Emodin: A Natural Anthraquinone — A Systematic and Comprehensive Review of Its Structural Attributes, Physicochemical Properties, Source Profiling, Extraction Techniques, Derivative Exploration, and Pharmacological Relevance

Many popular Chinese medicinal herbs, including Rheum palmatum, Polygonum cuspidatum, and Polygonum multiflorum, contain emodin, a natural anthraquinone derivative. For more than 2000 years, emodin has been utilized in traditional Chinese medicine. It is still included in a number of herbal remedies. There is growing evidence that emodin has a broad range of pharmacological characteristics, including as hepatoprotective, anti-inflammatory, antioxidant, antibacterial, and anticancer effects. However, emodin may also cause hepatotoxicity, renal toxicity, and reproductive toxicity, especially when used for extended periods of time and in large doses. Emodin has a low oral bioavailability in rats due to its substantial glucuronidation, according to pharmacokinetic studies. This review aims to comprehensively summarize the pharmacology, derivatives toxicity and pharmacokinetics of emodin reported to date with an emphasis on its biological properties and mechanisms of action. [1] Emodin itself is a tumor cell-growth inhibitor. Emodin possesses diuretic and vasodilatatoric effects as well as antibiotic,4,5 anti-viral, and anti-neoplastic activity. It is also supposed to sensitize cancer cells. Thus, emodin sensitizes HER/neu-overexpressing cancer cells to chemotherapeutic agents such as cisplatin, doxorubicin, etoposide, and paclitaxel.[2]               

FIG.1: Different plant species that are rich source of e

1. CHEMICAL CHARACTERSTICS OF EMODIN:

Rheum palmatum, Polygonum cuspidatum, Polygonum multiflorum, Aloe vera, and Cassia obtusifolia are among the Chinese herbs that contain emodin (1,3,8-trihydroxy-6-methylanthraquinone), a naturally occurring anthraquinone derivative. Many nations, particularly in eastern Asia, have made extensive use of these herbs as traditional treatments. Many studies are currently concentrating on this compound's pharmacological effects.

However, a growing number of newly published research have documented emodin's negative effects. The goal of this study is to present current, thorough data on the pharmacology, toxicology, and pharmacokinetics of emodin over the previous few decades in order to investigate the compound's therapeutic potential and assess prospective avenues for future research. [3] Emodin has a molecular weight of 270.23 and a formula of C15H10O5. The melting point of emodin, which is typically orange powder, is between 256 and 257 °C. One of the main causes of emodin's broad biological action is the numerous hydroxyl and carbonyl groups in its structure that can chelate with metal ions in biological target enzymes to form comparatively stable chelates. DNA can be bound by emodin in a reversible manner. Its aromatic ring plane structure can mix with DNA, interfere with the function of DNA binding enzymes like DNA topoisomerase and DNA polymerase, and be embedded and superimposed between the double helix base pairs of DNAS.  However, emodin's water solubility and bioavailability can be enhanced by chemically altering its structure by adding hydrophilic groups like -OH and -NH-. Emodin compounds with various amine substitutions at four locations showed greater antitumor fine activity than emodin itself, according to a 2004 study by Teich et al. on the production and action of emodin derivatives. By adding quaternary ammonium salts to emodin's sites 6 and 3, Shao et al. created a number of quaternary ammonium derivatives of emodin. They then tested the compounds' biological activities and discovered that the derivatives had low toxicity against the HELF normal cell line 33 but strong anti-proliferation ability against the HepG2 GC-823 AGS cell line.  Emodin sensitizes HER/neu-overexpressing cancer cells to chemotherapeutic agents such as cisplatin, doxorubicin, etoposide, and paclitaxel. It belongs to the anthraquinone group along with daunorubicin (II) and mitoxantrone (III), which constitute some of the most powerful cytostatics. [2]

According to a recent study, the inhibition rate of HT-29 and HeLa cells was considerably boosted by the new semisynthetic anthraquinone derivatives with the NαFmoc-l-Lys and ethynyl group NαFmoc-l-Lys generated from emodin. Emodin's pharmacological activity and bioavailability can potentially be enhanced via pharmaceutical dose form modification. Di et al. increased the bioavailability of emodin by using piperine as a bioenhancer to prevent its glucuronidation in the colon and liver.
Arginine-glycine-aspartic acid (RGD) modified emodin to create a targeted liposome that can effectively block the creation of vasculogenic mimicry (VM) channels and metastasis in breast cancer tumors, hence enhancing emodin's anticancer activity.  According to a recent study, a transfer form of nano-emodin—a novel sonar-responsive nanomaterial—was created to improve nanoparticle accumulation and penetration and may be a useful treatment for head and neck squamous cell cancer (HNSCC). Additionally, Krajnovi­c et al. transported emodin using mesoporous silica, which can guarantee emodin release in the stomach's highly acidic environment, stop emodin from photodecomposition, and enhance emodin's anti-proliferation and pro-apoptotic effects on a range of tumor cells. [4]

2. Extraction of Emodin:

The separation of emodin was achieved using a High-Performance Liquid Chromatography (HPLC) system with a specific column, mobile phases, and gradient elution program.

