Simultaneous determination of metolazone and losartan potassium in their binary mixtures using high-performance liquid chromatography with fluorimetric detection: application to combined tablets and spiked human plasma

Degradation of Losartan Potassium Highlighted by Correlated Studies of Photoluminescence, Infrared Absorption Spectroscopy and Dielectric Spectroscopy

In this paper, new results on the degradation of losartan potassium (LP, (1)), in the absence and presence of excipients, which was induced by UV light, the acid character of phosphate buffer solution (PBS) and alkaline medium, respectively, are reported through correlated studies of FTIR spectroscopy, photoluminescence and dielectric spectroscopy. The photoluminescence (PL) spectra of LP and the drug marked under the name Lorista (LO) are characterized by intense emission bands, peaking at 378 nm and 380 nm, respectively, accompanied by low intensity bands with a maximum at ~450–460 nm. Photodegradation of LO in a solid state is evidenced by a decrease in the intensity of the PL band at 380 nm, a variation that originates both in the adsorption of water vapors from the air and in the interaction of LP with excipients such as cornstarch, silicon dioxide and cellulose. The LP-water interaction is described, taking into account the main electrical parameters, i.e., complex dielectric permittivity and electrical conductivity. Photodegradation of LP and LO also induces an increase in the intensity of the emission band, at ~450–460 nm. The influence of acid and alkaline medium on the LO degradation is analyzed using phosphate buffer (PBS) and NaOH solutions, respectively. In both cases, a decrease in the intensity of the PL band, at 380 nm, is reported. The intensity diminution of the PL spectra of NaOH-reacted LP and LO is the result of the formation of the photodegradation product N-methanolamide-{[2′-(1H-tetrazol-5-yl)(1,1′-biphenyl)-4-yl]methyl} (2). This compound was proven by the studies of FTIR spectroscopy achieved on LP and NaOH-reacted LP. The appearance of the IR band at 1740 cm−1 and the increase in the absorbance in the IR band at 1423 cm−1 indicate that the photodegradation product (2) contains the C=O and C-OH functional groups.

First-order derivative synchronous spectrofluorimetric determination of two antihypertensive drugs, metolazone and valsartan, in pharmaceutical and biological matrices

In this study, a facile, rapid, and sensitive spectrofluorimetric method was evolved to analyse two antihypertensive drugs, namely, metolazone (MTZ) and valsartan (VST), in pharmaceutical and biological matrices. Both analytes exhibited intrinsic fluorescence activities which were significantly affected by environmental factors such as pH and solvent systems. However, simultaneous determination of MTZ and VST by conventional spectrofluorometry cannot be achieved simply because of the strong overlap between their fluorescence spectra. Thus, a combination of derivative and synchronous spectrofluorometry was conducted to overcome this dilemma. The proposed method relies on measurement of the first-order derivative of synchronous fluorescence intensity of the studied drugs at Δλ = 160 nm using 0.1 M acetic acid as the optimum solvent. The amplitudes of the first derivative synchronous fluorescence spectra of MTZ and VST were recorded at 236.0 nm (zero-crossing point of VST) and at 262.8 nm (zero-crossing point of MTZ) for simultaneous analysis of MTZ and VST, respectively. The fluorescent method was optimized efficiently to get the maximum selectivity and sensitivity by investigating different solvents, different buffer pHs, and different surfactants. The highest sensitivity and selectivity were achieved when 0.1 M acetic acid was used as a solvent. The method showed a linear concentration range of 10.0-100.0 ng mL^-1 and a limit of detection of <3.0 ng mL^-1 for each analyte. Statistical data analysis confirmed that no significant difference between the proposed spectrofluorometric method and the reference methods. The validity of the proposed spectrofluorometric method approved its suitability for quality control work. The proposed spectrofluorometric method was applied to assay the studied drugs in pharmaceutical dosage and in biological matrices with acceptable %recoveries and small RSD values.

Development and Validation of Two Robust Stability-Indicating Chromatographic Methods for Determination of Metolazone in Drug Substance and Pharmaceutical Dosage Form in the Presence of Its Degradation Products and Characterization of Main Degradation Products Based on LC-MS

Two robust and selective stability-indicating chromatographic methods were developed and validated for the determination of metolazone in drug substance and pharmaceutical dosage form in the presence of its degradation products. The HPLC method employed a Kromasil C18 (250 × 4.6,5 μm) column and a mobile phase of acetonitrile: 0.2% orthophosphoric acid (32:68 v/v) at a flow rate 2 mL/min and detection at 238 nm. The separation was performed in HPLC isocratic mode. The robustness of the suggested method was assessed using the Plackett-Burman design, parameters affecting system suitability were established and non-significant intervals for the significant parameters were considered. The HPTLC method employed Nano-SIL-20 UV254 HPTLC plates as adsorbent, ethyl acetate: toluene: acetic acid solution (4:4:0.5, v/v/v), as a developing solvent system and densitometric detection at 238 nm. Metolazone was exposed to different stress conditions, including acid and alkaline hydrolysis and oxidative and photolytic degradation. The main degradation products obtained have been characterized and interpreted based on LC-MS. The linearity of the suggested methods was proved in the concentration range of 20-75 μg/mL for the HPLC method and 100-900 ng/spot for the HPTLC method. The suggested methods were validated according to international conference on harmonization guidelines. These methods were successfully dedicated for the estimation of metolazone in drug substance and pharmaceutical dosage form in the presence of its degradation products. The results of the suggested methods were evaluated and compared statistically with results obtained by an official method without finding any significant difference.

A new spectrofluorimetric method for determination of losartan potassium in rabbit plasma and its application to pharmacokinetic study

A new spectrofluorimetric method to determine losartan potassium (LP) in rabbit plasma is described. The method was based on measuring the native fluorescence of LP in acidic medium. Optimum excitation and emission wavelengths were found to be 248 nm and 410 nm, respectively, in methanol that was diluted with a sulfurous acid solution LP was extracted from rabbit plasma by methyl-tertiary-butyl-ether in acidic media and then back extracted with NaOH. The calibration curves were linear between 0.025 and 0.5 µg/mL with a lower limit of detection 0.004 µg/mL. Precision and accuracy values of the method were calculated as lower than 4.97% and ± 5.68, respectively and the recovery of LP from rabbit plasma was higher than 91.1%. In addition, stability studies of LP in rabbit plasma were carried out and demonstrated its good stability at - 20 °C and at room temperature. The developed and validated method was successfully applied for estimating the pharmacokinetic parameters of LP following oral administrations of a single 10 mg LP/kg to rabbits and it could be concluded that the method can be applied to clinical trials.