The dendrimers were first synthesized by Tomalia et al. taking advantage of the architecture including monodispersity, extraordinary symmetry, hyper branch with tree-like structures (Tomalia et al., 1985). They consist of a central core and branches emanating from the core terminated with functional surface groups (Figure 1B). With the increase of generation, dendrimers form 3D spheres, thus creating supramolecular void spaces that can bind and transfer other molecules (Quadir and Haag 2012; Dzmitruk et al., 2018). In addition, it has multiple surface functional groups that can be modified, which is different from linear structures (Menjoge et al., 2010). A study had shown that linear structure had stronger DNA binding and cellular uptake, but dendritic structure mediated gene expression is higher than linear structure, which may be related to the escape of sufficient DNA amount of effective gene expression into the cytoplasm (Yamagata et al., 2007).

PAMAM is one of the most widely studied dendrimers. Because of the ability to combine with nucleic acid, numerous effort has been developed in applying PAMAM to the treatment of diseases including gene delivery and nucleic acid scavenge (Abedi-Gaballu et al., 2018). However, the binding efficiency is associated with the positive charge density. Higher generation PAMAM with more primary amines possess higher positive charge density on the surface (Jensen et al., 2011). G3-G10 PAMAM dendrimers are the optimal choices resulted from their remarkable stability to combine with nucleic acid and higher transfect efficiency (Palmerston Mendes et al., 2017). Successful attempts have been made to use PAMAM-G3 as antithrombotic agents and anti-metastatic agents. Due to the property of combination with nucleic acid, PAMAM-G3 attenuated the activation of blood coagulation and inflammation induced with nucleic acid through the Toll-like receptor (TLR) pathway, which was related to the thrombosis, lung metastasis in breast cancer, and liver metastasis in pancreatic cancer (Jain et al., 2012; Naqvi et al., 2018; Holl et al., 2020).

Furthermore, surface modification and tri-block modification have been applied to PAMAM. Compared with polyamidoamine (PAMAM), PEGylation modification of PAMAM could increase the transfection efficiency and stabilization with lower cytotoxicity (Reyes-Reveles et al., 2013; Sun et al., 2014). A novel tri-block nanocarriers consisting of PAMAM, poly(ethylene glycol) (PEG), and poly-l-lysine (PLL) were developed to deliver siRNA. PLL replaced the role of PAMAM to form polyplexes with siRNA and PAMAM severed as a proton sponge. PEG was used to stabilize nanocarriers in plasmas (Patil et al., 2011). Biswas et al. also developed another triblock nanocarrier PAMAM-G4-D-PEG-DOPE to deliver siRNA. Different from the former, PAMAM-G4 worked for efficient siRNA condensation (Biswas et al., 2013). Similarly, PAMAM, PEG, and lactobionic acid (Gal) were used to construct a delivery system to carry AEG-1 siRNA, and it was proved that PAMAM-AEG-1si nanoplexes restrain tumor growth (Rajasekaran et al., 2015).

As an important pH bioresponsive functional cationic nanomaterials, as well as the character of excellent stability and safety profile (Huang et al., 2018), PDMA (Figure 1C) has been widely applied as a delivery system. Possessing a high affinity for nucleic acid, PDMA is increasingly studied in the gene delivery and neutralization of nucleic acid. A biodegradable cationic micelles PDMAEMA-PCL-PDMAEMA was established to deliver siRNA and paclitaxel into cancer cells, which diminished the expression of VEGF (Zhu et al., 2010). Rungsardthong et al. demonstrated that the DNA affinity of PDMA could be fine-tuned by varying the PH and the polymer/DNA ratios (Rungsardthong et al., 2003).

