Covalent Grafting of Functionalized MEW Fibers to Silk Fibroin Hydrogels to Obtain Reinforced Tissue Engineered Constructs.

Hydrogels are ideal materials to encapsulate cells, making them suitable for applications in tissue engineering and regenerative medicine. However, they generally do not possess adequate mechanical strength to functionally replace human tissues, and therefore they often need to be combined with reinforcing structures. While the interaction at the interface between the hydrogel and reinforcing structure is imperative for mechanical function and subsequent biological performance, this interaction is often overlooked. Melt electrowriting enables the production of reinforcing microscale fibers that can be effectively integrated with hydrogels. Yet, studies on the interaction between these micrometer scale fibers and hydrogels are limited. Here, we explored the influence of covalent interfacial interactions between reinforcing structures and silk fibroin methacryloyl hydrogels (silkMA) on the mechanical properties of the construct and cartilage-specific matrix production in vitro. For this, melt electrowritten fibers of a thermoplastic polymer blend (poly(hydroxymethylglycolide-co-ε-caprolactone):poly(ε-caprolactone) (pHMGCL:PCL)) were compared to those of the respective methacrylated polymer blend pMHMGCL:PCL as reinforcing structures. Photopolymerization of the methacrylate groups, present in both silkMA and pMHMGCL, was used to generate hybrid materials. Covalent bonding between the pMHMGCL:PCL blend and silkMA hydrogels resulted in an elastic response to the application of torque. In addition, an improved resistance was observed to compression (∼3-fold) and traction (∼40-55%) by the scaffolds with covalent links at the interface compared to those without these interactions. Biologically, both types of scaffolds (pHMGCL:PCL and pMHMGCL:PCL) showed similar levels of viability and metabolic activity, also compared to frequently used PCL. Moreover, articular cartilage progenitor cells embedded within the reinforced silkMA hydrogel were able to form a cartilage-like matrix after 28 days of in vitro culture. This study shows that hybrid cartilage constructs can be engineered with tunable mechanical properties by grafting silkMA hydrogels covalently to pMHMGCL:PCL blend microfibers at the interface.

Mechanistic Study on the Degradation of Hydrolysable Core-Crosslinked Polymeric Micelles.

Core-crosslinked polymeric micelles (CCPMs) are an attractive class of nanocarriers for drug delivery. Two crosslinking approaches to form CCPMs exist: either via a low-molecular-weight crosslinking agent to connect homogeneous polymer chains with reactive handles or via cross-reactive handles on polymers to link them to each other (complementary polymers). Previously, CCPMs based on methoxy poly(ethylene glycol)-b-poly[N-(2-hydroxypropyl) methacrylamide-lactate] (mPEG-b-PHPMAmLacn) modified with thioesters were crosslinked via native chemical ligation (NCL, a reaction between a cysteine residue and thioester resulting in an amide bond) using a bifunctional cysteine containing crosslinker. These CCPMs are degradable under physiological conditions due to hydrolysis of the ester groups present in the crosslinks. The rapid onset of degradation observed previously, as measured by the light scattering intensity, questions the effectiveness of crosslinking via a bifunctional agent. Particularly due to the possibility of intrachain crosslinks that can occur using such a small crosslinker, we investigated the degradation mechanism of CCPMs generated via both approaches using various analytical techniques. CCPMs based on complementary polymers degraded slower at pH 7.4 and 37 °C than CCPMs with a crosslinker (the half-life of the light scattering intensity was approximately 170 h versus 80 h, respectively). Through comparative analysis of the degradation profiles of the two different CCPMs, we conclude that partially ineffective intrachain crosslinks are likely formed using the small crosslinker, which contributed to more rapid CCPM degradation. Overall, this study shows that the type of crosslinking approach can significantly affect degradation kinetics, and this should be taken into consideration when developing new degradable CCPM platforms.

Conversion of an Injectable MMP-Degradable Hydrogel into Core-Cross-Linked Micelles.

