Cerebral histamine H1 receptor binding potential measured with PET under a test dose of olopatadine, an antihistamine, is reduced after repeated administration of olopatadine.
Some antihistamine drugs that are used for rhinitis and pollinosis have a sedative effect as they enter the brain and block the H(1) receptor, potentially causing serious accidents. Receptor occupancy has been measured with PET under single-dose administration in humans to classify antihistamines as more sedating or as less sedating (or nonsedating). In this study, the effect of repeated administration of olopatadine, an antihistamine, on the cerebral H(1) receptor was measured with PET.


METHODS
A total of 17 young men with rhinitis underwent dynamic brain PET with (11)C-doxepin at baseline, under an initial single dose of 5 mg of olopatadine (acute scan), and under another 5-mg dose after repeated administration of olopatadine at 10 mg/d for 4 wk (chronic scan). The H(1) receptor binding potential was estimated using Logan graphical analysis with cerebellum as reference region input.


RESULTS
The acute scan showed a slight decrease in H(1) receptor binding potential across the cerebral cortex (by 15% in the frontal cortex), but the chronic scan showed a marked decrease (by 45% from the acute scan in the frontal cortex). Behavioral data before and after the PET scans did not reveal any sedative effect.


CONCLUSIONS
The results may be interpreted as either intracerebral accumulation of olopatadine or H(1) receptor downregulation due to repeated administration. The study shows feasibility and potential value for PET in evaluating the pharmacologic effect of a drug not only after a single dose but also after repeated administration.
Senda
Michio
M
Division of Molecular Imaging, Institute of Biomedical Research and Innovation, Kobe, Japan. senda@fbri.org
Kubo
Nobuo
N
Adachi
Kazuhiko
K
Ikari
Yasuhiko
Y
Matsumoto
Keiichi
K
Shimizu
Keiji
K
Tominaga
Hideyuki
H
D000328:Adult; D001921:Brain / Q000378:metabolism*; D002250:Carbon Radioisotopes; D003990:Dibenzoxepins / Q000494:pharmacology*; D004316:Doxepin / Q000378:metabolism*; D039563:Histamine H1 Antagonists, Non-Sedating / Q000494:pharmacology*; D006801:Humans; D008297:Male; D000069605:Olopatadine Hydrochloride; D049268:Positron-Emission Tomography / Q000379:methods*; D011969:Receptors, Histamine H1 / Q000378:metabolism*; D055815:Young Adult
D002250:Carbon Radioisotopes; D003990:Dibenzoxepins; D039563:Histamine H1 Antagonists, Non-Sedating; D011969:Receptors, Histamine H1; D004316:Doxepin; D000069605:Olopatadine Hydrochloride