Diabetes mellitus is a familial metabolic disorder with strong genetic and environmental etiology. Familial aggregation is more common in type 2 than in type 1 diabetes. Rema et al. [32] reported that familial clustering of diabetic retinopathy was 3 times higher in siblings of type 2 subjects with diabetic retinopathy. Presence or absence of genetic factors may play a fundamental role in determining specific pathways of vascular disease and, as a consequence, different progression patterns of diabetic retinal disease. It could be that certain polymorphisms would make the retinal circulation more susceptible to an early breakdown of the BRB (pattern B) or microthrombosis and capillary closure (pattern C). The absence of these specific genetic polymorphisms would lead to an evolving pattern of pattern A.

It is clear from this study and from previous large studies such as DCCT [33] and UKPDS [34] that hyperglycemia plays a determinant role in the progression of retinopathy. It is interesting to note that HbA1C levels are also largely genetically determined [35].

An interesting perspective of our observations, analyzed under the light of available literature, depicts diabetic retinopathy as a microvascular complication of diabetes mellitus conditioned in its progression and prognosis by a variety of different genetic polymorphisms and modulated in its evolution by HbA1C levels, partly genetically determined and partly dependent on individual diabetes management. The interplay of these multiple factors and the duration of this interplay would finally characterize different clinical pictures or phenotypes of diabetic retinopathy.

RELEVANCE FOR CLINICAL TRIAL DESIGN

It is crucial in order to design an appropriate clinical trial to test the efficacy of a drug, to identify not only the meaningful clinical endpoints but also the surrogate endpoints that may demonstrate efficacy of a drug in a realistic and feasible period of time [36].

It is clear that such process implies the validation of surrogate endpoints by the associated occurrence of hard clinical outcomes such as significant visual loss. It is here that the problem lies. Diabetic retinopathy progresses to irreversible stages of the disease with relatively little visual loss, and when macular edema or proliferative retinopathy is present, it becomes ethically mandatory to perform photocoagulation treatment.

The development of an effective drug must take into account the need to demonstrate efficacy on the earliest and reversible stages of diabetic retinal disease by demonstrating its effect on surrogate endpoints which can be followed for shorter periods of time. The assumption would be that those surrogate endpoints would ultimately be validated by association with more hard clinical outcomes.

It is therefore an urgent priority to identify endpoints which can be accepted as surrogates and be validated in longer natural history studies.

The candidates for surrogate endpoints in the initial stages of the retinal disease are not many: mean differences on the ETDRS retinopathy scale, reduction in fluorescein leakage, reduction in macular thickening, and microaneurysm turnover.

The problem of using the ETDRS retinopathy scale lies in the fact that in the initial stages of the retinopathy, even a two-step eye change means that we have to wait for an important worsening of the retinopathy and the presence of irreversible changes.

The second possibility, reduction in fluorescein leakage, evaluates one of the two main factors in the progression of the retinopathy, the alteration of the BRB permeability. It has, however, a major drawback; it involves intravenous fluorescein administration and is technically difficult.

The third candidate, reduction in macular thickening by measuring the mean change with dedicated instrumentation, has been shown to correlate poorly with visual acuity loss, and changes in retinal thickness do not necessarily correlate with progression of the retinopathy [29].

Finally, the fourth possibility, calculation of MA turnover from fundus photographs, taking into account every new MA according to their specific location in the eye fundus is noninvasive and has the potential to become an extremely informative marker of the overall progression of diabetic retinal vascular disease. By calculating MA turnover on digital fundus images, using appropriate software to identify the specific location of each MA, we may be able to measure the rate of progression of diabetic vascular retinal disease [23]. Our studies suggest that MA turnover may contribute decisively to design feasible clinical trials to test the efficacy of new drugs.

Another fundamental step in this procedure is the characterization of the different phenotypes of diabetic retinal disease.

The design of future clinical trials should consider only groups of patients characterized by their homogeneity: patients presenting a specific retinopathy phenotype (wet/ leaky or ischemic), with similar duration of diabetes and at similar levels of blood pressure and metabolic control (HbA1C values). Patients that have retinopathy characterized by low progression with low values of MA turnover, which are the majority, should not be included in relatively short-term clinical trials.

