In vitro bioactivity of a synthesized prostaglandin E1-heparin conjugate

Antithrombogenic Poly(vinyl chloride) with Heparin- and/or Prostaglandin I2-Immobilized in Hydrogels

Poly(vinyl chloride) (PVC) with heparin- and/or prostaglandin I2 (PGI2)- immobilized on crosslinked polyacrylamide hydrogels were prepared by the ad dition of the dry hydrogels to a PVC tetrahydrofuran solution. Immobilized heparin was found to be continuously released from the PVC matrix at 37 ° C in a physiological saline. Antithrombogenic activity of PVCs with heparin and/or PGI2 was evaluated by activated partial thromboplastin time (aPTT), whole blood clotting time, and inhibition of platelet aggregation. The heparin- immobilized PVC's exhibited excellent anticoagulant activity and the PGI2 immobilized PVC's completely inhibit platelet aggregation of human blood after sitting for 46 h in a physiological saline.

Water-soluble HPMA copolymer—prostaglandin E1conjugates containing a cathepsin K sensitive spacer

A novel bone targeting, N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer based, prostaglandin E1 (PGE1) delivery system was designed, synthesized and characterized. PGE1 was bound to the polymer backbone via a spacer, composed of a cathepsin K sensitive tetrapeptide (Gly-Gly-Pro-Nle) and a self-eliminating 4-aminobenzyl alcohol structure. The HPMA copolymer conjugates were prepared by photo-initiated free radical copolymerization of HPMA, PGE1-containing macromonomer, and optionally a comonomer containing a reactive p-nitrophenyl ester group. The latter group was used as attachment points for the D-aspartic acid octapeptide targeting moieties. Incubation of the PGE1-containing macromonomer and HPMA copolymer-PGE1 conjugates with cathepsin K resulted in release of unmodified PGE1. The rate of release depended on the composition of the conjugate. The higher the PGE1 content in the conjugate, the slower the PGE1 release. This appeared to be the result of association of hydrophobic side-chains in aqueous media, which rendered the formation of the enzyme substrate complex more difficult. The data seems to indicate that HPMA copolymer-PGE1 conjugates have a potential in the treatment of osteoporosis and other bone diseases.

Design of polymeric prodrugs of prostaglandin E(1) having galactose residue for hepatocyte targeting

Based on the relationship between in vivo disposition of macromolecules and their physicochemical and biological characteristics obtained through clearance concept-based pharmacokinetic analysis, polymeric prodrugs of prostaglandin E(1)(PGE(1)) were designed stepwise and evaluated on their targeting and therapeutic efficiencies. First poly-L-lysine (PLL) and poly-L-glutamic acid (PLGA) with an ethylenediamine (ED) spacer were modified with 2-imino-2-methoxyethyl 1-thiogalactoside to obtain galactosylated derivatives. After intravenous injection in mice, Gal-ED-PLGA was selectively taken up by the liver parenchymal cells via receptor-mediated endocytosis, while Gal-PLL accumulated in the liver as well as PLL mostly due to electrostatic interaction. Although Gal-ED-PLGA showed good targeting efficacy, its PGE(1) conjugate synthesized with activated PGE(1) by carbonyldiimidazole method failed to show therapeutic effects probably due to inactivation of PGE(1) during conjugation and lack of release in the tissue. In order to overcome these problems, we next conjugated PGE(1) to galactosylated poly-(L-glutamic acid) hydrazide (Gal-HZ-PLGA) in which PGE(1) was easily coupled to Gal-HZ-PLGA via a hydrazone bond in weak acidic solution (pH 5) at room temperature. The PGE(1)-Gal-HZ-PLGA conjugate labeled with [(111)In] or [(3)H]PGE(1) rapidly accumulated in the liver parenchymal cells. In addition, the PGE(1) conjugate effectively inhibited the increase of the GPT level in plasma, while free PGE(1) indicated no therapeutic efficacy even at more than ten times higher doses, in carbon tetrachloride-induced hepatitis mice. These findings suggest potentials of polymeric targeting systems of PGE(1) to hepatocyte utilizing galactose recognition.

Chemical and physical characterization of a novel poly(carbonate urea) urethane surface with protein crosslinker sites

