Mechanism of nitrogen-13-labeled ammonia formation in a cryogenic water target.

Methods for producing N-13 ammonia via the 16O(p,alpha)13N nuclear reaction utilizing a cryogenic target have been investigated. These targets included frozen carbon dioxide and pure frozen water. Results from these targets were compared with the more traditional liquid water target with and without additives. A very dramatic difference was found between the pure water target in the frozen state when compared with the liquid state. When frozen, more than 95% of the nitrogen-13 activity is in the chemical form of ammonia at all radiation doses. In contrast, the liquid water target yielded predominately nitrates and nitrites at high radiation doses. When frozen carbon dioxide was irradiated under these conditions, more than 95% of the nitrogen-13 activity was in the form of nitrate and nitrite. The nitrogen oxides remained on the surface of the target and could be easily removed from the surface with pure water. The wash solution was converted to [13N]ammonia using the DeVarda's alloy method for reduction. It was determined that levels of [13N]ammonia sufficient for diagnostic medical procedures could be produced directly using the frozen water targets or from frozen carbon dioxide with a wet chemical reduction. These results have significance particularly in the design of targetry for low-energy, high-beam current accelerators, because targets of this design can be used with either no vacuum isolation window or a very thin window. The substitution of carbon-13-enriched carbon dioxide for natural carbon dioxide gives access to the 13C(p,n)13N nuclear reaction, which allows protons energies as low as 6 MeV to be used to produce useable quantities of N-13 ammonia. The mechanism of these reactions has been explored to determine why there are such dramatic differences in the product distribution between the frozen state and the liquid or gaseous state.

Investigations of acetonitrile solvent cluster formation in supercritical carbon dioxide, and its impact on microscale syntheses of carbon-11-labeled radiotracers for PET.

A new strategy has been developed for synthesizing positron emission tomography (PET) radiotracers using [11C]methyl iodide. This strategy relies on the ability of organic co-solvents to cluster within mixtures of supercritical fluids resulting in localized regions of high density which can serve as microscopic pockets for reaction. We've shown that acetonitrile will cluster about dilute solutes when mixtures of this co-solvent with carbon dioxide are forced to behave as a homogeneous fluid at the critical point. We applied this strategy in a systematic investigation of the conditions for optimized reaction between methyl iodide and L-alpha-methyl-N-2-propynyl phenethylamine (nordeprenyl) to yield L-deprenyl. Variables such as temperature, ultraviolet light exposure, co-solvent concentration, system pressure, and methyl iodide concentration were explored. The synthesis of radioactive [11C]-L-deprenyl using no-carrier-added concentrations of [11C]methyl iodide was also tested. Results showed that greater than 90% radiochemical yield of the desired product could be attained using 40 times less labeling substrate than in conventional PET tracer syntheses.

Neuropharmacological actions of cigarette smoke: brain monoamine oxidase B (MAO B) inhibition.

We measured the concentration of brain monoamine oxidase B (MAO B; EC 1.4.3.4) in 8 smokers and compared it with that in 8 non-smokers and in 4 former smokers using positron emission tomography (PET) and deuterium substituted [11C]L-deprenyl ([11C]L-deprenyl-D2) as a radiotracer for MAO B. Smokers had significantly lower brain MAO B than non-smokers as measured by the model term lambda k3 which is a function of MAO B activity. Reductions were observed in all brain regions. Low brain MAO B in the cigarette smoker appears to be a pharmacological rather than a genetic effect since former smokers did not differ from non-smokers. Brain MAO B inhibition by cigarette smoke is of relevance in light of the inverse association between smoking and Parkinson's disease and a high prevalence of smoking in psychiatric disorders and in substance abuse. Though nicotine is at the core of the neuropharmacological actions of tobacco smoke, MAO B inhibition may also be an important variable in understanding and treating tobacco smoke addiction.

Yield measurements for the 11B(p,n)11C and the 10B(d,n)11C nuclear reactions.

