Application of active targeting nanoparticle delivery system for chemotherapeutic drugs and traditional/herbal medicines in cancer therapy: a systematic review

Introduction

Cancer remains one of the major causes of deaths worldwide. In 2017, approximately 1.7 million new cases and 600 thousand deaths were estimated to occur in the USA.¹ Most patients treated with conventional chemotherapy suffer from serious side effects due to non-selective action of chemotherapeutic drugs to normal cells. For a few decades, nanoparticles have been developed as a drug delivery system of various chemotherapeutic drugs to enhance drug efficacy and safety.^2–4 Nanoparticles play an important role in increasing drug concentration in cancer cells by enhancing drug accumulation by passive and active targeting mechanisms as well as by decreasing drug efflux from cancer cells. The passive targeting nanoparticle is the mechanism by which the drugs leak from blood vessels supplying cancer cells and accumulate in the cells by enhanced permeability and retention (EPR) effect.⁵ The active targeting nanoparticles, on the other hand, target ligands conjugated on the surface of nanoparticles, resulting in increasing cellular uptake by receptor-mediated endocytosis and therefore increased drug accumulation in cancer cells. This mechanism relies on the interaction between tumor ligands conjugated on the surface of nanoparticles and cell-surface receptors or antigens on cancer cell surfaces (Figure 1).⁵ Nanoparticles acting via both mechanisms have been shown to increase drug concentration in cancer cells. Active targeting nanoparticles have been shown in various studies to be more efficient in increasing drug accumulation in cancer cells and therefore play important role not only in modern cancer chemotherapy, but also in cancer therapy with traditional/herbal medicines.^6–11 A number of nanoparticle formulations derived from these active compounds have been developed for active targeting purpose to improve anticancer efficacy and to reduce side effects. The objective of this current review is to summarize the research articles relating to the application of active targeting nanoparticles delivering system for chemotherapeutic drugs derived from chemical synthesis as well as natural sources.

Figure 1 Passive targeting and active targeting mechanisms of nanoparticles.

Materials and methods

Study selection and inclusion and exclusion criteria

This systematic review was conducted through the search from PubMed database up to March 2017. The following keywords were used: nanoparticle, chemotherapy, traditional medicine, herbal medicine, natural medicine, natural compound, cancer treatment, and active targeting. Inclusion criteria for selection of the searched articles were 1) articles in full text and written in English; 2) articles with in vitro or in vivo investigations of effects of nanoparticles delivering chemotherapeutic drugs or traditional/herbal medicines on efficacy and/or safety; and 3) articles with investigations of targeting and receptor/antigen. The articles with insufficient data for extraction or those with application for radiotherapy, gene therapy, photodynamic therapy, or for diagnostic purpose, or duplicate articles, or review articles were excluded from the analysis.

Data extraction and collection

The titles and abstracts of articles searched from PubMed database using the above mentioned keywords were initially screened to obtain relevant original research articles according to the eligibility criteria. Thereafter, the full texts of all relevant articles were carefully examined in details to confirm their compliance with the defined eligibility criteria. The studies of active targeting nanoparticles applied for both chemotherapeutic drugs and traditional/herbal medicines for cancer were classified according to the types of targeting ligands.

Results

Study description

Twenty out of 695 research articles were initially excluded from the analysis during title screening for duplicate articles. The titles together with abstracts of the remaining articles were further checked for eligibility criteria and a total of 597 articles were excluded from the analysis. Finally, 61 out of 78 articles were included in the analysis, 17 articles being excluded due to unclear/inadequate information. The flow diagram of the search process is presented in Figure 2, and the effects of active targeting nanoparticles delivering modern chemotherapeutic drugs and traditional/herbal medicines for cancer are summarized in Tables 1 and 2.

Figure 2 Flow diagram showing the different phases of the systematic review.

