WO2013008250A2

WO2013008250A2 - Crystalline form of retigabine and processes for mixture of retigabine crystalline modifications

Info

Publication number: WO2013008250A2
Application number: PCT/IN2012/000468
Authority: WO
Inventors: Vishweshwar Peddy; Rajesham Boge; Venkata Ramana Reddy Chimala; Vamsi Krishna Mudapaka; Deepika PATHIVADA; Venu Nalivela; Ramya Kumar; Srividya Ramakrishnan; Srinivasulu Rangineni; Laxmi Charan SAMINENI
Original assignee: Dr.Reddys Laboratories Limited.
Priority date: 2011-07-01
Filing date: 2012-07-02
Publication date: 2013-01-17
Also published as: WO2013008250A3

Abstract

Aspects of the present invention relate to crystalline form of retigabine, process for its preparation and processes for preparation of mixture of known retigabine modifications and pharmaceutical composition comprising thereof.

Description

CRYSTALLINE FORM OF RETIGABINE AND PROCESSES FOR MIXTURE OF RETIGABINE CRYSTALLINE MODIFICATIONS

INTRODUCTION

Aspects of the present invention relate to a crystalline form of retigabine, process for its preparation and processes for the preparation of mixture of known retigabine modifications and pharmaceutical composition comprising thereof.

BACKGROUND

The drug compound having the adopted name "retigabine" has a chemical name 2-amino-4-(4-fluorobenzylamino)-1-ethoxycarbonylaminobenzene and is represented by structure of formula (I).

In the United States of America, the non-proprietary name retigabine has been superseded by ezogabine. Retigabine is a well-characterized white to slightly colored solid and it is the active ingredient in Trobalt^® (Europe) and Potiga^® (US) tablets sold for the treatment of adjunctive treatment of partial onset seizures in patients aged 18 years and older.

U.S. Patent No. 5,384,330 discloses retigabine and its pharmaceutically acceptable salts. In addition, it -discloses their properties as an anti-epileptic, muscle relaxing, fever reducing, peripheral analgesic, or anti convulsive agent. U.S. Patent No. 6,538,151 discloses modification A, modification B and modification C of retigabine and processes for the preparation thereof.

International Application Publication No. WO 2010/105823 A1 discloses retigabine in a non-crystalline form together with a surface stabilizer in the form of a stable intermediate and process for the preparation thereof. International Application Publication No. WO 2011/039369 A1 discloses an amorphous solid mixture of retigabine and at least one pharmaceutically acceptable carrier. Chinese patent application publication CN102241608A discloses crystalline retigabine hemi hydrate having moisture content 2.78 to 2.9%.

International Application Publication No. WO 20 1/039369 A1 discloses the preparation of mixture of crystalline modification A and modification B by dissolving retigabine in methanol and evaporating the solution in a rotary evaporator.

The occurrence of different crystal forms, i.e., polymorphism, is a property of some compounds. A single molecule may give rise to a variety of polymorphs having distinct crystal structures and physical properties, such as PXRD patterns, IR absorption spectra, melting points (MP), TGA curves, DSC curves, and solubility. Polymorphs are different solids having the same molecular structure, yet having distinct physical properties when compared to other polymorphs of the same structure. The discovery of new polymorphs and solvates of a pharmaceutical active compound provides an opportunity to improve the performance of a drug product in terms of its bioavailability or release profile in vivo, or it may have improved stability or advantageous handling properties. Polymorphism is an unpredictable property of any given compound.

Thus, the present invention presents new crystalline form of Retigabine and process for its preparation. It also describes the processes for the preparation of mixture of retigabine modifications.

SUMMARY

In an aspect, the application provides a crystalline form of retigabine, characterized by a PXRD pattern having peaks at about 14.2, 22.2, and 24.1°± 0.2° 2Θ.

In an aspect, the application provides a crystalline form of retigabine, characterized by a PXRD pattern having peaks substantially in accordance with the pattern of Fig.1.

In an aspect, the application provides a process for the preparation of a crystalline form of retigabine, characterized by a PXRD pattern having peaks at about 14.2, 22.2, and 24.1°± 0.2° 2Θ, comprising;

a) providing a solution of retigabine in a solvent selected from alcohols, esters, ethers, halogenated hydrocarbons or mixtures thereof; b) optionally adding the seed material to an anti-solvent;

c) adding the solution obtained in step a) to an anti-solvent or suspension obtained in step b).at below -20°C; and

d) isolating the crystalline form of retigabine.

In an aspect, the application provides a process for the preparation of mixture of retigabine crystalline modification A and modification C, comprising: a) providing a solution of retigabine in a solvent selected from esters and ketones;

b) optionally adding the seed material-1 to the solution obtained in step a); c) adding anti-solvent to the solution obtained in step a) or to the slurry obtained in step b);

d) adding the seed material-2 to the slurry obtained in step c) at 40 to 80°C; and

e) isolating the mixture of retigabine crystalline modification A and modification C.

In an aspect, the application provides a process for the preparation of mixture of retigabine crystalline modification A and modification B, comprising: a) providing a solution of retigabine in a solvent selected from ester and alcohols;

b) adding the solution obtained in step a) to an anti-solvent at - 5 to 35°C; and

c) isolating the mixture of retigabine crystalline modification A and modification B.

In an aspect, the application provides a process for the preparation of mixture of retigabine crystalline modification A and modification B, comprising: a) providing a solution of retigabine in methyl ethyl ketone;

b) adding the solution obtained in step a) to methylcyclohexane at -15 to 35°C; and

In an aspect, the application provides a process for the preparation of mixture of retigabine crystalline modification B and modification C, comprising: a) providing a solution of retigabine in a solvent selected from toluene and methyl ethyl ketone;

b) adding the solution obtained in step a) to an anti-solvent at -15 to 35°C; and

c) isolating the mixture of retigabine crystalline modification B and modification C.

wherein the anti-solvent is n-pentane and methylcyclohexane when toluene is used as a solvent and n-heptane when methyl ethyl ketone is used as a solvent.

In an aspect, the present invention provides a pharmaceutical composition comprising the new crystalline form of retigabine or mixture of crystalline modifications of retigabine produced according to the processes of the present application together with one or more pharmaceutically acceptable excipients.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig: 1 is an illustration of a powder X-ray diffraction (PXRD) pattern of a crystalline form of retigabine prepared according to Example 1.

Fig. 2 is an illustration of a powder X-ray diffraction (PXRD) pattern of mixture of retigabine modification A and modification C prepared according to Example 7.

Fig. 3 is an illustration of a powder X-ray diffraction (PXRD) pattern of mixture of retigabine modification A and modification B prepared according to Example 14.

Fig. 4 is an illustration of a powder X-ray diffraction (PXRD) pattern of mixture of retigabine modification B and modification C prepared according to Example 21.

DETAILED DESCRIPTION

In an aspect, the application provides a crystalline form of retigabine, characterized by a PXRD pattern having peaks at about 14.2, 22.2, and 24. ± 0.2° 2Θ.

In an aspect, the application provides a crystalline form of retigabine, characterized by a PXRD pattern having peaks at about 14.2, 22.2, and 24.1 °± 0.2° 2Θ. In an aspect, the crystalline form of retigabine is further characterized by peaks at about 21.3 and 23.4° ± 0.2° 2Θ. In an aspect, the application provides a. crystalline form of retigabine, further characterized by a PXRD pattern having peaks at about 5.0, 10.0, 15.1 , 18.9, and 19.3 + 0.2° 2Θ.

