Besides its important nature of donating hydrogen ions, curcuminoids also have a number of moieties (phenolic hydroxyl group, methoxy group and 1,3 β-diketone) with a potential to neutralize reactive oxygen intermediates. Curcuminoids can undergo nucleophilic addition and imparts its anti-oxidant property by a various, complex mechanism. These free radical scavenging and anti-oxidant activity counteract with reactive oxygen and nitrogen species and thereby protects cells from oxidative damage. In alkaline pH, curcumin is less stable but its stability markedly increases in acidic pH. These explain why curcumin is stable within the gastrointestinal tract (pH 1 to 6)[15,20].

Oxidative stress and oxidative damage play important role in pathogenesis of many diseases, e.g., cardiovascular diseases, diabetes, neurodegenerative diseases and cancer etc. Curcuminoids have been shown to have the potential to scavenge free radical and attenuate free radical mediated lipid peroxidation in a number of experimental systems[21]. Besides its direct anti-oxidant property, curcumin may function indirectly as anti-oxidant by inhibiting the activity of inflammatory enzymes.

Curcumin is known for its remarkable anti-inflammatory effect. Its anti-inflammatory property is comparable with the recognized steroidal and non-steroidal anti-inflammatory drugs, e.g., Indomethacin and Phenylbutazone which have dangerous side effects. For example, curcumin considerably inhibits carrageenan-induced paw edema in mice, acute lung injury of rats by cyclophosphamide and other inflammatory diseases (arthritis, ulcerative colitis or pancreatitis) in different animal models[12,22]. It’s anti-inflammatory mechanism is attributed through inhibition of inducible nitric oxide synthase (iNOS), Cycloxygenase-2 (COX-2), inhibition in production of pro-inflammatory cytokines [IL-1, IL-6, IL-8, IL-12, interferon γ (IFN-γ), tumor necrosis factor-α (TNF-α) etc.], monocyte chemoattractant protein (MCP), migration inhibitory protein (MIP) and down regulation of mitogen activated and Janus kinases[11]. Menon and Sudheer[23] have nicely reviewed the anti-inflammatory property of curcumin. Several reports have indicated that curcumin not only inhibits prostaglandins it also inhibits lipoxigenase (LOX) activity. Inhibition of pro-inflammatory cytokines, COX-2 and iNOS are mainly mediated via suppression of transcription factor nuclear factor κB (NF-κB) and activating protein (AP-1). Down regulation of intercellular signaling proteins (e.g., protein kinase C) is another way in which curcumin inhibits production of pro-inflammatory cytokines[11]. This molecular mechanism finally inhibits cell proliferation, cell invasion and apoptosis causing suppression of carcinogenesis.

Curcumin is well known for its anti-carcinogenesis activity and plays important role in all stages of cancer. Inflammation and carcinogenesis are broadly interlinked. Inhibition of I-κB kinase is required to inhibit potent inflammatory transcription factor NF-κB which is also involved in progression of carcinogenesis. Curcumin has the potential to decrease matrix metalo-protease activity which is thought to be a prognostic marker of neoplasm[24]. In in vitro studies, where bioavailability is not an issue curcumin plays its role like a master manipulator. It inhibits important transcription factors and thereby decreases production of pro-inflammatory cytokines which leads to reduced inflammation and cell survival. Curcumin also effectively prevented its anti-cancer activity in animal models of oral, esophageal, stomach, duodenal and colon cancer[13]. Plenty of articles are available for anti-cancer activity of curcumin. in vitro, pre-clinical and clinical studies on anti-carcinogenic property of curcumin was thoroughly reviewed by Sharma et al[19].

Curcumin can suppress the growth of a variety of parasite, bacteria and pathogenic fungi. It shows anti-microbial effect against Helicobacter pylori, Bacillus subtilis, Plasmodium falciparum etc[25]. In a study with chick infected with caecal parasite Eimera maxima showed that 1% dietary turmeric resulted in a decrease intestinal lesions and improved weight gain. Topically applied curcumin demonstrated its potential anti-microbial effect against either dermatophytes, pathogenic molds, or yeast in guinea pigs. Curcumin also possesses a pro-microbial effect against Leishmania and some species of Plasmodium[26].

