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Supercritical CO2 Based Production of Magnetically Responsive Micro- and Nanoparticles for Drug Targeting Pratibhash Chattopadhyay and Ram B. Gupta* Department of Chemical Engineering, Auburn University, Auburn, Alabama 36839-5127
Magnetically responsive, controlled-release drug formulations are needed for targeted drug delivery, especially for toxic drugs. A supercritical antisolvent (SAS) technique is used to produce magnetically responsive poly(methyl methacrylate), 50/50 DL-poly(lactide-co-glycolide), and Eudragit RS polymer particles via coprecipitation of the polymer with a suspension of magnetite particles in mineral oil and a fatty acid surfactant, using dichloromethane as the solvent. The SAS technique is again used to precipitate drug-loaded magnetically responsive polymer particles. A modification of the SAS process, the SAS-EM technique developed at Auburn University, is used to produce magnetite-encapsulated polymer nanoparticles of controllable sizes. In SASEM, atomization and increased mixing within the supercritical phase is brought about using a surface vibrating at an ultrasonic frequency. Size, morphology, and drug-release kinetics of the obtained particles are studied. Introduction Strategies involving the use of nano- and microparticulate drug delivery systems have been under investigation for several years now. Particulate drug delivery systems are micron- or submicron-size biodegradable polymer reservoirs in which the drug is either encapsulated or adsorbed or chemically associated. Advantages of these drug delivery systems include site-specific delivery of the therapeutic agent, modulation of pharmokinetics, and alteration of the distribution profile of the drug. Most of the existing drug substances can be administered using these delivery systems to ensure their better utilization with reduced adverse side effects and improved therapeutic efficacy. In recent years, polymer microparticles containing magnetite nanospheres have found application in different areas of biological/medical research and therapy. These particles have been used as contrasting agents for magnetic resonance imaging scans,1 for separation of biological products,2 and also more importantly for site-specific delivery of drugs using external magnetic fields.3,4 For example, in the administration of antineoplastic agents used in systemic chemotherapy, by targeting a local tumor site using an external magnetic field, magnetic drug carriers can be used to allow higher tumor drug concentrations to be attained while reducing the systemic toxicity.5 In fact, tail vein experiments conducted by Vyas and Malaiya6 have shown a 60-fold increase in the target concentrations in the presence of magnetic field for magnetite-encapsulated, indomathacin-loaded poly(methyl methacrylate) (PMMA) particles. The magnetized polymer particles possess different magnetic properties based on their size, surface properties, and method of preparation. Studies have shown that nanosize magnetic particles (10-50 nm) are superparamagnetic in nature. When these magnetic nanoparticles are incorporated inside the polymer microspheres, composite particles are formed that have a * To whom correspondence should be addressed. Phone: (334) 844-2013. Fax: (334) 844-2063. E-mail: gupta@ auburn.edu.
unique blend of properties of both the polymer and the magnetic material. Indeed, new areas of applications, for example, such as paint pigments, toners, or sound carriers, can open up after the production of these particles. Drug Targeting Using Polymer/Magnetite Particles. The main principle governing the delivery of drugs using magnetite-encapsulated biodegradable polymer particles (also referred to in this paper as polymer/ magnetite particles) is based on the attractive forces between the applied magnetic field at the target site and the magnetic material dispersed within the drugloaded polymer particles.5 The polymer/magnetite particles are injected into the blood supply of the target organ in the presence of an external magnetic field that drives the particles through the blood vessels and onto the targeted site. This, in turn, causes an increase in the localized drug concentration at the targeted site and reduces toxic side effects of the drug associated with other parts of the body. At the target site the magnetic field must be applied long enough to allow the particles to deliver the drug onto the affected cells. The drug is released from the polymer/magnetite particles by diffusion through the polymer matrix. Release of the drug can also be triggered by using high-frequency magnetic fields, by means of laser pulses, or by electromagnetic heating of the particles.7,8 In vivo studies by several researchers have shown that, by using polymer/magnetic drug targeting, a much higher amount of the drug is retained in the targeted area, leading to a drastic change in the distribution of the administered drug. The magnetite particles used to produce these polymer/ magnetite particles, in general, have mean sizes of less than 30-50 nm. In this size range the magnetite particles exhibit the phenomenon of superparamagnetism, in which the substances have characteristics of both ferromagnetic and paramagnetic substances. The small size of these materials enables them to have a single magnetic domain, and this prevents them from remaining magnetic in the absence of an applied magnetic field. When these magnetic particles are coated with a biodegradable polymer, they become
10.1021/ie020205b CCC: $22.00 © 2002 American Chemical Society Published on Web 10/25/2002
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biocompatible and are capable of being injected into our bodies as drug carriers.3,9 Several techniques have been developed in the past for manufacture of magnetic-polymer microparticles. The majority of these techniques involve first the precipitation of the magnetite particles and then subsequently their encapsulation using a suitable biodegradable polymer. Dresco et al.2 used the seed precipitation polymerization technique for the preparation of polymeric magnetic particles using water in oil microemulsions. Chu et al.3 prepared superparamagnetic magnetic particles encapsulated by homopolymers or copolymers using a water-in-oil microemulsion system. Liberti et al.10 first precipitated magnetite particles from ferric and ferrous salts and then coated them using different materials such as polymers, proteins, polypeptides, etc. Kawaguchi et al.11 prepared polymeric microspheres with an aqueous solution of Fe2+ and Fe3+, followed by base precipitation using ammonia to form