Synthesis and preclinical evaluation of [¹⁸F]FCHC for neuroimaging of fatty acid amide hydrolase

PURPOSE

Fatty acid amide hydrolase (FAAH), a catabolic enzyme which regulates lipid transmitters in the endocannabinoid system, is an avidly sought therapeutic and positron emission tomography (PET) imaging target for studies involving addiction and neurological disorders. We report the synthesis of a new fluorine-18-labeled FAAH inhibitor, trans-3-(4, 5-dihydrooxazol-2-yl)phenyl-4-[(18)F]fluorocyclohexylcarbamate ([(18)F]FCHC), and its evaluation in rat brain.

PROCEDURES

The synthesis of [(18)F]FCHC was conducted via a 3-step, 1-pot reaction, resulting in uncorrected radiochemical yields between 10 and 20% (n = 5) relative to [(18)F]fluoride, with specific activities of >5 Ci/μmol at the end of the synthesis. The radiosynthesis was seamlessly automated using a commercial radiofluorination apparatus. Ex vivo biodistribution and preliminary PET imaging studies were carried out in male Sprague-Dawley rats.

RESULTS

Rat brain biodistribution at 2 min post-injection showed a standard uptake value of 4.6 ± 0.1 in the cortex, which increased to 7.8 ± 0.1 at 40 min. Pretreatment with the selective FAAH inhibitor URB597 reduced uptake of radioactivity in all brain regions by >90%, with 98 % blockade in the FAAH-rich cortex. PET imaging was consistent with biodistribution studies.

CONCLUSIONS

[(18)F]FCHC appears to be a highly sensitive (18)F-labeled radiotracer for imaging FAAH in the central nervous system, and these results warrant further imaging in nonhuman primates.

Practical Radiosynthesis and Preclinical Neuroimaging of [11C]isradipine, a Calcium Channel Antagonist

In the interest of developing in vivo positron emission tomography (PET) probes for neuroimaging of calcium channels, we have prepared a carbon-11 isotopologue of a dihydropyridine Ca²⁺-channel antagonist, isradipine. Desmethyl isradipine (4-(benzo[c][1,2,5]oxadiazol-4-yl)-5-(isopropoxycarbonyl)-2,6-dimethyl-1,4-dihydropyridine-3-carboxylic acid) was reacted with [¹¹C]CH₃I in the presence of tetrabutylammonium hydroxide in DMF in an HPLC injector loop to produce the radiotracer in a good yield (6 ± 3% uncorrected radiochemical yield) and high specific activity (143 ± 90 GBq·µmol⁻¹ at end-of-synthesis). PET imaging of normal rats revealed rapid brain uptake at baseline (0.37 ± 0.08% ID/cc (percent of injected dose per cubic centimeter) at peak, 15–60 s), which was followed by fast washout. After pretreatment with isradipine (2 mg·kg⁻¹, i.p.), whole brain radioactivity uptake was diminished by 25%–40%. This preliminary study confirms that [¹¹C]isradipine can be synthesized routinely for research studies and is brain penetrating. Further work on Ca²⁺-channel radiotracer development is planned.

Initial in vivo PET imaging of 5-HT1A receptors with 3-[(18)F]mefway

4-trans-[(18)F]Mefway is a PET radiotracer with high affinity for 5-HT1A receptors. Our preliminary work indicated the positional isomer, 3-[(18)F]mefway, would be suitable for PET imaging of 5-HT1A receptors. We now compare the in vivo behaviour of 3-mefway with 4-mefway to evaluate 3-[(18)F]mefway as a potential 5-HT1A PET radiotracer. Two male rhesus macaques were given bolus injections of both 3- and 4-trans-[(18)F]mefway in separate experiments. 90 minute dynamic PET scans were acquired. TACs were extracted in the mesial temporal lobe (MTL) and caudal anterior cingulate gyrus (cACg). The cerebellum (CB) was used as a reference region. In vivo behavior of the radiotracers in the CB was compared based upon the ratio of normalized PET uptake for 3- and 4-trans-[(18)F]mefway. Specific binding was compared by examining MTL/CB and cACg/CB ratios. The subject-averaged ratio of 3-[(18)F]mefway to 4-trans-[(18)F]mefway in the cerebellum was 0.96 for 60-90 minutes. MTL/CB reached plateaus of ~2.7 and ~6 by 40 minutes and 90 minutes for 3- and 4-trans-[(18)F]mefway, respectively. cACg/CB reached plateaus of ~2.5 and ~6 by 40 minutes and 70 minutes for 3- and 4-trans-[(18)F]mefway, respectively. The short pseudoequilibration times and sufficient uptake of 3-[(18)F]mefway may be useful in studies requiring short scan times. Furthermore, the similar nondisplaceable clearance in the CB to 4-trans-[(18)F]mefway suggests the lower BPND of 3-[(18)F]mefway is due to a lower affinity. The lower affinity of 3-[(18)F]mefway may make it useful for measuring changes in endogenous 5-HT levels, however, this remains to be ascertained.

Simultaneous 99mTc-MDP/123I-MIBG tumor imaging using SPECT-CT: phantom and constructed patient studies

PURPOSE

Authors' goal is to evaluate the performance of simultaneous (99m)Tc-MDP/(123)I-MIBG tumor imaging with fast Monte-Carlo (MC) based joint iterative reconstruction as compared to sequential (99m)Tc-MDP and (123)I-MIBG tumor imaging.

METHODS

Noise-free (99m)Tc and (123)I SPECT projections were acquired separately using an anthropomorphic torso phantom modified to include a fillable tube around the lungs to mimic ribs. Additionally, (99m)Tc and (123)I projections were acquired separately using a 1-cm spherical "tumor" placed at various distances from one detector. Tumor-present data were generated by adding tumor projections to the torso phantom data, which were scaled to the total counts in typical clinical studies. Twenty-five noise realizations were generated by adding Poisson noise to the projection data for each radionuclide. Dual-radionuclide data were synthesized by summing the (99m)Tc and (123)I projections. Image reconstruction was performed using: (1) SR-OSEM, ordered subset expectation maximization (OSEM) without scatter correction (SC) using single-radionuclide (SR) data; (2) SR-MC-OSEM, OSEM with a fast MC-based SC using SR data; (3) DR-OSEM, OSEM without SC using dual-radionuclide (DR) data; and (4) DR-MC-JOSEM, joint OSEM with a fast MC-based SC using DR data. Ten (99m)Tc-MDP and ten (123)I-MIBG data sets, which had tumors mathematically inserted, were also used to evaluate the performance of authors' approach. For the phantom study, relative bias and relative standard deviation of tumor uptake were computed for each tumor using the tumor uptake in the noise-free single-radionuclide images, which were reconstructed by SR-MC-OSEM, as the gold standard. For both the phantom and constructed patient studies, mean contrast and standard deviation of contrast were computed for each tumor for both the single- and dual-radionuclide images. Additionally, contrast recovery was computed as the ratio between mean contrast and the mean contrast for SR-MC-OSEM.

RESULTS

For the phantom study, DR-MC-JOSEM yielded 2.7% on average relative bias of tumor uptake using the images, which were reconstructed from the noise-free SR data with SR-MC-OSEM, as the gold-standard. For both the phantom and constructed patient studies, DR-MC-JOSEM yielded 94.7% and 95.2% tumor contrast recovery on average using SR-MC-OSEM as the reference, in the phantom and constructed patient studies, respectively. DR-MC-JOSEM yielded comparable relative standard deviation of bias and standard deviation of contrast to SR-MC-OSEM.

CONCLUSIONS

Simultaneous (99m)Tc-MDP/(123)I-MIBG tumor imaging using authors' dual-radionuclide reconstruction approach yielded comparable image quality to sequential (99m)Tc-MDP and (123)I-MIBG imaging. For patients who need to undergo both scans, authors' approach offers perfectly registered dual-tracer images under identical conditions without compromising image quality, and reduces the imaging cost while increasing patient throughput.

Myocardial defect detection using PET-CT: phantom studies

It is expected that both noise and activity distribution can have impact on the detectability of a myocardial defect in a cardiac PET study. In this work, we performed phantom studies to investigate the detectability of a defect in the myocardium for different noise levels and activity distributions. We evaluated the performance of three reconstruction schemes: Filtered Back-Projection (FBP), Ordinary Poisson Ordered Subset Expectation Maximization (OP–OSEM), and Point Spread Function corrected OSEM (PSF–OSEM). We used the Channelized Hotelling Observer (CHO) for the task of myocardial defect detection. We found that the detectability of a myocardial defect is almost entirely dependent on the noise level and the contrast between the defect and its surroundings.

