Drug Phase Transformation and Water Redistribution during Continuous Tablet Manufacturing: A Case Study of Carbamazepine Dihydrate

In recent years, continuous tablet manufacturing technology has been used to obtain regulatory approval of several new drug products. While a significant fraction of active pharmaceutical ingredients exists as hydrates (wherein water is incorporated stoichiometrically in the crystal lattice), the impact of processing conditions and formulation composition on the dehydration behavior of hydrates during continuous manufacturing has not been investigated. Using powder X-ray diffractometry, we monitored the dehydration kinetics of carbamazepine dihydrate in formulations containing dibasic calcium phosphate, anhydrous (DCPA), mannitol, or microcrystalline cellulose. The combined effect of nitrogen flow and vigorous mixing during the continuous mixing stage of tablet manufacture facilitated API dehydration. Dehydration was rapid and most pronounced in the presence of DCPA. The dehydration product, amorphous anhydrous carbamazepine, sorbed a significant fraction of the water released by dehydration. Thus, the dehydration process resulted in a redistribution of water in the powder blend. The unintended formation of an amorphous dehydrated phase, which tends to be much more reactive than its crystalline counterparts, is of concern and warrants further investigation.