Study on brain uptake of local anesthetics in rats

Brain uptake of local anesthetics under steady-state plasma condition and/or following intravenous bolus administration was investigated in rats. All ester-type anesthetics examined such as ethyl (Et), propyl (Pr), butyl (Bu) esters of p-aminobenzoic acid (PABA), and procaine disappeared rapidly from plasma in a dose-dependent manner. Plasma profiles of these compounds were well explained by a 2-compartment model with a Michaelis-Menten type elimination process from a central compartment. On the other hand, lidocaine, amide-type anesthetic, showed a linear pharmacokinetic characteristic and its half life in the elimination phase was far longer than those of ester type agents. Extents of brain uptake (brain-to-plasma partition coefficient, Kp value) of these drugs were determined at 3 different steady-state plasma concentrations (1-15 microM). The Kp value of each drug was similar under the three different steady-state plasma concentrations. The Kp value increased in the following order; procaine (1.1) less than PABA-Et (1.9) less than lidocaine (2.2) less than PABA-Pr (2.7) less than PABA-Bu (3.6). A linear relationship was observed between the Kp value and the logarithmic value of the partition coefficient obtained in n-heptane/water or n-octanol/water partition system. The value of PABA-Et and PABA-Bu following intravenous bolus administration were varied with time elapsed but the mean values were almost same with those obtained under steady-state plasma conditions.