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This application claims priority to and benefits from Indian provisional patent application No. 202341054046 filed on August 11, 2023; the disclosure of which is incorporated herein by reference.
Field of the Invention
The present invention relates to a scalable and an improved process for the preparation of pure polyaminocarboxylate, particularly DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic Acid) that could be employed for the preparation of active pharmaceutical ingredients especially MRI contrast agents.
Background of the Invention
DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic Acid) belong to the class of polyaminocarboxylates that are useful in the preparation of active pharmaceutical ingredients in the field of contrast-enhanced magnetic resonance imaging. DOTA belongs to polyaminopolycarboxylic acid compounds that are exceptionally suitable for forming stable complexes with gadolinium ions. DOTA is used for the preparation of gadoterate megumine (DOTAREM). DOTA is also referred as tetraxetan and having the structure as mentioned below.

The compound is generally prepared from the macrocycle known as cyclen (1,4,7,10-tetraazacyclododecane). Other process for the preparation of DOTA is disclosed in the Patent US 9777022. DOTA exhibit both positive and negative charge depending on the pH; chelation property; and high solubility in water. As DOTA are employed mostly in the injectable formulations directly, the purity of DOTA is an essential requirement. The removal of the organic impurities and inorganic impurities formed during the preparation of DOTA are difficult due to nature of the compound.
Techniques such as nanofiltration, crystallization and purification using ion exchange may be employed to solve this problem. Nanofiltration is used to remove monovalent ions from DOTA depending on the pH of the compound. Rejection of low molecular weight organic compounds depends on the solute and membrane properties. Rejection of the polyaminocarboxylic acid may be increased with increasing pH from 4 to 7. Some purification process in the state of art is mentioned below.
The Publication Inorganic Chemistry 1980, 19, pages 1319-1324 disclose the purification process of DOTA, first by precipitation of DOTA from the reaction mixture by acidification at a pH of 2.5; and second by elution of the solution of DOTA in Dowex-50W-X4-cation exchange resin.
The Patent US 5208324 discloses the purification process of DOTA by mixing Dowex 50WX-4 acidic ion exchange resin for 1 hour and then release of DOTA from resin by addition of ammonia solution.
The Patent US 5428156 discloses the purification process of DOTA by precipitation of DOTA from a methanolic solution by the addition of acetone and a washing with acetone.
The Patent US 6162912 discloses purification process of DOTA by precipitation of DOTA by adjusting a pH of the mixture to 2 by concentrated hydrochloric acid; and percolation of a solution of DOTA through a polyvinlypyridine resin column.
The Patent US 10106489 discloses the purification process of DOTA involving the steps of: treating an aqueous solution of DOTA with an inorganic acid, wherein the pH of the solution is below 0.75; crystallizing the salt of DOTA as a solid from the aqueous solution; dissolving the crystallized salt of DOTA in water; treating the prepared aqueous solution of salt of DOTA with anionic resin or a basic solution at a pH between 1.5 and 3.0 to obtain an aqueous solution of DOTA; and precipitating DOTA from the solution in solid form.
The Patent US 10195295 discloses the purification process of DOTA involving the steps of: treating an aqueous solution of DOTA with an inorganic acid; crystallizing in an organic solvent or a mixture of organic solvent and water; dissolving the crystallized salt of DOTA in water; filtering the aqueous solution through a nanofiltration membrane having a molecular cut off in a range from 150 to 500 Daltons; and crystallizing the DOTA as a solid from the aqueous solution.
The Patent US 9458117 discloses the purification process of DOTA involving the steps of: treating an aqueous solution of DOTA with an inorganic acid at a pH less than or equal to 3; crystallizing in DOTA in water; dissolving the crystallized salt of DOTA in water; treating the solution with a cationic resin and then desorbing DOTA with a volatile base solution; further treating the solution with anionic resin and releasing the DOTA from anionic resin using first organic volatile acid.
The Patent US 10941124 discloses the purification process of DOTA involving the steps of: treating an aqueous solution of DOTA with an inorganic acid; crystallizing in an organic solvent or a mixture of organic solvent and water; dissolving the crystallized salt of DOTA in water; filtering the aqueous solution through a nanofiltration membrane; and crystallizing the DOTA as a solid from the aqueous solution at a pH between 3 and 3.5.
