Simultaneous determination of triprolidine and pseudoephedrine in human plasma by liquid chromatography–ion trap mass spectrometry

A highly efficient, selective and specific method for simultaneous quantitation of triprolidine and pseudoephedrine in human plasma by liquid chromatography–ion trap-tandem mass spectrometry coupled with electro spray ionization (LC–ESI-ion trap-tandem MS) has been validated and successfully applied to a clinical pharmacokinetic study. Both targeted compounds together with the internal standard (gabapentin) were extracted from the plasma by direct protein precipitation. Chromatographic separation was achieved on a C₁₈ ACE^® column (50.0 mm × 2.1 mm, 5 μm, Advance Chromatography Technologies, Aberdeen, UK), using an isocratic mobile phase, consisting of water, methanol and formic acid (55:45:0.5, v/v/v), at a flow-rate of 0.3 mL/min. The transition monitored (positive mode) was m/z 279.1 → m/z 208.1 for triprolidine, m/z 165.9 → m/z 148.0 for pseudoephedrine and m/z 172.0 → m/z 154.0 for gabapentin (IS). This method had a chromatographic run time of 5.0 min and a linear calibration curves ranged from 0.2 to 20.0 ng/mL for triprolidine and 5.0–500.0 ng/mL for pseudoephedrine. The within- and between-batch accuracy and precision (expressed as coefficient of variation, %C.V.) evaluated at four quality control levels were within 94.3–106.3% and 1.0–9.6% respectively. The mean recoveries of triprolidine, pseudoephedrine and gabapentin were 93.6, 76.3 and 82.0% respectively. Stability of triprolidine and pseudoephedrine was assessed under different storage conditions. The validated method was successfully employed for the bioequivalence study of triprolidine and pseudoephedrine formulation in twenty six volunteers under fasting conditions.     

Simultaneous determination of paracetamol,pseudoephedrine, dextrophan and chlorpheniramine in human plasma by liquid chromatography–tandem mass spectrometry

For the first time, a highly sensitive and simple LC–MS/MS method after one-step precipitation was developed and validated for the simultaneous determination of paracetamol (PA), pseudoephedrine (PE), dextrophan (DT) and chlorpheniramine (CP) in human plasma using diphenhydramine as internal standard (IS). The analytes and IS were separated on a YMC-ODS-AQ C₁₈ Column (100 mm × 2.0 mm, 3 μm) by a gradient program with mobile phase consisting of 0.3% (v/v) acetic acid and methanol at a flow rate of 0.30 mL/min. Detection was performed on a triple quadrupole tandem mass spectrometer via electrospray ionization in the positive ion mode. The method was validated and linear over the concentration range of 10–5000 ng/mL for PA, 2–1000 ng/mL for PE, 0.05–25 ng/mL for DT and 0.1–50 ng/mL for CP. The accuracies as determined from quality control samples were in range of ?8.37% to 3.13% for all analytes. Intra-day and inter-day precision for all analytes were less than 11.54% and 14.35%, respectively. This validated method was successfully applied to a randomized, two-period cross-over bioequivalence study in 20 healthy Chinese volunteers receiving multicomponent formulations containing 325 mg of paracetamol, 30 mg of pseudoephedrine hydrochloride, 15 mg of dextromethorphan hydrobromide and 2 mg of chlorphenamine maleate.     

Simultaneous determination of citalopram and its metabolite in human plasma by LC–MS/MS applied to pharmacokinetic study

A simple sensitive and robust method for simultaneous determination of citalopram and desmethylcitalopram was developed using liquid chromatography tandem mass spectrometry (LC–MS/MS). A 200 μL aliquot of plasma sample was employed and deproteinized with methanol and desipramine was used as the internal standard. After vortex mixing and centrifugation, the supernatant was diluted with water (1:1, v/v) and then directly injected to analysis. Analytes were separated by a Zorbax XDB C₁₈ column with the mobile phase composed of acetonitrile and water (30:70, v/v) with 0.25% formic acid and monitored in MRM mode using a positive electrospray source with tandem mass spectrometry detection. The total run time was 3.5 min. The dynamic range was 0.2–100 ng/mL for citalopram and 0.25–50 ng/mL for desmethylcitalopram, respectively. Compared to the best existing literatures for plasma samples, the same LOQ for CIT (0.5 ng/mL) and lower LOQ for DCIT (0.25 vs 5 ng/mL) were reached, and less sample preparation steps and runtime (3.5 vs 10 min) were taken for our method. Accuracy and precision was lower than 8% and lower than 11.5% for either target. Validation results and its application to the analysis of plasma samples after oral administration of citalopram in healthy Chinese volunteers demonstrated the method was applicable to pharmacokinetic studies.     

Simultaneous determination of clarithromycin,rifampicin and their main metabolites in human plasma by liquid chromatography–tandem mass spectrometry

The drug combination rifampicin and clarithromycin is used in regimens for infections caused by Mycobacteria. Rifampicin is a CYP3A4 inducer while clarithromycin is known to inhibit CYP3A4. During combined therapy rifampicin concentrations may increase and clarithromycin concentrations may decrease. Therefore a simple, rapid and easy method for the measurement of the blood concentrations of these drugs and their main metabolites (14-hydroxyclarithromycin and 25-desacetylrifampicin) is developed to evaluate the effect of the drug interaction. The method is based on the precipitation of proteins in human serum with precipitation reagent containing the internal standard (cyanoimipramine) and subsequently high-performance liquid chromatography (HPLC) analysis and tandem mass spectrometry (MS/MS) detection in an electron positive mode. The method validation included selectivity, linearity, accuracy, precision, dilution integrity, recovery and stability according to the “Guidance for Industry – Bioanalytical Method Validation” of the FDA. The calibration curves were linear in the range of 0.10–10.0 mg/L for clarithromycin and 14-hydroxyclarithromycin and 0.20–5.0 mg/L for rifampicin and 25-desacetylrifampicin, with within-run and between-run precisions (CVs) in the range of 0% to ?10%. The components in human plasma are stable after freeze–thaw (three cycles), in the autosampler (3 days), in the refrigerator (3 days) and at room temperature (clarithromycin and 14-hydroxyclarithromycin: 3 days; rifampicin and 25-desacetylrifampicin: 1 day). The developed rapid and fully validated liquid chromatography–tandem mass spectrometry (LC/MS/MS) method is suitable for the determination of clarithromycin, 14-hydroxyclarithromycin, rifampicin and 25-desacetylrifampicin in human plasma.     

A simple and sensitive HPLC–ESI-MS/MS method for the determination of andrographolide in human plasma

A liquid chromatography–electrospray ionization tandem mass spectrometry (HPLC–ESI-MS/MS) method for the determination of andrographolide in human plasma was established. Dehydroandrographolide was used as the internal standard (I.S.). The plasma samples were deproteinized with methanol and separated on a Hanbon C₁₈ column with a mobile phase of methanol–water (70:30, v/v). HPLC–ESI-MS/MS was performed in the selected ion monitoring (SIM) mode using target ions at [M?H₂O–H]^?, m/z 331.1 for andrographolide and [M?H]^?, m/z 331.1 for the I.S. Calibration curve was linear over the range of 1.0–150.0 ng/mL. The chromatographic separation was achieved in less than 6.5 min. The lower limits of quantification (LLOQ) was 1.0 ng/mL. The intra and inter-run precisions were less than 6.95 and 7.22%, respectively. The method was successfully applied to determine the plasma concentrations of andrographolide in Chinese volunteers.     

Simultaneous determination of histamine and Nτ-methylhistamine in human plasma and urine by gas chromatography-mass spectrometry

A new and sensitive method is described for the determination of histamine and N^τ-methylhistamine in human plasma and urine by gas chromatography-mass spectrometry. ¹⁵N₂-Labeled histamine and N^τ-[methyl-d₃]methylhistamine were used as internal standards. Histamine and N^τ-methylhistamine were converted to the derivatives N^α-heptafluorobutyryl-N^τ-ethoxycarbonylhistamine and N^α-heptafluorobutyryl-N^τ-methylhistamine, respectively. After these derivatives had been purified on a small column packed with CPG-10, the molecular ions were monitored during selected ion monitoring. Linear standard curves were obtained in the range of 0.5–10 ng/ml for both compounds. The reliability of the histamine analysis was demonstrated by using two different ion pairs, while a comparison with results from two different derivatizations on the same urine sample also established the specificity of the N^τ-methylhistamine analysis. An increase of 1 ng of histamine in the plasma could be precisely determined by the present method. The histamine content of plasma from five normal subjects was determined as 0.83 ÷ 0.37 (S.D.) ng/ml and the N^τ-methylhistamine content in most subjects was below the limits of this measurement. High excretion of histamine was noted in the urine collected in the early morning from a patient with nephritis.     

Stability study of Prulifloxacin and Ulifloxacin in human plasma by HPLC–DAD

A new and specific HPLC–DAD method for the direct determination of Prulifloxacin and its active metabolite, Ulifloxacin, in human plasma has been developed. Plasma samples were analysed after a simple solid phase extraction (SPE) clean-up using a new HILIC stationary phase based high-performance liquid chromatography (HPLC) column and an ammonium acetate buffer (5?mM, pH 5.8)/acetonitrile (both with 1% Et₃N, v/v) mobile phase in isocratic elution mode, with Danofloxacin as the internal standard. Detection was performed using DAD from 200 to 500?nm and quantitative analyses were carried out at 278?nm. The LOQ of the method was 1?μg/mL of the cited analytes and the calibration curve showed a good linearity up to 25?μg/mL. For both analytes the precision (RSD%) and the trueness (bias%) of the method fulfil with International Guidelines. The method was applied for stability studies, at three QC concentration levels, in human plasma samples stored at different temperature of?+?25,?+?4 and ?20?°C in order to evaluate plasma stability profiles.     

Simple,sensitive and rapid HPLC–MS/MS method for the determination of cepharanthine in human plasma

A rapid, sensitive and specific method for the determination of cepharanthine in human plasma using high performance liquid chromatography coupled with tandem mass spectrometry (HPLC–MS/MS) was described. Cepharanthine and the internal standard (I.S.), telmisartan, were extracted from human plasma by methanol to precipitate the protein. A centrifuged upper layer was then evaporated and reconstituted with 100 μL methanol. Chromatographic separation was performed on an AGILENT XDB-C₈ column (150 mm × 2.1 mm, 5.0 μm, Agilent, USA) using a gradient mobile phase with 1 mmol/L ammonium acetate in water with 0.05% formic acid and methanol. Detection and quantitation was performed by MS/MS using electrospray ionization (ESI) and multiple reaction monitoring (MRM) in the positive ion mode. The most intense [M+H]⁺ MRM transition of cepharanthine at m/z 607.3 → 365.3 was used for quantitation and the transition at m/z 515.5 → 276.4 was used to monitor telmisartan. The calibration curve was linear within the concentration range of 0.5–200.0 ng/mL (r = 0.9994). The limit of quantification (LOQ) was 0.5 ng/mL. The extraction recovery was above 81.1%. The accuracy was higher than 92.3%. The intra- and inter-day precisions were less than 9.66%. The method was accurate, sensitive and simple and was successfully applied to a pharmacokinetic study after single intravenous administration of 50 mg cepharanthine in 12 healthy Chinese volunteers.     

Simultaneous determination of cyclophosphamide and carboxyethylphosphoramide mustard in human plasma using online extraction and electrospray tandem mass spectrometry (HTLC–ESI-MS/MS)

A rapid and selective method for simultaneous determination of cyclophosphamide and its metabolite carboxyethylphosphoramide mustard (CEPM) was developed using online sample preparation and separation with tandem mass spectrometric detection. Diluted plasma was injected onto an extraction column (Cyclone MAX 0.5 mm × 50 mm, >30 μm), the sample matrix was washed with an aqueous solution, and retained analytes were transferred to an analytical column (Gemini 3 μm C18 110A, 100 mm × 2.0 mm) using a gradient mobile phase prior to detection by MS/MS. Analytes were detected in an API-3000 LC-MS/MS system using positive multiple-reaction monitoring mode (m/z 261/140 and 293/221 for CTX and CEPM, respectively). Online extraction recoveries were 76% and 72% for cyclophosphamide and CEPM. Within-day and between-day variabilities were <3.0%, and accuracies were between ?6.9% and 5.2%. This method has been used to measure plasma cyclophosphamide and CEPM concentrations in an ongoing Phase II study in children with newly diagnosed medulloblastoma.     

Simultaneous HPLC–UV analysis of rufinamide,zonisamide, lamotrigine,oxcarbazepine monohydroxy derivative and felbamate in deproteinized plasma of patients with epilepsy

We present an implementation of a method we previously reported allowing the newer antiepileptic drugs (AEDs) rufinamide (RFN) and zonisamide (ZNS) to be simultaneously determined with lamotrigine (LTG), oxcarbazepine's (OXC) main active metabolite monohydroxycarbamazepine (MHD) and felbamate (FBM) in plasma of patients with epilepsy using high performance liquid chromatography (HPLC) with UV detection. Plasma samples (250 μL) were deproteinized by 1 mL acetonitrile spiked with citalopram as internal standard (I.S.). HPLC analysis was carried out on a Synergi 4 μm Hydro-RP, 250 mm × 4.6 mm I.D. column. The mobile phase was a mixture of potassium dihydrogen phosphate buffer (50 mM, pH 4.5), acetonitrile and methanol (65:26.2:8.8, v/v/v) at an isocratic flow rate of 0.8 mL/min. The UV detector was set at 210 nm. The chromatographic run lasted 19 min. Commonly coprescribed AEDs did not interfere with the assay. Calibration curves were linear for both AEDs over a range of 2–40 μg/mL for RFN and 2–80 μg/mL for ZNS. The limit of quantitation was 2 μg/mL for both analytes and the absolute recovery ranged from 97% to 103% for RFN, ZNS and the I.S. Intra- and interassay precision and accuracy were lower than 10% at all tested concentrations. The present study describes the first simple and validated method for RFN determination in plasma of patients with epilepsy. By grouping different new AEDs in the same assay the method can be advantageous for therapeutic drug monitoring (TDM).     

Simultaneous determination of m-nisoldipine and its three metabolites in rat plasma by liquid chromatography–mass spectrometry

A simple, sensitive and selective liquid chromatography–tandem mass spectrometry (LC–MS/MS) method for the simultaneous determination of m-nisoldipine and its three metabolites in rat plasma has been developed using nitrendipine as an internal standard (IS). Following liquid–liquid extraction, the analytes were separated using an isocratic mobile phase on a reverse phase C₁₈ column and analyzed by MS in the multiple reaction monitoring (MRM) mode. To avoid contamination by residual sample in the injection syringe, a special injection protocol was developed. We found that m-nisoldipine, metabolite M1 and IS could be ionized under positive or negative electrospray ionization conditions, whereas metabolite M and M2 could only be ionized in the positive mode. The mass spectrometry fragmentation pathways for these analytes are analyzed and discussed herein. The total analysis time required less than 5 min per sample. We employed this method successfully to study the metabolism of m-nisoldipine when it was orally administered to rats at a dose of 9 mg/kg. Three metabolites of m-nisoldipine and an unknown compound of molecular weight 386 were found for the first time in rat plasma. The concentration of the potentially active metabolite was approximately equal to its parent compound concentration.     

Simultaneous determination of a novel antifibrotic agent and three metabolites in human urine by LC–MS/MS

A robust and validated high performance liquid chromatography tandem mass spectrometry (LC–MS/MS) method has been developed for simultaneous determination of F351 (5-methyl-1-(4-hydroxylphenyl)-2-(1H)-pyridone) and three major metabolites in human urine sample. This assay method has also been validated in terms of selectivity, linearity, lower limit of quantification (LLOQ), accuracy, precision, stability, matrix effect and recovery. Chromatography was carried out on an XTerra RP 18 column and mass spectrometric analysis was performed using an API 4000 mass spectrometer coupled with electro-spray ionization (ESI) source in the positive ion mode. The MRM transitions of m/z 202 → 109, 232 → 93, 282 → 202 and 378 → 202 were used to quantify F351 and three metabolites, respectively. Retention times for F351 and three metabolites were 2.54, 1.38, 1.53 and 1.34 min, respectively. The assay was validated from 20 to 4000 ng/mL for F351 and M1, from 80 to16,000 ng/mL for M2 and M3. Intra- and inter-day precision for all analytes was <6.3%, method accuracy was between −11.2 and 0.3%. This assay was used to support a clinical study where multiple oral doses were administered to healthy subjects to investigate the pharmacokinetics, safety, and tolerability of F351.     

Simultaneous determination of midazolam and its metabolites 1-hydroxymidazolam and 4-hydroxymidazolam in human serum using gas chromatography–mass spectrometry

A method for the quantitation of midazolam and its metabolites 1-hydroxymidazolam and 4-hydroxymidazolam from human serum capable of monitoring concentrations achieved under therapeutic conditions is presented. The substances were extracted under basic conditions with toluene and the hydroxy metabolites transformed to their tert-butyldimethylsilyl derivatives with N-(tert-butyldimethylsilyl)-N-methyltrifluoroacetamide. The samples were measured by gas chromatography–mass spectrometry. The limits of detection are 0.2 ng ml⁻¹ for midazolam and 0.1 ng ml⁻¹ for 1-hydroxy- and 4-hydroxymidazolam. The coefficients of variation are 3.9% at 5 ng ml⁻¹ for midazolam, 6.7% at 2 ng ml⁻¹ for 1-hydroxymidazolam and 8.8% (22.2%) at 0.5 (0.2) ng ml⁻¹ for 4-hydroxymidazolam.     

