Polyphenol nanoformulations for cancer therapy: experimental evidence and clinical perspective

Introduction

Cancer has always been a great health problem all over the world despite growing advances in its prevention and treatment strategies. This disease is characterized by abnormal proliferation of cells that cannot be controlled or stopped. In addition, cells of malignant tumors have a tendency to become metastatic and attack tissues other than the place in which the primary tumor was formed.¹ Common modalities in the treatment of cancer consist of nonpharmacological treatments, such as radiation therapy, surgical operation, stem cell therapy and hyperthermia, as well as pharmacological intervention, including immunotherapy, chemotherapy, and hormone therapy, as well as a combination of these methods.¹ Nevertheless, all of these methods, including chemotherapy, have significant limitations such as dissatisfying specificity, which can cause low concentrations of drugs at the tumor site, causing multiple side effects and off-target toxic effects.²

Nanotechnology is the study of particles within the range of ≤100 nm, and a nanometer is defined as one-billionth of a meter.³ High surface area-to-volume ratio of atoms or molecules is one of the significant advantages of nanoparticles. This property leads to boosting of the surface activity and changes the physical and biological properties of the nanomaterials.

Rapid diagnosis of cancer and exact targeted drug delivery to the site of the neoplasms with the least possible adverse effects on other normal tissues are the desired goals of anticancer therapies. Nanotechnology has shown promising effects in the treatment of cancer.⁴ Through targeted drug delivery via nanoformulations, improvement in drug delivery to the tumor site can be achieved, and better therapeutic responses are expected. Another parameter is the increased selectivity that reduces the adverse effects of chemotherapy drugs.⁵ Nanoformulations such as liposomes,⁶ micelles,⁷ natural and synthetic nanoparticles, metal nanoparticles, and microspheres are among the important nanoformulations. Selection of these methods for nanoformulation provides improvements in bioavailability, biodistribution, specificity, and pharmacokinetics of drugs delivered to the site of tumor. Recently, several liposomal formulations have become available in the market for treatment of cancer, such as doxorubicin (Doxil^®), cytarabine (Depocyt^®), daunorubicin (DaunoXome^®), and vincristine (Onco-TCS^®). Doxil has been examined for head and neck cancer, brain tumors, ovarian cancer, breast neoplasms, and acquired immunodeficiency syndrome (AIDS)-related Kaposi’s sarcoma. Liposomal drug delivery has shown great promise, but it still has obstacles to overcome, including its short shelf life, limited loading potential, insufficient bioavailability on oral administration, decomposition of the drug inside the liposome, lack of enough control of drug release, and the unpredictable clearance by the reticuloendothelial system (RES). Polymeric, metal-, or lipid-based nanoparticles have a fundamental advantage for systemic drug delivery due to their smaller size, which permits lower RES uptake, extended circulation time, and better ability of penetration into capillaries.^8,9

Polyphenolic compounds comprise one of the most diverse groups of plant secondary metabolites, with several health-promoting properties, including antioxidant, anti-inflammatory, and antineoplastic activities.¹⁰ Previous studies have discussed the anticancer effects of numerous natural polyphenols such as curcumin, resveratrol, and several flavonoids;¹¹ thus, these compounds have attracted the attention of scientists for more extensive research.

The aim of the current study is to review current available data on the nanoformulations of natural polyphenols as chemopreventive and chemotherapeutic agents and to discuss the molecular mechanisms of their anticancer action.