Considering the strong association of H. pylori and gastric cancer the authors of the study by Mahady et al[38] hypothesized that curcumin may exerts its chemopreventive activity by directly inhibiting H. pylori. They have showed that both curcumin and methanolic extract of turmeric rhizome inhibited the growth of 19 different strains of H. pylori (including cag positive isolates). MIC₅₀ and MIC₉₀ of the turmeric rhizome extract were 12.5 μg/mL and 25 μg/mL respectively where as curcumin at a concentration of 12.5 μg/mL inhibited 100% growth of all strains with a MIC range from 6.25-12.5 μg/mL. Later, a study by our group with a large number of clinical isolates (n = 65) was also found to be in agreement with regard to MIC result. We have reported that MIC of curcumin ranged from 5 μg/mL to 50 μg/mL and the majority of the strains (81%) showed a MIC of either 10 μg/mL (23%) or 15 μg/mL (58%)[7]. To understand the reason of this wide range in sensitivity, we have examined sequence uniformity of aroE gene encoding sikimate dehydrogenase (SDH) of H. pylori which is involved in shikimate pathway known to play crucial role in developing non-toxic antimicrobial agents. Although curcumin is a potent inhibitor of SDH, we were unable to establish any correlation in between aroE sequence and MIC of curcumin towards H. pylori. These results clearly confirmed that curcumin acts as a potent growth inhibitor for Indian H. pylori strains irrespective of different genetic makeup and disease status[7].

The mouse model of H. pylori infection has been extensively used in investigations of host responses to H. pylori infection as well as in eradication studies. In our study, we have shown that curcumin treatment completely eradicated H. pylori from infected mouse stomach. This eradication by curcumin was irrespective of the bacterial genotype, which is independent of the presence of the cag PAI. Histological analysis clearly showed that curcumin is remarkably effective in repairing damaged tissue. Our microscopical observations revealed substantial damage in the mouse gastric tissues infected with H. pylori in comparison to the control. Intact epithelial layer and glandular cells with continuous gastric pits in control gastric mucosa have damaged by denudation of the surface epithelial layer in H. pylori an infected mouse gastric tissue which was almost restored to normal upon treatment with curcumin. Inflammation in the gastric pit cells of infected tissue was significantly reduced by curcumin treatment. Disruption of submucosal, muscularis mucosal layers and Glandular atrophy of infected mouse gastric tissues was also considerably recovered by curcumin treatment. Negligible inflammatory cells in the control tissue infiltered more in the submucosal region of H. pylori-infected mouse gastric tissues and was prevented to a significant degree in curcumin-treated mice[7]. Hence, these results pointed towards that high efficacy of curcumin in healing the overall damage caused by H. pylori infection. These observations not only indicate the therapeutic potential of curcumin against H. pylori infections but also highlight the anti-inflammatory effect of curcumin and also draw our attention to its potential as an alternative therapy against H. pylori infection[7].

MMPs are a family of various zinc dependant endopeptidases that have wide substrate specificity and play a critical role in a variety of physiological processes including tissue inflammatory processes, remodeling, wound repair and organ development. Among them, gelatinases (MMP-2 and MMP-9) and stromelysin-1 (MMP-3) collectively cleave gelatins (types I and V), collagens (type IV, V, VII, IX and X), elastin, fibronectin, laminin and proteoglycan core proteins[8]. Apart from curcumin’s antioxidant, anti-inflammatory, anti-microbial and anti-carcinogenic properties, it has been shown to target a number of molecules like cytokines, growth factors, transcription factors and enzymes including MMPs, that are involved in the etiology of diverse diseases[32]. Several studies have demonstrated that H. pylori-infection induces the secretion of MMPs from a variety of gastric cells in vivo as well as in cultured cells, which in turn contributes to the pathogenesis of gastric ulcer and gastric cancer[8].

