Can Cataract Be Prevented?

Can cataract be JOHN J. HARDING prevented? Abstract The other major drawback of surgery is the risk of complications. In percentage terms the The feasibility of preventing cataract by risk of most complications is small, but given elimination of risk factors or by anti-cataract the enormous number of procedures - about 2 therapy is reviewed. Elimination of the major million per annum in both the United States and risk factors in Western countries would be India, for example - the overall number of even difficult, although it has potential in the serious complications is great. One developing world. Various approaches to drug complication, posterior capsular opacification therapy have been made but ran into (PCO), occurs in up to 50% of patients but in problems. The development of aldose 3 younger patients it affects almost 100%. In reductase inhibitors received by far the many of the countries with the greatest cataract greatest financial support but problems with toxicity were rapidly followed by the collapse problems the equipment to deal with this of the original hypothesis. The case of aspirin, complication is a rarity, a point which is ignored paracetamol and ibuprofen built up more by those advocating greater use of the latest slowly with no encouragement from the Western surgical techniques in developing pharmaceutical industry. Positive results have countries.4 In Nepal the outcome of surgery been achieved in vitro, and in the prevention with intraocular lenses was no better than that of diabetic cataract in rats, as well as the without; and 50% of the impaired vision in identification of an association with a operated eyes was attributed to surgical protective effect against cataract in human complications including PCO.s patients. A clinical trial of these compounds is Accepting that prevention of cataract is required. Other anti-cataract agents are also desirable, the next question is how to achieve being investigated. this objective. There would seem to be two major possibilities: elimination of risk factors and an anti-cataract drug. Abandoning the use Key words Aldose reductase inhibitors, Anti of phospholine iodide for the treatment of cataract therapy, Aspirin, Cataract, Ibuprofen, glaucoma has prevented tens of thousands of Paracetamol cataracts.6,7 The scope for further prevention in Western countries is limited because there is not a single predominant cause of cataract and even It has been suggested that there is no need to the major causes are somewhat intractable prevent cataract because there is a perfectly (Table 1). If diabetes accounts for about 12% of good surgical procedure to deal with it as and cataract in the West, only that proportion could when it arises. Cataract surgery has not, be eliminated by wiping out diabetes. Of course however, eliminated the problem; indeed it is likely that steps that fall short of elimination cataract remains the major cause of blindness could provide tangible benefits. Better control of and visual impairment world-wide. In India blood sugar levels would probably decrease the alone there are almost 4 million people newly amount of cataract in the following decades. blind from cataract every yearl in spite of enormous efforts from Indian ophthalmologists Table 1. Attributable risks for the risk factors for cataract and the government. A IS-year eye care identified in Oxfordshire programme in Nepal, with emphasis on cataract Factor Attributable risk (%) surgery, has not decreased the prevalence of Diabetes 12 cataract blindness significantly? Closer to home Myopia 7 there are waiting lists, with an IS-month wait Glaucoma 6 being commonplace after being listed. Most Steroids 5 Severe diarrhoea 4 patients will spend more than a tenth of their Heavy smoking 3 remaining life expectancy visually impaired on Heavy drinking 3 John J. Harding � the list; some die with their cataracts. The cost of Military work 3 Nuffield Laboratory of cataract surgery is a worry to some authorities, Spironolactone 2 Ophthalmology 2 accounting for 12% of the Medicare budget in Renal failure University of Oxford 8 Adapted from Harding and van Heyningen. Oxford OX2 6AW, UK the United States. 454 Eye (1999) 13, 454-456 © 1999 Royal College of Ophthalmologists ll Table 2. Problems with the aldose reductase (AR) hypothesis AR has never been shown to convert glucose stoichiometrically to sorbitol using only NADPH The usual assay with glyceraldehyde is invalid AR is remarkably inefficient AR is extremely sluggish AR is remarkably non-specific Different preparations have markedly different patterns of relative activities with different substrates AR activity is very low in human lens Osmotically significant levels of sorbitol have never been found in human lens Adult human lens placed in high-glucose medium does not accumulate sorbitol The X-ray structure of AR has no binding site for glucose Developing countries have different risk factors and The reason for looking at ibuprofen was because it, there is evidence that 50% of the cataract may be together with aspirin