Use of a column-switching high-performance liquid chromatography method to assess the presence of specific binding of (R)- and (S)-[(11)C]rolipram and their labeled metabolites to the phosphodiesterase-4 enzyme in rat plasma and tissues

INTRODUCTION: To complement recent studies using the high-affinity (11)C-labeled phosphodiesterase-4 (PDE4) inhibitor (R)-rolipram and the less active enantiomer (S)-[(11)C]rolipram for in vivo quantification of PDE4 levels, we evaluated the presence of radiolabeled metabolites and their potential binding to PDE4 in the rat plasma, brain, heart, pancreas, skeletal muscle and brown adipose tissue. METHODS: A reverse-phase capture and analytical HPLC column-switch method was used to detect (R)-[(11)C]rolipram, (S)-[(11)C]rolipram and their radiolabeled metabolites in rat plasma and tissue extracts. The relative proportion of PDE4-specific binding of the radiotracers and their labeled metabolites was analyzed following co-injections with a saturating dose of unlabeled (R)-rolipram at 45 min post-tracer injection in tissue extracts. RESULTS: Radiolabeled metabolites were found in the plasma (72-75% of total radioactive signal), and in the heart, skeletal muscle, pancreas and brown adipose tissue (44-52%), but not in the brain. In comparison to polar labeled metabolites, the proportion of unchanged (R)-[(11)C]rolipram was reduced in PDE4-rich organs by co-injection of unlabeled (R)-rolipram. Conversely, no changes were obtained in brown adipose tissue, or with (S)-[(11)C]rolipram, suggesting that radiolabeled metabolites of (R)-[(11)C]rolipram display no specific binding to PDE4. CONCLUSIONS: Radiolabeled hydrophilic metabolites are unlikely to compete with (R)-[(11)C]rolipram for PDE4-specific retention. However, due to the high proportion of the radioactive metabolites in the total radioactive signal, any kinetic modeling calculations in the peripheral tissues will need to take into account the presence of labeled metabolites.     

Dependence of cardiac 11C-meta-hydroxyephedrine retention on norepinephrine transporter density.

The norepinephrine analog (11)C-meta-hydroxyephedrine (HED) is used with PET to map the regional distribution of cardiac sympathetic neurons. HED is rapidly transported into sympathetic neurons by the norepinephrine transporter (NET) and stored in vesicles. Although much is known about the neuronal mechanisms of HED uptake and retention, there is little information about the functional relationship between HED retention and cardiac sympathetic nerve density. The goal of this study was to characterize the dependence of HED retention on nerve density in rats with graded levels of cardiac denervation induced chemically with the neurotoxin 6-hydroxydopamine (6-OHDA). METHODS: Thirty male Sprague-Dawley rats were divided into 6 groups, and each group was administered a different dose of 6-OHDA: 0 (controls), 7, 11, 15, 22, and 100 mg/kg intraperitoneally. One day after 6-OHDA injection, HED (3.7-8.3 MBq) was injected intravenously into each animal and HED concentrations in heart and blood at 30 min after injection were determined. Heart tissues were frozen and later processed by tissue homogenization and differential centrifugation into a membrane preparation for in vitro measurement of cardiac NET density. A saturation binding assay using (3)H-mazindol as the radioligand was used to measure NET density (maximum number of binding sites [B(max)], fmol/mg protein) for each heart. RESULTS: In control animals, NET B(max) was 388 +/- 23 fmol/mg protein and HED heart uptake (HU) at 30 min was 2.89% +/- 0.35 %ID/g (%ID/g is percentage injected dose per gram tissue). The highest 6-OHDA dose of 100 mg/kg caused severe cardiac denervation, decreasing both NET B(max) and HED HU to 8% of their control values. Comparing values for all doses of 6-OHDA, HED retention had a strong linear correlation with NET density: HU = 0.0077B(max) -0.028, r(2) = 0.95. CONCLUSION: HED retention is linearly dependent on NET density in rat hearts that have been chemically denervated with 6-OHDA, suggesting that HED retention is a good surrogate measure of NET density in the rat heart. This finding is discussed in relation to clinical observations of the dependence of HED retention on cardiac nerve density in human subjects using PET.     

