Modifications of Natural Peptides for Nanoparticle and Drug Design

Andrew P. Jallouk*; Rohun U. Palekar*; Hua Pan*; Paul H. Schlesinger†; Samuel A. Wickline*,†,1    * Consortium for Translational Research in Advanced Imaging and Nanomedicine, Department of Medicine, Division of Cardiology, Washington University in St. Louis, St. Louis, Missouri, USA
† Department of Cell Biology and Physiology, Washington University in St. Louis, St. Louis, Missouri, USA
1 Corresponding author: email address: wicklines@aol.com

1 Introduction

Since the dawn of medicine, natural products have served as a crucial source of therapeutic compounds for drug development. From the identification of salicylic acid as the active component of willow bark extract to the discovery of penicillin produced by the mold Penicillium rubens, natural compounds have formed the basis of many commonly used drugs throughout history. The classical paradigm of drug development from natural products includes screening of biological extracts and identification of active components, followed by structure determination and modification. In the past 20 years, this method of developing small-molecule drugs has largely been supplanted by high-throughput screening of synthetic chemical libraries, lead compound identification, and structural optimization (Koehn & Carter, 2005). During this same time period, peptides have emerged as an important new class of drugs due to their versatility and specificity for individual targets. An enormous number of potentially therapeutic natural peptides have been identified in the course of biological research. Their clinical translation, however, has been limited by the cost of large-scale synthesis and their inefficient delivery to therapeutic sites in vivo (Vlieghe, Lisowski, Martinez, & Khrestchatisky, 2010). Along with continual improvements in the efficiency of peptide synthesis, nanoparticle platforms have been proposed as a strategy to facilitate delivery of peptide drugs. Conversely, natural peptides with unique biological properties have been studied as tools to enhance delivery of nanoparticles bearing other therapeutic compounds. In this chapter, we will review the use of natural peptides in the design and creation of nanomedicines, with a particular focus on short cationic or amphipathic peptides. The sequence and natural origin of several of these peptides are listed in Table 1. These compounds, which may be classified as cell-penetrating peptides (CPPs), antimicrobial peptides (AMPs), and peptide toxins, exhibit interesting interactions with lipid membranes which influence their use as components of nanoparticle drug delivery systems. It is important to note that there is significant overlap between members of these classes and that a particular peptide may behave as a member of more than one class. As a result, the following sections have been constructed based primarily on the applications being discussed rather than individual peptide identities. For each section, we provide a brief review of the structure and function of peptides used for these purposes followed by a detailed description of their application in nanoparticle delivery systems.

2 Role of Nanoparticles in Peptide Drug Delivery

Despite a great deal of enthusiasm, there remain several key pharmacological limitations to the development of peptide-based pharmaceutical agents (Craik, Fairlie, Liras, & Price, 2013). Short peptides (< 50 amino acids) have poor pharmacokinetic profiles due to their degradation by serum proteases and rapid clearance by renal filtration. Additionally, most peptide drugs are limited to extracellular targets due to their inability to cross the plasma membrane and, like all drugs, may have off-target effects when administered systemically. Nanoparticle delivery platforms hold great promise as tools to overcome these limitations and enhance the efficacy and utility of peptide drugs. The term “nanoparticle” refers to an extremely broad range of constructs between 1 and 500 nm in diameter which possess unique physical, chemical, and biological properties as a result of their size (Caruthers, Wickline, & Lanza, 2007). Although most types of nanoparticles have been applied in some way for drug delivery, lipid and polymer nanoparticles are by far the most extensively studied and commonly used for this purpose. The rest of this chapter will focus on the use of these nanoparticles as a platform for the delivery of natural peptides and their derivatives.

Lipid nanoparticles as a class include liposomes and perfluorocarbon nanoparticles, as well as a number of other lipid-based constructs. Liposomes are among the most well-studied nanostructures for drug delivery and include several formulations currently approved for clinical use (Allen & Cullis, 2013). As shown in Fig. 1, liposomes are vesicles consisting of a phospholipid bilayer surrounding an aqueous core (Fig. 1A). Based on their hydrophobicity, small-molecule compounds may either be encapsulated within the aqueous region (e.g., liposomal doxorubicin, Doxil®) or incorporated into the lipid bilayer (e.g., liposomal amphotericin B, AmBisome®). Perfluorocarbon nanoparticles, in contrast, consist of a hydrophobic perfluorocarbon core stabilized by a phospholipid monolayer shell (Fig. 1B). These nanoparticles have been extensively used for targeted drug delivery and imaging via incorporation of targeting ligands into the phospholipid monolayer (Tran et al., 2007). For both liposomes and perfluorocarbon nanoparticles, peptides may be covalently attached to the phospholipid head groups or noncovalently associated with the lipid membrane. Due to their relatively large size, liposomes and perfluorocarbon nanoparticles alter the pharmacokinetic profile of attached peptides, preventing renal filtration and leading to an overall increase in the circulating peptide half-life. For instance, covalent attachment of the thrombin inhibitor d-phenylalanyl-l-prolyl-l-arginyl-chloromethylketone (PPACK) to the surface of liposomes (Palekar et al., 2013) or perfluorocarbon nanoparticles (Myerson, He, Lanza, Tollefsen, & Wickline, 2011) substantially increased its duration of antithrombotic activity relative to free PPACK. Additionally, insertion into the lipid monolayer of perfluorocarbon nanoparticles enhanced circulation time of melittin, a cytolytic peptide toxin derived from bee venom (Soman et al., 2009). Loading of melittin onto perfluorocarbon nanoparticles also protected melittin from cleavage by serum proteases and significantly reduced its hemolytic activity and toxicity when injected systemically. Interestingly, attempts to use liposomes as a melittin delivery vehicle resulted in membrane pore formation and complete disruption of liposome structure (Soman, Lanza, Heuser, Schlesinger, & Wickline, 2008). These findings illustrate the importance of selecting an...