Synthesis of (S)-N-[methyl-11C]nicotine and its regional distribution in the mouse brain: a potential tracer for visualization of brain nicotinic receptors by positron emission tomography.

A nicotine agonist, 11C-labeled (S)-nicotine, was synthesized by N-methylation of (S)-nornicotine with [11C]-methyl iodide in dimethylformamide-dimethylsulfoxide in order to study nicotinic receptors in the human brain by positron emission tomography. The radiochemical yield of this N-methylation reaction was more than 90% within 5 min. After purification by high performance liquid chromatography the radiochemical purity of the product was more than 99% and the specific radioactivity was 7.4-11.1 GBq/mumol. The regional distribution of (S)-[11C]nicotine in the mouse brain after intravenous injection was compared with that of (R)-[11C]nicotine. After injection of (S)-[11C]nicotine, the regional uptake of radioactivity was in the following order: cortex greater than thalamu approximately hippocampus greater than striatum greater than hypothalamus greater than cerebellum. Moreover, (S)-[11C]nicotine was displaced from the brain by unlabeled (S)-nicotine, but unlabeled (R)-nicotine caused no change in uptake. In contrast, (R)-[11C]nicotine showed a lower brain uptake and lesser regional differences in radioactivity.     

Synthesis of [125I]iodoclorgyline, a selective monoamine oxidase A inhibitor, and its biodistribution in mice.

A new radioiodinated monoamine oxidase A (MAO-A) specific inhibitor, [125I]iodoclorgyline, was synthesized from its tin precursor by iododestannylation reaction using sodium [125I]iodide and hydrogen peroxide with high yield and site specificity. The product possessed a high radiochemical purity as well as high specific activity. The method can be readily applicable for labeling with 123I, a very suitable radioisotope for in vivo imaging with single photon emission computer tomography (SPECT). Biodistribution studies of the [125I]iodoclorgyline in mice showed high initial uptake in the brain, and brain radioactivity reached a constant level at 60 min after intravenous injection. The results suggested that [125I]iodoclorgyline might have potential as a radiopharmaceutical for MAO-A studies in the brain with SPECT.     

Asymmetric synthesis of L-2-amino[3-11C]butyric acid, L-[3-11C]norvaline and L-[3-11C]valine

The short-lived radionuclide 11C (t1/2 = 20.4 min) has been used in the asymmetric synthesis of L-2-amino[3-11C]butyric acid, L-[3-11C]-norvaline and L-[3-11C]valine. The syntheses were performed by alkylation of [(+)-2-hydroxypinanyl-3-idene]-glycine tert-butyl ester under anhydrous conditions in tetrahydrofuran/1,3-dimethyl-3,4,5,6-tetrahydro-2-pyrimidinone with lithiated 2,2,6,6-tetramethylpiperidine as base, using the appropriate 11C-alkyl iodides prepared in a one-pot reactor from [11C]carbon dioxide. Following removal of the protecting groups, the -[3-11C]amino acids were obtained in 80-82% enantiomeric excess and in 9-25% radiochemical yields, decay corrected and calculated on the basis of the amount of [11C]carbon dioxide at the start of the syntheses within 50-55 min.     

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 ¹¹C radionuclide into various 2‐arylpropionic acids that have a common methylated structure, particularly among nonsteroidal anti‐inflammatory drugs (NSAIDs). Thus, we developed a novel ¹¹C‐radiolabeling methodology based on rapid C‐[¹¹C]methylation by the reaction of [¹¹C]CH₃I with enolate intermediates generated from the corresponding esters under basic conditions. One‐pot hydrolysis of the above [¹¹C]methylation products also allows the synthesis of desired ¹¹C‐incorporated acids. We demonstrated the utility of this method in the syntheses of six PET tracers, [¹¹C]Ibuprofen, [¹¹C]Naproxen, [¹¹C]Flurbiprofen, [¹¹C]Fenoprofen, [¹¹C]Ketoprofen, and [¹¹C]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 [¹¹C]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 ¹¹C‐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.     

