FIELD OF THE INVENTION

The present invention relates to a novel process for the preparation of pure Levothyroxine Sodium, i.e., (S)-2-amino-3-[4-(4-hydroxy-3,5-diiodophenoxy)-3,5-diiodophenyl]propanoic acid sodium salt via two process intermediates viz 3,5-Diiodo L-Tyrosine copper complex and novel Bis (p-anisyl) iodonium Iodide.

BACKGROUND OF THE INVENTION

Levothyroxine Sodium is a synthetic form of thyroid hormone Thyroxine, which is secreted from follicular cells of thyroid gland. Levothyroxine is ideally used for the treatment of thyroid hormone deficiencies such as hypothyroidism. Due to its ability to lower thyroid-stimulating hormone, Levothyroxine is also used in the treatment of goiter and also to prevent the recurrence of thyroid cancer.

Levothyroxine Sodium has the following chemical structure.

For the preparation of Levothyroxine Sodium several methods have been known. US2889363 demonstrates the use of animal natural sources as starting material for the preparation of thyroxine while US2889364 discloses an enzymatic or bio-mimetic method of synthesis for levothyroxine.

The invention disclosed in US2889363 provides a process for the production of esters of N-acylthyroxine with a substantial reduction in the time period and involves digestive coupling reactions in which alkyl esters of N-acyldiidotyrosine yield alkyl esters of N-acylthyroxine in a greatly reduced time in the presence of optimal catalyst concentration, pH range and alcohol concentration. This prior art further involves a process for production of thyroxine by incubating alkyl esters of N-acyldiidotyrosine derived from lower alkanols and lower alkanoic acids in aqueous solution of ethanol (70%) of pH of 9.5-10.5 and in the presence of between 1.5 and 5% by weight of a manganese containing catalyst while passing substantially pure oxygen through the solution and while maintaining the temperature in the range of 25°C to 78 C, and hydrolytically removing acyl and ester substituents from the ester of N-acylthyroxine obtained from the incubation step.

PCT publication No.WO 1996011904 discloses further improvements to a six stage process for production of sodium 1-thyroxine from 1-tyrosine as described in U.S. Pat. No. 2,889,363 and U.S. Pat. No. 2,889,364. These improvements comprise the oxidative coupling of a diiodo-1-tyrosine to form a biphenyl ether derivative, catalysed by a manganese salt in which the amine and acid functionality of the diiodo-1-tyrosine have been protected by suitable protecting groups, characterised in that the reaction is performed at a pressure of about 20 atmospheres in the presence of an organic amine additive using a gaseous oxidant comprising oxygen and optionally an inert diluent. The process optionally further comprises acid hydrolysis of the biphenyl ether derivative with hydrochloric acid to form 1-thyroxine hydrochloride salt and generation of sodium-1-thyroxine from the 1-thyroxine hydrochloride salt.

PCT publication number WO2009136249 provides a process for the preparation of levothyroxine sodium and comprises the steps of: iodinating 3, 5-diiodothyronine to obtain crude levothyroxine, followed by its conversion to disodium salt and acidifying the disodium salt to give pure levothyroxine. The purified levothyroxine is converted to levothyroxine scdium having reduced level of impurities. Levothyroxine sodium obtained by this invention is substantially free from d- enantiomer of thyroxine / 3, 5-Diiodothyronine impurity. The process also reports the d- enantiomer of thyroxine / 3, 5-Diiodothyronine levels below the limit of detection and liothyronine impurity below 0.5% wt / wt. The end product derived from this process is also free from coloured impurities.

Many patent and non-patent literature describe the use of an intermediate called Bis (p-anisyl)iodonium bromide in the preparation of Levothyroxine sodium. Hillmann (Z.Naturforch 1956;llb:424-425) describes a process for the assembly of the biphenyl-ether system present in Levothyroxine, wherein a key coupling reaction is initiated between N-Acetyl 3,5- diiodo-L-tyrosine ethyl ester, derived from the stepwise protective conversion of amine as amide and acid as ester of 3,5 diido-L-tyrosine, and Bis (p-anisyl)iodonium bromide in the presence of copper metal or powder as a catalyst to afford (S)-N-acetyl-3,5-diiodo-4-p-methoxyphenoxyphenylalanine ethyl ester, with 87% yield. All the three protective groups in (S)-N-acetyl-3,5-diiodo-4-p-methoxyphenoxyphenylalanine ethyl ester ,viz. acetamide, methyl ether, ethyl ester were cleaved using a mixture of Hydroiodic acid and Hydrobromic acid to give 3,5-Diiodothyronine. 3,5-Diiodothyronine on subsequent iodination with iodine gave L-thyroxine of an yield that corresponds to 92%.

