Abstract:
The present invention provides an apparatus and methods of producing particles that include a polymer, a wax and/or lipid and, optionally, a biologically active substance. In accordance with the methods of the invention, a load stock including a polymer, a wax and/or a lipid that is a solid at standard temperature and pressure and, optionally, a biologically active substance is provided. The load stock is contacted with a supercritical fluid to form a melt. The melt is contacted with a polar solvent under suitable conditions to form an emulsion. The emulsion is expanded across a pressure drop to form solid particles that include the load stock. The methods and apparatus facilitate the production of very small particles that have a narrow particle size distribution.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of Invention  
         [0002]     The present invention relates generally to an apparatus and methods for producing particles that include a polymer, a wax and/or a lipid and, optionally, a biologically active substance.  
         [0003]     2. Description of Related Art  
         [0004]     Several processing techniques utilize the enhanced mass-transfer properties and benign nature of supercritical fluids or near-critical fluids or compressed gases (hereinafter collectively referred to as “supercritical fluids”) to produce composite or single-material particles. One of the conventional supercritical fluid processing techniques, which is sometimes referred to as the Particles from Gas-Saturated Solutions (“PGSS”) processing technique, uses a supercritical fluid to melt a solid material into a fluid or semi-fluid mass that can be sprayed into a collection vessel. The term “melt” as used in this context denotes that the supercritical fluid reduces the viscosity of the solid material (e.g., via plasticization, swelling or dissolution) so as to render it fluid or semi-fluid, which can be further processed as such. In other words, the formerly solid material can be flowed, pumped or sprayed as a fluid or semi-fluid.  
         [0005]     The conventional PGSS particle production method exploits this characteristic by flowing the melt through a nozzle across a pressure drop into an expansion chamber that is maintained at a lower pressure than the vessel containing the melt. When the melt is sprayed through the nozzle into the expansion chamber, the supercritical fluid decompresses and rapidly expands into a gas. This causes the melt to undergo three significant changes that transform the melt into solid particles. First, as supercritical fluid expands from the melt, the remaining supercritical fluid loses its solvating power and the melt returns to a solid state. Second, the rapid expansion of the supercritical fluid into a gas results in a significant reduction in the temperature of the melt, which also assists in returning the melt to a solid phase. Third, the expansion of the supercritical fluid to a gas fractures the melt into small particles that solidify in the form of particles.  
         [0006]     While the conventional PGSS process provides a method of forming particles at mild operating temperatures without using any potentially destructive solvents, it does suffer from several disadvantages. For example, it is very difficult to obtain particles in the low micron particle size range that have a relatively narrow particle size distribution using the conventional PGSS process. Certain biologically active materials, especially those materials that do not form a melt upon contact with supercritical fluid at mild operating conditions, simply cannot be processed using the conventional PGSS process into small particles.  
         [0007]     Another problem with the conventional PGSS process is that it tends to form agglomerated particles, which is believed to occur as the result of the formation of bridges between particles during expansion. Moreover, the particles formed according to the conventional PGSS process tend to be irregular in shape.  
         [0008]     Keeping the above-mentioned limitations of the conventional PGSS process in mind, it would be highly desirable to have a method for producing particles that provides the benefits of conventional PGSS processing but does not suffer from the limitations of conventional PGSS processing. Such a process would preferably produce particles having a narrow particle size distribution. Moreover, the process should also provide for enhanced control over the size and morphology of the particles produced.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     The present invention provides an apparatus and methods for producing particles that include a polymer, a wax and/or lipid and, optionally, a biologically active substance. In accordance with the methods of the invention, a load stock comprising a polymer, a wax and/or a lipid that is a solid at standard temperature and pressure and, optionally, a biologically active substance is charged with a supercritical fluid to form a melt. The melt is then contacted with a polar solvent that further preferably comprises a suitable surfactant, under mixing conditions, to form an emulsion. The emulsion is then expanded across a pressure drop to form solid particles that include the load stock. In one embodiment of the invention, the emulsion includes a discontinuous phase that includes the melt and a continuous phase that includes the polar solvent. In another embodiment of the invention, the emulsion includes a discontinuous phase that includes the polar solvent and a continuous phase that includes the melt.  
