High reactivity of [11C]CH3I with thiol group in the synthesis of C-11 labeled radiopharmaceuticals

High reactivity of [¹¹C]-methyl iodide ([¹¹C]CH₃I) with the thiol group was demonstrated with cysteamine and other compounds containing a thiol and another functional groups in each structure. The methylation of the thiol group in cysteamine with [¹¹C]CH₃I was very rapid at 0°C with no catalyst, and gave a high radiochemical yield and purity without any detectable by-product. Moreover, this reaction was not disturbed by the other functional groups, such as -NH₂, -OH and -COOH in the same structure. This S-methylation reaction is very useful for producing a new radiopharmaceutical labeled with the short lived positron emitting nuclide C-11.     

Facile synthesis of carbon-11-labeled arylpiperazinylthioalkyl derivatives as new PET radioligands for imaging of 5-HT1AR

Carbon-11-labeled arylpiperazinylthioalkyl derivatives, 2-((4-(4-(2-[¹¹C]methoxyphenyl)piperazin-1-yl)butyl)thio)benzo[d]oxazole ([¹¹C]5a), 2-((4-(4-(2-[¹¹C]methoxyphenyl)piperazin-1-yl)butyl)thio)-5,7-dimethylbenzo[d]oxazole ([¹¹C]5c), 2-((4-(4-(2-[¹¹C]methoxyphenyl)piperazin-1-yl)butyl)thio)benzo[d]thiazole ([¹¹C]5e), 2-((6-(4-(2-[¹¹C]methoxyphenyl)piperazin-1-yl)hexyl)thio)benzo[d]oxazole ([¹¹C]5g), 2-((6-(4-(2-[¹¹C]methoxyphenyl)piperazin-1-yl)hexyl)thio)-5,7-dimethylbenzo[d]oxazole ([¹¹C]5i), and 2-((6-(4-(2-[¹¹C]methoxyphenyl)piperazin-1-yl)hexyl)thio)benzo[d]thiazole ([¹¹C]5k), were prepared from their corresponding phenol precursors with [¹¹C]CH₃OTf through O-[¹¹C]methylation and isolated by a simplified solid-phase extraction (SPE) method using a Sep-Pak Plus C18 cartridge in 50-60% (n=5) radiochemical yields based on [¹¹C]CO₂ and decay corrected to end of bombardment (EOB). The overall synthesis time from EOB was 23 min, the radiochemical purity was >99%, and the specific activity at end of synthesis (EOS) was 277.5±92.5 GBq/μmol (n=5).     

Synthesis and regional rat brain distribution of [11C]MDL 72222: a 5HT3 receptor antagonist

MDL 72222, an antagonist of 5HT₃ receptors, was labeled with a specific radioactivity of 340–400 mCi/μmol by alkylation of the nor-precursor with [¹¹C]CH₃I. The yield of the synthesis, starting from [¹¹C]methyliodide to the purified product and corrected for decay, was good ∼70–75%. After i.v. injection, [¹¹C]MDL 72222 diffuses readily in the central nervous system but is not detected as metabolites in brain and blood, during 1 h study carried out in rats. The time course and distribution of [¹¹C]MDL 72222 was assessed in various organs (liver, lung, kidney, heart, whole brain) and in blood; the organ uptake was rapid and large; the highest accumulation was found in the lung. The regional brain distribution shows initial uptake and subsequent retention of tracer in favor of the cerebral cortex. The level of brain radioactivity was not reduced by pretreatment with a 1000-fold excess of unlabeled MDL 72222. These results suggest that [¹¹C]MDL 72222 is of limited interest for 5HT₃ receptor binding studies in brain in vivo, presumably mainly because of large non-specific binding.     

Synthesis of carbon-11 labeled celecoxib derivatives as new candidate PET radioligands for imaging of inflammation

