Effects of ball-milling and cryomilling on sulfamerazine polymorphs: a quantitative study

The effects of ball-milling and cryomilling on sulfamerazine forms I and II (SMZ FI, FII) were investigated using X-ray powder diffraction, infrared and near-infrared (NIR) spectroscopy. Cryomilling resulted in a complete amorphization of both polymorphs. Milling at room temperature gave mixtures of amorphous SMZ (FA) and FII. Calibration models were developed for the quantitative analysis of binary (FI/FII, FI/FA, and FII/FA) and ternary (FI/FII/FA) mixtures using NIR spectroscopy combined with partial least-squares (PLS) regression. The PLS models for binary (0%-100%), ternary (0%-100%), and low-level (0%-10%) binary mixtures had root-mean-square errors of prediction of ≤1.8%, ≤5.1%, and ≤0.80%, respectively. The calibration models were used to obtain a detailed quantitative picture of solid-state transformations during milling and any subsequent recrystallizations. FA prepared by cryomilling FI for less than 60 min recrystallized to mixtures of FI and FII, whereas samples milled for more than 60 min crystallized to pure FII. The effect of comilling SMZ with stoichiometric amounts of additives was investigated. SMZ formed amorphous materials with oxalic, dl-tartaric, and citric acids that were more stable toward recrystallization than FA. Amorphous SMZ/oxalic acid was found to recrystallize to a 2:1 cocrystal during storage.