Table 1 Summary of research articles that investigated active targeting NPs delivering chemotherapeutic drugs in cancer therapy Abbreviations: ASGPR, asialoglycoprotein receptor; BSA, bovine serum albumin; cRGDyK, cyclic arginine-glycine-aspartic acid-tyrosine-lysine; DSPE-PEG2000, 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-methoxy polyethylene glycol-2000; EGF, epidermal growth factor; EGFR, epidermal growth factor receptor; FGFR, fibroblast growth factor receptor; HACE, hyaluronic acid-ceramide; HSA, human serum albumin; HPAEG, hyperbranched poly(2-((2-(acryloyloxy)ethyl)disulfanyl)ethyl 4-cyano-4-(((propylthio)-carbonothioyl)-thio)-pentanoate-co-polyethylene glycol methacrylate; IL, interleukin; LbL-LCNP, layer-by-layer-liquid crystalline nanoparticle; LHRH, luteinizing hormone-releasing hormone; LHRHR, luteinizing hormone-releasing hormone receptor; LPL, lithocholic acid-polyethylene glycol-lactobionic acid; LRP, low density lipoprotein-receptor related protein; Lys-LA10, L-lysine methyl ester-lipoic acid; mAb, monoclonal antibody; MIONP, magnetic iron oxide nanoparticle; MSN, mesoporous silica nanoparticle; NP, nanoparticle; γ-PGA-PLA, poly(gamma-glutamic acid)-poly(lactic acid); PBLG, poly(γ-benzyl L-glutamate); PBLG-LA, G-poly(c-benzyl-L-glutamate)-lipoic acid; PCL, polyethylene glycol-poly(ε-caprolactone); PEG; polyethylene glycol; PEI, polyethylenimine; PLGA, poly(lactic-co-glycolic acid); PTMC, poly(trimethylene carbonate); RGD, arginine–glycine–aspartic acid peptide; RGDS, arginine–glycine–aspartic acid–serine peptide; SPARC, secreted protein, acidic and rich in cysteine; TbFGF, truncated basic fibroblast growth factor; TPGS, tocopheryl polyethylene glycol 1000 succinate; UnTHCPSi, undecylenic acid modified, thermally hydrocarbonized porous silicon.

Table 2 Summary of research articles that investigated active targeting NP delivering traditional/herbal medicines in cancer therapy Abbreviations: AMPB, (3-aminomethylphenyl)boronic acid; cRGD, cyclic arginine–glycine–aspartic acid peptide; HACE, hyaluronic acid-ceramide; mPEG-PLGA-PLL, methoxy polyethylene glycol-poly(lactic-co-glycolic acid)-poly-L-lysine; NP, nanoparticle; PCEC, poly(ε-caprolactone)-polyethylene glycol-poly (ε-caprolactone); PLGA, poly(lactic-co-glycolic acid); RGD, arginine–glycine–aspartic acid peptide.

Of the 61 articles included in the analysis, 54 (88.5%) investigated nanoparticles delivering modern chemotherapeutic drugs; the majority was doxorubicin (40.7%), followed by paclitaxel (8.5%). Types of targeting ligand platforms used included proteins or small peptides (15 articles), hyaluronic acids (HAs; 10 articles), folic acids (9 articles), antibodies (5 articles), aptamers (5 articles), carbohydrates or polysaccharide (5 articles), and other molecules (5 articles). Seven articles (11.5%) investigated nanoparticles delivering traditional/herbal medicines; the majority was curcumin (42.9%). The ligand platforms used were proteins or small peptides (2 articles), HA (1 article), folic acid (1 article), antibody (1 article), aptamer (1 article), and other molecule (1 article).

