Coils and tubes releasing heparin. Studies on a new vascular graft prototype.

Coiled metallic guidewires find widespread use, for instance in interventional cardiology. It is known that release of heparin from the surface of guidewires is advantageous to prevent formation of thrombotic emboli. New coiled tubular structures, having larger inner and outer diameter as compared to guidewires, are presented. In theory these tubes can be used as interposition vascular grafts. Ten coiled tubes with an internal diameter of 690 microm were made. Five different adherent polymeric coatings with increasing hydrophilicity were used. Five tubes contained heparin in the coating and the other five were unheparinised controls. The five tubes containing heparin were studied with respect to heparin release in vitro (amount released, kinetics), and immobilised heparin that is exposed at the surface. All tubes were studied with a direct cell contact assay using 3T3 mouse fibroblast cells, a dynamic thrombin generation test, and endothelial cell growth onto the coils. It was found that the heparinised tubes lead to very little thrombin formation. It is argued that this is due to heparin that is immobilised and exposed at the inner surface of such tubes. Furthermore the coils showed to be cytocompatible and endothelial cells adhere and proliferate well onto the coils. This concept is believed to hold promise for further development of small vascular grafts.

Platelet adhesion studies on dipyridamole coated polyurethane surfaces.

Surface modification of polyurethanes (PUs) by covalent attachment of dipyridamole (Persantin) is known to reduce adherence of blood platelets upon exposure to human platelet rich plasma (PRP). This effect was investigated in further detail. First platelet adhesion under static conditions was studied with four different biomaterial surfaces: untreated PU, PU immobilised with conjugate molecule 1, PU immobilised with conjugate molecule 2, and PU immobilised with conjugate molecule 3. In PU immobilised with 1 dipyridamole is directly linked to the surface, in PU immobilised with 2 there is a short hydrophilic spacer chain in between the surface and the dipyridamole, while conjugate molecule 3 is merely the spacer chain. Scanning electron microscopy (SEM) was used to characterise platelet adhesion from human PRP under static conditions, and fluorescence imaging microscopy was used to study platelet adhesion from whole blood under flow. SEM experiments encompassed both density measurements and analysis of the morphology of adherent platelets. In the static experiments the surface immobilised with 2 showed the lowest platelet adherence. No difference between the three modified surfaces emerged from the flow experiments. The surfaces were also incubated with washed blood platelets and labeled with Oregon-Green Annexin V. No capture of Oregon-Green Annexin V was seen, implying that the adhered platelets did not expose any phosphatidyl serine at their exterior surface.

Performance of a polyurethane vascular prosthesis carrying a dipyridamole (Persantin) coating on its lumenal surface.

A porous polyurethane vascular prosthesis with an internal diameter of 5 mm was studied. The graft carries a coating of immobilized dipyridamole (Persantin(R)) on the surface of its lumen. Dipyridamole is a potent nontoxic inhibitor of platelet activation/aggregation, and also a strong inhibitor of vascular smooth muscle cell proliferation. The polyurethane material is also known as Chronoflex(R), and already finds use as a vascular access graft. The coated vascular graft was studied in vitro (hemocompatibility, interaction with blood platelets and cultured endothelial cells), as well as in two established in vivo models. In the first in vivo study, coated grafts were implanted in goats, as a bypass of the carotid artery (four animals, eight grafts, length of the graft was approximately 12 cm). Four uncoated grafts were used as controls in otherwise identical experiments. In the second in vivo experiment, eight sheep were used. Each animal received one coated and one uncoated prosthesis as an interposition graft in the carotid artery (length of the graft was 4 cm). The in vitro experiments revealed that the dipyridamole coating has three beneficial effects: reduced thrombogenicity, reduced adherence of blood platelets, and accommodation of a confluent monolayer of endothelial cells. The goat experiments showed patency of the coated grafts in three of the eight cases. The sheep experiments were not useful for the evaluation of the dipyridamole coating because deterioration of the polyurethane material was observed. The in vivo results indicate that the dipyridamole coating may positively influence the patency rate, probably because the coating promotes the growth of an endothelial cell lining. The sheep data show, however, that the limited stability of the Chronoflex(R) material precludes its issue for the construction of permanent small-bore vascular grafts.

