Controlled activity polymers. IX copolymers of acrylic acid and isomeric N-alkylacrylamide monomers with pendent β-naphthol ester moieties: Hydrolytic release studies

The hydrolytic release of the allelopathic compound, β-naphthol, from copolymers of acrylic acid and N-substituted acrylamide monomers β-naphthyl acrylate, 2-acrylamido(β-naphthyl)isovalerate, 5-acrylamido(β-naphthyl)valerate, 3-acrylamido-3-methyl(β-naphthyl)butanoate and 6-acrylamido(β-naphthyl)caproate was studied. Reversed-phase liquid chromatography with ultraviolet spectrophotometric detection was utilized to follow kinetics of naphthol release from series of well-characterized copolymers in aqueous media at three pH conditions. Release profiles of the totally soluble copolymers with 4 to 12 mol% naphthol ester monomers were compared to those of the constituent monomers or model compounds at pH values of 2, 6 and 10. Release rates and the extent of release depended upon the nature of the monomer, the spacing of the naphthol ester moiety from the polymer backbone and pH. The extent of hydrophilicity of the polymer backbone was the major factor as dictated by pH (degree of ionization of the acrylic acid mers) and by the copolymer microstructure. The substituted hydrophobic copolymers exhibited the slowest release rates. At high pH values, a maximum in naphthol ester hydrolysis was observed, apparently due to a thermodynamic limit on charge along the polyelectrolyte. Neighboring group assistance was observed at pH 6 and 2 values for the β-naphthol acrylate copolymers. The initial hydrolysis rates for all of the other copolymers in water and for the monomers in dioxane/water mixtures followed pseudo-first-order kinetics. The limited extent of hydrolysis, dependent upon pH and composition, was attributed to long-range associative effects in aqueous media.     

Modulated release of cyclosporine from soluble vinyl pyrrolidone--hydroxyethyl methacrylate copolymer hydrogels. A correlation of 'in vitro' and 'in vivo' experiments.

Soluble, uncrosslinked and high molecular weight copolymers of vinylpyrrolidone, VP, with 2-hydroxyethyl methacrylate, HEMA, prepared by free radical copolymerization, are proposed as supports for the modulated release of the immunosuppressor cyclosporine. Two copolymeric systems with copolymer compositions f(VP)=0.52 (namely VP--HEMA 60--40) and 0.42 (VP--HEMA 40--60) have been prepared and tested in vitro and in vivo using rats as animal model. Micellar electrokinetic capillary chromatography, MEKC, has been used for the simultaneous detection of the polymer reabsorption and the drug release for the in vitro experiments. The composition and microstructural distribution of the copolymer system controls the solubilization rate which modulates the in vitro release of the drug (with time profiles from a few days to several weeks for the VP--HEMA 60--40 and 40--60, respectively) and the in vivo response that correlates with the previous in vitro results: the more hydrophobic implant (VP--HEMA 40--60) reverts the immune response more slowly (2--4 weeks) compared to the more hydrophilic one (VP--HEMA 60--40, 1--2 weeks).     

Polymeric micellar pH-sensitive drug delivery system for doxorubicin.

A novel polymeric micellar pH-sensitive system for delivery of doxorubicin (DOX) is described. Polymeric micelles were prepared by self-assembly of amphiphilic diblock copolymers in aqueous solutions. The copolymers consist of a biocompatible hydrophilic poly(ethylene oxide) (PEO) block and a hydrophobic block containing covalently bound anthracycline antibiotic DOX. The starting block copolymers poly(ethylene oxide)-block-poly(allyl glycidyl ether) (PEO-PAGE) with a very narrow molecular weight distribution (Mw/Mn ca. 1.05) were prepared by anionic ring opening polymerization using sodium salt of poly(ethylene oxide) monomethyl ether as macroinitiator and allyl glycidyl ether as functional monomer. The copolymers were covalently modified via reactive double bonds by the addition of methyl sulfanylacetate. The resulting ester subsequently reacted with hydrazine hydrate yielding polymer hydrazide. The hydrazide was coupled with DOX yielding pH-sensitive hydrazone bonds between the drug and carrier. The resulting conjugate containing ca. 3 wt.% DOX forms micelles with Rh(a)=104 nm in phosphate-buffered saline. After incubation in buffers at 37 degrees C DOX was released faster at pH 5.0 (close to pH in endosomes; 43% DOX released within 24 h) than at pH 7.4 (pH of blood plasma; 16% DOX released within 24 h). Cleavage of hydrazone bonds between DOX and carrier continues even after plateau in the DOX release from micelles incubated in aqueous solutions is reached.     

