Effective drug delivery by PEGylated drug conjugates
Introduction
Poly (ethylene glycol) (PEG, Fig. 1) is a unique polyether diol, usually manufactured by the aqueous anionic polymerization of ethylene oxide, although other polymerization initiators can be employed. Initiation of ethylene oxide polymerization using anhydrous alkanols such as methanol or derivatives including methoxyethoxy ethanol results in a monoalkyl capped poly (ethylene glycol) such as methoxy PEG (mPEG). The polymerization reactions can be modulated and a variety of molecular weights (1000–50,000) can be obtained with low polydispersities (Mw/Mn), <1.05. These polymers are amphiphilic and dissolve in organic solvents as well as in water; they are also non-toxic and eliminated by a combination of renal and hepatic pathways thus making them ideal to employ in pharmaceutical applications. In fact the FDA has approved PEG for Human intravenous (i.v.), oral, and dermal applications. PEG has the lowest level of protein or cellular absorption of any known polymer [1]. These properties have been exploited in numerous ways including grafting PEG to surfaces to prevent deposition of proteinaceous material. This can be further extrapolated to prevention of bacterial surface growth as well as conjugation to proteins which prevents recognition by the immune system. These unusual properties of PEG may in part be due to the highly hydrated polyether backbone which is capable of acceptor hydrogen bonding, and which gives rise to a large exclusion volume (in solution PEG of a given molecular weight (Mw) is much larger than a protein of comparable Mw [2]). A host of other applications of PEG have been reported [3] making PEG and its derivatives one of the most utilized polymers in the vast array of macromolecular organic compounds available. In a rather surprising report by Roy [4] it was suggested that PEG itself induces apoptosis in HT-29 colorectal cancer cells. While the mechanism of this finding was not accounted for, a range of PEG Mws from 400 to 35,000 was examined and it was determined that Mw 8000 produced the greatest efficacy. Finally, another excellent use of PEG is as a soluble polymer-support in organic synthesis [5].
In this review, we will mainly focus on the use of PEG in drug delivery of organic molecules and briefly touch on protein delivery. Styrene-maleic anhydride neocarzinostatin (SMANCS) copolymer [6], hydroxypropyl methacrylamide (HPMA) copolymer [7], dextran [8], [9], polyglutamic acid [10], and polyaspartic acid [11] are but a few of other polymeric systems that have been employed to accomplish delivery in analogous ways. However, PEG conjugation has been most frequently approved for human use with therapeutic proteins, and offers the unique advantage of being a telechelic or semitelechelic polymer [12] and thus loaded quite predictably with organic species. Currently, new strategies have been developed which allow the delivery of several active classes of small molecule PEG-conjugates, whose loading can be accurately determined by UV methods. Anticancer agents have particularly benefited from this technology, but on-going investigations of other potent medicinal agents should soon extend these applications. Most of these recent approaches will be discussed in this review. When appropriate, applications to larger proteins will be presented as well.
Section snippets
Permanent PEGylation of proteins
The potential value of proteins as therapeutics has been recognized for years. Unfortunately, many therapeutic human proteins still suffer from short circulating t1/2 and low stability, and therefore require the use of high doses to maintain therapeutic efficacy. This in turn increases the chance for the development of an adverse immune response [13]. Abuchowski, Davis and co-workers first described a method for the covalent attachment of mPEG to proteins in 1977 [14], since termed PEGylation.
Releasable PEGylation–PEG prodrugs
Prodrug design comprises an area of drug research that is concerned with the optimization of drug delivery. A prodrug is a biologically inactive derivative of a parent drug molecule that usually requires an enzymatic transformation within the body in order to release the active drug, and has improved delivery properties over the parent molecule [55], [56], [57], [58]. Too rapid breakdown of the prodrug can lead to spiking of the parent drug and possible toxicity, while too slow a release rate
Hybrid rPEGylation of protein—a strategy
Typically, this technology involves rPEG-linkers that react with free amine groups present in the protein. In addition to serving as a releasable carrier, rPEG may also be used as a water soluble amino protecting group on or near the active site of the protein. This can be followed by the attachment of permanently bound PEG at other sites where PEGylation is less critical to the biological activity of the protein to produce what can be termed a “PEG-hybrid” (Fig. 17). Once permanently bonded
PEG prodrug targeting
Novel PEG-immunoconjugates, recently reported by Yamasaki’s group [132], [133], have demonstrated antigen specific targeting using anticancer agents, DU-257 and adriamycin, with mAb, KM231 and NL-1, respectively. The enzymatically cleavable linker, PEG-l-ala-l-val, was coupled with DU-257, a potent anticancer duocarmycin derivative, through an amide bond. Coupling of the PEG-DU-257 conjugate to the KM231 mAb which is specifically reactive to GD3 antigen, was then carried out. GD3 antigen is
Conclusion
For protein conjugation, early workers generally used LMw mPEG (2000–5000) attached to multiple sites leading to long-lived protein conjugates. The early development of LMw PEG proteins no doubt influenced small molecule drug delivery strategies, and probably accounts for HMw PEG rarely being considered as pertinent for drug conjugates. However, during the past five years the field of PEG-drug conjugates has metamorphosized into an important delivery methodology, stimulated by the use of higher
Acknowledgements
We wish to thank Carol Aitken for her efforts in helping to prepare this manuscript. We also wish to acknowledge the outstanding contributions made by Drs. Chyi Lee, Hong Zhao, Kwok Shum, Stanford Lee, and Karen Yang. Special thanks to the Analytical and Pharm/Tox Groups of Enzon, Inc. for their outstanding performance in making much of this work possible.
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