Abstract
Diphenhydramine, a sedative histamine H(1)-receptor (H(1)R) 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(m) = 2.99 mM and V(max) = 179.5 nmol s(-1) g(-1). In the pharmacological plasma concentration range the carrier-mediated component accounted for 77% of diphenhydramine influx while passive diffusion accounted for only 23%. [(14)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(1a/1b)/ABCG2) deficiency. Brain and plasma kinetics of [(11)C]diphenhydramine were measured by positron emission tomography imaging in rats. [(11)C]Diphenhydramine kinetics in different brain regions were not influenced by displacement with 1 mg kg(-1) 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. [(11)C]Diphenhydramine radiometabolites were not detected in the brain 15 min after injection, allowing for the reliable calculation of [(11)C]diphenhydramine brain uptake clearance (Cl(up) = 0.99 ± 0.18 mL min(-1 )cm(-3)). Diphenhydramine is a selective and specific H(+)-antiporter substrate. [(11)C]Diphenhydramine positron emission tomography imaging offers a reliable and noninvasive method to evaluate H(+)-antiporter function at the blood-brain barrier.