Since MMP-3 and -9 play a critical role in gastric ECM degradation during H. pylori induced pathogenesis, and are associated with various carcinomas including gastric cancer, attenuation of their increased secretion and synthesis is crucial for restoration of the gastric damage caused by H. pylori infection. In our recent study, we have tested the effect of curcumin on the activity of MMP-3 and -9 during protection against H. pylori infection in cultured cells and mice. We have also compared the efficiency of curcumin and conventional TT in stabilizing the balance between MMPs and its inhibitor[8]. It was the first report where we have confirmed that curcumin dose dependently suppressed the increased secretion of MMP-3 and MMP-9 in H. pylori infection. The more interesting part was that secreted MMP-3 and -9 were inhibited 90% by curcumin while 50% by TT and antibiotic alone. We demonstrate that curcumin, apart from eradication of H. pylori strains from infected mice, regulated the expressions and activities of MMP-3 and -9 in the gastric tissues. Furthermore, curcumin was highly effective in restoring the denudation of epithelial region, disruption in gastric mucosal layer and infiltration of inflammatory cells that occurred due to H. pylori infection in mouse gastric tissues. Curcumin is more effective than TT in restabilizing the altered balance between MMPs and TIMPs during protection against H. pylori-infection. This curcumin mediated down regulation of MMP-3 and -9 levels in H. pylori-infected mice and cultured cells suggest its immense therapeutic potential against H. pylori associated gastrointestinal diseases.

NF-κB plays an essential role in regulating human immune response. It is normally present as a cytosolic-inactive form, and upon stimulation by a huge varity of pathogenic agents (bacterial, viral or others), inflammatory cytokines, UV or γ-irradiation, NF-κB is activated. Those stimulants help phosphorylation and ubiquitination of the inhibitory subunit IκB, which leads to the release of NF-κB dimer. Released NF-κB dimer then rapidly translocates into the nucleus, where it activates transcription of target genes e.g., IL-1, IL-6, IL-8, TNF-α as well as other cytokines, IFNs, cell adhesion molecules and hemopoietic growth factors[39]. NF-κB is known to be inhibited by many anti-oxidants[48] and therefore, curcumin which have a strong anti-oxidant property can be considered as an inhibitor of this nuclear factor in H. pylori infection.

Münzenmaier et al[39] first reported the activation of NF-κB by H. pylori infection in gastric epithelial cells. They have demonstrated that ability to produce IL-8 by a variety of H. pylori strains correlates with the activation of NF-κB. IL-8 production requires NF-κB activation, and prevention by the anti-oxidant curcumin leads to suppression of cytokine production. Unlike many Gram-negative bacteria H. pylori does not use LPS to activate NF-κB, instead they use a secreted bacterial product[39].

Later, Foryst-Ludwig et al[40] described how curcumin blocks NF-κB and thus reduces gastric inflammatory response. Curcumin, a known inhibitor of AP-1, blocks the synthesis of H. pylori induced IL-8 production and inhibited the H. pylori-induced cell scattering in the gastric epithelial cells. Cucumin have the ability to reduce v-Src activity which might prevent Phosphorylation of cytotoxicity associated immunodominant antigen (CagA)[40]. The phenomenon of curcumin’s effect on inhibition of NF-κB and alleviating inflammatory response was also found to be effective in rat model of H. pylori infection[45]. Authors in that study have also described that supplementation of curcumin helped in suppression of macromolecular leakage. In a very recent study with mouse model also corroborate that curcumin treatment exerted a significant anti-inflammatory response in gastric mucosa. In treatment group, no sign of inflammation and decreased expression of different inflammatory mediators were demonstrated by histology[47]. H. pylori induced activation of signaling molecules e.g., mitogen-activated protein kinases (MAPK), extracellular signal-regulated kinases (ERK1/2) and p38 are not inhibited by curcumin[26] but inhibition potential of NF-κB and cell scattering justifies therapeutic properties of curcumin in H. pylori-associated stomach ailments[40].

