Quantification of co-trimoxazole in serum and plasma using MS/MS.

BACKGROUND: Co-trimoxazole is frequently used in the prophylaxis and treatment of Pneumocystis carinii pneumonia. High plasma concentrations of sulfamethoxazole or trimethoprim are correlated with toxicity. There is, however, a large variation in PK observed which can lead to underexposure or toxicity. RESULTS: We developed a novel LC-MS/MS method to analyze the components of co-trimoxazole, trimethoprim and sulfamethoxazole and its metabolite sulfamethoxazole-N-acetyl. This new method is expeditious due to its limited sample preprocessing and a relatively short run-time of only 3 min. CONCLUSION: This new method met the US FDA requirements on linearity, selectivity, precision, accuracy, matrix effects, recovery and stability and is suitable for routine analysis and future prospective studies.

The relation of the number of hydrogen-bond acceptors with recoveries of immunosuppressants in DBS analysis.

BACKGROUND: We investigated the influence of the number of hydrogen-bond acceptors on the recovery of immunosuppressant drugs and their structural analogs. This hypothesis was tested by evaluation of the extraction recoveries of tacrolimus, ascomycin, sirolimus, everolimus and temsirolimus with 12, 12, 13, 14 and 16 hydrogen-bond acceptors, respectively. RESULTS: With an increasing number of hydrogen-bond acceptors of sirolimus, everolimus and temsirolimus, a decrease in recoveries was found while ascomycin showed recoveries corresponding to those of tacrolimus. CONCLUSION: This study showed that the number of hydrogen-bond acceptors of the analyte of interest may influence the recoveries in dried blood spot analysis and is a relevant factor to be investigated during method development and validation.

Determination of bedaquiline in human serum using liquid chromatography-tandem mass spectrometry.

Bedaquiline, a diarylquinoline for the treatment of multidrug-resistant tuberculosis (TB), relies on exposure-dependent killing. As data on drug exposure in specific populations are scarce, pharmacokinetic studies may be of interest. No simple and robust validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) method has been reported to date. Therefore, a new method using a quadrupole mass spectrometer was developed for analysis of bedaquiline and N-monodesmethyl bedaquiline (M2) in human serum, using deuterated bedaquiline as the internal standard. The calibration curve was linear over a range of 0.05 (lower limit of quantification [LLOQ]) to 6.00 mg/liter for both bedaquiline and M2, with correlation coefficient values of 0.997 and 0.999, respectively. The calculated accuracy ranged from 1.9% to 13.6% for bedaquiline and 2.9% to 8.5% for M2. Within-run precision ranged from 3.0% to 7.2% for bedaquiline and 3.1% to 5.2% for M2, and between-run precision ranged from 0.0% to 4.3% for bedaquiline and 0.0% to 4.6% for M2. Evaluation of serum concentrations in a patient receiving bedaquiline showed high levels at the end of treatment, reflecting accumulation of the drug. More observational pharmacokinetic data are needed to relate altered drug concentrations to clinical outcome or adverse drug effects. A simple LC-MS/MS method to quantify bedaquiline and M2 levels in human serum using a deuterated internal standard has been validated. This method can be used in clinical studies and daily practice.

The performance of five different dried blood spot cards for the analysis of six immunosuppressants.

BACKGROUND: The relation between hematocrit, substance concentration, extraction recovery and spot formation of tacrolimus, sirolimus, everolimus, ascomycin, temsirolimus and cyclosporin A was investigated for Whatman 31 ET CHR, Whatman FTA DMPK-C, Whatman 903, Perkin Elmer 226 and Agilent Bond Elut DMS DBS cards. RESULTS & DISCUSSION: We found that all DBS cards showed the same hematocrit and concentration-dependent recovery patterns for sirolimus, everolimus and temsirolimus. At high concentrations, the total hematocrit effects were much more pronounced than at low concentrations for tacrolimus, sirolimus, everolimus, ascomycin and temsirolimus. CONCLUSION: The tested card types showed differences in performance, especially at extreme concentrations and hematocrit values. It may be useful to investigate the performance of different types of DBS cards prior to analytical method validation.

Analysis of remifentanil with liquid chromatography-tandem mass spectrometry and an extensive stability investigation in EDTA whole blood and acidified EDTA plasma.

BACKGROUND: Remifentanil is a µ-opioid receptor agonist that was developed as a synthetic opioid for use in anesthesia and intensive care medicine. Remifentanil is rapidly metabolized in both blood and tissues, which results in a very short duration of action. Even after blood sampling, remifentanil is unstable in whole blood and plasma through endogenous esterases and chemical hydrolysis. The instability of remifentanil in these matrices makes sample collection and processing a critical phase in the bioanalysis of remifentanil. METHODS: We have developed a fast and simple sample preparation method using protein precipitation followed by liquid chromatography-tandem mass spectrometry analysis. To improve the stability of remifentanil, citric acid, ascorbic acid, and formic acid were investigated for acidification of EDTA plasma. The stability of remifentanil was investigated in stock solution, EDTA whole blood, EDTA plasma, and acidified EDTA plasma at ambient temperature, 4 °C, 0 °C, and at -20 °C. RESULTS: The analytical method was fully validated based on the Food and Drug Administration guidelines for bioanalytical method validation with a large linear range of 0.20 to 250 ng/mL remifentanil in EDTA plasma acidified with formic acid. The stability results of remifentanil in EDTA tubes, containing whole blood placed in ice water, showed a decrease of approximately 2% in 2 hours. EDTA plasma acidified with citric acid, formic acid, and ascorbic acid showed 0.5%, 4.2%, and 7.2% remifentanil degradation, respectively, after 19 hours at ambient temperature. Formic acid was chosen because of its volatility and thus liquid chromatography-tandem mass spectrometry compatibility. The use of formic acid added to EDTA plasma improved the stability of remifentanil, which was stable for 2 days at ambient temperature, 14 days at 4 °C, and 103 days at -20 °C. CONCLUSIONS: The analytical method we developed uses a simple protein precipitation and maximal throughput by a 2-point calibration curve and short run times of 2.6 minutes. Best sample stability is obtained by placing tubes containing EDTA whole blood in ice water directly after sampling, followed by centrifugation and transfer of the EDTA plasma to tubes with formic acid. The stability of remifentanil in EDTA plasma was significantly improved by the addition of 1.5 µL formic acid per milliliter of EDTA plasma. This analytical method and sample pretreatment are suitable for remifentanil pharmacokinetic studies.

What is the right blood hematocrit preparation procedure for standards and quality control samples for dried blood spot analysis?

Remco Koster is a research analyst and PhD candidate at the University Medical Center Groningen and University of Groningen. He has been working in the field of bioanalysis for over 13 years, where he has developed numerous analytical methods using LC-MS/MS. His main research focus is the influence of various matrices on the development and performance of analytical methods using LC-MS/MS. The development of high-speed extraction and analysis methods for drugs and drugs of abuse in human matrices like blood, plasma, hair, saliva and dried blood spots often leads to improved procedures for preparation of standards and quality control samples, sample handling and validation. Two hematocrit preparation procedures for standards and quality control samples were evaluated in order to improve the quality of procedures for dried blood spot validation and analysis.

Dried blood spot analysis of creatinine with LC-MS/MS in addition to immunosuppressants analysis.

In order to monitor creatinine levels or to adjust the dosage of renally excreted or nephrotoxic drugs, the analysis of creatinine in dried blood spots (DBS) could be a useful addition to DBS analysis. We developed a LC-MS/MS method for the analysis of creatinine in the same DBS extract that was used for the analysis of tacrolimus, sirolimus, everolimus, and cyclosporine A in transplant patients with the use of Whatman FTA DMPK-C cards. The method was validated using three different strategies: a seven-point calibration curve using the intercept of the calibration to correct for the natural presence of creatinine in reference samples, a one-point calibration curve at an extremely high concentration in order to diminish the contribution of the natural presence of creatinine, and the use of creatinine-[(2)H3] with an eight-point calibration curve. The validated range for creatinine was 120 to 480 µmol/L (seven-point calibration curve), 116 to 7000 µmol/L (1-point calibration curve), and 1.00 to 400.0 µmol/L for creatinine-[(2)H3] (eight-point calibration curve). The precision and accuracy results for all three validations showed a maximum CV of 14.0% and a maximum bias of -5.9%. Creatinine in DBS was found stable at ambient temperature and 32 °C for 1 week and at -20 °C for 29 weeks. Good correlations were observed between patient DBS samples and routine enzymatic plasma analysis and showed the capability of the DBS method to be used as an alternative for creatinine plasma measurement.

