Photo‐ and pH‐Responsive Polycarbonate Block Copolymer Prodrug Nanomicelles for Controlled Release of Doxorubicin

Photo/pH dual‐responsive amphiphilic diblock copolymers with alkyne functionalized pendant o‐nitrobenzyl ester group are synthesized using poly(ethylene glycol) as a macroinitiator. The pendant alkynes are functionalized as aldehyde groups by the azide‐alkyne Huisgen cycloaddition. The anticancer drug doxorubicin (DOX) molecules are then covalently conjugated through acid‐sensitive Schiff‐base linkage. The resultant prodrug copolymers self‐assemble into nanomicelles in aqueous solution. The prodrug nanomicelles have a well‐defined morphology with an average size of 20–40 nm. The dual‐stimuli are applied individually or simultaneously to study the release behavior of DOX. Under UV light irradiation, nanomicelles are disassembled due to the ONB ester photocleavage. The light‐controlled DOX release behavior is demonstrated using fluorescence spectroscopy. Due to the pH‐sensitive imine linkage the DOX molecules are released rapidly from the nanomicelles at the acidic pH of 5.0, whereas only minimal amount of DOX molecules is released at the pH of 7.4. The DOX release rate is tunable by applying the dual‐stimuli simultaneously. In vitro studies against colon cancer cells demonstrate that the nanomicelles show the efficient cellular uptake and the intracellular DOX release, indicating that the newly designed copolymers with dual‐stimuli‐response have significant potential applications as a smart nanomedicine against cancer.     

Nucleobase‐Functionalized Supramolecular Micelles with Tunable Physical Properties for Efficient Controlled Drug Release

Complementary nucleobase‐functionalized polymeric micelles, a combination of adenine‐thymine (A‐U) base pairs and a blend of hydrophilic–hydrophobic polymer pairs, can be used to construct 3D supramolecular polymer networks; these micelles exhibit excellent self‐assembly ability in aqueous solution, rapid pH‐responsiveness, high drug loading capacity, and triggerable drug release. In this study, a multi‐uracil functionalized poly(ε‐caprolactone) (U‐PCL) and adenine end‐capped difunctional oligomeric poly(ethylene glycol) (BA‐PEG) are successfully developed and show high affinity and specific recognition in solution owing to dynamically reversible A‐U‐induced formation of physical cross‐links. The U‐PCL/BA‐PEG blend system produces supramolecular micelles that can be readily adjusted to provide the desired critical micellization concentration, particle size, and stability. Importantly, in vitro release studies show that doxorubicin (DOX)‐loaded micelles exhibit excellent DOX‐encapsulated stability under physiological conditions. When the pH value of the solution is reduced from 7.4 to 5.0, DOX‐loaded micelles can be rapidly triggered to release encapsulated DOX, suggesting these polymeric micelles represent promising candidate pH‐responsive nanocarriers for controlled‐release drug delivery and pharmaceutical applications.

     

pH‐Responsive Drug‐Delivery Systems

In many biomedical applications, drugs need to be delivered in response to the pH value in the body. In fact, it is desirable if the drugs can be administered in a controlled manner that precisely matches physiological needs at targeted sites and at predetermined release rates for predefined periods of time. Different organs, tissues, and cellular compartments have different pH values, which makes the pH value a suitable stimulus for controlled drug release. pH‐Responsive drug‐delivery systems have attracted more and more interest as “smart” drug‐delivery systems for overcoming the shortcomings of conventional drug formulations because they are able to deliver drugs in a controlled manner at a specific site and time, which results in high therapeutic efficacy. This focus review is not intended to offer a comprehensive review on the research devoted to pH‐responsive drug‐delivery systems; instead, it presents some recent progress obtained for pH‐responsive drug‐delivery systems and future perspectives. There are a large number of publications available on this topic, but only a selection of examples will be discussed.     

Acid‐Labile Thermoresponsive Copolymers That Combine Fast pH‐Triggered Hydrolysis and High Stability under Neutral Conditions

Biodegradable polymeric materials are intensively used in biomedical applications. Of particular interest for drug‐delivery applications are polymers that are stable at pH 7.4, that is, in the blood stream, but rapidly hydrolyze under acidic conditions, such as those encountered in the endo/lysosome or the tumor microenvironment. However, an increase in the acidic‐degradation rate of acid‐labile groups goes hand in hand with higher instability of the polymer at pH 7.4 or during storage, thus posing an intrinsic limitation on fast degradation under acidic conditions. Herein, we report that a combination of acid‐labile dimethyldioxolane side chains and hydroxyethyl side chains leads to acid‐degradable thermoresponsive polymers that are quickly hydrolyzed under slightly acidic conditions but stable at pH 7.4 or during storage. We ascribe these properties to high hydration of the hydroxy‐containing collapsed polymer globules in conjunction with autocatalytic acceleration of the hydrolysis reactions by the hydroxy groups.     

