Muscarinic subtype selectivity of tetrahydroaminoacridine: possible relationship to its capricious efficacy

Novel bipharmacophoric inhibitors of the cholinesterases with affinity to the muscarinic receptors M1 and M2

A set of hybrid compounds composed of the fragment of allosteric modulators of the muscarinic receptor, i.e. W84 and naphmethonium, and the well-known AChE inhibitor tacrine on the one hand, and the skeletons of the orthosteric muscarinic agonists, iperoxo and isox, on the other hand, were synthesized. The two molecular moieties were connected via a polymethylene linker of varying length. These bipharmacophoric compounds were investigated for inhibition of AChE (from electric eel) and BChE (from equine serum) as well as human ChEs in vitro and compared to previously synthesized dimeric inhibitors. Among the studied hybrids, compound 10-C10, characterized by a 10 carbon alkylene linker connecting tacrine and iperoxo, proved to be the most potent inhibitor with the highest pIC₅₀ values of 9.81 (AChE from electric eel) and 8.75 (BChE from equine serum). Docking experiments with compounds 10-C10, 7b-C10, and 7a-C10 helped to interpret the experimental inhibitory power against AChE, which is affected by the nature of the allosteric molecular moiety, with the tacrine-containing hybrid being much more active than the naphthalimido- and phthalimido-containing analogs. Furthermore, the most active AChE inhibitors were found to have affinity to M₁ and M₂ muscarinic receptors. Since 10-C10 showed almost no cytotoxicity, it emerged as a promising lead structure for the development of an anti-Alzheimer drug.

Allosteric ligands for G protein-coupled receptors: a novel strategy with attractive therapeutic opportunities

Allosteric receptor ligands bind to a recognition site that is distinct from the binding site of the endogenous messenger molecule. As a consequence, allosteric agents may attach to receptors that are already transmitter-bound. Ternary complex formation opens an avenue to qualitatively new drug actions at G protein-coupled receptors (GPCRs), in particular receptor subtype selective potentiation of endogenous transmitter action. Consequently, suitable exploitation of allosteric recognition sites as alternative molecular targets could pave the way to a drug discovery paradigm different from those aimed at mimicking or blocking the effects of endogenous (orthosteric) receptor activators. The number of allosteric ligands reported to modulate GPCR function is steadily increasing and some have already reached routine clinical use. This review aims at introducing into this fascinating field of drug discovery and at providing an overview about the achievements that have already been made. Various case examples will be discussed in the framework of GPCR classification (family A, B, and C receptors). In addition, the behavior at muscarinic receptors of hybrid derivatives incorporating both an allosteric and an orthosteric fragment in a common molecular skeleton will be illustrated.

Allosteric modulation of muscarinic acetylcholine receptors

Muscarinic acetylcholine receptors (mAChRs) are prototypical Family A G protein coupled-receptors. The five mAChR subtypes are widespread throughout the periphery and the central nervous system and, accordingly, are widely involved in a variety of both physiological and pathophysiological processes. There currently remains an unmet need for better therapeutic agents that can selectively target a given mAChR subtype to the relative exclusion of others. The main reason for the lack of such selective mAChR ligands is the high sequence homology within the acetylcholine-binding site (orthosteric site) across all mAChRs. However, the mAChRs possess at least one, and likely two, extracellular allosteric binding sites that can recognize small molecule allosteric modulators to regulate the binding and function of orthosteric ligands. Extensive studies of prototypical mAChR modulators, such as gallamine and alcuronium, have provided strong pharmacological evidence, and associated structure-activity relationships (SAR), for a "common" allosteric site on all five mAChRs. These studies are also supported by mutagenesis experiments implicating the second extracellular loop and the interface between the third extracellular loop and the top of transmembrane domain 7 as contributing to the common allosteric site. Other studies are also delineating the pharmacology of a second allosteric site, recognized by compounds such as staurosporine. In addition, allosteric agonists, such as McN-A-343, AC-42 and N-desmethylclozapine, have also been identified. Current challenges to the field include the ability to effectively detect and validate allosteric mechanisms, and to quantify allosteric effects on binding affinity and signaling efficacy to inform allosteric modulator SAR.

Mechanism of tacrine block at adult human muscle nicotinic acetylcholine receptors

We used single-channel kinetic analysis to study the inhibitory effects of tacrine on human adult nicotinic receptors (nAChRs) transiently expressed in HEK 293 cells. Single channel recording from cell-attached patches revealed concentration- and voltage-dependent decreases in mean channel open probability produced by tacrine (IC(50) 4.6 microM at -70 mV, 1.6 microM at -150 mV). Two main effects of tacrine were apparent in the open- and closed-time distributions. First, the mean channel open time decreased with increasing tacrine concentration in a voltage-dependent manner, strongly suggesting that tacrine acts as an open-channel blocker. Second, tacrine produced a new class of closings whose duration increased with increasing tacrine concentration. Concentration dependence of closed-times is not predicted by sequential models of channel block, suggesting that tacrine blocks the nAChR by an unusual mechanism. To probe tacrine's mechanism of action we fitted a series of kinetic models to our data using maximum likelihood techniques. Models incorporating two tacrine binding sites in the open receptor channel gave dramatically improved fits to our data compared with the classic sequential model, which contains one site. Improved fits relative to the sequential model were also obtained with schemes incorporating a binding site in the closed channel, but only if it is assumed that the channel cannot gate with tacrine bound. Overall, the best description of our data was obtained with a model that combined two binding sites in the open channel with a single site in the closed state of the receptor.

