Fabrication of multiresponsive shell cross-linked micelles possessing pH-controllable core swellability and thermo-tunable corona permeability

A double hydrophilic ABC triblock copolymer, poly(2-(diethylamino)ethyl methacrylate)-b-poly(2-(dimethylamino)ethyl methacrylate)-b-poly(N-isopropylacrylamide) (PDEA-b-PDMA-b-PNIPAM), containing the well-known pH-responsive PDEA block and thermoresponsive PNIPAM block, was synthesized by atom transfer radical polymerization via sequential monomer addition using ethyl 2-chloropropionate as the initiator. The obtained triblock copolymer exhibits interesting "schizophrenic" micellization behavior in aqueous solution, and supramolecularly self-assembles into three-layer "onion-like" PNIPAM-core micelles at acidic pH's and elevated temperatures and PDEA-core micelles with "inverted" structures at alkaline pH's and room temperature. In both cases, dynamic laser light scattering (LLS) and optical transmittance reveal the presence of near-monodisperse micelles, and the micelle formation/inversion process is fully reversible. Novel shell cross-linked (SCL) micelles with pH-responsive PDEA cores and thermoresponsive PNIPAM coronas were then facilely fabricated from the PDEA-b-PDMA-b-PNIPAM triblock copolymer by cross-linking the PDMA inner shells with 1,2-bis(2-iodoethoxy)ethane. The reversible pH-dependent swelling/shrinking of PDEA cores and thermosensitive collapse/aggregation of PNIPAM coronas of the obtained SCL micelles were investigated in detail by dynamic LLS, optical transmittance, and transmission electron microscopy. As the structurally stable SCL micelles possess pH-controllable core swellability and thermo-tunable corona permeability, the release profile of a model hydrophobic drug, dipyridamole, initially loaded within the hydrophobic PDEA core, can be dually controlled by both the solution pH and the temperature. This represents the first report of SCL micelles with multiresponsive cores and coronas, which may find practical applications in fields such as drug delivery and smart release.     

Facile preparation of well-defined AB2 Y-shaped miktoarm star polypeptide copolymer via the combination of ring-opening polymerization and click chemistry

Well-defined AB2 Y-shaped miktoarm star polypeptide copolymer, PZLL-b-(PBLG)2, was synthesized via a combination of ring-opening polymerization (ROP) of alpha-amino acid N-carboxyanhydride (NCA) and click chemistry, where PZLL is poly(epsilon-benzyloxycarbonyl-L-lysine) and PBLG is poly(gamma-benzyl-L-glutamate). First, two types of primary-amine-containing initiators, N-aminoethyl 3,5-bis(propargyloxyl)-benzamide and 3-azidopropylamine, were synthesized and employed for the ROP of NCA, leading to the formation of dialkynyl-terminated PZLL and azide-terminated PBLG, dialkynyl-PZLL and PBLG-N3, respectively. The subsequent copper(I)-catalyzed cycloaddition reaction between dialkynyl-PZLL and slightly excess PBLG-N3 led to facile preparation of PZLL-b-(PBLG)2 Y-shaped miktoarm star polypeptide copolymer. The excess PBLG-N3 was scavenged off by reacting with alkynyl-functionalized Wang resin. The obtained Y-shaped miktoarm star polypeptide copolymer was characterized by gel permeation chromatograph (GPC), Fourier transform-infrared spectroscopy (FT-IR), and (1)H NMR. Moreover, after the hydrolysis of protecting benzyl and benzyloxycarbonyl groups of PZLL-b-(PBLG)2, water-soluble pH-responsive Y-shaped miktoarm star polypeptide copolymer, PLL-b-(PLGA)2, was obtained, where PLL is poly(L-lysine) and PLGA is poly(L-glutamic acid). It can self-assemble into PLGA-core micelles at acidic pH and PLL-core micelles at alkaline pH, accompanied with the coil-to-helix transition of PLGA and PLL sequences, respectively. The spontaneous pH-responsive supramolecular assembly of PLL-b-(PLGA)2 miktoarm star polypeptide copolymer has been investigated via a combination of (1)H NMR, laser light scattering (LLS), transmission electron microscopy (TEM), and circular dichroism (CD) spectroscopy.     

