Obstacles against the Marketing of Curcumin as a Drug

Among the extensive public and scientific interest in the use of phytochemicals to prevent or treat human diseases in recent years, natural compounds have been highly investigated to elucidate their therapeutic effect on chronic human diseases including cancer, cardiovascular disease, and neurodegenerative disease. Curcumin, an active principle of the perennial herb Curcuma longa, has attracted an increasing research interest over the last half-century due to its diversity of molecular targets, including transcription factors, enzymes, protein kinases, growth factors, inflammatory cytokines, receptors, and it’s interesting pharmacological activities. Despite that, the clinical effectiveness of the native curcumin is weak, owing to its low bioavailability and rapid metabolism. Preclinical data obtained from animal models and phase I clinical studies done in human volunteers confirmed a small amount of intestinal absorption, hepatic first pass effect, and some degree of intestinal metabolism, might explain its poor systemic availability when it is given via the oral route. During the last decade, researchers have attempted with new pharmaceutical methods such as nanoparticles, liposomes, micelles, solid dispersions, emulsions, and microspheres to improve the bioavailability of curcumin. As a result, a significant number of bioavailable curcumin-based formulations were introduced with a varying range of enhanced bioavailability. This manuscript critically reviews the available scientific evidence on the basic and clinical effects and molecular targets of curcumin. We also discuss its pharmacokinetic and problems for marketing curcumin as a drug.


Introduction and History
Natural ingredients have been used during human history for numerous purposes, including alimentary usages but, due to their pharmacological effects, it has grown in the years the scientific interest to use these compounds to prevent or treat human diseases. Therefore, recent years have witnessed a growing understanding of the effect of natural products as sources of new supplements and drugs. Curcumin is one of such compounds with a history that goes back to 5000 years before [1]. Turmeric derived from the rhizome of the plant Curcuma longa (C. longa) has been used by the Indian people for centuries with no known side effects, not only as a food color and flavor but also to treat a wide variety of illnesses [2]. Turmeric was stated in the writings of Marco Polo regarding his journey

Antimicrobial Effect of Curcumin
As shown in Table 1, there are some in vitro studies with different kinds of curcumin extracts and few number of clinical studies indicating the antimicrobial property of curcumin against organisms like bacteria, fungi, parasite and virus. Antimicrobial studies started with anti-parasitic effect of curcumin and the anti-bacterial characteristic has attracted the most attention.
The stability of FtsZ protofilaments as a vital factor for bacterial cytokinesis has been introduced as a drug target for the antibacterial agents. Inhibition of assembly dynamics of FtsZ in the Z-ring may suppress the bacterial cell proliferation as one of the probable curcumin antibacterial mechanisms of action [15]. Accumulated evidence indicated curcumin plays an inhibitory role against numerous viral infections. The mechanisms involved in antiviral effect of curcumin are either a direct interference of viral replication machinery or suppression of cellular signaling pathways essential for viral replication, such as phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT), nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) [16].
Studies indicating antimicrobial effects of curcumin in the last decade focused on the pharmacokinetics of curcumin rather than pharmacodynamics, reflecting the concerns of researchers in this area. There is evidence suggesting that mode of antimicrobial activity of curcumin depends on the properties of the delivery system [17]. For instance, in an attempt to compare the antimicrobial activity toward Escherichia coli of two curcumin formulations: methyl-β-cyclodextrin supramolecular inclusion complex and polyelectrolyte-coated monolithic nanoparticles, researchers showed that while curcumin-β-cyclodextrin complexes exhibited a potent bactericidal activity, the curcumin nanoparticles were bacteriostatic [17].

Antimicrobial Effect of Curcumin
As shown in Table 1, there are some in vitro studies with different kinds of curcumin extracts and few number of clinical studies indicating the antimicrobial property of curcumin against organisms like bacteria, fungi, parasite and virus. Antimicrobial studies started with anti-parasitic effect of curcumin and the anti-bacterial characteristic has attracted the most attention.
The stability of FtsZ protofilaments as a vital factor for bacterial cytokinesis has been introduced as a drug target for the antibacterial agents. Inhibition of assembly dynamics of FtsZ in the Z-ring may suppress the bacterial cell proliferation as one of the probable curcumin antibacterial mechanisms of action [15]. Accumulated evidence indicated curcumin plays an inhibitory role against numerous viral infections. The mechanisms involved in antiviral effect of curcumin are either a direct interference of viral replication machinery or suppression of cellular signaling pathways essential for viral replication, such as phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT), nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) [16].
Studies indicating antimicrobial effects of curcumin in the last decade focused on the pharmacokinetics of curcumin rather than pharmacodynamics, reflecting the concerns of researchers in this area. There is evidence suggesting that mode of antimicrobial activity of curcumin depends on the properties of the delivery system [17]. For instance, in an attempt to compare the antimicrobial activity toward Escherichia coli of two curcumin formulations: methyl-β-cyclodextrin supramolecular inclusion complex and polyelectrolyte-coated monolithic nanoparticles, researchers showed that while curcumin-β-cyclodextrin complexes exhibited a potent bactericidal activity, the curcumin nanoparticles were bacteriostatic [17].  [21] Turmeric extract antifungal activity against Candida albicans at 1 µg/mL Anti-fungal Candida albicans In vitro Roth GN et al. 1998 [22] Turmeric oil Anti-bacterial activity in 50-200 ppm Anti-bacterial In vitro Negi P.S et al.1999 [23] Curcumin cytotoxicity against leishmania in vitro. The LD50 = 37.6 µM

