Potentiation of analgesic efficacy but not side effects: co-administration of an α4ß2 neuronal nicotinic acetylcholine receptor agonist and its positive allosteric modulator in experimental models of pain in rats.

Positive modulation of the neuronal nicotinic acetylcholine receptor (nAChR) α4ß2 subtype by selective positive allosteric modulator NS-9283 has shown to potentiate the nAChR agonist ABT-594-induced anti-allodynic activity in preclinical neuropathic pain. To determine whether this benefit can be extended beyond neuropathic pain, the present study examined the analgesic activity and adverse effect profile of co-administered NS-9283 and ABT-594 in a variety of preclinical models in rats. The effect of the combined therapy on drug-induced brain activities was also determined using pharmacological magnetic resonance imaging. In carrageenan-induced thermal hyperalgesia, co-administration of NS-9283 (3.5 µmol/kg, i.p.) induced a 6-fold leftward shift of the dose-response of ABT-594 (ED(50)=26 vs. 160 nmol/kg, i.p.). In the paw skin incision model of post-operative pain, co-administration of NS-9283 similarly induced a 6-fold leftward shift of ABT-594 (ED(50)=26 vs. 153 nmol/kg). In monoiodo-acetate induced knee joint pain, co-administration of NS-9283 enhanced the potency of ABT-594 by 5-fold (ED(50)=1.0 vs. 4.6 nmol/kg). In pharmacological MRI, co-administration of NS-9283 was shown to lead to a leftward shift of ABT-594 dose-response for cortical activation. ABT-594 induced CNS-related adverse effects were not exacerbated in presence of an efficacious dose of NS-9283 (3.5 µmol/kg). Acute challenge of NS-9283 produced no cross sensitization in nicotine-conditioned animals. These results demonstrate that selective positive allosteric modulation at the α4ß2 nAChR potentiates nAChR agonist-induced analgesic activity across neuropathic and nociceptive preclinical pain models without potentiating ABT-594-mediated adverse effects, suggesting that selective positive modulation of α4ß2 nAChR by PAM may represent a novel analgesic approach.

Synthesis, potency, and in vivo profiles of quinoline containing histamine H3 receptor inverse agonists.

A new structural series of histamine H3 receptor antagonist was developed. The new compounds are based on a quinoline core, appended with a required basic aminoethyl moiety, and with potency- and property-modulating heterocyclic substituents. The analogs have nanomolar and subnanomolar potency for the rat and human H3R in various in vitro assays, including radioligand competition binding as well as functional tests of H3 receptor-mediated calcium mobilization and GTPgammaS binding. The compounds possessed favorable drug-like properties, such as good PK, CNS penetration, and moderate protein binding across species. Several compounds were found to be efficacious in animal behavioral models of cognition and attention. Further studies on the pharmaceutic properties of this series of quinolines discovered a potential problem with photochemical instability, an issue which contributed to the discontinuation of this series from further development.

Novel heterocyclic-substituted benzofuran histamine H3 receptor antagonists: in vitro properties, drug-likeness, and behavioral activity.

Three novel heterocyclic benzofurans A-688057 (1), A-687136 (2), and A-698418 (3) were profiled for their in vitro and in vivo properties as a new series of histamine H(3) receptor antagonists. The compounds were all found to have nanomolar potency in vitro at histamine H(3) receptors, and when profiled in vivo for CNS activity, all were found active in an animal behavioral model of attention. The compound with the most benign profile versus CNS side effects was selected for greater scrutiny of its in vitro properties and overall drug-likeness. This compound, A-688057, in addition to its potent and robust efficacy in two rodent behavioral models at blood levels ranging 0.2-19 nM, possessed other favorable features, including high selectivity for H(3) receptors (H(3), K(i)=1.5 nM) versus off-target receptors and channels (including the hERG K(+) channel, K(i)>9000 nM), low molecular weight (295), high solubility, moderate lipophilicity (logD(pH7.4)=2.05), and good CNS penetration (blood/brain 3.4x). In vitro toxicological tests indicated low potential for phospholipidosis, genotoxicity, and CYP(450) inhibition. Even though pharmacokinetic testing uncovered only moderate to poor oral bioavailability in rat (26%), dog (30%), and monkey (8%), and only moderate blood half-lives after i.v. administration (t(1/2) in rat of 2.9h, 1.7h in dog, 1.8h in monkey), suggesting poor human pharmacokinetics, the data overall indicated that A-688057 has an excellent profile for use as a pharmacological tool compound.

Evidence for tolerance following repeated dosing in rats with ciproxifan, but not with A-304121.

