Preferential labeling of glial and meningial brain tumors with [2-(14)C]acetate.

Acetate is preferentially transported into and metabolized by astrocytes, rather than synaptosomes or neurons, and labeled acetate is used as a glial reporter molecule to assess glial metabolism and glial-neuronal interactions. Because monocarboxylic acid transporter specificity might confer a phenotype to help localize, detect, and characterize brain tumors of glial origin, use of [2-(14)C]acetate and [(14)C]deoxyglucose (a glucose analog metabolized by all brain cells) was compared in rat and human brain tumors. METHODS: Cultured C6 glioma or U-373 glioblastoma/astrocytoma tumor cells were injected into the caudate nucleus of anesthetized CDF Fisher rats; 2--3 wk later, an intravenous pulse of [2-(14)C]acetate or [(14)C]deoxyglucose was given, and timed blood samples were drawn during the 5- or 45-min experiment, respectively. Local (14)C levels in the brain were assayed by quantitative autoradiography, and acetate uptake or glucose use was calculated. Uptake and metabolism of the [(14)C]acetate was also assayed in C6 glioma and human surgical tumor samples in vitro. RESULTS: [(14)C]Acetate uptake into rat brain C6 tumors was 9.9 +/- 2.1 mL/100 g/min, compared with 3.9 +/- 1.0 mL/100 g/min in contralateral tissue (n = 6; P < 0.001), and was much higher than that into other brain structures (e.g., 5:1 for white matter and 2:1 for cortical gray matter). Glucose use in C6 tumors was 111 +/- 34 micromol/100 g/min, versus 81 +/- 5 micromol/100 g/min in contralateral tissue (n = 6; P = 0.08); no left-right differences in glucose use or acetate uptake were seen in other brain structures. The tumor-to-contralateral-tissue ratio for acetate (2.3 +/- 0.3) exceeded that for deoxyglucose (1.4 +/- 0.5) (P < 0.05), indicating that acetate is a sensitive C6 glioma marker. [(14)C]Acetate uptake also demarcated a few 3-wk-old C6 tumors that had unlabeled necrotic cores. U-373 tumors were smaller than C6 tumors in rat brain and were detected equally well with [(14)C]acetate and [(14)C]deoxyglucose. In vitro uptake of [(14)C]acetate into human glioblastoma or meningioma tumors was higher than uptake into pituitary adenoma. Rat C6 and human tumors with high uptake metabolized acetate to acidic compounds and amino acids. CONCLUSION: Tumor imaging with radiolabeled acetate can help to localize and classify brain tumors. Transporter and metabolic substrate specificity are traits that can be exploited further for in vivo imaging of brain glial tumors.     

Detection of [1,6-13C2]-glucose metabolism in rat brain by in vivo 1H-[13C]-NMR spectroscopy.

Localized, water-suppressed (1)H-[(13)C]-NMR spectroscopy was used to detect (13)C-label accumulation in cerebral metabolites following the intravenous infusion of [1,6-(13)C(2)]-glucose (Glc). The (1)H-[(13)C]-NMR method, based on adiabatic RF pulses, 3D image-selected in vivo spectroscopy (ISIS) localization, and optimal shimming, yielded high-quality (1)H-[(13)C]-NMR spectra with optimal NMR sensitivity. As a result, the (13)C labeling of [4-(13)C]-glutamate (Glu) and [4-(13)C]-glutamine (Gln) could be detected from relatively small volumes (100 microL) with a high temporal resolution. The formation of [n-(13)C]-Glu, [n-(13)C]-Gln (n = 2 or 3), [2-(13)C]-aspartate (Asp), [3-(13)C]-Asp, [3-(13)C]-alanine (Ala), and [3-(13)C]-lactate (Lac) was also observed to be reproducible. The (13)C-label incorporation curves of [4-(13)C]-Glu and [4-(13)C]-Gln provided direct information on metabolic pathways. Using a two-compartment metabolic model, the tricarboxylic acid (TCA) cycle flux was determined as 0.52 +/- 0.04 micromol/min/g, while the glutamatergic neurotransmitter flux equaled 0.25 +/- 0.05 micromol/min/g, in good correspondence with previously determined values.     

