WO2009003878A1 - Method for the preparation of mycophenolate mofetil - Google Patents

Abstract

The present invention provides a method for the preparation of mycophenolate mofetil wherein mycophenolic acid or an amine salt of mycophenolic acid is esterified with 2-morpholinoethanol followed by downstream processing in the presence of a chelating agent (formula (I)).

Description

METHOD FOR THE PREPARATION OF MYCOPHENOLATE MOFETIL

Field of the invention

The present invention relates to a method for the preparation of mycophenolate mofetil.

Background of the invention

Mycophenolic acid (MPA, also known as 6-(4-hydroxy-6-methoxy-7-methyl-3- oxo-5-phthalanyl)-4-methyl-4-hexenoic acid, 6-(1 ,3-dihydro-4-hydroxy-6-methoxy-7- methyl-3-oxo-5-isobenzofuranyl)-4-methyl-4-hexenoic acid, Ci₇H₂OO₆, CAS 24280-93-1 ) is a compound with various advantageous properties. Next to antibiotic activity, MPA also displays antifungal, antiviral and antitumor properties and the compound has been used in the treatment of psoriasis and recently as immunosuppressant. The 2-morpholinoethyl ester of MPA, also known as mycophenolate mofetil (MPM, C₂3H₃₁NO₇, CAS 128794-94-5), is a prodrug of MPA and has similar advantageous properties. The chemical structure of MPM is:

MPM can be prepared by esterification of MPA with 2-morpholinoethanol. In US 4,753,935 an acid halide condensation route has been described. This is a two-step process requiring toxic reagents for forming the halide of MPA and/or of 2-morpholinoethanol. In EP 649,422 B1 , an improved route was disclosed concerning refluxing MPA with 2-morpholinoethanol in an inert organic solvent capable of azeotropic removal of water, without the use of additional reagents. Although this method represents advantages over the method as described in US 4,753,935, it still has drawbacks, one of which is the unwanted formation of color in the end product as mentioned in WO 2002/100855, a problem that is particularly pronounced in industrial-scale synthesis. In WO 2002/100855, the light violet coloration of mycophenolate mofetil was prevented by using specific solvents during the esterification reaction, notably higher dialkyl ethers of which especially dibutyl ether proved to be particularly useful. Obviously, the number of solvents suitable for this method is limited to high-boiling ethers, a class of compounds that is subject to side reactions such as hydrolysis and/or unwanted esterification of MPA. In view of the problem of product coloration, there is room for improvement of the synthesis of MPM from MPA.

Detailed description of the invention

During esterification of MPA with 2-morpholinoethanol in a refluxing solvent followed by isolation of the resulting MPM, in many cases the product contains an unwanted color, for instance purple or violet. It was surprisingly found that said color could be eliminated by the addition of small amounts of a chelating agent.

In the first aspect of the invention MPM is prepared from MPA and 2-morpholinoethanol by esterification in a refluxing solvent followed by downstream processing in the presence of a chelating agent. Said chelating agent can be added before and/or during and/or after said esterification. The term "chelating agent" refers to a substance whose molecules can form several bonds to a positively charged ion. In other words, a chelating agent is a multidentate ligand. An example of a chelating agent is ethylenediamine, a single molecule of which can form two bonds to, for instance, a transition-metal ion such as Ni²⁺. Substituted derivatives of ethylenediamine can also serve as chelating agent and a well-known example that proved to be well-suited in the context of the present invention is ethylenediamine tetraacetic acid (EDTA) or a salt thereof. Suitable EDTA salts are alkaline or alkaline earth metal salts such as for instance the di- or tetra sodium salt. Other suitable examples of chelating agents are diamines wherein the nitrogen atoms are connected with 1-6 carbon atoms, preferably 2-4 carbon atoms and wherein the amine groups are independently substituted with formyl, acetyl, propionyl or similar substituents. Another suitable chelating agent in the context of the present invention is porphine and derivatives thereof such as the porphyrins. Still other suitable chelating agents are dithiols, notably 1 ,2-dithiols such as dimercaprol (2,3-dimercapto-1-propanol). The amount of chelating agent to be used is from 0.001 to 10 mmol per mol of MPA, preferably from 0.005 to 5 mmol per mol of MPA, more preferably from 0.01 to 1 mmol per mol of MPA, most preferably from 0.1 to 0.8 mmol per mol of MPA. The chelating agent can be added during the esterification reaction but can preferably also be added after the esterification reaction, i.e. to the reaction mixture after a certain degree of conversion has been reached or to any of the aqueous or organic phases in which MPM is present during downstream processing and/or crystallization.

