Description:FIELD OF THE INVENTION
The present invention relates to a process for purification of Risdiplam wherein the pure product obtained is characterized by the impurities Des -fluoro Impurity i.e. 2-(2,8-dimethylimidazo[1,2-b] pyridazin-6-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 1) and 2-(2,8-dimethylimidazo[1,2-b]pyridazin-6-yl)-7-(4,7-diazaspiro[2.5]octan-4-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 2) which are structurally related to Risdiplam and which are present NMT 0.10%.

BACKGROUND OF THE INVENTION
Risdiplam which is chemically 7-(4,7-Diazaspiro[2,5]octan-7-yl)-2-(2,8-dimethylimidazo[1,2-b]pyridazin-6-yl)-pyrido[1,2-a]pyrimidin-4-one (Formula-1) is a medication used to treat spinal muscular atrophy (SMA) and the first oral medication approved to treat this disease.

Various processes are known in the art for preparation of Risdiplam, however, the processes described in the art resulted in poor yield due to presence of closely related structures as impurities.
Purity of the API or its intermediates is of utmost importance in the field of pharmaceutical industry. It is well documented in the art that the chemical synthesis of the API is rarely a single compound with sufficient purity to comply with pharmaceutical standards. The impurities that can be present in pharmaceutical compounds are starting material by-products of the reaction or the products of side reactions or degradation products and may be structurally similar. Such presence of impurities is undesirable and may be harmful to a patient being treated with a dosage form containing the API.
The ICH guidelines clearly dictate that process impurities should be maintained below set limits by specifying the quality of raw materials, their stoichiometric ratios, controlling process parameters such as temperature, pressure, time and including purification steps in the manufacturing process. Typically these limits are less than about 0.15 % by weight of each identified impurity. Limits for unidentified and/or uncharacterized impurities are obviously lower typically less than 0.10 % by weight.
It is pertinent therefore for a pharmaceutical active compound to be free of impurities or contain impurities within the acceptable limits.
The present Applicants earlier filed patent application no. IN202321004252 discloses the preparation of Risdiplam and the novel intermediates in high yield and purity and with less effluent.
The present applicant however felt that there is a scope to provide Risdiplam with HPLC purity of >99.5%. Further, during working on the purification process of Risdiplam the Applicant surprisingly found the formation of two impurities that are structurally related to Risdiplam, however, within the limits of ICH guidelines.

OBJECT OF THE INEVNTION
The primary object of the present invention is to provide a simple cost effective process for purification of Risdiplam which can be scaled to the industrial level.
The other objective is to provide a process for purification of Risdiplam which is characterized by the impurities viz. Des-fluoro Impurity i.e. 2-(2,8-dimethylimidazo[1,2-b]pyridazin-6-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 1) and 2-(2,8-dimethylimidazo[1,2-b]pyridazin-6-yl)-7-(4,7-diazaspiro[2.5]octan-4-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 2).

SUMMARY OF THE INVENTION
Accordingly, the present invention provides a simple, cost effective process for purification of Risdiplam to obtain Risdiplam of HPLC purity =99.5% comprising;
i. Dissolving crude wet Risdiplam in a mixture of acid and alcohol to obtain the slurry followed by filtering and washing to obtain the wet product;
ii. Stirring the wet product of step (i) in a mixture of water and solvent; settling and separating the layers;
iii. Washing the aqueous layer with the solvent, separating and collecting the aqueous layer;
iv. Adjusting the pH to 10-12 with a base followed by filtering the solid and washing to obtain Risdiplam as a wet solid;
v. Crystallizing Risdiplam wet solid of step (iv) from lower alcohol;
wherein said Risdiplam is comprises the impurities 2-(2,8-dimethylimidazo[1,2-b] pyridazin-6-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 1) and 2-(2,8-dimethylimidazo[1,2-b]pyridazin-6-yl)-7-(4,7-diazaspiro[2.5]octan-4-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 2).

Formula 1 Formula 2

In an aspect, crude Risdiplam used in step (i) for purification process is prepared by the process disclosed in IN202321004252.
In yet another aspect, the present invention provides a novel impurity 2-(2,8-dimethylimidazo[1,2-b]pyridazin-6-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 1);
Formula 1

In another aspect, the present invention provides a novel impurity 2-(2,8-dimethylimidazo[1,2-b]pyridazin-6-yl)-7-(4,7-diazaspiro[2.5]octan-4-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 2);

Formula 2

DETAILED DESCRIPTION OF THE INVENTION
The invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be more fully understood and appreciated.
The present invention relates to a simple, cost effective process for purification of Risdiplam with HPLC purity >99.5% and which comprises the impurities of 2-(2,8-dimethylimidazo[1,2-b]pyridazin-6-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 1) and 2-(2,8-dimethylimidazo[1,2-b]pyridazin-6-yl)-7-(4,7-diazaspiro[2.5]octan-4-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 2). The impurities are controlled in the final product wherein said impurities are NMT 0.10% detected by HPLC and which are within the limits of ICH guidelines for the API.
In an embodiment, the present invention relates to a simple, cost effective process for purification of Risdiplam to obtain Risdiplam of HPLC purity =99.5% comprising;
i. Dissolving crude wet Risdiplam in a mixture of acid and alcohol to obtain the slurry followed by filtering and washing to obtain the wet product;
ii. Stirring the wet product of step (i) in a mixture of water and solvent; settling and separating the layers;
iii. Washing the aqueous layer with the solvent, separating and collecting the aqueous layer;
iv. Adjusting the pH to 10-12 with a base followed by filtering the solid and washing to obtain Risdiplam as a wet solid;
v. Crystallizing Risdiplam wet solid of step (iv) from lower alcohol ;
wherein said Risdiplam comprises the impurities 2-(2,8-dimethylimidazo[1,2-b] pyridazin-6-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 1) and 2-(2,8-dimethylimidazo[1,2-b]pyridazin-6-yl)-7-(4,7-diazaspiro[2.5]octan-4-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 2).

Formula 1 Formula 2

The alcohol for the process step (i) is selected from C1-C5 straight or branched chain alcohol.
The solvent for the process of the present invention includes but is not limited to lower alcohols such as methanol ethanol, isopropyl alcohol, butanol, t-butanol; halogenated hydrocarbons such as dichloromethane, chloroform, carbon tetrachloride, 1 2- dichloroethane; aliphatic or aromatic hydrocarbons such as hexane, heptane, toluene, xylene; water and the like alone or mixtures thereof.
The acid is selected from organic acids such as acetic acid, propionic acid, butyric acid and the like which optionally may be substituted organic acid.
The base is selected from organic or inorganic base which include but is not limited to aq. ammonia, hydroxides or carbonates or bicarbonates of alkali and alkaline earth metals such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate and the like alone or mixtures thereof.
According to the process of the present invention, crude wet Risdiplam of purity 90% was mixed in the mixture of acid and alcohol and stirred at a temperature ranging between 27 to 35°C to obtain uniform slurry. The slurry was filtered and washed with suitable solvent to obtain the wet product.
To said wet product was charged water, suitable solvent and stirred the mixture and maintained for about 30-35minutes. The mixture was allowed to settle and layers were separated. The aqueous layer was collected and washed with suitable solvent. Separated the layers and collected the aqueous layer. Added suitable amount of base to adjust the pH in the range of 10-12 and maintained for about 30 minutes. The solid was filtered and washed with water to obtain Risdiplam as wet solid.
The above wet solid was crystallized from lower alcohol under stirring and maintaining the mixture at 30-40°C for about an hour followed by filtering, washing the solid with alcohol to obtain Risdiplam of purity =97.0%.
The crystallized Risdiplam of purity =97.0% was subjected to the purification process steps as given above until Risdiplam having HPLC purity =99.5% is obtained.
The Risdiplam purified by the present process has a purity more than 99.5%.
The crude Risdiplam used in the present invention is prepared by the process disclosed in IN202321004252.
In another embodiment, the present invention discloses the impurity 2-(2,8-dimethylimidazo[1,2-b]pyridazin-6-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 1);

