Biljana Nigović

Voltammetric assay of azithromycin in pharmaceutical dosage forms

The oxidative behaviour of azithromycin was studied at s glassy carbon electrode in different buffer system using cyclic, linear sweep and differential pulse voltammetry. The oxidation process was shown to be irreversible over the entire pH range studied (5-11) and was diffusion-adsorption controlled. Analytical method with adequate precision and accuracy was developed for the determination of azithromycin in phosphate buffer at pH 7 as supporting electrolyte containing 10% methanol and 0.05 M ammonium acetate. The peak current varied linearly with azithromycin concentration in the range 1-15 microg/ml. The procedure was successfully applied for assay of the drug in the pharmaceutical dosage forms. The relative standard deviation (n = 5) of 2.18% was obtained.

Determination of 5-aminosalicylic acid in pharmaceutical formulation by differential pulse voltammetry

The oxidative behaviour of 5-aminosalicylic acid (5-ASA) has been investigated by differential pulse voltammetry using a glassy carbon electrode in different buffer systems. Linear sweep voltammetry was used to study the influence of pH on the peak current and peak potential. The solution conditions and instrumental parameters were optimized to obtain a good sensitivity. The Britton-Robinson buffer of pH 1.81 was selected as a suitable analytical medium in which 5-ASA exhibited a sensitive diffusion controlled oxidative peak at 0.564 V (vs. Ag/AgCl). The peak current varied linearly with drug concentration in the range between 1 x 10(-4) and 2 x 10(-6) M. The proposed voltammetric method has been applied to the determination of the drug in commercial delayed-release tablet forms. A mean recovery of 101.23% with a relative standard deviation of 1.35% was obtained.

Simultaneous determination of lovastatin and citrinin in red yeast rice supplements by micellar electrokinetic capillary chromatography

Lovastatin is a main component of Monascus purpureus fermented red rice contributing to the lipid-lowering effect. Citrinin is a toxic fermentation by-product which can be found as a contaminant. An accurate, simple and rapid micellar electrokinetic capillary chromatographic method was developed for the first time for simultaneous determination of lovastatin present in lactone and hydroxy acid forms and citrinin in red rice products provided by different manufacturers and formulated in various dosage forms. Separation was achieved within only 2 min using 20 mM of phosphate buffer at pH 7.0 and 30 mM of sodium dodecyl sulphate at an applied voltage of 25 kV. Sensitivity crucial for detecting citrinin was enhanced by using an extended light path capillary. The results showed that the content of lovastatin and its acid form in dietary supplements were considerably different indicating the need for improved standardization in order to ensure efficiency and safety of these products.

Development of a Rapid LC/DAD/FLD/MSn Method for the Simultaneous Determination of Monacolins and Citrinin in Red Fermented Rice Products

Red fermented rice is used worldwide by many patients as an alternative therapy for hyperlipidemia; however, the discovery of a toxic fermentation byproduct, citrinin, causes much controversy about the safety of red mold rice products. A new and fast high-performance liquid chromatography method was developed and validated for simultaneous determination of cholesterol-lowering compounds monacolin K (lovastatin), monacolin K hydroxy acid, and other monacolins present in red fermented rice as well as nephrotoxic mycotoxin citrinin in a single run using connected diode array and fluorescence and mass spectrometric detectors. The proposed method was successfully applied for the analysis of red fermented rice food samples and various dietary supplements also containing other natural lipid-lowering agents. The deviations between label content and levels of active compounds found in investigated samples as well as high batch-to-batch variation found in one product indicate that the regular quality control of red fermented rice products is of great importance.

Structural studies on monohalogenated derivatives of the phytohormone indole-3-acetic acid (auxin)

