2740
Biomacromolecules 2010, 11, 2740–2747
Bioactive Polymeric Systems with Platelet Antiaggregating Activity for the Coating of Vascular Devices G. Rodrı´guez,*,† M. Ferna´ndez-Gutie´rrez,† J. Parra,‡ A. Lo´pez-Bravo,‡ N. G. Honduvilla,§ J. Buja´n,§ M. Molina,| L. Duocastella,| and J. San Roma´n† Instituto de Ciencia y Tecnologı´a de Polı´meros, CSIC, and CIBER-BBN, c/Juan de la Cierva 3, 28006 Madrid, Spain, Servicio de Anatomı´a Patolo´gica, Unidad de Investigacio´n Clı´nica y Biopatologı´a Experimental, Hospital Provincial, Complejo Hospitalario de A´vila, SACYL and CIBER-BBN, c/Jesu´s del Gran Poder 42, 05003, A´vila, Spain, Departamento de Cirugı´a, Facultad de Medicina, UAH, and CIBER-BBN, Cra. Madrid-Barcelona km 33, 600, 28871 Alcala´ de Henares, Madrid, Spain, and Iberhospitex S.A. Av., Catalunya 4, 08185 Llic¸a de Vall, Barcelona, Spain Received July 16, 2010; Revised Manuscript Received September 8, 2010
The preparation, characterization, and analysis of physicochemical and biological properties of a new bioactive polymer system, based on a copolymer of an acrylic derivative of triflusal (a molecule with chemical structure related to aspirin with antiaggregating activity for platelets) is described and evaluated as thin bioactive coating for vascular grafts and coronary stents. The acrylic monomer derived from triflusal (THEMA) provides random copolymers when it is polymerized with butyl acrylate (BA), according to their reactivity ratios, rTHEMA ) 1.05 and rBA ) 0.33. The copolymer THBA70, containing a molar composition fTHEMA ) 0.45 and fBA ) 0.55 presents the optimal properties of stability, flexibility, and adhesion, with a Tg ) 21 ( 2 °C, to be applied as bioactive and biostable coatings for vascular grafts and coronary stents. Thin films of this copolymer system present an excellent biocompatibility and a good inherent antiaggregant activity for platelets.
1. Introduction Percutaneous transluminal coronary angioplasty (PTCA) is a minimally invasive procedure performed to reduce the narrowing or obstruction of coronary arteries. This involves steering a balloon catheter to the site of the blockage and inflating the balloon to compress the deposit that obstructs the artery and to re-establish the blood flow. The most important complication associated to this therapy is restenosis, that is, renarrowing of the vessel, which occurs in 3040% of coronary lesions within 6 months after this intervention.1 To improve the clinical outcome of PTCA, bare metallic stents have been used. Stents are small metal scaffolds that are placed on the balloon and then expanded into the damaged artery at the site of the stenosis. Although the use of these devices has reduced the restenosis rate to 20-30%, they often increase the incidence of inflammation, thrombosis, and fibromuscular proliferation.2 Several approaches have been described to optimize stent design, and these can be summarized in three groups: coated metallic stents, biodegradable stents, and drug-eluting stents (DES). In this sense, the most successful strategy has been the use of DES, which are stents coated with synthetic polymers that act as drug reservoirs and elute specific antiproliferative drugs over several weeks or months.3,4 Up to now, there are some DES products in the market that release drugs such as sirolimus, paclitaxel, zotarolimus, or everolimus. These products improve significantly the incidence of restenosis, although some adverse effects, such as * To whom correspondence should be addressed. Tel.: +34 915 618 806. Fax: +34 915 644 853. E-mail:
[email protected]. † Instituto de Ciencia y Tecnologı´a de Polı´meros. ‡ ´ vila. Complejo Hospitalario de A § UAH. | Iberhospitex S.A. Av.
