Recent advances in co-delivery systems based on polymeric nanoparticle for cancer treatment

Abstract

Cancer is a broad term for a class of prevalent diseases as one in three people develop cancer during their lifetime. Although, there are few success stories of cancer therapy, most of the existing medications do not lead to complete recovery. Because of the complexity of cancer, usually a single therapeutic approach is insufficient for the suppression of cancer growth and metastasis. Simultaneous loading and co-delivery of different agents with different physiochemical characteristics to the same tumors have been suggested for minimizing the dose of anticancer drugs and achieving the synergistic therapeutic impacts in cancers treatment. Intense work to develop nanotechnology-based systems as a suitable option for cancer treatment is currently underway. The purpose of this review is to provide an overview of the co-delivery systems based on polymeric nanoparticles including polymeric micelles, dendrimers, poly-d,l-lactide-co-glycolide, polyethylenimine, poly(l-lysine) and chitosan for efficacious cancer therapy.

Keywords:

• Co-delivery
• cancer therapy
• polymeric nanoparticles
• gene delivery
• drug delivery

Introduction

Recently, cancer is considered as a prominent cause of death worldwide and many researches have been undertaken to overcome this public health problem. Although so much efforts have been made to design new chemotherapy drugs only some of them have shown significant effects on destroying cancer cells, the inability of these drugs to distinguish the differences between healthy and cancer cells leads to systemic toxicity and undesirable side effects [1]. Moreover, resistance against chemotherapy is another difficulty to the prosperous control of tumors [2].

On the other hand, since the causes of cancer are complex, usually a single therapeutic approach is insufficient for the suppression of cancer cells proliferation growth and migration. In this way, co-delivery systems delivering anticancer drugs have been developed as an effective strategy for reducing the dose of anticancer drugs achieved by the synergistic effects of the co-delivered agents [3].

Recent researches have revealed that nanoparticles (NPs) have taken a significant role in the improvement of co-delivery methods efficiency. The NPs have been employed as special tools with considerable benefits including the easy penetration through cell membranes because of their very small sizes, decrease in off-target effects, enhanced drug kinetics and lysosomal escape after endocytosis [4–6]. Also, the co-delivery of anticancer drugs by NPs has been developed to overcome multi-drug resistance (MDR) and enhanced drug concentration at tumor sites through the improved permeability and retention effects [7].

The NPs have been utilized for the co-administration of drug and drug or gene. The greatest obstacle facing co-delivery systems is finding applicable carriers for simultaneous delivery of drugs with different properties or genes with high molecular weights and negatively charged surfaces [8]. So far, many modified carriers have been designed to enhance the efficiency of co-delivery systems such as liposome, micelle and polymeric NPs.

In this way, Yang et al. suggested that co-delivery of doxorubicin (DOX) and Bmi1siRNA by folate receptor-targeted liposomes is an effective therapeutic approach for cancer treatment. Deregulation of Bmi1 expression, which involves in the regulation of development, stem cell self-renewal, cell cycle and senescence, has been reported in various forms of tumor [9]. In order to develop the delivery of docetaxel and matrix metalloproteinase 9 (MMP-9) siRNA, Zhou et al. utilized a star-shaped copolymer with amphiphilic characteristics. In vivo and in vitro investigations have shown more remarkable apoptosis in breast cancer cells and tumor inhibition compared to docetaxel (DOC) or MMP-9, alone [10]. In another study, DOC micelles were embedded into DOX thermosensitive hydrogels (TSHs) to develop a co-delivery system with minimal side effects and attenuate the chemoresistance [11]. In the current review, recent findings on polymeric NP-based co-delivery systems for cancer treatment with high efficiencies are presented.

Polymeric NPs used in co-delivery systems

Polymeric NPs have a major role in co-delivery systems because of their efficient capacity for drugs and/or condensed genes loadings. These polymers offer important advantages including protection of the contents against degradation, controlled release of the therapeutic agents and enhancement of the site-specific delivery. Furthermore, frameworks of polymeric vehicles can be modified with functional groups including PEG moieties and targeting agents to improve the delivery of the drug and/or gene cargos to tumors [12]. Various polymers with different physicochemical characteristics have been offered for co-delivery of cytotoxic drugs or other therapeutic agents (e.g. chemosensitizers, differentiation-inducing and neovasculature disruption agents). This type of NPs are made of synthetic polymers (e.g. dendrimers), poly(dl-lactide co-glycolide) (PLGA), poly(ethylenimine) (PEI), poly(l-lysine) (PLL), poly(ethylene glycol) (PEG), polymeric micelles and natural polymers (e.g. chitosan (CS)) [13] (Table 1).

Table 1. Some examples of polymeric nanoparticles employed as co-delivery systems.

