Comparison of [3,4‐13C2]glucose to [6,6‐2H2]glucose as a tracer for glucose turnover by nuclear magnetic resonance

A recently introduced tracer, [3,4‐¹³C₂]glucose, was compared to the widely used tracer, [6,6‐²H₂]glucose, for measurement of whole‐body glucose turnover. The rate of glucose production (GP) was measured in rats after primed infusions of [3,4‐¹³C₂]glucose, [6,6‐²H₂]glucose, or both tracers simultaneously followed by a constant infusion of tracer(s) over 90 min. Blood glucose was purified and converted into monoacetone glucose for analysis by ¹³C NMR (for [3,4‐¹³C₂]glucose) or ¹H and ²H NMR (for [6,6‐²H₂]glucose). The values of GP measured during infusion of each single tracer were not significantly different. In rats infused with both tracers simultaneously, GP was identical as reported by each tracer, 42 ± 4 μmol/kg/min. Since ²H and ¹³C enrichment in glucose is typically much less than 2% for in vivo studies, [3,4‐¹³C₂]glucose does not interfere with measurements of ¹³C or ²H enrichment patterns and therefore is valuable when multiple metabolic pathways are being evaluated simultaneously. Magn Reson Med 53:1479–1483, 2005. © 2005 Wiley‐Liss, Inc.     

Metabolic imaging in the anesthetized rat brain using hyperpolarized [1‐13C] pyruvate and [1‐13C] ethyl pyruvate

Formulation, polarization, and dissolution conditions were developed to obtain a stable hyperpolarized solution of [1‐¹³C]‐ethyl pyruvate. A maximum tolerated concentration and injection rate were determined, and ¹³C spectroscopic imaging was used to compare the uptake of hyperpolarized [1‐¹³C]‐ethyl pyruvate relative to hyperpolarized [1‐¹³C]‐pyruvate into anesthetized rat brain. Hyperpolarized [1‐¹³C]‐ethyl pyruvate and [1‐¹³C]‐pyruvate metabolic imaging in normal brain is demonstrated and quantified in this feasibility and range‐finding study. Magn Reson Med 63:1137–1143, 2010. © 2010 Wiley‐Liss, Inc.     

Synthesis of 1-11C-labeled fatty acid from [11C]HCN

A new chemical approach to the preparation of [1-¹¹C]fatty acids from [¹¹C]HCN has been developed. Four straight chain [1-¹¹C]fatty acids (C₁₅C₁₈) and one branched chain [1-¹¹C]fatty acid (3-methylheptadecanoic acid) were produced by the reaction of alkyl bromides with ¹¹CN⁻ and subsequent hydrolysis. The synthesis time was 47–67 min after the end of bombardment, with radiochemical yields of 16–20 mCi (70–83%) for a series of straight chain [1-¹¹C]fatty acids and 4–8 mCi (33–42%) for 3-methyl [1-¹¹C]heptadecanoic acid (from 120 mCi of trapped [¹¹C]HCN). This method has some advantages, such as no requirement of absolutely anhydrous solvent, easy ¹¹C-labeling using an extremely small amount of substrate, and good reproducibility.     

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.     

Production of [2-11C]thymidine for quantification of cellular proliferation with PET

A three-step synthesis of [2-¹¹C]thymidine, using [¹¹C]urea as precursor, is described. [2-¹¹C]Thymine was obtained by cyclization of [¹¹C]urea and diethyl β-methylmalate in fuming sulfuric acid. After purification of the reaction mixture, [2-¹¹C]thymine and 2′-deoxyribose-1-phosphate were incubated in the presence of thymidine phosphorylase to form [2-¹¹C]thymidine. The whole synthesis procedure, including purification and pharmaceutical package, was achieved within 60 min with a decay corrected yield of 30–35%.     