Blockade of presynaptic histamine H(3) receptors with potent and selective ligands improves cognitive function in rodents and there is significant interest in developing such drugs for long-term symptomatic treatment of CNS disorders such as attention deficit hyperactivity disorder (ADHD). Unfortunately, little is known about the effects of repeated exposure to H(3) receptor antagonists/inverse agonists. We therefore investigated the effects of acute and repeated daily administration of two potent, brain penetrating H(3) receptor antagonists/inverse agonists, ciproxifan and A-304121, on rat body weight, food and water intake, core temperature and locomotor activity, as well as H(3) receptor density and gene expression levels. Methylphenidate, used clinically for the treatment of ADHD, was included as an additional comparator. Ciproxifan, an imidazole-based compound, decreased food intake over the first 10 days and locomotor activity acutely, but these effects were lost after further repeated administration. The ex vivo binding studies revealed increased H(3) receptor density in rats following repeated administration of ciproxifan for 10 or 15 days; however, H(3) receptor gene expression was not changed. In contrast, rats treated with the non-imidazole, A-304121, did not differ from controls on any measure during the observation period, while rats treated with methylphenidate exhibited hyperthermia and hyperactivity. The implications for potential long-term treatment with H(3) receptor antagonists in CNS disorders such as ADHD are discussed.

Preclinical profiling and safety studies of ABT-769: a compound with potential for broad-spectrum antiepileptic activity.

PURPOSE: The objective of this study was to characterize the antiseizure and safety profiles of ABT-769 [(R)-N-(2 amino-2-oxoethyl)spiro[2,5]octane-1-carboxamide]. METHODS: ABT-769 was tested for protection against maximal electroshock and pentylenetetrazol-induced seizures in the mouse and for suppression of electrically kindled amygdala seizures and spontaneous absence-like seizures in the rat. The central nervous system safety profile was evaluated by using tests of motor coordination and inhibitory avoidance. The potential for liver toxicity was assessed in vitro by using a mitochondrial fatty acid beta-oxidation assay. Teratogenic potential was assessed in the mouse. RESULTS: ABT-769 blocked maximal electroshock, subcutaneous pentylenetetrazol and intravenous pentylenetetrazol-induced seizures with median effective dose (ED50) values of 0.25, 0.38, and 0.11 mmol/kg, p.o., respectively. No tolerance was evident in the intravenous pentylenetetrazol test after twice-daily dosing of ABT-769 (0.3 mmol/kg, p.o.) for 4 days. ABT-769 blocked absence-like spike-wave discharge (ED50, 0.15 mmol/kg, p.o.) and shortened the cortical and amygdala afterdischarge duration of kindled seizures (1 and 3 mmol/kg, p.o.). The protective indices (ED50 rotorod impairment/ED50 seizure protection) were 4.8, 3.2, and 10.9 in the maximal electroshock, subcutaneous pentylenetetrazol and intravenous pentylenetetrazol seizure tests, respectively. ABT-769 did not affect inhibitory avoidance performance (0.1-1 mmol/kg, p.o.). ABT-769 did not affect mitochondrial fatty acid beta-oxidation or induce neural tube defects. CONCLUSIONS: ABT-769 is an efficacious antiseizure agent in animal models of convulsive and nonconvulsive epilepsy and has a favorable safety profile. ABT-769 has a broad-spectrum profile like that of valproic acid. Its profile is clearly different from those of carbamazepine, phenytoin, lamotrigine, topiramate, vigabatrin, and tiagabine.

A-412997, a selective dopamine D4 agonist, improves cognitive performance in rats.

The recent development of a highly selective dopamine D4 receptor agonist, A-412997 (2-(3',4',5',6'-tetrahydro-2'H-[2,4'] bipyridinyl-1'-yl)-N-m-tolyl-acetamide), has provided a pharmacological tool with which to conduct systematic investigations into the putative role for dopamine D4 receptors in the central nervous system. These present studies evaluated the potential cognitive enhancing properties of A-412997 in rat models of ADHD (5-trial repeated acquisition inhibitory avoidance in Spontaneous Hypertensive Rat pups) and short-term memory (Social Recognition), in comparison with the less selective dopamine D4 receptor agonists PD168077 and CP226269. A-412997 showed significant dose-dependent efficacy in both models. PD168077 repeatedly improved acquisition in the 5-trial inhibitory avoidance model but failed to reach significance at any dose tested, although significantly improved social recognition was observed (albeit less potent than A-412997). CP226269 showed a significant enhancement in the 5-trial inhibitory avoidance model. These results support a role for the dopamine D4 receptor subtype in cognition.

Effects of histamine H3 receptor antagonists in two models of spatial learning.