Description of study selection

Electronic databases including “Scopus”, “PubMed”, and “ScienceDirect” were searched with the keywords “cancer” in title/abstract, along with “plant”, “phytochemical”, “extract”, and “herb” in the whole text. Data were collected from the inception date until August 2016. Only English language papers were included. Primarily obtained articles were screened by two independent investigators. Articles that had assessed nanoformulations of polyphenolic compounds in an in vitro or in vivo model of cancer were selected for this study, and conventional formulations (without using a nanonization technique), preparations of phytochemicals other than polyphenols, eg, terpenoids or alkaloids, and animal or cellular models of disease other than cancer were excluded. References of the retrieved studies were also screened for relevant articles. Included articles were screened for the name of phytochemical, nanonization technique, and the type of cancer in animal or cellular models. From a total of 1,939 results, 1,033 were excluded because of duplication, 138 for being reviews, and 713 being irrelevant judged on the title and/or abstract. From the 55 primarily selected papers, 15 were excluded based on the full texts (four were excluded because they were not on polyphenols, four because no anticancer effects were assessed, three because the cell cultures were not cancerous cell lines, three because the polyphenol was not prepared in the form of a nanoformulation, and 1 because the full-text was in Chinese). Finally, 40 articles were included in this review. Table 1 shows the summary of the obtained results.

Table 1 Polyphenols with anticancer activity and their mechanisms of action Abbreviations: ALP, alkaline phosphatase; AST, aspartate aminotransferase; ALT, alanine aminotransferase; CAT, catalase; cyt c, cytochrome c; 5-FU, 5-fluorouracil; GSH, glutathione; HPV, human papillomavirus; IL, interleukin; iNOS, inducible nitric oxide synthase; miRNA, microRNA; MMP, mitochondrial membrane potential; MPEG, methoxy polyethylene glycol; MPEG-PLA-PAE, (MPEG)-poly(lactide)-poly(b-amino ester); OMe, methoxy group; PAE, poly(b-amino ester); PARP, poly(ADP-ribose) polymerase; PCL, poly(ε-caprolactone); PEG, polyethylene glycol; pegylated, conjugated with PEG; PHEMA, poly(2-hydroxyethyl methacrylate); PLA, polylactic acid; PLGA, poly(lactic-co-glycolic acid); PNIPAAm, poly(N-isopropylacrylamide); ROS, reactive oxygen species; SOD, superoxide dismutase; TNF, tumor necrosis factor.

Nanostructures and polymers

Several polymers and nanostructures have been used for the preparation of polyphenolic compounds as anticancer agents. Poly(lactic-co-glycolic acid) (PLGA) nanoparticles are a group of hydrophobic, biocompatible, and biodegradable polymers that have attracted immense interest since being approved by the US Food and Drug Administration as a safe drug delivery system. This polymer consists of lactic acid and glycolic acid, which can be metabolized by the body, and the degradation rate depends on their ratio.¹² Polylactic acid (PLA) is another biodegradable and biocompatible polymer that is metabolized into monomeric units of lactic acid in the body. The preparation method for PLA nanoparticles is mostly via solvent evaporation, solvent displacement,¹³ salting out,¹⁴ and solvent diffusion.¹⁵ Recently, researchers have focused on polymeric systems activated by a water-soluble stimulus, which represents a phase transition in response to external stimuli such as pH, specific ions, temperature, and electrical field.

Dendrosome is an amphipathic, neutral, and biodegradable nanostructure that has been used for delivering genes into diverse cell lines.¹⁶ Dendrosomes are liposomes encapsulating dendrimers in their aqueous core and can be functionalized using different functional groups.¹⁷

Poly(β-amino ester) (PAE) is a biodegradable and pH-responsive polymer that is not soluble at pH of 7.4, but it turns into a soluble form at a lower pH (<6.8) through the protonation process of tertiary amino groups.¹⁸ Another popular polymer is poly(N-isopropylacrylamide) (PNIPAAm) that is used for the preparation of temperature- and pH-sensitive structures for biomedical applications.¹⁹ Cyclodextrins (CDs) consist of cyclic oligosaccharide molecular structures including six (a-), seven (b-), or eight (c-) D-glucopyranose units linked together by α-(1,4) glycosidic linkages. These structures are popular to form molecular complexes loaded with other molecules, eg, drugs.²⁰ β-CD is the most commonly used type because of the easier synthesis, low price, and large loading capacity for polar molecules that can be loaded into the internal cavity.

