Diastereomerically enriched complexes of gadolinium and PCTA-based chelating ligands and methods for their synthesis

Abstract

The present invention relates to a complex of formula (II) consisting of a diastereomeric excess of at least 80%, said diastereomeric excess comprising a mixture of isomers II-RRR and II-SSS having the following formula: The present invention also relates to a process for preparing said complex of formula (II) and also to two synthetic intermediates.

Description

[0001] Related Applications

[0002] The present invention is a divisional application of a patent application with an application date of January 17, 2020, an application number of 202080009659.7, and an invention name of “Diastereoisomer-enriched complexes of gadolinium and PCTA-based chelating ligands and synthesis methods”. Technical Field

[0003] The present invention relates to a novel process for the synthesis of complexes of gadolinium and chelating ligands based on PCTA, which process makes it possible to preferentially obtain stereoisomers of said complexes which have particularly advantageous physicochemical properties for use as contrast agents in the field of medical imaging, in particular in the field of magnetic resonance imaging. The invention also relates to the diastereomerically enriched complex itself and also to two synthetic intermediates which optionally contain gadolinium. Background Art

[0004] Numerous contrast agents based on chelates of lanthanides (paramagnetic metals), in particular gadolinium (Gd), are known, as described, for example, in US Pat. No. 4,647,447. These products are generally grouped together under the term GBCA (gadolinium-based contrast agents). Several products are on the market, among which are macrocyclic chelates, such as meglumine gadolinium salt based on DOTA (1,4,7,10-tetraazacyclododecane-N,N',N",N'"-tetraacetic acid), gadobutrol based on DO3A-butrol, gadoteridol based on HPDO3A, and also linear chelates, in particular based on DTPA (diethylenetriaminepentaacetic acid) or DTPA-BMA (gadolinium diamide ligand).

[0005] Other products, some of which are under development, represent a new generation of GBCAs. They are essentially complexes of macrocyclic chelates, such as bicyclic polyazamacrocyclic carboxylic acid complexes (EP 0 438 206) or PCTA derivatives (i.e. derivatives containing at least the chemical structure of 3,6,9,15-tetraazabicyclo[9,3,1]pentadeca-1(15),11,13-triene-3,6,9-triacetic acid), as described in EP 1 931 673.

[0006] The PCTA-based chelating ligand complexes described in EP 1 931 673 have the advantages, inter alia, of being relatively easy to chemically synthesize and also having a relaxivity superior to that of other GBCAs currently on the market (relaxivity r1 in water up to 11-12 mM -1 .s -1 ), this relaxation rate corresponds to the efficiency of these products and therefore to their contrast power.

[0007] In living organisms, chelates (or complexes) of lanthanides (and especially gadolinium) are in a state of chemical equilibrium (measured by their thermodynamic constants, K therm Characterization), which may lead to the undesired release of the lanthanide (see reaction 1 below):

[0008]

[0009] The complexation chemical equilibrium between the chelate or ligand (Ch) and the lanthanide (Ln) produces the complex Ch-Ln.

[0010] Since 2006, a disease called NSF (nephrogenic systemic fibrosis or fibrogenic dermopathy), which is at least partly linked to the release of free gadolinium in the body, has led health authorities to issue alerts against the marketing of gadolinium-based contrast agents to certain categories of patients.

[0011] Therefore, several strategies have been adopted to solve the complex problem of patient tolerance in a completely safe manner and to limit or even eliminate the risk of undesired release of lanthanides after administration. This problem is made more difficult by the fact that contrast agents are often repeatedly administered, both during diagnostic examinations and during dose adjustments and monitoring of therapeutic effects.

[0012] In addition, since 2014, it has been mentioned that gadolinium may be deposited in the brain after repeated administration of gadolinium-based products, especially linear gadolinium chelates, especially gadolinium macrocyclic chelates (e.g. ) have been reported little or not at all. Therefore, due to the lack of stability, countries have decided to withdraw most linear chelators from the market or to significantly restrict their indications for use.

[0013] Therefore, a strategy to limit the risk of lanthanide release into the body is to choose complexes characterized by the highest possible thermodynamic and / or kinetic stability. The reason is that the more stable the complex, the more limited the release of lanthanide will be over time.

[0014] However, complexes containing PCTA-based chelating ligands of the pyclene-type structure described in EP 1 931 673, although having good kinetic stability, generally have thermodynamic constants lower than those of other cycloolefin-based macrocycle complexes.

[0015] This is particularly the case for the complexes of formula (II) represented by:

[0016]

[0017] Specifically, as described in particular in WO 2014 / 174120, the thermodynamic equilibrium constant (also called stability constant) corresponding to the reaction for forming the complex having formula (II) is 10 14.9 (i.e. log(K therm )=14.9). For comparison purposes, the stability constant of the gadolinium complex of 1,4,7,10-tetraazacyclododecane-N,N',N",N'-tetraacetic acid (DOTA-Gd) is 10 25.6 (i.e. log(K therm )=25.6).

[0018] However, it should be noted that the complex of formula (II) corresponds to several stereoisomers, in particular due to the presence of three asymmetric carbon atoms in the side chain of the complex, located in the alpha position relative to the nitrogen atom of the macrocycle to which the side chain is grafted. These three asymmetric carbons are marked with an asterisk (*) in the formula (II) represented above.

[0019] Thus, the synthesis of the complex of formula (II) as described in EP 1 931 673 results in the production of a mixture of stereoisomers.

[0020] The aminopropanediol group of the side chain of the complex of formula (II) also includes an asymmetric carbon. Therefore, the complex of formula (II) contains six asymmetric carbons in total, and therefore exists in the form of 64 configurational stereoisomers. However, in the remainder of this specification, for simplicity, the only source of stereoisomers considered for a given side chain will be the source corresponding to the asymmetric carbon with a carboxylate group, which is marked with an asterisk (*) in the above formula (II).

[0021] Since each of the three asymmetric carbons can have an absolute configuration of R or S, the complexes of formula (II) exist in the form of eight families of stereoisomers, hereinafter referred to as II-RRR, II-SSS, II-RRS, II-SSR, II-RSS, II-SRR, II-RSR and II-SRS. More precisely, according to the usual nomenclature in stereochemistry, the complexes of formula (II) exist in the form of eight families of diastereomers.

[0022] As mentioned before, the use of the term "family" is justified since each of these families groups together several stereoisomers, in particular due to the presence of an asymmetric carbon in the aminopropanediol group.

[0023] However, since stereoisomers associated with an asymmetric carbon of a given aminopropanediol group will not be considered in the remainder of the present specification, the terms isomer, stereoisomer or diastereomer II-RRR, II-SSS, II-RRS, II-SSR, II-RSS, II-SRR, II-RSR and II-SRS will be used indiscriminately without stating that each corresponds to a family of stereoisomers.

[0024] The inventors have succeeded in separating and identifying by high performance liquid chromatography (HPLC) and ultra high performance liquid chromatography (UHPLC) four unresolved peaks or isomeric groups of the complex of formula (II) obtained according to the methods of the prior art, corresponding to four different elution peaks characterized by their retention times on the chromatogram, which will be referred to as iso1, iso2, iso3 and iso4 in the remainder of this specification. By carrying out the method described in EP 1 931 673, the respective contents of the iso1, iso2, iso3 and iso4 groups in the obtained mixture are as follows: 20%, 20%, 40% and 20%.

[0025] They then found that these different isomeric groups had different physicochemical properties and determined that an isomeric group called iso4, comprising a mixture of isomers II-RRR and II-SSS having the formulas (II-RRR) and (II-SSS) represented below, proved to be the most advantageous medical imaging contrast agent.

[0026]

[0027]

[0028] Thus, surprisingly, iso4 is characterized by a thermodynamic stability significantly superior to that of the diastereomeric mixture in the form of the complex of formula (II) obtained by carrying out the process described in EP 1 931673. In particular, its equilibrium thermodynamic constant K therm iso4 ​​is equal to 10 18.7 (i.e. log(K therm iso4)=18.7), which value has been determined by carrying out the method described in Pierrard et al., Contrast Media Mol. Imaging, 2008, 3, 243-252 and Moreau et al., Dalton Trans., 2007, 1611-1620.

[0029] Moreover, iso4 is the isomer group with the best kinetic inertia (also called kinetic stability) among the four groups separated by the inventors. Specifically, the inventors evaluated the kinetic inertia by studying the decomplexation kinetics of the four groups of isomers in an acidic aqueous solution (pH = 1.2) at 37°C. The half-life time values ​​(T 1 / 2 ), which half-life time corresponds to the time when 50% of the amount of complex initially present dissociates according to the following decomplexation reaction (reaction formula 2):

[0030]

[0031] (Reaction 2)

[0032] Isomer group <![CDATA[T 1 / 2 (pH 1.2-37℃)]]> Iso1 18 hours Iso2 6 hours Iso3 8 days Iso4 27 days

[0033] [Table 1]: Decomplexation kinetics of isomers iso1 to iso4

[0034] For comparison purposes, gadobutrol or gadolinate, which are macrocyclic gadolinium complexes, have kinetic inertias of 18 hours and 4 days, respectively, under the same conditions, whereas linear gadolinium complexes (such as gadodiamide or gadopentetate) dissociate instantaneously.

