Influence of ultrafiltration conditions on the measurement of unbound drug concentrations: flucloxacillin as an example.

BACKGROUND: When performing therapeutic drug monitoring (TDM) for flucloxacillin, it is advised to measure the unbound, not the total, flucloxacillin concentration. To be able to accurately quantify unbound flucloxacillin concentrations, a reliable analytical method is indispensable. OBJECTIVE: To determine the influence of temperature and pH of the sample during ultrafiltration on the measured unbound fraction of flucloxacillin. MATERIALS AND METHODS: We performed three different experiments. In a single laboratory experiment, we investigated the influence of ultrafiltration temperature (10°C, room temperature and 37°C) on the measured unbound fraction of flucloxacillin for three concentration levels. In a multiple laboratory experiment, the results of eight laboratories participating in an international quality control programme measuring unbound flucloxacillin concentrations were analysed. In the third experiment, patient samples were ultrafiltrated using four different conditions: (i) physiological pH and room temperature; (ii) unadjusted pH (pH 9 after freezing) and room temperature; (iii) physiological pH and 37°C and (iv) unadjusted pH and 37°C. RESULTS: For all experiments, measurement of samples that were ultrafiltrated at room temperature resulted in a substantially lower unbound fraction compared to samples that were ultrafiltrated at 37°C. Adjusting the pH to physiological pH only had a minimal impact on the measured unbound fraction. CONCLUSIONS: On the basis of these findings and considering the need for fast, simple and reproducible sample pretreatment for TDM purposes, we conclude that ultrafiltration of flucloxacillin should be performed at physiological temperature (37°C), but adjustment of pH does not seem to be necessary.

Highly sensitive quantification of pemetrexed in human plasma using UPLC-MS/MS to support microdosing studies.

Pemetrexed is an antifolate drug approved for the treatment of non-small-cell lung cancer and mesothelioma. Assessing pemetrexed pharmacokinetics after administration of a microdose (100 µg) may facilitate drug-drug interaction and dose individualization studies with cytotoxic drugs, without causing harm to patients. Therefore, a highly sensitive bioanalytical assay is required. A reversed-phase ultra-high performance liquid chromatography method was developed to determine pemetrexed concentrations in human ethylenediaminetetraacetic acid-plasma after microdosing. [13 C5 ]-Pemetrexed was used as the internal standard. The sample preparation involved solid-phase extraction from plasma. Detection was performed using MS/MS in a total run time of 9.5 min. The assay was validated over the concentration range of 0.0250-25.0 µg/L pemetrexed. The average accuracies for the assay in plasma were 96.5 and 96.5%, and the within-day and between-day precision in coefficients of variations was <8.8%. Extraction recovery was 59 ± 1 and 55 ± 5% for pemetrexed and its internal standard. Processed plasma samples were stable for 2 days in a cooled autosampler at 10°C. The assay was successfully applied in a pharmacokinetic curve, which was obtained as a part of an ongoing clinical microdosing study.

Protein binding of rifampicin is not saturated when using high-dose rifampicin.

BACKGROUND: Higher doses of rifampicin are being investigated as a means to optimize response to this pivotal TB drug. It is unknown whether high-dose rifampicin results in saturation of plasma protein binding and a relative increase in protein-unbound (active) drug concentrations. OBJECTIVES: To assess the free fraction of rifampicin based on an in vitro experiment and data from a clinical trial on high-dose rifampicin. METHODS: Protein-unbound rifampicin concentrations were measured in human serum spiked with increasing total concentrations (up to 64 mg/L) of rifampicin and in samples obtained by intensive pharmacokinetic sampling of patients who used standard (10 mg/kg daily) or high-dose (35 mg/kg) rifampicin up to steady-state. The performance of total AUC0-24 to predict unbound AUC0-24 was evaluated. RESULTS: The in vitro free fraction of rifampicin remained unaltered (∼9%) up to 21 mg/L and increased up to 13% at 41 mg/L and 17% at 64 mg/L rifampicin. The highest (peak) concentration in vivo was 39.1 mg/L (high-dose group). The arithmetic mean percentage unbound to total AUC0-24in vivo was 13.3% (range = 8.1%-24.9%) and 11.1% (range = 8.6%-13.6%) for the standard group and the high-dose group, respectively (P = 0.214). Prediction of unbound AUC0-24 based on total AUC0-24 resulted in a bias of -0.05% and an imprecision of 13.2%. CONCLUSIONS: Plasma protein binding of rifampicin can become saturated, but exposures after high-dose rifampicin are not high enough to increase the free fraction in TB patients with normal albumin values. Unbound rifampicin exposures can be predicted from total exposures, even in the higher dose range.