Anti-parasitic against leishmania
In vitro Koide T et al. 2002 [24] Curcumin extract Anti-fungal at 50-500 mg/L Anti-fungal In vitro Kim MK et al. 2003 [25] Curcumin In Table 1, there is an overview of the multipotent antimicrobial character of curcumin that would be useful for investigators to additional studies on curcumin's antimicrobial role against new microorganisms.

Gastrointestinal Effect of Curcumin
Similar to the antimicrobial studies, the gastrointestinal activity of curcumin has been tested in animal models and there are also few human clinical studies. Curcumin has been suggested as a remedy for liver and digestive diseases such as irritable bowel syndrome, colitis, Crohn's disease and bacterial and parasitic diseases. Data in Table 2 shows that most of the reports are related to liver protection and some limited studies indicate the intestine and colon protection properties of curcumin. On the other hand, some reports are about its beneficial effects in animal models recapitulating inflammatory bowel disease. In addition to these effects, in several animal studies, curcumin has been revealed to influence the composition of gut microbiome. For instance, adding the curcumin to a high-fat diet in animals, prevented the fat-induced changes of the gut microbiota [34].
Several lines of evidence support the hepatoprotective effect of curcumin. The findings in this area indicated that curcumin exerts notable protective and therapeutic effects in oxidative liver diseases through numerous molecular mechanisms including the pro-inflammatory cytokines and lipid perodixation products suppression, activation of PI3K/AKT and hepatic stellate cells and ameliorating cellular responses to oxidative stress [35]. As shown in Table 2, the data is mostly obtained from animal studies and further clinical studies are needed to clarify the mechanisms of curcumin in oxidative associated liver diseases.
Research on colitis in animal models indicate a significant decrease in pro-inflammatory biomarkers and modulation of inflammatory cells [36][37][38]. In a study, it has been strongly suggested that curcumin diminishes neutrophil recruitment to the inflammatory sites through interference in chemokine gradient formation in addition to the direct effect on neutrophil polarization, chemotaxis, and chemokinesis [38]. These mechanisms likely significantly contribute to the described protective and potent effect of curcumin in ulcerative colitis. In patients with ulcerative colitis, curcumin has been well-tolerated and led to ameliorate the symptoms and inflammatory markers in clinical trials [39,40].
Curcumin has been reported to be promising in the treatment of intestinal inflammatory diseases (IBD). It provides some beneficial effects on the microbiome, inhibition of toll-like receptors (TLR4)/NF-κB/activator protein1 (AP-1) signal transduction, alterations in cytokine profiles and immune cell maturation and differentiation. These are possible mechanisms contributing to IBD pathology thatare affected by curcumin to strengthen the intestinal barrier [41].
Effects of curcumin on the microbiome have been widely studied and it has been reported that these effects are different based on the disease characteristics. For instance, in a research on mice living in specific-pathogen-free conditions, curcumin reduced the microbial richness and diversity [42]. In another study on rat model of hepatic steatosis, administration of curcumin decreased species richness and diversity induced notable microbiota compositional changes compared to both high-fat diet and control groups. Also, it shifted the structure of the gut microbiota [43]. These microbiome changes inhibit intestinal inflammation. In fact, curcumin maintains short-chain fatty acid-producing bacteria, which acts to provide intestinal mucosal protection [44,45]. Table 2 shows some studies indicating the effect of curcuminoids and curcumin extract on different pathological conditions in the gastrointestinal system ranging from cell line to clinical studies.

