P-glycoprotein function at the blood-brain barrier in humans can be quantified with the substrate radiotracer 11C-N-desmethyl-loperamide.
Permeability-glycoprotein (P-gp), an efflux transporter in several organs, acts at the blood-brain barrier to protect the brain from exogenous toxins. P-gp almost completely blocks brain entry of the PET radiotracer (11)C-N-desmethyl-loperamide ((11)C-dLop). We examined the ability of (11)C-dLop to quantify P-gp function in humans after increasing doses of tariquidar, an inhibitor of P-gp.


METHODS
Seventeen healthy volunteers had a total of 23 PET scans with (11)C-dLop at baseline and after increasing doses of tariquidar (2, 4, and 6 mg/kg intravenously). A subset of subjects received PET with (15)O-H(2)O to measure cerebral blood flow. Brain uptake of (11)C-dLop was quantified in 2 ways. Without blood data, uptake was measured as area under the time-activity curve in the brain from 10 to 30 min (AUC(10-30)). With arterial blood data, brain uptake was quantified with compartmental modeling to estimate the rates of entry into (K(1)) and efflux from (k(2)) the brain.


RESULTS
Brain uptake of radioactivity was negligible at baseline and increased only slightly (approximately 30%) after 2 mg of tariquidar per kilogram. In contrast, 4 and 6 mg of tariquidar per kilogram increased brain uptake 2- and 4-fold, respectively. Greater brain uptake reflected greater brain entry (K(1)), because efflux (k(2)) and cerebral blood flow did not differ between tariquidar-treated and untreated subjects. In the subjects who received the highest dose of tariquidar (and had the highest brain uptake), regional values of K(1) correlated linearly with absolute cerebral blood flow, consistent with high single-pass extraction of (11)C-dLop. AUC(10-30) correlated linearly with K(1).


CONCLUSIONS
P-gp function at the blood-brain barrier in humans can be quantified using PET and (11)C-dLop. A simple measure of brain uptake (AUC(10-30)) may be used as a surrogate of the fully quantified rate constant for brain entry (K(1)) and thereby avoid arterial sampling. However, to dissect the function of P-gp itself, both brain uptake and the influx rate constant must be corrected for radiotracer delivery (blood flow).
Molecular Imaging Branch, National Institute of Mental Health, Bethesda, Maryland 20892, USA.
William C
WC
Kreisl
Jeih-San
JS
Liow
Nobuyo
N
Kimura
Nicholas
N
Seneca
Sami S
SS
Zoghbi
Cheryl L
CL
Morse
Peter
P
Herscovitch
Victor W
VW
Pike
Robert B
RB
Innis
D018435:ATP Binding Cassette Transporter, Subfamily B / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000328:Adult; D001692:Biological Transport / Q000187:drug effects; D001812:Blood-Brain Barrier / Q000000981:diagnostic imaging / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D002560:Cerebrovascular Circulation / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D064420:Drug-Related Side Effects and Adverse Reactions; D005260:Female; D006801:Humans; D007700:Kinetics; D008139:Loperamide / Q000031:analogs & derivatives* / Q000378:metabolism; D008279:Magnetic Resonance Imaging; D008297:Male; D049268:Positron-Emission Tomography; D011804:Quinolines / Q000494:pharmacology; D011849:Radioactive Tracers
D018435:ATP Binding Cassette Transporter, Subfamily B; D011804:Quinolines; D011849:Radioactive Tracers; C419149:N-demethylloperamide; D008139:Loperamide; C402343:tariquidar