83

CHAPTER-III IMPURITY PROFILE OF RABEPRAZOLE SODIUM

3.1

Introduction Impurities in drugs are the unwanted chemicals that remain with the

active pharmaceutical ingredients (APIs), or develop during formulation, or upon aging of both API and formulation of medicines. The presence of these unwanted chemicals even in small amount may influence the efficacy and safety of pharmaceutical products. Impurity profiling study (i.e., the identity as well as the quantity of impurity in the pharmaceuticals) is now receiving important attention from regulatory authorities. The different pharmacopoeias,

84

such as the British Pharmacopoeia (BP) and the United States Pharmacopoeia (USP), are slowly incorporating limits to allowable levels of impurities presenting in the APIs of formulations. Also, the International Conference on Harmonization (ICH) has published guidelines on impurities in new drug substances,1 products2 and residual solvents.3 In addition, some books have been published covering different aspects of impurities, including the governmental regulations, guidelines, the identification and monitoring of impurities in drug products.4-5 There is significant demand for the impurity/reference standards along with the API reference standards for both regulatory authorities and pharmaceutical companies. A number of recent articles6 have described a designed approach and guidance for isolating and identifying process related impurities and degradation products using mass spectrometry, Nuclear Magnetic Resonance (NMR), High- Performance Liquid Chromatography (HPLC), Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR-MS), and tandem mass spectrometry for pharmaceutical substances.

As

per ICH guidelines on impurities in new drug products,2 identification of impurities below the 0.1% level is not considered to be necessary unless the potential impurities are expected to be unusually potent or toxic. In all the cases, impurities should be qualified. If data are not available to qualify the proposed specification level of an impurity, studies to obtain such data may be needed (when the usual qualification threshold limits given below are exceeded). According to ICH, the < 2g/day 0.1 % or 1 mg per day intake (whichever is lower) > 2g/day 0.05%.7

85

The control of pharmaceutical impurities is currently a critical issue to the pharmaceutical industry. The regulatory-pharmacopoeial aspects of related organic impurities present above the threshold limits in drugs must be identified and quantified by sufficient methods are mandatory. In the majority of cases this limit is 0.1%,

but

drug

registration

authorities,

synthetic

research

chemists,

technologists and customers of bulk drug materials are increasingly interested in impurities/degradation products in the range of 0.01-0.1%. 3.1.1 Source of impurities Synthesis-related impurities can begin from various sources and from various phases of the synthesis of bulk drugs and the preparation of pharmaceutical dosage forms. Degradation products can be formed during the synthesis, isolation of the end product, storage of the bulk drug, during formulation and storage. 3.1.2 Formation of impurities  Formation of impurities due to incomplete reaction  Impurities originating from impurities in the starting material of the synthesis  Impurities originating from solvent of the reaction  Impurities originating from the Catalyst  Formation of impurities due to side reactions  Impurities due to degradation of API (Active pharmaceutical ingredient)  Impurities in Chiral drugs  Impurities formed during formulation and  Impurities while storage

86

3.1.3 Detection of impurities The detection of impurities in drugs is rapidly developing. The main impurities in drugs were identified already in the process development research, production and formulation. The entry of thin-layer chromatography (TLC) and High-performance liquid chromatography (HPLC) in the early 1960‟s respectively and the appearance of sophisticated techniques mainly HPLC-MSMS and HPLC-NMR (MS) in 1990s has created an entirely new revolution in the field of impurity profiling. The primary step of the impurity profile is the detection of the impurities. It is very important to use as many separation techniques with different mechanisms as possible to obtain the number of impurities to be dealt with during the course of impurity profiling. The high performance liquid chromatographic (TLC and HPLC) techniques (possibly both in the normal and reverse phase mode) is mandatory, but supercritical fluid chromatography (SFC) in the case of charged molecules capillary electrophoresis (CE) and for sufficiently volatile and thermally stable materials gas chromatography (GC) can also afford useful data. Another important aspect of the success the detection of all impurities at the 0.015 level if the above-mentioned techniques are used under optimized conditions. The first spectroscopic data which are usually obtained during the course of the application of chromatographic and spectroscopic techniques in drug impurity profiling are the Ultra Violet Spectroscopy (UV spectra) of the impurities. When HPLC or one of the capillary electrophoretic techniques are used for the separation of the impurities, rapid scanning (usually diode-array) UV detectors can produce good-quality UV spectra.

