A novel anticholinesterase THB013: Biochemical and behavioural studies

Summary Clinical trials with tacrine (THA) have resulted in elevations of liver enzymes in Alzheimer patients that showed improvement. In an effort to minimize these side effects several THA analogues were synthesized. These analogues were compared to THA in biochemical as well as behavioural studies. In this study, the biochemical effects of THA and one of these analogs, THB 013, on plasma cholinesterase activity, cholinergic receptors as well as the effect of these drugs on spatial learning in adult rats were examined. THB 013 was, at lower concentration, more efficacious in inhibiting plasma cholinesterase as well as blocking the scopolamine induced disruption of spatial learning when administered 10 min before the scopolamine injection. It is possible that THB 013 with more potent cholinergic effects than THA might be useful in the treatment of Alzheimer's disease.     

Anatomy of cholinesterase inhibition in Alzheimer's disease: effect of physostigmine and tetrahydroaminoacridine on plaques and tangles

The histochemical distribution of cholinesterases in the cerebral cortex and their response to cholinesterase inhibitors such as physostigmine and tetrahydroaminoacridine (THA) were investigated in brains from patients with Alzheimer's disease and control subjects. In the temporal neocortex of the control subjects, most of the cholinesterase activity was located within axons and cell bodies belonging to cholinergic pathways. In keeping with their well-known cholinomimetic effects, physostigmine and THA effectively inhibited this cholinesterase activity. Cholinesterase-containing normal axons (and in some cases cells) were severely depleted in the cerebral cortex of patients with Alzheimer's disease. Although the cerebral cortex of these patients continued to display abundant cholinesterase activity, the location of this enzyme was largely shifted to the neuritic plaques and neurofibrillary tangles. In fact, the majority of these pathological structures demonstrated intense acetylcholinesterase and butyrylcholinesterase activities. Physostigmine and THA were potent inhibitors of these plaque- and tangle-bound cholinesterases as well. In patients with Alzheimer's disease, cholinesterase inhibitors would therefore appear to have a major and widespread effect directly upon the enzymatic activity of plaques and tangles. Consequently, the clinical effects of anticholinesterases in Alzheimer's disease may be based on mechanisms that are different from those that apply to the normal brain.     

Synergistic effects of tetrahydroaminoacridine and lithium on cholinergic function after excitotoxic basal forebrain lesions in rat

Effects of lithium and tetrahydroaminoacridine (THA), either alone or in combination, were tested in an animal model of excitotoxic cholinergic deafferentation of the cerebral cortex. Rats received ibotenic acid lesions of cholinergic basal forebrain nuclei resulting in a 30% to 40% depletion of both cortical choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activity. Lithium as well as THA, given separately either prior or subsequently to the development of the lesion, had small but significant effects on the recovery of cortical ChAT and AChE activity. Applied in combination, these drugs clearly showed synergistic effects. These potentiating actions might be due to neuroprotective/ neurotrophic mechanisms as well as to effects on acetylcholine turnover and muscarinic receptor-coupled phosphoinositide turnover. Similar approaches of combination therapy might prove useful for the management of mental disorders associated with cholinergic dysfunction.     

Responses induced by tacrine in neuronal and non-neuronal cell lines

Alzheimer's disease (AD) is associated with a reduction in cholinergic activity as a result of specific neuronal loss. Current potential treatments for the disease include both cholinomimetic drugs and anti-cholinesterase inhibitors. One of the drugs approved by the FDA is tacrine (9-amine-1,2,3,4 tetrahydroacridine; THA), a strong acetylcholinesterase (AChE) inhibitor. We have studied the effects of tacrine on glial and neuronal cells in culture assessing cell survival and viability and morphology. Lactate dehydrogenase (LDH) activity and methylthiazol-diphenyltetrazolium (MTT) reduction were used as toxicity indicators. We found that tacrine toxicity on rat B12 glial cells and mouse Neuro 2A cells was strongly dependent on its concentration (up to 500 μM) and time of exposure. The toxic effect was not prevented by serum factors nor by bovine serum albumin. Fluorescein-conjugated phalloidin was used to examine the arrangement of actin filaments at substrate adhesion regions and cell-cell contacts. Primary events following exposure to tacrine included changes in cell morphology, disappearance of actin filament bundles, and disruption of focal adhesion contacts. At concentrations between 10 and 50 μM, tacrine induced neurite outgrowth in Neuro 2A cells, an effect that was not observed in B12 cells, suggesting that certain tacrine effects could be specific for neuronal cells. Although similar trends of response were observed for both cell types, some differences between undifferentiated and differentiated cells were apparent. J. Neurosci. Res. 52:435–444, 1998. © 1998 Wiley-Liss, Inc.     

