FORM 2
THE PATENT ACT, 1970
(39 of 1970)
&
The Patent Rules, 2006
COMPLETE SPECIFICATION
(See section 10; rule 13)
1. TITLE OF THE INVENTION: PROCESS FOR THE PREPARATION STABLE
LAMIVUDINE FORM -1
2. Applicant
(a) NAME: ARCH PHARMALABS LIMITED
(b) NATIONALITY: INDIAN
(c) ADDRESS: "H" Wing, 4th floor, Tex Centre, Off Saki Vihar Road,
Chandivali, Andheri (East), Mumbai-400072, India.
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed.

TITLE
Process for the preparation of stable Lamivudine Form-I
FIELD OF TECHNOLOGY
Disclosed herein is an efficient and industrially viable process for the preparation of stable crystalline Form-I of Lamivudine comprising heating lamivudine in solid powdered, amorphous or any other crystalline form in an alcoholic solvent preferably methanol upto its reflux temperature to obtain a suspension and to the said suspension whilst heating about 0.5% to about 5% (based on quantity of alcoholic solvent) of water is added to obtain a clear solution which is cooled to separate stable crystalline lamivudine Form I which is filtered and dried.
BACKGROUND
Lamivudine is a substituted 1,3-oxathiolane and has been described as having antiviral activity in particular against the human immunodeficiency viruses (HIVs) which are responsible for causing AIDS.

Compound of Formula-1 is a racemic mixture of represented by Formula-2 and Formula-3


Both the enantiomers of Formula-1 are equipotent against HIV but the (-) enantiomer represented by Formula-3 has considerably lower toxicity than the other enantiomer and is thus the preferred compound as an antiviral agent and is marketed as lamivudine.lt is a cis(-) isomer and has chemical name(2R,cis)-4-amino-l-(2-hydroxymethyl-l,3-oxathiolan-5-yl)-(lH)-pyrimidin-2-one represented by compound of Formula-3.
US 5905082 (hereinafter referred as '082) discloses in example-1 a process for the preparation of needle shaped crystals (Form-I) comprising dissolving lamivudine compound in 3 volumes of water which on cooling gives a unstirrable mass which after processing results in a very thick mass which is not easy to isolate by filtration at industrial scale. The process disclosed therein in '082 is unviable at industrial scale, therefore, there is a dire need to overcome the problem associated with process disclosed therein in '082 by which formation of unfilterable thick mass can be avoided and instead a easily filterable reaction mass is obtained. Limitation associated with this prior art is that the quantity of water can not be increased as it will result into lower yield since lamivudine has good solubility in water.
J. Chem. Soc, Perkin Trans., (1997), 2: 2653-2659 discloses that Form I is crystallized as needles whereas Form II is obtained as tetragonal bipyramids on slow recrystallization from dry ethanol, n-

propanol or mixtures of ethanol and less polar organic solvents. However, J. Chem. Soc, Perkin Trans. (1997), 2: 2653-2659 also does not disclose any method to obtain starting substrate of lamivudine, and also the crystallization conditions and process for the preparation of Form-I. It also does not give any indication in respect of crystalisation process to prepare Form-I.
J.Pharm.Sci.(1996),85(2), 193-99 discloses that Form-I is prepared by dissolving lamivudine in hot water, adding equal volume of methanol and cooling in a refrigerator. It does not specify crystalisation conditions and also no teaching, indication or motivation to a person skilled in the art to suspend lamivudine in alcohol as a main solvent which generally in such a case is taken as an antisolvent in which lamivudine is insoluble or less soluble instead of water in which it is highly soluble and treating the said suspension with a very small quantity of water to completely solubilise the product thereby avoiding the high voluminous mass at plant scale. There is no indication, teaching or motivation to a person skilled in the art to try a concept which is contrary to generally practiced concept of solvent and antisolvent combination for the purification/crystalisation of organic compounds.
US6329522 (hereinafter referred as '522) discloses a process for purification of lamivudine by the formation of salicylate salt and a crystalisation method for lamivudine using isopropyl actetate. The crystalisation process disclosed therein in '522 requires seeding and does not disclose process to obtain seed crystals.
WO03/027106 discloses a process for preparing Form II comprising using ethyl acetate and acetonitrile as solvents and triethylamine as a base.
WO2007119248 (hereinafter referred as '248) discloses a process for the preparation of Form I comprising dissolving lamivudine in water

