Chimerism of avian IgY-scFv and truncated IgG-Fc: A novel strategy in cross-species antibody generation and enhancement

Chicken single-chain fragment variable (IgY-scFv) is a functional fragment and an emerging development in genetically engineered antibodies with a wide range of biomedical applications. However, scFvs have considerably shorter serum half-life due to the absence of antibody Fc region compared with the full-length antibody, and usually requires continuous intravenous administration for efficacy. A promising approach to overcome this limitation is to fuse scFv with immunoglobulin G (IgG) Fc region, for better recognition and mediation by the neonatal Fc receptor (FcRn) in the host. In this study, engineered mammalian ΔFc domains (CH2, CH3, and intact Fc region) were fused with anti-canine parvovirus-like particles avian IgY-scFv to produce chimeric antibodies and expressed in the HEK293 cell expression system. The obtained scFv-CH2, scFv-CH3, and scFv-Fc can bind with antigen specifically and dose-dependently. Surface plasmon resonance investigation confirmed that scFv-CH2, scFv-CH3, and scFv-Fc had different degrees of binding to FcRn, with scFv-Fc showing the highest affinity. scFv-Fc had a significantly longer half-life in mice compared with the unfused scFv. The identified ΔFcs are promising for the development of engineered Fc-based therapeutic antibodies and proteins with longer half-lives. The avian IgY-scFv-mammalian IgG Fc region opens up new avenues for antibody engineering, and it is a novel strategy to enhance the rapid development and screening of functional antibodies in veterinary and human medicine.

Reducing neonatal Fc receptor binding enhances clearance and brain-to-blood ratio of TfR-delivered bispecific amyloid-β antibody

Recent development of amyloid-β (Aβ)-targeted immunotherapies for Alzheimer's disease (AD) have highlighted the need for accurate diagnostic methods. Antibody-based positron emission tomography (PET) ligands are well suited for this purpose as they can be directed toward the same target as the therapeutic antibody. Bispecific, brain-penetrating antibodies can achieve sufficient brain concentrations, but their slow blood clearance remains a challenge, since it prolongs the time required to achieve a target-specific PET signal. Here, two antibodies were designed based on the Aβ antibody bapineuzumab (Bapi) - one monospecific IgG (Bapi) and one bispecific antibody with an antigen binding fragment (Fab) of the transferrin receptor (TfR) antibody 8D3 fused to one of the heavy chains (Bapi-Fab8D3) for active, TfR-mediated transport into the brain. A variant of each antibody was designed to harbor a mutation to the neonatal Fc receptor (FcRn) binding domain, to increase clearance. Blood and brain pharmacokinetics of radiolabeled antibodies were studied in wildtype (WT) and AD mice (AppNL-G-F). The FcRn mutation substantially reduced blood half-life of both Bapi and Bapi-Fab8D3. Bapi-Fab8D3 showed high brain uptake and the brain-to-blood ratio of its FcRn mutated form was significantly higher in AppNL-G-F mice than in WT mice 12 h after injection and increased further up to 168 h. Ex vivo autoradiography showed specific antibody retention in areas with abundant Aβ pathology. Taken together, these results suggest that reducing FcRn binding of a full-sized bispecific antibody increases the systemic elimination and could thereby drastically reduce the time from injection to in vivo imaging.

Exploiting the neonatal crystallizable fragment receptor to treat kidney disease

The neonatal Fc receptor (FcRn) was initially discovered as the receptor that allowed passive immunity in newborns by transporting maternal IgG through the placenta and enterocytes. Since its initial discovery, FcRn has been found to exist throughout all stages of life and in many different cell types. Beyond passive immunity, FcRn is necessary for intrinsic albumin and IgG recycling and is important for antigen processing and presentation. Given its multiple important roles, FcRn has been utilized in many disease treatments including a new class of agents that were developed to inhibit FcRn for treatment of a variety of autoimmune diseases. Certain cell populations within the kidney also express high levels of this receptor. Specifically, podocytes, proximal tubule epithelial cells, and vascular endothelial cells have been found to utilize FcRn. In this review, we summarize what is known about FcRn and its function within the kidney. We also discuss how FcRn has been used for therapeutic benefit, including how newer FcRn inhibiting agents are being used to treat autoimmune diseases. Lastly, we will discuss what renal diseases may respond to FcRn inhibitors and how further work studying FcRn within the kidney may lead to therapies for kidney diseases.

