Paclitaxel Drug Delivery from Cardiovascular Stent

Transcription

1 88 Trends Biomater. Artif. Devesh Organs, Kothwala, Vol 19(2), Ankur pp Raval, (2006) Animesh Choubey 1, Chhaya Engineer and Haresh Kotadia Paclitaxel Drug Delivery from Cardiovascular Stent Devesh Kothwala*, Ankur Raval*, Animesh Choubey 1, Chhaya Engineer* and Haresh Kotadia* *Research and Development Division Sahajanand Medical Technologies, Surat , India 1University of Texas at San Antonio, TX-78249, USA The study was undertaken to assess the feasibility of sustained intracoronary delivery of paclitaxel from a polymer-coated stent. Characterization of the coating morphology and its correlation with the mechanism of drug release is critical for the development and understanding of controlled drug delivery coatings. In the present study, three successive layers of drug mixed with biodegradable polymer solutions were applied on SS 316 LVM stents using air suspension spray coating technique. In-vitro release of paclitaxel at regular intervals for 38 days from stents was analyzed using high performance liquid chromatography (HPLC). Scanning electron microscopy (SEM) was used to characterize the mechanism of drug delivery from multilayered biodegradable polymer based stent and it was observed that the drug particles were released owing to swollen polymeric matrix and bulk erosion. Introduction Stent implantation has become the major method of percutaneous myocardial revascularization [1]. However, in-stent restenosis continues to limit the long-term success of this approach [2]. The concept of local drug delivery via coated stents, couples the biological and mechanical solutions necessary to maximize the angiographic results and facilitate the recovery of the vessel from the injury caused by the stent implantation. At the same time local drug delivery using a drugeluting stent offers the advantage of allowing high local concentrations of drug at the treatment site while minimizing systemic toxic effects. Biodegradable polymers have been used in controlled drug delivery for many years [3]. These biodegradable polymers degrade within the body as a result of natural biological processes, eliminating the need to remove a drug delivery system after release of the active agent has been completed. The reported mechanism of erosion, which controls the release of drugs from cardiovascular stents, is insufficiently investigated. The current research work aims at probing the mechanism of paclitaxel drug release from multilayer drug-polymer coated stent. The surface morphology of drug-coated stent was observed using scanning electron microscopy before and after incubation in phosphate buffer saline solution (ph 7.4) at 37 C for 38 days to understand the in-vitro drug release mechanism. High pressure liquid chromatography was used to explore the drug release kinetics. 2. Materials and Methods Materials The SS 316 LVM 16 mm MATRIX stents (Sahajanand Medical Technologies, India) were used in the study. Paclitaxel was obtained from Bioxel Pharma Inc., Canada and used without further purification. Polymers; 50/50 Poly DL lactide-co-glycolide, 75/25 Poly L lactide-cocaprolactone (Alkermes Inc., USA), Poly L- lactide (Purac Inc., USA) having inherent viscosity (IV) 0.60 dl/g, 1.2 dl/g and 1.63 dl/g respectively were used as carriers for drug. Poly vinyl pyrrolidone (PVP K-90/D) was procured from ISP Technologies Inc., Wayne, NJ, USA. The typical molecular weight for PVP K-90/D was Da. The solvent dichloromethane

