Stability and Release Performance of a Series of Pegylated Copolymeric Micelles

Purpose. The aim of this work is to evaluate the capability of a series of biocompatible amphiphilic copolymers as a nano-sized drug carrier. Methods. The influences of the type of lactone monomer, the feed molar ratios of lactone/PEG, and the molecular weight of PEG on the performance and release behavior of micelles are investigated. Results. These pegylated amphiphilic copolymers efficiently form micelles with a low CMC value in the range of 10^–6-10^–7 M. The average particle size of micelles is 100 nm. The phenomenon of increasing particle size as increasing the chain length of poly(lactone) block is observed. The different hydrophobicity, based on chemical structure of poly(lactone), accounts for different interaction strength between indomethacin and hydrophobic inner core, which further influences the drug loading in copolymeric micelles and their release character. In addition, the PCL/PEG/PCL micellar solutions maintain their sizes at 4°C for 8 weeks without occurring significant aggregation or dissociation. Conclusions. A series of biocompatible pegylated amphiphilic copolymers have been elucidated possessing micellization potential to form nano-sized micelles in an aqueous environment, which enable incorporate hydrophobic drug and regulate drug release.     

Copolymer micelles and nanospheres with different in vitro stability demonstrate similar paclitaxel pharmacokinetics

Paclitaxel loaded amphiphilic block copolymer nanoparticles have been demonstrated to enhance the aqueous solubility and improve the toxicity profile as compared to the commercially available product Taxol; however, in many cases long circulation of the drug is not achieved due to rapid partitioning of the drug from the carrier and/or carrier instability upon injection. In this work we investigated the effect of increasing the hydrophobic block length of methoxy poly(ethylene glycol)-block-poly(ε-caprolactone) (MePEG-b-PCL) copolymers on the physicochemical properties and in vitro stability of the formed nanoparticles as well as the pharmacokinetics and biodistribution of both the copolymer and solubilized drug. We hypothesized that copolymers composed of high molecular weight hydrophobic blocks (MePEG???-b-PCL???) that form nanoparticles with a kinetically "frozen core" (which we term nanospheres) would better retain their PTX payload as compared to micelles composed of shorter hydrophobic blocks (MePEG???-b-PCL??), thus leading to prolonged drug circulation. Nanospheres solubilized PTX more efficiently, released the drug in a more sustained fashion and were characterized by enhanced stability and drug retention in the presence of plasma proteins as compared to micelles. Using radiolabeled copolymers and PTX, it was found that, upon injection, MePEG???-b-PCL??? circulated for longer than MePEG???-b-PCL??; however, the drug was rapidly eliminated from the blood regardless of the formulation. These results suggest that, despite formulation in more stable nanospheres, PTX was still rapidly extracted from these nanoparticles.     

Solubilization of hydrophobic drugs by methoxy poly(ethylene glycol)-block-polycaprolactone diblock copolymer micelles: theoretical and experimental data and correlations

The solubilization of five model hydrophobic drugs by a series of micelle-forming, water-soluble methoxy poly(ethylene glycol)-block-polycaprolactone diblock copolymers (MePEG-b-PCL) with varying methoxy poly(ethylene glycol) (MePEG) and polycaprolactone (PCL) block lengths was investigated. Variation of the feed weight ratio of MePEG to caprolactone resulted in the synthesis of copolymers with predictable block lengths. The micelle diameter and pyrene partition coefficient (Kv) were directly related to the PCL block length whereas the critical micelle concentrations (CMC) were inversely related to the PCL block length. The aqueous solubilities of the model hydrophobic drugs, indomethacin, curcumin, plumbagin, paclitaxel, and etoposide were increased by encapsulation within the micelles. Drug solubilization was directly related to the compatibility between the solubilizate and PCL as determined by the Flory-Huggins interaction parameter (chisp). Furthermore, the concentration of solubilized drug was also directly related to the PCL block length.     