In an aspect, the crystalline form of retigabine characterized by a PXRD pattern having peaks located substantially as shown in Fig.1.

In an aspect, the crystalline form of retigabine as described above can be designated as Form D or modification D.

In an aspect, the application provides a process for the preparation of a crystalline form of retigabine (modification D), characterized by a PXRD pattern having peaks at about 14.2, 22.2, and 24.1°± 0.2° 2Θ, comprising;

(a) providing a solution of retigabine in a solvent selected from alcohols, esters, ethers, halogenated hydrocarbons or mixtures thereof;

(b) optionally adding the seed material to an anti-solvent;

(c) adding the solution obtained in step a), to an anti-solvent or suspension obtained in step b), at below -20°C; and

(d) isolating the crystalline form of retigabine.

Step a) involves providing a solution of retigabine in a solvent or mixture of solvents. Providing a solution in step a) includes:

i) solution obtained from reaction mixtures in the final stage of processes for preparing the retigabine or

ii) dissolving retigabine in a suitable solvent or mixture of solvents.

In embodiments of step a) retigabine can be dissolve in a suitable solvent to provide a solution. Suitable solvents that may be used in step a) include, but are not limited to, alcohol solvents; ester solvents; ether solvents; halogenated hydrocarbon solvents; or mixtures thereof. Preferably the solvents used for providing a solution of retigabine can be selected from isopropyl alcohol, ethyl acetate, propyl acetate, 2-methyl tetrahydrofuran, cyclopentyl methyl ether, dichloromethane or mixtures thereof.

In embodiment of step a), the solution of retigabine can be prepared at any suitable temperatures, such as from about 20°C to about the reflux temperature of the solvent used for dissolution, as long as a clear solution of retigabine is obtained without affecting its quality. Mixing may be used to reduce the time required for the dissolution process. In embodiments, a solution of retigabine may be filtered to make it clear, free of undissolved particles. In embodiments, the obtained solution may be optionally treated with a decojorizing agent or an adsorbent material, such as carbon and/or hydrose, flux-calcined diatomaceous earth (Hyflo), or any other suitable material to remove colored components, etc., before filtration.

In embodiment of step b), optionally the seed material can be added to the anti-solvent to initiate the nucleation of crystalline form. In embodiment, the seed material may be either in micronized or unmicronized form. In embodiments the seed material can be prepared according to the procedure as described in this application for the preparation of crystalline form of retigabine. In embodiments, the seed material can be added to the anti-solvent before the addition of the solution obtained in step a) or after the addition of the solution obtained in step a). Preferably the seed material can be added to the anti-solvent before the addition of the solution obtained in step a). In embodiment, the seed material can be added to the anti-solvent at temperature from about -80°C to about -20°C, preferably about -70°C to about -20°C.

In embodiment of step c), the solution obtained in step a), can be added to an anti-solvent or a suspension obtained in step b). Preferably, the anti-solvent can be hydrocarbon. More preferably, the examples of anti-solvents include, but are not limited to: saturated or unsaturated, linear or branched, cyclic or acyclic hydrocarbons, such as n-pentane, n-hexane, n-heptane, petroleum ether or any mixtures thereof.

In embodiment of step c), the solution obtained in step a), can be added to an anti-solvent or a suspension obtained in step b), at temperature from about - 80°C to about -20°C, preferably about -70°C to about -20°C. In embodiments, the anti-solvent or suspension obtained in step b) can be cooled to at temperature from about -80°C to about -20°C, preferably about -70°C to about -20°C before the addition of the solution obtained in step a). In embodiments, the retigabine solution obtained in step a), can be added to the anti-solvent or suspension obtained in step b) in one lot or more lots. Preferably the retigabine solution obtained in step a) can be added in one lot. In embodiments, the slurry comprising precipitated crystalline retigabine can be maintained at any suitable temperatures, such as from about -80°C to about -20°C for 10 minutes to 5 hours or longer. ln embodiments of step d), crystalline retigabine can be isolated using any techniques, such as decantation, filtration by gravity or suction, centrifugation, or the solvent can be evaporated from the mass to obtain the desired product, and optionally the solid can be washed with a solvent, such as an anti-solvent or the solvent, to reduce the amount of entrained impurities. In embodiments, the crystalline retigabine can be isolated by filtering the slurry comprising precipitated retigabine.

In embodiments, crystalline retigabine that is isolated can be dried at suitable temperatures such as 25°C to about 70°C under atmospheric or reduced pressures, for about 10 minutes to about 50 hours, or longer, using any types of drying equipment, such as a tray dryer, vacuum oven, air oven, fluidized bed dryer, spin flash dryer, flash dryer, or the like. Drying temperatures and times will be sufficient to achieve desired product purity.

Fig. 1 provides the PXRD pattern of crystalline form of retigabine obtained by the process of the present application. The crystalline form of retigabine as described in Fig. can be designated as Form D or modification D.

In embodiments, the crystalline form of retigabine as described above can be used in the pharmaceutical composition together with one or more pharmaceutical excipients.

In embodiments, the starting material which can be used for the preparation of Form D or modification D of retigabine as described above can be amorphous or mixture of crystalline and amorphous forms or modification A or modification B or modification C or mixture of modification A and modification B or mixture of modification B and modification C or mixture of modification A and modification C or mixture of modification A, modification B and modification C.

b) optionally adding the seed material-1 to the solution obtained in step a); c) adding anti-solvent to the solution obtained in step a) or to the slurry obtained in step b); d) adding the seed material-2 to the slurry obtained in step c) at 40 to 80°C; and

ii) dissolving retigabine in a suitable solvent or mixture of solvents.

In embodiments of step a), retigabine can be dissolved in any suitable solvent to provide a solution. Preferably, suitable solvents include but are not limited to: esters, such as methyl acetate, ethyl acetate, methyl formate, ethyl formate, propyl acetate, isopropyl acetate, or butyl acetate; ketones, such as acetone, methyl ethyl ketone, methyl isobutyl ketone, or diethyl ketone; or any mixtures thereof.

In embodiments of step a), a solution of retigabine can be prepared at any suitable temperatures, such as from about 20°C to about the reflux temperature of the solvent used for dissolution, as long as a clear solution of retigabine is obtained without affecting its quality. Mixing may be used to reduce the time required for the dissolution process. In embodiments, a solution of retigabine may be filtered to make it clear and free of undissolved particles. In embodiments, the obtained solution may be optionally treated with a decolorizing agent or an adsorbent material, such as carbon and/or hydrose, flux-calcined diatomaceous earth (Hyflo), or any other suitable material to remove colored components, etc., before filtration.

In embodiment of step a), a small amount of base can be added to the retigabine solution. The base can be added to the solution of retigabine or before making the solution. The bases may be selected from organic or inorganic bases, preferably organic bases. The bases that may be useful included but are not limited to triethylamine, Ν,Ν-dimethylethanolamine, N,N-diethy!ethanolamine, 4- ethylmorpholine, N-methylmorpholine, dimethylaminopyridine, diisopropylamine, diisopropylethylamine, or the like. The use of base in this step may be useful to remove certain impurities if present in the retigabine sample. The amount of base can be from 0.01 to 0.0001 moles with respect to one mole of retigabine.