FcRN receptor antagonists in the management of myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disorder characterized by autoantibodies specifically directed against proteins located within the postsynaptic membrane of the neuromuscular junction. These pathogenic autoantibodies can be reduced by therapies such as plasma exchange, IVIG infusions and other immunosuppressive agents. However, there are significant side effects associated with most of these therapies. Since there is a better understanding of the molecular structure and the biological properties of the neonatal Fc receptors (FcRn), it possesses an attractive profile in treating myasthenia gravis. FcRn receptors prevent the catabolism of IgG by impeding their lysosomal degradation and facilitating their extracellular release at physiological pH, consequently extending the IgG half-life. Thus, the catabolism of IgG can be enhanced by blocking the FcRn, leading to outcomes similar to those achieved through plasma exchange with no significant safety concerns. The available studies suggest that FcRn holds promise as a versatile therapeutic intervention, capable of delivering beneficial outcomes in patients with distinct characteristics and varying degrees of MG severity. Efgartigimod is already approved for the treatment of generalized MG, rozanolixizumab is under review by health authorities, and phase 3 trials of nipocalimab and batoclimab are underway. Here, we will review the available data on FcRn therapeutic agents in the management of MG.

Comparative Molecular Characterization and Pharmacokinetics of IgG1-Fc and Engineered Fc Human Antibody Variants to Insulin-like Growth Factor 2 Receptor (IGF2R)

Novel therapeutic approaches are much needed for the treatment of osteosarcoma. Targeted radionuclide therapy (TRT) and radioimmunotherapy (RIT) are promising approaches that deliver therapeutic radiation precisely to the tumor site. We have previously developed a fully human antibody, named IF3, that binds to insulin-like growth factor 2 receptor (IGF2R). IF3 was used in TRT to effectively inhibit tumor growth in osteosarcoma preclinical models. However, IF3's relatively short half-life in mice raised the need for improvement. We generated an Fc-engineered version of IF3, termed IF3δ, with amino acid substitutions known to enhance antibody half-life in human serum. In this study, we confirmed the specific binding of IF3δ to IGF2R with nanomolar affinity, similar to wild-type IF3. Additionally, IF3δ demonstrated binding to human and mouse neonatal Fc receptors (FcRn), indicating the potential for FcRn-mediated endocytosis and recycling. Biodistribution studies in mice showed a higher accumulation of IF3δ in the spleen and bone than wild-type IF3, likely attributed to abnormal spleen expression of IGF2R in mice. Therefore, the pharmacokinetics data from mouse xenograft models may not precisely reflect their behavior in canine and human patients. However, the findings suggest both IF3 and IF3δ as promising options for the RIT of osteosarcoma.

Megalin Orchestrates FcRn Endocytosis and Trafficking

The neonatal Fc receptor (FcRn) is highly expressed in the renal proximal tubule and is important for the reclamation of albumin by cellular transcytosis to prevent its loss in the urine. The initial event of this transcellular transport mechanism is the endocytosis of albumin by the apical scavenger receptors megalin and cubilin. An interaction of megalin and FcRn was postulated, however, evidence is still missing. Similarly, the intracellular trafficking of FcRn remains unknown and shall be identified in our study. Using a Venus-based bimolecular fluorescence complementation system, we detected an interaction between megalin and FcRn in the endosomal compartment, which significantly increased with the induction of endocytosis using albumin or lactoglobulin as a ligand. The interaction between megalin and FcRn occurred at a neutral and acidic pH between the extracellular domains of both proteins. Amnionless, another transmembrane acceptor of cubilin, revealed no interaction with FcRn. With the induction of endocytosis by albumin or lactoglobulin, super resolution microscopy demonstrated a redistribution of megalin and FcRn into clathrin vesicles and early endosomes. This trafficking into clathrin vesicles was impaired in megalin-deficient cells upon albumin-induced endocytosis, supporting the role of megalin in FcRn redistribution. Our results indicate that megalin and FcRn specifically bind and interact within their extracellular domains. The availability of megalin is necessary for the redistribution of FcRn. Megalin, therefore, orchestrates FcRn endocytosis and intracellular trafficking as an early event intranscytosis.