Cardiovascular Protective Effect of Curcumin
The cardiovascular protective effects of curcumin have been extensively studied and its use as an adjuvant or therapeutic agent to alleviate cardiovascular disease and other vascular dysfunctions is currently being investigated. As shown in Table 3, the most evaluations on cardiovascular effects of curcumin have been made in the last decade and cardioprotective and anti-hyperlipidemia have more considered for curcumin. The majority of studies are in animal models using curcumin extract, while there are few assessments with nanoparticles.
Curcumin at a dose of 200 mg/kg, nine days (7 days before and 2 days following Adriamycin), significantly prevented adriamycin-induced cardiotoxicity. The mechanism of cardioprotective effects of curcumin is unclear and multiple mechanisms have been proposed in this area including (1) curcumin prevents lipid peroxidation through free radicals scavenging, leading to a blocking of the lipid chain reaction. (2) Curcumin exerts membrane-stabilizing effect, which is supported by the fact that it could prevent the ECG changes induced by adriamycin [58].
In diabetic patients with cardiovascular complications, curcumin has been shown to downregulate nitric oxide synthase (NOS) and reduce NO oxidation, which plays a key role in the pathogenesis of cardiovascular complications in diabetes [59]. In an animal study, researchers demonstrated that the myocardial tissue from diabetic rats exhibited increased levels of NOS enzyme mRNA as compared to control rats which was prevented by curcumin treatment showing a decrease in the oxidative DNA damage [60].
Cardiac hypertrophy is the heart's response to some variety stimuli such as workload or myocardial infarction that increase biomechanical stress. It is characterized by an increase in the size of the individual cardiac myocytes and enlargement of the myocardium and the whole heart [61]. This disorder threatens affected patients with progression to overt heart failure or sudden death.
Histone acetylation is one of the important control points for the regulation of genes in the hypertrophic myocardium [62]. Curcumin has been reported to be an inhibitor of histone acetyltransferases (HATs), p300-HAT, which perform the acetylation of histone tails [63], therefore, it may have an effect in the prevention of the cardiac hypertrophy and heart failure [64].
There is also limited evidence on anti-hyperchlostrolemia effect of curcumin. In an animal study, administration of curcumin extract had hypolipidemic effects in high cholesterol-induced hypercholesterolemic mice. The authors reported that a mixture of Nelumbo nucifera leaf (NL) and C. longa provided more advanced protection against high cholesterol diet-related lipid accumulation and liver dysfunction and may be a more effective functional food for the management of hypercholesterolemia [65]. In another animal study, curcumin has been found to have hypocholesterolemic effect on both normal and hypercholesterolemic rats and was more effective in hypercholesterolemic animals. The possible proposed mechanism for this effect was, interfering with intestinal cholesterol uptake, augmentation of the conversion of cholesterol into bile acids, and increasing the excretion of bile acids [66].
Despite the extensive research regarding the effect of curcumin on the cardiovascular diseases in animals, additional studies in humans on curcumin's cardiovascular role and mechanisms using new formulations exerting a high degree of bioavailability are required. Curcumin Curcumin (0.05-g/100-g diet) for 10 weeks Anti-hyperlipidemia: curcumin exhibits an obvious hypolipidemic effect by increasing plasma paraoxonase activity, ratios of high-density lipoprotein cholesterol to total cholesterol and of apo A-I to apo B, and hepatic fatty acid oxidation activity with simultaneous inhibition of hepatic fatty acid and cholesterol biosynthesis in high-fat-fed hamsters.

Anti-Cancer Effect of Curcumin
Numerous molecular targets have been proposed concerning the chemotherapeutical effect of curcumin against different types of cancer. Extensive studies, mainly in vitro or in animal models, indicate that it can modulate all kinds of cancer hallmarks, including uncontrolled cell proliferation, cancer-associated inflammation, cancer cell death, signaling pathways, cancer angiogenesis, and metastasis [81]. According to a report on curcumin in 2019, 37% of all researches on curcumin focus on its anti-cancer effect or relevant mechanisms. As shown in Table 4, effect of curcumin in different organ cancers have been widely studied, almost all studies have been done in vitro or in animal models. Based on these reports, curcumin, through different signaling pathways, represents a promising candidate as an effective anti-cancer agent to be used alone or in combination with other drugs. It affects molecular targets involved in the development of several cancers.
Several lines of evidence revealed that inflammatory pathways disorder plays a key role in cancer development [82]. Increase in the production of pro-inflammatory cytokines such as tumor necrosis factor alpha (TNF-α) and interleukins, transcription factors including nuclear factor κB (NF-κB), ROS, cyclooxygenase (COX-2), protein kinases B (AKT), activator protein 1 (AP1), signal transducer and activator of transcription 3 (STAT3), causing the initiation and development of cancer [83,84].
Curcumin exerts its immunomodulatory characteristics by interacting with above-mentioned immune mediators, hence its anti-cancer property.
There are few clinical trials for use of curcumin in cancers, either as a monotherapy or in combination with other anti-cancer agents. In a phase I clinical trial, curcumin was administered orally in 15 colorectal cancer patients. The findings reveal that there was not a significant toxicity except a diarrhea symptom reported in two patients, and two patients showed stable disease after two months of curcumin treatment [85]. Another monotherapy phase II clinical trial study of oral formulation of curcumin administered to 25 advanced pancreatic cancer patients showed low levels of curcumin in plasma (22-41 ng/mL) but two patients showed clinical biological activity. In one patient, it was reported disease stability for >18 months. In another patient, a brief but marked tumor regression (73%) was found [86].
There are some combination therapy studies using curcumin in combination with other chemotherapeutic agents used in standard treatments of cancer disease. In a clinical study, a combination therapy of curcumin with imatinib (tyrosine kinase inhibitor) has been evaluated in 50 chronic myeloid leukemia patients in which the synergic action of the two drugs was more efficient than imatinib alone, although additional studies are needed to confirm this efficacy [87]. Furthermore, a combination of curcumin with anti-EGFR (epidermal growth factor receptor) monoclonal antibodies in cutaneous squamous cell carcinoma patients has been shown as a highly effective strategy in disease control in another clinical [88]. Table 4 summarized some of studies indicating anti-cancer effects of curcumin in different organs in the last two decades.

