Polyelectrolytes are an intriguing group of macromolecules that contain repeating units with either positive or negative charges. They possess unique theoretical and practical properties that distinguish them from uncharged polymers (Förster and Schmidt 1995; Dobrynin and Rubinstein 2005; Barrat and Joanny 2007). Even the dissociation of a small fraction of ionic units can significantly alter their properties, such as chain conformation, viscosity, diffusion coefficients, polarizability, and counter ion condensation. Polyelectrolytes have found various applications in different industries, including water treatment, cosmetics, pharmaceuticals, mineral separation, paper coating, and paints (Sato et al. 1979; Ito et al. 1986; Senuma et al. 1986; Vergaro et al. 2011; De Geest et al. 2012). Depending on their origin and charge type, polyelectrolytes can be classified as natural, modified natural, synthetic, cationic, anionic, or polyampholyte. Their potential applications are diverse and depend on factors such as charge density, dissociation constant of ionic groups, and type of charge. Polyelectrolytes have recently garnered attention in drug delivery applications. Their charged repeating segments make them soluble in aqueous conditions, which allows for complex formation with oppositely charged drugs or polymers via electrostatic interaction, resulting in the formation of nanoparticles with core and shell structures, also known as PICs. These nano vehicles can enhance drug bioavailability by increasing cellular uptake via endocytosis pathways and protecting the complexed drug from enzymatic degradation in vivo conditions (Shu and Zhu 2000; Kim et al. 2008b; Liu et al. 2008; Hamman 2010). Additionally, hydrophobically modified polyelectrolytes can self-assemble to form nanoparticles with a surface charge that can encapsulate hydrophobic drugs in the core (Shuai et al. 2003; Lim et al. 2016). Some polyelectrolytes also exhibit various biological functions, such as endosome disruption, antimicrobial activity, and immune system activation (Cakmak et al. 2004; Dyakonova et al. 2004; Müller et al. 2011). Thus, the pharmaceutical industry has shown great interest in studying different types of polyelectrolytes for drug delivery applications. In this review, we focus on the use of polyelectrolytes as bioavailability enhancers for anticancer treatment.

Mechanism for the formation of PIC

The formation of PICs occurs when oppositely charged polyelectrolytes come into contact with each other (Fig. 1). The interaction between the polyelectrolytes is driven by electrostatic forces, which cause the charged segments of the polyelectrolytes to attract each other. The interaction can lead to the formation of a nanoparticle with a core-shell structure, in which one polyelectrolyte forms the core and the other forms the shell. The formation of PICs is influenced by several factors, including the charge density, chain length, and degree of ionization of the polyelectrolytes, as well as the ionic strength and pH of the solution. In general, higher charge densities and longer chains lead to stronger interactions between the polyelectrolytes and the formation of more stable nanoparticles. The degree of ionization of the polyelectrolytes also affects the formation of PICs, as the dissociation of the charged segments can affect the electrostatic interaction between the polyelectrolytes. As shown in Fig. 1, the mechanism of PIC formation can be further divided into three stages: (1) initial contact between the polyelectrolytes, forming the primary complex, (2) formation of a diffuse polyelectrolyte cloud around the oppositely charged polyelectrolyte, and (3) coacervation, in which the polyelectrolyte chains come into close proximity and form a stable nanoparticle, resulting in the secondary complex. The secondary aggregates are composed of about 100 primary complex particles, and the inter-complex aggregation process is mainly progressed by hydrophobic interactions (Chornet and Dumitriu 1998; Starchenko et al. 2008; Müller et al. 2011).

Figure 1. Mechanism for formation of polyelectrolyte complexes (PICs).

Parameters in preparation of polyelectrolyte nanoparticles

Polyelectrolyte nanoparticles are formed by the interaction of oppositely charged polyelectrolytes, and several parameters can be controlled during their preparation to optimize their physical properties and drug delivery efficiency. These parameters include the true charge and charge density of the polyelectrolytes, the mixing ratio of the polyelectrolytes, the order of addition of the polyelectrolytes, and the effect of salt concentration in the solution.

