DESC:
FIELD OF THE INVENTION
The present invention relates to processes for the preparation of substantially pure Letermovir. The present invention further relates to processes for the preparation of substantially pure Letermovir essentially free from N-oxide impurity. The present invention further relates to processes for the preparation of substantially pure amorphous Letermovir. The present invention further relates to use of the substantially pure amorphous Letermovir API for the preparation of formulations for oral administration.

BACKGROUND
The present invention relates to processes for the preparation of substantially pure Letermovir. The present invention also relates to processes for the preparation of the substantially pure Letermovir essentially free from N-oxide impurity. Letermovir was approved for the treatment for prophylaxis of cytomegalovirus (CMV) infection and is currently marketed under the proprietary name Prevymis in USA. The chemical name of Letermovir is (S)-{8-Fluoro-2-[4-(3-methoxyphenyl)-1-piperazinyl]-3-[2-methoxy-5-(trifluoromethyl) phenyl]-3,4-dihydro-4-quinazolinyl}acetic acid and the structure of Letermovir is as per Formula-I as depicted below.

Letermovir is described in US 2005/0065160 for use in methods of prophylaxis or methods of treatment of viral diseases, HCMV infections. The synthesis of Letermovir is also disclosed in the examples 14 and 15 of US’160 and in this, Letermovir is isolated using chromatography followed by acid/base precipitation process. However, US’160 is silent about purity and other physicochemical properties of the compound.

The synthesis of Letermovir is also disclosed in the example 11 of US 2009/0221822 and in this, Letermovir is isolated in amorphous solid by evaporation of ethanolic solution of the product up to dryness. However, US’822 is silent about purity of the compound.

The synthesis of Letermovir is also disclosed in the example 6A of US 2015/0045371 and in this, Letermovir is precipitated in amorphous solid by adding water to ethanolic solution of the product. However, US’371 is also silent about purity of the compound.

The isolation of amorphous Letermovir is also disclosed in the examples of US 2016/0145216. The process involves hydrolysis reaction of Letermovir ester derivative to Letermovir at first place and then Letermovir is extracted in methyl tert-butyl ether (MTBE) solvent. The solvent is switched from MTBE to acetone/acetonitrile and finally the amorphous Letermovir was isolated by either use of i) a roller dryer, or ii) by precipitation of an acetonic or acetonitrile solution of Letermovir into an excess of stirred water. However, US’216 is also silent about purity of the compound.

WO 2013/127968 discloses purification processes of Letermovir. The purification processes involve the preparation of besylate salts or tosylate salts of Letermovir to remove impurities. Finally, the salts are converted back into Letermovir. The process involves addition steps, incurring cost and loss in the yield percentage.

These documents are incorporated herein by reference in entirety for all the purposes.

Hence, a need exists in the art for a process that allows for preparation of substantially pure Letermovir on an industrial scale by sufficient yield, purity and have residual solvents in accordance with ICH guideline.

Surprisingly, the inventors of the present application have unexpectedly found processes for the preparation of substantially pure Letermovir as describe below in detail.

OBJECT OF THE INVENTION
The present invention relates to processes for the preparation of substantially pure Letermovir. The substantially pure Letermovir is essentially free from N-oxide impurity. The substantially pure Letermovir is in amorphous state. The present processes make substantially pure amorphous Letermovir API, is ready to be formulated in a solid pharmaceutical formulation for oral administration and for use in methods of prophylaxis or methods of treatment of viral diseases.

Specifically, the objective of present invention is summarized as follows:

(1) Objective of the present invention is related to substantially pure Letermovir.

(2) Second objective of the present invention is related to substantially pure amorphous Letermovir essentially free from N-oxide impurity.

(3) Third objective of the present invention is related to processes for the preparation of substantially pure Letermovir.

(4) Another objective of the present invention is related to processes for the preparation of substantially pure amorphous Letermovir essentially free from N-oxide impurity.

(5) Another objective of the present invention is related to purification of Letermovir by applying DMF:water washing to Letermovir solution for the preparation of the substantially pure Letermovir.

(6) Another objective of the present invention is related to a process for the preparation of substantially pure Letermovir comprising:
a) providing solution of Letermovir in organic solvent, containing N-oxide impurity,
b) washing the solution of step a) by DMF:water,
c) mixing the solution of step b) with heptane to precipitate solid,
d) isolating the solid from step c) via filtration or centrifugation,
e) optionally, drying the solid of step d) to obtain the substantially pure Letermovir.

(7) Another objective of the present invention is related a process the preparation of substantially pure amorphous Letermovir comprising:
a) providing solution of Letermovir in organic solvent, containing N-oxide impurity,
b) washing the solution of step a) by DMF:water,
c) mixing the solution of step b) with heptane at less than 0 degree Celsius to precipitate solid,
d) isolating the solid from step c) via filtration or centrifugation,
e) drying the solid of step d) to obtain the substantially pure amorphous Letermovir.

(8) Another objective of the present invention is related a process the preparation of substantially pure amorphous Letermovir comprising:
a) providing solution of Letermovir in organic solvent, containing N-oxide impurity,
b) washing the solution of step a) by DMF:water,
c) mixing the solution of step b) with heptane to precipitate solid,
d) isolating the solid from step c) via filtration or centrifugation,
e) drying the solid of step d) to obtain the substantially pure amorphous Letermovir,
wherein the substantially pure amorphous Letermovir contains residual solvents in accordance with ICH guidelines.

(9) Another objective of the present invention is related to a process for isolating substantially pure amorphous Letermovir comprising:
a) providing MTBE solution of Letermovir essentially free from N-oxide impurity,
b) mixing the solution of step a) with heptane at temperature less than 0 degree Celsius to precipitate solid,
c) isolating the solid from step b) via filtration or centrifugation to obtain the substantially pure amorphous Letermovir,
wherein the substantially pure amorphous Letermovir contains residual solvents in accordance with ICH guidelines.

