PD-137889 (N-methylhexahydrofluorenamine) is a chemical compound that is active as an NMDA receptor antagonist in the central nervous system at roughly 30 times the potency of the "flagship" of its class, ketamine, and substitutes for phencyclidine in animal studies. K<sub>i</sub> [<sup>3</sup>H]TCP binding = 27 nM versus ketamine's K<sub>i</sub> = 860 nM.