The yields from the 11B(p,n)11C nuclear reactions have been measured over the energy range of 0-28.6 MeV. The yields from the 10B(d,n)11C nuclear reactions have been measured over the energy range of 0-21.5 MeV. Cross-section values were derived from the yield data. The yield measurements were carried out on sealed samples of boron powder isotopically enriched to greater than 93%. The target holders were constructed of aluminum and each contained 50 mg of boron powder to achieve high energy resolution, but with sufficient material to ensure accurate cross-section measurements. 11C yields were determined by decay-curve analysis. Our data show that at energies below 5 MeV, the 11B(p,n)11C nuclear reaction can be expected to produce 250 mCi of 11C (based on a 30 min irradiation at a beam current of 100 microA).

Long-term stability of neurotransmitter activity investigated with 11C-raclopride PET.

There is evidence for the shift of regulatory setpoints of functionally linked neurotransmitter systems as a basis of psychiatric disorders. 11C-raclopride PET, which has been shown to be sensitive to changes in endogenous dopamine and has a high short-term test-retest reliability, can be used to investigate different regulatory states of the dopaminergic system with respect to psychiatric diseases and pharmacological influences. Prior to these studies, the reliability of the method over time has to be established. The current study was performed in order to evaluate the long-term stability of the striatal dopaminergic system. Eight normal healthy subjects (mean age: 48.1 years; range: 24-75) were studied twice with 11C-raclopride PET two times under resting conditions with a mean time interval between the scans of 11.3 months (range: 1-19). The ratio of basal ganglia (BG) to cerebellar (CB) distribution volumes (DVs) revealed a mean absolute change of 6.94 (range: 0.0-12.87%) between study A and B. BG DVs mean absolute change was 6.30% (range: 0.55-30.46%), CB DVs mean absolute change was 8.65% (range: 3.51-16.33%). The mean change of the BG/CB ratio was -0.33% (range: 12.87-12.34%). BG DVs mean change was 4.55% (range: 4.2-30.46%), CB DVs mean change was 5.10% (range: -10.71-16.33%). The intraindividual differences between the two scans in our study were not significantly different as compared to the 24 hour interval test-retest data, which have been published earlier (repeated measures ANOVA with df = 11; F = 0.49; P = 0.50) [Volkow et al. (1993) J. Nucl. Med., 34:609-613]. The intraclass correlation of the DV ratio index was r = 0.81. The binding potential in the baseline scans and repeated scans showed a non-significant correlation with age (r = -0.58, P = 0.13). Interindividually, the DV ratio index revealed a mean of 3.18 (range = 2.55-3.68, SD = 0.42 in study A and of 3.16 (range 2.37-3.57, SD = 0.41) in study B. The intrasubject stability of the 11C-raclopride binding over a long-term period in normal human subjects suggests the feasibility of study designs investigating the long-term changes of the dopaminergic responsivity after pharmacological challenges. The baseline stability will also serve as a necessary reference for further dose-response studies and investigations of subchronical pharmacological interventions.

Carbon-11 labelled ketamine-synthesis, distribution in mice and PET studies in baboons.

No-carrier-added (NCA)[11C](+/-)-ketamine (2a) and its enantiomers (+)-2b and (-)-2c were synthesized by methylation of the corresponding norketamine (1a-c) with [11C]H3I in an overall radiochemical yield of 20% (EOB) with specific activities of 0.35-0.45 Ci/mumol at EOB in a synthesis time of 40 min from EOB. Compound 2a was metabolized rapidly in mouse brain and labeled metabolites appeared in baboon plasma. PET studies of compounds 2a-c in a baboon showed that influx of compounds 2a-c into the brain was high for the first few min but radioactivity then declined rapidly. Although the retention of radioactivity in the baboon striatum was not significantly different for 2a-c 20 min post-injection, graphical analysis of time-activity data for each enantiomer and for the racemate in baboon striatum suggested that (+)-ketamine may interact with receptors slightly more effectively than its (-)-enantiomer or racemate. However, due to its rapid metabolism in the brain and a similar uptake in the striatum and cerebellum, [11C]ketamine may not be an ideal tracer for studying NMDA receptor with PET.

