Discovery of 2‑(Imidazo[1,2‑b]pyridazin-2-yl)acetic Acid as a New Class of Ligands Selective for the γ‑Hydroxybutyric Acid (GHB) High-Affinity Binding Sites

Gabazine, a γ-aminobutyric acid type A (GABA_A) receptor antagonist, has previously been reported to inhibit the binding of [³H]­NCS-382, a representative ligand of the high-affinity binding site for the neuroactive substance γ-hydroxybutyric acid (GHB). We herein report a study on the structural determinants of gabazine for binding to (i) the orthosteric binding site of the GABA_A receptor and (ii) the high-affinity GHB binding site. Expanding the structural diversity of available ligands for the high-affinity GHB binding sites, this study identified 2-(imidazo­[1,2-b]­pyridazin-2-yl)­acetic acid as a novel ligand-scaffold leading to analogues with relatively high affinity (K_i 0.19–2.19 μM) and >50 times selectivity for the [³H]­NCS-382 over [³H]­muscimol binding sites. These results highlight that gabazine interacts with the high-affinity GHB and orthosteric GABA_A receptor binding sites differently and that distinct analogues can be generated to select between them. To facilitate further in vivo studies, a promising prodrug candidate for brain delivery was identified.