Differential inhibition of foot tapping and chromodacryorrhoea in gerbils by CNS penetrant and non-penetrant tachykinin NK1 receptor antagonists

Prokinetic effects of the neurokinin NK2 receptor agonist [Lys5,MeLeu9,Nle10]-NKA(4-10) on bladder and colorectal activity in minipigs

The effects of the neurokinin NK2 receptor agonist [Lys⁵,MeLeu⁹,Nle¹⁰]-NKA_(4-10) (LMN-NKA) on bladder and colorectal function were examined in minipigs. In anesthetized animals, subcutaneous (SC) administration of 30-100 μg/kg increased peak bladder and colorectal pressures. Increases in bladder and colorectal pressure were inhibited by a 15 min pretreatment with the NK2 receptor antagonist GR 159897 (1 mg/kg intravenously (IV)). Bladder and colorectal pressures were also increased after IV (0.3 μg/kg), intranasal (IN; 100 μg/kg) and sublingual administration (SL; 5 mg/kg). There was a nonsignificant trend for hypotension (16 or 12% decrease in mean arterial pressure) after 100 μg/kg SC and 0.3 μg/kg IV, respectively, but not after 100 μg/kg IN or 5 mg/kg SL. In conscious minipigs, 30-300 μg/kg SC caused a dose-related increase in defecation that was accompanied by emesis in 38% of subjects receiving 300 μg/kg. Urination was increased after 100 μg/kg SC but not lower or higher doses. The peak plasma exposure (Cmax) after 100 μg/kg SC was 123 ng/mL, and area under the curve (AUC) was 1790 min * ng/mL. Defecation response rates (~82%) were maintained after SC administration of LMN-NKA (30 μg/kg) given 3 times daily over 5 consecutive days. Defecation rates were higher after a single dose of 100 μg/kg IN compared with vehicle, but this did not reach significance. After 7-10 mg/kg SL, 83% of animals urinated and defecated, and none had emesis. The data support the feasibility of developing a convenient and well-tolerated route of administration of LMN-NKA for human use. Minipigs may be a suitable species for toxicology studies with LMN-NKA due to the relatively low rate of emesis in this species.

[Lys5,MeLeu9,Nle10]-NKA(4-10) Elicits NK2 Receptor-Mediated Micturition and Defecation, and NK1 Receptor-Mediated Emesis and Hypotension, in Conscious Dogs

Tachykinin neurokinin 2 (NK2) receptor agonists may have potential to alleviate clinical conditions associated with bladder and gastrointestinal underactivity by stimulating contraction of visceral smooth muscle. The ability of [Lys⁵,MeLeu⁹,Nle¹⁰]-neurokinin A_(4-10) (LMN-NKA) to elicit micturition and defecation was examined after repeated administration in groups of 2-10 conscious dogs. Administration of 10-100 μg/kg, i.v., four times daily for six consecutive days, reliably elicited micturition after ≥90% of doses and defecation after ≥50% of doses. Voiding occurred <4 minutes after dosing and was short lasting (<10 minutes). LMN-NKA was well tolerated, with emesis after ∼25% of doses at 100 μg/kg, i.v. Hypotension was induced by 100 μg/kg, i.v., of LMN-NKA but not by lower doses. Administration of 30-300 μg/kg, s.c., twice daily for seven consecutive days, reliably elicited both urination and defecation after 88%-100% of doses, and was accompanied by a high rate of emesis (50%-100%). The onset of voiding was rapid (<7 minutes) but was more prolonged than after intravenous administration (30-60 minutes). Emesis induced by 30 or 300 μg/kg, s.c., of LMN-NKA was significantly reduced (from 58% to 8% and from 96% to 54%, respectively) by a 30-minute pretreatment with the neurokinin 1 (NK1) receptor antagonist, (2S,3S)-N-(2-methoxybenzyl)-2-phenylpiperidin-3-amine (CP-99,994; 1 mg/kg, s.c.). The ability of selective NK2 receptor agonists to elicit on-demand voiding could potentially address a major unmet need in people lacking voluntary control of micturition and/or defecation. LMN-NKA unexpectedly activated NK1 receptors at doses that stimulated voiding, causing emesis and hypotension that may limit the clinical utility of nonselective NK2 receptor agonists.

2-[(3aR,4R,5S,7aS)-5-{(1S)-1-[3,5-bis(trifluoromethyl)phenyl]-2-hydroxyethoxy}-4-(2-methylphenyl)octahydro-2H-isoindol-2-yl]-1,3-oxazol-4(5H)-one: a potent human NK1 receptor antagonist with multiple clearance pathways

Hydroisoindoline 2 has been previously identified as a potent, brain-penetrant NK1 receptor antagonist with a long duration of action and improved profile of CYP3A4 inhibition and induction compared to aprepitant. However, compound 2 is predicted, based on data in preclinical species, to have a human half-life longer than 40 h and likely to have drug-drug-interactions (DDI), as 2 is a victim of CYP3A4 inhibition caused by its exclusive clearance pathway via CYP3A4 oxidation in humans. We now report 2-[(3aR,4R,5S,7aS)-5-{(1S)-1-[3,5-bis(trifluoromethyl)phenyl]-2-hydroxyethoxy}-4-(2-methylphenyl)octahydro-2H-isoindol-2-yl]-1,3-oxazol-4(5H)-one (3) as a next generation NK1 antagonist that possesses an additional clearance pathway through glucuronidation in addition to that via CYP3A4 oxidation. Compound 3 has a much lower propensity for drug-drug interactions and a reduced estimated human half-life consistent with once daily dosing. In preclinical species, compound 3 has demonstrated potency, brain penetration, and a safety profile similar to 2, as well as excellent pharmacokinetics.

In vitro and in vivo pharmacological characterization of the novel NK₁ receptor selective antagonist Netupitant

The novel NK(1) receptor ligand Netupitant has been characterized in vitro and in vivo. In calcium mobilization studies CHO cells expressing the human NK receptors responded to a panel of agonists with the expected order of potency. In CHO NK(1) cells Netupitant concentration-dependently antagonized the stimulatory effects of substance P (SP) showing insurmountable antagonism (pK(B) 8.87). In cells expressing NK(2) or NK(3) receptors Netupitant was inactive. In the guinea pig ileum Netupitant concentration-dependently depressed the maximal response to SP (pK(B) 7.85) and, in functional washout experiments, displayed persistent (up to 5h) antagonist effects. In mice the intrathecal injection of SP elicited the typical scratching, biting and licking response that was dose-dependently inhibited by Netupitant given intraperitoneally in the 1-10mg/kg dose range. In gerbils, foot tapping behavior evoked by the intracerebroventricular injection of a NK(1) agonist was dose-dependently counteracted by Netupitant given intraperitoneally (ID(50) 1.5mg/kg) or orally (ID(50) 0.5mg/kg). In time course experiments in gerbils Netupitant displayed long lasting effects. In all the assays Aprepitant elicited similar effects as Netupitant. These results suggest that Netupitant behaves as a brain penetrant, orally active, potent and selective NK(1) antagonist. Thus this molecule can be useful for investigating the NK(1) receptor role in the control of central and peripheral functions. Netupitant has clinical potential in conditions such as chemotherapy induced nausea and vomiting, in which the blockade of NK(1) receptors has been demonstrated valuable for patients.