Pharmacokinetics and disposition of nanomedicine using biodegradable PEG/PCL polymers as drug carriers

Micelles assembled from amphiphilic poly(ethylene glycol)/poly(-caprolactone) (PEG/PCL) copolymers are promised as safe and effective drug delivery systems. They offer the potential to achieve high solubility of hydrophobic drugs, long blood circulation time and effective delivery to target organs. These advantages contribute to their application as vehicles of a broad variation of therapeutic compounds. In this review, we discussed the safety of the copolymers, release behavior of PEG/PCL micelles in vitro, and pharmacokinetic profiles referring to the optimized fate in vascular system and targeting biodistribution.     

The characterization of paclitaxel-loaded microspheres manufactured from blends of poly(lactic-co-glycolic acid) (PLGA) and low molecular weight diblock copolymers

Paclitaxel-loaded biodegradable drug delivery systems manufactured from poly(lactic-co-glycolic acid) (PLGA) are known to release the drug at extremely slow rates. The objective of this study was to characterize paclitaxel-loaded microspheres composed of blends of PLGA with low molecular weight ampipathic diblock copolymers. The encapsulation and release of a series of poly(epsilon-caprolactone) (PCL)- or poly(D,L-lactic acid) (PDLLA)-co-methoxypolyethylene glycol (MePEG) diblock copolymers was measured using quantitative gel permeation chromatography. Polymeric miscibility was determined by glass transition temperature measurements using differential scanning calorimetry and paclitaxel release was measured using HPLC methods. The PCL- and PDLLA-based diblock copolymers encapsulated at high efficiency and were miscible in PLGA microspheres (30-120m microm size range). The burst phase of paclitaxel release was increased up to 20-fold by the inclusion of diblock copolymers in PLGA microspheres. Approximately 10% of the more hydrophobic PCL-based copolymers released from the microspheres in a short burst over 3 days followed by very slow release over the following 10 weeks. Only the PDLLA-based copolymer released from the PLGA microspheres in a controlled manner over 10 weeks. All microspheres containing PEG were found to have more hydrophilic surfaces (as measured by contact angle) with improved biocompatibility (reduced neutrophil activation) compared to PLGA only microspheres. These results indicate that low molecular weight polyester-based diblock copolymers may be effectively encapsulated in PLGA microspheres to increase paclitaxel release (probably through a micellization process) and improve biocompatibility.     

Preparation,characterization, and drug release behaviors of drug nimodipine-loaded poly(epsilon-caprolactone)-poly(ethylene oxide)-poly(epsilon-caprolactone) amphiphilic triblock copolymer micelles

Amphiphilic triblock copolymers, poly(epsilon-caprolactone)-poly(ethylene oxide)-poly(epsilon-caprolactone) (PCL-PEO-PCL), were synthesized by ring opening polymerization of epsilon-caprolactone initiated with the hydroxyl functional groups of poly(ethylene glycol) at both ends of the chain. The micelles composed of this type of copolymer had such a structure that both ends of the PEO chain were anchored to the micelle. The critical micelle concentration of the block copolymer in distilled water was determined by a fluorescence probe technique using pyrene. As the hydrophobic components of the block copolymer increased, the critical micelle concentration value decreased. To estimate the feasibility as novel drug carriers, the block copolymer micelles were prepared by precipitation of polymer from acetone solution into water. From the observation of transmission electron microscopy, the micelles exhibited a spherical shape. Nimodipine was incorporated into the hydrophobic inner core of micelles as a lipophilic model drug to investigate the drug release behavior. The PEO/PCL ratio of copolymer was a main factor in controlling micelle size, drug-loading content, and drug release behavior. As PCL weight ratio increased, the micelle size and drug-loading content increased, and the drug release rate decreased.     

Optimization of self-assembling properties of fatty acids grafted to methoxy poly(ethylene glycol) as nanocarriers for etoposide

The objective of this work was to study the effect of fatty acid chain length grafted to methoxy poly(ethylene glycol) (mPEG) on self assembling properties of micelles for etoposide delivery. Three amphiphilic copolymers were synthesized using mPEG, myristic acid, stearic acid and behenic acid through an esteric linkage. The particle size and zeta potential of the micelles were determined by the dynamic light scattering method. Etoposide was loaded into micelles by film casting using various drug/polymer ratios. Drug release was studied by the dialysis method. The structure of copolymers was confirmed by (1)H NMR and FTIR. Central micellar concentration (CMC) measurements showed that the longer hydrophobic chains formed more thermodynamically stable micelles. Among the prepared copolymers, etoposide showed the highest solubility in the mPEG-behenic copolymer. Drug loading efficiency depended on the hydrophobic chain length and drug/polymer ratio. The highest drug loading efficiency was found in mPEG-myristic micelles with 1:20 drug/polymer ratio. Micelles released 80 % of loaded drug within about 5 h.     

