WO2015004029A1 - 18f-labeling of aromatic and heteroaromatic molecules with unprotected carboxylic acid groups - Google Patents

Abstract

The present invention provides a method for the ¹⁸F labeling of aromatic or heteroaromatic compounds containing an unprotected carboxylic acid group to yield labeled molecules which can be used as radiotracers. In addition, the invention provides precursor compounds containing an unprotected carboxylic acid group which can be used in such a method.

Description

F-Labeling of aromatic and heteroaromatic molecules with unprotected carboxylic acid groups

The present invention provides methods for the nucleophilic aromatic and heteroaromatic radiofluorination of molecules with aliphatic or aromatic carboxylic acid groups.

Scheme 1 : Exemplary structures for aromatic or heteroaromatic compounds for n.c.a. nucleophilic ¹⁸F-fluorination (n > 0) wherein X may be a group CH or a heteroatom, and Y may be CH₂, substituted CH or a heteroatom.

Molecules which are labeled by radioisotopes play an important role as biomarkers in Positron Emission Tomography (PET). Radioisotopes widely used in PET imaging are ¹¹C, ¹⁸F or ⁶⁸Ga, to mention only a few. Among these, fluorine-18 turned out to have several advantages. However, the direct introduction of ^1SF into appropriate targeted molecules is often challenging due to the presence of functional groups, such as phenol, amide, amine or carboxylic acids which are considered to lower the radiochemical yields or even avoid direct nucleophilic radiofluorination on the no-carrier-added level. Thus, nucleophilic aromatic and heteroaromatic radiofluorination of molecules bearing such functional groups is commonly only performed on protected precursors. Often, such molecules are labelled with fluorine-18 via time-consuming and laborious multi-step reactions including the deprotection of these functional groups after radiofluorination. Thus, e.g. current production methods for n.c.a. (no carrier added) ¹⁸F-labeled amino acids carrying ¹⁸F on an aromatic ring, e.g. 6-[¹⁸F]-fluoro-L- dopa (F-DOPA) and L-2-[¹⁸F]fluorophenylalanine via nucleophilic aromatic substitution rely on multi step syntheses comprising at least two steps, i.e. the ¹⁸F-fluorination of protected precursors and subsequent deprotection. In view of the half-life of ⁸F of about 1 10 minutes, fast and simple synthetic strategies are required that can improve the availability and facilitate the automation of a synthesis for such compounds, i.e. under GMP conditions.

The present invention provides a method for the nucleophilic aromatic or heteroaromatic ¹⁸F labeling of molecules with aliphatic and aromatic carboxylic acids. Tracers and radiopharmaceuticals produced by the described method made accessible by the invention can be used for non-invasive depiction and quantitation of biochemical targets in animals or humans.

In particular, the invention provides, in a first aspect, a method for the radiofluorination of an aromatic or heteroaromatic compound containing a carboxylate group, comprising the steps of: a) providing a precursor compound comprising an aromatic or heteroaromatic ring, said aromatic or heteroaromatic ring bearing a leaving group for nucleophilic aromatic substitution reactions as a substituent L, such as nitro, bromo, chloro, fluoro, trialkylamino, etc., and at least one further substituent which is selected from a carboxylate group -COO^" and a saturated or unsaturated aliphatic group or an aromatic group, which aliphatic or aromatic group carries at least one carboxylate group -COO^", wherein the carboxylate group -COO^" is in the form of a carboxylate salt with a cation selected from (i) a cationic chelate composed of a metal cation and a chelating agent such as a cryptand or a crown ether, and (ii) a quaternary ammonium cation; b) reacting the precursor compound containing at least one carboxylate group -COO^" in the form of a carboxylate salt with a cation selected from a cationic chelate composed of a metal cation and a chelating agent and a quaternary ammonium cation, with a ¹⁸F^" anion in the presence of a phase transfer catalyst, such as typically used cryptates (kryptofix, (K/2.2.2)⁺, a quaternary alkyl ammonium ion etc., to replace the leaving group of the aromatic or heteroaromatic ring with a ¹⁸F substituent, to yield an aromatically or heteroaromatically radiofluorinated compound which comprises an aromatic or heteroaromatic ring, said aromatic or heteroaromatic ring bearing the ¹⁸F substituent at the position held by the leaving group in the precursor compound, and at least one further substituent which is selected from a carboxylic acid group and a saturated or unsaturated aliphatic group or an aromatic group, which aliphatic or aromatic group carries at least one carboxylic acid group, wherein the carboxylic acid group may be in the form of a free acid or in the form of a carboxylate salt as defined above. Moreover, the invention provides, in a further aspect, a precursor compound comprising an aromatic or heteroaromatic ring, said aromatic or heteroaromatic ring bearing a leaving group for nucleophilic aromatic substitution reactions as a substituent L, such as nitro, bromo, chloro, fluoro, trialkylamino, etc., and at least one further substituent which is selected from a carboxylate group -COO^" and a saturated or unsaturated aliphatic group or an aromatic group, which aliphatic or aromatic group carries at least one carboxylate group -COO^", wherein the carboxylate group -COO^" is in the form of a carboxylate salt with a cation selected from (i) a cationic chelate composed of a metal cation and a chelating agent, such as a cryptand or a crown ether, and (ii) a quaternary ammonium cation.

Preferably, the precursor compound provided by the invention is not a carboxylate salt of 5- methyl-isophthalic acid with a cationic chelate of Cs⁺ and 18-crown-6, and not a carboxylate salt of 5-methoxy-isophthalic acid with a cationic chelate of Cs⁺ and 18-crown-6. In particular, it is preferred that the precursor compound provided by the invention is not a monocarboxylate salt of 5-methyl-isophthalic acid with a cationic chelate of Cs⁺ and 18- crown-6 (i.e. the 5-methyl-isophthalic acid mono-(Cs⁺/18-crown-6)-salt), and not a monocarboxylate salt of 5-methoxy-isophthalic acid with a cationic chelate of Cs⁺ and 18- crown-6 (i.e. the 5-methoxy-isophthalic acid mono-(Cs l 8-crown-6)-salt).

It is widely accepted and state-of-the-art in radiochemistry that introduction of ⁸F-fluoride by n.c.a nucleophilic aromatic substitution into molecules carrying a carboxylic acid group can only be performed using precursors wherein the carboxylic acid group is protected by a suitable protecting group. It has been unexpectedly found that aromatic and heteroaromatic molecules with a suitable leaving group L for nucleophilic ¹⁸F-for-L substitution carrying one or more unprotected aromatic or aliphatic carboxylic acid group(s) can be subjected to direct radiofluorination if the fluorination reaction is carried out as a nucleophilic aromatic substitution reaction wherein the precursor contains certain salt(s) of the carboxylic acid(s).