by heating to 45 C and subsequent cooling to 30 C in 15 min to obtain unstirrable mass which needs to be broken, stirred to prepare a filterable mass. The said process also involves seeding with Form I crystal. The said process is not industrially friendly as the mass obtained needs breaking up and not filterable. Furthermore, reducing
the temperature from 45 C to 30 C in 15 min is not suitable at
industrial scale. '248 also states that when water is used as a solvent there is a risk factor as it forms Form II as an impurity.
US20100324290 discloses a process for the preparation of lamivudine Form I comprising dissolving lamivudine in water at 38-45 C to obtain a solution which is cooled to 30°C in about 10 minutes or less to obtain solid which is filtered. The process comprising cooling from 45 C to less than 30 C in less than 10 minutes is not industrially viable. Moreover, the yield obtained is 72%.
Processes for the preparation of lamivudine Form I disclosed therein in the prior art references cited hereinabove comprises formation of thick solid mass which requires breaking into stirrable reaction mass, instant cooling from 45°c to 30°C in less than 10 minutes, seeding with Form I crystal are not industrially viable, therefore, there is a dire need to develop a industrially viable process which can overcome problems associated with the processes for the preparation of lamivudine Form I disclosed therein in the prior art.
TECHNICAL PROBLEMS IN PRIOR ART:
1) Thickening of the reaction mass making it difficult to stir and isolate form I.
2) Limitation in using quantity of water owing to its solubility that will result into lower yield.
3) Instant cooling at plant level.
4) Seeding with Form I crystal.

TECHNICAL SOLUTION TO THE PROBLEM:
1) Use of limited quantity of methanol as a main solvent in which lamivudine has got restricted solubility hence can be used in appropriate volume that will provide stirrable and filterable mass.
2) Use of small quantity of water that results into enhancement in yield.
3) Process neither requires dissolving of lamivudine as suspension works well and gives a clear solution when treated with small quantity of water while stirring and heating.
4) Process does not require instant cooling.
5) Process does not require seeding with lamivudine Form I crystal.
SUMMARY
Disclosed herein is a process for the preparation of crystalline and stable Form I of lamivudine free from Form II comprising heating lamivudine in an alcoholic solvent preferably methanol upto its reflux temperature to obtain a suspension and to the said suspension whilst heating about 0.5% to about 5% (based on quantity of alcoholic solvent) of water is added slowly to obtain a clear solution which is cooled to separate crystalline lamivudine Form I which is filtered and dried. Advantageous feature of the lamivudine form I made by the said process is not only thermally stable but is also chemically stable. In the present invention lamivudine form I made by the process under the invention is subjected to continuous exposure to abnormal environment at about 80 °C to ascertain the chemical and polymorphic stability of form I. The studies reveal that it remains unaffected on both aspects even at 80°C. Figures and tables given hereinbelow highlight these features.
The present invention does not use the concept of solvent and antisolvent as it does not comprise dissolving lamivudine in a solvent in which it is soluble and then precipitating form I by the addition of

some other solvent in which it is insoluble. In the present invention inventors have used the concept of solubility difference of lamivudine in an alcoholic solvent and water and the said concept is quite contrary to the solvent/antisolvent combination concept. As per the observation lamivudine has a greater solubility in water than that of alcohols particularly methanol. Inventors of the present invention have treated the suspension of lamivudine in an alcoholic solvent preferably methanol with a small quantity of waters that results into higher yield and also solves the problem of mass thickening. The main feature of the invention is to use alcoholic solvent preferably methanol as main solvent despite the fact that lamivudine is not soluble in the said solvent rather than using the said solvent as anti solvent as reported in the prior art. Moreover suspension obtained from alcoholic solvent particularly methanolic solvent turns into clear solution when treated with a small and limited quantity of water results into form I and this solves all the difficulties encountered in the processes reported therein in the prior art.
BRIEF DESCRIPTION OF FIGURES AND TABLES FOR FORM I MADE BY THE PROCESS OF THE PRESENT INVENTION:
Figure 1 represents needle shaped crystals confirming form I
Figure 2 represents IR at ambient temperature showing the peak at 1109 cm-1 and not at 918 cm-1 and 852 cm-1 confirming form I.
Figure 3 represents IR at 80 °C showing the peak at 1109 cm-1 and not at 918 cm-1 and 852 cm-1 confirming form I.
Figure 4 represents Raman spectrogram of crystalline Form I at ambient temperature
Figure 5 represents HPLC chromatograph of crystalline Form I of lamivudine at ambient temperature

