Measurement of Hepatic ABCB1 and ABCG2 Transport Activity with [11C]Tariquidar and PET in Humans and Mice

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 [11C]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. [11C]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 [11C]tariquidar. PET data in transporter knockout mice revealed that both ABCB1 and ABCG2 mediated biliary excretion of [11C]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 [11C]tariquidar can be used to measure hepatic canalicular ABCB1/ABCG2 transport activity without a confounding effect of uptake transporters

Assessing the Functional Redundancy between P-gp and BCRP in Controlling the Brain Distribution and Biliary Excretion of Dual Substrates with PET Imaging in Mice

P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) are co-localized at the blood–brain barrier, where they display functional redundancy to restrict the brain distribution of dual P-gp/BCRP substrate drugs. We used positron emission tomography (PET) with the metabolically stable P-gp/BCRP substrates [11C]tariquidar, [11C]erlotinib, and [11C]elacridar to assess whether a similar functional redundancy as at the BBB exists in the liver, where both transporters mediate the biliary excretion of drugs. Wild-type, Abcb1a/b(−/−), Abcg2(−/−), and Abcb1a/b(−/−)Abcg2(−/−) mice underwent dynamic whole-body PET scans after i.v. injection of either [11C]tariquidar, [11C]erlotinib, or [11C]elacridar. Brain uptake of all three radiotracers was markedly higher in Abcb1a/b(−/−)Abcg2(−/−) mice than in wild-type mice, while only moderately changed in Abcb1a/b(−/−) and Abcg2(−/−) mice. The transfer of radioactivity from liver to excreted bile was significantly lower in Abcb1a/b(−/−)Abcg2(−/−) mice and almost unchanged in Abcb1a/b(−/−) and Abcg2(−/−) mice (with the exception of [11C]erlotinib, for which biliary excretion was also significantly reduced in Abcg2(−/−) mice). Our data provide evidence for redundancy between P-gp and BCRP in controlling both the brain distribution and biliary excretion of dual P-gp/BCRP substrates and highlight the utility of PET as an upcoming tool to assess the effect of transporters on drug disposition at a whole-body level

Strategies to Inhibit ABCB1- and ABCG2-Mediated Efflux Transport of Erlotinib at the Blood–Brain Barrier: A PET Study on Nonhuman Primates

International audienceThe tyrosine kinase inhibitor erlotinib poorly penetrates the blood-brain barrier (BBB) because of efflux transport by P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2), thereby limiting its utility in the treatment of non-small cell lung cancer metastases in the brain. Pharmacologic strategies to inhibit ABCB1/ABCG2-mediated efflux transport at the BBB have been successfully developed in rodents, but it remains unclear whether these can be translated to humans given the pronounced species differences in ABCG2/ABCB1 expression ratios at the BBB. We assessed the efficacy of two different ABCB1/ABCG2 inhibitors to enhance brain distribution of 11C-erlotinib in nonhuman primates as a model of the human BBB

Inhibition of ABCB1 and ABCG2 at the mouse blood-brain barrier with marketed drugs to improve brain delivery of the model ABCB1/ABCG2 substrate [11C]erlotinib

P-Glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2) are two efflux transporters at the blood–brain barrier (BBB), which effectively restrict brain distribution of diverse drugs, such as tyrosine kinase inhibitors. There is a crucial need for pharmacological ABCB1 and ABCG2 inhibition protocols for a more effective treatment of brain diseases. In the present study, seven marketed drugs (osimertinib, erlotinib, nilotinib, imatinib, lapatinib, pazopanib, and cyclosporine A) and one nonmarketed drug (tariquidar), with known in vitro ABCB1/ABCG2 inhibitory properties, were screened for their inhibitory potency at the BBB in vivo. Positron emission tomography (PET) using the model ABCB1/ABCG2 substrate [11C]erlotinib was performed in mice. Tested inhibitors were administered as i.v. bolus injections at 30 min before the start of the PET scan, followed by a continuous i.v. infusion for the duration of the PET scan. Five of the tested drugs increased total distribution volume of [11C]erlotinib in the brain (VT,brain) compared to vehicle-treated animals (tariquidar, + 69%; erlotinib, + 19% and +23% for the 21.5 mg/kg and the 43 mg/kg dose, respectively; imatinib, + 22%; lapatinib, + 25%; and cyclosporine A, + 49%). For all drugs, increases in [11C]erlotinib brain distribution were lower than in Abcb1a/b(−/−)Abcg2(−/−) mice (+149%), which suggested that only partial ABCB1/ABCG2 inhibition was reached at the mouse BBB. The plasma concentrations of the tested drugs at the time of the PET scan were higher than clinically achievable plasma concentrations. Some of the tested drugs led to significant increases in blood radioactivity concentrations measured at the end of the PET scan (erlotinib, + 103% and +113% for the 21.5 mg/kg and the 43 mg/kg dose, respectively; imatinib, + 125%; and cyclosporine A, + 101%), which was most likely caused by decreased hepatobiliary excretion of radioactivity. Taken together, our data suggest that some marketed tyrosine kinase inhibitors may be repurposed to inhibit ABCB1 and ABCG2 at the BBB. From a clinical perspective, moderate increases in brain delivery despite the administration of high i.v. doses as well as peripheral drug–drug interactions due to transporter inhibition in clearance organs question the translatability of this concept