Cyclooxygenase (prostaglandin endoperoxide synthase or COX) enzyme represents a particularly attractive target in inflammation processes for the development of both therapeutic agents and imaging agents. This study was designed to develop new radioligands for imaging of inflammation using the biomedical imaging technique positron emission tomography (PET). Carbon-11 labeled celecoxib derivatives, [¹¹C]methyl 2-(4-(5-p-tolyl-3-(trifluoromethyl)-1H-pyrazol-1-yl)phenylsulfonamidooxy)acetate ([¹¹C]6e), [¹¹C]methyl 2-methyl-2-(4-(5-p-tolyl-3-(trifluoromethyl)-1H-pyrazol-1-yl)phenylsulfonamidooxy)propanoate ([¹¹C]6f), [¹¹C]methyl 2-(4-(5-(4-methoxyphenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl)phenylsulfonamidooxy)acetate ([¹¹C]6g), and [¹¹C]methyl 2-methyl-2-(4-(5-(4-methoxyphenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl)phenylsulfonamidooxy)propanoate ([¹¹C]6h), were prepared by O-[¹¹C]methylation of their corresponding precursors using [¹¹C]CH₃OTf under basic condition and isolated by a simplified solid-phase extraction (SPE) method in 50–60% radiochemical yields based on [¹¹C]CO₂ and decay corrected to end of bombardment (EOB). The overall synthesis time from EOB was 15–20 min, the radiochemical purity was >99%, and the specific activity at end of synthesis (EOS) was 111–185 GBq/μmol.     

Synthesis of carbon-11-labeled 4-aryl-4H-chromens as new PET agents for imaging of apoptosis in cancer

Carbon-11-labeled 4-aryl-4H-chromenes, 2-amino-7-(dimethylamino)-4-(3-[¹¹C]methoxy-5-methoxyphenyl)-4H-chromene-3-carbonitrile ([¹¹C]6a), 2-amino-4-(3-bromo-4-[¹¹C]methoxy-5-methoxyphenyl)-7-(dimethylamino)-4H-chromene-3-carbonitrile ([¹¹C]6c), 2-amino-4-(3-[¹¹C]methoxy-5-methoxyphenyl)-4,7-dihydropyrano[2,3-e]indole-3-carbonitrile ([¹¹C]6d), 2-amino-4-(3-bromo-4-[¹¹C]methoxy-5-methoxyphenyl)-4,7-dihydropyrano[2,3-e]indole-3-carbonitrile ([¹¹C]6f), 2-amino-4-(3-[¹¹C]methoxy-5-methoxyphenyl)-4,9-dihydropyrano[3,2-g]indole-3-carbonitrile ([¹¹C]6g), 2-amino-4-(3-bromo-4-[¹¹C]methoxy-5-methoxyphenyl)-4,9-dihydropyrano[3,2-g]indole-3-carbonitrile ([¹¹C]6i), 2-amino-4-(3-[¹¹C]methoxy-5-methoxyphenyl)-7-methyl-4,7-dihydropyrano[2,3-e]indole-3-carbonitrile ([¹¹C]6j) and 2-amino-4-(3-bromo-4-[¹¹C]methoxy-5-methoxyphenyl)-7-methyl-4,7-dihydropyrano[2,3-e]indole-3-carbonitrile ([¹¹C]6l), were prepared by O-[¹¹C]methylation of their corresponding precursors using [¹¹C]CH₃OTf under basic conditions and isolated by a simplified solid-phase extraction (SPE) method in 30–50% radiochemical yields based on [¹¹C]CO₂ and decay corrected to end of bombardment (EOB). The overall synthesis time from EOB was 15–20 min, the radiochemical purity was >99%, and the specific activity at end of synthesis (EOS) was 111–185 GBq/μmol.     

Simple and rapid preparation of [C]DASB with high quality and reliability for routine applications

[¹¹C]DASB combines all major prerequisites for a successful SERT-ligand, providing excellent biological properties and in-vivo behaviour. Thus, we aimed to establish a fully automated procedure for the synthesis and purification of [¹¹C]DASB with a high degree of reliability reducing the overall synthesis time while conserving high yields and purity. The optimized [¹¹C]DASB synthesis was applied in more than 60 applications with a very low failure rate (3.2%). We obtained yields up to 8.9 GBq (average 5.3±1.6 GBq). Radiochemical yields based on [¹¹C]CH₃I, (corrected for decay) were 66.3±6.9% with a specific radioactivity (A_s) of 86.8±24.3 GBq/μmol (both at the end of synthesis, EOS). Time consumption was kept to a minimum, resulting in 43 min from end of bombardment to release of the product after quality control.Form our data, it is evident that the presented method can be implemented for routine preparations of [¹¹C]DASB with high reliability.     