In the context of the present invention, the term "radiofluorination" refers to n.c.a. ⁸F-labeling of an aromatic or heteroaromatic compound by a reaction which leads to the formation of a covalent bond between the ¹⁸F isotope and the compound to be labeled. The corresponding term "radiofluorinated" thus refers to the state of a compound in which an ¹⁸F isotope is covalently bound within the compound. Advantageously, the method is a no-carrier-added synthesis, i.e. no unlabelled (i.e. non-radioactive or "cold") fluorine containing compound is added as a carrier.

The term "precursor compound" is used in the context of the invention to refer to a compound which is intended for radiofluorination and comprises a suitable leaving group L, such as nitro, bromo, chloro, trialkyl amino etc, together with a carboxylate group in the form of a salt with a cation selected from a cationic chelate composed of a metal cation and a chelating agent such as a cryptand (such as the potassium cryptate of kryptofix 222, [K/2.2.2]⁺), or a crown ether (such as K⁺/18-crown-6), and a quaternary ammonium cation (e.g. ⁺NR'_4l wherein the R' groups may be the same or different alkyl or aryl groups, such as tetrabutylammonium). In view of the fact that the ¹⁸F isotope has a half life of about 1 10 min, a precursor compound or composition is generally prepared and optionally stored in a form ready for reaction with the ¹⁸F fluoride anion.

In the context of the present invention, the precursor compound to be subjected to n.c.a. radiofluorination comprises a combination of an aromatic or heteroaromatic ring with an unprotected carboxylate group in the form of a salt as defined above. The aromatic or heteroaromatic precursor compound thus comprises a substituent L which represents a leaving group and a substituent R which bears or represents the carboxylic acid group in the form of a carboxylate salt as defined above. In addition to the first substituent L and the second substituent R, further substituents may be present, as stability and the number of available valencies permits. This includes the possibility that the aromatic or heteroaromatic ring comprises more than one substituent R.

Thus, the radiofluorinated compound as a target compound of the method in accordance with the invention, which is obtainable by a radiofluorination reaction of the precursor compound, thus comprises a substituent R which bears or represents a carboxylic acid group, a substituent ¹⁸F, and optionally further substituents, as stability and the number of available valencies permits. All these substituents are attached to the aromatic or heteroaromatic ring. As will be understood, the substituent ¹⁸F in the radiofluorinated target compound replaces the leaving group L of the precursor compound.

The aromatic or heteroaromatic ring is preferably a monocyclic ring or an annellated bi- or trycyclic ring system having 5 to 14 ring members. More preferred is a monocyclic ring or a bicyclic ring. The aromatic or heteroaromatic ring fulfills the requirements of aromaticity, i.e. it possesses a delocalized ττ-electron system. Heteroatoms in the heteroaromatic ring are preferably selected from N, O or S, and it is particularly preferred that a heteroaromatic ring contains only nitrogen atoms as heteroatoms. The number of heteroatoms in the heteroaromatic ring or ring system lies typically between 1 and 4, preferably between 1 and 2, and is most preferably 1. Particularly preferred as aromatic or heteroaromatic rings are benzene, pyridine, naphthalene, quinoline, isoquinoline, anthracene, acridine, pyrimidine, and purine. Most preferred is a benzene ring or a pyridine ring.

The substituent R comprises one or more carboxylate groups. This encompasses the possibility of the substituent R being a) an unprotected carboxylate group directly attached to the aromatic or heteroaromatic ring or b) one or more unprotected carboxylate groups located in an aliphatic group R which may be saturated or unsaturated, or an aromatic group R as a side chain of the aromatic or heteroaromatic ring, including the aliphatic residues of aromatic amino acids derivatives. The substituent R as a side chain of the aromatic or heteroaromatic ring may also comprise more than one, e.g. two carboxylic acid groups.

In the precursor compound which is reacted with an ¹⁸F^" anion, the carboxylate group(s) represents a group -COO^" in its deprotonated anionic form, with a counter-ion as defined above. In the radiofluorinated target compound, the carboxylate group(s) may be present as the corresponding protonated carboxylic acid or in the depotonated anionic form, depending on the conditions, especially the pH of a surrounding solution. For the skilled reader, it will be apparent in which form the carboxylic acid group will preferably exist under defined conditions, i.e. under physiological conditions (pH ca. 7). However, unless indicated otherwise in any specific context, the terms "carboxylate group" and "carboxylic acid group" do not encompass a protected carboxylic acid group, e.g. the case where it has been transformed into an ester. As noted above, it is an important aspect of the invention that the carboxylate group(s) in the aromatic or heteroaromatic molecule used as a precursor compound in the method of the invention is (are) present in an unprotected form during the reaction with the ¹⁸F^" anion.

In addition to the carboxylate group -COO^", the core aromatic or heteroaromatic ring as well as the substituent R of the precursor compound may comprise other chemical groups. Preferably, the substituent R represents either a carboxylate group -COO^" as such, or a linear or branched alkyl or alkenyl group which is substituted with a carboxylate group -COO^" . The alkyl or alkenyl group may be interspersed with one or more moieties independently selected from -NH-, -N(CH₃)-, -0-, -S-, -S(O)-, -S(0)₂-, -C(0)-0-, -O-C(O)-, -C(0)-NH-, -NH- C(O)-, or -C(O)- and a phenylene group. Furthermore, it may carry one or more substituents e.g. selected from-NR⁷R⁸, -CONR⁷R⁸, -COR⁷, -S0₂NR⁷R⁸, -NR⁷COR⁸, -NR⁷S0₂R⁸, cycloalkyl, aryl, aryloxy, and heterocyclyl, wherein each R⁷ and each R⁸ is independently selected from hydrogen or C1 -6 alkyl. It will be understood that the carboxylate group may represent a terminal group in a linear substituent R, and may represent one of several groups in a branched group R. In any case, the carboxylate group may be present in the proximity of the aromatic or heteroaromatic ring or distanced, and separated from the aromatic or heteroaromatic ring to which L is attached.

The substituent L represents a leaving group. In the context of the invention, such a leaving group is a group which can be replaced by an ¹⁸F^" fluoride anion in a nucleophiiic aromatic substitution reaction. Exemplary leaving groups, which are suitable for this purpose, can be selected from a nitro group, chloro, bromo, iodo, and -N(R⁹)₃ ⁺, with R⁹ being selected, independently for each occurrence, from C1 -6 alkyl. A fluorine (i.e. ¹⁹F) substituent can also act as a leaving group. However, this is not preferred due to the fact that the equilibrium between the ¹⁸F and the ¹⁹F substituents may not allow an optimum specific activity in the product to be obtained. Preferred leaving groups are selected from a nitro group, chlorine, bromine and -N(CH₃)₃ ⁺. Particularly preferred are the nitro group or the group -N(CH₃)₃ ⁺.

