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P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2) restrict at the blood-brain barrier (BBB) the brain distribution of the majority of currently known molecularly targeted anticancer drugs. To improve brain delivery of dual ABCB1/ABCG2 substrates, both ABCB1 and ABCG2 need to be inhibited simultaneously at the BBB. We examined the feasibility of simultaneous ABCB1/ABCG2 inhibition with i.v. co-infusion of erlotinib and tariquidar by studying brain distribution of the model ABCB1/ABCG2 substrate [ 11 C]erlotinib in mice and rhesus macaques with PET. Tolerability of the erlotinib/tariquidar combination was assessed in human embryonic stem cell-derived cerebral organoids. In mice and macaques, baseline brain distribution of [ 11 C]erlotinib was low (brain distribution volume, V T,brain  < 0.3 mL/cm 3 ). Co-infusion of erlotinib and tariquidar increased V T,brain in mice by 3.0-fold and in macaques by 3.4- to 5.0-fold, while infusion of erlotinib alone or tariquidar alone led to less pronounced V T,brain increases in both species. Treatment of cerebral organoids with erlotinib/tariquidar led to an induction of Caspase-3-dependent apoptosis. Co-infusion of erlotinib/tariquidar may potentially allow for complete ABCB1/ABCG2 inhibition at the BBB, while simultaneously achieving brain-targeted EGFR inhibition. Our protocol may be applicable to enhance brain delivery of molecularly targeted anticancer drugs for a more effective treatment of brain tumors.

Introduction: Tau deposition is one of the hallmarks of Alzheimer's disease (AD) and can be visualized and quantified using [ 18 F]THK-5317 together with kinetic modeling. To determine the feasibility of this approach, we measured blood/plasma pharmacokinetics and radiotracer metabolism in female and male rats., Methods: Female and male rats (n = 11-12) were cannulated via the femoral artery for continuous blood sampling. Blood sampling was performed at regular intervals after intravenous injection of [ 18 F]THK-5317. After collection of the last blood sample, animals were sacrificed, and organs were excised. Blood from minute 5, 20 and 60 was centrifuged to obtain plasma. Radiolabeled metabolites in plasma, brain, liver and urine were analyzed by radio-thin-layer chromatography (radio-TLC)., Results: Plasma pharmacokinetics and metabolism were significantly different between female and male rats. [ 18 F]THK-5317 plasma clearance was faster in female (0.66 ± 0.08 mL/h/kg BW) than in male (0.52 ± 0.11 mL/h/kg BW) rats (p = .005). The percentage of unmetabolized parent was significantly different between both sexes at 20 min and 60 min p.i. In the liver, a 1.6-fold higher radioactivity concentration was found in male versus female animals and in addition also the percentage of unmetabolized parent was different., Conclusion: Our results show pronounced sex differences in blood/plasma pharmacokinetics and metabolism of [ 18 F]THK-5317 in rats. Female animals showed a faster plasma clearance compared to males. These results underline the importance of investigating both sexes and also support the notion that individual input functions or sex-specific population-based input functions are needed for kinetic modeling analyses., Advances in Knowledge: First preclinical study in rats showing pronounced sex differences in blood/plasma pharmacokinetics and metabolism of [ 18 F]THK-5317., Implications for Patient Care: Sex-specific differences might also be present in humans and thus clinical trials should have adequate sample size to account for effects in men and women separately., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

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.

Brain penetration of radiopharmaceuticals or therapeutic drugs may be restricted by adenosine triphosphate-binding cassette (ABC) transporters, such as P-glycoprotein (Pgp), breast cancer resistance protein (BCRP), or the multidrug resistance-associated proteins. These transporters are expressed in the luminal membrane of brain capillary endothelial cells forming the blood-brain barrier (BBB), where they actively efflux a wide range of chemically unrelated compounds from the brain back into the blood. Most efforts to visualize ABC transporters at the BBB with positron emission tomography have concentrated on Pgp. Pgp imaging probes can be classified as radiolabeled substrates or inhibitors. The radiolabeled substrates (R)-[(11) C]verapamil and [(11) C]-N-desmethyl-loperamide have been successfully used to assess Pgp function at the BBB of animals and humans. Radiolabeled Pgp inhibitors, such as [(11) C]tariquidar, [(11) C]elacridar, or [(11) C]laniquidar, were developed to measure Pgp expression levels at the BBB, which has so far remained unsuccessful as these probes were unexpectedly recognized at tracer concentrations by Pgp and BCRP as substrates resulting in low brain uptake. Studies on positron emission tomography tracers for other ABC transporters than Pgp (BCRP and multidrug resistance-associated proteins) are still in their infancy. It is hoped that the experience gained with the imaging of Pgp will be successfully translated to the development of radiotracers to visualize other ABC transporters., (Copyright © 2013 John Wiley & Sons, Ltd.)