¹¹C-labeled analogs of indomethacin esters and amides for brain cyclooxygenase-2 imaging: radiosynthesis, in vitro evaluation and in vivo characteristics in mice

There is great potential in the use of positron emission tomography (PET) and suitable radiotracers for the study of cyclooxygenase type 2 (COX-2) enzyme in living subjects. In the present study, we prepared and evaluated five 11C-labeled ester and amide analogs derived from indomethacin as potential PET imaging agents for the in vivo visualization of the brain COX-2 enzyme. Five 11C-labeled COX-2 inhibitors, with different lipophilicities and moderate COX-2 inhibitory activity, were prepared by treatment of the corresponding O-desmethyl precursors with [11C]methyl triflate and purified by HPLC (radiochemical yields of 55-71%, radiochemical purity of >93%, and the specific activities of 22-331 GBq/μmol). In mice, radioactivity in the brain for all radiotracers was low, with very low brain-to-blood ratios. A clear inverse relationship was observed between brain uptake at 1 min postinjection and the lipophilicity (experimental log P?.?) of the studied 11C-radiotracers. Pretreatment of mice with cyclosporine A to block P-glycoproteins caused a significant increase in brain uptake of radioactivity following injection of the 11C-radiotracer compared to control. HPLC analysis showed that each radiotracer was rapidly metabolized, and a few metabolites, which were more polar than the original radiotracers, were found in both plasma and brain. No specific binding of the tracers towards the COX-2 enzyme in the brain was clearly revealed by in vivo blocking study. Further structural refinement of the tracer agent is necessary for better enhancement of brain uptake and for sufficient metabolic stability.     

Positron-emitting N-[18F]fluoroalkyl and [18F]fluoropyrrolidinyl analogues of eticlopride as potential in vivo radioligands for dopamine D2 receptors.

N-Fluoroalkyl and 4-fluoropyrrolidinyl eticlopride analogues with high affinity toward central nervous system dopamine D2 receptors in vitro were labelled with positron emitting fluorine-18 (t1/2 = 110 min), and their in vivo biodistribution was investigated in rats. N-[18F]Fluoro-ethyl and -propyl eticlopride derivatives showed poor in vivo selectivity in the rat brain. On the other hand, 4-[18F]fluoropyrrolidinyl eticlopride exhibited almost constant and relatively high striatal concentration. The striatal/cerebellar radioactivity ratio, which corresponds to the ratio of a brain D2 receptor-rich to poor region, gradually increased to 5.2-6.4, 90 min after the injection. The striatal accumulation was selectively inhibited by pre-injection of haloperidol, a dopamine D2 antagonist, without affecting accumulation in other tissues. Thus, the selective striatal accumulation of 4-[18F]fluoropyrrolidinyl eticlopride in striatal tissue appears to be due to the specific binding to dopamine D2 receptors.     

Synthesis of 11C-labelled N,N'-diphenylurea and ethyl phenylcarbamate by a rhodium-promoted carbonylation via [11C]isocyanatobenzene using phenyl azide and [11C]carbon monoxide

The reaction with phenyl azide and [11C]carbon monoxide to give N,N'-diphenyl[11C]urea and ethyl phenyl[11C]carbamate has been studied with the aim of development of a new methodology for carbonylation using [11C]carbon monoxide with high specific radioactivity. The synthesis of 11C-labelled N,N'-diphenylurea from phenyl azide and [11C]carbon monoxide, with 1,2-bis(diphenylphosphino)ethane-bound Rh(I) complex at 120 degrees C at a pressure of 35 MPa in the presence of aniline was accomplished in 82% trapping efficiency and 82% conversion yield. This approach was also useful for the synthesis of ethyl phenyl[11C]carbamate with lithium ethoxide as a nucleophilic reagent giving 90% trapping efficiency and 76% conversion yield. These reactions can be considered to proceed via a [11C]isocyanate or a [11C]isocyanate-coordinated Rh complex to give the corresponding 11C-products. This protocol provides the chemical basis for the synthesis of [11C]urea and [11C]carbamate derived from [11C]isocyanates.     