The use of Bis (p-anisyl)iodonium bromide in the synthesis of thyroxine leads to the formation of an impurities called monobromo triiodo tyrosine and dibromo triiodo tyrosine which may be categorized as "Genotoxic impurities" based on the structure alerts. The levothyroxine compositions thus prepared are not generally preferred due to their genotoxic impurity content associated with purified Levothyroxine.

The aforesaid prior art processes cannot be considered for industrial scale preparation in view of their shortcomings such as less yield of the product, high expenses involved, pressure reactions which are less feasible for commercialization of the product, more number of reaction steps, cycle time and concerns about the purity of the endproduct. Hence there is a long-felt need for the development of an improved process that circumvents the above disadvantages and provides an industrially feasible, cost effective process involving the use of commercially available, simple raw materials in the process. At the same time it is also required that the process or technology developed should be easily adaptable for multikilo manufacturing plants for scaling-up the most demanding API product, Levothyroxine sodium at a very cheap cost, compared to the current import costs of the material and contemporary technologies in vogue.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide an improved and industrially realisable process for preparing Levothyroxine Sodium.

Another object of the present invention is to provide an improved process for preparing Levothyroxine Sodium,which is simple, convenient and economical with commercial feasibility.

Yet another object of the present invention is to provide an improved process for preparing Levothyroxine Sodium by involving two new process intermediates namely 3,5-Diiodo L-Tyrosine copper complex and novel Bis (p-anisyl)iodonium Iodide.

A further object of the present invention is to provide an improved process for preparing Levothyroxine Sodium by using unprotected L-Thyrosine in the reaction. Yet another object of the invention is to provide a process for preparing pure Levothyroxine Sodium that is devoid of any genotoxic impurities generated in the process.

Another object of the invention is to provide a process for preparing pure Levothyroxine Sodium obtained with liothyronine impurities <0.04%.

These and other objects of the present invention will be realized by way of practice of the invention described hereinafter.

SUMMARY OF THE INVENTION

According to an aspect of the present invention there is provided a process for the preparation of Levothyroxine Sodium comprising the steps of:

a. Coupling reaction between 3, 5-diiodo L-Tyrosine copper complex and novel Bis (p- anisyl) iodonium iodide in the presence of Diisopropylamine or other organic bases in n-Butanol or other alcoholic solvents to obtain 2-Amino-3-(3,5-diiodo-4-(4- methoxy phenoxy)phenyl)propanoic acid;

b. Demethylation of 2-Amino-3-(3,5-diiodo-4-(4- ethoxyphenoxy)phenyl)propanoic acid using a mixture of Acetic acid and Hydroiodic acid to obtain 3,5- diiodothyronine;

c. Iodinizing 3,5-diiodothyronine with methanolic or other alcoholic monomethylamine and Iodine to obtain Levothyroxine;

d. Converting Levothyroxine to Levothyroxine Sodium in the presence of sodium hydroxide and n-butanol or other alcololic solvents.

DETAILED DESCRIPTION OF THE INVENTION

It has been surprisingly found that by using two simple intermediates namely 3,5-Diiodo L-Tyrosine copper complex and novel Bis (p-anisyl)iodonium Iodide the process can be made economically and industrially feasible. The use of 3,5-Diiodo L-Tyrosine copper complex reduces the number of steps and increases the yield of the product while the use of novel Bis (p-anisyl)iodonium Iodide reduces the probable genotoxic impurities formation. In this method highly pure levothyroxine (99.9%) Sodium is obtained and the Liothyronine impurities observed, corresponds to only 0.04%.

