Dual pH-Responsive Shell-Cleavable Polycarbonate Micellar Nanoparticles for in Vivo Anticancer Drug Delivery.

To exploit tumor and intracellular microenvironments, pH-responsive diblock copolymers of poly(ethylene glycol) and catechol-functionalized polycarbonate with acid-labile acetal bond as the linker are synthesized to prepare micellar nanoparticles that shed the shell at acidic tumor tissues and inside cancer cells, hence accelerating drug release at the target. The pH-dependent cleavage of the shell is demonstrated at pH 5.0 and 6.5 using 1H NMR. Bortezomib (BTZ, an anticancer drug containing a phenylboronic acid group) is conjugated to the polymers through formation of pH-responsive boronate ester bond between boronic acid and catechol in the polymers. Dual pH-responsive bortezomib-polymer conjugates (BTZ-PC) self-assemble into micellar nanoparticles of small size (<110 nm) with narrow size distribution and high drug loading capacity. Acidic pH accelerates BTZ release from BTZ-PC micelles and enhances intracelluar uptake of the micelles, hence increasing in vitro cytotoxicity against human breast cancer cells. More importantly, the BTZ-PC micelles achieve a stronger antitumor effect in a human breast cancer BT-474 xenograft mouse model than free BTZ and mitigate in vivo hepatotoxicity of BTZ. These dual pH-responsive shell-cleavable nanoparticles are a potentially promising carrier for BTZ delivery.

Injectable Coacervate Hydrogel for Delivery of Anticancer Drug-Loaded Nanoparticles in vivo.

In this study, bortezomib (BTZ, a cytotoxic water-insoluble anticancer drug) was encapsulated in micellar nanoparticles having a catechol-functionalized polycarbonate core through a pH-sensitive covalent bond between phenylboronic acid (PBA) in BTZ and catechol, and these drug-loaded micelles were incorporated into hydrogels to form micelle/hydrogel composites. A series of injectable, biodegradable hydrogels with readily tunable mechanical properties were formed and optimized for sustained delivery of the BTZ-loaded micelles through ionic coacervation between PBA-functionalized polycarbonate/poly(ethylene glycol) (PEG) "ABA" triblock copolymer and a cationic one having guanidinium- or thiouronium-functionalized polycarbonate as "A" block. An in vitro release study showed the pH dependence in BTZ release. At pH 7.4, the BTZ release from the micelle/hydrogel composite remained low at 7%, whereas in an acidic environment, ∼85% of BTZ was released gradually over 9 days. In vivo studies performed in a multiple myeloma MM.1S xenograft mouse model showed that the tumor progression of mice treated with BTZ-loaded micelle solution was similar to that of the control group, whereas those treated with the BTZ-loaded micelle/hydrogel composite resulted in significant delay in the tumor progression. The results demonstrate that this hydrogel has great potential for use in subcutaneous and sustained delivery of drug-loaded micelles with superior therapeutic efficacy.

Biodegradable Strain-Promoted Click Hydrogels for Encapsulation of Drug-Loaded Nanoparticles and Sustained Release of Therapeutics.

Biodegradable polycarbonate-based ABA triblock copolymers were synthesized via organocatalyzed ring-opening polymerization and successfully formulated into chemically cross-linked hydrogels by strain-promoted alkyne-azide cycloaddition (SPAAC). The synthesis and cross-linking of these polymers are copper-free, thereby eliminating the concern over metallic contaminants for biomedical applications. Gelation occurs rapidly within a span of 60 s by simple mixing of the azide- and cyclooctyne-functionalized polymer solutions. The resultant hydrogels exhibited pronounced shear-thinning behavior and could be easily dispensed through a 22G hypodermic needle. To demonstrate the usefulness of these gels as a drug delivery matrix, doxorubicin (DOX)-loaded micelles prepared using catechol-functionalized polycarbonate copolymers were incorporated into the polymer solutions to eventually form micelle/hydrogel composites. Notably, the drug release rate from the hydrogels was significantly more gradual compared to the solution formulation. DOX release from the micelle/hydrogel composites could be sustained for 1 week, while the release from the micelle solution was completed rapidly within 6 h of incubation. Cellular uptake of the released DOX from the micelle/hydrogel composites was observed at 3 h of incubation of human breast cancer MDA-MB-231 cells. A blank hydrogel containing PEG-(Cat)12 micelles showed almost negligible toxicity on MDA-MB-231cells where cell viability remained high at >80% after treatment. When the cells were treated with the DOX-loaded micelle/hydrogel composites, there was a drastic reduction in cell viability with only 25% of cells surviving the treatment. In all, this study introduces a simple method of formulating hydrogel materials with incorporated micelles for drug delivery applications.

