Peptide-based drugs have slowly begun migrating from laboratories into pharmacies and now there are several on the market. However, currently only one gene based therapy that is relies on a viral delivery vector has been approved. The long-term goal of our research is to leverage the cell-penetrating peptide (CPP) technology into a potent, safe and simple delivery vector for oligonucleotide (ON) based therapies.

Cell-penetrating peptides have been actively researched for more than 20 years, and many CPPs have been discovered. However, it is not fully understood how the peptides are able to enter cells. In this thesis we present a novel receptor for CPP:ON complexes. Pharmacological inhibition and siRNA knockdown of the class A scavenger receptors (SCARAs) demonstrate that these receptors are the main pathway by which CPP:ON complexes are taken up. As the intracellular fate of particles taken up by (receptor mediated) endocytosis is entrapment in endosomes this thesis also presents a new peptide for ON delivery that has endosomolytic properties. Additionally this new peptide (PepFect 15) is also taken up via receptor-mediated endocytosis by the SCARAs. 

The use of oligonucleotides for gene therapy has the potential to efficiently treat a plethora of diseases with minimal side effects. However, the use of oligonucleotides is hampered by the properties of these molecules, which make it essentially impossible for them to permeate the cellular membrane. Therefore, a great deal of research has been focused on developing delivery vectors, which can efficiently and safely deliver oligonucleotides into cells. Cell-penetrating peptides (CPPs) constitute a class of delivery vectors that have received much attention since they were discovered over 20 years ago. CPPs can deliver a wide variety of cargos into cells, such as small molecules, proteins, oligonucleotides and particles, in an efficacious and non-toxic manner.

In this thesis two new CPPs for oligonucleotide delivery were designed. The purpose of the design was to create CPPs, which form stable complexes with oligonucleotides and have endosomolytic properties. The new peptides showed superior potency in intracellular oligonucleotide delivery compared to previously reported CPPs. These results demonstrate that it is possible to drastically improve the efficiency of existing CPPs with relatively simple modifications.

It is well known that CPPs use endocytosis to gain entry into cells, however, why cells endocytose CPPs has never been clearly established. In this thesis we determine that several CPP:oligonucleotide complexes interact with scavenger receptors, and that this interaction leads to endocytosis. The results presented in this thesis provides a deeper understanding of how CPPs function and thereby insights how to improve CPP design.

Regulation of biological processes through the use of genetic elements is a central part of biological research and also holds great promise for future therapeutic applications. Oligonucleotides comprise a class of versatile biomolecules capable of modulating gene regulation. Gene therapy, the concept of introducing genetic elements in order to treat disease, presents a promising therapeutic strategy based on such macromolecular agents. Applications involving charged macromolecules such as nucleic acids require the development of the active pharmaceutical ingredient as well as efficient means of intracellular delivery. Cell-penetrating peptides are a promising class of drug delivery vehicles, capable of translocation across the cell membrane together with molecules otherwise unable to permeate cells, which has gained significant attention. In order to increase the effectiveness of cell-penetrating peptide-mediated delivery, further understanding of the mechanisms of uptake is needed in addition to improved design to make the cell-penetrating peptides more stable and, in some cases, targeted.

This thesis encompasses four scientific studies aimed at investigating cell-penetrating peptide and oligonucleotide designs amenable to therapeutic applications as well as elucidating the mechanisms underlying uptake of cell-penetrating peptide:oligonucleotide nanoparticles. It also includes an example of a therapeutic application of cell-penetrating peptide-mediated delivery of oligonucleotides. Paper I presents a study evaluating a range of chemically modified anti-miRNAs for use in the design of therapeutic oligonucleotides. All varieties of oligonucleotides used in the study target miRNA-21 and are evaluated using a dual luciferase reporter system. Paper II introduces a novel cell-penetrating peptide, PepFect15, aiming at combining the desirable properties of improved peptide stability and efficient cellular uptake with a propensity for endosomal escape, to produce a delivery vector well suited for delivery of oligonucleotides. The performance of this new cell-penetrating peptide was evaluated based on its delivery capabilities pertaining to splice-correcting oligonucleotides and anti-miRNAs. Paper III investigates the involvement of scavenger receptor class A in the uptake of various cell-penetrating peptides together with their oligonucleotide cargo. Finally, paper IV aims at using cell-penetrating peptide-mediated delivery to improve the efficiency of telomerase inhibition by antisense oligonucleotides targeting the telomerase enzyme ribonucleotide component.