Elsevier

Journal of Controlled Release

Volume 192, 28 October 2014, Pages 67-81
Journal of Controlled Release

Review
PEG — A versatile conjugating ligand for drugs and drug delivery systems

https://doi.org/10.1016/j.jconrel.2014.06.046Get rights and content

Abstract

Polyethylene glycol (PEG) conjugation is a rapidly evolving strategy to solve hurdles in therapeutic delivery and is being used as an add-on tool to the traditional drug delivery methods. Chemically, PEGylation is a term used to denote modification of therapeutic molecules by conjugation with PEG. Efforts are constantly being made to develop novel strategies for conjugation of PEG with these molecules in order to increase its current applications. These strategies are specific to the therapeutic system used and also depend on the availability of activated PEGylating agents. Therefore, a prior knowledge is essential in selecting appropriate method for PEGylation. Once achieved, a successful PEGylation can amend the pharmacokinetic and pharmacodynamic outcomes of therapeutics. Specifically, the primary interest is in their ability to decrease uptake by reticuloendothelial system, prolong blood residence, decrease degradation by metabolic enzymes and reduce protein immunogenicity. The extensive research in this field has resulted into many clinical studies. The knowledge of outcome of these studies gave a good feedback and lessons which helped researchers to redesign PEG conjugates with improved features which can increase the chance of hitting the market. In light of this, the current paper highlights the approaches, novel strategies and the utilization of modern concept for PEG conjugation with respect to various bioactive components of clinical relevance. Moreover, this review also discusses potential clinical outcomes of the PEG conjugation, regulatory approved PEGylated product, clinical trials for newer formulations, and also provides future prospects of this technology.

Introduction

Over the decade, research was focused on discovery of novel therapeutics, and to improve the utility of therapeutics that are otherwise limited by suboptimal pharmacokinetic properties like poor absorption, stability, distribution and elimination. Attempts like chemical modification of drug or use of novel drug delivery systems have been proposed to overcome these inconveniencies. Although, chemical modification and novel delivery systems such as liposomes, microspheres, nanoparticles, are able to enhance stability and decrease the clearance of drugs and diagnostics, they are not easily applicable for proteins and peptides and other biologicals [1].

Potential approaches were sought to overcome these constraints which led to evolution of PEGylation as a strategy to impart favorable pharmacokinetic and pharmacodynamic properties to therapeutics. PEGylation involves modification of the therapeutics by linking one or more poly-ethylene glycol (PEG) molecules to it. Polyethylene glycol, a polymer of ethylene oxide monomers, being safe and non-toxic, has been approved by FDA for human use [2].

PEGylation emerged gradually with progress in field of biologicals and polymer chemistry and was exploited to improve pharmaceutical applications of a wide range of therapeutics. First reports of modification of albumin and catalase by Abuchowski were a breakthrough in PEGylation [3], [4]. Later, these efforts have successfully culminated into various FDA approved PEGylated products like enzymes (bovine adenosine deaminase, urate oxidase and l-asparginase), cytokines (interferon-α2a, interferon-α2b), granulocyte colony stimulating factors, hormones (epoetin-β), antibodies and their fragments and other organic molecules (pegvisomant, pegatinib) and several are in clinical trials [5], [6]. Besides enzymes and proteins, small drug molecules and drug delivery systems like microparticles, hydrogels, liposomes, nanoparticles of polymeric and inorganic materials can also be PEGylated in order to improve their in-vivo performance [7].

The widespread use of PEG owes to its unique physicochemical characteristics i.e. low polydispersity in molecular weight and solubility in aqueous as well as in many organic solvents. It entangles 2–3 water molecules per oxyethylene unit, which increases its apparent molecular weight 5–10 times that of a globular protein of similar molecular weight. The hydrated state also enhances product solubility and diminishes product aggregation and immunogenicity [3], [7]. However, accomplishing successful PEGylation depends on substrate compatibility, selection of complimentary PEGylating agent and method for PEGylation. A number of PEGylating agents are available in the market which range from monofunctional to homo-/hetero-bifunctional PEG products. Therefore, this review focuses on the role of PEGylation in drug delivery with emphasis on understanding the chemistry of substrate and the PEGylating agents and conditions used for conjugation. Further, since the successful transformation of these PEGylated compounds in clinical practice is mainly dependent on an accurate quantitative analysis of their pharmacokinetic and pharmacodynamics parameters in in-vitro and in-vivo samples, a brief overview of such techniques is given. Additionaly discussed here are pharmacotherapeutic outcomes of the PEGylated products in market and under clinical evaluation so as to act as input for future developments.

