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Abstract Background P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2) are two efflux transporters expressed at the blood–brain barrier which effectively restrict the brain distribution of the majority of currently known anticancer drugs. High-grade brain tumors often possess a disrupted blood–brain tumor barrier (BBTB) leading to enhanced accumulation of magnetic resonance imaging contrast agents, and possibly anticancer drugs, as compared to normal brain. In contrast to high-grade brain tumors, considerably less information is available with respect to BBTB integrity in lower grade brain tumors. Materials and methods We performed positron emission tomography imaging with the radiolabeled ABCB1 inhibitor [11C]tariquidar, a prototypical ABCB1/ABCG2 substrate, in seven patients with non-contrast -enhancing brain tumors (WHO grades I–III). In addition, ABCB1 and ABCG2 levels were determined in surgically resected tumor tissue of four patients using quantitative targeted absolute proteomics. Results Brain distribution of [11C]tariquidar was found to be very low across the whole brain and not significantly different between tumor and tumor-free brain tissue. Only one patient showed a small area of enhanced [11C]tariquidar uptake within the brain tumor. ABCG2/ABCB1 ratios in surgically resected tumor tissue (1.4 ± 0.2) were comparable to previously reported ABCG2/ABCB1 ratios in isolated human micro-vessels (1.3), which suggested that no overexpression of ABCB1 or ABCG2 occurred in the investigated tumors. Conclusions Our data suggest that the investigated brain tumors had an intact BBTB, which is impermeable to anticancer drugs, which are dual ABCB1/ABCG2 substrates. Therefore, effective drugs for antitumor treatment should have high passive permeability and lack ABCB1/ABCG2 substrate affinity. Trial registration European Union Drug Regulating Authorities Clinical Trials Database (EUDRACT), 2011-004189-13. Registered on 23 February 2012, https://www.clinicaltrialsregister.eu/ctr-search/search?query=2011-004189-13.

Background: P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2) are two efflux transporters expressed at the blood–brain barrier which effectively restrict the brain distribution of the majority of currently known anticancer drugs. High-grade brain tumors often possess a disrupted blood–brain tumor barrier (BBTB) leading to enhanced accumulation of magnetic resonance imaging contrast agents, and possibly anticancer drugs, as compared to normal brain. In contrast to high-grade brain tumors, considerably less information is available with respect to BBTB integrity in lower grade brain tumors. Materials and methods: We performed positron emission tomography imaging with the radiolabeled ABCB1 inhibitor [11C]tariquidar, a prototypical ABCB1/ABCG2 substrate, in seven patients with non-contrast -enhancing brain tumors (WHO grades I–III). In addition, ABCB1 and ABCG2 levels were determined in surgically resected tumor tissue of four patients using quantitative targeted absolute proteomics. Results: Brain distribution of [11C]tariquidar was found to be very low across the whole brain and not significantly different between tumor and tumor-free brain tissue. Only one patient showed a small area of enhanced [11C]tariquidar uptake within the brain tumor. ABCG2/ABCB1 ratios in surgically resected tumor tissue (1.4 ± 0.2) were comparable to previously reported ABCG2/ABCB1 ratios in isolated human micro-vessels (1.3), which suggested that no overexpression of ABCB1 or ABCG2 occurred in the investigated tumors. Conclusions: Our data suggest that the investigated brain tumors had an intact BBTB, which is impermeable to anticancer drugs, which are dual ABCB1/ABCG2 substrates. Therefore, effective drugs for antitumor treatment should have high passive permeability and lack ABCB1/ABCG2 substrate affinity. Trial registration: European Union Drug Regulating Authorities Clinical Trials Database (EUDRACT), 2011-004189-13. Registered on 23 February 2012, https://www.clinicaltrialsregister.eu/ctr-search/search?query=2011-004189-13. [ABSTRACT FROM AUTHOR]