Figure 6 represents HPLC chromatograph of crystalline Form I of lamivudine after storage at 80° C.
Table 1 represents characteristic values of crystalline Form I of lamivudine at ambient temperature
Table 2 represents characteristic values of crystalline Form I of lamivudine after storage at 80 °C


DETAILED DESCRIPTION OF THE INVENTION
One aspect of the invention is to disclose an industrially viable process for the preparation of free flowing stable crystalline Form I of lamivudine avoiding obtaining unstirrable thick mass which needs breaking into stirrable powder, abrupt cooling from 45 °C to less than 30°C in less than 10 minutes and seeding with crystals of Form I.
Another aspect of the invention is to disclose an industrially viable process for the preparation of free flowing stable crystalline Form I of lamivudine free from crystalline Form II of lamivudine. The crystalline Form II of lamivudine is present in crystalline Form I of lamivudine based on the process disclosed herein in the present invention is maximum upto about 2% preferably less than about 0.5%.
One more aspect of the present invention is to disclose an industrially viable process for the preparation of crystalline thermally stable form I of lamivudine with higher yield.
Yet another aspect of the invention is a process for the preparation of stable crystalline form I of lamivudine that does not involve the crystalisation concept of solvent and antisolvent combination.
In one aspect of the invention disclosed herein is a process for the preparation of stable crystalline lamivudine Form I comprising:
a) contacting lamivudine with an alcoholic solvent preferably with methanol at a temperature upto reflux temperature of the solvent to obtain a suspension;
b) add about 0.5 to about 5% by weight of water compared to quantity of solvent to the suspension of step b) whilst heating to obtain a solution;

c) cooling the solution from step b to about 45°C to about 5°C to obtain crystalline lamivudine;
d) filter the crystalline reaction mass from the step c
e) washing the crystalline solid from step d with solvent
f) dried the crystalline solid from step e to obtain stable
lamivudine form I.
In another preferred aspect disclosed herein is a process for the preparation of stable crystalline lamivudine Form I comprising:
a) contacting lamivudine with methanol at a temperature upto reflux temperature of the methanol to obtain a suspension;
b) add about 0.5 to about 5% by weight of water compared to quantity of methanol to the suspension of step b) whilst heating to obtain a solution;
c) cooling the solution from step b to about 45°C to about 5°C to obtain crystalline lamivudine;
d) filter the crystalline reaction mass from the step c
e) washing the crystalline solid from step d with methanol
f) dried the crystalline solid from step e to obtain stable
lamivudine form L
In yet another aspect disclosed herein does a process for the preparation of crystalline Form I of lamivudine comprises the use of salicylic acid salt of lamivudine of formula 4 as a starting material;
a) contacting lamivudine salicylate of Formula 4 with a base in a solvent to obtain pure lamivudine compound of Formula 3 in solid form;
b) contacted the solid content of lamivudine obtained in step a) with an alcoholic solvent preferably with methanol upto reflux temperature of the solvent to obtain a suspension;

c) add about 0.5 to 5% by weight of water compared to quantity of solvent to the suspension of step b) whilst heating to obtain a solution;
d) cooling the solution obtained in step c) to about 45° to about -5 C to obtain crystalline lamivudine;
e) filter crystalline reaction mass obtained in step d);
f) washing the crystalline solid reaction mass of step e) with solvent;
g) dried the crystalline lamivudine obtained in step d) to obtain stable crystalline lamivudine Form I.
In preferred aspect disclosed herein is a process for the preparation of crystalline lamivudine Form I comprising:
a) contacting lamivudine salicylate of Formula 4 with a base in methanol to obtain pure lamivudine compound of Formula 3 in solid form;
b) contacted the reaction mass of step a) containing solid content of lamivudine with methanol upto reflux temperature of the solvent to obtain a suspension;
c) add about 0.5 to about 5% by weight of water compared to quantity of methanol to the suspension of step b) whilst heating to obtain a solution;
d) cooling the solution obtained in step c) to about 45 to about -5 C to obtain crystalline lamivudine;
e) filter crystalline reaction mass obtained in step d);
f) washing the crystalline solid reaction mass of step e) with methanol;
g) dried the crystalline lamivudine obtained in step f) to obtain stable crystalline lamivudine Form I
In yet another aspect disclosed herein is a process for the preparation of stable crystalline lamivudine Form I comprising:

a) contacting impure lamivudine obtained by any of the process known in the prior art with salicylic acid in a solvent;
b) isolating lamivudine salicylate;
c) contacting optionally purified lamivudine salicylate of formula
4 with a base in a solvent to obtain pure lamivudine compound of
Formula 3 in solid form;
d) contacting lamivudine from step c) with alcoholic solvent
preferably with methanol upto reflux temperature of the solvent to
obtain a suspension;
e) add about 0.5 to about 5% by weight of water compared to
quantity of solvent to the suspension of step d) whilst heating to
obtain a solution;
f) cooling the solution from step b to about 45 °C to about 5°C to obtain crystalline lamivudine;
g) filter the crystalline reaction mass from the step f
h) washing the crystalline solid from step g) with solvent
i) dried the crystalline solid from step e to obtain stable
lamivudine form I.
In another preferred aspect disclosed herein is a process for the preparation of stable crystalline lamivudine Form I comprising:
a) contacting impure lamivudine obtained by any of the process disclosed in the prior art with salicylic acid in water;
b) isolating lamivudine salicylic acid salt;
c) contacting optionally purified lamivudine salicylate of formula
4 with a base in a methanol to obtain pure lamivudine compound
of Formula 3 in solid form;
d) contacting lamivudine from step c) with methanol at a
temperature upto reflux temperature to obtain a suspension;
e) add about 0.5 to about 5% by weight of water compared to
quantity of methanol to the suspension of step d) whilst heating to
obtain a solution;

f) cooling the solution from step b to about 45°C to about 5°C to obtain crystalline lamivudine;
g) filter the crystalline reaction mass from the step f;
h) washing the crystalline solid from step g) with methanol;
i) dried the crystalline solid from step e to obtain stable
lamivudine form I.
Contacting hereinabove and hereinbelow refers to mixing, reacting, treating, mixing, heating, stirring, refluxing and the like,
The pure lamivudine used as a starting material can exist in any solid form. The lamivudine used as a starting material has a chemical purity of about 98% and a chiral purity of about 99%.

The impure lamivudine obtained by any process disclosed therein in the prior art is converted into its salicylic acid salt by contacting with salicylic acid in a solvent. Preferably solvent for the preparation of lamivudine salicylate is water. The isolation of said lamivudine salicylate comprises stirring the reaction mixture at temperature about 20°C to about 70°C for about 10 hrs to about 25 hrs. Preferably the said reaction mixture is stirred at temperature about 25°C to about 30°C for about 10 hrs. The pure lamivudine salicylic acid salt obtained is treated with a base in a solvent. The solvent is selected from the group comprising water, alkanols, ethers, esters and mixture thereof. The solvent is preferably selected from the group comprising alkanol, tetrahydrofuran, dioxane, isopropyl acetate, ethyl acetate and mixture thereof. The solvent more preferably is methanol. The base is preferably an amine, more preferably a tertiary amine. The treatment

of lamivudine salicylic acid salt with base in a solvent comprises stirring the said reaction mixture at a temperature about 40°C to about 70°C for about 1 hr preferably for about 30 minutes. Cooled gradually the said reaction mixture to about 15-30°C in about 3 to about 5 hours and maintain at the said temperature range for about 10 hrs. Lamivudine base separates out as a solid mass having chemical purity of about 98% and a chiral purity of about 99%. The solid lamivudine so obtained is taken in a solvent .The solvent for the said reaction is selected from alcohols. Preferably solvent is methanol. The said reaction mixture containing lamivudine in solid form is heated up to reflux temperature of the solvent to obtain a reaction mass with some undissolved solid lamivudine in the form of a suspension and to the said suspension about 0.5 wt % to about 5wt% of water compared to the weight % of the solvent is added slowly to the said suspension whilst heating and stirring for about 0.5 to about 1 hr. Preferably about 1 wt% to about 2wt% of water compared to wt% of the solvent is used, cooled the reaction mixture to about 45° to about 5°C separate the crystalline lamivudine Form I. the solid obtained is filtered, washed with the same solvent and dried to obtain stable pure crystalline lamivudine Form I.
In a preferred embodiment solid lamivudine is taken in a solvent selected from aliphatic alcohols. Preferably solvent is methanol. The said reaction mixture containing lamivudine in solid form is heated up to reflux temperature of the solvent to obtain a reaction mass with undissolved solid lamivudine in the form of a suspension and to the said suspension about 0.5 wt % to about 5wt% of water compared to the weight % of the solvent is added slowly to the said suspension whilst heating and stirring for about 0.5 to about 1 hr to get a clear solution which upon cooling to about 45° to about 5°C to separate the crystalline lamivudine Form I. Preferably about 1 wt% to about 2 wt % of water compared to wt% of the solvent is used. The solid obtained is filtered washed with the same solvent and dried to obtain stable pure crystalline lamivudine Form I.

