Cyclen based compounds and their gd(III) complexes for use as multimodal pet / mri contrast agents

Abstract

The present invention relates to the use of cyclen based compound of general formula (I) wherein R is NO2 or F; A is independently selected from -CH2COOH; -CH(CH3)COOH;; -CH((CH2)nCH3)COOH, wherein n is an integer in the range of from 1 to 3; and -CH2P(=O)(OH)Ph; Z is H or A; for preparation of a multimodal PET / MRI contrast agents. The invention also relates to Gd(III) complexes of compound of general formula (I), wherein R is 18F, as multimodal PET / MRI contrast agents.

Description

[0001]Cyclen based compounds and their Gd(III) complexes for use as multimodal PET / MRI contrast agents Technical field This invention relates to cyclen based compounds, their Gd(III) complexes, and their use as multimodal PET / MRI contrast agents, containing18F and Gd3+within one complex molecule. Background art Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET) are among the most powerful tools in clinical diagnostic imaging. The information that these techniques provide is to a great degree complementary. The latest and most advanced instruments combine advantages of both methods into a single hybrid PET / MRI technology. MRI is appreciated for providing an excellent anatomical image of soft tissue with high resolution. It is based on the magnetic resonance signal of1H nuclei in water molecules, which constitute about 60 - 70 % of body weight. Organs can be easily distinguished in MRI owing to different water content or relaxation rate of the magnetic resonance (MR) signal. In cases when the native contrast is insufficient, exogenous contrast agents can be intravenously applied. These are mostly based on gadolinium(III) chelates, known as gadolinium-based contrast agents (GBCAs). Typical examples are the chelates of the macrocyclic ligand DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and the open-chain ligand DTPA (diethylenetriaminepentaacetic acid). The purpose of the paramagnetic Gd(III) ion is to induce relaxation of nearby1H nuclei (of water), while the ligand serves to mask toxicity of this metal ion. In general, the macrocyclic DOTA-like chelators are known to provide Gd(III) chelates with much higher kinetic inertness compared to the acyclic DTPA-like chelators. High kinetic inertness is desirable for in-vivo applications, because it prevents the toxic Gd(III) ion from escaping from the chelate. Thus, macrocyclic chelators are preferred. Currently used GBCAs are non-specific compounds, i.e. they do not bind to a specific molecular target. Yet, they provide important diagnostic information based on differences in diffusion and perfusion characteristics of the tissue, which are often linked to pathological changes. Resulting differences in biodistribution of the GBCAs are visualized in MRI and reveal the pathologies. PET is valued for providing specific information about molecular targets with very high sensitivity. This technique is based on administration of tracer molecules labelled with a positron-emitting radionuclide. When the radionuclide decays, the emitted positron annihilates with a nearby electron, creating two gamma photons that travel in opposite direction (180 degrees apart). An external array of detectors registers both photons and reveals their point of origin, allowing to construct an image of the tracer molecules in the body. Thanks to this photon-counting principle, PET is a very sensitive and quantitative technique. Extremely low amounts of the tracer molecules are needed for detection, allowing targeting and imaging of low- abundance molecular targets. A disadvantage is the lack of anatomical image. For this reason, PET is combined with computed tomography (CT) or (more recently) with MRI that both provide the anatomical image for reference. Numerous PET tracers are clinically approved or in development, using various PET radionuclides, such as11C,13N,18F,64Cu,68Ga,89Zr and others. Among these,18F is the most widely used, because it is cost-effectively prepared in large quantities by cyclotron irradiation of18O-enriched water. With a moderate half-life of 110 minutes,18F is amenable for synthesis of a broad range of small-molecular organic tracer molecules, including the most common clinical PET tracer, [18F]FDG (fluorodeoxyglucose). Combined PET / MRI instruments provide significant advantages over separate PET and MRI and other imaging techniques. Mainly, examination with PET / MRI decreases the radiation burden to the patient compared to PET / CT, while providing superior anatomical image of soft tissue. Obtaining temporally and spatially matched images from both modalities simultaneously offers opportunities for increasing diagnostic accuracy, speed and discovery of new diagnostic imaging markers. However, the current practice of PET / MRI examinations is to administer two separate contrast agents: the radioactive tracer for PET and the GBCA for MRI. These have completely independent pharmacokinetic behaviour and provide independent diagnostic information. There are no clinically approved agents that would concurrently provide the PET signal and MRI contrast from a single compound. In fact, only a few such compounds have been reported in literature so far. Combining the Gd(III) chelate with the PET radionuclide into a single stable and well-defined molecule presents significant synthetic challenges that hamper the progress. One of the earliest examples [Frullano, L. et al., Angew. Chem. Int. Ed.2010, 49 (13), 2382–2384] reported a pH responsive PET / MRI contrast agent based on a Gd(III) chelate that was equipped with an18F-bearing prosthetic group via a click reaction, using multi-step synthesis with only 0.6% overall yield. Later, the same group prepared a PET / MRI version of the fibrin-targeted MRI agent EP-2104R by partial dechelation of one of its four Gd(III) chelates and subsequent complexation of64Cu(II) [Uppal, R. et al., Radiology 2011, 258 (3), 812–820]. However, the lack of control over the decomplexation / complexation strategy yielded a mixture of positional isomers, not a single well-defined compound. Others have tried to create dendrimer-like multi-metallic structures by decorating a single68Ga(III) chelate with multiple Gd(III) chelates, creating rather large and complicated molecules [Kumar, A. et al., Bioconjugate Chem. 2015, 26 (3), 549–558; Notni, J. et al., Chem. Eur. J. 2013, 19 (38), 12602–12606]. Translating such complicated molecules to clinical practice is typically hampered by poor reproducibility of the synthesis. Another challenge faced in preparing combined PET / MRI contrast agent is that the amount of radionuclide needed for the PET signal is about 9 - 10 orders of magnitude lower than the Gd(III) chelate needed for the MRI contrast. To match these conflicting quantity requirements, mixture of two isotopically different versions of the agent can be prepared. The agent containing the active PET isotope is present only in trace quantities needed to obtain the PET signal, while majority of the agent is a chemically identical molecule containing a stable (non-radioactive) isotope. This bulk of non-radioactive version is needed to achieve the desired MRI contrast. Since the two versions are chemically identical (only different in isotopes), they possess the same pharmacokinetic properties. However, in cases when the agent utilizes metal PET radionuclides this means co-administration of a relatively large amount of non-radioactive exogenous metal, which presents a significant metal-induced toxicity risk. From this point of view, the use of18F is advantageous over the metal PET radionuclides. Not only is18F widely available and cheap isotope, but it forms stable covalent C-F bonds and fluorinated compounds are generally well tolerated. Despite of the favorable properties of the18F isotope, challenges remain to achieve its fast and efficient incorporation into the PET / MRI agent, especially in relation to stability of the Gd(III) chelate as the source of MRI contrast. In order to supress the losses of the18F isotope due to radioactive decay, it is desirable to connect it to the molecule ideally in the last synthetic step. Methods have been developed to achieve this for purely organic PET tracers, but the presence of Gd(III) chelates in PET / MRI agents is incompatible with many synthetic and purification conditions. For example, Gd(III) chelates are generally poorly soluble in water-free organic solvents required for the18F labelling chemistry. They are also unstable under acidic conditions, which may be required for chromatographic or ion-exchange purification of the final products. Thus, the conditions developed for synthesis of purely organic PET tracers are not straightforwardly transferrable to chemistry of Gd(III) chelates. The background art thus lacks compounds and methods for fast and efficient labelling of Gd(III) chelates with18F radionuclide that would provide access to well-defined small-molecular PET / MRI agents akin to the currently clinically used MRI contrast agents. Disclosure of Invention In the background art, a modest number of bimodal PET / MRI agents have been prepared, based on rather complicated bimetallic compounds or multi-step synthesis to achieve labelling with the PET isotope. These examples fail to provide a simple and synthetically easily achievable molecular PET / MRI agent akin to the current clinically used non-specific MRI contrast agents. In contrast to this, we have found that macrocyclic Gd(III) chelates bearing a pyridine arm equipped with a suitable leaving group can be conveniently labelled with the18F-fluoride anion via a nucleophilic attack. This single-step reaction is fast and proceeds directly on the pre-formed Gd(III) chelate. There is therefore no need for additional complexation step. The macrocyclic chelator molecule is structurally closely related to DOTA, which has a long proven history of safe clinical use as an MRI contrast agent in the form of its Gd(III) chelate. Surprisingly, we found that18F labelling is also possible by19F to18F isotopic exchange on the pyridine moiety of the Gd(III) chelate, but this reaction does not take place at all, or proceeds with much lower yields, if performed on the free macrocyclic chelator. This ensures that the Gd(III) chelate is labelled preferentially, leading to a cleaner desired product that contains both Gd(III) and18F to act as a multimodal PET / MRI agent. Moreover, this synthetic approach allows to adjust the molar ratio between the PET-active (radiolabeled) version and the non-radioactive version of the agent to tune the desired PET signal and MRI contrast to the particular PET / MRI instrument and application. Thus, these compounds and the method of their radiolabeling overcome the problems of background art mentioned above. The object of the present invention is the use in pharmacy, preferably the use for preparation of a multimodal PET / MRI contrast agent, of cyclen based compound of general formula (I) wherein R is NO2or F; A is independently selected from –CH2COOH; –CH(CH3)COOH; –CH((CH2)nCH3)COOH, wherein n is an integer in the range of from 1 to 3; –CH2P(=O)(OH)Ph; Z is H or A. The compounds of general formula (I) can be synthetised from nitro- or fluoro-pyridine intermediates of general formula (II) (II), wherein R is NO2or F; and X is halogen, preferably Cl or Br. The intermediates of general formula (II) react with cyclen-based intermediate of general formula (III) (III), wherein A’ is A (as defined above) in which OH group is protected, preferably as methoxy, ethoxy or tert-butoxy group; and Z’ is hydrogen or A’. The resulting precursor of general formula (IV) (IV), wherein Z’ and A’ are as defined above; is then deprotected (the protection of OH groups is removed), preferably by using trifluoroacetic acid, giving the compound of general formula (I) as defined above. In one preferred embodiment of cyclen based compound of general formula (I), A and Z are the same, preferably selected from –CH2P(=O)(OH)Ph; and –CH(CH3)COOH. In one preferred embodiment of cyclen based compound of general formula (I), A are the same, preferably selected from –CH2P(=O)(OH)Ph; and –CH(CH3)COOH; and Z is hydrogen. In one preferred embodiment, R is located in para-position in relation to the nitrogen atom of the pyridyl pendant arm. More preferably, R is F. In one embodiment, the cyclen based compound of general formula (I) is selected from the group comprising: 2,2',2''-(10-((4-nitropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (L1) 2,2',2''-(10-((6-nitropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (L2) 2,2',2''-(10-((3-fluoropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (L3) 