Purpose: One important mechanism for chemoresistance of tumours is overexpression of the adenosine triphosphate-binding cassette transporter P-glycoprotein (Pgp). Pgp reduces intracellular concentrations of chemotherapeutic drugs. The aim of this study was to compare the suitability of the radiolabelled Pgp inhibitors [(11)C]tariquidar and [(11)C]elacridar with the Pgp substrate radiotracer (R)-[(11)C]verapamil for discriminating tumours expressing low and high levels of Pgp using small-animal PET imaging in a murine breast cancer model., Methods: Murine mammary carcinoma cells (EMT6) were continuously exposed to doxorubicin to generate a Pgp-overexpressing, doxorubicin-resistant cell line (EMT6AR1.0 cells). Both cell lines were subcutaneously injected into female athymic nude mice. One week after implantation, animals underwent PET scans with [(11)C]tariquidar (n = 7), [(11)C]elacridar (n = 6) and (R)-[(11)C]verapamil (n = 7), before and after administration of unlabelled tariquidar (15 mg/kg). Pgp expression in tumour grafts was evaluated by Western blotting., Results: [(11)C]Tariquidar showed significantly higher retention in Pgp-overexpressing EMT6AR1.0 compared with EMT6 tumours: the mean ± SD areas under the time-activity curves in scan 1 from time 0 to 60 min (AUC(0-60)) were 38.8 ± 2.2 min and 25.0 ± 5.3 min (p = 0.016, Wilcoxon matched pairs test). [(11)C]Elacridar and (R)-[(11)C]verapamil were not able to discriminate Pgp expression in tumour models. Following administration of unlabelled tariquidar, both EMT6Ar1.0 and EMT6 tumours showed increases in uptake of [(11)C]tariquidar, [(11)C]elacridar and (R)-[(11)C]verapamil., Conclusion: Among the tested radiotracers, [(11)C]tariquidar performed best in discriminating tumours expressing high and low levels of Pgp. Therefore [(11)C]tariquidar merits further investigation as a PET tracer to assess Pgp expression levels in solid tumours.

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 [(11)C]methyl triflate to afford [(11)C]-1. Small-animal PET imaging of [(11)C]-1 was performed in naïve rats, before and after administration of unlabeled 1 (15 mg/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 [(11)C]-1 using brain sections of all four mouse types, with and without co-incubation with unlabeled 1 or elacridar (1 microM). 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 25 min 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 [(11)C]-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 [(11)C]-1 interacts specifically with P-gp and BCRP in the BBB. However, further investigations are needed to assess if [(11)C]-1 behaves in vivo as a transported or a non-transported inhibitor., (Copyright (c) 2010 Elsevier Ltd. All rights reserved.)

Introduction: The multidrug efflux transporter breast cancer resistance protein (BCRP) is highly expressed in the blood-brain barrier (BBB), where it limits brain entry of a broad range of endogenous and exogenous substrates. Methyl is a recently discovered BCRP-selective inhibitor, which is structurally derived from the potent P-glycoprotein (P-gp) inhibitor tariquidar. The aim of this study was to develop a new PET tracer based on 1 to map BCRP expression levels in vivo., Methods: Compound 1 was labelled with (11)C in its methyl ester function by reaction of the corresponding carboxylic acid 2 with [(11)C]methyl triflate. Positron emission tomography (PET) imaging of [(11)C]-1 was performed in wild-type, Mdr1a/b((-/-)), Bcrp1((-/-)) and Mdr1a/b((-/-))Bcrp1((-/-)) mice (n=3 per mouse type) and radiotracer metabolism was assessed in plasma and brain., Results: Brain-to-plasma ratios of unchanged [(11)C]-1 were 4.8- and 10.3-fold higher in Mdr1a/b((-/-)) and in Mdr1a/b((-/-))Bcrp1((-/-)) mice, respectively, as compared to wild-type animals, but only modestly increased in Bcrp1((-/-)) mice. [(11)C]-1 was rapidly metabolized in vivo giving rise to a polar radiometabolite which was taken up into brain tissue., Conclusion: Our data suggest that [(11)C]-1 preferably interacts with P-gp rather than BCRP at the murine BBB which questions its reported in vitro BCRP selectivity. Consequently, [(11)C]-1 appears to be unsuitable as a PET tracer to map cerebral BCRP expression., (Copyright 2010 Elsevier Inc. All rights reserved.)