Application of Functional Biocompatible Nanomaterials to Improve Curcumin Bioavailability

Curcumin is a lipophilic natural product extracted from turmeric and commonly used as a dietary spice. Being multi-functional, curcumin has been proposed in the prevention and treatment of a broad spectrum of diseases. However, due to unsatisfactory aqueous solubility and hence low bioavailability, clinical application of curcumin has been greatly restrained. To break these limitations, biocompatible nanoformulation, such as liposomes, nanoparticles, micelles, nanoemulsions and conjugates has been employed as alternatives to improve in vivo delivery of curcumin. In this scenario, in order to enhance bioavailability of curcumin, the choice of effective molecules as facilitators is of prominence. In this review, we focus on functional biocompatible materials, including polymers, protein molecules, polysaccharide, surface stabilizers and phospholipid complexes, and decipher their potential applications as how they assist to promote medicinal performance of curcumin.


Polymeric Curcumin Nanoparticles
Polymeric nanoparticles (NPs) have been widely studied in the field of drug delivery due to their high capacity of drug loading, controlled release, and efficient degradation. Curcumin-loaded polymeric NPs are generally colloidal particles in spherical or core-shell structures with a size range of 60-1,000 nm. Both natural and synthetic polymers have been intensively investigated for NPs formulation, in which natural polymers, such as polysaccharides [alginate, starch, chitosan, hyaluronic acid (HA), etc.] and proteins (collagen, albumin, fibrin, silk, etc.), showed high biocompatibility and biodegradability in vivo. Nevertheless, their significant batch-to-batch variation makes the extensive following purification procedure unavoidable to reach bio-similarities (Karlsson et al., 2018). In this respect, biodegradable synthetic polymers such as polyethylene glycol (PEG), polylactide (PLA), polyglycolide acid (PGA), poly (εcaprolactone) (PCL), and polylactic-co-glycolic acid (PLGA), with the advantage of simple control in large-scale production are gaining an increasing importance. Although holding a promise, other interfering factors, such as potential cytotoxicity or immunogenicity have been shown introduced by products or metabolites from their unexpected degradation (Sun et al., 2012). Typically, zein is a water-insoluble amphiphilic protein from the corn, which is commonly used in curcumin encapsulation (Hong et al., 2018). Chen et al. developed curcumin-zein-HA NPs by incorporation of curcumin in a hydrophobic zein core with a hydrophilic HA shell using layer-by-layer electrostatic deposition. This formulation prolonged the release of curcumin with the increased HA surface density, and concomitantly offered a 3-fold increase in light and thermal stability compared to free curcumin. However, the HA hydrophilic coating surface may result in insufficient NPs cellular uptake and transport (Chen et al., 2019). To solve this problem, Akhtar et al. prepared curcumin-chitosan NPs by ionic gelation, in which curcumin cellular uptake was triggered by positive chitosan electrostatic interaction with negative intestinal mucin glycoproteins. Curcumin-chitosan NPs by oral administration have led to 6.4-fold increase in blood half-life and excellent penetration ability across transmucosal barrier (Akhtar et al., 2012). Another example falls on PEG, which is the most widely used hydrophilic polymer to prepare curcumin-loaded NPs for tumor therapy. Generally, PEG conjugates take the advantage of enhanced permeability and retention (EPR) effect and get accumulated in tumor vessels through leaky vasculature and poor lymphatic drainage. To increase tumor targeting specificity and avoid highdose curcumin-induced toxicity in non-tumor tissues, Guo et al. fabricated a dual functional MMPs-responsive curcumin-loaded nanoparticles (Cur-P-NPs) based on a tri-block biomaterial (mPEG-Peptide-PCL) through solvent evaporation. Besides PEGylation, peptide (ACP)-GPLGIAGQr9-(ACP) was also designed to target matrix metalloproteinases-2 (MMP-2), which is often up-regulated in tumor cells. In this study, Cur-P-NPs with a diameter of 159.7 nm and encapsulation efficiency of 80.12% selectively targeted and penetrated A549 non-small cell lung cancer cells, when compared with that in non-targeted L929 mouse fibroblast cells (Guo et al., 2018). Further efforts to improve the accuracy of curcumin delivering have been made by designing polymers whose target or release is environmentally triggered (such as pH, temperature, light, enzymes, and biomolecules). Poly (2, 4, 6-trimethoxybenzylidene-1,1,1tris(hydroxymethyl)ethane methacrylate) (PTTMA) are pH-sensitive polymers undergoing hydrophobic-to-hydrophilic transition in mildly acidic environment due to the hydrolysis reaction of acid-labile cyclic benzylidene acetal groups (CBAs). Disulfide bridge (SS) between polymer chains are used as a reduction-responsive linkage for structure disassembles. Zhao et al. prepared curcumin-loaded PTTMA-g-SS-PEG NPs by reversible addition-fragmentation chain transfer polymerization (RAFT) copolymerization and coupling reaction. A particle of 130.2 nm in size and encapsulation efficiency of 96% was obtained. The curcumin-loaded PTTMA-g-SS-PEG NPs were found more stable (< 15% release) in normal physiological conditions, whereas upto 76.8 and 94.3% release of curcumin were achieved in acidic and acidic with reductive condition, respectively. Functionally, curcumin-loaded PTTMA-g-SS-PEG NPs effectively inhibited the proliferation of both esophageal carcinoma cells and human liver cancer cells (HepG-2). Therefore, these data point out that dual pH and reductiveresponsive materials could facilitate a rapid release of curcumin in tumor cells, which is more acidic than normal cells (Zhao et al., 2013).