2,2',2''-(10-((4-fluoropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (L4) 2,2',2''-(10-((5-fluoropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (L5) 2,2',2''-(10-((6-fluoropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (L6) 2,2'-(4-((4-fluoropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,7-diyl)diacetic acid (L7) (2S,2'S,2''S)-2,2',2''-(10-((3-fluoropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7- triyl)tripropionic acid (L8) ((4-((3-fluoropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,7- diyl)bis(methylene))bis(phenylphosphinic acid) (L9) 2,2'-(4-((4-nitropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,7-diyl)diacetic acid (L10) (2S,2'S,2''S)-2,2',2''-(10-((4-fluoropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7- triyl)tripropionic acid (L11) (2S,2'S,2''S)-2,2',2''-(10-((4-nitropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7- triyl)tripropionic acid (L12) The object of the present invention is also a compound of general formula (Ia) (Ia), wherein R is NO2or F; preferably R is F; A is independently selected from –CH(CH3)COOH; –CH((CH2)nCH3)COOH, wherein n is an integer in the range of from 1 to 3; and –CH2P(=O)(OH)Ph; and Z is H or A, preferably Z is A. Preferably, the R substituent is located in para-position in relation to the nitrogen atom of the pyridyl pendant arm. More preferably, A is –CH(CH3)COOH. In the most preferred embodiment, the compound according to the present invention has the following formulae (Va, Vb), wherein the stereoconfiguration of all–C*H(CH3)COOH groups is the same (either all have S configuration or all have R configuration, (Va) (Vb). Preferably, R substituent in the compound of formula (Va) or (Vb) is located in para-position in relation to the nitrogen atom of the pyridyl pendant arm. More preferably, R is F. Yet another object of the present invention is a coordination compound of Gd(III) ion and the cyclen based compound of general formula (I) or (Ia) as defined above, and its use in medicine for preparation of a multimodal PET / MRI contrast agent. The coordination compound can be prepared by reacting the cyclen based compound of general formula (I) or (Ia) with Gd3+salt (e.g. GdCl3), thereby forming a Gd(III) chelate of the compound of general formula (I) or (Ia). Such Gd(III) chelate is a suitable contrast agent for MRI imaging because of its high thermodynamic stability and kinetic inertness and favourable relaxivity properties. The multimodal PET / MRI contrast agent may be prepared from the above described coordination compound of Gd(III) ion and the cyclen based compound of general formula (I) or (Ia), by radiolabelling the Gd (III) chelate with18F. The radiolabelling may be performed either using19F to18F isotopic exchange on the pyridine moiety of the Gd(III) chelate, wherein R is F, or using nucleophilic aromatic substitution of nitro group with18F, wherein R is NO2. A combination of both radiolabelling methods may also be used for Gd(III) chelates containing R being NO2, wherein a nucleophilic aromatic substitution of nitro group with19F is performed, followed by19F to18F isotopic exchange. In all cases, the radiolabelling is performed on the Gd(III) chelate, not on the ligand itself. The product thus contains both Gd(III) and18F and is suitable to act as a multimodal PET / MRI agent. Preferably, the radiolabelling is performed using a solution containing18F- produced on a cyclotron, mixed with the Gd(III) chelate at 90 °C, preferably for at least 5 minutes. Another object of the present invention is thus a multimodal PET / MRI contrast agent of general formula (VI) (VI), wherein A is independently selected from –CH2COOH; –CH(CH3)COOH; –CH((CH2)nCH3)COOH, wherein n is an integer in the range of from 1 to 3; and –CH2P(=O)(OH)Ph; Z is H or A. Preferably,18F is in para-position in relation to the nitrogen atom of the pyridyl pendant arm. In one embodiment, the multimodal PET / MRI contrast agent has general formula (VIa) (VIa), wherein A is independently selected from –CH2COOH; –CH(CH3)COOH; –CH((CH2)nCH3)COOH, wherein n is an integer in the range of from 1 to 3; and –CH2P(=O)(OH)Ph; preferably, A is –CH(CH3)COOH or –CH2P(=O)(OH)Ph. Preferably, if18F is in para-position in relation to the nitrogen atom of the pyridyl pendant arm, then A is not –CH2COOH. In one preferred embodiment, Z is A, more preferably, A is –CH(CH3)COOH, even more preferably, stereoconfiguration of all A (–C*H(CH3)COOH) is the same (either all A have S configuration or all A have R configuration). Most preferably, all A have the same stereoconfiguration and18F is in para-position in relation to the nitrogen atom of the pyridyl pendant arm. In one embodiment, the multimodal PET / MRI contrast agent has general formula (VIb) (VIb), wherein A is independently selected from –CH2COOH; –CH(CH3)COOH; –CH((CH2)nCH3)COOH, wherein n is an integer in the range of from 1 to 3; and –CH2P(=O)(OH)Ph; preferably, A is –CH(CH3)COOH or –CH2P(=O)(OH)Ph. More preferably, A is –CH(CH3)COOH, even more preferably, stereoconfiguration of both A (–C*H(CH3)COOH) is the same. In the most preferred embodiment, the multimodal PET / MRI contrast agent has general formula (VI), (VIa) or (VIb), wherein18F is in para-position in relation to the nitrogen atom of the pyridyl pendant arm. The object of the present invention is also a method of preparing the multimodal PET / MRI contrast agent of general formula (VI) as defined above, the method comprising the following steps: a) providing the cyclen based compound of general formula (I) as defined above; b) forming a Gd(III) chelate of the compound of general formula (I); c) radiolabelling the Gd (III) chelate with18F. In one embodiment, step b) is achieved by reacting the cyclen based compound of general formula (I) with Gd3+ion, thereby forming a Gd(III) chelate of the compound of general formula (I). Preferably, the conditions of the chelation reaction are the following: A solution containing Gd3+ion in the form of salt (e.g. chloride, bromide, sulfate, nitrate, methanesulfonate, trifluoromethanesulfonate, formate, acetate, lactate, malate, citrate, 2-hydroxyisobutyrate, mandelate, diglycolate, tartarate) is mixed with a solution of the compound of general formula (I), preferably in molar ratio of Gd3+ion to compound of general formula (I) in the range of from 1:0.5 to 1:100, more preferably from 1:0.7 to 1:50, even more preferably from 1:0.9 to 1:10. Concentrations of the soluble components may be selected from the concentration range permitted by solubility of such compounds in a given solvent at a given temperature, preferably in the concentration range 0.000001 – 0.5 mol / L. The solvent may be water, a water-miscible organic solvent such as methanol, ethanol, propanol, isopropanol, acetone, acetonitrile, N,N-dimethylformamide, dimethylsulfoxide, tetrahydrofurane, or a mixture thereof. An organic or inorganic base, such as LiOH, NaOH, KOH, aqueous NH3, triethylamine, N,N-diisopropylethylamine or pyridine, is added to the reaction mixture in order to compensate for protons released during the complexation, and the complexation takes place in the solution. Preferably, from 1 to 10 molar equivalents of base are added per molecule of the compound of general formula (I). Eventually, the reaction can take place in a buffer. In such case there is no need of adding organic or inorganic base to the reaction mixture. The mixture is stirred or shaken at room temperature or elevated temperature for up to 24 hours to afford complete complexation. Preferably, the mixture is stirred or shaken at 40 °C for 15 minutes. A reasonable excess of the compound of general formula (I) may be used to accelerate the complexation and to shift the equilibrium towards formation of the chelates. The resulting Gd(III) chelate of the compound of general formula (I) may optionally be desalted and purified from excess of the compound of general formula (I) and other impurities. The purification may be achieved, for example, by column chromatography or high- performance liquid chromatography (HPLC). Preferably, the chromatographic separation is achieved using HPLC on C8, C18 or phenyl-hexyl reversed phase. The mobile phase used for chromatographic purification may contain water and 3 – 40 vol. % of methanol, ethanol or acetonitrile. Optionally, 0.01 – 0.1 mol / L of a buffer is used in the mobile phase, wherein the buffer comprises ammonium formate pH = 7.0 or ammonium acetate pH = 7.0. Fractions containing the desired metal chelate are collected and evaporated or lyophilized. In one embodiment, step c) of radiolabelling the Gd(III) chelate from step b) with18F is performed by using either19F to18F isotopic exchange on the pyridine moiety of the Gd(III) chelate, wherein R is F; or by using nucleophilic aromatic substitution of nitro group with18F on the pyridine moiety of the Gd(III) chelate, wherein R is nitro group. Preferably, step c) is performed under the following conditions: The Gd(III) chelate from step b) is dissolved in a solvent, preferably selected from the group comprising acetone, acetonitrile, dichloromethane, N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), dimethylpropyleneurea (DMPU), dimethylsulfoxide (DMSO), sulfolane, ethyl acetate, hexamethylphosphoramide (HMPA), pyridine, tetrahydrofuran (THF), methanol, ethanol, propanol, isopropanol, butanol, tert-butanol, N-methyl-2-pyrrolidone (NMP), 1,3-dimethyl-2-imidazolidinone, tert- amyl alcohol, water or a mixture thereof. Preferably, the final concentration of Gd(III) chelate in the solvent or solvent mixture is in the range of from 10-12M to 2 M. Optionally, a source of non-radioactive F- ions is added to achieve a concentration in the range of from 10−12M to 2 M. The source of non-radioactive F- ions may be, for example: tetramethylammonium fluoride, tetraethylammonium fluoride, tetrapropylammonium fluoride, tetrabutylammonium fluoride, tetrahexylammonium fluoride, NaF, KF, RbF, CsF, [Na(15-crown- 5)+]F−, [K(18-crown-6)+]F−, [Cs(21-crown-7)+]F−, [Na(cryptand[2.2.1])+]F−, [K(cryptand[2.2.2])+]F−or [Cs(cryptand[3.2.2])+]F−. The source of non-radioactive F- ions is added in order to achieve a nucleophilic aromatic substitution of NO2 group with19F. If the Gd(III) chelate from step b) does not contain any NO2 group, then addition of non-radioactive F- ions can be omitted. Analogously, source of radioactive18F- ions is added, in the amount that corresponds to the desired activity. In terms of molar equivalents, this represents only a fraction of the amount of the Gd(III) chelate and / or of the non-radioactive F- ions. The amount (activity) of18F- ions would be known to a person skilled in the art without exerting any inventive activity. The source of radioactive18F- ions is analogous to the sources of non-radioactive F- ions listed above (e.g. [18F]tetramethylammonium fluoride, [18F]tetraethylammonium fluoride, [18F]tetrapropylammonium fluoride, [18F]tetrabutylammonium fluoride, [18F]tetrahexylammonium fluoride, Na18F, K18F, Rb18F, Cs18F, [Na(15-crown-5)+]18F−, [K(18-crown- 6)+]18F−, [Cs(21-crown-7)+]18F−, [Na(cryptand[2.2.1])+]18F−, [K(cryptand[2.2.2])+]18F−or [Cs(cryptand[3.2.2])+]18F−). The reaction mixture is then stirred at a temperature in the range of from -50 °C to 250 °C, preferably in the range of from 5 °C to 200 °C, for at least 0.1 s, preferably for at most 8 hours. In a more preferred embodiment, step c) is performed in the solvent selected from acetonitrile, DMF, DMAC, DMPU, DMSO, sulfolane, HMPA, THF, tert-butanol, NMP, 1,3-Dimethyl-2-imidazolidinone, tert-amyl alcohol, water or a mixture thereof. The final concentration of Gd(III) chelate in the solvent or solvent mixture is in the range of from 10-6M to 0.5 M. Optionally, a source of non-radioactive F- ions is added to achieve a concentration in the range of from 10−6M to 0.5 M. The source of non-radioactive F- ions is preferably selected from: tetramethylammonium fluoride, tetrabutylammonium fluoride, NaF, KF, CsF, [Na(15-crown-5)+]F−, [K(18-crown-6)+]F−, [Na(cryptand[2.2.1])+]F−and [K(cryptand[2.2.2])+]F−. A source of radioactive18F- ions is added, in the amount that corresponds to the desired activity. The source of radioactive18F- ions is analogous to the sources of non-radioactive F- ions listed above (e.g. [18F]tetramethylammonium fluoride, [18F]tetrabutylammonium fluoride, Na18F, K18F, Cs18F, [Na(15- crown-5)+]18F−, [K(18-crown-6)+]18F−, [Na(cryptand[2.2.1])+]18F−or [K(cryptand[2.2.2])+]18F−. The reaction mixture is then stirred at a temperature in the range of from 25 °C to 200 °C for at least 1 minute, preferably for at most 2 hours. Most preferably, the solvent is selected from acetonitrile, DMF, DMSO or mixtures thereof. The final concentration of Gd(III) chelate is in the range of from 