Oxytetracycline compositions
BACKGROUND OF THE INVENTION 
     This invention relates to antibiotic compositions suitable for pharmaceutical use. More particularly, it relates to magnesium-calcium-oxytetracycline mixed chelates in aqueous 2-pyrrolidone solution. 
     U.S. Pat. No. 4,018,899 discloses oxytetracycline aqueous solutions containing 2-pyrrolidone as a cosolvent. The oxytetracycline is present as a magnesium chelate. 
     U.S. Pat. No. 3,017,323 discloses oxytetracycline in aqueous glycol solutions containing calcium and magnesium. A ratio of 1:1:3 of calcium:magnesium:oxytetracycline is employed. 
     U.S. Pat. No. 3,929,989 discloses suspensions of a calcium-magnesium-oxytetracycline complex having a molar ratio of combined calcium and magnesium to oxytetracycline of about 4 to 1 in aqueous 1,2-propanediol. 
     SUMMARY OF THE INVENTION 
     It has now been found that stable clear high potency solutions of oxytetracycline producing prolonged blood levels can be provided by means of a novel pharmaceutical composition comprising a solution in water of from about 20 to 30% w/v of oxytetracycline, from about 0.8 to 0.95 molar proportions based on said oxytetracycline of magnesium oxide, from about 0.15 to 0.3 molar proportions based on said oxytetracycline of a pharmaceutically acceptable calcium compound soluble in said solution and from about 40 to 60% w/v 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 
     Oxytetracycline, the therapeutically-active component of this invention, is a widely used tetracycline-type antibiotic. It is particularly described in U.S. Pat. No. 2,516,081. An effective concentration range for oxytetracycline in the solutions of this invention is generally from about 20 to 30% w/v expressed as the free base. The preferred concentration is from about 20 to 25% w/v. 
     Magnesium ions and calcium ions combine with oxytetracycline in solution to form magnesium-calcium-oxytetracycline chelates. Magnesium oxide is the source of magnesium ions and is present in a concentration of from about 0.8 to 0.95 molar proportions based on said oxytetracycline. Calcium acetate is a convenient and preferred source of calcium ions, but other soluble, pharmaceutically acceptable calcium compounds may be employed such as calcium lactate and calcium propionate. The calcium compound is present in a concentration of from about 0.15 to 0.3 molar proportions based on said oxytetracycline. 
     2-Pyrrolidone is present as a co-solvent in a concentration of from about 40 to 60 percent w/v. The preferred concentration is from about 50 to 55% w/v. 2-Pyrrolidone is also known as 2-pyrrolidinone, 2-oxopyrrolidine, alpha-pyrrolidone and 2-ketopyrrolidine. It has an oral LD 50  of 8 g/kg in rats and 3.8 g/kg by intraperitoneal injection in mice. Its use allows for minimum volume per dose and excellent syringability due to the low viscosity of the resultant composition. 
     As an optional ingredient polyvinylpyrrolidone having a molecular weight of between about 5000 and 100,000 (K-12 to 30) may also be present in a concentration of from about 1 to 15% w/v. The polyvinylpyrrolidone preferred for this invention is one having an average molecular weight of about 10,000-17,000 (K-12 to 17). It is also present in part as a co-solublizer and may improve tissue toleration. 
     As optional co-solvents ingredients such as propylene glycol and glycerol formal may be present in concentrations of up to 15% w/v. 
     The stability of these solutions for therapeutic administration is still further enhanced by the use of anti-oxidants such as sodium or magnesium formaldehyde sulfoxylate and monothioglycerol at levels of from about 0.01 to 1% w/v. 
     The pH value is adjusted if necessary to pH 7.5 to 9.5. The preferred range is pH 8.5 to 9.0. The pH can be adjusted with organic bases such as monoethanolamine, dimethylaminoethanol, dimethylamine and so forth. Of these compounds, monoethanolamine is the preferred compound. 
     The compositions of this invention are readily prepared by mixing and dissolving the calcium compound in a portion of the water. The 2-pyrrolidone is then mixed with the major portion of the water and polyvinylpyrrolidone, if present in the formulation, is added and dissolved. The solution is heated to 45°-75° C. and the antioxidants added with stirring. The magnesium oxide is slurried with this solution and the oxytetracycline is slowly added with stirring until solution is obtained. The calcium compound in solution is then slowly added to the magnesium oxytetracycline solution with stirring until a clear solution is formed. After the solution cools to room temperature the pH is adjusted if necessary. 
     Solution is then brought up to viscosity with water. 
     These compositions are easy to syringe over a wide temperature range, have acceptable animal tissue toleration and give therapeutic blood levels for a period for up to 15 days. The mixed magnesium-calcium chelate produces on injection a controlled amount of precipitated antibiotic which acts as a depot and brings about unusually prolonged blood levels which are not obtained with the use of the magnesium chelate alone. 
    
