WO2007142804A2

WO2007142804A2 - Preparation of complexing agents and metal ion complexes in aqueous media

Info

Publication number: WO2007142804A2
Application number: PCT/US2007/012002
Authority: WO
Inventors: Bradley J. Rowe; Rebecca A. Wallace; David H. White; Derick L. Kaspar
Original assignee: Mallinckrodt Inc.
Priority date: 2006-06-01
Filing date: 2007-05-18
Publication date: 2007-12-13
Also published as: WO2007142804A3

Abstract

The invention relates to methods of preparation for bis-amide complexing agents, as well as paramagnetic metal ion complexes that include such agents. To prepare these complexing agents, a reaction mixture is formed that includes a bis-anhydride, a nitrogen-containing compound, and an aqueous solvent system. The nitrogen-containing compound reacts with the bis-anhydride in the reaction mixture to form the bis-amide complexing agent. The molar ratio of water to the nitrogen containing compound in the reaction mixture is greater than 3:1, respectively. In addition, the molar ratio of the nitrogen-containing compound to the bis-anhydride in the reaction mixture is no greater than 2.5:1, respectively. In some embodiments, a base other than the nitrogen-containing compound may be included in the reaction mixture (e.g., to buffer the reaction mixture). In some embodiments, the bis-amide complexing agent may be contacted with a paramagnetic metal ion salt to form a paramagnetic metal ion complex of the invention.

Description

PREPARATION OF COMPLEXING AGENTS AND METAL ION COMPLEXES IN AQUEOUS MEDIA

FIELD OF THE INVENTION

[0001] The present invention generally relates to processes for preparing complexing agents and metal ion complexes in aqueous media.

BACKGROUND

[0002] Magnetic Resonance Imaging (MRI) encompasses the detection of certain atomic nuclei utilizing magnetic fields and radio-frequency radiation. It is similar in some respects to X-ray computed axial tomography (CAT) in providing a cross-sectional display of the body organ anatomy with excellent resolution of soft tissue detail. As currently used, the images produced constitute a map of the proton density distribution and/or the relaxation times in organs and tissues. MRI is believed by some to be advantageous in that it allows for medical imaging without the use of ionizing radiation. It is known to administer divalent and trivalent paramagnetic ions in the form of complexes with organic complexing agents as magnetic resonance contrast agents. Such complexes provide the paramagnetic ions in a soluble, non-toxic form, and facilitate their rapid clearance from the body following the imaging procedure. Gries et. al., U.S. Pat. No. 4,647,447, disclose complexes of various paramagnetic ions with conventional aminocarboxylic acid complexing agents. A preferred complex disclosed by Gries et. al. is the complex of gadolinium(III) with diethylenetriaminepentaacetic acid ("DTPA"). Paramagnetic ions, such as gadolinium(III), have also been found to form strong complexes with ethylenediaminetetraacetic acid ("EDTA") and with tetraazacyclododecane-N, N', N", N'"- tetraacetic acid ("DOTA").

[0003] These resulting complexes do not dissociate substantially in physiological aqueous fluids. The gadolinium complex of DTPA has a net charge of -2, whereas the gadolinium complex of each of EDTA and DOTA has a net charge of -1, and both are generally administered as soluble salts. Typical such salts are sodium and N-methylglucamine. The administration of such salts is attended by certain disadvantages. These salts can raise in vivo ion concentration and cause localized disturbances in osmolality, which in turn, can lead to edema and/or other undesirable reactions.

[0004] Efforts have been made to synthesize new ionic and neutral paramagnetic metal complexes which avoid or minimize the above-mentioned disadvantages. In general, this goal can be achieved by converting one or more of the free carboxylic acid groups of the complexing agent to neutral, non-ionizable groups. For example, S. C. Quay, in U.S. Pat. Nos. 4,687,658 and 4,687,659, discloses the syntheses of alkylester and alkylamide derivatives, respectively, of DTPA complexes. Similarly, Dean et al., U.S. Pat. No. 4,826,673, disclose syntheses of mono- and polyhydroxyalkylamide derivatives of DTPA and their use as complexing agents for paramagnetic ions. R. W. Webber, in U.S. Pat. No. 5,130,120, and Webber et al., in U.S. Pat. No. 5,137,711 disclose syntheses of paramagnetic DTPA and EDTA alkoxyalkylamide complexes as magnetic resonance imaging agents.

[0005] Conventional syntheses of paramagnetic metal ion complexes of the type described in the above-mentioned patents can employ environmentally stressful and relatively expensive organic solvents. Besides the purchase cost, organic solvents can generate excessive waste and disposal costs. In addition, some organic solvents may contain trace impurities, which may adversely affect yield and purity of the final product.

SUMMARY

[0006] Among the various aspects of the present invention may be noted the provision of a process for the preparation of bis-amides in an aqueous solvent system, and the provision of a process for the preparation of complexes, such as gadoversetamide, derived from such bis-amides and paramagnetic metals such as gadolinium. Certain exemplary aspects of the invention are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be explicitly set forth below.

[0007] In one aspect, the invention is directed to a method for the preparation of a bis-amide complexing agent. In this method, a reaction mixture that includes a bis-anhydride, a nitrogen- containing compound, and an aqueous solvent system is formed. The nitrogen-containing compound reacts with the bis-anhydride in the reaction mixture to form the bis-amide complexing agent. The molar ratio of water combined with the nitrogen-containing compound in the reaction mixture is greater than 3:1, respectively. In addition, the molar ratio of the nitrogen-containing compound combined with the bis-anhydride in the reaction mixture is no greater than 2.5:1, respectively. In some embodiments, a base other than the nitrogen-containing compound may be included in the reaction mixture (e.g., to buffer the reaction mixture).

[0008] In a related aspect, the invention is directed to a method for the preparation of a paramagnetic metal ion complex. In this method, a bis-amide complexing agent prepared in a manner described herein is contacted with a paramagnetic metal ion salt to form a paramagnetic metal ion complex.

[0009] Numerous refinements exist of the features noted above in relation to the various aspects of the present invention. Further features may also be incorporated in these various aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to one or more of the exemplary embodiments may be incorporated into any of the above-described aspects of the present invention alone or in any combination. Accordingly, the brief summary presented above is intended to familiarize the reader with certain aspects and contexts of the present invention without limitation to the claimed subject matter.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

[0010] As mentioned above, among the various aspects of the present invention is a method for the preparation of a bis-amide complexing agent, such as versetamide. In this method, a bis- anhydride is combined with a nitrogen-containing compound in an aqueous solvent system to form a reaction mixture, the nitrogen-containing compound reacting with the bis-anhydride in the reaction mixture to form the bis-amide complexing agent. The bis-amide complexing agent, in turn, may then be complexed with a paramagnetic metal ion, such as gadolinium(III), to form a paramagnetic metal ion complex. The paramagnetic metal ion complex has particular application as a contrast agent in an imaging procedure. In one embodiment, the bis-amide and the paramagnetic metal ion are combined in the reaction mixture without prior isolation of the bis-amide complexing agent from its reaction mixture.

[0011] In one embodiment, the bis-amide complexing agent is a bis-amide corresponding to Formula I:

wherein

Ri is ethylene, propylene, butylene, N,N-bis-ethylenegIycine, N,N-bis-ethylene-3- aminopropionic acid (N,N-bis-ethylene-β-aIanine), N,N-bis-ethylene-4-amino-butyric acid (N,N-bis- ethylene-γ-aminobutyric acid), N,N-bis-propyleneglycine, N,N~bis-propylene-3-aminopropionic acid (N,N-bis-propylene-β-alanine), N,N-bis-propylene-4-amino-butyric acid (N,N-bis-propylene-γ- aminobutyric acid), N,N-bis-butyleneglycine, N,N-bis-butyleπe-3-aminopropionic acid (N,N-bis- butylene-β-alanine), or N,N-bis-butylene-4-amino-butyric acid (N,N-bis-butylene-γ-aminobutyric acid);

R₂ and R₃ are independently hydrogen or a substituted or unsubstituted hydrocarbyl; and each R₄ is independently methylene, ethylene, or propylene.

[0012] When R₂ or R₃ is substituted or unsubstituted hydrocarbyl, it may be alkyl, alkenyl, alkynyl, aryl, or a combination thereof, such as alkaryl. Exemplary substituents include hydroxyl, alkoxy, and aromatic substituents. In one embodiment, the substituted or unsubstituted hydrocarbyl is a substituted or unsubstituted alkyl, which may be selected from, for example, Ci_-I0 alkyl, C_M0 monohydroxylalkyl, Ci-io polyhydroxylalkyl, C_M0 alkoxyalkyl, or C_MO polyalkoxyalkyl. In another embodiment, the substituted or unsubstituted hydrocarbyl is a substituted or unsubstituted alkyl, which may be selected from, for example, Ci_-8 alkyl, Ci_-8 monohydroxylalkyl, C|.g polyhydroxylalkyl, Ci-8 alkoxyalkyl, or C).₈ polyalkoxyalkyl. In yet another embodiment, the substituted or unsubstituted hydrocarbyl is a substituted or unsubstituted alkyl, which may be selected from, for example, C_]-6 alkyl, Ci-₆ monohydroxylalkyl, C^ polyhydroxylalkyl, Cj.₆ alkoxyalkyl, and Ci-β polyalkoxyalkyl. Exemplary hydrocarbyls include methyl, ethyl, n-propyl, isopropyl, branched or straight chained butyl, hydroxylmethyl, hydroxylethyl, methoxyethyl, and ethoxyethyl.

[0013] In one embodiment, the bis-amide complexing agent is a bis-amide corresponding to Formula II:

wherein

R.₂ and R₃ are independently hydrogen or a substituted or unsubstituted hydrocarbyl. [0014] When R₂ or R₃ is substituted or unsubstituted hydrocarbyl, it may be an alkyl, alkenyl, alkynyl, aryl, or a combination thereof, such as alkaryl. Exemplary substituents include hydroxyl, alkoxy, and aromatic substituents. In one embodiment, the substituted or unsubstituted hydrocarbyl is a substituted or unsubstituted alkyl, which may be selected from, for example, C_MO alkyl, Cj.₁₀ monohydroxylalkyl, C_MO polyhydroxylalkyl, C_M0 alkoxyalkyl, or Q.₁₀ polyalkoxyalkyl. In another embodiment, the substituted or unsubstituted hydrocarbyl is a substituted or unsubstituted alkyl, which may be selected from, for example, Ci._s alkyl, Ci.β monohydroxylalkyl, Ci_-8 polyhydroxylalkyl, Ci.₈ alkoxyalkyl, or Ci.₈ polyalkoxyalkyl. In yet another embodiment, the substituted or unsubstituted hydrocarbyl is a substituted or unsubstituted alkyl, which may be selected from, for example, Ci^ alkyl, C1.6 monohydroxylalkyl, Ci-β polyhydroxylalkyl, C₁.₆ alkoxyalkyl, and C₁.₆ polyalkoxyalkyl. Exemplary hydrocarbyls include methyl, ethyl, n-propyl, isopropyl, branched or straight chained butyl, hydroxylmethyl, hydroxylethyl, methoxyethyl, and ethoxyethyl. [0015] In one embodiment, the Rj bridging group is ethylene, each R₄ is methylene, and the bis-amide complexing agent corresponds to Formula III:

wherein R₂ and R₃ are as defined in connection with Formula I. The bis-amide complexing agent corresponding to Formula III is a bis-amide of ethylenediaminetetraacetic acid ("EDTA") and is referred to herein as an EDTA bis-amide complexing agent.

