Abstract:
Aqueous solutions of tetracycline or salts thereof, a pharmaceutically acceptable soluble magnesium compound and 2-pyrrolidone as a co-solvent, said solution having a pH of 7.5 to 9.5 and being useful as an injectable composition combining low viscosity, high potency, good clarity and good stability.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of my copending application Ser. No. 646,295, filed Jan. 2, 1976, now U.S. Pat. No. 4,018,889. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to antibiotic compositions suitable for pharmaceutical use. More particularly, it relates to tetracycline solutions containing 2-pyrrolidone. 
     Previous efforts made to prepart high concentration tetracycline solutions have been unsuccessful. This is of particular importance in the case of veterinary parenteral compositions for administration to large animals. 
     U.S. Pat. No. 2,990,331 discloses parenteral solutions of tetracycline hydrochloride, containing about 50 mg./ml having a pH value between 5 and 7, containing magnesium ions, an alkali bisulfite and a carboxylic acid amide, such as lactic acid-hydroxyethyl amide. 
     Japanese Patent Publication No. Sho 47-303 discloses stable aqueous solutions of p-biphenylmethyl (dl-tropylα-tropinium) bromide, 2.5%, in which 2-pyrrolidone is present in a concentration of 20%. The use of polyvinylpyrrolidone at a concentration of 30% is also disclosed. The pH of these solutions is less than 7, the preferred range being 3-4. 
     Japanese Patent Publication No. Sho 43-1758 discloses insecticidal solutions containing hexachlorcyclohexane in alcohol and 2-pyrrolidone as solvents. The use of N-methyl pyrrolidone as a co-solvent is also disclosed. 
     British Patent Specification No. 802,111 discloses pesticidal compositions containing 2-pyrrolidone or N-methyl pyrrolidone as solvents for DDT, dieldrin, aldrin and similar insecticides. The use of 67-82% of 2-pyrrolidone is exemplified. 
     British Patent Specification No. 805,026 discloses the use of N-methyl pyrrolidione in concentrations of 40% as a solvent for various medicaments intended to parenteral administration, such as chloramphenicol, N,N&#39;-dibenzyl ethylenediamine-diencillin G and procaine penicillin. 
     U.S. Pat. No. 2,987,437 discloses nematocidal compositions of 3,4-dichlorotetrahydrothiophene, 1,1-dioxide in 2-pyrrolidone. 
     German Patent No. 1,091,287 discloses stable aqueous solutions of tyrothricin 0.25% or subtilin 0.2% for nasal or otic use prepared with the aid or pyrrolidone and/or polyvinylpyrrolidone as solubilizers. Pyrrolidone is used in a concentration of 0.5% and polyvinylpyrrolidone can be used up to 10%. 
     U.S. Pat. No. 3,062,717 discloses aqueous parenteral solutions of tetracycline calcium complexes containing 35-80% of an amide of acetic or lactic acid, such as N,N-dimethylacetamide or N-(β-hydroxyethyl) lactamide, at a pH of 7 to 9.5. Concentrations of 10 to 100 mg/ml are disclosed. J. Pharm. Sci. 46, p.458 (1957) discloses that oxytetracycline forms soluble complexes with N-methyl pyrrolidone in aqueous solution. The degree of interaction is limited by pH and solubility considerations. 
     SUMMARY OF THE INVENTION 
     It has now been found that stable high potency solutions of tetracycline can be provided by means of a novel pharmaceutical composition comprising an aqueous solution of from about 1 to 15% by weight of an antibiotic compound selected from tetracycline and the pharmaceutically acceptable acid addition salts thereof, about 0.8 to 1.3 molar proportions based on said antibiotic of a pharmaceutically acceptable magnesium compound soluble in said solution, and from about 10 to 70% by weight of 2-pyrrolidone, said composition having a pH value in the range of from about 7.5 to 9.5. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Tetracycline, the therapeutically-active component of this invention, is a widely used tetracycline-type antibiotic. It is particularly described in U.S. Pat. No. 2,699,054. An effective concentration range for tetracycline in the solutions of this invention is generally from about 1 to 15% by weight of the total in the form of the free base or a pharmaceutically acceptable acid addition salt. The preferred form is the free base with the preferred concentration being from about 5 to 15% by weight, with the especially preferred concentration being from about 5 to 10% by weight. 
     Examples of suitable tetracycline acid addition salts which can be used include such pharmaceutically acceptable acid addition salts as hydrochloride, hydrobromide and sulfate. However, the preferred acid addition salt is tetracycline hydrochloride. 
     Magnesium ions combine with tetracycline in solution to form magnesium-tetracycline chelates. Magnesium oxide is a convenient and preferred source of magnesium ions, but other magnesium compounds useful for the purpose of this invention include magnesium chloride, magnesium acetate and magnesium sulfate. The molar ratio of magnesium to tetracycline in these compositions is about from 0.8 to 1.3 mole. This ratio is advisable to produce clear stable solutions. 
     2-Pyrrolidone is present as a co-solvent in a concentration of from about 10 to 70%, and preferably from about 60 to 70%, based on the total weight of the composition. 2-Pyrrolidone is also known as 2-pyrrolidinone, 2-oxopyrrolidine, α-pyrrolidone and 2-ketopyrrolidine. It has an oral LD 50  of 8 gm/kg in rats and 3.8 gm/kg by intraperitoneal injection in mice. Its use allows for minimum volume per dose and excellent satisfactory due to low viscosity of the resultant composition. 
     As an optional ingredient polyvinylpyrrolidone may also be present in a concentration of from about 1 to 7% by weight. The polyvinylpyrrolidone preferred for this invention is one having an average molecular weight of between about 5,000 and 100,000 (K-12 to 30) and especially between about 10,000 and 17,000 (K - 17). It is present in part as a cosolubilizer and may improve tissue toleration. 
     The stability of these solutions for therapeutic administration is still further enchanced by the use of antioxidants such as sodium or magnesium formaldehyde sulfoxylate and monothioglycerol at levels of from about 0.01 to 1.0% by weight. 
     The pH value is adjusted if necessary to pH 7.5 to 9.5. The preferred range is pH 8 to 9. The pH can be adjusted with an organic base such as monoethanolamine, or with an acid that is pharmaceutically acceptable, such as hydrochloric acid. 
     The compositions of this invention are readily prepared by mixing the magnesium compound with the 2-pyrrolidone and water at about 50° C and slowly adding the tetracycline antibiotic with stirring until dissolved. The pH is then adjusted to the desired range. If polyvinylpyrrolidone is to be included it is added to the 2-pyrrolidone and water before the addition of the magnesium compound as previously described. 
     These compositions are also easy to syringe over a wide temperature range and are satisfactory from a physical and chemical stability standpoint. 
     The use of these high potency tetracycline compositions enables a reduction of the number of injections that must be administered to large animals, such as steers, in order to receive an effective dose. 
     The primary application is as a parenteral composition but the new compositions can also be used for topical or oral application. 
     EXAMPLE 1 
     The following solution containing 100 mg/ml of tetracycline hydrochloride activity was prepared. 
     
