The present invention relates to a new process for the synthesis of a gadolinium complex and of a chelating ligand derived from PCTA, which makes it possible to obtain preferentially the stereoisomers of said complex which exhibit physicochemical properties which are very particularly advantageous for applications as a contrast agent in the field of medical imaging, in particular for Magnetic Resonance Imaging. The present invention also relates to the diastereoisomerically enriched complex as such, as well as two synthetic intermediates, which may or may not contain gadolinium.

Many contrast agents are known based on lanthanide (paramagnetic metal) chelates, in particular gadolinium (Gd), described for example in US Pat. No. 4,647,447. These products are often grouped under the term GBCA

(Gadolinium-based Contrast Agent, gadolinium-based contrast agents). Several products are marketed, among which are macrocyclic chelates, such as meglumine gadoterate based on DOTA (1, 4,7,10-tetraazacyclododecane-N, N ', N ", N"' - tetraacetic acid), gadobutrol based on D03A-butrol, gadoteridol based on HPD03A, as well as linear chelates, in particular based on DTPA (diethylenetriaminepentaacetic acid) or DTPA-BMA (gadodiamide ligand).

Other products, some of which are in development, represent a new generation of GBCA. They are essentially complexes of macrocyclic chelates, such as the complexes of bicyclopolyazamacrocyclocarboxylic acid (EP 0 438 206) or derivatives of PCTA (that is to say comprising at least the chemical structure of acid 3 , 6,9,15-tetraazabicyclo [9,3,1] pentadéca-1 (15), 11,13-triene-3,6,9-triacetic), as described in document EP 1 931 673.

The complexes of chelating ligands derived from PCTA described in document EP 1 931 673 have in particular the advantage of being relatively easy to chemically synthesize, and, in addition, of having a relaxivity greater than other GBCA (relaxivity which can go up to 11 -12 rnM Ls 1 in water) currently on the market, this relaxivity corresponding to the effectiveness of these products and therefore to their contrasting power.

In the body, the chelates (or complexes) of lanthanide - and in particular of gadolinium - are in a situation of chemical equilibrium (characterized by its constant

SUBSTITUTE SHEET (RULE 26)

thermodynamics Ktherm), which can lead to an unwanted release of said lanthanide (see equation 1 below):

(equation 1)

Chemical equilibrium of complexation between the chelate or ligand (Ch) and the lanthanide (Ln) to lead to the Ch-Ln complex.

Since 2006, a condition called NSF (Nephrogenic Systemic Fibrosis) has been linked, at least in part, to the release of free gadolinium in the body. This disease has led to an alert from the health authorities vis-à-vis gadolinized contrast agents marketed for certain categories of patients.

Strategies have therefore been put in place to resolve the complex problem of patient tolerance in a completely safe manner, and to limit, or even eliminate, the risk of unwanted release of lanthanide after administration. This problem is all the more difficult to solve as the administration of contrast agents is often repeated, whether during diagnostic examinations, or for adjusting the doses and monitoring the effectiveness of a drug. therapeutic treatment.

In addition, since 2014, a possible brain deposit of gadolinium has been mentioned after repeated administrations of gadolinium products, more particularly of linear gadolinium chelates, such a deposit having not, or little, been reported with macrocyclic gadolinium chelates, such as Dotarem. ® Consequently, various countries have decided either to withdraw most of the linear chelates from the market, or to drastically limit their indications for use, given their stability considered insufficient.

One strategy for limiting the risk of lanthanide release in the body thus consists in opting for complexes which are distinguished by the highest possible thermodynamic and / or kinetic stabilities. In fact, the more stable the complex, the more the quantity of lanthanide released over time will be limited.

However, the complexes of chelating ligands derived from PCTA comprising a structure of pyclene type described in document EP 1 931 673, while having good stability

kinetics, have, in general, a thermodynamic constant lower than that of the complexes of other macrocycles derived from cyclene.

This is particularly the case with the complex of formula (II) shown below:

Indeed, as described in particular in document WO 2014/174120, the thermodynamic equilibrium constant corresponding to the formation reaction of the complex of formula (II), also called the stability constant, is 10149 (i.e. log (Ktherm) = 14.9). For comparison, the stability constant of the gadolinium complex of 1, 4, 7, 10 tetra-azacyclododecane N, N ', N ", N'" tetraacetic acid (DOTA-Gd), is 10256 (ie log (Ktherm) = 25.6).

