Abstract:
This invention relates to the synthesis of iodixanol (1,3-bis(acetamido)-N,N′-bis[3,5-bis(2,3-dihydroxypropylaminocarbonyl)-2,4,6-triiodophenyl]-2-hydroxypropane), more specifically to the dimerization of 5-acetamido-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodo-isophthalamide with a solvent mixture comprising 1-methoxy-2-propanol and water or methanol.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims benefit of priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 61/227,097 filed Jul. 21, 2009, the entire disclosure of which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates to the synthesis of iodixanol (1,3-bis(acetamido)-N,N′-bis[3,5-bis(2,3-dihydroxypropylaminocarbonyl)-2,4,6-triiodophenyl]-2-hydroxypropane), more specifically to the dimerisation of 5-acetamido-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodo-isophthalamide with a solvent mixture comprising 1-methoxy-2-propanol and water or methanol. 
     BACKGROUND OF THE INVENTION 
     Iodixanol is the non-proprietory name of the chemical drug substance of a non-ionic X-ray contrast agent marketed under the trade name Visipaque™. Visipaque™ is one of the most used agents in diagnostic X-ray procedures and is manufactured in large quantities. 
     The manufacture of such non-ionic X-ray contrast agents involves the production of the chemical drug substance (referred to as primary production) followed by formulation into the drug product (referred to as secondary production). Primary production of iodixanol involves a multi step chemical synthesis and a thorough purification process. For a commercial drug product it is important for the primary production to be efficient and economical and to provide a drug substance fulfilling the specifications, e.g. as expressed in the US Pharmacopea. 
     A number of methods are known for the preparation of iodixanol. These are all multi step chemical synthetic processes and the cost of the final formulated product thus mainly depends on these processes. It is therefore important to optimize the processes both for economic and environmental reasons. 
     In a preferred method for the preparation of iodixanol described in EP 108638, which document is hereby incorporated by reference, the final intermediate 5-acetamido-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodo-isophthalamide (hereinafter “Compound A”) is reacted with a dimerisation agent such as epichlorohydrin to yield the drug substance, see Scheme I. 
     
       
                 
         
             
             
         
      
     
     The reaction is usually carried out in the non-aqueous solvent 2-methoxyethanol and generally results in the conversion of 40 to 60% of Compound A to iodixanol. The product contains large amounts of impurities and is normally purified by crystallization. Too large amounts of impurities make the purification difficult and to achieve the desired purity, the crude iodixanol produced by the synthetic chemical process is crystallized twice. The process is time consuming and takes about 3 days for the first crystallization and about 2 days for the second one. Hence, the crystallization process is very demanding in terms of time and equipment size, it will take several days to perform and is often a bottleneck in industrial scale processes. 
     It is hence a desire to identify alternative low-cost and easily accessible solvents that can be used in the dimerisation step and that fulfill the above-mentioned criteria. 
     WO 99/18054 describes a method of crystallization of iodixanol where a solvent mixture comprising methanol and propan-2-ol is used in the example. A vast number of other general solvents are suggested, however many of these will not be feasible in an industrial scale and there are no suggestions regarding how to perform the crystallization in terms ratios or process parameters. 
     WO 98/23296 and U.S. Pat. No. 5,349,085 describe the synthesis of iodixanol and mention possible solvents in the same general way as in WO 99/18054, including alcohols and glycol respectively. 
     WO 2006/016815 describes the purification of iodixanol by crystallization using 1-methoxy-2-propanol as solvent. 
     It has now surprisingly been found that a solvent mixture comprising 1-methoxy-2-propanol and water or methanol can be used in the dimerisation step of Compound A in an industrial scale and will fulfill the requirements listed above. 
     SUMMARY OF THE INVENTION 
     The present invention provides a large scale dimerisation process of 5-acetamido-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodo-isophthalamide to iodixanol. 
     Thus, the invention provides a process for the dimerisation of 5-acetamido-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodo-isophthalamide in a temperature range of about 10 to about 20° C. using a solvent mixture comprising about 70 to about 90 vol % 1-methoxy-2-propanol and about 10 to about 30 vol % water or methanol in a concentration of about 0.8 to about 2.0 ml solvent per g 5-acetamido-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodo-isophthalamide in the presence of about 0.30 to about 0.40 mole equivalents of epichlorohydrin. 
     The instant process uses a low cost solvent mixture that is environmentally friendly and provides high enough yields and purity in the final product to make the manufacturing process of iodixanol economically feasible in an industrial scale. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Crude iodixanol is obtained from the processes known from the state of the art, e.g. from the dimerisation process illustrated in Scheme I above. The dimerisation step itself may be carried out as described in EP 108638 and WO 98/23296, for example using epichlorohydrin as the dimerisation agent. The reaction is usually carried out in the non-aqueous solvent 2-methoxyethanol and generally results in the conversion of 40 to 60% of Compound A to iodixanol. 
     As explained above the dimerisation generally results in the conversion of 40 to 60% of Compound A. However, the product contains large amounts of impurities and needs to undergo costly work-up procedures, like, for example, multiple crystallizations. 
     The most important impurities in the reaction with regard to work-up consequences are the so-called backpeaks. This term refers to retention times in reversed phase HPLC, where the backpeaks have slightly longer retention times than iodixanol itself. Most of the backpeaks are either trimers or O-alkylated dimers. Two examples are given below: 
     
       
                 
         
             
             
         
      
     
     Other by-products of importance are e.g. iohexol and N-acetyl cyclised iodixanol, whose structures are shown below. Iohexyl is fairly easy to remove in the subsequent crystallization of iodixanol, even when present in several weight percent. 
     
