CLIAMS:1. A process for the preparation of Pidotimod of formula II

Formula II
which includes steps of:

a) condensing L-pyroglutamic acid of formula III

Formula III
with ethyl-L-thiazolidine-4-carboxylate hydrochloride of formula IV

Formula IV
in the presence of a condensing agent and a base in a suitable organic solvent to get 3-(L-pyroglutamyl)-L-thiazolidine-4- carboxylate of formula V

Formula V

b) hydrolysing 3-(L-pyroglutamyl)-L-thiazolidine-4-carboxylate of formula V in aqueous alkali to get alkali metal salt of 3-(L-pyroglutamyl)-L-thiazolidine-4-carboxylate of formula VI

Formula VI
c) extraction of the solution of step (b)with suitable organic solvent.
d) precipitation and isolation of Pidotimod of formula II using concentrated
acid.

2. The process of claim 1, wherein the condensing agent and base in step (a) is N,N’-dicyclohexylcarbodiimide and potassium carbonate respectively.

3. The process of claim 1, wherein the suitable solvent in step (a) and step (c) is dichloromethane.

4. The process of claim 1, wherein the alkali in step (b) is sodium hydroxide and the acid in step (d) is concentrated hydrochloric acid.

5. A process of purification of Pidotimod of formula II

Formula II
which includes steps of:
a) providing a clear solution of Pidotimod of formula II

Formula II
in a polar organic solvent.
b) cooling the clear solution of step (a) and
c) isolating the pure Pidotimod of formula I

Formula I

6. The process of claim 5, wherein the Pidotimod of formula II is having HPLC purity greater than or equal to 91% with N,N’-dicyclohexylurea in the range of 5% to 9%.

7. The process of claim 5, wherein the polar solvent in step (a) is methanol.

8. The process of claim 5, wherein the cooling in step (b) is done at a temperature range of -5°C to 10°C.

9. The process of claim 5, wherein the pure Pidotimod in step (c) is having HPLC purity greater than or equal to 99.5% and melting range of 196-198°C.
,TagSPECI:Field of Invention

The present invention relates to an industrially scalable process for the preparation of Pidotiomod substantially free from impurities.

Background of the invention

The drug compound having the name “Pidotimod” has chemical name (4R)-3-[[(2S)-5-Oxo-2-pyrrolidinyl]carbonyl]-4-thiazolidine carboxylic acid and has the structural formula I:

Formula I

Pidotimod (3-L-pyroglutamyl-L-thiaziolidine-4carboxylic acid) is a synthetic dipeptide molecule with immunomodulatory properties.

The compound patent US 4,839,387 describes process of preparation of Pidotimod (3-L-pyroglutamyl-L-thiaziolidine-4-carboxylic acid) involving reaction of pentachlorophenyl-L-pyroglutamate with 4-thiazolidine carboxylic acid in presence of triethylamine and dimethylformamide (DMF) solvent, followed by base hydrolysis, acidification with hydrochloric acid and crystallization from water to get a crystalline product (70% yield and melting point 192-194°C).

The above prior art process involves difficulty in handling of toxic substances like pentachlorophenyl-L-pyroglutamate, poor stability and less yield due to solubility of Pidotimod in water.

The US Pat. No 5,110,936 describes the process of preparation of Pidotimod by condensation of ethyl L-thiazolidine-4-carboxylate hydrochloride and L-pyroglutamic acid using ethyl acetate solvent in presence of potassium carbonate and N,N’-dicyclohexylcarbodiimide to get ethyl 3-(L-pyroglutamyl)-L-thiazolidine-4-carboxylate which is further hydrolysed using sodium hydroxide solution and precipitated with hydrochloric acid to get Pidotimod (87.6%, m.p 193°C).

The US Pat. ’936 described process for the preparation of Pidotimod overcomes the drawback of handling of toxic compounds by use of ethyl L-thiazolidine-4-carboxylate hydrochloride. However, the process suffers in removal of various impurities like L-pyroglutamic acid, Ethyl-L-thiazolidine-4-carboxylate and other coloring impurities, as well as complete removal of N,N’-dicyclohexylurea which is a by-product formed in the reaction due to use of N,N’-dicyclohexylcarbodiimide.

Therefore, there is a need to develop an industrially scalable process for the preparation of pure Pidotimod, substantially free of impurities, to get high yields and purity.

The inventors of the present invention surprisingly found that pure Pidotimod could be prepared in high yield and purity by using pure intermediates.

Summary of the Invention

The present invention provides an industrially scalable process for the preparation of pure Pidotimod, substantially free of impurities, to get high yields and purity.

