This side effect is more likely to occur in people with pre-existing cardiac disease, or with the use of other medicines known to prolong the QT interval. Pharmacology
Pharmacodynamics
Hydroxyzines predominant mechanism of action is as a potent and selective histamine H1 receptor inverse agonist. This action is responsible for its antihistamine and sedative effects. Unlike many other first-generation antihistamines, hydroxyzine has a lower affinity for the muscarinic acetylcholine receptors, and in accordance, has a lower risk of anticholinergic side effects. In addition to its antihistamine activity, hydroxyzine has also been shown to act more weakly as an antagonist of the serotonin 5-HT2A receptor, the dopamine D2 receptor, and the α1-adrenergic receptor. The weak antiserotonergic effects of hydroxyzine likely underlie its usefulness as an anxiolytic, as other antihistamines without such properties have not been found to be effective in the treatment of anxiety.Hydroxyzine crosses the blood–brain barrier easily and exerts effects in the central nervous system. A positron emission tomography (PET) study found that brain occupancy of the H1 receptor was 67.6% for a single 30 mg dose of hydroxyzine. In addition, subjective sleepiness correlated well with the brain H1 receptor occupancy. PET studies with antihistamines have found that brain H1 receptor occupancy of more than 50% is associated with a high prevalence of somnolence and cognitive decline, whereas brain H1 receptor occupancy of less than 20% is considered to be non-sedative. Pharmacokinetics
Hydroxyzine can be administered orally or via intramuscular injection. When given orally, hydroxyzine is rapidly absorbed from the gastrointestinal tract.
Monocytic leukemia is a type of myeloid leukemia characterized by a dominance of monocytes in the marrow. When the monocytic cells are predominantly monoblasts, it can be subclassified into acute monoblastic leukemia. Monocytic leukemia is almost always broken down into "acute" and "chronic":

acute monocytic leukemia
chronic myelomonocytic leukemia

References


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