Aldose reductase ALR EC the first enzyme in the
Aldose reductase (ALR2, EC22.214.171.124), the first enzyme in the polyol pathway, is a monomeric oxidoreductase that catalyses the NADPH-dependent reduction of a wide variety of carbonyl compounds, especially glucose. In this metabolism pathway, glucose is firstly reduced to sorbitol catalyzed by ALR2 with NADPH as coenzyme, and subsequently sorbitol dehydrogenase converts sorbitol into fructose with concomitant reduction of NAD (). In normal circumstances, glucose is predominantly converted to glucose-6-phosphate by hexokinase and then enters the glycolytic pathway. However, at high glucose concentration, particularly in diabetics, the combining capacity of ALR2 to glucose is motivated and about one third of the total glucose is metabolized through the polyol pathway in tissues such as lens, retina, kidney, and peripheral nerves. As a consequence, the increased polyol pathway flux directly leads to the accumulation of sorbitol in the cells, which is hardly to penetrate through cellular membranes, eventually resulting in osmotic imbalance, cell swelling and membrane permeability changes, mainly in the lens. In addition, the dramatic reduction of NADPH and NAD gives rise to changes in cellular redox potentials, and deteriorates the activity of TC-I 2000 such as nitric oxide synthase (NOS) and glutathione reductase, further exacerbating intracellular oxidative stress. Also, several radical precursor molecules generated downstream of the polyol pathway, including the formation of advanced glycation end products (AGEs), protein kinase C (PKC) isomer, mitogen-activated protein kinase (MAPK), and poly-ADP-ribose polymerase (PARP), which contribute to free radicals and lead to oxidative stress. Excessively high levels of free radicals will cause damage to multiple tissues, and eventually diabetic complications. Accordingly, all these ALR2-mediated oxidative stress reactions along with the polyol pathway represent important pathogenesis of diabetic complications. Therefore, aldose reductase inhibitors (ARIs), which can restrain the abnormal accumulation of sorbitol and indirectly inhibit oxidative stress, have potential as therapeutic drugs. To date, numerous structurally different ARIs have been developed but only epalrestat is available on the market in Japan and more recently in China and India (). Most of those ARIs have been renounced in the clinical trials mainly for low in vivo efficacies, adverse side effects, or pharmacokinetic drawbacks. The side effects are the result of inhibiting aldehyde reductase (ALR1, EC 126.96.36.199), an enzyme that is closely related to ALR2 and plays a detoxification role in specifically metabolizing toxic aldehydes. Although at present the underlying mechanism of the low efficacy is not clear, it is speculated that only inhibiting the accumulation of sorbitol is not enough to prevent and treat pathological changes in all kind of tissues. As the pathogenesis of diabetic complications and oxidative stress always promote to each other, development of multifunctional ARIs that combining both the inhibition of aldose reductase and antioxidation into an organic whole may be a feasible therapy strategy for increasing efficacy. We have recently developed several groups of ARIs (),, and among them quinoxalinone derivatives were proved to be potent on ALR2 inhibition and selectivity. In addition, our previous 3D-QSAR studies on aromatic thiazine derivatives demonstrated that introduction of hydrophilic and less bulky substituents (hydroxyl and halogen) at the benzothiazine-1,1-dioxide core might be favorable for improving the ALR2 inhibitory activity, and this could be helpful for us in the rational design of novel quinoxalinone based ARIs. Indeed, the halogen substituents at quinoxalinone scaffold largely increased the ALR2 inhibitory activity,, but there have been no study about the hydroxyl substituent. To provide more insight into active conformations, here we introduced the phenolic hydroxyl structure not only to the C3 side chain but also to the C6 or C7 position of the core structure, resulting in a new series of quinoxalinone based ARIs to identify the effect of phenolic hydroxyl both on ALR2 inhibition and antioxidant activities.