ReviewPEG — A versatile conjugating ligand for drugs and drug delivery systems
Graphical abstract
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.
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