The diverse biological roles of natural macromolecules have evolved over millennia and inspire the development of synthetic analogs that can mimic the structure, function or biological role of their natural congeners. The goal of this program is to develop a new class of responsive biocompatible macromolecules bearing dithiolane heterocycles. Organocatalytic ring-opening polymerization of trimethylene carbonate monomers is a versatile strategy for the generation of synthetic, biodegradable polymers with diverse functional groups that mimic the function of natural polymers. The Waymouth group at Stanford has recently discovered an organocatalytic synthetic strategy for generation of water-soluble polymers containing pendant dithiolane heterocycles. Dithiolanes are five-membered rings with disulfide bonds analogous to that in Lipoic acid cofactors. Dithiolanes exchange readily with thiols and are reversibly reduced to dithiols with excess thiols, such as glutathione.I propose that the reversible and rapid exchange of diothiolanes with thiols could provide a responsive control element that could facilitate release of drugs or probes in the reducing environment of the cell, as well as provide new dynamic covalent networks as adaptive matrices for cell culture and tissue engineering. The first goal of this project is to develop a reliable method for rapid, controlled synthesis of polymers and hydrogels incorporating dithiolane functionalities. Other functional groups, such as guanidinium groups (GD) for drug transport across cell membranes and lipids (L) to induce micelle formation, will be incorporated alongside dithiolane groups (DT) in block polymers. The synthesis of dithiolane polymers will be tuned and refined to control physical properties such as molecular weight, polymer charge, and water solubility. The next goal is to generate an array of block polymer architectures targeted for mRNA delivery, including sequences such as GD-DT-PEG-DT-GD, PEG-GD-DT, and DT-GD-PEG-L. These responsive materials will be evaluated for their ability to bind, transport, and release messenger RNA (mRNA) in cells. Trimethylene carbonate oligomer/polymer uptake in cells can be monitored by attaching a fluorescent label to mRNA, and gene expression will be evaluated to determine the degree of mRNA release. The last goal described in this proposal is to evaluate hydrogels as covalent adaptable networks for tissue engineering. Hydrogel formation will be investigated by addition of dithiol cross linkers (HS-X-SH) to DT-PED-DT polymer architectures. Hydrogel properties such as gelation time, modulus, and stress relaxation can be controlled by varying the degree of dithiolane polymerization, the ratio of the dithiol cross-linker to the dithiolane polymer, and by adding a PEG- acrylate cross-linker capable of forming permanent covalent cross-links. These properties will be optimized for hydrogel injection. Cell viability and proliferation will be evaluated in vitro in collaboration wih Professor Fan Yang (Stanford Orthopaedic Surgery and Bioengineering). Other biomedical applications will be guided by new results and insights on the physical and dynamic properties of these adaptable and responsive macromolecules.
A strategy for synthesis of multi-functional, biocompatible polymers incorporating dithiolanes will be developed. Synthetic polymers with dithiolanes offer a reductive-release mechanism for targeted drug delivery in cells. Hydrogels capable of restructuring their shape are also formed when a dithiol cross-linker is added to dithiolane polymers, which shows great promise for an array of biomedical materials.
