Medicinal attributes of nitrogen heterocycles directing aldose reductase selectivity and potency

Abstract

Diabetic complications arise primarily from hyperglycemia-induced metabolic disturbances, among which the polyol pathways play a significant role through the overactivation of aldose reductase. ARIs have therefore emerged as a potent therapeutic approach to prevent or delay diabetes-associated microvascular oxidative damage. In recent years, nitrogen-based heterocyclic compounds have gained prominence as potent ARIs due to their structural variability and pharmacodynamic profiles. This review comprehensively analyzes the structural features and target interactions of various nitrogen-containing scaffolds, including quinoxalines, pyrazolines, imidazoles, oxadiazoles, pyrazoles, triazoles, pyrimidines, thiazoles, and purines, as ARIs. SAR analysis reveals that acidic or bioisosteric head groups and electrostatic interactions with key ALR2 residues, such as Tyr48, His110, and Trp111, thereby increase potency and selectivity over related aldo–keto reductases. Similarly, physicochemical parameters, including lipophilicity, pK_a, and H-bond donor and acceptor properties, influence tissue penetration and pharmacokinetic behavior. Additionally, molecular docking and binding analyses reveal common interaction patterns in the anion-binding and specificity pockets of ALR2, providing mechanistic insight into heterocycle-mediated inhibition. Overall, this review identifies design trends and limitations in nitrogen heterocycle-based ARIs, offering valuable guidance for structure-guided drug design of selective, potent, and clinically therapeutic agents for managing diabetic complications.

This review describes the medicinal attributes of nitrogen heterocycles, including SAR and target-interaction studies that guide their selectivity and potency for aldose reductase.

1. Introduction

Heterocyclic compounds are cyclic organic molecules in which oxygen, nitrogen, or sulfur replaces a carbon atom in the ring. The presence of heteroatoms alters molecular electronics, aromaticity, and polarity, thereby modulating hydrogen-bond donor/acceptor capacity and dipole moment.^1–3 Nitrogen atoms introduce basicity and lone-pair interactions that facilitate selective recognition by biological macromolecules. Oxygen and sulfur atoms further contribute to electrostatic and hydrophobic modulation; these electronic and stereoelectronic features critically influence ligand–enzyme interactions, metabolic stability, and overall drug likeness.⁴ The incorporation of these heteroatoms, along with variations in ring size, significantly influences the compound's physical and chemical properties.^5,6 Heterocyclic compounds represent the most significant classes of organic compounds, widely used in various biological fields due to their effectiveness in treating multiple diseases.^7,8 Heterocycle frameworks are crucial in medicinal chemistry and are commonly found in numerous biomolecules, including enzymes, vitamins, and natural products.^9,10

Many heterocyclic compounds possess important therapeutic relevance in the management of common diseases.¹¹ For example, triazine derivatives have been extensively investigated for their antimicrobial,¹² herbicidal, urinary antiseptic,¹³ and anti-inflammatory¹⁴ activities. Similarly, benzimidazole derivatives have been reported to exhibit a wide range of biological activities, including antibacterial,¹⁵ antifungal, antiviral,¹⁶ and anthelmintic¹⁷ properties. Nitrogen-containing heterocyclic compounds are abundant and form the basis of various substances, including alkaloids, vitamins, hormones, dyes, antibiotics, herbicides, and various pharmaceuticals.¹⁸ Nitrogen-containing heterocyclic compounds also play a crucial role as ARIs in medicinal chemistry, particularly in the management of diabetic complications, such as neuropathy,¹⁹ retinopathy,²⁰ and nephropathy.²¹ ALR2 and ALR1 share similar catalytic cores and NADPH-binding sites; there is a small difference in the substrate-binding regions of ALR2, which contains a distinct, specific pocket formed by residues such as Leu300, Trp219, and Phe122. Potent, selective ARIs typically use this pocket via suitably oriented aromatic or heterocyclic moieties. Nitrogen-containing heterocycles, including pyridines, quinolines, indoles, oxazoles, thiazoles, and imidazoles, enable strong H-bonding and electrostatic interactions with Tyr48 and His110, together with π–π stacking against Trp111, thereby stabilizing ALR2-selective binding conformation.²² Oxygen atoms often contribute to interaction with the anion binding site, whereas sulfur atoms modulate hydrophobic contacts and lipophilicity. These compounds showed improved bioavailability, metabolic stability, and selective binding affinity, making them promising compounds for drug development as aldose reductase inhibitors to manage diabetic complications.^22–24

1.1. Aldose reductase in diabetic complications

Aldose reductase (AR), also known as AKR1B1, is a key enzyme within the aldo–keto reductase (AKR) superfamily.^25,26 This monomeric, NADPH-dependent cytosolic enzyme was first identified in the eye's lens and has been found to play a crucial role in various biological tissues.^27–29 The aldo–keto-superfamily comprises a broad range of NADPH-dependent oxidoreductases that catalyze the reduction of aldehydes and ketones to their corresponding alcohols. Enzymes of the AKR superfamily share a conserved (α/β)₈-barrel structural fold and use NADPH as a hydride donor to catalyze redox reactions involving endogenous and xenobiotic carbonyl substrates.³⁰ More than 15 aldo–keto-reductase superfamilies have been identified, involved in the metabolism of sugars, steroids, xenobiotics, and prostaglandin biosynthesis.^31,32 Under physiological conditions, AR contributes to detoxifying lipid peroxidation products, including 4-hydroxynonenal and glutathione conjugates.^33–35 Inhibition of AR (ALR2) presents a promising strategy for preventing or managing diabetes-related complications.^36,37

The first and rate-limiting enzyme of the polyol pathway is aldose reductase (AR), which catalyzes the NADPH-dependent reduction of glucose to sorbitol, leading to oxidative stress and cellular damage in hyperglycemic states.^38,39 Under normal blood glucose conditions, this pathway plays a minor physiological role in redox homeostasis and detoxification. Meanwhile, under hyperglycemic conditions, increased glucose flux through aldose reductase leads to intracellular sorbitol accumulation due to sorbitol's poor membrane permeability, resulting in osmotic imbalance, cellular swelling, and tissue dysfunction.^38,40,41 The subsequent reactions are catalyzed by sorbitol dehydrogenase (SD), which oxidizes sorbitol to fructose using NAD⁺ as the cofactor. This further enhances the NADH/NAD⁺ imbalance and increases glucose flux.^42–45 When glucose flux increases, it forms advanced glycation end products (AGEs),⁵¹ which subsequently promote the production of reactive oxygen species (ROS) and initiate a cascade of oxidative and inflammatory signaling.^43,46 This indicates a complex interaction between AR-mediated and non-AR-mediated oxidative stress pathways, complicating the evaluation of their roles in oxidative damage (Fig. 1).^47,48 This effect is particularly observed in insulin-independent tissues such as peripheral nerves, retina, and kidney, where excessive sorbitol accumulation leads to microvascular damage and diabetic complications.^43,49–51 These mechanistic insights provide a strong rationale for targeting aldose reductase with heterocyclic inhibitors that suppress AR-mediated oxidative stress under hyperglycemic conditions.

