Chemistry and Pharmacological Potential of Oxindole Nucleus: A Review

Oxindoles are bicyclic aromatic heterocyclic organic moieties in various plant natural products and mammals' tissues and body fluids ¹. Oxindole derivatives were first isolated from the bark of Uncaria tomentosa. These derivatives were widely used in traditional medicinal systems to treat malignancy, sepsis, inflammatory diseases, arthritis, and gastric ulcers ². The chemical structure of oxindole (Fig. 2) consists of a pyrrole ring, which is a 5-membered heterocyclic ring system bearing a carbonyl group (-C=O) at position C-2, fused with a benzene ring, and the nomenclature of oxindole derivatives is 1,3-dihydro-2H-indole-2-one. This scaffold shows tautomerism and has a convertible hydroxyl group. Oxindole bears the general chemical formula C₈H₇NO ³. Besides natural methods for synthesizing oxindole nuclei, several synthetic approaches are documented to synthesize numerous derivatives based upon these oxindole nuclei, along with various pharmacological activities. ⁴.

Figure 1. Uses of oxindole

For medicinal chemists, oxindole-based compounds have emerged as a pharmacologically important class, using molecular hybridization. Due to this hybridization, the covalently fused two or more pharmacophores result in the development of potent and safer compounds with multiple pharmacological activities ⁵. A recently approved drug named ‘Nintedanib’ is a synthetic drug (Fig. 2) approved in the United States in March 2020, which constitutes an oxindole nucleus, which is clinically used to treat chronic pulmonary diseases like idiopathic pulmonary fibrosis (IPF) and other forms of pulmonary fibrosis. It has also proved as an effective drug against cancer as it limits the growth factor through angiokinase inhibition and is sold under common trade names such as "Ofev" and "Vargatef" ^6-8.

Figure 2. Structure of oxindole and important marketed drugs

Sunitinib is a well-known oxindole derivative (Fig. 2), a small molecule that primarily serves as an anti-cancer drug in the category of tyrosine kinase inhibitors. The drug was approved in the year 2006 by the USFDA for the cancer of renal and stromal GIT cancer. Sunitinib was the first medication that was approved for use against different cancers simultaneously. Its mechanism of action involves the indirect inhibition of multiple receptors that belong to the family of tyrosine kinases and the suppression of cellular signalling ^9-11. Semaxanib is another anti-cancer drug that targets angiogenesis by inhibiting tyrosine kinase for colorectal cancer; however, the phase III trials were not successful ^12,13. Ropinirole is a commonly prescribed drug having oxindole as the core nucleus against Parkinson’s Disease (PD) and restless legs syndrome (Fig. 1), and it acts as a complete agonist of dopamine receptors, mainly D₂, D₃, and D₄. However, it shows a relatively lower affinity towards D₁ and D₅ receptors. The structural characteristics like low molecular weight, a non-stereo-centric chemical structure, and accessibility of this drug are favourable for its functionality, and make it a successful medicine ^14,15. Another novel oxindole-based antipsychotic drug, Ziprasidone (Fig. 2), acts by inhibiting the dopamine receptor D₂. It was manufactured by Pfizer, a US-based pharmaceutical company and sold commercially under the brand name "Geodon." Since that time, this drug has received approval from the USFDA for the treatment of mental disorders such as mania and schizophrenia ¹⁶.

Chemistry of oxindole

The International Union of Pure and Applied Chemistry (IUPAC) has officially designated oxindole as 1,3-dihydro-2H-indole-2-one. This scaffold is known to exist in two hydroxyl groups containing tautomeric forms (Fig. 3). This tautomerism is commonly known as lactam-lactim form, depending on the migration of hydrogen i.e. when the hydrogen of nitrogen tautomerizes, then it is lactim form and when the hydrogen of CH₂ tautomerizes then it is enol form (Kaur et al 2016, Hachula et al 2018).