Membrane potential-dependent uptake of 18F-triphenylphosphonium--a new voltage sensor as an imaging agent for detecting burn-induced apoptosis

BACKGROUND

Mitochondrial dysfunction has been closely related to many pathologic processes, such as cellular apoptosis. Alterations in organelle membrane potential are associated with mitochondrial dysfunction. A fluorine-18 labeled phosphonium compound: (18)F-triphenylphosphonium ((18)F-TPP) was prepared to determine its potential use as a mitochondria-targeting radiopharmaceutical to evaluate cellular apoptosis.

METHODS

Studies were conducted in both ex vivo cell lines and in vivo using a burned animal model. Uptake of (18)F-TPP was assessed in PC-3 cells by gamma counting under the following conditions: graded levels of extracellular potassium concentrations, incubation with carbonyl cyanide m-chlorophenylhydrazone and staurosporine. Apoptosis was studied in a burn animal model using terminal deoxynucleotidyl transferase dUTP nick end labeling staining and simultaneous assessment of (18)F-TPP uptake by biodistribution.

RESULTS

We found that stepwise membrane depolarization by potassium (K) resulted in a linear decrease in (18)F-TPP uptake, with a slope of 0.62 ± 0.08 and a correlation coefficient of 0.936 ± 0.11. Gradually increased concentrations of m-chlorophenylhydrazone lead to decreased uptake of (18)F-TPP. Staurosporine significantly decreased the uptake of (18)F-TPP in PC-3 cells from 14.2 ± 3.8% to 5.6 ± 1.3% (P < 0.001). Burn-induced significant apoptosis (sham: 4.4 ± 1.8% versus burn: 24.6 ± 6.7 %; P < 0.005) and a reduced uptake of tracer in the spleens of burn-injured animals as compared with sham burn controls (burn: 1.13 ± 0.24% versus sham: 3.28 ± 0.67%; P < 0.005). Biodistribution studies demonstrated that burn-induced significant reduction in (18)F-TPP uptake in spleen, heart, lung, and liver, which were associated with significantly increased apoptosis.

CONCLUSIONS

(18)F-TPP is a promising new voltage sensor for detecting mitochondrial dysfunction and apoptosis in various tissues.

Bias atlases for segmentation-based PET attenuation correction using PET-CT and MR

This study was to obtain voxel-wise PET accuracy and precision using tissue-segmentation for attenuation correction. We applied multiple thresholds to the CTs of 23 patients to classify tissues. For six of the 23 patients, MR images were also acquired. The MR fat/in-phase ratio images were used for fat segmentation. Segmented tissue classes were used to create attenuation maps, which were used for attenuation correction in PET reconstruction. PET bias images were then computed using the PET reconstructed with the original CT as the reference. We registered the CTs for all the patients and transformed the corresponding bias images accordingly. We then obtained the mean and standard deviation bias atlas using all the registered bias images. Our CT-based study shows that four-class segmentation (air, lungs, fat, other tissues), which is available on most PET-MR scanners, yields 15.1%, 4.1%, 6.6%, and 12.9% RMSE bias in lungs, fat, non-fat soft-tissues, and bones, respectively. An accurate fat identification is achievable using fat/in-phase MR images. Furthermore, we have found that three-class segmentation (air, lungs, other tissues) yields less than 5% standard deviation of bias within the heart, liver, and kidneys. This implies that three-class segmentation can be sufficient to achieve small variation of bias for imaging these three organs. Finally, we have found that inter- and intra-patient lung density variations contribute almost equally to the overall standard deviation of bias within the lungs.

Reprogramming of intestinal glucose metabolism and glycemic control in rats after gastric bypass

The resolution of type 2 diabetes after Roux-en-Y gastric bypass (RYGB) attests to the important role of the gastrointestinal tract in glucose homeostasis. Previous studies in RYGB-treated rats have shown that the Roux limb displays hyperplasia and hypertrophy. Here, we report that the Roux limb of RYGB-treated rats exhibits reprogramming of intestinal glucose metabolism to meet its increased bioenergetic demands; glucose transporter-1 is up-regulated, basolateral glucose uptake is enhanced, aerobic glycolysis is augmented, and glucose is directed toward metabolic pathways that support tissue growth. We show that reprogramming of intestinal glucose metabolism is triggered by the exposure of the Roux limb to undigested nutrients. We demonstrate by positron emission tomography-computed tomography scanning and biodistribution analysis using 2-deoxy-2-[18F]fluoro-D-glucose that reprogramming of intestinal glucose metabolism renders the intestine a major tissue for glucose disposal, contributing to the improvement in glycemic control after RYGB.

Functional genomics of attention-deficit/hyperactivity disorder (ADHD) risk alleles on dopamine transporter binding in ADHD and healthy control subjects

BACKGROUND

The main aim of this study was to examine the relationship between dopamine transporter (DAT) binding in the striatum in individuals with and without attention-deficit/hyperactivity disorder (ADHD), attending to the 3'-untranslated region of the gene (3'-UTR) and intron8 variable number of tandem repeats (VNTR) polymorphisms of the DAT (SLC6A3) gene.

METHODS

Subjects consisted of 68 psychotropic (including stimulant)-naïve and smoking-naïve volunteers between 18 and 55 years of age (ADHD n = 34; control subjects n = 34). Striatal DAT binding was measured with positron emission tomography with 11C altropane. Genotyping of the two DAT (SLC6A3) 3'-UTR and intron8 VNTRs used standard protocols.

RESULTS

The gene frequencies of each of the gene polymorphisms assessed did not differ between the ADHD and control groups. The ADHD status (t = 2.99; p<.004) and 3'-UTR of SLC6A3 9 repeat carrier status (t = 2.74; p<.008) were independently and additively associated with increased DAT binding in the caudate. The ADHD status was associated with increased striatal (caudate) DAT binding regardless of 3'-UTR genotype, and 3'-UTR genotype was associated with increased striatal (caudate) DAT binding regardless of ADHD status. In contrast, there were no significant associations between polymorphisms of DAT intron8 or the 3'-UTR-intron8 haplotype with DAT binding.

CONCLUSIONS

The 3'-UTR but not intron8 VNTR genotypes were associated with increased DAT binding in both ADHD patients and healthy control subjects. Both ADHD status and the 3'-UTR polymorphism status had an additive effect on DAT binding. Our findings suggest that an ADHD risk polymorphism (3'-UTR) of SLC6A3 has functional consequences on central nervous system DAT binding in humans.

Brown adipose tissue and its modulation by a mitochondria-targeted peptide in rat burn injury-induced hypermetabolism

Hypermetabolism is a prominent feature of burn injury, and altered mitochondria function is presumed to contribute to this state. Recently, brown adipose tissue (BAT) was found to be present not only in rodents but also in humans, and its activity is associated with resting metabolic rate. In this report, we elucidate the relationship between burn injury-induced hypermetabolism and BAT activity and the possible role of the mitochondria-targeted peptide SS31 in attenuating burn injury-induced hypermetabolism by using a rat burn injury model. We demonstrate that burn injury induces morphological changes in interscapular BAT (iBAT). Burn injury was associated with iBAT activation, and this effect was positively correlated with increased energy expenditure. BAT activation was associated with augmentation of mitochondria biogenesis, and UCP1 expression in the isolated iBAT mitochondria. In addition, the mitochondria-targeted peptide SS31 attenuated burn injury-induced hypermetabolism, which was accompanied by suppression of UCP1 expression in isolated mitochondria. Our results suggest that BAT plays an important role in burn injury-induced hypermetabolism through its morphological changes and expression of UCP1.

5-HT2 receptor distribution shown by [18F] setoperone PET in high-functioning autistic adults

The serotonergic system is implicated in disordered emotional behavior. Autism is characterized by impaired processing of emotional information. The serotonergic (5-HT) system is also critically involved in brain development, and abnormal brain synthesis of serotonin is observed in autism. Furthermore, whole blood and platelet serotonin have been reported to be elevated in autism. The authors examined the CNS serotonin system in autism in vivo. 5-HT2 receptors were visualized by PET imaging of [18F]setoperone-binding in this pilot study of 6 high-functioning autistic adults and 10 matched-control participants. Autism subjects had less thalamic [18F]setoperone binding than controls, when covaried for age, but no difference reached significance in other areas. A negative relationship between thalamic binding and history of language impairment was also observed. Further studies will be needed to gain a clearer picture of the role of the 5-HT system in autism.