Some Actions of 9-Amino-1,2,3,4-Tetrahydroacridine (THA) on Cholinergic Transmission and Membrane Currents in Rat Sympathetic Ganglia

THA (Tacrine) is an anticholinesterase drug reported to alleviate cognitive deficit in Alzheimer's disease. We have used rat isolated superior cervical sympathetic ganglia as a model mammalian cholinergic neural system to study effects of THA on cholinergic synaptic transmission and postsynaptic membrane currents. At 0.1 - 3 microM, THA augmented the postsynaptic depolarizations and inward clamp currents produced by acetylcholine but not by the cholinesterase-resistant analogue, DMPP. Higher concentrations depressed these responses to both acetylcholine and DMPP, and reduced the acetylcholine-induced increase in membrane current noise. At 1 microM, THA did not affect the amplitude or time-course of fast (nicotinic) excitatory postsynaptic currents (epscs) evoked by single orthodromic volleys, but higher concentrations induced a biphasic epsc decay. In contrast, low concentrations of THA (1 - 3 microM) greatly augmented and prolonged the muscarinic slow epsc evoked by repetitive orthodromic volleys: this effect was blocked by 1 microM atropine. Concentrations above 0.1 mM produced a membrane depolarization and inhibited a variety of membrane ionic currents, including voltage-gated Ca current and subsequent Ca-activated K currents, and voltage-gated M- and A-type K currents. It is concluded that the principal effect of THA is to inhibit cholinesterase, and that the main consequence of this is to augment and prolong the muscarinic slow epsc. In contrast, the nicotinic fast epsc is not increased but instead may be reduced through a nicotinic channel-blocking action. Although THA could also block several other ion channels the concentrations required were too high to contribute significantly to its principal pharmacological actions on ganglionic transmission.     

Long-term tetrahydroaminoacridine treatment and quantitative EEG in Alzheimer's disease

The development of therapies for Alzheimer's disease (AD) has focused on drugs designed to correct the loss of cholinergic function within the central nervous system. Quantitative EEG (qEEG) changes associated with AD consist of background slowing. One way to study the effects of cholinergic drugs may be through assessment of their qEEG effects. The aim of the current work was to evaluate the effect of long-term treatment with tetrahydroaminoacridine (THA) on qEEG in AD patients.     

Side effects of long acting cholinesterase inhibitors

This paper reviews the side effects of the two long acting cholinesterase inhibitors tacrine and velnacrine. It will focus on tacrine which has been most studied. The two drugs are special as the dosage of them is guided mainly by side effects and not by therapeutic effects in contrast to most drugs used for psychiatric disorders.     

Clinical experiences and biochemical findings with tacrine (THA)

A clinical comparison of tacrine (THA) and placebo was performed in 15 Alzheimer patients using a double blind crossover technique over 4 plus 4 weeks with one drug-free week in between. Treatment results, as evaluated by clinical rating scales and neuropsychological tests, were mostly negative. Side effects were few, except for elevation liver enzymes which occured in one third of the patients. CSF levels of the monoamine metabolites HVA and 5-HIAA increased on tacrine as evidence for activation of dopamine and serotonin pathways through cholinergic receptors. Pharmacokinetic investigations showed that the oral bioavailability of tacrine was low and greatly varying between subjects. Patients with high bioavailability of the drug tended to improve more, and also to have more liver enzyme elevations, than those with low bioavailability. A gel preparation for rectal administration was manufactured for comparison of plasma levels attained during one week's treatment with levels attained with oral capsules. Preliminary results indicate that the dose of tacrine can be reduced to 50 per cent when administered rectally, probably as by this route the rapid first-pass metabolism of the drug in the liver is diminished. A clinical trial of tacrine via the rectal route would be justified as this could decrease the number of patients with liver side effects and increase the number of patients improving on the treatment.     

Effect of long-term treatment with tacrine (THA) in Alzheimer's disease as visualized by PET

Among various attempts to enhance cholinergic neurotransmission in AD clinical trials with cholinesterase inhibitors have been most promising. In this study positron emission tomography (PET) was used to investigate how long-term treatment with cholinesterase inhibitors like tacrine could induce changes in the functional activity of Alzheimer brains. PET investigations measuring cerebral blood flow, glucose metabolism, nicotinic and muscarinic receptors have repeatedly been performed in patients treated with tacrine up to 2.5 years. Changes in nicotinic receptors and blood flow were observed after 3 weeks of treatment while changes in glucose metabolism were measured after 3 months of treatment. Following longer period of treatments and increase in dose of tacrine improvements were measured by PET. The most significant effects were found in patients with early forms of the dementia. The findings suggest that longer treatment may not only be symptomatic but might slow down the disease process.     

Mechanism of action of cholinesterase inhibitors in Alzheimer's disease

Cholinesterase inhibitors, such as physostigmine and tacrine, have lately gained interest as potential drugs in the treatment of Alzheimer's disease. Already in the 1950ths, it was discovered that physostigmine and tacrine were potent inhibitors of acetylcholinesterase and butyrylcholinesterase. However, later studies have shown that cholinesterase inhibitors also interact with cholinergic receptors, with sodium and potassium ion channels and effect the uptake, synthesis and release of neurotransmitters. In summary, cholinesterase inhibitors are drugs with many modes of action, which may be of advantage in the treatment of a complex disorder such as Alzheimer's disease.     

The cerebrovascular role of the cholinergic neural system in Alzheimer's disease

The intrinsic cholinergic innervation of the cortical microvessels contains both subcortical pathways and local cortical interneurons mediated by muscarinic and nicotinic acetylcholine receptors. Stimulation of this system leads to vasodilatation. In the extrinsic innervation, choline acts as a selective agonist for the α7-nicoticinic acetylcholine receptor on the sympathetic nerves to cause vasodilatation, and through this mechanism, cholinergic modulation may affect this sympathetic vasodilatation. Alzheimer's disease is characterized by a cerebral cholinergic deficit and cerebral blood flow is diminished. Cholinesterase inhibitors, important drugs in the treatment of Alzheimer's disease, could influence the cerebral blood flow through stimulation of the intrinsic cholinergic cerebrovascular innervation. Indeed, cholinesterase inhibitors improve cerebral blood flow in Alzheimer patients who respond to treatment. Further, cerebrovascular reactivity and neurovascular coupling are impaired in Alzheimer's disease and both can be improved by cholinesterase inhibitors. Conversely, cholinesterase inhibitors inhibit the α7-nicoticinic acetylcholine receptor on extrinsic sympathetic nerves and thus may impair vasodilatation. The net outcome of these opposing effects in clinical practice remains unknown. Moreover, it is uncertain whether the regulation of cerebral blood flow during blood pressure changes (cerebral autoregulation) is impaired in patients with Alzheimer's disease. Technological developments now allow us to dynamically measure blood pressure, cerebral blood flow, and cerebral cortical oxygenation. Using simple maneuvers like single sit-stand and repeated sit-stand maneuvers, the regulation of cerebral perfusion in patients with Alzheimer's disease can easily be measured. Sit-stand maneuvers can be considered as a provocation test for cerebral autoregulation, and provide excellent opportunities to study the cerebrovascular effects of cholinesterase inhibitors.     

Long-term treatment with tacrine (THA) in Alzheimer's disease—evaluation of neuropsychological data

Long-term effects of tacrine (THA) on cognitive functions of very mild AD patients were studied. The stability of possible positive changes following prolonged treatment and the effect of increased dose was also studied. Three patients were treated with tacrine (80 mg/day) and the effect on cognitive functions was measured with a neuropsychological test battery. Two of the patients (Pats 1 and 4) showed clear positive changes in all parameters measured. The third patient (Pat 5) did not show as clear positive responses. The effect of the initial treatment dose diminished over time. After raising the dose two of the patients showed improvement in cognitive tests reaching their initial level of performance or even better in most of the tests. This positive effect was not as clear in patient 5. After 13 months of tacrine all patients still showed positive changes in some of the tests. Compared to a hypothetical progression curve for untreated AD patients the patients treated with tacrine seemed to have slower progression. In conclusion, it seems that long-term positive effects on cognitive functions of AD patients can be reached with tacrine and it seems to be possible to slow down the progression of the disease. However, to reach long-term positive effects increasing doses seem to be needed. AD patients seem to differ in their response to tacrine.     