1. 1. Preparation of Crude Extract for HPLC Loading

 Solution Preparation

• Dissolve 1 g dried methanolic extract in 5 mL methanol
• Sonicate for 10 min to ensure complete dissolution.
• Filter through 0.45 µm PTFE membrane filter to remove insolubles.

 Preparative HPLC System for Physical Separation of emodin

To collect pure emodin, use a prep/semi-prep HPLC with the following:

 Chromatographic System

Table: 2

Use the same chromatographic conditions as analytical HPLC (scaled for prep use).

Mobile Phase and Gradient Program:

Mobile Phase: A = Methanol (HPLC grade)       B = 2% acetic acid in water

Gradient Program (same as article)

Flow rate (prep scale): 8–10 mL/min (analytical was 1 mL/min), Column Temp: 30°C, Detection wavelength: 254 nm

Injection volume: 50–200 µL depending on column size

Separation and Fraction Collection Procedure

Sample Injection: Inject filtered crude extract into preparative HPLC. Monitor chromatogram at 254 nm.

Identification of Target Peak: According to article

Collect fractions corresponding to ~25 min retention time = Emodin.

Fraction Collection: Program automatic fraction collector to collect eluate from minute 24 to 27 (adjust based on real retention time). Each collected fraction contains emodin + mobile phase

 Recovery of Pure Emodin from Fractions

Pooling: Combine all fractions containing the emodin peak (verified by analytical HPLC).

Solvent Removal: Concentrate pooled fractions using rotary evaporator at 40°C. Remove remaining solvent under nitrogen stream or vacuum desiccator. Drying: Dry the residue to constant weight. Observed appearance: yellow-orange crystalline powder.

Confirmation of Purity: Analytical HPLC Check: Inject 10 µL of isolated compound on analytical C18 column (same gradient). A single sharp peak at Rt ~25 min confirms purity (>95%).[6]

3.2 Column chromatography (isolation of emodin):

Column chromatography (emodin isolation): Silica gel (60–120 mesh, for example) is the stationary phase.
• Apply a slurry or pre-adsorb 1–5 g of fraction to the silica.
• Hexane → Hexane:EtOAc (9:1 → 7:3 → 1:1) → EtOAc → EtOAc:MeOH (95:5 → 9:1) OR CHCl3 → CHCl3: MeOH (99:1 → 95:5 → 9:1) are the gradient elutes.
• Gather fractions (20–50 mL each) and use TLC to monitor them (see with UV 254/366 nm; anthraquinones can also be seen with anisaldehyde/NP-PEG spray). In CHCl3: MeOH solvent systems, emodin displays a distinctive Rf and usually fluoresces orange or red when exposed to UV light. [7]

3.3 Supercritical CO₂ extraction of emodin:

Emodin and were effectively extracted using supercritical CO2 + ethanol modifier following consistent design-sequential adjustment of the extraction conditions. The concentration of the ethanol modifier was shown to be the primary factor in the successful extraction of the emodin. Emodin yields were 0.616 and 0.178 g/100 g, respectively, at the ideal extraction conditions of 20 MPa, 30°C, and 95% ethanol. [8]

3.4 Microwave-assisted, aqueous two-phase extraction:

To obtain effective ingredient emodin, aqueous two-phase extraction with microwave assistance was studied. In comparison to microwave-assisted extraction and heat reflux extraction, an aqueous two-phase solution comprising 25% (w/w) ethanol and 21% (w/w) (NH₄)₂SO₄ produced identical quantities of piceid and 1.1and 1.9 times higher yields of resveratrol and emodin, respectively. To get greater yields at a lower cost, three distinct processes—extraction, clarity, and concentration—are combined into a single phase. Therefore, this approach has the potential to be helpful for the extraction and purification of target products.[9]

