Improving the pharmacokinetic properties of biologics by fusion to an anti-HSA shark VNAR domain

Advances in recombinant antibody technology and protein engineering have provided the opportunity to reduce antibodies to their smallest binding domain components and have concomitantly driven the requirement for devising strategies to increase serum half-life to optimise drug exposure, thereby increasing therapeutic efficacy. In this study, we adopted an immunization route to raise picomolar affinity shark immunoglobulin new antigen receptors (IgNARs) to target human serum albumin (HSA). From our model shark species, Squalus acanthias, a phage display library encompassing the variable binding domain of IgNAR (VNAR) was constructed, screened against target, and positive clones were characterized for affinity and specificity. N-terminal and C-terminal molecular fusions of our lead hit in complex with a naïve VNAR domain were expressed, purified and exhibited the retention of high affinity binding to HSA, but also cross-selectivity to mouse, rat and monkey serum albumin both in vitro and in vivo. Furthermore, the naïve VNAR had enhanced pharmacokinetic (PK) characteristics in both N- and C-terminal orientations and when tested as a three domain construct with naïve VNAR flanking the HSA binding domain at both the N and C termini. Molecules derived from this platform technology also demonstrated the potential for clinical utility by being available via the subcutaneous route of delivery. This study thus demonstrates the first in vivo functional efficacy of a VNAR binding domain with the ability to enhance PK properties and support delivery of multifunctional therapies.     

Single-chain Variable Fragment Albumin Fusions Bind the Neonatal Fc Receptor (FcRn) in a Species-dependent Manner: IMPLICATIONS FOR IN VIVO HALF-LIFE EVALUATION OF ALBUMIN FUSION THERAPEUTICS*

Albumin has a serum half-life of 3 weeks in humans. This has been utilized to extend the serum persistence of biopharmaceuticals that are fused to albumin. In light of the fact that the neonatal Fc receptor (FcRn) is a key regulator of albumin homeostasis, it is crucial to address how fusion of therapeutics to albumin impacts binding to FcRn. Here, we report on a detailed molecular investigation on how genetic fusion of a short peptide or an single-chain variable fragment (scFv) fragment to human serum albumin (HSA) influences pH-dependent binding to FcRn from mouse, rat, monkey, and human. We have found that fusion to the N- or C-terminal end of HSA only slightly reduces receptor binding, where the most noticeable effect is seen after fusion to the C-terminal end. Furthermore, in contrast to the observed strong binding to human and monkey FcRn, HSA and all HSA fusions bound very poorly to mouse and rat versions of the receptor. Thus, we demonstrate that conventional rodents are limited as preclinical models for analysis of serum half-life of HSA-based biopharmaceuticals. This finding is explained by cross-species differences mainly found within domain III (DIII) of albumin. Our data demonstrate that although fusion, particularly to the C-terminal end, may slightly reduce the affinity for FcRn, HSA is versatile as a carrier of biopharmaceuticals.     

Human single-chain antibodies specific to the tumor necrosis factor

Six unique phage antibodies against human tumor necrosis factor (TNF) were selected from the earlier constructed naïve combinatorial library of single-chain antibodies by affinity selection. The TNF binding of these antibodies was examined by enzyme immunoassay and Western blot analysis. The specificity of the selected antibodies was determined from their binding to interferons alpha and gamma, bovine serum albumin, ovalbumin, and ubiquitin. Two antibodies (sA1 and sB3) were converted into a soluble single-chain antibodies as individual molecules. Their affinity proved to be 2.5 and 13.7 nM, respectively.     

Mutational analysis of the interaction between albumin-binding domain from streptococcal protein G and human serum albumin

Streptococcal protein G (SpG) is a bacterial cell surface receptor exhibiting affinity to both human immunoglobulin (IgG) and human serum albumin (HSA). Interestingly, the serum albumin and immunoglobulin-binding activities have been shown to reside at functionally and structurally separated receptor domains. The binding domain of the HSA-binding part has been shown to be a 46-residue triple alpha-helical structure, but the binding site to HSA has not yet been determined. Here, we have investigated the precise binding region of this bacterial receptor by protein engineering applying an alanine-scanning procedure followed by binding studies by surface plasmon resonance (SPR). The secondary structure as well as the HSA binding of the resulting albumin-binding domain (ABD) variants were analyzed using circular dichroism (CD) and affinity blotting. The analysis shows that the HSA binding involves residues mainly in the second alpha-helix.     