Long-term stabilizing effect of cholinesterase inhibitors in the therapy of Alzheimer' disease

During the last decade, a systematic effort to develop a pharmacological treatment for Alzheimer disease (AD) resulted into three drugs being registered for the first time in USA and Europe. All three compounds are cholinesterase inhibitors (ChEI). The major therapeutic effect of ChEI on AD patients is to maintain cognitive function at a stable level during a 6 months to one year period of treatment as compared to placebo. Additional drug effects are slowing cognitive deterioration and improving behavioral and daily living activity. Recent studies show that in many patients the cognitive stabilization effect can be prolonged up to 24 months. This long-lasting effect suggests a mechanism of action other than symptomatic and direct cholinergic. In vitro and in vivo studies have consistently demonstrated a link between cholinergic activation and APP metabolism. Lesions of cholinergic nuclei cause a rapid increase in cortical APP and CSF. The effect of such lesions can be reversed by ChEI treatment. Reduction in cholinergic neurotransmission experimental or pathological (AD) leads to amyloidogenic metabolism and contributes to the neuropathology and cognitive dysfunction. In order to explain the long-term effect of ChEI, a mechanism based on beta-amyloid metabolism, is postulated. Evidence for such an effect is available at experimental as well as at clinical level. Does cholinergic stabilization imply slowing down disability or delaying disease progression?     

Functional and pharmacokinetic studies of tetrahydroaminoacridine in patients with amyotrophic lateral sclerosis

Assuming the presence of clinically significant cholinergic hypofunction in amyotrophic lateral scleroses (ALS), seven patients with ALS were treated with 100-200 mg tetrahydroaminoacridine (THA) together with 11 g lecithin daily for up to 7 weeks. In a separate experiment pharmacokinetics and effects on muscle strength and neurophysiological parameters were studied following the injection of 30 mg THA intravenously. Following the injection of THA an increase in muscle strength was observed in two patients. There were no consistent pharmacokinetic differences that could explain the effect on intravenous THA on muscle strength in these two patients. The plasma clearance of THA was high and the oral bioavailability low with large interindividual differences (6-36%). No beneficial effect was seen during oral medication and side-effects were common. There were no conclusive changes observed regarding neurophysiological parameters after drug administration. THA has probably no place in the treatment of ALS.     

Effects of cholinesterase inhibitors on learning and memory in rats: A brief review with special reference to THA

Research on memory enhancing effects of acetylcholinesterase (AChE) inhibitors was stimulated by the finding of diminished cholinergic markers in patients with Alzheimer's disease, and the correlation of cognitive impairment to cholinergic deficits in these patients. The rationale for the use of AChE inhibitors is based on their abilities to prevent breakdown of acetylcholine released from surviving nerve terminals. In experimental animals the AChE inhibitor has been found by some investigators to be efficacious in improving cognitive function. Recent work has focused more on the performance and memory enhancing effects of tetrahydroaminoacridine (THA). THA has been found to improve performance in experimental animals with cognitive impairments induced by a variety of experimental manipulations such as by pharmacological blockade, cholinergic lesions, chronic alcohol or barbital treatment and ischemic lesion. These findings are compatible with the view that AChE inhibitors can be efficacious in "restoration" of some cholinergic functions.     

Progress in clinical neurosciences: Treatment of Alzheimer's disease and other dementias--review and comparison of the cholinesterase inhibitors

BACKGROUND: Alzheimer's disease (AD) is the most common cause of dementia in older adults. Acceptance of the cholinergic hypothesis led to a search for medications which could enhance central cholinergic activity in this condition. There are now three cholinesterase inhibitors available for the treatment of AD in Canada. OBJECTIVES: To review the currently available cholinesterase inhibitors approved for the treatment of AD in Canada and to provide guidance on who and how to treat with these agents. RESULTS: Donepezil, rivastigmine, and galantamine are approved for the treatment of AD in Canada. In clinical trails, patients with mild to moderate AD treated with these agents experienced modest improvements in cognition, function, behaviour, and/or global clinical state. The magnitude of benefits seen with each agent appeared to be similar. While to date, there is no convincing evidence that one is more efficacious or effective, they do differ in their pharmacokinetics, additional mechanisms of action, and side effect profiles. Therefore, the selection of agent will be based on considerations such as side effect profiles, ease of administration, personal familiarity/experience, and beliefs about the importance of the noted differences in their pharmacokinetics and additional mechanisms of action. CONCLUSION: We believe that these agents should be offered to all individuals with a mild to moderate dementia where Alzheimer's pathology is felt to be a contributing factor. We view all three available cholinesterase inhibitors as first-line drugs.     

Effects of tetrahydro-9-aminoacridine on cortical and hippocampal neurons in the rat: an in vivo and in vitro study

The effects of tetrahydro-9-aminoacridine (THA), an anticholinesterase drug, have been studied in the rat both in vivo (cerebral cortex) and in vitro (CA1 field of the hippocampus) and compared with those of physostigmine. In the cerebral cortex THA potentiated the excitatory effect of acetylcholine in most neurons, including cortical neurons recorded from chronic unanesthetized animals. In vitro, THA (but not physostigmine) had a depolarizing, atropine- and tetrodotoxin-insensitive effect. This effect is associated with an increase in membrane resistance which suggests a direct effect of THA on hippocampal neurons. In addition THA blocked the slow inhibitory postsynaptic potential. At the same concentration THA potentiated the slow cholinergic excitatory postsynaptic potential produced by electrical stimulation of the cholinergic afferents. Its potency was, however, about 10 times lower than that of physostigmine. These results show that THA: (1) is an anticholinesterase much less potent than physostigmine; but (2) has also direct effects on central neurons, not observed with physostigmine and unrelated to its anticholinesterase activity.     