True charge

The true charge of polyelectrolytes refers to the actual charge of the repeating units. When preparing oppositely charged polyelectrolytes in stoichiometric ratios to form PIC nanoparticles, the determination of true charge and charge density is very important. As mentioned above, the driving force for PIC formation is the electrostatic interaction between oppositely charged macromolecules. However, the true charge or charge density of polyelectrolytes could be affected by the colloidal pH, and the aggregation formation may not be identical to the experimental design. Wang et al. (2006) reported the effect of pH on linear poly (ethyleneimine) (PEI) affinity for complex formation. Under acidic conditions, the PEI is a strong cationic polymer, and the binding affinity with anionic surfactants is very high. However, at pH 7, the electrostatic attraction becomes weaker, and at pH 10, PEI is essentially nonionic, and there is no electrostatic attraction anymore (Wang et al. 2006). In our previous study, we also reported the pH-sensitive PEI/PASP complex system for anticancer drug delivery. At pH 7.4, the PEI/PASP complex system formed a compacted nanoparticle with a narrow size distribution. However, under acidic conditions, the true charge of PASP decreased, and the PASP dissociated from the PEI-based platform, resulting in accelerated drug release (Lim et al. 2016).

Charge density

The charge density of polyelectrolytes is also related to the formation of PICs. When preparing complex nanoparticles, core compacted structures can be obtained with similar charge densities of the polyelectrolytes. Conversely, loose and unstable structures become prevalent in systems with strongly deviating charge densities of the polyelectrolytes (Dautzenberg and Jaeger 2002; Mende et al. 2002). Mende et al. (2002) investigated the role of charge density in the formation of PICs. Among various charge densities of polymers, the most stable and smallest complex systems were obtained from oppositely charged polyelectrolytes with similar charge densities (Mende et al. 2002).

Mixing ratio

The mixing ratio of charges between polyelectrolytes and oppositely charged macromolecules is a crucial parameter that should be considered to create small-sized nanoparticles with good stability. Depending on the cationic/anionic charge ratio (C/A ratio), the particle size and surface charge can be dramatically changed. Zelphati et al. (1998) investigated the change in size and zeta charge of liposome-gene complexes depending on the mixing ratio. At a charge ratio near 1, the surface charge of the PIC became about zero and exhibited a significantly increased particle size with poor physical stability due to increased hydrophobicity (Zelphati et al. 1998). Kabanov and Zezin (1984) also reported the change in size and aggregation number of PDMAEMA/phosphate polymers at different C/A ratios. The size of the PIC dropped from 49 nm at a C/A value of 0.067 down to about 30 nm at C/A = 0.5, and the aggregation number of the polymer increased due to the change in structures (Kabanov and Zezin 1984).

Order of addition

The order of polymer addition for PIC formation can affect the particle size and stability. In principle, the smallest particle size of PIC can be achieved by adding the minority component into the majority component. Zelphati et al. (1998) studied the particle size of PIC with different orders of addition. To prepare the cationic lipid/DNA complex (C/A ratio > 1), DNA was added to excess lipid materials, and a stable and small-sized nanoparticle was formed. However, large aggregates were produced when lipid materials were added to DNA (Zelphati et al. 1998). In the former case, the 1:1 stoichiometry between lipid and DNA cannot be achieved, and they form homogeneously complexed PIC nanoparticles. However, in the latter case, the binding of DNA to cationic lipid is not homogeneous because the 1:1 complex ratio must be exceeded, resulting in the formation of aggregates. Similar studies were also reported by Müller et al. (2011) and Sæther et al. (2008). All the studies reveal that adding the minor component to the major solution can produce more equilibrated and compacted PIC nanoparticles.

The structure of polyelectrolytes has been tailor-made to correspond with the physico-chemical properties required for the target drug and disease environment conditions. In this chapter, we will provide an overview of these polyelectrolyte types for various drug formulations (Fig. 2) and summarize the PICs in Table 1.