Rifampicin Alters Metformin Plasma Exposure but Not Blood Glucose Levels in Diabetic Tuberculosis Patients.

The pharmacokinetic (PK) and clinical implications of combining metformin with rifampicin are relevant to increasing numbers of patients with diabetic tuberculosis (TB) across the world and are yet unclear. We assessed the impact of rifampicin on metformin PKs and its glucose-lowering effect in patients with diabetic TB by measuring plasma metformin and blood glucose during and after TB treatment. Rifampicin increased metformin exposure: plasma area under the plasma concentration-time curve from time point 0 to the end of the dosing interval (AUC0-τ ) and peak plasma concentration (Cmax ) geometric mean ratio (GMR; during vs. after TB treatment) were 1.28 (90% confidence interval (CI) 1.13-1.44) and 1.19 (90% CI 1.02-1.38; n = 22). The metformin glucose-lowering efficacy did not change (Δglucose - Cmax ; P = 0.890; n = 18). Thus, we conclude that additional glucose monitoring in this population is not warranted. Finally, 57% of patients on metformin and rifampicin, and 38% of patients on metformin alone experienced gastrointestinal adverse effects. Considering this observation, we advise patients to take metformin and rifampicin with food and preferably separated in time. Clinicians could consider metoclopramide if gastrointestinal adverse effects occur.

Determination of protein-unbound, active rifampicin in serum by ultrafiltration and Ultra Performance Liquid Chromatography with UV detection. A method suitable for standard and high doses of rifampicin.

Rifampicin is the most important antibiotic in use for the treatment of tuberculosis (TB). Preclinical and clinical data suggest that higher doses of rifampicin, resulting in disproportionally higher systemic exposures to the drug, are more effective. Serum concentrations of rifampicin are the intermediary link between the dose administered and eventual response and only protein-unbound (free) rifampicin is pharmacologically active. The objective of this work was to develop an ultra performance liquid chromatography assay for protein-unbound rifampicin in serum with ultrafiltration, carried out at a sample temperature of 37°C, suitable for measurement of concentrations achieved after currently used and higher doses of rifampicin. Human serum was equilibrated at 37°C and ultrafiltrated at the same temperature in a Centrifree YM-30 ultrafiltration device, followed by dilution of the ultrafiltrate with methanol and ascorbic acid. Unbound rifampicin was analyzed using ultra performance liquid chromatography with a BEH C18 column, isocratic elution and ultra-violet (UV) detection. The run time was 5min. The assay was linear over the concentration range of 0.065-26mg/L rifampicin in ultrafiltrate. Accuracies for measurement of rifampicin in ultrafiltrate were 97% and 102% at the higher and lower limits of quantitation. Accuracy of the ultrafiltration process cannot be established, as it is not possible to spike blank serum with known amounts of protein-unbound rifampicin. Within- and between-day precision of the method including ultrafiltration as well as after ultrafiltration were within prespecified limits (CV<10%). Dilution of the ultrafiltrate with methanol and ascorbic acid kept rifampicin in solution and prevented it from degradation. Rifampicin loss during the ultrafiltration process and variation in analytical results when using two different batches of ultrafiltration devices were both limited. Processed ultrafiltrate samples were stable for 3days in the autosampler. The developed method can be applied in pharmacokinetic research, studying exposure-response relationships for rifampicin when administered at higher than currently used doses.

Drug-drug interactions between raltegravir and pravastatin in healthy volunteers.

BACKGROUND: To evaluate the potential drug-drug interaction between raltegravir and pravastatin. METHODS: This was an open-label, randomized, 3-period, cross-over, single-centre trial in 24 healthy volunteers. Subjects received the following treatments: pravastatin 40 mg every day for 4 days, raltegravir 400 mg twice a day for 4 days, and pravastatin 40 mg every day + raltegravir 400 mg twice a day for 4 days. The treatments were separated by washout periods of 10 days. On day 4 of each treatment period, blood samples for pharmacokinetics were collected throughout a 24-hour period. RESULTS: Geometric mean ratios (90% confidence interval) for pravastatin + raltegravir versus pravastatin alone were 0.96 (0.83 to 1.11) for AUC0-24 and 1.04 (0.85 to 1.26) for Cmax. The mean low-density lipoprotein cholesterol decrease after 4 days of pravastatin was 0.42 mmol/L both in the presence and the absence of raltegravir. The geometric mean ratio (90% confidence interval) AUC0-12, Cmax, and C12 for raltegravir + pravastatin versus raltegravir alone were 1.13 (0.77 to 1.65), 1.31 (0.81 to 2.13), and 0.59 (0.39 to 0.88), respectively. CONCLUSIONS: Raltegravir did not influence the pharmacokinetics or the short-term lipid-lowering effects of pravastatin, whereas pravastatin increased the Cmax but decreased the C12 of raltegravir. The effects of pravastatin on raltegravir pharmacokinetics are not likely to be clinically relevant.

