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.

Influence of ABC transporters on the excretion of ciprofloxacin assessed with PET imaging in mice.

Ciprofloxacin is a commonly prescribed fluoroquinolone antibiotic which is cleared by active tubular secretion and intestinal excretion. Ciprofloxacin is a known substrate of the ATP-binding cassette (ABC) transporters breast cancer resistance protein (BCRP) and multidrug resistance-associated protein 4 (MRP4). In this work, we used positron emission tomography (PET) imaging to investigate the influence of BCRP, MRP4, MRP2 and P-glycoprotein (P-gp) on the excretion of [18F]ciprofloxacin in mice. Dynamic 90-min PET scans were performed after intravenous injection of [18F]ciprofloxacin in wild-type mice without and with pre-treatment with the broad-spectrum MRP inhibitor MK571. Moreover, [18F]ciprofloxacin PET scans were performed in Abcc4(-/-), Abcc2(-/-), Abcc4(-/-)Abcg2(-/-) and Abcb1a/b(-/-)Abcg2(-/-) mice. In addition to non-compartmental pharmacokinetic (PK) analysis, a novel three-compartment PK model was developed for a detailed assessment of the renal disposition of [18F]ciprofloxacin. In MK571 pre-treated mice, a significant increase in the blood exposure to [18F]ciprofloxacin was observed along with a significant reduction in the renal and intestinal clearances. PK modelling revealed a significant reduction in renal radioactivity uptake (CL1) and in the rate constants for transfer of radioactivity from the corticomedullary renal region into blood (k2) and urine (k3), respectively, after MK571 administration. No changes in the renal clearance or in the estimated kidney PK model parameters were observed in any of the studied knockout models, while a significant reduction in the intestinal clearance was observed in Abcc2(-/-) and Abcc4(-/-)Abcg2(-/-) mice. Our data failed to reveal a role of any of the studied ABC transporters in the tubular secretion of ciprofloxacin. This may indicate that ciprofloxacin is handled in the kidneys by more than one transporter family, most likely with a great degree of mutual functional redundancy. Our study highlights the potential of PET imaging for an assessment of transporter-mediated renal excretion of radiolabelled drugs.

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.

Reproducibility and Comparability of Preclinical PET Imaging Data: A Multicenter Small-Animal PET Study.

The standardization of preclinical imaging is a key factor to ensure the reliability, reproducibility, validity, and translatability of preclinical data. Preclinical standardization has been slowly progressing in recent years and has mainly been performed within a single institution, whereas little has been done in regards to multicenter standardization between facilities. This study aimed to investigate the comparability among preclinical imaging facilities in terms of PET data acquisition and analysis. In the first step, basic PET scans were obtained in 4 different preclinical imaging facilities to compare their standard imaging protocol for 18F-FDG. In the second step, the influence of the personnel performing the experiments and the experimental equipment used in the experiment were compared. In the third step, the influence of the image analysis on the reproducibility and comparability of the acquired data was determined. Distinct differences in the uptake behavior of the 4 standard imaging protocols were determined for the investigated organs (brain, left ventricle, liver, and muscle) due to different animal handling procedures before and during the scans (e.g., fasting vs. nonfasting, glucose levels, temperature regulation vs. constant temperature warming). Significant differences in the uptake behavior in the brain were detected when the same imaging protocol was used but executed by different personnel and using different experimental animal handling equipment. An influence of the person analyzing the data was detected for most of the organs, when the volumes of interest were manually drawn by the investigators. Coregistration of the PET to an MR image and drawing the volume of interest based on anatomic information yielded reproducible results among investigators. It has been demonstrated that there is a huge demand for standardization among multiple institutions.

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 Multidrug Resistance-Associated Proteins on the Excretion of the ABCC1 Imaging Probe 6-Bromo-7-[11C]Methylpurine in Mice.

