11C-labeled Tracers Research Articles

11C]CO2 BOP fixation with amines to access 11C-labeled ureas for PET imaging.

Carbon-11 (11C) is a widely used radionuclide for positron emission tomography (PET) owing to the omnipresence of carbon atoms in organic molecules. While its half-life of 20.4 min is ideal for imaging and dosimetry, it also limits the synthetic possibilities. As such, the development of fast and easy, high-yielding synthesis methods is crucial for the application of 11C-labeled tracers in humans. In this study, we present a novel and efficient method for the reaction of [11C]CO2 with amine precursors using benzotriazole-1-yl-oxy-tris-(dimethylamino)-phosphonium hexafluorophosphate (BOP) to access 11C-labeled ureas. Our method is extremely fast as it only requires transfer of [11C]CO2 into a solution with precursor and BOP at room temperature, where it reacts momentary into the desired 11C-labeled urea. This simple procedure makes it possible to radiolabel urea directly from [11C]CO2 without the need for advanced equipment, making the method applicable for all laboratories where [11C]CO2 is available. We synthesized a small series of aliphatic symmetrical and non-symmetrical 11C-labeled ureas using this method, and achieved good to excellent yields. The novelty of our study lies in the fact that peptide coupling reagent BOP is used for the first time in radiochemistry to activate [11C]CO2, facilitating its reaction with amines to obtain 11C-labeled ureas.

Impact of TOF on Brain PET With Short-Lived 11C-Labeled Tracers Among Suspected Patients With AD/PD: Using Hybrid PET/MRI.

ObjectiveTo explore the impact of the time-of-flight (TOF) reconstruction on brain PET with short-lived 11C-labeled tracers in PET magnetic resonance (PET/MR) brain images among suspected patients with Alzheimer's and Parkinson's disease (AD/PD).MethodsPatients who underwent 11C-2-ß-carbomethoxy-3-b-(4-fluorophenyl) tropane (11C-CFT) and 2-(4-N-[11C] methylaminophenyl)-6-hydroxybenzothiazole (11C-PiB) PET/MRI were retrospectively included in the study. Each PET LIST mode data were reconstructed with and without the TOF reconstruction algorithm. Standard uptake values (SUVs) of Caudate Nucleus (CN), Putamen (PU), and Whole-brain (WB) were measured. TOF and non-TOF SUVs were assessed by using paired t-test. Standard formulas were applied to measure contrast, signal-to-noise ratio (SNR), and percentage relative average difference of SUVs (%RAD-SUVs).ResultsTotal 75 patients were included with the median age (years) and body mass index (BMI-kg/m2) of 60.2 ± 10.9 years and 23.9 ± 3.7 kg/m2 in 11C-CFT (n = 41) and 62.2 ± 6.8 years and 24.7 ± 2.9 kg/m2 in 11C-PiB (n = 34), respectively. Higher average SUVs and positive %RAD-SUVs were observed in CN and PU in TOF compared with non-TOF reconstructions for the two 11C-labeled radiotracers. Differences of SUVmean were significant (p < 0.05) in CN and PU for both 11C-labeled radiotracers. SUVmax was enhanced significantly in CN and PU for 11C-CFT and CN for 11C-PiB, but not in PU. Significant contrast enhancement was observed in PU for both 11C-labeled radiotracers, whereas SNR gain was significant in PU, only for 11C-PiB in TOF reconstruction.ConclusionTime-of-flight leads to a better signal vs. noise trade-off than non-TOF in 11C-labeled tracers between CN and PU, improving the SUVs, contrast, and SNR, which were valuable for reducing injected radiation dose. Improved timing resolution aided the rapid decay rate of short-lived 11C-labeled tracers, and it shortened the scan time, increasing the patient comfort, and reducing the motion artifact among patients with AD/PD. However, one should adopt the combined TOF algorithm with caution for the quantitative analysis because it has different effects on the SUVmax, contrast, and SNR of different brain regions.