Nanoparticle	Polymer	Drug-drug or Drug-gene	Assay condition	Reference
Polymeric micelles	P(MDS-co-CES)	Paclitaxel and Herceptin	in vitro	[14]
	EG-acid diblock copolymer PEG-PAC (or PEG-P(MTC-OH))	Thioridazine and doxorubicin	in vitro and in vivo	[15]
	A ternary block copolymer PEG- PAsp(AED)-PDPA	Doxorubicin and BCL2 siRNA	in vivo	[8]
	Pluronic(®) micelles	Resveratrol and curcumin	in vitro	[16]
	PDMAEMA-PCL-PDMAEMA triblock copolymers	Paclitaxel and VEGF SiRNA	in vitro	[17]
	PDP-PDHA	Doxorubicin and Survivin-targeting shRNA	in vitro and in vivo	[18]
	POSS-based star-shaped	Doxorubicin and P53 plasmid DNA	in vitro and in vivo	[19]
	PEG-pp-PEI-PE	Paclitaxel and free-SiRNA	in vitro and in vivo	[20]
	A ternary block copolymer PEG-PAsp(AED)-PDPA	Doxorubicin and BCL2 SiRNA	in vitro and in vivo	[21]
Dendrimers	MPEG-b-PAMAM	10-hydroxy-camptothecin and doxorubicin	in vitro and in vivo	[22]
	A three-layered telodendrimer micelles (TM)	Cisplatin and paclitaxel	in vitro and in vivo	[23]
	PAMAM-PEG-T7	Doxorubicin and plasmid pORF-hTRAIL	in vitro and in vivo	[24]
	G(4)-PAMAM	Docetaxel and SiRNA	in vitro	[25]
	PAMAM	5-Fluorouracil and miR-21	in vitro	[26]
	PAMAM-HA	Doxorubicin and MVP-siRNA	in vivo	[27]
	CD-PLLD	Docetaxel and MMP-9 siRNA	in vitro and in vivo	[28]
Poly(dl-lactide co-glycolide) (PLGA)	PLGA	Docetaxel and vitamin E TPGS	in vitro and in vivo	[29]
	PEG-PLGA copolymer	Doxorubicin and paclitaxel	in vitro	[30]
	PLGA-TPGS	Docetaxel and poloxamers 235	in vitro and in vivo	[31]
	PLGA	Rapamycin and piperine	in vitro and in vivo	[32]
	mPEG-PLGA	SH-aspirin and curcumin	in vitro	[33]
	MLNP	Camptothecin and pTRAIL	in vitro and in vivo	[34]
	PCNPs	Doxorubicin and free-SiRNA	in vitro and in vivo	[35]
	PLGA	Paclitaxel and Stat3 SiRNA	in vitro	[36]
	PLGA-b-PEG	Antisense-miR-10b and antisense-miR-21	in vitro and in vivo	[37]
	PLGA/PF127	Doxorubicin and miR-542-3P	in vitro	[38]
	PLGA/PEI	Doxorubicin and anti Bcl-xL shRNA	in vitro	[39]
	PLGA/PEG	Metformin and curcumin	in vitro	[40]
Chitosan (CS)	CS	Camptothecin and curcumin	in vitro	[41]
	PEGylated CS	Methotrexate and mitomycin	in vitro and in vivo	[42]
	Pluronic F127-chitosan	Doxorubicin and paclitaxel	in vitro and in vivo	[43]
	CPC	Candesartan and Wt-p53	in vitro and in vivo	[44]
	CLP	Paclitaxel and free-SiRNA	in vitro	[45]
	CS	Doxorubicin and mi-34a	in vitro and in vivo	[46]
	CS	Gefitinib and shMDR1	in vitro	[47]
	CS	Doxorubicin and IGF-1R siRNA	in vitro	[48]
poly(ethylenimine) (PEI)	mPEG-PCL-g-PEI	Doxorubicin and plasmid DNA	in vitro and in vivo	[49]
	PELG/PEI	Cis-aconityl-doxorubicin and P53 plasmid DNA	in vitro and in vivo	[50]
	PEI-SA	Doxorubicin and VEGF SiRNA	in vivo	[51]
	LMw-PEI/PCL	Doxorubicin and P53 plasmid DNA	in vitro	[52]
Poly(l-lysine) (PLL)	mPEsG-b-PLG-b-PLL/DOCA	Doxorubicin and paclitaxel	in vitro and in vivo	[53]
	G3-[PEG-RGD]	Doxorubicin and SiRNA	in vivo	[54]
	PEG-PLL-PL-Leu	Docetaxel and SiRNA-BCL2	in vitro and in vivo	[55]
	CD-PLLD	Docetaxel and MMP-9 SiRNA plasmid	in vitro and in vivo	[28]
	PP-PLLD-Arg	Docetaxel and MMP-9 shRNA	in vitro and in vivo	[56]

Polymeric micelles (PMs)

Polymeric micelles are self-assembled nanostructures composed of a core with hydrophobic properties that encapsulates the slightly water-soluble drugs , a hydrophilic coating to protect the drug in the aqueous environment and to stabilize the PMs against recognition in vivo using the reticuloendothelial system (RES) (Figure 1) [57,58].

Figure 1. Schematic representation of polymeric micelles. Adapted from the published works of Xu et al. [57].

Other basic properties of PMs include (1) small size of the micelles (10–100 nm) that helps to be accumulated in cancerous tissue, (2) non-specific interactions with biological components because of the biocompatible polymer shell, (3) the capability of controlled drug release and (4) ability to modify the structure and improve their characteristics [59].

Micelles are the most reported instances of co-delivery carriers, although, their capabilities are required to be improved by developing new synthesis routes for polymers by introducing new technology to solve the immature drug-incorporation [60].

Lee et al. reported micellar NPs constructed of a biodegradable and amphiphilic copolymer, poly [(N-methyldietheneamine sebacate)-co-[(cholesteryl oxocarbonylamido ethyl) methyl bis (ethylene) ammonium bromide] sebacate], P(MDS-co-CES) delivered the paclitaxel (PTX) and Herceptin to human epidermal growth factor receptor-2 (HER2/neu), overexpressing human breast cancer cells led to enhanced cytotoxicity through synergistic activities [14].

Ke et al. combined the therapeutic effects of thioridazine (THZ) and DOX to restrain cancer cells and DOX-resistant cancer stem cells, simultaneously. To construct the self-assembly micelles and to demonstrate their applicability, they mixed poly(ethylene glycol) diblock copolymer (PEG-PAC), acid-functionalized poly(carbonate), PEG diblock copolymer (PEG-PUC) and urea-functionalized poly(carbonate) (PUC). Results showed that the co-delivery of DOX-loaded mixed micelles (DOX-MM) and thioridazine mixed micelles (THZ-MM) by these PMs could serve as a potential strategy to treat breast cancer by targeting both breast cancer and cancer stem cells (Figure 2) [15].

Figure 2. Co-delivery of thioridazine and doxorubicin using polymeric micelles. Adapted from the published works of Ke et al. [15].

Carlson et al. also reported the effective co-delivery of resveratrol and curcumin (CUR) by Pluronic® micelles which were co-administered with DOX. The system, as well as its maintaining/improving DOX anti-proliferative effects, demonstrated efficient cardioprotective properties in vitro [16]. Ma et al. developed a co-delivery system based on hyaluronic acid-vitamin E succinate (HA-VES) graft copolymeric micelles loaded by DOX and CUR to improve the therapeutic effects of DOX. In vitro cytotoxicity studies showed that the NPs has significant advantages including uniform particle size, high encapsulation efficacy, enhancement of the cellular uptake of DOX and sustained release profile and good colloidal stability, which resulted in highest cell apoptosis-inducing activities and reversed MDR effects as well as released synergic effect on CUR release. Furthermore, in vivo studies revealed higher tumor targeting and tumor accumulation of the system, as well as its less pathological damage to the cardiac tissue [61].

Bae et al. investigated a novel mixed polymeric micelles, including a combination of DOX and wortmannin (WOR), as a phosphatidylinositol-3 kinase inhibitor, which conjugated onto poly (ethylene glycol)-poly (aspartate hydrazide) block copolymers. Cytotoxicity assay against a human breast cancer MCF-7 cell showed that, with controlling the DOX/WOR ratios, the DOX amounts could be reduced while the biological activity of the polymer is maintained due to enhancing effect of WOR on the DOX efficiency. In other words, concentration and distribution of multiple drugs could be controlled while identical pharmacokinetic profiles is protected [62].

Zhu et al. fabricated a biodegradable nano-sized micelle with a positively charged surface using poly[2-(dimethylamino) ethyl methacrylate] (PDMAEMA)-PCL-PDMAEMA triblock copolymer. In order to demonstrate its efficiency, they utilized the as-obtained micelle for the co-delivery of PTX and vascular endothelial growth factor targeted siRNA (VEGF siRNA). The results showed that co-delivery of these agents led to an efficient knockdown of VEGF expression (Figure 3) [17].