Highly potent antimicrobial polyionenes with rapid killing kinetics, skin biocompatibility and in vivo bactericidal activity.

Effective antimicrobial agents are important arsenals in our perennial fight against communicable diseases, hospital-acquired and surgical site multidrug-resistant infections. In this study, we devise a strategy for the development of highly efficacious and skin compatible yet inexpensive water-soluble macromolecular antimicrobial polyionenes by employing a catalyst-free, polyaddition polymerization using commercially available monomers. A series of antimicrobial polyionenes are prepared through a simple polyaddition reaction with both polymer-forming reaction and charge installation occurring simultaneously. The compositions and structures of polymers are modulated to study their effects on antimicrobial activity against a broad spectrum of pathogenic microbes. Polymers with optimized compositions have potent antimicrobial activity with low minimum inhibitory concentrations of 1.95-7.8 µg/mL and high selectivity over mammalian cells. In particular, a killing efficiency of more than 99.9% within 2 min is obtained. Moreover, the polymers demonstrate high antimicrobial efficacy against various clinically-isolated multidrug-resistant microbes, yet exhibit vastly superior skin biocompatibility in mice as compared to other clinically used surgical scrubs (chlorhexidine and betadine). Microbicidal activity of the polymer is mediated via membrane lysis as demonstrated by confocal microscopy. Unlike small molecular antibiotics, repeated use of the polymer does not induce drug resistance. More importantly, the polymer shows excellent bactericidal activity in a P. aeruginosa-contaminated mouse skin model. Given their rapid and efficacious microbicidal activity and skin compatibility, these polymers have tremendous potential to be developed as surgical scrubs/hand sanitizers to prevent multidrug-resistant infections.

Tuning the Selectivity of Biodegradable Antimicrobial Cationic Polycarbonates by Exchanging the Counter-Anion.

There is a growing interest in modern healthcare to develop systems able to fight antibiotic resistant bacteria. Antimicrobial cationic biodegradable polymers able to mimic antimicrobial peptides have shown to be effective against both Gram-positive and Gram-negative bacteria. In these systems, the hydrophilic-hydrophobic ratio and the cationic charge density play a pivotal role in defining the killing efficiency. Nevertheless, many of these antimicrobial polymers show relatively low selectivity as defined by the relative toxicity to mammalian cells or hemolysis relative to pathogens. In this study, a series of polycarbonates containing pendant quaternary ammoniums are used to understand the role of different counter-anions including chloride, citrate, malonate, benzoate, acetate, lactate and trifluoroacetate, and the antibiotic penicillin on antimicrobial efficacy and selectivity. Interestingly, it is found that in spite of the strong antimicrobial activity of trifluoroacetate and benzoate anions, they prove to be much less hemolytic than chloride anion. It is believed that the proper selection of the anion could enhance the potential of antimicrobial polymers to fight against clinically relevant pathogenic infections, while concurrently mitigating harmful side effects.

Expanding the Cationic Polycarbonate Platform: Attachment of Sulfonium Moieties by Postpolymerization Ring Opening of Epoxides.

Postpolymerization modification is a critical strategy for the development of functional polycarbonate scaffolds for medicinal applications. To expand the scope of available postpolymerization functionalization methods, polycarbonates containing pendant thioether groups were synthesized by organocatalyzed ring-opening polymerization. The thioether group allowed for the postpolymerization ring-opening of functional epoxides, affording a wide variety of sulfonium-functionalized A-B diblock and A-B-A triblock polycarbonate copolymers. The pendant thioether groups were found to be compatible with previously developed postsynthesis functionalization methods allowing for selective and orthogonal modifications of the polycarbonates.

An insight into non-emissive excited states in conjugated polymers.

Conjugated polymers in the solid state usually exhibit low fluorescence quantum yields, which limit their applications in many areas such as light-emitting diodes. Despite considerable research efforts, the underlying mechanism still remains controversial and elusive. Here, the nature and properties of excited states in the archetypal polythiophene are investigated via aggregates suspended in solvents with different dielectric constants (ɛ). In relatively polar solvents (ɛ>∼ 3), the aggregates exhibit a low fluorescence quantum yield (QY) of 2-5%, similar to bulk films, however, in relatively nonpolar solvents (ɛ<∼ 3) they demonstrate much higher fluorescence QY up to 20-30%. A series of mixed quantum-classical atomistic simulations illustrate that dielectric induced stabilization of nonradiative charge-transfer (CT) type states can lead to similar drastic reduction in fluorescence QY as seen experimentally. Fluorescence lifetime measurement reveals that the CT-type states exist as a competitive channel of the formation of emissive exciton-type states.