A major complication which occurs with implantable polyurethane biomaterials is bioincompatibility between blood and the biomaterial surface. Development of a novel biodurable polyurethane surface to which biological agents, such as growth factors or anticoagulants could be covalently bound, would be beneficial. The purpose of this study was to synthesize a novel poly(carbonate urea) urethane polymer with carboxylic acid groups which would serve as "anchor" sites for protein attachment. Physical characteristics such as tensile strength, initial modulus, ultimate elongation, tear strength, water/alcohol uptake and water vapor permeation were then evaluated and compared to other biomedical-grade polyurethanes. Covalent linkage of the blood protein albumin to this novel surface was then examined. A biodurable polycarbonate-based polyurethane containing carboxylic acid groups (cPU) was synthesized using a two step procedure incorporating the chain extender 2,2-bis(hydroxymethyl)-propionic acid (DHMPA). Tensile strength of this cPU film was 2.7 and 2.6 fold greater than both a polycarbonate-based polyurethane synthesized with a 1,4-butanediol chain extender (bdPU) and Mitrathane (Mit) controls, respectively. The cPU polymer also possessed 7.8 and 31 fold greater structural rigidity upon evaluation of initial modulus as compared to the bdPU and Mit, respectively. Ultimate elongation for the bdPU films was slightly higher than the cPU and Mit films, which had comparable elongation properties. The force required to tear the bdPU film was 1.9 and 32 fold greater than the cPU and Mit films, respectively. Alcohol solution uptake by all of the polyurethane segments increased with increasing alcohol concentrations, with the cPU having the greatest uptake. Water uptake was minimal for all the polyurethanes examined and was not affected by altering pH. Water vapor permeation was lowest for the cPU films as compared to both bdPU and Mit. Swelling the cPU in 50% ethanol prior to evaluation slightly increased water vapor permeation through the films. Covalent linkage of the radiolabelled blood protein albumin (125I-BSA) to the cPU segments incubated with the heterobifunctional crosslinker 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) was greatest in the higher percent of ethanol as compared to controls. These results serve as foundation for developing a novel poly(carbonate urea) urethane with physical characteristics comparable to other medical-grade polyurethanes while having protein binding capabilities.

Covalent linkage of recombinant hirudin to poly(ethylene terephthalate) (Dacron): creation of a novel antithrombin surface

Thrombus formation and intimal hyperplasia on the surface of implantable biomaterials such as poly(ethylene terepthalate) (Dacron) vascular grafts are major concerns when utilizing these materials in the clinical setting. Thrombin, a pivotal enzyme in the blood coagulation cascade primarily responsible for thrombus formation and smooth muscle cell activation, has been the target of numerous strategies to prevent this phenomenon from occurring. The purpose of this study was to covalently immobilize the potent, specific antithrombin agent recombinant hirudin (rHir) to a modified Dacron surface and characterize the in vitro efficacy of thrombin inhibition by this novel biomaterial surface. Bovine serum albumin (BSA), which was selected as the "basecoat' protein, was reacted with various molar ratios of the cross-linker sulphosuccinimidyl 4-(N-maleimidomethyl) cyclohexane-1-carboxylate (sulpho-SMCC; 1:5-1:50). These BSA-SMCC complexes were then covalently linked to sodium hydroxide-hydrolysed Dacron (HD) segments via the cross-linker 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC). Covalent linkage of these complexes to HD (HD-BSA-SMCC) was not affected by any of the sulpho-SMCC cross-linker ratios assayed. rHir, which was initially reacted with 2-iminothiolane hydrochloride (Traut's reagent) in order to create sulphydryl groups, was then covalently bound to these HD-BSA-SMCC surfaces (HD-BSA-SMCC-S-rHir). The 1:50 (BSA: sulpho-SMCC) HD-BSA-SMCC-S-rHir segments bound 22-fold more rHir (111 ng per mg Dacron) compared to control segments and also possessed the greatest thrombin inhibition of the segments evaluated using a chromogenic substrate assay for thrombin. Further characterization of the HD-BSA-SMCC-S-rHir segments demonstrated that maximum thrombin inhibition was 20.43 NIHU, 14.6-fold greater inhibition than control segments (1.4 NIHU). Thrombin inhibition results were confirmed by 125I-thrombin binding experiments, which demonstrated that the 1:50 HD-BSA-SMCC-S-rHir segments had significantly greater specific thrombin adhesion compared to control segments. Non-specific 125I-thrombin binding to and release from the 1:50 HD-BSA-SMCC-S-rHir segments was also significantly less than the control segments. Thus, these results demonstrate that rHir can be covalently bound to a clinically utilized biomaterial (Dacron) while still maintaining its ability to bind and inhibit thrombin.

