9-Amino-1,2,3,4-tetrahydroaminoacridine (THA) in combination with
lecithin has been reported to improve the memory of
Alzheimer's disease patients. We have examined some properties of THA in vitro and in vivo so as to define some of the mechanism(s) by which THA might produce its
therapeutic effects. In vitro, THA was more potent at inhibiting human plasma
cholinesterase (IC50 = 0.03 microM) than human erythrocyte
acetylcholinesterase (IC50 = 0.3 microM) and rat brain
acetylcholinesterase (IC50 = 0.32 microM). Radioligand binding studies indicated that THA binds reversibly and competitively to primary M1 and M2 human cortical
muscarinic receptors with similar affinities. Moreover, THA showed similar affinity for temporal cortices
muscarinic receptors from Alzheimer and non-Alzheimer (control) brains. In vivo, subcutaneous administration of THA (1-8 mg/kg
body weight) to adult rats (6 months old) produced a dose dependent decrease in general activity compared to saline-treated rats. However, at a concentration of 0.5 mg/kg
body weight, the general activity of the rats was increased compared to saline-treated rats. The cognitive function of the THA-treated adult rats (subcutaneously 2 mg/kg
body weight) was not significantly improved compared to saline-treated rats. It is concluded that the mechanisms of action of THA on the
cholinergic system involve reversible inhibition of
cholinesterases and reversible and competitive interaction with
muscarinic acetylcholine receptors. These effects might be of therapeutic value in the treatment of
Alzheimer's disease.