[0035] In addition, iso4 is chemically more stable than iso3 in particular. The reason for this is that the amide functional group of the complex of formula (II) is susceptible to hydrolysis. The hydrolysis reaction of the amide functional group (reaction formula 3) leads to the formation of a double coupled impurity, which is accompanied by the release of 3-amino-1,2-propanediol. The inventors studied the hydrolysis reaction kinetics of the complex of formula (II) in an aqueous solution at pH 13 and observed that the amide functional group of iso4 is more stable in terms of hydrolysis than the amide functional group of iso3.

[0036]

[0037] With regard to the relaxivities of the various groups of isomers (i.e. their efficiency as contrast agents), the measurements performed showed that the contrast capabilities of the groups iso1, iso2 and iso4 were relatively comparable, whereas iso3 showed reduced efficiency (see Table 2).

[0038]

[0039] [Table 2] Relaxation rates of isomeric groups iso1 to iso4 at 37°C

[0040] The inventors have succeeded in developing a novel process for the preparation and purification of a complex of formula (II) which makes it possible to preferably obtain the diastereomers II-RRR and II-SSS of said complex having particularly advantageous physicochemical properties. The process according to the invention comprises a step of enriching the isomers by converting the least stable stereoisomer into the most stable stereoisomer, which surprisingly makes it possible in the vast majority of cases to obtain the most stable isomer of the complex of formula (II) when carried out on a hexaacid intermediate complex rather than on the final complex.

[0041] It is undoubtedly advantageous to implement a process that makes it possible in most cases to obtain the target diastereomer, compared to the alternative of preparing a mixture of stereoisomers, in order to subsequently attempt to separate the diastereomers according to conventional techniques, thereby isolating the target isomer. In particular, in addition to the fact that processes that do not involve a diastereoisomer separation step are easier to carry out on an industrial scale, the absence of a separation allows, on the one hand, a considerable saving of time and, on the other hand, enables the overall yield of the process to be increased by limiting as much as possible the production of the unwanted diastereomers that will ultimately be eliminated. In addition, conventional separation techniques generally involve the use of large amounts of solvents, which, beyond the financial costs, are undesirable for environmental reasons. In addition, the use of silica chromatography should be particularly avoided, given the health risks inherent in occupational exposure to silica, which is classified as carcinogenic to humans (group 1) by the International Agency for Research on Cancer.

[0042] As mentioned above, the method developed by the inventors for preparing the complex having formula (II) is based on the step of isomer enrichment of the intermediate gadolinium hexahydrate complex having formula (I) represented below:

[0043]

[0044] The complex of formula (I) corresponds to several stereoisomers due to the presence of three asymmetric carbon atoms in the side chain of the complex, located in the alpha position relative to the nitrogen atom of the macrocycle to which the side chain is grafted. These three asymmetric carbons are marked with asterisks (*) in the formula (I) shown above.

[0045] Since each of the three asymmetric carbons carrying the carboxylate functional group can have an absolute configuration of R or S, the complex of formula (I) exists in the form of eight stereoisomers, hereinafter referred to as I-RRR, I-SSS, I-RRS, I-SSR, I-RSS, I-SRR, I-RSR and I-SRS. More precisely, according to the usual nomenclature in stereochemistry, the complex of formula (I) exists in the form of four pairs of enantiomers that are diastereomers of each other.

[0046] The inventors have succeeded in separating and identifying by high performance liquid chromatography (HPLC) and ultra high performance liquid chromatography (UHPLC) four unresolved peaks or groups of isomers of the complex of formula (I) obtained according to the method described in EP 1 931 673, corresponding to four distinct elution peaks characterized by their retention times on the chromatogram, which will be referred to in the remainder of this specification as isoA, isoB, isoC and isoD.

[0047] IsoD crystallizes in water. X-ray diffraction analysis allowed the inventors to determine the crystal structure of this group of isomers and thus found that it comprises diastereomers I-RRR and I-SSS of the complex with formula (I-RRR) and (I-SSS) represented below.

[0048]

[0049] It should be noted that the diastereomers I-RRR and I-SSS of the complex having formula (I) are enantiomers of each other.

[0050] The isomer enrichment step of the method of the present invention involves enriching the intermediate gadolinium hexaphosphate complex having formula (I) in isoD.

[0051] The synthesis of the complex of formula (II) specifically involves converting the carboxylic acid functional group of the intermediate hexaacid complex of formula (I) into an amide functional group. This amidation reaction does not change the absolute configuration of the three asymmetric carbon atoms of the complex of formula (I).

[0052] Therefore, when the hexaacid complex having the formula (I) enriched in isoD obtained previously is subjected to an amidation reaction, the complex having the formula (II) enriched in iso4 can be obtained. Summary of the invention

[0053] Gadolinium hexaoxide complex having formula (I)

[0054] Therefore, the present invention firstly relates to a hexacarboxylic acid gadolinium complex having the formula (I):

[0055]

[0056] It consists of a diastereomeric excess of at least 80% comprising a mixture of isomers I-RRR and I-SSS having the following formula:

[0057]

[0058] In the context of the present invention, the term "diastereomeric excess" is intended to represent the fact that the hexa-acid gadolinium complex of formula (I) exists mainly in the form of an isomer or isomer group selected from the following diastereoisomers: I-RRR, I-SSS, I-RRS, I-SSR, I-RSS, I-SRR, I-RSR and I-SRS. The diastereomeric excess is expressed as a percentage and corresponds to the amount represented by the main isomer or isomer group relative to the total amount of the hexa-acid gadolinium complex of formula (I). It should be understood that the percentage can be based on moles or on mass, since, by definition, isomers have the same molar mass.

[0059] In one embodiment, the complex of formula (I) according to the invention has a diastereomeric excess of at least 85%, in particular at least 90%, specifically at least 95%, preferably at least 97%, advantageously at least 98%, more advantageously at least 99% of the mixture comprising the isomers I-RRR and I-SSS.

[0060] Preferably, the diastereomeric excess consists of at least 70%, in particular at least 80%, advantageously at least 90%, preferably at least 95% of the mixture of the isomers I-RRR and I-SSS.

[0061] Advantageously, said diastereomeric excess consists of a mixture of the isomers I-RRR and I-SSS.

[0062] By extension, the term "mixture of isomers I-RRR and I-SSS" also covers the situation where only one isomer is present, whether I-RRR or I-SSS. However, the term "mixture of isomers I-RRR and I-SSS" preferably represents all situations where each of the isomers I-RRR and I-SSS is present in variable but non-zero amounts.

[0063] In a preferred embodiment, the isomers I-RRR and I-SSS are present in the mixture in a ratio between 65 / 35 and 35 / 65, in particular between 60 / 40 and 40 / 60, specifically between 55 / 45 and 45 / 55. Advantageously, the mixture of isomers I-RRR / I-SSS is a racemic (50 / 50) mixture.

[0064] More specifically, the diastereomeric excess as defined above corresponds to peak 4 (i.e. the fourth peak in the order of elution and corresponding to isoD) in the HPLC profile obtained using the HPLC method described below, characterized by a retention time of between 33.9 and 37.5 minutes, typically around 35.7 minutes.

[0065] Within the meaning of the present invention, the term "HPLC graph" refers to the curve of concentration changes over time measured by the detector after the passage and separation of a mixture of compounds (in this case isomers of compounds) on a stationary phase for a given composition and a given eluent flow rate. The HPLC graph consists of various peaks or unresolved peaks characteristic of the analyzed compound or mixture of compounds.

[0066] HPLC Method:

[0067] Waters C18-250x4.6mm-5μm column.

[0068] This is a reverse phase UPLC column containing spherical silica particles that have C18 (octadecyl) grafted and silanol treated with a capping agent (end capping). It also features a length of 250 mm, an inner diameter of 4.6 mm, a particle size of 5 μm, porosity and 19% carbon content.

[0069] Preferably, the stationary phase used will be compatible with aqueous mobile phases.

[0070] Analysis conditions:

[0071]

[0072] -Mobile phase gradient (by volume):

[0073]

[0074]

[0075] The complex having formula (II)

[0076] Secondly, the present invention relates to a complex having formula (II):

[0077]

[0078] It consists of a diastereomeric excess of at least 80% comprising a mixture of isomers II-RRR and II-SSS having the following formula:

[0079]

[0080]

[0081] In the context of the present invention, the term "diastereomeric excess" is intended to represent the fact that, with respect to the complex of formula (II), the complex is present predominantly in the form of an isomer or isomer group selected from the following diastereomers: II-RRR, II-SSS, II-RRS, II-SSR, II-RSS, II-SRR, II-RSR and II-SRS. The diastereomeric excess is expressed as a percentage and corresponds to the amount represented by the predominant isomer or isomer group relative to the total amount of the complex of formula (II). It should be understood that the percentage can be based on moles or on masses, since, by definition, isomers have the same molar mass.

[0082] In one embodiment, the complex of formula (II) according to the invention has a diastereomeric excess of at least 85%, in particular at least 90%, specifically at least 92%, preferably at least 94%, advantageously at least 97%, more advantageously at least 99% of the mixture comprising isomers II-RRR and II-SSS.

[0083] Preferably, the diastereomeric excess consists of at least 70%, in particular at least 80%, advantageously at least 90%, preferably at least 95% of the mixture of isomers II-RRR and II-SSS.

