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

Extension of the circulatory half-life of recombinant ecallantide via albumin fusion without loss of anti-kallikrein activity

Ecallantide comprises Kunitz Domain 1 of Tissue Factor Pathway Inhibitor, mutated at seven amino acid positions to inhibit plasma kallikrein (PK). It is used to treat acute hereditary angioedema (HAE). We appended hexahistidine tags to the N- or C-terminus of recombinant Ecallantide (rEcall) and expressed and purified the resulting proteins, with or without fusion to human serum albumin (HSA), using Pichia pastoris. The inhibitory constant (Ki) of rEcall-H6 or H6-rEcall for PK was not increased by albumin fusion. When 125I-labelled rEcall proteins were injected intravenously into mice, the area under the clearance curve (AUC) was significantly increased, 3.4- and 3.6-fold, for fusion proteins H6-rEcall-HSA and HSA-rEcall-H6 versus their unfused counterparts but remained 2- to 3-fold less than that of HSA-H6. The terminal half-life of H6-rEcall-HSA and HSA-H6 did not differ, although that of HSA-rEcall-H6 was significantly shorter than either other protein. Receptor Associated Protein (RAP), a Low-density lipoprotein Receptor-related Protein (LRP1) antagonist, competed H6-rEcall-HSA clearance more effectively than intravenous immunoglobulin (IVIg), a neonatal Fc receptor (FcRn) antagonist. HSA fusion decreases rEcall clearance in vivo, but LRP1-mediated clearance remains more important than FcRn-mediated recycling for rEcall fusion proteins. The properties of H6-rEcall-HSA warrant investigation in a murine model of HAE.

In vivo intranasal delivery of coagulation factor IX: a proof-of-concept study

BACKGROUND

Hemophilia B (HB) is a bleeding disorder characterized by coagulation factor (F) IX (FIX) deficiency. The current standard-of-care for severe HB is prophylaxis with long-term repetitive intravenous (i.v.) infusions of recombinant FIX (rFIX) with standard half-life or extended half-life. Unmet needs remain regarding the development of non-invasive administration routes for coagulation factors. The aim of this study was to evaluate the effectiveness of intranasal delivery (IND) of rFIX and rFIX fused to Fc fragment (rFIX-Fc) in mice.

METHODS

Drops of rFIX and rFIX-Fc were deposited in the nostrils of wild-type, FcRn knock-out, FcRn humanized, and FIX knock-out mice. rFIX mucosal uptake was evaluated by measuring plasma FIX antigen and FIX activity (FIX:C) levels, and by performing histologic analysis of the nasal mucosa following IND.

RESULTS

After IND, both rFIX and rFIX-Fc were equally delivered to the blood compartment, irrespective of the mouse strain studied, mostly through a passive mechanism of transportation across the mucosal barrier, independent of FcRn receptor. Both plasma FIX antigen and FIX:C activity levels increased following IND in FIX knock-out mice.

CONCLUSION

This proof-of-concept study describes evidence supporting the nasal route as an alternative to FIX i.v. infusion for the treatment of HB.

The endosomal system of primary human vascular endothelial cells and albumin-FcRn trafficking

Human serum albumin (HSA) has a long circulatory half-life owing, in part, to interaction with the neonatal Fc receptor (FcRn or FCGRT) in acidic endosomes and recycling of internalised albumin. Vascular endothelial and innate immune cells are considered the most relevant cells for FcRn-mediated albumin homeostasis in vivo. However, little is known about endocytic trafficking of FcRn-albumin complexes in primary human endothelial cells. To investigate FcRn-albumin trafficking in physiologically relevant endothelial cells, we generated primary human vascular endothelial cell lines from blood endothelial precursors, known as blood outgrowth endothelial cells (BOECs). We mapped the endosomal system in BOECs and showed that BOECs efficiently internalise fluorescently labelled HSA predominantly by fluid-phase macropinocytosis. Pulse-chase studies revealed that intracellular HSA molecules co-localised with FcRn in acidic endosomal structures and that the wildtype HSA, but not the non-FcRn-binding HSAH464Q mutant, was excluded from late endosomes and/or lysosomes. Live imaging revealed that HSA is partitioned into FcRn-positive tubules derived from maturing macropinosomes, which are then transported towards the plasma membrane. These findings identify the FcRn-albumin trafficking pathway in primary vascular endothelial cells, relevant to albumin homeostasis.