PET imaging of brain aromatase in humans and rhesus monkeys by 11C-labeled cetrozole analogs

Aromatase is an estrogen synthetic enzyme that plays important roles in brain functions. To quantify aromatase expression in the brain by positron emission tomography (PET), we had previously developed [11C]cetrozole, which showed high specificity and affinity. To develop more efficient PET tracer(s) for aromatase imaging, we synthesized three analogs of cetrozole. We synthesized meta-cetrozole, nitro-cetrozole, and iso-cetrozole, and prepared the corresponding 11C-labeled tracers. The inhibitory activities of these three analogs toward aromatase were evaluated using marmoset placenta, and PET imaging of brain aromatase was performed using the 11C-labeled tracers in monkeys. The most promising analog in the monkey study, iso-cetrozole, was evaluated in the human PET study. The highest to lowest inhibitory activity of the analogs toward aromatase in the microsomal fraction from marmoset placenta was in the following order: iso-cetrozole, nitro-cetrozole, cetrozole, and meta-cetrozole. This order showed good agreement with the order of the binding potential (BP) of each 11C-labeled analog to aromatase in the rhesus monkey brain. A human PET study using [11C]iso-analog showed a similar distribution pattern of binding as that of [11C]cetrozole. The time–activity curves showed that elimination of [11C]iso-cetrozole from brain tissue was faster than that of 11C-cetrozole, indicating more rapid metabolism of [11C]iso-cetrozole. [11C]Cetrozole has preferable metabolic stability for brain aromatase imaging in humans, although [11C]iso-cetrozole might also be useful to measure aromatase level in living human brain because of its high binding potential.

Human Biodistribution and Radiation Dosimetry of the P-Glycoprotein Radiotracer [11C]Metoclopramide

PurposeTo assess in healthy volunteers the whole-body distribution and dosimetry of [11C]metoclopramide, a new positron emission tomography (PET) tracer to measure P-glycoprotein activity at the blood-brain barrier.ProceduresTen healthy volunteers (five women, five men) were intravenously injected with 387 ± 49 MBq of [11C]metoclopramide after low dose CT scans and were then imaged by whole-body PET scans from head to upper thigh over approximately 70 min. Ten source organs (brain, thyroid gland, right lung, myocardium, liver, gall bladder, left kidney, red bone marrow, muscle and the contents of the urinary bladder) were manually delineated on whole-body images. Absorbed doses were calculated with QDOSE (ABX-CRO) using the integrated IDAC-Dose 2.1 module.ResultsThe majority of the administered dose of [11C]metoclopramide was taken up into the liver followed by urinary excretion and, to a smaller extent, biliary excretion of radioactivity. The mean effective dose of [11C]metoclopramide was 1.69 ± 0.26 μSv/MBq for female subjects and 1.55 ± 0.07 μSv/MBq for male subjects. The two organs receiving the highest radiation doses were the urinary bladder (10.81 ± 0.23 μGy/MBq and 8.78 ± 0.89 μGy/MBq) and the liver (6.80 ± 0.78 μGy/MBq and 4.91 ± 0.74 μGy/MBq) for female and male subjects, respectively.Conclusions[11C]Metoclopramide showed predominantly renal excretion, and is safe and well tolerated in healthy adults. The effective dose of [11C]metoclopramide was comparable to other 11C-labeled PET tracers.

Development of a non-radiometric method for measuring the arterial input function of a 11C-labeled PET radiotracer

Positron emission tomography (PET) uses radiotracers to quantify important biochemical parameters in human subjects. A radiotracer arterial input function (AIF) is often essential for converting brain PET data into robust output measures. For radiotracers labeled with carbon-11 (t1/2 = 20.4 min), AIF is routinely determined with radio-HPLC of blood sampled frequently during the PET experiment. There has been no alternative to this logistically demanding method, neither for regular use nor validation. A 11C-labeled tracer is always accompanied by a large excess of non-radioactive tracer known as carrier. In principle, AIF might be obtained by measuring the molar activity (Am; ratio of radioactivity to total mass; Bq/mol) of a radiotracer dose and the time-course of carrier concentration in plasma after radiotracer injection. Here, we implement this principle in a new method for determining AIF, as shown by using [11C]PBR28 as a representative tracer. The method uses liquid chromatography-tandem mass spectrometry for measuring radiotracer Am and then the carrier in plasma sampled regularly over the course of a PET experiment. Am and AIF were determined radiometrically for comparison. The new non-radiometric method is not constrained by the short half-life of carbon-11 and is an attractive alternative to conventional AIF measurement.

Synthesis of (R,S)-isoproterenol, an inhibitor of tau aggregation, as an 11C-labeled PET tracer via reductive alkylation of (R,S)-norepinephrine with [2-11C]acetone.