Figure 3. Co-delivery of siRNA and paclitaxel by biodegradable cationic micelles composed of PDMAEMA–PCL–PDMAEMA copolymers Adapted from the published works of Zhu et al. [17].

Tang et al. synthesized a new amphiphilic poly (b-amino ester), poly[(1,4-butanediol)-diacrylate-b-5-polyethylenimine]-block-poly[(1,4-butanediol)-diacrylate-b-5-hydroxy amylamine] (PDP-PDHA) and utilized it for concomitant delivery of DOX and RNA. The system demonstrated a synergistic impact on prevailing of breast cancer with multidrug resistance [18].

Cao et al. developed a diblock copolymer consisting of linear polyethylenimine (PEI) and poly (ε-caprolactone)(PEI-PCL), which was assembled into micelles for BCL2 siRNA and DOX co-delivery [63]. In another work, dual-functional PEI-poly (γ-cholesterol-l-glutamate)(PEI-PCHLG) copolymer for the co-delivery of gene and drug such as amphiphilic chimeric peptide, e.g. (Fmoc)2KH₇-TAT and Ac-(AF)₆-H₅-K₁₅-NH₂(FA32) has been employed [8,64].

Li et al. constructed a star-shaped polymer consisting of a cationic poly (2-dimethylaminoethyl methacrylate) (PDMAEMA) shell and a zwitterionic poly[N-(3-(methacryloylamino) propyl)-N,N-dimethyl-N-(3-sulfopropyl) ammonium hydroxide] (PMPD) corona. They also employed oligomeric silsesquioxanes (POSS)-based initiators to synthesis a polyhedral via atomic transfer radical polymerization (ATRP). Afterwards, both DOX and p53 gene were loaded on the constructed polyhedral during a micelle formation process. The results showed that the polyplex was highly efficient and biocompatible for drug and gene delivery [19].

Zhu et al. reported the construction of a simple micellar platform by a matrix metalloproteinase 2 (MMP2)–sensitive copolymer (PEG-pp-PEI-PE) through the self-assembly for targeted siRNA delivery to tumor and co-delivery of drugs. Further investigations showed the system has excellent stability and tumor-targeting activity triggered by the up-regulated tumoral MMP2 [20].

Furthermore, a reduction and pH dual-sensitive nanocarrier for the co-delivery of BCL2 siRNA and DOX was developed by Chen et al. The obtained results demonstrated that the co-delivery system successfully increased apoptosis of human ovarian cancer SKOV 3 cells thereby resulting in the inhibition of tumor growth [8,21].

Lee et al., developed a multifunctional nanocarrier including cationic PDMA-block-poly(ε-caprolactone) (PDMA-b-PCL) and mPEG-PCL copolymers to target human vascular endothelial growth factor (VEGF) thorough dual therapeutic effect of SN-38 (7-ethyl-10-hydroxycamptothecin) and gene (VEGF siRNA) delivery. In order to enhance in vivo bio-safety and bio-stability of the co-delivery system, mPEG-PCL was mixed with PDMA-b-PCL copolymer that resulted in fast in vitro and in vivo enzymatic degradation, facilitation of VEGF silencing and tumor growth suppressing. Furthermore, the system could be applied as a negative magnetic resonance imaging (MRI) contrast agent, due to use of ultra-small superparamagnetic iron oxide (USPIO) nanoparticles [65].

Dendrimers

For the first time, Tomalia et al. introduced dendrimers in the 1980s [66]. These nano-sized polymers exhibited super biological characteristics including small size, high water solubility, regularand a highly branched three-dimensional architecture and high payload with almost perfect monodispersible origin (Figure 4) [67]. Dendrimers, highly branched globular macromolecules, are synthesized in a iterative and stepwise fashion [68].

Figure 4. Schematic presentation of dendrimers. Adapted from the published works of Debnath et al. [128].

Dendrimer-based NPs are known to be robust, covalently fixed with three dimensional structures possessing three distinct architectural regions as a focal moiety or a core, layers of branched repeat units originating from the core and functional end groups on the external layer of repeat units [69–71]. A variety of dendrimers have been extensively utilized as delivery systems such as poly(amidoamine) (PAMAM), poly(etherhydroxylamine) (PEHAM) and poly(propyleneimine) (PPI) dendrimers [72]. However, application of dendrimers in delivery systems poses some safety- toxicity limitations according to the comprehensive statistics [73].

Linear dendritic prodrugs with amphiphilic properties were developed by Zhang et al. through conjugation with DOX. Afterwards, a hydrophobic anticancer drug, 10-hydroxycamptothecin (HCPT), was encapsulated with these conjugated prodrugs. In vitro assays demonstrated that NPs suppressed cancer cell growth with higher efficiency than the free HCPT, MPEG-b-PAMAM-DOX prodrugs and physical mixture of MPEG-b-PAMAM-DOX and HCPT at equivalent DOX or HCPT concentrations [22].

Cai et al. developed a three-layered dendrimer (telodendrimer) by introducing carboxylic acid groups in the adjacent layer via peptide chemistry and the thiol-ene click chemistry for co-delivery of cisplatin (CDDP) through carboxylic chelating and encapsulation of PTX in the core of the micelles. Based on the obtained results, co-delivery of CDDP with low dose of PTX not only decreased their toxic side effects but also enhanced the treatment efficacy [23].

Clementi et al. developed a carrier including PEG-dendrimer, H₂N-PEG-dendrimer-(COOH)₄, for co-delivery of PTX and alendronate (ALN). The PTX-PEG-ALN was exploited for active and passive targeting by ALN molecule and through the enhanced permeability and retention effect, respectively. Pharmacokinetic profile of as-mentioned conjugated polymeric carrier showed lower half-life as well as higher binding affinity for a strong bone targeting, compared to that of the free drugs. Additionally, PTX-PEG-ALN exhibited lower cytotoxic activity than free PTX/ALN against prostate cancer cells [74].

Tekade et al. investigated a co-delivery system including a loaded PAMAM dendrimer with methotrexate (MTX) and all-trans retinoic acid (ATRA). After optimization of pH and dialysis time, it was found that the degree of dendrimer protonation is important in the release kinetics. In order to decrease the haemolytic toxicity of the dendrimer formulation, less haemolytic bioactive was used in a dendrimer terminal loading process. (3–(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) MTT assay on the HeLa cell lines revealed that the as-noted co-delivery system could be more effective than the free drug combination [75].

