Lipophilicities of baclofen ester prodrugs correlate with affinities to the ATP-dependent efflux pump P-glycoprotein: relevance for their permeation across the blood-brain barrier?

PURPOSE: 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).
METHODS: 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.
RESULTS: 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.
CONCLUSION: 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.