Triply triggered doxorubicin release from supramolecular nanocontainers

The synthesis of a supramolecular double hydrophilic block copolymer (DHBC) held together by cucurbit[8]uril (CB[8]) ternary complexation and its subsequent self-assembly into micelles is described. This system is responsive to multiple external triggers including temperature, pH and the addition of a competitive guest. The supramolecular block copolymer assembly consists of poly(N-isopropylacrylamide) (PNIPAAm) as a thermoresponsive block and poly(dimethylaminoethylmethacrylate) (PDMAEMA) as a pH-responsive block. Moreover, encapsulation and controlled drug release was demonstrated with this system using the chemotherapeutic drug doxorubicin (DOX). This triple stimuli-responsive DHBC micelle system represents an evolution over conventional double stimuli-responsive covalent diblock copolymer systems and displayed a significant reduction in the viability of HeLa cells upon triggered release of DOX from the supramolecular micellar nanocontainers.     

Phosphorylcholine-based pH-responsive diblock copolymer micelles as drug delivery vehicles: light scattering, electron microscopy, and fluorescence experiments

The micellization behavior of a diblock copolymer comprising a highly hydrophilic and biocompatible poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) corona-forming block and a pH-sensitive poly(2-(diisopropylamino)ethyl methacrylate) (PDPA) core-forming block (PMPC-b-PDPA) has been studied by static and dynamic light scattering (SDLS), transmission electron microscopy (TEM), and potentiometry. Self-assembly of PMPC-b-PDPA copolymers with two different DPA volume fractions (phi(DPA)) leads to narrowly distributed and structurally distinct spherical micelles, as evidenced by their molecular weight (M(w,mic)), aggregation number (N(agg)), hydrodynamic radius (R(H)), corona width (W), and core radius (R(c)). The excellent potential of these pH-responsive micelles as nanosized drug delivery vehicles was illustrated by the encapsulation of dipyridamole (DIP), a model hydrophobic drug that dissolves in acid solutions and becomes insoluble above pH 5.8, which is comparable to the pK(a) of the PDPA block. The influence of micelle structure (namely M(w,mic), N(agg), R(H), W, and R(c)) on drug loading content, drug loading efficiency, partition coefficient, and release kinetics was investigated and confirmed by fluorescence spectroscopy studies. The maximum dipyridamole loadings within PMPC(30)-b-PDPA(30) (R(H) = 14.0 nm; W = 4.8 nm; R(c) = 9.2 nm) and PMPC(30)-b-PDPA(60) (R(H) = 27.1 nm; W = 11.0 nm; R(c) = 16.1 nm) micelles were 7 and 12% w/w(p), respectively. This preferential solubilization of DIP into micelles formed by copolymer chains having longer core-forming blocks (i.e., possessing larger core volumes) reflects the larger partition coefficient (K(V)) of DIP between the aqueous phase and PMPC(30)-b-PDPA(60) micelles (K(V) = 5.7 x 10(4)) compared to PMPC(30)-b-PDPA(30) micelles (K(V) = 1.1 x 10(4)). This enhanced ability of PMPC(30)-b-PDPA(60) aggregates to entrap/stabilize small hydrophobic molecules also produces slower release kinetics. Rapid release can be triggered by lowering the pH to induce micellar dissociation.     

Novel polymer-DNA hybrid polymeric micelles composed of hydrophobic poly(D,L-lactic-co-glycolic acid) and hydrophilic oligonucleotides.

Biodegradable poly(D,L-lactic-co-glycolic acid) (PLGA) was chemically conjugated to oligonucleotide (ODN) to form an amphiphatic structure which is similar to an A-B type block copolymer. A terminal end of PLGA was activated and reacted with primary amine-terminated ODN. The ODN/PLGA conjugates self-assembled in aqueous solution to form a micellar structure by serving PLGA segments as a hydrophobic core and ODN segments as a surrounding hydrophilic corona. Critical micelle concentration was determined by a spectroflurometric method. Atomic force microscopic observation revealed that the micelle size was around 80 nm. These micelles could release ODN in a sustained manner by controlled degradation of hydrophobic PLGA chains. Compared to unconjugated ODN, the ODN/PLGA micelles could be more efficiently transported within cells, presumably by endocytosis. This study proposes a potential delivery method of ODN into cells by forming hybrid ODN/PLGA micelles.     