Figure 2. Various type of drug delivery platform using polyelectrolytes. (a) Polymer-drug complex via electrostatic interaction. (b) Polymer-drug complex via electrostatic and hydrophobic interactions. (c) Polymer-polymer-drug complex via electrostatic and hydrophobic interactions. (d) Layer-by-layer formation of a polymer-polymer-drug complex via electrostatic and hydrophobic interactions.

Table 1 . Application of PICs as a drug delivery system.

Polyelecrolyte	Property	Types of interactions between polymers and drugs	References
Poly (ethyleneimine) (PEI)	Hydrophilic cationic	Electrostatic interaction	Fischer et al. (1999), Kunath et al. (2003), Lungwitz et al. (2005), Ogris et al. (1999)
Polyamidoamine (PAMAM)	Hydrophilic cationic	Electrostatic interaction	Haensler and Szoka (1993)
Poly-β-Aminoesters (PBAE)	Hydrophilic cationic	Electrostatic interaction	Rui et al. (2019)
Poly-L-Lysine (PLL)	Hydrophilic cationic	Electrostatic interaction	Zhang et al. (2010)
Chitosan	Hydrophilic cationic	Electrostatic interaction	Loo et al. (2022), Bowman and Leong (2006), Riezk et al. (2020)
Glycol chitosan modified with 5B cholanic acid	Amphiphilic cationic	Electrostatic and hydrophobic interactions	Park et al. (2004)
Poly (lactic acid)–poly (ethylene glycol)–poly (histidine) (PLA-b-PEG-b-P (His))	Amphiphilic pH-responsive	Hydrophobic interaction	Lee et al. (2007)
Poly (ethylene glycol)-block-poly (4-vinylbenzylphosphonate) (PEG-PVBP)	Amphiphilic anionic	Hydrophobic interaction	Kamimura et al. (2012)
Poly (ethylene glycol)–poly (lactic acid)–poly (ethyleneimine) (PEG-PLA-PEI)	Amphiphilic cationic	Electrostatic and hydrophobic interactions	Lim et al. (2016)
Poly (aspartic acid) (P (Asp))	Hydrophilic anionic
Poly-alkyl amine derivative	Amphiphilic cationic	Electrostatic and hydrophobic interactions	Fatima et al. (2016)
P (Asp)	Hydrophilic anionic
Carboxymethyl cellulose	Hydrophilic anionic	Electrostatic and hydrophobic interactions	Zhao et al. (2007)
Chitosan	Hydrophilic cationic	Electrostatic and hydrophobic interactions	Chen et al. (2009)
Alginate	Hydrophilic anionic

Hydrophilic polyelectrolyte

Polymer-drug complex via electrostatic interaction

A wide range of polyelectrolytes have been synthesized and used as materials for drug delivery systems. Among them, copolymers containing cationic groups have been extensively investigated (Kabanov et al. 1996; Kataoka et al. 1996; Kabanov et al. 2004). As shown in fig. 2a, the cationic segments can bind to oppositely charged drugs by electrostatic interaction, and they can spontaneously form nanosized PICs.

1) Poly (ethyleneimine)

The representative cationic polyelectrolyte for PIC is PEI, which has been known as the most effective non-viral vector due to its condensation and enhanced transfection ability for complexed genetic materials (Fischer et al. 1999; Kunath et al. 2003; Lungwitz et al. 2005). Ogris et al. (1999) investigated the in vitro and in vivo properties of pegylated gene/PEI complexes. Pegylated PEI condenses the DNA into nanosized particles by forming complexes and protects it from enzymatic degradation. They demonstrated that the pegylated gene/PEI complex system could reduce the plasma protein binding into the nanoparticle and prolong blood circulation time with enhanced gene expression levels at the tumor site (Ogris et al. 1999).

2) Polyamidoamine

Polyamidoamine (PAMAM) dendrimers, which contain a large number of secondary and tertiary amines, have also been widely used for gene or cytokine delivery. Haensler et al. originally reported the use of PAMAM dendrimers as gene delivery agents. They demonstrated that the sixth generation PAMAM dendrimer exhibits improved gene delivery efficacy with good biocompatibility. These synthetic non-viral vectors using PIC systems provide opportunities for simple preparation, improved safety, and greater flexibility in the gene therapeutic area (Haensler and Szoka 1993).

