FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
PROVISIONAL SPECIFICATION
(See sectionlO and rule 13]

1. Title of the invention: "A novel crystalline form of Linezolid"
2. Macleods Pharmaceuticals Ltd., an Indian Company, having its Registered Office at 304 - Atlanta Arcade, Opp. Leela Hotel, Marol Church Road, Andheri (East), Mumbai - 400 059, Maharashtra, India.
3. The following specification describes the invention.

A NOVEL CRYSTALLINE FORM OF LINEZOLID
FIELD OF THE INVENTION
The present invention relates to a cost effective and industrially advantageous process for the isolation of Linezolid of Formula I substantially free from compounds of Formula II and III. The invention also relates to novel polymorphic Form M of Linezolid substantially free from Linezolid Form II, process for its preparation, pharmaceutical composition comprising it and its method of use for the treatment of infection caused by multi-resistant bacteria.


BACKGROUND OF THE INVENTION
Linezolid is chemically, (S)-N-[[3-[3-Fluoro-4-(4-morpholinyl) phenyl]-2-oxo-5-oxazolidinyl] methyl] acetamide and is represented by Formula I. Linezolid is known from U.S. Patent No. 5,688,792 and is used as an antibacterial agent.

Formula I
Several processes for the preparation of Linezolid are known in literature such as those described in U.S. Patent Nos. 5,688,792, 6,887,995, 7,291,614, 7,307,163 and 7,429,661, PCT Publication No. WO 2007/ 116284 and Tetrahedron Letters 40 (1999) 4855-4856, which are herein incorporated by reference.
U.S. Patent No. 5,688,792 describes a process for the preparation of Linezolid
comprising reducing (R)-N-[[3-[3-fluoro-4-morpholinyl]phenyl]-2-oxo-5-
oxazolidinyl]methy(]azide with hydrogen in the presence of 10% palladium / carbon in ethyl acetate to produce S-N-(4-morpholinyi-3-fluorophenyl)-2-oxo-5-oxazolidinyl methyl amine followed by reacting with acetic anhydride to produce Linezolid, which is being isolated by the tedious and cumbersome procedures of chromatography.
Tetrahedron Letters 40 (1999) 4855-4856 describes a process for the preparation of Linezolid by treating (R)-N-[[3-[3-fluoro-4-morpholinyI] phenyl]-2-oxo-5-oxazolidinyl] methyl] azide with thioacetic acid without mentioning the isolation
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process. The said process leads to formation of an undesirable level of reaction by¬products such as compound of Formula II and III.

In light of the above drawbacks in the prior art processes, there is a need for the development of a process for the isolation of Linezolid which is convenient to operate on an industrial scale and gives substantially pure product in good yield.
Linezolid is known to exhibit polymorphism and several crystalline forms are so far known in literature such as those described in U.S. Patent No. 6,559,305, U.S. Publication Nos. 2006/0128703, 2006/0111350, 2006/0142283 and 2007/0020329 which are herein incorporated by reference.
U.S. Patent. No. 6,559,305 discloses Linezolid crystalline Form II, which is being characterized by IR spectrum having bands at 3364, 1748, 1675, 1537, 1517, 1445, 1410, 1401, 1358, 1329, 1287, 1274,1253, 1237, 1221, 1145, 1130, 1123, 1116, 1078, 1066, 1049, 907, 852 and 758 cm"1 and powder X-ray diffraction spectrum having 2-theta values at 7.10, 9.54, 13.88, 14.23, 16.18, 16.79, 17.69, 19.41, 19.69, 19,93,21.61, 22.39, 22.84, 23.52,24.16, 25.28, 26.66; 27.01 and 27.77 degrees. Linezolid Form II is the stable form below about 85°C.
The Linezolid Form II having needle shaped crystals, poor flowability and compressibility and therefore difficult to handle during formulation.
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The new polymorphic form of a pharmaceutically useful compound provides a new opportunity to improve the performance characteristics of a pharmaceutical product. Therefore there is a need in the art for the development of new polymorphic form of Linezolid.
SUMMARY OF THE INVENTION
A first aspect of the present invention is to provide a process for the isolation of Linezolid substantially free from compounds of Formula II and III.