In this study, a new type of injectable hydrogel called "HyMic" that can convert into core cross-linked (CCL) micelles upon exposure to matrix metalloproteinases (MMP's), was designed and developed for drug delivery applications. HyMic is composed of CCL micelles connected via an enzyme cleavable linker. To this end, two complementary ABA block copolymers with polyethylene glycol (PEG) as B block were synthesized using atom transfer radical polymerization (ATRP). The A blocks were composed of a random copolymer of N-isopropylacrylamide (NIPAM) and either N-(2-hydroxypropyl)methacrylamide-cysteine (HPMA-Cys) or N-(2-hydroxypropyl) methacrylamide-ethylthioglycolate succinic acid (HPMA-ETSA). Mixing the aqueous solutions of the obtained polymers and rising the temperature above the cloud point of the PNIPAM block resulted in the self-assembly of these polymers into flower-like micelles composed of a hydrophilic PEG shell and hydrophobic core. The micellar core was cross-linked by native chemical ligation between the cysteine (in HPMA-Cys) and thioester (in HPMA-ETSA) functionalities. A slight excess of thioester to cysteine groups (molar ratio 3:2) was used to allow further chemical reactions exploiting the unreacted thioester groups. The obtained micelles displayed a Z-average diameter of 80 ± 1 nm (PDI 0.1), and ζ-potential of -4.2 ± 0.4 mV and were linked using two types of pentablock copolymers of P(NIPAM-co-HPMA-Cys)-PEG-peptide-PEG-P(NIPAM-co-HPMA-Cys) (Pep-NC) to yield hydrogels. The pentablock copolymers were synthesized using a PEG-peptide-PEG ATRP macroinitiator and the peptide midblock (lysine-glycine-proline-glutamine-isoleucine-phenylalanine-glycine-glutamine-lysine (Lys-Gly-Pro-Gln-Gly-Ile-Phe-Gly-Gln-Lys)) consisted of either l- or d-amino acids (l-Pep-NC or d-Pep-NC), of which the l-amino acid sequence is a substrate for matrix metalloproteases 2 and 9 (MMPs 2 and 9). Upon mixing of the CCL micelles and the linker (l/d-Pep-NC), the cysteine functionalities of the l/d-Pep-NC reacted with remaining thioester moieties in the micellar core via native chemical ligation yielding a hydrogel within 160 min as demonstrated by rheological measurements. As anticipated, the gel cross-linked with l-Pep-NC was degraded in 7-45 days upon exposure to metalloproteases in a concentration-dependent manner, while the gel cross-linked with the d-Pep-NC remained intact even after 2 months. Dynamic light scattering analysis of the release medium revealed the presence of nanoparticles with a Z-average diameter of ∼120 nm (PDI < 0.3) and ζ-potential of ∼-3 mV, indicating release of core cross-linked micelles upon HyMic exposure to metalloproteases. An in vitro study demonstrated that the released CCL micelles were taken up by HeLa cells. Therefore, HyMic as an injectable and enzyme degradable hydrogel displaying controlled and on-demand release of CCL micelles has potential for intracellular drug delivery in tissues with upregulation of MMPs, for example, in cancer tissues.

Membrane Solubilization by Styrene-Maleic Acid Copolymers: Delineating the Role of Polymer Length.

Styrene-maleic acid (SMA) copolymers have attracted interest in membrane research because they allow the solubilization and purification of membrane-spanning proteins from biological membranes in the form of native-like nanodisks. However, our understanding of the underlying SMA-lipid interactions is hampered by the fact that SMA preparations are very polydisperse. Here, we obtained fractions of the two most commonly used SMA preparations: SMA 2:1 and SMA 3:1 (both with specified Mw ∼10 kD), with different number-average molecular weight (Mn) and styrene content. The fractionation is based on the differential solubility of styrene-maleic anhydride (SMAnh) in hexane and acetone mixtures. SMAnh fractions were hydrolyzed to SMA and added to lipid self-assemblies. It was found that SMA fractions inserted in monolayers and solubilized vesicles to a different extent, with the highest efficiency being observed for low-Mn SMA polymers. Electron microscopy and dynamic light scattering size analyses confirmed the presence of nanodisks independent of the Mn of the SMA polymers forming the belt, and it was shown that the nanodisks all have approximately the same size. However, nanodisks bounded by high-Mn SMA polymers were more stable than those bounded by low-Mn polymers, as indicated by a better retention of the native lipid thermotropic properties and by slower exchange rates of lipids between nanodisks. In conclusion, we here present a simple method to separate SMAnh molecules based on their Mn from commercial SMAnh blends, which allowed us to obtain insights into the importance of SMA length for polymer-lipid interactions.

