The NK3 receptor is a GPCR that is prominently expressed in limbic areas of the 
brain, many of which have been implicated in schizophrenia. Phase II clinical 
trials in schizophrenia with two selective NK3 antagonists (osanetant and 
talnetant) have demonstrated significant improvement in positive symptoms. The 
objective of this study was to characterize the properties of a novel dual 
NK2/NK3 antagonist, RO5328673. [(3)H]RO5328673 bound to a single saturable site 
on hNK2, hNK3 and gpNK3 with high-affinity. RO5328673 acted as an insurmountable 
antagonist at both human and guinea-pig NK3 receptors in the [(3)H]IP 
accumulation assay. In binding kinetic analyses, [(3)H]RO5328673 had fast 
association and dissociation rates at hNK2 while it had a fast association rate 
and a remarkably slow dissociation rate at gp and hNK3. In electrophysiological 
recordings of gp SNpc, RO5328673 inhibited the senktide-induced potentiation of 
spontaneous activity of dopaminergic neurons with an insurmountable mechanism of 
action. RO5328673 exhibited in-vivo activity in gerbils, robustly reversing the 
senktide-induced locomotor activity. The TM2 residue gpNK3-A114(2.58) (threonine 
in all other species) was identified as the critical residue for the RO5328673's 
slower dissociation kinetics and stronger insurmountable mode of antagonism in 
the guinea-pig as compared to hNK3-T139(2.58). Using site-directed mutagenesis, 
[(3)H]RO5328673 binding and rhodopsin-based modeling, the important molecular 
determinants of the RO5328673-binding pocket of hNK3 were determined. A 
comparison of the RO5328673-binding pocket with that of osanetant showed that 
two antagonists have similar contact sides on hNK3 binding crevice except for 
three mutations V95L(1.42), Y247W(5.38), V255I(5.46), which behaved differently 
between interacting modes of two antagonists in hNK3.