LC-MS/MS method for simultaneous quantification of ten antibiotics in human plasma for routine therapeutic drug monitoring.

Background: Optimizing antimicrobial therapy to attain drug exposure that limits the emergence of resistance, effectively treats the infection, and reduces the risk of side effects is of a particular importance in critically ill patients, in whom normal functions are augmented or/and are infected with pathogens less sensitive to treatment. Achievement of these goals can be enhanced by therapeutic drug monitoring (TDM) for many antibiotics. A liquid chromatography tandem mass spectrometry (LC-MS/MS) method is presented here for simultaneous quantification of ten antimicrobials: cefazolin (CZO), cefepime (CEP), cefotaxime (CTA), ceftazidime (CTZ), ciprofloxacin (CIP), flucloxacillin (FLU), linezolid (LIN), meropenem (MER), piperacillin (PIP) and tazobactam (TAZ) in human plasma. Methods: Plasma samples were precipitated with acetonitrile and injected into the LC-MS/MS. Chromatographic separation was on a Waters Acquity BEH C18 column. Compounds were eluted with water and acetonitrile containing 0.1 % formic acid, using a gradient (0.5-65 % B), in 3.8 min. The flow rate was 0.4 mL/min, and the run time was 5.8 min. Results: The calibration curves were linear across the tested concentration ranges (0.5-250, CZO, CEP, CTA, CTZ and FLU; 0.2-100, MER and TAZ; 0.1-50, CIP and LIN and 1-500 mg/L, PIP). The intra and inter-day imprecision was < 11 %. Accuracy ranged from 95 to 114 %. CTZ and MER showed ionization suppression while CIP showed ionization enhancement, which was normalized with the use of the internal standard. Conclusion: An LC-MS/MS method for simultaneous quantification of ten antimicrobials in human plasma was developed for routine TDM.

Mitigating analyte to stable isotope labelled internal standard cross-signal contribution in quantitative liquid chromatography-tandem mass spectrometry.

Background: Utilising stable isotope labelled internal standards (SIL-IS) in quantitative LC-MS/MS drug analysis is the most widely used approach to normalise for variability during sample quantification processes. However, compounds containing atoms such as Sulphur, Chlorine or Bromine, could potentially cause cross-signal contribution to the SIL-IS from the naturally occurring isotopes, resulting in non-linear calibration curves. A simple, novel method of mitigating the effect is presented here. It entails monitoring of a less abundant SIL-IS isotope, as the precursor ion, of a mass that has no/minimal isotopic contribution from the analyte isotopes. Methods: Experiments were conducted on two LC-MS/MS analysers: Waters Xevo TQ-S and Shimadzu 8050. Flucloxacillin (FLX) was used as an example. Two transitions were selected for FLX (m/z 454 â†’ 160 â†’ 295) and one for each of the SIL-IS isotopes (m/z 458 â†’ 160 for the isotope 457 g/mol and m/z 460 â†’ 160 for the isotope 459 g/mol). Assay biases were assessed at three SIL-IS concentrations: 0.7, 7 and 14 mg/L for each isotope. Results: When using the SIL-IS isotope m/z 458 â†’ 160 at a concentration of 0.7 mg/L, biases were up to 36.9 % on both instruments. Increasing the SIL-IS concentration to 14 mg/L, reduced the bias to 5.8 %. Using the less abundant isotope, m/z 460 â†’ 160, resulted in biases of 13.9 % at an SIL-IS concentration of 0.7 mg/L. Conclusions: Applying this method will mitigate cross-signal contribution from the analyte isotopes to the corresponding SIL-IS, minimise the use of SIL-IS, and, thereby, reduce overall cost.

Current fluconazole treatment regimens result in under-dosing of critically ill adults during early therapy.

PURPOSE: To evaluate current fluconazole treatment regimens in critically ill adults over the typical treatment course. METHODS: Data from critically ill adults treated with fluconazole (n=30) were used to develop a population pharmacokinetic model. Probability of target attainment (PTA) (fAUC24/MIC >100) was determined from simulations for four previously proposed treatment regimens: (i) 400 mg once daily, (ii) an 800 mg loading dose followed by 400 mg once daily, (iii) 400 mg twice daily, and (iv) a 12 mg/kg loading dose followed by 6 mg/kg once daily. The effect of body weight (40, 70, 120 kg) and renal function (continuous renal replacement therapy (CRRT); 20, 60, 120, 180 mL/min creatinine clearance) on PTA was assessed. RESULTS: Early (0-48 h) fluconazole target attainment for infections with a minimum inhibitory concentration (MIC) of 2 mg/L was highly variable. PTA was highest with an 800 mg loading dose for underweight (40 kg) patients and with a 12 mg/kg loading dose for the remainder. End-of-treatment PTA was highest with the 400 mg twice daily maintenance dosing for patients who were under- or normal weight and 6 mg/kg maintenance dosing for overweight (120 kg) patients. None of the fluconazole regimens reliably attained early targets for MICs of ≥4 mg/L. CONCLUSION: Current fluconazole dosing regimens do not achieve adequate early target attainment in critically ill adults, particularly in those who are overweight, have higher creatinine clearance, or are undergoing CRRT. Current fluconazole dosing strategies are generally inadequate to treat organisms with an MIC of ≥4 mg/L.

Determination of febuxostat in human plasma by high performance liquid chromatography (HPLC) with fluorescence-detection.

Febuxostat prevents gout attacks by lowering serum urate. Aspects of the pharmacokinetic-pharmacodynamic relationship of febuxostat concentrations to urate in gout patients need further elucidation. In order to undertake these studies, the assay methodology for febuxostat has been enhanced and validated to meet FDA standards. An HPLC method with fluorescence-detection has been modified to increase sensitivity, reduce complexity, shorten the sample preparation process and improve the inter-day coefficient of variation of the lowest quality control sample (0.03 µg/L). Protein in plasma samples (200 µL) is now precipitated with acetonitrile (400 µL) containing the internal standard (2-naphthoic acid). The supernatant is analysed at excitation and emission wavelengths of 320 and 380 nm, respectively as in the previous method. A Luna C18 column (Phenomenex, Australia) at 40 °C with mobile phase of glacial acetic acid (0.032%) in acetonitrile:water (60:40, v:v), an injection volume of 10 µL and a flow rate of 1.5 mL/min is employed. Analysis time is 8 min. Calibration curves in drug-free plasma range from 0.005 to 10.00 µg/mL. Data points are fitted using linear regression with a weighting factor of 1/concentration. The inter-day accuracy and imprecision of the quality control samples (0.0075, 0.015, 3.00 and 9.80 µg/mL) is 90-115% and ≤ 14.5%, respectively.

Preliminary Evidence for Enhanced Thymine Absorption: A Putative New Phenotype Associated With Fluoropyrimidine Toxicity in Cancer Patients.

BACKGROUND: Chemotherapy for colorectal, head and neck, and breast cancer continues to rely heavily on 5-fluorouracil and its oral prodrug capecitabine. Associations of serious fluoropyrimidine adverse effects have focused on inherited deficiency of the catabolic enzyme, dihydropyrimidine dehydrogenase. However, abnormal dihydropyrimidine dehydrogenase activity accounts for only about one-third of observed toxicity cases. Thus, the cause of most fluorouracil toxicity cases remains unexplained. METHODS: For this small cohort study, thymine (THY) 250 mg was administered orally to 6 patients who had experienced severe toxicity during treatment with 5FU or capecitabine. Plasma and urine were analyzed for THY and its catabolites dihydrothymine (DHT) and ß-ureidoisobutyrate. RESULTS: Of the 6 patients, 2 had decreased THY elimination and raised urinary THY recovery consistent with inherited partial dihydropyrimidine dehydrogenase deficiency, confirmed by DPYD sequencing. Unexpectedly, 3 patients displayed grossly raised plasma THY concentrations but normal elimination profiles (compared with a normal range for healthy volunteers previously published by the authors). DPYD and DPYS sequencing of these 3 patients did not reveal any significant loss-of-activity allelic variants. The authors labeled the phenotype in these 3 patients as "enhanced thymine absorption". Only 1 of the 6 cases of toxicity had a normal postdose plasma profile for THY and its catabolites. Postdose urine collections from all 6 patients had THY/DHT urinary ratios above 4.0, clearly separated from the ratios in healthy subjects that were all below 3.0. CONCLUSIONS: This small cohort provided evidence for a hypothesis that fluorouracil toxicity cases may include a previously undescribed pyrimidine absorption variant, "enhanced thymine absorption," and elevated THY/DHT ratios in urine may predict fluorouracil toxicity. A prospective study is currently being conducted.