87

3.1.4 Synthesis of impurities Synthesis of the impurities is more difficult task than that of the main component and may involve multi step synthesis. Before synthesizing the impurities the thorough study and understanding of the structure of the impurities is required. The synthesis of a proposed structure on the basis of UV, IR, NMR and mass spectrum is easier and less time consuming than its preparation by preparative HPLC for further NMR studies. After the preparation of impurity, the full spectroscopic and analytical investigation of the synthesized compound will undergo chromatographic and spectral matching with the impurity found in the drug material. The identification of real impurity on the basis of chromatographic

and

on-line

spectral

matching

with

known

potential

impurities should be carried out. After matching the impurity structure the complete structure elucidation should be given. The small quantity of the synthesized impurity can be used as impurity standard. This impurity is used for the development of selective analytical methods for the quantitative determination of the impurity. When such a selective method becomes part of the analytical testing protocol for every batch this impurity standard has to use routinely. Therefore it is necessary to submit this impurity to the drug authorities and customers of bulk drugs, which will be used as impurity standard for regulation analysis. When the synthesis of the impurity is very problematic, moreover impossible. In these exceptional cases the synthesis can be omitted from the protocol of impurity profiling and the impurity standard can be prepared using preparative HPLC. Taking an important view of impurity profile and these

88

stringent levels in Active Pharmaceutical Ingredient, begin to study the impurity levels in rabeprazole sodium 1. In the literature there are six impurities reported for rabeprazole sodium.8-9 The structure of these six impurities are as follows: O

O

OCH3 H N

H3 C

H N

S N

O S

N

N

6 O

OCH3

O

H3 C O S

N

7

N

O

4

H N

OCH3

H3 C

H N

O S

N

N

O

O

OCH3

H3 C

9

N O OCH3

OCH3 O

O H3C

H3 C N H3C

N

S N

8

O

OCH3 N

N H3C S

N

N

O

O

OCH3

N

10

Chart: 3.1 Rabeprazole reported impurities

3.2 Present Work During the preparation of rabeprazole sodium 1 in the laboratory, two unknown impurities were detected in HPLC consistently along with known impurities. These two potential impurities in HPLC, ranging from 0.05 – 0.8 % (Fig: 3.1).

89

Fig: 3.1 HPLC chromatogram of Rabeprazole sodium 1 The traditional approach for the synthesis of rabeprazole sodium 1 as shown in scheme 3.1, with slight modifications to make it simple and commercially viable.10 Reaction of 4-nitro-2,3-dimethylpyridine-1-oxide (11) with POCl3 to give 4-chloro-2,3-dimethylpyridine-1-oxide (12), which up on condensation

with

methoxy

propanol

yielded

4-methoxy

propoxy-2,3-

dimethylpyridine-1-oxide (13), which on further reaction with acetic anhydride give 2-acetoxymethyl-4-(methoxypropoxy)-3-methylpyridine (14). Hydrolysis of 14 with sodium hydroxide obtained 2-hydroxymethyl-4-(methoxypropoxy)-3methyl pyridine (15), which on chlorination with thionyl chloride yielded 2chloromethyl-4-(methoxypropoxy)-3-methylpyridine

hydrochloride

(3).

Condensation of 3 with 2-mercapto-1H-benzimidazole (2) in the presence of aqueous sodium hydroxide yielded

[(4-methoxypropoxy-3-methylpyridine-2-

yl]methyl]-thio]-1H-benzimidazole (4), which on oxidation with afforded

m-CPBA

2-[[[4-methoxypropoxy-3-methyl-2-pyridinyl]methyl]sulphinyl]-1H-

90

bezimidazole (5) free base, further it was converted to sodium salt in the presence of sodium hydroxide and methanol to give rabeprazole sodium 1. Scheme: 3.1 NO2

Cl CH3

N

CH3

POCl3

CH3

O 11

O-(CH2)3-OCH3

CH3

CH3

OH-(CH2)3-OCH3

CH3

N

O-(CH2)3-OCH3

N

O 12

(CH3CO)2O

CH3

O

N 14

O 13

CH3 O

NaOH/H2O

N N H

H3C S

O-(CH2)3-OCH3 N N H 2

m-CPBA

N N H

H3C O S

O-(CH2)3-OCH3

CH3

CH3

NaOH/H2O N

4

O-(CH2)3-OCH3

SH

O-(CH2)3-OCH3 NaOH/MeOH N

5

Cl

N HCl 3

SOCl2 N

OH

15

Na+ H3C N- O S N 1

O-(CH2)3-OCH3

N

3.2.1 Formation of impurities During the preparation of rabeprazole sodium 1, the small amount of unreacted 2-chloromethyl-3-methyl-4-chloro pyridine hydrochloride (12) is carry forward to further reaction and reacts with compound 2 resulted in the formation of

2-[[[4-chloro-3-methyl-2-pyridinyl]methyl]thio]-1H-bezimidazole

19 which undergoes oxidation with m-CPBA to give 2-[[[4-chloro-3-methyl-2pyridinyl]methyl]thio]-1H-bezimidazole 20. The schematic diagram for the formation of impurity as shown in Scheme 3.2. The impurity 21 is formed during the condensation of 4-chloro-2,3dimethylpyridine-1-oxide (12) with 1-methoxy-2-propanol in the presence of

91

dimethyl sulfoxide and NaOH. Traces of methanol present in 1-methoxy-2propanol, also participates in the reaction which results in the formation of 22. The compound 22 is carry forward for further reaction and reacts with compound 2 to form 2-[[(4-methoxy-3-methyl-2-pyridinyl) methyl] thio]-1Hbenzimidazole 26 which undergoes oxidation with m-CPBA to give 2-[[(4methoxy-3-methyl-2-pyridinyl)methyl]

sulphinyl]-1H-benzimidazole

21

(Scheme: 3.3). A comprehensive study was undertaken to synthesize and characterize these impurities by spectroscopic techniques. An impurity profile study is necessary for any final product to identify and characterize all the unknown impurities that are present at levels even below 0.05%. This became essential in the wake of stringent purity requirements from regulatory authorities.

3.2.2 Synthesis of impurity 20 Reaction of compound 12 with acetic anhydride at 120 oC for 5 h give the product 16, which on hydrolysis with aqueous sodium hydroxide and followed by chlorination with thionyl chloride give the compound 18. The compound 18 is condensed with benzimidazole 2 in the presence of aqueous sodium hydroxide to give 19, which is further oxidation with m-CPBA give 2-[[[4-chloro3-methyl-2-pyridinyl]methyl]sulphinyl]-1H-bezimidazole characterized on the basis of its spectral data.

20

which

was

92

Cl

Cl CH3

N

N

CH3

O 12

Cl

CH3

(CH3CO)2O

NaOH/H2O

O

CH3

CH3 O

16

OH

N 17 SOCl2 N

N N H

H3C O S

Cl m-CPBA N

N N H

20

H3C S

Cl

N

N H 2

SH

NaOH/H2O

19

Cl CH3 N HCl

Cl

18

Scheme: 3.2: Formation of impurity 20 Thus, its IR spectrum showed the characteristic absorption at 3350 cm-1 assignable to the tautomeric N-H-group of the imidazole, 1037 cm-1 assignable to the S=O. (Fig: 3.2). In the 1H NMR spectrum (Fig: 3.4) up filed region with signals due to S-alkylated methylene protons show two doublets at δ 4.8. In the positive mode mass spectrum (Fig: 3.3) of 20 M+1 peak at m/z 306 corresponds to the molecular weight, 305 of 20 and thus further confirms its assigned structure. (Scheme: 3.2). 3.2.3 Synthesis of impurity 21 Reaction of compound 12 with sodium hydroxide in methanol at 65 oC give the compound 22, which on reaction with acetic anhydride at 120 oC for 5 h give the product 23, which is on hydrolysis with aqueous sodium hydroxide and followed by chlorination with thionyl chloride give the compound 25. The compound 25 is condensed with benzimidazole 2 in the presence of aqueous sodium hydroxide to give 26, which is on further oxidation with m-CPBA to give 2-[[[4-methoxy-3-methyl-2-pyridinyl]methyl]sulphinyl]-1H-bezimidazole which was characterized on the basis of its spectral data.

21

93

Cl CH3 MeOH/ NaOH N

CH3

OCH3 CH3

N N H

21

23

OCH3 m-CPBA N

N N H

H3C S

OCH3

N

CH3

NaOH/H2O

O

N

O 22

H3C O S

OCH3

CH3

(CH3CO)2O

CH3

N

O 12

OCH3 CH3 O

OH

N 24 SOCl2

N N H 2

SH

NaOH/H2O

26

OCH3 CH3 N HCl

Cl

25

Scheme: 3.3: Formation of Impurity 21

Thus, its IR spectrum showed the characteristic absorption at 3310 cm-1 assignable to the tautomeric N-H group of the imidazole, 1080 cm-1 assignable to the S=O, 1043 assignable to the methoxy group attached to pyridine. (Fig: 3.5). In 1H NMR spectrum (Fig: 3.7) the down field region was characterized by the presence of signals up filed region with signals due to s-alkylated methylene protons show two doublets at δ 4.75. In the positive mode mass spectrum (Fig: 3.6) of 21 M+1 peak at m/z 302 corresponds to the molecular weight, 301 of 21 and thus further confirms its assigned structure. (Scheme: 3.3). The present study describes the synthesis and characterization of the potential impurities of 1. The structures were unambiguously established by independently synthesizing them and co-injecting in HPLC with compound 1, to confirm its presence as an impurity Fig: 3.8 and Fig: 3.9. Based on the spectral data, the two impurities were characterized as 2[[(4-chloro-3-methyl-2-pyridinyl)methyl]sulfinyl]-1H-bezimidazole

(20,

chloro

94

analogue

of

Rabeprazole

1)

and

pyridinyl)methyl]sulfinyl]-1H-benzimidazole

2-[[(4-methoxy-3-methyl-2-

(21,

methoxy

analogue

Rabeprazole 1). Table: 3.1. Molecular weight, chemical structure of the impurities.

S. No

01

Compound

M.W

Rabeprazole

381

Structure

Nature

H3C O

sodium 1

API

O H C Na 3 O N S N

N

H3 C

02

Compound-4

O

343

Process H3 C H N

S

O

Related

N

N

H3 C O

03

Compound-6

O

375 H H3 C O N S N O

04

Compound-7

391

Process related N

Process

of

95

H3C

H N

O H3C S

N

O

related

O

N O

H3C O

05

Compound-9

552

Process

O

related

H H3C O N S N O

N O

O CH3

06

Compound-10

301

Process

O CH3

O CH3

N N N

07

Compound-20

O H3C S

Cl

375

N

Compound-21

O

N

H N

08

related

O S

H3C

Process related

OCH3

305

H N N

O S

H3C

Process related

96

3.3 Experimental section [[[4-Chloro-3-methyl-2-pyridinyl]methyl]sulfinyl]-1H-bezimidazole (20) 2-Acetoxymethyl-4(chloro)-3-methylpyridine (16): A mixture of 4-chloro-2,3dimethylpyridine-1-oxide (12, 17 g, 0.11 mol) and acetic anhydride (33 g, 0.32 mol) were stirred at 120 oC for 5 h, and the excess acetic anhydride was distilled completely at the same temperature. The obtained compound 16 as residue. This residue was used further with out purification. 2-Hydroxymethyl-4(chloro)-3-methylpyridine

(17):

A

mixture

of

2-

acetoxymethyl-4-(chloro)-3-methylpyridine (16,

18 g, 0.09 mol), sodium

hydroxide (15 g, 0.37 mol) and water (150 ml) was stirred at 25 oC for 1 h. The reaction mass was extracted with dichloro methane (2 X 50 mL) and the organic layer was distilled completely. A black residue was obtained, which was extracted with petroleum ether (3 X 50 mL), and excess solvent was distilled, resulting light brown crude of 17. 2-Chloromethyl-4-(chloro)-3-methylpyridinehydrochloride (18): The above crude (17, 14 g, 0.09 mol) and dichloromethane (100 mL), thionyl chloride (15.8 g, 0.13 mol) was added slowly at 25 oC for 1 h. After 1 h, excess thionyl chloride and dichloro methane were distilled under vacuum and the product was isolated on triturating with petroleum ether (50 mL) yielded 18 as a fine solid, 15 g, yield: 80% [[(4-Chloro-3-methylpyridine-2-yl]methyl]-thio]-1H-benzimidazole (19):

To

a mixture of 2-mercapto-1H-benzimidazole (2, 11.8 g, 0.08 mol), sodium hydroxide (14 g, 0.35 mol) and water (100 ml), a solution of 2-chloromethyl-4(chloro)-3-methylpyridine hydrochloride (18, 14 g, 0.065 mol) in water (25 ml)

97

at 25 oC for 3 h. After stirring for1 h at the same temperature, solid was separated, filtered and dried under vacuum (19): 18 g, yield: 94 %. 2[[[4-Chloro-3-methyl-2-pyridinyl]methyl]sulfinyl]-1H-bezimidazole To

a

solution

of

[[(4-chloro-3-methyl

(20):

pyridine-2-yl]methyl]-thio]-1H-

benzimidazole (19, 16 g, 0.05 mol) and dichloromethane (160 ml) was added mchloro per benzoic acid (10.8 g, 0.06 mol) in dichloro methane (100 mL) at -10 to -15 oC for 1 h. The reaction mass was quenched by adding sodium hydroxide (15 g) and water (25 ml). The pH of the reaction mass was adjusted to 8.0-8.5 with acetic acid and was extracted with dichloro methane (3 X 50 mL). The separated organic layer was distilled, and the obtained residue was triturated with petroleum ether at 10 oC until solid separates. The separated solid was filtered, dried and characterized as compound 20. Yield: 10 g (60%). FTIR (KBr) cm-1: 3350 (N-H); 3067, 2985 (Ar-H, C-H); 1558 (C=C), 1037 cm-1 (S=O). 1H

NMR (CDCl3; δ ppm; 400 MHz): 2.37 (s, 3H, CH3), 4.8 (dd, 2H, CH2), 7.30-

7.71 (m, 5H, Ar-H), 8.26 (d, 1H, Ar-H). Mass: (m/z): 306(M++H). 2-[[(4-Methoxy-3-methyl-2-pyridinyl)methyl}-sulfinyl]-1H-benzimidazole (21): 2,3-Dimethyl-4(methoxy)pyridine-1-oxide

(22):

A

mixture

of

sodium

hydroxide (17 g, 0.42 mol), dimethyl sulfoxide (34 mL) and methanol (51 g, 1.59 mol) was stirred at 60 oC for 1 h, 4-chloro-2,3-dimethylpyridine-1-oxide (12, 17g, 0.107 mol) was added slowly at same temperature for 1 h and maintained for 3 h. The reaction mass was quenched with water (100 mL) and

98

extracted with dichloromethane (2 X 75 mL). Dichloromethane was evaporated resulting in 15 g of compound 22 as residue. 2-Acetoxymethyl-4(methoxy)-3-methylpyridine (23): A mixture of 2,3dimethyl-4(methoxy)pyridine-1-oxide (22, 14 g, 0.01 mol ) and acetic anhydride (27.9g, 0.28 mol) was stirred at 120 oC for 5 h. The excess acetic anhydride was distilled completely at same temperature resulting in 15 g of compound 23 as residue. 2-Hydroxymethyl-4(methoxy)-3-methylpyridine

(24):

A

mixture

of

2-

acetoxymethyl-4(methoxy)-3-methylpyridine (23, 14 g, 0.07 mol), sodium hydroxide (14 g, 0.35 mol) and water was stirred at 25 oC for 1 h. The reaction mass was extracted with dichloromethane and organic layer was distilled completely to get the residue and was extracted with petroleum ether. The organic layer was distilled resulting in 10 g, of compound (24), as light brown residue. 2-Chloromethyl-4(methoxy)-3-methylpyridine hydrochloride (25): To a mixture of 2-hydroxymethyl-4(methoxy)-3-methylpyridine (24, 9 g, 0.06 mol) and dichloromethane (70 mL), thionyl chloride (8.4 g, 0.07 mol) was added slowly at 25 oC for 1 h. The excess thionyl chloride and dichloromethane were distilled from the reaction mass and the product 25 was isolated in petroleum ether (50 mL), 11 g, yield: 90%. [[(4-Methoxy-3-methylpyridine-2-yl]methyl]-thio]-1H-benzimidazole

(26):

To a mixture of 2-mercapto-1H-benzimidazole (2, 8.6 g, 0.06 mol), sodium hydroxide (10 g, 0.25 mol) and water (100 ml) was added a solution of 2chloromethyl-4(methoxy)-3-methylpyridine hydrochloride (25, 10 g, 0.05 mol)

99

in water (25 mL) at 25 oC for 3 h. Solid separated after 1 h, was filtered to yield a compound 26, 12 g, yield: 88 %. 2-[[(4-Methoxy-3-methyl-2-pyridinyl)methyl}-sulfinyl]-1H-benzimidazole (21): To a solution of [[(4-methoxy-3-methyl pyridine-2-yl]methyl]-thio]-1Hbenzimidazole (12, 10g, 0.04 mol) and dichloro methane (100 mL) was added m-chloro per benzoic acid (8.5 g, 0.05 mol) in dichloromethane (85 mL) at -10 to -15 oC for 1 h. The reaction mass was quenched by adding sodium hydroxide (12 g), water (25 mL). The pH of the reaction mass was adjusted to 8.0-8.5 with acetic acid and was extracted with dichloromethane (2 X 100 mL). The separated organic layer was distilled and the obtained residue was triturated with petroleum ether at 10 oC until the solid separates. The separated solid was filtered, dried and characterized as compound 21, 6 g, yield: 57%. FTIR (KBr) cm-1: 3310 (N-H), 2967, 2843 (Ar-H), 1587 (C=C), 1080 (S=O), 1043 (C-O-C). 1H

NMR (CDCl3; δ ppm; 400 MHz): 2.13 (s, 3H, CH3), 3.85 (s, 3H, OCH 3),

4.75 (dd, 2H, CH2), 6.96 (d, 1H, Ar-H), 7.29-7.31 (m, 4H, Ar-H), 8.23 (d, 1H, ArH). Mass: (m/z): 302(M++H).

3.4

References

1.

International Conference on Harmonization, Draft Revised

on Impurities in New Drug Substances. 65(140), 2000, 45085.

Q3A(R),

Federal

Guidance Register.,

100

2.

International Conferences on Harmonization, Draft Revised on Impurities in New Drug Substances.

Guidance

Q3B(R),

Federal

Register., 65(139), 2000, 44791. 3.

International Conferences on Harmonization, Impurities-

for Residual solvents.

Guidelines

Q3C, Federal Register.,

62(247), 1997, 67377. 4.

Ahuja S,

Impurities

Evaluation

of

Pharmaceuticals,

New

York,

Marcel Dekker, 1998. 5.

Gorog S, Identification and Determination of Impurities in Drugs, Amsterdam., Elsevier Science Publishing Company, 2000.

6.

Alsante K. M, Hatajik T. D, Lohr L. L, Sharp T. R, American Pharmaceutical Review., 4(1), 2001, 70.

7.

ICH harmonized tripartite guideline, Impurities in new drug substances Q3A (R1), current step 4 version, 2002.

8.

Souda S, Ueda N, Miyazawa S, Tagami K, Nomoto S, Okita M, Shimomura N, Kaneko T, Fujimoto M, Murakami M, Oketani K, Fujisaki H, Shibata H, Wakabayashi T, US 5,045,552, 1991.

9.

Ganta M. R, Boluggodu V. B, Padi P. R, Sudhakar P, Babu J. M, Vyas K, Pingili R. R, Mukkanti

K,

J. Pharm. Biomed. Anal., 43, 2007, 1262. 10.

Sharpless K. B, Thomas V. R, Aldrichimica Acta., 12, 1979, 63.