The present invention relates to a process for the preparation of iodinated X-ray contrast agents and in particular to key intermediates thereof. More particularly, the invention relates to preparation of a compound mixture comprising 1 -formylamino- 3,5-bis(2,3-bis(formyloxy)propan-1 -ylcarbamoyl)-2,4,6-trioodobenzene, a key intermediate in the process for preparing loforminol. Further, the invention relates to a process for preparing loforminol, a contrast agent useful in X-ray imaging.

For the last 50 years the field of X-ray contrast agents has been dominated by soluble iodine containing compounds. Commercial available contrast media containing iodinated contrast agents are usually classified as ionic monomers such as diatrizoate (Gastrografen™), ionic dimers such as ioxaglate (Hexabrix™), nonionic monomers such as iohexol (Omnipaque™), iopamidol (Isovue™), iomeprol (lomeron™) and the non-ionic dimer iodixanol (Visipaque™). The most widely used commercial non-ionic X-ray contrast agents such as those mentioned above are considered safe. Contrast media containing iodinated contrast agents are used in more than 20 million of X-ray examinations annually in the USA and the number of adverse reactions is considered acceptable. However, since a contrast enhanced X- ray examination will require up to about 200 ml contrast media administered in a total dose, there is a continuous drive to provide improved contrast media.

The part of the patient population considered as high risk patients is increasing. To meet the need for continuous improvement of in vivo X-ray diagnostic agents for the entire patient population, there is a continuous drive in finding X-ray contrast agents that have improved properties, also with regards to contrast induced nephrotoxicity (CIN).

X-ray contrast media containing a chemical compound as the active pharmaceutical ingredient(s) having two triiodinated phenyl groups linked by a linking group are usually referred to as dimeric contrast agents or dimers. During the years a wide variety of iodinated dimers have been proposed. Currently, one contrast medium having an iodinated non-ionic dimer as the active pharmaceutical ingredient is on the market _^ the product Visipaque™ containing the compound iodixanol. In WO2009/008734 of the applicant a novel dimeric contrast agent named loforminol is disclosed. The properties of this is described in more detail in the publications Chai et al. "Predicting cardiotoxicity propensity of the novel iodinated contrast medium GE-145: ventricular fibrillation during left coronary arteriography in pigs", Acta Radiol, 2010, and in Wistrand, L.G., et al "GE-145, a new low-osmolar dimeric radiographic contrast medium", Acta Radiol, 2010. loforminol (GE-145) is named Compound 1 herein and has the following structure:

Compound 1 :

5,5'-(2-Hydroxypropane-1 ,3-diyl)bis(formylazanediyl)bis(N ¹ ,N ³ -bis(2,3- dihydroxypropyl)-2,4,6-triiodoisophthalamide)

The manufacture of non-ionic X-ray contrast media involves the production of the chemical drug, the active pharmaceutical ingredient (API), i.e. the contrast agent, followed by the formulation into the drug product, herein denoted the X-ray composition. WO2009/008734 of the applicant provides a synthetic route for preparing the API loforminol. loforminol can e.g., as provided by the general preparation description and Example 1 of WO2009/008734, be synthesized from 5- amino-N,N'-bis-(2,3-dihydroxy-propyl)-2,4,6-triiodo-isophtha lamide (compound (4)), which is commercially available. The preparation of this compound is known from the synthesis of both iohexol and iodixanol and can also be prepared from 5- nitroisophthalic acid for instance as described in WO2006/016815, including hydrogenation and subsequent iodination e.g. by iodine chloride, I CI. Alternatively, 5-amino-2,4,6-triiodoisophthalic acid may be used, which is commercially available precursor, e.g. from Sigma-Aldrich. The free amino group of the isophthalamide compound (compound (4)) is then acylated and the hydroxyl groups in the substituents may also be protected by acylation. The protecting groups may be removed for example by hydrolysis to give N ¹ ,N ³ -bis(2,3-dihydroxypropyl)-5- formylamino-2,4,6-triiodoisophthalamide. In a dimerization step this is reacted e.g. with epichlorohydrin to provide the loforminol contrast agent compound. The state of the art synthesis of loforminol, as disclosed in examples 1 and 2 of WO2009/008734, is shown in Scheme 1 below.