Isolation rearing significantly perturbs brain metabolism in the thalamus and hippocampus

Psychosocial neglect during childhood severely impairs both behavioral and physical health. The isolation rearing model in rodents has been employed by our group and others to study this clinical problem at a basic level. We previously showed that immediate early gene (IEG) expression in the hippocampus and medial prefrontal cortex (mPFC) is decreased in isolation-reared (IR) compared to group-reared (GR) rats. In the current study, we sought to evaluate: (1) whether these changes in IEG expression would be detected by the measurement of brain glucose metabolism using positron emission tomography (PET) with fluorodeoxyglucose (FDG) and (2) whether PET FDG could illuminate other brain regions with different glucose metabolism in IR compared to GR rats. We found that there were significant differences in FDG uptake in the hippocampus that were consistent with our findings for IEG expression (decreased mean FDG uptake in IR rats). In contrast, in the mPFC, the FDG uptake between IR and GR rats did not differ. Finally, we found decreased mean FDG uptake in the thalamus of the IR rats, a region we had not previously examined. The results suggest that PET FDG has the potential to be utilized as a biomarker of molecular changes in the hippocampus. Further, the differences found in thalamic brain FDG uptake suggest that further investigation of this region at the molecular and cellular levels may provide an important insight into the neurobiological basis of the adverse clinical outcomes found in children exposed to psychosocial deprivation.

Understanding the central pharmacokinetics of spheroidal oral drug absorption system (SODAS) dexmethylphenidate: a positron emission tomography study of dopamine transporter receptor occupancy measured with C-11 altropane

OBJECTIVE

Pediatric studies of the long-acting formulation (spheroidal oral drug absorption system [SODAS]) of the isomer dexmethylphenidate have shown a dose-dependent efficacy through 12 hours. However, there are no studies of central nervous system (CNS) dopamine transporter occupancies.

METHOD

Eighteen healthy volunteers underwent positron emission tomography (PET) imaging with C-11 altropane before and after administration of oral doses of SODAS dexmethylphenidate. Each group of 6 subjects received 1 of 3 doses (20 mg, 30 mg, 40 mg) before PET imaging at 1, 8, 10, 12 (20 mg and 30 mg), or 1, 8, 10, and 14 (40 mg) hours after dosing. Transporter occupancy was calculated by standard methods. The study was conducted from January 2007 through December 2007.

RESULTS

For all doses, plasma dexmethylphenidate levels and CNS dopamine transporter occupancies were greatest at 8 hours and decreased over time at 10, 12, and 14 hours. Plasma dexmethylphenidate levels were correlated to dose (P < .003). Mean plasma levels were ≥ 6 ng/mL to at least 8 hours with 20 mg (5.7 ng/mL), 10 hours with 30 mg, and 12 hours (extrapolated) with 40 mg. Dopamine transporter occupancies in the right caudate were 47% at 8 hours with 20 mg, 42% at hour 10 with 30 mg, and 46% (extrapolated) at hour 12 with 40 mg. Dopamine transporter occupancy was significantly correlated with plasma concentration of dexmethylphenidate (P < .001).

CONCLUSIONS

These results confirm the study hypothesis that central dopamine transporter occupancy parallels peripheral pharmacokinetic findings in orally administered long-acting dexmethylphenidate in later hours after administration.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT00593138.

Association of heat production with 18F-FDG accumulation in murine brown adipose tissue after stress

Previous studies have demonstrated that cold stress results in increased accumulation of (18)F-FDG in brown adipose tissue (BAT). Although it has been assumed that this effect is associated with increased thermogenesis by BAT, direct measurements of this phenomenon have not been reported. In the current investigation, we evaluated the relationship between stimulation of (18)F-FDG accumulation in BAT by 3 stressors and heat production measured in vivo by thermal imaging. Male SKH-1 hairless mice were subjected to full-thickness thermal injury (30% of total body surface area), cold stress (4°C for 24 h), or cutaneous wounds. Groups of 6 animals with each treatment were kept fasting overnight and injected with (18)F-FDG. Sixty minutes after injection, the mice were sacrificed, and biodistribution was measured. Other groups of 6 animals subjected to the 3 stressors were studied by thermal imaging, and the difference in temperature between BAT and adjacent tissue was recorded (ΔT). Additional groups of 6 animals were studied by both thermal imaging and (18)F-FDG biodistribution in the same animals. Accumulation of (18)F-FDG in BAT was significantly (P < 0.0001) increased by all 3 treatments (burn, ∼5-fold; cold, ∼15-fold; and cutaneous wound, ∼15-fold), whereas accumulation by adjacent white adipose tissue was unchanged. Compared with sham control mice, in animals exposed to all 3 stressors, ΔTs showed significant (P < 0.001) increases. The ΔT between stressor groups was not significant; however, there was a highly significant linear correlation (r(2) = 0.835, P < 0.0001) between the ΔT measured in BAT versus adjacent tissue and (18)F-FDG accumulation. These results establish, for the first time to our knowledge, that changes in BAT temperature determined in vivo by thermal imaging parallel increases in (18)F-FDG accumulation.

Evaluation of the antioxidant peptide SS31 for treatment of burn-induced insulin resistance

After severe burn injury and other major traumas, glucose tolerance tests demonstrate delayed glucose disposal. This 'diabetes of injury' could be explained by insulin deficiency, and several studies have shown that soon after trauma (ebb phase) insulin concentrations are reduced in the face of hyperglycemia. After resuscitation of trauma patients (flow phase), β-cell responsiveness normalizes and plasma insulin levels are appropriate or even higher than expected, however, glucose intolerance and hyperglycemia persist. In the acute care setting, several approaches have been used for treating insulin resistance, including insulin infusion, propranolol and glucagon-like-peptide-1 (GLP-1). Recently, it was demonstrated that a tetrapeptide with antioxidant properties D-Arg-Dmt-Lys-Phe-NH2 (SS31), but not its inactive analogue Phe-D-Arg-Phe-Lys-NH2 (SS20) attenuates insulin resistance in mice maintained on a high fat diet. In this report the effects of SS31 and SS20 on burn-induced insulin resistance was studied in mice. Oral glucose tolerance tests (OGTT) were performed in 4 groups of 6 mice with thermal injury with or without pre-treatment with SS31 or SS20 and sham controls. In addition, biodistribution of 18FDG was measured in burned mice with and without SS31 treatment and shams (subsets of these animals were also studied by µPET). For comparison purposes, groups of 6 cold-stressed mice with and without SS31 treatment were also studied. The results of these studies demonstrate that SS31 but not SS20 ameliorated burn-induced insulin resistance. In addition, SS31 treatment resulted in marked reduction in the increased 18FDG uptake by brown adipose tissue (BAT) in burned but not cold-stressed animals; suggesting that the stressors act by different mechanisms. Overall, these studies confirmed that SS31 can be used to reverse burn-induced insulin resistance and provide a firm pre-clinical basis for future clinical trials of SS31 for the treatment of insulin resistance in patients with burn injury.

Effects of burn injury, cold stress and cutaneous wound injury on the morphology and energy metabolism of murine brown adipose tissue (BAT) in vivo

AIMS

Cold stress has been shown to produce dramatic increases in 2-fluoro-2-deoxy-D-Glucose ((18)FDG) accumulation by brown adipose tissue (BAT) in rodents. However, neither the effects of other types of stress on (18)FDG accumulation nor the effects of stressors on the accumulation of tracers of other aspects of energy metabolism have been evaluated. In this report we studied the effects of cold stress, burn injury and cutaneous wounds on murine BAT at the macroscopic, microscopic and metabolic level.

MAIN METHODS

Glucose metabolism was studied with (18)FDG, fatty acid accumulation was evaluated with trans-9(RS)-(18)F-fluoro-3,4(RS,RS)-methyleneheptadecanoic acid (FCPHA) and tricarboxcylic acid cycle (TCA) activity was evaluated with (3)H acetate.

KEY FINDINGS

All three stressors produced dramatic changes in BAT at the macroscopic and microscopic level. Macroscopically, BAT from the stressed animals appeared to be a much darker brown in color. Microscopically BAT of stressed animals demonstrated significantly fewer lipid droplets and an overall decrease in lipid content. Accumulation of (18)FDG by BAT was significantly (p<0.01) increased by all 3 treatments (Cold: ~16 fold, burn ~7 Fold and cutaneous wound ~14 fold) whereas uptake of FDG by white fat was unchanged. This effect was also demonstrated non invasively by μPET imaging. Although less prominent than with (18)FDG, BAT uptake of FCPHA and acetate were also significantly increased by all three treatments. These findings suggest that in addition to cold stress, burn injury and cutaneous wounds produce BAT activation in mice.

