The capsule with S-309309 was added to 50 mL of 0.1N hydrochloric acid in a smallvolume vessel equipped with a small paddle stirring at 50 rpm. The mixture was heated to 37 °C and then 3 mL of aliquots were withdrawn after 5, 10, 15, 20, and 30 min. Each aliquot was filtered through a 0.45-µm filter and then diluted 2 times with 100 mM carbonate buffer (pH 9.7) to inject into the UPLC system. The amount of the degradation product compound 8 was measured by UPLC at a wavelength of 267 nm using a YMC-Triart C18 ExRS column (1.9 µm, 3.0 × 100 mm) at 60°C using a column heater. The mobile phase consisted of 0.1% formic acid and acetonitrile. The separation was achieved in 35 min using the gradient program summarized in Table . The flow rate was 0.6 mL/min throughout the run. The injection volume was 10 µL. The amount of compound 8 was calculated as a percentage (%) of the total peak area of the chromatogram, which was set to 100%.