A new drug delivery system for anticancer drugs is based on cellulose nanocrystals (CNCs) consisting of uniform nanorods with liquid crystalline features, high mechanical resistance, high surface area-to-volume ratio, ability for sustained drug release, biodegradability, and biocompatibility.²¹ This structure is obtained by the acid hydrolysis of cotton fibers, and the surface can be easily modified.²²

Micelles are lipid structures that are converted into a spherical form in water. This reaction is the result of the amphipathic properties of fatty acid molecules, which contain both hydrophilic polar heads and long hydrophobic regions. The hydrophilic head groups of micelles usually form the outside surface of micelles, and the hydrophobic tails are retained inside because they are nonpolar. The core–shell structure of the polymeric micelles is the result of the amphiphilic block, which is useful for solubilizing water-insoluble or poorly soluble drugs. Some advantages of the use of micelles as a drug delivery system include conjugation with the targeted molecules via surface modification, trapping of hydrophobic drugs into a hydrophobic core that protects drugs from fast degradation, and reduction of nonspecific uptake by RES.²³

Poly(ε-caprolactone) (PCL) is a biodegradable and biocompatible polymer that is extensively studied for controlled drug delivery systems.²⁴ A wide range of drugs are compatible with this polymer, which provides homogeneous drug dispersal in the formulation and lower degradation rate.²⁴ Poly(2-hydroxyethyl methacrylate) (PHEMA) is an artificially synthesized polymer with numerous applications in medical instruments and devices, such as soft contact lenses and artificial cornea, as well as in drug delivery systems.²⁵ The polar hydroxyl groups and carbonyl functional groups on the repeating units of PHEMA make it a water-compatible polymer. Hydrophobic α-methyl groups in the PHEMA backbone result in better hydrolytic stability of the polymer.²⁶ Among the different drug delivery systems, lipid nanoparticles, such as nanostructured lipid carriers, can be applied for the delivery of many types of drugs.²⁷ Large-scale production in high quality and lack of need for organic solvent during the production are advantages of lipid-based nanocarriers. They have good stability during long storage and can be either lyophilized or steam sterilized.²⁸ Nanostructured lipid carriers consist of standard ingredients for pharmaceutical use in humans and are recognized as safe structures.²⁹

Polyphenol nanoformulations for cancer therapy

Coumarins

Coumarins are polyphenols with appetite-suppressing properties that discourage animals from eating plants containing them. They can also be used in the treatment of lymphedema.³⁰ They can also cause bleeding, which is their most well-known feature.³¹ Fabaceae, Lamiaceae,³² and Rosaceae³³ are among the natural sources of coumarins. These phytochemicals have shown anticancer properties via several mechanisms of action.

4-Methyl-7-hydroxycoumarin is a synthetic coumarin that is made by methylation of umbelliferone (7-hydroxycoumarin). PLGA nanoparticles of 4-methyl-7-hydroxy coumarin have demonstrated anticancer effects in melanoma A375 cell cultures by increasing cell apoptosis, DNA fragmentation, caspase-3, and p53 (tumor suppressor factors) and by decreasing cell viability.³⁴ Farnesiferol C is another coumarin extracted from plant species such as Ferula asafoetida. Dendrosomal nanoformulation of farnesiferol C exhibited antineoplastic activity by decreasing cell proliferation in AGS gastric cancer cell line. The expression of Bax (an antiapoptotic marker) and Bcl-2 (a proapoptotic factor) was modified so that the Bax/Bcl-2 ratio increased as a result of treatment with dendrosomal farnesiferol C.³⁵

Diarylheptanoid (curcumin)

Curcumin is the principal diarylheptanoid polyphenolic structure extracted from turmeric (Curcuma longa) rhizome, and it has numerous biological and pharmacological properties such as antioxidant, anti-inflammatory, and anticarcinogenic activities. It has been thoroughly studied in the field of cancer therapeutics.³⁶

