Solubility in Phosphate buffer and Fed State Simulated Intestinal Fluid (FeSSIF) 8 µl of reference or test compound stock solution (10 mM in DMSO) was added into 792 µl of 100 mM phosphate buffer (pH 7.4) or FeSSIF (pH 5.8). The resulting mixture was shaken for 1h (1000 rpm) at room temperature, then centrifuged for 10 min (12000 rpm) to remove un-dissolved particles. The supernatant was collected and diluted 10 times and 100 times separately with 100 mM phosphate buffer or FeSSIF. 5 µl of the supernatant samples (no diluted, 10 times diluted, 100 times diluted) were mixed with 95 µl of acetonitrile (containing internal standard) separately before injecting into LC-MS/MS for analysis.