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-[11C]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-[11C]methylpurine. PROCEDURES: We performed PET scans with 6-bromo-7-[11C]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-[11C]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 (kelimination,blood). In the kidneys, there were significant decreases in the rate constant for radioactivity uptake from the blood (kuptake,kidney), kelimination,kidney, and the rate constant for tubular secretion of radioactivity (kurine). Experiments in Abcc4(-/-) mice indicated that Abcc4 contributed to renal excretion of 6-bromo-7-[11C]methylpurine-derived radioactivity. CONCLUSIONS: Our data suggest that 6-bromo-7-[11C]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-[11C]methylpurine-derived radioactivity.

Effect of Rifampicin on the Distribution of [11C]Erlotinib to the Liver, a Translational PET Study in Humans and in Mice.

Organic anion-transporting polypeptides (OATPs) mediate the uptake of various drugs from blood into the liver in the basolateral membrane of hepatocytes. Positron emission tomography (PET) is a potentially powerful tool to assess the activity of hepatic OATPs in vivo, but its utility critically depends on the availability of transporter-selective probe substrates. We have shown before that among the three OATPs expressed in hepatocytes (OATP1B1, OATP1B3, and OATP2B1), [11C]erlotinib is selectively transported by OATP2B1. In contrast to OATP1B1 and OATP1B3, OATP2B1 has not been thoroughly explored yet, and no specific probe substrates are currently available. To assess if the prototypical OATP inhibitor rifampicin can inhibit liver uptake of [11C]erlotinib in vivo, we performed [11C]erlotinib PET scans in six healthy volunteers without and with intravenous pretreatment with rifampicin (600 mg). In addition, FVB mice underwent [11C]erlotinib PET scans without and with concurrent intravenous infusion of high-dose rifampicin (100 mg/kg). Rifampicin caused a moderate reduction in the liver distribution of [11C]erlotinib in humans, while a more pronounced effect of rifampicin was observed in mice, in which rifampicin plasma concentrations were higher than in humans. In vitro uptake experiments in an OATP2B1-overexpressing cell line indicated that rifampicin inhibited OATP2B1 transport of [11C]erlotinib in a concentration-dependent manner with a half-maximum inhibitory concentration of 72.0 ± 1.4 µM. Our results suggest that rifampicin-inhibitable uptake transporter(s) contributed to the liver distribution of [11C]erlotinib in humans and mice and that [11C]erlotinib PET in combination with rifampicin may be used to measure the activity of this/these uptake transporter(s) in vivo. Furthermore, our data suggest that a standard clinical dose of rifampicin may exert in vivo a moderate inhibitory effect on hepatic OATP2B1.

Influence of breast cancer resistance protein and P-glycoprotein on tissue distribution and excretion of Ko143 assessed with PET imaging in mice.

Ko143 is a reference inhibitor of the adenosine triphosphate-binding cassette (ABC) transporter breast cancer resistance protein (humans: ABCG2, rodents: Abcg2) for in vitro and in vivo use. Previous in vitro data indicate that Ko143 binds specifically to ABCG2/Abcg2, suggesting a potential utility of Ko143 as a positron emission tomography (PET) tracer to assess the density (abundance) of ABCG2 in different tissues. In this work we radiolabeled Ko143 with carbon-11 (11C) and performed small-animal PET experiments with [11C]Ko143 in wild-type, Abcg2(-/-), Abcb1a/b(-/-) and Abcb1a/b(-/-)Abcg2(-/-) mice to assess the influence of Abcg2 and Abcb1a/b on tissue distribution and excretion of [11C]Ko143. [11C]Ko143 was extensively metabolized in vivo and unidentified radiolabeled metabolites were found in all investigated tissues. We detected no significant differences between wild-type and Abcg2(-/-) mice in the distribution of [11C]Ko143-derived radioactivity to Abcg2-expressing organs (brain, liver and kidney). [11C]Ko143 and possibly its radiolabeled metabolites were transported by Abcb1a and not by Abcg2 at the mouse blood-brain barrier. [11C]Ko143-derived radioactivity underwent both hepatobiliary and urinary excretion, with Abcg2 playing a possible role in mediating the transport of radiolabeled metabolites of [11C]Ko143 from the kidney into urine. Experiments in which a pharmacologic dose of unlabeled Ko143 (10 mg/kg) was co-administered with [11C]Ko143 revealed pronounced effects of the vehicle used for Ko143 formulation (containing polyethylene glycol 300 and polysorbate 80) on radioactivity distribution to the brain and the liver, as well as on hepatobiliary and urinary excretion of radioactivity. Our results highlight the challenges associated with the development of PET tracers for ABC transporters and emphasize that inhibitory effects of pharmaceutical excipients on membrane transporters need to be considered when performing in vivo drug-drug interaction studies. Finally, our study illustrates the power of small-animal PET to assess the interaction of drug molecules with membrane transporters on a whole body level.

