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Dendrobium nobile (D. nobile), a traditional herb known for its immunomodulatory and neuroprotective properties, contains characteristic alkaloids and sesquiterpene glycosides. While alkaloids have been extensively studied, research on sesquiterpene glycosides remains limited. This study established and validated a UHPLC-Q-Trap-MS/MS method for detecting six sesquiterpene glycosides in D. nobile, applying it to pharmacokinetic and tissue distribution studies in rats following oral administration of the D. nobile aqueous extract. Plasma and tissue samples were prepared using methanol for protein precipitation and separated on a Waters Acquity UPLC BEH C18 column. Quantification was performed using multiple reaction monitoring (MRM) in negative electrospray ionization (ESI) mode. Method validation demonstrated specificity, selectivity, precision, accuracy, stability, matrix effects, and recovery rates meeting the criteria for in vivo drug analysis. Pharmacokinetic results indicated that dendronobiloside A, dendronobiloside C, and dendronobiloside D were rapidly absorbed with low plasma concentrations and quick elimination. In contrast, dendronobiloside E, dendroside G, and dendromoniliside D were rapidly absorbed with higher plasma concentrations but also eliminated quickly. Tissue distribution studies revealed that dendronobiloside A, C, and D were detectable in the heart, liver, spleen, lungs, kidneys, stomach, large intestine, small intestine, thymus, and pancreas, but almost undetectable in the brain. And dendronobiloside E, dendroside G, and dendromoniliside D were detectable in all tissues. Overall, the six sesquiterpene glycosides reached various tissues within 2 h of administration, with distribution levels ranked as follows: small intestine > stomach > large intestine > pancreas > lungs > kidneys > liver > heart > thymus > spleen > brain. These findings provide insights into the immunomodulatory mechanisms of D. nobile sesquiterpene glycosides and inform clinical dosing considerations., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier B.V.)

Nardostachyos Radix et Rhizoma (NR) is a valuable medicinal herb widely used in Korea, India, and China for the treatment of many diseases. Desoxo-narchinol A (DA) and nardosinonediol (ND) are the two main bioactive compounds belonging to the sesquiterpene group. Desoxo-narchinol A possesses anti-inflammatory activity while ND exhibits anti-depressant and cardioprotective activities. A pharmacokinetic study is important to decide whether the isolated compounds or the NR extract have better pharmacological activity. Hence, we developed an analytical method for studying the pharmacokinetics of DA and ND after oral administration of the pure compounds and herbal extract. An optimized liquid chromatography-mass spectrometry method (LC-MS/MS) with solid-phase extraction (SPE) for sample preparation was developed. A ZORBAX Extend C18 column (2.1 × 50 mm, 3.5 μm) was used under gradient elution with acetonitrile and 0.1% formic acid in water as the mobile phase. Validation experiments assessing accuracy, precision, and stability were satisfactory; the lower limit of quantification was 5 ng/mL. For the pharmacokinetic study, three groups of rats were administrated pure DA, pure ND, or NR extract orally. Concentrations of DA and ND in their plasma were determined by the developed method. Pharmacokinetic parameters, including the time to achieve maximum plasma concentration (T max ) and the area under the plasma concentration curve from time zero to infinity (AUC 0-∞ ), were compared for the herbal extract and pure compounds. The T max of the pure compound and the NR extract for DA was 7.50 and 8.33 min, respectively, compared to 5.00 and 5.83 min for the pure compound and the NR extract for ND, respectively. The AUC 0-∞ of the pure compound and the NR extract for DA was 156.34 and 133.90 μg min/mL, respectively, and that for the NR extract for ND was 6.42 and 4.15 μg min/mL, respectively. LC-MS/MS was used to determine DA and ND in rat plasma. The pharmacokinetic profile of each pure compound and those in the extract were characterized and compared., (Copyright © 2018. Published by Elsevier B.V.)

Guaiol has been used for thousands of years as a traditional Uygur medicine and is the primary active component found in Ferula ferulaeoides (Steud.) Korov (F. ferulaeoides). In our present study, a rapid, selective, and sensitive method of monitoring selected ions was established based on gas chromatography-mass spectrometry. This method was optimized for the quantification and pharmacokinetic analysis of guaiol in rat plasma following oral administration of chloroform extract from Ferula ferulaeoides. Plasma was extracted using liquid-liquid extraction with ethyl acetate and was analyzed on a HP-5MS column (30 m × 250 μm × 0.25 μm) with a mass selective detector. Detection was carried out under selected ion monitoring mode, and three selected ion monitoring ions (m/z 59.1, 107.1, and 161.1 for guaiol) were used for the quantitative determination of that under investigation. The assay demonstrated excellent linearity in the range of 1-200 ng/mL (r = 0.9993, n = 8) in the case of guaiol measured in rat plasma. The limit of detection and the limit of quantification for guaiol in rat plasma were found to be 0.25 ng/mL and 1 ng/mL, respectively. Intra-day and inter-day precisions were expressed as the relative standard deviation for the method and were in the range of 97.49%-106.16% and 97.04%-105.91%, respectively. Extraction efficiencies were all determined to be >90%, and recoveries were ranged from 91.25% to 96.24%. This method has been successfully applied for the pharmacokinetic evaluation of chloroform extract isolated from F. ferulaeoides following a single oral administration dose (157.5 mg/kg) in rats. The guaiol pharmacokinetic study demonstrated that the half-life of guaiol was 9.18 ± 3.75 h, the mean residence time was 9.07 ± 3.86 h, the maximum guaiol concentration in the plasma was 28.63 ± 6.82 ng/mL, and the maximum time guaiol was in the plasma was 0.50 h., (Copyright © 2018. Published by Elsevier B.V.)

Alantolactone (AL) and isoalantolactone (IAL), two major active sesquiterpene lactones isolated from Radix Inulae extract, have a wide range of pharmacological activities. The predominant metabolic pathway of AL and IAL observed was glutathione (GSH) conjugation in vitro, which could occur in the absence of metabolic enzymes. Non-enzymatic conjugation with cysteine (Cys) couldalso be observed. Four metabolites (AL-GSH, AL-Cys, IAL-GSH, IAL-Cys) were subsequently isolated and confirmed by nuclear magnetic resonance (NMR). The results indicated that the thiol of GSH or Cys can be reacted with the exomethylene carbon atoms of α, β-unsaturated carbonyl of AL and IAL. After intravenous administration in rats, AL and IAL were extensively metabolized, and the exposure, as measured by area under the concentration-time curve (AUC), for AL-GSH, AL-Cys, IAL-GSH, and IAL-Cys was approximately 1.54-, 0.96-, 1.50-, and 0.91-fold that of the parent drug, respectively. The AUC ratio of metabolites to parent compounds of oral administration was 3.66-, 9.19-, 12.97-, and 9.92-fold that of the parent drug for the above metabolites, respectively. The bioavailability of AL-total (AL, AL-GSH, AL-Cys) and IAL-total (IAL, IAL-GSH, IAL-Cys) was, respectively, 8.39% and 13.07%, which was 3.62- and 6.95- fold that of AL (2.32%) and IAL (1.88%), respectively. The oral exposure will be underestimated if the parent drugs are tested alone. These findings provide useful information for preclinical safety evaluation, and for predicting AL and IAL metabolism in humans., (Copyright © 2017. Published by Elsevier B.V.)

Medicarpin is the active phytoalexin found in the stem bark of Butea monosperma having potent osteogenic activity. An LC-ESI-MS/MS was developed and validated for quantification of medicarpin in rat plasma using liquid-liquid extraction technique and diethyl ether as the extraction solvent. Medicarpin was separated on RP18 column (4.6mm×50mm, 5.0μm) using methanol and 10mM ammonium acetate (pH 4.0) in the ratio of 80:20 (v/v) as mobile phase. The method was linear within the concentration range of 1-500ng/mL and its sensitivity was 1ng/mL. The precision value for intra- and inter-day assays and stability assays was within 0.88-14.22% while the accuracy ranged between 87.46-116.0% at all four QC levels. The validated method was successfully applied to study the preclinical pharmacokinetics of medicarpin in rats. Medicarpin showed multiple peak phenomenon upon oral administration. Its oral bioavailability was 17.43%. It was found to be a rapidly absorbed (Tmax=15min), 81.61% protein bound and pH stable compound. The present study provides important information regarding preliminary pharmacokinetics of medicarpin for its further exploration as a potential therapeutic agent., (Copyright © 2015 Elsevier B.V. All rights reserved.)

A simple and rapid ultra high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method was developed for the simultaneous determination of Atractylenolide I, II and III in rat plasma. Plasma samples were processed by liquid-liquid extraction with ethyl acetate, using schisandrin as internal standard (IS). Chromatographic separation was accomplished on a Thermo Hypersil GOLD C18 column (2.1mm×50mm, 1.9μm) with mobile phase consisting of acetonitrile and 0.1% formic acid-water (50:50, v/v). The detection was carried out by ESI-MS (positive ionization mode) and low-energy collision dissociation tandem mass spectrometric analyses using the multiple-reaction monitoring (MRM) scan mode. The quantification was performed using the transitions of the protonated molecule→product ion at m/z 231.0→185.1 for Atractylenolide I, at m/z 233.1→187.1 for Atractylenolide II and at m/z 249.1→231.1 for Atractylenolide III, respectively. Method validation revealed excellent linearity over investigated range together with satisfactory intra- and inter-day precision, accuracy, matrix effects and extraction recoveries. This method was successfully applied to the comparative pharmacokinetic study of Atractylenolide I, II and III in rat plasma after intragastric administration of Baizhufuling extract and Atractylodis extract., (Copyright © 2015 Elsevier B.V. All rights reserved.)

Ptaquiloside (PTA) is a toxin from bracken fern (Pteridium sp.) with genotoxic effects. Hydrolysis of PTA leads to the non-toxic and aromatised indanone, pterosin B (PTB). Here we present a sensitive, fast, simple and direct method, using SPE cartridges to clean and pre-concentrate PTA and PTB in plasma, urine and milk followed by LC-MS quantification. The average recovery of PTA in plasma, urine, and milk was 71, 88 and 77%, respectively, whereas recovery of PTB was 75, 82 and 63%. The method LOQ for PTA and PTB in plasma was 1.2 and 3.7ngmL(-1), 52 and 33ngmL(-1) for undiluted urine and 5.8 and 5.3ngmL(-1) for milk. The method is repeatable within and between days, with RSD values lower than 15% (PTA) and 20% (PTB). When PTA and PTB spiked samples were stored at -18°C for 14 days both compounds remained stable. In contrast, the PTA concentration was reduced by 15% when PTA spiked plasma was left for 5h at room temperature before SPE clean-up, whereas PTB remained stable. The method is the first to allow simultaneous quantification of PTA and PTB in biological fluids in a relevant concentration range. After intravenous administration of 0.092mg PTA per kgbw in a heifer, the plasma concentration was more than 300ngmL(-1) PTA and declined to 9.8ngmL(-1) after 6h, PTB was determined after 10min at 50ngmL(-1.), (Copyright © 2014 Elsevier B.V. All rights reserved.)

A highly sensitive and rapid ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) has been developed and validated for simultaneous quantification of the three main bioactive compounds, i.e., ligustilide, dehydrocostuslactone and α-cyperone in dog plasma after oral administration of the essential oil of Xiang-Fu-Si-Wu Formula (XEO). Clarithromycin was used as an internal standard (IS). Plasma samples were processed by protein precipitation with methanol. The separation was performed on an Acquity BEH C18 column (100mm×2.1mm, 1.7μm) at a flow rate of 0.4mLmin(-1), using 0.1% formic acid-acetonitrile as mobile phase. The MS/MS ion transit ions monitored were 190.5→90.9 for ligustilide, 231.1→185.1 for dehydrocostuslactone, 219.2→123.0 for α-cyperone and 748.5→158.1 for IS. Method validation was performed as per Food and Drug Administration guidelines and the results met the acceptance criteria. The lower limit of quantification (LLOQ) achieved was 5.12ng/mL for ligustilide, 1.06ng/mL for dehydrocostuslactone and 1.89ng/mL for α-cyperone, respectively, and the calibration curves obtained were linear (r>0.99) over the concentration range approximately 1-1000ng/mL. The intra- and inter-day precision was less than 15% and the accuracy was within ±9.2%. After validation, this method was successfully applied to a pharmacokinetic study where dogs were orally given 0.3g/kg XEO, equivalent to 183.6mg/kg of ligustilide, 5.0mg/kg of dehydrocostuslactone and 26.2mg/kg of α-cyperone, respectively., (Copyright © 2013 Elsevier B.V. All rights reserved.)

A sensitive and selective high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method was developed for the determination of buagafuran in human plasma. The analyte was extracted from plasma samples with hexane after addition of isotopic internal standard and chromatographed on a RP-C(8) column. The mobile phase consisted of methanol-water (90:10, v/v) and the flow rate was 0.2 mL/min. The detection was performed on a triple quadrupole tandem mass spectrometer in multiple reactions monitoring (MRM) mode using positive electrospray ionization (ESI). The method was validated over the concentration range of 0.5-200 ng/mL. Inter- and intra-day precision (RSD%) were all within 15% and the accuracy (RE%) was equal or lower than 9.5%. The lower limit of quantitation (LLOQ) was 0.5 ng/mL. The extraction recovery was on average 38.1% and the detection was not affected by the matrix. The method was successfully applied to the pharmacokinetic study of buagafuran in healthy Chinese volunteers., (Copyright © 2011 Elsevier B.V. All rights reserved.)

In the present studies, to give momentum to traditionally low throughput pharmacokinetic screening, a bioanalytical method based on the concept of sample pooling for simultaneous bioanalysis of multiple compounds is discussed. A sensitive, selective, specific and rapid HPLC/ESI-MS/MS assay method was developed and validated for the simultaneous quantitation of three novel trioxane antimalarials (99-357, 99-408 and 99-411) in rat plasma using trioxane analogue as internal standard. The suitably validated bioanalytical method was then further extrapolated to rabbit and monkey plasma by performing partial validation. Extraction from the plasma involves a simple two-step liquid-liquid extraction with n-hexane. The analytes were chromatographed on a cyano column by isocratic elution with acetonitrile:ammonium acetate buffer (pH 6) (85:15, v/v) and analyzed by mass spectrometry in multiple reaction-monitoring (MRM) mode. The chromatographic run time was 5.5 min and the weighted (1/x(2)) calibration curves were linear over a range of 1.56-200 ng/ml. The limit of detection (LOD) and lower limit of quantification (LLOQ) in rat plasma, rabbit plasma and monkey plasma were 0.78 and 1.56 ng/ml, respectively, for all three analytes. The intra- and inter-batch accuracy and precision in terms of % bias and % relative standard deviation were found to be well within the acceptable limits (< 15%). The average absolute recoveries of 99-357, 99-408 and 99-411 from spiked plasma samples were > 90%, > 70% and > 60%, respectively. The assay method described here could be applied to study the pharmacokinetics of 99-357, 99-408 and 99-411 using sample-pooling technique.

The aim of this paper is to investigate the characteristics of atractylenolide I (AO-I) in the body by a GC-MS method. All bio-samples were cleared up with a liquid-liquid extraction procedure. The calibration curves were linear within a range of 5-1000 ng/mL for plasma samples, 0.06-16.00 microg/g for cerebellum samples, and 0.03-8.00 microg/g for other tissue samples. The limit of quantification (LOQ) for AO-I was 1.0 ng/mL or 1.0 ng/g (S/N>micro=10) in the bio-samples. In the applications, the main pharmacokinetic parameters were firstly obtained as follows: Tmax=0.37+/-0.19 h, Cmax=0.26+/-0.05 microg/mL, AUC=1.95+/-0.30 microgh/mL and ka=10.08+/-5.60 h(-1). The tissue distribution of AO-I in rats after the oral administration of 50.0mg/kg was from 0.225 to 0.031microg/g with a decreasing tendency in different tissues like liver>kidney>spleen>cerebellum>heart>cerebrum>lung. The protein binding in rat plasma, human plasma and bovine serum albumin was 80.8+/-3.9, 90.6+/-3.1 and 60.9+/-5.1%, respectively.

