In agreement with our in vitro studies using celecoxib, we found that the microparticulate system of celecoxib used in this study was effective in reducing retinal VEGF protein levels. The reduction of the blood–retinal barrier breakdown can be partly explained by the reduction in the retinal VEGF expression. Several lines of evidence suggest an important role for VEGF in the breakdown of the blood–retinal barrier.
15 57 68 It is a potent vasopermeabilizing agent,
69 70 71 with efficacy 4 to 5 orders of magnitude higher than histamine.
72 How hyperglycemia leads to increased VEGF is still open to investigation. There is some evidence of direct elevation of VEGF by hyperglycemia in culture systems. However, the major stimuli are considered to be inflammatory cytokines and oxidative stress that is generated due to the hyperglycemia. Our present and previous studies demonstrate that the celecoxib or celecoxib-PLGA microparticulate systems do not have an effect on the hyperglycemia or body weights in rats.
10 Thus, reduction in VEGF is an outcome of other mechanisms. These could involve a reduction in the production of inflammatory prostaglandins, which is a direct consequence of the Cox-2 inhibitory effect of celecoxib. PGE
2, a major prostaglandin produced by the Cox enzyme is known to stimulate VEGF in several cancer cells as well as in certain normal cells, including the retina.
16 17 18 19 20 21 22 In addition, PGE
2 has a direct effect on vascular permeability and can cause a breakdown of the blood–retinal barrier in rats and rabbits.
8 73 74 In addition, studies from our laboratory have demonstrated that celecoxib can cause a reduction in diabetes-induced retinal oxidative stress.
31 Thus, multiple pathways could be involved in the inhibition of VEGF by celecoxib. A recent important discovery of antiangiogenic isoforms of VEGF has brought to light a reason for possible failure of long-term anti-VEGF therapies.
75 It is hypothesized that these antiangiogenic isoforms of VEGF may be inhibited by anti-VEGF therapies.
75 76 77 If celecoxib inhibits antiangiogenic isoforms of VEGF, it could be a potential limitation of this drug. However, the present study demonstrated a 60% inhibition in PGE
2, a 40% inhibition in VEGF, and a 40% to 50% inhibition of vascular leakage. Thus, the inhibition of vascular leakage might be due to the inhibition of VEGF and other possible pathways, including the direct vasopermeability effect of PGE
2. Our studies to date suggest that, in diabetic conditions, the Cox-2 expression and activity, oxidative stress, VEGF expression, and vascular leakage are elevated
(Fig. 8)and that all these parameters except Cox-2 expression can be reduced by celecoxib, a Cox-2 inhibitor.