Pharmacokinetics and toxicology of therapeutic proteins: Advances and challenges

Interference in immunoassays to support therapeutic antibody development in preclinical and clinical studies

During preclinical and clinical studies, immunoassays are used to measure the concentration of the therapeutic antibody, anti-drug antibodies and soluble protein biomarkers. The reliability of these assays is crucial since the results are routinely used for safety assessment and dose selection. Furthermore, soluble protein biomarkers can provide information about target engagement, proof of mechanism, proof of principle and prediction of response. Study samples mostly consist of complex matrices that can exhibit considerable interference, resulting in inaccurate measurements. This perspective discusses the source of interference and strategies to mitigate or eliminate interference in immunoassays used during preclinical and clinical drug development of drugs with a focus on the development of therapeutic antibodies.

Assessment of the metabolism of therapeutic proteins and antibodies

INTRODUCTION

In the last decade, our increased knowledge of factors governing the pharmacokinetics and metabolism of biologics (recombinant therapeutic proteins) has driven, and will continue to support, biological engineering and the design of delivery systems for more efficient biologics. Further research in analytical methods for assessing their in vitro and/or in vivo metabolism will also support these developments.

AREAS COVERED

In this review we will discuss the main components affecting the metabolism of biologics, and try to demonstrate how novel analytical evaluations will facilitate their future development. We will focus on the use of radiolabeled drugs, ligand-binding assays and mass spectrometry.

EXPERT OPINION

Future marketed biologics will be complex structures, such as glycoengineered, fused, or chemically modified proteins. Their in vivo efficiencies will be strongly dependent on their metabolic stabilities. Similarly to small molecular drugs, for which in vitro and in vivo biochemical platforms and analytical techniques have helped to rationalize preclinical and clinical developments, we would expect this also to translate to effective approaches to study the metabolism of biologics in the near future. Mass spectrometry should emerge as a standard technique for in vivo characterization of the biotransformation products of biologics.

Nonclinical Safety Biomarkers of Drug-induced Vascular Injury

Better biomarkers are needed to identify, characterize, and/or monitor drug-induced vascular injury (DIVI) in nonclinical species and patients. The Predictive Safety Testing Consortium (PSTC), a precompetitive collaboration of pharmaceutical companies and the U.S. Food and Drug Administration (FDA), formed the Vascular Injury Working Group (VIWG) to develop and qualify translatable biomarkers of DIVI. The VIWG focused its research on acute DIVI because early detection for clinical and nonclinical safety monitoring is desirable. The VIWG developed a strategy based on the premise that biomarkers of DIVI in rat would be translatable to humans due to the morphologic similarity of vascular injury between species regardless of mechanism. The histomorphologic lexicon for DIVI in rat defines degenerative and adaptive findings of the vascular endothelium and smooth muscles, and characterizes inflammatory components. We describe the mechanisms of these changes and their associations with candidate biomarkers for which advanced analytical method validation was completed. Further development is recommended for circulating microRNAs, endothelial microparticles, and imaging techniques. Recommendations for sample collection and processing, analytical methods, and confirmation of target localization using immunohistochemistry and in situ hybridization are described. The methods described are anticipated to aid in the identification and qualification of translational biomarkers for DIVI.

Site-specific functionalization of proteins and their applications to therapeutic antibodies

Protein modifications are often required to study structure and function relationships. Instead of the random labeling of lysine residues, methods have been developed to (sequence) specific label proteins. Next to chemical modifications, tools to integrate new chemical groups for bioorthogonal reactions have been applied. Alternatively, proteins can also be selectively modified by enzymes. Herein we review the methods available for site-specific modification of proteins and their applications for therapeutic antibodies.

Monoclonal antibody disposition: a simplified PBPK model and its implications for the derivation and interpretation of classical compartment models

The structure, interpretation and parameterization of classical compartment models as well as physiologically-based pharmacokinetic (PBPK) models for monoclonal antibody (mAb) disposition are very diverse, with no apparent consensus. In addition, there is a remarkable discrepancy between the simplicity of experimental plasma and tissue profiles and the complexity of published PBPK models. We present a simplified PBPK model based on an extravasation rate-limited tissue model with elimination potentially occurring from various tissues and plasma. Based on model reduction (lumping), we derive several classical compartment model structures that are consistent with the simplified PBPK model and experimental data. We show that a common interpretation of classical two-compartment models for mAb disposition-identifying the central compartment with the total plasma volume and the peripheral compartment with the interstitial space (or part of it)-is not consistent with current knowledge. Results are illustrated for the monoclonal antibodies 7E3 and T84.66 in mice.

