Polymethylene tetraamines: a novel class of cardioselective M2-antagonists

Discovery of Multi-Target Agents for Neurological Diseases via Ligand Design

The incidence of neurological disorders in the developed world is rising in concert with an increase in human life expectancy, due in large part to better nutrition and health care. Even as drug discovery efforts are refocused on these disorders, there has been a dearth in the introduction of new disease-modifying therapies to prevent or delay their onset, or reverse their progression. Mounting evidence points to complex and heterogeneous etiopathologies that underlie these diseases. Therefore, it is unlikely that disorders in this class will be mitigated by any single drug that acts exclusively on a single pathway or target. The rational design of novel drug entities with the ability to simultaneously address multiple drug targets of a complex pathophysiology has recently emerged as a new paradigm in drug discovery. Similarly to the concept of multi-target agents within the psychopharmacology field, ligand design has gained an increasing prominence within the medicinal chemistry community. In this chapter we discuss several examples of select chemical scaffolds (polyamines, alkylxanthines, and propargyl carbamates) wherein these concepts were applied to develop novel drug candidates for Alzheimer's disease and Parkinson's disease.

Structure–activity relationships of methoctramine-related polyamines as muscarinic antagonist: Effect of replacing the inner polymethylene chain with cyclic moieties

The aim of the present paper was to investigate the role of the octamethylene spacer of methoctramine (1) on the biological profile. Thus, this spacer was incorporated into a dianiline or dipiperidine moiety to determine whether flexibility and the basicity of the inner nitrogen atoms are important determinants of potency with respect to muscarinic receptors. The most potent compound was 4, which displayed, in the functional assays, a comparable potency at muscarinic M(2) receptors with respect to 1, and, in the binding assays, a loss of potency and selectivity toward muscarinic M(1) and M(3) receptor subtypes. Both compounds were endowed with antinociceptive activity. Furthermore, in microdialysis tests in rat parietal cortex, they enhanced acetylcholine release, most likely by antagonizing presynaptic muscarinic receptor subtypes.

Mode of action of novel polyamines increasing the permeability of bacterial outer membrane

We have clarified how the polyamines naphthylacetylspermine and methoctramine (N,N'-bis[6-[[(2-methoxyphenyl)methyl]amino]hexyl]-1,8-octanediamine), originally developed as a synthetic analogue of joro spider toxin and a muscarinic receptor antagonist, respectively, can increase the permeability of the outer membrane of Escherichia coli. These polyamines were recently found to be outer membrane permeabilisers, based on investigations of the structure-activity relationship using ion-selective electrodes. In a standard microbiological assay examining membrane-permeabilising ability, these polyamines enhanced the action of hydrophobic antibiotics such as novobiocin and erythromycin, which ineffectively traverse the outer membrane of E. coli, to inhibit the growth of E. coli. This result substantiated the outer membrane-permeabilising ability of these polyamines demonstrated by using ion-selective electrodes. We observed the release of lipopolysaccharide from the outer membrane in the concentration range causing permeabilisation, showing that the action of the polyamines is attributable to disruption of the outer membrane structure.

Polymethylene tetraamine backbone as template for the development of biologically active polyamines

The concept that polyamines may represent a universal template in the receptor recognition process is embodied in the design of ligands for different biological targets. As a matter of fact, the insertion of different pharmacophores onto the polymethylene tetraamine backbone can tune both affinity and selectivity for any given receptor. The application of this approach provided a prospect of modifying benextramine (1). structure to achieve specific recognition of muscarinic receptors that led to the discovery of methoctramine (2). which is widely used as a pharmacological tool for muscarinic receptor characterization. In turn, appropriate structural modifications performed on the structure of methoctramine led to the discovery of new polyamines endowed with high affinity and selectivity for (a). muscarinic receptor subtypes, (b). G(i) proteins, and (c). muscle-type nicotinic receptors. Thus, polyamines tripitramine (9) and spirotramine (33), among others, were designed, which were shown to be highly selective for muscarinic M(2) and M(1) receptors, respectively. Several polyamines have been discovered, which inhibit noncompetitively a closed state of the nicotinic receptor. These ligands, such as 66, resulted in important tools for elucidating the mode and site of interaction of polyamines with the ion channel. It was discovered that reducing the flexibility of the diaminohexane spacer of methoctramine led to polyamines, such as 70, which are endowed with a biological profile significantly different from that of the prototype. Most likely, tetraamine (70) is a potent activator of G(i) proteins. Finally, the universal template approach formed the basis for modifying benextramine (1) structure to the design of ligands, which display affinity for acetylcholinesterase and muscarinic M(2) receptors. Thus, these polyamines, such as caproctamine (78), could have potential in the investigation of Alzheimer disease.