(10) Another objective of the present invention is related to a process for isolating substantially pure amorphous Letermovir comprising:
a) providing MTBE solution of Letermovir essentially free from N-oxide impurity,
b) mixing the solution of step a) with heptane at temperature less than 0 degree Celsius to precipitate solid,
c) isolating the solid from step b) via filtration or centrifugation,
d) drying the solid of step c) to obtain the substantially pure amorphous Letermovir,
wherein the substantially pure amorphous Letermovir contains residual solvents in accordance with ICH guidelines.

(11) Another objective of the present invention is related to a process for preparation of Letermovir essentially free from N-oxide impurity comprising:
a) providing solution of Letermovir in solvent, containing N-oxide impurity,
b) treating the solution of step a) with catalyst(s) to reduce the N-oxide impurity,
c) removing solvent of step b),
d) isolating the Letermovir essentially free from N-oxide impurity.

(12) Another objective of the present invention is related to a process for preparation of substantially pure Letermovir comprising:
a) providing solution of Letermovir in solvent, containing N-oxide impurity,
b) treating the solution of step a) with catalyst(s) to reduce the N-oxide impurity,
c) mixing the solution of step b) with heptane at temperature less than 0 degree Celsius to precipitate solid,
d) isolating the solid from step c) via filtration or centrifugation,
e) optionally, drying the solid of step d) to obtain the substantially pure amorphous Letermovir.

(13) Another objective of the present invention is related to a process for preparation of substantially pure amorphous Letermovir comprising:
a) providing solution of Letermovir in solvent, containing N-oxide impurity,
b) treating the solution of step a) with catalyst(s) to reduce the N-oxide impurity,
c) mixing the solution of step b) with heptane at temperature less than 0 degree Celsius to precipitate solid,
d) isolating the solid from step c) via filtration or centrifugation,
e) drying the solid of step d) to obtain the substantially pure amorphous Letermovir,
wherein the substantially pure amorphous Letermovir contains residual solvents in accordance with ICH guidelines.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1: PXRD pattern of substantially pure amorphous Letermovir of example 1.
FIG. 2: Certification of analysis of substantially pure amorphous Letermovir of example 1.
FIG. 3A & 3B: HPLC result of substantially pure amorphous Letermovir of example 1.
FIG. 4: PXRD pattern of amorphous Letermovir of example 7.
FIG. 5: PXRD pattern of amorphous Letermovir of example 8.
FIG. 6: PXRD pattern of amorphous Letermovir of example 18.
FIG. 7: Certification of analysis of amorphous Letermovir of example 19.
FIG. 8A & 8B: HPLC result of amorphous Letermovir of example 19.

DESCRIPTION OF THE INVENTION
The present invention relates to processes for the preparation of substantially pure Letermovir. The present invention further relates to processes for the preparation of substantially pure Letermovir in amorphous state. Specifically, the present invention further relates to processes for the preparation of substantially pure Letermovir essentially free from N-oxide impurity. The present invention further relates to purification of Letermovir containing N-oxide impurity. The present invention further relates to processes for the reducing the content of N-oxide impurity in Letermovir API. The present invention provides a cost effective and industrially viable processes for obtaining substantially pure amorphous Letermovir with higher yield.

The present invention relates to processes for the preparation of substantially pure Letermovir essentially free from N-oxide impurity and the structure of the N-oxide impurity is as depicted below:

In one aspect, substantially pure Letermovir is prepared by
a) providing solution of Letermovir in organic solvent, containing N-oxide impurity,
b) washing the solution of step a) by DMF:water,
c) mixing the solution of step b) with heptane to precipitate solid,
d) isolating the solid from step c) via filtration or centrifugation,
e) optionally, drying the solid of step d) to obtain the substantially pure Letermovir.

In second aspect, substantially pure Letermovir is prepared by
a) providing solution of Letermovir in organic solvent, containing N-oxide impurity,
b) washing the solution of step a) by DMF:water,
c) mixing the solution of step b) with heptane at less than 0 degree Celsius to precipitate solid,
d) isolating the solid from step c) via filtration or centrifugation,
e) drying the solid of step d) to obtain the substantially pure Letermovir.

In third aspect, Letermovir of the present invention is amorphous in state and having the residual solvents in accordance with ICH guidelines.

In another aspect, a process the preparation of substantially pure amorphous Letermovir comprising:
a) providing solution of Letermovir in organic solvent, containing N-oxide impurity,
b) washing the solution of step a) by DMF:water,
c) mixing the solution of step b) with heptane to precipitate solid,
d) isolating the solid from step c) via filtration or centrifugation,
e) drying the solid of step d) to obtain the substantially pure amorphous Letermovir,
wherein the substantially pure amorphous Letermovir contains residual solvents in accordance with ICH guidelines.

In another aspect, a process for isolating substantially pure amorphous Letermovir comprising:
a) providing MTBE solution of Letermovir essentially free from N-oxide impurity,
b) mixing the solution of step a) with heptane at temperature less than 0 degree Celsius to precipitate solid,
c) isolating the solid of step b) via filtration or centrifugation to obtain the substantially pure amorphous Letermovir,
wherein the substantially pure amorphous Letermovir contains residual solvents in accordance with ICH guidelines.

In another aspect, a process for isolating substantially pure amorphous Letermovir comprising:
a) providing MTBE solution of Letermovir essentially free from N-oxide impurity,
b) mixing the solution of step a) with heptane at temperature less than 0 degree Celsius to precipitate solid,
c) isolating the solid of step b) via filtration or centrifugation,
d) drying the solid of step c) to obtain the substantially pure amorphous Letermovir,
wherein the substantially pure amorphous Letermovir contains residual solvents in accordance with ICH guidelines.

In another aspect of the present invention, the organic solvent is water immiscible organic solvent(s). Preferably MTBE. Volume of the organic solvent is 1 volume to 5 volume with respect to Letermovir.