Drug release from biodegradable injectable thermosensitive hydrogel of PEG-PLGA-PEG triblock copolymers.

An aqueous solution of newly developed low-molecular-weight PEG-PLGA-PEG triblock copolymers with a specific composition is a free flowing sol at room temperature but becomes a gel at body temperature. Two model drugs, ketoprofen and spironolatone, which have different hydrophobicities, were released from the PEG-PLGA-PEG triblock copolymer hydrogel formed in situ by injecting the solutions into a 37 degrees C aqueous environment. Ketoprofen (a model hydrophilic drug) was released over 2 weeks with a first-order release profile, while spironolactone (a model hydrophobic drug) was released over 2 months with an S-shaped release profile. The release profiles were simulated by models considering degradation and diffusion, and were better described by a model assuming a core-shell structure of the gel.     

Modified guar gum matrix tablet for controlled release of diltiazem hydrochloride.

Polyacrylamide-grafted-guar gum (pAAm-g-GG) was prepared by taking three different ratios of guar gum to acrylamide (1:2, 1:3.5 and 1:5). Amide groups of these grafted copolymers were converted into carboxylic functional groups. Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry were used to characterize copolymers. Tablets were prepared by incorporating an antihypertensive drug viz., diltiazem hydrochloride. In-vitro drug release was carried out in simulated gastric and intestinal conditions. Effect of drug loading on release kinetics was evaluated. Release continued up to 8 and 12 h, respectively, for pAAm-g-GG and hydrolyzed pAAm-g-GG copolymers. Nature of drug transport through the polymer matrices was studied by comparing with Higuchi, Hixson-Crowell and Kopcha equations. Drug release was found to be dissolution-controlled in case of unhydrolyzed copolymer. With hydrolyzed copolymers, drug release was swelling-controlled initially (i.e., in 0.1 N HCl), but at later stage, it became dissolution-controlled in pH 7.4. Hydrolyzed pAAm-g-GG matrices are pH sensitive and can be used for intestinal drug delivery.     

Incorporation and release behavior of hydrophobic drug in functionalized poly(D,L-lactide)-block-poly(ethylene oxide) micelles.

The poly(ethylene oxide)-poly(lactide) (PEO-PLA) block copolymers containing a small quantity of carboxylic acid in the PLA block were synthesized. The microscopic characteristics of nanoparticles with carboxylic acid content in the copolymer were analyzed, and the effect of specific interactions between the copolymer and the model drug on the drug loading capacity and the release behavior were investigated systematically. The sizes of nanoparticles prepared by a dialysis method are within the range of 30-40 nm. The nanoparticles prepared from functionalized block copolymers have a very low critical micelle concentration (CMC) value as low as approximately 10(-3) mg/ml, which indicates a good stability of the nanoparticles in spite of the presence of carboxylic acid. The drug loading efficiency of nanoparticles dramatically increased when carboxylic acid content was increased in the block copolymer. This result may be attributed to the increase of interactions between the copolymer and the drug. The release rate of the drug was much slower from nanoparticles containing higher amounts of carboxylic acid in the copolymer, which might be associated with the enhanced interaction between the carboxylic group of copolymers and the drug. These experimental results suggest that the nanoparticles prepared from functionalized PEO-PLA block copolymers could be a good candidate for an injectable drug delivery carrier.     

Design of novel bioconjugates for targeted drug delivery.

This paper summarizes recent work on the design and development of targeted polymeric bioconjugates based on N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers. Polymerizable antibody Fab' fragment (MA-Fab') has been developed and used in the preparation of targeted HPMA copolymer-mesochlorin e6 conjugates for the treatment of human ovarian carcinomas. The reactivity of the MA-Fab' in copolymerization with HPMA depended on the length of the spacer between the monomer double bond and the antibody Fab' fragment. The biological activity of the antibody Fab' fragment was maintained after incorporation into the HPMA copolymer. Novel aromatic azo spacers were designed and incorporated into HPMA copolymer-drug (cyclosporin A, 9-aminocamptothecin) conjugates for the colon-specific drug delivery and for the treatment of colon diseases. The colon-specific drug release from the conjugates was controlled by the structures of both drug and spacers. Lectins, wheat germ agglutinin (WGA) and peanut agglutinin (PNA), were conjugated to the colon-specific polymer drug conjugates to enhance specific adhesion onto colon tissues.     