New biocompatible polymer surface coating for stents results in a low neointimal response.

Clinical studies indicate a more pronounced endothelial response after stent implantation than after balloon inflation. This might be related to the metal surface of the stent, and therefore it is speculated that coating of the stent might partially prevent hyperplasia. One coated and one noncoated Palmaz-Schatz stent were implanted in two separate coronary arteries of seven pigs. The coating was composed of methylmethacrylate (MMA) (hydrophobic, 70 mol %) and 2-hydroxyethyl methacrylate (HEMA) (hydrophilic 30 mol %). After sacrifice (3 weeks), cross sections were made of the stented areas. Vessel wall reaction was calculated both independently and dependently of local vessel wall injury due to the stent struts. Overall, vessel wall reaction of the coated stents was lower than that of the noncoated stents. The degree of hyperplasia was linearly related to the degree of stent-induced vessel wall injury. Analyses of all the struts showed that significantly less hyperplasia occurred in the coated versus noncoated stents. In this porcine coronary artery model, the MMA/HEMA stent coating resulted in significantly reduced vessel wall response. However, it remains to be determined whether this favorable outcome will also be present in humans.

Biodegradable three-dimensional networks of poly(dimethylamino ethyl methacrylate). Synthesis, characterization and in vitro studies of structural degradation and cytotoxicity.

In ophthalmology, there is a need for novel degradable biomaterials for e.g. controlled drug release in the vitreous body. These degradable materials should feature both excellent biocompatibility, and well-defined kinetics of degradation. In most cases, poly(D,L-lactic acid), or poly(lactic-co-glycolic acid) are used. These materials, however, suffer from some serious drawbacks, since the degradation kinetics are difficult to control, especially since the so-called 'burst-degradation' occurs. Here, we describe a set of novel polymeric networks which largely consist of poly(dimethylamino ethyl methacrylate) (poly(DMAEMA)); these materials are crosslinked via a dimethacrylate molecule that contains two carbonate groups. This system is susceptible to hydrolytic scission. The degradation products do not exert a catalytic effect on the ongoing degradation reaction (i.e. there is no burst effect). We describe the synthesis of three of these materials, which differ merely with regard to the crosslinker content. These materials were characterized through DMTA, 1H NMR and FT-IR spectroscopy, and scanning electron microscopy. The reaction DMAEMA + 2-hydroxyethyl methacrylate (HEMA) was studied in detail, using 1H NMR spectroscopy, and these experiments revealed that the reaction of DMAEMA and HEMA produces a random (Bernouillian-type) copolymer. From this, we contend that the new materials have more or less uniform distribution of the crosslinks throughout their volume. Structural degradation of the three materials was studied in vitro, at pH 7.4, 9.1 and 12.0. It is found that the materials exhibit smooth hydrolysis, which can be controlled via the crosslink density and the pH, as was expected a priori. It should be noted that degradation of these materials produces non-hydrolysable, but water-soluble, oligo(DMAEMA) and poly(DMAEMA) molecules. We subsequently performed in vitro studies on the biocompatibility of these materials. The MTT cytotoxicity assay revealed that the materials were cytotoxic to chondrosarcoma cells. This is most probably due to local increase of the pH due to the basic character of the pending dimethylamino groups. Cytotoxicity remained virtually unchanged after extended washing with water. This indicates that the cytotoxicity is an intrinsic property of the material and was not caused by remnants of free monomer. Cytotoxicity was also seen in cell cultures (human fibroblasts isolated from donor corneas) which were grown in contact with the materials. It is concluded that the new materials have attractive degradation characteristics, but their cytotoxicity makes them unsuitable for applications in ophthalmology.

Tailoring of new polymeric biomaterials for the repair of medium-sized corneal perforations.

The aim of this study was to investigate whether polymeric biomaterials can be designed such that they become suitable for surgical closure of medium-sized perforations in the cornea, the transparent tissue in the front of the eye. Such a biomaterial must meet stringent requirements in terms of hydrophilicity, strength, transparency, flexibility, and biocompatibility. Four different copolymers of n-butyl methacrylate (BMA) and hexa(ethylene glycol) methacrylate (HEGMA) were prepared and characterized. Poly(BMA) was made as a reference material. Physicochemical properties were measured (contact angles, glass-transition temperatures, thermal degradation, water uptake and swelling), and cytotoxicity in vitro was assessed with a MTT test. Moreover, the interaction between the materials and cultured human corneal epithelial cells was studied. The copolymers may be useful for temporary closure of corneal perforations.