Clinical experience of switching patients with severe hemophilia to rVIII-SingleChain or rIX-FP

OBJECTIVE

Prophylaxis treatment is the current standard of care for patients with severe hemophilia. Factor concentrates with improved pharmacokinetics have offered more options for individualizing treatment. The treatment focus may be on increased protection, aiming for higher trough factor levels or longer dosing intervals to reduce the burden of hemophilia. Both aspects can have long-term effects on joint health. Products, such as rVIII‑SingleChain and rIX-FP have been developed to reduce the treatment burden for patients with hemophilia and optimize prophylactic efficacy. The objective of this report is to provide a summary of the clinical experience of different Hemophilia Treatment Centers in managing the switch to rVIII-SingleChain or rIX-FP in patients with hemophilia.

METHODS

This report summarizes a selection of patient cases presented at the 3rd Alliance for Coagulation Academy Meeting in October 2020. The cases from the participating centers provide examples of the clinical experience in managing patients' switch to rVIII-SingleChain and rIX‑FP, including which types of patients are suitable for switching, and practical steps in managing a switch.

RESULTS

It is important to take into consideration the physical and social fulfillment of the patient when deciding to switch to rVIII-SingleChain or rIX-FP. The physician plays an important role in the motivation of patients as they understand not only the patient's needs but the potential benefits of the new treatment.

CONCLUSION

The selected patient cases reported here demonstrate that patients may wish to switch factor products for a variety of reasons; therefore, it is critical to understand why patients switch and what they expect from switching.

Albumin-Fusion Recombinant FIX in the Management of People with Hemophilia B: An Evidence-Based Review

Albutrepenonacog-alfa (Idelvion®, CSL Behring) is a recombinant fusion protein in which the recombinant FIX (rFIX) links a recombinant human albumin, extending the half-life of rFIX even beyond 100 hours. In 2016, this drug was approved worldwide for the treatment of pediatric and adult persons with hemophilia B (PWH-B). Its efficacy and safety were described in the PROLONG-9FP program and subsequently confirmed in the real-world practice, even if to date there are not many manuscripts that extensively and completely deal with the use of albutrepenonacog-alfa in daily practice, also evaluating its impact on the quality of life of patients treated with this drug; this review therefore aims to analyze all the publications currently available regarding the real-world use of this extended half-life concentrate, also noting which topics need further study and research.

Functionalized FcRn-targeted nanosystems for oral drug delivery: A new approach to colorectal cancer treatment

Colorectal cancer (CRC) is the second type of cancer with the highest lethality rate. The current chemotherapy to treat CRC causes systemic toxicity, unsatisfying response rate, and low tumor-specific selectivity, which is mainly administered by invasive routes. The chronic and aggressive nature of cancers may require long-term regimens. Thus, the oral route is preferred. However, the orally administered drugs still need to surpass the harsh environment of the gastrointestinal tract and the biological barriers. Nanotechnology is a promising strategy to overcome the oral route limitations. Targeted nanoparticle systems decorated with functional groups can enhance the delivery of anticancer agents to tumor sites. It is described in the literature that the neonatal Fc receptor (FcRn) is expressed in cancer tissue and overexpressed in CRC epithelial cells. However, the impact of FcRn-targeted nanosystems in the treatment of CRC has been poorly investigated. This review article discusses the current knowledge on the involvement of the FcRn in CRC, as well as to critically assess its relevance as a target for further localization of oral nanocarriers in CRC tumor cells. Finally, a brief overview of cancer therapeutics, strategies to design the nanoparticles of anticancer drugs and a review of decorated nanoparticles with FcRn moieties are explored.