Discussion

Ligands for nanoparticle platform

Proteins or small peptides

Various types of proteins or small peptides were used to conjugate on the surface of nanoparticles to improve selectivity of chemotherapeutic drugs or traditional/herbal medicines to cancer cells. Transferrin, a serum glycoprotein, was one of the widely used targeting ligands. It plays a role in transferring iron from blood stream into the cells by binding to transferrin receptor on the cell surface. Upregulation of transferrin receptor has been reported in metastatic and drug-resistant cancer cells.⁶⁷ The transferrin-conjugating nanoparticles delivering chemotherapeutic drugs have been shown to improve cellular uptake of the drugs by cancer cells and enhance in vitro and in vivo cytotoxicity. For instance, the transferrin-conjugated polyethylene glycol (PEG) nanoparticle delivering hydroxycamptothecin was shown to provide longer retention time of drug in blood circulation, higher drug accumulation in cancer cells, and higher in vivo growth inhibitory activity against S180 tumor compared with non-targeted nanoparticles.¹⁶ In the study of transferrin-conjugated chitosan-PEG nanoparticles delivering paclitaxel, the targeted nanoparticles also exhibited higher cytotoxic activity to transferrin-overexpressing human non-small cell lung cancer cells (HOP-62). The respective half-maximal inhibitory concentrations (ICs₅₀) were 0.3 μM and 2.0 μM.¹⁷ Apart from transferrin, arginine–glycine–aspartic acid (RGD) peptide has been used as targeting ligand to conjugate on the surface of nanoparticles to specifically target integrin α_vβ₃ receptor. This receptor is expressed on the surface of tumor vessels and various types of cancer cells and plays important roles in tumor growth promotion, metastasis, and angiogenesis.¹⁸ A number of RGD-conjugated nanoparticles delivering chemotherapeutic drugs or traditional/herbal medicines have been developed and demonstrated to promote their delivery to the cancer cells. The cyclic arginine–glycine–aspartic acid–tyrosine–lysine c(RGDyK)-conjugated poly(trimethylene carbonate)-PEG micellar nanoparticle delivering paclitaxel was shown to enhance cytotoxic activity of the drug to integrin α_vβ₃-overexpressing human glioblastoma cells (U87MG) compared with non-targeted nanoparticles and free drugs (mean IC₅₀: 0.022 μg/mL, 0.051 μg/mL, and 0.058 μg/mL, respectively). The targeted nanoparticles also exhibited greater activity on cell apoptosis (11.23% vs 8.31% and 8.03% vs 5.38% inhibition, for early and late apoptosis, respectively). The percentages (mean values) of free drug were 6.67% and 4.32%, respectively. In addition, cellular drug uptake by U87MG cells was significantly increased.¹⁸ Superior cytotoxic potency against integrin α_vβ₃-overexpressing human cervical carcinoma cells (HeLa) together with increased cellular uptake was also demonstrated with RGD-conjugated magnetic iron oxide nanoparticles (MIONPs)-PEG delivering doxorubicin compared with free drug and non-targeted MIONPs.¹⁹ In another study, improved cytotoxic activity by the cRGDyK-conjugated poly(2-ethyl-2-oxazoline)-poly(D,L-lactide) nanoparticles delivering paclitaxel over the non-targeted nanoparticles and free drug was reported (mean IC₅₀: 51.16 ng/mL, 64.53 ng/mL, and 62.95 ng/mL, respectively). The enhanced activity was through direct targeting of the integrin α_vβ₃-overexpressing prostate cancer cells (PC-3), as well as increasing of cellular uptake by PC-3 cells. Moreover, the targeted nanoparticle was also shown to enhance in vivo tumor growth inhibition rate in PC-3 tumor-bearing mice.²⁰

