Bisquaternary caracurine V and iso-caracurine V salts as ligands for the muscle type of nicotinic acetylcholine receptors: SAR and QSAR studies.

The binding constants (K(i) values) of 24 caracurine V and 6 iso-caracurine V analogues for the muscle type of nicotinic ACh receptors (nAChR) from Torpedo californica were determined in a binding assay using (+/-)-[(3)H]epibatidine as a radioligand. The allyl alcohol group present in the iso-caracurine V ring system was found to be essential for high binding affinity. The most potent compounds are the dimethyl and di-(4-nitrobenzyl)-iso-caracurinium V salts 29 (18 nM), and 31 (79 nM), respectively. Compound 29 and the corresponding diallyl analogue 30 (350 nM) exhibited similar binding affinities as the equally substituted neuromuscular-blocking agents toxiferine I (14 nM) and alcuronium (234 nM), respectively. The SAR results were confirmed by QSAR studies, which additionally revealed that the presence of hydrogen-bond acceptor groups close to the quaternary nitrogen, is detrimental for the nicotinic binding affinity. The diallyl- and dimethylcaracurinium V salts 13 and 27, respectively, which are known to be among the most potent allosteric modulators of M(2) receptors (EC(50)=10 and 8nM, respectively), exhibited rather low nicotinic binding affinities for muscle type nAChR (K(i)=1.5 and 5.2 microM, respectively). Such a large difference in affinity suggests that it is possible to develop compounds with high muscarinic allosteric potency and low or negligible affinities for (alpha1)(2)beta1gammadelta nAChR. Additionally, the iso-caracurine V analogues with binding affinities comparable to those of (+)-tubocurarine and alcuronium could become a new class of neuromuscular-blocking agents.

Prenylated indole alkaloids from Flustra foliacea with subtype specific binding on NAChRs.

Several brominated indole alkaloids and a diterpene (1-7) were isolated from the dichloromethane extract of the North Sea Bryozoan Flustra foliacea. Alkaloid 4 is a new natural product, whose structure was elucidated by interpretation of spectroscopic data (NMR, mass, UV, and IR). All compounds were tested for their in vitro affinity towards the alpha4beta2* and alpha7* subtype of the neuronal nicotinic acetylcholine receptor (nAChR) using radioligand binding assays. Deformylflustrabromine (3) and deformylflustrabromine B (4) were shown to have affinities in the lower micromolar range for nAChRs, differing in their subtype preference.

Novel enantiopure ferrugininoids active as nicotinic agents: synthesis and radioligand binding studies.

A series of hitherto unknown enantiopure (-)-ferruginine analogues of type 8 and 9 was prepared and tested for the affinity toward the nicotinic acetylcholine receptor (nAChR) subtypes (alpha4)2(beta2)3, alpha7*, alpha3beta4*, and (alpha1)2beta1gamma delta. The stereoconservative asymmetric syntheses started with (-)-cocainhydrochloride (10) from the chiral pool which was transformed into the chiral building blocks (+)-2-tropanone (11) and to (-)-anhydroecgonine (18). Key steps of the syntheses are novel extensions to existing methodology e.g. a Suzuki Pd(0)-mediated cross-coupling of vinyl triflate (12) with the heteroaryl organoboranes 13-15 and an inverse type [4+2]-cycloaddition with 1,2,4,5-tetrazine (21). The bioisosteric replacement of the 3-acetyl pharmacophoric element of the lead 6 by a 3-pyridyl, 5-chloropyridyl, 5-pyrimidinyl, 2-pyrazinyl, or 4-pyridazinyl moiety resulted in nAChR ligands with Ki-values ranging from 1.1-713 nM toward the (alpha4)2(beta2)3 subtype combined with significant differentiation among the nAChR subtypes when tested in vitro by radioligand binding studies. Generally the ferrugininoids are less potent than the corresponding norferrugininoids. Similar to results of the norferrugininoid series the novel azine substituted ferrugininoids 8 proved to be more potent than the diazine analogues 9; both exhibited higher affinities compared to the lead 6. The 5-chloropyridyl-containing variant 8b [1R, 5S)-enantiomer] turned out to be the most active nAChR ligand with a 12-fold higher affinity toward the (alpha4)2(beta2)3 subtype than the corresponding (1S, 5R)-form ent-8b.

Synthesis and nicotinic binding studies on enantiopure pinnamine variants with an 8-azabicyclo[3.2.1]octane moiety.

Bioisosteric replacement of the 9-azabicyclo[4.2.1]nonane pharmacophoric element of the novel alkaloidal marine toxine pinnamine (5) by the 8-azabicyclo[3.2.1]octane moiety resulted in conformationally restricted analogues 6a and 6c of (-)-ferruginine (4). Key step in the diastereoselective synthesis of these pyranotropanes was the condensation of enantiopure ecgonine methyl ester (9) from the "chiral pool" with the lithium anion of N-tert-butylbutyraldimin and subsequent cyclisation with TFA. The potential nAChR ligands were tested for their in vitro affinity for the (alpha4)2(beta2)3 and the alpha7* nAChR subtypes. Despite obvious structural similarities with the potent alkaloids pinnamine (5) and (-)-ferruginine (4) the pyranotropanes 6a and 6c exhibited distinctly lower nAChR affinities.