Besides different processes exist for the preparation of DOTA involving the purification steps, there exists a need to provide an improved process for the preparation of pure DOTA that is technically advanced and commercially significant.
Summary of the Invention
The present invention provides a scalable and improved process for the preparation of pure DOTA comprising the steps of:
(i) treating an aqueous solution of DOTA with an inorganic acid, wherein the pH of the solution is below 0.75;
(ii) crystallizing the salt of DOTA as a solid from the aqueous solution obtained in step (i);
(iii) recrystallizing the crystallized salt of DOTA obtained in step (ii) in an organic solvent or a mixture of organic solvent and water at a pH below 0.75;
(iv) preparing aqueous solution of the recrystallized salt of DOTA obtained in step (iii) at a pH between 3.0 and 5.0;
(v) treating the prepared aqueous solution of salt of DOTA obtained in step (iv) with anionic resin at a pH between 1.0 and 5.0 to obtain an aqueous solution of DOTA; and
(vi) adding an organic solvent to the filtered aqueous solution of DOTA obtained in step (v); and
(vii) precipitating DOTA from the solution obtained in step (vi) in solid form.
(viii) optionally repeating the process step (vii)

Brief Description of the Drawings of the Invention
Figure-1: HPLC Chromatogram of the compound of DOTA hydrochloride obtained in Step-A of the Example-2 of the present invention.
Figure-2: HPLC Chromatogram of the compound of DOTA hydrochloride obtained in Step-B of the Example-2 of the present invention.
Figure-3: HPLC Chromatogram of the compound of DOTA hydrochloride obtained in Step-C of the Example-2 of the present invention.
Figure-4: HPLC Chromatogram of the compound of DOTA obtained in Step-D of the Example-2 of the present invention.
Figure-5: HPLC Chromatogram of the compound of DOTA obtained in Step-E of the Example-2 of the present invention.
Detailed Description of the Invention
One embodiment of the present invention provides a scalable and improved process for the preparation of pure DOTA comprising the steps of:
(i) treating an aqueous solution of DOTA with an inorganic acid, wherein the pH of the solution is below 0.75;
(ii) crystallizing the salt of DOTA as a solid from the aqueous solution obtained in step (i);
(iii) recrystallizing the crystallized salt of DOTA obtained in step (ii) in a mixture of organic solvent and water at a pH below 0.75;
(iv) preparing aqueous solution of the recrystallized salt of DOTA obtained in step (iii) at a pH between 3.0 and 5.0;
(v) treating the prepared aqueous solution of salt of DOTA obtained in step (iv) with anionic resin at a pH between 3.0 and 5.0 to obtain an aqueous solution of DOTA; and
(vi) adding an organic solvent to the filtered aqueous solution of DOTA obtained in step (v); and
(vii) precipitating DOTA from the solution obtained in step (vi) in solid form.
(viii) optionally repeating the process step (vii)

DOTA may be prepared in different ways as described in the state of art. Preferably DOTA may be prepared by reacting cyclen with monochloroacetic acid in the presence of sodium hydroxide. The present inventors identified two impurities formed during the preparation of DOTA, that are (i) an impurity at 1.23 RRT and other impurity at 0.80 RRT. Preferably, the impurity at 1.23 RRT is not more than 11% by HPLC and other impurity at 0.80 RRT is not more than 1% by HPLC in the reaction mass resulted from the reaction of cyclen with monochloroacetic acid in the presence of sodium hydroxide.
The step (i) of treating an aqueous solution of DOTA with an inorganic acid according to the process of the present invention is carried out in pH is below 0.75, that is in the range of 0 to 0.75, preferably below 0.5.
The inorganic acid according to the step (i) of the process of the present invention is selected from the group comprising of hydrochloric acid, hydrobromic acid and likes, preferably hydrochloride.
Preferably the salt of DOTA as a solid resulted in the step (ii) of the process of the present invention is DOTA hydrochloride, more preferably DOTA hydrochloride hydrate.
