Abstract:
An injectable solution suitable for the systemic treatment of anaerobic infections is prepared by solubilization of 500-650 mg. metronidazole in 10 ml. of a solvent system comprised of: 
     25-35% v/v N,N-dimethylacetamide 
     40-60% v/v ethanol 
     10-30% v/v aqueous buffer 
     Or, 
     15-25% v/v N,N-dimethylacetamide 
     40-60% v/v ethanol 
     10-30% v/v aqueous buffer 
     And including one of the following: 
     1-5% v/v nicotinamide 
     5-15% v/v propylene glycol 
     5-15% v/v 2,2-dimethyl-1,3-dioxolane-4-methanol, 
     The aqueous buffer maintaining the pH at 5.0-7.5.

Description:
BACKGROUND OF THE INVENTION 
     In the systemic treatment of anaerobic infections a parenteral mode of administration is advantageous for patients who are seriously ill and for whom oral administration is not feasible or may be considered to be insufficiently incisive. Heretofore, a practical way of administering a parenteral dose of metronidazole was not available since the aqueous solubility of metronidazole is only about 100 mg./10 ml. and a practical dosage is in the 500-650  mg./10 ml. range. Those skilled in the art recognize the practical advantages of a single 10 ml. injectable dosage form as opposed to the large volume of solution which would be needed to administer a 500-650 mg. dose of metronidazole in an aqueous solution. Thus, this invention provides a convenient and practical dosage form for the systemic treatment of anaerobic infections. 
     Generally, parenteral solutions are preferable for injections since they avoid the disadvantages inherent in suspensions, such as nonuniform dosage, caking, and possible slow release of the medicament when it is not desired. However, when wholly aqueous solvent systems are unsuitable, the choice of a nonaqueous system requires consideration of numerous parameters. The chosen solvent system must be nontoxic, nonirritating, and nonsensitizing. It also must exert no pharmacologic activity of its own, nor adversely affect the action of the drug. Additionally, the system must be stable under normal conditions of pharmaceutical use and not adversely affect the stability of the drug. The viscosity must be such as to allow for ease of injection, and the solvent must remain fluid over a fairly wide temperature range. Other considerations are water and body fluid miscibility, the degree of flammability, availability, source of supply and constant purity. 
     SUMMARY OF THE INVENTION 
     The present invention is concerned with an injectable solution suitable for the systemic treatment of anaerobic infections. More particularly, such an injectable solution is prepared by solubilization of 500-650 mg. metronidazole in 10 ml. of a solvent system comprised of 
     15-35% v/v N,N-dimethylacetamide 
     40-60% v/v ethanol 
     10-30% v/v aqueous buffer 
     Which may include one of the following: 
     1-5% w/v nicotinamide 
     5-15% v/v propylene glycol 
     5-15% v/v 2,2-dimethyl-1,3-dioxolane-4-methanol 
     The aqueous buffer maintaining the pH at 5.0-7.5, preferably 5.0-6.0. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph illustrating the solubility of metronidazole in various mixtures of water and ethanol. 
     FIG. 2 is a graph illustrating the solubility of metronidazole in various mixtures of water and ethanol with a constant composition of 20% v/v N,N-dimethylacetamide. 
     FIG. 3 is a graph illustrating the solubility of metronidazole in various mixtures of water and ethanol with a constant composition of 30% v/v N,N-dimethylacetamide. 
     FIG. 4 is a graph illustrating the solubility of metronidazole in various mixtures of water and ethanol with a constant composition of 20% v/v N,N-dimethylacetamide and 10% v/v propylene glycol. 
     FIG. 5 is a graph illustrating the solubility of metronidazole in various mixtures of water and ethanol with a constant composition of 20% v/v N,N-dimethylacetamide and 10% v/v 2,2-dimethyl-1,3-dioxolane-4-methanol. 
     FIG. 6 is a graph illustrating the solubility of metronidazole in various mixtures of water and ethanol with a constant composition of 20% v/v N,N-dimethylacetamide and 2% w/v nicotinamide. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the present invention, it has now been found that the required concentration of 500-650 mg./10 ml. of metronidazole can be attained by utilizing a solvent system having the above properties. Thus, a solvent system suitable for an injectable solution containing 500-650 mg./10 ml. of metronidazole is comprised of: 
     25-35% v/v N,N-dimethylacetamide 
     40-60% v/v ethanol 
     10-30% v/v aqueous buffer 
     or 
     15-25% v/v N,N-dimethylacetamide 