In a more preferred embodiment solid lamivudine is taken in methanol. The said reaction mixture containing lamivudine in solid form is heated up to reflux temperature of ethanol to obtain a reaction mass with undissolved solid lamivudine in the form of a suspension and to the said suspension about 0.5wt % to about 5wt% of water compared to the weight % of methanol is added slowly to the said suspension whilst heating and stirring for about 0.5 to about 1 hr to get a clear solution which upon cooling to about 45 °C to about 5°C to separate the crystalline lamivudine Form I. Preferably about lwt % to about 2wt % of water compared to wt% of methanol is used. The solid obtained is filtered washed with the same solvent and dried to obtain stable pure crystalline lamivudine Form I.
The present invention is best understood by the following non limiting examples:
Example 1: Solid lamivudine 10 g is taken in 80 mL methanol and contents are heated to refluxion resulting into suspension. 1.0 mL water is then added dropwise whilst refluxing to obtain a clear solution. Stirring still continued for another half an hour and the mass is gradually cooled to about 45° to about 5°C to obtain the stable crystalline form I. Product is filtered off and dried under vacuum. Weight of the Lamivudine form I is 8.5 g Yield: 85%
Example 2: Lamivudine salicylate (120 g) was added to a mixture of 720 mL ethyl acetate and 6 mL water at 25° to 35°C. The reaction mixture is heated to 45° to 50°C, followed by the addition of 104.76 g triethylamine over 30 minutes at 45° to 50°C. The reaction mixture is stirred for 4 hours at the same temperature and cooled to 25° to 30°C. The reaction mixture is stirred for further 30 minutes at 25° to 30°C, filtered and dried by suction. The solid obtained is washed with ethyl acetate. 600 mL ethyl acetate is added to the washed solid and heated

to 50° to 55°C. The mixture is stirred at 50° to 55°C for 15 minutes, cooled to 25° to 30°C and stirred for further 30 minutes. The solid is filtered at 25° to 30°C, washed with 60 mL ethyl acetate and dried under vacuum at 45° to 50°C to obtain Lamivudine as solid. This is further converted into form I by the method as described under example 1.

CLAIMS:
We claim:
1. A process for the preparation of stable crystalline lamivudine Form I comprising:
a) contacting solid form of lamivudine with a solvent optionally in a single lot to obtain partially dissolved reaction mixture;
b) heated the reaction mass obtained in step a) at a temperature upto reflux temperature of the solvent to obtain a suspension;
c) add about 0.5 to about 5% by weight of water compared to quantity of solvent wt % to the suspension of step b) whilst heating to obtain a solution;
d) cooling the solution obtained in step c) to about 45 "C to about -5 C to obtain stable crystalline lamivudine Form I.
2. A process for the preparation of stable crystalline lamivudine Form I

comprising :
a) contacting lamivudine salicylic acid salt of Formula 4 with a base in a solvent to obtain pure lamivudine compound of Formula I in solid form;
b) contacted the solid content of lamivudine obtained in step a) without isolation at a temperature upto reflux temperature of the solvent to obtain a suspension;

c) add about 0.5 to about 5% by weight of water compared to quantity of solvent by wt % to the suspension of step b) whilst heating to obtain a solution;
d) cooling the solution obtained in step c) to about 45°C to about -5 C to obtain stable crystalline lamivudine Form I.
3. Process of claims 1 and 2 wherein solvent is selected from
alcohols.
4. Process of claim 3 wherein solvent is methanol.
5. A process for the preparation of stable crystalline lamivudine Form I comprising :

e) contacting solid form of lamivudine with methanol optionally in a single lot to obtain partially dissolved reaction mixture;
f) heated the reaction mass obtained in step a) at a temperature upto reflux temperature of methanol to obtain a suspension;
g) add about 0.5 to about 5% by weight of water compared to quantity of solvent wt % to the suspension of step b) whilst heating to obtain a solution;
h) cooling the solution obtained in step c) to about 45 °C to about -5 C to obtain stable crystalline lamivudine Form I.
6. Process of claims 1, 2 and 3 wherein wt% of water is about 1% to
about 2% against the wt% of the solvent.