A Liquid Chromatographic Method for Quantifying Unbound Micafungin in Human Plasma and the Relationship between the Concentrations of Unbound and Total Micafungin in Plasma

This report describes a modified method for the quantitative determination of unbound micafungin (MCFG) in human plasma that is unrelated to chemical methods currently in use, and the relationship between the concentration of unbound and total MCFG in plasma of the patients. The mobile phase consisted of 50 mm phosphate buffer:tetrahydrofuran (65:35, v/v). Samples were fractionated on a C-18 column. The fluorescence detection wavelengths of excitation and emission were set at 273 nm and 464 nm, respectively. The retention times of MCFG and 1-hydroxy-2-naphtoeic acid (internal standard: IS) were 10.5 min and 7.3 min, respectively. For each concentration of MCFG/IS, the peak height ratio on a 5-point calibration curve was linear from 0.004 to 0.08 mu g/mL (r=0.999, p<0.001). In addition, the concentrations of unbound and total MCFG were measured in the plasma of 11 patients treated with MCFG for fungal infection. In total, 99 samples were collected. The concentration of unbound and total MCFG in plasma correlated with one another (r=0.896, p<0.001). These concentrations were not affected by serum albumin, total bilirubin, blood urea nitrogen, or creatinine clearance. There were small differences of [fu (unbound MCFG/total MCFG) X 100%] in the every term after start of treatment of MCFG. Further, there was no difference in the unbound and total concentration of MCFG in plasma between the effective group and the ascertained effectiveness group. The concentration of MCFG in plasma could be used as an indicator of clinical effect as a substitute for the concentration of unbound MCFG in plasma.