Scheme 1 .

As described in WO2009/008734 compound 3 is a mixture comprising 1 - formylamino-3,5-bis(2,3-bis(formyloxy)propan-1 -ylcarbamoyl)-2,4,6-trioodobenzene, and X is then a formyl group. In each synthetic step it is important to optimize the yield and minimize the production of impurities. The problem to be solved by the present invention may be regarded as the provision of optimizing the process for preparation of compound mixture (3) of scheme 1 , i.e. a mixture comprising 1 -formylamino-3,5-bis(2,3- bis(formyloxy)propan-1 -ylcarbamoyl)-2,4,6-trioodobenzene. The process is hence directed to the preparation of compound mixture (3) by the formylation of the amino function of 5-amino-N ¹ ,N ³ -bis(2,3-dihydroxypropyl)-2,4,6-triiodoisophthalamide (4), including a work-up procedure.

In the state of the art process, as disclosed in WO2009/008734, Example 2, procedure B, 1 -formylamino-3,5-bis(2,3-bis(formyloxy)propan-1 -ylcarbamoyl)-2,4,6- trioodobenzene (compound 3 wherein X is a formyl-group) is prepared by formylation of compound (4) using formic acid and acetic anhydride. The reaction mixture was evaporated in vacuum to a moist mass which was further dried in vacuum. The product was used in the next step without purification. The work up process of the state of the art hence consisted of a concentration of the reaction mixture to dryness under reduce pressure, and the product was recovered as a plaster like material with a high content of formic acid and acetic acid. A challenge with this work-up process has been the removal of formic acid and acetic acid from the reaction suspension which is difficult due to their high boiling points. The hard lumpy material obtained by the state of the art process was difficult to purify and also needed milling before the next synthetic step.

An improved work-up process has been sought for preparation of the acylated compound mixture (3) from compound (4), wherein the product is obtained in powder form in a high yield and of high purity. We have now found that compound mixture (3) can be prepared in powder form in high purity and high yield by performing a work-up procedure wherein a short chain alcohol is used as an antisolvent. It has been found that by using such alcohol as an antisolvent compound mixture (3) can be crystallised out of the solution in high yields allowing for a filtration work-up.

Accordingly, in a first aspect the invention provides a process for preparation of compound mixture (3) as a powder

Compound mixture 3

wherein each X individually denotes hydrogen, a formyl group (-CO-H) or an acetyl group (-CO-CH ₃ )

the process comprises the steps of:

i) formylating the amino function of 5-amino-N ¹ ,N ³ -bis(2,3- dihydroxypropyl)-2,4,6-triiodoisophthalamide (compound (4)); ii) performing a work-up procedure wherein a short chain alcohol is added to the product solution of step i).

In the synthetic step i) the free amino function of the iodinated phenyl group of compound (4) is formylated to prepare the corresponding formanilide group. The formylation may be effected by any convenient method, e.g. by use of activated formic acid such as mixed anhydrides as the formylating agent, which can be prepared by a variety of methods described in the literature. A convenient method of preparing mixed anhydrides is to add a carboxylic acid anhydride to an excess of formic acid under controlled temperature. Preferably, a mixture of formic acid and acetic anhydride is used in this step. Preferably, formic acid is cooled, e.g. to 8- 15 °C, such as to about 10 °C, and acetic acid anhydride is added slowly, such as drop wise, keeping the temperature down, such as below about 15 °C. As the formation of mixed anhydride is an exotermic reaction, and high temperatures decompose the anhydrides, the reagents should be cooled prior to mixing. This mixed anhydride solution may then be added to the compound (4), which is preferably solved in formic acid. Alternatively, compound (4) is added to the mixed anhydride solution. It is also possible to make mixed anhydrides by addition of a carboxylic acid chloride to a solution of a formic acid salt. Formyl-mixed anhydrides may include acetyl, isobutyryl, pivaloyl, benzoyl etc. As a result of the formylating step (i) using mixed anhydrides, compound mixture (3) will be a mixture of different compounds with both formyl and acetyl protecting groups. Varies degree of O-formylation is seen, but a high degree of N-formylation takes place, ensuring a high yield of the N-formylated compound mixture (3). In one embodiment compound mixture (3) comprises a mixture of compounds wherein all X groups are individually formyl or acetyl. The main component of compound mixture (3) is 1 -formylamino-3,5-bis(2,3-bis(formyloxy)propan-1 -ylcarbamoyl)-2,4,6- trioodobenzene.