0.1 mM to 100 mM. Optionally, a source of non- radioactive F- ions is added to achieve a concentration in the range of from 1 mM to 300 mM. The source of non-radioactive F- ions is preferably selected from tetrabutylammonium fluoride or [K(cryptand[2.2.2])+]F−. A source of radioactive18F- ions is added, in the amount that corresponds to the desired activity. The source of radioactive18F- is selected from [18F]tetrabutylammonium fluoride and [K(cryptand[2.2.2])+]18F−. The reaction mixture is then stirred at a temperature in the range of from 25 °C to 150 °C for at least 1 minute, preferably for at most 100 minutes. The object of the present invention is also a method of imaging a region of interest in a patient using the multimodal PET / MRI imaging, the method comprising: - administering to a patient a pharmaceutical formulation comprising the multimodal PET / MRI contrast agent of general formula (VI), (VIa) or (VIb); - detecting arrival of the multimodal PET / MRI contrast agent present in the formulation in the region of interest by comparing the PET and / or MRI signals in said region of interest during or after administering said pharmaceutical formulation; - collecting PET and MRI image data of the region of interest; and - constructing a multimodal PET / MRI image of said region of interest using the PET and MRI image data, wherein the region of interest appears distinct from the background tissue. The pharmaceutical formulation comprising the multimodal PET / MRI contrast agent of general formula (VI), (VIa) or (VIb) may further contain conventionally used pharmaceutically acceptable auxiliary substances. The dosage form of the pharmaceutical preparation is a form for administration by injection, most often as a bolus or as an infusion, preferably intravenously. Suitable pharmaceutically acceptable auxiliary substances are preferably selected from the group containing solvents (especially aqueous or saline solution), buffers (especially phosphate buffer, HEPES = 2-[4-(2-hydroxyethyl)piperazine-1- yl]ethanesulfonic acid), ionization additives, antioxidants, antimicrobial additives. A person skilled in the art would be able, without exerting inventive activity, to determine which auxiliary substance to choose. In one embodiment, the pharmaceutical formulation further contains the coordination compound of Gd(III) ion and the cyclen based compound of general formula (I) or (Ia) as defined above. The presence of said coordination compound, containing Gd(III) and19F, provides for the MRI signal, while the presence of the multimodal PET / MRI contrast agent of general formula (VI), (VIa) or (VIb), bearing18F, provides for the PET and MRI signal. As the amount of18F radionuclide needed for the PET signal is about 9 - 10 orders of magnitude lower than the amount of Gd(III) chelate needed for the MRI contrast, MRI contrast of the pharmaceutical formulation may be enhanced by the presence of non-radioactive Gd(III) chelate of the compound of general formula (I) or (Ia) as defined above. It is preferred if the radioactive and non- radioactive Gd(III) chelates differ only in the fluorine isotope (meaning that the coordination compounds are structurally identical, differing only in whether bearing19F or18F isotope). The concentrations of the non-radioactive Gd(III) chelate should be sufficient to achieve MRI contrast, the concentration of the PET / MRI contrast agent should be sufficient to achieve PET contrast. The concentrations of Gd(III) chelate to achieve MRI contrast, and of the18F radionuclide to achieve the PET contrast, would be known to the skilled person. In general, the molar ratio of non-radioactive Gd(III) chelate of compound of general formula (I) or (Ia) to the PET / MRI contrast agent of general formula (VI), (VIa) or (VIb) should preferably be from about 1:10-10to about 1:10-8. The object of the present invention is also the multimodal PET / MRI contrast agent of general formula (VI), (VIa) or (VIb) and / or the pharmaceutical formulation containing thereof, for use in medicine, preferably in combined PET / MRI diagnostic methods. Brief description of Drawings Figure 1: Chromatograms from radiolabelling experiment from Example 4 with [Gd(L1)] as precursor. Product corresponds to peak at 7.431 minutes in UV detector. In gamma detector, product corresponds to peak at 7.767 minutes. Figure 2: Chromatograms from radiolabelling experiment from Example 4 with [Gd(L4)] as precursor. Product corresponds to peak at 7.343 minutes in UV detector. In gamma detector, product corresponds to peak at 7.700 minutes. Figure 3: Chromatograms from radiolabelling experiment from Example 4 with L4as a precursor. It can be seen that no radiolabelled product is present, the only peak in gamma detector corresponds to the free18F-. The main peak in UV detector corresponds to unlabelled starting material L4. Figure 4: Determination of relaxivities of [Gd(L4)] and [Gd(L7)] by linear regression of their concentration versus longitudinal relaxation rate at 37 °C and 0.47 T. Figure 5: Comparison of kinetic inertness of [Gd(L4)], [Gd(L7)] and Magnevist by acid-assisted decomplexation in 0.1M HCl at 37 °C by measurement of relaxation rate on a relaxometer. The top graph shows data taken during the first 4 hours of the experiment and the bottom graph adds a data point after 24 hours. The R1pvalue should be the same for all measured complexes after reaching thermodynamic equilibrium but elemental analysis confirmed minor differences in concentrations of the samples (pipetting error) that explain the difference. Data for unchelated GdCl3solution of the same molar concentration is shown for reference. The curves demonstrate the differences in kinetic inertness of the metal chelates. While clinically used Magnevist is decomposed almost immediately (< 5 min), [Gd(L7)] takes longer time to reach complete decomplexation. Finally, the [Gd(L4)] chelate is the most inert and even after 24 hours under these conditions the curve is far from reaching the thermodynamic equilibrium, demonstrating that most of the chelate remains intact. Figure 6: In-vitro simultaneous PET / MRI imaging of radiolabelled [Gd(L4)] in phantoms. (A) T1- weighted MRI and PET images obtained simultaneously on an array of vials containing different concentrations of radiolabelled [Gd(L4)], different concentrations of Magnevist as a reference, and two samples of pure H2O and PBS (phosphate-buffered saline). The numbers denote the concentrations of the contrast agents in mM. (B) Concentrations of [Gd(L4)] in samples from panel A quantified by ICP-OES (gadolinium content) and gamma counting (radioactivity) show excellent linear correlation. Quantification of the PET signal from panel A also shows excellent linear correlation with the MRI signals determined from T1-weighted images (C) and with relaxation rate determined from T1-maps (D). Figure 7: Pharmacokinetics of radiolabelled [Gd(L4)] in mice (N = 3) from PET / MRI showing rapid uptake and clearance through kidneys. (A) Pharmacokinetic curves obtained from MRI T1-maps, expressed as 1 / T1 change in %. (B) Pharmacokinetic curves obtained from PET images, expressed as % injected dose per mL. The right axis relates to the bladder data. Figure 8: Biodistribution of radiolabelled [Gd(L4)] in mice (N = 6). (A) Biodistribution based on the measurement of radioactivity (gamma counter) expressed as % injected dose per gram of wet tissue. (B) Biodistribution based on the measurement of gadolinium (ICP-OES) expressed as % injected dose per gram of wet tissue. Figure 9: In-vivo simultaneous PET / MRI imaging of radiolabelled [Gd(L4)] in a mouse. From left to right the images show time points: pre-injection image, injection, 10 minutes post-injection and 60 minutes post- injection. Top row: T1-weighted dynamic contrast enhancement (DCE) MRI. Middle row: PET images. Bottom row: merged MRI and PET images. Notable hot spots are visible in kidneys in PET images, while the same areas are over-saturated and appear dark in MRI. Examples Example 1: synthesis of intermediates Synthesis of intermediate (1): In a 250 mL round bottom glass flask, 2-methyl-4- nitropyridine N-oxide (5.15 g, 33.4 mmol, 1.0 equiv.) was dissolved in DCM (200 mL). Solution of TFAA (14mL, 101 mmol, 3.0 equiv.) in DCM (35 mL) was added dropwise and the resulting suspension was stirred for 96 h at RT. Saturated aqueous solution of K2CO3 (40 mL) was added in four portions over period of 1 h and reaction mixture was stirred until bubbling stopped. Water phase was extracted with DCM (2×200 mL) and solvent was evaporated on rotary evaporator. The residue was purified by preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions with product were joined and lyophilized. The resulting white solid was dissolved in DCM (340 mL) followed by addition of DMF (46 µL, 0.60 mmol, 0.02 eqiv.) and SOCl2 (5.5 mL, 75.1 mmol, 2.25 eqiv.) dropwise. After 6 hours of stirring at RT the reaction mixture was put on ice bath and was quenched by careful addition of saturated aqueous solution of NaHCO3 (200 mL). Phases were separated and water phase was extracted with DCM (200 mL). Organic phase was dried over Na2SO4, filtered and liquids were evaporated on rotary evaporator. The residue was purified by column chromatography (SiO2, 100 % DCM to 10% MeOH in DCM). Fractions containing product were pooled and evaporated to dryness to yield the product as yellow oil. Yield: 2.2 g (38%; 2 steps; based on 2-methyl-4-nitropyridine N-oxide). NMR (CDCl3):1H δH 4.80 (CH2–arom, s, 2H); 7.98 (arom, dd,3JHH=5,4JHH= 2, 1H); 8.24 (arom, d,4JHH=2, 1H); 8.87 (arom, d,3JHH=5, 1H);13C{1H} δC 45.8 (CH2–arom, s); 115.6 (arom, s); 115.8 (arom, s); 151.8 (arom, s); 154.8 (arom, s); 160.2 (arom, s).EI-HRMS: 172.0033 [M]+(theor. [C6H5O2N2Cl1]+= 172.0034). Synthesis of intermediate (2): In a 50 mL pear-shaped glass flask, 2-methyl-6- nitropyridine (224 mg, 1.62 mmol, 1.0 equiv.), NBS (577 mg, 3.24 mmol, 2.0 equiv.) and AIBN (42 mg, 0.26 mmol, 0.16 equiv.) were dissolved in CCl4 (6 mL). The reaction mixture was heated under reflux for 19 hours. The solvent was evaporated on rotary evaporator. The residue was purified by preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions containing the product were pooled and DCM was added until separation of phases occurred. Organic phase was separated and washed with saturated aqueous NaHCO3. Organic phase was concentrated on rotary evaporator, giving product as colourless solid. Yield: 79 mg (23 %, 1 step, based on 2-methyl-6-nitropyridine). NMR (CDCl3):1H δH 4.63 (CH2–arom, s, 2H); 7.87 (arom, dd,2JHH=7.6,3JHH= 0.9, 1H); 8.07 (arom, t,2JHH=7.8, 1H); 8.19 (arom, dd,2JHH=8.0,3JHH= 0.8, 1H);13C{1H} δC 31.7 (CH2–arom, s); 117.3 (arom, s); 129.3 (arom, s); 141.1 (arom, s); 156.2 (arom, s); 157.5 (arom, s).CI-HRMS: 216.9605 [M+H]+(theor. [C6H6O2Br1N2]+= 216.9607). Synthesis of intermediate (3): In a 50 mL round bottom glass flask, 3-fluoropicolinic acid (813 mg, 5.76 mmol, 1.0 equiv.) was dissolved in MeOH (12 mL) and H2SO4(300 µL, 5.40 mmol, 0.94 eqiv.) was added dropwise. The reaction mixture was heated under reflux for 41 hours. The reaction mixture was cooled to RT and quenched with saturated aqueous NaHCO3(1 mL). Reaction mixture was concentrated to approximately 2 ml on rotary evaporator and was let stand until crystals started to appear. Crystals were washed with H2O (2 × 2 mL) and dried under vacuum. The resulting crystals were dissolved in MeOH (30 mL) and NaBH4(370 mg, 9.78 mmol, 1.70 eqiv.) was added in three portions while stirring on ice bath. After 30 minutes reaction mixture was allowed to reach RT and continued stirring for another 1.5 hours. Reaction was quenched with H2O (1 mL) and liquids were evaporated on rotary evaporator. Residue was suspended in saturated aqueous K2CO3(50 mL) and extracted with DCM (2×50 mL). The solvent was evaporated on rotary evaporator and residue was dissolved in DCM (7 mL) followed by addition of DMF (5 µL, 0.06 mmol, 0.01 eqiv.) and SOCl2 (0.523 mL, 1.17 mmol, 0.20 eqiv.) was added dropwise. After 1 hour of stirring at RT the reaction mixture was quenched by careful addition of saturated aqueous solution of NaHCO3 (10 mL). Phases were separated and water phase was extracted with DCM (10 mL). Organic phase was dried over Na2SO4, filtered and liquids were evaporated on rotary evaporator to yield the product as yellowish oil. Yield: 186 mg (22%; 3 steps; based on 3-fluoropicolinic acid). NMR (CDCl3):1H δH 4.68 (CH2–arom, s, 2H); 7.19–7.29 (arom., m, 1H); 7.32–7.42 (arom., m, 1H); 8.31–8.40 (arom., m, 1H);13C{1H} δC 40.6 (CH2–arom, s); 123.9 (arom, d,2JCF = 18.6 Hz); 125.4 (arom, d,3JCF = 4.0 Hz); 144.9 (arom, d,2JCF = 14.2 Hz); 145.5 (arom, d,4JCF = 5.4 Hz); 157.8 (arom, d,1JCF = 260.8 Hz).19F{1H} δF –123.2 (s). CI-HRMS: 146.0174 [M+H]+(theor. [C6H6Cl1N1F1]+= 146.0167). Synthesis of intermediate (4): In a 50 mL round bottom glass flask, (4-fluoropyridin-2- yl)methanol (227 mg, 1.79 mmol, 1.0 equiv.) was dissolved in DCM (35 mL) and DMF (4 µL, 0.05 mmol, 0.01 equiv.) was added. The reaction mixture was put on ice bath. After 20 minutes on ice bath, thionyl chloride (400 µL, 5.36 mmol, 3.0 equiv.) was added dropwise and reaction mixture was let stir on ice bath. After 15 minutes, the reaction mixture was allowed to let warm to RT. After 1 hour of stirring at RT the reaction mixture was put on ice bath and quenched by careful addition of saturated aqueous solution of NaHCO3(10 mL). Phases were separated, and the water phase was extracted with DCM (2 × 40 mL). Organic phase was dried over Na2SO4, filtered and liquids were evaporated on rotary evaporator to yield the product as colorless oil. Yield: 217 mg (83%; 1 step; based on (4-fluoropyridin-2-yl)methanol) NMR (CDCl3):1H (400.1 MHz, T = 300 K) δH4.69 (CH2–arom, s, 2H); 7.00 (arom., ddd, J = 8, 6, 2 Hz 1H); 7.24 ̶ 7.30 (arom., m, 1H); 8.55 (arom., dd, J = 8, 6 Hz H).13C{1H} (100.6 MHz, T = 300 K) δC70.8 (CH2– arom, d,4JCF= 3 Hz); 110.9 (arom, d,2JCF= 18 Hz); 111.2 (arom, d,2JCF= 17 Hz); 151.8 (arom, d,3JCF= 7 Hz); 160.0 (arom, d,3JCF = 7 Hz); 169.4 (arom, d,1JCF = 263 Hz).19F{1H}(376.5 MHz, T = 300.0 K)δF – 100.9 (s). ESI-MS: 146.0 [M+H]+(theor. [C6H6N1F1Cl1]+= 146.0). Synthesis of intermediate (5): Pear-shaped glass flask (100 mL) was charged with benzyl l-lactate (900 mg; 4.99 mmol; 1.00 equiv.) and magnetic stirrer and three times briefly secured with argon. Under constant flow of argon was then added dry DCM (20 mL) through septum and the mixture was cooled with an ice bath (5 °C) followed by dropwise addition of triflic anhydride (0.88 mL; 5.23 mmol; 1.05 equiv.) and immediately followed by dropwise addition of dry pyridine (0.42 ml; 5.25 mmol; 1.05 equiv.). The resulting mixture was stirred at 5 °C for 30 min. Resulting suspension was then directly purified by column chromatography (30 g SiO2, DCM). Combined fractions with product were evaporated to dryness and briefly dried on high vacuum to give product in the form of free base as nearly colourless oil. Yield: 1.12 g (77%; 1 step; based on benzyl l-lactate). NMR (DMSO-d6):1H δH1.53 (CH3, d, 3H,3JHH= 7); 5.27 (CH2, s, 2H); 5.34 (CH, q, 1H,3JHH= 7); 7.17–7.65 (Ph, m, 5H).19F{1H} δF−77.7 (s). ESI- HRMS: 335.0175 [M+Na]+(theor. [C11H11O5F3S1Na1]+= 335.0172). Synthesis of intermediate (6): In a glass vial (20 mL), cyclen (free base; 100 mg; 580 µmol; 1.0 equiv.) and K2CO3(160 mg; 1.16 mmol; 2.0 equiv.) were suspended in dry MeCN (6 mL) followed by addition of 5 (362 mg; 1.16 mmol; 2.0 equiv.) in dry MeCN (4 mL). The resulting mixture was stirred at 40°C for 1 h. Solids were filtered off using syringe microfilter (PTFE) and filtrate was evaporated to dryness. Residue was purified by preparative HPLC (C18; H2O‒MeCN gradient with TFA additive). Fractions with product were joined and lyophilized to give product in the form of TFA salt as brown honey. Yield: 106 mg (25 %, 1 step, based on cyclen). NMR (CD3CN):1H δH1.20 (CH3, d, 3H,3JHH= 7 Hz); 1.26 (CH3, d, 6H,3JHH= 7 Hz); 2.30–2.96 (mc, m, 16H); 3.59 (CHCH3–COOBn, q, 2H,3JHH= 7 Hz); 3.77 (CHCH3–COOBn, q, 1H,3JHH= 7 Hz); 5.09 (CH2–arom, s, 2H); 5.10 (CH2–arom, s, 4H); 7.29–7.41 (arom, m, 15H). ESI-HRMS: 659.3797 [M+H]+(theor. [C38H51N4O6]+= 659.3803). EA (C38H50N4O6∙0.3TFA∙2.0H2O, MR= 729.0): C 63.6 (63.0); H 7.5 (6.9); N 7.7 (7.4); F 2.4 (2.2). Synthesis of intermediate (7): In a glass vial (20 mL), Cbz2cyclen (free base; 242 mg; 549 µmol; 1.0 equiv.) was dissolved in MeCN (12 mL) followed by addition of solid (CH2O)n (50 mg; 1.67 mmol; 3.0 equiv.) and PhP(OMe)2 (350 µL; 2.2 mmol; 4.0 equiv.). The resulting suspension was stirred 24 h at 80°C. Reaction mixture was then filtered through syringe microfilter (PTFE) and the filtrate was evaporated to dryness. Residue was purified by preparative HPLC (C18; H2O‒MeCN gradient with TFA additive). Fractions with product were combined, neutralized with dil. aq. NaHCO3 and evaporated to dryness. Residue was dissolved in DCM (75 mL) and H2O (75 mL) and transferred to a separatory funnel. After shaking, the bottom phase was separated. Aqueous phase was further extracted with DCM (3 × 25 mL). Combined organic layers were dried with anhydrous Na2SO4, filtered through glass frit (S3) and evaporated to dryness. The residue was further dried on high vacuum overnight to give product in the form of free base as nearly colourless oil. Yield: 259 mg (61%; 1 step; based on Cbz2cyclen). NMR (CD3CN):1H δH 2.30‒3.38 (mc, CH2‒P, m, 16+4H); 3.47 (CH3, d, 6H,3JHP = 11); 5.00‒5.12 (CH2‒Ph, m, 4H); 7.27‒7.40 (Ph, m, 10H); 7.40‒7.51 (Ph, m, 4H); 7.51‒7.60 (Ph, m, 2H); 7.60‒7.82 (Ph, m, 4H).31P δP42.8 (m). ESI-HRMS: 777.3178 [M+H]+(theor. [C40H51N4O8P2]+= 777.3177). Synthesis of intermediate (8): Pear-shaped glass flask (50 mL) was charged with 7 (250 mg; 322 µmol) and magnetic stirrer and three times secured with argon. Solid Pd@C (50 mg) was then added followed by another securing with argon (three times). Under constant flow of argon was then added MeOH (25 mL) through septum. Argon input was then removed and H2 gas (from balloon) was allowed to bubble through the mixture for 2 h at RT. Catalyst was then filtered off using syringe microfilter (PTFE; filter was further washed with MeOH). Filtrate was evaporated to dryness and once co-evaporated with DCM. The residue was further dried on high vacuum overnight to give product in the form of free base as nearly colourless oil. Yield: 161 mg (98%; 1 step; based on 8). NMR (CD3CN):1H δH2.28‒3.14 (mc, CH2‒P, m, 16+4H); 3.53 (CH3, d, 3H,3JHP= 11); 3.54 (CH3, d, 3H,3JHP= 11); 7.46‒ 7.63 (Ph, m, 6H); 7.71‒7.83 (Ph, m, 4H).31P δP43.3 (m). ESI-HRMS: 509.2438 [M+H]+(theor. [C24H39N4O4P2]+= 509.2441). Synthesis of intermediate (9): In a 50 mL round bottom glass flask, methyl 4- fluoropicolinate (250 mg, 1.61 mmol, 1.0 equiv.) was dissolved in MeOH (16 mL) and NaBH4(180 mg, 4.78 mmol, ~3.0 equiv.) was added while stirring. The reaction was stirring for 3 hours at RT after which H2O (1 mL) was added. Liquids were evaporated on rotary evaporator and residue was suspended in saturated aqueous solution of K2CO3(25 mL). Water phase was extracted with DCM (2×25 mL) and solvent was evaporated on rotary evaporator. The residue was dissolved in DCM (11 mL) and Et3N (280 µL, 2.02 mmol, 1.3 eqiv.) was added. The reaction mixture was put on ice bath. After 20 minutes on ice bath methanesulfonyl chloride (146 µL, 1.88 mmol, 1.23 eqiv.) was added and reaction mixture was stirred on ice bath. After 15 minutes the reaction mixture was let to warm to RT a was let stirring for 1 hour. Reaction mixture was diluted with DCM (40 mL) and washed with H2O (2×40 mL), brine (40 mL). Organic phase was dried over Na2SO4, filtered and liquids were evaporated on rotary evaporator to yield the product as dark orange oil. Yield: 240 mg (77%; 2 steps; based on methyl 4-fluoropicolinate) NMR (CDCl3):1H δH 3.12 (CH3, s, 3H); 5.33 (CH2–arom, s, 2H); 7.04 (arom.,ddd, J = 8, 6, 3 Hz 1H); 7.24 (arom., dd, J = 9, 3 Hz, 1H); 8.58 (arom., dd, J = 8, 6 Hz, 1H);13C{1H} δC 38.2 (CH3, s); 70.4 (CH2–arom, s); 110.5 (arom, d,2JCF = 18 Hz); 111.7 (arom, d,2JCF = 17 Hz); 152 (arom, d,3JCF = 7 Hz); 157.2 (arom, d,3JCF = 7 Hz); 169.5 (arom, d,1JCF = 265 Hz).19F{1H} δF –99.3 (s). CI-HRMS: 206.0281 [M+H]+(theor. [C7H9O3N1F1S1]+= 206.0282). Synthesis of intermediate (10): In a 50 mL round bottom glass flask, (5-fluoropyridin- 2-yl)methanol (173 mg, 1.36 mmol, 1.0 equiv.) was dissolved in DCM (10 mL) and Et3N (250 µL, 1.80 mmol, 1.3 eqiv.) was added. The reaction mixture was put on ice bath. After 20 minutes on ice bath methanesulfonyl chloride (142 µL, 1.84 mmol, 1.35 eqiv.) was added and reaction mixture was stirred on ice bath. After 15 minutes the reaction mixture was let warm to RT a was let stirring for another 1 hour. Reaction mixture was diluted with DCM (40 mL) and washed with H2O (2×40 mL), brine (40 mL). Organic phase was dried over Na2SO4, filtered and liquids were evaporated on rotary evaporator to yield the product as orange solid. Yield: 219 mg (78%; 1 steps; based on (5-fluoropyridin-2-yl)methanol) NMR (CDCl3):1H δH 3.08 (CH3, s, 3H); 5.31 (CH2–arom, s, 2H); 7.42 – 7.53 (arom.,m, 2H); 8.47 (arom., dd, J = 3, 1 Hz, 1H);13C{1H} δC 38.2 (CH3, s); 70.8 (CH2–arom, s); 124.1 (arom, d,3JCF = 3 Hz); 124.2 (arom, d,2JCF = 11 Hz); 138 (arom, d,2JCF = 24 Hz); 149.6 (arom, d,4JCF = 4 Hz); 159.4 (arom, d,1JCF = 258 Hz).19F{1H} δF – 125.8 (s). CI-HRMS: 206.0284 [M+H]+(theor. [C7H9O3N1F1S1]+= 206.0282). Synthesis of intermediate (11): Pear-shaped glass flask (500 mL) was charged with ethylL-lactate (6.70 g; 56.7 mmol; 1.00 equiv.) and magnetic stirrer and three times briefly secured with argon. Under constant flow of argon was then added dry DCM (200 mL) through septum and the mixture was cooled with an ice bath (5 °C) followed by dropwise addition of triflic anhydride (10.0 mL; 59.4 mmol; 1.05 equiv.) and immediately followed by dropwise addition of dry pyridine (4.80 ml; 59.6 mmol; 1.05 equiv.). The ice bath was then removed and the mixture was further stirred at RT for 1 h. Resulting suspension was then directly purified by column chromatography (250 g SiO2, DCM). Combined fractions with product were evaporated to dryness and briefly dried on high vacuum to give product as faint brown oil. Yield: 12.02 g (85%; 1 step; based on ethyl L-lactate). NMR (DMSO-d6):1H δH1.26 (CH3‒CH2, t, 3H,3JHH= 7); 1.51 (CH3‒CH, d, 3H,3JHH= 7); 4.19–4.31 (CH2, m, 2H); 5.27 (CH, q, 1H,3JHH= 7).19F{1H} δF−77.7 (s). CI-HRMS: 251.0193 [M+H]+(theor. [C6H10O5S1F3]+= 251.0196). Synthesis of intermediate (12): In a pear shaped glass flask (25 mL), cyclen (free base; 200 mg; 1160 µmol; 1.0 equiv.) was dissolved in dry DCM (12 mL) followed by dropwise addition of tert-butyl 4-nitrophenyl carbonate (278 mg; 1.16 mmol; 1.0 equiv.) in dry DCM (4 mL). The resulting mixture was stirred at RT for 30 minutes. Liquids were evaporated and residue was purified by preparative HPLC (C18; H2O‒MeCN gradient with TFA additive). Fractions with product were joined and lyophilized to give product in the form of TFA salt as transparent solid. Yield: 245 mg (42 %, 1 step, based on cyclen assuming 12∙2TFA). NMR (CD3CN):1H δH 1.44 (CH3, s, 9H); 3.01–3.08 (mc, m, 4H); 3.11–3.18 (mc, m, 4H); 3.23–3.29 (mc, m, 4H); 3.39–3.48 (mc, m, 4H).13C{1H} (100.6 MHz, T = 300 K) δC 27.48 (CH3, s); 44.14 (mc, s); 44.70 (mc, s); 47.69 (mc, s); 47.96 (mc, s); 80.57 (C–CH3, s); 155.66 (CO, s). ESI-HRMS: 273.2284 [M+H]+(theor. [C13H29N4O2]+= 273.2285). Synthesis of intermediate (13): In a glass vial (20 mL), intermediate 12 (100 mg; 200 µmol; 1.0 equiv.) and 11 (367 mg; 1.47 mmol; 7.4 equiv.) were dissolved in dry MeCN (15 mL) followed by addition of K2CO3(254 mg; 1.84 mmol; 9.2 equiv.). The resulting mixture was stirred at RT for 15 h. Solids were filtered off using syringe microfilter (PTFE) and filtrate was evaporated to dryness. Residue was purified by preparative HPLC (C18; H2O‒MeCN gradient with FA additive). Fractions containing pure product in the form of ethyl ester were pooled, evaporated to dryness. The residue was dissolved in neat TFA (3 mL) and stirred for 15 h at RT. Volatiles were removed on rotary evaporator and the residue was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions with product were joined and lyophilized to give product as transparent honey. Yield: 21 mg (20 %, 2 steps, based on 12 assuming 13∙FA). NMR (DMSO-d6):1H δH 1.17–1.24 (CH3, m, 18H); 2.40 (mc, m, 2H); 2.55 (mc, m, 2H); 2.73–2.94 (mc, m, 10H); 3.21 (mc, m, 2H); 3.63 (CHCH3–COOEt, q, 2H,3JHH = 7 Hz); 3.74 (CHCH3–COOBn, q, 1H,3JHH = 7 Hz); 4.07–4.13 (O-CH2–CH3, m, 6H); 8.8 (NH, bs, 1H).13C{1H} δC 14.1 (CH3, s); 14.22 (CH3, s); 14.24 (CH3, s); 15.2 (CH3, s); 44.8, 45.2, 46.6, 50.0 (mc); 54.8 (CCH3H‒COCH2CH3, s); 57.8 (CCH3H‒COCH2CH3, s); 60.0 (CCH3H‒COCH2CH3, s); 60.04 (CCH3H‒COCH2CH3, s); 172.5 (CO, s); 172.8 (CO, s). ESI-HRMS: 473.3329 [M+H]+(theor. [C23H45N4O6]+= 473.3334). Example 2: synthesis of ligands of ligand L1(2,2',2''-(10-((4-nitropyridin-2-yl)methyl)- 1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid): Solution 1 (168 mg, 0.97 mmol, 1.1 equiv.) was added to a solution of t-BuDO3A·HBr (527 mg, 0.89 mmol, 1.0 equiv.) in MeCN (18 mL) followed by addition of dried K2CO3(489 mg, 3.54 mmol, 4.0 equiv.) and the resulting suspension was stirred for 24 hours at RT. The solids were filtered off and the filtrate was concentrated on rotary evaporator. Resulting oil was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions containing pure product in the form of tert-butyl ester were pooled, evaporated