TECHNICAL FIELD OF THE INVENTION

The present invention relates to an improved process for the preparation of (S)-N-[(3-(3-fluoro- 4-morpholinophenyl)-2-oxo-5-oxazolidinyl)methyl]acetamide, known as Linezolid or salts thereof in crystalline form and in particular to a cost effective process for large scale production of Linezolid or salts thereof and pharmaceutical preparations containing said compounds. BACKGROUND OF THE INVENTION

Linezolid is a synthetic antibacterial agent of the oxazolidinone class used for the treatment of serious infections caused by Gram-positive bacteria that are resistant to several other antibiotics. Linezolid is active against most Gram-positive bacteria that cause disease, including streptococci, vancomycin-resistant enterococci, and methicillin-resistant Staphylococcus aureus.

As a protein synthesis inhibitor, it stops the growth of bacteria by disrupting their production of proteins. Although many antibiotics work this way, the exact mechanism of action of Linezolid appears to be unique to the oxazolidinone class.

Linezolid is chemically designated as (S -N-[(3-(3-fluoro-4-morpholinophenyl)-2-oxo-5- oxazolidinyl)methyl]acetamide and is represented by the chemical structure of Formula 1.

(1)

It is already known that Linezolid exists in different polymorphic forms.

Polymorphism is the occurrence of different crystalline forms of a single compound and each polymorph has distinct physical properties, such as different solubility profiles, melting points and flow properties, which greatly affect the development of pharmaceutical formulations.

Various methods are already known for the preparation of Linezolid in crystalline form due to its useful properties. Linezolid and methods for the preparation thereof were first disclosed in EP-B- 0717738 and J. Med. Chem. 1996, 39, 673, wherein Linezolid is obtained in a crystalline form characterized by a melting point at 181.5-182.5°C and a set of IR spectrum peaks, said crystalline form is also know as Form I.

EP-B-1255754 discloses a distinct crystalline form of Linezolid, known as Form II, characterized by different IR peaks and a table of X-Ray diffraction (XRD) degrees 2Θ values, wherein said Form II is obtained by agitating a solution of the known Form I at a temperature below 80°C for not less than 60 minutes. In said document, it is stated that the one crystalline form (Form I) converts into the other crystalline form (Form II), and that the new form, Form II, is the most stable form below 85°C. The interconversion of these two crystalline forms has been confirmed later by several other publications.

In WO-A-2005/035530 and WO-A-2006/004922 additional crystalline forms of Linezolid are disclosed, however, in said documents, there is no information to exclude the interconversion of one crystalline form into the other crystalline form and no evidence of crystalline form stability during standard industrial procedures is provided. In fact, it is questionable whether all the additional crystalline forms are actually different than the one originally described in EP-B- 0717738.

In contrast, it is widely acknowledged that a crystalline product is more suitable for pharmaceutical use if prepared in a single, specific form, which remains stable under the conditions required for formulation.

Therefore, the desirable process for the preparation of Linezolid would have to be effective, comprising a synthetic procedure that provides enantiomerically pure material of a single and stable form. In order to be consistent, said process must also control the parameters that may result in crystalline form interconversion during scale-up, precipitation, filtration and particle size reduction procedures, as well as confirm the crystalline form stability of the material during storage and handling. And in order to be efficient, the process has to be scalable, employing industrial practice that confines manufacturing costs.

According to EP-B-1673370, Linezolid is either heated directly or suspended and refluxed in toluene or xylene, then cooled to 25°C and filtered. The resulting material is characterized by lists of XRD and E peaks, not the actual spectra. However, such analytical data is not sufficient to demonstrate the purity and the stability of the obtained crystalline form. Additionally, the reported examples only involve lOg batches and details of the filtration process are not given, although they were found to significantly affect crystalline form stability during scale-up. It is, therefore, questionable whether this process would consistently provide Linezolid in a single and stable form when applied in industrial scale. In US 7 649 096, Linezolid is dissolved in 10 to 15 volumes of organic solvent (such as chloroalkane, alcohol, aliphatic or cyclic ether) then 4 to 5 volumes of anti-solvent (such as xylenes, mesitylene, diphenylether, n-heptane) are added. In the 30g-scale examples given, the solvent is distilled off in a rotor evaporator at a preferred temperature of 60 to 70°C. The material obtained is characterized by detailed XRD and IR spectra and is further brought to various levels of particle size by known techniques. However, no specific analytical data corresponding to the material after particle size reduction is disclosed, which is a procedure that it has also been found to result in crystalline form interconversion. Most importantly, the lack of information on scale- up procedures in the above process and the large volume of organic solvent mixtures distilled off the Linezolid solution are expected to reduce its cost-efficiency and presumably hamper its industrial applicability.