Determination of age-related changes in structure and function of skin, adipose tissue, and skeletal muscle with computed tomography, magnetic resonance imaging, and positron emission tomography

In this article, we report quantitative preliminary data obtained from retrospective analysis of (18)F-fluorodeoxyglucose positron emission tomography (FDG-PET) and combined PET-computed tomography (PET/CT) examinations in subjects ages 3 to 84 years pertaining to changes in the metabolism of skin, subcutaneous adipose tissue, visceral adipose tissue, and skeletal muscle with age, as well as age-related changes in skeletal muscle attenuation. We also propose a new method for identifying hypermetabolic brown fat on FDG-PET. Finally, we present a review of the literature regarding reported age-related structural and functional changes that occur in skin, fat, and skeletal muscle. Using FDG-PET, We evaluated 213 subjects for changes in the metabolism of skin, adipose tissue, and skeletal muscle with aging. Thirty-two separate subjects were chosen to measure maximum standardized uptake value (SUV) of hypermetabolic brown fat on dual-time point PET imaging. Finally, 15 subjects evaluated by PET/CT were selected to measure changes in metabolism and attenuation of skeletal muscle, and changes in metabolism of adipose tissue with aging. We found that skin, fat, and skeletal muscle all demonstrate significant (P < 0.05) increases in SUV with increasing age on PET imaging. Dual-time point PET imaging demonstrates increasing FDG uptake of hypermetabolic brown fat in various regions studied. Finally, our PET/CT studies revealed statistically insignificant (P > 0.05) decreases in SUV of adipose tissue with aging and the opposite trend in skeletal muscles (P > 0.05). Skeletal muscle attenuation in the various regions studied was found to significantly decrease with age (P < 0.05). Our study shows notable trends in metabolism and attenuation of skeletal muscle and metabolism of skin and adipose tissue that occur with normal aging. We hope that the methodologies and data we present here will serve as a useful starting point for those interested in conducting future prospective research on age-related changes in these structures.     

Evaluation of radioiodinated (2S,alphaS)-2-(alpha-(2-iodophenoxy)benzyl)morpholine as a radioligand for imaging of norepinephrine transporter in the heart

INTRODUCTION: The norepinephrine transporter (NET) is located presynaptically on noradrenergic nerve terminals and plays a critical role in the regulation of the synaptic norepinephrine (NE) concentration via the reuptake of NE. Changes in NET have been recently reported in several cardiac failures. Therefore, a NET-specific radioligand is useful for in vivo assessment of changes in NET density in various cardiac disorders. Recently, we developed a radioiodinated reboxetine analogue, (2S,alphaS)-2-(alpha-(2-iodophenoxy)benzyl)morpholine ((S,S)-IPBM), for NET imaging. In the current study, we assessed the applicability of radioiodinated (S,S)-IPBM to NET imaging in the heart. METHODS: The NET affinity and selectivity were measured from the ability to displace specific [3H]nisoxetine and (S,S)-[125 I]IPBM binding to rat heart membrane, respectively. To evaluate the distribution of (S,S)-[125 I]IPBM in vivo, biodistribution experiment was performed in rats. With the use of several monoamine transporter binding agents, pharmacological blocking experiments were performed in rats. RESULTS: In vitro binding assays showed that the affinity of (S,S)-IPBM to NET was similar to those of the well-known NET-specific binding agents, nisoxetine and desipramine. Furthermore, (S,S)-[125 I]IPBM binding was inhibited by nisoxetine and desipramine, but not by dopamine or serotonin transporter binding agents. These data indicated that (S,S)-IPBM had high affinity and selectivity for NET in vitro. Biodistribution studies in rats showed rapid and high uptake of (S,S)-[125 I]IPBM by the heart and rapid clearance from the blood. The heart-to-blood ratio was 31.9 at 180 min after the injection. The administration of nisoxetine and desipramine decreased (S,S)-[125 I]IPBM accumulation in the heart, but injection of fluoxetine and GBR12909 had little influence. CONCLUSIONS: Radioiodinated (S,S)-IPBM is a potential radioligand for NET imaging in the heart.     

Biodistribution of the fatty acid analogue 18F-FTHA: plasma and tissue partitioning between lipid pools during fasting and hyperinsulinemia.

