Structural Determinants for the Mode of Action of Imidazopyridine DS2 at δ-Containing γ-Aminobutyric Acid Type A Receptors

Abstract

Despite the therapeutic relevance of δ-containing γ-aminobutyric acid type A receptors (GABAARs) and the need for δ-selective compounds, the structural determinants for the mode and molecular site of action of δ-selective positive allosteric modulator imidazo­[1,2-a]­pyridine DS2 remain elusive. To guide the quest for insight, we synthesized a series of DS2 analogues guided by a structural receptor model. Using a fluorescence-based fluorometric imaging plate reader membrane potential assay, we found that the δ-selectivity and the pharmacological profile are severely affected by substituents in the 5-position of the imidazopyridine core scaffold. Interestingly, the 5-methyl, 5-bromo, and 5-chloro DS2 analogues, 30, 35, and 36, were shown to be superior to DS2 at α4β1δ as mid-high nanomolar potency δ-selective allosteric modulators, displaying 6–16 times higher potency than DS2. Of these, 30 also displayed at least 60-fold selectivity for α4β1δ over α4β1γ2 receptor subtypes representing a potential tool for the selective characterization of δ-containing GABAARs in general.

Relating neurosteroid modulation of inhibitory neurotransmission to behaviour

“…173 Unfortunately, DS2 has limited brain penetration 64 but ongoing analogue studies may resolve this current limitation. 174,175 The development of brain penetrant δ-GABA A R selective compounds would be invaluable in clarifying the role of these receptors in the behavioural effects of neurosteroids but may additionally pave the way for novel small molecule therapeutics with an improved side effect profile.…”

“…Although there is functional selectivity for both recombinant and native δ‐GABA A Rs, mutagenesis studies have highlighted key amino acid interaction sites with DS2 located not on the δ subunit per se, but instead residing in transmembrane pockets formed between the α subunit (Falk‐Petersen 2021 α4) and the β subunit 173 . Unfortunately, DS2 has limited brain penetration 64 but ongoing analogue studies may resolve this current limitation 174,175 . The development of brain penetrant δ‐GABA A R selective compounds would be invaluable in clarifying the role of these receptors in the behavioural effects of neurosteroids but may additionally pave the way for novel small molecule therapeutics with an improved side effect profile.…”