Influence of PEGylation of Vitamin-K-Loaded Mixed Micelles on the Uptake by and Transport through Caco-2 Cells.

The aim of the study is to investigate the uptake by and transport through Caco-2 cells of two mixed micelle formulations (based on egg phosphatidylcholine and glycocholic acid) of vitamin K, i.e., with and without DSPE-PEG2000. The uptake of vitamin K and fluorescently labeled mixed micelles with and without PEG coating showed similar kinetics and their uptake ratio remained constant over time. Together with the fact that an inhibitor of scavenger receptor B1 (BLT-1) decreased cellular uptake of vitamin K by ∼80% compared to the uptake in the absence of this inhibitor, we conclude that both types of micelles loaded with vitamin K can be taken up intactly by Caco-2 cells via this scavenger receptor. The amount of vitamin K in chylomicrons fraction from Caco-2 cell monolayers further indicates that mixed micelles (with or without PEGylation) are likely packed into chylomicrons after internalization by Caco-2 cells. Uptake of vitamin K from PEGylated mixed micelles increased four- to five-fold at simulated gastrointestinal conditions. In conclusion, PEGylated mixed micelles are stable upon exposure to simulated gastric conditions, and as a result, they do show overall a higher cellular uptake efficiency of vitamin K as compared to mixed micelles without PEG coating.

Versatile Supramolecular Gene Vector Based on Host-Guest Interaction.

It is a great challenge to arrange multiple functional components into one gene vector system to overcome the extra- and intracellular obstacles for gene therapy. In this study, we developed a supramolecular approach for constructing a versatile gene delivery system composed of adamantyl-terminated functional polymers and a ß-cyclodextrin based polymer. Adamantyl-functionalized low molecular weight PEIs (PEI-Ad) and PEG (Ad-PEG) as well as poly(ß-cyclodextrin) (PCD) were synthesized by one-step chemical reactions. The supramolecular inclusion complex formed from PCD to assemble LMW PEI-Ad4 via host-guest interactions can condense plasmid DNA to form nanopolyplexes by electrostatic interactions. The supramolecular polyplexes can be further PEGylated with Ad-PEG to form inclusion complexes, which showed increased salt and serum stability. In vitro experiments revealed that these supramolecular assembly polyplexes had good cytocompatibility and showed high transfection activity close to that of the commercial ExGen 500 at high dose of DNA. Also, the supramolecular vector system exhibited about 60% silencing efficiency as a siRNA vector. Thus, a versatile effective supramolecular gene vector based on host-guest complexes was fabricated with good cytocompatbility and transfection activity.

Nanoparticles based on a hydrophilic polyester with a sheddable PEG coating for protein delivery.

PURPOSE: To investigate the effect of polyethylene glycol (PEG) in nanoparticles based on blends of hydroxylated aliphatic polyester, poly(D,L-lactic-co-glycolic-co-hydroxymethyl glycolic acid) (PLGHMGA) and PEG-PLGHMGA block copolymers on their degradation and release behavior. METHODS: Protein-loaded nanoparticles were prepared with blends of varying ratios of PEG-PLGHMGA (molecular weight of PEG 2,000 and 5,000 Da) and PLGHMGA, by a double emulsion method with or without using poly(vinyl alcohol) (PVA) as surfactant. Bovine serum albumin and lysozyme were used as model proteins. RESULTS: PEGylated particles prepared without PVA had a zeta potential ranging from ~ -3 to ~-35 mV and size ranging from ~200 to ~600 nm that were significantly dependent on the content and type of PEG-block copolymer. The encapsulation efficiency of the two proteins however was very low (<30%) and the particles rapidly released their content in a few days. In contrast, all formulations prepared with PVA showed almost similar particle properties (size: ~250 nm, zeta potential: ~-1 mV), while loading efficiency for both model proteins was rather high (80-90%). Unexpectedly, independent of the type of formulation, the nanoparticles had nearly the same release and degradation characteristics. NMR analysis showed almost a complete removal of PEG in 5 days which explains these marginal differences. CONCLUSIONS: Protein release and particle degradation are not substantially influenced by the content of PEG, likely because of the fast shedding of the PEG blocks. These PEG shedding particles are interesting system for intracellular delivery of drugs.