Alterations of free fatty acid (FA) metabolism in several organs are implicated in the pathogenesis of chronic disorders. The aim of this study was to investigate the biodistribution and partitioning of the FA analog, 14(R,S)-(18)F-fluoro-6-thia-heptadecanoic acid ((18)F-FTHA), across different lipid pools in plasma and in metabolically important organs and its response to insulin. METHODS: Eight anesthetized pigs were studied during fasting or euglycemic insulin stimulation. Plasma samples from the carotid artery, hepatic vein, and portal vein were collected at 10 and 40 min after (18)F-FTHA injection via indwelling catheters. The animals were then sacrificed and tissue biopsies rapidly obtained from the heart, brain, liver, subcutaneous and visceral fat, pancreas, intestine, and skeletal muscle. Radioactivity was assessed in the FA, phospholipid, and triglyceride or glycerol ester pools. RESULTS: The tissue-to-plasma intact (18)F-FTHA ratio was high in all tissues, with the highest values being in the heart and liver; (18)F-FTHA accumulated in the brain to a significant extent. Hyperinsulinemia was associated with higher plasma (18)F-FTHA clearance (P < 0.05) and lower labeled triglyceride appearance (P 相似文献   

Reappearance of cardiac presynaptic sympathetic nerve terminals in the transplanted heart: correlation between PET using (11)C-hydroxyephedrine and invasively measured norepinephrine release.

Previously, sympathetic reinnervation of the transplanted heart has been described using invasive catheterization techniques and noninvasive radionuclide imaging techniques. However, little is known about the agreement between these 2 methods. Thus, correlation between (11)C-hydroxyephedrine (HED) PET and invasively measured norepinephrine (NE) release was investigated in transplant recipients in this study. METHODS: Using PET and the catecholamine analog HED, 17 patients were studied between 2 mo and 13.6 y after transplantation. Based on results in completely denervated hearts, areas with HED retention >7%/min were defined as reinnervated. Additionally, transcardiac NE release induced by intravenous tyramine (55 microg/kg) was measured by coronary sinus and aortic catheterization within 1 wk of the PET study. NE levels between coronary sinus and aortic root, DeltaNE(CS-AO), were calculated at baseline and after tyramine administration. Differences of more than 3 SD of baseline (>163 pg/mL) were interpreted as reinnervation. RESULTS: HED retention indicated reinnervation in 10 patients. Maximal HED retention ranged from 4.3%/min to 16.4%/min. DeltaNE(CS-AO) 1 min after tyramine administration ranged between -10 pg/mL and 1157 pg/mL, and 8 patients were above the reinnervation threshold. Fisher's exact test demonstrated good agreement between results of PET and DeltaNE(CS-AO) measurements (P = 0.002). Maximal HED retention was also significantly correlated with NE release (r = 0.69; P = 0.001). CONCLUSION: Results of invasively measured NE release and noninvasive (11)C-HED PET are well correlated. This study further supports the usefulness of PET as a noninvasive approach for detection of reappearance of catecholamine uptake sites after heart transplantation.     

Metabolism of 15 (p 123I iodophenyl-)pentadecanoic acid in heart muscle and noncardiac tissues

The uptake and turnover of omega(p 123I iodophenyl-)pentadecanoic acid (I-PPA), a radioiodinated free-fatty-acid analog, was examined in the heart, lung, liver, kidneys, spleen, and skeletal muscle of rats. At 2 min post injection, a high cardiac uptake of 4.4% dose per gram had already been achieved; this was followed by a rapid, two-component, tracer clearance. The kinetics of tissue concentrations of labeled hydrophilic catabolites indicated a rapid oxidation of I-PPA and the subsequent washout of I-PPA catabolites from heart-muscle tissue. The fractional distribution of the labeled cardiac lipids compared favorably with previously reported values for 3H-oleic- or 14C-palmitic-acid-labeled myocardial lipids. Typical patterns of I-PPA metabolism were observed in tissues depending on primary fatty-acid oxidation, lipid metabolism regulation, or I-PPA-catabolite excretion. The tissue concentrations and kinetics of I-PPA and its metabolites in the heart muscle indicated that general pathways of cardiac-lipid metabolism are traced by this new gamma-emitting isotope-labeled radiopharmaceutical.     

In vivo PET imaging of cardiac presynaptic sympathoneuronal mechanisms in the rat.

The sympathetic nervous system of the heart plays a key role in the pathophysiology of various cardiac diseases. Small-animal models are valuable for obtaining further insight into mechanisms of cardiac disease and therapy. To determine the translational potential of cardiac neuronal imaging from rodents to humans, we characterized the rat sympathetic nervous system using 3 radiotracers that reflect different subcellular mechanisms: (11)C-meta-hydroxyephedrine (HED), a tracer of neuronal transport showing stable uptake and no washout in healthy humans; (11)C-phenylephrine (PHEN), a tracer of vesicular leakage and intraneuronal metabolic degradation with initial uptake and subsequent washout in humans; and (11)C-epinephrine (EPI), a tracer of vesicular storage with stable uptake and no washout in humans. METHODS: We used a small-animal PET system to study healthy male Wistar rats at baseline, after desipramine (DMI) pretreatment (DMI block), and with DMI injection 15 min after tracer delivery (DMI chase). The rats were kept under general isoflurane anesthesia while dynamic emission scans of the heart were recorded for 60 min after radiotracer injection. A myocardial retention index was determined by normalizing uptake at 40 min to the integral under the arterial input curve. Washout rates were determined by monoexponential fitting of myocardial time-activity curves. RESULTS: At baseline, HED showed high myocardial uptake and sustained retention, EPI showed moderate uptake and significant biphasic washout, and PHEN showed moderate uptake and monoexponential washout. The average (+/- SD) left ventricular retention index for HED, PHEN, and EPI was 7.38% +/- 0.82%/min, 3.43% +/- 0.45%/min, and 4.24% +/- 0.59%/min, respectively; the washout rate for HED, PHEN, and EPI was 0.13% +/- 0.23%/min, 1.13% +/- 0.35%/min, and 0.50% +/- 0.24%/min, respectively. The DMI chase resulted in increased washout only for HED. DMI block decreased myocardial uptake of all tracers by less than 90%. CONCLUSION: Kinetic profiles of HED in the rat myocardium were similar to those of HED in humans, suggesting comparable neuronal transport density. Unlike in humans, however, significant washout of EPI and faster washout of PHEN were encountered, consistent with high intraneuronal metabolic activity, high catecholamine turnover, and reduced vesicular storage. This evidence of increased neuronal activity in rodents has implications for translational studies of cardiac neuronal biology in humans.     

Radioiodinated metaiodobenzylguanidine (MIBG): radiochemistry, biology, and pharmacology

As an analogue of adrenergic neurotransmitter norepinephrine (NE), metaiodobenzylguanidine (MIBG) demonstrates high uptake both in normal sympathetically innervated tissues, such as the heart and salivary glands, and in tumors that express the NE transporter (NET), specifically those of neural crest and neuroendocrine origin. In 1994, (131)I-MIBG, also known as iobenguane I-131 intravenous, received Food and Drug Administration (FDA) approval as an imaging agent. In 2008, (123)I-MIBG was also approved by FDA as a tumor imaging agent. Commercial formulations of radioiodinated MIBG are prepared on the basis of radioiodide exchange reaction with unlabeled MIBG as a precursor and contain large mass amounts of unlabeled MIBG, or "cold carrier," molecules. Because the cold MIBG molecules competitively inhibit the uptake of radiolabeled MIBG molecules by adrenergic and neuroendocrine cells expressing NET, no-carrier-added (n.c.a.), high specific activity (SA) radioiodinated MIBG preparations have been developed on the basis of electrophilic radioiodination reaction and solid-phase technology by using dibutylstanyl benzylguanidine precursor linked to polymers. On the basis of n.c.a. synthetic procedures, therapeutic doses of [(131)I]MIBG can be administered with very high SA (1600 mCi/μmol or 5734 mCi/mg). The very high SA of n.c.a. [(131)I]MIBG drug would increase the specific cellular uptake of adrenergic neurons and neuroendocrine tumor cells expressing NET.     