Despite the well-described attention and short-term memory enhancing effects of H3 receptor antagonists, and evidence to suggest a close relationship between central histaminergic and cholinergic systems, there is a paucity of evidence for a role for H3 receptor blockade in spatial learning. To address this, we investigated two H3 receptor antagonists in a visual discrimination water maze in rats, and in a Barnes circular maze in mice. Thioperamide and ciproxifan significantly attenuated a scopolamine-induced deficit in the water maze task, while only ciproxifan showed a modest attenuation in the Barnes maze. Taken together, these data suggest a role for H3 receptors in spatial learning that appears to be task-dependent.

Pharmacological properties of ABT-239 [4-(2-{2-[(2R)-2-Methylpyrrolidinyl]ethyl}-benzofuran-5-yl)benzonitrile]: II. Neurophysiological characterization and broad preclinical efficacy in cognition and schizophrenia of a potent and selective histamine H3 receptor antagonist.

Acute pharmacological blockade of central histamine H3 receptors (H3Rs) enhances arousal/attention in rodents. However, there is little information available for other behavioral domains or for repeated administration using selective compounds. ABT-239 [4-(2-{2-[(2R)-2-methylpyrrolidinyl]ethyl}-benzofuran-5-yl)benzonitrile] exemplifies such a selective, nonimidazole H3R antagonist with high affinity for rat (pK(i) = 8.9) and human (pK(i) = 9.5) H3Rs. Acute functional blockade of central H3Rs was demonstrated by blocking the dipsogenia response to the selective H3R agonist (R)-alpha-methylhistamine in mice. In cognition studies, acquisition of a five-trial, inhibitory avoidance test in rat pups was improved with ABT-239 (0.1-1.0 mg/kg), a 10- to 150-fold gain in potency, with similar efficacy, over previous antagonists such as thioperamide, ciproxifan, A-304121 [(4-(3-(4-((2R)-2-aminopropanoyl)-1-piperazinyl)propoxy)phenyl)(cyclopropyl) methanone], A-317920 [N-((1R)-2-(4-(3-(4-(cyclopropylcarbonyl) phenoxy)propyl)-1-piperazinyl)-1-methyl-2-oxoethyl)-2-furamide], and A-349821 [(4'-(3-((R,R)2,5-dimethyl-pyrrolidin-1-yl)-propoxy)-biphenyl-4-yl)-morpholin-4-yl-methanone]. Efficacy in this model was maintained for 3 to 6 h and following repeated dosing with ABT-239. Social memory was also improved in adult (0.01-0.3 mg/kg) and aged (0.3-1.0 mg/kg) rats. In schizophrenia models, ABT-239 improved gating deficits in DBA/2 mice using prepulse inhibition of startle (1.0-3.0 mg/kg) and N40 (1.0-10.0 mg/kg). Furthermore, ABT-239 (1.0 mg/kg) attenuated methamphetamine-induced hyperactivity in mice. In freely moving rat microdialysis studies, ABT-239 enhanced acetylcholine release (0.1-3.0 mg/kg) in adult rat frontal cortex and hippocampus and enhanced dopamine release in frontal cortex (3.0 mg/kg), but not striatum. In summary, broad efficacy was observed with ABT-239 across animal models such that potential clinical efficacy may extend beyond disorders such as ADHD to include Alzheimer's disease and schizophrenia.

ABT-963 [2-(3,4-difluoro-phenyl)-4-(3-hydroxy-3-methyl-butoxy)-5-(4-methanesulfonyl-phenyl)-2H-pyridazin-3-one], a highly potent and selective disubstituted pyridazinone cyclooxgenase-2 inhibitor.

Nonsteriodal anti-inflammatory drugs (NSAIDs) are efficacious for the treatment of pain associated with inflammatory disease. Clinical experience with marketed selective cyclooxygenase-2 (COX-2) inhibitors (celecoxib, rofecoxib, and valdecoxib) has confirmed the utility of these agents in the treatment of inflammatory pain with an improved gastrointestinal safety profile relative to NSAID comparators. These COX-2 inhibitors belong to the same structural class. Each contains a core heterocyclic ring with two appropriately substituted phenyl rings appended to adjacent atoms. Here, we report the identification of vicinally disubstituted pyridazinones as potent and selective COX-2 inhibitors. The lead compound in the series, ABT-963 [2-(3,4-difluoro-phenyl)-4-(3-hydroxy-3-methyl-butoxy)-5-(4-methanesulfonyl-phenyl)-2H-pyridazin-3-one], has excellent selectivity (ratio of 276, COX-2/COX-1) in human whole blood, improved aqueous solubility compared with celecoxib and rofecoxib, high oral anti-inflammatory potency in vivo, and gastric safety in the animal studies. After oral administration, ABT-963 reduced prostaglandin E2 production in the rat carrageenan air pouch model (ED50 of 0.4 mg/kg) and reduced the edema in the carrageenan induced paw edema model with an ED30 of 1.9 mg/kg. ABT-963 dose dependently reduced nociception in the carrageenan hyperalgesia model (ED50 of 3.1 mg/kg). After 14 days of dosing in the adjuvant arthritis model, ABT-963 had an ED(50) of 1.0 mg/kg in reducing the swelling of the hind paws. Magnetic resonance imaging examination of the diseased paws in the adjuvant model showed that ABT-963 significantly reduced bone loss and soft tissue destruction. ABT-963 is a highly selective COX-2 inhibitor that may have utility in the treatment of the pain and inflammation associated with arthritis.