According to WO-A-2011/050826, WO- A-2011/050865 and WO-A-2011/051384, Linezolid is dissolved in 10 to 100 volumes of a solvent or mixture of solvents, heated and then added to a 2 to 7 times larger volume of a pre-cooled, pre-seeded anti-solvent in a specific time range and manner to avoid nucleation prior to the contact of the Linezolid solution with the anti-solvent. A set of XRD peaks for the obtained material is provided. As mentioned above, such analytical data is not sufficient to demonstrate its crystalline form purity and stability. Moreover, the 0.5g-scale examples provided for the intricate procedure described above hardly ensure the industrial applicability of the process, while the large volume of solvent and anti- solvent used, questions its cost-efficiency.

WO-A-2006/004922 discloses detailed analytical and stability data for the material obtained by three different processes, wherein the processes that involve direct heating of Linezolid or refluxing a toluene slurry before filtration have been previously disclosed in other documents. The improved process described in said document consists of dissolving Linezolid in methanol to obtain Linezolid crystals by spray-drying, a highly demanding technique, which is known to increase manufacturing costs during industrial application.

Although each of the above patents represents an attempt to overcome the deficiencies of the prior art processes, there still exists a need for a cost-effective process for large scale production which consistently provides Linezolid in a stable crystalline form, thus enabling the use of the obtained product in pharmaceutical formulation.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide an improved process for the preparation of Linezolid or pharmaceutically acceptable salts or derivatives thereof, which overcomes the deficiencies of the prior art processes and results to a cost effective industrial production without sacrificing the yield and quality of the product.

Another object of the present invention is to provide an improved method for the preparation of Linezolid or salts thereof in crystalline form having physical properties that render it suitable for pharmaceutical preparations and wherein no conversion to any other crystalline form occurs.

A further object of the present invention is to provide a process for the preparation pure Linezolid in a crystalline form, which is stable when subjected to particle size reduction method and having a D ₉₀ particle size distribution suitable for pharmaceutical preparations.

In accordance with the above objects of the present invention, a process for the preparation of Linezolid in crystalline form is provided comprising the following steps:

a) Dissolving Linezolid in ethyl acetate and filtering the resulting solution;

b) Removing ethyl acetate until the total volume is reduced to about 2-4 volumes;

c) Heating the obtained mixture in a sealed reactor to about 95-1 15°C for about 60-120 minutes;

d) Removing solvent at a temperature below 60°C under reduced pressure optionally before adding about 10-15 volumes of toluene at 30-40°C;

e) Stirring the reaction mass for 30-60 minutes at 20-30°C;

f) Filtering the reaction mass and washing with toluene;

g) Drying the wet cake to obtain a Linezolid crystalline form.

According to another embodiment of the present invention, a crystalline Linezolid or salts thereof is provided characterised by an X-ray powder diffraction pattern having characteristic reflections at approximately 13.4, 18.0, 18.4, 18.7, 20.9 and 22.1± 0.2 2Θ degrees and having an XRD pattern in accordance to Figure 1. Preferred embodiments of the present invention are set out in dependent claims 2 to 8. Other objects and advantages of the present invention will become apparent to those skilled in the art in view of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows an X-RD spectrum of Linezolid active ingredient prepared according to the present invention.

Fig. 2 shows an FT-IR spectrum of Linezolid active ingredient prepared according to the present invention.

Fig. 3 shows a DSC spectrum of Linezolid active ingredient prepared according to the present invention.

Fig. 4 shows an X-RD spectrum of Linezolid active ingredient prepared according to example 1 of the present invention.

Fig. 5 shows an FT-IR spectrum of Linezolid active ingredient prepared according to example 1 of the present invention.

Fig. 6 shows a DSC spectrum of Linezolid active ingredient prepared according to example 1 of the present invention.

Fig. 7 shows an X-RD spectrum of Linezolid active ingredient prepared according to example 2 of the present invention.

Fig. 8 shows an FT-IR spectrum of Linezolid active ingredient prepared according to example 2 of the present invention.