Pharmacokinetics of anti-IL17A and anti-IL22 peptide-antibody bispecific genetic fusions in mice

The peptide-antibody (Ab) genetic fusion is a promising technology for targeting multiple antigens in a single Ab-like molecule. We have recently described generation and in vitro characterization of several such genetic fusions, using an interleukin (IL)-17A binding peptide and an anti-IL-22 Ab as a model system. In this study we assessed pharmacokinetic profiles of these model genetic fusions in mice. Specifically an IL-17A binding peptide was fused to either the heavy chain or both the heavy and the light chains of an anti-IL22 human IgG1 (referred to Compounds 1 or 2, respectively). Swiss Webster mice were given a single 10 mg/kg IV dose of Compound 1 or Compound 2 and serum concentrations were measured by a fused molecule immunoassay, in which IL-17A was used as a capture and anti-human IgG was used as a detector. In addition, serum samples were assayed using a total human IgG immunoassay. PK parameters were calculated by non-compartmental modeling. The two genetic fusions had similar PK profiles, with total body clearance of ~0.9-1.0 mL/h/kg, volume of distribution at steady-state of ~63-65 mL/kg, and elimination half-life of ~40 h. Our study provides the first characterization of the PK properties of peptide-Ab genetic fusions and suggests that although these genetic fusions appear to be eliminated faster than a typical Ab, the PK profile may be suitable for preclinical and clinical testing.

Tissue bioanalysis of biotherapeutics and drug targets to support PK/PD

Contemporary drug discovery leverages quantitative modeling and simulation with increasing emphasis, both to gain deeper knowledge of drug targets and mechanisms as well as improve predictions between preclinical models and clinical applications, such as first-in-human dose projections. Proliferation of novel biotherapeutic modalities increases the need for applied PK/PD modeling as a quantitative tool to advance new therapies. Of particular relevance is the understanding of exposure, target binding and associated pharmacology at the target site of interest. Bioanalytical methods are key to informing PK/PD models and require assessment of both PK and PD end points. Where targets are sequestered in tissues (noncirculating), the ability to quantitatively measure drug or biomarker in tissue compartments becomes particularly important. This perspective provides an overview of contemporary applications of quantitative bioanalysis in tissue compartments as applied to PK and PD assessments associated with novel biotherapeutics. Case studies and key references are provided.

Pharmacokinetics of Peptide-Fc fusion proteins

Peptide-Fc fusion proteins (or peptibodies) are chimeric proteins generated by fusing a biologically active peptide with the Fc-domain of immunoglobulin G. In this review, we describe recent studies that have evaluated the absorption, distribution, metabolism, and excretion characteristics of peptibodies. Key features of the pharmacokinetics of peptibodies include their extended half-life due to recycling by the neonatal Fc receptor (FcRn), a substantial contribution by renal excretion to total clearance and, for certain peptibodies, target-mediated drug disposition. The prolonged half-life of peptibodies permits less-frequent dose administration compared with small therapeutic peptides, thereby supporting patient convenience and compliance. Hence, a considerable number of peptibodies are currently in preclinical and clinical development. Investigation of the metabolism (biotransformation) of biologics is an evolving area of research: ligand-binding mass spectrometry techniques have been employed for the characterization of the peptibody romiplostim, providing a new approach to evaluation of the degradation products of biologics. Pharmacokinetic/pharmacodynamic modeling and simulation techniques have been used to predict the pharmacokinetics of peptibodies which can inform clinical decision-making, particularly selection of dosing regimens. This integrated review highlights the distinct pharmacokinetic characteristics of peptibodies and their influence on the drug development process for this emerging family of therapeutics.

Identification and elimination of target-related matrix interference in a neutralizing anti-drug antibody assay

Biopharmaceuticals administered to the human body have the potential to trigger the production of anti-drug (also called anti-therapeutic) antibodies (ADA) that can neutralize the therapeutic activity. For antibody therapeutics, cell-based neutralizing ADA assays are frequently used to evaluate ADA in clinical studies. We developed a method to detect neutralizing antibodies against MEDI-575, a fully human IgG2κ antagonistic antibody against PDGFR-α. We evaluated three assay formats, two of which measured late responses, cell proliferation and apoptosis, whereas the third assay detected an early signaling event, phosphorylation of PDGFR-α. Measuring phosphorylation provided a superior assay window and therefore was developed as a neutralizing ADA (NAb) assay. Matrix interference, however, was significant, and could be identified to be caused by PDGF-AA and PDGF-AB, apparently the two most abundant ligands of PDGFR-α present in human serum samples. A simple pre-treatment step, addition of an inhibitory antibody to PDGF-A, a subunit present in PDGF-AA and PDGF-AB, was found to eliminate matrix interference, increasing assay reliability and sensitivity. We integrated the pre-treatment step into assay development and qualified a robust NAb assay.

