Simultaneous Quantification of Linezolid, Tinidazole, Norfloxacin, Moxifloxacin, Levofloxacin, and Gatifloxacin in Human Plasma for Therapeutic Drug Monitoring and Pharmacokinetic Studies in Human Volunteers

Background:Linezolid may be administered in combination with norfloxacin, gatifloxacin, levofloxacin, moxifloxacin, and tinidazole for the treatment of various infections, such as urinary and respiratory tract infections, to improve the efficacy of the treatment or to reduce the duration of therapy. Knowledge of the antibiotic plasma concentrations combined with bacterial susceptibility evaluated in terms of minimum inhibitory concentration would optimize treatment efficacy while limiting the risk of dose-related adverse effects and avoiding suboptimal concentrations.Methods:A new high-performance liquid chromatography assay method was developed and validated for determination of the above-mentioned drugs in small samples of human plasma. After protein precipitation with acetonitrile:methanol (1:1, vol/vol), satisfactory separation was achieved on a Hypersil BDS C-18 column (250 x 4.6 mm, 5 m) using a mobile phase comprising 20 mM sodium dihydrogen phosphate-2 hydrate (pH = 3.2) and acetonitrile at a ratio of 75:25, vol/vol; the elution was isocratic at ambient temperature with a flow rate of 1.5 mL/min. The ultraviolet detector was set at 260 nm. The validated method was applied to assay real plasma samples used for pharmacokinetic studies and therapeutic drug monitoring of the selected drugs.Results:The assay method described was found to be rapid, sensitive, reproducible, precise, and accurate. Linearity was demonstrated over the concentration ranges as follows: 0.1-30 g/mL for linezolid and tinidazole; 0.05-5 g/mL for norfloxacin; and 0.1-10 g/mL for moxifloxacin, levofloxacin, and gatifloxacin (mean r(2) = 0.9999, n = 12). The observed within- and between-day assay precisions were within 12.5%, whereas accuracy ranged between 92.0% and 112% for all the analytes. The lower limit of quantification was 0.1 g/mL for all the analytes except norfloxacin which was 0.05 g/mL.Conclusions:This assay method was valid within a wide range of plasma concentrations and may be proposed as a suitable method for pharmacokinetic studies, therapeutic drug monitoring implementation, and routine clinical applications, especially for some populations of patients who receive a combination of these drugs.