Effect of molecular size on interstitial pharmacokinetics and tissue catabolism of antibodies

Advances in antibody engineering have enabled the construction of novel molecular formats in diverse shapes and sizes, providing new opportunities for biologic therapies and expanding the need to understand how various structural aspects affect their distribution properties. To assess the effect of antibody size on systemic pharmacokinetics (PK) and tissue distribution with or without neonatal Fc receptor (FcRn) binding, we evaluated a series of non-mouse-binding anti-glycoprotein D monoclonal antibody formats, including IgG [~150 kDa], one-armed IgG [~100 kDa], IgG-HAHQ (attenuated FcRn binding) [~150 kDa], F(ab')₂ [~100 kDa], and F(ab) [~50 kDa]. Tissue-specific concentration-time profiles were corrected for blood content based on vascular volumes and normalized based on interstitial volumes to allow estimation of interstitial concentrations and interstitial:serum concentration ratios. Blood correction demonstrated that the contribution of circulating antibody on total uptake was greatest at early time points and for highly vascularized tissues. Tissue interstitial PK largely mirrored serum exposure profiles. Similar interstitial:serum ratios were obtained for the two FcRn-binding molecules, IgG and one-armed IgG, which reached pseudo-steady-state kinetics in most tissues. For non-FcRn-binding molecules, interstitial:serum ratios changed over time, suggesting that these molecules did not reach steady-state kinetics during the study. Furthermore, concentration-time profiles of both intact and catabolized molecule were measured by a dual tracer approach, enabling quantification of tissue catabolism and demonstrating that catabolism levels were highest for IgG-HAHQ. Overall, these data sets provide insight into factors affecting preclinical distribution and may be useful in estimating interstitial concentrations and/or catabolism in human tissues.

Oral Fc-Coupled Preproinsulin Achieves Systemic and Thymic Delivery Through the Neonatal Fc Receptor and Partially Delays Autoimmune Diabetes

Tolerogenic vaccinations using beta-cell antigens are attractive for type 1 diabetes prevention, but clinical trials have been disappointing. This is probably due to the late timing of intervention, when multiple auto-antibodies are already present. We therefore devised a strategy to introduce the initiating antigen preproinsulin (PPI) during neonatal life, when autoimmunity is still silent and central tolerance mechanisms, which remain therapeutically unexploited, are more active. This strategy employs an oral administration of PPI-Fc, i.e. PPI fused with an IgG Fc to bind the intestinal neonatal Fc receptor (FcRn) that physiologically delivers maternal antibodies to the offspring during breastfeeding. Neonatal oral PPI-Fc vaccination did not prevent diabetes development in PPI T-cell receptor-transgenic G9C8.NOD mice. However, PPI-Fc was efficiently transferred through the intestinal epithelium in an Fc- and FcRn-dependent manner, was taken up by antigen presenting cells, and reached the spleen and thymus. Although not statistically significant, neonatal oral PPI-Fc vaccination delayed diabetes onset in polyclonal Ins2^-/-.NOD mice that spontaneously develop accelerated diabetes. Thus, this strategy shows promise in terms of systemic and thymic antigen delivery via the intestinal FcRn pathway, but the current PPI-Fc formulation/regimen requires further improvements to achieve diabetes prevention.

