[Show abstract][Hide abstract] ABSTRACT: A previous pharmacokinetic study on dosing of colistin methanesulfonate (CMS) at 240 mg (3 million units [MU]) every 8 h indicated that colistin has a long half-life, resulting in insufficient concentrations for the first 12 to 48 h after initiation of treatment. A loading dose would therefore be beneficial. The aim of this study was to evaluate CMS and colistin pharmacokinetics following a 480-mg (6-MU) loading dose in critically ill patients and to explore the bacterial kill following the use of different dosing regimens obtained by predictions from a pharmacokinetic-pharmacodynamic model developed from an in vitro study on Pseudomonas aeruginosa. The unbound fractions of colistin A and colistin B were determined using equilibrium dialysis and considered in the predictions. Ten critically ill patients (6 males; mean age, 54 years; mean creatinine clearance, 82 ml/min) with infections caused by multidrug-resistant Gram-negative bacteria were enrolled in the study. The pharmacokinetic data collected after the first and eighth doses were analyzed simultaneously with the data from the previous study (total, 28 patients) in the NONMEM program. For CMS, a two-compartment model best described the pharmacokinetics, and the half-lives of the two phases were estimated to be 0.026 and 2.2 h, respectively. For colistin, a one-compartment model was sufficient and the estimated half-life was 18.5 h. The unbound fractions of colistin in the patients were 26 to 41% at clinical concentrations. Colistin A, but not colistin B, had a concentration-dependent binding. The predictions suggested that the time to 3-log-unit bacterial kill for a 480-mg loading dose was reduced to half of that for the dose of 240 mg.
Antimicrobial Agents and Chemotherapy 05/2012; 56(8):4241-9. · 4.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) method for the quantification of the opioid peptide DAMGO in rat plasma, as well as DAMGO and the microdialysis recovery calibrator [(13)C(2),(15)N]-DAMGO in microdialysis samples, is described. The microdialysis samples consisted of 15 μL Ringer solution containing 0.5% bovine serum albumin. Pretreatment of the samples involved protein precipitation with acetonitrile followed by dilution with 0.01% formic acid. The lower limits of quantification were 0.52 ng/mL and 0.24 ng/mL for DAMGO and [(13)C(2),(15)N]-DAMGO respectively and the response was linear up to 5000 fold higher concentrations. The plasma samples (50 μL) were precipitated with acetonitrile containing the isotope labeled analog [(13)C(2),(15)N]-DAMGO as internal standard. The method was linear in the range of 11-110,000 ng/mL. The separations were conducted on a HyPurity C18 column, 50×4.6 mm, 3 μm particle size, with a mobile phase consisting of acetonitrile, water and formic acid to the proportions of 17.5:82.5:0.01. Low energy collision dissociation tandem mass spectrometric (CID-MS/MS) analysis was carried out in the positive ion mode using multiple reaction monitoring (MRM) of the following mass transitions: m/z 514.2→453.2 for DAMGO and m/z 517.2→456.2 for [(13)C(2),(15)N]-DAMGO. The intra-day precision and accuracy did not exceed 5.2% and 93-104% for both compounds and sample types described. The inter-day precision an accuracy were <6.8% and 95-105% respectively. The method described is simple, reproducible and suitable for the analysis of small sample volumes at low concentrations.
Journal of chromatography. B, Analytical technologies in the biomedical and life sciences 05/2012; 900:11-7. · 2.78 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Diphenhydramine (DPHM) and oxycodone are weak bases that are able to form cations. Both drugs show active uptake at the blood-brain barrier (BBB). There is thus a possibility for a pharmacokinetic interaction between them by competition for the same uptake transport system. The experiments of the present study were designed to study the transport of DPHM across the BBB and its interaction with oxycodone in vitro and in vivo. In vitro, the interaction between the drugs was studied using conditionally immortalized rat brain capillary endothelial cells (TR-BBB13 cells). The in vivo relevance of the in vitro findings was studied in rats using brain and blood microdialysis. DPHM was actively transported across the BBB in vitro (TR-BBB13 cells). Oxycodone competitively inhibited DPHM uptake with a K(i) value of 106 μM. DPHM also competitively inhibited oxycodone uptake with a K(i) value of 34.7 μM. In rats, DPHM showed fivefold higher unbound concentration in brain interstitial fluid (ISF) than in blood, confirming a net active uptake. There was no significant interaction between DPHM and oxycodone in vivo. This accords with the results of the in vitro experiments because the unbound plasma concentrations that could be attained in vivo, without causing adverse effects, were far below the K(i) values.