[0084] Advantageously, said diastereomeric excess consists of a mixture of isomers II-RRR and II-SSS.

[0085] By extension, the term "mixture of isomers II-RRR and II-SSS" also covers the situation where only one isomer is present, whether II-RRR or II-SSS. However, the term "mixture of isomers II-RRR and II-SSS" preferably represents all situations where each of the isomers II-RRR and II-SSS is present in a variable but non-zero amount.

[0086] In a preferred embodiment, the isomers II-RRR and II-SSS are present in the mixture in a ratio between 65 / 35 and 35 / 65, in particular between 60 / 40 and 40 / 60, specifically between 55 / 45 and 45 / 55. Advantageously, the isomers II-RRR and II-SSS are present in the mixture in a ratio of 50 / 50.

[0087] More specifically, the diastereomeric excess as defined previously corresponds to peak 4 (i.e., the fourth unresolved peak of the isomers arranged in order of elution and corresponding to iso4) in the UHPLC profile, which is characterized by a retention time between 6.0 and 6.6 minutes, typically around 6.3 minutes, said profile being obtained using the UHPLC method described below.

[0088] Within the meaning of the present invention, the term "UHPLC profile" refers to the plot of concentration changes over time as measured by the detector after a mixture of compounds (in this case isomers of compounds) has passed and separated on a stationary phase for a given composition and a given eluent flow rate. The UHPLC profile consists of various peaks or unresolved peaks characteristic of the analyzed compound or mixture of compounds.

[0089] UHPLC Method:

[0090] Waters UPLC T3 150x2.1mm-1.6μm column.

[0091] This is a reverse phase UPLC column containing spherical particles consisting of a preferably very hard core made of silica surrounded by trifunctional C18 (octadecyl) grafted porous silica and silanol treated with a capping agent (end capping). It is also characterized by a length of 150 mm, an inner diameter of 2.1 mm, a particle size of 1.6 μm, porosity and 4.7% carbon content.

[0092] Preferably, the stationary phase used will be compatible with aqueous mobile phases.

[0093] Analysis conditions:

[0094]

[0095] Mobile phase gradient (% v / v):

[0096]

[0097] In a preferred embodiment, the complex of formula (II) according to the invention is obtained by amidation with 3-amino-1,2-propanediol in racemic or enantiomerically pure form, preferably in racemic form, starting from the complex of formula (I) according to the invention as defined above.

[0098] Within the meaning of the present invention, the term "amidation" refers to a reaction for converting a carboxylic acid function into an amide function by reaction with an amine function.

[0099] This reaction can in particular be carried out after activation of the carboxylic acid function, as described in detail later in the description.

[0100] Process for preparing a complex having formula (II)

[0101] The present invention also relates to a method for preparing a complex having formula (II), comprising the following consecutive steps:

[0102] a) a hexacarboxylic acid having the following formula (III):

[0103]

[0104] complexed with gadolinium to obtain a gadolinium hexacarboxylate complex having formula (I) as defined above,

[0105] b) isomerizing the hexaoxato gadolinium complex of formula (I) by heating in aqueous solution at a pH between 2 and 4 to obtain a diastereoisomerically enriched complex consisting of at least 80% of diastereoisomer excess comprising a mixture of isomers I-RRR and I-SSS of the hexaoxato gadolinium complex of formula (I), and

[0106] c) Starting from the diastereomerically enriched complex obtained in step b), forming the complex of formula (II) by reaction with 3-amino-1,2-propanediol.

[0107] In this specification, unless otherwise specified, the terms "Gd", "gadolinium" and "Gd 3+ "To represent Gd 3+ ions. By extension, it may also be a source of free gadolinium, such as gadolinium chloride (GdCl3) or gadolinium oxide (Gd2O3).

[0108] In the present invention, the term "free Gd" represents the non-complexed form of gadolinium, which is preferably available for complexation. Typically, Gd dissolved in water 3+ ions. By extension, it may also be a source of free gadolinium, such as gadolinium chloride (GdCl3) or gadolinium oxide.

[0109] ■Step a)

[0110] During this step, a complexation reaction occurs between the hexaacid of formula (III) and gadolinium, which makes it possible to obtain the hexaacid gadolinium complex of formula (I) as defined previously.

[0111] According to one specific embodiment, step a) comprises the reaction between the hexaacid of formula (III) and a source of free Gd in water.

[0112] In a preferred embodiment, the source of free Gd is GdCl3 or Gd2O3, preferably Gd2O3.

[0113] Preferably, the reagents used in step a), ie the source of gadolinium (typically gadolinium oxide), the hexaacid of formula (III) and water, are as pure as possible, in particular with respect to metallic impurities.

[0114] Thus, the source of gadolinium will advantageously be gadolinium oxide, preferably with a purity greater than 99.99%, even more preferably greater than 99.999%.

[0115] The water used in the process preferably contains less than 50 ppm calcium, more preferably less than 20 ppm and most preferably less than 15 ppm calcium. Typically, the water used in the process is deionized water, water for injection (injection grade water) or purified water.

[0116] Advantageously, the amounts of reagents (hexacid of formula (III) and gadolinium) used in this step a) correspond to or are close to the stoichiometric proportions, as determined by the equilibrium equation of the complexation reaction occurring in this step.

[0117] The term "close to the stoichiometric ratio" means that the difference between the molar ratio of the introduced reagents and the stoichiometric ratio is less than 15%, in particular less than 10%, preferably less than 8%.

[0118] It is worth noting that the introduction of gadolinium may be slightly excessive relative to the stoichiometric ratio. Then, the ratio of the amount of material introduced as gadolinium to the amount of material introduced as hexaacid having formula (III) is greater than 1, but typically less than 1.15, particularly less than 1.10, and advantageously less than 1.08. In other words, relative to the amount of hexaacid having formula (III) introduced (which itself corresponds to 1 equivalent), the amount of gadolinium introduced is greater than 1 equivalent (eq.), but typically less than 1.15 eq., particularly less than 1.10 eq., and advantageously less than 1.08 eq. In a preferred embodiment in which the source of free gadolinium is Gd2O3, relative to the amount of hexaacid having formula (III) introduced (1 eq.), the amount of Gd2O3 introduced is typically greater than 0.5 eq., but less than 0.575 eq., particularly less than 0.55 eq., and advantageously less than 0.54 eq.

[0119] According to a specific embodiment, step a) comprises the following consecutive steps:

[0120] a1) preparing an aqueous solution of a hexaacid of formula (III), and

[0121] a2) adding a source of free gadolinium to the aqueous solution obtained in step a1).

[0122] In this embodiment, the content of hexaacid of formula (III) in the aqueous solution prepared in step a1) is typically between 10 and 60% by weight, in particular between 15 and 45%, preferably between 20 and 35%, advantageously between 25 and 35% and even more advantageously between 25 and 30% by weight relative to the total weight of the aqueous solution.

[0123] Preferably, steps a) and b) are carried out according to a one-pot process, ie in the same reactor without intermediate steps of separation or purification.

[0124] Therefore, in this preferred embodiment, the hexacarboxylic gadolinium complex of formula (I) formed in step a) is directly subjected to the isomerization step b) without isolation or purification and is carried out in the same reactor as used in step a).

[0125] ■Step b)

[0126] The hexaacid gadolinium complex of formula (I) formed by the complexation reaction between the hexaacid of formula (III) and gadolinium in step a) is first obtained as a mixture of diastereomers.

[0127] Step b) involves enriching the mixture of diastereomers in the isomers I-RRR and I-SSS to obtain a diastereomerically enriched gadolinium hexahydrate complex having formula (I), which consists of a diastereomeric excess of at least 85%, in particular at least 90%, specifically at least 95%, preferably at least 97%, advantageously at least 98%, more advantageously at least 99% of the mixture comprising isomers I-RRR and I-SSS.

[0128] Preferably, the diastereomeric excess consists of at least 70%, in particular at least 80%, advantageously at least 90%, preferably at least 95% of the mixture of the isomers I-RRR and I-SSS.

[0129] Advantageously, said diastereomeric excess consists of a mixture of the isomers I-RRR and I-SSS.

[0130] In fact, the inventors have found that factors such as pH and temperature of the solution of the gadolinium hexahydrate complex of formula (I) obtained at the end of step a) have an influence on the ratio of the various isomers of the complex of formula (I) present in the mixture of diastereomers. Over time, the mixture tends to be enriched in a group of isomers that surprisingly contains the most thermodynamically stable but also the most chemically stable isomers, in this case the isomers I-RRR and I-SSS.

[0131] By extension, the term "mixture of isomers I-RRR and I-SSS" also covers the situation where only one isomer is present, whether I-RRR or I-SSS. However, the term "mixture of isomers I-RRR and I-SSS" preferably represents all situations where each of the isomers I-RRR and I-SSS is present in variable but non-zero amounts.

[0132] In a preferred embodiment, the isomers I-RRR and I-SSS are present in the mixture in a ratio between 65 / 35 and 35 / 65, in particular between 60 / 40 and 40 / 60, specifically between 55 / 45 and 45 / 55. Advantageously, the mixture of isomers I-RRR / I-SSS is a racemic (50 / 50) mixture.

[0133] Step b) of isomerization of the hexaacid gadolinium complex of formula (I) in aqueous solution is typically carried out at a pH between 2 and 4, in particular between 2 and 3, advantageously between 2.2 and 2.8.

