Modifications at C(5) of 2-(2-Pyrrolidinyl)-Substituted 1,4-Benzodioxane Elicit Potent α4β2 Nicotinic Acetylcholine Receptor Partial Agonism with High Selectivity over the α3β4 Subtype

Abstract

A series of diastereomeric 2-(2-pyrrolidinyl)-1,4-benzodioxanes bearing a small, hydrogen-bonding substituent at the 7-, 6-, or 5-position of benzodioxane have been studied for α4β2 and α3β4 nicotinic acetylcholine receptor affinity and activity. Analogous to C(5)H replacement with N and to a much greater extent than decoration at C(7), substitution at benzodioxane C(5) confers very high α4β2/α3β4 selectivity to the α4β2 partial agonism. Docking into the two receptor structures recently determined by cryo-electron microscopy and site-directed mutagenesis at the minus β2 side converge in indicating that the limited accommodation capacity of the β2 pocket, compared to that of the β4 pocket, makes substitution at C(5) rather than at more projecting C(7) position determinant for this pursued subtype selectivity.

Determinants for α4β2 vs. α3β4 Subtype Selectivity of Pyrrolidine-Based nAChRs Ligands: A Computational Perspective with Focus on Recent cryo-EM Receptor Structures

“…The much higher difference in affinity between the human α3β4 and the rat α3β4 of sazetidine-A compared to that of TMAQ and AT-1001, (192-times vs. 14-and 20-times), also reflected in the 115 times higher α4β2 vs. α3β4 selectivity at the rat compared to the human subtypes, prompted us to hypothesize that other structural inter-species differences of the α3β4 subtype could be involved. Differently, the similar binding affinities at the human and rat α4β2 subtypes of sazetidine-A and of our in-house ligands, among which (S,R)-2, suggest a very high degree of inter-species binding site similarity of the α4β2 subtype [33].…”

“…b Tested at membranes of human α3β4 transfected cells. c Data from Bavo et al [33]. d Data from Bolchi et al [32].…”

“…In detail, since none of the compounds contained a pyridine nitrogen as a suitable hydrogen bond partner for the structural water molecule, and all except (S)-4 presented a substituent in the same (meta) position instead, we removed the water molecule from the α3β4 binding pocket. This approach was previously successful in describing the binding mode of (S,R)-2 at α4β2 and α3β4 nAChRs [33].…”

“…(S)-3 binds similarly at the interfaces of both subtypes and coherently overlays the pyrrolidine ring with that of (S)-nicotine and (S,R)-2 (Figure 5A). Furthermore, the m-hydroxyl group of (S)-3 superimposes with the structural water molecule, and interacts with the backbone carbonyl of β2-Asn109/β4-Asn111 and the backbone NH of β2-Leu121/β4-Leu123 within the small and hydrophilic pocket known to be critical for nAChR affinity [33]. The 3-times higher affinity of (S)-3 than (S)-nicotine might be attributed to the replacement of a flexible water-mediated H-bond network with the more rigid and stable direct interaction between the m-OH and the receptor.…”

“…In the last 15 years, we have designed and developed some series of chiral α4β2 ligands, full and partial agonists and antagonists [22], initially linking the N-methyl-2pyrrolidinyl residue, typical of nicotinoids, to C(2) of 1,4-benzodioxane [23,24], a scaffold widely employed to design bioactive molecules [25][26][27][28][29] and, in this instance, to mimick the aryloxymethyl portion of prolinol aryl ethers, well known high-affinity α4β2 ligands such as A-84543 [30] (Figure 1B; for benzodioxane scaffold numbering see (S,R)-21 formula). Successive steps were the decoration of the benzodioxane by introducing substituents at its C (7) [31], deconstruction of the dioxane ring to give new phenyl and pyridyl ethers of prolinol [7,32], replacement of benzene with pyridine to give the four regioisomeric pyridodioxanes [5], and again benzodioxane decoration with substituents at C(6) and C(5) [33]. In each of these series of prolinol aryl ethers or benzodioxane/pyridodioxane derivatives some compounds, as exemplified in Figure 1, exhibited one to hundred nanomolar α4β2 affinity and some of these also from good to high functional and binding selectivity over the α3β4 subtype.…”