1National College of Pharmacy, Manassery, Mukkam, calicut, Kerala, pin:673602, India
2Professor and HOD, Department of Pharmaceutical Chemistry, National College of Pharmacy, Kerala University of Health Sciences, Manassery, Kozhikode, Kerala -673602, India
Isatin (1H-Indole-2,3-dione) is a versatile indole-based heterocyclic compound exhibiting a wide range of biological activities, particularly neuroprotection. Neurodegenerative disorders such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and Huntington’s disease (HD) involve progressive neuronal loss, oxidative stress, protein misfolding, and inflammation. Recent studies highlight that isatin derivatives can modulate multiple pathogenic mechanisms, including inhibition of monoamine oxidase (MAO) and acetylcholinesterase (AChE), suppression of neuroinflammation, and protection against oxidative stress-induced apoptosis. This review discusses the physicochemical properties, reactivity, reactions, synthetic strategies, and neuroprotective potential of isatin-based molecules. These findings highlight the promise of isatin scaffolds as lead structures for designing multifunctional neuroprotective agents [1-50].
Neurodegenerative disorders such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and Huntington’s disease (HD) are characterized by the gradual loss of neurons, leading to memory problems, motor dysfunctions, and changes in behaviour [1,2]. Globally, more than 50 million people are affected by dementia, including Alzheimer’s, and the number is expected to rise to 152 million by 2050 [4]. Parkinson’s disease affects approximately 1–2% of people over 60 years old [5], while Huntington’s disease, though rarer, leads to severe cognitive and motor decline [6]. These diseases arise from a group of factors, including oxidative stress, mitochondrial dysfunction, abnormal protein aggregation, and inflammation [3,4]. Current treatments mainly manage symptoms and do little to stop disease progression, highlighting the need for multi targeted compounds to protect neurons and slow down disease progression [5,10,11].One promising class of molecule is isatin (1H?indole?2,3?dione) a naturally occurring molecule found in mammals and plants [5]. Isatin has a fused benzene-pyrrole?2,3?dione structure with adjacent carbonyl groups and a nitrogen-hydrogen bond allows it to form hydrogen bonds, interact with enzymes, proteins and receptors in the brain [15,16]. Research has shown that isatin derivatives can act on multiple targets relevant to neurodegeneration. They inhibit enzymes like monoamine oxidase (MAO) and acetylcholinesterase (AChE) [17], reduce oxidative stress and protect neurons from free radical damage [21,22], modulate neuroinflammation by reducing pro-inflammatory molecules like TNF- α and IL-6 [23,24] and prevent protein aggregation, such as amyloid-β plaques in AD [25,26]. In preclinical studies, these compounds have been shown to protect neurons from damage, reduce pro-inflammatory cytokines, and support mitochondrial function. Because of these multifunctional properties, isatin is considered a valuable scaffold for designing new neuroprotective agents capable of targeting several pathological mechanisms at once [27–30].
ISATIN
PHYSICOCHEMICAL PROPERTIES
Isatin (C?H?NO?, MW = 147.13 g/mol) is a planar heterocyclic compound composed of a fused benzene ring and a pyrrole-2,3-dione ring system, forming a planar structure [1,2]. This framework contains two adjacent carbonyl groups at positions 2 and 3 and an NH group at position 1[3,4]. This allows keto-enol tautomerism. Isatin is slightly soluble in water but dissolves well in organic solvents such as ethanol, methanol, and dimethyl sulfoxide (DMSO) [7,8]. It appears as orange-red crystals with a melting point of around 203.5 °C and exhibits weakly acidic behaviour [9]. These physicochemical characteristics are important because they influence how derivatives of isatin interact with enzymes like MAO and AChE or cross the blood–brain barrier (BBB) [10-12]. It also serves as a precursor for various derivatives of pharmacological importance [7].
REACTIVITY OF ISATIN
Isatin is reactive at three main positions on its structure: C-2, C-3, and C-5 [13-15].
1.C-5 position (aromatic ring):
Substitutions at C-5, such as halogens or alkyl groups, can enhance biological activity and improve enzyme inhibition [16,17].
2.C-3 position (carbonyl group):
This site allows N-alkylation and condensation reactions, which are commonly used to develop MAO inhibitors or anti-Alzheimer compounds [18-20].