Researchers anticipated that the properties of PDMA could be optimized by modifying different groups. Deshpande et al. utilized 2-(dimethylamino) ethyl methacrylate (DMAEMA) to construct three different polymers: DMAEMA-PEG (a diblock copolymer), DMAEMA-OEGMA 7 (a brush-type copolymer), and DMAEMA-stat-PEGMA (a comb-type copolymer). Compared with PDMAEMA, all of them exhibited more excellent binding ability with oligonucleotide while DMAEMA-stat-PEGMA showed the best. But DMAEMA-PEG and DMAEMA-OEGMA 7 own better long-term colloidal stability (Deshpande et al., 2002). PEO-PPO-PEO-pDMAEMA (L92-pDMAEMA) and PEO-pDMAEMA copolymers basing on PDMA, poly (propylene oxide) (PPO), and poly (ethylene oxide) (PEO) were also developed to deliver genes. It has been reported that modification with PEO could reduce the unfavorable interactions with complement factors or cellular components (Bromberg et al., 2005). Furthermore, PEO-b-PDMAEMA could form soluble complexes with DNA of a much smaller size resulted from the amphiphilic nature of the polymer (Tan et al., 2006).

Polyphosphoramidate (PPA) (Figure 1D), a biodegradable cationic material, has been investigated as drug delivery and gene delivery for years. Their structures, different side chains, molecular weight, and positive charge on the surface can influence the complexation with nucleic acids. PPA owned higher DNA binding efficiency when molecular weight and positive charge density increased (Ren et al., 2010). Zhang et al. developed a series of cationic polymers which had an identical backbone and different side chains including primary, secondary, tertiary, and quaternary amino groups, and demonstrated that PPA with primary amino group possessed uppermost ability to complex with nucleic acids (Wang et al., 2004). Furthermore, the same team synthesized ternary complexes, consisting of PPA backbone, primary and tertiary amino group, and quaternary complexes, containing PPA, primary, secondary, and tertiary amino groups. And the results showed that the coexistence of primary and other amino groups could elevate the combination with the nucleic acid (Zhang PC. et al, 2005).

PEGylation modification is also applied in PAA. PEG-b-PPA/DNA micelles with lower surface charge and smaller particle size ranging from 80 to 100 nm maintained similar transfection efficiency while showing lower cytokines and better biocompatibility compared with PPA/DNA (Jiang et al., 2007). Moreover, galactosylated PPA was prepared to enhance the targeted capacity as a delivery system. However, the transfection efficiency of gal-PPA reduced with the increase of galactose substitution degree, presumably resulting from the decreased DNA binding capacity and particle stability (Zhang XQ. et al, 2005). Hence, modification of PPA needs to be further explored.

Hexadimethrine bromide (HDMBR) (Figure 1E) has been used as an antiheparin agent for many years (Pai and Crowther 2012) and has been rediscovered in recent years to neutralize nucleic acids and deliver genes due to its positive charge. A combination of HDMBr and dimethyl sulfoxide facilitated DNA transfection into chicken embryo fibroblast cells and human fibroblast (Kawai and Nishizawa 1984; Aubin et al., 1997). After intraperitoneally administered, HDMBR neutralized extracellular nucleic acids and thereby reduced lung injury, restrained disruption of alveolar-capillary barrier, and increased blood oxygenation in acute respiratory distress syndrome (ARDS) model rats exposed to CEES, a toxic chemical (Mariappan et al., 2020). Similarly, HDMBr could scavenge mitochondrial DNA (mtDNA) in an in vivo model of trauma hemorrhage, and the ability to inhibit inflammation and apoptotic cell death emerged (Aswani et al., 2018). But a crucial concern was the toxicity of HDMBr, including the nephrotoxicity and neurotoxicity, which may be a critical challenge for its biomedicine application (Pai and Crowther 2012; Bao et al., 2018).

β-cyclodextrin-containing polycation (CDP) (Figure 1F), a classical cationic compound, is widely studied in gene delivery. The introduction of β-cyclodextrin, which is itself a large carbohydrate, reduced the toxicity of the polymer (Reineke and Davis 2003). A delivery system, consisting of a CDP, a polyethylene glycol (PEG) steric stabilization agent, and human transferrin (Tf) encapsulated ribonucleotide reductase subunit M2 siRNA, was administrated in non-human primates and the result showed that the nanoparticles could be safely used in non-human primates (Heidel et al., 2007). The same team conducted a phase I clinical trial and the nanoparticle indeed diminished the expression of mRNA (Davis et al., 2010).