Effect of Curcumin on Skin Diseases
Growing evidence suggests that curcumin may show an effective role in the treatment of several skin disorders [103] as shown in the provided in the table below. The studies imply on the anti-inflammatory effect of curcumin except those that show apoptotic effects of curcumin for cancer indication (Table 5). Study on the possible interactions between curcumin and other chemicals, commonly used in topical skin treatments, may provide useful insights for the development of new effective preparations, tailored for different conditions.

Neuroprotective Effect of Curcumin
Curcumin has several appropriate characteristics for a neuroprotective agent, including potent antioxidant, anti-inflammatory, and anti-protein-aggregate activities that have been previously reviewed [116,117]. Due to its above-mentioned pluripotent benefits, curcumin has countless potential for the prevention of many neurological conditions that existing therapeutics are less than optimal. These disorders include Parkinson's, Alzheimer's, Huntingtin's, brain injury, stroke, and aging [118]. Data in Table 6 show a summary of the main findings on the potential beneficial effects of curcumin against neurodegeneration, and principal attention has been given to Alzheimer's and Parkinson's disease (PD).
Alzheimer's disease (AD) is the most prevalent form of age-related dementia and currently, there are millions of AD patients in the world and this number is expected to increase dramatically with the demographic shift toward a more aged population, unless preventive processes would be achieved [119]. Classically, AD is characterized by the accumulation of amyloid and tau aggregation with the development of neurodegeneration and cognitive defects.
Curcumin has been tested in animal model of Alzheimer's disease in which not only reduced oxidative stress damage and inflammation markers, but it also mitigates amyloid plaques accumulation and cognitive deficits in rats [120,121]. The possible mechanism for curcumin protection against tau aggregates might be related to its antioxidant characteristics since the initial step for tau dimerization is driven also by oxidative damage, lipid peroxidation, or redox-regulated disulfides [118,122].
Evidence in recent years supports the efficacy of curcumin in PD. In both in vitro and in vivo models of PD curcumin could prevent oxidative stress toxicity by reducing the production of ROS and malondialdehyde and restoring GSH levels [123][124][125] which shields against alpha-synuclein-induced toxicity in the brain [123]. More specifically, antioxidative and anti-apoptotic activity of curcumin has been reported in in vitro studies, indicating the neuroprotective effect of curcumin on dopaminergic neurons [124,125].
More recently, we found that curcumin, at the concentration of 5 µM, protected neuroblastoma (SH-SY5Y) cells against H2O2-induced cell death by modulating of Small Ubiquitin-like Modifier (SUMO)-1-JNK(c-Jun N-terminal kinases)-TAU axis, indicating that curcumin might be a promising therapeutic agent against not only oxidative stress, but also pathologies characterized through SUMO, JNK, and Tau alterations in many neurodegenerative diseases [126].
Other mechanisms of the neuroprotective effect of curcumin are from oxidative damage by restoring mitochondrial membrane potential, upregulation of Cu-Zn superoxide dismutase, and inhibiting the production of intracellular ROS [127].
Although, in all these studies, authors are pleased with their findings, adapted from in vitro and animal models studies, more preclinical and clinical trials are needed to verify the exact effect of curcumin in neurodegenerative disease using different formulations.   Table 7 summarized some studies suggesting the efficiency of curcumin in eye diseases. All the studies were carried out on experimental models using curcumin except one in which researcher used nanoparticles. Curcumin beneficial effects have been proved for major eye diseases [150] such as glaucoma [151,152], age-related macular degeneration [153], diabetic retinopathy [154], cataract [155,156], corneal neovascularization [157], dry eye disease [158] and conjunctivitis [159]. In general, a wide variety of mechanisms have been raised for these effects of which antioxidative stress, anti-angiogenesis, anti-inflammatory and anti-apoptotic are more important [150].

Cellular and Molecular Targets of Curcumin
The cellular and molecular targets of curcumin have been summarized in Figure 2 [162][163][164][165]. It has been categorized based on the role of targets and the effect of curcumin on those. Curcumin clearly diminishes mRNA production of pro-inflammatory mediators, including cytokines and relative enzymes such as cyclooxygenase (COX)-2, and inducible nitric oxide synthase (iNOS) [166][167][168]. Apparently, this is due to it inhibitor effect of transcription factors such as activator protein (AP)-1 and nuclear factor (NF)-κB-mediated gene [143,169]; however, the direct molecular targets at low doses are not entirely clear.

