WO2003027106A1

WO2003027106A1 - Process for the preparation of crystalline polymorph ii of lamivudine

Info

Publication number: WO2003027106A1
Application number: PCT/IN2001/000222
Authority: WO
Inventors: Chirag Surendrabhai Shah; Suresh Devjibhai Sorathiya; Virendra Kumar Agarwal; Venkata Satya Ramakrishna Nirogi; Kanwal Pandita; Pankaj Ramanbhai Patel
Original assignee: Cadila Healthcar Limited
Priority date: 2001-09-25
Filing date: 2001-12-21
Publication date: 2003-04-03

Abstract

A process for the direct preparation of Lamivudine polymorph of Form II is disclosed. The process of the invention involves the direct preparation of Lamivudine polymorph of form II from its salicylate salt thereby avoiding the formation or use of Form I. The process comprises suspending Lamivudine salicylate monohydrate in a solvent selected from the group consisting of aliphatic ketones, esters, either straight chain or branched chain ethers containing 1 - 8 carbon atoms, refluxing the mixture to at the reflux temperature of the solvent, adding an organic base at said reflux temperature, cooling it and filtering out the product Form II of Lamivudine. The process is economical, and is capable of operation on a commercial scale with minimum contamination with Form I polymorph of Lamivudine. The yield obtained is about 93 % with high HPLC purity above 99.5 % which is isolated from the aqueous medium.

Description

PROCESS FOR THE PREPARATION OF CRYSTALLINE POLYMORPH - Π OF LAMIVUDINE

Field of the invention The present invention relates to a process for the preparation of crystalline polymorph - II of Lamivudine. Background of the invention

Lamivudine is a 1, 3 - oxathiolane nucleoside analog. The chemical structure of Lamivudine is (~)cis - 4 - amino - 1 - (2 - hydroxymethyl - 1, 3 - oxathioian - 5 - yl) - (1H) - pyrimidine - 2 - one of the formula I,

HO /^* ""..

CD

Lamivudine is a nucleoside analog and is a reverse transcriptase inhibitor. It is now being investigated for potential use against HIV and Hepatitis B viruses. The molecule has two chiral centers. Lamivudine specifically refers to the (-) enantiomer of the cis racernate and is also known as 3TC.

WO 91/17159 describes the preparation of Lamivudine, its anti-viral activity and its use in medicine. Lamivudine as prepared in this reference is obtained as a freeze dried powder.

Prior art also discloses the preparation of lamivudine by reducing (2R, 5R)-5-(4- amino-2-oxo-2H-pyrimidin-l-yl)-(l,3) oxathiolan-2-carboxylic acid, 2S-isopropyl-5R- methyl-lR-cyclohexyl ester. The product is obtained in aqueous solution and since Lamivudine is highly soluble in water, it is isolated from aqueous solution as insoluble Lamivudine salicylate (1 :1) salt which is obtained as monohydrate. Free Lamivudine is liberated from Lamivudine salicylate monohydrate in non aqueous solvent by treating with organic base.

It has been reported in literature [J. Pharm. Sci., 1996, 85, 193 - 199; US Patent

5905082] that Lamivudine exists in two plymorphs known as polymorphic Form I and Form π. Form I is obtained by crystallisation from aqueous solution, in the form of needle shaped crystals. However, Lamivudine in this form is not suitable for pharmaceutical formulations in the form of solid dosage forms because of their physical properties, such as poor flow characteristics. Modification of the crystals habit using alcohols in which the compound is less soluble results in the formation of the second crystal form (Form π) of preferred morphology. Form II of Lamivudine is a bipyramide with improved flow characteristics and is thus preferred in the manufacture of solid dosage forms. In addition, Form I crystals are a less stable polymorphic form and certain pharmaceutical unit operations such as milling may cause conversion of Form I to Form II which is an undesirable characteristic for manufacture of solid dosage forms, Lamivudine in the form of bipyramidyl crystals has a melting point of 177 - 178°C when pure and a melting point of 124 - 127°C when in the form of needle shaped crystals.

Lamivudine in tlje form of preferred bipyramidyl shaped crystals can be obtained by recrystallisation from non-aqueous media, in particular a lower alcohol such as ethanol,

IMS, propan-1-ol. It can also be obtained from Form I by aging in IMS or ethanol at about

50°C for about 1 hour. Lamivudine of Form π can also be prepared by heating Form I to about 177 - 178°C and allowing the melt to cool.

All the above discussed methods are not suitable for commercial scale operations and inevitably contamination of Form II crystals with some Form I crystals occurs. Another significant disadvantages of prior art processes for the preparation of Form π of Lamivudine is that they require high temperatures and large amounts of solvent. Accordingly, it is important to develop a process that is suitable on a commercial scale for the preparation of Form H polymorph of Lamivudine, which overcomes the disadvantages of the prior art methods enumerated above. Objects of the invention

It is, therefore, an object of the invention to provide a process for the preparation of Form II of Lamivudine directly during the manufacture of Lamivudine that overcomes the disadvantages of the prior art.