It should however be noted that the complex of formula (II) corresponds to several stereoisomers, in particular due to the presence of the three asymmetric carbon atoms located in position a on the side chains of the complex, relative to the nitrogen atoms of the macrocycle onto which said side chains are grafted. These three asymmetric carbons are marked with an asterisk (*) in formula (II) shown above.

Thus, the synthesis of the complex of formula (II) as described in document EP 1 931 673 results in the production of a mixture of stereoisomers.

The aminopropanediol groups of the side chains of the complex of formula (II) also contain an asymmetric carbon. Thus, the complex of formula (II) comprises a total of 6 asymmetric carbons, and therefore exists in the form of 64 stereoisomers of configuration. However, in the remainder of the description, the only source of stereoisomerism considered for a given side chain will be, for the sake of simplicity, that corresponding to the asymmetric carbon bearing the carboxylate group, marked with an asterisk (*) in formula (II ) shown above.

Insofar as each of these 3 asymmetric carbons can be of absolute configuration R or S, the complex of formula (II) exists in the form of 8 families of stereoisomers, hereinafter referred to as II-RRR, II-SSS, II- RRS, ll-SSR, ll-RSS, ll-SRR, ll-RSR and ll-SRS. More precisely, according to the usual stereochemical nomenclature, the complex of formula (II) exists in the form of 8 families of diastereoisomers.

The use of the term "family" is justified in that each of these families groups together several stereoisomers, in particular due to the presence of an asymmetric carbon within the aminopropanediol group, as mentioned above.

Nevertheless, insofar as, in the remainder of the description, the stereoisomerism linked to the asymmetric carbon of a given aminopropanediol group will not be considered, we will speak indifferently of isomers, stereoisomers or even diastereoisomers II-RRR, II-SSS, II-RRS, II-SSR, II-RSS, II-SRR, II-RSR and II-SRS, without specifying that each corresponds to a family of stereoisomers.

The inventors succeeded in separating and identifying by high performance liquid chromatography (HPLC, more commonly referred to by the acronym HPLC) and by ultra high performance liquid chromatography (HPLC, more commonly referred to by the acronym. English UHPLC) 4 solid masses or groups of isomers of the complex of formula (II) obtained according to the process of the prior art, corresponding to 4 different elution peaks characterized by their retention time on the chromatogram, which will be called iso1, iso2, iso3 and iso4 in the remainder of the description. By implementing the process described in document EP 1 931 673, the respective contents of the iso1, iso2, iso3 and iso4 groups in the mixture obtained are as follows: 20%, 20%, 40% and 20%.

They then discovered that these different groups of isomers exhibited distinct physicochemical properties, and determined that the group of isomers called iso4, which comprises a mixture of the ll-RRR and ll-SSS isomers of formulas (ll-RRR ) and (II-SSS) shown below, proves to be the most interesting as a contrast agent for medical imaging.

CLAIMS

1. Gadolinium hexaacid complex of formula (I):

consisting of at least 80% of a diastereomeric excess comprising a mixture of l-RRR and l-SSS isomers of the formulas:

2. Complex according to claim 1, characterized in that it consists of at least 90% of said diastereoisomeric excess.

3. Complex according to claim 1 or 2, characterized in that said diastereoisomeric excess corresponds to peak 4 of the HPLC trace, characterized by a retention time of approximately 35.7 minutes, said trace being obtained by implementing the method HPLC following:

• Symmetry® RP18 column - 250 x 4.6 mm - 5 pm from Waters,

• analysis conditions:

- sample: aqueous solution of the complex of formula (I) at 10 mg / ml_,

- column temperature: 25 ° C,

- sample temperature: ambient temperature (20-25 ° C),

- flow rate: 1.0 mL / min,

- injection volume: 20 pl_

- UV detection: 200 nm,

• mobile phase gradient (volume):

- at 0 min, 1% acetonitrile and 99% of an aqueous solution of hkSC at 0.1% v / v, - at 10 min, 5% of acetonitrile and 95% of an aqueous solution of hkSC at 0.1% v / v,

- at 40 min, 10% acetonitrile and 90% of a 0.1% v / v aqueous solution of hkSC.

