Polymeric nanoparticle-encapsulated curcumin ("nanocurcumin"): a novel strategy for human cancer therapy
© Bisht et al; licensee BioMed Central Ltd. 2007
Received: 20 December 2006
Accepted: 17 April 2007
Published: 17 April 2007
Curcumin, a yellow polyphenol extracted from the rhizome of turmeric (Curcuma longa), has potent anti-cancer properties as demonstrated in a plethora of human cancer cell line and animal carcinogenesis models. Nevertheless, widespread clinical application of this relatively efficacious agent in cancer and other diseases has been limited due to poor aqueous solubility, and consequently, minimal systemic bioavailability. Nanoparticle-based drug delivery approaches have the potential for rendering hydrophobic agents like curcumin dispersible in aqueous media, thus circumventing the pitfalls of poor solubility.
We have synthesized polymeric nanoparticle encapsulated formulation of curcumin – nanocurcumin – utilizing the micellar aggregates of cross-linked and random copolymers of N-isopropylacrylamide (NIPAAM), with N-vinyl-2-pyrrolidone (VP) and poly(ethyleneglycol)monoacrylate (PEG-A). Physico-chemical characterization of the polymeric nanoparticles by dynamic laser light scattering and transmission electron microscopy confirms a narrow size distribution in the 50 nm range. Nanocurcumin, unlike free curcumin, is readily dispersed in aqueous media. Nanocurcumin demonstrates comparable in vitro therapeutic efficacy to free curcumin against a panel of human pancreatic cancer cell lines, as assessed by cell viability and clonogenicity assays in soft agar. Further, nanocurcumin's mechanisms of action on pancreatic cancer cells mirror that of free curcumin, including induction of cellular apoptosis, blockade of nuclear factor kappa B (NFκB) activation, and downregulation of steady state levels of multiple pro-inflammatory cytokines (IL-6, IL-8, and TNFα).
Nanocurcumin provides an opportunity to expand the clinical repertoire of this efficacious agent by enabling ready aqueous dispersion. Future studies utilizing nanocurcumin are warranted in pre-clinical in vivo models of cancer and other diseases that might benefit from the effects of curcumin.
Curcumin or diferuloylmethane is a yellow polyphenol extracted from the rhizome of turmeric (Curcuma longa), a plant grown in tropical Southeast Asia . For centuries, turmeric has been used as a spice and coloring agent in Indian food, as well as a therapeutic agent in traditional Indian medicine. Enthusiasm for curcumin as an anti-cancer agent evolved based on the wealth of epidemiological evidence suggesting a correlation between dietary turmeric and low incidence of gastrointestinal mucosal cancers [2, 3]. A plethora of experimental data has unequivocally established that free curcumin induces cell cycle arrest and/or apoptosis in human cancer cell lines derived from a variety of solid tumors including colorectal, lung, breast, pancreatic and prostate carcinoma, amongst others [4–12]. In addition to a potential application in cancer therapy, studies in numerous experimental (chemical) carcinogenesis models [13–17], and more recently in a clinical trial performed in patients with familial adenomatous polyposis , have confirmed that curcumin can also ameliorate the progression to cancer in a variety of organ sites, reiterating this agent's potential as a tool for chemoprevention.
Despite the considerable promise that curcumin is an efficacious and safe compound for cancer therapy and chemoprevention, it has by no means been embraced by the cancer community as a "panacea for all ills". The single most important reason for this reticence has been the reduced bioavailability of orally administered curcumin, such that therapeutic effects are essentially limited to the tubular lower GI tract (i.e., colorectum) [19, 20]. For example, in a Phase I clinical trial, patients with hepatic colorectal cancer metastases were administered 3600 mg of oral curcumin daily, and levels of curcumin and its metabolites measured by HPLC in portal and peripheral blood . Curcumin was poorly available following oral administration, with low nanomolar levels of the parent compound and its glucuronide and sulphate conjugates found in the peripheral or portal circulation. In another Phase I study, patients were required to partake 8000 mg of free curcumin orally per day, in order to achieve detectable systemic levels; beyond 8 grams, the bulky volume of the drug was unacceptable to patients . A third human Phase I trial involving curcumin dose escalation found no trace of this compound at doses of 500–8,000 mg/day, and only trace amounts in a minority of patients at 10–12 grams of curcumin intake per day . The development of a delivery system that can enable parenteral administration of curcumin in an aqueous phase medium will significantly harness the potential of this promising anti-cancer agent in the clinical arena.
We report the synthesis, physico-chemical characterization, and cancer-related application of a nanoparticle-encapsulated formulation of curcumin, "nanocurcumin". Cross-linked polymeric nanoparticles with a hydrophobic core and a hydrophilic shell were used for encapsulation of curcumin, generating drug-encapsulated nanoparticles consistently in size less than 100 nm.
