RT Journal Article SR Electronic T1 Assessment of P-Glycoprotein Transport Activity at the Human Blood–Retina Barrier with (R)‐11C-Verapamil PET JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 678 OP 681 DO 10.2967/jnumed.116.182147 VO 58 IS 4 A1 Martin Bauer A1 Rudolf Karch A1 Nicolas Tournier A1 Salvatore Cisternino A1 Wolfgang Wadsak A1 Marcus Hacker A1 Peter Marhofer A1 Markus Zeitlinger A1 Oliver Langer YR 2017 UL http://jnm.snmjournals.org/content/58/4/678.abstract AB P-glycoprotein (ABCB1) is expressed at the blood–retina barrier (BRB), where it may control distribution of drugs from blood to the retina and thereby influence drug efficacy and toxicity. Methods: We performed PET scans with the ABCB1 substrate (R)-11C-verapamil on 5 healthy male volunteers without and with concurrent infusion of the ABCB1 inhibitor tariquidar. We estimated the rate constants for radiotracer transfer across the BRB (K1, k2) and total retinal distribution volume VT. Results: During ABCB1 inhibition, retinal VT and influx rate constant K1 were significantly, by 1.4 ± 0.5-fold and 1.5 ± 0.3-fold, increased compared with baseline. Retinal efflux rate constant k2 was significantly decreased by 2.8 ± 1.0-fold. Conclusion: We found a significant increase in (R)-11C-verapamil distribution to the retina during ABCB1 inhibition, which provides first in vivo evidence for ABCB1 transport activity at the human BRB. The increase in retinal distribution was approximately 2.5-fold less pronounced than previously reported for the blood–brain barrier.