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

Therapeutic Potential of Histamine H3 Receptors in Substance Use Disorders

Substance use disorders are a leading cause of morbidity and mortality, and available pharmacological treatments are of modest efficacy. Histamine is a biogenic amine with four types of receptors. The histamine H₃ receptor (H₃R) is an autoreceptor and also an heteroreceptor. H₃Rs are highly expressed in the basal ganglia, hippocampus and cortex, and regulate a number of neurotransmitters including acetylcholine, norepinephrine, GABA and dopamine. Its function and localization suggest that the H₃R may be relevant to a number of psychiatric disorders and could represent a potential therapeutic target for substance use disorders. The purpose of the present review is to summarize preclinical studies investigating the effects of H₃R agonists and antagonists on animal models of alcohol, nicotine and psychostimulant use. At present, the effects of H₃R antagonists such as thioperamide, pitolisant or ciproxifan have been investigated in drug-induced locomotion, conditioned place preference, drug self-administration, reinstatement, sensitization and drug discrimination. For alcohol and nicotine, the effects of H₃R ligands on two-bottle choice and memory tasks, respectively, have also been investigated. The results of these studies are inconsistent. For alcohol, H₃R antagonists generally decreased the reward-related properties of ethanol, which suggests that H₃R antagonists may be effective as a treatment option for alcohol use disorder. However, the effects of H₃R antagonists on nicotine and psychostimulant motivation and reward are less clear. H₃R antagonists potentiated the abuse-related properties of nicotine, but only a handful of studies have been conducted. For psychostimulants, evidence is mixed and suggests that more research is needed to establish whether H₃R antagonists are a viable therapeutic option. The fact that different drugs of abuse have different brain targets may explain the differential effects of H₃R ligands.

International Union of Basic and Clinical Pharmacology. XCVIII. Histamine Receptors

Histamine is a developmentally highly conserved autacoid found in most vertebrate tissues. Its physiological functions are mediated by four 7-transmembrane G protein-coupled receptors (H1R, H2R, H3R, H4R) that are all targets of pharmacological intervention. The receptors display molecular heterogeneity and constitutive activity. H1R antagonists are long known antiallergic and sedating drugs, whereas the H2R was identified in the 1970s and led to the development of H2R-antagonists that revolutionized stomach ulcer treatment. The crystal structure of ligand-bound H1R has rendered it possible to design new ligands with novel properties. The H3R is an autoreceptor and heteroreceptor providing negative feedback on histaminergic and inhibition on other neurons. A block of these actions promotes waking. The H4R occurs on immuncompetent cells and the development of anti-inflammatory drugs is anticipated.

Assessment of the abuse liability of ABT-288, a novel histamine H₃ receptor antagonist

RATIONALE

Histamine H3 receptor antagonists, such as ABT-288, have been shown to possess cognitive-enhancing and wakefulness-promoting effects. On the surface, this might suggest that H3 antagonists possess psychomotor stimulant-like effects and, as such, may have the potential for abuse.

OBJECTIVES

The aim of the present study was to further characterize whether ABT-288 possesses stimulant-like properties and whether its pharmacology gives rise to abuse liability.

METHODS

The locomotor-stimulant effects of ABT-288 were measured in mice and rats, and potential development of sensitization was addressed. Drug discrimination was used to assess amphetamine-like stimulus properties, and drug self-administration was used to evaluate reinforcing effects of ABT-288. The potential development of physical dependence was also studied.

RESULTS

ABT-288 lacked locomotor-stimulant effects in both rats and mice. Repeated administration of ABT-288 did not result in cross-sensitization to the stimulant effects of d-amphetamine in mice, suggesting that there is little overlap in circuitries upon which the two drugs interact for motor activity. ABT-288 did not produce amphetamine-like discriminative stimulus effects in drug discrimination studies nor was it self-administered by rats trained to self-administer cocaine. There were no signs of physical dependence upon termination of repeated administration of ABT-288 for 30 days.

CONCLUSIONS

The sum of these preclinical data, the first of their kind applied to H3 antagonists, indicates that ABT-288 is unlikely to possess a high potential for abuse in the human population and suggests that H3 antagonists, as a class, are similar in this regard.

Pharmacological characterization of A-960656, a histamine H₃ receptor antagonist with efficacy in animal models of osteoarthritis and neuropathic pain

Histamine H(3) receptor antagonists have been widely reported to improve performance in preclinical models of cognition, but more recently efficacy in pain models has also been described. Here, A-960656 ((R)-2-(2-(3-(piperidin-1-yl)pyrrolidin-1-yl)benzo[d]thiazol-6-yl)pyridazin-3(2H)-one) was profiled as a new structural chemotype. A-960656 was potent in vitro in histamine H(3) receptor binding assays (rat K(i)=76 nM, human K(i)=21 nM), and exhibited functional antagonism in blocking agonist-induced [(35)S]GTPγS binding (rat H(3) K(b)=107 nM, human H(3) K(b)=22 nM), and was highly specific for H(3) receptors in broad screens for non-H(3) sites. In a spinal nerve ligation model of neuropathic pain in rat, oral doses of 1 and 3mg/kg were effective 60 min post dosing with an ED(50) of 2.17 mg/kg and a blood EC(50) of 639 ng/ml. In a model of osteoarthritis pain, oral doses of 0.1, 0.3, and 1mg/kg were effective 1h post dosing with an ED(50) of 0.52 mg/kg and a blood EC(50) of 233 ng/ml. The antinociceptive effect of A-960656 in both pain models was maintained after sub-chronic dosing up to 12 days. A-960656 had excellent rat pharmacokinetics (t(1/2)=1.9h, 84% oral bioavailability) with rapid and efficient brain penetration, and was well tolerated in CNS behavioral safety screens. In summary, A-960656 has properties well suited to probe the pharmacology of histamine H(3) receptors in pain. Its potency and efficacy in animal pain models provide support to the notion that histamine H(3) receptor antagonists are effective in attenuating nociceptive processes.

