Synthesis and radiolabeling of N-[4-[4-(2-[11C]methoxyphenyl)piperazin-1-yl]butyl]benzo[b]thiophene-2-carboxamide -- a potential radiotracer for D3 receptor imaging with PET

FAUC346 (N-[4-[4-(2-methoxyphenyl)piperazin-1-yl]butyl]benzo[b]thiophene-2-carboxamide), an in vitro D(3)-selective ligand, and its normethyl derivative have been synthesized from commercially available 1-(2-substituted-phenyl)piperazines. FAUC346 has been labeled using [(11)C]methyl triflate in acetone containing aqueous NaOH (5 Eq) at -10 degrees C for 1 min, purified on semipreparative reverse-phase high-performance liquid chromatography (HPLC) and formulated as an intravenous injectable solution using a Sep-Pak Plus C(18) device. Up to 5.5 GBq of [(11)C]FAUC346 (N-[4-[4-(2-[methyl-(11)C]methoxyphenyl)piperazin-1-yl]butyl]benzo[b]thiophene-2-carboxamide), with a specific radioactivity of 45-75 GBq/micromol, could be obtained in 30-35 min, including HPLC purification and formulation starting from 44.4 GBq of [(11)C]carbon dioxide. Preliminary pharmacological evaluation of [(11)C]FAUC346 in rat brain clearly demonstrated in vivo selectivity for D(3) receptors and the absence of radiolabeled metabolite within the brain. These encouraging results, however, could not be confirmed in nonhuman primates; therefore, this radioligand does not appear to have the required pharmacological profile for a positron emission tomography probe for imaging D(3) receptors.     

Syntheses of racemic [1-11C]-alanine and partially resolved [3-11C]-alanine

Irradiation of N₂(g) with 10 MeV protons gave [¹¹C]-carbon dioxide which was used as such in the synthesis of racemic [1-¹¹C]-alanine. Alternatively it was transformed to [¹¹C]-methyl iodide for the asymmetric synthesis of [3-¹¹C]-alanine, giving the L(+) form in excess, the ratio L(+)/D(−) being 2.9. The syntheses were completed in 20 and 60 min, respectively.     

A semi-automated synthesis system for routine preparation of [11C]YM-09151-2 and [11C]pyrilamine from [11C]methyl iodide

A synthesis system has been developed for routine preparation of the ¹¹C-labeled receptor ligands, [¹¹C]YM-09151-2 and [¹¹C]pyrilamine, from [¹¹C]methyl iodide produced automatically. The system features semi-automated operation, from the reaction of the desmethyl derivative with [¹¹C]methyl iodide to filtration with a specifically developed syringe pump of the final product in saline into a sterile vial. The preparations were completed within 45 min after irradiation and approx. 1 GBq (27 mCi) of [¹¹C]YM-09151-2 or [¹¹C]pyrilamine was obtained with a radiochemical and chemical purity of >99% and an average specific activity of 44 GBq/μmol (1.2 Ci/μmol) at the end of synthesis. Sterile and pyrogen-free ¹¹C-labeled receptor ligands suitable for human injection are routinely prepared using the present synthesis system.     

Imaging of I2-imidazoline receptors by small-animal PET using 2-(3-fluoro-[4-11C]tolyl)-4,5-dihydro-1H-imidazole ([11C]FTIMD)

IntroductionImidazoline receptors (IRs) have been established as distinct receptors, and have been categorized into at least two subtypes (I₁R and I₂R). I₂Rs are associated with depression, Alzheimer's disease, Huntington's disease and Parkinson's disease. A few positron emission tomography (PET) probes for I₂Rs have been synthesized, but a selective PET probe has not been evaluated for the imaging of I₂Rs by PET. We labeled a selective I₂R ligand 2-(3-fluoro-4-tolyl)-4,5-dihydro-1H-imidazole (FTIMD) with ¹¹C and performed the first imaging of I₂Rs by PET using 2-(3-fluoro-[4-¹¹C]tolyl)-4,5-dihydro-1H-imidazole ([¹¹C]FTIMD).Methods[¹¹C]FTIMD was prepared by a palladium-promoted cross-coupling reaction of the tributylstannyl precursor and [¹¹C]methyl iodide in the presence of tris(dibenzylideneacetone)dipalladium(0) and tri(o-tol)phosphine. Biodistribution was investigated in rats by tissue dissection. [¹¹C]FTIMD metabolites were measured in brain tissues and plasma. Dynamic PET scans were acquired in rats, and the kinetic parameters estimated.Results[¹¹C]FTIMD was successfully synthesized with a suitable radioactivity for the injection. Co-injection with 0.1 mg/kg of cold FTIMD and BU224 induced a significant reduction in the brain-to-blood ratio 15 and 30 min after the injection. In metabolite analysis, unchanged [¹¹C]FTIMD in the brain was high (98%) 30 min after the injection. In PET studies, high radioactivity levels were observed in regions with a high density of I₂R. The radioactivity levels and V_T values in the brain regions were prominently reduced by 1.0 mg/kg of BU224 pretreatment as compared with control.Conclusion[¹¹C]FTIMD showed specific binding to I₂Rs in rat brains with a high density of I₂R.     