to dryness and several times co-evaporated with MeOH to remove MeCN. The residue was dissolved in neat TFA (6 mL) and stirred for 16 h at RT. Volatiles were removed on rotary evaporator and the residue was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % TFA in the mobile phase). Fractions with product were joined and lyophilized to give product in the form of TFA salt as yellowish solid. Yield: 342 mg (56 %, 2 steps, based on t-BuDO3A·HBr). NMR (D2O):1H δH3.09–3.70 (mc, CH2–COOH, m, 22H); 4.35 (CH2–arom, bs, 2H); 8.14 (arom, dd, 1H,3JHH= 6 Hz,4JHH= 2 Hz); 8.40 (arom, d, 1H,3JHH= 2 Hz); 8.87 (arom, d, 1H,3JHH= 6Hz);13C{1H} δC49.7 (mc, s); 50.6 (mc, s); 51.0 (mc, s); 54.7 (mc, s); 55.8 (CH2‒ COOH, s); 57.8 (CH2‒arom, s); 117.4 (arom, s); 118.6 (arom, s); 152.0 (arom, s); 155.8 (arom, s); 173.0 (CO, s); 175.5 (CO, s). ESI-HRMS: 483.2195 [M+H]+(theor. [C20H31N6O8]+= 483.2198). EA (C20H30N6O8∙1.8TFA∙1.0H2O, MR = 687.7): C 41.2 (42.0); H 4.7 (5.2); N 12.2 (12.8); F 14.9 (15.9). Synthesis of ligand L2(2,2',2''-(10-((6-nitropyridin-2-yl)methyl)-1,4,7,10- tetraazacyclododecane-1,4,7-triyl)triacetic acid): Solution 2 (74 mg, 0.34 mmol, 1.1 equiv.) in MeCN (2 mL) was added to a solution of t-BuDO3A·HBr (185 mg, 0.31 mmol, 1.0 equiv.) in MeCN (42 mL) followed by addition of dried K2CO3 (215 mg, 1.55 mmol, 5 equiv.) and the resulting suspension was stirred for 24 hours at RT. The solids were filtered off and the filtrate was concentrated on rotary evaporator. Resulting oil was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions containing pure product in the form of tert-butyl ester were pooled, evaporated to dryness and several times co-evaporated with MeOH to remove MeCN. The residue was dissolved in neat TFA (3 mL) and stirred for 16 h at RT. Volatiles were removed on rotary evaporator and the residue was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions with product were joined and lyophilized to give product in the form of FA salt as yellowish solid. Yield: 50 mg (31 %, 2 steps, based on t-BuDO3A·HBr). NMR (D2O):1H δH 2.94–3.18 (mc, m, 8H); 3.33–3.51 (mc, m, 8H); 3.54 (CH2–COOH, s, 2H); 3.61–3.82 (CH2–COOH, m, 4H); 4.05 (CH2–arom, s, 2H); 8.16 (arom, t, 1H,3JHH = 8 Hz); 8.24–8.33 (arom, m, 2H);13C{1H} δC 48.6 (mc, s); 51.1 (mc, s); 52.2 (mc, s); 54.1 (CH2‒ COOH, s); 56.8 (CH2‒COOH, s); 58.2 (CH2‒arom, s); 118.5 (arom, s); 132.2 (arom, s); 143.6 (arom, s); 155.9 (arom, s); 157.8 (arom, s); 169.9 (CO, s); 175.3 (CO, s). ESI-HRMS: 483.2196 [M+H]+(theor. [C20H32N5O6]+= 483.2198). EA (C20H30N6O8∙0.8FA∙0.2H2O, MR= 522.9): C 47.8 (47.8); H 6.2 (6.2); N 16.1 (16.1). Synthesis of ligand L3(2,2',2''-(10-((3-fluoropyridin-2-yl)methyl)-1,4,7,10- tetraazacyclododecane-1,4,7-triyl)triacetic acid): Solution of 3 (106 mg, 0.74 mmol, 1.1 equiv.) in MeCN (1 mL) was added to a solution of t-BuDO3A·HBr (400 mg, 0.67 mmol, 1.0 equiv.) in MeCN (13.4 mL) followed by addition of dried K2CO3(464 mg, 3.36 mmol, 5.0 equiv.) and the resulting suspension was stirred for 19 hours at RT. The solids were filtered off and the filtrate was concentrated on rotary evaporator. Resulting oil was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions containing pure product in the form of tert-butyl ester were pooled, evaporated to dryness and several times co-evaporated with MeOH to remove MeCN. The residue was dissolved in neat TFA (9 mL) and stirred for 19 h at RT. Volatiles were removed on rotary evaporator and the residue was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions with product were joined and lyophilized to give product in the form of FA / TFA salt as white solid. Yield: 273 mg (70 %, 2 steps, based on t-BuDO3A·HBr). NMR (ref tBuOH) (D2O):1H δH 3.00–3.30 (mc, m, 8H); 3.30–3.40 (mc, m, 4H); 3.40–3.56 (mc, m, 4H); 3.61 (CH2–COOH, s, 2H); 3.68 (CH2–COOH, s, 4H); 4.36 (CH2– arom, s, 2H); 7.74 (arom, dt, 1H,3JHH = 9 Hz,3JHH = 5 Hz); 8.00 (arom, t, 1H,3JHH = 9 Hz); 8.51–8.57 (arom, m, 1H);13C{1H} δC 49.8(mc, s); 50.0 (mc, s); 50.6 (mc, s); 50.9 (mc, s); 51.1 (CH2‒arom, s); 54.9 (CH2‒COOH, s); 55.9 (CH2‒COOH, s); 127.5 (arom, d,3JCF = 6 Hz ); 128.4 (arom, d,2JCF = 18 Hz ); 140.7 (arom, s); 144.6 (arom, s); 158.8 (arom, d,1JCF = 255 Hz ); 171.3 (CO, s); 173.4 (CO, s).19F{1H} δF –122.2 (s). ESI-HRMS: 456.2254 [M+H]+(theor. [C20H31N5O6F1]+= 456.2253). EA (C20H30N5O6F1∙0.7TFA∙1.1FA, MR = 585.9): C 46.1 (46.4); H 5.7 (5.6); N 12.0 (12.2); F 10.1 (9.7). Synthesis of ligand L4(2,2',2''-(10-((4-fluoropyridin-2-yl)methyl)- 1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid): Solution of 4 (70 mg, 0.48 mmol, 1.02 equiv.) in MeCN (0.7 mL) was added to a solution of t-BuDO3A·HBr (281 mg, 0.47 mmol, 1.0 equiv.) in MeCN (9 mL) followed by addition of dried K2CO3(266 mg, 1.92 mmol, 4 equiv.) and the resulting suspension was stirred for 24 hours at RT. The solids were filtered off and the filtrate was concentrated on rotary evaporator. Resulting oil was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions containing pure product in the form of tert-butyl ester were pooled, evaporated to dryness and several times co-evaporated with MeOH to remove MeCN. The residue was dissolved in neat TFA (5 mL) and stirred for 16 h at RT. Volatiles were removed on rotary evaporator and the residue was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % TFA in the mobile phase). Fractions with product were joined and lyophilized to give product in the form of TFA salt as white solid. Yield: 208 mg (58 %, 2 steps, based on t-BuDO3A·HBr). NMR (ref tBuOH) (D2O): ):1H δH2.95–3.98 (mc, CH2–COOH, m, 22H); 4.20 (CH2–arom, bs, 2H); 7.80 (arom, m, 1H); 7.96 (arom, m, 1H); 8.85 (arom, m, 1H);13C{1H} δC49.1(mc, s); 50.9 (mc, s); 52.6 (mc, s); 53.6 (mc, s); 54.4 (CH2‒arom, s); 56.2 (mc, s); 115.9 (arom, m); 117.7 (arom, m); 148.0 (arom, m); 155.6 (arom, m); 158.8 (arom, d,1JCF= 255 Hz ); 175.4 (CO, s); 176.1 (CO, s).19F{1H} δF–79.3 (s). ESI-HRMS: 456.2250 [M+H]+(theor. [C20H31N5O6F1]+= 456.2253). EA (C20H30F1N5O6∙2.6TFA∙0.3H2O, MR = 757.3): C 40.0 (39.9); H 4.4 (4.5); N 9.3 (9.1); F 22.1 (22.2). Synthesis of ligand L5(2,2',2''-(10-((5-fluoropyridin-2-yl)methyl)- 1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid): Solution of 10 (107 mg, 0.52 mmol, 1.1 equiv.) in MeCN (2 mL) was added to a solution of t-BuDO3A·HBr (282 mg, 0.47 mmol, 1.0 equiv.) in MeCN (9.5 mL) followed by addition of dried K2CO3 (327 mg, 2.37 mmol, 5.0 equiv.) and the resulting suspension was stirred for 20 hours at RT. The solids were filtered off and the filtrate was concentrated on rotary evaporator. Resulting oil was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions containing pure product in the form of tert-butyl ester were pooled, evaporated to dryness and several times co-evaporated with MeOH to remove MeCN. The residue was dissolved in neat TFA (6 mL) and stirred for 19 h at RT. Volatiles were removed on rotary evaporator and the residue was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions with product were joined and lyophilized to give product in the form of FA / TFA salt as white solid. Yield: 165 mg (63 %, 2 steps, based on t-BuDO3A·HBr). NMR (ref tBuOH) (D2O):1H δH 2.90–3.91 (mc, CH2–COOH, m, 22H); 4.30 (CH2–arom, s, 2H); 7.67–8.05 (arom, m, 2H); 8.58 (arom, s, 1H);13C{1H} δC 50.0, 50.1, 50.2 (mc); 50.5 (CH2‒COOH, s); 55.3 (CH2‒COOH, s); 56.7 (CH2‒arom, s); 127.6 (arom, s); 128.5 (arom, d,2JCF = 17 Hz ); 137.1 (arom, d,2JCF = 28 Hz); 148.4 (arom, s); 160.1 (arom, d,1JCF= 255 Hz ); 171.3 (CO, s); 173.4 (CO, s).19F{1H} δF–122.0 (s). ESI-HRMS: 456.2254 [M+H]+(theor. [C20H31N5O6F1]+= 456.2253). EA (C20H30N5O6F1∙0.7TFA∙1.3FA, MR= 595.1): C 45.8 (45.6); H 5.6 (5.6); N 11.8 (11.7); F 9.9 (10.1). Synthesis of ligand L6(2,2',2''-(10-((6-fluoropyridin-2-yl)methyl)-1,4,7,10- tetraazacyclododecane-1,4,7-triyl)triacetic acid): Solution 2-(chloromethyl)-6-fluoropyridine (81mg, 0.55 mmol, 1.1 equiv.) in MeCN (1 mL) was added to a solution of t-BuDO3A·HBr (300 mg, 0.50 mmol, 1.0 equiv.) in MeCN (10 mL) followed by addition of dried K2CO3 (278 mg, 2.02 mmol, 4 equiv.) and the resulting suspension was stirred for 16 hours at RT. The solids were filtered off and the filtrate was concentrated on rotary evaporator. Resulting oil was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions containing pure product in the form of tert-butyl ester were pooled, evaporated to dryness and several times co-evaporated with MeOH to remove MeCN. The residue was dissolved in neat TFA (7 mL) and stirred for 16 h at RT. Volatiles were removed on rotary evaporator and the residue was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions with product were joined and lyophilized to give product in the form of FA / TFA salt as white solid. Yield: 210 mg (73 %, 2 steps, based on t-BuDO3A·HBr). NMR (D2O):1H δH3.01– 3.65 (mc, CH2–COOH, m, 22H); 3.98 (CH2–arom, s, 2H); 7.06 (arom, m, 1H); 7.58 (arom, m, 1H); 7.96 (arom, m, 1H);13C{1H} δC48.8 (mc, s); 48.9 (mc, s); 51.2 (mc, s); 51.8 (mc, s); 54.5 (CH2‒COOH, s); 56.8 (CH2‒COOH, s); 58.0 (CH2‒arom, s); 109.7 (arom, d,2JCF= 35 Hz ); 123.0 (arom, d,4JCF= 4 Hz); 144.7 (arom, d,3JCF= 8 Hz); 154.8 (arom, s); 163.3 (arom, d,1JCF= 239 Hz); 170.2 (CO, s); 174.5 (CO, s).19F{1H} δF–82.1 (s). ESI-HRMS: 478.2069 [M+Na]+(theor. [C20H30N5O6F1Na1]+= 478.2072). EA (C20H30N5O6F1∙0.5TFA∙1.4FA, MR= 576.9): C 46.6 (46.9); H 5.8 (5.8); N 12.1 (12.4); F 8.2 (8.0). Synthesis of ligand L7(2,2'-(4-((4-fluoropyridin-2-yl)methyl)-1,4,7,10- tetraazacyclododecane-1,7-diyl)diacetic acid): Solution of 4 (127 mg, 0.87 mmol, 1.00 equiv.) in MeCN (1 mL) was added to a solution of t-BuDO2A (350 mg, 0.87 mmol, 1.0 equiv.) in MeCN (44 mL) followed by addition of dried K2CO3(121 mg, 0.87 mmol, 1 equiv.) and the resulting suspension was stirred for 24 hours at RT. The solids were filtered off and the filtrate was concentrated on rotary evaporator. Resulting oil was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % TFA in the mobile phase). Fractions containing pure product in the form of tert-butyl ester were pooled, evaporated to dryness and several times co-evaporated with MeOH to remove MeCN. The residue was dissolved in neat TFA (5 mL) and stirred for 16 h at RT. Volatiles were removed on rotary evaporator and the residue was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % TFA in the mobile phase). Fractions with product were joined and lyophilized to give product in the form of TFA salt as white solid. Yield: 206 mg (36 %, 2 steps, based on t-BuDO2A). NMR (D2O):1H δH2.96–3.60 (mc, CH2–COOH, m, 20H); 4.70 (CH2–arom, s, 2H); 7.29 (arom, ddd, 1H,3JHF= 8 Hz,3JHH= 6 Hz,4JHH= 3 Hz); 7.34 (arom, dd, 1H,3JHF= 9 Hz,4JHH= 3 Hz); 8.51 (arom, dd, 1H,4JHF= 8 Hz,3JHH= 6 Hz);13C{1H} δC43.0 (mc, s); 49.2 (mc, s); 50.0 (mc, s); 52.4 (mc, s); 54.5 (CH2‒COOH, s); 57.8 (CH2‒arom, d,4JCF= 3 Hz); 112.5 (arom, d,2JCF= 19 Hz ); 113.5 (arom, d,2JCF= 17 Hz); 152.6 (arom, d,3JCF= 8 Hz); 153.0 (arom, d,3JCF= 8 Hz); 170.3 (arom, d,1JCF = 264 Hz); 174.9 (CO, s).19F{1H} δF –100.1 (s). ESI-HRMS: 398.2197 [M+H]+(theor. [C18H29N5O4F1]+= 398.2198). EA (C18H28F1N5O4∙2.2TFA∙0.9H2O, MR = 664.5): C 40.0 (39.9); H 4.4 (4.5); N 9.3 (9.1); F 22.1 (22.2). Synthesis of ligand L8((2S,2'S,2''S)-2,2',2''-(10-((3-fluoropyridin-2- yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)tripropionic acid): Solution of 3 (13 mg, 92.2 µmol, 1.5 equiv.)) was added to a solution of 6 (45 mg, 61.7 µmol, 1.0 equiv.) in MeCN (1.2 mL) followed by addition of dried K2CO3 (43 mg, 307 µmol, 5 equiv.) and the resulting suspension was stirred for 16 hours at RT. The solids were filtered off and the filtrate was concentrated on rotary evaporator. Resulting oil was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions containing pure product in the form of tert-butyl ester were pooled, evaporated to dryness and several times co-evaporated with MeOH to remove MeCN. The residue was dissolved in MeOH (3 mL) followed by addition of Pd@C catalyst (10%, 10 mg). The flask was briefly washed with argon and then H2 gas (from balloon) was introduced. After 30 min, the catalyst was filtered off and the mixture was evaporated to dryness. The residue was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % TFA in the mobile phase). Fractions with product were joined and lyophilized to give product in the form of TFA salt as white solid. Yield: 4.2 mg (13 %, 2 steps, based on 6). NMR (D2O):1H δH 1.26–1.30 (CH3, m, 3H); δH 1.49 (CH3, bs, 6H); δH2.70–3.95 (mc, CH2–COOH, m, 19H); 4.23–4.30 (CH2–arom, m, 2H); 7.60 (arom, m, 1H); 7.84 (arom, m, 1H); 8.47 (arom, m, 1H);13C{1H} δC8.6 (CH3, s); 10.3 (CH3, s); 47.7, 47.8 (mc); 50.7 (CH2‒arom, s); 56.4 (CCH3H‒COOH, s); 61.9 (CCH3H‒COOH, s); 126.6 (arom, d,3JCF= 6 Hz ); 127.4 (arom, d,2JCF= 20 Hz); 142.8 (arom, d,2JCF= 19 Hz); 144.7 (arom, d,4JCF= 5 Hz); 159.1 (arom, d,1JCF= 254 Hz); 177.2 (CO, s).19F{1H} δF–123.2 (s). ESI-HRMS: 498.2719 [M+H]+(theor. 