     40-60% v/v ethanol 
     10-30% v/v aqueous buffer 
     and including one of the following: 
     1-5% w/v hydrotropic agent 
     5-15% v/v polyhydroxyl alcohol 
     5-15% v/v dioxolane derivative 
     said aqueous buffer maintaining the pH at 5.0-7.5. 
     As metronidazole is most stable at a pH range of 5.0-7.5, stability and shelf life are enhanced by using an appropriate aqueous buffer. Any non-toxic, pharmaceutically acceptable, aqueous buffer capable of buffering in the pH 5.0-7.5 range is operable in this invention. A particularly preferred aqueous buffer is Tris(hydroxymethyl)aminomethane buffer [Tris] such as that available from Sigma Chemical Co., St Louis, Mo., marketed under the trademark Trizma®. 
     Desirable polyhydroxyl alcohols include, but are not limited to, propylene glycol (1,2-propanediol), and 1,3-butylene glycol (1,3-butanediol). 
     The term &#34;hydrotropic agent&#34; refers to an agent which forms a solubility-increasing complex. Hydrotropic agents which are useful in the present invention include, but are not limited to, nicotinamide (niacin), ascorbic acid, 3-pyridylcarbinol and 4-pyridylcarbinol. A particularly preferred hydrotropic agent is nicotinamide. 
     Typical dioxolanes contemplated for use in this invention include 2,2-dimethyl-1,3-dioxolane-4-methanol Solketal®, isopropylidene glycerol, glycerol dimethylketal) and glycerol formal (25:75 3-hydroxymethyl-1,3-dioxolane: 5-hydroxydioxane). 
     The addition of a polyhydroxyl alcohol, hydrotropic agent and/or a dioxolane has the advantage of enabling one to decrease the dialkylacetamide percentage in the solvent system, while still retaining ability to dissolve the 500-650 mg./10 ml. concentration of metronidazole. This decreased percentage of the dialkylacetamides is particularly advantageous since these solvents exhibit undesirable side effects in high doses, e.g., liver damage and hallucinations. See, for instance, Horn, Toxicol. Appl. Pharmacol., 3,12 (1961) and Weiss et. al., Science, 136, 751 (1962). While concentrations of N,N-dimethylacetamide of 50% or less are widely used in formulations, [see, for instance, U.S. Pat. Nos. 2,980,584 and 2,990,333, and British Pat. No. 886,996] any minimization of concentration is advantageous. 
     Thus, a particularly preferred solvent system for dissolving 500-650 mg./10 ml. of metronidazole comprises: 
     15-25% v/v N,N-dimethylacetamide 
     40-60% v/v ethanol 
     10-30% v/v aqueous buffer 
     and includes one of the following: 
     1-5% w/v nicotinamide 
     5-15% v/v propylene glycol 
     5-15% v/v 2,2-dimethyl-1,3-dioxolane-4-methanol 
     said aqueous buffer maintaining the pH of 5.0-7.5. 
     The critical features of the solubility of metronidazole, [1-(2-hydroxyethyl)-2-methyl-5-nitroimidazole] are evident from FIGS. 1 through 6. 
     FIG. 1 indicates that 100% water, 100% ethanol and all mixtures of water and ethanol do not dissolve a suitable amount of metronidazole. It is observed that the optimum solubility in water-ethanol mixtures occur at about 80% ethanol. Due to the low solubility of metronidazole in both 100% water and in 100% ethanol, one skilled in formulation arts would be led away from using these solvents as major components in a solvent system. 
     It is considered desirable to maintain N,N-dimethylacetamide concentrations in any formulation at a relatively low level. In FIG. 2, it is shown that 20% N,N-dimethylacetamide is not a suitable solvent system in combination with water or ethanol alone. Those satisfied with less desirable formulation could use a solvent system of 20% N,N-dimethylacetamide, 40% water, and 40% ethanol. But as before, the low solubility of metronidazole in 20% N,N-dimethylacetamide-water alone or 20% N,N-dimethylacetamide-ethanol alone would lead one skilled in the art to abandon a combination of water-ethanol and 20% N,N-diemthylacetamide. One would also believe this for combinations of 30% N,N-dimethylacetamide and water and ethanol. 
     It can be observed from FIG. 3 that, unexpectedly, a solvent system of 30% N,N-dimethylacetamide in combination with the indicated proportions of ethanol and water solubilizes more than 500 mg./10 ml. of metronidazole. 
     Also, unexpectedly, FIGS. 4, 5 and 6 illustrate that the combination of ethanol, water and N,N-dimethylacetamide can be made to dissolve more than 500 mg. of metronidazole by the addition of nicotinamide, propylene glycol or 2,2-dimethyl-1,3-dioxolane-4-methanol. These systems have the advantages of an even lower percentage composition of N,N-dimethylacetamide. Optimum systems and the solubility of metroinidazole therein are illustrated in Table 1. 
     