It’s has been reported that the DNA binding efficiency of CDP was related to the structure. Hwang et al. synthesized five compounds composed of dicysteamine-β-cyclodextrin and other difunctionalized comonomers. And DNA affinity, DNA protective ability, and the toxicity of polymers altered with the number of methylene groups within the difunctionalized comonomers. When the number of methylene groups was six, the spacing between the cationic amidine groups is optimal for DNA binding (Hwang et al., 2001). Besides, maintaining stability in vivo is also a concern for CDP. Researchers have found that the introduction of PEG or Adamantane (AD) could enhance the stability of CDP (Heidel 2011).

SLE is characterized by increased apoptosis and impaired clearance of apoptotic cells. Many factors can influence the clearance of the cell-free DNA in SLE patients, including the abnormalities of DNase activity to clear cf DNA, the combination of the cfDNA with the antibodies, proteins, and nucleosomes, and thus potentially activate inflammatory pathways (Courtney et al., 1999; Duvvuri and Lood 2019). PAMAM-G3 was proved that local administration of it facilitated wound healing in a cutaneous lupus erythematosus (CLE) prone animal by diminishing extracellular nucleic acids and inhibiting TLR7 and TLR9 activation (Holl et al., 2016). Furthermore, researchers also evaluated the ability of PAMAM-G3 to reduce glomerulonephritis and circulating autoantibody levels in MRLlpr mice (Holl et al., 2016).

The elevated level of cf DNA was discovered in Rheumatoid arthritis patients and whole-genome shotgun sequencing showed SFcfDNAs in RA are enriched with specific CMR sequences, which are hypomethylated (Dong et al., 2020). Therefore, neutralizing cfDNA may be a potential treatment for Rheumatoid arthritis. In a recent study, the researchers prepared a self-assembly PLGA-block-PDMA block copolymer, PLGA-b-PDMA463, and they discovered that it could neutralize cfDNA derived from RA patients and inhibit nucleic acid-mediated activation of primary synovial fluid monocytes and fibroblast-like synoviocytes by restraining the activation of TLR9 (Figure 5). After intravenous injecting PLGA-b-PDMA463 into a CpG-induced mouse model or collagen-induced arthritis rat model (CIA model), successful prevention of RA symptoms, which was evaluated by inflammation, swelling, and deformities of the paws, was achieved and it might be attributed to capacity to scavenge cfDNA and a more favorable biodistribution (Liang H. et al, 2018). With the intent to boost the binding affinity and avoid potential systemic toxicities of PDMA-based cationic nanoparticles (cNPs), the same team tuned the proportion of PLGA and PDMA and introduced poly (ethylene glycol) (PEG) segments to the cNPs’ PDMA shell. The introduction of PEG segments translated into a lower DNA binding efficacy while preserving the ability to hamper joint inflammation. Moreover, due to a greater accumulation and longer retention at the inflamed joints, new NPs were allowed for a lower frequency of administration (Wu JJ. et al, 2020). And another cationic nanoparticle, PCL-g-PAMAM, was also developed to inhibit synovial inflammation and relieve joint inflammation and damage in the CIA mouse model (Peng et al., 2019). Moreover, differences in surface groups of cationic nanomaterials could change their DNA scavenging ability and anti-inflammatory effect in RA, which may be related to different adsorption of opsonin protein. Hydroxylated nanoparticles could prolong the retention in joints and enhance anti-inflammatory effects (Liu. et al., 2021).

It’s has been reported that a significantly elevated level of cfDNA in psoriasis patients (Beranek et al., 2017). Topical administration of PLGA-b-PDMA on psoriasiform skin of an IMQ-induced mouse model could alleviate psoriatic symptoms by efficiently competing for DNA from the DNA-LL37 immunocomplex and suppressing DNA-LL37-induced cell activation. Consistent with this result, the application of PLGA-b-PDMA in a cynomolgus monkey model relieved the symptoms of psoriasis (Liang et al., 2020) (Figure 6). A series of cationic materials, poly (2-(dimethylamino) ethyl methacrylate) grafted hairy silica particles (cSPs), with different PDMA lengths and different particle sizes had been studied. These cationic materials also had the ability to scavenge cfDNA and effectively inhibited psoriatic skin inflammation and inflammatory cytokines secretion. In addition, they showed that different particle sizes and the ratio of PDMA affect DNA binding affinity, which was related to anti-inflammatory effects and the ability to enter the dermis (Yan et al., 2021).

