Diphenhydramine as a selective probe to study H-antiporter function at the blood-brain barrier: Application to [C]diphenhydramine positron emission tomography imaging.

Diphenhydramine, a sedative histamine H-receptor (HR) antagonist, was evaluated as a probe to measure drug/H-antiporter function at the blood-brain barrier. In situ brain perfusion experiments in mice and rats showed that diphenhydramine transport at the blood-brain barrier was saturable, following Michaelis-Menten kinetics with a K = 2.99 mM and V = 179.5 nmol s g. In the pharmacological plasma concentration range the carrier-mediated component accounted for 77% of diphenhydramine influx while passive diffusion accounted for only 23%. [C]Diphenhydramine blood-brain barrier transport was proton and clonidine sensitive but was influenced by neither tetraethylammonium, a MATE1 (SLC47A1), and OCT/OCTN (SLC22A1-5) modulator, nor P-gp/Bcrp (ABCB/ABCG2) deficiency. Brain and plasma kinetics of [C]diphenhydramine were measured by positron emission tomography imaging in rats. [C]Diphenhydramine kinetics in different brain regions were not influenced by displacement with 1 mg kg unlabeled diphenhydramine, indicating the specificity of the brain positron emission tomography signal for blood-brain barrier transport activity over binding to any central nervous system target in vivo. [C]Diphenhydramine radiometabolites were not detected in the brain 15 min after injection, allowing for the reliable calculation of [C]diphenhydramine brain uptake clearance (Cl = 0.99 ± 0.18 mL mincm). Diphenhydramine is a selective and specific H-antiporter substrate. [C]Diphenhydramine positron emission tomography imaging offers a reliable and noninvasive method to evaluate H-antiporter function at the blood-brain barrier.