Fig. 1. The polyol pathway contributes to the development and progression of diabetic complications.

The clinical development of ARIs has been limited by several challenges. Earlier ARIs often showed poor penetration into target tissues, such as peripheral nerves, the kidney, and the retina, thereby reducing their effectiveness against diabetic microvascular complications.⁵² Additionally, structural homology within the aldo–keto reductase superfamilies often led to off-target inhibition of ALR1. In addition, physicochemical properties play an important role in AR selectivity and in vivo efficacy. Moderate lipophilicity (log P value 2–4) is favourable for tissue penetration, and balanced pK_a values enable optimal interactions within the anion-binding site without increasing ionization. Controlled hydrogen-bond donors and acceptors are essential to maintain binding affinity while avoiding poor membrane permeability. These parameters underscore the importance of designing selective, tissue-penetrant nitrogen-heterocycle-based inhibitors with optimized pharmacokinetic properties.^40,53

Elevated blood glucose levels promote oxidative stress through the polyol pathway via three potential mechanisms.^54–56 Firstly, under hyperglycaemic conditions, approximately 30% of intracellular glucose is directed into the aldose reductase (AR)-dependent polyol pathway, leading to depletion of nicotinamide adenine dinucleotide phosphate (NADPH) and a consequent reduction in glutathione (GSH) levels. The increased utilization of NADPH by aldose reductase reduces its availability for glutathione regeneration via glutathione reductase.^57–59 This results in an imbalance between oxidant production and antioxidant defence, which promotes excessive generation of reactive oxygen species (ROS).^39,60,61

Secondly, oxidative stress arises during the subsequent oxidation of sorbitol to fructose by sorbitol dehydrogenase (SDH), where NAD⁺ is reduced to NADH.^62,63 NADH, in turn, serves as a substrate for NADH oxidase, promoting the generation of superoxide anions.^64–66 Thirdly, the metabolic conversion of glucose to fructose via the polyol pathway generates fructose-3-phosphate and 3-deoxyglucosone, highly reactive intermediates that drive non-enzymatic glycation.^67–69 Consequently, increased glucose flux through the polyol pathway promotes the formation of advanced glycation end products (AGEs). The accumulation of AGEs activates oxidative and inflammatory signalling pathways through interactions with AGE receptors, thereby further enhancing reactive oxygen species (ROS) production.^70–75 The degree of oxidative stress resulting from these mechanisms is tissue and cell-specific, indicating complex interactions between these processes in different biological contexts.^76–78

2. Development of nitrogen-containing heterocycles as ARIs

Nitrogen-containing heterocycles are fundamental scaffolds in medicinal chemistry and represent the backbone of bioactive molecules. The nitrogen atoms make the compound more flexible and reactive, and help form strong bonds, acting like natural molecules in the body.^79,80 Among nitrogenous scaffolds, quinoxalines (1,4-diazanaphthalenes) have been extensively explored for their antitumor, antimicrobial, and CNS activities. One approved drug of this class, brimonidine, is an α₂-adrenergic receptor agonist for glaucoma management.^81,82 Pyrazolines and pyrazoles constitute privileged scaffolds in the design of anti-inflammatory and kinase inhibitors; the COX-2 inhibitor celecoxib demonstrates a clinically validated example of this structural class.^83,84 Imidazole-based frameworks displayed antibacterial, anticancer, anti-inflammatory, antiviral, antihypertensive, and anticonvulsant activities. The imidazole nucleus is also integral to antifungal pharmacophores such as ketoconazole, clotrimazole, and miconazole.^85,86 Oxadiazole, mainly the 1,2,4-isomer, is a metabolically stable bioester and a key structural moiety in the marketed HIV integrase inhibitor raltegravir. These structural scaffolds appear across antibacterial, antimicrobial, and anticancer activities.^87,88 Triazole scaffolds are metabolically stable and have transformed antifungal therapy with drugs such as fluconazole and itraconazole, as well as serving as kinase inhibitors and antiviral leads.^89,90 Pyrimidine and purine derivatives are leads for numerous antiviral and anticancer therapeutics that mimic nucleobases; 5-fluorouracil and mercaptopurine are representative examples.^91–93 In thiazole scaffolds, the sulfur atom modulates lipophilicity and electronic character, as seen in both natural molecules, such as thiamine (vitamin B1), and in a multikinase inhibitor, dasatinib. Additionally, this structural scaffold exhibits broad pharmacological profiles, including anticancer, antimicrobial, anti-inflammatory, antidiabetic, and antitubercular activities.^94–97 Recent reviews and designed papers have reported that nitrogen-containing scaffolds, such as benzothiazole, oxadiazole, triazole, pyrazole derivatives, and natural flavonoids, act as ARIs. Additionally, different combination strategies (ARI + antioxidant + glucose-lowering therapy) have been reported to increase protection against diabetic complications.^98–100

2.1. Quinoxaline derivatives

A class of quinoxaline-2-(1H)-one derivatives, multifunctional ARIs with both enzyme-inhibitory and antioxidant activities, was identified by Hao and coworkers. They synthesised several derivatives incorporating a 1-hydroxypyrazole head group as a bioisosteric replacement for the carboxylic acid and introducing different aromatic substituents at the C-3 position of the quinoxaline structure. They evaluated enzyme-based in vitro assay against the aldose reductase (ALR2) and (ALR1) enzymes. Structure analysis (Fig. 2) demonstrated that the presence of hydroxy at the para position, fluoro at the C-7 of the quinoxalinone scaffold, and ethene at X leads to the formation of the most potent molecule 1a, with an IC₅₀ value of 0.107 μM. No substitution at R₁, dihydroxy at para and meta positions, and ethene at position X exhibited the excellent activity of compound 1b with an IC₅₀ value of 0.148 μM. Additionally, no substitution at the phenyl ring, and the quinoxalinone scaffold exhibited a less active compound, 1c, with an IC₅₀ value of 6.329 μM. Although compound 1a exhibited the highest in vitro potency, target interaction analysis highlighted compound 1b as showing superior binding interactions within the active site of human aldose reductase (ALR2) co-crystallized with NADP+ and minalrestat (PDB ID: 1PWL). Compound 1b was observed to bind strongly within the ALR2 active site, which includes the anion-binding site, hydrophobic pocket, and specificity pocket. The 1-hydroxy pyrazole ring formed three hydrogen bonds with Tyr48 and one with His110, while the C2-carbonyl group established an additional hydrogen bond with Trp111. The C3-styryl ring's 3-hydroxyl group interacted with Leu300 in the specificity pocket, contributing to binding stability. Furthermore, the C3-styryl ring aligned with the indole ring of Trp111, forming π–π stacking interactions that optimized the inhibitor's positioning. The quinoxalinone core was accommodated within the hydrophobic pocket, further stabilizing the inhibitor's binding within the enzyme's active site. Compounds 1a and 1b exhibited significant aldose reductase inhibitory activity compared to other synthesized compounds, although their potency was still lower than that of the reference inhibitor, epalrestat (IC₅₀ = 0.083 μM).¹⁰¹

Fig. 2. Structure analysis of quinoxalinone derivatives.

