Modular Synthetic Approach for Adjusting the Disassembly Rates of Enzyme-Responsive Polymeric Micelles

Polymeric architecture as a tool for controlling the reactivity of palladium(II) loaded nanoreactors

Self-assembled systems, like polymeric micelles, have become great facilitators for conducting organic reactions in aqueous media due to their broad potential applications in green chemistry and biomedical applications. Massive strides have been taken to improve the reaction scope of such systems, enabling them to perform bioorthogonal reactions for prodrug therapy. Considering these significant advancements, we sought to study the relationships between the architecture of the amphiphiles and the reactivity of their PdII loaded micellar nanoreactors in conducting depropargylation reactions. Towards this goal, we designed and synthesized a series of isomeric polyethylene glycol (PEG)-dendron amphiphiles with different dendritic architectures but with an identical degree of hydrophobicity and hydrophilic to lipophilic balance (HLB). We observed that the dendritic architecture, which serves as the main binding site for the PdII ions, has greater influence on the reactivity than the hydrophobicity of the dendron. These trends remained constant for two different propargyl caged substrates, validating the obtained results. Density functional theory (DFT) calculations of simplified models of the dendritic blocks revealed the different binding modes of the various dendritic architectures to PdII ions, which could explain the observed differences in the reactivity of the nanoreactors with different dendritic architectures. Our results demonstrate how tuning the internal architecture of the amphiphiles by changing the orientation of the chelating moieties can be used as a tool for controlling the reactivity of PdII loaded nanoreactors.

Enzyme and pH dual responsive linear-dendritic block copolymer micelles based on a phenylalanyl-lysine motif and peripherally ketal-functionalized dendron as potential drug carriers

Stimuli-responsive linear-dendritic block copolymers (LDBCs) have attracted significant research attention as novel drug carriers. We report here three generations of new enzyme and pH dual responsive linear-dendritic block copolymers (LDBCs) with a phenylalanyl-lysine (Phe-Lys) dipeptide linking hydrophilic linear poly(N-vinylpyrrolidone) (PNVP) and a hydrophobic peripherally ketal-functionalized dendron derived from 2,2'-bis(hydroxymethyl)propionic acid (bis-MPA). The LDBCs are synthesized via a combination of interchange of xanthates/reversible addition-fragmentation chain transfer (MADIX/RAFT) polymerization of N-vinylpyrrolidone (NVP) and "chain-first" strategy. Their structures are confirmed by ¹H NMR spectra. The gel permeation chromatograph (GPC) analysis revealed that the LDBCs have a narrow molecular weight distribution (PDI ≤ 1.25). The amphiphilic LDBCs can self-assemble into spherical nanomicelles in aqueous solution. The presence of enzyme or/and the change of pH cause disassembly of micelles to release encapsulated cargos. The release rates of the guest molecules are faster in buffer solution at pH 5.0 than those upon the addition of the activating enzyme and can be fine-tuned by changing the generation of bis-MPA dendrons. The combination of enzyme and pH dual stimuli results in significantly accelerated and more complete release of the loaded hydrophobic guests. The cell viability assay confirmed the favorable biocompatibility until the LDBC micelle concentration reached 800 μg mL^-1. These results indicate that the LDBCs can be considered as a good candidate for targeting drug delivery.

Tuning the Reactivity of Micellar Nanoreactors by Precise Adjustments of the Amphiphile and Substrate Hydrophobicity

Polymeric assemblies, such as micelles, are gaining increasing attention due to their ability to serve as nanoreactors for the execution of organic reactions in aqueous media. The ability to conduct organic transformations, which have been traditionally limited to organic media, in water is essential for the further development of important fields ranging from green catalysis to bioorthogonal chemistry. Considering the recent progress that has been made to expand the range of organometallic reactions conducted using nanoreactors, we aimed to gain a deeper understanding of the roles of the hydrophobicity of both the core of micellar nanoreactors and the substrates on the reaction rates in water. Toward this goal, we designed a set of five metal-loaded micelles composed of polyethylene glycol-dendron amphiphiles and studied their ability to serve as nanoreactors for a palladium-mediated depropargylation reaction of four substrates with different log P values. Using dendrons as the hydrophobic block, we could precisely tune the lipophilicity of the nanoreactors, which allowed us to reveal linear correlations between the rate constants and the hydrophobicity of the amphiphiles (estimated by the dendron's cLog P). While exponential dependence was obtained for the lipophilicity of the substrates, a similar degree of rate acceleration was observed due to the increase in the hydrophobicity of the amphiphiles regardless of the effect of the substrate's log P. Our results demonstrate that while increasing the hydrophobicity of the substrates may be used to accelerate reaction rates, tuning the hydrophobicity of the micellar nanoreactors can serve as a vital tool for further optimization of the reactivity and selectivity of nanoreactors.

