NKP608: a selective NK-1 receptor antagonist with anxiolytic-like effects in the social interaction and social exploration test in rats

NKP608 is a non-peptidic derivative of 4-aminopiperidine which acts as a selective, specific and potent antagonist at the neurokinin-1 (NK-1) receptor both in vitro and in vivo. In vitro, the binding of NKP608 to bovine retina was characterized by an IC50 of 2.6+/-0.4 nM, whereas the compound's affinity to other receptor binding sites, including NK-2 and NK-3, was much lower. Species differences in IC(50) values with NKP608 were less pronounced than with previously described NK-1 receptor antagonists, being 13+/-2 and 27+/-2 nM in gerbil midbrain and rat striatum, respectively. In vivo, using the hind foot thumping model in gerbils, NKP608 exhibited a potent NK-1 antagonistic activity following oral administration (ID(50)=0.23 mg/kg; 2 h pretreatment), supporting a central activity of NKP608. The compound had a long duration of action with an ID(50) value of 0. 15 mg/kg p.o. and 0.38 mg/kg p.o. following a pretreatment of 5 and 24 h, respectively. Following a subchronic administration for 7 consecutive days (once daily) there was no evidence for the development of tolerance or accumulation. In the social interaction test performed in a highly illuminated, unfamiliar test arena, NKP608 specifically increased the time the two rats spent in social contact, and there was no concomitant increase in parameters reflecting general activity, i.e. ambulation (number of square entries) or the number of rearings. Active social time was maximally increased at a dose range of 0.01-1 mg/kg p.o. NKP608, the effect being weaker or absent at both lower (0.001 mg/kg p.o.) and higher (10 mg/kg p.o.) doses. A comparable bell-shaped dose-response relation was seen in the social exploration test in rats. In this modified resident/intruder paradigm, maximal increase in social contact of the intruder rat directed towards the resident rat was seen at a similar dose range (0.03-3 mg/kg p.o.) The effects observed following an acute oral administration of NKP608 were comparable to those seen following a treatment with the well-known benzodiazepine, chlordiazepoxide, in both these tests. These findings indicate that NKP608 exhibits an anxiolytic-like effect and that this effect, as concluded from the observed antagonism of the hind foot thumping induced by i.c.v. administration of the NK-1 receptor agonist SPOMe, is centrally mediated. This makes this compound a potentially promising candidate for treating anxiety-related disorders in humans.