In embodiment of step b), optionally the seed material-1 can be added to the retigabine solution obtained in step a) to initiate nucleation of modification A. In embodiment, the seed material-1 may be reitgabine modification A or a mixture of modification A and modification C. In embodiment, the seed material-1 may be either in micronized or unmicronized form. In embodiment, the seed material-1 can be added at temperature about 20 to about 40°C, preferably about 25 to about 35°C.

In embodiment of step c), precipitation of retigabine can be obtained by combining the retigabine solution obtained in step a) or retigabine suspension/slurry obtained in step b) with an anti-solvent. Preferably, examples of anti-solvents include, but are not limited to: saturated or unsaturated, linear or branched, cyclic or acyclic hydrocarbons. More preferably the anti- solvents can be selected from n-pentane, n-hexane, n-heptane, cyclohexane, methylcyclohexane, or any mixtures thereof.

In embodiments of step c), an anti-solvent can be added to retigabine solution obtained in step a) or retigabine suspension/slurry obtained in step b). In embodiments, the anti-solvent can be added to the retigabine solution or slurry in one lot or in many lots. In a preferred embodiment, anti-solvent can be added slowly to the retigabine solution/slurry. In embodiments, anti-solvent can be added to retigabine solution/slurry at a temperature from about 10°C to about 50°C. Preferably the anti-solvent may be added to the reitgabine solution/slurry at a temperature from about 20°C to about 40°C.

In embodiments of step c) the ratio of the solvent to anti-solvent can be used from about 1 :1 to about 1 :20.

In embodiments of step c), slurry comprising precipitated retigabine can be heated to about 40°C to about 80°C. In a preferred embodiment slurry comprising precipitated retigabine can be heated to about 45°C to about 70°C or from about 50°C to about 65°C or from about 55°C to about 60°C.

In embodiments of step d), the retigabine slurry obtained in step c) can be combined with the seed material-2. The seed material-2 can be crystalline modification C or mixture of modification A and modification C. In embodiments, the seed material-2 may be added in solid form or by making slurry in anti-solvent. The seed material-2 may be added to the retigabine slurry at a temperature from about 40°C to about 80°C. Preferably the seed material-2 may be added at a temperature from about 55°C to about 60°C. The seed material-2 may be either in micronized or unmicronized form. The seed material-2 used in the present application can be prepared from any of the process known in the art or according to the procedure described in the present application. The temperature during this step plays critical role in obtaining the ratio of mixture of modification A and modification C.

In embodiments of step d), slurry comprising precipitated retigabine can be maintained for about 10 minutes to about 10 hours, or longer at a temperature from about 40°C to about 80°C, preferably at a temperature from about 55°C to about 60°C.

In embodiments of step e), the crystalline retigabine mixture of modification A and modification C can be isolated using any techniques, such as decantation, filtration by gravity or suction, centrifugation, or the solvent can be evaporated from the mass to obtain the desired product, and optionally the solid can be washed with a solvent, such as an anti-solvent or the solvent, to reduce the amount of entrained impurities. In embodiments, crystalline retigabine mixture of modification A and modification C can be isolated by filtering the slurry comprising precipitated retigabine.

In embodiments of step e), retigabine mixture of modification A and modification C that is isolated can be dried at suitable temperatures such as about 25°C to about 80°C under atmospheric or reduced pressures, for about 10 minutes to about 50 hours, or longer, using any types of drying equipment, such as a tray dryer, vacuum oven, air oven, fluidized bed dryer, spin flash dryer, flash dryer, or the like. Drying temperatures and times will be sufficient to achieve desired product purity.

The mixture of retigabine modification A and modification C as described herein contains from about 1% to about 99% by weight of modification A or about 1% to about 99% by weight of modification C. According to one embodiment, the mixture of retigabine modification A and modification C as described herein contains from about 10% to about 90% by weight of modification A or about 10% to about 90% by weight of modification C.

Fig. 2 provides the PXRD pattern of retigabine mixture of modification A and modification C obtained by the process of the present application.

During the development of retigabine mixture of modification A and modification C, it was found that the ratio of modification A and modification C that results from the processes of the present application also depends on the other parameters such as particle size of seed material-2, speed (rate) of addition of anti-solvent in step c), and stirring rate (rotations per minute, RPM). The following table provides the effect of these parameters on the ratio of mixture of modification A and modification C of the resulted product. The content of modification C in the obtained mixture of modification A and modification C are after one hour of stirring at the given temperature.

Table-1

Temperature Seed used Addition rate of RPM Content of in step (d) anti-solvent modification C

(in %)

Fast High 21

Slow low 2

56°C

Fast low 22

Unmicronized

Slow high 16 (coarse)

Fast high 73 d₉o>20p

Slow low 29

60°C

Fast low 47

Slow high 59

Table-2

(in %)

Fast High 95

Micronized Slow low 33

60°C

d₉o<20p Fast low 93

Slow high 73 Fast: anti solvent is added at one lot.

Slow: anti solvent is added in about 1 hour.

RPM: Rotations per minute

High: 60-96 RPM in the reactor

Low: 30-50 RPM in the reactor

In embodiments, the starting material which can be used for the preparation of mixture of modification A and modification C as described above can be amorphous or mixture of crystalline and amorphous form or modification D of the present application or modification A or modification B or modification C or mixture of modification A and modification B or mixture of modification B and modification C or mixture of modification A, modification B and modification C.

ii) dissolving retigabine in a suitable solvent or mixture of solvents.

In embodiments of step a), retigabine can be dissolved in any suitable solvent to provide a solution. Suitable solvents include but are not limited to: esters, such as methyl acetate, ethyl acetate, methyl formate, ethyl formate, propyl acetate, isopropyl acetate, or butyl acetate; alcohols, such as methanol, ethanol, ethylene glycol, 1-propanol, 2-propanol, 2-methoxyethanol, 1-butanol, 2- butanol, i-butyl alcohol, t-butyl alcohol, 2-ethoxyethanol, or any mixtures thereof.

In embodiments of step a), a solution of retigabine can be prepared at any suitable temperatures, such as from about 20°C to about the reflux temperature of the solvent used for dissolution, as long as a clear solution of retigabine is obtained without affecting its quality. Mixing may be used to reduce the time required for the dissolution process. In embodiments, a solution of retigabine may be filtered to make it clear and free of undissolved particles. In embodiments, the obtained solution may be optionally treated with a decolorizing agent or an adsorbent material, such as carbon and/or hydrose, flux-calcined diatomaceous earth (Hyflo), or any other suitable material to remove colored components, efc, before filtration.

In embodiment of step b), precipitation of retigabine can be obtained by adding the solution obtained in step a) to an anti-solvent. Preferably the anti- solvent is hydrocarbon. More preferably the anti-solvents include, but are not limited to: saturated or unsaturated, linear or branched, cyclic or acyclic hydrocarbons, such as n-pentane, n-hexane, n-heptane, cyclohexane, methylcyclohexane, or any mixtures thereof. In embodiments, retigabine solution can be added to the anti-solvent in one lot or in many lots. In a preferred embodiment, retigabine solution may be added to the anti-solvent slowly. In embodiments, retigabine solution may be added to the anti-solvent at a temperature from about -15°C to about 35°C. Preferably, retigabine solution may be added to the anti-solvent at a temperature from about -10°C to about 30°C.

In embodiments of step b) the ratio of the solvent to anti-solvent can be used from about 1 :1 to about 1 :20.

In embodiments of step b), slurry comprising precipitated retigabine can be maintained for about 10 minutes to about 10 hours, or longer at a temperature from about -15 to about 35°C, preferably at a temperature from about -10°C to about 30°C.