         [0010]     Unlike the conventional PGSS process, which involves expanding a melt rather than an emulsion across a pressure drop to form solid particles, the present methods provide for substantially greater control over factors such as particle size, particle size distribution, and particle morphology. Furthermore, the methods according to the invention make it possible to produce very small particles from materials that cannot be formed into small particles using the conventional PGSS process.  
         [0011]     The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a schematic drawing of an apparatus for implementing a method according to the invention.  
         [0013]      FIG. 2  is a block diagram of a method according to the invention.  
         [0014]      FIG. 3  is graph showing the particle size distribution of particles formed in accordance with Example 1.  
         [0015]      FIG. 4  is Scanning Electron Micrograph (SEM) of tripalmitin encapsulated Ketoprofen particles formed in Example 2.  
         [0016]      FIG. 5  is an X-ray Diffraction (XRD) plot of the tripalmitin encapsulated Ketoprofen particles formed in Example 2.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     An exemplary apparatus  100  for implementing the methods according to the invention is shown in  FIG. 1 . The apparatus  100  includes an emulsification and mixing apparatus  102  and an expansion apparatus  104 . The emulsification apparatus  102  includes an emulsification vessel  110 , a polar solvent pump  112 , a supercritical fluid pump  114 , and an emulsification assembly  116 .  
         [0018]     The emulsification vessel  110  is preferably tubular and defines an axis  120 , and has first and second ends  122 ,  124  that are spaced axially apart. Preferably, the axis  120  is oriented vertically such that the first end  122  is below the second end  124 . That is, the second end  124  is UP and the first end  122  is DOWN when moving along the axis  120 . The emulsification vessel  110  has an inner surface that defines an emulsification chamber  130 . The pressure in the emulsification chamber  130  is denoted with the reference number P 1 . The mixing apparatus  102  includes means for accessing the interior of the emulsification vessel  110  so as to charge the interior with a load stock.  
         [0019]     The load stock comprises a polymer, a wax and/or a lipid that is a solid at standard temperature and pressure. Throughout the instant specification and in the appended claims, the phrase “standard temperature and pressure” means 25° C. and  1  atmosphere pressure. Suitable polymers for use in the invention include, for example, polysaccharides, polyesters, polyethers, polyanhydrides, polyglycolides (PLGA), polylactic acid (PLA), polycaprolactone (PCL), polyethylene glycol (PEG) and polypeptides. Suitable lipids include, for example, glycerides.  
         [0020]     The load stock can further optionally comprise a biologically active substance such as, for example, a drug, a pharmaceutical, or a therapeutic agent. In the preferred embodiments of the invention, the load stock does comprise a biologically active substance, which is incorporated with the polymer, wax and/or lip in a desired manner to form a coated, encapsulated or taste-masked formulation, or a controlled prolonged or sustained release formulation. It will be appreciated that the load stock can also further comprise other substances such as, for example, pigments, sugars, diagnostic aids and/or markers, nutritional materials, proteins, peptides, animal and/or plant extracts, dyes, antigens, catalysts, nucleic acids and combinations thereof.  
         [0021]     The load stock must be capable of forming a melt  111  when contacted with a supercritical fluid under pressure. Throughout the instant specification and in the appended claims, the term “melt” denotes that the supercritical fluid reduces the viscosity of the load stock (e.g., via plasticization, swelling or dissolution) so as to render it a fluid or semi-fluid that can be processed as such. In other words, the load stock can be flowed, pumped or sprayed as a fluid or semi-fluid. In addition, the melt  111  must be generally insoluble in the polar solvent selected for use in the invention, at least insofar as is necessary to form an emulsion.  