IDELVION: A Comprehensive Review of Clinical Trial and Real-World Data

Hemophilia B is a bleeding disorder caused by a deficiency of coagulation factor IX (FIX). Treatment with FIX replacement products can increase FIX activity levels to minimize or prevent bleeding events. However, frequent dosing with standard-acting FIX products can create a high treatment burden. Long-acting products have been developed to maintain bleed protection with extended dosing intervals. Recombinant factor IX–albumin fusion protein (rIX-FP) is a long-acting product indicated for the treatment and prophylaxis of bleeding events and perioperative management in adult and pediatric patients. This review outlines data from all previously treated patients in the Prophylaxis and On-Demand Treatment using Longer Half-Life rIX-FP (PROLONG-9FP) clinical trial program and summarizes real-world data evaluating the use of rIX-FP in routine clinical practice. In the PROLONG-9FP program, rIX-FP demonstrated effective hemostasis in all patients at dose regimens of up to 21 days in patients aged ≥ 18 years and up to 14 days in patients aged < 12 years. rIX-FP has a favorable pharmacokinetic profile and an excellent safety and tolerability profile. Extended dosing intervals with rIX-FP led to high levels of adherence and reduced consumption compared with other FIX therapies. Data from real-world practice are encouraging and reflect the results of the clinical trials.

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.

Increased potency of recombinant VWF D'D3 albumin fusion proteins engineered for enhanced affinity for coagulation factor VIII

BACKGROUND

We have recently reported on a recombinant von Willebrand factor (VWF) D'D3 albumin fusion protein (rD'D3-FP) developed to extend the half-life of coagulation factor VIII (FVIII) for the treatment of hemophilia A. Based on predictive modelling presented in this study, we hypothesized that modifying rD'D3-FP to improve FVIII interaction would reduce exchange with endogenous VWF and provide additional FVIII half-life benefit.

OBJECTIVES

The aim of this study was to identify novel rD'D3-FP variants with enhanced therapeutic efficacy in extending FVIII half-life.

METHODS

Through both directed mutagenesis and random mutagenesis using a novel mammalian display platform, we identified novel rD'D3-FP variants with increased affinity for FVIII (rVIII-SingleChain) under both neutral and acidic conditions and assessed their ability to extend FVIII half-life in vitro and in vivo.

RESULTS

In rat preclinical studies, rD'D3-FP variants with increased affinity for FVIII displayed enhanced potency, with reduced dose levels required to achieve equivalent rVIII-SingleChain half-life extension. In cell-based imaging studies in vitro, we also demonstrated reduced dissociation of rVIII-SingleChain from the rD'D3-FP variants within acidic endosomes and more efficient co-recycling of the rD'D3-FP/rVIII-SingleChain complex via the FcRn recycling system.

CONCLUSIONS

In summary, at potential clinical doses, the rD'D3-FP variants provide marked benefits with respect to dose levels and half-life extension of co-administered FVIII, supporting their development for use in the treatment of hemophilia A.

Prevention of diabetes-associated fibrosis: Strategies in FcRn-targeted nanosystems for oral drug delivery

Diabetes mellitus is a chronic disease with an elevated risk of micro- and macrovascular complications, such as fibrosis. To prevent diabetes-associated fibrosis, the symptomatology of diabetes must be controlled, which is commonly done by subcutaneous injection of antidiabetic peptides. To minimize the pain and distress associated with such injections, there is an urgent need for non-invasive oral transmucosal drug delivery strategies. However, orally administered peptide-based drugs are exposed to harsh conditions in the gastrointestinal tract and poorly cross the selective intestinal epithelium. Thus, targeting of drugs to receptors expressed in epithelial cells, such as the neonatal Fc receptor (FcRn), may therefore enhance uptake and transport through mucosal barriers. This review compiles how in-depth studies of FcRn biology and engineering of receptor-binding molecules may pave the way for design of new classes of FcRn-targeted nanosystems. Tailored strategies may open new avenues for oral drug delivery and provide better treatment options for diabetes and, consequently, fibrosis prevention.