In embodiments of step c), the crystalline retigabine mixture of modification A and modification B can be isolated using any techniques, such as decantation, filtration by gravity or suction, centrifugation, or the solvent can be evaporated from the mass to obtain the desired product, and optionally the solid can be washed with a solvent, such as an anti-solvent or the solvent, to reduce the amount of entrained impurities.

In embodiments, crystalline retigabine mixture of modification A and modification B can be isolated by filtering the slurry comprising precipitated retigabine. ln embodiments of step c), retigabine mixture of modification A and modification B that is isolated can be dried at suitable temperatures such as about 25°C to about 80°C under atmospheric or reduced pressures, for about 10 minutes to about 50 hours, or longer, using any types of drying equipment, such as a tray dryer, vacuum oven, air oven, fluidized bed dryer, spin flash dryer, flash dryer, or the like. Drying temperatures and times will be sufficient to achieve desired product purity.

The mixture of retigabine modification A and modification B as described herein contains from about 1% to about 99% by weight of modification A or about 1 % to about 99% by weight of modification B. According to one embodiment, the mixture of retigabine modification A and modification B as described herein contains from about 10% to about 90% by weight of modification A or about 10% to about 90% by weight of modification B.

Fig. 3 provides the PXRD pattern of mixture of retigabine modification A and modification B obtained by the process of the present application.

In embodiments, the starting material which can be used for the preparation of mixture of modification A and modification B as described above can be amorphous or mixture of crystalline and amorphous forms or modification D of the present application or modification A or modification B or modification C or mixture of modification A and modification C or mixture of modification B and modification C or mixture of modification A, modification B and modification C.

In embodiments of step a) retigabine can be dissolved in methyl ethyl ketone to provide a solution. In embodiments, a solution of retigabine can be prepared at any suitable temperatures, such as from about 20°C to about the reflux temperature of the solvent used for dissolution, as long as a clear solution of retigabine is obtained without affecting its quality. Mixing may be used to reduce the time required for the dissolution process. In embodiments, a solution of

retigabine may be filtered to make it clear and free of undissolved particles. In

embodiments, the obtained solution may be optionally treated with a decolorizing

agent or an adsorbent material, such as carbon and/or hydrose, flux-calcined

diatomaceous earth (Hyflo), or any other suitable material to remove colored

components, etc., before filtration.

In embodiment of step b), precipitation of retigabine can be obtained by

adding the solution obtained in step a) to methylcyclohexane. In embodiments,

retigabine solution obtained in step a) can be added to methylcyclohexane in one

lot or in many lots. In a preferred embodiment, retigabine solution may be added

to methylcyclohexane slowly. In embodiments, retigabine solution may be added

to methylcyclohexane at a temperature from about -15°C to about 35°C.

Preferably, retigabine solution may be added to methylcyclohexane at a

temperature from about -10°C to about 30°C.

In embodiments of step b) the ratio of the methyl ethyl ketone to

methylcyclohexane can be used from about 1 :1 to about 1:20.

In embodiments of step b), slurry comprising precipitated retigabine can be

maintained for about 10 minutes to about 10 hours, or longer at a temperature

from about -15 to about 35°C, preferably at a temperature from about -10°C to / about 30°C.

In embodiments of step c), the crystalline retigabine mixture of modification

A and modification B can be isolated and dried by using the procedures known in

the art or the procedures described in this application.

In an aspect, the application provides a process for the preparation of

mixture of retigabine crystalline modification B and modification C, comprising:

a) providing a solution of retigabine in a solvent selected from toluene and

methyl ethyl ketone;

b) adding the solution obtained in step a) to an anti-solvent at -15 to 35°C;

and

c) isolating the mixture of retigabine crystalline modification B and

modification C.

wherein the anti-solvent is n-pentane and methylcyclohexane when toluene is

used as a solvent and n-heptane when methyl ethyl ketone is used as a solvent. Step a) involves providing a solution of retigabine in a solvent or mixture of solvents. Providing a solution in step a) includes:

ii) dissolving retigabine in a suitable solvent or mixture of solvents.

In embodiments of step a), retigabine can be dissolved in any suitable solvent to provide a solution. In embodiments, retigabine can be dissolved in toluene or methyl ethyl ketone to provide a solution.

In embodiment of step b), precipitation of retigabine can be obtained by adding the solution obtained in step a) to an anti-solvent. In embodiments, wherein the anti-solvent is n-pentane and methylcyclohexane when toluene is used as a solvent and n-heptane when methyl ethyl ketone is used as a solvent. In embodiments, retigabine solution can be added to the anti-solvent in one lot or in many lots. In a preferred embodiment, retigabine solution may be added to the anti-solvent slowly. In embodiments, retigabine solution may be added to the anti- solvent at a temperature from about -15°C to about 35°C. Preferably, retigabine solution may be added to the anti-solvent at a temperature from about -10°C to about 30°C.

In embodiments of step c), the crystalline retigabine mixture of modification B and modification C can be isolated using any techniques, such as decantation, filtration by gravity or suction, centrifugation, or the solvent can be evaporated from the mass to obtain the desired product, and optionally the solid can be washed with a solvent, such as an anti-solvent or the solvent, to reduce the amount of entrained impurities.

In embodiments, crystalline retigabine mixture of modification B and modification C can be isolated by filtering the slurry comprising precipitated retigabine.

In embodiments of step c), retigabine mixture of modification B and modification C that is isolated can be dried at suitable temperatures such as about 25°C to about 80°C under atmospheric or reduced pressures, for about 10 minutes to about 50 hours, or longer, using any types of drying equipment, such as a tray dryer, vacuum oven, air oven, fluidized bed dryer, spin flash dryer, flash dryer, or the like. Drying temperatures and times will be sufficient to achieve desired product purity.

The mixture of retigabine modification B and modification C as described herein contains from about 1% to about 99% by weight of modification B or about 1% to about 99% by weight of modification C. According to one embodiment, the mixture of retigabine modification B and modification C as described herein contains from about 10% to about 90% by weight of modification B or about 10% to about 90% by weight of modification C.

Fig. 4 provides the PXRD pattern of mixture of retigabine modification B and modification C obtained by the process of the present application.

In embodiments, the starting material which can be used for the preparation of mixture of modification B and modification C as described above can be amorphous or mixture of crystalline and amorphous forms or modification D of the present application or modification A or modification B or modification C or mixture of modification A and modification B or mixture of modification A and modification C or mixture of modification A, modification B and modification C. In embodiments, the new retigabine crystalline modification D of the present application can be used as an intermediate for the preparation of modification A or modification B or modification C or mixture of modification A and modification B or mixture of modification A and modification C or mixture of modification B and modification C or mixture of modification A, modification B and modification C or any other crystalline form or amorphous form.

In embodiments, the mixture of retigabine crystalline forms i.e., mixture of modification A and modification B or mixture of modification A and modification C or mixture of modification B and modification C produced according to the processes of the present application can be used as an intermediate for the preparation of modification A or modification B or modification C or modification D of the present application or any other crystalline form or amorphous form.

It is known from the prior art that retigabine is light sensitive in solid form, since it is described that retigabine is preferably isolated from light on storage. The present inventors have observed that retigabine in solid state, especially retigabine-free base, is susceptible to oxidation upon contact with the atmosphere.