Π-π stacking increases the stability and loading capacity of thermosensitive polymeric micelles for chemotherapeutic drugs.

Thermosensitive amphiphilic block copolymers self-assemble into micelles above their lower critical solution temperature in water, however, the micelles generally display mediocre physical stability. To stabilize such micelles and increase their loading capacity for chemotherapeutic drugs, block copolymers with novel aromatic monomers were synthesized by free radical polymerization of N-(2-benzoyloxypropyl methacrylamide (HPMAm-Bz) or the corresponding naphthoyl analogue (HPMAm-Nt), with N-(2-hydroxypropyl) methacrylamide monolactate, using a polyethylene glycol based macroinitiator. The critical micelle temperatures and critical micelle concentrations decreased with increasing the HPMAm-Bz/Nt content. The micelles of 30-50 nm were prepared by heating the polymer aqueous solutions from 0 to 50 °C and were colloidally stable for at least 48 h at pH 7.4 and 37 °C. Paclitaxel and docetaxel encapsulation was performed by mixing drug solutions in ethanol with polymer aqueous solutions and heating from 0 to 50 °C. The micelles had a drug loading capacity up to 34 wt % for docetaxel, which is among the highest loadings reported for polymeric micelles, with loaded micelle sizes ranging from 60 to 80 nm. The micelles without aromatic groups almost completely released loaded paclitaxel in 10 days, whereas the HPMAm-Bz/Nt containing micelles released 50% of the paclitaxel at the same time, which showed a better retention for the drug of the latter micelles. (1)H solid-state NMR spectroscopy data are compatible with π-π stacking between aromatic groups. The empty micelles demonstrated good cytocompatibility, and paclitaxel-loaded micelles showed high cytotoxicity to tumor cells. In conclusion, the π-π stacking effect introduced by aromatic groups increases the stability and loading capacity of polymeric micelles.

Controlled release of octreotide and assessment of peptide acylation from poly(D,L-lactide-co-hydroxymethyl glycolide) compared to PLGA microspheres.

PURPOSE: To investigate the in vitro release of octreotide acetate, a somatostatin agonist, from microspheres based on a hydrophilic polyester, poly(D,L-lactide-co-hydroxymethyl glycolide) (PLHMGA). METHODS: Spherical and non-porous octreotide-loaded PLHMGA microspheres (12 to 16 µm) and loading efficiency of 60-70% were prepared by a solvent evaporation. Octreotide release profiles were compared with commercial PLGA formulation (Sandostatin LAR(®)); possible peptide modification with lactic, glycolic and hydroxymethyl glycolic acid units was monitored. RESULTS: PLHMGA microspheres showed burst release (~20%) followed by sustained release for 20-60 days, depending on the hydrophilicity of the polymer. Percentage of released loaded peptide was high (70-90%); > 60% of released peptide was native octreotide. PLGA microspheres did not show peptide release for the first 10 days, after which it was released in a sustained manner over the next 90 days; > 75% of released peptides were acylated adducts. CONCLUSIONS: PLHMGA microspheres are promising controlled systems for peptides with excellent control over release kinetics. Moreover, substantially less peptide modification occurred in PLHMGA than in PLGA microspheres.

Tuning the size of all-HPMA polymeric micelles fabricated by solvent extraction.