Influence of OATPs on hepatic disposition of erlotinib measured with positron emission tomography

To assess the hepatic disposition of erlotinib, we performed positron emission tomography (PET) scans with [C]erlotinib in healthy volunteers without and with oral pretreatment with a therapeutic erlotinib dose (300mg). Erlotinib pretreatment significantly decreased the liver exposure to [C]erlotinib with a concomitant increase in blood exposure, pointing to the involvement of a carriermediated hepatic uptake mechanism. Using cell lines overexpressing human organic aniontransporting polypeptides (OATPs) 1B1, 1B3, or 2B1, we show that [C]erlotinib is selectively transported by OATP2B1. Our data suggest that at PET microdoses hepatic uptake of [C]erlotinib is mediated by OATP2B1, whereas at therapeutic doses OATP2B1 transport is saturated and hepatic uptake occurs mainly by passive diffusion. We propose that [C]erlotinib may be used as a hepatic OATP2B1 probe substrate and erlotinib as an OATP2B1 inhibitor in clinical drugdrug interaction studies, allowing the contribution of OATP2B1 to the hepatic uptake of drugs to be revealed.(VLID)481531

Factors Governing P‑Glycoprotein-Mediated Drug–Drug Interactions at the Blood–Brain Barrier Measured with Positron Emission Tomography

The adenosine triphosphate-binding cassette transporter P-glycoprotein (ABCB1/Abcb1a) restricts at the blood–brain barrier (BBB) brain distribution of many drugs. ABCB1 may be involved in drug–drug interactions (DDIs) at the BBB, which may lead to changes in brain distribution and central nervous system side effects of drugs. Positron emission tomography (PET) with the ABCB1 substrates (<i>R</i>)-[¹¹C]­verapamil and [¹¹C]-<i>N</i>-desmethyl-loperamide and the ABCB1 inhibitor tariquidar has allowed direct comparison of ABCB1-mediated DDIs at the rodent and human BBB. In this work we evaluated different factors which could influence the magnitude of the interaction between tariquidar and (<i>R</i>)-[¹¹C]­verapamil or [¹¹C]-<i>N</i>-desmethyl-loperamide at the BBB and thereby contribute to previously observed species differences between rodents and humans. We performed <i>in vitro</i> transport experiments with [³H]­verapamil and [³H]-<i>N</i>-desmethyl-loperamide in ABCB1 and Abcb1a overexpressing cell lines. Moreover we conducted <i>in vivo</i> PET experiments and biodistribution studies with (<i>R</i>)-[¹¹C]­verapamil and [¹¹C]-<i>N</i>-desmethyl-loperamide in wild-type mice without and with tariquidar pretreatment and in homozygous <i>Abcb1a/1b^(−/−)</i> and heterozygous <i>Abcb1a/1b^(+/−)</i> mice. We found no differences for <i>in vitro</i> transport of [³H]­verapamil and [³H]-<i>N</i>-desmethyl-loperamide by ABCB1 and Abcb1a and its inhibition by tariquidar. [³H]-<i>N</i>-Desmethyl-loperamide was transported with a 5 to 9 times higher transport ratio than [³H]­verapamil in ABCB1- and Abcb1a-transfected cells. <i>In vivo</i>, brain radioactivity concentrations were lower for [¹¹C]-<i>N</i>-desmethyl-loperamide than for (<i>R</i>)-[¹¹C]­verapamil. Both radiotracers showed tariquidar dose dependent increases in brain distribution with tariquidar half-maximum inhibitory concentrations (IC₅₀) of 1052 nM (95% confidence interval CI: 930–1189) for (<i>R</i>)-[¹¹C]­verapamil and 1329 nM (95% CI: 980–1801) for [¹¹C]-<i>N</i>-desmethyl-loperamide. In homozygous <i>Abcb1a/1b^(−/−)</i> mice brain radioactivity distribution was increased by 3.9- and 2.8-fold and in heterozygous <i>Abcb1a/1b^(+/−)</i> mice by 1.5- and 1.1-fold, for (<i>R</i>)-[¹¹C]­verapamil and [¹¹C]-<i>N</i>-desmethyl-loperamide, respectively, as compared with wild-type mice. For both radiotracers radiolabeled metabolites were detected in plasma and brain. When brain and plasma radioactivity concentrations were corrected for radiolabeled metabolites, brain distribution of (<i>R</i>)-[¹¹C]­verapamil and [¹¹C]-<i>N</i>-desmethyl-loperamide was increased in tariquidar (15 mg/kg) treated animals by 14.1- and 18.3-fold, respectively, as compared with vehicle group. Isoflurane anesthesia altered [¹¹C]-<i>N</i>-desmethyl-loperamide but not (<i>R</i>)-[¹¹C]­verapamil metabolism, and this had a direct effect on the magnitude of the increase in brain distribution following ABCB1 inhibition. Our data furthermore suggest that in the absence of ABCB1 function brain distribution of [¹¹C]-<i>N</i>-desmethyl-loperamide but not (<i>R</i>)-[¹¹C]­verapamil may depend on cerebral blood flow. In conclusion, we have identified a number of important factors, i.e., substrate affinity to ABCB1, brain uptake of radiolabeled metabolites, anesthesia, and cerebral blood flow, which can directly influence the magnitude of ABCB1-mediated DDIs at the BBB and should therefore be taken into consideration when interpreting PET results