3.C-2 position (carbonyl group):
Reactions here include reduction, oxidation, spiro-annulation, and nucleophilic attack, enabling the design of derivatives with antioxidant and neuroprotective properties [21-23].
TAUTOMERISM
Isatin exists in two tautomeric forms: the lactam and the lactim form. Tautomerism involves the transfer of a proton between the nitrogen atom (N-H) and the oxygen atom at the C-2 position, resulting in interconversion between these two forms [24,26].The lactam form predominates in the solid state and has a carbonyl group at C-2. The lactim form has an enol-like structure (C-OH) at C-2 and is observed in some derivatives, such as O-alkylated isatins or isatin-α-chloride [24,26]. This tautomerism affects reactivity, hydrogen bonding, and biological activity, making it a key feature when designing neuroprotective derivatives [27-30].
SYNTHESIS OF ISATIN
Several classical and modern strategies enable the preparation of isatin and its functionalized derivatives. The Sandmeyer, Stolle, and oxidative cyclization methods are widely used due to their versatility and adaptability, providing flexible routes for synthesizing diverse isatin derivatives.
1.Sandmeyer Synthesis
In this method, aniline derivatives react with chloral hydrate and hydroxylamine hydrochloride to form an O-amino aryl oxime intermediate. This intermediate then undergoes cyclization in concentrated sulfuric acid followed by oxidation to yield isatin [3,4]. This method is versatile and allows the introduction of substituents on the benzene ring, which can enhance enzyme inhibition and neuroprotective activity [5].
2.Stolle Synthesis
Here, an aniline derivative reacts with oxalyl chloride to produce a phenylglyoxalyl intermediate. Cyclization under Lewis acid catalysis converts it into isatin [6,7]. This method is particularly useful for preparing C-3 substituted derivatives that show selective MAO-B inhibition [8].
3.Oxidation of Indoles
Indole derivatives can be oxidized using suitable oxidizing agents, such as oxone or sodium hypochlorite, to produce isatin [9,10]. This method is simple and effective for generating various functionalized isatin derivatives, which can be further modified for anti-Alzheimer or anti-Parkinson activity [11,12].
REACTIONS OF ISATIN
1.N-Alkylation
The NH group at position 1 can react with alkyl halides under basic conditions, producing N-substituted isatin derivatives with improved blood–brain barrier permeability [16,17].
2.Pfitzinger Reaction
Isatin reacts with ketones in alkaline conditions to form quinoline-4-carboxylic acids, which have potential MAO and AChE inhibitory activity [18,19].
3.Oxidation
Isatin can be oxidized to isatonic anhydride, which serves as a building block for further
derivatization [20].
4.Reduction
Carbonyl groups can be reduced to form oxindole derivatives, which have shown antioxidant and neuroprotective effects [21].
5.Friedel–Crafts Reaction
Asymmetric Friedel-Crafts alkylation of isatin can produce 3-aryl-3-hydroxy-2-oxindoles, which are valuable in designing multi-targeted neuroprotective agents [22,23].
NEUROPROTECTIVE ACTIVITY OF ISATIN
Isatin derivatives have demonstrated multitarget neuroprotective effects, including inhibition of monoamine oxidase (MAO), acetylcholinesterase (AChE), reduction of neuroinflammation, antioxidant activity, and prevention of protein aggregation [1-3].
Key studies are summarized below:
1. Monoamine Oxidase (MAO) Inhibition
Monoamine oxidase (MAO) enzymes, particularly MAO-B, are responsible for the breakdown of neurotransmitters like dopamine. Overactivity of MAO-B contributes to oxidative stress and neuronal damage, especially in Parkinson’s disease [4–6].
Several studies have shown that isatin derivatives act as selective MAO-B inhibitors. For example:
1.Benny et al., 2023
Benny et al. synthesized a series of isatin-based benzyloxybenzene derivatives as potent monoamine oxidase (MAO) inhibitors with neuroprotective potential. Biological evaluation revealed strong MAO-B selectivity, with compounds ISB1 and ISFB1 showing IC?? values of 0.124 μM and 0.135 μM, respectively. Kinetic and docking studies confirmed competitive, reversible inhibition mediated by hydrogen bonding and π–π stacking interactions. These findings highlight that para-benzyloxy substitution on the benzene ring is favorable for selective MAO-B inhibition, positioning the isatin–benzyloxybenzene scaffold as a promising lead for neurodegenerative therapy [7].
