The therapeutic potential of curcumin and its related substances in turmeric: From raw material selection to application strategies

Abstract

Turmeric (Curcuma longa L.) is a medicinal plant used extensively in Chinese and Indian traditional medicine as a home remedy for various diseases. It has been used for medical purposes for centuries. Today, turmeric has become one of the most popular medicinal herbs, spices, and functional supplements worldwide. Curcuminoids are linear diary-lheptanoids from the rhizomes that include curcumin and two related compounds: demethoxycurcumin and bisdemethoxycurcumin, which are the active components of the C. longa plant, play a crucial role in numerous functions. This review summarises the composition of turmeric and the properties of curcumin regarding its antioxidant, anti-inflammatory, anti-diabetic, anti-colorectal cancer, and other physiological activity. In addition, the dilemma of the application of curcumin due to its low water solubility and bioavailability was discussed. Finally, this article provides three novel application strategies based on previous studies: using curcumin analogues and related substances, gut microbiota regulation, and using curcumin-loaded exosome vesicles and turmeric-derived exosome-like vesicles to overcome application limitations.

1. Introduction

Turmeric (Curcuma longa L.) is a medicinal plant that is used extensively in Chinese and Indian traditional medicine as a home remedy for various diseases. It is botanically related to ginger (Zingiberaceae family). It is a perennial plant with a short stem, large leaves, and rhizomes of various shapes that are often branched and have a brownish-yellow peel [1]. Today, turmeric is grown in several parts of the world, including Southeast Asia, China, and South America [2]. It has been used for medical purposes in India and China for many centuries to treat liver ailments [3] and other inflammation-related symptoms [4]. Turmeric has become one of the most popular medicinal herbs, spices, and functional supplements. The popularity of turmeric can be attributed to its pharmacological activity, including it antioxidant [5], antibacterial [6], anti-inflammatory [7], anti-tumour [8], and antiaging properties [9]. These functional activities of turmeric have been attributed to its rich curcuminoid content [1,2]. Curcuminoids (Fig. 1) are linear diary-lheptanoids from the rhizomes that include curcumin (CUR) and two related compounds: demethoxycurcumin (DMC) and bisdemethoxycurcumin (BDMC) [2]. Curcumin is a crystalline compound with a bright orange-yellow colour. It is often used as a food colouring and additive [2]. In the current international regulations, curcumin is considered a safe food additive. The World Health Organization (WHO) evaluated the acceptable daily intake (ADI) of curcumin as a food colouring additive in the range of 0–3 mg/kg [10]. The United States Food and Drug Administration (USFDA) has declared curcumin as ‘generally regarded as safe’ (GRAS). It is worth noting that most researchers consider curcumin to exhibit a wide variety of pharmacological properties and is relatively safe in animals and humans [11]; however, high doses (>50,000 ppm) ingested in order to overcome the low bioavailability of curcumin have some side effects and safety concerns [12]. Overall, if the bioavailability of curcumin can be effectively improved, it has the potential to improve or even cure diseases [13].

Fig. 1.

Curcuminoids found in Curcuma longa, curcumin metabolites (reduction), and its degradation products (cleavage).

2. Composition of turmeric

Curcumin (C₂₁H₂₀O₅) is also known as diferuloylmethane or (1E,6E )-1,7-bis(4-hydroxy-3-methoxyphenyl)hepta-1,6-diene-3,5-dione, DMC (C₂₀H₁₈O₅) is also known as (1E,6E )-1-(4-hydroxy-3-methoxyphenyl)-7-(4-hydroxyphenyl)hepta-1,6-diene-3,5-dione, and BDMC (C₁₉H₁₆O₄) is also known as (1E,6E )-1,7-bis(4-hydroxyphenyl)hepta-1,6-diene-3,5-dione. These three major curcuminoids are also found in other Curcuma species at different concentrations and proportions. Curcuminoids were only detected in a few species, such as C. longa and C. phaeocaulis, and geographical differences affect its content. In China, C. longa (Jianghuang) rhizomes from Pengzhou Sichuan contain the highest amount of curcuminoids (40.36 mg/g), which are almost 20-fold higher than those found in C. longa (Huangsi Yujin) collected from the same area [14]. In another study, C. longa rhizomes from the Zhengzhou pharmacies market exhibited a higher amount of curcuminoids (172.6 mg/g) [15]. Of note, curcuminoid content is related to geographical location; in Nepal, turmeric that is cultivated in warmer climates (Southern Nepal) has higher curcuminoid content than turmeric samples from cooler climates (Northern Nepal) [16]. In India, samples collected from different geographical regions also have different curcuminoid content; the main reason is the different environmental conditions across the Indian subcontinent. The maximum and minimum amount of curcuminoids were found to be present in samples of turmeric from Erode (South province, 50.27 mg/g) and Surat (West province, 14.08 mg/g) [17]. In Brazil, different samples in the same area contained different amounts of curcuminoids in the rhizome, from 18.2 mg/g to 23.3 mg/g. The curcumin content in the two samples is not much different and is mainly due to the difference in the content of DMC and BDMC [18]. Different Curcuma species may have different curcuminoid content, curcuminoids, and xanthorrhizol (XNT) content, which can be used as a specific marker to differentiate Curcuma xanthorriza and C. longa [19]. The guidelines of the Taiwan Herbal Pharmacopeia [20] and The Pharmacopoeia of the People's Republic of China 2020 Edition [21], two official compendiums of Chinese drugs, state that medicinal turmeric must contain more than 1% curcumin, and almost all turmeric rhizome samples meet this specification. However, this guideline is limited to the rhizome of turmeric. The curcumin content of the root is much lower than that of the rhizome [14]; therefore, the plant localisation or extract of medicinal turmeric must be considered for a standardised dose of curcumin [22]. Furthermore, ingested turmeric products may contain metabolites and degradation products of curcumin. Curcumin will easily degrade in neutral to alkaline solutions and is cleaved into ferulic acid, feruloyl methane, and vanillin (Fig. 1) [23]. Tetrahydrocurcumin (THC), hexahydrocurcumin (HHC), and octahydrocurcumin (OHC) (Fig. 1) are hydrogenated derivatives of curcumin [24] that appear as metabolites after curcumin is metabolised by cells or animals. Because of their structure, they are functionally closely related to curcumin [25]. In addition to the extraction and isolation of curcuminoids from turmeric, chemical synthesis is a strategy for obtaining curcuminoids [26,27]. The earliest record of chemical synthesis of curcumin was in 1913 [28]; since then, many studies have explored preparation methods of curcumin by chemical synthesis [26,29,30]. Unlike curcumin extracted from turmeric, which often contains a variety of curcuminoids, chemical synthesis can prepare specific, high-purity curcuminoids by adjusting conditions and precursors. This makes it possible to examine the chemical structure and mechanism of action of curcuminoids more precisely when discussing the function of curcuminoids.

