"Selective" serotonin 5-HT2A receptor antagonists

Abstract

Blockade of the serotonin 5-HT_2A G protein-coupled receptor (5-HT_2AR) is a fundamental pharmacological characteristic of numerous antipsychotic medications, which are FDA-approved to treat schizophrenia, bipolar disorder, and as adjunctive therapies in major depressive disorder. Meanwhile, activation of the 5-HT_2AR by serotonergic psychedelics may be useful in treating neuropsychiatric indications, including major depressive and substance use disorders. Serotonergic psychedelics and other 5-HT_2AR agonists, however, often bind other receptors, and standard 5-HT_2AR antagonists lack sufficient selectivity to make well-founded mechanistic conclusions about the 5-HT_2AR-dependent effects of these compounds and the general neurobiological function of 5-HT_2ARs. This review discusses the limitations and strengths of currently available "selective" 5-HT_2AR antagonists, the molecular determinants of antagonist selectivity at 5-HT_2ARs, and the utility of molecular pharmacology and computational methods in guiding the discovery of novel unambiguously selective 5-HT_2AR antagonists.

Keywords: 5-HT(2A); Antagonist; Antipsychotic; GPCR; Selectivity.

Figure 1:

Structures of antagonists, with varying degrees of selectivity, commonly employed to probe the 5-HT_2AR.

Figure 2:

[³H]Ketanserin binds with high affinity (1 nM) a large population of targets in the striatum that are not 5-HT₂-type receptors. Note that co-incubation with the putatively selective 5-HT_2AR inverse agonist, M100,907 (1 μM, a concentration chosen to occupy and block the vast majority of 5-HT_2ARs, in order to reveal off-target binding of [³H]ketanserin), blocked binding of [³H]ketanserin in the cortex, but not the striatum. [³H]ketanserin (42.5 Ci/mmol) autoradiography (10-week film exposure) of coronal brain sections from FVB.129P2-Pde6b⁺ Tyr^c-ch/AntJ (sighted FVB) mice.

Figure 3:. Molecular determinants of nonselective binding at aminergic receptors.

Structures of a risperidone-bound 5-HT_2AR (light green, PDB: 6A93) and ritanserin-bound 5-HT_2CR (dark green, PDB: 6BQH) show aromatic and hydrophobic interactions with conserved residues forming a hydrophobic cleft deep in the binding pocket, and an ionic bond between the ligand’s protonated amine and the negatively charged side chain of D^3.32. The whole 5-HT_2AR is shown (left) for perspective, with each transmembrane domain numbered.

Figure 4:

Structures of ergoline-based 5-HT_2AR antagonists variably substituted at the N(1) position (denoted by *).

Figure 5:. Putative molecular determinants of subtype-selective binding at 5-HT₂-type receptors.

Superimposition of 5-HT₂-type receptors suggests that a “side extended cavity” is formed in the 5-HT_2AR (light blue color) by a unique rotamer of F^5.38, making hydrophobic interactions with the non-conserved residue F^4.63 (purple circle, top left). Structures of the 5-HT_2AR (PDB: 6A93, top right), 5-HT_2BR (PDB: 6DS0, bottom left), and 5-HT_2CR (PDB: 6BQH, bottom right) indicate that ECL2 of the 5-HT_2BR exhibits greater conformational freedom compared 5-HT_2A and 5-HT_2CRs, wherein K^ECL2 is electrostatically constrained (orange dashed lines). The side chain of G^5.42 is shown for reference to the side cavity.

Figure 6:. Proposed binding mode of pimavanserin at 5-HT_2A (light green) and 5-HT_2CRs (dark green).

Molecular docking and molecular dynamics simulations indicate that pimavanserin engages nearly identical amino acid residues within the binding pocket of 5-HT_2A and 5-HT_2CRs. The primary determinant of selectivity, the extension of the isobutoxybenzyl moiety into the side-extended cavity between TM4 and TM5 (purple circle), as allowed by the small side chain of G^5.42, is conserved in both receptors and is accompanied by a raised rotamer conformation of F^5.38 in both receptor subtypes.

Figure 7:

Molecular dynamics simulation results of an unbound, antagonist-, and agonist-bound 5-HT_2AR (APO, methiothepin, and ergotamine, respectively). Ionic lock (E318^6.30—R173^3.50) distances are shown as a function of simulation time.