Oxidation of lactate in rats after short-term strenuous exercise

Oxidation of lactate and glucose was investigated in rats after short-term strenuous running to exhaustion at a speed of 80-110 m.min-1, lasting about 100 sec. Immediately after the exercise, 4 microCi of [U-14C]lactate (LA and AR) or 9.4 microCi of [U-14C]glucose (GL) was injected into the aorta through an indwelling catheter. In AR, the rats ran at a speed of 25 m.min-1 for 20 min after injection of [U-14C]lactate. Expired gas was collected by a bottomless metabolism chamber while the rats were on the treadmill for 120 min. Blood lactate concentration tended to decrease faster in AR than in LA. Peak evolution of 14CO2 expiration occurred at 12.5 min recovery in LA, 7.5 min of recovery in AR, and 35 min of recovery in GL. Cumulative percent recovery of 14C as 14CO2 was 48.5% +/- 2.8% in LA, 74.0% +/- 2.9% in AR, and 18.6% +/- 1.6% (mean +/- SE) in GL. Significant differences were found in these rates between groups (P less than 0.01). It was suggested that a great deal of lactate was oxidized directly, not after conversion to glucose in rats after short-term strenuous exercise to exhaustion and mild exercise following strenuous exercise (active recovery) enhanced lactate oxidation.     

Carbon-13 chemical shift imaging of [1-13C] glucose under metabolism in the rat head in vivo.

Carbon-13 chemical shift images (metabolic maps) of [1-13C] glucose in the heads of rats were obtained and compared with proton images of the same rats in terms of signal allocation. Wistar rats were kept awake or anesthetized. [1-13C] glucose was injected intravenously in a dose of 1 g per kg of body weight. The head of the Wistar rat was placed on or into circular coils. Carbon-13 images were obtained using a 7.05 Tesla system. A simple spin echo sequence was used with a chemical shift selective (CHESS) pulse. The frequency band width was set to cover the spectral breadth of the carbon-13 signal of [1-13C] glucose. The slice thickness of the image was 4 mm or 6 mm, and the field of view (FOV) was 60 mm x 60 mm, with a matrix size of 64 x 64. The total acquisition time was 36 minutes. Strong signals were observed from the scalp muscles and tissues outside the brain, but signal strength from the brain itself was minimal. This was presumably due to the metabolism of [1-13C] glucose in the brain. Little difference was recognized between [1-13C] glucose images of the heads of rats with and without anesthesia. Chemical shift imaging of carbon-13 could be useful methods for the in vivo study of physiochemical structures and metabolic pathways of living organs.     

Incorporation of blood lactate and glucose into tissues in rats after short-term strenuous exercise

The purpose of this study was to investigate the main site of removal of blood lactate and glucose and which is the more important substrate for muscle glycogen resynthesis in rats after short-term strenuous exercise, to exhaustion. Male Wistar rats ran to exhaustion at a speed of 70-100m.min-1. Immediately after the exercise the rats received an injection of [U-14C]lactate (LA, 0.025 microCi.g-1, n = 5) or [U-14C]glucose (GL, 0.015 microCi.g-1, n = 5) into the aorta through an indwelling catheter. The rats were sacrificed after 40 min of recovery. During 40 min of recovery, 20.4% +/- 2.0% (mean +/- SE) of 14C injected was recovered as 14CO2 in LA, while 4.1% +/- 0.4% of 14C was recovered as 14CO2 in GL. In LA, the content of 14C incorporated per tissue weight in the vastus lateralis was significantly greater than that in the kidney, heart, and blood, while in GL that in the vastus lateralis was significantly greater than in any other tissues measured. The incorporation of 14C-glucose into muscle glycogen (vastus lateralis) was about five times greater than that of 14C-lactate. Data from this study indicate that lactate and glucose are incorporated from the blood into the skeletal muscle which was active during exercise and that blood glucose is a more preferred substrate for muscle glycogen resynthesis in rats after strenuous exercise to exhaustion.     

Hepatic UDP-glucose 13C isotopomers from [U-13C]glucose: a simple analysis by 13C NMR of urinary menthol glucuronide.