Recombinant receptor-binding domain of diphtheria toxin increases the potency of curcumin by enhancing cellular uptake

Diphtheria toxin (DT) binds to a specific cell surface receptor, gets internalized, and causes cytotoxicity through its catalytic domain. The toxicity of DT is used in several therapeutic molecules. Here, we have exploited the receptor-binding ability of DT to increase cellular uptake of curcumin, a hydrophobic molecule with low bioavailability and cellular uptake. We have expressed only the receptor-binding domain of DT (RDT) in Escherichia coli. Purified RDT binds to the receptor with an affinity equivalent to that of full-length DT. It also binds to curcumin forming a curcumin-RDT complex, and this increases the fluorescence intensity and fluorescence lifetime of curcumin. The curcumin-RDT complex binds to the receptor and associates with human glioblastoma cells (U-87 MG) expressing the receptor. The cellular uptake of curcumin is higher for the curcumin-RDT complex than curcumin alone. This increase in uptake enhances the antiproliferative effect of curcumin and induces apoptosis of these cells even at a lower dose.

Investigation of the influence of nephropathy on monoclonal antibody disposition: a pharmacokinetic study in a mouse model of diabetic nephropathy

PURPOSE

This study employed a mouse model to evaluate the effects of diabetic nephropathy on the pharmacokinetics of 8C2, a murine monoclonal antibody (mAb).

METHODS

Streptozotocin (STZ) was administered to mice to induce diabetic nephropathy (125 mg/kg/day × 2). Mice were grouped (n = 8-10) based on time after STZ-treatment (control, 1, 2, 3, 4, or 6 weeks), and injected intravenously with 10 mg/kg 8C2. Blood samples were collected up to 7 days, and 8C2 plasma concentrations were determined via immunoassay. Inulin clearance and urinary albumin excretion rate (UAE) were determined to assess renal function.

RESULTS

UAE, inulin clearance, and 8C2 clearance increased significantly following STZ. Comparing control and 6 week STZ-treatment groups, UAE and inulin clearance increased from 25.7 ± 3.3 to 99.3 ± 13.7 μg/day, and from 421 ± 31 to 584 ± 78 μl/min. 8C2 clearance increased from 121 ± 12.5 to 228 ± 61 μl/hr/kg (p < 0.01). 8C2 clearance was highly correlated with UAE (r(2): 0.731). Inclusion of UAE as a covariate in population modeling explained significant residual variability in 8C2 clearance.

CONCLUSIONS

The clearance of 8C2 increased significantly in STZ-treated mice. Population pharmacokinetic modeling suggests that UAE has potential for use in predicting mAb clearance in subjects with diabetic nephropathy, possibly assisting in the individualization of mAb dosing.

Life cycle management of critical ligand-binding reagents

Bioanalytical laboratories develop and validate ligand-binding assays (LBA) used to quantify the concentration of analytes of interest in various buffers and relevant biological matrices. The building blocks of LBA are reagents that recognize molecular and structural motifs on ligands, which are combined in various LBA formats to minimize biological matrix interferences and specifically detect and quantify the analyte of interest. The use of these LBA-requiring critical reagents, can span decades as programs mature to commercialization. Since critical reagents are generated mostly from biological systems, attention to their life cycle management, quality, characterization and sustainability are vital to the success of bioanalytical laboratories. Integrating de novo reagent generation, reagent biophysical characterization, LBA development, validation, and use, with reagent resupply processes leverages interdisciplinary activities and ensures smooth operations of a bioanalytical laboratory.

Are endosomal trafficking parameters better targets for improving mAb pharmacokinetics than FcRn binding affinity?