Journal of Pharmaceutical Sciences 04/2011; 100(9):3912-23. · 3.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using a liquid chromatography-tandem mass spectrometry method, the serum and cerebrospinal fluid (CSF) concentrations of colistin were determined in patients aged 1 months to 14 years receiving intravenous colistimethate sodium (60,000 to 225,000 IU/kg of body weight/day). Only in one of five courses studied (a 14-year-old receiving 225,000 IU/kg/day) did serum concentrations exceed the 2 microg/ml CLSI/EUCAST breakpoint defining susceptibility to colistin for Pseudomonas and Acinetobacter. CSF colistin concentrations were <0.2 microg/ml but increased in the presence of meningitis (approximately 0.5 microg/ml or 34 to 67% of serum levels).
Antimicrobial Agents and Chemotherapy 09/2010; 54(9):3985-7. · 4.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An analytical method for quantitation of colistin A and colistin B in plasma and culture medium is described. After protein precipitation with acetonitrile (ACN) containing 0.1% trifluoroacetic acid (TFA), the supernatants were diluted with 0.03% TFA. The compounds were separated on an Ultrasphere C18 column, 4.6 mm x 250 mm, 5 microm particle size with a mobile phase consisting of 25% ACN in 0.03% TFA and detected with tandem mass spectrometry. The instrument was operating in ESI negative ion mode and the precursor-product ion pairs were m/z 1167.7-->1079.6 for colistin A and m/z 1153.7-->1065.6 for colistin B. The lower limit of quantification (LLOQ) for 100 microL plasma was 19.4 and 10.5 ng/mL for colistin A and B, respectively, with CV <6.2% and accuracy <+/-12.6%. For culture medium (50 microL+50 microL plasma), LLOQ was 24.2 and 13.2 ng/mL for colistin A and B, respectively, with CV <11.4% and accuracy <+/-8.1%. The quick sample work-up method allows for determination of colistin A and B in clinical samples without causing hydrolysis of the prodrug colistin methanesulfonate (CMS).
Journal of pharmaceutical and biomedical analysis 12/2008; 49(3):760-7. · 2.45 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The aim was to develop a model to describe the population pharmacokinetics of nevirapine in South African human immunodeficiency virus (HIV)-infected patients who were taking nevirapine-based antiretroviral therapy concomitantly or in the absence of rifampicin-based tuberculosis therapy.
Patients were divided into two groups: (1) patients receiving nevirapine-containing antiretroviral regimen (200 mg twice daily) and continuation phase rifampicin-containing tuberculosis therapy (n = 27) in whom blood samples were obtained before and not less than 14 days after they completed tuberculosis therapy; (2) patients without tuberculosis who were receiving a nevirapine-containing antiretroviral regimen for at least 3 weeks (n = 26). The population pharmacokinetics of nevirapine was described using nonlinear mixed effects modelling with NONMEM software. Based on the developed model, plasma concentration profiles after 300, 400 and 500 mg of nevirapine twice daily were simulated.
Concomitant administration of rifampicin increased nevirapine oral clearance (CL/F) by 37.4% and reduced the absorption rate constant (k(a)) by almost sixfold. Rifampicin reduced the nevirapine average minimum concentration by 39%. Simulated doses of 300 mg twice daily elevated nevirapine concentrations above subtherapeutic levels in most patients, with minimum exposure above the recommended maximum concentration. The area under the concentration-time curve of 12-hydroxynevirapine was not different in the presence of rifampicin. 2-, 3- and 8-Hydroxynevirapine were not detectable (LLOQ = 0.025 mg/L).
The developed model adequately describes nevirapine population pharmacokinetics in a South African population when taken with/and in the absence of rifampicin treatment. The simulations suggest that an increased dose of 300 mg twice daily would achieve adequate nevirapine concentrations in most patients during rifampicin-containing treatment for tuberculosis.
European Journal of Clinical Pharmacology 09/2008; 65(1):71-80. · 2.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The aim of this study was to investigate which principal human cytochrome P450 (CYP450) enzymes are affected by artemisinin and to what degree the artemisinin derivatives differ with respect to their respective induction and inhibition capacity. Seventy-five healthy adults were randomized to receive therapeutic oral doses of artemisinin, dihydroartemisinin, arteether, artemether or artesunate for 5 days (days 1-5). A six-drug cocktail consisting of caffeine, coumarin, mephenytoin, metoprolol, chlorzoxazone and midazolam was administered orally on days -6, 1, 5 and 10 to assess the activities of CYP1A2, CYP2A6, CYP2C19, CYP2D6, CYP2E1 and CYP3A, respectively. Four-hour plasma concentrations of parent drugs and corresponding metabolites and 7-hydroxycoumarin urine concentrations were quantified by liquid chromatography-tandem mass spectrometry. The 1-hydroxymidazolam/midazolam 4-h plasma concentration ratio (CYP3A) was increased on day 5 by artemisinin [2.66-fold (98.75% CI: 2.10-3.36)], artemether [1.54 (1.14-2.09)] and dihydroartemisinin [1.25 (1.06-1.47)] compared with day -6. The S-4'-hydroxymephenytoin/S-mephenytoin ratio (CYP2C19) was increased on day 5 by artemisinin [1.69 (1.47-1.94)] and arteether [1.33 (1.15-1.55)] compared with day -6. The paraxanthine/caffeine ratio (CYP1A2) was decreased on day 1 after administration of artemisinin [0.27 (0.18-0.39)], arteether [0.70 (0.55-0.89)] and dihydroartemisinin [0.73 (0.59-0.90)] compared with day -6. The alpha-hydroxymetoprolol/metoprolol ratio (CYP2D6) was lower on day 1 compared with day -6 in the artemisinin [0.82 (0.70-0.96)] and dihydroartemisinin [0.83 (0.71-0.96)] groups, respectively. In the artemisinin-treated subjects this decrease was followed by a 1.34-fold (1.14-1.58) increase from day 1 to day 5. These results show that intake of artemisinin antimalarials affect the activities of several principal human drug metabolizing CYP450 enzymes. Even though not significant in all treatment groups, changes in the individual metrics were of the same direction for all the artemisinin drugs, suggesting a class effect that needs to be considered in the development of new artemisinin derivatives and combination treatments of malaria.
Fundamental and Clinical Pharmacology 07/2007; 21(3):307-16. · 1.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A sensitive method using enantiospecific liquid chromatography/tandem mass spectrometry detection for the quantitation of S- and R-mephenytoin as well as its metabolites S- and R-nirvanol and S- and R-4'-hydroxymephenytoin in plasma and urine has been developed and validated. Plasma samples were prepared by protein precipitation with acetonitrile, while urine samples were diluted twice with the mobile phase before injection. The analytes were then separated on a chiral alpha(1)-acid glycoprotein (AGP) column and thereafter detected, using electrospray ionization tandem mass spectrometry. In plasma, the lower limit of quantification (LLOQ) was 1 ng/mL for S- and R-4'-hydroxymephenytoin and S-nirvanol and 3 ng/mL for R-nirvanol and S- and R-mephenytoin. In urine, the LLOQ was 3 ng/mL for all compounds. Resulting plasma and urine intra-day precision values (CV) were <12.4% and <6.4%, respectively, while plasma and urine accuracy values were 87.2-108.3% and 98.9-104.8% of the nominal values, respectively. The method was validated for plasma in the concentration ranges 1-500 ng/mL for S- and R-4'-hydroxymephenytoin, 1-1000 ng/mL for S-nirvanol, and 3-1500 ng/mL for R-nirvanol and S- and R-mephenytoin. The validated concentration range in urine was 3-5000 ng/mL for all compounds. By using this method, the metabolic activities of two human drug-metabolizing enzymes, cytochrome P450 (CYP) 2C19 and CYP2B6, were simultaneously characterized.
