Site-Specific Bioconjugation Approaches for Enhanced Delivery of Protein Therapeutics and Protein Drug Carriers

Proteins have the capacity to treat a multitude of diseases both as therapeutics and as drug carriers due to their complex functional properties, specificity toward binding partners, biocompatibility, and programmability. Despite this, native proteins often require assistance to target diseased tissue due to poor pharmacokinetic properties and membrane impermeability. Functionalizing therapeutic proteins and drug carriers through direct conjugation of delivery moieties can enhance delivery capabilities. Traditionally, this has been accomplished through bioconjugation methods that have little control over the location or orientation of the modification, leading to highly heterogeneous products with varying activity. A multitude of promising site-specific protein conjugation methods have been developed to allow more tailorable display of delivery moieties and thereby enhance protein activity, circulation properties, and targeting specificity. Here, we focus on three particularly promising site-specific bioconjugation techniques for protein delivery: unnatural amino acid incorporation, Sortase-mediated ligation, and SpyCatcher/SpyTag chemistry. In this review, we highlight the promise of site-specific bioconjugation for targeted drug delivery by summarizing impactful examples in literature, considering important design principles when constructing bioconjugates, and discussing our perspectives on future directions.