Enhancement of prepulse inhibition of startle in mice by the H3 receptor antagonists thioperamide and ciproxifan.

Histamine H3 receptor antagonists/inverse agonists have been proposed as potential therapeutic agents for the treatment of a number of neurological disorders ranging from attention deficit hyperactivity disorder and Alzheimer's disease to narcolepsy and schizophrenia. With respect to the latter, schizophrenic patients typically exhibit impaired prepulse inhibition (PPI) of startle, a reflex that can be modeled in many animal species. Certain strains of mice naturally display poor PPI and it was recently suggested that these mice might offer a new way to screen for novel antipsychotic compounds. To examine whether H3 receptor antagonists might enhance PPI in mice with naturally occurring deficits, DBA/2 and C57BL/6 were tested in a startle paradigm with three prepulse intensities: 5, 10 and 15 dB above background. Both thioperamide and ciproxifan enhanced PPI in the DBA/2 strain; thioperamide also showed a trend towards enhancing PPI in C57BL/6. Risperidone, an atypical antipsychotic, enhanced PPI in both the DBA/2 and the C57BL/6 strain. These data confirm previous reports describing a natural deficit in PPI in some mouse strains that is amenable to enhancement with known antipsychotics. Further, these data suggest that H3 receptor antagonists/inverse agonists have anti-psychotic potential for disorders such as schizophrenia.

Identification and characterization of novel antagonists of the CCR3 receptor.

Eotaxin, an inducer of eosinophil migration and activation, exerts its activity by binding to CCR3, the C-C chemokine receptor 3. An inhibitor of the eotaxin-CCR3 binding interaction may have potential as an anti-inflammatory drug for treatment of asthma, parasitic infections, and allergic disorders. A radioligand binding assay was developed using HEK cells transfected with CCR3, with (125)I eotaxin as the ligand. Whole cells grown on polylysine-coated plates were used as the receptor source for the screen. Screening of more than 200,000 compounds with this assay yielded a number of screening hits, and of these, 2 active novel antagonists were identified. These compounds showed inhibitory effects on eosinophil chemotaxis in both in vitro and in vivo assays.

H3 receptor blockade by thioperamide enhances cognition in rats without inducing locomotor sensitization.

RATIONALE: Attention deficit hyperactivity disorder (ADHD) is currently treated with psychomotor stimulants, including methylphenidate and amphetamine. Several adverse effects are associated with these drugs, however, such as agitation and abuse. H(3) receptor antagonists are under clinical investigation for ADHD. OBJECTIVES: To investigate the potential of thioperamide, a prototypical H(3) receptor antagonist, to enhance learning and attention while inducing no effects on locomotor stimulation and sensitization, or alterations in ACTH levels. METHODS: Thioperamide (1, 3, 10, 30 mg/kg) was administered prior to testing in a multi-trial, inhibitory avoidance response in rat pups (five trials separated by 1 min) to evaluate attention/cognition. Locomotor sensitization and cross-sensitization was assessed following administration of methylphenidate (3 mg/kg), cocaine (10 mg/kg), or thioperamide (1, 3, 10 mg/kg). RESULTS: Thioperamide significantly enhanced performance of the five-trial inhibitory avoidance response with efficacy similar to that previously reported for methylphenidate. Administration of amphetamine, methylphenidate and cocaine produced significant locomotor sensitization, however. In contrast, thioperamide did not induce locomotor stimulation or sensitization, nor did it cross-sensitize to the stimulant effects of amphetamine or cocaine. The repeated administration of methylphenidate significantly elevated ACTH levels, while thioperamide did not affect this neuroendocrine endpoint. CONCLUSIONS: H(3) receptor blockade may offer a safer alternative to psychomotor stimulants for the treatment of ADHD.