A series of sixteen methylbenzopyrazine derivatives were synthesized and evaluated for their inhibitory potential against ALR2 and ALR1 enzymes by Khan and co-researchers. The ALR2 enzyme was isolated and purified from calf lenses, and the ALR1 enzyme was isolated from calf kidneys. Structure analysis revealed that a hydroxy group at the para position led to compound 2a (Fig. 3), the most potent derivative in the series, with an IC₅₀ of 1.34 μM. Insertion of bromine at the meta position formed compound 2b, which exhibited a moderate IC₅₀ value of 3.48 μM. In contrast, introducing a methyl group at the meta position significantly decreased activity, as observed with compound 2c, which had an IC₅₀ value of 66.11 μM. Compounds 2a and 2b showed higher activity than the standard inhibitor sorbinil (IC₅₀ = 3.14 μM).¹⁰²

Fig. 3. Benzopyrazine derivatives as ARIs.

Additionally, a series of quinoxalinone scaffold-based acyl sulfonamide derivatives as multifunctional ARIs, with both enzyme-inhibitory and antioxidant activities, were reported by Ji and colleagues (2019). These derivatives were tested for their inhibitory activity against ALR2 and ALR1 using epalrestat as the standard. Structure analysis indicated that compound 3a, featuring m, p-dihydroxy substitution and the presence of ethene at X, displayed the maximum inhibition with an IC₅₀ value of 0.100 μM. Compound 3b, incorporating a hydroxy group at the para position, also demonstrated moderate activity, with an IC₅₀ value of 0.627 μM. Compound 3c displayed the least potent activity due to the lack of substitution, with an IC₅₀ value of 75.19 μM compared to the reference drug epalrestat with an IC₅₀ value of 0.031 μM against ALR2 (Fig. 4). The target interaction studies revealed that compound 3a exhibited strong binding affinity to the active site of ALR2 (PDB ID: 1PWL). It formed hydrogen bonds with key residues Tyr48, His110, and Cys298, as well as a stable electrostatic interaction with the NADP⁺ cofactor. The C2-carbonyl oxygen of 3a forms additional hydrogen bonds with Trp111. The 3,4-dihydroxystyryl moiety at the C3 side chain engaged in π–π stacking interactions with Trp111 and fitted into a defined pocket formed by various residues. The 3-hydroxy and 4-hydroxy groups of the C3 side chain also formed hydrogen bonds with Tyr309 and Thr113, respectively. Furthermore, the quinoxalinone scaffold interacted with a hydrophobic pocket formed by several residues.¹⁰³

Fig. 4. Quinazolinone-based rhodanine-3-acetic acid derivatives as ARIs.¹⁰³.

Moreover, El Sayed and colleagues (2017) reported a series of quinazolinone-based rhodanine-3-acetic acid derivatives with aldose reductase (ALR2) inhibitory and antioxidant activities. They developed 23 derivatives and tested them for inhibition of ALR2, an enzyme closely associated with diabetic complications such as retinopathy, neuropathy, and nephropathy, isolated from rat lenses. Structure analysis revealed that the substitution of the methoxy and no substitution on the quinazolinone scaffold demonstrated enhanced activity of compound 4a with an IC₅₀ value of 0.0497 μM. An electron-withdrawing fluorine group at the para position on the phenyl ring and no substitution at the quinazolinone ring formed good potent compounds 4b and 4c with IC₅₀ values of 0.0556 and 0.0609 μM, respectively. Compounds 4a, 4b, and 4c exhibited nearly threefold greater inhibitory potency than the reference agent, epalrestat (IC₅₀ = 0.17 μM). Substitution of the bromine at the para position showed the 10-fold decreased activity of compound 4d with an IC₅₀ value of 0.711 μM. Compound 4e, containing m-methyl on the phenyl ring, showed moderate activity with an IC₅₀ value of 0.574 μM (Fig. 5). The molecular docking analysis of rhodanine-3-acetic acid derivatives, particularly compound 4a, revealed its favourable binding affinity to the ALR2 enzyme, with a strong binding energy of −11.05 kcal mol⁻¹. Compound 4a exhibited efficient interaction within the enzyme's polar anion-binding pocket, stabilizing hydrogen bonds with Tyr48 and Phe122 and electrostatic interactions with the NADP⁺ cofactor (PDB ID: 1US0). The rhodanine core effectively aligns the quinazolinone fragments into the specificity pocket, facilitating hydrophobic interactions with Leu300. These findings indicate that compound 4a exhibits superior binding stability compared to other derivatives.¹⁰⁴

Fig. 5. Quinazolinone-based rhodanine-3-acetic acid derivatives as ARIs.¹⁰⁴.

Furthermore, a series of ARIs were designed and synthesized by Hao and colleagues (2017) by incorporating phenolic hydroxyl groups at the C3 side chain and at the C6 or C7 position of the quinoxalinone core. These structural modifications resulted in a new class of ARI agents with notable antioxidant activity. SAR analysis demonstrated that the para-hydroxyl group substitution and the presence of a 7-methoxy group, combined with an ethene moiety at position X, showed excellent activity of compound 5a with an IC₅₀ value of 0.059 μM (Fig. 6). Compound 5b having 4-hydroxyl group substitution at R₂, 6-methoxy group at R₁, and the presence of an ethene moiety in place of X, showed good potency with an IC₅₀ value of 0.098 μM. Compounds 5c and 5d, characterized by a 7- methoxy group and hydroxyl substitution at the ortho and para position, exhibited moderate inhibitory activity with an IC₅₀ value of 0.116 μM and 0.179 μM, respectively. In contrast, substitution with a 6-methoxy group yielded compound 5e, which had the lowest potency (IC₅₀ = 6.825 μM). Among the synthesized compounds, 5a exhibited the highest aldose reductase inhibitory activity, surpassing that of the reference drug epalrestat (0.086 μM). In comparison, the other derivatives showed lower inhibition.¹⁰⁵

Fig. 6. Structure analysis of phenolic hydroxyl derivatives clubbed with quinoxalinone.