Characterization of RO4583298 as a novel potent, dual antagonist with in vivo activity at tachykinin NK₁ and NK₃ receptors

BACKGROUND AND PURPOSE

Clinical results of osanetant and talnetant (selective-NK₃ antagonists) indicate that blocking the NK₃ receptor could be beneficial for the treatment of schizophrenia. The objective of this study was to characterize the in vitro and in vivo properties of a novel dual NK₁/NK₃ antagonist, RO4583298 (2-phenyl-N-(pyridin-3-yl)-N-methylisobutyramide derivative).

EXPERIMENTAL APPROACH

RO4583298 in vitro pharmacology was investigated using radioligand binding ([³H]-SP, [³H]-osanetant, [³H]-senktide), [³H]-inositol-phosphate accumulation Schild analysis (SP- or [MePhe⁷]-NKB-induced) and electrophysiological studies in guinea-pig substantia nigra pars compacta (SNpc). The in vivo activity of RO4583298 was assessed using reversal of GR73632-induced foot tapping in gerbils (GFT; NK₁) and senktide-induced tail whips in mice (MTW; NK₃).

KEY RESULTS

RO4583298 has a high-affinity for NK₁ (human and gerbil) and NK₃ (human, cynomolgus monkey, gerbil and guinea-pig) receptors and behaves as a pseudo-irreversible antagonist. Unusually it binds with high-affinity to mouse and rat NK₃, yet with a partial non-competitive mode of antagonism. In guinea-pig SNpc, RO4583298 inhibited the senktide-induced potentiation of spontaneous activity of dopaminergic neurones with an apparent non-competitive mechanism of action. RO4583298 (p.o.) robustly blocked the GFT response, and inhibited the MTW.

CONCLUSIONS AND IMPLICATIONS

RO4583298 is a high-affinity, non-competitive, long-acting in vivo NK₁/NK₃ antagonist; hence providing a useful in vitro and in vivo pharmacological tool to investigate the roles of NK₁ and NK₃ receptors in psychiatric disorders.

Potent, brain-penetrant, hydroisoindoline-based human neurokinin-1 receptor antagonists

3-[(3aR,4R,5S,7aS)-5-{(1R)-1-[3,5-bis(trifluoromethyl)phenyl]ethoxy}-4-(4-fluorophenyl)octahydro-2H-isoindol-2-yl]cyclopent-2-en-1-one (17) is a high affinity, brain-penetrant, hydroisoindoline-based neurokinin-1 (NK(1)) receptor antagonist with a long central duration of action in preclinical species and a minimal drug-drug interaction profile. Positron emission tomography (PET) studies in rhesus showed that this compound provides 90% NK(1) receptor blockade in rhesus brain at a plasma level of 67 nM, which is about 10-fold more potent than aprepitant, an NK(1) antagonist marketed for the prevention of chemotherapy-induced and postoperative nausea and vomiting (CINV and PONV). The synthesis of this enantiomerically pure compound containing five stereocenters includes a Diels-Alder condensation, one chiral separation of the cyclohexanol intermediate, an ether formation using a trichloroacetimidate intermediate, and bis-alkylation to form the cyclic amine.

The discovery of potent, selective, and orally bioavailable hNK1 antagonists derived from pyrrolidine

SAR studies on amides, ureas, and vinylogous amides derived from pyrrolidine led to the discovery of several potent hNK(1) antagonists. One particular vinylogous amide (45b) had excellent potency, selectivity, pharmacokinetic profile, and functional activity in vivo. An in vivo rhesus macaque brain receptor occupancy PET study for compound 45b revealed an estimated Occ(90) approximately 300 ng/ml.

[3H]GR205171 displays similar NK1 receptor binding profile in gerbil and human brain

1 In this study, [(3)H]GR205171 (3(S)-(2-methoxy-5-(5-trifluoromethyltetrazol-1-yl)-phenylmethylamino)-2(S)-phenylpiperidine), a potent and selective NK1 receptor antagonist, was characterised in autoradiographic studies in gerbil brain and in binding experiments on homogenates from gerbil and human brain cortex and striatum. 2 In autoradiographic studies in gerbil brain, highest levels of [(3)H]GR205171 binding sites were observed in caudate putamen, nucleus accumbens, medial and cortical nuclei of the amygdala and intermediate levels were detected in the hypothalamus, basolateral amygdala, septum, and cortex. 3 Saturation experiments in homogenates of brain striatum from gerbil showed that [(3)H]GR205171 binds to a single receptor population with a pK(d) value of 10.8+/-0.2 and a B(max) value of 607+/-40 fmol mg(-1). A lower number of NK1 receptor sites was found in cortex, where a B(max) of 94+/-6 fmol mg(-1) protein was obtained. Saturation experiments performed on homogenates from brain striatum of two human subjects and brain cortex of three human subjects showed that [(3)H]GR205171 binds with pK(d) values not different from gerbil and B(max) values ranging from 318+/-51 to 432+/-27 fmol mg(-1) protein in striatum and from 59+/-1 to 74+/-21 fmol mg(-1) protein in cortex. The natural ligand [(3)H]Substance P (SP) bound with sub-nanomolar affinity to 15 and 6% sites compared to [(3)H]GR205171 in gerbil and human striatum, respectively. 4 In competition binding experiments, GR205171 and the NK1 receptor antagonists aprepitant (MK-869), L-733,060 and NKP-608 bound with similar pK(i) values in gerbil and human striatum, irrespective of the use of [(3)H]GR205171 or [(3)H]SP as radioligand. The following rank order was found in terms of pK(i) values: GR205171>aprepitant> or =L-733,060>NKP-608. In homologous displacement experiments in gerbil and human striatum, SP showed nanomolar affinity, whereas in [(3)H]GR205171 competition experiments SP bound with pIC(50) values in the micromolar range and Hill slopes significantly lower than one. 5 It is concluded that the similarities of [(3)H]GR205171 binding characteristics and pharmacology between gerbil and human in cortex and striatum support the use of gerbil in preclinical models to study the effects of NK1 receptor antagonists in the central nervous system.

The neurokinin-1 antagonist activity of maropitant, an antiemetic drug for dogs, in a gerbil model

Maropitant is a novel synthetic nonpeptide neurokinin type 1 (NK1) selective receptor antagonist, recently developed for use in the dog as an antiemetic. The in vivo functional activity of maropitant was investigated in the gerbil foot-tapping model, to determine the ability of maropitant to penetrate the central nervous system and inhibit foot-tapping induced by the selective NK1 agonist GR73632. In comparison with CP-122,721, a previously characterized NK1 receptor antagonist, maropitant (1 mg/kg by s.c. injection) was found to inhibit foot-tapping for significantly longer (P < 0.01). Inhibition of foot-tapping by maropitant was 100% at 2 h and approximately 50% at 8 h postdosing. The mean brain:plasma concentration ratio at 8 h post-treatment was 3.59. These data demonstrate the central functional action of maropitant as a selective and potent NK1 receptor antagonist and help to support and explain its clinical potential as a broad-spectrum antiemetic agent.