(R,S)-Isoproterenol inhibits the formation of toxic granular tau oligomers associated with neuronal loss and development of cognitive disorders, and is an attractive drug candidate for Alzheimer's disease. To elucidate its behavior in the brain by positron emission tomography, we synthesize (R,S)-[11C]isoproterenol by reductive alkylation of (R,S)-norepinephrine with [2-11C]acetone, which was in turn synthesized in situ under improved conditions afforded a decay-corrected radiochemical yield of 54%. The reductive alkylation using NaBH(OAc)3 as reducing agent in the presence of benzoic acid in DMSO/DMF (60:40 v/v) at 100 °C for 10 min gave (R,S)-[11C]isoproterenol in an 87% radio-high performance liquid chromatography (HPLC) analytical yield. HPLC separation using a strong cation exchange column, followed by pharmaceutical formulation in the presence of d/l-tartaric acid, afforded (R,S)-[11C]isoproterenol with a total radioactivity of 2.0 ± 0.2 GBq, a decay-corrected radiochemical yield of 19 ± 2%, chemical and radiochemical purities of 71% and >99%, respectively, and a molar activity of 100 ± 13 GBq/μmol (n = 3). The overall synthesis time from the end of the bombardment to pharmaceutical formulation was 48 min. A preliminary preclinical PET study in a rat demonstrated the potential of the radioligand for the evaluation of the penetration of (R,S)-isoproterenol in human brain.

Fluorine-18 (18F)-labeled retinoid x receptor (RXR) partial agonist whose tissue transferability is affected by other RXR ligands.

Bexarotene (1), a retinoid X receptor (RXR) agonist approved for the treatment of cutaneous T cell lymphoma (CTCL), was reported to migrate into baboon brain based on findings obtained by positron emission tomography (PET) with a 11C-labeled tracer. However, co-administration of non-radioactive 1 had no effect on the distribution of [11C]1, probably due to non-specific binding of 1 as a result of its high lipophilicity. Here, we report a fluorine-18 (18F)-labeled PET tracer [18F]6 derived from RXR partial agonist CBt-PMN (2), which has lower lipophilicity and weaker RXR-binding ability than [11C]1. The concomitant administration of 1 or 2 with [18F]6 with resulted in decreased accumulation of [18F]6 in liver, together with increased brain uptake and increased accumulation in kidney and muscle, as visualized by PET. A plausible explanation of these findings is the inhibition of [18F]6 uptake into the liver by concomitantly administered 1 or 2, leading to an increase in blood concentration of [18F]6 followed by increased accumulation in other tissues.

Synthesis and biodistribution of novel dansyl derivative 11C-DSB

A novel 11C-labeled dansyl 1, 2-diphenylethene derivative for positron emission tomography (PET) imaging of β-amyloid (Aβ) plaques and apoptosis in the brain of Alzheimer’s disease (AD) was designed and synthesized. The 11C-labeled tracer, (E)-5-(dimethylamino)-N-(4-(4-11C-methoxystyryl)phenyl)-naphthalene-1-sulfonamide (11C-DSB), was synthesized. The decay-corrected radiochemistry yield was 20 ± 5% (n = 5) from 11C–CO2 within 20 min and the specific activity of 1.6 GBq/µmol. Moreover, in vivo biodistribution and preliminary PET imaging of normal mice were performed to show that 11C-DSB had moderate brain penetration and washout. But11C-DSB has high log P value, showing that its structure needs to be further improved to increase its water solubility. The results provide the basis for further research on PET and fluorescence dual-modality imaging of AD models.

New horizons in cardiac innervation imaging: introduction of novel 18F-labeled PET tracers.

Cardiac sympathetic nervous activity can be uniquely visualized by non-invasive radionuclide imaging techniques due to the fast growing and widespread application of nuclear cardiology in the last few years. The norepinephrine analogue 123I-meta-iodobenzylguanidine (123I-MIBG) is a single photon emission computed tomography (SPECT) tracer for the clinical implementation of sympathetic nervous imaging for both diagnosis and prognosis of heart failure. Meanwhile, positron emission tomography (PET) imaging has become increasingly attractive because of its higher spatial and temporal resolution compared to SPECT, which allows regional functional and dynamic kinetic analysis. Nevertheless, wider use of cardiac sympathetic nervous PET imaging is still limited mainly due to the demand of costly on-site cyclotrons, which are required for the production of conventional 11C-labeled (radiological half-life, 20min) PET tracers. Most recently, more promising 18F-labeled (half-life, 110min) PET radiopharmaceuticals targeting sympathetic nervous system have been introduced. These tracers optimize PET imaging and, by using delivery networks, cost less to produce. In this article, the latest advances of sympathetic nervous imaging using 18F-labeled radiotracers along with their possible applications are reviewed.