Functional humanization of immunoglobulin heavy constant gamma 1 Fc domain human FCGRT transgenic mice

A major asset of many monoclonal antibody (mAb)-based biologics is their persistence in circulation. The MHC class I family Fc receptor, FCGRT, is primarily responsible for this extended pharmacokinetic behavior. Engagement of FCGRT with the crystallizable fragment (Fc) domain protects IgG from catabolic elimination, thereby extending the persistence and bioavailability of IgG and related Fc-based biologics. There is a need for reliable in vivo models to facilitate the preclinical development of novel IgG-based biologics. FcRn-humanized mice have been widely accepted as translationally relevant surrogates for IgG-based biologics evaluations. Although such FCGRT-humanized mice, especially the mouse strain, B6.Cg-Fcgrt^tm1Dcr Tg(FCGRT)32Dcr (abbreviated Tg32), have been substantially validated for modeling humanized IgG-based biologics, there is a recognized caveat – they lack an endogenous source of human IgG that typifies the human competitive condition. Here, we used CRISPR/Cas9-mediated homology-directed repair to equip the hFCGRT Tg32 strain with a human IGHG1 Fc domain. This replacement now results in mice that produce human IgG1 Fc-mouse IgG Fab₂ chimeric antibodies at physiologically relevant levels, which can be further heightened by immunization. This endogenous chimeric IgG1 significantly dampens the serum half-life of administered humanized mAbs in an hFCGRT-dependent manner. Thus, such IgG1-Fc humanized mice may provide a more physiologically relevant competitive hFCGRT-humanized mouse model for the preclinical development of human IgG-based biologics.

Relevance of the Materno-Fetal Interface for the Induction of Antigen-Specific Immune Tolerance

In humans, maternal IgGs are transferred to the fetus from the second trimester of pregnancy onwards. The transplacental delivery of maternal IgG is mediated by its binding to the neonatal Fc receptor (FcRn) after endocytosis by the syncytiotrophoblast. IgGs present in the maternal milk are also transferred to the newborn through the digestive epithelium upon binding to the FcRn. Importantly, the binding of IgGs to the FcRn is also responsible for the recycling of circulating IgGs that confers them with a long half-life. Maternally delivered IgG provides passive immunity to the newborn, for instance by conferring protective anti-flu or anti-pertussis toxin IgGs. It may, however, lead to the development of autoimmune manifestations when pathological autoantibodies from the mother cross the placenta and reach the circulation of the fetus. In recent years, strategies that exploit the transplacental delivery of antigen/IgG complexes or of Fc-fused proteins have been validated in mouse models of human diseases to impose antigen-specific tolerance, particularly in the case of Fc-fused factor VIII (FVIII) domains in hemophilia A mice or pre-pro-insulin (PPI) in the case of preclinical models of type 1 diabetes (T1D). The present review summarizes the mechanisms underlying the FcRn-mediated transcytosis of IgGs, the physiopathological relevance of this phenomenon, and the repercussion for drug delivery and shaping of the immune system during its ontogeny.

Pemphigus: Current and Future Therapeutic Strategies

Pemphigus encompasses a heterogeneous group of autoimmune blistering diseases, which affect both mucous membranes and the skin. The disease usually runs a chronic-relapsing course, with a potentially devastating impact on the patients' quality of life. Pemphigus pathogenesis is related to IgG autoantibodies targeting various adhesion molecules in the epidermis, including desmoglein (Dsg) 1 and 3, major components of desmosomes. The pathogenic relevance of such autoantibodies has been largely demonstrated experimentally. IgG autoantibody binding to Dsg results in loss of epidermal keratinocyte adhesion, a phenomenon referred to as acantholysis. This in turn causes intra-epidermal blistering and the clinical appearance of flaccid blisters and erosions at involved sites. Since the advent of glucocorticoids, the overall prognosis of pemphigus has largely improved. However, mortality persists elevated, since long-term use of high dose corticosteroids and adjuvant steroid-sparing immunosuppressants portend a high risk of serious adverse events, especially infections. Recently, rituximab, a chimeric anti CD20 monoclonal antibody which induces B-cell depletion, has been shown to improve patients' survival, as early rituximab use results in higher disease remission rates, long term clinical response and faster prednisone tapering compared to conventional immunosuppressive therapies, leading to its approval as a first line therapy in pemphigus. Other anti B-cell therapies targeting B-cell receptor or downstream molecules are currently tried in clinical studies. More intriguingly, a preliminary study in a preclinical mouse model of pemphigus has shown promise regarding future therapeutic application of Chimeric Autoantibody Receptor T-cells engineered using Dsg domains to selectively target autoreactive B-cells. Conversely, previous studies from our group have demonstrated that B-cell depletion in pemphigus resulted in secondary impairment of T-cell function; this may account for the observed long-term remission following B-cell recovery in rituximab treated patients. Likewise, our data support the critical role of Dsg-specific T-cell clones in orchestrating the inflammatory response and B-cell activation in pemphigus. Monitoring autoreactive T-cells in patients may indeed provide further information on the role of these cells, and would be the starting point for designating therapies aimed at restoring the lost immune tolerance against Dsg. The present review focuses on current advances, unmet challenges and future perspectives of pemphigus management.