53.0 (53.4); H 7.2 (7.0); 2-yl)methyl)-1,4,7,10- tetraazacyclododecane-1,7-diyl)bis(methylene))bis(phenylphosphinic acid)): Solution of 3 (6 mg, 40.0 µmol, 0.5 equiv.)) was added to a solution of 8 (41 mg, 80.6 µmol, 1.0 equiv.) in MeCN (1 mL) followed by addition of dried K2CO3 (11 mg, 80.6 µmol, 1 equiv.) and the resulting suspension was stirred for 16 hours at RT. The solids were filtered off and the filtrate was concentrated on rotary evaporator. Resulting oil was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions containing pure product in the form of methyl ester were pooled, evaporated to dryness and several times co-evaporated with MeOH to remove MeCN. The residue was dissolved in mixture of MeOH (1 mL) and H2O (0.2 mL) followed by addition of 1M NaOH (2 mL). The reaction mixture was neutralized with formic acid after 4 days of stirring at RT and the mixture was evaporated to dryness. The residue was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % TFA in the mobile phase). Fractions with product were joined and lyophilized to give product in the form of white solid. Yield: 9.1 mg (18 %, 2 steps, based on 8). NMR (D2O):1H δH 2.97–3.24 (mc, CH2‒PhPOOH m, 20H); 4.54 (CH2–arom, d, 2H,5JHF = 2 Hz,); 7.46–7.81 (arom, m, 12H); 8.42–8.47 (arom, m, 1H);13C{1H} δC 44.0 (mc), 50.3, 51.6, 51.9, 54.3, 55.1 (mc, CH2‒PhPOOH); 52.4 (CH2‒arom, s); 125.9 (arom, d,2JCF = 19 Hz ); 127.9 (arom, d,3JCF = 5 Hz); 129.2 (arom, d,2JCP = 11 Hz); 131.2 (arom, d,2JCP = 10 Hz); 131.9 (arom, d,4JCP = 3 Hz); 137.5 (arom, d,2JCF = 15 Hz); 137.6 (arom, d,1JCP = 60 Hz); 146.4 (arom, d,4JCF = 5 Hz); 159.4 (arom, d,1JCF = 258 Hz); 177.2 (CO, s).19F{1H} δF –122.9 (s);31P{1H} δ 30.05. ESI-HRMS: 590.2454 [M+H]+(theor. [C28H39N5O4F1P2]+= 590.2456). EA (C28H38N5O4F1P2∙0.1TFA∙0.8H2O, MR = 615.4): C 55.0 (55.0); H 6.5 (6.3); N 4.0 (4.1); F 11.4 (11.2). Synthesis of ligand L10(2,2'-(4-((4-nitropyridin-2-yl)methyl)-1,4,7,10- tetraazacyclododecane-1,7-diyl)diacetic acid): Solution of 1 (376 mg, 2.18 mmol, 1.0 equiv.) was added to a solution of t-BuDO2A (873 mg, 2.18 mmol, 1.0 equiv.) in MeCN (109 mL) followed by addition of dried K2CO3(301 mg, 2.18 mmol, 1.0 equiv.) and the resulting suspension was stirred for 39 hours at RT. The solids were filtered off and the filtrate was concentrated on rotary evaporator. Resulting oil was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % TFA in the mobile phase). Fractions containing pure product in the form of tert-butyl ester were pooled, evaporated to dryness and several times co- evaporated with MeOH to remove MeCN. The residue was dissolved in neat TFA (6 mL) and stirred for 14 h at RT. Volatiles were removed on rotary evaporator and the residue was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % TFA in the mobile phase). Fractions with product were joined and lyophilized to give product in the form of TFA salt as yellowish solid. Yield: 358 mg (26 %, 2 steps, based Synthesis of ligand L11((2S,2'S,2''S)-2,2',2''-(10-((4-fluoropyridin-2-yl)methyl)-1,4,7,10- tetraazacyclododecane-1,4,7-triyl)tripropionic acid): Intermediate 9 (18.7 mg, 91.2 µmol, 1.5 equiv.)) was mixed with intermediate 6 (44.5 mg, 60.8 µmol, 1.0 equiv.) in tBuOH (2.4 mL) followed by addition of DIPEA (53 µl, 304 µmol, 5 equiv.) and the resulting solution was stirred for 16 hours at 80°C. The liquids were evaporated on rotary evaporator and resulting oil was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions containing pure product in the form of benzyl ester were pooled, evaporated to dryness and several times co-evaporated with MeOH to remove MeCN. The residue was dissolved in MeOH (3 mL) followed by addition AcOH (14 µl, 246 µmol, 4 eqiv.) and Pd@C catalyst (10%, 5 mg). The flask was briefly washed with argon and then H2 gas (from balloon) was introduced. After 30 min, the catalyst was filtered off and the mixture was evaporated to dryness. The residue was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % TFA in the mobile phase). Fractions with product were joined and lyophilized to give product in the form of TFA salt as white solid. Yield: 4.1 mg (13 %, 2 steps, based on 6 assuming L11∙0.1TFA). NMR (D2O, ref. tBuOH):1H δH1.11 (CH3, d, 3H,3JHH= 7 Hz); 1.22 (CH3, d, 3H,3JHH= 7 Hz); 1.46 (CH3, d, 3H,3JHH= 7 Hz,); 2.49–2.56 (mc, m, 2H); 2.70 (mc, m, 1H); 2.82–3.16 (mc, m, 9H); 3.27–3.46 (mc, m, 5H); 3.77 (CHCH3COOH, q, 1H,3JHH= 7 Hz ); 4.07 (CHCH3COOH, q, 1H,3JHH= 8 Hz); 4.13 (m, 1H); 4.50 (CH2–arom, d, 1H,2JHH= 13 Hz); 7.10 (arom, ddd, 1H,3JHF= 8 Hz,3JHH= 6 Hz,4JHH= 3 Hz); 7.60 (arom, dd, 1H,3JHF= 8 Hz,4JHH= 3 Hz);; 8.36 (arom, m, 1H);13C{1H} δC9.0 (CH3, s); 10.5 (CH3, s); 11.0 (CH3, s); 44.0, 46.4, 47.1, 47.4, 47.8, 48.2, 49.0 (mc, s); 58.1 (CH2‒arom, s); 57.4 (CCH3H‒COOH, s); 61.3 (CCH3H‒COOH, s); 62.3 (CCH3H‒ COOH, s); 112.8 (arom, d,2JCF= 18 Hz ); 115.1 (arom, d,2JCF= 18 Hz); 150.9 (arom, d,3JCF= 8 Hz); 160.0 (arom, s); 170.8 (arom, d,1JCF= 266 Hz); 172.0 (CO, s); 174.9 (CO, s); 180.2 (CO, s).19F{1H} δF– 96.4 (s). ESI-HRMS: 498.2724 [M+H]+(theor. [C23H37N5O6F1]+= 498.2722). Synthesis of ligand L12((2S,2'S,2''S)-2,2',2''-(10-((4-nitropyridin-2- yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)tripropionic acid): Intermediate 1 (8.2 mg, 47.6 µmol, 1.6 equiv.)) was mixed with intermediate 13 (15.0 mg, 28.9 µmol, 1.0 equiv.) in tBuOH (1.3 mL) followed by addition of DIPEA (28 µl, 158 µmol, 5.5 equiv.) and the resulting solution was stirred for 16 hours at 80°C. The liquides were evaporated on rotary evaporator and resulting oil was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions containing pure product in the form of ethyl ester were pooled, evaporated to dryness and several times co-evaporated with MeOH to remove MeCN. The residue was dissolved in mixture of MeCN (500 µl) and H2O (330 µl) followed by addition of 1M solution of LiOH (296 µl, 296 µmol, 10 equiv.) After 47 hours, the reaction mixture was quenched with FA (10 µl, 220 µmol, 7.6 equiv.) and the mixture was evaporated to dryness. The residue was purified on preparative HPLC (C18, MeCN / H2O gradient with 0.1 % FA in the mobile phase). Fractions with product were joined and lyophilized to give product in the form of FA salt as yellowish solid. Yield: 6.0 mg (33 %, 2 steps, based on 13). NMR (D2O, ref. tBuOH):1H δH1.24 (CH3, d, 3H,3JHH= 7 Hz); 1.25 (CH3, d, 3H,3JHH= 7 Hz); 1.59 (CH3, d, 3H,3JHH= 7 Hz); 2.60–2.64 (mc, m, 2H); 2.83–3.05 (mc, CHCH3COOH, m, 6H); 3.10–3.31 (mc, m, 5H); 3.41–3.55 (mc, CH2–arom, m, 4H); 3.98 (CHCH3COOH, q, 1H,3JHH = 7 Hz ); 4.20 (CHCH3COOH, q, 1H,3JHH = 8 Hz); 4.27 (m, 1H); 4.78 (CH2– arom, m, 1H); 8.07 (arom, dd, 1H,3JHH = 6 Hz,4JHH = 2 Hz); 8.52 (arom, d, 1H,4JHH = 2 Hz); 8.75 (arom, d, 1H,3JHH = 6 Hz);13C{1H} δC 8.0 (CH3, s); 10.0 (CH3, s); 10.7 (CH3, s); 43.5, 46.1, 46.9, 47.4, 47.5, 47.8, 48.1, 49.2 (mc, s); 55.7 (CCH3H‒COOH, s); 58.2 (CH2‒arom, s); 60.9 (CCH3H‒COOH, s); 62.1 (CCH3H‒ COOH, s); 117.2 (arom); 120.1 (arom); 151. (arom); 156.0 (arom, s); 160.3 (arom); 171.6 (CO, s); 174.7 (CO, s); 177.1 (CO, s). ESI-HRMS: 525.2667 [M+H]+(theor. [C23H37N6O8]+= 525.2667). EA (C23H36N6O8∙2.2FA∙0.6H2O, MR= 627.4): C 47.5 (47.6); H 6.6 (6.7); N 13.2 (13.1). Example 3: synthesis of metal chelates Synthesis of complex [Gd(L1)]: In a glass vial (25 mL), L1∙1.8TFA∙1.0H2O (138 mg, 0.20 mmol, 1.0 equiv.) was dissolved in aq. MOPS / NaOH buffer (3M, pH 7.0, 2 mL, 6 mmol, 30 equiv.) followed by addition of aq. GdCl3(100 mM, 2.1 mL, 0.21 mmol, 1.05 equiv.) and the resulting solution was stirred at RT for 15 mins. The mixture was then purified by preparative HPLC (C18, H2O / MeCN gradient with 0.1% FA additive). Fractions with product were joined and lyophilized to give product as a white solid with yellow tint. Yield: 111 mg (81%, 1 step, based on L1∙1.8TFA∙1.0H2O). ESI-HRMS: 638.1203 [M+H]+(theor. [C20H28N6O8Gd]+= 638.1204). EA (C20H27N6O8Gd∙2.5H2O, MR = 687.0): C 35.0 (35.4); H 4.7 (4.4); N 12.2 (11.9); Gd 22.9 (19.4). Synthesis of complex [Gd(L2)]: In a glass vial (4 mL), L2∙0.8FA∙0.2H2O (37.4 mg, 71.5 µmol, 1.0 equiv.) was dissolved in aq. MOPS / NaOH buffer (3M, pH 7.0, 0.95 mL, 2.85 mmol, 40 equiv.) followed by addition of aq. GdCl3 (100 mM, 0.78 mL, 78 µmol, 1.1 equiv.) and the resulting solution was stirred at RT for 1 hour. The mixture was then purified by preparative HPLC (C18, H2O / MeCN gradient with 0.1% FA additive). Fractions with product were joined and lyophilized to give product as a solid with yellow tint. Yield: 33 mg (62 %, 1 step, based on L2∙0.8FA∙0.2H2O). ESI-HRMS: 638.1201 [M+H]+(theor. [C20H28N6O8Gd]+= 638.1204). EA (C20H27N6O8Gd∙2.1FA∙1.4H2O, MR= 758.6): C 35.0 (34.9); H 4.5 (4.4); N 11.1 (11.2); Gd 20.7 (20.8). Synthesis of complex [Gd(L3)]: In a glass vial (25 mL), L3∙0.7TFA∙1.1FA (100 mg, 0.17 mmol, 1.0 equiv.) was dissolved in aq. MOPS / NaOH buffer (3M, pH 7.0, 1.7 mL, 5.1 mmol, 30 equiv.) followed by addition of aq. GdCl3(100 mM, 1.9 mL, 0.19 mmol, 1.1 equiv.) and the resulting solution was stirred at RT for 1 hour. The mixture was then purified by preparative HPLC (C18, H2O / MeCN gradient with 0.1% FA additive). Fractions with product were joined and lyophilized to give product as white solid. Yield: 98 mg (88 %, 1 step, based on L3∙0.7TFA∙1.1FA). ESI-HRMS: 611.1257 [M+H]+(theor. [C20H28N5O6Gd1F1]+= 611.1259). EA (C20H27N5O6Gd1F1∙2.5H2O, MR= 654.7): C 36.7 (37.2); H 4.9 (4.9); N 10.7 (10.6); Gd 24.0 (21.4); F 2.9 (2.6). Synthesis of complex [Gd(L4)]: In a glass vial (25 mL), L4∙2.6TFA∙0.3H2O (156 mg, 0.21 mmol, 1.0 equiv.) was dissolved in aq. MOPS / NaOH buffer (3M, pH 7.0, 1.9 mL, 5.8 mmol, 28 equiv.) followed by addition of aq. GdCl3 (100 mM, 2.16 mL, 0.22 mmol, 1.05 equiv.) and the resulting solution was stirred at RT for 15 mins. The mixture was then purified by preparative HPLC (C18, H2O / MeCN gradient with 0.1% FA additive). Fractions with product were joined and lyophilized to give product as white solid. Yield: 82 mg (59 %, 1 step, based on L4∙2.6TFA∙0.3H2O). ESI-HRMS: 611.1275 [M+H]+(theor. [C20H28N5O6Gd1F1]+= 611.1259). EA (C20H27N5O6Gd1F1∙2.5H2O, MR = 654.7): C 36.7 (36.2); H 4.9 (5.0); N 10.7 (10.5); Gd 24.0 (19.8); F 2.9 (2.8). Synthesis of complex [Gd(L5)]: In a glass vial (25 mL), L5∙0.7TFA∙1.3FA (100 mg, 0.17 mmol, 1.0 equiv.) was dissolved in aq. MOPS / NaOH buffer (3M, pH 7.0, 1.7 mL, 5.1 mmol, 30 equiv.) followed by addition of aq. GdCl3 (100 mM, 1.9 mL, 0.19 mmol, 1.1 equiv.) and the resulting solution was stirred at RT for 1 hour. The mixture was then purified by preparative HPLC (C18, H2O / MeCN gradient with 0.1% FA additive). Fractions with product were joined and lyophilized to give product as white solid. Yield: 95 mg (82 %, 1 step, based on L5∙0.7TFA∙1.3FA). ESI-HRMS: 611.1257 [M+H]+(theor. [C20H28N5O6Gd1F1]+= 611.1259). EA (C20H27N5O6Gd1F1∙4H2O, MR = 681.8): C 35.2 (35.4); H 5.2 (5.0); N 10.3 (10.1); Gd 23.1 (20.8); F 2.8 (2.4). Synthesis of complex [Gd(L6)]: In a glass vial (25 mL), L6∙0.5TFA∙1.4FA (150 mg, 0.26 mmol, 1.0 equiv.) was dissolved in aq. MOPS / NaOH buffer (3M, pH 7.0, 2.6 mL, 7.8 mmol, 30 equiv.) followed by addition of aq. GdCl3(100 mM, 3.1 mL, 0.31 mmol, 1.2 equiv.) and the resulting solution was stirred at RT for 1 hour. The mixture was then purified by preparative HPLC (C18, H2O / MeCN gradient with 0.1% FA additive). Fractions with product were joined and lyophilized to give product as white solid. Yield: 151 mg (86 %, 1 step, based on L6∙0.5TFA∙1.4FA). ESI-HRMS: 611.1256 [M+H]+(theor. [C20H28N5O6Gd1F1]+= 611.1259). EA (C20H27N5O6Gd1F1∙1.5H2O∙0.8FA, MR= 673.5): C 37.1 (37.3); H 4.7 (4.6); N 10.4 (10.7); Gd 23.4 (23.4); F 2.8 (2.5). (C20H30N5O6F1∙0.5TFA∙1.4FA, MR= 576.9) Synthesis of complex [Gd(L7)]: In a glass vial (25 mL), L7∙2.2TFA∙0.9H2O (152 mg, 0.22 mmol, 1.0 equiv.) was dissolved in aq. MOPS / NaOH buffer (3M, pH 7.0, 1.64 mL, 4.9 mmol, 21 equiv.) followed by addition of aq. GdCl3 (100 mM, 2.34 mL, 0.23 mmol, 1.05 equiv.) and the resulting solution was stirred at RT for 15 mins. The mixture was then purified by preparative HPLC (C18, H2O / MeCN gradient with 0.1% FA additive). Fractions with product were joined and lyophilized to give product as white solid. Yield: 116 mg (77 %, 1 step, based on L7∙2.2TFA∙0.9H2O). ESI-HRMS: 553.1214 [M+H]+(theor. [C18H26N5O4Gd1F1]+= 553.1204). EA (C18H26N5O4Gd1F1∙3.1H2O, MR = 681.2): C 33.9 (34.5); H 4.7 (4.5); N 10.3 (9.9); Gd 23.1 (19.6); F 7.8 (7.4). Synthesis of complex [Gd(L8)]: In a glass vial (25 mL), L8∙0.1TFA∙0.6H2O (4 mg, 8.1 µmol, 1.0 equiv.) was dissolved in aq. MOPS / NaOH buffer (0.5M, pH 7.0, 1.00 mL, 0.5 mmol, 62 equiv.) followed by addition of aq. GdCl3(100 mM, 97.0 µL, 9.7 µmol, 1.2 equiv.) and the resulting solution was stirred at RT for 1 hour. The mixture