Quantification of amikacin and kanamycin in serum using a simple and validated LC-MS/MS method.

BACKGROUND: Amikacin and kanamycin are frequently used in the treatment of multidrug-resistant TB. The current commercially available immunoassay is unable to analyze kanamycin and trough levels of amikacin. The objective was therefore to develop a LC-MS/MS method for the quantification of amikacin and kanamycin in human serum. MATERIALS & METHODS: Using apramycin as internal standard, selectivity, accuracy, precision, recovery, matrix effects and stability were evaluated. RESULTS: The presented LC-MS/MS method meets the recommendations of the US FDA with a low LLOQ of 250 ng/ml for amikacin and 100 ng/ml for kanamycin. No statistical significant difference was found between the LC-MS/MS method and the immunoassay of amikacin (Architect(®) assay, p = 0.501). CONCLUSION: The low LLOQ of amikacin and the ability to analyze kanamycin makes the LC-MS/MS method the preferred method for analyzing these aminoglycosides.

Application of sweat patch screening for 16 drugs and metabolites using a fast and highly selective LC-MS/MS method.

BACKGROUND: To facilitate the monitoring of drug abuse by patients, a method was developed and validated for fast and highly selective screening for amphetamine, methamphetamine, 3,4-methylenedioxymethamphetamine, methylenedioxyamphetamine, methylenedioxyethylamphetamine, methylphenidate, cocaine, benzoylecgonine, morphine, codeine, heroin, 6-monoacteylmorphine, methadone, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine, nicotine, and cotinine in PharmCheck sweat patches. The analysis of sweat patches would provide a noninvasive alternative matrix to urine or blood samples. METHODS: The sweat patches were extracted during vigorous shaking for 10 minutes with 1.5 mL of 20 mmol/L ammonium formate, pH 7, and methanol (50%:50% vol/vol). The extracts were cleaned up by filtering through Whatman mini-Uniprep syringeless filter vials before injection. The method uses a single injection to detect and confirm all 16 drugs and metabolites within 9.6 minutes. RESULTS: The validated substances have a linear range of 3.0-300 nanograms per patch, except for nicotine which has a linear range of 30-3750 nanograms per patch. Stabilities of all substances in worn sweat patches were validated at room temperature for 7 days and as a processed sample in the autosampler at 10°C for 5 days. Only heroin was unstable, with high individual variability and reported bias and coefficient of variation of, respectively, -30.6% and 22.1% in worn sweat patches at room temperature. The monitoring of ion ratios was added to the validation criteria. This resulted in analytical cutoff concentrations of 3.0 and 60 nanograms per patch for nicotine with validated qualifier/quantifier ratios. All analytical cutoff concentrations were lower than the cutoff concentrations proposed by the Substance Abuse and Mental Health Services Administration. CONCLUSIONS: The method uses validated cutoff concentrations, qualifier/quantifier ratios, and a simple extraction without extensive sample treatment for the analysis of 16 drugs and metabolites with a runtime of 9.6 minutes. This method was successfully applied for the analysis of 96 worn sweat patches to monitor patients for drug abuse. The results provided the physician or health-care professional with information about drug abuse and could be used to improve patient care with patient-specific therapy.

Fast and highly selective LC-MS/MS screening for THC and 16 other abused drugs and metabolites in human hair to monitor patients for drug abuse.