Routine synthesis of N-[11C-methyl]scopolamine by phosphite mediated reductive methylation with [11C]formaldehyde

A synthesis of [¹¹C]scopolamine capable of clinical delivery of this agent in high specific activity is described. The precursor [¹¹C]formaldehyde was produced by catalytic oxidation of [¹¹C]CH₃OH over metallic silver and was used to N-¹¹C-methylate norscopolamine using aqueous neutral potassium phosphite as the reducing agent. The labeling reaction was complete after 5 min at 75–80°C and the [¹¹C]scopolamine (99% radiochemical purity) was isolated by preparative HPLC. Total synthesis time is less than 45 min. Decay corrected radiochemical yields from [¹¹C]CO₂ are presently 20–43%.     

Synthesis of carbon-11-labeled piperidine ring of N-[ω-(6-methoxynaphthalen-1-yl)alkyl] derivatives as new selective PET σ1 receptor probes

Carbon-11-labeled piperidine ring of N-[ω-(6-methoxynaphthalen-1-yl)alkyl] derivatives were first designed and synthesized as new selective PET σ₁ receptor probes. The target tracers were prepared by O-[¹¹C]methylation of their corresponding phenolic hydroxyl precursors using [¹¹C]CH₃OTf under basic conditions and isolated by a simplified SPE method in 40–50% radiochemical yields based on [¹¹C]CO₂ and decay corrected to EOB. The overall synthesis time from EOB was 15–20 min, the radiochemical purity was >99%, and the specific activity at EOS was 111–185 GBq/μmol.     

Synthesis of carbon-11-labeled tricyclic necroptosis inhibitors as new potential PET agents for imaging of tumor necrosis factor α (TNF-α)

Carbon-11-labeled tricyclic necroptosis inhibitors were first designed and synthesized as new potential PET agents for imaging of tumor necrosis factor α (TNF-α). The target tracers were prepared by O-[¹¹C]methylation of their corresponding precursors using [¹¹C]CH₃OTf under basic conditions and isolated by a simplified SPE method in 50–60% radiochemical yields based on [¹¹C]CO₂ and decay corrected to end of bombardment (EOB). The overall synthesis time from EOB was 15–20 min, the radiochemical purity was >99%, and the specific activity at end of synthesis (EOS) was 111–185 GBq/μmol.     

Dopamine D1 agonist R-[11C]SKF 82957: synthesis and in vivo characterization in rats

The active enantiomer R-SKF 82957 was labeled with ¹¹C by N-[¹¹C]methylation of the full dopamine (D₁) agonist R-SKF 81297, using [¹¹C]methyl iodide in the presence of N-ethyldiisopropylamine, in high specific activity, radiochemical purity and yields. Compared with the D₁ agonist R/S-[¹¹C]SKF 82957, R-[¹¹C]SKF 82957 showed higher binding in the D₁ rich regions, such as striatum and olfactory tubercles (∼1.7 times), thereby improving the tissue contrast. R-[¹¹C]SKF 82957 exhibited high in vivo binding selectivity for D₁ receptors in rats, because only high doses of D₁ competitors, but not D₂ or serotonin (5-HT₂) blockers, significantly reduced the radioactivity levels in all brain areas. No labeled metabolites were detected in rat brain. These results indicate that R-[¹¹C]SKF 82957 will provide more sensitive measurements of D₁ receptors in in vivo studies than the racemic mixture.     

Dopamine transporter binding in rat striatum: a comparison of [O-methyl-11C]β-CFT and [N-methyl-11C]β-CFT

IntroductionPositron emission tomography scanning with radiolabeled phenyltropane cocaine analogs is important for quantifying the in vivo density of monoamine transporters, including the dopamine transporter (DAT). [¹¹C]β-CFT is useful for studying DAT as a marker of dopaminergic innervation in animal models of psychiatric and neurological disorders. [¹¹C]β-CFT is commonly labeled at the N-methyl position. However, labeling of [¹¹C]β-CFT at the O-methyl position is a simpler procedure and results in a shorter synthesis time [desirable in small-animal studies, where specific activity (SA) is crucial]. In this study, we sought to validate that the O-methylated form of [¹¹C]β-CFT provides equivalent quantitative results to that of the more commonly reported N-methyl form.MethodsFour female Sprague–Dawley rats were scanned twice on the IndyPET II small-animal scanner, once with [N-methyl-¹¹C]β-CFT and once with [O-methyl-¹¹C]β-CFT. DAT binding potentials (BP≡B′_avail/K_d) were estimated for right and left striata with a nonlinear least-squares algorithm, using a reference region (cerebellum) as the input function.Results[N-Methyl-¹¹C]β-CFT and [O-methyl-¹¹C]β-CFT were synthesized with 40–50% radiochemical yields (HPLC purification). Radiochemical purity was >99%. SA at end of bombardment was 258±30 GBq/μmol. Average BP values for right and left striata with [N-methyl-¹¹C]β-CFT were 1.16±0.08 and 1.23±0.14, respectively. BP values for [O-methyl-¹¹C]β-CFT were 1.18±0.08 (right) and 1.22±0.16 (left). Paired t tests demonstrated that labeling position did not affect striatal DAT BP.ConclusionsThese results suggest that [O-methyl-¹¹C]β-CFT is quantitatively equivalent to [N-methyl-¹¹C]β-CFT in the rat striatum.     