Metallo‐Supramolecular Nanogels for Intracellular pH‐Responsive Drug Release

A simple process is developed to fabricate metallo‐supramolecular nanogels (MSNs) by the metallo‐supramolecular‐coordinated interaction between histidine and iron‐meso‐tetraphenylporphin. MSNs are composed of histidine‐modified dextran (DH) and iron‐meso‐tetraphenylporphin (Fe–Por) and exhibit excellent biocompatibility and stability. MSNs show pH responsiveness in the intracellular mildly acidic environment, which has great potential for acid‐triggered drug release delivery. In vitro drug release profiles demonstrate that the pH‐dependent disassembly of MSNs to histidine and Por results in a quicker release rate of loaded‐DOX at pH 5.3, while at pH 7.4 MSNs could hinder the release of loaded‐DOX due to the enhanced stability of MSNs.

     

pH‐responsive polymer core–shell nanospheres for drug delivery

Stimuli‐responsive polymer nanoparticles are playing an increasingly more important role in drug delivery applications. However, limited knowledge has been accumulated about processes which use stimuli‐responsive polymer nanospheres (matrix nanoparticles whose entire mass is solid) to carry and deliver hydrophobic therapeutics in aqueous solution. In this research, pyrene was selected as a model hydrophobic drug and a pyrene‐loaded core‐shell structured nanosphere named poly(DEAEMA)‐poly(PEGMA) was designed as a drug carrier where DEAEMA and PEGMA represent 2‐(diethylamino)ethyl methacrylate and poly(ethylene glycol) methacrylate, respectively. The pyrene‐loaded core‐shell nanospheres were prepared via an in situ two‐step semibatch emulsion polymerization method. The particle size of the core‐shell nanosphere can be well controlled through adjusting the level of surfactant used in the polymerization where an average particle diameter of below 100 nm was readily achieved. The surfactant was removed via a dialysis operation after polymerization. Egg lecithin vesicles (liposome) were prepared to mimic the membrane of a cell and to receive the released pyrene from the nanosphere carriers. The in vitro release profiles of pyrene toward different pH liposome vesicles were recorded as a function of time at 37 °C. It was found that release of pyrene from the core‐shell polymer matrix can be triggered by a change in the environmental pH. In particular the pyrene‐loaded nanospheres are capable of responding to a narrow window of pH change from pH = 5, 6, to 7 and can achieve a significant pyrene release of above 80% within 90 h. The rate of release increased with a decrease in pH. A first‐order kinetic model was proposed to describe the rate of release with respect to the concentration of pyrene in the polymer matrix. The first‐order rate constant of release k was thus determined as 0.049 h^?1 for pH = 5; 0.043 h^?1 for pH = 6; and 0.035 h^?1 for pH = 7 at 37 °C. The release of pyrene was considered to follow a diffusion‐controlled mechanism. The synthesis and encapsulation process developed herein provides a new approach to prepare smart nanoparticles for efficient delivery of hydrophobic drugs. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4440–4450     

Porous Microstructured Surfaces with pH‐Triggered Antibacterial Properties

New antibacterial films are designed with the capability to reversibly regulate their killing and repelling functions in response to variations in environmental pH. These systems consist of porous polystyrene surfaces as the main components and a copolymer bearing pH‐sensitive thiazole and triazole groups as the minor components. These pH‐sensitive groups, located on the surfaces, can be partially protonated at acidic pH levels, increasing the positive charge density of the surfaces and their antibacterial activity. Similarly, their bacterial adhesion and killing efficiencies in response to changes in pH are evaluated by analyzing the bacterial viability of Staphylococcus aureus bacteria on the surfaces under acidic and neutral pH values. It is demonstrated that after only 1 h of incubation with the bacterial suspension in acidic conditions, the surfaces killed the bacteria, while at pH = 7.4, some of the adhered bacteria are removed. Furthermore, the surface topography exerts an important role by intensifying this response.     

pH‐Triggered Sustained Drug Delivery from a Polymer Micelle having the β‐Thiopropionate Linkage

