Semisynthetic analogues of toxiferine I and their pharmacological properties at α7 nAChRs, muscle-type nAChRs, and the allosteric binding site of muscarinic M2 receptors

J Nat Prod. 2014 Sep 26;77(9):2006-13. doi: 10.1021/np500259j. Epub 2014 Sep 5.

Abstract

A new series of analogues of the calabash curare alkaloid toxiferine I was prepared and pharmacologically evaluated at α7 and muscle-type nAChRs and the allosteric site of muscarinic M2 receptors. The new ligands differ from toxiferine I by the absence of one (2a-c) or two (3a-c) hydroxy groups, saturation of the exocyclic double bonds, and various N-substituents (methyl, allyl, 4-nitrobenzyl). At the muscle-type nAChRs, most ligands showed similar binding to the muscle relaxant alcuronium, indicating neuromuscular blocking activity, with the nonhydroxylated analogues 3b (Ki = 75 nM) and 3c (Ki = 82 nM) displaying the highest affinity. At α7 nAChRs, all ligands showed a moderate to low antagonistic effect, suggesting that the alcoholic functions are not necessary for antagonistic action. Compound 3c exerted the highest preference for the muscle-type nAChRs (Ki = 82 nM) over α7 (IC50 = 21 μM). As for the allosteric site of M2 receptors, binding was found to be dependent on N-substitution rather than on the nature of the side chains. The most potent ligands were the N-allyl analogues 2b and 3b (EC0.5,diss = 12 and 36 nM) and the N-nitrobenzyl derivatives 2c and 3c (EC0.5,diss = 32 and 49 nM). The present findings should help delineate the structural requirements for activity at different types of AChRs and for the design of novel selective ligands.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Alkaloids / chemistry
  • Allosteric Site
  • Humans
  • Ligands
  • Molecular Structure
  • Muscles / metabolism
  • Nicotinic Antagonists / pharmacology
  • Nuclear Magnetic Resonance, Biomolecular
  • Receptor, Muscarinic M2 / chemistry*
  • Toxiferine* / analogs & derivatives
  • Toxiferine* / chemical synthesis
  • Toxiferine* / chemistry
  • Toxiferine* / pharmacology
  • alpha7 Nicotinic Acetylcholine Receptor / chemistry*

Substances

  • Alkaloids
  • Ligands
  • Nicotinic Antagonists
  • Receptor, Muscarinic M2
  • alpha7 Nicotinic Acetylcholine Receptor
  • Toxiferine