Conformational Polymorphism in a Non-steroidal Anti-inflammatory Drug, Mefenamic Acid

For several years, the non-steroidal anti-inflammatory
drug mefenamic
acid, MA, has been known to exist as dimorphs (I and II). We report a new metastable polymorph
(III) of MA obtained during attempted co-crystallization
experiments and establish its stability relationship with existing
forms. At elevated temperatures I and III convert to II, as evident from DSC experiments. On
the basis of the lattice energy calculations in conjunction with thermal
analysis, the stability order is proposed to be I > II > III at ambient conditions, whereas at
elevated
temperature the order is II > I > III. In either condition III is a metastable
form and hence transforms to I at ambient conditions
and to II at higher temperatures. Also we report the
structural studies of a DMF solvate and a cytosine complex.