Approximately 6 mg of the S-892216 drug substance was added to each buffer to prepare 30-mL suspensions. The prepared suspensions were placed in centrifuge tubes and shaken at ≥100 rpm for 60 minutes. Next, 5 mL of the suspension was immediately filtered through a 0.45-µm filter, then diluted in methanol twice to prepare the sample solution. To measure the S-892216 content in the suspension, an isocratic reverse-phase UV-HPLC method was used. The samples were transferred to a volumetric flask, and 50% acetonitrile was added. The flask was shaken for 5 minutes, then subjected to ultrasonication for 15 minutes. The solution volume was adjusted to 100 mL by adding 50% acetonitrile and then filtered through a 0.45-µm membrane filter. The absorbance of the sample solution at a wavelength of 299 nm was measured using a UV-HPLC system (Alliance2695, Waters Corporation, Milford, MA, USA) equipped with a reverse-phase ODS column (X Bridge C18, 3.