Pharmacokinetics and brain dispositions of tacrine and its major bioactive monohydroxylated metabolites in rats.

The current study aims to investigate the pharmacokinetics and brain disposition of tacrine and its three major bioactive monohydroxylated metabolites (1-hydroxytacrine, 2-hydroxytacrine and 4-hydroxytacrine). An assay for simultaneous quantification of tacrine and three above metabolites in rat plasma and brain tissue was developed. Four analytes together with internal standard were extracted from rat plasma or brain tissue homogenate by liquid-liquid extraction using ethyl acetate. Baseline separation of the four studied compounds was achieved by a Thermo Hypersil BDS C(18) column with gradient elution using acetonitrile and ammonium formate-triethylamine (pH 4.0) under fluorescence detection. Extraction recoveries of all analytes ranged from 82.1% to 93.2% in both rat plasma and brain tissue. The intra- and inter-day precision and accuracy of each analyte at lower limit of quantification (LLOQ) and three quality control (QC) concentrations (low, middle and high) was within 12% RSD and within 11% bias in both biological matrices. The LLOQ for tacrine, 1-hydroxytacrine, 2-hydroxytacrine and 4-hydroxytacrine were found to be 2.5, 6.7, 2.1 and 2.1 ng/ml in plasma, and 12.3, 33.5, 10.6 and 10.5 ng/g in brain tissue, respectively. All four analytes were stable during analysis. The developed method provides a simple, sensitive and reproducible procedure for pharmacokinetics and brain disposition study of tacrine and its three major metabolites in rats after oral administration of tacrine. Pharmacokinetic study demonstrated that tacrine could be quickly absorbed and extensively metabolized to its monohydroxylated metabolites in vivo. In addition, rat brain disposition study showed that tacrine and its monohydroxylated metabolites were evenly distributed in different brain regions except for a slight lower concentration of tacrine in olfactory region. Moreover, tacrine and 4-hydroxytacrine exhibited a much higher brain-to-plasma ratio than that of 1-hydroxytacrine and 2-hydroxytacrine.