Unravelling cytosolic delivery of endosomal escape peptides with a quantitative endosomal escape assay (SLEEQ)

Endosomal escape is an essential requirement but a major obstacle to efficient delivery of therapeutic peptides, proteins and nucleic acids. Current understanding of endosomal escape mechanisms remains limited due to significant number of conflicting reports, which are compounded by low sensitivity and indirect assays. To resolve this, we developed a highly sensitive Split Luciferase Endosomal Escape Quantification (SLEEQ) assay to probe mechanisms of cytosolic delivery. We applied SLEEQ to evaluate the endosomal escape of a range of widely studied putative endosomal escape peptides (EEPs). We demonstrated that positively-charged EEPs enhanced cytosolic delivery as a result of increased non-specific cell membrane association, rather than increased endosomal escape efficiency. These findings transform our current understanding of how EEPs increase cytosolic delivery. SLEEQ is a powerful tool that addresses fundamental questions in intracellular drug delivery and will significantly improve the way materials are engineered to increase therapeutic delivery to the cytosol.