Abstract:
The present invention is concerned with the simultaneous formation and encapsulation of small particles of organic compounds whose solubility in water is greater at a first pH than at a second pH by concurrently precipitating said organic compounds as small particles and forming a coacervate of an anionic (or cationic) surfactant and an amphoteric surfactant. The process is preferably used to prepare a readily soluble encapsulated pharmaceutically active compound.

Description:
This application is a continuation-in-part of U.S. patent application Ser. No. 506,598 filed June 22, 1983 and now abandoned. 
    
    
     The present invention is concerned with the formation of small particles of organic compounds upon precipitation at a selected temperature in the presence of a surfactant mixture, induced by pH change from a first pH at which their solubility in water is greater to a second pH at which it is lower. In this application, a small particle refers to a particle size of less than 2 μm. The object of the invention is to provide a process for the formation of small particles of organic compounds especially pharmaceutically active compounds. 
     The rate and extent of absorption of a pharmaceutically active compound by a patient is dependent on the particle size of the compound. The administration of pharmaceutically active compounds having smaller particles makes it possible to give a reduced dosage at lower cost and results in fewer side effects. 
     BACKGROUND OF THE INVENTION 
     From a pharmaceutical point of view, the smaller the particle size of a relatively insoluble drug, the greater is its rate of solution and as a rule, the greater is its bioavailability, (J. H. Fincher, J. Pharm. Sci., 57, 1825 (1968)). To this end, small particles are conventionally formed by mechanical subdivision of bulk matter or by aggregation of small molecules or ions, (D. J. Shaw, &#34;Introduction to Colloid and Surface Chemistry&#34; 3rd Edition, Butterworths, London, 1980, Chapter 1). The initial rate of nucleation depends on the relative degree of supersaturation of the solute, while the rate of particle growth depends on several factors, including the amount of material available, the viscosity of the medium, adsorption of impurities onto the particle surface and particle-particle interaction, (D. J. Shaw, &#34;Introduction to Colloid and Surface Chemistry&#34;, 3rd Edition, Butterworths, London, 1980, Chapter 1). The coacervation of ionic dyes with ionic surfactants has been reported, (S. P. Moulik, S. Ghosh and A. R. Das, Colloid &amp; Polymer Sci., 257, 645 (1979); B. W. Barry and G. F. J. Russell, J. Pharm. Sci., 61, 502 (1972)). 
     SUMMARY OF THE INVENTION 
     A method has now been found which is useful for forming small particles of weakly acidic and weakly basic organic compounds upon precipitation at a selected temperature in the presence of a surfactant mixture, induced by pH change from a first pH at which their solubility in water is greater to a second pH at which it is lower. The method comprises the steps of: 
     (a) dissolving the compound in water, when said compound is weakly acidic, in the presence of sufficient base to raise the pH of the solution to said first pH, and above the pKa of the compound, preferably about 2 pH units together with an anionic surfactant which maintains its ionic condition between said first pH and said second pH and an amphoteric surfactant which is anionic at said first pH and whose cationic nature increases as the pH is changed from the first pH to said second pH. When said compound is weakly basic, in the presence of sufficient acid to lower the pH to said first pH below the pKa of said compound, preferably about 2 pH units together with a cationic surfactant which maintains its ionic condition between said first pH and said second pH, and an amphoteric surfactant which is cationic at said first pH and whose anionic nature increases as the pH is changed from the first pH to said second pH;) 
     (b) stirring and titrating the solution, with a suitable acid titrant (if the starting solution is basic) or a suitable basic titrant (if the starting solution is acidic) in an amount effective to alter the pH from said first pH to said second pH and thereby cause the concurrent formation of a coacervate of the surfactants, and precipitation of the compound as small particles. 
     The said second pH may be about 2 pH units above or below the pKa of the compound to precipitate the free acid, free base or the salt forms of the compound. 
     It is believed that as the pH of the solution changes, the compound&#39;s solubility is altered and a coacervate forms between the anionic or cationic surfactant (as the case may be) and the amphoteric surfactant simultaneously with the precipitation of the compound. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This process is preferably used to form small particles of organic compounds whose solubility in water is greater at a first pH than at a second pH. Such compounds are commonly found in the pharmaceutical industry and are preferably used in small-particle form as explained above. 
     Depending on the protolytic properties of such an organic compound it can be dissolved in either an alkaline (weakly acidic compound) or acidic solution (weakly basic compound) and precipitated by the subsequent titration with either an acid or alkaline titrant, respectively. The starting pH should preferably be 2 pH units above the pKa of a weakly acid compound and preferably 2 pH units below the pKa of a weakly basic compound. 
     Suitable pharmaceutically active compounds which can be used in this process are, for example, sulfadiazine, lidocaine, salicyclic acid, felodipine, sulbactam pivoxil, chlorzoxazone, theophylline and erythromycin. Suitable amphoteric surfactants which change ionic character between the first and second pH are, for example, surfactants derived from fatty imidazolines (Miranols ), particularly monocarboxylated compounds, such as Miranol SM, which is a clear, aqueous, amphoteric solution, derived from 99% capric acid; the surfactant is a monocarboxylated derivative of a capryl imidazoline. Other suitable amphoteric surfactants are, for example, betaines, such as cocamidopropyl betaine, lauramidopropyl, betaine, amino acid amphoterics such as disodium lauriminodipropionate and imidazoline derived amphoterics such as Miranol SM and other members of these general classes. 
     Suitable anionic surfactants which maintain their ionic condition between the first and second pH of the weakly acidic organic compounds are, the common salts of natural and synthetic organic carboxylates, sulfonates and sulfates, such as for example, sodium or potassium stearates, sodium lauryl sulfate, sodium or potassium alkyl sulfates having alkyl groups with 8-18 carbon atoms and dialkyl sodium sulfosuccinates having alkyl groups with 6-8 carbon atoms. 
     Suitable cationic surfactants which maintain their ionic condition between the first and second pH of the weakly basic organic compounds are common surface-active derivatives of ammonium and various amines, for example, alkyltrimethylammonium halides containing alkyl groups with 11-18 carbon atoms, alkylpyridinium halides containing alkyl groups with 8-18 carbon atoms, benzylalkyldimethylammonium halides containing alkyl groups with 8-18 carbon atoms, and alkyldimethylethylammonium halides containing alkyl groups with 8-18 carbon atoms. 
     A suitable molar ratio of the pharmaceutically active compound to amphoteric surfactant and the anionic or cationic surfactant is for example 0.15:1:1 to 4.4:1:1, up to the maximum solubilizing capacity for a particular system. 
     The alkaline solution used to dissolve the weakly acidic compounds can be, for example, sodium hydroxide or potassium hydroxide solutions. The alkaline solution should be about 0.05-5.0 N, preferably 0.05 N or 0.1 N in order to obtain a pH preferably 2 units above the pKa of the compound. For dissolving the weakly basic compounds, the acidic solutions should be 0.05-5.0N, preferably 0.05N or 0.lN in order to obtain a pH preferably 2 units below the pKa of the compound. 
     The titrations are performed with stirring using a suitable acid titrant, such as hydrochloric acid to reduce the pH of the solution to anywhere below pH 9 to pH 1.5, or in the case of an alkaline titrant, to a pH anywhere above pH 2 up to pH 12 and to cause the concurrent formation of a coacervate of the surfactants and precipitation of the compounds as small particles. 
     The molarity of the acid titrant should be in the range 0.05-5.0N, preferably 0.1N or 1.0N, and that of the alkaline titrant should be in the range of 0.05-5.0N, preferably 0.2N or 1.0N. Higher normalities can be used as well to obtain the desired pH. 
     The titration should be preformed within the temperature range of 0°-50° C., usually at about 22° C. 
     While the invention is described with particular reference to pharmaceutical manufacture, it should be understood that the basic principals are not so limited. Obviously when applied to pharmaceuticals, the surfactants, acids and bases used should not leave pharmaceutically objectionable residues. 
     EXAMPLE 1 
     Appropriate molar amounts of sulfadiazine, sodium lauryl sulfate and Miranol SM (42-44% solids by weight) as indicated in Table 1 were dissolved in sodium hydroxide solution, 0.05N NaOH, when 0.044M or 0.0044M sulfadiazine was used or 0.1N, for 0.088M sulfadiazine. The solutions were then stirred at constant speed with a magnetic stirrer and sulfadiazine was precipitated upon dropwise titration of the solutions with 1.0N hydrochloric acid solution. 
     The effect of several different composite ratios of sulfadiazine, Miranol SM and sodium lauryl sulfate on the precipitation of sulfadiazine is summarized in Table 1. As a general rule, precipitation of the sulfadiazine began when the pH reached 8.5-8.6, as indicated by increasing turbidity. Samples 1-5 represent the process of this invention while Sample A does not. 
     