Structure-activity relationships of methoctramine-related polyamines as muscular nicotinic receptor noncompetitive antagonists. 3. Effect of inserting the tetraamine backbone into a macrocyclic structure

The present article expands on the study of another aspect of structure-activity relationships of the polymethylene tetraamines, namely, the effect of inserting the tetraamine backbone into a macrocyclic structure. To this end, compounds 8-12 were designed by linking the two terminal nitrogen atoms of prototype methoctramine 2 to an aryl moiety. Alternatively, 2 was first modified to achieve compounds 6 and 7, which in turn were cyclized by linking the two terminal primary amine functions to a polyphenyl spacer, affording 13-20. All the compounds were tested on muscle-type nAChRs and most of them as well on AChE. Furthermore, selected compounds were tested also on peripheral M(2) and M(3) mAChRs. All these cyclic derivatives, like prototypes, were potent noncompetitive antagonists at both frog and Torpedo nAChRs, suggesting that polyamines do not need to be linear or in extended conformation to optimally interact with the nicotinic channel; rather, they may bind in a U-shaped conformation. Relative to muscarinic activity, macrocyclic compounds 10, 13, 14, and 20, in contrast with the profile displayed by 2, were almost devoid of affinity. It is derived that an aryl spacer is detrimental to the interaction of polyamines with mAChRs. Finally, all the diamine diamides investigated in this study were much less potent in inhibiting AChE activity than prototype 3, suggesting that a macrocyclic structure may not be suitable for AChE inhibition.

Interaction between polyamines and bacterial outer membranes as investigated with ion-selective electrodes

We analyzed the interaction between polyamines and the outer membrane of Escherichia coli cells using potentiometric measurements with Ca(2+), tetraphenylphosphonium (TPP(+)), and K(+) electrodes. The Ca(2+) electrode was used to examine the ability of the polyamines to release Ca(2+) from the outer membrane. The TPP(+) electrode was used to examine the ability to permeabilize the outer membrane, since the uptake of TPP(+) was enhanced when the permeability barrier of the outer membrane was disrupted. The K(+) electrode was used to examine permeabilization in the cytoplasmic membrane by monitoring the efflux of K(+) in cytosol. Although Ca(2+) release was remarkably enhanced by increasing the number of amino groups in polyamines, no TPP(+) uptake was observed with polyamines of a simple structure, such as ethylenediamine, spermidine, and spermine. TPP(+) uptake was observed when appropriate lipophilic moieties were further attached to the polyamines with three or four amino groups, indicating that the existence of bulky moieties as well as the number of amino groups is important to induce outer membrane permeabilization. Thus, 1-naphthylacetylspermine and N,N'-bis[6-[[(2-methoxyphenyl)methyl]amino]hexyl]-1,8-octanediamine (methoctramine) were especially effective in increasing the permeability of the outer membrane of E. coli cells, being comparable to polymyxin B nonapeptide, a well-known cationic peptide showing such action.

The design of novel methoctramine-related tetraamines as muscarinic receptor subtype selective antagonists

Several novel methoctramine-related tetraamines were designed, and their biological profiles at muscarinic receptor subtypes were assessed by functional experiments in isolated guinea pig and rat atria (M2) and smooth muscle (ileum and trachea, M3) and by binding assays in rat cortex (M1), heart (M2), and submaxillary gland (M3) homogenates and NG 108-15 cells (M4). Tripitramine, a nonsymmetrical tetraamine, resulted in the most potent and the most selective muscarinic M2 receptor antagonist of the series (pA2 = 9.14-9.85; pKi = 9.54). Spirotramine (FC 15-94), a symmetrical tetraamine, was able to differentiate between muscarinic M1 receptors (pKi = 7.88) and the other subtypes (M2, pKi = 6.20; M3, pKi = 5.81; M4, pKi = 6.27). Thus, tripitramine and spirotramine could be valuable tools for the pharmacological classification and characterization of muscarinic receptor subtypes.

