FORM 2
THE PATENTS ACT 1970
(Act 39 of 1970)
&
THE PATENTS RULE, 2003
COMPLETE SPECIFICATION
(SECTION 10 and Rule 13) TITLE OF THE INVENTION
Process for purification of 2-alkyl-6-oxo-l,6-dihydro-3,4'-bipyridine-5-carbonitrile
Emcure Pharmaceuticals Limited, an Indian company, registered under the Indian Company's Act 1957 and
having its registered office at
Emcure House, T-184, M.I.D.C, Bhosari, Pune-411026, India.
THE FOLLOWING SPECIFICATION DESCRIBES THE NATURE OF THE INVENTION AND THE
MANNER IN WHICH IT IS PERFORMED.

FIELD OF THE INVENTION
The present invention relates to a process for purification of 2-alkyl-6-oxo-l,6-dihydro-3,4'-bipyridine-5-carbonitrile derivatives (I) wherein associated impurities are filtered in the pH range of 9 to 10 and the resulting filtrate, on acidification, provides purified compound (I) conforming to regulatory specifications.
BACKGROUND OF THE INVENTION
Milrinone of formula (la), chemically known as 2-methyl-6-oxo-l,6-dihydro-3,4'-bipyridine-5-carbonitrile, is a phosphodiesterase 3 inhibitor, employed as a cardiotonic for increasing the heart's contractility. Milrinone, administered as its lactate salt, was approved in the United States by the Food and Drug Administration (FDA) in 1987 for the treatment of heart failure.

Milrinone (la) was first disclosed in US 4,313,951 wherein the synthetic process comprised reaction of l-(4-pyridinyl)-2-propanone and hexamethylphosphoramide in presence of dimethylformamide dimethyl acetal to give l-(4-pyridinyl)-2-(dimethylamino)ethenyl methyl ketone, which was reacted with a-cyanoacetamide employing sodium methoxide and dimethylformamide as solvent.
US 4,413,127 discloses reaction of 1-(4-pyridinyl) propane-2-one with triethyl orthoformate in presence of acetic anhydride and acetic acid to give l-ethoxy-2-(4-pyridinyl)ethenyl methyl ketone; which, after reaction with malononitrile in ethyl alcohol gave Milrinone.
Various methods for the synthesis of Compound (I) and Milrinone (la) are reported in the literature. However, almost all the synthetic methods have a major disadvantage of formation of associated impurities, especially the amide impurities (II), which are extremely difficult to

separate due to their close similarity to Milrinone with respect to structural features and solubility properties. On the other hand, the formation of associated impurities in the synthetic processes is also unavoidable and hence extensive measures such as repeated crystallization processes or chromatographic separations are needed to obtain pure Milrinone (la), which is suitable for use in drug product.

An attempt to tackle this problem was disclosed in US 6,118,002 wherein the Milrinone amide impurity (Ha) was derivatized by acylating agents. It was believed that the derivatized impurity had a different solubility that would facilitate separation of the impurity. However, the use of a reagent like acetic anhydride for derivatization, which is strictly regulated by authorities for its possible use in narcotics rendered the process unsuitable for commercial purpose. Further, due to additional unit steps of derivatization followed by hydrolysis and purification by recrystallization ultimately resulted in a yield of only 42%, which was not viable for commercial purpose.
Thus, there still exists a need for a convenient, commercially viable process for preparation of pure compound (I) and Milrinone (la), which avoids lengthy purification procedures or derivatization reactions involving various unit operations and is yet capable of providing the final compounds having desired purity. It is well known that purity criteria for active pharmaceutical ingredients like Milrinone are very stringent and are rigorously enforced by regulatory agencies.
While working on problem of controlling and / or removing associated amide impurity (II), either in reaction or during purification, the present inventors carried out a series of

experiments aimed at selectively separating the amide impurity by avoiding additional unit operations. Surprisingly, the inventors observed that the basicity of alkali-metal salt of the amide impurity in aqueous medium was significantly different from that of the parent compound (I). As a result, the amide impurity could be preferentially separated in a certain pH range by addition of acid to the aqueous alkaline solution.
Thus, the pure compounds (I) and Milrinone (la) were obtained without resorting to additional steps of derivatization, hydrolysis and purification thereby making the process cost-effective and advantageous for industrial manufacture. The purification method is carried out at room temperature and involves simple reagents and helps in separating the desired compound (I) from the associated amide impurity, which is formed to the extent of 10-12% during the course of synthesis, during the course of synthesis.
OBJECT OF THE INVENTION
An objective of the present invention is to provide a convenient and cost-effective process for preparation of 2-alkyl-6-oxo-l,6-dihydro-3,4'-bipyridine-5-carbonitrile derivatives (I) having desired purity without additional steps of purification.
SUMMARY OF THE INVENTION
The present invention relates to a method for purification of 2-alkyl-6-oxo 1,6 dihydro-3,4'-bipyridine-5-carbonitrile derivatives (I).
An aspect of the invention relates to a method for purification of 2-alkyl-6-oxo 1,6 dihydro-3,4'-bipyridine-5-carbonitrile derivatives (I), comprising of dissolving compound (I) in aqueous alkaline solution, gradually adding an acid to get a pH range of 9 to 10, filtering the amide impurity (II), followed by acidification of the filtrate to provide compound (I) conforming to regulatory specifications.
Another aspect of the invention relates to a method for purification of Milrinone (la), comprising of dissolving compound (la) in aqueous alkaline solution, gradually adding an acid to get a pH range of 9 to 10, filtering the amide impurity (Ha), followed by acidification of the filtrate to provide compound (la) conforming to regulatory specifications.