The solvent used for esterification of MPA can be a solvent such as benzene and substituted benzenes like ethyl benzene, mefa-xylene, orf/?o-xylene, para-xylene and toluene, chloroform, methylene chloride, ethers such as dialkyl ethers like dibutyl ether and diisopropyl ether, ketones such as acetone, cyclohexanone, cyclopentanone, dipropyl ketone, methylisobutyl ketone, methylpropyl ketone and mixtures of these solvents. Preferred solvents are xylene, dibutyl ether and cyclohexanone. Preferably the esterification is carried out under azeotropic separation of water and under use of an excess of 2-morpholinoethanol, for instance 1.00 to 20 molar equivalents, preferably 1.01 to 10 molar equivalents, more preferably 1.02 to 5 molar equivalents, most preferably 1.03 to 3 molar equivalents, still most preferably 1.04 to 2 molar equivalents. Although the present invention is most suitably carried out under refluxing conditions, also esterification reactions carried out at lower temperatures than the boiling point can be further optimized by the presence of the chelating agent. The advantages of esterification at a temperature below the boiling point are that equipment for condensing solvent vapors and returning these condensed vapors are no longer required and the energy input required for reaching and maintaining the boiling point, which normally is substantial, can be circumvented. Furthermore, formation of unwanted by-products generally is lower at lower reaction temperatures. Following conversion of MPA to MPM, downstream processing can be carried out, for instance by direct crystallization of the product from the reaction mixture.

Alternatively, the MPM is extracted from the reaction mixture to water at low pH and back-extracted to an organic solvent at higher pH after which crystallization is effected.

In a first embodiment of the present invention MPA is used in the form of a salt. Suitable salts are amines and alkali metal salts. In case of alkali metal salts, also an acid should be present in a molar amount that is at least equal to that of the molar amount of the MPA alkali metal salt. In case of amine salts, addition of acid is not mandatory, although acid can also be added in order to decrease conversion times and/or increase yields. Examples of suitable amine salts of MPA are, but are not limited to, salts from amines such as te/t-butylamine, cyclohexylamine, dibenzylamine, N,N-di/sopropyl- ethylamine, N^-dimethylcyclohexylamine, N,N-dimethylisopropylamine, N-methyl- piperidine, morpholine, te/t-octylamine, piperidine, /so-propylamine, N,N,N',N'-tetra- methylbutylenediamine, N,N,N',N'-tetramethylethylenediamine, tributylamine, triethyl- amine and tripropylamine. Suitable alkali metal salts of MPA are salts from lithium and potassium, preferably from sodium.

In a second embodiment of the present invention, esterification of MPA can be positively influenced (Ae. reduction of reaction time, increase of maximum conversion) by the addition of substances that are capable of absorbing water. These substances can be present in the mixture of MPA, solvent and 2-morpholinoethanol. However, these substances may also be present in the vapor phase of said mixture; despite the fact that the present invention deals with a method for esterification in non-boiling mixtures, a vapor phase nevertheless is usually present above such non-boiling mixtures. Substances that are capable of absorbing water are for instance salts of alkali and earth alkali metals and usually these salts are carbonates, halides or sulfates. Suitable examples are CaCI₂, CaSO₄, K₂CO₃, K₂SO₄, MgSO₄, Na₂CO₃, Na₂SO₄ and the like. Preferred other substances are molecular sieves, preferably those with pore sizes ranging from 0.1-0.6 nm, more preferably ranging from 0.2-0.5 nm, most preferably ranging from 0.3-0.4 nm. In a second aspect of the present invention, MPM obtained according to the first aspect can be used in pharmaceutical compositions, for instance in antifungal, antiviral and/or antitumor compositions, but also in compositions useful in the treatment of psoriasis and as immunosuppressant. Accordingly, said pharmaceutical compositions have the advantage that color-generating components are absent resulting in the absence of unwanted impurities but also in the absence of color.