Synthesis of [1-11C]iodoethane for the preparation of [11C]ethyl labelled radiopharmaceuticals

A method for the synthesis of [¹¹C]iodoethane, a useful precursor for labelling of molecules of biomedical interest, has been developed. ¹¹CO₂ prepared by irradiation of nitrogen gas with 18 MeV protons, is converted into [¹¹C]ethanol by a Grignard reaction with methylmagnesium bromide, followed by reduction with lithium aluminum hydride and hydrolysis with hydrochloric acid. [¹¹C]Iodoethane is produced by reaction of [¹¹C]ethanol with hydriodic acid. The purification is performed by gaschromatography on Porapak Q. For a 20 min irradiation at, 15 μA intensity the method yields about 370 mCi2 of radiochemically pure [¹¹C]iodoethane with a specific activity of 180–300 mCi/μmol at the end of synthesis, 23 min after irradiation.     

Syntheses of [11C]methyl esters of prostaglandins D2 and E2

Prostaglandins (PG) are endogenous compounds that have been extensively studied by various techniques. In this paper, the syntheses of methyl esters of PGD₂ and PGE₂, labelled with ¹¹C for PET investigations, are reported. The prostaglandin was esterified via its carboxylate anion, with [¹¹C]methyl iodide as alkylating reagent, in a dimethyl sulphoxide-dimethylformamide mixture. To minimize the base-catalysed degradations, the carboxylate anion was generated in situ by use of tetramethylpiperidine. The radio-chemical yield, including purification, was 70%, with a total synthesis time of 20–25 min counted from the end of the [¹¹C]methyl iodide synthesis.     

Synthesis of racemic [3-11C]phenylalanine and [3-11C]DOPA

The synthesis of racemic [3-¹¹C]phenylalanine and [3-¹¹C]DOPA is reported. The [¹¹C]benzaldehyde and [¹¹C]veratraldehyde prepared in a two-step reaction from the corresponding [¹¹C]acid salt and [¹¹C]alcohol, by means of selective oxidation with tetrabutylammonium hydrogen chromate, were reacted with 2-phenyl-5-oxazolone or 2-(4-chloro)phenyl-5-oxazolone in the presence of a tertiary amine to give the corresponding [α-¹¹C]-4-arylene-2-aryl-5-oxazolones. Ring opening of these olefins, hydrogenation, and removal of protecting groups was carried out in one step using hydroiodic acid/phosphorus, with the production of the racemic [3-¹¹C]amino acids in 8–30% radiochemical yield (starting with ¹¹CO₂) within 52–60 min (including LC separation).     

Synthesis and initial PET imaging of new potential NK1 receptor radioligands 1-[2-(3,5-bis-trifluoromethyl-benzyloxy)-1-phenyl-ethyl]-4-[11C]methyl-piperazine and {4-[2-(3,5-bis-trifluoromethyl-benzyloxy)-1-phenyl-ethyl]-piperazine-1-yl}-acetic acid [11C]methyl ester

The NK(1) receptor radioligands 1-[2-(3,5-bis-trifluoromethyl-benzyloxy)-1-phenyl-ethyl]-4-[(11)C]methyl-piperazine ([(11)C]BMP, [(11)C]) and {4-[2-(3,5-bis-trifluoromethyl-benzyloxy)-1-phenyl-ethyl]-piperazine-1-yl}-acetic acid [(11)C]methyl ester ([(11)C]BME, [(11)C]) were synthesized for evaluation as new potential PET imaging agents for brain NK(1) receptors. The new tracers [(11)C]BMP and [(11)C]BME were prepared by N-[(11)C]methylation and O-[(11)C]methylation of corresponding precursors 1-[2-(3,5-bis-trifluoromethyl-benzyloxy)-1-phenyl-ethyl]-piperazine and {4-[2-(3,5-bis-trifluoromethyl-benzyloxy)-1-phenyl-ethyl]-piperazine-1-yl}-acetic acid using [(11)C]methyl triflate and isolated by solid-phase extraction (SPE) purification procedure with 40-55% radiochemical yields, decay corrected to end of bombardment, and a synthesis time of 15-20 min. The initial PET dynamic studies of the tracers [(11)C] and [(11)C] in rats were performed using an animal PET scanner, IndyPET-II, developed in our laboratory. The results show the tracer [(11)C]BMP had better uptake in the animal brain than the tracer [(11)C]BME and gave higher quality rat brain images. Blocking studies by intravenous coinjection of hot tracer [(11)C]BMP with cold drug BMP had no effect on [(11)C]BMP-PET rat brain imaging. Likewise, blocking studies by intravenous coinjection of hot tracer [(11)C]BME with cold drug BME also showed no effect on [(11)C]BME-PET rat brain imaging. These results suggest that the localization of [(11)C]BMP and [(11)C]BME in rat brain is mediated by nonspecific processes, and the visualization of [(11)C]BMP-PET and [(11)C]BME-PET on rat brain is related to nonspecific binding.     

Enzymatic synthesis of [1-11C]pyruvic acid, L-[1-11C]lactic acid and L-[1-11C]alanine via DL-[1-11C]alanine

L-[1-11C]Lactic acid was prepared enzymatically from [1-11C]pyruvic acid by way of DL-[1-11C]alanine, using remote, semiautomated procedures. The DL isomers of alanine were prepared by a modification of the Bucherer-Strecker reaction from no-carrier-added (NCA) hydrogen [11C]cyanide. The enantiomer mixture was transformed to [1-11C]pyruvic acid by successive elution through columns of (a) immobilized D-amino acid oxidase (D-AAO)/catalase and (b) immobilized L-alanine dehydrogenase (L-AID) or L-amino acid oxidase (L-AAO/catalase). [1-11C]-Pyruvic acid was subsequently converted to L-[1-11C]lactic acid by passage through a L-lactic dehydrogenase (L-LDH) column. L-[1-11C]Alanine and [1-11C]-pyruvic acid were separated chromatographically by way of a cation-exchange column (AG50W-X2, H+ form). Typically the synthesis time was 35-40 min after cyclotron production of hydrogen [11C]cyanide (400 mCi), with radiochemical yields of 25 mCi (25%) for L-[1-11C]lactic acid, 35 mCi (29%) for [1-11C]pyruvic acid, and 20 mCi (20%) for L-[1-11C]alanine. The use of immobilized enzymes eliminates the possibility of protein contamination and assures the production of sterile, pyrogen-free products, allowing for rapid and effective regio- and stereo-specific transformations.     

Synthesis and evaluation of 6-[11C]methoxy-3-[2-[1-(phenylmethyl)-4-piperidinyl]ethyl]-1,2- benzisoxazole as an in vivo radioligand for acetylcholinesterase

6-Methoxy-3-[2-[1-(phenylmethyl)-4-piperidinyl]ethyl]-1,2-benzisoxazole is a high affinity (K(i) = 8.2 nM) reversible inhibitor of acetylcholinesterase (AChE). The carbon-11 labeled form was prepared in high (>97%) radiochemical purity and with specific activities of 37+/-20 GBq/micromol at end of synthesis, by the alkylation of the desmethyl precursor with [11C]methyl trifluoromethanesulfonate in N,N-dimethyl-formamide at room temperature. In vivo studies in mice demonstrated good blood brain permeability but essentially uniform regional brain distribution. Thus, despite in vitro and in vivo activity as an AChE inhibitor, 6-[11C]methoxy-3-[2-[1-(phenylmethyl)-4-piperidinyl]ethyl]-1,2-benzis oxa zole does not appear to be a good candidate for in vivo imaging studies of AChE in the mammalian brain.     

Synthesis and in vivo evaluation of the putative breast cancer resistance protein inhibitor [11C]methyl 4-((4-(2-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl)phenyl)amino-carbonyl)-2-(quinoline-2-carbonylamino)benzoate

IntroductionThe multidrug efflux transporter breast cancer resistance protein (BCRP) is highly expressed in the blood-brain barrier (BBB), where it limits brain entry of a broad range of endogenous and exogenous substrates. Methyl 4-((4-(2-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl)phenyl)amino-carbonyl)-2-(quinoline-2-carbonylamino)benzoate (1) is a recently discovered BCRP-selective inhibitor, which is structurally derived from the potent P-glycoprotein (P-gp) inhibitor tariquidar. The aim of this study was to develop a new PET tracer based on 1 to map BCRP expression levels in vivo.MethodsCompound 1 was labelled with ¹¹C in its methyl ester function by reaction of the corresponding carboxylic acid 2 with [¹¹C]methyl triflate. Positron emission tomography (PET) imaging of [¹¹C]-1 was performed in wild-type, Mdr1a/b^(?/?), Bcrp1^(?/?) and Mdr1a/b^(?/?)Bcrp1^(?/?) mice (n=3 per mouse type) and radiotracer metabolism was assessed in plasma and brain.ResultsBrain-to-plasma ratios of unchanged [¹¹C]-1 were 4.8- and 10.3-fold higher in Mdr1a/b^(?/?) and in Mdr1a/b^(?/?)Bcrp1^(?/?) mice, respectively, as compared to wild-type animals, but only modestly increased in Bcrp1^(?/?) mice. [¹¹C]-1 was rapidly metabolized in vivo giving rise to a polar radiometabolite which was taken up into brain tissue.ConclusionOur data suggest that [¹¹C]-1 preferably interacts with P-gp rather than BCRP at the murine BBB which questions its reported in vitro BCRP selectivity. Consequently, [¹¹C]-1 appears to be unsuitable as a PET tracer to map cerebral BCRP expression.     

Synthesis and preclinical evaluation of carbon-11 labelled N-((5-(4-fluoro-2-[11C]methoxyphenyl)pyridin-3-yl)methyl)cyclopentanamine as a PET tracer for NR2B subunit-containing NMDA receptors

IntroductionThe N-methyl-D-Aspartate (NMDA) receptor plays an important role in learning and memory. Overactivation is thought to play an important role in neurodegenerative disorders such as Alzheimer's disease. Currently, it is not possible to assess N-methyl-D-aspartate receptor (NMDAr) bio-availability in vivo. The purpose of this study was to develop a positron emission tomography (PET) ligand for the NR2B binding site of the NMDA receptor.MethodsN-((5-(4-fluoro-2-methoxyphenyl)pyridin-3-yl)methyl)cyclopentanamine was radiolabelled with carbon-11 in the phenyl moiety. Biodistribution and blocking studies were carried out in anaesthetized mice and in non-anaesthetized rats.ResultsN-((5-(4-fluoro-2-[¹¹C]methoxyphenyl)pyridin-3-yl)methyl)cyclopentanamine was prepared in 49 ± 3% (decay-corrected) yield, affording 4.1 ± 0.3 GBq of formulated product at the end of synthesis with a radiochemical purity of > 99% and with a specific activity of 78 ± 10 GBq/μmol.ConclusionA new NR2B PET ligand was developed in high yield. [¹¹C]4 readily enters the brain and binds to the NR2B subunit-containing NMDAr in the rodent brain. High sigma-1 receptor binding may, however, limit its future application as a PET probe for imaging the NR2B subunit-containing NMDAr. Anaesthesia has an effect on NMDAr function and therefore can complicate interpretation of preclinical in vivo results. In addition, effects of endogenous compounds cannot be excluded. Despite these potential limitations, further studies are warranted to investigate the values of [¹¹C]4 as an NR2B PET ligand.     

Upregulation of putaminal dopamine D2 receptors in early Parkinson's disease: a comparative PET study with [11C] raclopride and [11C]N-methylspiperone.

Dopamine D2 receptor function was assessed in a PET study with 2 dopamine D2 receptor PET ligands, [11C]raclopride (RAC) and [11C]N-methylspiperone (NMSP), in early Parkinson's disease. METHODS: Seven patients with early Parkinson's disease and 5 healthy volunteers were studied. Each underwent PET both with reversible [11C]RAC and with irreversible [11C]NMSP. RESULTS: Upregulation of dopamine D2 receptors in the putamen contralateral to the predominant symptoms of Parkinson's disease was confirmed using both [11C]RAC and [11C]NMSP. Uptake of [11C]RAC in the contralateral putamen was 105% of uptake in the opposite putamen (P = 0.020). For [11C]NMSP, uptake in the contralateral putamen was 105% of uptake in the ipsilateral putamen (P = 0.011). No significant differences between Parkinson's disease patients and healthy volunteers were detected in any of the studied brain regions using either [11C]RAC or [11C]NMSP. No significant differences between [11C]RAC and [11C]NMSP uptake were detected in the striatum, whereas in the extrastriatal regions, [11C]NMSP showed significantly higher uptake than [11C]RAC both in healthy volunteers and in Parkinson's disease patients. CONCLUSION: This study confirms an increase in dopamine D2 receptors in the putamen contralateral to the predominant symptoms, compared with the ipsilateral putamen, in early Parkinson's disease. This increase was seen both with reversible ligand [11C]RAC and with irreversible ligand [11C]NMSP and thus does not seem a consequence of depleted endogenous dopamine.     