[0016] In an alternative embodiment, the R] bridging group is N,N-bisethyleneglycine, each R₄ is methylene, and the bis-amide complexing agent corresponds to Formula IV:

wherein R₂ and R₃ are as defined in connection with Formula I. The bis-amide complexing agent . corresponding to Formula IV is a bis-amide of diethylenetriaminepentaacetic acid ("DTPA") and is referred to herein as a DTPA bis-amide complexing agent.

[0017] In a preferred embodiment, the Ri bridging group is N,N-bisethyleneglycine, each R₄ is methylene, one of each R₂ or R₃ is methoxyethyl, and the complexing agent is DTPA bis-(2- methoxyethylamide), commonly known as versetamide. DTPA bis-(2-methoxyethylamide) has the following structure:

[0018] Bis-amide complexing agents corresponding to Formulae I, II, III, and IV may be, in accordance with the present invention, the product of the reaction of a nitrogen-containing compound and the corresponding bis-anhydride in an aqueous solvent system. 10019] In general, the nitrogen-containing compound corresponds to the formula HNR₂R3 wherein R₂ and R₃ are as defined in connection with Formulae I and II. In one embodiment, therefore, the nitrogen-containing compound is ammonia. In another embodiment, at least one of R₂ and R₃ is hydrocarbyl or substituted hydrocarby!, and the nitrogen-containing compound is a primary or secondary amine. If one (or both) of R₂ and R₃ is hydrocarbyl or substituted hydrocarbyl, for example, R₂ and/or R₃ may be alkyl or substituted alkyl. Exemplary substituents include hydroxyl, alkoxy, and aromatic substituents. In one embodiment, the amine is a primary amine, one OfR₂ or R₃ is the substituted or unsubstituted hydrocarbyl, and one of R₂ or R₃ is hydrogen. The primary amine may be selected from, for example, methylamine, ethylamine, n-propylamine, isopropylamine, branched or straight chained butylamines, hydroxylmethylamine, hydroxylethylamine, 2-methoxy ethylamine, and 2- ethoxyethylamine. In a preferred embodiment, one OfR₂ or R₃ is methoxyethyl and one OfR₂ or R₃ is hydrogen. Accordingly, in this embodiment, the primary amine is 2-methoxyethylamine.

[0020] In one embodiment, the bis-anhydride corresponds to Formula V:

Formula V wherein Ri and R₄ are as defined in connection with Formula I.

[0021] In one embodiment, the bis-anhydride corresponds to Formula VI:

Formula VI wherein Rj is as defined in connection with Formula II.

[0022] In one embodiment, the Ri bridging group is ethylene, each R₄ is methylene, and the bis-anhydride is EDTA-bisanhydride (4,4'-Ethylenebis(2,6-morpholinedione)), which has the following structure:

[0023] In an alternative embodiment, the Ri bridging group is N,N-bisethyleneglycine, each R₄ is methylene, and the bis-anhydride is DTPA-bisanhydride (N,N-Bis[2-(2,6- dioxomorpholino)ethyl]glycine), which has the following structure:

[0024] The bis-amide complexing agents corresponding to Formulae I, II, III, and IV are useful in the preparation of paramagnetic metal ion complexes which have particular application as contrast agents. In one embodiment, the paramagnetic metal ion complex can be formed by combining a bis-amide complexing agent of Formula I with a paramagnetic metal ion salt to form the paramagnetic metal ion complexes corresponding to Formula VII:

Formula VII wherein Ri, R₂, R₃, and R₄ are as defined in connection with Formula I; M²⁺ is a paramagnetic metal ion with an atomic number of 21-29, 42-44, or 58-70, and a valence number, Z, of 2 or 3.

[0025] Exemplary paramagnetic metal ions having a 2+ charge include iron(II), cobalt(II), manganese(II), nickel(II), copper(II), chromium(II), rhodium(II), and iridium(II). Exemplary paramagnetic metal ions having a 3+ charge include gadolinium(III), chromium(III), manganese(III), iron(III), praseodymium(III), europium(III), neodymium(III), samarium(III), ytterbium(III), terbium(III), dysprosium(III)₅ holmium(III), and erbium(III).

[0026] In one embodiment, the paramagnetic metal ion complex of Formula VII comprises a bis-amide complexing agent of Formula I wherein the Ri bridging group is ethylene, propylene, or butylene. In this embodiment, the bis-amide complexing agent of Formula I has two carboxylate groups available for complexation. The paramagnetic metal ion complex of Formula VII has an overall neutral charge when the complex comprises a bis-amide complexing agent having two carboxylate groups and a paramagnetic metal ion having a 2+ charge. Alternatively, the paramagnetic metal ion complex of Formula VII may have an overall positive charge when the complex comprises a bis-amide complexing agent having two carboxylate groups and a paramagnetic metal ion having a 3+ charge. A paramagnetic metal ion complex having an overall positive charge may be charged balanced with an appropriate anion, such as citrate, tartrate, hydroxide, or a halide selected from among chloride, bromide, and iodide.

[0027] In an alternative embodiment, the paramagnetic metal ion complex of Formula VII comprises a bis-amide complexing agent of Formula I wherein the Ri bridging group is N,N-bis- ethyleneglycine, N,N-bis-ethylene-3-aminopropionic acid (N,N-bis-ethylene-β-alanine), N,N-bis- ethylene-4-amino-butyric acid (N,N-bis-ethylene-γ-aminoburyric acid), N,N-bis-propyleneglycine, N,N-bis-propylene-3-aminopropionic acid (N,N-bis-propylene-β-alanine), N,N-bis-propylene-4-amino- butyric acid (N,N-bis-propylene-γ-aminobutyrϊc acid), N,N-bis-butyleneglycine, N,N-bis-butylene-3- aminopropionic acid (N,N-bis-butylene-β-alanine), or N,N-bis-butylene-4-amino-butyric acid (N,N-bis- butylene-γ-aminobutyric acid). In this embodiment, the bis-amide complexing agent of Formula I has three carboxylate groups available for complexation. The paramagnetic metal ion complex of Formula VII has an overall negative charge when the complex comprises a bis-amide complexing agent having three carboxylate groups and a paramagnetic metal ion having a 2+ charge. The paramagnetic metal ion complex having an overall negative charge may be charged balanced with an appropriate cation, such as an alkali metal ion selected from among lithium, sodium, and potassium or an amine, such as N-methyl-D-glucamine. Alternatively, the paramagnetic metal ion complex of Formula VII has an overall neutral charge when the complex comprises a bis-amide complexing agent having three carboxylate groups and a paramagnetic metal ion having a 3+ charge.

[0028] In one embodiment, the paramagnetic metal ion complexes can be formed by combining a bis-amide complexing agent of Formula II with a paramagnetic metal ion salt to form the paramagnetic metal ion complexes corresponding to Formula VIU:

Formula VIII wherein R₁, R₂, and R₃are as defined in connection with Formula II; M²⁺ is a paramagnetic metal ion with an atomic number of 21-29, 42-44, or 58-70, and a valence number, Z, of 2 or 3.

[0029] Exemplary paramagnetic metal ions having a 2+ charge include iron(II), cobalt(II), manganese(II), nickel(II), copper(II), chromium(II), rhodium(II), and iridium(II). Exemplary paramagnetic metal ions having a 3+ charge include gadolinium(III), chromium(III), manganese(III), iron(III), praseodymium(III), europium(III), neodymium(III), samarium(III), ytterbium(III), terbium(III), dysprosium(III), holmium(III), and erbium(III). Gadolinium(III) ion coupled with this complexing agent has been particularly preferred as a MRI contrast agent.

[0030] The paramagnetic metal ion complex of Formula VIII may have an overall neutral charge. In one embodiment, a neutral paramagnetic metal ion ion complex of Formula VIII comprises an EDTA bis-amide complexing agent of Formula III and a paramagnetic metal ion having a 2+ charge. For example, the neutral paramagnetic metal ion complex comprises an EDTA bis-amide complexing agent and a paramagnetic metal ion selected from among iron(II), cobalt(II), manganese(II), nickel(ll), copper(II), chromium(II), rhodium(II), and iridium(II). In an alternative embodiment, a neutral paramagnetic metal ion complex of Formula VIII comprises a DTPA-bisamide complexing agent of Formula IV and a paramagnetic metal ion having a 3+ charge. For example, a neutral paramagnetic metal ion complex comprises a DTPA bis-amide and a paramagnetic metal ion selected from among gadolinium(III), chromium(III), manganese(III), iron(III), praseodymium(III), europium(III), neodymium(III), samarium(III), ytterbium(III), terbium(III), dysprosium(III), holmium(III), and erbium(III).

[0031] The paramagnetic metal ion complex of Formula VIII may have an overall positive charge. In one embodiment, a positively-charged paramagnetic metal ion complex of Formula VIII comprises an EDTA bis-amide complexing agent of Formula III and a paramagnetic metal ion having a 3+ charge. For example, a positively-charged paramagnetic metal ion complex comprises an EDTA bis-amide complexing agent and a paramagnetic metal ion selected from among gadolinium(III), chromium(III), manganese(III), iron(III), praseodymium(III), europium(III), neodymium(III), samarium(III), ytterbium(III), terbium(III), dysprosium(III), holmium(III), and erbium(III). Such a paramagnetic metal ion complex has a charge of +1. A positively-charged paramagnetic metal ion complex may be charged balanced with an appropriate anion, such as citrate, tartrate, hydroxide, or a halide selected from among chloride, bromide, and iodide.

[0032] The paramagnetic metal ion complex may have an overall negative charge. In one embodiment, a negatively-charged paramagnetic metal ion complex of Formula VIII comprises a DTPA-bisamide complexing agent of Formula IV and a paramagnetic metal ion having a 2+ charge. For example, a negatively-charged paramagnetic metal ion complex comprises a DTPA bis-amide complexing agent and a paramagnetic metal ion selected from among iron(II), cobalt(II), manganese(II), nickel(II), copper(II), chromium(II), rhodium(II), and iridium(II). Such a paramagnetic metal ion complex has a charge of -1. A negatively-charged paramagnetic metal ion complex may be charged balanced with an appropriate cation, such as an alkali metal ion selected from among lithium, sodium, and potassium or an amine, such as N-methyl-D-glucamine.

