Amorphous form of rifaximin and processes for its preparation

1. A compound, which is an amorphous form of rifaximin having substantially the same X-ray powder diffraction pattern as shown in FIG. 1 .

2. A process for the preparation of an amorphous form of rifaximin, said process comprising:

i. dissolving rifaximin in a solvent selected from the group consisting of C 3 to C 6 alkyl esters, C 2 to C 4 alcohols, C 3 to C 7 alkyl ketones, and mixtures thereof to form a solution;

ii. optionally distilling said solution;

iii. optionally filtering said solution;

iv. precipitating said solution by adding an anti-solvent selected from the group consisting of C 5 to C 10 hydrocarbons, C 4 to C 8 alkyl ethers, and mixtures thereof, or optionally precipitating by adding the solution to the anti-solvent,

v. isolating of said precipitated amorphous rifaximin; and

vi. optionally drying the amorphous rifaximin.

3. The process of claim 2 wherein said solvent is selected from methyl acetate, ethyl acetate, isopropyl acetate, isobutyl acetate, ethanol, isopropanol, 2-butanol, acetone, methyl iso-butyl ketone or mixtures thereof.

4. The process of claims 2 or 3 wherein the amount of said solvent is 1 to 25 volumes.

5. The process of claims 2 or 3 wherein the amount of solvent is 1.5 to 12 volumes.

6. The process of claims 2 or 3 wherein the amount of solvent is 2 to 6 volumes.

7. The process of claim 2 wherein the anti-solvent is selected from heptanes, methyl tert-butyl ether, or mixtures thereof.

8. The process of claims 2 or 7 wherein the amount of anti-solvent is 1 to 36 volumes.

9. The process of claims 2 or 7 wherein the amount of anti-solvent is 2 to 20 volumes.

10. The process of claims 2 or 7 wherein the amount of anti-solvent is 3 to 10 volumes.

11. The process of claim 2 wherein step (i) solution is maintained at a temperature of between 0° C. to reflux temperature.

12. The process of claim 2 wherein step (i) solution is maintained at a temperature of between 10° to 55° C.

13. The process of claim 2 step (i) solution is maintained at a temperature of between 15° to 45° C.

14. The process of claim 2 wherein the addition and precipitation temperature is −20° to 55° C.

15. The process of claim 2 wherein the addition and precipitation temperature is 0° to 45° C.

16. The process of claim 2 wherein the addition and precipitation temperature is 10° to 35° C.