[0134] The pH is preferably adjusted with an acid, preferably an inorganic acid, such as hydrochloric acid, hydrobromic acid, sulfuric acid, nitric acid or phosphoric acid, for example with hydrochloric acid.

[0135] It is completely surprising that under such pH conditions, an enrichment of the mixture, in particular of the isomers, occurs, in this case the isomers I-RRR and I-SSS, since it is known in the art that gadolinium chelates are characterized by low kinetic inertia in acidic media. + The higher the concentration of ions, the greater the probability that a proton will be transferred to one of the donor atoms of the ligand, thereby causing the complex to dissociate. Therefore, one skilled in the art would expect that placing the hexa-acid gadolinium complex of formula (I) in an aqueous solution at a pH between 2 and 4 would cause the complex to dissociate, rather than isomerize to I-RRR and I-SSS.

[0136] It should be noted that the pH range recommended by EP 1 931 673 for the complexation of the hexaacid of formula (III), ie 5.0-6.5, does not allow the complex of formula (I) to be obtained enriched in its isomers I-RRR and I-SSS.

[0137] Step b) is typically carried out at a temperature between 80°C and 130°C, in particular between 90°C and 125°C, preferably between 98°C and 122°C, advantageously between 100°C and 120°C, typically for a time between 10 hours and 72 hours, in particular between 10 hours and 60 hours, advantageously between 12 hours and 48 hours.

[0138] Contrary to all expectations, such temperature conditions used in combination with the above mentioned pH conditions should favour the instability of the gadolinium chelate, not leading to its decomplexation or formation of any other impurities, but rather to its isomerization into I-RRR and I-SSS.

[0139] In a particular embodiment, the aqueous solution of step b) comprises acetic acid.Step b) is then advantageously carried out at a temperature between 100°C and 120°C, in particular between 110°C and 118°C, typically for a time between 12 hours and 48 hours, in particular between 20 hours and 30 hours, in particular between 24 hours and 26 hours.

[0140] Preferably, an amount of acetic acid is added before heating the solution of the hexaacid gadolinium complex of formula (I) obtained in step a) so that the content of acetic acid is between 25% and 75% by mass, in particular between 40% and 50% by mass, relative to the mass of the hexaacid of formula (III) used in step a).

[0141] When the aqueous solution is heated to a temperature advantageously between 100 and 120° C., typically between 110 and 118° C., acetic acid is gradually added as the water evaporates so as to maintain a constant volume of solution.

[0142] According to a preferred embodiment, at the end of step b), the diastereomerically enriched complex is isolated by crystallization, preferably by seeded crystallization.

[0143] In this embodiment, step b) comprises the following consecutive steps:

[0144] b1) isomerizing the hexaoxato gadolinium complex of formula (I) by heating it in aqueous solution at a pH between 2 and 4 to obtain a diastereoisomerically enriched complex consisting of at least 80% of diastereoisomer excess, the diastereoisomer excess comprising a mixture of isomers I-RRR and I-SSS of the hexaoxato gadolinium complex of formula (I), and

[0145] b2) isolating the diastereomerically enriched complex by crystallization, preferably by seeded crystallization.

[0146] The crystallization step b2) involves firstly the removal of any impurities present in the aqueous solution, which may have resulted from the previous step, so as to obtain a decolorized product of higher purity in the form of crystals, and secondly the continuation of the diastereomeric enrichment of the gadolinium hexaacid complex of formula (I) so as to obtain a mixture comprising the isomers I-RRR and I-SSS of the complex in a diastereomeric excess higher than that obtained at the end of step b1).

[0147] Specifically, isomers I-RRR and I-SSS of the hexaacid complex of formula (I) crystallize from water. On the other hand, the hexaacid gadolinium complex of formula (I) that is not enriched in said isomers does not crystallize.

[0148] The fact that the isomers I-RRR and I-SSS in which the complex tends to be enriched during step b) (and contrary to all expectations, according to the conditions under which it is carried out) are the only isomers of the complex that crystallize from water is a completely unexpected result. Isomerization and crystallization therefore act synergistically to contribute to the enrichment of the isomers I-RRR and I-SSS and therefore to the overall efficiency of the process according to the invention.

[0149] Furthermore, it should be noted that the crystallization of the target isomer of the gadolinium hexaoxide complex of formula (I) in water allows avoiding the addition of solvent as described in Example 7 of EP 1 931 673, which involves a step of precipitating the trisodium salt of the complex from ethanol.

[0150] Step b2) is advantageously carried out at a temperature between 10 and 70°C, in particular between 30 and 65°C, specifically between 35 and 60°C.

[0151] According to a variation, after reducing the temperature of the aqueous solution so that it is within the above range, a crystallization process is induced by seeding. "Crystallization by seeding", also referred to as "crystallization by seeding", includes introducing a known amount of crystals (referred to as "seeds" or "primers") into a reactor (also referred to as a crystallization vessel) in which crystallization is performed. This makes it possible to reduce the crystallization time. Crystallization by seeding is well known to those skilled in the art. In the method according to the present invention, a primer, in the present case a diastereoisomer-enriched crystal of a hexacarboxylic acid gadolinium complex with formula (I) added to an aqueous solution of a diastereoisomer-enriched complex having previously reduced the temperature, is used to crystallize so that nucleation can be obtained, thereby initiating crystallization. The duration of crystallization by seeding is advantageously between 2 hours and 20 hours, and preferably between 6 hours and 18 hours; typically 16 hours.

[0152] The diastereomerically enriched crystals of the gadolinium hexahydrate complex having formula (I) are then isolated by filtration and drying, typically by any technique known to those skilled in the art.

[0153] Advantageously, the diastereomerically enriched gadolinium hexahydrate complex of formula (I) isolated at the end of step b2) has a purity greater than 95%, in particular greater than 98%, advantageously greater than 99%, expressed as a percentage by mass of the complex of formula (I) relative to the total mass obtained at the end of step b2).

[0154] In one embodiment, the diastereomerically enriched complex from step b) separated by crystallization is purified again by recrystallization to obtain a diastereomerically enriched and purified complex.

[0155] In this embodiment, in addition to the previously described consecutive steps b1) and b2), step b) further comprises a step b3) of purifying the diastereomerically enriched gadolinium hexahydrate complex having formula (I) separated by recrystallization.

[0156] The recrystallization step b3) is, like the crystallization step b2), firstly concerned with obtaining a product of higher purity and secondly with continuing the diastereomeric enrichment of the gadolinium hexaoxide complex of formula (I) in order to obtain a diastereomeric excess of a mixture comprising the isomers I-RRR and I-SSS of the complex which is higher than that obtained at the end of step b2).

[0157] Step b3) typically comprises the following successive sub-steps:

[0158] suspending the diastereomerically enriched gadolinium hexahydrate complex of formula (I) separated in step b2) in an aqueous solution, preferably in water,

[0159] • dissolving the complex by heating to a temperature advantageously between 80° C. and 120° C., for example to a temperature of 100° C.,

[0160] recrystallization, preferably by seeding, advantageously at a temperature between 10°C and 90°C, in particular between 20°C and 87°C, in particular between 55°C and 85°C, typically for a time between 2 hours and 20 hours, in particular between 6 hours and 18 hours, and

[0161] - Isolating diastereomerically enriched and purified crystals of the gadolinium hexaoxide complex of formula (I), for example by filtration and drying.

[0162] The purity of the purified diastereomerically enriched gadolinium hexahydrate complex of formula (I) isolated at the end of step b3) is typically greater than 98%, in particular greater than 99%, advantageously greater than 99.5%, expressed as a percentage by mass of the complex of formula (I) relative to the total mass obtained at the end of step b2).

[0163] In another embodiment, the diastereoisomerically enriched complex from step b) is further enriched by selective decomplexation of diastereoisomers of the complex having formula (I) other than the diastereoisomers I-RRR and I-SSS, i.e., by selective decomplexation of the diastereoisomers I-RSS, I-SRR, I-RSR, I-SRS, I-RRS and I-SSR.

[0164] In this embodiment, in addition to the previously described consecutive steps b1) and b2), step b) also comprises a step b4) of selective decomplexation of diastereomers of the complex of formula (I) other than the diastereomers I-RRR and I-SSS). In this variant, step b) may also comprise the previously described step b3), which is carried out between steps b2) and b4) or after b4).

[0165] The selective decomplexation step b4) involves continuing the diastereomeric enrichment of the gadolinium hexaoxide complex of formula (I) so as to obtain a diastereomeric excess comprising a mixture of isomers I-RRR and I-SSS of the complex, which is higher than the diastereomeric excess obtained at the end of step b2) or at the end of step b3) when said step is carried out before step b4).

[0166] Step b4) typically comprises the following successive sub-steps:

[0167] ● suspending the diastereomerically enriched gadolinium hexahydrate complex of formula (I) separated in step b2) or step b3) in water,

[0168] ● Adding a base, such as sodium hydroxide,

[0169] heating to a temperature advantageously between 30°C and 60°C, in particular between 35°C and 55°C, for example 40°C, typically for a time between 2 hours and 20 hours, in particular between 10 hours and 18 hours,

[0170] - cooled to a temperature advantageously between 10°C and 30°C, for example to 30°C, and

[0171] - Isolating the diastereomerically enriched and purified gadolinium hexacarboxylate complex of formula (I), for example by filtration and drying.

