Abstract
Distribution to the effect site is a prerequisite for the therapeutic effect and determined by physicochemical properties and affinities to inside- and outside-directed membrane transporters. Based on the hypothesis that lipophilic esters of the GABA-derivative baclofen have a higher affinity to brain tissue, baclofen esters (methyl, ethyl, 1-propyl, 2-propyl, butyl) were studied regarding their penetration through the blood-brain barrier and their affinities to P-glycoprotein (P-gp). Octanol-water distribution coefficients (D) served as lipophilicity parameters. Blood and brain concentrations of baclofen and its methyl ester were determined in vivo in rats following intraperitoneal administration. Affinities to P-gp were evaluated using a radioligand binding assay based on P-gp-overexpressing cells and [3H]-talinolol as radioligand. Log D values for baclofen and ester derivatives were -0.96 (baclofen), 0.48 (methyl), 0.77 (ethyl), 1.31 (1-propyl), 1.27 (2-propyl), and 1.42 (butyl). In-vitro studies yielded negligible affinity of baclofen to P-gp, whereas IC50-values for the esters ranged between 1300 microM (methyl) and 290 microM (2-propyl). Affinity parameters correlated well with the lipophilicity parameters. Despite the P-gp affinity, brain concentrations of methyl ester were significantly higher than those of baclofen, however, baclofen levels following administration of the ester were smaller than with baclofen administration indicating only partial hydrolysis.
Published Version
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