In addition, a set of quinoxaline-2(1H)-one-based compounds was synthesized, focusing on altering the C3 side chain and substituting the phenyl ring of the quinoxaline core to optimize and enhance the activity of an ALR2 inhibitor, as reported by Qin and coworkers (2014). Structure analysis revealed that compounds 6a and 6b were identified as the most active bearing substituents of 7-fluoro at the R₁ position, di-hydroxy at the ortho and para positions, and chlorine at the para position, leading to excellent inhibition of ALR2 with an IC₅₀ value of 0.032 μM and 0.056 μM compared to the reference drug epalrestat (IC₅₀ = 0.084 μM) (Fig. 7). No substitution at R₁ and substitution of a methoxy group at the para position and the presence of an ethene chain at the X position led to compounds 6c and 6d exhibiting the least potency with an IC₅₀ value of 4.181 μM and 5.981 μM, respectively. An in silico study of the most active compound 6a showed strong binding within the active site of ALR2 (PDB ID: 1Z3N), forming H-bonds with Tyr48 and His110 and π–π stable stacking interactions with the indole ring of Trp111. The quinoxaline core aligned effectively within the hydrophobic pocket formed by Leu 300, Trp219, Phe122, Trp20, and Trp79, contributing to the overall binding stability and affinity of the inhibitor.¹⁰⁶

Fig. 7. Structure analysis of quinoxaline-2(1H)-one based derivatives.

Additionally, to explore the influence of structural modification on enzyme selectivity and potency, SAR studies have been discussed for the nitroquinoxalinone moiety. It revealed that no substitution at R₁ and R₂ yields the most potent inhibitor, 7a, with an IC₅₀ value of 1.54 μM (Fig. 8). Substitution of the nitro and dimethyl groups at the meta position on R₂ decreased activity in derivative 7b, with an IC₅₀ value of 7.1 μM. In contrast, substituting the fluoro group at the para position of the nitroquinoxalinone moiety yielded the less active compound 7c, with an IC₅₀ of 11.81 μM. The presence of fluorine at the para position decreases the compounds' activity. Substitution of the bromo group at the ortho position and 8-nitro at the R₁ position showed moderate activity of compound 7d with an IC₅₀ value of 5.8 μM. Introducing the 7-bromo and di-nitro groups at the meta or ortho positions resulted in inferior activity for compound 7e, with an IC₅₀ value of 18.17 μM. All compounds were less active than the standard compound, epalrestat (IC₅₀ = 0.12 μM). Docking studies revealed that compound 7a exhibited the most favorable interactions at the ALR2 target site (PDB ID: 1Z3N). The carboxylate group exhibited three H-bonds, two with the hydroxyl groups of Tyr48 and another one with the N2 atom of His110. In contrast, the C2 carbonyl oxygen formed a hydrogen bond with N1 of Trp111, and the C3-phenethyl ring formed a stable π–π stacking interaction with the quinoxaline ring of Trp111, which together enabled the best fit and enhanced stabilization within the binding site. The presence of multiple nitro groups leads to potential toxicity, metabolic instability, and off-target effects. Such electron-deficient moieties, although beneficial for enzyme binding, may limit their potential due to safety concerns.¹⁰⁷

Fig. 8. Structural analysis of nitro-quinoxalinone derivatives as ARIs.

Furthermore, Wu and coworkers (2013) designed and synthesized quinoxaline derivatives as ARIs. Structure analysis indicated that substituting a 7-fluoro group on the quinoxaline core and the presence of bromine at the para position and fluorine at the ortho position of an aromatic ring resulted in compound 8a exhibiting high activity with an IC₅₀ value of 0.0032 μM (Fig. 9). Similarly, insertion of a 7-fluoro group at the quinoxaline ring and bromine at the para position led to the formation of compound 8b, which also demonstrated good activity with an IC₅₀ value of 0.0279 μM. However, introducing a 7,4-fluorobenzyl group on the quinoxaline ring and a para-fluorine substituent significantly reduced potency, as observed with compound 8c, which had an IC₅₀ value of 1.427 μM. Target interaction studies revealed that compound 8a interacts with the aldose reductase active site by forming hydrogen bonds with the side chains of Trp48 and His110, as well as electrostatic interactions with the NADP⁺ moiety (PDB ID: 1IEI). Additionally, the oxygen atom of the C2-carbonyl group in the quinoxaline core was predicted to establish a hydrogen bond with the side chain of Trp111, further stabilizing the binding. Most synthesized compounds exhibit greater activity than the reference drug epalrestat (IC₅₀ = 0.0856 μM).¹⁰⁸

Fig. 9. Structure analysis of quinoxaline derivatives.

Similarly, Yang and colleagues (2012) reported an efficient synthesis of the same quinoxalinone derivatives as potential ARIs. Structure analysis showed that the presence of an amine group reduces the potency of the compounds, while an oxygen atom at position X enhances the activity (Fig. 10). Substituting 7-fluorine on the quinoxalinone scaffold and introducing a bromine group at the para position of the phenyl ring resulted in the most active compound 9a with an IC₅₀ value of 0.0114 μM. Similarly, incorporating a bromine atom at the ortho position of the quinoxalinone ring and a bromine atom at the para position of the phenyl ring resulted in compound 9b, which exhibited good activity with an IC₅₀ value of 0.0163 μM. No substitution in R₁, and substitution of the difluoro at the ortho and para positions in R₂ showed moderate activity, as observed in compound 9c, exhibiting an IC₅₀ value of 1.203 μM. The order of activity enhancement at the C6 position was determined to be Br > Cl > F. All these derivatives are compared with the reference drug epalrestat, with an IC₅₀ value of 14.32 μM. The docking analysis of compound 9a (PDB ID: 1Z3N) reveals that the carboxylate group fits deeply within the anion binding site, forming hydrogen bond interactions with the side chains of key residues, such as Tyr48, His110, and Trp11. Additionally, it interacts electrostatically with the positively charged C4N atom of the nicotinamide moiety in the NADP⁺ cofactor. The carbonyl oxygen atom forms a hydrogen bond with the side chain of Cys298, indicating the crucial role of the carbonyl group in the strong interaction between the inhibitor and ALR2.¹⁰⁹

Fig. 10. Quinoxalinone derivatives as ARIs.

2.2. Pyrazoline derivatives

A series of 1,3-diaryl-5-(4-fluorophenyl)-2-prazoline derivatives were designed by Sever and research associates (2021) as potent ARIs using a microwave-assisted technique. All 20 synthesized derivatives were subsequently evaluated for their AR inhibitory potential and cytotoxicity against L929 mouse fibroblast cells. A structure–activity relationship study revealed that introducing a methyl and oxygen at the X position and the presence of the bromine atom at the meta position in the phenyl ring significantly enhanced the potency of compounds 10a and 10b, having IC₅₀ values of 0.160 μM and 0.152 μM, respectively (Fig. 11). Substituting a methyl group at the X position and a methyl substitution at the para position of the phenyl ring reduced the activity of compound 10c, resulting in an IC₅₀ value of 0.503 μM. On the other hand, compound 10b showed the highest potency based on the IC₅₀ value, and compound 10a showed a strong binding affinity with a K_i value of 0.019 μM. Both compounds showed significantly greater inhibitory activity than the reference drugs epalrestat (IC₅₀ = 0.278 μM, K_i = 0.801 μM) and quercetin (IC₅₀ = 4.120 μM, K_i = 6.082 μM). Target interaction analysis revealed that compound 10a (PDB ID: 6KIY) forms π–π stacking interactions with the Trp21 residue. Similarly, compound 10b exhibited π–π stacking and π–cation interactions with Arg203 through the phenyl ring attached to the morpholine moiety. These interactions are likely responsible for their enhanced inhibitory effects against AR.¹¹⁰

Fig. 11. Structure analysis of pyrazoline derivatives.