Metallic wires with an adherent lubricious and blood-compatible polymeric coating and their use in the manufacture of novel slippery-when-wet guidewires: possible applications related to controlled local drug delivery.

A new procedure was developed for the controlled application of adherent hydrophilic and biocompatible coatings onto the surface of "endless" metallic wires. Use of copolymers of 1-vinyl-2-pyrrolidinone and alkylmethacrylates provided coatings with excellent adherence and lubricity, and markedly low thrombogenicity. Coated wires could be spiralized without damaging the coating; the resulting coils are potentially useful as lubricious guidewires for use in, for example, interventional cardiology or urology. This study demonstrates that the lubricity of the coating is dependent on the composition (hydrophilicity) of the coating biomaterial, as well as on the thickness of the coating. Furthermore, the results imply that the adherence of the hydrophilic coating is essentially due to entanglement of the binder polymer chains and the hydrophilic copolymer chains. Moreover, the idea to use the hydrophilic coating on the wire as a temporary depot for controlled local drug delivery was explored. The coating was loaded with the dye rhodamine, and release of the dye upon immersion of the coated wire in water was studied. This work revealed that release of the drug is dependent on the composition of the coating. The potential utility of such wires with a drug-charged coating for controlled local drug delivery is discussed briefly.

Photoimmobilisation of poly(N-vinylpyrrolidinone) as a means to improve haemocompatibility of polyurethane biomaterials.

A novel method to improve the haemocompatibility of polymeric biomaterials (in particular: polyurethane elastomers) is reported. The new approach essentially rests upon photochemical immobilisation of the highly biocompatible polymer poly(N-vinylpyrrolidinone) (poly(NVP)) onto the biomaterial's surface. One of the key steps in the surface modification procedure is the preparation of a copolymer of NVP and the photoreactive building block 4-[4'-azidobenzoyl]-oxo-n-butylmethacrylate (1). This copolymer is first dissolved in a volatile solvent, then sprayed onto the biomaterial's surface, and subsequently immobilised via irradiation with ultraviolet light. The paper describes: (i) preparation of 1, (ii) preparation of the copolymer (NVP + 1), (iii) physico-chemical characterisation of the modified surfaces, and (iv) results of two in vitro haemocompatibility assays (i.e. thrombin generation and adhesion of blood platelets from recalcified human platelet-rich plasma). Furthermore, the surface modification was performed with a microporous polyurethane vascular graft (Chronoflex), which is already in clinical use. The in vitro experiments revealed that significant improvement of the haemocompatibility of polyurethanes can be achieved through this method.

New biodegradable networks of poly(N-vinylpyrrolidinone) designed for controlled nonburst degradation in the vitreous body.

Polymers of N-vinylpyrrolidinone (NVP) are known to have excellent biocompatibility when implanted in the vitreous body or used as a vitreous substitute. Although poly(NVP) is capable of absorbing relatively large amounts of water, it is not prone to hydrolysis. Yet intraocular degradation of several crosslinked poly(NVP) hydrogels has been reported recently, but some ambiguity remains about the exact mechanism of degradation of these materials. To date there is no biomaterial that combines the excellent intraocular biocompatibility on the one hand and controlled kinetics of degradation on the other hand. We attempted to design and prepare such materials through the chemical synthesis of a novel dimethacrylate crosslinker molecule. The essential feature of this molecule is that its core contains two carbonate groups, which are evidently susceptible to hydrolytic scission. We studied a series of 3-dimensional networks of poly(NVP), which were crosslinked by this molecule. This approach offers several advantages: the hydrolysis of the carbonate groups in the crosslinks leads to liberation of poly(NVP) and/or oligo(NVP) chains that can probably be cleared from the eye via phagocytosis; hydrolysis generates two alcohols and CO(2) (i.e., there is no catalytic burst effect); when these materials are implanted in dry form, swelling and degradation will progress from the exterior of the material toward its interior. Therefore, these materials can be designed such that surface degradation rather than bulk degradation occurs; the hydrolysis rate can be controlled via the crosslink density or through synthesis of other crosslink molecules with either more (>2) or less (1) carbonate groups or alternatively with one or more other labile groups. We report on the chemical synthesis of the crosslinker molecule, as well as the preparation and degradation of a series of poly(NVP)-based hydrogels in vitro and in vivo (rabbit eyes). We found that these materials indeed displayed excellent biocompatibility in the rabbit eye. Further, the experiments confirmed that degradation occurs without the burst effect. The results are in line with the idea that the rate of intraocular swelling and degradation depends on the crosslink density, but this is only a preliminary conclusion that must be strengthened by much more experimental work. Nonetheless, we foresee several applications of these or related materials in ophthalmology, for example, as biodegradable matrix materials for controlled drug delivery of ganciclovir in the vitreous body.

