Ifenprodil Stereoisomers: Synthesis, Absolute Configuration, and Correlation with Biological Activity

Ifenprodil (1) is a potent GluN2B-selective N-methyl-d-aspartate (NMDA) receptor antagonist that is used as a cerebral vasodilator and has been examined in clinical trials for the treatment of drug addiction, idiopathic pulmonary fibrosis, and COVID-19. To correlate biological data with configuration, all four ifenprodil stereoisomers were prepared by diastereoselective reduction and subsequent separation of enantiomers by chiral HPLC. The absolute configuration of ifenprodil stereoisomers was determined by X-ray crystal structure analysis of (1R,2S)-1a and (1S,2S)-1d. GluN2B affinity, ion channel inhibitory activity, and selectivity over α, σ, and 5-HT receptors were evaluated. (1R,2R)-Ifenprodil ((1R,2R)-1c) showed the highest affinity toward GluN2B-NMDA receptors (K_i = 5.8 nM) and high inhibition of ion flux in two-electrode voltage clamp experiments (IC₅₀ = 223 nM). Whereas the configuration did not influence considerably the GluN2B-NMDA receptor binding, (1R)-configuration is crucial for elevated inhibitory activity. (1R,2R)-Configured ifenprodil (1R,2R)-1c exhibited high selectivity for GluN2B-NMDA receptors over adrenergic, serotonergic, and σ₁ receptors.