In vivo fate of unimers and micelles of a poly(ethylene glycol)-block-poly(caprolactone) copolymer in mice following intravenous administration.

Methoxy poly(ethylene glycol)-b-poly(caprolactone) (MePEG-b-PCL) copolymers with varying PEG block lengths and a constant PCL block length were synthesized by cationic ring-opening polymerization and used to form nano-sized micelles. Due to their small size and superior in vitro stability, the MePEG(5000)-b-PCL(5000) micelles were selected for further in vitro characterization and an in vivo evaluation of their fate and stability following intravenous (i.v.) administration. Specifically, (3)H-labelled MePEG(5000)-b-PCL(5000) micelles were i.v. administered to Balb/C mice at copolymer doses of 250, 2 and 0.2 mg/kg in order to examine the distribution kinetics of (1) copolymer assembled as thermodynamically stable micelles, (2) copolymer assembled as thermodynamically unstable micelles and (3) copolymer unimers, respectively. Overall, it was found that when the copolymer is assembled as thermodynamically stable micelles the material is effectively restricted to the plasma compartment. Interestingly, the copolymer was found to have a relatively long circulation half-life even when administered at a dose that would likely fall to concentrations below the CMC following distribution. Analysis of plasma samples from this group revealed that even 24 h post-administration a significant portion of the copolymer remained assembled as intact micelles. In this way, this study demonstrates that the hydrophobic and semi-crystalline nature of the PCL core imparts a high degree of kinetic stability to this micelle system.     

Improving physicochemical properties and doxorubicin cytotoxicity of novel polymeric micelles by poly(ε-caprolactone) segments

This study constructed a series of novel micelles based on star-shaped amphiphilic copolymers (sPEC/CDs), and aimed to confirm the important role poly(ε-caprolactone) (PCL) segments played to improve the various properties of micelles. sPEC/CDs, consisting of β-cyclodextrin (β-CD) as a core and monomethoxy poly(ethylene glycol) (mPEG) and PCL diblock copolymers as arms, were synthesized by arm-first method. The critical micelle concentrations (CMC) of sPEC/CDs were determined by fluorescence spectrophotometry using pyrene as a probe. 3-(4, 5-dimethylthiazol-2-yl)- 2,5-diphenyltetrazolium bromide and flow cytometry were used to detect drug cytotoxicity and cellular uptake of the doxorubicin-loaded micelles. Rhodamine-123 cellular accumulation was examined to evaluate the polymer action to P-glycoprotein. It was revealed that, once PCL segment was inserted between β-CD and mPEG, the CMC can be significantly decreased, the drug loading capability greatly improved, and the drug resistance of MCF-7/ADR cells effectively reversed. These findings suggest that sPEC/CDs own potential superiority for cancer therapy as drug carriers.     

In vitro human plasma distribution of nanoparticulate paclitaxel is dependent on the physicochemical properties of poly(ethylene glycol)-block-poly(caprolactone) nanoparticles