Extending the half-life of a fab fragment through generation of a humanized anti-human serum albumin Fv domain: An investigation into the correlation between affinity and serum half-life

We generated an anti-albumin antibody, CA645, to link its Fv domain to an antigen-binding fragment (Fab), thereby extending the serum half-life of the Fab. CA645 was demonstrated to bind human, cynomolgus, and mouse serum albumin with similar affinity (1–7 nM), and to bind human serum albumin (HSA) when it is in complex with common known ligands. Importantly for half-life extension, CA645 binds HSA with similar affinity within the physiologically relevant range of pH 5.0 – pH 7.4, and does not have a deleterious effect on the binding of HSA to neonatal Fc receptor (FcRn). A crystal structure of humanized CA645 Fab in complex with HSA was solved and showed that CA645 Fab binds to domain II of HSA. Superimposition with the crystal structure of FcRn bound to HSA confirmed that CA645 does not block HSA binding to FcRn. In mice, the serum half-life of humanized CA645 Fab is 84.2 h. This is a significant extension in comparison with < 1 h for a non-HSA binding CA645 Fab variant. The Fab-HSA structure was used to design a series of mutants with reduced affinity to investigate the correlation between the affinity for albumin and serum half-life. Reduction in the affinity for MSA by 144-fold from 2.2 nM to 316 nM had no effect on serum half-life. Strikingly, despite a reduction in affinity to 62 µM, an extension in serum half-life of 26.4 h was still obtained. CA645 Fab and the CA645 Fab-HSA complex have been deposited in the Protein Data Bank (PDB) with accession codes, 5FUZ and 5FUO, respectively.     

Interaction with Both Domain I and III of Albumin Is Required for Optimal pH-dependent Binding to the Neonatal Fc Receptor (FcRn)

Albumin is an abundant blood protein that acts as a transporter of a plethora of small molecules like fatty acids, hormones, toxins, and drugs. In addition, it has an unusual long serum half-life in humans of nearly 3 weeks, which is attributed to its interaction with the neonatal Fc receptor (FcRn). FcRn protects albumin from intracellular degradation via a pH-dependent cellular recycling mechanism. To understand how FcRn impacts the role of albumin as a distributor, it is of importance to unravel the structural mechanism that determines pH-dependent binding. Here, we show that although the C-terminal domain III (DIII) of human serum albumin (HSA) contains the principal binding site, the N-terminal domain I (DI) is important for optimal FcRn binding. Specifically, structural inspection of human FcRn (hFcRn) in complex with HSA revealed that two exposed loops of DI were in proximity with the receptor. To investigate to what extent these contacts affected hFcRn binding, we targeted selected amino acid residues of the loops by mutagenesis. Screening by in vitro interaction assays revealed that several of the engineered HSA variants showed decreased binding to hFcRn, which was also the case for two missense variants with mutations within these loops. In addition, four of the variants showed improved binding. Our findings demonstrate that both DI and DIII are required for optimal binding to FcRn, which has implications for our understanding of the FcRn-albumin relationship and how albumin acts as a distributor. Such knowledge may inspire development of novel HSA-based diagnostics and therapeutics.     

A small bispecific protein selected for orthogonal affinity purification

A novel protein domain with dual affinity has been created by randomization and selection. The small alkali-stabilized albumin-binding domain (ABD*), used as scaffold to construct the library, has affinity to human serum albumin (HSA) and is constituted of 46 amino acids of which 11 were randomized. To achieve a dual binder, the binding site of the inherent HSA affinity was untouched and the randomization was made on the opposite side of the molecule. Despite its small size and randomization of almost a quarter of its amino acids, a bifunctional molecule, ABDz1, with ability to bind to both HSA and the Z2 domain/protein A was successfully selected using phage display. Moreover, the newly selected variant showed improved affinity for HSA compared to the parental molecule. This novel protein domain has been characterized regarding secondary structure and affinity to the two different ligands. The possibility for affinity purification on two different matrices has been investigated using the two ligands, the HSA matrix and the protein A-based, MabSelect SuRe matrix, and the new protein domain was purified to homogeneity. Furthermore, gene fusions between the new domain and three different target proteins with different characteristics were made. To take advantage of both affinities, a purification strategy referred to as orthogonal affinity purification using two different matrices was created. Successful purification of all three versions was efficiently carried out using this strategy.     

