Epidural and intrathecal n-butyl-p-aminobenzoate solution in the rat. Comparison with bupivacaine.

Background: Epidural administration of an aqueous suspension of n-butyl-p-aminobenzoate (BAB) to humans results in long-lasting sensory blockade without motor block. The dose-response of BAB administered epidurally and intrathecally as a solution was studied in rats to define the local anesthetic properties in an established animal model.
Methods: The time course of changes in tail withdrawal latency and motor function were determined in rats after epidural or intrathecal administration of solutions of BAB or bupivacaine. The dose-response relation was determined and median effective dose values were calculated.
Results: After epidural and intrathecal administration of BAB solutions, the onset and duration of the antinociceptive action were comparable to bupivacaine. Median effective dose values for tail-withdrawal latency of 6 s or more were significantly greater for BAB. After both routes of administration, BAB clearly affected motor function.
Conclusions: When administered epidurally and intrathecally as a solution, BAB is a local anesthetic of relative low potency with onset and duration of action comparable to those of bupivacaine. These findings suggest that the long-lasting action obtained after applying BAB suspension results from the slow dissolution (continuous release) of the solid BAB deposited in the epidural space.