3) Poly-β-Aminoesters

Poly-β-Aminoesters (PBAEs) are versatile polymers composed of repeating units with varying charge states. Their flexibility in design during synthesis is a key attribute. PBAEs are primarily used to interact with anionic drugs. These can include small-molecule drugs, nucleic acids (such as siRNA or DNA), and proteins (such as antibodies or enzymes). The electrostatic interaction between PBAEs and these drug molecules allows for the formation of nanoparticles or the encapsulation of drugs. PBAE-based drug delivery systems find applications in cancer therapy, gene therapy, and vaccination. Rui et al. (2019) reported PBAE-based protein delivery system for efficient cellular uptake of protein therapeutics. The delivery system enabled rapid cellular uptake, efficient endosomal escape, and nanoparticles loading CRISPR-Cas9 ribonucleoproteins induced strong gene editing (Rui et al. 2019).

4) Poly-L-Lysine

Poly-L-Lysine (PLL) is a cationic polypeptide polymer made up of repeating units of L-lysine. It possesses high biocompatibility, biodegradability, and antimicrobial properties. PLL is frequently used to interact with anionic drugs such as nucleic acids (DNA or RNA), peptides, and proteins. The electrostatic interactions between PLL and these drug molecules enable the formation of nanoparticles or the surface binding of drugs. PLL-based drug delivery systems are applied in gene therapy, protein delivery, and vaccine development. Zhang et al. (2010) designed PLL-based nanoparticle co-loading a plasmid DNA and a peptide hormone. Zhang’s data showed that melanin secretion was induced by efficient gene transfer and hormone stimulation (Zhang et al. 2010).

5) Chitosan

Chitosan is a natural polymer derived from chitin, found in crustacean shells. Its amino groups confer a cationic nature. Chitosan interacts strongly with anionic drugs, which can include small molecules, peptides, proteins, and even nucleic acids. This electrostatic interaction is leveraged for encapsulating drugs within its matrix or forming drug-loaded nanoparticles. Chitosan-based drug delivery systems are utilized in various applications, such as wound healing (Loo et al. 2022), oral drug delivery (Bowman and Leong 2006), and dermal drug delivery (Riezk et al. 2020). In particular, its versatility extends even further as it can be extensively modified for a wide range of applications. Modified Chitosan derivatives, such as carboxymethyl chitosan, thiolated chitosan, or pegylated chitosan, offer enhanced properties and functionalities. These modifications allow Chitosan to be tailored for specific drug delivery applications, such as targeted drug delivery, sustained release, and improved stability.

Hydrophobically modified polyelectrolytes

As mentioned earlier, the formation of PICs involves not only electrostatic interactions but also hydrophobic interactions. It has been reported that the presence of a hydrophobic moiety in the polyelectrolyte can enhance the colloidal stability of particles and increase tissue permeability (Shuai et al. 2003; Kamimura et al. 2012; Xun et al. 2013). Additionally, it is known to assist in the encapsulation of hydrophobic drugs (Yun et al. 2012; Hoang et al. 2016). In this chapter, we introduce polyelectrolytes that incorporate hydrophobic interactions for drug formulation.

Polymer-drug complex via hydrophobic and electrostatic interaction

Using the mechanism shown in Fig. 2b, Park et al. (2004) developed self-assembled nanoparticles based on glycol chitosan bearing a hydrophobic moiety. The glycol chitosan was hydrophobically modified with 5B cholanic acid (HGC) to have self-assembling properties, and RGD peptides labeled with fluorescein isothiocyanate (FITC) were loaded into the nano-sized HGC. FITC-labeled peptides have several potential interactions with other molecules, including hydrophobic, hydrogen bonding, and electrostatic interactions. These interactions allow the self-assembled HGC to form a compact particle with a small size. The FITC-RGD peptides were successfully loaded into HGC with high loading efficiency and exhibited enhanced biological activity in vivo conditions (Park et al. 2004).

Polymer-drug complex via hydrophobic interaction

Polymer-drug complexes via hydrophobic interaction can be used as smart drug delivery systems for insoluble drug formulations. Lee et al. (2007) reported enhanced antitumor activity with pH-responsive polyelectrolytes. They prepared self-assembled PLA-b-PEG-b-P (His) triblock copolymers, optimized Dox formulation, and reported its pH-dependent physical changes and cell cytotoxicity. The protonation of the imidazole ring in P (His) can be changed in acidic conditions, showing pH-induced micelle interior structural changes from shrinking to swelling. This provides a mechanism for accelerated drug release and enhanced cytotoxicity in acidic pH conditions. The hydrophobic moiety in polyelectrolytes can increase colloidal stability and enhance tissue permeability due to hydrophobic interactions. Kamimura et al. (2012) reported a highly stable and pH-responsive PIC system formed by poly (ethylene glycol)-block-poly (4-vinylbenzylphosphonate) (PEG-b-PVBP) and cationic surfactants. The polyanionic segment in PEG-b-PVBP contained a hydrophobic styrene moiety in the repeating unit, which could strongly affect the colloidal stability of the PIC system against changes in ionic strength and dilution. They revealed that additional hydrophobic interactions of benzyl groups in PEG-b-PVBP contribute to the highest stability of PEG-b-PVBP/surfactants among PIC systems in vivo conditions. Accelerated drug release from PEG-b-PVBP/surfactants systems was also observed at acidic pH conditions due to the protonation of phosphate groups in PVBP chains (Fig. 3).

Figure 3. Illustration of self-assembly of the block ionomer complexes. Reused from Kamimura et al. (2012), with permission from Elsevier (license number: 5633941492156). PEG-b-PVBP, poly (ethylene glycol)-block-poly (4-vinylbenzylphosphonate); DTAB, dodecyltrimethylammonium bromide; C12Py, Dodecylpyridinium chloride; HTAB, hexadecyltrimethylammonium bromide; C16Py, cetylpyridinium chloride; DOX, Doxorubicin hydrochloride.

PICs with oppositely charged polymer

Nanoparticles formed by complexation of amphiphilic polyelectrolyte and oppositely charged surfactant or polymers can be used as a strategy to control the drug release profile and toxicity (Fig. 2c). In a previous study, we developed a polyelectrolyte ionomer complex composed of positively charged poly (ethylene glycol)–poly (lactic acid)–poly (ethyleneimine) (PEG-PLA-PEI) triblock copolymer and a negatively charged poly (aspartic acid) (P (Asp)) polymer. These polyelectrolytes formed stable aggregates via electrostatic interaction between PEI and PASP blocks and showed pH sensitivity by protonation and deprotonation of the carboxyl groups in P (Asp) for targeting the extracellular pH of cancers. At acidic conditions, the release of Dox loaded in the platforms was accelerated, improving the antitumor activity (Lim et al. 2016). The surfactant complex with polyelectrolyte can also be used to reduce the toxicity of charged platforms themselves. Fatima et al. (2016) developed a PIC system with amphiphilic poly-alkyl amine derivatives and poly-aspartic acid for anticancer drug delivery. Due to its unique cell permeating property, the poly-alkyl amine derivatives have been widely explored as drug delivery platforms. However, the amphiphilic polyelectrolyte has membrane-disrupting activity, which can induce hemolysis in blood circulation. They demonstrated that the inherent cytotoxicity of the polymer can be overcome by complexing polyelectrolytes with anionic-charged P (Asp) to form a PIC system. The cytotoxicity of poly-alkyl amine derivative was significantly decreased, and the aqueous stability of curcumin, a model hydrophobic drug, was significantly improved with enhanced antitumor activity (Fatima et al. 2016).