Brain monoamine oxidase A inhibition in cigarette smokers.

Several studies have documented a strong association between smoking and depression. Because cigarette smoke has been reported to inhibit monoamine oxidase (MAO) A in vitro and in animals and because MAO A inhibitors are effective antidepressants, we tested the hypothesis that MAO A would be reduced in the brain of cigarette smokers. We compared brain MAO A in 15 nonsmokers and 16 current smokers with [11C]clorgyline and positron emission tomography (PET). Four of the nonsmokers were also treated with the antidepressant MAO inhibitor drug, tranylcypromine (10 mg/day for 3 days) after the baseline PET scan and then rescanned to assess the sensitivity of [11C]clorgyline binding to MAO inhibition. MAO A levels were quantified by using the model term lambda k3 which is a function of brain MAO A concentration. Smokers had significantly lower brain MAO A than nonsmokers in all brain regions examined (average reduction, 28%). The mean lambda k3 values for the whole brain were 0.18 +/- 0.04 and 0.13 +/- 0.03 ccbrain (mlplasma)-1 min-1 for nonsmokers and smokers, respectively; P < 0.0003). Tranyl-cypromine treatment reduced lambda k3 by an average of 58% for the different brain regions. Our results show that tobacco smoke exposure is associated with a marked reduction in brain MAO A, and this reduction is about half of that produced by a brief treatment with tranylcypromine. This suggests that MAO A inhibition needs to be considered as a potential contributing variable in the high rate of smoking in depression and in the development of more effective strategies for smoking cessation.

Glucose metabolic changes in nontumoral brain tissue of patients with brain tumor following radiotherapy: a preliminary study.

PURPOSE: Our goal was to measure the effect of radiotherapy on the brain glucose metabolism of tumoral and nontumoral tissue of patients with brain malignancies. METHOD: Fifteen patients with primary or metastatic brain tumors were studied with 2-deoxy-2-[18F]fluoro-D-glucose and PET prior to radiotherapy, and nine of them were rescanned 1 week after completing radiotherapy. RESULTS: Brain metabolism in patients (all brain regions except for tumoral and edematous tissue) was lower than that of matched controls (34.0 +/- 8.3 vs. 46.5 +/- 6.4 mumol/100 g/min; p < or = 0.0001). Five of the nine patients retested after radiotherapy showed decrements in tumor metabolism (47 +/- 10%; p < or = 0.05) and increases in brain metabolism (10 +/- 4%; p < or = 0.004), and the other four showed no changes in tumor or in brain metabolism. Radiotherapy-induced changes in tumor metabolism were negatively correlated with changes in brain metabolism (r = 0.85, p < or = 0.004), but not with changes in tumor volume (assessed with MR images). CONCLUSION: The study indicates that radiotherapy-induced increases in metabolism of nontumoral tissue are secondary to decreased tumor metabolic activity and not just due to volume reduction.

Measuring age-related changes in dopamine D2 receptors with 11C-raclopride and 18F-N-methylspiroperidol.

This study investigates the rate of age-related dopamine D2 receptor loss as determined by positron emission tomography (PET) and 11C-raclopride and compares it with D2 loss previously estimated with 18F-N-methylspiroperidol (NMS). Dopamine D2 receptors were measured with 11C-raclopride in 24 healthy volunteers (24-73 years of age) using the ratio of the distribution volume in striatum to that in cerebellum (Bmax/Kd + 1). The results were compared with those obtained in 20 healthy male volunteers (20-49 years of age) in whom D2 receptors were measured with NMS using the ratio index (slope of the striatum-to-cerebellum ratio as a function of time). Findings of correlational analysis between age and dopamine D2 receptor availability were significant for both ligands. Estimates of dopamine D2 receptor loss per decade corresponded to 7.9% for the 11C-raclopride study and 7.8% for the NMS study. Both ligands documented significant age-related decreases in dopamine D2 receptors that occurred relatively early in life (40 years of age).