According to the process of the present invention, the crystallized salt of DOTA formed after the step (iii) of recrystallizing the crystallized salt of DOTA in a mixture of organic solvent and water, comprises the impurity at 1.23 RRT is less than 7% by HPLC and the impurity at 0.80 RRT is less than 0.5%, preferably crystallized salt of DOTA formed after the step (iii) comprises the impurity at 1.23 RRT is less than 6% by HPLC and the impurity at 0.80 RRT is less than 0.1%.
The organic solvent is selected from the group comprising ethanol, methanol, propanol, isopropanol, acetone and mixtures thereof, preferably acetone.
The anionic resin is selected from a group comprising of Amberlyst A126OH, Lewaitite M600OH or M800OH, Purolite UCW5072OH and resin based on crosslinked polystyrene matrix that has quaternary ammonium functional group such as INDION810(OH) resin; preferably INDION810(OH) resin.
According to the step (v) of the present invention, the absorption of the impurity at 1.23 RRT from the solution is effectively carried out at a pH between 3.0 and 5.0, preferably at a pH between 3.5 and 4.5.
According to the process of the present invention, the precipitated DOTA after the step (vii) comprises the impurity at 1.23 RRT is less than 1% by HPLC. The impurity at 0.80 RRT in the precipitated DOTA after the step (vii) is significantly low or not detected by HPLC.
The “Pure DOTA” according to the present invention is DOTA has the purity more than 98.0% by HPLC, preferably more than 99% by HPLC and more preferably more than 99.5% by HPLC.
Surprisingly, the inventors of the present invention found an improved process for the preparation of pure DOTA that is simple and commercially suitable for large scale preparations. The sequence of the steps and the steps performed during the process are essential in the present process for preparing pure DOTA. The step of obtaining DOTA free base after the treatment of aqueous solution of salt of DOTA with anionic resin at a pH between 3 and 5 removes the organic and inorganic impurities significantly. Particularly the absorption of the impurity at 1.23 RRT by the anionic resin at the pH 3 to 5 was more efficient than the absorption at pH more than 5 and less than 3.
Table-1: Percentage of purity of DOTA free base after the treatment of aqueous solution of salt of DOTA with anionic resin in different pH range.
SNO The pH of the treatment of aqueous solution of salt of DOTA with anionic resin % Purity of DOTA free base by HPLC % of Impurity at 1.23 RRT in HPLC chromatogram of DOTA free base

1 2.38 pH
B. No: 1800-001 RD2319219) 98.49 0.99
2 3.67 pH
B. No: 1800-001 RD2319219) 99.20 0.43
3 3.879 pH
B. No: 1860-097 99.79 0.21
4 3.856 pH
Batch No: 1877-001 99.67 0.13

Preferred embodiment of the present invention provides a scalable and an improved process for the preparation of pure DOTA comprising the steps of:
(i) treating an aqueous solution of DOTA with an hydrochloric acid, wherein the pH of the solution is below 0.75;
(ii) crystallizing the hydrochloride salt of DOTA as a solid from the aqueous solution obtained in step (i);
(iii) recrystallizing the crystallized salt of DOTA obtained in step (ii) in a mixture of acetone and water at a pH below 0.75;
(iv) preparing aqueous solution of the recrystallized hydrochloride salt of DOTA obtained in step (iii) at a pH between 3.0 and 5.0;
(v) treating the prepared aqueous solution of salt of DOTA obtained in step (iv) with anionic resin at a pH between 3.0 and 5.0 to obtain an aqueous solution of DOTA; and
(vi) adding an organic solvent to the filtered aqueous solution of DOTA obtained in step (v); and
(vii) precipitating DOTA from the solution obtained in step (vi) in solid form.
(viii) optionally repeating the process step (vii)
Certain specific aspect and embodiment of the present invention will be explained in detail with reference to the following examples, which are provided only for purposes of illustration and should not be construed as limiting the scope of the invention in any manner.
