Radiofluorinated histamine H3 receptor antagonist as a potential probe for in vivo PET imaging: Radiosynthesis and pharmacological evaluation

Chemical Probes for Histamine Receptor Subtypes

Ligands with different properties and different selectivity are highly needed for in vitro and in vivo studies on the (patho)physiological influence of the chemical mediator histamine and its receptor subtypes. A selection of well-described ligands for the different receptor subtypes and different studies is shown with a particular focus on affinity and selectivity. In addition, compounds with radioactive or fluorescence elements will be presented with their beneficial use for other species or different investigations.

In Vivo Receptor Visualization and Evaluation of Receptor Occupancy with Positron Emission Tomography

Positron emission tomography (PET) is useful for noninvasive in vivo visualization of disease-related receptors, for evaluation of receptor occupancy to determine an appropriate drug dosage, and for proof-of-concept of drug candidates in translational research. For these purposes, the specificity of the PET tracer for the target receptor is critical. Here, we review work in this area, focusing on the chemical structures of reported PET tracers, their K_i/K_d values, and the physical properties relevant to target receptor selectivity. Among these physical properties, such as cLogP, cLogD, molecular weight, topological polar surface area, number of hydrogen bond donors, and pK_a, we focus especially on LogD and LogP as important physical properties that can be easily compared across a range of studies. We discuss the success of PET tracers in evaluating receptor occupancy and consider likely future developments in the field.

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.

Role of the Histamine H3 Receptor in the Central Nervous System

The G_i/o protein-coupled histamine H₃ receptor is distributed throughout the central nervous system including areas like cerebral cortex, hippocampus and striatum with the density being highest in the posterior hypothalamus, i.e. the area in which the histaminergic cell bodies are located. In contrast to the other histamine receptor subtypes (H₁, H₂ and H₄), the H₃ receptor is located presynaptically and shows a constitutive activity. In detail, H₃ receptors are involved in the inhibition of histamine release (presynaptic autoreceptor), impulse flow along the histaminergic neurones (somadendritic autoreceptor) and histamine synthesis. Moreover, they occur as inhibitory presynaptic heteroreceptors on serotoninergic, noradrenergic, dopaminergic, glutamatergic, GABAergic and perhaps cholinergic neurones. This review shows for four functions of the brain that the H₃ receptor represents a brake against the wake-promoting, anticonvulsant and anorectic effect of histamine (via postsynaptic H₁ receptors) and its procognitive activity (via postsynaptic H₁ and H₂ receptors). Indeed, H₁ agonists and H₃ inverse agonists elicit essentially the same effects, at least in rodents; these effects are opposite in direction to those elicited by brain-penetrating H₁ receptor antagonists in humans. Although the benefit for H₃ inverse agonists for the symptomatic treatment of dementias is inconclusive, several members of this group have shown a marked potential for the treatment of disorders associated with excessive daytime sleepiness. In March 2016, the European Commission granted a marketing authorisation for pitolisant (Wakix^R) (as the first representative of the H₃ inverse agonists) for the treatment of narcolepsy.

Benzylpiperidine variations on histamine H3 receptor ligands for improved drug-likeness

Several hH3R antagonists/inverse agonists entered clinical phases for a broad spectrum of mainly centrally occurring diseases. Nevertheless, many promising candidates failed due to their pharmacokinetic profile, mostly because of their strong lipophilicity and their dibasic character. Analysis of previously, as potential PET ligands synthesized compounds (ST-889, ST-928) revealed promising results concerning physicochemical properties and drug-likeness. Herein, the synthesis, the evaluation of the binding properties at the hH3R and the estimation of different physicochemical and drug-likeness properties of further novel benzylpiperidine variations on H3R antagonists is described. Due to the introduction of various small hydrophilic moieties in the structure, drug-likeness parameters have been improved. For instance, compound 12 (ST-1032) showed in addition to high affinity at the H3R (pKi (hH3R)=9.3) clogS, clogP, LE, LipE, and LELP values of -2.48, 2.18, 0.44, 7.14, and 4.95, respectively. Also, the keto derivative 5 (ST-1703, pKi (hH3R)=8.6) revealed LipE and LELP values of 5.25 and 6.84, respectively.