Cellular and Molecular Targets of Curcumin
The cellular and molecular targets of curcumin have been summarized in Figure 2 [162][163][164][165]. It has been categorized based on the role of targets and the effect of curcumin on those. Curcumin clearly diminishes mRNA production of pro-inflammatory mediators, including cytokines and relative enzymes such as cyclooxygenase (COX)-2, and inducible nitric oxide synthase (iNOS) [166][167][168]. Apparently, this is due to it inhibitor effect of transcription factors such as activator protein (AP)-1 and nuclear factor (NF)-κB-mediated gene [143,169]; however, the direct molecular targets at low doses are not entirely clear. There are some controversies regarding the effect of curcumin on these factors. To our knowledge, some of these effects are probably dose-dependent. For example, curcumin increase and decrease the apoptosis proteins and markers depending on its dose or concentration. In fact, in order to clarify the exact mechanism, further investigations using a wide range of doses are needed to explore the dose-dependent effects of curcumin. This hypothesis has been supported by the results of previous studies for instance, curcumin at low doses (<100 nM) prevents AP-1 and NF-κB-mediated transcription, which might rely on the inhibition effect on histone acetylase (HAT) or activation of histone deacetylase (HDAC) activity [170] while at high doses (>3 µM) curcumin can act as an alkylating agent in a study on colon cancer [171]. In fact, some of these curcumin effects at high doses in vitro are obviously toxic and beyond its usage in cancer therapy.

Curcumin Metabolism and Degradation
Following oral administration of curcumin, it is metabolized extensively. It has been reported that the phase I reduction reaction and phase II conjugation reaction were the major metabolic pathways of curcumin in animals, and dihydrocurcumin, tetrahydrocurcumin, octahydrocurcumin, and hexahydrocurcumin were the main metabolites in phase I [172].
There is some evidence indicating that metabolites are the major contributors of the pharmacological activity of curcumin. For instance, dihydrocurcumin diminished lipid accumulation, oxidative stress, and insulin resistance in oleic acid-induced L02 and HepG2 cells [173].
Tetrahydrocurcumin, another metabolite of curcumin, has been reported to show a wide range of therapeutic properties [174,175]. The properties of this metabolite are comparable to those of curcumin. However, studies revealed that Tetrahydrocurcumin is more potent than curcumin as an anti-inflammatory, antioxidant, neuroprotective agent and anti-cancer [174]. Several lines of evidence demonstrated that this curcumin metabolite has higher antioxidant activity and upregulates the antioxidant enzymes in different pathological conditions including atherosclerosis [176], diabetes [177], hyperlipidemia [178], and neurotoxicity [179]. Having additional hydrogen molecules, Tetrahydrocurcumin is more hydrophilic than curcumin [180] and pharmacokinetic assessments reveal that it is more stable than curcumin in 0.1 M phosphate buffers at neutral pH and plasma [181]. Moreover, Its half-life in in vitro studies is significantly longer than that of curcumin [182]. Together, these properties suggest that THC may be more potent and efficacious against human diseases than curcumin, owing to its distinct chemical properties and stability. Recent studies showed that curcumin quickly degrades in aqueous buffer and some degradation compounds are produced [183] comprising alkaline hydrolysis products (ferulic acid, vanillin, ferulaldehyde, and feruloyl methane) which are formed through ahydroxyl ion (OH-)-mediated hydrolysis reaction [184] and autoxidation products (bicyclopentadione), are formed through a radical-mediated process [185]. Evidence suggests that curcumin mostly degrades via oxidation pathway rather than through hydrolysis [183].
Although some of these degradation products are biologically active, they are noticeably less-active in comparison to curcumin [183], supporting the idea that chemical degradation of curcumin has a limited contribution to its biological and pharmacological activities.

Curcumin Toxicity