F.W.R. Brambell deduced the existence of a protective receptor for IgG, the neonatal Fc receptor (FcRn), long before its discovery in the early to mid-1990s. With the coincident, explosive development of IgG-based drugs, FcRn became a popular target for tuning the pharmacokinetics of monoclonal antibodies (mAbs). One aspect of Brambell's initial observation, however, that is seldom discussed since the discovery of the receptor, is the compliance in the mechanism that Brambell observed (saturating at 10s-100s of μM concentration), vs. the comparative stiffness of the receptor kinetics (saturating in the nM range for most species). Although some studies reported that increasing the already very high Fc-FcRn affinity at pH 6.0 further improved mAb half-life, in fact the results were mixed, with later studies increasingly implicating non-FcRn-dependent mechanisms as determinants of mAb pharmacokinetics. Mathematical modelling of the FcRn system has also indicated that the processes determining the pharmacokinetics of mAbs have more nuances than had at first been hypothesised. We propose, in keeping with the latest modelling and experimental evidence reviewed here, that the dynamics of endosomal sorting and trafficking have important roles in the compliant salvage mechanism that Brambell first observed nearly 50 years ago, and therefore also in the pharmacokinetics of mAbs. These ideas lead to many open questions regarding the endosomal trafficking of both FcRn and mAbs and also to what properties of a mAb can be altered to achieve an improvement in pharmacokinetics.

Biological Insights into Therapeutic Protein Modifications throughout Trafficking and Their Biopharmaceutical Applications

Over the lifespan of therapeutic proteins, from the point of biosynthesis to the complete clearance from tested subjects, they undergo various biological modifications. Therapeutic influences and molecular mechanisms of these modifications have been well appreciated for some while remained less understood for many. This paper has classified these modifications into multiple categories, according to their processing locations and enzymatic involvement during the trafficking events. It also focuses on the underlying mechanisms and structural-functional relationship between modifications and therapeutic properties. In addition, recent advances in protein engineering, cell line engineering, and process engineering, by exploring these complex cellular processes, are discussed and summarized, for improving functional characteristics and attributes of protein-based biopharmaceutical products.

Impact of physiochemical properties on pharmacokinetics of protein therapeutics

Physicochemical properties, such as molecular weight, size, partition coefficient, acid dissociation constant and solubility have a great impact on pharmacokinetics of traditional small molecule drugs and substantially used in development of small drugs. However, predicting pharmacokinetic fate (absorption, distribution, metabolism and elimination) of protein therapeutics from their physicochemical parameters is extremely difficult due to the macromolecular nature of therapeutic proteins and peptides. Their structural complexity and immunogenicity are other contributing factors that determine their biological fate. Therefore, to develop generalized strategies concerning development of therapeutic proteins and peptides are highly challenging. However, reviewing the literature, authors found that physiochemical properties, such as molecular weight, charge and structural modification are having great impact on pharmacokinetics of protein therapeutics and an attempt is made to provide the major findings in this manuscript. This manuscript will serve to provide some bases for developing protein therapeutics with desired pharmacokinetic profile.

Issues, challenges, and opportunities in model-based drug development for monoclonal antibodies

Over the last two decades, there has been a simultaneous explosion in the levels of activity and capability in both monoclonal antibody (mAb) drug development and in the use of quantitative pharmacologic models to facilitate drug development. Both of these topics are currently areas of great interest to academia, the pharmaceutical and biotechnology industries, and to regulatory authorities. In this article, we summarize convergence of these two areas and discuss some of the current and historical applications of the use of mathematical-model-based techniques to facilitate the discovery and development of mAb therapeutics. We also consider some of the current issues and limitations in model-based antibody discovery/development and highlight areas of further opportunity.

Nonclinical development of a biosimilar: the current landscape

Preclinical studies have always been a critical component in the development program of a biopharmaceutical. With the advent of biosimilars the traditional preclinical program has changed to a new paradigm that integrates the concept of comparability with existing knowledge of the biopharmaceutical reference drug. Recently, the recommended preclinical program espoused by the European Medicines Agency has been modified to an abbreviated one that now emphasizes in vitro studies in lieu of in vivo for monoclonal antibody biosimilars. Likewise, the US FDA guidance on biosimilars suggests a flexible approach rather than the 28day comparative toxicology studies that have historically been conducted for worldwide marketing. For now, structure and function studies will continue to be the foundation of the overall analytical assessment of biosimilarity. Traditional, comparative animal safety assessments will have limited value in determination of biosimilarity and in an abbreviated design they may have most value in providing assurance of safety in first-in-human trials when structural attributes are not indistinguishable. Unless this value can be proven, particularly as analytical technology improves in sensitivity, accuracy and precision, the need for these animal safety studies will diminish. Thus, the future lies in the ever evolving and sophisticated analytical studies that will replace the current in vivo studies for biosimilar products.

