Kathleen M. Krueger

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 (pKi = 8.9) and human (pKi = 9.5) H3Rs. Acute functional blockade of central H3 Rs was demonstrated by blocking the dipsogenia response to the selective H3R agonist (R)-α-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 Alzheimers disease and schizophrenia.

Pharmacological Properties of ABT-239: 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 (pKi = 8.9) and human (pKi = 9.5) H3Rs. Acute functional blockade of central H3 Rs was demonstrated by blocking the dipsogenia response to the selective H3R agonist (R)-α-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 Alzheimers disease and schizophrenia.

Species-related pharmacological heterogeneity of histamine H3 receptors

We compared radioligand binding and functional data for histamine H(3) receptor ligands across different tissues or species to evaluate the basis for pharmacological evidence of receptor heterogeneity previously reported. Agonist binding affinities showed correlation coefficients near unity in comparing human, dog, rat, and guinea pig cerebral cortical histamine H(3) receptors. Antagonist binding affinities revealed lower correlations for human compared to dog, rat, or guinea pig, suggesting species-based pharmacological differences. The functional potencies of histamine H(3) receptor antagonists in field-stimulated guinea pig ileum were highly correlated to binding affinities for guinea pig, dog, and, to a lesser extent, rat cerebral cortex. However, antagonist binding affinity at human cerebral cortex did not correlate well with guinea pig ileum functional potency. These results suggest significant interspecies histamine H(3) receptor heterogeneity, consistent with recent receptor gene sequence data. Therefore, genetic heterogeneity, rather than peripheral and central histamine H(3) receptor diversity, is responsible for the pharmacological differences observed.

An 80-Amino Acid Deletion in the Third Intracellular Loop of a Naturally Occurring Human Histamine H3 Isoform Confers Pharmacological Differences and Constitutive Activity

In this article, we pharmacologically characterized two naturally occurring human histamine H3 receptor (hH3R) isoforms, hH3R(445) and hH3R(365). These abundantly expressed splice variants differ by a deletion of 80 amino acids in the intracellular loop 3. In this report, we show that the hH3R(365) is differentially expressed compared with the hH3R(445) and has a higher affinity and potency for H3R agonists and conversely a lower potency and affinity for H3R inverse agonists. Furthermore, we show a higher constitutive signaling of the hH3R(365) compared with the hH3R(445) in both guanosine-5′-O-(3-[35S]thio) triphosphate binding and cAMP assays, likely explaining the observed differences in hH3R pharmacology of the two isoforms. Because H3R ligands are beneficial in animal models of obesity, epilepsy, and cognitive diseases such as Alzheimers disease and attention deficit hyperactivity disorder and currently entered clinical trails, these differences in H3R pharmacology of these two isoforms are of great importance for a detailed understanding of the action of H3R ligands.

Pharmacological Properties and Procognitive Effects of ABT-288, a Potent and Selective Histamine H3 Receptor Antagonist

Blockade of the histamine H3 receptor (H3R) enhances central neurotransmitter release, making it an attractive target for the treatment of cognitive disorders. Here, we present in vitro and in vivo pharmacological profiles for the H3R antagonist 2-[4′-((3aR,6aR)-5-methyl-hexahydro-pyrrolo[3,4-b]pyrrol-1-yl)-biphenyl-4-yl]-2H-pyridazin-3-one (ABT-288). ABT-288 is a competitive antagonist with high affinity and selectivity for human and rat H3Rs (Ki = 1.9 and 8.2 nM, respectively) that enhances the release of acetylcholine and dopamine in rat prefrontal cortex. In rat behavioral tests, ABT-288 improved acquisition of a five-trial inhibitory avoidance test in rat pups (0.001–0.03 mg/kg), social recognition memory in adult rats (0.03–0.1 mg/kg), and spatial learning and reference memory in a rat water maze test (0.1–1.0 mg/kg). ABT-288 attenuated methamphetamine-induced hyperactivity in mice. In vivo rat brain H3R occupancy of ABT-288 was assessed in relation to rodent doses and exposure levels in behavioral tests. ABT-288 demonstrated a number of other favorable attributes, including good pharmacokinetics and oral bioavailability of 37 to 66%, with a wide central nervous system and cardiovascular safety margin. Thus, ABT-288 is a selective H3R antagonist with broad procognitive efficacy in rodents and excellent drug-like properties that support its advancement to the clinical area.

Detection of multiple H3 receptor affinity states utilizing [3H]A-349821, a novel, selective, non-imidazole histamine H3 receptor inverse agonist radioligand.