The synthesis, micellar aggregation, and pH‐triggered intracellular drug delivery ability of an amphiphilic statistical copolymer (P2) are studied. Two methacrylate derivatives, one containing a hydrophilic pendant and the other containing a hydrophobic pendant chain, are copolymerized to produce P2. The hydrophobic pendant chain is linked to the polymer backbone by a β‐thiopropionate linkage, known to undergo slow hydrolysis at mild acidic pH. P2 forms a multimicellar cluster in water with a critical aggregation concentration of 0.02 mg mL⁻¹ and encapsulates a hydrophobic guest such as pyrene, Nile red, or the anti‐cancer drug doxorubicin (Dox). Sustained release of the entrapped Dox (80% after 100 h) is noticed at pH 5.2, while release is significantly slower (35% after 100 h) at pH 7.4. Acidic hydrolysis of the β‐thiopropionate linkage leading to the reduction of the hydrophobicity is established as the cause for micellar disassembly and triggered drug release. Cell‐culture studies with the human breast cancer cell line, MCF‐7, reveal biocompatibility of P2 (below 150 μg mL⁻¹). It is further tested for intracellular delivery of Dox. MCF‐7 cells remain healthy at pH 7.4 but become unhealthy at pH 5.2 when treated with a Dox‐loaded P2 micelles.

     

Reactive Oxygen,Nitrogen, and Sulfur Species (RONSS)‐Responsive Polymersomes for Triggered Drug Release

Reactive oxygen, nitrogen, and sulfur species (RONSS) are cross‐reacting and involved in a myriad of physiological and pathological processes. Similar to acidic pH, overexpressed enzymes, and other specific stimuli found in pathological microenvironments, RONSS are recognized as a category of emerging triggering events and have been employed to design activatable theranostic nanomaterials. In this regard, a plethora of RONSS‐responsive nanovectors including polymeric micelles and vesicles (also referred to as polymersomes) are constructed. In comparison with micelles, polymersomes comprising aqueous interiors enclosed by hydrophobic membranes show intriguing applications in synergistic delivery of both hydrophobic and hydrophilic drugs, nanoreactors, and artificial organelles. This feature article focuses on the recent developments in the fabrication of RONSS‐responsive polymersomes and their potential biomedical applications in terms of triggered drug delivery.

     

pH‐Disintegrable Polyelectrolyte Multilayer‐Coated Mesoporous Silica Nanoparticles Exhibiting Triggered Co‐Release of Cisplatin and Model Drug Molecules

We report on the fabrication of pH‐disintegrable polyelectrolyte multilayer‐coated mesoporous silica nanoparticles (MSN) capable of triggered co‐release of cisplatin and model drug molecules. The outer polyelectrolyte multilayer was assembled from permanently cationic polyelectrolyte, poly(allyl amine hydrochloride) (PAH), and negatively charged polyelectrolyte, P(DMA‐co‐TPAMA), consisting of N,N‐dimethylacrylamide (DMA) and 3,4,5,6‐tetrahydrophthalic anhydride‐functionalized N‐(3‐aminopropyl)methacrylamide (TPAMA) monomer units, which exhibits pH‐induced charge conversion characteristics. Thus, the subtle alteration of solution pH from 7.4 to ≈5–6 can lead to the disintegration of outer polyelectrolyte multilayers, accompanied with the co‐release of cisplatin and RhB.

     

Light and pH Dual‐Degradable Triblock Copolymer Micelles for Controlled Intracellular Drug Release

A novel amphiphilic ABA‐type triblock copolymer poly(ethylene glycol)‐b‐poly(ethanedithiol‐alt‐nitrobenzyl)‐b‐poly(ethylene glycol) (PEG‐b‐PEDNB‐b‐PEG) is successfully prepared by sequential thiol‐acrylate Michael addition polymerization in one pot. PEG‐b‐PEDNB‐b‐PEG is designed to have light‐cleavable o‐nitrobenzyl linkages and acid‐labile β‐thiopropionate linkages positioned repeatedly in the main chain of the hydrophobic block. The light and pH dual degradation of PEG‐b‐PEDNB‐b‐PEG is traced by gel permeation chromatography (GPC). Such triblock copolymer can self‐assemble into micelles, which can be used to encapsulate anticancer drug doxorubicin (DOX). Because of the different degradation chemistry of o‐nitrobenzyl linkages and β‐thiopropionate linkages, DOX can be released from the micelles by two different manners, i.e., light‐induced rapid burst release and pH‐induced slow sustained release. Confocal laser scanning microscopy (CLSM) results indicated that DOX‐loaded micelles exhibited faster drug release in A549 cells after UV irradiation. Furthermore, 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide (MTT) results show that the DOX‐loaded micelles under UV light degradation exhibit better anticancer activity against A549 cells than that of the nonirradiated ones.

