Some effects of aldose reductase inhibition upon the eyes of long-term streptozotocin-diabetic rats

Diabetic and galactosaemic cataracts

An increased prevalence of cataract is associated with diabetes. Biochemical studies of diabetic lenses have revealed a variety of metabolic abnormalities including changes in the levels of electrolytes, glutathione, nucleotides and sugars. Similar biochemical changes have also been observed in cataracts associated with galactosaemia, suggesting that these sugar cataracts have a common biochemical aetiology. The common biochemical factor found to initiate both types of sugar cataract is the formation of sugar alcohols (polyols) from either glucose or galactose by the enzyme aldose reductase (alditol: NADP+ 1-oxidoreductase, EC 1.1.1.21). Increased intracellular levels of these polar alcohols have a hyperosmotic effect which leads to lens fibre swelling, vacuole formation and subsequent opacification. The process of sugar cataract formation in animals can be prevented by inhibiting aldose reductase.

Pharmacological treatment strategies in age-related cataracts

Cataract is the major cause of blindness worldwide and at present the only approved treatment in many countries including the UK and USA is surgical removal of the lens. In other countries various anti-cataract drugs are available without proof of their efficacy. Research is continuing into the possible benefits of several groups of drugs and some vitamins. The first to be studied were sorbitol-lowering agents (aldose reductase inhibitors) based on the sorbitol hypothesis for diabetic cataract. Sorbitol-lowering agents have distinct effects in vitro and many of them delay the development of cataract in galactose-fed rats. A few delay cataract in diabetic rats but none have been proved effective in clinical trials, although these continue. Aspirin, paracetamol (acetaminophen) and ibuprofen delay diabetic cataract in rats, and have been shown to delay other experimental cataracts. Case-control studies from 3 continents indicate that these drugs, or at least aspirin, protect against cataract. Results of studies on all 3 drugs indicate a benefit even at low doses. Population-based studies did not identify any protection against early lens opacities but tiny opacities that do not impair vision are not a problem. Bendazac protects lens proteins in vitro and delays cataractogenesis in x-irradiated rats. In humans, it reached the clinical trial stage but most trials have been small and with subjective criteria of opacification. One objectively monitored trial suffered from a high drop-out rate. Other preparations studied less extensively include vitamins, aminoguanidine to prevent protein cross-linking in diabetes and agents designed to boost glutathione levels. It is probable that some agents which may delay or prevent cataract will be proved effective soon, and in the end there may be different drugs to delay cataract in different high risk groups. This is what might be expected of a multifactorial disease, although compounds that intervene in the final common pathways to cataract could have a broad efficacy.

Aldose reductase inhibitors and diabetic complications

Aldose reductase inhibitors impede flux of glucose through the sorbitol pathway in diabetes mellitus. They therefore reduce the accumulation of the pathway metabolites, sorbitol and fructose, reduce the impact of the flux on the cofactors used by the pathway and reduce other derived phenomena, such as osmotic stress and myo-inositol depletion. As drugs, their targets are the chronic complications of diabetes--neuropathy, retinopathy, nephropathy and vasculopathy. In experimental models there is proof of activity against biochemical, functional and structural defects in all of the involved tissues, but we await full clinical verification of this potential.

Effect of piroxicam on the blood-retina barrier in experimentally induced diabetes in rats

The effect of piroxicam on the blood-retina barrier was evaluated in rats with experimentally induced diabetes. Diabetes was induced in rats by intraperitoneal injection of streptozocin (STZ). Diabetic rats were divided into two equal groups: those treated with piroxicam, a long-acting platelet inhibitor, and an untreated control group. Vitreous fluorophotometry (VFP) was performed both before and two weeks after induction of diabetes and piroxicam intake. Streptozocin-induced diabetes caused an alteration in the blood-retinal barrier evidenced by an increase in vitreous fluorescein concentration in diabetic rats compared with normal rats. Piroxicam intake did not lead to significant change in vitreous fluorescein concentrations. However, the examination had to be terminated at two weeks because of cataract formation. The piroxicam treated group showed less incidence of lens opacity formation (59.1% compared to 81.8% in the untreated group, p = 0.0006). Piroxicam administration appears to protect the diabetic rat eye against lens opacification.

Sorbitol metabolism in retina studied in vitro

A new method is described which allows study of the retinal sorbitol metabolism in vitro. Bovine retinal tissue was incubated in tissue culture medium for 24 h and after this time showed only small morphological changes. The sorbitol content doubled at a glucose level of 5.5 mM compared with freshly prepared noncultured retina. Increasing glucose concentrations led to a gradual increase of the sorbitol content. Supplementation of the culture medium with fructose likewise enhanced the retinal sorbitol accumulation. An aldose reductase inhibitor (ICI 128436; Statil) significantly decreased the sorbitol content.

Inhibition of aldose reductase from human retina

Aldose reductase was prepared from a pool of 21 male and 16 female human retinas by ammonium sulphate fractionation (40-75% saturation) and chromatography on DEAE-Sephacel and Matrex-OA. The overall purification was 132-fold with 50% recovery of enzyme activity. The concentrations of the aldose reductase inhibitors Sorbinil, Statil and M79175 required to give 50% inhibition (IC50 value) of enzyme activity with the model substrate 4-nitrobenzaldehyde (4NB) were 3.4 microM, 2.3 microM and 0.22 microM respectively. This indicated that M79175 was the most effective inhibitor tested of aldose reductase with 4NB in vitro. These inhibitors were more effective when tested against aldose reductase activity with glucose, the substrate which might play a role in the pathogenesis of diabetic complications. Sorbinil gave an IC50 (glucose) of 0.40 microM; M79175 and Statil were more effective. At an inhibitor concentration of 0.1 microM the %-inhibitions observed were: Sorbinil 20% M79175 55%, Statil 76%. Thus Statil was the most potent compound tested against human retinal enzyme using the more physiological substrate in vitro. This report provides the first direct evidence that human retinal aldose reductase is susceptible to inhibition by compounds designed for chemotherapy of diabetic complications, and indicates that the concentrations of inhibitor required for a substantial block of activity in vitro are lower than those attained in plasma in man.