Examples of Invention
Example-1: Preparation of DOTA
Step-A: Preparation of DOTA hydrochloride: To a mixture of cyclen hydrochloride/free base (50 gm) and water (50 ml), aqueous chloroacetic acid (123.4 gm of chloroacetic acid dissolved in 50 ml of water) was added at 0-10? for a period of 20-30 minutes. The pH of the reaction mixture was adjusted to 10-10.5 by using 60% sodium hydroxide solution (116.08 gm of sodium hydroxide dissolved in 200 ml of water at 25-30?) at 0-10?. The reaction mixture was heated to 25-30? and then adjusted the pH of the reaction mixture to 10-10.5 by using 60% sodium hydroxide solution. The temperature of the reaction mixture was gradually raised 70-75? and maintained at the same temperature for 16-18 hours at a pH of 10-10.5 throughout the reaction. The progress of the reaction was monitored by HPLC. After completion of the reaction, the reaction mass was cooled to 30-35? and then to 5-10?. The pH of the cooled reaction mass was adjusted below 0.5 by the addition of concentrated hydrochloric acid. The pH adjusted reaction mass was heated to 20-25? and maintained at the same temperature for 2-3 hours. The resultant solid was filtered under vacuum and washed with acetone (50 ml). The washed solid was dissolved in water (150 ml) at 25-30? for 30-40 minutes. The contents were then heated to 65-70?. The pH of the cooled contents was adjusted below 0.5 by the addition of concentrated hydrochloric acid at 65-70?. The pH adjusted contents were maintained at the same temperature for 30-40 minutes and then cooled to 0-5?. The cooled contents were stirred for 60-80 minutes at 0-5?. The resultant solid was filtered under vacuum and washed with acetone (50 ml). The washed solid was dissolved in water (250 ml) at 25-30? for 30-40 minutes. The resultant solution was filtered through hyflo bed. To the filtrate, concentrated hydrochloric acid (25 ml) was added at 25-30? and stirred for 30-40 minutes at the same temperature. To the stirred contents, acetone (650 ml) was added at 25-30? and stirred for 45-60 minutes at the same temperature. The resultant solid was filtered under vacuum; washed with acetone (50 ml) and dried under vacuum at 50-55?. Yield: 103 gm
Step-B: Preparation of DOTA from DOTA hydrochloride: To a solution of DOTA hydrochloride obtained from Step-A (40 gm) and water (320 ml), pretreated INDION810(OH) resin (120 gm) [180 gm of INDION810(OH) resin pretreated with 400 ml of water] was added at 25-30? and the pH of the reaction mixture was adjusted to 3.72 at the same temperature. The reaction mixture was stirred for 5-10 minutes at 25-30? at a pH of 3.67 The reaction mixture was filtered under vacuum. The filtrate was concentrated under vacuum below 60? and then cooled to 25-30?. To the cooled filtrate, acetone (320 ml) was added at 25-30? and stirred for 60-80 minutes at the same temperature. The resultant solid was filtered under vacuum. To the obtained solid, water (140 ml) was added at 25-30? and then heated to 40-45?. The heated contents were stirred at 40-45? till the formation of a clear solution. The formed solution was filtered through filter having 0.2-micron filters at 40-45?. The filtrate was then cooled to 25-30?, followed by the addition of acetone (280 ml) at the same temperature. The contents were then stirred for 60-80 minutes at 25-30?. The resultant solid was filtered under vacuum; washed with acetone (25 ml) and dried under vacuum at 60-65? for 3 hours. Yield: 23.7 gm

Example-2: Preparation of DOTA free base
Step-A: Preparation of DOTA hydrochloride: To a mixture of cyclen free base (200 gm) and water (200 ml), aqueous chloroacetic acid (548.6 gm of chloroacetic acid dissolved in 200 ml of water) was added at 42.5±2.5? for a period 1 to 2 hours. The pH of the reaction mixture was adjusted to 10-12.5 by adding sodium hydroxide solution (510.91 gm of sodium hydroxide dissolved in 800 ml of water) at 42.5±2.5? for 1 to 2 hours. The reaction mixture was heated to 72.5±2.5? at a pH of 10-12.5 and maintained at the same temperature at a pH of 10-10.5 for 2.0 to 2.5 hours. The progress of the reaction was monitored by HPLC. After completion of the reaction, the reaction mass was cooled to 30±5? and then to 5-10?. The pH of the cooled reaction mass was adjusted below 0.5 by the addition of concentrated hydrochloric acid. The pH adjusted reaction mass was heated to 22.5±2.5? and maintained at the same temperature for 2 to 3 hours. The resultant solid was filtered under vacuum at 22.5±2.5? and washed with acetone (200 ml) at 30±5?. Yield: 901 gm (Wet solid); % Purity by HPLC: 89.89%; % Impurity at 1.23 RRT: 9.85% by HPLC; % Impurity at 0.80 RRT: 0.18% by HPLC.