The precursor compound may comprise one substituent L or more than one, such as two, substituents L as activator. However, it is generally preferred if one substituent L is attached to the aromatic or heteroaromatic ring of the precursor compound independently of the position of the carboxylate group.

As noted above, further substituents apart from the substituents R and L may be contained in the precursor compound as far as free valencies are available. Exemplary optional substituents are selected from -NR ⁰R¹¹, -OH, alkoxy, in particular methoxy, alkyl, in particular methyl, aryloxy, in particular phenoxy, -CF₃, phenyl, aralkyl, in particular benzyl, - COR¹⁰, and heterocyclyl, wherein each R¹⁰ and R ¹ is independently selected from hydrogen or C1 -6 alkyl. Preferred optional substituents are electron withdrawing groups, such as COR¹⁰. For example the precursor may comprise one or more other substituents e.g. an electron withdrawing group or donor group, protic or aprotic group etc. which may or may not influence radiofluorination.

In the context of the present invention it was found to be favourable when all substituents of the aromatic or heteroaromatic precursor compound generate an overall electron withdrawing effect, thus activating and facilitating the aromatic nucleophiiic substitution of L by ¹⁸F-fluoride.

Furthermore, it was found that the precursor compound in the method in accordance with the invention may comprise a free (i.e. unprotected) amino group -NH₂ or a mono or dialkylated amino group, e.g. as a part of R or of a further substituent attached to the aromatic or heteroaromatic ring of the aromatic carboxylic acid. Unexpectedly, the presence of such an additional functional group did not have a negative impact on the nucieophilic aromatic substitution.

As used herein, "alky!" represents a straight or branched chain saturated hydrocarbon residue which does not comprise any carbon-to-carbon double bonds or carbon-to-carbon triple bonds. As exemplary groups, methyl, ethyl, propyl and butyl are mentioned.

As used herein, "alkenyl" represents a straight or branched chain unsaturated hydrocarbon residue comprising one or more than one (such as two or three) carbon-to-carbon double bond(s) which does not comprise any carbon-to-carbon triple bonds.

As used herein, "aryl" represents an aromatic hydrocarbon ring, in particular a 6 to 10 membered ring (unless a different number of ring members is indicated in a specific context), including bridged ring or fused ring systems containing at least one aromatic ring. Preferred as aryl groups are monocyclic groups with 6 ring members or fused bicyclic groups with 9 or 10 ring members. Thus, a generally preferred embodiment of "aryl" is phenyl.

As used herein, "aralkyl" represents an alkyl group as defined above, wherein at least one hydrogen atom is replaced by an aryl group. A preferred embodiment is a benzyl group.

As used herein, "cycloalkyl" represents a saturated hydrocarbon ring, preferably a 3-1 1 membered ring (unless a different number of ring members is indicated in a specific context), including bridged ring, spiro ring or fused ring systems. "Cycloalkyl" may, for example, refer to cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, or cycloheptyl. Preferred as cycloalkyl groups is a monocyclic group with 5 or 6 ring members.

As used herein, a "heterocyclic group" or "heterocycle" represents a ring containing carbon atoms and one or more (such as, e.g., one, two, or three) heteroatoms independently selected from O, S, and N as ring members, preferably a 3-14 membered ring, including bridged ring, spiro ring or fused ring systems. The ring members may be linked by single bonds or double bonds, including aromatic bonds. Preferred are monocyclic groups with 5 or 6 ring members. It may, for example, refer to thienyl (thiophenyl), benzo[b]thienyl, naphtho[2,3-b]thienyl, thianthrenyl, furyl (furanyl), isobenzofuranyl, chromenyl, xanthenyl, phenoxathiinyl, pyrrolyl (including, without limitation, 2H-pyrrolyl), imidazolyl, pyrazolyl, pyridyl (pyridinyl; including, without limitation, 2-pyridyl, 3-pyridyl, and 4-pyridyl), pyrazinyl, pyrimidinyl, pyridazinyl, indolizinyl, isoindolyl, indolyl (including, without limitation, 3H-indolyl), indazolyl, purinyl, isoquinolyl, quinolyl, phthalazinyl, naphthyridinyl, quinoxalinyl, cinnolinyl, pteridinyl, carbazolyl, beta-carbolinyl, phenanthridinyl, acridinyl, phenanthrolinyl (including, without limitation, [1 , 10]phenanthrolinyl, [1 ,7]phenanthro-linyl, and [4,7]phenanthrolinyl), phenazinyl, isothiazolyl, phenothiazinyl, oxazolyl, isoxazolyl, furazanyl, phenoxazinyl, pyrazolo[1 ,5-a]pyrimidinyl (including, without limitation, pyrazolo[1 ,5-a]pyrimidin-3-yl), 1 ,2- benzoisoxazol-3-yl, benzimidazolyl, oxetanyl, tetrahydrofuranyl, piperidinyl, piperazinyl, aziridinyl, azetidinyl, pyrrolidinyl, imidazolidinyl, morpholinyl, pyrazolidinyl, tetrahydrothienyl, octahydroquinolinyl, octahydroisoquinolinyl, oxazolidinyl, isoxazolidinyl, azepanyl, diazepanyl, oxazepanyl or 2-oxa-5-aza-bicyclo[2.2.1 ]hept-5-yl.

In view of the above, preferred structures of the aromatic or heteroaromatic compound which can be used as a precursor compound in the context of the invention can be illustrated by the following formulae (I) and (II),

wherein R and L are as defined above, including preferred embodiments; X is CH or N; R¹ is independently selected, for each occurrence, from -NR¹⁰R¹¹ , alkoxy, in particular methoxy, aryloxy, alkyl, in particular methyl, -CF₃, phenyl, and -COR¹⁰, wherein each R¹⁰ and R¹ is independently selected from hydrogen or C1 -6 alkyl, and is preferably selected from alkoxy, in particular methoxy, and -COR¹⁰; and n is an integer from 0 to 3, and is preferably 0, 1 or 2. In line with the common understanding in the art, the bond crossing the C-C-bond of the six membered ring to which R¹ is attached indicates that R can replace any remaining H-atom bound to a ring member at any position.

More preferred as aromatic or heteroaromatic compound which can be used as a precursor compound in the context of the invention are the compounds of the following formulae (III) to

(VII),

wherein L is defined as above, including preferred embodiments; X is CH or N; and R² to R⁶ are independently selected from H and alkoxy, in particular H and methoxy; and the carboxylate group -COO^" is in the form of a carboxylate salt as defined above.

