Nanoparticle interaction with plasma proteins as it relates to particle biodistribution, biocompatibility and therapeutic efficacy

Abstract

Proteins bind the surfaces of nanoparticles, and biological materials in general, immediately upon introduction of the materials into a physiological environment. The further biological response of the body is influenced by the nanoparticle–protein complex. The nanoparticle's composition and surface chemistry dictate the extent and specificity of protein binding. Protein binding is one of the key elements that affects biodistribution of the nanoparticles throughout the body. Here we review recent research on nanoparticle physicochemical properties important for protein binding, techniques for isolation and identification of nanoparticle-bound proteins, and how these proteins can influence particle biodistribution and biocompatibility. Understanding the nanoparticle–protein complex is necessary for control and manipulation of protein binding, and allows for improved engineering of nanoparticles with favorable bioavailability and biodistribution.

Introduction

Nanotechnology has recently gained attention as one of the critical research endeavors of the 21st century [1]. According to the National Nanotechnology Initiative, nanotechnology is the research and development of nanosystems, such as nanoparticles, on the scale of 1–100 nm [1]. Particles of this size are potentially useful tools for medicine and biology, as they are of commensurate size to important biological components (e.g. DNA, proteins, cell membranes) and thus able to interact in a sophisticated and controlled way at the cellular level. The ability to manipulate particular nanoparticle features, such as their physical, chemical, and biological properties, opens up a host of possibilities for researchers interested in rationally designing these nanoparticles for use in drug delivery, as image contrast agents, and for diagnostic purposes.

Nanoparticles for such medical applications are frequently given via parenteral administration. As with any foreign material, the body mounts a biological response to an administered nanoparticle. This response is the result of a complex interplay of factors, not just the intrinsic characteristics of the nanoparticle. In particular, most materials, upon contact with biological matrices, are immediately coated by proteins, leading to a protein “corona” [2], [3], [4]. Protein coronas are complex and variable. The complete plasma proteome is expected to contain as many as 3700 proteins [5], [6], of which approximately 50 have been identified in association with various nanoparticles [7], [8], [9], [10].

Certain components of the nanoparticle corona, called opsonins, may enhance uptake of the coated material by cells of the reticuloendothelial system (RES) [11], [12]. The presence of opsonins on the particle surface creates a “molecular signature” which is recognized by immune cells and determines the route of particle internalization [13], [14]. The route of internalization, then, may affect the eventual fate of the nanoparticle in the body (i.e. its rate of clearance from the bloodstream, volume of distribution, organ disposition, and rate and route of clearance from the body) [15], [16].

The effect of this protein corona may have special significance for nanoparticles, due to the increased importance of surface effects for particles of this size. Several studies have shown that biological responses to nanoparticles tend to scale with surface area rather than mass [17], [18], [19], [20], [21]. As things become smaller, their surface areas shrink much more slowly than their volumes, causing nanoscale materials to have far greater surface-to-volume ratios than larger particles. A larger surface-to-volume ratio also implies more proteins will bind a nanoparticle (relative to its mass) than a particle of larger size.

Even for larger particles, protein binding is established as one of the most important factors influencing biodistribution [22], [23], [24], [25]. In current preclinical testing of pharmaceutical molecules, evaluation of plasma protein binding is recognized as an important element in an assessment of drug efficacy, safety, and disposition [26], [27]. It has been shown to be important for understanding the pharmacokinetics and pharmacodynamics of the drug inside the body [26], [28], [29], [30]. It is also used to extrapolate preclinical data to models that predict potential drug efficacy and/or toxicity in humans [26]. For biomaterials used as medical implants, it has long been understood that the nature of the deposited protein layer onto these medical devices is responsible for the early immunological response in patients [6]. In fact, even late stage biological responses are influenced and dictated by the surface composition of the material and how this surface interacts with surrounding tissue.

Of course, there are other factors that play a role in determining how nanoparticles distribute within the body. These include particle properties such as size, shape, surface charge (zeta potential), solubility, surface modifications (including targeting), and route of administration. There have been numerous review articles dwelling on the importance of these factors and how each one can influence biodistribution [11], [31], [32], [33], [34], [35], [36], [37], [38]. What is missing, however, is an understanding of how these factors influence plasma protein binding to nanoparticles, and the importance of nanoparticle–protein interactions to biodistribution, biocompatibility, and therapeutic efficacy of nanoparticles. The mechanism of protein binding is not well understood, nor is it currently known how more or less protein binding influences the biological response to a given nanoparticle (e.g. uptake by phagocytic cells of the RES and clearance). However, it is clear that both the amount and identities of proteins on the particle surface play a part in affecting the biodistribution of nanoparticles. To fully understand the protein corona, one must understand not only which proteins are attached to the particle, but the kinetics, affinities, and stoichiometries of protein association and dissociation with the nanoparticle [2].

This review article will cover these aspects of protein binding to nanoparticles. We will discuss methods used to isolate and identify proteins bound to nanoparticles, what factors influence protein binding, how these factors can be manipulated to enhance or decrease protein binding for drug development, the kinetics of protein binding, and the importance of protein binding to drug delivery and distribution throughout the body.