3232 Adrjanowicz et al. Imp.1 Furosemide Fig. 9 Increment of Imp.1 in acidic environment for cryomilled furosemide (the sample cryomilled for 60 min). The kinetics of the buildup of Imp. 1(CSA) was monitored by UPLC technique. The solid-state degradation study of the cryomilled furosemide extended up to 6 months. As has been found, in a cryomilled powder (one cryomilled for 180 min) stored at room temperature for 6 months, an increase in the main degradation product within that period was less than 0.5% (data not shown herein). Thus, the degradation in the solid state certainly proceeds, but not fast enough to be the reason of a sudden growth of Imp.1 upon cryomilling. It was also found that in organic solvents, such as methanol, the cryomilled drug is quite stable (no significant increment in the impurity level was noticed), while in acidic solutions furose- mide underwent acid-catalyzed hydrolysis. In this condition, the linear upturn of Imp. 1 (sample milled 60 min) within 12000 min was monitored. The results are shown in Figs. 9 and 10 (in the latter, only the first 4200 min are shown). As the hydrolysis progressed, a significant growth of the main impurity was observed. This indicates that the cryomilled furosemide is very unstable at lower pH values, and similarly to crystalline one, CSA is expected to be the main degradation product. It is worth emphasizing that the growth of Imp.1 from 1.68% to ~6% required approxi- mately 3600 min in acidic solution, while the same effect was attained after just 180 min of cryomilling. This implies that chemical degradation induced and proceeding in the high- energy milling has a surprisingly high rate. For comparison, acid-catalyzed hydrolysis of crystalline furosemide in water solution was carried out. The results in acidic medium are presented in the inset of Fig. 10. We Fig. 10 Buildup of the impurity with relative retention time of 0.26 (Imp.1) in acidic solution.