Dendron-Polymer Hybrids as Tailorable Responsive Coronae of Single-Walled Carbon Nanotubes

Functional composite materials that can change their spectral properties in response to external stimuli have a plethora of applications in fields ranging from sensors to biomedical imaging. One of the most promising types of materials used to design spectrally active composites are fluorescent single-walled carbon nanotubes (SWCNTs), noncovalently functionalized by synthetic amphiphilic polymers. These coated SWCNTs can exhibit modulations in their fluorescence spectra in response to interactions with target analytes. Hence, identifying new amphiphiles with interchangeable building blocks that can form individual coronae around the SWCNTs and can be tailored for a specific application is of great interest. This study presents highly modular amphiphilic polymer-dendron hybrids, composed of hydrophobic dendrons and hydrophilic polyethylene glycol (PEG) that can be synthesized with a high degree of structural freedom, for suspending SWCNTs in aqueous solution. Taking advantage of the high molecular precision of these PEG-dendrons, we show that precise differences in the chemical structure of the hydrophobic end groups of the dendrons can be used to control the interactions of the amphiphiles with the SWCNT surface. These interactions can be directly related to differences in the intrinsic near-infrared fluorescence emission of the various chiralities in a SWCNT sample. Utilizing the susceptibility of the PEG-dendrons toward enzymatic degradation, we demonstrate the ability to monitor enzymatic activity through changes in the SWCNT fluorescent signal. These findings pave the way for a rational design of functional SWCNTs, which can be used for optical sensing of enzymatic activity in the near-infrared spectral range.

Supramolecular Functionalizable Linear-Dendritic Block Copolymers for the Preparation of Nanocarriers by Microfluidics

Hybrid linear-dendritic block copolymers (LDBCs) having dendrons with a precise number of peripheral groups that are able to supramolecular bind functional moieties are challenging materials as versatile polymeric platforms for the preparation of functional polymeric nanocarriers. PEG2k-b-dxDAP LDBCs that are based on polyethylene glycol (PEG) as hydrophilic blocks and dendrons derived from bis-MPA having 2,6-diacylaminopyridine (DAP) units have been efficiently synthesized by the click coupling of preformed blocks, as was demonstrated by spectroscopic techniques and mass spectrometry. Self-assembly ability was first checked by nanoprecipitation. A reproducible and fast synthesis of aggregates was accomplished by microfluidics optimizing the total flow rate and phase ratio to achieve spherical micelles and/or vesicles depending on dendron generation and experimental parameters. The morphology and size of the self-assemblies were studied by TEM, Cryogenic Transmission Electron Microscopy (cryo-TEM), and Dynamic Light Scattering (DLS). The cytotoxicity of aggregates synthesized by microfluidics and the influence on apoptosis and cell cycle evaluation was studied on four cell lines. The self-assemblies are not cytotoxic at doses below 0.4 mg mL-1. Supramolecular functionalization using thymine derivatives was explored for reversibly cross-linking the hydrophobic blocks. The results open new possibilities for their use as drug nanocarriers with a dynamic cross-linking to improve nanocarrier stability but without hindering disassembly to release molecular cargoes.

Using High Molecular Precision to Study Enzymatically Induced Disassembly of Polymeric Nanocarriers: Direct Enzymatic Activation or Equilibrium-Based Degradation?

Enzyme-responsive polymers and their assemblies offer great potential to serve as key materials for the design of drug delivery systems and other biomedical applications. However, the utilization of enzymes to trigger the disassembly of polymeric amphiphiles, such as micelles, also suffers from the limited accessibility of the enzyme to moieties that are hidden inside the assembled structures. In this Perspective, we will discuss examples for the utilization of high molecular precision that dendritic structures offer to study the enzymatic degradation of polymeric amphiphiles with high resolution. Up to date, several different amphiphilic systems based on dendritic blocks have all shown that small changes in the hydrophobicity and amphiphilicity strongly affected the degree and rate of enzymatic degradation. The ability to observe the huge effects due to relatively small variations in the molecular structure of polymers can explain the limited enzymatic degradation that is often observed for many reported polymeric assemblies. The observed trends imply that the enzymes cannot reach the hydrophobic core of the micelles, and instead, they gain access to the amphiphiles by the unimer–micelle equilibrium, making the unimer exchange rate a key parameter in tuning the enzymatic degradation rate. Several approaches that are aimed at overcoming the stability–responsiveness challenge are discussed as they open the way to the design of stable and yet enzymatically responsive polymeric nanocarriers.