Other types of peptides that have been applied for conjugation on the surface of nanoparticles to increase selectivity of chemotherapeutic drugs to cancer cells include bombesin peptide-conjugated poly(lactic-co-glycolic acid) (PLGA) and NR7 peptide-conjugated PLGA-PEG nanoparticles. Bombesin-conjugated nanoparticles delivering docetaxel specifically bind to gastrin-releasing peptide receptor, which is overexpressed on cell surfaces of prostate, breast, ovarian, pancreatic, and colorectal cancers.^22,68 This targeted nanoparticle was shown to enhance cytotoxic activity of the drug to the gastrin-releasing peptide receptor overexpressing human breast cancer cells (MDA-MB-231) compared with non-targeted nanoparticles (mean IC₅₀: 35.53 ng/mL and 142.23 ng/mL, respectively).²² The NR7 peptide-conjugated PLGA-PEG nanoparticle delivering doxorubicin was used for specific drug binding to epidermal growth factor receptor (EGFR) on the cancer cell surface.²³ The EGFR is a known receptor that is overexpressed on various types of cancer cell surfaces including head and neck, renal, ovarian, breast, and non-small-cell lung cancer.^47,69 Activation of this receptor results in inhibition of cell apoptosis, promotion of cell proliferation, triggering of angiogenesis, and enhancement of tumor survival and metastasis. Therefore, inhibition of the function of this receptor would be expected to benefit cancer treatment. The NR7 peptide-conjugated PLGA-PEG nanoparticles exhibited higher cytotoxic activity against human ovarian carcinoma cells (SKOV3) compared with non-targeted nanoparticles (mean IC₅₀: 0.05 μg/mL and 3.12 μg/mL, respectively).²³ Although most studies demonstrated satisfactory outcomes of peptide- or protein-conjugated nanoparticles on targeting cancer cells, one study reported that H2009.1 peptide-conjugated liposome delivering doxorubicin to cancer cells expressing integrin α_vβ₆ receptor could not improve the efficacy of the drug. This was due to the liposome platform preventing the targeting ligand from binding to the receptor on the cancer cell surface, and resulted in relatively low drug accumulation in cancer cells.¹²

Hyaluronic acid

HA is a negatively charged linear glycosaminoglycan that consists of D-glucuronic acid and N-acetyl-D-glucosamine. It can specifically bind to CD44 receptor that is overexpressed on the cell surface of various types of cancer including lung, breast, colon, prostate, gastric, and head and neck cancers.⁷⁰ HA is a widely used targeting ligand to conjugate on the surface of nanoparticles to improve selectivity of chemotherapeutic drugs to cancer cells and enhance drug efficacy and safety. In one study, HA with the two molecular weights, ie, 9.5 kDa and 35 kDa, was used to conjugate polymeric micelles delivering paclitaxel. The conjugate using 9.5 kDa HA was found to effectively increase drug cellular uptake by CD44-overexpressing human breast adenocarcinoma cells (MCF-7) compared with 35 kDa HA. In murine hepatic carcinoma (Heps), it also exhibited tumor growth inhibition at a higher rate and greater accumulation at the tumor site compared with other nanoparticle formulations.²⁷ These results suggest that the molecular weight of HA directly influenced the efficacy of drug-loaded active targeting nanoparticles. The HA-conjugated chitosan nanoparticle delivering cisplatin was shown to increase drug cellular uptake by CD44-positive human lung cancer cells (A549) and effectively enhance cytotoxic activity of the drug, compared with non-targeted nanoparticles.²⁸ The HA-conjugated liquid crystalline nanoparticle delivering rapamycin was reported to increase cytotoxic activity of the drug to CD44-overexpressing human breast adenocarcinoma cells (MDA-MB-231) compared with non-targeted nanoparticles (mean IC₅₀: 18 μg/mL and 24.3 μg/mL, respectively). Moreover, the targeted nanoparticles also enhanced in vivo tumor growth inhibition rate in Ehrlich ascites tumor-bearing mice.²⁹

For traditional/herbal medicines, HA has also been used for conjugation on the surface of nanoparticles delivering 3,4-difluorobenzylidene curcumin resulting in increased cellular uptake and cytotoxic activity of the drug against human pancreatic cancer cells (MiaPaCa-2 and AsPC-1) compared with non-targeted nanoparticles (mean IC₅₀: 140 nM, 160 nM, and 245 nM, respectively). Interestingly, when the CD44 receptor was blocked by free soluble HA, the cytotoxic activity to MiaPaCa-2 cells was comparable between the targeted and non-targeted nanoparticles (mean IC₅₀: 234 nM and 245 nM, respectively).⁶³ These results confirm that targeting ligand-conjugated nanoparticles enhances drug efficacy by improving cellular uptake through receptor-mediated endocytosis mechanism.

