KA-672 inhibits rat brain acetylcholinesterase in vitro but not in vivo.

KA-672, a lipophilic benzopyranone derivative which is currently under development as a cognitive enhancer and antidementia drug, has previously been shown to have facilitatory effects on learning and memory in rats at doses of 0.1-1 mg/kg. We now report that KA-672 inhibited the activity of acetylcholinesterase (AChE), measured in vitro in rat brain cortical homogenate, with an IC50 value of 0.36 microM indicating that KA-672 may improve cognitive functions as a consequence of AChE inhibition. However, when we employed the microdialysis procedure to monitor acetylcholine (ACh) release from rat hippocampus, no effect of KA-672 (0.1-10 mg/kg) was found, indicating a lack of inhibition of brain AChE under in vivo-conditions. [14C]-labelled KA-672 was found to easily penetrate the blood-brain barrier, and an apparent concentration of 0.22 nmol/g brain (equivalent to 0.39 microM tissue concentration) was calculated following an i.p. injection of 1 mg/kg KA-672. However, no labelled substance could be detected in hippocampal microdialysates or in cerebrospinal fluid (CSF) taken from the cisterna magna, indicating that the concentration of KA-672 in brain extracellular fluid must have been below 0.01 microM. We conclude that KA-672 is a potent AChE inhibitor, an activity which, however, does not contribute to its behavioural effects in vivo because the lipophilic drug does not reach sufficient concentrations in the extracellular fluid, apparently due to cellular sequestration.