Thermoresponsive nanostructured polycarbonate block copolymers as biodegradable therapeutic delivery carriers.

Water-soluble, thermoresponsive block copolymers based on a biodegradable platform were synthesized by the ring opening polymerization of cyclic carbonate monomers functionalized with hydrophilic and hydrophobic groups for application as nanocarriers in medicine. The approach based on cyclic carbonate monomers derived from 2,2-bis(methylol)propionic acid (bis-MPA) allowed a simple and versatile route to functional monomers capable of undergoing ring opening polymerization (ROP). The resulting polymers possessed the predicted molecular weights based on the molar ratio between monomers to initiators and the narrow molecular weight distributions. Transmittance measurement for aqueous polymer solutions provided an evidence for temperature-responsiveness with lower critical solution temperature (LCST) in the range of 36 °C-60 °C, depending on the molecular weight of hydrophilic poly(ethylene glycol) (PEG) chains, compositions of copolymers, molar ratios of hydrophilic to hydrophobic monomers in the corona, and the hydrophobic core. This study showed synthetic advancement toward the design and preparation of biodegradable thermoresponsive polymers with extremely low CMC values for injectable drug delivery systems. TRC350-10,30,60, which possessed an LCST of 36 °C in PBS, was identified as a useful model polymer. Paclitaxel, an anti-cancer drug, was loaded into the micelles efficiently, giving rise to nano-sized particles with a narrow size distribution. Paclitaxel release from the micelles was faster, and cellular uptake of the micelles was higher at the body temperature (i.e. 37 °C) as compared to a temperature below the LCST. While the polymer was not cytotoxic, paclitaxel-loaded micelles killed HepG2 human liver carcinoma cells more efficiently at the body temperature as compared to free paclitaxel and paclitaxel-loaded micelles at the temperature below the LCST. These micelles are ideally suited to deliver anti-cancer drugs to tumor tissues through local injection.

Biodegradable nanostructures with selective lysis of microbial membranes.

Macromolecular antimicrobial agents such as cationic polymers and peptides have recently been under an increased level of scrutiny because they can combat multi-drug-resistant microbes. Most of these polymers are non-biodegradable and are designed to mimic the facially amphiphilic structure of peptides so that they may form a secondary structure on interaction with negatively charged microbial membranes. The resulting secondary structure can insert into and disintegrate the cell membrane after recruiting additional polymer molecules. Here, we report the first biodegradable and in vivo applicable antimicrobial polymer nanoparticles synthesized by metal-free organocatalytic ring-opening polymerization of functional cyclic carbonate. We demonstrate that the nanoparticles disrupt microbial walls/membranes selectively and efficiently, thus inhibiting the growth of Gram-positive bacteria, methicillin-resistant Staphylococcus aureus (MRSA) and fungi, without inducing significant haemolysis over a wide range of concentrations. These biodegradable nanoparticles, which can be synthesized in large quantities and at low cost, are promising as antimicrobial drugs, and can be used to treat various infectious diseases such as MRSA-associated infections, which are often linked with high mortality.

Hydrogen bonding-enhanced micelle assemblies for drug delivery.

Ring-opening polymerization (ROP) of functionalized cyclic carbonates derived from 2,2-bis(methylol)propionic acid (bis-MPA) allows for incorporation of H-bonding urea-functional groups into block copolymers with a potential application of supramolecular drug-delivery systems. The strong H-bonding functionalities of poly(ethylene glycol)-block-poly(ethyl-random-urea carbonate) (PEG-P(E(1-x)-U(x))C) block copolymers not only lowered critical micelles concentration (cmc) of the block copolymer (to 1/4x) in aqueous environment compared to conventional PEG-poly(trimethylene carbonate) (PEG-PTMC) block copolymer without the non-covalent stabilization, but also improved kinetic stability of micelles and Dox-loaded micelles in the presence of a destabilizing agent. It was observed that the incorporation of anticancer drug doxorubicin affected the micellization process of block copolymers in water and caused a sudden increase in sizes of drug-loaded micelles above 200 nm. This phenomenon that can be a significant drawback in drug delivery applications was considerably mitigated in urea-bearing block copolymer/Dox micelles with simultaneously accompanying a significant improvement in drug loading. In vitro drug release profile showed that the increase in urea content led to a slight decrease in Dox release rate. Block copolymer did not have any significant cytotoxicity against HEK293 and HepG2 cells up to 400 mg/L. Importantly, Dox-loaded micelles exerted cytotoxic effect against HepG2 cells.