Adsorption of proteins onto poly(ether urethane) with a phosphorylcholine moiety and influence of preadsorbed phospholipid.

In a previous report we demonstrated that the blood compatibility of poly(ether urethane) (PEU) was improved by grafting phosphorylcholine (PC) groups on the surface. The improved blood compatibility was indicated by decreased platelet adsorption/activation and reduced thrombin formation at the polymer surface in experiments in which the surfaces were contacted with platelet-rich plasma in vitro. In the present study, we investigated the effect of grafted PC groups at a PEU surface on protein and phospholipid adsorption. Adsorption of human fibrinogen (Fg), human serum albumin (Alb), human high-molecular-weight kininogen (HMWK), and dioleoyl phosphatidylcholine (DOPC) vesicles was measured by ellipsometry. For this purpose, thin PEU films were cast on silicon wafers. The polymer film was photochemically modified with a PC-containing aryl azide. The presence of PC groups on the polymer surface was demonstrated by ESCA (Electron Spectroscopy for Chemical Analysis). The hydrophilicity of the polymer surface increased by the surface modification, as indicated by a decrease of the contact angle from 59 degrees before to 43 degrees after modification. Our data show that the presence of PC groups has little effect on the adsorption of proteins to a PEU surface. The highest adsorption was observed for Fg (0.49 microgram/cm2 on PC-modified PEU and 0.50 microgram/cm2 on PEU), followed by HMWK (0.28 microgram/cm2 on both PC-modified PEU and PEU), and Alb (0.16 microgram/cm2 on PC-modified PEU and 0.18 microgram/cm2 on PEU). Protein adsorption was further studied on a "biomembrane-like" DOPC bilayer formed on hydrophilic silicon. We found no protein adsorption on this DOPC bilayer. The adsorption of small unilamellar DOPC vesicles on the polymer surfaces amounted to about 0.06 microgram/cm2 (corresponding to circa 30% of monolayer coverage) and was similar for PC-modified PEU and PEU. Despite this partial surface coverage, preadsorbed DOPC on the polymer surface diminished the subsequent adsorption of proteins considerably. These results show that the mere presence of phosphorylcholine groups on a PEU surface is insufficient to suppress protein adsorption. The highly ordered structure of natural phospholipid bilayers seems to be required to suppress protein adsorption effectively.

Sustained local drug delivery from a radiopaque implanted reservoir.

A new polymeric biomaterial that contains covalently bound iodine, and is therefore radiopaque, was used to construct a sustained local drug-delivery device. A polymeric wall was designed to be porous (i.e., passage of low-molecular-weight molecules across the wall is possible), self-healing, and biocompatible. Once implanted, the sphere cavity can be filled and refilled with a concentrated solution of a (cytostatic) drug, which is subsequently released by slow diffusion into the tissue region surrounding the sphere. This principle of sustained local drug delivery is shown by a series of in vitro experiments on the release of 5-fluorouracil, and in vivo animal experiments, using x-ray fluoroscopic and scintigraphic techniques.

A photochemical method for the surface modification of poly(etherurethanes) with phosphorylcholine-containing compounds to improve hemocompatibility.