Layer by layer polyelectrolyte

Layer-by-layer (LbL) polyelectrolyte assembly is also widely used in the drug delivery area. As shown in Fig. 2d, a LbL nanoparticle is a thin-film consecutive absorption technique using positively and negatively charged polymers on the surface. This multi-layered nanoparticle can be used to control the release profile, increase the stability, and improve the solubility of various drugs (Johnston et al. 2006; Kim et al. 2008a; Ariga et al. 2011). Zhao et al. (2007) investigated the encapsulation of the anticancer drug Dox in LbL nanoparticles and validated their therapeutic efficacy under in vitro conditions (Fig. 4). The positively charged Dox was successfully accumulated in negatively charged carboxymethyl cellulose-based LbL nanoparticles, and the drug-loaded LbL nanoparticle exhibited enhanced antitumor activity (Zhao et al. 2007). Chen et al. (2009) also studied anticancer drug-loaded LbL nanoparticles with chitosan, gelatin, and alginate. They demonstrated the feasibility of controlling the drug release profile by changing several parameters, such as polyelectrolyte type, concentration of polymer, and number of coating layers (Chen et al. 2009).

Figure 4. Illustration of LBL nanoparticle formation and in vivo antitumor activity. Reused from Zhao et al. (2007), with permission from Elsevier (license number: 5633951000634).

Polyelectrolytes possess a variety of preferential biological properties as well as physicochemical characteristics for nanoparticle formation, making them useful for drug delivery purposes. In this chapter, we will provide a representative overview of the biological properties of polyelectrolytes that can maximize the therapeutic efficacy of drugs in anticancer treatment (Table 2).

Table 2 . Polyelectrolytes according to biological function.

Polyelecrolytes	Biological function	Property	References
Chitosan	Enhanced cellular uptake	Electrostatic interaction between positively charged polymer and negatively charged cell membrane	Yue et al. (2011)
Poly (l-lysine)-poly (l-leucine)-based block copolymer			Han et al. (2015)
PEI	Endosomal escape	Proton-sponge effect	Han et al. (2012)
pH-sensitive methacrylic acid copolymers		Destabilization of membrane bilayers	Yessine et al. (2003)
Oligoarginine		Leakiness and rupture of membrane bilayers	Hitz et al. (2006)
Polyoxidonium	Immune stimulator	Antigen-conjugated polyoxidonium	Dyakonova et al. (2004)
Pyran (divinyl ether-maleic anhydride) copolymer		Immunoactive properties	Mohr et al. (1975)
Chondroitin sulfate		Strong immune response activator	Chaurasia et al. (2015)
Cationic polyelectrolytes	Antitumor activity	Stimulating secretion of several antitumor cytokines and promoting Th1 and NK cell infiltration	Chen et al. (2010)
Chitosan derivatives		Hydrophobic and electrostatic interaction between the chitosan derivatives and the tumor cell surface	Lee et al. (2002)
PEI		Cell membrane damage due to its high charge density	Yim et al. (2014)

Enhanced cellular uptake

Cellular uptake of nanoparticles is influenced by several factors, including size, shape, surface charge, and hydrophobicity (González et al. 1996; Win and Feng 2005; Chithrani et al. 2006). Surface charge plays an especially important role in the cellular uptake of nanoparticles. Thus, polyelectrolyte-based nanoplatforms with a positive charge exhibit a stronger affinity to the negatively charged cell membrane by electrostatic interaction and are generally better taken up than negatively charged particles. Yue et al. (2011) demonstrated the effect of nanoparticle surface charge on cellular uptake with chitosan-based polyelectrolytes. They prepared three kinds of chitosan-based polyelectrolytes with different surface charges and evaluated the effect of surface charge on cellular uptake profiles. The results showed that a positive charge promotes the internalization rate and increases the internalization amount of nanoparticles in different eight cell lines (Yue et al. 2011). Han et al. (2015) reported charge-reversal micelles with Poly (l-lysine)-poly (l-leucine)-based block copolymer for tumor targeting. PLLeu-PLYS (DMA)-Tat (SA) has β-carboxylic amide, which allows the polypeptides to self-assemble into negatively charged micelles. At acidic conditions, the amide can be cleaved, and the micelles switch to a positively charged state, significantly enhancing their cellular uptake (Han et al. 2015).

