Abstract:
A solute is precipitated from a solution of a solute, a solvent, and a non-solvent by preparing the solution and introducing the solution into a precipitation chamber contain a supercritical fluid.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a method for precipitating a solute from a solution. More particularly the invention relates to a method for prepare fine particles, especially the preparation of drug and/or polymer microparticles or nanoparticles, by precipitating a solute from a solution. 
     BACKGROUND TO THE INVENTION 
     Precipitation is the formation of a solid in a solution during a chemical reaction. The solid is called the precipitate, and the liquid remaining above the solid is called the supernate. Solvent/non-solvent precipitation is a technique commonly employed to isolate a substance in solid form from a solution. In this process the substance particles are obtained by dissolving the substance in a solvent and then precipitating the substance by combining the substance solution with a non-solvent. Adding a non-solvent to a substance solution increases the saturation ratio of the solution causing precipitation of the solute. The solvent and the non-solvent must be at least partially soluble in each other. U.S. Pat. No. 3,953,383, U.S. Pat. No. 4,166,898, U.S. Pat. No. 3,900,424, U.S. Pat. No. 4,500,704, U.S. Pat. No. 4,066,630, U.S. Pat. No. 4,145,525, U.S. Pat. No. 3,706,713 and U.S. Pat. No. 4,975,525 teach to dissolve polymers in certain solvents and to precipitate the polymers by adding the polymer solutions to certain non-solvents. 
     CO2 is non-toxic, non-flammable, relatively inexpensive and recyclable gass. Its critical temperature and pressure (Tc=31.1 deg-C, Pc=7.38 MPa) are relatively mild. Supercritical CO2—where CO2 is in a fluid state while also being at or above its critical temperature and pressure—has several desirable features such as low viscosity and high solvent power for most organic solvents. Because most organic (carbon-containing) solvents are miscible (mix to foam a homogeneous solution) in supercritical CO2, it acts a non-solvent in solvent/non-solvent precipitation. Technologies based on a supercritical anti-solvent (SAS) process have been wildly used for engineering microparticles and nanoparticles. Based on employing CO2 as an anti-solvent for particle fabrication, the SAS methods include the follows: supercritical anti-solvent (GAS/SAS), precipitation with a compressed anti-solvent (PCA), aerosol solvent extraction system (ASES), solution-enhanced dispersion by supercritical CO2 (SEDS), and supercritical anti-solvent with enhanced mass transfer (SAS-EM). These method re well known in he art. 
     In a SAS process, the substance organic solution is sprayed into a precipitation chamber where a supercritical fluid (antisolvent) is present causing rapid contact between the two media. This generates a higher super-saturation ratio of the solution resulting in fast nucleation and growth. 
     The U.S. Pat. No. 5,725,836, U.S. Pat. No. 6,998,051, U.S. Pat. No. 5,803,966, U.S. Pat. No. 7,250,152, U.S. Pat. No. 6,063,910, U.S. Pat. No. 6,830,714 and U.S. Pat. No. 5,864,923 teach to use supercritical fluid as anti-solvent to precipitate or co-precipitate drugs and/or polymers. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for precipitating a solute from a solution which produces fine particles having a smaller particle size, narrower particle size distribution and/or higher yield than methods use hitherto, or at least to provide the public with a useful choice. 
     The object is achieved by adding an organic non-solvent to a supercritical anti-solvent process. Accordingly, the invention provides a method for precipitating a solute from a solution comprising preparing a solution of a solute, a solvent and a non-solvent, and introducing the solution containing the solvent and a non-solvent into a precipitation chamber containing a supercritical anti-solvent. 
     In a particular aspect the invention provides a method comprising:
         (a) Dissolving a substance in a suitable organic solvent to form a solution. The solution containing the substance is preferably an organic solvent selected to be soluble with supercritical CO2, and selected to dissolve the substance. In one aspect of the invention the a solution has a concentration preferably between 0.1% (wt/v) and 5.0% (wt/v), more preferably between 0.5% (wt/v) and 2.0% (wt/v).   (b) Adding a suitable organic non-solvent to the solution with agitation to increase the saturation ratio of the solution. The organic non-solvent is preferably one in which the substance is insoluble, and is selected to be miscible with the solvent in step (a), and also miscible in supercritical CO2. In one aspect of the invention the volume ratio of non-solvent to solvent ranges from 0.2 to 5. In another aspect of the invention the saturation ratio of the solvent/non-solvent solution increases with the decreasing of concentration. Preferably the solvent/non-solvent solution is very close to saturated state.   (c) Using a supercritical anti-solvent process to precipitate the substance from the solvent/non-solvent solution of step (B). The solvent/non-solvent solution is sprayed through a stainless steel coaxial nozzle by an HPLC pump into the high-pressure vessel where a supercritical CO2 is present causing rapid contact between the supercritical CO2 and the solvent/non-solvent solution. The solvent/non-solvent solution is expanded greatly and instantaneously generating higher super-saturation ratio of the solution resulting in fast nucleation and growth, which consequently creates smaller particles. Preferably, the supercritical anti-solvent process employs a supercritical fluid as an anti-solvent, and includes but not limited to GAS/SAS, PCAS ASES, SEDS, and SAS-EM. In on aspect of the invention the supercritical anti-solvent process has a temperature range from 304.1 K to 354.1 K, a pressure range from 7.38 MPa to 30 MPa, a flow rate of solvent/non-solvent solution in the range from 0.02 ml/min to 9.98 ml/min and a flow rate of CO2 in the range from 10 NL/h to 1000 NL/h.       