Memantine for the treatment of dementia

BACKGROUND: The use of cholinesterase inhibitors to correct the cholinergic deficit in patients with mild to moderate Alzheimer's disease (AD) is well established. However, the treatment is only effective in about half of the patients for whom it is prescribed. Vascular dementia may respond, at least to some extent, to these drugs (T Erkinjuntti and colleagues, Lancet 2002; 359: 1283-90). In 2002, the Committee of Proprietary Medicinal Products recommended that memantine-a drug that acts on the glutamatergic system rather than the cholinergic system-be approved by the European Commission for the treatment of moderately severe to severe AD. Clinical trials have shown some effectiveness of memantine in the treatment of vascular dementia, although it has not been approved for use in this disorder. RECENT DEVELOPMENTS: The results of a study of the effects of memantine on moderate to severe AD have recently been published (B Reisberg and colleagues, N Engl J Med 2003; 348: 1333-41). Reisberg and colleagues treated their patients for 28 weeks, assessed several outcome variables, and found that memantine reduced clinical deterioration without significant adverse effects. This study is important as memantine is the only treatment licensed for patients with more advanced AD. WHERE NEXT? Several questions about the use of memantine as a treatment for AD remain to be answered. How beneficial is memantine treatment in routine clinical practice compared with clinical trials? What is the best way to assess treatment effects? How long do the beneficial effects last? Does memantine have neuroprotective, rather than just symptomatic, effects? In addition, we need to know when to switch from cholinesterase inhibitors to memantine or when to co-prescribe memantine with cholinesterase inhibitors. The efficacy of memantine in vascular dementia also requires further investigation.     

Tetrahydroaminoacridine (THA) in Alzheimer's disease: An assessment of attentional and mnemonic function using CANTAB

A placebo-controlled cross-over trial (n=89) investigated the use of a chronic dose of the cholinesterase inhibitor THA, as a treatment for dementia of the Alzheimer type (DAT). Effects on both subjective clinical rating scales and objective computerised tests were assessed. In regard to the former, analysis of the three main clinical outcome measures showed statistically significant effects of the drug on the Mini-Mental State Examination (MMSE) and the Abbreviated Mental Test Score (AMTS), but not on the Activities of Daily Living scale (ADL). Using the objective computerised CANTAB tests, sensitive to specific aspects of memory and attention, evidence was found for improvements in attentional function rather than memory, in patients with mild to moderate DAT. Although these improvement: were significant, they were small and restricted to certain tests of attentional function. Nevertheless, they add to the growing body of evidence that the cholinergic system is involved in the control of attentional processes: and are substantiated by the findings of a second study examining the effects of an acute dose of nicotine on attentional and mnemonic performance in patients with DAT. This study found significant improvements in cognitive performance in patients receiving nicotine, in objective tests of attention but not of short-term memory. These data will clearly provide important comparative data for future investigations of putative cognitive enhancing drugs in DAT sufferers.     

Learning deficits in aged rats pretreated chronically with barbital and tested late in abstinence: alleviation by tetrahydroaminoacridine

Summary Physostigmine and tetrahydroaminoacridine (THA) have been reported to improve cognitive function in patients with Alzheimer's disease. Two experiments were conducted to examine the effects of these anticholinesterase agents on learning in aged rats pretreated chronically with barbital. In the first experiment animals received barbital in their drinking water for 46 weeks. Controls were given only water. On days 100–104 of abstinence, when the animals were 20 months old, acquisition of the Morris maze task was initiated after treatment with physostigmine. It was found that physostigmine improved learning of the maze task in control but not barbital treated rats. In the second experiment animals received barbital solution or water as in experiment one. On days 100–103 of abstinence they were injected with THA before being tested in the Morris water maze. It was found that THA improved learning in both barbital treated and control rats. These results corroborate clinical findings of improved cognitive function following treatment with THA, and suggest that the therapeutic effects of THA may be mediated by mechanisms distinct from cholinesterase inhibition. Furthermore chronic barbital treatment could be used as a model to study cognitive disturbances in experimental animals.