In addition to the wide variety therapeutic effects of curcumin discussed above, there are some reports about its toxic potential that could be discussed from 2 aspects; first, even in the therapeutic doses its reactivity against a number of enzymes which act through different cellular mechanisms, provides adverse effects. For instance, its glutathione S-transferase (GST) inhibition can lead to impaired detoxification and potential toxic drug−drug contraindications [186] or the human ether-a-go-go-related gene (hERG) channel inhibition which may lead to cardiotoxicity [187,188]. Second, high doses of curcumin have been reported to be toxic for cells inducing apoptosis [189]. In some studies of therapeutic utility, it has been shown as cytotoxic against a number of important cancer cell lines as mentioned before or even cytotoxic against normal human lymphocytes [190] and noncancerous cell lines [191].
Despite these effects, curcumin is known to be safe by Food and Drug Administration (FDA). Several studies including preclinical and clinical evaluated the safety of this compound [192][193][194]. The maximum proposed dose of curcumin varies, ranging from daily consumption of 3 mg/kg to 10 g [195]. In a clinical study, no serious adverse effect was observed in any of the healthy subjects who used a daily dose of 12 g [196].
According to the literature, curcumin didn't show mutagenic and genotoxic effects and is safe in pregnant animals. However, more studies in humans are needed. In addition, animal studies didn't report reproductive toxicity of curcumin following oral administration. Curcumin was safe even at the high doses: in a phase I human trial 25 subjects used up to 8000 mg of curcumin per day for 3 months without showing toxicity. Five other human trials using 1125-2500 mg of curcumin per day have also found it to be safe [197] as well as in another study in which it was used 6 g/day orally for 4-7 weeks.
In addition, a good safety profile has been reported for curcumin in patients with cardiovascular risk factors or pre-malignant lesions of internal organs who took a dose of curcumin ranging from 500 to 8000 mg/day for a 3 months period [192,198]. This safety has been observed in patients with different cancers, including colorectal (taking, ranging from 36 to 180 mg/day for up to 4 months), breast (taking up to 6000 mg/day) and pancreatic cancer (taking 8000 mg/day of curcumin for 2 months) [86,199,200].
Having in mind that the majority of studies reporting curcumin safety has been performed for short periods of time so far and, there is not complete evidence regarding the consequences of chronic administration of this compound. Therefore, trials and more studies are needed specially on novel formulations and in long term use to find out all aspects of toxicity of curcumin.
However, only limited adverse effects such as gastrointestinal upsets have been stated in human. In addition, abdominal pain has also been reported followed by curcumin consumption at a dose of 8000 mg/day in patients with pancreatic cancer [201]. These gastrointestinal side effects might be related to the curcumin-induced COX inhibition and the subsequent inhibition of prostaglandin (PG) synthesis and/or its influence on the composition of the gut microbiome, which needs further study to be established. In addition to gastrointestinal side effects, a mild headache or nausea have been reported in few patients affected by primary sclerosing cholangitis taking curcumin in dose of up to 1400 mg/day [202].
Moreover, recent reports of liver diseases related to curcumin attracted the medical community's attention to its possible hepatotoxicity [203]. Whether this effect belongs to curcumin molecule or other possible contamination has to be elucidated.
Evaluating the other routes of administration, in a short-term intravenous dosing of liposomal formulation, curcumin has been shown to be safe up to a dose of 120 mg/m 2 on healthy subjects in a clinical trial, while in a dose escalation study in patients with metastatic cancer a dose of 300 mg/m 2 over 6 h reported to be the maximum tolerated dosage [204,205]. Besides, one case of hemolysis and one death associated with intravenous curcumin preparations were reported, indicating that regarding the safety of intravenous administration of curcumin, further studies and data are required [205][206][207].