SIGNIFICANCE

This study demonstrates brown fat activated by several stressors leads to increased uptake of various substrates.

Iodine-124 as a label for pharmacological PET imaging

With the growing number of biotechnology products and drug delivery systems entering preclinical and clinical studies, pharmacological imaging studies with PET play an increasingly significant role. Such studies often require investigation of slow and complex pharmacokinetics (PK). This suggests labeling of the drug candidate with radionuclides that have long physical half-lives. Among the currently available PET positron emitters, ¹²⁴I has the longest physical half-life (4.2 days). This, combined with the well-investigated behavior of iodine in vivo, makes ¹²⁴I very attractive for pharmacological studies. However, the high energy of the positrons emitted by ¹²⁴I and the presence of single photons in the ¹²⁴I emission can potentially introduce limitations in the quantitative analysis of the images. The objective of this research was to determine whether the use of ¹²⁴I as a PET label provides data quality suitable for PK studies. The study was carried out using MicroPET P4 scanner (Siemens/Concorde Microsystems). Spatial resolution, count-rate performance, sensitivity and scatter fraction were measured using a line source and a cylindrical phantom. Model animal studies in rats and cynomolgus monkeys were carried out using human recombinant proteins. The proteins were labeled with ¹²⁴I, up to 185 MBq/mg. The transaxial and axial spatial resolutions in the center of the camera were satisfactory and higher for OSEM3D/MAP than FORE-2DFBP (FWHM 2.52 vs 3.31 mm, and 3.10 vs 3.69 mm). Linearity of the true coincidence count-rate was observed up to 44 MBq. Animal studies demonstrated excellent delineation and resolution of even very small organs. At optimal doses, 2-10 MBq per animal for rodents and 4-10 MBq per kg of body weight for larger animals, the quality of numerical data was appropriate for PK analysis in all experimental timeframes from minutes (dynamic studies) to 10 days. Overall, the data suggest that ¹²⁴I is an excellent label for quantitative pharmacological PET imaging studies.

Effect of simvastatin on burn-induced alterations in tissue specific glucose metabolism: implications for burn associated insulin resistance

In addition to their primary role in lowering plasma cholesterol, statins have a variety of other actions. We studied the effect of simvastatin treatment on burn injury-induced changes in regional glucose metabolism. Groups of six CD-1 mice (male, approximately 25 g) were subjected to full thickness 30% total body surface area (TBSA) burn injury. The animals were treated with simvastatin at various doses (0.02, 0.2 and 2.0 microg/kg, i.p.) for seven days. The following morning, mice were injected with 18F labeled 2-fluoro-2-deoxy-D-glucose (18FDG) (50 microCi) via the tail vein. Approximately 60 min after tracer injection, the animals were sacrificed and biodistribution was measured. A sub-set of burned mice with and without statin treatment and sham controls was injected with approximately 1.0 mCi of FDG and tracer distribution was evaluated by microPET. In addition, oral glucose tolerance tests (OGTT) were performed in other groups of burned mice with and without statin treatment and sham controls. In the heart and brown adipose tissue (BAT), burn injury produced a highly significant increase in 18FDG accumulation (p<0.01), whereas tracer accumulation in brain was markedly reduced (p<0.01). In the heart and BAT, simvastatin treatment produced dose-dependent reductions in 18FDG accumulation. In contrast, simvastatin did not affect 18FDG accumulation in the brain. There was no effect of simvastatin treatment on 18FDG accumulation in the heart, BAT or brain of sham-treated mice. Less pronounced effects were detected in other tissues that were studied. All animals had normal plasma glucose levels (approximately 90 mg/dl). The OGTTs demonstrated insulin resistance in burn injured mice which was reversed by statin treatment. Our results indicate that simvastatin reverses burn-induced increases in 18FDG accumulation by the heart and BAT in a dose-dependent manner but does not affect burn-induced reductions of 18FDG accumulation by the brain. These findings suggest that statins exert some of their effects by tissue specific modulation of glucose metabolism.

A PET study examining pharmacokinetics and dopamine transporter occupancy of two long-acting formulations of methylphenidate in adults

The delivery systems of two long-acting formulations of methylphenidate (MPH) were designed for different durations. Diffucaps bead-delivery system (DBDS)-MPH was designed to last 8 h and osmotically controlled-release oral delivery system (OROS)-MPH was designed to last 12 h. While the plasma pharmacokinetics and timing of efficacy have been studied, the corresponding central nervous system dopamine transporter (DAT) occupancies are unknown. In this study, 21 healthy volunteers underwent PET imaging with 11C Altropane before and after administration of oral doses of DBDS-MPH and OROS-MPH. Each subject received 40 mg DBDS-MPH and 36 mg OROS-MPH on different days. PET imaging occurred at 10 h after dosing. Each subject was injected with 5 mCi of 11C Altropane and serial images of the brain were acquired over 60 min with a Siemens HR+ PET camera. Binding potentials (BP, k3/k4) were calculated from time-activity curves using the simplified reference region method with cerebellum as reference. Transporter occupancy was calculated by standard methods. At 10 h, plasma d-MPH levels were lower (3.8+/-1.2 vs. 5.2+/-2.0) and brain DAT occupancy was lower (34.8+/-12.9 vs. 44.3+/-11.8) for DBDS-MPH than OROS-MPH. Across the range of values, for each unit of change in plasma d-MPH level there was a larger change in DAT occupancy with the DBDS-MPH formulation than with the OROS-MPH formulation. As predicted from previous pharmacokinetic and efficacy data, the average plasma level and DAT occupancy of 36 mg OROS-MPH was >40 mg DBDS-MPH at 10 h. Moreover, a relatively small difference in plasma levels (1.4 ng/ml at 10 h) was associated with a more impressive difference in DAT occupancy ( approximately 10% at 10 h).

Further evidence of dopamine transporter dysregulation in ADHD: a controlled PET imaging study using altropane

BACKGROUND

The dopamine transporter (DAT) is known to be a key regulator of dopamine, and recent studies of genetics, treatment, and imaging have highlighted the role of DAT in attention-deficit/hyperactivity disorder (ADHD). The findings of in vivo neuroimaging of DAT in ADHD have been somewhat discrepant, however.

METHOD

Dopamine transporter binding was measured using a highly selective ligand (C-11 altropane) and positron emission tomography (PET). The sample consisted of 47 well-characterized, treatment-naïve, nonsmoking, non-comorbid adults with and without ADHD. Additionally, control subjects had few symptoms of ADHD.

RESULTS

Results showed significantly increased DAT binding in the right caudate in adults with ADHD compared with matched control subjects without this disorder.

CONCLUSIONS

These results confirm abnormal DAT binding in the striatum of adults with ADHD and provide further support that dysregulation of DAT may be an important component of the pathophysiology of ADHD.

PET/CT imaging for treatment verification after proton therapy: a study with plastic phantoms and metallic implants

The feasibility of off-line positron emission tomography/computed tomography (PET/CT) for routine three dimensional in-vivo treatment verification of proton radiation therapy is currently under investigation at Massachusetts General Hospital in Boston. In preparation for clinical trials, phantom experiments were carried out to investigate the sensitivity and accuracy of the method depending on irradiation and imaging parameters. Furthermore, they addressed the feasibility of PET/CT as a robust verification tool in the presence of metallic implants. These produce x-ray CT artifacts and fluence perturbations which may compromise the accuracy of treatment planning algorithms. Spread-out Bragg peak proton fields were delivered to different phantoms consisting of polymethylmethacrylate (PMMA), PMMA stacked with lung and bone equivalent materials, and PMMA with titanium rods to mimic implants in patients. PET data were acquired in list mode starting within 20 min after irradiation at a commercial luthetium-oxyorthosilicate (LSO)-based PET/CT scanner. The amount and spatial distribution of the measured activity could be well reproduced by calculations based on the GEANT4 and FLUKA Monte Carlo codes. This phantom study supports the potential of millimeter accuracy for range monitoring and lateral field position verification even after low therapeutic dose exposures of 2 Gy, despite the delay between irradiation and imaging. It also indicates the value of PET for treatment verification in the presence of metallic implants, demonstrating a higher sensitivity to fluence perturbations in comparison to a commercial analytical treatment planning system. Finally, it addresses the suitability of LSO-based PET detectors for hadron therapy monitoring. This unconventional application of PET involves countrates which are orders of magnitude lower than in diagnostic tracer imaging, i.e., the signal of interest is comparable to the noise originating from the intrinsic radioactivity of the detector itself. In addition to PET alone, PET/CT imaging provides accurate information on the position of the imaged object and may assess possible anatomical changes during fractionated radiotherapy in clinical applications.