In one aspect, the present invention provides a process for the preparation of Pidotimod of formula II

Formula II
which includes steps of:

a) condensing L-pyroglutamic acid of formula III

Formula III
with ethyl-L-thiazolidine-4-carboxylate hydrochloride of formula IV

Formula IV
in the presence of a condensing agent and a base in a suitable organic solvent to get 3-(L-pyroglutamyl)-L-thiazolidine-4- carboxylate of formula V

Formula V

b) hydrolysing 3-(L-pyroglutamyl)-L-thiazolidine-4-carboxylate of formula V in aqueous alkali to get alkali metal salt of 3-(L-pyroglutamyl)-L-thiazolidine-4-carboxylate of formula VI

Formula VI
c) extraction of the solution of step (b)with suitable organic solvent.
d) precipitation and isolation of Pidotimod of formula II using concentrated
acid.

In another aspect, the invention also relates to process of purification of Pidotimod of formula II
which includes steps of:
a) providing a clear solution of Pidotimod of formula II

Formula II
in a polar organic solvent.
b) cooling the clear solution of step (a) and
c) isolating the pure Pidotimod of formula I

Formula I
Description of the Invention

In one aspect, the present invention relates to a process for the preparation of Pidotimod of formula II

Formula II
which includes steps of:

a) condensing L-pyroglutamic acid of formula III

Formula III
with ethyl-L-thiazolidine-4-carboxylate hydrochloride of formula IV

Formula IV
in the presence of a condensing agent and a base in a suitable organic solvent to get 3-(L-pyroglutamyl)-L-thiazolidine-4- carboxylate of formula V

Formula V

b) hydrolysing 3-(L-pyroglutamyl)-L-thiazolidine-4-carboxylate of formula V in aqueous alkali to get alkali metal salt of 3-(L-pyroglutamyl)-L-thiazolidine-4-carboxylic acid of formula VI

Formula VI
c) extraction of the solution of step (b)with suitable organic solvent.
d)precipitation and isolation of Pidotimod of formula II using concentrated
acid.

The condensation of L-pyroglutamic acid with ethyl-L-thiazolidine-4-carboxylate hydrochloride may be carried out at a temperature range of 0°C to 30°C, in a suitable organic solvent which includes, but is not limited to halogenated solvents like dichloromethane, chloroform or mixtures thereof in the presence of a condensing agent and a base. The condensing agent used for this step includes, but is not limited to basic carbodiimides like N,N’-dicyclohexylcarbodiimide, N,N’-diisopropylcarbodiimide .The base used includes, but is not limited to organic bases like triethylamine and N,N-diethylamine or inorganic bases like sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate.
The condensation reaction may be carried out at a temperature below 30°C preferably in the range of 0°C-25°C.
The by-product of the reaction maybe filtered and the filtrate concentrated to get 3-(L-pyroglutamyl)-L-thiazolidine-4-carboxylate.

The obtained 3-(L-pyroglutamyl)-L-thiazolidine-4-carboxylate maybe hydrolysed in aqueous alkali at a temperature range of 20°C to 30°C for a period of 30 minutes to 3hours to get alkali metal salt of 3-(L-pyroglutamyl)-L-thiazolidine-4-carboxylic acid. The alkali used for the hydrolysis includes, but is not limited to sodium hydroxide, potassium hydroxide and the like.
The aqueous solution containing alkali metal salt of 3-(L-pyroglutamyl)-L-thiazolidine-4-carboxylic acid maybe further extracted with suitable organic solvent to remove any colored impurities. The suitable organic solvent includes, but is not limited to halogenated solvents like dichloromethane, chloroform and the mixtures thereof.

The aqueous solution containing alkali metal salt of 3-(L-pyroglutamyl)-L-thiazolidine-4-carboxylic acid maybe precipitated using concentrated acid at a temperature range of 0°C-10°C, preferably at 3°C-8°C.The concentrated acid used includes, but is not limited to hydrochloric acid, sulphuric acid, acetic acid and the like.

In another aspect, the invention also relates to process of purification of Pidotimod of formula II
which includes steps of:
a) providing a clear solution of Pidotimod of formula II

Formula II
in a polar organic solvent.
b) cooling the clear solution of step (a) and
c) isolating the pure Pidotimod of formula I

Formula I
The purification of Pidotimod of formula II involves providing a clear solution of Pidotimod of formula II in a polar solvent. The polar solvent includes, but is not limited to water, methanol, ethanol, isopropanol or mixtures thereof.
The clear solution of Pidotimod may be provided at a temperature ranging from 30°C to boiling point of the solvent.