Half-life-extended recombinant coagulation factor IX-albumin fusion protein is recycled via the FcRn-mediated pathway

The neonatal Fc receptor (FcRn) has a pivotal role in albumin and IgG homeostasis. Internalized IgG captured by FcRn under acidic endosomal conditions is recycled to the cell surface where exocytosis and a shift to neutral pH promote extracellular IgG release. Although a similar mechanism is proposed for FcRn-mediated albumin intracellular trafficking and recycling, this pathway is less well defined but is relevant to the development of therapeutics exploiting FcRn to extend the half-life of short-lived plasma proteins. Recently, a long-acting recombinant coagulation factor IX–albumin fusion protein (rIX-FP) has been approved for the management of hemophilia B. Fusion to albumin potentially enables internalized proteins to engage FcRn and escape lysosomal degradation. In this study, we present for the first time a detailed investigation of the FcRn-mediated recycling of albumin and the albumin fusion protein rIX-FP. We demonstrate that following internalization via FcRn at low pH, rIX-FP, like albumin, is detectable within the early endosome and rapidly (within 10–15 min) traffics into the Rab11+ recycling endosomes, from where it is exported from the cell. Similarly, rIX-FP and albumin taken up by fluid-phase endocytosis at physiological pH traffics into the Rab11+ recycling compartment in FcRn-positive cells but into the lysosomal compartment in FcRn-negative cells. As expected, recombinant factor IX (without albumin fusion) and an FcRn interaction–defective albumin variant localized to the lysosomal compartments of both FcRn-expressing and nonexpressing cells. These results indicate that FcRn-mediated recycling via the albumin moiety is a mechanism for the half-life extension of rIX-FP observed in clinical studies.

Loss of expression of the recycling receptor, FcRn, promotes tumor cell growth by increasing albumin consumption

Tumor cells rely on high concentrations of amino acids to support their growth and proliferation. Although increased macropinocytic uptake and lysosomal degradation of the most abundant serum protein, albumin, in Ras-transformed cells can meet these demands, it is not understood how the majority of tumor cells that express wild type Ras achieve this. In the current study we reveal that the neonatal Fc receptor, FcRn, regulates tumor cell proliferation through the ability to recycle its ligand, albumin. By contrast with normal epithelial cells, we show that human FcRn is present at very low or undetectable levels in the majority of tumor cell lines analyzed. Remarkably, shRNA-mediated ablation of FcRn expression in an FcRn-positive tumor cell line results in a substantial growth increase of tumor xenografts, whereas enforced expression of this receptor by lentiviral transduction has the reverse effect. Moreover, intracellular albumin and glutamate levels are increased by the loss of FcRn-mediated recycling of albumin, combined with hypoalbuminemia in tumor-bearing mice. These studies identify a novel role for FcRn as a suppressor of tumor growth and have implications for the use of this receptor as a prognostic indicator and therapeutic target.

