Crystal forms of rifaximin and their effect on pharmaceutical properties

Five distinct crystal forms of rifaximin (alpha, beta, gamma, delta and epsilon) have been identified and characterised by X- ray powder diffraction, solid state C-13 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 C-max values of the crystal forms range from 1.1 to 1085.31 ng ml(-1) and the AUC(0-24) h values range from 10 to 4795 ng h ml(-1). These findings are relevant to the therapeutic use of rifaximin.