Silva EA, Mooney DJ. Spatiotemporal control of vascular endothelial growth factor delivery from injectable hydrogels enhances

angiogenesis. J Thromb Haemost 2007; 5: 590–8.

Ischemic tissue and damaged tissue due to myocardial infarctions require angiogenesis in order to provide more oxygen to local tissue. Promotion of angiogenesis involves the release of the growth factor, VEGF, to the site of damaged tissue. Hydrogels are often used as the delivery vehicle because of their biocompatible nature. Hydrogel’s high water composition and ease of delivery due to non-invasive injections makes it a useful vehicle for drug delivery. Despite this, however, hydrogels have irregular degradation rates. This paper studies injectable alginate hydrogel that can provide sustained delivery and it looks at spatial control of VEGF in ischemic hindlimbs. In the presence of divalent cations, alginate, a polysaccharide, forms cross-links which cause gelation. The degradation rate of alginate hydrogels are studied in vivo and in vitro as treatment for ischemic tissues.

Hindlimb ischemia was induced in mice via unilateral external iliac, femoral artery, and vein ligation. Gels with VEGF protein incorporated into alginate solutions were cast and analyzed for release kinetics. The mice were then injected with alginate gel containing VEGF, without VEGF, or PBS solution with VEGF near areas of ligation. The tissues were fixed, paraffin embedded, and stained for mouse CD31.

In vitro experiments shows VEGF had sustained released and 60% of loaded VEGF was released over 7 days after the initial burst. VEGF delivered from on-oxidized alginated, which can hasten degradation of hydrogels, showed slower release and only 40% was released over 30 days.

In vivo localization of VEGF, supernatant from the tissue samples were collected and stored for VEGF quantification using human ELISA. VEGF quantification was taken at various time points to measure the effectiveness of the hydrogel. Under high-power fields, capillaries can be counted and measurements of ischemic/normal limb blood flow ratio were performed using Periscan system blood perfusion monitor laser Doppler equipment. In the first 24 hours, ELISA showed that VEGF concentrations in injection area were 95% of what was initially loaded via hydrogel. Hydrogels were able to maintain the remaining VEGF concentration while bolus delivery without the hydrogel led to almost zero levels of VEGF after the first 12 hours. Without the hydrogel, VEGF concentrations spread throughout regions near the injection site.

Animals were spared limbs from necrosis because of the sustained release of VEGF using alginate hydrogels. Without such, animals received an initial burst of VEGF and then a constant release for a short period of time. This lack of vascularization led to the loss of blood flow to the limbs and therefore VEGF wasn’t capable of preventing necrosis and loss of limb.

The significance of this study shows a multitude of possibilities for the alginate hydrogel in the future. Being able to spatially and temporally regulate the delivery of VEGF could provide more specific degradation rates as needed according to physiological needs.