Expert opinion paper on the treatment of hemophilia B with albutrepenonacog alfa

Introduction: Current guidelines recommend prophylactic treatment of hemophilia B with the missing coagulation factor IX, either with standard half-life or extended half-life products. Extended half-life products have half-lives three to six times longer than the former, allowing a reduction in the number of weekly injections and therefore, potentially impacting on treatment adherence and quality of life. Albutrepenonacog alfa is an extended half-life fusion protein of coagulation factor IX with recombinant human albumin, indicated for both on-demand and prophylactic treatment for bleeding in patients with hemophilia B of all ages.Areas covered: The authors review the clinical and pharmacokinetic characteristics of albutrepenonacog alfa, as well as the available information regarding trough levels and real-world evidence. Given the availability of other factor IX products in the market, indirect comparisons of clinical and pharmacokinetic characteristics are presented.Expert opinion: The authors exhibit their expert opinion on which patient profiles are candidates for prophylactic treatment with albutrepenonacog alfa, and on the management of patients in terms of dosing, regimens of administration and protocols for switching the treatment.

Fusion of engineered albumin with factor IX Padua extends half-life and improves coagulant activity

The short half‐life of coagulation factor IX (FIX) for haemophilia B (HB) therapy has been prolonged through fusion with human serum albumin (HSA), which drives the neonatal Fc receptor (FcRn)‐mediated recycling of the chimera. However, patients would greatly benefit from further FIX‐HSA half‐life extension. In the present study, we designed a FIX‐HSA variant through the engineering of both fusion partners. First, we developed a novel cleavable linker combining the two FIX activation sites, which resulted in improved HSA release. Second, insertion of the FIX R338L (Padua) substitution conferred hyperactive features (sevenfold higher specific activity) as for FIX Padua alone. Furthermore, we exploited an engineered HSA (QMP), which conferred enhanced human (h)FcRn binding [dissociation constant (K_D) 0·5 nM] over wild‐type FIX‐HSA (K_D 164·4 nM). In hFcRn transgenic mice, Padua‐QMP displayed a significantly prolonged half‐life (2·7 days, P < 0·0001) versus FIX‐HSA (1 day). Overall, we developed a novel FIX‐HSA protein with improved activity and extended half‐life. These combined properties may result in a prolonged functional profile above the therapeutic threshold, and thus in a potentially widened therapeutic window able to improve HB therapy. This rational engineering of both partners may pave the way for new fusion strategies for the design of engineered biotherapeutics.

Challenges and advancements in the pharmacokinetic enhancement of therapeutic proteins

Nowadays, proteins are frequently administered as therapeutic agents in human diseases. However, the main challenge regarding the clinical application of therapeutic proteins is short circulating plasma half-life that leads to more frequent injections for maintaining therapeutic plasma levels, increased therapy costs, immunogenic reactions, and low patient compliance. So, the development of novel strategies to enhance the pharmacokinetic profile of therapeutic proteins has attracted great attention in pharmaceuticals. So far, several techniques, each with their pros and cons, have been developed including chemical bonding to polymers, hyper glycosylation, Fc fusion, human serum albumin fusion, and recombinant PEG mimetics. These techniques mainly classify into three strategies; (i) the endosomal recycling of neonatal Fc receptor which is observed for immunoglobulins and albumin, (ii) decrease in receptor-mediated clearance, and (iii) increase in hydrodynamic radius through chemical and genetic modifications. Recently, novel PEG mimetic peptides like proline/alanine/serine repeat sequences are designed to overcome pitfalls associated with the previous technologies. Biodegradability, lack of or low immunogenicity, product homogeneity, and a simple production process, currently make these polypeptides as the preferred technology for plasma half-life extension of therapeutic proteins. In this review, challenges and pitfalls in the pharmacokinetic enhancement of therapeutic proteins using PEG-mimetic peptides will be discussed in detail.