BACKGROUND: To facilitate the monitoring of drug abuse by patients, a method was developed and validated for the analysis of amphetamine, methamphetamine, 3,4-methylenedioxymethamphetamine, methylenedioxyamphetamine, methylenedioxyethylamphetamine, methylphenidate, cocaine, benzoylecgonine, morphine, codeine, heroin, 6-monoacteylmorphine, methadone, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP), delta-9-tetrahydrocannabinol (THC), nicotine, and cotinine in human hair. METHODS: The hair preparation method contains a 3-step wash procedure with dichloromethane followed by a simultaneous hair pulverization and extraction procedure with disposable metal balls. The developed liquid chromatography tandem mass spectrometry method uses a single injection to detect and confirm all 17 abused drugs, including THC, within 4.8 minutes. RESULTS: Nicotine was validated with a linear range of 800-25,000 pg/mg hair, and all other substances were validated with a linear range of 30.0-2500 pg/mg hair. For inaccuracy and imprecision, the overall bias did not exceed -8.2% and the overall coefficient of variation did not exceed 17.7%. Autosampler stability was proven for 48 hours at 10°C for all substances. Analytical cutoff concentrations were defined for each substance at the lowest validated inaccuracy and imprecision concentration with a bias and coefficient of variation within 15% and qualifier/quantifier ratios within 20% of the set ratio. The analytical cutoff concentrations were 200 pg/mg for codeine and 80.0 pg/mg for 6-MAM, heroin, EDDP, and THC. The analytical cutoff concentration for nicotine was 800 pg/mg and for all other validated substances 30.0 pg/mg. This method was successfully applied to analyze hair samples from patients who were monitored for drug abuse. Hair samples of 47 subjects (segmented into 129 samples) showed 3,4-methylenedioxymethamphetamine, methylphenidate, cocaine, benzoylecgonine, codeine, methadone, EDDP, THC, nicotine, and cotinine above the analytical cutoff. CONCLUSIONS: The method was fully validated, including the validation of the qualifier/quantifier ratios. The analysis of real hair samples proved the efficacy of the developed method for monitoring drug abuse. The results obtained by this method provide the physician or health-care professional with extensive information about actual drug abuse or relapse and can be used for patient-specific therapy.

Simultaneous quantification of anidulafungin and caspofungin in plasma by an accurate and simple liquid chromatography tandem mass-spectrometric method.

INTRODUCTION: Echinocandins are a valuable addition for the treatment of invasive fungal infections, as they are efficacious, demonstrate low toxicity, and have limited drug-drug interactions. In specific clinical situations when altered pharmacokinetics can be expected or dosing guidelines are conflicting, it may be useful to measure concentrations. For this purpose, a liquid chromatography tandem mass-spectrometric method to measure anidulafungin and caspofungin in ethylenediaminetetraacetic acid plasma was developed. METHODS: The method was developed on a Thermo Fisher TSQ Quantum LC-MS/MS. For separation, a BetaBasic C4 (100 mm × 3.0 mm; 5 µm) analytical column was used. Sample preparation consisted of protein precipitation directly in the autosampler vial. The internal standard aculeacin A is structurally related, not used in humans, and commercially available. The method was validated according to the guidelines for bioanalytical method validation of the Food and Drug Administration. RESULTS: The method was accurate (bias ranging from -3.0% to 1.9%) and precise (within-run and between-run coefficients of variation of 2.2% to 7.7% and 1.6% to 9.0%, respectively). All calibration curves were linear over a range of 0.5-10.0 mg/L for anidulafungin and 0.1-20.0 mg/L for caspofungin, and if necessary, samples can be diluted 10-fold. The samples were stable for 3 freeze-thaw cycles, with a bias ranging from 0.6% to 11%. The maximum bias from the worst storage condition, 72 hours at room temperature, was -14.7%. In patient samples, anidulafungin peak concentrations ranged from 2.8 to 8.6 mg/L (n = 20) and trough concentrations ranged from 1.0 to 4.7 mg/L (n = 79). The measured caspofungin concentrations ranged from 1.9 to 7.3 mg/L (n = 20). CONCLUSIONS: The method developed has a straightforward sample preparation and uses a structural analog as the internal standard. This method has been applied successfully for the measurement of anidulafungin and caspofungin concentrations in patient samples, both for clinical practice and for research.

Fast LC-MS/MS analysis of tacrolimus, sirolimus, everolimus and cyclosporin A in dried blood spots and the influence of the hematocrit and immunosuppressant concentration on recovery.