The size of polymeric micelles crucially affects their tumor accumulation, penetration and antitumor efficacy. In the present study, micelles were formed based on amphiphilic poly(N-2-hydroxypropyl methacrylamide)-block-poly(N-2-benzoyloxypropyl methacrylamide) (p(HPMAm)-b-p(HPMAm-Bz)) via the solvent extraction method, and factors impacting micelle size were systematically studied, including the molecular weight of the polymers, homopolymer content, and processing methods (i.e., batch process versus continuous microfluidics). The formation of core-shell structured micelles was demonstrated by light scattering, sedimentation velocity and electron microscopy analysis. Micellar size and aggregation number increased with decreasing the molecular weight ratio of the hydrophilic/hydrophobic block. The presence of hydrophobic p(HPMAm-Bz) homopolymer and high copolymer concentration increased micelle size, while the presence of hydrophilic p(HPMAm) homopolymer did not affect micellar size. Regarding processing conditions, it was found that the use of tetrahydrofuran and acetone as solvents for the polymers resulted in larger micelles, likely due to their relatively high water-solvent interaction parameters as compared to other solvents tested, i.e., dimethylformamide, dimethylacetamide, and dimethyl sulfoxide. Among the latter, only dimethylformamide led to micelles with a narrow polydispersity. Addition of dimethylformamide to an aqueous solvent and faster mixing of two solvents using microfluidics favored the formation of smaller micelles. In conclusion, our results show that the size of all-HPMA polymeric micelles can be easily tailored from 40 to 120 nm by varying the formulation properties and processing parameters.

Post-loading of proangiogenic growth factors in PLGA microspheres.

Active self-encapsulation (ASE) is a recently developed post-loading method based on absorption of (positively charged) proteins in microporous PLGA microspheres loaded with negatively charged polysaccharides (trapping agents). The aim of this study was to investigate ASE for simultaneous loading and controlled release of multiple growth factors. For this purpose, vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF) and insulin-like growth factor (IGF) were loaded in microspheres containing high molecular weight dextran sulfate (HDS) as trapping agent; loading was performed in a concentrated growth factor solution of low ionic strength and of pH 5 under conditions at which the proteins are positively charged. Subsequent pore closure was induced by incubation of the growth factor-loaded microspheres at 42.5 °C, i.e. above the Tg of (hydrated) PLGA (~30 °C). A 1:1:1 combination of VEGF, FGF and IGF was loaded with high loading (4.3%) and loading efficiency (91%). The in vitro release kinetics and bioactivity of loaded growth factors were studied for 4 weeks using ELISA and an endothelial cell proliferation assay, respectively. While IGF was released quickly, VEGF and FGF were continuously released for 4 weeks in their bioactive form, whereby a growth factor combination had a synergistic angiogenic effect. Therefore, ASE is a suitable method for co-loading growth factors which can provide sustained release profiles of bioactive growth factors, which is attractive for vascularization of biomaterial implants.

Internalization and Transport of PEGylated Lipid-Based Mixed Micelles across Caco-2 Cells Mediated by Scavenger Receptor B1.

The aim of this study was to get insight into the internalization and transport of PEGylat-ed mixed micelles loaded by vitamin K, as mediated by Scavenger Receptor B1 (SR-B1) that is abundantly expressed by intestinal epithelium cells as well as by differentiated Caco-2 cells. Inhibition of SR-B1 reduced endocytosis and transport of vitamin-K-loaded 0%, 30% and 50% PEGylated mixed micelles and decreased colocalization of the micelles with SR-B1. Confocal fluorescence microscopy, fluorescence-activated cell sorting (FACS) analysis, and surface plasmon resonance (SPR) were used to study the interaction between the mixed micelles of different compositions (varying vitamin K loading and PEG content) and SR-B1. Interaction of PEGylated micelles was independent of the vitamin K content, indicating that the PEG shell prevented vitamin K exposure at the surface of the micelles and binding with the receptor and that the PEG took over the micelles' ability to bind to the receptor. Molecular docking calculations corroborated the dual binding of both vita-min K and PEG with the binding domain of SR-B1. In conclusion, the improved colloidal stability of PEGylated mixed micelles did not compromise their cellular uptake and transport due to the affinity of PEG for SR-B1. SR-B1 is able to interact with PEGylated nanoparticles and mediates their subsequent internalization and transport.