Self-assembly of polypeptide-containing ABC-type triblock copolymers in aqueous solution and its pH dependence

Self-assembling of novel biodegradable ABC-type triblock copolymer poly(ethylene glycol)-poly(L-lactide)-poly(L-glutamic acid) (PEG-PLLA-PLGA) is studied. In aqueous media, it self-assembles into a spherical micelle with the hydrophobic PLLA segment in the core and the two hydrophilic segments PEG and PLGA in the shell. With the lengths of PEG and PLLA blocks fixed, the diameter of the micelles depends on the length of the PLGA block and on the volume ratio of H(2)O/dimethylformamide (DMF) in the media. When the PLGA block is long enough, morphology of the self-assembly is pH-dependent. It assembles into the spherical micelle in aqueous media at pH 4.5 and into the connected rod at or below pH 3.2. The critical micelle concentration (cmc) of the copolymer changes accordingly with decreasing solution pH. Both aggregation states can convert to each other at the proper pH value. This reversibility is ascribed to the dissociation and neutralization of the COOH groups in the LGA residues. When the PLGA block is short compared to the PEG or PLLA block, it assembles only into the spherical micelle at various pH values.     

Synthesis of amphiphilic tadpole-shaped linear-cyclic diblock copolymers via ring-opening polymerization directly initiating from cyclic precursors and their application as drug nanocarriers

We report on the facile synthesis of well-defined amphiphilic and thermoresponsive tadpole-shaped linear-cyclic diblock copolymers via ring-opening polymerization (ROP) directly initiating from cyclic precursors, their self-assembling behavior in aqueous solution, and the application of micellar assemblies as controlled release drug nanocarriers. Starting from a trifunctional core molecule containing alkynyl, hydroxyl, and bromine moieties, alkynyl-(OH)-Br, macrocyclic poly(N-isopropylacrylamide) (c-PNIPAM) bearing a single hydroxyl functionality was prepared by atom transfer radical polymerization (ATRP), the subsequent end group transformation into azide functionality, and finally the intramacromolecular ring closure reaction via click chemistry. The target amphiphilic tadpole-shaped linear-cyclic diblock copolymer, (c-PNIPAM)-b-PCL, was then synthesized via the ROP of ε-caprolactone (CL) by directly initiating from the cyclic precursor. In aqueous solution at 20 °C, (c-PNIPAM)-b-PCL self-assembles into spherical micelles consisting of hydrophobic PCL cores and well-solvated coronas of cyclic PNIPAM segments. For comparison, linear diblock copolymer with comparable molecular weight and composition, (l-PNIPAM)-b-PCL, was also synthesized. It was found that the thermoresponsive coronas of micelles self-assembled from (c-PNIPAM)-b-PCL exhibit thermoinduced collapse and aggregation at a lower critical thermal phase transition temperature (T(c)) compared with those of (l-PNIPAM)-b-PCL. Temperature-dependent drug release profiles from the two types of micelles of (c-PNIPAM)-b-PCL and (l-PNIPAM)-b-PCL loaded with doxorubicin (Dox) were measured, and the underlying mechanism for the observed difference in releasing properties was proposed. Moreover, MTT assays revealed that micelles of (c-PNIPAM)-b-PCL are almost noncytotoxic up to a concentration of 1.0 g/L, whereas at the same polymer concentration, micelles loaded with Dox lead to ～60% cell death. Overall, chain topologies of thermoresponsive block copolymers, that is, (c-PNIPAM)-b-PCL versus (l-PNIPAM)-b-PCL, play considerable effects on the self-assembling and thermal phase transition properties and their functions as controlled release drug nanocarriers.     

Solubilization and controlled release of a hydrophobic drug using novel micelle-forming ABC triblock copolymers