Endosomal escape

The endocytic pathway is the major uptake mechanism of cells. Particles or drugs entering the cells via the endocytic pathway become entrapped in endosomes and are degraded by specific enzymes in the lysosome. Thus, a limiting step in achieving effective therapy is to facilitate endosomal escape and ensure cytosolic delivery of the therapeutics (Panyam et al. 2002; Nel et al. 2009; Dominska and Dykxhoorn 2010). There are several ways to use polyelectrolytes to destabilize the lipid membrane, releasing the endocytosed active compounds into the cytoplasm. The most extensively studied way for endosomal escape is using a “proton-sponge effect” (Yezhelyev et al. 2008; Chou et al. 2011). The proton-sponge effect has been observed in several cationic polymers containing protonable residues with pKa close to endosomal/lysosomal pH. When polymers with the proton-sponge effect are taken up into the acidic environment of endosomes, the polymer residue becomes protonable and resists the acidification of endosomes. As a result, more protons are continuously pumped into endosomes, promoting endosome osmotic swelling, rupture of endosomes, and intracellular release of their contents to the cytosol. Polyethyleneimine (PEI) is one of the most widely used polyelectrolytes for endosomal escape. Due to the amine residue in PEI, it exhibits considerable buffering capacity and subsequently leads to endosome disruption. Han et al. reported on PEI endosomal escape with PEI and PAH-Cit polymer (Han et al. 2012). They demonstrated enhanced gene expression efficiency with polyelectrolyte-coated gold nanoparticles through layer-by-layer techniques. PEI and charge reversion polymer are protonated in acidic conditions, facilitating the escape of the platform from the endosome/lysosome and releasing siRNA into the cytoplasm. Since endosomes have an acidic environment, anionic carboxylated polymers with pH-sensitive properties could also be used as a membrane-disrupting agent. The anionic polymer can destabilize membrane bilayers by pH-triggered conformational change. Yessine et al. (2003) reported intracellular delivery of biomacromolecules from endosomes into the cytoplasm with pH-sensitive methacrylic acid copolymers. They demonstrated that negatively charged polymers with a sharp phase transition property have high hemolytic activity and can efficiently destabilize membrane bilayers (Yessine et al. 2003). Hitz et al. (2006) studied the interaction of oligoarginine with anionic lipid vesicles. They demonstrated that firstly, the positively charged guanidine group in oligoarginine binds to the lipid membrane via electrostatic interaction and forms a nonpolar ion pair with negatively charged components of cell membranes. Secondly, the oligoarginine attaches to the lipid membrane more strongly through non-electrostatic interaction, and the bilayer becomes rigidified, resulting in leakiness and rupture of the lipid membrane (Hitz et al. 2006).

Immune stimulator

Individual bacterial or viral antigens, which may not be sufficiently active alone, can induce a specific strong immune response when chemically conjugated to specific polyelectrolytes (Kabanov et al. 2005; Kabanov 2006). Therefore, synthetic polyelectrolytes combined with specific antigens can enhance the immune response and be successfully used to develop a synthetic vaccine. Synthetic polyelectrolytes, such as polyacrylic acid, dextran sulfate, polyvinylpyridine, and polyvinylimidazole, have no structural similarity with antigens but when complexed with specific antigens, they enhance the immune response and increase antibody production several times over. A representative polyelectrolyte as an immune stimulator is polyoxidonium. Polyoxidonium is an N-oxidized polyethylene-piperazine derivative, a water-soluble high-molecular synthetic immune modulator. Antigen-conjugated polyoxidonium can enhance the specific immune response by orders of magnitude, and many studies have reported its efficacy in the immunotherapy area (Dyakonova et al. 2004). Mohr et al. (1975) reported significantly increased antitumor activity with polyanion as an effective adjuvant to chemotherapy. The pyran (divinyl ether-maleic anhydride) copolymer has highly immunoactive properties, and when it is co-treated with chemotherapeutic drugs, it exhibits significantly enhanced antiproliferative activity against cancer cells via immune activation (Mohr et al. 1975). Chaurasia et al. (2015) also reported the function of polyelectrolyte as a strong immune response activator. Chondroitin sulfate-based nanocapsules exhibit enhanced Dox-mediated apoptosis via Th1 immune response as a selective immune stimulator, resulting in improved chemotherapeutic efficacy (Chaurasia et al. 2015).