Investigation of the in vitro release of gentamicin from a polyanhydride matrix.

Septacin?trade mark omitted? is a sustained release formulation consisting of gentamicin sulfate dispersed in a biodegradable polyanhydride matrix. The polyanhydride matrix is a copolymer of erucic acid dimer (EAD) and sebacic acid in a 1:1 weight ratio. In vitro drug release was performed in both water and pH 7.4 phosphate buffer. The drug release in water was faster than that in the buffer, which was the opposite of what would be expected based upon a faster polymer hydrolysis rate in the buffer. Theoretical treatment of the data using the Peppas model revealed that release in water was anomalous, while the release in pH 7.4 phosphate buffer was diffusion-controlled. Profound bead morphology differences were observed between beads in these two in vitro release media. Cracking was observed in beads in water and swelling with no apparent cracking was seen in beads in buffer. Concurrent monitoring of drug and sebacic acid release indicated that drug release is not via surface erosion. Osmotic effects were found to play little role in the in vitro drug release. There was no spectroscopic evidence of amide formation between the drug and copolymer. Sulfate release was monitored along with drug release and the results indicate that there is ion-exchange occurring during the pH 7.4 in vitro release. It was subsequently demonstrated that gentamicin can form an insoluble salt with EAD. This salt formation explains the slower drug release in pH 7.4 phosphate buffer.     

Self-aggregates of poly(2-hydroxyethyl aspartamide) copolymers loaded with methotrexate by physical and chemical entrapments.

Amphiphilic copolymers based on poly(2-hydroxyethyl aspartamide) (PHEA) formed self-aggregates for the entrapment and release of methotrexate (MTX) by physical entrapment and chemical conjugation. In physical entrapment, MTX was partitioned into hydrophobic domains in self-aggregates of PHEA grafted with octadecyl chains (PHEA-C18) and the amount of the entrapped drug increased linearly by 3.39 mg per the degree of substitution of grafted octadecyl groups. The amphiphilic nature of the drug induced a large initial release in the buffer medium, irrespective of the amount of octadecyl chains. However, PEG-grafted PHEA-C18 copolymers conjugated with MTX, ConG, formed a micelle-like structure by self-association of the conjugates and suppressed the initial large release. The alkyl grafting lowered the CAC, meaning enhancement of aqueous stability. The release was accelerated in pH 10.0 by rapid hydrolysis of ester linkage by base-catalyzed cleavage, while it was significantly reduced at pH 5.0.     

Indomethacin-loaded polymeric nanocarriers based on amphiphilic polyphosphazenes with poly (N-isopropylacrylamide) and ethyl tryptophan as side groups: Preparation, in vitro and in vivo evaluation.

The effects of copolymer composition, drug structure and initial drug feed on drug loading of polymeric micelles based on amphiphilic polyphosphazenes were investigated. It was found that the drug loading capacity of micelles based on this type of amphiphilic copolymers was mainly determined by copolymer composition and the chemical structure of drug. In addition to the compatibility between drug and micellar core, hydrogen bonding interaction between drug and hydrophilic corona may significantly influence drug loading as well. In vitro drug release in 0.1 M PBS (pH 7.4) suggested that indomethacin (IND) in the micelles was released through Fickian diffusion, and no significant difference in release rate was observed for micelles based on copolymers with various EtTrp content. Compared with in vitro IND release profile, in vivo pharmacokinetic study after subcutaneous administration provides a more sustained release behavior. Additionally, in comparison with free drug solution at the same dose, IND concentration in rat plasma showed a prolonged retention when the drug was delivered through polymeric micelles. In vivo pharmacodynamic study based on both carrageenan-induced acute and complete Freund's adjuvant-induced adjuvant arthritis model indicated that sustained therapeutic efficacy could be achieved through intraarticular injection of IND-loaded micelles. Most importantly, local delivery of IND can avoid the severe gastrointestinal stimulation, which was frequently associated with oral administration.     

