Cell-Penetrating Peptides Work by Sticking to Membranes, Not by Escaping Endosomes — Overturning a Key Assumption

Researchers developed the SLEEQ assay using split luciferase complementation to directly and quantitatively measure cytosolic delivery of protein cargo. Multiple widely studied CPPs were fused to a model protein and tested in cell culture. The assay distinguished between total cellular uptake and actual cytosolic delivery, allowing researchers to calculate endosomal escape efficiency independently of uptake. This resolved conflicting results from previous less sensitive and indirect assays.

Billions of research dollars have been spent trying to improve endosomal escape as the key bottleneck in intracellular drug delivery. This study suggests that for CPPs, the bottleneck is actually membrane association — getting more cargo to stick to cells in the first place. This fundamentally redirects drug delivery research: instead of engineering better endosomal escape, scientists should focus on enhancing membrane binding for CPP-based therapies.

This study is a cornerstone for the intracellular drug delivery field because it provides a reliable assay to resolve long-standing mechanistic debates. The SLEEQ assay can be applied beyond CPPs — to lipid nanoparticles, polymeric carriers, and any other delivery system claiming cytosolic delivery. By establishing that CPPs work through membrane association rather than endosomal escape, it reshapes the design principles for an entire class of delivery systems used in gene therapy, peptide therapeutics, and RNA medicine.

All experiments were performed in cell culture, which may not capture the complexity of in vivo delivery where protein corona formation, immune clearance, and tissue barriers are present. Only one model protein cargo was tested — different cargo types (nucleic acids, smaller peptides) may interact differently with CPPs. The SLEEQ assay requires genetic modification of cells, which limits its application to laboratory settings. The findings may not apply to all CPP types or all cell types.