Amphiphilic ABC triblock copolymers composed of monomethoxy-capped poly(ethylene glycol) (MPEG), poly(2-(dimethylamino)ethyl methacrylate) (DMA), and poly(2-(diethylamino)ethyl methacrylate) (DEA) have been synthesized by atom transfer radical polymerization (ATRP). These copolymers dissolve molecularly in acidic aqueous media at room temperature due to protonation of the tertiary amine groups on the DMA and DEA residues. On adjusting the pH with base, micellization occurred at pH 8, with the water-insoluble, deprotonated DEA block forming the hydrophobic cores and the MPEG and DMA blocks forming the hydrophilic micellar coronas and inner shells, respectively. This pH-induced micellization has been exploited to develop a solvent-free protocol for drug loading. A model hydrophobic drug, dipyridamole (DIP), which dissolves in acid but is insoluble above pH 5.8, was incorporated into the micelles by increasing the pH of an aqueous drug/copolymer mixture to 9. Both the empty and the drug-loaded micelles were characterized by dynamic light scattering and fluorescence studies. The interaction of both pyrene and DIP with the MPEG-DMA-DEA micelles was studied by fluorescence; both compounds had relatively high partition coefficients into the micelles, 4.5 x 10(5) and 1.5 x 10(4), respectively. Intensity-average micelle diameters ranged from 20 to 90 nm, depending on the polymer composition and concentration. Shorter MPEG blocks (Mn = 2000) produced larger micelles than longer MPEG blocks (Mn = 5000) due to the shift in the hydrophilic-hydrophobic balance of the copolymer. Transmission electron microscopy studies of the drug-loaded micelles indicated spherical morphologies and reasonably uniform particle size distributions, which is in marked contrast to the needlelike morphology observed for pure DIP in the absence of the copolymer. Experiments on controlled release demonstrated that DIP-loaded MPEG-DMA-DEA micelles act as a drug carrier, giving slow release to the surrounding solution over a period of days. Rapid release can be triggered by reducing the pH to reverse the micellization.     

Synthesis, characterization, and bioavailability of mannosylated shell cross-linked nanoparticles

Saccharide-functionalized shell cross-linked (SCK) polymer micelles designed as polyvalent nanoscaffolds for selective interactions with receptors on Gram negative bacteria were constructed from mixed micelles composed of poly(acrylic acid-b-methyl acrylate) and mannosylated poly(acrylic acid-b-methyl acrylate). The mannose unit was conjugated to the hydrophilic chain terminus of the amphiphilic diblock copolymer precursor, from which the SCK nanoparticles were derived, by the growth of the diblock copolymer from a mannoside functionalized atom transfer radical polymerization (ATRP) initiator. Mixed micelle formation between the amphiphilic diblock copolymer and mannosylated amphiphilic diblock copolymer was followed by condensation-based cross-linking between the acrylic acid residues present in the periphery of the polymer micelles to afford SCK nanoparticles. SCKs presenting variable numbers of mannose functionalities were prepared from mixed micelles of controlled stoichiometric ratios of mannosylated and nonmannosylated diblock copolymers. The polymer micelles and SCKs were characterized by dynamic light scattering (DLS), electrophoretic light scattering, atomic force microscopy (AFM), transmission electron microscopy (TEM), and analytical ultracentrifugation (AU). Surface availability and bioactivity of the mannose units were evaluated by interactions of the nanostructures with the model lectin Concanavalin A via DLS studies, with red blood cells (rabbit) via agglutination inhibition assays and with bacterial cells (E. coli) via TEM imaging.     

Studies on the mechanism of interaction of a bioresponsive endosomolytic polyamidoamine with interfaces. 1. Micelles as model surfaces

Polymers are appealing as pH-responsive elements of multicomponent systems designed to promote cytosolic delivery of macromolecular drugs (including proteins and genes), but so far the delivery efficiency achieved has been relatively modest. Therefore, the aim of this study was to apply several physicochemical techniques that are well established in the colloid field (surface tension measurements, small-angle neutron scattering (SANS), and electron paramagnetic resonance (EPR)) to probe the mechanism of endosomolytic polymer-surface interaction over the pH range 7.4 to 5.5 using the poly(amidoamine) (PAA) ISA23 x HCl and a series of "model" micelle surfaces. These micellar models were chosen to represent increasing complexity from simple, single surfactant sodium dodecylsulfate (SDS) micelles, surfactant mixtures containing bulky malono-bis-N-methylglucamide headgroups, or highly extended ethylene oxide headgroups. Spherical micelles composed of 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (lyso-PC) were also used. Changes in the onset of micellization, micelle surface fluidity, and in selected cases, the overall micelle shape and size were all quantified as a function of pH in the presence and absence of ISA23 x HCl. This amphoteric PAA is negatively charged at pH 7.4 and becomes gradually more protonated on exposure to lower pH values representative of the endosomal-lysosomal pathway. As expected, the strength of polymer interaction with anionic micelles increased with a decrease in pH, while for cationic micelles the opposite was observed. Addition of bulky, nonionic surfactant headgroups led to weaker interactions. The observations from surface tension and SANS studies showed a complex pattern of interaction with both an electrostatic and hydrophobic component. Using EPR it was confirmed that ISA23 x HCl perturbed the micelle palisade layer leading to a decrease in fluidity of the interface with a lower degree of headgroup hydration, and a significant change in micelle morphology. Surprisingly, there was no interaction between ISA23 x HCl and globular micelles formed from lyso-PC (a more biologically relevant model), and this suggests that the PAA structure could be better optimized to promote rapid interaction with endosomal membranes at the physiologically relevant pH 6.5.     