A series of pyrazolyl-thiazole derivatives were synthesized and evaluated for inhibitory activity against AR and α-glucosidase by Demir and team members (2020), representing promising scaffolds for the treatment of diabetic complications. Structure analysis revealed that compound 11a, with a 4-bromophenyl substituent, was the most potent in the series, exhibiting an IC₅₀ value of 8.52 μM. Similarly, in compound 11b, substitution of the 3-chlorobenzene group showed moderate activity against the AR enzymes, with an IC₅₀ of 10.76 μM (Fig. 12). Both compounds exhibited greater potency than the standard compound quercetin (IC₅₀ = 11.44 μM). Compound 11c showed the lowest inhibitory effect, containing the thiophene group, with an IC₅₀ value of 23.73 μM. A molecular docking study revealed that compound 11a (PDB ID: 3 V36) interacted strongly through hydrogen bonding with an oxygen atom in the pyrrolidine-2,5-dione moiety, also π–π stacking and halogen–π interactions within the NADPH binding cavity, engaging with the key residues Lys21 and Lys262.¹¹¹

Fig. 12. Structure analysis of pyrazolyl-thiazole derivatives.

Notably, a series of pyrazoline-substituted benzene sulfonylurea and thiourea derivatives were synthesized by Ovais and colleagues (2014), and their antihyperglycemic and ARI activities were evaluated. Structure–activity relationship analysis revealed that incorporating the chlorophenyl and phenyl at the para position of the pyrazoline moiety, along with butyl and hexyl chains, significantly enhanced the activity of compounds 12a and 12b, with an IC₅₀ value of 0.0175 and 0.0178 μM, respectively (Fig. 13). Substituting a phenyl group at X, a butyl chain at R resulted in good activity for compound 12c with an IC₅₀ value of 0.0430 μM. In contrast, the presence of the benzyl group at the R position led to compound 12d, which showed reduced inhibitory activity (IC₅₀ = 0.0901 μM) compared to the most potent compound. The target interaction (PDB ID: 2FZD) analysis revealed that compounds 12a and 12b were located within the anion-binding site formed by Trp48, His110, and Trp111 and formed hydrogen bonds with Trp111. The pyrazoline scaffold exhibited strong binding interactions with the hydrophobic pocket formed by Phe122, Leu300, Leu301, Trp219, and Trp295 residues. Furthermore, the m-phenyl core of the pyrazoline moiety formed a π–π interaction with Trp295. All synthesized compounds exhibited excellent inhibitory activity compared with the reference compound, sorbinil (IC₅₀ = 8 μM).¹¹²

Fig. 13. Structure analysis of pyrazoline-substituted benzene sulfonylurea and thiourea derivatives.

2.3. Imidazole derivatives

Moreover, Morikawa and coworkers (2019) synthesized 2-aroyl-4-aryl-1H-imidazole derivatives and evaluated their in vitro AR inhibitory activity. They aimed to replace the conventional glycine and spirohydantoin pharmacophores with phenolic hydroxyl or catechol moieties to improve AR inhibition while avoiding pharmacokinetic drawbacks. Structure–activity relationship analysis showed that substituting (meta, para-dihydroxyphenyl)-λ³-methanone at the R₃ position increases the activity. Compound 13a, substituted with a catechol group at R₂ and 4λ³-benzene-1,2-diol at R₁, showed an improved AR inhibitory activity with an IC₅₀ value of 0.44 μM. Compound 13a is more effective than the reference compound sorbinil (IC₅₀ = 1.3 μM) (Fig. 14).

Fig. 14. Structure analysis of 2-aroyl-4-aryl-1H-imidazoles.

The introduction of quinolin-2-yl and quinolin-3-yl at R₁ led to compounds 13b and 13c, which exhibited moderate activity with IC₅₀ values of 1.92 μM and 1.66 μM, respectively. The absence of a substituent at R₃, the presence of a catechol group at R₂, and 4λ3-benzene-1,2-diol at R₁ resulted in compound 13d, which exhibited inferior AR inhibitory activity with an IC₅₀ value of 68.3 μM.¹¹³

A series of N-(imidazole)-2H-chromene-3-carboxamide analogs were synthesized by Gopinath and colleagues (2016), and their biological activities and protein–ligand interactions were evaluated. Structure analysis revealed that compound 14a (Fig. 15), which incorporates a methoxy group on the chromene ring, a para-chloro substituent on the imidazole ring, and a pyrrolidine linker between the chromene and imidazole moieties, exhibited the highest potency against aldose reductase (ALR2) inhibition, with an IC₅₀ value of 0.031 μM. Compound 14b, featuring a chlorine substituent on the phenyl group of the imidazole and a methoxy group on the chromene nucleus, demonstrated slightly lower activity (IC₅₀ = 0.065 μM) compared to 14a. The reduced potency was attributed to the presence of a carboxamide spacer between the chromene and imidazole moieties. Further modification, such as replacing the chlorine atom with bromine or fluorine, or removing the methoxy group from the chromene core, resulted in diminished activity, likely due to weaker interactions with the ALR2 active site. Notably, compounds lacking substituents on the phenyl ring of the imidazole fragment and the chromene nucleus (compound 14c) displayed significantly lower activity, with an IC₅₀ value of 2.122 μM. Additionally, introducing a 2λ³-propane moiety at R₄ (compound 14d) further reduced potency, yielding an IC₅₀ value of 4.29 μM. All these derivatives were compared with the reference drug sorbinil, which has an IC₅₀ of 0.43 μM.

Fig. 15. Structure analysis of chiral 2H-chromene-N-imidazolo-amino acid.

Target interaction analysis identified compound 14a as the optimal binder within the ALR2 active site, achieving a docking score of −9.3 (PDB ID: 1AHO). It formed hydrogen bonds with key residues, including Trp20, Val47, His110, Trp111, and Leu300. In contrast, compound 15e exhibited electrostatic interactions with Trp20, Val47, Trp111, Thr113, and Pro218, along with van der Waals interactions involving Lys21, Tyr48, Gln49, Trp79, Cys80, His110, Phe122, Trp219, Cys298, Cys303, Thr309, and Phe311. Overall, nearly all synthesized compounds demonstrated superior aldose reductase inhibitory activity compared to the reference compound, sorbinil.¹¹⁴

A series of arylsulfonylspiro [fluorene-9,5′-imidazolidine]-2′,4′-diones were reported for hypoglycemic and aldose reductase inhibitory activities for both ALR2 and ALR1 enzymes by Iqbal and coworkers (2015). As the structure analysis (Fig. 16) indicates, the arylsulfonyl (–ArO₂S) group is essential for the activity. The substitution of a para-chlorobenzene group at the R₂ position significantly enhanced activity, with compound 15a displaying an IC₅₀ value of 0.89 μM. Incorporating a para-methylbenzene group at R₁ in compound 15b showed moderate activity, yielding an IC₅₀ value of 2.29 μM. In contrast, the introduction of an ortho-naphthyl group resulted in a marked reduction in potency, as observed with compound 15c (IC₅₀ = 27.3 μM) compared with the standard drug sorbinil (IC₅₀ = 3.14 μM). Furthermore, the flavin-like moieties of these compounds were found to interact with several hydrophobic residues within the ALR2 active site, including Trp20, Trp49, Trp79, Trp111, Trp119, Val47, Phe122, and Leu300. Notably, nearly all synthesized compounds exhibited superior ALR2 inhibitory activity compared to the standard drug sorbinil.¹¹⁵

Fig. 16. Structure analysis of spiro[fluorene-9,5′-imidazolidine]-2′,4′-dione derivatives.