       
                 
         
             
             
         
      
     
     A typical selectivity required to be able to run an economically feasible work-up and obtain the required product quality is that the amount of backpeaks should not exceed 2% at 55-60% conversion of Compound A to iodixanol. It is even more advantageous if the backpeaks do not exceed 1.4% at this conversion. 
     It has now surprisingly been found that under specific conditions the dimerisation step can be carried out with a solvent mixture comprising 1-methoxy-2-propanol and water or methanol resulting in a product that meets the requirements in order to make the overall process of manufacturing iodixanol feasible. 
     Thus the invention provides a process for the dimerisation of 5-acetamido-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodo-isophthalamide in a temperature range of about 10 to about 20° C. using a solvent mixture comprising about 70 to about 90 vol % 1-methoxy-2-propanol and about 10 to about 30 vol % water or methanol in a concentration of about 0.8 to about 2.0 ml solvent per g 5-acetamido-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodo-isophthalamide in the presence of about 0.30 to about 0.40 mole equivalents of epichlorohydrin. 
     The process according to the present invention is carried out with a concentration of about 0.8 to about 2.0 ml solvent per g Compound A, preferably about 0.8 to about 1.0 and even more preferably about 0.85 to about 1.0. 
     The temperature during the dimerisation should be in the range of about 10 to about 20° C., with about 10° C. being most preferred. The temperature can be constant throughout the dimerisation or varied within the specified range, preferably the temperature is lowered throughout the dimerisation. 
     The dimerisation agent used in the present invention is epichlorohydrin which is added in about 0.3 to about 0.4 mole equivalents, with about 0.3 being most preferred. 
     The pH value in the reaction solution can preferably be about 11.0 to about 12.5, more preferably about 11.5 to about 12 and most preferably about 11.7 to about 11.9. The pH value can preferably be varied throughout the dimerisation having a higher value at the start of the dimerisation than at the end. 
     The base used to raise the pH value of the reaction solution can be any base suitable. Preferably the base is sodium hydroxide (NaOH) or potassium hydroxide (KOH), with sodium hydroxide being most preferred. 
     For further adjustment of the pH value of the reaction solution any suitable acid can be used, preferably concentrated hydrochloric acid (HCl). 
     The dimerisation step will be allowed to proceed for several hours with a preferred reaction time of 12 to 48 hours and particularly preferred from 24 to 48 hours. The reaction may be terminated by quenching with any acid, preferably hydrochloric acid. The reaction may be monitored, e.g. by HPLC, to determine the appropriate stage at which quenching should take place. 
     The invention is illustrated further by the following examples that are not to be construed as limiting the invention in scope to the specific procedures or products described in them. 
     EXAMPLES 
     Example 1 
     5-acetylamino-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodoisophthalamide (Compound A) (205.8 g, 0.268 mole) was dissolved in a solution of 1-methoxy-2-propanol (140 mL, 0.70 mL/g Compound A), water (60 mL, 0.30 mL/g Compound A) and sodium hydroxide (13.92 g, 0.348 mole, 1.30 eq) at 45° C. The mixture was cooled to 10° C. and conc. hydrochloric acid (18 g, 0.173 mole, 0.65 eq) was added followed by epichlorohydrin (7.43 g, 0.080 mole, 0.30 eq) added in one portion. After 40 hours an HPLC analysis showed the following composition: 52.8% iodixanol, 1.10% backpeaks and 2.9% iohexol. 
     Example 2 
     5-acetylamino-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodoisophthalamide (Compound A) (205.8 g, 0.268 mole) was dissolved in a solution of 1-methoxy-2-propanol (180 mL, 0.90 mL/g Compound A), water (20 mL, 0.10 mL/g Compound A) and sodium hydroxide (13.92 g, 0.348 mole, 1.30 eq) at 45° C. The mixture was cooled to 15° C. and conc. hydrochloric acid (18.46 g, 0.178 mole, 0.65 eq) was added followed by epichlorohydrin (7.93 g, 0.085 mole, 0.32 eq) added in one portion. After 24 hours an HPLC analysis showed the following composition: 50.7% iodixanol, 1.33% backpeaks and 3.1% iohexol 
     Example 3 
     5-acetylamino-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodoisophthalamide (Compound A) (205.8 g, 0.268 mole) was dissolved in a solution of 1-methoxy-2-propanol (140 mL, 0.70 mL/g Compound A), methanol (60 mL, 0.30 mL/g Compound A) and sodium hydroxide (13.92 g, 0.348 mole, 1.30 eq) at 45° C. The mixture was cooled to 10° C. and conc. hydrochloric acid (18.48 g, 0.178 mole, 0.67 eq) was added followed by epichlorohydrin (7.43 g, 0.080 mole, 0.30 eq) added in one portion. After 48 hours an HPLC analysis showed the following composition: 50.0% iodixanol, 1.08% backpeaks and 3.17% iohexol. 
     Example 4 
     5-acetylamino-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodoisophthalamide (Compound A) (200 kg, 0.268 kmole) was dissolved in a solution of 1-methoxy-2-propanol (112 L, 0.56 L/g Compound A), water (48 L, 0.24 mL/g Compound A) and sodium hydroxide (13.92 kg, 0.348 kmole, 1.30 eq) at 55° C. The mixture was cooled to 15° C. and conc. hydrochloric acid (18 kg, 0.173 kmole, 0.65 eq) was added followed by epichlorohydrin (8.425 kg, 0.091 kmole, 0.34 eq) added in one portion. 10.9M hydrochloric acid (0.05 L, 0.54 mole, 0.002 eq) was added after 18.5 hours. After 34.5 hours an HPLC analysis showed the following composition: 59.4% iodixanol, 2.03% backpeaks, 0.14% N-acetyl cyclized iodixanol and 3.81% iohexol. 
     All patents, journal articles, publications and other documents discussed and/or cited above are hereby incorporated by reference.