Positive allosteric modulators as an approach to nicotinic acetylcholine receptor-targeted therapeutics: advantages and limitations

Abstract

Neuronal nicotinic acetylcholine receptors (nAChR), recognized targets for drug development in cognitive and neuro-degenerative disorders, are allosteric proteins with dynamic interconversions between multiple functional states. Activation of the nAChR ion channel is primarily controlled by the binding of ligands (agonists, partial agonists, competitive antagonists) at conventional agonist binding sites, but is also regulated in either negative or positive ways by the binding of ligands to other modulatory sites. In this review, we discuss models for the activation and desensitization of nAChR, and the discovery of multiple types of ligands that influence those processes in both heteromeric nAChR, such as the high-affinity nicotine receptors of the brain, and homomeric α7-type receptors. In recent years, α7 nAChRs have been identified as a potential target for therapeutic indications leading to the development of α7-selective agonists and partial agonists. However, unique properties of α7 nAChR, including low probability of channel opening and rapid desensitization, may limit the therapeutic usefulness of ligands binding exclusively to conventional agonist binding sites. New enthusiasm for the therapeutic targeting of α7 has come from the identification of α7-selective positive allosteric modulators (PAMs) that work effectively on the intrinsic factors that limit α7 ion channel activation. While these new drugs appear promising for therapeutic development, we also consider potential caveats and possible limitations for their use, including PAM-insensitive forms of desensitization and cytotoxicity issues.

Figure 1

The energy landscapes for nAChR state transitions. A) Hypothetical energy wells and barriers for the conformational states of heteromeric nAChR (e.g. muscle-type or neuronal α4*nAChR) as functions of the level of agonist occupancy (see description in text). Under equilibrium conditions, the distributions of receptors into the resting closed (C), brief open (O*), long-lived open (O′), and desensitized (D) states will be determined by the relative free energy of the states (represented by vertical displacements). Dynamically, the transition rates between the states will be inversely related to the log of the energy barriers between the states. B) Two ways in which PAMs may operate on the energy profile of receptors in the doubly liganded state to increase the probability of channel opening: either transiently, after a jump in agonist concentration (left schematic) or under steady-state conditions (right schematic). C) Hypothetical scheme representing state transitions of α7 nAChR under various levels of agonist occupancy. In the absence of PAMs, α7 nAChR do not exhibit a long-lived open state, but do show a unique desensitized state (D_s) that is preferentially favored at higher levels of agonist occupancy. They also have an intrinsic desensitized state (D_i), which is analogous to the D state of heteromeric receptors (Figure 1A). Very efficacious type II PAMs, such as PNU-120596, appear to either destabilize the D_s state, or hypothetically may convert it into a conducting state similar to the O′ state of heteromeric receptors. This hypothesis is represented by (O′) symbols inserted into the panel.

Figure 2

Structures of α7-active PAMs. The PAMs classified as type I, which increase the amplitude but do not strongly alter the kinetics of a7-mediated agonist evoked responses, are shown on the left. They include: CCMI, N-(4-chlorophenyl)-alpha-[[(4-chloro-phenyl)amino]methylene]-3-methyl-5-isoxazoleacet-amide; NS-1738, 1-(5-chloro-2-hydroxy-phenyl)-3-(2-chloro-5-trifluoromethyl-phenyl)-urea; Galantamine; 5-HI, 5-hydroxyindole; LY-2087101, [2-(4-fluoro-phenylamino)-4-methyl-thiazol-5-yl]-thiophen-3-yl-methanone; Genistein; and Ivermectin. Type II PAMs, which appear to slow or reverse α7 desensitization are shown on the right and include: PNU-120596, 1-(5-chloro-2,4-dimethoxy-phenyl)-3-(5-methyl-isoxazol-3-yl)-urea; TQS, 4-naphthalene-1-yl-3a,4,5,9b-tetrahydro-3-H-cyclopenta[c]quinoline-8-sulfonic acid amide; and A-867744, 4-(5-(4-chlorophenyl)-2-methyl-3-propionyl-1H-pyrrol-1-yl)benzenesulfonamide. PAMs proposed to be intermediate in their activity are shown in the center and include: JNJ-1930942, 2-[[4-fluoro-3-(trifluoromethyl)phenyl]amino]-4-(4-pyridinyl)-5-thiazolemethanol; and SB-206553, 3,5-dihydro-5-methyl-N-3-pyridinylbenzo [1,2-b:4,5-b′]-di pyrrole-1(2H)-carboxamide.

Figure 3

α7 PAM structures and putative binding sites. A) Pharmacophore orientation comparison between LY-2087101 and JNJ-1930942. The structures of the two compounds were optimized using Chem 3D Pro v.12.0 (CambridgeSoft, Cambridge, MA): their energies were first minimized by molecular mechanics to minimum RMS gradient of 0.001 separately. Then LY-2087101 and JNJ-1930942 were overlaid on top of each other and shown in green and magenta, respectively. B) Molecular electrostatic potential surfaces of NS-1738 (left) and PNU-120595 (right). The structures of the two compounds were optimized using Gaussian09 (Gaussian, Inc., Wallingford, CT). The electrostatic potential on a total electron density isosurface (0.002 Bohr/ų) of the two compounds is displayed in GaussView (Gaussian, Inc., Wallingford, CT) with red color indicating the most negative electrostatic potential and blue color indicating the most positive electrostatic potential, respectively. The structures of the two compounds are displayed on top of each electrostatic potential surface. C) Structural domains of nAChR regulating PAM activity and/or binding. The key elements of the α7 receptor were modeled using the 2BG9 template for the transmembrane domain fused to the N-terminal extracellular domain made from the 2PGZ template. The C-loop is shown in yellow, and the M2 and M3 linker loop is shown in green. The view was made from the outside of the channel pore, and the four transmembrane helixes (M1 to M4) were lined clockwise as shown, putting the M2 helix toward the channel pore. Residues involved for different α7 PAMs potentiation are displayed as spheres of different colors as indicated: calcium, tan; galantamine, cyan; common residues for type I PAMs Ivermectin and LY-2087101 and the type II PAM PNU120596, blue; residues only for Ivermectin, orange; residues only for PNU-120596, red. Leucine 248 is also shown in magenta in both subunits. D) A close-up view of the transmembrane domain, made by rotating the view in part C 90° vertically away from the plane of the paper.