Population pharmacokinetic modelling of doxorubicin and doxorubicinol in children with cancer: is there a relationship with cardiac troponin profiles?

PURPOSE: Anthracyclines are a mainstay of the treatment of several childhood malignancies, but their utility is limited by dose-related cardiotoxicity. This study is aimed to explore the link between exposure of paediatric cancer patients to doxorubicin and its metabolite doxorubicinol, and cardiac troponin I (cTnI). METHODS: In a prospective pilot study plasma doxorubicin, doxorubicinol, and cTnI concentrations were measured in samples from children undergoing cancer chemotherapy. A mixed-effects population pharmacokinetic model for doxorubicin and doxorubicinol and in combination with a turn-over model for cTnI were developed. RESULTS: Seventeen patients, aged 3.4-14.7 year, treated for a variety of cancers had 99 doxorubicin and 119 doxorubicinol concentrations analysed from samples drawn between 0.5 and 336 h after the start of the infusion. Eleven patients had received previous doses of anthracyclines, with a median cumulative prior dose of 90 mg/m2 (range 0-225 mg/m2). The median administered doxorubicin dose was 30 mg/m2 (range 25-75 mg/m2). Doxorubicin disposition was described by a three-compartment model with first-order elimination and metabolism to doxorubicinol. Body surface area was related to all clearance and distribution parameters and age further influenced clearance (CL, 58.7 L/h/1.8 m2 for an average 8.4-year-old patient). Combined doxorubicin and metabolite exposure stimulated a temporary increase in cTnI in plasma, with a concentration of 11.8 µg/L required to achieve half-maximal effect. Prior cumulative anthracycline dosage received by patients was predictive of an increased cTnI baseline prior to a new doxorubicin dose. CONCLUSION: Prior anthracycline exposure increased baseline cTnI in a dose-dependent manner, consistent with the known cumulative risk of anthracycline exposure-induced cardiotoxicity.

Bayesian Estimation of Tobramycin Exposure in Patients with Cystic Fibrosis.

Fixed tobramycin (mg/kg) dosing is often inappropriate in patients with cystic fibrosis (CF), as pharmacokinetics are highly variable. The area under the concentration-time curve (AUC) is an exposure metric suited to monitoring in this population. Bayesian strategies to estimate AUC have been available for over 20 years but are not standard practice in the clinical setting. To assess their suitability for use in clinical practice, three AUC estimation methods using limited sampling were compared to measured true exposure by using intensive sampling tobramycin data. Adults prescribed once daily intravenous tobramycin had eight concentrations taken over 24 h. An estimate of true exposure within one dosing interval was calculated using the trapezoidal method and compared to three alternate estimates determined using (i) a two-sample log-linear regression (LLR) method (local hospital practice); (ii) a Bayesian estimate using one concentration (AUC1); and (iii) a Bayesian estimate using two concentrations (AUC2). Each method was evaluated against the true measured exposure by a Bland-Altman analysis. Twelve patients with a median (range) age and weight of 25 (18 to 36) years and 66.5 (51 to 76) kg, respectively, were recruited. There was good agreement between the true exposure and the three alternate estimates of AUC, with a mean AUC bias of <10 mg/liter · h in each case, i.e., -8.2 (LLR), 3.8 (AUC1), and 1.0 (AUC2). Bayesian analysis-based and LLR estimation methods of tobramycin AUC are equivalent to true exposure estimation. All three methods may be suitable for use in the clinical setting; however, a one-sample Bayesian method may be most useful in ambulatory patients for which coordinating blood samples is difficult. Suitably powered, randomized clinical trials are required to assess patient outcomes.

Towards a test to predict 5-fluorouracil toxicity: Pharmacokinetic data for thymine and two sequential metabolites following oral thymine administration to healthy adult males.