Heparin release from thermosensitive polymer coatings: in vivo studies

Biomer/poly(N-isopropylacrylamide)/[poly(NiPAAm)] thermosensitive polymer blends were prepared and their application as heparin-releasing polymer coatings for the prevention of surface-induced thrombosis was examined. The advantage of using poly (NiPAAm)-based coatings as heparin-releasing polymers is based on the unique temperature-dependent swelling of these materials. At room temperature, i.e., below the lower critical solution temperature (LCST) of poly (NiPAAm), the Biomer/(poly(NiPAAm) coatings are highly swollen. The high swelling enables fast loading of hydrophilic macromolecules (e.g., heparin) into the coating by a solution sorption technique. At a body temperature, i.e., above the LCST of poly (NiPAAm) the coatings are in a deswollen state and the absorbed macromolecules may be slowly released from a dense coating via a diffusion controlled mechanism. Biomer/poly(NiPAAm) coatings were obtained by blending and coprecipitation of the two linear polymers, Biomer and (poly(NiPAAm). The structure and water-swelling properties of the coatings were examined. Significant differences in water swelling at room temperature (RT) and 37 degrees C were observed as a result of the thermosensitivity of poly (NiPAAm). The surface structure of the coatings in dry and swollen states at RT and 37 degrees C was examined by scanning electron microscopy. Heparin was loaded into the coatings via a solution sorption at room temperature. Kinetic studies of heparin loading demonstrated that maximum loading was obtained within 1 h. The in vitro (37 degrees C) release profiles were characterized by a rapid initial release due to the squeezing effect of the collapsing polymer network, followed by a slower release phase controlled by heparin diffusion through the dense coating. The short-term antithrombogenicity of intravenous polyurethane catheters coated with heparin-releasing Biomer/poly(NiPAAm) thermosensitive coating was evaluated in a canine animal model. The results show that the heparin release from Biomer/poly(NiPAAm)-coated surfaces resulted in a significant reduction of thrombus formation on test surfaces in contact with venous blood as compared to control surfaces.

Antithrombogenic surfaces: characterization and bioactivity of surface immobilized PGE1-heparin conjugate

A covalently bonded conjugate of commercial grade heparin and prostaglandin E1 (PGE1) was synthesized to prevent both fibrin formation and platelet aggregation during thrombus formation. The PGE1-heparin conjugate was immobilized on an imidazole carbamate derivatized sepharose bead surface through hydrophilic spacer groups (diamino-terminated polyethylene oxides). One end of the spacer group was coupled to the derivatized surface through a urethane bond between the amine group of the spacer and the derivatized surface. The free amine group of the immobilized spacers was coupled to a carboxylic group of the PGE1-heparin conjugate through an amide bond. Bioactivity of the immobilized conjugate (heparin activity) was measured in terms of increased clotting times (thrombin time assay) and for the inactivation of Factor Xa. Bioactivity of the immobilized compound (PGE1 activity) was analyzed by platelet adhesion and platelet release reactions using C14-5-hydroxytryptamine (5-HT). The conjugate immobilized via the C2 spacer showed the highest incidence of platelet adhesion, 5-HT released and the lowest activity for coagulation factors. In contrast, the 1000 and 4000 immobilized systems showed a significant reduction in platelet activation, while having the greatest effect on coagulation factors. The results of these experiments imply that the immobilized conjugate is active in preventing both pathways of thrombus formation, and the efficacy is improved through the use of long-chain hydrophilic spacer groups.

The preparation of a urokinase-AT-III-PGE1-methyldopa complex, and its effects on platelet adhesion, coagulation times, protein adsorption, and fibrinolysis

Modifications of urokinase by substances possessing useful therapeutic activity permit combined action preparations to be obtained. Here an attempt was made to develop a complex having combined action for therapeutic activity. The possibility of repeatedly modified urokinase with antithrombin-III-methyldopa-prostaglandin E1 had been experimentally demonstrated. The complex was immobilized on albuminated substrate, which showed fibrinolytic, anticoagulant, and antiplatelet effects simultaneously, in addition to the normal antihypertensive action of methyldopa. The complex immobilized substrate also demonstrated an increase in albumin-surface attachment and a reduction in fibrinogen binding. This may be one of the parameters for a reduced platelet-surface attachment, which may also improve the blood compatibility of the substrate. The approaches suggested indicate the possible new ways of creating nonthrombogenic surfaces with wider applications. A better understanding of the mechanism of these complexes are needed in in vivo conditions to correlate these findings.

Surface modification for improved blood compatibility

Several surface modification techniques are currently being used to improve the biocompatibility of blood-contacting devices. These include the immobilization of bioactive materials to prevent thrombus generation and platelet activation, the incorporation of hydrophilic grafts onto practical hydrophobic surfaces (polyurethanes) to reduce protein adsorption, and the concept of microdomain-phase separated surfaces to regulate cellular and protein adhesion.

Platelet adhesion and prevention at blood-polymer interface

It has been proposed that adsorbed glycoproteins such as fibrinogen and gamma-globulin induce platelet adhesion at blood-polymer interfaces. The importance of oligosaccharide groups in the glycoproteins proved to be responsible for platelet adhesion and aggregation via possible complex formation. Several studies have provided evidence that the proposed mechanism was involved in platelet adhesion on polymer surfaces. To minimize or prevent platelet adhesion on polymers, prostaglandins (PGs), potent inhibitors of platelet aggregation and PG-heparin (HEP) conjugate, were combined with polymers via physical dispersion or chemical immobilization on the surfaces. Albumin-HEP conjugate-adsorbed surfaces also showed significant reduction of platelet adhesion.