Decreased striatal D1 binding as measured using PET and [11C]SCH 23,390 in patients with major depression with anger attacks

This study assessed striatal dopamine 1 (D1) receptor binding in patients with major depressive disorder and anger attacks (MDD+A) and healthy volunteers. We used positron emission tomography with [(11)C]SCH 23,390 to compare 10 patients with MDD+A to 10 healthy volunteers. [(11)C]SCH 23,390 binding in bilateral striata was significantly lower in the MDD+A group when compared to healthy volunteers. These results implicate striatal D1 receptor dysfunction in MDD+A and further suggest an association between dopaminergic transmission and anger or aggression.

Dopamine Transporter (DAT) Inhibitors Alleviate Specific Parkinsonian Deficits in Monkeys: Association with DAT Occupancy in Vivo

Viable dopamine neurons in Parkinson's disease express the dopamine transporter (DAT) and release dopamine (DA). We postulated that potent DAT inhibitors, with low affinity for the serotonin transporter (SERT), may elevate endogenously released extracellular dopamine levels to provide therapeutic benefit. The therapeutic potential of eight DAT inhibitors was investigated in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated cynomolgus monkeys (Macaca fascicularis), with efficacy correlated with DAT occupancy as determined by positron emission tomography imaging in striatum. Four potent DAT inhibitors, with relatively high norepinephrine transporter, but low SERT affinities, that occupied the DAT improved activity in parkinsonian monkeys, whereas three high-affinity DAT inhibitors with low DAT occupancy did not. 2beta-Carbomethoxy-3alpha-(3,4-dichlorophenyl)-7beta-hydroxy-8-methyl-8-azabicyclo[3.2.1.]octane (O-1163) occupied the DAT but had short-lived pharmacological effects. The benztropine analog difluoropine increased general activity, improved posture, reduced body freeze, and produced sleep disturbances at high doses. (1R)-2beta-(1-Propanoyl)-3alpha-(4-fluorophenyl)tropane (O-1369) alleviated parkinsonian signs in advanced parkinsonian monkeys, by increasing general activity, improving posture, reducing body freeze, and sedation, but not significantly reducing bradykinesia or increasing locomotor activity. In comparison with the D(2)-D(3) DA receptor agonist quinelorane, O-1369 elicited oral/facial dyskinesias, whereas quinelorane did not improve posture or reduce balance and promoted stereotypy. In conclusion, DAT inhibitors with therapeutic potential combine high DAT affinity in vitro and high DAT occupancy of brain striatum in vivo with enduring day-time effects that do not extend into the nighttime. Advanced parkinsonian monkeys (80% DAT loss) respond more effectively to DAT inhibitors than mild parkinsonian monkeys (46% DAT loss). The therapeutic potential of dopamine transport inhibitors for Parkinson's disease warrants preclinical investigation.

Modafinil Occupies Dopamine and Norepinephrine Transporters in Vivo and Modulates the Transporters and Trace Amine Activity in Vitro

2-[(Diphenylmethyl) sulfinyl]acetamide (modafinil), prescribed principally to treat narcolepsy, is undergoing assessment for other neuropsychiatric disorders and medical conditions. The neurochemical substrates of modafinil are unresolved. We postulated that modafinil enhances wakefulness by modulating dopamine (DAT), norepinephrine (NET), or serotonin (SERT) transporter activities. In vivo, we determined DAT and NET occupancy by modafinil by positron emission tomography imaging; in vitro, we determined modafinil activity at the DAT, NET, SERT, and rhesus monkey trace amine receptor 1 (TA1). In rhesus monkey, modafinil occupancy of striatal DAT was detected by [(11)C]2beta-carbomethoxy-3beta-4-(fluorophenyl)tropane and of thalamic NET by [(11)C](S,S)-2-(alpha-(2-methoxyphenoxy)-benzyl)morpholine. In vitro, modafinil effects in DAT-human embryonic kidney (HEK), NET-HEK, and SERT-HEK cells were investigated alone or combined with the TA1 receptor. Modafinil (i.v.) occupied striatal DAT sites (5 mg/kg: 35 +/- 12%, n = 4; 8 mg/kg: 54 +/- 3%, n = 3). In thalamus, modafinil occupied NET sites (5 mg/kg: 16 +/- 7.8%, n = 6; 8 mg/kg: 44 +/- 12%; n = 2). In vitro, modafinil inhibited [(3)H]dopamine (IC(50) = 6.4 microM), [(3)H]norepinephrine (IC(50) = 35.6 microM), and [(3)H]serotonin (IC(50) > 500 microM) transport via the human DAT, NET, and SERT. Modafinil did not activate the TA1 receptor in TA1-HEK cells, but it augmented a monoamine transporter-dependent enhancement of phenethylamine activation of TA1 in TA1-DAT and TA1-NET cells, but not in TA1-SERT cells. The present data provide compelling evidence that modafinil occupies the DAT and NET in living brain of rhesus monkeys and raise the possibility that modafinil affects wakefulness by interacting with catecholamine transporters in brain.

PET study examining pharmacokinetics, detection and likeability, and dopamine transporter receptor occupancy of short- and long-acting oral methylphenidate

OBJECTIVE

The abuse potential of methylphenidate has been related to the drug's capacity to produce a rapid onset of blockade of the presynaptic dopamine transporter in the brain. An oral once-a-day osmotic controlled-release formulation of methylphenidate produces a more gradual rise in plasma methylphenidate concentration, compared with immediate-release methylphenidate. The authors hypothesized that osmotic-release methylphenidate would also produce a slower onset of blockade of the presynaptic dopamine transporter and would be associated with a lower risk for detection and likeability, compared to immediate-release methylphenidate.

METHOD

Twelve healthy adults were randomly assigned to receive single doses of immediate-release methylphenidate or osmotic-release methylphenidate. Doses predicted to produce equivalent maximum concentration (C(max)) values were selected (40 mg of immediate-release methylphenidate and 90 mg of osmotic-release methylphenidate). Plasma d-methylphenidate levels and responses to detection/likeability questionnaire items were obtained hourly for 10 hours after administration of methylphenidate on two separate occasions for each subject. Dopamine transporter receptor occupancies were measured at hours 1, 3, 5, and 7 by using a carbon-11-labeled imaging agent (Altropane) and positron emission tomography.

RESULTS

Despite similar C(max) values for both formulations, osmotic-release methylphenidate was associated with a longer time to maximum concentration, longer time to maximum CNS dopamine transporter occupancy, and no detection/likeability, compared with immediate-release methylphenidate.

CONCLUSIONS

The findings suggest that the abuse potential of oral methylphenidate is strongly influenced by the rate of delivery and not solely by the magnitude of plasma concentration or brain transporter occupancy. These results advance understanding of the underlying central effects of methylphenidate in humans and identify a potentially less abusable methylphenidate formulation.

Inflammation and infection: imaging properties of 18F-FDG-labeled white blood cells versus 18F-FDG

(18)F-FDG and (18)F-FDG-labeled white blood cells ((18)F-FDG-WBCs) are valuable radiopharmaceuticals for imaging focal sites of inflammation and infection. In the present study, the imaging properties of both radiotracers were compared in sterile and septic inflammation models.

METHODS

Groups of adult male Sprague-Dawley rats (100-120 g) were injected in the left posterior thigh muscle with saline solution (group 1: controls, n = 15), 0.100 mL of turpentine oil (group 2: sterile inflammation, n = 26), 10(9) viable Escherichia coli bacteria (group 3: E. coli septic inflammation, n = 29), or 10(8) viable Pseudomonas aeruginosa bacteria (group 4: P. aeruginosa septic inflammation, n = 25). Twenty-four hours later, the animals were divided into 2 groups: One received (18)F-FDG intravenously and the other received human white blood cells (WBCs) labeled in vitro with (18)F-FDG injected intravenously. Biodistribution and microPET studies were performed 1 h after radiotracer injection. One hour after injection with cell-associated or free (18)F-FDG, phosphorimaging of abscess and contralateral muscle was performed in specimens collected from animals in groups 1, 2, and 3. The region of interest was selected within the abscess wall and values were converted to kBq/g using a (14)C calibration standard curve. Thin-layer radiochromatography (TLRC) was performed to study the chemical forms of (18)F within the WBCs.

RESULTS

Whole-body biodistribution demonstrated a significantly higher uptake ratio of (18)F-FDG-WBCs compared with (18)F-FDG in all sterile and septic inflammation models (t test: sterile, P = 0.048; E. coli, P = 0.040; P. aeruginosa, P = 0.037). microPET imaging confirmed the greater performance of (18)F-FDG-WBCs versus (18)F-FDG in the sterile inflammation model and in both E. coli and P. aeruginosa septic models. Phosphorimaging analysis showed higher (18)F-FDG-WBC uptake than (18)F-FDG in the sterile inflammation and P. aeruginosa septic models and similar tissue uptake in the E. coli septic model. Time course labeling and TLRC of lysed WBCs demonstrated that (18)F-FDG was retained as (18)F-FDG-6-phosphate inside WBCs for at least 2 h, corresponding to the time frame of analysis.

CONCLUSION

(18)F-FDG-WBCs gave better results compared with (18)F-FDG in all sterile and septic inflammation models. These data suggest that (18)F-FDG-WBC PET may be a useful technique for tracking focal inflammatory lesions in the body.

In vivo neuroreceptor imaging in attention-deficit/hyperactivity disorder: a focus on the dopamine transporter

There is converging evidence of the role of catecholamine dysregulation in the underlying pathophysiology of attention-deficit/hyperactivity disorder (ADHD). The dopamine transporter (DAT) is known to be a key regulator of dopamine, and recent genetic, treatment, and imaging studies have highlighted the role of DAT in ADHD. There is an emerging literature on in vivo neuroreceptor imaging of DAT in ADHD and control subjects reported by a number of groups internationally. A comprehensive review of existing imaging studies of DAT binding in ADHD shows that six of eight independent studies by six different groups have reported increased DAT binding in (mostly) treatment-naïve children and adults with ADHD. Although there is fair agreement regarding the presence and direction of abnormal DAT binding, there remains disagreement as to the magnitude of the finding and the importance of many potentially confounding variables, including clinical characteristics and imaging methodology. Three studies by three different groups have reported decreased DAT binding after methylphenidate treatment. Interpretation of the latter finding awaits clarification of the issue of timing of drug administration and imaging to disentangle receptor occupancy from downregulation.

Evaluation of trans-9-18F-fluoro-3,4-Methyleneheptadecanoic acid as a PET tracer for myocardial fatty acid imaging

This study describes the radiosynthesis and preliminary biologic evaluation of trans-9(RS)-(18)F-fluoro-3,4(RS,RS)-methyleneheptadecanoic acid ((18)F-FCPHA) as a new potential probe for assessing myocardial fatty acid metabolism by PET. This fatty acid, containing a cyclopropyl moiety in the beta,gamma-position, was designed to enter the myocardium by the same mechanism as natural fatty acids and to undergo partial metabolism before being trapped in the cell.

METHODS

(18)F-FCPHA and the beta-methyl analog 8(RS)-(18)F-fluoro-3(RS)-methylheptadecanoic acid ((18)F-FBMHA) were prepared from their corresponding mesylate precursors by nucleophilic substitution. The precursors used for labeling were fully characterized, and the data were consistent with the proposed structures. Biodistribution studies of each tracer were performed with Sprague-Dawley rats at 5 and 60 min after injection. Sequential imaging of a rhesus monkey injected with 222 MBq of (18)F-FCPHA was performed by use of a microPET camera.

RESULTS

At 5 and 60 min, heart uptake values measured as mean +/- SD percentage injected dose per gram (%ID/g) in rats for (18)F-FCPHA were 1.55 +/- 0.72 and 1.43 +/- 0.14, respectively. The heart-to-blood ratios at 5 and 60 min, an indication of target definition, were 25.8 and 20.4, respectively. The heart-to-lung ratios at 5 and 60 min were 3.3 and 4.6, respectively. Bone accumulation (%ID/g), an indication of defluorination, was 0.16 +/- 0.03 at 5 min and increased to 0.70 +/- 0.39 at 60 min. The heart-to-blood ratio obtained with (18)F-FBMHA was 2.6 at 5 min and did not change significantly at 60 min. Imaging of the monkey heart after injection of (18)F-FCPHA showed an initial spike of activity corresponding to blood flow followed by a plateau at 10 min.

CONCLUSION

The cyclopropyl moiety in (18)F-FCPHA does have a significant influence on heart accumulation, as suggested by the high heart-to-blood ratio and the fast blood clearance in rats. These results, along with the remarkable quality of the PET images, indicate the potential of this new class of labeled fatty acids for use in studying heart disease by PET.

Non-amine-based dopamine transporter (reuptake) inhibitors retain properties of amine-based progenitors

Without exception, therapeutic and addictive drugs that produce their primary effects by blocking monoamine transporters in brain contain an amine nitrogen in their structure. This fundamental canon of drug design was based on a prevailing premise that an amine nitrogen is required to mimic the structures of monoamine neurotransmitters and other natural products. Non-amines, a novel class of compounds that contain no amine nitrogen, block monoamine transporters in the nM range and display markedly high selectivity for monoamine transporters, but not for receptors. Non-amines retain the spectrum of biochemical and pharmacological properties characteristic of amine-bearing counterparts. These novel drugs compel a revision of current concepts of drug-monoamine transporter complex formation and open avenues for discovery of a new generation of therapeutic drugs.

A second-generation 99m technetium single photon emission computed tomography agent that provides in vivo images of the dopamine transporter in primate brain

The dopamine transporter (DAT), located presynaptically on dopamine neurons, provides a marker for Parkinson's disease (Pd) and attention deficit hyperactivity disorder (ADHD). In ADHD, DAT density levels are elevated, while in Pd these levels are depleted. The depletion of DAT levels also corresponds with the loss of dopamine. We now describe the design, synthesis, biology, and SPECT imaging in nonhuman primates of second-generation (99m)technetium-based tropane ligands that bind potently and selectively to the DAT. We demonstrate that improved selectivity and biological stability allows sufficient agent to enter the brain and label the DAT in vivo to provide a quantitative measure of DAT density in nonhuman primates. We introduce FLUORATEC (N-[(2-((3'-N'-propyl-(1"R)-3"alpha-(4-fluorophenyl)tropane-2"beta-1-propanoyl)(2-mercaptoethyl)amino)acetyl)-2-aminoethanethiolato]technetium(V) oxide), a DAT imaging agent that has emerged from these studies and is now in phase 1 clinical trials in the U.S.

[(11)C, (127)I] Altropane: a highly selective ligand for PET imaging of dopamine transporter sites

The E isomer of (123)I-2beta-carbomethoxy-3beta-(4-fluorophenyl)-N-(1-iodoprop-1-en-3-yl)nortropane (Altropane(R)) shows high affinity (IC(50) = 6.62 +/- 0.78 nmol) and selectivity (DA/5-HT = 25) for DAT sites in the striatum. Recently, dynamic SPECT studies in healthy volunteers and patients with Parkinson disease demonstrated that the kinetics of striatal accumulation followed a pattern that is characteristic of a reversible tracer with maximal accumulation within 30 min after injection. These findings suggested that radiolabeling Altropane with [(11)C] might provide an equivalent and complementary tracer for PET studies. [(127)I] Altropane was treated with HCl to hydrolyze the methyl ester bond and yield a precursor for [(11)C] labeling. Introduction of an [(11)C] methyl ester group was achieved by treatment with [(11)C] CH(3)I followed by HPLC purification. Five healthy rhesus monkeys were injected with approximately 10 mCi of [(127)I,(11)C] Altropane and dynamic PET images were acquired over 90 min. Arterial blood samples were collected in parallel with imaging and metabolite analysis was performed by HPLC. The PET and metabolite corrected arterial blood data were to calculate k(3)/k(4) by two methods: 1) nonlinear least-squares fitting, and 2) a linear graphical method for reversible ligands. The synthetic procedure yielded high specific activity tracer, >1,000 mCi/micro mole, with radiochemical purity >95%. Synthesis time was approximately 30 min. The PET images revealed excellent striatal definition, with clear separation of caudate nucleus and putamen and minimal accumulation in brain regions with high 5HT transporter density. Metabolite analysis demonstrated that at 60 min after injection, approximately 80% of circulating tracer was intact [(127)I,(11)C] Altropane and the remainder was converted to polar metabolites. Values for k(3)/k(4) calculated by two analysis methods were remarkably similar: Method 1, 3.48 +/- 0.41; Method 2, 3.77 +/- 0.45 (mean +/- SEM, t = 2.31, df = 8, P = 0.64). These results establish that Altropane has the important characteristics of: 1) rapid and specific striatal binding; 2) high selectivity for DA vs. 5-HT transporter sites; 3) reversible binding kinetics; 4) potential for multiple injection studies; 5) high efficiency labeling with either [(11)C] or [(123)I]; 6) applicability for both PET and SPECT. These properties make Altropane an important DAT ligand for both research and clinical applications.