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 [11 C]erlotinib in healthy volunteers without and with oral pretreatment with a therapeutic erlotinib dose (300 mg). Erlotinib pretreatment significantly decreased the liver exposure to [11 C]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 [11 C]erlotinib is selectively transported by OATP2B1. Our data suggest that at PET microdoses hepatic uptake of [11 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 [11 C]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.

Hepatocyte-Specific Deletion of EGFR in Mice Reduces Hepatic Abcg2 Transport Activity Measured by [11C]erlotinib and Positron Emission Tomography.

The epidermal growth factor receptor (EGFR) regulates cellular expression levels of breast cancer resistance protein (humans: ABCG2, rodents: Abcg2) via its downstream signaling pathways. Drugs that inhibit EGFR signaling (e.g., tyrosine kinase inhibitors, antibodies) may lead to ABCG2-mediated drug-drug interactions (DDIs) by changing the disposition of concomitantly administered ABCG2 substrate drugs. In this study, we used positron emission tomography and magnetic resonance imaging to compare disposition of the model Abcg2 substrate [11C]erlotinib in a mouse model of hepatocyte-specific deletion of EGFR (EGFR∆hep mice, n = 5) with EGFRfl/fl control mice (n = 6), which have normal EGFR expression levels in all tissues. Integration plot analysis was used to estimate the rate constants for transfer of radioactivity from the liver into bile (kbile) and from the kidney into urine (kurine). EGFR∆hep mice showed significantly lower radioactivity concentrations in the intestine (1.6-fold) and higher radioactivity concentrations in the urinary bladder (3.2-fold) compared with EGFRfl/fl mice. Kbile was significantly decreased (3.0-fold) in EGFR∆hep mice, whereas kurine was by 2.2-fold increased. Western blot analysis of liver tissue confirmed deletion of EGFR and showed significant decreases in Abcg2 and increases in P-glycoprotein (Abcb1a/b) expression levels in EGFR∆hep versus EGFRfl/fl mice. Our data show that EGFR deletion in hepatocytes leads to a reduction in Abcg2-mediated hepatobiliary clearance of a probe substrate accompanied by a shift to renal excretion of the drug, which raises the possibility that EGFR-inhibiting drugs may cause ABCG2-mediated DDIs.

[11C]Erlotinib PET cannot detect acquired erlotinib resistance in NSCLC tumor xenografts in mice.