         [0022]     The polar solvent pump  112  is preferably a high-pressure liquid chromatography (HPLC) reciprocating pump such as the model PU-2080, which is commercially available from Jasco Inc. (Easton, Md.). Suitable alternative pumps include syringe type pumps, such as the 1000D or 260D pumps, which are commercially available from Isco Inc. (Lincoln, Nebr.). The polar solvent pump  112  is in fluid communication with the emulsification chamber  130  via a liquid inlet nozzle that extends though a sidewall of the emulsification vessel  110 .  
         [0023]     The supercritical fluid pump  114  is preferably a P-200 high-pressure reciprocating pump commercially available from Thar Technologies, Inc. (Pittsburgh, Pa.). Suitable alternative pumps include diaphragm pumps and air-actuated pumps that provide a continuous flow of supercritical fluid. The supercritical fluid pump  114  supplies supercritical fluid into a surge tank  140  and a metering valve  142  so as produce a pulse-free flow. The supercritical fluid pump  114  is in fluid communication with the emulsification chamber  130 , and thus supplies supercritical fluid through the surge tank  140  and into the chamber  130 . As used herein, “supercritical fluid” includes fluids in supercritical and near-critical states, as well as compressed and liquefied gases.  
         [0024]     The emulsification assembly  116  includes a motor  150 , a shaft  152  extending from the motor  150  through the second end  116  of the emulsification vessel  110  and into the chamber  130 , and a rotor  154  disposed at a distal end of the shaft  152  and located in the chamber  130 . The mixing rate is controlled by the rotation speed and geometry (type and diameter) of the rotor  154 . In this embodiment, the rotor  154  is a propeller shaped two-bladed emulsifier device. Additional, supplemental and alternative shearing methods include both static and moving emulsification devices, such as baffles, rotors, turbines, shear-mixers, homogenizers or microfluidizers, ultrasonic devices, and other devices or mechanisms used to emulsify or homogenize the contents of the emulsification apparatus  102 .  
         [0025]     With reference to the expansion apparatus  104 , the expansion apparatus  104  includes a receiving or expansion vessel  160 , a backpressure regulator  162 , a nozzle  164 , and a filter  166 . The expansion vessel  160  is in fluid communication with the emulsification vessel  110  via a release valve  168 , which is disposed between the emulsification vessel  110  and the expansion vessel  160 , and is attached to the nozzle  164 . The filter  166  is adjacent an outlet from the expansion vessel  160  to the backpressure regulator  162 .  
         [0026]     The expansion vessel  160  is preferably tubular, and has an inner surface that defines an expansion chamber  170 . The pressure inside the expansion chamber is denoted with reference number P 2 . The expansion vessel  160  preferably has means, not shown, to access the expansion chamber  170  so as to remove the contents subsequent to an expansion operation.  
         [0027]     The backpressure regulator  162  is preferably a 26-1700-type regulator, which is commercially available from Tescom, USA (Elk River, Minn.). The backpressure regulator  162  maintains the pressure P 2  in the expansion chamber  170  in a predetermined range of pressures. The release valve  168  is preferably a standard commercially available valve and is interchangeable with other like valves that are known to those of ordinary skill in the art. The release valve  168  controls the rate of flow of the emulsion from the emulsification chamber  130  through the nozzle  164  and into the expansion chamber  170 . Accordingly, the release valve  168  and the backpressure regulator  162  cooperate to maintain the desired pressure P 2  in the expansion chamber  170  during operation.  
         [0028]     A thermostat  180  communicates with heating elements (not shown) that are located proximate to the emulsification vessel  110 , the expansion vessel  160 , and the release valve  168 . A controller  184  communicates with and controls the polar solvent pump  112 , the supercritical fluid pump  114 , the thermostat  180 , the emulsification assembly  116 , the backpressure regulator  162 , and the relief valve  168 . Suitable alternative controllers are interchangeable therewith.  
         [0029]     With reference to the polar solvent that the polar solvent pump  112  supplies to the chamber  130 , the polar solvent is selected based on its ability to form an emulsion with the melt  111  (i.e., the plasticized load stock). Suitable surfactants can be added to the polar solvent in order to aid in the formation of the emulsion. Accordingly, the relative solubility of the substances in the load stock must be compared to the possible polar solvents. In some instances, some solubility of the load stock in the polar solvent may be desired. In addition, the polar solvent can have some solubility in the supercritical fluid. Sufficient polar solvent remains undissolved in the supercritical fluid, and undissolved with the load stock so as to form an emulsion with the melt  111 . The emulsion, or reverse emulsion depending on the relative quantities of the materials used, reduces the viscosity of the melt  111 .  
