[¹¹C]metoclopramide is a sensitive radiotracer to measure moderate decreases in P-glycoprotein function at the blood-brain barrier

Abstract

The efflux transporter P-glycoprotein (P-gp) at the blood-brain barrier limits the cerebral uptake of various xenobiotics. To assess the sensitivity of [11C]metoclopramide to measure decreased cerebral P-gp function, we performed [11C]metoclopramide PET scans without (baseline) and with partial P-gp inhibition by tariquidar in wild-type, heterozygous Abcb1a/b(+/−) and homozygous Abcb1a/b(−/−) mice as models with controlled levels of cerebral P-gp expression. Brains were collected to quantify P-gp expression with immunohistochemistry. Brain uptake of [11C]metoclopramide was expressed as the area under the brain time-activity curve (AUCbrain) and compared with data previously obtained with ( R)-[11C]verapamil and [11C] N-desmethyl-loperamide. Abcb1a/b(+/−) mice had intermediate P-gp expression compared to wild-type and Abcb1a/b(−/−) mice. In baseline scans, all three radiotracers were able to discriminate Abcb1a/b(−/−) from wild-type mice (2.5- to 4.6-fold increased AUCbrain, p ≤ 0.0001). However, only [11C]metoclopramide could discriminate Abcb1a/b(+/−) from wild-type mice (1.46-fold increased AUCbrain, p ≤ 0.001). After partial P-gp inhibition, differences in [11C]metoclopramide AUCbrain between Abcb1a/b(+/−) and wild-type mice (1.39-fold, p ≤ 0.001) remained comparable to baseline. There was a negative correlation between baseline [11C]metoclopramide AUCbrain and ex-vivo-measured P-gp immunofluorescence (r = −0.9875, p ≤ 0.0001). Our data suggest that [11C]metoclopramide is a sensitive radiotracer to measure moderate, but (patho-)physiologically relevant decreases in cerebral P-gp function without the need to co-administer a P-gp inhibitor.

Performance and Sensitivity of [99mTc]Tc-sestamibi Compared with Positron Emission Tomography Radiotracers to Measure P-glycoprotein Function in the Kidneys and Liver

“…17 Using the same mouse models that were used in the present study, we could show that [ 11 C]metoclopramide has a better sensitivity than [ 11 C]N-desmethyl-loperamide and (R)-[ 11 C]verapamil to detect an approximately 30% reduction in cerebral P-gp abundance as it occurs in Abcb1a/b (+/−) mice relative to wild-type mice. 17 To examine the suitability of these mouse models for measuring changes in renal and hepatic P-gp function, we performed immunofluorescence labeling and Western blot analysis of P-gp in the kidneys and liver. These experiments revealed a 50% decrease in the abundance of P-gp in the kidneys of Abcb1a/b (+/−) mice relative to wild-type mice, while no P-gp was detected in Abcb1a/b (−/−) mice (Figure 1).…”

“…This is in good agreement with the differences in cerebral P-gp abundance observed in the same mouse models. 17,23 Regrettably, the employed anti-P-gp antibody proved to be unsuitable for detecting P-gp in the liver (Figure S2), so hepatic P-gp abundance could not be quantified in the employed mouse models. An important consideration for the use of PET and SPECT in the assessment of renal and hepatic P-gp function is the inability of these imaging methods to distinguish the radiolabeled parent drug from radiolabeled metabolites.…”

“…Wild-type, Abcb1a/b ( + / – ) , and Abcb1a/b ( – / – ) mice underwent under isoflurane/air anesthesia 60 min dynamic PET scans after i.v. administration of [ 11 C] N -desmethyl-loperamide, ( R )-[ 11 C]­verapamil, or [ 11 C]­metoclopramide using a microPET Focus 220 scanner (Siemens Medical Solutions) as previously described. ,, At the end of the PET scan, a blood sample was collected from the retro-bulbar plexus, and animals were killed by cervical dislocation while still under deep anesthesia. Radioactivity in weighted blood aliquots was measured on a gamma counter.…”

“…The aim of this study was to compare the performance and sensitivity of three radiolabeled P-gp substrates for PET (i.e., PET Imaging. The PET data sets analyzed in this study were previously published by Wanek et al, 22 Zoufal et al, 23 and Mairinger et al 17 In these previous studies, only the brain distribution of the PET radiotracers was analyzed in different mouse models. An overview of the mouse groups included in this study is given in Table 1.…”

“…The aim of this study was to compare the performance and sensitivity of three radiolabeled P-gp substrates for PET (i.e., [ 11 C] N -desmethyl-loperamide, ( R )-[ 11 C]­verapamil, and [ 11 C]­metoclopramide) with [ 99m Tc]­Tc-sestamibi for measuring P-gp function in the kidneys and liver of mice. Since a complete loss of P-gp, as it occurs in Abcb1a/b (−/−) mice, represents a drastic scenario that does not reflect a clinically realistic reduction in P-gp function caused by DDIs or disease, we included heterozygous Abcb1a/b knockout ( Abcb1a/b () ) mice into our evaluation as a model of moderately reduced P-gp function …”