Thermodynamic and Structural Powder Diffraction Studies of the Polymorphism of Florfenicol.

Florfenicol is an antimicrobial drug used in veterinary medicine and aquaculture. Two polymorphic forms called A and B have been reported in literature, but the relation between these two forms are unknown. In order to get a better understanding of the behavior of solid florfenicol and the possible evolution from a metastable form to a stable one, an accurate thermodynamic study has been carried out by calorimetric measurements. For this purpose, temperatures and enthalpies of transition and of fusion of the stable and metastable forms have been measured by DSC. TGA has been used in view to detect the eventual existence of solvates which does not occur. In view to confirm the kind of transition, cp measurements of the two forms have been performed with a C80 calorimeter. With these cp values, it has been possible to determine the function of the variation of enthalpies as a function of temperature, ΔH = f (T). A study of the kinetic of transformation has been realized and is presented as well as the patterns of the X-ray powder diffraction from 295 to 426 K. This last approach confirms the crystal structure of form A of florfenicol previously reported in literature.