The fluoropyrimidine drugs 5-fluorouracil and its oral prodrug capecitabine remain first line therapy for solid tumours of the neck, breast and colon. However, significant and unpredictable toxicity affects about 10-25% of patients depending upon the mode of 5-fluorouracil delivery. The pharmacokinetics of thymine (5-methyluracil) may provide an approach for screening for 5-fluorouracil toxicity, based on the rationale that thymine is a close structural analogue of 5-fluorouracil and is catabolized by the same enzymatic pathway. Oral thymine loading tests were performed on 12 healthy volunteers. Each subject was given a single oral dose of 250mg thymine in capsule form. Blood, urine and saliva samples were collected pre-dose and up to 5h post-dose. Concentrations of thymine, and its catabolites dihydrothymine and ß-ureidoisobutyrate were analysed by HPLC-tandem mass spectrometry in plasma, urine and saliva. The pharmacokinetic data of healthy volunteers were analysed assuming a non-compartmental model. Thymine peaked quickly (30-45min) in plasma to a maximum concentration of 170±185µg/L (mean±SD). Clearance was high (mean 57.9L/h/kg) exceeding normal human liver blood flow, suggesting low systemic bioavailability; urinary recovery of the thymine dose was low (<1%). Apparent formation rate-limited kinetics were observed for dihydrothymine, and the plasma concentration of dihydrothymine was consistently 10-fold higher than that of thymine. Plasma ß-ureidoisobutyrate concentrations, on the other hand, were similar to that of thymine. Genotyping confirmed that pathological mutations of the DPYD gene were absent. The urinary excretion ratio of thymine/dihydrothymine was informative of the maximum concentration. Saliva thymine was highly variable. These data are potentially useful as a basis for developing of a screening procedure to prospectively identify patients who are at risk of toxicity from fluoropyrimidine drugs.

Clinical consequences of a miscalibrated digoxin immunoassay.

BACKGROUND: A routine audit revealed that the analytical method used to measure digoxin concentrations by our statewide pathology provider in 2009 was underestimating digoxin concentrations by 10%. The assay was recalibrated by the manufacturer in 2010, but clinical outcomes of the underestimation were never measured. This is a pilot study to describe the prescribing behavior around out-of-range digoxin concentrations and to assess whether miscalibrated digoxin immunoassays contribute to clinically relevant effects, as measured by inappropriate alterations in digoxin doses. METHODS: About 30,000 digoxin concentrations across the State Hospital system were obtained in 2 periods before and after recalibration of the digoxin assay. Digoxin concentration means were calculated and compared and were statistically significantly different. Subsequently, a single-centered retrospective review of 50 randomly chosen charts was undertaken to study the clinical implications of the underestimated concentrations. RESULTS: Mean digoxin concentrations for 2009 and 2011 were significantly different by 8.8% (confidence interval, 7.0%-10.6%). After recalculating the 2009 concentrations to their "corrected" values, there was a 16% increase in the number of concentrations within the range when compared with the 2011 concentrations (41.48% versus 48.04%). However, overall, this did not cause unnecessary dose changes in patients who were "borderline" or outside the therapeutic range when compared with controls (P = 0.10). The majority of decisions were based on the clinical impression rather than concentration alone (85.1% versus 14.9%), even when the concentration was outside the "therapeutic range." CONCLUSIONS: Although recalculating digoxin concentrations measured during 2009 to their corrected values produced a significant change in concentration and values inside and outside the range, this does not seem to have had an influence on patient treatment. Rather, clinicians tended to use the clinical impression to dose digoxin.

Monitoring salivary melatonin concentrations in children with sleep disorders using liquid chromatography-tandem mass spectrometry.