INTRODUCTION: [11C]Erlotinib PET has shown promise to distinguish non-small cell lung cancer (NSCLC) tumors harboring the activating epidermal growth factor receptor (EGFR) mutation delE746-A750 from tumors with wild-type EGFR. To assess the suitability of [11C]erlotinib PET to detect the emergence of acquired erlotinib resistance in initially erlotinib-responsive tumors, we performed in vitro binding and PET experiments in mice bearing tumor xenografts using a range of different cancer cells, which were erlotinib-sensitive or exhibited clinically relevant resistance mechanisms to erlotinib. METHODS: The following cell lines were used for in vitro binding and PET experiments: the epidermoid carcinoma cell line A-431 (erlotinib-sensitive, wild-type EGFR) and the three NSCLC cell lines HCC827 (erlotinib-sensitive, delE746-A750), HCC827EPR (erlotinib-resistant, delE746-A750 and T790M) and HCC827ERLO (erlotinib-resistant, delE746-A750 and MET amplification). BALB/c nude mice with subcutaneous tumor xenografts underwent two consecutive [11C]erlotinib PET scans, a baseline scan and a second scan in which unlabeled erlotinib (10mg/kg) was co-injected. Logan graphical analysis was used to estimate total distribution volume (VT) of [11C]erlotinib in tumors. RESULTS: In vitro experiments revealed significantly higher uptake of [11C]erlotinib (5.2-fold) in the three NSCLC cell lines as compared to A-431 cells. In all four cell lines co-incubation with unlabeled erlotinib (1µM) led to significant reductions in [11C]erlotinib uptake (-19% to -66%). In both PET scans and for all four studied cell lines there were no significant differences in tumoral [11C]erlotinib VT values. For all three NSCLC cell lines, but not for the A-431 cell line, tumoral VT was significantly reduced following co-injection of unlabeled erlotinib (-20% to -35%). CONCLUSIONS: We found no significant differences in the in vitro and in vivo binding of [11C]erlotinib between erlotinib-sensitive and erlotinib-resistant NSCLC cells. Our findings suggest that [11C]erlotinib PET will not be suitable to distinguish erlotinib-sensitive NSCLC tumors from tumors with acquired resistance to erlotinib.

A Prediction Method for P-glycoprotein-Mediated Drug-Drug Interactions at the Human Blood-Brain Barrier From Blood Concentration-Time Profiles, Validated With PET Data.

The purpose of this study was to establish physiologically based pharmacokinetic models to predict in humans the brain concentration-time profiles and P-glycoprotein (Pgp)-mediated brain drug-drug interactions between the model Pgp substrate (R)-[11C]verapamil (VPM), the model dual Pgp/breast cancer resistance protein (BCRP) substrate [11C]tariquidar (TQD), and the Pgp inhibitor tariquidar. The model predictions were validated with results from positron emission tomography studies in humans. Using these physiologically based pharmacokinetic models, the differences between predicted and observed areas under the concentration-time curves (AUC) of VPM and TQD in the brain were within a 1.2-fold and 2.5-fold range, respectively. Also, brain AUC increases of VPM and TQD after Pgp inhibitor administration were predicted with 2.5-fold accuracy when in vitro inhibition constant or half-maximum inhibitory concentration values of tariquidar were used. The predicted rank order of the magnitude of AUC increases reflected the results of the clinical positron emission tomography studies. Our results suggest that the established models can predict brain exposure from the respective blood concentration-time profiles and rank the magnitude of the Pgp-mediated brain drug-drug interaction potential for both Pgp and Pgp/BCRP substrates in humans.

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

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 11C-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 11C-erlotinib under baseline conditions (n = 4) and during intravenous infusion of high-dose erlotinib (10 mg/kg/h, n = 4) or elacridar (12 mg/kg/h, n = 3). RESULTS: Under baseline conditions, 11C-erlotinib distribution to the brain (total volume of distribution [VT], 0.22 ± 0.015 mL/cm3) was markedly lower than its distribution to muscle tissue surrounding the skull (VT, 0.86 ± 0.10 mL/cm3). Elacridar infusion resulted in a 3.5 ± 0.9-fold increase in 11C-erlotinib distribution to the brain (VT, 0.81 ± 0.21 mL/cm3, P < 0.01), reaching levels comparable to those in muscle tissue, without changing 11C-erlotinib plasma pharmacokinetics. During high-dose erlotinib infusion, 11C-erlotinib brain distribution was also significantly (1.7 ± 0.2-fold) increased (VT, 0.38 ± 0.033 mL/cm3, P < 0.05), with a concomitant increase in 11C-erlotinib plasma exposure. CONCLUSION: We successfully implemented ABCB1/ABCG2 inhibition protocols in nonhuman primates resulting in pronounced increases in brain distribution of 11C-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.

Synthesis and preclinical characterization of 1-(6'-deoxy-6'-[18F]fluoro-ß-d-allofuranosyl)-2-nitroimidazole (ß-6'-[18F]FAZAL) as a positron emission tomography radiotracer to assess tumor hypoxia.

