Crystal forms of rifaximin and their effect on pharmaceutical properties

Five distinct crystal forms of rifaximin have been identified and characterised by X-ray powder diffraction, solid state 13C NMR, and HATR-IR spectroscopy. Changes in the crystal structure may produce differences of two to three orders of magnitude in the rate of intrinsic dissolution, solubility and bioavailability of rifaximin. Alteration of the pharmacokinetic parameters is of particular interest; the Cmax values of the crystal forms range from 1.1 to 1085.31 ng ml-1 and the AUC0-24 h values range from 10 to 4795 ng h ml-1. These findings are relevant to the therapeutic use of rifaximin.