It is another object of the invention to provide a process for the preparation of Form II of Lamivudine that avoids isolation of Form I, high temperature and large amounts of solvent. It is a further object of the invention to provide a process for the preparation of Form

II of Lamivudine that is economical and capable of operation in the commercial scale.

These and other objects of the invention are achieved by the novel process for the preparation of Form II of Lamivudine directly during the manufacture of Lamivudine. Summary of the invention

The process of the invention involves the direct preparation of Lamivudine polymorph of Form II from its salicylate salt thereby avoiding the formation or use of Form I. As a result the process is economical, and is capable of operation on a commercial scale with minimum contamination with Form I polymorph of Lamivudine. The yield obtained is about 93% with high HPLC purity above 99.5% which is isolated from the aqueous medium.

Accordingly the present invention provides a process for the manufacture of Lamivudine polymorph of Form II comprising suspending Lamivudine salicylate monohydrate in a solvent selected from the group consisting of aliphatic ketones, esters, either straight chain or branched chain ethers containing 1 - 8 carbon atoms, refluxing the mixture to at the reflux, temperature of the solvent, adding an organic base at said reflux temeprature, cooling it and filtering out the product Form II of Lamivudine.

Preferably, said reaction mixture is cooled to about 30 to 35°C before filtering out the product Form II of Lamivudine.

In a preferred embodiment, the solvent employed is ethyl acetate and the reflux temperature is about 77°C,

In another preferred embodiment, the solvent employed is acetonitrile arid the reflux temperature is about 82 to 84°C. In one embodiment of the invention, the amount of ethyl acetate or acetonitrile used is used as solvent at 3 to 15 volumes, preferably 6 to 6.5 volumes per volume of Lamivudine salicylate monohydrate.

In another embodiment of the invention, a temperature of at least 50°C to the reflux temperature of about 77°C for ethyl acetate and about 84°C for acetonitrile is achieved before the addition of the organic base.

In another embodiment of the invention, the organic base is selected from an aliphatic and aromatic amine.

In a further embodiment of the invention, the organic base comprises a tertiary aliphatic or aromatic amine. In a further embodiment of the invention, the organic base comprises a secondary aliphatic or aromatic amine.

In a further embodiment of the invention, the organic base comprises a primary aliphatic or aromatic amine.

In a further embodiment of the invention, the organic base comprises triethylamine or diisopropyl ethyl amine, preferably ethyl amine. In another embodiment of the invention, the organic base is used in a molar ratio of 1 to 5, preferably a molar ratio of 2.2.

In a further embodiment of the invention, the triethylamine is added to Lamivudine salicylate monohydrate in ethyl acetate or acetonitrile at reflux temperature over a time period in the range of 15 minutes to 3 hours, preferably 1 hour. Brief description of the accompanying drawings

Figure 1 shows the IR spectral data of Form II polymorph of Lamivudine when prepared according to the process of Example 1 below.

Figure 2 shows the DSC thermogram of Form II polymorph of Lamivudine when prepared according to the process of Example 1 below.

Figure 3 shows the X-ray powder diffractogram of Form II polymorph of Lamivudine when prepared according to the process of Example 1 below. The horizontal axis presents 20 and the vertical axis corresponds to the peak intensity of Form II.

Figure 4 shows the IR spectral data of Form II polymorph of Lamivudine when prepared according to the process of Example 2 below.

Figure 5 shows the DSC thermogram of Form II polymorph of Lamivudine when prepared according to the process of Example 2 below.

Figure 6 shows the X-ray powder diffractogram of Form II polymorph of Lamivudine when prepared according to the process of Example 2 below. The horizontal axis presents 2Θ and the vertical axis corresponds to the peak intensity of Form π. Detailed description of the invention

(2R, 5R)-5-(4-amino-2-oxo-2H-pyrirnidin-l-yl)-[l,3] oxathiolan-2-carboxylic acid, 2S-isopropyl-5R-methyl-lR-cyclohexyl ester on reduction provides 4-amino-l-(2R- hydroxyme yl-[l,3]-oxatMolan-5S-yl)-lH-pyrimidin-2-one, which is isolated as a salt with salicylic acid (1:1) from aqueous solution of Lamivudine.

The salt thus obtained is treated with a suitable amine selected from the class of aromatic or aliphatic, primary, secondary, or tertiary amines which are capable of forming a salt with salicylic acid and remain soluble in the reaction mixture, while the Lamivudine is precipitated and is isolated by any suitable method such as filtration or centrifugation. The amine is preferably triethyl amine.