Step-B: First purification of DOTA hydrochloride: The wet solid obtained in step-A (901 gm) was mixed with water (500 ml) at 30±5? and heated to 67.5±2.5?. Hydrochloric acid (100 ml) was added to the contents at 67±2.5? and stirred for 30 to 40 minutes at the same temperature. The stirred contents were then cooled to 4.5±2.5? and stirred for 60 to 80 minutes at the same temperature. The resultant solid was filtered under vacuum and washed with acetone (200 ml) at 30±5?. Yield: 609 gm (Wet solid); % Purity by HPLC: 92.77%; % Impurity at 1.23 RRT: 7.16% by HPLC; % Impurity at 0.80 RRT: 0.07% by HPLC.
Step-C: Second purification of DOTA hydrochloride: The wet solid obtained in step-B (609 gm) was mixed with water (600 ml) at 30±5? and heated to 45±5?. The heated contents were stirred for 30 to 60 minutes at 45±5?. The heated contents were filtered through hyflo bed (40 gm) under vacuum and then hyflo bed washed with water (200 ml) at 45±5?. The filtrate and the washings were combined. Hydrochloric acid (100 ml) was added to the combined contents at 45±5? and stirred for 30 to 40 minutes at the same temperature. The stirred contents were cooled to 30±5?. To the cooled contents, acetone (2200 ml) was added at 30±5? and stirred for 45-60 minutes at the same temperature. The resultant solid was filtered under vacuum and washed with acetone (200 ml) at 30±5? and dried under vacuum. Yield: 436 gm (Wet solid); % Purity by HPLC: 92.98%; % Impurity at 1.23 RRT: 6.99% by HPLC; % Impurity at 0.80 RRT: 0.04% by HPLC.
Step-D: Preparation of DOTA free base from DOTA hydrochloride: To an aqueous solution of solid obtained in step-B (400 gm) in water (3200 ml), the washed resin (The Indion 810 (OH) resin (1200 ml) was mixed with water (3000 ml) at 30±5? and stirred for 20-30 minutes at the same temperature. The resin was separated by filtration; and washed with water (800 ml) at 30±5?.) was slowly added for 50 to 60 minutes at 30±5? to adjust the pH of the reaction mixture between 3.8 and 4.2. The reaction mixture was stirred for 10-20 minutes and
maintained at a pH between 3.8 and 4.2 using water washed Indion 810 (OH) resin at 30±5?. The reaction mass was filtered under vacuum at 30±5? and washed with water (400 ml). The filtrate and the washings were combined; and filtered through hyflo bed (80 gm) at 30±5? under vacuum and washed with water (800 ml). The filtrate obtained from hyflo and washing of the hyflo bed are combined and filtered through a 0.2-micron filter at 30±5?. The filtrate obtained from micron filter was concentrated in vacuum at a temperature not more than 65? to obtain a residue. The residue was cooled to 42.5±2.5? and mixed slowly with acetone (3200 ml) at the same temperature for 10 to 20 minutes, then stirred for 60-80 minutes at 42.5±2.5?. The resultant solid was filtered under vacuum and washed with acetone (400 ml) at 42.5±2.5?. Yield: 472 gm (Wet solid); % Purity by HPLC: 99.94%; % Impurity at 1.23 RRT: 0.02% by HPLC; % Impurity at 0.80 RRT: Not Detected by HPLC.