2 Paclitaxel Drug Delivery from Cardiovascular Stent 89 (DCM) and other chemicals used in the current investigation were of HPLC grade procured from Ranbaxy Fine Chemicals Ltd, India. Methods Biodegradable polymers; 50/50 Poly DL lactideco-glycolide, 75/25 Poly L lactide-cocaprolactone, Poly L-lactide and Poly vinyl pyrrolidone were dissolved in HPLC grade dichloromethane (DCM). Various compositions of these polymers with paclitaxel were formulated to achieve 42% of total drug in layer A (base layer), 28% of drug in layer B (middle layer) and 30% of total drug in layer C (top layer) followed by a polymeric layer (layer D) of Poly vinyl pyrrolidone for protection against moisture and to prevent premature drug release at the time of implantation. Some stents were also coated with similar polymeric matrix without any drug using identical coating parameters. Stents were stored in amber colored glass vials after washing with de-ionized water and hot air drying to avoid any possible particle deposition. Before coating, the stents were weighed using analytical balance (Citizen CX-265) having 0.01 mg accuracy. They were coated by an indigenous coating machine using modified air suspension technique. Programmable controller box having fixed coating and drying periods, 50 and 20 seconds respectively, was used to coat successive layers (A, B, C and D). Stents were dried in ambient conditions for about hours. All the coating procedure was performed in class 100 clean room having temperature 25±3 C and relative humidity 50±10%. The HPLC system used for paclitaxel drug analysis was LC-10ATVP pump (Shimadzu, Japan) equipped with UV-VIS detector: SPD- 10AVP (Shimadzu, Japan), Rheodyne 1303 integrator (Rheodyne, USA). The column used was ODS PR C-18 (5µ pore size) 250x4.6 mm (Phenomenex). Gravimetrically assessed stents having weight 200±20 µg were evaluated using HPLC for precise qualitative and quantitative analysis. Stents were analyzed for drug content using mobile phase consisting of Water:Acetonitrile: Methanol (35:5:60 v/v) at flow rate of 1.0 ml/min. Detector wavelength was set at 227 nm. Two stents were evaluated for paclitaxel content and two stents for in-vitro paclitaxel release kinetics from biodegradable polymer matrix over 38 days in phosphate buffer saline (PBS) solution (ph 7.4) at 37 C with constant agitation at 55 rpm. Both the stents were removed at 1, 7, 14, 28 and 38 days from their release vials and analyzed for amount of paclitaxel release in PBS. Scanning electron microscopy (SEM) was done to analyze the coating morphology and degradation mechanism. 3. Results and Discussion Paclitaxel content and in-vitro release kinetics Paclitaxel drug content on two stents was found to be µg and µg. Similarly; the release kinetics of stents was evaluated using HPLC. Figure 1 represents the in-vitro paclitaxel release kinetics for 38 days at regular intervals. For coated stent, t1/2 (i.e. period of time required for half of the quantity of active substance to be consumed from the reservoir) was initial 7 days time period, which can be noted from the cumulative release profile. Figure 1 also depicts that the release rate is time dependent i.e. decreasing as the time progresses which is the characteristic of the polymer matrix system where the drug is directly dispersed or dissolved in the polymeric blend. Paclitaxel Release (µg) Time (days) Stent 1 Stent 2 Fig. 1: Cumulative paclitaxel release profile of stents in PBS (ph 7.4) for 38 days at 37 C Coating characteristics and drug release mechanism Representative SEM images of the surface of polymer coated stent and stent containing 29% paclitaxel and 71% polymer matrix can be seen in Figure 2 and 3 respectively. Morphology of the drug-polymer coated surface reveals a discrete particulate phase in contrast to the

3 90 Devesh Kothwala, Ankur Raval, Animesh Choubey 1, Chhaya Engineer and Haresh Kotadia Fig. 2: Scanning electron micrograph of polymer coated stent from any irregularities such as cracking, flacking and delamination. The SEM image of cardiovascular stent in Figure 4 was obtained following its 38 days incubation in PBS at 37 C. Small voids observed on the surface, were regions previously occupied by paclitaxel particles that were released from the coating. It is evident from in-vitro release profile (Figure 1) that release of paclitaxel shows zero order release kinetics for initial burst phase of 7 days from layer C and B that consists of 50/50 Poly DL lactide-co-glycolide. This indicated that early release of drug is via dissolution of the paclitaxel particles from the surface of the coated stents and rapid bulk erosion of the polymer. After the disappearance of the initial burst effect, second phase release begins and as evident from the release profile, the release rate is slow. (a) (a) (b) Fig. 3: Scanning electron micrograph of drugpolymer coated stent polymer coated stent surface that reveals a smooth appearance. This particulate phase may be the result of incorporation of paclitaxel drug in the polymeric matrix as no such morphology could be observed in case of polymer coated stent surface (Figure 2). The surface of the coated stent was found to be free (b) Fig. 4: SEM micrograph of drug-polymer coated stent after incubating at 37 C in PBS (ph 7.4) for 38 days