3.5. Ultrasound-assisted extraction:

UAE) has been used to extract components from a variety of sources and is thought to be preferable for natural product extraction. Its primary benefits over conventional extraction techniques are its greater efficiency and reduced cost [10]. The foundation of UAE is the idea of acoustic cavitation, which can weaken the plant matrix's cell walls and promote the release of bioactive substances [11]. Phenolic compounds are one prominent example of the several phytochemicals that can be obtained with this approach. [12].
Nevertheless, it is still difficult to improve compound yields by optimizing extraction conditions. In this work, we used response surface methodology (RSM) to examine the ideal extraction conditions for UAE of emodin. RSM, which was first presented by Box and Wilson in 1951 [13] is a set of mathematical and statistical methods that have been effectively applied to create, enhance, and optimize processes. [14,15].

Fig:2 ultrasound assisted extraction

3. Derivatives of emodin with synthetic pharmacophore:
   1. Emodin derivatives with amino acid.

In this work, a novel series of emodin derivatives were designed and synthesized by binding emodin to an amino acid using linkers of different lengths and compositions. The anti-proliferative properties of these derivatives were assessed using human normal liver L02 cells, human hepatic carcinoma HepG2 cells, and human breast cancer MCF-7 cells.
 A unique set of emodin derivatives connected to amino acids was created. the methods used to synthesize these emodin derivatives. Compound 1 was alkylated with 2-iodoethanol and Cs2CO3 in DMF to produce compound 2, which was then used to synthesize compounds 3a–3z. Condensation products were then produced through coupling reactions between compound 2 and other commercially available N-Boc protected amino acids under the standard coupling condition (DCC/DMAP). After removal of the protecting group using 20% TFA in DCM, the target compounds 3a–3z were obtained as TFA salts.[16]

Scheme 1. Synthesis of compounds 3a–3z. Reagents and conditions: (a) 2-iodoethanol, Cs₂CO₃, DMF, 60 °C, 65%; (b) (i) various N-Boc amino acids, DCC, DCM, 0 °C; (ii) 20% TFA in DCM, r.t., 20%–50% over two steps.

Scheme 2. Synthesis of compounds 4a, 4b, 5a and 5b. Reagents and conditions: (a) (i) Boc-N-Me-R/S-Ala-OH, DCC, DCM, 0 °C; (ii) 20% TFA in DCM, r.t., 40%–50% over two steps; (b) formaldehyde (37% aqueous solution), NaBH₃CN, MeOH, RT, 85%–89

1. 2.  Emodin Derivatives with halogen group

Due to its low potency, poor water solubility, and possible hepatotoxicity and nephrotoxicity, emodin, a component of traditional Chinese medicine with an anthraquinone structure, exhibits a variety of biological activities but faces clinical limitations. As a result, structural modifications are of great interest. This review examines the biological activities of emodin derivatives, emphasizing the advancements in structural modification research.[17]

Table: 3

1. 3. Emodin with polyamine derivatives:

Emodin has been shown to prevent the SARS-CoV-2 virus from entering human cells and may also prevent the creation of cytokines, which would lessen the damage that SARS-CoV-2 causes to the lungs. The pharmacophore linked to emodin was either a diphenylmethylpiperazine derivative of the norchlorcyclizine series (emoxyzine series) or a polyamine residue (emodin-PA series), which was chosen due to the fact that natural alkyl PA like spermine and spermidine play regulatory roles in immune cell functions. Diphenylmethylpiperazine antagonists of the H1 histamine receptor actually exhibit activity against a number of viruses through a variety of interconnected pathways. With an EC50 value of 1.9 μM, which is comparable to that of the reference medication remdesivir, (R)-emoxyzine-2 was the most effective medication in the emoxyzine series against SARS-CoV-2.[18]

Scheme1: Emodin and polyamine pharmacophores, or emoxyzine and emodin-PA hybrid compounds with emodin and norchlorcyclizine, respectively.

1. 4.  Emodin Derivatives as Multi-Target-Directed Ligands Blocking Vasopressin V1A Receptors and Monoamine Oxidase