Fig. 9 shows a DSC spectrum of Linezolid active ingredient prepared according to example 2 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an improved process for large scale preparation of Linezolid in crystalline form and pharmaceutically acceptable salts thereof, which has characteristic peaks as depicted in XRD pattern as in Figure 1, an IR spectrum in accordance to Figure 2 and a DSC curve in accordance to Figure 3. The process for the preparation of Linezolid or salts thereof according to the present invention comprises the following steps:

a) Dissolving Linezolid in ethyl acetate and filtering the resulting solution;

b) Removing ethyl acetate until the total volume is reduced to 2-4 volumes;

c) Heating the obtained mixture in a sealed reactor to about 95-115°C for about 60-180 minutes;

d) Removing solvent at a temperature below 60°C under reduced pressure optionally before adding about 10-15 volumes of toluene at 30-40°C;

e) Stirring the reaction mass for 30-60 minutes at 20-30°C;

f) Filtering the reaction mass and washing with toluene;

g) Drying the wet cake to obtain a Linezolid crystalline form.

Linezolid used as starting material may be obtained according to any known prior art processes.

A solution of Linezolid is heated to about 75-80 °C in ethyl acetate until dissolution. The resulting hot solution is filtered through celite and washed with hot ethyl acetate, which is then distilled until the total volume of the reaction mass reaches about 2-4 volumes, preferably 3 volumes. The reactor is sealed and heated to about 95-115 °C, preferably 105-110 °C for 60-180 minutes, preferably 90-120 minutes.

According to a first embodiment of the present invention, the solvent is removed completely under reduced pressure and the reaction mass is then cooled to 20-30°C under stirring. About 10- 15 volumes, preferably 12 volumes of toluene are added. The material is further cooled to about 20-30°C for about 15-30 minutes, maintained under stirring for another 15-30 minutes, then filtered, washed with toluene and dried by conventional means.

The obtained crystalline form has an XRD pattern in accordance to Figure 1, and it has the following characteristic peaks and intensities:

In addition, the crystalline form has an IR spectrum in accordance to Figure 2 and having bands at 3338, 1741, 1662, 1546, 1517, 1471, 1453, 1425, 1334, 1274, 1228, 1198, 1177, 1117, 1050, 936, 923, 904, 870, 824, 755 and 661 cm ^"1 and a DSC curve in accordance to Figure 3 having an endothermic peak at 181.24°C and does not contain any other crystalline form of Linezolid.

According to another embodiment of the present invention, the mixture obtained from step (c) is cooled to about 80°C and about 10-15 volumes, preferably 12 volumes of toluene are added. At least 10 volumes of solvent are removed under reduced pressure at a temperature below 60°C. The material is further cooled to 20-30°C for about 15-30 minutes, maintained under stirring for another 15-30 minutes, then filtered, washed with toluene and dried by conventional means. The obtained crystalline form has an XRD pattern in accordance to Figure 1, an IR spectrum in accordance to Figure 2, a DSC curve in accordance to Figure 3 and does not contain any traces of other crystalline form of Linezolid. In a further aspect of the present invention, the material obtained from the process of the present invention is subjected to standard holding-time studies, stability and accelerated stability tests. The XRD, IR and DSC spectra obtained after the tests are in accordance to Figures 1, 2 and 3, respectively.

Subsequently, the material obtained according to the process of the present invention is subjected to a particle size reduction method, such as sifting, milling and jet milling, and the XRD, IR and DSC analytical data obtained after particle size reduction are still in accordance to Figures 1 , 2 and 3 respectively.

By way of comparison, Linezolid according to the process of J. Med. Chem. 1996, 39, 673 has been prepared and the analytical data of the resulting material before and after particle size reduction (XRD, IR and DSC) were identical to the analytical data of the material obtained by the process of the present invention.

Moreover, Linezolid has been prepared in large scale (10kg) according to steps (a), (b) and (c) of the process of the present invention, then it has been directly filtered, the wet cake washed with ethyl acetate, and dried. XRD and DSC analysis of the obtained material showed traces of another crystalline form, referred to as Form II in the prior art. When this material was subjected to stability tests mentioned above and/or to known methods of particle size reduction, XRD and DSC analysis showed an increase of the Form II content.

The process for the preparation of Linezolid or pharmaceutically acceptable salts thereof according to the present invention will be described in detail with reference to the following examples, which are provided by way of illustration only and should not be construed as limit to the scope of the reaction in any manner.

Analytical data information:

FT-IR transmittance spectrum was performed on a Perkin Elmer Instrument model Spectrum One (16 scans from 4000 to 450cm ^'1 ).