Step-E: Purification of DOTA free base: The wet solid (472 gm) was added to the water (1400 ml) at 30±5? and then heated to 42.5±2.5?. The heated contents were stirred for 15-20 minutes at 42.5±2.5?. To the stirred contents, acetone (2800 ml) was added at 42.5±2.5? for 10-20 minutes and stirred for 60-80 minutes at 42.5±2.5?. The stirred contents were cooled to 30±5? and stirred for 20-30 minutes at the same temperature. The resultant solid was filtered under vacuum and washed with acetone (400 ml) at 30±5?; and dried under vacuum for 12 to 15 hours at 55-65?. Yield: 243 gm; % Purity by HPLC: 99.91%; % Impurity at 1.23 RRT: Not Detected by HPLC; % Impurity at 0.80 RRT: Not Detected by HPLC. ,CLAIMS:1. A scalable process for the preparation of DOTA having purity more than 98.0% by HPLC, preferably more than 99% by HPLC and more preferably more than 99.5% by HPLC;
comprising the steps of:
(i) treating an aqueous solution of DOTA with an inorganic acid, wherein the pH of the solution is in the range of 0.00 to 0.75;
(ii) crystallizing the salt of DOTA as a solid from the aqueous solution obtained in step (i);
(iii) recrystallizing the crystallized salt of DOTA obtained in step (ii) in a mixture of organic solvent and water at a pH below 0.75;
(iv) preparing aqueous solution of the recrystallized salt of DOTA obtained in step (iii) at a pH between 3.0 and 5.0;
(v) treating the prepared aqueous solution of salt of DOTA obtained in step (iv) with anionic resin at a pH between 3.0 and 5.0 to obtain an aqueous solution of DOTA; and
(vi) adding an organic solvent to the filtered aqueous solution of DOTA obtained in step (v); and
(vii) precipitating DOTA from the solution obtained in step (vi) in solid form.
2. The process as claimed in claim 1 wherein pH of step (i) is preferably maintained at below 0.5
3. The process as claimed in claim 1 wherein inorganic acid of step (i) is selected from hydrohalogenic acids, preferably from hydrochloric acid and hydrobromic acid but most preferably is hydrochloric acid.
4. The process as claimed in claim 1 wherein a solid resulted in the step (ii) is DOTA hydrochloride, preferably a hydrate form of DOTA hydrochloride.
5. The process as claimed in claim 1 wherein the crystallized salt of DOTA of step (iii) comprises the first impurity at 1.23 RRT is less than 7% by HPLC and the second impurity at 0.80 RRT is less than 0.5%.
6. The process as claimed in claim 1 wherein the crystallized salt of DOTA of step (iii) comprises the first impurity at 1.23 RRT is less than 6% by HPLC and the second impurity at 0.80 RRT is less than 0.1%.
7. The process as claimed in steps (iii) and (vi) of claim 1 wherein the organic solvent is selected from ethanol, methanol, propanol, isopropanol, acetone and mixtures thereof.
8. The process as claimed in steps (iii) and (vi) of claim 1 wherein the organic solvent is preferably acetone.
9. The process as claimed in step (v) of claim 1 wherein the anionic resin is selected from a group comprising of Amberlyst A126OH, Lewaitite M600OH or M800OH, Purolite UCW5072OH and resin based on crosslinked polystyrene matrix that has quaternary ammonium functional group such as INDION810(OH) resin.
10. The process as claimed in step (v) of claim 1 wherein pH of the reaction is carried out at a pH range from 3.5 to 4.5.
11. The process as claimed in claim 1 wherein the crystallized salt of DOTA of step (vii) comprises the first impurity at 1.23 RRT is less than 1% by HPLC and the second impurity at 0.80 RRT is not detectable.
12. The process as claimed in claim 1 wherein the crystallized salt of DOTA of step (vii) comprises the first impurity at 1.23 RRT is 0.02% by HPLC and the second impurity at 0.80 RRT is not detectable.
13. the process step of (vii) of claim 1 is further repeated wherein the crystallized salt of DOTA of repeated step (vii) comprises the first impurity at 1.23 RRT and the second impurity at 0.80 RRT are not detectable.