Opioid receptor imaging and displacement studies with [6-O-[11C] methyl]buprenorphine in baboon brain.

Buprenorphine (BPN) is a mixed opiate agonist-antagonist used as an analgesic and in the treatment of opiate addiction. We have used [6-O-[11C]methyl]buprenorphine ([11C]BPN) to measure the regional distribution in baboon brain, the test-retest stability of repeated studies in the same animal, the displacement of the labeled drug by naloxone in vivo, and the tissue distribution in mice. The regional distribution of radioactivity in baboon brain determined with PET was striatum > thalamus > cingulate gyrus > frontal cortex > parietal cortex > occipital cortex > cerebellum. This distribution corresponded to opiate receptor density and to previously published data (37). The tracer uptake in adult female baboons showed no significant variation in serial scans in the same baboon with no intervention in the same scanning session. HPLC analysis of baboon plasma showed the presence of labeled metabolites with 92% +/- 2.2% and 43% +/- 14.4% of the intact tracer remaining at 5 and 30 min, respectively. Naloxone, an opiate receptor antagonist, administered 30-40 min after tracer injection at a dose of 1.0 mg/kg i.v., reduced [11C]BPN binding in thalamus, striatum, cingulate gyrus, and frontal cortex to values 0.25 to 0.60 of that with no intervention. There were minimal (< 15%) effects on cerebellum. Naloxone treatment significantly reduced the slope of the Patlak plot in receptor-containing regions. These results demonstrate that [11C]BPN can be displaced by naloxone in vivo, and they affirm the feasibility of using this tracer and displacement methodology for short-term kinetics studies with PET. Mouse tissue distribution data were used to estimate the radiation dosimetry to humans. The critical organ was the small intestine, with a radiation dose estimate to humans of 117 nrad/mCi.

The prevalence of agitation and brain injury in skilled nursing facilities: a survey.

All skilled nursing facilities (SNF) in the state of Connecticut were surveyed to determine the number of residents with a primary diagnosis of brain injury, the incidence of agitation among these residents, and management strategies utilized for agitation in these residents, including consultation with brain injury specialists. One hundred and sixty-two, or 64%, of the SNF responded to the survey, and reported on 140 individuals with a primary diagnosis of brain injury. These individuals were residing in 39, or 24%, of the 162 facilities. Forty-five percent of these 39 facilities had brain-injured residents who met the definition of agitation presented in the survey. Management of agitation was equally distributed between environmental manipulation, formal behaviour care plans and medications. The availability of specialist consultation was variable and included physiatrists (38%), psychiatrists (67%), neurologists (41%), and psychologists (36%). This survey study defined the prevalence of brain-injured residents in Connecticut's SNF, and examined management strategies for agitation in this patient population.

Cocaine uptake is decreased in the brain of detoxified cocaine abusers.

Binding of [11C]cocaine in brain was measured with positron emission tomography in 12 detoxified cocaine abusers and in 20 controls to evaluate if there were changes in cocaine binding and in dopamine (DA) transporter availability associated with chronic cocaine use. Nine controls and 10 cocaine abusers had an additional scan with [18F]N-methylspiroperidol to measure dopamine D2 receptors. Cocaine abusers had significantly lower uptake of [11C]cocaine in brain (6.2 +/- 1% dose/cc tissues) than controls (7.7 +/- 2%). The distribution volumes (DV) for [11C]cocaine were reduced in basal ganglia (BG), cortex, thalamus, and cerebellum (CB) of cocaine abusers. However there were no differences in the ratio of the DV in BG to that in CB, which is an estimate of DA transporter availability. Values for DA D2 receptor availability were decreased in cocaine abusers and did not correlate with estimates of dopamine transporter availability. In summary, detoxified cocaine abusers showed decreased uptake of cocaine in brain but did not show changes in DA transporter availability.