was then purified by preparative HPLC (C18, H2O / MeCN gradient with 0.1% FA additive). Fractions with product were joined and lyophilized to give product as white solid. Yield: 4.6 mg. ESI-HRMS: 653.1728 [M+H]+(theor. [C23H34N5O6Gd1F1]+= 653.1729). Synthesis of complex [Gd(L9)]: In a glass vial (25 mL), L9∙0.1TFA∙0.8H2O (6 mg, 9.18 µmol, 1.0 equiv.) was dissolved in aq. MOPS / NaOH buffer (0.5M, pH 7.0, 1.00 mL, 0.5 mmol, 51 equiv.) followed by addition of aq. GdCl3 (100 mM, 117.0 µL, 11.7 µmol, 1.2 equiv.) and the resulting solution was stirred at RT for 20 hours. The mixture was then purified by preparative HPLC (C18, H2O / MeCN gradient with 0.1% AcOH additive). Fractions with product were joined and lyophilized to give product as white solid. Yield: 5.4 mg. ESI-HRMS: 745.1461 [M+H]+(theor. [C28H36N5O4Gd1F1P2]+= 745.1462). Synthesis of complex [Gd(L10)]: In a glass vial (25 mL), L10∙1.7TFA∙1.4H2O (129 mg, 0.20 mmol, 1.0 equiv.) was dissolved in aq. MOPS / NaOH buffer (3M, pH 7.0, 2. mL, 8.02 mmol, 40 equiv.) followed by addition of aq. GdCl3(100 mM, 2.2 mL, 0.22 mmol, 1.1 equiv.) and the resulting solution was stirred at RT for 1 hour. The mixture was then purified by preparative HPLC (C18, H2O / MeCN gradient with 0.1% FA additive). Fractions with product were joined and lyophilized to give product as a solid with yellow tint. Yield: 92 mg (64 %, 1 step, based on L10∙1.7TFA∙1.4H2O). ESI-HRMS: 580.1152 [M+H]+(theor. [C18H26N6O6Gd1]+= 580.1149). EA (C18H26N6O6Gd1∙3FA, MR = 717.8): C 35.1 (34.9); H 4.5 (4.1); N 11.7 (12.1); Gd 21.9 (19.9). Synthesis of complex [Gd(L11)]: In a glass vial (4 mL), L11∙0.1TFA (3.1 mg, 6.0 µmol, 1.0 equiv.) was dissolved in aq. MOPS / NaOH buffer (0.5M, pH 7.0, 0.738 mL, 369 µmol, 62 equiv.) followed by addition of aq. GdCl3 (100 mM, 72.0 µL, 7.2 µmol, 1.2 equiv.) and the resulting solution was stirred at RT for 1 hour. The mixture was then purified by preparative HPLC (C18, H2O / MeCN gradient with 0.1% FA additive). Fractions with product were joined and lyophilized to give product as white solid. Yield: 3.5 mg. ESI-HRMS: 653.1731 [M+H]+(theor. [C23H34N5O6Gd1F1]+= 653.1729). Synthesis of complex [Gd(L12)]: In a glass vial (4 mL), L12∙2.2FA∙0.6H2O (3 mg, 4.8 µmol, 1.0 equiv.) was dissolved in aq. MOPS / NaOH buffer (0.5M, pH 7.0, 0.706 mL, 353 µmol, 73 equiv.) followed by addition of aq. GdCl3(100 mM, 69.0 µL, 6.9 µmol, 1.4 equiv.) and the resulting solution was stirred at RT for 15 minutes. The mixture was then purified by preparative HPLC (C18, H2O / MeCN gradient with 0.1% FA additive). Fractions with product were joined and lyophilized to give product as white solid. Yield: 4.1 mg. ESI-HRMS: 680.1679 [M+H]+(theor. [C23H34N6O8Gd1]+= 680.1674). Radiolabelling experiments Example 4: radiolabelling with18F- anion General procedure A solution containing18F- produced on a cyclotron was absorbed on the QMA column (Sep-Pak, Waters) in a CO3- cycle and eluted with an eluting solution of 0.8 ml (8.3 mg TBAHCO3 in a mixture of 0.4 ml H2O and 0.4 ml MeCN). A volume containing the desired activity was transferred to a 4 ml glass vial with a stirring bar preheated for 120 °C and a stream of argon was introduced. The aqueous solution was evaporated and 300 µl of dry MeCN was introduced and evaporated again. This step was performed twice. 100 µl of 15 mM solution of the material for radiolabelling ([Gd(L1)], [Gd(L4)] or L4) in a dry DMSO was introduced, followed by the 4 µl of 1 M solution of a TBAF in a THF. The vial was transferred to an aluminium heat block preheated to 90°C and the reaction mixture was stirred for 5 minutes. The reaction mixture was afterwards analysed on a HPLC equipped with the gamma and UV detector. Radiolabelling of [Gd(L1)] by nucleophilic aromatic substitution Scheme 1: Schematic depiction of [Gd(L1)] radiolabelling experiment Table 1: Radiolabelling experiments with [Gd(L1)] as a precursor. All the reactions were done in dry solvents. The amount of precursor used was 1 mg. Experiment number Radiochemical yield (%) Chemical conversion (%) 1 11 78 2 17 76 Chromatograms from this radiolabelling experiment are depicted in Figure 1. Radiochemical yield is defined as an amount of activity in the product expressed as the percentage of related starting activity. Radiolabelling of [Gd(L4)] by18F<–>19F isotopic exchange Scheme 2: Schematic depiction of [Gd(L4)] radiolabelling experiment performed by18F<–>19F isotopic exchange Table 2: Radiolabelling experiments with [Gd(L4)] as a precursor. All the reactions were done in dry solvents. The amount of precursor used was 1 mg. Experiment number Radiochemical yield (%) Chemical conversion (%) 1 17 83 2 17 85 3 13 82 Chromatograms from this radiolabelling experiment are depicted in Figure 2. Radiolabelling of free ligand L4by18F<–>19F isotopic exchange The experiment with free ligand L4was performed to demonstrate that the chelated metal is crucial for the radiolabelling reaction to proceed. The experiment was carried out using the same conditions as the other radiolabelling reactions. Scheme 3: Schematic depiction of L4radiolabelling experiment performed by18F<–>19F isotopic exchange Chromatograms from this radiolabelling experiment are depicted in Figure 3. Table 3: Radiolabelling experiments with free ligand L4as a precursor. All the reactions were done in dry solvents. The amount of precursor used was 1 mg. Experiment number Radiochemical yield (%) 1 0 2 0 Example 5: Relaxivity of [Gd(L4)] and [Gd(L7)] The value of relaxivity is directly proportional to the effectiveness of a compound as an MRI contrast agent. For the relaxivity determination, samples of [Gd(L4)] and [Gd(L7)] were prepared at 0, 0.2, 0.5, 1, 2 mM concentrations in a MOPS / NaOH buffer (50 mM, pH 7.0, 150 µL). The samples thus prepared were used for the determination of the longitudinal relaxation time T1 values on the relaxometer at 37 °C and 0.47 T. From these values, it was possible to determine that the [Gd(L4)] has a relaxivity of 3.43 mM-1s-1and [Gd(L7)] has a relaxivity of 5.33 mM-1s-1(Table 4). The relaxivities were determined by linear regression of their concentration versus longitudal relaxation rate as can be seen in Figure 4. For comparison, the relaxivity of most clinically used MRI contrast agents at comparable conditions is within the range 3 – 4 mM-1s-1(Rohrer, M.; Bauer, H.; Mintorovitch, J.; Requardt, M.; Weinmann, H.-J. Comparison of Magnetic Properties of MRI Contrast Media Solutions at Different Magnetic Field Strengths. Investigative Radiology 2005, 40 (11), 715–724. https: / / doi.org / 10.1097 / 01.rli.0000184756.66360.d3). Table 4: Determination of the longitudinal relaxation time T1 values of [Gd(L4)] and [Gd(L7)]. Compound Relaxivity (mM-1s-1) [Gd(L4)]3.43[Gd(L7)] 5.33 Example 6: Kinetic inertness of [Gd(L4)] and [Gd(L7)] The kinetic inertness of [Gd(L4)] and [Gd(L7)] was tested using acid-assisted decomplexation with HCl (0.1M). Magnevist®, gadolinium-DTPA (diethylenetriamine penta-acetic acid), a clinically used contrast agent was used as a reference and solution of GdCl3(1mM) in HCl (0.1M) was used as an example of fully released gadolinium from chelate. All measured complexes were used as 1 mM solutions with a volume of 150 µL and incubated at 37 °C. During decomplexation, the gadolinium ion detaches from the complex and becomes fully solvated. T1of the sample decreases during this process since solvated gadolinium shortens T1relaxation time to a greater degree than chelated gadolinium. The reaction reaches equilibrium when the complex is fully decomposed, and all the gadolinium is solvated. The T1relaxation time isn’t decreasing in this equilibrium state anymore. The T1relaxation time values were determined on a relaxometer at 37 °C and a field strength of 0.47 T. The kinetic inertness was compared in paramagnetic relaxation rate: R1p= 1 / T1- 1 / T1d. The longitudinal relaxation time of the HCl (0.1M) corresponds to T1d. A slow increase of T1values can be observed in the case of [Gd(L4)] during the measurement, which suggests that the complex is slowly decomposed. Over a 240-minute period, the T1value of [Gd(L4)] increased by 12 %. Situation is different in the case of Magnevist, where the reaction reaches its equilibrium already after 6 minutes (Table 5). The R1pvalues of the Magnevist sample didn’t change over time, so it can be assumed that the complex fully decomposed. In the case of [Gd(L7)] the increase of R1pis slower in comparison to Magnevist, which suggest that the [Gd(L7)] is more kinetically inert. The measurements show that [Gd(L4)] and [Gd(L7)] are more kinetically inert than the clinically used Magnevist and thus suitable for in vivo applications. Graphs showing measurements of kinetic inertness are shown in Figure . Table 5: Comparison of kinetic inertness of [Gd(L4)], [Gd(L7)], Magnevist and solution of GdCl3 by acid- assisted decomplexation in 0.1M HCl. Time 6 min 8 min 12 min 4h 24h R1p of [Gd(L7)] (s-1) 7.2 7.9 8.8 9.5 9.5 R1pof GdCl3(s-1) 9.0 No data No data 9.1 9.0 R1pof Magnevist(s-1) 8.8 8.7 8.7 8.8 8.7 R1p of [Gd(L4)] (s-1) 3.4 No data No data 3.8 5.3 Example 7: Simultaneous PET / MRI imaging of radiolabelled [Gd(L4)] In-vitro imaging of phantoms Simultaneous PET / MRI was performed on a 7T preclinical PET-MRI scanner (Bruker Clinscan with PET insert). Sample of18F-radiolabeled [Gd(L4)] of 700 MBq activity was prepared on an automatic radiosynthesis module by isotopic exchange of19F for18F analogously to Example 4. A stock solution of 0.5 mM solution of non-radioactive [(Gd(L4)] in phosphate buffer saline (PBS) was then enriched with18F- radiolabeled [Gd(L4)] to prepare phantoms (vials) with concentrations ranging from 0.048 to 0.489 mM and activities from 0.033 to 0.490 MBq. Vials containing only aqueous solutions of Magnevist ([Gd(DTPA)] complex) ranging in concentrations between 0.1 and 0.5 mM served as a reference contrast for MR images. Simultaneous PET / MRI imaging was performed by acquiring T1-weighted MR images of the phantoms using a 3D FLASH sequence and 600 seconds static PET scan (Figure 6, panel A). Quantification of18F activity based on gamma counting showed excellent linear correlation with quantification of Gd with ICP-OES (Figure 6, panel B). Similarly, radioactivity measured with PET showed excellent linear correlation with signal obtained from the T1-weighted MR images (Figure 6, panel C) and relaxation rate obtained from MRI T1-maps (Figure 6, panel D). Overall, the data confirms that18F- radiolabeled [Gd(L4)] provides consistent signal in both imaging modalities and is therefore suitable for use as a bimodal PET / MRI contrast agent. In-vivo imaging of mice Sample of18F-radiolabeled [Gd(L4)] was prepared on an automatic radiosynthesis module by isotopic exchange of19F for18F analogously to Example 4 and prepared as a saline solution with molar activity of 518 MBq / µmol. Anesthetised healthy C57BL / 6 mice were injected with 0.09 ± 0.03 mmol / kg of [Gd(L4)] with activity 0.91 ± 0.16 MBq (N = 6). The dose was administered as a bolus at the beginning of the PET / MRI scan via a catheter placed in the tail vein. Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and PET images were acquired simultaneously for 62 minutes to dynamically follow the distribution of [Gd(L4)] in both MRI and PET modalities (N = 3). In addition, in another cohort of N = 3 mice dynamic T1 maps were acquired for 55 minutes and followed by a 5 minutes PET scan. The dynamics of MR signal (Figure 7, panel A) and PET signal (Figure 7, panel B) followed the same pattern with a fast renal excretion typical for small Gd-based agents. This was also confirmed by biodistribution determined by ex-vivo analysis of radioactivity (gamma counter) and gadolinium (ICP-OES), which showed the highest uptake in kidneys, bladder, and urine (Figure 8, panels A and B, respectively). Figure 9 shows series of whole-body images separately for T1-weighted MRI, PET, and their overlay. These images demonstrate that the contrast agent consistently provides signal in both modalities. Notably, the advantage of having simultaneous signal from a single agent in both MRI and PET modalities is apparent in the renal pelvis, which develops prominent dark spots in the MRI after injection of the bimodal agent due to contrast over-saturation caused by high local concentration of the agent in this organ of excretion. Such dark spots could be easily missed, neglected, mistaken for imaging artefacts, or falsely attributed to malignancies (necrosis, poorly perfused areas etc.). However, the simultaneously obtained PET images undoubtedly confirm strong accumulation of the tracer in the same areas that become visible as "hot spots" in the PET, therefore improving diagnostic accuracy and interpretation of the images.