Results and discussion
Synthesis and detailed physico-chemical characterization of NIPAAM/VP/PEG-A copolymeric nanoparticles (FT-IR, 1H-NMR, DLS, TEM and Release Kinetics)
In vitro and in vivo toxicity studies of void polymeric nanoparticles
Nanocurcumin inhibits the growth of pancreatic cancer cell lines and abrogates colony formation
Nanocurcumin inhibits NFκB function in pancreatic cancer cell lines and downregulates multiple pro-inflammatory cytokines
In the course of the past decade, the field of drug delivery has been revolutionized with the advent of nanotechnology, wherein biocompatible nanoparticles have been developed as inert systemic carriers for therapeutic compounds to target cells and tissues [33–38]. A recent example of the impact of nanomedicine in drug delivery is underscored by the success of Abraxane™, an albumin nanoparticle conjugate of paclitaxel, and the first FDA-approved anti-cancer agent in this emerging class of drug formulations . In a quest for developing stable and efficient systemic carriers for hydrophobic anti-cancer compounds, our laboratory has developed cross-linked polymeric nanoparticles comprised of N-isopropylacrylamide (NIPAAM), N-vinyl-2-pyrrolidinone (VP) and poly(ethyleneglycol) acrylate (PEG-A). We demonstrate the essential non-toxicity of the void polymeric formulation in vitro and in vivo, underscoring the potential of these nanoparticles as a carrier for hydrophobic drugs.
Peer reviewed publications numbering in the 100 s have reiterated the potency of curcumin against a plethora of human cancer lines in the laboratory (selected reviews include [1, 4–6, 40, 41]). Equally important, free curcumin was shown not to be cytotoxic to normal cells, including hepatocytes, mammary epithelial cells, kidney epithelial cells, lymphocytes, and fibroblasts at the dosages required for therapeutic efficacy against cancer cell lines [42–46]; these in vitro findings are underscored by the limited human clinical trials performed with oral curcumin, wherein doses up to 10 grams per day have had minimal adverse effects, even to the highly exposed gastrointestinal mucosa [18–22]. Nevertheless, few clinical trials have been performed with this agent.
A liposomal curcumin formulation was recently described that demonstrates comparable potency to free curcumin, and which can be administered via the parenteral route . Even as further studies with this liposomal formulation are awaited, it is emphasized that liposomes, which are metastable aggregates of lipids, tend to be more heterogeneous, and larger in size (typically 100–200 nm) than most nanoparticles. We have synthesized a nanoparticulate formulation of curcumin – nanocurcumin – wherein the polymeric nanoparticles formed are consistently less than 100 nm in size (mostly in the 50 nm size range), as stated in the National Nanotechnology Initiative's (NNI's) definition of "nanomaterials". We have demonstrated that our nanocurcumin formulation has comparable efficacy to free curcumin against pancreatic cancer cell lines in vitro, by inhibiting cell viability and colony formation in soft agar. Further, our studies confirm that nanocurcumin retains the mechanistic specificity of free curcumin, inhibiting the activation of the seminal transcription factor NFκB, and reducing steady state levels of pro-inflammatory cytokines like interleukins and TNFα.
Nanocurcumin opens up avenues for systemic therapy of human cancers, as well as other human maladies like Alzheimer disease [48–51] and cystic fibrosis [52–54], wherein the beneficial effects of curcumin have been propounded. Future studies using relevant experimental models will enable addressing these scenarios in an in vivo setting, and should facilitate the eventual clinical translation of this well known but under-utilized therapeutic agent.
Preparation of polymeric nanoparticles
A co-polymer of N-isopropylacrylamide (NIPAAM) with N-vinyl-2-pyrrolidone (VP) poly(ethyleneglycol) monoacrylate (PEG-A) was synthesized through free radical polymerization as shown in the accompanying flowchart (Figure 1). NIPAAM, VP and PEG-A were obtained from Sigma chemicals (St. Louis, MO). NIPAAM was recrystallized using hexane, VP was freshly distilled before use, and PEG-A was washed with n-hexane three times to remove any inhibitors; Millipore water and other chemicals were used as-is. Thereafter, the water-soluble monomers – NIPAAM, VP and PEG-A were dissolved in water in 90: 5: 5 molar ratios. The polymerization was initiated using ammonium persulphate (APS, Sigma) as an initiator in a nitrogen (N2) atmosphere. Ferrous ammonium sulphate (FAS, Sigma) was added to activate the polymerization reaction, and also to ensure complete polymerization of the monomers. In a typical experimental protocol, 90 mg NIPAAM, 5 μl freshly distilled VP, and 500 μl PEG-A (1% w/v) were added in 10 ml of water. To cross-link the polymer chains, 30 μl of N,N'-Methylene bis acrylamide (MBA, Sigma, 0.049 g/ml) was added to the aqueous solution of monomers. The dissolved oxygen was removed by passing nitrogen gas for 30 minutes. Thereafter, 20 μl of FAS (0.5% w/v), 30 μl of APS and 20 μl of TEMED (Invitrogen, Carlsbad CA, USA) were added to initiate the polymerization reaction. The polymerization was performed at 30°C for 24 hours in a N2 atmosphere. After the polymerization was complete, the total aqueous solution of polymer was dialyzed overnight using a Spectrapore® membrane dialysis bag (12 kD cut off) to remove any residual monomers. The dialyzed solution was then lyophilized immediately to obtain a dry powder for subsequent use, which was easily re-dispersible in aqueous media. The yield of the polymeric nanoparticles was typically more than 90% with this protocol.