We developed a method for the analysis of four immunosuppressants in dried blood spot (DBS) samples to facilitate therapeutic drug monitoring for transplant patients outside the hospital. An 8mm disc from the central part of the DBS was punched, extracted and followed by LC-MS/MS analysis. The method was validated with ranges from 1.00-50.0 µg/L for tacrolimus, sirolimus and everolimus, and from 20.0-2000 µg/L for cyclosporin A. The validation showed a maximum overall bias of 13.0% for the sirolimus LLOQ, while the maximum overall CV was 15.7% for the everolimus LLOQ. All four immunosuppressants showed to be stable in DBS for at least 7 days at 22°C. The volume of the blood spot showed to have minor effect on measured concentrations. A cross-validation test between the 31 ET CHR paper and the Whatman FTA DMPK-C cards showed no significant difference between the two types of paper. During validation the hematocrit (HT) showed to have significant influence on the analytical results. When the measured concentrations were corrected for the effect of the HT, biases improved significantly. Additional recovery tests proved that the combination of especially low HT and high concentration does not only affect the spot size but can also affect the extraction recoveries of sirolimus and especially everolimus. Although the tested parameters like HT and concentrations are extreme and unlikely for routine analysis of outpatients, the fundamental effect of the combination of these parameters on extraction recoveries are proven with this research. The protein binding in the blood and hydrogen binding to the cellulose of the paper is suggested to influence extractions and gives new insights in the extraction methodology of DBS samples. The observed HT effect during the validation appeared to be negligible during the correlation study as no concentration corrections for the HT values were needed. Nevertheless, results from DBS samples with extremely high concentrations combined with extremely low HT values should be interpreted with caution. The patient correlation study showed good correlations with R(2) values higher than 0.87 between venous whole blood and venous DBS samples were observed for all four immunosuppressants. The Passing & Bablok plots showed positive biases of the slopes of 18% for tacrolimus and less than 12% for sirolimus, everolimus and cyclosporin A. The validated method, proved stability of the immunosuppressants in DBS, and the correlation study showed the capability of the DBS method to be used as an alternative for whole blood analysis in therapeutic drug monitoring.

Dried blood spot analysis suitable for therapeutic drug monitoring of voriconazole, fluconazole, and posaconazole.

Invasive aspergillosis and candidemia are important causes of morbidity and mortality in immunocompromised and critically ill patients. The triazoles voriconazole, fluconazole, and posaconazole are widely used for the treatment and prophylaxis of these fungal infections. Due to the variability of the pharmacokinetics of the triazoles among and within individual patients, therapeutic drug monitoring is important for optimizing the efficacy and safety of antifungal treatment. A dried blood spot (DBS) analysis was developed and was clinically validated for voriconazole, fluconazole, and posaconazole in 28 patients. Furthermore, a questionnaire was administered to evaluate the patients' opinions of the sampling method. The DBS analytical method showed linearity over the concentration range measured for all triazoles. Results for accuracy and precision were within accepted ranges; samples were stable at room temperature for at least 12 days; and different hematocrit values and blood spot volumes had no significant influence. The ratio of the drug concentration in DBS samples to that in plasma was 1.0 for voriconazole and fluconazole and 0.9 for posaconazole. Sixty percent of the patients preferred DBS analysis as a sampling method; 15% preferred venous blood sampling; and 25% had no preferred method. There was significantly less perception of pain with the DBS sampling method (P = 0.021). In conclusion, DBS analysis is a reliable alternative to venous blood sampling and can be used for therapeutic drug monitoring of voriconazole, fluconazole, and posaconazole. Patients were satisfied with DBS sampling and had less pain than with venous sampling. Most patients preferred DBS sampling to venous blood sampling.

Serum ibuprofen levels of extremely preterm infants treated prophylactically with oral ibuprofen to prevent patent ductus arteriosus.

AIM: The aim of this study was to explore the effects of early oral ibuprofen administration on the incidence of hemodynamically significant patent ductus arteriosus (hsPDA) and define the association between serum ibuprofen levels and ductal closure. METHOD: Preterm infants with a gestational age of <28 weeks and/or birth weight of <1,000 g were randomized either to the intervention (ibuprofen prophylaxis) or control group. The intervention group received oral ibuprofen 10 mg/kg within 12-24 h after birth followed by 5 mg/kg at 24 and 48 h. Serum ibuprofen levels after the treatment were analyzed in the intervention group, and the incidence of hsPDA and complication rates were compared between two groups. RESULTS: Nineteen infants who received one course (three doses) of prophylactic ibuprofen in the intervention group and 17 infants in the control group who underwent an echocardiographic examination on the fourth day of life were analyzed. hsPDA was observed in five (26 %) infants in the intervention group and ten (58 %) infants in the control group (p = 0.09). In the intervention group two infants experienced gastrointestinal bleeding two infants had spontaneous intestinal perforation, and two infants developed acute kidney failure. Mean serum ibuprofen level was 28.7 ± 16.9 mg/L in the intervention group, and there was no correlation between ibuprofen level obtained on the fourth day and ductal closure. CONCLUSION: Oral ibuprofen prophylaxis reduces the rates of hsPDA even it is not statistically significant. The ductal closure rate did not correlate with serum ibuprofen levels. Due to high prevalence of adverse events observed, our data do not support the use of oral ibuprofen for prophylaxis of hsPDA.