Development and evaluation of muscarinic cholinergic receptor ligands N-[11C]ethyl-4-piperidyl benzilate and N-[11C]propyl-4-piperidyl benzilate: a PET study in comparison with N-[11C]methyl-4-piperidyl benzilate in the conscious monkey brain

The muscarinic cholinergic receptor ligands N-[¹¹C]ethyl-4-piperidyl benzilate (4-EPB) and N-[¹¹C]propyl-4-piperidyl benzilate (4-PPB) were developed and evaluated in comparison with N-[¹¹C]methyl-4-piperidyl benzilate (4-MPB) in the conscious monkey brain using positron emission tomography (PET). Time-activity curves of [¹¹C]4-EPB, unlike [¹¹C]4-MPB, showed peaks within 91 min in regions rich in muscarinic receptors. [¹¹C]4-PPB showed no specific binding even in the regions rich in these receptors. These observation demonstrated that increases in [¹¹C]alkyl chain length could alter the kinetic properties of receptor ligands for PET.     

Synthesis of [11C]SCH 23390 for dopamine D1 receptor studies

The optimal conditions for the synthesis of ¹¹C-labeled SCH 23390 by radio-methylation of its desmethyl precursor, SCH 24518, with [¹¹C]iodomethane are described. Isocratic reversed phase HPLC was used for the purification of [¹¹C]SCH 23390. The specific activity range in 30 runs was 10–235 Ci/mmol and average radiochemical yield was 72% based on [¹¹C]iodomethane. Mean synthesis time was 40–60 min from the end of bombardment. Preliminary animal studies indicate that [¹¹C]SCH 23390 would be useful in visualizing D1 receptors in a living brain by positron tomography.     

Synthesis and evaluation of 11C- and 18F-labeled 1-[2-(4-alkoxy-3-methoxyphenyl)ethyl]-4-(3-phenylpropyl)piperazines as sigma receptor ligands for positron emission tomography studies

We prepared sigma(1)-receptor selective 1-([4-methoxy-(11)C]-3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine ([(11)C]SA4503) and its fluorinated analog 1-([4-methoxy-(11)C]3,4-dimethoxyphenethyl)-4-[3-(4-fluorophenyl)propyl]piperazine ([(11)C]SA5845), and their [(11)C]ethoxy and [(18)F]fluoroethoxy analogs, and evaluated their potential for positron emission tomography studies. [(11)C]SA4503 is most selective for sigma(1) receptors, and the other five showed affinities for sigma(1) and sigma(2) receptors with a different extent. All radioligands showed the receptor-specific binding in the brain, and visualized similar regional brain distributions by ex vivo autoradiography. The [(11)C]ethoxy analogs were relatively labile for metabolism.     

Carbon-11 labelling of 8[[3-[4-(2-[(11)C]methoxyphenyl)piperazin-1-yl]-2-hydroxypropyl]oxy]thiochroman,a presynaptic 5-HT(1A) receptor agonist,and its in vivo evaluation in anaesthetised rat and in awake cat

A new compound, 8[[3-[4-(2-[(11)C]methoxyphenyl)piperazin-1-yl]-2-hydroxypropyl]oxy]thiochroman was labeled via O-methylation with [(11)C]methyl iodide in good yield and specific activity. Original biological evaluations included (i) the study in anesthetized rat with a beta-sensitive intracerebral probe (beta-Microprobe), allowing to measure locally the kinetic of the new PET ligand, and (ii) a PET-scan on a conditioned cat maintained awake during the acquisition. In both in vivo techniques, the new ligand did not reveal any specific binding in hippocampus indicating that this radiotracer is not suitable for mapping 5HT(1A) receptors using positron emission tomography.     