A robust and high-capacity [(35)S]GTPgammaS binding assay for determining antagonist and inverse agonist pharmacological parameters of histamine H(3) receptor ligands

Guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding assays were established and utilized as a reliable and high-capacity functional assay for determining antagonist and inverse agonist pharmacological parameters of novel histamine H(3) ligands, at the recombinant human H(3) receptor. [(35)S]GTPgammaS binding assays were performed with membranes prepared from human embryonic kidney 293 cells stably expressing the full-length (445 amino acids) human H(3) receptor isoform, at approximately 1 pmol/mg of protein. Utilizing robotic liquid handling, assay filtration, and scintillation counting in a 96-well format, concentration-response curves were determined for up to 40 compounds per assay. The imidazole-containing H(3) receptor antagonist ciproxifan and the non-imidazole antagonist ABT-239 inhibited (R)-alpha-methylhistamine (RAMH)-stimulated [(35)S]GTPgammaS binding in a competitive manner, and negative logarithm of the dissociation equilibrium constant (pK(b)) values determined for nearly 200 structurally diverse H(3) antagonists were very similar to the respective negative logarithm of the equilibrium inhibition constant values from N-alpha-[(3)H]methylhistamine competition binding assays. H(3) antagonists also concentration-dependently decreased basal [(35)S]GTPgammaS binding, thereby displaying inverse agonism at the constitutively active H(3) receptor. At maximally effective concentrations, non-imidazole H(3) antagonists inhibited basal [(35)S]GTPgammaS binding by approximately 20%. For over 100 of these antagonists, negative logarithm of the 50% effective concentration values for inverse agonism were very similar to the respective pK(b) values. Both H(3) receptor agonist-dependent and -independent (constitutive) [(35)S]GTPgammaS binding were sensitive to changes in assay concentrations of sodium, magnesium, and the guanine nucleotide GDP; however, the potency of ABT-239 for inhibition of RAMH-stimulated [(35)S]GTPgammaS binding was not significantly affected. These robust and reliable [(35)S]GTPgammaS binding assays have become one of the important tools in our pharmacological analysis and development of novel histamine H(3) receptor antagonists/inverse agonists.

The histamine H3 receptor: an attractive target for the treatment of cognitive disorders

The histamine H3 receptor, first described in 1983 as a histamine autoreceptor and later shown to also function as a heteroreceptor that regulates the release of other neurotransmitters, has been the focus of research by numerous laboratories as it represents an attractive drug target for a number of indications including cognition. The purpose of this review is to acquaint the reader with the current understanding of H3 receptor localization and function as a modulator of neurotransmitter release and its effects on cognitive processes, as well as to provide an update on selected H3 antagonists in various states of preclinical and clinical advancement. Blockade of centrally localized H3 receptors by selective H3 receptor antagonists has been shown to enhance the release of neurotransmitters such as histamine, ACh, dopamine and norepinephrine, among others, which play important roles in cognitive processes. The cognitive-enhancing effects of H3 antagonists across multiple cognitive domains in a wide number of preclinical cognition models also bolster confidence in this therapeutic approach for the treatment of attention deficit hyperactivity disorder, Alzheimer's disease and schizophrenia. However, although a number of clinical studies examining the efficacy of H3 receptor antagonists for a variety of cognitive disorders are currently underway, no clinical proof of concept for an H3 receptor antagonist has been reported to date. The discovery of effective H3 antagonists as therapeutic agents for the novel treatment of cognitive disorders will only be accomplished through continued research efforts that further our insights into the functions of the H3 receptor.

In vitro SAR of pyrrolidine-containing histamine H3 receptor antagonists: trends across multiple chemical series

Structure-activity relationships (SAR) were analyzed within a library of diverse yet simple compounds prepared as histamine H3 antagonists. The libraries were constructed with a variety of low molecular weight pyrrolidines, selected from (R)-2-methylpyrrolidine, (S)-2-methylpyrrolidine, and pyrrolidine.

Histamine H3 receptor ligands break ground in a remarkable plethora of therapeutic areas

The neurotransmitter histamine exerts its action through four distinct histamine receptors. The histamine H(1) and H(2) receptor are well established drug targets, whereas the histamine H(4) receptor is undergoing rigorous characterisation at present. The histamine H(3) receptor (H(3)R) is a G(i/o)-protein coupled receptor and is mostly expressed in the CNS. A remarkably large and different array of therapeutic areas, in which ligands for the H(3)R may prove useful, has been identified and a massive research undertaking is underway to substantiate the high expectations for H(3)R ligands. At present, several ligands for the H(3)R are being evaluated in clinical studies. In this review, the many potential therapeutic areas for H(3)R antagonists, inverse agonists and agonists is discussed. Promising medicinal chemistry and toxicological developments, as well as the advancement of several H(3)R ligands into the clinic, will be highlighted. This review also describes the problems that have been overcome and the questions that remain in developing H(3)R-related drugs. Considering the tremendous efforts by industry, it can be expected that the first H(3)R drugs will reach the market soon.