Oxidatively activated hydrogels for molecular cargo release

Abstract

Non-communicable diseases (NCDs) are responsible for more than 70% of all deaths globally and more than 80% of premature deaths in developing countries. Many NCDs, including asthma, cancer, and rheumatoid arthritis, are complicated by chronic inflammatory responses and exhibit high levels of reactive oxygen species (ROS) that damage essential cellular components of healthy cells, resulting in tissue and organ damage. While conventional drugs can treat a wide range of inflammatory symptoms, their systemic delivery can cause off-target effects. To achieve site-specific delivery of drugs, arylboronic acid-based hydrogels have been explored as drug reservoirs. In one approach, arylboronic acids were used to build arylboronate ester-crosslinked hydrogels that degrade upon oxidation. Ideally, these hydrogels lie dormant until activated by the high levels of ROS at inflammatory sites, at which point, they deliver their therapeutic cargo. However, because this design uses non-covalently attached cargo, significant leakage of the therapeutic occurs, even in the absence of an oxidative trigger. Moreover, arylboronate ester is a dynamic covalent bond that can dissociate at lower pH, at higher temperature, or in the presence of cis-diol-containing molecules, such as glucose, which causes undesirable background degradation in the absence of ROS. Such shortcomings can lead to non-specific delivery prior to target inflammatory events. In the other approach, arylboronic acids have been designed to degrade upon oxidation and were used to covalently attach cargoes to polymeric scaffolds of hydrogels. This moiety is resistant to non-specific hydrolysis caused by dynamic covalent exchanges because the cargo is linked at a site separate from the aryl boronic acid/boronate ester. In our studies, we have designed oxidatively degradable arylboronic acid molecular switches that can release payloads from hydrogels in pathophysiological conditions. We first explored diazaborine (DAB) moiety that had been proposed to be a stable and oxidizable linkage. After performing in-depth physical organic analyses of DAB formation, hydrolysis, and oxidative reactivity, we found that many DABs are susceptible to hydrolysis and exchange at the carbonyl position in physiological conditions. Furthermore, DAB oxidations were found to be undesirably slow in pathophysiologically relevant ROS concentrations. Next, we explored para-benzyl functionalized arylboronic acids that can be designed to undergo 1,6-elimination upon oxidation. When oxidized, the resulting phenol can undergo subsequent electron rearrangement into para-quinone methide (p-QM) and release the cargo attached at the benzyl position. To better understand the design principles, we conducted a systematic study by comparing arylboronic acids loaded with analogous amine cargoes. We found that the use of a carbamate bridge can drastically increase the oxidative cargo release efficacy in physiological conditions and that amine cargoes of low pKa can generate undesirable side products. We then designed and synthesized rhodamine-loaded arylboronic acid switch that can be attached to polymeric scaffold of hydrogels. Our findings set stage for hydrolytically stable hydrogels that can be used to deliver cargoes under pathophysiological conditions.