Oligomeric interface modifiers in hybrid polymer solar cell prototypes investigated by fluorescence voltage spectroscopy.

Carboxylated oligothiophenes were evaluated as interfacial modifiers between the organic poly(3-hexylthiophene) (P3HT) and inorganic TiO2 layers in bilayer hybrid polymer solar cells. Carboxylated oligothiophenes can be isolated using conventional purification techniques resulting in pure, monodisperse molecules with 100% carboxylation. Device prototypes using carboxylated oligothiophenes as interfacial modifiers showed improved performance in the open-circuit voltage and fill factor over devices using unmodified oligothiophenes as interfacial modifiers. X-ray photoelectron spectroscopy (XPS) studies supported the idea that interface layer adhesion was improved by functionalizing oligothiophenes with a carboxyl moiety. Wide-field fluorescence images revealed that devices made using carboxylated oligothiophenes had fewer aggregates in the P3HT layers atop the modified TiO2 surface. Hysteresis seen in the fluorescence intensity as a function of applied bias, obtained from In-Device Fluorescence Voltage Spectroscopy (ID-FVS), was found to be a diagnostic criterion of the quality of the hybrid interface modification. The best interfaces were found using oligothiophenes functionalized with carboxylates, which created smooth layers on TiO2, and showed no hysteresis, suggesting elimination of interfacial charge traps. However, this hysteresis could be re-introduced by increasing the scan rate of the applied bias, suggesting that smooth P3HT layers created by carboxylated oligothiophene interface modifiers were necessary but not sufficient for sustaining improved photovoltaic properties especially during long-term device operation.

Biodegradable Block Copolyelectrolyte Hydrogels for Tunable Release of Therapeutics and Topical Antimicrobial Skin Treatment.

Biodegradable polycarbonate-based ABA triblock copolyelectrolytes were synthesized and formulated into physically cross-linked hydrogels. These biocompatible, cationically, and anionically charged hydrogel materials exhibited pronounced shear-thinning behavior, making them useful for a variety of biomedical applications. For example, we investigated the antimicrobial activity of positively charged thiouronium functionalized hydrogels by microbial growth inhibition assays against several clinically relevant Gram-negative and Gram-positive bacteria. It is noteworthy that these hydrogels exhibited broad spectrum killing efficiencies approaching 100%, thereby rendering these thixotropic materials attractive for treatment of skin and other surface bound infections. Finally, cationic trimethylammonium containing hydrogels and anionic carboxylic acid functionalized hydrogels were utilized to sustain the release of negatively charged (diclofenac) and positively charged (vancomycin) therapeutics, respectively. Collectively, the present work introduces a simple method for formulating charged hydrogel materials that are capable of interacting with various analytes of interest through noncovalent interactions.

Antimicrobial hydrogels: a new weapon in the arsenal against multidrug-resistant infections.

The rapid emergence of antibiotic resistance in pathogenic microbes is becoming an imminent global public health problem. Treatment with conventional antibiotics often leads to resistance development as the majority of these antibiotics act on intracellular targets, leaving the bacterial morphology intact. Thus, they are highly prone to develop resistance through mutation. Much effort has been made to develop macromolecular antimicrobial agents that are less susceptible to resistance as they function by microbial membrane disruption. Antimicrobial hydrogels constitute an important class of macromolecular antimicrobial agents, which have been shown to be effective in preventing and treating multidrug-resistant infections. Advances in synthetic chemistry have made it possible to tailor molecular structure and functionality to impart broad-spectrum antimicrobial activity as well as predictable mechanical and rheological properties. This has significantly broadened the scope of potential applications that range from medical device and implant coating, sterilization, wound dressing, to antimicrobial creams for the prevention and treatment of multidrug-resistant infections. In this review, advances in both chemically and physically cross-linked natural and synthetic hydrogels possessing intrinsic antimicrobial properties or loaded with antibiotics, antimicrobial polymers/peptides and metal nanoparticles are highlighted. Relationships between physicochemical properties and antimicrobial activity/selectivity, and possible antimicrobial mechanisms of the hydrogels are discussed. Approaches to mitigating toxicity of metal nanoparticles that are encapsulated in hydrogels are reviewed. In addition, challenges and future perspectives in the development of safe and effective antimicrobial hydrogel systems especially involving co-delivery of antimicrobial polymers/peptides and conventional antimicrobial agents for eventual clinical applications are presented.

