Synthesis and pharmacological evaluation of hexahydrofluorenamines as noncompetitive antagonists at the N-methyl-D-aspartate receptor.

S J Hays, P M Novak, D F Ortwine, C F Bigge, N L Colbry, G Johnson, L J Lescosky, T C Malone, A Michael, M D Reily

The noncompetitive (PCP) site of the N-methyl-D-aspartate (NMDA) receptor complex has been implicated in a number of pathologies, including the etiology of ischemic stroke. Recent testing has shown that cis-1,2,3,4,9,9a-hexahydro-N-methyl-4aH-fluoren-4a-amine (1), a rigid analog of PCP, is a potent antagonist at this site (IC50 = 30 nM for displacement of [3H]TCP). On the basis of this finding, a number of derivatives encompassing variations in stereochemistry, amine substitution and position, aromatic and aliphatic ring substitution, and heteroatom ring substitution have been prepared to explore the structure-activity relationships around this ring system. All compounds were evaluated for their PCP receptor affinity; potent compounds were also tested in vitro (cultured neurons) and in vivo (prevention of NMDA-induced lethality in mice). The present hexahydrofluorenamines demonstrated a wide range of potencies, with optimal affinity concentrated in analogs containing a heteroatom (sulfur) in the B ring (IC50 of 11 nM versus [3H]TCP for 16b), methyl substitution on the amine, and R stereochemistry at the 4a position. No significant improvement in affinity was seen with aromatic ring substitution. Aliphatic ring substitution, large amine substituents, and alterations in the position of amine substitution on the ring system resulted in a loss of potency. To explore the effect of simultaneous hydrogen bonding with a putative receptor atom from two directions, the 2-hydroxymethyl derivatives were prepared. This substitution resulted in a loss in receptor binding affinity. Molecular modeling, X-ray, and NMR studies have been used to determine an optimal conformation of the hexahydrofluoreneamines at the receptor site.