4. Complex of formula (II):

consisting of at least 80% of a diastereomeric excess comprising a mixture of II-RRR and II-SSS isomers of the formulas:

5. Complex according to claim 4, characterized in that said diastereoisomeric excess corresponds to peak 4 of the UHPLC trace, characterized by a retention time of approximately 6.3 minutes, said trace being obtained by implementing the following UHPLC method :

• CORTECS® UPLC T3 150 x 2.1 mm - 1.6 pm column from Waters, · analysis conditions:

- sample: aqueous solution of the complex of formula (II) at 2.0 mg / mL,

- column temperature: 40 ° C,

- sample temperature: ambient temperature (20-25 ° C),

- flow rate: 0.3 mL / min,

- injection volume: 1 pL

- UV detection: 200 nm,

• mobile phase gradient (volume):

- at 0 min, 1% acetonitrile and 99% of an aqueous solution of hkSC at 0.0005% v / v,

- at 3 min, 5% acetonitrile and 95% of an aqueous solution of hkSC at 0.0005% v / v,

- at 12 min, 10% acetonitrile and 90% of a 0.0005% v / v aqueous solution of hkSC.

6. Complex according to claim 4 or 5, obtained by amidification from the gadolinium hexaacid complex of formula (I) according to any one of claims 1 to 3 and 3-amino-1, 2-propanediol.

7. Process for preparing the complex of formula (II) below:

comprising the following successive steps:

a) Complexation of the following hexaacid of formula (III):

with gadolinium to obtain the gadolinium hexaacid complex of formula (I) as defined in claim 1,

b) Isomerization by heating the gadolinium hexaacid complex of formula (I) in an aqueous solution at pH between 2 and 4, to obtain a diastereomerically enriched complex consisting of at least 80% of an excess

diastereomeric comprising a mixture of the 1-RRR and 1-SSS isomers of said gadolinium hexaacid complex of formula (I), and

c) Formation, from the diastereoisomerically enriched complex obtained in step b), of the complex of formula (II), by reaction with 3-amino-1, 2-propanediol.

8. Method according to claim 7, characterized in that the aqueous solution of step b) comprises acetic acid.

9. The method of claim 7 or 8, characterized in that, at the end of step b), the diastereomerically enriched complex is isolated by crystallization.

10. The method of claim 9, characterized in that the diastereomerically enriched complex of step b) isolated by crystallization is purified by recrystallization, to obtain a diastereomerically enriched and purified complex.

1 1. The method of claim 9 or 10, characterized in that the diastereoisomerically enriched complex of step b) is further enriched by selective decomplexation of the diastereomers of the complex of formula (I) other than the l-RRR and l- diastereomers. SSS, ie by selective decomplexing of the diastereoisomers l-RSS, l-SRR, l-RSR, l-SRS, l-RRS and l-SSR.

12. Method according to any one of claims 7-1 1, characterized in that step c) comprises the following successive steps:

c1) formation of a triester of formula (VIII),

in which Ri represents a (Ci-Ce) alkyl group, in particular by reaction in alcohol of formula RiOH in the presence of an acid such as hydrochloric acid, and

c2) aminolysis of the triester of formula (VIII) with 3-amino-1, 2-propanediol, in particular in alcohol of formula R1OH in the presence of an acid such as hydrochloric acid.

13. The method of claim 12, characterized in that the triester of formula (VIII) is not isolated between steps c1) and c2).

14. The method of claim 12 or 13, characterized in that R 1 represents a methyl group.

15. Process is lon any one of claims 7 to 14, characterized in that the hexacid of formula (III) as defined in claim 7 is obtained by alkylation of pyclene of formula

with dibutyl 2-bromoglutarate, to obtain the butyl hexaester of formula (VI)

followed by a hydrolysis step, leading to said hexacid of formula (III).

16. Complex of formula (II):

consisting of at least 80% of a diastereomeric excess comprising a mixture of l l-RRR and l-SSS isomers of the formulas:

obtainable by the process according to any one of claims 7 to 15.

17. Butyl hexaester of formula (VI) below: