NOVEL PROCESS FOR THE PREPARATION OF CLOPIDOGREL
BESYLATE
Field of the Invention:
The present invention relates to a novel process for the preparation of clopidogrel besylate. The compound of the invention referred herein, is clopidogrel benzene sulfonic acid chemically known as methyl (+)-(S)-a-(o-chlorophenyl)-6,7-dihydrothieno-[3,2-c]pyridine-5(4H)"acetate benzene sulfonic acid represented as a compound of formula-1.

Clopidogrel acts as an inhibitor of platelet aggregation. It is marketed as an antianginal agent, anti platelet agent and is found to decrease morbid events in people with established atherosclerotic cardiovascular diseases and cerebrovascular diseases. It is used for the treatment or preventing thromboembolic events such as atherosclerosis, myocardial infraction, strokes and vascular death. Presently clopidogrel is marketed as (S)-(+)-clopidogrel bisulfate which is endowed with a few disadvantages.
Background of the invention:
Clopidogrel is disclosed as a active ingredient in EP-A-0 009 802. The therapeutic application of clopidogrel as blood-platelet aggregation inhibiting agent and its preparation is disclosed in US 4,529,596. US Patent No 4,847,265 describes the process for the preparation of the hydrogen sulfate salt of (S)-(+)"Clopidogrel. It discloses that the dextrorotatory enantiomer of formula-1 of clopidogrel has an excellent antiagregant platelet activity; where as the corresponding levorotatory enantiomer of formula-1 is less tolerated of the two enantiomers and is less active. It also describes various other salts of the compound of formula-1, like its hydrochloride, carboxylic acid and sulfonic acid salts, specifically, salts of acetic, benzoic, fumaric, maleic, citric, tartaric, gentisic,

methanesulfonic, ethanesulfonic, benzenesulfonic and laurylsulfonic acids were prepared. However, according to this patent these salts usually precipitated in amorphous form and/or they were hygroscopic, making them difficult to handle in industrial scale.
Polymorphs of clopidogrel bisulfate have been described in US Patent Nos. 6,504,040 and 6,429,210. EP-A-0281459 proposes the use in pharmaceutical formulations of inorganic salts of (S)-(4-)-clopidogrel, in particular (S)-(+)-Clopidogrel hydrogen sulfate. This document also discloses organic salts of clopidogrel, but these are described as amorphous and/or hygroscopic, and difficult to purify. The (S)-(+)-clopidogrel hydrogen sulfate employed in the pharmaceutical formulations has the disadvantage that concentrated sulfuric acid is necessary to prepare it, and that corresponding products have a very strong acidic nature because of the proton. These acidic properties have an adverse effect on the compatibility with many pharmaceutical excipients and the stability of corresponding drug forms.
There is thus a need for stable forms of clopidogrel, which can be purified and which are easy to process with various pharmaceutical excipients such as medicament carriers and additives. Contrary to the teaching of EP-A-0 281 459, it has been found surprisingly that the salt of sulfonic acid with clopidogrel is suitable under certain conditions for preparing pharmaceutical formulations. It is possible to incorporate the salt of a sulfonic acid with clopidogrel into pharmaceutical formulation and especially into pharmaceutical formulations for oral administration.
Clopidogrel besylate amorphous form and crystalline (solvated) forms have been disclosed by Helm in their published application WO 2004 072084 (US 2005/ 0256152, EP 1480985 Bl) and WO 2004 072085. Subsequently, Helm disclosed non solvated forms in their application No US 2005 0203122. It discloses crystalline clopidogrel besylate form-Ill.
New polymorphic amorphous and crystalline forms of clopidogrel mesylate, clopidogrel besylate and clopidogrel tosylate have been disclosed in US application No. 2004/106344.

