PEGylation as a strategy for improving nanoparticle-based drug and gene delivery

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Abstract

Coating the surface of nanoparticles with polyethylene glycol (PEG), or “PEGylation”, is a commonly used approach for improving the efficiency of drug and gene delivery to target cells and tissues. Building from the success of PEGylating proteins to improve systemic circulation time and decrease immunogenicity, the impact of PEG coatings on the fate of systemically administered nanoparticle formulations has, and continues to be, widely studied. PEG coatings on nanoparticles shield the surface from aggregation, opsonization, and phagocytosis, prolonging systemic circulation time. Here, we briefly describe the history of the development of PEGylated nanoparticle formulations for systemic administration, including how factors such as PEG molecular weight, PEG surface density, nanoparticle core properties, and repeated administration impact circulation time. A less frequently discussed topic, we then describe how PEG coatings on nanoparticles have also been utilized for overcoming various biological barriers to efficient drug and gene delivery associated with other modes of administration, ranging from gastrointestinal to ocular. Finally, we describe both methods for PEGylating nanoparticles and methods for characterizing PEG surface density, a key factor in the effectiveness of the PEG surface coating for improving drug and gene delivery.

Introduction

In order to deliver adequate concentrations of systemically administered therapeutics to target tissues, these materials must circulate in the blood stream for as long as possible. However, proteins and peptides are rapidly degraded and cleared from the blood stream, necessitating approaches for increasing circulation time. One such approach is to coat the surface of the therapeutic with an inert polymer that resists interactions with components of the blood stream, imparting “stealth” properties. Polyethylene glycol (PEG) is the most widely used “stealth” polymer in the drug delivery field, due to its long history of safety in humans and classification as Generally Regarded as Safe (GRAS) by the FDA. Considered the first reports of PEGylation for drug delivery, Davis and Abuchowski described in 1977 the covalent attachment of PEG to bovine serum albumin and liver catalase proteins [1]. They found that by optimizing the PEGylation chemistry and the extent of PEGylation, they could increase the systemic circulation time and decrease the immunogenicity of the proteins without significantly compromising activity. In 1990, the FDA approved the first PEGylated protein product, Adagen®, a PEGylated adenosine deaminase enzyme for severe combined immunodeficiency disease [2]. Since then, 8 other PEGylated protein therapeutics have been FDA approved for treatment of diseases ranging from rheumatoid arthritis to age-related macular degeneration [2].

The success of protein PEGylation as a method for producing longer circulating, and thus, more efficacious intravenous therapies led to investigations of nanoparticle (NP) PEGylation for systemic applications in the early 80s and 90s [3], [4], [5]. Recognized as foreign objects, NPs are readily cleared from systemic circulation by the cells of the mononuclear phagocyte system (MPS), precluding accumulation in target cells and tissues. However, similar to what was observed with PEGylated proteins, PEG coatings on NPs shield the surface from aggregation, opsonization, and phagocytosis, thereby prolonging circulation time. The first FDA approval of a PEGylated nanoparticle (NP) product, Doxil®, came in 1995. Doxil “Stealth®” liposomes increased doxorubicin bioavailability nearly 90-fold at 1 week from injection versus free drug, with a drug half-life of 72 h and circulation half-life of 36 h [6], [7], [8]. In the years since, PEGylation has become a mainstay in NP formulation. Although much of the initial development of PEGylated NPs focused on systemic administration, in this review we also highlight the benefits of NP PEGylation for overcoming biological barriers to effective delivery associated with numerous modes of delivery, ranging from injection into the eye to topical mucosal applications. Special emphasis is given to studies that directly compare PEGylated to non-PEGylated formulations to specifically demonstrate the benefits of NP PEGylation. Further, we discuss common methods for PEGylating NPs, as well as quantifying a critical parameter that influences the efficiency of delivery, the surface PEG density.

Section snippets

The potential fates of systemically administered nanoparticles

Systemically administered NPs can potentially reach and deliver therapeutic payloads to every vascularized organ/tissue in our body. Prolonging the retention time in the blood has been accepted as the frontline strategy, since it provides higher probability of circulating NPs to encounter, and partition into, the targets of interest. However, this task has been challenging primarily due to the presence of the MPS. The MPS consists of dendritic cells, blood monocytes, granulocytes, and

Non-systemic applications for improved delivery with PEGylated nanoparticles

For many applications, local rather than systemic delivery can improve efficacy while minimizing off-target side effects, but every mode of administration has associated barriers to effective delivery. Although PEGylation was first employed to increase circulation time, improve stability in circulation, and reduce interactions with serum components, benefits of coating NPs with PEG have also be observed with various other non-systemic modes of administration. Discussed in this section, PEG

Methods for nanoparticle PEGylation and quantification of PEG surface density

In this section, we highlight the various techniques that can be employed for formulating PEGylated NPs with a primary focus on polymeric formulations followed by a brief discussion of lipid-based and micelle type NPs. In general, the methods either involve self-assembly of PEG-containing molecules, or surface modification of pre-formed NPs with PEG. As described in previous sections, the surface density of PEG on NPs is a key factor that influences delivery, so here we also describe methods

Conclusion

Building on the success of protein PEGylation for improved systemic delivery of therapeutic proteins, PEGylation of NPs has also proven to be a highly effective approach for improving systemic delivery of therapeutic cargo. Furthermore, we highlighted here how NP PEGylation has also been used as an approach to overcome various extracellular barriers associated with other modes of administration, ranging from mucosal delivery to delivery to brain tissue. Undoubtedly, PEG coatings will continue

Acknowledgments

This work was supported by National Institutes of Health grants U19AI133127 (J.H., L.M.E.), R01HL127413 (J.H., J.S.S.), R33AI094519 (J.H., L.M.E.), and R01EB020147 (J.H.); the Johns Hopkins University Center for AIDS Research P30AI094189 (L.M.E.); the 2015 Burroughs Wellcome Fund Preterm Birth Initiative (L.M.E); the Raymond Kwok Family Research Fund, USA (J.H. and Q.X.); and the Cystic Fibrosis Foundation (J.H., J.S.S.).

The mucus-penetrating particle technology described in this publication is

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