Noninvaive observation of hepatic glycogen formation in man by13C MRS after Orl and intravenous glucose administration

The formation of glycogen in the liver of normal volunteers was followed noninvasively with ¹³C manetic resonance spectroscopy (MRS) under tow different conditions; a) intravenous infusion of [₁-¹³C] glucose under hyperglycemic and hyperinsullinemic clamp conditions, and b) oral Intake of glucose in the form of a bolus. For the intravenous infusion, [₁-¹³C]glucose with an enrichment level of 99% was employed. The C₁ signals of α- and β-glucose could be detected in the human liver already after an infusion period of 8 min. However, an increase in the glycogen signal was observed only after a prolonged infusion of about 60 min. Changes in the glycogen signal correlated well with the time course of insulin and glucagon during the measurement. Experiments showed also that liver glycogen formation in man can be followed noninvasively by¹³C-MRS using nonlabeled glucose or [₁-¹³C]glucose with a low level of enrichment (6.6%). The use of nonlabeled glucose may therefore simplify the quantitation of net liver glycogen synthesis since it can be based directly on changes in the natural abundance ¹³C MRS glycogen signal, avoiding label dilution through the various metabolic pathways of glucose. The glucose uptake, estimated from the increase in the glycogen signal, was consistent with findings from more complex and invasive studies of glucose uptake in the liver. The average liver glycogen concentration in 12 h overnight fasted volunteers (n = 18) without any special dietary preparation was assessed to be 229 ± 34 mM (minimum = 160 mM; maximum = 274 mM).     

Remote-controlled production of [1-11C]-d-glucose and evaluation of the effect of labelling position on loss of [11C]CO2

A remote-controlled apparatus is described for the routine production of [1-¹¹C]-d-glucose by the reaction of [¹¹C]cyanide with d(−)arabinose and reduction of the intermediate [1-¹¹C]aldonitriles with Raney nickel in formic acid. The total time of synthesis, including HPLC separation, is 55–60 min from the end-of-trapping. The decay-corrected isolated radiochemical yield of [1-¹¹C]-d-glucose is 8–11% (based on trapped [¹¹C]cyanide) and the radiochemical purity is >99%. Studies of regional cerebral metabolic rate of glucose using [1-¹¹C]-d-glucose and positron emission tomography indicate that better contrast between areas of high and low metabolic rate is obtained when glucose is selectively rather than uniformly labelled. Arteriovenous determinations showed that the egress of [¹¹C]carbon dioxide from the brain was reduced and delayed compared to that previously observed for [U-¹¹C]-d-glucose.     

Synthesis and preclinical evaluation of [11C]D617, a metabolite of (R)-[11C]verapamil

Objectives(R)-[¹¹C]verapamil is widely used as a positron emission tomography (PET) tracer to evaluate P-glycoprotein (P-gp) functionality at the blood–brain barrier in man. A disadvantage of (R)-[¹¹C]verapamil is the fact that its main metabolite, [¹¹C]D617, also enters the brain. For quantitative analysis of (R)-[¹¹C]verapamil data, it has been assumed that the cerebral kinetics of (R)-[¹¹C]verapamil and [¹¹C]D617 are the same. The aim of the present study was to investigate whether the cerebral kinetics of (R)-[¹¹C]verapamil and [¹¹C]D617 are indeed similar and, if so, whether [¹¹C]D617 itself could serve as an alternative PET tracer for P-gp.Methods[¹¹C]D617 was synthesized and its ex vivo biodistribution was investigated in male rats at four time points following intravenous administration of [¹¹C]D617 (50 MBq) without (n=4) or with (n=4) pretreatment with the P-gp inhibitor tariquidar (15 mg·kg^?1, intraperitoneally). Brain distribution was further assessed using consecutive PET scans (n=8) before and after pretreatment with tariquidar (15 mg·kg^?1, intravenously), as well as metabolite analysis (n=4).ResultsThe precursor for the radiosynthesis of [¹¹C]D617, 5-amino-2-(3,4-dimethoxy-phenyl)-2-isopropyl-pentanitrile (desmethyl D617), was synthesized in 41% overall yield. [¹¹C]D617 was synthesized in 58%–77% decay-corrected yield with a radiochemical purity of ≥99%. The homogeneously distributed cerebral volume of distribution (V_T) of [¹¹C]D617 was 1.1, and this increased 2.4-fold after tariquidar pretreatment.ConclusionV_T of [¹¹C]D617 was comparable to that of (R)-[¹¹C]verapamil, but its increase after tariquidar pretreatment was substantially lower. Hence, (R)-[¹¹C]verapamil and [¹¹C]D617 do not show similar brain kinetics after inhibition of P-gp with tariquidar.     

