FIELD OF THE INVENTION
The present invention relates to an isolated impurity of rimonabant, referred to as "Bis impurity" and removal thereof.
BACKGROUND OF THE INVENTION
Rimonabant of formula I is a selective CB 1 endocannabinoid receptor antagonist and is chemically known as 5-(4-chlorophenyl)-l-(2,4-dichlorophenyl)-4-methyl-7V-(piperidin-l-yl)pyrazole-3-carboxamide.(Formula Removed)
Formula-I
It is indicated for the treatment of obesity, smoking cessation, overweight and related diseases.
Like any synthetic compound, rimonabant can contain extraneous compounds or impurities that can come from many sources. They can be starting materials, byproducts of the reaction, products of side reactions, or degradation products. Impurities in rimonabant or any active pharmaceutical ingredient (API) are undesirable and, in extreme cases, might even be harmful to a patient being treated with a dosage form containing the API. It is also known in the art that impurities in an API may arise from degradation of the API itself, which is related to the stability of the pure API during storage, and the manufacturing process, including the chemical synthesis. Process impurities include unreacted starting materials, chemical
derivatives of impurities contained in starting materials, synthetic by-products and degradation products.
In addition to stability, which is a factor in the shelf life of the API, the purity of the API produced in the commercial manufacturing process is clearly a necessary condition for commercialization. Impurities introduced during commercial manufacturing processes must be limited to very small amounts, and are preferably substantially absent. For example, the ICH Q7A guidance for API manufacturers requires that process impurities be maintained below set limits by specifying the quality of raw materials, controlling process parameters, such as temperature, pressure, time, and stoichiometric ratios, and including purification steps, such as crystallization, distillation, and liquid-liquid extraction, in the manufacturing process.
The product mixture of a chemical reaction is rarely a single compound with sufficient purity to comply with pharmaceutical standards. Side products and byproducts of the reaction and adjunct reagents used in the reaction will, in most cases, also be present in the product mixture. At certain stages during processing of an API, such as rimonabant, it must be analyzed for purity, typically, by High Performance Liquid Chromatography (HPLC) analysis, to determine if it is suitable for continued processing and, ultimately, for use in a pharmaceutical product. The API need not be absolutely pure, as absolute purity is a theoretical idea that is typically unattainable. Rather, purity standards are set with the intention of ensuring that an API is as free of impurities as possible, and, thus, are as safe as possible for clinical use. As discussed above, in the United States, the Food and Drug Administration guidelines recommend that the amounts of some impurities be limited to less than 0.1 percent.
Generally, side products, by-products and adjunct reagents (collectively "impurities") are identified spectroscopically and/or with another physical method, and then associated with a peak position, such as that in a chromatogram, or a spot on a TLC
plate. Thereafter, the impurity can be identified, e.g., by its relative position on the TLC plate and, wherein the position on the plate is measured in cm from the base line of the plate or by its relative position in the chromatogram of the HPLC, where the position in a chromatogram is conventionally measured in minutes between injection of the sample on the column and elution of the particular component through the detector. The relative position in the chromatogram is known as the "retention time."
The retention time can vary about a mean value based upon the condition of the instrumentation, as well as many other factors. To mitigate the effects such variations have upon accurate identification of an impurity, practitioners use the "relative retention time" ("RRT") to identify impurities. It may be advantageous to select a compound other than the API that is added to, or present in, the mixture in an amount sufficiently large to be detectable and sufficiently low as not to saturate the column, and to use that compound as the reference marker for determination of the RRT.
Those skilled in the art of drug manufacturing research and development understand that a compound in a relatively pure state can be used as a "reference standard." A reference standard is similar to a reference marker, which is used for qualitative analysis only, but is used to quantify the amount of the compound of the reference standard in an unknown mixture, as well. A reference standard is an "external standard," when a solution of a known concentration of the reference standard and an unknown mixture are analyzed using the same technique. The amount of the compound in the mixture can be determined by comparing the magnitude of the detector response.
As is known by those skilled in the art, the management of process impurities is greatly enhanced by understanding their chemical structures and synthetic pathways, and by identifying the parameters that influence the amount of impurities in the final product.
The detection or quantification of the reference standard serves to establish the level of purity of the API or intermediates thereof. Use of a compound as a standard requires recourse to a sample of substantially pure compound.
SUMMARY OF THE INVENTION
In one aspect, the present invention provides an isolated bis impurity, 5-(4-Chloro-phenyl)-1 -(2,4-dichloro-phenyl)-4-methyl-1 H-pyrazole-3-carboxylic acid [5-(4-chloro-phenyl)-1 -(2,4-dichloro-phenyl)-4-methyl-1 H-pyrazole-3-carbonyl]-piperidin-1-yl-amide of the formula II:
(Formula Removed)Formula-II
The present invention further provides a process of determining the presence of bis impurity in a sample of rimonabant comprising carrying out HPLC or TLC.
In another aspect, the present invention also provides a process for preparing rimonabant from 5-(4-chlorophenyl)-1 -(2,4-dichlorophenyl)-4-methyl-1 H-pyrazole-3-carboxylic acid having less than about 0.10% area by HPLC of bis impurity which comprises:
(a) reacting 5-(4-chlorophenyl)-l- l-(3,4-dichlorophenyl)-4-methyl- lH-pyrazole-3-
carboxylic acid with thionyl chloride to give 5-(4-chlorophenyl)-l-(2,4-
dichlorophenyl)-4-methyl- lH-pyrazole-3-carbonyl chloride,
(b) reacting 5-(4-chlorophenyl)- l-(2,4-dichlorophenyl)-4-methyl- lH-pyrazole-3-
carbonyl chloride with 1-aminopiperidine in the presence of base to obtain
rimonabant,
(c) converting rimonabant to its hydrochloride salt;
(d) and further converting it to rimonabant.
In yet another aspect, the present invention provides a HPLC method used to determine the presence of bis impurity in a rimonabant sample comprising: combining a rimonabant sample with acetonitrile and diluent to obtain a solution; injecting the obtained solution into a 250.X.4.6 mm, 3.5µm Xterra RP18 (or equivalent) column; eluting the sample from the column at about 90 minutes using mobile phase (gradient) as an eluent, and measuring the bis impurity content in the relevant sample with a UV detector (preferably at a 254 nm wavelength).
DETAILED DESCRIPTION OF THE INVENTION
As used herein, the term "substantially pure" is used in reference to rimonabant containing less than about 0.10% area by HPLC of bis impurity.
The present invention provides an isolated impurity, 5-(4-Chloro-phenyl)-l-(2,4-dichloro-phenyl)-4-methyl-1 H-pyrazole-3-carboxylic acid [5-(4-chloro-phenyl)-1 -(2,4-dichloro-phenyl)-4-methyl-1 H-pyrazole-3-carbonyl]-piperidin-1 -yl-amide of formula II
(Formula Removed)

Formula-II
This impurity, referred to as "bis impurity", contaminates rimonabant of formula I.

(Formula Removed)Formula-I
It can be characterized by data selected from the group consisting of an RRT, 1HNMR, or API-MS with an m/z peak at about 825.07.
Bis impurity may be isolated by column chromatography using a mixture of hexane and ethylacetate as an eluent. Preferably, the eluent contains hexane and ethylacetate gradient 9.9:0.1 to 8:2. Preferably, bis impurity contains about 0% to about 8% area by HPLC of rimonabant.
It is found by the present inventors that bis impurity is formed in scale up batches and can be removed during the course of converting rimonabant to its hydrochloride salt.
The present invention further provides a process of determining the presence of a compound in a sample comprising carrying out HPLC or TLC with bis impurity as a reference marker.
The present invention also provides a process of determining the presence of bis impurity in a sample comprising carrying out HPLC or TLC with the bis impurity.
The present invention provides a process of determining the amount of bis impurity in a rimonabant sample comprising carrying out HPLC or TLC.
In another aspect, the present invention also provides a process for preparing rimonabant from 5-(4-chlorophenyl)-l-(2,4-dichlorophenyl)-4-methyl-lH-pyrazole-3-carboxylic acid having less than about 0.10% area by HPLC of bis impurity.
Specifically, 5-(4-chlorophenyl)-1 -(2,4-dichlorophenyl)-4-methyl-1 H-pyrazole-3-carboxylic acid is converted to acid chloride derivative by reacting with thionyl chloride at a temperature of 20-40°C and further heating at reflux temperature for a period of 3-7 hours. The reaction can be conducted in the presence or absence of suitable organic solvent. Preferably the organic solvent used is aromatic hydrocarbon selected from amongst toluene, xylenes and the like.
The acid chloride so formed is dissolved in halogenated solvent selected from amongst methylene chloride, chloroform, 1,2-dichloroethane, etc. The mixture is cooled to a temperature of about 10-15°C and a solution of 1 -aminopiperidine in the halogenated solvent and a base is added to the mixture. Generally, base may include inorganic or organic base. Typically the inorganic base may include alkali metal carbonates, and bicarbonates. The alkali metal carbonates, and bicarbonates may be sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate and the like, the organic base may include triethylamine, diisopropylethylamine etc. The reaction mixture is stirred at 0-40°C, preferably at 20-30°C. The progress of the reaction is monitored by high performance liquid chromatography. After completion of the reaction, the reaction mass is filtered, washed with demineralized water and dried over anhydrous sodium sulphate to yield rimonabant. The rimonabant so obtained is preferably isolated in the form of the free base. Optionally rimonabant so
obtained is purified by suspending in ethereal solvents such as isopropyl ether, methyl tert-butyl ether. It is observed that in lab scale batches, bis impurity of formula II does not form, but in scale up batches the rimonabant is contaminated with bis impurity in up to 8% area by HPLC.
Rimonabant is further converted to its hydrochloride salt by contacting with alcoholic-hydrochloride at a temperature of 20-40°C in the presence of suitable solvent to crystallize the hydrochloride salt. Rimonabant hydrochloride so formed is further basified with base such as sodium or ammonium hydroxide, triethylamine or an alkali metal carbonate or bicarbonate like sodium or potassium carbonate or bicarbonate. The reaction is further worked up to give highly pure rimonabant having less than about 0.10% area by HPLC of bis impurity.
The inventors also observed that 1-aminopiperidine is very sensitive and easily converts to piperidine even during handing and leads to the formation of impurity of formula III, (Formula Removed)
Formula III
during condensation reaction with 5-(4-chlorophenyl)-l-(2,4-dichlorophenyl)-4-methyl-lH-pyrazole-3-carboxylic acid chloride. This impurity is removed during workup and purification steps.

The present invention provides an HPLC method used to determine the presence and amount of bis impurity in a rimonabant sample comprising: combining a rimonabant sample with acetonitrile and diluent to obtain a solution; injecting the obtained solution into a 250.X.4.6 mm, 3.Sum Xterra RP18 (or equivalent) column; eluting the sample from the column at about 90 minutes using mobile phase (gradient) as an eluent, and measuring the bis impurity content in the relevant sample with a UV detector (preferably at a 254 nm wavelength). Preferably, the eluent used may be a mixture of eluent A and eluent B, wherein the ratio of them varies over the time, i.e. a gradient eluent.
Having described the invention with reference to certain preferred embodiments, other embodiments will become apparent to one skilled in the art from consideration of the specification. The invention is further defined by reference to the following examples describing in detail the preparation of the compound of the present invention. It will be apparent to those skilled in the art that many modifications, both to materials and methods, may be practiced without departing from the scope of the invention.
EXAMPLES
The Analysis of bis impurity in rimonabant sample is done using the HPLC: Column & Packing: Xterra RP18 (or equivalent); 250 mm x 4.6 mm, Eluent A: mixture of water:triethylamine(950:5) and pH adjusted to4.0+ O.OSwith dilute phosphoric acid and make up the volume up to 1000ml with water. Eluent B: Acetonitrile, gradient and stop time: 90 minutes; Equilibrium time: 5 minutes; Flow Rate: 1.0 ml/min; Detector: UV at 254 nm.
The Mobile phase composition and flow rate may be varied in order to achieve the required system suitability.
Example 1: Preparation of Rimonabant
Thionyl chloride (670g) was added to a suspension of 5-(4-chlorophenyl)-l-(2,4-dichlorophenyl)-4-methyl-lH-pyrazole-3-carboxylic acid (1.0 Kg,) in toluene (5.0 It) at 25-28°C. The reaction mass was stirred for 5 hours at reflux temperature and then toluene and thionyl chloride were evaporated to under vacuum. The resulting residue was taken up in toluene (1000 ml) and the solvent was again evaporated under vacuum. The acid chloride so obtained was dissolved in methylene chloride (5.0 It), the mixture was cooled to 10-15°C and was added to a stirred mixture of a solution of 1-aminopiperidine (408g) in methylene chloride (5.0 It) and anhydrous powdered potassium carbonate (720g) at 10-15°C. The reaction mixture was stirred at 25-30°C and progress of the reaction was monitored by high performance liquid chromatography. After completion of the reaction, the reaction mass was filtered and the filtrate was successively washed with demineralized water and 20% brine solution and dried over anhydrous sodium sulphate. The organic layer was evaporated under vacuum to give title compound having purity 93.5%; bis impurity 3.5 %.
Example 2: Purification of Rimonabant
A solution of rimonabant (50g) in dichloromethane (150ml) was acidified with methanolic-hydrochloride (20ml) at 25-30°C which was saturated with isopropyl ether (200ml) to crystallize its hydrochloride salt. The salt was taken in dichloromethane (500ml) and the suspension was basified with 20% aqueous sodium hydroxide (l00ml).The immiscible layers were separated and the aqueous layer was again extracted with dichloromethane. The separated dichloromethane layer was washed with 20% aqueous sodium chloride (150ml) and was concentrated under vacuum to give solid which was stirred with isopropyl ether (200ml) under reflux condition for 2 hours. The reaction mass was cooled, filtered,
washed with isopropyl ether and dried under vacuum to give 35g of purified title compound (yield 75%) having purity 99.76% by HPLC; bis impurity 0.05%
Example 3: Purification of Rimonabant
A solution of rimonabant (50g) in dichloromethane (150ml) was acidified with methanolic-hydrochloride (20ml) at 25-30°C which was saturated with isopropyl ether (200ml) to crystallize its hydrochloride salt. The salt was taken in dichloromethane (500ml) and the suspension was basified with 20% aqueous sodium hydroxide (100ml). The separated dichloromethane layer was washed with 20% aqueous sodium chloride (150ml) and was concentrated under vacuum to give 35g of purified title compound (yield 75%) having purity 99.50% by HPLC; bis impurity 0.07%
Example 4: Isolation of bis impurity
A sample of rimonabant containing bis impurity was purified by flash column chromatography eluting with a mixture of hexane/ethyl acetate gradient 9.9:0.1 to 8.0:2.0 and analyzing the fractions with TLC/ HPLC. Fractions containing bis impurity with purity greater than 97.0% were pooled and the solvent was removed under vacuum to isolate the title compound.
'H-NMR (CDC13) (5): 1.38-1.39 (2H, brs, piperidine protons); 1.60-1.63 (4H,brs, piperidine protons); 2.10 (6H, s, methyl groups); 3.30-3.32 (4H, t, piperidine protons); 6.86-6.90 (4H, m, aromatic protons); 7.01-7.04 (2H, s, aromatic protons); 7.11-7.14 (2H, d, aromatic protons); 7.17-7.21 (4H, m, aromatic protons); 7.23-7.24 (2H, s, aromatic protons) ppm.
APCI-MS (m/z): [M.F.: C39H3oCl635N6O2] 825.07[M+1

WE CLAIM:
1. An isolated 5-(4-Chloro-phenyl)-l-(2,4-dichloro-phenyl)-4-methyl-lH-pyrazole-3-carboxylic acid [5-(4-chloro-phenyl)-1 -(2,4-dichloro-phenyl)-4-methyl-1 H-pyrazole-3-carbonyl]-piperidin-l-yl-amide ( Bis impurity), of the following formula(Formula removed)
Formula II
2. The isolated impurity of claim 1, wherein it contaminates rimonabant.
3. A process of determining the presence of bis impurity in a sample comprising
carrying out HPLC or TLC with bis impurity as reference marker.
4. A process for preparing rimonabant having less than about 0.10% area by HPLC
of bis impurity which comprises:
(b) reacting 5-(4-chlorophenyl)-l-l-(3,4-dichlorophenyl)-4-methyl-lH-pyrazole-3-carboxylic acid with thionyl chloride to give 5-(4-chlorophenyl)-l-(2,4-dichlorophenyl)-4-methyl- lH-pyrazole-3-carbonyl chloride,
(e) reacting 5-(4-chlorophenyl)-1 -(2,4-dichlorophenyl)-4-methyl-lH-pyrazole-3-
carbonyl chloride with 1-aminopiperidine in the presence of inorganic base to
obtain rimonabant;
(f) converting rimonabant to its hydrochloride salt;
(g) and further converting it to rimonabant.
5. The process of claim 4, wherein the process comprises purification of rimonabant
wherein the level of bis impurity is about 5.0 % by HPLC.
6. The process of claim 4, wherein the process comprises, purification of rimonabant
wherein the level of bis impurity is about 1.0 % by HPLC.
7. Rimonabant and its pharmaceutically acceptable salts containing less than
approximately 0.1% area by HPLC of bis impurity of formula II or a salt thereof.