Several studies have assessed the antineoplastic properties of curcumin-loaded PLGA nanoparticles on different cancerous cell lines. The anticancer activity of nanocurcumin in cervical cancer has been investigated, wherein CaSki and SiHa cells were treated with nanocurcumin, which resulted in reduction of cell growth and cellular proliferation. The formulation also induced apoptosis via G1/S cell cycle arrest.³⁷ Beta-catenin is a protein involved in transcription and cell proliferation, whose phosphorylation is controlled by glycogen synthase kinase 3 (GSK3); thus, any malfunction in β-catenin signaling can result in abnormal cell proliferation.³⁸ Small noncoding sequences of RNA, known as microRNAs (miRNAs), are found to have regulatory effects on oncogenic or tumor suppressor genes and to be involved in tumorigenesis. Investigations have suggested miRNAs as novel therapeutic targets for cancer. The potential therapies of miRNAs with oncogenic activities include miRNA masking, microRNA sponges, and anti-miRNA oligonucleotides. Natural agents that block or enhance the expression of specific miRNAs can lead to the suppression of oncogenic effects.^39,40 Nanocurcumin has been shown to diminish the levels of miRNA-2, nuclear β-catenin, and E6/E7 human papillomavirus (HPV) oncoproteins in an orthotopic mouse model of cervical cancer.³⁷ Using the same formulation, Xie et al⁴¹ showed the anticancer effects in human colorectal cancer HCT116 cells via decrease in cell viability, increase in apoptosis by G2/M phase cell cycle arrest, and decrease in the toxicity for normal cells. In another study, PLGA nanocurcumin showed antineoplastic properties for cisplatin-resistant A2780CP ovarian cancer cells, as well as metastatic MDA-MB-231 breast cancer cell cultures by increasing apoptosis and decreasing tumor cell proliferation and colony formation.⁴² The same formulation could elevate mitochondrial cytochrome (cyt) C release and reduce both mitochondrial reactive oxygen species (ROS) generation as well as levels of inducible nitric oxide synthase (iNOS), alkaline phosphatase (ALP), aspartate aminotransferase (AST), and alanine aminotransferase (ALT), in diethylnitrosamine-induced hepatocellular carcinoma in Swiss albino rat.⁴³

PLGA nanoparticles loaded with curcumin and conjugated with anti-P-glycoprotein could exhibit cytotoxic effects in human cervical carcinoma KB-V1 and KB-3-1 cells, which resulted in increased curcumin solubility and cellular uptake as well as decreased cell viability.⁴⁴ A micellar system based on methoxy polyethylene glycol (MPEG)-poly(lactide)-poly(b-amino ester) (MPEG-PLA-PAE) copolymers loaded with curcumin was assessed for its anticancer activity in MCF-7 human breast cancer cell cultures and MCF-7 tumor-bearing mice. The study showed better cellular internalization than free curcumin and significantly diminished tumor cell growth both in vitro and in vivo.⁴⁵ A nanoformulation of thermoresponsive chitosan-g-poly(N-isopropylacrylamide) co-polymeric nanoparticles (TRC-NPs) loaded with curcumin was tested for anticancer effects in human prostate cancer PC3, breast cancer MCF-7 cells, and human nasopharyngeal cancer KB cells.⁴⁶ The formulation yielded a better uptake of curcumin by cancerous cells, decreased the cell viability, and augmented apoptosis in PC3 cells, as well as diminished the mitochondrial membrane potential in PC3 cells.⁴⁶ In a recent study,⁴⁷ CD/CNCs loaded with curcumin were examined. The formulation improved the cellular uptake of curcumin and could inhibit cell proliferation in human prostate cancer PC-3 and DU145 cells and in human colorectal carcinoma HT-29 cells.⁴⁷ In another study, folate-modified PLA-PEG micelles loaded with curcumin were examined in human hepatocellular carcinoma HepG2 cells, which resulted in decreased cell growth.²³ Dextran micelles loaded with curcumin were studied as a pH-sensitive drug delivery system in C6 glioma cells, which demonstrated improved cellular uptake and reduction of cell proliferation.²³ Dendrosomal nanoparticles loaded with curcumin exhibited antineoplastic activity in metastatic breast cancer cells in BALB/c mice by reducing the tumor size and expression of the STAT3, interleukin-10 (IL-10), and arginase-1 genes, as well as by augmentation of STAT4 and IL-12 gene expression.⁴⁸ Curcumin-loaded caprolactone was also found to decrease tumor growth in S180 cancer-bearing mice.⁴⁹ Curcumin-loaded PHEMA nanoparticles could increase apoptosis and inhibit tumor cell growth in ovarian cancer SKOV-3 cells.⁴⁹ The colloidal system of curcumin nanoparticles in esophageal Barrett cancer OE19 and OE33 cells was tested.⁵⁰ The results showed a decrease in proliferation rate of cancer cells, but not in normal cells. The formulation also activated apoptosis of T cells and reduced the production of IL-1β, IL-6, IL-8, IL-10, and tumor necrosis factor (TNF)-α.⁵⁰ Nanoparticles of curcumin prepared by ultrasonic spray also showed anticancer properties in human PC3 prostate cancer and human embyronic kidney HEK cell lines.⁵¹ Nanostructured lipid carriers loaded with curcumin were examined in human neuroblastoma LAN5 cells, which showed diminishing cell viability and increasing Hsp70 activity.⁵² Polymer-coated magnetic nanoparticles of curcumin could decrease cell viability in human ovarian carcinoma SKOV-3 cells.⁵³ Lipid–polymer hybrid nanoparticles loaded with a combination of docetaxel and curcumin were studied in human PC3 prostate adenocarcinoma cells, which resulted in better adherence of the nanoparticles to the cell membrane and antitumor effects in PC3 cells.⁵⁴ In vivo tumor growth was also inhibited by a docetaxel–curcumin nanoformulation in a BALB/c nude mouse model of the disease.⁵⁴