According to the present invention, the solution of Letermovir in the organic solvent is washed by DMF:water to remove and/or to decrease the content of the impurity, specifically the N-oxide impurity, wherein the ratio of DMF and water is from 1:99 % v/v to 50:50 % v/v.

According to the present invention, the reaction mixture is stirred for enough time to enable the removal of the impurity during the washing, preferably about 30 min. to 5 hours.

According to the present invention, the reaction mixture is washed by DMF:water for one time or more than one time to enable the removal of the impurity.

In another aspect, a process for preparation of Letermovir essentially free from N-oxide impurity comprising:
a) providing solution of Letermovir in solvent, containing N-oxide impurity,
b) treating the solution of step a) with catalyst(s) to reduce the N-oxide impurity,
c) removing solvent of step b),
d) isolating the Letermovir essentially free from N-oxide impurity.

In another aspect, a process for preparation of substantially pure amorphous Letermovir comprising:
a) providing solution of Letermovir in solvent, containing N-oxide impurity,
b) treating the solution of step a) with catalyst(s) to reduce the N-oxide impurity,
c) mixing the solution of step b) with heptane at temperature less than 0 degree Celsius to precipitate solid,
d) isolating the solid from step c) via filtration or centrifugation.
e) optionally, drying the solid of step d) to obtain the substantially pure amorphous Letermovir.

In another aspect, a process for preparation of substantially pure amorphous Letermovir comprising:
a) providing solution of Letermovir in solvent, containing N-oxide impurity,
b) treating the solution of step a) with catalyst(s) to reduce the N-oxide impurity,
c) mixing the solution of step b) with heptane at temperature less than 0 degree Celsius to precipitate solid,
d) isolating the solid from step c) via filtration or centrifugation,
e) drying the solid of step d) to obtain the substantially pure amorphous Letermovir,
wherein the substantially pure amorphous Letermovir contains residual solvents in accordance with ICH guidelines.

According to the present invention, the catalyst acts as reducing agent and is capable to reduction of the N-oxide impurity. The impurity, after reaction with the catalyst, are converted back into Letermovir.

According to the present invention, the solvent is organic solvent(s). Preferably tetrahydrofuran, acetonitrile, MTBE, or mixture thereof.

According to the present invention, the catalyst is selected from borohydride compounds and triphenyl phosphine for the reduction the N-oxide impurity.

According to the present invention, the catalyst borohydride is preferably sodium borohydride or lithium borohydride.

According to the present invention, the reaction mixture is treated with the catalyst for enough time and temperature to enable the reduction of the N-oxide impurity during the reaction, preferably about 30 min. to 5 hours and at 30 degree Celsius to boiling point of reaction solvent(s).

According to the present invention, Letermovir is isolated in amorphous state by precipitation process from mixing of the MTBE solution of Letermovir with heptane as anti-solvent at temperature less than about 0 degree Celsius.

According to the present invention, the precipitation temperature is about less than 0 degree Celsius, preferably at about 0 to -10 degree Celsius.

According to the present invention, the reaction mixture is stirred for enough time to complete the precipitation, preferably about 30 min. to 5 hours.

In another aspect of the present invention, the precipitated amorphous Letermovir is isolated via filtration or centrifugation.

In another aspect of the present invention, the amorphous Letermovir produced by present invention is substantially pure and contains the residual solvents in accordance with ICH guidelines.

According to the present invention, the substantially pure amorphous Letermovir is further dried under vacuum at about 40 degree Celsius to 100 degree Celsius for 2 - 24 hours, preferably at about 50 degree Celsius to 80 degree Celsius for 5 - 20 hours.

In another aspect of the invention, the isolated substantially pure amorphous Letermovir is essentially free from N-oxide impurity.

According to the present invention, purity of the Letermovir is measured as per below HPLC method.
Column: Ascentis Express C18 (4.6 x 150) mm, 2.7µm; Flow rate: 1.0 ml/min.; Wavelength: UV at 210 nm; Injection volume: 5µL; Column oven temperature: 25 degree Celsius; Auto sampler temperature: 25 degree Celsius; Run time: 50 minutes; Eluent A: a mixture of Buffer, acetonitrile, and methanol (80:10:10); Eluent B: a mixture of Buffer, acetonitrile, and methanol (35:55:10); Buffer: a mixture of 1.56 gm Sodium dihydrogen orthophosphate dehydrate and 1000ml water and then adjust pH to 3.5 by orthophosphoric acid; Gradient: 0.01 min 35% B, 43 min 90% B, 44 min 36% B.

According to the present invention, the substantially pure amorphous Letermovir, contains residual solvents in accordance with ICH guidelines as per below static headspace Gas chromatography method.
Weigh 100mg of Letermovir sample and transfer to a 5ml volumetric flask. Dissolve and dilute to the volume with 1-Methyl-2-pyrrolidinone. Pipette out 1.0ml of this solution into a headspace vial. Seal with a septum and crimp cap. The analysis is performed by using Instrument: Perkin Elmer Gas Chromatograph; Column: DB-624 (30 meters length, 0.32mm inner diameter, 1.8µm Capillary Column); Detector: Flame Ionization Detector; Carrier gas: Nitrogen; Carrier gas Flow: 2.2 ml/min; Detector Temperature: 250°C.; Injector Temperature: 200°C.; Detector Range: 1; Attenuation: 32; Split: Split-less; Hydrogen Gas Flow: 45.0 ml/min.; Zero Air Gas Flow: 450.0 ml/min.; Equilibration Time: 0.5 min.; FID Filter Time Constant: 200 ms; Carrier Gas Flow Offset (Fixed): 3.0ml/min.; Detector Offset: 5.0 mV; Injection volume calc. flow rate (HS): 2 ccm.