[0033] In a preferred embodiment, DTPA bis-(2-methoxyethylamide) is used as a complexing agent for gadolinium(III) and the paramagnetic metal ion complex for use as a contrast agent is a gadolinium(III) complex with DTPA bis-(2-methoxyethylamide), commonly known as gadoversetamide. Gadoversetamide has the structure:

[0034] The method for the preparation of a bis-amide complexing agent of the present invention comprises combining a bis-anhydride, a nitrogen-containing compound, and an aqueous solvent system to form a reaction mixture in which the nitrogen-containing compound reacts with the bis-anhydride in the reaction mixture to form the bis-amide complexing agent.

[0035] The bis-amide is prepared in an aqueous solvent system because the reaction between the nitrogen-containing compound and the bis-anhydride is exothermic, such that a substantial amount of heat evolves. Water acts advantageously as a heat sink, absorbing heat evolved from the reaction and enhancing temperature control. Accordingly, in one embodiment, the molar ratio of water in the aqueous solvent system to the nitrogen-containing compound is greater than 3:1. In another embodiment, the molar ratio of water to the nitrogen-containing compound is at least about 5:1. In another embodiment, the molar ratio of water to the nitrogen-containing compound is at least about 10:1. In yet another embodiment, the molar ratio of water to the nitrogen-containing compound is at least about 15:1. In some embodiments, the molar ratio of water to the nitrogen-containing compound is at least 70:1, such as about 130:1. Although there is some tolerance for the use of organic solvents, other than the nitrogen-containing compound, in the aqueous solvent system, alcohols are desirably avoided because alcohols may react with the bis-anhydride, yielding undesired bis-alkylester products. In a preferred embodiment, the aqueous solvent system excludes organic solvents altogether other than the nitrogen-containing compound. The lack of organic solvents in the aqueous solvent system is advantageous from both cost and purity standpoints. Organic solvents can be environmentally stressful and relatively expensive. Additionally, these organic solvents generate excessive waste and disposal costs, and the organic solvents may contain trace impurities, which adversely affect yield and purity of the final product.

[0036] In one embodiment, the nitrogen-containing compound is dissolved in the aqueous solvent system prior to combining it with the bis-anhydride. The nitrogen-containing compound may be added in an initial concentration as high as about 8M, more preferably between about 0.3 M and about 6 M. In one embodiment, the concentration is between about 0.3 M and about 0.6 M, such as about 0.45M. In another embodiment, the concentration is between about 0.6M and about 1.0M, such as about 0.8M. In yet another embodiment, the concentration is between about 3.2 M and about 4.0M, such as about 3.6M. In yet another embodiment, the concentration is between about 5.5M and about 6.0M, such as about 5.7M. It has been discovered that good results can be attained by adding the nitrogen-containing compound in both relatively low concentrations between about 0.6M and about 1.0M, such as about 0.8M and at relatively high concentrations between about 5.5M and about 6.0M, such as about 5.7M at the preferred slightly alkaline pH range between about 7.0 and about 9.0, such as about 8.0.

[0037] The aqueous solvent system is preferably brought to a desired temperature prior to adding any of the reactants. For example, the aqueous solvent system temperature may be between about 0⁰C and about 40⁰C, such as between about 15°C and about 25°C. Typically, the solvent system temperature will be slightly below room temperature. In one embodiment, the aqueous solvent system temperature is maintained relatively constant throughout the bis-amide complexing agent preparation. In one embodiment, the nitrogen-containing compound is dissolved in the aqueous solvent system prior to adding any of the bis-anhydride. Dissolution of the nitrogen-containing compound in water is an exothermic reaction. Accordingly, in this approach, before adding the bis-anhydride to react with the nitrogen-containing compound, the aqueous solvent system temperature will typically be returned to the desired temperature.

[0038] In one embodiment, the reaction mixture is buffered to a desired pH and maintained at the desired pH throughout the bis-amide complexing agent preparation. At highly acidic pH, it has been observed that the desired bis-amide product hydrolyzes into nitrogen-containing reactant starting material and the hydrolysis product of the bis-anhydride. At highly alkaline pH, hydroxyl ions present in solution may compete with the nitrogen-containing compound in wasteful side reactions. Accordingly, the reaction mixture is preferably buffered to a pH which is at least about 3 to avoid undesirable hydrolysis, and the reaction mixture is preferably buffered to a pH which is less than about 13 to avoid wasteful side reactions. In one embodiment, the pH is buffered between about 3 and about 4. In another embodiment, the pH is buffered to a pH of at least about 7, such as between about 12 and about 13 or between about 7 and about 9. In a preferred embodiment, the pH is buffered to a slightly alkaline pH between about 7 and about 9, such as about 8. pH buffering within this range typically requires the use of less acid and base pH adjusting agents. This is often desirable because it results in less cationic and anionic impurity in the aqueous solvent system, which eases purification. pH- adjusting and buffering agents useful for attaining the desired solution pH include hydrochloric acid, acetic acid, and acidic ion exchange resins for acidic pH adjustment and sodium hydroxide, triethylamine, pyridine, poly(4-vinylpyridine), and basic ion exchange resins for alkaline pH adjustment. Alkaline pH adjustment is with a base other than the nitrogen-containing compound. Moreover, the use of nitrogen-containing buffering agent such as ammonia, primary amines, and secondary amines is preferably avoided because of the possibility of side reactions with the bis- anhydride which compete with the nitrogen-containing compound.

[0039] In one embodiment, after dissolving the nitrogen-containing compound in the aqueous solvent system, allowing the solution temperature to equilibrate to the desired preparation temperature, and adjusting the pH, if necessary, to the desired solution pH, the bis-anhydride is added to the aqueous solvent system comprising the nitrogen-containing compound. The molar amount of added bis- anhydride may be determined based on the molar amount of the nitrogen-containing compound added. According to the stoichiometry of the bis-amide complexing agent synthesis, two moles of nitrogen- containing compound react with one mole of bis-anhydride to yield the bis-amide complexing agent. Accordingly, the molar amount of bis-anhydride is typically about half the molar amount (not initial molar concentration) of the nitrogen-containing compound. Stated another way, the moϊar ratio of the nitrogen-containing compound to the bis-anhydride combined in the aqueous solvent system can be about 2:1, respectively. To achieve a greater product yield, the nitrogen-containing compound may be added in a stoichiometric excess, such that the molar ratio of the nitrogen-containing compound to the bis-anhydride combined in the aqueous solvent system is greater than the 2:1 ratio. In one embodiment, the molar ratio is no greater than about 2.5: 1. In one embodiment, the molar ratio is between about 2: 1 and about 2.5: 1. In one embodiment, the molar ratio is between about 2:1 and about 2.2:1, such as about 2.04: 1. The molar ratio is preferably no greater than about 2.5:1 because at greater ratios, the excess nitrogen-containing compound complicates purification of the final paramagnetic metal ion complex and thereby lowers yields.

[0040] The bis-anhydride may be fed to the aqueous solvent system at a controlled feeding rate, which depends on such factors as the molar amount of bis-anhydride to be added, the molar amount of water in the aqueous solvent system, and the volume of water in the aqueous solvent system. Controlled feeding over a desired duration is preferred for several reasons. For example, although the nitrogen-containing compound is a stronger nucleophile than water and therefore reacts preferentially with the bis-anhydride at the desired solution pH, rapid feeding of the bis-anhydride may result in undesired hydrolysis reactions with water and thereby lower the reaction yield. Moreover, the reaction of the bis-anhydride with the nitrogen-containing compound to yield the bis-amide is exothermic, and adding the bis-anhydride rapidly can cause a rapid temperature increase, which may result in splattering. Splattering of the high temperature solution, which presents a potentially dangerous condition, can also reduce the reaction yield. Finally, the bis-anhydride, being an acid, lowers the pH of the aqueous solvent system. By controlling the feeding rate of the bis-anhydride, the aqueous solvent system can more easily be adjusted to and maintained at the desired pH. It has been observed that rapid addition of the bis-anhydride as well as addition of the bis-anhydride without pH adjustment and buffering can cause the pH of the solution to drop below about 3. This is disadvantageous because at pH below about 3, the desired bis-amide reaction product hydrolyzes to the nitrogen-containing reactant starting material and the hydrolysis product of the bis-anhydride. Accordingly, the bis- anhydride is preferably added at a controlled rate to allow a more complete reaction between the bis- anhydride and nitrogen-containing compound, to allow for better control of the solvent system temperature, and to allow for better control of the solvent system pH. Since water in the aqueous solvent system acts as a heat sink for the exothermic reaction between the bis-anhydride and the nitrogen-containing compound, the rate of addition of the bis-anhydride may be expressed in terms of molar amount of bis-anhydride per unit of time per molar amount of water in the aqueous solvent system. To achieve adequate heat dissipation, the rate of addition of the bis-anhydride will generally not exceed about 2 millimole of bis-anhydride per min per mole of water (2 mmol/min*mol). Although slower addition rates are applicable, an exemplary commercially practicable addition rate is at least about 0.2 mmol/min*mol. In one embodiment, the rate of addition may be between about 0.4 mmol/min*mol and about 1.6 mmol/min*mol. In one embodiment, the rate of addition may be between about 0.6 mmol/min* mol and about 0.8 mmol/min*mol. Stated in terms of mass of bis- anhydride per volume of water in the aqueous solvent system, the rate of addition of the bis-anhydride will generally not exceed about 25 grams of bis-anhydride per minute per Liter of water in the aqueous solvent system (25 g/min*L). A commercially practicable rate of addition of the bis-anhydride is at least about 2.5 g/min*L. In one embodiment, the feed rate is between about 5 grams/min*L and about 15 grams/min*L. In one embodiment, the rate of bis-anhydride feeding may add between about 7.5 grams/min*L and about 10 grams/min*L of bis-anhydride to the aqueous solvent system.

