Structure−Activity Relationships of Methoctramine-Related Polyamines as Muscular Nicotinic Receptor Noncompetitive Antagonists. 2. Role of Polymethylene Chain Lengths Separating Amine Functions and of Substituents on the Terminal Nitrogen Atoms

Abstract

Polymethylene tetraamine methoctramine (1) is a prototypical antimuscarinic ligand endowed with a significant affinity for muscular nAChRs. Thus, according to the universal template approach, structural modifications were performed on 1 in order to improve affinity and selectivity for the muscle-type nAChR. The polyamine derivatives synthesized were tested at both frog rectus and Torpedo nAChRs and at guinea pig left atria (M(2)) and ileum longitudinal muscle (M(3)) mAChRs. All of the compounds, like prototype 1, were noncompetitive antagonists of nicotinic receptors while being competitive antagonists at M(2) and M(3) mAChRs. The biological profile of polyamines 4-7 revealed that increasing the number of amine functions and the chain length separating these nitrogen atoms led to a significant improvement in potency at nAChRs. Moreover, the role of the number and type of amine functions in the interaction with nAChRs was further investigated through the synthesis of compounds 9 and 10. Tetraamines 8 and 11, bearing a rather rigid spacer between the nitrogen atoms instead of the very flexible polymethylene chain, displayed a profile similar to that of 1 at nAChRs, whereas a significant decrease in potency was observed at mAChRs. Tetraamine 12, bearing a 2-methoxyphenethyl group, was less potent than 1, whereas tetraamine 13, carrying a diphenylethyl moiety, was more potent than 1, confirming that an increase in size of the hydrophobic group on the terminal nitrogen atoms increases significantly the binding affinity for nAChRs. Tetraamines 14-17 were significantly more potent than prototype 2 at both frog rectus and Torpedo nAChRs, confirming that an increase in the distance between the amine functions results in a parallel increase in the affinity for nAChRs. To gain insight into the mode of interaction of polymethylene tetraamines with nAChRs, photolabile (19 and 20) and fluorescent (21 and 22) compounds were synthesized. A most intriguing finding was the observation that 19, which bears two identical azido groups on the terminal nitrogen atoms, was found to bind the Torpedo nAChR with a 1:1 stoichiometry, suggesting a U-shaped conformation in the receptor interaction. Moreover, the high potency shown by fluorescent compounds 21 and 22 appears promising for a further characterization of the polymethylene tetraamines binding site with the muscle-type nAChR.

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

“…They turned out to be potent noncompetitive antagonists of the nicotinic receptor, while they were competitive muscarinic receptor antagonists (Table V). 76,77 All of the compounds were significantly more potent than prototype PhTX-343 at both frog rectus and Torpedo nicotinic receptors, confirming that an increase in the distance between the amine functions results in a parallel increase in the affinity for nicotinic receptors. However, the higher homologues 39 and 41 were, like PhTX-343 but to a greater extent, more potent at frog rectus versus Torpedo nicotinic receptors.…”

“…76,77 Tetraamine 51, which carries a (4 00 -aminomethyl-[1,1 0 ;4 0 ,1 00 ]terphenyl-4-yl)-methylamine function instead of the 1,12-dodecanediamine fragment of 34 showed the same potency at both frog rectus and Torpedo nicotinic receptors and was 5-fold less potent than 34 at the frog rectus nicotinic receptor and 5-fold more potent than 34 at the Torpedo nicotinic receptor. Furthermore, 51, unlike 34, was devoid of activity at both M 2 and M 3 muscarinic receptors.…”

“…76,77 To this end the butyryltyrosyl moiety of philanthotoxin-343 (PhTX-343), a synthetic analogue of a wasp (Philanthus triangulum) toxin PhTX-433 (Fig. 10), 78 was chosen because it is known that an aromatic moiety at one end and a primary amine function at the other end of the molecule are important for philanthotoxin binding at the Torpedo nicotinic receptor.…”

“…The introduction of two iodine atoms in 66, affording 125 I 2 -66, resulted in a moderate increase in the binding affinity of 66, an effect that was previously observed when iodinating the asymmetrical MR44. 86 The 77 clearly demonstrate that symmetrically substituted tetraamines interact, at least with the Torpedo nicotinic receptor, differently from mono-substituted tetraamines bearing a terminal primary amine function. It was found that two molecules of 125 I-MR44 label one receptor monomer with a 2:1 stoichiometry, whereas 125 I 2 -66 interacted with the receptor with a 1:1 stoichiometry.…”

“…Iodine was introduced into the aryl moiety of 68, affording 69, because it is known that this element may determine quenching of the fluorescence when facing the fluorophor group. 77 Consequently, in a conformation in which the two aromatic moieties of compound 69 would be in close proximity, i.e., if the two aromatic moieties of 69 would refold generating a polyamine ring structure with the two aromatic moieties of 69 interacting by stacking forces, the fluorescence intensity of 69 should be lower than that of 68, which lacks iodine. Preliminary results have shown that the presence of iodine in 69 did not affect the intensity of fluorescence relative to 68 (unpublished results).…”