Polymeric Curcumin Micelles
Polymer micelles are generally formed by amphiphilic block copolymers due to hydrophobic interaction in a core-shell structure with a very narrow size range from 10 to 100 nm in aqueous media (Torchilin, 2007). Based on these properties, polymer micelles served as transporters for curcumin to evade the mononuclear phagocyte system (MPS) and increase curcumin accumulation at diseased tissues (Moghimi et al., 2005;Oerlemans et al., 2010;Hanafy et al., 2018;Hussein and Youssry, 2018). Aiming to achieve long blood-circulating micelles, Kumar et al. fabricated a mixed micellar system encapsulation of curcumin, in which short chain of PEG15-hydroxystearate together with long PEG chain of D-α-tocopheryl polyethylene glycol succinate1000 (TPGS1000) were used to create a stronger interfacial bond (Kumar et al., 2017). Although promising, the actual shear flow led to alterations of polymer structure or conformation in blood, which drove micelles dissociation and thus drug release before reaching the targeted organs (Almeida et al., 2013). In order to obtain more stable curcumin micelles, Li et al. formulated amphiphilic Pluronic F-127 (PF127) micelles to encapsulate curcumin by thin-film evaporation method. PF127curcumin-micelles are spherical, with a diameter of 25.4 ± 1.8 nm and 91.76 ± 5.93% encapsulate efficiency. In vitro drug release showed a biphasic release profile with an initial release of 18% at 24 h and 50% sustained drug release over 9 days. In vivo study in mice further confirmed that the bioavailability of PF127curcumin-micelles was enhanced up to 1.69-fold than that of free curcumin (Li et al., 2014).

Polymeric Curcumin Conjugates
In addition to physical encapsulation, chemical modifications of curcumin by polymer conjugates have also been investigated, which are advantageous in efficient curcumin loading and controlled release (Alves et al., 2018). The appropriate choice of polymer and the drug conjugate linkage can be manipulated to control curcumin's solubility and stability. Chemically, the two phenolic rings and active methylene groups of curcumin provide potential sites to conjugate with water soluble polymers, including HA, alginate and hydroxyethyl starch (HES) via esterification reactions (Yallapu et al., 2012). Dey and Sreenivasan developed alginate-curcumin conjugate by covalently conjugating curcumin to the C-6 carboxylate functional group of hydrophilic sodium alginate via an ester linkage to enhance solubility and stability at physiological pH of curcumin (Dey and Sreenivasan, 2014). Likewise, the ester linkage can be cleaved quickly by acid or esterase, thus the release pattern of curcumin can be achieved, and therefore fulfill the effectiveness of the curcumin (Fleige et al., 2012). For instance, recent work by Chen et al. fabricated a HEScurcumin conjugates through conjugating HES to curcumin via an acid-labile ester linker. This formulation increased solubility of curcumin to a thousand times higher than free curcumin, meanwhile offered curcumin with protections against UV and thermal degradation. In addition, adjusting aqueous media to a lower pH led to increased curcumin release and improved efficacy correspondingly, as evidenced by its anticancer and antioxidant activity in HeLa cervical carcinoma cells and Caco-2 human colorectal adenocarcinoma cells (Chen et al., 2020).