Positron emission tomography (PET) using fluorine-18 (18F)-labeled 2-nitroimidazole radiotracers has proven useful for assessment of tumor oxygenation. However, the passive diffusion-driven cellular uptake of currently available radiotracers results in slow kinetics and low tumor-to-background ratios. With the aim to develop a compound that is actively transported into cells, 1-(6'-deoxy-6'-[18F]fluoro-ß-d-allofuranosyl)-2-nitroimidazole (ß-[18F]1), a putative nucleoside transporter substrate, was synthetized by nucleophilic [18F]fluoride substitution of an acetyl protected labeling precursor with a tosylate leaving group (ß-6) in a final radiochemical yield of 12±8% (n=10, based on [18F]fluoride starting activity) in a total synthesis time of 60min with a specific activity at end of synthesis of 218±58GBq/µmol (n=10). Both radiolabeling precursor ß-6 and unlabeled reference compound ß-1 were prepared in multistep syntheses starting from 1,2:5,6-di-O-isopropylidene-α-d-allofuranose. In vitro experiments demonstrated an interaction of ß-1 with SLC29A1 and SLC28A1/2/3 nucleoside transporter as well as hypoxia specific retention of ß-[18F]1 in tumor cell lines. In biodistribution studies in healthy mice ß-[18F]1 showed homogenous tissue distribution and excellent metabolic stability, which was unaffected by tissue oxygenation. PET studies in tumor bearing mice showed tumor-to-muscle ratios of 2.13±0.22 (n=4) at 2h after administration of ß-[18F]1. In ex vivo autoradiography experiments ß-[18F]1 distribution closely matched staining with the hypoxia marker pimonidazole. In conclusion, ß-[18F]1 shows potential as PET hypoxia radiotracer which merits further investigation.

Breast Cancer Resistance Protein and P-Glycoprotein Influence In Vivo Disposition of 11C-Erlotinib.

UNLABELLED: (11)C-erlotinib is a PET tracer to distinguish responders from nonresponders to epidermal growth factor receptor-targeted tyrosine kinase inhibitors and may also be of interest to predict distribution of erlotinib to tissues targeted for treatment. The aim of this study was to investigate if the known interaction of erlotinib with the multidrug efflux transporters breast cancer resistance protein (humans, ABCG2; rodents, Abcg2) and P-glycoprotein (humans, ABCB1; rodents, Abcb1a/b) affects tissue distribution and excretion of (11)C-erlotinib and has an influence on the ability of (11)C-erlotinib PET to predict erlotinib tissue distribution at therapeutic doses. METHODS: Wild-type and Abcb1a/b or Abcg2 knockout mice underwent (11)C-erlotinib PET/MR scans, with or without the coinjection of a pharmacologic dose of erlotinib (10 mg/kg) or after pretreatment with the ABCB1/ABCG2 inhibitor elacridar (10 mg/kg). Integration plot analysis was used to determine organ uptake (CLuptake) and biliary excretion (CLbile) clearances of radioactivity. RESULTS: (11)C-erlotinib distribution to the brain was restricted by Abcb1a/b and Abcg2, and CLuptake into the brain was only significantly increased when both Abcb1a/b and Abcg2 were absent (wild-type mice, 0.017 ± 0.004 mL/min/g of tissue; Abcb1a/b((-/-))Abcg2((-/-)) mice, 0.079 ± 0.013 mL/min/g of tissue; P < 0.001). The pretreatment of wild-type mice with elacridar increased CLuptake into the brain to levels comparable to Abcb1a/b((-/-))Abcg2((-/-)) mice (0.090 ± 0.007 mL/min/g of tissue, P < 0.001). The absence of Abcb1a/b and Abcg2 led to a 2.6-fold decrease in CLbile (wild-type mice, 0.025 ± 0.005 mL/min/g of tissue; Abcb1a/b((-/-))Abcg2((-/-)) mice, 0.0095 ± 0.001 mL/min/g of tissue; P < 0.001). There were pronounced differences in distribution of (11)C-erlotinib to the brain, liver, kidney, and lung and hepatobiliary excretion into intestine between animals injected with a microdose and pharmacologic dose of erlotinib. CONCLUSION: ABCG2, ABCB1, and possibly other transporters influence in vivo disposition of (11)C-erlotinib and thereby affect its distribution to normal and potentially also tumor tissue. Saturable transport of erlotinib leads to nonlinear pharmacokinetics, possibly compromising the prediction of erlotinib tissue distribution at therapeutic doses from PET with a microdose of (11)C-erlotinib. The inhibition of ABCB1 and ABCG2 is a promising approach to enhance brain distribution of erlotinib to increase its efficacy in the treatment of brain tumors.