An intact C-terminal end of albumin is required for its long half-life in humans

Albumin has an average plasma half-life of three weeks and is thus an attractive carrier to improve the pharmacokinetics of fused therapeutics. The half-life is regulated by FcRn, a cellular receptor that protects against intracellular degradation. To tailor-design the therapeutic use of albumin, it is crucial to understand how structural alterations in albumin affect FcRn binding and transport properties. In the blood, the last C-terminal residue (L585) of albumin may be enzymatically cleaved. Here we demonstrate that removal of the L585 residue causes structural stabilization in regions of the principal FcRn binding domain and reduces receptor binding. In line with this, a short half-life of only 3.5 days was measured for cleaved albumin lacking L585 in a patient with acute pancreatitis. Thus, we reveal the structural requirement of an intact C-terminal end of albumin for a long plasma half-life, which has implications for design of albumin-based therapeutics.

Nilsen et al. show that structural alterations in the last C-terminal α-helix of albumin strongly reduce its binding to the neonatal Fc receptor, decreasing the half-life of albumin in humans. This study suggests the structural requirement of the C-terminal of albumin for its long plasma half-life, providing insights into the design of albumin used to carry drugs.

Extended half-life recombinant products in haemophilia clinical practice - Expectations, opportunities and challenges

Extended half-life (EHL) products have shown robust efficacy in clinical trials, whilst allowing for less intense treatment regimens when compared with standard half-life products. Regimen optimisation with EHL products could lead to further improvements in bleeding rates, quality of life and reductions in treatment burden. Patients now expect good efficacy, a lower treatment burden and equivalent safety when compared with standard half-life products. As our knowledge base grows these expectations have evolved and targeting an annualised bleeding rate of zero has become a more realistic clinical goal. Personalised prophylaxis can help patients achieve these goals. However, a number of challenges still remain, including cost, challenges in predicting outcomes for patients and differences in patients' and clinicians' expectations. When switching a patient, comprehensive patient care can reduce the impact of these issues. This review presents in brief the protein therapeutics with an extended half-life, including key trial results, challenges of chronic care that impact on patients' outcomes and how the modified proteins might help address some of these issues. In addition, practical steps for managing the switch to EHL products are presented.

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.

The Neonatal Fc Receptor (FcRn): A Misnomer?

Antibodies are essential components of an adaptive immune response. Immunoglobulin G (IgG) is the most common type of antibody found in circulation and extracellular fluids. Although IgG alone can directly protect the body from infection through the activities of its antigen binding region, the majority of IgG immune functions are mediated via proteins and receptors expressed by specialized cell subsets that bind to the fragment crystallizable (Fc) region of IgG. Fc gamma (γ) receptors (FcγR) belong to a broad family of proteins that presently include classical membrane-bound surface receptors as well as atypical intracellular receptors and cytoplasmic glycoproteins. Among the atypical FcγRs, the neonatal Fc receptor (FcRn) has increasingly gained notoriety given its intimate influence on IgG biology and its ability to also bind to albumin. FcRn functions as a recycling or transcytosis receptor that is responsible for maintaining IgG and albumin in the circulation, and bidirectionally transporting these two ligands across polarized cellular barriers. More recently, it has been appreciated that FcRn acts as an immune receptor by interacting with and facilitating antigen presentation of peptides derived from IgG immune complexes (IC). Here we review FcRn biology and focus on newer advances including how emerging FcRn-targeted therapies may affect the immune responses to IgG and IgG IC.