Modulation of the pH-responsive properties of poly(L-lysine iso-phthalamide) grafted with a poly(ethylene glycol) analogue

Advanced Delivery Systems Based on Lysine or Lysine Polymers

Polylysine and materials that integrate lysine form promising drug delivery platforms. As a cationic macromolecule, a polylysine polymer electrostatically interacts with cells and is efficiently internalized, thereby enabling intracellular delivery. Although polylysine is intrinsically pH-responsive, the conjugation with different functional groups imparts smart, stimuli-responsive traits by adding pH-, temperature-, hypoxia-, redox-, and enzyme-responsive features for enhanced delivery of therapeutic agents. Because of such characteristics, polylysine has been used to deliver various cargos such as small-molecule drugs, genes, proteins, and imaging agents. Furthermore, modifying contrast agents with polylysine has been shown to improve performance, including increasing cellular uptake and stability. In this review, the use of lysine residues, peptides, and polymers in various drug delivery systems has been discussed comprehensively to provide insight into the design and robust manufacturing of lysine-based delivery platforms.

Temperature and CO2 Dual-Responsive Pickering Emulsions Using Jeffamine M2005-Modified Cellulose Nanocrystals

Cellulose nanocrystals (CNCs) with excellent biodegradability are promising biomaterials for use as responsive Pickering emulsifiers. However, the high hydrophilicity of CNCs limits their emulsification ability. Some existing studies have utilized complicated covalent modification procedures to increase the hydrophobicity of CNCs. To simplify the modification process, we prepared hydrophobically modified CNCs (CNCs-M2005) via simple and controllable electrostatic interactions with thermosensitive M2005. The obtained CNCs-M2005 exhibited temperature and CO₂ dual-responsive properties. Subsequently, stable oil/water Pickering emulsions were prepared using the partially hydrophobic CNCs-M2005 at 20 °C. However, demulsification occurred when the temperature increased to 60 °C. This temperature-induced demulsification resulted from the dehydration of polyethylene oxide and polypropylene oxide, causing the aggregation of the CNCs-M2005, as shown by dynamic light scattering and transmission electron microscopy experiments. In addition, demulsification was also achieved after bubbling CO₂, which was attributed to the dissociation of the partially hydrophobic CNCs-M2005. The temperature and CO₂ dual-responsive biosafe Pickering emulsions open up opportunity for the design of intelligent food, cosmetic, and drug delivery systems.

pH-Responsive Polypeptide-Based Smart Nano-Carriers for Theranostic Applications

Smart nano-carriers have attained great significance in the biomedical field due to their versatile and interesting designs with different functionalities. The initial stages of the development of nanocarriers mainly focused on the guest loading efficiency, biocompatibility of the host and the circulation time. Later the requirements of less side effects with more efficacy arose by attributing targetability and stimuli-responsive characteristics to nano-carriers along with their bio- compatibility. Researchers are utilizing many stimuli-responsive polymers for the better release of the guest molecules at the targeted sites. Among these, pH-triggered release achieves increasing importance because of the pH variation in different organ and cancer cells of acidic pH. This specific feature is utilized to release the guest molecules more precisely in the targeted site by designing polymers having specific functionality with the pH dependent morphology change characteristics. In this review, we mainly concert on the pH-responsive polypeptides and some interesting nano-carrier designs for the effective theranostic applications. Also, emphasis is made on pharmaceutical application of the different nano-carriers with respect to the organ, tissue and cellular level pH environment.

Enhancing Membrane-Disruptive Activity via Hydrophobic Phenylalanine and Lysine Tethered to Poly(aspartic acid)

Amphiphilic polymers with pH-responsive abilities have been widely used as carriers for intracellular delivery of bioactive substances, while their membrane-disruptive activity exerted on cells is a critical characteristic that determines delivery efficiency. Herein, we present a novel method to prepare amphiphilic and pH-responsive polymers by chemically tethering l-phenylalanine methyl ester and followed by N_ε-carbobenzyloxy-l-lysine benzyl ester to the side carboxylic acid groups of poly(aspartic acid). The obtained phenylalanine- and lysine-grafted polymer (PAsp- g-Phe)- g-Lys demonstrated enhanced membrane-disruptive activity at pH 7.4 in comparison with that of PAsp- g-Phe. Moreover, the pH-responsive behavior of the grafted polymers caused by the significantly intensified hydrophobicity could be modulated by the tethered amount of hydrophobic amino acids with phenyl groups. The prepared amphiphilic (PAsp- g-Phe)- g-Lys could facilitate entry of calcein into NIH/3T3 and HeLa cells at physiological pH values, possibly due to local chemical destabilization of cell membranes by the interaction between the polymer and membrane bilayers. Therefore, we have provided a feasible approach to prepare pH-responsive polymers with enhanced membrane-disruptive activity, and the phenylalanine- and lysine-grafted polymers could be a potential candidate for intracellular delivery of bioactive molecules in biomedical applications.

