Patent ID: 8518949

Claim:
Rifaximin in polymorphic form δ, wherein the polymorph δ is obtained by the process of: reacting a molar equivalent of rifamycin O with an excess of 2-amino-4-methylpyridine in a first solvent mixture, comprising water and ethyl alcohol in volumetric ratios between 1:1 and 2:1, for between 2 and 8 hours at a temperature between 40° C. and 60° C. to obtain a first reaction mixture; treating the first reaction mixture at room temperature with a solution of ascorbic acid in a mixture of water, ethyl alcohol and concentrated aqueous hydrochloric acid to obtain a second reaction mixture; adding concentrated aqueous solution of hydrochloric acid to the second reaction mixture to bring the pH to 2.0 thereby obtaining a first suspension; filtering the first suspension to obtain a first solid; washing the first solid with the first solvent mixture to obtain raw rifaximin; dissolving the raw rifaximin in ethyl alcohol at a temperature between 45° C. and 65° C.; forming a precipitate by adding water and lowering the temperature of the mixture to between 50° C. and 0° C. while stirring for between 4 and 36 hours to obtain a second suspension; filtering the second suspension to obtain a second solid; washing the second solid with water and drying it under vacuum or under normal pressure, with or without a drying agent, at a temperature between room temperature and 105° C., for between 2 and 72 hours to a water content of between 2.5% and 6% (w/w), wherein the rifaximin polymorphic form δ is free from other polymorphic forms of rifaximin and has x-ray powder diffraction pattern peaks at about 5.7°±0.2, 12.1°±0.2, and 17.0°±0.2 2-θ.