Test-retest reproducibility of dopamine D2/3 receptor binding in human brain measured by PET with [11C]MNPA and [11C]raclopride

Purpose

Dopamine D_2/3 receptors (D_2/3Rs) have two affinity states for endogenous dopamine, referred to as high-affinity state (D_2/3 ^HIGH), which has a high affinity for endogenous dopamine, and low-affinity state (D_2/3 ^LOW). The density of D_2/3 ^HIGH can be measured with (R)-2-¹¹CH₃O-N-n-propylnorapomorphine ([¹¹C]MNPA), while total density of D_2/3 ^HIGH and D_2/3 ^LOW (D_2/3Rs) can be measured with [¹¹C]raclopride using positron emission tomography (PET). Thus, the ratio of the binding potential (BP) of [¹¹C]MNPA to that of [¹¹C]raclopride ([¹¹C]MNPA/[¹¹C]raclopride) may reflect the proportion of the density of D_2/3 ^HIGH to that of D_2/3Rs. In the caudate and putamen, [¹¹C]MNPA/[¹¹C]raclopride reflects the proportion of the density of D₂ ^HIGH to that of D₂Rs. To evaluate the reliability of the PET paradigm with [¹¹C]MNPA and [¹¹C]raclopride, we investigated the test-retest reproducibility of non-displaceable BP (BP _ND) measured with [¹¹C]MNPA and of [¹¹C]MNPA/[¹¹C]raclopride in healthy humans.

Methods

Eleven healthy male volunteers underwent two sets of PET studies on separate days that each included [¹¹C]MNPA and [¹¹C]raclopride scans. BP _ND values in the caudate and putamen were calculated. Test-retest reproducibility of BP _ND of [¹¹C]MNPA and [¹¹C]MNPA/[¹¹C]raclopride was assessed by intra-subject variability (absolute variability) and test-retest reliability (intraclass correlation coefficient: ICC).

Results

The absolute variability of [¹¹C]MNPA BP _ND was 5.30?±?3.96 % and 12.3?±?7.95 % and the ICC values of [¹¹C]MNPA BP _ND were 0.72 and 0.82 in the caudate and putamen, respectively. The absolute variability of [¹¹C]MNPA/[¹¹C]raclopride was 6.11?±?3.68 % and 11.60?±?5.70 % and the ICC values of [¹¹C]MNPA/[¹¹C]raclopride were 0.79 and 0.80 in the caudate and putamen, respectively.

Conclusion

In the present preliminary study, the test-retest reproducibility of BP _ND of [¹¹C]MNPA and of [¹¹C]MNPA/[¹¹C]raclopride was reliable in the caudate and putamen.     

Radiosynthesis and in vivo evaluation of [11C]-labelled pyrrole-2-carboxamide derivates as novel radioligands for PET imaging of monoamine oxidase A