Synthesis and evaluation of 5,7-dihydro-3-[2-[1-(4-[18F]-fluorobenzyl)-4-piperidinyl]ethyl]-6H-pyrrolo[3,2-f]-1,2-benzisoxazol-6-one for in vivo mapping of acetylcholinesterase

OBJECTIVES: Acetylcholinesterase (AChE) is an important cholinergic marker for the diagnosis of Alzheimer's disease (AD). A recent study has demonstrated that C-labelled 5,7-dihydro-7-methyl-3-[2-[1-(phenylmethyl)-4-piperidinyl]ethyl]-6H-pyrrolo[3,2-f]-1,2-benzisoxazol-6-one (CP-126,998) shows promising results. The demethylated form of this ligand (CP-118,954) is a more potent and selective inhibitor than CP-126,998. In this study, therefore, CP-118,954 was labelled with F and evaluated for the in vivo mapping of AChE. METHODS: The 4-fluoro (1). and 2-fluoro (2). derivatives of CP-118,954 were synthesized from 4-methyl-3-nitroanisole in 11 steps. Their in vitro binding affinities to AChE were measured using Ellman's method. The preparation of [F]-1 was carried out by reductive alkylation of the piperidine precursor with 4-[F]-fluorobenzaldehyde, followed by high-performance liquid chromatography (HPLC) purification. In vitro autoradiography was performed by incubating rat brain coronal slices with [F]-1. Tissue distribution studies were performed in mouse brain and the data were expressed as the percentage of the injected dose per gram of tissue (%ID x g). RESULTS: Two fluorine-substituted AChE inhibitors were synthesized and their in vitro binding data showed that the 4-fluoro and 2-fluoro derivatives (1 and 2) had similar or superior binding affinity to that of the unsubstituted ligand, CP-118,954. The F-labelled ligand was synthesized in 20-35% radiochemical yield (EOS) and with high effective specific activity (36-42 GBq x micromol). Autoradiography showed high uptake of [F]-1 in the striatum and this striatal uptake was completely inhibited by the unlabelled ligand 1. Tissue distribution studies demonstrated that high radioactivity was accumulated in the striatum, an AChE-rich region. CONCLUSIONS: This study demonstrates that [F]-1 may hold promise as a radioligand for the in vivo mapping of AChE.     

Equivalence of arterial and venous blood for [11C]CO2-metabolite analysis following intravenous administration of 1-[11C]acetate and 1-[11C]palmitate

PurposeSampling of arterial blood for metabolite correction is often required to define a true radiotracer input function in quantitative modeling of PET data. However, arterial puncture for blood sampling is often undesirable. To establish whether venous blood could substitute for arterial blood in metabolite analysis for quantitative PET studies with 1-[¹¹C]acetate and 1-[¹¹C]palmitate, we compared the results of [¹¹C]CO₂-metabolite analyses performed on simultaneously collected arterial and venous blood samples.MethodsPaired arterial and venous blood samples were drawn from anesthetized pigs at 1, 3, 6, 8, 10, 15, 20, 25 and 30 min after i.v. administration of 1-[¹¹C]acetate and 1-[¹¹C]palmitate. Blood radioactivity present as [¹¹C]CO₂ was determined employing a validated 10-min gas-purge method. Briefly, total blood ¹¹C radioactivity was counted in base-treated [¹¹C]-blood samples, and non-[¹¹C]CO₂ radioactivity was counted after the [¹¹C]-blood was acidified using 6 N HCl and bubbled with air for 10 min to quantitatively remove [¹¹C]CO₂.ResultsAn excellent correlation was found between concurrent arterial and venous [¹¹C]CO₂ levels. For the [¹¹C]acetate study, the regression equation derived to estimate the venous [¹¹C]CO₂ from the arterial values was: y = 0.994x + 0.004 (r² = 0.97), and for the [¹¹C]palmitate: y = 0.964x ? 0.001 (r² = 0.9). Over the 1–30 min period, the fraction of total blood ¹¹C present as [¹¹C]CO₂ rose from 4% to 64% for acetate, and 0% to 24% for palmitate. The rate of [¹¹C]CO₂ appearance in venous blood appears similar for the pig model and humans following i.v. [¹¹C]-acetate administration.ConclusionVenous blood [¹¹C]CO₂ values appear suitable as substitutes for arterial blood samples in [¹¹C]CO₂ metabolite analysis after administration of [¹¹C]acetate or [¹¹C]palmitateAdvances in Knowledge and Implications for Patient CareQuantitative PET studies employing 1-[¹¹C]acetate and 1-[¹¹C]palmitate can employ venous blood samples for metabolite correction of an image-derived tracer arterial input function, thereby avoiding the risks of direct arterial blood sampling.