In this study, we synthesized and characterized two methoxy poly(ethylene glycol)-block-poly(caprolactone) (MePEG-b-PCL) amphiphilic diblock copolymers, both based on MePEG with a molecular weight of 5000 g/mol (114 repeat units) and PCL block lengths of either 19 or 104 repeat units. Nanoparticles were formed from these copolymers by a nanoprecipitation and dialysis technique. The MePEG₁₁₄-b-PCL₁₉ copolymer was water soluble and formed micelles that had a hydrodynamic diameter of 40 nm at all copolymer concentrations tested, and displayed a relatively low core microviscosity. The practically water insoluble MePEG₁₁₄-b-PCL₁₀₄ copolymer formed nanoparticles with a larger hydrodynamic diameter, which was dependent on copolymer concentration, and possessed a higher core microviscosity than the MePEG₁₁₄-b-PCL₁₉ micelles, characteristic of nanospheres. The micelles solubilized a maximum of 1.6% w/w of the hydrophobic anticancer agent, paclitaxel (PTX), and released 92% of their drug payload over 7 days, as compared to the nanospheres, which solubilized a maximum of 3% w/w of PTX and released 60% over the same period of time. Both types of nanoparticles were found to be hemocompatible, causing only minimal hemolysis and no changes in plasma coagulation times as compared to control. Upon in vitro incubation in human plasma, PTX solubilized by micelles had a plasma distribution similar to free drug. The majority of PTX was associated with the lipoprotein deficient plasma (LPDP) fraction, which primarily consists of albumin and alpha-1-acid glycoprotein. In contrast, nanospheres were capable of retaining more of the encapsulated drug with significantly less PTX partitioning into the LPDP fraction.     

Preparation and characterization of novel polymeric micelles for 9-nitro-20(S)-camptothecin delivery.

9-Nitro-20(S)-camptothecin (9-NC) has achieved remarkable curative effect in anticancer research. However, the clinical application of 9-NC is largely hampered by its poor solubility and stability. In this paper, novel amphiphilic block copolymers derived from d,l-lactide, trimethylene carbonate, and methylated poly(ethylene glycol) (mPEG) (PECA) with different molecular weight were synthesized and characterized. Self-assembly PECA micelles loaded with 9-NC were prepared. The micelles were regular spheres with a diameter ranged from 20 to 120 nm. The critical micelle concentration (CMC) decreased with the increase of the hydrophobic components. The solubility of 9-NC was improved obviously with micelle encapsulation. The stability experiments proved that over 90% of 9-NC could keep its lactone form in micelle solution after incubating in phosphate-buffered saline for 100 min, while the corresponding proportion for free drug solution was 25%. The release of 9-NC was nearly zero-order after the burst release, and the long hydrophobic chain length led to slower release rate. The novel PECA copolymer micelles could be effective carriers to improve the solubility, stability, and release performance of 9-NC.     

Ketal containing amphiphilic block copolymer micelles as pH-sensitive drug carriers

pH-Responsive linkages have been widely exploited in the development of polymeric drug delivery systems, which trigger drug release selectively at tumor tissues or endosomes and lysosomes of cells. Herein we report new pH-sensitive amphiphilic poly(ketal adipate)-co-poly(ethylene glycol) block copolymers (PKA-PEG), which have acid-cleavable ketal linkages in their hydrophobic backbone. PKA-PEG copolymers self-assemble to form stable micelles with a mean diameter of ~175 nm, which can encapsulate a payload of anticancer drugs and rapidly dissociate to release drug payload at the acid environment. The micelles are biocompatible and exhibit abilities to disrupt endosomes to enhance the cytosol drug delivery. Taken together, we anticipate that the pH-sensitive PKA-PEG micelles have great potential as anticancer drug carriers.     

Folate-conjugated methoxy poly (ethylene glycol)/poly (L-Alanine) amphiphilic block copolymeric micelles for targeted delivery of paclitaxel

Folate has been used as a targeting moiety of various anticancer agents to increase their cellular uptake within target cells since folate receptors(FR) are vastly overexpressed in many tumors. In this study, amphiphilic block copolymers composed of methoxy poly (ethylene glycol)(MPEG) and poly(L-Alanine)(PALA) were synthesized and then conjugated with folate to produce a folate receptor-targeted drug carrier for tumor-specific drug delivery. The structure of the copolymers was confirmed by 1HNMR. The CMC values of MPEG-PALA(PLAM) and FOL-PALA-MPEG(FOL-PLAM) were 0.678?×?10?? mol/L and 0.864?×?10?? mol/L, respectively. The paclitaxel loaded micelles prepared from PLAM and FOL-PLAM both exhibited spherical shapes and nano-scale dimensions. The average diameter, encapsulation efficiency(EE), drug loading efficiency(LC) were 55 nm, 80.6%, 20.2% for the PLAM micelles and 75 nm, 69.7%, 17.4% for FOL-PLAM micelles. Furthermore, in vitro release study indicated that the release rate of paclitaxel from both drug-loaded micelles was slow and sustained.     