Pharmacokinetic Deficiency of Curcumin and Current Attempts
As described previously, the major constituent of extracts of C. longa are called curcuminoids, which includes curcumin demethoxycurcumin, and bisdemethoxycurcumin, along with numerous and less abundant secondary metabolites [208]. According to our literature review, many in vitro studies have used synthetic curcumin, while most animal studies and clinical trials used the curcuminoids mixture. In fact, these compounds are completely different in pharmacokinetic parameters.
Gastrointestinal absorption and bioavailability are two most important and critical characteristics in the pharmacokinetics of any compound. From the preclinical and clinical studies, we have found that curcumin is poorly absorbed following oral administration. It has been reported that oral bioavailability of curcumin was only 1% and the highest amount of plasma concentration was 0.051 µg/mL from 12 g curcumin in human, 1.35 µg/mL from 2 g/kg in rat, and 0.22 µg/mL from 1 g/kg in mouse [196]. Pharmacokinetic assessments showed that there were just negligible quantities detected in liver and kidney (<20 µg/tissue) following curcumin oral administration. These studies indicated the nano-molar plasmatic concentration of this compound, with limited biological effects [209][210][211][212][213].
Several studies, including clinical trials, have been performed using curcumin in a wide variety of oral formulations. From low oral doses to high amount of 12 g/day have been given and tested [196]. Data shown in Table 8 summarize some of formulations and their pharmacokinetic properties. In order to increase curcumin bioavailability, different pharmaceutical strategies, including combination with adjuvant substances such as piperine, encapsulation, formulation in nanoparticles, liposome, micelles, nanomicellizing solid dispersion etc., have been proposed, shedding new light to overcome this pivotal limitation [214][215][216][217].
As presented below, there is a remarkable difference between the amount of plasma level of curcumin in different products. Interestingly, new formulations including nanoparticles, liposomes, and micelles exert a higher amount of C-max, the maximum (or peak) serum concentration that a drug achieves, compared to the other formulations (Table 8).
Based on the Nutraceutical Bioavailability Classification Scheme (NuBACS), curcumin exerts poor bio-accessibility, due to its low solubility in water and low stability [218].
It is well documented that water-solubility has a crucial role in oral absorption of compounds. Pharmacokinetics assessments revealed that curcumin is a poorly water-soluble agent (about 11 ng/mL) [219] and considerable efforts have been made to improve this characteristic.
Another important point that has to be taken into account in pharmacokinetic of curcumin, is its susceptibility to degradation, which has been shown to be pH-dependent. Indeed, under the alkaline conditions (pH > 7), curcumin degrades to Trans-6-(40-hydroxy-30-methoxyphenyl)-2, 4-dioxo-5-hexanal, ferulic acid, feruloylmethane, and vanillin within 30 min. while, in acidic medium, curcumin degradation is much slower, with less than 20% of total curcumin at 60 min [220,221].
Following oral administration, the major portion of curcumin is excreted through the feces while a small portion, which is absorbed within the intestine, rapidly metabolized in the livers and plasma [222]. Curcumin is extensively converted to its water-soluble metabolites (glucuronides and sulfates) and excreted through urine [222,223].
Curcumin also undergoes extensive hepatic first-pass metabolism through glucuronidation and sulfation, with the metabolites that exerts a remarkably lower biological function in comparison with parent curcumin [214]. Taking advantage of extensive research on curcumin metabolism, a curcuminconverting enzyme named "Nicotinamide adenine dinucleotide phosphate (NADPH)-dependent curcumin/dihydrocurcumin reductase" has been known which has purified from E. coli, clarifying the role of human intestinal microorganisms in the metabolism of curcumin in vivo [224].
The table provided below shows the studies on curcumin formulations, routes of administration, dose Plasma/tissue level (Cmax) and the time to maximum concentration (Tmax, min) in animals and humans. There is a significant variation in the doses used from the formulations included in this review as well as their Cmax and Tmax were really different.
Having two phenolic hydroxyls and one enolic hydroxyl group, which can form hydrogen bonds with phospholipids polar groups, in one formulation named Meriva, curcumin has complexed with phosphatidylcholine, and this structure protects it from degradation and augmenting the cellular uptake across lipophilic cell membranes by facilitated diffusion mechanism [225,226]. Taking advantage of this formulation, curcumin absorption and bioavailability were significantly increased compared to unformulated curcumin in a randomized, double-blind, study in human [225]. The Tmax has been reported 15 to 30 min, which showed a considerable increase in speed of absorption compared to the other formulations.
In another compound named LongVida ® , curcumin is included in a pharmaceutical form of solid lipid particle (SLP)-based formulation. This complex keeps the curcumin from rapid degradation and excretion so improving the systemic curcumin plasma concentration and half-life and eventually improved bioavailability compared to unformulated curcumin. In a clinical trial, oral administration of 650 mg in human exerts 22.43 ng/mL plasma level, which was shown to be higher than curcuma extract and unformulated curcumin [227].
There are also some micronized formulations for curcumin, which were reported to have 9-fold increased bioavailable than unformulated. The micronized curcumin has a smaller diameter of drug particles, which increases the surface area to drug ratio and finally augmentation in the dissolution rate that directly increase the bioavailability of curcumin [228]. Interestingly in a human study, administration of 500 mg of a micronized formulation exerts a considerable 0.60 µg/mL plasma level, which was a promising results for this compound [229].
One of the early and basic strategies for increasing the bioavailability of curcumin was its combination with other compounds that could change its characteristics; among them piperine was known to improve the oral absorption of curcumin in humans. It is a P-glycoprotein inhibitor and enhances the curcumin absorption in the intestine by reducing the efflux phenomenon [230].
P-glycoprotein (P-gp) is a drug transporter that effluxes drugs and other foreign substances out of cells from gastrointestinal tract, brain, liver, and kidney and exerts an important role in drug pharmacokinetics and pharmacoresistance. Piperine has been reported to enhance the bioavailability of curcumin, as a substrate of P-gp by at least 2000% [231].
Piperine also inhibits uridine diphosphate-glucuronosyltransferase (UGT) thus increasing the curcumin availability in the systemic circulation. As shown in Table 8, adding piperine 20 mg/kg to curcumin 2 g/kg in rats exerts a C-max of 1.55 µg/mL in animals following oral administration [231].
In another attempt, micellization technique was employed to improve the solubility of curcumin in a compound named NovaSol ® . In this formulation, curcumin incorporated in a nonionic surfactant Tween 80, caused the formation of liquid micelles, which increased dissolution and improved its absorption. This compound has been studied in a single-blind crossover study in healthy subjects and interestingly they showed a 185 folds higher than that of unformulated curcumin [232].
As previously described in this review, nanoparticle-based formulations have been extensively tried to improve the bioavailability and reduction of adverse drug reactions. In this regard, a colloidal nanoparticle dispersion of curcumin has been produced and its pharmacokinetic characteristic has been studied in humans. This formulation which is called Theracurmin™ has been showed to have relative bioavailability almost 16 time as compared to unformulated curcumin. In fact, the colloidal nanoparticle dispersion improves the solubility of curcumin and its oral bioavailability in healthy volunteers. In one study on this formulation, authors stated that two factors including improved solubility by adding a compound named "gum ghatti" and reduced particle size improved the clinical bioavailability of Theracurmin™ [233].