The process for the preparation of Levothyroxine Sodium comprises the steps of:

a. Reacting 3,5-Diiodotyrosine dihydrate with aqueous copper sulphate solution to give 3, 5-diiodo L-Tyrosine copper complex

b. Initiating coupling reaction between 3, 5-diiodo L-Tyrosine copper complex and Bis (p-anisyl)iodonium iodide to give 2-Amino-3-(3,5-diiodo-4-(4-methoxy phenoxy)phenyl)propanoic acid in the presence of Diisopropylamine in n-Butanol.

c. Demethylation of 2-Amino-3-(3,5-diiodo-4-(4- ethoxyphenoxy)phenyl)propanoic acid using a mixture of Acetic acid and Hydroiodic acid to give 3,5-diiodothyronine, an intermediate of L-Thyroxine and

d. Iodinizing 3,5-diiodothyronine with methanolic monomethylamine and Iodine to obtain Levothyroxine

e. Converting Levothyroxine to its sodium salt by using sodium hydroxide solution and n-butanol followed by filtering and drying to give pure Levothyroxine Sodium.

It will be apparent to the skilled person that certain changes and modifications may be practiced within the scope of the invention. Following examples illustratively demonstrate the process of the present invention, without posing any limitation to it.

Example-1 Preparation of 3.5-Diiodo L-Tyrosine copper complex:

RBF was charged with 1200ml of water and 15.0Ignis of sodium hydroxide at 20-25°C and stirred to get a clear solution. lOOgms of 3,5-Diiodotyrosine dihydrate was added and to the above mixture and stirred for an hour to get a clear solution.Copper sulphate solution (35.94gms dissolved in 225ml of water at 40°C) was then added into the reaction mixture over a period of lhr and maintained at 20-25°C for lhr. The resultant reaction mixture is filtered and washed with 1500ml of DM water and dried under vacuum at 55-60°C to get 95gms of 3,5-Diiodo L-Tyrosine copper complex.The purity of obtained complex is NLT 97% with yield of 93-98%.

Example-2: Preparation of 3,5-Diiodothvronine:

RBF was charged with lOOgms of 3,5-Diiodo L-Tyrosine copper complex and 1200ml of water to which 1460ml of n-Butanol was slowly added over a period of 30-45min at 20-25°C. RBF was then charged with 36.6gms of diisopropylamine at 20°C and 92gms of Bis(p- anisyl)iodonium iodide was added into the reaction mixture at 20-25°C. The reaction mixture was heated to 90°C and maintained for 2hrs at 90°C, then cooled to 20°C, followed by addition of 278ml of toluene. To the above mixture 226ml of 10% aqueous citric acid solution was added at 20-3 0°C and maintained for 2hrs at 20°C followed by filtration and washing with 41ml of water followed by 181ml of Methyl isobutyl ketone wash to get 230gms of wet 2-Amino-3-(3,5-diiodo-4-(4-methoxyphenoxy)phenyl) propanoic acid. The above wet cake was stirred with 400ml of water and 40gms of citric acid for an hour at 20-25°C, filtered and washed with dilute citric acid solutions to get 160gms of wet and pure 2-Amino-3-(3,5-diiodo-4-(4-methoxyphenoxy)phenyl)propanoic acid. The above wet cake was charged with 400ml of acetic acid and 300ml of Hydroiodic acid and the mixture was heated to 100°C and maintained for 5hrs and then cooled to 25-30°C. The reaction mixture was quenched in 3000ml of 5% potassium bisulfite solution and 5% potassium dihydrogen phosphate mixture solution. The pH of the reaction mixture was adjusted to 4 using 250ml of 50% aqueous lithium hydroxide solution and maintained for 30min at 25-30°C. The reaction mixture was then filtered and washed with 500ml of water followed by 200ml of 1-Propanol (2 times), dried under vacuum at 50-55°C for 12-15hrs to get 80gms of 3,5-
Diiodothyronine of purity NLT 98% and yield corresponding to 71.5-76%.

