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To assess the hepatic disposition of erlotinib, we performed positron emission tomography (PET) scans with [11C]erlotinib in healthy volunteers without and with oral pretreatment with a therapeutic erlotinib dose (300 mg). Erlotinib pretreatment significantly decreased the liver exposure to [11C]erlotinib with a concomitant increase in blood exposure, pointing to the involvement of a carrier‐mediated hepatic uptake mechanism. Using cell lines overexpressing human organic anion‐transporting polypeptides (OATPs) 1B1, 1B3, or 2B1, we show that [11C]erlotinib is selectively transported by OATP2B1. Our data suggest that at PET microdoses hepatic uptake of [11C]erlotinib is mediated by OATP2B1, whereas at therapeutic doses OATP2B1 transport is saturated and hepatic uptake occurs mainly by passive diffusion. We propose that [11C]erlotinib may be used as a hepatic OATP2B1 probe substrate and erlotinib as an OATP2B1 inhibitor in clinical drug–drug interaction studies, allowing the contribution of OATP2B1 to the hepatic uptake of drugs to be revealed.

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 (R)-[11C]verapamil and [11C]-N-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 (R)-[11C]verapamil or [11C]-N-desmethyl-loperamide at the BBB and thereby contribute to previously observed species differences between rodents and humans. We performed in vitro transport experiments with [3H]verapamil and [3H]-N-desmethyl-loperamide in ABCB1 and Abcb1a overexpressing cell lines. Moreover we conducted in vivo PET experiments and biodistribution studies with (R)-[11C]verapamil and [11C]-N-desmethyl-loperamide in wild-type mice without and with tariquidar pretreatment and in homozygous Abcb1a/1b(−/−) and heterozygous Abcb1a/1b(+/−) mice. We found no differences for in vitro transport of [3H]verapamil and [3H]-N-desmethyl-loperamide by ABCB1 and Abcb1a and its inhibition by tariquidar. [3H]-N-Desmethyl-loperamide was transported with a 5 to 9 times higher transport ratio than [3H]verapamil in ABCB1- and Abcb1a-transfected cells. In vivo, brain radioactivity concentrations were lower for [11C]-N-desmethyl-loperamide than for (R)-[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 (R)-[11C]verapamil and 1329 nM (95% CI: 980–1801) for [11C]-N-desmethyl-loperamide. In homozygous Abcb1a/1b(−/−) mice brain radioactivity distribution was increased by 3.9- and 2.8-fold and in heterozygous Abcb1a/1b(+/−) mice by 1.5- and 1.1-fold, for (R)-[11C]verapamil and [11C]-N-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 (R)-[11C]verapamil and [11C]-N-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]-N-desmethyl-loperamide but not (R)-[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]-N-desmethyl-loperamide but not (R)-[11C]verapamil may depend on cerebral blood flow. In conclusion, we have identified a number of important factors, i.e., substrate affinity to ABCB1...

The 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 11 C-erlotinib in nonhuman primates as a model of the human BBB. Methods: Papio anubis baboons underwent PET scans of the brain after intravenous injection of 11 C-erlotinib under baseline conditions (n 5 4) and during intravenous infusion of high-dose erlotinib (10 mg/kg/h, n 5 4) or elacridar (12 mg/kg/h, n 5 3). Results: Under baseline conditions, 11 C-erlotinib distribution to the brain (total volume of distribution [V T ], 0.22 6 0.015 mL/cm 3 ) was markedly lower than its distribution to muscle tissue surrounding the skull (V T , 0.86 6 0.10 mL/cm 3 ). Elacridar infusion resulted in a 3.5 6 0.9-fold increase in 11 C-erlotinib distribution to the brain (V T , 0.81 6 0.21 mL/cm 3 , P , 0.01), reaching levels comparable to those in muscle tissue, without changing 11 Cerlotinib plasma pharmacokinetics. During high-dose erlotinib infusion, 11 C-erlotinib brain distribution was also significantly (1.7 6 0.2-fold) increased (V T , 0.38 6 0.033 mL/cm 3 , P , 0.05), with a concomitant increase in 11 C-erlotinib plasma exposure. Conclusion: We successfully implemented ABCB1/ABCG2 inhibition protocols in nonhuman primates resulting in pronounced increases in brain distribution of 11 C-erlotinib. For patients with brain tumors, such inhibition protocols may ultimately be applied to create more effective treatments using drugs that undergo efflux transport at the BBB.

Purpose: Multidrug resistance-associated proteins (MRPs) mediate the hepatobiliary and renal excretion of many drugs and drug conjugates. The positron emission tomography (PET) tracer 6-bromo-7-[11 C]methylpurine is rapidly converted in tissues by glutathione-S-transferases into its glutathione conjugate, and has been used to measure the activity of Abcc1 in the brain and the lungs of mice. Aim of this work was to investigate if the activity of MRPs in excretory organs can be measured with 6-bromo-7-[ 11 C]methylpurine.Procedures: We performed PET scans with 6-bromo-7-[ 11 C]methylpurine in groups of wild-type, Abcc4(−/−) and Abcc1 (−/−) mice, with and without pre-treatment with the prototypical MRP inhibitor MK571.Results: 6-Bromo-7-[ 11 C]methylpurine-derived radioactivity predominantly underwent renal excretion. In blood, MK571 treatment led to a significant increase in the AUC and a decrease in the elimination rate constant of radioactivity (k elimination,blood ). In the kidneys, there were significant decreases in the rate constant for radioactivity uptake from the blood (k uptake,kidney ), k elimination,kidney , and the rate constant for tubular secretion of radioactivity (k urine ). Experiments in Abcc4 (−/−) mice indicated that Abcc4 contributed to renal excretion of 6-bromo-7-[ 11 C]methylpurine-derived radioactivity.Conclusions: Our data suggest that 6-bromo-7-[ 11 C]methylpurine may be useful to assess the activity of MRPs in the kidneys as well as in other organs (brain, lungs), although further work is needed to identify the MRP subtypes involved in the disposition of 6-bromo-7-[ 11 C]methylpurinederived radioactivity.Electronic supplementary material The online version of this article (https://