Flavonolignans

Flavonolignans are natural compounds of flavonoids and lignans with antioxidant and hepatoprotective properties. Silibinin, silychristin, silydianin, dehydrosilybin, and deoxysilycistin are the examples of flavonolignans that can be found in sylimarin, the extract of milk thistle (Sylibum marianum), and in other plants of the Asteraceae family.⁵⁵

Silibinin in combination with glycyrrhizic acid (a triterpene from licorice), formulated as nanoliposomes containing its PEG conjugate (pegylated nanoliposomes), demonstrated its anticancer effect by decreasing the cell viability in human hepatocellular carcinoma HepG2 cells. The nanoformulation was about 10-fold more potent than the free molecules.⁵⁵

Lignans

Lignans are plant chemical compounds that are found in dietary fiber; thus, fiber-rich food items can be a good source for lignans. Plants of Pedaliaceae⁵⁶ and Linaceae⁵⁷ families are among the well-known sources of lignans. Lignans have several therapeutic activities such as anti-inflammatory and antioxidant properties, which are beneficial for the management of pathologic conditions in humans.⁵⁸

Honokiol is a lignan extracted from the seed cones, bark, and leaves of trees of the Magnoliaceae family.⁵⁹ A nanoformulation of honokiol caused an anticancer effect in the mouse Lewis lung cancer LL/2 cell lines by induction of cell cycle arrest at the G0/G1 phase.⁶⁰

Naphthoquinones

Naphthoquinones are a subcategory of phenolic compounds derived from naphthalene. Alkanin from Alkanna tinctoria⁶¹ and juglone from Juglans regia62 are well-known examples of naphthoquinones. Naphthoquinones have an important role in blood coagulation and ossification. Overuse of naphthoquinones can cause internal bleeding and peptic ulcers.⁶³ Naphthoquinones have also demonstrated acceptable anticancer properties and have attracted the attention of scientists to be formulated as a novel drug delivery system such as nanoparticles.