3. Function of curcumin and curcuminoids

3.1. Antioxidant

As cells grow, oxygen consumption leads to the production of reactive oxygen species (ROS) [31]. ROS are forms of activated oxygen that include free radicals, such as superoxide anion radicals ( ${\text{O}}_2^{·-}$), hydroxyl radicals (^•OH), and non-free-radical species, such as H₂O₂ and singlet oxygen (¹O₂) [32]. ROS in cells cause the peroxidation of membrane lipids, leading to lipid peroxides. Furthermore, ROS can damage intracellular chemical composition, such as nucleic acids, lipids, proteins, and carbohydrates [25], which affect the inner workings of the cell. Generally speaking, antioxidant systems (e.g., the glutathione cycle and superoxide dismutase) in cells scavenge ROS and free radicals [33]; however, as the oxidative pressure increases, these systems may be overloaded and become ineffective. It has been confirmed that ROS production is directly related to many diseases [34]. Dietary antioxidants can protect the human body from free radicals and the effects of ROS. The curcuminoid structure contains several functional groups, including the β-diketone group, carbon–carbon double bonds, and phenyl rings containing hydroxyl and methoxy substituents [35]. Interestingly, the demethoxy and hydrogenated derivatives of curcumin, such as THC, HHC, and OHC, were remarkably more potent than curcumin in in vitro antioxidant assays [24]. Due to the loss of the ortho-methoxyphenolic group, BDMC does not have hydrogen donating activity [36].

3.2. Anti-inflammatory

Inducible nitric oxide synthase (iNOS) is one of the critical enzymes producing nitric oxide (NO) from the amino acid L-arginine, and iNOS-derived NO plays a crucial role in blood pressure regulation, inflammation, infection, and the progression of malignant diseases [37]. In macrophages, iNOS serves as a mediator of non-specific host defence and plays an essential role in clearing bacterial, viral, fungal, and parasitic infections [38]. When macrophages are stimulated, iNOS produces NO, which regulates blood pressure and has antibacterial activity. However, when the iNOS-derived NO is overproduced, it causes excessive expansion of blood vessels and tissue damage [39]. In this case, inducible haem oxygenase 1 (HO-1) plays a regulatory role as a rate-limiting enzyme; it catalyses the metabolism of haem into bilirubin, carbon monoxide (CO), and iron ions to regulate iNOS-mediated production of NO [40,41]. HO-1 and CO suppress the expression of iNOS and NO production in activated macrophages by deactivating nuclear factor-kappa B (NF-κB) [38,40,42]. Surprisingly, curcumin has a similar function; it indirectly inhibits NO production via inhibition of iNOS through suppression of NF-κB [36,43,44]. The NO-scavenging activity of curcumin and its derivatives, curcumin and THC, is potent, and the IC₅₀ values are: curcumin ≈ THC > DMC > BDMC ≈ HHC > OHC [36]. Cyclooxygenase (COX) has two isozymes: COX-1 and COX-2. Its function is to catalyse the synthesis of prostaglandin from arachidonic acid. The role of COX-1 is to protect the gastric mucosa and maintain kidney function; the role of COX-2 is to promote prostacyclin (PGI₂) production, which causes inflammation [45]. Curcumin, DMC, and BDMC are reported to inhibit COX-1 and COX-2 (32.0%, 38.5%, and 39.2% COX-1 inhibitory activity at a dose of 125 μg/mL; 89.7%, 82.0%, and 58.9% COX-2 inhibitory activity at a dose of 125 μg/mL, respectively). The COX-1 inhibitory effect of these compounds was slightly below that of the positive controls Aspirin (μg/mL), ibuprofen (2.06 μg/mL), and naproxen (2.52 μg/mL); however, they exhibited higher COX-II inhibitory activity than positive controls [45]. Furthermore, RAW264.7 macrophages treated with curcumin exhibited increased expression of HO-1 and decreased expression of iNOS protein and decreased iNOS promoter activity, which reduced NO production [38]. Moreover, curcumin can block lipopolysaccharide (LPS)-mediated expression of tumour necrosis factor α (TNF-α), interleukin (IL)-1β, and IL-6. Curcumin may alleviate inflammation through the nuclear factor-E2-related factor 2 (Nrf2)/HO-1 pathway [46]. Furthermore, the anti-inflammatory effect may be associated with activator protein-1 (AP-1) [47]. NF-κB and AP-1 are two transcription factor genes that are crucial to the LPS-induced inflammatory response and are commonly overexpressed in cancer cells [48]. In this situation, by inhibiting NF-κB, curcumin treatment results in the death of malignant cells and inhibits inflammation. Of note, one study suggested that the role of curcumin in the inflammatory system was like a pro-drug that requires oxidative activation to a reactive metabolite to exert anti-inflammatory effects [49]. Stably transfected RAW264.7 cells showed reduced luciferase activity expressed downstream of an NF-κB response element when treated with Curcuminoids and induced with LPS. Furthermore, NF-κB inhibition by curcumin and DMC involves the oxidation of these compounds into reactive electrophiles (Fig. 2) [50]. In contrast, BDMC is less likely to undergo spontaneous oxidative transformation due to the lack of ortho-methoxy groups that can accelerate the oxidation of the phenolic hydroxyl [51]. This also explains why curcumin and DMC have similar inhibitory pathways [36,45], while BDMC is different. In view of the lack of uncertainty of curcumin oxidative in vivo, the anti-inflammatory potential of oxidation-independent BDMC may be a new research direction.

