Brain histamine H1 receptor occupancy of orally administered antihistamines, bepotastine and diphenhydramine, measured by PET with 11C-doxepin.

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT: 'Bepotastine besilate' is a novel second-generation antihistamine developed in Japan and its antiallergic effects have already been demonstrated by various studies. However, only a few clinical studies regarding its sedative property are available. In addition, histamine H(1) receptor occupancy (H(1)RO) of this new antihistamine has never been measured by positron emission tomography (PET).

WHAT THIS STUDY ADDS: This paper provides the first measurement result of cerebral H(1)RO of bepotastine besilate (approximately 15%) as determined by PET. This result is in accordance with the clinical classification of bepotastine as a second-generation antihistamine. In addition, the relationship between subjective sleepiness and cerebral H(1)RO of this second-generation antihistamine is demonstrated for the first time using a placebo-controlled crossover study design. AIMS Antihistamines are frequently used for treating various allergic diseases, but often induce sedation. The degree of sedation can be evaluated by measuring histamine H(1) receptor occupancy (H(1)RO) in the brain using positron emission tomography (PET). The aim was to measure H(1)RO of bepotastine, a new second-generation antihistamine, and to compare it with that of diphenhydramine.

METHODS: Eight healthy male volunteers (mean age +/- SD 24.4 +/- 3.3 years) were studied after single oral administration of bepotastine (10 mg), diphenhydramine (30 mg) or placebo, by PET imaging with (11)C-doxepin in a crossover study design. Binding potential ratio and H(1)ROs were calculated using placebo data and were compared between bepotastine and diphenhydramine in the anterior and posterior cingulate gyri (ACG and PCG, respectively), superior and inferior frontal cortices (SFC and IFC, respectively), orbitofrontal cortex (OFC), insular cortex (IC), lateral and medial temporal cortices (LTC and MTC, respectively), parietal cortex (PC), occipital cortex (OC) and sensorimotor cortex (SMC). Plasma concentration of each antihistamine was measured, and its correlation to H(1)RO was examined.

RESULTS: H(1)RO after bepotastine treatment was significantly lower than that after diphenhydramine treatment in all cortical regions (P < 0.001). Mean H(1)ROs of bepotastine and diphenhydramine were 14.7% and 56.4%, respectively. H(1)ROs of both bepotastine and diphenhydramine correlated to their respective drug plasma concentration (P < 0.001).

CONCLUSION: Oral bepotastine (10 mg), with its relatively low H(1)RO and thus minimal sedation, has the potential for use as a mildly or slightly sedative antihistamine in the treatment of various allergic disorders.