The combination of two sensitive, selective and reproducible reversed phase liquid chromatographic (RP-HPLC) methods was developed for the determination of artesunate (AS), its active metabolite dihydroartemisinin (DHA) and mefloquine (MQ) in human plasma. Solid phase extraction (SPE) of the plasma samples was carried out on Supelclean LC-18 extraction cartridges. Chromatographic separation of AS, DHA and the internal standard, artemisinin (QHS) was obtained on a Hypersil C4 column with mobile phase consisting of acetonitrile-0.05 M acetic acid adjusted to pH 5.2 with 1.0M NaOH (42:58, v/v) at the flow rate of 1.50 ml/min. The analytes were detected using an electrochemical detector operating in the reductive mode. Chromatography of MQ and the internal standard, chlorpromazine hydrochloride (CPM) was carried out on an Inertsil C8-3 column using methanol-acetonitrile-0.05 M potassium dihydrogen phosphate adjusted to pH 3.9 with 0.5% orthophosphoric acid (50:8:42, v/v/v) at a flow rate of 1.00 ml/min with ultraviolet detection at 284 nm. The mean recoveries of AS and DHA over a concentration range of 30-750 ng/0.5 ml plasma and MQ over a concentration of 75-1500 ng/0.5 ml plasma were above 80% and the accuracy ranged from 91.1 to 103.5%. The within-day coefficients of variation were 1.0-1.4% for AS, 0.4-3.4% for DHA and 0.7-1.5% for MQ. The day-to-day coefficients of variation were 1.3-7.6%, 1.8-7.8% and 2.0-3.4%, respectively. Both the lower limit of quantifications for AS and DHA were at 10 ng/0.5 ml and the lower limit of quantification for MQ was at 25 ng/0.5 ml. The limit of detections were 4 ng/0.5 ml for AS and DHA and 15 ng/0.5 ml for MQ. The method was found to be suitable for use in clinical pharmacological studies.

Dihydroartemisinin (DHA) is a sesquiterpene used in the world as an antimalarial. To evaluate the pharmacokinetics of dihydroartemisinin in rats, a sensitive and specific liquid chromatography/tandem mass spectrometric (LC-MS/MS) method was developed and validated for the quantitation of dihydroartemisinin in rat plasma. For detection, a Sciex API 4000 LC-MS/MS with a TurboIonSpray ionization (ESI) inlet in the positive ion-multiple reaction monitoring (MRM) mode was used. The plasma samples were pre-treated by a simple liquid-liquid extraction with diethyl ether. The statistical evaluation for this method reveals excellent linearity, accuracy and precision for the range of concentrations 0.2-100.0 ng/mL. The method had a lower limit of quantification (LLOQ) of 0.2 ng/mL for beta-dihydroartemisinin in 100 microL of plasma. The method was successfully applied to the characterization of the pharmacokinetic profile of beta-dihydroartemisinin in rats after oral administration.

Atractylenolide III is a major active component in Atractylodes macrocephala. This paper describes a simple, rapid, specific and sensitive method for the quantification of atractylenolide III in rat plasma using a liquid-liquid extraction procedure followed by liquid chromatography mass spectrometric (LC-MS) analysis. A Kromasil 3.5 microm C(18) column (150 mm x 2.00 mm) was used as the analytical column. Linear detection responses were obtained for atractylenolide III concentration ranging from 5 to 500 ng L(-1). The precision and accuracy data, based on intra-day and inter-day variations over 5 days were within 10.29%. The lower limit of quantitation for atractylenolide III was 5 ng mL(-1), using 0.1 mL plasma for extraction and its recoveries were greater than 85% at the low, medium and high concentrations. The method has been successfully applied to a pharmacokinetic study in rats after an oral administration of atractylenolide III with a dose of 20.0 mg kg(-1). With the lower limits of quantification at 5 ng mL(-1) for atractylenolide III, this method was proved to be sensitive enough for the pharmacokinetics study of atractylenolide III.

ZT-1 is a novel acetylcholinesterase (AChE) inhibitor. It is rapidly transformed to Huperzine A (Hup A) in vitro. A simple and rapid HPLC-UV method for the simultaneous determination of ZT-1 and its metabolite Hup A in plasma is described. The chromatographic separations were achieved on a C(18) ODS column (250 mm x 4.6 mm ID) using methanol-1 mmol/L ammonium acetate (70:30,v/v) as mobile phase. The flow rate was 0.7 mL/min, the detection wavelength was 313 nm and the column temperature was kept at 35 degrees C. Plasma samples were prepared as rapidly as possible and extracted immediately with 5 mL of chloroform:iso-propyl alcohol mixture (v/v, 9:1). The retention times of ZT-1 and Huperzine A (Hup A) were 18.7 and 14.4 min, respectively. The mean absolute recoveries of two analytes were >90%. Quantification limits were all 0.02 nmol/mL for ZT-1 and Hup A. This analytical method was reliable and convenient procedure that meets the criteria for the pharmacokinetic evaluation of ZT-1 on experimental animals.