Pharmacokinetic properties of IgG and various Fc fusion proteins in mice

Fusion to an IgG Fc region is an established strategy to extend the half-life of therapeutic proteins. Most Fc fusion proteins, however, do not achieve the long half-life of IgGs. Based on findings that scFv-Fc fusion proteins exhibit a shorter half-life than the corresponding IgG molecules, we performed a comparative study of different antibody-derived Fc fusion proteins. We could confirm that fusion of single-chain Fv (scFv) and single-chain diabody (scDb) molecules to an Fc region yields in fusion proteins with substantially extended half-lives compared with the single-chain versions. However, even fusion proteins with a size similar to that of IgG, e.g., scDb-Fc, did not have a half-life as long as an IgG molecule. Binding to the neonatal Fc receptor (FcRn) under acidic and neutral conditions was similar for IgG and all Fc fusion proteins. However, we observed differences between IgG and the Fc fusion proteins for dissociation of FcRn-bound proteins induced by shifting from acidic to neutral pH, reflecting the physiological release mechanism, further supporting a contribution of the kinetics of pH-dependent release from FcRn to the pharmacokinetic properties of IgG and Fc fusion proteins.

Target-independent variable region mediated effects on antibody clearance can be FcRn independent

The importance of the neonatal Fc receptor (FcRn) in extending the serum half-life of monoclonal antibodies (mAbs) is well demonstrated, and has led to the development of multiple engineering approaches designed to alter Fc interactions with FcRn. Recent reports have additionally highlighted the effect of nonspecific interactions on antibody pharmacokinetics (PK), suggesting an FcRn-independent mechanism for mAb clearance. In this report we examine a case study of 2 anti-interleukin-12/23 antibodies, ustekinumab and briakinumab, which share the same target and Fc, but differ in variable region sequences. Ustekinumab displayed near baseline signal in a wide range of early stage developability assays for undesirable protein/protein interactions, while briakinumab showed significant propensity for self- and cross-interactions. This phenotypic difference correlates with faster clearance rates for briakinumab in both human FcRn transgenic and FcRn knockout mice. These findings support a dominant contribution for FcRn-independent clearance for antibodies with high nonspecificity, and highlight a key role for early stage developability screening to eliminate clones with such high nonspecific disposition PK.

A novel in vitro assay to predict neonatal Fc receptor-mediated human IgG half-life

Immunoglobulin G (IgG) has an unusually long serum half-life in comparison to proteins of a similar size. It is well-known that this phenomenon is due to IgG's ability to bind the neonatal Fc receptor (FcRn) in a pH-dependent manner. FcRn binding properties can vary among IgGs, resulting in altered in vivo half-lives, and therefore it would be beneficial to accurately predict the FcRn binding properties of therapeutic IgG monoclonal antibodies (mAbs). Here we describe the development of an in vitro model capable of predicting the in vivo half-life of human IgG. Using a high-throughput biolayer interferometry (BLI) platform, the human FcRn association rate at acidic pH and subsequent dissociation rate at physiological pH was determined for 5 human IgG1 mAbs. Comparing the combined FcRn association and dissociation rates to the Phase 1 clinical study half-lives of the mAbs resulted in a strong correlation. The correlation was also verified in vivo using mice transgenic for human FcRn. The model was used to characterize various factors that may influence FcRn-mAb binding, including mAb variable region sequence differences and constant region glycosylation patterns. Results indicated that the complementarity-determining regions of the heavy chain significantly influence the mAb's FcRn binding properties, while the absence of glycosylation does not alter mAb-FcRn binding. Development of this high-throughput FcRn binding model could potentially predict the half-life of therapeutic IgGs and aid in selection of lead candidates while also serving as a screening tool for the development of mAbs with desired pharmacokinetic properties.

Eosinophilic gastroenteritis in a patient with Bruton's tyrosine kinase deficiency

Eosinophilic gastrointestinal diseases (EGID) are relatively rare diseases characterized by eosinophilic infiltration of the gastrointestinal tract resulting in various gastrointestinal symptoms. EGID are often caused by allergic reactions or systemic eosinophilic disorders, but their comorbidity with Bruton's tyrosine kinase (BTK) deficiency has not been previously documented. Here, we report a case of eosinophilic gastroenteritis (EG) in a patient with BTK deficiency. Despite adequate replacement immunoglobulin (Ig) therapy, trough serum IgG was not maintained. To identify the underlying cause of the low trough level and chronic diarrhea, the intestine was investigated on endoscopy. We also screened for the variable number of tandem repeat polymorphism in FCGRT. Genetic analysis could not explain the low trough IgG, but endoscopy indicated eosinophilic enterocolitis. EG may be an important differential diagnosis when primary immunodeficiency patients have chronic diarrhea or continued low serum IgG.