pH-responsive three-layered PEGylated polyplex micelle based on a lactosylated ABC triblock copolymer as a targetable and endosome-disruptive nonviral gene vector

Nonviral vectors for gene therapy have recently received an increased impetus because of the inherent safety problems of the viral vectors, while their transfection efficiency is generally low compared to the viral vectors. The lack of the ability to escape from the endosomal compartments is believed to be one of the critical barriers to the intracellular delivery of noviral gene vectors. This study was devoted to the design and preparation of a novel ABC triblock copolymer for constructing a pH-responsive and targetable nonviral gene vector. The copolymer, lactosylated poly(ethylene glycol)-block-poly(silamine)-block-poly[2-(N,N-dimethylamino)ethyl methacrylate] (Lac-PEG-PSAO-PAMA), consists of lactosylated poly(ethylene glycol) (A-segment), a pH-responsive polyamine segment (B-segment), and a DNA-condensing polyamine segment (C-segment). The Lac-PEG-PSAO-PAMA spontaneously associated with plasmid DNA (pDNA) to form three-layered polyplex micelles with a PAMA/pDNA polyion complex (PIC) core, an uncomplexed PSAO inner shell, and a lactosylated PEG outer shell, as confirmed by 1H NMR spectroscopy. Under physiological conditions, the Lac-PEG-PSAO-PAMA/pDNA polyplex micelles prepared at an N/P (number of amino groups in the copolymer/number of phosphate groups in pDNA) ratio above 3 were found to be able to condense pDNA, thus adopting a relatively small size (< 150 nm) and an almost neutral surface charge (zeta approximately +5 mV). The micelle underwent a pH-induced size variation (pH = 7.4, 132.6 nm --> pH = 4.0, 181.8 nm) presumably due to the conformational changes (globule-rod transition) of the uncomplexed PSAO chain in response to pH, leading to swelling of the free PSAO inner shell at lowered pH while retaining the condensed pDNA in the PAMA/pDNA PIC core. Furthermore, the micelles exhibited a specific cellular uptake into HuH-7 cells (hepatocytes) through asialoglycoprotein (ASGP) receptor-mediated endocytosis and achieved a far more efficient transfection ability of a reporter gene compared to the Lac-PEG-PSAO/pDNA and Lac-PEG-PAMA/pDNA polyplex micelles composed of the diblock copolymers and pDNA. The effect of hydroxychloroquine as an endosomolytic agent on the transfection efficiency was not observed for the Lac-PEG-PSAO-PAMA/pDNA polyplex micelles, whereas the nigericin treatment of the cell as an inhibitor for the endosomal acidification induced a substantial decrease in the transfection efficiency, suggesting that the protonation of the free PSAO inner shell in response to a pH decrease in the endosome might lead to the disruption of the endosome through buffering of the endosomal cavity. Therefore, the polyplex micelle composed of ABC (ligand-PEG/pH-responsive segment/DNA-condensing segment) triblock copolymer would be a promising approach to a targetable and endosome disruptive nonviral gene vector.     

Synthesis and micellization of amphiphilic brush-coil block copolymer based on poly(epsilon-caprolactone) and PEGylated polyphosphoester

A novel biodegradable amphiphilic brush-coil block copolymer consisting of poly(epsilon-caprolactone) and PEGylated polyphosphoester was synthesized by ring opening polymerization. The composition and structure of the copolymer were characterized by 1H NMR, 13C NMR, and FT-IR, and the molecular weight and molecular weight distribution were analyzed by gel permeation chromatograph (GPC) measurements to confirm the diblock structure. These amphiphilic copolymers formed micellar structures in water, and the critical micelle concentrations (CMCs) were around 10(-3) mg/mL, which was determined using pyrene as a fluorescence probe. Transmission electron microscopy (TEM) images showed that the micelles took an approximately spherical shape with core-shell structure, which was further demonstrated by laser light scattering (LLS) technique. The degradation behavior of the polymeric micelle was also investigated in the presence of Pseudomonas lipase and characterized by GPC measurement. Such polymer micelles from brush-coil block copolymers are expected to have wide utility in the field of drug delivery.     