Regional pharmacokinetics of orally administered PET tracers

Positron emission tomography (PET) is currently the most useful imaging technique for noninvasive measurement of drug pharmacokinetics regionally in a variety of tissues. Over the past decade, PET measurements have provided many critical insights about the tissue distribution of several classes of drugs; neuroleptics, antimicrobials, antineoplastics, etc. PET measurements can be performed after any route of drug administration, intravenous, inhalation or oral, however, intravenously administered drugs have been the most extensively evaluated. Studies of orally administered drugs are clearly of great interest; however, formulation issues have precluded widespread applications in these areas. In this report, we discuss the unique problems associated with studying orally administered drugs and review the results of recent studies performed in our laboratory.

Comparison of 4 methods for quantification of dopamine transporters by SPECT with [123I]IACFT

2Beta-carbomethoxy-3beta-(4-fluorophenyl)-n-(1-iodoprop-1-en -3-yl) nortropane (IACFT) is a highly selective ligand for dopamine transporter (DAT) sites in the striatum. Recent reports have described the basic kinetics, neurobiology, and imaging properties of [123I]IACFT. This report focuses on the structural (i.e., the ability to produce consistent binding estimates) validity of 4 methods to quantify striatal binding potential (BP) for IACFT.

METHODS

Seven healthy volunteers and 8 patients with Parkinson's disease were subjects for this study. Dynamic SPECT images and arterial blood samples were acquired during the 1.5-2 h after injection of 185-370 MBq [123I]IACFT. Plasma radioactivity was analyzed chromatographically to obtain metabolite-corrected arterial input functions. The k3/k4 ratio (BP) for striatal DAT sites was calculated by 4 methods. In the first method, tissue time-activity curves and metabolite-corrected arterial input functions were analyzed by a linear graphic method developed for reversible receptor ligands. The second method was also graphic; however, the occipital cortex time-activity curve was used as the input function. In the third method, the difference between the striatal and occipital cortex time-activity curves at secular equilibrium was taken to represent bound tracer, the occipital cortex time-activity curve was used to represent tracer in the free and nonspecifically bound state, and equilibrium receptor equations were used to determine BP. The fourth method used the occipital cortex time-activity curve to mathematically derive an input function for fitting the striatal time-activity curve and to determine BP.

RESULTS

Analysis of the dynamic SPECT data by methods 1 and 2 resulted in highly linear plots (after approximately 15 min), supporting the reversibility of the tracer. A high linear correlation was found for BP determined by all 4 methods. ANOVA showed that methods 1-3 were indistinguishable; method 4 yielded lower BPs than did methods 1-3.

CONCLUSION

These results show that BP can be estimated consistently using 4 different methods. This finding lends support to the modeling assumptions and provides methods suitable for clinical investigation.

Dopamine transporter density in patients with attention deficit hyperactivity disorder

Dopamine transporter density was measured in vivo in six adult patients with attention deficit hyperactivity disorder. We have shown a 70% increase in age-corrected dopamine transporter density in patients with attention hyperactivity disorder compared with healthy controls.

Measurement of muscle protein synthesis by positron emission tomography with L-[methyl-11C]methionine: effects of transamination and transmethylation

BACKGROUND

Positron emission tomography with L-[methyl-11C]methionine provides a measure of regional protein synthesis rate (PSR) in skeletal muscle. However, the validity of the method depends on incorporation of methionine into protein with minimal transamination, transmethylation, or both. To test directly these assumptions, uptake of L-[methyl-14C]methionine in skeletal muscle was measured in control and cycloheximide-treated rats.

METHODS

Normal and cycloheximide-treated rats (n = 8/group) were injected with 50 microCi of L-[methyl-14C]methionine and arterial blood sampled over 90 minutes. After killing, thigh muscle was homogenized, centrifuged, and treated with trichloroacetic acid. PSR from circulating methionine was estimated from trichloroacetic acid-precipitable radioactivity, arterial time-activity curves, and plasma methionine concentrations.

RESULTS

In normal rats, approximately 70% of the tissue radioactivity was precipitated with trichloroacetic acid. In normal animals, PSR was 0.22 nmoles x min(-1) x g(-1), in excellent agreement with previous results. In the cycloheximide-treated group, PSR was 0.0032 nmoles x min(-1) x g(-1); approximately 98% reduction compared with controls.

CONCLUSION

These studies support the hypothesis that L-[methyl-11(14C]methionine accumulates in skeletal muscle as 11(14)C-labeled protein.

Loss of D2 receptor binding with age in rhesus monkeys: importance of correction for differences in striatal size

The relation between striatal dopamine D2 receptor binding and aging was investigated in rhesus monkeys with PET. Monkeys (n = 18, 39 to 360 months of age) were scanned with 11C-raclopride; binding potential in the striatum was estimated graphically. Because our magnetic resonance imaging analysis revealed a concomitant relation between size of striatum and age, the dynamic positron emission tomography (PET) data were corrected for possible partial volume (PV) artifacts before parameter estimation. The age-related decline in binding potential was 1% per year and was smaller than the apparent effect if the age-related change in size was ignored. This is the first in vivo demonstration of a decline in dopamine receptor binding in nonhuman primates. The rate of decline in binding potential is consistent with in vitro findings in monkeys but smaller than what has been measured previously in humans using PET. Previous PET studies in humans, however, have not corrected for PV error, although a decline in striatal size with age has been demonstrated. The results of this study suggest that PV correction must be applied to PET data to accurately detect small changes in receptor binding that may occur in parallel with structural changes in the brain.

Pharmacokinetics of [18F]trovafloxacin in healthy human subjects studied with positron emission tomography

Tissue pharmacokinetics of trovafloxacin, a new broad-spectrum fluoroquinolone antimicrobial agent, were measured by positron emission tomography (PET) with [18F]trovafloxacin in 16 healthy volunteers (12 men and 4 women). Each subject received a single oral dose of trovafloxacin (200 mg) daily beginning 5 to 8 days before the PET measurements. Approximately 2 h after the final oral dose, the subject was positioned in the gantry of the PET camera, and 1 h later 10 to 20 mCi of [18F]trovafloxacin was infused intravenously over 1 to 2 min. Serial PET images and blood samples were collected for 6 to 8 h, starting at the initiation of the infusion. Drug concentrations were expressed as the percentage of injected dose per gram, and absolute concentrations were estimated by assuming complete absorption of the final oral dose. In most tissues, there was rapid accumulation of the radiolabeled drug, with high levels achieved within 10 min after tracer infusion. Peak concentrations of more than five times the MIC at which 90% of the isolates are inhibited (MIC90) for most members of Enterobacteriaceae and anaerobes (>10-fold for most organisms) were achieved in virtually all tissues, and the concentrations remained above this level for more than 6 to 8 h. Particularly high peak concentrations (micrograms per gram; mean +/- standard error of the mean [SEM]) were achieved in the liver (35.06 +/- 5.89), pancreas (32.36 +/- 20. 18), kidney (27.20 +/- 10.68), lung (22.51 +/- 7.11), and spleen (21. 77 +/- 11.33). Plateau concentrations (measured at 2 to 8 h; micrograms per gram; mean +/- SEM) were 3.25 +/- 0.43 in the myocardium, 7.23 +/- 0.95 in the lung, 11.29 +/- 0.75 in the liver, 9.50 +/- 2.72 in the pancreas, 4.74 +/- 0.54 in the spleen, 1.32 +/- 0.09 in the bowel, 4.42 +/- 0.32 in the kidney, 1.51 +/- 0.15 in the bone, 2.46 +/- 0.17 in the muscle, 4.94 +/- 1.17 in the prostate, and 3.27 +/- 0.49 in the uterus. In the brain, the concentrations (peak, approximately 2.63 +/- 1.49 microg/g; plateau, approximately 0.91 +/- 0.15 microg/g) exceeded the MIC90s for such common causes of central nervous system infections as Streptococcus pneumoniae (MIC90, <0.2 microg/ml), Neisseria meningitidis (MIC90, <0.008 microg/ml), and Haemophilus influenzae (MIC90, <0.03 microg/ml). These PET results suggest that trovafloxacin will be useful in the treatment of a broad range of infections at diverse anatomic sites.