Excitonic energy migration in conjugated polymers: the critical role of interchain morphology.

Excitonic energy migration was studied using single molecule spectroscopy of individual conjugated polymer (CP) chains and aggregates. To probe the effect of interchain morphology on energy migration in CP, tailored interchain morphologies were achieved using solvent vapor annealing to construct polymer aggregates, which were then studied with single aggregate spectroscopy. We report that highly ordered interchain packing in regioregular poly(3-hexylthiophene) (rr-P3HT) enables long-range interchain energy migration, while disordered packing in regiorandom poly(3-hexylthiophene) (rra-P3HT), even in aggregates of just a few chains, can dramatically impede the interchain mechanism. In contrast to rr-P3HT, interchain energy migration in poly(3-(2'-methoxy-5'-octylphenyl)thiophene) (POMeOPT), a polythiophene derivative with bulky side chains, can be completely inhibited. We use simulated structures to show that the reduction in interchain coupling is not due simply to increased packing distance between backbones of different chains, but reflects inhibition of stacking due to side-chain-induced twisting of the contours of individual chains. A competition from intrachain coupling has also been demonstrated by comparing POMeOPT aggregates with different polymer chain sizes.

Brush-like polycarbonates containing dopamine, cations, and PEG providing a broad-spectrum, antibacterial, and antifouling surface via one-step coating.

An antibacterial and antifouling surface is obtained by simple one-step immersion of a catheter surface with brush-like polycarbonates containing pendent adhesive dopamine, antifouling polyethylene glycol (PEG), and antibacterial cations. This coating demonstrates excellent antibacterial and antifouling activities against both Gram-positive (S. aureus) and Gram-negative (E. coli) bacteria, proteins, and platelets, good stability under simulated blood-flow conditions, and no toxicity.

Role of non-covalent and covalent interactions in cargo loading capacity and stability of polymeric micelles.

Polymeric micelles self-assembled from biodegradable amphiphilic block copolymers have been proven to be effective drug delivery carriers that reduce the toxicity and enhance the therapeutic efficacy of free drugs. Several reviews have been reported in the literature to discuss the importance of size/size distribution, stability and drug loading capacity of polymeric micelles for successful in vivo drug delivery. This review is focused on non-covalent and covalent interactions that are employed to enhance cargo loading capacity and in vivo stability, and to achieve nanosize with narrow size distribution. In particular, this review analyzes various non-covalent and covalent interactions and chemistry applied to introduce these interactions to the micellar drug delivery systems, as well as the effects of these interactions on micelle stability, drug loading capacity and release kinetics. Moreover, the factors that influence these interactions and the future research directions of polymeric micelles are discussed.

Synthesis of a donor-acceptor diblock copolymer via two mechanistically distinct, sequential polymerizations using a single catalyst.

Treatment of a Ni-terminated poly(3-hexylthiophene) (P3HT), generated in situ from 5-chloromagnesio-2-bromo-3-hexylthiophene and Ni(1,3-bis(diphenylphosphino)propane)Cl2, with a perylene diimide-functionalized arylisocyanide monomer effects a chain-extension polymerization to afford a donor-acceptor diblock copolymer using a single catalyst and in a single reaction vessel. The two mechanistically distinct polymerizations proceed in a controlled, chain growth fashion, allowing the molecular weight of both the P3HT and poly(isocyanide) blocks to be tuned by adjusting the initial monomer-to-catalyst ratios. The resulting materials are found to self-assemble into crystalline, lamellar stacks of donor and acceptor components in the solid state, and also exhibit fluorescence quenching in thin films, properties which poise these materials for use in organic photovoltaic applications.

Effect of the side-chain-distribution density on the single-conjugated-polymer-chain conformation.

The spatial arrangement of the side chains of conjugated polymer backbones has critical effects on the morphology and electronic and photophysical properties of the corresponding bulk films. The effect of the side-chain-distribution density on the conformation at the isolated single-polymer-chain level was investigated with regiorandom (rra-) poly(3-hexylthiophene) (P3HT) and poly(3-hexyl-2,5-thienylene vinylene) (P3HTV). Although pure P3HTV films are known to have low fluorescence quantum efficiencies, we observed a considerable increase in fluorescence intensity by dispersing P3HTV in poly(methyl methacrylate) (PMMA), which enabled a single-molecule spectroscopy investigation. With single-molecule fluorescence excitation polarization spectroscopy, we found that rra-P3HTV single molecules form highly ordered conformations. In contrast, rra-P3HT single molecules, display a wide variety of different conformations from isotropic to highly ordered, were observed. The experimental results are supported by extensive molecular dynamics simulations, which reveal that the reduced side-chain-distribution density, that is, the spaced-out side-chain substitution pattern, in rra-P3HTV favors more ordered conformations compared to rra-P3HT. Our results demonstrate that the distribution of side chains strongly affects the polymer-chain conformation, even at the single-molecule level, an aspect that has important implications when interpreting the macroscopic interchain packing structure exhibited by bulk polymer films.