Activation induced cytidine deaminase (AICD) is an enzyme encoded by aicd gene in human which creates mutations in DNA by deamination of cytosine base by turning it into uracil. This also creates immune diversity by inducing somatic hypermutations and class-switch recombinations in human immunoglobulin genes. By somatic mutation in several host genome of nonlymphoid tissues like TP53 tumor suppressor gene it can cause tumorigenesis[43]. An unusual expression of AICD in H. pylori infected gastric epithelial cells is induced by NF-κB[49]. NF-κB-regulated gene products e.g., AICD, have found to be been closely associated with H. pylori-induced gastric carcinogenesis. Therefore, agents that inhibits NF-κB pathway might have potential in preventing H. pylori-related tumorogenesis[43]. Study by Zaidi et al[43] tested the effect of curcumin on the expression of AICD in H. pylori-infected gastric epithelial cells and the role of NF-κB. They have demonstrated, low concentration of curcumin down regulates H. pylori induced AICD expression possibly by inhibiting NF-κB pathway in MKN-45 cells. Consequently, this study also suggested that curcumin can serve as potentially alternative treatment option.

It is always essential to carefully evaluate the toxicity of any compound in preclinical and early clinical studies. We can’t assume plant derived product which have been continuously in dietary use will be always harmless, as the doses may exceed or the formulation could differ. Early studies in rat model have demonstrated no significant toxicity at doses upto 5 g/kg body weight (BW) when given orally[50]. The safety issues have been lucidly elaborated in a review “Curcumin: The story so far” by Sharma et al[19]. One more organized pre-clinical study by prevention division of National Cancer Institute (NCI, US) did not find any side effects/toxicity in rat, dogs or monkeys using the doses upto 3.5 g/kg BW for 3 mo. Although, some infrequent adverse reactions (e.g., gastrointestinal upset or ulcerogenic response) reported were either spontaneously healed or not confirmed by subsequent studies. Some NCI reports confirmed this gastrointestinal disturbances in patients with different curcumin doses (0.45-3.6 g/d)[19]. Conversely, in another study where patients with advanced colorectal cancer were treated with curcumin upto a dose of 3.6 g daily for 4 mo and found to be well tolerated[51]. Moreover, three different phase I clinical trials showed that curcumin is extremely safe and well tolerated at very high doses (12 g/d)[18]. This pharmacological safety and potential efficacy have prompted investigators for further analysis.

Even though the toxicity study of curcumin assures an immense possibility for treatment and prevention of a variety of diseases, the relatively low bioavailability of curcumin is the main difficulty in drug development. The bioavailability depends on absorption, metabolism, tissue distribution and excretion of that compound. Studies over the past few decades have established that curcumin undergoes poor absorption, fast metabolism and rapid clearance which severely reduced it’s bioavailability. Early study by Wahlstrom et al[50] showed that almost 75% of the dose (1 g/kg BW) was excreted in the faeces of rats and proved that curcumin is poorly absorbed in the gut.

A study of oral curcumin administration (400 mg) to rats demonstrated 60% absorption of curcumin, absence of any traces in heart blood and as a small amount (less than 5 μg/mL) was found in portal blood[52]. Same investigators, using tritiated curcumin showed majority of the oral dose being excreted in faeces, including one-third dose was excreted unchanged[53]. Pharmacokinetics of curcumin was also studied in mice either orally or intraperitonealy (ip) by Pan et al[54]. With oral administration of 1 g/kg of curcumin, low plasma level (0.22 μg/mL) was obtained at 1 h which was declined below the detection limit as soon as after 6 h. Although some studies have reported different results but all are in agreement that curcumin exhibits low bioavailability. In rodents it undergoes intestinal metabolism and excreted rapidly[18]. Result from pilot studies and phase I clinical trial confirmed first-pass and some degree of intestinal metabolism of curcumin, specially glucuronidation and sulphation, which explain curcumin’s low bioavailability[19]. The liver and intestinal mucosa were found to be the major organs responsible for metabolism of curcumin. Tissue distribution studies for curcumin showed its better accumulation in the intestine, liver, and colon and might be the most important cause why its most promising in vivo effects have been found in gastrointestinal diseases when compared with other organ systems[33]. H. pylori mediated gastrointestinal ailments can be treated with curcumin and the above phenomenon is the rationale behind its potential.