Cyclopentane-based human NK1 antagonists. Part 2: development of potent, orally active, water-soluble derivatives

The synthesis and optimization of a cyclopentane-based hNK1 antagonist scaffold 3, having four chiral centers, will be discussed in the context of its enhanced water solubility properties relative to the marketed anti-emetic hNK1 antagonist EMEND (Aprepitant). Sub-nanomolar hNK1 binding was achieved and oral activity comparable to Aprepitant in two in vivo models will be described.

The role of substance P in stress and anxiety responses

Substance P (SP) is one of the most abundant peptides in the central nervous system and has been implicated in a variety of physiological and pathophysiological processes including stress regulation, as well as affective and anxiety-related behaviour. Consistent with these functions, SP and its preferred neurokinin 1 (NK1) receptor has been found within brain areas known to be involved in the regulation of stress and anxiety responses. Aversive and stressful stimuli have been shown repeatedly to change SP brain tissue content, as well as NK1 receptor binding. More recently it has been demonstrated that emotional stressors increase SP efflux in specific limbic structures such as amygdala and septum and that the magnitude of this effect depends on the severity of the stressor. Depending on the brain area, an increase in intracerebral SP concentration (mimicked by SP microinjection) produces mainly anxiogenic-like responses in various behavioural tasks. Based on findings that SP transmission is stimulated under stressful or anxiety-provoking situations it was hypothesised that blockade of NK1 receptors may attenuate stress responses and exert anxiolytic-like effects. Preclinical and clinical studies have found evidence in favour of such an assumption. The status of this research is reviewed here.

Cyclobutane derivatives as potent NK1 selective antagonists

A series of novel cyclobutane derivatives as potent and selective NK1 receptor antagonists is described. Several compounds in this series exhibited high in vitro binding affinity (Ki <or=1 nM), and potent inhibition of central NK1 receptor following oral administration. Syntheses of these compounds are also described herein.

Investigating the effect of bilateral amygdala lesions on fear conditioning and social interaction in the male Mongolian gerbil

Identification of the selective neurokinin NK(1) receptor antagonist, 2-(R)-(1-(R)-3,5-Bis(trifluromethyl)phenylethoxy)-3-(S)-(4-fluoro)phenyl-4-(3-oxo-1,2,4-triazol-5yl)methylmor-phine (MK-869), as a novel therapeutic approach for anxiety/depression has led to increased use of the Mongolian gerbil in behavioural studies since the gerbil NK(1) receptor pharmacology is similar to human, but not rat or mouse. Within this species, foot tapping and immobility elicited by aversive conditioning, as well as social interaction have been shown to be sensitive to clinically used anxiolytic and antidepressant agents and also NK(1) receptor antagonists. The high levels of NK(1) receptor binding in the amygdala as well as preclinical studies demonstrating increased release of substance P and corresponding internalisation of NK(1) receptors in the basolateral amygdala in response to stressful stimuli suggest that the BLA may represent a potential site of action for NK(1) receptor antagonists in anxiety and/or depression. Therefore, in the current study, we assessed the effect of bilateral BLA lesions in male Mongolian gerbils on footshock-induced foot tapping and immobility, social interaction, and NK(1)-agonist-induced foot tapping. Lesioned gerbils exhibited reduced immobility time during fear conditioning, a non-significant reduction in immobility time when re-exposed to the conditioned stimulus (CS) 24 h later, and increased social interaction in the gerbil social interaction task. In contrast, BLA lesions had no effect on NK(1)-agonist-induced foot tapping. These data provide further support that the gerbil BLA is a potential site for NK(1) receptor antagonists to attenuate anxiety-related behaviours.

Cyclic urea derivatives as potent NK1 selective antagonists. Part II: Effects of fluoro and benzylic methyl substitutions

A series of novel five-membered urea derivatives as potent NK1 receptor antagonists is described. The effects of substitution of a 4-fluoro group at the phenyl ring and the introduction of an alpha-methyl group at the benzylic position to improve potency and duration of in vivo activity are discussed. Several compounds with high affinity and sustained in vivo activity were identified.

Cyclic urea derivatives as potent NK1 selective antagonists

A series of novel five- and six-membered ring urea derivatives have been described as potent and selective NK1 receptor antagonists. Several compounds in this series exhibited good oral activity and brain penetration. Syntheses of these compounds are also described herein.

Comparison of the functional blockade of rat substance P (NK1) receptors by GR205171, RP67580, SR140333 and NKP-608

Extensive screening of compound libraries was undertaken to identify compounds with high affinity for the rat NK(1) receptor based on inhibition of [(125)I]-substance P binding. RP67580, SR140333, NKP-608 and GR205171 were selected as compounds of interest, with cloned rat NK(1) receptor binding K(i) values of 0.15-1.9 nM. Despite their high binding affinity, NKP-608 and GR205171 exhibited only a moderate functional antagonism of substance P-induced inositol-1-phosphate accumulation and acidification rate at 1 microM using cloned or native rat NK(1) receptors in vitro. The ability of the compounds to penetrate the CNS was determined by inhibition of NK(1) agonist-induced behaviours in gerbils and rats. GR205171 and NKP-608 potently inhibited GR73632-induced foot drumming in gerbils (ID(50) 0.04 and 0.2 mg/kg i.v., respectively). In contrast, RP67580 and SR140333 were poorly brain penetrant in gerbils (no inhibition at 10 mg/kg i.v.) and were not examined further in vivo. In rats, only high doses of GR205171 (10 or 30 mg/kg s.c.) inhibited NK(1) agonist-induced sniffing and hypertension, whilst NKP-608 (1 or 10 mg/kg i.p.) was without effect. GR205171 (3-30 mg/kg s.c.) caused only partial inhibition of separation-induced vocalisations in rat pups, a response that is known to be NK(1) receptor mediated in other species. These observations demonstrate the shortcomings of currently available NK(1) receptor antagonists for rat psychopharmacology assays.