Section snippets

PEGylation strategies

To date, researchers described a number of different approaches for PEGylation. The first generation of PEGylated products largely involved the nonspecific and irreversible PEGylation of target proteins with linear PEG chains. The demand for bio-responsive drug delivery systems led to the development of second generation PEGylated products, having covalently bound branched PEG chains at specific positions of the molecule, which are able to release drug on stimulation from surrounding

Analytical techniques for quantification

Although several PEGylated therapeutics have entered into clinical trials their successful translation to the clinical use requires accurate assessment of their pharmacokinetic and pharmacodynamic parameters in animals and patients. Sensitive in-vivo quantification is important for evaluating stability, metabolism, and bioavailability of PEGylated compounds. The methods used for quantification include colorimetry, chromatography, radiolabeling, biological, and enzyme-linked immunosorbent

PEGylation of drugs and proteins

PEGylation transforms physicochemical properties of parent molecules like molecular weight, size, hydrophilicity, conformation, steric hindrance, and ionic properties which lead to altered elimination kinetics (Fig. 2). The increase in molecular weight of the parent molecule leads to increase in its size. However, increase in size is several folds, than that predicted based on actual molecular weight of PEG due to extensive hydration of polar oxyethylene units of PEG. This increase in molecular

Clinical status of PEGylated products

Even though PEG is devoid of therapeutic efficacy, it is no longer deliberated as a simple ingredient. This fact implies that after coupling to chemical moiety the conjugate must be considered as a new chemical entity. It can redefine the pharmacokinetics and pharmacodynamics of conjugated chemical moiety. Hence, PEGylated product becomes part of the process that will carry out all the approval steps needed for new drugs. Stability, toxicology and biological fate of the PEGylated product must

Marketed PEGylated products

The value of PEGylated products in pharmaceutical and biomedical industries is now well established. PEGylated bovine adenosine deaminase (Adagen®) marketed by Enzon Pharmaceuticals was the first USFDA approved PEGylated protein for the treatment of immunogenicity syndrome. A large number of protein and peptide pharmaceuticals have entered in the market following Adagen®. Many of the PEGylated proteins and delivery systems are under clinical trials or at the developmental stages as described

Potential alternatives to PEG

Increasing application of PEG in pharmaceutical, clinical and biomedical research not only provides new intuition into the fundamental mechanism of the beneficial properties of PEG, but also increases the probability of encountering potential side effects. The potentially unfavorable effects of PEG includes hypersensitivity reaction in body due to PEG or its side products formed during synthesis and which can also provoke anaphylactic shock [3]. Hypersensitivity reactions not only occur after

Conclusion and future perspective

Over the last two decades, PEGylation has emerged as a promising approach to overcome various physicochemical and biological problems associated with drugs and drug delivery systems in order to develop potential therapeutics for clinical use. The technological advances have resulted in novel method of preparation of PEGylated products. Much of them were to meet the demands for quality and purity of substance mandated by the strict regulatory markets. In this regard the development in rDNA

Acknowledgment

We are grateful to financial support from Indian Council of Medical Research (ICMR, grant no. 45136/2011-Nan/BMS), New Delhi, India, the Department of Foreign Affairs and International Trade (DFAIT), Canadian Commonwealth Scholarship Program (CCSP) by the Canadian Bureau of International Education (CBIE) grant no. M235572C03.

References (166)

  • C. Mueller et al.

    Noncovalent pegylation by dansyl-poly(ethylene glycol)s as a new means against aggregation of salmon calcitonin

    J. Pharm. Sci.

    (2011)
  • C. Mueller et al.

    Tryptophan-mPEGs: novel excipients that stabilize salmon calcitonin against aggregation by non-covalent PEGylation

    Eur. J. Pharm. Biopharm.

    (2011)
  • J.C. Neal et al.

    In vitro displacement by rat serum of adsorbed radiolabeled poloxamer and poloxamine copolymers from model and biodegradable nanospheres

    J. Pharm. Sci.

    (1998)
  • K. Yoncheva et al.

    Pegylated nanoparticles based on poly(methyl vinyl ether-co-maleic anhydride): preparation and evaluation of their bioadhesive properties

    Eur. J. Pharm. Sci.

    (2005)
  • F.K. Bedu-Addo et al.

    Interaction of PEG–phospholipid conjugates with phospholipid: implications in liposomal drug delivery

    Adv. Drug Deliv. Rev.

    (1995)
  • Y. Bae et al.

    Intelligent polymeric micelles from functional poly(ethylene glycol)-poly(amino acid) block copolymers

    Adv. Drug Deliv. Rev.

    (2009)
  • R.B. Greenwald et al.

    Effective drug delivery by PEGylated drug conjugates

    Adv. Drug Deliv. Rev.

    (2003)
  • P. Caliceti et al.

    Pharmacokinetic and biodistribution properties of poly(ethylene glycol)-protein conjugates

    Adv. Drug Deliv. Rev.

    (2003)
  • D. Filpula et al.

    Releasable PEGylation of proteins with customized linkers

    Adv. Drug Deliv. Rev.

    (2008)
  • J. Khandare et al.

    Polymer–drug conjugates: progress in polymeric prodrugs

    Prog. Polym. Sci.

    (2006)
  • T. Minko

    Soluble polymer conjugates for drug delivery

    Drug Discov. Today

    (2005)
  • T. Yamaoka et al.