    
     EXAMPLE 1 
     
         ______________________________________                                    
                      gm./100ml.                                          
______________________________________                                    
Oxytetracycline (based on a potency                                       
of 927 mcg./mg. plus a 2% overage)                                        
                        27.51                                             
Magnesium oxide         1.844                                             
Calcium acetate         1.931                                             
2-pyrrolidone           50.00                                             
Polyvinylpyrrolidone, K-17                                                
                        5.00                                              
Sodium formaldehyde sulfoxylate                                           
                        0.30                                              
Monoethanolamine, to adjust pH to 8.5                                     
Water q.s. to           100 ml.                                           
______________________________________                                    
 
    
     The calcium acetate was dissolved in 10 ml. of water. The 2-pyrrolidone was mixed with 30 ml. of water. The polyvinylpyrrolidone was added and dissolved. The solution was heated to 45° C. and the sodium formaldehyde sulfoxylate was added and dissolved with stirring. The magnesium oxide was then slurried with the solution. The oxytetracycline was slowly added with stirring until solution was attained. The calcium acetate solution was slowly added to the magnesiumoxytetracycline solution with stirring until a clear solution is formed. The solution was then 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. 
     The above solution containing 250 mg./ml. of oxytetracycline activity had a viscosity of 45 cts. at 25° C. 
     EXAMPLE 2 
     The following solution containing 250 mg./ml. of oxytetracycline activity was prepared using the procedure described in Example 1. 
     
         ______________________________________                                    
                      gm./100ml.                                          
______________________________________                                    
Oxytetracycline (based on a potency                                       
of 927 mcg./mg. plus a 2% overage)                                        
                        27.51                                             
Magnesium oxide         1.844                                             
Calcium acetate         1.931                                             
2-pyrrolidone           55.00                                             
Sodium formaldehyde sulfoxylate                                           
                        0.30                                              
Monoethanolamine, to adjust pH to 8.5                                     
water q.s. to           100 ml.                                           
______________________________________                                    
 
    
     The viscosity was 49 cts at 25° C. 
     EXAMPLE 3 
     The following solution containing 300 mg./ml. of oxytetracycline activity was prepared using the procedure described in Example 1. 
     
         ______________________________________                                    
                      gm./100ml.                                          
______________________________________                                    
Oxytetracycline (based on a potency                                       
of 927 mcg./mg. plus a 2% overage)                                        
                        33.00                                             
Magnesium oxide         2.221                                             
Calcium acetate         2.111                                             
2-Pyrrolidone           50.00                                             
Polyvinylpyrrolidone, K-17                                                
                        3.00                                              
Sodium formaldehyde sulfoxylate                                           
                        0.30                                              
Monoethanolamine, to adjust pH to 8.5                                     
water q.s. to           100 ml.                                           
______________________________________                                    
 
    
     The viscosity was 173 cts. at 25° C. 
     EXAMPLE 4 
     The following solution containing 300 mg./ml. of oxytetracycline activity was prepared using the procedure described in Example 1. 
     
         ______________________________________                                    
                      gm./100ml.                                          
______________________________________                                    
Oxytetracycline (based on a potency                                       
of 927 mcg./mg. plus a 2% overage)                                        
                        33.00                                             
Magnesium oxide         2.221                                             
Calcium acetate         2.111                                             
2-Pyrrolidone           55.00                                             
Sodium formaldehyde sulfoxylate                                           
                        0.30                                              
Monoethanolamine, to adjust pH to 8.5                                     
water q.s. to           100 ml.                                           
______________________________________                                    
 
    
     The viscosity was 138 cts. at 25° C. 
     EXAMPLE 5 
     A solution containing 250 mg./ml. of oxytetracycline activity was prepared using the procedure described in Example 1. 
     