     

A pH‐responsive self‐fluorescent polymeric micelle as a potential optical imaging probe

A series of methoxypolyethylene glycol‐terminated self‐fluorescent polyurethane multi‐block copolymers with excellent pH‐responsivity, self‐fluorescence, and biocompatibility are designed and synthesized. In our design, 1, 4‐bis (hydroxyethyl) piperazine is chosen as a pH‐responsive segment which can donate or accept protons in response to the change of environmental pH, and fluorescein isothiocyanate is used as a fluorescent dye conjugated into the micelles to offer self‐fluorescence. The chemical structure of the polyurethane multi‐block copolymers is characterized by nuclear magnetic resonance and Fourier transform infrared spectroscopy. The results of the acid‐based titration, the fluorescence spectrometry, and the ultraviolet visible spectroscopy indicate that the polyurethane multi‐block copolymers own an excellent pH‐buffering capacity responded to the change of pH values and the favorable self‐fluorescence property in an aqueous solution. And the ultraviolet absorption peaks of samples are strengthened with increasing of pH values, indicating that methoxypolyethylene glycol‐terminated self‐fluorescent polyurethane multi‐block copolymer can be a pH‐dependent fluorescent probe in a broad pH range. In addition, the in vitro cytotoxicity test showed that the polyurethane multi‐block copolymer has low cytotoxicity and good biocompatibility, which make it a promising nanoplatform for molecular imaging, diagnosis, and treatment of disease.     

Au(III) complexes loaded pH‐responsive magnetic nanogels for cancer therapy

A novel pH‐responsive magnetic nanogels were developed with the aim of targeted delivering and simultaneously releasing of newly synthesized Au(III)‐based anticancer drug, Au(1,7‐Phen)Br₃. The obtained nanogels were characterized by FT‐IR, DLS, EDAX, TEM, XRD, ICP‐Ms and MRI. The TEM images showed that the nanogels had a spherical shape with a mean diameter of 20 nm. The in vitro release studies of Au (III)‐loaded nanogels showed a pH‐triggered controlled release of drugs. The in vitro cytotoxicity assay of samples to human cervical cancer HeLa cell lines indicated that the Au(III)‐loaded magnetic nanogels exert higher cytotoxicity in comparison with free Au(III) complex. Fluorescent microscope images indicated that these magnetic nanogels possessed notable cell specific targeting in vitro in the presence of an external magnetic field. The results show that this superparamagnetic nanocarrier is a promising candidate for inhibiting growth of tumor cells.     

Fabrication and characterization of gold nanospheres‐cored pH‐sensitive thiol‐ended triblock copolymer: A smart drug delivery system for cancer therapy

Conventional chemotherapy suffers lack of multidrug resistance (MDR), lack of bioavailability, and selectivity. Nano‐sized drug delivery systems (DDS) are developing aimed to solve several limitations of conventional DDS. These systems have been offered for targeting tumor tissue owing to enhanced long circulation time, drug solubility, their retention effect, and improved permeability. As a result, the aim of this project was the design and development of DDS for biomedical applications. For this purpose, gold nanospheres (GNSs) covered by pH‐sensitive thiol‐ended triblock copolymer [poly(methacrylic acid) ‐b‐poly(acrylamide) ‐b‐poly(ε‐caprolactone)‐SH; PMAA‐b‐PAM‐b‐PCL‐SH] for delivery of anticancer drug doxorubicin (DOX). The chemical structures of triblock copolymer were investigated by proton nuclear magnetic resonance (¹H NMR) and Fourier transform infrared (FTIR) spectroscopies. ¹H NMR spectroscopy and gel permeation chromatography (GPC) were used for calculating the molecular weights of each part in the nanocarrier. The success of coating, GNSs with triblock copolymer was considered by means of dynamic light scattering (DLS), FTIR, ultraviolet‐visible (UV‐Vis), and transmission electron microscopy (TEM) measurement. The pH‐responsive drug release ability, (DOX)‐loading capacity, biocompatibility, and in vitro cytotoxicity effects of the nanocarriers were also studied. As a result, it is expected that the synthesized GNSs@polymer‐DOX considered as a potential application in nanomedicine demand like smart drug delivery, imaging, and chemo‐photothermal therapy.