Cardiovascular effects elicited by central administration of physostigmine via M2 muscarinic receptors in conscious cats

The cardiovascular effects of an intracerebroventricular (i.c.v.) injection of physostigmine were studied using conscious cats. Physostigmine (5-25 micrograms: 5 microliters) caused a dose-dependent increase in mean arterial pressure (MAP) and heart rate (HR). The highest dose (25 micrograms) increased MAP and HR by 32 +/- 3 mmHg and 45 +/- 5 beats/min, respectively (n = 5). Pre-administration of the muscarinic receptor antagonist, atropine (25 micrograms; i.c.v.) blocked the effects of physostigmine (25 micrograms; i.c.v.). Also, the pre-administration of the M2 muscarinic antagonist, methoctramine (25 micrograms; i.c.v.), antagonized the cardiovascular effects of physostigmine without altering the baseline variables. However, the M1 muscarinic antagonist, pirenzepine (100 micrograms; i.c.v.) did not alter baseline MAP or HR, and also failed to inhibit the cardiovascular responses to physostigmine. Similarly, the M3 muscarinic blocker, 4-diphenyl-acetoxy-N-methylpiperidine methiodide (50 micrograms; i.c.v.), neither changed baseline cardiovascular variables nor blocked the effects of physostigmine. When the same cats were anesthetized with intravenous injection of sodium pentobarbital (25-30 mg/kg), physostigmine (25 micrograms; i.c.v.) evoked a decrease in MAP and HR of 13 +/- 6 mmHg and 15 +/- 6 bpm, respectively (n = 5). These results demonstrate that the increases in MAP and HR to the i.c.v. administration of physostigmine in conscious cats are possibly mediated through stimulation of central M2 muscarinic receptors. In addition, anesthesia reverses the effects elicited by the central administration of physostigmine to a decrease in MAP and HR.

In vitro characterization of tripitramine, a polymethylene tetraamine displaying high selectivity and affinity for muscarinic M2 receptors

1. The antimuscarinic effects of tripitramine were investigated in vitro in isolated driven left (force) and spontaneously beating right (force and rate) atria as well as in the ileum of guinea-pig and rat and in the trachea and lung strip of guinea-pig and compared with the effects of methoctramine. 2. Tripitramine was a potent competitive antagonist of muscarinic M2 receptors in right and left atria. The pA2 values ranged from 9.14 to 9.85. However, in the guinea-pig and rat left atria but not in guinea-pig right atria, tripitramine at lower concentrations (3-10 nM) produced a less than proportional displacement to the right of agonist-induced responses owing to the presence of a possible saturable removal process. 3. Tripitramine was about three orders of magnitude less potent in ileal and tracheal than in atrial preparations (pA2 values ranging from 6.34 to 6.81) which makes it more potent and more selective than methoctramine. 4. Another intriguing finding was the observation that the pA2 value of 7.91 observed for tripitramine in guinea-pig lung does not correlate with that found at both muscarinic M2 and M3 receptor subtypes, which clearly indicates that the contraction of guinea-pig lung strip is not mediated by these muscarinic receptor subtypes. 5. A combination of tripitramine with atropine resulted in addition of the dose-ratios for left atria as required for two antagonists interacting competitively with the same receptor site, whereas the same combination gave a supra-additive antagonism on guinea-pig ileum which suggests that tripitramine interacts with a second interdependent site. 6. Tripitramine was more specific than methoctramine since, in addition to muscarinic receptors, it inhibited only frog rectus abdominis muscular (pIC50 value of 6.14) and rat duodenum neuronal (pIC50 value of 4.87) nicotinic receptors among receptor systems investigated, namely alpha 1-, alpha 2-, and beta 1-adrenoceptors, H1- and H2-histamine receptors, and muscular and neuronal nicotinic receptors.

Muscarinic M1 and M3 receptor antagonist effects of a new pirenzepine analogue in isolated guinea-pig ileal longitudinal muscle-myenteric plexus

The new pirenzepine analogue DF 545 has been tested for its muscarinic M1 and M3 receptor antagonist properties in guinea-pig longitudinal muscle-myenteric plexus preparations. McN-A-343-induced inhibition of twitch contractions was taken as a parameter for muscarinic M1 receptor activation while electrical and acetylcholine-induced contractions were considered as a model for muscarinic M3 receptor stimulation. An unexpected contractile effect evoked by McN-A-343 was also investigated. In contrast to pirenzepine, DF 545 only weakly counteracted the M1-mediated McN-A-343 inhibitory effect but blocked M3-related twitch- or acetylcholine-stimulated responses with a 2-fold higher affinity than pirenzepine. Therefore, in this preparation, our findings suggest that DF 545 does not share the selectivity profile exhibited by pirenzepine at ileal muscarinic receptors. Studies on the McN-A-343 contractile effect provide evidence that this agonist may interact with ileal muscarinic effector sites in a different way from other cholinergic agents.