EXAMPLES

General Methods

HPLC analysis was performed on a Waters HPLCWIS system (Alliance HT 2795 separation module; Diode array detector, model 996) with the following specifics:

Column: Waters Sunfire C18, 150x4.6mm, 3.5 μm Column temp: 4O⁰C Flow rate: 1.0 ml/min UV-detection 251 nm (and 214 nm for the determination of xylene) Injection volume: 5 μl (use fixed loop) Mobile phase A: Sorensen buffer/water (30/70) Mobile phase B: Sorensen buffer/ACN (30/70) Gradient: T= 0 min. 50 % B

T= 12 min. 70 % B T= 16 min 100 % B T= 21.4 min 100 % B T= 21.5 min 50 % B T= 28 min 50% B

The chemicals are water (MiIIi-Q purified or HPLC grade), acetonitrile (ACN, gradient grade, Merck 1.00030), KH₂PO₄ (p.a., Merck 1.04873), Na₂HPO₄.2H₂O (p.a., Merck 1.06580). Mobile phases:

• Phosphate solution A: 3.026 g of KH₂PO₄ was dissolved in 1 L MiIIiQ water.

• Phosphate solution B: 3.9587 g of Na₂HPO₄.2H₂O was dissolved in 1 L MiIIiQ water.

• Sorensen buffer (0.022 M, pH 6.4): 700 mL phosphate solution A was mixed with 300 mL phosphate solution B.

References, standards and controls:

Standard: PH Eur reference for peak identification CRS (contains mycophenolate mofetil with impurities A, B, D, E, F, G and H). Comparative Example Preparation of MPM from MPA-triethylamine salt (MPA-TEA)

In a stainless steel 316 vessel, a mixture of MPA-TEA salt (10 kg), xylene (40 L), and 2-morpholinoethanol (3.7 kg) was stirred at 125-130⁰C for 65 h under a positive nitrogen flow. The mixture was cooled to 10-15⁰C and water (40 L) was added. The pH was adjusted to 2 with 6N H₂SO₄ and the mixture was stirred for 15 min at 10-15°C. The phases were separated. Under stirring ethyl acetate (26 L) was added to the aqueous phase and the mixture was stirred for 15 min after which the phases were separated. This washing was repeated twice. Under stirring ethyl acetate (1 10 L) was added to the aqueous phase. The pH was adjusted to 4.2 with 4N NaOH and a purple color developed in the organic phase. After stirring for 15 min the phases were separated. The organic phase was washed with water (50 L) at pH 4.2. After stirring for 15 min the phases were separated. Water (50 L) was added to the organic phase and the pH was adjusted to 8 with 4N NaOH. After stirring for 15 min the layers were separated. Water (50 L) was added to the organic phase and the pH was adjusted to 7 with diluted H₂SO₄. After stirring for 15 min the phases were separated. The solution was partially decolorized with active carbon. Under stirring the extract was concentrated under vacuum to dryness. The residue was dissolved in a mixture of ethyl acetate (10 L) and 1-propanol (50 L) at 50-60⁰C. Under stirring the mixture was cooled to 0-5⁰C in 4 h and stirring at 0-5°C was continued for 4 h. The crystals were filtered off, washed with 1-propanol (20 L) of 0-5⁰C, and dried under vacuum at 40-45⁰C, giving 7.0 kg of MPM as crystals having a purple tinge.

