Characterization of piroxicam crystal modifications.

Piroxicam polymorphism was extensively studied in the past. The objective of the present work was to evaluate polymorphism of piroxicam once again and to characterize the obtained crystal forms. Three polymorphic forms and one monohydrate form were obtained by crystallization from saturated solutions in various solvents. Polarity of solvents and crystallization rate defined by temperature of crystallization were found to be critical parameters in determining the polymorphic form. A new polymorphic form designated as form III was obtained by forced crystallization using dry ice. Only form I with the highest melting point was found to be stable under mechanical and thermal stress. Differences in IR spectra were attributed mainly to the differences in number and positions of H-bonds in the piroxicam crystal forms. Slow crystallization of piroxicam from absolute ethanol solution resulted in a mixture of form II and monohydrate. Crystal structure analysis proved that form II represents form alpha(2) already proposed in the literature. Differences in dissolution rates among crystal forms of piroxicam were attributed to differences in their wettability, where highest wettability was obtained for monohydrate and the lowest for form III.