4 Paclitaxel Drug Delivery from Cardiovascular Stent 91 This slow release of paclitaxel compared to the initial phase takes place as layer A incorporates highly hydrophobic and crystalline (@37%) polymer, Poly L lactide [4,5]. Poly L lactide and 50/50 Poly DL lactide-co-glycolide are bulk eroding polymers where ingress of water is faster than the rate of degradation [6]. This degradation occurs throughout the polymer matrix and proceeds until a critical molecular weight is reached where degradation products become small enough to be solubilised. At this juncture, the structure starts to become significantly more porous and hydrated leading to the release of drug dissolved in the polymer. Drug can only be released from the porous surface regions, where diffusion of drug into the degradation medium is possible. Therefore, release begins when the reaction-erosion fronts are set-up, and continues as the fronts move, until they meet in the cellular environment, by which time all drug has been released. As swelling fronts are clearly observed in Figure 4a, it can be assumed that non-homogenous uptake of water by polymer matrix might cause polymer chains to degrade unevenly along the matrix cross sections. This creates free carboxylic acid groups which lead to a decrease of ph inside the swollen polymer matrix [7]. As the surface of the polymer is kept at neutral ph, a ph gradient develops that slows down the degradation of the polymer matrix surface compared to the core. The surface layer breaks at some point when a critical osmotic pressure builds up inside the matrix due to the accumulation of degradation products which is evident from the Figure 4b. Percolation phenomena are based on the fact that degradation products can apparently not leave the matrix prior to the erosion onset [8]. Only after a critical degree of degradation is reached, the polymers form a network of pores as could be observed from Figure 4a, that allows for the release of degradation products and drug. Based on the predicted mechanism of drug release from cardiovascular stent a schematic could be drawn as illustrated in Figure 5. Figure 5a shows drug dissolved in the polymeric matrix which is released to the external environment after swelling and bulk erosion (Figure 5b). 4. Conclusion Present research work investigates the drug release mechanism from multilayered drugpolymer coated cardiovascular stent using SEM and HPLC. It was observed that release of drug was due to intake of fluid from simulated biological environment (PBS). The swelling increases the aqueous solvent content within Fig. 5: Schematic of drug delivery from matrix swelling-controlled release system

5 92 Devesh Kothwala, Ankur Raval, Animesh Choubey 1, Chhaya Engineer and Haresh Kotadia the formulation as well as the polymer mesh size, enabling the drug to diffuse through the swollen network into the external environment. In-vitro release profile confirms an early burst release due to surface paclitaxel followed by a sustained slower release from the bulk coating that characterizes the polymer matrix system. Acknowledgments Authors wish to thank Mr. Rajesh Vaishnav, Technical Director, Sahajanand Medical Technologies and Dr. Vandana Patravale, UICT Mumbai, for providing the technical support. The authors also express their sincere gratitude to Mr. Dhirajlal Kotadia, Chairman, Sahajanand Group of Companies, for providing financial assistance to carry out the research work. Contributions from Mr. Kamlesh Tailor are also gratefully acknowledged. References [1] Serruys PW, et. al. A comparison of balloon expandable stent implantation with balloon angioplasty in patients with coronary artery disease. N Engl J Med.1994; 331: [2] Nobuyoshi M, Kimura T, Ohishi H et al. Restenosis after percutaneous transluminal coronary angioplasty: pathologic observations in 20 patients, J Am Coll. Cardiol.1991; 17(2): [3] Panyam J, Labhasetwar V. Biodegradable nanoparticles for drug and gene delivery to cells and tissue, Adv. Drug. Deliv. Rev. 55 (2002) [4] Cutright DE, Bienvenido P, Beasly JD, Larsen WJ. Degradation rates of polymers and copolymers of polylactic and polyglycolic acids. Oral Surg. Oral Med. Oral Pathol.1974;37:142. [5] Agrawal CM, Niederauer GG, Micallef DM, Athanasiou KA. The use of PLA-PGA polymers in orthopedics. In: Wise DL, Trantolo DJ, Altobelli DE, Yaszemski MJ, Greser JD, Schwartz ER. editors. Encyclopedic handbook of biomaterials and bioengineering Part A. Materials, vol. 2. New York: Marcel Dekker; p [6] Dickers KJ, Milroy GE, Huatan H and Cameron RE. The Use of Biodegradable Polymers in Drug Delivery Systems to Provide Pre-Programmed Release. Cambridge Centre for Medical Materials, Department of Materials Science and Metallurgy, University of Cambridge; The Drug Delivery Companies Report Autumn/ Winter [7] Sykes PM. A guidebook to mechanism in organic chemistry, 4th ed. London, Longman; [8] Gopferich A. Langer R. Modeling of polymer erosion. Macromolecules 1993; 26: