Chemical investigation on Sijunzi decoction and its two major herbs Panax ginseng and Glycyrrhiza uralensis by LC/MS/MS

Sijunzi decoction consists of Panax ginseng, Poria cocos, Atractylodes macrocephala and Glycyrrhiza uralensis. High performance liquid chromatography coupled with tandem mass spectrometry (LC/MSⁿ) was applied to identify and characterize three types of active components, ginsenoside (from P. ginseng), flavonoid and triterpenoid (from G. uralensis) in Sijunzi decoction. Spectra of MS and MS/MS from [M + Na]⁺ ions of ginsenosides were acquired and interpreted for their identification. Fragmentations with losing masses of 194 or 176 Da were the characteristic ions of triterpenoids in the MS/MS analysis. A characteristic fragment ion of the aglycon moiety at m/z 257 from source collision-induced dissociation was observed for flavonoid. LC/MS was also applied for the comparison of relative peak area of major active components between Sijunzi decoction and the single herb extracts. The concentration ratios of major active components detected in the individual herbs of P. ginseng and G. uralensis were found different from those in Sijunzi decoction. The experimental data indicated that the decocting process could result in the difference in the amount of active components.     

Intracerebral microdialysis coupled to LC–MS/MS for the determination tramadol and its major pharmacologically active metabolite O‐desmethyltramadol in rat brain microdialysates

A rapid and sensitive method involving liquid chromatography electrospray tandem mass spectrometry (LC‐ESI‐MS/MS) coupled to an intracerebral microdialysis technique was developed for the determination and pharmacokinetic investigation of tramadol and its major active metabolite O ‐desmethyltramadol (ODT) in rat brain. The microdialysis samples were separated on a C18 column and eluted with a mobile phase of acetonitrile‐water‐formic acid (50:50:0.1; v/v/v ) at a flow rate of 0.3 mL/min. The ESI‐MS/MS spectra were performed in electrospray positive ion mode, and the analytes were detected by multiple reaction monitoring (MRM) of the transitions m/z [M + H]⁺ 264.3 → 58.2 for tramadol, m/z [M + H]⁺ 250.3 → 58.3 for ODT, and m/z [M + H]⁺ 379.4 → 264.0 for ambroxol (internal standard; IS). The total run time was 4.0 min. A lower limit of quantitation (LLOQ) was achieved as 1 ng/mL for tramadol and 0.5 ng/mL for ODT, with excellent linearity over a concentration range of 1 ~ 500 ng/mL (r  > 0.99) for tramadol and 0.5 ~ 50 ng/mL for ODT (r  > 0.99), respectively. The proposed method was successfully applied to the pharmacokinetic studies of tramadol and ODT in rat brain. Copyright © 2017 John Wiley & Sons, Ltd.     

Two-dimensional LC–MS/MS determination of antiretroviral drugs in rat serum and urine

A simple, rapid, reliable and highly sensitive on-line two-dimensional reversed-phase liquid chromatography–tandem mass spectrometric (2D-LC/MS/MS) method to determine antiretroviral drugs viz., abacavir (ABC), nevirapine (NVP) and indinavir (IDV) in rat serum and urine was developed and validated. The analytes were extracted on-line from rat serum and urine by a restricted access material (RAM) column and back-flushed into the reversed-phase C₁₈ column for separation by LC. Detection was carried out by ESI-MS/MS. The developed method showed good selectivity, accuracy and precision for quantification of the antiretroviral drugs in rat serum and urine. Quantification limits for abacavir and nevirapine were 4.0 ng ml⁻¹, whereas for indinavir 4.7 ng ml⁻¹. The calibration graphs were linear in the range of 4–50 ng ml⁻¹for abacavir, nevirapine and indinavir. The method was successfully applied to study the pharmacokinetics of antiretroviral in rats.     

Cocaine adulteration with the anthelminthic tetramisole (levamisole/dexamisole): Long‐term monitoring of its intake by chiral LC–MS/MS analysis of cocaine‐positive hair samples

Recent studies indicate that not only the anthelminthic levamisole but also the racemate tetramisole (R‐/S‐phenyltetraimidazothiazole, PTHIT) was found as an adulterant for cocaine. We herein report on the investigation of the prevalence of PTHIT among cocaine‐positive hair samples and the discrimination of the presence of its stereoisomers levamisole and dexamisole. Cocaine‐positive hair samples were collected in a forensic context in 2015 and mainly 2017 (n = 724). Cocaine and PTHIT concentrations have been determined by achiral liquid chromatography–tandem mass spectrometry (LC–MS/MS). For distinction of levamisole/dexamisole chiral LC–MS/MS was performed. Cocaine hair concentrations ranged from 500 (cut‐off) to approximately 800 000 pg/mg. The study demonstrates a strong prevalence of PTHIT in cocaine users' hair (87%, n = 627). PTHIT hair concentrations ranged from below LLOQ 3.5 to approximately 61 000 pg/mg (median: 260 pg/mg). Surprisingly, enantiomeric ratios of levamisole/dexamisole ranged from 0.17 to 1.34 (median: 0.63). Therefore, PTHIT‐adulterated street cocaine samples (n = 24) seized between 2013 and 2016 were tested. Samples mainly contained racemic tetramisole (87.5%), only one sample contained levamisole only and two samples contained non‐racemic PTHIT. Our experiments suggest that the presence of tetramisole in biological samples may have hitherto been underestimated. Most probably higher dexamisole than levamisole concentrations in hair specimens arise from stereoselective metabolism and/or elimination. This is particularly important in light of the different pharmacological activities of the two enantiomers and potentially different adverse effects. Toxicological interpretations in intoxication cases with adulterated cocaine should not only consider levamisole but also tetramisole and terminology in scientific contributions should be used accordingly.     

Metabolism of the tryptamine‐derived new psychoactive substances 5‐MeO‐2‐Me‐DALT, 5‐MeO‐2‐Me‐ALCHT,and 5‐MeO‐2‐Me‐DIPT and their detectability in urine studied by GC–MS,LC–MSn,and LC‐HR‐MS/MS

Many N,N‐dialkylated tryptamines show psychoactive properties and were encountered as new psychoactive substances. The aims of the presented work were to study the phase I and II metabolism and the detectability in standard urine screening approaches (SUSA) of 5‐methoxy‐2‐methyl‐N,N‐diallyltryptamine (5‐MeO‐2‐Me‐DALT), 5‐methoxy‐2‐methyl‐N‐allyl‐N‐cyclohexyltryptamine (5‐MeO‐2‐Me‐ALCHT), and 5‐methoxy‐2‐methyl‐N,N‐diisopropyltryptamine (5‐MeO‐2‐Me‐DIPT) using gas chromatography–mass spectrometry (GC–MS), liquid chromatography coupled with multistage accurate mass spectrometry (LC–MSⁿ), and liquid chromatography‐high‐resolution tandem mass spectrometry (LC‐HR‐MS/MS). For metabolism studies, urine was collected over a 24 h period after administration of the compounds to male Wistar rats at 20 mg/kg body weight (BW). Phase I and II metabolites were identified after urine precipitation with acetonitrile by LC‐HR‐MS/MS. 5‐MeO‐2‐Me‐DALT (24 phase I and 12 phase II metabolites), 5‐MeO‐2‐Me‐ALCHT (24 phase I and 14 phase II metabolites), and 5‐MeO‐2‐Me‐DIPT (20 phase I and 11 phase II metabolites) were mainly metabolized by O‐demethylation, hydroxylation, N‐dealkylation, and combinations of them as well as by glucuronidation and sulfation of phase I metabolites. Incubations with mixtures of pooled human liver microsomes and cytosols (pHLM and pHLC) confirmed that the main metabolic reactions in humans and rats might be identical. Furthermore, initial CYP activity screenings revealed that CYP1A2, CYP2C19, CYP2D6, and CYP3A4 were involved in hydroxylation, CYP2C19 and CYP2D6 in O‐demethylation, and CYP2C19, CYP2D6, and CYP3A4 in N‐dealkylation. For SUSAs, GC–MS, LC‐MSⁿ, and LC‐HR‐MS/MS were applied to rat urine samples after 1 or 0.1 mg/kg BW doses, respectively. In contrast to the GC–MS SUSA, both LC–MS SUSAs were able to detect an intake of 5‐MeO‐2‐Me‐ALCHT and 5‐MeO‐2‐Me‐DIPT via their metabolites following 1 mg/kg BW administrations and 5‐MeO‐2‐Me‐DALT following 0.1 mg/kg BW dosage. Copyright © 2017 John Wiley & Sons, Ltd.     

Fatal intoxication by 5F–ADB and diphenidine: Detection,quantification, and investigation of their main metabolic pathways in humans by LC/MS/MS and LC/Q‐TOFMS

Despite the implementation of a new blanket scheduling system in 2013, new psychoactive substance (NPS) abuse remains a serious social concern in Japan. We present a fatal intoxication case involving 5F–ADB (methyl 2‐[1‐(5‐fluoropentyl)‐1H–indazole‐3‐carboxamido]‐3,3‐dimethylbutanoate) and diphenidine. Postmortem blood screening by liquid chromatography/quadrupole time‐of‐flight mass spectrometry (LC/Q‐TOFMS) in the information‐dependent acquisition mode only detected diphenidine. Further urinary screening using an in‐house database containing NPS and metabolites detected not only diphenidine but also possible 5F–ADB metabolites; subsequent targeted screening by LC/tandem mass spectrometry (LC/MS/MS) allowed for the detection of a very low level of unchanged 5F–ADB in postmortem heart blood. Quantification by standard addition resulted in the postmortem blood concentrations being 0.19 ± 0.04 ng/mL for 5F–ADB and 12 ± 2.6 ng/mL for diphenidine. Investigation of the urinary metabolites revealed pathways involving ester hydrolysis (M1) and oxidative defluorination (M2), and further oxidation to the carboxylic acid (M3) for 5F–ADB. Mono‐ and di‐hydroxylated diphenidine metabolites were also found. The present case demonstrates the importance of urinary metabolite screening for drugs with low blood concentration. Synthetic cannabinoids (SCs) fluorinated at the terminal N‐alkyl position are known to show higher cannabinoid receptor affinity relative to their non‐fluorinated analogues; 5F–ADB is no exception with high CB₁ receptor activity and much greater potency than Δ⁹‐THC and other earlier SCs, thus we suspect its acute toxicity to be high compared to other structurally related SC analogues. The low blood concentration of 5F–ADB may be attributed to enzymatic and/or non‐enzymatic degradation, and further investigation into these possibilities is underway.     

LC/MS/MS的多反应监测方法定量测定灯盏乙素

目的：建立一种可靠的灯盏乙素定量分析方法。方法：用三级四极串联质谱(MS/MS)作为HPLC的检测器,其中MS/MS使用了多反应监测(MRM)扫描方式。选择母→子离子对m/z -461→m/z -285作为MRM监测的离子对;HPLC流动相为100％甲醇,流速0.9 mL.min^-1,色谱柱Beckman ODS-1。以测定短葶飞蓬提取物的灯盏乙素含量为例,对此方法进行了应用。结果：灯盏乙素在短葶飞蓬提取物中含量为6.98%。方法线性范围20～160 ng.mL^-1 (γ=0.999);加入灯盏乙素标准品20,60和160 ng的加样回收率分别为：96.5%,97.4%和97.3%。检测限为1 ng,每个样品的分析时间为4 min。结论：此法灵敏、快速、准确,可应用于灯盏乙素的各种药剂、药代的研究。     

Determination of ranolazine in human plasma by LC-MS/MS and its application in bioequivalence study

A simple, sensitive and specific liquid chromatography–tandem mass spectrometry (LC–MS/MS) method was developed and validated for quantification of ranolazine in human plasma. The analytical method consists in the precipitation of plasma sample with methanol, followed by the determination of ranolazine by an LC–MS/MS. The analyte was separated on a Peerless Cyano column (33 mm × 4.6 mm, 3 μm) an isocratic mobile phase of methanol–water containing formic acid (1.0%, v/v) (65:35, v/v) at a flow rate of 1.0 ml/min. Protonated ions formed by a turbo ionspray in positive mode were used to detect analyte and internal standard (IS). The MS/MS detection was made by monitoring the fragmentation of m/z 428.20 → 279.50 for ranolazine and m/z 448.30 → 285.20 for internal standard on a triple quadrupole mass spectrometer. The method was validated over the concentration range of 5–2000 ng/ml for ranolazine in human plasma with correlation coefficient of 0.9937 (S.D.: ±0.00367, range: 0.9895–0.9963). The accuracy and precision values obtained from six different sets of quality control samples analyzed in separate occasions ranged from 94.53 to 117.86 and 0.14% to 4.56%, respectively. Mean extraction recovery was 82.36–94.25% for three quality control (QC) samples and 88.37% for IS. Plasma samples were stable for three freeze–thaw cycles, or 24 h ambient storage, or 1 and 3 months storage at −20 °C. Processed samples (ready for injection) were stable up to 72 h at autosampler (4 °C). The developed method was successfully applied for analyzing ranolazine in plasma samples for a bioequivalence study with 12 healthy volunteers.     

LC/MS/MS evaluation of cocaine and its metabolites in different brain areas,peripheral organs and plasma in cocaine self-administering rats

BackgroundWe employed a cocaine intravenous self-administration model based on positive reinforcement of animals' instrumental reactions (i.e., lever pressing) rewarded by a dose of the drug. We also carried out simultaneous characterization of the phar-macokinetics of cocaine and its metabolites in rats during withdrawal; in this part of the experiments, we investigated the cocaine (2 mg/kg, iv)-induced changes in the distribution, rate constant, clearance and t_1/2 of the parent drug and its metabolites in different structures of the brain and in peripheral tissues.MethodsBy using liquid chromatography-tandem mass spectrometry (LC/MS/MS) we measured the levels of cocaine and its major metabolites.ResultsOur results demonstrate differences in the levels of cocaine after cocaine self-administration in the rat, with the highest concentration seen in the striatum and the lowest in the cerebellum. Cocaine metabolites determined in the rat brain remained at very low levels (benzoylecgonine), irrespectively of the brain area, whereas the norcocaine concentration varied from 1.56 μg/g (the nucleus accumbens) to 2.73 μg/g (the striatum).ConclusionAtandem LC/MS/MS is a valid method for evaluation of brain and peripheral levels ofcocaine and its metabolites. Our results demonstrate brain area-dependent differences in the levels of cocaine after its self-administration in the rat. There were also differences in pharmacokinetic parameters among the brain areas and peripheral tissues following a bolus iv injection of cocaine to rats withdrawn from cocaine; among brain structures the slowest metabolic rate was detected for the striatum.     

UHPLC–MS/MS quantitation of porphyroxine in opium and application of porphyroxine‐acetylated products as signature markers for heroin

Porphyroxine, a trace alkaloid in opium, was identified in the early 1800s and isolated/characterized in the 1960s. Recently, two significant porphyroxine‐related byproducts found in the acidic and neutral extracts of illicit heroin were characterized by this laboratory as the N‐acetyl‐O¹⁴‐desmethyl‐epi‐porphyroxine ( B ) and N,O⁸‐diacetyl‐O¹⁴‐desmethyl‐epi‐porphyroxine ( C ). The prevalence of the B and C compounds has been consistent in the following order of abundance for the thousands of authentic heroin samples analyzed: Southwest Asia (SWA) > South America (SA) > Southeast Asia (SEA) > Mexico (MEX). In this research, a rapid and efficient ultra‐high performance liquid chromatography–tandem mass spectrometry (UHPLC–MS/MS) method was developed to determine the content of porphyroxine and five primary alkaloids (morphine, codeine, thebaine, noscapine, and papaverine) in opium after extraction with methanol/water (50/50). The method was validated in terms of linearity, accuracy, recovery, and precision for porphyroxine. The limit of quantitation (LOQ) for porphyroxine was 2.5 ng/mL. The developed method was successfully applied to a total of 114 authentic opium samples from the major poppy‐growing regions. The amount of porphyroxine was determined at the level of part per thousand (‰) and the relative concentrations to morphine were in the range of 1x10^?4 and 1x10^?2 with an order of SWA > SEA, SA > MEX for its average abundance, which is consistent with the order of the average abundance of its acetylated products ( B , C ) in illicit heroin. This study reveals the significance of porphyroxine and its acylated compounds in classifying heroin and opium samples to major geographical regions of production.     

The preparation of genistein and LC‐MS/MS on‐line analysis

Genistein, a promising agent for cancer chemoprevention or treatment, can be obtained from soybean seeds, via a complex isolation procedure. The objective of this study was to investigate whether natural and high purity genistein could be acquired from Sophora japonica L. via a simple approach. High‐performance liquid chromatography electrospray ionization combined with tandem mass spectrometry (LC‐ESI‐MS/MS) was used to monitor the whole process of genistein preparation. The results showed there were at least 13 types of flavonoid in the crude extracts from sophora fruits with sophoricoside being the predominant component. The products obtained from our preparation were unambiguously identified as genistein based on molecular ion [M+H]⁺, UV spectra, retention time, IR spectra, ¹HNMR, ¹⁴CNMR spectra, and tandem mass spectrometric analysis. It demonstrated that 99.6% (w/w) genistein could be obtained via our preparation protocol. Drug Dev. Res. 61: 6–12, 2004. © 2004 Wiley‐Liss, Inc.     