An alternative synthesis of DL-[1-11C]Alanine from [11C]HCN

The synthesis of dl-[1-¹¹C]alanine from [1-¹¹C]HCN via dl-[1-¹¹C]2-aminopropanenitrile and its use in an enzymatic synthesis of [1-¹¹C]pyruvic acid is reported. The purified dl-[1-¹¹C]alanine was obtained in a radiochemical yield of 75% with a radiochemical purity higher than 98% within 40 min after end of bombardment (EOB). [1-¹¹C]Pyruvic acid was prepared by enzymatic synthesis from crude dl-[1-¹¹C]alanine using a previously reported procedure for the synthesis of [3-¹¹C]pyruvic acid. dl-[1-¹¹C]Alanine and [1-¹¹C]pyruvic acid prepared by these methods have been found to be sterile and free of pyrogens, and can thus be used in studies of the distribution of these radiopharmaceuticals in man using positron emission tomography (PET).     

Detecting response of rat C6 glioma tumors to radiotherapy using hyperpolarized [1‐13C]pyruvate and 13C magnetic resonance spectroscopic imaging

We show here that hyperpolarized [1‐¹³C]pyruvate can be used to detect treatment response in a glioma tumor model; a tumor type where detection of response with ¹⁸fluoro‐2‐deoxyglucose, using positron emission tomography, is limited by the high background signals from normal brain tissue. ¹³C chemical shift images acquired following intravenous injection of hyperpolarized [1‐¹³C]pyruvate into rats with implanted C6 gliomas showed significant labeling of lactate within the tumors but comparatively low levels in surrounding brain.Labeled pyruvate was observed at high levels in blood vessels above the brain and from other major vessels elsewhere but was detected at only low levels in tumor and brain.The ratio of hyperpolarized ¹³C label in tumor lactate compared to the maximum pyruvate signal in the blood vessels was decreased from 0.38 ± 0.16 to 0.23 ± 0.13, (a reduction of 34%) by 72 h following whole brain irradiation with 15 Gy. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

PET studies with l-[1-11C]tyrosine,l-[methyl-11C]methionine and 18F-fluorodeoxyglucose in prolactinomas in relation to bromocryptine treatment

Aspects of metabolism in prolactinomas were investigated by positron emission tomography using l-[1-¹¹C]tyrosine, l-[methyl-¹¹C]methionine and ¹⁸F-fluorodeoxyglucose (¹⁸FDG). Using l-[1-¹¹C]tyrosine, four patients were monitored prior to and 18 h after an injection of 50 mg bromocryptine. At 18 h after bromocryptine intervention, l-[1-¹¹C]tyrosine uptake into tumour was reduced with 28% (P<0.07). A correlation analysis of the bromocryptine-induced decrease in l-[1-¹¹C]tyrosine uptake and the reduction of serum prolactin levels indicated that the action of bromocryptine on prolactin synthesis and prolactin release is not coupled. In the untreated situation, the four patients were investigated with ¹⁸FDG as well, but the prolactinomas could not be visualized. Three untreated patients were studied with l-[methyl-¹¹C]methionine. The tumour-imaging potential of l-[methyl-¹¹C]methionine and l-[1-¹¹C]tyrosine appeared to be nearly equivalent for prolactinomas. Unlike prolactinoma tissue, the salivary glands showed a pronounced preference for l-[1-¹¹C]tyrosine as compared to l-[methyl-¹¹C]methionine. l-[1-¹¹C]tyrosine is a valuable tool to obtain information on the metabolism and treatment of prolactinomas.     

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.     

Magnetization transfer measurements of exchange between hyperpolarized [1‐13C]pyruvate and [1‐13C]lactate in a murine lymphoma

Measurements of the conversion of hyperpolarized [1‐¹³C]pyruvate into lactate, in the reaction catalyzed by lactate dehydrogenase, have shown promise as a metabolic marker for the presence of disease and response to treatment. However, it is unclear whether this represents net flux of label from pyruvate to lactate or exchange of isotope between metabolites that are close to chemical equilibrium. Using saturation and inversion transfer experiments, we show that there is significant exchange of label between lactate and pyruvate in a murine lymphoma in vivo. The rate constants estimated from the magnetization transfer experiments, at specific points during the time course of label exchange, were similar to those obtained by fitting the changes in peak intensities during the entire exchange time course to a kinetic model for two‐site exchange. These magnetization transfer experiments may therefore provide an alternative and more rapid way of estimating flux between pyruvate and lactate to serial measurements of pyruvate and lactate ¹³C peak intensities following injection of hyperpolarized [1‐¹³C]pyruvate. Magn Reson Med 63:872–880, 2010. © 2010 Wiley‐Liss, Inc.     