Further preferred as aromatic carboxylic acid for use in the context of the present invention are compounds (VIII) to (XIII),

wherein L is defined as above, including preferred embodiments, and X is CH or N; and the carboxylate group -COO^" is in the form of a carboxylate salt as defined above.

The aromatic or heteroaromatic compounds used as precursor compounds in the context of the invention, and consequently also the radiofluorinated target compounds, can contain optically active centres, and may thus exist in the form of different stereoisomers (including enantiomers and diastereomers). All such isomers are contemplated for use in the context of the present invention, either in admixture or in pure or substantially pure form. In particular, mixtures (such as racemic forms) and the isolated optical isomers of the aromatic or heteroaromatic compounds may be used in the context of the invention. The racemic forms can be resolved by physical methods, such as, e.g., fractional crystallization, separation or crystallization of diastereomeric derivatives or separation by chiral column chromatography. The carboxyiic acid group(s) in the precursor compound for use in accordance with the invention are in the form of a carboxylate salt with a cation selected from (i) a cationic chelate composed of a metal cation and a chelating agent, such as a cryptand (e.g. as [K/2.2.2]⁺), or a crown ether (e.g. K7l 8-crown-6), and (ii) a quaternary ammonium cation (e.g. ⁺NR'₄. wherein the R' groups may be the same or different alkyl or aryl groups, such as tetrabutylammonium). The different cations may be used singly or in combination. Among them, the cryptate cations are preferred.

Preferred cationic chelates composed of a metal cation and a cryptand comprise an alkali metal cation, in particular a Li, Na or K cation, or an alkaline earth metal cation, in particular a Mg or Ca cation, complexed by a cryptand. More preferred are monovalent cations, and the most preferred cation is K⁺.

Cryptands are azapolyethers, wherein two nitrogen atoms as bridgeheads are linked by three bridges, each of which contains one or more ether bonds. A well known and preferred cryptand for use in the context of the invention is cryptand [2.2.2] (i.e. 1 ,10-diaza- 4,7, 13, 16,21 ,24-hexaoxabicyclo[8.8.8]hexacosane). The numbers 2.2.2 indicate the number of oxygen atoms in each of the bridges. It is commercially available under the name "Kryptofix®". Particularly preferred carboxylate salts are salts with K⁺ cations complexed by cryptand [2.2.2], which are also referred to as [K⁺c 2.2.2] or [K/2.2.2]⁺, as counter-ions.

Preferred ammonium cations as counter-ions for the carboxylate salts in the precursor compound for use in the context of the invention are formed with one or more types of the ammonium cation of the following structure:

wherein R^a to R are independently selected from a linear or branched alkyl group or phenyl. Preferably, R^a to R^d are methyl or butyl and are preferably methyl.

A precursor compound containing a carboxylate group for use in accordance with the invention can be prepared, e.g., via the addition of a salt selected from a salt of a cationic chelate, such as a cryptate salt or a crown ether salt, and an ammonium salt to a compound comprising an aromatic or heteroaromatic ring, said aromatic or heteroaromatic ring bearing a leaving group for nucleophilic aromatic substitution reactions as a substituent L and at least one further substituent which is selected from a carboxyiic acid group and a saturated or unsaturated aliphatic or an aromatic group, which aliphatic or aromatic group carries at least one carboxylic acid group, (also referred to herein as "aromatic or heteroaromatic compound with a carboxylic acid group"). The carboxylic acid may be in free form or in the form of an inner salt, e.g. if the aromatic or heteroaromatic compound comprises a basic group, such as an amino group). In view of the fact that the precursor compound used in the context of the invention is to be subjected to a nucleophilic substitution reaction, the anion which is present in salt selected from a salt of a cationic chelate, such as a cryptate salt or a crown ether salt, and an ammonium salt which may be used in the preparation of a precursor compound is preferably selected such that it does not have strong nucleophilic properties. Such ions are known to the skilled person, and can be selected, e.g., from oxalate ions, carbonate ions, hydrogen carbonate ions and trifluoromethanesulfonate (triflate) ions, or combinations thereof.

The aromatic or heteroaromatic compound with a carboxylic acid group and a salt selected from a salt of a cationic chelate and an ammonium salt may be combined in a solvent. If desired, e.g. in order to facilitate the storage of the precursor compound, the solvent can be subsequently removed. Alternatively, the precursor compound can be subjected to the reaction with the ¹⁸F^" anion without being isolated as a solid intermediate. Suitable solvents are in particular dry polar, non-protic solvents, such as acetonitrile, or DMSO, or mixtures of such solvents.

Thus, the precursor compound for use in the context of the present invention may be in the form of a solid compound, or in the form of a precursor composition including, e.g. a dry solvent, in particular a polar, non-protic solvents, such as acetonitrile or DMSO.

The aromatic or heteroaromatic compound with a carboxylic acid group, and the salt selected from a salt of a cationic chelate and an ammonium salt are typically combined in amounts such that the formal charge of the cations in the salt selected from a salt of a cationic chelate and an ammonium salt balances at least the number of carboxylic acid groups provided by the aromatic or heteroaromatic compound with a carboxylic acid group. Typically, an excess of the salt selected from a salt of a cationic chelate and an ammonium salt is used, e.g. 1 to 10, preferably 5 to 10, and most preferably 4 to 8 equivalents of salt. In a preferred case, where the salt contains a monovalent cation, the molar ratio of carboxylic acid groups in the aromatic or heteroaromatic compound with a carboxylic acid group to the salt selected from a salt of a cationic chelate and an ammonium salt ranges from 1 to 10, preferably from 5 to 10, and more preferably from 4 to 8. Thus, in the combination of the aromatic or heteroaromatic compound with a carboxylic acid group with a salt selected from a salt of a cationic chelate and an ammonium salt, the anions of the salt(s) can act as basic compounds to deprotonate the carboxylic acid groups, whereas the chelated cations and/or ammonium cations act as counterions for the carboxylate groups which are formed. In other words, the combination of the aromatic or heteroaromatic compound with a carboxylic acid group with a salt selected from a salt of a cationic chelate and an ammonium salt contains a salt formed from carboxylate anions of the aromatic or heteroaromatic compound with a carboxylic acid group and cationic chelates and/or ammonium cations, optionally in combination with an excess of the salt selected from a salt of a cationic chelate and an ammonium salt in an equilibrium. The aromatic or heteroaromatic compound with a carboxylic acid group can be conveniently combined with a salt selected from a salt of a cationic chelate and an ammonium salt at room temperature.