Huperzine-A (Hup-A), a biologically potent, reversible acetylcholinesterase inhibitor for the treatment of Alzheimer disease (AD) in China, has very low blood concentration. In order to study the pharmacokinetics of newly developed Hup-A transdermal patches in animal, a rapid and sensitive ion-pair reverse-phase high performance liquid chromatography (RP-HPLC) method for the determination of Hup-A in beagle dog serum using mebendazole as internal standard has been developed and validated. The analyte and internal standard were extracted from serum using chloroform-isopropanol (95:5, v/v), analyzed on a C (18) column (5 microm, 150 mm x 4.6 mm i.d.) with a mobile phase consisting of methanol-water-glacial acetic acid (50:48.5:1.5, v/v/v), using sodium dodecylsulfonate as an ion-pair reagent, and detected with UV detector at 313 nm. The chromatographic run time was within 15 min. The assay was linear over the concentration range of 1-12 ng/ml and intra- and inter-day precision over this range was not more than 12.8%. The limit of quantification in serum was 1 ng/ml. The method was successfully applied to characterize the Hup-A concentration-time profiles and study the single and multiple doses phamacokinetics of Hup-A transdermal patches in beagle dogs. The pharmacokinetic study results showed that Hup-A patches has the characteristic of sustained or controlled drug release in vivo.

This study reports the development and validation of a sensitive and selective assay method for the determination of alpha-,beta-arteether in rat serum by liquid chromatography-mass spectrometry. The mobile phase was composed of methanol-0.1 mM sodium acetate (pH 5) (80:20%) at a flow-rate of 1 ml min(-1) and chromatographic separations were achieved on a Ultracarb, 5 ODS 20, Phenomenex column (5 micrometer, 30 mmx4.6 mm I.D.). The total effluent from the column was split so that one-tenth was injected into the electrospray LC-MS interface. ESI-MS analysis was carried out using a Micromass Quattro II Triple Quadrupole Mass Spectrometer equipped with an electrospray source. The MS analysis was carried out at a cone voltage of 52 V with a scan range of 100-400 Da. The analytes were quantified from the [M+Na](+) ion chromatograms of alpha-,beta-arteether at m/z 335 and artemisinin at m/z 305. A simple liquid-liquid extraction with 2x2 ml n-hexane was used to isolate alpha-,beta-arteether from rat serum. The method was validated in terms of recovery, linearity, accuracy and precision (within- and between-assay variation). The recovery from spiked control samples ranged from 88.41 to 96.17% with a maximum CV of 10.8% for alpha-arteether and 69.83-79.69% with a maximum CV of 17.06% for beta-arteether. Linearity in serum was observed over the range 20-320 ng ml(-1). Percent bias (accuracy) was well within the acceptable range. Within- and between-assay precision were less than 15%. The assay method described here is being applied to study the pharmacokinetics of CDRI developed intramuscular formulation Emal (alpha-/beta-arteether in the ratio of 30:70) in rats. The method is sensitive enough to monitor alpha-,beta-arteether up to 24 h after a single 30 mg kg(-1) i.m. dose.

A sensitive and selective method is described for the determination of artemether and its active dihydroartemisinin metabolite in human plasma using artemisinin as internal standard. The method consists of a liquid-liquid extraction with subsequent evaporation of the supernatant to dryness followed by the analysis of the reconstituted sample by liquid chromatography-mass spectrometry (LC-MS) in single ion monitoring mode using atmospheric pressure chemical ionization (APCI) as an interface. Chromatography was performed on a C(18) reversed-phase column using acetonitrile-glacial acetic acid 0.1% (66:34) as a mobile phase. The method was fully validated over a concentration range of 5-200 ng/ml using 0.5 ml of human plasma per assay. Stability assessment was also included. The method was applied to the quantification of artemether and its metabolite in human plasma of healthy volunteers participating in pharmacokinetic drug-drug interaction studies.