Claims

CLAIMS 1. Use of cyclen based compound of general formula (I)(I), wherein R is NO2or F; A is independently selected from –CH2COOH; –CH(CH3)COOH; ; –CH((CH2)nCH3)COOH, wherein n is an integer in the range of from 1 to 3; and –CH2P(=O)(OH)Ph; Z is H or A; in pharmacy for preparation of a multimodal PET / MRI contrast agent.

2. Use according to claim 1, wherein A and Z of the cyclen based compound of general formula (I) are the same, preferably selected from –CH2P(=O)(OH)Ph; and –CH(CH3)COOH.

3. Use according to claim 1, wherein all A of the cyclen based compound of general formula (I) are the same, preferably selected from –CH2P(=O)(OH)Ph; and –CH(CH3)COOH; and Z is hydrogen.

4. Use according to claim 1, wherein R of the cyclen based compound of general formula (I) is located in para-position in relation to the nitrogen atom of the pyridyl pendant arm.

5. Use according to claim 1, wherein the cyclen based compound of general formula (I) is selected from the group comprising: 2,2',2''-(10-((4-nitropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (L1) 2,2',2''-(10-((6-nitropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (L2) 2,2',2''-(10-((3-fluoropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (L3) 2,2',2''-(10-((4-fluoropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (L4) 2,2',2''-(10-((5-fluoropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (L5) 2,2',2''-(10-((6-fluoropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (L6) 2,2'-(4-((4-fluoropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,7-diyl)diacetic acid (L7)(2S,2'S,2''S)-2,2',2''-(10-((3-fluoropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7- triyl)tripropionic acid (L8) ((4-((3-fluoropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,7- diyl)bis(methylene))bis(phenylphosphinic acid) (L9) 2,2'-(4-((4-nitropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,7-diyl)diacetic acid (L10) (2S,2'S,2''S)-2,2',2''-(10-((4-fluoropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7- triyl)tripropionic acid (L11) (2S,2'S,2''S)-2,2',2''-(10-((4-nitropyridin-2-yl)methyl)-1,4,7,10-tetraazacyclododecane-1,4,7- triyl)tripropionic acid (L12) 6. Use of a coordination compound of Gd(III) ion with the cyclen based compound of general formula (I) as defined in any one of the preceding claims 1 to 5, for preparation of a multimodal PET / MRI contrast agent.

7. Cyclen based compound of general formula (Ia)(Ia), wherein R is NO2 or F; preferably R is F; A is independently selected from –CH(CH3)COOH; –CH((CH2)nCH3)COOH, wherein n is an integer in the range of from 1 to 3; and –CH2P(=O)(OH)Ph; preferably A is –CH(CH3)COOH; Z is H or A, preferably Z is A.

8. Cyclen based compound of general formula (Ia) according to claim 7, wherein the R substituent is located in para-position in relation to the nitrogen atom of the pyridyl pendant arm.

9. Cyclen based compound of general formula (Ia) according to claim 7 or 8, wherein Z is A, and all A are –CH(CH3)COOH of the same stereoconfiguration.

10. A multimodal PET / MRI contrast agent of general formula (VI)(VI), wherein A is independently selected from –CH2COOH; –CH(CH3)COOH; –CH((CH2)nCH3)COOH, wherein n is an integer in the range of from 1 to 3; and –CH2P(=O)(OH)Ph; Z is H or A; preferably18F is in para-position in relation to the nitrogen atom of the pyridyl pendant arm.

11. A method of synthesis of the multimodal PET / MRI contrast agent of general formula (VI) as defined in claim 10, comprising the following steps: a) providing the cyclen based compound of general formula (I) as defined in claims 1 to 5; b) reacting the cyclen based compound of general formula (I) with Gd3+ion, thereby forming a Gd(III) chelate of the compound of general formula (I); c) radiolabelling the Gd(III) chelate from step b) with18F.

12. The method according to claim 11, wherein step c) is performed using19F to18F isotopic exchange on the pyridine moiety of the Gd(III) chelate, wherein R is F.

13. The method according to claim 11, wherein step c) is performed using nucleophilic aromatic substitution of nitro group with18F on the pyridine moiety of the Gd(III) chelate, wherein R is nitro group.

14. A pharmaceutical formulation comprising the multimodal PET / MRI contrast agent of general formula (VI) according to claim 10, which further contains at least one conventionally used pharmaceutically acceptable auxiliary substance, preferably selected from the group containing solvents, buffers, ionization additives, antioxidants, antimicrobial additives.

15. The multimodal PET / MRI contrast agent of general formula (VI) according to claim 10 and / or the pharmaceutical formulation according to claim 14, for use in medicine, preferably in multimodal PET / MRI diagnostic methods.

Images