25-deacetylrifampicin and 14-hydroxyclarithromycin

Clarithromycin (CLA) is a well established macrolide antibiotic which is frequently used in therapy of airway diseases in foals. It is extensively metabolized by CYP3A4 resulting in the antimicrobial active metabolite 14-hydroxyclarithromycin (OH-CLA). Rifampicin (RIF) is often comedicated to prevent resistance and augment therapy. RIF is a known inducer for metabolizing enzymes and transporter proteins. Therefore, comedication might bare the risks of pharmacokinetic drug interactions which were investigated in a clinical trial. As no adequate method to determine CLA, RIF and their main metabolites OH-CLA and 25-O-desacetylrifampicin (DAc-RIF) were described so far, we developed a selective and sensitive assay to measure concentrations of all four substances simultaneously in plasma, epithelial lining fluid (ELF) and broncho-alveolar cells (BAC) of foals. Drugs were measured after extraction with methyl tert-butyl ether using roxithromycin as internal standard and liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) for detection. The chromatography was done isocratically using 25mM ammonium acetate buffer (pH 4)/acetonitrile (45%/55%, flow rate 200μl/min). The MS/MS analysis was performed in the positive ion mode (m/z transitions: CLA, 748.5-590.1; OH-CLA, 764.1-606.1; RIF, 823.1-791.2; DAc-RIF, 781.1-749.1 and 837.3-679.2 for the internal standard). The method was validated according to selectivity, linearity, accuracy, precision, recovery, matrix effects and stability. The validation ranges for all substances were 2.5-25 for the low and 25-250ng/ml for the high validation range. The described assay was shown to be valid and successfully applied to measure disposition of CLA, OH-CLA, RIF and DAc-RIF in plasma, ELF and BAC of foals in a clinical trial.

Topics: Animals; Anti-Bacterial Agents; Biotransformation; Bronchioles; Bronchoalveolar Lavage Fluid; Chromatography, High Pressure Liquid; Clarithromycin; Drug Interactions; Drug Stability; Horse Diseases; Horses; Limit of Detection; Pulmonary Alveoli; Reproducibility of Results; Respiratory Tract Infections; Rifampin; Spectrometry, Mass, Electrospray Ionization; Tandem Mass Spectrometry

Clarithromycin and rifampicin are used for the treatment of Mycobacteria. Pharmacokinetic drug interaction is possibly due to the influence of the two drugs on the liver enzymes. Using a Hypurity Aquastar C18 column (50mm×2.1mm×5μm) for liquid chromatography including a polar end-capped phase for the determination of clarithromycin, rifampicin and their metabolites together in plasma using LC-MS/MS resulted in a substantial carry-over. As a consequence, the throughput of the method is not assured. Using a step-by-step troubleshooting procedure, such carry-over was found originating from column memory effect. With the use of another type of C18 column, the carry-over is eliminated. Due to the absence of carry-over, the analytical concentration ranges are extended and are therefore more appropriate for the analysis of patient samples. The method was re-validated for linearity, reproducibility and dilution integrity.

Topics: Chromatography, Liquid; Clarithromycin; Humans; Linear Models; Reproducibility of Results; Rifampin; Sensitivity and Specificity; 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.

Topics: Anti-Bacterial Agents; Chromatography, Liquid; Clarithromycin; Humans; Rifampin; Tandem Mass Spectrometry