The ¹⁸F^" fluoride anion can be prepared by methods known in the art using a nuclear reaction of ¹⁸0 atoms (generally by irradiation of highly enriched [ ⁸0]H₂0 with protons (¹⁸0(p,n) ⁸F)) induced by cyclotron irradiation. In accordance with a preferred approach established in the art, the ¹⁸F^" fluoride is provided after irradiation and post-processing (anion exchange extraction and purification of ¹⁸F-fuoride from the target water) as a salt with a cation of a phase transfer catalyst. Particularly preferably, but not solely, the cation is selected from the same cations disclosed above as suitable cations for the cryptate salt or the quaternary ammonium salts. Particularly preferred cations for the ¹⁸F^" fluoride are selected from complexes of K⁺ and cryptand [2.2.2], and tetramethylammonium hydroxide, carbonate or hydrogencarbonate. It will be understood that any reference to the addition of the ¹⁸F^" fluoride to another reactant, to the combination of the ¹⁸F^" fluoride with another reactant, etc., implies the presence of such a counterion. However, since the use of such ¹⁸F^" fluoride salts for nucleophilic substitution reactions is well established in the art, and since the ¹⁸F^" fluoride anion plays the more important role of a reactant in the context of the method in accordance with the invention, the cation is not always mentioned.

Typically and preferably, the ¹⁸F^" fluoride anion used in the method of the invention is no carrier added ("n.c.a.") ¹⁸F^" fluoride, i.e. the fluoride is essentially free of intentionally added cold (¹⁹F) fluoride, resulting in specific activities of >37 GBq/μιηοΙ, preferably >1 1 1 GBq/pmol, most preferably >370 GBq/pmol.

In order to react the precursor compound with ¹⁸F^" fluoride, the precursor compound and the ¹⁸F^" fluoride typically react in homogeneous solution. Typically, a non protic, polar solvent such as DMSO, DMF or acetonitrile is used. Preferred are solvents or solvent mixtures with a high boiling point, e.g. a boiling point above 150 °C. The reaction is preferably carried out at temperatures between 80 and 180 °C, in particular from 100 to 150 °C. The reaction time generally ranges from 1 min to 60 min, preferably from 3 min to 30 min, and in particular from 5 min to 20 min.

The ⁸F^" fluoride is typically used in amounts 10 MBq to 400 GBq, preferably 370MBq to 100 GBq of ¹⁸F fluoride to be introduced into the aromatic or heteroaromatic compound with carboxylic acid.

The reaction between the precursor compound and the ¹⁸F^~ fluoride in the context of the method of the present invention is carried out in the presence of a phase transfer catalyst. Conventional phase transfer catalysts can be used for this purpose, including typically used cryptates, quaternary alkyl ammonium compounds, etc. Examples are ([K⁺c 2.2.2])₂C0₃, [K⁺ C 2.2.2]0H, [K⁺ C 2.2.2]HC0₃, or ([K⁺ C 2.2.2])₂C₂0₄, wherein [K⁺ C 2.2.2] designates a potassium cation complexed by cryptand [2.2.2].

The phase transfer catalyst can be used e.g. in amounts of 5-50 mg in a volume of the reaction of 500 to 1500 μΙ, preferably 10-30 mg, and most preferably 5-30 mg.

The radiofluorination method in accordance with the invention is advantageously a no- carrier-added method, i.e. no unlabelled (i.e. non-radioactive or "cold") fluorine containing compound is intentionally added as a carrier for the labeling of the precursor compound. Also, it is not necessary to add any compounds apart from the precursor composition and the ¹⁸F^" anion together with a suitable cation as described above to the reaction mixture in a solvent as described above in order to promote the reaction, such as a transition metal complex.

In a preferred embodiment of the method in accordance with the invention, the aromatic or heteroaromatic compound with a carboxylic acid group and the salt selected from a salt of a cationic chelate and an ammonium salt are first combined in a solution, and subsequently the solvent is removed to prepare a solid precursor compound. For the reaction of the precursor compound with the ¹⁸F^" anion, the precursor compound can be dissolved again, and can be reacted in a further step in the solution with the ¹⁸F^" anion in the presence of a phase transfer catalyst. Examples

The invention will be further illustrated in the following by means of examples which are not intended to impose any limitations on its scope.

General procedure for the preparation of ¹⁸F-fluoride from target irradiated ⁸0-H₂0 for nucleophilc substitution:

Target water containing ¹⁸F-fluoride prepared by irradiation of [¹⁸0]H₂0 is loaded onto a solid phase extraction cartridge, i.e. an anion exchange cartridge (e.g. QMA cartridges from Waters). The excess of water is removed by a stream of gas (e.g. air, oxygen or nitrogen). Optionally, the cartridge is subsequently purged with deionized water (e.g. ultrapure water). The elution of ¹⁸F-fluoride from the cartridge is carried out by using a water/acetonitrile mixture (with a minimum of 5% water) containing carbonate, hydrogencarbonate, hydroxide or other commonly and less commonly used anions to exchange with ¹⁸F-fluoride on the solid phase matrix of the extraction cartridge and thus used to mobilize and elute ¹⁸F-fluoride from the cartridge. To the eluates formed thereby, a phase transfer catalyst (PTS) is added, such as the commonly used PTCs e.g. kryptofix 2.2.2, or tetraalkylammonium salts (carbonates, hydrogencarbonates, hydroxides etc.) followed by azeotropic drying of the mixture. Often, azeotopic drying is repeated one to three times. By this procedure, ¹⁸F-fluoride is converted into salts, such as [K⁺C 2.2.2]¹⁸F in presence of ([K⁺C 2.2.2])₂C0₃, [K⁺C2.2.2]HC0₃, [K⁺ c2.2.2]OH, [K⁺/c2.2.2]OTf or ([K⁺C 2.2.2])₂C₂0₄, that are soluble in polar organic solvents, such as MeCN, DMF, fBuOH or DMSO, to mention only a few.

General procedure for the formation of cryptates and ammonium salts:

([K⁺ c2.2.2])₂C₂0₄ is prepared by addition of a minimum of 2, e.g. 2.4 equivalents of kryptofix 2.2.2 to one equivalent of K₂C₂0₄ in water/MeCN (50 vol%/ 50 vol%). The solvent is evaporated under reduced pressure and freeze dried.

([K⁺ c2.2.2])₂C0₃ is prepared by addition of a minimum of 2, e.g. 2.4 equivalents of kryptofix 2.2.2 to one equivalent of K₂C0₃ in water/MeCN (50 vol%/ 50 vol%). The solvent is evaporated under reduced pressure and freeze dried.

[K⁺ 2.2.2]0H is prepared by addition of a minimum of 2, e.g. 2.4 equivalents of kryptofix 2.2.2 to one equivalent of KOH in water/MeCN (50 vol%/ 50 vol%). The solvent is evaporated under reduced pressure and freeze dried. [K⁺c2.2.2]HC0₃ is prepared by addition of a minimum of 1 , e.g. 1.2 equivalents of kryptofix 2.2.2 to one equivalent of KHC0₃ in water/MeCN (50 vol%/ 50 vol%). The solvent is evaporated under reduced pressure and freeze dried.