comprising the steps of:
a. extracting crude Linezolid with aqueous acidic solution.
b. isolating Linezolid substantially free from compound of Formula II.
c. treating Linezolid substantially free from compound of Formula II with hydrogen
peroxide and
d. isolating Linezolid substantially free from compounds of Formula II and III
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A second aspect of the present invention is to provide a novel polymorphic Form M of Linezolid, which having a good flowability and stability.
A third aspect of the present invention is to provide novel polymorphic Form M of Linezolid substantially free of Linezolid Form II.
A fourth aspect of the present invention is to provide a process for the preparation of novel polymorphic Form M of Linezolid substantially free of Linezolid Form II.
A fifth aspect of the present invention is to provide pharmaceutical composition comprising novel polymorphic Form M of Linezolid substantially free of Linezolid Form II and its method of use for the treatment of infection caused by multi-resistant bacteria.
DETAILED DESCRIPTION OF THE INVENTION
The crude Linezolid may be obtained by processes known in literature such as those described in U.S. Patent Nos. 5,688,792, 6,887,995, 7,291,614, 7,307,163 and 7,429,661, PCT Publication No, WO 2007/ 116284 and Tetrahedron Letters 40 (1999) 4855-4856, which are herein incorporated by reference.
The crude Linezolid may be obtained by reacting (R)-5-(azidomethyl)-3-(3-fluoro-4-morpholinophenyl) oxazolidin-2-one of Formula IV with thioacetic acid.