Dynamic studies of the interaction of a pH responsive, amphiphilic polymer with a DOPC lipid membrane

Deeper understanding of the molecular interactions between polymeric materials and the lipid membrane is important across a range of applications from permeation for drug delivery to encapsulation for immuno-evasion. Using highly fluidic microcavity supported lipid bilayers, we studied the interactions between amphiphilic polymer PP50 and a DOPC lipid bilayer. As the PP50 polymer is pH responsive the studies were carried out at pH 6.5, 7.05 and 7.5, corresponding to fully, partly protonated (pH = pK_a = 7.05) and fully ionized states of the polymer, respectively. Fluorescence correlation spectroscopy (FCS) using both labelled lipid and polymer revealed the PP50 associates with the bilayer interface across all pHs where its diffusion along the interface is impeded. Both FCS and electrochemical impedance spectroscopy (EIS) data indicate that the PP50 does not penetrate fully into the bilayer core but rather forms a layer at the bilayer aqueous interface reflected in increased resistance and decreased capacitance of the bilayer on PP50 binding. The extent of these effects and the dynamics of binding are influenced by pH, increasing with decreasing pH. These experimental trends concurred with coarse grained Monte Carlo simulations of polymer-bilayer interactions wherein a model hydrophilic polymer backbone grafted with side chains of varying hydrophobicity, to mimic the effect of varying pH, was simulated based on the bond fluctuation model with explicit solvent. Simulation results showed that with increasing hydrophobicity, the polymer penetrated deeper into the contacting bilayer leaflet of the membrane suppressing, consistent with EIS data, solvent permeation and that a full insertion of the polymer into the bilayer core is not necessary for suppression of permeability.

Stimuli-responsive nanomaterials for therapeutic protein delivery

Protein therapeutics have emerged as a significant role in treatment of a broad spectrum of diseases, including cancer, metabolic disorders and autoimmune diseases. The efficacy of protein therapeutics, however, is limited by their instability, immunogenicity and short half-life. In order to overcome these barriers, tremendous efforts have recently been made in developing controlled protein delivery systems. Stimuli-triggered release is an appealing and promising approach for protein delivery and has made protein delivery with both spatiotemporal- and dosage-controlled manners possible. This review surveys recent advances in controlled protein delivery of proteins or peptides using stimuli-responsive nanomaterials. Strategies utilizing both physiological and external stimuli are introduced and discussed.

Effect of L-leucine graft content on aqueous solution behavior and membrane-lytic activity of a pH-responsive pseudopeptide

A series of pH-responsive polymers have been synthesized by grafting L-leucine onto the pendant carboxylic acid groups of the linear pseudopeptide, poly(L-lysine iso-phthalamide). The effect of the degree of grafting on aqueous solution properties, cell membrane-disruptive activity, and in vitro cytotoxicity was examined by UV-visible and fluorescence spectroscopy, hemolysis, alamar blue staining, and propidium iodide fluorescence assays. Modification of poly(L-lysine iso-phthalamide) with < or =23.6 mol % L-leucine caused a marginal effect on the pH-mediated hydrophobic association and hemolytic activity. Increasing the degree of grafting from 31.9 to 61.2 mol % resulted in polymers with progressively enhanced hydrophobic association and cell membrane disruption, thus confirming that the pH responsiveness and the extent of hydrophobic association and membrane disruption of the polymers can be modulated by varying the degree of grafting with hydrophobic amino acids. The pH responses were demonstrated to be concentration-dependent. At certain degrees of leucine grafting, the polymers were nonmembrane-lytic at physiological pH but mediated considerable membrane lysis at endosomal pH values (5.0-6.8), a feature critical for potential drug delivery applications.

Release mechanisms for polyelectrolyte capsules

Polyelectrolyte capsules have recently been introduced as new microscopic vehicles which could have high potential in the biomedical field. In this critical review we give an introduction to the layer-by-layer (LbL) technique which is used to fabricate these polyelectrolyte capsules as well as to the different triggers that have been exploited to obtain drug release from these capsules. Furthermore, other types of triggered delivery systems are compared and critically discussed with regard to their clinical relevance. (171 references.).

Modulation of cell membrane disruption by pH-responsive pseudo-peptides through grafting with hydrophilic side chains

The effect of grafting an amphiphilic pseudo-peptide, poly (L-lysine iso-phthalamide), with poly (ethylene glycol) or a hydrophilic poly (ethylene glycol) analogue, Jeffamine M-1000, on the pH-dependent erythrolytic activity and in vitro cytotoxicity have been studied together with the concentration-dependent haemolysis of the polymers with different degrees of grafting. PEGylated polymers showed pH-dependent membrane-disruptive ability similar to the parent poly (L-lysine iso-phthalamide). The polymers showed a better ability to haemolyse the erythrocyte membrane at mildly acidic pHs with increasing degree of PEGylation (up to 17.0 wt.%). Further increasing the degree of PEGylation resulted in a decrease in haemolytic ability. Grafting poly (L-lysine iso-phthalamide) with the lower molecular weight Jeffamine M-1000 had little effect on the haemolytic ability. Finally, the in vitro cytotoxicity of the grafted polymers was assessed by MTT assay, LDH assay and viable cell counts. At pH 7.4, these polymers were well tolerated by a range of mammalian cell lines and grafting reduced the cytotoxicity of polymers. However, at pH 5.5, relative to poly (L-lysine iso-phthalamide), the grafted polymers displayed a better ability to rupture the outer membranes of these cells.