late stent thrombosis, have been described recently.5,6 According to this behavior, we consider that the application of bioactive coatings with inherent antithrombogenic activity can improve the stent applications and guarantee the nonthrombogenic activity at long-term. We consider “bioactive polymer coatings” as polymeric systems that display a specific biological or pharmacological activity. The polymeric formulations described in this article present an antiaggregating activity for platelets, and therefore, they can be considered as “bioactive systems”. This work describes the preparation and physicochemical characterization of new polymeric systems for stent coating, which contains triflusal covalently attached to the polymer backbone. Triflusal, 2-acetoxy-4-trifluoromethylbenzoic acid, is a powerful platelet aggregation inhibitor. With a chemical structure closely related to aspirin, this drug prevents platelet aggregation by an irreversible inhibition of platelet cyclooxigenase activity.7 In vitro platelet adhesion test, as well as in vitro biological response in human umbilical vein endothelial cell (HUVEC) cultures, have also been evaluated for a model triflusal containing polymer, THBA70, prepared at high conversion from a feed molar fraction of 0.45 of THEMA, giving rise to a molar fraction of THEMA ) 0.45 ( 0.02 in the copolymer system. This polymer presents excellent properties for the development of stent coating with inherent antiaggregating modulation of platelets, as well as support for the release of an antiproliferative drug to prevent restenosis.
2. Materials and Methods 2.1. Copolymer Synthesis. Poly(THEMA-co-BA) copolymer synthesis was carried out from a methacrylic derivative of triflusal, THEMA, 2-metacryloyloxyethyl [2-(acetyloxy)-4-(trifluoromethyl)] benzoate, synthesized in our laboratory as described elsewhere.8
10.1021/bm100801k 2010 American Chemical Society Published on Web 09/24/2010
Systems with Platelet Antiaggregating Activity
Biomacromolecules, Vol. 11, No. 10, 2010
Table 1. THEMA Molar Fraction in the Feed (FTHEMA) and in the Copolymer (fTHEMA), Conversion, and Molecular Weight Obtained for the Copolymer Synthesized To Determine the Reactivity Ratios of the Copolymer System Poly(THEMA-co-BA) THEMA molar fraction in the feed, FTHEMA
THEMA molar fraction in the copolymer, fTHEMA
conversion, wt %
Mn × 103 g/mol
Mw/Mn
0.2 0.4 0.6 0.8
0.36 0.53 0.66 0.85
4.3 6.2 8.6 7.4
54.5 52.8 56.2 55.0
2.1 1.8 1.9 2.0
Triflusal was kindly provided by Laboratorios Uriach. Butyl acrylate (Aldrich) and 1,4-dioxane (Panreac) were vacuum distilled. 2,2Azobis(isobutyronitrile) (AIBN, Merck) was recrystallized twice from ethanol. Copolymers were synthesized by free radical polymerization of mixtures of the corresponding monomers ([M] ) 0.95 M) in 1,4-dioxane using AIBN as initiator (1.5 × 10-2 M). The polymer containing a THEMA molar fraction of 0.45 (THBA70) was selected as the optimal polymer system for the coating of stents because of its physicochemical properties and bioactivity. This system was obtained by the polymerization of a 70:30 mixture (wt %) of monomers THEMA and BA. Reactions were carried out in the absence of oxygen by bubbling nitrogen for 30 min before sealing the system. The solution was introduced in a thermostatic bath at 60 °C and maintained 72 h to reach high conversion (90% by weight). The copolymer was isolated by precipitation in an excess of ethanol using an ice bath and dried under reduced pressure until constant weight. To evaluate the influence of the polymeric backbone on platelet adhesion, poly(HEMA-co-BA) was also synthesized (HBA). Polymer synthesis was carried out by free radical polymerization of a 45:55 molar mixture of HEMA and BA. Experimental conditions were similar to those described previously for poly(THEMA-co-BA). HBA copolymer corresponds to the THBA70 but without the triflusal residue. 2.2. Polymer Characterization. Structural characterization of the polymers was carried out by NMR and FTIR. 1H and 13C NMR analyses were performed using a Varian XL-300 spectrometer; spectra were recorded at room temperature with deuterated chloroform solutions prepared at 5 and 15% for 1H and 13C NMR, respectively. Polymer compositions were determined from the 1H NMR spectra by the integrated intensities of proton NMR units. Fourier transform infrared spectra in attenuated total reflection mode (ATR-FTIR) were recorded in a Spectrum One FT-IR spectrometer, Perkin-Elmer. Polymer samples were analyzed without further treatment at room temperature by 32 scans and with a resolution of 4 cm-1. 2.3. Determination of the Monomer Reactivity Ratios. Reactivity ratios of the polymeric system were determined from copolymers prepared at low conversion (