Gas chromatographic–tandem mass spectrometric determination of acetylsalicylic acid in human plasma after oral administration of low-dose aspirin and guaimesal

A fully validated gas chromatographic–tandem mass spectrometric (GC–MS–MS) method is described for the accurate determination of acetylsalicylic acid (ASA) in human plasma after a single low-dose oral administration of aspirin or guaimesal, an ASA releasing prodrug. ASA and the newly prepared O-[²H₃]-acetylsalicylic acid (d3-ASA) used as internal standard were determined in 100-μl aliquots of plasma by extractive pentafluorobenzyl (PFB) esterification using PFB bromide and tetrabutylammoniumhydrogen sulphate as the esterifying and ion-pairing agent, respectively, and by GC–MS–MS analysis in the negative-ion chemical ionization mode. The overall relative standard deviations were below 8% for ASA levels in the range 0–1 μg/ml plasma. Mean accuracy was 3.8% for ASA levels within the range 0–100 ng/ml. The limit of quantitation of the method was determined as 200 pg/ml ASA at an accuracy of 5.5% and a precision of 15.2%. The limit of detection was determined as 546 amol of ASA at a signal-to-noise ratio of 10:1.     

Simultaneous and sensitive determination of xanthotoxin,psoralen, isoimpinellin and bergapten in rat plasma by liquid chromatography–electrospray ionization mass spectrometry

A sensitive, specific and rapid liquid chromatography–mass spectrometry (LC–MS) method has been developed and validated for the simultaneous determination of xanthotoxin (8-methoxypsoralen), psoralen, isoimpinellin (5,8-dimethoxypsoralen) and bergapten (5-methoxypsoralen) in rat plasma using pimpinellin as an internal standard (IS). The plasma samples were pretreated by protein precipitation with methanol and chromatographic separation was performed on a C₁₈ column with a mobile phase composed of 1 mmol ammonium acetate and methanol (30:70, v/v). The detection was accomplished by multiple-reaction monitoring (MRM) scanning via electrospray ionization (ESI) source operating in the positive ionization mode. The optimized mass transition ion-pairs (m/z) for quantitation were 217.1/202.1 for xanthotoxin, 187.1/131.1 for psoralen, 247.1/217.0 for isoimpinellin, 217.1/202.1 for bergapten, and 247.1/231.1 for IS. The total run time was 6 min between injections. The calibration curves were linear over the investigated concentration range with all correlation coefficients higher than 0.998. The lower limits of quantitation (LLOQ) of these analytes were less than 1.21 ng/ml. The intra- and inter-day RSD were no more than 9.7% and the relative errors were within the range of ?8.1% to 4.5%. The average extraction recoveries for all compounds were between 90.7% and 106.2%. The proposed method was further applied to the determination of actual plasma samples from rats after oral administration of Radix Glehniae extract.     

Simultaneous determination of the enantiomers of leucine and [2H7]leucine in plasma by capillary gas chromatography–mass spectrometry

A method for the stereoselective assay of d- and l-enantiomers of both leucine and [²H₇]leucine in rat plasma was developed using gas chromatography–mass spectrometry–selected-ion monitoring. dl-[²H₃]leucine was used as an internal standard. The method involved purification by cation-exchange chromatography using BondElut SCX cartridge and derivatization with hydrochloric acid in methanol to form methyl ester followed by subsequent chiral derivatization with (+)-α-methoxy-α-trifluoromethylphenylacetyl chloride to form diastereomeric amide. The derivatization made the separation of the leucine enantiomers possible with good gas chromatographic behavior. Quantitation was performed by selected-ion monitoring of the quasi-molecular ions of the diastereomers on the chemical ionization method. The sensitivity, specificity, accuracy and reproducibility of the method were demonstrated to be satisfactory for application to pharmacokinetic studies of leucine enantiomers.     

Simultaneous determination of F-β-alanine and β-alanine in plasma and urine with dual-column reversed-phase high-performance liquid chromatography

F-β-Alanine and β-alanine were detected in plasma and urine samples with fluorescence detection of orthophthaldialdehyde derivatives of F-β-alanine and β-alanine after separation with dual-column reversed-phase HPLC. The detection limits of F-β-alanine and β-alanine in the HPLC system were approximately 0.3 and 0.7 pmol, respectively. The procedure proved to be very reproducible with intra-assay RSDs and inter-assay RSDs being less than 8%. The usefulness of the method was demonstrated by the analysis of the F-β-alanine and β-alanine concentrations in plasma and urine samples from tumor patients treated with S-1 (Tegafur, 5-chloro-2,4-dihydroxypyridine and potassium oxonate in a molar ratio of 1:0.4:1).     