When using the work-up procedure of the invention, compound mixture (3) can be recovered by filtration as a fine powder with low acid content. The short chain alcohol has several functions in the procedure; it is initially used to quench excess anhydride in the reaction solution, in addition to that it acts as an antisolvent that lowers the solubility and crystallizes compound mixture (3). The crystallisation gives small crystals that aggregate into robust clusters that give an excellent rapid filtration, and an easy wash of the crystals. The work-up procedure has been repeated multiple times and provides yields of the N-formylated compound mixture (3) above 95 %, more preferably above 98 %, and most preferably 99 % or above, in both small scale and large scale, such as in 100 kg scale. The main impurity is the N-acylated compound in approx. 0.5% abundance.

The short chain alcohol used in step (ii) is selected from C1 -C6 straight or branched alcohols and may be a mixture of such. The alcohol may be monohydroxylated or dihydroxylated. Methanol, ethanol and propanols are preferred alcohols, with propanols, particularly iso-propanol, being most preferred.

After step i) has been performed, i.e. when all the formylating agent has been added to the compound (4) solution, this reaction solution is preferably kept at a low temperature, such as 10-40 °C, e.g. about 15 °C, under stirring for a period to allow the reaction to completion, such as for at least an hour. In one embodiment, this reaction solution is seeded with compound mixture (3) and left under stirring for an additional period, such as for one hour or more. In step ii) the alcohol is then added carefully to the reaction solution to quench any excess anhydride and to lower the solubility of compound mixture (3), to provide a thicker suspension. This suspension is preferably left under stirring at ambient temperature. It has been found useful adding the alcohol as an antisolvent in a concentration of about 0.3 to about 2.0 ml per gram of compound (4). More preferably the concentration is about 0.5-1 .0 ml per gram of compound (4), and most preferably 0.6-0.7 ml per gram of compound (4).

To provide a complete precipitation the reaction slurry of step (ii) is left under stirring for a period of e.g. 5-25 hours, such as 10-20 hours before compound mixture (3) is collected and optionally purified. In one embodiment the process comprises such further step of collecting the product preferably by filtration, e.g. by using a nutch filter, such as a vacuum nutch filter or pressure nutch, or a combination thereof, also optionally combined with heating. The product is preferably subsequently washed with a short chain alcohol, preferably the same alcohol as used in step (ii), in one or more portions, such as in 1 to 5 portions and preferably with 3 portions providing compound mixture (3) as a dense white powder, optionally being vacuum-dried. The amount of alcohol used in the washing may be about the same amount as used in the precipitation, divided on the number of portions. It has been found that it is beneficially for the further synthesis that some alcohol is left in the compound mixture (3) after the work-up procedure has finished, hence the drying should be adjusted accordingly. Preparation of compound mixture (3) with a rest alcohol content of 0-7 % has been found appropriate.

The compound mixture (3) as provided by the process of the invention is a dense white powder, in contrast to the hard lumpy material that was obtained by the state of the art process. In a further aspect the invention provides compound mixture (3) as a white powder, preferably prepared according to the process of the first aspect.

