The hygroscopic behavior of five polymorphic forms (forms I, II, III, a monohydrate and an amorphous form) of tulobuterol hydrochloride was compared under various relative humidities (R.H.) at 30°C. Form I uptook moisture above 91% R.H. while forms II and III uptook it above 75% R.H. They immediately transformed into the monohydrates after uptaking moisture. The transformation kinetics was represented by the model of Avrami-Erofeev and it was found that form I was highly dependent on the relative humidity than forms II and III. No difference in the hygroscopic rate constants was found among them. The monohydrate was stable under pretty high humidity, i.e. 75-91% R.H., but was dehydrated under less than 59% R.H. The dehydration kinetics at 0% R.H. was described by the first-order rate process, the activation energy of which was 20.0 kcal/mol. The amorphous form revealed the characteristic behavior of hygroscopicity. This form uptook moisture rapidly above 59% R.H. at the initial stage, then transformed into form II at 59% R.H. Above 75% R.H., the amorphous form transformed into monohydrate via form II. In conclusion, form I was the most stable over the wide range of humidity.