Menthol glucuronide was isolated from the urine of a healthy 70-kg female subject following ingestion of 400 mg of peppermint oil and 6 g of 99% [U-(13)C]glucose. Glucuronide (13)C-excess enrichment levels were 4-6% and thus provided high signal-to-noise ratios (SNRs) for confident assignment of (13)C-(13)C spin-coupled multiplet components within each (13)C resonance by (13)C NMR. The [U-(13)C]glucuronide isotopomer derived via direct pathway conversion of [U-(13)C]glucose to [U-(13)C]UDP-glucose was resolved from [1,2,3-(13)C(3)]- and [1,2-(13)C(2)]glucuronide isotopomers derived via Cori cycle or indirect pathway metabolism of [U-(13)C]glucose. In a second study, a group of four overnight-fasted patients (63 +/- 10 kg) with severe heart failure were given peppermint oil and infused with [U-(13)C]glucose for 4 hr (14 mg/kg prime, 0.12 mg/kg/min constant infusion) resulting in a steady-state plasma [U-(13)C]glucose enrichment of 4.6% +/- 0.6%. Menthol glucuronide was harvested and glucuronide (13)C-isotopomers were analyzed by (13)C NMR. [U-(13)C]glucuronide enrichment was 0.6% +/- 0.1%, and the sum of [1,2,3-(13)C(3)] and [1,2-(13)C(2)]glucuronide enrichments was 0.9% +/- 0.2%. From these data, flux of plasma glucose to hepatic UDPG was estimated to be 15% +/- 4% that of endogenous glucose production (EGP), and the Cori cycle accounted for at least 32% +/- 10% of GP.     

Comparison of technetium-99m MAG3 kit with HPLC-purified technetium-99m MAG3 and OIH in rats

Technetium-99m (99mTc) mercaptoacetylglycylglycylyglycine (MAG3) in high (greater than or equal to 95%) radiochemical purity is prepared from lyophilized kits containing benzoylMAG3, sodium tartrate, lactose, and stannous chloride by adding sodium [99mTC]pertechnetate and heating the contents briefly. Constant-infusion renal whole-blood clearance obtained with [99mTc] MAG3 kits was compared with that obtained with high performance liquid chromatography (HPLC) pure [99mTc]MAG3 and with co-infused iodine-131 (131I) iodohippurate (OIH) in anesthetized rats. Average renal whole-blood clearance of [99mTc]MAG3 from kits was 3.9 +/- 0.4 ml/min/100 g body weight (mean +/- s.e.m. n = 5) and that for HPLC-pure [99mTc]MAG3 was 4.6 +/- 0.3 (n = 3). Renal whole-blood clearance ratios for [99mTc]MAG3 to co-infused iodine-131 (131I) OIH were greater than unity for both kit formulation (1.7 +/- 0.1) and HPLC-pure [99mTc]MAG3 (1.9 +/- 0.2). Differences in these two measures were not significant. Plasma binding (determined from blood drawn at the end of the infusion) of [99mTc]MAG3 prepared from both kits (75 +/- 2%, n = 4) and HPLC-separation (76 +/- 4%) were greater than that of [131I]OIH in corresponding plasma samples (31 +/- 1% and 32 +/- 2%) respectively). Renograms performed in anesthetized rats revealed no statistically significant differences between kit-prepared [99mTc]MAG3 and [131I]OIH in terms of time-to-peak renal activity (5.0 +/- 1.7 min, n = 6; and 2.2 +/- 0.2 min, n = 3, mean +/- s.e.m. for [99mTc]MAG3 and [131I]OIH, respectively), in terms of time to fall to half-maximal activity (15.3 +/- 2.4 min and 9.6 +/- 2.1 min, respectively), or in terms of fraction of peak radioactivity in right kidney (0.53 +/- 0.01 for both substances). To assess possible interference from hepatobiliary uptake and excretion in renal failure, radioactivity in liver regions of interest was followed by gamma camera scintigraphy for 30 min after intravenous injection of [131I]OIH and kit and HPLC-purified [99mTc]MAG3 in anesthetized rats rendered anephric by ligating renal peduncles. Liver activity was 25% of total for both preparations of [99mTc]MAG3 and was 22% of total for [131I]OIH. There were no significant differences among the substances.     