Automated C-11 Methyl Iodide/Triflate Production: Current State of the Art

[11C]methyl iodide was originally produced by using LiAlH4 to reduce [11C]CO2 to [11C]methanol, followed by reaction with liquid hydriodic acid. However, it is difficult to obtain the desired high specific radioactivity of the end-product 11C-methylated tracer because the lithium reagent solution is so easily contaminated by atmospheric [12C]CO2. The introduction of high temperature gas-phase methodologies greatly enhanced the specific activity of 11C-labeled PET tracers by eliminating the lithium reagent entirely. Details for three commercially available gas-phase synthesis modules are provided and discussed with respect to ease-of-operation and preventative maintenance concerns. Lastly, C-11 target performance is discussed with respect to the in-target release of carrier [12C]carbon, the remaining obstacle to ultra-high specific activity for 11C-labeled PET tracers. Keywords: Automation, Gas-phase production, Methyl iodide, Methyl triflate, Specific activity.

Characterization of fast-decaying PET radiotracers solely through LC-MS/MS of constituent radioactive and carrier isotopologues

BackgroundThe characterization of fast-decaying radiotracers that are labeled with carbon-11 (t1/2 = 20.38 min), including critical measurement of specific radioactivity (activity per mole at a specific time) before release for use in positron-emission tomography (PET), has relied heavily on chromatographic plus radiometric measurements, each of which may be vulnerable to significant errors. Thus, we aimed to develop a mass-specific detection method using sensitive liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) for identifying 11C-labeled tracers and for verifying their specific radioactivities.MethodsThe LC-MS/MS was tuned and set up with methods to generate and measure the product ions specific for carbon-11 species and M + 1 carrier (predominantly the carbon-13 isotopologue) in four 11C-labeled tracers. These radiotracers were synthesized and then analyzed before extensive carbon-11 decay. The peak areas of carbon-11 species and M + 1 carrier from the LC-MS/MS measurement and the calculated abundances of carbon-12 carrier and M + 1 radioactive species gave the mole fraction of carbon-11 species in each sample. This value upon multiplication with the theoretical specific radioactivity of carbon-11 gave the specific radioactivity of the radiotracer.ResultsLC-MS/MS of each 11C-labeled tracer generated the product ion peaks for carbon-11 species and M + 1 carrier at the expected LC retention time. The intensity of the radioactive peak diminished as time elapsed and was undetectable after six half-lives of carbon-11. Measurements of radiotracer-specific radioactivity determined solely by LC-MS/MS at timed intervals gave a half-life for carbon-11 (20.43 min) in excellent agreement with the value obtained radiometrically. Additionally, the LC-MS/MS measurement gave specific radioactivity values (83 to 505 GBq/μmol) in good agreement with those from conventional radiometric methods.Conclusions11C-Labeled tracers were characterized at a fundamental level involving isolation and mass detection of extremely low-abundance carbon-11 species along with the M + 1 carrier counterpart. This LC-MS/MS method for characterizing fast-decaying radiotracers is valuable in both the development and production of PET radiopharmaceuticals.

From Alois to Amyvid

When Alois Alzheimer first described the neuropathologic hallmarks of his eponymous disease (Alzheimer disease [AD]) in 1906,1 the possibility of detecting amyloid plaques in vivo was unimaginable. About a century later, this became a reality when Klunk and colleagues2 demonstrated that the 11C-labeled PET tracer Pittsburgh compound B (PiB) selectively bound to fibrillar β-amyloid (Aβ) and revealed amyloid pathology noninvasively in subjects with AD. However, due to its short half-life of 20 minutes and difficult manufacturing process, 11C-PiB had no prospect for widespread use. 18F-labeled Aβ ligands are easier …

Suggested pathway to assess radiation safety of 11C-labeled PET tracers for first-in-human studies

Suggested pathway to assess radiation safety of 11C-labeled PET tracers for first-in-human studies

Synthesis of an acromelic acid A analog-based 11C-labeled PET tracer for exploration of the site of action of acromelic acid A in allodynia induction

A novel (11)C-labeled PET (positron emission tomography) tracer, which was designed based on the (phenylthio)pyrrolidine derivative that can competitively block the acromelic acid A-induced allodynia, was synthesized. A protocol in which methylation by palladium-mediated coupling of the boronate derivative with [(11)C]CH(3)I and deprotection of the protected amino acid moiety are successively performed in one-pot within 5 min was established for the synthesis of the tracer. The tracer is potentially useful as a tool to investigate the mode of action of acromelic acid A in the induction of allodynia.