Folate

Folate or vitamin B9 is a stable and poorly immunogenic water-soluble vitamin. It is essential for DNA synthesis and replication, methylation, cell division and growth, and cell survival, particularly in rapidly dividing cells or cancer cells.⁷¹ Folic acid receptor is overexpressed on several cancer cell surfaces including ovarian, cervical, breast, lung, kidney, colorectal, and brain cancers.⁷¹ Using folic acid as cancer cell targeting of chemotherapeutic drug nanocarriers has been demonstrated in various studies to improve drug efficacy and safety profiles. The folic acid-conjugated PEG-PLGA nanoparticle delivering docetaxel was shown to increase drug cellular uptake by human cervical carcinoma cells (HeLa) with enhanced cytotoxic activity compared with free drug (mean IC₅₀: 0.69 μg/mL and 8.29 μg/mL, respectively). It also significantly inhibited tumor growth in HeLa tumor-bearing mice.³⁶ The folic acid-conjugated albumin nanoparticle delivering gemcitabine was shown to enhance cytotoxic activity of the drug against folic acid receptor-overexpressing human breast adenocarcinoma cells (MCF-7) compared with non-targeted nanoparticles (mean IC₅₀: 0.175 μM and 0.240 μM, respectively). Similarly, in folic acid receptor-overexpressing human ovarian cancer cells (Ovcar-5), the targeted nanoparticles exhibited superior cytotoxic activity over the non-targeted nanoparticles (mean IC₅₀: 0.173 μM and 0.279 μM, respectively). On the other hand, activity of the targeted nanoparticles was found similar to that of non-targeted nanoparticles against folate receptor expressing human pancreatic cancer cells (MIAPaCa-2) (mean IC₅₀: 0.166 μM and 0.169 μM, respectively).³⁸ In one study, the folic acid-conjugated PEG-PLGA nanoparticle delivering paclitaxel was shown to increase drug cellular uptake by folic acid-overexpressing human endometrial carcinoma cells (HEC-1A) with superior cytotoxic activity over the non-targeted nanoparticle (mean IC₅₀: 3.43 μg/mL and 8.81 μg/mL, respectively). Moreover, it also produced significantly higher cell apoptotic activity compared with non-targeted and free drug (35.94%, 23.97% and 19%, respectively). In vivo, it produced significant tumor growth inhibition in HEC-1A tumor-bearing mice.⁴³ For traditional/herbal medicines, folic acid-conjugated poly(epsilon-caprolactone)-PEG-poly (epsilon-caprolactone) nanoparticle delivering honokiol, a traditional Chinese medicine, was shown to increase compound cellular uptake by folic acid-overexpressing human nasopharynx carcinoma cells (HNE-1) with enhanced cytotoxic activity compared with free drug (mean IC₅₀: 18.41 μg/mL and 38.59 μg/mL, respectively). Furthermore, the targeted nanoparticles also resulted in significant cell apoptotic activity compared with non-targeted nanoparticles (86.07% and 70.89%, respectively) and prolongation of median survival time compared with non-targeted nanoparticles and free drug (median survival time: 57.5 days, 42.5 days, and 34 days, respectively).¹¹

Antibodies or antibody fragments

Antibodies or antibody fragments are one of the first targeting ligands used for conjugation on the surface of nanoparticles to target cancer cells due to their potential to specifically bind to antigens or receptors on cancer cell surfaces with high affinity. Various types of antibodies or antibody fragments have been used as targeting agents including anti-CD20 monoclonal antibody, anti-CD47 monoclonal antibody, EGFR antibody, and anti-Fas monoclonal antibody. These targeted nanoparticles have been shown to improve cellular uptake by cancer cells and enhance cytotoxic activity of the drugs to cancer cells. For instance, anti-CD20 monoclonal antibody-conjugated 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-methoxypolyethylene-glycol-2000 delivers doxorubicin active carbon nanoparticles to the target CD20 receptor. It exhibited higher cytotoxic activity against CD20-positive human Burkitt’s lymphoma cells (Raji) compared with non-targeted nanoparticles.⁴⁵ The anti-CD47 monoclonal antibody-conjugated iron oxide magnetic nanoparticle delivering gemcitabine to the target CD47 receptor was shown to increase drug cellular uptake by human pancreatic ductal adenocarcinoma cells (Panc215 and Panc354). Their cytotoxic activity was also significantly enhanced.⁴⁶ The EGFR antibody-conjugated PLGA nanoparticle delivering rapamycin was shown to exhibit higher cellular uptake by EGFR-overexpressing human breast adenocarcinoma cells (MCF-7) with enhanced cell apoptotic activity against all cell stages.⁴⁷ For traditional/herbal medicines, the anti-annexin A2 antibody-conjugated PLGA nanoparticle delivering curcumin was shown to significantly increase compound cellular uptake by human breast adenocarcinoma cells (MDA-MB-231).⁶⁴