[0172] Step b4) is made possible by the fact that the isomers I-RRR and I-SSS are the most stable in alkaline medium. Such alkaline conditions promote the formation of gadolinium hydroxide and therefore the decomplexation of the least stable isomer.

[0173] It should therefore be noted that, surprisingly, the isomers I-RRR and I-SSS are more stable both in an acidic medium allowing the isomerization step b1) and in a basic medium allowing the selective decomplexation step b4).

[0174] In a preferred embodiment, the diastereomerically enriched complex obtained at the end of step b) according to any of the above-described variants has a diastereomeric excess of at least 85%, in particular at least 90%, specifically at least 95%, preferably at least 97%, advantageously at least 98%, more advantageously at least 99% over the mixture comprising the isomers I-RRR and I-SSS.

[0175] Preferably, the diastereomeric excess consists of at least 70%, in particular at least 80%, advantageously at least 90%, preferably at least 95% of the mixture of the isomers I-RRR and I-SSS.

[0176] Advantageously, said diastereomeric excess consists of a mixture of the isomers I-RRR and I-SSS.

[0177] By extension, the term "mixture of isomers I-RRR and I-SSS" also covers the situation where only one isomer is present, whether I-RRR or I-SSS. However, the term "mixture of isomers I-RRR and I-SSS" preferably represents all situations where each of the isomers I-RRR and I-SSS is present in variable but non-zero amounts.

[0178] In a preferred embodiment, the isomers I-RRR and I-SSS are present in the mixture in a ratio between 65 / 35 and 35 / 65, in particular between 60 / 40 and 40 / 60, specifically between 55 / 45 and 45 / 55. Advantageously, the mixture of isomers I-RRR / I-SSS is a racemic (50 / 50) mixture.

[0179] ■Step c)

[0180] Step c) involves the formation of the complex of formula (II) from its precursor, ie the diastereomerically enriched gadolinium hexacarboxylate complex of formula (I) obtained in step b).

[0181] In this step, the three carboxylic acid functional groups of the hexaacid complex of formula (I) carried by the carbon atom located in the γ position on the side chain of the complex relative to the nitrogen atom of the macrocycle to which the side chain is grafted are converted into amide functional groups by an amidation reaction with 3-amino-1,2-propanediol in racemic or enantiomerically pure form, preferably in racemic form.

[0182] This amidation reaction does not change the absolute configuration of the three asymmetric carbon atoms located in the alpha position on the side chain relative to the nitrogen atom of the macrocycle grafted with the side chain. Thus, step c) makes it possible to obtain a complex of formula (II) with a diastereomeric excess comprising a mixture of isomers II-RRR and II-SSS, which is identical to the diastereomeric excess of a mixture comprising isomers I-RRR and I-SSS, the diastereomeric enriched gadolinium hexahydrate complex of formula (I) obtained at the end of step b) being obtained with a diastereomeric excess of at least 80% of the mixture comprising isomers I-RRR and I-SSS.

[0183] In a preferred embodiment, the complex of formula (II) obtained at the end of step c) has a diastereomeric excess of at least 85%, in particular at least 90%, specifically at least 92%, preferably at least 94%, advantageously at least 97%, more advantageously at least 99% of the mixture comprising isomers II-RRR and II-SSS.

[0184] Preferably, the diastereomeric excess consists of at least 70%, in particular at least 80%, advantageously at least 90%, preferably at least 95% of the mixture of isomers II-RRR and II-SSS.

[0185] Advantageously, said diastereomeric excess consists of a mixture of isomers II-RRR and II-SSS.

[0186] By extension, the term "mixture of isomers II-RRR and II-SSS" also covers the situation where only one isomer is present, whether II-RRR or II-SSS. However, the term "mixture of isomers I-RRR and I-SSS" preferably represents all situations where each of the isomers I-RRR and I-SSS is present in variable but non-zero amounts.

[0187] In a preferred embodiment, the isomers II-RRR and II-SSS are present in the mixture in a ratio between 65 / 35 and 35 / 65, in particular between 60 / 40 and 40 / 60, specifically between 55 / 45 and 45 / 55. Advantageously, the isomers II-RRR and II-SSS are present in the mixture in a ratio of 50 / 50.

[0188] The amidation reaction can be carried out according to any method known to those skilled in the art, in particular in the presence of reagents for activating the carboxylic acid function and / or by acid catalysis.

[0189] It can be carried out in particular according to the method described in EP 1 931 673, in particular in paragraph

[0027] of said patent.

[0190] In one embodiment, step c) comprises activating the carboxylic acid (-COOH) functional group carried by the carbon atom located in the γ position on the side chain of the hexaacid complex of formula (I) relative to the nitrogen atom of the macrocycle on which the side chain is grafted, in the form of a derivative functional group, including a carbonyl (C=O) group, so that the carbon atom of the carbonyl group is more electrophilic than the carbon atom of the carbonyl group of the carboxylic acid functional group. Therefore, according to this embodiment, the carboxylic acid functional group can be activated in particular in the form of an ester, acyl chloride or anhydride functional group, or in any activated form that can produce an amide bond. Activated forms that can produce amide bonds are well known to those skilled in the art and can be obtained, for example, by a group of methods known in peptide chemistry for forming peptide bonds. Examples of such methods are given in the publication Synthesis of peptides and Peptidomimetics, Vol. E22a, pp. 425-588, Houben-Weyl et al., ed. Goodman, Thieme-Stuttgart-New York (2004), and among these examples mention may be made in particular of methods of activating carboxylic acids by azides (acyl azides), for example by the action of reagents such as diphenylphosphoryl azide (commonly abbreviated to DPPA), the use of carbodiimides, alone or in the presence of catalysts such as N-hydroxysuccinimide and its derivatives, the use of carbonyldiimidazoles (1,1′-carbonyldiimidazole, CDI), the use of phosphonium salts such as benzotriazol-1-yloxytris(dimethylamino)phosphonium hexafluorophosphate (commonly abbreviated to BOP), or other uroniums, for example 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (commonly abbreviated to HBTU).

[0191] Preferably, step c) comprises activating the above-mentioned carboxylic acid (—COOH) functional group in the form of an ester, acid chloride or anhydride functional group.

[0192] This embodiment is more preferred than the use coupling agent (e.g. EDCI / HOBT) described in EP 1 931 673 by activating the carboxylic acid functional group to carry out peptide coupling. Specifically, this coupling results in the formation of an equivalent of 1-ethyl-3-[3-(dimethylamino)propyl]urea, which must be removed, particularly by silica gel chromatography or by adding a solvent to carry out liquid / liquid extraction to remove. As previously discussed, independent of the increased complexity of the method caused by such additional steps, it is not desirable to use such purification methods. In addition, the use of HOBT itself is problematic because it is an explosive product.

[0193] Within the meaning of the present invention, the term "ester function" is intended to represent a -C(O)O- group. It may in particular be a group -C(O)O-R1, wherein R1 corresponds to a (C1-C6)alkyl group.

[0194] Within the meaning of the present invention, the term "(C1-C6)alkyl group" refers to a linear or branched saturated hydrocarbon chain containing 1 to 6, preferably 1 to 4, carbon atoms. Examples that may be mentioned include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl and hexyl groups.

[0195] Within the meaning of the present invention, the term "acyl chloride function", also called "acid chloride function", is intended to represent a -CO-Cl group.

[0196] Within the meaning of the present invention, the term "anhydride function" is intended to represent a -CO-O-CO- group. It may in particular be a group -CO-O-CO-R2, wherein R2 corresponds to a (C1-C6)alkyl group.

[0197] Reactions for converting a carboxylic acid function into an ester, acid chloride or anhydride function are well known to the person skilled in the art who will be able to carry out the reaction according to any conventional method with which he is familiar.

[0198] The complex of formula (II) is then obtained by aminolysis of the activated carboxylic acid function in the form of an ester, chloride or anhydride function, in particular an ester or anhydride, preferably an ester, by reaction with 3-amino-1,2-propanediol in racemic or enantiomerically pure form, preferably in racemic form.

[0199] Preferably, the steps of activating the carboxylic acid function and the aminolysis are carried out according to a one-pot embodiment, i.e. in the same reactor, without intermediate steps of isolating or purifying intermediates comprising the carboxylic acid function activated in the form of an ester, chloride or anhydride function, in particular an ester or anhydride, preferably an ester.

[0200] According to a specific embodiment, step c) comprises the following consecutive steps:

[0201] c1) forming an activated complex having formula (VII),

[0202]

[0203] wherein Y represents a chlorine atom, an -OR1 or an -OC(O)-R2 group; preferably, Y represents an -OR1 or -OC(O)-R2 group, wherein R1 and R2 independently of one another correspond to a (C1-C6)alkyl group, and

[0204] c2) Aminelysis of the activated complex of formula (VII) with 3-amino-1,2-propanediol.