HPLC–MS/MS method for the intracellular determination of ribavirin monophosphate and ribavirin triphosphate in CEMss cells

A sensitive and specific method using high performance liquid chromatography–tandem mass spectrometry (HPLC–MS/MS) for the determination of ribavirin monophosphate (RBV-MP) and ribavirin triphosphate (RBV-TP) in cells has been developed and validated. In this method, ribavirin phosphorylated metabolites were extracted and separated by anion exchange solid phase extraction (SPE). The RBV-MP and RBV-TP fractions were dephosphorylated using acid phosphatase and further purified by phenyl boronate SPE prior to HPLC–MS/MS analysis. ¹³C₅-uridine was added as internal standard to obtain better accuracy and precision of the analysis. The MS/MS detector was optimized at multiple reaction monitoring (MRM) using positive electrospray ionization to detect 245 → 113 and 250 → 133 transitions for ribavirin and internal standard, respectively. The calibration curve was linear over a concentration range of 0.01–10 μg/mL with a limit of quantitation of 0.01 μg/mL. Mean inter-assay accuracy and precision for RBV-MP and RBV-TP quality control samples at 0.03, 0.3 and 8 μg/mL were 5% and 10%, respectively. This method was successfully used for the in vitro determination of RBV-MP and RBV-TP in CEM_ss cells cultured with RBV.     

LC/MS/MS analysis of vesnarinone and its principal metabolites in plasma and urine

An LC/MS/MS assay was developed and successfully used to quantitate vesnarinone and its principal metabolites (OPC-8230, OPC-18136, and OPC-18137) in human plasma and urine. Samples were pre-treated with liquid–solid extraction followed by simultaneous monitoring of primary and daughter ions which were used for the identification and quantitation of the analytes on LC/MS/MS. This assay offers advantages of specificity, speed and greater sensitivity over the previously developed HPLC-UV assay. The lower limit of quantitation is 500 ng ml⁻¹ for vesnarinone and 20 ng ml⁻¹ for OPC-8230, OPC-18137, and OPC-18136 in plasma. Methodology is similar for the estimation of these analytes in urine with the lower limit of quantitation being 500 ng ml⁻¹ for vesnarinone and 100 ng ml⁻¹ for each metabolite. Ascorbic acid was added to stabilize the analytes from degradation. This LC/MS/MS method was developed to overcome many practical problems associated with the HPLC method. The LC/MS/MS method offers the flexibility of analyzing additional metabolites and changing the linearity range to accommodate the differences in linear range (200–10 000 ng ml⁻¹ for vesnarinone and 20–1000 for metabolites) for the analytes.     

Development and validation of a high-performance liquid chromatography/tandem mass spectrometry method for the determination of artemether and its active metabolite dihydroartemisinin in human plasma

To study the pharmacokinetic profile of artemether in children and in the context of antiviral drugs for HIV infected patients co-infected with malaria, an LC-MS/MS method was developed and validated to simultaneously determine artemether and its metabolite dihydroartemisinin in human plasma. Using artemisinin as the internal standard, 0.5 mL samples were processed with solid phase extraction (Waters Oasis^® HLB column), the elutes were directly injected onto a C₁₈ LC column (Waters, Symmetry^®, 150 mm × 4.6 mm, 5 μm). Mass detection utilized ESI⁺ as the ionization mode and MRM as the quantitation mode. In respect to the low ionization capacity of artemether, ammonium formate was added to the LC mobile phase to facilitate ionization (M+NH₄⁺). The calibration range was 2–200 ng/mL. The recovery was 73–81% for artemether and 90–99% for dihydroartemisinin. The validated method was applied to analysis of clinical samples with results in good agreement with an existing method.     

Ultra‐fast LC–MS/MS in therapeutic drug monitoring: Quantification of clozapine and norclozapine in human plasma

A novel approach to high‐throughput, targeted liquid chromatography–tandem mass spectrometry (LC–MS/MS) analysis has been developed. A single chromatographic system can be used for the analysis of a range of 20 drugs and metabolites with a total analysis time of 36 s (one 96‐well plate of prepared samples per hour). To demonstrate the applicability of this approach to quantitative analysis, a method has been validated for the therapeutic drug monitoring of clozapine and norclozapine following automated extraction from human plasma. Chromatographic retention times were 11.4 and 12.4 s for norclozapine and clozapine, respectively (for both analytes the chromatographic peak width was less than 1 s). Comparison with a conventional LC–MS/MS method (5 min analysis time) showed excellent agreement. This new approach offers analysis times more akin to flow‐injection analysis, but is likely to be more widely applicable because of chromatographic resolution from residual matrix components and isobaric interferences.     

