H3 receptor antagonist

The histamine H3 receptor (H3R) was discovered in 1983 and was one of the last receptors that were discovered using conventional pharmacological methods. Its structure was discovered later as a part of an effort to identify a commonly expressed G-protein-coupled receptor (GPCR) in the central nervous system (CNS). The pharmacology of H3R is very complicated which has made drug development difficult. Many different functional isoforms of the H3R exist which means it could theoretically be possible to target a single isoform specifically. That may, however, be difficult due to genetic variability of the isoforms as well as differing functionality of each one. H3R ligands have now been classified as agonists, antagonists or inverse agonists, depending on the signaling assay used. == Mechanism of action ==

The H3R is a GPCR and it has been described as a presynaptic autoreceptor, regulating the release of histamine and also as a heteroreceptor, regulating neurotransmitters such as acetylcholine, dopamine, serotonin, norepinephrine and GABA. The receptor has a high constitutive activity which means that it can signal without being activated by an agonist. H3Rs are mostly expressed on the histaminergic neurons of the CNS but can also be found in various areas of the peripheral nervous system. The H3R has been found in high densities in the basal ganglia, hippocampus and cortical areas which are all regions of the brain associated with cognition. The histaminergic system has been described as having a role in the pathophysiology of cognitive symptoms of diseases such as Alzheimer's, schizophrenia and narcolepsy. == Development ==

Early pharmacophore In the beginning of development for H3R ligands the focus was on the agonist histamine which contains an imidazole ring in its structure. The structural diversity among H3R is limited and all known H3R agonists today contain an imidazole ring. New pharmacophore The focus turned to non-imidazole H3R antagonists. They do not seem to interact with the CYP family on the same level as imidazole-based H3R antagonists and can reach the CNS more easily. Unfortunately other problems have come up such as strong binding to hERG K+ channel, phospholipidosis as well as problems with P-gp substrate. Strong binding to hERG K+ channel can lead to QT prolongation. It has been proved to be useful for maintaining waking-state in the daytime for people with narcolepsy. Structure activity relationship A general structural pattern that is necessary for the antagonist affinity for H3R has been described. An H3R antagonist needs to have a basic amine group which is linked to an aromatic/lipophilic region that is connected to either a polar group or another basic group or a lipophilic region. == Clinical significance ==

H3R antagonists/inverse agonists demonstrate a possible way to treat diseases of the CNS for example Alzheimer's disease (AD), attention deficit hyperactivity syndrome (ADHD), schizophrenia (SCH), pain, and narcolepsy. Narcolepsy Narcolepsy is a sleeping disorder which is characterised by chronic sleepiness. Cataplexy, hypnagogic hallucinations and sleep paralysis can also be present in narcolepsy. H3R antagonism leads to histamine release into the cerebrospinal fluid which promotes wakefulness. Therefore, H3R antagonists have been studied in the hope of treating narcolepsy. Pitolisant has been approved for treatment of narcolepsy and other H3R antagonists are in clinical trials. Alzheimer's disease (AD) Alzheimer's disease is a progressive neurodegenerative disease of the brain. Though histamine plays a well documented role in AD, the varying levels of histamine in different areas of the brain make it hard to demonstrate a direct link between histaminergic neurotransmission and pathology of AD. In vivo studies have shown that a number of H3R antagonists facilitate learning and memory. Thioperamide blocks H3R and causes an increase in neuronal histamine release which then modifies cognition processes through H1R and H2R and other receptors (e.g. cholinergic and GABA). Degeneration of histaminergic neurons in AD doesn't correlate to H3R expressions since a large portion of H3R in the brain are located elsewhere deep in cortical and thalamocortical neurons among others. Attention deficit hyperactivity disorder (ADHD) ADHD is a neurodevelopmental disorder which is most pronounced in children. Current pharmacological treatments consist of stimulant medications (e.g. methylphenidate), non-stimulant medication (e.g. atomoxetine) and α2 agonists. These medications can cause adverse effects and some types have the potential to cause addiction. Developing alternative treatments is therefore desirable. In vivo studies show potential of using H3R antagonists in ADHD to aid in attention and cognitive activity by elevating release of neurotransmitters such as acetylcholine and dopamine. Schizophrenia In schizophrenia, dopaminergic pathways, among other neurotransmitter systems, play a significant role in the development of the disease. Current treatments are based on first and second generation antipsychotics. These drugs are principally dopamine antagonists, and they can cause many undesirable side-effects. Histaminergic neurons also seem to play a role in schizophrenia, and H3 receptors are co-localized with dopamine receptors in GABAergic neurons. H3 receptor antagonists may be useful in treating the negative and cognitive symptoms of schizophrenia, even if they are not effective in the treatment of its positive symptoms. == See also ==