Unpredictable tensile strength biomechanics may limit thawed cadaver use for simulant research

A series of tensile strength experiments were conducted using an un-embalmed elderly 91-year-old female cadaver, who had been frozen and thawed on five occasions. One sample was taken from the heart, kidney, oesophagus, skeletal muscle, ascending aorta, trachea, spleen, liver, lung, pancreas, pericardium, skin (abdomen) and skin (thorax) and the tensile strengths were measured using universal test equipment – Hounsfield H50KM (Hounsfield Test Equipment Ltd, Surrey, UK). Tensile strengths were: heart 34.9 Pa, spleen 45.6 Pa, kidney 100.7 Pa, liver 106.2 Pa, pancreas 148.9 Pa, oesophagus 216.5 Pa, skeletal muscle 288.9 Pa, lung 293.6 Pa, ascending aorta 588.2 Pa, pericardium 1341.9 Pa, trachea 1523.9 Pa, skin (abdomen) 3483.9 Pa, and skin (thorax) 3999.9 Pa. Compared with published values for fresh cadavers (that are only available for certain tissues), some organs and tissues had tensile strengths that fell well below the normal range (heart, oesophagus and ascending aorta), compared with others where the tensile strengths were well above the normal range (kidney and skeletal muscle). Only one tissue, from the trachea, fell within the normal range. The remainder of the data gave relative tensile strengths of other organs and tissues with the spleen being the least, and skin being the most, elastic tissue. Freezing and thawing cadaveric organs and tissues may alter their physical properties in ways that are not predicable, with both increases and decreases in tensile strength. Although this pilot study allows the relative tensile strengths of such tissues to be compared, it also demonstrates that physical changes following freezing and thawing alter properties in such a way that its usefulness as a simulant for normal tissues may be limited.     

Glucose uptake of the muscle and adipose tissues in diabetes and obesity disease models: evaluation of insulin and β3-adrenergic receptor agonist effects by <Superscript>18</Superscript>F-FDG

Objective

One of the major causes of diabetes and obesity is abnormality in glucose metabolism and glucose uptake in the muscle and adipose tissue based on an insufficient action of insulin. Therefore, many of the drug discovery programs are based on the concept of stimulating glucose uptake in these tissues. Improvement of glucose metabolism has been assessed based on blood parameters, but these merely reflect the systemic reaction to the drug administered. We have conducted basic studies to investigate the usefulness of glucose uptake measurement in various muscle and adipose tissues in pharmacological tests using disease-model animals.

Methods

A radiotracer for glucose, ¹⁸F-2-deoxy-2-fluoro-d-glucose (¹⁸F-FDG), was administered to Wistar fatty rats (type 2 diabetes model), DIO mouse (obese model), and the corresponding control animals, and the basal glucose uptake in the muscle and adipose (white and brown) tissues were compared using biodistribution method. Moreover, insulin and a β3 agonist (CL316,243), which are known to stimulate glucose uptake in the muscle and adipose tissues, were administered to assess their effect. ¹⁸F-FDG uptake in each tissue was measured as the radioactivity and the distribution was confirmed by autoradiography.

Results

In Wistar fatty rats, all the tissues measured showed a decrease in the basal level of glucose uptake when compared to Wistar lean rats. On the other hand, the same trend was observed only in the white adipose tissue in DIO mice, while brown adipose tissue showed increments in the basal glucose uptake in this model. Insulin administration stimulated glucose uptake in both Wistar lean and fatty rats, although the responses were inhibited in Wistar fatty rats. The same tendency was shown also in control mice, but clear increments in glucose uptake were not observed in the muscle and brown adipose tissue of DIO mice after insulin administration. β3 agonist administration showed the similar trend in Wistar lean and fatty rats as insulin, while the responses were inhibited in the adipose tissues of Wistar fatty rats.

Conclusion

A system to monitor tissue glucose uptake with ¹⁸F-FDG enabled us to detect clear differences in basal glucose uptake between disease-model animals and their corresponding controls. The responses in the tissues to insulin or β3 agonist could be identified. Taken as a whole, the biodistribution method with ¹⁸F-FDG was confirmed to be useful for pharmacological evaluation of anti-diabetic or anti-obesity drugs using disease-model animals.

     

Impact of animal handling on the results of 18F-FDG PET studies in mice.