LC/MS/MS与GC/MS法分析鉴定小鼠尿中沙美特罗的代谢产物

目的:鉴定沙美特罗在小鼠尿中的主要代谢产物.方法:ig给药后,收集小鼠尿液,经固相提取,葡萄糖醛酸酶水解,进行LC/MS/MS分析和硅烷化后进行GC/MS分析同时分离鉴定沙美特罗代谢产物.结果和结论:在给药后尿样中发现沙美特罗原型和4种代谢产物M1～M4,其结构推测为19-羟基沙美特罗(M1)、2-羰基沙美特罗(M2)、19-羰基沙美特罗(M3)和19-羟基-8-甲氧基沙美特罗(M4).     

中药复方四逆汤化学成分的RRLC-TOF/MS分析

目的：通过高分离度快速液相-飞行时间质谱（RRLC—TOF／MS）联用技术定性分析中药复方四逆汤中的主要化学成分。方法：色谱分离采用资生堂CAPCELLPAK C18反相柱（50mm×2．0mm,2μm）,流动相组成分别为0．1％甲酸水溶液和乙腈,梯度洗脱,流速为0．25mL／min;质谱定性采用飞行时间质谱,正离子模式扫描。结果：在优化的液质联用条件下,通过飞行时间质谱鉴定出四逆汤中34个成分,并对其药材来源进行了归属。结论：通过RRLC—TOF／MS联用技术,为鉴定四逆汤中的化学成分建立起了一种快速、高效的分析方法。     

A direct LC/MS/MS method for the determination of ciclopirox penetration across human nail plate in in vitro penetration studies

Due to severe chelating effect caused by N-hydroxylpyridone group of ciclopirox, there is no published direct HPLC or LC/MS/MS method for the determination of ciclopirox in any in vitro or in vivo matrix. Instead, the time-consuming pre-column derivatization methods have been adapted for indirect analysis of ciclopirox. After overcoming the chelating problem by using K₂EDTA coated tubes, a direct, sensitive and high-throughput LC/MS/MS method was successfully developed and validated to determine the amount of ciclopirox that penetrated across the nail plate during in vitro nail penetration studies. The method involved adding a chemical analog, chloridazon as internal standard (IS) in K₂EDTA coated tubes, mixing IS with ciclopirox in a 96-well plate and then proceeding to LC/MS/MS analysis. The MS/MS was selected to monitor m/z 208.0 → 135.8 and 221.8 → 77.0 for ciclopirox and IS, respectively, using positive electrospray ionization. The method was validated over a concentration range of 8–256 ng/mL, yielding calibration curves with correlation coefficients greater than 0.9991 with a lower limit of quantitation (LLOQ) of 8 ng/mL. The assay precision and accuracy were evaluated using quality control (QC) samples at three concentration levels. Analyzed concentrations ranged from 101% to 113% of their respective nominal concentration levels with coefficients of variation (CV) below 10.6%. The average recovery of ciclopirox from nail matrix was 101%.     

Determination of dexmedetomidine in human plasma using high performance liquid chromatography coupled with tandem mass spectrometric detection: Application to a pharmacokinetic study

A rapid, sensitive and selective high performance liquid chromatography-electrospray ionization-tandem mass spectrometry method (HPLC-ESI-MS/MS) was developed and validated for the determination and pharmacokinetic investigation of dexmedetomidine (DMED) in human plasma. Dexmedetomidine and the internal standard (ondansetron) were extracted in a single step with diethyl-ether from 1.0 mL of alkalinized plasma. The mobile phase was a mixture of acetonitrile and 0.5% formic acid solution (30:70, v/v) at a flow rate of 0.2 mL min⁻¹. The detection was performed on a triple quadrupole tandem mass spectrometer in the selected reaction monitoring (SRM) mode using the respective [M+H]⁺ ions m/z 201.0 → 95.1 for DMED and m/z 294.1 → 170.1 for the IS. The assay exhibited a linear dynamic range of 5–5000 pg mL⁻¹ with the correlation coefficient above 0.9995. The lower limit of quantification (LLOQ) was 5 pg mL⁻¹ with a relative standard deviation of less than 15%. Acceptable precision and accuracy were obtained for concentrations over the standard curve range. The validated HPLC-MS/MS method has been successfully applied to study the pharmacokinetics of three level doses of DMED in Chinese healthy volunteers.