The effect of raltegravir on the glucuronidation of lamotrigine.

The authors studied the effect of raltegravir on the pharmacokinetics of the antiepileptic agent lamotrigine. Twelve healthy volunteers (group A) received 400 mg raltegravir twice daily from days 1 to 5. On day 4, a single dose of 100 mg lamotrigine was administered. After a washout period, participants received a second single dose of 100 mg of lamotrigine but now without raltegravir (day 32). In group B, 12 participants received the same treatment as in group A but in reverse order. On days 4 and 32, 48-hour pharmacokinetic curves were drawn. Geometric mean ratios (+90% confidence intervals [CIs]) of lamotrigine area under the plasma concentration-time curve (AUC(0-->48)) and peak plasma concentration (C(max)) for raltegravir + lamotrigine versus lamotrigine alone were 0.99 (0.96-1.01) and 0.94 (0.89-0.99), respectively. The mean ratio of the AUC(0-->48) of lamotrigine-2N-glucuronide to lamotrigine was similar when lamotrigine was taken alone (0.35) or when taken with raltegravir (0.36). Raltegravir does not influence the glucuronidation of lamotrigine.

Lopinavir/ritonavir reduces lamotrigine plasma concentrations in healthy subjects.

BACKGROUND: Limited data are available about the effect of lopinavir and low-dose ritonavir on glucuronidation. Lamotrigine undergoes glucuronidation. We studied the effect of lopinavir/ritonavir on the pharmacokinetics of lamotrigine and vice versa. METHODS: Twenty-four healthy subjects received 50 mg lamotrigine once daily on days 1 and 2 and 100 mg twice daily on day 3 through day 23. Lopinavir (400 mg twice daily)/ritonavir (100 mg twice daily) was added on day 11. Depending on the decrease in lamotrigine trough level between days 10 and 20, either the study was stopped (<20% decrease) or a dose increase was applied from day 23 to day 31, as follows: increase to 150 mg lamotrigine twice daily if there was a 20% to 33% decrease, increase to 200 mg twice daily if there was a 34% to 66% decrease, and increase to 300 mg twice daily if there was a greater than 66% decrease. On days 10, 20, and 31, 12-hour pharmacokinetic curves were drawn. RESULTS: The mean decrease in lamotrigine trough level between days 10 and 20 was 55.4% (n = 18). A dose increment to 200 mg lamotrigine twice daily was used in all subjects. The area under the plasma concentration-time curve (AUC) values of lamotrigine on day 20 (with lopinavir/ritonavir) and day 10 (without lopinavir/ritonavir) were bioinequivalent, with a point estimate of 0.50 (90% confidence interval, 0.47-0.54). After dose adjustment of lamotrigine to 200 mg twice daily, the AUC on day 31 (n = 15) was bioequivalent to that on day 10, with a point estimate of 0.91 (90% confidence interval, 0.82-1.02). The median AUC ratios of lamotrigine 2N-glucuronide to lamotrigine on day 10 and day 20 were 0.57 (interquartile range, 0.39-0.75) and 1.12 (interquartile range, 0.87-1.31). Pharmacokinetic parameters for lopinavir/ritonavir were similar to historical controls. CONCLUSION: Lopinavir/ritonavir decreases the AUC of lamotrigine, probably by induction of glucuronidation. A dose increment to 200% of the initial lamotrigine dose is needed to achieve concentrations similar to those with lamotrigine alone. Lamotrigine does not appear to affect the pharmacokinetics of lopinavir/ritonavir.

International interlaboratory quality control program for measurement of antiretroviral drugs in plasma.

An international interlaboratory quality control program for measurement of antiretroviral drugs was initiated. The first round was confined to protease inhibitors and showed large variability in the performance of participating laboratories. The results demonstrate the need for and utility of an ongoing quality control program in this area of bioanalysis.