In recent decades many new methods have been developed to cure or treat genetical disorders such as cancer, viral infections or inheritable diseases. The problem is that the nucleic acids and their synthetic analogs, oligonucleotides, are not able to cross the cell membrane due to their physicochemical properties like high negative charge and size. Therefore they need assistance to reach their intracellular target.

Cell-penetrating peptides (CPPs) are a class of versatile delivery vectors that can be used to transport various types of bioactive molecules inside the cells, including proteins, small molecules and also nucleic acids like plasmid DNA (pDNA), splice-correcting oligonucleotides (SCO), small interfering RNA (siRNA) and messenger RNA (mRNA).

A well-known method to improve CPPs in non-covalent delivery of nucleic acids is to modify them N-terminally with fatty acids such as stearic acid (C18:0). In this thesis we have studied the role of N-terminal acylation and the length of the carbon chain in the delivery of short SCO as well as larger plasmid DNA. In paper I we varied the N-terminal acyl chain length of a well-studied stearylated CPP, PepFect14, from 2-22 carbons. The delivery efficiency of SCO was dependent on the acyl chain length and it was found to be proportional to the increased association of peptide/oligonucleotide complexes to the cell membrane. In paper II the versatility of PepFect14 as a non-covalent nucleic acid delivery vector was validated using plasmid DNA. Compared to its non-stearylated counterpart, PepFect14 was able to condense pDNA into stable nanoparticles and mediate high gene expression both in regular adherent cell lines as well as difficult-to-transfect primary cells.