating the manufacture of these antibodies as products. Here we propose a general method to remove Fv glycans and use this method to develop engineered versions of these four antibodies with Fv glycans removed. When germline residues were introduced to remove each glycan, antibody properties between wild type and mutant were not significantly altered for CAP256-VRC26.25 and PGT121; however, germline mutants for N6 and VRC07-523 showed increased polyreactivity, which is known to correlate with unfavorable in vivo pharmacokinetics. To reduce polyreactivity induced by removal of Fv glycan, we mutated aromatic residues and arginines structurally proximal to the removed glycan and identified Fv glycan-removed variants with low polyreactivity for N6 and VRC07-523. Two such variants, N6-N72_LCQ-R18_LCD and VRC07-523-N72_LCQ-R24_LCD, showed thermostability, neutralization potency and breadth, and half-life in humanized FcRn mice that were similar to their wild-type Fv-glycosylated counterparts. The removal of Fv glycan and reduction of chemical heterogeneity were confirmed by liquid chromatography-mass spectrometry. With reduced heterogeneity, the Fv-glycan-removed variants developed here may have utility as products for treating or preventing infection by HIV-1.

Fc-optimized antibodies elicit CD8 immunity to viral respiratory infection

Antibodies against viral pathogens represent promising therapeutic agents for the control of infection, and their antiviral efficacy has been shown to require the coordinated function of both the Fab and Fc domains¹. The Fc domain engages a wide spectrum of receptors on discrete cells of the immune system to trigger the clearance of viruses and subsequent killing of infected cells^1–4. Here we report that Fc engineering of anti-influenza IgG monoclonal antibodies for selective binding to the activating Fcγ receptor FcγRIIa results in enhanced ability to prevent or treat lethal viral respiratory infection in mice, with increased maturation of dendritic cells and the induction of protective CD8⁺ T cell responses. These findings highlight the capacity for IgG antibodies to induce protective adaptive immunity to viral infection when they selectively activate a dendritic cell and T cell pathway, with important implications for the development of therapeutic antibodies with improved antiviral efficacy against viral respiratory pathogens.

An antibody Fc domain variant with enhanced binding to an activating Fc receptor on dendritic cells promotes the induction of a protective CD8 T cell response.

Improved Detection of in vivo Human NK Cell-Mediated Antibody-Dependent Cellular Cytotoxicity Using a Novel NOG-FcγR-Deficient Human IL-15 Transgenic Mouse

We generated an NOD/Shi-scid-IL2Rγ^null (NOG) mouse deficient for the Fcer1g and Fcgr2b genes (NOG-FcγR^−/− mice), in which monocytes/macrophages do not express activating (FcγRI, III, and IV) or inhibitory (FcγRIIB) Fcγ receptors. Antibody-dependent cellular cytotoxicity (ADCC) by innate immune cells was strongly reduced in this strain. Thus, while the growth of xenogeneic human tumors engrafted in conventional NOG mice was suppressed by innate cells upon specific antibody treatment, such growth inhibition was abrogated in NOG-FcγR^−/− mice. Using this novel strain, we further produced NOG-FcγR^−/−-mice expressing human IL-15 (NOG-FcγR^−/−-hIL-15 Tg). The mice inherited unique features from each strain, i.e., the long-term sustenance of human natural killer (NK) cells, and the elimination of mouse innate cell-mediated ADCC. As a result, segregation of human NK cell-mediated ADCC from mouse cell-mediated ADCC was possible in the NOG-FcγR^−/−-hIL-15 Tg mice. Our results suggest that NOG-FcγR^−/−-hIL-15 Tg mice are useful for validating the in vivo function of antibody drug candidates.

FcRn, but not FcγRs, drives maternal-fetal transplacental transport of human IgG antibodies

The IgG Fc domain has the capacity to interact with diverse types of receptors, including the neonatal Fc receptor (FcRn) and Fcγ receptors (FcγRs), which confer pleiotropic biological activities. Whereas FcRn regulates IgG epithelial transport and recycling, Fc effector activities, such as antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis, are mediated by FcγRs, which upon cross-linking transduce signals that modulate the function of effector leukocytes. Despite the well-defined and nonoverlapping functional properties of FcRn and FcγRs, recent studies have suggested that FcγRs mediate transplacental IgG transport, as certain Fc glycoforms were reported to be enriched in fetal circulation. To determine the contribution of FcγRs and FcRn to the maternal-fetal transport of IgG, we characterized the IgG Fc glycosylation in paired maternal-fetal samples from patient cohorts from Uganda and Nicaragua. No differences in IgG1 Fc glycan profiles and minimal differences in IgG2 Fc glycans were noted, whereas the presence or absence of galactose on the Fc glycan of IgG1 did not alter FcγRIIIa or FcRn binding, half-life, or their ability to deplete target cells in FcγR/FcRn humanized mice. Modeling maternal-fetal transport in FcγR/FcRn humanized mice confirmed that only FcRn contributed to transplacental transport of IgG; IgG selectively enhanced for FcRn binding resulted in enhanced accumulation of maternal antibody in the fetus. In contrast, enhancing FcγRIIIa binding did not result in enhanced maternal-fetal transport. These results argue against a role for FcγRs in IgG transplacental transport, suggesting Fc engineering of maternally administered antibody to enhance only FcRn binding as a means to improve maternal-fetal transport of IgG.