The physiological properties of the phytohormone (auxin) indole-3-acetic acid (IAA) and its ring substituted derivatives have so far been rationalized by a number of contradictory hypotheses based on incomplete structural data deduced mainly by inspection of molecular models. In order to give more evidence for structure–activity relationships of monohalogenated IAAs, the molecular structures of the natural auxin 4-CI-IAA as well as 5-CI-IAA, 6-CI-IAA, 7-CI-IAA and 5-Br-IAA have been compared, as revealed by X-ray analysis, and molecular mechanics and dynamics. The influence of the substitution site and the size of the halogen atom and bioactivity is discussed. The typical structural feature of the molecules studied is the slight distortion of part of the indole nucleus around C7: bond length C6—C7 1.368 (6) A and C6—C7—C71 117.6 (3)° (average values of five structures and seven molecules). The conformations of monohalogenated indole-3-acetic acid molecules, characteristic for auxins, are defined by rotations about two bonds only: one describes the relative orientation of a side chain towards the indole moiety and the second the orientation of the carboxylic group. The results of X-ray structure analysis, and molecular mechanics and dynamics revealed the folded shape of the molecules in all compounds studied. Ab initio calculations showed that the planar conformation can be adopted as well. Crystal data at 297 K for 4-CI-IAA, 6-CI-IAA, 7-CI-IAA and 5-Br-IAA, and at 220 K for 5-CI-IAA, using Mo Kα radiation (λ = 0.71073 A), and CuKα (λ = 1.5418 A) for 6-CI-IAA, are as follows: 4-CI-IAA, C10H8ClNO2, Mr = 209.63, monoclinic, P21/c, a = 7.313 (4), b = 17.156 (4), c = 7.640 (4) A, β = 92.71 (5)°, V = 957.5 (1) A3, Z = 4, Dx = 1.454 g cm−3, μ = 3.7 cm−1, F(000) = 432, R = 0.037, wR = 0.039 for 1040 symmetry-independent [I ≥ 3σ(I)] reflections; 5-CI-IAA, C10H8ClNO2, monoclinic, P21/c, a = 19.141 (4), b = 5.154 (2), c = 10.323 (3) A, β = 116.23 (2)°, V = 913.5 (1) A3, Z = 4, Dx = 1.524  g cm−3, μ = 3.8 cm−1, F(000) = 432, R = 0.039, wR = 0.042 for 1184 symmetry-independent [I≥ 3σ(I)] reflections; 6-CI-IAA, C10H8ClNO2, orthorhombic, Pbca, a = 61.08 (1), b = 12.115 (7), c = 7.674 (5) A, V = 5679 (5) A3, Z = 24, Dx = 1.471 g cm−3, μ = 33.9 cm−1, F(000) = 2592, R = 0.052, wR = 0.052 for 3030 symmetry-independent [I ≥ σ(I)] reflections; 7-CI-IAA, C10H8ClNO2, monoclinic, P21/c, a = 20.244 (5), b = 4.829 (2), c = 10.728 (4) A, β = 116.30 (1)°, V = 940 (1) A3, Z = 4, Dx = 1.481 g cm−3, μ = 3.7 cm−1, F(000) = 432, R = 0.042, wR = 0.029 for 889 symmetry-independent [I ≥ 3σ(I)] reflections; 5-Br-IAA, C10H8BrNO2, Mr = 254.08, triclinic, P{\bar 1}, a = 5.645 (3), b = 9.713 (4), c = 10.019 (4) A, α = 116.02 (3), β = 92.67 (5), γ = 100.12 (4)°, V = 481.2 (5) A3, Z = 2, Dx = 1.754 g cm−3, μ = 42.0 cm−1, F(000) = 252, R = 0.029, wR = 0.020 for 1865 symmetry-independent [I ≥ 3σ(I)] reflections.

Correlation of structural and physico-chemical parameters with the bioactivity of alkylated derivatives of indole-3-acetic acid, a phytohormone (auxin)

As part of molecular recognition studies on the phytohormone indole-3-acetic acid (IAA) a series of alkylated IAAs has been examined. Phenyl-ring substitution (alkyl = methyl and ethyl) at positions 4-, 6- or 7- as well as pyrrole substitution at the 2-site resulted in the six compounds which are analyzed: 2-Me-IAA, 4-Me-IAA, 6-Me-IAA, 7-Me-IAA, 4-Et-IAA and 6-Et-IAA. The structure-activity relationships investigated include those between the geometrical parameters of the molecular structures determined by X-ray analysis, the growth-promoting activities in the Avena coleoptile straight-growth bioassay and relative lipophilicities calculated from retention times on a reversed-phase HPLC column and from R(F) values in reversed-phase TLC. Lipophilicities are correlated with the moments of inertia, average polarizability, molecular mass, and the van der Waals radii of the ring substituents. The influence of substitution on the electronic properties of the indole ring and its geometry is discussed on the basis of the UV and 1H NMR spectra.

Multi-walled carbon nanotubes/Nafion composite film modified electrode as a sensor for simultaneous determination of ondansetron and morphine.