Figure 3. Tautomerism in oxindole

Synthetic methods of preparing the oxindole nucleus

This scaffold is considered a modified derivative of indole and is often known as indolin-2-one, also known as 2-oxindole. The physicochemical properties of this scaffold are also interesting and well-explored. Oxindole-containing compounds typically exist as crystalline substances or off-white coarse powders with an approximate melting point range between 127 °C to 130 °C. The ESI-MS of oxindole exhibits an m/z value of 133.15 as a base peak, with 100% abundance ^1,2.

In 1866, Bayer and Knop reported the first synthetic molecule of the 2-oxindole nucleus (Fig. 3) using isatin (1a) as a starting material and reduced with Na-Hg (sodium amalgam) to produce di-oxindole in an appropriate diluent. This was followed by the reduction using acidic Stannum (II) to generate the oxindole molecule (2) (Scheme 1-1). In parallel, Marschalk et al. also produced oxindole (2) directly from isatin (2a) using sodium hydrosulfite (Na₂S₂O₄), involving the utilization of Na-Hg under the influence of carbon dioxide (CO₂) (Schemes 1-2)

Figure 4. Synthetic Schemes (1-1 to 1-2) for 2-oxindole

Bayer employed yet another effective synthetic approach (shown in Fig. 5) to produce the target 2-oxindole scaffold by the reduction of 2-nitrophenylacetic acid (3a) to 2-oxindole (2) using tin and hydrochloric acid as reducing agents (Scheme 2-1). Suida and colleagues constructed oxindole (2) by the reduction of 2-acetaminomandelic acid (4a) in the presence of Na-Hg (Scheme 2-2). Meanwhile, Brunner utilized a reaction between slaked lime and β-acetylphenylhydrazine (5a) by heating at a high temperature (200–220 °C) to generate oxindole (2) (Scheme 2-3). Chloroacetyl chloride is condensed with an aromatic amine (6a) using aluminium chloride (AlCl₃) to form an amide (6b) which is categorically cyclized to oxindole (2). This is another important method for making oxindole (Scheme 2-4).

Figure 5. Schemes (2-1 to 2-4) for the synthesis of 2-oxindole nucleus

In 1952, o-chlorophenylacetic acid (7a) and ammonium hydroxide were heated to synthesize oxindole (shown in Fig. 6)  (Scheme 3-1).

Figure 6. Schemes (3-1 to 3-4) for the synthesis of the oxindole nucleus

To make 2-oxindole (2), Gassman and Bergen simultaneously treated aniline (8a) with ethyl methyl thioacetate, tert-butyl hypochlorite, and organic base triethylamine to produce amino ester (8b). The product obtained was subsequently acidified, and treated with a catalyst Raney Nickel (Fig. 6)  to afford oxindole (Scheme 3-2) ¹⁹. Similarly, oxindole was also produced when N-acyl-o-chloroanilines (9a) were treated with lithium diisopropylamide (LDA) in the presence of a mixture of tetrahydrofuran-hexane (Scheme 3-3) ²⁰. This straightforward oxindole scaffold was generated through the interaction of ortho-bromo-N-methylanilides (10a) with tributyl stannane at high temperatures (160 °C), which involved sequential translocation and homolytic aromatic translocation (Scheme 3-4) ^1,21. The reaction scheme 2-1 reveals the most practical, simple, and cost-effective method to synthesize 2-oxindole, because of the easy and economical availability of the starting material (2-nitrophenylacetic acid). Despite being a stepwise reaction, this process has an exceptional overall yield of approximately 80%. Therefore, scheme 2-1 encompasses all the feasible reactions for the synthesis of oxindole ².

Chemical Reactions of Oxindoles

Chemical reactions and their feasibility help to design various compounds for a particular pharmacological action. The carbon atom adjacent to the amide group in the access point is present at the lactam isomers of oxindole. , which is a frequently exploited reactive site except in cases of the amine deprotonation reaction. The resulting anion is stabilized due to the oxindole enolate's stable aromatic nature. This is critical in various oxindole and its derivatives reactions, such as acylation, alkylation, and fast condensation reactions with diverse types of ketones and aldehydes. Fig. 7 briefly summarizes the typical chemical processes of oxindole-based compounds, but it has been reported that preference for the C-5 the C-3 position by the electrophilic aromatic substitution (EAS) process ^1,22.

Figure 7. Chemical reactions of oxindole

Bromination of oxindole with either N-bromosuccinimide (NBS) or bromine water will result in the production of the bromo derivative of indolin-2-one at the C-5 position, i.e., 5-bromo-indolin-2-one, rather than the C-3 position, i.e., 3-bromo-indolin-2-one. When concentrated HNO₃ and H₂SO₄ are used simultaneously, nitration leads to the formation of 5-nitro-2-oxindole, and HNO₃ alone produces 3-nitro-2-oxindole ^1,23.

Chlorinated 2-oxindole is a major product obtained when phosphorous oxychloride is treated with dimethylformamide as a solvent and diluent. In Vilsmeier-Haak formylation, 2-chloro-3-formylindole is generated as a main product, but at reflux temperature, it results in the production of 2-chloroindole instead of 2-chloro-3-formylindole.

Base-catalyzed reaction conditions cause substitution at the C-3 position rather than the C-1 position of the oxindole moiety ²⁴. The lactim form of oxindole reacts with a strong base like sodium hydroxide (NaOH) to form the sodium-IH-indole-2-olate salt. The base-catalyzed reaction with aryl-substituted aldehydes at high temperature (140 °C), and the aldol condensation reaction led to the formation of 3-arylide 2-oxindole derivatives ¹. Fig. 7 provides an overview of the common chemical reactions that can be carried out on oxindole rings.

PHARMACOLOGICAL POTENTIAL OF SOME OXINDOLE DERIVATIVES:

Oxindole derivatives from natural and synthetic origins have emerged as an important scaffold bearing oxygen and nitrogen atoms which are adaptable substances used in a wide range of biochemical applications and display a diverse array of biological impacts. The development, design, synthesis, and testing of oxindole derivatives have made them applicable in various  medical treatments like anticancer ²⁵, antimicrobial ²⁶, anti-inflammatory ²⁷, anti-glucosidase activity ²⁸, anti-rheumatoid arthritis activities ²⁹, management of glaucoma ³⁰, anti-tyrosinase activity ³¹, and radical-scavenging ³², anti-leishmanial activity ³³ and anti-HIV properties ³⁴.

Anticancer activities of oxindole derivatives

Cancer medically known as malignancy is a class of diseases in which the body experiences aberrant and uncontrolled growth of blood or tissue cells that can be localized to one area or spread across the entire body because of certain DNA mutations. A tumor can be benign or malignant depending on the rate and area of cell division (Leber et al 2009). A benign tumor is a primary stage when the cells are healthy, non-invasive, and have simply overgrown to form a lump, whereas a malignant shows abnormally uncontrolled proliferation of cells that become invasive to adjacent tissues as well as organs (Boutry et al 2022).

Cancer is a rapidly spreading disease and causes may be but not limiting to  sedentary lifestyles, genetic and  environmental factors. ³⁷. In 2020, there were around 19.3 million cancer cases reported as new and nearly 10.0 million cancer deaths globally. Among cancers diagnosed, the most frequent were 2.26 million cases of breast cancer in women, lung cancer (2.21 million cases), and 1.41 million cases of prostate cancer. Among these reported cancers, lung cancer was the most common cause of death (1.79 million), 830000 deaths were due to liver cancer, and stomach cancers (769000 deaths) ³⁸.Despite significant advancements in medical technology, treatment and diagnosis of cancer remains a challenging task. Chemotherapy is the main effective approach in mild cases and in severe cases, it prolongs life expectancy with presence of side effects. The identification of potential anti-cancer agents with minimal toxicity to non-cancerous cells is a major goal for drug discovery researchers ³⁹. Several research groups have explored and documented the anti-cancer activity of novel chemical entities and molecules containing the oxindole ring nucleus ⁴⁰.The new class of compounds with indole-3-ylidene acetate and indole-3-carbohydrazide scaffold was a potent antiproliferative derivative when tested in vitro against different cancer cell lines ⁴¹. Screening of synthesized compounds reported the most active compound 11 that induced apoptosis by blocking the Trx R (thioredoxin reductase) protein in the HCT-116 cancer cell line. The preliminary structure-activity relationships studies indicated that replacing a double bond and an ester group on the C3 carbon significantly increased the anti-proliferative efficacy of tested compounds ⁴². To assess the cytotoxic effect on various human cancer cell lines, like MCF-7, A549, DU-145, and HT-29, new oxindole derivatives were synthesized as antimitotic agents. Among the tested derivatives, two compounds 12a and 12b exhibit potent IC₅₀ values on A549 cell lines (Human Lung Cancer) and HT-29 cell lines (Human Colon Cancer). Mechanistic research revealed that both compounds led to apoptotic cell death by causing a G2/M phase arrest. Further investigation of these compounds exhibited substantial tubulin protein inhibition with IC₅₀ values in the nanomolar range ⁴³.