Improving PET Receptor Binding Estimates from Logan Plots Using Principal Component Analysis

This work reports a principal component analysis (PCA)-based approach for reducing bias in distribution volume ratio (DVR) estimates from Logan plots in positron emission tomography (PET). Comparison has been made of all existing bias-removal methods with the proposed PCA method, for both single-estimate PET studies and intervention studies where pre- and post-intervention estimates are made. Bias in Logan-based DVR estimates is because of the noise in the PET time-activity curves (TACs) that propagates as correlated errors in dependent and independent variables of the Logan equation. Intervention studies show this same bias but also higher variance in DVR estimates. In this work, noise in the TACs was reduced by fitting the curves to a low-dimension PCA-based linear model, leading to reduced bias and variance in DVR. For validating the approach, TACs with realistic noise were simulated for a 11C-labeled tracer with carfentanil (CFN)-like kinetics for both single-measurement and intervention studies. Principal component analysis and existing methods were applied to the simulated data and their performance was compared by statistical analysis. The results indicated that existing methods either removed only part of the bias or reduced bias at the expense of precision. The proposed method removed approximately 90% of the bias while also improving precision in both single- and dual-measurement simulations. After validation of the proposed method in simulations, PCA, along with the existing methods, was applied to human [11C]CFN data acquired for both single estimation of DVR and dual-estimation intervention studies. Similar results were observed in human scans as were seen in the simulation studies.

Radical Carbonylation with [11C]Carbon Monoxide Promoted by Oxygen-Centered Radicals: Experimental and DFT Studies of the Mechanism

Photoinitiated carbonylation of alkyl iodides with [11C]carbon monoxide (11C t1/2=20.4 min) is enhanced by ketones that have lowest-lying excited triplet state of npi* character. For example, adding 5 mol % of acetophenone increases radiochemical yields from 3 to 59% in brief 6-min long reactions. Similar or higher yields were achieved by adding di-tert-butyl peroxide. Since radicaloid npi* exited-state ketones and tert-butoxyl radicals have similar reactivity, the photosensitization proceeds most likely via a H-atom transfer mechanism rather than via energy transfer. We propose a mechanism that can account for the enhancement as well as for the formation of observed byproducts. The energy profile obtained by DFT calculations support the feasibility of the mechanism, and observed experimental differences in reactivity could be well rationalized by the calculated data. NBO calculations were performed to further analyze the obtained energetics. Various [carbonyl-11C]esters and some [carbonyl-11C]amides were synthesized in good radiochemical yields from primary and secondary alkyl iodides illustrating the utility of dialkyl peroxides to accelerate the carbonylations. These findings have potential in elaborating new synthetic protocols for the production of 11C-labeled tracers for positron emission tomography.

Synthesis and evaluation of vesamicol analog (-)-o-[11C]methylvesamicol as a PET ligand for vesicular acetylcholine transporter