         [0030]     Preferred polar solvents include water and polar alcohols. The most preferred polar solvent is water. Other materials can be added to the polar solvent to form a solution, an emulsion, a suspension or a mixture. If additional materials are dissolved, suspended or dispersed in the polar solvent, the resultant solid particles can contain both the constituents of the melt as well as the additional materials carried by the polar solvent. Alternatively, the additional materials mixed with the polar solvent can be washed or carried away from the particles by a subsequent separation of the polar solvent from the solid particles.  
         [0031]     Surfactants or modifiers can be added to the polar solvent and/or the melt, as desired, so as to affect properties of the emulsion. The affected properties can include, for example, the rheology, the atomization, the particle stability properties, and/or the interaction of the supercritical fluid with the load stock or polar solvent.  
         [0032]     With reference to the supercritical fluid that the supercritical fluid pump  114  supplies to the chamber  130 , the supercritical fluid is preferably supercritical carbon dioxide (“CO 2 ”). Suitable alternative supercritical fluids include water, nitrous oxide, dimethylether, straight chain or branched C1-C6-alkanes, alkenes, alcohols, and combinations thereof. Preferable alkanes and alcohols include ethane, ethanol, propane, propanol, butane, butanol, isopropane, isopropanol, and the like. The supercritical fluid is chosen generally with reference to the ability of the supercritical fluid to melt, plasticize or swell the load stock during operation.  
         [0033]     The supercritical fluid contacts the load stock and forms the melt and the polar solvent contacts the melt and forms an emulsion. It will be appreciated that materials that are partially soluble in each other may form emulsions under proper conditions, and are thus such materials may also be suitable for use with the present invention.  
         [0034]     During operation of the apparatus  100  (step  200 ) and with reference to  FIG. 2 , the emulsification vessel  110  is charged with a quantity of the load stock (step  202 ). The emulsification vessel  110  is closed and sealed. The controller  184  activates the supercritical fluid pump  114  to supply a quantity of supercritical fluid through the surge tank  140 , through the metering valve  142 , and into the emulsification chamber  130  (step  204 ). The supercritical fluid pump  114  increases the pressure P 1  in the emulsification chamber  130  to be in a predetermined range of pressures.  
         [0035]     The supercritical fluid contacts the load stock. The supercritical fluid acts on the load stock to form the melt  111  (step  206 ). The controller  180  controls the emulsification assembly  116  to engage the motor  150  so as to rotate the shaft  152  and the rotor  154 .  
         [0036]     The thermostat  180  and the supercritical fluid pump  114  cooperate to maintain the temperature and the pressure P 1  in a generally constant operating range. The temperature and the pressure P 1  are maintained about constant. Accordingly, the pressure P 1  is generally in a range that has an increased pressure relative to atmospheric pressure.  
         [0037]     In this particular embodiment, the controller  184  controls the polar solvent pump  112  to supply the polar solvent into the emulsification chamber  130  (step  208 ). Alternatively, the polar solvent can be added together with the load stock so as to be present during the formation of the melt. The rotor  154  spins to emulsify the melt  111  with the polar solvent to form an emulsion (step  210 ). In one embodiment of the invention, the discontinuous phase of the emulsion comprises the melt and the continuous phase of the emulsion comprises the polar solvent. In another embodiment of the invention, the discontinuous phase of the emulsion comprises the polar solvent and the continuous phase comprises the melt.  
         [0038]     The pressure P 2  in the expansion vessel  160  is preferably maintained at about atmospheric pressure. Optionally, the pressure P 2  can be controlled by the backpressure regulator  162  to be increased relative to atmospheric pressure, and to reduce the difference between the pressures P 1 , P 2  in the vessels  110 ,  160 .  