Phosphorylcholine groups attached to polymer surfaces are known to improve hemocompatibility. A photochemical method is presented to couple phosphorylcholine-containing aryl azides to poly(etherurethane) surfaces (PEUs). Two aryl azides that consist of a photoactivatable 4-azidobenzoyl group, a short spacer chain, and a phosphorylcholine endgroup were synthesized. The two compounds differ only in the type of spacer used: triethylene glycol for compound 1 and hexanediol for compound 2. These compounds were physically adsorbed to PEU surfaces. Upon UV irradiation, reactive intermediates are formed that react with nucleophilic groups on the polymer surface. The modified surfaces showed decreased underwater contact angles, indicating that hydrophilic phosphorylcholine groups are present at the surface. ESCA measurements showed the presence of phosphorus and positively charged nitrogen atoms in the outermost polymer layers (analyzed depth about 50 A), which is a strong indication of the presence of phosphorylcholine groups. Hemocompatibility in vitro was tested with thrombin generation assays and platelet adhesion tests. In thrombin generation assays the clotting time of platelet-rich plasma in contact with the polymer surface is determined. Clotting times were clearly prolonged for the modified surfaces. Surfaces modified with compound 2 showed slightly higher clotting times than those modified with compound 1. Repeated surface modification with compound 2 further increased the clotting time. For the tested surfaces an increase in the clotting time corresponds to an increase in the concentration of phosphorylcholine groups at the surface (as measured by ESCA and contact angle). Platelet adhesion studies with scanning electron microscopy demonstrated that fewer platelets (showing less activation) adhered to the modified surfaces than to the unmodified polyurethane.

Synthesis of a new photoreactive derivative of dipyridamole and its use in the manufacture of artificial surfaces with low thrombogenicity.

Photoimmobilization of dipyridamole (Persantin) was accomplished through the use of a new synthetic conjugate molecule, 1. Persantin is a powerful inhibitor of platelet activation and aggregation and is widely used as a vasodilator. Conjugate 1 consists of triply protected dipyridamole [three of the four hydroxyl groups carry a tert-butyldimethylsilyl (TBDMS) protective group) and the photoreactive 4-azidobenzoyl group. A short hydrophilic spacer chain, derived from triethylene glycol, separates the protected dipyridamole system and the photoreactive group. Compound 1 was immobilized on polyurethane sheets (Pellethane D-55) through irradiation with ultraviolet (UV) light, and the protective groups were removed afterward. The resulting modified polyurethane surfaces were characterized by different physicochemical techniques: UV extinction, contact angle measurements (captive bubble technique), and X-ray photoelectron spectroscopy (XPS). The UV extinction measurements showed the presence of 13 +/- 1 nmol of immobilized dipyridamole/cm2. The contact angle measurements revealed that the modified surface was markedly more hydrophilic than the control (i.e. unmodified polyurethane). XPS measurements clearly established the presence of immobilized dipyridamole in the outermost layers of the modified surface. This was especially clear from the XPS spectra recorded at a low take-off angle (approximately 6 degrees). Furthermore, the XPS spectra showed that the TBDMS protective groups had been quantitatively removed during the deprotection/washing treatment. The in vitro blood compatibility of the modified surface was studied with the thrombin generation assay as developed in our group, as well as with scanning electron microscopy. The thrombin generation test produced a lag time of 1275 s for the modified surface, as opposed to 569 s for the control. Scanning electron microscopy showed that far fewer platelets adhere to the modified surface (approximately 7 x 10(3)/mm2) as compared to the control (approximately 6 x 10(2)/mm2). Taken together, the experimental data reveal that the modified surface has excellent blood compatibility in vitro. It is discussed that the use of conjugate 1 leads to simultaneous exposure of dipyridamole at the modified surface and to a marked increase of the surface hydrophilicity, which is likely to hamper adsorption of plasma proteins. The combination of these effects is uniquely related to the molecular buildup of 1. Conjugate 1 will be used in future work that is aimed at preparing small-caliber polyurethane vascular grafts with a blood compatible lumenal surface.

Photo-immobilization of dipyridamole (Persantin) at the surface of polyurethane biomaterials: reduction of in-vitro thrombogenicity.