     Further objects and aspects of the invention will become apparent from the following description which is given by way of example only to help illustrate the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of apparatus for a supercritical antisolvent process according to the invention, and 
         FIG. 2  are scanning electron microscope (SEM) images of puerarin particles prepared in different parameters according to Table 1. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates apparatus for practicing the invention. Precipitation takes place in a high pressure vessel supplied with CO2 from a CO2 cylinder. The vessel is incubated in a gas bath to keep the temperature constant. The COs is feed into the high pressure via a cooler, pump and heat exchanger and is maintained in a supercritical state. Temperature and pressure sensors are provided in the vessel for monitoring the temperature and pressure of the CO2. During the precipitation reaction an organic solution can be feed into the high pressure vessel from a solution container using a HPLC pump. A stainless steel coaxial nozzle is used to disburse the organic solution within the high pressure vessel and ensure rapid contact with the supercritical within the vessel. The precipitate and supernate are extracted out of the vessel into a separation container via a filter and valve in the botton of the vessel. The supernate can then be drawn off the percipitate. 
     The method of the invention will be illustrated by the production of fine particles of drug and/or polymer. The method comprises four main steps. Firstly, a substance is dissolved in a suitable organic solvent to form a solution. A suitable organic non-solvent is added into the solution with agitation to increase the saturation ratio of solution. When the solution reaches a saturated state, the organic non-solvent can not be added to keep the solution homogenous and prevent the precipitation of the substance. Thirdly, a supercritical anti-solvent process is used to precipitate the substance from the solution. As shown in  FIG. 1 , the said solution is sprayed through a stainless steel coaxial nozzle by an HPLC pump into the high-pressure vessel containing supercritical CO2. This causes rapid contact between supercritical CO2 and the mixture solvent/non-solvent. The solvent/non-solvent solution is expanded greatly and instantaneously generating a higher super-saturation ratio of the solution, resulting in fast nucleation and growth, and consequently creates smaller particles. Finally, when the spraying was finished, fresh CO2 is used continually to wash the products to remove the residual organic solvent for about 30 minutes. During washing temperature and pressure are kept constant. After washing, the high pressure vessel was slowly depressurized and the products were collected. 
     It is important that the organic solvent, organic non-solvent and supercritical CO2 are soluble in each other. It is preferred that the solution with a relatively low concentration be near saturated state by adding organic non-solvent to prepare fine particles of the substance with a smaller particle size and narrower particle size distribution. 
     The supercritical anti-solvent methods should be chosen based on employing CO2 as an anti-solvent. Suitable methods known in the art include, but are not limited to, GAS/SAS, PCA, ASES, SEDS, and SAS-EM. 
     The particle size and particle size distribution are controlled by varying the solvent/non-solvent ratio, solution concentration, flow rate of solution, flow rate of CO2, temperature and pressure of the CO2. The inventors have found that when keeping the other parameters constant, but increasing of non-solvent ratio in the mixture to increase the saturation ratio of the solution, the method generates a faster precipitation during the supercritical anti-solvent process, the resulting particle size is greatly decreased, and the higher yield is obtained. 
     The following examples are meant to further illustrate, but not limit, the invention. 
     Example 1 
     Preparation of Puerarin Fine Particles 
     Table 1 below illustrates the yield, particle size and particle size distribution of puerarin particles prepared using different parameters of A: solvent/non-solvent, B: concentration (wt/v %), C: flow rate (mi/mm). 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                   
                 Particle size and particle size distribution 
               
             
          
           
               
                   
                   
                   
                   
                 Yield 
                 Mean Size 
                   
                   
                   
                   
               
               
                 Run 
                 A 
                 B 
                 C 
                 (%) 
                 (um) 
                 D10 
                 D50 
                 D90 
                 Span 
               
               
                   
               
             
          
           
               
                 a 
                 2:1 
                 1.0 
                 1.0 
                 40.69 
                 1.989 
                 1.315 
                 1.942 
                 2.667 
                 0.6962 
               
               
                 b 
                 1:2 
                 1.0 
                 1.0 
                 72.73 
                 0.4340 
                 0.3334 
                 0.4289 
                 0.5774 
                 0.5689 
               
               
                 c 
                 2:1 
                 1.0 
                 0.5 
                 60.65 
                 1.844 
                 1.191 
                 1.787 
                 2.546 
                 0.7583 
               
