Chemically Circumventing the Oxidative Instability of Boronic Acids for Biological Applications

Abstract

Boronic acids are more than just chemical reagents used in synthetic organic chemistry. They are life-saving chemo- and radiotherapeutics, optical imaging agents, chemical sensors of biological stress, hydrogels, nanomaterials, and so much more. While the literature is rich with applications of boronic acids, there is much room for improvement. From the environment we live in, to the biochemical transformations that drive cellular respiration, oxidation is a common thread. Boronic acids are highly susceptible to oxidation by reactive oxygen species that are continuously generated in these environments, and though boronic acids have found wide use, their utility is limited by their short lifetimes due to this form a degradation. We sought to find chemical solutions for this instability. Benzoxaborolone, a boralactone, was previously developed in the Raines lab as an oxidatively stable arylboronic acid. In this current work, we demonstrate that this oxidative stability is enhanced by electron-withdrawing functional groups and reduced by electron-donating functional groups appended to the aryl ring. Applying principles of physical organic chemistry, we reasoned that this electronic impact is predictable depending on the identity of the functional group, with valuable insights for use in medicinal chemistry and chemical biology. Next, we demonstrated that a benzoxaborolone–fluorophore conjugate is an equally effective glycan imaging reagent compared to the commonly used phenylboronic acid-bearing reagent, yet is more stable and thus versatile. We designed this reagent to be a general-use minimalistic glycan-binding reagent with high diffusivity and modularity. We then used this reagent to explore the diverse organ-specific glycome in mice, leveraging recently reported techniques in expansion microscopy to acquire fluorescence images in nanoscale resolution. Finally, we took a new approach to circumventing the chemical instability of boronic acids in biological settings. Bortezomib is a highly potent alkylboronic acid chemotherapeutic that has been a mainstay in the clinic for the treatment of multiple myeloma for nearly twenty-five years. Unfortunately, it is highly susceptible to oxidation and thus prone to inactivation during synthesis, storage, and administration. Substituting in an aromatic, oxidatively stable benzoxaborolone is not an effective strategy for bortezomib, as it would completely change the chemical structure and abrogate activity. Instead, we leveraged a popular boronic acid protecting group, Nmethyliminodiacetic acid (MIDA), used primarily in organic chemistry, to create a bench-stable, oxidation-resistant bortezomib that retains its cytotoxic potency toward cancer cells. Furthermore, we uncovered for the first time that the MIDA protecting group may be susceptible to esterase-mediated hydrolysis, paving a path towards potential prodrug applications in the future.