Rapid Communications in Mass Spectrometry 02/2006; 20(3):463-72. · 2.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A sensitive and reproducible method for the determination of morphine and the metabolites morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) was developed. The method was validated for perfusion fluid used in microdialysis as well as for sheep and human plasma. A C18 guard column was used to desalt the samples before analytical separation on a ZIC HILIC (hydrophilic interaction chromatography) column and detection with tandem mass spectrometry (MS/MS). The mobile phases were 0.05% trifluoroacetic acid (TFA) for desalting and acetonitrile/5 mM ammonium acetate (70:30) for separation. Microdialysis samples (5 microL) were directly injected onto the system. The lower limits of quantification (LLOQ) for morphine, M3G and M6G were 0.50, 0.22 and 0.55 ng/mL, respectively, and the method was linear from LLOQ to 200 ng/mL. For plasma, a volume of 100 microL was precipitated with acetonitrile containing internal standards (deuterated morphine and metabolites). The supernatant was evaporated and reconstituted in 0.05% TFA before the desalting process. The LLOQs for sheep plasma were 2.0 and 3.1 ng/mL and the ranges were 2.0-2000 and 3.1-3100 ng/mL for morphine and M3G, respectively. For human plasma, the LLOQs were 0.78, 1.49 and 0.53 ng/mL and the ranges were 0.78-500, 1.49-1000 and 0.53-500 ng/mL for morphine, M3G and M6G, respectively.
Rapid Communications in Mass Spectrometry 02/2005; 19(15):2116-22. · 2.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Sensitive enantioselective liquid chromatographic assays using tandem mass spectrometric detection were developed and validated for the determination of S-cetirizine (S-CZE) and R-cetirizine (R-CZE) in guinea pig plasma, brain tissue, and microdialysis samples. Enantioselective separation was achieved on an alpha1-acid glycoprotein column within 14 min for all methods. A cetirizine analog, ucb 20028, was used as internal standard. Cetirizine and the internal standard were detected by multiple reaction monitoring using transitions m/z 389.1 --> 200.9 and 396.1 --> 276.1, respectively. The samples were prepared using protein precipitation with acetonitrile. For guinea pig plasma, the assay was linear over the range 0.25-5000 ng/mL for both S-CZE and R-CZE, with a lower limit of quantification (LLOQ) of 0.25 ng/mL. For the brain tissue and microdialysis samples, the assays were linear over the range 2.5-250 ng/g and 0.25-50 ng/mL, respectively, and the LLOQ values were 2.5 ng/g and 0.25 ng/mL, respectively. The intra- and inter-day precision values were < or =7.1% and < or =12.6%, respectively, and the intra- and inter-day accuracy varied by less than +/-8.0% and +/-6.0% of the nominal value, respectively, for both enantiomers in all the matrices investigated.
Rapid Communications in Mass Spectrometry 01/2005; 19(12):1749-57. · 2.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Sensitive and reproducible methods for the determination of oxycodone, oxymorphone and noroxycodone in Ringer solution, rat plasma and rat brain tissue by liquid chromatography/mass spectrometry are described. Deuterated analogs of the substances were used as internal standards. Samples in Ringer solution were analyzed by direct injection of 10 microL Ringer solution diluted by an equal volume of water. The limit of quantification was 0.5 ng/mL and the method was linear in the range of 0.5-150 ng/mL for all substances. To analyze oxycodone and oxymorphone in rat plasma, 50 microL of plasma were precipitated with acetonitrile, and the supernatant was directly injected onto the column. To analyze oxycodone, oxymorphone and noroxycodone in rat plasma, 100 microL of rat plasma were subjected to a C18 solid-phase extraction (SPE) procedure, before reconstituting in mobile phase and injection onto the column. For both methods the limit of quantification in rat plasma was 0.5 ng/mL and the methods were linear in the range of 0.5-250 ng/mL for all substances. To analyze the content of oxycodone, oxymorphone and noroxycodone in rat brain tissue, 100 microL of the brain homogenate supernatant were subjected to a C18 SPE procedure. The limit of quantification of oxycodone was 20 ng/g brain, and for oxymorphone and noroxycodone 4 ng/g brain, and the method was linear in the range of 20-1000 ng/g brain for oxycodone and 4-1000 ng/g brain for oxymorphone and noroxycodone. All methods utilized a mobile phase of 5 mM ammonium acetate in 45% acetonitrile, and a SB-CN column was used for separation. The total run time of all methods was 9 min. The intra-day precision and accuracy were <11.3% and <+/-14.9%, respectively, and the inter-day precision and accuracy were <14.9% and <+/-6.5%, respectively, for all the concentrations and matrices described.