Similarly, Da Settimo and colleagues (2001) synthesized acetic acid derivatives of the^1,2,4 triazino[4,3-a] benzimidazole (TBI) nucleus as a class of selective ARIs. According to the structure analysis, compound 16a, the most potent derivative of the series, exhibited ALR2 inhibitory activity and contained a methylbenzene moiety, with an IC₅₀ of 0.36 μM (Fig. 17), similar to the standard drug sorbinil, which has an IC₅₀ of 0.65 μM. Additionally, compound 16b, with an IC₅₀ value of 4.15 μM, and compound 16c, with an IC₅₀ value of 4.58 μM, displayed moderate activity due to substitution of p-chloro methylbenzene or p-fluoro methylbenzene. In contrast, compound 16d displayed the least potent activity due to substitution of p-methoxybenzyl with an IC₅₀ value of 236.0 μM.

Fig. 17. Structure analysis of [1,2,4] triazino[4,3-a] benzimidazole acetic acid derivatives.

Compound 16e showed moderate activity due to substitution of 1-methyl-4-(trifluoromethyl) benzene with an IC₅₀ value of 2.63 μM. Compound 16f also displayed moderate potency with methylbenzene substitution, with an IC₅₀ value of 4.50 μM. In contrast, compound 16g was the least potent, with an IC₅₀ of 236.0 μM due to the presence of acetic acid. The target interaction analysis demonstrated that compound 16a forms a stable complex with ALR2. The carboxylate oxygens of compound 16a interact with the anion-binding site via hydrogen bonds to key residues, including Tyr48, His110, and Trp111, indicating that the carboxylate core is required for its inhibitory activity. A further hydrogen bond was identified between the carbonyl oxygen at the para position of the TBI moiety and the N1 hydrogen of Trp20. In contrast, the benzyl moiety binds into the specific pocket formed by Trp79, Trp111, Phe115, Phe122, Val130, and Leu300. However, this bond was less stable during the simulation.¹¹⁶

2.4. Oxadiazole derivatives

A series of oxadiazole-sulfonamide hybrid compounds were synthesized by Javid and coworkers (2020) to identify a therapeutic agent to combat diabetic complications, and the compounds were evaluated for inhibitory activity against ALR2 and ALR1. Based on the structural analysis, compound 17a was found to be the most effective in the series, bearing a methoxy group at the para position, with an IC₅₀ value of 3.06 μM. Additionally, introducing the nitro group at the para position resulted in moderate inhibitory activity for compound 17b, with an IC₅₀ value of 3.56 μM. Alternatively, substituting the carbomethoxy group at the ortho position decreased the activity of compound 17c, having an IC₅₀ value of 24.8 μM. Substitution of the carboxylic group at the para position reduced the activity of compound 17d, exhibiting an IC₅₀ value of 30.6 μM (Fig. 18). In target interaction studies, compound 17a exhibits strong binding interactions within the active site of ALR2; the o-methoxyphenyl group showed a hydrogen bonding interaction (PDB ID: 1US0) with the key residues Tyr50 and Trp22, which also interact with the anion pocket or catalytic pocket of ALR2 containing His110, Tyr48, Trp111. All synthesized derivatives show better inhibition than the reference drug sorbinil (IC₅₀ = 3.14 μM).¹¹⁷

Fig. 18. Structure analysis of oxadiazole-sulfonamide hybrids.

Moreover, a series of 1,2,4-oxadiazol-5-yl-acetic acid and oxazol-4-yl-acetic acid derivatives were synthesized and further evaluated for their potential to inhibit aldose reductase (ALR2) by La Motta and research associates (2008). They designed eighteen compounds, incorporating different withdrawing and electron-donating substituents to enhance inhibitory potential. According to structure analysis (Fig. 19), compound 18a exhibited remarkable in vitro activity, with an IC₅₀ value of 0.27 μM. The methoxy group at the para position enhances the potency of compound 18a. Compounds 18b and 18c, which featured a nitro group substituent at the meta position, also demonstrated significant inhibitory activity, with an IC₅₀ value of 0.30 μM. These compounds showed superior potency, similar to the standard drug tolrestat, which had an IC₅₀ value of 0.05 μM. In contrast, compound 18d, with no substitutions, exhibited reduced inhibitory activity (IC₅₀ = 28.4 μM). All these derivatives were compared with tolrestat (IC₅₀ = 0.05 μM). An in silico docking study demonstrates that compound 18a exhibits strong interactions with the ALR2 active sites, anchoring its carboxylate in the anion-binding pocket via hydrogen bonds to Tyr48, His110, and Trp111, and engaging the hydrophobic residues Trp79, Phe115, Phe122, and Leu300 via π–π stacking. Additionally, the methoxy group formed a hydrogen bond with Thr113; as a result, it showed higher potency than methyl- or unsubstituted analogues.¹¹⁸

Fig. 19. Structure analysis of 1,2,4-oxadiazol-5-yl-acetic acids and oxazol-4-yl-acetic acids derivatives.

2.5. Pyrazole derivatives

A class of pyrazole-rhodanine-3-hippuric acid derivatives was synthesized by Celestina and coworkers (2020) and evaluated as ALR2 inhibitors to improve diabetic complications. From structural analysis, compounds 19a and 19b showed greater inhibitory activity than the standard drug epalrestat (IC₅₀ = 0.87 μM). Substitution of the fluorine group at the meta position resulted in excellent inhibitory activity for compound 19a, with an IC50 of 0.04 μM, 21.5-fold more active than epalrestat. The insertion of the methyl group at the ortho position and the nitro group at the meta position resulted in compounds 19b and 19c exhibiting good inhibitory activity, with IC₅₀ values of 0.06 μM and 0.08 μM, respectively. The presence of the chlorine at the para position decreased the activity of compound 19d with an IC₅₀ value of 1.36 μM (Fig. 20). Target interaction analysis of compound 19a exhibited strong binding energies of −11.4 kcal mol⁻¹, (PDB Id: 2FZD) and presence of electronegative oxygen atom in the acid group showed hydrogen bonding interaction in the active site of Try48, Gln183, and Lys77 residues. The electronegative oxygen atom in the amide group binds to the hydrogen atom on His110. Trp20 binds to the thiocarbonyl group. Trp219, Phe122, and Trp20 bind the target with hydrophobic interactions at the active sites.¹¹⁹

Fig. 20. Structure analysis of pyrazole-rhodanine-3-hippuric acid derivatives.