Aptamers

Aptamers are short single-stranded DNA or RNA sequences that can fold to three-dimensional structure and bind to specific targets on the cancer cell surfaces that express specific targets for different aptamers. For instance, nucleolin receptor is specific for AS14111 aptamer and EpCAM protein is specific for EpCAM aptamer. The aptamer AS14111-conjugated PEG-PLGA nanoparticle delivering gemcitabine to target nucleolin receptor was shown to increase drug cellular uptake (36%) by nucleolin-overexpressing human lung cancer cells (A549) compared with non-targeted nanoparticles, with enhanced cytotoxic activity (IC₅₀: 4.9 μg/mL and 28.9 μg/mL, respectively).⁴⁹ The AS14111-conjugated undecylenic acid modified, thermally hydrocarbonized porous silicon (UnTHCPSi) nanoparticle delivering methotrexate was shown to increase drug cellular uptake by nucleolin-overexpressing human breast adenocarcinoma cells (MDA-MB-231) with enhanced cytotoxic activity compared with non-targeted nanoparticles and nucleolin-negative fibroblasts cells (NIH 3T3).⁵⁰ The aptamer AS14111-conjugated polymersome delivering doxorubicin was shown to increase drug cellular uptake by nucleolin-overexpressing human breast adenocarcinoma cells (MCF-7) with enhanced cytotoxic activity compared with mutated aptamer-conjugated nanoparticles (mean IC₅₀: 210.9 ng/mL and 369.4 ng/mL, respectively). In addition, it also produced significant tumor growth inhibition in MCF-7 tumor-bearing mice compared with mutated aptamer-conjugated nanoparticles.⁵² For traditional/herbal medicines, the EpCAM aptamer-conjugated lipid-polymer-lecithin hybrid delivering curcumin was shown to increase compound cellular uptake by EpCAM-overexpressing human colon adenocarcinoma cells (HT29) compared with EpCAM-negative human embryonic kidney cells (HEK293T), (58.9%±2.6% and 72.4%±1.3%, respectively.⁶⁵

Carbohydrates or polysaccharides

Galactose is one of targeting ligands in group of carbohydrates that is widely used as a targeting agent for nanoparticles. It is specifically recognized by the asialoglycoprotein receptor (ASGPR), which is overexpressed on liver cancer cell surface.⁵⁴ The galactose-conjugated lithocholic acid-PEG-lactobionic acid nanoparticles delivering doxorubicin was shown to increase drug cellular uptake by human liver cancer cells (SK-HEP-1) leading to massive cell death and tumor growth inhibition compared with non-targeted nanoparticles.⁵⁴ The galactose-conjugated pectin nanoparticle delivering 5-fluorouracil was shown to increase drug cellular uptake by human hepatocellular liver carcinoma cells (HepG2) with enhanced cytotoxic activity compared with free drug (mean IC₅₀: 0.17×10⁻⁴ mol/L and 0.45×10⁻⁴ mol/L, respectively). Moreover, the targeted nanoparticle also improved pharmacokinetic profile of the drugs.⁵⁵ On the other hand, the lactose-conjugated nanoparticle delivering doxorubicin was shown to improve drug efficacy, but not as good as galactose.⁵³ The galactose conjugates not only specifically bind to ASGPR but also to lectin receptor, which is overexpressed on the alveolar macrophages, liver endothelial Kupffer cells, splenic macrophages, peritoneal macrophages, and macrophages of brain. The galactose-conjugated solid lipid nanoparticles delivering doxorubicin specifically targeted human lung cancer cells (A549) resulting in higher cellular uptake, enhanced cytotoxic activity, and improved pharmacokinetic profiles compared with non-targeted nanoparticles and free drug.⁵⁷