2. Sunil Kumar et al., 2023
Sunil Kumar et al. synthesized sixteen halogen-substituted isatin–acylhydrazone derivatives (IS1–IS16) as dual MAO inhibitors with neuroprotective potential. Among these, IS7 exhibited potent MAO-B inhibition (IC?? = 0.082 μM), while IS15 was the most effective MAO-A inhibitor (IC?? = 1.85 μM), both acting as competitive and reversible inhibitors. The active compounds demonstrated good blood–brain barrier (BBB) permeability and significantly reduced ROS, TNF-α, IL-6, and NF-κB levels in LPS-challenged SH-SY5Y cells, while enhancing antioxidant enzyme activity. These results suggest that para-bromo substitution and a hydrophobic isatin core improve MAO-B selectivity and neuroprotective efficacy [8].
IS15
These findings suggest that C-3 substitutions and para-halogen groups enhance MAO-B selectivity and neuroprotective efficacy [9,10].
2. Acetylcholinesterase (AChE) Inhibition
Acetylcholinesterase breaks down acetylcholine, a neurotransmitter essential for memory and cognition. In Alzheimer’s disease, reduced acetylcholine levels contribute to cognitive decline [11].
1.Mavroidi et al., 2023
Mavroidi et al. synthesized two isatin–thiosemicarbazone derivatives (M and FMp) and evaluated their multimodal neuroprotective effects against Alzheimer’s disease. Both compounds at 1 μM reversed Aβ??-induced toxicity in neuronal cells by restoring Akt and GSK-3β phosphorylation, in addition to exhibiting moderate AChE and LOX inhibition (IC?? ≈ 54–60 μM). They also demonstrated strong antioxidant properties and favorable drug-likeness, suggesting that isatin–thiosemicarbazone scaffolds are promising prophylactic neuroprotective agents for Alzheimer’s therapy [12].
FMP
2.da Silva et al., (2025)
da Silva et al. (2025) developed isatin-pyridine oxime hybrids showing potential AChE inhibition for neuroprotection against neurotoxins [13].
These studies indicate that isatin scaffolds can be tuned to target both MAO and AChE, creating multi-targeted neuroprotective agents [14,15].
3. Antioxidant and Anti-Inflammatory Effects
Oxidative stress and inflammation are key drivers of neuronal damage. Isatin derivatives reduce oxidative stress by scavenging free radicals and increasing antioxidant enzyme activity [16–21].
1.Cenalmor et al., 2023
Cenalmor et al. synthesized a series of N′-alkylated and halogen-substituted isatin derivatives and evaluated their anti-inflammatory activity using LPS-activated BV2 microglial cells. Biological testing revealed that compound 10 (N′-alkyl derivative) and compound 20 (5-chloro substituted isatin) at 25 μM significantly reduced nitric oxide (NO) release and decreased the levels of pro-inflammatory mediators IL-6 and TNF-α, while maintaining low cytotoxicity. These results demonstrate that N′-alkylation combined with 5-halogen substitution on the isatin core favorably modulates microglial inflammation, positioning this scaffold as a promising lead for neuroprotective anti-inflammatory drug development [21].
2.Wang and Wang et al., 2025
Wang et al. designed and synthesized three series of isatin–chalcone hybrid derivatives aimed at modulating neuroinflammatory activity in microglial cells. The compounds were evaluated in LPS-activated BV2 microglia for nitric oxide (NO) release, cell viability, and pro-inflammatory cytokine production (TNF-α, IL-6), along with in-silico docking to key inflammation-related proteins.Among the series, compound 4B exhibited the most potent activity (IC?? = 1.6 μM, therapeutic index = 21.6) and significantly reduced TNF-α and IL-6 levels (p < 0.0001). Structure–activity relationship analysis indicated that a phenyl penta-1,4-diene-3-one chalcone moiety at the ketone position, combined with favorable lipophilicity (logP ≈ 3.36, logBB ≈ –0.32)[20].