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The (R)-5-(azidomethyi)-3-(3-fluoro-4-morpholinophenyl) oxazolidin-2-one of Formula IV may be prepared by processes known in literature such as those described in U.S. Patent Nos. 5,688,792, 7,291.614 and 7,307,163, which are herein incorporated by reference.
The thioacetic acid may be used in the ratio of 0.5 volume / weight to 10 volume / weight of (R)-5-(azidomethyl)-3-(3-fluoro-4-morpholinophenyl) oxazolidin-2-one.
The reaction of (R)-5-(azidomethyl)-3-(3-fluoro-4-morpholinophenyl) oxazolidin-2-one with thioacetic acid may be carried out at a temperature in the range of -10°C to 50°C
The reaction of (R)-5-(azidomethyl)-3-(3-fluoro-4-morpholinophenyl) oxazolidin-2-one with thioacetic acid may be carried out for 2 hours to 10 hours.
After completion of reaction of (R)-5-(azidomethyl)-3-(3-fluoro-4-morpholinophenyl) oxazolidin-2-one of Formula IV with thioacetic acid, excess thioacetic acid may be removed by vacuum distillation at a temperature in the range of 45°C to 60°C to get crude Linezolid.
The crude Linezolid may contain compound of Formula 0 in the range of 1.5 % to 3.5% weight / weight.
The crude Linezolid may contain compound of Formula III in the range of 4 % to 8 % weight / weight.
The Linezolid substantially free of compound of Formula II may be isolated by diluting crude Linezolid with organic solvent, extracting with aqueous acidic solution followed by basifying aqueous layer with base.
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The term "substantially free" refer hereafter to an amount of less than 0.1 % weight / weight.
The organic solvent used for diluting crude Linezolid may be halogenated hydrocarbons selected from the group comprising of methylene chloride or ethylene dichloride.
The aqueous acidic solution consisting of a mixture of acid and water.
The acid may be selected from the group consisting of hydrochloric acid, acetic acid or sulphuric acid.
The pH of aqueous layer may be adjusted in the range of 6-10 with base.
The base used for the basifying aqueous layer may be selected from the group consisting of aqueous solution of sodium carbonate; potassium carbonate, sodium bicarbonate and potassium bicarbonate or ammonia.
The Linezolid substantially free of compound of Formula 11 may be isolated by filtration.
The Linezolid substantially free from compound of Formula II obtained from step b may contain compound of Formula III in the range of 2.0 % to 3.5 % weight / weight.
The Linezolid substantially free of compound of Formula II obtained from step b may be further treated with hydrogen peroxide to get Linezolid substantially free from compounds of Formula II and III.
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The Linezolid substantially free of compound of Formula II obtained from step b may be further treated with hydrogen peroxide in organic solvent at a temperature in the range of 40°C to 60°C for 2 hours to 6 hours.
The hydrogen peroxide used in step c may be diluted with water.
The organic solvent may be selected from the group consisting of methanol, ethanol, 1-propanol, 2-propanoL tetrahydrofuran, 1, 4-dioxane, diglyme, 1, 2-dimethoxyethane or 2-methoxyethanol.
The Linezolid substantially free from compounds of Formula II and III may be isolated by neutralizing excess hydrogen peroxide with sodium thiosulphate solution followed by filtering the reaction mixture.
The Linezolid substantially free from compounds of Formula II and III may be isolated by crystallizing Linezolid from organic solvent and acetic acid solution to get a novel polymorphic Form M of Linezolid.
The organic solvent used for crystallizing Linezolid may be selected from the group comprising of toluene, xylenes or mixture(s) thereof.
The crystallization of Linezolid may be carried out at a temperature in the range of 50°C tol 10°C for 1 hour to 24 hours.
A novel polymorphic Form M of Linezolid may be isolated by filtration at a temperature in the range of 40°C to 60°C.
A novel polymorphic Form M of Linezolid may be dried at a temperature in the range of 45°C to 70°C under reduced pressure.
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A novel polymorphic Form M of Linezolid obtained by present invention may be characterized by Powder X-ray Diffraction pattern as depicted in Figure 1.
A novel polymorphic Form M of Linezolid obtained by present invention may be characterized by Powder X-ray Diffraction spectrum having 2-theta values at 6.65, 7.39, 9.01, 9.38, 12.17, 13.53, 14.25, 14.75, 15.16, 15.37, 16,3, 16.86, 18.03, 18.46, 18.77, 19.86, 20.69, 21.05, 21.6, 22.2, 22.74, 23.05, 25.42, 25.83, 26.81, 27.15, 27.68, 28.35, 28.66, 29.66 and 30.19 ±0.1.
The Powder X-ray Diffraction data may be obtained by the known methods by using PANALYTICAL powder X-ray diffractometer model XPERT-PRO equipped with solid-state detector. Copper radiation of 1.54 A may be used for the analysis.
A novel polymorphic Form M of Linezolid may be further characterized by Differential Scanning Calorimetry (DSC) thermogram as depicted in Figure 2.
A novel polymorphic Form M of Linezolid may be further characterized by single sharp endotherm at 178°C ± 1°C in Differential Scanning Calorimetry (DSC) thermogram.
DSC analysis may be done using Metier Toledo apparatus. The sample's weight may be about 3 mg and scanning may be done at a rate of 10°C/minute from 35°C to 250°C. The oven may be constantly purged with N2 gas with flow rate of 60 ml/minute. The aluminium crucible of 40 (iL may be used for the sampling of material.
A novel polymorphic Form M of Linezolid obtained by present invention is consistently reproducible, stable during the formulation studies and having good flowability.
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A novel polymorphic Form M of Linezolid obtained by present invention is substantially free from Linezolid Form II.
LOD (Limit of Detection) and LOQ (Limit of Quantification) values as determined using Powder X-ray Diffraction Method is 0.10%) (weight / weight) and 0.25%o respectively.
The Linezolid substantially free from compounds of Formula II and III obtained by the present invention may be formulated into dosage forms such as, for example, tablets, injection, suspension etc. In these cases, the medicaments may be prepared by conventional methods with conventional pharmaceutical excipients.
The Linezolid substantially free from compounds of Formula II and III dosage forms described herein may be used in a method for treatment of infection caused by multi-resistant bacteria. The method of treatment includes administering to a mammal in need of treatment a dosage form that includes a therapeutically effective amount of the Linezolid substantially free from compounds of Formula II and III.
v DESCRIPTION OF THE DRAWINGS
Figure 1 shows representative Powder X-ray diffraction pattern of novel polymorphic Form M of Linezolid.
Figure 2 shows representative Differential Scanning Calorimetry (DSC) thermogram of novel polymorphic Form M of Linezolid.
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While the present invention has been described in terms of its specific embodiments, certain modifications and equivalents will be apparent to those skilled in the art and are intended to be included within the scope of the present invention.
EXAMPLE
EXAMPLE-1: PREPARATION OF LINEZOLID POLYMORPHIC FORM M
(R)-5-(azidomethy])-3-(3-fluoro-4-rnorpho]inophenyl) oxazohdin-2-one (100 gm) was added to thioacetic acid (200 ml) at 25°C and resulting reaction mixture was stirred till the completion of reaction. Excess thioacetic acid was removed by vacuum distillation to get oily residue. The oily residue was diluted with methylene chloride and then extracted with aqueous hydrochloric acid followed by the basification with sodium carbonate to get crude Linezolid. Crude Linezolid was dissolved in 1, 2-dimethoxyethane and then aqueous hydrogen peroxide was added and resulting reaction mixture was stirred at 60°C for 4 hours. The reaction mixture was quenched with sodium thiosulphate solution (48 ml) and resulting solids were filtered to get pure Linezolid. The pure Linezolid was dissolved in toluene and acetic acid and provided carbon treatment. The resulting solution was stirred at 95-100°C and resulting solids were filtered at 50°C to get Linezolid polymorphic Form M, which was washed with hot toluene (50 ml) and dried at 60-65°C under reduced pressure. Yield: 60 gm Purity: 99.9% (By HPLC) Compound of Formula II: 0.01 Compound of Formula III: 0.02
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To
The Controller of Patents The patent Office, At Mumbai
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