and paracetamol, had been attributable to life-threatening diarrhoea.9 In this case the associated with a protective effect against cataract in a possibility of eliminating this proportion by prevention population in Oxfordshire.22 This result came from a of diarrhoea arises. The great advantage of elimination of case-control study of 300 cataract patients and 609 risk factors is that this would not normally lead to side controls,23 and was confirmed in a second case-control effects. Indeed better control of diabetes, and elimination study in Oxfordshire24 and two others in Germany and of diarrhoea, heavy drinking and smoking could only India?5,26 At this stage a carefully controlled randomised bring additional benefits. clinical trial of these drugs against early cataract was Anti-cataract drugs are the alternative option for required but none has been carried out. Aspirin, cataract prevention and have been investigated for many paracetamol and ibuprofen are out of patent and cheap. years, but as yet none is licensed for use in the United Population studies attempting to test the hypothesiS have Kingdom or the United States. Drugs to prevent cataract mostly been add-ons to other studies and have been too are sold in many countries. Some are known to be small with subjects too young for a worthwhile cataract ineffective, while for others there is no evidence either study. A trial is necessary because case-control studies way. The first compounds to be studied systematically cannot prove a causal relationship, only demonstrate were the aldose reductase inhibitors, or ARIS.lO associations. However, they have not proved very successful and the Causal relationships can be demonstrated in animal hypothetical basis for their use has been undermined.1o,n experiments and these have established the beneficial The most serious problem with the sorbitol! osmotic effects of these three drugs. Aspirin appears to delay stress hypothesis for cataractogenesis is that the pure cataract in naphthalene- and galactose-fed rats and in enzyme, isolated or recombinant, has never been shown diabetic rats?7,28 Paracetamol and ibuprofen, as well as to convert glucose stoichiometrically to sorbitol (Table 2). aspirin, protected against diabetic cataract in rats?9 The That is, the 'enzyme' cannot perform the reaction that drugs were given in the drinking solution and protected gave it its name. It is not a reductase for aldoses. for up to 160 days. They decreased glycation of lens The next group of potential anti-cataract drugs - proteins and maintained glutathione levels. aspirin, paracetamol (acetaminophen) and ibuprofen - The potential of the aspirin-like drugs to prevent arrived on the scene by a series of fortuitous cataract is summarised in Table 3. Clinical trials would observationsY Cotlier and Sharma13 first suggested that be the logical next step. aspirin might be the cause of the decreased lens Other agents have been studied as potential anti opacification in rheumatoid arthritis patients. One cataract therapy and some have come to human studies. important mechanism for cataractogenesis is by chemical These include bendazac, pantethine and vitamins A, C modification of lens proteins,12 so it seemed at first and E. In general the results have been disappointing. unlikely that aspirin would protect because it is known However a recent report indicates that poor nutrition in to acetylate proteins. Very recently acetylation of a single early life could enhance the risk of lens opacities six or amino group of a major human lens protein was seven decades later.30 The results of a full clinical trial demonstrated.14 However, it was shown that aspirin can conducted on the vitamins are awaited with diminishing protect lens proteins against attack by other modifiers, patience. such as cyanate,15 prednisolone,16 glucose and other Prevention of cataract has been achieved in many sugars.17-19 Ibuprofen also protected lens crystallins animal models by different chemical compounds, against attack by sugars and cyanate.19-21 showing the possibilities of prevention.12 Only a few of Table 3. Anti-cataract potential of the aspirin-like drugs Aspirin and ibuprofen decrease the binding of harmful sugars, steroids and cyanate to lens proteins Aspirin and ibuprofen protect some enzymes against chemically induced inactivation Aspirin and ibuprofen protect incubated rat lenses against cyanate-induced opacification Aspirin, paracetamol and ibuprofen prevent cataract in diabetic rats Aspirin, paracetamol and ibuprofen were all associated with a protective effect against cataract in human populations in case-control studies in Oxford (UK), Germany and India 455 these molecules have been taken into clinical trials 16. Bucala R, Manabe S, Urban RC, Cerami A. Nonenzymatic modification of lens crystallins by prednisolone induces designed to test whether or not they can prevent cataract sulfhydryl oxidation and aggregate formation: in vitro and in in man. There is the opportunity to test others now the vivo studies.

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