Delivery of anticancer drugs using polymeric micelles stabilized by hydrogen-bonding urea groups.

Polymeric micelles comprising a hydrogen-bonding core were formed from block copolymers with pendant urea groups and explored as drug delivery vehicles. The amphiphilic block copolymers were synthesized by organocatalytic ring opening polymerization (ROP) of urea-functionalized cyclic carbonates from a poly(ethylene glycol) macroinitiator. The urea functionality was incorporated because its ability to increase the hydrophobic core's affinity toward polar organic compounds through intermolecular hydrogen bonding. Doxorubicin (DOX), a lipophilic anticancer drug with hydrogen-bonding functionalities, was systematically incorporated into the micelle's hydrophobic interior via hydrogen bonding to the functionalized monomers. Micelles employing urea groups were found to more efficiently interact with DOX thus allowing increased drug loading capacity while maintaining a desirable micellular size. More importantly, while DOX-loaded micelles were shown to kill HepG2 human liver carcinoma cell lines efficiently, all of the polymers were non-cytotoxic.

Oligocarbonate molecular transporters: oligomerization-based syntheses and cell-penetrating studies.

A new family of guanidinium-rich molecular transporters featuring a novel oligocarbonate backbone with 1,7-side chain spacing is described. Conjugates can be rapidly assembled irrespective of length in a one-step oligomerization strategy that can proceed with concomitant introduction of probes (or by analogy drugs). The new transporters exhibit excellent cellular entry as determined by flow cytometry and fluorescence microscopy, and the functionality of their drug delivery capabilities was confirmed by the delivery of the bioluminescent small molecule probe luciferin and turnover by its intracellular target enzyme.

A supramolecularly assisted transformation of block-copolymer micelles into nanotubes.

Once around the block: Incorporation of a rigid hydrogen-bonding benzamide unit, placed at the interface between two polymer blocks, in poly(ethylene glycol) (PEG)-(thio)urea-poly(L-lactide) (PLLA) block copolymers transforms the morphology of the block copolymers, from spherical micelles, as formed by PEG-PLLA diblock copolymers, into nanotubes in solution.

Simple approach to stabilized micelles employing miktoarm terpolymers and stereocomplexes with application in paclitaxel delivery.

A simple and versatile approach to miktoarm co- and terpolymers from carbonate functional oligomers is described. The key building block employed is a carboxylic acid functional cyclic carbonate, derived from 2,2-bis(methylol)propionic acid, that was readily coupled to a hydroxyl functional monomethylether poly(ethylene glycol) oligomer. Ring-opening of the cyclic carbonate using functional amines generates a carbamate linkage bearing a functional group capable of initiating either controlled radical or ring-opening polymerization, together with a primary hydroxyl group for ring-opening polymerization. Two tandem polymerization steps were possible which add the second two arms, thus generating the targeted ABC miktoarm terpolymer. The resulting amphiphilic miktoarm terpolymers containing poly(D- and L-lactide) formed polylactide stereocomplexes in the bulk. In aqueous solution, the stereocomplex mixture of Y-shaped miktoarm copolymers, poly(ethylene glycol)-poly(D-lactide)-poly(D-lactide) and poly(ethylene glycol)-poly(L-lactide)-poly(L-lactide), or the stereoblock miktoarm poly(ethylene glycol)-poly(D-lactide)-poly(L-lactide) form stabilized micelles with a significantly lower critical micelle concentration than those derived from conventional stereo regular linear or Y-shaped amphiphiles. This simple and versatile approach provides a useful synthetic route to complex macromolecular architectures that can assemble into stable micelles. These micelles provide high capacity for loading of the anticancer drug paclitaxel and possess narrow size distribution as well as unique structure, leading to sustained and near zero-ordered release of drug without significant initial burst.

Organocatalytic approach to amphiphilic comb-block copolymers capable of stereocomplexation and self-assembly.

