Histamine H1-receptor
Levocetirizine targets the histamine H1 receptor, where it acts as a highly selective antagonist. By binding to the H1 receptor, it prevents the binding of its endogenous ligand histamine. The failure of histamine to bind to the receptor reduces the typical symptoms of allergic reactions, such as pruritus, vasodilation, and increased vascular permeability. Levocetirizine has greater affinity and selectivity for the H1 receptor than cetirizine.

Eosinophils are well known to play essential roles in the development and maintenance of allergic diseases. However, the influence of histamine H1 receptor antagonists on eosinophil functions, especially chemokine production, are not well-defined. Therefore, in the present study, we examined the influence of histamine H1 receptor antagonist on chemokine production by eosinophils through the use of levocetirizine in vitro and in vivo. Eosinophils prepared from mice were stimulated with specific antigens in the presence of different concentrations of levocetirizine. After 24 h, regulated on activation normal T cell expressed and secreted (RANTES) and eotaxin levels in culture supernatants were measured by enzyme-linked immunosorbent assay (ELISA). Patients with Japanese cedar pollinosis were treated with 5 mg levocetirizine once a day for four weeks during the pollen season (February 2012 to April 2012). RANTES and eotaxin levels in nasal secretions were also examined by ELISA. The addition of levocetirizine to eosinophil cultures caused a dose-dependent decrease in the ability of cells to produce RANTES and eotaxin in response to antigen stimulation, and the minimum concentration that caused a significant decrease was 0.05 μM. Although cetirizine also exerted suppressive effects on the production of RANTES and eotaxin by eosinophils, the minimum concentration that caused significant suppression was 0.15 μM, which was three-times higher than that of levocetirizine. Oral administration of levocetirizine for four weeks also reduced RANTES and eotaxin levels in nasal secretions from patients with pollinosis, along with attenuation of clinical symptoms. The ability of levocetirizine to reduce RANTES and eotaxin levels may account, at least in part, for the clinical efficacy of the agent for allergic disorders, including allergic rhinitis[2].
In vitro, Levocetirizine has been shown to have a higher affinity for the histamine H1 receptor than cetirizine and its S-enantiomer. It effectively antagonizes histamine-induced responses in various in vitro models. The compound's high selectivity for the H1 receptor contributes to its favorable side effect profile, with minimal anticholinergic or sedative effects. Levocetirizine is commonly used as a reference compound in in vitro assays to study H1 receptor antagonism.

Levocetirizine (0.4 mg/kg; oral; male Sprague-Dawley rats) treatment revealed Cmax, AUC0-t, AUC0-∞, and t1/2 of 0.34 μg/mL, 3.26 μg h/mL, and 3.67 μg, respectively. h/mL in Sprague-Dawley rats and 2.34 hours respectively [1].
In vivo, Levocetirizine has been shown to be effective in the treatment of allergic rhinitis and chronic idiopathic urticaria. Clinical studies have demonstrated its efficacy in reducing symptoms such as sneezing, itching, rhinorrhea, and nasal congestion. Levocetirizine has a rapid onset of action and provides sustained relief of allergic symptoms. It is well-tolerated with a low incidence of sedation, making it suitable for once-daily dosing.

Assay for transcription factor activation. NF-κB activity in cultured eosinophils was analyzed by commercially available NF-κB ELISA test kits that contained sufficient reagents and a monoclonal antibody against p65, according to the manufacturer's recommendations. In brief, nuclear extract (5.0 mg of protein) from eosinophils was introduced into each well of 96-well microplates precoated with oligonucleotide containing NF-κB consensus site (5’-GGGACTTTCC-3’) in a volume of 20.0 μl, followed by incubation for one hour at 25°C. After washing three times, 100 μl of monoclonal antibody against p65 was added to the appropriate wells and incubated for a further one hour at 25°C. Anti-IgE horseradish peroxidase (HRP) conjugate in a volume of 100 μl was then added and the plates incubated for a further one hour at 25°C. The absorbance at 450 nm was measured after the addition of tetramethylbenzidine (TMB) solution. AP-1 activity was also measured with commercially available AP-1 ELISA test kit in a similar manner[2].
In vitro receptor binding assays for Levocetirizine typically involve measuring its affinity for the histamine H1 receptor using radioligand binding. The compound is incubated with membrane preparations from cells expressing the H1 receptor and a radiolabeled ligand such as [3H]mepyramine. The concentration of Levocetirizine required to displace 50% of the radioligand (IC50) is determined, and the Ki value is calculated. The selectivity of Levocetirizine for H1 over other receptors, including H2, H3, and H4, can be assessed using similar binding assays.