There is need for a process for the synthesis of clopidogrel benzene sulfonic acid ash which is pure, easy to process with various pharmaceutical excipients such as medicament carriers and additives.
Many chemical substances can crystallize in different crystal forms, a phenomenon known as polymorphism. These crystal forms differ in packing, thermodynamic, spectroscopic, kinetic, surface and mechanical properties. Different polymorphs of the same active pharmaceutical ingredient (API) display distinct physical properties, such as melting point, solubility, dissolution rate, hygroscopic, or stability. The ability to successfully produce and reproduce specific stable polymorphs is intricately correlated with the efficiency and speed of drug development, the robustness of manufacturing process, and - ultimately - the stability and quality of APIs.
Crystallization is a separation and purification technique commonly used in the chemical industry. Crystallization of API is one of the most critical steps during pharmaceutical manufacturing\processes. It improves the purity of the compound and also sets the physical properties which are determining factors for bioavailability, formulation and stability. There are three key crystal attributes that are commonly of interest: morphology (crystal shape), particle size distribution and polymorphic form (crystal form). These key attributes can be controlled during crystallization.
From a pharmaceutical point of view, bioavailability, the ability to process and stability of the product are influenced by the existence of varied physical and chemical properties of these solid-state forms. Particle size (PS) and morphology of active pharmaceutical ingredient (API) critically influences dissolution rate, bioavailability (BA), and bioequivalence of generic formulations, especially for insoluble drugs The increasing prevalence of poorly soluble drugs in development provides notable risk of new products demonstrating low and erratic bioavailability with consequences for safety and efficacy, particularly for drugs delivered by the oral route of administration.
There is a need to develop capabilities to characterize, control and optimize particle properties at every step of the manufacturing process.

In the present invention the size of the particles of the clopidogrel besylate was influenced by the technique and the solvent used for crystallization. It also provided clopidogrel besylate with morphology which is pure, free flowing solid and easy to process with various pharmaceutical excipients such as medicament carriers and additives.
Brief description of the invention:
Accordingly the present invention provides a novel process for the preparation of clopidogrel besylate. The crystalline clopidogrel besylate of the present invention is stable, free flowing solid which is characterized by powder X-ray diffraction pattern with peaks at about 10.8, 11.7, 13.6, 14.3, 16.3, 17.1, 18.0, 21.0, 21.4, 21.9, 22.5, 23.2, 24.5, 28.6 ± 0.2 degrees two theta.
The process of preparation of crystalline clopidogrel besylate comprises of:
a) suspending (S)-(+)-clopidogrel free base in an first organic solvent, heating the reaction mixture,
b) adding benzene sulfonic acid dissolved in a first organic solvent,
c) adding a second organic solvent and slowly cooling the reaction mixture to low temperatures,
d) filtering the solid crystallized out, washing and drying it to obtain crystalline clopidogrel besylate.
In another embodiment of the invention the process of preparation of crystalline clopidogrel besylate comprises of:
a) suspending (S)-(+)-clopidogrel free base in an organic solvent, heating the reaction mixture,
b) adding benzene sulfonic acid dissolved in a organic solvent, and slowly cooling the reaction mixture to low temperatures,
c) filtering the solid crystallized out, washing and drying it to obtain crystalline clopidogrel besylate.

Brief Description of the Drawings:
Figure-1: Illustrates the powder X-ray diffraction pattern of crystalline clopidogrel
besylate Figure-2: Illustrates the IR spectrum of crystalline clopidogrel besylate Figure-3: Illustrates the DSC of crystalline clopidogrel besylate Figure-4: Illustrates the photographs of crystalline clopidogrel besylate recorded by a
Microscope
Detailed description of the invention:
Accordingly the present invention provides a novel process for the preparation of clopidogrel besylate. The crystalline clopidogrel besylate of the present invention is stable, free flowing solid which is characterized by powder X-ray diffraction pattern with peaks at about 10.8, 11.7, 13.6, 14.3, 16.3, 17.1, 18.0, 21.0, 21.4, 21.9, 22.5, 23.2, 24.5, 28.6 ± 0.2 degrees two theta.
The process of preparation of crystalline clopidogrel besylate comprises of:
a) suspending (S)-(+)-clopidogrel free base in an first organic solvent, heating the reaction mixture,
b) adding benzene sulfonic acid dissolved in a first organic solvent,
c) adding a second organic solvent and slowly cooling the reaction mixture to low temperatures,
d) filtering the solid crystallized out, washing and drying it to obtain crystalline clopidogrel besylate.
The suitable first organic solvent used in step a) and step b) is selected from a group of ester solvents which include but are not limited to methyl acetate, isopropyl acetate, tert-butyl acetate, isobutyl acetate, isopropenyl acetate, n-butyl acetate and the like and their mixtures thereof, preferably isopropyl acetate .The mixture is heated to a temperature in the range of 40-85°C.
The second organic solvent used in step c) is selected from a group of alcohol solvents which include but are not limited to methanol, ethanol, n-propane, isopropanol.