1 A‐349821 is a selective histamine H3 receptor antagonist/inverse agonist. Herein, binding of the novel non‐imidazole H3 receptor radioligand [3H]A‐349821 to membranes expressing native or recombinant H3 receptors from rat or human sources was characterized and compared with the binding of the agonist [3H]N‐α‐methylhistamine ([3H]NαMH). 2 [3H]A‐349821 bound with high affinity and specificity to an apparent single class of saturable sites and recognized human H3 receptors with 10‐fold higher affinity compared to rat H3 receptors. [3H]A‐349821 detected larger populations of receptors compared to [3H]NαMH. 3 Displacement of [3H]A‐349821 binding by H3 receptor antagonists/inverse agonists was monophasic, suggesting recognition of a single binding site, while that of H3 receptor agonists was biphasic, suggesting recognition of both high‐ and low‐affinity H3 receptor sites. 4 pKi values of high‐affinity binding sites for H3 receptor competitors utilizing [3H]A‐349821 were highly correlated with pKi values obtained with [3H]NαMH, consistent with labelling of H3 receptors by [3H]A‐349821. 5 Unlike assays utilizing [3H]NαMH, addition of GDP had no effect on saturation parameters measured with [3H]A‐349821, while displacement of [3H]A‐349821 binding by the H3 receptor agonist histamine was sensitive to GDP. 6 In conclusion, [3H]A‐349821 labels interconvertible high‐ and low‐affinity states of the H3 receptor, and displays improved selectivity over imidazole‐containing H3 receptor antagonist radioligands. [3H]A‐349821 competition studies showed significant differences in the proportions and potencies of high‐ and low‐affinity sites across species, providing new information about the fundamental pharmacological nature of H3 receptors.

The alkaloid conessine and analogues as potent histamine H3 receptor antagonists.

The naturally occurring alkaloid, conessine (6), was discovered to bind to histamine H3 receptors in a radioligand-based high-throughput screen. Conessine displayed high affinity at both rat and human H3 receptors (pKi = 7.61 and 8.27) and generally high selectivity against other sites, including histamine receptors H1, H2, and H4. Conessine was found to efficiently penetrate the CNS and reach very high brain concentrations. Although the very slow CNS clearance and strong binding to adrenergic receptors discouraged focus on conessine itself for further development, its potency and novel steroid-based skeleton motivated further chemical investigation. Modification based on introducing diversity at the 3-nitrogen position generated a new series of H3 antagonists with higher in vitro potency, improved target selectivity, and more favorable drug-like properties. One optimized analogue (13c) was examined in detail and was found to be efficacious in animal behavioral model of cognition.

Distinctions and contradistinctions between antiobesity histamine H3 receptor (H3R) antagonists compared to cognition-enhancing H3 receptor antagonists

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Antiobesity evaluation of histamine H3 receptor (H3R) antagonist analogs of A-331440 with improved safety and efficacy

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Differential activation of dual signaling responses by human H1 and H2 histamine receptors.

Abstract Stimulation of human H1 and H2‐histamine receptors (HRs) primarily activates signaling pathways to increase intracellular calcium [Ca2+]i and cyclic AMP (cAMP), respectively. Activation of H2‐HR in human embryonic kidney (HEK) cells by histamine and dimaprit increases both cAMP formation and [Ca2+]i, as determined by cAMP‐scintillation proximity assays and fluorescence imaging plate reader (FLIPR) assays. In HEK cells expressing relatively high levels of H2‐HR (Bmax = 26 pmol/mg protein), histamine and dimaprit are full agonists in eliciting cAMP responses with pEC50 values of 9.30 and 7.72 that are 1000‐fold more potent than their respective pEC50 values of 6.13 and 4.91 for increasing [Ca2+]i. The agonist potencies decrease for both responses at lower H2‐HR density (5 pmol/mg protein) and dimaprit exhibits partial agonist behavior for the [Ca2+]i response. The inverse agonists ranitidine and cimetidine more potently inhibit cAMP production in the higher expressing H2‐HR line. Histamine also activated both signaling pathways via human H1‐HRs highly expressed (Bmax = 17 pmol/mg protein) in HEK cells, with a 1000‐fold greater potency for [Ca2+]i vs. cAMP responses (pEC50 = 7.86 and 4.82, respectively). These studies demonstrate a markedly different potency for activation of multiple signaling pathways by H1‐ and H2‐HRs that may contribute to the selectivity of histamine responses in vivo.