[0205] It is obvious to a person skilled in the art that the reaction to form the activated complex of formula (VII) does not change the absolute configuration of the three asymmetric carbon atoms located in the alpha position on the side chain relative to the nitrogen atom of the macrocycle grafted with the side chain. Thus, step c1) makes it possible to obtain an activated complex of formula (VII) with a diastereomeric excess comprising a mixture of isomers VII-RRR and VII-SSS having the formula (VII-RRR) and (VII-SSS) represented below, which is the same as the diastereomeric excess of a mixture comprising isomers I-RRR and I-SSS, and the diastereomeric enriched gadolinium hexahydrate complex of formula (I) obtained at the end of step b) has a diastereomeric excess of at least 80% of the mixture comprising isomers I-RRR and I-SSS.

[0206]

[0207]

[0208] In the case where Y represents a chlorine atom, step c1) is typically carried out by reaction between the diastereomerically enriched gadolinium hexahydrate complex of formula (I) obtained in step b) and thionyl chloride (SOCl2).

[0209] In the case where Y represents a -OC(O)-CH3 group, step c1) is typically carried out by reaction of the diastereomerically enriched gadolinium hexahydrate complex of formula (I) obtained in step b) with acetyl chloride.

[0210] In an advantageous embodiment, step c) comprises activating the above-mentioned carboxylic acid (—COOH) functions in the form of ester functions.

[0211] According to this embodiment, step c) may more specifically comprise the following consecutive steps:

[0212] c1) forming a triester having the formula (VIII),

[0213]

[0214] wherein R1 represents a (C1-C6)alkyl group, and

[0215] c2) Aminelysis of the triester of formula (VIII) with 3-amino-1,2-propanediol.

[0216] Step c1) is typically carried out in the presence of an acid, such as hydrochloric acid, in an alcohol of formula R1OH acting both as solvent and reagent.

[0217] In the first stage, a hexa-acid gadolinium complex of formula (I) and an alcohol R1OH are placed in a reactor. The reaction medium is then cooled to a temperature below 10°C, particularly below 5°C, typically to 0°C, and then an acidic solution of alcohol R1OH, typically a solution of hydrochloric acid in R1OH, is gradually added. The reaction medium is kept stirred at room temperature (i.e., at a temperature between 20°C and 25°C) for typically more than 5 hours, preferably for a time between 10 hours and 20 hours. Prior to step c2), the reaction medium is cooled to a temperature below 10°C, particularly between 0°C and 5°C.

[0218] Step c2) is also typically carried out in the presence of an acid, such as hydrochloric acid, in an alcohol of formula R1OH.

[0219] Thus, according to the one-pot embodiment, steps c1) and c2) can be easily performed. Advantageously, the triester of formula (VII) is not isolated between steps c1) and c2).

[0220] However, in order to promote the aminolysis reaction, in step c2), the alcohol of formula R1OH is preferably removed by vacuum distillation.

[0221] Within the meaning of the present invention, the term "vacuum distillation" refers to the distillation of the mixture carried out at a pressure between 10 and 500 mbar, in particular between 10 and 350 mbar, preferably between 10 and 150 mbar, in particular between 50 and 100 mbar.

[0222] Similarly, in order to promote the aminolysis reaction, in step c2), a large amount of 3-amino-1,2-propanediol is introduced. Typically, relative to the amount of material of the diastereoisomer-enriched gadolinium hexahydrate complex of formula (I) initially introduced in step c) (which itself corresponds to 1 equivalent), the amount of material of the introduced 3-amino-1,2-propanediol is greater than 4 eq., particularly greater than 7 eq., advantageously greater than 10 eq.

[0223] Surprisingly, despite the acidic conditions typically employed in steps c1) and c2) which increase the kinetic instability of the gadolinium complex, no decomplexation or isomerization of the triester of formula (VIII) was observed. The desired triamide was obtained with a very good degree of conversion, and the absolute configuration of the three asymmetric carbon atoms located in the alpha position of the side chain relative to the nitrogen atom of the macrocycle was preserved.

[0224] Furthermore, it should be noted that amidation reactions via direct reaction between esters and amines are generally rarely described in the literature (on this subject see KC Nadimpally et al., Tetrahedron Letters, 2011, 52, 2579-2582).

[0225] In a preferred embodiment, step c) comprises the following consecutive steps:

[0226] c1) forming a methyl triester of formula (IV),

[0227]

[0228] In particular, the reaction is carried out in methanol in the presence of an acid such as hydrochloric acid, and

[0229] c2) Aminelysis of the methyl triester of the formula (IV) with 3-amino-1,2-propanediol, in particular in the presence of an acid such as hydrochloric acid in methanol.

[0230] Advantageously, the methyl triester of formula (IV) is not isolated between steps c1) and c2).

[0231] In a preferred embodiment, in step c2), the methanol is removed by vacuum distillation until a temperature of typically above 55°C, in particular between 60°C and 65°C, is reached, and the reaction medium is kept at this temperature under vacuum for a period of typically greater than 5 hours, in particular between 10 and 20 hours, followed by cooling to room temperature and dilution with water.

[0232] The present invention includes all combinations of the above-mentioned particular, advantageous or preferred embodiments relating to each step of the method.

[0233] The present invention also relates to a triester gadolinium complex having the formula (VIII):

[0234]

[0235] It consists of a diastereomeric excess of at least 80% comprising a mixture of isomers VIII-RRR and VIII-SSS having the following formula:

[0236]

[0237] In the context of the present invention, the term "diastereomeric excess" is intended to represent the fact that, with respect to the triester gadolinium complex of formula (VIII), the complex exists mainly in the form of an isomer or isomer group selected from the following diastereomers: VIII-RRR, VIII-SSS, VIII-RRS, VIII-SSR, VIII-RSS, VIII-SRR, VIII-RSR and VIII-SRS. The diastereomeric excess is expressed as a percentage and corresponds to the amount represented by the main isomer or isomer group relative to the total amount of triester complex of formula (VIII). It should be understood that the percentage can be based on moles or on mass, since, by definition, isomers have the same molar mass.

[0238] In a specific embodiment, the triester gadolinium complex of formula (VIII) according to the present invention has a diastereomeric excess of at least 85%, in particular at least 90%, specifically at least 95%, preferably at least 97%, advantageously at least 98%, more advantageously at least 99% of a mixture comprising isomers VIII-RRR and VIII-SSS.

[0239] Preferably, the diastereomeric excess consists of at least 70%, in particular at least 80%, advantageously at least 90%, preferably at least 95% of the mixture of isomers VIII-RRR and VIII-SSS.

[0240] Advantageously, said diastereomeric excess consists of a mixture of isomers VIII-RRR and VIII-SSS.

[0241] The term "mixture of isomers VIII-RRR and VIII-SSS" also encompasses the situation where only one isomer is present, whether VIII-RRR or VIII-SSS is present. However, the term "mixture of isomers VIII-RRR and VIII-SSS" preferably represents all situations where each of isomers VIII-RRR and VIII-SSS is present in variable but non-zero amounts.

[0242] In a preferred embodiment, isomers VIII-RRR and VIII-SSS are present in the mixture in a ratio between 65 / 35 and 35 / 65, in particular between 60 / 40 and 40 / 60, specifically between 55 / 45 and 45 / 55. Advantageously, the mixture of isomers VIII-RRR / VIII-SSS is a racemic (50 / 50) mixture.

[0243] In a preferred embodiment, the triester gadolinium complex of formula (VIII) according to the present invention is a trimethyl gadolinium complex, ie a triester gadolinium complex of formula (VIII) in which R1 is a methyl group (CH3).

[0244] ■ Preparation of hexacarboxylic acid having formula (III)

[0245] The hexaacid of formula (III) which participates in step a) of the process for preparing the complex of formula (II) according to the invention can be prepared according to any known method, in particular according to the method described in EP 1 931 673.

[0246] However, according to a preferred embodiment, the hexaacid of formula (III) is obtained by: a pyclene of formula (V):

[0247]

[0248] With the formula R3OOC-CHG p -(CH2)2-COOR4(IX) compounds are alkylated,

[0249] in:

[0250] - R3 and R4 independently of one another represent a (C3-C6)alkyl group, in particular a (C4-C6)alkyl group, for example a butyl, isobutyl, sec-butyl, tert-butyl, pentyl or hexyl group, and

[0251] -Gp represents a leaving group, such as a tosylate or triflate group, or a halogen atom, preferably a bromine atom,

[0252] To obtain hexaester having formula (X)

[0253]

[0254] A hydrolysis step is then carried out to obtain the hexaacid of formula (III).

[0255] In a preferred embodiment, R3 and R4 are the same.

[0256] According to an advantageous embodiment, the hexaacid of formula (III) is obtained by: a pyclene of formula (V):

[0257]

[0258] Alkylation with dibutyl 2-bromoglutarate to give the butyl hexaester having the formula (VI):

[0259]

[0260] A hydrolysis step is then carried out to obtain the hexaacid of formula (III).

[0261] The dibutyl 2-bromoglutarate is used in racemic or enantiomerically pure form, preferably in racemic form.

[0262] Compared with using the 2-bromoglutarate ethyl ester described in EP 1 931 673, using 2-bromoglutarate dibutyl ester is particularly advantageous. Specifically, the commercially available 2-bromoglutarate diethyl ester is a relatively unstable compound, which degrades over time and under the effect of temperature. More precisely, this ester has a tendency to be hydrolyzed or cyclized and therefore lose its bromine atom. Purification of the commercially available 2-bromoglutarate diethyl ester or developing new synthetic routes to obtain this ester with higher purity and therefore prevent the attempt of its degradation are all unsuccessful.