Blunted change in cerebral glucose utilization after haloperidol treatment in schizophrenic patients with prominent negative symptoms.

OBJECTIVE: The purpose of this report was to determine 1) the effects of chronic haloperidol treatment on cerebral metabolism in schizophrenic patients, 2) the relation between negative symptoms and haloperidol-induced regional changes in cerebral glucose utilization, and 3) the relation between metabolic change and clinical antipsychotic effect. METHOD: Cerebral glucose utilization, as determined by position emission tomography (PET), was studied in 18 male schizophrenic subjects before and after chronic treatment with haloperidol at a standardized plasma level. RESULTS: Overall, haloperidol caused a widespread decrease in absolute cerebral glucose metabolism. The cerebral metabolic response to haloperidol was blunted in patients with high pretreatment negative symptom scores. CONCLUSIONS: Taken together with the results from a previously reported PET study of the effects of an acute amphetamine challenge (in which 14 of the current subjects participated), these data suggest that the negative symptom complex is associated with diminished cerebral response to change in dopaminergic activity. This deficit cannot be solely accounted for by structural differences.

Time-dependent effects of a haloperidol challenge on energy metabolism in the normal human brain.

Positron emission tomography and the fluorodeoxyglucose method were used to measure regional brain metabolism before and 2 h after haloperidol (5 mg, i.m.) in 11 young normal men. These data were compared with measures obtained from nine previously studied normal men who had received no drug intervention. Although a previously published study had demonstrated significantly decreased metabolism in whole brain, neocortex, limbic cortex, thalamus, and caudate nucleus 12 h after a 5-mg dose of haloperidol, the present 2-h study did not show significant metabolic changes despite the fact that significant extrapyramidal effects occurred. Taken together, these studies demonstrate differences in the temporal organization of behavioral and metabolic responses to haloperidol challenge.

Inhibition of monoamine oxidase B in the brains of smokers.

The massive health problem associated with cigarette smoking is exacerbated by the addictive properties of tobacco smoke and the limited success of current approaches to cessation of smoking. Yet little is known about the neuropharmacological actions of cigarette smoke that contribute to smoking behaviour, or why smoking is so prevalent in psychiatric disorders and is associated with a decreased risk of Parkinson's disease. Here we report that brains of living smokers show a 40% decrease in the level of monoamine oxidase B (MAO B; EC 1.4.3.4) relative to non-smokers or former smokers. MAO B is involved in the breakdown of dopamine, a neurotransmitter implicated in reinforcing and motivating behaviours as well as movement. MAO B inhibition is therefore associated with enhanced activity of dopamine, as well as with decreased production of hydrogen peroxide, a source of reactive oxygen species. We propose that reduction of MAO B activity may synergize with nicotine to produce the diverse behavioural and epidemiological effects of smoking.

[18F]haloperidol binding in baboon brain in vivo.

The binding of [18F]haloperidol to dopamine D2 and to sigma recognition sites in baboon brain was examined using positron emission tomography (PET). Studies were performed at baseline and after treatment with either haloperidol (to evaluate saturability), (+)-butaclamol (which has specificity for dopamine D2 receptors) or (-)-butaclamol (which has specificity for sigma sites). Binding was widespread. Treatment with (-)-butaclamol had no effect, whereas (+)-butaclamol selectively reduced the uptake in striatum. Haloperidol increased the clearance rate from all brain regions. These results indicate that the binding profile of [18F]haloperidol does not permit the selective examination of either dopamine D2 or sigma sites using PET.

Plasma input function determination for PET using a commercial laboratory robot.