A class of pyrazolone and pyrazole carbothioamide derivatives was synthesized by Kadam and colleagues (2014) and evaluated in vitro. Subsequently, in silico analysis demonstrated that, among ligands, hydrophobicity alone is not sufficient for effective inhibition of aldose reductase (AR). The structure analysis findings suggested that the amino-λ³-methanethione is crucial for the AR inhibitory activity. Compound 20a exhibited the most potent inhibitory activity profile in the series, with an IC₅₀ value of 6.30 μM (Fig. 21). This potency was attributed to the substitution of 3-(4-chlorophenyl)-1-phenyl-1H-pyrazole-5-carbaldehyde. In contrast, compound 20b, containing the substituent of 3-(4-methoxyphenyl)-1-phenyl-1H-pyrazole-5-carbaldehyde, demonstrated moderate inhibitory activity with an IC₅₀ value of 8.10 μM. On the other hand, compound 20c, substituted with fluoro benzaldehyde at the para position, showed significantly less potency with an IC₅₀ value of 18.80 μM. All these derivatives are compared with the standard drug quercetin, which has an IC₅₀ value of 3.12 μM. The target interaction analysis of compounds 20a and 20b showed binding energies −10.58 and −9.35 kcal mol⁻¹, respectively. The S atom of the carbothioamide in 20a formed a hydrogen bond with Ser159 and Asn160 (PDB Id: 2DUX) and the polar interactions with the P2 lining pocket of the residue. A salt bridge between Cys298 and the key oxygen of the amino-pyrazole was formed, while N3 and N6 atoms formed H-bonds with His110, Tyr48, and Gln183, stabilising the ligand–enzyme complex.¹²⁰

Fig. 21. Structure analysis of a pyrazolone and pyrazole carbothioamide derivatives.

2.6. Triazole derivatives

Five-membered heterocyclic triazole is known for its stability and diverse biological activities. Additionally, a series of 18 4H-1,2,4-triazole derivatives conjugated with thiazole and benzothiazole scaffolds were reported and evaluated for their potential as ARIs (Sever et al., 2020). As per structural analysis (Fig. 22), compound 21a, the presence of a methoxyphenyl group at the para position of the benzothiazole scaffold and insertion of chlorobenzothiazol-2-yl at the ortho position of R₂, exhibited the highest potency with an IC₅₀ value of 0.205 μM. Introducing a methoxyphenyl at the para position of the benzothiazole ring and substitution of the methylbenzothiazole-2-yl group at the ortho position of R₂ led to compound 21b showing moderate inhibitory activity with an IC₅₀ value of 0.232 μM. In contrast, compound 21c, containing a methylphenyl group at the para position of R₁ and substituting methylbenzothiazole-2-yl, exhibited lower activity, with an IC₅₀ value of 0.346 μM. All compounds showed greater activity than the standard, quercetin. In vitro cytotoxicity (MTT assay) using L929 mouse fibroblast cells showed that all synthesized compounds were non-toxic. A molecular docking study revealed that the most potent inhibitor, 21d, exhibited the most favourable docking score (−6.74 kcal mol⁻¹), compared with standard inhibitors quercetin (−6.34 kcal mol⁻¹) and sornibil (−5.94 kcal mol⁻¹). The triazole scaffold facilitates strong π–π stacking interactions with Trp 219 and Phe122, whether substituents at the triazole ring facilitated additional π–π stacking with Trp111.¹²¹

Fig. 22. Structure analysis of 4H-1,2,4-triazole derivatives conjugated with thiazole and benzothiazole scaffolds.

2.7. Pyrimidine derivatives

Pyrimidine derivatives exhibit a wide spectrum of biological activities due to their electron-rich nitrogen framework. La Motta and coworkers (2007) discovered pyrido[1,2-a]pyrimidin-4-one derivatives as a class of selective ARIs and antioxidant compounds. Structure–activity relationship investigations showed that the insertion of the 4λ₃-benzene-1,2-diol at R exhibited excellent activity of compound 22a with an IC₅₀ value of 0.10 μM. Substitution of the 4λ₃-phenol showed moderate AR inhibitory activity in compound 22b with an IC₅₀ value of 0.42 μM. Compound 22c displayed decreased inhibitory activity, which is due to the substitution of 1,2-dimethoxy-4λ³-benzene at R and the IC₅₀ value of 100 μM (Fig. 23). Target interaction studies revealed that in compound 22a (PDB ID: 2FZD), the hydroxy group at the para position of the phenyl moiety interacts with Tyr48 and Trp111 in the ALR2 and NADP⁺ complex. The hydroxyl group at the meta position also formed hydrogen bonds with Cys298. Almost all synthesized compounds showed better activity than quercetin (IC₅₀ = 0.67 μM).¹²²

Fig. 23. Structure analysis of pyrido[1,2-a] pyrimidin-4-one derivatives.

2.8. Thiazole derivatives

The B. Sever research group (2020) reported the synthesis of a class of twelve compounds via the ring closure reaction of 3,4-dihydro-2H-1,5-benzodioxepine-7-carbaldehyde thiosemicarbazone with 2-bromo-1-arylethanones as ARIs. Five-membered heterocyclic thiazole containing both electron-donating sulfur and electron-accepting (C N) groups. In addition to their antidiabetic activity, they exhibit very potent AR inhibition. According to the docking analysis, compound 23a, which featured an m-nitrophenyl group at R, was the most potent, with an IC₅₀ value of 0.983 μM. Furthermore, the introduction of a morpholinophenyl group at the para position led to the formation of compound 23b, which exhibited moderate inhibitory activity (IC₅₀ = 1.936 μM). In contrast, substitution with a phenyl group yielded the least potent compound, 23c, with an IC₅₀ of 3.330 μM (Fig. 24). All compounds were promising AR inhibitors compared with quercetin. The target interaction analysis demonstrated that the thiazole moiety of compound 23a (PDB ID: 1 T41) interacts through π–π stacking with Trp20 and Phe122, and that it also interacts with Trp111 via its 3-nitrophenyl substituent. According to both in vitro and in silico assays, these potential inhibitors may be of great importance in preventing diabetic complications.¹²³

Fig. 24. Structure analysis of 3,4-dihydro-2H-1,5-benzodioxepine-7-carbaldehyde thiosemicarbazone compounds.