Biswas et al. modified PAMAM with another amphiphilic block copolymer, poly (ethyleneglycol)-1,2-dioleoyl-sn-glycero-3-phospho-ethanol-amine, to form a new construct G(4)-D-PEG-2 K-DOPE. This nanocarrier revealed a more efficient cellular uptake of siRNA, excellent serum stability, efficient micellization and higher DOX-loading efficiency [25]. It has been demonstrated that siRNAs and miRNAs could be deregulated into different types of cancer. PAMAM dendrimer was exploited as a carrier to co-deliver 5-FU and antisense-miR-21 oligonucleotide (as-miR-21) into glioblastoma cells, leading to repression of cell growth. Co-delivery of as-miR-21 eminently enhanced the cytotoxicity of 5-FU and significantly improved the apoptosis of the human brain glioma cells, whereas the tumor cell migration ability reduced [26]. Han et al. functionalized PAMAM dendrimers with a polysaccharide, hyaluronic acid (HA), to efficiently co-deliver DOX and small-interfering RNA (MVP-siRNA) silencing major vault protein (MVP) production, which is abundantly present in the cytoplasm of eukaryotic cells. As a consequence, the expression of MVP decreased, whereas the chemotherapy effects of DOX increased in MCF7/ADR cells. In addition to improved tumor targeting, greater intracellular accumulation, increased blood circulating time and less in vivo toxicity, DOX PAMAM-HA conjugate showed an increased cytotoxicity effect. Co-delivery of DOX and siRNA by PAMAM-HA demonstrated satisfying gene silencing effect as well as improved stability and efficient intracellular delivery of siRNA which permitted DOX access to the nucleus and induced higher cytotoxicity than in the cases where siRNA was absent to knockdown the expression of MVP [27]. Furthermore, using a transferrin receptor-specific peptide, a tumor-targeting carrier, HAIYPRH (T7) and a conjugated polyethylene glycol-modified polyamidoamine dendrimer (PAMAM-PEG-T7), Han et al. designed a co-delivery system for simultaneous delivery of therapeutic gene encoding human tumor necrosis factor-related apoptosis-inducing ligand (pORF-hTRAIL) and DOX to investigate a combination cancer therapy strategy. Compared to either single DOX or pORF-hTRAIL, the co-delivery system enforced apoptosis of human liver cancer Bel-7402 cells and more efficiently suppressed cancer growth under in vivo conditions. Furthermore, compared to high doses of free DOX, lower doses of the co-delivery system suppressed tumor growth in mice bearing Bel-7402 xenografts, effectively [24]. These results suggest that dendrimer-based co-delivery systems are easy to prepare combined delivery platforms that can considerably enhance the anti-tumor effect.

Poly-d,l-lactide-co-glycolide (PLGA)-based NPs

In late mid-1990 s, long-circulating PLGA polymer NPs have been accessible, introducing great opportunities for the delivery of drugs. The US Food and Drug Administration (FDA) approved PLGA polymer because of its biodistribution, biocompatibility and biodegradability properties [76,77]. PLGA degradation realizes by an autocatalytic cleavage of ester bonds through spontaneous hydrolysis to oligomers and D, L-lactic and glycolic acid monomers (Figure 5), which are the substrates in the Krebs cycle [78,79].

Figure 5. Hydrolysis of poly-d,l-lactide-co-glycolide nanoparticles. Adapted from the published works of Kumari et al. [82].

Reports have been proved the PLGA NPs can be employed with great success for gene therapy purposes and in bioactive agents delivery such as proteins, vitamins and drugs [80,81]. Although PLGA is among the most important biodegradable NPs for the development of anticancer drugs [82], the acidic origin of PLGA monomers is not appropriate for drugs or bioactive molecules [76]. To overcome this problem, the PLGA NP formulation has been blended with alginate, chitosan, pectin [83], poly(propylene fumarate) [84], polyvinyl acohol [85], poly(orthoester) [82], and so on.

Zhu et al. introduced a PLGA-based co-delivery system loaded with DTX as an exemplary anticancer drug together with vitamin E, d-α-tocopheryl polyethylene glycol succinate (TPGS). Results showed that the DTX encapsulated TPGS can act as a pore-forming agent with dual functions for producing porous NPs which could lead to smaller particle size, higher drug encapsulation efficiency and faster drug release, as well as an active matrix component providing P-glycoproteins (P-gp)ATPase and drug efflux inhibition to overcome MDR of the cancer cells; both contributing to the therapeutic effects in vitro and in vivo [29]. Wang et al. synthesized amphiphilic copolymer, methoxy PEG-PLGA (mPEG-PLGA) NPs for the co-delivery of hydrophilic DOX and hydrophobic PTX [30]. In another work, Tang et al. demonstrated the feasibility of DTX-loaded PLGA–TPGS/poloxamer NPs to overcome MDR in DTX-resistant human breast cancer cell line. The PLGA–TPGS/poloxamer NPs produced significantly higher toxicity than both of PLGA–TPGS nanoparticles and Taxotere®, either in vitro or in in vivo conditions [31]. Also, they indicated that by blocking the P-gp pump, poloxamers could reverse the MDR. Katiyar et al. evaluated NPs of rapamycin (RPM) along with piperine (PIP) as a chemosensitizer for improved oral bioavailability and efficacy. The PLGA NPs were selected because PLGA has moderate MDR reversal activity, which could serve additional benefits. An in vitro study indicated higher efficacy of PLGA NPs containing RPM compared to the free drug solution [32].

Songa and co-workers synthesized and examined a co-delivery system including PLGA nanoparticles loaded with vincristine (VCR) and verapamil (VRP), using the O/W emulsion solvent evaporation and salting-out method. A systematic manner was applied for co-administration of chemosensitizer and cytotoxic drugs. The results revealed the highest reversal efficacy and fewer normal tissue drug toxicity [86].

Cui et al developed a dual-targeting system through the co-encapsulation of the hydrophobic magnetic PLGA nanoparticles and a combination of PTX and CUR agents. The obtained results showed that the synergistic effects of the combined drugs on the inhibition of tumor growth are more effective than each drug. Orthotropic glioma model was used to study the in vivo condition and anti-glioma treatment efficacy, indicating higher efficacy of the system through enhancing brain delivery efficiency as well as reduced adverse toxicity [87].

Zhou et al. co-encapsulated H₂S-releasing prodrug SH-aspirin (SH-ASA) and CUR into methoxy-PEG-PLGA NPs which indicated obvious synergistic anticancer effects on ES-2 and SKOV3 human ovarian carcinoma cells in vitro, in addition to the activation of the mitochondrial apoptosis pathway [33].

Farajzadeh and co-workers investigated a co-encapsulated Metformin (Met) and CUR in PEGylated PLGA nanoparticles and evaluated their therapeutic efficacy against T47D breast cancer cells. More synergistic antiproliferative and anti-cancer effects proved the significant performance of the synthesized nanoparticles. The results demonstrated synergy between Met and CUR in a co-delivery system that could provide a promising nano-combinational therapy, considering more future clinical trials [40].

