Engineering therapeutic proteins for cell entry: the natural approach

• Toxins and viruses provide natural templates for the design of intracellular biologics.
• Endosomal entrapment is a major issue for cytoplasmic delivery.
• Antibodies are effective in redirecting toxin catalytic domains for therapy.
• Protein engineering can address intracellular stability and potency.

Owing to the challenges of cell entry, protein-based therapies have so far been restricted to extracellular targets, whereas intracellular targets have been almost exclusively addressed by small molecules. The specificity and potency of proteins would enable them to be effective intracellular drugs, provided that the proteins are delivered efficiently to appropriate intracellular compartments within specific cell types. By mimicking the natural mechanisms of toxins and other natural proteins, new intracellular delivery systems are being developed, the first of which are showing clinical efficacy. This review highlights a range of ingenious approaches designed to adapt natural entry mechanisms to facilitate delivery of proteins and open up a range of validated intracellular targets to modulation by potent and specific therapeutic drugs.