[0041] In general, the bisanhydride may be fed to the aqueous solvent system on an intermittent or continuous basis. For example, the feed method can be intermittent in that a discrete amount of bis-anhydride is added at desired intervals. In an exemplary method of intermittent feeding of about 100 grams of bis-anhydride to an aqueous solvent system having a volume of about 100 mL, about 5 g of bis-anhydride can be added once every 5 minutes. The exemplary feed rate is an acceptable compromise between adding the bis-anhydride rapidly enough to achieve acceptable throughput, while adding the bis-anhydride slowly enough to avoid excessive temperature increases and for maintaining control of the pH. In continuous feeding, one exemplary method of adding the bis- anhydride to the aqueous solution continuously is by a mechanical feeder. The mechanical feeder can be set to feed bis-anhydride (about 100 grams, for example) into an aqueous solvent system (about 100 mL, for example) at a rate between about 0.5 grams/min and about 2.5 grams/min. In this example, the feed rate is preferably set at the lower end of the range, such as about 0.5 g/min or about 0.75 g/min, which is rapid enough to balance acceptable throughput with acceptable purity. In this example, the feed rate is preferably no higher than about 2.5 g/min to achieve acceptable purity and avoid excessive temperature increases. Continuous feeding is currently preferred over intermittent feeding. The feed rates exemplified above can be scaled based on the total amount of bis-anhydride to be added and the total aqueous solvent system volume. Stated another way, as the total amount of bis-anhydride to be added and the total aqueous solvent system volume increase, the feed rate can be adjusted, i.e., increased, to properly balance throughput, purity, pH control, and temperature control.

[0042] Bis-anhydride feeding decreases solution pH. Therefore, in one embodiment, the reaction mixture pH is adjusted during the course of controlled feeding to maintain the reaction mixture at the desired pH. Exemplary pH adjusting agents include sodium hydroxide, potassium hydroxide, tertiary amines such as triethylamine, aromatic amines such as pyridine and poly(4-vinylpyridine), and basic ion exchange resins for alkaline pH adjustment. Ammonia, primary amines, and secondary amines are avoided for alkaline adjustment because these could cause unwanted side reactions with the bis-anhydride. Hydrochloric acid, acetic acid, and acidic ion exchange resins are applicable for acidic pH adjustment. The pH adjusting agents are typically added as concentrated solutions, so the volume used may be minor in comparison to the initial volume of the aqueous solvent system. However, in some embodiments, pH adjustment can appreciably change the total system volume, which changes the concentrations of the reactants during the course of the reaction.

[0043] Preferred bis-anhydrides for combining with the nitrogen-containing compounds in the aqueous solvent system include those shown in Formulae V and VI, which include EDTA bis- anhydride and DTPA bis-anhydride. In a preferred bis-amide complexing agent preparation, the bis- anhydride is DTPA bis-anhydride (M. W. 357.32 g/mol) which is combined with preferred primary amine, 2-methoxyethylamine (M.W. 75.11 g/mol), to form DTPA bis-(2-methoxyethylamide) complexing agent (M.W. 504.52 g/mol), commonly known as versetamide. In an exemplary preparation using 2-methoxyethylamine (2-MEA) and DTPA bis-anhydride to form DTPA bis-(2- methoxyethylamide) complexing agent, the reactants and conditions are as shown in the following Table I:

TABLE I. REACTANTS AND CONDITIONS

In a quantitative synthesis under these conditions, about 0.285 mols (about 154 grams) of DTPA bis-(2- methoxyethylamide) complexing agent are synthesized. Several parameters, i.e., initial solvent volume, molar amounts of 2-MEA and DTPA bis-anhydride, and feed rates, may be scaled to achieve lesser or greater amounts of DTPA bis-(2-methoxyethylamide) complexing agent.

[0044] The method of preparing the paramagnetic metal ion complex of the present invention comprises combining the bis-amide complexing agent and a paramagnetic metal ion salt. In one embodiment, the bis-amide complexing agent may be isolated from the reaction mixture and purified prior to combining the complexing agent with the paramagnetic metal ion salt. In one embodiment of the method of the present invention, the paramagnetic metal ion complex may be prepared without prior isolation of the bis-amide complexing agent from the reaction mixture used to prepare the bis-amide complexing agent. In this embodiment, prior to adding the paramagnetic metal ion salt into the reaction mixture containing bis-amide complexing agent, the pH may be lowered to between about 1 and about 5, such as between about 3 and about 5, preferably about 4. Exemplary acids for acidic pH adjustment include hydrochloric acid, acetic acid, and acidic ion exchange resins. The reaction mixture is preferably lowered to a pH within this range because at a more acidic pH, the paramagnetic metal ion may not complex with the bis-amide complexing agent. At pH above the preferred range, the paramagnetic metal ion may not dissolve. Additionally, the reaction mixture may be heated to a temperature in excess of about 70⁰C, such as about 8O⁰C, prior to the introduction of the paramagnetic metal ion salt.

[0045] In one embodiment, the amount of paramagnetic metal ion salt is added such that there is a slight stoichiometric excess of bis-amide complexing agent. Typically, the molar ratio of the bis- amide to paramagnetic metal ion combined in the aqueous solvent system may be between about 1 : 1 and about 1.15:1, respectively, such as about 1.07:1.

[0046] The paramagnetic metal ion may be selected from among iron(II), cobalt(II), manganese(II), nickel(II), copper(II), chromium(II), rhodium(II), iridium(II), gadolinium(III), chromium(III), manganese(III), iron(III), praseodymium(III), europium(III), neodymium(III), samarium(ΪII), ytterbium(IH), terbium(III), dysprosium(III), holmium(IH), and erbium(IIl). In one embodiment, the paramagnetic metal ion is gadolinium(III). Sources of gadoIinium(III) ion include gadolinium oxide (Gd₂O₃, M. W. 362.50 g/mol), gadolinium nitrate hexahydrate (Gd(NO₃)₃*6H₂O, M.W. 451.36 g/mol), gadolinium chloride (GdCl₃, M. W. 263.61 g/mol), gadolinium chloride hexahydrate (GdCl₃ ^«6H₂O, M.W. 371.70 g/mol), gadolinium acetate hydrate (Gd(CH₃CO₂)₃*xH₂O, M.W. 334.38 g/mol), gadolinium oxalate (Gd₂(C₂O₄)₃, M.W. 578.56 g/mol), gadolinium(ITI) bromide (GdBr₃ M.W. 396.96 g/mol), and gadolinium carbonate (Gd₂(CO₃)₃, M.W. 494.53). Gadolinium oxide (Gd₂O₃) is a preferred source of gadolinium because its use does not introduce extraneous anions which may interfere with the reaction. Gadolinium nitrate hexahydrate (Gd(NO₃)₃ ^«6H₂O) is another preferred source of gadolinium because of its high solubility. However, the use of the nitrate introduces extraneous anions, which may be removed by reverse osmosis.

[0047] After introduction of the gadolinium salt, the reaction mixture can be refluxed at elevated temperature for at least about 2 hours, preferably between about 2 and about 5 hours, such as about 2 hours. In an exemplary synthesis using gadolinium oxide and DTPA bis-(2- methoxyethylamide) complexing agent, the reactants and conditions are as shown in the following Table II:

TABLE II. REACTANTS AND CONDITIONS

The molar amounts of reactants may be scaled to achieve lesser or greater amounts of gadolinium(III) complex with DTPA bis-(2-methoxyethylamide).

[0048] In another exemplary synthesis using gadolinium nitrate hexahydrate and DTPA bis-(2- methoxyethylamide) complexing agent, the reactants and conditions are as shown in the following Table III:

TABLE III. REACTANTS AND CONDITIONS

[0049] When the preferred paramagnetic metal ion gadolinium is used, the resulting contrast agent is gadolinium(III) complex of DTPA-bis(methoxyethylamide), commonly known as gadoversetamide. After completion of the reaction, the complex may be isolated from the reaction mixture by rotary evaporation, spray drying, precipitation, crystallization, or a combination thereof. If necessary, the gadolinium(III) complex of DTPA-bis(methoxyethylamide) can be purified by redissolving the paramagnetic metal ion complex in water and using reverse osmosis. [0050] In a reverse osmosis procedure, the gadolinium(III) complex of DTPA- bis(methoxyethylamide) is dissolved in water (purified using MiIIiQ) which is poured into the reservoir on the reverse osmosis unit. High impurity (salt) content is determined by measuring the conductivity of the solution with a conductivity meter. A high salt content results in a reading being off scale. This impure solution is then pumped through a cylinder containing a reverse osmosis membrane. Small molecules such as some residual solvents and salts pass through the membrane (called permeate) while larger compounds are retained and recirculated back into the reservoir (retentate) where more fresh water is added (essentially washing the salts away). This is repeated until an aliquot of the permeate shows low conductivity (usually ~100-l 50 μS (micro Siemens)). The compound may be isolated using the various methods discussed previously.

[0051] The following examples further illustrate the present invention.

Example 1. Synthesis of Diethylenetriaminepentaacetic acid bis-anhydride (DTPA bis- anhydride)

[0052] Diethylenetriaminepentaacetic acid (1006 g, DTPA, Aldrich), acetonitrile (391.94 g, EMD Chemicals), acetic anhydride (777.23 g, Mallinckrodt), and pyridine (900.96 g, Mallinckrodt) were added, in that order, to a 5 L reaction flask, equipped with an overhead stirrer, a temperature probe, and a reflux condenser. The flask was placed on a heating mantle, and the reaction mixture was heated to 6O⁰C and refluxed at that temperature for 18 hours.

[0053] The mixture was allowed to cool to below 45°C. Cooling, which took about 1.5 hours, caused the formation of a precipitate. The solids were isolated by vacuum filtration, and the product was washed, first with isopropyl alcohol (3249 g), and next with acetonitrile (3501 g). The washed solid cake was dried at 45°C to 50⁰C under 22 mmHg vacuum for 18 hours.

[0054] A total of 873.2 g DTPA bis-anhydride was synthesized by this reaction, representing a 95.9% yield based on DTPA starting material. The DTPA bis-anhydride purity was 100% as determined by gas chromatography.

Example 2. Synthesis of Versetamide in Alkaline Aqueous Solvent System

[0055] DTPA bis-(2-methoxyethylamide) (versetamide) was synthesized in an aqueous solvent system which was maintained at a slightly alkaline pH of about 8.

[0056] Purified water (730 mL purified using MiIIiQ, Millipore, Billerica, MA) was added to a 1 Liter 3-neck round bottom flask equipped with a mechanical overhead stirrer, a pH meter, and digital temperature probe. 2-Methoxyethylamine (42.88 g, 2-MEA, BASF) was added to the water, which caused an increase in temperature to about 23°C. The solution pH upon addition of 2-MEA was 11.97. The flask was placed in an ice bath to lower the solution temperature and maintain it between about 10⁰C and about 20⁰C. DTPA bis-anhydride (100.04 g) was added to a mechanical powder feeder, which was set to continuously feed about 0.5 g DTPA bis-anhydride per minute to the flask. Upon initiation of DTPA bis-anhydride feeding, the solution pH decreased. The decrease in pH was allowed to continue until the pH was slightly below about 8. Thereafter, the solution pH was maintained between about 8 and about 10 throughout the course of the reaction using a 10 N NaOH solution. Additionally, the solution temperature was maintained between about 17°C and about 19°C using the ice bath. Aliquots were taken periodically and analyzed by HPLC (Altima Cl 8 column, 4.6 x 250 mm, 10 μm particles, 1 mL/min flow rate using 93:7 v:v pH 2.5 phosphate buffeπacetonitrile buffer solution) to monitor the formation of versetamide. Upon completion of the reaction, it was determined that about 97 g DTPA bis-anhydride had reacted with the 2-MEA to form versetamide. Chromatographic analysis indicated the versetamide product was 98.04% pure.