The antidepressant-like effects of neurokinin NK1 receptor antagonists in a gerbil tail suspension test

Recent clinical evidence supports the potential of neurokinin NK1 receptor antagonists as novel antidepressant drugs. A number of NK1 antagonists have reduced affinity for rat and mouse NK1 receptors compared to human, making it difficult to test for efficacy in traditional animal models. NK1 antagonists, in general, have similar affinity at gerbil and human NK1 receptors. The aims of these studies were first, to validate the gerbil tail suspension test, a test used frequently to demonstrate antidepressant drug efficacy in mice, and second, to determine whether the test could be used to demonstrate the antidepressant potential of NK1 antagonists. Immobility time was reduced by oral administration of the antidepressants imipramine (3-30 mg/kg), desipramine (1-30 mg/kg), amitriptyline (30 mg/kg), fluoxetine (1-30 mg/kg), paroxetine (3-10 mg/kg), citalopram (0.1-3 mg/kg), sertraline (1-30 mg/kg), venlafaxine (1-30 mg/kg) and nefazodone (100 mg/kg). Furthermore, oral administration of the NK1 antagonists MK-869 (10 mg/kg), L-742694 (10 mg/kg), L-733060 (10 mg/kg), CP-99994 (30 mg/kg), and CP-122721 (3-30 mg/kg) reduced immobility time. Diazepam (1-10 mg/kg), chlordiazepoxide (1-10 mg/kg), buspirone (3-30 mg/kg), FG-7142 (1-30 mg/kg), and haloperidol (1-10 mg/kg) did not reduce immobility. Amphetamine (0.3-10 mg/kg) and atropine (0.3-10 mg/kg) reduced immobility, suggesting susceptibility to false positives, e.g. compounds that affect locomotion. Compounds were therefore tested in a gerbil locomotor activity (LMA) test to ensure that the antidepressant-like effects were not secondary to effects on activity. Antidepressant drugs and NK1 antagonists had no effect on LMA at doses that reduced immobility, whereas amphetamine and atropine induced marked hyperactivity. These studies support both the utility of gerbils in behavioral pharmacology and the antidepressant potential of selective NK1 antagonists.

Dual NK(1) antagonists--serotonin reuptake inhibitors as potential antidepressants. Part 2: SAR and activity of benzyloxyphenethyl piperazine derivatives

The synthesis, structure-affinity relationship and activity of benzyloxyphenethyl piperazine derivatives combining NK(1) antagonism and serotonin reuptake inhibition is described. Compound 7u was shown to be active in animal models of 5-HT reuptake inhibition and central NK(1) receptor blockade, and was demonstrated to be orally active in an integrated model sensitive to both mechanisms. This class of compounds potentially represents a new generation of antidepressants.

Correlation of neurokinin (NK) 1 receptor occupancy in gerbil striatum with behavioral effects of NK1 antagonists

Interest in central neurokinin (NK) 1 receptors has increased based on reports of the therapeutic potential for NK1 antagonists in anxiety and depression. In these studies, an ex vivo binding procedure was used to correlate NK1 receptor occupancy in striatum by NK1 antagonists with their potency to inhibit NK1 agonist-induced foot tapping in gerbils (GFT). The following compounds were administered orally: CP-99,994 [(+)-cis-n-[(2-methoxyphenyl)methyl]-2-phenyl-3-piperidinamine), L-742,694 [5-[[2(S)-[[3,5-bis(trifluoromethyl)phenyl]methoxy]-3(S)-phenyl-4-morpholinyl]methyl]-2,4-dihydro-3H-1,2,4-triazol-3-one]), MK-869 [5-[[2(R)-[1(R)-[3,5-bis(trifluoromethyl)phenyl]ethoxy]-3(S)-(4-fluorophenyl)-4-morpholinyl]methyl]-2,4-dihydro-3H-1,2,4-triazol-3-one], CP-122,721 [cis-n-[[2-methoxy-5-(trifluoromethoxy)phenyl]methyl]-2-phenyl-3-piperidinamine], L-760,735-F [4-[[2(R)-[1(R)-[3,5-bis(trifluoromethyl)phenyl]ethoxy]-3(S)-(4-fluorophenyl)-4-morpholinyl]methyl]-N,N-dimethyl-1H-1,2,3-triazole-5- methanamine], GR205171 [N-[[2-methoxy-5-[5-(trifluoromethyl)-1H-tetrazol-1-yl]phenyl]methy]-2(S)-phenyl-3(S)-piperidinamine], L-733,060 [(2S,3S)3-([3,5-bis(trifluoro methyl)phenyl]methoxy)-2-phenylpiperidine], and L-733,061 [(2R,3R)-3-([3,5-bis(trifluoromethyl)phenyl]methoxy)-2-phenylpiperidine]. Two hours later, gerbils received the NK1 agonist GR73632 [H(2)N-(CH(2))(4)-CO-Phe-Pro-NMe-Leu-Met-NH(2)] i.c.v. and foot tapping was measured for 5 min. The same procedure was used for ex vivo binding studies except that saline, rather than agonist, was administered i.c.v. before dissection of the striatum. The tissue homogenate was then used in an equilibrium radioligand binding assay. When IC(50) values for inhibition of ex vivo (125)I-substance P binding by NK1 antagonists were compared with the corresponding EC(50) values for inhibition of GFT, a significant positive correlation was observed (r(2) = 0.97, p < 0.001). This result indicates that increased NK1 receptor occupancy in striatum by NK1 antagonists parallels the inhibition of agonist-mediated GFT. For all compounds, the dose that produced the maximum inhibition of GFT resulted in less than 100% ex vivo receptor occupancy in striatum. When gerbils did not receive the i.c.v. saline injection before ex vivo binding, thereby leaving the blood-brain barrier (BBB) intact, the IC(50) values for antagonists were unchanged, suggesting that potential damage to the BBB caused by the i.c.v. injection did not affect determinations of antagonist potency in the GFT model.

An orally active, water-soluble neurokinin-1 receptor antagonist suitable for both intravenous and oral clinical administration

1-(5-[[(2R,3S)-2-([(1R)-1-[3,5-Bis(trifluoromethyl)phenyl]ethyl]oxy)-3-(4-fluorophenyl)morpholin-4-yl]methyl]-2H-1,2,3-triazol-4-yl)-N,N-dimethylmethanamine hydrochloride 3 is a high affinity, orally active, h-NK(1) receptor antagonist with a long central duration of action and a solubility in water of >100 mg/mL. The construction of the 5-dimethylaminomethyl 1,2,3-triazol-4-yl unit, which incorporates the solubilizing group of 3, was accomplished by thermal rearrangement of a propargylic azide in the presence of dimethylamine. Compound 3 is highly effective in pre-clinical tests that are relevant to clinical efficacy in emesis and depression.

Comparison of the phenotype of NK1R-/- mice with pharmacological blockade of the substance P (NK1 ) receptor in assays for antidepressant and anxiolytic drugs

The phenotype of NK1R-/- mice was compared with that of acute pharmacological blockade of the tachykinin NK1 receptor on sensorimotor function and in assays relevant to depressive illness and anxiety. The dose range for L-760735 and GR205171 that was associated with functional blockade of central NK1 receptors in the target species was established by antagonism of the behavioural effects of intracerebroventricular NK1 agonist challenge in gerbils, mice and rats. The caudal grooming and scratching response to GR73632 was absent in NK1R-/- mice, confirming that the receptor had been genetically ablated. There was no evidence of sedation or motor impairment in NK1R-/- mice or following administration of L-760735 to gerbils, even at doses in excess of those required for central NK1 receptor occupancy. In the resident-intruder and forced swim test, the behaviour of NK1R-/- mice, or animals treated acutely with L-760735 or GR205171, resembled that seen with the clinically used antidepressant drug fluoxetine. However, the effects of GR205171 were not clearly enantioselective in mice. In contrast, although NK1R-/- mice also exhibited an increase in the duration of struggle behaviour in the tail suspension test, this was not observed following pharmacological blockade with L-760735 in gerbils or GR205171 in mice, suggesting that this may reflect a developmental alteration in the knockout mouse. There was no effect of NK1 receptor blockade with L-760735 in guinea-pigs or GR205171 in rats, or deletion of the NK1 receptor in mice, on behaviour in the elevated plus-maze test for anxiolytic activity. These findings extend previous observations on the phenotype of the NK1R-/- mouse and establish a broadly similar profile following acute pharmacological blockade of the receptor. These studies also serve to underscore the limitations of currently available antagonists that are suitable for use in rat and mouse behavioural assays.