Radiosynthesis of [2-11C-carbonyl]dantrolene using [11C]phosgene for PET

Automated radiosynthesis of [2-¹¹C-carbonyl]dantrolene, the substrate of breast cancer resistance protein (BCRP/ABCG2), was performed for the first time through a multi-step/one-pot labeling sequence that started with ethyl 2-{2-[5-(4-nitrophenyl)furfurylidene]hydrazino}acetate and used [¹¹C]phosgene as a labeling agent. After optimization of the automated synthesis conditions and parameters, [2-¹¹C-carbonyl]dantrolene was obtained at a radiochemical yield of 34.0±8.4% (decay-corrected). The radiochemical purity was greater than 98% and the specific activity was 46.8±15.2 GBq/μmol at the end of the synthesis.     

Localized proton NMR observation of [3-13C]lactate in stroke after [1-13C]glucose infusion.

To assess whether elevated lactate in stable stroke is being actively produced from blood glucose localized 1H NMR stimulated echo spectra were obtained from a patient in the region of a 32-day-old cortical infarct before and 60-100 min after infusion of [1-13C]glucose. Prior to the infusion the spectrum from the region of the infarct contained an elevated resonance from C3 lactate and a greatly reduced resonance from N-acetyl groups relative to an unaffected contralateral region. After the infusion two additional resonances were observed at 62 and -64 Hz relative to the unlabeled resonance of C3 lactate which were assigned on the basis of chemical shift and relative intensity to [3-13C]lactate. The [3-13C]lactate fractional enrichment in the infarct region was measured to be 32% which is within error one-half the average [1-13C]plasma glucose enrichment during the postinfusion NMR measurement. The result suggests that the stroke lactate pool was completely derived from infused glucose.     

[11C]octopamine synthesis using [11C]cyanide: Chemical and enzymatic approaches for the [11C]cyanohydrin synthesis

[¹¹C]-p- and m-octopamine hydrochloride were synthesized from [¹¹C]HCN in a two-step sequence. Chemical and enzymatic approaches were used for the formation of the [¹¹C]cyanohydrin intermediates as the key step. Isolated radiochemical yields of 0.7–2.3% at the end-of-synthesis were obtained with an overall preparation time of 40–60 min. The enantiomeric purity of the [¹¹C]-p-octopamine obtained through the enzymatic process was 92% e.e. in the (S)-enantiomer, whereas that of the [¹¹C]-m-octopamine was 42% e.e. in the (R)-enantiomer, as determined by HPLC without any derivatization.     

Efficient and reproducible synthesis of [1-11C]acetyl chloride using the loop method

[1-¹¹C]Acetyl chloride ([¹¹C]AcCl), an important [¹¹C]acylating agent, was synthesized by reacting [¹¹C]CO₂ with methylmagnesium bromide coated on the inner surface of a polyethylene loop (loop method). By optimizing the reaction conditions and synthesis parameters, [1-¹¹C]phenylacetate and [1-¹¹C]benzylacetate were produced from [¹¹C]AcCl in high radiochemical yield and specific activity.     

Preparation of some NCA [1-11C]acid chlorides as labelling agents

The [¹¹C]carbonation of simple Grignard reagents (RMgX, R = Et, Pr, cyclo-Bu; X = Br) with cyclotron-produced [¹¹C]carbon dioxide followed by direct treatment of the ¹¹C adducts with phthaloyl dichloride and 2,6-di-t-butylpyridine affords [1-¹¹C]acid chlorides in useful radiochemical yields (> 10–30%, decay-corrected from ¹¹CO₂), as assessed by HPLC and TLC of the labelled amides formed by reaction with 1,2,3,4-tetrahydroisoquinoline. With suitable precuations these [1-¹¹C]acid chlorides can be isolated at high specific activity within 15 min from radionuclide production as labelling agents for potential radiopharmaceuticals for PET. The preparation of [1-¹¹C]benzoyl chloride without isolation has also been demonstrated.