Objective To determine the solubility of rhein in different vehicles and its partition coefficients in the n-octanol-water system for designing new formulations. Methods High performance liquid chromatography(HPLC) method was established to determine the concentration of rhein in water,different pH solutions and different solvents;the partition coefficients for the n-octanol- buffer solution systems were determined by shaking flask method. Results The equilibrium solubility of Rhein was 3.89 μg·mL-1and the lgP was 2.79 in water at 37 ℃.The solubility of rhein was increased with the raise of pH, which could reach 362.20 and 431.65 μg·mL-1in phosphate buffer solution at pH 6.8 and 7.4,respectively.Solubility of rhein in the semi polar solvent was relatively good,which can up to 2 971.74 μg·mL-1in PEG400.The oil-water partition coefficients of rhein were decreased with the increase of pH,and were 0.83 and 0.54 in phosphate buffer solution at pH 6.8 and 7.4,respectively. Conclusion Rhein is almost not soluble in water.With the raise of pH,the solubility is increased but the oil-water partition coefficient is decreased.

Background: Twenty-four-hour urinary free cortisol (UFC) measurement is the initial diagnostic test for Cushing’s syndrome (CS). We compared UFC determination by both direct and extraction immunoassays using Abbott Architect, Siemens Atellica Solution, and Beckman DxI800 with liquid chromatography-tandem mass spectrometry (LC-MS/MS). In addition, we evaluated the value of 24-hr UFC measured by six methods for diagnosing CS. Methods: Residual 24-hr urine samples of 94 CS and 246 non-CS patients were collected.A laboratory-developed LC-MS/MS method was used as reference. UFC was measured by direct assays (D) using Abbott, Siemens, and Beckman platforms and by extraction assays (E) using Siemens and Beckman platforms. Method was compared using Passing–Bablok regression and Bland–Altman plot analyses. Cut-off values for the six assays and corresponding sensitivities and specificities were calculated by ROC analysis. Results: Abbott-D, Beckman-E, Siemens-E, and Siemens-D showed strong correlations with LC-MS/MS (Spearman coefficient r = 0.965, 0.922, 0.922, and 0.897, respectively), while Beckman-D showed weaker correlation (r = 0.755). All immunoassays showed proportionally positive bias. The areas under the curve were 0.975 for Abbott-D, 0.972 for LCMS/MS, 0.966 for Siemens-E, 0.948 for Siemens-D, 0.955 for Beckman-E, and 0.877 for Beckman-D. The cut-off values varied significantly (154.8–1,321.5 nmol/24 hrs). Assay sensitivity and specificity ranged from 76.1% to 93.2% and from 93.0% to 97.1%, respectively. Conclusions Commercially available immunoassays for measuring UFC show different levels of analytical consistency compared to LC-MS/MS. Abbott-D, Siemens-E, and Beckman-E have high diagnostic accuracy for CS.