Anxiolytic actions of the substance P (NK1) receptor antagonist L-760735 and the 5-HT1A agonist 8-OH-DPAT in the social interaction test in gerbils

The gerbil social interaction test has previously detected anxiolytic effects of nicotine and diazepam. In the present study, the high affinity substance P (NK(1)) receptor antagonist L-760735 (3 mg/kg) significantly increased the time spent in social interaction, whereas its low affinity analogue L-781773 (3 mg/kg) was without effect. Diazepam (0.1 mg/kg) and the 5-HT(1A) receptor agonist 8-OH-DPAT (0.003 and 0.01 mg/kg) also increased social interaction, whereas an acute dose of the selective serotonin re-uptake inhibitor fluoxetine (10 mg/kg) decreased the time spent in social interaction. Diazepam (0.1 mg/kg) significantly increased locomotor activity, but this effect was independent of the increase in social interaction. The other drugs tested were without effect on locomotor activity. The present findings suggest that the gerbil social interaction may well provide a useful assay for detecting both anxiolytic and anxiogenic compounds, and suggests that the high affinity NK(1) receptor antagonist L-760735 may prove to be useful as an anxiolytic therapy.

Pharmacology of MEN 11467: a potent new selective and orally- effective peptidomimetic tachykinin NK(1) receptor antagonist

We have investigated the pharmacological properties of MEN 11467, a novel partially retro-inverse peptidomimetic antagonist of tachykinin NK(1) receptors. MEN 11467 potently inhibits the binding of [(3)H] substance P (SP) to tachykinin NK(1) receptors in the IM9 limphoblastoid cell line (pK(i) = 9.4 +/- 0.1). MEN 11467 is highly specific for the human tachykinin NK(1) receptors, since it has negligible effects (pK(i) <6) on the binding of specific ligands to tachykinin NK(2) or NK(3) receptors and to a panel of 30 receptors ion channels unrelated to tachykinin receptors. The antagonism exerted by MEN 11467 at tachykinin NK(1) receptors is insurmountable in saturation binding experiments, both K(D) and B(max) of SP were significantly reduced by MEN 11467 (0.3-10 nM). In the guinea-pig isolated ileum, MEN 11467 (0.03-1 nM) produced a nonparallel rightward shift of the concentration-response curve to SP methylester with a concomitant reduction of the Emax to the agonist (pK(B) = 10.7 +/- 0.1). Moreover the antagonist activity of MEN 11467 was hardly reversible despite prolonged washout. In vivo, MEN 11467 produced a long lasting (> 2-3h) dose-dependent antagonism of bronchoconstriction induced by the selective tachykinin NK(1) receptor agonist, [Sar(9), Met(O(2))(11)]SP in anaesthetized guinea-pigs (ID(50)s' = 29+/-5, 31+/-12 and 670+/-270 microg/kg, after intravenous, intranasal and intraduodenal administration, respectively), without affecting bronchoconstriction induced by methacholine. After oral administration MEN 11467 produced a dose-dependent inhibition of plasma protein extravasation induced in guinea-pig bronchi by [Sar(9), Met(O(2))(11)] (ID(50) = 6.7 +/- 2 mg/kg) or by antigen challenge in sensitized animals (ID(50) = 1.3 mg/kg). After i.v. administration MEN 11467 weakly inhibited the GR 73632-induced foot tapping behaviour in gerbil (ED(50) = 2.96 +/- 2 mg/kg), indicating a poor ability to block central tachykinin NK(1) receptors. These results demonstrate that MEN 11467 is a potent, highly selective and orally effective insurmountable pseudopeptide antagonist of peripheral tachykinin NK(1) receptors with a long duration of action.

Inhibition of shock-induced foot tapping behaviour in the gerbil by a tachykinin NK1 receptor antagonist

The selective tachykinin NK1 receptor antagonist, 2-(R)-(1-(R)-3,5-Bis(trifluoromethyl)phenylethoxy)-3-(S)-(4-fluoro)phenyl-4-(3-oxo-1,2,4-triazol-5-yl)methylmorpholine (MK-869), has been recently described as a novel therapeutic approach for anxiety/depression. A frequently used model to establish the central nervous system (CNS) activity of tachykinin NK1 receptor antagonists is the inhibition of NK1 agonist-induced foot tapping in gerbils. In the present study, we demonstrate that foot tapping can also be induced in most, but not all, gerbils by footshock and associated cues. MK-869 (0.3-3 mg/kg, i.p.) dose-dependently blocked this foot tapping response. This effect was further shown to be due to selective NK1 receptor blockade, since (2S,3S)-cis-3(2-methoxybenzylamino)-2-phenylpiperidine (CP-99,994; 3 mg/kg, i.p.) inhibited foot tapping, whereas its less active enantiomer (2R,3R)-cis-3(2-methoxybenzylamino)-2-phenylpiperidine (CP-100,263; 3 mg/kg, i.p.) had no effect. Diazepam (1-10 mg/kg, i.p.) also inhibited foot tapping, whereas fluoxetine (10-30 mg/kg, i.p.) markedly increased this behaviour. The present data support the view that foot tapping in the gerbil is a behavioural response to an aversive stimulus, and is robustly inhibited by two NK1 receptor antagonists. The data support a role for tachykinin NK1 receptor antagonists as novel anxiolytic/antidepressants.

Role of central and peripheral tachykinin NK1 receptors in capsaicin-induced pain and hyperalgesia in mice

Substance P and its receptor (NK1) are thought to play an important role in pain and hyperalgesia. Here we have further examined this role by comparing the behavioural responses to intradermal capsaicin of mutant mice with a disruption of the NK1 receptor (NK1 KO) and wild-type (WT) mice. We have also evaluated the contribution of peripheral NK1 receptors to capsaicin-evoked behaviour by selective blockade of peripheral NK1 receptors in WT mice using a non-brain penetrant NK1 receptor antagonist. Injection of 6 microg capsaicin into the heel evoked paw licking with the same latency in WT and KO mice, but a significantly longer duration in WT mice. A higher dose (30 microg) evoked a similar duration of licking in both groups. There were no differences in mechanical sensitivity tested with von Frey hairs between WT and KO mice before capsaicin. Both capsaicin doses resulted in pronounced increases in responses to von Frey hairs (hyperalgesia) and novel responses to cotton wisps (allodynia) applied to the digits of the injected paw in WT mice, but no significant changes from baseline in KO mice. Selective blockade of peripheral NK1 receptors in WT mice resulted in a complete inhibition of capsaicin-evoked plasma extravasation, but the mechanical hyperalgesia induced by 30 microg capsaicin intraplantar was still significantly greater than that seen in KO mice. We conclude that the response to intradermal capsaicin is still present but abbreviated in mice lacking NK1 receptors, such that secondary hyperalgesia is not observed even after a high dose. Further, the lack of secondary hyperalgesia in NK1 KO mice is largely due to the loss of central rather than peripheral NK1 receptors. The phenotype of the NK1 KO mice is consistent with a loss of function of mechanically-insensitive nociceptors, and thus we propose that substance P may be expressed by this group of primary sensory neurones and required for their function.

