Contrast agent preparation

In the metallation of complexing agents such as DTPA-BMA with a lanthanide using a lanthanide oxide such as the lanthanide source, oxalic acid is used as a reaction accelerator.

This invention relates to a process for the metallation of complexing
 agents with lanthanides, e.g. gadolinium, and in particular to the
 preparation of lanthanide chelates such as those suitable for use as
 contrast agents in diagnostic imaging modalities such as magnetic
 resonance (MR) imaging.
 In MR imaging, the use of lanthanide chelates as contrast agents has become
 well established. Several such agents (eg. Gd DTPA, Gd DTPA-BMA and Gd
 HP-DO3A, available under the trade marks Magnevist, Omniscan and
 Pro-Hance) are already commercially available, while still others are in
 early, middle and late stages of development. Such contrast agents are
 complexes of lanthanide ions with various different complexing agents
 (ligands) and a key stage of their production is the metallation of the
 ligand with a lanthanide. In general this is the last stage of primary
 production, ie. the production of the chemical drug substance that is
 subsequently formulated into the drug product in the secondary production
 phase.
 Between metallation and secondary production the lanthanide complex must be
 thoroughly purified to remove unwanted impurities. As with any commercial
 drug synthesis, it is important to optimize yield of the desired product,
 reduce the levels of impurities produced during the various synthetic
 steps, and reduce process duration (and so. optimize the efficiency of
 reactor usage).
 Metallation with lanthanides is normally performed by reacting the ligand
 with a lanthanide oxide (e.g. Gd.sub.2 O.sub.3) in a heated aqueous
 medium. If this reaction takes too long, decomposition of the ligand can
 occur, resulting in reduction in yield and increased levels of impurities
 in the end product.
 Thus for example in the metallation of DTPA-BMA
 (diethylene-triaminepentaacetic acid-N,N'-bis(methylamide) with gadolinium
 oxide, where the metallation proceeds too slowly some breakdown of the
 ligand to the mono-methylamide DTPA-MMA occurs. The reaction product then
 includes both Gd DTPA-BMA and a salt, eg. the sodium salt, of Gd DTPA-MMA.
 As a result NaGd DTPA-MMA must be removed by a recrystallization
 procedure.
 The lanthanide oxide used in the metallation process is produced
 commercially by thermal decomposition of a lanthanide oxalate.
 It has now surprisingly been found that the rate of the ligand metallation
 reaction is increased if the reaction medium includes oxalic acid or
 derivatives (eg. salts thereof).
 Thus viewed from one aspect the invention provides a process for the
 preparation of a lanthanide complex by reaction of a lanthanide oxide with
 a complexing agent in an aqueous reaction medium, characterised in that
 oxalic acid or a salt or derivative thereof is used as a reaction
 accelerator.
 When the ligand is subject to thermal decay, the process of the invention
 will represent an improvement in terms of speed of reaction as well as
 reduction in by-product formation; however, even where the ligand is
 thermally stable an improvement in speed of reaction will still be
 achieved.
 The lanthanide used according to the invention may be any lanthanide but
 preferably is Eu, Th, Tm, Yb, Er or Ho, more preferably Dy, and most
 preferably Gd.
 In this process where oxalic acid or a salt or derivative thereof is used
 as a reaction accelerator, this relates to further oxalic acid and not
 simply to the oxalate residue in the lanthanide oxide, even though this
 residue will of course contribute to the acceleration of the reaction.
 The total amount of oxalic acid (or salt or derivative) added as a reaction
 accelerator is conveniently at least 10 .mu.g oxalic acid/g L.sub.2
 O.sub.3 (where L is the lanthanide, e.g. Gd), preferably at least 50
 .mu.g/g, especially at least 100 .mu.g/g, particularly at least 200
 .mu.g/g and more particularly at least 400 .mu.g/g, eg. about 500 .mu.g/g.
 The amount added will preferably be less than 2000 .mu.g/g, particularly
 less than 1000 .mu.g/g, preferably less than 800 .mu.g/g.
 The oxalic acid reaction accelerator can be added to the metallation
 reaction mixture as a separate reagent. However in alternative aspects of
 the invention some or all of the oxalic acid/oxalate may derive from
 oxalate impurity in the lanthanide oxide.