Plumbagin is a naphthoquinone that is extracted from Plumbago spp, family Plumbaginaceae, and it has demonstrated anticancer activity in human cancerous cell lines.⁶⁴ Silver nanoparticles of plumbagin induced apoptosis in cancerous human skin HaCaT and A431 cells by producing free radicals. The compound also showed anticancer effect by increasing pyruvate kinase activity (an enzyme that catalyzes pyruvate and ATP synthesis in glycolysis and has a crucial role in the metabolism of normal as well as cancerous cells).^65,66

Stilbenes

Stilbenes are E- or Z-isomers of 1,2-diphenylethene derivatives. These compounds are distributed in a few number of plant families, especially Vitaceae (grapes family), which is the source of the most well-known stilbene, resveratrol.⁶⁷ There are numerous studies on the anticancer properties of resveratrol. Nanoformulated resveratrol using PLGA-PEG demonstrated anticancer effects by decreasing cell growth and proliferation in human prostate cancer DU-145, PC-3, and LNCaP cell lines.⁶⁸ Another study also reported the antineoplastic activity of resveratrol PCL nanocapsule formulation on cultures of mouse B16F10 skin melanoma cells.⁶⁹ In rat C6 glioma cells, resveratrol nanoformulated with MPEG–PCL polymer exhibited cytotoxic effects.⁷⁰ Bovine serum albumin nanoparticles of resveratrol also inhibited tumor growth in a nude mouse model of human ovarian cancer SKOV3 cells.⁷¹ Resveratrol coencapsulated with 5-fluorouracil (5-FU) using PEG polymer showed mixed cytotoxicity in human NT8e head and neck cancer, because the effect was synergistic at high concentrations of resveratrol, whereas an antagonistic effect was observed with low resveratrol concentrations.⁷² This shows that the use of resveratrol in cancer should be closely monitored; otherwise, it may cause undesirable results.

Flavonoids

Flavonoids are a group of polyphenols that have several subclasses, such as chalcones, flavones, isoflavones, flavanones, flavonols, and anthocyanins.⁷³ Many pharmacological effects of flavonoids have been reported, such as antioxidant,⁷⁴ anti-inflammatory,⁷⁵ immunomodulatory,⁷⁶ and antineoplastic activities.⁷⁷ Flavonoids can be extracted from higher plants. They can be found in yellow, orange, or red colors and are widely available as colorful fruits and vegetables in the human diet: Apiaceae (parsley), Ericaceae (blueberries),⁷⁸ Rutaceae (citrus fruits), Rosaceae (apple), as well as the popular delicious product of Theobroma cacao, namely, dark chocolate.⁷⁹ Flavonoids have demonstrated antineoplastic activity in several studies.⁸⁰