Dipyridamole is a well-known vasodilator and a powerful inhibitor of activation and aggregation of blood platelets. Moreover, dipyridamole is essentially non-toxic. The drug is used extensively in clinical anti-coagulation regimes, for example pre- and post-coronary angioplasty procedures. Recently, we have found that photochemical, covalent coupling of dipyridamole to polyurethane surfaces leads to improved thromboresistance in vitro. This phenomenon is now studied in more detail. Both qualitative and more quantitative biochemical experiments were performed in order to characterize the in vitro blood compatibility of a set of polyurethane surfaces onto which dipyridamole was immobilized. First, scanning electron microscopy was used to examine the morphology of platelets which adhered during incubation with platelet-rich plasma. These experiments showed that immobilization of dipyridamole leads to a clearly decreased number of adherent platelets and to a largely diminished propensity of the surface to activate adherent platelets. Secondly, an in vitro thrombogenicity assay was run. These experiments showed that the thromboresistance increased with increasing surface density of immobilized dipyridamole. A short spacer chain separating dipyridamole from the polymer surface, was found to improve the thromboresistance further. Such a spacer chain apparently increases the efficacy of the immobilized drug. Collectively, the present results further substantiate the idea that dipyridamole retains its inhibitory activity with respect to activation and aggregation of blood platelets, when the compound is covalently attached to a polymer surface. The possible utility of these findings with respect to the development of an artificial blood vessel prosthesis is discussed briefly.

In vivo tissue compatibility of two radio-opaque polymeric biomaterials.

Polymeric biomaterials featuring intrinsic radio-opacity continue to attract considerable scientific attention. This work focusses on two polymers that contain covalently bound iodine, rendering the materials radio-opaque. The first material is hard, transparent and glass-like, and consists of methyl methacrylate, 2-(2'-iodobenzoyl)-ethyl methacrylate (1) and 2-hydroxyethyl methacrylate (HEMA), in the molar ratio 65:20:15, respectively. The second material is a cross-linked hydrophilic network, consisting of HEMA and 1, in the molar ratio 80:20, respectively. Both materials were characterized by means of different physico-chemical techniques, including magic-angle-spinning solid state NMR spectroscopy, infrared spectroscopy and differential scanning calorimetry. Moreover, both materials were implanted subcutaneously in rats for 24 days. Upon explanation and histological examination, it appeared that both materials were well tolerated. No tissue necrosis, abscess formation or inflammation were observed. The samples were found to be surrounded by a vascularized capsule consisting of connective tissue cells. The results reveal excellent tissue compatibility for both materials. This is an important observation, since tissue compatibility is absolutely necessary for the applications which are foreseen for this type of radio-opaque biomaterials.

A versatile three-iodine molecular building block leading to new radiopaque polymeric biomaterials.

A methacrylic monomer containing three iodine atoms, 2- [2',3',5'-triiodobenzoyl]-ethyl methacrylate (compound 1), was prepared in pure form. Compound 1 can be reacted with other methacrylates, such as methyl methacrylate (MMA), and 2-hydroxyethyl methacrylate (HEMA) with high conversion. Typically, less than 0.5% of free monomer is left after polymerization. For example, compound 1 was reacted with MMA and HEMA in the molar ratio 7:73:20, respectively. This yielded a terpolymer with Tg = 86 degrees C, Mw = 47,000 g/ mol and Mn = 22,800 g/mol. This material was characterized by various physicochemical techniques, including gel permeation chromatography, differential scanning calorimetry, thermogravimetric analysis, and nuclear magnetic resonance (NMR) spectroscopy (1H at 400 MHz, DMSO-d6 solution). In addition the material was found to exhibit low surface thrombogenicity in vitro and a low propensity to activate contacting blood platelets. Furthermore it was found that the terpolymer is markedly radiopaque: even thin objects (< 0.5 mm) could be easily visualized using X-ray fluoroscopic techniques as are routinely used in the clinic, e.g., during coronary angiography. The combined results obtained with the present terpolymer (particularly its in vitro hemocompatibility and its radiopacity) leads to the suggestion that this type of polymer could be used as cardiovascular biomaterials, for instance for the construction of a new type of endovascular stents. These would be expected to show improved biocompatibility if compared with metallic stents which are currently used, for instance in conjunction with percutaneous transluminal coronary angioplasty (PTCA). A stent prototype, constructed from the present radiopaque terpolymer, is shown and discussed briefly.

Studies on radio-opaque polymeric biomaterials with potential applications to endovascular prostheses.