Example-3 Preparation of Levothvroxine:

R3F was charged with lOOgms of 3,5-Diiodothyronine and 1000ml of methanolic monomethylamine at 25-30°C and stirred for 15-20min to get clear solution. The reaction mixture was then cooled to -8 to 0°C, and added with Iodine solution at -8 to 0°C over a period of 2hrs and maintained for 30-45min at -8 to 0°C.The temperature of the reaction mixture was slowly brought to 15-20°C and charged with lOOgms of sodium bisulphite followed by 300gms of Potassium dihydrogen phosphate.The temperature of the reaction mixture was adjusted to 25-30°C and maintained for 30 - 45min at 25-30°C. The resultant mixture was filtered and washed with 200ml of water followed by 200ml of acetonitrile, dried under vacuum for 12-15hrs at 55-60°C to get 130gms of Levothyroxine with purity NLT 98.5% and yield of 84.5-91.2%.

Example-4 Preparation of Levothvroxine Sodium:

RBF was charged with 1000ml of n-Butanol and lOOgms of Levothyroxine, cooled the contents to 5°C and charged with 10ml of concentrated sulphuric acid to get clear solution. The reaction mixture was filtered through 0.45 micron membrane filter and the filtrate was purged with ammonia gas to attain pH 8. The mixture was then heated to 60-65°C and maintained for 1 hr at 60-65°C, followed by cooling to 40-45°C and filtration at 45°C.The filtrate was then washed with 200ml of hot n-Butanol. The wet cake is then charged with 250ml of 1.5% aqueous Sodium hydroxide solution at 25-30°C (pH maintained at 10), Followed by Slow addition of 800ml of 1-propanol over a period of 10-15min at 25-30°C, heated to 60-65°C and maintained for 1 hour. Further the reaction mixture is cooled to 25-30°C and maintained for lhour, filtered and washed with 100ml of 1-propanol followed by vacuum drying at 45-50°C for 12-15hrs to get 85gms of pure Levothyroxine sodium of 78-83% yield.

Example-5 Chemical analysis of Levothyroxine Sodium:

Three consecutive bulk batches of Levothyroxine sodium were prepared through the process described in the present invention and their yields were determined (Table-1). The product was chemically characterized by purity analysis through USP method-I and Method-II (depicted in Tables 2 and 3), lH NMR (depicted in Structure -1 and Table-4), 13C NMR (Structure-2 and Table-5), Mass (Table-6), SOR (as per USP method) (Table-7), PSD(to show the consistency) (Table-8), Elemental analysis (Table-9).

Table-1:

Foot notes for table:

Input: 3,5-Diiodotyrosine dihydrate
Out Put: Levothyroxine sodium pentahydrate
Table-2 Purity by USP Method-I;

Table-3 Purity bv USP Method-II;
Structure 1:

Table-4 Proton NMR:
ed in DMSO-d6 solvent at 400 MHz Bruker NMR spectrometer Structure 2

Table-5:13C NMR

The spectrum was obtained in DMSO-d6 solution at 100 MHz Bruker Avance NMR Spectrometer.

Table-6: Mass Analysis:
TabIe-7: Specific Optical rotation:
Table-8: PSD results:
Table-9: Elemental analysis: (on basis of pentahydrate)

WE CLAIM:

1. A process for the preparation of Levothyroxine Sodium comprising the steps of:

a. Coupling reaction between 3, 5-diiodo L-Tyrosine copper complex and novel Bis (p-anisyl) iodonium iodide in the presence of Diisopropylamine or other organic bases in n-Butanol or other alcoholic solvents to obtain 2-Amino-3- (3,5-diiodo-4-(4-methoxy phenoxy)phenyl)propanoic acid;

b. Demethylation of 2-Amino-3-(3,5-diiodo-4-(4- methoxyphenoxy)phenyl)propanoic acid using a mixture of Acetic acid and Hydroiodic acid to obtain 3,5-diiodothyronine;

c. Iodinizing 3,5-diiodothyronine with methanolic or alcoholic monomethylamine and Iodine to obtain Levothyroxine;

d. Converting Levothyroxine to Levothyroxine Sodium in the presence of sodium hydroxide and n-butanol or other alcoholic solvents

2. The process as claimed in claim 1 wherein levothyroxine sodium is obtained with
liothyronine levels below 0.04% wt/wt.