The testosterone binding mechanism of an antibody derived from a naïve human scFv library

A testosterone binding scFv antibody was isolated from a naïve human library with a modest size of 10⁸ clones. The crystal structure of the Fab fragment form of the 5F2 antibody clone complexed with testosterone determined at 1.5 Å resolution shows that the hapten is bound deeply in the antibody binding pocket. In addition to the interactions with framework residues only CDR‐L3 and CDR‐H3 loops interact with testosterone and the heavy chain forms the majority of the contacts with the hapten. The testosterone binding site of the 5F2 antibody with a high abundance of aromatic amino acid residues shows similarity with an in vitro affinity matured antibody having around 300 times higher affinity. The moderate affinity of the 5F2 antibody originates from the different orientation of the hapten and few light chain contacts. This is the first three‐dimensional structure of a human steroid hormone binding antibody that has been isolated from a naïve human repertoire. Copyright © 2010 John Wiley & Sons, Ltd.     

Species-dependent stereoselective drug binding to albumin: a circular dichroism study

Drug binding to albumins from different mammalian species was investigated to disclose evidence of species-dependent stereoselectivity in drug-binding processes and affinities. This aspect is important for evaluating the reliability of extrapolating distribution data among species. The circular dichroism (CD) signal induced by drug binding to the albumins [human serum albumin (HSA), bovine serum albumin (BSA), rat serum albumin (RSA), and dog serum albumin (DSA)] were measured and analyzed. The binding of selected drugs and metabolites to HSA significantly differed from the binding to the other albumins in terms of affinity and conformation of the bound ligands. In particular, phenylbutazone, a marker of site one on HSA, showed a higher affinity for binding to BSA with respect to RSA, HSA, and DSA, respectively. In the case of diazepam, a marker of site two on HSA, the affinity decreased in order from HSA to DSA, RSA, and BSA. The induced CD spectra were similar in terms of energy and band signs, suggesting almost the same conformation for the bound drug to the different albumins. Stereoselectivity was high for the binding of ketoprofen to HSA and RSA. A different sign was observed for the CD spectra induced by the drug to the two albumins because of the prevalence of a different conformation of the bound drug. Interestingly, the same induced CD spectra were obtained using either the racemic form or the (S)-enantiomer. Finally, significant differences were observed in the affinity of bilirubin, being highest for BSA, then decreasing for RSA, HSA, and DSA. A more complex conformational equilibrium was observed for bound bilirubin.     

Enhancement of the binding affinity of methylene blue to site I in human serum albumin by cupric and ferric ions

In this work, the binding characteristics of methylene blue (MB) to human serum albumin (HSA) and the influence of Cu²⁺ and Fe³⁺ on the binding affinity of MB to HSA were investigated using fluorescence, absorption, circular dichroism (CD) spectroscopy and molecular modelling. The results of competitive binding experiments using the site probes ketoprofen and ibuprofen as specific markers suggested that MB was located in site I within sub‐domain IIA of HSA. The molecular modelling results agreed with the results of competitive site marker experiments and the results of CD spectra indicated that the interaction between MB and HSA caused the conformational changes in HSA. The binding affinity of MB to HSA was enhanced but to a different extent in the presence of Cu²⁺ and Fe³⁺, respectively, which indicated that the influence of different metal ions varied. Enhancement of the binding affinity of MB to HSA in the presence of Cu²⁺ is due to the formation of Cu²⁺–HSA complex leading to the conformational changes in HSA, whereas in the presence of Fe³⁺, enhancement of the binding affinity is due to the greater stability of the Fe³⁺–HSA–MB complex compared with the MB–HSA complex. Copyright © 2015 John Wiley & Sons, Ltd.     