Fig. 2.

Mechanistic differences in the inhibition of NF-κB by curcumin, DMC, and BDMC.

3.3. Anti-diabetes

Diabetes is a chronic metabolic disease. According to the guidelines published by the American Diabetes Association in 2021, diabetes is diagnosed under one of the following conditions: glycosylated haemoglobin (HbA1c) −6.5%, fasting plasma glucose −126 mg/dL, or 2-h plasma glucose −200 mg/dL, or random blood glucose −200 mg/dL [52]. Diabetes can be categorised into four types: insulin-dependent type 1 diabetes, non-insulin-dependent type 2 diabetes, gestational diabetes, and diabetes caused by other factors. Among these types, type 2 diabetes accounts for most cases of diabetes. In addition, due to the functional loss of blood sugar utilisation, diabetes often causes dyslipidaemia, which promotes mitochondria to produce ROS and causes macrophages to produce pro-inflammatory factors, such as iNOS and ROS, eventually leading to complications, such as heart, kidney, and liver disease [53]. Incretin is a collective term for peptide hormones that act on pancreatic β cells to stimulate insulin secretion [54]. Research has discovered that curcumin can inhibit gluconeogenesis enzymes glucose-6-phosphatase (G6Pase), phosphoenolpyruvate carboxykinase (PEPUC), and adenosine 5′-monophosphate-activated protein (AMP) kinase activity to suppress liver gluconeogenesis and glycolysis. Moreover, curcumin can increase the effect of insulin in inhibiting glycolysis [55]. In an animal model, treatment with 15 mg/kg and 30 mg/kg curcumin significantly reduced blood sugar, vasoconstrictor pressure, proteinuria, polyuria, serum creatinine, and blood urea nitrogen (BUN) in male diabetic Sprague–Dawley rats; furthermore, it reduced the production of kidney lipid peroxidation products (malondialdehyde) and increased the activity of antioxidant enzymes glutathione, catalase, and superoxide dismutase [56]. Recent research highlighted that curcumin could significantly increase GLP-1 secretion in GLUTag cells [57]. The secretion of GLP-1 requires a β-diketone structure and an aromatic ring with at least one methoxy group; curcumin has a β-diketone structure with two methoxy groups, making it the most potent candidate. In contrast, BDMC, which lacks a methoxy group, and THC, which lacks the β-diketone structure, cannot stimulate GLP-1 secretion [57,58]. In a clinical trial, 240 subjects who met the pre-diabetes criteria were divided into control and placebo groups. After treatment with 1.5 g of curcuminoids daily for 12 months, there was a significant decrease in the subjects' diabetes indicators (glycated haemoglobin, fasting blood glucose, and glucose tolerance), improved pancreatic β-cell function, and increased performance of anti-inflammatory factor adiponectin [59]. Together, these data support the therapeutic potential of curcumin in controlling diabetes.

3.4. Anti-colorectal cancer

One of the basic concepts of cancer is the balance between cell proliferation and cell death [60]. When the apoptotic signals lose their function, cells proliferate out of control, leading to cancer [61]. Apoptotic signals are generated through two main pathways: in the intrinsic pathway, the mitochondrial membrane inhibits the expression of anti-apoptotic proteins Bcl-2 and Bcl-xL; in contrast, increasing the death receptors in the extrinsic apoptotic pathway triggers TNF-related apoptosis [62]. A study highlighted that curcumin disturbed the balance of mitochondrial membrane potential and increased the inhibition of Bcl-xL protein [63]. Furthermore, a study has revealed that curcumin significantly increases the expression of death receptor 5 (DR5) on both the mRNA and protein level [64]. Curcumin modulates tumour cell growth by regulating multiple cell signalling pathways. In addition to the cell survival pathway (Bcl-2, Bcl-xL) and death receptor pathway (DR4, DR5), there is the cell proliferation pathway (cyclin D1, c-myc), caspase activation pathway (caspase-8, -3, -9), tumour suppressor pathway (p53, p21), mitochondrial pathways, and protein kinase pathway (JNK, Akt, and AMPK) [65]. In the aforementioned pathways (Fig. 3), curcumin has shown significant anticancer effects in vitro and in vivo against several types of cancer, including prostate cancer, breast cancer, and colorectal cancer.

Fig. 3.

Mechanism of induction of apoptosis by curcumin in colorectal cancer.

