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Cetirizine




Cetirizine, sold under the brand name Zyrtec among others, is a second-generation antihistamine used to treat hay fever, allergies, angioedema, and urticaria.[5]

It acts as a selective antagonist of the histamine H1 receptor. Second-generation antihistamines like cetirizine are less able to cross the blood–brain barrier and therefore have diminished effects on the central nervous system compared to first-generation drugs. Because of this, they are less likely to cause drowsiness or memory impairment.

In 2016 it was the 74th most prescribed medication in the United States with more than 10 million prescriptions.[6]

Medical uses
Allergies
Cetirizine's primary indication is for hay fever and other allergies. Because the symptoms of itching and redness in these conditions are caused by histamine acting on the H1 receptor, blocking those receptors temporarily relieves those symptoms.
Cetirizine is also commonly prescribed to treat acute and (in particular cases) chronic urticaria, more efficiently than any other second-generation antihistamine.
Rhinovirus infection
Interleukin 6 and interleukin 8 have been shown to be elevated in acute respiratory distress syndrome.[7] One recent study of airway epithelial cells showed that levocetirizine, the active enantiomer of cetirizine, may have beneficial effects on the pathophysiologic changes related to human rhinovirus (HRV) infection.[8]
Kimura's disease
Cetirizine is an effective agent in treating the symptoms of Kimura's disease which predominantly affects the lymph nodes and soft tissue of the head and neck in the form of tumor-like lesions. Cetirizine's properties of being effective both in the treatment of pruritus (itching) and as an anti-inflammatory agent make it suitable for the treatment of the pruritus associated with these lesions.[9] In a 2005 study, the American College of Rheumatology conducted treatments initially using prednisone, followed by steroid dosages and azathioprine, omeprazole, and calcium and vitamin D supplements over the course of two years.[9] The skin condition of the patient began to improve and the skin lesions lessened. However, there were symptoms of cushingoid and hirsutism observed before the patient was removed from the courses of steroids and placed on 10mg/day of cetirizine to prevent skin lesions;[9] an agent suitable for the treatment of pruritus associated with such lesions.[9] Asymptomatically, the patient's skin lesions disappeared after treatment with cetirizine, blood eosinophil counts became normal,[9] corticosteroid effects were resolved,[9] and a remission began within a period of two months.[9] It is also thought that the inhibition of eosinophils may be the key to treatment of Kimura's disease due to the role of eosinophils, rather than other cells with regards to the lesions of the skin.[9]
Available forms
Cetirizine is available over-the-counter in the form of 5 and 10mg tablets. A 20mg strength is available by prescription only.[2]
Adverse effects
Commonly reported side effects of cetirizine include headache (16%), dry mouth (5.7%), drowsiness (5–20%), and fatigue (5.6%), while more serious but rare side effects include cardiac failure, tachycardia, and edema.[10]
Insatiable, generalized itching is the most commonly-reported withdrawal side effect, though others have reported severe rebound of original, pre-treatment systems as well, including sneezing and runny nose.[11][12]
Pharmacology