A commercial laboratory robot system (Zymate PyTechnology II Laboratory Automation System) was interfaced to standard and custom laboratory equipment and programmed to perform rapid radiochemical assays necessary for plasma input function determination in quantitative PET studies in humans and baboons. A Zymark XP robot arm was used to carry out two assays: (1) the determination of total plasma radioactivity concentrations in a series of small-volume whole blood samples and (2) the determination of unchanged (parent) radiotracer in plasma using only solid phase extraction methods. Steady state robotic throughput for determination of total plasma radioactivity in whole blood samples (0.350 mL) is 14.3 samples/h, which includes automated centrifugation, pipetting, weighing and radioactivity counting. Robotic throughput for the assay of parent radiotracer in plasma is 4-6 samples/h depending on the radiotracer. Percents of total radioactivities present as parent radiotracers at 60 min, postinjection of 25 +/- 5.0 (N = 25), 26 +/- 6.8 (N = 68), 13 +/- 4.4 (N = 30), 32 +/- 7.2 (N = 18), 16 +/- 4.9 (N = 20), were obtained for carbon-11 labeled benztropine, raclopride, methylphenidate, SR 46349B (trans, 4-[(3Z)3-(2-dimethylamino-ethyl) oxyimino-3 (2-fluorophenyl)propen-1-yl]phenol), and cocaine respectively in baboon plasma and 84 +/- 6.4 (N = 9), 18 +/- 11 (N = 10), 74 +/- 5.7 (N = 118) and 16 +/- 3.7 (N = 18) for carbon-11 labeled benztropine, deprenyl, raclopride, and methylphenidate respectively in human plasma. The automated system has been used for more than 4 years for all plasma analyses for 7 different C-11 labeled compounds used routinely in our laboratory. The robotic radiotracer assay runs unattended and includes automated cleanup procedures that eliminates all human contact with plasma-contaminated containers.

Sinemet and brain injury: functional versus statistical change and suggestions for future research designs.

During the past decade the usage of dopaminergic agonists for the brain-injured population has become a more common treatment option during both the acute and subacute phases of recovery. We attempted to use Sinemet to address the functional limitations of a 74-year-old woman who was 7 months status post-traumatic brain injury secondary to a motor vehicle accident. The patient was administered a 3-month trial of Sinemet while continuing to receive structured sensory stimulation. Her baseline performance using Rappaport's Coma/Near-Coma (CNC) scale yielded a score in the moderate coma range. Weekly evaluations were conducted throughout the drug trial period using the CNC scale. Although the patient displayed modest improvement in her total score, functional change was negligible. Long-term follow-up suggested an absence of retention of earlier gains obtained on the CNC scale. The authors suggest that flawed research methodologies, heterogeneous patient populations, and potential difficulties obtaining reliable dependent measures makes the interpretation of brain injury research findings equivocal. However, given the many limitations, the current research design suggested that the long-term practical utility of Sinemet was unremarkable. Suggestions for future pharmacological research designs with the brain-injured population are discussed.

Depression of thalamic metabolism by lorazepam is associated with sleepiness.

Though it is well recognized that the pharmacological actions of benzodiazepines are mediated by facilitation of GABAergic neurotransmission, the consequences of these changes in regional brain function are not well understood. This study measured regional brain glucose metabolism using Positron Emission Tomography and 2-deoxy-2[18F]fluoro-D-glucose in normal controls (n = 21) investigated with and without lorazepam (30 micrograms/kg IV) and with flumazenil given after lorazepam (n = 9). Lorazepam markedly decreased metabolism in thalamus (23 +/- 8%) and occipital cortex (19 +/- 8%), and flumazenil partially reversed these changes. Changes in metabolic activity in thalamus were significantly correlated with lorazepam-induced sleepiness (r = .69, df 20, p < .0005) and there was a trend of an association between the reversal by flumazenil of lorazepam-induced change in thalamus and in sleepiness (r = .63, df 8, p = .07). Benzodiazepine-induced changes in thalamic activity may account for their sedative properties.