Therefore, there is need to provide an improved packing conditions for retigabine to control the degradation of retigabine.

In an aspect, the application provides improved packing conditions for retigabine which controls the degradation of retigabine comprises of the following steps;

a) packing the retigabine into a clear low-density polyethylene bag which is flushed with nitrogen;

b) placing the above polyethylene bag in a black polyethylene bag along with oxygen absorbent, filled with nitrogen and sealing the bag with vacuum sealer;

c) placing the above polyethylene bag obtained in step b) in a triple laminated bag along with silica pouch (1 pouch, which is previously dried for 3 hours under vacuum at 105°C) and seal it with VNS sealer;

d) placing the above bag obtained in step c) in HDPE container and sealing the container;

e) storing the container in controlled environment chamber at room temperature. In embodiments, retigabine obtained by a method of the present disclosure having a chemical purity greater than about 97%, greater than about 98%, greater than about 99%, greater than about 99.5%, or greater than about 99.9%, by weight, as determined using high performance liquid chromatography (HPLC). In embodiments, the process described in this application also can be useful to enhance the chemical purity of the retigabine. In embodiments, the same process described in this application can be repeated to enhance the chemical purity of the retigabine. In embodiments, retigabine obtained by a method of the present disclosure having a chemical purity greater than about 97%, greater than about 98%, greater than about 99%, greater than about 99.5%, or greater than about 99.9%, by weight, as determined using high performance liquid chromatography (HPLC).

Retigabine obtained according to the process of the present application can be milled or micronized by any process known in the art, such as ball milling, jet milling, wet milling etc., to produce desired particle sizes and particle size distributions. Particle size distributions can be determined using any means, including laser light diffraction equipment sold by Malvern Instruments limited, Malvern, Worcestershire, United Kingdom, Coulter counters, microscopic procedures, etc. The term d(x) means that a particular fraction has particles with a maximum size being the value given; 0.5 represents 50% of the particles and 0.9 represents 90% of the particles.

In another aspect of the application retigabine obtained according to certain processes of the present application has a particle size distribution wherein: mean particle size is less than about 150 μαι or less than about 100 pm; d(0.5) is less than about 200 pm or less than about 25 pm; and d(0.9) is less than about 250 pm or less than about 50 μην In another aspect of the application retigabine obtained by the processes herein described, having a mean particle size of about less than about 00 μηι.

In another aspect of the application retigabine obtained by the processes herein described, having a specific surface area of less than 5 m²/g or about 0.5 m²/g to about 5 m²/g as measured by B.E.T. (Brunauer-Emmett-Teller), preferably from about 0.5 m²/g to about 4 m²/g. - ZU -

In another aspect of the application retigabine obtained by the processes herein described, having bulk density of about 0.1 to 1.0, preferably from about 0.1 to 0.6 and tapped bulk density 0.1 to 1.0, preferably from about 0.2 to 0.8. In another aspect of the application retigabine obtained by the processes herein described, having bulk volume of about less than 5 mLJg or from about 0.5 mL/g to about 5 mlJg, preferably from about 1 mlJg to about 4 mL/g and Hausner ratio of about 1 to about 5, preferably from about 1 to about 4.

In another aspect of the application the crystal particles of retigabine obtained by the processes herein described, can be in needle shape, rod shape, and flake shape or in any other regular shape or mixture of the shapes.

In embodiments, retigabine which is used as the starting material can be prepared by any methods, including methods known in the art or methods described in the present application. In embodiments, retigabine which is used as the starting material can be in any crystalline form, mixture of crystalline form, hydrate, solvate, anhydrous or amorphous.

X-ray powder diffraction patterns described herein were generated using a Bruker AXS D8 Advance powder X-ray diffractometer, with a copper K-alpha radiation source. Generally, a diffraction angle (2Θ) in powder X-ray diffractometry may have an error in the range of ±0.2°. Therefore, the aforementioned diffraction angle values should be understood as including values in the range of about ± 0.2°. Accordingly, the present invention includes not only crystals whose peak diffraction angles in powder X-ray diffractometry completely coincide with each other, but also crystals whose peak diffraction angles coincide with each other with an error of about ± 0.2°. Therefore, in the present specification, the phrase "having a diffraction peak at a diffraction angle (2Θ) ±0.2° of 5.0°" means "having a diffraction peak at a diffraction angle (2Θ) of 4.8° to 5.2°. Although the intensities of peaks in the x-ray powder diffraction patterns of different batches of a compound may vary slightly, the peak relationships and the peak locations are characteristic for a specific polymorphic form. The relative intensities of the PXRD peaks can vary somewhat, depending on factors such as the sample preparation technique, crystal size distribution, various filters used, the sample mounting procedure, and the particular instrument employed. Moreover, instrumental variation and other factors can slightly affect the 2-theta values. Therefore, the term "substantially" in the context of PXRD is meant to encompass that peak assignments can vary by plus or minus about 0.2°. Moreover, new peaks may be observed or existing peaks may disappear, depending on the type of the machine or the settings (for example, whether a filter is used or not).

An aspect of the present application provides pharmaceutical compositions containing a therapeutically effective, amount of retigabine obtained according to the present invention, together with one or more pharmaceutically acceptable excipients. The pharmaceutical compositions comprising retigabine of the application together with one or more pharmaceutically acceptable excipients may be formulated as: solid oral dosage forms, such as, but not limited to: powders, granules, pellets, tablets, and capsules; liquid oral dosage forms such as but not limited to syrups, suspensions, dispersions, and emulsions; and injectable preparations such as, but not limited to, solutions, dispersions, and freeze-dried compositions. Formulations may be in the form of immediate release, delayed release or modified release. Further, immediate release compositions may be conventional, dispersible, chewable, mouth dissolving, or flash melt preparations, and modified release compositions may comprise hydrophilic or hydrophobic, or combinations of hydrophilic and hydrophobic, release rate-controlling substances to form matrix or reservoir systems, or combinations of matrix and reservoir systems. The compositions may be prepared using any one or more of techniques such as direct blending, dry granulation, wet granulation, and extrusion and spheronization. Compositions may be presented as uncoated, film coated, sugar coated powder coated, enteric coated, or modified release coated.

Pharmaceutically acceptable excipients that are useful in the present application include, but are not limited to, any one or more of: diluents such as starches, pregelatinized starches, powdered cellulose, microcrystalline cellulose, dicalcium phosphate, tricalcium phosphate, mannitol, sorbitol, sugar, or the like; binders such as acacia, guar gum, tragacanth, gelatin, polyvinylpyrrolidones, hydroxypropyl celluloses, hydroxypropyl methylcelluloses, pregelatinized starches, or the like; disintegrants such as starches, sodium starch glycolate, pregelatinized starches, crospovidones, croscarmellose sodium, colloidal silicon dioxide, or the like; lubricants such as stearic acid, magnesium stearate, zinc stearate, or the like; glidants such as colloidal silicon dioxide or the like; solubility or wetting enhancers such as anionic, cationic, and neutral surfactants; complex forming agents such as various grades of cyclodextrins and resins; and release rate controlling agents such as hydroxypropyl celluloses, hydroxymethyl celluloses, hydroxypropyl methylcelluloses, ethylcelluloses, methylcelluloses, various grades of methyl methacrylates, waxes, or the like. Other pharmaceutically acceptable excipients that are useful include, but are not limited to, film-formers, plasticizers, colorants, flavoring agents, sweeteners, viscosity enhancers, preservatives, antioxidants, or the like.