3.Bakir et al., 2024
Bakir et al. synthesized a series of schiff-base derivatives of five substituted isatins and a monothiocarbohydrazone, which were characterized by FTIR, NMR, and elemental analysis. Antioxidant activity, evaluated using the DPPH assay, revealed that five iodo-isatin derivatives containing a 3-methoxy-4-hydroxy-benzothiocarbohydrazone moiety exhibited the highest activity (IC?? = 9.76 μM).DFT calculations indicated that smaller HOMO–LUMO gaps and higher bond delocalization correlate with stronger radical-scavenging activity, suggesting a single-electron transfer mechanism. These results highlight that substituent on both the isatin core and hydrazone unit enhance antioxidant potential, positioning this scaffold as a promising lead for further development [19].
4.Olga Buneeva et al., 2023
Buneeva et al. investigated the effects of the neurotoxin 1?methyl?4?phenyl?1,2,3,6?tetrahydropyridine (MPTP) and the endogenous neuroprotector isatin on the ubiquitinated brain mitochondrial proteome. Administration of MPTP in mice caused significant changes in the ubiquitination profile of brain mitochondrial proteins, while pre-treatment with isatin significantly ameliorated locomotor deficits and modulated the mitochondrial ubiquitinated proteome. Specifically, MPTP reduced the total number of ubiquitinated mitochondrial proteins from 75 to 49, whereas isatin altered this profile and increased the proportion of oxidized-plus-ubiquitinated proteins.
5.Uvarov et al., 2025
Uvarov et al. synthesized a panel of tricyclic isatin–oxime derivatives and evaluated their anti-inflammatory activity and kinase binding affinity. Biological assays showed that compounds 5A and 5D inhibited LPS-induced NF-κB or AP-1 transcription and IL-6 production in THP1-Blue and Monomax-6 cells, with IC?? values in the low micromolar range. Further profiling revealed that 5D exhibited nanomolar to submicromolar Kd values toward 12 kinases, including DYRK1A, DYRK1B, PIM1, HIPK1–3, NEK10, and DAPK1–3.These results indicate that unsubstituted oxime isatin cores in a tricyclic framework strongly favor dual anti-inflammatory and kinase inhibitory activity, positioning this scaffold as a promising lead for neuroinflammation-targeted therapeutics.
4. Protein Aggregation Inhibition
Protein aggregation, such as amyloid-β plaques in AD, leads to neuronal toxicity. Isatin derivatives can prevent or reverse aggregation, protecting neurons [22–24].
1.Purgatorio et al., 2023
Purgatorio et al. performed pharmacophore modeling and 3D-QSAR studies on a series of 36 indole- and isatin-based derivatives designed to inhibit β-amyloid (Aβ) aggregation as potential therapeutics for Alzheimer’s disease. Their models exhibited acceptable predictive statistics (q² ≈ 0.596, r²??? ≈ 0.695) and identified key physicochemical features correlating with anti-amyloidogenic potency. These findings highlight structural motifs favorable for Aβ aggregation inhibition and provide a useful computational framework for optimizing indole/isatin scaffolds in Alzheimer’s drug discovery [22].
2.Maliyakkal et al., 2024
Maliyakkal et al. synthesized ten isatin-based hydrazone derivatives, divided into sub-series 1A (isatin + acetophenone) and 1B (isatin + benzaldehyde), to investigate inhibition of monoamine oxidase A (MAO-A) and monoamine oxidase B (MAO-B). Biochemical assays revealed that compounds in the 1B series were the most potent, with 1B4 showing IC?? = 0.15 μM for MAO-A and 1B3 exhibiting IC?? = 0.068 μM for MAO-B. Both compounds which acted as competitive MAO inhibitors while stabilizing protein interactions and preventing aggregation [23].Molecular docking and dynamics studies indicated stable hydrogen bonding between isatin-NH and Asn-181 of MAOs. In-silico ADME profiling suggested blood-brain barrier (BBB) permeability for the lead compounds. These findings highlight that 4-chloro or 4-bromo substituent.
Recent Advances in Isatin Derivative Research
In recent years, there has been significant progress in designing isatin-based molecules with enhanced neuroprotective activity. Researchers have focused on modifying the isatin scaffold to improve enzyme inhibition, antioxidant activity, anti-inflammatory effects, and blood–brain barrier (BBB) permeability [1–3].
