Abstract
Although therapeutic proteins have achieved recognized clinical success, they are inherently membrane impermeable, which limits them to acting only on extracellular or membrane-associated targets. Developing an efficient protein delivery method will provide a unique opportunity for intracellular target-related therapeutic proteins. In this review article, we summarize the different pathways by which cells take up proteins. These pathways fall into two main categories: One in which proteins are transported directly across the cell membrane and the other through endocytosis. At the same time, important features to ensure successful delivery through these pathways are highlighted. We then provide a comprehensive overview of the latest developments in the transduction of covalent protein modifications, such as coupling cell-penetrating motifs and supercharging, as well as the use of nanocarriers to mediate protein transport, such as liposomes, polymers, and inorganic nanoparticles. Finally, we emphasize the existing challenges of cytoplasmic protein delivery and provide an outlook for future progress.