Controlled drug release of active targeting nanoparticles

Controlled drug release is a property of drug delivery systems in cancer therapy. Drugs are delivered and released at specific location to avoid side effects to normal cells.⁷² Most studies included in this review showed biphasic characteristics of drugs released from both targeted and non-targeted nanoparticles, ie, initial burst release, followed by sustained release. For instance, about 48% and 46% of gemcitabine were released from folic acid-conjugated bovine serum albumin nanoparticles and non-targeted nanoparticles during the first 2 hours, respectively. Sustained release of up to 99% and 94% was observed at 36 hours and pH 7.4 after burst release of targeted and non-targeted nanoparticles, respectively.³⁸ About 22% and 29% of doxorubicin were shown to release from galactose-conjugated solid lipid nanoparticles and non-targeted nanoparticles during the first 8 hours, respectively. After burst release, sustained released was observed up to 76% and 93% at 144 h and pH 7.4 for targeted and non-targeted nanoparticles, respectively.⁵⁷ Moreover, in some cases, the amount of drug released from nanoparticles at endolysosomal environment (pH 5.5) or cancer cell environment (pH 6.8) was shown to be higher than that from physiological environment (pH 7.4). Up to 60% of doxorubicin was released from anti-CD20-conjugated active carbon nanoparticles and non-targeted nanoparticles at 12 hours and at pH 5.5. At pH 7.4, on the other hand, the release from nanoparticles was only 20%.⁴⁵ Similarly, about 28% and 24% of gemcitabine burst were released during the first 24 hours from AS1411 aptamer-conjugated PEG-PLGA nanoparticles and non-targeted nanoparticles, respectively. After burst release, up to 44% and 41% sustained release were observed in both formulations at 120 hours and pH 5.5 for targeted and non-targeted nanoparticles, respectively. Only 30% release was observed at pH 7.4.⁴⁹ In another study, doxorubicin released from chondroitin sulfate A-deoxycholic acid at day 6 was 92%, 53%, and 34% for pH 5.5, 6.8, and 7.4, respectively.⁶⁰ These results suggested that conjugation of targeting ligands on the surface of nanoparticles did not affect drug release from nanoparticles. Furthermore, higher amount of drug released at acidic pH would benefit the delivery of cancer chemotherapeutic agents to cancer cells with lower side effects to normal cells.

Conclusion

Active targeting nanoparticles of chemotherapeutic drugs or traditional/herbal medicines have been demonstrated in various studies both in vitro and in vivo to improve selectivity of cellular uptake of drugs to cancer cells through receptor-mediated endocytosis and/or cytotoxicity. They provide several advantages over the conventional chemotherapeutic drugs and non-targeted nanoparticle platform, particularly in regard to enhancement of drug efficacy and safety. Active targeting nanoparticles possess several advantages in cancer therapy including enhancement of selectivity of drugs to cancer cells to avoid side effects to normal cells, enhancement of drug accumulation and anticancer activity in cancer cells, and efficiency in control of drug release. Nevertheless, some disadvantages of active targeting nanoparticles include their limitation of clinical uses in only certain types of cancer that express specific receptors on the cell surfaces. Moreover, manufacturing of nanoparticle platforms is costly and requires sophisticated technology. Selection of the types of targeting nanoparticles is determined by the types of target proteins or receptors expressed on cancer cell surfaces. Clinical studies are required to confirm their application in cancer patients.

Acknowledgments

The authors would like to thank the Center of Excellence in Pharmacology and Molecular Biology of Malaria and Cholangiocarcinoma, Chulabhorn International College of Medicine at Thammasat University, Rangsit Center, for providing all the necessary support in conducting this systematic review.

Disclosure

The authors report no conflicts of interest in this work.

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