Colorectal (colon) cancer is one of the leading tumours globally. It is considered among the third most common cancers worldwide, with an array of high morbidity and mortality, being the fourth highest cause death [66,67]. Unfortunately, although treatment is mainly based on surgical resection, many patients continue to have a high risk of tumour recurrence [68]. COX-2 overexpression is observed in up to 90% of sporadic colon cancers and 40% of colon adenomas [69]. Therefore, specific COX-2 inhibitors have been clinically studied as agents for colon cancer chemoprevention; however, there are still doubts about their safety as they may increase cardiovascular risk [70,71]. Studies have shown that administration of curcumin can reduce the levels of oxidative DNA adduct 3-(2-deoxy-β-dierythro-pentafuranosyl)-pyr[1,2-α]-purin-10(3H) one (M₁G) in malignant colorectal cells without changing the level of COX-2 protein (Table 1.) [72].

Table 1.

Effects of curcumin in colon cancer.

Model	Curcumin dose/concentration	Affected signalling pathways	Mechanism	Reference
Clinical trial	450, 1800, 3600 mg/day	Curcumin reduces the adenoma burden in patients with colorectal cancer.	M1G ↓ COX-2 -	[72]
In vivo  Female F344 rats	2% mixed with daily diet	Curcumin prevents colon cancer in rodent models.	M1G ↓ COX-2 ↓	[73]
In vitro  HCT116	10 μM	Curcumin regulates miR-21, tumour growth, invasion, and metastasis of	AP-1 binding ↓ miR-21 ↓ Pdcd4 ↑	[75]
In vitro  HCT116	10 μM	Curcumin induces cell cycle arrest and cellular senescence and down-regulates autophagosome	ATG5 protein ↓ Cleavage of PARP ↑ Cell viability ↓ p53 ↑	[76]
In vivo   Specific pathogen-free wild-type (WT) 129/SvEv mice Germ-freeIl10−/−mice	0.05% and 1% diets	Curcumin reduced or eliminated colonic tumour burden in AOM-induced colon cancer.	Tumour burden ↓ Colon weight/length ratio ↓ bacterial richness ↑	[78]
In vivo  Swiss-Webster male mice	60 mg/kg	Curcumin modulates polyamines synthesis, morphological changes, oxidative stress, and alters homeostasis and tumour development.	Arginase Activity ↓ ACF formation ↓ Nitrotyrosine ↑ formation↓ Apoptosis ↑ HES-1 ↑ TGF-β ↑ Lipid peroxides ↓	[77]
In vivo  HCT116 xenograft in nude mice	1 g/kg	Curcumin targets NF-κB to improve the response of radiation therapy.	Tumour regrowth ↓ Ki-67 proliferation index ↓ NF-κB activity ↓	[81]
In vitro  HCT116  HT29  SW620	25 μM	Curcumin inhibits the proliferation and post-irradiation clonogenic survival of multiple colo-rectal cancer cell lines.	NF-kB activity ↓ BCL-xL ↓ BCL-1 ↓ Cyclin D1 ↓ COX-2 ↓	[82]
In vitro  HCT116	20 μM	Curcumin enhances anti-proliferation and induces apoptosis in 5-FU treatment.	Caspase-8 ↑ Caspase-9 ↑ Caspase-3 ↑ Bax ↑ PARP ↑ BCL-xL ↓ Cyclin D1 ↓	[83]
In vitro  HCT116	20 μM	Curcumin enhances chemosensitivity in 5-FU treatment.	Cell growth ↓ Colonspheres ↓ Apoptosis ↑	[84]

Unlike in clinical studies, both M₁G and COX-2 protein are reduced in animal models following treatment with curcumin [73]. This outcome is hypothesised to be due to the ability of curcumin to inhibit COX-2, which is mainly caused by the reduction or conjugation of generated species [74]. In HCT 116 colorectal cancer cells treated with curcumin, the cell cycle was arrested in the G2/M phase via miR-21 gene regulation, which inhibited tumour tissue growth [75]. In another study, curcumin induced senescence and inhibited the growth of HCT116 colorectal cancer cells [76]. In an in vivo model, curcumin significantly reduced aberrant crypt foci (ACF) and iNOS expression and arginase activity in 1,2-dimethylhydrazine (DMH)-induced ACF mice [77]. Furthermore, a curcumin-supplemented diet increased survival, decreased colon weight/length ratio, and decreased tumour burden in rats with AOM-induced colon cancer [78]. Although curcumin seems effective in preventing or treating colon cancer in vivo and in vitro, oral administration of curcumin results in its rapid metabolism, and approximately 60–70% of the compound is excreted in the faeces [67]. In clinical trials, quantifiable serum levels are not achieved until high doses are administered [69]. This observation can be attributed to the low bioavailability of curcumin. Because of the limited clinical effects of curcumin alone [79], current studies tends to increase the bioavailability of curcumin or administer it as an adjuvant treatment [80]. In an in vivo study in HCT116 xenograft nude mice, curcumin targeted NF-κB and improved the response of radiation therapy to colorectal cancer [81]. Another in vitro model study confirmed that curcumin prevented the proliferation and post-irradiation clonogenic survival of multiple colorectal cancer cell lines by suppressing radiation-induced NF-κB activation [82]. In addition to radiation therapy, curcumin combined with chemotherapy has been extensively studied. In a study evaluating a combination treatment regimen of 5-fluorouracil (5-FU) and curcumin in colorectal cancer cells, cell cycle analysis revealed that treatment with curcumin and 5-FU led to accumulation of colorectal cancer cells in the S cell cycle phase and induction of apoptosis [83]. In another study, when compared with treatment with 5-FU alone, pre-treatment with curcumin significantly enhanced the effect of 5-FU on colorectal cancer cells [84].