         [0039]     The release valve  168  opens and the emulsion, under the influence of the pressure difference between the chambers  130 ,  170 , flows through the release valve  168  and further though the nozzle  164 . The pressurized emulsion is sprayed from the nozzle  164  as indicated by the directional arrows  172  into the chamber  170  (step  212 ). Because of the pressure drop of the emulsion during spraying (from P 1  to the relatively lower pressure P 2 ), the supercritical fluid contained therein flashes from a liquid or compressed state to a gaseous or relatively uncompressed state. The loss of the supercritical fluid from the emulsion increases the melt point and/or glass transition temperature of the load stock.  
         [0040]     Further, the phase change of the supercritical fluid from liquid to gas reduces the localized temperature of emulsion adjacent to the expansion location (i.e., the nozzle outlet). If a nozzle heater is present, the nozzle can be heated to reduce the level of polar solvent in the particles  174 , and to affect particle characteristics, such as size and morphology.  
         [0041]     As the melt solidifies or precipitates from the emulsion into a plurality of particles  174 , any dissolved or suspended materials, if present, precipitate at substantially the same time. The particles  174  can thus be in the form of composite particles, homogenous or single component materials and crystals or, alternatively, microspheres or microcapsules or the like. Rather than discrete particles, the expanded material can also be precipitated as a suspension, a foam or a gel. Further, the particles can have different surface profiles or morphologies, and can be discrete or can be grouped or agglomerated.  
         [0042]     If the polar solvent is not removed during the expansion step, the particles  174  may be obtained as a suspension in the polar solvent. For example, if the continuous phase of the emulsion comprises water, and the melt comprises a water-insoluble polymer, wax and/or lipid and a water-insoluble biologically active substance, the resulting solid particles formed by expanding the emulsion may comprise an aqueous suspension of composite solid polymer, wax and/or lipid/biologically active substance particles. The suspension can be subsequently processed to separate the solid particles  174  from the polar solvent. The particles can be washed and filtered to remove surfactants and other material residues. An additional processing step can be implemented such as spray-drying or freeze-drying, by which the polar solvent can be removed from the suspension. A parallel spray-drying can be implemented simultaneously with the expansion by heating the expansion vessel or by supplying a stream of inert heating gas or air in the expansion vessel. A parallel freeze-drying can be implemented simultaneously with the expansion vessel by attaching a vacuum device to the collection vessel and keeping the temperature within this vessel at or below the freezing temperature of the polar solvent.  
         [0043]     The following examples are intended only to illustrate the invention and should not be construed as imposing limitations upon the claims. Unless specified otherwise, all ingredients are commercially available from such common chemical suppliers as Sigma Aldrich, Inc. (St. Louis, Mo.) and/or Fisher Scientific International, Inc. (Hanover Park, Ill.).  
       EXAMPLE 1  
       [0044]     Preparation.  
         [0045]     Initially, 6 grams (g) of tripalmitin (a lipid, model compound) was charged to an emulsification vessel. The emulsification vessel was closed and pressurized with carbon dioxide gas (CO 2 ) to an operating pressure of  300  bar.  
         [0046]     The thermostat was set at a predetermined temperature, and the temperature was ramped to 318 Kelvin (K). The thermostat monitored and maintained the temperature at a constant temperature of 318 K. At the predetermined temperature and pressure, the carbon dioxide became supercritical. The controller was set to maintain the emulsifier device to rotate the emulsifier blade at a constant agitation speed of 4000 revolutions per minute (rpm).  
         [0047]     The tripalmitin and CO 2  mixture was allowed to equilibrate and mix at 4000 rpm for one hour (hr).  
         [0048]     A solution pump was activated and pumped an aqueous solution of TWEEN-80 (1% w/w) into the emulsification vessel. The agitation speed of 4000 rpm was maintained during the addition of the aqueous phase. The aqueous solution of TWEEN-80, the tripalmitin and the supercritical carbon dioxide formed an emulsion.  
         [0049]     Expansion of Particles.  
