Novel zwitterionic vectors: Multi-functional delivery systems for therapeutic genes and drugs

Abstract

Zwitterions consist of equal molar cationic and anionic moieties and thus exhibit overall electroneutrality. Zwitterionic materials include phosphorylcholine, sulfobetaine, carboxybetaine, zwitterionic amino acids/peptides, and other mix-charged zwitterions that could form dense and stable hydration shells through the strong ion-dipole interaction among water molecules and zwitterions. As a result of their remarkable hydration capability and low interfacial energy, zwitterionic materials have become ideal choices for designing therapeutic vectors to prevent undesired biosorption especially nonspecific biomacromolecules during circulation, which was termed antifouling capability. And along with their great biocompatibility, low cytotoxicity, negligible immunogenicity, systematic stability and long circulation time, zwitterionic materials have been widely utilized for the delivery of drugs and therapeutic genes. In this review, we first summarized the possible antifouling mechanism of zwitterions briefly, and separately introduced the features and advantages of each type of zwitterionic materials. Then we highlighted their applications in stimuli-responsive "intelligent" drug delivery systems as well as tumor-targeting carriers and stressed the multifunctional role they played in therapeutic gene delivery.

Keywords: Antifouling; Biomaterials; Drug delivery; Gene delivery; Zwitterion.

Graphical abstract

Fig. 1

Schematic diagram of hydration shell formed by PEG and zwitterions separately. (A) PEG interact with H₂O molecules via hydrogen bonding, while zwitterions attract H₂O molecules through the powerful ion–dipole interaction, so that to form stronger hydration layers to prevent bio-adherent. (B) Incorporation of salt disrupts the previous electrostatic attraction of the charge pairs of intra-monomer, intra-chain and inter-chain, making the conformation of zwitterionic brushes from shrinking to stretching relatively.

Fig. 2

Models and structures of zwitterionic materials. (A) Chemical structures of the common zwitterionic groups. (B) Zwitterionic poly(amino acids) and polypeptide. (C) Mixed-charge zwitterionic polymers that have balanced cationic and anionic groups in different monomer units, and pseudo-zwitterionic materials with equimolar negative and positive charge binding to the same medium.

Fig. 3

The multistage pH-responsive process of the smart liposomes (HHG2C₁₈-L). HHG2C₁₈-L kept negative charged in bloodstream. When the liposomes arrived at tumor site by EPR effect, their surface charge would convert to positive through the protonation of the imidazole group in histidine thus to promote endocytosis. As the proton swarmed into endo/lysosomes, the proton sponge effect led to endo/lysosomal bursting and facilitated liposome escape. The following hydrolysis of hexahydrobenzoic amide brought about an extremely strong positive surface charge of by abandoning the carboxy groups in liposomes, which obstructed the charge reverse again when escaped to cytoplasm. Eventually, the positive charged HHG2C₁₈-L might accumulate at the mitochondria via electrostatic interaction attributing to the mitochondrial transmembrane potential.

Fig. 4

The process of drug loading and release from PMPC-PLA and PMPC-Blink-PLA. (A) Hydrophobic DOX and hydrophilic DOX·HCl was loaded in the membrane and interior of the PMPC-PLA vesicles respectively. When pH decreased from 7.4 to 5.0, the hydrolysis of PLA segments would lead to morphology transformation from vesicles to micelle, and finally extruded the encapsulated drugs. (B) The PMPC-Blink-CPLA self-assembled into micelles and encapsulated PTX by CPLA core via hydrophobic interactions as well as π-π stacking. The pH-sensitive benzoyl imine linkages would be cut off in acid environment and the disruption of micelles finally resulted in drug release.

Fig. 5

The forming process and intracellular drug release of the PCL-ss-PDEASB/DOX system. With the shield of zwitterionic SB shell, these micelles could avoid undesired absorption, and internalized by tumor cells effectively. After endocytosis, the loaded DOX would be release rapidly as the result of PDEA protonation in response to low pH and disulfide bonds cleavage triggered by cytosol high GSH concentration.

Fig. 6

The tumor targeting cRGD-PCSSL micelles guided by cRGDfc peptide. The functional zwitterionic micelles could accumulate around tumor cells leading by cRGD ligands. Through the interaction between cRGD and α_vβ₃ integrin receptor, the uptake rate of micelles by tumor cells would be enhanced.

Fig. 7

The reduction and pH dual-responsive behavior of the tumor targeting RGD-PCSSD shell-cross-linked nanoparticles. When the pH value was 7.4, the liner RGD-PCSSD copolymers could assemble into NPs using the hydrophobic PDPA core to encapsulate DOX. Treating with ultrasound promoted the formation of disulfide bonds to get compacted cross-linked NPs and held DOX tightly. When the pH value decreased, the PDPA moieties would become protonated which made the NPs swell and a part of drug leak slowly. And as the concentration of GSH in the environment increased, the cross-linked disulfide bonds would be cut off to disrupt the whole system and release the cargoes rapidly.

Fig. 8

The “gatekeepers” of Z-MSN and the structural model of MSN-HA-SiO₂-TSA/DMA. (A) The carboxylic groups and quaternary amino groups were anchored on the surface of Z-MSN to keep DOX in the caves under the physical condition. And the “doors” would be open after hydrolysis of their carboxylic groups in tumor microenvironment. (B) During the blood circulation, the outermost mix-charged zwitterionic layer could keep away from biomolecules with their dense hydration shell. While the pH decreased from 7.4 to 6.5, the surface charge would shift to cationic hence to promote cellular uptake. After the second GSH-sensitive layer taken off in cytoplasm by GSH with high concentration, the innermost FITC-HA layer would be degraded by HAase to release the therapeutics quickly. And the whole process could be tracked in real-time by fluorescence.

Fig. 9

The derivatives of methacrylate and methacrylamine are widely used as cationic moieties to build cationic and zwitterionic gene vectors.

Fig. 10

Schematic diagram of PSBMA-PAA/PDMAEMA (or PEI)/DNA core–shell polyplexes. Cationic PDMAEMA (or PEI) first interacted with DNA molecules electrostatically. In physical condition, the diblock polymer PSBMA-PAA served as electroneutral shells to protect the polyplexes from biosorption and fell off around tumor cells to expose the positive charges of the core for enhanced cellular uptake.