Wang et al. developed a PLGA nanoparticle co-encapsulated with an anticancer paclitaxel (PTX) and antiangiogenesis agent combretastatin A4 (CA4), with a peptide comprising Arg-Gly-Asp (RGD) to modify the nanoparticle surface. The cellular uptake was found to be a receptor-mediated endocytosis pathway. Cellular apoptosis by PTX and cytoskeleton disruption by CA4 could be respectively induced using single-agent-encapsulated nanoparticle. The co-delivery system was applied to in vivo studies via high cellular proliferation inhibition, significant vascular disruption and eminent apoptosis induction in tumor medium that led to tumor suppression when compared to the control sample [88].

He et al. synthesized a modified PLGA with folic acid (FA-PEG-PLGA) copolymer and encapsulated it with cis-diaminodichloroplatinum (CDDP) and PTX inside the hydrophobic inner core and chelated to the middle shell, respectively. The co-encapsulated nanoparticles showed significant synergistic inhibitory effects on A549 and M109, lung cancer cell lines which act as FA receptor negative and positive, respectively. Prevention of an initial burst release of CDDP, longer circulation time in the blood, better targeting and obvious tumor inhibiting effect (based on in vivo studies) and reduced side effects are some major advantages of the as-mentioned system. As a consequence, the as-mentioned combinational therapy showed greater anti-tumor effects in folic acid receptor positive cells than negative ones [89].

Wang et al., examined a combinatorial chemotherapy, including co-loaded PLGA nanoparticles with PTX and etoposide (ETP) for the treatment of osteosarcoma. After assessment of different physicochemical and biological properties, the nanoparticles were engaged in a release study. The results showed no sign of initial burst release and the nanoparticles exhibited a sustained release profile for both anticancer agents. The combinational drug-loaded PLGA NPs showed an enhanced time and concentration dependent anticancer effect in MG63 and Saos-2 cancer cells and as well as a significant uptake in MG63 cells. Additionally, the results proved the enhanced therapeutic index and greater inhibitory effect of loaded nanoparticles that would be of great advantage during systemic cancer therapy [90]. Wu et al. synthesized a co-delivery system including PLGA and soybean lecithin, monomethoxy-poly(ethylene glycol)-S-S-hexadecyl (mPEG-S-S-C₁₆), loaded with a hydrophobic Chinese herb extract triptolid (TPL) and DOX combination to develop a reduction-sensitive lipid-polymer hybrid nanoparticle. The in vitro and in vivo studies of the system showed a high level of synergistic activation with low combination index, at the ratio of 0.2:1 TPL/DOX. The experiments on the human oral cavity squamous cell carcinoma cells (KB cells) demonstrate the enhanced effect of TPL on the DOX uptake. It seems that the examined nanoparticle could be effectively applied as a co-encapsulated reduction-sensitive system for cancer treatment [91].

Muntimadugu and co-workers developed a co-delivery system based on co-encapsulation of salinomycin (SLM) and PTX in PLGA NPs by emulsion solvent diffusion method in order to kill cancer stem and cancer cells, respectively. The highest cytotoxicity and minimum IC50 on MCF-7 cells as well as enhanced cellular uptake on breast cancer cells and CD44 receptors were achieved by coated SLM nanoparticles with hyaluronic acid. The results showed the specificity of simultaneous application of both drugs towards the targeted cells. Furthermore, longer circulation periods was achieved which indicated the enhanced bioavailability of drugs from nanoparticles. As a result, this type of combinational therapy could be one of the best approaches in overcoming resistant cancer stem cells (CSCs) [92].

Ediriwickrema et al. developed a PLGA based multi-layered polymer NPs (MLNPs) which was covered with PEI and functional peptides for drug and gene co-delivery purposes. Furthermore, they demonstrated the capability of the particles for suppressing the tumor growth through the interactive function of the drug and gene product. MLNPs exhibited transfection levels similar to Lipofectamine while maintaining minimal cytotoxicity [34]. Cationic lipids together with siRNA-phospholipids and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[amino((polyethylenelycol)-2000] (DSPE-PEG2000) were fused around PLGA to form siRNA-phospholipids enveloped NPs (siRNA-PCNPs), which can deliver siRNAs and hydrophobic anticancer drugs into the tumor. Studies on animal models proved that co-delivery of Dox and siRNA-PCNPs has significant influence on the tumor growth inhibition [35].

Su et al. used PLGA-PEI NPs for PTX and Stat3 siRNA co-delivery, which could be absorbed by cancer cells to suppress the Stat3 protein expression and subsequently killed lung cancer cells effectively [36]. In another work, Devulapally et al. synthesized the antisense-miR-10 b and antisense-miR-21-loaded PLGA-b-PEG NPs and showed that multi-target inhibition of endogenous miRNAs could efficiently generate targeting metastasis, as well as anti-apoptosis, in the metastatic cancer therapy [37].

In an another study, a hyaluronic acid (HA)-decorated polyethylenimine-poly (d,l-lactide-co-glycolide) (PEI-PLGA) nanoparticle was developed for the targeted co-delivery of doxorubicin and miR-542-3p. These nanoparticles with average particle size of 131 nm can be employed for the treatment of triple negative breast cancer (TNBC). On the other hand, they can prevent the miR-542-3pdegradation in the serum, due to high drug encapsulation efficiency. In comparison with MCF-7 cells with lower CD44 level, increase in both drug uptake and cytotoxicity in MDA-MB-231 cells was observed using HA/PEI-PLGA nanoparticles. Besides, intracellular restoration of miR-542-3p enhanced TNBC cell apoptosis tby activating p53 and suppressing survivin expression. As a consequent, the results showed that the HA/PEI-PLGA nanoparticles possessed the potential capability to co-deliver chemotherapeutic agents and tumor suppressive miRNAs in combinatorial TNBC therapy [38].

Li et al. targeted the hypoxic cancer area and gene silencing to predominate the hypoxia and treat the tumors. They utilized the hypricin-encapsulated nanocomplexes (Hy-HPP-NPs) as carriers for the transfection of hypoxic human nasopharyngeal carcinoma (CNE2) cells with hypoxia-inducible factor 1 alpha (HIF-1a) small interfering siRNA. In this way, the beneficial effects of hypericin (Hy) and gene therapy on tumor cells were combined. Hy-loaded nanoparticles could increase the accumulation in the tumors by enhanced permeability and retardation impact. At the end, the in vivo investigations demonstrated that co-delivery of Hy and HIF-1a siRNA (siHIF-1a) could efficiently decrease the level of HIF-1a and increase the affinity toward necrotic tissues [93].

Chitosan (CS)-based NPs

Chitosan (CS), a partially deacetylated polymer, obtained from the alkaline deacetylation of chitin, is a glucose-based un-branched polysaccharide (Figure 6), which is extensively spread out in the nature as the principal element of exoskeletons of crustaceans and insects and as well in the cell walls of some bacteria and fungi [94].