Determination of ribavirin in human serum using liquid chromatography tandem mass spectrometry.

A method has been developed for the determination of ribavirin in human serum for therapeutic drug monitoring purposes, using liquid chromatography electrospray ionization mass spectrometry. Separation was obtained with a mobile phase gradient starting and ending in 100% aqueous conditions using a Waters Atlantis® T3 column (100×2mm, 3µm). The entire sample preparation consisted of dilution, followed by ultrafiltration. From the clear ultrafiltrate 5µL was injected on the LC-MS/MS system. The calibration curves were linear in the range of 0.2-10mg/L with within-run and between-run precisions (CVs) in the range of 0-10%. The method was validated with respect to specificity, selectivity, linearity, accuracy, precision, recovery and stability and meets the requirements of the FDA. The method was extensively tested for matrix effects by determining the variation of the slopes of calibration curves in different sources of serum and plasma. This method is suitable for the determination of ribavirin in human serum for therapeutic drug monitoring.

Determination of moxifloxacin in human plasma, plasma ultrafiltrate, and cerebrospinal fluid by a rapid and simple liquid chromatography- tandem mass spectrometry method.

Moxifloxacin (MFX) is a useful agent in the treatment of multi-drug-resistant tuberculosis (MDR-TB). At Tuberculosis Centre Beatrixoord, a referral center for tuberculosis in the Netherlands, approximately 36% of the patients have received MFX as treatment. Based on the variability of MFX AUC, the variability of in vitro susceptibility to MFX of M. tuberculosis, and the variability of penetration into sanctuary sites, measuring the concentration of MFX in plasma and cerebrospinal fluid (CSF) could be recommended. Therefore, a rapid and validated liquid chromatography-tandem mass spectrometry (LC-MS-MS) analyzing method with a simple pretreatment procedure was developed for therapeutic drug monitoring of MFX in human plasma and CSF. Because of the potential influence of protein binding on efficacy, we decided to determine both bound and unbound (ultrafiltrate) fraction of MFX. The calibration curves were linear in the therapeutic range of 0.05 to 5.0 mg/L plasma and CSF with CV in the range of -5.4% to 9.3%. MFX ultrafiltrate samples could be determined with the same method setup for analysis of MFX in CSF. The LC-MS-MS method developed in this study is suitable for monitoring MFX in human plasma, plasma ultrafiltrate, and CSF.

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.

Gas chromatography-mass spectrometric assay for propofol in cerebrospinal fluid of traumatic brain patients.

A sensitive gas chromatography-mass spectrometry method for measuring propofol in cerebrospinal fluid is described, validated and applied to four patients after traumatic brain injury. The limit of quantitation was 2 microg/L using a volume of 0.5 mL. The inter- and intra-assay coefficients of variation were 5.9 and 5.1%, respectively. The assay covers the linear concentration range of 2-50 microg/L, which is relevant in clinical pharmacokinetic studies when propofol is used in the Intensive Care Unit as a sedative agent or to lower the intracranial pressure.

A simple and sensitive fully validated HPLC-UV method for the determination of 5-fluorouracil and its metabolite 5,6-dihydrofluorouracil in plasma.

The authors developed a simple and sensitive, fully validated HPLC-UV method for the determination of both 5-FU and its metabolite DHFU in small-volume plasma samples. The analytes were separated on a 4.6 x 250 mm ID Atlantis dC18 5-microm column with isocratic elution at room temperature. Chlorouracil was used as internal standard. The analytes were detected with an UV diode array detector. DHFU was detected at 205 nm, 5-FU at 266 nm, and chlorouracil at both wavelengths. The limits of quantification in plasma were 0.040 mug /mL for 5-FU and 0.075 microg/mL for DHFU. Linearity, accuracy, precision, recovery, dilution, freeze-thaw stability, and stability in the sample compartment were evaluated. The method appeared linear over a range from 0.04 to 15.90 microg/mL for 5-FU and from 0.075 to 3.84 microg/mL for DHFU. The method appeared very suitable for therapeutic drug monitoring and pharmacokinetic studies of 5-FU because of its simple extraction and small sample volume. Problems in earlier published methods with interfering peaks and variable retention times were overcome. The method appeared also to be suitable for detection of uracil and its metabolite dihydrouracil in plasma.