Synthesis and release of 5-fluorouracil from poly(N-vinylpyrrolidinone) bearing 5-fluorouracil derivatives.

1-beta-allyloxycarbonyloxymethyl-5-fluorouracil (4) and 1,3-bis(beta-allyloxycarbonyloxymethyl)-5-fluorouracil (5) were synthesised by reacting 5-fluorouracil with formaldehyde followed by treating the product with isopropenyl chloroformate. The monomers 4 and 5 were copolymerized separately with N-vinylpyrrolidinone to form linear copolymers and cross-linked polymer networks, respectively. The monomer reactivity ratios in the copolymerization of 4 with NVP were evaluated by both linear and non-linear methods and the effect of monomer feed composition on copolymer molecular weight was examined. The degradation of the polymer networks in phosphate buffer (pH 7.4) was investigated. The hydrolytic scission of the carbonate groups resulted in release of 5-fluorouracil and a decrease in cross-linking density. The time-dependent fractional release of the 5-FU could be fitted by a power relationship with exponents between 0.10 and 0.25.     

Validation and optimization of a polymer system for potential use as a controlled drug-delivery system

A room temperature polymerizing system consisting of polyethylmethacrylate (PEM) and tetrahydrofurfuryl methacrylate (THFMA) monomer was validated for use as a drug-delivery system. The effect of gelling PEM/THFMA copolymers with a more hydrophilic monomer, hydroxyethyl methacrylate (HEMA), was also investigated. The release of growth hormone (GH) and bovine serum albumin (BSA) protein from this polymer has been studied. The polymer has the advantage of high water absorption and low shrinkage properties. Changes in release profiles have been studied by introducing structural differences in the polymer by changing the mixing technique. The stability and bioactivity of the GH incorporated have been examined at various temperatures. To optimize release profiles further, the possibility of attaining a more sustained and controllable release by varying the ratios of polymer used has also been investigated.     

Sustained release ketoprofen microparticles with ethylcellulose and carboxymethylethylcellulose.

Microparticulate systems for sustained release of ketoprofen were prepared and evaluated by monitoring drug release in the JP XIII second fluid, pH 6.8. All the microparticulate dosage forms were prepared using ketoprofen in the form of calcium salt (KP-Ca). Simple ethylcellulose microparticles of KP-Ca (EC-MP) exhibited the fairly rapid release in the first phase with slower release in the late period. Most of the drug was released from EC-MP showing high drug content. For polymer-coated microparticles of ketoprofen, Eudragit microparticles of KP-Ca (ER-MP) were first prepared, and then coated with ethylcellulose or with a mixture of carboxymethylethylcellulose and ethylcellulose to produce ethylcellulose-coated (EC-coat) and the mixture-coated microparticles (CMEC/EC-coat), respectively. Some polymer-coated microparticles showed drug release at nearly zero-order rate. Especially, CMEC/EC-coat prepared at a CMEC:EC ratio of 1:1 (w/w), named formation I, could supply the drug constantly and efficiently for about half a day except for an initial rapid release. When formation I was administered intraduodenally to rats, the plasma concentration of ketoprofen could be maintained at a nearly constant level. Kinetic analysis demonstrated that formation I showed a nearly zero-order release rate in vivo consistent with that observed in vitro.     

Block copolymer micelles with acid-labile ortho ester side-chains: Synthesis, characterization, and enhanced drug delivery to human glioma cells

