On the prodrug potential of novel aldose reductase inhibitors with diphenylmethyleneaminooxycarboxylic acid structure

Abstract

Diphenylmethyleneaminooxycarboxylic acids were found to represent novel type inhibitors of the enzyme aldose reductase. Ester derivatives of the most active compound (3c) (IC₅₀=33 μM) were prepared as potential prodrugs and the rate of degradation was studied by treatment with buffers, plasma, and various hydrolytic enzymes. Whereas all compounds were not hydrolysed at physiological pH, incubation in the presence of enzyme led to hydrolysis. The rate of enzymatic degradation, however, depended on the nature of the ester function. Whereas the isopropyl ester (4) turned out to be the most stable compound, the ethyl ester (2c) could be cleaved in the presence of esterase and lipase, respectively. The benzylic and aromatic esters were found to be hydrolysed rapidly in the presence of lipase (benzyl ester, 7), or in plasma, by cholinesterase and esterase (phenyl ester, 6), respectively.

Introduction

Most diabetic patients suffer from so-called long-term complications such as neuropathy, nephropathy, retinopathy and cataracts. These complications arise from chronic hyperglycaemia, which causes damage to blood vessels and peripheral nerves, greatly increasing the risk of heart attack, stroke, blindness, amputation, and kidney failure (Porte and Schwartz, 1996). Several metabolic pathways have been implicated in the aetiology of secondary complications, such as excess flux through the polyol pathway, elevated nonenzymatic glycation and increased glycolytic metabolism (i.e. abnormally activated intracellular messenger systems, particularly diacylglycerol and protein kinase C) (Sarges and Oates, 1993). To date, several studies in diabetic animals and man have clearly shown a link between glucose metabolism via the polyol pathway and diabetic complications (Kador et al., 1985, Kador, 1988, Lipinski and Hutson, 1984). Inhibition of aldose reductase, which is the rate limiting enzyme of the polyol pathway, should therefore provide a pharmacological approach to the prevention and treatment of these complications.

Although numerous compounds have been selected as inhibitors of aldose reductase (for a selection of the most potent substances, see Anonymous, 1984, Anonymous, 1987a, Anonymous, 1987b, Anonymous, 1989, Anonymous, 1995, Ashizawa and Aotsuka, 1998, Brittain and Wood, 1980, Itoh et al., 1992, Mylari et al., 1991, Oku et al., 1987, Sestanj et al., 1984, Terashima et al., 1984), the most potent compounds are mostly limited to cyclic imides and carboxylic acids with the acidic proton being necessary for inhibition of the enzyme.

Recently, we have prepared a series of diazinyl substituted methanonoximethers of type A (Scheme 1) as potent inhibitors of aldose reductase (Rakowitz et al., 2000). For this novel class of aldose reductase inhibitors, the following structure–activity relationships can be established: the carboxylic acid function seems to be essential for enzyme inhibition since compounds lacking the terminal carboxylic group were found to be inactive. On the contrary, the geometry around the CN double bond did not affect the biological properties. The length of the alkyl spacer between the oxime ether and the carboxylic acid function has an effect on the activity, the hexanoic acid derivatives being the most potent (IC₅₀=10–110 μM). However, up to the present time the influence of the heterocyclic moiety is not known since only compounds bearing either the pyrimidine or pyridazine ring system have been investigated. In order to gain additional insight into the structure–activity relationships, further structural modifications of compounds of type A became the object of our interest. In the course of these studies, congeners characterised by isosteric replacement of the diazine moiety by a benzene nucleus were prepared. It should be noted that this structural modification leads to simplification with respect to synthesis and purification.

Due to the fact that aldose reductase inhibitors bearing a carboxylic acid function are generally less active in vivo than in vitro (Costantino et al., 1997), ester derivatives of the target compounds are also of interest. It is known that esterification of drugs bearing a carboxylic acid function leads to increased bioavailability (Bundgaard, 1985). In order to investigate the prodrug potential of these ester derivatives we have studied whether they could be transformed, under physiological conditions, into the corresponding carboxylic acid. It should be emphasised that the enzymatic hydrolysis has to take place primarily at the ester group and not at the oxime function.