Photo-Responsive Supramolecular Micelles for Controlled Drug Release and Improved Chemotherapy

Development of stimuli-responsive supramolecular micelles that enable high levels of well-controlled drug release in cancer cells remains a grand challenge. Here, we encapsulated the antitumor drug doxorubicin (DOX) and pro-photosensitizer 5-aminolevulinic acid (5-ALA) within adenine-functionalized supramolecular micelles (A-PPG), in order to achieve effective drug delivery combined with photo-chemotherapy. The resulting DOX/5-ALA-loaded micelles exhibited excellent light and pH-responsive behavior in aqueous solution and high drug-entrapment stability in serum-rich media. A short duration (1-2 min) of laser irradiation with visible light induced the dissociation of the DOX/5-ALA complexes within the micelles, which disrupted micellular stability and resulted in rapid, immediate release of the physically entrapped drug from the micelles. In addition, in vitro assays of cellular reactive oxygen species generation and cellular internalization confirmed the drug-loaded micelles exhibited significantly enhanced cellular uptake after visible light irradiation, and that the light-triggered disassembly of micellar structures rapidly increased the production of reactive oxygen species within the cells. Importantly, flow cytometric analysis demonstrated that laser irradiation of cancer cells incubated with DOX/5-ALA-loaded A-PPG micelles effectively induced apoptotic cell death via endocytosis. Thus, this newly developed supramolecular system may offer a potential route towards improving the efficacy of synergistic chemotherapeutic approaches for cancer.

Supramolecular antibiotics: a strategy for conversion of broad-spectrum to narrow-spectrum antibiotics for Staphylococcus aureus

The propensity of broad-spectrum antibiotics to indiscriminately kill both pathogenic and beneficial bacteria has a profound impact on the spread of resistance across multiple bacterial species. Alternative approaches that narrow antibacterial specificity towards desired pathogenic bacterial population are of great interest. Here, we report an enzyme-responsive antibiotic-loaded nanoassembly strategy for narrow delivery of otherwise broad-spectrum antibiotics. We specifically target Staphylococcus aureus (S. aureus), an important blood pathogen that secretes PC1 β-lactamases. Our nanoassemblies selectively eradicate S. aureus grown in vitro with other bacteria, highlighting its potential capability in targeting the desired pathogenic bacterial population.

Architectural Change of the Shell-Forming Block from Linear to V-Shaped Accelerates Micellar Disassembly, but Slows the Complete Enzymatic Degradation of the Amphiphiles

Tuning the enzymatic degradation and disassembly rates of polymeric amphiphiles and their assemblies is crucial for designing enzyme-responsive nanocarriers for controlled drug delivery applications. The common methods to control the enzymatic degradation of amphiphilic polymers are to tune the molecular weights and ratios of the hydrophilic and hydrophobic blocks. In addition to these approaches, the architecture of the hydrophilic block can also serve as a tool to tune enzymatic degradation and disassembly. To gain a deeper understanding of the effect of the molecular architecture of the hydrophilic block, we prepared two types of well-defined PEG-dendron amphiphiles bearing linear or V-shaped PEG chains as the hydrophilic blocks. The high molecular precision of these amphiphiles, which emerges from the utilization of dendrons as the hydrophobic blocks, allowed us to study the self-assembly and enzymatic degradation and disassembly of the two types of amphiphiles with high resolution. Interestingly, the micelles of the V-shaped amphiphiles were significantly smaller and disassembled faster than those of the amphiphiles based on linear PEG. However, the complete enzymatic cleavage of the hydrophobic end groups was significantly slower for the V-shaped amphiphiles. Our results show that the V-shaped architecture can stabilize the unimer state and, hence, plays a double role in the enzymatic degradation and the induced disassembly and how it can be utilized to control the release of encapsulated or bound molecular cargo.