The biodistribution of therapeutic proteins: Mechanism, implications for pharmacokinetics, and methods of evaluation

Therapeutic proteins (TPs) are a diverse drug class that include monoclonal antibodies (mAbs), recombinantly expressed enzymes, hormones and growth factors, cytokines (e.g. chemokines, interleukins, interferons), as well as a wide range of engineered fusion scaffolds containing IgG1 Fc domain for half-life extension. As the pharmaceutical industry advances more potent and selective protein-based medicines through discovery and into the clinical stages of development, it has become widely appreciated that a comprehensive understanding of the mechanisms of TP biodistribution can aid this endeavor. This review aims to highlight the literature that has advanced our understanding of the determinants of TP biodistribution. A particular emphasis is placed on the multi-faceted role of the neonatal Fc receptor (FcRn) in mAb and Fc-fusion protein disposition. In addition, characterization of the TP-target interaction at the cell-level is discussed as an essential strategy to establish pharmacokinetic-pharmacodynamic (PK/PD) relationships that may lead to more informed human dose projections during clinical development. Methods for incorporation of tissue and cell-level parameters defining these characteristics into higher-order mechanistic and semi-mechanistic PK models will also be presented.

Antibody Structure and Function: The Basis for Engineering Therapeutics

Antibodies and antibody-derived macromolecules have established themselves as the mainstay in protein-based therapeutic molecules (biologics). Our knowledge of the structure–function relationships of antibodies provides a platform for protein engineering that has been exploited to generate a wide range of biologics for a host of therapeutic indications. In this review, our basic understanding of the antibody structure is described along with how that knowledge has leveraged the engineering of antibody and antibody-related therapeutics having the appropriate antigen affinity, effector function, and biophysical properties. The platforms examined include the development of antibodies, antibody fragments, bispecific antibody, and antibody fusion products, whose efficacy and manufacturability can be improved via humanization, affinity modulation, and stability enhancement. We also review the design and selection of binding arms, and avidity modulation. Different strategies of preparing bispecific and multispecific molecules for an array of therapeutic applications are included.

FcRn mediates fast recycling of endocytosed albumin and IgG from early macropinosomes in primary macrophages

The neonatal Fc receptor (FcRn) rescues albumin and IgG from degradation following endocytosis and thereby extends the half-life of these plasma proteins. However, the pathways for the uptake of these soluble FcRn ligands, and the recycling itinerary of the FcRn-ligand complexes, have not been identified in primary cells. Here, we have defined the recycling of human albumin and IgG in primary mouse macrophages selectively expressing the human FcRn. Albumin is internalised by macropinocytosis; in the absence of FcRn, internalised albumin is rapidly degraded, while in the presence of FcRn albumin colocalises to SNX5-positive membrane domains and is partitioned into tubules emanating from early macropinosomes for delivery in transport carriers to the plasma membrane. Soluble monomeric IgG was also internalised by macropinocytosis and rapidly recycled by the same pathway. In contrast, the fate of IgG bound to surface Fcγ receptors differed from monomeric IgG endocytosed by macropinocytosis. Overall, our findings identify a rapid recycling pathway for FcRn ligands from early macropinosomes to the cell surface of primary cells.

Mouse Strains Influence Clearance and Efficacy of Antibody and Antibody-Drug Conjugate Via Fc-FcγR Interaction

To provide a better understanding of the pharmacokinetics-pharmacodynamics relationships of antibody-based drugs, we analyzed several chimeric and humanized monoclonal antibodies or antibody-drug conjugates (ADC) for PK and efficacy among four strains of mice. Notably, antibodies and ADCs displayed a dose-dependent drug disposition profile in the plasma of NSG mice. The increased clearance rate in NSG mice resulted in the reduction of antitumor activity of ADCs. Furthermore, we identified that the abnormal clearance was mediated by Fc-FcγR interaction by comparing antibodies that lack FcγR binding capacity. We also found a high percentage of FcγR-expressing macrophages in the bone marrow, spleen, and liver of NSG mice, which may be responsible for the abnormal distribution of antibodies. Overall, these findings suggest that preclinical evaluation of efficacy and pharmacokinetics of antibodies and ADCs need to consider mouse strain-induced variations.

Simple and rapid analysis of tocilizumab using HPLC-fluorescence detection method

We developed a novel assay using high-performance liquid chromatography (HPLC) with fluorescence detection for the determination of tocilizumab (TCZ), after it has undergone a facile and rapid pretreatment. TCZ belongs to the same subclass as IgG1 (Immunoglobulin G subclass 1), and we could separate TCZ from IgG1 without antigen-antibody reactions, with the novel detection method. The separation of these antibodies was achieved by pretreatment with an organic solvent containing a base, such as trimethylamine and triethylamine. The effect of these bases on the separation of TCZ is related to the hydrophobicity of the base rather than the electrostatic charge. The results indicated that the surface charge of antibodies changed because of the structural change, even though the difference in the amino acid sequences of the antibodies was very low. Our method is available for the separation of the antibody subclasses, and it would be useful to assay TCZ in blood.

The neonatal Fc receptor is a pan-echovirus receptor

Echoviruses are amongst the most common causative agents of aseptic meningitis worldwide and are particularly devastating in the neonatal population, where they are associated with severe hepatitis, neurological disease, including meningitis and encephalitis, and even death. Here, we identify the neonatal Fc receptor (FcRn) as a pan-echovirus receptor. We show that loss of expression of FcRn or its binding partner beta 2 microglobulin (β2M) renders cells resistant to infection by a panel of echoviruses at the stage of virus attachment, and that a blocking antibody to β2M inhibits echovirus infection in cell lines and in primary human intestinal epithelial cells. We also show that expression of human, but not mouse, FcRn renders nonpermissive human and mouse cells sensitive to echovirus infection and that the extracellular domain of human FcRn directly binds echovirus particles and neutralizes infection. Lastly, we show that neonatal mice expressing human FcRn are more susceptible to echovirus infection by the enteral route. Our findings thus identify FcRn as a pan-echovirus receptor, which may explain the enhanced susceptibility of neonates to echovirus infections.