                                           TABLE 1__________________________________________________________________________Precipitation of Sulfadiazine Upon Acidification of AlkalineSolutions Containing SurfactantsSulfadiazine:Miranol SM:Sodium Lauryl Sulfate                      pH of appearance                               Observations of precipitateSample    Molar ratio       Concentration ratio                      of turbidity                               upon acidification to pH__________________________________________________________________________                               4.1   1:1:1   0.044 M:0.045 M:0.045 M                      8.5-8.6  Rod-shaped particles and                               needles, 1-12 μm                               Oval-shaped particles &lt;1 μm                               Droplets of coacervate                               phase entrapping some                               particles2   1:2:2   0.044 M:0.09 M:0.09 M                      8.5-8.6  Rod- and oval-shaped                               particles &lt;1 μm. Larger                               rods up to 4 μm                               Droplets of coacervate phase                               entrapping some particles3   2:2:2   0.088 M:0.09 M:0.09 M                      ˜8.9                               small oval- or rod-shaped                               particles 1 μm4   2:1:1   0.088 M:0.045 M:0.045 M                      ˜8.9                               small oval- or rod-shaped                               particles &lt;1 μm5   4.4:1:1 0.088 M:0.02 M:0.02 M                      ˜8.9                               small oval- or rod-shaped                               particles &lt;1 μmA   0.1:1:1 0.0044 M:0.045 M:0.045 M                      6.8-7.0  large needle shaped crystal                               of sulfadiazine (10-30__________________________________________________________________________                               μm)