An engineered human follistatin variant: insights into the pharmacokinetic and pharmocodynamic relationships of a novel molecule with broad therapeutic potential

Human follistatin is a regulatory glycoprotein with widespread biologic functions, including antiinflammatory activities, wound-healing properties, and muscle-stimulating effects. The role of follistatin in a wide range of biologic activities shows promise for potential clinical application, which has prompted considerable interest in the investigation of the protein as a potential disease-modifying agent. In spite of this potential, the development of follistatin as a broad use biotherapeutic has been severely hindered by a poor understanding and characterization of its pharmacokinetic/pharmacodynamic (PK/PD) relationships. Therefore, to better define these relationships, we performed in-depth analyses of the PK/PD relationships of native follistatin-315 (FST315). Our data indicate that the intrinsic PK/PD properties of native FST315 are poorly suited for acting as a parentally administered biotherapeutic with broad systemic effects. Here, we leveraged protein engineering to modify the PK characteristics of the native molecule by fusing FST315 to a murine IgG(1) Fc and removing the intrinsic heparan sulfate-binding activity of follistatin. The engineered variant molecule had ~100- and ~1600-fold improvements in terminal half-life and exposure, respectively. In contrast to the native FST315, the variant showed a robust, dose-dependent pharmacological effect when administered subcutaneously on a weekly basis in mouse models of muscle atrophy and degeneration. These studies highlight the underappreciated and critical relationship between optimizing multiple physical and chemical properties of follistatin on its overall PK/PD profile. Moreover, our findings provide the first documented strategy toward the development of a follistatin therapeutic with potential use in patients affected with skeletal muscle diseases.

Identification and multidimensional optimization of an asymmetric bispecific IgG antibody mimicking the function of factor VIII cofactor activity

In hemophilia A, routine prophylaxis with exogenous factor VIII (FVIII) requires frequent intravenous injections and can lead to the development of anti-FVIII alloantibodies (FVIII inhibitors). To overcome these drawbacks, we screened asymmetric bispecific IgG antibodies to factor IXa (FIXa) and factor X (FX), mimicking the FVIII cofactor function. Since the therapeutic potential of the lead bispecific antibody was marginal, FVIII-mimetic activity was improved by modifying its binding properties to FIXa and FX, and the pharmacokinetics was improved by engineering the charge properties of the variable region. Difficulties in manufacturing the bispecific antibody were overcome by identifying a common light chain for the anti-FIXa and anti-FX heavy chains through framework/complementarity determining region shuffling, and by pI engineering of the two heavy chains to facilitate ion exchange chromatographic purification of the bispecific antibody from the mixture of byproducts. Engineering to overcome low solubility and deamidation was also performed. The multidimensionally optimized bispecific antibody hBS910 exhibited potent FVIII-mimetic activity in human FVIII-deficient plasma, and had a half-life of 3 weeks and high subcutaneous bioavailability in cynomolgus monkeys. Importantly, the activity of hBS910 was not affected by FVIII inhibitors, while anti-hBS910 antibodies did not inhibit FVIII activity, allowing the use of hBS910 without considering the development or presence of FVIII inhibitors. Furthermore, hBS910 could be purified on a large manufacturing scale and formulated into a subcutaneously injectable liquid formulation for clinical use. These features of hBS910 enable routine prophylaxis by subcutaneous delivery at a long dosing interval without considering the development or presence of FVIII inhibitors. We expect that hBS910 (investigational drug name: ACE910) will provide significant benefit for severe hemophilia A patients.

Characterization of critical reagents in ligand-binding assays: enabling robust bioanalytical methods and lifecycle management

The effective management of validated ligand-binding assays used for PK, PD and immunogenicity assessments of biotherapeutics is vital to ensuring robust and consistent assay performance throughout the lifetime of the method. The structural integrity and functional quality of critical reagents is often linked to ligand-binding assay performance; therefore, physicochemical and biophysical characterization coupled with assessment of assay performance can enable the highest degree of reagent quality. The implementation of a systematic characterization process for monitoring critical reagent attributes, utilizing detailed analytical techniques such as LC–MS, can expedite assay troubleshooting and identify deleterious trends. In addition, this minimizes the potential for costly delays in drug development due to reagent instability or batch-to-batch variability. This article provides our perspectives on a proactive critical reagent QC process. Case studies highlight the analytical techniques used to identify chemical and molecular factors and the interdependencies that can contribute to protein heterogeneity and integrity.