3.5. Other physiological activities

Curcumin has potential in the treatment of a variety of cancers in addition to colorectal cancer. Prostate cancer is the second most commonly diagnosed cancer in men, and ranks fifth as the leading cause of death globally [85]. The current mainstream treatment methods of localised and androgen-dependent prostate cancer (ADPC) include hormonal treatment, surgery, and radiotherapy. However, these cancerous cells progress to androgen-independent prostate cancer (AIPC) over time and are no longer dependent on hormones [86]. Research has shown that curcumin inhibits the proliferation of prostate cancer cells and induces apoptosis [87] by interfering with several cellular pathways, including mitogen-activated protein kinase (MAPK), epidermal growth factor receptor (EGFR), and NF-κB [86,88]. The dilemmas faced in the treatment of breast cancer and prostate cancer are similar. After lumpectomy, radiotherapy, chemotherapy, and endocrine therapy, the recurrence rate of breast cancer remains high [89]. A study evaluated the effects of curcumin on cell cycle regulatory proteins, matrix metalloproteinases (MMPs), and NF-κB in MDA-MB-231 and BT-483 breast cancer cells. The results indicated that curcumin exhibited antiproliferative activity by downregulating NF-κB [90]. Furthermore, curcumin has in vivo and in vitro effects in head and neck squamous cell carcinoma [91,92] and ovarian cancer through similar pathways [93,94].

In addition to the aforementioned functionalities, curcumin can reduce mouse oxidized protein, cytokine IL-1β, astrocyte skeleton protein glial fibrillary acidic protein (GFAP), insoluble and soluble amyloid β-amyloid expression, and inhibit microgliosis to prevent Alzheimer's disease [95]. Furthermore, curcumin has in vivo antibacterial activity that can prevent and restore damage to the stomach caused by Helicobacter pylori [96]. Recent studies have reported the antiparasitic properties of curcuminoids (DMC and BDMC), which inhibited TGR activity, giving them the ability to fight Taenia crassiceps cysticercosis. Moreover, curcuminoids (including curcumin, DMC, and BDMC) can effectively inhibit neuraminidase activity to inhibit influenza viruses H1N1 and H9N2. Of note, several studies have stated that curcumin has the potential to treat severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection [97,98]. In addition, studies have mentioned that curcumin can inhibit autoimmune deficiency syndrome (AIDS), Parkinson's disease, allergies, cardiovascular diseases, and other physiological activities [11,99–102].

4. Application dilemma

Although curcumin has been shown to exhibit therapeutic and protective effects in various diseases, studies have shown that oral administration of 8 g/day of curcumin will only results in a plasma concentration of about 2.5 ng/mL [103]. This is due to the low water solubility and bioavailability of curcumin, and it is also degraded by the digestive system or converted into metabolites [104]. In general, higher doses may be more effective, but further consideration should be given to potential side effects.

Turmeric and curcumin have been used for decades as food additives, supplements, and medicines. According to reports by the Joint United Nations and World Health Organization Expert Committee on Food Additives (JECFA) and European Food Safety Authority (EFSA), the ADI of curcumin is 0–3 mg/kg body weight [10,105,106]. A long-term curcumin carcinogenicity study by the National Toxicology Program (NTP) revealed that the incidence of malignant neoplastic lesions (carcinomas) did not reach statistical significance and the observed effects were not dose-dependent, were in agreement with historical control values, and were not consistent across sexes and/or species; these observations eliminated concerns of genotoxicity [107]. Nevertheless, there are still some records of side effects in previous research. For example, diarrhoea, headache, rash, and yellow stool have been reported after oral administration of 500–12000 mg of curcumin; however, the same study stated that a daily intake of up to 12000 mg of curcumin has no harmful effects on individuals [108]. In another study, dose-limiting toxicity was not observed up to 3600 mg of curcumin. Still, patients reported two types of gastrointestinal adverse events, and a rise in serum alkaline phosphatase levels was also observed [109]. These concerns make administering high doses of curcumin less optimal. At present, the most studied strategy is the modification of curcumin and its delivery systems, using nanoparticles [110–112], micellization [113–115], and chemical conjugation [104,116–118]. The application of these technologies can improve the stability, solubility, in vivo absorption, biological activity, and safety of curcumin without increasing the dosage. Recent studies have begun to explore more strategies that can help improve the application of curcumin.

5. Novel application strategies

5.1. Curcumin analogues and related substances

The main functionalities of curcumin are derived from three reactive functional groups on the chemical structure, including one diketone moiety and two phenolic groups. These groups determine the biological activity of curcumin, including hydrogen donation reactions, reversible and irreversible nucleophilic addition reactions, hydrolysis, degradation, and enzymatic reactions [27]. Analogues or degradation products that are different from curcumin can also promote or reduce its functionality. Therefore, direct supplementation of curcumin analogues or degradation products may be a potential strategy to improve its effectiveness (Table 2).

Table 2.

Activities of curcumin analogues, metabolites, and degradation products.