Assay for mRNA expression. Poly A+ mRNA was separated from cultured cells with oligo(dT)-coated magnetic micro beads. The first-strand cDNA was synthesized from 1.0 mg of PolyA+ mRNA using a Superscript cDNA synthesis kit according to the manufacturer's instructions. Polymerase chain reaction (PCR) was then carried out using a GeneAmp 5700 Sequence Detection Syste. The PCR mixture consisted of 2.0 μl of sample cDNA solution (100 ng/μl), 25.0 μl of SYBR-Green Mastermix, 0.3 μl of both sense and antisense primers, and distilled water to give a final volume of 50.0 μl. The reaction was conducted as follows: 4 min at 94°C, followed by 40 cycles of 15 s at 95°C and 60 s at 60°C. Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) was amplified as an internal control. mRNA levels were calculated by using the comparative parameter threshold cycle and normalized for GAPDH. The nucleotide sequences of the primers were as follows: for RANTES, 5’-CCTCACCATCATCCTCACTGCA-3’ (sense) and 5’-TCTTCTCTGGGTTGGCACACAC-3’ (antisense), for eotaxin, 5’-CCCTTTTCTGTTCTGCTGACAAG-3’ (sense) and 5’-GAAGAGTCCCTCGATGTGGCTA-3’ (antisense), and for GAPDH, 5’-GTCTTCTGGGTGGCAGTGAT-3’ (sense) and 5’-CCCTTT TCTGTTCTGCTGACAAG-3’ (antisense)[2].
In vitro cell-based studies with Levocetirizine typically involve cultured cells expressing the histamine H1 receptor, such as human mast cells or basophils. Cells are treated with Levocetirizine at various concentrations and stimulated with histamine or an allergen. The inhibition of histamine-induced calcium mobilization, chemotaxis, or cytokine production is measured. The IC50 value for inhibition of these responses is determined from dose-response curves. The effect of Levocetirizine on cell viability and receptor internalization can also be evaluated.

Animal/Disease Models: 30 male SD (SD (Sprague-Dawley)) rats (8 weeks old; 200-250 g) [1]
Doses: 0.4 mg/kg
Route of Administration: Oral (pharmacokinetic/PK/PK analysis)
Experimental Results: Cmax, AUC0-t, AUC0-∞ and t1 /2 were 0.34μg/mL, 3.26μg·h/mL, 3.67μg·h/mL and 2.34 hrs (hrs (hours)) respectively.
In vivo animal studies with Levocetirizine are typically conducted in rodent models of allergic rhinitis or urticaria. The compound is administered orally. The effect of Levocetirizine on histamine-induced cutaneous reactions, such as wheal and flare, is assessed. The compound's ability to inhibit allergen-induced nasal symptoms, including sneezing and nasal rubbing, is also evaluated. Pharmacokinetic studies are performed to evaluate the absorption, distribution, and elimination of the compound.

