Anhydrous crystalline forms of I and II of 1-cyclopropyl-6-fluoro-8-methoxy-7-(3-methyl-1- piperazinyl) 4-oxo-1, 4-dihydroquinoline-3-carboxylic acid (gatifloxacin)

A process for preparing anhydrous crystalline Form I of 1-Cyclopropyl-6-fluoro-8-methoxy-7-(3-methyl-1-piperazinyl)-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (Gatifloxacin), which comprises the steps of:

• i) refluxing azeotropically a hydrated form of Gatifloxacin in a water-immiscible aromatic or aliphatic hydrocarbon solvent or a ketone solvent;
• ii) cooling the reaction mixture of step i) accompanied by stirring of the mixture until the solid mass crystallizes;
• iii) isolating the solid obtained in step ii) by conventional methods;
• iv) drying the solid obtained in of step iii) at 30-70° C.;
• v) dissolving the solid isolated in step iv) in a linear or branched chain substituted or unsubstituted alkanone solvent at reflux temperature;
• vi) cooling to 0-5° C.;
• vii) filtering and washing with a linear or branched chain substituted or unsubstituted alkanone solvent;
• and
• viii) drying at 30-90° C. to obtain anhydrous crystalline Form I of Gatifloxacin.

A process for preparing anhydrous crystalline polymorph Form II of 1-Cyclopropyl-6-fluoro-8-methoxy-7-(3-methyl-1-piperazinyl)-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (Gatifioxacin), which comprises the steps of:

• i. refluxing azeotropically a hydrated form of Gatifloxacin in a water-immiscible aliphatic hydrocarbon solvent;
• ii. cooling the reaction mixture of step (i) accompanied by stirring of the mixture until the solid mass crystallizes;
• iii. isolating the solid obtained in step (ii) by conventional methods;
• and
• iv. drying the isolated solid of step (iii) with or without vacuum at a temperature of 30-70° C., to obtain anhydrous crystalline polymorph Form II of Gatifloxacin.