Structure-Activity Studies of 3,9-Diazaspiro[5.5]undecane-Based γ-Aminobutyric Acid Type A Receptor Antagonists with Immunomodulatory Effect

The 3,9-diazaspiro[5.5]undecane-based compounds 2027 and 018 have previously been reported to be potent competitive γ-aminobutyric acid type A receptor (GABA_AR) antagonists showing low cellular membrane permeability. Given the emerging peripheral application of GABA_AR ligands, we hypothesize 2027 analogs as promising lead structures for peripheral GABA_AR inhibition. We herein report a study on the structural determinants of 2027 in order to suggest a potential binding mode as a basis for rational design. The study identified the importance of the spirocyclic benzamide, compensating for the conventional acidic moiety, for GABA_AR ligands. The structurally simplified m-methylphenyl analog 1e displayed binding affinity in the high-nanomolar range (K_i = 180 nM) and was superior to 2027 and 018 regarding selectivity for the extrasynaptic α₄βδ subtype versus the α₁- and α₂- containing subtypes. Importantly, 1e was shown to efficiently rescue inhibition of T cell proliferation, providing a platform to explore the immunomodulatory potential for this class of compounds.