Absorption, Distribution and Excretion
Following oral administration of 5 mg levocetirizine, the peak plasma concentration (Cmax) was 0.27 ± 0.04 µg/mL, and the time to peak concentration (Tmax) was 0.75 ± 0.50 h. The area under the curve (AUC) of levocetirizine was 2.31 ± 0.50 µgh/mL. Co-administration with food did not affect the AUC, but delayed Tmax by 1.25 hours and reduced Cmax by 36%. At 168 hours post-administration, an average of 85.4% of the radiolabeled dose was recovered, with an average of 80.8% excreted in the urine and 9.5% in the feces. In urine, 77% of the dose was recovered unchanged, 0.5% as metabolites M8 and M9, 0.4% as metabolites M10a, 0.4% as metabolites M10b, 0.3% as metabolites M3, 0.3% as metabolites M4 and M5, 0.2% as metabolites M2, and 0.1% as metabolites M1. In feces, 9.0% of the dose was excreted unchanged, 1.0% as metabolites M4 and M5, and 0.1% as metabolites M1. The volume of distribution of levocetirizine is 0.33 ± 0.02 L/kg. The mean clearance of levocetirizine is 0.57 ± 0.18 mL/min/kg. Levocetirizine is poorly metabolized, with 85.8% of the oral dose excreted unchanged. Levocetirizine is metabolized to dihydrodiol (M2), N-oxide (M3), hydroxymethoxy derivative (M4), hydroxy derivative (M5), O-dealkylated derivative (M6), taurine conjugate (M8), and N-dealkylated aromatic hydroxylated derivative (M9). Metabolite M5 can be glucuronidated to generate metabolite M1, and metabolite M9 can generate 4-chloro-4'-hydroxydibenzoyl mercaptourate (M10a and M10b). The mean half-life of levocetirizine is 7.05 ± 1.54 hours.
Levocetirizine is a well-characterized drug with established pharmacokinetic properties. Following oral administration, it is rapidly absorbed from the gastrointestinal tract, reaching peak plasma concentrations within 0.9 hours. Levocetirizine has a bioavailability of approximately 100%. It is not extensively metabolized in the liver, and is primarily excreted unchanged in the urine. The elimination half-life is approximately 8 hours. Levocetirizine is not significantly protein-bound.

Use of Levocetirizine During Pregnancy and Lactation ◉ Overview of Use During Lactation
Levocetirizine is the R-enantiomer of cetirizine. Based on limited information regarding cetirizine and levocetirizine, levocetirizine appears to be acceptable during lactation. Higher doses or longer use may cause infant drowsiness and other adverse reactions, or reduced milk production, especially when used in combination with sympathomimetic drugs such as pseudoephedrine or before lactation is fully established. International guidelines recommend that cetirizine (the racemic form of the drug) is an acceptable option if antihistamines are needed during lactation.
◉ Effects on Breastfed Infants
As of the revision date, no published information was found regarding levocetirizine. In a telephone follow-up study, mothers reported irritability and colic symptoms in 10% of infants exposed to various antihistamines, and drowsiness in 1.6% of infants. None of these reactions required medical attention.
A breastfeeding mother who took 5 mg of levocetirizine daily reported no adverse reactions in her breastfed infant.
◉ Effects on Lactation and Breast Milk
Injected relatively high doses of antihistamines can lower basal serum prolactin levels in non-lactating women and early postpartum women. However, pre-administration of antihistamines by postpartum mothers does not affect suckling-induced prolactin secretion. Whether lower oral doses of levocetirizine have the same effect on serum prolactin, and whether the effect on prolactin has any impact on breastfeeding success, is currently unstudied. For mothers who have established lactation, prolactin levels may not affect their ability to breastfeed.
Protein Binding

The plasma protein binding rate of levocetirizine was 96.1% on average at 1 hour after administration and 91.9% on average at 6 hours after administration.
Levocetirizine is a clinically approved drug with a well-established safety profile. The most common adverse effects are mild and include headache, drowsiness, fatigue, and dry mouth. Levocetirizine has a lower incidence of sedation compared to first-generation antihistamines due to its reduced penetration of the blood-brain barrier. Serious adverse effects are rare. Levocetirizine is generally well-tolerated at therapeutic doses.

[1]. Simultaneous bioanalysis and pharmacokinetic interaction study of acebrophylline, levocetirizine and pranlukast in Sprague-Dawley rats. Biomed Chromatogr. 2019 Dec;33(12):e4672.