Curcumin Liposome Complex
Liposomes are nanosized spherical vesicles comprising phospholipid bilayers, and are created by physiologically accepted natural or synthetic phospholipids (Papahadjopoulos and Kimelberg, 1974). In aqueous media, phospholipid could self-assemble into a bilayer structure due to itself amphiphilic property with the basis of a hydrophilic head group and two hydrophobic acyl chains. This architecture endowed liposomes to be able to load with both lipophilic and hydrophilic drugs within phospholipid bilayers and aqueous core, respectively (Frolov et al., 2011). The selection of liposome composition and preparation method have found highly relevant to curcumin's bioavailability and efficacy. In this regard, a variety of lipids have been tested in liposomes preparations of curcumin. Thangapazham et al. prepared curcumin-loaded liposomes with lipid composition of 1,2-Dimyristoyl-sn-glycero-3-phosphocholine (DMPC), dipalmitoyl phosphatidylcholine (DPPC), or egg phosphatidylcholine (egg PC) (Thangapazham et al., 2008). Authors showed that DMPC-based liposomes allowed the greatest amount of curcumin entrapment, indicating curcumin preferentially partitioned into liposomes prepared from DMPC other than DPPC or egg PC. Besides lipids, cholesterol as a membrane constituent is widely incorporated in liposome composition to improve the liquidity of lipid membrane. Xu et al. prepared curcumin-loaded liposomes from soybean phosphatidylcholine (SoyPC) and cholesterol using the conventional thin-film dispersion method, and showed potential therapeutic effect in inhibiting cancer cells proliferation in vitro and in vivo (Xu et al., 2018). However, the clinical use of cholesterol as part of a drug is controversial due to the potential adverse effect of its oxidized products to human. In another study, Hamano et al. prepared curcumin liposomal formulation with DMPC and 5 mol% Tween 80 using microfluidic and thinfilm hydration, respectively. Though, no significant difference was observed in the particle size (117.1 ± 4.57 nm) and polydispersity index (PDI, 0.120 ± 0.005) between these two liposomes preparation methods, vesicles obtained from thin-film hydration tended to form aggregates after 24 h storage at 4 • C, whereas curcumin-liposome from microfluidics remained highly stable for at least 3 weeks. Moreover, in comparison to standard curcumin suspension, this curcumin-liposome improved its aqueous solubility by 700-fold, which corresponds to 8-20-fold bioavailability increased in tumor-bearing mice (Hamano et al., 2019).

