Tadayuki Takashima

mRNA Expression and Transport Characterization of Conditionally Immortalized Rat Brain Capillary Endothelial Cell Lines; a New in vitro BBB Model for Drug Targeting

Abstract Brain capillary endothelial cell lines (TR-BBB) were established from a recently developed transgenic rat harboring temperature-sensitive simian virus 40 (ts SV 40) large T-antigen gene (Tg rat) and used to characterize the endothelial marker, transport activity, and mRNA expression of transporters and tight-junction strand proteins at the blood-brain barrier (BBB). These cell lines expressed active large T-antigen and grew well at 33°C with a doubling-time of about 22-31 hr, but did not grow at 39°C. TR-BBBs expressed the typical endothelial marker, von Willebrand factor, and exhibited acetylated low-density lipoprotein uptake activity. Although the γ-glutamyltranspeptidase activity in TR-BBBs was ~13% of that of the brain capillary fraction of a normal rat, it was localized in the apical side, suggesting that it reflects the functional polarity of the in vivo BBB. The mRNA of tight-junction strand proteins such as claudine-5, occludin, and junctional adhesion molecule are expressed in TR-BBB 13. Drug efflux transporter, P-glycoprotein, with a molecular weight of 170 kDa was expressed in all TR-BBBs and mdr 1a, mdr 1b, and mdr 2 mRNA were detected in TR-BBBs using RT-PCR. Moreover, mrp1 mRNA was expressed in all TR-BBBs. Influx transporter, GLUT-1, expressed at 55 kDa was revealed by Western blot analysis. It had 3-O-memyl-D-glucose (3-OMG) uptake activity which was concentration-dependent with a Michaelis-Menten constant of 9.86 ± 1.20 mM. The mRNA of large neutral amino acid transporter, which consists of LAT-1 and 4F2hc was expressed in TR-BBBs. In conclusion, the conditionally immortalized rat brain capillary endothelial cell lines (TR-BBB) had endothelial makers, expressed mRNA for tight-junction strand proteins and the influx and efflux transporters and produced GLUT-1, which is capable of 3-OMG transport activity.

PET Imaging–Based Evaluation of Hepatobiliary Transport in Humans with (15R)-11C-TIC-Me

It is well accepted that drug transporters play a pivotal role in hepatobiliary excretion of anionic drugs, in which drug–drug interactions and genetic polymorphisms are known to cause variations. However, PET probes for in vivo functional characterization of these transporters have not been established yet. We used PET to investigate hepatic uptake and subsequent canalicular efflux of 11C-labeled (15R)-16-m-tolyl-17,18,19,20-tetranorisocarbacyclin methyl ester [(15R)-11C-TIC-Me)] in healthy subjects. Methods: Serial PET scans of the abdominal region in healthy male subjects were obtained with or without the organic anion–transporting polypeptide (OATP) inhibitor rifampicin after intravenous injection of (15R)-11C-TIC-Me as a radiotracer. Venous blood samples and PET images were obtained at frequent intervals up to 30 min after administration of the PET tracer. Dynamic imaging data were evaluated by integration plots of data collected for 2–10 min and for 10–30 min after tracer administration for the determination of tissue uptake clearance and biliary efflux clearance, respectively. Results: After rapid hydrolysis in blood, the acid form—11C-labeled (15R)-16-m-tolyl-17,18,19,20-tetranorisocarbacyclin [(15R)-11C-TIC]—accumulated in the liver (37% of the dose by 17 min), and the radioactivity was then excreted into the bile (6.2% by 30 min). Rifampicin (600 mg by mouth), a potent OATP inhibitor, significantly reduced the radioactivity excreted into the bile (by 44%) by inhibiting both uptake (by 45%) and subsequent canalicular efflux (by 62%). (15R)-11C-TIC is an in vitro substrate of OATP1B1 and OATP1B3, and clinically relevant concentrations of rifampicin inhibited uptake by OATP1B1 and OATP1B3. These results demonstrated that in humans, (15R)-11C-TIC–associated radioactivity is excreted into the bile by organic anion transport systems. Conclusion: We demonstrated that PET image analysis with (15R)-11C-TIC-Me is useful for investigating variations in OATP function in the human hepatobiliary transport system.