A new polymeric biomaterial, which uniquely combines radio-opacity (X-ray visibility) and low thrombogenicity, is described. First, preparation, purification, and identification of the essential monomeric building block, 2-[2'-iodobenzoyl]-ethyl methacrylate (3), are outlined. Second, [Figure: See text] the synthesis of the biomaterial, a terpolymer with composition MMA: HEMA: 3 = 65:15:20 (mole/mole/mole) is described. Third, the physico-chemical characteristics of the polymer (e.g. NMR spectroscopy, thermal behaviour) are given. Fourth, the in vitro thrombogenicity of the material was characterized by means of recent test assay. The combined results reveal that the terpolymer is very suitable for prosthetic applications in the cardiovascular system. A new prototype of an endovascular stent, made from the terpolymer, is presented. Stents find clinical use in interventional cardiology, in conjunction with percutaneous transluminal coronary angioplasty (PTCA). It is put forward that the stent prototype presented herein has, at least in principle, some advantages over existing (metallic) stents; these advantages are primarily owing to the unique combination of X-ray visibility and haemocompatibility which is presently achieved.

Preparation, characterization and radiolabeling of a new amphiphilic derivative of DTPA.

A new derivative (1) of diethylenetriamine pentaacetic acid (DTPA) is described. Compound 1 contains a hydrophobic unit (a triphenyl methyl group) and a hydrophilic unit (a DTPA ester), and therefore behaves as an amphiphile in aqueous solution. Compound 1 appears to form a stable inclusion complex with 99mTc. A rabbit model was used in a scintigraphic study of the biodistribution of the complex 99mTc-1, using 99mTc-DTPA as a control. The resulting images revealed marked differences: 99mTc-1 showed rapid uptake in the liver followed by excretion in the gallbladder and intestines within 1 h, whereas 99mTc-DTPA appears, as expected, in the renal-pelvico system and in the bladder. These findings are significant as they provide more insight into the complex relationship between structural and physicochemical properties of radiopharmaceuticals and their biodistribution. Knowledge of such relationships is absolutely mandatory with respect to the development of new radiopharmaceuticals with increased efficacy and/or specificity.

Studies on a new strategy for surface modification of polymeric biomaterials.

A new methodology to improve the hemocompatibility of polyurethane (medical grade Pellethane D-55) surfaces is reported. The approach is essentially based on a photochemical immobilization reaction. Two new conjugate molecules, compounds 2 and 3, were prepared. They consist of (i) dipyridamole, a well-known inhibitor of platelet activation, and a vasodilating drug with clinical application, for instance before and during pecutaneous transluminar coronary angioplasty (Dottering); and (ii) an aryl azide, a moiety that exhibits marked photoreactivity. In 2, the dipyridamole unit is directly linked to the aryl azide (via an ester bond), while a short spacer chain separates both units in 3. Upon irradiation of 2 or 3, adsorbed onto the polyurethane foil, the aryl azide is converted into a highly reactive species which reacts with a nucleophilic group on the polymer surface. In this way, the dipyridamole is covalently linked to the polymer. The underlying principle is also used in photoaffinity labeling, a well-known technique in biochemical studies on enzyme structure and function. From UV extinction experiments it could be deduced that the surface-density of immobilized 2 is 4.9 nmol/cm2. The surface density for 3 was 14.6 nmol/cm2. The surfaces were subjected to an in vitro thrombin generation assay. This assay gives a valuable impression about the hemocompatibility of artificial surfaces. These experiments revealed that the clotting times were substantially prolonged as a result of the photoimmobilization of dipyridamole. This was especially the case for immobilized 3. This effect cannot be readily explained. Possibly, the enhanced activity of immobilized 3 is due to the spacer chain. An alternative explanation is that the surface density is larger for 3 than for 2. In addition, the photomodified surfaces were incubated with platelet-rich blood plasma (37 degrees C, 30 min) and subsequently examined by scanning electron microscopy. The morphology of the blood platelets adhered to the surface also showed that hemocompatibility increased in the order untreated polyurethane < polyurethane with immobilized 2 < polyurethane with immobilized 3. Future work will concentrate on evaluation of the role of the spacer (length, hydrophilicity, etc.), as well as on the possible use of this approach with respect to the construction of biomaterials with improved in vivo biocompatibility, in particular hemocompatibility.