Curcumin analogues/related substances	Bioactivity	Reference
Demethoxycurcumin (DMC)	• Suppressed migration and invasion in MDA-MB-231 human breast cancer cell line.	[120]
	• Induced Bcl-2-mediated G2/M arrest and apoptosis in human glioma U87 cells.	[121]
	• Exerted a cytostatic effect at G2/M in MCF-7 human breast tumour cells.	[122]
	• Inhibited cell proliferation and induced apoptosis in HER2-overexpressing bladder cancer cells.	[125]
	• Increased the sensitivity of cisplatin-resistant cancer cells.	[124]
	• The inhibition and degradation activity of bacteria.	[126,127]
Bisdemethoxycurcumin (BDMC)	• Cytotoxic against human ovarian cancer OVCAR-3 cells.	[128]
	• Prevented DMH-induced colon carcinogenesis.	[129]
	• Accelerated gastric ulcer healing.	[130]
	• Induced apoptotic cell death in Hep 3B cells.	[131]
	• Inhibited MCF-7 breast cancer cell proliferation.	[132]
	• Inhibited adipogenesis in 3T3-L1 preadipocytes and suppressed obesity.	[133]
	• Used as an antibacterial agent to relieve antibiotic resistance.	[134]
Tetrahydrocurcumin (THC)	• Ameliorated oxidative stress-induced renal injury.	[139]
	• Alleviated the oxidative stress caused by cholesterol intake.	[140]
	• Antioxidant and vascular protective effects in L-NAME-induced hypertension.	[141]
	• Ameliorated insulin resistance in fatty acid-induced hepatic steatosis.	[142]
	• Preventative effects on azoxymethane-induced colon carcinogenesis.	[143]
	• Anti-angiogenic effects on implanted hepatocellular carcinoma.	[144]
Ferulic acid	• Improved cardiovascular and kidney structure and function in hypertensive rats.	[148]
	• Attenuated acute renal injury induced by cisplatin.	[149]
	• Improved cognitive function.	[150–152]
	• Inhibited cell proliferation and invasion in HeLa and CaSki cervical cancer cells.	[153]
Vanillin	• Induced G0/G1 arrest and apoptosis in human HT-29 colon cancer cells.	[156]
	• Induced apoptosis in human hepatic carcinoma HepG2 and neuroblastoma SH-SY5Y cells.	[157]
	• Reduced apoptosis and exerted neuroprotective effects in rats with spinal cord injury.	[158]
	• Promoted early neurofunctional development in neonatal rats.	[159]
Calebin A	• Protected cells from beta-amyloid insult.	[161]
	• Inhibited cell growth and induced apoptosis in drug-resistant human gastric cells.	[162]
	• Regulated survival and inflammatory gene products, leading to inhibition of cell growth and chemosensitisation.	[160]
	• Suppressed NF-κB-mediated proliferation, invasion, and metastasis of human colorectal cancer cells.	[163]

5.1.1. Curcumin analogues

DMC and BDMC are the most abundant curcuminoids after curcumin, and research has shown that their functions may be better than curcumin [119]. For example, in the MDA-MB-231 human breast cancer cell line, DMC exhibited anti-invasive activity by modulating the expression of invasion-associated proteins [120]. In human glioma U87 cells, DMC bound more efficiently to the Bcl-2 putative active site and induced Bcl-2-mediated G2/M arrest and apoptosis [121]; similar results were observed in MCF-7 cells [122]. In HER2-overexpressing bladder cancer cells, it significantly suppressed the expression of HER2, preferentially inhibited cell proliferation, and induced apoptosis. In human cervical cancer HeLa cells, it suppressed migration and invasion via inhibition of NF-κB pathways [123]. Furthermore, some studies reported that DMC increased the sensitivity of cisplatin-resistant cancer cells, such as A549 human alveolar basal epithelial adenocarcinoma cells [124] and HER2-overexpressing bladder cancer cells [125]. In addition to its antagonistic effects on specific cancer cells, DMC has potential as an antimicrobial against pathogens such as Candida albicans ATCC 10231 [126] and methicillin-resist Staphylococcus aureus [127].

Compared with DMC, BDMC has two fewer methyl groups, which means it has better water solubility and bioavailability. The structure of BDMC is responsible for its loss of antioxidant activity [36]; however, it has an increased anti-inflammatory effect [50]. However, there appears to be a trend in other specific conditions. In human ovarian cancer OVCAR-3 cells, BDMC exhibited higher cytotoxic activity than curcumin and DMC against ovarian cancer cells [128]. In a DMH-induced carcinogenesis in vivo model, intragastric BDMC significantly reduced the number and size of tumours in the colon in addition to hepatic oxidative stress [129]. In a gastric ulcer model, BDMC suppressed iNOS-mediated inflammation and directly accelerated gastric ulcer healing [130]. In human liver cancer Hep3B cells, BDMC decreased cell viability and induced S phase arrest, DNA damage, and cell apoptosis [131]. In human breast cancer MCF-7 cells, BDMC increased the concentration of ROS in the cells, inhibiting cancer cell proliferation [132]. In addition to its anti-cancer effects, BDMC has anti-obesity effects; it inhibited adipogenesis in 3T3-L1 preadipocytes and suppressed obesity in a high-fat-diet-induced in vivo model [133]. Finally, BDMC has antimicrobial effects and may be a potential natural antibacterial agent to ameliorate antibiotic resistance [134]. DMC and BDMC are potent and are comparable to curcumin in many studies; however, their properties are more anti-inflammatory than purely anti-oxidative due to chemical structural factors, and both have better water solubility and bioavailability than curcumin [135]. These two compounds can be used as alternative strategies where curcumin fails.

THC is one of the primary metabolites of curcumin; it lacks an α,β-unsaturated carbonyl moiety [136] and has different spectral properties from curcumin, usually producing white crystals at room temperature. Due to its higher bioavailability and different biological activity and molecular mechanisms than curcumin, some studies suggest that it has more potential for development than curcumin [137]. THC is well known for its anti-oxidative activity. During the anti-oxidative process, the β-iketone moiety C–C bond will be cleaved and exhibit anti-oxidative activity [138]. THC can be used against ferric nitrilotriacetate (Fe-NTA)-induced oxidative renal damage in male ddY mice [139], and it can effectively alleviate oxidative stress in cholesterol-fed rabbits [140]. Furthermore, THC has shown antioxidative and vascular protective effects in L-NAME-induced hypertension in rats [141]. In fatty acid-induced hepatic steatosis, THC ameliorated insulin resistance in HepG2 cells [142]. Some studies have shown that THC has partial anticancer effects, such as preventing azoxymethane-induced colon carcinogenesis [143], and anti-angiogenic effects on implanted hepatocellular carcinoma in nude mice [144]. Overall, THC is more effective than curcumin in oxidative stress-related diseases due to its excellent antioxidant effect. Overall, the chemical structure of THC enhances its antioxidant effects; therefore, it can be more effective in oxidative stress-related diseases.