Example 1

Preparation of MPM from MPA-triethylamine salt (MPA-TEA) in the presence of

EDTA-disodium salt

In a stainless steel 316 vessel, a mixture of MPA-TEA salt (10 kg), xylene (40 L), and 2-morpholinoethanol (3.7 kg) was stirred at 125-130°C for 52 h under a positive nitrogen flow. The reaction mixture was cooled to 10-15°C and under stirring water (40 L) and a solution of EDTA-diNa salt (6 g in 600 mL water) were added. The pH was adjusted to 2 with 6N H₂SO₄ and the mixture was stirred for 15 min at 10-15°C. The phases were separated. Under stirring ethyl acetate (26 L) was added to the aqueous phase and the mixture was stirred for 15 min after which the phases were separated. This washing was repeated twice. Under stirring ethyl acetate (110 L) was added to the aqueous phase. The pH was adjusted to 4.2 with 4N NaOH. After stirring for 15 min the phases were separated. The organic phase was washed with water (50 L) at pH 4.2. After stirring for 15 min the phases were separated. Water (50 L) was added to the organic phase and the pH was adjusted to 8 with 4N NaOH. After stirring for 15 min the phases were separated. Water (50 L) was added to the organic phase and the pH was adjusted to 7 with diluted H₂SO₄. After stirring for 15 min the layers were separated. The solution was decolorized with active carbon. Under stirring the decolorized extract was concentrated under vacuum to dryness. The residue was dissolved in a mixture of ethyl acetate (10 L) and 1-propanol (40 L) at 50-60⁰C. A solution of EDTA-diNa (0.7 g in 70 mL hot distilled water) was added and the hot solution was filtered. The filter was washed with 1-propanol (10 L) of 50-60⁰C. Under stirring the mixture was gradually cooled to 0-5⁰C in 4 h and stirring at 0-5°C was continued for 4 h. The crystals were filtered off, washed with 1-propanol (20 L) of 0-5°C, and dried under vacuum at 40-45°C, giving 7.1 kg of MPM as white to off-white crystals.

Example 2

Crystallization of MPM in the presence of EDTA

In a stainless steel 316 vessel, MPM (7.0 kg) with a purple tinge was dissolved in a mixture of 1-propanol (43.75 L) and ethyl acetate (8.75 L) at 55-60⁰C under nitrogen. A solution of EDTA-diNa (4.2 g in 500 mL hot distilled water) was added and the pink color disappeared. The hot solution was filtered and the filter was washed with a hot mixture of 1-propanol (8.75 L) and ethyl acetate (1.75 L). The combined filtrate was slowly cooled to 0-5⁰C and stirring was continued for 1 h. The crystals were filtered off, washed with 1-propanol (14 L) of 0-5 °C, and dried under vacuum at 40-45 °C, giving 6.1 kg MPM as white to off-white crystals. Example 3 Crystallization of MPM with seeding in the presence of EDTA

In a stainless steel 316 vessel, MPM (7.0 kg) with a purple tinge was dissolved in 1-propanol (65 L) at 55-60⁰C under nitrogen. A solution of EDTA-diNa (0.7 g in 70 ml_ hot distilled water) was added and the pink color disappeared. The hot solution was filtered and the filter was washed with hot 1-propanol (14 L). The combined filtrate was slowly cooled to 40⁰C, seeded and cooled to 0-5°C and stirring was continued for 4 h.

The crystals were filtered off, washed with 1-propanol (21 L) of 0-5 ⁰C, and dried under vacuum at 40-45 ⁰C, giving 6.5 kg MPM as white to off-white crystals.

Claims

1. Method for the preparation of mycophenolate mofetil by esterification of mycophenolic acid or an amine salt of mycophenolic acid with 2-morpholinoethanol characterized in that a chelating agent is added before and/or during and/or after said esterification.

2. Method according to claim 1 wherein the amount of chelating agent is from 0.001 to 10 mmol per mol of mycophenolic acid or per mol of an amine salt of mycophenolic acid.

3. Method according to any one of claims 1 to 2 wherein said amine is triethylamine.

4. Method according to any one of claims 1 to 3 wherein said esterification is carried out in xylene.

5. Method according to any one of claims 1 to 4 wherein said chelating agent is ethylene diamine tetraacetic acid or a salt thereof.

6. A pharmaceutical composition comprising mycophenolate mofetil obtained according to any one of claims 1 to 5.

7. Use of mycophenolate mofetil obtained according to anyone of claims 1 to 5 in the preparation of a medicament.