Conformational Destabilization of Immunoglobulin G Increases the Low pH Binding Affinity with the Neonatal Fc Receptor

Crystallographic evidence suggests that the pH-dependent affinity of IgG molecules for the neonatal Fc receptor (FcRn) receptor primarily arises from salt bridges involving IgG histidine residues, resulting in moderate affinity at mildly acidic conditions. However, this view does not explain the diversity in affinity found in IgG variants, such as the YTE mutant (M252Y,S254T,T256E), which increases affinity to FcRn by up to 10×. Here we compare hydrogen exchange measurements at pH 7.0 and pH 5.5 with and without FcRn bound with surface plasmon resonance estimates of dissociation constants and FcRn affinity chromatography. The combination of experimental results demonstrates that differences between an IgG and its cognate YTE mutant vary with their pH-sensitive dynamics prior to binding FcRn. The conformational dynamics of these two molecules are nearly indistinguishable upon binding FcRn. We present evidence that pH-induced destabilization in the CH2/3 domain interface of IgG increases binding affinity by breaking intramolecular H-bonds and increases side-chain adaptability in sites that form intermolecular contacts with FcRn. Our results provide new insights into the mechanism of pH-dependent affinity in IgG-FcRn interactions and exemplify the important and often ignored role of intrinsic conformational dynamics in a protein ligand, to dictate affinity for biologically important receptors.

Improved in vivo anti-tumor effects of IgA-Her2 antibodies through half-life extension and serum exposure enhancement by FcRn targeting

Antibody therapy is a validated treatment approach for several malignancies. All currently clinically applied therapeutic antibodies (Abs) are of the IgG isotype. However, not all patients respond to this therapy and relapses can occur. IgA represents an alternative isotype for antibody therapy that engages FcαRI expressing myeloid effector cells, such as neutrophils and monocytes. IgA Abs have been shown to effectively kill tumor cells both in vitro and in vivo. However, due to the short half-life of IgA Abs in mice, daily injections are required to reach an effect comparable to IgG Abs. The relatively long half-life of IgG Abs and serum albumin arises from their capability of interacting with the neonatal Fc receptor (FcRn). As IgA Abs lack a binding site for FcRn, we generated IgA Abs with the variable regions of the Her2-specific Ab trastuzumab and attached an albumin-binding domain (ABD) to the heavy or light chain (HCABD/LCABD) to extend their serum half-life. These modified Abs were able to bind albumin from different species in vitro. Furthermore, tumor cell lysis of IgA-Her2-LCABD Abs in vitro was similar to unmodified IgA-Her2 Abs. Pharmacokinetic studies in mice revealed that the serum exposure and half-life of the modified IgA-Her2 Abs was extended. In a xenograft mouse model, the modified IgA1 Abs exhibited a slightly, but significantly, improved anti-tumor response compared to the unmodified Ab. In conclusion, empowering IgA Abs with albumin-binding capacity results in in vitro and in vivo functional Abs with an enhanced exposure and prolonged half-life.