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 (R)-[(11)C]verapamil and [(11)C]-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)-[(11)C]verapamil or [(11)C]-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 [(3)H]verapamil and [(3)H]-N-desmethyl-loperamide in ABCB1 and Abcb1a overexpressing cell lines. Moreover we conducted in vivo PET experiments and biodistribution studies with (R)-[(11)C]verapamil and [(11)C]-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 [(3)H]verapamil and [(3)H]-N-desmethyl-loperamide by ABCB1 and Abcb1a and its inhibition by tariquidar. [(3)H]-N-Desmethyl-loperamide was transported with a 5 to 9 times higher transport ratio than [(3)H]verapamil in ABCB1- and Abcb1a-transfected cells. In vivo, brain radioactivity concentrations were lower for [(11)C]-N-desmethyl-loperamide than for (R)-[(11)C]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)-[(11)C]verapamil and 1329 nM (95% CI: 980-1801) for [(11)C]-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)-[(11)C]verapamil and [(11)C]-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)-[(11)C]verapamil and [(11)C]-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 [(11)C]-N-desmethyl-loperamide but not (R)-[(11)C]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 [(11)C]-N-desmethyl-loperamide but not (R)-[(11)C]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.

[(18)F]FDG is not transported by P-glycoprotein and breast cancer resistance protein at the rodent blood-brain barrier.

INTRODUCTION: Transport of 2-[(18)F]fluoro-2-deoxy-d-glucose ([(18)F]FDG) by the multidrug efflux transporters P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2) at the blood-brain barrier (BBB) may confound the interpretation of [(18)F]FDG brain PET data. Aim of this study was to assess the influence of ABCB1 and ABCG2 at the BBB on brain distribution of [(18)F]FDG in vivo by performing [(18)F]FDG PET scans in wild-type and transporter knockout mice and by evaluating changes in [(18)F]FDG brain distribution after transporter inhibition. METHODS: Dynamic small-animal PET experiments (60min) were performed with [(18)F]FDG in groups of wild-type and transporter knockout mice (Abcb1a/b((-/-)), Abcg2((-/-)) and Abcb1a/b((-/-))Abcg2((-/-))) and in wild-type rats without and with i.v. pretreatment with the known ABCB1 inhibitor tariquidar (15mg/kg, given at 2h before PET). Blood was sampled from animals from the orbital sinus vein at the end of the PET scans and measured in a gamma counter. Brain uptake of [(18)F]FDG was expressed as the brain-to-blood radioactivity concentration ratio in the last PET time frame (Kb,brain). RESULTS: Kb,brain values of [(18)F]FDG were not significantly different between different mouse types both without and with tariquidar pretreatment. The blood-to-brain transfer rate constant of [(18)F]FDG was significantly lower in tariquidar-treated as compared with vehicle-treated rats (0.350±0.025mL/min/g versus 0.416±0.024mL/min/g, p=0.026, paired t-test) but Kb,brain values were not significantly different between both rat groups. CONCLUSION: Our results show that [(18)F]FDG is not transported by Abcb1 at the mouse and rat BBB in vivo. In addition we found no evidence for Abcg2 transport of [(18)F]FDG at the mouse BBB. ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE: Our findings imply that functional activity of ABCB1 and ABCG2 at the BBB does not need to be taken into account when interpreting brain [(18)F]FDG PET data.