Self-Assembly and Enzyme Responsiveness of Amphiphilic Linear-Dendritic Block Copolymers Based on Poly(N-vinylpyrrolidone) and Dendritic Phenylalanyl-lysine Dipeptides

In this study, we present the synthesis, self-assembly, and enzyme responsive nature of a unique class of well-defined amphiphilic linear-dendritic block copolymers (PNVP-b-dendr(Phe-Lys)n, n = 1-3) based on linear poly(N-vinylpyrrolidone) (PNVP) and dendritic phenylalanyl-lysine (Phe-Lys) dipeptides. The copolymers were prepared via a combination ofreversible addition-fragmentation chain transfer (RAFT) /xanthates (MADIX) polymerization of N-vinylpyrrolidone and stepwise peptide chemistry. The results of fluorescence spectroscopy, 1H NMR analyses, transmission electron microscopy (TEM), and particle size analysis demonstrated that the copolymers self-assemble in aqueous solution into micellar nanocontainers that can disassemble and release encapsulated anticancer drug doxorubicin or hydrophobic dye Nile red by trigger of a serine protease trypsin under physiological conditions. The disassembly of the formed micelles and release rates of the drug or dye can be adjusted by changing the generation of dendrons in PNVP-b-dendr(Phe-Lys)n. Furthermore, the cytocompatibility of the copolymers have been confirmed using human lung epithelial cells (BEAS-2B) and human liver cancer cells (SMMC-7721). Due to the fact of their enzyme responsive properties and good biocompatibility, the copolymers may have potential applicability in smart controlled release systems capable of site-specific response.

Recent advances in stimuli-responsive polymeric micelles via click chemistry

Stimuli-responsive polymeric micelles via click chemistry are divided into six major sections (temperature, light, ultrasound, pH, enzymes, and redox).

Impact of Glycolipid Hydrophobic Chain Length and Headgroup Size on Self-Assembly and Hydrophobic Guest Release

Encapsulation of a hydrophobic guest molecule inside a micelle and its stimuli-sensitive release is a useful strategy for target-specific drug delivery. Herein, nine biobased glycolipids were derived from plant sources. The influence of headgroup on the stability and aggregation pattern in water with different alkyl chain lengths was investigated to deduce the structure-property relationship. External factors, such as temperature, pH, and NaCl and urea concentrations, were employed to explore stimuli response of glycolipid nanoassemblies. Furthermore, solvatochromic dyes, such as pyrene, N-phenyl-1-naphthylamine, and curcumin, were utilized to examine hydrophobe loading capacities of these glycolipid assemblies. A fluorescence study was performed to investigate the enzyme-sensitive hydrophobe release. Interestingly, the pH-sensitive hydrophobic guests showed pH-responsive release from dynamic micelles. Finally, the synthesized glycolipids revealed their nanoassemblies as smart carriers for hydrophobic cargo.

Tuning the molecular weight of polymeric amphiphiles as a tool to access micelles with a wide range of enzymatic degradation rates

Tuning the molecular weight of polymeric amphiphiles allows access to polymeric micelles with extremely diverse enzymatic degradation and disassembly rates.

Micellar Stability in Biological Media Dictates Internalization in Living Cells

The dynamic nature of polymeric assemblies makes their stability in biological media a crucial parameter for their potential use as drug delivery systems in vivo. Therefore, it is essential to study and understand the behavior of self-assembled nanocarriers under conditions that will be encountered in vivo such as extreme dilutions and interactions with blood proteins and cells. Herein, using a combination of fluorescence spectroscopy and microscopy, we studied four amphiphilic PEG-dendron hybrids and their self-assembled micelles in order to determine their structure-stability relations. The high molecular precision of the dendritic block enabled us to systematically tune the hydrophobicity and stability of the assembled micelles. Using micelles that change their fluorescent properties upon disassembly, we observed that serum proteins bind to and interact with the polymeric amphiphiles in both their assembled and monomeric states. These interactions strongly affected the stability and enzymatic degradation of the micelles. Finally, using spectrally resolved confocal imaging, we determined the relations between the stability of the polymeric assemblies in biological media and their cell entry. Our results highlight the important interplay between molecular structure, micellar stability, and cell internalization pathways, pinpointing the high sensitivity of stability-activity relations to minor structural changes and the crucial role that these relations play in designing effective polymeric nanostructures for biomedical applications.

The power of the ring: a pH-responsive hydrophobic epoxide monomer for superior micelle stability

We developed micelles with superior stability by integrating a novel hydrophobic, pH-responsive epoxide monomer, tetrahydropyranyl glycidyl ether.