Hisashi Doi

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.

Palladium(0)‐Mediated Rapid Methylation and Fluoromethylation on Carbon Frameworks by Reacting Methyl and Fluoromethyl Iodide with Aryl and Alkenyl Boronic Acid Esters: Useful for the Synthesis of [11C]CH3C‐ and [18F]FCH2C‐Containing PET Tracers (PET=Positron Emission Tomography)

A new synthetic methodology for the rapid methylation and fluoromethylation on aryl and alkenyl frameworks by using methyl and fluoromethyl iodide with an organoboronic acid ester has been developed under the simple and mild conditions of [Pd(2)(dba)(3)]/P(o-CH(3)C(6)H(4))(3)/K(2)CO(3) (dba= dibenzylideneacetone) in DMF at 60 degrees C for 5 min (see scheme). This boron protocol provides a firm chemical basis for the synthesis of (11)C- and (18)F-incorporated PET tracers.The rapid methylation and fluoromethylation on aryl and alkenyl carbon frameworks by reacting methyl and fluoromethyl iodide with aryl and alkenyl boronates have been studied with the focus on the realization of the synthesis of [(11)C]CH(3)- and [(18)F]FCH(2)-labeled positron emission tomography (PET) tracers. The coupling of methyl iodide and pinacol phenylboronate (40 equiv) is accomplished in >91 % yield within 5 min at 60 degrees C under the conditions of [Pd(2)(dba)(3)]/P(o-CH(3)C(6)H(4))(3)/K(2)CO(3) (0.5:2:2; dba=dibenzylideneacetone) in DMF. The reaction shows a high generality and is applicable to various types of aryl and alkenyl boronates, giving the corresponding methylated derivatives in high yields (80-99 %). This reaction is also useful for the rapid incorporation of the fluoromethyl group. Thus, this boron protocol provides a firm chemical basis for the synthesis of (11)C- and (18)F-incorporated PET tracers and can be used as a complementary method for [(11)C]methylation together with our previous and ongoing processes using organotributylstannanes.

Rapid Methylation for the Synthesis of a 11C-Labeled Tolylisocarbacyclin Imaging the IP2 Receptor in a Living Human Brain

Rapid Methylation for the Synthesis of a 11C-labeled Tolylisocarbacyclin Imaging the IP2 Receptor in a Living Human Brain

Synthesis of a 11C-labelled prostaglandin F2α analogue using an improved method for stille reactions with [11C]methyl iodide

17-(3-[11C]methylphenyl)-18,19,20-trinor-PGF2αisopropyl ester 2 was synthesised using an improved method for cross-coupling reactions with [11C]methyl iodide. The decay-corrected radiochemical yield of 2 was 34 % based on [11C]methyl iodide in a synthesis time of 30 min from end of radionuclide production. The specific radioactivity was approximately 100 GBq/µmol and the radiochemical purity was higher than 95 % as determined by analytical LC. In a typical experiment 1.3 GBq of 2 was obtained from 11 GBq of [11C]methyl iodide. Copyright

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.

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.

Mapping of serotonin transporters by positron emission tomography with [11C]DASB in conscious common marmosets: comparison with rhesus monkeys.

The common marmoset (Callithrix jacchus) is unique among the primates in its small body size, reproductive efficacy, and characteristic social behavior, making it useful as an animal model in neuroscientific research. To assess the brain serotonergic systems, we investigated the binding of [11C]‐3‐amino‐4‐(2‐dimetylaminomethyl‐phenylsulfanyl)‐benzonitrile ([11C]DASB) to brain serotonin transporter (SERT) in conscious common marmosets using positron emission tomography (PET), and compared with findings for rhesus monkeys. Both species showed globally similar distribution patterns of [11C]DASB uptake in the brain, with highest activity in the midline of the brain and lowest in the cerebellum, and higher activity in some subcortical regions than in surrounding cortex, while the common marmoset brain showed almost equal or rather higher binding potential (BP) values (BPND) in cortical regions and hippocampus, and lower BPND than the rhesus monkey brain in some subcortical regions. Test‐retest reproducibility of BPND at an interval of several months was high, indicating reliable and stable measurements of serotonin transporters in both species. These results suggest that SERT imaging by PET with [11C]DASB under conscious state is valuable for investigating the physiological serotonergic functions in common marmosets (182). Synapse 2010.

Rapid methylation of terminal acetylenes by the Stille coupling of methyl iodide with alkynyltributylstannanes: a general protocol potentially useful for the synthesis of short-lived 11CH3-labeled PET tracers with a 1-propynyl groupElectronic supplementary information (ESI) available: general experimental remarks and synthetic methods and characterization of tributylalkynylstannanes and the corresponding methylacetylenes. See http://www.rsc.org/suppdata/ob/b3/b311532a/

The Pd(0)-mediated rapid coupling (trapping) reaction of methyl iodide with an excess amount of alkynyltributylstannane has been developed with the aim to incorporate a short-lived (11)C-labeled methyl group into biologically active organic compounds with a 1-propynyl structural unit.

11C-PK11195 PET for the In Vivo Evaluation of Neuroinflammation in the Rat Brain After Cortical Spreading Depression

Neurogenic inflammation triggered by extravasation of plasma protein has been hypothesized as a key factor in the generation of the pain sensation associated with migraine. The principal immune cell that responds to this inflammation is the parenchymal microglia of the central nervous system. Methods: Using a PET technique with 11C-(R)-[1-(2-chlorophenyl)-N-methyl-N-(1-methyl-propyl)-3-isoquinolinecarboxamide] (11C-PK11195), a PET ligand for peripheral type–benzodiazepine receptor, we evaluated the microglial activation in the rat brain after generation of unilateral cortical spreading depression, a stimulation used to bring up an experimental animal model of migraine. Results: We found a significant increase in the brain uptake of 11C-PK11195, which was completely displaceable by the excess amounts of unlabeled ligands, in the ipsilateral hemisphere of the spreading depression–generated rats. Moreover, the binding potential of 11C-PK11195 in the spreading depression–generated rats was significantly higher than that in the sham-operated control rats. Conclusion: These results suggest that as an inflammatory reaction, microglial cells are activated in response to the nociceptive stimuli induced by cortical spreading depression in the rat brain. Therefore, the 11C-PK11195 PET technique could have a potential for diagnostic and therapeutic monitoring of neurologic disorders related to neuroinflammation such as migraine.