Gordana V. Popović

Ultra-thin-layer chromatography mass spectrometry and thin-layer chromatography mass spectrometry of single peptides of angiotensin-converting enzyme inhibitors

The separation of structurally related angiotensin-converting enzyme (ACE) inhibitors lisinopril, cilazapril, ramipril and quinapril and their corresponding active diacid forms (prilates) by conventional TLC silica gel 60 plates was contrasted with that afforded by monolithic ultra-thin-layer chromatographic (UTLC) plates. For the use of UTLC plates technical modifications of the commercially available equipments for the sample application, development and detection were made. Plates were developed in modified horizontal developing chamber using ethyl acetate-acetone-acetic acid-water (4:1:0.25:0.5, v/v). Detection of the separated compounds was performed densitometrically in absorption/reflectance mode at 220 nm and after exposure to iodine also by image analysis. The obtained results showed that monolithic layer is more efficient for the separation of structurally similar polar compounds, such as prilates than conventional silica layers. Identification of the compounds was confirmed by ESI-MS after their on-line extraction from the UTLC and TLC plates by means of Camag TLC-MS interface.

Acid–base equilibria and solubility of loratadine and desloratadine in water and micellar media

Acid-base equilibria in homogeneous and heterogeneous systems of two antihistaminics, loratadine and desloratadine were studied spectrophotometrically in Britton-Robinsons buffer at 25 degrees C. Acidity constant of loratadine was found to be pK(a) 5.25 and those of desloratadine pK(a1) 4.41 and pK(a2) 9.97. The values of intrinsic solubilities of loratadine and desloratadine were 8.65x10(-6) M and 3.82x10(-4) M, respectively. Based on the pK(a) values and intrinsic solubilities, solubility curves of these two drugs as a function of pH were calculated. The effects of anionic, cationic and non-ionic surfactants applied in the concentration exceeding critical micelle concentration (cmc) on acid-base properties of loratadine and desloratadine, as well as on intrinsic solubility of loratadine were also examined. The results revealed a shift of pK(a) values in micellar media comparing to the values obtained in water. These shifts (DeltapK(a)) ranged from -2.24 to +1.24.

Determination of bifonazole in creams containing methyl- and propyl p-hydroxybenzoate by derivative spectrophotometric method

A second order derivative spectrophotometric method for the determination of bifonazole in the presence of methyl- and propyl p-hydroxybenzoate as preservatives has been developed. The determination was performed in a 0.1 M HCl solution at 241.5 nm, a wavelength corresponding to the intersection of the second order derivative spectra (2D) of methyl- and propyl p-hydroxybenzoate with the axis (zero-crossing point). On the basis of the knowledge of acidity constants and solubility, as well as of the investigations of zero-order and 2D spectra of bifonazole and preservatives, these conditions were chosen as optimal ones. A calibration curve constructed for bifonazole concentrations ranging from 1.5 to 15 microg/ml had a correlation coefficient of 0.9998. Reliability and reproducibility of the method was checked by analyzing laboratory mixtures of bifonazole and preservatives (recovery 99.97-102.7%; RDS 0.48-1.46%). The proposed method was applied for the determination of bifonazole in a commercial cream formulation. The mean value of bifonazole obtained per 100 g cream was 1.029 g (102.9% of the labeled claim) with a RSD of 0.60%.

Study of heterogeneous equilibria in saturated aqueous solutions of some 7-chloro-1,4-benzodiazepines

Acid-base equilibria in heterogeneous and homogeneous systems of some sparingly soluble benzodiazepines (diazepam, prazepam, chlordiazepoxide and medazepam) were investigated in aqueous solution and the corresponding equilibrium constants were determined by the application of pH-metric, spectrophotometric and solubility methods. The sites of protonation were predicted on the basis of the analysis of the corresponding carbon-13 nuclear magnetic resonance spectra. In addition, buffer characteristics of saturated aqueous solutions of these drugs in the presence of the solid phase were investigated and it was established that they possess a very high buffer capacity over a specific pH range.

Study of acid-base equilibria of fleroxacin.

The acid-base equilibria of fleroxacin were studied by means of potentiometry and spectrophotometry. It was established that fleroxacin undergoes a complex acid-base equilibrium due to its zwitterionic nature and two proton-binding sites of similar acidity. The stoichiometric equilibrium constants were determined at 25 degrees C and constant ionic strength 0.1 M (NaCl). The acidity constants pK1 = 5.59 +/- 0.01 and pK2 = 8.08 +/- 0.04 were found by potentiometry, and pK1 = 5.61 +/- 0.03 and pK2 = 8.11 +/- 0.06 by spectrophotometry. The distribution diagram of the corresponding ionic species is given.