                       TABLE 1______________________________________                SolubilitySolvent System       (mg./10 ml.)______________________________________30% v/v N,N-dimethylacetamide52.3% v/v ethanol17.7% v/v aqueous buffer                642.220% v/v N,N-dimethylacetamide52.3% v/v ethanol25.7% v/v aqueous buffer2% w/v nicotinamide  631.020% v/v N,N-dimethylacetamide49% v/v ethanol21% v/v aqueous buffer10% v/v propylene glycol                595.720% v/v N,N-dimethylacetamide47.5% v/v ethanol22.5% v/v aqueous buffer10% v/v 2,2-dimethyl-1,3-dioxo- lane-4-methanol     575.4______________________________________ 
    
     Thus, critical solubility ranges for metronidazole which provide for a pharmaceutically acceptable and injectable unit dose form of this antimicrobial agent are provided by the present invention. 
     The following examples describe in detail compositions illustative of the present invention and methods which have been devised for their preparation. It will be apparent to those skilled in the art that modifications, both of materials and methods, may be practiced without departing from the purpose and intent of this disclosure. Throughout the examples hereinafter set forth, relative amounts are given in percent by volume, except as otherwise noted. The term &#34;q.s.&#34; refers to &#34;quantum sufficit&#34;. 
     EXAMPLE 1 
     A solution of 2.0 parts of N,N-dimethylacetamide and 5.23 parts of ethanol are mixed together with 2.0 parts of Tris buffer at room temperature with agitation or stirring. Dissolve nicotinamide (200 mg. per 10 ml. of the final solution) into such solution with agitation. After the solution has cleared, metronidazole (500 mg. per 10 ml. of the final solution) is added and dissolved with continuous stirring, and buffer is added q.s. to 10 parts by volume. The resulting solution is stirred for one hour, filtered, filled into ampules, sealed and sterilized. The resulting formulation has the following composition: 
     
         ______________________________________metronidazole          500 mg.N,N-dimethylacetamide  2.0 ml.ethanol                5.23 ml.nicotinamide           200 mg.Tris buffer q.s. to make                  10 ml.pH of the final solution is 5.0 - 6.0______________________________________ 
    
     EXAMPLE 2 
     When the procedure of Example 1 is repeated using only the appropriate amounts of metronidazole, N,N-dimethylacetamide, ethanol and Tris buffer, there is obtained a formulation having the following composition: 
     
         ______________________________________metronidazole          500 mg.N,N-dimethylacetamide  3.0 ml.ethanol                5.23 ml.Tris buffer q.s. to make                  10 ml.pH of the final solution is 5.0 - 6.0______________________________________ 
    
     EXAMPLE 3 
     When the procedure of Example 1 is repeated using propylene glycol or 2,2-dimethyl-1,3-dioxolane-4-methanol, respectively, in place of the nicotanamide, there are obtained formulations of the following compositions: 
     
         ______________________________________ metronidazole         500 mg.N,N-dimethylacetamide  2.0 ml.ethanol                4.9 ml.propylene glycol       1.0 ml.Tris buffer q.s.       10 ml.metronidazole          500 mg.N,N-dimethylacetamide  2.0 ml.ethanol                4.75 ml.2,2-dimethyl-1,3-dioxolane- 4-methanol            1.0 ml.Tris buffer q.s.       10 ml.______________________________________