Biodegradable Tetra-PEG Hydrogels as Carriers for a Releasable Drug Delivery System

We have developed an approach to prepare drug-releasing Tetra-PEG hydrogels with exactly four cross-links per monomer. The gels contain two cleavable β-eliminative linkers: one for drug attachment that releases the drug at a predictable rate, and one with a longer half-life placed in each cross-link to control biodegradation. Thus, the system can be optimized to release the drug before significant gel degradation occurs. The synthetic approach involves placing a heterobifunctional connector at each end of a four-arm PEG prepolymer; four unique end-groups of the resultant eight-arm prepolymer are used to tether a linker-drug, and the other four are used for polymerization with a second four-arm PEG. Three different orthogonal reactions that form stable triazoles, diazines, or oximes have been used for tethering the drug to the PEG and for cross-linking the polymer. Three formats for preparing hydrogel–drug conjugates are described that either polymerize preformed PEG–drug conjugates or attach the drug postpolymerization. Degradation of drug-containing hydrogels proceeds as expected for homogeneous Tetra-PEG gels with minimal degradation occurring in early phases and sharp, predictable reverse gelation times. The minimal early degradation allows design of gels that show almost complete drug release before significant gel-drug fragments are released.