Loading of curcumin
Curcumin was a kind gift of Indsaff, Inc. (Batala, Punjab, India). Curcumin loading in the polymeric nanoparticles was done by using a post-polymerization method. In this process of loading, the drug is dissolved after the co-polymer formation has taken place. The physical entrapment of curcumin in NIPAAM/VP/PEG-A polymeric nanoparticles was carried out as follows: 100 mg of the lyophilized powder was dispersed in 10 ml distilled water and was stirred to re-constitute the micelles. Free curcumin was dissolved in chloroform (CHCl3; 10 mg/ml) and the drug solution in CHCl3 was added to the polymeric solution slowly with constant vortexing and mild sonication. Curcumin is directly loaded into the hydrophobic core of nanoparticles by physical entrapment. The drug-loaded nanoparticles are then lyophilized to dry powder for subsequent use.
Entrapment efficiency (E %)
Fourier Transform Infrared (FT-IR) studies of polymeric nanoparticles
Mid infra red (IR) spectrum of NIPAAM, VP and PEG-A monomers, as well as the void polymeric nanoparticles were taken using Bruker Tensor 27 (FT-IR) spectrophotometer (Bruker Optics Inc., Billerica, MA, USA).
1-H Nuclear Magnetic Resonance (NMR) studies
The NMR spectrum of monomers NIPAAM, VP and PA, as well as void polymeric nanoparticles were taken by dissolving the samples in D2O as solvent using Bruker Avance 400 MHz spectrometer (Bruker BioSpin Corporation, Billerica, MA, USA).
Dynamic light scattering (DLS) measurements
DLS measurements for determining the average size and size distribution of the polymeric micelles were performed using a Nanosizer 90 ZS (Malvern Instruments, Southborough, MA). The intensity of scattered light was detected at 90° to an incident beam. The freeze-dried powder was dispersed in aqueous buffer and measurements were done, after the aqueous micellar solution was filtered with a microfilter having an average pore size of 0.2 mm (Millipore). All the data analysis was performed in automatic mode. Measured size was presented as the average value of 20 runs, with triplicate measurements within each run.
Transmission electron microscopy (TEM)
TEM pictures of polymeric nanoparticles were taken in a Hitachi H7600 TEM instrument operating at magnification of 80 kV with 1 K × 1 K digital images captured using an AMT CCD camera. Briefly, a drop of aqueous solution of lyophilized powder (5 mg/ml) was placed on a membrane coated grid surface with a filter paper (Whatman No. 1). A drop of 1% uranyl acetate as immediately added to the surface of the carbon coated grid. After 1 min excess fluid was removed and the grid surface was air dried at room temperature before loaded in the microscope.
In vitro release kinetics of nanocurcumin
where, [Curcumin]rel is the concentration of released curcumin collected at time t and [Curcumin]tot is the total amount of curcumin entrapped in the nanoparticles.
In vitro and vivo toxicity studies with void polymeric nanoparticles
In order to exclude the possibility of de novo toxicity from the polymeric constituents, we utilized void nanoparticles against a panel of eight human pancreatic cancer cell lines (MiaPaca2, Su86.86, BxPC3, Capan1, Panc1, E3LZ10.7, PL5 and PL8). These cells were exposed to void nanoparticles for 96 hours across a 20-fold concentration range (93 – 1852 μg/mL) and cell viability measured by MTS assay, as described below. Further, limited in vivo toxicity studies were performed in athymic (nude) mice by intraperitoneal injection of void polymeric nanoparticles at a considerably high dosage of 720 mg/kg twice weekly, for a period of three weeks. Mice receiving intra-peritoneal nanocurcumin (N = 4) were weighed weekly during the course of therapy and average weight compared to that of control littermate nude mice (N = 4). At the culmination of the three week course, mice were euthanized and necropsy performed to exclude any intraperitoneal deposition of polymers, or gross organ toxicities.