DEFINITIONS

The following definitions are used in connection with the disclosure of the present application, unless the context indicates otherwise. In general, the number of carbon atoms present in a given group or compound is designated "C_x-C_y", where x and y are the lower and upper limits, respectively. For example, a group designated as ' ι-Οβ" contains from 1 to 6 carbon atoms. The carbon number as used in the definitions herein refers to carbon backbone and carbon branching, but does not include carbon atoms of the substituents, such as alkoxy substitutions and the like.

The following abbreviations and acronyms are used herein and have the indicated definitions:

An "alcoholic solvent" is an organic compound containing a carbon bound to a hydroxyl group. "Ci-C₆ alcohols" include, but are not limited to, methanol, ethanol, 2-nitroethanol, 2-fluoroethanol, 2,2,2-trifluoroethanol, hexafluoroisopropyl alcohol, ethylene glycol, 1-propanol, 2-propanol, 2-methoxyethanol, 1-butanol, 2- butanol, i-butyl alcohol, t-butyl alcohol, 2-ethoxyethanol, diethylene glycol, 1-, 2-, or 3-pentanol, neo-pentyl alcohol, t-pentyl alcohol, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, cyclohexanol, phenol, glycerol, or the like.

An "aliphatic hydrocarbon or aromatic hydrocarbon" is a liquid hydrocarbon compound, which may be linear, branched, or cyclic and may be saturated or have as many as two double bonds. A liquid hydrocarbon compound that contains a six-carbon group having three double bonds in a ring is called "aromatic." Examples of "C₅-C8 aliphatic or aromatic hydrocarbons" include, but are not limited to, n-pentane, isopentane, neopentane, n-hexane, isohexane, 3-methylpentane, - ά -

2,3-dimethylbutane, neohexane, n-heptane, isoheptane, 3-methylhexane, neoheptane, 2,3-dimethylpentane, 2,4-dimethylpentane, 3,3-dimethylpentane, 3- ethylpentane, 2,2,3-trimethylbutane, n-octane, isooctane, 3-methylheptane, neooctane, cyclohexane, methylcyclohexane, cycloheptane, petroleum ethers, benzene toluene, ethylbenzene, m-xylene, o-xylene, p-xylene, trimethylbenzene, chlorobenzene, fluorobenzene, trifluorotoluene, anisole, and any mixtures thereof.

"Petroleum ether" is a group of various volatile, liquid hydrocarbon mixtures used chiefly as nonpolar solvents. Chemically, it is not ether like diethyl ether, but a light hydrocarbon. Petroleum ether is commonly available as 30 to 40°C, 40 to 60°C, 60 to 80°C, 80 to 100°C, 80 to 120°C, and sometimes 100 to 120°C boiling fractions. The 60 to 80°C fraction is often used as a replacement for hexane. Petroleum ether is mostly used by pharmaceutical companies in the manufacturing process. The 30-40°C fraction of petroleum ether consists mainly of pentane, and is sometimes used instead of pentane due to its lower cost. Petroleum ether should not be confused with the class of organic compounds called ethers, which contain the R-O-R functional group.

An "ester" is an organic solvent containing a carboxyl group -(C=0)-0- bonded to two other carbon atoms. "C₃-C₆ Esters" include, but are not limited to, ethyl acetate, n-propyl acetate, n-butyl acetate, isobutyl acetate, t-butyl acetate, ethyl formate, methyl acetate, methyl propanoate, ethyl propanoate, methyl butanoate, ethyl butanoate, or the like.

An "ether" is an organic solvent containing an oxygen atom -O- bonded to two other carbon atoms. "C_2-6Ether solvents" include, but are not limited to, diethyl ether, diisopropyl ether, methyl t-butyl ether, glyme, diglyme, tetrahydrofuran, 2-methyltetrahydrofuran, ,4-dioxane, dibutyl ether, dimethylfuran, 2-methoxyethanol, 2-ethoxyethanol, anisole, or the like.

A "halogenated hydrocarbon" is an organic solvent containing a carbon bound to a halogen. "Halogenated hydrocarbon solvent" include, but are not limited to, dichloromethane, ,2-dichloroethane, trichloroethylene, perchloroethylene, 1 , ,1-trichloroethane, , ,2-trichloroethane, chloroform, carbon tetrachloride, or the like. A "ketone" is an organic solvent containing a carbonyl group -(C=0)- bonded to two other carbon atoms. "C_3-6Ketones" include, but are not limited to, acetone, ethyl methyl ketone, diethyl ketone, methyl isobutyl ketone, or the like.

A "decolorizing agent" removes colored impurities by adsorption on the surface of the "decolorizing agent" particles. Examples of a "decolorizing agent" include, decolorizing carbon, activated alumina, activated clay, or silica gel.

Certain specific aspects and embodiments of the present application will be explained in greater 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 application in any manner. Variations of the described procedures, as will be apparent to those skilled in the art, are intended to be within the scope of the present application.

EXAMPLES

Example 1 : Preparation of a crystalline form of retigabine (modification D):

Retigabine (10 g) and isopropyl alcohol (450 mL) were charged into a round bottom flask at 27°C. The mixture was heated to 58°C to obtain a solution and filtered to obtain clear solution and cooled to 30°C. N-heptane (1500 mL) was taken in another flask and cooled to -65°C. The clear solution of retigabine in isopropyl alcohol was added slowly to the flask containing n-heptane at -65°C in 15 minutes and stirred for 15 minutes at the same temperature. The precipitated solid was collected by filtration under vacuum and washed with n-heptane (100 mL). The material is dried under suction for 10 minutes and further dried under vacuum at 55°C for 30 minutes. Yield: 9.3 g; HPLC purity: 99.44%; The PXRD pattern is same as shown in Fig. 1.

Example 2: Preparation of a crystalline form of retigabine (modification D):

Retigabine (1 g) and isopropyl alcohol (45 mL) were charged into a round bottom flask at 26°C. The mixture was heated to 60°C to obtain a solution. The solution was filtered at 60°C to obtain clear solution and cooled to 29°C. N-pentane (150 mL) was taken in another flask and cooled to -70°C. The clear solution of retigabine in isopropyl alcohol was added slowly to the flask containing n-pentane at -70°C and stirred for 15 minutes. The precipitated solid was collected by filtration under vacuum and washed with chilled n-pentane (10 mL). The material was dried under suction for 10 minutes and further dried under vacuum at 55°C for 30 minutes. Yield: 0.92 g; HPLC purity: 99.37%; Example 3: Preparation of a crystalline form of retigabine (modification D):

Retigabine (0.5 g) and isopropyl alcohol (22 mL) were charged into a round bottom flask at 27°C. The mixture was heated to 50°C to obtain a solution. The solution was filtered at 50°C to obtain a clear solution and cooled to 27°C. Petroleum ether (125 mL) was taken in another flask and cooled to -37°C. The clear solution of retigabine in isopropyl alcohol was added slowly to the flask containing petroleum ether at -37°C and stirred for 10 minutes at the same temperature. The precipitated solid was collected by filtration and dried under suction. Yield: 0.49 g; HPLC purity: 99.30%; Example 4: Preparation of a crystalline form of retigabine (modification D):