Characterization of the Amino Acid Transport of New Immortalized Choroid Plexus Epithelial Cell Lines: A Novel In Vitro System for Investigating Transport Functions at the Blood-Cerebrospinal Fluid Barrier

Purpose. To establish and characterize a choroid plexus epithelial cell line (TR-CSFB) from a new type of transgenic rat harboring the temperature-sensitive simian virus 40 (ts SV 40) large T-antigen gene (Tg rat).Methods. Choroid plexus epithelial cells were isolated from the Tg rat and cultured on a collagen-coated dish at 37°C during the first period of 3 days. Cells were subsequently cultured at 33°C to activate large T-antigen. At the third passage, cells were cloned by colony formation and isolated from other cells using a penicillin cup.Results. Five immortalized cell lines of choroid plexus epithelial cells (TR-CSFB 1∼5) were obtained from two Tg rats. These cell lines had a polygonal cell morphology, expressed the typical choroid plexus epithelial cell marker, transthyretin, and possessed Na+, K+-ATPase on their apical side. TR-CSFBs cells expressed a large T-antigen and grew well at 33°C with a doubling-time of 35∼40 hr. [3H]-L-Proline uptake by TR-CSFB cells took place in an Na+-dependent, ouabain-sensitive, energy-dependent, and concentration-dependent manner. It was also inhibited by α-methylaminoisobutylic acid, suggesting that system A for amino acids operates in TR-CSFB cells. When [3H]-L-proline uptake was measured using the Transwell device, the L-proline uptake rate following application to the apical side was fivefold greater than that following application to the basal side. In addition, both Na+-dependent and Na+-independent L-glutamic acid (L-Glu) uptake processes were present in TR-CSFB cells.Conclusions. Immortalized choroid plexus epithelial cell lines were successfully established from Tg rats and have the properties of choroid plexus epithelial cells, and amino acid transport activity was observed in vivo.

The involvement of organic anion transporting polypeptide in the hepatic uptake of telmisartan in rats: PET studies with [¹¹C]telmisartan.

Telmisartan, a selective angiotensin II receptor antagonist, is primarily excreted via hepatobiliary transport. The predominant contribution of organic anion transporting polypeptide (OATP) 1B3 in its hepatic uptake of telmisartan has been demonstrated by in vitro transport studies. In the present study, a quantitative positron emission tomography (PET) methodology was developed for in vivo kinetic assessment of hepatobiliary transport of telmisartan. Serial abdominal PET scans were performed in rats following intravenous administration of [(11)C]telmisartan as a radiotracer. PET scans revealed that [(11)C]telmisartan was localized primarily in the liver and some of the radioactivity moved to the intestine, which corresponds to biliary excretion. Radiometabolite analysis by radiometric HPLC showed that [(11)C]telmisartan was converted to its acylglucuronide, which was mainly detected in bile, but little in plasma and liver. Integration plot analysis revealed that [(11)C]telmisartan was taken up into the liver as rapidly as the hepatic blood flow rate, and the radiometabolite was subsequently excreted into the bile. When rifampicin, a typical Oatp inhibitor, was coadministered with [(11)C]telmisartan in rats, hepatic uptake clearance of [(11)C]telmisartan was significantly decreased, whereas biliary efflux clearance was not changed. Coinjection with unlabeled telmisartan (4 and 10 mg/kg) also decreased hepatic uptake clearance of [(11)C]telmisartan. On the other hand, PET imaging analysis revealed a significant increase of biliary efflux when telmisartan dose was increased to more than 4 mg/kg. These results suggested that the hepatic uptake of [(11)C]telmisartan mainly consists of a saturable process mediated by Oatps in rats, according to noninvasive real-time measurement of tissue radioactivity with the use of PET. The present study with rats is expected to provide the feasibility of PET imaging study to quantitatively estimate OATP1B3 function in humans.

Positron Emission Tomography Studies Using (15R)-16-m-[11C]tolyl-17,18,19,20-tetranorisocarbacyclin Methyl Ester for the Evaluation of Hepatobiliary Transport

A quantitative positron emission tomography (PET) methodology was developed for in vivo kinetic analysis of hepatobiliary transport. Serial abdominal PET scans were performed on normal and multidrug resistance-associated protein 2 (Mrp2)-deficient rats after intravenous injection of (15R)-16-m-[11C]tolyl-17,18,19,20-tetranorisocarbacyclin methyl ester (15R-[11C] TIC-Me) as a radiotracer. 15R-[11C]TIC-Me was rapidly converted to its acid form in blood within 10 s. PET scans revealed that 15R-[11C]TIC was localized mainly in the liver within 5 min of injection. By 90 min, total radioactivity in bile of Mrp2-deficient rats was significantly reduced compared with controls. Metabolite analysis by thin-layer chromatography autoradiography showed that 15R-[11C]TIC is converted to at least three metabolites (M1, M2, and M3), and M2 and M3 are the major metabolites in plasma and bile, respectively. Hepatic uptake clearance of total radioactivity in normal rats was close to the hepatic blood flow rate and slightly higher than that in Mrp2-deficient rats. The intrinsic canalicular efflux clearance of M3 (CLint,bile,M3) in Mrp2-deficient rats was decreased to 12% of controls, whereas clearance of M2 was moderately decreased (54%). An in vitro transport assay detected ATP-dependent uptake of both M2 and M3 by rat Mrp2-expressing membrane vesicles. These results demonstrated that M3 is excreted primarily into the bile by Mrp2 in normal rats. We conclude that PET studies using 15R-[11C]TIC-Me could be useful for in vivo analyses of Mrp2-mediated hepatobiliary transport.