Allosteric and competitive displacement of drugs from human serum albumin by octanoic acid, as revealed by high-performance liquid affinity chromatography, on a human serum albumin-based stationary phase

A chiral stationary phase for high-performance liquid chromatography, based upon immobilized human serum albumin (HSA), was used to investigate the effect of octanoic acid on the simultaneous binding of a series of drugs to albumin. Octanoic acid was found to bind with high affinity to a primary binding site, which in turn induced an allosteric change in the region of drug binding Site II, resulting in the displacement of compounds binding there. Approximately 80% of the binding of suprofen and ketoprofen to HSA was accounted for by binding at Site II. Octanoic acid was found to also bind to a secondary site on HSA, with much lower affinity. This secondary site appeared to be the warfarin—azapropazone binding area (drug binding Site I), as both warfarin and phenylbutazone were displaced in a competitive manner by high levels of octanoic acid. The enantioselective binding to HSA exhibited by warfarin, suprofen and ketoprofen was found to be due to differential binding of the enantiomers at Site I; the primary binding site for suprofen and ketoprofen was not enantioselective.     

Human Serum Albumin and p53-Activating Peptide Fusion Protein Is Able to Promote Apoptosis and Deliver Fatty Acid-Modified Molecules

Therapeutic peptides offer a high degree of specificity, potency, and low toxicity; making them promising candidates for cancer therapy. Despite these advantages, a number of hurdles, such as poor serum stability and inefficient cellular penetration, must be overcome. Fusing a therapeutic peptide to human serum albumin (HSA) is a common approach to extend the serum stability of a peptide that binds to extracellular receptors. However, no study has shown that this approach can be applied to target intracellular proteins. Here we demonstrate the feasibility of using a recombinant human serum albumin (rHSA) fusion protein to simultaneously deliver two types of molecules: a peptide capable of binding an intracellular target, as well as fatty acid (FA)-modified FITC (FA-FITC). Two peptides reported to disrupt the intracellular p53 and MDM2/MDMX interaction were fused to the C-terminal of HSA. Cellular and biochemical studies indicate that rHSA fusion proteins were efficiently taken up by SJSA-1 cells and retained MDM2- and MDMX-binding activity. By inducing the accumulation of p53, both fusion proteins promoted efficient cytotoxicity in SJSA-1 cells via caspase activation. Long chain fatty acid (LCFA) transportation is an essential endogenous function of HSA. This study also demonstrates that rHSA fusion proteins formed highly stable complexes with FA-FITC via non-covalent interactions. FA-FITC complexed with HSA could be internalized efficiently and rHSA-P53i and rHSA-PMI retained apoptotic activity as complex components. It is expected that such an approach can ultimately be used to facilitate intracellular delivery of two anticancer therapeutics, each with distinct but complimentary mechanisms, to achieve synergistic efficacy.     

Binding of δ9-tetrahydrocannabinol and diazepam to human serum albumin

Cannabis is the most commonly used illicit drug worldwide. Cannabis users also appear to use other psychoactive drugs more frequently than noncannabis users. Here, Δ9-tetrahydrocannabinol (THC) and diazepam binding to human serum albumin (HSA) and HSA-heme is reported. THC binds to two different binding sites of HSA (K(d1) ≤ 10(-7) M and K(d2) = 10(-3)M) without affecting diazepam binding (K(d) = 1.2 × 10(-5) M). THC binding to the high-affinity site accounts for the low free fraction of the drug in plasma. Moreover, THC increases the affinity of heme for HSA. Accordingly, the affinity of THC for HSA-heme is higher than that for HSA. THC could bind to FA2 and FA7 sites, as substantiated by docking simulations; nevertheless, the observed allosteric effect(s) suggests that the primary binding site of THC is the FA2 cleft that positively modulates heme affinity. Possibly, the HSA conformational transition(s) induced by THC binding could account for drug delivery to the liver through receptor- mediated endocytosis.     

Live isolation of naïve ESCs via distinct glucose metabolism and stored glycogen

Naïve and primed pluripotent stem cells recapitulate the peri- and post-implantation development, respectively. Thus, investigation of distinct traits between each pluripotent stem cell type would shed light on early embryonic processes. Herein, by screening a fluorescent probe library, we found that intracellular glycogen led to specific reactivity to CDg4, a glycogen fluorescence sensor, in both human and mouse naïve embryonic stem cells (ESCs). The requirement of constant inhibition of Gsk3β as well as high oxidative phosphorylation (OxPHOS) in naïve compared to primed ESCs was closely associated to high level of intracellular glycogen in naïve ESCs. Both capacity of OxPHOS and stored glycogen, rescued naïve ESCs by transient inhibition of glycolysis, which selectively eliminated primed ESCs. Additionally, naïve ESCs with active OxPHOS were enriched from a mixture with primed ESCs by high reactivity to ATP-Red1, a mitochondrial ATP fluorescence probe. These results indicate the active OxPHOS and high intracellular glycogen as a novel “biomarker” delineating metabolic remodeling during the transition of naïve pluripotency.     

Binding to human serum albumin of zidovudine (AZT) and novel AZT derivatives. Experimental and theoretical analyses

This work presents the binding of AZT and nine novel AZT derivatives to human serum albumin (HSA), both defatted (HSA(D)) and complexed with fatty acids (HSA(FA)). The bound fractions and binding site were determined by applying an ultrafiltration procedure, with an increased affinity for the majority of these derivatives to HSA(D) being found with respect to that of AZT, while only one derivative exhibited an increased affinity for HSA(FA). By means of computational methods, we observed that specific electrostatic interactions are responsible for the increased affinity for HSA(D), while the presence of fatty acids complexed to HSA caused an intense electrostatic repulsion with negatively charged ligands located in Sudlow site I, thus diminishing their bound fractions. A strong relationship between the calculated energetic components and the observed experimental affinity was identified.     

Isolation and characterization of an IgNAR variable domain specific for the human mitochondrial translocase receptor Tom70.

The new antigen receptor (IgNAR) from sharks is a disulphide bonded dimer of two protein chains, each containing one variable and five constant domains, and functions as an antibody. In order to assess the antigen-binding capabilities of isolated IgNAR variable domains (VNAR), we have constructed an in vitro library incorporating synthetic CDR3 regions of 15-18 residues in length. Screening of this library against the 60 kDa cytosolic domain of the 70 kDa outer membrane translocase receptor from human mitochondria (Tom70) resulted in one dominant antigen-specific clone (VNAR 12F-11) after four rounds of in vitro selection. VNAR 12F-11 was expressed into the Escherichia coli periplasm and purified by anti-FLAG affinity chromatography at yields of 3 mg x L(-1). Purified protein eluted from gel filtration columns as a single monomeric protein and CD spectrum analysis indicated correct folding into the expected beta-sheet conformation. Specific binding to Tom70 was demonstrated by ELISA and BIAcore (Kd = 2.2 +/- 0.31 x 10(-9) m-1) indicating that these VNAR domains can be efficiently displayed as bacteriophage libraries, and selected against target antigens with an affinity and stability equivalent to that obtained for other single domain antibodies. As an initial step in producing 'intrabody' variants of 12F-11, the impact of modifying or removing the conserved immunoglobulin intradomain disulphide bond was assessed. High affinity binding was only retained in the wild-type protein, which combined with our inability to affinity mature 12F-11, suggests that this particular VNAR is critically dependent upon precise CDR loop conformations for its binding affinity.     

Comparing CDRH3 diversity captured from secondary lymphoid organs for the generation of recombinant human antibodies

The plasticity of natural immunoglobulin repertoires can be exploited for the generation of phage display libraries. Secondary lymphoid organs, such as the spleen and the lymph nodes, constitute interesting sources of diversity because they are rich in B cells, part of which can be affinity matured. These organs, however, differ in their anatomical structure, reflecting the different fluids they drain, which affects the B cell repertoires. The CDRH3 repertoires from these organs, extracted from naïve or immunized mice, were compared in the context of phage display libraries using human antibody framework families. Deep sequencing analysis revealed that all libraries displayed different CDRH3 repertoires, but the one derived from lymph nodes of naïve mice was the most diverse. Library performance was assessed by in vitro selection. For both organs, immunization increased substantially the frequency of molecules able to bind to the immunogen. The library derived from lymph nodes from naïve mice, however, was the most effective in generating diverse and high affinity candidates. These results illustrate that the use of a biased CDRH3 repertoire increases the performance of libraries, but reduces the clonal diversity, which may be detrimental for certain strategies.     

The CH3PH2 and CH3PH isomers: isomerization,hydrogen release,thermodynamic, and spectroscopy properties

Curcumin has been reported to be therapeutically active but has poor bioavailability, half life, and high rate of metabolic detoxifcation. Most of the hydrophobic and acidic drugs get transported through human serum albumin (HSA). Binding of drugs to serum protein increases their half-life. The present study is focused to analyze interaction of curcumin with HSA by NMR and docking studies. In order to investigate the binding affinity of curcumin with HSA, NMR based diffusion techniques and docking study have been carried out. We report that curcumin has shown comparable binding affinity value vis-a-vis standard, the accessible surface area (ASA) of human serum albumin (uncomplexed) and its docked complex with curcumin at both binding sites was calculated and found to be close to that of warfarin and diazepam respectively. Conclusion drawn from our study demonstrates that curcumin interacts with HSA strongly thereby its poor half life is due to high rate of its metabolic detoxification as reported in literature.

Proton exchange coupled to the specific binding of alkylsulfonates to serum albumins

We have applied isothermal titration calorimetry to investigate the linkage between ligand binding and the uptake or release of protons by human serum albumin (HSA) and bovine serum albumin (BSA). The ligands were sodium decyl sulfate (SDeS) and sodium dodecyl sulfate (SDS). Within a certain temperature range, the binding isotherm could be clearly resolved into two classes of sites (high affinity and low affinity) and modeled assuming independence and thermodynamic equivalence of the sites within each class. Measurements at pH 7.0 in different buffer systems revealed that the binding of SDS to the high affinity sites did not couple to any exchange of protons in either of the proteins. Saturation of the 6-8 low affinity sites for SDS, on the other hand, brought about the release of two protons from both HSA and BSA. In addition to elucidating the pH dependence of ligand binding, this analysis stressed that binding enthalpies for the low affinity sites measured by calorimetry must be corrected for effects due to the concomitant protonation of the buffer. The shorter ligand SDeS bound to HSA with a comparable stoichiometry but with four times lower affinity. Interestingly, no proton linkage was observed for the binding of SDeS. An empirical structural analysis suggested that His 242 in site 7 (of HSA) is a likely candidate for one of the proton donors.     

Elucidation of structural and functional properties of albumin bound to gold nanoparticles

Nanoparticle–albumin complexes are being designed for targeted drug delivery and imaging. However, the changes in the functional properties of albumin due to adsorption on nanoparticles remain elusive. Thus, the objective of this work was to elucidate the structural and functional properties of human and bovine serum albumin bound to negatively charged gold nanoparticles (GNPs). Fluorescence data demonstrated static quenching of albumin by GNP with the quenching of buried as well as surface tryptophan in BSA. The binding process was enthalpy and entropy-driven in HSA and BSA, respectively. At lower concentrations of GNP there was a higher affinity for tryptophan, whereas at higher concentrations both tryptophan and tyrosine participated in the interaction. Synchronous fluorescence spectra revealed that the microenvironment of tryptophan in HSA turned more hydrophilic upon exposure to GNP. The α-helical content of albumin was unaltered by GNP. Approximately 37 and 23% reduction in specific activity of HSA and BSA was observed due to GNP binding. In presence of warfarin and ibuprofen the binding constants of albumin–GNP complexes were altered. A very interesting observation not reported so far is the retained antioxidant activity of albumin in presence of GNP i.e. we believe that GNPs did not bind to the free sulfhydryl groups of albumin. However enhanced levels of copper binding were observed. We have also highlighted the differential response in albumin due to gold and silver nanoparticles which could be attributed to differences in the charge of the nanoparticle.