Simultaneous determination of tolbutamide,omeprazole, midazolam and dextromethorphan in human plasma by LC–MS/MS—A high throughput approach to evaluate drug–drug interactions

Drug–drug interactions involving cytochrome P450 (CYP450s) are an important factor for evaluation of a new chemical entity (NCE) in drug development. To evaluate the potential inhibitory effects of a NCE on the pharmacokinetics of a cocktail of representative probes of CYP enzymes (midazolam for CYP3A4, tolbutamide for CYP2C9, omeprazole for CYP2C19 and dextromethorphan for CYP2D6) and the safety and tolerability of the NCE in the presence of probe substrates, a high throughput liquid chromatography/tandem mass spectrometry (LC–MS/MS) method was developed and validated for the simultaneous determination of tolbutamide, omeprazole, midazolam and dextromethorphan in human plasma using tolbutamide-d₉, midazolam-d₄, (±)-omeprazole-d₃, and dextromethorphan-d₃ as the internal standards (ISs). Human plasma samples of 50 μL were extracted by a simple protein-precipitation procedure and analyzed using a high performance liquid chromatography electrospray tandem mass spectrometer system. Reversed-phase HPLC separation was achieved with a Hypersil GOLD AQ column (50 mm × 4.6 mm, 5 μm). MS/MS detection was set at mass transitions of 271 → 172 m/z for tolbutamide, 346 → 198 m/z for omeprazole, 326 → 291 m/z for midazolam, 272 → 171 m/z for dextromethorphan, 280 → 172 m/z for tolbutamide-d₉ (IS), 349 → 198 m/z for (±)-omeprazole-d₃ (IS), 330 → 295 m/z for midazolam-d₄ (IS), and 275 → 171 m/z for dextromethorphan-d₃ (IS) in positive mode. The high throughput LC–MS/MS method was validated for accuracy, precision, sensitivity, stability, recovery, matrix effects, and calibration range. Acceptable intra-run and inter-run assay precision (<10%) and accuracy (<10%) were achieved over a linear range of 50–50,000 ng/mL for tolbutamide, 1–1000 ng/mL for omeprazole, 0.1–100 ng/mL for midazolam and 0.05–50 ng/mL for dextromethorphan in human plasma. Method robustness was demonstrated by the 100% pass rate of 24 incurred sample analysis runs and all of the 50 clinical study samples used for incurred sample reproducibility (ISR) test having met the acceptance criterion (%Diff within 20%). The overall ISR results for all compounds showed that over 95% of the samples had a %Diff of less than 10%. The method is simple, rapid and rugged, and has been applied successfully to sample analysis in support of a drug–drug interaction study.     

Rapid determination of finasteride in human plasma by UPLC–MS/MS and its application to clinical pharmacokinetic study

A rapid, specific, and sensitive method utilizing reversed-phase ultra-performance liquid chromatography tandem mass spectrometry (UPLC–MS/MS) was developed and validated to determine finasteride levels in human plasma. The plasma samples were prepared by liquid–liquid extraction with ethyl acetate, evaporation, and reconstitution. MS/MS analyses were performed on a triple–quadrupole tandem mass spectrometer by monitoring protonated parent → daughter ion pairs at m/z 373 → 305 for finasteride and m/z 237 → 194 for carbamazepine (internal standard, IS). The method was validated with respect to linearity, recovery, specificity, accuracy, precision, and stability. The method exhibited a linear response from 0.1 to 30 ng/mL (r² > 0.998). The limit of quantitation for finasteride in plasma was 0.1 ng/mL. The relative standard deviation (RSD) of intra- and inter-day measurements was less than 15% and the method was accurate within −6.0% to 2.31% at all quality-control levels. The mean extraction recovery was higher than 83% for finasteride and 84% for the IS. Plasma samples containing finasteride were stable under the three sets of conditions tested and the processed samples were stable up to 29 h in an autosampler at 5 °C. Detection and quantitation of both analytes within 3 min make this method suitable for high-throughput analyses. The method was successfully applied to a pharmacokinetic study of finasteride in healthy volunteers following oral administration.