Mitochondria have simply been known as the cell’s powerhouse for a long time, with its vital function of producing ATP. However, substantially more attention was directed towards these organelles once they were recognized to perform several essential functions having an impact in cell biology, pharmaceutics and medicine. Dysfunctions of these organelles have been linked to several diseases such as diabetes, cancer, neurodegenerative diseases and cardiovascular disorders. Mitochondrial medicine emerged once the relationship of reactive oxygen species and mutations of the mitochondrial DNA linked to diseases was shown, referred to as mitochondrial dysfunction. This has led to the need to deliver therapeutic molecules in their active form not only to the target cells but more importantly into the targeted organelles.

In this thesis, cell-penetrating peptides (CPPs) used as mitochondrial drug delivery system and the pathways involved in the uptake mechanisms of a CPP are described. In particular, Paper I describes a novel cell-penetrating peptide targeting mitochondria with intrinsic antioxidant properties. Paper II expands upon this first finding and show that the same peptide can carry a glutathione analogue peptide with improved radical scavenging ability into cytoplasm and mitochondria. Paper III introduces mitochondrial targeting peptides for delivery of therapeutic biomolecules to modify mitochondrial gene expression. In Paper IV, the uptake mechanisms of the CPP delivery strategy has been investigated to gain a better understanding of the used transfection system.

Overall, this thesis summarizes our current effort regarding cell-penetrating peptides delivery system to target mitochondria and the progress made towards a potential gene therapy. It contributes to the field of CPPs and drug delivery with a set of peptides with radical scavenging ability, a strategy to deliver oligonucleotides to mitochondria as proof-of-concept for mitochondrial gene therapy, and to help understanding the pathways involved in CPPs uptake.