Various classical techniques e.g., heat, pH, and complexations with metal ions, polymers or serum have been applied for enhancement of curcumin solubility and efficacy. It has been claimed that the solubility of curcumin can be increased by 12 fold by the use of heat[55]. Some novel approaches to overcome curcumin’s less bioavailability are adjuvants (which can block metabolic pathway of curcumin), effective delivery system e.g., phospholipid complexes, liposomes, micelles, and nanoparticles.

An adjuvant is a compound that enhances an existing medical regimen, as a pharmacological agent added to a drug to increase or aid its effect. Glucuronidation of curcumin is known to cease its effect. Metabolic conversation of curcumin can be inhibited by an adjuvant Piperine, a known inhibitor of hepatic and intestinal glucuronidation. Concomitant use of curcumin with piperine increased the bioavailability of curcumin by 154% in rats and 2000% in human volunteers[56]. Other adjuvants which showed synergistic effect when used in combination with curcumin are quercetin[57], genistein[58] and Eugenol/terpeneol[59].

Structure of curcumin plays crucial role in determining its biological activity. As a result, several attempts have been made using curcumin derivatives/analogue with a number of achievements. EF-24, a curcumin analogue, was reported to be a lead compound presenting increased anti-tumor activity both in vitro and in vivo in comparison to curcumin[60] and less toxic[61]. EF-24 showed its higher bioavailability than curcumin of 60% and 35% with oral and ip respectively[18]. A review[18] from a well known group, working with curcumin’s biochemical property, has lucidly explained different reports where investigators have studied different complexes of curcumin (e.g., copper, boron, manganese, vanadyl, indium and gallium complexes). Lead complexes may be tested for anti-H. pylori effect.

Curcumin phospholipid complexes exhibited better bioavailability, pharmacokinetics and hepato-protective property than free curcumin as describes by several researchers[32]. In a study with rat model of oral administration, curcumin phospholipid complex showed 1.5-fold more half-life over free cucumin[62]. A threefold high aqueous solubility and better efficacy in rat model was reported by Maiti et al[63].

Some important delivery system e.g., liposomes and micelles are also effectively used to study effect of curcumin. Liposomes are known drug delivery system due to the property of carrying both hydrophilic and hydrophobic molecules. Li et al[64] demonstrated that liposomal curcumin inhibits the growth of human pancreatic carcinoma cells and tumor angiogenic properties. Several other studies have reported the efficacy of liposomal curcumin in colorectal cancer or lymphoma cells but a controlled in vivo study is essential to check the effect of its bioavailability. On the other hand, use of micelles increases the solubility of curcumin in a number of systems. Letchford et al[65], showed a stiff increase in curcumin solubility using a polymeric micelle. Micelles inhibited curcumin uptake by liver/spleen, and also increased the distribution of curcumin in the other organs (e.g., lung and brain)[66].

Recently use of nanoparticle has been come out as a possible solution of compounds with compromised bioavailability. Nanoparticle-based delivery systems will most likely be appropriate for very hydrophobic agents like curcumin evading the drawbacks of poor aqueous solubility. Nanocurcumin has found to have more anti-inflammatory property and gives more protection to oxidative stress and apoptosis[67-69]. Main aim for using nanocarcumin is to have more dispersed and available curcumin in aqueous solution[70,71]. Use of nanocurcumin in different cancers is now a matter of regular discussion due to its better efficacy than native curcumin[67,72]. Oral bioavailability has been increased by 9 fold with nano-delivery system than piperine adjuvant administration. PLGA (polylactic-co-glycolic acid) nano formulation of curcumin have shown increased bioavailability upto 22 fold in rat model than free curcumin[73]. Better efficacy and solubility was described in a number of cancer cells and pre-clinical studies[74] but yet to analyze in H. pylori infection.