Solubilization of an amphiphilic drug by poly(ethylene oxide)-block-poly(ester) micelles.

The purpose of this study was to investigate the solubilization of an amphiphilic drug, i.e, amiodarone (AMI) in methoxy poly(ethylene oxide)-block-poly(ester) micelles of different core structure. The effect of core-forming block structure as well as molecular weight, applied drug to polymer ratios and assembly condition on AMI solubilization; stability of the solubilized formulation upon dilution in phosphate buffer and the hemolytic activity of solubilized AMI against rat red blood cells were assessed and compared to those parameters for the commercial intravenous formulation of AMI. In general, polymeric micelles of different core structure were found to be more efficient in retaining their AMI content upon dilution than surfactant micelles in the commercial formulation of AMI for injection. Micelles with a poly(epsilon-caprolactone) (PCL) core were more efficient than poly(D,L-lactide) and poly(L-lactide) cores in the solubilization and stabilization of encapsulated AMI within the carrier. Encapsulation of AMI by methoxy poly(ethylene oxide)-block-poly(epsilon-caprolactone) (MePEO-b-PCL) micelles having higher PCL chains increased the level of AMI solubilization and decreased its hemolytic activity. Compared to O/W emulsion, application of solvent evaporation method led to higher encapsulation efficiency and lower hemolytic activity for AMI in micelles. An increase in the level of AMI added to the co-solvent evaporation process led to an increase in the solubilized AMI levels, but made the formulation more hemolytic. In conclusion, PEO-b-PCL micelles, particularly those with longer PCL chains, were found to be efficient carriers in encapsulating amphiphilic AMI, retaining encapsulated AMI within the carrier and reducing its hemolytic activity.     

Block copolymer micelles as a solution for drug delivery problems

Micelles, nanosized colloidal particles with a hydrophobic core and hydrophilic shell, can be successfully used for the solubilisation of various poorly soluble pharmaceuticals, and demonstrate a series of attractive properties as drug carriers. Polymeric micelles, such as micelles formed by amphiphilic block copolymers, are of a special interest as they possess high stability both invitro and invivo, and good biocompatibility. Drug-loaded micelles can spontaneously accumulate in body areas with compromised vasculature (tumours, infarcts) via the enhanced permeability and retention (EPR) effect. Micelles made of stimuli-responsive (pH- or temperature-sensitive) amphiphilic block copolymers can release their contents in pathological areas demonstrating hyperthermia or acidosis. Various specific targeting ligand molecules, such as antibodies, can be attached to the micelle surface and bring drug-loaded micelles to, and into, target cells (cancer cells being a primary target). Micelles carrying various reporter (contrast) groups may become the imaging agents of choice in different imaging modalities. This review will consider some recent trends in using micelles as pharmaceutical carriers.     

A novel delivery system of doxorubicin with high load and pH-responsive release from the nanoparticles of poly (α,β-aspartic acid) derivative

A poly (amino acid)-based amphiphilic copolymer was utilized to fabricate a better micellar drug delivery system (DDS) with improved compatibility and sustained release of doxorubicin (DOX). First, poly (ethylene glycol) monomethyl ether (mPEG) and DOX were conjugated onto polyasparihyazide (PAHy), prepared by hydrazinolysis of the poly (succinimide) (PSI), to afford an amphiphilic polymer [PEG-hyd-P (AHy-hyd-DOX)] with acid-liable hydrazone bonds. The DOX, chemically conjugated to the PAHy, was designed to supply hydrophobic segments. PEGs were also grafted to the polymer via hydrazone bonds to supply hydrophiphilic segments and prolong its lifetime in blood circulation. Free DOX molecules could be entrapped into the nanoparticles fabricated by such an amphiphilic polymer (PEG-hyd-P (AHy-hyd-DOX)), via hydrophobic interaction and π-π stacking between the conjugated and free DOX molecules to obtain a pH responsive drug delivery system with high DOX loaded. The drug loading capacity, drug release behavior, and morphology of the micelles were investigated. The biological activity of micelles was evaluated in vitro. The drug loading capacity was intensively augmented by adjusting the feed ratio, and the maximum loading capacity was as high as 38%. Besides, the DOX-loaded system exhibited pH-dependent drug release profiles in vitro. The cumulative release of DOX was much faster at pH 5.0 than that at pH 7.4. The DOX-loaded system kept highly antitumor activity for a long time, compared with free DOX. This easy-prepared DDS, with features of biocompatibility, biodegradability, high drug loading capacity and pH-responsiveness, was a promising controlled release delivery system for DOX.     