n-butanol, isO butanol, tert-butanol, 1-pentanol, 2-pentanol, isopentanol, hexanes, cyclohexanol and the like and their mixtures thereof, preferably isopropyl alcohol..
In step c) the second organic solvent is added and the reaction mixture is slowly cooled to 25"30°C and stirred at that temperature for 45-60 min.
In step d) the crystalline clopidogrel besylate formed is filtered and dried at 50-60° C for constant weight.
In another embodiment of the invention the process of preparation of crystalline clopidogrel besylate comprises of:
a) suspending (S)-(+)-clopidogrel free base in an organic solvent, heating the reaction mixture,
b) adding benzene sulfonic acid dissolved in a organic solvent, and slowly cooling the reaction mixture to low temperatures,
c) filtering the solid crystallized out, washing and drying it to obtain crystalline clopidogrel besylate.
The suitable organic solvent used in step a) and step b) is selected from a group of ester solvents which include but are not limited to methyl acetate, isopropyl acetate, tart-butyl acetate, isobutyl acetate, isopropenyl acetate, n-butyl acetate and the like and their mixtures thereof, preferably isopropyl acetate. The mixture is heated to a temperature in the range of 40-85°C.
The starting material, Clopidogrel free base can be prepared by the process disclosed in patents US 4847265, EP 0281459 or by any other prior art references. Clopidogrel free base can be prepared from Clopidogrel hydrogen chloride and / or Clopidogrel hydrogen bromide and / or Clopidogrel hydrogen sulfate and / or from Clopidogrel camphor sulphonic acid salt and / or the above salt mixtures thereof as illustrated in Scheme-1.


In the present invention the size of the particles of the clopidogrel besylate was influenced by the technique and the solvent used for crystallization. Change in conditions in the process of crystallization, like the solvent used, the conditions employed, RPM of the stirrer and the sequence of the reaction steps etc., led to the formation of samples with

different particle size. Clopidogrel besylate with finer particles was obtained when an alcoholic solvent was used. But clopidogrel besylate with coarse particles was obtained when crystallization was carried out using ester solvent like isopropyl acetate. The use of minute quantities of alcohol along with ester solvent like isopropyl acetate induced formation of coarse particles which are uniform in size. The particle size of the clopidogrel besylate can be reduced by conventional methods.
The photographs of crystalline clopidogrel besylate recorded on a microscope show that the crystals of clopidogrel have hexagonal to irregular platelet shaped morphology. Clopidogrel besylate formed in the present invention is highly pure, free flowing solid and easy to handle during formulation as an active pharmaceutical ingredient. It has a greater advantage over the prior art forms.
Analysis of particle size distribution of clopidogrel besylate:
A Malvem laser diffraction instrument was used to characterize the particle size distribution of crystalline clopidogrel besylate. Instrument model: Malvem Mastersizer 2000 Technique used: wet method Instrument parameters:
i) Model: General purpose
ii) Handling unit: Hydro2000 S (A)
iii) Material RI :1.6
iv) Dispersant RI: 1.43
v) Dispersant: liquid paraffin
vi) Sensitivity: Normal
vii) Particle shape : Irregular
viii) Stirrer speed :3000 RPM
FI-IR spectrum of crystalline clopidogrel besylate was recorded on Thermo model Nicolet-380 as KBr pellet.

The thermal analysis of crystalline clopidogrel besylate was carried out on Waters DSC Q-10 model differential scanning calorimeter
Morphology of crystalline clopidogrel besylate was recorded in the following method: The samples are molded on alumina stubs using double adhesive tape, coated with gold using HUS-5GB vacuum evaporator and observed in Hitachi S-520 Scanning Electron Microscope at an accusations voltage of 10 KV.