         ______________________________________                  gm/100 ml______________________________________Tetracycline (based on atetracycline hydrochloride potency of955 γ/mg plus a 5% overage                    10.995Magnesium oxide          0.9582-Pyrrolidone            70.00Sodium formaldehyde sulfoxylate                    1.00Polyvinylpyrrolidone K-17                    5.00Monoethanolamine, to adjust pH to 8.5Water q.s. to            100 ml______________________________________ 
    
     The 2-pyrrolidone was mixed with water. Polyvinylpyrrolidone was then added and stirred until dissolved. The solution was heated to about 50° C and the sodium formaldehyde sulfoxylate was added and dissolved with stirring. The magnesium oxide was then slurried with the solution. The tetracycline was slowly added with stirring until a clear solution resulted. The solution was allowed to cool to room temperature and the pH adjusted to 8.5 with monoethanolamine. The solution was then brought up to volume with water. 
     A comparable solution was made by using 60.00 gm/100 ml of 2-pyrrolidone instead of 70.00 gm/100 ml. 
     EXAMPLE 2 
     The following solution containing 50 mg/ml of tetracycline hydrochloride activity was prepared using the procedure described in Example 1. 
     
         ______________________________________                  gm/100 ml______________________________________Tetracycline (based on atetracycline hydrochloride potency of955 γ/mg plus a 5% overage)                    5.496Magnesium oxide          0.4792-Pyrrolidone            70.00Sodium formaldehyde sulfoxylate                    1.00Polyvinylpyrrolidone K-17                    5.00Monoethanolamine, to adjust pH to 8.5Water q.s. to            100 ml______________________________________ 
    
     A solution comparable to the above was also made by using 50.00 g/100 ml of 2-pyrrolidone instead of 70.00 g/100 ml. 
     EXAMPLE 3 
     The following solution containing 50 mg/ml tetracycline hydrochloride activity was prepared using the procedure described in Example 1. 
     
         ______________________________________                  gm/100 ml______________________________________Tetracycline (based on atetracycline hydrochloride potency of955 γ/mg plus a 5% overage)                    5.496Magnesium oxide          0.4792-Pyrrolidone            60.00Magnesium formaldehyde sulfoxylate                    0.44Polyvinylpyrrolidone K-17                    5.00Monoethanolamine, to adjust pH to 8.5Water q.s. to            100 ml______________________________________ 
    
     EXAMPLE 4 
     The following solution containing 100 mg/ml of tetracycline hydrochloride activity was prepared using the procedure described in Example 1, except the polyvinylpyrrolidone is not present. 
     
         ______________________________________                  gm/100 ml______________________________________Tetracycline (based on atetracycline hydrochloride potency of955 γ/mg plus a 5% overage)                    10.995Magnesium oxide          0.9582-Pyrrolidone            60.00Magnesium formaldehyde sulfoxylate                    0.44Monoethanolamine, to adjust pH to 8.9Water q.s. to            100 ml______________________________________ 
    
     A comparable solution containing 70.00 g/ml of 2-pyrrolidone with the pH adjusted to 8.8 was also prepared. 
     EXAMPLE 5 
     The following solution containing 50 mg/ml of tetracycline hydrochloride activity was prepared using the procedure described in Example 4. 
     
         ______________________________________                  gm/100 ml______________________________________Tetracycline (based on atetracycline hydrochloride potency of955 γ/mg plus a 5% overage)                    5.496Magnesium oxide          0.4792-Pyrrolidone            60.00Magnesium formaldehyde sulfoxylate                    0.44Monoethanolamine, to adjust pH to 8.8Water q.s. to            100 ml______________________________________ 
    
     A comparable solution containing 70.00 g/ml of 2-pyrrolidone with the pH adjusted to 8.7 was also prepared. 
     EXAMPLE 6 
     The following solution containing 100 mg/ml of tetracycline hydrochloride activity was prepared using the procedure described in Example 4. 
     
         ______________________________________                  mg/100 ml______________________________________Tetracycline hydrochloride (basedon a tetracycline hydrochloridepotency of 980 γ/mg plus a 5% overage)                    10.714Magnesium oxide          0.9392-Pyrrolidone            60.00Sodium formaldehyde sulfoxylate                    1.00Monoethanolamine, to adjust pH to 8.5Water q.s. to            100 ml______________________________________ 
    
     EXAMPLE 7 
     The following solution containing 10 mg/ml of tetracycline hydrochloride activity was prepared following the procedure described in Example 4, except that the pH is adjusted with concentrated hydrochloric acid. 
     
         ______________________________________                  gm/100 ml______________________________________Tetracycline (based on atetracycline hydrochloride potencyof 955 γ/mg plus a 5% overage)                    1.099Magnesium oxide          0.0962-Pyrrolidone            30.00Sodium formaldehyde sulfoxylate                    1.00Concentrated hydrochloric acid, toadjust pH to 7.5Water q.s. to            100 ml______________________________________ 
    
     Solutions comparable to the above were also made by adjusting the pH to 6.5 and 5.2 respectively.