Oven Temp. Program:
Ramp °C/min Temperature (°C) Hold (min)
-- 40 5
40 220 2

Head space parameters:
HS Mode: Constant; Oven Temperature: 90°C; Needle Temperature: 100°C; Transfer line temperature: 110°C; GC Cycle time: 25 minutes; Thermostat time: 30 min.; Pressurization time: 3.0 min.; Injection Time: 0.05 min.; Injection volume: 0.1 ml; Withdrawal time: 0.1 min.; Headspace Pressure: 25 psig.

The inventors of present application have found that the isolation solvent(s) play major role in purity of amorphous Letermovir API and thus specific and adequate solvent(s) is(are) needed to obtain Letermovir in substantially pure Letermovir with chemically stable amorphous form.

Surprisingly, the inventors have found against expectation that washing of Letermovir solution by DMF:water is capable to remove and/or to reduce the content of the N-oxide impurity. The inventors have found that reagents like borohydride compounds or triphenyl phosphine are capable for the reduction of N-oxide impurity and the N-oxide impurity are converted back into Letermovir. Further, MTBE/heptane solvent combination is the most preferred solvent combination to be applied for precipitation of amorphous Letermovir.

Furthermore, in accordance with the invention, substantially pure Letermovir is essentially free from N-oxide impurity and is amorphous in state.

Furthermore, in accordance with the invention the residual solvents like MTBE and heptane can effectively be removed in vacuo at elevated temperature (40 - 100 degree Celsius) without loss of purity or change of physicochemical properties in regard to the amorphous state.

The inventors have found that the substantially pure amorphous Letermovir prepared as per present invention, has a content of MTBE less than 5000 ppm (pursuant to ICH guidelines), and/or a content of heptane less than 5000 ppm (pursuant to ICH guidelines).

The inventors have found that the substantially pure amorphous Letermovir prepared as per present invention, has a content of the N-oxide impurity less than 0.1% w/w, preferably less than 0.05% w/w, more preferably absent or not detectable as per the HPLC method.

Optionally, the substantially pure amorphous Letermovir is further stirred in hot water for 1 to 5 hours. The reaction mixture is then filtered and dried under vacuum.

In context with the stated above, particularly preferred subject matter of the present invention is provided by the following consecutively numbered and inter-related embodiments:

1. Letermovir according to Formula-I essentially free from the N-oxide impurity

which is in the amorphous state and suitable for use in solid oral dosage forms, wherein said Letermovir is characterized by having residual solvents in accordance with ICH guidelines.

2. Letermovir according to embodiment 1, wherein said Letermovir is substantially pure Letermovir.

3. Letermovir according to embodiment 1, wherein said Letermovir is substantially pure amorphous Letermovir.

4. Letermovir according to embodiment 1, wherein said Letermovir is essentially free from the N-oxide impurity.

5. Letermovir according to embodiment 1, wherein said Letermovir is characterized by contains the N-oxide impurity less than 0.1% w/w, preferably less than 0.05% w/w, more preferably absent or not detectable, when the purity of said Letermovir is determined by the HPLC method.

6. Letermovir according to embodiment 1, wherein said amorphous state is further characterized by no detectable crystalline content/signal within the limit of detection of 2%, when said Letermovir is determined by any of the standard XRPD methods. One of the exemplary methods of analysis is described below, but not limited to
Powder sample of Letermovir is prepared on a rotating sample holder with an effective surface area of 10 mm (in diameter); powder diffraction patterns were recorded using a PANalytical Empyrean diffractometer equipped with pixel detector and Nickel filter using CuKa radiation operated at 45 kV and 40 mA. The measurement was performed using a step size of 0.007° with a step time of 24 s.

7. Letermovir according to any of the preceding embodiments, obtainable by applying DMF:water washing to Letermovir solution containing the N-oxide impurity for the preparation of substantially pure Letermovir.

8. Letermovir according to any of the preceding embodiments, obtainable by the following process:
a) providing solution of Letermovir in organic solvent, containing N-oxide impurity,
b) washing the solution of step a) by DMF:water,
c) mixing the solution of step b) with heptane to precipitate solid,
d) isolating the solid from step c) via filtration or centrifugation,
e) optionally, drying the solid of step d) to obtain the substantially pure Letermovir.

9. Letermovir according to any of the preceding embodiments, obtainable by the following process:
a) providing solution of Letermovir in organic solvent, containing N-oxide impurity,
b) washing the solution of step a) by DMF:water,
c) mixing the solution of step b) with heptane at less than 0 degree Celsius to precipitate solid,
d) isolating the solid from step c) via filtration or centrifugation,
e) drying the solid of step d) to obtain the substantially pure Letermovir.

10. Letermovir according to any of the preceding embodiments, obtainable by the following process:
a) providing solution of Letermovir in organic solvent, containing N-oxide impurity,
b) washing the solution of step a) by DMF:water,
c) mixing the solution of step b) with heptane at less than 0 degree Celsius to precipitate solid,
d) isolating the solid from step c) via filtration or centrifugation,
e) optionally, stirring the solid from step d) in hot water,
f) isolating the solid from step e) via filtration or centrifugation,
g) drying the solid of step f) to obtain the substantially pure Letermovir.

11. Letermovir according to any of the preceding embodiments, obtainable by the following process:
a) providing solution of Letermovir in organic solvent, containing N-oxide impurity,
b) washing the solution of step a) by DMF:water,
c) mixing the solution of step b) with heptane to precipitate solid,
d) isolating the solid from step c) via filtration or centrifugation,
e) drying the solid of step d) to obtain the substantially amorphous pure Letermovir,
wherein the substantially pure amorphous Letermovir contains residual solvents in accordance with ICH guidelines.

12. Letermovir according to any of the preceding embodiments, obtainable by the following process:
a) providing MTBE solution of Letermovir essentially free from N-oxide impurity,
b) mixing the solution of step a) with heptane at temperature less than 0 degree Celsius to precipitate solid,
c) isolating the solid from step b) via filtration or centrifugation to obtain the substantially pure amorphous Letermovir,
wherein the substantially pure amorphous Letermovir contains residual solvents in accordance with ICH guidelines.

13. Another objective of the present invention is related to a process for isolating substantially pure amorphous Letermovir comprising:
a) providing MTBE solution of Letermovir essentially free from N-oxide impurity,
b) mixing the solution of step a) with heptane at temperature less than 0 degree Celsius to precipitate solid,
c) isolating the solid from step b) via filtration or centrifugation,
d) drying the solid of step c) to obtain the substantially pure amorphous Letermovir,
wherein the substantially pure amorphous Letermovir contains residual solvents in accordance with ICH guidelines.

14. Letermovir according to any of the preceding embodiments, obtainable by the following process:
a) providing solution of Letermovir in solvent, containing N-oxide impurity,
b) treating the solution of step a) with catalyst(s) to reduce the N-oxide impurity,
c) removing solvent of step b),
d) isolating the Letermovir essentially free from N-oxide impurity.

15. Letermovir according to embodiments 11 and 13, wherein the process according to last step has a final drying step.

16. Letermovir according to any of the preceding embodiments, obtainable by the following process:
a) providing solution of Letermovir in solvent, containing N-oxide impurity,
b) treating the solution of step a) with catalyst(s) to reduce the N-oxide impurity,
c) mixing the solution with heptane at temperature less than 0 degree Celsius to precipitate substantially pure amorphous Letermovir,
d) isolating the substantially pure amorphous Letermovir from step c) via filtration or centrifugation.
e) optionally, drying the substantially pure amorphous Letermovir of step d).

17. Letermovir according to any of the preceding embodiments, obtainable by the following process:
a) providing solution of Letermovir in solvent, containing N-oxide impurity,
b) treating the solution of step a) with catalyst(s) to reduce the N-oxide impurity,
c) mixing the solution of step b) with heptane at temperature less than 0 degree Celsius to precipitate substantially pure amorphous Letermovir,
d) isolating the substantially pure amorphous Letermovir from step c) via filtration or centrifugation,
e) drying the substantially pure amorphous Letermovir of step d),
wherein the substantially pure amorphous Letermovir contains residual solvents in accordance with ICH guidelines.

18. Letermovir according to any of the preceding embodiments, wherein said substantially pure amorphous Letermovir has a content of MTBE less than 5000 ppm and/or a content of heptane less than 5000 ppm, when said MTBE or heptane content is determined by the gas chromatography method.

19. Letermovir according to any of the preceding embodiments, wherein said substantially pure amorphous Letermovir is essentially free from the N-oxide impurity.

20. Letermovir according to any of the preceding embodiments, wherein said substantially pure amorphous Letermovir has a content of the N-oxide impurity less than 0.1% w/w, preferably less than 0.05% w/w, most preferably absent or not detectable as per the HPLC method.

21. Letermovir according to any of the preceding embodiments, wherein said substantially pure amorphous Letermovir is optionally stirred in hot water for 1 to 5 hours.

22. Solid pharmaceutical formulation comprising the amorphous Letermovir, wherein said solid pharmaceutical formulation is orally administrable.

23. Solid pharmaceutical formulation according to embodiment 21, wherein the amorphous Letermovir is isolated according to the preceding embodiments and contains residual solvents according to embodiment 17.

24. Solid pharmaceutical formulation according to embodiment 22 for use in a method for prophylaxis or method of treatment for diseases associated with the group of Herpesviridae, preferably associated with cytomegalovirus (CMV), even more preferably associated with human cytomegalovirus (HCMV).

DEFINITION
The term “metastable” with the context of amorphous Letermovir denotes a chemical state of temporary energy trap or a somewhat stable intermediate stage of a system the energy of which may be lost in discrete amounts.

The term “residual solvents” in terms of pharmaceuticals are defined here as organic volatile chemicals that are used or produced in the manufacture of drug substances or excipients, or in the preparation of drug products, as in the present case drugs substances based on Letermovir.

The term “amorphous” denotes the characteristic that no long-range order of neighboring molecular units is present or being a disorganized solid mass. Accordingly, the amorphous is exhibiting no detectable crystalline content/signal attributable when analyzed by an appropriate crystallographic method.

Accordingly, throughout the specification the expressions “amorphous, amorphous form, amorphous state” with the context of the present invention denotes material exhibiting no indication of crystallinity within the limit of detection of 2% by using standard PXRD methods and thus exhibits no detectable crystalline content/signal when analyzed by an appropriate crystallographic method. Typically Powder X-Ray Diffraction (PXRD) is used to determine the crystalline content of the material in accordance with the invention. One of the exemplary methods of analysis is described below, but not limited to:
The sample was prepared on a rotating sample holder with an effective surface area of 10 mm (in diameter). Powder diffraction patterns were recorded using a PANalytical Empyrean diffractometer equipped with pixel detector and Nickel filter using CuKa radiation operated at 45 kV and 40 mA. The measurement was performed using a step size of 0.007° with a step time of 24 s.

The term “ICH guideline(s)” within the scope of the invention denotes the International Conference on Harmonization of impurity: Guideline for residual solvents Q3C(R6). The objective of this guideline is to recommend acceptable amounts for residual solvents in pharmaceuticals for the safety of the patient. The guideline recommends use of less toxic solvents and describes levels considered to be toxicologically acceptable for some residual solvents. The guideline applies to all dosage forms and routes of administration. Higher levels of residual solvents may be acceptable in certain cases such as short term (30 days or less) or topical application.

The terms “pure/purified/substantially pure” in view of the API Letermovir characterizes the API in that it is essentially free from the N-oxide impurity and it contains residual solvents in accordance with ICH guidelines in accordance with the instant invention.

The terms “pharmaceutical grade” in view of the API Letermovir characterizes any active or inactive drug, biologic, reagent, etc., manufactured under Good Manufacturing Practices (GMP) which is approved, conditionally approved, or indexed by the Food and Drug Administration (FDA) or for which a chemical purity standard has been written or established by a recognized compendium (e.g., United States Pharmacopeia-National Formulary (USP/NF) or British Pharmacopeia (BP)).