The appearance of cationic nanomaterials gives a novel direction in the treatment of disease, which is closely linked to the broad range of properties they offer. However, the safety profiles of cationic nanomaterials have been a critical concern in therapeutic researches (Wu LP. et al, 2020).

Cytotoxicity, immune-related toxicity, and systemic toxicity are the main barriers to the application of cationic nanomaterials. The application of cationic nanomaterials was restricted due to toxicity such as cell necrosis, inflammatory toxicity, pulmonary toxicity, leukopenia, and thrombocytopenia (Liang X. et al, 2018).

The cytotoxicity of cationic nanomaterials is mainly attributed to the positive charges. Compared with the anion group, cationic nanomaterials exhibit higher cytotoxicity and lethal effects (Calienni et al., 2017; Pereira et al., 2019). Cationic nanomaterials destroy plasma membrane integrity, mitochondrial and lysosomal damage, and more autophagosomes (Mecke et al., 2005; Frohlich 2012). It has been reported that cationic surfactants incorporated into nanoparticles induced cell necrosis and the release of mediators, which resulted from accelerating cell membrane lysis and Ca²⁺ influx via the interaction with the cell membrane (Hwang et al., 2015). Another study demonstrated that cytotoxicity decreased in the presence of increases in serum which was based on serum masking of the PEI surface and decrease of the interaction with cell (McConnell et al., 2016). Both of them illustrated that high positive charge density could increase cytotoxicity. A study found that cationic nanomaterials induced cell necrosis rapidly through inhibition of Na⁺/K⁺-ATPase and subsequent leakage of mitochondrial DNA from necrotic cells. Mitochondrial DNA triggered severe inflammation in vivo by a pathway involving TLR9 and MyD88 signaling (Wei et al., 2015). But the same team also discovered that the inflammatory response induced by cationic nanocarriers was gradually and spontaneously regressed within 1 week. They hypothesized that cationic nanoparticles negatively regulated inflammation and the result demonstrated that leaked mtDNA altered the phenotype of monocyte via a STING- or TLR9 pathway and PEG2 secreted from Ly6C⁺mancytes inhibited neutrophil activation (Liu et al., 2018).

Similarly, Immunotoxicity limits the application of cationic nanomaterials. Cationic nanomaterials could alter the immune state via suppressing innate immunity such as inhibition of natural killer (NK) cell activity, reduction of CD4+/CD8+ ratio, and inflammation cytokines (Kim et al., 2014). Macrophages were also been proved that they could be activated by cationic nanomaterials depending on TLR4 (Toll-like receptor 4) and ROS (reactive oxygen species) signaling (Zhang et al., 2014; Mulens-Arias et al., 2015).

In animal models, the pulmonary toxicity of cationic nanomaterials was also described. Acute lung injury induced by the intratracheal instillation of cationic polyamidoamine dendrimer (PAMAM) nanoparticles had been reported and the model demonstrated cationic nanoparticles suppressed the activity of ACE2 via binding with ACE2, resulting in an imbalance of the renin-angiotensin system (Sun et al., 2015).

Although cationic nanomaterials have not yet entered the stage of clinical research in the treatment of autoimmune diseases, certain systemic toxicity has been found in clinical studies of cationic nanomaterials in tumors. Fatigue, chills, fever, and nausea were mostly described in clinical trials (Zuckerman et al., 2014; Autio et al., 2018). Cardiovascular symptoms such as sinus bradycardia, tachycardia, and hypotension have been reported in a phase Ia/Ib clinical data with polymer-based nanoparticle containing siRNA and a Phase I study of systemically delivering p53 nanoparticle in advanced solid tumors, respectively (Senzer et al., 2013; Zuckerman et al., 2014). In a phase I study for advanced solid tumors, patients experienced infusion-related hypersensitivity which could have been controlled by the frequency after pretreatment with drugs (Rudin et al., 2004). Besides, hematologic disorders including thrombocytopenia and lymphocytopenia occurred in the clinical trials of cationic nanomaterials (Rudin et al., 2004; Zuckerman et al., 2014).