1-13C]glucose MRS in chronic hepatic encephalopathy in man.

[1-13C]-labeled glucose was infused intravenously in a single dose of 0.2 g/kg body weight over 15 min in six patients with chronic hepatic encephalopathy, and three controls. Serial 13C MR spectra of the brain were acquired. Patients exhibited the following characteristics relative to normal controls: 1) Cerebral glutamine concentration was increased (12.6 +/- 3.8 vs. 6.5 +/- 1.9 mmol/kg, P < 0.006) and glutamate was reduced (8.2 +/- 1.0 vs. 9.9 +/- 0.6 mmol/kg, P < 0.02). 2) 13C incorporation into glutamate C4 and C2 positions was reduced in patients (80 min after start of infusion C4: 0.43 +/- 0.09 vs. 0.84 +/- 0.15 mmol/kg, P < 0.001; C2: 0.20 +/- 0.03 vs. 0.45 +/- 0.07 mmol/kg, P < 0.0001). 3) 13C incorporation into bicarbonate was delayed (90 +/- 21 vs. 40 +/- 10 min, P < 0.003), and the time interval between detection of glutamate C4 and C2 labeling was longer in patients (22 +/- 8 vs. 12 +/- 3 min, P < 0.03). 4) Glutamate C2 turnover time was reduced in chronic hepatic encephalopathy (17.1 +/- 6.8 vs. 49.6 +/- 8.7 min, P < 0.0002). 5) 13C accumulation into glutamine C2 relative to its substrate glutamate C2 increased progressively with the severity of clinical symptoms (r = 0.96, P < 0.01). These data indicate disturbed neurotransmitter glutamate/glutamine cycling and reduced glucose oxidation in chronic hepatic encephalopathy. [1-13C] glucose MRS provides novel insights into disease progression and the pathophysiology of chronic hepatic encephalopathy.     

Ventricular performance and glucose uptake in rats during chronic hypobaric hypoxia

To identify changes of ventricular performance and their relationship to myocardial glucose uptake in Sprague-Dawley rats exposed to hypobaric hypoxia, radionuclide angiocardiograms (n = 34) and 2-[14C]deoxyglucose (2-[14C]DG) autoradiography (n = 14) were performed on rats maintained either for two weeks in air at 380 mmHg (hypoxic group), two weeks in hypobaric hypoxia followed by two weeks of air (recovered group), or in air (control group). Right ventricular ejection fraction (RVEF) was 66% +/- 2% (mean +/- s.e.m.) in controls, 40% +/- 3% during hypoxia, and 60% +/- 2% in recovered rats. LVEF remained unchanged. In controls, RV 2-[14C]DG uptake was 77% +/- 3% of LV uptake. During hypoxia, 2-[14C]DG uptake increased. This increase was greater within the RV than the LV and septum (85 +/- 16% versus 51 +/- 10%, p less than 0.005). The alterations of RV 2-[14C]DG uptake correlated with systolic pulmonary artery pressure (r = 0.77, p = 0.002).     

Synthesis and in vivo evaluation of [11C]zolpidem,an imidazopyridine with agonist properties at central benzodiazepine receptors

The synthesis and evaluation of [(11)C]zolpidem, an imidazopyridine with agonist properties at central benzodiazepine receptors, is reported herein. The reaction of desmethylzolpidem with [(11)C] methyl iodide afforded the title compound [(11)C]zolpidem in a yield of 19.19 +/- 3.23% in 41 +/- 2 min in specific activities of 0.995-1.19 Ci/micromol (1.115 +/- 0.105 Ci/micromol) (n = 3; decay corrected, EOB). The amount of radioactivity in the brain after tail vein injection in male Wistar rats was low, and the regional distribution was homogeneous and not consistent with the known distribution of the central benzodiazepine receptors. The frontal cortex/cerebellum ratio was not significantly greater than one (1.007 +/- 0.266 at 5 min) and did not increase from 5 to 40 min post-injection. A PET brain imaging study in one baboon confirmed the results obtained in rats. Therefore, it can be concluded that [(11)C]zolpidem is not a suitable tracer for in vivo visualization of central benzodiazepine receptors.     

Nitrogen-13 glutamate uptake and perfusion in Walker 256 carcinosarcoma before and after single-dose irradiation

Nitrogen-13 (13N) glutamate uptake was recorded in 18 anesthetized rats, both before and at least once after intervention. Each investigation was immediately followed by imaging of blood flow distribution using [11C]butanol. All animals had Walker 256 carcinosarcoma implants in one hind leg. Tumors were locally irradiated with a dose of 800 rad in 14 rats; in four rats, the vasoactive substance 5-hydroxytryptamine (5-HT) was administered. Prior to interventions, the [13N]glutamate tumor-to-muscle uptake showed a linear correlation with blood flow close to identity (y = 0.117 + 0.915x, r = 0.97). After irradiation, a discordant pattern was observed: blood flow tended to increase, while [13N]glutamate tumor-to-muscle uptake dropped from 4.30 +/- 0.66 (s.e.m.) to 3.06 +/- 0.36 (p less than 0.005) during 30 min and attained 4.04 +/- 0.67 2 days later. If [13N]glutamate tumor-to-muscle uptake was related to that of [11C] butanol in each individual animal, this index dropped from 0.93 +/- 0.03 (s.e.m.) to 0.62 +/- 0.04 (p less than 0.001) 30 min after irradiation and attained 0.90 +/- 0.09 after 2 days. In animals treated with 5-HT, [13N]glutamate and [11C]butanol showed a parallel drop from 6.60 +/- 0.84 to 2.10 +/- 0.60 (p less than 0.05) and from 6.8 +/- 0.78 to 2.08 +/- 0.74 (p less than 0.05), respectively. Thus, single-dose irradiation causes [13N]glutamate uptake to be uncoupled with respect to flow, while [13N]glutamate uptake in untreated tumors is flow-limited and responds together with flow on vasomotion.     

N-[11C-Methyl]chlorphentermine and N,N-[11C-dimethyl]chlorphentermine as brain blood-flow agents for positron emission tomography

N-[11C-methyl]chlorphentermine ([11C]NMCP) and N,N-[11C-dimethyl]chlorphentermine ([11C]NDMCP) were prepared from chlorphentermine and 11CH3I in DMF and evaluated in rats as brain blood-flow agents for positron emission tomography (PET). Tissue distribution of [11C]NMCP showed that brain uptake was 2.70 +/- 0.40% of injected dose per organ at 5 min with no change in radioactivity concentration up to 30 min after i.v. injection. Approximately 80% of the initial brain uptake remained at 60 min. On the other hand, initial brain uptake of [11C] NDMCP (3.66 +/- 0.31 and 3.63 +/- 0.88% injected dose per organ at 5 and 15 min, respectively) was greater than that of [11C]NMCP. The brain activity however, rapidly decreased to 2.38 +/- 0.17 and 1.82 +/- 0.32% at 30 and 60 min, respectively. Because of its longer retention in the brain compared with [11C]NDMCP, [11C]NMCP would be a potential brain blood-flow agent for quantitative PET studies.     

Myocardial kinetics of the (11)C-labeled enantiomers of the Ca(2+) channel inhibitor S11568: an in vivo study.