General Method for the 11C‐Labeling of 2‐Arylpropionic Acids and Their Esters: Construction of a PET Tracer Library for a Study of Biological Events Involved in COXs Expression

Cyclooxygenase (COX) is a critical enzyme in prostaglandin biosynthesis that modulates a wide range of biological functions, such as pain, fever, and so on. To perform in vivo COX imaging by positron emission tomography (PET), we developed a method to incorporate (11)C radionuclide into various 2-arylpropionic acids that have a common methylated structure, particularly among nonsteroidal anti-inflammatory drugs (NSAIDs). Thus, we developed a novel (11)C-radiolabeling methodology based on rapid C-[(11)C]methylation by the reaction of [(11)C]CH(3)I with enolate intermediates generated from the corresponding esters under basic conditions. One-pot hydrolysis of the above [(11)C]methylation products also allows the synthesis of desired (11)C-incorporated acids. We demonstrated the utility of this method in the syntheses of six PET tracers, [(11)C]Ibuprofen, [(11)C]Naproxen, [(11)C]Flurbiprofen, [(11)C]Fenoprofen, [(11)C]Ketoprofen, and [(11)C]Loxoprofen. Notably, we found that their methyl esters were particularly useful as proradiotracers for a study of neuroinflammation. The microPET studies of rats with lipopolysaccharide (LPS)-induced brain inflammation clearly showed that the radioactivity of PET tracers accumulated in the inflamed region. Among these PET tracers, the specificity of [(11)C]Ketoprofen methyl ester was demonstrated by a blocking study. Metabolite analysis in the rat brain revealed that the methyl esters were initially taken up in the brain and then underwent hydrolysis to form pharmacologically active forms of the corresponding acids. Thus, we succeeded in general (11)C-labeling of 2-arylpropionic acids and their methyl esters as PET tracers of NSAIDs to construct a potentially useful PET tracer library for in vivo imaging of inflammation involved in COXs expression.

Evaluation of Oatp and Mrp2 Activities in Hepatobiliary Excretion Using Newly Developed Positron Emission Tomography Tracer [11C]Dehydropravastatin in Rats

We developed a pravastatin derivative, sodium (3R,5R)-3,5-dihydroxy-7-((1S,2S,6S,8S)-6-hydroxy-2-methyl-8-((1-[11C]-(E)-2-methyl-but-2-enoyl)oxy)-1,2,6,7,8,8a-hexahydronaphthalen-1-yl)heptanoate ([11C]DPV), as a positron emission tomography (PET) probe for noninvasive measurement of hepatobiliary transport, and conducted pharmacokinetic analysis in rats as a feasibility study for future clinical study. Transport activities of DPV in freshly isolated rat hepatocytes and rodent multidrug resistance–associated protein 2 (rMrp2; human, MRP2)-expressing membrane vesicles were similar to those of pravastatin. Rifampicin diminished the uptake of DPV and pravastatin by the hepatocytes, with similar inhibition potency. [11C]DPV underwent biotransformation to produce at least two metabolites in rat, but metabolism of [11C]DPV occurred negligibly in human hepatocytes during a 90-minute incubation. After intravenous injection, [11C]DPV was mainly distributed to the liver and kidneys, where the tissue uptake clearances (CLuptake,liver and CLuptake,kidney) were blood-flow–limited (73.6 ± 4.8 and 24.6 ± 0.6 ml/min per kilogram, respectively). Systemic elimination of [11C]DPV was delayed in rifampicin-treated rat and an Mrp2-deficient mutant rat, Eisai hyperbilirubinemic mutant rat (EHBR). Rifampicin treatment decreased both CLuptake,liver and CLuptake,kidney of [11C]DPV by 30% (P < 0.05), whereas these parameters were unchanged in EHBR. Meanwhile, the canalicular efflux clearance (CLint,bile) of [11C]DPV, which was 12.2 ± 1.5 ml/min per kilogram in the control rat, decreased by 60% and 89% in rifampicin-treated rat and EHBR (P < 0.05), respectively. These results indicate that [11C]DPV is taken up into the liver by organic anion-transporting polypeptides (rodent, Oatps; human, OATP) and excreted into bile by Mrp2 in rat, and that rifampicin may inhibit Mrp2 as well as Oatps, and consequently increase systemic exposure of [11C]DPV. PET using [11C]DPV is feasible for studies prior to the future clinical investigation of OATP and MRP2 functionality, especially for personalized medicine.

Whole-body distribution and radiation dosimetry of [11C]telmisartan as a biomarker for hepatic organic anion transporting polypeptide (OATP) 1B3.

INTRODUCTION Telmisartan, a nonpeptide angiotensin II AT1 receptor antagonist used as an antihypertensive drug, is specifically taken up by the liver through the OATP1B3. PET imaging with [(11)C]telmisartan is expected to provide information about the whole body pharmacokinetics of telmisartan as well as its transport property by OATP1B3. The purpose of the study was to determine the biodistribution and radiation dosimetry of [(11)C]telmisartan in humans. METHODS Biodistribution of [(11)C]telmisartan was measured in three rats and six healthy male human volunteers. In the rat study, a dynamic emission scan was performed for 90 min. In the human study, dynamic whole-body PET images were acquired after intravenous injection of [(11)C]telmisartan. ROIs were defined for source organs on the PET images to measure time-course of [(11)C]telmisartan uptake as percentage injected dose and the number of disintegration for each organ. Radiation dosimetry was calculated with OLINDA/EXM. RESULTS In the rat study, most radioactivity was rapidly taken up by the liver and part of it was excreted into the biliary tract and intestine. Extrapolating from the rat data, the effective dose for the adult human being was estimated to be 3.65±0.01 microSv/MBq (n=3). In the human study, most of the tracer was taken up by the liver as well, although not as rapidly as in the rat. The activity in the gall bladder and intestine increased gradually. The effective dose for the adult human being was 4.24±0.09 microSv/MBq (n=6). CONCLUSIONS [(11)C]Telmisartan is a safe PET tracer with a dosimetry profile comparable to other common (11)C PET tracers.

Developmental Changes in P-Glycoprotein Function in the Blood–Brain Barrier of Nonhuman Primates: PET Study with R-11C-Verapamil and 11C-Oseltamivir

P-glycoprotein (P-gp) plays a pivotal role in limiting the penetration of xenobiotic compounds into the brain at the blood–brain barrier (BBB), where its expression increases with maturation in rats. We investigated developmental changes in P-gp function in the BBB of nonhuman primates using PET with R-11C-verapamil, a PET radiotracer useful for evaluating P-gp function. In addition, developmental changes in the brain penetration of 11C-oseltamivir, a substrate for P-gp, was investigated as practical examples. Methods: PET studies in infant (age, 9 mo), adolescent (age, 24–27 mo), and adult (age, 5.6–6.6 y) rhesus monkeys (Macaca mulatta) were performed with R-11C-verapamil and also with 11C-oseltamivir. Arterial blood samples and PET images were obtained at frequent intervals up to 60 min after administration of the PET tracer. Dynamic imaging data were evaluated by integration plots using data collected within the first 2.5 min after tracer administration. Results: R-11C-verapamil rapidly penetrated the brain, whereas the blood concentration of intact R-11C-verapamil decreased rapidly in all subjects. The maximum brain uptake in infant (0.033% ± 0.007% dose/g of brain) and adolescent (0.020% ± 0.002% dose/g) monkeys was 4.1- and 2.5-fold greater, respectively, than uptake in adults (0.0082% ± 0.0007% dose/g). The clearance of brain R-11C-verapamil uptake in adult monkeys was 0.056 ± 0.010 mL/min/g, significantly lower than that in infants (0.11 ± 0.04 mL/min/g) and adolescents (0.075 ± 0.023 mL/min/g). 11C-oseltamivir showed little brain penetration in adult monkeys, with a clearance of R-11C-verapamil uptake of 0.0072 and 0.0079 mL/min/g, slightly lower than that in infant (0.0097 and 0.0104 mL/min/g) and adolescent (0.0097 and 0.0098 mL/min/g) monkeys. Conclusion: These results suggest that P-gp function in the BBB changes with development in rhesus monkeys, and this change may be closely related to the observed difference in drug responses in the brains of children and adult humans.

The synthesis and biodistribution of [11C]metformin as a PET probe to study hepatobiliary transport mediated by the multi-drug and toxin extrusion transporter 1 (MATE1) in vivo

In order to develop a new positron emission tomography (PET) probe to study hepatobiliary transport mediated by the multi-drug and toxin extrusion transporter 1 (MATE1), (11)C-labelled metformin was synthesized and then evaluated as a PET probe. [(11)C]Metformin ([(11)C]4) was synthesized in three steps, from [(11)C]methyl iodide. Evaluation by small animal PET of [(11)C]4 showed that there was increased concentrations of [(11)C]4 in the livers of mice pre-treated with pyrimethamine, a potential inhibitor of MATEs, inhibiting the hepatobiliary excretion of metformin. Radiometabolite analysis showed that [(11)C]4 was not degraded in vivo during the PET scan. Biodistribution studies were undertaken and the organ distributions were extrapolated into a standard human model. In conclusion, [(11)C]4 may be useful as a PET probe to non-invasively study the in vivo function of hepatobiliary transport and drug-drug interactions, mediated by MATE1 in future clinical investigations.