[K⁺ 2.2.2]OTf is prepared by addition of a minimum of 1 , e.g. 1 .2 equivalents of kryptofix 2.2.2 to one equivalent of KOTf in water/MeCN (50 vol%/ 50 vol%). The solvent is evaporated under reduced pressure and freeze dried.

NMe₄OTf is prepared by addition of a minimum of 1 , e.g. 1.2 equivalents of N e₄HC0₃ to one equivalent of KOTf in water/MeCN (50 vol%/ 50 vol%). The solvent is evaporated under reduced pressure and freeze dried.

Other ammonium salts are prepared according to the above described procedures. General procedure for the formation of a precursor composition:

The aromatic or heteroaromatic compound with a carboxylic acid group is added as free acid or, in the case of the use of amino acids, as inner salts to a minimum of 1 equivalent, typically 4-8 equivalents of the cryptates or ammonium salts e.g. ([K⁺ 2.2.2])₂C₂0₄, ([K⁺ c 2.2.2])₂C0₃ or combinations thereof as described above in solvent, typically in acetonitrile. The solvent is evaporated and the remaining substance carefully dried. The precursors prepared according to this method are stored under inert gas until they are used for ¹⁸F-fluorination.

Example 1 : N.c.a. nucleophilic ¹⁸F-fluorination of the precursor composition Tirofiban analogue 4 (scheme 2 and scheme 3)

N.c.a. ¹⁸F-fluorination of analogs of the precursor composition tirofiban by nucleophilic ¹⁸F- fluorination of the unprotected precursor compounds 4 and 7 (scheme 2) was carried out by nucleophilic substitution of the nitro group by [K⁺ C 2.2.2]¹⁸F^" of the precursor composition of 4 prepared following above mentioned procedure for the formation of a precursor composition with [K⁺ C 2.2.2]C0₃ or [K⁺ C 2.2.2]C0₃ and ([K⁺ C 2.2.2])₂C₂0₄ in CH₃CN, iBuOH/CH₃CN and DMSO at a temperature 100-150 °C for 10-60 min. Radiofluorination the precursor composition of 4 with [K⁺ C 2.2.2]C0₃ and ([K⁺ C 2.2.2])₂C₂0₄ by n.c.a. [K⁺ C 2.2.2]¹⁸F^" in DMSO at 150 °C for 20 min yielded the desired ¹⁸F-labeled product [¹⁸F]5 with radiochemical yields of 19.0 ± 7.4%. The ¹⁸F-labeling of the similarly prepared precursor composition of 7 with [K⁺ C 2.2.2]C0₃ and ([K⁺ c 2.2.2])₂C₂0₄ (N-Boc protected analogue compound) to yield

[¹⁸F]10 was performed with 10.0 ± 1 .6% radiochemical yields (not optimized condition). The direct one-step ¹⁸F-labeling of the the precursor composition of totally unprotected precursor 4 afforded [¹⁸F]5 was used also for further evaluation of the integrin tracer in vivo and in vitro. Here and in the following, [K⁺ C 2.2.2] indicates a K⁺ cation complexed by cryptand [2.2.2].

Scheme 2 shows the synthesis of para-nitro substituted precursors 3, 4 and 7 and ¹⁹F- reference compounds 5 and 6. Compound 1 was synthesized according to literature procedures (e.g. Bollinger et al. , J. Med. Chem. 2012, 55, 871 -882; Duggan et al. , J. Med. Chem. 2000, 43, 3736-3745). a) 1 . HATU, DIEA, DMF, RT (room temperature, 20 °C), 16 h; 2. 20% piperidine/DMF, RT, 2 h; b) 4-Nitrobenzenesulfonyl chloride, DIEA, DCM, RT, 5 h; c) HCI_(aq), dioxane, RT, 1 h; d) 1. 4-Fluorobenzenesulfonyl chloride, DIEA, DCM, RT, 5 h; 2. HCI_(aq₎, dioxane, RT, 1 h; e) 1 . HATU, 4-Fluorobenzoic acid, DIEA, DMF, RT, 16 h; 2. HCI_(aq), dioxane, RT, 1 h; f) 1 . 4-Nitrobenzenesulfonyl chloride, DIEA, DCM, RT, 5 h; 2. TFA, DCM, RT, 1 h; g) 1 . HATU, 7, DIEA, DMF, RT, 16 h; 2. LiOH, methanol, H₂0, RT, 16 h. All compounds were purified by RP-HPLC .

Scheme 3 illustrates the radiosynthesis of ¹⁸F-labeled compounds [¹⁸F]5 and [¹⁸F]10: a) 10μ mol 4 or 7 as their precursor composition (with 6 μιτιοΙ [K⁺C 2.2.2]C0₃ and 12 pmol [K⁺C 2.2.2]C₂0₄) in 150 μΙ DMSO (150 °C, 20min) in the presence of [K⁺C 2.2.2]¹⁸F^" .

Scheme 3

The radiofluorination of precursor composition of 4 was studied using different amounts of precursor at a reaction temperature of 150 °C. After 20 min reaction time the mixture was analyzed by HPLC (cf. Fig.1 ). 5 mg of precursor gave the highest radiochemical yield. Furthermore, the radiofluorination of 5 mg of 4 analysed after reaction times of 10-60 min by radio thin layer chromatography (TLC) revealed an optimal radiochemical yield after 20 min under these reaction conditions (cf. Fig. 2).

As a result, direct labeling of the precursor composition of unprotected precursor 4 with a carboxylic acid present as its [K⁺C 2.2.2]salt -and in this case also bearing an unprotected amino group (scheme 3) yielded up to 26 % [¹⁸F]5 depending on the amount of precursor 4 (1 -5 mg respectively).

Example 2: N.c.a. nucleophilic ⁸F-fluorination of 2-amino-3-(4,5-dimethoxy-2- nitrophenyl)propanoic acid 13.

Compound 13 was prepared by dropwise addition 1 mL of nitric acid (d = 1.4) to a solution of 12 (0.9 g, 4 mmol) in 10 mL of glacial acetic acid at 0°C. The reaction was stirred at room temperature for 2 h. The solution was poured into 100 mL water and the product was extracted with ethyl acetate. The organic phase was washed with water and 10% bicarbonate solution and brine, dried over anhydrous MgS0₄. The solvent was evaporated under reduced pressure, the residue was purified by RP HPLC to provide the nitrated product 13. [¹⁸F]14 was prepared by precursor 13 (5mg) as its precursor composition, [K⁺ C 2.2.2]salt after adding ([K⁺ C 2.2.2])₂C0₃ (0,012 mmol) and ([K⁺ C 2.2.2])₂C₂0₄ (0,032 mmol) in CH₃CN and evaporation of the solvent in DMSO (150°C, 20 min) with 7 % RCY (not optimized result).