Improvement of pharmacokinetic properties of therapeutic antibodies by antibody engineering

Monoclonal antibodies (mAbs) have become an important therapeutic option for several diseases. Since several mAbs have shown promising efficacy in clinic, the competition to develop mAbs has become severe. In efforts to gain a competitive advantage over other mAbs and provide significant benefits to patients, innovations in antibody engineering have aimed at improving the pharmacokinetic properties of mAbs. Because engineering can provide therapeutics that are more convenient, safer, and more efficacious for patients in several disease areas, it is an attractive approach to provide significant benefits to patients. Further advances in engineering mAbs to modulate their pharmacokinetics were driven by the increase of total soluble target antigen concentration that is often observed after injecting a mAb, which then requires a high dosage to antagonize. To decrease the required dosage, several antibody engineering techniques have been invented that reduce the total concentration of soluble target antigen. Here, we review the various ways that antibody engineering can improve the pharmacokinetic properties of mAbs.

FcRn Expression in Wildtype Mice, Transgenic Mice, and in Human Tissues

Quantitative real-time PCR and Western blot methods were developed to assess neonatal Fc-receptor (FcRn) mRNA and protein expression in human FcRn transgenic mice, Swiss Webster mice, and in select human tissues. Additionally, FcRn turnover was evaluated via pulse-chase. FcRn mRNA expression was significantly higher in transgenic mice when compared to mouse FcRn mRNA in Swiss Webster mice and it ranged from 184-fold higher in the kidney to 109,000-fold higher in the skin. FcRn protein expression was found to be 13-fold lower in kidney to 5.6-fold higher in lung obtained from transgenic mice compared to FcRn protein expression in lung samples obtained from Swiss Webster mice. FcRn protein expression in human liver and small intestine tissues matched more closely with FcRn expression in Swiss Webster mice but were significantly lower when compared to values found from Swiss Webster and transgenic mice. Although FcRn mRNA expression correlated significantly with protein expression (p < 0.0005), the correlation coefficient was only 0.113. As such, the measurement of FcRn protein may be preferred to FcRn mRNA for quantitative applications. Significant differences were found in FcRn expression in transgenic mice, Swiss Webster mice, and human tissues, which may have implications for the use of mouse models in the assessment of monoclonal antibody disposition, efficacy, and safety.

Extending human IgG half-life using structure-guided design

Engineering of antibodies for improved pharmacokinetics through enhanced binding to the neonatal Fc receptor (FcRn) has been demonstrated in transgenic mice, non-human primates and humans. Traditionally, such approaches have largely relied on random mutagenesis and display formats, which fail to address related critical attributes of the antibody, such as effector functions or biophysical stability. We have developed a structure- and network-based framework to interrogate the engagement of IgG with multiple Fc receptors (FcRn, C1q, TRIM21, FcγRI, FcγRIIa/b, FcγRIIIa) simultaneously. Using this framework, we identified features that govern Fc-FcRn interactions and identified multiple distinct pathways for enhancing FcRn binding in a pH-specific manner. Network analysis provided a novel lens to study the allosteric impact of half-life-enhancing Fc mutations on FcγR engagement, which occurs distal to the FcRn binding site. Applying these principles, we engineered a panel of unique Fc variants that enhance FcRn binding while maintaining robust biophysical properties and wild type-like binding to activating receptors. An antibody harboring representative Fc designs demonstrates a half-life improvement of > 9 fold in transgenic mice and > 3.5 fold in cynomolgus monkeys, and maintains robust effector functions such as antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity.

Linear pharmacokinetic parameters for monoclonal antibodies are similar within a species and across different pharmacological targets: A comparison between human, cynomolgus monkey and hFcRn Tg32 transgenic mouse using a population-modeling approach

The linear pharmacokinetics (PK) of therapeutic monoclonal antibodies (mAbs) can be considered a class property with values that are similar to endogenous IgG. Knowledge of these parameters across species could be used to avoid unnecessary in vivo PK studies and to enable early PK predictions and pharmacokinetic/pharmacodynamic (PK/PD) simulations. In this work, population-pharmacokinetic (popPK) modeling was used to determine a single set of ‘typical’ popPK parameters describing the linear PK of mAbs in human, cynomolgus monkey and transgenic mice expressing the human neonatal Fc receptor (hFcRn Tg32), using a rich dataset of 27 mAbs. Non-linear PK was excluded from the datasets and a 2-compartment model was applied to describe mAb disposition. Typical human popPK estimates compared well with data from comparator mAbs with linear PK in the clinic. Outliers with higher than typical clearance were found to have non-specific interactions in an affinity-capture self-interaction nanoparticle spectroscopy assay, offering a potential tool to screen out these mAbs at an early stage. Translational strategies were investigated for prediction of human linear PK of mAbs, including use of typical human popPK parameters and allometric exponents from cynomolgus monkey and Tg32 mouse. Each method gave good prediction of human PK with parameters predicted within 2-fold. These strategies offer alternative options to the use of cynomolgus monkeys for human PK predictions of linear mAbs, based on in silico methods (typical human popPK parameters) or using a rodent species (Tg32 mouse), and call into question the value of completing extensive in vivo preclinical PK to inform linear mAb PK.

