The present invention relates to a new process for the preparation and purification of a gadolinium complex and a chelating ligand derived from PCTA, which makes it possible to obtain, preferably, the stereoisomers of said complex which exhibit physical properties. particularly interesting chemicals 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 diastereomerically enriched complex as such, a composition comprising said complex, as well as a process for preparing the corresponding chelating ligand by decomplexing said complex, and the ligand as such.

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 together under the term GBCA (Gadolinium-based Contrast Agent). 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 bicyclopolyazamacrocyclocarboxylic acid (EP 0 438 206) or derivatives of PCTA (that is to say comprising at least the chemical structure of the 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 range up to 11- 12 mM Ls 1 in water) currently on the market,

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.

A first 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.

Other areas for improving the tolerance of lanthanide chelates (in particular gadolinium) are described in the prior art. Document US Pat. No. 5,876,695, dating back more than thirty years, reports, for example, formulations comprising, in addition to the chelate of

lanthanide, an additional complexing agent, intended to prevent an unwanted in-vivo release of the lanthanide, by complexing the lanthanide (metal ion Gd3 +) released. The additional chelating agent can be introduced into the formulation either in its free form or as a weak complex, typically of calcium, sodium, zinc or magnesium. If it can possibly be distinct from the ligand constitutive of the active complex, it is nevertheless important that the complex which it forms with the released lanthanide is less stable than the active complex, so as to prevent a transligation reaction between the active complex and the additional chelate, which would in particular have the effect of completely consume said additional ligand, which could therefore no longer trap the lanthanide released. This risk of consuming the additional chelating agent by transligation is more pronounced when it is added in free form than in the form of a calcium complex, for example.

Thus, in the two strategies described above, it is important that the active complex is as stable as possible.

However, the complexes of chelating ligands derived from PCTA comprising a pyclene-type structure described in document EP 1 931 673, while having good kinetic stability, generally have a lower thermodynamic constant than that of the complexes of other macrocycles. cyclene derivatives.

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 reaction for the formation of the complex of formula (II), also called the stability constant, is 10149 (ie log (Ktherm) = 14.9). For comparison, the stability constant of the gadolinium complex of 1, 4, 7, 10 tetraazacyclododecane N, N ', N ", N'" tetraacetic acid (DOTA-Gd), is 1025 6 (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, II-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.

(ll-SSS)

(ll-RRR)

Thus, iso4 stands out
e, surprisingly, by a thermodynamic stability markedly greater than that of the mixture of diastereoisomers in the form of which the complex of formula (II) is obtained by implementing the process described in document EP 1 931 673. In fact, its equilibrium thermodynamic constant Ktherm iso4 is equal to 10187 (ie log (Ktherm iso4) = 18.7) this value having been determined by implementing the method in Pierrard et al., Contrast Media Mol. Imaging, 2008, 3, 243-252 and Moreau et al., Dalton Trans., 2007, 1611-1620.

Furthermore, iso4 is the group of isomers which exhibits the best kinetic inertia (also called kinetic stability) among the four groups isolated by the inventors. Indeed, the inventors evaluated the kinetic inertia of the four groups of isomers by studying their decomplexation kinetics in acidic aqueous solution (pH = 1, 2), at 37 ° C. The values ​​of the half-life times (T1 / 2) which were determined for each of the groups of isomers are indicated in Table 1 below, the half-life time corresponding to the time after which 50% of the amount of complex initially present has been dissociated, according to the following decomplexation reaction (equation 2):

(equation 2)

Table 1: decomplexation kinetics of iso1 to iso4 isomer groups

By way of comparison, gadobutrol or gadoterate, macrocyclic gadolinium complexes, respectively exhibit a kinetic inertia of 18 hours and 4 days under the same conditions, while linear gadolinium complexes such as gadodiamide or gadopentetate dissociate instantaneously.

CLAIMS

1. Complex of formula (II) below:

consisting of at least 90% of a diastereomeric excess comprising a mixture of II-RRR and II-SSS isomers of the formulas:

2. Composition comprising the complex according to claim 1 and a free macrocyclic ligand, and advantageously having a free gadolinium concentration of less than 1 ppm (m / v).

3. Composition according to claim 2, characterized in that it comprises between 0.002 and 0.4% mol / mol of free macrocyclic ligand relative to the complex of formula (II).

4. Composition according to claim 2 or 3, characterized in that the free macrocyclic ligand is selected from the group consisting of DOTA, NOTA, D03A, BT-D03A, HP-D03A, PCTA, DOTA-GA and their derivatives.

5. Composition according to claim 4, characterized in that the free macrocyclic ligand is 1,4,7,10-tetraazacyclododecane-1, 4,7,10-tetraacetic acid (DOTA).

6. Process for the purification of the complex of formula (II) below:

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

comprising:

1) the combination of the following 2 steps:

l b) passage through ion exchange resin (s), and

lc) ultrafiltration of said complex, and

2) isolation of the purified complex thus obtained in solid form.

7. Method according to claim 6, characterized in that steps 1 b) and 1c) are further combined with a step 1a) of nanofiltration.

8. Method according to claim 6 or 7, characterized in that steps 1a) when the latter is present, 1b) and 1c) are carried out in this order.

9. A method according to any one of claims 6 to 8, characterized in that step 2) comprises atomization.

10. Method according to any one of claims 6 to 9, characterized in that the complex of formula (II) consisting of at least 80% of a diastereoisomeric excess comprising a mixture of isomers II-RRR and II-SSS on which the purification process is implemented has been prepared beforehand by the following successive steps:

a) complexation of the hexacid of the following formula (III):

with gadolinium to obtain the gadolinium hexaacid complex of the following formula (I):

b) isomerization by heating the hexaacid gadolinium 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 a diastereomeric excess comprising a mixture l-RRR and l-SSS isomers of said gadolinium hexaacid complex of formula (I), and

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

11. The method of claim 10, characterized in that:

- At the end of step b), the diastereoisomerically enriched complex is isolated by crystallization, purified by recrystallization, and further enriched by selective decomplexation of the diastereomers of the complex of formula (I) other than the diastereoisomers I- RRR and I- SSS, ie by selective decomplexing of the diastereoisomers l-RSS, l-SRR, l-RSR, l-SRS, l-RRS and l-SSR, and

- step c) comprises the following successive steps:

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

in which R 1 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 RiOH in the presence of an acid such as hydrochloric acid,

the triester of formula (VIII) not being isolated between steps c1) and c2).

12. 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:

obtainable by the method according to any one of claims 6 to 1 1.

13. Complex according to claim 12, characterized in that its degree of purity is greater than 95%.

14. A composition comprising the complex according to claim 12 or 13 and free DOTA, and advantageously having a free gadolinium concentration of less than 1 ppm (m / v).

15. Composition according to claim 14, characterized in that it comprises between 0.002 and 0.4% mol / mol of DOTA relative to the complex of formula (II).