A positron emission tomography (PET) study found that brain occupancy of the H1 receptor was 12.6% for 10 mg cetirizine, 25.2% for 20 mg cetirizine, and 67.6% for 30 mg hydroxyzine. (A 10 mg dose of cetirizine equals about a 30 mg dose of hydroxyzine in terms of peripheral antihistamine effect.) 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. In accordance, H1 receptor occupancy correlated well with subjective sleepiness for 30 mg hydroxyzine but there was no correlation for 10 or 20 mg cetirizine. As such, brain penetration and brain H1 receptor occupancy by cetirizine are dose-dependent, and in accordance, while cetirizine at doses of 5 to 10 mg have been reported to be non-sedating or mildly sedating, a higher dose of 20 mg has been found to induce significant drowsiness in other studies.Cetirizine also shows anti-inflammatory properties independent of H1 receptors. The effect is exhibited through suppression of the NF-κB pathway, and by regulating the release of cytokines and chemokines, thereby regulating the recruitment of inflammatory cells. It has been shown to inhibit eosinophil chemotaxis and LTB4 release. At a dosage of 20 mg, Boone et al. found that it inhibited the expression of VCAM-1 in patients with atopic dermatitis. Pharmacokinetics
Absorption
Cetirizine is rapidly and extensively absorbed upon oral administration in tablet or syrup form.
Some acne and wrinkle treatments reported to cause rosacea include microdermabrasion and chemical peels, as well as high dosages of isotretinoin, benzoyl peroxide, and tretinoin. Steroid-induced rosacea is caused by the use of topical steroids. These steroids are often prescribed for seborrheic dermatitis. Dosage should be slowly decreased and not immediately stopped to avoid a flare-up. Cathelicidins
In 2007, Richard Gallo and colleagues noticed that patients with rosacea had high levels of the antimicrobial peptide cathelicidin and elevated levels of stratum corneum tryptic enzymes (SCTEs). Antibiotics have been used in the past to treat rosacea, but they may only work because they inhibit some SCTEs. Demodex folliculitis and Demodex mites
Studies of rosacea and Demodex mites have revealed that some people with rosacea have increased numbers of the mite, especially those with steroid-induced rosacea. Demodex folliculitis (demodicidosis, also known as "mange" in animals) is a condition that may have a "rosacea-like" appearance.A 2007, National Rosacea Society-funded study demonstrated that Demodex folliculorum mites may be a cause or exacerbating factor in rosacea. The researchers identified Bacillus oleronius as distinct bacterium associated with Demodex mites. When analyzing blood samples using a peripheral blood mononuclear cell proliferation assay, they discovered that B. oleronius stimulated an immune system response in 79 percent of 22 patients with subtype 2 (papulopustular) rosacea, compared with only 29% of 17 subjects without the disorder. They concluded, "The immune response results in inflammation, as evident in the papules (bumps) and pustules (pimples) of subtype 2 rosacea.