Small-animal PET scanning with (18)F-FDG is increasingly used in murine models of human diseases. However, the impact of dietary conditions, mode of anesthesia, and ambient temperature on the biodistribution of (18)F-FDG in mice has not been systematically studied so far. The aim of this study was to determine how these factors affect assessment of tumor glucose use by (18)F-FDG PET and to develop an imaging protocol that optimizes visualization of tumor xenografts. METHODS: Groups of severe combined immunodeficient (SCID) mice were first imaged by microPET with free access to food, at room temperature (20 degrees C), and no anesthesia during the uptake period (reference condition). Subsequently, the impact of (a) fasting for 8-12 h, (b) warming the animals with a heating pad (30 degrees C), and (c) general anesthesia using isoflurane or ketamine/xylazine on the (18)F-FDG biodistribution was evaluated. Subcutaneously implanted human A431 epidermoid carcinoma and U251 glioblastoma cells served as tumor models. RESULTS: Depending on the study conditions, (18)F-FDG uptake by normal tissues varied 3-fold for skeletal muscle, 13-fold for brown adipose tissue, and 15-fold for myocardium. Warming and fasting significantly reduced the intense (18)F-FDG uptake by brown adipose tissue observed under the reference condition and markedly improved visualization of tumor xenografts. Although tumor (18)F-FDG uptake was not above background activity under the reference condition, tumors demonstrated marked focal (18)F-FDG uptake in warmed and fasted animals. Quantitatively, tumor (18)F-FDG uptake increased 4-fold and tumor-to-organ ratios were increased up to 17-fold. Ketamine/xylazine anesthesia caused marked hyperglycemia and was not further evaluated. Isoflurane anesthesia only mildly increased blood glucose levels and had no significant effect on tumor (18)F-FDG uptake. Isoflurane markedly reduced (18)F-FDG uptake by brown adipose tissue and skeletal muscle but increased the activity concentration in liver, myocardium, and kidney. CONCLUSION: Animal handling has a dramatic effect on (18)F-FDG biodistribution and significantly influences the results of microPET studies in tumor-bearing mice. To improve tumor visualization mice should be fasted and warmed before (18)F-FDG injection and during the uptake period. Isoflurane appears well suited for anesthesia of tumor-bearing mice, whereas ketamine/xylazine should be used with caution, as it may induce marked hyperglycemia.     

Brown adipose tissue: Evaluation with<Superscript>201</Superscript>Tl and<Superscript>99m</Superscript>Tc-sestamibi dual-tracer SPECT

Brown adipose tissue is one kind of adipose tissue and regulates body temperature and balance of energy via non-shivering thermogenesis. The authors present a case that strongly suggested the presence of activated brown adipose tissue in the neck, shoulders and axillary space by increased 18F-FDG uptake. 99mTc-sestamibi and 201Tl dual-tracer SPECT study showed increased 99mTc-sestamibi uptake and non-increased 201Tl uptake in the corresponding 18F-FDG uptake sites. Brown adipose tissue has dense mitochondria in the cells, which play an important role in thermogenesis. 99mTc-sestamibi uptake and retention depend on the mitochondrial activity but 201Tl uptake does not. Therefore, the activity of mitochondria in activated brown adipose tissue may explain the discrepant uptake between 99mTc-sestamibi and 201Tl.     

Rolipram depresses [(3)H]2-deoxyglucose uptake in mouse brain and heart in vivo

The effects of systemic administration of rolipram, a selective phosphodiesterase type 4 inhibitor, on [(3)H]2-deoxyglucose (DG) uptake in brain and peripheral tissues were examined. Rolipram significantly and dose-dependently decreased [(3)H]DG uptake in brain, heart and skeletal muscle. In contrast, the radioactivity concentrations in the plasma of rolipram-treated mice were significantly higher than those of control mice at all times after injection of the tracer. In the kinetic study, the initial uptake of [(3)H]DG in brain was decreased by rolipram, whereas no significant differences were observed in the uptake in heart and skeletal muscle. However, radioactivity concentrations in the brain, heart and skeletal muscle 30 min after the injection of [(3)H]DG were significantly lowered by rolipram to about 60%, 10% and 10% of control values, respectively. The uptake of [(13)N]ammonia in brain and heart of rolipram-treated mice was slightly decreased, which indicated that rolipram diminished both cerebral and cardiac blood flow. These results indicate that the phosphorylation process via hexokinase rather than the transport of [(3)H]DG might be depressed by rolipram. Together with the previous observations that inhibition of protein kinase A (PKA) markedly enhanced [(14)C]DG uptake in rat brain, these results indicate an important role of the cAMP/PKA systems in the regulation of glucose metabolism in the living brain as well as in peripheral tissues such as the heart and skeletal muscle.     

Technetium-99m labelled imidodiphosphate: an improved bone-scanning radiopharmaceutical.