The preferred embodiment illustrated in the foregoing section this described by way of example to illustrate the process of invention. However, it is not intended to limit the scope of the present invention and several variants of this example would be evident to persons skilled in the art.
EXAMPLE:
EXAMPLE 1: Preparation of crystalline clopidogrel besylate:
The (S)"(+)-clopidogrel free base (116g) was suspended in isopropyl acetate (232 ml). The reaction mixture was heated to 60- 65°C and a solution of benzene sulfonic acid (57.09g dissolved in 232 ml of isopropyl acetate) was added to it. Isopropyl alcohol (116 ml) was added to the reaction mixture. The reaction mixture was cooled to 25-30°C and stirred the mixture at this temperature for 45-60 min. The solid formed was filtered, washed with isopropyl acetate and dried to give the title compound as a crystalline solid. Yield: 145 g. Particle size distribution: D(0.1):99.37; D(0.5):212.21; D(0.9):373.5 ^m.
EXAMPLE 2: Preparation of crystalline clopidogrel besylate:
The (S)-(+)-clopidogrel free base (116 g) was suspended in isopropyl acetate (232 ml). The reaction mixture was heated to 40-45°C and a solution of benzene sulfonic acid (57.09g dissolved in 232 ml of isopropyl acetate) was added. The reaction mixture was cooled to 25-30°C and stirred the mixture at this temperature for 60 min. The solid formed was filtered, washed with isopropyl acetate and dried to give the title compound as a crystalline solid. Yield: 145 g.
Particle size distribution: D(0.1):79.37; D(0.5):201.01; D(0.9):387.23 |am. (Crystalline clopidogrel besylate prepared above was Micronized to provide a sample with mean particle size in the range of below 100 microns, which on further mercerization provided crystalline clopidogrel besylate with a mean particle size in the range of below 50 microns)

We Claim:
1. A process of preparation of crystalline clopidogrel besylate which comprises of:
a) suspending (S)-(+)-clopidogrel free base in an first organic solvent, heating the reaction mixture,
b) adding benzene sulfonic acid dissolved in a first organic solvent,
c) adding a second organic solvent and cooling the reaction mixture to low temperatures,
d) filtering the solid crystallized out, washing and drying it to obtain crystalline clopidogrel besylate.

2. The process of claim 1) wherein the first organic solvent is an ester solvent selected from a group which includes methyl acetate, isopropyl acetate, tert-butyl acetate, isobutyl acetate, isopropenyl acetate, n-butyl acetate and the like and their mixtures thereof.
3. The process of claim 1) wherein the second organic solvent is an alcohol solvent from a group which includes methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butanol, 1-pentanol, 2-pentanol, isopentanol, hexanol, cyclohexanol and the like and their mixtures thereof.
4. The process of claim 1) wherein the first organic solvent is a mixture ester and alcohol solvent selected from a group ester which includes methyl acetate, isopropyl acetate, tert-butyl acetate, isobutyl acetate, isopropenyl acetate and the like; and alcohol solvent which includes methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butanol, 1-pentanol, 2-pentanol, isopentanol, hexanol, cyclohexanol and the like; and their mixtures thereof.
5. Crystalline clopidogrel besylate according to claim 1) with a mean particle size in the range of 160-230 microns and D(v 0.9) in the range of 320-400 microns.
6. A process of preparation of crystalline clopidogrel besylate which comprises of:
a) suspending (S)-(+)-clopidogrel free base in an organic solvent, heating the reaction mixture.

b) adding benzene sulfonic acid dissolved in a organic solvent, and cooling the
reaction mixture to low temporalities,
c) filtering the solid crystallized out, washing and drying it to obtain crystalline
clopidogrel besylate.
7. The process of claim 6) wherein the organic solvent used in step a) and step b) is an ester solvent selected from a group which includes methyl acetate, isopropyl acetate, tert-butyl acetate, isobutyl acetate, isopropenyl acetate, n-butyl acetate and the like and their mixtures thereof.
8. Crystalline clopidogrel besylate according to claim 6) with a mean particle size in the range of 200-260 microns and D(v 0.9) in the range of 370-450 microns.
9. A process of preparation of crystalline clopidogrel besylate which comprises of:
a) suspending (S)-(+)-clopidogrel free base in isopropyl acetate, heating the reaction
mixture,
b) adding benzene sulfonic acid dissolved in a isopropyl acetate,
c) adding a isopropanol and cooling the reaction mixture to low temperatures,
d) filtering the solid crystallized out, washing and drying it to obtain crystalline
clopidogrel besylate.
10. Crystalline clopidogrel besylate having morphology as shown in figure-4.