         [0050]     A release valve was opened to communicate the emulsion from the emulsification vessel to an expansion vessel. Specifically, the release valve communicated the emulsion to a nozzle that opened into the interior of the expansion vessel. The nozzle was a multiple nozzle plate defining ten orifices, each orifice had a diameter of 180 micrometers (μm). The pressure in the interior of the expansion vessel was standard atmospheric pressure, and the pressure in the emulsification vessel was adjusted to remain at a constant 30 megapascals (MPa).  
         [0051]     The supercritical fluid flashed into a gas at atmospheric pressure. The particles were obtained in the form of concentrated liquid suspension of particles in the aqueous solution of TWEEN-80.  
         [0052]     Analysis of the Particles.  
         [0053]     Analysis of the particles was performed using a Scanning Electron Microscope (SEM) to determine size and morphology, using a laser diffraction particle analyzer to determine the particle size distribution in suspension, and using a Differential Scanning Calorimeter (DSC) to determine polymorphism and melting behavior of dried particles. The particles produced in Example 1 were compared against particles prepared by a conventional PGSS process using the same materials (i.e., no aqueous solution was pumped into the melt to form an emulsion). The particles obtained from the two processes had different morphologies. There were more platelets in the particles of Example 1 compared to the acicular shapes formed by the conventional method. The particles produced in Example 1 were sub-micron in size, and were less aggregated relative to the conventionally produced particles. In particular, the particles of Example 1 had a volume mean diameter that was 786 nanometers (nm).  FIG. 3  is graph showing the particle size distribution of particles formed in accordance with Example 1.  
       EXAMPLE 2  
       [0054]     Preparation.  
         [0055]     Initially 6 grams of tripalmitin was charged into the emulsification vessel, and 0.6 grams of ketoprofen were also charged into the emulsification vessel. The emulsification vessel was pressurized with CO 2  to the operating pressure  300  bar. The thermostat maintained the emulsification vessel at a constant temperature of 328 K. At the predetermined temperature and pressure, the carbon dioxide became supercritical. The controller was set to maintain the emulsifier device to rotate the emulsifier blade at a constant agitation speed of 4250 revolutions per minute (rpm).  
         [0056]     The ketoprofen and the tripalmitin dissolved in the supercritical carbon dioxide. The molten solution or melt was mixed and allowed to equilibrate for one hour. A solution pump was activated and pumped an aqueous solution of TWEEN-80 (2.5% w/w) into the emulsification vessel. The agitation speed of 4250 rpm was maintained during the addition of the aqueous phase. The aqueous solution of TWEEN-80, the tripalmitin, the ketoprofen and the supercritical carbon dioxide formed an emulsion.  
         [0057]     Expansion of Particles.  
         [0058]     A release valve was opened to communicate the emulsion from the emulsification vessel to an expansion vessel. Specifically, the release valve communicated the emulsion to a nozzle that opened into the interior of the expansion vessel. The nozzle was a multiple nozzle plate defining ten orifices; each orifice had a diameter of 180 micrometers (mm). The pressure in the interior of the expansion vessel was standard atmospheric pressure, and the pressure in the emulsification vessel was adjusted to remain at a constant 300 bar. The supercritical fluid flashed into a gas at atmospheric pressure. The particles were obtained in the form of concentrated liquid suspension of particle in the aqueous solution of TWEEN-80.  
         [0059]     Analysis of the Particles.  
         [0060]     Analysis of the particles formed in Example 2 was performed using a Scanning Electron Microscope (SEM) to determine size and morphology, using a laser diffraction particle analyzer to determine particle size distribution in the suspension, and using X-ray powder diffraction (XRPD) to determine crystallinity and drug content. The particles consisted of non-aggregated composite particles. The particles had a volume mean diameter of about 12-20 microns, as shown in  FIG. 4 . The chemical composition of the particles was about 9% w/w of ketoprofen. The ketoprofen was present in the form sub-micron sized crystalline particles embedded into or coated onto the tripalmitin, as shown in  FIG. 5 .  
         [0061]     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and illustrative examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.