Baicalein is a flavonoid that can be found in the roots of Scutellaria baicalensis and S. lateriflora.⁸¹ Baicalein nanoparticles with dual-targeted ligands of folate and hyaluronic acid demonstrated anticancer effect on human lung cancer A549 as well as paclitaxel-resistant lung cancer A549/PTX cells by decreasing cell viability and inhibiting tumor growth in xenograft mouse model of A549/PTX.⁸² Chrysin is another flavonoid that can be extracted from many plants such as Passiflora spp., as well as from some mushrooms such as Pleurotus ostreatus.⁸³ Chrysin has many pharmacological effects such as anti-inflammatory, antioxidant, and anticancer properties.⁸⁴ A nanosuspension of chrysin showed anticancer effects on human hepatocellular carcinoma HepG2 cells by inhibiting cell growth.⁸⁵ EGCG can be found in several plants, especially in green tea.⁸⁶ The flavonoid demonstrated anticancer effects on human and animal cells by several different mechanisms. EGCG formulated as Ca/Al-NO₃ layered double-hydroxide nanoparticles induced apoptosis, decreased cell viability, and inhibited colony formation in human prostate cancer PC-3 cells.⁸⁷ Another study reported that chitosan nanoparticles of EGCG exhibited anticancer effects on human melanoma Mel 928 cells by apoptosis via increase in Bax levels, increased poly (ADP-ribose) polymerase (PARP) cleavage, G2/M phase cell cycle arrest, inhibition of cyclin D1 and D3, induction of p21 and p27, decrease in Bcl-2, caspase-3 and caspase-9 protein expression, which resulted in the reduction of cancer cell viability.⁸⁸ The anticancer effect of EGCG chitosan nanoparticles on xenograft athymic mouse model of melanoma was shown by suppression of tumor growth and proliferation, inhibition of CDK4 and 6, and an increase in apoptosis.⁸⁸ EGCG core–shell PLGA–casein nanoparticles in combination with paclitaxel also showed anticancer activity on MCF-7 cells and human MDA-MB-231 breast cancer cells by increasing apoptosis and decreasing NF-κB activation.⁸⁹ Green tea polyphenols containing EGCG as one of the major components were nanoformulated by graphene nanosheets and showed anticancer effects on colon cancer HT29 and SW48 cells, via photothermal destruction of cells, assessed by high-efficiency near-infrared photothermal therapy.⁹⁰ Luteolin is a flavonoid with yellow crystalline appearance and is widely found in plants and vegetables of human diet.⁹¹ Phytosomes containing luteolin demonstrated anticancer effects on human MDA-MB-231 breast cancer cells by reducing cell viability and decreasing the expression of Nrf2 and its related downstream gene Ho1.⁹² The formulation also reduced sensitivity of cells to the chemotherapeutic agent doxorubicin.⁹² Another study⁹³ reported that nanoformulated luteolin with PLA-PEG polymer possesses anticancer effects against lung cancer H292 cells and TU212 head and neck squamous cell carcinoma, demonstrated by inhibition of tumor growth and colony formation. In addition, these effects were observable in xenograft mouse model of head and neck cancer, which resulted in reduced tumor growth and tumor size.⁹³ Quercetin is another widely distributed flavonoid that can be found in several fruits such as the apple. Quercetin supplements are used for their cancer-preventive effects.⁹⁴ Quercetin nanoformulated as phytosomes had anticancer effects on human breast cancer MCF-7 cells by increasing apoptosis and decreasing mRNA expression of Nrf2 downstream genes NQO1 and MRP1; however, quercetin did not make a significant change in Nrf2.⁹⁵ In another study, quercetin nanoformulated in liposomes showed anticancer effects on human breast cancer MCF-7 cells by reducing cancer cell proliferation and induction of antitoxic effects.⁹⁶

Concerns regarding the safety of nanopolyphenols

Although most of the current evidence supports the idea of anticancer activity of natural polyphenols,⁹⁷ there are some concerns regarding their antioxidant properties, which may not only help normal cells but also increase the surveillance of cancerous cells against oxidative stress caused by conventional anticancer agents. In other words, polyphenols should act in an intelligent manner in order to support the viability of only normal cells, not the cancerous ones, which is not currently guaranteed with respect to available evidence.⁹⁸

Another concern is with reference to the safety of the nanostructures used for formulation of polyphenols. As nanotechnology is rather a new field of science and research, many questions are still left unanswered. Some types of nanostructures can accumulate at the site of injection and may not be completely cleared from the blood circulation.⁹⁹ It should be mentioned that nanoformulations of a drug have different physicochemical properties in comparison to their simple preparations; thus, different pharmacokinetics and pharmacodynamics are expected. Hence, studies performed on the simple form of the drugs cannot totally reflect the toxicity of the nanoformulated products, and toxicological studies should be conducted for nanoformulations.^100,101

Discussion

Cancer has always been a major calamity for health care providers all over the world due to difficulties in the selection of a suitable treatment approach. Chemotherapy, radiotherapy, immunotherapy, and surgery are currently used in cancerous patients; nevertheless, fast cellular and molecular changes in the cells, which result in drug-resistant types of the disease, can prevent achieving the ideal results.¹⁰²

Severe adverse effects of chemotherapy such as gastrointestinal complications, alopecia, bone marrow suppression, and secondary malignancies have limited its use. Lower cellular permeability, unusual vascularization, and abnormal expression of surface molecules (such as drug-resistant pumps) have caused chemotherapeutic drugs to have a low concentration in these cells.¹¹ Nanonization, as a recently developed technique in drug delivery systems, can improve the bioavailability of antineoplastic agents via passive or active targeting.¹⁰³ Table 2 shows a summary of the advantages and disadvantages of nanoformulations in comparison to conventional drugs. In the current paper, we have discussed the nanoformulation of polyphenols and their application as anticancer agents. Figure 1 summarizes the main anticancer properties of nanoformulated polyphenols.