Example 3. Synthesis of Versetamide in Alkaline Aqueous Solvent System

[0057] DTPA bis-(2-methoxyethylamide) (versetamide) was synthesized in an aqueous solvent system which was maintained at a slightly alkaline pH of about 8.

[0058] Purified water (730 mL purified using MiIIiQ, Millipore, Billerica, MA) was added to a 1 Liter 3-neck round bottom flask equipped with a mechanical overhead stirrer, a pH meter, and digital temperature probe. 2-MEA (42.83 g) was added to the water. The solution was stirred using a stirrer rotation of 500 rpm. The addition of the 2-MEA raised the temperature of the solution from 17.8⁰C to 21.8°C and raised the pH to 11.69. Before the addition of DTPA bis-anhydride, the solution pH was lowered to.7.98 using 37% HCl, and the solution temperature was lowered to about 0⁰C using an ice bath. DTPA bis-anhydride was added to the reaction mixture over the course of 1.5 to 2 hours in approximately 5 g increments about every 5 minutes. The solution temperature was maintained between about 15⁰C and about 20⁰C during the course of the reaction, and the solution pH was maintained at about 8 using IO N NaOH solution and concentrated HCl. Aliquots were taken periodically and analyzed by HPLC (Altima Cl 8 column, 4.6 x 250 mm, 10 μm particles, 1 mL/min flow rate using 93:7 v:v pH 2.5 phosphate buffeπacetonitrile buffer solution) to monitor the formation of versetamide. Upon completion of the reaction, it was determined that 100.29 g DTPA bis-anhydride had reacted with the 2-MEA to form versetamide. Chromatographic analysis indicated the versetamide product was 73.36% pure.

Example 4. Synthesis of Versetamide in Alkaline Aqueous Solvent System

[0059] DTPA bis-(2-methoxyethyIamide) (versetamide) was synthesized in an aqueous solvent system which was maintained at a slightly alkaline pH of about 8.

[0060] Purified water (730 mL purified using MiUiQ, Millipore, Billerica, MA) was added to a 1 Liter 3-neck round bottom flask equipped with a mechanical overhead stirrer, a pH meter, and digital temperature probe. 2-MEA (42.89 g) was added to the water. The solution was stirred using a stirrer rotation of 500 rpm. The addition of the 2-MEA raised the temperature of the solution from 18.2°C to 22.2°C and raised the pH to 11.71. Before the addition of DTPA bis-anhydride, the solution pH was lowered to 7.99 using 37% HCl, and the solution temperature was allowed to reach 18°C. DTPA bis- anhydride was added to the reaction mixture over the course of 1.5 to 2 hours in approximately 5 g increments about every 5 minutes. The solution temperature was maintained between about 13°C and about 18⁰C during the course of the reaction, and the solution pH was maintained at about 8 using IO N NaOH solution and concentrated HCl. Aliquots were taken periodically and analyzed by HPLC (Altima C18 column, 4.6 x 250 mm, 10 μm particles, 1 mL/min flow rate using 93:7 v:v pH 2.5 phosphate buffeπacetonitrile buffer solution) to monitor the formation of versetamide. Upon completion of the reaction, it was determined that 100.15 g DTPA bis-anhydride had reacted with the 2- MEA to form versetamide. Chromatographic analysis indicated the versetamide product was 91.71% pure.

Example S. Synthesis of Versetamide in Strongly Alkaline Aqueous Solvent System

[0061] DTPA bis-(2-methoxyethylamide) (versetamide) was synthesized in an aqueous solvent system which was maintained at a strongly alkaline pH of about 13.

[0062] Purified water (160 mL purified using MiIIiQ, Millipore, Billerica, MA) was added to a 500 mL 3-neck round bottom flask equipped with a mechanical overhead stirrer, a pH meter, and digital temperature probe. 2-MEA (42.89 g) was added to the water, which caused the temperature to rise from 22.5°C to 39°C, and the solution pH to rise from 6.25 to 12.32. DTPA bis-anhydride (100.03 g) was added to a mechanical powder feeder, which was set to continuously feed about 0.5 g DTPA bis- anhydride per minute to the flask. After initiation of DTPA bis-anhydride feeding, the solution pH was raised to 13.23 using IO N NaOH solution and maintained at about 13 throughout the course of the reaction. Moreover, the solution temperature was maintained at about 35°C using a heating mantle. Aliquots were taken periodically and analyzed by HPLC (Altima Cl 8 column, 4.6 x 250 mm, 10 μm particles, 1 mL/min flow rate using 93:7 v:v pH 2.5 phosphate buffeπacetonitrile buffer solution) to monitor the formation of versetamide. Upon completion of the reaction, it was determined that about 97 g DTPA bis-anhydride had reacted with the 2-MEA to form versetamide. Chromatographic analysis indicated the versetamide product was 98.75% pure.

Example 6. Synthesis of Versetamide in Strongly Alkaline Aqueous Solvent System

[0063] DTPA bis-(2-methoxyethylamide) (versetamide) was synthesized in an aqueous solvent system which was maintained at an alkaline pH of about 13.

[0064] Purified water (160 mL purified using MiIIiQ₅ Millipore, Billerica, MA) was added to a 1 L 3-neck round bottom flask equipped with a mechanical overhead stirrer and digital temperature . probe. The flask was placed in an ice bath to lower the solution temperature to about 0⁰C. 2-MEA (42.91 g) was added to the water, which caused the temperature to rise to about 20⁰C and the pH to rise from 5.50 to 12.36. The pH was adjusted to 13.42 using IO N NaOH solution. DTPA bis-anhydride (100.08 g) was added to a mechanical powder feeder, which was set to continuously feed about 0.7 g DTPA bis-anhydride per minute to the flask. After initiation of DTPA bis-anhydride feeding, the solution pH gradually lowered. The solution pH was increased to and maintained at about 13 throughout the course of the reaction using IO N NaOH solution. Moreover, the solution temperature was maintained at around 0⁰C using the ice bath. Aliquots were taken periodically and analyzed by HPLC (Altima C18 column, 4.6 x 250 mm, 10 μm particles, 1 mL/min flow rate using 93:7 v:v pH 2.5 phosphate buffeπacetonitrile buffer solution) to monitor the formation of versetamide. Chromatographic analysis indicated the versetamide product was 99.25% pure.

Example 7. Synthesis of Versetamide in Strongly Alkaline Aqueous Solvent System

[0065] DTPA bis-(2-methoxyethylamide) (versetamide) was synthesized in an aqueous solvent system which was maintained at a strongly alkaline pH of about 13.

[0066] Purified water (1300 mL purified using MiIIiQ, Millipore, Billerica, MA) was added to a 3-neck round bottom flask equipped with a mechanical overhead stirrer and digital temperature probe.

The water was cooled to about 0⁰C using an ice bath. 2-MEA (42.92 g) was added to the water. The solution was stirred at stirrer rotation of 500 rpm. The addition of the 2-MEA raised the temperature of the solution from 1.9⁰C to 4.3°C and raised the solution pH to 11.57. Before the addition of DTPA bis- anhydride, the solution pH was raised to 13.06 using IO N NaOH solution, and the solution temperature was cooled to about 0⁰C. DTPA bis-anhydride was added to the reaction mixture over the course of 1.5 to 2 hours in approximately 5 g increments about every 5 minutes. The solution temperature was maintained between about 0⁰C and about 2°C during the course of the reaction, and the solution pH was maintained at about 13 using IO N NaOH solution. Aliquots were taken periodically and analyzed by HPLC (Altima Cl 8 column, 4.6 x 250 mm, 10 μm particles, 1 mL/min flow rate using 93:7 v:v pH 2.5 phosphate buffeπacetonitrile buffer solution) to monitor the formation of versetamide. Upon completion of the reaction, it was determined that 100.37 g DTPA bis-anhydride had reacted with the 2- MEA to form versetamide. Chromatographic analysis indicated the versetamide product was 95.50% pure.

Example 8. Synthesis of Versetamide in Strongly Alkaline Aqueous Solvent System

[0067] DTPA bis-(2-methoxyethylamide) (versetamide) was synthesized in an aqueous solvent system which was maintained at a strongly alkaline pH of about 13.

[0068] Purified water (1300 mL purified using MiIIiQ, Millipore, Billerica, MA) was added to a 3-neck round bottom flask equipped with a mechanical overhead stirrer, a pH meter, and digital temperature probe. The water was heated to about 35°C using a heating mantle. 2-MEA (42.92 g) was added to the water. The solution was stirred at stirrer rotation of 500 rpm. With the addition of the 2- MEA, the solution temperature increased to 37.2°C, and the solution pH increased to about 11. Before the addition of DTPA bis-anhydride, the solution pH was raised to 13.03 using ION NaOH solution, and the solution temperature was about 37.9°C. DTPA bis-anhydride was added to the reaction mixture over the course of 1.5 to 2 hours in approximately 5 g increments about every 5 minutes. The solution temperature was maintained between about 34°C and about 38°C during the course of the reaction, and the solution pH was maintained between about 12 and about 13 using 10 N NaOH solution. Aliquots were taken periodically and analyzed by HPLC (Altima Cl 8 column, 4.6 x 250 mm, 10 μm particles, 1 mL/min flow rate using 93:7 v:v pH 2.5 phosphate buffeπacetonitrile buffer solution) to monitor the formation of versetamide. Upon completion of the reaction, it was determined that 100.4 g DTPA bis-aπhydride had reacted with the 2-MEA to form versetamide. Chromatographic analysis indicated the versetamide product was 44.39% pure.

Example 9. Synthesis of Versetamide in Acidic Aqueous Solvent System

[0069] DTPA bis-(2-methoxyethylamide) (versetamide) was synthesized in an aqueous solvent system which was maintained at an acidic pH of about 3.