Figure 6. Chemical structure of chitosan. Adapted from the published works of Younes et al. [129].

The CS-based polymers have many applications in medicine such as drug delivery, tissue engineering scaffolds, dressings, coatings and sensors. In addition, CS has been employed in gene therapy as a non-viral vector because of its polycationic characteristics that allows it to make strong interactions with DNA. In spite of low transfection efficiency, many groups have demonstrated prosperous gene therapy approaches using CS-DNA polyplexes. In conjugation with other agents, such as polyethylenimine, CS can increase DNA release ability and transfection performance [95]. In addition, the primary hydroxyl and amine groups situated on the backbone of CS permit its chemical modifications to regulate its physical characteristics [96].

Xiao et al. evaluated a series of CS-functionalized camptothecin (CPT)/CUR-loaded polymeric NPs with various weight ratios of CPT to CUR. The obtained results revealed that the co-delivery of CPT and CUR using a single NP enhanced synergistic properties of both drugs [41].

Jia et al. successfully improved a drug delivery system including methotrexate (MTX) and mitomycin C (MMC) loaded PEGylated CS NPs. In this system, MTX which is a folic acid analogue, was also exploited as a tumor-targeting ligand. In comparison with delivery of either drug alone, the (MTX + MMC)-PEG-CS-NPs could co-deliver MTX and MMC to not only achieve higher accumulation in the tumor site, but also more considerably suppressed the tumor cells growth, indicating a synergistic effect (Figure 7) [42].

Figure 7. Development of both methotrexate and mitomycin C loaded PEGylated chitosan nanoparticles for targeted drug co-delivery and synergistic anticancer effect. Adapted from the published works of Jia et al. [42].

Figure 8. A chitosan-graft-PEI-candesartan conjugate for targeted co-delivery of drug and gene in anti-angiogenesis cancer therapy. Adapted from the published works of Bao et al. [44].

Ma et al. successfully synthesized a DOX-conjugated prodrug which incorporated acid-sensitive linkage between the drug and pluronic F127-CS polymer. Subsequently, a pH-sensitive polymeric system was designed based on the conjugated prodrugs (F127-CS-DOX) to co-deliver DOX and paclitaxel (PAX). The in vivo results showed that the PTX-loaded F127-CS-DOX NPs could be considered as a promising candidate for the co-delivery of DOX and PTX [43]. A copolymer conjugate chitosan-graft-polyethylenimine-candesartan (CPC) containing a low molecular weight chitosan backbone and PEI arms with candesartan (CD) was reported by Bao et al. Candesartan enhanced the endosomal buffering capacity and suppressed angiogenesis while wt-p53 gene delivered by this conjugate down-regulated the VEGF production and thereby angiogenesis was further suppressed. Thus, a strategy for tumor therapy was successfully constructed for the simultaneous tumor-targeted delivery of CD and wt-p53 gene (Figure 8) [44].

In another work, Shali et al. prepared a co-delivery system consisting of chitosan nanoparticles loaded with IGF-1 R siRNA and DOX to investigate their effects on the viability of A549 lung cancer cells line. The results showed that migration and expressions of mmp9, VEGF and signal transducer and activator of transcription 3 (STAT3) could be decreased in A549 cells, which could be useful in preventing the metastatic spread and in improving the treatment of lung cancer [48].

Alinejad et al. designed a co-delivery system including an anticancer drug DOX and a gene IL17RB siRNA co-encapsulated carboxymethyl dextran (CMD) chitosan nanoparticles (ChNPs) to investigate its efficacy on the viability and gene expression of MDA-MB361 cell lines. In this way, Annexin-V and wound healing assays were utilized to evaluate the system apoptosis induction and its migration inhibition in the MDA-MB361 cells. As a result, the co-delivery system exhibited a decrement in the viability, growth, proliferation of MDA-MB361 cells and increase of the apoptosis rate of such cells [97].

In another investigation by Yu et al. shMDR1 and gefitinib-encapsulating CS NPs were prepared and the results demonstrated that CS NPs entrapping gefitinib and shMDR1 have the potential to overcome the multidrug resistance and enhance the efficiency of cancer treatment, particularly in the resistant cells [47].

Polyethyleneimine (PEI)-based NPs

Polyethyleneimine (PEI) is a cationic polymer that contains primary, secondary and tertiary amino groups (Figure 9) [98,99]. The PEI polymers are formed as either linear or branched molecules [100]. PEI is a valuable cationic polyamine, which has been extensively utilized as an effective drug and gene nanocarrier [101,102]. Similar to non-viral delivery system, the cationic PEI could condense the nucleic acids with negative charge by electrostatic interactions [103]. It has been suggested that the high transfection efficiency of PEI-based vectors is because of their capability of avoiding trafficking toward degradative lysosomes. The degree of cellular uptake of PEI is dependent on heparan sulfate proteoglycans (HSPGs) that act as the interaction moieties for PEI on the cell surfaces [104–107]. According to the proton sponge hypothesis, the buffering capacity of PEI results in osmotic swelling and burst of endosomes, leading to the early release of the vector into the cytoplasm [108,109].

Figure 9. Chemical structure of polyethyleneimine. Adapted from the published works of Liu et al. [130].

Shi et al. synthesized a series of amphiphilic triblock copolymers based on polyethylene glycol-poly ε-caprolactone-polyethyleneimine (mPEG-PCL-g-PEI). The obtained results showed that the co-delivery of drug and gene into both B16-F10 and 293 T cells with these copolymers can be achieved as an effective vector [49].

In an investigation by Guan et al. a co-delivery of gene and drug was investigated by electrostatic binding of PEI-poly(l-lysine)-poly(l-glutamic acid) (PELG), PEI, cis-aconityl-DOX (CAD) and DNA. By the PELG/PEI/(p53 + CAD) co-delivery system, a significant enhancement of p53 gene expression was obtained in treated HepG2 cells. In addition, results obviously indicated that the co-delivery system could lead to an effective apoptosis in tumor cells (Figure 10) [50].

Figure 10. Co-delivery of antitumor drug and gene by a pH-sensitive charge-conversion system. Adapted from the published works of et al. [50].

Recently, Davoodi et al. synthesized a novel intelligent polymeric nano-system consisting of an amphiphilic copolymer by means of the conjugation of low molecular weight polyethylenimine (LMw-PEI) with poly(e-caprolactone) (PCL) to combine the chemotherapy with non-viral gene therapy contrary to human cancers. The obtained copolymer showed great tendency to form positively charged nanoparticles with hydrophobic core and hydrophilic shell compartments, implying its feasibility for anti-cancer drug and plasmid DNA encapsulation, respectively. The in vitro cytotoxicity tests and LDH assay confirmed its non-toxicity in different carcinoma cell lines in contrast to branched PEI 25 KDa. The synthesized copolymer was used for the co-delivery of both Dox and p53-pDNA. The obtained results revealed that it had higher cytotoxic impact compared with the Dox-loaded nanoparticle counterpart, demonstrated by cell viability and caspase 3 expression assay. Hence, the copolymer can be an inspiring candidate for the improvement of smart delivery systems [52].

