Therapeutic angiogenesis holds great potential for a myriad of tissue engineering and regenerative medicine approaches. While a number of peptides have been identified with pro-angiogenic behaviors, therapeutic efficacy is limited by poor tissue localization and persistence. Therefore, poly(ethylene glycol) hydrogels providing sustained, enzymatically-responsive peptide release were exploited for peptide delivery. Two pro-angiogenic peptide drugs, SPARC113 and SPARC118, from the Secreted Protein Acidic and Rich in Cysteine, were incorporated into hydrogels as crosslinking peptides flanked by matrix metalloproteinase (MMP) degradable substrates. In vitro testing confirmed peptide drug bioactivity requires sustained delivery. Furthermore, peptides retain bioactivity with residual MMP substrates present after hydrogel release. Incorporation into hydrogels achieved enzymatically-responsive bulk degradation, with peptide release in close agreement with hydrogel mass loss and released peptides retaining bioactivity. Interestingly, SPARC113 and SPARC118-releasing hydrogels had significantly different degradation time constants in vitro (1.16 and 8.77 x10−2 hour−1, respectively), despite identical MMP degradable substrates. However, upon subcutaneous implantation, both SPARC113 and SPARC118 hydrogels exhibited similar degradation constants of ~ 1.45 x10−2 hour−1, and resulted in significant ~ 1.65- fold increases in angiogenesis in vivo compared to controls. Thus, these hydrogels represent a promising pro-angiogenic approach for applications such as tissue engineering and ischemic tissue disorders.

Pro-angiogenic drugs hold great potential to promote reperfusion of ischemic tissues and in tissue engineering applications, but efficacy is limited by poor targeting and short half-lives. Methods to control release duration or provide enzymatically-responsive drug delivery have independently improved drug efficacy. However, no material has been developed to temporally control the rate of enzymatically-responsive drug release. To address this void, hydrogels were developed to provide sustained, tunable release of Qk, a pro-angiogenic peptide mimic of vascular endothelial growth factor, via tissue-specific enzymatic activity. After confirmation that sustained delivery of Qk is necessary for pro-angiogenic effects, a variety of previously-identified matrix metalloproteinase (MMP)-degradable linkers were used to tether Qk to hydrogels. Of these, three (IPES↓LRAG, GPQG↓IWGQ, and VPLS↓LYSG) showed MMP-responsive peptide release. These linkers provided tunable Qk release kinetics, with rates ranging from 1.64 to 19.9 × 10−3 hours−1 in vitro and 4.82 to 8.94 × 10−3 hours−1 in vivo. While Qk was confirmed to be bioactive as released, hydrogels releasing Qk failed to induce significant vascularization in vivo after one week, likely due to non-enzymatically degradable hydrogels employed. While Qk was the focus of this study, the approach could easily be adapted to control the delivery of a variety of therapeutic molecules.

On p. 2002, D.S.W. Benoit and co‐workers describe the first approach to temporally control enzymatically responsive drug release. The background shows the vascularization induced by the enzymatically responsive hydrogels, imaged using multiphoton fluorescence microscopy. Overlaid on this background are schematic depictions of hydrogels releasing drug slowly or quickly, depending on the enzymatically degradable linker use. A cell expressing enzymes is depicted showing the enzymatically mediated release.

No abstract