Cell-penetrating peptides have been discovered almost three decades ago and there are, nowadays, thousands of available sequences. They offer multiple applications in the field of drug delivery as they are able to carry therapeutic macromolecules across the plasma membrane. Throughout the years, new sequences have been developed and designed to achieve new applications such as specificity for certain kinds of cargoes, intrinsic therapeutic effects and targeted delivery.

In this thesis, we focused on a single most promising cell-penetrating peptide named PepFect14 and aimed at reaching a better understanding of the factors involved in the cellular uptake through paper I and paper II. Notably, in paper I we screened a library of small molecule drugs that influences signaling pathways and discovered that three drugs had an unreported influence on endocytosis. In paper II, After performing an RNA sequencing on cells treated with PepFect14, we demonstrated the involvement of autophagy in the intracellular trafficking of the cell-penetrating peptide. A second aim of this thesis, covered in paper III and paper IV, was to discover new applications for PepFect14 in order to broaden its potential. In paper III, we successfully used PepFect14 to mediate the intracellular delivery of heat shock protein 70kDa. This was the first protein delivery assisted by PepFect14. In paper IV, PepFect14 was covalently fused to mtCPP1, a cell-penetrating peptide that targets mitochondria and reduce the level of reactive oxygen species. The constructs showed the ability to keep the properties of both peptides and achieved a mitochondria-targeted antisense therapy.

Overall, this thesis summarizes our effort to develop and bring to their full potential already existing cell-penetrating peptides instead of developing new sequences for each new application.