Lyophilization stabilizes clinical-stage core-crosslinked polymeric micelles to overcome cold chain supply challenges.

BACKGROUND: CriPec technology enables the generation of drug-entrapped biodegradable core-crosslinked polymeric micelles (CCPM) with high drug loading capacity, tailorable size, and drug release kinetics. Docetaxel (DTX)-entrapped CCPM, also referred to as CPC634, have demonstrated favorable pharmacokinetics, tolerability, and enhanced tumor uptake in patients. Clinical efficacy evaluation is ongoing. CPC634 is currently stored (shelf life > 5 years) and shipped as a frozen aqueous dispersion at temperatures below -60°C, in order to prevent premature release of DTX and hydrolysis of the core-crosslinks. Consequently, like other aqueous nanomedicine formulations, CPC634 relies on cold chain supply, which is unfavorable for commercialization. Lyophilization can help to bypass this issue. METHODS AND RESULTS: Freeze-drying methodology for CCPM was developed by employing CPC634 as a model formulation, and sucrose and trehalose as cryoprotectants. We studied the residual moisture content and reconstitution behavior of the CPC634 freeze-dried cake, as well as the size, polydispersity index, morphology, drug retention, and release kinetics of reconstituted CPC634. Subsequently, the freeze-drying methodology was validated in an industrial setting, yielding a CPC634 freeze-dried cake with a moisture content of less than 0.1 wt%. It was found that trehalose-cryoprotected CPC634 could be rapidly reconstituted in less than 5 min at room temperature. Critical quality attributes such as size, morphology, drug retention, and release kinetics of trehalose-cryoprotected freeze-dried CPC634 upon reconstitution were identical to those of non-freeze-dried CPC634. CONCLUSION: Our findings provide proof-of-concept for the lyophilization of drug-containing CCPM and our methodology is readily translatable to large-scale manufacturing for future commercialization.

Hydrophilic polyester microspheres: effect of molecular weight and copolymer composition on release of BSA.

PURPOSE: To study the release of a model protein, bovine serum albumin (BSA), from microspheres of an hydroxylated aliphatic polyester, poly(lactic-co-hydroxymethyl glycolic acid) (PLHMGA). METHODS: BSA-loaded microspheres were prepared by a double emulsion solvent evaporation method. The effect of copolymer composition and the molecular weight of the copolymer on in vitro release and degradation were studied. The integrity of the released BSA was studied by fluorescence spectroscopy and size exclusion chromatography (SEC). RESULTS: Microspheres prepared from PLHMGA with 50% hydroxymethyl glycolic acid (HMG) showed a burst release followed by a sustained release in 5-10 days. PLHMGA microspheres prepared from a copolymer with 35% and 25% HMG showed a sustained release of BSA up to 80% for 30 and 60 days, respectively. The release of BSA was hardly affected by the molecular weight of the polymer. Fluorescence spectroscopy and SEC showed that the released BSA preserved its structural integrity. Microspheres were fully degradable, and the degradation time increased from approximately 20 days to 60 days when the HMG content decreased from 50% to 25%. CONCLUSIONS: Taking the degradation and release data together, it can be concluded that the release of BSA from PLHMGA microspheres is governed by degradation of the microspheres.

Porous titanium fiber mesh with tailored elasticity and its effect on stromal cells.

