Ординатура / Офтальмология / Английские материалы / Diabetic Retinopathy_Lang_2007

is important because the inhibition of PKC can normalize the retinal microvascular hemodynamics.

The adhesion of monocytes on endothelial cells is increased in diabetes mellitus. The membrane-associated activity of PKC in monocytes is markedly elevated in diabetics and leads to an increased adhesion of monocytes on endothelial vessel walls [13].

The activity of PKC plays an important role in the regulation of receptor density on the cell surfaces for hormones, in the intracellular signal response, the ion channel activity, the intracellular pH and the phosphorylation of proteins [14]. The increased reactive contractility of smooth muscles, which is observed in diabetic patients, is caused by hyperglycemia-induced PKC activation. Changes in the intracellular calcium concentration are associated with the PKC activation and modulate growth factor-induced mitogenesis and contraction. Finally, apoptosis of smooth vascular muscle cells are dependent on PKC [15].

The loss of endothelial cell barrier function is an early pathophysiological phenomenon in diabetic retinopathy. The PKC-mediated phosphorylation of junctional proteins and dissolution of tight junctions, as well as the relaxation of cytoskeletal and adhesion proteins like caldesmon, vimentin, talin and vinulin are responsible for increased vascular permeability caused by increased glucose levels.

VEGF is not only the primary mediator of disturbed vascular permeability, but also of neoangiogenesis. In eyes of diabetics, high VEGF levels were found. When glucose levels are increased, the VEGF gene expression is dependent on PKC. Diabetic macular edema and the majority of the neovascular response in the retina is mediated by VEGF. Activation of PKCis involved in mediating VEGF-induced intracellular signaling. The VEGF-induced disturbed bloodretinal barrier, endothelial cell proliferation and neoangiogenesis can be blocked by -specific PKC inhibition, although the PKCinhibitor is not primarily a VEGF inhibitor [16]. In cellular and animal models, its antiproliferative effect is weaker than its antipermeability activity [16, 17]. Over time, different growth factors and cytokines are involved in the pathogenesis of diabetic retinopathy.

The thickening of the capillary basement membrane and the increase in extracellular matrix are the predominant vascular changes in the early phase of diabetes mellitus. The basement membrane plays an important role concerning vascular permeability, cellular adhesion, proliferation, differentiation and gene expression. The production of collagen types IV and VI as well as fibronectin is enhanced in diabetics. PKC inhibitors can prevent these effects.

Transforming growth factorand connective tissue growth factor play a key role in the thickening of the basement membrane and the synthesis of extracellular matrix. The expression of these growth factors can be blocked by PKC inhibition [18].

Treatment of Diabetic Retinopathy with PKCInhibitors

Recently, an isoenzyme-selective PKC inhibitor, ruboxistaurin mesylate (RBX; Eli Lilly), was developed [19], which is orally administered. RBX is the first of a new class of compounds and the most potent and selective PKCinhibitor being investigated. The treatment of diabetic rats with RBX showed an amelioration of the retinal blood flow in a dose-responsive manner in parallel with its inhibition of the retinal PKC activity [20]. Aiello et al. [16] demonstrated that intravitreal injection of VEGF rapidly activates PKC in the retina at concentrations observed clinically and increases retinal vasopermeability in vivo by more than 3-fold. Intravitreal or oral administration of a PKCinhibitor resulted in 95% inhibition of VEGF-induced permeability. RBX reduces the VEGF-induced retinal blood-retinal barrier breakdown and neovascularization in animals. PKC inhibitors abolished both VEGF-induced PKC activation and endothelial cell proliferation. The mitogenic effect of VEGFs was inhibited by RBX in a concentration-dependent manner [17]. The PKCinhibitor is effective in preventing diabetes-induced retinal vascular leakage in animal models and in preventing retinal neovascularization. Danis et al. [21] found that the PKCinhibitor RBX effectively inhibited preretinal and optic nerve head neovascularization in a pig model of branch retinal vein occlusion. They found a significantly decreased degree of neovascularization in pig eyes with no apparent systemic toxicity. The ameliorative effect seems to be a result of disruption of a key element of the intracellular signal cascade by angiogenic growth factors, in particular VEGF. It has been shown that the PKC pathway lies downstream of the VEGF receptor ligand binding and is involved in mediating the proliferative response of endothelial cells to VEGF.

RBX treatment can reduce the retinal vascular leakage in eyes that have diabetic macular edema and markedly elevated leakage at doses between 4 and 32 mg/day. These data suggest that RBX treatment may be most prominent in patients with severe macular edema [22].

In patients receiving 16 mg RBX twice daily, the diabetes-induced increase in retinal circulation time was ameliorated. Increasing the RBX dose was linearly associated with greater effect on retinal circulation time. Similar results were observed with retinal blood flow [23].