Technetium-99m labelled imidodiphosphate was prepared in the presence of stannous ions. It was evaluated as a bone-scanning agent in animals and patients. Comparative tissue distribution studies in mice showed a relatively higher uptake of radioactivity in bone when 99Tcm -labelled pyrophosphate and diphosphonate. Accumulation of radioactivity in soft tissues, especially kidneys, was less with this radiopharmaceutical. Results in patients were most satisfactory in delineating skeletal bone and identifying bone lesions with relatively small tracer doses of the radiopharmaceutical.     

Reduction of FDG uptake in brown adipose tissue in clinical patients by a single dose of propranolol

Purpose Uptake in brown adipose tissue (hibernating fat) is sometimes seen at FDG-PET examinations. Despite a characteristic appearance, this may hide clinically relevant uptake. Stimulation of the sympathetic nervous system increases glucose uptake of brown fat. We now re-examine patients with brown fat activity that could disguise tumour uptake after pre-treatment with propranolol (a non-selective β-blocker) in order to reduce the uptake. Our first examinations of this kind are reported. Methods Eleven patients with strong brown fat uptake were studied. There was a mean of 5 days (range 2–8) between the examinations. At the second examination, 80 mg of propranolol was given orally 2 h before FDG administration. In addition to visual evaluation of the brown fat uptake, SUV assessments of the uptake in brown fat, lung, heart, liver, spleen and bone marrow were made. Results All patients showed complete or almost complete disappearance of the brown fat activity at the second examination (p < 0.001) both upon visual evaluation and when comparing SUVs. In seven patients there was also uptake in a known or strongly suspected malignancy, which remained unchanged between the examinations. Beyond an insignificant decrease in the myocardial uptake, there was no redistribution to the various examined organs at the second examination. Conclusion Pre-treatment with a single dose of propranolol blocks the FDG uptake in brown adipose tissue, thereby increasing the specificity of the examination. The tumour uptake seems not to be impaired.     

Metabolism of 15 (p123I iodophenyl-)pentadecanoic acid in heart muscle and noncardiac tissues

The uptake and turnover of (p¹²³I iodophenyl-) pentadecanoic acid (I-PPA), a radioiodinated free-fatty-acid analog, was examined in the heart, lung, liver, kidneys, spleen, and skeletal muscle of rats. At 2 min post injection, a high cardiac uptake of 4.4% dose per gram had already been achieved; this was followed by a rapid, two-component, tracer clearance. The kinetics of tissue concentrations of labeled hydrophilic catabolites indicated a rapid oxidation of I-PPA and the subsequent washout of I-PPA catabolites from heart-muscle tissue. The fractional distribution of the labeled cardiac lipids compared favorably with previously reported values for ³H-oleic-or ¹⁴C-palmitic-acid-labeled myocardial lipids. Typical patterns of I-PPA metabolism were observed in tissues depending on primary fatty-acid oxidation, lipid metabolism regulation, or I-PPA-catabolite excretion. The tissue concentrations and kinetics of I-PPA and its metabolites in the heart muscle indicated that general pathways of cardiac-lipid metabolism are traced by this new -emitting isotope-labeled radiopharmaceutical.     

Biodistribution and radiation dosimetry estimates of 1-(2'-deoxy-2'-(18)F-Fluoro-1-beta-D-arabinofuranosyl)-5-bromouracil: PET imaging studies in dogs.

This study reports on the biodistribution and radiation estimates of 1-(2'-deoxy-2'-(18)F-fluoro-1-beta-d-arabinofuranosyl)-5-bromouracil ((18)F-FBAU), a potential tracer for imaging DNA synthesis. METHODS: Three normal dogs were intravenously administered (18)F-FBAU and a dynamic PET scan was performed for 60 min over the upper abdomen followed by a whole-body scan for a total of 150 min. Blood samples were collected at stipulated time intervals to evaluate tracer clearance and metabolism. Tissue samples of various organs were analyzed for tracer uptake and DNA incorporation. Dynamic accumulation of the tracer in different organs was derived from reconstructed PET images. The radiation dosimetry of (18)F-FBAU was evaluated using the MIRD method. RESULTS: At 60 min after injection, blood analysis found >90% of the activity in unmetabolized form. At 2 h after injection, (18)F-FBAU uptake was highest in proliferating tissues (mean SUVs: marrow, 2.6; small intestine, 4.0), whereas nonproliferative tissues showed little uptake (mean SUVs: muscle, 0.75; lung, 0.70; heart, 0.85; liver, 1.28). Dynamic image analysis over 60 min showed progressive uptake of the tracer in marrow. Extraction studies demonstrated that most of the activity in proliferative tissues was in the acid-insoluble fraction (marrow, 83%; small intestine, 73%), consistent with incorporation into DNA. In nonproliferative tissue, most of the activity was not found in the acid-insoluble fraction (>84% for heart, muscle, and liver). CONCLUSION: These results demonstrate that (18)F-FBAU was resistant to metabolism, readily incorporated into DNA in proliferating tissues, and showed good contrast between organs of variable DNA synthesis. These findings indicate that (18)F-FBAU may find use in measuring DNA synthesis with PET.     