Inhibitory effects of tacrine and physostigmine on catecholamine secretion and membrane currents in guinea-pig adrenal chromaffin cells

The effects of tacrine and physostigmine on catecholamine secretion induced by veratridine and high K+, and on voltage-dependent Na+ and Ca2+ currents, were investigated in guinea-pig adrenal chromaffin cells. In perfused adrenal glands, tacrine (100 microM) caused an inhibition of veratridine-induced catecholamine secretion, but physostigmine (100 microM) did not. In dispersed cells, both tacrine (1 microM-1 mM) and physostigmine (1 microM-1 mM) decreased catecholamine secretion induced by veratridine in a dose-dependent manner. The inhibitory effect of tacrine was much greater than that of physostigmine. Tacrine alone at a high concentration (such as 1 mM) caused a substantial increase in catecholamine secretion by itself and completely abolished the veratridine-induced secretory response in dispersed cells. High-concentration physostigmine showed a similar effect, but to a much lesser extent. The high K+ (46.2 mM)-evoked catecholamine secretion from dispersed cells was not affected by tacrine (1-100 microM) or physostigmine (1 microM-1 mM). In fura-2 loaded cells, tacrine (100 microM) almost abolished [Ca2+]i rise induced by veratridine, but only slightly reduced that evoked by high K+. In voltage-clamped cells, tacrine (300 microM) depressed the voltage-dependent Na+ and Ca2+ current by about 93% and 69%, and physostigmine (300 microM) depressed them by about 30% and 17%, respectively. These results suggest that tacrine decreases the veratridine-induced catecholamine secretion primarily by inhibiting the voltage-dependent Na+ channels rather than the Ca2+ channels. Physostigmine acts in a manner similar to tacrine, but its potency is much lower than that of tacrine.

Effects of NIK-247 and tacrine on muscarinic receptor subtypes in rats

1. The purpose of this study was to compare the effect of NIK-247 on muscarinic receptor subtypes with that of tacrine (THA) in rats. 2. NIK-247 and tacrine dose dependently inhibited the binding of [3H]pirenzepine (M1), [3H]AF-DX 384 (M2), and [3H]4-DAMP (M3). The IC50 values for NIK-247 were 4.4 x 10(-6) M, 1.1 x 10(-5) M, and 1.5 x 10(-5) M, respectively, whereas those for tacrine were 5.8 x 10(-7) M, 2.0 x 10(-6) M, and 5.8 x 10(-6) M, respectively. 3. Gpp[NH]p, a GTP analogue, slightly shifted the curve of displacement of [3H]AF-DX. 384 binding for NIK-247 to the right. However, Gpp[NH]p did not shift the curve of displacement of [3H]pirenzepine and [3H]4-DAMP binding to the right. 4. NIK-247 moderately decreased the rate of beating in right atrial preparations, but did not decrease it below 50% of control level. 5. These findings indicate that NIK-247 is an M1 antagonist, M2 partial agonist, and M3 antagonist.

Allosteric interactions at muscarinic cholinoceptors

1. An allosteric interaction occurs when the binding of a ligand to its site on a receptor is able to modify the binding of another ligand to a topographically distinct site on the same receptor and vice versa. The muscarinic cholinoceptors represent the best-studied examples of allosteric phenomena among the G-protein-coupled receptor superfamily. 2. The simplest model describing allosteric interactions at muscarinic cholinoceptors is the ternary complex model, which allows for a three-way interaction between the receptor, a classical (orthosteric) ligand and an allosteric modulator. The interaction may be quantified using the dissociation constant of each ligand for its respective binding site on the free receptor and the 'co-operativity factor' alpha. This latter term is the ratio of affinities of a ligand for the occupied versus the unoccupied receptor and is a measure of the magnitude of the cooperativity between two concomitantly bound ligands. 3. Identification of allosteric phenomena requires the utilization of both radioligand binding and functional approaches. Manifestations of allosterism include: (i) a limited ability to influence radioligand binding as the concentration of the latter is increased; (ii) alterations in the dissociation rate of orthosteric ligands; (iii) curvilinear Schild regressions; and (iv) nonadditivity of agonist/orthosteric antagonist/allosteric modulator combination concentration ratios. 4. Allosteric modulators of muscarinic cholinoceptors represent a diverse range of compounds. Some of the most studied agents include gallamine, alcuronium and the bis-ammonium compounds, C7/3'-phth and W84. Alcuronium has proven a most useful pharmacological tool, as it has been shown to display both positive and negative co-operativity, depending on the receptor subtype and orthosteric ligand involved in the interaction. 5. Evidence has accumulated pointing to the existence of a common allosteric binding site on the muscarinic cholinoceptors, located close to the orthosteric site, but at a more extracellular level. However, the possibility of more than one accessory binding site on various receptor subtypes cannot be excluded. 6. Allosteric modulators offer a number of potential therapeutic advantages, including a ceiling level to their effects and the possibility of 'absolute selectivity' of action, based on the degree of co-operativity rather than the affinity of the modulator for any one receptor subtype.