Further said terms denote that residual MTBE content does not exceed 5000 ppm and residual heptane content does not exceed 5000 ppm when determined by Gas Chromatography (GC).

ABBREVIATIONS
Throughout the specification the following abbreviations apply:
“API” denotes active pharmaceutical ingredient
“MTBE” denotes Methyl tert-butyl ether, also known as methyl tertiary butyl ether, is an organic compound with molecular formula (CH3)3COCH3. MTBE is a volatile, flammable, and colorless liquid that is not readily soluble in water.
“DMF” denotes dimethylformamide.
“ACN” denotes acetonitrile.
“EA” or “EtOAc” denotes ethyl acetate.
“THF” denotes tetrahydrofuran.
“DMSO” denotes dimethyl sulfoxide.
“NMP” denotes N-methyl-2-pyrrolidone.
“IPAc” denotes isopropyl acetate
“MEK” denotes methyl ethyl ketone.
“XRPD” denotes X Ray Powder Diffraction.
“ppm” denotes part per million.
“PXRD” denotes Powder X-Ray Diffraction.
“HPLC” denotes High Performance Liquid Chromatography.

Present invention is further illustrated with the following non-limiting examples.

EXAMPLES
Example-1:
Letermovir methyl ester Di-p-toluoyl-D-tartaric acid salt (95.0 gm) was treated with 8.0 % aqueous sodium bicarbonate solution (950 ml) in isopropanol. Then after, 4.0 % aqueous sodium hydroxide solution and IPA (475 ml) were added to the reaction mixture. After completion of hydrolysis, the aqueous layer was washed with MTBE and further acidified using aqueous hydrochloric acid to pH 4 to 6. Hydrolyzed product was then extracted in MTBE. The MTBE solution was washed twice with 1000 ml DMF:Water (200:800). The MTBE layer was separated and was distilled out up to about 2 to 5 volume. The product is precipitated by mixing the MTBE solution with heptane at temperature less than 0 degree Celsius to yield amorphous solid having HPLC purity 99.85% and N-oxide impurity: 0.02%. The solid was further stirred in water at temperature more than 50 degree Celsius. The solid was filtered and was dried at between 50 degree Celsius and 80 degree Celsius under vacuum to yield substantially pure amorphous Letermovir having N-oxide impurity 0.02% w/w (RRT at about 0.54), analyzed by HPLC method; BD: 0.2845 gm/ml and TD: 0.4623 gm/ml.

Example-2:
Prepared solution of Letermovir (1.0 gm) containing N-oxide impurity (0.16 % w/w in MTBE (1:10 w/v) and the solution was washed twice with 20 ml DMF:Water (4:16) The MTBE layer was separated and was distilled out to yield substantially pure Letermovir (0.8 gm) having N-oxide impurity 0.04% w/w, analyzed by HPLC method.

Example-3:
The solution of Letermovir (1.0 gm) containing N-oxide impurity 0.16 % w/w in 5.0 ml THF was added to mixture of NaBH4 (30 mg), LiCl (30 mg) and THF (5.0 ml). After the consumption of the N-oxide impurity, brine solution was added to the reaction mixture and extracted the reaction mass with EtOAc. Separated organic layer was distilled out under vacuum to yield substantially pure Letermovir. N-oxide impurity was at not detectable level, analyzed by HPLC method.

Example-4:
The solution of Letermovir (5.0 g, containing N-oxide impurity 0.16 % w/win 25.0 ml THF was added to mixture of NaBH4 (1500 mg), LiCl (150 mg) and THF (5.0 ml). After the consumption of the N-oxide impurity, water was added to the reaction mixture and extracted with EtOAc. Separated organic layer, was distilled out to yield substantially pure Letermovir. N-oxide impurity was at not detectable level, analyzed by the HPLC method.

Example-5:
To solution of Letermovir (1.0 gm) containing N-oxide impurity 0.15% w/w in MTBE, triphenyl phosphine (100 mg) was charged and the reaction mass was stirred at 50 degree Celsius with monitoring. After the consumption of the N-oxide impurity, water was added to the reaction mixture and extracted with EtOAc. Separated organic layer was distilled out to substantially pure Letermovir. N-oxide impurity was at not detectable level, analyzed by the HPLC method.

Example-6:
To solution of Letermovir (1.0 gm) containing N-oxide impurity 0.15% w/w in acetonitrile, triphenyl phosphine (100 mg) was added and the reaction mass was stirred at 70 degree Celsius with monitoring. Water was added to the reaction mixture and extracted with EtOAc. Separated organic layer was distilled out to substantially pure Letermovir having N-oxide impurity 0.04% w/w, analyzed by the HPLC method.

Example 7: To 600 ml heptane, solution of 20 gm Letermovir in 140 ml MTBE was added at -5 °C. to -10 °C. temperature. The reaction mixture was further maintained for 1 hour at same temperature. The precipitate was filtered under vacuum and dried at 55°C. to 65°C. temperature in vacuum to yield Letermovir as an amorphous state. (Yield 88%; Residual Solvents: MTBE - 93ppm & heptane - 1283ppm).

Example 8: Letermovir of Example 2 was prepared as same manner with same quantities as in Example 1 except using heptane 300 ml. (Yield 85%; Residual Solvents: MTBE - 755ppm & heptane - 1087ppm).

Example 9: To 150 ml heptane, solution of 15 gm Letermovir in 60 ml MTBE was added at 0°C. to -5°C. temperature. The reaction mixture was further maintained for 1hour at same temperature. The precipitate was filtered under vacuum and dried at 55°C. to 65°C. temperature in vacuum to yield amorphous Letermovir. (Yield 87%; Residual Solvents: MTBE – 631ppm & heptane - 163ppm)

Example 10: To 60 ml heptane, solution of 4 gm Letermovir in 20 ml MTBE was added at -5°C. to -10°C. temperature., The reaction mixture was further maintained for 1 hour at same temperature. The precipitate was filtered under vacuum and dried at 65°C. temperature in vacuum to yield amorphous Letermovir. (Yield 85%; Residual Solvents: MTBE - 424ppm & heptane - 234ppm).

