The Chemistry of Boronic Acids in Nanomaterials for Drug Delivery

Interest in increasing drug delivery efficiency has risen over the past decade both as a means to improve efficacy of already clinically available drugs and due to the increased difficulties of approving new drugs. As a functional group for targeted drug delivery, boronic acids (BAs) have been incorporated in polymeric particles both as a stimuli-responsive functional group and as a targeting ligand. Here, BA chemistry presents a wealth of opportunities for biological applications. It not only reacts with several chemical markers of disease such as reactive oxygen species (ROS), adenosine triphosphate (ATP), glucose, and reduced pH, but it also acts as ligands for diols such as sialic acid. These stimuli-responsive drug delivery systems optimize delivery of therapeutics based on rational design and precise molecular engineering. When designing materials containing BA, the unique chemical properties are important to take into consideration such as its vacant p-orbital, its molecular geometry, and the designed acid's pK_a. Instead of behaving as most carboxylic acids that donate protons, BAs instead primarily act as Lewis acids that accept electrons. In aqueous solution, most polymers containing BA exist in an equilibrium between their triangular hydrophobic form and a tetrahedral hydrophilic form. The most common pK_a's are in the nonphysiological range of 8-10, and much ongoing research focuses on modifying BAs into materials sensitive to a more physiologically relevant pH range. So far, BA moieties have been incorporated into a stunning array of materials, ranging from small molecules that can self-assemble into higher order structures such as micelles and polymeric micelles, via larger polymeric assemblies, to large scale hydrogels. With the abundance of biological molecules containing diols and polyhydroxy motifs, BA-containing materials have proven valuable in several biomedical applications such as treatment of cancer, diabetes, obesity, and bacterial infections. Both materials functionalized with BA and boronic esters display good safety profiles in vitro and in vivo; thus, BA-containing materials represent promising carriers for responsive delivery systems with great potential for clinical translation. The intention of this Account is to showcase the versatility of BA for biomedical applications. We first discuss the chemistry of BA and what to consider when designing BA-containing materials. Further, we review how its chemistry recently has been applied to nanomaterials for enhanced delivery efficiency, both as a stimuli-responsive group and as a targeting ligand. Lastly, we discuss the current limitations and further perspectives of BA in biomaterials, based on the great benefits that can come from utilizing the unique BA chemistry to enhance drug delivery efficiency.