Porous titanium fiber mesh (TFM) is considered a suitable scaffold material for bone reconstruction. Also, TFM can be used to cover the surface of bone-anchored devices, that is, orthopedic or dental implants. The titanium fiber size has an effect of the stiffness as well as porosity of the titanium mesh, which can influence the behavior of bone forming cells. Therefore, the aim of this study was to vary TFM composition, in order to achieve different stiffness, and to assess the effects of such variation on the behavior of bone marrow-derived stromal cells (BMSCs). With that purpose, nine types of TFM (porosities 60-87%; fiber size 22-50 µm), were examined for their mechanical properties as well as their effect on the proliferation and differentiation of rat bone marrow-derived stromal cells (rBMSCs) up to 21 days. Dynamic mechanical analysis revealed that the stiffness of TFM were lower than of solid titanium and decreased with larger fiber sizes. The stiffness could effectively be tailored by altering fiber properties, which altered the pore simultaneously. For the 22 and 35 µm size fiber meshes with the highest porosity, the stiffness closely matched the value found in literature for cortical bone. Finally, all tested TFM types supported the growth and differentiation of rBMSCs. We concluded that TFM material has been proven cytocompatible. Further preclinical studies are needed to assess which TFM type is most suitable as clinical use for bone ingrowth and bone regeneration.

Dithiolane-Crosslinked Poly(ε-caprolactone)-Based Micelles: Impact of Monomer Sequence, Nature of Monomer, and Reducing Agent on the Dynamic Crosslinking Properties.

Dithiolanes are used to obtain dynamic and reversible crosslinks between polymer chains. Copolymers of two different dithiolane-containing cyclic carbonate monomers and ε-caprolactone (CL) were synthesized by ring-opening polymerization using a methoxy-poly(ethylene glycol) (mPEG) initiator and different catalysts (diphenyl phosphate (DPP), methanesulfonic acid (MSA), 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), or Sn(Oct)2). Each catalyst required a different temperature, which had a pronounced influence on the reactivity ratio of the monomers and occurrence of transesterification reactions and, therefore, the monomer sequence. Self-crosslinkable copolymers were obtained when the dithiolane units were connected closely to the polymer backbone, whereas the presence of a linker unit between the dithiolane and the backbone prevented self-crosslinking. The former amphiphilic PEGylated block copolymers formed micelles by nanoprecipitation in the aqueous environment and crosslinked spontaneously by disulfide exchange during subsequent dialysis. These dithiolane-crosslinked micelles showed reduction-responsive dissociation in the presence of 10 mM glutathione, making them promising drug delivery systems for the intracellularly triggered cargo release.

Biotin-decorated all-HPMA polymeric micelles for paclitaxel delivery.

To avoid poly(ethylene glycol)-related issues of nanomedicines such as accelerated blood clearance, fully N-2-hydroxypropyl methacrylamide (HPMAm)-based polymeric micelles decorated with biotin for drug delivery were designed. To this end, a biotin-functionalized chain transfer agent (CTA), 4-cyano-4-[(dodecylsulfanylthiocarbonyl)-sulfanyl]pentanoic acid (biotin-CDTPA), was synthesized for reversible addition-fragmentation chain-transfer (RAFT) polymerization. Amphiphilic poly(N-2-hydroxypropyl methacrylamide)-block-poly(N-2-benzoyloxypropyl methacrylamide) (p(HPMAm)-b-p(HPMAm-Bz)) with molecular weights ranging from 8 to 24 kDa were synthesized using CDTPA or biotin-CDTPA as CTA and 2,2'-azobis(2-methylpropionitrile) as initiator. The copolymers self-assembled in aqueous media into micelles with sizes of 40-90 nm which positively correlated to the chain length of the hydrophobic block in the polymers, whereas the critical micelle concentrations decreased with increasing hydrophobic block length. The polymer with a molecular weight of 22.1 kDa was used to prepare paclitaxel-loaded micelles which had sizes between 61 and 70 nm, and a maximum loading capacity of around 10 wt%. A549 lung cancer cells overexpressing the biotin receptor, internalized the biotin-decorated micelles more efficiently than non-targeted micelles, while very low internalization of both types of micelles by HEK293 human embryonic kidney cells lacking the biotin receptor was observed. As a consequence, the paclitaxel-loaded micelles with biotin decoration exhibited stronger cytotoxicity in A549 cells than non-targeted micelles. Overall, a synthetic pathway to obtain actively targeted poly(ethylene glycol)-free micelles fully based on a poly(HPMAm) backbone was established. These polymeric micelles are promising systems for the delivery of hydrophobic anticancer drugs.