5.1.2. Curcumin related substances

Ferulic acid and vanillin were structurally identified as curcumin-derived radical reaction products [145], and both have higher water solubility as curcumin degradation products [146]. Although studies have highlighted that the two compounds may not be major curcumin degradation products [147], as long-studied and biologically active phenolic compounds, they may still retain some functional properties of turmeric after degradation. Research has confirmed that ferulic acid improves cardiovascular and kidney structure and function in hypertensive rats [148] and has the ability to attenuate acute renal injury induced by cisplatin [149]. Furthermore, studies have shown that ferulic acid improves cognitive function [150–152]. Regarding anti-cancer activity, ferulic acid can significantly inhibit cell proliferation and invasion in HeLa and CaSki, two cervical cancer cells [153]. Ferulic acid may not possess as effective biological activity as curcumin, but its use as an adjunct to curcumin can compensate for its mechanical deficiencies [150].

Although vanillin and ferulic acid have similar structures, vanillin is more remarkable in food applications. Like curcumin, it is a common flavour additive and is probably the most widely used flavouring agent for sweet foods. Furthermore, its antioxidant activity helps stabilise the oxidation and degradation of food components [154]. Vanillin and ferulic acid have similar biological activity, including antioxidant, anti-inflammatory, neuroprotective, and anticancer properties [155]. In human colon cancer HT-29 cells, vanillin arrests the cell cycle in G0/G1 phase and significantly increases apoptosis in the sub-G0 phase [156]; this effect has also been reported in human hepatic carcinoma and neuroblastoma cells [157]. Recent studies have found that, in addition to its neuroprotective effects [158], vanillin promotes early neurological development and improves hypoxic-ischaemic brain damage in neonatal rats [159]. More notably, due to its lack of toxicity in rats [162], vanillin is well-suited for curcumin application strategies.

Calebin A (4-[3-methoxy-4-hydroxyphenyl]-2-oxo-3-enebutanyl-3-[3-methoxy-4-hydroxyphenyl] propenoate) is a compound that had not been isolated or identified in turmeric until recently [160]. It was originally identified to protect cells from beta-amyloid insult [161]. Follow-up studies found that it can inhibit cell growth and induce apoptosis in drug-resistant human gastric carcinoma MDR cell line SGC7901/VINCRISTINE. It resulted in a reduction in S phase and G2/M phase arrest and modulated the activity of MAPK family members [162]. Studies have reported that calebin A inhibits the NF-κB activation pathway via interaction with p65 and enhances multiple cancer cell apoptosis [160]. In addition, one study reported that calebin A had anti-proinflammatory and anti-tumour activity in TNF-β-stimulated colorectal cancer cells [163] and enhanced the effect of the anticancer drug 5-FU [164]. Despite the sparsity of research on calebin A, according to the current literature, it has potential in cancer prevention and treatment.

In conclusion, the strategy of applying curcumin analogues and related substances to replace curcumin is based on its structural activity. Ideally, it increases bioavailability, leaving the active structure unchanged. According to the reported results, curcumin analogues and related substances have many different efficacy aspects and application potential.

5.2. Gut microbiota regulation

The gut microbiota is an entire population of microorganisms located in the gut [165]. It is associated with a variety of human diseases, including intestinal disorders, such as inflammatory bowel disease (IBD) [166] and irritable bowel syndrome (IBS) [167], and metabolic diseases, such as obesity and diabetes [168–170]. The bioavailability of curcumin is extremely poor; however, studies have pointed out that with administered orally, curcumin has preferential distribution and accumulation in the intestinal tract and can reach the level of biological activity [171]. In the intestinal tract, curcumin has a regulatory impact on the gut microbiota, influencing microbial abundance, variety, and composition [172]. Curcumin significantly increases the proportion of beneficial microbiota relative to pathogenic microbiota by increasing the abundance of Bifidobacterium, Lactobacillus, and butyrate-producing bacteria and reducing the abundance of Prevotellaceae, Coriobacteriaceae, enterobacteria, and Enterococcus. In addition to its anti-inflammatory and anti-colorectal cancer activity, these changes in the intestinal microbiota can explain the immunomodulatory and anti-hyperlipidaemic effects of curcumin [173]. Furthermore, the reduction of Prevotellaceae, Bacteroidaceae, and Rikenellaceae, which are often linked to the onset of systemic diseases [172], demonstrates the potential of curcumin as a dietary supplement. One study reported that curcumin dramatically shifted the overall structure of a high-fat diet, disrupted gut microbiota towards that of lean rats fed a normal diet, and altered the gut microbial composition [174]. Some studies suggest that this can be attributed to the curcumin promotes a shift from pathogenic to beneficial bacterial strains and further affect intestinal metabolisms, such as fatty acids [175,176] and bile acid [176,177].