 Thus viewed from a further aspect the invention provides a process for the
 preparation of a lanthanide complex by reaction of lanthanide oxide with a
 complexing agent in an aqueous reaction medium, characterised in that said
 process comprises the steps of: (a) determining the level of impurity in
 the lanthanide oxide; and (b) mixing lanthanide oxide from batches with
 different determined levels of impurity and/or including in the reaction
 medium a predetermined quantity of oxalic acid or a salt or derivative
 thereof; whereby by virtue of step (b) the reagents used in the
 metallation reaction contain oxalic acid (or salt or derivative) or
 oxalate at a total level of at least 50 .mu.g oxalic acid per gram L.sub.2
 O.sub.3, preferably at least 100 .mu.g/g, more preferably at least 200
 .mu.g/g, especially at least 250 .mu.g/g and particularly preferably at
 least 400 .mu.g/g, eg. up to 1750 .mu.g/g, particularly 700 to 900
 .mu.g/g.
 Viewed from a yet further aspect the invention provides a process for the
 preparation of a lanthanide complex by reaction of a lanthanide oxide with
 a complexing agent in an aqueous reaction medium, characterised in that
 for use as said lanthanide oxide is selected a lanthanide oxide having
 (eg. pre-analysed to contain) an oxalate impurity level of at least 100
 .mu.g oxalic acid/g lanthanide oxide, preferably at least 200 .mu.g/, more
 preferably at least 250 .mu.g/g, especially preferably at least 400
 .mu.g/g, more especially at least 700 .mu.g/g.
 The oxalate impurity level of the L.sub.2 O.sub.3 may be inferred from its
 residue on ignition--the higher the residue the higher the oxalate
 content. Alternatively it can be determined by suitable analytical
 methods.
 Where oxalic acid is added to the reaction medium, with or without
 predetermination of oxalate impurity levels of the lanthanide oxide, it
 may be added as a salt (eg. an alkali metal or alkaline earth metal salt),
 an ester or an amide or as the free acid. Lanthanide oxalates themselves
 may be used. However, preferably the free acids are used.
 The use of oxalic acid (or salts or derivatives thereof) can reduce the
 metallation reaction time by a factor of two or more, eg. by a factor of
 up to 6.
 The ligand which is metallated may be any ligand capable of producing a
 highly stable lanthanide complex, eg. one with a dissociation content of
 at least 10.sup.12. Preferably it will be a linear, cyclic or branched
 chelating agent, eg. a linear mono- or polychelant, a macrocyclic chelant
 or a branched polychelant (eg. a dendrimeric polychelant). Preferably the
 ligand will be a polyaminopolyoxyacid (eg. polyaminopolycarboxylic acid),
 such as one of the mono and polychelants suggested for lanthanide
 chelation in the patent literature relating to MR contrast agents, eg. the
 patent publications of Nycomed (including Nycomed Imaging and Nycomed
 Salutar), Sterling Winthrop, Schering, Bracco, Squibb, Mallinckrodt,
 Guerbet and Metasyn, eg. US-A-4647447, EP-A-71564, WO96/03154, WO96/01655,
 EP-A-430863, WO96/41830, and WO93/10824. Thus by way of example the ligand
 may be of formula
EQU (Y) (X) N (CHR).sub.n (N(X) (CHR).sub.n).sub.m N(X) (Y)
 where m is 0, 1, 2, or 3; n is 2 or 3; y; each X is a hydrogen or a
 substituted C.sub.1-6 alkyl group; each Y is a group X or the two Y groups
 together represent a (CHR).sub.n bridge; and each R is hydrogen or a
 substituted C.sub.1-6 alkyl group or a CHR-N(X)-CHR moiety may represent
 an optionally substituted, saturated or unsaturated 5 to 7 membered
 heterocyclic ring or a CHRCHR moiety may represent an optionally
 substituted, saturated or unsaturated 5 to 7 membered homo- or
 heterocyclic ring; where at least two X groups are alkyl groups
 substituted by sulphur, phosphorus or carbon oxyacid groups or amides or
 esters thereof, and where alkyl group substitution is preferably by
 oxyacid or oxyacid derivative groups, by hydroxyl groups, by optionally
 substituted phenyl groups, or by directly or indirectly attached polymer
 forming or biotargeting groups, eg. polyaminoacids, dendrimeric polymers,
 polyalkylene oxide groups, antibodies, antibody fragments, drugs, site
 specific peptidic groups (eg. oligopeptide binding motifs), etc.
