| Background:Coronary stent implantation plays an important role in treatment of coronary heart disease. In-stent restenosis (ISR), has been resolved because of unceasing improvement of drug-eluting stent (DES). However, there are also many malpractices about the DES, especially the formation of advanced thrombus. The advanced thrombus has becomes the new major contradiction after stent implantation, which is caused by the endothelium injuries and unsuccessful reendothelialization. Multifactors take part in the process, such as nonspecific inhibition of normal endothelial cells, inflammation caused by the undegradable materials of stent platform and coating.Materials that have potential use for bioabsorbable stent preparation include various biodegradable polyesters, polyanhydrides, poly(orthoesters), polyurethanes, poly(ether amides), poly(amino acids) and tyrosine-derived polycarbonates. However, the problem of biodegradable stent about poor biocompatability and strength, contradiction between anti-proliferation and reendothelialization remains unsolved. Therefore, the main research direction of coronary stents is to develop biodegradable polymer coating materials of favourable biocompatibility, controllable drug release system and stent materials owning strong support.Calcium phosphates are a family of inorganic biodegradable polymers which has been widely applied as tissue engineering scaffold. Among the family members, amorphous calcium phosphate (ACP) has become increasingly significant in biomaterial tissue engineering. ACP was used mainly in the form of particles or powders, as an inorganic component incorporated into biopolymers, to adjust the mechanical properties, biodegradability, and bioactivity of the resulting composites. In this study, It was the first time we introduced the amorphous calcium phosphate into vascular stent application. ACP was blended with Poly(lactic-co-glycolic acid (PLGA) and Poly L Lactic Acid (PLLA) for degradable stent coating and platform useness, respectively. Degradation, drug release kinetics, biocompatibility, and radial strength were tested in vitro and/or in vivo.Objective:1. Building a novel biodegradable polymer PLGA/ACP for drug eluting stent (DES) applications, the purpose is to determine a best ratio of PLGA (Lactide/Glycolide) and PLGA/ACP for final product according the degradation testing in vitro and in vivo.2. The study focused on the investigation of paclitaxel/sirolimus combination release profiles from a novel biodegradable polymer PLGA/ACP coated stent both in vitro and in vivo. Meanwhile, in vivo biocompatibility of the material was tested in a rat stent model.3. Building a novel biodegradable polymer PLLA/ACP for drug eluting stent (DES) applications, the purpose is to test whether the strengh of PLLA/ACP is superior to simple PLLA before or after degradation in vitro.Methods:1. Degradation testing:Degradation profiles of different PLGA/ACP composites coated stents were studied both in vitro and in vivo for three months. For the in vitro study, stents were immersed into the phosphate buffered saline with constant shaking. The polymer weight loss was measured weekly and morphological changes were analyzed. Based on in vitro results,65%PLGA (Lactide/Glycolide=65/35, mw/mw) with35%ACP coated stents were selected and implanted into rat aortas for the in vivo study.2. The drug release experiment:The present work was focused on the investigation of paclitaxel/sirolimus combination release profiles from PLGA/ACP coated stent both in vitro and in vivo. For the in vitro study, the drug releasing profiles were characterized by measuring the drug concentration in a drug release medium at predetermined time points. For the in vivo study, a rat aorta stenting model was employed.3. Biocompatability in vivo study:316L metal stents were coated with polyethylene-co-vinyl acetate/poly-n-butyl methacrylate (PEVA/PBMA), PLGA or PLGA/ACP composites, and implanted into rat aortas for one and three months, damage fraction, restenosis rate, endothelial score and inflammation score were recorded.4. Physical and mechanical properties in vitro:Twenty-four equivalent extruded tube segments of PLLA and PLLA/ACP composite were incubated individually in sealed black containers with Dulbecco’s phosphate buffered saline solutions and then submerged in a37℃water bath shaken constantly at30rpm. Samples from each group were collected after6weeks,3months and6months, and vacuum dried at room temperature for48hours prior to radial strength testing using a catheter tensile testing machine. Results:1. Degradation testing:Approximately60%of polymer was degraded within the three-month period and there was no significant difference between the different PLGA/ACP composites. However, the composite of50%PLGA (Lactide/Glycolide=65/35, mw/mw) with50%ACP showed a slightly faster degradation rate than other composites. Microscopic observation showed that no composite was found on any of the implanted stents at12weeks post implantation in vivo study.2. The drug release experiment:Both paclitaxel and sirolimus had a two-phase release profile both in vitro and in vivo, which is similar to the drug release profile of their individual coated DESs, and there is no evident of interference between these two drugs.3. In vivo biocompatibility:Comparing with both PEVA/PBMA and PLGA groups after one month, the results showed that stents coated with PLGA/ACP had significantly reduced restenosis, reduced inflammation and increased speed of re-endothelialization. After three months, the PLGA/ACP group still displayed lower inflammation score and higher endothelial scores as compared with the PEVA/PBMA group.4. Physical and mechanical properties in vitro:From in-vitro studies, PLLA/ACP/PTX stent tube maintained significantly greater radial strength than control group during6months in-vitro degradation. Moreover, ACP facilitated the hydrolytic degradation of PLLA compared with control one, meanwhile, it also increased the crystallinity of PLLA at6months. From SEM observations, ACP created nanometer pores which enlarged gradually to a micrometer scale as degradation proceeded.Conclusions:1. The ideal ratio of PLGA/ACP in the final product should be the one having a balance between degradation, neutralization and viscosity. For this reseaon, the65%PLGA (Lactide/Glycolide=65/35, mw/mw) with35%ACP, was selected for coating polymer.2. Paclitaxel and sirolimus can be combined pharmacokinetically in a DES for the treatment of coronary arterial diseases; the65%PLGA (Lactide/Glycolide=65/35, mw/mw) with35%ACP composite was suitable for drug carrier as stent coating.3. No pathological evidence of toxicity was found in either PLGA/ACP. The co-formulation of ACP into PLGA resulted in improved biocompatibility without systemic toxicity.4. ACP strengthened the radial strength of PLLA stent tube. PLLA/ACP remained enough support during the degradation process in the first6months in vitro. PLLA/ACP may become a potential material for fully biodegradable stent. |