XRD measurements were performed on a Siemens D500 instrument using CuKa tube (40kV, 30mA, λ=1.54060). The diffractogram was collected from 2.0 to 60.0 2theta value using step size 0.040, time/step 2.0, peak width 0.30 and threshold 1.0.

DSC thermograms were obtained using a Perkin-Elmer Pyris Diamond differential scanning calorimeter. Samples were heated from 20 to 190°C at a heating rate of 20.0°C/min.

Particle size distribution was measured by laser diffraction with a Malvern Mastersizer 2000.

EXAMPLES

Example 1 : Preparation of Linezolid

10kg Linezolid is dissolved in 250L ethyl acetate by heating to about 75-80°C and the resulting hot solution is filtered through celite and washed with ethyl acetate. The reaction mass is distilled under atmospheric pressure at about 75-80°C until the total volume of the remaining reaction mass is reduced to 30L. The reactor is sealed and the mixture is heated at 106-108°C under stirring for 120 minutes. The reaction mass is then cooled to about 80°C under stirring within 15 minutes, 120L toluene are added within 15 minutes and the temperature of the reaction mass is 32°C. 102-105L of solvent are removed under reduced pressure at about 32-42°C within 100 minutes. The residual mass is cooled under stirring to 20-30°C within 20 minutes and left under stirring at 20-30°C for 30 minutes. The reaction mass is filtered and washed with 10L toluene. The wet cake is dried in a vacuum oven at 50-55°C for 10 hours to afford 9.8kg of Linezolid (yield 98%; and having a melting point: 181.53°C).

The material obtained above is subjected to sifting and jet-milling and subsequently submitted for analysis.

The Linezolid obtained after particle size reduction has D ₉₀ particle size distribution = 34.3 μηι, Melting point at 181.84°C; XRD as depicted in Figure 4 having characteristic reflections at least at 13.4, 18.0, 18.4, 18.7, 20.9 and 22.1± 0.2 2Θ degrees; FT-IR as depicted in Figure 5 and having bands at 3338, 1742, 1663, 1546, 1517, 1471, 1452, 1425, 1334, 1274, 1228, 1198, 1177, 1117, 1050, 936, 923, 904, 870, 825, 756 and 662 cm ^"1 ; DSC as depicted in Figure 6 having an endothermic peak at 181.84°C.

Example 2: Preparation of Linezolid 10kg Linezolid is dissolved in 250L ethyl acetate by heating to about 75-80°C and the resulting hot solution is filtered through celite and washed with ethyl acetate. The reaction mass is distilled under atmospheric pressure at about 75-80°C until the total volume of the remaining reaction mass is reduced to 30L. The reactor is sealed and the mixture is heated at 105-110°C under stirring for 120 minutes. The reaction mass is then cooled under stirring within 20 minutes and the solvent is distilled under reduced pressure at 58-76°C within 90 minutes. The residual mass is cooled to 30-40°C within 30 minutes under stirring and 120L toluene is added. Stirring is maintained at 20-30°C for 30 minutes, then the reaction mass is filtered and washed with 10L toluene. The wet cake is dried in a vacuum oven at 50-55°C for 10 hours to afford 9.7kg of Linezolid (yield: 97% with a melting point at 181.65°C).

The material obtained above is subjected to sifting and jet-milling, and subsequently submitted for analysis.

The Linezolid obtained after particle size reduction has D ₉₀ particle size distribution = 56.8um, Melting point at 182.21°C; XRD as depicted in Figure 7 having characteristic reflections at least at 13.5, 18.0, 18.4, 18.7, 20.9 and 22.1± 0.2 2Θ degrees; FT-IR as depicted in Figure 8 and having bands at 3338, 1742, 1663, 1546, 1517, 1471, 1452, 1424, 1334, 1274, 1228, 1198, 1177, 1117, 1050, 936, 923, 904, 870, 824, 755 and 662 cm ^'1 ; DSC as depicted in Figure 9 having an endothermic peak at 181.21 °C. The present invention describes a cost-effective process for the large-scale manufacture of Linezolid, which consistently provides the desired crystalline form. No conversion to any other crystalline form is observed, either during particle size reduction or under standard stability regimes. The material obtained by the described process has, therefore, physical properties that render it suitable for pharmaceutical preparations.

While the present invention has been described with respect to the particular embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made in the invention without departing from the spirit and scope thereof, as defined in the appended claims.