To minimize the toxicity, two strategies have been presented: designing and synthesizing biodegradable nanomaterials or masking of peripheral charge of nanomaterials by surface engineering (Jain et al., 2010).

It is generally accepted that poly (d,l-lactide-co-glycolide) (PLGA), taking advantage of remarkable biocompatibility, biodegradability, solubility, and stability, plays a pivotal role as delivery systems for drug and gene, scaffold in tissue engineer and drug in treatment. Aragao-Santiago et al. compared the toxicity of biodegradable and non-biodegradable nanoparticles via nebulization and discovered that biodegradable PLGA mostly accumulated in lung and eliminated to half in 17.5 to 19.9 h without an elevated level of IL-6 and TNF-α in bronchoalveolar lavage (BAL) supernatant while non-biodegradable nanoparticle induced overexpression of pro-inflammation cytokines and the recruitment of polymorphonuclear to BAL (Aragao-Santiago et al., 2016). Sun et al. constructed a biodegradable micellar nanoparticle consisting of monomethoxy poly (ethylene glycol), poly (epsilon-caprolactone) (PCL), and poly (2-aminoethyl ethylene phosphate) to deliver siRNA and the nanoparticles showed non-toxicity even at high concentrations (Sun et al., 2008). Biocompatible and biodegradable polymers provide non-toxic building blocks for the treatment of diseases, such as PLGA, PLA, PCL we have mentioned above (Hu et al., 2014). The toxicity of cationic materials can be effectively alleviated by introducing these groups.

Zhang et al. synthesized a series of terpolymer with low charge density and high molecular weight, which possessed low toxicity and high conversion efficiency (Zhou et al., 2011). Consistent with this finding, another study illustrated that the toxicity decreased with the increase of particle size (Yan et al., 2021). Using high molecular weight and increased hydrophobicity to compensate for low charge density may be a good strategy to balance performance and toxicity (Mastrobattista and Hennink 2011).

Another strategy for the reduction of toxicity is the modification of the nanoparticle surface. In addition to this, modification of nanomaterial surface possesses extra properties such as prolongation of the retention time, improvement of biodistribution and efficiency, and so on (Jain et al., 2010). Polyethylene glycol (PEG) is the most widely used to coat cationic nanomaterials. Karabasz et al. confirmed that five-layer positively charged poly-l-lysine-terminated nanocapsules (NC5) with rapid hematotoxicity did not show cytotoxicity after being incorporated with PEG (Karabasz et al., 2018). PEG could invest in cationic nanomaterials stealthiness without inducing blood, kidney, spleen, and liver acute and extended acute toxicity (Perret et al., 2018). However, it has been reported that modification of nanomaterials with PEG could trigger activation of both the complement and coagulation systems (Pham et al., 2011). But Pannuzzo et al. put forward the solution. They modified nanomaterials with appropriate combinations and proportions of carboxyPEG2000 and methoxyPEG550 can and indeed inhibited activation of complement (Pannuzzo et al., 2020). Besides, other polymers have gradually been developed as modifications to cationic materials such as poly [N-(2-hydroxypropyl)methacrylamide], poly (carboxybetaine), poly (hydroxyethyl-l-asparagine), or poly-l-glutamic acid (Hu et al., 2014). Toy et al. modified primary amines with imidazole-acetic-acid (IAA) to secondary and tertiary amines and demonstrated that introduction of IAA could abate toxicity and immunotoxicity from branched polyethylenimine (bPEI) and chiton through the TLR4 pathway (Toy et al., 2019). A biodegradable, polyelectrolyte m ultilayer shell consisting of poly-l-lysine (PLL) and poly-L-glutamic (PGA) acid was coated with PGA(NC-PGA) and PEG (NC-PEG), respectively. The biochemical and histopathological evaluation suggested that neither of them showed acute or chronic hematotoxicity, hepatotoxicity, or nephrotoxicity. Compared with NC-PEG, NA-PGA didn’t provoke activation of immune system (Karabasz et al., 2019).