(-)-O-Methylvesamicol ((-)-OMV) exhibited in vitro a high affinity for vesicular acetylcholine transporter (VAChT) (Ki, 6.7 nM) and a relatively low affinity for sigmal receptor (Ki, 33.7 nM). We prepared (-)-[11C]OMV by a palladium-promoted cross-coupling reaction using [11C]methyl iodide, in a radiochemical yield of 38 +/- 6.9% (n=3), a radiochemical purity of 98 +/- 2.3% (n = 5), and a specific activity of 11 +/- 7.0 TBq/mmol at 30 minutes after EOB (n=5). Then, we evaluated in vivo whether (-)-[11C]OMV has properties as a PET radioligand for mapping VAChT. In rats, the brain uptake of (-)-[11C]OMV was 1.1%ID/g at 5 minutes postinjection, and was retained of a high level for 60 minutes. The brain uptake was significantly inhibited by the co-injection (500 nmol/kg) of cold (-)-OMV (58-66%), (-)-vesamicol (57-65%), and two sigma receptor ligands with modulate affinities for VAChTs: SA4503 (56-71%) and haloperidol (39-64%) in all of the brain regions, including the cerebellum with a low density of VAChTs, but not of sigmal-selective ligand (+)-pentazocine. However, the pretreatment with a large excess amount of (+/-)-pentazocine (50 micromol/kg) reduced the uptake in a different manner in the brain regions: 25% reduction in the striatum with a high density of VAChTs, and a 50-55% reduction in the other regions with a lower density of VAChTs. Ex vivo autoradiography using (-)-[11C]OMV showed a similar regional brain distribution of [3H](-)-vesamicol. In the PET study of the monkey brain, the regional brain distribution pattern of (-)-[11C]OMV was different from that of [11C]SA4503. The uptake of (-)-[11C]OMV was relatively higher in the striatum, was reversible, and an apparent equilibrium state was found at 20-40 minutes. In conclusion, (-)-[11C]OMV exhibited appropriate brain kinetics during the time frame of 11C-labeled tracers and bound mainly to VAChTs; however, the binding to sigmal receptors was not disregarded. Therefore, (-)-[11C]OMV-PET together with help of [11C]SA4503-PET may evaluate VAChTs.

Simple automated system for simultaneous production of 11C-labeled tracers by solid supported methylation

We herein describe a simple setup for the automated simultaneous synthesis of l-[methyl- 11C]methionine and N-[methyl- 11C]choline by solid-supported methylation . The setup is extremely simple and easy to adapt to other automated systems and due to its versatility, the method can be utilized for the production of other radiopharmaceuticals requiring a simple [ 11C]methylation step. Furthermore, it can be used for multiple simultaneous synthesis.

The Synthesis and application of 11C-labeled PET tracers

正电子核素:碳-11广泛应用于氨基酸、脂肪酸、受体的配体和神经递质等分子标记,其标记的PET显像剂在生物医学和药理学研究中实现真正意义上的分子生物学显像.利用PET显像临床应用于探查肿瘤、脑血流、代谢、蛋白质合成和神经递质功能活动,在基础科学和临床研究中具有非常重要的意义。

Rapid methylation on carbon frameworks leading to the synthesis of a PET tracer capable of imaging a novel CNS-type prostacyclin receptor in living human brain

To develop novel short-lived 11C radionuclide-incorporated PET tracers with high metabolic tolerance, we performed rapid methylation using 11CH 3I, a frequently used precursor for 11C-labeled tracers, on sp- and sp 2-carbon frameworks. Methyl iodide was trapped using an excess of tributylphenylstannane for 5 min at 60 °C in the presence of Pd[P( o-tolyl) 3] 2, a copper(I) salt (CuCl or CuBr), and K 2CO 3, resulting in a greater than 90% yield of toluene. A trapping reaction with tributyl-1-hexynylstannane for 5 min at 60 °C in the presence of Pd[P( t-C 4H 9) 3] 2 and a fluoride ion (KF or CsF) gave a high yield (>90%) of 2-heptyne. Since these reactions are applicable to a wide range of organic molecules, we synthesized 15 R-[ 11C]TIC methyl ester, a PET tracer targeting the CNS-specific prostacyclin receptor (IP 2), by rapid sp 2–sp 3 coupling with some modifications. Radiolabeled 11CH 3I was trapped with the methyl ester of the corresponding stannyl precursor via two steps. In the first, 11CH 3I and Pd[P( o-tolyl) 3] 2 were mixed to form a methylpalladium complex, and in the second, the methylpalladium complex was mixed with the other reagents necessary for the coupling reaction. The yield of radiolabeled 15 R-[ 11C]TIC methyl ester was 80–90%, and it had a radioactivity of 2.0–2.5 GBq, sufficient for an in vivo human PET study with high reproducibility. We used this tracer in PET studies imaging the IP 2 receptor in living monkey and human brains. Following its penetration of the blood–brain barrier, the tracer underwent ester hydrolysis and bound to its specific receptor in the brain. A protocol for sp–sp 3 coupling would be used to synthesize [ 11C]iloprost methyl ester, a PET tracer for the peripheral-type prostacyclin receptor (IP 1). A methyl group is the least bulky, unpolarized organic group, suggesting that it would have little effect on the biological activity of its parent compound. Thus, [ 11C]methylated compounds may be applicable to a wide range of biologically active molecules, converting them to PET tracers that can be utilized for in vivo molecular imaging.