 Particular examples of appropriate ligands include DTPA, DTPA-BMA, DOTA,
 DO3A, HP-DO3A, BOPTA, PAMAM-polyDTPA, and PAMAM-polyDOTA. Especially
 preferred ligands include DTPA, DTPA-BMA, DOTA, and HP-DO3A.
 The metallation reaction is preferably performed in aqueous solution, eg.
 in distilled water optionally containing a miscible cosolvent, at an
 elevated temperature, eg. 70.degree. to 95.degree. C., preferably
 80.degree.-90.degree. C. During the reaction the pH is preferably 3 to 6.
 The pH may be controlled by addition of an acid or base, preferably an
 acid or base which produces pharmaceutically acceptable neutralisation
 products, such as hydrochloric acid and sodium hydroxide.
 The progress of the metallation reaction will generally be monitored to
 determine the residual quantities of unreacted lanthanide oxide or ligand,
 with extra portions of oxide or ligand optionally being added until the
 reaction is deemed to be complete, eg. when a stable low concentration of
 ligand and negligible free lanthanide is detected. The reaction mixture
 will then be cooled, eg. to below 25.degree. C. If necessary the pH of the
 reaction mixture is then adjusted, eg. about 6, for example using sodium
 hydroxide. The solution is then filtered and the lanthanide complex is
 isolated, eg. by crystallisation.
 Using this procedure, the metallation reaction time for a ligand such as
 DTPA-BMA may be reduced from 2 to 3 hours to 1 hour or below, eg. 30
 minutes.
 Viewed from a further aspect the invention provides the use of oxalic acid
 (or a salt or derivative thereof) and/or a lanthanide oxide having a
 oxalate content of at least 100 .mu.g oxalic acid/g lanthanide oxide,
 preferably at least 200 .mu.g/g, as a reaction accelerator in the
 lanthanide metallation of a ligand.

The invention will now be described further with reference to the following
 non-limiting Examples.
 EXAMPLE 1
 A reactor vessel is charged with 180 mL of distilled water. After cooling
 to below 50.degree. C., 43.2 g (119.17 mmoles) gadolinium oxide and 30.2
 mg oxalic acid dihydrate are added. (The oxalic acid represents 500 ppm
 relative to the gadolinium oxide). During stirring, 100 g (238.42 mmoles)
 DTPA-BMA is added in one portion and the mixture is heated to
 80.degree.-90.degree. C. After 0.5 hours, a sample of the reaction mixture
 is taken and analysed for the content of unreacted DTPA-BMA. If DTPA-BMA
 content is below 1% (w/v) a new sample is taken and analysed to confirm
 that the DTPA-BMA content is low and stable. If the DTPA-BMA content is
 above 1% w/v, the reaction mixture is stirred until sampling and analysis
 shows DTPA-BMA content to have stabilized below 1% w/w. (Optionally
 further DTPA-BMA or gadolinium oxide may be added to complete the
 reaction).
 After complexation is complete, the reaction mixture is cooled to below
 25.degree. C. If necessary the pH is adjusted to about 6.1 to 6.4 by the
 addition of aqueous sodium hydroxide. The solution is filtered and
 GdDTPA-BMA is crystallized out.
 EXAMPLE 2
 An oxalate-contaminated batch of gadolinium oxide was analysed for oxalic
 acid content by titration with potassium permanganate in sulphuric acid.
 The content was found to be 270 .mu.g oxalic acid per gram Gd.sub.2
 O.sub.3.
 A sample of this batch was heated to 1100.degree. C. for 2 hours to
 decompose the oxalate contamination.
 Three metallation reactions were carried out using (i) heat treated
 contaminated gadolinium oxide, (ii) contaminated gadolinium oxide and
 (iii) contaminated gadolinium oxide with the further addition of 230
 .mu.g/g Gd.sub.2 O.sub.3 of oxalic acid. The ligand was DTPA-BMA and the
 metallation reaction was carried out in aqueous solution at 80.degree. C.
 The times required for the reactions to go to completion were respectively
 2.5 hours, 1 hour and less than 1/2 hour.