         ______________________________________                                    
                      gm./100ml.                                          
______________________________________                                    
Oxytetracycline (based on a potency                                       
of 927 mcg./mg. plus a 2% overage)                                        
                        27.51                                             
Magnesium oxide         1.844                                             
Calcium acetate         1.931                                             
2-Pyrrolidone           50.00                                             
Polyvinylpyrrolidone, K-17                                                
                        3.00                                              
Sodium formaldehyde sulfoxylate                                           
                        0.30                                              
Monoethanolamine, to adjust pH to 8.5                                     
water q.s. to           100 ml.                                           
______________________________________                                    
 
    
     The viscosity was 69 cts. at 25° C. 
     EXAMPLE 6 
     The following solution containing 250 mg./ml. of oxytetracycline activity was prepared using the procedure described in Example 1. 
     
         ______________________________________                                    
                      gm./100ml.                                          
______________________________________                                    
Oxytetracycline (based on a potency                                       
of 927 mcg./mg. plus a 2% overage)                                        
                        27.51                                             
Magnesium oxide         1.844                                             
Calcium acetate         3.379                                             
2-Pyrrolidone           50.00                                             
Polyvinylpyrrolidone, K-17                                                
                        3.00                                              
Sodium formaldehyde sulfoxylate                                           
                        0.30                                              
Monoethanolamine, to adjust pH to 8.5                                     
water q.s. to           100 ml.                                           
______________________________________                                    
 
    
     The viscosity was 72 cts. at 25° C. 
     EXAMPLE 7 
     The following solution containing 250 mg./ml. of oxytetracycline activity was prepared using the procedure described in Example 1. 
     
         ______________________________________                                    
                      gm./100ml.                                          
______________________________________                                    
Oxytetracycline (based on a potency                                       
of 927 mcg./mg. plus a 2% overage)                                        
                        27.51                                             
Magnesium oxide         2.075                                             
Calcium acetate         1.931                                             
2-Pyrrolidone           50.00                                             
Polyvinylpyrrolidone, K-17                                                
                        3.00                                              
Sodium formaldehyde sulfoxylate                                           
                        0.30                                              
Monoethanolamine, to adjust pH to 8.5                                     
water q.s. to           100 ml.                                           
______________________________________                                    
 
    
     The viscosity was 58 cts. at 25° C. 
     EXAMPLE 8 
     The following solution containing 250 mg./ml. of oxytetracycline activity was prepared using the procedure described in Example 1. 
     
         ______________________________________                                    
                      gm./100ml.                                          
______________________________________                                    
Oxytetracycline (based on a potency                                       
of 927 mcg./mg. plus a 2% overage)                                        
                        27.51                                             
Magnesium oxide         1.844                                             
Calcium acetate         3.379                                             
2-Pyrrolidone           50.00                                             
Polyvinylpyrrolidone, K-12                                                
                        5.00                                              
Sodium formaldehyde sulfoxylate                                           
                        0.30                                              
Monoethanolamine, to adjust pH to 8.5                                     
water q.s. to           100 ml.                                           
______________________________________                                    
 
    
     The viscosity was 75 cts. at 25° C. 
     EXAMPLE 9 
     The following solution containing 250 mg./ml. of oxytetracycline activity was prepared using the procedure described in Example 1. 
     
         ______________________________________                                    
                      gm./100ml.                                          
______________________________________                                    
Oxytetracycline (based on a potency                                       
of 927 mcg./mg. plus a 2% overage)                                        
                        27.51                                             
Magnesium oxide         1.844                                             
Calcium acetate         2.897                                             
2-Pyrrolidone           5.00                                              
Polyvinylpyrrolidone, K-12                                                
                        7.50                                              
Sodium formaldehyde sulfoxylate                                           
                        0.30                                              
Monoethanolamine, to adjust pH to 8.5                                     
water q.s. to           100 ml.                                           
______________________________________                                    
 
    
     The viscosity was 115 cts. at 25° C. 
     EXAMPLE 10 
     The following solution containing 250 mg./ml. of oxytetracycline activity was prepared using the procedure described in Example 1, except that the glycerol formal was added to the 2-pyrrolidone before the addition of water. 
     
         ______________________________________                                    
                      gm./100ml.                                          
______________________________________                                    
Oxytetracycline (based on a potency                                       
of 927 mcg./mg. plus a 2% overage)                                        
                        27.51                                             
Magnesium oxide         1.844                                             
Calcium acetate         2.897                                             
2-Pyrrolidone           5.00                                              
Polyvinylpyrrolidone, K-12                                                
                        7.50                                              
Glycerol formal         15.00                                             
Sodium formaldehyde sulfoxylate                                           
                        0.50                                              
Monoethanolamine, to adjust pH to 8.5                                     
water q.s. to           100 ml.                                           
______________________________________                                    
 
    
     The viscosity was 138 cts. at 25° C. 
     EXAMPLE 11 
     The following solution containing 250 mg./ml. of oxytetracycline activity was prepared using the procedure described in Example 1. 
     