Abstract: Introduction: The adenosine triphosphate-binding cassette (ABC) transporter P-glycoprotein (Pgp) protects the brain from accumulation of lipophilic compounds by active efflux transport across the blood–brain barrier. Changes in Pgp function/expression may occur in neurological disorders, such as epilepsy, Alzheimer’s or Parkinson’s disease. In this work we investigated the suitability of the radiolabeled Pgp inhibitors [11C]elacridar and [11C]tariquidar to visualize Pgp density in rat brain with PET. Methods: Rats underwent a first PET scan with [11C]elacridar (n =5) or [11C]tariquidar (n =6) followed by a second scan with the Pgp substrate (R)-[11C]verapamil after administration of unlabeled tariquidar at a dose which half-maximally inhibits cerebral Pgp (3mg/kg). Compartmental modeling using an arterial input function and Logan graphical analysis were used to estimate rate constants and volumes of distribution (V T) of radiotracers in different brain regions. Results: Brain PET signals of [11C]elacridar and [11C]tariquidar were very low (~0.5 standardized uptake value, SUV). There was a significant negative correlation between V T and K 1 (i.e. influx rate constant from plasma into brain) values of [11C]elacridar or [11C]tariquidar and V T and K 1 values of (R)-[11C]verapamil in different brain regions which was consistent with binding of [11C]inhibitors to Pgp and efflux of (R)-[11C]verapamil by Pgp. Conclusion: The small Pgp binding signals obtained with [11C]elacridar and [11C]tariquidar limit the applicability of these tracers to measure cerebral Pgp density. PET tracers with higher (i.e. subnanomolar) binding affinities will be needed to visualize the low density of Pgp in brain. [Copyright &y& Elsevier]

Abstract: The aim of this study was to develop a positron emission tomography (PET) tracer based on the dual P-glycoprotein (P-gp) breast cancer resistance protein (BCRP) inhibitor tariquidar (1) to study the interaction of 1 with P-gp and BCRP in the blood–brain barrier (BBB) in vivo. O-Desmethyl-1 was synthesized and reacted with [11C]methyl triflate to afford [11C]-1. Small-animal PET imaging of [11C]-1 was performed in naïve rats, before and after administration of unlabeled 1 (15mg/kg, n =3) or the dual P-gp/BCRP inhibitor elacridar (5mg/kg, n =2), as well as in wild-type, Mdr1a/b (−/−), Bcrp1 (−/−) and Mdr1a/b (−/−) Bcrp1 (−/−) mice (n =3). In vitro autoradiography was performed with [11C]-1 using brain sections of all four mouse types, with and without co-incubation with unlabeled 1 or elacridar (1μM). In PET experiments in rats, administration of unlabeled 1 or elacridar increased brain activity uptake by a factor of 3–4, whereas blood activity levels remained unchanged. In Mdr1a/b (−/−), Bcrp1 (−/−) and Mdr1a/b (−/−) Bcrp1 (−/−) mice, brain-to-blood ratios of activity at 25min after tracer injection were 3.4, 1.8 and 14.5 times higher, respectively, as compared to wild-type animals. Autoradiography showed approximately 50% less [11C]-1 binding in transporter knockout mice compared to wild-type mice and significant displacement by unlabeled elacridar in wild-type and Mdr1a/b (−/−) mouse brains. Our data suggest that [11C]-1 interacts specifically with P-gp and BCRP in the BBB. However, further investigations are needed to assess if [11C]-1 behaves in vivo as a transported or a non-transported inhibitor. [ABSTRACT FROM AUTHOR]

P-Glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2) in the canalicular membrane of hepatocytes mediate the biliary excretion of drugs and drug metabolites. To measure hepatic ABCB1 and ABCG2 activity, we performed positron emission tomography (PET) scans with the ABCB1/ABCG2 substrate [ 11 C]tariquidar in healthy volunteers and wild-type, Abcb1a/b (-/-) , Abcg2 (-/-) , and Abcb1a/b (-/-) Abcg2 (-/-) mice without and with coadministration of unlabeled tariquidar. PET data were analyzed with a three-compartment pharmacokinetic model. [ 11 C]Tariquidar underwent hepatobiliary excretion in both humans and mice, and tariquidar coadministration caused a significant reduction in the rate constant for the transfer of radioactivity from the liver into bile (by -74% in humans and by -62% in wild-type mice), suggesting inhibition of canalicular efflux transporter activity. Radio-thin-layer chromatography analysis revealed that the majority of radioactivity (>87%) in the mouse liver and bile was composed of unmetabolized [ 11 C]tariquidar. PET data in transporter knockout mice revealed that both ABCB1 and ABCG2 mediated biliary excretion of [ 11 C]tariquidar. In vitro experiments indicated that tariquidar is not a substrate of major hepatic basolateral uptake transporters (SLCO1B1, SLCO1B3, SLCO2B1, SLC22A1, and SLC22A3). Our data suggest that [ 11 C]tariquidar can be used to measure hepatic canalicular ABCB1/ABCG2 transport activity without a confounding effect of uptake transporters.