Ebru Çubuk Demiralay

Combined effect of polarity and pH on the chromatographic behavior of some angiotensin II receptor antagonists and optimization of their determination in pharmaceutical dosage forms

In the present study, the combined effect of mobile phase polarity and pH on retention behavior of some ARA-IIs (irbesartan, losartan, valsartan and telmisartan) is investigated. The linear relationships established between retention factors of the species and the polarity parameter of the mobile phase has proved to predict accurately retention in LC as a function of the acetonitrile content (50%, 55%, 60%, v/v). The suggested model uses the pH value in the acetonitrile-water mixture as mobile phase instead of pH value in water and takes into account the effect of activity coefficients. Moreover, correlation between retention and the mobile phase pH can be established allowing prediction of the retention behavior as a function of the mobile phase pH. The model can be used to estimate the pKa in an acetonitrile percentage between 50% and 60%, at 30 degrees C. The developed method was successfully applied to both the simultaneous separation of these drug-active compounds and individual determination in their commercial pharmaceutical dosage forms.

Determination of pKa values of nonsteroidal antiinflammatory drug-oxicams by RP-HPLC and their analysis in pharmaceutical dosage forms.

In this study, pK(a) values were determined by using the dependence of the capacity factor on the pH of the mobile phase for four ionizable substances, namely, tenoxicam, piroxicam, meloxicam, and naproxen (I.S.). The effect of the mobile phase composition on the ionization constant was studied by measuring the pK(a) at different ACN concentrations, ranging from 30 to 40%. The adequate condition for the chromatographic determination of these compounds in pharmaceutical dosage forms was established based on the different retention behaviors of the species. An octadecylsilica Nucleosil C18 column (150 x 4.6 mm, 5 microm) was used for all the determinations. The chromatographic separation of oxicams was carried out using acetonitrile (ACN)/water at 35% v/v, containing 65 mM phosphoric acid and UV detection at a wavelength of 355 nm. The method developed was successfully applied to the simultaneous determination of these drug compounds in laboratory-prepared mixtures and their commercial pharmaceutical dosage forms. Each analysis requires no longer than 12 min.

Determination of chromatographic and spectrophotometric dissociation constants of some beta lactam antibiotics

In this work, dissociation constants values of seven beta lactam antibiotics in water and acetonitrile-water mixtures using spectrophotometric and reversed phase liquid chromatography methods were determined. The dissociation constant values of these compounds were calculated by NLREG and STAR programs. Aqueous pK(a) values of beta lactam antibiotics were calculated with extrapolation by means of the Yasuda-Shedlovsky and mole fraction equations. Finally, application of the different techniques was compared to the determination of aqueous pK(a) values of investigated compounds.

Estimation of thermodynamic acidity constants of some penicillinase-resistant penicillins.

In this work, thermodynamic acidity constants (pssKa) of methicillin, oxacillin, nafcillin, cloxacilin, dicloxacillin were determined with reverse phase liquid chromatographic method (RPLC) by taking into account the effect of the activity coefficients in hydro-organic water-acetonitrile binary mixtures. From these values, thermodynamic aqueous acidity constants of these drugs were calculated by different approaches. The linear relationships established between retention factors of the species and the polarity parameter of the mobile phase (ET(N)) was proved to predict accurately retention in LC as a function of the acetonitrile content (38%, 40% and 42%, v/v).

Determination of chromatographic dissociation constants of some carbapenem group antibiotics and quantification of these compounds in human urine

The dissociation constant values (s (s) pKa ) of some carbapenem group drugs (ertapenem, meropenem, doripenem) in different percentages of methanol-water binary mixtures (18, 20 and 22%, v/v) were determined from the mobile phase pH dependence of their retention factor. Evaluation of these data was performed using the NLREG program. From calculated pKa values, the aqueous pKa values of these subtances were calculated by different approaches. Moreover, the correlation established between retention factor and the pH of the water-methanol mobile phase was used to determine the optimum separation conditions. In order to validate the optimized conditions, these drugs were studied in human urine. The chromatographic separation was realized using a Gemini NX C18 column (250 × 4.6 mm i.d., 5 µm particles) and UV detector set at 220 and 295 nm.

Effect of Organic Solvent Composition on Dissociation Constants of Some Reversible Acetylcholinesterase Inhibitors

Reversible acetylcholinesterase inhibitors are an important group of drug compounds that are used medicinally to treat Alzheimers disease. In this study, dissociation constant values (pKa) of some reversible acetylcholinesterase inhibitors (donepezil, rivastigmine and galantamine) having different functional groups were determined in different percentages of acetonitrile (MeCN)-water and methanol (MeOH)-water binary mixtures using reversed phase liquid chromatography (RPLC) method. In this way, a complete description of the retention behavior of each solute in the space defined by the pH and organic modifier percentages variables was obtained. The linear relationships established between retention factors of the species and the polarity parameter of the mobile phase (ETN) was proved to predict accurately retention in liquid chromatography (LC) as a function of the acetonitrile and methanol content. In result, the estimated aqueous pKa values of studied basic compounds were assessed by comparing them with literature data.

Investigation of Chromatographic Behavior of Leflunomide and its Identification in Pharmaceutical Dosage form by Reversed Phase Liquid Chromatography

Dissociation constant value ( ) is key parameter for predicting the extent of the ionization of a drug molecule at different pH. In this study, pK a values of leflunomide in different percentages of acetonitrile–water binary mixtures (50%, 55%, and 60%, v/v) were determined from the mobile phase pH dependence of retention factor with reverse phase liquid chromatographic method (RPLC). From calculated pK a values, the aqueous pK a value of leflunomide was calculated by various approaches. Moreover, the correlation established between retention factor and the pH of the water–acetonitrile mobile phase was used to determination of optimum separation condition. In order to validate the optimized condition, leflunomide was studied in pharmaceutical dosage form. A Gemini NX C18 column (250 × 4.6 mm I.D., 5 µm particles) was preferred to carry out the developed method. The parameters, linearity, precision, accuracy, specificity, limit of detection, and limit of quantitation were studied according to U.S. Pharmacopeia.

Identification of Penicillinase-Resistant Penicillins in Human Urine by RPLC Method

In this study, the combined effect of the organic modifier content and mobile phase pH has been used to predict the chromatographic behavior of methicillin, oxacillin, nafcillin, cloxacilin and dicloxacillin. The optimum separation condition of these drugs in acetonitrilewater binary mixture was determined from the mobile phase pH dependence of their retention factor. A Synergy Fusion-RP C18 analytical column (150 mm x 4.6 mm I.D., 4 μm) was preferred to carry out the developed method. Chromatographic measurements were done at 30C with an eluent flow rate of 1 mL min. In order to validate the optimized conditions, these drugs were applied successfully in spiked samples of human urine. Valsartan was chosen as an internal standard. The described method was linear over a concentration range of 2-32 μg mL and 3-48 μg mL for the assay of methicillin, nafcillin, dicloxacillin and oxacillin, cloxacillin respectively. Good linear relationships over the investigated concentration ranges were observed with values of correlation coefficient higher than 0.999 for all of the drugs. The intra-day and inter-day precisions of this method were evaluated with RSD (%) values less than 0,561 and 1,622%, respectively. Optimized assay method was validated according to United States Pharmacopeia (USP) guidelines to confirm linearity, accuracy and precision.