Ca(2+) channels play a key role in the basic working of the heart. There is one particular type of Ca(2+) channel in cardiac cells (L-type) whose gating is affected in different ways by beta-adrenoceptors and 1,4-dihydropyridines. In this study, we used ex vivo studies and PET to evaluate and compare the myocardial kinetics of the enantiomers labeled with (11)C (the more active: S12968, absolute configuration S; the less active: S12967, absolute configuration R) of the L-type Ca(2+) channel antagonist S11568 (3-ethyl 5-methyl (+/-)-2-[(2-(2-aminoethoxy)ethoxy) methyl]-4-(2,3-dichlorophenyl)-6-methyl-1,4-dihydropyridine-3,5-dicarboxylate). METHODS: [(11)C]S12968 was injected into the tail vein of rats (0.22 kBq--5.92 MBq) to assess the relationship between injected dose and myocardial uptake. A series of 5 rats was pretreated with 4 micromol unlabeled S12968 5 min before injection of 2.2 kBq [(11)C]S12968. In another series of 5 rats, unlabeled S12698 (4 micromol) was injected 5 min after injection of 2.2 kBq [(11)C]S12968. The animals were killed 15 min later, and the myocardial radioactivity was assessed in a gamma well counter. Beagle dogs received injections of 5-15 nmol [(11)C]S12968 or [(11)C]S12967 and were imaged with PET. Presaturation and displacement experiments using 2 micromol/kg unlabeled S12968 or 6 mol/kg S12967 were performed. RESULTS: In rats, a statistically significant relationship between myocardial uptake and injected dose of S12968 was observed. Pretreatment or displacement with unlabeled S12968 reduced myocardial radioactivity by 75% and 70%, respectively. In dogs, after injection of 5 nmol of each enantiomer, myocardial radioactivity plateaued within 3 min and the clearance from blood was rapid. Injection of 13--15 nmol [(11)C]S12968 led to a higher myocardial uptake and a more rapid washout, which were related to an increased coronary blood flow as shown by the linear relationship between k(1)--an estimate of coronary blood flow--and the mass of S12968 injected. Presaturation and displacement experiments showed that 70%--80% of S12968 binding was specific. This specificity was not observed with S12967. Plasma metabolite analysis showed that 70% of the compound was unchanged 20 min after injection. CONCLUSION: These results show the feasibility of imaging myocardial L-type Ca(2+) channels in vivo using [(11)C]S12968.     

Radiochemical synthesis and tissue distribution of p-[18F]DMPPF, a new 5-HT1A ligand for PET, in rats

Several studies have demonstrated the potential of p-[(18)F]MPPF as a radiopharmaceutical to study the 5-HT(1A) receptor family in animals and humans. A structural modification leading to a higher radioactive signal at an equipotent dose would greatly enhance this potential. With this goal, the desmethylated 4-(2'-methoxyphenyl)-1-[2'-[N-(2'-pyridinyl)-p-fluorobenzamido]ethyl]-piperazine (p-MPPF), identified as p-DMPPF, was synthesized, labeled with fluorine-18 and evaluated through ex vivo tissue distribution in rats. The new compounds p-DMPPF, p-DMPPNO(2), MEM-p-MPPF and MEM-p-MPPNO(2) were isolated and fully identified ((1)H and (13)C NMR, LC-MS). The final compound, p-[(18)F]DMPPF, was obtained ready for injection, with an overall radiochemical yield of 10% (EOB corrected) within 90 min and a specific activity of 62 GBq/mumol. Tissue distributions showed that the carbon-fluorine bond was stable in vivo and that this compound could cross the blood-brain barrier. For kidney, lung, heart, spleen, bone, testicle, liver and muscle, the percentage of injected dose per gram of tissue obtained with p-[(18)F]DMPPF was of the same order of magnitude as that of p-[(18)F]MPPF. The amount of radioactivity reaching the brain was much higher (approximately fivefold at 60 min) for p-[(18)F]DMPPF compared with p-[(18)F]MPPF, which was taken as reference. The distribution and specificity were in total agreement with the known localization of 5-HT(1A) receptors in rats. The radioactivity increase was more important for specific tissues (hippocampus and frontal cortex) than for cerebellum or striatum, leading to better contrast (hippocampus/cerebellum=5.8 at 60 min). The levels of metabolites found in plasma showed that p-[(18)F]DMPPF appears to be less metabolized than p-[(18)F]MPPF. p-[(18)F]DMPPF deserves further evaluation as a radiopharmaceutical candidate.     

In vivo 13C saturation transfer effect of the lactate dehydrogenase reaction.

Lactate dehydrogenase (LDH, EC 1.1.1.27) catalyzes an exchange reaction between pyruvate and lactate. It is demonstrated here that this reaction is sufficiently fast to cause a significant magnetization (saturation) transfer effect when the 13C resonance of pyruvate is saturated by a continuous-wave (CW) RF pulse. Infusion of [2-(13)C]glucose was used to allow labeling of pyruvate C2 at 207.9 ppm to determine the pseudo first-order rate constant of the unidirectional lactate-->pyruvate flux in vivo. During systemic administration of GABAA receptor antagonist bicuculline, this pseudo first-order rate constant was determined to be 0.08+/-0.01 s-1 (mean+/-SD, N=4) in halothane-anesthetized adult rat brains. In 9L and C6 rat glioma models, the 13C saturation transfer effect of the LDH reaction was also detected in vivo. Our results demonstrate that the 13C magnetization transfer effect of the LDH reaction may be useful as a novel marker for utilizing noninvasive in vivo MRS to study many physiological and pathological conditions, such as cancer.     

In vivo evaluation in rats of [(18)F]1-(2-fluoroethyl)-4-[(4-cyanophenoxy)methyl]piperidine as a potential radiotracer for PET assessment of CNS sigma-1 receptors

INTRODUCTION: Sigma-1 receptors are expressed throughout the mammalian central nervous system (CNS) and are implicated in several psychiatric disorders, including schizophrenia and depression. We have recently evaluated the high-affinity (K(D)=0.5+/-0.2 nM, log P=2.9) sigma-1 receptor radiotracer [(18)F]1-(3-fluoropropyl)-4-(4-cyanophenoxymethyl)piperidine, [(18)F]FPS, in humans. In contrast to appropriate kinetics exhibited in baboon brain, in the human CNS, [(18)F]FPS does not reach pseudoequilibrium by 4 h, supporting the development of a lower-affinity tracer [Waterhouse RN, Nobler MS, Chang RC, Zhou Y, Morales O, Kuwabara H, et al. First evaluation of the sigma-1 receptor radioligand [(18)F]1-3-fluoropropyl-4-((4-cyanophenoxy)-methyl)piperidine ([(18)F]FPS) in healthy humans. Neuroreceptor Mapping 2004, July 15-18th, Vancouver, BC Canada 2004]. We describe herein the in vivo evaluation in rats of [(18)F]1-(2-fluoroethyl)-4-[(4-cyanophenoxy)methyl]piperidine ([(18)F]SFE) (K(D)=5 nM, log P=2.4), a structurally similar, lower-affinity sigma-1 receptor radioligand. METHODS: [(18)F]SFE was synthesized (n=4) as previously described in good yield (54+/-6% EOB), high specific activity (2.1+/-0.6 Ci/micromol EOS) and radiochemical purity (98+/-1%) and evaluated in awake adult male rats. RESULTS: Similar to [(18)F]FPS, regional brain radioactivity concentrations [percentage of injected dose per gram of tissue (%ID/g), 15 min] for [(18)F]SFE were highest in occipital cortex (1.86+/-0.06 %ID/g) and frontal cortex (1.76+/-0.38 %ID/g), and lowest in the hippocampus (1.01+/-0.02%ID/g). Unlike [(18)F]FPS, [(18)F]SFE cleared from the brain with approximately 40% reduction in peak activity over a 90-min period. Metabolite analysis (1 h) revealed that [(18)F]SFE was largely intact in the brain. Blocking studies showed a large degree (>80%) of saturable binding for [(18)F]SFE in discrete brain regions. CONCLUSIONS: We conclude that [(18)F]SFE exhibits excellent characteristics in vivo and may provide a superior PET radiotracer for human studies due to its faster CNS clearance compared to [(18)F]FPS.     