A new type of block copolymer micelles for pH-triggered delivery of poorly water-soluble anticancer drugs has been synthesized and characterized. The micelles were formed by the self-assembly of an amphiphilic diblock copolymer consisting of a hydrophilic poly(ethylene glycol) (PEG) block and a hydrophobic polymethacrylate block (PEYM) bearing acid-labile ortho ester side-chains. The diblock copolymer was synthesized by atom transfer radical polymerization (ATRP) from a PEG macro-initiator to obtain well-defined polymer chain-length. The PEG-b-PEYM micelles assumed a stable core-shell structure in aqueous buffer at physiological pH with a low critical micelle concentration as determined by proton NMR and pyrene fluorescence spectroscopy. The hydrolysis of the ortho ester side-chain at physiological pH was minimal yet much accelerated at mildly acidic pHs. Doxorubicin (Dox) was successfully loaded into the micelles at pH 7.4 and was released at a much higher rate in response to slight acidification to pH 5. Interestingly, the release of Dox at pH 5 followed apparently a biphasic profile, consisting of an initial fast phase of several hours followed by a sustained release period of several days. Dox loaded in the micelles was rapidly taken up by human glioma (T98G) cells in vitro, accumulating in the endolysosome and subsequently in the nucleus in a few hours, in contrast to the very low uptake of free drug at the same dose. The dose-dependent cytotoxicity of the Dox-loaded micelles was determined by the MTT assay and compared with that of the free Dox. While the empty micelles themselves were not toxic, the IC₅₀ values of the Dox-loaded micelles were approximately ten-times (by 24 h) and three-times (by 48 h) lower than the free drug. The much enhanced potency in killing the multi-drug-resistant human glioma cells by Dox loaded in the micelles could be attributed to high intracellular drug concentration and the subsequent pH-triggered drug release. These results establish the PEG-b-PEYM block copolymer with acid-labile ortho ester side-chains as a novel and effective pH-responsive nano-carrier for enhancing the delivery of drugs to cancer cells.     

Mechanistic relationships between polymer microstructure and drug release kinetics in bioerodible polyanhydrides.

This work investigates the relationship between polymer microstructure and drug release kinetics in the bioerodible polyanhydride system, poly[(1,6-bis-p-carboxyphenoxy hexane)-co-(sebacic anhydride)] (CPH-SA). Model drugs, p-nitroaniline (PNA) and disperse yellow 3 (DY), were selected based on compatibility with CPH and SA, respectively. The polymer microstructure and compatibility of the drug with the constituent monomers were determined to have significant influence over the release kinetics of the drugs studied. Polymer systems with homogeneous microstructure, poly(SA) and 50:50 CPH-SA, showed simultaneous polymer degradation and drug release, although the solubility of the drug in the polymer influenced the shape of the release profiles. For the heterogeneous copolymers, 20:80 and 80:20 CPH-SA, individual monomer release kinetics demonstrated the effects of drug partitioning within a phase-separated microstructure. The PNA molecules partition preferentially into the CPH microdomains in the 20:80 CPH-SA copolymer while the DY molecules partition preferentially into the SA microdomains in the 80:20 CPH-SA copolymer. These studies suggest that the drug release mechanism is driven by polymer microstructure, compatibility of the drug with the constituent polymer phases, and solubility of the drug within the polymer. A thorough understanding of drug-polymer interactions as well as the polymer microstructure will pave the way for more accurate predictions of drug release from bioerodible polyanhydrides.     

Conformational consequences of cooperative binding of a coiled-coil peptide motif to poly(N-(2-hydroxypropyl) methacrylamide) HPMA copolymers

Small-angle neutron scattering and pulsed-gradient spin-echo NMR have been used to examine the solution conformation of a series of water soluble poly(N-(2-hydroxypropyl) methacrylamide) P(HPMA) co-polymer drug delivery vehicles incorporating a coiled-coil peptide motif as a novel pH sensitive non-covalent linker. The conformation of the HPMA homopolymer is well-described by a Gaussian coil model and changing pH from pH 7 to pH 5 has little effect on the solution conformation, as quantified via the radius of gyration. Copolymerisation with 5-10 mol% of the K3 peptide bearing methacrylate monomer (K3-MA), gave a series of copolymers that exhibited an increase in radius of gyration at both pH's, despite being typically 30% lower in molecular weight, indicating that the K3-MA causes a perturbation (expansion) of the copolymer conformation. Subsequent addition of an equimolar amount of the complementary peptide E3 makes little further difference to the conformation, indicative of the intimate binding (coiled-coil motif) between the two peptides. Again, the effects of pH are small. Only the addition of a large aromatic structure such as methotrexate causes a further perturbation of the structure — the hydrophobic interaction between the MTX units causes a significant collapse of the polymer coil. These findings further elaborate the understanding of those factors that determine the solution conformation of novel polymer therapeutics.     