Processes which take the compound mixture (3) as prepared by the claimed process and react this further, e.g. to produce loforminol, are deemed to fall within the scope of the invention. Hence, in a further aspect the invention provides a process for preparing loforminol comprising the work-up procedure for preparation of compound mixture (3) as described in the first aspect. Such process could comprise the additional steps of removal of the protecting groups of compound mixture (3) and dimerization to provide loforminol. The protecting acyl groups (formyl and acetyl) of compound mixture (3) may conveniently be removed by standard methods, for example by hydrolysis, such as in aqueous basic media, or by alcoholysis.

In a final step for preparation of loforminol a bis alkylation via a 2-hydroxypropane bridge takes place. This step may be carried out as described in European patent 108638 and WO 98/23296, for example using epichlorohydrin, 1 ,3-dichloro-2- hydroxypropane or 1 ,3-dibromo-2-hydroxypropane as the dimerization agent. This dimerization is preferably effected in the presence of an acid binding agent, for example an organic or inorganic base; an alkali metal alkoxide such as sodium methoxide or an alkali metal hydroxide such as sodium and potassium hydroxide may be used as base.

Compound mixture (3) and compound (1 ) as prepared by the claimed process comprise optical active isomers and will exist in several isomeric forms due to chiral carbon atoms. In addition, the compounds exhibit exo/endo isomerism due to the restricted rotation of the N-CO bond in the formyl function caused by the proximity of the bulk iodine atom. Both preparation of enantiomerically pure products as well as mixtures of optical isomers are encompassed by the process of the invention.

The compounds prepared, such as compounds (1 ), (2) and (3), may be purified in any convenient manner, e.g. by preparative chromatography, by recrystallization or ultra/nano-filtration.

The compounds prepared according to the invention may be used as contrast agents and may be formulated with conventional carriers and excipients to produce diagnostic contrast media. Thus viewed from a further aspect the invention provides a diagnostic composition comprising loforminol prepared according to the process of the invention, together with at least one physiologically tolerable carrier or excipient, e.g. in aqueous solution for injection optionally together with added plasma ions or dissolved oxygen. The contrast agent composition of the invention may be in a ready to use concentration or may be a concentrate form for dilution prior to administration. Hence, the invention further embraces use of loforminol prepared according to the process of preparation, and a diagnostic composition containing such, in X-ray contrast examinations. The invention is illustrated with reference to the following non-limiting examples.

Examples

Example 1 : Preparation of compound mixture (3) comprising 1-formylamino- 3,5-bis(2,3-bis(formyloxy)propan-1-ylcarbamoyl)-2,4,6-triood obenzene

5-amino-N1 ,N3-bis(2,3-dihydroxypropyl)-2,4,6-triiodoisophthalamide (compound (4)) (7.5 kg, 10.6 moles) was dissolved in formic acid (4.9 I) and heated to 45 until a clear solution was obtained (~4 hours), then the thick amber solution was cooled to 10 °C.

Formic acid (9.4 I) was charged into a different reactor and cooled to 10 ^< €, after reaching the target temperature acetic anhydride was added at such a rate that the temperature did not exceeded 15 ^< €.

After 2.5 hours all acetic anhydride was added to the formic acid and the mixed anhydride solution was added drop wise to the compound (4) solution. The rate of addition was adjusted so that the temperature never exceeded 20 °C. After 2 hours all mixed anhydride had been added and the reaction was left stirring at 15 °C for additional 1 hour. Isopropanol (4.9 I) was added carefully and the suspension became noticeable thicker and was left stirring at ambient temperature. After 16 hours the reaction slurry was filtered on a vacuum nutch and washed with isopropanol (3 ^* 1 .5 I) to give compound mixture (3) comprising 1 -formylamino-3,5- bis(2,3-bis(formyloxy)propan-1 -ylcarbamoyl)-2,4,6-trioodobenzene as a dense white powder (7.98kg). The quantitative yield with regards to N-formylation was > 99 %.