Blood–brain and retinal barriers show dissimilar ABC transporter impacts and concealed effect of P‐glycoprotein on a novel verapamil influx carrier

Abstract

BACKGROUND AND PURPOSEThe respective impact and interplay between ABC (P-glycoprotein/P-gp/Abcb1a, BCRP/ABCG2, MRP/ABCC) and SLC transporter functions at the blood-brain barrier (BBB) and blood-retinal barriers (BRB) are incompletely understood. EXPERIMENTAL APPROACHWe measured the initial cerebral and retinal distribution of selected ABC substrates by in situ carotid perfusion using P-gp/Bcrp knockout mice and chemical ABC/SLC modulation strategies. P-gp, Bcrp, Mrp1 and Mrp4 were studied by confocal retina imaging. KEY RESULTSChemical or physical disruption of P-gp increased [ ]-zidovudine transport suggested, respectively, that Bcrp efflux was less involved at the BRB than BBB, whereas Mrps were significantly and similarly involved at both barriers. Confocal imaging showed that P-gp and Bcrp were expressed in intra-retinal vessels (inner BRB/iBRB) but absent from the blood/basal membrane of cells of the retinal pigment epithelium (outer BRB/oBRB/RPE) where, in contrast, Mrp1 and Mrp4 were localized. CONCLUSIONS AND IMPLICATIONSP-gp, Bcrp, Mrp1 and Mrp4 are differentially expressed at the outer and inner BRB, resulting in an altered ability to limit substrate distribution at the retina as compared with the BBB. [ 3 H]-Verapamil distribution is not P-gp-specific and involves a proton antiporter at both the BBB and BRB. However, this transport is concealed by P-gp at the BBB, but not at the BRB, where P-gp activity is reduced.

Influx Transport of Cationic Drug at the Blood–Retinal Barrier: Impact on the Retinal Delivery of Neuroprotectants

“…21) In the further assessment of differences in the BRB and BBB, a study with P-gp knockout rats showed the negligible impact of P-gp on K in, verapamil, retina in spite of a marked alteration of the K in, verapamil, brain 22) ; similar results were reported in a study using P-gp/Bcrp knockout mice. 23) These findings strongly support differences in transport properties of the BRB and BBB, and suggest the dominant influx transport of verapamil at the BRB (Fig. 3B).…”

P-gp/ABCB1 exerts differential impacts on brain and fetal exposure to norbuprenorphine

“…Recently, Eyal et al showed that chemical inhibition of P-gp in non-human primate increased maternal brain distribution of [ 11 C]-verapamil ∼4-fold, but its fetal liver distribution (as a surrogate of fetal exposure) was increased only ∼2-fold [52]. Chapy et al reported that the knockout of Abcb1a/1b increased [ 3 H]-verapamil distribution across the BBB and blood-retinal barrier (BRB) ∼10- and ∼1.5-fold, respectively [34]. Thus, our results regarding dissimilar impacts of P-gp on NBUP distribution across the BBB and BPB are consistent with previous studies.…”

“…Therefore, the aim of this study was to investigate whether P-gp and BCRP affect maternal pharmacokinetics of NBUP and its metabolite NBUP-G and whether P-gp and BCRP play important roles in restricting fetal penetration of NBUP and NBUP-G, using wild-type, Abcb1a -/- / 1b -/- , and Abcb1a -/- / 1b -/- / Abcg2 -/- pregnant mice. It has previously been shown that P-gp in the blood-brain barrier (BBB) and the blood- retinal barrier (BRB) of mouse models has dissimilar impacts on tissue exposure to a substrate drug verapamil, with a much greater effect on limiting brain exposure than retinal exposure [34]. Hence, to investigate whether P-gp and BCRP in the BBB and the blood-placental barrier (BPB) display differential roles in restricting brain and fetal drug exposure, we also examined and compared brain and fetal exposure to NBUP and NBUP-G in the same wild-type and transporter knockout dams.…”

Imaging techniques to study drug transporter function in vivo

“…Invasive experiments in rodents have revealed the functional impact of P-gp at this specific barrier (Chapy et al, 2016). Compared with the BBB, mBCRP-mediated efflux was shown to be less involved at the BRB, whereas mMRPs were involved to a similar degree at both barriers (Chapy et al, 2016).…”

“…Invasive experiments in rodents have revealed the functional impact of P-gp at this specific barrier (Chapy et al, 2016). Compared with the BBB, mBCRP-mediated efflux was shown to be less involved at the BRB, whereas mMRPs were involved to a similar degree at both barriers (Chapy et al, 2016). Using (R)-[ 11 C]verapamil PET imaging in healthy volunteers, Bauer et al reported a significant increase in radiotracer distribution to the retina during P-gp inhibition, which provided first in vivo evidence for P-gp transport activity at the human BRB.…”