Treatment of diabetic macular edema patients for 3 months with multitargeted kinase inhibitor, which also acts as a nonspecific PKC inhibitor, led to reduction in retinal thickening as evaluated by optical coherence tomography. The systemic applicability of this nonselective compound was limited by gastrointestinal side effects and dose-related problems with tolerability, glycemic control and liver toxicity [24].

In a mulitcenter, double-masked, randomized, placebo-controlled study, the safety and efficacy of the orally administered PKCisoform selective inhibitor RBX was evaluated in subjects with moderately severe to very severe nonproliferative diabetic retinopathy. Two hundred and fifty-two subjects received placebo or RBX (8, 16 or 32 mg/day) for 36–46 months. Patients had an Early Treatment Diabetic Retinopathy Study retinopathy severity level between 47B and 53E inclusive, an Early Treatment Diabetic Retinopathy Study visual acuity of 20/125 or better, and no history of scatter photocoagulation. Efficacy measurements included progression of diabetic retinopathy, moderate visual loss and sustained moderate visual loss. RBX was well tolerated without significant adverse events, but had no significant effect on the progression of diabetic retinopathy. Compared with placebo, 32 mg/day RBX was associated with a delayed occurrence of moderate visual loss (p 0.038) and sustained moderate visual loss (p 0.226). This was evident only in eyes with definite diabetic macular edema at baseline (p 0.017). In multivariable Cox proportional hazard analysis, 32 mg/day RBX significantly reduced the risk of moderate visual loss compared with placebo (p 0.012) [25]. In this clinical trial, RBX was well tolerated and reduced the risk of visual loss but did not prevent diabetic retinopathy progression. The beneficial effect of RBX on moderate visual loss might be the result of improved retinal cell viability resulting from PKCinhibition. Reduction in PKCactivity might result in greater resistance of retinal vascular and neural cells to the pathologic stress of hyperglycemia and changes in hemodynamics like blood flow.

Further multicenter trials investigate if RBX can reduce the progression of diabetic macular edema and diabetic retinopathy. One of the phase 3 clinical trials was finished recently, and it was announced by Eli Lilly and Company that RBX significantly reduced the occurrence of sustained vision loss in patients with moderately severe to very severe nonproliferative diabetic retinopathy, showing a beneficial effect on the functional outcome. RBX reduced sustained moderate vision loss by 40%. Twice as many RBX-treated eyes gained 15 or more letters of visual acuity from baseline to endpoint. The detailed results of the study are expected to be published this year. RBX would be the first oral medication for treatment of diabetic retinopathy.

Side Effects

When considering systemic therapy, the safety profile of a compound is of substantial importance. This is particularly true when inhibiting a key signaling enzyme such as PKC, where substantial toxicity might be expected. Treatment of diabetic macular edema patients with an inhibitor of multiple kinases and PKC isoforms resulted in liver enzyme elevations, nausea, vomiting and diarrhea [24].

In contrast, RBX is selective for the -isoform of PKC and highly selective for PKC as compared with other kinases [25]. Indeed, RBX is very well tolerated without significant adverse events over 52 months of treatment. Mild side effects are rare. Only nine adverse events occurred, with an incidence exceeding 1%, that were statistically different among the groups. No serious adverse events were reported more frequently in the RBX treatment groups. The frequency of nonserious adverse event occurrence of diarrhea, flatulence, nephropathy, proteinuria and coronary artery disease was highest among patients in the 16-mg/day treatment group; there did not appear to be a RBX dose-response effect. In addition, the small number of events makes it likely that any disparity in the 16-mg group was due to chance. Patients taking the highest RBX dose of 32 mg/day did not experience these same events more frequently than placebo patients. To date, over 1,400 patients have been exposed to RBX, and no clinically significant increase in adverse effects has been identified [25].

In a small study with 29 persons, Aiello et al. [23] found a statistically significant difference among treatment groups, showing that abdominal pain was more common in the placebo group compared with RBX-treated persons.

Conclusions

New treatment modalities for diabetic retinopathy are clearly needed. The next years will demonstrate exciting new therapies for diabetic microvascular complications, the orally active RBX (Eli Lilly) for PKC inhibition possibly being one of them. RBX provides an effective blockade of hyperglycemiainduced vascular injury and is safe for administration in humans. RBX treatment reduces visual loss in patients with moderately severe to very severe nonproliferative diabetic retinopathy.

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Prof. Dr. Gabriele E. Lang Universitätsklinikum Ulm, Augenklinik Prittwitzstrasse 43

DE–89075 Ulm (Germany)

Tel. 49 731 500 27551, Fax 49 731 500 27549, E-Mail gabriele.lang@uniklinik-ulm.de

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