RELEVANCE FOR CLINICAL MANAGEMENT

It is accepted that in the initial stages of diabetic retinopathy when the fundus alterations detected by ophthalmoscopy or slit-lamp examination are limited to MA, hemorrhages and hard or soft exudates, i.e., mild or NPDR, an annual examination is indicated to every patient with 5 or more years of duration of their diabetes. This is the recommendation of the American Academy of Ophthalmology Guidelines for Diabetic Retinopathy [37].

Our observations and the identification of different diabetic retinopathy phenotypes in the initial stages of diabetic retinopathy, i.e., mild or moderate NPDR, characterized by different rates of progression of the retinopathy suggest that specific approaches should be used when managing these different retinopathy phenotypes.

A patient with mild or moderate NPDR, presenting retinopathy phenotype B (wet/ leaky), characterized by marked breakdown of the BRB, identified by high MA formation rates and increased values of fluorescein leakage into the vitreous, registered during a period of 1–2 years of follow-up, and indicating rapid retinopathy progression, should be watched more closely and examined at least at 6-month intervals. Furthermore, blood pressure values and metabolic control should be closely monitored at least at 3-month intervals and paying close attention to HbA1C levels. Communication channels should be rapidly established between ophthalmologist and their diabetologist, internist, or general

health care provider. Information should be given indicating that the chances of rapid retinopathy progression to more advanced stages of disease are in these patients relatively high, calling for immediate tighter control of both glycemia and blood pressure.

A patient with mild or moderate NPDR presenting retinopathy phenotype C, ischemic, characterized by high MA formation and disappearance rates and signs of capillary closure would similarly indicate the need for shorter observation intervals than 1 year with particular attention for other systemic signs of microthrombosis. Here, however, control of hyperglycemia and blood pressure must be addressed with some degree of caution. Improved metabolic and blood pressure control must be progressive and less aggressive than with phenotype B. It is realized that the ischemia that characterizes phenotype C may become even more apparent in eyes submitted to rapid changes in metabolic control, and lowering rapidly the blood pressure may increase the retinal damage associated with ischemia.

Finally, a patient with mild or moderate NPDR, presenting phenotype A, identified by low levels of fluorescein leakage, no signs of capillary closure, low MA formation rates, and with a diabetes duration of more than 10 years, all signs indicating a slowly progression subtype of diabetic retinopathy, may be followed at intervals longer than 1 year. If the examination performed at 2-year intervals confirms the initial phenotype characterization, the patient and his diabetologist, internist, or general health care provider should be informed of the good prognosis associated with this retinopathy phenotype.

TARGETED TREATMENTS

It would be of great benefit to have a drug available which would prevent the need for destructive photocoagulation of the retina. Furthermore, many diabetic patients are not well controlled, they do not come to the doctor often, and they become blind because they do not get medical attention in time for photocoagulation.

The major large clinical trials have shown that tight glycemic control slows the development and progression of diabetic retinopathy. But the constantly increasing incidence of type 2 diabetes and the evidence that retinal damage begins early on underscore the need for a medical treatment that is targeted to the initial retinal alterations.

Several key pathways have been incriminated in the process of triggering diabetic retinal disease, and they may play specific roles in the development of specific retinopathy phenotypes. Four candidates, the polyol pathway, nonenzymatic glycosylation, growth factors, and protein kinase C, may be playing leading roles in the development of diabetic retinal disease.

It is possible that all these different mechanisms of disease play complementary roles in the progression of diabetic retinal disease.

The identification of different retinopathy phenotypes characterized by different rates of progression and different dominant retinal alterations may indicate that different disease processes predominate in specific retinopathy phenotypes.

Identification of well-defined retinopathy phenotypes may be an essential step in the quest for a successful treatment of diabetic retinopathy. After the characterization of

specific retinopathy phenotypes, the predominant disease mechanisms involved may be identified, and drugs directly targeted at the correction of these disease mechanisms may be used with greater chances of success.

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