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.

Oral anti-hyperalgesic and anti-inflammatory activity of NK(1) receptor antagonists in models of inflammatory hyperalgesia of the guinea-pig

The oral analgesic and anti-inflammatory activity of NK(1) antagonists with species preference for the human receptor were assessed in (1) the carrageenan-induced inflammatory hyperalgesia and (2) Freund's complete adjuvant (FCA)-induced extravasation in the knee joint models of the guinea-pig, respectively. Mechanical hyperalgesia was determined by measuring the withdrawal threshold to a noxious mechanical stimulus applied to the paw and thermal hyperalgesia as the withdrawal latency to a noxious thermal stimulus applied to the plantar surface. A concentration of 1.0% carrageenan (intraplantar) reduced mechanical thresholds from 124+/-5 to 63+/-3 g and thermal latencies from 19+/-0.4 to 4.7+/-0.9 s as determined 4 h after injection. The hyperalgesia persisted for over 24 h. The NK(1) receptor antagonists, SDZ NKT 343, RPR100893 and SR140333, reduced mechanical hyperalgesia by 68, 36 and 27% at a dose of 30 mg kg(-1) p.o., respectively. No further reduction was noted at higher doses (maximum 100 mg kg(-1) p.o.). The anti-hyperalgesic effect of SDZ NKT 343 and RPR100893 peaked at 3 h while SR140333 produced maximal reversal at 1 h after oral administration. D(30) values indicated significant differences between the potency of these compounds. SDZ NKT 343 was by far the most potent anti-hyperalgesic agent (D(30): 1.1 mg kg(-1)). The D(30) values for RPR100893 and SR140333 were estimated to be 17 and >100 mg kg(-1), respectively. In thermal hyperalgesia, SDZ NKT 343 produced a significantly weaker anti-hyperalgesic effect with a peak of 25% reversal. The D(30) value for SDZ NKT 343 was 3.89 mg kg(-1). For comparison, morphine inhibited the carrageenan-induced mechanical and thermal hyperalgesia with an ED(50) of 1.85 and 2.51 mg kg(-1) s.c., respectively. When tested up to 300 mg kg(-1) p.o., aspirin reduced carrageenan-induced mechanical and thermal hyperalgesia by 55.0 and 45.2%, respectively. In addition to the anti-hyperalgesic effects of NK(1) receptor antagonists, the effects of SDZ NKT 343 and RPR100893 on plasma protein extravasation were measured in the FCA-treated knee joint of the guinea-pig. SDZ NKT 343 reversed plasma protein extravasation 2 h after administration by 60% at the oral dose of 30 mg kg(-1). RPR100893 was significantly less effective with a maximum reversal of 30% at 100 mg kg(-1). In comparison, indomethacin produced a 50% reversal at a 10 mg kg(-1) dose. These experiments indicate that the carrageenan-induced hyperalgesia in the guinea-pig may be predictive of analgesic activity of NK(1) receptor antagonists in man. NK(1) receptor antagonists are active anti-hyperalgesic drugs in both mechanical and thermal hyperalgesia in the guinea-pig. In addition they inhibit plasma protein extravasation in the same species. The variability of in vivo potency and efficacy of the NK(1) receptor antagonists in the mechanical hyperalgesia model is difficult to interpret as all compounds are highly effective at blocking the NK(1) receptor in guinea-pig tissues. Amongst several possibilities, differences in pharmacokinetics may explain discrepancies.

The emetic and anti-emetic effects of the capsaicin analogue resiniferatoxin in Suncus murinus, the house musk shrew

1. In SUNCUS: murinus the ultrapotent capsaicin analogue resiniferatoxin (RTX) induced an emetic response in the dose range 1 - 1000 microg kg(-1), s.c. The latency was inversely related to dose and ranged from 41.2+/-4.4 min. (1 microg kg(-1), s.c.) to 2.7+/-0.6 min. (1000 microg kg(-1), s.c.). 2. The emetic response to RTX (10 or 100 microg kg(-1), s.c.) was blocked or markedly reduced by pre-treatment with RTX (100 microg kg(-1), s.c.), 8-OH-DPAT (100 microg kg(-1), s.c.), morphine (2 mg kg(-1), s.c.), neonatal capsaicin (100 mg kg(-1), s.c.) and the NK(1) receptor antagonist CP-99,994 (10 - 20 mg kg(-1), s.c.) but not by the 5-HT(3) receptor antagonist tropisetron (200 microg kg(-1), s.c.). 3. RTX (100 microg kg(-1), s.c.) induced c-fos-like immunoreactivity in the area postrema and parts of the nucleus tractus solitarius. This pattern is consistent with the proposal that the emetic effect is mediated via one or both of these structures and an involvement of substance P is discussed. 4. RTX (10 and 100 microg kg(-1), s.c.) had broad-spectrum antiemetic effects in Suncus as indicated by its ability to block or markedly reduce the emetic response to motion (1 Hz, 4 cm lateral, 10 min.), cisplatin (20 mg kg(-1), i.p.), intragastric copper sulphate (40 mg kg(-1), p.o.), nicotine (10 mg kg(-1), s.c.) and RTX (100 microg kg(-1), s.c.) itself. 5. It is proposed that the site of the anti-emetic effect is in the nucleus tractus solitarius and mechanisms involving the modulation of substance P release are discussed. 6. The general utility of SUNCUS: for investigations of vanilloid receptors is reviewed in the light of the exquisite sensitivity of the emetic reflex in this species to resiniferatoxin.

Substance P antagonists: the next breakthrough in treating depression?

Several lines of evidence implicate the neuropeptide substance P in depression, either in the pathogenesis or as a novel target for amelioration of symptoms. NK1 (substance P) receptor antagonists have been reported to have antidepressant-like actions in animal models. The first clinical trial of an NK1 antagonist showed promising results. A second trial, using a more potent compound, is underway. If the clinical trials show that NK1 (substance P) antagonism represents a well-tolerated, distinct mechanism for antidepressant activity, novel antidepressant agents will emerge as mono- or adjunct-therapy.

