Functionalization of Hyaluronic Acid with Chemoselective Groups via a Disulfide-Based Protection Strategy for In Situ Formation of Mechanically Stable Hydrogels

Functionalization of hyaluronic acid (HA) with chemoselective groups enables in situ (in vivo) formation of HA-based materials in minimally invasive injectable manner. Current methods of HA modification with such groups primarily rely on the use of a large excess of a reagent to introduce a unique reactive handle into HA and, therefore, are difficult to control. We have developed the new protective group strategy based on initial mild cleavage of a disulfide bond followed by elimination of the generated 2-thioethoxycarbonyl moiety ultimately affording free amine-type functionality, such as hydrazide, aminooxy, and carbazate. Specifically, new modifying homobifunctional reagents have been synthesized that contain a new divalent disulfide-based protecting group. Amidation of HA with these reagents gives rise to either one-end coupling product or to intra/intermolecular cross-linking of the HA chains. However, after subsequent treatment of the amidation reaction mixture with dithiothreitol (DTT), these cross-linkages are cleaved, ultimately exposing free amine-type groups. The same methodology was applied to graft serine residues to the HA backbone, which were subsequently oxidized into aldehyde groups. The strategy therefore encompasses a new approach for mild and highly controlled functionalization of HA with both nucleophilic and electrophilic chemoselective functionalities with the emphasis for the subsequent conjugation and in situ cross-linking. A series of new hydrogel materials were prepared by mixing the new HA-aldehyde derivative with different HA-nucleophile counterparts. Rheological properties of the formed hydrogels were determined and related to the structural characteristics of the gel networks. Human dermal fibroblasts remained viable while cultured with the hydrogels for 3 days, with no sign of cytotoxicity, suggesting that the gels described in this study are candidates for use as growth factors delivery vehicles for tissue engineering applications.