Quantifying tumour hypoxia with fluorine-18 fluoroerythronitroimidazole ([<Superscript>18</Superscript>F]FETNIM) and PET using the tumour to plasma ratio

Fluorine-18 fluoroerythronitroimidazole ([(18)F]FETNIM) is a nitroimidazole compound that is potentially useful as a hypoxia marker in positron emission tomography (PET) studies of oncological patients. Our aim was to develop a simple protocol to quantitate uptake of [(18)F]FETNIM in hypoxic tumours. Dynamic imaging data from ten patients with head and neck cancer undergoing [(18)F]FETNIM PET was used in simulations and model fits to assess hypoxia marker uptake under different levels of blood flow. The distribution volume determined from dynamic PET study was compared with simple tumour to plasma and tumour to muscle ratios at 90-120 min. In skeletal muscle having a low but variable blood flow [2-6 ml/(100 gxmin)], differences in hypoxia-specific uptake of [(18)F]FETNIM remain small and may be hard to detect with PET. At higher blood flow [>20 ml/(100 gxmin)], the retention of [(18)F]FETNIM reflects the oxygenation status well and results in satisfactory contrast between hypoxic and well-oxygenated tissue. A good estimate of tissue hypoxia is accomplished by measuring the tissue to plasma [(18)F]FETNIM activity ratio using only a few late time points. The increased hypoxia-specific retention of [(18)F]FETNIM in tissues with high blood flow, such as malignant tumours, may facilitate application of [(18)F]FETNIM as a hypoxia marker in oncological patients. In the assessment of the tumour to non-target uptake ratio, plasma is the preferred reference tissue rather than muscle, which may show a more heterogeneous tracer uptake not easily controlled for.     

High specific radioactivity (1R,2S)-4-[(18)F]fluorometaraminol: a PET radiotracer for mapping sympathetic nerves of the heart

The radiolabeled catecholamine analogue (1R, 2S)-6-[(18)F]fluorometaraminol (6-[(18)F]FMR) is a substrate for the neuronal norepinephrine transporter. It has been used as a positron emission tomography (PET) ligand to map sympathetic nerves in dog heart. 6-[(18)F]FMR could be only synthesized with low specific radioactivity, which precluded its use in human subjects. We have recently prepared (1R,2S)-4-[(18)F]fluorometaraminol (4-[(18)F]FMR), a new fluoro-analogue of metaraminol, with high specific radioactivity (56-106 GBq/micromol). In the present study, we demonstrate in rats that 4-[(18)F]FMR possesses similar affinity toward myocardial norepinephrine transport mechanisms as 6-[(18)F]FMR. When compared with control animals, an 80% and 76% reduction in myocardial uptake was observed in animals pretreated with desipramine (an inhibitor of the neuronal norepinephrine transporter) and with reserpine (a blocker of the vesicular storage of monoamines), respectively. The entire radioactivity in rat myocardium represented unmetabolized parent tracer as determined by high performance liquid chromatography analysis of tissue extracts. In dogs, myocardial kinetics of 4-[(18)F]FMR were assessed using PET. A rapid and high uptake was observed, followed by prolonged cardiac retention. A heart-to-lung ratio of 15 was reached 10 min after injection of the radiotracer. Pretreatment with desipramine reduced the heart half-life of 4-[(18)F]FMR by 90% compared with control. Moreover, an infusion of tyramine caused a rapid decline of radioactivity in the heart. This demonstrates that 4-[(18)F]FMR specifically visualizes sympathetic neurons in dog heart. High specific radioactivity 4-[(18)F]FMR is a promising alternative to 6-[(18)F]FMR for myocardial neuronal mapping with PET in humans.