Oral delivery of therapeutics is the preferred route of administration due to ease of administration which is associated with greater patient medication adherence. One major barrier to oral delivery and intestinal absorption is rapid clearance of the drug and the drug delivery system from the gastrointestinal (GI) tract. To address this issue, researchers have investigated using GI mucus to help maximize the pharmacokinetics of the therapeutic; while mucus can act as a barrier to effective oral delivery, it can also be used as an anchoring mechanism to improve intestinal residence. Nano-drug delivery systems that use materials which can interact with the mucus layers in the GI tract can enable longer residence time, improving the efficacy of oral drug delivery. This review examines the properties and function of mucus in the GI tract, as well as diseases that alter mucus. Three broad classes of mucus-interacting systems are discussed: mucoadhesive, mucus-penetrating, and mucolytic drug delivery systems. For each class of system, the basis for mucus interaction is presented, and examples of materials that inform the development of these systems are discussed and reviewed. Finally, a list of FDA-approved mucoadhesive, mucus-penetrating, and mucolytic drug delivery systems is reviewed. In summary, this review highlights the progress made in developing mucus-interacting systems, both at a research-scale and commercial-scale level, and describes the theoretical basis for each type of system.

In this work, poly(N-isopropyl acrylamide-co-acrylamide) [P(NIPAAm-co-AAm)] nanogels were modified by hydrolysis above the lower critical solution temperature (LCST) to localize carboxylic acid functional groups at the surface (surface hydrolysis). PNIPAAm copolymerized with 15% and 20% nominal AAm in the feed were prepared and compared to equivalent hydrogels with acrylic acid. The effect and extent of surface hydrolysis was confirmed by potentiometric titration and zeta potential. These surface modified nanogels were then modified with primary amine functionalized PEG chains. Surface hydrolysis-mediated PEGylation had little effect on the swelling response of the nanogels, while also preventing adsorption of model proteins in physiological relevant conditions. While both 15% and 20% AAm gels both decreased protein adsorption, only the 20% AAm gels resulted in fully preventing protein adsorption. The results presented here point to surface hydrolysis as a new route to passivate nanogels for use in vivo.