Folate-conjugated methoxy poly (ethylene glycol)/poly (L-Alanine) amphiphilic block copolymeric micelles for targeted delivery of paclitaxel

Folate has been used as a targeting moiety of various anticancer agents to increase their cellular uptake within target cells since folate receptors(FR) are vastly overexpressed in many tumors. In this study, amphiphilic block copolymers composed of methoxy poly (ethylene glycol)(MPEG) and poly(L-Alanine)(PALA) were synthesized and then conjugated with folate to produce a folate receptor-targeted drug carrier for tumor-specific drug delivery. The structure of the copolymers was confirmed by ¹HNMR. The CMC values of MPEG-PALA(PLAM) and FOL-PALA-MPEG(FOL-PLAM) were 0.678?×?10^?5?mol/L and 0.864?×?10^?5?mol/L, respectively. The paclitaxel loaded micelles prepared from PLAM and FOL-PLAM both exhibited spherical shapes and nano-scale dimensions. The average diameter, encapsulation efficiency(EE), drug loading efficiency(LC) were 55nm, 80.6%, 20.2% for the PLAM micelles and 75nm, 69.7%, 17.4% for FOL-PLAM micelles. Furthermore, in vitro release study indicated that the release rate of paclitaxel from both drug-loaded micelles was slow and sustained.     

Safety Evaluation of Amphiphilic Three-Armed Star-Shaped Copolymer Micelles

In our previous work, we had prepared a biodegradable amphiphilic three-armed star-shaped copolymers (SPCE) based on poly(ε-caprolactone) (PCL) and poly(ethylene glycol) (PEG), which could form micelles by self-assembly method and it was a potential carrier for hydrophobic drug. For further application, the safety of SPCE micelles was evaluated in vitro and in vivo here. ¹³C-NMR was used to confirm the formation of the micelles in aqueous solution, and the morphology was observed on transmission electron microscope (TEM). Also, thermostability of blank SPCE micelles was determined by Malvern Nano-ZS 90 laser particle size analyzer. In vitro toxicity evaluation included hemolytic test and cytotoxicity. In vivo acute toxicity tests and histopathological study of SPCE micelles were carried out on BALB/C mice which were administrated SPCE micelles (1 g/kg b.w.) intravenously. In acute toxicity test, the mice were observed continuously for 7 days, obtained their body weight every day, at last the mice was sacrificed for the following study: the blood of the mice was assigned for blood chemistry and routine analysis, the heart, liver, spleen, lung, and kidneys were used for histopathological study. All results indicated that the biodegradable self-assembled SPCE micelles were nontoxic; therefore, it might be used as a safe candidate for drug delivery system     

PCL/PEG copolymeric nanoparticles: potential nanoplatforms for anticancer agent delivery

Nanotechnology provides researchers with new tools for cancer treatment. Biodegradable polymeric nanoparticles, as an advanced drug delivery system, have promising applications in cancer treatment. Poly(ε-caprolactone)/poly(ethylene glycol) (PCL/PEG) copolymers are biodegradable and amphiphilic, and show potential application in drug delivery. In recent years, PCL/PEG copolymeric nanoparticles, as a potential nanoplatform for anticancer agent delivery, received increasing attention. This paper reviews PCL/PEG copolymer nanoparticles for anticancer agent delivery, including overcoming water insolubility of hydrophobic drug, targeting chemotherapeutic drug to tumor, and delivering genes, vaccines, and diagnostic agents.