Challenges and opportunities in absorption, distribution, metabolism, and excretion studies of therapeutic biologics

With the advancement of biotechnology in the last two decades, optimized and novel modalities and platforms of biologic moieties have emerged rapidly in drug discovery pipelines. In addition, new technologies for delivering therapeutic biologics (e.g., needle-free devices, nanoparticle complexes), as well as novel approaches for disease treatments (e.g., stem cell therapy, individualized medicine), continue to be developed. While pharmacokinetic studies are routinely carried out for therapeutic biologics, experiments that elucidate underlying mechanisms for clearance and biodistribution or identify key factors that govern absorption, distribution, metabolism, and excretion (ADME) of biologics often are not thoroughly conducted. Realizing the importance of biologics as therapeutic agents, pharmaceutical industry has recently begun to move the research focus from small molecules only to a blended portfolio consisting of both small molecules and biologics. This trend brings many opportunities for scientists working in the drug disposition research field. In anticipation of these opportunities and associated challenges, this review highlights impact of ADME studies on clinical and commercial success of biologics, with a particular focus on emerging applications and technologies and linkage with mechanistic pharmacokinetic/pharmacodynamic modeling and biomarker research.

Absorption, distribution, metabolism, and excretion (ADME) studies of biotherapeutics for autoimmune and inflammatory conditions

Biotherapeutics are becoming an increasingly common drug class used to treat autoimmune and other inflammatory conditions. Optimization of absorption, distribution, metabolism, and excretion (ADME) profiles of biotherapeutics is crucial for clinical, as well as commercial, success of these drugs. This review focuses on the common questions and challenges in ADME optimization of biotherapeutics for inflammatory conditions. For these immunomodulatory and/or immunosuppressive biotherapeutics, special consideration should be given to the assessment of the interdependency of ADME profiles, pharmacokinetic/pharmacodynamic (PK/PD) relationships, and immunogenicity profiles across various preclinical species and humans, including the interdependencies both in biology and in assay readouts. The context of usage, such as dosing regimens, extent of disease, concomitant medications, and drug product characteristics may have a direct or indirect (via modulation of immunogenicity) impact on ADME profiles of biotherapeutics. Along these lines, emerging topics include assessments of preexisting reactivity to a biotherapeutic agent, impact of immunogenicity on tissue exposure, and analysis of penetration to normal versus inflamed tissues. Because of the above complexities and interdependences, it is essential to interpret PK, PD, and anti-drug antibody results in an integrated manner. In addition, because of the competitive landscape in autoimmune and inflammatory markets, many pioneering ADME-centric protein engineering and subsequent in vivo testing (such as optimization of novel modalities to extend serum and tissue exposures and to improve bioavailability) are being conducted with biotherapeutics in this therapeutic area. However, the ultimate challenge is demonstration of the clinical relevance (or lack thereof) of modified ADME and immunogenicity profiles.

Pharmacokinetic, biodistribution, and biophysical profiles of TNF nanobodies conjugated to linear or branched poly(ethylene glycol)

Covalent attachment of poly(ethylene glycol) (PEG) to therapeutic proteins has been used to prolong in vivo exposure of therapeutic proteins. We have examined pharmacokinetic, biodistribution, and biophysical profiles of three different tumor necrosis factor alpha (TNF) Nanobody-40 kDa PEG conjugates: linear 1 × 40 KDa, branched 2 × 20 kDa, and 4 × 10 kDa conjugates. In accord with earlier reports, the superior PK profile was observed for the branched versus linear PEG conjugates, while all three conjugates had similar potency in a cell-based assay. Our results also indicate that (i) a superior PK profile of branched versus linear PEGs is likely to hold across species, (ii) for a given PEG size, the extent of PEG branching affects the PK profile, and (iii) tissue penetration may differ between linear and branched PEG conjugates in a tissue-specific manner. Biophysical analysis (R(g)/R(h) ratio) demonstrated that among the three protein-PEG conjugates the linear PEG conjugate had the most extended time-average conformation and the most exposed surface charges. We hypothesized that these biophysical characteristics of the linear PEG conjugate accounts for relatively less optimal masking of sites involved in elimination of the PEGylated Nanobodies (e.g., intracellular uptake and proteolysis), leading to lower in vivo exposure compared to the branched PEG conjugates. However, additional studies are needed to test this hypothesis.