Rapid Communications in Mass Spectrometry 01/2004; 18(21):2565-76. · 2.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cefuroxime is a second-generation cephalosporin used against different kinds of bacterial infections. To be able to optimize the dosing it is necessary to characterize the pharmacokinetics of cefuroxime which requires a selective and sensitive analytical method for cefuroxime in plasma or serum. A new rapid liquid chromatography/electrospray tandem mass spectrometry (LC/MS/MS) method, using cefotaxime as internal standard, was developed for analysis of cefuroxime in human serum. The work-up procedure consisted of protein precipitation with acetonitrile/cefotaxime, and after centrifugation the supernatant was dissolved in mobile phase. The sample was injected on a SB-CN column and the detection was performed using tandem mass spectrometry (MS/MS). The limit of quantification was determined to 0.025 microg/mL. The method was linear in the range 0.025-50 microg/mL with a coefficient of correlation >0.999. The limit of quantification and intra-day variability were found to be the same for plasma samples, which indicates that the method is valid for serum as well as plasma samples.
Rapid Communications in Mass Spectrometry 01/2004; 18(6):707-10. · 2.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Our goal was to investigate whether artemisinin autoinduction is caused by an increase in cytochrome P450 (CYP) 2B6 or CYP2C9 activities, we evaluated the effects of multiple-dose artemisinin administration on S-mephenytoin N-demethylation in healthy subjects.
Fourteen subjects, 6 poor metabolizers of CYP2C19 and 8 extensive metabolizers, received a single oral dose of 200 mg racemic mephenytoin (CYP2B6 in vivo marker) before (day -28) and during multiple-dose artemisinin administration (250 mg/d orally for 9 days and 500 mg on the tenth day). A 500-mg single dose of artemisinin was administered on day -28. The CYP2C9 in vivo marker tolbutamide was administered on day -28 and on days 7, 12, and 17 to monitor the minor involvement of CYP2C9 in S-mephenytoin N-demethylation.
Artemisinin oral clearance increased 5.3-fold (P <.001) by the tenth day of administration. Its pharmacokinetics was not different in the 2 CYP2C19 phenotypes. The oral clearance of R-mephenytoin increased 1.7-fold (P <.05) in both phenotypes during the period of artemisinin administration. The area under the concentration-time curve ratio of S-nirvanol/S-mephenytoin, an index of CYP2B6 activity, increased 1.9-fold (P <.05) in CYP2C19 poor metabolizers during artemisinin multiple-dose administration, whereas the urinary excretion ratio of hydroxytolbutamide plus carboxytolbutamide/tolbutamide remained constant during the study period.
These results indicate that artemisinin induces the N-demethylation of S-mephenytoin probably by an increased capacity of CYP2B6. The autoinduction phenomenon of artemisinin may, therefore, be attributed, at least in part, to induction of CYP2B6, because this is the isozyme primarily involved in its metabolism. In addition, artemisinin alters the disposition of R-mephenytoin by an unidentified isozyme.
[Show abstract][Hide abstract] ABSTRACT: A high-performance liquid chromatographic method for the enantiospecific quantitation of S- and R-mephenytoin and its metabolites S- and R-nirvanol and S- and R-4'-hydroxymephenytoin in plasma is described. The compounds were separated using a reversed-phase C(2) column in tandem with a chiral alpha(1)-acid glycoprotein column and were detected using ultraviolet detection at 205 nm. The lower limit of quantification was 10 ng/ml for all compounds using 0.5 ml human plasma (intra-day coefficient of variation <13%, accuracy <+/-20%). The method was validated for human plasma in the concentration range 10-2000 ng/ml for each of the six compounds. The method allows for the simultaneous characterisation of the metabolic capacity of two human drug-metabolising enzymes, CYP2C19 and CYP2B6, and may be used when investigating polymorphisms or changes in activity of these two enzymes.
Journal of Chromatography B 07/2003; 791(1-2):179-91. · 2.49 Impact Factor