Retention mechanism study of imidazole derivatives on a beta-cyclodextrin-bonded stationary phase. Thermal analysis contributions

The high-performance liquid chromatography retention mechanism of a series of six imidazole derivatives was investigated over a wide range of mobile-phase compositions, pH, and column temperatures using a beta-cyclodextrin (beta-CD)-bonded chiral stationary phase. Thermodynamic constants for the transfer of a solute from the mobile to the beta-CD stationary phase were determined. Different van't Hoff plot shapes were observed with mobile-phase pH values, indicating a change in the retention mechanism. Enthalpy-entropy compensation revealed that the solute retention mechanism was independent of the compound molecular structure, the same at pH 7 and 7.5, but changed at pH 6.5, 8, and 8.5. Differential scanning calorimetry and thermogravimetric analysis were used to show different thermal features for the beta-CD stationary phase at pH 6.5, 8, and 8.5 and at pH 7 and 7.5. A new theory was presented to explain the beta-CD cavity structure balance between an ordered and disordered state. Variations of column temperature and mobile-phase pH tend to cause this phase transition between these two states, explaining the thermodynamic constant variations with pH and temperature.

Multicentre evaluation of the DCA 2000 system for measuring glycated haemoglobin. DCA 2000 Study Group

The recommended method for assessing long-term blood glucose control in diabetic patients is the measurement of glycated haemoglobin (Hb). The Ames DCA 2000 system for assaying glycated Hb uses an immunoassay with a monoclonal antibody specific for an aminoacid sequence within the HblAc molecule. This study compared the performance of the DCA 2000 system for HblAc measurement with that of high-performance liquid chromatography (HPLC). A total of 1.016 insulin-dependent and non-insulin-dependent diabetic patients from 5 outpatient clinics took part. The correlation coefficients between DCA 2000 and HPLC data ranged between 0.94 and 0.98, depending on site. The mean variations and 95% confidence intervals for the differences between the results for each sample were: site A 0.172 (-1.186 to 1.53), site B -0.275 (-1.317 to 0.767), site C -0.146 (-0.868 to 0.576), site D -0.088 (-0.864 to 0.688), and site E -0.251 (-1.099 to 0.597). The sensitivity of the DCA 2000 assay ranged between 80 and 94%, and the specificity between 88 and 100%, depending on site. For pooled results, the correlation coefficient assayed by the two methods was 0.95. The mean variation was -0.116 and the 95% confidence interval -1.23 to 0.998. The sensitivity of DCA 2000 was 91%, and the specificity 94%. DCA tended to underestimate HbAlc slightly as compared to HPLC. This study confirms the reliability of DCA 2000 for measuring glycated Hb. The system is easy to use and provides valuable information for the care of the diabetic patients.

Lipophilicity determination of psoralens used in therapy through solubility and partitioning: comparison of theoretical and experimental approaches

The aim of this study was to determine and to compare experimental and theoretical solubilities (S) as well as partition coefficients (PC) in an octanol/water system of psoralen (P), 8-methoxypsoralen (8-MOP), 5-methoxypsoralen (5-MOP) and 4,5',8-trimethylpsoralen (TMP). For each psoralen, experimental results were performed in triplicate with a spectrofluorimetric technique. The measurements were achieved 10 times for each solution. The obtained order of the solubilities in pure octanol was 5-MOP approximately TMP > P > 8-MOP, while in water-saturated octanol it was expressed as follows: TMP approximately 5-MOP > P > 8-MOP. However, the following order was found for hydrophobicity: TMP > 5-MOP > 8-MOP > P. The solubility ratios (SR) in pure octanol and water were assessed (mean +/- SD): 3.13 +/- 0.01 (P), 2.60 +/- 0.01 (8-MOP), 3.75 +/- 0.01 (5-MOP), and 5.11 +/- 0.01 (TMP). In saturated phases, they were 3.27 +/- 0.01, 2.63 +/- 0.01, 3.85 +/- 0.01, and 5.32 +/- 0.01, respectively. The PCs were determined with low concentrations according to the Dearden and Bresnen32 method and they were 1.67 +/- 0.01, 1.93 +/- 0.01, 2.00 +/- 0.01, and 3.14 +/- 0.01, respectively. Solubility parameters (delta), in Hildebrand unit (H) or in (cal/cm3)1/2, were evaluated. They confirmed the polarity of psoralens, previously expressed through the PC, although the positional isomers (5-MOP and 8-MOP) revealed no difference. Hildebrand's approach to the solubility of regular solutions and Yalkowsky's concept of the solubility of nonelectrolytes and weak electrolytes in an octanol/water system permitted a comparison of the theoretical and experimental results. The perspective of this work is to use the physicochemical properties of the psoralens in practice for insuring convenient experimental assays and the prediction, in vitro, of the percutaneous absorption of these compounds.

Ibuprofen-loaded ethylcellulose microspheres: analysis of the matrix structure by thermal analysis

The determination of the drug dispersion state in microspheres prepared by the solvent evaporation method is essential to foresee the stability of the particles and the drug release behavior. The present work deals with ibuprofen-loaded ethylcellulose microspheres, that are characterized by a lower drug melting point than the polymer glass transition temperature. Although annealing experiments were not possible, the study has evidenced the presence of a metastable molecular dispersion for intermediate loadings, coexisting with a solid solution and a crystalline dispersion of the drug in the polymer matrix. In addition, differential scanning calorimetry helped to distinguish between surface and inner ibuprofen crystals, which interact differently with the polymer matrix and therefore have different melting points.