Inhibition of hexonate dehydrogenase and aldose reductase from bovine retina by sorbinil, statil, M79175 and valproate

Aldose reductase inhibitors (A.R.I.s), developed as potentially therapeutic agents for the treatment of complications of long-term diabetes, were found to be potent inhibitors of aldose reductase (ALR2) partially purified from bovine retina (IC50 values: Statil 0.89 microM, Sorbinil 2 microM, M79175 greater than 1 microM). These compounds varied, however, in their ability to inhibit hexonate dehydrogenase (ALR1), a closely related enzyme isolated from the same source (IC50 values: Statil greater than 1 microM, Sorbinil 3.9 microM, M79175 0.18 microM). Statil and Sorbinil were active against ALR2 at very low concentrations (approx. 5% inhibition at 100 pM), but did not inhibit ALR1 at less than or equal to 10 nM. In contrast, M79175 (structurally very similar to Sorbinil) and M7HEQ (a flavonoid) were preferential inhibitors of ALR1. Valproate, a compound of value in the treatment of epilepsies, was a poor inhibitor of ALR2 (18% at 1 mM). Furthermore, valproate was found to be a relatively poor inhibitor of ALR1, particularly in comparison with M79175.

The effects of a new aldose reductase inhibitor (tolrestat) in galactosemic and diabetic rats

Tolrestat(N-[[5-(trifluoromethyl)-6-methoxy-1-naphthalenyl] thioxomethyl]-N-methylglycine; AY-27,773; Alredase) is a potent, structurally novel inhibitor of aldose reductase (AR). In vitro, tolrestat inhibited in dose-dependent fashion the AR from bovine lenses (IC50, 3.5 X 10(-8) mol/L) and the formation of sorbitol in human RBC incubated with glucose (IC50, 3 X 10(-8) mol/L). Upon administration with the diet to rats made galactosemic or diabetic, tolrestat decreased, in a dose-related fashion, the accumulation of galactitol or sorbitol in the sciatic nerve and lens. The effectiveness of tolrestat depended upon the experimental conditions and tended to be higher in less severe galactosemia and after suitable pretreatment, particularly in galactosemic rats, resulting in ID50 of 5 mg/kg/d in the sciatic nerve and 12-15 mg/kg/d in the lens. Tolrestat also decreased, in dose-related manner, the RBC sorbitol levels in normal and in streptozotocin diabetic rats; in the latter, at less than 2 mg/kg/d, the RBC sorbitol was reduced to control levels.

The effects of long-term treatment of streptozotocin-diabetic rats with an aldose reductase inhibitor

To test the possible involvement of the sorbitol pathway in the pathogenesis of retinopathy in long-term experimentally-diabetic rats, streptozotocin-diabetic and normal rats were dosed orally (50 mg/kg body weight daily) for up to 373 days with an aldose reductase inhibitor (ICI 105552) or a placebo. Long-term treatment with ICI 105552 (1,(3,4-dichlorobenzyl)-3-methyl-1,2-dihydro-2-oxoquinol-4-ylaceti c acid; sodium salt) markedly reduced the normal accumulations of sorbitol and fructose in the sciatic nerves (86 and 69% reductions) and seminal vesicles with coagulating glands (75 and 49% reductions). Thus, by these criteria, the inhibitor was as effective after several months of dosing as after three weeks. There was no evidence that treatment with this aldose reductase inhibitor had any protective effect against the development of pathological changes in the retina and kidney of these rats.

The effects of an aldose reductase inhibitor upon the sorbitol pathway, fructose-1-phosphate and lactate in the retina and nerve of streptozotocin-diabetic rats

To investigate the aetiology of complications secondary to experimental diabetes in the rat, the concentrations of glucose, sorbitol, fructose, fructose-1-phosphate, lactate and inositol were measured in the retina and sciatic nerve of streptozotocin-diabetic and normal rats treated for 22 days with ICI 105552 (1-(3,4-dichlorobenzyl)-3-methyl-1,2-dihydro-2-oxoquinol-4-ylacetic acid, sodium salt; 50 mg/kg body wt daily), an inhibitor of aldose reductase (E.C. 1.1.1.21) the first enzyme of the sorbitol pathway. In the diabetic nerve, where accumulation of sorbitol may be pathogenic, treatment with ICI 105552 reduced the accumulations of sorbitol (70%), fructose (47%) and lactate (34%) without affecting glucose, fructose-1-phosphate or inositol. In the nerves of controls, the inhibitor reduced both sorbitol (23%) and fructose (20%) levels without other effects. In the diabetic retina where accumulation of sorbitol or lactate might be pathogenic, treatment with ICI 105552 had no statistically significant effect upon the concentrations of glucose, sorbitol, fructose, fructose-1-phosphate or inositol. However, the inhibitor reduced the concentration of lactate to below the non-diabetic level. In the retinas of controls, dosage with ICI 105552 reduced the concentration of sorbitol by 36% without other effects.

THE RESULTS

(1) demonstrated that ICI 105552 was a potent inhibitor of aldose reductase in sciatic nerve and suggested that a proportion of nerve lactate in diabetes could result from sorbitol pathway activity; (2) implied that flux through the sorbitol pathway, eventually to form lactate, increases in diabetic retina; and (3) indicated, by the drug's reduction of retinal lactate concentration, that inhibitors of aldose reductase might be of potential use in diabetic retinopathy.