TETRAHYDROAMINACRIN AS A DECURARISING AGENT

The pharmacology of tacrine at N-methyl-d-aspartate receptors

The mechanism of tacrine as a precognitive drug has been considered to be complex and not fully understood. It has been reported to involve a wide spectrum of targets involving cholinergic, gabaergic, nitrinergic and glutamatergic pathways. Here, we review the effect of tacrine and its derivatives on the NMDA receptors (NMDAR) with a focus on the mechanism of action and biological consequences related to the Alzheimer's disease treatment. Our findings indicate that effect of tacrine on glutamatergic neurons is both direct and indirect. Direct NMDAR antagonistic effect is often reported by in vitro studies; however, it is achieved by high tacrine concentrations which are not likely to occur under clinical conditions. The impact on memory and behavioral testing can be ascribed to indirect effects of tacrine caused by influencing the NMDAR-mediated currents via M1 receptor activation, which leads to inhibition of Ca²⁺-activated potassium channels. Such inhibition prevents membrane repolarization leading to prolonged NMDAR activation and subsequently to long term potentiation. Considering these findings, we can conclude that tacrine-derivatives with dual cholinesterase and NMDARs modulating activity may represent a promising approach in the drug development for diseases associated with cognitive dysfunction, such as the Alzheimer disease.

Enzyme Inhibiting Action of Tetrahydroaminoacridine and its Structural Fragments

Tetrahydro-5-aminoacridine and four compounds representing its structural fragments have been compared as inhibitors of acetylcholinesterase and of monoamine oxidase. The entire structure of tetrahydro-5-aminoacridine appears to be essential for optimal inhibition of the esterase, less than 10−6 m concentration showing a 50 per cent inhibition of the enzyme, with the inhibition constant Ki as 1 × 10−4. For optimum inhibition of monoamine oxidase, the 4-aminoquinoline part of the acridine molecule appears to be a structural requirement. 4-Arninoquinoline shows a stronger monoamine oxidase inhibition than any known therapeutically used inhibitor. It gives a 50 per cent inhibition of the oxidase in 10−6 m concentration, with ki as 1.1 × 10−5.

Estimation and Urinary Excretion of Tetrahydroaminoacrxdine

Two methods for the quantitative determination of tetrahydroamino-acridine in aqueous solutions and in urine are described. Four metabolites have been isolated from the rat urine. Two of these, constituting the major proportion of the total metabolites, have also been isolated from human urine and partially characterised.

Distribution of tacrine across the blood-brain barrier in awake, freely moving rats using in vivo microdialysis sampling

Microdialysis was used to sample simultaneously the distribution of THA (9-amino-1,2,3,4-tetrahydroacridine; Tacrine), a potential anti-Alzheimer agent, both in blood and across the blood-brain barrier of anesthetized and awake, freely moving rats. Microdialysis probes were implanted in the jugular vein and dorsal hippocampus and dialysis samples were simultaneously collected from both sites. Dialysis samples were analyzed using a microbore column chromatographic assay with a detection limit of 0.3 ng/ml. Pharmacokinetic parameters were calculated after a 1 mg/kg intravenous dose of THA. Plasma pharmacokinetics followed a biexponential mode, with t1/2(dis.) = 8.4 +/- 2.7 min and t1/2(elim.) = 76.7 +/- 24.2 min for awake, freely moving rats. THA rapidly penetrated the blood-brain barrier, with maximum concentrations attained within 60 min post-dose. In the brain of awake, freely moving rats t1/2(abs.) was 26.0 +/- 5.2 min and t1/2(elim.) was 99.1 +/- 17.7 min. THA levels in hippocampus extracellular fluid were 10 times lower than those in plasma. For anesthetized rats, the t1/2(elim.) in blood was 154.8 +/- 46.8 min, while in the hippocampus t1/2(elim.) was 159.5 +/- 31.7 min. The binding of THA in rat plasma was 56.2 +/- 5.0%, while the fraction bound to rat whole blood was 73.3 +/- 4.1% as determined by microdialysis and ultrafiltration.

Identification of 9-hydroxylamine-1,2,3,4-tetrahydroacridine as a hepatic microsomal metabolite of tacrine by high-performance liquid chromatography and electrochemistry

Amperometric detection using a dual-electrode thin-layer cell in the series configuration can aid in the identification of unknown components in complicated samples by voltammetric characterization. This is shown by studying the metabolism of tacrine by rat hepatic microsomes using high-performance liquid chromatography with electrochemical detection. The major metabolite detected in microsomal incubations did not co-elute with any standard acridine available and was produced in too small a quantity for mass spectral characterization. Tentative identification of this metabolite as 9-hydroxylamine-1,2,3,4-tetrahydroacridine was made by electrochemical characterization. The electrochemistry of the metabolite was compared to that of the hydroxylamine produced and studied by cyclic voltammetry.

Further analysis of the neuropharmacological profile of 9-amino-1,2,3,4-tetrahydroacridine (THA), an alleged drug for the treatment of Alzheimer's disease

In a recent study we have documented the acetylcholinesterase and outward K+-current inhibiting activity of 9-amino-1,2,3,4-tetrahydroacridine (THA), a drug reportedly active in the treatment of Alzheimer patients. In the present study we investigated the effects of THA on the uptake and release of radiolabeled NA, DA and 5-HT. THA concentration-dependently inhibited the uptake of these monoamines with IC-50 values of approximately 1, 7 and 2 microM respectively. Release studies of these radiolabeled monoamines from control and reserpine pretreated tissue revealed that the THA-induced uptake inhibition does not occur at the level of the axonal membrane but at the level of the monoaminergic storage granules. In addition the affinity of THA for alpha-1, alpha-2 and beta-adrenoceptors, for D-2 dopamine, S-1a and S-2 serotonin and for muscarinic receptors was investigated. It appeared that in concentrations up to 1 microM THA did not display any affinity towards these receptors. It is concluded from these experiments that the effects of THA on monoaminergic neurotransmission might contribute to the alleged therapeutic action of THA in Alzheimer's disease.

Suxamethonium apnoea masked by tetrahydroaminacrine

A case is reported of a patient who was given THA with suxamethonium and who demonstrated a prolonged apnoea. This patient was later shown to have a variant cholinesterase enzyme. The different techniques of THA and suxamethonium administration are discussed and it is suggested that an initial dose of suxamethonium should precede a THA-suxamethonium combination in order to try and avoid a prolonged apnoea.

Specific drug interactions in anaesthesia

The sites and mechanisms of the interaction of drugs have been discussed with special relevance to the work of the anaesthetist.

Anticurare activity of tacrine (THA) in vitro

Tacrine (tha) has been found to be a relatively weak anticurare agent having a significant anticurare activity on the rat isolated phrenic nerve diaphragm preparation in concentrations above 10−7 m. It is about 20 times less potent than neostigmine. Between 10−5 m and 10−4 m, anticurare activity of tha ceases to be reproducible, and in concentrations above 10−4 m the compound exerts a depressant action directly on the muscle. The qualitative and quantitative differences between the anticurare activities of tha and neostigmine are discussed.