[0263] The alkylation reaction is typically carried out in a polar solvent, preferably in water, in particular in deionized water, advantageously in the presence of a base such as potassium carbonate or sodium carbonate.

[0264] For obvious reasons, preference is given to using water and particular preference is given to using acetonitrile, as described in EP 1 931 673.

[0265] The reaction is advantageously carried out at a temperature between 40 and 80°C, typically between 50 and 70°C and in particular between 55 and 60°C, for a time between 5 and 20 hours, specifically between 8 and 15 hours.

[0266] The hydrolysis step is advantageously carried out in the presence of an acid or a base, advantageously in the presence of a base such as sodium hydroxide. The hydrolysis solvent may be water, an alcohol such as ethanol or a water / alcohol mixture. The step is advantageously carried out at a temperature between 40° C. and 80° C., typically between 40° C. and 70° C. and in particular between 50° C. and 60° C., typically for a time between 10 hours and 30 hours, in particular between 15 hours and 25 hours.

[0267] Furthermore, the present invention relates to butyl hexaesters having the formula (VI):

[0268]

[0269] In particular, the hexaester is distinguished by a significantly improved stability relative to esters with shorter alkyl chains, in particular relative to the ethyl hexaester described in EP 1 931 673. BRIEF DESCRIPTION OF THE DRAWINGS

[0270] Figure 1: Degradation of the isomeric group iso1 to iso4 of the complex of formula (II) under alkaline conditions, expressed as the area percentage of a given isomeric group over time. DETAILED DESCRIPTION

[0271] The examples given below are presented as non-limiting illustrations of the invention.

[0272] Separation of the isomeric groups isoA, isoB, isoC and isoD of the gadolinium hexachloride complex having the formula (I) by HPLC

[0273] A HPLC machine consisting of a pump system, a sample injector, a chromatography column, a UV spectrophotometer and a data processing and control station was used. The chromatography column used was a C18-250×4.6 mm-5 μm column (from Waters scope).

[0274] -Mobile phase:

[0275] Route A: 100% acetonitrile and Route B: 0.1% v / v H2SO4 (96%) in water

[0276] -Preparation of test solution:

[0277] 10 mg / mL solution of the gadolinium hexahydrate complex of formula (I) in purified water

[0278] -Analysis conditions:

[0279]

[0280]

[0281] -gradient:

[0282] time %Acn <![CDATA[%H2SO4 0.1%]]> 0 1 99 10 5 95 40 10 90 50 25 75 55 1 99 60 1 99

[0283] %Acn: %v / v of acetonitrile in mobile phase

[0284] %H2SO4 0.1%: % v / v of 0.1% v / v H2SO4 solution in mobile phase

[0285] Four main peaks were obtained. Peak 4 (i.e., isoD) of the HPLC graph corresponds to a retention time of 35.7 minutes. The isomeric groups isoA, isoB, isoC, and isoD of the hexa-acid gadolinium complex of formula (I) were separated by UHPLC.

[0286] A UHPLC machine consisting of a pump system, an injector, a chromatography column, a UV detector and a data station was used. The chromatography column used was a UHPLC 150×2.1 mm-1.8 μm column (Waters Acquity UPLC HSS T3 column). This is a reverse phase UPLC column containing spherical particles consisting of silica with trifunctional C18 (octadecyl) grafts and silanols that have been treated with a capping agent (end capping). It is also characterized by a length of 150 mm, an inner diameter of 2.1 mm, a particle size of 1.8 μm, porosity and 11% carbon content.

[0287] Preferably, the stationary phase used will be compatible with aqueous mobile phases.

[0288] -Mobile phase:

[0289] Route A: 100% acetonitrile and Route B: 0.1% v / v H2SO4 (96%) in water

[0290] -Preparation of test solution:

[0291] 0.8 mg / mL solution of the gadolinium hexahydrate complex of formula (I) in purified water

[0292] -Analysis conditions:

[0293]

[0294]

[0295] -gradient:

[0296] time %Acn <![CDATA[%H2SO4 0.1%]]> 0 1 99 14 8 92 20 11 89 25 25 75 27 1 99 32 1 99

[0297] Four main peaks were obtained. Peak 4 (i.e., isoD) of the UHPLC graph corresponds to a retention time of 17.4 minutes. The isomeric groups iso1, iso2, iso3, and iso4 of the complex having formula (II) were separated by UHPLC.

[0298] A UHPLC machine consisting of a pump system, an injector, a chromatography column, a UV detector, and a data station was used. The chromatography column used was a UHPLC 150×2.1 mm-1.6 μm column (Waters UPLC T3 column).

[0299] -Mobile phase:

[0300] Route A: 100% acetonitrile and Route B: 0.0005% v / v H2SO4 (96%) in water

[0301] -Preparation of test solution:

[0302] 2 mg / mL solution of the complex of formula (II) in purified water

[0303] -Analysis conditions:

[0304]

[0305] -gradient:

[0306]

[0307]

[0308] Four main peaks were obtained. Peak 4 (i.e., iso4) of the UHPLC profile corresponded to a retention time of 6.3 minutes.

[0309] Relaxation rate measurement

[0310] By operating at 20 MHz (0.47 T), at 60 MHz (1.41 T) and 37 °C The relaxation times T1 and T2 were determined by standard procedures on an mq20 machine (Brüker). The longitudinal relaxation time T1 was measured using a recovery sequence and the transverse relaxation time T2 was measured by the CPMG (Carr-Purcell-Meiboom-Gill) technique.

[0311] At 37°C, the total metal concentrations in aqueous solution ranged from 0.5x10 -3 Up to 5x10 -3 The relaxation rates R1 (=1 / T1) and R2 (=1 / T2) are calculated from the concentration of R1 or R2. The correlation between the changes in R1 or R2 with concentration is linear, and the slope represents the relaxation rate r1 (R1 / C) or r2 (R2 / C), expressed as (1 / sec) x (1 / mMol / L), i.e. (mM -1 .s -1 )express.

[0312] Measurement of the kinetic inertia of the isomer group of the complex having the formula (II) in acidic medium

[0313] The dissociation of the gadolinium complex present in the four unresolved peaks of isomers iso1 to iso4 was studied under pseudo-first order kinetics in hydrochloric acid solution at 37°C, pH 1.2 without controlling the ionic strength (C = 8 × 10 -6 M). After adding the solution of arsenazo III, the amount of free gadolinium was determined spectroscopically at 654 nm (C = 5.3 × 10 -4 M).

[0314] The half-life (T) determined for each of the isomeric groups 1 / 2 ) are organized in the following table:

[0315] Isomer group <![CDATA[T 1 / 2 (pH 1.2-37℃)]]> Iso1 18 hours Iso2 6 hours Iso3 8 days Iso4 27 days

[0316] Study on the degradation of the isomeric group of the complex having formula (II) under alkaline conditions

[0317] The complex having formula (II) is referred to as AP in the remainder of this example.

[0318] The degradation kinetics of the unresolved peaks of isomers iso1 to iso4, referred to as the generic term isoX, were evaluated by measuring the HPLC purity and by monitoring the area of ​​each unresolved peak of the isomers over time. The magnitude measured was therefore:

[0319] -P HPLC (time), and

[0320] -[Mathematical formula 1]

[0321]

[0322] The chosen degradation conditions were the following: [AP] = 1 mM in 0.1 N sodium hydroxide. Under these dilution conditions, the impact of AP degradation on the experimental medium was low. The degradation products did not change the pH of the medium, a parameter that is critical in the study of degradation kinetics. This was confirmed experimentally by measuring the initial pH and the pH at the end of degradation (72 hours, 37° C.):

[0323]

[0324] The method used to prepare the solution is described below:

[0325] - Weigh approximately 0.05 g of each product, an appropriate amount of 10 mL of mQ water, to obtain solution A such that [AP] A =5mM,

[0326] - Dilution: 2 mL of solution A, appropriate amount of 10 mL of NaOH (0.1 N) to obtain solution B, such that [AP] B =1 mM and [NaOH] = 0.08 M,

[0327] - the solution was aliquoted into HPLC flasks, and

[0328] - HPLC flasks containing AP solutions in NaOH were incubated at the study temperature (37°C).

[0329] For each point, an aliquot was taken and analyzed by HPLC (ammonium acetate method) without diluting the sample.

[0330] The results obtained in Figure 1 Given in.

[0331] Preparation of butyl hexaester having formula (VI)

[0332] 184 kg (570 mol) of dibutyl 2-bromoglutarate and 89 kg (644 mol) of potassium carbonate were mixed in a reactor and heated to 55-60° C. An aqueous solution of 29.4 kg (143 mol) of pyclene in 24 kg of water was added to the aforementioned preparation. The reaction mixture was maintained at 55-60° C. and then refluxed for about 10 hours. After the reaction, the medium was cooled, diluted with 155 kg of toluene, and then washed with 300 liters of water. Butyl hexaester was extracted into the aqueous phase with 175 kg (1340 mol) of phosphoric acid (75%). It was then washed 3 times with 150 kg of toluene. The butyl hexaester was extracted into the toluene phase by diluting with 145 kg of toluene and 165 kg of water, and then alkalized with 30% sodium hydroxide (m / m) to reach a pH of 5-5.5. The lower aqueous phase was removed. Butyl hexaester was obtained by vacuum concentration to dryness at 60° C. with a yield of about 85%.