Conclusions and Future Prospects
Similar to any drug, curcumin still requires extensive preclinical and clinical assessments to clarify its pharmacokinetics, dosing, and potential toxicity, the issues that are of essential concern of drug companies.
According to the therapeutic effects that were summarized in this review, curcumin plays different roles through apposite mechanisms such as cytotoxic and cytoprotective role in different disease. For understanding this controversy and diversity of effects, it is crucial to pay attention to the doses and concentrations that have been used in different situations. It seems that curcumin's behavior changes in various doses and conditions, and this issue should be highly considered in clinical trials, especially in new formulations with higher bioavailability characteristics. Confirming this idea about the dose-dependent effect of curcumin, recently, we showed that low concentration of curcumin (5 µM) exerts cytoprotective effect in SH-SY5Y neuroblastoma cells while higher concentration (15 µM) induced apoptosis and cell death [126].
Despite having a wide range of pharmacological activity, there is still a key question on curcumin to be answered, that is, why it remains as a supplement and couldn't play a higher-order role as a drug in treatment of diseases?
In response to this essential concern, we believe that pharmacokinetic deficiency is the main obstacle and studies to address this issue should be more considered before doing further preclinical and clinical studies. Confirming this idea, researchers have discussed that given its unstable, reactive, and poor bioavailability nature, further clinical trials on curcumin are unwarranted [34,191]. Therefore, this decade has witness an escalation in curcumin formulations addressing its pharmacokinetics (Figure 3). clinical trials, especially in new formulations with higher bioavailability characteristics. Confirming this idea about the dose-dependent effect of curcumin, recently, we showed that low concentration of curcumin (5 µM) exerts cytoprotective effect in SH-SY5Y neuroblastoma cells while higher concentration (15 µM) induced apoptosis and cell death [126].
Despite having a wide range of pharmacological activity, there is still a key question on curcumin to be answered, that is, why it remains as a supplement and couldn't play a higher-order role as a drug in treatment of diseases?
In response to this essential concern, we believe that pharmacokinetic deficiency is the main obstacle and studies to address this issue should be more considered before doing further preclinical and clinical studies. Confirming this idea, researchers have discussed that given its unstable, reactive, and poor bioavailability nature, further clinical trials on curcumin are unwarranted [34,191]. Therefore, this decade has witness an escalation in curcumin formulations addressing its pharmacokinetics (Figure 3). A wide variety of compounds have been formulated and tried to improve the pharmacokinetic problems, especially absorption, which is the first step in oral administration of drugs.
Although the absorption and bioavailability are the crucial issues, curcumin distribution in body tissues should be take into account for its biological activity. In fact, there are just limited number of investigations have addressed this problem so far. In this regard, researchers reported that oral administration of curcumin (400 mg) in rats exerts only traces of unchanged drug in other organs such as liver and kidney [209].
Novel delivery strategies including nanoparticles, liposome, phospholipid complex, microemulsions, or polymer micelles suggest significant promise and are worthy of further investigation to enhance the oral bioavailability of curcumin. Here we reviewed some new formulations on the market, and some of their pharmacokinetic characteristics have been compared in Table 8. Most clinical studies on these new formulations were conducted randomized and blinded (neither researchers nor volunteers were aware of the treatment). However, the most important limitation of these investigations was the low number of subjects were recruited in the studies. In addition, there are remarkable differences in the sampling time following oral administration of curcumin that may affect the pharmacokinetic parameters including Tmax and Cmax. In fact, a wide-ranging of variability in this research, including route of administration, study design, sample size, and even the type of reports, makes it very difficult to directly compare and conclude which formulation is the best in pharmacokinetic characteristics overall. However, an overall view to these results shows that novel formulations involving nanoparticles and micronized products showed improved solubility Figure 3. Obstacles against the marketing of curcumin as a drug. Challenges in curcumin oral bioavailability, distribution and metabolism, the main pharmacokinetic parameters, that emerged as major obstacles limiting the therapeutic efficacy and marketing of curcumin as a drug.
A wide variety of compounds have been formulated and tried to improve the pharmacokinetic problems, especially absorption, which is the first step in oral administration of drugs.
Although the absorption and bioavailability are the crucial issues, curcumin distribution in body tissues should be take into account for its biological activity. In fact, there are just limited number of investigations have addressed this problem so far. In this regard, researchers reported that oral administration of curcumin (400 mg) in rats exerts only traces of unchanged drug in other organs such as liver and kidney [209].
Novel delivery strategies including nanoparticles, liposome, phospholipid complex, micro-emulsions, or polymer micelles suggest significant promise and are worthy of further investigation to enhance the oral bioavailability of curcumin. Here we reviewed some new formulations on the market, and some of their pharmacokinetic characteristics have been compared in Table 8. Most clinical studies on these new formulations were conducted randomized and blinded (neither researchers nor volunteers were aware of the treatment). However, the most important limitation of these investigations was the low number of subjects were recruited in the studies. In addition, there are remarkable differences in the sampling time following oral administration of curcumin that may affect the pharmacokinetic parameters including Tmax and Cmax. In fact, a wide-ranging of variability in this research, including route of administration, study design, sample size, and even the type of reports, makes it very difficult to directly compare and conclude which formulation is the best in pharmacokinetic characteristics overall. However, an overall view to these results shows that novel formulations involving nanoparticles and micronized products showed improved solubility and a relatively more amount of blood concentration following oral administration compared to the others.
With regards to the fact that most of the investigations reported in this review have been designed and performed in vitro, extensive harmonized large clinical study including different formulations in humans with the same design will make great sense and is needed to compare the bioavailability and efficacy of these curcumin formulations.
Considering the pharmacokinetic properties of curcumin metabolites and their beneficial effects, such as regulation of oxidative stress, prevention of neurodegenerative disorders and their potential mechanism to prevent or treat human diseases, developing new formulations and performing clinical trials using these compounds is also suggested.
Finally, bypassing the limitations of curcumin mentioned in this review, such as low oral bioavailability, distribution, and metabolism, will pave the way to do more applicative research and clinical studies on curcumin as a drug in the near future.

Conflicts of Interest:
The authors declare no conflict of interest.