Retigabine modification A (5 g) and ethyl acetate (125 mL) were charged into a round bottom flask at 25°C. The mixture was heated 35°C to obtain a solution and filtered to obtain clear solution. N-heptane (625 mL) was taken in another flask and cooled to -60°C and seed of reitgabine crystalline material (500 mg) was added. The clear solution of retigabine in ethyl acetate was added slowly to the flask containing n-heptane at -60°C and stirred for 25 minutes at -60°C. The precipitated solid was collected by filtration at -60°C and dried under suction. Yield: 4.4 g; Example 5: Preparation of a crystalline form of retigabine (modification D):

Retigabine (5 g), isopropyl alcohol (20 mL) and dichloromethane (80 mL) were charged into a round bottom flask at 25°C. The mixture was stirred to obtain a solution and filtered to obtain clear solution. N-heptane (500 mL) was taken in another flask and cooled to -45°C. The clear solution of retigabine in isopropyl alcohol and dichloromethane was added slowly to the flask containing n-heptane at -45°C and stirred for 25 minutes at -45°C. The precipitated solid was collected by filtration at -45°C and dried under suction. Yield: 4.1 g; Example 6: Preparation of a crystalline form of retigabine (modification D):

Retigabine (mixture of modification A and modification C) (5 g), isopropyl alcohol (25 mL) and ethyl acetate (100 mL) were charged into a round bottom flask at 27°C. The mixture was stirred to obtain a solution and filtered to obtain clear solution. N-heptane (625 mL) was taken in another flask and cooled to -60°C and seed of reitgabine crystalline material (500 mg) was added. The clear solution of retigabine in isopropyl alcohol and ethyl acetate was added slowly to the flask containing n-heptane at -60°C in 15 minutes and stirred for 5 minutes at -60°C. The precipitated solid was collected by filtration at -60°C and dried under suction. Yield: 4.0 g;

Example 7: Preparation of mixture of retigabine modification A and modification C: Retigabine (55 g), triethyl amine (1.1 mL) and ethyl acetate (330 mL) were charged into a round bottom flask at 28°C. The obtained reaction mixture was heated to 46°C to produce a solution. Activated carbon (1.1 g) was added to the solution and stirred for 30 minutes at 46°C. The reaction mass was filtered on hyflo at 46°C and washed with ethyl acetate (110 mL). The seed material of retigabine (mixture of modification A and modification C) (0.55 g) was added to the filtrate at 32°C and stirred for 15 minutes, n-heptane (1650 mL) was added slowly to the filtrate at 28°C in about 1 hour. The reaction mass was heated to 59°C and stirred for 30 minutes. The seed material of retigabine (mixture of modification A and modification C) (2.75 g) was added to the reaction mass at 59°C and stirred for 2 hours at 59°C. The precipitated solid was collected by filtration at 59°C under vacuum and washed with n-heptane (110 mL). The product was dried under vacuum at 32°C for 4 hours 30 minutes! Yield: 52.3 g; HPLC purity: 99.90%; The PXRD pattern of the obtained product was same as shown in Fig. 2.

Example 8: Preparation of mixture of retigabine modification A and modification C: Retigabine (5 g) and methyl isobutyl ketone (65 mL) were charged into a round bottom flask at 28°C and stirred for 15 minutes. The reaction mixture was heated to 42°C to obtain a solution. Activated carbon (250 mg) was added to the solution at 42°C and stirred for 30 minutes and filtered at 42°C on hyflo. The filtrate was transferred to another flask and cooled to 30°C and retigabine modification A (50 mg) was added to the filtrate and stirred for 30 minutes, n-heptane (150 mL) was added slowly to the flask at 26 °C. The reaction mass was heated to 59°C and stirred for 30 minutes at the same temperature. Retigabine modification C (250 mg) was added to the reaction mass at 59°C and stirred for 2 hours 30 minutes. The precipitated solid was collected by filtration under vacuum and washed with n-heptane (10 mL). The material was dried under vacuum at 35°C for 4 hours 30 minutes. Yield: 2.5 g; HPLC purity: 99.79%; Example-9: Preparation of mixture of Retigabine modification A and modification C: Retigabine modification C (5 g) and methyl ethyl ketone (25 mL) were charged into a round bottom flask at 25°C and stirred for 15 minutes. The obtained suspension was heated to 60°C under stirring for 1 hour to obtain a clear solution. The solution was cooled to 25°C and cyclohexane (50 mL) was added slowly under stirring. The reaction mass was further heated to 58°C and stirred for 30 minutes. Retigabine crystalline modification C (0.5 g) was added to the reaction mass at 58°C and stirred for 1 hour at the same temperature. The precipitated solid was collected by filtration at 58°C. The material was dried under vacuum at 55°C for 4 hours. Yield: 2.8 g; HPLC Purity: 99.63%.

Example-10: Preparation of mixture of Retigabine modification A and " modification C: Retigabine modification C (2.5 g) and ethyl acetate (20 mL) were charged into a round bottom flask at 25°C and stirred for 10 minutes. The obtained slurry was heated to 60°C under stirring to obtain a clear solution. The solution was cooled to 25°C and n-heptane (25 mL) was added slowly under stirring. The reaction mass was further heated to 58°C and stirred for 15 minutes.

Retigabine crystalline modification C (0.25 g) was added to the reaction mass and stirred for 5 hour at 58°C. The precipitated solid was collected by filtration at 58°C.

The material was dried under vacuum at 55°C for 8 hours. Yield: 1.2 g; HPLC Purity: 99.57%.

Example-11: Preparation of mixture of Retigabine modification A and modification C: Retigabine (3 g) and methyl acetate (12 mL) were charged into a round bottom flask at 28°C and stirred for 5 minutes. The obtained slurry was heated to 45°C under stirring to obtain a clear solution. The clear solution was cooled to 30°C and n-heptane (80 ml_) added slowly under stirring. The reaction mass was further heated to 58°C and stirred for 1 hour. Retigabine crystalline modification C (150 mg) was added to the reaction mass and stirred for 3 hours at 58°C. The precipitated solid was collected by filtration and washed with n-heptane (10 mL). The product was dried under vacuum at 35°C for 5 hours. Yield: 1.4 g; HPLC Purity: 99.87%. Examples 12-18: Preparation of mixture of retigabine modification A and modification B: Retigabine (500 mg) and a solvent were charged into a round bottom flask. The mixture was stirred to obtain a clear solution. If required, the mixture was heated to obtain a clear solution. The clear solution was added to an anti-solvent at -10 to 30°C. The mass was maintained to precipitate the solid (if required, using cooling) and the formed solid was collected by filtration, washed with a small quantity of anti-solvent, and then dried to produce mixture of retigabine modification A and modification B. The following examples illustrate the preparation of mixture of retigabine modification A and modification B following this procedure.

The Powder X-ray diffraction (PXRD) pattern of mixture of retigabine modification A and modification B obtained in example-14 is in accordance with Figure 3. Examples 19-21 : Preparation of mixture of retigabine modification B and modification C: Retigabine (500 mg) and a solvent were charged into a round bottom flask. The mixture was stirred to obtain a clear solution. If required, the mixture was heated to obtain a clear solution. The clear solution was added to an anti-solvent at -10°C. The mass was maintained to precipitate the solid and the formed solid was collected by filtration, washed with a small quantity of anti- solvent, and then dried to produce mixture of retigabine modification B and modification C. The following examples illustrate the preparation of mixture of retigabine modification B and modification C following this procedure.

The Powder X-ray diffraction (PXRD) pattern of mixture of retigabine modification B and modification C obtained in example-21 is in accordance with Figure 4.