Table 2 Advantages and disadvantages of conventional formulations versus nanoformulations

Figure 1 Polyphenol nanoformulations and their mechanisms as antineoplastic agents. Abbreviations: Casp-3 and -9, caspase-3 and caspase-9; MAPK/ERK, mitogen-activated protein kinases/extracellular signal-regulated kinases.

Polyphenols, as a group of phytochemicals with a wide range of pharmacological and therapeutic activities, have been extensively studied in the area of oncology. Nanoformulations of polyphenols have been prepared using a wide variety of techniques that use carriers such as liposomes, dendrosomes, nanocapsules, and nanosheets. In addition, micelles of polymers such as PLA, PLGA, PEG, PHEMA, CD, dextran, and chitosan, as well as bovine serum albumin and silver nanoparticles, have been used. These polymers were activated by using different functional groups such as carboxyl (COOH) or methoxy (OMe) groups. Most of these nanostructures were successful in demonstrating anticancer activity. Curcumin, resveratrol, EGCG, chrysin, baicalein, luteolin, quercetin, honokiol, silibinin, and coumarin derivatives are among the polyphenols assessed for their anticancer properties in nanoformulated forms (Table 1).

Polyphenol nanoformulations perform their anticancer activity via several cellular mechanisms, including induction of cell cycle arrest at different phases of cancer cell cycle, activation of caspase enzymes, reduction of tumor vascularization, reducing tumor cell invasion and metastasis, induction of mitochondrial damage, as well as apoptosis in the neoplasm. Apoptosis, an important indicator for anticancer therapy, is one of the main functions of nanoformulated polyphenols in cancer cells. Many natural products exert their anticancer effect through apoptosis, which is the programmed cell death responsible for removal of unwanted cells. Among the Bcl-2 family proteins that regulate apoptosis, Bcl-2 and Bax possess opposite actions in the apoptosis process. Bcl-2 blocks apoptosis, while Bax promotes apoptosis.^104,105 The elevation of the ratio of Bax/Bcl-2 has a pivotal contribution to apoptosis, which is considered as one of the main mechanisms of these drugs in the induction of apoptosis in cancer cells.

Natural remedies have been broadly administered as complementary and alternative medicines for thousands of years for the management of different types of malignancies to achieve optimal body performance. The conventional medicines delivered via currently available dosage forms provide anticancer potential only up to a suboptimal degree. Hence, the existence of limitations in the management of malignancies is predictable. This requires exploring novel drug delivery strategies for the development of targeted and safe drug delivery medicines with enhanced therapeutic activity. The most important improvement in nanoformulated polyphenols, in comparison to their free molecules, is their higher antineoplastic activity and better bioavailability, which can improve passive targeting of cancerous cells. In the meantime, this scenario requires lower doses of drugs to obtain optimum response while bypassing the pharmacokinetic problems of conventional formulations. Several studies have suggested that phytochemicals, including polyphenols, have anticancer properties exploitable in an intelligent manner, ie, showing cytotoxicity only against neoplasms, not against normal cells, whereas no such intelligence has been detected for conventional anticancer agents.^43,52,106

Conclusion

Thus, polyphenols are suitable candidates to be studied as future anticancer agents. Nanoformulation techniques are suggested to be promising alternatives relative to the conventional drug delivery systems as they encompass exclusive drug delivery properties. It is concluded that nanonization can potentiate the beneficial effects of natural polyphenols against cancer by different mechanisms, hence opening a new field of research in oncology. However, it should be mentioned that all of the abovementioned formulations were evaluated in cellular and/or animal models of malignancies; thus, a long path lies ahead of commercializing these agents, including assessment of their safety and efficacy in healthy as well as cancerous subjects in preclinical and clinical settings.

Author contributions

All authors contributed toward data analysis, drafting and critically revising the paper and agree to be accountable for all aspects of the work.

Disclosure

The authors report no conflicts of interest in this work.

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