Distinct mechanism for antidepressant activity by blockade of central substance P receptors

The localization of substance P in brain regions that coordinate stress responses and receive convergent monoaminergic innervation suggested that substance P antagonists might have psychotherapeutic properties. Like clinically used antidepressant and anxiolytic drugs, substance P antagonists suppressed isolation-induced vocalizations in guinea pigs. In a placebo-controlled trial in patients with moderate to severe major depression, robust antidepressant effects of the substance P antagonist MK-869 were consistently observed. In preclinical studies, substance P antagonists did not interact with monoamine systems in the manner seen with established antidepressant drugs. These findings suggest that substance P may play an important role in psychiatric disorders.

Further evidence for the involvement of tachykinin receptor subtypes in formalin and capsaicin models of pain in mice

The intradermal (i.d.) injection of NK1 receptor antagonists GR 82334 and FK 888 (1-50 pmol/paw), in association with formalin, produced graded inhibition of the early but not the late phase of the formalin test. The NK2, SR 48968 and NK3 SR 142801 receptor antagonists (1-50 pmol/paw) were effective in inhibiting both phases of the formalin model. Co-injection of NK1, (FK 888, GR 82334), NK2 (SR 48968) or NK3 (SR 142801) receptor antagonists with capsaicin dose-dependently attenuated capsaicin-induced licking. In addition, all antagonists were more efficacious when compared with response in the formalin test. The antinociception caused by i.d. injection of the NK3 receptor antagonist SR 142801 against both phases of the formalin test, but not that of NK1 and NK2 receptor antagonists, was significantly reversed by intraperitoneal (i.p.) injection of naloxone (5 mg/kg). Intracerebroventricular (i.c.v.) injection of NK1, NK2 or NK3 receptor antagonists (15-500 pmol/site), all produced significant and dose-dependent inhibition of both phases of the formalin and capsaicin tests. With the exception of the response of SR 48968, which was equipotent in both models of nociception, FK 888, GR 82334 and SR 142801 were about 2-25-fold less potent at the ID50 level against the capsaicin-induced pain. The antinociception caused by i.c.v. injection of NK1, NK2 or NK3 receptor antagonists was reversed by i.p. injection of naloxone (5 mg/kg). These results indicate that tachykinin receptor antagonists, acting through NK1, NK2 and NK3 receptors, produce powerful antinociception when injected i.d. or by i.c.v. route against both formalin- and capsaicin-induced licking, being more efficacious against the latter model of nociception. The action of NK3 receptor antagonist given i.d. was mediated through an opioid mechanism sensitive to naloxone. However, when injected i.c.v., the antinociception caused by NK1, NK2 or NK3 receptor antagonists was largely reversed by naloxone when assessed in the formalin test, suggesting a distinct mechanism of action.

In vitro and in vivo predictors of the anti-emetic activity of tachykinin NK1 receptor antagonists

The ability of tachykinin NK1 receptor antagonists to inhibit GR73632 (D-Ala-[L-Pro9,Me-Leu8]substance P-(7-11))-induced foot tapping in gerbils was employed as an indirect measure of brain penetration and this was compared with their ability to prevent acute emesis induced by cisplatin in ferrets. (+)-GR203040 ((2S,3S and 2R,3R)-2-methoxy-5-tetrazol-1-yl-benzyl-(2-phenyl-piperidin- 3-yl)-amine), CP-99,994 ((2S,3S)-cis-3-(2-methoxybenzylamino)-2-phenyl piperidine) dihydrochloride), and L-742,694 (2-(S)-(3,5-bis(trifluoromethyl)benzyloxy)-3-(S)-phenyl-4-(5-(3-oxo-1,2, 4-triazolo)methylmorpholine) potently inhibited GR73632-induced foot tapping (ID50 < or = 0.85 mg/kg), and acute retching induced by cisplatin (ID50 < or = 0.18 mg/kg). RPR100893 ((3aS,4S,7aS)-7,7-diphenyl-4-(2-methoxyphenyl)-2-[(S)-2-(2-m ethoxyphenyl)proprionyl] perhydroisoindol-4-ol) was not a potent antagonist of retching (ID50 4.1 mg/kg) or foot tapping (ID50 > 10 mg/kg). High doses (3-10 mg/kg) of CGP49823 ((2R,4S)-2-benzyl-1-(3,5-dimethylbenzoyl)-N-[(4-quinolinyl)methyl] -4-piperineamine) dihydrochloride), FK888 (N2-[(4R)-4-hydroxy-1-(1-methyl-1H-indol-3-yl)carbonyl-L-propyl]-N-methy l-N-phenylmethyl-L-3-(2-naphthyl)-alaninamide), and LY303870 ((R)-1-[N-(2-methoxybenzyl)acetylamino]-3-(1H-indol-3-yl)-2-[N-(2-(4-(pi peridinyl)piperidin-1-yl)acetyl)amino]propane) were required to inhibit foot tapping; these agents were not anti-emetic in this dose range. SR140333 ((S)-1-[2-[3-(3,4-dichlorphenyl)-1 (3-isopropoxyphenylacetyl)piperidin-3-yl] ethyl]-4-phenyl-1 azaniabicyclo [2.2.2]octane; 3-10 mg/kg) failed to inhibit foot tapping or emesis. Affinities for the human and ferret tachykinin NK1 receptor were highly correlated (r = 0.93, P = 0.0008). Inhibition of foot tapping in gerbils, but not NK1 receptor binding affinity, predicted anti-emetic activity in ferrets (r = 0.75, P < 0.01). These findings confirm that the anti-emetic activity of tachykinin NK1 receptor antagonists is dependent on brain penetration.

The tachykinin NK1 receptor antagonist PD 154075 blocks cisplatin-induced delayed emesis in the ferret

The activity of a selective tachykinin NK1 receptor antagonist, PD 154075 ([(2-benzofuran)-CH2OCO]-(R)-alpha-MeTrp-(S)-NHCH(CH3) Ph), was examined in radioligand binding studies, in a [Sar9,Met(O2)11]substance P-induced foot-tapping model in the gerbil, and in cisplatin-induced acute and delayed emesis in the ferret. In radioligand binding studies, PD 154075 showed nanomolar affinity for the human, guinea-pig, gerbil, dog and ferret NK1 receptors with an approximate 300 times lower affinity for the rodent NK1 receptor. Using NK2,NK3 receptors and a range of other receptor ligands, PD 154075 was shown to exhibit a high degree of selectivity and specificity for the human type NK1 receptor. Following subcutaneous administration PD 154075 dose dependently (1-100 mg/kg) antagonised the centrally mediated [Sar9,Met(O2)11] substance P-induced foot tapping in the gerbil with a minimum effective dose (MED) of 10 mg/kg. The ability of PD 154075 to readily penetrate into the brain following oral administration was confirmed by its extraction and high performance liquid chromatography assay from the rat brain. PD 154075 was shown to achieve a relatively fast and sustained brain concentration (brain/plasma ratios ranged from 0.27 to 0.41 during the time period of 0.25-12 h). Further pharmacokinetic studies revealed that the absolute oral bioavailability of PD 154075 in the rat was (mean +/- S.D.) 49 +/- 15%. PD 154075 (1-30 mg/kg, i.p.) dose dependently antagonised the acute vomiting and retching in the ferret measured for 4 h following administration of cisplatin (10 mg/kg, i.p.) with a MED of 3 mg/kg. The administration of a lower dose of cisplatin (5 mg/kg, i.p.) in the ferret induces both an acute (day 1) and delayed (days 2 and 3) phase of emesis. The i.p. administration of PD 154075, 10 mg/kg three times a day for 3 days, almost completely blocked both the acute and delayed emetic responses. In the same study, the 5-HT3 receptor antagonist ondansetron (1 mg/kg, i.p., t.i.d.) was also very effective against the acute emetic response observed during the first 4 h following cisplatin, but it was only weakly active against the delayed response. In conclusion, PD 154075 is a selective and specific high affinity NK1 receptor antagonist with good oral bioavailability which is effective against both acute and delayed emesis induced by cisplatin in the ferret.