The interplay of non-specific binding, target-mediated clearance and FcRn interactions on the pharmacokinetics of humanized antibodies

The application of protein engineering technologies toward successfully improving antibody pharmacokinetics has been challenging due to the multiplicity of biochemical factors that influence monoclonal antibody (mAb) disposition in vivo. Physiological factors including interactions with the neonatal Fc receptor (FcRn) and specific antigen binding properties of mAbs, along with biophysical properties of the mAbs themselves play a critical role. It has become evident that applying an integrated approach to understand the relative contribution of these factors is critical to rationally guide and apply engineering strategies to optimize mAb pharmacokinetics. The study presented here evaluated the influence of unintended non-specific interactions on the disposition of mAbs whose clearance rates are governed predominantly by either non-specific (FcRn) or target-mediated processes. The pharmacokinetics of 8 mAbs representing a diverse range of these properties was evaluated in cynomolgus monkeys. Results revealed complementarity-determining region (CDR) charge patch engineering to decrease charge-related non-specific binding can have a significant impact on improving the clearance. In contrast, the influence of enhanced in vitro FcRn binding was mixed, and related to both the strength of charge interaction and the general mechanism predominant in governing the clearance of the particular mAb. Overall, improved pharmacokinetics through enhanced FcRn interactions were apparent for a CDR charge-patch normalized mAb which was affected by non-specific clearance. The findings in this report are an important demonstration that mAb pharmacokinetics requires optimization on a case-by-case basis to improve the design of molecules with increased therapeutic application.

Alternative functional in vitro models of human intestinal epithelia

Physiologically relevant sources of absorptive intestinal epithelial cells are crucial for human drug transport studies. Human adenocarcinoma-derived intestinal cell lines, such as Caco-2, offer conveniences of easy culture maintenance and scalability, but do not fully recapitulate in vivo intestinal phenotypes. Additional sources of renewable physiologically relevant human intestinal cells would provide a much needed tool for drug discovery and intestinal physiology. We compared two alternative sources of human intestinal cells, commercially available primary human intestinal epithelial cells (hInEpCs) and induced pluripotent stem cell (iPSC)-derived intestinal cells to Caco-2, for use in in vitro transwell monolayer intestinal transport assays. To achieve this for iPSC-derived cells, intestinal organogenesis was adapted to transwell differentiation. Intestinal cells were assessed by marker expression through immunocytochemical and mRNA expression analyses, monolayer integrity through Transepithelial Electrical Resistance (TEER) measurements and molecule permeability, and functionality by taking advantage the well-characterized intestinal transport mechanisms. In most cases, marker expression for primary hInEpCs and iPSC-derived cells appeared to be as good as or better than Caco-2. Furthermore, transwell monolayers exhibited high TEER with low permeability. Primary hInEpCs showed molecule efflux indicative of P-glycoprotein (Pgp) transport. Primary hInEpCs and iPSC-derived cells also showed neonatal Fc receptor-dependent binding of immunoglobulin G variants. Primary hInEpCs and iPSC-derived intestinal cells exhibit expected marker expression and demonstrate basic functional monolayer formation, similar to or better than Caco-2. These cells could offer an alternative source of human intestinal cells for understanding normal intestinal epithelial physiology and drug transport.

Minipig as a potential translatable model for monoclonal antibody pharmacokinetics after intravenous and subcutaneous administration

Subcutaneous (SC) delivery is a common route of administration for therapeutic monoclonal antibodies (mAbs) with pharmacokinetic (PK)/pharmacodynamic (PD) properties requiring long-term or frequent drug administration. An ideal in vivo preclinical model for predicting human PK following SC administration may be one in which the skin and overall physiological characteristics are similar to that of humans. In this study, the PK properties of a series of therapeutic mAbs following intravenous (IV) and SC administration in Göttingen minipigs were compared with data obtained previously from humans. The present studies demonstrated: (1) minipig is predictive of human linear clearance; (2) the SC bioavailabilities in minipigs are weakly correlated with those in human; (3) minipig mAb SC absorption rates are generally higher than those in human and (4) the SC bioavailability appears to correlate with systemic clearance in minipigs. Given the important role of the neonatal Fc-receptor (FcRn) in the PK of mAbs, the in vitro binding affinities of these IgGs against porcine, human and cynomolgus monkey FcRn were tested. The result showed comparable FcRn binding affinities across species. Further, mAbs with higher isoelectric point tended to have faster systemic clearance and lower SC bioavailability in both minipig and human. Taken together, these data lend increased support for the use of the minipig as an alternative predictive model for human IV and SC PK of mAbs.