Figure 8. Some potential anticancer oxindole derivatives

Sharma et al. discovered that novel indolin-2-one derivatives induced apoptosis in various cancer cell lines, including non-small lung (A549), DU-(145), prostate (PC-3), gastric (HGC) cancer, breast (BT-549) cancer cells, and breast epithelial cells (MCF-10A). Among the tested compounds, analogue 13 showed a remarkable cytotoxic effect with IC₅₀ of 3.26 μM against MDA-MB-231 (breast cancer cell line) ⁴⁴. Some potential anticancer oxindole derivatives are shown in Fig. 8.

Anti-HIV activities of oxindole derivatives

Human Immunodeficiency Virus (HIV) which causes AIDS currently impacts approximately 38 million individuals and poses a threat to world health. The disease is caused by two distinct lentiviruses, namely HIV-1 and HIV-2. Several studies have since demonstrated the presence of HIV-1 in Africa during the 1950s and in America during the 1970s. Subsequently, it rapidly spread across the globe, resulting in tens of millions of cases.

In 1985, HIV-2 was identified as a contributing factor to the AIDS pandemic, albeit causing a less severe form of immunodeficiency ^45-47.

Utilizing oxindoles and cyclic derivatives of oxindoles, one can prevent HIV replication and treat infections which are due to different forms of the virus (drug-resistant form, drug-sensitive form, and mutated form). Some novel oxindole derivatives operate as non-nucleoside reverse transcriptase inhibitors (NNRTIs), one of the most often recommended families of medications used against viral diseases ⁴⁸. It is discovered that the oxindoles' interactions with reverse transcriptase are what give rise to their antiviral activity ⁴⁹. Chander et al. reported the synthesis of novel 2-oxindole derivatives and screened them for in vitro anti-HIV potential. Results of anti-HIV activity showed three compounds 14a, 14b, and 14c as highly potent derivatives with a micromolar range of IC₅₀ values of 0.82 µM, 0.27 µM, and 0.76 µM, respectively. Further investigations showed that the levo form of 14b is the best active derivative ⁵⁰. Potential anti-HIV agents are shown in Fig. 9.

Figure 9. Oxindole derivatives as a potential anti-HIV agent

Antimicrobial resistance (AMR) has emerged as a critical global clinical and public health challenge, advancing at an alarming pace and posing a serious threat to effective antibacterial therapies in the coming years. Multidrug-resistant (MDR) infections, for which no viable treatment options exist, are becoming increasingly prevalent. In response, structurally novel antibacterial agents with unique mechanisms of action have gained significant importance in combating bacterial infections^51-54. Among these, oxindole oxazolidinone derivatives represent a new generation of synthetic antimicrobials, demonstrating efficacy against a broad spectrum of serious infections caused by both human and animal pathogens, including MDR bacterial strains. These derivatives exert their antibacterial effects by inhibiting bacterial protein synthesis and selectively targeting critical enzymes essential for the survival of drug-resistant pathogens ^55-58.