"Schizophrenic" Hemocompatible Copolymers via Switchable Thermoresponsive Transition of Nonionic/Zwitterionic Block Self-Assembly in Human Blood

"Schizophrenic" diblock copolymers containing nonionic and zwitterionic blocks were prepared with well-controlled molecular weights via atom-transfer radical polymerization (ATRP). In this work, we report a systematic study of how morphological changes of poly(N-isopropylacrylamide)-block-poly(sulfobetaine methacrylate) (PNIPAAm-b-PSBMA) copolymers affect hemocompatibility in human blood solution. The "schizophrenic" behavior of PNIPAAm-b-PSBMA was observed by (1)H NMR, dynamic light scattering (DLS), and turbidity measurement with double morphological transition, exhibiting both lower critical solution temperature (LCST) and upper critical solution temperature (UCST) in aqueous solution. Below the UCST of PSBMA block, micelles were obtained with a core of insoluble PSBMA association and a shell of soluble PNIPAAm, whereas the opposite micelle structure was observed above the LCST of PNIPAAm block. In between the UCST and LCST, unimers with both soluble blocks were detected. Hydrodynamic size of prepared polymers and copolymers is determined to illustrate the correlations between intermolecular nonionic/zwitterionic associations and blood compatibility of PNIPAAm, PNIPAAm-b-PSBMA, and PSBMA suspension in human blood. Human fibrinogen adsorption onto the PNIPAAm-b-PSBMA copolymers from single-protein solutions was measured by DLS to determine the nonfouling stability of copolymer suspension. The new nonfouling nature of PNIPAAm-b-PSBMA copolymers was demonstrated to show extremely high anticoagulant activity and antihemolytic activity in human blood over a wide range of explored temperatures from 4 to 40 °C. The temperature-independent blood compatibility of nonionic/zwitterionic block copolymer along with their schizophrenic phase behavior in aqueous solution suggests their potential in blood-contacting applications.     

Size control of styrene oxide-ethylene oxide diblock copolymer aggregates with classical surfactants: DLS, TEM, and ITC study

The interactions between the diblock copolymer S(15)E(63) and the surfactants sodium dodecyl sulfate (SDS), sodium decyl sulfate (SDeS), and sodium octyl sulfate (SOS) have been investigated by dynamic light scattering (DLS), transmission electron microscopy (TEM), and isothermal titration calorimetry (ITC). The surfactants with the same headgroup differentiate in their chain length. At 20 degrees C, the block copolymer is associated into micelles with a hydrodynamic radius of 11.6 nm, which is composed of a hydrophobic styrene oxide (S) core and a water-swollen oxypolyethylene (PEO or E) corona. The different copolymer/surfactant systems have been studied at a constant copolymer concentration of 2.5 g dm(-3) and in a vast range of surfactant concentrations, from 7.5 x 10(-6) up to 0.75 M. When SDS and SDeS are added to the block copolymer solution, different regions are observed in the DLS data: at low surfactant concentrations (c < 1.0 x 10(-4) M), single surfactant molecules associate with the copolymer micelle, probably the former being solubilized in the micelle core, leading to a certain disruption of the mixed micelle due to repulsive electrostatic interactions between surfactant headgroups followed by a stabilization of the mixed micelle. At higher concentrations (1.0 x 10(-4) < c < 0.1 M), two types of copolymer-surfactant complexes coexist: one large copolymer-rich/surfactant complex and one small complex consisting of one or a few copolymer chains and rich in surfactants. At higher SDS and SDeS concentrations, complete disintegration of mixed micelles takes place. In contrast, SOS-S(15)E(63) interactions are less important up to surfactant concentrations of 0.05 M due to its higher hydrophilicity, reducing the hydrophobic interactions between surfactant alkyl chains and copolymer micelles. At concentration larger than the critical aggregation concentration (cac) of the system, 0.05 M, disruption of copolymer micelles occurs. These regions have been confirmed by transmission electron microscopy. On the other hand, the titration calorimetric data for SDS and SDeS present an endothermic increase indicating the formation of mixed copolymer-rich-surfactant micelles. From that point, important differences in the ITC plot for both surfactants are present. However, the ITC curve obtained after titration of a SOS solution in the copolymer solution is quite similar to that of its titration in water.     