The solvent is selected from the group consisting of aliphatic ketones, esters, either straight chain or branched chain ethers containing 1 - 8 carbon atoms. The most preferred solvents are ethyl acetate and acetonitrile. When the salicylate salt in the solvent is treated at

25 to 35°C with a triethyl amine, the precipitated Lamivudine is obtained having Form I crystallinity. Where the solvent employed is ethyl acetate and triethylamine is added at reflux temperature of ethyl acetate, i.e., about 77°C, the product obtained substantially comprises Lamivudine polymorph of Form π. Similarly, where the solvent employed is acetonitrile, and triethylamine is added at reflux temperature of acetonitrile, i.e., about 82 to 84°C, the product obtained substantially comprises Lamivudine polymorph of Form π When alcohol such as methanol, ethanol, or isopropanol are used, Form 1 is obtained. However, it has now been found that when benzene or toluene are used as solvent a mixture of both Forms I and II are obtained. As a result it is clear that the solvents and temperature play an important role in determining the crystallinity of Lamivudine.

The reaction scheme of the process of the invention is given below:

Scheme - 1

CO 00

LAMIVUDINE

Example 1

4-amino-l-(2R-hydroxymethyl-[l,3]-oxathiolan-5S-yl)-lH-pyrimidin-2-one monosalicylate (100 gms) and ethyl acetate (600 ml) are charges and the mass stirred for 10 minutes at 30 - 35°C while heating slowly to reflux temperature of ethyl acetate. A solution of triethylamine (60g) in ethyl acetate (50ml) was added dropwise at reflux temperature in 1 hour. The reflux was maintained for 30 minutes. The reaction was then slowly cooled to 30°C and stirred for 1 hour at this temperature. The solid was filtered, washed with ethyl acetate (2x50ml) and dried at 70 - 75°C to a constant weight of 58.5 g. The product is identified as Form II by IR spectral data (Figure 1), DSC Thermogram (Figure 2) and X-ray powder diffractogram (Figure 3). Example 2

The same process was followed as in Example 1 under similar conditions except that acetonitrile was used as solvent instead of ethyl acetate. The product is identified as Form II by IR spectral data (Figure 4), DSC Thermogram (Figure 5) and X-ray powder diffractogram (Figure 6).

Claims

We claim:

1. A process for the manufacture of Lamivudine polymorph of Form II comprising suspending Lamivudine salicylate monohydrate in a solvent selected from the group consisting of aliphatic ketones, esters, either straight chain or branched chain ethers containing 1 - 8 carbon atoms, refluxing the mixture to the reflux temperature of the solvent, adding an organic base at said reflux temperature, cooling the reaction mixture and filtering out the product Form II of Lamivudine.

2. A process as claimed in claim 1 wherein said solvent is ethyl acetate.

3. A process as claimed in claim 2 wherein the amount of ethyl acetate used is 3 to 15 volumes volumes per volume of said Lamivudine salicylate monohydrate.

4. A process as claimed in claim 1 wherein said solvent is acetonitrile.

5. A process as claimed in claim 4 wherein the amount of acetonitrile used is 3 to 15 volumes volumes per volume of said Lamivudine salicylate monohydrate.

6. A process as claimed in claim 3 or 5 wherein said amount is 6 to 6.5 volumes per volume of said Lamivudine salicylate monohydrate.

7. A process as claimed in any one of claims 2 or 3 wherein said reflux temperature is about 77°C.

8. A process as claimed in any one of claims 4 or 5 wherein said reflux temperature is about 84°C.

9. A process as claimed in any preceding claim wherein a temperature between 50°C. to said reflux temperature is achieved before the addition of the organic base.

10. A process as claimed in any preceding claim wherein the organic base is selected from an aliphatic and aromatic amine.

11. A process as claimed in claim 10 wherein the organic base comprises a tertiary aliphatic or aromatic amine.

12. A process as claimed in claim 10 wherein the organic base comprises a secondary aliphatic or aromatic amine.

13. A process as claimed in claim 10 wherein the organic base comprises a primary aliphatic or aromatic amine.

14. A process as claimed in claim 11 wherein said tertiary aliphatic amine is selected from triethylamine or diisopropyl ethyl amine, preferably triethyl amine.

15. A process as claimed in any preceding claim wherein the organic base is used in a molar ratio of 1 to 5.

16. A process as claimed claim 15 wherein the organic base is used in a molar ratio of 2.2.

17. A process as claimed in claim 14 wherein the triethylamine is added to Lamivudine salicylate monohydrate in ethyl acetate or acetonitrile at reflux temperature over a time period in the range of 15 minutes to 3 hours, preferably 1 hour.

18. A process as claimed in any preceding claim wherein said reaction mixture is cooled to about 30 to 35°C before filtering out the product Form II of Lamivudine

19. A process for the manufacture of Lamivudine polymorph of Form II substantially as herein described and illustrated in the foregoing examples.