Preliminary Assessment of the Anti-inflammatory Activity of New Structural Honokiol Analogs with a 4′-O-(2-Fluoroethyl) Moiety and the Potential of Their 18F-Labeled Derivatives for Neuroinflammation Imaging

Neolignans honokiol and 4′-O-methylhonokiol (MH) and their derivatives have pronounced anti-inflammatory activity, as evidenced by numerous pharmacological studies. Literature data suggested that cyclooxygenase type 2 (COX-2) may be a target for these compounds in vitro and in vivo. Recent studies of [¹¹C]MPbP (4′-[¹¹C]methoxy-5-propyl-1,1′-biphenyl-2-ol) biodistribution in LPS (lipopolysaccharide)-treated rats have confirmed the high potential of MH derivatives for imaging neuroinflammation. Here, we report the synthesis of four structural analogs of honokiol, of which 4′-(2-fluoroethoxy)-2-hydroxy-5-propyl-1, 1′-biphenyl (F-IV) was selected for labeling with fluorine-18 (T_1/2 = 109.8 min) due to its high anti-inflammatory activity confirmed by enzyme immunoassays (EIA) and neuromorphological studies. The high inhibitory potency of F-IV to COX-2 and its moderate lipophilicity and chemical stability are favorable factors for the preliminary evaluation of the radioligand [¹⁸F]F-IV in a rodent model of neuroinflammation. [¹⁸F]F-IV was prepared with good radiochemical yield and high molar activity and radiochemical purity by ¹⁸F-fluoroethylation of the precursor with Boc-protecting group (15) with [¹⁸F]2-fluoro-1-bromoethane ([¹⁸F]FEB). Ex vivo biodistribution studies revealed a small to moderate increase in radioligand uptake in the brain and peripheral organs of LPS-induced rats compared to control animals. Pretreatment with celecoxib resulted in significant blocking of radioactivity uptake in the brain (pons and medulla), heart, lungs, and kidneys, indicating that [¹⁸F]F-IV is likely to specifically bind to COX-2 in a rat model of neuroinflammation. However, in comparison with [¹¹C]MPbP, the new radioligand showed decreased brain uptake in LPS rats and high retention in the blood pool, which apparently could be explained by its high plasma protein binding. We believe that the structure of [¹⁸F]F-IV can be optimized by replacing the substituents in the biphenyl core to eliminate these disadvantages and develop new radioligands for imaging activated microglia.     

Examination of the in vitro degradation of [14C] pentaerythritol tetranitrate in rat and human blood with an improved thin-layer radiochromatographic procedure

Improvements were made on a reported thin-layer radiochromatographic assay for the determination of [14C]pentaerythritol tetranitrate (PETN) and its metabolites in whole blood, using methanol instead of dioxane as the extracting solvent. Recovery of total radioactivity for the entire work-up procedure was greater than 90%, and the distribution of PETN and its metabolites in degraded blood samples was found to be reproducible. This modified method appeared simpler and yielded better recovery of radioactivity than the literature method. In vitro metabolism of [14C]PETN in rat and human blood was examined by incubation of the drug with fresh blood at 37 degrees C for 60 min. In rat blood, the half-life of PETN degradation was about 15 min producing the trinitrate, dinitrate and mononitrate metabolites. Human blood was also capable of degrading PETN in vitro, but at a lower rate than rat blood, yielding only the trinitrate metabolite in quantifiable amounts during the incubation period. Equilibrium of PETN between plasma and red blood cells was observed within 1 min after PETN addition to both rat and human blood. The apparent plasma/red blood cells partition ratios of PETN were 1.1 and 1.7 for rat and human blood, respectively. PETN degradation was approximately ten times slower in rat plasma than in rat blood, suggesting that enzymes in erythrocytes are important for PETN metabolism in rat whole blood.     

Liposomes prepared from synthetic amphiphiles. II. Their interaction with Ehrlich ascites tumor cells and tissue distribution in Ehrlich solid tumor-bearing mice

The interaction of 99mTc-labeled liposomes prepared from synthetic amphiphiles containing amino acid residues with Ehrlich ascites tumor cells in vitro and their tissue distributions in Ehrlich solid tumor-bearing mice were investigated. The amphiphiles used were N,N-didodecyl-N alpha-[6-(trimethylammonio)hexanoyl]-L-alaninamide bromide N+C5Ala2C12), N,N-didodecyl-N alpha-(6-[dimethyl(2-carboxyethyl)ammonio]hexanoyl)-L- alaninamide bromide (CAC2N+C5Ala2C12) and S-[1-carboxy-2- ([2,3-bis(hexadecyloxy)propoxy]carbonyl)ethyl]homocysteine (HcyM-G2C1 6). Most of the radioactivity of N+C5Ala2C12 and CAC2N+C5Ala2C12 liposomes was firmly bound to Ehrlich ascites tumor cells in vitro. On the other hand, the accumulation of three 99mTc-labeled liposomes in the tumor of Ehrlich solid tumor-bearing mice was low (about 1% dose per gram of tissue), and most of the liposomes were taken up highly in the liver and spleen of the tumor-bearing mice. However, the radioactivity of the liposomes in the tumor, especially that of N+C5Ala2C12 and CAC2N+C5Ala2C12 liposomes, decreased more slowly with time than in the liver in up to 24 h after injection, suggesting that these liposomes were hard to separate from the tumor cells.     

Determination of enantiomeric purity in biogenic11C-labelled amino acids by aminoacylation of tRNA

In syntheses of naturally occurring amino acids labelled with short-lived radionuclides, such as¹¹C, the determination of the enantiomeric purity presents problems. The aminoacyl coupling of L-amino acids to tRNA followed by a separation with gel filtration of the tRNA-amino acid complex and free amino acid, was here shown to be an adequate method for determining the enantiomeric purity of [methyl-¹¹C]-methionine even in the pmol range. This method has also been used for the determination of the specific radioactivity of the¹¹C-labelled methionine. The method is probably valid for other naturally occurring amino acids and it might also be of interest for enantiomeric separations of L- and D-amino acids of high specific radioactivity.     

Reactions of M[CH(SiMe3)2] (M = Na or K) with PhCN, and related chemistry

A number of metal complexes containing one of the following ligands: the 1-azaallyl [N(R)C(Ph)C(H)R]- ([triple bond]L-), the 1,3-diazaallyl([triple bond]LL'-) and the isomeric beta-diketiminate [{N(R)C(Ph)]}2CH]- ( identical with LL-) have been prepared (R = SiMe(3)). These are the crystalline compounds H(LL) (2), Na(LL) (3), [Na(LL)(thf)2] (4), Na(L) (6), [Na(mu-LL')]8 (7), [K(mu-L)(eta6-C6H6)]2 (8), [K(mu-LL')(thf)]2 (9), [K(thf)2(mu-LL)](infinity) (10) and [Ni(LL')2] (11). A new synthesis of Na[C(H)R2] (1) involved Hg[C(H)R2]2 and Na/Hg as reagents. The beta-diketimine 2 was obtained from Li(LL) and cyclopentadiene. Under different conditions compounds 3, 6 and 7 were isolated from 1 and benzonitrile, and compounds 8, 9 and 10 from K[C(H)R2] and PhCN. Complex 11 was derived from [Li(LL')]2 and [NiBr(2)(dme)]. The solution obtained from 1 + 2 PhCN in Et2O at ambient temperature was a mixture (5) of 3 (predominantly) and 7. The 1-azaallyl complex 8 has the ligand bound to the metal as the enamide, and this is also probably (NMR) the case for 6. The molecular structures of the crystalline complexes 7, 8 and 11 are presented; that of 10 was published earlier. Compound 7, a cyclooctamer, is particularly interesting, in that each LL'- ligand is bridging via one of its N atoms to two neighbouring sodium ions and is not only N,N'- but also (eta2-C[=]C)-chelating to one of them.     

Simple, rapid and reliable preparation of [¹¹C]-(+)-α-DTBZ of high quality for routine applications

[11C]-(+)-α-DTBZ has been used as a marker of dopaminergic terminal densities in human striatum and expressed in islet beta cells in the pancreas. We aimed to establish a fully automated and simple procedure for the synthesis of [11C]-(+)-α-DTBZ for routine applications. [11C]-(+)-α-DTBZ was synthesized from a 9-hydroxy precursor in acetone and potassium hydroxide with [11C]-methyl triflate and was purified by solid phase extraction using a Vac tC-18 cartridge. Radiochemical yields based on [11C]-methyl triflate (corrected for decay) were 82.3% ± 3.6%, with a specific radioactivity of 60 GBq/μmol. Time elapsed was less than 20 min from end of bombardment to release of the product for quality control.     

Biosynthesis of juvenile hormone in the Cecropia moth. Labelling pattern from 1-[14C]-propionate through degradation to single carbon atom derivatives.

In the adult made cecropia moth, 1-[¹⁴C]-propionate is incorporated specifically into juvenile hormone I ( 1 ). By chemical degradation it was found that only C(7) and C(11) are labelled, each carbon atom bearing 50% of the radioactivity originally present in JH-I. It is concluded that propionate serves as a precursor of homomevalonate, which in turn is a precursor for JH-I. Application of 2-[¹⁴C]-propionate and 3-[¹⁴C]-propionate leads to extensive randomization of the label. Apparently propionate is metabolized such that C(2) and C(3) can be reused as smaller fragments–probably acetate–while C(1) is either highly diluted or removed from the propionate in a metabolically inactive form.     

Mono-, di-, and Ttianionic beta-diketiminato ligands: a computational study and the synthesis and structure of [(YbL)(3)(THF)], L = [[N(SiMe(3))C(Ph)](2)CH

A trinuclear Yb beta-diketiminato cluster [(YbL)3(THF)] (1) (L = {N(SiMe3)C(Ph)}2CH), containing L-1 and L-3 as well as Yb(II) and Yb(III) centers, was obtained by treatment of [YbL2] with Yb-naphthalene and was characterized by X-ray crystallography. The electron distribution in 1 and the Yb(II)/L-2 complex [Yb{(mu-L)Li(THF)}2] (2) was analyzed by DFT and ONIOM (QM/MM) calculations.     

Evaluation of 13C-phenylalanine and 13C-tyrosine breath tests for the measurement of hepatocyte functional capacity in patients with liver cirrhosis.

Liver disease is associated with an abnormal elevation of the plasma concentrations of the aromatic amino acids phenylalanine and tyrosine. The liver is the main site of aromatic amino acid metabolism, particularly the hydroxylation of phenylalanine to tyrosine and further tyrosine degradation. In the present study, we have examined the usefulness of the L-[1-13C]phenylalanine breath test (13C-PheBT) and L-[13C]tyrosine breath test (13C-TyrBT) for the detection of hepatic damage in patients with liver cirrhosis. First, the time courses of 13CO2 excretion after the administration of L-[1-13C]phenylalanine and L-[1-13C]tyrosine were compared. The peak times (the time expressed in minutes at which 13CO2 excretion was maximal) were 20 min in both breath tests, but 13C-TyrBT gave a higher peak than 13C-PheBT. Next, the parameters of 13C-PheBT and 13C-TyrBT were compared with biochemical liver function test values. These parameters were well correlated with several liver blood test values conventionally regarded as measures of hepatocyte functional reserve. Therefore, 13C-PheBT and 13C-TyrBT may be useful to assess the degree and progression of hepatic dysfunction.     