2-[2-[4-[(R)-(4-chlorophenyl)-phenylmethyl]-1-piperazinyl]ethoxy]acetic acid is a diarylmethane. Levocetirizine is a selective histamine H1 receptor antagonist used to treat a variety of allergic symptoms. It is the R enantiomer of cetirizine. Levocetirizine has a higher affinity for histamine H1 receptors than cetirizine. Levocetirizine was approved by the FDA in 1995. Levocetirizine is a histamine-1 receptor antagonist. The mechanism of action of levocetirizine is as a histamine H1 receptor antagonist. Levocetirizine is a third-generation, non-sedating, selective histamine H1 receptor antagonist with antihistamine, anti-inflammatory, and potential anti-angiogenic activities. Levocetirizine competes with endogenous histamine for binding to peripheral H1 receptor sites on the surface of effector cells. This can prevent adverse symptoms associated with histamine release and allergic reactions. Furthermore, since histamine plays a crucial role in angiogenesis during allergic inflammatory responses, blocking histamine's action may regulate the expression of pro-angiogenic factors, thereby inhibiting angiogenesis. As a third-generation histamine H1 receptor antagonist, levocetirizine has fewer side effects than most second-generation antihistamines.
See also: levocetirizine hydrochloride (in salt form); cetirizine (note moved to).
Indications
Leviticus is indicated for the treatment of symptoms of perennial allergic rhinitis and uncomplicated cutaneous manifestations of chronic idiopathic urticaria. It can also be used to treat a variety of mild allergy symptoms and is available without a prescription.
FDA Label
Mechanism of Action
Leviticus selectively inhibits histamine H1 receptors. This action prevents histamine from activating these receptors, thus avoiding effects such as smooth muscle contraction, increased vascular endothelial permeability, basophil uptake of histidine, stimulation of cough receptors, and stimulation of inflammatory responses in the nervous system.
Levocetirizine is a clinically approved antihistamine used for the treatment of allergic rhinitis and chronic idiopathic urticaria. It is available in various formulations, including tablets and oral solutions. Levocetirizine is marketed under various brand names, including Xyzal. It is a prescription medication in some countries and is available over-the-counter in others. Levocetirizine is the active R-enantiomer of cetirizine and provides effective relief of allergic symptoms with a favorable safety profile.

C21H25N2O3CL

388.8878

388.155

C, 64.86; H, 6.48; Cl, 9.12; N, 7.20; O, 12.34

130018-77-8

Cetirizine;83881-51-0;Cetirizine dihydrochloride;83881-52-1;Cetirizine-d4;1219803-84-5;Cetirizine-d8;774596-22-4;Levocetirizine dihydrochloride;130018-87-0;Cetirizine-d4 dihydrochloride;Cetirizine-d8 dihydrochloride;2070015-04-0;Levocetirizine-d4 dihydrochloride;Levocetirizine-d4;1133210-23-7

1549000

Typically exists as white to off-white solids at room temperature

1.2±0.1 g/cm3

542.1±45.0 °C at 760 mmHg

205-208°C (dec.)

281.6±28.7 °C

0.0±1.5 mmHg at 25°C

1.589

2.16

1

5

8

27

443

1

C1=CC=C(C=C1)[C@H](C2=CC=C(C=C2)Cl)N3CCN(CC3)CCOCC(=O)O

ZKLPARSLTMPFCP-OAQYLSRUSA-N

InChI=1S/C21H25ClN2O3/c22-19-8-6-18(7-9-19)21(17-4-2-1-3-5-17)24-12-10-23(11-13-24)14-15-27-16-20(25)26/h1-9,21H,10-16H2,(H,25,26)/t21-/m1/s1

Acetic acid, (2-(4-((R)-(4-chlorophenyl)phenylmethyl)-1-piperazinyl)ethoxy)-

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Note: Please store this product in a sealed and protected environment (e.g. under nitrogen), avoid exposure to moisture and light.

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

DMSO : ~100 mg/mL (~257.14 mM)

Solubility in Formulation 1: ≥ 2.5 mg/mL (6.43 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 2: ≥ 2.5 mg/mL (6.43 mM) (saturation unknown) in 10% DMSO + 90% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of 20% SBE-β-CD physiological saline solution and mix evenly.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

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Solubility in Formulation 3: ≥ 2.5 mg/mL (6.43 mM) (saturation unknown) in 10% DMSO + 90% Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

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 (Please use freshly prepared in vivo formulations for optimal results.)