Crystal form conversion of nevirapine solvates subjected to elevated temperature and humidity: a qualitative study

Some known nevirapine solvates have been reported to undergo solvent exchange in aqueous media to form a stable hemihydrate. This study aimed to determine the effects of atmospheric moisture on said nevirapine solvates and to gain insight into which factors determine the end product of transformation. Solvates were prepared by solvent recrystallisation and stored, together with the anhydrous and hemihydrate forms, in a climate chamber at 40 °C and 75% RH for a period of 28 days. Samples were analyzed using DSC, TGA, FT-IR, PXRD and Karl Fischer titration. Some solvates were observed to undergo desolvation to the anhydrous form of nevirapine (Form I), whilst others converted to the hemihydrate. It was found that water miscibility of the guest solvent determined the stable form of nevirapine, anhydrous or hemihydrate, to which each solvate eventually transformed. Transformation to the hemihydrate only occurred if the guest solvent was sufficiently water soluble to allow water molecules to enter solvent channels and displace the original guest. Solvates with hydrophobic guests desolvated to the anhydrous form. We concluded that, in the absence of a guest, solvent channels are lost during transformation to the monoclinic crystal system and space group P21/c (Form I) so that water cannot enter after desolvation.