Oxindole derivatives as cardiovascular agents

Some of the antihypertensives and cardiotonic are made from oxindole derivatives. Adibendan and Indolidan are non-steroidal oxindole derivatives that are used to treat an incapacitating condition called congestive heart failure ^59,60. In contrast to Adibendan, which has the dihydropyridazinone moiety coupled to a tricyclic 5-6-5 fused heterocycle, Indolidan comprising of the dihydropyridazinone entity joined with benzo-fused heterocycle nucleus. The primary mechanism by which these drugs work is by inhibiting cAMP, which is a secondary messenger that coordinates the cell responses to numerous hormones as well as neurotransmitters. Additionally, these oxindole derivatives have very potent inotropic, vasodilators, and other beneficial effects on the cardiovascular system. Aminomethylene oxindole analogues are effective antihypertensive medications ^58,61.

Oxindole derivatives as kinase inhibitors

The human genome encodes over 700 protein kinases and 100 protein phosphatases ⁶². These enzymes are crucial in signalling pathways within the cells and regulation of signal transduction pathways as they facilitate protein phosphorylation and dephosphorylation, respectively ⁶³. Their catalytic activities have a pivotal role in mediating cellular communication and coordinating various biological processes. Cellular signalling processes, governing vital aspects such as cell growth, differentiation, migration, and metabolism, heavily rely on protein tyrosine kinases (PTKs) ⁶⁴. These enzymes play a very crucial role in orchestrating intracellular communication and mediating key events that dictate cell behaviour and function ⁶⁵. The protein kinases include extracellular signal-regulated kinases (ERKs),  cyclin-dependent kinases (CDKs), epidermal growth factor receptors (EGFR), vascular endothelial growth factor receptors (VEGF-R1, VEGF-R2), fibroblast growth factor receptors (FGFR1, FGFR2), Janus kinase (JAK1 and JAK2) ^66,67. The neurological disorders of pain perception, Alzheimer's disease (AD), osteoporosis, inflammatory diseases (arthritis), angiogenesis, cancer, chronic obstructive pulmonary disease (COPD), psoriasis, restenosis, and atherosclerosis have also been linked to them as targets ^68,69. The protein serine kinase and protein tyrosine kinase receptors are inhibited by some oxindoles, and tumour growth has also been shown to be inhibited by preventing tumour-related angiogenesis. They are also utilized to treat a variety of cellular proliferation-related clinical conditions including atherosclerosis, thrombosis, and restenosis. By blocking tyrosine kinase receptors like VEGF-R, platelet-derived growth factor receptors (PDGFRs), CDK, etc., several oxindoles have also been proclaimed to possess anticancer activity ^58,70-72.

This review study has highlighted the wide range of biological activities of oxindole, including its potential as an antioxidant, anticancer, anti-leishmanial, anti-rheumatoid arthritis, antiviral, anti-tubercular, α-glucosidase inhibitor, PAK4 inhibitor, and intraocular pressure reducer. However, further investigation is necessary to determine the efficacy of this scaffold to proceed in the next phase of drug development.

APPROVED ANTI-CANCER DRUGS COMPRISING OF OXINDOLE NUCLEUS:

Several anti-cancer drugs that have been approved contain the oxindole nucleus, underscoring its significance in cancer therapeutics. One such drug is Sunitinib, which targets tyrosine kinases involved in angiogenesis and cancer cell growth. Sunitinib has gained approval for treating various cancers, including renal cell carcinoma and gastrointestinal stromal tumours ⁷³. Semaxanib, also referred to as SU5416, is another oxindole-based drug that focuses on inhibiting VEGF receptors, thereby obstructing angiogenesis and tumour progression. Semaxanib has been investigated for its potential in managing solid tumours, particularly advanced colorectal cancer ⁷⁴. Additionally, Nintedanib, an oxindole-containing drug, acts as a triple angiokinase inhibitor by targeting FGFR, VEGF receptors, and PDGFRs. Nintedanib has obtained approval for treating IPF and non-small cell lung cancer ⁷⁵. These approved anti-cancer drugs highlight the significance of the oxindole nucleus in developing targeted cancer therapies. By capitalizing on the unique properties of the oxindole scaffold, these drugs offer valuable treatment options for individuals combating cancer. A summary of clinical trials based upon the above drugs is provided in Table No. 1. Ongoing research endeavours aim to further explore the potential of oxindole-based compounds in the discovery of anti-cancer agents.

Table I Clinical drug candidates with oxindole nucleus