[0070] Purified water (160 mL purified using MiIIiQ, Millipore, Billerica, MA) was added to a 500 mL 3-neck round bottom flask equipped with a mechanical overhead stirrer, a pH meter, and digital temperature probe. 2-MEA (42.92 g) was added to the water, which caused the temperature to rise from 24.0⁰C to 48°C and the solution pH to increase from 5.30 to 11.89. Concentrated HCl (56.88 g) was added to lower the solution pH to 2.50. IO N NaOH and concentrated HCl solutions were added to achieve pH 3.01. DTPA bis-anhydride (100.06 g) was added to a mechanical powder feeder, which was set to continuously feed about 0.5 g DTPA bis-anhydride per minute to the flask. After initiation of DTPA bis-anhydride feeding, the solution pH gradually lowered. Accordingly, the solution pH was maintained at about 3 throughout the course of the reaction using IO N NaOH solution. Moreover, the solution temperature was maintained at around 35°C using a heating mantle. Aliquots were taken periodically and analyzed by HPLC (Altima Cl 8 column, 4.6 x 250 mm, 10 μm particles, 1 mL/min flow rate using 93:7 v:v pH 2.5 phosphate buffeπacetonitrile buffer solution) to monitor the formation of versetamide. Upon completion of the reaction, it was determined that about 95 g DTPA bis- anhydride had reacted with the 2-MEA to form versetamide. The solution was cooled and filtered through a Buchner funnel. The filter cake was washed twice with acetonitrile (300 g each wash). The cake was transferred to a recrystallization dish and dried overnight in a vacuum oven at 55°C. Chromatographic analysis indicated the versetamide product was 98.93% pure.

Example 10. Synthesis of Versetamide in Acidic Aqueous Solvent System

[0071] DTPA bis-(2-methoxyethyIamide) (versetamide) was synthesized in an aqueous solvent system which was maintained at an acidic pH of about 3.

[0072] Purified water (160 mL purified using MiIIiQ, Millipore, Billerica, MA) was added to a 500 mL 3-neck round bottom flask equipped with a mechanical overhead stirrer and digital temperature probe. The flask was placed in an ice bath to lower the solution temperature at about 0⁰C. 2-MEA (42.90 g) was added to the water, which caused the temperature to rise to 18.2°C and the solution pH to increase to 12.02. DTPA bis-anhydride (100.20 g) was added to a mechanical powder feeder, which was set to continuously feed about 0.5 g DTPA bis-anhydride per minute to the flask. After initiation of DTPA bis-anhydride feeding, the solution pH gradually lowered. The solution pH was lowered to 2.07 using concentrated HCl. Feeding was stopped and concentrated HCl and IO N NaOH solutions were used to attain a pH of 3.11. Feeding continued, which lowered the solution pH. IO N NaOH solution was used to maintain the solution pH at about 3 during the course of the reaction, and the solution temperature was maintained at around 0⁰C using the ice bath. Aliquots were taken periodically and analyzed by HPLC (Altima Cl 8 column, 4.6 x 250 mm, 10 μm particles, 1 mL/min flow rate using 93:7 v:v pH 2.5 phosphate buffeπacetonitrile buffer solution) to monitor the formation of versetamide. Upon completion of the reaction, it was determined that about 95 g DTPA bis-anhydride had reacted with the 2-MEA to form versetamide. Chromatographic analysis indicated the versetamide product was 12.01% pure.

Example 11. Synthesis of Versetamide in Acidic Aqueous Solvent System

[0073] DTPA bis-(2-methoxyethylamide) (versetamide) was synthesized in an aqueous solvent system which was maintained at an acidic pH of about 3.

[0074] Purified water (1300 mL purified using MiIIiQ, Millipore, Billerica, MA) was added to a 3 -neck round bottom flask equipped with a mechanical overhead stirrer, a pH meter, and digital temperature probe. The water was cooled to about 0⁰C using an ice bath. 2-MEA (42.89 g) was added to the water. The solution was stirred using a stirrer rotation of 500 rpm. With the addition of the 2- MEA, the solution temperature increased from 0.5⁰C to 3°C, and the solution pH increased to 1 1.50. Before the addition of DTPA bis-anhydride, the solution pH was lowered to 2.75 using concentrated HCl and ION NaOH solutions, and the solution temperature was about 0.2⁰C. DTPA bis-anhydride was added to the reaction mixture over the course of 1.5 to 2 hours in approximately 5 g increments about every 5 minutes. The solution temperature was maintained between about O⁰C and about 1°C during the course of the reaction, and the solution pH was maintained between about 2 and about 3 using concentrated HCl and IO N NaOH solutions. Aliquots were taken periodically and analyzed by HPLC (Altima Cl 8 column, 4.6 x 250 mm, 10 μm particles, 1 mL/min flow rate using 93:7 v:v pH 2.5 phosphate buffeπacetonitrile buffer solution) to monitor the formation of versetamide. Upon completion of the reaction, it was determined that 100.29 g DTPA bis-anhydride had reacted with the 2- MEA to form versetamide. Chromatographic analysis indicated the versetamide product was 87.76% pure.

Example 12. Synthesis of Versetamide in Acidic Aqueous Solvent System

[0075] DTPA bis-(2-methoxyethylamide) (versetamide) was synthesized in an aqueous solvent system which was maintained at an acidic pH of about 3. [0076J Purified water (1300 mL purified using MiIIiQ, Millipore, Billerica, MA) was added to a 3-neck round bottom flask equipped with a mechanical overhead stirrer and digital temperature probe. The water was heated to about 35⁰C using a heating mantle. 2-MEA (42.87 g) was added to the water. The solution was stirred at stirrer rotation of 500 rpm. With the addition of the 2-MEA, the solution temperature increased from 36.6°C to 38.2⁰C, and the solution pH increased to 1 1.17. Before the addition of DTPA bis-anhydride, the solution pH was lowered to 2.80 using concentrated HCl and ION NaOH solutions, and the solution temperature was 34⁰C. DTPA bis-anhydride was added to the reaction mixture over the course of 1.5 to 2 hours in approximately 5 g increments about every 5 minutes. The solution temperature was maintained between about 33°C and about 36°C during the course of the reaction, and the solution pH was maintained between about 2 and about 3 using concentrated HCI and IO N NaOH solutions. Aliquots were taken periodically and analyzed by HPLC (Altima Cl 8 column, 4.6 x 250 mm, 10 μm particles, 1 mL/min flow rate using 93:7 v:v pH 2.5 phosphate buffeπacetonitrile buffer solution) to monitor the formation of versetamide. Upon completion of the reaction, it was determined that 100.25 g DTPA bis-anhydride had reacted with the 2- MEA to form versetamide. Chromatographic analysis indicated the versetamide product was 90.73% pure.

Example 13. Synthesis of Gadoversetamide in Aqueous Solvent System from Versetamide

[0077] Gadolinium(III) complex of DTPA-bis(methoxyethylamide) (gadoversetamide) was synthesized from versetamide in an aqueous solvent system which was maintained at an acidic pH of less than about 1.

[0078] Purified water (500 mL purified using MiIIiQ, Millipore, Billerica, MA) was added to a 1 L 3-neck round bottom flask equipped with a mechanical overhead stirrer, pH meter, and reflux condenser. Versetamide (20.00 g) was added to the reaction flask, which lowered the solution pH to 2.87. The solution was further acidified using concentrated HCl to a solution pH of 0.50. Gadolinium oxide, Gdaθ₃, (6.40 g, Rhodia) was added, which did not result in a pH change. The flask was placed on a heating mantle, which heated the solution to about 100⁰C after about half an hour. The solution was heated and refluxed for 16 hours. The solution pH remained at about 0.5. Aliquots were taken periodically and analyzed by HPLC (Waters Xterra RPl 8, 4.6 x 150 mm, 5 μm particles, 0.8 mL/min flow rate using aqueous buffer solution (0.5 mg/L CaCl₂, adjusted to pH 8 +/- 0.2 with HCl and NH₄OH)) to monitor the formation of gadoversetamide. Chromatographic analysis indicated the gadoversetamide product was 80.90% pure.

Example 14. Synthesis of Gadoversetamide in Aqueous Solvent System from Versetamide

[0079] Gadolinium(III) complex of DTPA-bis(methoxyethylamide) (gadoversetamide) was synthesized from versetamide in an aqueous solvent system having a starting pH of about 4, which drifted to about 10 during the course of the reaction. [0080] Purified water (500 mL purified using MiIIiQ, Millipore, Billerica, MA) was added to a 1 L 3-neck round bottom flask equipped with a mechanical overhead stirrer, pH meter, and reflux condenser. Versetamide (20.00 g) was added to the reaction flask, which lowered the solution pH to 2.877. The solution pH was increased to 4.04 using 50% NaOH solution. Gadolinium oxide, Gd₂O₃, (6.40 g) was added, which did not result in a pH change. The flask was placed on a heating mantle, which heated the solution to about 100⁰C after about half an hour. The solution was heated and refluxed for 18 hours. The solution pH increased during the course of refluxing to about 10. AHquots were taken periodically and analyzed by HPLC (Waters Xterra RP18, 4.6 x 150 mm, 5 μm particles, 0.8 mL/min flow rate using aqueous buffer solution (0.5 mg/L CaCl₂, adjusted to pH 8 +/- 0.2 with HCl and NH₄OH)) to monitor the formation of gadoversetamide. Chromatographic analysis indicated the gadoversetamide product was 80.9% pure.

Example 15. Synthesis of Gadoversetamide in Aqueous Solvent System from Versetamide

[0081] Gadolinium(III) complex of DTPA-bis(methoxyethylamide) (gadoversetamide) was synthesized from versetamide in an aqueous solvent system which was maintained at an acidic pH of less than about 1.

[0082] Versetamide (20.06 g) was added to water (12 mL purified using MiIIiQ, Millipore, Billerica, MA) contained in a reaction flask. The pH of the solution was 2.49 upon the addition of the versetamide. The solution pH was lowered further to about 0.5, using concentrated HCl and IO N NaOH solutions. Gadolinium nitrate hexahydrate, Gd(NOs)₃ *6H₂O, (15.92 g, Aldrich) was added to the reaction mixture with additional water (5 mL). The reaction progressed at room temperature for about 1 hour. Aliquots were taken periodically and analyzed by HPLC (Waters Xterra RPl 8, 4.6 x 150 mm, 5 μm particles, 0.8 mL/min flow rate using aqueous buffer solution (0.5 mg/L CaCl₂, adjusted to pH 8 +/- 0.2 with HCl and NH₄OH)) to monitor the formation of gadoversetamide. Upon completion of the reaction, the solution pH was raised to 7.38 using concentrated HCl and IO N NaOH solutions. Chromatographic analysis indicated the gadoversetamide product was 62.4% pure. The solution contained 32.5 % of an unknown compound that was believed to be some intermediate in the versetamide to gadoversetamide complexation process. When the pH was adjusted, the unknown disappeared and the gadoversetamide purity increased.

Example 16. Synthesis of Gadoversetamide in Aqueous Solvent System from Versetamide

[0083] Gadolinium(III) complex of DTPA-bis(methoxyethylamide) (gadoversetamide) was synthesized from versetamide in an aqueous solvent system which was maintained at an acidic pH of about 2.5.