2.9. Purine derivatives

A series of 9H-purin-6-amine derivatives as inhibitors of aldose reductase (ALR) were designed and synthesized by Zhu and colleagues (2022). According to the SAR study, compound 24a was the most potent inhibitor among this series, with an IC₅₀ value of 0.038 μM. This compound demonstrated greater potency than the standard drug epalrestat (IC₅₀ = 0.045 μM). The presence of hydroxy-λ³-methanone structures is essential for the compounds' activity. The enhanced inhibitory activity of compound 24a is due to the fluoro at the ortho position and the bromo at the para position on the benzylamine side chain.

Furthermore, compounds 24b and 24c, with fluorine and chlorine at the para position on the benzylamine side chain, showed good inhibition, with IC50 values of 0.085 μM and 0.127 μM, respectively. Compound 24d, which contained hydroxy-λ³-methanone at R₁, displayed the lowest inhibitory effect, with an IC₅₀ value of 72.653 μM (Fig. 25). According to the molecular interaction studies, it was observed that compound 24a (PDB ID: 1Z3N) strongly binds with the active site of the ALR2. The carboxylate group was inserted in the anion-binding site by strong hydrogen bonding with the side chain of Tyr48 and His110. Furthermore, the para-bromo and meta-fluoro-benzylamine ring showed stable stacking interactions with Trp111.¹²⁴

Fig. 25. Structure analysis of 9H-purine-6-amine derivatives.

3. Future perspectives

The development of nitrogen-containing heterocyclic derivatives as ARIs has led to improved therapies for diabetic complications. Future research should focus on optimizing scaffold strategies to enhance ALR2 selectivity while maintaining favorable physicochemical properties that facilitate penetration into peripheral tissues. Integrating in vitro and computational studies with in vivo models is essential for validating pharmacodynamic efficacy and safety, ensuring a smoother path toward clinical applications. A major future direction is the design of multifunctional ARIs that simultaneously modulate interconnected pathways, such as protein kinase C and advanced glycation end-product (AGE) formation, to achieve superior therapeutic benefits. Additionally, integrating green chemistry approaches, such as microwave-assisted synthesis, can improve sustainability and scalability in ARI production. Advancements in computational techniques, including quantum mechanics/molecular mechanics (QM/MM) and free-energy perturbation (FEP) analyses, will further refine molecular docking accuracy and structure analysis predictions, accelerating drug discovery. Comprehensive pharmacokinetic (ADME) and toxicity assessments are critical to optimizing bioavailability, metabolic stability, and safety. Adopting these strategies is expected to facilitate the transition from preclinical to clinical research, ultimately leading to next-generation ARIs with improved efficacy, safety, and clinical relevance. These advancements will not only enhance the management of diabetic complications but also open new avenues for therapeutic innovations in metabolic disorders.

4. Conclusions

Over the past 24 years, exploring nitrogen-containing heterocyclic derivatives as ARIs has led to significant advancements in combating diabetes-related complications. Structure analysis has illuminated the important role of various functional groups in enhancing the potency, selectivity, and pharmacokinetics of ARIs. Nitrogen-containing heterocycles, such as quinoxaline, pyrazoline, triazole, and imidazole derivatives, include numerous compounds that demonstrate superior efficacy compared with existing reference drugs, including epalrestat, sorbinil, and tolrestat (Table 1).

Table 1. A comparative overview of potency, binding interaction, and selectivity of heterocyclic scaffolds as ARIs.

Heterocyclic scaffolds	IC50 ranges (μM)	Key enzyme interactions	Impact on ALR2 selectivity
Quinoxazoline derivatives	0.0032–75.19	Trp111, Tyr48, His110, Thr113, Tyr309, Leu300, Phe122	High to moderate selectivity via H-bonding and π–π stacking in a specific pocket
Pyrazoline derivatives	0.0175–23.73	Trp111, Trp48, His110, Trp21, Arg203, Lys21, Lys 262	High to moderate selectivity through H-bonding and hydrophobic interactions
Imidazole derivative	0.031–236.0	Trp111, Trp20, Tyr48, His110, Phe122, Phe121	Highest-to-lowest selectivity through ionic and H-bond interactions
Oxadiazole derivatives	0.27–30.6	Trp111, Trp20, Trp79, Trp22, Tyr50, Tyr48, His110, Thr113	Moderate selectivity via anion-binding, dipolar interaction, and H-bonding
Pyrazole derivatives	0.04–18.80	Trp20, His110, Tyr48, Gln183, Lys77, Asn160, ser159, Cys 298	High to moderate specificity by H-bond and polar interactions
Triazole derivatives	0.205–0.311	Trp111, Phe122, Trp219	Moderate selectivity via π–π stacking interaction
Pyrimidine derivatives	0.10–100	Trp111, Tyr48, Cys298	Moderate ALR2 selectivity by hydrogen bonding and planar stacking
Thiazole derivatives	0.983–3.33	Trp111, Phe122, Trp20	Moderate preference through hydrophobic and π–π stacking interaction
Purine derivatives	0.03–72.65	Trp111, Tyr48, His110	High to moderate selectivity by strong H-bonding in the anion binding site and stable stacking interaction

The reported compounds exhibit significant aldose reductase inhibitory (ARI) activity and can be modified to enhance their potency and selectivity. They were evaluated in vitro against ALR2 and ALR1, enzymes originally isolated from rat lenses and rat kidneys. Despite these advances, the clinical success of aldose reductase inhibitors has been limited by challenges including inadequate tissue penetration, suboptimal isoform selectivity, and unfavorable pharmacokinetic profiles. Ongoing clinical interest in targeting AR through optimization of existing inhibitors, repurposing, and combination therapies highlights the sustained relevance of this enzyme in diabetic drug discovery. Further advances will depend on integrating structure-guided design, physicochemical optimization, and early translation evaluation. Furthermore, the structural analysis of ALR2 inhibitors reviewed in this article may be useful to medicinal chemists in designing new antidiabetic agents with potent ALR2 inhibition.

Author contributions

Writing – original draft preparation, AK; conceptualization, writing – review, editing, final draft preparation, SKV; methodology, SKM, KR, RK, & GDG; editing, proofreading the final draft; SKV and GDG. All authors have read and agreed to the submitted manuscript for publication in Medicinal Chemistry Research.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Abbreviations

AGEs

Advanced glycation end products

AKR

Aldo–keto reductase

ALR2

Aldose reductase

ARI

Aldose reductase inhibitor

DAG

Diacylglycerol

DM

Diabetes mellitus

GSH

Glutathione

GSSG

Oxidized glutathione

IC₅₀

Half-maximal inhibitory concentration

IDF

International diabetes federation

NADP

Nicotinamide adenine dinucleotide phosphate

NADPH

Nicotinamide adenine dinucleotide phosphate hydrogen

PKC

Protein kinase C

PLC

Phospholipase C

PDB

Protein data bank

ROS

Reactive oxygen species

SD

Sorbitol dehydrogenase

T1DM

Type 1 diabetes mellitus

T2DM

Type 2 diabetes mellitus

Data availability

No data were used for the research described in this article.