The electrochemical behavior of ondansetron was studied on the multi-walled carbon nanotubes/Nafion polymer composite modified glassy carbon electrode (MWCNTs-Nafion/GCE). The oxidation peak potential was shifted from 1.32 V to 1.18 V compared to the bare electrode indicating excellent electrocatalytic activity of immobilized film toward drug molecule. The modified electrode exhibited a remarkable enhancement effect on voltammetric response due to the synergistic effect of nanomaterial and cation-exchange polymer on the electron transfer rate, the effective electrode area and the accumulation capability. After optimizing the experimental parameters, adsorptive stripping procedure was used for the determination of ondansetron in pharmaceutical formulation. The results were satisfactory in comparison with those obtained by high-performance liquid chromatography. In addition, the MWCNTs-Nafion/GCE exhibited high selectivity in the voltammetric measurements of ondansetron and co-administrated drug morphine with potential difference of 430 mV. The response peak currents had linear relationship with drug concentration in the range of 1.0 × 10(-7)-5.0 × 10(-6)M and 1.0 × 10(-7)-4.0 × 10(-6)M with detection limits 3.1 × 10(-8) and 3.2 × 10(-8)M for ondansetron and morphine, respectively. The electrode was successfully applied for simultaneous electrochemical sensing of both drugs in human serum samples after selective accumulation at the electrode surface.

N-(indol-3-ylacetyl)amino acids as sources of auxin in plant tissue culture

N-(Indol-3-ylacetyl) derivatives (IAA conjugates) of aliphatic amino acids with a two- to six-carbon backbone including α-l-amino acids, (ω-amino acids, and the α,ω-diamino acids ornithine and lysine were prepared, chemically characterized, and tested as sources of auxin in plant tissue culture. Stimulation of unorganized growth in Solanum nigrum L. callus and callus induction and developmental effects in tomato (Lycopersicon esculentum Mill. cv. Marglobe) hypocotyl explants were studied systematically. Relative auxin activities were estimated by comparing physiologically equivalent concentrations, in the optimal and suboptimal range, of the individual IAA conjugates. While the growth-promoting properties of some of the conjugates were species-dependent, those containing straight-chain two- to four-carbon α-l-amino acid moieties were generally up to 100 times more active than those of their five- to six-carbon homologues. Branching of the amino acid backbone at C-β (norvaline vs. valine and norleucine vs. isoleucine) and C-γ (norleucine vs. leucine) had a minor effect, but substitution of H-α by a methyl group (α-amino-l-butyric vs. α-aminoisobutyric acids) almost completely blocked growth-promoting activity. IAA conjugates of ω-amino acids were, in most cases, nearly as active as those of their α-amino-l-isomers. Among the conjugates of α,ω-diamino acids Nδ-(IAA) ornithine was less active than Nε-(IAA)lysine. The activity of Nα-(IAA)lysine was less than for the ε-(IAA) isomer, and that of Nα,Nε-(IAA)2-lysine was different in tomato and Solanum nigrum. The l-alanine and ε-lysine conjugates were also found to be useful for induction and development of Oenothera leaf callus and in tomato cell-suspension culture, two systems which require highly active sources of auxin.

Voltammetric studies of 2-hydroxy-5-[(4-sulfophenyl)azo]benzoic acid as a novel prodrug of 5-aminosalicylic acid.

The electrochemical properties of a colon-targeted prodrug of 5-aminosalicylic acid (5-ASA), 2-hydroxy-5-[(4-sulfophenyl)azo]benzoic acid (SPSA), were investigated in aqueous solutions at glassy carbon electrodes using cyclic voltammetry and controlled potential electrolysis. The influence of the pH and experimental time domain on the reaction pathway has been studied. The electrochemical reduction of SPSA is identified as an ECE process always leading to the cleavage of azo bond. In an acidic media SPSA is reduced in a 4e(-)/4H(+) process yielding 5-ASA and sulfanilic acid. In neutral and weakly basic media SPSA is reduced in 2e(-)/2H(+) process resulting in the hydrazo intermediate that is stable enough to enable its reoxidation back to SPSA in the time scale of the cyclic voltammetry.

A novel electrochemical sensor for assaying of antipsychotic drug quetiapine

A novel electrochemical sensor based on poly(2-hydroxy-5-[(4-sulfophenyl)azo]benzoic acid) film modified glassy carbon electrode for fast and simple quantification of trace amount of quetiapine fumarate (QF) was developed. It exhibits excellent enhancement effects on the electrooxidation of QF facilitating preconcentration of drug molecules on the electrode surface. Based on its strong adsorptive activity, the concentration of QF in pharmaceutical formulations and biological fluids was determined directly by voltammetry with excellent sensitivity and high selectivity. The introduction of carboxylated and sulfonated functionalities in polymer film improves the uniform selectivity for positively charged target QF molecules. The calibration curve is linear in QF concentration range of 8.0 × 10(-8) to 7.5 × 10(-6)M with detection limit 1.9 × 10(-8) and sensitivity 8.96 × 10(5) μA M(-1). The presented sensor has long term stability and good reproducibility with benefits of fast response time, ease of preparation and regeneration of the surface that makes the proposed method useful in the determination of QF in real samples.