[0084] Versetamide (20.03 g) was added to water (12 mL purified using MiIIiQ, Millipore,

Billerica, MA) contained in a reaction flask. The pH of the solution was 2.70 upon the addition of the versetamide. The solution pH was lowered further to 2.34, using concentrated HCl. Gadolinium nitrate, Gd(NO₃)₃ '6H₂O, (15.93 g, Aldrich) was added to the reaction mixture with additional water. The reaction progressed at room temperature for about 1 hour, and volumes of concentrated HCl and 10 N NaOH solutions were added to maintain the solution pH at about 2.5 throughout the course of the reaction. Aliquots were taken periodically and analyzed by HPLC (Waters Xterra RPl 8, 4.6 x 150 mm, 5 μm particles, 0.8 mL/min flow rate using aqueous buffer solution (0.5 mg/L CaCl₂, adjusted to pH 8 +/- 0.2 with HCl and NH₄OH)) to monitor the formation of gadoversetamide. Upon completion of the reaction, the solution pH was raised to 7.8 using concentrated HCl and IO N NaOH solutions. Chromatographic analysis indicated the gadoversetamide product was 60.3% pure. The material contained an unknown compound that was believed to be some intermediate in the versetamide to gadoversetamide complexation process. When the pH was adjusted, the unknown disappeared and the gadoversetamide purity increased.

Example 17. Synthesis of Gadoversetamide in Aqueous Solvent System from Versetamide

Ϊ0085] Gadolinium(III) complex of DTPA-bis(methoxyethylamϊde) (gadoversetamide) was synthesized from versetamide in an aqueous solvent system which was maintained at an acidic pH of about 4.

[0086] Versetamide (20.06 g) was added to water (12 mL purified using MiIIiQ, Millipore, Billerica, MA) contained in a reaction flask. The pH of the solution was 2.77 upon the addition of the versetamide. The solution pH was raised to 3.9, using IO N NaOH solution. Gadolinium nitrate, Gd(NO₃)₃ ¹OH₂O, (15.94 g, Aldrich) was added to the reaction mixture with additional water (5 mL). The reaction progressed at room temperature for about 1 hour, and volumes of concentrated HCl and 10 N NaOH solutions were used to maintain the solution pH at about 4 throughout the course of the reaction. Aliquots were taken periodically and analyzed by HPLC (Waters Xterra RPl 8, 4.6 x 150 mm, 5 μm particles, 0.8 mL/min flow rate using aqueous buffer solution (0.5 mg/L CaCb, adjusted to pH 8 +/- 0.2 with HCl and NH₄OH)) to monitor the formation of gadoversetamide. Upon completion of the reaction, the solution pH was raised to 8.18 using concentrated HCl and IO N NaOH solutions. Chromatographic analysis indicated the gadoversetamide product was 60.0% pure. The material contained an unknown compound that was believed to be some intermediate in the versetamide to gadoversetamide complexation process. When the pH was adjusted, the unknown disappeared and the gadoversetamide purity increased.

Example 18. Synthesis of Gadoversetamide in Aqueous Solvent System from Versetamide

[0087] Gadolinium(III) complex of DTPA-bis(methoxyethylamide) (gadoversetamide) was synthesized from versetamide in an aqueous solvent system. During the course of the reaction, the solution pH was not maintained at a target pH but was allowed to drift.

[0088] Versetamide (20.01 g) was added to water (20 mL purified using MiHiQ, Millipore,

Billerica, MA) contained in a reaction flask. The pH of the solution was 2.89 upon the addition of the versetamide. The reaction flask holding the solution was equipped with an overhead stirrer, a temperature probe, and a pH probe. The flask was placed on a heating mantle, which heated the solution to 80⁰C in about 10 minutes. Gadolinium oxide, Gd₂O₃, (7.23 g) was added, which raised the pH slightly to 3.04. The solution was refluxed at 80⁰C for about 2 hours, during which the pH of the solution increased from about 3 to a final pH of 1.12. 2.5 hours after the addition of the Gd₂O₃, concentrated HCl solution was added to lower the solution pH to 3.52. The solution was allowed to react for an additional half hour. Aliquots were taken periodically and analyzed by HPLC (Waters Xterra RP 18, 4.6 x 150 mm, 5 μm particles, 0.8 mL/min flow rate using aqueous buffer solution (0.5 mg/L CaCl₂, adjusted to pH 8 +/- 0.2 with HCl and NH₄OH)) to monitor the formation of gadoversetamide. Chromatographic analysis indicated the gadoversetamide product was 98.96% pure.

Example 19. Synthesis of Gadoversetamide in Aqueous Solvent System from Versetamide

[0089] Gadolinium(III) complex of DTPA-bis(methoxyethylamide) (gadoversetamide) was synthesized from versetamide in an aqueous solvent system which was maintained at an acidic pH of about 3.

[0090] Versetamide (20.05 g) was added to water (160 mL purified using MiUiQ, Millipore, Billerica, MA) contained in a reaction flask. The pH of the solution was 2.67 upon the addition of the versetamide. The reaction flask holding the solution was equipped with an overhead stirrer, a temperature probe, and a pH probe. The flask was placed on a heating mantle, which heated the solution to 80⁰C in about 10 minutes. Gadolinium oxide, Gd₂O₃, (7.25 g) was added, which raised the pH slightly to 2.78. The solution was refluxed at 80⁰C for about 2 hours. During refluxing, the pH of the solution increased after about an hour to 6.90. The solution pH was lowered to 2.89 using concentrated HCl solution. The solution pH continued to increase during the second hour, to a final pH of 3.12 upon completion of the reflux. Aliquots were taken periodically and analyzed by HPLC (Waters Xterra RPl 8, 4.6 x 150 mm, 5 μm particles, 0.8 mL/min flow rate using aqueous buffer solution (0.5 mg/L CaCl₂, adjusted to pH 8 +/- 0.2 with HCl and NH₄OH)) to monitor the formation of gadoversetamide. Chromatographic analysis indicated the gadoversetamide product was 98.70% pure.

Example 20. Total Synthesis of Gadoversetamide in Aqueous Solvent System

[0091] Gadolinium(III) complex of DTPA-bis(methoxyethylamide) (gadoversetamide) was synthesized from DTPA bis-anhydride and 2-MEA starting materials in aqueous solvent system without prior isolation and purification of versetamide.

[0092] To prepare versetamide, 2-methoxyethylamine (85.1 g 2-MEA, BASF) was added to water (700 mL) contained in a reaction flask. The pH of the solution after adding the 2-MEA was about 13. The solution was cooled to 10⁰C using an ice bath. DTPA bis-anhydride (200 g) was added to the reaction flask incrementally over the course of 1.5 to 2 hours to maintain the solution temperature at about 10⁰C. Because the aqueous solution was non-buffered, the addition of the DTPA bis- anhydride lowered the solution pH to about 3.2. Aliquots of the reaction composition were taken periodically and analyzed by HPLC (Altima Cl 8 column, 4.6 x 250 mm, 10 μm particles, 1 mL/min flow rate using 93:7 v:v pH 2.5 phosphate buffeπacetonitrile buffer solution) to monitor the formation of versetamide.

[0093] Upon completion of the versetamide reaction, gadolinium nitrate, Gd(NO₃)₃ '6H₂O solution (28.85g in 250 mL water) was added to the reaction flask containing the versetamide. The pH decreased to about 0.7 upon addition of the gadolinium nitrate. After 50 minutes, the pH of the solution was raised to about 7.7 using IO N NaOH solution. Water was removed by rotary evaporation, leaving solid gado versetamide product and nitrate salts. The product was purified by redissolving the solids in water and reverse osmosis.

Example 21. Total Synthesis of Gadoversetamide in Aqueous Solvent System

[0094] Gadolinium(III) complex of DTPA-bis(methoxyethylamide) (gadoversetamide) was synthesized from DTPA bis-anhydride and 2-MEA starting materials in aqueous solvent system without prior isolation and purification of versetamide.

[0095] Water (160 mL) was added to a 3-neck round bottom flask equipped with an overhead stirrer, a temperature probe, and a pH probe. The water temperature was 18.6°C. 2-MEA (42.88 g) was added to the water, and the stirrer was set to stir the solution at 500 rpm. Upon addition of the 2- MEA, the solution temperature was 19.4°C, and the solution pH was 12.14. Concentrated HCl solution was added to lower the pH to 8.02, and the solution was allowed to cool to about 18°C. DTPA bis- anhydride (100.86 g) was added to a powder feeder set at a rate to add the powder to the reaction solution over the course of 1.5 to 2 hours. The addition of the DTPA bis-anhydride lowered the solution pH to 2.19 and raised the solution temperature to 30.1⁰C. IO N NaOH solution was used to raise the pH back to about 8 and maintain the solution pH at about 8 throughout the reaction. Additionally, an ice bath was used to maintain the solution temperature between about 18°C and about 20⁰C. Upon addition of all of the DTPA bis-anhydride, an aliquot was taken and analyzed by liquid chromatography .

[0096] The solution containing versetamide was divided into two solutions of roughly equal volume. The first versetamide solution was added to a round bottom flask equipped with an overhead stirrer, a temperature probe, and a reflux condenser. This solution had a volume of about 215 mL and a pH of 8.35. The solution pH was lowered to 2.00 with concentrated HCI solution. Gadolinium oxide, Gd₂O₃, (24.12 g) was added, and the solution was heated to 100⁰C and refluxed at that temperature for over 16 hours.

[0097] The second versetamide solution was added to a round bottom flask equipped with an overhead stirrer, a temperature probe, and a pH probe. This solution had a volume of about 220 mL and a pH of 8.45. Gadolinium nitrate hexahydrate, Gd(NO₃)₃ -6H₂O, (15.99 g) was added with water (5 mL), which lowered the solution pH slightly to 7.03. The pH was lowered to about 2.0 using concentrated HCl and 10 N NaOH solutions. The solution was stirred for 1 hour and upon completion of the reaction, the pH of the solution was raised to about 7 to 8 using I O N NaOH solution.

Example 22. Total Synthesis of Gadoversetamide in Aqueous Solvent System

[0100] Gadolinium(III) complex of DTPA-bis(methoxyethylamide) (gadoversetamide) was synthesized from DTPA bis-anhydride and 2-MEA starting materials in aqueous solvent system without prior isolation and purification of versetamide.

[0101] Water (100 mL) was added to a 3-neck round bottom flask equipped with an overhead stirrer, a temperature probe, and a pH probe. 2-MEA (42.88 g) was added to the flask, and the stirrer was set to stir the solution at about 54 rpm. Addition of the 2-MEA raised the solution temperature from 21.7°C to 36.8°C, and the solution pH increased to 13.77. Concentrated HCl solution was added to lower the pH to 7.99. Adding the HCl raised the solution temperature to 47.5°C, so the solution was cooled to 32.3°C using an ice bath. DTPA bis-anhydride (103.24 g) was added to a powder feeder set at a rate to feed the powder to the reaction mixture over about 2.5 hours. The addition of the DTPA bis- anhydride lowered the solution pH and raised the solution temperature. 10 N NaOH solution was used to maintain the solution pH at about 8 throughout the reaction. Additionally, an ice bath was used to maintain the solution temperature between about 18°C and about 26°C. Aliquots were taken periodically and analyzed by liquid chromatography to monitor the formation of versetamide. Upon completion of the reaction, the solution was filtered through a 0.45 μm filter and returned to the reaction flask.