Example 11: To 30 ml heptane, solution of 2 gm Letermovir in 14 ml MTBE was added at -5°C. to -10°C. temperature. The reaction mixture was further maintained at same temperature for 1 hour. The precipitate was filtered under vacuum and dried at 65°C. temperature for 24 hours to yield Letermovir in amorphous state. (Yield 85%; Residual Solvents: MTBE - 139ppm & heptane - 837ppm).

Example 12: Letermovir of Example 6 was prepared as same manner as in Example 5, except addition of Letermovir solution is performed at 40°C. to 45°C. temperature with maintaining at same temperature for 30 min. followed by maintaining at 20°C. temperature for 1 hour. (Yield 95%; Residual Solvents: MTBE - 27719ppm & heptane - 54270ppm).

Example 13: To 60 ml heptane, solution of 2 gm Letermovir in 14 ml MTBE was added at -5°C. to -10°C. temperature. The reaction mixture was further maintained at same temperature for 1 hour. The precipitate was filtered under vacuum and dried at 65°C. temperature for 24 hours to yield Letermovir in amorphous state. (Yield 85%; Residual Solvents: MTBE - 1336ppm & heptane - 923ppm).

Example 14: Letermovir of Example 8 was prepared as same manner as in Example 7, except addition of the Letermovir solution is performed at 30°C. to 35°C. temperature and with maintaining the solution at same temperature for 1 hour. (Yield 90%; Residual Solvents: MTBE - 2521ppm & heptane - 17381ppm).

Example 15: Letermovir of Example 9 was prepared as same manner as in Example 8 except using 30 ml of heptane (Yield 80%; Residual Solvents: MTBE - 320ppm & heptane - 18494ppm).

Example 16: To 60 ml heptane, solution of 2 gm Letermovir in 8 ml MTBE was added at 35°C. to 40°C. temperature. The reaction mixture was further maintained at same temperature for 1 hour. The precipitate was filtered under vacuum and dried at 65°C. temperature for 24 hours to yield amorphous Letermovir. (Yield 90%; Residual Solvents: MTBE - 1242ppm & heptane - 23344ppm).

Example 17: Amorphous Letermovir of Example 11 was prepared as same manner as in Example 10 (Yield 90%; Residual Solvents: MTBE - 2087ppm & heptane - 33724ppm).

Example 18: To 45 ml heptane, solution of 2 gm Letermovir in 10 ml MTBE was added at 15°C. to 20°C. temperature. The reaction mixture was further maintained at same temperature for 1 hour. The precipitate was filtered under vacuum and dried at 65°C. temperature for 24 hours to yield amorphous Letermovir. (Yield 85%; Residual Solvents: MTBE - 326ppm & heptane - 8226ppm).

Example 19: To 21.0 liters heptane, solution of 700 gm Letermovir in 4.9 liters MTBE was added at -5 °C. to -10 °C. temperature. The reaction mixture was further maintained for 1-2 hour at same temperature. The precipitate was filtered under vacuum and dried at 65°C. to 70°C. temperature in vacuum to yield Letermovir as an amorphous state. Purity by HPLC 99.85 (Yield 93%; Residual Solvents: MTBE - 166ppm & heptane - 166ppm).

Example 20: To 20 ml heptane, solution of 1 gm Letermovir in 3 ml ethyl acetate was added at 30°C. to 35°C. temperature. The reaction mixture was further maintained at 25°C. temperature for 1 hour. The precipitate was filtered under vacuum and dried at 50°C. temperature for 13 hours to yield Letermovir. (Yield 90%; Residual Solvents: Ethyl acetate - 38902ppm & heptane - 8318ppm).

Example 21: To 30 ml heptane, solution of 1 gm Letermovir in 3 ml isopropyl acetate was added at 30°C. to 35°C. temperature. The reaction mixture was further maintained at 20°C. temperature for 1 hour. The precipitate was filtered under vacuum and dried at 50°C. temperature for 13 hours to yield Letermovir. (Yield 90%; Residual Solvents: isopropyl acetate - 137944ppm & heptane - 36ppm).

Example 22: To 60 ml heptane, solution of 2 gm Letermovir in 14 ml isopropyl acetate was added at -5°C. to -10°C temperature. The reaction mixture was further maintained at same temperature for 1 hour. The precipitate was filtered under vacuum and dried at 65°C. temperature to yield Letermovir. (Yield 92%; Residual Solvents: isopropyl acetate - 7764ppm & heptane - 16135ppm)

Example 23: To 60 ml heptane, solution of 2 gm Letermovir in 14 ml Ethyl acetate was added at -5°C. to -10°C temperature. The reaction mixture was further maintained at same temperature for 1 hour. The precipitate was filtered under vacuum and dried at 65°C temperature to yield Letermovir. (Yield 92%; Residual Solvents: Ethyl acetate - 1662ppm & heptane - 21238ppm)

Reference example-1: Letermovir methyl ester (+) Di-p-toluoyl-D-tartaric acid salt (108 gm) was treated with aqueous sodium bicarbonate solution in ethyl acetate and Letermovir methyl ester free base was isolated from the reaction mixture. Letermovir methyl ester was further hydrolyzed using aqueous NaOH solution in IPA at ambient temperature under inert atmosphere. After completion of reaction, water as added and acidified the reaction mixture in between pH 5-7 using aqueous hydrochloric acidsolution. The product was extracted in MTBE solvent. The separated organic layer was separated and wasdistilled out to get foamy solid. The solid was dissolved in IPA and precipitated out in water at room temperature to give Letermovir having N-oxide impurity 0.22% w/w, analyzed by HPLC method.
,CLAIMS:
1. Substantially pure Letermovir according to Formula-I, essentially free from N-oxide impurity of Formula-II.

