The work described in this dissertation present a novel technique utilizing multivalent hyaluronic acid bioconjugates with an anti-VEGF protein for improving the action of drugs in the vitreous. This technology, which has shown efficacy both in vitro and in vivo has the potential to enhance the bioactivity of drugs used for treating patients with diseases including diabetic retinopathy, wet AMD and other neovascular diseases of the retina.
Chapter 2 described our initial efforts in creating multivalent conjugates of the anti-VEGF protein, sFlt. Before beginning in vivo studies, we wanted to determine what parameters would maximize the bioactivity of mvsFlt. We investigated the use of several HyA molecular weights and valencies of sFlt molecules to HyA chains. The characterization and in vitro experiments were carried out with 6 mvsFlt conjugates of 300 kDa, 650 kDa and 1 MDa molecular weights with feed ratios of 10 sFlt per 1 HyA chain (termed low conjugation ratio (LCR)) and 30 sFlt per 1 HyA chain (termed high conjugation ratio (HCR)). SDS-PAGE and SEC-MALS enabled us to examine the conjugation efficiency following the reaction as well as to investigate the composition of the conjugates focusing specifically on the contribution of unbound sFlt that remained in solution. The in vitro experiments were crucial in determining whether the conjugation of sFlt to the HyA resulted in a decrease in the affinity of sFlt for VEGF. Using an ELISA and a cell-based survival assay, we determined that all the conjugates, irrespective of their molecular weight and valency, equally inhibited VEGF activity and were unaffected by conjugation. Using HyA crosslinked gels, we created an in vitro model of the vitreous to study how the increase in size impacted movement of the mvsFlt conjugates through the gel. The largest mvsFlt conjugates (650 kDa and 1 MDa) were significantly slower in their movement through the gel. The 650 kDa mvsFlt conjugate was then used in the in vivo studies described in Chapter 3. Furthermore, this conjugate was used to confirm the protective effect of HyA on sFlt degradation by a protease that specifically targets sFlt, matrix metalloproteinase-7. Taken together, the work in this chapter demonstrated the efficacy of conjugation, characterization, in vitro bioactivity, slowed diffusion and protection from proteases. The progress demonstrated in this chapter enabled the studies presented in Chapter 3.
The work in Chapter 3 details the in vivo studies used to show mvsFlt efficacy in two models of in vivo angiogenesis and a half-life model in the rat vitreous. As shown in Chapter 2, all the mvsFlt conjugates performed equally at inhibiting VEGF-dependent processes. Thus, we chose the conjugate that displayed slowest diffusion in the crosslinked HyA vitreous model, the 650 kDa mvsFlt conjugate. The first question we tried to address was whether the mvsFlt conjugate remained bioactive in vivo. The corneal angiogenesis model enabled us to examine the effect of sFlt and mvsFlt treatment on the growth of blood vessels in a corneal injury model of angiogenesis. Due to the fact that VEGF is one of the main mediators of angiogenesis in vivo, we expected that the addition of sFlt and mvsFlt would inhibit the formation of new blood vessels. Our data showed that both the sFlt and mvsFlt were equally capable of inhibiting angiogenesis in this model, indicating that the conjugation did not decrease the ability of sFlt to inhibit angiogenesis in vivo. We next demonstrated that the conjugation of sFlt to HyA significantly enhanced the half-life of sFlt in the vitreous of the rat eye by an order of magnitude. Finally, we utilized an oxygen-induced retinopathy rat model to examine the indirect effect of increased half-life on the prolonged anti-angiogenic effect of mvsFlt on neovascularization in the retina. mvsFlt was more effective than sFlt at inhibiting retinal neovascularization, likely due to its increased half-life in the vitreous. Taken together, the mvsFlt conjugate showed superior activity to sFlt in vivo, results that could substantially improve upon currently available drugs for treating retinal disorders.
Chapter 4 gives an overall conclusion and details future directions the project can go in. There are some questions that remain unanswered and this chapter is a guideline to help fill in those gaps.