[0102] The solution pH was lowered to 3.75 using concentrated HCl solution. The flask was placed on a heating mantle, and the solution was heated to about 80⁰C, which took about 10 minutes. Gadolinium oxide, Gd₂O₃ (48.19 g) was added to the reaction mixture, and the heated solution was refluxed for about 2 hours. Aliquots were taken periodically and analyzed by liquid chromatography to monitor the formation of gadoversetamide. The pH was monitored during the reaction and maintained at about 4 using concentrated HCl solution. HPLC analysis indicated the gadoversetamide product was 96.69% pure.

Example 23. Total Synthesis of Gadoversetamide in Aqueous Solvent System

[0103] Gadolinium(III) complex of DTPA-bis(methoxyethylamide) (gadoversetamide) was synthesized from DTPA bis-anhydride and 2-MEA starting materials in aqueous solvent system without prior isolation and purification of versetamide.

[0104] Water (100 mL) was added to a 3-neck round bottom flask equipped with an overhead stirrer, a temperature probe, and a pH probe. 2-MEA (42.88 g) was added to the water, and the stirrer was set to stir the solution at about 54 rpm. Addition of the 2-MEA raised the solution temperature from 19.1⁰C to 35.8°C, and the solution pH increased to 13.46. Concentrated HCl solution was added to lower the pH to 7.98. Adding the HCl raised the solution temperature to about 52°C, so the solution was cooled to 35.3°C using an ice bath. DTPA bis-anhydride (102.33 g) was added to a powder feeder set at a rate to feed the powder to the reaction mixture over about 2 hours. The addition of the DTPA bis-anhydride lowered the solution pH to 3.47 and raised the solution temperature to 44.6⁰C. IO N NaOH solution was used to maintain the solution pH at about 8 throughout the reaction. Additionally, an ice bath was used to maintain the solution temperature between about 13⁰C and about 26°C. Aliquots were taken periodically and analyzed by liquid chromatography to monitor the formation of versetamide. Upon completion of the reaction, the solution was filtered through a 0.45 μm filter and returned to the reaction flask.

[0105] The solution pH was lowered to 3.99 using concentrated HCl solution. The flask was placed on a heating mantle, and the solution was heated to about 80⁰C, which took about 10 minutes. Gadolinium oxide, Gd₂C^, (48.22 g) was added to the reaction flask, and the heated solution was refluxed for about 2 hours. Aliquots were taken periodically and analyzed by liquid chromatography to monitor the formation of gadoversetamide. The pH was monitored during the reaction and maintained at about 4 using concentrated HCl solution. HPLC analysis indicated the gadoversetamide product was 98.1% pure.

[0106] In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

[0107] When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles "a", "an", "the" and "said" are intended to mean that there are one or more of the elements. The terms "comprising", "including" and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0108] As various changes could be made in the above without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

What is claimed is:

1. A process comprising: forming a reaction mixture comprising a bis-anhydride, a nitrogen-containing compound, and an aqueous solvent system, wherein the nitrogen-containing compound reacts with the bis-anhydride in the reaction mixture to form a bis-amide compound of the following structure,

wherein the molar ratio of water to the nitrogen-containing compound in the reaction mixture is greater than 3:1, and the molar ratio of the nitrogen-containing compound to the bis-anhydride in the reaction mixture is no greater than 2.5:1, and wherein: the nitrogen-containing compound is ammonia, a primary amine, or a secondary amine;

the bis-anhydride is

Ri is ethylene, propylene, butylene, N,N-bis-ethyleneglycϊne, N,N-bis-ethylene-3- aminopropionic acid, N,N-bis-ethylene-4-arnino-butyric acid, N,N-bis-propyleneglycine, N,N-bis- propylene-3-aminopropionic acid, N,N-bis-propylene-4-amino-butyric acid, N,N-bis-butyleneglycine, N,N-bis-butylene-3-aminopropionic acid, orN,N-bis-butylene-4-amino-butyric acid; each R.₂ and R.₃ is independently hydrogen or a substituted or unsubstituted hydrocarbyl; and each R» is independently methylene, ethylene, or propylene.

2. The process of claim 1, wherein R₄ is methylene.

3. The process of claim 1 or 2, wherein Ri is ethylene.

4. The process of claim 1 or 2, wherein Ri is N,N-bis-ethyleneglycine.

5. The process of any of claims 1-4, wherein the reaction mixture is buffered to a pH of at least about 3 with a base other than the nitrogen-containing compound.

6. The process of any of claims 1-4, wherein the reaction mixture is buffered to a pH between about 7 and about 9 with a base other than the nitrogen-containing compound.

7. The process of any preceding claim, wherein the molar ratio of the water in the reaction mixture to the nitrogen-containing compound in the reaction mixture is at least about 5:1.

8. The process of any preceding claim, wherein the molar ratio of the nitrogen-containing compound in the reaction mixture to the bis-anhydride in the reaction mixture is between about 2:1 and about 2.5: 1.

9. The process of any preceding claim, wherein the molar ratio of the nitrogen-containing compound in the reaction mixture to the bis-anhydride in the reaction mixture is between about 2:1 and about 2.2:1.

10. The process of any preceding claim, wherein the substituted or unsubstituted hydrocarbyl is a substituted or unsubstituted alkyl selected from Ci_-Io alkyl, Cj.io monohydroxylalkyl, Ci.io polyhydroxylalkyl, C_MO alkoxyalkyl, and Ci.io polyalkoxyalkyl.

11. The process of any preceding claim, wherein the substituted or unsubstituted hydrocarbyl is a substituted or unsubstituted alkyl selected from Ci_-8 alkyl, Ci_-8 monohydroxylalkyl, C_]-g polyhydroxylalkyl, Ci_₈ alkoxyalkyl, and Ci_-8 polyalkoxyalkyl.

12. The process of any preceding claim, wherein the substituted or unsubstituted hydrocarbyl is a substituted or unsubstituted alkyl selected from

alkyl, Ci_-6 monohydroxylalkyl, C|.₆ polyhydroxylalkyl, C₁^ alkoxyalkyl, and Ci.β polyalkoxyalkyl.

13. The process of any preceding claim, wherein the nitrogen-containing compound is a primary amine.

14. The process of claim 13, wherein the primary amine is selected from methylamine, ethylamine, n- propylamine, isopropylamine, branched or straight chained butylamine, hydroxylmethylamine, hydroxylethylamine, methoxyethylamine, and ethoxyethylamine.

15. The process of any preceding claim, further comprising: forming a paramagnetic metal ion complex, wherein the forming of the paramagnetic ion complex comprises contacting the bis-amide compound with a paramagnetic metal ion salt, the resulting paramagnetic metal ion complex having the following structure,

wherein M is a paramagnetic metal having an atomic number of 21, 22, 23, 24, 25, 26, 27, 28, 29, 42, 43, 44, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, or 70; and Z is 2 or 3.

16. The process of claim 15, wherein the molar ratio of the bis-amide compound to the paramagnetic metal ion is between about 1 :1 and about 1.15:1.

17. The process of claims 15 or 16, wherein the paramagnetic metal ion is selected from iron(II), cobalt(II), manganese(II), nickel(II), copper(II), chromium(II), rhodium(II), iridium(II), gadolinium(III), chromium(III), manganese(I II), iron(III), praseodymiurn(III), europium(III), neodymium(III), samarium(III), ytterbium(III), terbium(III), dysprosium(III), hoImium(III)₅ and erbium(III).

18. The process of claims 15 or 16, wherein the paramagnetic metal ion is selected from iron(II), cobalt(II), manganese(II), nickel(II), copper(II), chromium(II), rhodium(II), and iridium(II).

19. The process of claims 15 or 16, wherein the paramagnetic metal ion is selected from gadolinium(III), chromium(III), manganese(III), iron(III), praseodymium(III), europium(III), neodymium(III), samaπum(III), ytterbium(III), terbium(HI), dysprosium(III), holmium(III), and erbium(III).

20. The process of claims 15 or 16, wherein the paramagnetic metal ion is gadolinium(III).

21. The process of claims 15 or 16, wherein the paramagnetic metal ion salt is selected from gadolinium oxide, gadolinium nitrate hexahydrate, gadolinium chloride, gadolinium chloride hexahydrate, gadolinium acetate hydrate, gadolinium oxalate, gadolinium bromide, and gadolinium carbonate.

22. The process of any of claims 15-21, wherein the bis-amide compound is contacted with the paramagnetic metal ion salt in the reaction mixture of any of claims 1-14.

23. The process of claim 22, further comprising buffering the pH of the reaction mixture to between about 3 and about 5 prior to contacting the bis-amide compound with the paramagnetic metal ion salt.

24. The process of claims 22 or 23, further comprising buffering the pH to between about 7 and about 8 after contacting the bis-amide compound with the paramagnetic metal ion salt.

25. The process of any of claims 22-24, further comprising refluxing the reaction mixture after contacting the bis-amide compound with the paramagnetic metal ion salt.

26. The process of any of claims 22-25, further comprising isolating the paramagnetic metal ion complex from the reaction mixture by rotary evaporation of the solvent, spray drying, precipitation, crystallization, or a combination thereof.

27. The process of claim 27, further comprising: dissolving the isolated paramagnetic metal ion complex in water; and purifying the paramagnetic metal ion complex by reverse osmosis, ion exchange, crystallization, precipitation, or a combination thereof.

28. The process of any of claims 15-21, wherein the bis-amide compound is isolated from the reaction mixture of any of claims 1-14 prior to contacting the bis-amide compound with the paramagnetic metal ion salt.

29. The process of claim 28, wherein the bis-amide compound and paramagnetic metal ion salt are contacted in a solution buffered to a pH between about 3 and about 5.

30. The process of claim 29 wherein the solution is buffered to a pH between about 7 and about 8 after contacting the bis-amide compound with the paramagnetic metal ion salt.

31. The process of claims 29 or 30 wherein the solution is refluxed after contacting the bis-amide compound with the paramagnetic metal ion salt.

32. The process of any of claims 29-31 , further comprising isolating the paramagnetic metal ion complex from the solution by rotary evaporation of the solvent, spray drying, precipitation, crystallization, or a combination thereof.

33. The process of claim 32, further comprising: dissolving the isolated paramagnetic metal ion complex in water; and purifying the paramagnetic metal ion complex by reverse osmosis, ion exchange, crystallization, precipitation, or a combination thereof.