Pharmacodynamics
Cetirizine acts as a highly selective antagonist of the histamine H1 receptor.[2] The Ki values for the H1 receptor are approximately 6nM for cetirizine, 3nM for levocetirizine, and 100nM for dextrocetirizine, indicating that the levorotatory enantiomer is the main active form.[2] Cetirizine has 600-fold or greater selectivity for the H1 receptor over a wide variety of other sites, including muscarinic acetylcholine, serotonin, dopamine, and α-adrenergic receptors, among many others.[2] The drug shows 20,000-fold or greater selectivity for the H1 receptor over the five muscarinic acetylcholine receptors, and hence does not exhibit anticholinergic effects.[13][14] It shows negligible inhibition of the hERG channel (IC50 > 30µM)[15] and no cardiotoxicity has been observed with cetirizine at doses of up to 60mg/day, six times the normal recommended dose[2] and the highest dose of cetirizine that has been studied in healthy subjects.[16]
Cetirizine crosses the blood–brain barrier only slightly, and for this reason, it produces minimal sedation compared to many other antihistamines.[17] A positron emission tomography (PET) study found that brain occupancy of the H1 receptor was 12.6% for 10mg cetirizine, 25.2% for 20mg cetirizine, and 67.6% for 30mg hydroxyzine.[18] (A 10mg dose of cetirizine equals about a 30mg dose of hydroxyzine in terms of peripheral antihistamine effect.)[19] 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.[20] In accordance, H1 receptor occupancy correlated well with subjective sleepiness for 30mg hydroxyzine but there was no correlation for 10 or 20mg cetirizine.[18] As such, brain penetration and brain H1 receptor occupancy by cetirizine are dose-dependent, and in accordance, while cetirizine at doses of 5 to 10mg have been reported to be non-sedating or mildly sedating, a higher dose of 20mg has been found to induce significant drowsiness in other studies.[18]
Cetirizine has been shown to inhibit eosinophil chemotaxis and LTB4 release.[21] At a dosage of 20mg, Boone et al. found that it inhibited the expression of VCAM-1 in patients with atopic dermatitis.[21]
Pharmacokinetics
Absorption
Cetirizine is rapidly and extensively absorbed upon oral administration in tablet or syrup form.[2] The oral bioavailability of cetirizine is at least 70% and of levocetirizine is at least 85%.[1] The Tmax of cetirizine is approximately 1.0hour regardless of formulation and its onset of action has been reported to be as early as 20minutes.[3] The pharmacokinetics of cetirizine have been found to increase linearly with dose across a range of 5 to 60mg.[2] Its Cmax following a single dose has been found to be 257ng/mL for 10mg and 580ng/mL for 20mg.[3] Food has no effect on the bioavailability of cetirizine but has been found to delay the Tmax by 1.7hours (i.e., to approximately 2.7hours) and to decrease the Cmax by 23%.[2][3][22] Similar findings were reported for levocetirizine, which had its Tmax delayed by 1.25hours and its Cmax decreased by about 36% when administered with a high-fat meal.[22] Steady-state levels of cetirizine occur within 3days and there is no accumulation of the drug with chronic administration.[3] Following once-daily administration of 10mg cetirizine for 10days, the mean Cmax was 311ng/mL.[23]
Distribution
The mean plasma protein binding of cetirizine has been found to be 93 to 96% across a range of 25 to 1,000ng/mL independent of concentration.[3] Plasma protein binding of 88 to 96% has also been reported across multiple studies.[1] The drug is bound to albumin with high affinity, while α1-acid glycoprotein and lipoproteins contribute much less to total plasma protein binding.[1] The unbound or free fraction of levocetirizine has been reported to be 8%.[1] The true volume of distribution of cetirizine is unknown but is estimated to be 0.3 to 0.45L/kg.[2][1] Cetirizine poorly and slowly crosses the blood–brain barrier, which is due mainly to its chemical properties but also to a minor extent to its activity as a P-glycoprotein substrate.[1]
Metabolism
Cetirizine does not undergo extensive metabolism.[2] It is notably not metabolized by the cytochrome P450 system.[24] Because of this, it does not interact significantly with drugs that inhibit or induce cytochrome P450 enzymes such as theophylline, erythromycin, clarithromycin, cimetidine, or alcohol.[2] While cetirizine does not undergo extensive metabolism or metabolism by the cytochrome P450 enzyme, it does undergo some metabolism by other means, the metabolic pathways of which include oxidation and conjugation.[2][3] Plasma radioactivity attributed to unchanged cetirizine is more than 90% at 2hours, 80% at 10hours, and 70% at 24hours, indicating limited and slow metabolism.[3] The enzymes responsible for transformation of cetirizine have not been identified.[2]
Elimination
Cetirizine is eliminated approximately 70 to 85% in the urine and 10 to 13% in the feces.[2] About 50 or 60% of cetirizine eliminated in the urine is unchanged.[2][3] It is eliminated in the urine via an active transport mechanism.[3] The elimination half-life of cetirizine ranges from 6.5 to 10hours in healthy adults, with a mean across studies of approximately 8.3hours.[2][3] Its duration of action is at least 24hours.[3] The elimination half-life of cetirizine is increased in the elderly (to 12hours), in hepatic impairment (to 14hours), and in renal impairment (to 20hours).[3]
Chemistry
Cetirizine contains L- and D-stereoisomers. Chemically, levocetirizine is the active L-enantiomer of cetirizine. The drug is a member of the diphenylmethylpiperazine group of antihistamines. Analogues include cyclizine and hydroxyzine.
Synthesis

The 1-[(4-chlorophenylmethyl]-piperazine is alkylated with methyl (2-chloroethoxy)-acetate in the presence of sodium carbonate and xylene solvent to produce the Sn2 substitution product in 28% yield. Saponification of the acetate ester is done by refluxing with potash in absolute ethanol to afford a 56% yield of the potassium salt intermediate. This is then hydrolyzed with aqueous HCl and extracted to give an 81% yield of the carboxylic acid product.
Society and culture

Brand names
Cetirizine is marketed under the brand names Alatrol, Alerid, Alzene, Cetirin, Cetzine, Cezin, Cetgel, Histazine, Humex, Letizen, Reactine, Razene, Rigix, Triz, Zetop, Zirtec, Zirtek, Zodac, Zyllergy, Zynor, Zyrlek, and Zyrtec among others.
Availability
Formerly prescription-only in many countries, cetirizine is now available without prescription in most countries. In some countries it is available over-the-counter only in packages containing seven or ten 10mg doses.
Like many other antihistamine medications, cetirizine is commonly prescribed in combination with pseudoephedrine, a decongestant. These combinations are often marketed using the same brand name as the cetirizine with a "-D" suffix (Zyrtec-D, Virlix-D, etc.)