The present study aims at developing a simple, sensitive and specific liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for simultaneous quantification of sildenafil and its metabolite N-desmethyl sildenafil in human plasma using sildenafil-d8, N-desmethyl sildenafil-d8 as internal standards (IS). Chromatographic separation was performed on Zorbax SB C18, 4.6 Â 75 mm, 3.5 mm column with an isocratic mobile phase composed of 10 mM ammonium acetate and acetonitrile (5/95 v/v), at a flow-rate of 0.6 ml min À1 . Sildenafil, sildenafil-d8, N-desmethyl sildenafil and N-desmethyl sildenafil-d8 were detected with proton adducts at m/z 475.2 / 283.4, 483.4 / 283.4, 461.3 / 283.4 and 469.4 / 283.4 in multiple reaction monitoring (MRM) positive mode respectively. Both drug, metabolite and internal standards were extracted by liquid-liquid extraction. The method was validated over a linear concentration range of 1.0-1000.0 ng ml À1 for sildenafil and 0.5-500.0 ng ml À1 for N-desmethyl sildenafil with correlation coefficient (r 2 ) $ 0.9998 for sildenafil and (r 2 ) $ 0.9987 for N-desmethyl sildenafil. This method demonstrated intra and inter-day precision within 1.5 to 5.1 and 2.2 to 3.4% for sildenafil and within 1.3 to 3.1 and 2.8 to 4.3% for N-desmethyl sildenafil. This method demonstrated intra and inter-day accuracy for sildenafil within 97.3 to 98.3 and 96.7 to 97.2% and for N-desmethyl sildenafil within 95.3 to 96.3 and 95.0 to 97.2%. Both analytes were found to be stable throughout three freeze/thaw cycles, bench top and postoperative stability studies. This method was used successfully for the analysis of plasma samples following oral administration of 100 mg in 43 healthy Indian male human volunteers under fasting conditions.

A bioequivalence study was proved of generic Febuxostat 80 mg tablets (T) in healthy volunteers.For this purpose, Authors developed a simple, sensitive, selective, rapid, rugged and reproducible liquid chromatography–tandem mass spectrometry method for the quantification of Febuxostat (FB) in human plasma using Febuxostat D7 (FBD7) as an internal standard (IS) was used. Chromatographic separation was performed on Ascentis Express C18 (50x4.6 mm, 3.5 μ) column. Mobile phase composed of 10 mM Ammonium formate: Acetonitrile (20:80 v/v), with 0.8 mL/min flow-rate. Drug and IS were extracted by Liquid- liquid extraction. FB and FBD7 were detected with proton adducts at m/z 317.1→261.1 and 324.2→262.1 in multiple reaction monitoring (MRM) positive mode respectively. The method was validated with the correlation coefficients of (r2) ≥ 0.9850 over a linear concentration range of 1.00-8000.00 ng/mL. This method demonstrated intra and inter-day precision within 2.64 to 3.88 and 2.76 to 8.44% and accuracy within 97.33 to 99.05 and 100.30 to 103.19% for FB. This method is successfully applied in the Bioequivalence study of 9 human volunteers.

A novel simple, sensitive, selective, and rapid high-performance liquid chromatography coupled with tandem mass spectrometry method was developed and validated for quantification of riluzole in human plasma. The chromatography was performed by using a Zorbax-SB-C18 (4.6 x 75 mm, 3.5 microm) column , isocratic mobile phase 0.1% formic acid/acetonitrile (10:90 v/v), and an isotope-labeled internal standard (IS), [(13)C,(15)N(2)]riluzole. The extraction of drug and internal standard was performed by liquid-liquid extraction and analyzed by MS in the multiple reaction monitoring (MRM) mode using the respective [M+H](+) ions, m/z 235.0/165.9 for riluzole and m/z 238.1/169.0 for the IS. The calibration curve was linear over the concentration range 0.5-500.0 ng/ml for riluzole in human plasma. The limit of quantification (LOQ) was demonstrated at 0.5 ng/ml. The within-batch and between-batch precision were 0.6-2.3% and 1.4-5.7%, and accuracy was 97.1-101.1% and 98.8-101.2% for riluzole respectively. Drug and IS were eluted within 3.0 min. The validated method was successfully applied in a bioequivalence study of riluzole in human plasma.

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A simple, sensitive, selective, rapid, rugged, reproducible and specific liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was used for quantitative estimation of rizatriptan (RZ) in human plasma using rizatriptan-d 6 (RZD6) as internal standard (IS). Chromatographic separation was performed on Ascentis Express RP Amide C18, 50 9 4.6 mm, 2.7 lm column with isocratic mobile phase composed of 10 mM ammonium formate:acetonitrile (20:80 v/v) at flow rate of 0.5 mL min -1 . RZ and RZD6 were detected with proton adducts at m/z (amu) 270.2 ? 201.2 and 276.1 ? 207.1, respectively, in multiple reaction monitoring (MRM) positive mode. Liquid-liquid extraction was used and validated over a linear concentration range of 0.1-100.0 ng mL -1 with correlation coefficient r 2 C 0.9981. The limit of quantification (LOQ) and limit of detection (LOD) were found to be 0.1 ng mL -1 and 12.5 fg, respectively. Intra-and inter-day precision were within 1.7-3.1% and 2.8-3.7%, and accuracy within 96.0-101.7% and 99.7-101.4% for RZ. Drug was found to be stable throughout three freeze-thaw cycles. The method was successfully employed for analysis of plasma samples following oral administration of RZ (10 mg) in 25 healthy Indian male human volunteers under fasting conditions.