The pathology of many neurodegenerative diseases is linked to the level of brain neurotransmitters. Age-dependent increases in blood vasopressin levels, oxidative stress, human brain monoamine oxidase (hMAO) levels, and imbalances in aminergic signaling are common disease-modifying factors that cause a variety of neurodegenerative disorders. In this investigation, we synthesized six emodin derivatives and assessed their effects on MAO activity and G protein-coupled receptors based on reports of emodin's hMAO suppression and antagonist impact on the vasopressin V1A receptor. Among these, 2-hydroxyemodin and 5-hydroxyemodin were effective V1AR antagonists and 4-hydroxyemodin and 5-hydroxyemodin were strong hMAO inhibitors.  The in vitro impact was confirmed by in silico molecular docking modeling, which showed that the hydroxyl groups at C2, C4, and C5 of the corresponding compounds interacted with prime residues. It was also anticipated that these three compounds would have favorable drug-like qualities. Overall, our work shows that emodin's hydroxyl derivatives are multi-target-directed ligands that could serve as leads for the creation of a treatment for illnesses of the central nervous system. [19]

1. 5.  Emodin pyrazole derivative:

By adding a pyrazole ring to the anthraquinone chromophore and then attaching different cationic amino side chains to the pyrazole ring, this work attempts to create a class of novel emodin derivatives with enhanced DNA binding affinity and anticancer activity. Three types of tumor cell lines—mouse melanoma B16, human hepatocellular carcinoma HepG2, and Lewis lung carcinoma (LLC) cells—were used to measure the cytotoxicity of the novel anthrapyrazoles derived from emodin and their DNA binding ability based on interaction with calf thymus (CT) DNA. There was discussion of the links between molecular structure and bioactivity. The compounds with a mono-cationic alkyl side chain showed the greatest cytotoxicity and affinity for binding DNA. Under the given circumstances, the initial molecule emodin 1 was methylated to the equivalent di-O-methyl emodin 2. 6, 8-O-dimethyl emodin 2 and a very tiny quantity of 3, 8-O-dimethyl isomer were the main products of the methylation of 1. Then, flash chromatography and crystallization were used to purify 6, 8-O-dimethyl emodin.[20]

1. 6.  Thioether derivatives containing 1,3,4-oxadiazole/thiadiazole and emodin moieties

A number of new thioether derivatives with emodin and 1,3,4-oxadiazole/thiadiazole moieties were created. 1H NMR, 13C NMR, infrared, and elemental analysis were used to confirm the target compounds' structures. The majority of the target compounds had moderate to good antiviral activity against tobacco mosaic virus at a concentration of 500 mg/L, according to the bioactivity assay results. Y2, Y8, and Y10 in particular shown significant curative action in vivo, with inhibition rates of 50.51, 52.08, and 54.62%, respectively, comparable to Ningnanmycin's (53.40%). [21]

Target compound shows good antiviral activity

4. Pharmacological Uses:

5.1 Lung cancer

Lung cancer continues to be the primary cause of cancer-related mortality and is currently the most commonly diagnosed malignant cancer type. A study by Zhang and Hung revealed that emodin has strong anti-proliferative activity and may also reverse the drug resistance of HER-2/neu-overexpressing lung cancer cells to chemotherapeutic drugs (cisplatin, doxorubicin, or VP16) by inhibiting the protein tyrosine kinase. There is currently growing scientific evidence that emodin exhibits potential anticancer effects against lung cancers in vivo and in vitro through inhibition of cell proliferation and cell cycle arrest, as well as an increase in ROS. [4]

                        Fig:3

MTA3

Heart failure has been found to be caused by cardiac fibrosis. Emodin can alleviate fibrosis in multiple tissues. Consequently, the positive benefits and possible mechanisms of emodin on cardiac fibrosis were assessed. First, it was found that emodin can lessen TAC-induced cardiac fibrosis and Ang II-induced cardiac fibroblast activation. The emodin/MTA3 axis was then discovered to be essential in controlling cardiac fibrosis. This is in line with MTA3's known function in controlling a number of cellular processes. As a transcription factor, MTA3 suppresses cancer by preventing cancer cells from proliferating, invading, and migrating. Furthermore, MTA3 has an anti-fibrosis action. research revealed that emodin may reduce myocardial fibrosis through molecular pathways including the overexpression of MTA3.[22]

5.3 Breast cancer

One of the most prevalent cancers in women and the second most common cause of cancer-related deaths globally is breast cancer (BC). Zhang et al. discovered that emodin might function as a tyrosine kinase inhibitor to stop HER-2/neu tyrosine kinase activity in MDA-MB453 cells, stop cancer cells from growing, cause lipid droplets to form, and encourage BC cells to differentiate into mature cells. Additionally, they discovered that emodin can prevent BC cells with HER-2/neu overexpression from transforming and spreading. Furthermore, emodin and paclitaxel together have been shown to reduce tumor drug resistance, raise tumor sensitivity to paclitaxel, and synergistically limit BC cell proliferation and survival. [4]

                     Fig:4