In addition to directly regulating the intestinal microbiota, leading to changes in microbial richness, diversity, and composition, curcumin is bio-transformed into various metabolites, such as dihydrocurcumin (DHC), THC, and ferulic acid by intestinal microbiota via demethylation, hydroxylation, demethoxylation, and decomposition [175,178]. For example, studies have shown that Escherichia coli from the intestinal tract will sequentially convert curcumin into DHC and THC [179]. Take the THC mentioned in Section 5.1.1 as an example, the amount of THC and its conjugates (as sulfates and glucuronides) were higher in the liver and serum after dietary administration of either curcumin or THC compared to the amount of curcumin and its conjugates. Therefore, THC is more readily absorbed from the gastrointestinal tract than curcumin [139]. In addition to this, some studies have reported that many of these metabolites are more biologically active and bioavailable than curcumin [175,180,181]. The above research note that curcumin not only achieves health benefits by regulating gut microbiota, but also by being metabolized by these microorganisms to produce bioavailable metabolites with similar effects to curcumin, thus increasing health benefits [173].

5.3. Exosome vesicles

Previous studies on improving curcumin bioavailability have focused on chemical or microbial modification, binding phospholipids to form complexes, entrapment using liposomes, and nanoparticles as carriers [80,104,119,182–184]. Exosomes are extracellular microvesicles with a particle size between 30 nm and 150 nm that carry a large number of proteins, lipids, RNA, and DNA and can be used as intercellular messaging tools [182]. Because exosomes have the ability to shuttle in and out of cells, the use of exosomes as nano-drug carriers has potential for new therapeutic applications [185]. In addition, many studies have suggested that exosomes as drug carriers have the potential to overcome the technology-related limitations of traditional nanoparticles [186]. For example, exosomes have a longer circulating half-life, are more easily internalised by cells, and can be linked to one or more tumour-recognition ligands to enhance their targeting capabilities, which make exosomes ideal nanoparticle drug delivery vesicles [187]. Many studies have used exosomes to coat curcumin and deliver it to targeted cells via membrane fusion [182,188].

The coating of functional components with exosomes can be divided into two methods: active and passive. The passive method only requires co-culture with functional components and cells or purification of exosomes from the culture medium of the cell culture and mixing them so that functional components can diffuse into exosomes via concentration difference (Fig. 4) [189]. In 2010, a study combined exosomes from mouse lymphoma with curcumin to produce curcumin-coated exosomes [182]. Subsequent studies have confirmed that curcumin treatment of exosomes produced by chronic myelogenous leukaemia (CML) can attenuate their ability to promote angiogenesis and regulate endothelial barrier tissue, thereby affecting tumour progression [190]. Another study used curcumin to intervene in mouse brain endothelial cells (MBECs) to produce curcumin-containing exosomes. It was confirmed that curcumin could ameliorate oxidative stress during endothelial cell damage and regulate tightness. Expression of connexins (ZO-1, claudin-5, and occludin) and adhesion junction protein (VE-cadherin) and increase endothelial cell permeability [191]. In contrast, the active method is more complicated and involves techniques such as ultrasound, extrusion, membrane perforation, or repeated freezing and thawing [192].

Fig. 4.

Curcumin-loaded exosome vesicle preparation and its advantages.

Studies have shown that curcumin coating of exosomes effectively increases the concentration of curcumin in the plasma and improve bioavailability. Injection of 100 mg/kg body weight of curcumin in exosomes can reach the highest plasma concentration in 30 min (1250 ng/mL), which is 5–10-fold that of curcumin alone [182]. Other studies have confirmed that exosome coating effectively improves the solubility, stability, and bioavailability of curcumin [183,184]. In recent years, the exosome drug delivery system has been an emerging research field that is a promising and novel concept. Existing literature clarifies that exosome-coated curcumin has anti-inflammatory [182,184], endothelial protective [183,191], neuroprotective [193], and anti-cancer properties [194]. Turmeric-derived exosome-like vesicle-related research has begun to attract attention. Turmeric-derived nanoparticles (TDNPs) or turmeric-derived nanovesicles (TNVs) isolated from turmeric can effectively alleviate colitis. Research indicates that oral administration of TDNPs prevents colitis and promotes wound repair in colitis [195]. Meanwhile, another study reported that oral administration of TNVs restored the damaged gut barrier, modulated gut microbiota, reshaped the macrophage phenotype, then increased its anti-inflammatory effect [196]. Compared with nanoparticle delivery systems, TDNPs and TNVs are natural colon-targeting therapeutics that have the advantages of low toxicity and ease of large-scale production. Although there are very few related studies, this is a topic with great potential for future research based on the current results.

6. Conclusion

Curcumin, the active component of C. longa extract, has been extensively studied in recent decades. These studies have confirmed its antioxidant, anti-inflammatory, anti-diabetes, and anticancer effects. However, the application of curcumin has been restricted by its low water solubility, which results in low cellular uptake, poor oral bioavailability, and low chemical stability. These factors make its clinical effectiveness and in vivo efficacy lower than its in vitro activity. Structural modification, synergistic combination therapy, and drug delivery systems are currently the most common solutions that are proven to increase the bioavailability of curcumin and improve its effectiveness. With the development of innovative technologies, there are an increasing number of strategies to solve this problem. For example, after passing through the digestive system and undergoing microbial metabolism, curcumin may be metabolised into a more biologically active form, thereby affecting overall body functions and organs. Therefore, directly involving these metabolites or degradants may achieve more direct effects. In addition, curcuminoids and curcumin analogues provide similar biological activity but higher bioavailability because of their structural similarity to curcumin. We can also take advantage of the low bioavailability of curcumin and allow it to directly regulate gut microbiota, leading to changes in microbial richness, diversity, and composition. Finally, the application of an exosome vesicle delivery system, greatly improves the bioavailability of curcumin and provides the possibility of targeted therapy. Overall, these successful findings provide valuable information for the future study of curcumin. Furthermore, the potential function and development of curcumin should be established.

Funding Statement

This study was supported by the Ministry of Science and Technology [109-2320-B-002 -012 -MY3 and 110-2320-B-002 -019 -MY3].