Process for the preparation of Retigabine:

Example 22: Preparation of 2-amino-5-(4-fluorobenzylamino)-nitrobenzene:

2-nitro-p-phenylenediamine (10 g), methanol (50 mL) and N, N- dimethylformamide (50 mL) were charged into round bottom flask at 25°C under stirring and cooled the reaction mass to 20°C for 15 minutes. 4- fluorobenzaldehyde (7.7 mL) was added to the reaction mass at 20°C. The resultant reaction mass was stirred for 30 minutes at 30°C. After completion of the reaction, the reaction mass was cooled to 5°C in 15 minutes. Sodium borohydride (3.45 g) was added portion wise to the reaction mass at 5°C and maintained for 30 minutes at 5°C. The temperature of the reaction mass was further raised to 30°C and maintained for 30 minutes at the same temperature. Water (100 mL) was charged in another flask and cooled to 17°C. After completion of the reaction, reaction mass was added slowly to the water at 17°C. The obtained reaction mass was stirred for 30 minutes at 35°C. The precipitated solid was collected by filtration and washed with water (50 mL) and dried under suction at 35°C. The wet material was further dried at 60°C to afford 15.1 g; of the title compound.

HPLC purity: 99.41%. Example 23: Preparation of retigabine: Dichloromethane (375 mL), 2-amino-5- (4-fluorobenzylamino)-nitrobenzene (75 g), sodium dithionite (225 g) and triethylamine (168.75 mL) were charged into a round bottom flask at 26°C. The reaction mass was stirred and cooled to 0°C. Water (375 mL) was added slowly to the reaction mass at 0 to 5°C in 35 minutes. The temperature of the reaction mass was slowly raised to 34°C in 45 minutes and stirred for 1 hour 45 minutes at the same temperature. After completion of the reaction, water (750 mL) was added to the reaction mass and stirred for 15 minutes at 34°C. Aqueous and organic layers were separated. The aqueous layer was extracted with dichloromethane (187.5 mL) and combined the organic layers (about 1800 g).

The organic layer (600 g) as obtained above was charged into a round bottom flask at 26°C and cooled to -5°C. Dimethylaminopyridine (1.17 g) and potassium carbonate (13.2 g) were charged into the reaction mass at -5°C. The solution of ethylchloroformate (prepared by diluting 8.33 mL of ethylchloroformate in 62.5 mL dichloromethane) was slowly added to the reaction mass at -5°C in 40 minutes and stirred for 45 minutes at the same temperature. After completion of the reaction, Hydrochloric acid 0% solution 125 mL (prepared by diluting 12.5 ml concentrated hydrochloric acid in 125 mL water) was added to the reaction mass at -5°C. The reaction mass was stirred for 45 minutes at 5°C. The temperature of the reaction mass was raised to 25°C and 10% potassium carbonate solution (100 mL) was added and stirred for 20 minutes at 25°C. Aqueous and organic layers were separated. The organic layer was washed with water (125 mL). The activated carbon (1.25 g) was added to the organic layer at 25°C and stirred for 20 minutes. The resulting suspension was filtered on hyflo and washed with dichloromethane (50 mL). Distilled out the dichloromethane under vacuum at 30°C and Isopropyl alcohol (100 mL) was added. The obtained solution was further distilled to remove the traces of dichloromethane at 35°C and cooled to 28°C and stirred for 40 minutes. The precipitated solid product was obtained by filtration and washed with Isopropyl alcohol (37.5 mL). The product was dried under suction at 27°C. The wet solid (15.1 g) and N, N-dimethylformamide (15 mL) were charged into a round bottom flask at 26°C. The obtained suspension was heated to 30 C to get clear solution and Isopropyl alcohol (150 mL) added to the clear solution at 30°C. The obtained suspension was stirred for 15 minutes at 30°C and cooled to 5°C. The resulted slurry was stirred for 30 minutes at 5°C. The solid was obtained by filtration and washed with Isopropyl alcohol (22.5 mL), dried under suction to obtained the title compound. The product was dried under vacuum at 33°C for 5 hours. Yield: 12.7 g; HPLC purity: 99.83%.

N-acetyl impurity: not detected; Desfluoro impurity: 0.017%; 2-fluoro impurity: not detected; 3-fluoro impurity: 0.033%; 1 , 4-Dicarbamate impurity: 0.036%, Methylene Dimer: 0.023%; Dimer- : 0.0 5%; Dimer-2: not detected:

Claims

CLAIMS:

1. A crystalline form of retigabine, characterized by a PXRD pattern having peaks at about 14.2, 22.2 and 24.1 ± 0.2° 2Θ.

2. A crystalline form of retigabine according to claim 1 , further characterized 5 by a PXRD pattern having peaks at about 5.0, 10.0, 15.1 , 18.9, 19.3, 21.3 and

23.4 ± 0.2° 2Θ.

3. A crystalline form of retigabine according to claims 1 and 2, characterized in that it provides PXRD pattern substantially in accordance with Fig.1.

4. A process for preparation of crystalline form of retigabine according to ' 10 claims 1 to 3 comprising:

c) adding the solution obtained in step a), to an anti-solvent or suspension 15 obtained in step b), at below -20°C; and

d) isolating the crystalline form of retigabine.

5. A process according to claim 4 wherein the solvent used in step a) are selected from isopropyl alcohol, ethyl acetate, propyl acetate, 2-methyl tetrahydrofuran, dichloromethane or mixtures thereof.

20 6. A process according to claim 4, wherein the anti-solvent used in step b) is hydrocarbon.

7. A process according to claims 4, 5 and 6, wherein the solution obtained in step a), is added to the anti-solvent or suspension obtained in step b), at a temperature from -70°C to -20°C.

25 8. A process for the preparation of mixture of retigabine crystalline modification A and modification C, comprising:

a) providing a solution of retigabine in a solvent selected from esters and ketones;

b) optionally adding the seed material-1 to the solution obtained in step a);

30 c) adding anti-solvent to the solution obtained in step a) or to the slurry obtained in step b);

d) adding the seed material-2 to the slurry obtained in step c) at 40 to 80°C; and e) isolating the mixture of retigabine crystalline modification A and modification C.

9. A process according to claim 8, wherein the solvent used in step a) are selected from methyl acetate, ethyl acetate, methyl, ethyl ketone, methyl isobutyl ketone.

10. A process according to claim 8, wherein the anti-solvent used in step c) is hydrocarbon.

11. A process according to claims 8, 9 and 10, wherein the seed material-2 in step d) is added at a temperature from 50°C to 70°C.

12. A process for the preparation of mixture of retigabine crystalline modification A and modification B, comprising:

a) providing a solution of retigabine in a solvent selected from ester and alcohols;

13. A process for the preparation of mixture of retigabine crystalline modification A and modification B, comprising:

a) providing a solution of retigabine in methyl ethyl ketone;

14. A process for the preparation of mixture of retigabine crystalline modification B and modification C, comprising:

a) providing a solution of retigabine in a solvent selected from toluene and methyl ethyl ketone;

c) isolating the mixture of retigabine crystalline modification B and modification C. wherein the anti-solvent is n-pentane and methylcyclohexane when toluene is used as a solvent and n-heptane when methyl ethyl ketone is used as a^■< solvent.

15. A pharmaceutical composition comprising the crystalline form of retigabine according to claims 1 to 7 or mixture of crystalline retigabine modifications according to any of the claim 8 to 14 and one or more pharmaceutically acceptable carrier.