IntroductionSince MAO-A is an enzyme involved in the metabolism of neurotransmitters, fluctuations in MAO-A functionality are associated with psychiatric and neurological disorders as well as with tobacco addiction and behaviour. This study reports the radiolabelling of two [¹¹C]-labelled pyrrole-2-carboxamide derivates, RS 2315 and RS 2360, along with the characterization of their in vivo properties.MethodsThe radiolabelling of [¹¹C]-RS 2315 and [¹¹C]-RS 2360 was accomplished by alkylation of their amide precursors with [¹¹C]CH₃I. Biodistribution, blocking and metabolite studies of both tracers were performed in NMRI mice. Finally, a PET study in Sprague-Dawley rats was performed for [¹¹C]-RS 2360.ResultsBoth tracers were obtained in a radiochemical yield of approximately 30% with radiochemical purity of >98%. Biodistribution studies showed high brain uptake followed by rapid brain clearance for both radiotracers. In the brain, [¹¹C]-RS 2360 was more stable than [¹¹C]-RS 2315. Blocking studies in mice could not demonstrate specificity of [¹¹C]-RS 2315 towards MAO-A or MAO-B. The blocking and imaging study with [¹¹C]-RS 2360 on the other hand indicated specific binding in MAO-A at the earliest time points.Conclusions[¹¹C]-RS 2315 displayed a high nonspecific binding and is therefore not suitable for visualization of MAO-A in vivo. [¹¹C]-RS 2360 on the other hand has potential for mapping MAO-A since specific binding is demonstrated.     

In-target produced [11C]methane: Increased specific radioactivity

The ¹⁴N(ρ, α)¹¹C reaction on N₂–O₂ or N₂–H₂ gaseous systems as targets in proton bombardment allows for the production of [¹¹C]CO₂ and [¹¹C]CH_4. We report the target production of [¹¹C]CH₄ and the gas phase iodination to produce [¹¹C]CH₃I with high specific radioactivity (SA). SA was calculated for four different radiopharmaceuticals produced in-house from both target produced [¹¹C]CO₂ and [¹¹C]CH_4. For [¹¹C]raclopride we obtained an average SA of 3908 GBq/μmol (106 000 Ci/mmol) at the end of bombardment for the last 52 productions, which is a 32-fold increase compared to when using the in-house [¹¹C]CO₂ target.     

Synthesis and evaluation of [11C]XR9576 to assess the function of drug efflux transporters using PET

Objective

XR9576 (tariquidar) is an anthranilic acid derivative and potent P-glycoprotein (P-gp) inhibitor. XR9576 has undergone phase I and II studies as combined chemotherapy against cancer. XR9576 has been developed as a useful therapeutic agent but not as a PET probe. We therefore developed [¹¹C]XR9576 as a PET probe and assessed whether PET studies using [¹¹C]XR9576 are a promising approach to assess P-gp function primarily.

Methods

We synthesized [¹¹C]XR9576 by methylation of 7-O-desmethyl XR9576 with [¹¹C]methyl iodide. In in vivo tissue distribution, the effects of co-injection with XR9576 on the uptake of [¹¹C]XR9576 in mice were investigated. PET studies using [¹¹C]XR9576 were performed in P-gp and/or Bcrp knockout mice as well as in wild-type mice. Metabolites of [¹¹C]XR9576 were measured in the brain and plasma of mice.

Results

[¹¹C]XR9576 was successfully synthesized with suitable radioactivity for injection as well as appropriate radiochemical purity and stability. In in vivo tissue distribution, the brain uptake of [¹¹C]XR9576 significantly increased about tenfold of control on co-injection with >10 mg/kg of XR9576. In PET studies, the AUC_{brain [0–60 min]} in P-gp and P-gp/Bcrp knockout mice was 2- and 11-fold higher than that in wild-type mice. [¹¹C]XR9576 showed a high metabolic stability (>90% unchanged form) in the brain and plasma of mice 30 min after injection. These results suggest that a tracer amount of [¹¹C]XR9576 behave as the P-gp and Bcrp substrate, and the increased brain uptake or AUC_brain of [¹¹C]XR9576 correlates with P-gp and Bcrp functions.

Conclusions

PET studies using [¹¹C]XR9576 may be a promising approach for evaluating deficiency of the function of drug efflux transporters targeting intracranial diseases and tumors.     

Validation of two fluoro-analogues of N,N-dimethyl-2-(2′-amino-4′-hydroxymethyl-phenylthio)benzylamine as serotonin transporter imaging agents using microPET