Biocompatible amphiphilic block copolymers comprised of poly(ethylene glycol) (PEG) as the hydrophilic component and a poly(methylcarboxytrimethylene carbonate) (PMTC) as a hydrophobic backbone having either poly(L-lactide) (L-PLA) or poly(D-lactide) (D-PLA) branches were prepared by organocatalytic ring-opening polymerization (ROP). The polycarbonate backbone was prepared by copolymerization of two different MTC-type monomers (MTCs) including a tetrahydropyranyloxy protected hydroxyl group, a masked initiator for a subsequent ROP step. Interestingly, the organic catalyst used in the ROP of MTCs was also effective for acetylation of the hydroxyl end-groups by the addition of acetic anhydride added after polymerization. Acidic deprotection of the tetrahydropyranyloxy (THP) protecting group on the carbonate chain generated hydroxyl functional groups that served as initiators for the ROP of either D- or L-lactide. Comb-shaped block copolymers of predictable molecular weights and narrow polydispersities (approximately 1.3) were prepared with up to 8-PLA branches. Mixtures of the D- and L-lactide based copolymers were studied to understand the effect of noncovalent interactions or stereocomplexation on the properties.

Tagging alcohols with cyclic carbonate: a versatile equivalent of (meth)acrylate for ring-opening polymerization.

Cyclic carbonate monomers based on a single biocompatible scaffold allow for incorporation of a wide range of functional groups into macromolecules via ring-opening polymerization.

Hierarchical assembly of nanostructured organosilicate networks via stereocomplexation of block copolymers.

The effect of the stereochemistry of polylactide (PLA)-based block copolymers on templated inorganic nanostructures has been investigated from the self-assembly of a stereoisomer pair/organosilicate mixture followed by organosilicate vitrification and copolymer thermolysis. Isomeric PLA homopolymers, block copolymers, and a stereoblock copolymer were prepared by ring-opening polymerization of D-, L-, or rac-lactide using an organocatalytic catalyst. Both differential scanning calorimetry and atomic force microscopy showed the formation of a stereocomplex between enantiomeric stereoisomers, that is, block copolymer/block copolymer and block copolymer/homopolymer mixtures as well as a stereoblock copolymer. The unique noncovalent interactions driven by stereocomplexation of D- and L-lactide provided supramolecular structures with a hierarchical order as characterized by distinctive vertical and horizontal growth of toroidal nanostructured inorganic features. This study demonstrates the potential of hierarchically assembling suprastructures that bridge the nano- to mesoscale feature sizes in the design of tunable functional nanomaterials suitable for future applications of microelectronics, material science, and bioengineering.

New ground for organic catalysis: a ring-opening polymerization approach to hydrogels.

Herein, we describe an organocatalytic living polymerization approach to network and subsequent hydrogel formation. Cyclic carbonate-functionalized macromolecules were ring-opened using an alcoholic initiator in the presence of an organic catalyst, amidine 1,8-diazabicyclo[5.4.0]undec-7-ene. A model reaction for the cross-linking identified monomer concentration-dependent reaction regimes, and enhanced kinetic control was demonstrated by introducing a co-monomer, trimethylene carbonate. The addition of the co-monomer facilitated near-quantitative conversion of monomer to polymer (>96%). Resulting poly(ethylene glycol) networks swell significantly in water, and an open co-continuous (water-gel) porous structure was observed by scanning electron microscopy. The organocatalytic ring-opening polymerization of cyclic carbonate functional macromonomers using alcoholic initiators provides a simple, efficient, and versatile approach to hydrogel networks.

Molecular dynamics of a biodegradable biomimetic ionomer studied by broadband dielectric spectroscopy.

Broadband dielectric spectroscopy was used to investigate the bulk molecular dynamics of a recently developed biodegradable biomimetic ionomer potentially useful for biomedical applications. Isothermal dielectric spectra were gathered for a phosphoryl choline (PC)-functionalized poly(trimethylene carbonate) (PTMC) ionomer and unfunctionalized PTMC at temperatures ranging from 2 to 60 degrees C over a broad frequency range of 10(-3) to 10(6) Hz. Four relaxations were clearly identified, two of which were shown to stem from the PTMC polymer backbone. A detailed analysis showed that the formation of zwitterionic aggregates was responsible for the material's bulk functionality and that bulk conduction processes may provide useful information for assessing the PC ionomer as a candidate for drug delivery applications. Finally, it was concluded that absorbed water concentrates around the aggregates, resulting in an increased mobility of the PC end-groups.

Organocatalytic ring opening polymerization of trimethylene carbonate.