[0333] Preparation of hexacarboxylic acid having formula (III)

[0334] 113 kg (121 mol) of butyl hexaester was placed in a reactor together with 8 kg of ethanol. The medium was raised to 55 ± 5 ° C, and then 161 kg (1207.5 mol) of 30% sodium hydroxide (m / m) was added within 3 hours. The reaction mixture was kept at this temperature for about 20 hours. Butanol was then removed by decanting the reaction medium. The hexaacid of formula (III) obtained in the form of a sodium salt was diluted with water to obtain an aqueous solution of about 10% (m / m). The solution was treated on an acidic cationic resin. The hexaacid of formula (III) in an aqueous solution was obtained with a yield of about 90% and a purity of 95%.

[0335] Preparation of gadolinium hexahydrate complex having formula (I)

[0336] ■Experimental plan

[0337] ● Complexation and isomerization

[0338] - No acetic acid

[0339] 418 kg (117 kg of pure hexaacid of formula (III) / 196 mol) of a 28% by weight aqueous solution of hexaacid of formula (III) are placed in a reactor. The pH of the solution is adjusted to 2.7 by adding hydrochloric acid, and then 37 kg (103.2 mol) of gadolinium oxide are added. The reaction medium is heated at 100-102° C. for 48 hours to achieve the desired isomer distribution of hexaacid of formula (III).

[0340] -Contains acetic acid

[0341] Gadolinium oxide (0.525 molar equivalents) was suspended at 28.1% by mass in a solution of the hexaacid of formula (III).

[0342] 99-100% acetic acid (50% by mass / pure hexaacid of formula (III)) is poured into the medium at room temperature.

[0343] The medium is heated to reflux and then distilled up to 113° C. by gradually refilling the medium by mass with acetic acid while removing the water. Once a temperature of 113° C. has been reached, sufficient acetic acid is added to reach the starting volume.

[0344] The medium is kept at 113° C. overnight.

[0345] ●Crystallization, recrystallization

[0346] -crystallization

[0347] The hexa-acid gadolinium complex of formula (I) in solution is cooled to 40°C, the primer is added, and the reagents are allowed to contact for at least 2 hours. The product is then isolated by filtration at 40°C and washed with permeate water.

[0348] - Recrystallization

[0349] 180 kg of the previously obtained hexacarboxylic acid gadolinium complex of formula (I) (solid content of about 72%) are suspended in 390 kg of water. The medium is heated to 100° C. to dissolve the product and then cooled to 80° C. to pre-treat by adding a small amount of primer. After cooling to room temperature, the hexacarboxylic acid gadolinium complex of formula (I) is isolated by filtration and drying.

[0350] ●Selective decomplexation

[0351] The dried product is placed in a reactor together with 20° C. permeate water. The mass of water added is equal to twice the theoretical mass of the hexaacid gadolinium complex of formula (I). 30.5% sodium hydroxide (m / m) (6.5 equivalents) is poured into the medium at 20° C. At the end of the addition of NaOH, the medium is kept in contact at 50° C. for 16 hours. The medium is cooled to 25° C. and the product is filtered on a Clarcel pad.

[0352] ■ Content of diastereomeric excess of a mixture containing diastereoisomers I-RRR and I-SSS

[0353] The ratios in which the various isomers of the complex having formula (I) are present in the mixture of diastereoisomers depend on the conditions under which the complexation and isomerization steps are carried out, as can be seen in Table 3 below.

[0354]

[0355] [Table 3]: Content of I-RRR and I-SSS mixtures as a function of complexation / isomerization conditions

[0356] Additional steps of recrystallization and selective decomplexation made it possible to increase the diastereomeric excess of the mixture of I-RRR and I-SSS (see Table 4).

[0357]

[0358] [Table 4]: Diastereoisomer excess content of the mixture containing I-RRR and I-SSS after crystallization / recrystallization / selective decomplexation

[0359] Preparation of the complex having formula (II)

[0360] 90 kg (119 mol) of the hexaacid complex of formula (I) and 650 kg of methanol are placed in a reactor. The mixture is cooled to about 0°C and then 111 kg (252 mol) of a methanol solution of hydrochloric acid (8.25% HCl in methanol) are poured, while maintaining the temperature at 0°C. The reaction medium is allowed to reach room temperature and then stirring is continued for 16 hours. After cooling to 0-5°C, 120 kg (1319 mol) of 3-amino-1,2-propanediol are added. The reaction medium is then heated while methanol is distilled off under vacuum until a temperature of 60-65°C is reached. The concentrate is kept under vacuum at this temperature for 16 hours. At the end of the contact, the medium is diluted with 607 kg of water while cooling to room temperature. The solution of the crude complex of formula (II) is neutralized with 20% hydrochloric acid (m / m). 978.6 kg of a solution having a concentration of 10.3% is thus obtained, representing 101 kg of material. The yield obtained is 86.5%.

[0361] Tests for isomer conversion starting from the complex of formula (II)

[0362] The isomers of the complex of formula (II) were synthesized from the isomeric groups isoA, isoB, isoC and isoD of the hexaacid complex of formula (I) separated by preparative HPLC. Four isomeric groups were separated and then amidated with R and S 3-amino-1,2-propanediol (APD). Eight isomers were thus obtained:

[0363] isoA+APD(R) and isoA+APD(S),

[0364] isoB+APD(R) and isoB+APD(S),

[0365] isoC+APD(R) and isoC+APD(S), and

[0366] isoD+APD(R) and isoD+APD(S).

[0367] Each of these isomers is subjected to conditions that allow the gadolinium hexacarate complex of formula (I) to isomerize.

[0368] Therefore, a pH 3 HCl solution was prepared by diluting 1 mL of 1 N HCl in 1 liter of water. The isomers were added to the pH 3 HCl solution at a concentration of 1 mM. 10 mg of the powder was dissolved in 10 mL of the solution. The eight solutions obtained were heated to 100° C. and then analyzed by HPLC at T0 and T0+23 hours.

[0369] The percent purity measured by HPLC is given in the table below.

[0370]

[0371] The loss of purity is due to chemical degradation of the product (hydrolysis of the amide functionality) due to the conditions resulting from the isomerization reaction.

[0372] The complex of formula (II) obtained according to the process described in EP 1 931 673 cannot be directly isomerized in a clean and selective manner, since the conditions allowing the isomerization of the various compounds lead to a pronounced chemical degradation of the product by hydrolysis of the amide function.

Claims

1. A gadolinium hexaoxide mixture having formula (I): Having a diastereomeric excess of at least 95% of a mixture of isomers I-RRR and I-SSS comprising the formula:

2. The mixture according to claim 1, characterized in that The diastereomeric excess is at least 97%.

3. The mixture according to claim 1, characterized in that The isomers I-RRR and I-SSS are present in the mixture in a ratio between 55 / 45 and 45 / 55.

4. The mixture according to claim 1, characterized in that The diastereomeric excess corresponds to peak 4, characterized by a retention time of about 35.7 minutes, of the HPLC profile obtained using the following HPLC method: ·Waters RP18-250x 4.6mm-5μm column, Analytical conditions: - Sample: 10 mg / mL aqueous solution of a mixture of formula (I), - Column temperature: 25°C, - Sample temperature: 20℃-25℃, - Flow rate: 1.0 mL / min, -Injection volume: 20μL -UV detection: 200nm, Mobile phase gradient by volume: - At 0 min, 1% acetonitrile and 99% 0.1% v / v H2SO4 aqueous solution, - At 10 min, 5% acetonitrile and 95% 0.1% v / v H2SO4 aqueous solution, - At 40 min, 10% acetonitrile and 90% 0.1% v / v H2SO4 in water.

5. A mixture having formula (II): Having a diastereomeric excess of at least 92% of a mixture of isomers II-RRR and II-SSS of the formula:

6. The mixture according to claim 5, characterized in that The diastereomeric excess is at least 94%.

7. The mixture according to claim 5, characterized in that The diastereomeric excess is at least 97%.

8. The mixture according to claim 5, characterized in that Isomers II-RRR and II-SSS are present in the mixture in a ratio between 55 / 45 and 45 / 55.

9. The mixture according to claim 5, characterized in that The diastereomeric excess corresponds to peak 4 of the UHPLC profile, characterized by a retention time of approximately 6.3 minutes, obtained using the following UHPLC method: ·Waters UPLC T3 150x 2.1mm - 1.6μm column, Analytical conditions: - Sample: 2.0 mg / mL aqueous solution of a mixture of formula (II), - Column temperature: 40°C, - Sample temperature: 20℃-25℃, - Flow rate: 0.3mL / min, -Injection volume: 1μL -UV detection: 200nm, Mobile phase gradient by volume: - At 0 min, 1% acetonitrile and 99% 0.0005% v / v H2SO4 aqueous solution, - At 3 min, 5% acetonitrile and 95% 0.0005% v / v H2SO4 aqueous solution, - At 12 min, 10% acetonitrile and 90% 0.0005% v / v H2SO4 in water.

10. A mixture having formula (VIII): Having a diastereomeric excess of at least 90% of a mixture of isomers VIII-RRR and VIII-SSS of the formula: in, R1 is a methyl group.

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