Scheme 4 illustrates the direct n.c.a. ¹⁸F-fluorination of 13 as its precursor composition yielded [¹⁸F]-2-amino-3-(4,5-dimethoxy-2-fluorophenyl) propanoic acid, [¹⁸F]14.

Sche

Example 3: N.c.a. nucleophilic F-fluorination of the precursor composition of amino acid analogue (2S)-2-amino-3-(6-chloropyridin-3-yl)propanoic acid 15

(2S)-2-amino-3-(6-chloropyridin-3-yl)propanoic acid 15 (3,2 mg, 0,0135 mmol) was converted to its precursor composition [K⁺C 2.2.2]salt by ([K⁺C 2.2.2])₂C0₃ (0,012 mmol) and ([K⁺C 2.2.2])₂C₂0₄ (0,032 mmol) and fluorinated by n.c.a. [K⁺C 2.2.2]¹⁸F at 150°C in DMSO. Compound [¹⁸F]16 was obtained after a reaction time of 20 min with 6 % RCY (not optimized).

Scheme 5 illustrates the n.c.a. ¹⁸F-fluorination of (2S)-2-amino-3-(6-chloropyridin-3- yl)propanoic acid (15) yielded [¹⁸F]-2-amino-3-(4,5-dimethoxy-2-fluorophenyl) propanoic acid [¹⁸F]16 .

Scheme 5:

15 [¹⁸F]16

Examples 4-8: [¹⁸F]18, [¹⁸F]20, [¹⁸F]22, [¹⁸F]24, and [¹⁸F]26 was prepared following similar procedure as above by converting precursor 17, 19, 21 , 23, or 25 (5mg) as its [K⁺c 2.2.2]salt after adding ([K⁺ C 2.2.2])₂C0₃ (0,012 mmol) and ([K⁺ C 2.2.2])₂C₂0₄ (0,032 mmol) and reacting with [K⁺ C 2.2.2]¹⁸F in DMSO (150°C, 20 min) with 6-26 % RCY (not optimized result). These examples for the n.c.a. nucleophilic ¹⁸F-fluorination of aromatic compounds with free carboxylic acids, i.e. benzoic acid analogues (17, 19, 21), a phenyl acetic acid (23) and a pyridine carboxylic acid (25) are illustrated below.

* as measured by Radio-TLC Examples 9 and 10: The [¹⁸F]-fluorinated compounds shown in the following table were prepared following similar procedure as in Examples 4 to 8 by converting the corresponding precursors also indicated in the table in the form of their [K⁺C 2.2.2] salt. The [¹⁸F]-fluorinated products were obtained with a RCY of 1 1 and 12 %, respectively.

* as measure y a o-

Description of the Figures

Figure 1 shows the result of the radiochemical labeling yield of precursor 4 in example 1 as a function of its concentration: a) 150 μΙ DMSO (150 °C, 20min) in the presence of [K⁺ C 2.2.2]¹⁸F and 6 pmol ([ ⁺ C 2.2.2])₂C0₃ and 12 pmol ([K⁺ C 2.2.2])₂C₂0₄.

Figure 2 shows the result of the radiochemical labeling yield of precursor 4 in example 1 as a function of the reaction time: a) 10 pmol (5 mg) 4 in 150 μΙ DMSO (150 °C) in the presence of [K⁺ C 2.2.2]¹⁸F and 6 μηιοΙ ([K⁺ C 2.2.2])₂C0₃ and 12 Mmol ([K⁺ C 2.2.2])₂C₂0₄.

Claims

Claims

A method for the radiofluorination of an aromatic or heteroaromatic compound containing a carboxylate group, said method comprising the steps of: a) providing a precursor compound comprising an aromatic or heteroaromatic ring, said aromatic or heteroaromatic ring bearing a leaving group for nucleophilic aromatic substitution reactions as a substituent L, and at least one further substituent which is selected from a carboxylate group -COO^" and a saturated or unsaturated aliphatic group or an aromatic group, which aliphatic or aromatic group carries at least one carboxylate group -COO^", wherein the carboxylate group -COO^" is in the form of a carboxylate salt with a cation selected from (i) a cationic chelate composed of a metal cation and a chelating agent and (ii) a quaternary ammonium cation; b) reacting the precursor compound containing at least one carboxylate group -COO^" in the form of a carboxylate salt with a cation selected from a cationic chelate composed of a metal cation and a chelating agent and a quaternary ammonium cation with a ¹⁸F^" anion in the presence of a phase transfer catalyst, to replace the leaving group of the aromatic or heteroaromatic ring with a ¹⁸F substituent, to yield a radiofluorinated compound which comprises an aromatic or heteroaromatic ring, said aromatic or heteroaromatic ring bearing the ¹⁸F substituent at the position held by the leaving group in the precursor compound, and at least one further substituent which is selected from a carboxylic acid group and a saturated or unsaturated aliphatic group or an aromatic group, which aliphatic or aromatic group carries at least one carboxylic acid group, wherein the carboxylic acid group may be in the form of a free acid or in the form of a carboxylate salt as defined above..

The method of claim 1 , wherein the carboxylate group(s) of the precursor compound is (are) in the form of a carboxylate salt with a cation selected from (i) a cationic chelate composed of a metal cation and a cryptand and (ii) a cationic chelate composed of a metal cation and a crown ether. The method of claim 2, wherein the carboxylate group(s) of the precursor compound is (are) in the form of a carboxylate salt with a potassium cation chelated by cryptand [2.2.2].

The method of claim 1 , wherein the carboxylate group(s) of the precursor compound is (are) in the form of a carboxylate salt with a quaternary ammonium cation.

The method of any of claims 1 to 4, wherein the phase transfer catalyst is selected from ([K⁺ C 2.2.2])₂C0₃, [K⁺ C 2.2.2]OH, [K⁺ C 2.2.2]HC0₃, and ([K⁺ C 2.2.2])₂C₂0₄.

The method of any of claims 1 to 5, wherein the leaving group L of the precursor compound is selected from nitro, chloro, bromo, iodo, and -N(R⁹)₃ ⁺, with R⁹ being selected, indepently for each occurrence, from C1-6 alkyl.