Xu et al. conjugated DOX onto PEI via hydrazine bond and combined it with survivin siRNA through electrostatic interaction, to develop a pH-sensitive co-delivery system. Simultaneous application of DOX and survivin siRNA could be efficiently enhance the cytotoxicity in B16F10 cells and considerably their accumulation in tumor tissues of the lungs. In comparison with mono-delivery of each drug, the antitumor efficiency of the co-delivery system was enhanced, while its side effects was reduced [110].

Zhang et al. synthesized a co-delivery system based on hierarchical targeted nanocarriers of siRNA and lonidamine (LND) via the mitochondria-targeted ligand (triphenylphosphine) conjugated to the PEI in chitosan-graft-PEI and then coated with poly(acrylic acid)-polyethylene glycol-folic acid copolymer. The system was evaluated for tumor accumulation ability and cancer growth inhibition effect via a H22 subcutaneous sarcoma model as well as its therapeutic efficacy via hematoxylin and eosin and transferase-mediated dUTP nick end-labeling staining of tumor sections. In addition to decreased hepatotoxicity of LND and satisfactory in vivo biocompatibility, the results proved the higher tumor accumulation and inhibition growth of the system [111].

Poly(l-lysine) (PLL)-based NPs

Poly(l-lysine) (PLL) is a polycation widely used to co-deliver the gene and drug [54]. The PLL is a peptide having excellent structural precision in terms of molecular weight and secondary structure elements [112,113]. It is a non-viral transfection agent for gene delivery and DNA condensation because of the existence of amine groups which promote cell adhesion. To overcome the PLL cytotoxicity problems, PLL was chemically modified by covalent conjugation with compounds such as tyrosine-amide-triantennary oligosaccharide, asialoglycoprotein, N-glutarylphosphatidyl-ethanolamine, transferrin, fusogenic peptides, and antibodies for use as a delivery vector [114]. However, it is unclear whether PLL particles could be taken up by cells [115].

An amphiphilic methoxy poly(ethylene glycol)-b-poly(l-glutamic acid)-b-PLL triblock copolymer decorated with deoxycholate (mPEsG-b-PLG-b-PLL/DOCA) was introduced and employed by Lv et al. as a nano-vehicle for the DOX and PTX co-delivery. In vitro cytotoxicity evaluations contrary to the A549 human lung adenocarcinoma cell line showed that the co-delivery of DOX-PTX loaded NPs exhibited a synergistic effect in inducing cancer cell apoptosis [53].

A novel class of dendrimers based on PLL with a silsesquioxane cubic core (named as nanoglobules), were developed by Kaneshiro and Lu. These PLL-based NPs have highly functionalized surfaces, with a compact globular structure which could be employed as versatile carriers for biomedical utilizations. They used these NPs for the DOX and siRNA co-delivery in U87 glioblastoma cells [54].

Zheng et al. prepared NPs of a triblock copolymer poly(ethyleneglycol)-b-PLL-b-poly(L-leucine) (PEGePLLePLLeu) to systemically co-deliver DTX and siRNA-BCl-2 as an influential drug-gene carrier. The results showed that the BCl2mRNA and protein production was blocked by a complex of drug and siRNA BCl2. Moreover, the co-delivery of DTX and siRNA-BCl-2 (DTXesiRNAeNPs) obviously down-regulated the anti-apoptotic BCl2gene and enhanced the antitumor activity of DTX, resulting in significant tumor growth inhibition of the MCF-7 xenograft murine model compared with the free siRNA and DTX treatments [55].

Ma et al. designed a star-shaped porphyrin-arginine-functionalized PLL copolymer (PP-PLLD-Arg) for the drug and gene co-delivery. Results with this copolymer showed that PP-PLLD-Arg together with irradiation was a promising non-toxic and photo-inducible drug and gene delivery strategy [56].

A new cyclodextrin derivative (CD-PLLD), consisting of a sz-cyclodextrin core and PLL NPs arms, was prepared by the click conjugation of per-6-azido-sz-cyclodextrin with the propargyl focal point third generation PLL NPs and then used for the co-delivery of DTX and the siRNA plasmid targeting MMP-9 (pMR3). It was recognized that CD-PLLD/pMR3 nanocomplex had desirable in vitro gene transfection efficiency and can reduce the MMP-9 protein level in HNE-1cells [28].

Other polymeric co-delivery systems

Nowadays, several novel biodegradable copolymers are used for the co-delivery of anticancer agents. A previously published research has confirmed that DTX and endostatin-loaded TPGS-b-(PCL-ran-PGA) NPs could be efficiently taken up by cervical cancer cells and effectively inhibit the promotion of growth of tumor cells [116].

Zakerzadeh et al. designed and characterized a pH-triggered multi-potent nanocarrier, as a multi block silica based polymer, for the smart co-delivery of DOX and MTX. The excellent stimuli-responsive co-delivery ability as well as its significant simultaneous antibacterial and anti-cancer activities are eminent advantages which represent a more efficient drug delivery system [117].

Miller et al. developed a targeted co-delivery system containing PTX and ALN conjugated to the N-(2-hydroxypropyl) methacrylamide (HPMA)-copolymer macromolecule, for the neoplastic bone metastases treatment. The system exhibited a significant cytotoxic and antiangiogenic effects. The authors claimed that if the results of in vivo assays is satisfactory, the system could be a realistic strategy for the treatment of prostate and breast cancer bone metastases and osteosarcomas [118].

Soma et al. developed a polyalkylcyanoacrylate (PACA) nanoparticle loaded with DOX and cyclosporin A (CyA), utilizing an emulsion polymerization process with no precipitation and polymer aggregation. Since CyA is released rapidly from the NPs, it can possibly be suggested that it is loaded onto the nanoparticle surface. The results indicated that compared to individual drugs and other combination of both drugs, simultaneous association of CyA and doxorubicin within a single nanoparticle can exhibit the most effective growth rate inhibition for overcoming the multi-drug resistance [119].

Wong et al. investigated an encapsulated polymer-lipid hybrid nanoparticle (PLN) with a combination of DOX and GG918 (Elacridar) as a chemosensitizer for MDR cancer treatment. Clonogenic and trypan blue exclusion assays were used to evaluate the acute and long-term anticancer activities of different combination of both drugs, along with the nanoparticle, in a human MDR breast cell line. Of different combinational formulations, PLN loaded with either DOX and GG918 or each agent alone revealed the highest and lowest anticancer activities in MDR cells, respectively. The obtained results revealed that the maximum therapeutic effects depend on the proximal spatial distribution of both chemosensitizer and cytotoxic drug [120].