Determination of iodide and thiocyanate in powdered milk and infant formula by on-line enrichment ion chromatography with photodiode array detection

Automated ultra-performance liquid chromatography (LC) with a radiometric detection system has been developed to analyze compounds labeled with short-lived positron (beta(+))-emitting radionuclides in microdialysates. This system involves (1) on-line microdialysis sampling to submit dialysates directly into the LC, (2) ultra-performance LC for improving sensitivity, resolution and speed of analysis, and (3) tandem flow-through of beta(+) and electrochemical detectors for real-time monitoring of radioactive and endogenous compounds simultaneously. This system permitted ultrasensitive measurement of radioactive compounds at the lowest detection limit of about 1Bq with a high-temporal resolution of sampling and less operator attention. The developed system was used for the continuous monitoring of l-[beta-(11)C]-3,4-dihydroxyphenylalanine (l-[beta-(11)C]DOPA) metabolites in the rat brain.     

Radical-mediated carboxylation of alkyl iodides with [11C]carbon monoxide in solvent mixtures

[reaction: see text] [carboxyl-(11)C]Carboxylic acids were prepared from alkyl iodides via photoinitiated radical reactions using 10(-)(8) mol of [(11)C]carbon monoxide in binary and ternary homogeneous solvent mixtures. Short- (isobutyric), medium-, and long-chain saturated fatty acids (heptadecanoic) were labeled with isolated decay-corrected radiochemical yields ranging from 55% to 70% in 5-7-min reactions. The conversion of [(11)C]carbon monoxide to products reached 80-90%. To obtain good yields in the reactions performed in water-acetonitrile and water-THF mixtures, the addition of tetrabutylammonium hydroxide or potassium hydroxide was essential. The carboxylation was efficient for primary and secondary alkyl iodides. The carboxylation of tertiary iodides was feasible for 1-iodoadamantane but not for tert-butyl iodide. The dependence of the radiochemical yields on reaction time, photoirradiation conditions, and organic and inorganic additives was studied. The method provides a one-step route to [carboxyl-(11)C]carboxylic acids; traditional methods, in contrast, would require several steps. For example, using the devised reaction conditions, 3.19 GBq of purified [1-(11)C]1,10-decanedicarboxylic acid (specific radioactivity 188 GBq/mumol) was obtained within 35 min of the end of 10 muAh bombardment. (1-(13)C)4-Phenylbutyric acid was synthesized using ((13)C)carbon monoxide for identifying the labeling position with (1)H and (13)C NMR.     

Dry-distillation of experimental animal wastes containing radioisotopes

Mice were dry-distilled at 800 degrees C for 10 min, after [32P]orthophosphate or L-[4,5-3H]leucine was intraperitoneally administered. Phosphorus-32 was quantitatively recovered in the residual solid, whereas 95% of 3H was found int he distillate (condensed water). When 14C (L-[U-14C]malic acid or L-[U-14C]leucine) was administered to mice and they were dry-distilled, 14C-radioactivity was distributed into two fractions; residual solid and exhaust gas. In these cases, the recovery percentage of 14C in residual solid was not very high but increased as the treating temperature decreased. It reached about 40% of the administered 14C at 400 degrees C for 120 min. By the dry-distillation of animals, their weight was reduced to about 10% in every animal tested (mice, rats and rabbits). The volume was reduced to about 20% in cases of mice and rats, and about 30% in case of rabbits. It was concluded that the dry-distillation can be useful as a pre-treatment for disposal of animal wastes containing radioisotopes.     

Die Struktur von Cyclosporin C

The Structure of Cyclosporin C The structure of cyclosporin C ( 2 ), a minor antifungal metabolite from Trichoderma polysporum (Link ex Pers.) RIFAI has been elucidated. Hydrolytic cleavage and spectroscopic evidence show that cyclosporin C is a neutral oligopeptide of 11 amino acids linked together in a 33-membered ring. Cyclosporin C ( 2 ) differs from the main metabolite cyclosporin A ( 1 ) [2] [4] only by containing L-threonine in the place of L-α-aminobutyric acid as has been shown by the conversion of 2 into 1 . ¹³C-NMR. spectra and study of molecular models suggest that cyclosporin C ( 2 ) has the same molecular conformation as 1 , which is best described as a twisted β-pleated sheet held together in a conformationally stable form by intramolecular hydrogen bonding.