1. Multi-Targeted Derivatives
Multi-target directed ligands are important because neurodegenerative diseases involve multiple pathogenic pathways. Several studies report successful multi-target directed ligands designs using isatin:
1.Benny et al., 2023 synthesized isatin–benzyloxybenzene derivatives as selective MAO-B inhibitors with neuroprotective potential in oxidative stress models [4].
2.Sunil Kumar et al., 2023 created halogen-substituted isatin–acylhydrazones, which inhibited both MAO-A and MAO-B, reduced ROS, TNF-α, and IL-6, and showed good BBB permeability [5].
3.Mavroidi et al., 2023 developed isatin–thiosemicarbazone hybrids, which reversed Aβ-induced toxicity and moderately inhibited AChE and LOX enzymes, demonstrating multimodal neuroprotection [6].
2. Preclinical and Proteomic Studies
Several studies have used animal models and proteomic analysis to evaluate neuroprotective effects of isatin:
1.Buneeva et al., 2023 investigated isatin in MPTP-induced Parkinsonism. Pre-treatment with isatin ameliorated locomotor deficits and modified the mitochondrial ubiquitinated proteome, protecting neurons from oxidative damage [12].
2.Medvedev et al., 2020 studied isatin’s effects on the brain proteome, showing multilevel neuroprotective changes in mice [13].
3.Maliyakkal et al., 2024 performed in vitro and in silico analyses to confirm MAO inhibition and BBB permeability for lead isatin derivatives [14].
These findings confirm that isatin derivatives are effective in vivo and in vitro, supporting their potential translation to therapeutic applications.
3.Hybrid molecules-potential scaffolds
Recent research emphasizes hybrid molecules combining isatin with other bioactive scaffolds:
1.Zhong et al., 2025 developed melatonin–isatin hybrids, showing potential for Alzheimer’s therapy due to combined antioxidant and MAO-inhibiting properties [15].
2.Uvarov et al., 2025 synthesized tricyclic isatin–oxime derivatives, which acted as dual anti-inflammatory and kinase inhibitors, targeting multiple neurodegenerative pathways [16].
3.Dimkovski et al., 2025 produced coumarin–triazole–isatin hybrids as selective butyrylcholinesterase inhibitors, providing new avenues for Alzheimer’s treatment [17].
These hybrid approaches enhance bioavailability, multi-target activity, and therapeutic efficacy.
CONCLUSION
Isatin and its related compounds have shown great promise as potential protectors of nerve cells. Because of their flexible chemical structure, these molecules can be modified in many ways to act on different biological targets linked with brain diseases. Research has shown that isatin-based compounds can reduce oxidative stress and inflammation, block harmful enzymes like monoamine oxidase and acetylcholinesterase, and help prevent nerve cell damage.Many of these molecules are also able to cross the blood–brain barrier, which is very important for treating brain disorders such as Alzheimer’s and Parkinson’s diseases. The overall findings suggest that isatin works through several pathways at once, offering a multitarget approach rather than acting on a single cause of disease.
FUTURE PERSPECTIVES
1.Most current studies are preclinical. Future research should focus on safety, pharmacokinetics, and efficacy in humans.
2.Modifying C-3, C-5 can improve enzyme inhibition, BBB permeability, and antioxidant activity.
3.Hybrid strategies: Combining isatin with other bioactive scaffolds may enhance multitarget effects and reduce side effects.
4.Proteomic and computational approaches: Integrating in silico modeling, docking, and proteomics can accelerate the identification of lead compounds.
In conclusion, isatin derivatives represent a versatile and promising scaffold for developing next-generation neuroprotective agents. Their multitarget potential, combined with favorable pharmacokinetic properties and modifiable structure, makes them highly suitable for tackling the complex biology of neurodegenerative disorders. Continued research could lead to effective, safe, and clinically relevant therapeutics for AD, PD, HD, and related disorders.
REFERENCES
Fathima Hasbuna KA1, Dr. Deepika P.2*, An Overview of Isatin Molecules as Neuroprotective Agents in Neurodegenerative Disorders, Int. J. of Pharm. Sci., 2026, Vol 4, Issue 1, 2865-2878. https://doi.org/10.5281/zenodo.18365451
10.5281/zenodo.18365451