characterized by IR bands at 2907cm-1, 3074cm-1, 1481cm-1, 1445cm-1, 1669cm-1, 1623cm-1; Mass spectrum at (M+H) at 292 m/z. The said impurity is present in the final product NMT 0.10% indicated by HPLC.
In another embodiment, the present invention disclose the impurity 2-(2,8-dimethylimidazo[1,2-b]pyridazin-6-yl)-7-(4,7-diazaspiro[2.5]octan-4-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 2),

characterized by as LCMS: 402 (M+H+) wherein said impurity is present in the final product NMT 0.10% indicated by HPLC.
The impurities were characterized by IR, 1H-NMR and Mass spectroscopy indicated in the tables below which confirm their structure.
In an embodiment, the present process for purification of Risdiplam is simple, cost effective and can be scaled to industrial process. The process is safe and operationally environment friendly.
The following examples illustrate the present invention in detail but the examples are not intended in any way to limit the scope of the present invention

Examples:
Example 1:
Into a 1.0 Lit 4 neck round bottom flask was charged crude wet Risdiplam (37.0 g) with purity of about 90.0%, Acetic Acid (37.0 mL) and methanol (200.ml) under stirring at 25-35°C to obtain uniform slurry. Filtered the slurry, washed with Methanol (25 mL), unloaded the wet product.
Yield: (30-40 gm).
Charged the above wet Risdiplam (35.0 g), water (200.0 mL), methylene dichloride (MDC) (200.0 mL) under stirring and maintained for 30 min. Settled and separated the layers. Collected aqueous layer and washed with MDC (200 ml). Separated the layers and collected the aqueous layer and adjusted pH of the aqueous layer to 10-12 with aqueous ammonia. Maintained for 30 min, filtered the solid and washed with water to obtain Risdiplam as a wet solid.
Yield: 30-40 gm.
Purity: =95.0%
To the above wet Risdiplam (35.0 gm) was charged methanol (70 ml) under stirring and maintained at 30-40°C form one hour. Filtered the solid and washed with methanol to obtain Risdiplam as wet solid 28-38 gm.
Purity: 97.0%

Example 2:
To the Risdiplam (27.0 gm) of purity NLT 97.0% obtained in Example-1, acetic acid (27.0 ml) and methanol (150ml) were added under stirring at 30-35°C. Maintained for one hour, filtered and washed the solid with methanol (25 ml) to obtain the wet solid 35-40 gm.
Charged above wet Risdiplam (35.0 g), water (150.0 mL), MDC (150.0 mL) under stirring and maintained for 30 min. Settled and separated the layers. Collected aqueous layer and washed twice with MDC (150 ml). Separated the layers and collected aqueous layer. Collected aqueous layer and washed with ethyl acetate (150 ml). Separated the layers and collected aqueous layer. To the aqueous layer charged toluene (150 ml). Separated the layers and collected aqueous layer. To the aqueous layer charged activated charcoal (0.3gm) and stirred for one hour. Filtered the charcoal and collected the filtrate. Added aqueous ammonia and pH was adjusted to 10-12. Maintained for 30 min., filtered the solid and washed with water to obtain Risdiplam as a wet solid 30-40 gm.
To the wet solid (34.0 gm) charged methanol (70 ml) under stirring at 30-35°C. Maintained for one hour and filtered and dried at 60-70°C to obtain pure Risdiplam of ICH grade.
(HPLC purity: > 99.5%).

