Abstract:
The present invention relates to an improved process for the purification of polyaminocarboxylates such as DOTA, DTPA, DO3A-butrol, BOPTA.

Description:
BACKGROUND 
       [0001]    The invention relates to the purification of polyaminocarboxylate chelating agents which can be used as MRI contrast agents. Polyaminocarboxylates are part of several drugs which include gadoteric acid, gadobutrol, gadobenic acid, gadopentetate, gadoversetamide, gadodiamide. The key ingredient to all the gadolinium based drugs is the chelate to which the gadolinium binds. Having a highly pure chelate free of any salts and metals is the basis for the preparation of the pure gadolinium based MRI agents. 
         [0002]    Some of the chelates include 1,4,7,10-tetraazocyclododecane-, 1,4,7,10-tetraacetic acid (DOTA) for gadoteric acid, Pentetic acid (DTPA) for gadopentetate, BOPTA for gadobenic acid, 10-(1-hydroxymethyl-2,3-dihydroxypropyl)-1,4,7-tris(carboxymethyl)-1,4,7,10 tetraazocyclododecane (DO3A-butrol) for gadobutrol. 
       STATE OF THE ART 
       [0003]    Some of the chelates reported in the literature involve complex and expensive purification methods which involve resins of various kinds. 
         [0004]    U.S. Pat. No. 5,922,862 describes a procedure for the purification of DOTA and other cyclen substituted derivatives. It describes a procedure for the removal of inorganic salts by elution of the crude product dissolved in water over PVP resin. 
         [0005]    Inorg. Chem. 1980, 19, 1319 describes a procedure for the purification of DOTA using Dowex 50W-X4 resin. 
         [0006]    Inorg. Chem. 1997, 36, 6086 describes a procedure for the purification of DO3A-butrol with IR120H+resin to remove salts. 
         [0007]    Likewise several other resins are reported for the purification of the chelates to remove inorganic salts. The resin methods involved are expensive and some of the chelates bind to the resins, which then require washing with ammonia solution or formic acid solutions. Removal of ammonium ions becomes particularly difficult as the ligand forms ammonium salts. 
         [0008]    The manufacturing of the chelates becomes difficult when large ion-exchange columns are required at commercial scale manufacturing. Since most of the chelates have high water solubility the distillation of water becomes time consuming and also leads to potential degradation products. 
       SUMMARY OF THE INVENTION 
       [0009]    One of the objects of the invention was to provide a process for the preparation of the chelates in the pure form without the presence of impurities such as inorganic salts and other ions such as chloride, bromide, sulphate or the like. 
         [0010]    Yet another object of the invention was to develop a process with low cycle time, which is commercially viable, inexpensive and which utilizes none or minimal amount of resin. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    The purification procedure of chelates without ion-exchange column purifications are particularly advantageous as they are less time consuming and are rather inexpensive. 
         [0012]    It has now surprisingly been found that polyaminocarboxylate compounds when isolated under highly acidic conditions preferably below 0.75 are isolated as their acid salts and upon purification of these salts by recrystallization from water or water solvent mixtures result in polyaminocarboxylate with a content of any inorganic salts or ions such as sodium, potassium or the like below 200 ppm. The acid employed for the salt formation can be hydrochloric acid, hydrobromic acid, sulfuric acid and the like. The acid is preferably hydrochloric acid and the salt isolated is as hydrochloride salt. 
         [0013]    The water solvent mixtures employed are water/acetone, water/ethanol, water/methanol or the like to obtain a product free of undesired chloride or bromide ions, which have very stringent specifications. 
         [0014]    The salt thus obtained is dissolved in water and is then adjusted to the desired pH either with an aqueous basic solution or alternatively a resin. It is noteworthy to mention here that the amount of resin employed is minimal and is used to adjust the pH of the solution about 1.5-3.0, thereby eliminating the chance for any additional inorganic salt formation. As the resin employed is in minimal amounts the process does not suffer from the drawbacks of the prior art. The pH is preferably adjusted with a basic anionic resin. 
         [0015]    The resins that are employed in the current process are resin in the hydroxide form such as Amberlyst A26 OH resin and the like. The bases that can be employed for the pH adjustment are potassium hydroxide, sodium hydroxide, ammonium hydroxide, triethyl amine or the like. 
         [0016]    An example of the process schematics is shown below for DOTA. The example described for DOTA is not limiting and a similar purification process can be applied to any polyaminocarboxylate. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0017]    The following examples describe the process in greater detail. 
       EXAMPLE 1 
     Synthesis of DOTA 
       [0018]    Charge 100 ml of water and 100 g cyclen HCI to a flask. Cool the reaction mass to 0-10 ° C. and adjust the pH of the solution to 10-10.5 with sodium hydroxide. Charge chloroacetic acid to the reaction with continuous pH adjustment to 10-10.5 with sodium hydroxide solution. Slowly raise the reaction to 70-75 ° C. and maintain until the completion of reaction. Cool the reaction mass and adjust the pH of the reaction to below 0.75 with cone. HCI and filter the solid separated which is DOTA hydrochloride. Recrystallize the solid from water and check the content of sulfated ash to be below 0.10%. The solid is then dissolved in 800 ml water and the pH of the solution adjusted to around 2.5 to 3.0 with amberlyst A26 OH resin to obtain a chelate free of hydrochloride salt. The mass is then filtered, and the filtrate obtained is distilled to 200-300 ml water and the solid is precipitated by adding acetone. The solid is filtered and dried to give the title product with desired purity having sulfated ash content below 0.1% and the sodium and chloride content below 200 ppm. 
       EXAMPLE 2 
     Synthesis of BOPTA 
       [0019]    To a flask were charged 100 ml of water and 50 g of N-[2-[(2-aminoethyl)amino]ethyl]-O-(phenylmethyl)serine. Cool the reaction mass to 0-10° C. and adjust the pH of the solution to 10.5-11.5 with sodium hydroxide solution. Charge chloroacetic acid to the reaction with continuous pH adjustment to 10.5-11.5 with sodium hydroxide solution. Slowly raise the reaction temperature to 70-75° C. and maintain until the completion of reaction. Cool the reaction mass and adjust the pH of the reaction to below 0.75 with Cone. HCI and charge acetone. Filter the solid separated which is BOPTA hydrochloride salt. Recrystallize the solid from water and check the sulfated ash content to be below 0.10%. The solid is then dissolved in 800 ml water and the pH of the solution adjusted to around 2.0 with amberlyst A26 OH resin. The mass is then filtered and the filtrate is distilled to 200-300 ml water and the solid is precipitated by adding acetone. The solid thus obtained is filtered and dried to give the product with desired purity having sulfated ash below 0.1%.