The preparation of the chronic hyper-endotoxemia experimental animal model by means of a drug delivery system.

A drug delivery system (DDS) consisting of lipopolysaccharide (LPS) as a drug and 2-hydroxyethyl methacrylate (HEMA)-diethylene glycol dimethacrylate (2G) or -polyethylene glycol dimethacrylate (4G, 9G) copolymer was prepared, and used for the efficient preparation of an experimental animal model of chronic hyper-endotoxemia. The release profiles of LPS in the in-vitro test were greatly influenced by the composition of HEMA-2G, 4G, 9G in the copolymer. It was found that LPS release from the DDS continued gradually and constantly throughout 2 weeks. In the in-vivo experiment with rats, the DDS maintained a high blood concentration level of LPS for 3 days. These results strongly suggest the possibility of convenient and reproducible preparation of a chronic hyper-endotoxemia animal model.     

Micelle-like nanoparticles of star-branched PEO-PLA copolymers as chemotherapeutic carrier.

Four-armed (star-branched) block copolymers of l-PLA and PEO were synthesized using ring opening polymerization with different LA/EO ratio. Micellar aggregates were prepared from these block copolymers and characterized. Some surface segregation of PEG was found : the extent depends on the state of the material (whether it is in film or particle form), as well as on molecular geometry. The degradation behavior of star-shaped copolymer was studied over a three week period and compared to its linear counterpart. Anti-cancer drugs 5-FU and paclitaxel were loaded into the micellar nanoparticles. The drug release profile showed that the release of paclitaxel from these polymers could be controlled over 2 weeks. The kinetics of drug release for star-branched, tri- and di-block copolymers were compared. The micelles from star-shaped branch showed more complete release of drug than the diblock copolymers; also, the lower hydrodynamic radius of star-shaped polymers may result in better clearance of the carrier polymer from the body.     

Properties of HPMA copolymer-doxorubicin conjugates with pH-controlled activation: effect of polymer chain modification.

Various conjugates of anticancer drug doxorubicin (DOX) covalently attached via hydrolytically degradable hydrazone bond to water-soluble N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer drug carriers were synthesized. Three types of precursors containing either positively or negatively charged groups or a hydrophobic substituent were employed. In vitro incubation of the conjugates in buffers showed relative stability at pH 7.4 (modelling blood) and a fast DOX release at pH 5 (modelling intracellular environment). The presence of carboxylic groups in the copolymer structure resulted in an increase in the DOX release rate of 15-20% while no effect of the introduction of positively charged groups was observed if compared with the unmodified conjugate. Self-assembling of the oleoyl groups-containing conjugate led into formation of polymeric micelles with high apparent molecular weight (M(w)=170,000) in aqueous solution and resulted in a decrease in the DOX release rate of approximately 20%. The cytostatic activity of the conjugates tested on several cancer cell lines was comparable with that of free DOX.HCl, depending on the sensitivity of a particular cell line to DOX. All the conjugates showed a much higher antitumour activity in vivo than the free drug tested in mice bearing EL4 T-cell lymphoma and treated using the therapeutic regime of drug administration. The highest activity (100% long-term survivors) exhibited polymer-DOX conjugate containing negatively charged GFLG sequences.     

Bioadhesive grafted starch copolymers as platforms for peroral drug delivery: a study of theophylline release.

Nonirritant bioadhesive drug release systems based on starch-acrylic acid graft copolymers prepared by radiation of starch and acrylic acid mixtures with (60)Co were developed for buccal application. The release rate of theophylline (TPL), used as a model drug, depended on the ratio of starch to acrylic acid and on the presence of cations in the graft copolymers, but was practically not affected by the pH (between pH 3 and 7) of the dissolution medium nor by the type of starch used (corn, rice, or potato). Possible release mechanisms are discussed for specific conditions. In general, the release behavior of the graft copolymers was found to be non-Fickian, n value being between 0.6 and 0.96, suggesting that the release was controlled by a combination of tablet erosion and the diffusion of the drug from the swollen matrix. Incorporation of divalent cations into the graft copolymers led to a significant decrease in swelling erosion of the tablets as well as a substantial retardation of drug release. Highest work of adhesion was obtained with graft copolymers containing calcium ions as well as longer time of adhesion on dogs' gingiva.