Synthesis of well-defined amphiphilic block copolymers having phospholipid polymer sequences as a novel biocompatible polymer micelle reagent

To realize safer and effective drug administration, novel well-defined and biocompatible amphiphilic block copolymers containing phospholipid polymer sequences were synthesized. At first, the homopolymer of 2-methacryloyloxyethylphosphorylcholine (MPC) was synthesized in water by reversible addition-fragmentation chain transfer (RAFT) controlled radical polymerization. The "living" polymerization was confirmed by the fact that the number-average molecular weight increased linearly with monomer conversion while the molecular weight distribution remained narrow independent of the conversion. The poly(MPC) thus prepared is end-capped with a dithioester moiety. Using the dithioester-capped poly(MPC) as a macro chain transfer agent, AB diblock copolymers of MPC and n-butyl methacrylate (BMA) were synthesized. Associative properties of the amphiphilic block copolymer (pMPC(m)-BMA(n)) with varying poly(BMA) block lengths were investigated using NMR, fluorescence probe, static light scattering (SLS), and quasi-elastic light scattering (QELS) techniques. Proton NMR data in D2O indicated highly restricted motions of the n-butyl moieties, arising from hydrophobic associations of poly(BMA) blocks. Fluorescence spectra of N-phenyl-1-naphthylamine indicated that the probes were solubilized in the polymer micelles in water. The formation of polymer micelles comprising a core with poly(BMA) blocks and shell with hydrophilic poly(MPC) blocks was suggested by SLS and QELS data. The size and mass of the micelle increased with increasing poly(BMA) block length. With an expectation of a pharmaceutical application of pMPC(m)-BMA(n), solubilization of a poorly water-soluble anticancer agent, paclitaxel (PTX), was investigated. PTX dissolved well in aqueous solutions of pMPC(m)-BMA(n) as compared with pure water, implying that PTX is incorporated into the hydrophobic core of the polymer micelle. Since excellent biocompatible poly(MPC) sequences form an outer shell of the micelle, pMPC(m)-BMA(n) may find application as a promising reagent to make a good formulation with a hydrophobic drug.     

Shell-cross-linked micelles containing cationic polymers synthesized via the RAFT process: toward a more biocompatible gene delivery system

Block copolymers poly(2-(dimethylamino) ethyl methacrylate)-b-poly(polyethylene glycol methacrylate) (PDMAEMA-b-P(PEGMA)) were prepared via reversible addition fragmentation chain transfer polymerization (RAFT). The polymerization was found to proceed with the expected living behavior resulting in block copolymers with varying block sizes of low polydispersity (PDI <1.3). The resulting block copolymer was self-assembled in an aqueous environment, leading to the formation of pH-responsive micelles. Further stabilization of the micellar system was performed in water using ethylene glycol dimethacrylate and the RAFT process to cross-link the shell. The cross-linked micelle was found to have properties significantly different from those of the uncross-linked block copolymer micelle. While a distinct critical micelle concentration (CMC) was observed using block copolymers, the CMC was absent in the cross-linked system. In addition, a better stability against disintegration was observed when altering the ionic strength such as the absence of changes of the hydrodynamic diameter with increasing NaCl concentration. Both cross-linked and uncross-linked micelles displayed good binding ability for genes. However, the cross-linked system exhibited a slightly superior tendency to bind oligonucleotides. Cytotoxicity tests confirmed a significant improvement of the biocompatibility of the synthesized cross-linked micelle compared to that of the highly toxic PDMAEMA. The cross-linked micelles were taken up by cells without causing any signs of cell damage, while the PDMAEMA homopolymer clearly led to cell death.     

pH-Responsive copolymer assemblies for controlled release of doxorubicin

pH-Responsive drug carriers have the potential to provide selective drug release at therapeutic targets including tumors and in acidic intracellular vesicles such as endosomes and lysosomes. We have developed a new approach to the design of acid-sensitive micelles by incorporating hydrophobic acetal groups on the core block of a micelle-forming block copolymer. Hydrolysis of the acetals at mildly acidic pH is designed to reveal polar groups on the core-forming block, thus changing its solubility and disrupting the micelle, triggering drug release. The anticancer drug doxorubicin (DOX) was encapsulated in these pH-sensitive micelles, and the acetal hydrolysis rates and DOX release rates were determined in the pH range of 4.0 to 7.4 and were compared to those of control systems. The micelle disruption was investigated by dynamic light scattering. The in vitro toxicities of the empty and DOX-loaded micelles were determined, and the intracellular fate of the encapsulated DOX was compared to free DOX using fluorescence confocal microscopy.     