Solid Lipid Nanocurcumin
Solid lipid nanoparticles (SLNs) are a new generation nanoemulsions developed to avoid residual organic solvents. SLNs are able to encapsulate both hydrophobic and hydrophilic drugs. Production of SLNs can be realized by solid lipids and surfactants using high-pressure homogenization (HPH), and can be modified to yield particles ranging from 10-1,000 nm in size for a large-scale production (Jenning et al., 2002).
Considering the thermal effect that is likely to impact the curcumin's chemical stability, lipids with low melting point and being solid state at room or body temperature, such as monostearin, glyceryl monostearate, precirol ATO 5 (mono, di, triglycerides of C16-C18 fatty acids), compritol ATO 888, stearic acid, and glyceryl trioleate would be optimal to use (Manjunath et al., 2005). Considering surfactants involved in SLNs, by reducing interfacial tension between hydrophobic surface of lipid and aqueous environment, surfactants act as surface stabilizers. In studies to date, the most preferred surfactants in clinical applications are poloxamer 188, Tween 80 and dimethyl dioctadecyl ammonium bromide (DDAB) (Manjunath et al., 2005;Tapeinos et al., 2017), which facilitate curcumin SLNs with increased bioavailability and improved pharmacological activity. In addition to production time and speed, a proper selection of lipids (amount) and surfactants (concentration) would have significant impacts on their physiological performance. Ban et al. prepared (Ban et al., 2020). In another study, to overcome the initial burst release of SLNs, Huang et al. applied sodium caseinate (NaCas) and sodium caseinate-lactose (NaCas-Lac) conjugates as bioemulsifiers to stabilize curcumin SLNs. The presence of the glycated lactose on surface provided steric hindrance, which therefore enabled the dispersibility and enhanced stability of curcumin at acidic pH. By doing this, curcumin loaded NaCas-Lac SLNs enhanced antioxidant activity by 3-4-folds in relative to that of free curcumin .

Nanostructured Lipid Curcumin
Nanostructured lipid carriers (NLCs), also known as hybrid lipid nanoparticles (HLNs), are composed of solid and liquid lipids mixture (Selvamuthukumar and Velmurugan, 2012). By addition of liquid lipid, the melting point will be decreased, and an amorphous solid state will appear at ambient temperature. This unique asymmetric lipid matrix structure provides a greater load of drug as well as minimized drug expulsion (Weber et al., 2014). Sadegh et al. prepared NLCs loading curcumin with solid lipid (cetyl palmitate and cholesterol) and liquid lipid (oleic acid) using homogenization and ultrasonication, with resultant nanoparticles being 117.36 ± 1.36 nm in size and 94 ± 0.74% in encapsulation efficiency. The particle size was not physically changed after 3 months of storage, demonstrating a great storage stability of this formulation. In addition, antioxidant activity of curcumin-NLCs as evaluated by DPPH (2, 2-diphenyl-1picrylhydrazyl) free radical scavenging efficiency was unaltered compared with free curcumin, indicating the biological property of curcumin was not affected by the production process. Moreover, in vivo pharmacokinetic studies in brain showed that the area under the curve (AUC) for curcumin-NLCs (AUC = 505.76 ng/g·h) was significantly higher than both free curcumin (AUC = 0.00 ng/g·h) and curcumin-SLNs (AUC = 116.31 ng/g·h) in rats exposed to 4 mg/kg curcumin intravenously (Sadegh Malvajerd et al., 2018).

CONCLUSION
Great progress has been made in the development of functional biocompatible nanomaterials contributing to the versatility of nano-curcumin formulations. In particular, biodegradable materials used in nanocurcumin are advantageous in biocompatibility and biosafety, showing greater potential to achieve rapid clinical application. In each formulation, materials are chosen, and used for specific purpose in order to improve curcumin bioavailability, such as enhanced water solubility, increased loading or encapsulation efficiency, safe degradation, controlled release, and targeted therapy. Among these, it is noted that a dual or multi-purpose can be achieved with the use of same material in different formulations. In particular, biodegradable materials used in nanocurcumin have the advantages of biocompatibility and biosafety, considered to held great promise in clinical application. Meanwhile, diverse approaches to prepare nanocurcumin have been established to construct nanoparticles of different physiochemical features. Therefore, the selection of an optimal method in practice requires considerations of both strengths and weaknesses of each technique, and physiochemical properties of the desired products. In spite of complexity in formulations, low yield, and high cost are of prime importance that influence clinical translation of nanocurcumin. Thus, further investigations will be needed in order to better understand the interaction of materials with curcumin prior to developing nanomedicine in clinics.

AUTHOR CONTRIBUTIONS
ZL and MS contributed equally to the literature search, data integration, and writing. RX and NL instructed, wrote, and review the manuscript. All authors contributed to the article and approved the submitted version.