Effects of central administration of tachykinin receptor agonists and antagonists on plus-maze behavior in mice

This study assessed the effects of intracerebroventricular administration of selective agonists and antagonists for tachykinin NK1 and NK2 receptors on performance of mice in the elevated plus-maze, an ethological model of anxiety. Mice were treated with either vehicle (5 microliters) or 1, 10, 100 or 500 pmol of substance P, neurokinin A, the selective NK1 receptor agonist substance P methyl ester, or the selective NK2 receptor agonist, [beta-Ala8]neurokinin A-(4-10). Other mice received similar doses of FK 888, i.e., N2-[(4R)-4-hydroxy-1-(1-methyl-1 H-indol-3-y)carbonyl-L-prolyl]-N-methyl-N-phenylmethyl-3-(2-naphthyl)-L- alaninamide, or SR 48968, i.e., (S)-N-methyl-(N-[4-acetylamine-4-phenylpiperidine)-2-(3, 4-dichlorophenyl)buthyl]benzamide, selective antagonists of tachykinin NK1 and NK2 receptors, respectively. Injections of substance P, neurokinin A, substance P methyl ester or [beta-Ala8]neurokinin A-(4-10) significantly reduced the frequency of open arm entries, and [beta-Ala8]neurokinin A-(4-10) also enhanced the percentage of entries into enclosed arms. Conversely, the NK1 antagonist FK 888 and the NK2 antagonist SR 48968 each increased the time spent in the open arms, and SR 48968 also increased the frequency of entries into the open arms. None of the tachykinin receptor agonists or antagonists modified motor performance and coordination on the rotarod apparatus or ambulation in an activity cage. Together, these results suggest that centrally administered NK1 and NK2 receptor agonists and antagonists can modulate anxiety, as evaluated in the elevated plus-maze test in mice. Stimulation of either tachykinin NK1 or NK2 receptors induces anxiogenic-like responses, whereas the reverse occurs following their blockade. The anxiolytic-like profiles of action of both tachykinin NK1 and NK2 receptor antagonists suggest that central tachykinin mechanisms are tonically involved in the modulation of anxiety.

Enantioselective inhibition of the formalin paw late phase by the NK1 receptor antagonist L-733,060 in gerbils

Intravenous administration of the NK1 receptor antagonist L-733,060 to gerbils 3 h before intraplantar injection of formalin caused a dose-dependent and complete inhibition of the late, but not early, phase nociceptive response (paw licking). The ID50 for L-733,060 (0.17 mg/kg) revealed a greater than 50-fold separation in potency over its less active enantiomer L-733,061 (ID50 > or = 10 mg/kg). In contrast, the non-brain penetrant quaternary ketone NK1 receptor antagonist, L-743,310 (3 mg/kg), did not attenuate the response to formalin, indicating that the antinociceptive effect of blockade of NK1 receptors by L-733,060 in this assay is centrally-mediated. These findings add to the preclinical evidence that NK1 receptor antagonists may be of therapeutic use as centrally-acting analgesics.

Electrophysiological, behavioural and biochemical evidence for activation of brain noradrenergic systems following neurokinin NK3 receptor stimulation

The objective of the present in vitro and in vivo experiments was to examine the involvement of neurokinin NK3 receptors in the regulation of the noradrenergic function in gerbils and guinea-pigs. Application of senktide, a peptide NK3 receptor agonist, on guinea-pig locus coeruleus slices increased the firing rate of presumed noradrenergic neurons (EC50 = 26 nM) in a concentration-dependent manner. Given i.c.v., senktide (0.5-2 micrograms) and (MePhe7)neurokinin B (1-10 micrograms), another NK3 receptor agonist, reduced exploratory behaviour in gerbils in a dose-dependent manner (2 micrograms of senktide producing a 50% reduction of locomotor activity and rearing). In vivo microdialysis experiments in urethane-anaesthetized guinea-pigs showed that senktide (2-8 micrograms i.c.v.) induced a dose-dependent increase in norepinephrine release in the medial prefrontal cortex. The electrophysiological, behavioural and biochemical changes elicited by senktide were concentration- or dose-dependently reduce by SR 142801, the selective non-peptide NK3 receptor antagonist. In the locus coeruleus slice preparation, complete antagonism of senktide (30 nM) was observed with 50 nM of SR 142801, while injected i.p. (0.1-1 mg/kg) it abolished the senktide-induced norepinephrine release in guinea-pigs. In gerbils, SR 142801 (1-10 mg/kg i.p.) reversed the reduction of exploratory behaviour induced by senktide (1 microgram). By contrast, the 100-fold less active enantiomer, SR 142806, did not exert any antagonism in these models. Finally, the reduction of exploratory behaviour in gerbils was found to be reversed by prazosin (0.25-2.56 micrograms/kg i.p.) and to some extent by clonidine, drugs known to depress noradrenergic function. All these experiments strongly support the hypothesis that brain noradrenergic neurons can be activated by stimulation of neurokinin NK3 receptors.

Effects of specific tachykinin receptor antagonists on citric acid-induced cough and bronchoconstriction in unanesthetized guinea pigs

We compared the effects of a tachykinin NK1 receptor antagonist, FK888 (N2-[(4R)-4-hydroxy-1-(1-methyl-1H-indol-3-yl)carbonyl-L-prolyl]-N-methy l-N -phenylmethyl-3-(2-naphthyl)-L-alaninamide), and a tachykinin NK2 receptor antagonist, SR48968 ((S)-N-methyl-N[4-(4-acetylamino-4-phenyl piperidino)-2-(3,4-dichlorophenyl)butyl]benzamide]), on citric acid-induced cough and bronchoconstriction in conscious guinea pigs. FK888 and SR48968 inhibited the cough dose dependently. Combination of FK888 and SR48968 showed a small additive effect compared with that of FK888 or SR48968 alone. SR48968 but not FK888 inhibited the bronchoconstriction dose dependently. These results indicate that tachykinin NK1 receptors as well as tachykinin NK2 receptors are involved in the citric acid-induced cough response. The antitussive activity of the tachykinin NK1 receptor antagonist appeared not to depend on the anti-bronchoconstrictor effects.