Controlled catalyst transfer polycondensation and surface-initiated polymerization of a p-phenyleneethynylene-based monomer.

Herein, we describe a catalyst transfer polycondensation that enabled access to well-defined poly(p-phenyleneethynylene) (PPE), a prominent conjugated polymer. Treatment of a stannylated 4-iodophenylacetylene derivative with PhPd(t-Bu3P)Br afforded the corresponding PPE in up to 94% yield. Under optimized conditions, the molecular weight of the polymer increased linearly with monomer consumption, and was controlled by adjusting the initial monomer-to-catalyst ratio. The chain-growth nature of the polymerization reaction was utilized to produce well-defined PPE-containing block copolymers, as well as to grow PPE brushes from silica nanoparticles via a surface-initiated polymerization process.

Mimicking conjugated polymer thin-film photophysics with a well-defined triblock copolymer in solution.

Conjugated polymers (CPs) are promising materials for use in electronic applications, such as low-cost, easily processed organic photovoltaic (OPV) devices. Improving OPV efficiencies is hindered by a lack of a fundamental understanding of the photophysics in CP-based thin films that is complicated by their heterogeneous nanoscale morphologies. Here, we report on a poly(3-hexylthiophene)-block-poly(tert-butyl acrylate)-block-poly(3-hexylthiophene) rod-coil-rod triblock copolymer. In good solvents, this polymer resembles solutions of P3HT; however, upon the addition of a poor solvent, the two P3HT chains within the triblock copolymer collapse, affording a material with electronic spectra identical to those of a thin film of P3HT. Using this new system as a model for thin films of P3HT, we can attribute the low fluorescence quantum yield of films to the presence of a charge-transfer state, providing fundamental insights into the condensed phase photophysics that will help to guide the development of the next generation of materials for OPVs.

Conformational effect on energy transfer in single polythiophene chains.

Herein we describe the use of regioregular (rr-) and regiorandom (rra-) P3HT as models to study energy transfer in ordered and disordered single conjugated polymer chains. Single molecule fluorescence spectra and excitation/emission polarization measurements were compared with a Förster resonance energy transfer (FRET) model simulation. An increase in the mean single chain polarization anisotropy from excitation to emission was observed for both rr- and rra-P3HT. The peak emission wavelengths of rr-P3HT were at substantially lower energies than those of rra-P3HT. A simulation based on FRET in single polymer chain conformations successfully reproduced the experimental observations. These studies showed that ordered conformations facilitated efficient energy transfer to a small number of low-energy sites compared to disordered conformations. As a result, the histograms of spectral peak wavelengths for ordered conformations were centered at much lower energies than those obtained for disordered conformations. Collectively, these experimental and simulated results provide the basis for quantitatively describing energy transfer in an important class of conjugated polymers commonly used in a variety of organic electronics applications.

Synthesis and study of redox-active acyclic triazenes: toward electrochromic applications.

Coupling of various 4-substituted phenyl azides with two distinct quinone-containing N-heterocyclic carbenes (NHCs) afforded the respective mono- and ditopic 1,3-disubstituted acyclic triazenes in moderate to excellent yields (38-92%). Depending on their pendant substituents (derived from the azides), the acyclic triazenes exhibited intense absorptions in the visible spectrum (359-428 nm), which were bathochromically shifted by up to Δλ=68 nm upon reduction of the quinone moiety on the component derived from the NHC. Cyclic voltammetry confirmed that the aforementioned redox processes were reversible, and a related set of UV-vis spectroelectrochemical experiments revealed that bulk electrolysis may also be used to switch reversibly the colors exhibited by these triazenes.

Synthesis and self-assembly of poly(3-hexylthiophene)-block-poly(acrylic acid).

A modular and convenient synthesis of ethynyl end functionalized poly(3-hexylthiophene) in high purity is reported; this material facilitated access to poly(3-hexylthiophene)-block-poly(acrylic acid) which self-assembled into hierarchical structures.