Rational design of purely peptidic amphiphiles for drug delivery applications

Abstract

A broad range of new properties is emerging from supramolecular aggregates. Self-assembled structures of purely peptidic amphiphiles exploit these properties to produce biocompatible, biodegradable, smart materials for drug administration. This thesis explores the design, synthesis, purification, characterization of purely peptidic amphiphiles, and evaluates potential applications. The first chapter provides a general introduction to the field of self-assembly, and of drug delivery as compared to nature’s delivery mechanisms. The benefit of amino acid-based molecules in producing smart materials for drug delivery applications is highlighted via biocompatibility and biodegradability. Synthetic strategies and purification methods are discussed. Finally, gramicidin A (gA) – a naturally occurring, short, hydrophobic, membrane-integrated peptide for producing the amphiphilic peptides presented here – is introduced. Chapter 2 presents an initial approach to produce self-assembled structures from purely peptidic amphiphiles. The undecamer used features a repetitive L-tryptophan and D-leucine [LW-DL] motif, representing the hydrophobic block, and an N-terminally attached hydrophilic (lysine or acetylated lysine) section. Besides solid-phase peptide synthesis and purification, the process that self-assembles micelles and spherical peptide particles, or “peptide beads”, was characterized as a function of temperature and solvent composition by electron paramagnetic resonance, dynamic and static light scattering, fluorimetry and electron microscopy. Equilibrium between single peptide molecules, micelles and peptide beads is then presented. Chapter 3 examines the structure of self-assembled peptide beads of diameters from 200 – 1500 nm. The beads were analyzed by electron- and atomic force microscopy, static and dynamic light-, and small angle X-ray scattering. The peptide beads result from hierarchical organization of micellar-like subunits and confirm the concept of multicompartment micelles. An improved understanding of the beads’ capacity to embed hydrophobic and hydrophilic payloads emerges and provides perspectives for drug delivery applications. Chapter 4 presents a library of longer peptides, based on the full sequence of gA. The peptide design includes three parts: (a) a charged lysine part, (b) an acetylated lysine part and (c) a constant hydrophobic rod-like helix, based on gA. Stepwise replacement of lysine (K) with acetylated lysine (X) generated ten peptides: Ac-X8-gA and KmX8-m-gA (m ranging from 0 to 8). A change in the primary sequence caused a change in secondary structure. The transition reflected a change in the self-assembled structures from fibers to micelles. This demonstrates how even small point mutations influence the supramolecular outcome and serve as an important step to understanding and controlling self-assembly. In Chapter 5, knowledge gained on gA-based peptides is applied to produce purely peptidic vesicles. The work here demonstrates that, to form such structures with short amphiphiles, additional stabilizing factors are necessary. Thus, we exploited different dimerization strategies to form stable peptide membranes and developed a general recipe to form purely peptidic vesicles. The vesicles demonstrated pH responsiveness and the capacity to embed hydrophilic and hydrophobic payloads in its structure. Chapter 6 presents the potential of the beads in drug delivery applications. Hydrophobic and hydrophilic payload-filled beads are internalized by human cells. Further, a method to increase embedding efficiency for RNA/DNA payloads to 99% is presented. The internalization of the gene delivery vehicle into cells led to specific gene silencing. Delivery of co-embedded paclitaxel and doxorubicin was proven effective. The results also demonstrate that the new class of drug delivery material caused no measurable toxicity in the experiments. Thus, the material is suggested as a biocompatible drug delivery vehicle for gene therapy and multi-drug delivery. In Chapter 7, self-assembly of the peptide is used to template-pack gold nanoparticles. The C-terminally cysteinated peptide Ac-X3-gT-C was used to coat gold nanoparticles and form gold core micelles. These micelles aggregate into composite peptide-gold nanoparticles in which the individual gold nanoparticles remain separated. Dense packing of the gold nanoparticles offers opportunities for new optical- and electronic properties and use in potential payload release from the beads by the typical gold nanoparticle radiation absorption effect. The last chapter summarizes and discusses the achievements of this work. It gives an overview of on-going work and the prospect of worthwhile research. This includes, e.g. development of drug delivery systems, use of the presented peptidic self-assembly system as template material in nanosciences, and the use of the material to investigate cell uptake pathways of nano-sized objects.