Fluorescence microscopy for nanocurcumin uptake by pancreatic cancer cells
Curcumin is naturally fluorescent in the visible green spectrum. In order to study uptake of curcumin encapsulated in nanoparticles, BxPC3 cells were plated in 100 mm dishes, and allowed to grow to sub-confluent levels. Thereafter, the cells were incubated with nanocurcumin for 2–4 hours, and visualized in the FITC channel.
Cell viability (MTS) assays in pancreatic cancer cell lines exposed to nanocurcumin
Growth inhibition was measured using the CellTiter 96® Aqueous Cell Proliferation Assay (Promega), which relies on the conversion of a tetrazolium compound (MTS) to a colored formazan product by the activity of living cells. Briefly, 2000 cells/well were plated in 96 well plates, and were treated with 0, 5, 10, 15 and 20 μM concentrations of free curcumin and equivalent nanocurcumin, for 72 hours, at which point the assay was terminated, and relative growth inhibition compared to vehicle-treated cells measured using the CellTiter 96® reagent, as described in the manufacturer's protocol. A panel of ten human pancreatic cancer cell lines were examined (BxPC3, AsPC1, MiaPaca, XPA-1, XPA-2, PL-11, PL-12, PL-18, PK-9 and Panc 2.03) in the MTT assays; the sources and culture conditions of these ten lines have been previously described . All experiments were set up in triplicates to determine means and standard deviations.
Colony assays in soft agar
Colony formation in soft agar was assessed for therapy with free curcumin and equivalent dosage of nanocurcumin. Briefly, 2 ml of mixture of serum supplemented media and 1 % agar containing 5, 10 or 15 μM of free curcumin and equivalent nanocurcumin was added in a 35 mm culture dish and allowed to solidify (base agar) respectively. Next, on top of the base layer was added a mixture of serum supplemented media and 0.7 % agar (total 2 mL) containing 10,000 MiaPaca2 cells in the presence of void polymer, free or nano-curcumin, and was allowed to solidify (top agar); a fourth set of plates contained MiaPac2 cells without any additives. Subsequently, the dishes were kept in tissue culture incubator maintained at 37°C and 5 % CO2 for 14 days to allow for colony growth. All assays were performed in triplicates. The colony assay was terminated at day 14, when plates were stained and colonies counted on ChemiDoc XRS instrument (Bio-Rad, Hercules, CA).
Electrophoretic mobility shift assay (EMSA)
Nuclear extracts were prepared as described . Briefly, double-stranded oligonucleotides containing a consensus binding site for c-Rel (5'-GGG GAC TTT CCC-3') (Santa Cruz Biotechnology) were 5' end-labeled using polynucleotide kinase and [32P]dATP. Nuclear extracts (2.5–5 μg) were incubated with ≈1 μl of labeled oligonucleotide (20,000 c.p.m.) in 20 μl of incubation buffer (10 mM Tris-HCl, 40 mM NaCl, 1 mM EDTA, 1 mM β-mercaptoethanol, 2% glycerol, 1–2 μg of poly dI-dC) for 20 min at 25°C. DNA-protein complexes were resolved by electrophoresis in 5% non-denaturing polyacrylamide gels and analyzed by autoradiography.
Determination of IL-6, IL-8 and TNF-alpha synthesis
IL-6, IL-8 and TNF-alpha mRNA levels were assessed as described previously . Briefly, peripheral blood mononuclear cells (PBMC) from a healthy donor were isolated by centrifugation on a Ficoll Hypaque density gradient (GE Healthcare Biosciences) and washed twice with phosphate buffered saline (PBS; Invitrogen, Carlsbad, CA). Next, 500,000 cells per well of a 24 well plate in 1 ml of RPMI (Invitrogen) supplemented with 10% FBS (Invitrogen) and 1× Pen/Strep (Biofluids, Camarillo, CA) were co-stimulated with 2% phytohaemagglutinin (PHA M-Form, liquid; Invitrogen) and 1 μg/ml lipopolysaccharide (LPS; Sigma-Aldrich, St. Louis, MO) in the presence of free or nanocurcumin, using solvent and void nanoparticles as controls, respectively. Cells were lysed after 24 hours incubation at 37°C and 5% CO2 and RNA extracted using the RNeasy Mini Kit (Qiagen, Valencia, CA). Relative fold steady-state mRNA levels were determined on a 7300 Real time PCR System (Applied Biosystems, Foster City, CA,) by RT-PCR as described .
N-isopropylacrylamide = NIPAAM
N-vinyl-2-pyrrolidone = VP
poly (ethyleneglycol) monoacrylate = PEG-A
nuclear factor kappa B = NFκB
The Sol Goldman Pancreatic Cancer Research Center, Michael Rolfe Foundation for Pancreatic Cancer Research. AM and SB are supported by NIH R01CA119397. GF was supported by a fellowship grant within the postdoc programme of the German Academic Exchange Service (DAAD).
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