Zhang et al. investigated a pH-sensitive amphiphilic poly (sz-amino ester) copolymer NP for co-delivery of DOX and CUR. After drug ratio optimization, co-loaded nanoparticles showed some advantages including high encapsulation efficacy and low polydispersity. In acidic environment, the NPs released the drugs, which results in inhibiting cancer cell proliferation and angiogenesis. Furthermore, improved antitumor efficacy and reduced toxicity from off-target exposure were achieved because of precise intracellular target site and optimized drug combination concentration [121].

Guo et al. investigated the superiority of a self-assembled polycurcumin NP in the presence of a chemosensitizer. In this way, DOX and CUR were respectively co-encapsulated in the intracellular cleavable backbone and inner core of an amphiphilic poly(curcumin-dithiodipropionic acid)- sz-poly(ethylene glycol)-biotin (PCDA-PEG-Biotin), respectively to maximize the synergistic effect of chemosensitization and chemotherapy in a multidrug resistant MCF-7/ADR xenografted nude mice model. The results showed that ATP activity and suppression of P-gp expression induced by polycurcumin nanoparticles could efficiently reverse the cell resistance to DOX. Due to the hydrophobicity of the polycurcumin inner core, the authors suggested that other hydrophobic drugs could also be loaded into the NP which could be released efficiently like DOX [122].

Yao et al. developed a nanocarrier containing an amphiphilic hyaluronic acid (HA)-g-all-trans retinoid acid (HRA) conjugate loaded with PTX and all-trans retinoid acid (ATRA). Cell viability study proved the time- and concentration-dependent cytotoxicity of the so-mentioned nanoparticles, as well as their superiority in inducing the tumor cell apoptosis. Cellular uptake analysis revealed the efficient uptake of the nanoparticles through the transport into the nucleus and endocytic pathway. Compared to free DiR-loaded, in vivo studies showed that the increase of DiR-loaded HRA nanoparticles accumulation in tumor after intravenous administration, assisted the PTX targeting to the tumor. Higher inhibition of tumor growth and reduction in the toxicity are two advantages of the system [123].

Zhang et al. applied methoxy PEG poly(epsilon-caprolactone) (mPEG-PCL) nanoparticles, which have been co-loaded with tetrandrine (Tet) and PTX and were encapsulated into the physically cross-linked gelatin hydrogel for continuous drug release purposes. In vitro study exhibited efficient activity of the co-delivery system on the growth and aggressive ability of BGC-823 cells. In vivo study showed that the co-delivery system had the inducing and inhibiting effects through the pro-apoptotic Bax and p-Akt activations. Additionally, the monitoring of the volumes and weights of tumor demonstrated the molecular-modulating effect of the system on the variation of related protein [124].

Poon et al. synthesized carboplatin (Carbo) and gemcitabine (GMP) co-encapsulated in a self-assembled core-shell nanoscale coordination polymer nanoparticle (NCP-Carbo/GMP(. In vitro studies showed high synergistic effect of the system against SKOV-3, A2780/CDDP as platinum-resistant ovarian cancer cells. In comparison with control group, the system showed higher tumor uptake of drug as well as prolonged the blood circulation half-life, demonstrating the superiority of tumor regression based on the in vivo studies. Consequently, the co-loaded NP exhibited high efficacy of antitumor therapeutic and low systematic toxicity effects [125].

Li et al. successfully developed dual sensitive and temporally controlled CPT and siPlk1 co-delivery by a smart zwitterionic polymer, poly(carboxybetaine) (PCB)-based pH and esterase-sensitive CPT prodrug. Constructed cationic NPs with cationic dimethyldioctadecylammonium bromide (DDAB) were applied for siRNA co-delivery in a temporally controlled release manner for the efficient cancers treatment. The so-obtained dual sensitive CPT-PCB/siPlk1 lipoplexes was suitable for the combined impacts of cancer therapy with siRNA [126]. Lee et al. constructed self-assembly cationic NPs from a biodegradable and amphiphilic copolymer poly[(N-methyldietheneamine sebacate)-co-[(cholesteryl oxocarbonylamido ethyl) methyl bis(ethylene) ammonium bromide] sebacate] P(MDS-co-CES) which can carry both PTX and Herceptin simultaneously [14]. Tang et al. showed that a new amphiphilic poly (sz-amino ester), poly[(1,4-butanediol)-diacrylate-b-5-polyethylenimine]-block-poly[(1,4-butanediol)-diacrylate-b-5-hydroxy amylamine] (PDP-PDHA) enhanced DOX and survivin-targeting shRNA co-delivery to adverse breast cancers multiple drug resistance [18].

Liu and co-workers developed a co-loaded fasudil, as a mimicry vasculature suppressor and miRNA-195 in ST21-H3R5-PEG–based nanosystem with a novel aptamer-functionalized cationic peptide. The system showed some main advantages which included high loading capacity, adjustable dosage ratio of fasudil and fast release ability for both fasudil and miR195 in a reducing environment. In comparison with FasudilH3R5/PEGmiR195, FasudilST21-H3R5-PEGmiR195 had stronger silencing activity of ROCK2 protein and VEGF. In vitro and in vivo studies exhibited that the modified FasudilST21-H3R5-PEGmiR195 nanoparticle had significantly higher cellular uptake and antitumor efficacy compared with its unmodified counterparts, attributing to aptamer-mediated active targeting [127].

Conclusions and future perspectives

The co-delivery of anticancer drugs can help preventing adverse side effects of single chemotherapy such as drug resistance in cancer. It is now clear that the co-delivery of drug and gene utilizing nanocarriers is more efficient in cancer treatment compared to single chemotherapy. However, many challenges have yet remained associated with the co-delivery approaches including loading, capacity, stability, release kinetics, biocompatibility and tumor targeting efficacy [3]. Nowadays, different polymeric NPs have been constructed for the co-delivery of anticancer drugs and each has its own particular advantages. However, these polymeric-based NPs have to be optimized for their specific application where the outcome will be better-controlled therapy as a consequence of targeted delivery of lower quantities of effective anticancer agents. In addition, the development of new copolymers as co-delivery systems afford the opportunity to deal with the challenges of currently available polymeric NPs. Concerning future trends in the development of co-delivery systems, it is assumed that detailed systematic investigations for understanding and optimizing the conditions for efficient loading and sequential release of the respective therapeutic agents will be performed. By increasing the findings about the parameters that govern the success of the co-delivery strategy, rational and systematic improvements of the co-delivery systems for both drug and gene will be achieved in the near future, with enhanced therapeutic effectiveness. It seems that further studies should mainly focus on either the interaction between therapeutic agents or the interactions between carriers and therapeutic agents. Taken together, the improvement of co-delivery systems will ultimately lead towards the availability of highly effective therapies for cancer patients.

Disclosure statement

The authors have no conflict of interest.