2. Letermovir according to claim 1, obtainable by applying DMF:water washing to Letermovir solution containing N-oxide impurity of Formula-II.

3. A process for preparing substantially pure Letermovir of Formula-I comprising:
a) providing solution of Letermovir in organic solvent, containing N-oxide impurity of Formula-II;
b) washing the solution of step a) by DMF:water;
c) mixing the solution of step b) with heptane to precipitate solid;
d) isolating the solid from step c) via filtration or centrifugation; and
e) optionally, drying the solid of step d) to obtain the substantially pure Letermovir.

4. A process for preparing substantially pure Letermovir of Formula-I comprising:
a) providing solution of Letermovir in organic solvent, containing N-oxide impurity of Formula-II;
b) washing the solution of step a) by DMF:water;
c) mixing the solution of step b) with heptane at less than 0 degree Celsius to precipitate solid;
d) isolating the solid from step c) via filtration or centrifugation; and
e) drying the solid of step d) to obtain the substantially pure Letermovir.

5. A process for preparing substantially pure Letermovir of Formula-I comprising:
a) providing solution of Letermovir in organic solvent, containing N-oxide impurity of Formula-II;
b) washing the solution of step a) by DMF:water;
c) mixing the solution of step b) with heptane at less than 0 degree Celsius to precipitate solid;
d) isolating the solid from step c) via filtration or centrifugation;
e) optionally, stirring the solid from step d) in hot water;
f) isolating the solid from step e) via filtration or centrifugation; and
g) drying the solid to obtain the substantially pure Letermovir.

6. A process for preparing substantially pure Letermovir of Formula-I comprising:
a) providing solution of Letermovir in organic solvent, containing N-oxide impurity of Formula-II;
b) washing the solution of step a) by DMF:water;
c) mixing the solution of step b) with heptane to precipitate solid;
d) isolating the solid from step c) via filtration or centrifugation; and
e) drying the solid of step d) to obtain the substantially pure Letermovir,
wherein the substantially pure Letermovir contains residual solvents in accordance with ICH guidelines.

7. The processes according to claims 3-6, wherein said organic solvent is water immiscible organic solvent(s).

8. The process according to claim 7, wherein the water immiscible organic solvent is MTBE.

9. Letermovir according to any of the preceding claims, wherein the ratio of DMF and water is in the range of 1:99 % v/v to 50:50 % v/v.

10. A process for preparing substantially pure Letermovir of Formula-I comprising:
a) providing MTBE solution of Letermovir, essentially free from N-oxide impurity of Formula-II;
b) mixing the solution of step a) with heptane at temperature less than 0 degree Celsius to precipitate solid;
c) isolating the solid from step b) via filtration or centrifugation; and
d) drying the solid of step c) to obtain the substantially pure Letermovir,
wherein the substantially pure Letermovir contains residual solvents in accordance with ICH guidelines.

11. Letermovir according to claim 1, obtainable by a process comprising:
treating Letermovir solution containing N-oxide impurity of Formula (II) with catalyst(s) to reduce the N-oxide impurity.

12. A process for preparing substantially pure Letermovir of Formula-I, essentially free from N-oxide impurity of Formula-II comprising:
a) providing solution of Letermovir in solvent, containing the N-oxide impurity;
b) treating the solution of step a) with catalyst(s) to reduce the N-oxide impurity;
c) removing solvent of step b); and
d) isolating the Letermovir, essentially free from the N-oxide impurity.

13. The process according to claim 12, comprises a final drying step.

14. A process for preparing substantially pure Letermovir of Formula-I comprising:
a) providing solution of Letermovir in solvent, containing N-oxide impurity of Formula-II;
b) treating the solution of step a) with catalyst(s) to reduce the N-oxide impurity;
c) mixing the solution of step b) with heptane at temperature less than 0 degree Celsius to precipitate the substantially pure Letermovir;
d) isolating the substantially pure Letermovir from step c) via filtration or centrifugation; and
e) optionally, drying the substantially pure Letermovir of step d).

15. A process for preparing substantially pure Letermovir of Formula-I comprising:
a) providing solution of Letermovir in solvent, containing N-oxide impurity of Formula-II;
b) treating the solution of step a) with catalyst(s) to reduce the N-oxide impurity;
c) mixing the solution of step b) with heptane at temperature less than 0 degree Celsius to precipitate the substantially pure Letermovir;
d) isolating the substantially pure Letermovir from step c) via filtration or centrifugation; and
e) drying the substantially pure Letermovir of step d),
wherein the substantially pure Letermovir contains residual solvents in accordance with ICH guidelines.

16. The processes according to claims 12, 14 and 15, wherein said solvent is selected from a group of THF, ACN, MTBE and mixture thereof.

17. The processes according to claims 12, 14 and 15, wherein the catalyst(s) is selected from a group of borohydride compound(s) and triphenyl phosphine.

18. The process according to claim 17, wherein the borohydride compound(s) is selected from a group of sodium borohydride and lithium borohydride.

19. Letermovir according to any of the preceding claims is in amorphous state.

20. Letermovir according to any of the preceding claims contains residual solvents in accordance with ICH guidelines.

21. Letermovir according to any of the preceding claims, wherein said substantially pure Letermovir has a content of MTBE less than 5000 ppm and a content of heptane less than 5000 ppm, when said MTBE and heptane content are determined by the gas chromatography method.

22. Letermovir according to any of the preceding claims, wherein the substantially pure Letermovir has a content of the N-oxide impurity less than 0.1% w/w.

23. Solid pharmaceutical formulation comprising the substantially pure Letermovir according to claim 1, wherein said solid pharmaceutical formulation is orally administrable.

24. Solid pharmaceutical formulation according to claim 23 for use in a method for prophylaxis or method of treatment for diseases selected from a group of Herpesviridae, cytomegalovirus (CMV) and human cytomegalovirus (HCMV).