Folate decorated polymeric micelles for targeted delivery of the kinase inhibitor dactolisib to cancer cells.

One of the main challenges in clinical translation of polymeric micelles is retention of the drug in the nanocarrier system upon its systemic administration. Core crosslinking and coupling of the drug to the micellar backbone are common strategies to overcome these issues. In the present study, polymeric micelles were prepared for tumor cell targeting of the kinase inhibitor dactolisib which inhibits both the mammalian Target of Rapamycin (mTOR) kinase and phosphatidylinositol-3-kinase (PI3K). We employed platinum(II)-based linker chemistry to couple dactolisib to the core of poly(ethylene glycol)-b-poly(acrylic acid) (PEG-b-PAA) polymeric micelles. The formed dactolisib-PEG-PAA unimers are amphiphilic and self-assemble in an aqueous milieu into core-shell polymeric micelles. Folate was conjugated onto the surface of the micelles to yield folate-decorated polymeric micelles which can target folate receptor over-expressing tumor cells. Fluorescently labeled polymeric micelles were prepared using a lissamine-platinum complex linked in a similar manner as dactolisib. Dactolisib polymeric micelles showed good colloidal stability in water and released the coupled drug in buffers containing chloride or glutathione. Folate decorated micelles were avidly internalized by folate-receptor-positive KB cells and displayed targeted cellular cytotoxicity at 50-75 nM IC50. In conclusion, we have prepared a novel type of folate-receptor targeted polymeric micelles in which platinum(II) linker chemistry modulates drug retention and sustained release of the coupled inhibitor dactolisib.

Phenylglyoxaldehyde-Functionalized Polymeric Sorbents for Urea Removal from Aqueous Solutions.

For realization of a wearable artificial kidney based on regeneration of a small volume of dialysate, efficient urea removal from dialysate is a major challenge. Here a potentially suitable polymeric sorbent based on phenylglyoxaldehyde (PGA), able to covalently bind urea under physiological conditions, is described. Sorbent beads containing PGA groups were obtained by suspension polymerization of either styrene or vinylphenylethan-1-one (VPE), followed by modification of the aromatic groups of poly(styrene) and poly(VPE) into PGA. It was found that PGA-functionalized sorbent beads had maximum urea binding capacities of 1.4-2.2 mmol/g and removed ∼0.6 mmol urea/g in 8 h at 37 °C under static conditions from urea-enriched phosphate-buffered saline, conditions representative of dialysate regeneration. This means that the daily urea production of a dialysis patient can be removed with a few hundred grams of this sorbent which, is an important step forward in the development of a wearable artificial kidney.

Synthesis and characterization of hydroxyl-functionalized caprolactone copolymers and their effect on adhesion, proliferation, and differentiation of human mesenchymal stem cells.

The aim of this study was to develop new hydrophilic polyesters for tissue engineering applications. In our approach, poly(benzyloxymethyl glycolide-co-epsilon-caprolactone)s (pBHMG-CLs) were synthesized through melt copolymerization of epsilon-caprolactone (CL) and benzyl-protected hydroxymethyl glycolide (BHMG). Deprotection of the polymers yielded copolymers with pendant hydroxyl groups, poly(hydroxymethylglycolide-co-epsilon-caprolactone) (pHMG-CL). The synthesized polymers were characterized by GPC, NMR, and DSC techniques. The resulting copolymers consisting of up to 10% of HMG monomer were semicrystalline with a melting temperature above body temperature. Water contact angle measurements of polymeric films showed that increasing HMG content resulted in higher surface hydrophilicity, as evidenced from a decrease in receding contact angle from 68 degrees for PCL to 40 degrees for 10% HMG-CL. Human mesenchymal stem cells showed good adherence onto pHMG-CL films as compared to the more hydrophobic PCL surfaces. The cells survived and were able to differentiate toward osteogenic lineage on pHMG-CL surfaces. This study shows that the aforementioned hydrophilic polymers are attractive candidates for the design of scaffolds for tissue engineering applications.