               
                 d 
                 1:2 
                 1.0 
                 0.5 
                 80.58 
                 0.3339 
                 0.2384 
                 0.3334 
                 04732 
                 0.7043 
               
               
                 e 
                 2:1 
                 0.5 
                 1.0 
                 59.77 
                 1.650 
                 062 
                 1.541 
                 2.490 
                 0.9267 
               
               
                 f 
                 1:2 
                 0.5 
                 1.0 
                 74.91 
                 0.3705 
                 0.3069 
                 0.3694 
                 0.4606 
                 0.4161 
               
               
                 g 
                 2:1 
                 0.5 
                 0.5 
                 69.38 
                 0.7658 
                 0.4520 
                 0.7358 
                 1.024 
                 0.7774 
               
               
                 h 
                 1:2 
                 0.5 
                 0.5 
                 84.06 
                 0.2562 
                 0.1974 
                 0.2632 
                 0.3260 
                 0.4886 
               
               
                   
               
             
          
         
       
     
     Run (a) was conducted according to the following method. Three-hundred (300) mg of puerarin was dissolved in 20 ml ethanol, as an organic non-solvent of puerarin, 10 ml dichloromethane (DCM) was added into the solution of puerarin in ethanol to obtain a homogenous solution with a higher saturation ratio. In the running of supercritical anti-solvent process, the CO2 fed from a CO2 cylinder was cooled down to around 0° C. by the cooler in order to ensure the liquefaction of the gas and also to prevent cavitations. Then a high pressure meter pump was used to deliver liquefied CO2 to the high pressure vessel. After leaving the pump head, the liquefied CO2 was pre-heated to desired operating temperature by using a heat exchanger. The high pressure vessel was incubated in a gas bath to keep the temperature constant during the experiment. When the desired pressure of the high pressure vessel was reached, a steady flow of CO2 was maintained, and the system pressure was controlled by adjusting a downstream valve and monitored by a pressure gauge to keep the pressure constant. When the desired pressure and temperature were stabilized, the puerarin solution was delivered into the high pressure vessel through a stainless steel coaxial nozzle by using an HPLC pump at a flow rate of 1.0 ml/min. During the process, the pressure, temperature and flow rate of CO2 were kept as 12 MPa, 306 K and 25 NL/h, respectively. When the spraying was finished, fresh CO2 was used continually to wash the products to remove the residual organic solvent for about 30 mm. During the process of washing, the system operating conditions were maintained as described before. After washing, the high pressure vessel was slowly depressurized and the products were collected. 
     The run was repeated for the different parameters (b) through (h). Scanning electron microscope (SEM) images of puerarin particles prepared in different runs is illustrated in  FIG. 2 . It is evident that while the other parameters were kept in constant, with the increasing of non-solvent ratio in the puerarin solution, puerarin fine particle with much smaller particle size, much narrower particle size distribution and much higher yield were produced. 
     Example 2 
     60 mg poly(L-lactide) (PLLA) with a Mw of 100 KDa was dissolved in 20 ml DCM, then the said solution was added into 40 ml acetone which was a non-solvent for PLLA with high Mw, and agitating to resolve the precipitation as a homogeneous solution with a higher saturation ratio, then the PLLA solution was delivered into the high pressure vessel through a stainless steel coaxial nozzle by using an HPLC pump at a flow rate of 5.0 ml/mm, the pressure, temperature and flow rate of CO2 were set as 30 MPa, 304.1 K and 1000 NL/h, respectively. After spraying and washing by CO2, the high pressure vessel was slowly depressurized and the PLLA fine particles were collected. 
     Example 3 
     500 mg poly(L-lactide) (PLLA) with a Mw of 100 KDa was dissolved in mixture of 20 ml DCM and 5 ml acetone which was a non-solvent for PLLA with high Mw to obtain a homogeneous solution with a higher saturation ratio, then the PLLA solution was delivered into the high pressure vessel through a stainless steel coaxial nozzle by using an HPLC pump at a flow rate of 0.02 ml/mm, the pressure, temperature and flow rate of CO2 were set as 7.38 MPa, 323.0 K and 10 NL/h, respectively. After spraying and washing by CO2, the high pressure vessel was slowly depressurized and the PLLA fine particles were collected. 
     Example 4 
     50 mg 5-Fluorouracil was dissolved in 30 ml ethanol, then 20 ml DCM which was a non-solvent for 5-Fluorouracil was added in the said solution to obtain a homogeneous solution with a higher saturation ratio, then the 5-Fluorouracil solution was delivered into the high pressure vessel through a stainless steel coaxial nozzle by using an HPLC pump at a flow rate of 0.5 ml/min, the pressure, temperature and flow rate of CO2 were set as 7.38 MPa, 304.1 K and 50 NL/h, respectively. After spraying and washing by CO2, the high pressure vessel was slowly depressurized and the 5-Fluorouracil fine particles were collected.