The method of any of claims 1 to 6, wherein the at least one further substituent of the precursor compound is a substituent R which either represents a carboxylate group -COO^", or a linear or branched alkyl or alkenyl group which is substituted with a carboxylate group -COO^", which alkyl or alkenyl group may be interspersed with one or more moieties independently selected from -NH-, -N(CH₃)-, -0-, -S-, -S(O)-, -S(0)₂-, -C(0)-0-, -O-C(O)-, -C(0)-NH-, -NH-C(O)-, or -C(O)- and a phenylene group, and which may carry one or more substituents selected from-NR⁷R⁸, -CONR⁷R⁸, -COR⁷, -S0₂NR⁷R⁸, -NR⁷COR⁸, -NR⁷S0₂R⁸, cycloalkyl, aryl, aryloxy, and heterocyclyl, wherein each R⁷ and each R⁸ is independently selected from hydrogen or C1 -6 alkyl.

The method of any one of claims 1 to 6, wherein the precursor compound is selected from a compound of formula (I) or (II),

wherein

L is selected from nitro group, chloro, bromo, iodo, and -N(R⁹)₃ ⁺, with R⁹ being selected, indepently for each occurrence, from C1-6 alkyl; R either represents a carboxylate group -COO^", or a linear or branched alkyl or alkenyl group which is substituted with a carboxylate group -COO-, which alkyl or alkenyl group may be interspersed with one or more moieties independently selected from -NH-, -N(CH₃)-, -0-, -S-, -S(O)-, -S(0)₂-, -C(0)-0-, -O-C(O)-, -C(0)-NH-, -NH-C(O)-, or -C(O)- and a phenylene group, and which may carry one or more substituents selected from -NR⁷R⁸, -CONR⁷R⁸, -COR⁷, -S0₂NR⁷R⁸, -NR⁷COR⁸, -NR⁷S0₂R⁸, cycloalkyl, aryl, aryloxy, and heterocyclyl, wherein each R⁷ and each R⁸ is independently selected from hydrogen or C1 -6 alkyl;

X is CH or N;

R¹ is independently selected, for each occurrence, from -NR¹⁰R¹¹, alkoxy, aryloxy, alkyl, -CF₃, phenyl, and -COR¹⁰, wherein each R ⁰ and R is independently selected from hydrogen and C1-6 alkyl; and n is an integer from 0 to 3.

9. The method of any one of claims 1 to 6, wherein the precursor compound is selected from a compound of any one of formulae (III) to (XI):

wherein L is is selected from nitro group, chloro, bromo, iodo, and -N(R⁹)₃ ⁺, with R⁹ being selected, indepently for each occurrence, from C1-6 alkyl; X is CH or N; and R² to R⁶ are independently selected from H and alkoxy.

10. The method of any one of claims 1 to 6, wherein the precursor compound is selected from a com ound of formula (XII) or (XIII):

wherein L is is selected from nitro group, chloro, bromo, iodo, and -N(R⁹)₃ ⁺, with R⁹ being selected, indepently for each occurrence, from C1 -6 alkyl, and X is CH or N.

1 1. A precursor compound comprising an aromatic or heteroaromatic ring, said aromatic or heteroaromatic ring bearing a leaving group for nucleophilic aromatic substitution reactions as a substituent L, and at least one further substituent which is selected from a carboxylate group -COO^" and a saturated or unsaturated aliphatic group or an aromatic group, which aliphatic or aromatic group carries at least one carboxylate group -COO^", wherein the carboxylate group -COO^" is in the form of a carboxylate salt with a cation selected from (i) a cationic chelate composed of a metal cation and a chelating agent and (ii) a quaternary ammonium cation,

with the proviso that the precursor compound is not 5-methyl-isophthalic acid mono- (Cs⁺/18-crown-6)-salt, and not 5-methoxy-isophthalic acid mono-(Cs⁺/18-crown-6)-salt.

12. The precursor compound of claim 1 1 , wherein the carboxylate group(s) of the precursor compound is (are) in the form of a carboxylate salt with a cation selected from (i) a cationic chelate composed of a metal cation and a cryptand and (ii) a cationic chelate composed of a metal cation and a crown ether.

13. The precursor compound of claim 1 1 or 12, wherein the at least one further substituent of the precursor compound is a substituent R which either represents a carboxylate group -COO^", or a linear or branched alkyl or alkenyl group which is substituted with a carboxylate group -COO^" , which alkyl or alkenyl group may be interspersed with one or more moieties independently selected from -NH-, -N(CH₃)-, -0-, -S-, -S(O)-, -S(0)₂-, -C(0)-0-, -O-C(O)-, -C(0)-NH-, -NH-C(O)-, or -C(O)- and a phenylene group, and which may carry one or more substituents selected from-NR⁷R⁸, -CONR⁷R⁸, -COR⁷, -S0₂NR⁷R⁸, -NR⁷COR⁸, -NR⁷S0₂R⁸, cycloalkyl, aryl, aryloxy, and heterocyclyl, wherein each R⁷ and each R⁸ is independently selected from hydrogen or C1 -6 alkyl, wherein the carboxylate group -COO^" is in the form of a carboxylate salt with a cation selected from (i) a cationic chelate composed of a metal cation and a chelating agent and (ii) a quaternary ammonium cation.

14. The precursor compound of claim 1 1 or 12, wherein the precursor compound is selected from a compound of formula (I) or (II),

wherein

L is selected from nitro group, chloro, bromo, iodo, and -N(R⁹)₃\ with R⁹ being selected, indepently for each occurrence, from C1 -6 alkyl;

R either represents a carboxylate group -COO-, or a linear or branched alkyl or alkenyl group which is substituted with a carboxylate group -COO^", which alkyl or alkenyl group may be interspersed with one or more moieties independently selected from - NH-, -N(CH₃)-, -0-, -S-, -S(O)-, -S(0)₂-, -C(0)-0-, -O-C(O)-, -C(0)-NH-, -NH-C(O)-, or - C(O)- and a phenylene group, and which may carry one or more substituents selected from -NR⁷R⁸, -C0NR⁷R⁸, -COR⁷, -S0₂NR⁷R⁸, -NR⁷COR⁸, -NR⁷S0₂R⁸, cycloalkyl, aryl, aryloxy, and heterocyclyl, wherein each R⁷ and each R⁸ is independently selected from hydrogen or C1 -6 alkyl;

X is CH or N;

R is independently selected, for each occurrence, from -NR¹⁰R¹¹, alkoxy, aryloxy, alkyl, -CF₃, phenyl, and -COR¹⁰, wherein each R¹⁰ and R¹ is independently selected from hydrogen and C1 -6 alkyl; and n is an integer from 0 to 3.

15. The precursor compound of claim 1 1 or 12, wherein the precursor compound is selected from a compound of any one of formulae (Mi) to (XIII):

(Xiii), wherein L is is selected from nitro, chloro, bromo, iodo, and -N(R⁹)₃ ⁺, with R⁹ being selected, indepently for each occurrence, from C1 -6 alkyl; X is CH or N; and R² to R^s are independently selected from H and alkoxy.