Development of M1 mAChR allosteric and bitopic ligands: prospective therapeutics for the treatment of cognitive deficits

Abstract

Since the cholinergic hypothesis of memory dysfunction was first reported, extensive research efforts have focused on elucidating the mechanisms by which this intricate system contributes to the regulation of processes such as learning, memory, and higher executive function. Several cholinergic therapeutic targets for the treatment of cognitive deficits, psychotic symptoms, and the underlying pathophysiology of neurodegenerative disorders, such as Alzheimer's disease and schizophrenia, have since emerged. Clinically approved drugs now exist for some of these targets; however, they all may be considered suboptimal therapeutics in that they produce undesirable off-target activity leading to side effects, fail to address the wide variety of symptoms and underlying pathophysiology that characterize these disorders, and/or afford little to no therapeutic effect in subsets of patient populations. A promising target for which there are presently no approved therapies is the M1 muscarinic acetylcholine receptor (M1 mAChR). Despite avid investigation, development of agents that selectively activate this receptor via the orthosteric site has been hampered by the high sequence homology of the binding site between the five muscarinic receptor subtypes and the wide distribution of this receptor family in both the central nervous system (CNS) and the periphery. Hence, a plethora of ligands targeting less structurally conserved allosteric sites of the M1 mAChR have been investigated. This Review aims to explain the rationale behind allosterically targeting the M1 mAChR, comprehensively summarize and critically evaluate the M1 mAChR allosteric ligand literature to date, highlight the challenges inherent in allosteric ligand investigation that are impeding their clinical advancement, and discuss potential methods for resolving these issues.

Figure 1

Common signal transduction pathways of the five muscarinic acetylcholine receptors.

Figure 2

Structure of the M₁/M₄ mAChR-preferring agonist xanomeline (1).

Figure 3

Two pathophysiological pathways underlying Alzheimer’s disease.

Figure 4

Structures of two M₁ mAChR selective agonists, talsaclidine (2) and AF267B (3).

Figure 5

Three modes by which allosteric ligands exert their effects. (1) Orthosteric ligand affinity modulation; (2) orthosteric ligand efficacy modulation; and (3) direct allosteric agonism.

Figure 6

Structures of the muscarinic orthosteric agonist acetylcholine (4) and the muscarinic orthosteric antagonist atropine (5), highlighting the positively charged and ionizable head groups, respectively.

Figure 7

Structures of the two “common” muscarinic allosteric site binders, gallamine (6) and alcuronium (7).

Figure 8

Structures of the second muscarinic allosteric site binders, KT5720 (8) and WIN62,577 (9).

Figure 9

Simulated example of a common screening method for putative allosteric ligands. (a) A concentration–response curve of a known orthosteric ligand (e.g., ACh) used to derive an EC₂₀ concentration value; log τ_A is the operational efficacy parameter of the orthosteric ligand (the higher the value, the greater the efficacy). (b) How a concentration response curve of a putative allosteric ligand might look in the presence of the derived EC₂₀ concentration of orthosteric ligand.

Figure 10

Simulated example of interaction studies between an orthosteric agonist and (a) a positive allosteric modulator (PAM), (b) an allosteric agonist, and (c) a competitive agonist, with broken lines indicating the intersection of the concentrations of test ligand with the EC₂₀ of the orthosteric ligand. (d) An overlay of the data points from graphs (a)–(c) which intersect with the EC₂₀ of the orthosteric ligand; log τ_B is the operational efficacy parameter of the allosteric ligand (the higher the value, the greater the efficacy; the high negative value represents a complete absence of allosteric ligand efficacy); log β is the functional cooperativity factor between the orthosteric and allosteric ligands (log β > 0 = positive cooperativity, log β = 0 = neutral cooperativity or, in this instance, direct competition between two orthosteric ligands).

Figure 11

Structure of the M₄ mAChR allosteric ligand LY2033298 (10).

Figure 12

Advantages and disadvantages of orthosteric and allosteric ligands, and the theoretical fusion of their respective advantages by bitopic ligands: high potency, high subtype selectivity, and the potential to elicit distinct signaling profiles (Ps) that may be of therapeutic value.

Figure 13

Structure of the M₂ mAChR bitopic ligand McN-A-343 (11).

Figure 14

Structure of the M₁-preferring allosteric ligand brucine (12).

Figure 15

Structure of the breakthrough M₁-selective mAChR PAM BQCA (13).

Figure 16

Structure of compound 14.

Figure 17

Summary of the pharmacophoric features reported by Yang et al.

Figure 18

Summary of the pharmacophoric features reported by Kuduk et al.

Figure 19

Structures of the structurally diverse muscarinic allosteric ligands VU0119498 (22), VU0027414 (23), VU0090157 (24), and VU0029767 (25).

Figure 20

Structure of the BQCA-like PAMs VU0366369 (26), VU0448350-1 (27), and compound 28.

Figure 21

Structure of the novel scaffold compound 29.

Figure 22

Structures of the novel scaffold analogues VU0405652 (30) and VU0456940 (31).

Figure 23

Structures of AC-42 (32), AC-260584 (33), and 77-LH-28-1 (34); R-SAT functional assays measuring β-galactosidase activity gave estimates of potency, EC₅₀ (converted from pEC₅₀), and efficacy, %Eff (normalized to the maximum response to the muscarinic agonist carbachol).

Figure 24

Structure of N-desmethylclozapine (35).

Figure 25

Structure of TBPB (36). Calcium mobilization assays in rat M₁-transfected CHO-K1 cells gave an estimate of potency, EC₅₀, and efficacy, %Eff (normalized to the maximum response to the muscarinic agonist carbachol).

Figure 26

Structures of VU0184670 (37), VU0357017 (38), and VU0364572 (39). Calcium mobilization assays in rat M₁-transfected CHO-K1 cells gave an estimate of potency, EC₅₀, and efficacy, %Eff (normalized to the maximum response to the muscarinic agonist ACh).

Figure 27

Structure of Lu AE51090 (40).

Figure 28

Structure of compound 41.

Figure 29

Structures of GSK compounds 42–45, with the motifs common to other M₁ mAChR-selective ligands highlighted in red.