BACKGROUND: Melatonin is synthesized in the pineal gland and is an important circadian phase marker, especially in the determination of sleep patterns. Both temporary and permanent abnormal sleep patterns occur in children; therefore, it is desirable to have methods for monitoring melatonin in biological fluids in the diagnosis and treatment of such disorders. OBJECTIVE: The objective of the study is to develop a liquid chromatography-tandem mass spectrometry method for the determination of melatonin in saliva and to apply it to monitoring salivary concentrations in children with sleep disorders. METHODS: A deuterated internal standard (d7-melatonin) was added to a diluted saliva sample (20 µL) in an autosampler vial insert, and 50 µL were injected. Plasticware was strictly avoided, and all glassware was scrupulously cleaned and then baked at 120°C for at least 48 hours to obtain satisfactory performance. Reverse-phase chromatography was performed on a C8 column using a linear gradient elution profile comprising mobile phases A (0.1% aqueous formic acid) and B (15% methanol in acetonitrile containing 0.1% formic acid), pumped at a total flow rate of 0.8 mL/min. The run time was 8 minutes. After atmospheric pressure chemical ionization, mass spectrometric detection was in positive ion mode. Mass detection was by selected reaction monitoring mode with the following mass transitions used for quantification: melatonin, m/z 233.0 → 173.8 and d7-melatonin, m/z 240.0 → 178.3. RESULTS: Linearity (r > 0.999) was established from 3.9 to 1000 pg/mL. Imprecision (coefficient of variation percent) was less than 11%, and accuracy was 100-105% (7.0-900 pg/mL). The method was selective, and the mean (range) ratio of the slopes of calibrations in water to those in daytime saliva samples collected from 10 healthy adult subjects was 0.989 (0.982-0.997), indicating negligible matrix effects. The application of the assay was demonstrated in healthy adults and in children being clinically investigated for sleep disturbances. CONCLUSIONS: A validated liquid chromatography-tandem mass spectrometry method suitable for monitoring salivary melatonin in children with circadian rhythm sleep disorders is reported. The method also has potential application to pediatric population pharmacokinetic studies using sparse sampling of saliva as the biological sample matrix.

Current status of therapeutic drug monitoring in Australia and New Zealand: a need for improved assay evaluation, best practice guidelines, and professional development.

The measurement of drug concentrations, for clinical purposes, occurs in many diagnostic laboratories throughout Australia and New Zealand. However, the provision of a comprehensive therapeutic drug monitoring (TDM) service requires the additional elements of pre- and postanalytical advice to ensure that concentrations reported are meaningful, interpretable, and clinically applicable to the individual patient. The aim of this project was to assess the status of TDM services in Australia and New Zealand. A range of professions involved in key aspects of TDM was surveyed by questionnaire in late 2007. Information gathered included: the list of drugs assayed; analytical methods used; interpretation services offered; interpretative methods used; and further monitoring advice provided. Fifty-seven responses were received, of which 42% were from hospitals (public and/or private); 11% a hospital (public and/or private) and pathology provider; and 47% a pathology provider only (public and/or private). Results showed that TDM is applied to a large number of different drugs. Poorly performing assay methods were used in some cases, even when published guidelines recommended alternative practices. Although there was a wide array of assays available, the evidence suggested a need for better selection of assay methods. In addition, only limited advice and/or interpretation of results was offered. Of concern, less than 50% of those providing advice on aminoglycoside dosing in adults used pharmacokinetic tools with six of 37 (16.2%) respondents using Bayesian pharmacokinetic tools, the method recommended in the Australian Therapeutic Guidelines: Antibiotic. In conclusion, the survey highlighted deficiencies in the provision of TDM services, in particular assay method selection and both quality and quantity of postanalytical advice. A range of recommendations, some of which may have international implications, are discussed. There is a need to include measures of impact on clinical decision-making when assessing assay methodologies. Best practice guidelines and professional standards of practice in TDM are needed, supported by an active program of professional development to ensure the benefits of TDM are realized. This will require significant partnerships between the various professions involved.

Simultaneous liquid chromatographic determination of doxorubicin and its major metabolite doxorubicinol in parrot plasma.

A new microscale method is reported for the determination of doxorubicin and its active metabolite, doxorubicinol, in parrot plasma. Sample workup involved acetonitrile protein precipitation, ethyl acetate extraction, followed by back extraction into HCl. Separations were achieved on a phenyl-hexyl column at 30 degrees C using acetonitrile (17%, v/v) in 0.01 M orthophosphoric acid (83%, v/v) delivered via a linear flow program. Fluorometric detection wavelengths were 235 nm (excitation) and 550 nm (emission). Calibration plots were linear (r2>0.999), and recoveries were 71-87% from 20 to 400 ng/mL. Assay imprecision was