Antitumor activity

Positively charged polyelectrolytes are promising candidates as antimicrobial agents. The cationic polyelectrolytes interact with the phospholipid component of the cell membrane, penetrate into the cell wall, and cause cytoplasmic membrane disorganization and wall lysis due to autolytic enzymes (Zhang et al. 2001; Bechinger and Lohner 2006). Cationic antimicrobial polypeptides can also be used as novel cytotoxic agents for cancer treatment. The antitumor effect of antimicrobial cationic peptides occurs through a cell membrane lytic effect and apoptosis via mitochondrial membrane disruption. This happens via electrostatic interaction due to the more negatively charged tumor cell membrane than neutrally charged healthy cell membranes, which results from the greater expression of anionic molecules such as heparin sulfate, phosphatidylserine, or sialic acid-rich glycoproteins in tumor cells. In addition, the activation of toll-like receptors is known as a potential strategy for immuno-cancer therapy, and cationic polyelectrolytes were reported to have immunological activity mediated by toll-like receptors (TLRs) (Chen et al. 2010). Thus, cationic polyelectrolytes could stimulate the secretion of several antitumor cytokines, including IL2 and LPS, and could also promote Th1 and NK cell infiltration and suppression of tumor angiogenesis, which are directly related to antitumor activity (Huang et al. 2013). Lee et al. (2002) reported the cytotoxic activity of cationic chitosan derivatives against various cancer cell lines. They demonstrated the role of charge density and hydrophobicity against cancer cell viability with different types of chitosan derivatives. The antiproliferative activity was enhanced with increasing charge density and hydrophobicity of chitosan derivatives, indicating the hydrophobic interaction as well as electrostatic interaction between the chitosan derivatives and the tumor cell surface are closely related to cancer cell-killing activity. Yim et al. (2014) also revealed the antitumor activity with the all-trans-retinoic acid-g-polyethyleneimine/hyaluronic acid complex system (ATRA-PEI/HA) against various cancer cells. The PEI could result in cell membrane damage due to its high charge density. The toxicity of PEI could be controlled by hyaluronic acid complex via electrostatic interaction, and it minimizes systemic toxicity after IV administration. At the tumor site, the HA/ATRA-PEI is unshielded by hyaluronidase, and PEI could induce specific cell death. It also exhibited synergistic effects with ATRA-induced immune response to kill the cancer cells.

This review paper provides a brief overview of the use of polyelectrolytes in anticancer therapy. Polyelectrolytes are widely used in drug delivery as they can form nanoplatforms through self-assembly or by mixing with oppositely charged polymers. The physical properties of these nanoplatforms can be optimized by controlling various parameters during preparation, including particle size, surface charge, and platform stability. Additionally, polyelectrolytes have demonstrated promising biological functions, making them ideal therapeutic carriers for a variety of drugs.

Furthermore, PIC is used not only for therapeutic carriers but also for the development of biosensors, vaccines, and dialysis membranes. There is also a trend toward the application of polyelectrolytes in the field of tissue engineering materials, where complex constructs made of charged biocompatible polymers can be used to create scaffolds similar to biological matrices. In the future, PIC will have multiple applications in various fields. In conclusion, the use of polyelectrolytes with advantageous properties has the potential to significantly improve drug delivery systems, and comprehensive and systematic characterization should be performed to maximize their efficacy.

The authors declare that they have no conflict of interest.

This work was supported by the National Research Foundation of Korea (NRF), which is funded by the Korean government (MSIT) (grant nos. 2021R1A2C2008519, 2022R1C1C2002949, and 2022R1A5A600076012).