Hematopoietic cells as site of first-pass catabolism after subcutaneous dosing and contributors to systemic clearance of a monoclonal antibody in mice

The neonatal Fc receptor (FcRn) has been demonstrated to contribute to a high bioavailability of monoclonal antibodies (mAbs). In this study, we explored the cellular sites of FcRn-mediated protection after subcutaneous (SC) and intravenous (IV) administration. SC absorption and IV disposition kinetics of a mAb were studied in hFcRn transgenic (Tg) bone marrow chimeric mice in which hFcRn was restricted to radioresistant cells or hematopoietic cells. SC bioavailabilities close to 90% were observed in hFcRn Tg mice and chimeric mice with hFcRn expression in hematopoietic cells, whereas SC bioavailabilities were markedly lower when FcRn was missing in hematopoietic cells. Our study demonstrates: 1) FcRn in radiosensitive hematopoietic cells is required for high SC bioavailability, indicating first-pass catabolism after SC administration by hematopoietic cells; 2) FcRn-mediated transcytosis or recycling by radioresistent cells is not required for high SC bioavailability; and 3) after IV administration hematopoietic and radioresistent cells contribute about equally to clearance of the mAb. A pharmacokinetic model was devised to describe a mixed elimination via radioresistent and hematopoietic cells from vascular and extravascular compartments, respectively. Overall, the study indicates a relevant role of hematopoietic cells for first-pass clearance of mAbs after SC administration and confirms their role in the overall clearance of mAbs.

Design and preclinical characterization of ALXN1210: A novel anti-C5 antibody with extended duration of action

Eculizumab, a monoclonal antibody (mAb) directed against complement protein C5, is considered to be the current standard of care for patients with paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome. This study describes the generation and preclinical attributes of ALXN1210, a new long-acting anti-C5 mAb, obtained through select modifications to eculizumab to both largely abolish target-mediated drug disposition (TMDD) and increase recycling efficiency via the neonatal Fc receptor (FcRn). To attenuate the effect of TMDD on plasma terminal half-life (t_1/2), histidine substitutions were engineered into the complementarity-determining regions of eculizumab to enhance the dissociation rate of the mAb:C5 complex in the acidic early endosome relative to the slightly basic pH of blood. Antibody variants with optimal pH-dependent binding to C5 exhibited little to no TMDD in mice in the presence of human C5. To further enhance the efficiency of FcRn-mediated recycling of the antibody, two additional substitutions were introduced to increase affinity for human FcRn. These substitutions yielded an additional doubling of the t_½ of surrogate anti-mouse C5 antibodies with reduced TMDD in transgenic mice expressing the human FcRn. In conclusion, ALXN1210 is a promising new therapeutic candidate currently in clinical development for treatment of patients with PNH and atypical hemolytic uremic syndrome.

Evaluation of an FcRn affinity chromatographic method for IgG1-type antibodies and evaluation of IgG variants

AIM

The neonatal Fc-receptor (FcRn) mediates long serum half-life of therapeutic IgG-type antibodies. This interaction represents a critical quality attribute in terms of pharmacokinetics and should be covered by respective quality control strategies. Antibodies are taken up by cells unspecifically and can bind to FcRn in early endosomes preventing lysosomal degradation and allowing release back into circulation. Reflecting this complex cycle in an in vitro assay strategy represents a challenging task.

METHODOLOGY

We report the qualification of an FcRn affinity chromatographic method and, for the first time, establish a noncriticality window. We analyzed different IgG-type antibodies, subtypes, glycoforms as well as mutants.

CONCLUSION

The FcRn affinity chromatographic method allows the assessment of mAb samples with respect to their pH-dependent FcRn interaction. Furthermore, the method's capabilities and current limitations are discussed.

Functional, Biophysical, and Structural Characterization of Human IgG1 and IgG4 Fc Variants with Ablated Immune Functionality

Engineering of fragment crystallizable (Fc) domains of therapeutic immunoglobulin (IgG) antibodies to eliminate their immune effector functions while retaining other Fc characteristics has numerous applications, including blocking antigens on Fc gamma (Fcγ) receptor-expressing immune cells. We previously reported on a human IgG2 variant termed IgG2σ with barely detectable activity in antibody-dependent cellular cytotoxicity, phagocytosis, complement activity, and Fcγ receptor binding assays. Here, we extend that work to IgG1 and IgG4 antibodies, alternative subtypes which may offer advantages over IgG2 antibodies. In several in vitro and in vivo assays, the IgG1σ and IgG4σ variants showed equal or even lower Fc-related activities than the corresponding IgG2σ variant. In particular, IgG1σ and IgG4σ variants demonstrate complete lack of effector function as measured by antibody-dependent cellular cytotoxicity, complement-dependent cytotoxicity, antibody-dependent cellular phagocytosis, and in vivo T-cell activation. The IgG1σ and IgG4σ variants showed acceptable solubility and stability, and typical human IgG1 pharmacokinetic profiles in human FcRn-transgenic mice and cynomolgus monkeys. In silico T-cell epitope analyses predict a lack of immunogenicity in humans. Finally, crystal structures and simulations of the IgG1σ and IgG4σ Fc domains can explain the lack of Fc-mediated immune functions. These variants show promise for use in those therapeutic antibodies and Fc fusions for which the Fc domain should be immunologically "silent".