Selective blockade of different brain stem muscarinic receptor subtypes: effects on the sleep-wake cycle

Changes induced in the sleep-wake cycle by pontine microinjections of muscarinic antagonists were studied in freely moving rats, instrumented for chronic polygraphic recordings. Pirenzepine (PIR), methoctramine (MET) and p-fluoro-hexahydro-siladifenidol (p-F-HHSiD), which are highly selective M1, M2 and M3 antagonists, respectively, were dissolved in 0.1 microliter of sterile isotonic saline (0.2 microliter of distilled water for p-F-HHSiD) and injected into the pontine reticular nucleus, where the administration of 0.5 microgram carbachol (a mixed muscarinic agonist) induced a 52% increase in the amount of desynchronized sleep (DS) over a 6 h recording period. The blockade of M2 receptors was shown to (i) antagonize DS, by increasing its latency and decreasing its percentage, (ii) decrease slow wave sleep, and (iii) enhance wakefulness. These effects were dose-dependent. No changes in the sleep-wake cycle were observed following microinjection of M1 or M3 antagonists. The results support the hypothesis that at the brain stem level only M2 receptors are involved in sleep mechanisms and, particularly, in the generation and maintenance of DS.

Postsynaptic effects of methoctramine at the mouse neuromuscular junction

Functional studies were performed to evaluate the effects of methoctramine at the neuromuscular junction of the mouse. The presynaptic control of acetylcholine release and the postsynaptic activation of the nicotinic receptor have been analysed by means of the extracellular recording with an EPC7 Patch Clamp amplifier. This electrophysiological method revealed a dose-related inhibitory effect of methoctramine on the studied parameters. The dramatic reduction of the kinetics of the quantal conductance change indicates an action at the postsynaptic level. The effects of methoctramine have been compared with those of the muscarinic agonist oxotremorine. Concentration/response curves for the two drugs were obtained and the apparent EC50 values calculated. The effects of oxotremorine were not antagonized by 1 microM methoctramine. These findings suggest an interaction of some muscarinic agents on the postsynaptic receptor-ion-channel complex at the mouse neuromuscular junction.

Binding of the muscarine receptor antagonist heptane-1,7-bis(dimethyl-3'-phthalimidopropyl)ammonium bromide at cholinoceptor sites

The binding of the bisquaternary muscarine receptor antagonist heptane-1,7-bis(dimethyl-3'-phthalimidopropyl)-ammonium bromide (C7/3-phth) was investigated at a number of cholinergic binding sites using (-)-[3H]nicotine, [3H]pirenzepine and (-)-[3H]quinuclidinyl benzilate ([3H]QNB) in both central and peripheral tissues. C7/3-phth displayed an affinity for muscarine M2 receptors in rat atria (70.1 nM) which was 1.6-fold greater than for putative M4 receptors in rabbit lung, and 4- to 5-fold greater than for M1 receptors in rat cerebral cortex. Its affinity for nicotine receptors in the cortex was low, being 808-fold lower than its affinity for the M2 receptor. Although the displacement of (-)-[3H]nicotine and [3H]pirenzepine binding in rat cortex by C7/3-phth was best described in terms of one-site modelling, low Hill coefficients were observed with C7/3-phth in displacement studies using [3H]QNB in this tissue. The possibility of allosteric interactions or multiple receptor subtype interactions is discussed.

Selective antagonists provide evidence that M1 muscarinic receptors may mediate carbachol-induced drinking in the rat

The present study served to investigate the ability of seven selective muscarinic antagonists to inhibit carbachol-induced drinking in the rat. The muscarinic antagonists were given by intracerebroventricular (i.c.v.) injection 1 min before the i.c.v. injection of carbachol (1 microgram/rat). The M2 antagonist, methoctramine, was inactive up to 80.3 nmol/rat. The M3 antagonist, p-fluoro-hexahydro-sila-difenidol, elicited a modest (42%) but statistically significant inhibition of drinking only at 80 nmol/rat. On the other hand, the selective M1 antagonists, (R)-trihexphenidyl, o-methoxy-sila-hexocyclium and pirenzepine, produced a marked and dose-dependent inhibition of carbachol-induced drinking, their ID50 values being 0.51, 7.36 and 9.31 nmol/rat. Also the M1/M3 antagonists, 4-diphenylacetoxy-N-methylpiperidine methiodide and hexahydro-sila-difenidol, were potent inhibitors of carbachol-induced drinking, their ID50 values (0.28 and 11.09 nmol/rat) being related to their pA2 values for M1 receptors in rabbit vas deferens. These data suggest that carbachol-induced drinking may be mediated by activation of muscarinic M1 receptors.