A variety of organocatalysts has been surveyed in the ring opening polymerization of trimethylene carbonate. Excellent control was found for several of these catalysts yielding well-defined polycarbonates with molecular weights up to 50,000 g mol(-1), polydispersities below 1.08, and high end-group fidelity. Melt or bulk polymerization was accomplished without loss of control of molecular weight or polydispersity, and random ester-carbonate bulk polymerizations were also demonstrated. Furthermore, by combining disparate polymerization techniques using bifunctional initiators, the mild polymerization conditions allow for the preparation of new block copolymers. Hydrogen-bond activation of monomer and initiator/propagating species is proposed as the underlying mechanism, which can be tuned to mitigate adverse side reactions.

Photoelectron spectroscopy studies of the functionalization of a silicon surface with a phosphorylcholine-terminated polymer grafted onto (3-aminopropyl)trimethoxysilane.

The structure of a biomimetic phosphorylcholine (PC)-functionalized poly(trimethylene carbonate) (PC-PTMC-PC), linked to a silicon substrate through an aminolysis reaction at 120 degrees C with (3-aminopropyl)trimethoxysilane (APTMS), was studied using photoelectron spectroscopy. Two chemical states were found for the unreacted APTMS amine, a neutral state and a protonated state, where the protonated amine on average was situated closer to the silicon substrate than the neutral amine. The experiments also indicated the presence of a third chemical state, where amines interact with unreacted silanol groups. The PTMC chains of the grafted films were found to consist of only 2-3 repeat units, with the grafted chains enriched in the zwitterionic end group, suggesting that these groups are attracted to the surface. This was further supported by the experiments showing that the PC groups were situated deeper within the film.

Organocatalytic living ring-opening polymerization of cyclic carbosiloxanes.

[reaction: see text] An organocatalytic route to narrowly dispersed poly(carbosiloxanes) of predictable molecular weight and end group fidelity is described. N-Heterocyclic carbenes (NHC) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) catalyze the ring opening of cyclic carbosiloxanes. The pK(b) of the catalyst is important in preventing adverse transetherification reactions and obtaining well-defined polymers. Mechanistic studies indicate that hydrogen bonding to TBD or the NHC activates alcohols or silanols for ring-opening reactions.

Biocompatible and biodegradable phosphorylcholine ionomers with reduced protein adsorption and cell adhesion.

In this paper a recently developed biodegradable phosphorylcholine ionomer (PC ionomer) was evaluated in different biological environments with a focus on the adsorption of proteins (fibrinogen) and the adhesion of cells. Our results have shown that the polar phosphoryl choline (PC) group may be enriched at the surface of cast films with an added hydrophilic environment. X-ray photoelectron spectroscopy confirmed the surface depletion of PC groups in dry conditions, as nitrogen and phosphorous atoms were found in the bulk of the material but not at the outermost surface layer. The surface enrichment leads to a strongly hydrophilic surface that prevents the adsorption of proteins and reduces the adhesion of cells. The non-functional and hydrophobic reference poly(trimethylene carbonate) (PTMC) adsorbs both proteins and cells, thus the wetting and low adhesion behavior of the PC ionomer can be attributed to the introduced PC functionality. Since the in vivo acceptance of biomaterials is determined by their ability to withstand protein adsorption the PC ionomer described in this paper is highly interesting for a number of in vivo applications in which the adsorption of proteins may be critical, for example, blood contact events.

Synthetic biodegradable ionomers that engulf, store, and deliver intact proteins.

Telechelic anionic and cationic biodegradable ionomers capable of loading, storing, and releasing proteins are presented. Two different ionomers have been synthesized with either anionic or cationic end groups. The reaction was done quantitatively as shown by (1)H NMR. The swelling properties of the hydrophobic poly(trimethylene carbonate) polymer are contributed to the ionic end groups that display hydrophilic properties. Depending on the molecular weight of the ionomer, and also on the ionic charge, the materials swell differently in water, from approximately 50% for M(w) = 12 000 g/mol to approximately 500% when dealing with 2000 g/mol. The high swelling led us to believe that it would be possible to load and release proteins preferably in a still active form. As models, two different proteins were chosen: hemoglobin and cytochrome c. The swelling and release study shows that both ionomers possess the capability to adsorb and later release the proteins with retained structure. Release measurements from both the swollen and dried states have been evaluated with similar results, showing that the dried state seems to release a little bit less than the swollen one. These kinds of materials should be interesting for a wide variety of applications where drug and protein release is wanted, as well as in applications such as protein separation media.