Rapid detection of Parkinson's disease by SPECT with altropane: a selective ligand for dopamine transporters

Increasing evidence indicates that dopamine (DA) transporter density declines in Parkinson's disease (PD). 2Beta-carbomethoxy-3beta-(4-fluorophenyl)-n-(1-iodoprop-1-en -3-yl) nortropane (IACFT, Altropane) is a cocaine analog with high affinity and selectivity for dopamine transporter (DAT) sites in the striatum. In this study, single photon emission computed tomography (SPECT) with [123I]altropane was used to measure DAT density in seven healthy volunteers (five males, age 37-75, and two females, ages 26 and 39) and eight male patients with Parkinson's disease (age 14-79, Hoehn and Yahr stage: 1.5-3 (n = 5) and 4-5 (n = 3)). Dynamic SPECT images and arterial blood samples were acquired over 1.5-2 hr and plasma radioactivity was analyzed chromatographically to obtain metabolite corrected arterial input functions. Binding potential (BP, B'max/KD) for striatal (Str) DAT sites was calculated by two methods using occipital cortex (Occ) as a reference. In the first method, tissue time-activity curves (TAC) and metabolite corrected arterial input functions were analyzed by a linear graphical method developed for reversible receptor ligands. In the second method, the expression (Str(TAC) - Occ(TAC)) was fitted to a gamma variate function and the maximum divided by Occ(TAC) at the same time was used to estimate BP. In five of the PD patients, the SPECT data were compared with the results of PET with [18F] 6-fluoro DOPA (FD-PET). Plasma analysis indicated that [123I]altropane is rapidly converted to polar metabolites. SPECT images in healthy volunteers showed that [123I] altropane accumulated rapidly and selectively in the striatum and yielded excellent quality images within 1 h after injection. Both methods of analysis revealed a 7.6%/decade reduction in BP and average striatal values (corrected to age 25) were 1.83 +/- 0.22 and 2.09 +/- 0.20 by methods 1 and 2. In all the PD patients, striatal accumulation was markedly reduced and the pattern of loss was similar to that reported for DA; most profound in the posterior putamen with relative sparing of the caudate nuclei. A comparable pattern was observed with FD-PET. For total striatum, age-corrected BP was significantly (P < 0.001) reduced; 0.83 +/- 0.06 (method 1), 0.84 +/- 0.07 (method 2). BPs measured by the two methods were remarkably similar and highly correlated r2 = 0.88, (P < 0.001). These results indicate that [123I]altropane is an excellent SPECT ligand for imaging the DAT/DA neurons in human brain. The high selectivity and rapid striatal accumulation of the ligand allows for accurate quantitation of DAT sites in less than 2 hr. The results further demonstrate that [123I]altropane is an effective marker for PD.

Positron emission tomographic analysis of central dopamine D1 receptor binding in normal subjects treated with the atypical neuroleptic, SDZ MAR 327

SDZ MAR 327 is a new neuroleptic agent with high in vitro affinity for dopamine D1 and D2 receptors. The goal of this study was to determine the effect of time after SDZ MAR 327 administration on central dopamine D1 receptor occupancy in healthy humans. Positron emission tomography (PET) with the dopamine D1 receptor ligand, [11C] SCH 23390, was performed in 6 male volunteers (age 22-34), in both the drug naive state and at 1, 2 and 4 h after a single oral dose of SDZ MAR 327 (9 mg). The pre and post drug treatment [11C] SCH 23390 dynamic data were analyzed using two different methods, each yielding a parameter proportional to the receptor density: i) a simple regional comparison approximating the specifically bound to free fraction, B/F; and ii) a two compartment, two parameter model yielding the apparent distribution volume DV". With both methods, a metabolite corrected arterial input function was used and the vascular fraction of tissue (Vb) was fixed at a previously determined value of 4%. Method I served as a qualitative comparison of the paired studies and demonstrated little difference between the pre and post drug conditions, method II also confirmed that there was no significant change in binding of [11C] SCH 23390 in the striatum. These data indicate that SDZ MAR 327 produces little if any effect on dopamine D1 receptor binding at the dose used.

Quantification of dopamine transporter density in monkeys by dynamic PET imaging of multiple injections of 11C-CFT

Idiopathic Parkinson's disease (PD) is characterized by loss of dopaminergic terminals in the basal ganglia. The cocaine analog, CFT (WIN 35,428), has been shown to bind selectively to the pre-synaptic dopamine transporters and thus represents an important probe for monitoring disease progression. In this study, we evaluated [11C] labeled CFT as a PET ligand for the quantitative in vivo assay of dopamine transporter density in three normal rhesus monkeys (Macaca mulatta). One of the animals was studied after treatment with the neurotoxin, MPTP. Simulation studies demonstrated that a three injection protocol is necessary for quantitation of dopamine transporter density. The protocol consists of an initial high specific activity injection, a low specific activity "displacement dose" at 30 min, and a final high specific activity injection at approximately 90 min. Dynamic PET imaging and arterial blood sampling were started immediately before the first injection and continued for 2 h. Blood data were corrected for [11C] labeled CFT metabolites. Compartmental models describing the dynamics of labeled and the unlabeled ligand explicitly were fitted to the PET and metabolite corrected blood data. Prior to MPTP treatment, modeling of the striatal data required a saturable receptor term and yielded mean estimates of: B'max = 113 pmol/g and KD = 33 nm (n = 3). These values for B'max are in reasonable agreement with published values for [3H] CFT binding in vitro. After multiple treatments with MPTP (0.6 mg/kg x 3), B'max in one of the animals was reduced from 122 to 10.2 pmol/g. KD was relatively unaffected by MPTP treatment. These data provide additional basis for the use of [11C] CFT in monitoring the progression of Parkinson's disease and other conditions that are associated with the loss of dopaminergic nerve terminals.

Positron emission tomographic analysis of central 5-hydroxytryptamine2 receptor occupancy in healthy volunteers treated with the novel antipsychotic agent, ziprasidone

Ziprasidone is a novel antipsychotic agent, with high affinity for dopamine D2 and serotonin (5-HT2) receptors in vitro and in animal models. The goal of this study was to determine the time course of 5-HT2 receptor occupancy (%RO) in healthy humans after a single p.o. dose. Positron emission tomography with the 5-HT2 ligand, [18F]setoperone, was performed in eight male volunteers, in the drug-naive, base-line (BL) state and 4 to 18 hr after ziprasidone (40 mg). Cerebral cortical binding potential [BP, maximum number of available receptors/KD or association rate for specific binding (k3)/dissociation rate for specific binding (k4)] was estimated using the cerebellum as reference. Transport rate from plasma to brain (K1), transport rate from brain to plasma (k2), association rate of nonspecific binding (k5) and dissociation rate of nonspecific binding (k6) were derived by fitting cerebellar time-activity curves to a three-compartment model. Fitting of cortical data to a 4-compartment model with K1/k2, k5 and k6, fixed at cerebellar values, was used to determine k3 and k4. %RO was calculated using the relation: %RO = [(BPBL-BPDRUG)/BPBL] x 100%. At BL, cortical parameter (mean +/- S.E.M) were: K1 = 0.121 +/- 0.0072 ml.min-1.g-1; k2 = 0.0581 +/- 0.004 min-1; k3 = 0.321 +/- 0.0026 min-1; k4 = 0.0957 +/- 0.0059 min-1; k5 = 0.0147 +/- 0.00066 min-1; and k6 = 0.0059 +/- 0.00042 min-1. Ziprasidone did not effect K1, k2, k5 or k6; however, k3 was reduced and k4 was elevated (P < .01). RO was nearly complete at 4 hr after dosing (98%) and remained elevated at 18 hr (46%). Plasma concentrations were well described by a biexponential function and decreased much more rapidly than RO. These results establish that ziprasidone has high potency for blocking 5-HT2 receptors in healthy humans; a potentially important characteristic of atypical antipsychotic agents.