         ______________________________________                                    
                      gm./100ml.                                          
______________________________________                                    
Oxytetracycline (based on a potency                                       
of 927 mcg./mg. plus a 2% overage)                                        
                        22.008                                            
Magnesium oxide         1.844                                             
Calcium acetate         2.897                                             
2-Pyrrolidone           55.00                                             
Polyvinylpyrrolidone, K-12                                                
                        10.00                                             
Sodium formaldehyde sulfoxylate                                           
                        0.30                                              
Monoethanolamine, to adjust pH to 8.0                                     
water q.s. to           100 ml.                                           
______________________________________                                    
 
    
     The viscosity was 84 cts. at 25° C. 
     EXAMPLE 12 
     The following solution containing 250 mg./ml. of oxytetracycline activity was prepared using the procedure described in Example 8. 
     
         ______________________________________                                    
                      gm./100ml.                                          
______________________________________                                    
Oxytetracycline (based on a potency                                       
of 927 mcg./mg. plus a 2% overage)                                        
                        22.008                                            
Magnesium oxide         1.844                                             
Calcium acetate         2.897                                             
2-Pyrrolidone           50.00                                             
Polyvinylpyrrolidone, K-12                                                
                        7.50                                              
Glycerol formal         15.0                                              
Sodium formaldehyde sulfoxylate                                           
                        0.30                                              
Monoethanolamine, to adjust pH to 8.5                                     
water q.s. to           100 ml.                                           
______________________________________                                    
 
    
     The viscosity was 83 cts. at 25° C. 
     EXAMPLE 13 
     The following solution containing 250 m.g./ml. of oxytetracycline activity was prepared using the procedure described in Example 1. 
     
         ______________________________________                                    
                      gm./100ml.                                          
______________________________________                                    
Oxytetracycline (based on a potency                                       
of 927 mcg./mg. plus a 2% overage)                                        
                        27.51                                             
Magnesium oxide         1.844                                             
Calcium acetate         1.931                                             
2-Pyrrolidone           50.00                                             
Polyvinylpyrrolidone, K-17                                                
                        3.00                                              
Sodium formaldehyde sulfoxylate                                           
                        0.30                                              
Monothioglycerol        1.00                                              
water q.s. to           100 ml.                                           
______________________________________                                    
 
    
     The viscosity was 45 cts. at 25° C. 
     EXAMPLE 14 
     The following solution containing 250 mg./ml. of oxytetracycline activity was prepared using the procedure described in Example 8, except that propylene glycol is used instead of glycerol formal. 
     
         ______________________________________                                    
                      gm./100ml.                                          
______________________________________                                    
Oxytetracycline (based on a potency                                       
of 927 mcg./mg. plus a 2% overage)                                        
                        27.51                                             
Magnesium oxide         1.844                                             
Calcium acetate         2.897                                             
2-Pyrrolidone           50.00                                             
Polyvinylpyrrolidone, K-12                                                
                        7.50                                              
Propylene glycol        15.00                                             
Sodium formaldehyde sulfoxylate                                           
                        0.50                                              
Monoethanolamine, to adjust pH to 8.5                                     
water q.s. to           100 ml.                                           
______________________________________                                    
 
    
     The viscosity was 135 cts. at 25° C. 
     EXAMPLE 15 
     A mixed magnesium-calcium-oxytetracycline chelate solution and a magnesium-oxytetracycline chelate solution were each administered intramuscularly to cattle at two sites on the same leg at a dosage level of 30 mg. oxytetracycline/kg. Blood samples were taken at the indicated time intervals and plasma oxytetracycline levels were determined. 
     
         ______________________________________                                    
        mcg./ml. oxytetracycline                                          
        Post Injection                                                    
               1      4      7    10   13   15                            
        4 Hrs. Days   Days   Days Days Days Days                          
______________________________________                                    
Magnesium-                                                                
calcium                                                                   
chelate                                                                   
(Ex. 6)   1.72     1.30   0.38 0.35 0.31 0.26 0.23                        
Magnesium-                                                                
chelate   5.07     2.27   0.27 0.20 0    0    0                           
(Ex. 1 of                                                                 
U.S. Pat. No.                                                             
4,018,889)                                                                
______________________________________                                    
 
    
     The above results clearly illustrate the prolonged blood levels achievable with the mixed chelate formulation.