Pharmacokinetic de-risking tools for selection of monoclonal antibody lead candidates

Pharmacokinetic studies play an important role in all stages of drug discovery and development. Recent advancements in the tools for discovery and optimization of therapeutic proteins have created an abundance of candidates that may fulfill target product profile criteria. Implementing a set of in silico, small scale in vitro and in vivo tools can help to identify a clinical lead molecule with promising properties at the early stages of drug discovery, thus reducing the labor and cost in advancing multiple candidates toward clinical development. In this review, we describe tools that should be considered during drug discovery, and discuss approaches that could be included in the pharmacokinetic screening part of the lead candidate generation process to de-risk unexpected pharmacokinetic behaviors of Fc-based therapeutic proteins, with an emphasis on monoclonal antibodies.

Identification of human IgG1 variant with enhanced FcRn binding and without increased binding to rheumatoid factor autoantibody

Various studies have demonstrated that Fc engineering to enhance neonatal Fc receptor (FcRn) binding is effective for elongating half-life or increasing cellular uptake of IgG. A previous study has shown that a N434H mutation to enhance FcRn binding resulted in increased binding to rheumatoid factor (RF) autoantibody, which is not desirable for therapeutic use in autoimmune disease. In this study, we first showed that all the existing Fc variants with enhanced FcRn binding also show increased RF binding, and then identified specific mutations that could be introduced to those Fc variants to reduce the RF binding. Furthermore, we generated novel Fc variants that do not increase RF binding and show half-lives of 45 d in cynomolgus monkey, which is longer than those of previously reported Fc variants. In addition, we generated novel Fc variants with antigen sweeping activity that do not increase RF binding. We expect that these novel Fc variants will be useful as antibody therapeutics against autoimmune diseases.

Utility of immunodeficient mouse models for characterizing the preclinical pharmacokinetics of immunogenic antibody therapeutics

Prior to clinical studies, the pharmacokinetics (PK) of antibody-based therapeutics are characterized in preclinical species; however, those species can elicit immunogenic responses that can lead to an inaccurate estimation of PK parameters. Immunodeficient (SCID) transgenic hFcRn and C57BL/6 mice were used to characterize the PK of three antibodies that were previously shown to be immunogenic in mice and cynomolgus monkeys. Four mouse strains, Tg32 hFcRn SCID, Tg32 hFcRn, SCID and C57BL/6, were administered adalimumab (Humira®), mAbX and mAbX-YTE at 1 mg/kg, and in SCID strains there was no incidence of immunogenicity. In non-SCID strains, drug-clearing ADAs appeared after 4-7 days, which affected the ability to accurately calculate PK parameters. Single species allometric scaling of PK data for Humira® in SCID and hFcRn SCID mice resulted in improved human PK predictions compared to C57BL/6 mice. Thus, the SCID mouse model was demonstrated to be a useful tool for assessing the preclinical PK of immunogenic therapeutics.

Utility of a human FcRn transgenic mouse model in drug discovery for early assessment and prediction of human pharmacokinetics of monoclonal antibodies

Therapeutic antibodies continue to develop as an emerging drug class, with a need for preclinical tools to better predict in vivo characteristics. Transgenic mice expressing human neonatal Fc receptor (hFcRn) have potential as a preclinical pharmacokinetic (PK) model to project human PK of monoclonal antibodies (mAbs). Using a panel of 27 mAbs with a broad PK range, we sought to characterize and establish utility of this preclinical animal model and provide guidance for its application in drug development of mAbs. This set of mAbs was administered to both hemizygous and homozygous hFcRn transgenic mice (Tg32) at a single intravenous dose, and PK parameters were derived. Higher hFcRn protein tissue expression was confirmed by liquid chromatography-high resolution tandem mass spectrometry in Tg32 homozygous versus hemizygous mice. Clearance (CL) was calculated using non-compartmental analysis and correlations were assessed to historical data in wild-type mouse, non-human primate (NHP), and human. Results show that mAb CL in hFcRn Tg32 homozygous mouse correlate with human (r(2) = 0.83, r = 0.91, p < 0.01) better than NHP (r(2) = 0.67, r = 0.82, p < 0.01) for this dataset. Applying simple allometric scaling using an empirically derived best-fit exponent of 0.93 enabled the prediction of human CL from the Tg32 homozygous mouse within 2-fold error for 100% of mAbs tested. Implementing the Tg32 homozygous mouse model in discovery and preclinical drug development to predict human CL may result in an overall decreased usage of monkeys for PK studies, enhancement of the early selection of lead molecules, and ultimately a decrease in the time for a drug candidate to reach the clinic.

Albumin-deficient mouse models for studying metabolism of human albumin and pharmacokinetics of albumin-based drugs

Serum albumin is the major determinant of blood colloidal osmotic pressure acting as a depot and distributor of compounds including drugs. In humans, serum albumin exhibits an unusually long half-life mainly due to protection from catabolism by neonatal Fc receptor (FcRn)-mediated recycling. These properties make albumin an attractive courier of therapeutically-active compounds. However, pharmaceutical research and development of albumin-based therapeutics has been hampered by the lack of appropriate preclinical animal models. To overcome this, we developed and describe the first mouse with a genetic deficiency in albumin and its incorporation into an existing humanized FcRn mouse model, B6.Cg-Fcgrt(tm1Dcr) Tg(FCGRT)32Dcr/DcrJ (Tg32). Albumin-deficient strains (Alb(-/-)) were created by TALEN-mediated disruption of the albumin (Alb) gene directly in fertilized oocytes derived from Tg32 mice and its non-transgenic background control, C57BL/6J (B6). The resulting Alb(-/-) strains are analbuminemic but healthy. Intravenous administration of human albumin to Tg32-Alb(-/-) mFcRn(-/-) hFcRn(Tg/Tg)) mice results in a remarkably extended human albumin serum half-life of ∼24 days, comparable to that found in humans, and in contrast to half-lives of 2.6-5.8 d observed in B6, B6-Alb(-/-) and Tg32 strains. This striking increase can be explained by the absence of competing endogenous mouse albumin and the presence of an active human FcRn. These novel albumin-deficient models provide unique tools for investigating the biology and pathobiology of serum albumin and are a more appropriate rodent surrogates for evaluating human serum albumin pharmacokinetics and albumin-based compounds.

