Glucose and other reducing sugars react and bind covalently to protein, lipoproteins, and DNA by a process known as non-enzymatic glycation. Over time, these adducts form structures called advanced glycation end products (AGEs), a form of protein aging. These crosslinked proteins stiffen connective tissues and lead to tissue damage in the kidney, retina, vascular walls, and nerves. The formation of AGEs on long-lived connective tissue accounts for the increase in collagen cross-linking that accompanies normal aging, a process that occurs at an accelerated rate in diabetes. AGEs have also been implicated in the pathology of Alzheimer’s disease. Increasing evidence identifies the formation of AGEs as the major pathogenic link between hyperglycemia and the long-term complications of diabetes, namely nephropathy, neuropathy, and retinopathy.
In human diabetic patients, and in animal models of diabetes, these non-enzymatic reactions are accelerated and cause accumulation of glycation products on long-lived structural proteins such as collagen, fibronectin, tubulin, lens crystalline, myelin, laminin, actin, and other important biological molecules such as hemoglobin, albumin, LDL-associated lipids, and apoprotein. The structural and functional integrity of the affected molecules, which often play major roles in cellular functions, are modified by these AGE proteins, resulting in severe consequences for affected organs such as the kidneys, eyes, nerves, and micro-vascular vessels. Recent reports indicate that glycation affects metabolic enzymes, high-density lipoproteins, and IgG molecules. A number of inhibitors of glycation, AGE-formation, and AGE-protein crosslinking have been reported recently by our laboratory and others. These inhibitors may find therapeutic applications to prevent diabetic complications and to delay normal aging.
Recently, pharmacological strategies have begun for selectively cleaving and breaking the existing AGE-derived crosslinks on tissue proteins. N-phenacylthiazolium bromide (PTB) and ALT 711 have been reported to break AGE crosslinks in vitro and in vivo. In the course of our investigation on a large number of inhibitors of glycation and AGE formation, we investigated our previously reported inhibitor compounds for possible cleaving and breaking properties on AGE-protein crosslinks. Several compounds demonstrated AGE-breaking properties on AGE crosslinks that form in vitro with collagen and ?-amyloid peptides, and in vivo with collagen and IgG on the surface of red blood cells of STZ-induced diabetic rats. These compounds include LR-20: L-bis-[4-(4-chlorobenzamideophenoxyisobutyryl) cystine, LR-23: 4-(3,5-dichlorophenylureido)-phenoxyisobutyryl-l-amidocyclohexane-1-carboxylic acid, LR-90: Methylene bis [4,4’-(2-chlorophenyureidophenoxyisobutyric acid)], LR-102: 4-[4-isobutyrylphenylamino)methyl] phenylmethylaminophenoxyisobutyric acid, SMR-5* and SMR-12*. These “AGE breakers” may find therapeutic use in the treatment of diseases associated with AGE formation and accumulation, such as diabetes, Alzheimer’s rheumatoid arthritis, and atherosclerosis. To combat these diseases, both strategies – inhibition of AGE formation on tissues and organs, and cleaving and reversing exciting AGE damage on tissues by AGE-breakers – are promising.
We have recently investigated the efficacy of using one of the novel AGE-inhibitors, LR-90 in the treatment of experimental diabetes to determine its potential in preventing the development of renal disease in a diabetic animal model. The results showed significant improvement in renal function in terms of development of proteinuria and reduction in creatinine excretion. Histochemical observations indicate the LR-90 reduced the incidence of glomerulosclerosis, cortical tubule degeneration and collagen deposition in the kidneys compared to untreated diabetic rats. LR-90 treatment also effectively inhibited the increase of serum AGE and the in situ accumulation of immunoreactive AGE in collagen tissues and kidneys of diabetic rats.
* SMR-5 is 5-aminosalicyclic acid (5-ASA); SMR-12 is metformin (dimethylbiguanide)