magnetite. Hsieh and Langer12 produced these particles by encapsulating a small magnetic steel bead or a hemispheric magnetic pellet, along with the drug, inside a polymer matrix. The main disadvantage of these techniques is that encapsulation of the drug inside the polymer/magnetite particles is difficult, and even if drug encapsulation is possible, the manufacturing method has to be tailored for each individual drug. Other problems of using these techniques may include proper size control of the nanoparticles and production of dry particles. Particulate Formation Using Supercritical Fluids. In recent years, the use of supercritical fluids as a media for the formation of microparticles has shown tremendous success.13 Supercritical fluids offer significant advantages over other conventional routes of microparticulate formation, namely, mild operating temperatures, high purity of products, and production of solvent-free, dry particles.14,15 One of the popular methods of particle formation using supercritical fluids is the supercritical antisolvent (SAS) technique. This technique has been successfully used for the size reduction of many pharmaceutical substances such as proteins, antibiotics, and steroids.16-18 Apart from micronization of pharmaceuticals, this technique has also been successfully used for the encapsulation of various drugs inside biodegradable polymers to form particles that can be used for drug delivery and controlled release.19 Mueller and Fischer20 utilized the SAS technique for the preparation of clonidine hydrochloride particles microencapsulated inside a poly(L-lactic acid) matrix, of sizes around 100 µm, showing an in vitro release of over 40% of the drug in the first 1.5 h. Falk et al.21 used the SAS technique for the encapsulation of gentamycin, naloxone, and naltrexone inside poly(L-lactide) microspheres. The drug/polymer particles obtained were spherical in shape and between 0.2 and 1.0 µm in diameter. Release rate studies showed a linear release profile from the polymer for more than 7 weeks, even at drug loadings of near 25 wt % in the case of gentamycin. Naltrexone also exhibited similar release characteristics. In this paper, we report the formation of magnetiteencapsulated biodegradable polymer particles of controllable sizes for drug targeting, using the SAS and the SAS-EM techniques. The SAS technique is used to produce polymer/magnetite microparticles. A solution of the biodegradable polymer and the magnetite nano-
particulate suspension in mineral oil and a fatty acid surfactant, in a suitable solvent, is used as the organic phase, and this solution is injected into the supercritical phase to precipitate the polymer/magnetite microparticles. The SAS technique is also used to encapsulate a model drug, indomethacin, into these polymer/magnetite particles by coprecipitation of the drug, the polymer, and the magnetite nanoparticles. The choice of indomethacin as the model drug was made because of its excellent potential as a candidate for drug delivery by means of magnetically responsive polymer/magnetite particles. Indomethacin is a poorly water-soluble, nonsteroidal, antiinflammatory drug having an acidic function.22 It is used for the treatment of rheumatoid arthritis, which is a painful chronic disease and involves humoral and cellular immune responses resulting in tissue destruction and inflammation. The main problem that limits extensive use of the drug is the side effects caused by the drug such as ulceration and kidney and CNS toxicity.23 An effective way of overcoming this problem is by using magnetically responsive drug-delivery systems that would increase local concentrations of indomethacin at the inflamed site and hence reduce the systemic side effects. Dissolution kinetic experiments were also carried out to obtain the release rate profiles of indomethacin encapsulated inside the polymer/ magnetite particles. The SAS-EM technique developed at Auburn University24-27 is used to produce polymer/magnetite nanoparticles. The SAS-EM overcomes certain limitations of the SAS technique and can be used for the precipitation of particles of controllable sizes in the nanometer range. Like the conventional SAS technique, the SAS-EM utilizes supercritical CO2 as the antisolvent, but the solution jet is deflected by a surface vibrating at an ultrasonic frequency that atomizes the jet into much smaller droplets. The ultrasound field generated by the vibrating surface enhances mass transfer and prevents agglomeration through increased mixing. Particles of different sizes in the nanometer range are obtained through variation of the vibration intensity of the deflecting surface by changing the power supplied to the attached ultrasound transducer. In two previous SAS studies, ultrasonic nozzles have been used: (a) Randolph et al.28 used a Sonotek atomizer having a capillary tube, which was vibrated at 120 kHz to produce a narrow cylindrical spray; (b) Subramanian et al.29 used Sonomist model 600-1 to generate and focus the high-frequency sonic waves for atomization. The SAS-EM technique is different because it does not use any specialized nozzle. The solution is sprayed on a ultrasonically vibrating surface which atomizes the solution into ultrafine droplets. Experimental Section Materials. CO2 and N2 (both 99.9% pure) were supplied by Airco. Indomethacin (99% pure, lot no. 60K0745) was purchased from Sigma. Dichloromethane (DCM; 99.9% pure) and phosphate buffer (pH 7.0) were obtained from Fisher Scientific and used as received. PMMA was purchased from Aldrich, Eudragit RS100 was supplied by Ro¨hm America, and poly(50/50-DLlactide-co-glycolide) (PLGA; lot no. S0163S114) was supplied by Medisorb. Magnetite suspension (EMG 905, lot no. F032400C) was purchased from Ferrofluidics Corp. The carrier liquid used to prepare the suspension was a light hydrocarbon mineral oil. The magnetite
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Figure 1. Schematic representation of the SAS apparatus used for the formation of polymer/magnetite particles using supercritical CO2 as the antisolvent: A, B, nitrogen and carbon dioxide cylinders, respectively; SP, ISCO syringe pump; V, control valves; ID, solution injection system, a piston device; R, precipitation cell; P, pressure gauge.
Figure 2. Schematic representation of the precipitation cell used for the SAS-EM experiment for the formation of PLGA/magnetite particles using supercritical CO2 as the antisolvent.
particles in the suspension had a mean particle size of 10 nm and were coated with a fatty acid surfactant. The saturation magnetization of the fluid at 25 °C was 400 G ((10%), and the viscosity was