Improving target cell specificity using a novel monovalent bispecific IgG design

Monovalent bispecific IgGs cater to a distinct set of mechanisms of action but are difficult to engineer and manufacture because of complexities associated with correct heavy and light chain pairing. We have created a novel design, “DuetMab,” for efficient production of these molecules. The platform uses knobs-into-holes (KIH) technology for heterodimerization of 2 distinct heavy chains and increases the efficiency of cognate heavy and light chain pairing by replacing the native disulfide bond in one of the C_H1-C_L interfaces with an engineered disulfide bond. Using two pairs of antibodies, cetuximab (anti-EGFR) and trastuzumab (anti-HER2), and anti-CD40 and anti-CD70 antibodies, we demonstrate that DuetMab antibodies can be produced in a highly purified and active form, and show for the first time that monovalent bispecific IgGs can concurrently bind both antigens on the same cell. This last property compensates for the loss of avidity brought about by monovalency and improves selectivity toward the target cell.

The neonatal Fc receptor (FcRn) binds independently to both sites of the IgG homodimer with identical affinity

The neonatal Fc receptor (FcRn) is expressed by cells of epithelial, endothelial and myeloid lineages and performs multiple roles in adaptive immunity. Characterizing the FcRn/IgG interaction is fundamental to designing therapeutic antibodies because IgGs with moderately increased binding affinities for FcRn exhibit superior serum half-lives and efficacy. It has been hypothesized that 2 FcRn molecules bind an IgG homodimer with disparate affinities, yet their affinity constants are inconsistent across the literature. Using surface plasmon resonance biosensor assays that eliminated confounding experimental artifacts, we present data supporting an alternate hypothesis: 2 FcRn molecules saturate an IgG homodimer with identical affinities at independent sites, consistent with the symmetrical arrangement of the FcRn/Fc complex observed in the crystal structure published by Burmeister et al. in 1994. We find that human FcRn binds human IgG1 with an equilibrium dissociation constant (K_D) of 760 ± 60 nM (N = 14) at 25°C and pH 5.8, and shows less than 25% variation across the other human subtypes. Human IgG1 binds cynomolgus monkey FcRn with a 2-fold higher affinity than human FcRn, and binds both mouse and rat FcRn with a 10-fold higher affinity than human FcRn. FcRn/IgG interactions from multiple species show less than a 2-fold weaker affinity at 37°C than at 25°C and appear independent of an IgG's variable region. Our in vivo data in mouse and rat models demonstrate that both affinity and avidity influence an IgG's serum half-life, which should be considered when choosing animals, especially transgenic systems, as surrogates.

Inhibitory FcγRIIb-Mediated Soluble Antigen Clearance from Plasma by a pH-Dependent Antigen-Binding Antibody and Its Enhancement by Fc Engineering

Fc engineering can modulate the Fc-FcγR interaction and thus enhance the potency of Abs that target membrane-bound Ags, but it has not been applied to Abs that target soluble Ags. In this study, we revealed a previously unknown function of inhibitory FcγRII in vivo and, using an Ab that binds to Ag pH dependently, demonstrated that the function can be exploited to target soluble Ag. Because pH-dependent Ab dissociates Ag in acidic endosome, its Ag clearance from circulation reflects the cellular uptake rate of Ag/Ab complexes. In vivo studies showed that FcγR but not neonatal FcR contributes to Ag clearance by the pH-dependent Ab, and when Fc binding to mouse FcγRII and III was increased, Ag clearance was markedly accelerated in wild-type mice and FcR γ-chain knockout mice, but the effect was diminished in FcγRII knockout mice. This demonstrates that mouse FcγRII efficiently promotes Ab uptake into the cell and its subsequent recycling back to the cell surface. Furthermore, when a human IgG1 Fc variant with selectively increased binding to human FcγRIIb was tested in human FcγRIIb transgenic mice, Ag clearance was accelerated without compromising the Ab half-life. Taken together, inhibitory FcγRIIb was found to play a prominent role in the cellular uptake of monomeric Ag/Ab immune complexes in vivo, and when the Fc of a pH-dependent Ab was engineered to selectively enhance human FcγRIIb binding, the Ab could accelerate soluble Ag clearance from circulation. We assume such a function would enhance the therapeutic potency of Abs that target soluble Ags.

Genetically engineered humanized mouse models for preclinical antibody studies

The use of genetic engineering has vastly improved our capabilities to create animal models relevant in preclinical research. With the recent advances in gene-editing technologies, it is now possible to very rapidly create highly tunable mouse models as needs arise. Here, we provide an overview of genetic engineering methods, as well as the development of humanized neonatal Fc receptor (FcRn) models and their use for monoclonal antibody in vivo studies.

Charge-mediated influence of the antibody variable domain on FcRn-dependent pharmacokinetics

Here, we investigated the influence of the variable fragment (Fv) of IgG antibodies on the binding to the neonatal Fc receptor (FcRn) as well as on FcRn-dependent pharmacokinetics (PK). FcRn plays a key role in IgG homeostasis, and specific manipulation in the crystallizable fragment (Fc) is known to affect FcRn-dependent PK. Although the influence of the antigen-binding fragment (Fab) on FcRn interactions has been reported, the underlying mechanism is hitherto only poorly understood. Therefore, we analyzed the two IgG1 antibodies, briakinumab and ustekinumab, that have similar Fc parts but different terminal half-lives in human and systematically engineered variants of them with cross-over exchanges and varied charge distribution. Using FcRn affinity chromatography, molecular dynamics simulation, and in vivo PK studies in human FcRn transgenic mice, we provide evidence that the charge distribution on the Fv domain is involved in excessive FcRn binding. This excessive binding prevents efficient FcRn-IgG dissociation at physiological pH, thereby reducing FcRn-dependent terminal half-lives. Furthermore, we observed a linear correlation between FcRn column retention times of the antibody variants and the terminal half-lives in vivo. Taken together, our study contributes to a better understanding of the FcRn-IgG interaction, and it could also provide profound potential in FcRn-dependent antibody engineering of the variable Fab region.

Unraveling the Interaction between FcRn and Albumin: Opportunities for Design of Albumin-Based Therapeutics

The neonatal Fc receptor (FcRn) was first found to be responsible for transporting antibodies of the immunoglobulin G (IgG) class from the mother to the fetus or neonate as well as for protecting IgG from intracellular catabolism. However, it has now become apparent that the same receptor also binds albumin and plays a fundamental role in homeostatic regulation of both IgG and albumin, as FcRn is expressed in many different cell types and organs at diverse body sites. Thus, to gain a complete understanding of the biological function of each ligand, and also their distribution in the body, an in-depth characterization of how FcRn binds and regulates the transport of both ligands is necessary. Importantly, such knowledge is also relevant when developing new drugs, as IgG and albumin are increasingly utilized in therapy. This review discusses our current structural and biological understanding of the relationship between FcRn and its ligands, with a particular focus on albumin and design of albumin-based therapeutics.

Maternal Antiasthma Simplified Herbal Medicine Intervention therapy prevents airway inflammation and modulates pulmonary innate immune responses in young offspring mice

BACKGROUND

Maternal asthma is a risk factor for asthma in offspring; however, transmission of the risk for allergic asthma without direct offspring sensitization has not been explored.

OBJECTIVE

To determine whether offspring from mothers with ovalbumin (OVA)-sensitized asthma would develop airway disease at first-ever exposure to OVA and whether preconception maternal treatment with the Antiasthma Simplified Herbal Medicine Intervention (ASHMI) or dexamethasone (DEX) could modify this risk in offspring.

METHODS

Female BALB/c mice (F0) with OVA-induced asthma were generated using established protocols. Mice with asthma were treated with ASHMI, DEX, or water for 6 to 7 weeks. Naive mice served as controls. Subsequently, mice were mated. Twelve-day-old F1 offspring received 3 consecutive intranasal low- or high-dose OVA exposures without sensitization. Forty-eight hours later, airway inflammation, mucus hypersecretion, serum antibodies, and cytokines were evaluated.

RESULTS

Offspring from OVA-sensitized mothers, but not naive mothers, showed eosinophilic and neutrophilic airway inflammation, and mucus hyperplasia after OVA exposure and he presence of OVA-specific IgG1 and IgG2a. Offspring of ASHMI- and DEX-treated mothers showed decreased airway inflammation and mucus hypersecretion after low-dose OVA (P < .05-.001 for the 2 comparisons vs offspring of OVA/Sham mothers). Offspring of ASHMI-treated, but not DEX-treated, mothers were protected after the high-dose OVA challenge (P < .05-.01 vs offspring OVA/Sham). Maternal ASHMI therapy was associated with increased IgG2a (P < .01 vs offspring of OVA/Sham mothers) and decreased bronchoalveolar lavage fluid CXCL-1 and eotaxin-1 levels (P < .01 and P < .05, respectively, vs offspring of OVA/Sham mothers).

CONCLUSION

Offspring of mothers with OVA-induced asthma developed airway inflammation and mucus to first-ever OVA exposure without prior sensitization. Maternal therapy with ASHMI was superior to DEX in decreasing offspring susceptibility to airway disease and could be a strategy to lower asthma prevalence.

A novel approach to investigate the effect of methionine oxidation on pharmacokinetic properties of therapeutic antibodies

Preserving the chemical and structural integrity of therapeutic antibodies during manufacturing and storage is a major challenge during pharmaceutical development. Oxidation of Fc methionines Met252 and Met428 is frequently observed, which leads to reduced affinity to FcRn and faster plasma clearance if present at high levels. Because oxidation occurs in both positions simultaneously, their individual contribution to the concomitant changes in pharmacokinetic properties has not been clearly established. A novel pH-gradient FcRn affinity chromatography method was applied to isolate three antibody oxidation variants from an oxidized IgG1 preparation based on their FcRn binding properties. Physico-chemical characterization revealed that the three oxidation variants differed predominantly in the number of oxMet252 per IgG (0, 1, or 2), but not significantly in the content of oxMet428. Corresponding to the increase in oxMet252 content, stepwise reduction of FcRn affinity in vitro, as well as faster clearance and shorter terminal half-life, in huFcRn-transgenic mice were observed. A single Met252 oxidation per antibody had no significant effect on pharmacokinetics (PK) compared with unmodified IgG. Importantly, only molecules with both heavy chains oxidized at Met252 exhibited significantly faster clearance. In contrast, Met428 oxidation had no apparent negative effect on PK and even led to somewhat improved FcRn binding and slower clearance. This minor effect, however, seemed to be abrogated by the dominant effect of Met252 oxidation. The novel approach of functional chromatographic separation of IgG oxidation variants followed by physico-chemical and biological characterization has yielded the first experimentally-backed explanation for the unaltered PK properties of antibody preparations containing relatively high Met252 and Met428 oxidation levels.

Application of human FcRn transgenic mice as a pharmacokinetic screening tool of monoclonal antibody

1. For drug discovery, useful screening tools are essential to select superior candidates. Here, we evaluated the applicability of transgenic mice expressing human neonatal Fc receptor (FcRn) (hFcRn Tgm) as a pharmacokinetic screening tool of therapeutic monoclonal antibodies (mAbs) and Fc-fusion proteins that overcomes the species difference in FcRn binding. 2. Marketed 11 mAbs and 2 Fc-fusion proteins were intravenously administered to hFcRn Tgm and WT mice. The half-lives in hFcRn Tgm and WT mice were compared with those in human obtained from literature. The linear half-lives in human and monkey were also calculated by nonlinear pharmacokinetic analysis. For comparison, correlations of half-lives between monkey and human were also evaluated. 3. The half-lives of mAbs and Fc-fusion proteins after intravenous administration ranged from 1.1 to 13.2 days in hFcRn Tgm and from 1.2 to 30.3 days in WT mice. The half-lives in human correlated more closely with those in hFcRn Tgm than in WT mice and monkey. 4. Our results suggest that hFcRn Tgm are a valuable and useful tool for pharmacokinetic screening of mAbs and Fc-fusion proteins in the preclinical stage. Furthermore, we believe that hFcRn Tgm are broadly applicable to preclinical pharmacokinetic screening of mAbs-based therapeutics.

Humanized FcRn mouse models for evaluating pharmacokinetics of human IgG antibodies

A key element for the successful development of novel therapeutic antibodies is to fully understand their pharmacokinetic and pharmacodynamic behavior before performing clinical trials. While many in vitro modeling approaches exist, these simply cannot substitute for data obtained from appropriate animal models. It was established quite early that the unusual long serum half-life of immunoglobulin G's (IgGs) and Fc domains are due to their rescue and recycling by the neonatal Fc receptor (FcRn). The diverse roles of FcRn became apparent after isolation and cloning. Interesting are the significant species differences between rodent and human FcRn reactivity, rendering wild type rodents an inadequate model for studying IgG serum half-life. With the advance of genetic engineering mouse models have been established expressing human FcRn, and lacking mouse FcRn protein. These models have become highly relevant tools for serum half-life analysis of Fc-containing compounds.

Analytical FcRn affinity chromatography for functional characterization of monoclonal antibodies

The neonatal Fc receptor (FcRn) is important for the metabolic fate of IgG antibodies in vivo. Analysis of the interaction between FcRn and IgG in vitro might provide insight into the structural and functional integrity of therapeutic IgG that may affect pharmacokinetics (PK) in vivo. We developed a standardized pH gradient FcRn affinity liquid chromatography method with conditions closely resembling the physiological mechanism of interaction between IgG and FcRn. This method allows the separation of molecular IgG isoforms, degradation products and engineered molecules based on their affinity to FcRn. Human FcRn was immobilized on the column and a linear pH gradient from pH 5.5 to 8.8 was applied. FcRn chromatography was used in comparison to surface plasmon resonance to characterize different monoclonal IgG preparations, e.g., oxidized or aggregated species. Wild-type and engineered IgGs were compared in vitro by FcRn chromatography and in vivo by PK studies in huFcRn transgenic mice. Analytical FcRn chromatography allows differentiation of IgG samples and variants by peak pattern and retention time profile. The method can distinguish: 1) IgGs with different Fabs, 2) oxidized from native IgG, 3) aggregates from monomer and 4) antibodies with mutations in the Fc part from wild-type IgGs. Changes in the FcRn chromatographic behavior of mutant IgGs relative to the wild-type IgG correlate to changes in the PK profile in the FcRn transgenic mice. These results demonstrate that FcRn affinity chromatography is a useful new method for the assessment of IgG integrity.

Engineered monoclonal antibody with novel antigen-sweeping activity in vivo

Monoclonal antibodies are widely used to target disease-related antigens. However, because conventional antibody binds to the antigen but cannot eliminate the antigen from plasma, and rather increases the plasma antigen concentration by reducing the clearance of the antigen, some clinically important antigens are still difficult to target with monoclonal antibodies because of the huge dosages required. While conventional antibody can only bind to the antigen, some natural endocytic receptors not only bind to the ligands but also continuously eliminate them from plasma by pH-dependent dissociation of the ligands within the acidic endosome and subsequent receptor recycling to the cell surface. Here, we demonstrate that an engineered antibody, named sweeping antibody, having both pH-dependent antigen binding (to mimic the receptor-ligand interaction) and increased binding to cell surface neonatal Fc receptor (FcRn) at neutral pH (to mimic the cell-bound form of the receptor), selectively eliminated the antigen from plasma. With this novel antigen-sweeping activity, antibody without in vitro neutralizing activity exerted in vivo efficacy by directly eliminating the antigen from plasma. Moreover, conversion of conventional antibody with in vitro neutralizing activity into sweeping antibody further potentiated the in vivo efficacy. Depending on the binding affinity to FcRn at neutral pH, sweeping antibody reduced antigen concentration 50- to 1000-fold compared to conventional antibody. Thereby, sweeping antibody antagonized excess amounts of antigen in plasma against which conventional antibody was completely ineffective, and could afford marked reduction of dosage to a level that conventional antibody can never achieve. Thus, the novel mode of action of sweeping antibody provides potential advantages over conventional antibody and may allow access to the target antigens which were previously undruggable by conventional antibody.