IntroductionCarbon-11 (C-11) N,N-dimethyl-2-(2′-amino-4′-hydroxymethyl-phenylthio)benzylamine ([¹¹C]HOMADAM) has been reported as highly specific and selective positron emission tomography (PET) radiotracer showing fast kinetics for the human brain serotonin transporter (SERT). In our continued effort to develop appropriate PET SERT radioligand that can be labeled with either C-11 or fluorine-18 (F-18), two new C-11 labeled analogues of HOMADAM, [¹¹C]-N,N-dimethyl-2-(2′-amino-5′-fluoro-4′-hydroxymethyl-phenylthio)benzylamine ([¹¹C]-(2)) and [¹¹C]-N,N-dimethyl-2-(2′-amino-4-fluoro-4′-hydroxymethyl-phenylthio)benzylamine ([¹¹C]-(3)) have been synthesized and evaluated along the previously reported [¹¹C]-N,N-dimethyl-2-(2′-amino-5-fluoro-4′-hydroxymethyl-phenylthio)benzylamine ([¹¹C]-(1)).MethodsThe in vitro competitive binding assays were performed in cells transfected with human SERT (hSERT), human dopamine transporter (hDAT), and human norepinephrine transporter (hNET). [¹¹C]-(2) and [¹¹C]-(3) were prepared by methylation of their monomethylbenzylamine precursors 13 and 22 with cyclotron produced [¹¹C]iodomethane ([¹¹C]CH₃I), respectively. Uptake and kinetics of [¹¹C]-(2) and [¹¹C]-(3) in the brain regions of interest were determined in anesthetized rhesus monkeys using Concorde microPET P4.Results2 and 3 displayed moderate and high affinity for the SERT with Kis (SERT) = 5.45 and 1.10 nM (vs [³H]citalopram), respectively. After High Performance Liquid Chromatography (HPLC) purification, [¹¹C]-(2) and [¹¹C]-(3) were obtained in 23 and 9% radiochemical yield (RCY) and log Ps_7.4 of 1.77 and 1.91, respectively. The microPET images of [¹¹C]-(2) and [¹¹C]-(3) showed clear localization in the monkey brain regions rich in SERT with midbrain to cerebellum ratios of 1.75 and 3.86 at 85 min post injection, respectively, comparing to 3.40 for [¹¹C]-(1), at the same time. [¹¹C]-(3) was selected for further examination and showed to be specific to the SERT as displacement with citalopram (a potent SERT ligand) reduced radioactivity in SERT rich regions, such as midbrain, to the cerebellum level.ConclusionsCompound 2, the 5′-fluoro-analogue of HOMADAM, had the lowest brain uptake and target to non-target ratios. Compound 3, the 4-fluoro-analogue of HOMADAM, had good brain uptake and higher midbrain and thalamus to cerebellum ratios than compound 1, the 5-fluoro-analogue of HOMADAM. Although 1 and 3 presented better imaging properties than 2, none of the three candidates was suitable to surpass the binding or distributional qualities of the parent HOMADAM. Alternative fluoro-analogues of HOMADAM will soon be characterized, in future work, as SERT radioimaging agents.     

11C-labelling of the analgesic tramadol and its major metabolites by selective O- and N-methylation

For in vivo pharmacokinetic studies with PET, the analgesic Tramadol(1-(3-methoxyphenyl)-2-dimethylaminomethyl-cyclohexan-1-ol) and its major O- and N-desmethylated metabolites M1 and M2 were labelled with carbon-11. Starting with the corresponding desmethyl precursors, (1R, 2R)-(+), (1S, 2S)-(−)-[O-methyl-¹¹C]Tramadol and racemic-[N-methyl-¹¹C]Tramadol were prepared by methylation with n.c.a. [¹¹C]methyl iodide in DMSO with radiochemical yields of 85 and 90%, respectively. Specific n.c.a. N-methylation of bis-desmethyl-Tramadol (M5) was achieved without base obtaining ¹¹C-labelled (1R, 2R)-(+)- and (1S, 2S)-(−)-M1 with 90% radiochemical yield. However, a selective O-methylation of (+)- and (−)-M5 was not possible even with an excess of NaOH, and only 70% of (1R, 2R)-(+)- and (1S, 2S)-(−)-[O-methyl-¹¹C]M2 was obtained. Quaternization of Tramadol or M1 was > 15 times slower than O-methylation, and was only observed in the presence of added CH₃I carrier.     