Impurity profile:
Novel Impurity

Des -fluoro Impurity i.e. 2-(2,8-dimethylimidazo[1,2-b] pyridazin-6-yl)-4H-pyrido[1,2-a]pyrimidin-4-one

Formula 1
Controlled in final product by HPLC NMT 0.10 %
2-(2,8-dimethyl imidazo[1,2-b] pyridazin-6-yl)-7-(4,7-diazaspiro[2.5]octan-4-yl)-4H-pyrido[1,2-a]pyrimidin-4-one


Formula 2 Controlled in final product by HPLC NMT 0.10 %

Spectral studies:
a) Impurity of formula 1;

Characterization Data: 1H-NMR (CDCl3, 400 MHz): 9.17-9.16 (d, 1H), 8.54 (s, 1H), 7.93-7.88 (m, 2H), 7.85-7.82 (d, 1H), 7.47 (s, 1H), 7.32-7.29 (m, 1H), 3.12 (s, 3H), 2.83 (s, 3H); LCMS: 292 (M+H+).

Impurity of formula 2

Characterization Data: LCMS: 402 (M+H+)

Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims. , Claims:1. A simple, cost effective process for purification of Risdiplam to obtain Risdiplam of HPLC purity =99.5% comprising;
i. Dissolving crude wet Risdiplam in a mixture of acid and alcohol to obtain the slurry followed by filtering and washing to obtain the wet product;
ii. Stirring the wet product of step (i) in a mixture of water and solvent; settling and separating the layers;
iii. Washing the aqueous layer with the solvent, separating and collecting the aqueous layer;
iv. Adjusting the pH to 10-12 with a base followed by filtering the solid and washing to obtain Risdiplam as a wet solid; and
v. Crystallizing Risdiplam wet solid of step (iv) from lower alcohol;
wherein said Risdiplam comprises the impurities 2-(2,8-dimethylimidazo[1,2-b] pyridazin-6-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 1) and 2-(2,8-dimethylimidazo[1,2-b] pyridazin-6-yl)-7-(4,7-diazaspiro[2.5]octan-4-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 2).

Formula 1 Formula 2

2. The process as claimed in claim 1, wherein the alcohol for the process step (i) is selected from C1-C5 straight or branched chain alcohol.

3. The process as claimed in claim 1, wherein the solvent includes but is not limited to lower alcohols such as methanol ethanol, isopropyl alcohol, butanol, t-butanol; halogenated hydrocarbons such as dichloromethane, chloroform, carbon tetrachloride, 1 2- dichloroethane; aliphatic or aromatic hydrocarbons such as hexane, heptane, toluene, xylene; water and the like alone or mixtures thereof

4. The process as claimed in claim 1, wherein the acid is selected from organic acids such as acetic acid, propionic acid, butyric acid and the like which may be substituted organic acid.

5. The process as claimed in claim 1, wherein the base is selected from organic or inorganic base which include but is not limited to aq. ammonia, hydroxides or carbonates or bicarbonates of alkali and alkaline earth metals such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate and the like alone or mixtures thereof.

6. The process as claimed in claim 1, wherein the crude Risdiplam is prepared by the known process.

7. An impurity 2-(2,8-dimethylimidazo[1,2-b] pyridazin-6-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 1);

characterized by 1H-NMR (CDCl3, 400 MHz): 9.17-9.16 (d, 1H), 8.54 (s, 1H), 7.93-7.88 (m, 2H), 7.85-7.82 (d, 1H), 7.47 (s, 1H), 7.32-7.29 (m, 1H), 3.12 (s, 3H), 2.83 (s, 3H); LCMS: 292 (M+H+)

8. An impurity 2-(2,8-dimethylimidazo[1,2-b] pyridazin-6-yl)-7-(4,7-diazaspiro[2.5]octan-4-yl)-4H-pyrido[1,2-a]pyrimidin-4-one (Formula 2),
Formula 2
characterized by LCMS: 402 (M+H+)