Micellization of casein-graft-dextran copolymer prepared through Maillard reaction

Casein is almost insoluble at around pH 4.6, which is its isoelectric point (pI). Grafting copolymer, casein-g-dextran, was prepared through the Amadori rearrangement of the Maillard reaction. The copolymer has a reversible pH sensitive property: micellization at the pI of casein forming a casein core and dextran shell structure and dissociation when pH differs from the pI. The micelles produced at pH 4.6 have a spherical shape and their size is dependent on the Maillard reaction: reaction time, molar ratio of casein to dextran, and molecular weight of dextran used. Typically, the hydrodynamic diameter of the micelles is about 100 nm and the critical micelle concentration is about 10 mg/L. The micelles are very stable in aqueous solution and can be stored as lyophiled powder. The micelles are able to encapsulate hydrophobic compounds such as pyrene.     

Oxime linkage: a robust tool for the design of pH-sensitive polymeric drug carriers

Oxime bonds dispersed in the backbones of the synthetic polymers, while young in the current spectrum of the biomedical application, are rapidly extending into their own niche. In the present work, oxime linkages were confirmed to be a robust tool for the design of pH-sensitive polymeric drug delivery systems. The triblock copolymer (PEG-OPCL-PEG) consisting of hydrophilic poly(ethylene glycol) (PEG) and hydrophobic oxime-tethered polycaprolactone (OPCL) was successfully prepared by aminooxy terminals of OPCL ligating with aldehyde-terminated PEG (PEG-CHO). Owing to its amphiphilic architecture, PEG-OPCL-PEG self-assembled into the micelles in aqueous media, validated by the measurement of critical micelle concentration (CMC). The MTT assay showed that PEG-OPCL-PEG exhibited low cytotoxicity against NIH/3T3 normal cells. Doxorubicin (DOX) as a model drug was encapsulated into the PEG-OPCL-PEG micelles. Drug release study revealed that the DOX release from micelles was significantly accelerated at mildly acid pH of 5.0 compared to physiological pH of 7.4, suggesting the pH-responsive feature of the drug delivery systems with oxime linkages. Flow cytometry and confocal laser scanning microscopy (CLSM) measurements indicated that these DOX-loaded micelles were easily internalized by living cells. MTT assay against HeLa cancer cells showed DOX-loaded PEG-OPCL-PEG micelles had a high anticancer efficacy. All of these results demonstrate that these polymeric micelles self-assembled from oxime-tethered block copolymers are promising carriers for the pH-triggered intracellular delivery of hydrophobic anticancer drugs.     

Thermoresponsive self-assemblies of cyclic and branched oligosaccharide-block-poly(N-isopropylacrylamide) diblock copolymers into nanoparticles

This paper discusses the thermoresponsive nanoparticles obtained by self-assemblies of nonlinear oligosaccharide-based diblock copolymer systems. These diblock copolymers were synthesized by Cu(I)-catalyzed 1,3-dipolar azide/alkyne cycloaddition ("click" reaction) of propargyl-functionalized β-cyclodextrin (βCyD) and xyloglucooligosaccharide (XGO) with poly(N-isopropylacrylamide) (PNIPAM) having a terminal azido group prepared by atom transfer radical polymerization (ATRP). Elastic and quasi-elastic light scattering analysis of the dibock copolymers in H(2)O indicated that thermodynamic phase transitions of the PNIPAM blocks at their cloud points (T(cp)s ≈ 34 °C), around lower critical solution temperatures (LCSTs), triggered their self-assemblies into the nanoparticles. These nanoparticles had narrow size distributions and small interphases (i.e., sharp boundaries). The mean hydrodynamic radii (R(h)s) of the βCyD and XGO-based nanoparticles were determined to be around 150 and 250 nm upon slow heating (i.e., step-by-step heating), and 364 and 91.5 nm upon fast heating, respectively, depending on a predominance of the interchain association or the intrachain contraction. Transmission electron microscope (TEM) and field emission gun-scanning electron microscopy (FEG-SEM) images of the nanoparticles clearly showed compact spherical nanoparticles whose cores are mainly made with the PNIPAM blocks, whereas the rough shells consist in the oligosaccharidic blocks.