Synthesis of DL-[3-11C]valine using [2-11C]isopropyl iodide,and preparation of L-[3-11C]valine by treatment with D-amino acid oxidase

DL-[3-¹¹C]Valine, synthesized by phase-transfer alkylation of N-(diphenylmethylene)glycine t-butyl ester with [2-¹¹C]isopropyl iodide, followed by acidic hydrolysis, was obtained in 20–30% radiochemical yield (decay corrected and calculated on the amount of [¹¹C]carbon dioxide used) and with 93–99% radiochemical purity with a total synthesis time of 50 min. Following treatment with immobilized D-amino acid oxidase, L-[3-¹¹C]valine was obtained in 90–99% enantiomeric excess with a total synthesis time of 85 min. [2-¹¹C]Isopropyl iodide was obtained in 40 and 90% radiochemical yield and purity respectively, within 12 min calculated from [¹¹C]carbon dioxide. In a typical experiment starting with 150 mCi of [¹¹C]carbon dioxide, 7 mCi of DL-[3-¹¹C]valine and 0.8 mCi of L-[3-¹¹C]valine were obtained.     

Synthesis and evaluation of PET probes for the imaging of I2 imidazoline receptors in peripheral tissues

IntroductionTo explore the possible use of positron emission tomography (PET) probes for imaging of I₂-imidazoline receptors (I₂Rs) in peripheral tissues, we labeled two new I₂R ligands, 2-[2-(o-tolyl)vinyl]-4,5-dihydro-1H-imidazole (K_i for I₂Rs, 3.7 nM) and 2-[2-(o-tolyl)ethyl]-4,5-dihydro-1H-imidazole (K_i for I₂Rs, 1.7 nM) with ¹¹C ([¹¹C]metrazoline and [¹¹C]TEIMD), respectively, and evaluated these ligands and the recently developed I₂R ligand 2-[3-fluoro-[4-¹¹C]tolyl]-4,5-dihydro-1H-imidazole ([¹¹C]FTIMD) by in vivo studies.Methods[¹¹C]Metrazoline and [¹¹C]TEIMD were prepared by a palladium-promoted cross-coupling reaction of the tributylstannyl precursor and [¹¹C]methyl iodide. Their biodistribution in mice was investigated by tissue dissection. In addition, PET scans and metabolite analysis were performed.Results[¹¹C]Metrazoline and [¹¹C]TEIMD were successfully synthesized with a suitable radioactivity for injection. In the liver and pancreas expressing I₂Rs, coinjection with the high-affinity I₂R ligand, BU224, induced a reduction in the radioactivity level at 30 min after injection of [¹¹C]metrazoline and [¹¹C]FTIMD. However, the radioactivity level after injection of [¹¹C]TEIMD was unchanged. In the PET study, coinjection with BU224 induced a decrease in the radioactivity level in the liver and pancreas after more than 15 min of injection of [¹¹C]metrazoline and [¹¹C]FTIMD as compared with the results obtained for controls. In metabolite analysis, coinjection with BU224 induced a significant reduction in the percentage of unchanged [¹¹C]metrazoline at 30 min after injection as compared with that in the control, although no significant difference was observed in the percentage of unchanged [¹¹C]FTIMD.Conclusion[¹¹C]Metrazoline may be a more useful PET probe than [¹¹C]FTIMD for imaging of I₂Rs in peripheral tissues.     

Imaging of peripheral-type benzodiazepine receptor in tumor: in vitro binding and in vivo biodistribution of N-benzyl-N-[C]methyl-2-(7-methyl-8-oxo-2-phenyl-7,8-dihydro-9H-purin-9-yl)acetamide

IntroductionThe aim of this study was to evaluate N-benzyl-N-[¹¹C]methyl-2-(7-methyl-8-oxo-2-phenyl-7,8-dihydro-9H-purin-9-yl)acetamide ([¹¹C]DAC) as a novel peripheral-type benzodiazepine receptor (PBR) ligand for tumor imaging.Methods[¹¹C]DAC was synthesized by the reaction of a desmethyl precursor with [¹¹C]CH₃I. In vitro uptake of [¹¹C]DAC was examined in PBR-expressing C6 glioma and intact murine fibrosarcoma (NFSa) cells. In vivo distribution of [¹¹C]DAC was determined using NFSa-bearing mice and small-animal positron emission tomography (PET).Results[¹¹C]DAC showed specific binding to PBR in C6 glioma cells, a standard cell line with high PBR expression. Specific binding of [¹¹C]DAC was also confirmed in NFSa cells, a target tumor cell line in this study. Results of PET experiments using NFSa-bearing mice, showed that [¹¹C]DAC was taken up specifically into the tumor, and pretreatment with PK11195 abolished the uptake.Conclusions[¹¹C]DAC was taken up into PBR-expressing NFSa. [¹¹C]DAC is a promising PET ligand that can be used for imaging PBR in tumor-bearing mice.