In vivo 13CNMR spectroscopy of glucose metabolism of RIF-1 tumors

An efficient method for measuring in vivo 13C NMR spectra of tumors has been developed and employed to monitor glucose metabolism in radiation-induced fibrosarcomas (RIF-1) subcutaneously implanted in C3H/HeN mice. [1-13C]Glucose was injected directly into the tumors at a dose of 1 g/kg body wt. Spectra were obtained with a Bruker AM 360-WB spectrometer (8.4 T/8.9 cm bore) employing a homebuilt probe equipped with a four-turn solenoidal coil (1.5 cm outer diameter) for detection of 13C signals and a Helmholtz coil (two 3-cm turns separated by a 3-cm gap, oriented orthogonally to the 13C coil) for 1H decoupling. In addition to the natural abundance 13C resonances of the tumors, signals were detected from the alpha- and beta-anomers of labeled glucose. Within 15 min following injection of labeled glucose [3-13C]lactate and [3-13C]alanine were detected. Lactate labeling approached steady state levels within about 50 min after glucose injection: in contrast, alanine labeling increased continuously over the duration of the experiment (70 min). Sixty minutes after glucose injection, the ratio of the intensity of [3-13C]lactate to the principal lipid methylene resonance (30 ppm from external tetramethylsilane), which served as an internal intensity reference, was correlated with tumor size, whereas the corresponding ratio of the [3-13C]alanine resonance was not. Labeling of glutamate was below the level of detection in the in vivo spectra; however, labeling of C4-glutamate at a level approximately 50-fold lower than the level of [3-13C]lactate was detected in perchloric acid extracts. Incorporation of 13C label into C2- and C3-glutamate and C2-lactate was also observed.     

In vivo imaging of insulin receptors by PET: preclinical evaluation of iodine-125 and iodine-124 labelled human insulin.

[A(14)-*I]iodoinsulin was prepared for studies to assess the suitability of labeled iodoinsulin for positron emission tomography (PET). Iodine-125 was used to establish the methods and for preliminary studies in rats. Further studies and PET scanning in rats were carried out using iodine-124. Tissue and plasma radioactivity was measured as the uptake index (UI = [cpm x (g tissue)(-1)]/[cpm injected x (g body weight)(-1)]) at 1 to 40 min after intravenous injection of either [A(14)-(125)I]iodoinsulin or [A(14)-(124)I]iodoinsulin. For both radiotracers, initial clearance of radioactivity from plasma was rapid (T(1/2) approximately 1 min), reaching a plateau (UI = 2.8) at approximately 5 min which was maintained for 35 min. Tissue biodistributions of the two radiotracers were comparable; at 10 min after injection, UI for myocardium was 2.4, liver, 4.0, pancreas, 5.4, brain, 0.17, kidney, 22, lung, 2.3, muscle, 0.54 and fat, 0.28. Predosing rats with unlabelled insulin reduced the UI for myocardium (0.95), liver (1.8), pancreas (1.2) and brain (0.08), increased that for kidney (61) but had no effect on that for lung (2.5), muscle (0.50) or fat (0.34). Analysis of radioactivity in plasma demonstrated a decrease of [(125)I]iodoinsulin associated with the appearance of labeled metabolites; the percentage of plasma radioactivity due to [(125)I]iodoinsulin was 40% at 5 min and 10% at 10 min. The heart, liver and kidneys were visualized using [(124)I]iodoinsulin with PET.     

Biodistribution of [14C]PnAO in rats

The biodistribution of [14C]3,3'-(1,3-propanediyldiimino)-bis(3-methy 1-2-butanone)-dioxime, [( 14C]PnAO) was determined in anesthetized rats. The results of this study show that there is no significant brain accumulation or retention of the uncomplexed ligand in the brain.     

In vivo detection of cortical GABA turnover from intravenously infused [1-13C]D-glucose.

In this study [2-(13)C] gamma-aminobutyric acid (GABA) was spectrally resolved in vivo and detected simultaneously with [4-(13)C]glutamate (Glu) and [4-(13)C]glutamine (Gln) in the proton spectra obtained from a localized 40 microL voxel in rat neocortex with the use of an adiabatic (1)H-observed, (13)C-edited (POCE) spectroscopy method and an 89-mm-bore vertical 11.7 Tesla microimager. The time-resolved kinetics of (13)C label incorporation from intravenously infused [1-(13)C]glucose into [4-(13)C]Glu, [4-(13)C]Gln, and [2-(13)C]GABA were measured after acute administration of gabaculine, a potent and specific inhibitor of GABA-transaminase. In contrast to previous observations of a rapid turnover of [2-(13)C]GABA from [1-(13)C]glucose in intact rat brain, the rate of (13)C incorporation from [1-(13)C]glucose into [2-(13)C]GABA in the gabaculine-treated rats was found to be significantly reduced as a result of the blockade of the GABA shunt.