Tacrine. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic efficacy in Alzheimer's disease

Tacrine is a centrally acting cholinesterase inhibitor with additional pharmacological activity on monoamine levels and ion channels. It has been postulated that some or all of these additional properties may also be relevant to the mode of action of the drug. There are wide interindividual variations in pharmacological and clinical response to tacrine, possibly related to interindividual variation in bioavailability. Tacrine appears to improve cognitive function and behavioural deficits in a proportion of patients with Alzheimer's disease, at dosages of 80 to 160 mg/day. In the best designed trials, 30 to 51% of evaluable patients showed an improvement of at least 4 points on the cognitive subscale of the Alzheimer's Disease Assessment Scale, versus 16 to 25% of placebo recipients. A similar proportion of tacrine recipients were judged to have improved when global assessment scales were used. There was a significant dose-response relationship up to 160 mg/day. However, large numbers of patients were withdrawn during the trials, many because of tacrine-associated increases in transaminase levels. Elevated liver enzyme levels occurred in about 50% of tacrine recipients (reaching clinical significance in about 25%). Cholinergic symptoms also occurred more often in tacrine recipients than in those receiving placebo. A gradual increase in tacrine dosage, at 6-week intervals, is recommended when initiating therapy, and weekly serum transaminase monitoring is required for 6 weeks after each dosage increase. Despite the limitations implied by the low proportion of responders and high incidence of hepatic adverse effects associated with therapy, tacrine appears to make a measurable difference in both cognitive and behavioural function in a proportion of patients with Alzheimer's disease--a welcome advance in an area previously devoid of acceptable treatment options.

Pharmacological characterization of acetylcholine-stimulated [35S]-GTP gamma S binding mediated by human muscarinic m1-m4 receptors: antagonist studies

1. We have used dose-ratio analysis to estimate functionally the affinity constants (pKb) and Schild slope factors of a range of selective or atypical antagonists at human muscarinic m1-m4 receptors. 2. The functional response was the stimulation by acetylcholine of [35S]-GTP gamma S binding to membranes from Chinese hamster ovary (CHO) cells stably expressing individual receptor subtypes. 3. A novel experimental design and analysis was used which allowed the estimation of affinity and Schild slope factor from a single antagonist inhibition curve, and the results were compared with other methods of analysis, both theoretically valid and invalid. 4. In general, the affinity estimates were very similar to previously reported values obtained in binding studies with animal tissues and cloned human receptors and the Schild slope factors were close to unity. 5. These results demonstrate the validity of the assay and provide no evidence for species differences in antagonist affinity for muscarinic receptor subtypes. 6. The results confirm both the utility of himbacine in distinguishing between m1 and m4 receptors and a previously reported modest m4-selectivity for tropicamide and secoverine. 7. The cholinesterase inhibitor, tacrine (THA), had a potency profile similar to that of gallamine but with less selectivity. Its affinity could not be determined since it had Schild slope factors of about 2 at all subtypes. 8. o-Methoxy-sila-hexocyclium had only a modest selectivity for the m1 subtype.

Tetrahydroaminoacridine attenuates dizocilpine-induced behavioural changes

The acetylcholinesterase inhibitor tetrahydroaminoacridine (THA) is known to interact with neurotransmitter systems other than the cholinergic, e.g. several studies have shown interaction of THA with the N-methyl-D-aspartate (NMDA) receptor subtype of the glutamatergic system. We therefore investigated whether behavioral changes in rats, caused by the non-competitive NMDA-antagonist dizocilpine, were altered by THA. Spontaneous locomotion was measured in an open field and learning behaviour was evaluated in a spatial learning task in the 8-arm radial maze. Hyperactivity in the open field caused by 0.08 mg/kg i.p. dizocilpine was reversed by 5 mg/kg i.p. THA. Hyperactivity caused by 0.16 mg/kg i.p. dizocilpine was reversed by two THA doses (2.5 mg/kg, 5 mg/kg i.p.). A dizocilpine-induced acquisition deficit in the 8-arm radial maze was attenuated by THA (2.5 mg/kg) on the 6th and 7th day of testing. The effects of THA are discussed with reference to a possible functional glutamate agonism.