The objectives of the present invention will become more apparent from the following detailed description.
DETAILED DESCRIPTION OF THE INVENTION
Comparison of the structures of 2-alkyl-6-oxo-l,6-dihydro-3,4'-bipyridine-5-carbonitrile derivatives (I) and the amide impurity (II) reveals that the amide (CONH2) functional group is the only differentiating feature in the impurity in place of cyano group in (I). The cyano and amide groups exert similar inductive effects and both compounds (I) and (II) exist in keto-enol tautomeric forms wherein the enol form is the respective phenol. All these facts add to the difficulties in separating the amide impurity from (I), wherein separation based on differences between solubility profiles or other physical properties is extremely difficult due to the structural similarity. For the reasons discussed earlier, application of derivatization-type techniques for purification of the active pharmaceutical ingredient (API) is a cumbersome task especially on industrial scale.
However, while carrying out extensive experimentation aimed at development of a convenient and economically viable process for purification of Milrinone, the present inventors surprisingly observed that the alkali metal salts of Milrinone and other derivatives of compound (I) and their corresponding amide impurities (II), exhibited substantial differences in their relative basicities. As a result, when an aqueous alkaline solution of (I) containing the amide impurity was acidified by gradual addition of an acid, the impurity precipitated out as a solid in the pH range of 9 to 10 indicating preferential separation of the amide impurity from the mixture. The filtrate containing the pure compound (I or la) was acidified further to provide compound (I or la) of the desired purity.
Thus the method did not involve any derivatization procedures or hazardous reagents and was simple, convenient, and economical for obtaining Milrinone (I) on a commercial scale.

Additional steps of crystallization(s) either with single solvents or solvent mixtures were circumvented in this method. Significantly, the method provided compounds (I) and Milrinone (la) having desired purity, which was suitable for their application as pharmaceutical agents.
In an embodiment, Milrinone (la) containing around 8-10% of the amide impurity was dissolved in an alkaline aqueous solution at room temperature. The pH of the solution was gradually adjusted between 9 and 10 by addition of an acid to separate out the amide impurity (II), which was filtered. The filtrate was further acidified till pH range of 5 to 7 to provide compound of formula (I) having purity conforming to regulatory specifications.
The alkaline aqueous solution was obtained by dissolving in water a base selected from the group comprising of alkali metal hydroxide, alkali metal carbonates, and alkaline buffers selected from hydroxides, carbonates, phosphates of lithium sodium, potassium and cesium etc.
The acid was selected from the group comprising of mineral acids and organic acids, but preferably organic acids.
The organic acid was selected from the group comprising of acetic acid, formic acid, propionic acid, citric acid etc.
The following examples are meant to be illustrative of the present invention. These examples exemplify the invention and are not to be construed as limiting the scope of the invention.
EXAMPLES
Example 1: Purification of Milrinone (la)
Crude Milrinone (I; 10.3g) containing 7 to 8 % of amide impurity was dissolved in aqueous sodium hydroxide solution (1.9g sodium hydroxide in 50 ml water) at 25-30°C. Acetic acid (0.7 g in 14 ml water) was gradually added till pH range of 9 to 10 and the solid (Ha) separating out was filtered.
The filtrate was acidified to pH range of 6 to 6.5 with acetic acid (0.7 g in 14 ml water) to obtain compound (I), which was filtered and dried.

Yield: 55.6 %; Purity>99.5%
Example 2: Purification of Milrinone (la)
Crude Milrinone (I; 190.2g) containing 0.9% of amide impurity (Ha) was dissolved in aqueous potassium hydroxide solution (31 g potassium hydroxide in 1200 ml water) at 25-30°C. Acetic acid (16.2 g in 300 ml water) was gradually added to the stirred reaction mass till pH 9-10 to separate out compound (Ha), which was filtered.
The filtrate was further acidified further with acetic acid (36 g in 200 ml water) to obtain Milrinone (I), which was filtered and dried. Yield: 122.3 g Purity: >99.5%

Claims:
1. A process for purification of 2-alkyl-6-oxo 1,6 dihydro-3,4'-bipyridine-5-carbonitrile (I), comprising of dissolving compound (I) in an aqueous alkaline solution, adjusting pH between 9 and 10 by addition of an acid, filtering compound (II) and further acidifying the filtrate to obtain compound (I).
2. A process for purification of Milrinone (la), comprising of dissolving compound (la) in an aqueous alkaline solution, adjusting pH between 9 and 10 by addition of an acid, filtering compound (Ha) and further acidifying the filtrate to obtain compound (la).
3. A process as claimed in claim 1 and 2, wherein the acid is either an inorganic or organic acid.
4. A process as claimed in claim 1, 2 and 3, wherein the acid is preferably an organic acid and is selected from the group comprising of acetic acid, formic acid, propionic acid • and citric acid.
5. A process as claimed in claim 1, wherein compound of formula (I) is isolated by addition of an acid to the filtrate, till pH range of 5 to 7.