    Distribution and tissue uptake of poly(ethylene glycol) with different molecular weights after intravenous administration to mice

    J. Pharm. Sci.

    (1994)
  • D.E. Owens et al.

    Opsonization, biodistribution, and pharmacokinetics of polymeric nanoparticles

    Int. J. Pharm.

    (2006)
  • A. Nag et al.

    A colorimetric estimation of polyethyleneglycol-conjugated phospholipid in stealth liposomes

    Anal. Biochem.

    (1997)
  • T.W. Chung et al.

    A colorimetric method for determining distearoylphosphatidylethanolamine–polyethylene glycol 2000 in blood suspension

    Anal. Biochem.

    (2000)
  • A. Nag et al.

    A colorimetric assay for estimation of polyethylene glycol and polyethylene glycolated protein using ammonium ferrothiocyanate

    Anal. Biochem.

    (1996)
  • X. Liu et al.

    A HPLC-UV method for the determination of puerarin in rat plasma after intravenous administration of PEGylated puerarin conjugate

    J. Chromatogr. B Anal. Technol. Biomed. Life Sci.

    (2010)
  • C.M. Ryan et al.

    Separation and quantitation of polyethylene glycols 400 and 3350 from human urine by high-performance liquid chromatography

    J. Pharm. Sci.

    (1992)
  • X. Chen et al.

    Pharmacokinetics and tumor retention of 125I-labeled RGD peptide are improved by PEGylation

    Nucl. Med. Biol.

    (2004)
  • A. Malek et al.

    In vivo pharmacokinetics, tissue distribution and underlying mechanisms of various PEI(-PEG)/siRNA complexes

    Toxicol. Appl. Pharmacol.

    (2009)
  • K.A. Poschel et al.

    Pharmacodynamics and pharmacokinetics of polyethylene glycol-hirudin in patients with chronic renal failure

    Kidney Int.

    (2000)
  • Y.W. Jo et al.

    Long-acting interferon-alpha 2a modified with a trimer-structured polyethylene glycol: preparation, in vitro bioactivity, in vivo stability and pharmacokinetics

    Int. J. Pharm.

    (2006)
  • F.W. Holtsberg et al.

    Poly(ethylene glycol) (PEG) conjugated arginine deiminase: effects of PEG formulations on its pharmacological properties

    J. Control. Release

    (2002)
  • X.H. Song et al.

    Quantitative determination of PEG–hirudin in human plasma using a competitive enzyme-linked immunosorbent assay

    Thromb. Res.

    (2000)
  • H.A. Myler et al.

    Troubleshooting PEG–hGH detection supporting pharmacokinetic evaluation in growth hormone deficient patients

    J. Pharmacol. Toxicol. Methods

    (2010)
  • R. Federico et al.

    Histaminase PEGylation: preparation and characterization of a new bioconjugate for therapeutic application

    J. Control Release

    (2006)
  • V. Gaberc-Porekar et al.

    Obstacles and pitfalls in the PEGylation of therapeutic proteins

    Curr. Opin. Drug Discov. Devel.

    (2008)
  • J.M. Harris et al.

    Pegylation: a novel process for modifying pharmacokinetics

    Clin. Pharmacokinet.

    (2001)
  • J.M. Harris et al.

    Effect of pegylation on pharmaceuticals

    Nat. Rev. Drug Discov.

    (2003)
  • S.S. Wong

    Reactive Groups of Proteins and Their Modifying Agents

    (1991)
  • F.F. Schumacher et al.

    In situ maleimide bridging of disulfides and a new approach to protein PEGylation

    Bioconjug. Chem.

    (2011)
  • S. Shaunak et al.

    Site-specific PEGylation of native disulfide bonds in therapeutic proteins

    Nat. Chem. Biol.

    (2006)
  • H. Cho et al.

    Optimized clinical performance of growth hormone with an expanded genetic code

    Proc. Natl. Acad. Sci.

    (2011)
  • D.G. Longstaff et al.

    A natural genetic code expansion cassette enables transmissible biosynthesis and genetic encoding of pyrrolysine

    Proc. Natl. Acad. Sci. U. S. A.

    (2007)
  • W. Ou et al.

    Site-specific protein modifications through pyrroline-carboxy-lysine residues

    Proc. Natl. Acad. Sci.

    (2011)
  • Y. Anraku et al.

    Protein splicing: its discovery and structural insight into novel chemical mechanisms

    IUBMB Life

    (2005)
  • J. Thom et al.

    Recombinant protein hydrazides: application to site-specific protein PEGylation

    Bioconjug. Chem.

    (2011)
  • A. Mero et al.

    Multivalent and flexible PEG–nitrilotriacetic acid derivatives for non-covalent protein pegylation

    Pharm. Res.

    (2011)
  • S. DeFrees et al.

    GlycoPEGylation of recombinant therapeutic proteins produced in Escherichia coli

    Glycobiology

    (2006)
  • H.R. Stennicke et al.

    Generation and biochemical characterization of glycoPEGylated factor VIIa derivatives

    Thromb. Haemost.

    (2008)
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