Arteriovenous Fistula Aneurysm in Patients on Regular Hemodialysis: Prevalence and Risk Factors

Background/Aims: Compared to all other complications, literature data about vascular access aneurysm (VAA) are the scarcest. The aim of this cross-sectional study was to evaluate the prevalence of arteriovenous fistula (AVF) aneurysms and to confirm the risk factors for their appearance. Methods: The presence, number and morphological characteristics of AVF aneurysms were confirmed, and according to the score of AVF aneurysm (the sum of the length and width in cm), patients were classified into group 1 (score ≤12) and group 2 (score >12). Analysis included the last data from the medical records including vascular calcifications score. Results: Out of 181 patients, 150 with native fistula were included in this study. Aneurysmatic changes were detected in 90 (60%) patients, and the majority had two or more aneurysms. VAA were more frequent in patients with adult polycystic kidney disease (ADPKD) than in other diagnostic categories. By using forward stepwise logistic regression, we confirmed that patients on high-flux hemodialysis (HD) had 5.3-fold higher risk, and patients with diabetes mellitus had 5.8-fold less risk for developing AVF aneurysm. While vascular calcification score did not influence the incidence of VAA, higher PWV had significant negative influence on formation of AVF aneurysm (OR 1.25, 95% CI 1.003-1.56, p = 0.047). By ROC curve analysis, it was determined that patients who were longer than 5.7 years on HD had greater risk for developing VAA (area = 0.741, p = 0.000). Conclusion: This single-center study confirmed the very high prevalence of VAA (60%). Aneurysms were more frequent in patients with ADPKD and in those who had longer dialysis vintage on high-flux membranes with higher blood flow rate.

Study on protolytic equilibria of lorazepam and oxazepam by UV and NMR spectroscopy

Protolytic equilibria in homogeneous and heterogeneous systems of lorazepam and oxazepam, which are sparingly soluble ampholytes from the class of 1,4-benzodiazepines, were studied at 25 degrees C and ionic strength of 0.1 M. Acidity constants and equilibrium constants in a heterogeneous system were determined. On the basis of the analysis of the corresponding 13C- and 1H-NMR spectra, deprotonation site in the molecules of the investigated compounds was predicted. Finally, the correlation between chemical shifts in the 1H-NMR spectra and the acidity of the amide proton of 1,4-benzodiazepines was established.

Study of protolytic equilibria of flurazepam

Protolytic equilibria of flurazepam, a diprotic base which is sparingly soluble in water, were investigated. The investigations were carried out in the pH range 0–12, at constant ionic strength (0.1 M NaCl) and 25 ± 0.1 °C, by the application of spectrophotometric, pH-metric and solubility methods. The acidity, hydrolysis and equilibrium constants in the heterogeneous system were determined. In addition, a method, based on the application of a formation function to the hydrolytic process, is proposed. Finally, buffer characteristics of flurazepam in the homogeneous and heterogeneous systems were studied.

Comparison of HPTLC and HPLC for determination of econazole nitrate in topical dosage forms

HPTLC and HPLC methods have been established for separation and quantitative determination of econazole nitrate. HPTLC was performed on silica gel plates with n-butyl acetate–carbon tetrachloride–methanol–diethylamine, 3 + 6 + 2.5 + 0.5 (v/v), as mobile phase. Chromatographic zones, or spots, of econazole base and nitrate were used to quantify econazole nitrate. HPLC was performed on amino and C8 columns with acetonitrile–water, 30 + 70 (v/v), pH 2.5 (adjusted with phosphoric acid), as a mobile phase. The chromatograms were characterized by single peaks of econazole (amino column) or nitrate (C8 column). The methods were successfully used for determination of econazole nitrate in spray solution and vaginal pessaries.

Derivative spectrophotometric method for determination of acidity constants of single step acid-base equilibria.

A general derivative spectrophotometric method for determination of acidity constants is developed. The method appears suitable in cases when classical spectrophotometry cannot be employed due to little differences in the absorption spectra of conjugated acid-base pairs. Based on theoretical considerations, seven variants of the method have been established and their validity was checked, determining acidity constants of lorazepam and flurazepam as model compounds.