The clear solution containing Pidotimod may be cooled at a temperature range of -10°C to 10°C.The cooling may be maintained at the preferred temperature range for a period of 30 minutes to 2 hours.

The obtained pure Pidotimod of formula I may be isolated by using filtration techniques like filtration by gravity or by suction, centrifugation, decantation, and the like.

The pure Pidotimod of formula I may be dried in a tray dryer, vacuum oven, air oven, fluidized bed dryer, spin flash dryer, flash dryer, and the like. The drying may be carried out at temperatures in the range of 30 °C to about 85 °C, optionally under reduced pressure. The drying may be carried out for any time periods, such as, for example, for about 1 to about 10 hours, or longer, to give the desired pure compound of Formula I.

Formula I

The term “condensing agent” as used herein, unless defined, refers to basic carbodiimides such as N,N’-dicyclohexylcarbodiimide, N,N’-diisopropylcarbodiimide and the like.

The term “base” as used herein, unless defined, refers to organic and inorganic bases like triethylamine, N,N-dimethylamine and the like and alkali metal carbonates and bicarbonates such as sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate and the like.

The term “suitable organic solvent” as used herein, unless defined, refers to halogenated solvents like dichloromethane, chloroform or mixtures thereof.

The term “alkali” as used herein, unless defined, refers to soluble hydroxides of alkali and alkaline earth metals like sodium hydroxide, potassium hydroxide and the like.

The term “acid” refers to hydrochloric acid, sulphuric acid, acetic acid and the like.

The term “polar organic solvent” as used herein, unless defined, refers to alcoholic solvents like water, methanol, ethanol, isopropanol or mixtures thereof.

The term “precipitation” as used herein, unless defined, refers to pH adjustment in the range of 1-2.

The term “isolating” as used herein, unless defined, refers to the filtration techniques like filtration by gravity or by suction, centrifugation, decantation, and the like.
The term “pure Pidotimod” as used herein, unless defined, refers to Pidotimod , having HPLC purity equal to or greater than 99.5% and melting range 196-198°C.

The term “substantially free from impurities” as used herein, refers to pure Pidotimod having total impurities equal to or less than 0.3% by HPLC.

The term ”impurities” refers to known impurities such as Ethyl-L-thiazolidine carboxylate, L-pyroglutamic acid, N,N’-dicyclohexylurea,L-thioproline and other unknown impurities.

The present invention may further be illustrated by the following examples which may be provided merely to be exemplary of the invention and do not limit the scope of the invention. Certain modifications and equivalents may be apparent to those skilled in the art and may be intended within the scope of the present invention.

EXAMPLES

Example 1: Process for the preparation of ethyl 3-(L-pyroglutamyl)-L-thiazolidine-4-carboxylate.

A solution of of ethyl L-thiazolidine-4-carboxylate hydrochloride (1.0 kg) in water was treated with potassium carbonate (1.0kg) and extracted with dichloromethane. The organic phase was cooled and L-pyroglutamic acid (0.71 kg) and N,N’-dicyclohexylcarbodiimide (1.14kg) dissolved in dichloromethane was added. The reaction mixture was stirred at 25°C-30°C. After the completion of reaction, the N,N’-dicyclohexylurea was filtered off and filtrate concentrated to get an oily mass.

Example 2: Process for the preparation of crude Pidotimod.

The oily mass consisting of ethyl 3-(L-pyroglutamyl)-L-thiazolidine-4-carboxylate with traces of N,N-dicyclohexylurea was taken up in an aqueous solution of sodium hydroxide, stirred for 30 minutes and the solution was extracted with dichloromethane. After layer separation, aqueous layer was acidified with concentrated hydrochloric acid. The crude compound was filtered, and washed with water and the wet cake was taken for further purification.

Crude weight: 1.14 kg
HPLC purity of crude Pidotimod : 93.54%
%N,N-dicyclohexylurea by HPLC : 6.85%
Water content : 15 -35%
Melting range : 191-194°C

Example 3: A process for preparation of pure Pidotimod

Methanol (2.8 L) was added to the crude pidotimod (1.14kg) and was refluxed to get a clear solution. The reaction mass was cooled and maintained for 30 minutes. It was then filtered and washed with cold methanol to get pure Pidotimod.

Yield (w/w) : 0.80
HPLC purity : 99.75%
%N,N-dicyclohexylurea by HPLC : nil
Melting range : 196-198°C