PHARMACOLOGY OF OCULAR COMPLICATIONS

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Diabetes is a rapidly increasing public health problem and the long-term complications such as neuropathy, nephropathy, microangiopathy, retinopathy and cataract associated with diabetes result in loss of vision, decreased quality of life, loss of limbs and motor function and increased mortality. Ocular diabetic complications include cataract, keratopathy and retinopathy. Blindness due to retinopathy is the leading cause of blindness in of adults industrialized countries, and diabetics undergo cataract surgery more often than non-diabetics. Studies conducted over the last 30 years have established a clear link between the excess accumulation of intracellular sorbitol levels and the onset of diabetic complications. Sorbitol is a sugar alcohol formed from glucose by the enzyme aldose reductase (AR). We have discovered that mammalian tissues contain an intrinsic aldose reductase inhibitor (IARI). The discovery of an IARI is significant because this represents a new class of nontoxic (or significantly less toxic) inhibitor. Studies indicate that the IARI is a heat-stable compound which inhibits aldose reductase in apparently the pico- to nanomolar range. Its molecular weight appears to be less than 1,000 and evidence suggests that it is a small polypeptide. In vitro lens culture studies with partially purified IARI indicate that this compound can cross membranes to inhibit the intralenticular production of sorbitol. Preliminary rat studies also indicate that this IARI is active in vivo. When the partially purified extract containing the IARI from bovine lenses was injected intraperitoneally into 24 hr galactose-fed rats, galactitol formation was inhibited by 94 % in the three rats receiving ca. 0.3 mL injection of extract and 54% in the rat receiving ca. 0.1 mL injection. Estimating that less than 0.1% of the material represents the tight-binding inhibitor, the injected level was under 6 ?g/kg. Studies are also being conducted on the pathophysiological mechanism of how aldose reductase initiates diabetic complications. Since no sequence information on the enzymes of the polyol pathway in the dog is available for basic molecular biological studies, canine aldose reductase, aldehyde reductase and sorbitol dehydrogenase have been cloned and sequenced. In addition, to develop new methods of inhibition of aldose reductase, a number of antisense oligomers to suppress human and rat AR gene expression in cell cultures have been designed and screened. Magnetic resonance imaging studies are also being conducted to determine if aldose reductase initiates human sugar cataracts. Using the noninvasive tool of magnetization transfer contrast (MTC) enhanced magnetic resonance imaging (MRI), we have obtained high contrast images of the eyes of galactose-fed dogs that indicate that osmotic changes linked to aldose reductase occur in the lenses of these dogs during cataract formation. This technique is now being applied to a clinical setting using normal volunteers ranging from 28 to 72 years in age. Preliminary studies indicate that the high contrast images obtained by MTC-MRI are a good tool for clinically investigating cataractous changes.